mono-concerted and systematic effort to create photoactive molecular wires that can serve asmodel systems for the understanding of electron and charge transfer through pseudo-one-dimensi
Carbon Nanotubes ees kg HH thớt 3
A “carbon nanotube” is a tube shape material, made of carbon, with the diameter on the nanometer scale Carbon nanotubes were firstly discovered by Japanese scientist,
In 1991, Sumio Lijima discovered carbon nanotubes while using the arc discharge method to produce carbon soot, leading to the accidental identification of a needle-shaped material Carbon nanotubes are categorized into two types: multi-walled nanotubes (MWNT).
Figure 1.1 MWNTs and SWNTs images (From REF-16 and 19)
Multi-walled nanotubes (MWNTs) consist of coaxially arranged layers ranging from 2 to 50, showcasing significant variations in electrical properties influenced by interlayer interactions Discovered in 1993, single-walled nanotubes (SWNTs) have gained attention as promising candidates for molecular wires and nanoscale devices due to their uniform structure, excellent electrical conductivity, and rigidity.
Carbon nanotubes are cylindrical structures formed by rolling up graphitic sheets with a hexagonal lattice Their electrical conductance can range from metallic to semiconducting, depending on the wrapping angle and diameter of the tubes.
Scanning tunneling microscopy and atomic force microscopy are essential tools for investigating the relationship between the structure and electronic properties of carbon nanotubes, enabling predictions about their electronic behavior Typically, carbon nanotubes are placed on two electrodes, such as platinum or gold, to measure conductance and resistance Due to their highly symmetrical structure, exceptional mechanical stiffness, and unique electrical characteristics, the molecular wavefunctions can extend along the entire length of the nanotubes, allowing them to function as true wires in a classical sense.
Figure 1.2 AFM image of a 3 pm long single wall carbon nanotube (From REF-20)
Professor Cees Dekker’s group (Delft University of Technology) have grown a SWNT
A nanotube, measuring approximately 3 wm in length and 1 nm in diameter, was positioned between two platinum electrodes separated by 140 nm At room temperature, the resistance measured between the contact points ranged from 300 to 550 kΩ, significantly lower than the typical resistance of 1 MΩ.
Figure 1.3 Schematic Design of Random Access Memory based on SWNTs (From
The three-dimensional view of a suspended crossbar array illustrates four junctions, with two elements in the ON (contact) state and two in the OFF (separated) state The substrate features a conducting layer, such as highly doped silicon (dark gray), topped with a thin dielectric layer like SiO2 (light gray) The lower nanotubes rest directly on the dielectric film, while the upper nanotubes are suspended by periodic inorganic or organic supports (gray blocks) Each nanotube is connected to a metal electrode represented by yellow blocks.
> fe (B) Top view of an n by m device array TÌ nanotubes in this view are represented by blac crossing lines, and the support blocks for tt
4 —————Ƒ as i TT ~~~~~~ —— | suspended SWNTs are indicated by light gre squares The electrodes used to address tt nanotubes are indicated by yellow squares.
Following the successful generation of single-walled carbon nanotubes (SWNTs) with metallic properties, research shifted towards utilizing carbon nanotubes in transistors and memory devices Dekker's group announced the development of a logic circuit using field-effect transistors (FETs) based on single carbon nanotubes, highlighting their advantageous features such as high gain, substantial on-off ratio, room-temperature functionality, and the capability for integration on a single chip This advancement marked a significant improvement, as previous nanotube FETs lacked the ability for individual on-off control.
A research team headed by Professor Charles M Lieber at Harvard University has developed innovative molecular devices and wires capable of reading and writing information These devices utilize suspended, crossed nanotubes that can be electrostatically toggled on and off, enabling the writing, reading, and erasing of data The schematic circuit design is illustrated in Figure 1.3.
Carbon nanotubes hold great potential for revolutionizing circuit miniaturization, particularly in the construction of logic gates However, several practical challenges persist, including the lack of uniform physical and electrical properties across synthesized nanotubes, which complicates their selection for industrial applications Additionally, the absence of specific binding sites for further functionalization limits their versatility in various applications.
DNA Molecules . càng HH HH HH rệt 6
DNA molecules are being explored as potential molecular wires due to their unique structural properties It is suggested that electrons and holes may transfer through the orbitals of the stacked bases in this highly symmetrical configuration One significant advantage of DNA is its considerable length, reaching up to 15 micrometers However, current research primarily addresses fundamental questions, such as whether DNA can conduct electricity and the underlying mechanisms of electron conduction Existing studies on charge transport through DNA have yielded conflicting results.
Recent research highlights the transition of DNA molecules from semiconducting to insulating properties, with ongoing efforts to enhance their metallic conductivity Despite the instability in predicting these conducting properties, advancements are being made to improve the conductivity of DNA Additionally, the ability of DNA to self-assemble at the micro-scale offers a distinct advantage for developing nanoscale devices compared to other materials.
Figure 1.4 AFM Image of an M-DNA Bundle on the Surface of Gold Electrode
(scale bar: 1 um) (From REF-21)
Conjugated Hydrocarbon Molecules Systems with Metal Centers
Homo-Bimetallic Systems Ăn 9
The Ru-B-Ru complex, where B represents the bridge ligand, was initially investigated by Creutz and Taube in 1969 This complex features Ruthenium metal centers coordinated with penta-amines, bridged by pyrazine The study explores the interactions between the two Ruthenium centers in a mixed oxidation state.
Ru”-Ru'” states showed a charge transfer band in the UV-Vis electronic absorption spectrum.
Recent advancements in synthetic methods have resulted in the creation of novel ruthenium-acetylide complexes Research by Beljonne et al focused on dinuclear ruthenium systems, examining the impact of various bridging ligands on their properties, particularly concerning the conjugated nature of the ligands and the extent of metal-to-metal interactions.
6 The series of the dinuclear ruthenium-acetylide complexes have the Cl-Ru- coupling
The study investigates the B-Ru-Cl architecture, focusing on the bridge component B, which includes various ligands such as 1,4-benzene, 1,3-benzene, 2,5-thiophene, and 2,5-pyridine The coupling strength between the two metal centers is quantified using the comproportionation constant, K, derived from cyclic voltammetry measurements: logk = 0.9 (E; -E\) Experimental findings reveal that the coupling strength varies with the type of bridge ligands, following the order: K(2,5-thiophene) > K(2,5-pyridine) = K(1,4-benzene) > K(1,3-benzene) Integrating these results with semi-empirical quantum modeling, the authors propose a three-step mechanism, beginning with the oxidation of one metal site to form Ru”.
The intervalence charge transfer state (IVCT) in Ru" induces significant electronic and geometric deformation, resulting in a quinoid character in the central aromatic ring Additionally, the oxidation of the mixed valenced complex leads to the formation of Ru”Ru” species.
The second step is highly sensitive to the nature of the bridge ligand the one with the largest contribution of quinoid type resonance gives the highest K, value.
Researchers in Tong Ren's group synthesized a series of polyyn-diyls, specifically [Ru2(2-anilinopyridinate)4](U-C,C’-Crm)[Ru2(2-anilinopyridinate)4] with varying m values (1-4 and 6) The synthesis utilized either a metathesis reaction between Ru2(ap)4Cl and LiC2,Li or a Glaser oxidative coupling reaction of Ru2(ap)4(CnH) under Eglinton/Hay conditions X-ray diffraction studies indicated that the species with C4 and Cs bridges exhibit a highly linear structure Furthermore, electrochemical and spectroelectrochemical analyses demonstrated that the species with C2, C4, Cg, and Cio bridges function as donor-acceptor systems, with the degree of Ru termini coupling being dependent on the length of the polyyn-diyl bridges.
Pt-B-Pt represents a unique homo-bimetallic centered system documented in the literature, with the ability to form polymers from Pt molecules A E Dray et al introduced an innovative synthetic method for producing Cl-Pt-C=C-Ph-C=C-Pt-Cl and its polymer -[-Pt-C=C-Ph-C=C- High-resolution electron microscopy of the cast films reveals the presence of linear-shaped microcrystallites, while the absorption spectra indicate specific bands originating from these structures.
The transition in absorption between monomers and polymers is influenced by transition metals, highlighting the evidence of metal hybridization At a temperature of 10 K, the luminescence emission spectra displayed a prominent peak at 2.4 eV, accompanied by a vibrational fine structure ranging from 1.8 eV to 2.3 eV.
Platinum complexes with diverse structures have garnered significant interest due to their potential applications in molecular devices, nonlinear optics, and liquid crystals R D Markwell and colleagues synthesized and studied the vibrational properties of various polymers.
Figure 1.5 Molecular Structures of Pt Containing Polymers (From REF-38) ly ly
The molecules exhibit either C2 or Dz local symmetry, with -conjugation in the backbone being monitored through changes in vibrational frequencies, particularly the vcac modes FTIR spectra reveal that the polymer backbones primarily feature single/triple bond alternation (-C=C-) without contributions from allene-type structures (=C=C=) Increased -conjugation leads to a reduction in C=C vibrational frequency due to diminished triple bond character Notably, Vczc frequencies are significantly lower in compounds with only acetylene units compared to those with acetylene moieties spaced between phenyl rings, where the phenyl group reduces the extent of -conjugation Brozik’s group investigated the monomer using cryo-luminescence, finding that the excited state of the compound was reduced from D2 to Cry, with the excited electron localized on one side of the diethynlbenzene ligand system The luminescent spectrum closely resembles that of the polymer developed by Markwell et al., and based on spectral shape, emission lifetime, and computational results, this emission is attributed to 3mMLCT mixed manifold phosphorescence, suggesting that the polymer's behavior reflects the repetitive nature of the individual units Other di-platinum centered monomers or polymers share a similar structure but differ in their substituents.
Re-B-Re represents a significant category of homo-bi-metallic centered systems, which is a focal point of this dissertation Typically, the bridging ligands in these systems consist of triple bonds, cyanides, 4,4'-bipyridines, and thiolates.
Yam’s group pioneered the synthesis of Re-C=C-C=C-Re type molecules using the Eglinton synthetic route, which involves homo-coupling terminal acetylene to create a butadiyne rhenium compound with anhydrous copper(II) acetate in pyridine This methodology is also employed in our research The Re center exhibits a slightly distorted octahedral symmetry, with three carbonyl groups arranged facially The molecule functions as a rigid linear wire along the axial direction, displaying bond angles in the O=C-Re-C=C-C=C-Re-C=O units that range from 175° to 178° Additionally, the authors conducted photophysical measurements, revealing a significant solvatochromic effect, with transition energy varying according to solvent polarity, following the order EO < THF.
Solvatochromism is commonly associated with metal to ligand charge transfers; however, the authors did not attribute this transition solely to pure MLCT absorption This decision was based on two main factors: the comparison of the monomer (C=C-H) with the butadiynyl-terminated version (C=C-C=C-H) and the specific characteristics of the C=C-C=C-Re complex.
The compound (‘Buzbpy)(CO)3 is expected to exhibit a weaker o-donor ability, potentially due to the Re center attracting electrons, which should lead to similar or higher absorption energies compared to C=C-C=C-H However, the authors noted a red-shift in the low-energy band relative to its monomer, indicating a discrepancy that may be influenced by the extinction coefficient.
A pure MLCT (dite — 1 ppy) would typically exhibit an extinction coefficient twice that of its monomer, yet the observed coefficient is even lower Additionally, luminescent rigidochromism is evident in the emission spectrum, with a notable red-orange emission at 690 nm in a solvent mixture of EtOH/MeOH (4:1 v/v).
At 298 K, the luminescence shifts from blue to yellow-orange (590 nm) in glass at 77 K, with the emission spectrum in degassed THF showing only a broad band and no fine structure The lifetimes in solution are short (< 0.1 ns) and exhibit a single exponential decay, indicating a luminescence profile likely associated with metal-to-ligand charge transfer (MLCT), although this assignment remains uncertain While the authors did not present a luminescence spectrum at 77 K, they noted a longer biexponential lifetime in an EtOH/MeOH glass (4:1 v/v) at this temperature (3.00 ns and 0.45 ns), akin to our findings in solid poly(methyl methacrylate) (PMMA) matrices, which will be detailed in this dissertation Additionally, the study reveals two distinct emissions depending on the excitation source; exciting the molecule with 350-400 nm light yields a long-lived luminescent band at 630 nm in degassed THF, while excitation above 450 nm produces a lower-lying emission band at 750 nm This observation suggests the presence of two competing processes post-excitation, challenging the Crosby-Kasha rule that states emission should be independent of the excitation wavelength.
630 nm band And the 730 nm band, the authors proposed to be either a “MLCT (dttre > TỶcsc.c=c.ge) Or a ÌLLCT (ffc=c.c=c.Re—> Ttuạy) transition, but excluded a IL
Hetero-Bimetallic Systems ônen 17
Molecular wires featuring polypyridines and transition metals such as Ru(1), Os(II), Rh(II), and Re(I) demonstrate strong luminescence These materials can function as either donors or acceptors, making them ideal for investigating charge and electron transfer processes.
Ru-B-Os represents a classic example of a hetero-bimetallic complex, leveraging the favorable donor-acceptor properties of ruthenium (Ru) and osmium (Os) Barigelletti et al were pioneers in synthesizing a range of Ru/Os dinuclear compounds, utilizing biscyclometalating bridging ligands that feature dipyridylbenzene fragments These fragments are interconnected by phenylene spacers, with variations in length (n = 0-2), culminating in a terminal di-2-pyridyl-1,3-benzene group The resulting systems exhibit metal-to-metal distances of 11, 15.5, and 20 Å.
