Luminescent metal complexes containing naphthalene and anthracene units 3

Chapter The Spectroscopy and Photochemistry of Pt(II) Complexes 72 3.1 Spectroscopy of d8 Metal Complexes Luminescent coordinatively unsaturated metal complexes are appealing from a photochemical perspective. Saturated congeners such as [Ru(bpy)3]2+ are restricted to outersphere interactions with substrates. Chromophores that allow inner-sphere electron-transfer reactions and applications for chemical sensing,1 solar energy conversion, and photocatalysis2 have been developed. Investigation into square planar d8 platinum(II) compounds have been prominent since this class of molecules can mediate excited-state atom transfer reactions and bond activation. In particular, the prolific excited-state chemistry of the binuclear derivative [Pt2(µ-P2O5H2)4]4- has been demonstrated.3 The triplet (dσ*, pσ) excited state, which is a manifestation of the d8-d8 interaction between the diplatinum centers is capable of C-H, and Chalogen bond cleavage and electron-transfer reactions. Understanding the nature of the exited state of platinum is useful for the future development of photocatalysts for light to chemical energy conversion as well as other possible practical uses. Reviewing many of the d8 complexes, especially those of Pt(II), it can be determined that there are several types of absorptions observed in the near UV-visible region such as intraligand π→π* (IL) and metal-to-ligand charge transfer (MLCT) transitions. Each of these types of absorptions can have luminescence associated with the lowest spin-forbidden state if it is the lowest in energy. If both metal-metal and ligand-ligand interactions exist in dinuclear platinum(II) complexes, these interactions could give rise to a number of electronic transitions. A simplified molecular-orbital diagram is shown in Figure 3.1. The HOMO is a σ*(π) orbital from the ligand, but the energy of the dσ* and the σ*(π) is very close. Sometimes the low-energy absorption is mixed with the 1MMLCT (the metal-metal-to-ligand charge transfer) and 1IL. Most of the information regarding the excited states of Pt(II) complexes have been obtained from studies of luminescence from both solid state and solution. 73 pσ* σ*(π*) pz pσ π* pz π* σ(π*) i 5dz π ii iii i) MMLCT dσ*→ σ(π*) ii) metal centered dσ*→pσ iii) ligand-ligand σ*(π)→ σ(π*) dσ* σ*(π) 5dz2 dσ π σ*(π) Figure 3.1 Simplified schematic molecular-orbital diagram of platinum(II) complexes of polypyridine ligands with metal-metal and ligand-ligand interaction The nature and energy of the excited state in platinum complexes varies with the ligands. In the majority of complexes formed by low-valent transition metal atoms and ligands with low lying empty orbitals, in particular α-diimine, the lowest excited state is for the metal–to-ligand charge transfer, however, co-ligands with relatively high lying filled orbitals may participate in the orbital from which low lying electronic transitions originated (highest occupied molecular orbital HOMO). For instance, in Pt(α-diimine)X2 (X = CN), the energy and nature of the emitting state were dependent on the anionic ligand. The emission was proposed to originate from a state involving Pt(dz2) and diimine (π*) orbitals,5 and excimer emission has been detected from concentrated solutions. For the dithiolate and tridentate complexes, the lowest energy excited state has been attributed to either a triplet metal-to-ligand charge transfer state (3MLCT) or a ligand-based state; either a ligand-ligand charge transfer (LLCT), or single ligand centered triplet excited state 3LC.6 However, in complexes of metals in low oxidation states like Pd(II) and Pt(II), ligand-to-metal charge transfer transitions cannot be responsible for the observed luminescence. It is easy to distinguish LC and MLCT excited states: 1) LC luminescence is only slightly red shifted from the emission of the corresponding free-protonated ligand. The larger the energy-shift, 74 the higher is the MLCT character of the emitting excited state; 2) LC luminescence is less sensitive to the presence of the metal atom than MLCT emission. Thus, the longer the radiative lifetime, the purer is the LC character of the emitting excited state; and 3) The energy of MLCT transitions is expected to depend on the solvent polarity because of the change in the dipole moment of the molecule, whereas the energy of the LC transition is expected to be less influenced by the solvent. Platinum(II) diimine solids exhibited unusual