The study examines the effects of varying phenyl spacer numbers on the electronic transitions in dinuclear species, revealing distinct UV-Vis spectral features Ligand-centered transitions dominate the UV region, while metal-to-ligand charge transfer occurs in the 500-550 nm range Notably, the absorption spectra of dinuclear complexes do not merely reflect the sum of their mononuclear forms, indicating significant interactions between the two metal centers in solution Additionally, luminescence spectroscopy demonstrates photoinduced Ru-Os energy transfer, with dual emissions from both Ru and Os centers mirroring their mononuclear counterparts The observed quenching of Ru-based luminescence allows for the calculation of the charge transfer rate using the formula k = [(1/t) - (1/%)] where t represents the lifetime of the Ru-based luminescence.
The lifetime of dinuclear compounds is characterized by energy transfer rates of 1.4 x 10°, 2.2 x 10', and 3.6 x 10° s⁻¹ for 0, 1, and 2 phenyl bridging groups, respectively Energy transfer mechanisms include Dexter and Förster; the Dexter mechanism involves simultaneous double electron exchange between donor and acceptor, while the Förster mechanism focuses on the interaction of transition dipoles for excited state deactivation and excitation For molecules with phenyl spacers, energy transfer is primarily attributed to electron exchange, with an attenuation factor B of approximately 0.33 Å⁻¹, aligning closely with McLendon's findings of B = 0.4 Å⁻¹ for phenylene spacers.
Benedikt Schlicke et al reported on longer systems with Ru/Os distances ranging from 24 to 42 Å, specifically Ru(bpy):”-(Ph)a-Os(bpy)s, where n = 3, 5, 7 Their study investigated the absorption spectra, luminescent properties, and energy transfer rates of these compounds The structural similarity of these molecules to those referenced in previous studies led to comparable absorption and emission spectra, indicating similar electronic transitions.
The study examined Ru-based and Os-based complexes, highlighting the quenching effects observed in the Ru-based complex Energy transfer rates were measured at 4.5 x 10°, 1.1 x 10’, and 1.4 x 10° s” for phenylene bridging groups of n = 3, 5, and 7, respectively Additionally, a linear relationship was established by plotting the natural logarithm of the rate constant (k) against the length of the molecules, resulting in an attenuation coefficient (B) of 0.32 A’.
Recent studies have highlighted various systems featuring multichelating ligands, primarily tris-bipyridyl and di-tripyridyl, coordinated to Ru and Os metal centers that serve as photoactive components These systems utilize diverse bridging ligands, including methylene, acetylene, phenylene, naphthyl, and anthryl, all of which are highly conjugated, except for the methylene group Notably, the alkane chain's limited electronic interaction leads to a Féster-type dipole-dipole mechanism, and its free rotation contributes to a floppy structure, which is less desirable for quantum wire applications that require rigidity In contrast, systems utilizing acetylene as a bridge demonstrate a remarkable increase in energy transfer efficiency, being 100 times more effective Among over 40 Ru/Os hetero-bimetallic molecules studied, the top three charge transfer molecules exhibit energy transfer rates of 500 x 10°, 250 x 10°, and 100 x 10° s⁻¹, respectively.
A through-bond Dexter-type mechanism is essential for efficient energy transfer in molecular systems, with an attenuation factor B < 0.32 A’ being optimal for long-distance energy transfer Ru(II) and Os(II) rods serve not only as molecular wires and models for studying energy and electron transfer but also as versatile molecular devices when the bridge group is selected carefully Notably, an azo bridge can function as a redox-responsive molecular switch, acting as an energy trap during excitation from terminal chromophores Additional applications, such as anion sensors and multi-photo harvesting systems, exist but are beyond the scope of this discussion.
Figure 1.8 Three Most Effective Charge Transfer Ru-Os Molecules (From REF-50)
While various hetero-bimetallic compounds have been synthesized, they lack the extensive study seen in the systematic Ru(ID) / Os(II) systems The research group led by V W-W Yam has been at the forefront of this chemistry, successfully synthesizing and characterizing compounds such as Re-Cu, Re-Ag, and Re-Zn.
The study investigates the basic spectroscopic properties of d°-d’ and d®°-d!° mixed rhenium (I) complexes, which exhibit complex luminescence characteristics Notably, when the acetylide ligand interacts with Cu(I) and Ag(), a blue shift is observed in the emission spectra compared to the monometallic components This shift may result from the reduced electron donor ability of the acetylide upon coordination, leading to a decrease in the energy of dm and an increase in the energy difference.
In the Re(I)-Zn(II) and Re(I)-Cd(II) complexes, solid-state luminescence was observed at 520-630 nm when measured at 77 K, indicating that the emission is likely due to 3MLCT (dre → T bpy) transitions Excitation spectra revealed bands at 450-500 nm when monitoring emission at 560-625 nm, suggesting the presence of dual luminescence The broadness of the emission bands and their dependence on the excitation wavelength imply that the higher energy emission could stem from either IL (bpy) or *LLCT (pter → Tuy), while the lower energy emission is attributed to MLCT (dre).
A novel synthetic method utilizing KPFa, KOfBu, and MeOH successfully synthesized a mixed-based metal center Re(I)-Fe(II) complex featuring a -C=C-Phenylene-C=C- bridge In the absence of the base KO’Bu, the resulting complex was identified as Re-C=C-Phenylene-C=C, rather than the expected Re-C=C-Phenylene-C=C-Fe The Re center exhibited a distorted octahedral geometry, while the bridging ligands maintained slightly distorted linear configurations Density functional theory (DFT) computations indicated that the hetero-bimetallic structure demonstrates enhanced properties.
The HOMO/LUMO energy levels are 1.53 eV, with Re-spacer and Fe-spacer at 2.17 eV and 4.16 eV, respectively Molecular diagrams reveal that iron and carbon ligands predominantly contribute to the HOMO, while the lower occupied molecular orbitals are characterized by a deficiency of Fe and an abundance of Re and alkynyl ligands This suggests moderate electronic communication between the two metal centers Additionally, the electronic spectrum displays a high-energy band at 290 nm and a low-energy band.
The absorption spectrum features a prominent peak at 390 nm, accompanied by a shoulder at 420 nm This high-energy band is attributed to an intraligand (IL) x — Tử transition involving 1,4-diethynylbenzene and bipyridine Additionally, the low-energy absorptions are tentatively identified as a combination of three metal-to-ligand charge transfer (MLCT) transitions.
The molecule in question exhibits no emission in either the solid state or in solution, likely due to an intramolecular quenching mechanism Based on computational and electrochemical analyses, the authors suggest that the quencher, [(CsMes)(dppe)Fe(C=C-CeH4-C=C)], absorbs light at both the excitation and emission wavelengths of [Re(CO)3(bpy)(C=C-CsH4-C=C)].
Several Ru(II)-Re(I) bi- and tetranuclear complexes were synthesized by B.
Multiple Metallic Šystems cà ceineeee 24
A Aviram and M A Ratner, “Molecular Rectifiers”, Chemical Physics Letters,
L A Bumm, J J Arnold, M T Cygan, T D Dunbar, T P Burgin, L Jones II,
D L Allara, J M Tour and P S Weiss, “Are Single Molecular Wires Conducting?” Science, 1996, Vol 271, p1705-1707;
In their 2001 article published in Chemistry - A European Journal, James M Tour and colleagues, including Adam M Rawlett and Masatoshi Kozaki, explore the synthesis and preliminary testing of molecular wires and devices The research focuses on the development of innovative molecular structures that could enhance electronic applications, highlighting the potential of these materials in advancing technology The study presents significant findings that contribute to the understanding of molecular electronics, paving the way for future advancements in the field.
Z J Donhauser, B A Mantooth, K F Kelly, L A Bumm, J D Monnell, J J. Stapleton, D W Price Jr., A M Rawlett, D L Allara, J M Tour, and P S. Weiss, “Conductance Switching in Single Molecules Through Conformational Changes”, Science, 2001, Vol 292, p2303-2307;
Yuji Okawaand and Masakazu Aono, “Materials Science: Nanoscale Control of Chain Polymerization”, Natutre, 2001, Vol 409, p683-684;
Philip G Collins, Michael S Arnold and Phaedon Avouris, “Engineering Carbon Nanotubes and Nanotube Circuits Using Electrical Breakdown” Science, 2001, Vol 292, p706-709;
Henk W C Postma, Tijs Teepen, Zhen Yao, Milena Grifoni, Cees Dekker,
“Carbon Nanotube Single-Electron Transistors at Room Temperature” Science,
M A Reed, J Chen, A M Rawlett, D W Price and J M Tour, “Molecular Random Access Momery Cell”, Applied Physics Letters, 2001, Vol 78, p3735- 3737;
Thomas Rueckes, Kyoungha Kim, Ernesto Joselevich, Greg Y Tseng, Chin-Li Cheung and Charles M Liber, “Carbon Nanotube-Based Nonvolatile Random Access Memory for Meolecular Computing”, Science, 2000, Vol 289, p94-97;
V Derycke, R Martel, J Appenzeller, and Ph Avouris “Carbon Nanotube Inter- and Intramolecular Logic Gates” Nano Letters, 2001, Vol 1, p453-456;
Sample Preparation 0n
Sample solutions for UV-Visible and liquid phase luminescence measurements were prepared as 1X10° M solutions in freshly distilled and degassed THF FTIR measurements were conducted at room temperature using a Thermo Nicolet AVATAR 360-FTIR spectrophotometer The compounds were dissolved in acetone, and a small amount was placed on the sample holder, allowing the acetone to evaporate while the spectra were collected from the resulting thin film.
For 77 K spectroscopic studies, all solid samples were prepared using poly(methyl methacrylate) (PMMA) matrices The PMMA, with a molecular weight of approximately 120,000, was sourced from Aldrich and utilized without further processing A sample weighing about 1 mg was combined with 100 mg of PMMA and dissolved in methylene chloride until the solution reached a noticeably viscous consistency.
h)230 n6 e
Absorption spectra were measured with a Shimadzu, UV-2401PC UV-Vis recording spectrophotometer using 1 cm quartz cells.
Steady state emission spectra were acquired using a 408 nm photodiode laser and a 488 nm Ar+ ion laser as excitation sources, with the 408 nm beam filtered through a 405 nm band-pass filter and a 500 nm short-pass filter before focusing on the sample in a liquid nitrogen bath The 488 nm excitation was directly focused onto the sample without filtration Emitted luminescence was monitored at a 90° angle to the excitation beam, filtered through a 500 nm long-pass filter, dispersed by an Acton 500i monochromator, and detected by a thermoelectrically cooled Hamamatsu R943-02 photomultiplier tube The detector signal was amplified and sent to a Stanford Research System SR400 photo counter, with parameters detailed in table 2.1, and data was transmitted to a PC for further analysis All spectra were corrected using a quartz-tungsten-halogen spectral irradiance standard lamp, with correction procedures outlined in appendix II.
The luminescence decay time measurement system utilizes a 488 nm excitation laser, modulated by an electronic-optic modulator (Conoptics, model M380C) to gate the beam A Stanford Research Systems DS245 function generator, operating at 300MHz, synchronizes both the modulator and the photon counter with a generated square wave controlling the chop frequency The parameters for the photon counter's lifetime measurements are detailed in table 2.2 Data from the photon counter is transferred to a PC for further analysis, with all lifetime data processed using a non-linear least squares curve-fitting program (IGOR, Wavemetrics Inc.).
Figure 2.4 Schematic Diagram for Steady State Emission Apparatus
405nm bandpass filter 500 nm shortpass filter
500 nm longpass filter FL.048mm FL = 152.4 mm
Table 2.1 Photon Counter Parameters Setup for Steady State Emission Experiments
COUNT=A, BRT PRESET TRIGSLOPEL AGATE=CW
TMEz TSET)s ADISC=HIXED BGATE=CW
ATN=STOP DAHL+3 BDISCSLOPEL BWDIHE*e*
DISPLAY=HOLD TDISCSLOPE= FALL COMMENU
PORT] = HXED RS2B2 BALD=%0 § SETUP MENU PORT 1 LVL@.000V RS22H1-8
LODCONIRAST PORT 2 = FIXED min >SIORE=I PO®T2LMW.=+00WWV 7
EXBOUIE RS32HHO=ŒFRECALL EXKOUIE=—> DATAZ
Figure 2.5 Schematic Diagram for Luminescence Lifetime Apparatus
500 nm ơ— | pe ——~-~” longpass filter F.L = 304.8 mm F.L = 152.4 mm
Table 2.2 Photon Counter Parameters Setup for Lifetime Experiment
TRIG SLOPE = RISE TRIG LVL =+0.100 V
B DISC SLOPE = FALL BDISC= FIXED BDISCLVL=-100.0 mV
T DISCLVL=+100.0 mV PORT | = FIXED PORT i LVL=+0,000 V PORT 2 = FIXED PORT 2 LVL =+0,000V
AGATE=SCAN =1.000 s ADELAY0 s AWDTH=1.0 s BGATE=SCAN =1.000 s BDELAY0 s BWIDTH= 1.0 s
(Note: A, B delay and width can be changed for different samples B s delay and width must be the same as those of A s.)
RS 232 BITS=8RS232 PARITY =NONERS232 WAIT =6RS232 ECHO = OFFDATA=
The apparatus utilized a Xe arc lamp (PTI, model A1010) as the excitation source, directing the beam onto a Chromax 500IS monochromator Emitted luminescence was collected and refocused onto a Roper Scientific Sp-150 monochromator, with detection performed by a liquid nitrogen-cooled CCD camera (Princeton Instruments, model 1340/100E/1) managed by a PT-138 camera controller The detector's signal was processed by a computer for analysis, enabling rapid spectrum acquisition by varying the excitation wavelength and capturing the emissive spectra with the camera.
2.4.5 77 K Fourier Transformed Infrared (FTIR) and Time-Resolved Infrared (TRIR) Spectroscopy
Low temperature FTIR spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer, with PMMA samples housed in a Janis cryostat During the FTIR collection process, the sample chamber was consistently purged with purified nitrogen gas, and all low temperature FTIR spectra underwent background correction.
Figure 2.6 Schematic Diagram for Excitation Experiments Apparatus
PIIPS2ĐAc Oe ees lam3pdy
Princeton Instrurrents FEV 100 1340F LV CCD PC
Figure 2.7 Schematic Diagram for Time-Resolved Infrared Experiments Apparatus
Nicolet Nexus 870 Step Temperature Controller
Time-resolved infrared spectra (TRIR) were acquired using 408 nm light from a Molectron POL-3 dye laser, which was pumped by a XeF excimer laser at 451 nm This method of "soft" excitation addresses the limitations of directly employing the higher-energy XeF excimer laser, which can easily damage the sample The samples were positioned in a 3 mm hole of a custom bronze sample holder, which was then affixed to a thermal base at 77 K.
The K cryostat (Janis Inc Model VPF-100) was outfitted with CaF2 windows for UV-Vis and IR experiments, ensuring optimal performance Temperature control was achieved using a calibrated silicon cryodiode (Lake Shore Cryotronics Inc model DT-470-SC-13-1.4L) in conjunction with a Lake Shore Cryotronics Inc Model 330 temperature controller The cryostat was precisely mounted in the spectrometer's sample compartment with a custom-made plate and aligned using a mechanical state The TRIR spectroscopic experiment environment was sealed around the cryostat and continuously purged with dry nitrogen Transient IR signals were detected using a Nicolet Nexus 870 step-scan FTIR spectrometer, with careful alignment of the excitation pulse and IR probe The IR probe's spectral range was constrained by a long pass filter (Janos F1305L300) and the CaF2 windows of the cryostat.
In a study utilizing a liquid nitrogen-cooled MCT detector, two sets of data were gathered through step-scan spectroscopy: ac-coupled and dc-coupled outputs The dc-coupled signal, recorded prior to laser activation, captured the static background spectrum, while the ac-coupled signal, synchronized with the laser's pulse train, provided dynamic time-resolved infrared spectral profiles The time-resolved spectra were represented as changes in absorbance (AA), calculated using the formula AA = - log (1 + Tayn/Istatic).
References cccccccccccsccssscssevcvssseeccsesecceeeussssevccsseescsunscsuvsececeseseauaesceeeeensssensss 60
1 Arnold J Gordon, “The Chemists Companion: A Handbook of Practical Data, Techniques and References”, John Wiley & Sons Inc., In experimental techniques, p429;
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Vivian Wing-Wah Yam, Victor Chor-Yue Lau, and Kung-Kai Cheung explore the synthesis and photophysical properties of luminescent rhenium(I) acetylide precursors, contributing to the development of organometallic rigid-rod materials Their study includes the X-ray crystal structure analysis of [Re(‘Buzbpy)(CO)3(BuC=C)] and Re(‘Buzbpy)(CO);C)], providing valuable insights into the structural characteristics of these compounds The findings highlight the potential applications of rhenium-based materials in advanced photonic technologies.
Vivian Wing-Wah Yam, Victor Chor-Yue Lau and Kung-Kai Cheung,
“Luminescent Rhenium(I) Carbon Wires:Synthesis, Photophysics, and Electrochemistry X-ray Crystal Structure of [ReCBu;bpy)(CO);(CC)Re(Bu;bpy)(CO):]”, Organometallics, 1996, Vol.
Vivian Wing-Wah Yam, Samuel Hung-Fai Chong and Kung-Kai Cheung,
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The article titled "Synthesis and Luminescence Behavior of Rhenium(II) Trinyl Complexes" by Vivian Wing-Wah Yam, Samuel Hung-Fai Chong, Chi-Chiu Ko, and Kung-Kai Cheung, published in Organometallics in 2000, explores the synthesis and luminescent properties of specific rhenium complexes The study presents detailed X-ray structures of two complexes: [Re('Busbpy)(CO)(C=CPh)] and [Re(Me;bpy)(CO)(CSCCCSiMe:)], contributing valuable insights into their chemical behavior and structural characteristics.
Vivian Wing-Wah Yam, Victor Chor-Yue Lau, and Kung-Kai Cheung,
“Synthesis, Photophysics and electrochemistry X-ray Crystal Structures of [Re(‘Buzbpy)(CO)3(CC) Re(Bu;bpy)], Organometallics, 1996, Vol 15. p1740-1744;
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9 The mass spectrometric analysis was done by the chemistry department of OhioState University.
Absorption and Emission ŠD€CfTOSCODY ánh ng na 62
3.1.1 Jablonski Diagram for General Cases
A Jablonski diagram illustrates the energy transfer processes in a molecule when excited by light When an electron transitions from the ground state (S0) to a higher excited state, this occurs as a vertical transition due to the Frank-Condon principle, which assumes nuclei remain stationary during the transition The excited state quickly decays to the lowest excited state (S1) through a rapid radiationless internal conversion Energy in higher vibrational states is released to the lowest vibrational state (v = 0) via vibrational relaxation Electron absorption can occur from the lowest vibrational level in the ground state to various vibrational levels in excited states, creating vibrational fine structure in the absorption band The excited state can return to the ground state (S1 — S0 or T1 — S0) by emitting a photon or through a radiationless process Due to the rapid internal conversion, vibrational relaxation, and intersystem crossing processes, fluorescence and phosphorescence originate from the lowest vibrational state (v = 0) in S1 and T1, respectively, indicating that emission occurs regardless of the initial excitation point.
The Jablonski Diagram illustrates transition processes in molecular systems, highlighting the distinction between radiative and non-radiative processes Solid arrows represent radiative processes such as absorption (A), fluorescence (E), and phosphorescence (F), while dashed arrows indicate non-radiative processes including internal conversion (B), vibrational relaxation (C), and intersystem crossing (D) The diagram features singlet electronic states (S₁, S₂, S₃) and a triplet electronic state (T₁), along with vibrational states (ν) that exist within each electronic state, providing a comprehensive overview of energy transitions in molecules.
Kasha's rule states that fluorescence transitions occur independently of the excitation wavelength, while the Kasha-Crosby rule applies to phosphorescence In fluorescence, the transition from the ground state (S0) to the excited state (S1) involves spin-allowed transitions without a change in multiplicity In contrast, phosphorescence involves radiative spin-forbidden transitions that are significantly influenced by spin-orbit coupling, with lifetimes ranging from several microseconds to tens of seconds.
The Jablonski diagram illustrates that both fluorescence and phosphorescence emissions exhibit lower energy than the absorption process Analyzing typical UV-Vis and luminescence spectra reveals that these spectra are mirror images of each other, with differences in the excitation and emission peak energies, known as the Stokes shift However, this mirror image rule can be violated when significant geometric changes in the molecular arrangement occur while in an excited state.
3.1.2 Energy Transfer Processes in the Organometallic Complexes
Luminescent complexes featuring transition metals, particularly nd° transition metals like Re(I), Ru(II), Os(II), and Rh(III), have garnered significant interest over the past thirty years These metals exhibit octahedral geometry, where the interaction with ligands causes the five d orbitals to split into a triply degenerate t2g level and a doubly degenerate eg level In these complexes, all six electrons are paired, occupying the three t2g orbitals.
Most luminescent complexes feature œ,œ -diimine ligands linked to metal centers The classification of their spectroscopic states is based on the promotion of orbitals from initial to final states Typical organometallic compounds and transition metal complexes with an octahedral structure exhibit various possible transitions, as illustrated in Figure 3.2 Transitions that occur solely within the metal-centered d orbitals are referred to as dd transitions, while charge transfer transitions involve interactions between the metal center's d-orbitals.
Figure 3.2 Schematic Orbital Diagram of Available Transitions of d° Metal in the
The octahedral crystal ligand field encompasses several key interactions: a) metal-centered dd transitions, b) metal-to-ligand charge transfer, c) ligand-to-metal charge transfer, and d) ligand mm transitions, which may occur within the same ligand or involve charge transfer between different ligands.
Vị! tư a b d ty ty Tt 1t
The promotion of an electron from a metal orbital to a ligand antibonding orbital results in metal-to-ligand charge transfer (MLCT), while the reverse process leads to ligand-to-metal charge transfer (LMCT) Transitions within the same organic ligand involve promoting an electron from a bonding to an antibonding orbital, whereas transitions between different ligands bound to the same metal center are classified as ligand-to-ligand charge transfer (LLCT) O,a’-diimine ligands are easily reduced, and backbonding from carbonyl ligands stabilizes the t; orbitals, resulting in intense MLCT transitions with extinction coefficients between 2000 and 6000 M^-1cm^-1 Although dd transitions are formally parity-forbidden and long-lived, they are weak and sensitive to environmental quenching In rhenium(I) complexes, the close energy levels of mm and MLCT excited states enable frequent transitions between them with minor changes in environmental conditions, such as ligand type, solvent, or temperature The significant atomic mass of Re enhances spin-orbit coupling, which increases the likelihood of phosphorescence and complicates state assignments due to the mixing of MLCT and mm states.
The emission characteristics of a molecule are influenced by the energy gap law, which states that a small energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) enhances nonradiative emission efficiency As a result, this leads to a decrease in the molecule's emissive properties, causing the luminescence bands to shift toward longer wavelengths (red shift).
The spin selection rule states that AS = 0, indicating that transitions with the same spin multiplicity are permitted Consequently, transitions from singlet (S) to singlet (S) and triplet (T) to triplet (T) are allowed, while transitions from singlet (S) to triplet (T) are forbidden Despite this, many triplet to singlet phosphorescence processes are observed, highlighting exceptions in practical applications.
Vibrational SD€CfTOSCODV SH HH HH Hit 67
Transition metals exhibit stronger spin-orbit coupling compared to lighter atoms, leading to distinct emission characteristics For instance, rhenium, being heavier than ruthenium, demonstrates a relaxation of the spin selection rule This results in rhenium having a more allowed emission with a shorter lifetime compared to ruthenium, despite their similar structures.
Light consists of transverse electric and magnetic fields, and its interaction with molecules occurs when it passes through them This interaction, specifically between the molecule's dipole moment and the electric field of the incident light, is the basis for infrared vibrational spectroscopy In quantum mechanics, this interaction can be accurately described through specific principles.
In quantum mechanics, the wavefunctions of the initial and final states are represented as | ®¡> and | ®;>, respectively The Hamiltonian operator, denoted as H, is defined as H - E, where represents the molecular dipole moment and E signifies the electric field of the incident radiation.
H=-e uEose!# 3.3 Removing the constant associated with the amplitude of the electric filed and the time- dependent part, the transition dipole moment is often written in the simple form:
For vibrational transitions to occur, the transition dipole integral must be nonzero, which can be determined using group theory and parity considerations The transition dipole operator possesses odd parity, necessitating that the wavefunctions involved have opposite parity to yield a nonzero integral In systems exhibiting inversion symmetry, transitions between states of the same parity are prohibited, allowing only transitions such as g to u or u to g, where g denotes even parity and u denotes odd parity, in accordance with the Laporte or parity rule However, the occurrence of vibronic coupling can distort the center of symmetry, enabling slight transitions between g and g, as well as u and u states.
“allowed”, thus the violation of the Laporte rule is sometimes observed.
In quantum theory, the harmonic oscillator model effectively describes vibrational processes by illustrating vibrational states that are evenly spaced The energy levels of these states are quantized and determined by this model.
The energy of a harmonic oscillator is given by E=hvo(v + 1/2), where hvo represents the zero-point energy and v is the vibrational quantum number, taking values of 0, 1, 2, and up to 3.5 Two key selection rules arise from this model: first, any transition must involve a change in dipole moment, and second, the change in vibrational quantum number must be AV = +1 While the harmonic oscillator serves as an idealization, real molecular systems exhibit anharmonicity, which slightly modifies these selection rules This anharmonicity allows for the observation of transition overtones, such as v=0 to v=2 and v=0 to v=3, as well as combination bands.
Raman spectroscopy, alongside infrared spectroscopy, is a significant type of vibrational spectroscopy According to the "exclusion rule," in systems with inversion symmetry, a vibrational transition can be either infrared active or Raman active, but not both simultaneously.
A molecule with N atoms exhibits three translational and three rotational degrees of freedom (two for linear molecules), alongside 3N - 6 fundamental vibrational degrees of freedom (or 3N - 5 for linear molecules), with each vibrational degree known as a normal mode The vibrational spectrum of a symmetrical system reflects its symmetry through corresponding normal modes, making group theory a valuable tool for analysis By identifying the point group of the molecule, one can apply symmetric operations to derive the character table, which aids in calculating normal modes using the formula n’ =(1/h) =x'(s)x(s) Here, n represents the number of normal modes, ẽ indicates the normal mode in the irreducible character table, h denotes the number of symmetry elements, and x'(s) and x(s) are the characters from the irreducible and reproducible tables, respectively.
OC———Re———CE—c==cT———R‡——CO co ZN }
The molecule Di-[(4,4' -Di-tert-butyl-2,2' -bipyridyl) tricarbonyl Rhenium acetylene] (Dimer) is classified within the Cạn point group Our focus is to determine the number of normal modes specifically associated with the acetylene stretches The irreducible character table relevant to this analysis is provided below.