colors, strong emission, and highly anisotropic properties as a result of intermolecular stacking interactions. A series of square plane of Pt(diimine) complexes with anionic ligands such as arylacetylides were synthesized and their absorption and emission were compared. Their emissions will shift to lower energy with the increase of electron-withdrawing ability of the diimine substitute and the increase donation of the arylacetylide ligands. The behavior is consistent with a mainly metal-based HOMO and a π*diimine LUMO. The assignment is consistent with the notion that variation of the diimine affects the energy of the lowest unoccupied molecular orbital, and that the variation of the arylacetylide leads to only minor change in the Pt-based HOMO.5 Similarly, the luminescent properties of [PtII(tpy)X]n- ( tpy = terpyridine) derivatives and oligomers have been extensively investigated. Such processes were found to give low energy triplet emission in the 600-700 nm region (MMLCT: dσ*-π*) and the excimeric ligand-to-ligand (π-π*) excited state.7 A series of discrete d8-d8 complexes, namely, [Pt(tpy)]2(µ-L)n+ ( L = bidentate ligand) and [{Pt(CNN)2}(µ-L)]n( CNN=6-pheny-2,2’-bipyridine) have been prepared to model the low-energy [dσ(dz2)(Pt)π*(diimine)] and excimeric ligand-to-ligand emissions. Some other factors also affect the nature of the excited state of platinum(II) complexes as below: 1) Temperature. The red-shift of the solid-state emission upon cooling can be rationalized by the shortening of intermolecular Pt-Pt and π- π separations in the crystal lattice, which results in [dσ*, π*] emissions of lower energy. However, the blue shift of the emission is observed in solution of a Pt complex that exhibits 3MLCT. 75 2) Concentration. The red-shifted emission at higher concentration is assigned to an excimer emission or oligomer emission. 7, 10 3) Solvent. The solvatochromic shift of the lower-energy absorption indicates the 1MLCT nature. Unlike the 1MLCT absorption band which displays discernible solvatochromic behavior, the MLCT excited state shows minimal variation in different solvents. MMLCT also shows the solvatochromic shift. The respective emission energy, lifetime, and quantum yield are highly sensitive to the solvent polarity but insensitive to the complex concentration. The energy decreases from aqueous solutions to chloroform solutions. 11 4) Solvent acidity, basicity and other reasons. For example, to the ligand of Pt complexes with N, S atoms which keep uncoordinated proton, changing the pH is attributed to protonation of N or S and can affect the ability of energy of the π*diimine LUMO. The different protonation behaviors between the two complexes results from the difference in their electronic structure.12 The energy of 3MMLCT is highly dependent on the counterion (PF6-, ClO4-, Cl-, CF3SO3-) in line with the different colors of the various salts.13 There are several ways to clarify Pt(II) complexes. One is to use the Pt-Pt distance in the solid state crystal structure. A complex is considered to be monomeric if it is in a lattice where the nearest Pt-Pt distance is larger than 4.5 Å and if the solid state emission spectrum is not significantly different from that in dilute solution or a glass. A linear chain structure is one in which the Pt(II) complexes are stacked equidistantly along an axis that is usually but not always perpendicular to the plane of the complex. Typically, the Pt-Pt distance is from 3.2 to 3.4 Å. A dimeric structure contains pairs of complexes with an intermolecular distance that is clearly shorter than the distance of the next pair. 14 However, Pt-Pt distance and π-π stacking of the ligand are different in solid and solution. In this chapter Pt(II) complexes are separated into two classes according to the structure and whether a bridge connects the two Pt units. 