The first step is to assign appropriate internal coordinates to the individual acetylene unit. Count the C=C as a whole unit and operate on it with each individual symmetry element.
If the C=C unit stays at the same position, it is counted as 1, and counts as 0 if it moves in space Thus, we have the reproducible table as:
Next the reducible representation can be decomposed into its irreducible characters by: n^#= (1/4)[(2X1) + (0X1) + (0X1) + (2X1)] = 1
In the analysis of acetylene stretches, it is established that n^" = 0, nŠ = 0, and n”" = 1, indicating one A and one By mode present According to the Laport rule, the symmetric Ag mode should not appear in the infrared spectrum However, a peak at 1981 cm⁻¹ in the FTIR spectrum is attributed to the antisymmetric stretch normal mode (B₁) The By stretching mode of the acetylene units is illustrated in figure 3.3, while the exclusion rule indicates that this mode is not Raman active, contrasting with the missing symmetric Ag mode, which is Raman active.
Figure 3.3 The Antisymmtric Stretch Mode for the Acetylene Groups
Time-resolved infrared (TRIR) spectra can be obtained using two primary techniques: interferometric and noninterferometric methods While noninterferometric laser-based approaches can investigate systems with lifetimes ranging from nanoseconds to femtoseconds, interferometric techniques remain the most widely used Within interferometric methods, there are four mechanisms of motion: rapid scanning, ultrarapid scanning, stroboscopic sampling, and step-scan, with overlapping capabilities for system lifetimes The first three methods are limited to dynamic ranges longer than microseconds, while the step-scan technique, developed in the 1960s and applied to TRIR spectroscopy in the 1990s, can probe faster transients up to the nanosecond level In scanning methods, a movable mirror operates at a constant velocity, whereas the step-scan technique involves the mirror remaining fixed for a specific duration before jumping to the next position This allows for the separation of time-dependent IR measurements from the dynamic events being studied The detector signals are processed through DC and AC coupled outputs, where the DC output provides the ground state spectrum and the AC output captures transient changes in the IR spectrum at various retardation time intervals.
In the 1970s, time-resolved Raman spectroscopy was already advanced, while time-resolved infrared (TRIR) experiments have emerged more recently, particularly for investigating charge transfer excited states generated by light absorption This technique is crucial for analyzing photo-induced energy transfer systems involving carbonyl (-CO), cyanic (-CN), and acetylene (-C=C) organic ligands, known for their significant oscillator strength and sensitivity to molecular structure and environmental changes TRIR has been widely utilized in organic and organometallic photo-luminescent systems with these ligands bonded to transition metal centers such as Re, Ru(I), Pt(I), Ti(IV), and W(0) The frequency shifts of these ligands in their excited states provide insights into the charge transfer direction and the system's behavior upon light absorption Typically, samples are prepared in condensed phases, either as liquids or solids, while the fundamental selection rules for vibrational spectroscopy remain applicable to TRIR spectra.
This technique measures the absorbance difference (AA) between static and dynamic signals The incident light of infrared (IR) is denoted as lọ, while I represents the ground state output IR beam that passes through the sample, collected in the DC current The difference between the excited state and the ground state is indicated as AI, which is derived from the AC current with the equation AI = I, — I, This process is grounded in the principles of the Beers-Lambert law.
After substitute the AI=1,— ly os _ Al it gives us the result AA = - log(1 + 7?
= -log (1 + AC output / DC output) 3.9
In the spectrum, the DC output signal is the static single beam and the AC output signal is the transient single beam Consequently, the absorbance difference can be calculated
The equation AA = -log [1 + (dynamic single beam / static single beam)] 3.10 illustrates the relationship between absorbance and beam dynamics The spectrum of absorbance versus wavenumbers reveals both peaks that grow in and those that bleach The bleaching effect results in a negative signal, indicating a depletion of species upon electronic excitation, while the grow-in effect produces a positive signal, signifying the formation of transient species This understanding allows for a more precise investigation into the excited states of complex chemical species.
Donald A McQuarrie, and John D Simon, Physical Chemistry, A Molecular Approach, University Science Books(1997);
E Roland Menzel, Practical Spectroscopy Serires, Volume 18: Laser Spectroscopy Techniques and Applications, Marcel Dekker, Inc., (1995);
J M Chalmers, and P R Griffiths (editors), Handbook of Vibrational Spectroscopy, Volume 1: Theory and Instrumentation, John Wiley & Sons, Ltd.(2002);
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Ott6 Horvath and Kenneth L Stevenson, Charge Transfer Photochemistry of Coordination Compounds, John Wiley & Sons, Litd(1992);
S D Ross, Inorganic Infrared and Raman Spectra, McGraw-Hill Book Company (UK), Ltd.(1972);
J N Demas, and B A DeGraff, “Design and Applications of Highly Luminescent Transition Metal Complexes”, Analytical Chemistry, 1991, Vol 63,
Derek Steele, Theory of Vibrational Spectroscopy, W B Saunders Company(1971);
James R Durig (Editor), A Series of Advances, Volume 14: Vibrational Spectra and Structure, E/sevier(1985);
Jon R Schoonover, and Geoffrey F Strouse, “Time-resolved VibrationalSpectroscopy of Electronically Excited Inorganic Complexes in Solution”,Chemical Reviews, 1998, Vol 98, p1335-1355.
Chapter 4Introduction of ab initio Quantum Chemistry
Fundamental of Molecular Orbital TheOry sành 76
The Schrödinger equation serves as the cornerstone of quantum chemistry, establishing a crucial link between the wavefunction and the energy of a given system Introduced by Erwin Schrödinger in 1916, this time-dependent equation plays a vital role in understanding quantum mechanics.
H is the Hamiltonian operator, and if it is invariant in terms of time, the time-independent Schrédinger equation collapses to the following form:
In equation 4.2, the first term represents kinetic energy, while the second term denotes potential energy, which encompasses the attractive forces between nuclei and electrons, as well as the repulsive forces among nuclei and electrons In multi-electron systems, the Born-Oppenheimer Approximation effectively simplifies calculations by allowing electrons to respond swiftly to nuclear motion due to the significantly larger mass of nuclei compared to electrons This approximation enables the decoupling of nuclear and electronic motions, allowing for the computation of electronic energies at fixed nuclear positions Consequently, the nuclear kinetic energy is considered independent of electronic motion, eliminating correlations in the attractive nuclear-electron potential energy, and treating the repulsive nuclear-nuclear potential energy as a constant for a specific geometry Thus, the electronic Hamiltonian can be reformulated accordingly.
Ha = Ke + Ven + Vee (4.3) And the electronic Schréndinger equation including the Born-Oppenheimer approximation is:
(Hat Vary By = (Kay t+ Vig + + Vn) Ea = EU (4.4)
In molecular quantum mechanics, the nuclear-nuclear repulsion energy (VNN) remains constant for fixed nuclear coordinates, allowing for the determination of wavefunctions without considering the Van der Waals term However, accurately addressing electron-electron correlation (Vee) poses significant challenges, leading to the adoption of a one-electron approximation that neglects the electron-electron repulsive interactions As a result, the wavefunction relies solely on fixed nuclear coordinates and the three Cartesian coordinates of a single electron, resulting in a total wavefunction represented as a product of individual electron wavefunctions, known as molecular orbitals (MOs) To derive MOs, trial wavefunctions or a "basis set" are constructed using a linear combination of atomic orbitals (LCAO), where © signifies the initial guess wavefunction and the @;’s represent atomic wavefunctions The collection of N @; functions constitutes the basis set, with coefficients aj indicating the contribution of each atomic orbital While a full basis set theoretically offers the highest accuracy, practical limitations necessitate truncating the set to a manageable number of basis functions.
With a truncated basis set in hand, we turn to evaluating the energy levels associated with our wavefunctions From the definition,
Variation theory allows for minimizing energy in quantum systems by adjusting coefficients in the Linear Combination of Atomic Orbitals (LCAO), leading to optimal variational wavefunctions Consequently, the condition 2 =o should hold true for all indices i By substituting equation 4.6 into the associated partial differential equation, we can construct a secular determinant.
To analyze the determinant, we must identify the values of Hii, Hy, Si, and Sj, subsequently solving for the N roots of E and all eigenfunctions using the LCAO-MO method In systems with planar unsaturated or aromatic hydrocarbons, the secular equation simplifies to Hückel theory, where Hy is equal to œ, and Hụ equals B for neighboring carbons, while remaining zero otherwise The term Sj represents the orbital energy of the singly occupied 2p orbital with sp’ carbon hybridization, where Sj equals 1 when i = j and 0 when i ≠ j Both B, the stabilization energy for the m bond, and the orbital energy are negative and not defined by specific numerical values, instead being semi-empirical in nature.
Hartree-Fock TheOrV - óc 5 HH HH ng ng HH ĐH cư 79
The formalism discussed relies on the one-electron approximation, which simplifies the analysis of many-electron systems by assuming that the total energy is the sum of the energies of occupied one-electron orbitals This method, known as the “effective Hamiltonian” approach, posits that the orbitals remain unchanged regardless of the electron count in the system Consequently, the wavefunctions derived from this technique are represented by the Hartree-product wavefunction.
For each electron, ỉ = Êỉ,, thus, the total energy and the whole wavefunction are given by:
The Hamiltonian derived from equation 4.8 lacks interelectronic repulsion, leading to significant systematic errors in the theoretical model In reality, the system's behavior is influenced by simultaneous pairwise interactions Consequently, the corrected Hartree Hamiltonian for a one-electron system can be reformulated to account for these interactions.
The V\(j\) term represents the interaction potential of a single electron with all other electrons in orbital j, correlating with the charge density of these electrons Despite the inclusion of electron-electron repulsion in the individual h; terms, the separable Hamiltonian operator remains valid, allowing the total energy to be expressed as the sum of one-electron wavefunction energies This approach treats the electrons as "non-interacting," implying that each electron experiences a constant potential from its counterparts.
In 1928, Douglas Hartree introduced the iterative "self-consistent field" method to solve quantum equations and determine energies and wavefunctions This approach begins with an initial guess of a wavefunction, which is used to evaluate the one-electron operator and update the guess in subsequent cycles The process continues until the energy differences between successive iterations are minimal or meet predefined convergence criteria However, a significant limitation of Hartree's method is that it did not enforce antisymmetry in trial wavefunctions To address this, three spatial quantum numbers (principal quantum number n, angular momentum quantum number l, and magnetic quantum number m) and one spin quantum number (mₛ) must be considered, leading to the construction of antisymmetric wavefunctions that reflect the fermionic nature of electrons Slater devised a practical method for creating these antisymmetric wavefunctions.
Each electron possesses distinct quantum numbers, adhering to the Pauli Exclusion Principle A key aspect of the Slater Determinant is its incorporation of quantum mechanical exchange By considering interelectronic repulsion energy and the antisymmetric characteristics of the wavefunction, two integrals emerge: the Coulomb integral (Jụ) and the exchange integral (Ky).
Vladimir Fock advanced Hartree’s Self-Consistent Field (SCF) methods by incorporating Slater determinantal wavefunctions, allowing Hartree-Fock (HF) molecular orbitals to account for both the Hartree-product component—representing the interaction of each electron with the static field of other electrons—and the exchange effect due to Coulomb repulsion in the context of the Slater determinant C C J Roothaan was the first to introduce matrix algebraic equations that extended the Hartree-Fock approach for application in molecular systems.
1951 Fora spin-restricted or closed-shell system (the spin multiplicity is one and all electrons are paired into atomic orbitals), “restricted Hartree-Forck” method is
The Hartree-Fock (HF) potential, V,"”(/), is set at 2 —- Kj To derive molecular orbitals (MOs), one must follow a process similar to simple molecular orbital theory and self-consistent field (SCF) theory This involves establishing a secular equation akin to Hückel theory, evaluating each matrix element, and employing SCF procedures to identify optimal wavefunctions Despite its theoretical foundation through Roothaan's approach, Hartree-Fock theory has significant limitations: it overlooks all electron interactions except for exchange in the one-electron Fock operator, and it generates a four-index integral for matrix element determination, necessitating N^4 total numerical integral solutions based on the chosen basis set (N).
The Hartree-Fock (HF) theory faces significant challenges when applied to molecular systems, primarily due to its computational complexity and inherent systematic errors It often overestimates the occupation of bonding orbitals, resulting in inaccurately short bond lengths, particularly with saturated basis sets Additionally, HF dipole moment calculations show insensitivity to basis set size beyond valence double-zeta (DZ) and typically overestimate dipole moments by 10-25% The theory's inability to account for electron correlation limits its effectiveness in modeling non-bonding interactions such as dispersion, hydrogen bonds, and electrostatic interactions, ultimately leading to a flawed representation of molecular-molecular interactions.
Density Functional Theory (DFT) is an ab initio quantum chemical modeling method that utilizes electronic density p(r) to determine the external potential, Hamiltonian, wavefunctions, and molecular orbital energies This approach improves upon the Hartree-Fock (HF) method by focusing on physical observables for energy determination The Hamiltonian is derived from the positions and atomic numbers of nuclei, along with the total number of electrons By integrating the known electronic density p(r) over all space, the total electron count (N = | p(r)dr) can be established, allowing for the calculation of the Hamiltonian Once the Hamiltonian is defined, the Schrödinger equation can be solved to obtain the wavefunctions and energies of the system.
The formalism of DFT was first proposed by Hohenberg and Kohn in 19640),
The existence theorems established that a functional can accurately describe the non-degenerate ground state and electronic density of a molecular system In 1976, Gunnarsson and Lundqvist demonstrated that Hohenberg-Kohn's existence theorem supports the application of the variation theorem to density functional theory, adhering to the variation principle of molecular orbital theory, which states that the energy calculated from a candidate wavefunction must be equal to or greater than the true ground state energy.
The application of the above theorem to the Schrödinger equation faces challenges primarily due to electron-electron interactions in the Hamiltonian, lacking the simplifications found in the Hartree-Fock (HF) method Kohn and Sham (1965) advanced the Density Functional Theory (DFT) by introducing a non-interacting electron system within the Hamiltonian This approach allows the Hamiltonian to be represented as a sum of one-electron operators, leading to eigenvalues that are simply the sum of these one-electron eigenvalues Consequently, these eigenvalues create a density functional by partitioning the energy into multiple components.
In the context of electronic structure calculations, ET represents electronic kinetic energy, while EY encompasses nuclear-nuclear and nuclear-electron potential energy E’ denotes electron-electron repulsion potential energy, and E*© reflects exchange energy and dynamic electron correlation energy Although E*© enhances the Hartree-Fock (HF) method, it poses significant challenges in density functional theory (DFT) calculations due to the unknown exact functional A comparison of HF and DFT reveals distinct differences: HF is an intentionally approximate theory that allows for exact solutions of the Schrödinger equations, whereas DFT is an exact theory that can only be solved approximately due to the absence of a crucial operator.
Up to now, the art in the DFT method is to develop more exact functionals of the
The E* term plays a crucial role in this study, which has involved significant effort In practical applications, E* is divided into exchange and correlation terms, represented as EXC = E* + E° Two primary approximations are utilized in the development of E*©: the local spin density approximation (LSDA) and the generalized gradient approximation (GGA) While LSDA relies solely on the local density value (p), GGA incorporates additional factors for a more comprehensive approach.
The "non-local" Density Functional Theory (DFT) significantly enhances the Local Spin Density Approximation (LSDA) by incorporating both local density and gradient density (p & Ap) A key advancement in this area is the Generalized Gradient Approximation (GGA) exchange functional developed by Axel Becke in 1988, commonly referred to as "B." The naming convention for exchange or correlation functionals typically includes the initials of the authors' last names, often accompanied by the year of development Other notable exchange functionals include B86 (Becke, 1986) and PW (Perdew and Wang, 1986).
The exchange and correlation functionals in Density Functional Theory (DFT) are crucial for accurate computations, with notable examples including LG (Lacks and Gordan, 1993) and PBE (Perdew, Burke, and Ernzerhof, 1996, 1997) Popular Generalized Gradient Approximation (GGA) functionals such as P86, PW91, B95, and LYP are designed to compute full correlation energy The naming convention for these functionals combines the acronyms, as seen in the BLYP functional, which integrates Becke’s GGA exchange with the correlation functional from Lee, Yang, and Parr DFT is classified as an ab initio method because these functionals utilize GGA exchange and correlation in a 1:1 ratio, categorizing them as pure DFT methods.
Basis Sets nh ố ố ố ố ằ
In Hartree-Fock (HF) theory, molecular orbitals (MOs) are represented as linear combinations of atomic orbitals, with coefficients derived from the iterative solution of the HF self-consistent field (SCF) equations While an infinite set of atomic orbitals would provide the highest computational accuracy, practical limitations prevent their use Consequently, quantum chemists must select a smaller, yet chemically relevant, subset of atomic orbitals for effective calculations.
Molecular Orbital (MO) theory offers attractive features similar to hydrogen-like orbitals, but it faces significant challenges in ab initio methods due to the complexity of solving four-index integrals numerically To address this issue, Gaussian-type orbitals (GTOs) are employed, which are constructed from atomic orbital (AO)-like functions GTOs include s-type, p-type, and d-type orbitals, with s-type GTOs corresponding to one regular s orbital and p-type GTOs representing three p orbitals (px, py, pz) In contrast, d-type GTOs consist of six Cartesian d functions: x', y', z', xy, xz, and yz, rather than the commonly used xy functions.
Gaussian-type orbitals (GTOs) offer computational simplicity; however, their radial distribution shape limits their effectiveness To leverage the advantages of both GTOs and Slater-type orbitals (STOs), GTOs can be combined to create a new function that, when optimized, achieves the accuracy of an STO In 1969, Hehre, Stewart, and Poplet® developed the STO-MG basis set, which stands for a Slater-type orbital approximated by M Gaussians, with M=3 providing the best balance of speed and accuracy.
The STO-3G basis set, also known as the single-É or minimum basis set, features one basis function for each type of orbital from core to valence, such as 1s functions for H and He, and 1s, 2s, and 2p functions for Li to Ne For enhanced accuracy, double-É or triple-¢ basis sets can be constructed, increasing the basis set size toward the Hartree-Fock (HF) limit, exemplified by correlation-consistent polarized Core and Valence Double/Triple Zeta basis sets (cc-pCVDZ or cc-pCVTZ) Given that electrons exhibit greater flexibility in the valence shell compared to the core, "split-valence" basis sets were created, which utilize single basis functions for core orbitals while dividing valence orbitals into multiple functions Common split-valence basis sets include 3-21G, 6-21G, 6-31G, and 6-311G.
Molecular properties such as vibrational energy and inversion barriers are sensitive to polarization, and traditional AO-like Gaussian-type orbitals (GTOs) fall short in accurately representing these properties due to the atom-centered nature of s and p orbitals To address this limitation, incorporating a polarization function is essential Pople and colleagues established a straightforward nomenclature for these additional polarization functions, where the inclusion of d functions to enhance the p functions in a basis set is denoted by an asterisk (*) following the basis set name.
“* indicates the addition of a set of p functions on H and He For example, 6-31*, and 6-31**,
In supermolecular complexes and certain excited electronic states, the electron density tends to be spatially diffuse due to their loose structural arrangements, which can lead to inaccurate predictions of chemical properties sensitive to this diffusion To address this, diffuse basis functions are added to standard basis sets The notation “+” before the standard basis set signifies that heavy atoms are enhanced with an additional s and p function Additionally, for properties such as acidity and electron affinities, the inclusion of diffuse functions is essential for accurate calculations.
Effective Core Potentials (ECPS) - Q SH HH ng ng ng nh kg xà 90
Predicting and addressing heavy atoms poses significant challenges to molecular orbital (MO) theory due to the large number of electrons and the extensive basis sets required The complexity of electron-electron correlations leads to substantial computational demands To mitigate these issues, Hellmann proposed effective core potential (ECP) functions in 1935, which accurately and efficiently represent the interaction between the nuclear-electron core and the remaining electrons ECPs account for Coulomb repulsion effects and adhere to the Pauli Exclusion Principle, allowing for the removal of relativistic effects when determining an appropriate wavefunction for the valence electrons.
A “large-core” ECP includes everything except for the valence shell, while a
Small-core effective core potentials (ECPs) omit the valence and sub-valence shells, yet the polarization of the sub-valence shell is crucial for many heavy atoms As a result, small-core ECPs are more frequently utilized and rigorously tested compared to large-core ECPs Currently, the most prevalent ECP is the LANL (Los Alamos National Laboratory) model.
Alamos National Lab) ECPs developed by Hay and Wadt in 19853, All quantum chemical modeling in this dissertation employed the LANL2DZ ECP and pseudopotential.
Electronic Excited States CalculafIOTS án rườ 91 “Ẵ: ch
This dissertation employs time-dependent density functional theory (TDDFT) as the primary method for calculating excited states TDDFT utilizes the rapid phase approximation (RPA) and the polarization propagator (PP) technique to analyze the evolution of time-dependent properties By leveraging the PP technique, it is possible to predict the poles of frequency-dependent polarizability using only the ground-state wavefunction, which reacts to a time-dependent external electronic field, without requiring knowledge of excited state wavefunctions or their energies The PP Hamiltonian is typically truncated to a finite order, often first-order, making it particularly efficient for calculating single electron excitations The poles of the PP, which define the valid range for perturbations, are determined by the predicted ionization potential and electronic affinity derived from ground-state calculations Notably, TDDFT is the sole excited state technique applicable within the DFT framework, as it relies on the polarization propagator for excited state energy calculations, lacking a generalized approach for excited-state frequency assessments.
Configuration-Interaction (CI) calculations serve as an alternative method for modeling excited states by promoting a single electron from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO) This approach enhances the accuracy of predicted excited state energies and properties through electron correlation among excited state configurations While CI can effectively predict optimized geometries and frequencies unattainable by Time-Dependent Density Functional Theory (TDDFT), it faces limitations due to Hartree-Fock (HF) constraints In particular, for the systems examined in this dissertation, CI calculations were hindered by the extensive basis set required.
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W Kohn, and L J Sham, “Self-Consistent Equations Including Exchange and Correlation Effects”, Phys Rev., 1965, Vol 140, pA1133-1138;
A D Becke, “Density-Functional Exchange-Energy Approximation with
C J Cramer, Essentials of Computational Chemistry, Theories and Models, Chapter 8, “Density Functional Theory”, John Willy & Sons, Ltd (2002);
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W J Hehre, R F Stewart, and J A Pople, “Self-Consistent Molecular-Orbital Methods I Use of Gaussian Expansions of Slater-Type Atomic Orbitals”, J. Chem Phys., 1969, Vol 51, p2657-2664;
R Krishnan, M J Frisch, and J A Pople, “Contribution of triple substitutions to the electron correlation energy in fourth order perturbation theory”, J Chem. Phys., 1980, Vol 72, p4244-4245;
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P Jeffrey Hay, and Willard R Wadt, “Ab Initio Effective Core Potentials for Molecular Calculations Potentials for the Transition Metal Atoms Sc to Hg”,
Chapter 5 Spectroscopic Study and Computational Results of fac-Rhenium(I)(CO),(4,4-2-butyl-2,2'-bipyridine)Cl
Research into the spectroscopy, photophysics, and photochemistry of low-spin d° Re(I)(œ,œ-diimine)(CO)aL complexes remains a focal point for scientists due to their fascinating optical luminescence properties and unique excited state characteristics.
The charge transfer excited states of Re(I) diimine complexes exhibit properties that make them suitable for facilitating charge separation in complex chemical systems Extensive research has been conducted on molecules featuring 2,2'-bipyridine or 1,10-phenanthroline as ancillary ligands, which influence the classification of the five metal d orbitals into two sets: e and t In the ground state, six d electrons occupy the t orbitals, while the diimine ligands can easily accept electrons, promoting transitions from metal to ligand orbitals Low-temperature emission studies reveal a predominant metal-to-ligand charge transfer (MLCT) configuration, with phosphorescence being enhanced by strong spin-orbit coupling, particularly in heavy atoms like rhenium The temperature dependence of phosphorescence lifetimes provides insights into spin-orbit coupling and associated rate constants Re(I)(diimine)(CO)3Cl complexes demonstrate effective photocatalytic reduction of CO2 to CO and have been pivotal in validating the energy gap law Additionally, pyridine and its derivatives serve as ancillary ligands, with the non-radiative rate's natural logarithm showing a linear relationship to the maximum energies of MLCT luminescence This chapter presents a foundational overview of the photophysics of these compounds, comparing them to those discussed in subsequent chapters.
The color of fac-Re(CO);(4,4'-¢-butyl-2,2'-bipyridine)Cl in THF solution is yellowish.
As illustrated in figure 5.1, the electronic absorption spectrum exhibits two absorption regions The low energy broad band centered at 379 nm has an extinction coefficient of
The absorption band at 3250 dm⁻¹ mol⁻¹ cm⁻¹ indicates a metal-to-ligand charge transfer (MLCT), aligning with earlier findings that compounds with MLCT as their lowest excited state typically exhibit strong absorption bands ranging from 2000 to 6000 dm⁻¹ mol⁻¹ in the visible spectrum Additionally, within the 250-300 nm range, significant absorptions are attributed to spin-allowed ligand-centered transitions, including a notable bpy 2m absorption peak at 292 nm.
Figure 5.1 Room Temperature Absorption Spectrum in THF
Re(CO)3(4,4' butyl-2,2'-bpy)Cl1 exhibits exceptional luminescence at both room temperature and lower temperatures Its photochemical stability is notable in both fluid solutions and PMMA plastic films, as demonstrated in the emission spectrum illustrated in figure 5.2.
Figure 5.2 Excitation and Emission Spectra in PMMA at 77 K Excitation: solid line, monitoring at 580 nm; Emission spectrum: dashed line, excited at 408 nm.
Figure 5.3 Lifetime Measurement Result ọ aa ÁÁ A? 10 b rN BA a ahaa
Cay ar a Cay ,ÀA a^ a ama ar elie -20 a Me A Aa
0.4 structureless band centered at 547 nm, which is a nice mirror image of its excitation and absorption spectra.
Figure 5.3 illustrates the lifetime measurement of fac-Rhenium(I)(CO)3(4,4'-c-butyl-2,2'-bipyridine)Cl in PMMA at 77 K, showcasing experimental results represented by hollow round dots The red curve indicates the best exponential fit using Igor Pro from Wavemetrics Inc., while the black triangles display the distribution of the residues The analysis reveals a lifetime of 3.77 µs, which is comparable to the 3.4 µs lifetime of fac-Rhenium(I)(CO)3(2,2'-bipyridine)Cl and the 3.3 µs lifetime of another related compound.
Rhenium(1)(CO);(4,4'-methyl-2,2'-bipyridine)Cl This long-lived lifetime is consistent with the nature of a spin-orbit perturbed phosphorescence.
The FTIR spectrum of compound Re(CO)₃(4,4'-butyl-2,2'-bpy)Cl, both in its ground state and after laser excitation at 408 nm, is depicted in Figure 5.5 The average AA spectrum shown in Figure 5.5(a) is derived from the first 1.0 µs slices of five scans across two experiments, totaling 75 laser shots per mirror position and an overall data collection duration of 80 minutes for each experiment The spectral resolution achieved was 8 cm⁻¹.
The absorbance difference is calculated using the formula AA = —log (1 + AI / I), where I represents the static IR spectrum of the IR source with the sample, and AI denotes the dynamic IR spectrum influenced by photo-excitation This process results in a depletion of the ground state distribution, leading to negative peaks known as grow-ins In the ground state absorbance spectrum of Re(CO):(4,4'-butyl-2,2'-bpy)Cl, three carbonyl (CO) stretching vibrations are observed at 1893, 1916, and 2019 cm⁻¹, corresponding to the A", A'(2), and A(1) stretching modes in the Cs point group The TRIR spectrum reveals three bleaches (negative peaks) at the same energies as the ground state vibrations, indicating a reduction in ground state molecule concentration due to laser excitation, alongside the emergence of three distinct peaks at 1951, 1974, and 2044 cm⁻¹.
They are the three v(CO) vibrational modes in the excited state.
Figure 5.4 Three Carbonyl Stretching Modes in Cs Point Group oy a a
`“ Ny aa N= N Quy ` om ` nh ` "Re Re
Re - L oom ƒ N oe 4 Na of N oc At
Figure 5.5 FTIR (bottom) and TRIR (top, 1 1s time slice) Spectra
The quantum chemical modeling was achieved with the Gaussian 98W version 5.2 revision A.7 computational chemistry software suite Computations were performed on a
The PC operates on Microsoft Windows 98 and features a 1.6 GHz AMD Athlon XP 1900+ microprocessor, 512 MB of Corsair PC2400 Cas2 DDR SDRAM, and a 17 GB Seagate 318452LW SCSI hard drive.
Ground state calculations were conducted using the B3LYP hybrid functional from Density Functional Theory (DFT), optimizing the molecular geometry with either the LANL2DZ basis set, which incorporates an electron core potential, or a mixed basis set of {[8s7p6d]/[6s5p3d]}-GTO for the valence shell of Re, alongside 3-21G for lighter atoms, complemented by the Stuttgart-Dresden (SDD) effective core potential for Re Additionally, FTIR frequencies and relative intensities were calculated using the FREQ keyword with the NoRaman switch in the Route Section of G98W.
Figure 5.6 illustrates the perspective drawing of Re(CO)3(4,4'-t-butyl-2,2'-bpy)Cl, with atomic labels, while hydrogen atoms are omitted for clarity Key results, including selected bond angles, bond lengths, and Mulliken charge distributions, are summarized in Table 5.1 Additionally, the frontier molecular orbital diagrams are shown in Figure 5.7.
The analysis of data in Table 5.1 reveals that while neither method achieves complete accuracy, the mixed basis set yields results that are more aligned with experimental data compared to the LANL2DZ basis set used across all atoms Most calculated properties show comparable outcomes, with the exception of the Mulliken charge distribution Additionally, the bond angles and lengths indicate a distorted octahedral geometry, characterized by three carbonyl ligands arranged in a facial configuration.
The optimized geometry reveals that the bipyridine ring surface bends towards the chlorine atom, with a Cl-Re-N angle of approximately 80-82° Additionally, the N-Re-N bond angle measures around 74-75°, significantly lower than the 90° needed to accommodate the steric demands of the chelating bipyridyl ligand, resulting in a pseudo Cs point group structure Frequency calculations indicate that the LANL2DZ method significantly underestimates experimental frequencies, while a mixed basis set provides a more accurate estimation due to the incorporation of ECP pseudo potentials.
B3LYP/LANL2DZ | B3LYP/ SDD-3-21G | Experimental
*The data of experimental selected bond angels and experimental bond lengths are from the reference 9. Š The experimental AE is from the electronic absorption spectrum.
Figure 5.7 Molecular Orbital Diagrams Constructed from Geometries Optimized by
DFT Calculations on Two Kinds of Basis Sets
B3LYP/ LANL2DZ B3LYP / SDD-3-21G
The lowest electronic excitation can be understood as the transition of an electron from an occupied molecular orbital to a virtual molecular orbital, typically moving from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) By analyzing the HOMO and LUMO, insights into the nature of the lowest transition can be gained Notably, the HOMOs and LUMOs obtained from two different calculations exhibit similarities.
HOMOs are composed of 5dpe orbitals that are 7 bond bounded to the co orbitals and
Introduction ccceeescecessseeteceseccssseeeceesesssesseesssesssssseessesseessaeesssensesessessees 111 1/1 1
Several _fac-Re(I)(CO)3(4,4'-t-butyl-2,2'-bpy)(C=C),nL, momo-metalic centered carbyne complexes have been synthesized according to the literature methods.
This chapter examines the spectroscopic properties of compounds with varying numbers of acetylene ligands (n = 1 - 3) and different terminal groups (L = H or —Si(CH3)3) Our findings reveal that these compounds exhibit significantly different spectroscopic behaviors compared to the parent compound, /zc-Re()(CO):(4,4-/-butyl-2,2-bpy)Cl The incorporation of acetylene ligands enhances the accessibility of excited states and alters their characteristics, while the terminal groups also significantly influence the emission properties.
Figure 6.1 Structures of Mono-Metallic Compounds oc——Re-———= SiMe oc oc co
OC—Re—==—==——==—SiMe; oa oc co x x = Ss ooTRE ==-—==H Re(CC);SiMe; 2) ÓC CO Re(CC);H (5)
Figure 6.2 Absorption Spectra of Compounds 1 and 2 in THE Solution Compound
(1): solid line; Compound (2): dashed line.
3 £ dm mol +> 20 oOo Sa aa
Figure 6.3 Absorption Spectra of Compounds 3, 4 and 5 in THF Solution Compound
(3): solid line; Compound (4): dashed line; Compound (5): dotted line.
The complexes exhibit yellowish colors with slight variations, as illustrated in the absorption spectra of five compounds shown in figures 6.2 and 6.3 The spectra are characterized by a low-energy broad band and intricate high-energy bands around 300 nm, attributed to the long-axis mm transition of the Re-acetylide chromophore Notably, compound 1 displays a broad band centered at 403 nm with an extinction coefficient of 11,480 dm*mol'cm, while compound 2, Re(CC)3SiMe3, has a smaller band center at a similar wavelength with an extinction coefficient of 3,561 dm*mol'cm This broad band is likely due to a metal-to-ligand charge transfer electronic transition, with spin-allowed ligand-centered transitions occurring in the 220-300 nm range, including a peak at 296 nm corresponding to the mm tụy transition.
Tm cac It is noteworthy that in the spectrum of compound 1, the TT tụy transition is stronger with a weak shoulder at 279 nm, which is assigned to a TK c=c transition.
In contrast to compound 1, compound 2 exhibits a more pronounced peak at 277 nm, while the peak at 296 nm appears as a shoulder This change is attributed to the influence of longer conjugated ligands found in the extended acetylene structure.
The spectra of compounds 3, 4, and 5 exhibit similarities in both high-energy and low-energy regions In the high-energy region, the spin-allowed lan transition peaks linked to acetylene ligands show a noticeable red shift as the acetylene bridges increase in length Conversely, the low-energy region features a weak yet distinct band ranging from 350 nm to 450 nm, attributed to a spin-forbidden So → Ti charge transfer transition Notably, the maximum peak positions of the broad bands and their extinction coefficients remain relatively constant.
Five mono-metallic compounds exhibit high emissivity in solution and PMMA plastic film at both ambient and cryogenic temperatures Figures 6.4(a)-(e) illustrate the normalized luminescence excitation and emission spectra of these complexes at 77 K within PMMA plastic film The luminescence profiles for compounds 1, 3, 4, and 5 are characterized by a single, broad, and structureless band, which is typical for this class of materials.
The excitation and emission spectra of target compounds in PMMA matrices at 77 K are illustrated in Figure 6.4 Solid lines represent the excitation spectra monitored at emission maximum positions, while dashed lines depict the emission spectra excited at 408 nm The compounds analyzed include ReCCSiMe, Re(CC)SiMe, ReCCH, and Re(CC)H.
Re(CO)s(œ,œ-diimine)L compounds®® and generally assigned to a “MLCT — ‘gs.
The transition of MLCT (dare — Tùạy) for the parent compound fac-Re(I)(CO)3(4,4'-t-butyl-2,2'-bpy)Cl can be effectively analyzed using DFT quantum chemical modeling The emission spectrum of compound 2 is complex, featuring three prominent peaks within a broad band envelope Additionally, quantum chemical modeling and luminescence lifetime measurements indicate that the nature of the orbital transitions is more intricate than initially anticipated Notably, all excitation spectra exhibit similar maxima around 390-410 nm, with most showing a good mirror image of their corresponding emission spectra.
Table 6.1 presents the maxima for the excitation and emission spectra, along with the phosphorescence lifetimes of various compounds in PMMA plastic films Most decay curves, excluding compound 2, were fitted to a single exponential model Notably, compound 2 exhibited a luminescence decay at 500 nm with a lifetime of 632 µs, while at the broad band envelope of 620 nm, it predominantly followed a single exponential decay with a lifetime of 4.96 µs, comparable to the other compounds.
Table 6.1 Photophysical Data for Mono-Metallic Compounds in PMMA at 77 K.
Compound EX-Aimax /nm EM-A max /nm Tv, Us
Re(CC):H 410 577 3.17 a, measured at 500 nm peak position, fit into a single exponential; b, measured at
620 nm, also fit into a single exponential; all other lifetimes were measured at theirEM-Amax positions.
The FTIR spectra of mono-metallic compounds, measured in super-thin films from freshly dried acetone solutions and PMMA plastic films, are presented in figures 6.5 - 6.9 The IR spectra from both media are nearly identical, with minor peak position shifts likely attributed to rigidochromism in PMMA Within the 1800 cm⁻¹ to 2300 cm⁻¹ range, the spectra exhibit carbonyl stretching and carbon-carbon triple bond stretching modes Group theory analysis indicates these molecules possess C; symmetry, allowing for three carbonyl normal modes—two symmetric (A'(1), A2) and one antisymmetric (A")—to be observed Furthermore, each acetylene unit in the complexes displays one distinct band in accordance with the selection rules.
Calculation Results .cccccecscccscessseceresseeeesscessesssesesssesssesesaesssenesssseessees 118
The computations for complexes 1 to 5 were conducted in the gas phase using the LAN2DZ basis set, employing both the restricted Hartree-Fock method and the B3LYP functional within the G98W package Time-dependent DFT calculations provided insights into the lowest three singlet and triplet states derived from the DFT optimized singlet ground state geometry Notably, the calculated triplet excited states are multiplicity forbidden, resulting in an oscillator strength (f) of zero; however, this still offers valuable insights into the nature of triplet transitions The chemical modeling outcomes for these five compounds are detailed and compared in tables 6.2 to 6.6, while Jablonski diagrams and frontier molecular orbital diagrams illustrating the lowest three singlet and triplet transitions from DFT computations are presented in figures 6.10 to 6.14.
Figure 6.5 FTIR Spectrum of Compound ReCCSiMe; at Room Temperature (a) in acetone film, (b) in PMMA film
Figure 6.6 FTIR Spectrum of Compound Re(CC)3SiMe; at Room Temperature (a) in acetone film, (b) in PMMA film
Figure 6.7 FTIR Spectrum of Compound ReCCH at Room Temperature (a) in acetone film, (b) in PMMA film
Figure 6.8 FTIR Spectrum of Compound Re(CC)2H at Room Temperature (a) in acetone film, (b) in PMMA film
Figure 6.9 FTIR Spectrum of Compound Re(CC)3H at Room Temperature (a) in acetone film, (b) in PMMA film
Table 6.2 Selected Calculated Results of Compound ReCCSiMe:.
RHF/LAN2D | B3LYP/LAN2DZ Experimetnal Z
AE noMO/LUMO 8.353 eV 2.697 eV 3.08 eV
IR Frequency in KBr in PMMAV(CO) 2047.6 (1) 1875.3 (1) 1872.1 (1) 1889.9 (0.89)VCO) 2057.5 (0.83) 1887.7 (0.87) 1899.7 (0.75) 1905.4 (0.9)
Excited States (Based on DFT singlet geometry)
Excited State E /eV ® + 9, Assignment f T¡ 1.9369 H—>L *“MLCT + °LLCT
Figure 6.10 Jablonski Diagram and Molecular Orbital Diagrams of Compound
ReCCSiMe; Based on the DFT Calculation Optimized Geometry
Table 6.3 Selected Calculated Results of Compound Re(CC)3SiMe3.
RHF/LAN2D | B3LYP/LAN2DZ Experimetnal Z
AE noMo1UMO 7.800 eV 2.509 eV 3.08 eV
IR Frequency in KBr in PMMA Vico) 2057.5 (1) 1884.1 (0.84) 1889.7 (1) 1909.2 (1) Vico) 2065.2 (0.8) 1894.1 (0.66) 1932.8 (0.49) | 1915.0 (0.99) Vico) 2170.3 (0.86) 1969.9 (1) 1995.2 (0.6) | 1992.1 (0.96) Vcc) 2277.4 (0.03) 2074.5 (0.08) 2024.7 (0.5) | 2023.0 (0.78) V(CC) 2540.1 (0.37) 2275.2 (0.47) 2154.1 (0.19) | 2055.8 (0.61) Vicc) 2422.5 (0.0) 2219.8 (0.0) 2095.8 (0.05) | 2146.4 (0.64)
Excited States (Based on DFT singlet geometry) Excited State E /eV ®, > %, Assignment f
Figure 6.11 Jablonski Diagram and Molecular Orbital Diagram of Compound Re(CC);SiMe; Based on the DFT Calculation Optimized Geometry
Table 6.4 Selected Calculated Results of Compound ReCCH.
RHF/LAN2D | B3LYP/LAN2DZ Experimetnal
AE noMo/LUMO 8.355 eV 2.715 eV 3.27 eV
IR Frequency in KBr in PMMA V(CO) 2044.8 (1) 1874.9 (1) 1883.3 (1) 1895.7 (1) V(CO 2057.6 (0.88) 1887.9 (0.95) 1905.4 (0.99) V(CO) 2157.0 (0.52) 1965.5 (0.98) 2014.1 (0.68) | 2011.4 (0.82) V(CC) 2182.2 (0.19) 2029.9 (0.03) 1948.7 (0.13) | 1955.5 (0.52)
Excited States (Based on DFT singlet geometry)
Excited State E /eV % + ®, Assignment f T¡ 1.9360 H—L 3IMLCT+*LLCT
Figure 6.12 Jablonski Diagram and Molecular Orbital Diagrams of Compound
ReCCH Based on the DFT Calculation Optimized Geometry
Table 6.5 Selected Calculated Results of Compound Re(C©);H.
RHF/LAN2D | B3LYP/LAN2DZ Experimetnal Z
AE HoMO/LUMO 8.071 eV 2.554 eV 3.08 eV
IR Frequency in KBr in PMMA V(CO) 2053.4 (1) 1881.1 (0.97) 1884.2 (1) 1903.4 (0.98) Vico) 2063.3 (0.85) 1892.3 (0.86) 1909.7 (0.84) | 1911.1 (0.99) Vico) 2169.5 (0.76) 1970.1 (1) 2011.4 (0.95) 2013.4 (1) Vice) 2234.8 (0.01) 2060.2 (0.02) 1985.9 (0.2) | 1988.3 (0.5) VựCC 2452.5 (0.1) 2244.4 (0.14) 2141.7 (0.12) | 2136.8 (0.36)
Excited States (Based on DFT singlet geometry)
Excited State E /eV o 7 % Assignment f T¡ 1.8528 H—›L 3LLCT+ÌMLCT
Figure 6.13 Jablonski Diagram and Molecular Orbital Diagram of Compound
Re(CC);H Based on the DFT Calculation Optimized Geometry
—_—z— 3LLCT +3MLCT (2) eee TL LCT +°MLCT (1)
Table 6.6 Selected Calculated Results of Compound Re(CC)3H.
RHF/LAN2D | B3LYP/LAN2DZ Experimetnal Z
AE noMO/LUMO 7.873 eV 2.464eV 3.02 eV
IR Frequency in KBr in PMMA Vico) 2058.0 (1) 1884.7 (0.88) 1885.8 (1) 1909.2 (1) Vico) 2065.6 (0.82) 1894.5 (0.72) 1923.9 (0.36) | 1915.0 (0.99) Vico) 2170.7 (0.83) 1970.5 (1) 2019.5 (0.63) | 2015.3 (0.92) Vico) 2255.2 (0.01) 2057.3 (0.03) 1985.5 (0.27) | 1982.5 (0.46) Vicc) 2383.5 (0.01) 2192.9 (0.01) 2100.8 (0.02) | 2094.4 (0.17) Vic) 2542.4 (0.27) 2286.3 (0.36) 2159.4 (0.14) | 2156.1 (0.27)
Excited States (Based on DFT singlet geometry)
Figure 6.14 Jablonski Diagram and Molecular Orbital Diagrams of Compound
Re(CC)3H Based on the DFT Calculation Optimized Geometry
DISCUSSIONS 1n Ầ.Ố
6.4.1 State Assignments, Acetylene Length Effect and Terminal Group Effect
The five monometallic complexes in PMMA at 77 K exhibit long lifetimes and significant Stokes shifts, indicating that their lowest emitting states have a triplet spin parity Luminescence lifetime measurements and quantum chemical studies show that the highest occupied molecular orbitals (HOMOs) consist of closely aligned symmetry-adapted molecular orbitals situated on the acetylene ligands and the d orbitals of the Re metal center.
The electron density on the trimethylsilyl end group significantly influences the lowest unoccupied molecular orbitals (LUMOs), which exhibit antibonding characteristics on the bipyridine rings with some d orbital contribution from the Re center The phosphorescence observed in the five compounds arises from spin-forbidden transitions involving the oxidation of the metal center and acetylene ligands, while the bipyridine ligand undergoes reduction Time-dependent DFT calculations indicate that the lowest emitting triplet manifold is primarily derived from *LLCT and *MLCT transitions, resulting from the promotion of an electron from a bonding orbital to an antibonding orbital The similarity in electron density on the Re center within the LUMOs suggests that the *LLCT transition is more significant when considering the overlap between the metal d orbitals and acetylene p orbitals This interaction leads to the formation of M-C=C bonding and antibonding orbitals, resulting in a repulsive M-C=C interaction that positions the antibonding orbital as the HOMO As the number of acetylene units increases, the energy of the p orbitals rises due to enhanced conjugation, leading to a greater acetylene contribution in the HOMO Consequently, the bipyridine ligand is the most easily reduced, with the lowest electronic excitation occurring through the HOMO to π* transition The mixed M-C=C character of the HOMO indicates that the excited state hole is a combination of both MLCT and LLCT contributions, transitioning from a more MLCT character to a more LLCT character as the acetylene units lengthen, which aligns with the observed emission spectra.
The end group significantly influences emission properties, with the trimethylsilyl [-Si(CH3)3] group exhibiting a stronger o-donating ability compared to the hydrogen (—H) atom This enhanced donation enriches the acetylene bridge with electrons, elevating the energy of the mc=c orbitals and facilitating the 3LLCT (Ttc=c >).
T bpy) component much stronger in the samples with the same number of acetylene units,but with the trimethylsilyl group (e.g more SLLCT (ttc=c —> Tuy) in the ReCCSiMe;
The schematic MO diagrams illustrate how the interaction between the Re metal center and axial acetylene ligands influences the character of the lowest electronic transition As the number of acetylene units increases from one to three, the highest occupied molecular orbital (HOMO) increasingly exhibits ligand-to-ligand charge transfer (LLCT) characteristics.
MLCT + LLCT , th Prd jp debs fo a 2 ‘ : Pr \ by / Pr +d,
The extreme example is compound 2 In the whole emission spectrum, the effect of
3LLCT (Ttc=c > Tbpy) transition is predominant and overwhelms the 3MLCT (dre TỪ ppy
The end-group effect significantly influences system stability, with the trimethylsilyl group enhancing molecular stability The stability of the molecules decreases in the following order: Re(CC)3SiMe (-1581 a.u.) is the most stable, followed by Re(CC)3H (-1458 a.u.), ReCCSiMe (-1429 a.u.), Re(CC)H (-1382 a.u.), and finally ReCCH (-1306 a.u.).
In terms of the luminescent lifetimes, all compounds, except for compound 2, display single exponential behavior with values of several microseconds These observations are
The findings align with typical Re(I) diimine complexes and quantum chemical modeling, indicating that the lowest emitting triplet manifold for these compounds features a mixed ILCT (®csc —> T'bpy) and MLCT (dre 3’ bpy) state Notably, compound 2, Re(CC)3SiMe3, exhibits two distinct single exponential regions: one corresponding to a sharp peak at 500 nm and another appearing on the broad band envelope's right side, devoid of peaks, with an intermediate spectrum observed between 500 nm.
— 620 nm displays double exponential behavior Fortunately, these two regions are so apart that they can be easily separated The lifetime measured at 500 nm is pretty long,
The observed ligand-centered 3am transition, typical for acetylene ligands, is influenced by the extended conjugation from three acetylene units and the enhanced o-donating ability of the silyl end group However, this behavior was not anticipated in our DFT quantum chemical studies, highlighting discrepancies between computational predictions and experimental outcomes, particularly in bi-metallic compounds Compound 2 exhibits a short single decay time consistent with other mono-metallic compounds and features two accessible emitting states: a mixed LLCT & MLCT state and a π c=c state.
The TDDFT calculations reveal that the molecular orbital energies of Tì, T2, and S are closely aligned Spin-orbital coupling lifts the degeneracy of each triplet manifold, resulting in nine excited states that are in close proximity Consequently, at temperatures of 77 K and higher, all ten excited states—nine from the triplets and one from S¡—become thermally accessible This suggests that the observed luminescence likely arises from a weighted average of these ten states, in accordance with Boltzmann statistics.
We employed restricted Hartree-Fock (HF) and density functional theory (DFT) to investigate the characteristics and spectroscopic properties of five monometallic compounds The methodologies for HF and DFT calculations are comparable, particularly in the geometry optimization process, where the self-consistent field (SCF) method minimizes energy concerning the 3N nuclear coordinates At equilibrium geometry, the condition f = aR = 0 is achieved, indicating optimal structural configuration.
When the SCF energy of the system has been minimized, the equilibrium force constants for the 3N-6 vibrational frequencies can also be calculated by
HF methods overlook electron-electron repulsion, leading to systematic errors and potentially divergent orbital assignments, highlighting significant differences compared to DFT methods.
Firstly, the energies from the HF calculations are always a little higher than those fromDFT In the DFT calculation, we chose the B3LYP functional, which calculates the
The HF method exhibits a larger error in determining the energy difference between molecular orbitals The maximum peak position of the vertical transition from So to Tị in the electronic absorption spectra was used to identify the HOMO-LUMO energy difference Consequently, all five compounds show an AEyomo.umo of approximately 3 eV In comparison, DFT results range from 2.5 to 2.7 eV, while HF results are significantly higher, ranging from 7.8 to 8.3 eV.
The molecular structures derived from HF and DFT methods exhibit slight variations, both displaying a distorted octahedral geometry around the Re center, with carbonyl groups arranged facially and acetylene groups extending out of the octahedral plane Notably, the HF method presents a more distinct bond pattern of 1.20-1.38-1.20 Å for single-triple-single bonds, indicating a greater triple bond character, while DFT calculations yield shorter single bonds (1.35-1.36 Å) and longer triple bonds (1.24-1.25 Å), resembling allene-like characteristics As the long axis increases, these differences become more pronounced, with HF showing longer Re-CO, Re-N, and Re-C=C bonds compared to DFT The bond angles also differ; in the octahedral structure, N-Re-N angles are less than 90°, with DFT measuring around 75° and HF below 74° Additionally, the C-Re-C=C angle in DFT approaches 180°, indicating more linear bridging acetylenes Despite the absence of crystal structures for these compounds, comparisons with similar Re diimine complexes suggest that DFT provides geometries that are better optimized and closer to actual molecular structures.
The distinction becomes evident in the comparison of frequency calculations, where the calculated frequencies for carbonyl groups (-CO) and acetylene groups (-CC) are significantly higher, ranging from 1.06 to 1.15 times greater than the experimental results.
The application of optimal scaling factors (0.9085 for HF / 3-21G and 0.8929 for HF / 6-31G) resulted in calculated frequencies that were significantly lower than the experimentally observed values In contrast, Density Functional Theory (DFT) provided results for the carbonyl frequency that closely matched experimental data, while the acetylene frequency results were slightly off Both methods exhibited a common issue with the ordering of normal modes, listing frequencies from lowest to highest energy: antisymmetric stretch of two carbonyl groups (A"), symmetric stretch of three carbonyl groups (A'(2)), symmetric stretch of three carbonyl groups (A'(1)), and antisymmetric stretches of acetylene groups However, the first antisymmetric stretch of acetylene groups interfered with the intensity and population of the A'(1) symmetric stretch of carbonyl groups, leading to a rearrangement in the FTIR spectra This analysis indicates that while neither method perfectly addresses the complexities of highly conjugated systems, DFT demonstrates greater accuracy and precision compared to the HF method.
A series of rodlike mono-rhenium(I) carbynes, specifically Re(I) compounds Re(Bu;bpy)(CO)s(C=C)n (where n = 1, 2, and 3, and L = H or SiMe), were synthesized and characterized The UV-Vis absorption spectra revealed that the lowest energy absorption corresponds to the So — Tj charge transfer transition, with strong spin-allowed ligand transitions showing a red-shift as the acetylene length increases Steady-state emission spectra and luminescent lifetime measurements at 77 K, alongside quantum chemical modeling, indicated that the lowest emitting triplet manifold for these compounds is a mix of *LLCT and “MLCT states following laser excitation Notably, compound 2, Re(CC)3SiMe3, exhibited unique properties due to enhanced conjugation and a strong o-donating end group, allowing for ligand-centered emission not predicted by quantum modeling Additionally, comparisons of calculation methods showed that the DFT method provides more accuracy and precision than the HF method, although both have limitations for these highly conjugated systems.
Summary and ConcÌuSIOTI - - ¿<2 + 211 3E 9 9t 2 HH th rệt 146
6) and comparing with those of the bimetallic complexes, there are profound differences in the absorption spectra of the bimetallic species This indicates that the electronic interaction between the two metal-based counterparts in the bimetallic complexes is strong.
7.2.2 Excitation Wavelength Dependence in PMMA Matrices
This investigation aimed to understand the orbital characteristics of the lowest emitting triplet state and the influence of 7-conjugation on it We analyzed the spectroscopic properties of these materials across different excitation wavelengths, temperatures, and matrices.
According to the Crosby-Kasha rule, phosphorescence in transition metal complexes originates from the lowest vibrational level in the lowest triplet state, suggesting a single phosphorescent state with a consistent exponential lifetime and excitation spectrum However, our research indicates that these materials, when embedded in solid matrices, exhibit phosphorescence from at least two distinct states, independent of temperature Additionally, we observe a significant excitation dependence in the luminescent spectra Steady-state emission spectra in solid PMMA matrices at 77 K and room temperature, under different excitation wavelengths (408 nm and 488 nm), reveal a remarkable shift in the phosphorescence spectra.
The steady-state emission spectra of bimetallic compounds embedded in PMMA matrices at 77 K are depicted in Figure 7.2 The solid lines represent emissions under 408 nm excitation, while the dashed lines illustrate emissions under 488 nm excitation for three compounds: (a) Re(CC)4Re, (b) Re(CC)sRe, and (c) Re(CC)ôRe.
The steady-state emission spectra of bimetallic compounds embedded in PMMA matrices at 298 K are depicted in Figure 7.3 Solid lines represent the spectra obtained under 408 nm excitation, while dashed lines correspond to 488 nm excitation The compounds analyzed include Re(CCRe) and Re(CC)sRe, showcasing distinct emission characteristics based on the excitation wavelength.
Normalized Intensity "500 550 600 650 700 750 800 850 with respect to the observed peak maximum and the band shape with the excitation wavelength.
Compared to their monometallic counterparts and under similar experimental conditions, the luminescence of the bimetallic complexes in PMMA solid plastic films at
At 77 K and room temperature, phosphorescence exhibits weaker intensity but displays a complex vibrational fine structure The key characteristic of the spectra is a broad phosphorescence band attributed to a charge transfer (ÌLLCT) transition, with peak wavelengths (Amax) ranging from 560-570 nm at 77 K and 550-610 nm at 298 K Notably, the band shapes and luminescence lifetimes closely resemble those found in Re diimine systems.
The study reveals that the compounds exhibit characteristics of MLCT, supported by a comprehensive spectroscopic and theoretical analysis leading to an assignment of ÌLLCT (đãc=c — T'tpy) as the ground state This conclusion is reinforced by luminescence lifetime measurements, quantum chemical modeling, and detailed time-resolved infrared results Notably, the luminescence data shows vibrational fine structures on the red side of the dominant broad charge transfer band, with specific peaks observed at 587 nm, 671 nm, and 778 nm under 408 nm excitation at 77 K These peaks are tentatively assigned as Sat" coc transitions due to their resemblance to typical Kưui transitions found in literature, and the energy differences of 2131 cm” and 2050 cm” align with the vibrational energy of the acetylene group and its first overtone.
At 77 K, vibrational fine structure is clearly evident in the phosphorescence spectra of compound 1 When excited with 408 nm laser excitation, Amax of the broad emission band is at 567 nm and three additional peaks occur 587 nm, 669 nm, and 778 nm. However, when the excitation source is changed to 488 nm laser excitation, the intensity of the broad structureless band is dramatically reduced and does not have an obvious maximum The three peaks corresponding to the vibrational fine structure are only slightly blue-shifted to 583 nm, 665 nm, and 774 nm and in comparison to the room temperature data, they are much more discernable At ambient temperature, the most prominent property is still the broad structureless band that originates from the "LLCT (dftc=c > TỦ by) — Ì g.s transition However, the intensity of the two peaks at 590 nm and 664 nm are dramatically reduced, especially under 408 nm excitation.
The emission spectra of Re(CC)sRe at 77 K show similarities under different excitation wavelengths, with a notable difference in the broad luminescence band Under 488 nm excitation, the band is significantly broader compared to that under 408 nm excitation The maximum wavelength (Amax) of the broad band shifts from 587 nm at 488 nm excitation to 561 nm at 408 nm excitation Additionally, two vibrational peaks observed at longer wavelengths (661 nm and 767 nm) under 488 nm excitation are slightly blue-shifted to 657 nm and 759 nm under 408 nm excitation, with an energy separation of approximately 2050 cm⁻¹ between them.
Compound 2 exhibits a greater dependence on excitation wavelength for its phosphorescence band at room temperature compared to compound 1 When excited at 408 nm, only a single broad luminescence band is detected In contrast, excitation at 488 nm reveals both a broad structureless band and a structured luminescence featuring two distinct vibrational peaks, consistent with the observations made at 77 K.
The emission spectrum of compound 3 lacks a well-defined vibrational fine structure, consistently displaying a single broad band regardless of temperature or excitation wavelength At 77 K, the emission peaks at 568 nm with 408 nm excitation and 598 nm with 488 nm excitation, resulting in a 30 nm difference At room temperature, both emission bands red-shift, narrowing the difference to 15 nm, with peaks at 586 nm for 408 nm excitation and 601 nm for 488 nm excitation.
The relative emission intensity of the compounds is observed to follow this order: all monomers >> Re(CC)4Re > Re(CC)sRe 2 Re(CC)¿Re This trend can be attributed to the energy gap law, which indicates that the natural logarithm of the non-radiative rate, In kn, decreases linearly with increasing emission energy.
(34-36) and seems to hold true point of fact has been validated in many other Re(I) systems in the present case as well.
After analyzing the emission spectra, key questions emerge: (1) Is there a single coupled excited state or two distinct excited states after photo excitation and relaxation to the final emitting state(s)? (2) In addition to the 3LLCT, what other mechanisms are involved?
T upy) luminescence transitions, what other luminescent process(es) are likely involved?
(3) Why are the observed emissions so dependent an excitation wavelength?
7.2.3 Wavelength Dependence of the Excitation Spectra in PMMA Matrices
The excitation spectra of the bimetallic compounds in solid matrices at both cryogenic and ambient temperatures are also dependent on the monitoring emission wavelength as
Figure 7.4 Excitation Spectra of Bimetallic Compounds in PMMA Matrices at 77 K: (a) compound Re(CC),Re, solid line, monitored at 567 nm; dotted line, monitored at
669 nm; (b) compound Re(CC)sRe, solid line, monitored at 561 nm; dotted line, monitored at 661 nm; (c) compound Re(CC)¢Re, solid line, monitored at 568 nm; dotted line, monitored at 590 nm.
The excitation spectra of bimetallic compounds in PMMA matrices at 298 K reveal distinct characteristics for various compounds For compound Re(CC);Re, the solid line indicates monitoring at 590 nm, while the dotted line is observed at 665 nm Similarly, in compound Re(CC)sRe, the solid line is monitored at 590 nm, with the dotted line at 660 nm These findings highlight the spectral behavior of the compounds within the PMMA environment.
590 nm; dotted line, monitored at 660 nm.
The excitation spectra, shown in figures 7.4 and 7.5, reveal two significant characteristics Firstly, the spectra exhibit distinct shapes when monitored at the Amx of the broad band compared to one of the prominent vibrational peaks at longer wavelengths, indicating the presence of two separate triplet excited-state manifolds contributing to luminescence Secondly, the wavelength dependence diminishes notably at room temperature, mirroring the trend observed in the emission spectra This phenomenon in both the excitation and emission spectra is likely due to the "luminescence rigidochromic effect," predominantly influenced by conformational heterogeneity.
7.2.4 Steady-State Emission Spectra in THF Solution
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This study presents the synthesis of conjugated organometallic molecules featuring mono- and bi-Re(I) centers, highlighting the potential for innovative molecular designs in this field Future research can be enhanced by exploring various metal centers, adjusting the number of bridging acetylene units, and incorporating diverse bridging ligands.
Utilizing current synthetic skills, we can design molecules featuring Ru(II), Os(II), and Pt(II), such as Ru(CC), Os(CC), and Pt(CC) Additionally, we can enhance this series by combining these metal centers to form novel hetero-bimetallic molecules, including Re(CC)nRu, Re(CC)Os, and Re(CC)nPt.
Recent studies on bi-metallic complexes reveal the presence of 2, 4, 5, and 6 acetylene units between metal centers A key question arises: can these bridging chains be extended further? The primary challenge lies in synthesizing longer monomers, specifically M(CC)nH Currently, the longest achievable monomer is Re(CC)3H The increasing reactivity and instability of the acetylene chain as it lengthens pose significant hurdles, hindering advancements in this area.
One effective approach to addressing this issue is to modify the bridge ligand Potential candidates for this modification include oligo(cyanide) (CN)n, oligo(phenyl), oligo(phenyl-ethynylene), K2, oligo(phenyl-butadiyne), and various other aromatic rings.
The steady-state emission spectra, luminescent decay lifetimes, low-temperature FTIR, and time-resolved infrared measurements are valuable for analyzing highly conjugated systems When combined with TRIR and chemical modeling, it becomes evident that the complexity of transitions increases with longer acetylene bridging units Consequently, advanced time-resolved techniques, such as femto- and pico-second TRIR, may be essential for understanding the time evolution of the lowest excited states in these systems.
Temperature-dependent lifetime measurements and time-resolved emission determination are crucial for analyzing Zero-Field Splitting (ZFS) parameters By fitting curves with the ZFS equation, significant insights can be gained regarding the orbital parentage of transitions between excited and ground states The parameters Ae; and Ag, along with ko, ki, and kạ, provide valuable information about the allowed emitting states in these transitions, enhancing our understanding of the underlying mechanisms.
Some mono- and bi-metallic complexes exhibit biexponential lifetimes, indicating the presence of multiple emitting excited states Consequently, time-resolved emission techniques are valuable for distinguishing the emission spectra arising from these different states.
Advanced chemical modeling tools enable the analysis of complex photophysical properties observed in luminescence measurements, revealing the orbital nature of ground and excited states that optical measurements alone cannot distinguish However, these calculations face limitations, including inaccuracies in frequency calculations, which overestimate acetylene vibrational stretches and misorder carbonyl and acetylene vibrations Additionally, calculations are conducted in the gas phase, while spectroscopic properties are typically measured in solution or solid matrices, where solvent interactions, such as solvatochromism and rigidochromism, are not accounted for Furthermore, the absence of spin-orbit coupling in the computational package is significant, particularly for highly conjugated and symmetric Re(I) compounds, leading to discrepancies between computational results and optical experiments Exploring more sophisticated excited state calculations, such as Configuration Interaction Singles (CIS), may provide further insights.
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Appendix II Relative Spectral Irradiance Standardization
To accurately measure spectral irradiance, it is essential to use a calibrated standard that corrects for instrument response Various standards are available for different applications, and for our purposes, we utilized a NIST calibrated quartz-tungsten-halogen lamp (Oriel Instruments, model 63355, 200W, 6.5A) This lamp features a coiled tungsten filament within a quartz envelope, which contains a small amount of halogen, providing a standard spectral irradiance across a wavelength range of 250 nm to 2400 nm.
This appendix aims to provide detailed, step-by-step instructions for creating correction files essential for spectral irradiance calibrations using our current laboratory equipment Additionally, it outlines the process for utilizing these files to accurately correct the spectra.
The optical arrangement is depicted in figure ApII.1 The lamp was mounted in the matched mount with the glass “nipple” pointed away from the target It was then placed
The calibration of the spectrometer requires precise placement of the lamp filament, ensuring it is centered at the same height as the monochromator slit and positioned 50 cm from the focus plane Accurate distance measurement is crucial, as a mere 2.5 mm error can lead to a 1% calibration discrepancy To minimize secondary reflections from the lamp, all nearby surfaces were covered with black velvet, and a black shield was positioned 1 meter behind the lamp to block reflective flux from other angles.
Figure ApH 1 Optical Arrangement for The Spectral Irradiance Calibration
II.3 Collecting Data From the Standard Lamp
Data was collected from the standard lamp under identical conditions to our experiments, involving the calibration of grating 2 and grating 3 in the 0.5 m Acton monochromator This process included tests with and without filters, specifically using a KNO3 filter, a 441 nm notch filter, a 475 nm long pass filter, a 488 nm notch filter, a 500 nm long pass filter, and a 514 nm notch filter Each configuration was tested five times, and the averaged data was normalized to create observed intensity vs wavelength spectra (O(A)) An example of the O(A) spectrum from the emission setup of grating 2 with the 475 nm long pass filter is shown in Figure AplI 2.
Figure ApII 2 Observed Emission Data from the Grating 2 with 475 nm Long Pass
II.4 Creating the Correction Factor Files
The correction factor file, F(A), was created by using the formula:
The formula FA) = T(QA)/O(@) represents the relationship between relative true irradiance data, T(4), provided by Oriel Instruments Inc.'s standard lamp This data is characterized by a polynomial derived from a calibration conducted by NIST.
The actual NIST calibration data are tabulated in the table ApII.1 and the true irradiance spectrum is depicted in the figure Ap II 3.
Table ApII 1 Spectra Irradiance of Quartz Tungsten Halogen Lamp Model 63355 at a Distance of 50 cm When Operated at 6.5 Amperes
Wavelength /nm Irradiance (uW/ cm* nm)
Figure ApII 3 NIST Calibrated Irradiance Data Points for the Q-T-H Standard
Lamp in the Range of 250 nm to 2400 nm
Be sure to use the same range and resolution to generate the correction factors, F(A),when use the T (A) dividing the O (1) One of the factor examples is shown below.
Figure ApII 4 Irradiance Correction Factors for the Grating 2 with 475 nm Long
II.5 Use the Correction Factor Files to Correct Your Spectra
An experimental data, E(A), can be easily corrected with these correction factors by simply multiplying E(X) with F(A) So the real experimental spectrum, R(A), can be described as :
An example is shown here.