76 3.1.1 Mononuclear Platinum Complexes In this section, the case that is presented contains only a single platinum center with primarily the ligand 6-phenyl-2, 2’-bipyridine (CNN: this indicates C and two N atoms are the coordination sites). Figure 3.2 R l : R=H :R=Ph 3: R=4-ClPh 4: R=4-MePh 5: R=MeOPh 6: R=3,4,5-(MeO)3Ph N Pt N Cl Complexes Pt(L1-6)Cl, 1-615 (Figure 3.2) had similar absorptions in CH2Cl2. The absorbance λ < 370 nm, was dominated by 1IL (π→π*), while the absorbance around 438 nm was assigned to the 1MLCT [(5d)Pt→π*(L)] transition and the weak absorption tails at 519 - 525 nm (ε < 60 dm3mol-1cm-1) wais attributed to the 3MLCT. They also exhibited emission in solution or in the solid state. They exhibited an emission about 565 nm at 298 K and shifted somewhat at 77 K in CH2Cl2 and were assigned to the 3MLCT. In the solid state, the emission about 565 nm at 298 K in CH2Cl2 was due to a 3MLCT and changed a little in a glass at 77 K. The emission maximum in complexes 1, 5, and were shifted to 700 nm, 716 nm, 722 nm respectively and assigned to 3[dσ*, π*] because of the shortening of the intermolecular Pt-Pt and π-π separations in the crystal lattice which resulted in 3[dσ*, π*] emissions of low energy. In contrast complexes 2-4 exhibited a blue shift of the 3MLCT emission at 77 K. Other compounds with a CNN type ligand also showed similar photophysical properties as complexes 1-6. With the increase of electrophilicity of the metal center, the emission is expected to be shifted to higher energy. 77 Figure 3.3 N Pt N N Pt C C N O R 12 N R=iBu, ; nBu, ; iPr, 9; Cy, 10; 2,6-Me2Ph, 11 In CH2Cl2 or CH3CN solutions (concentration between 10-6 - 10-3 M), in the higherenergy region, the vibronically structured absorption of complexes - 11 (Figure 3.3) centered at 340 nm (ε ≈ 104 M-1cm-1) were assigned to an interligand 1IL (π- π*) of the CNN group. The absorptions at 390-430 nm (ε ≈ 102 M-1cm-1) were attributed to 1MLCT and the 529 nm band was assigned to a 3MLCT, which can be self quenching when the concentration was increased from 10-5 M to 10-3 M. When the C > 10-3 M, the band at λmax = 510 nm (ε ≈ 120 M-1cm-1) was assigned to 1(dσ*→π*) (1MMLCT) transitions. The emissions below 600 nm were assigned to MLCT whatever in solution or solid phrase and emissions above 700 nm were assigned to 3[dσ*- π*] (MMLCT). At higher concentration at 77 K, the complexes and 10 exhibited peaks at 500 nm and 710 nm respectively which were attributed to 3MLCT (8) and 3MMLCT (10). Peaks were observed at 600 - 625 nm and were tentatively ascribed to the excimeric intraligand excited state arising from a weak π-π stacking interactions of the CNN ligand. The emissions below 600 nm in the solid were attribute to 3MLCT with excimeric character due to weak CNN ligand π-π interactions. The orange cyclohexyl isocyanide complex exhibited a broad structureless emission at λmax = 625 nm which was an excimeric 3IL transition resulting from π-stacking of the ligand and emissions above 700 nm were assigned to the 3[dσ*-π*]. For complexes and 11 at 77 K, a dramatic change occurred in the band shape when the concentration was increased. The λmax shifted from 505 nm to 627 nm for complex and 519 nm to 730 nm for complex 11. The former was tentatively ascribed to an excimeric intraligand emission arising from weak π- 78 stacking of the CNN ligand and the latter can be attributed to a MMLCT excited state resulting from oligomerization of the Pt(II) centers in a glassy matrix.14, assigned to a IL transition.14, ethanol:methanol:DMF) of 17 The 16 Emissions below 500 nm were low-temperature emission spectra (5:5:1 [Pt(tpy)Cl]+ revealed a 740 nm band indicative of M-M oligomerization (3MMLCT), a 650 nm band attributable to tpy π-π interactions (3MLCT) and a 470 nm band characteristic of mononuclear [Pt(tpy)Cl]+ π-π* emission at the higher concentration. At low temperature in the solid state only a 3MMLCT emission was observed. From these results, it can be concluded that the platinum photoluminescent properties can be systematically tuned through modification of the diimine or cyclometalated ligand. Their square-planar geometry confers different photophysical and photochemical properties from those of the octahedral [RuII(bpy)3]2+ complexes. The red shift of the MLCT transition of Pt(II)derivative is through metal-metal and ligand-ligand interactions. By recording the excitation spectrum, a well-resolved absorption band at 500 nm is substantially red-shifted from the absorption spectrum of monomeric derivatives. Che18 also attempted to change the σ –donating strength of the alkynyl ligand of Pt(CNN)(C≡CR), which destabilized the dπ(Pt)→HOMO of the relatively low-energy MLCT 5d(Pt)→π*(CNN) transitions. Electron-withdrawing groups stabilized the Pt-based HOMO to yield blue-shifted emissions. Other Pt complexes with an α-diimine ligand and an acetylene ligand19 had the same nature of the excited state. Because of the electron-withdrawing nature of the substituted bipyridine and phenanthroline ligands, the emissions exhibit a red shift, although Pt(4,4’-dtbpy)(C≡C-C≡CPh) (13, Figure 3.4) displayed a strong solid-state and solution phosphorescence at 77 K and 298 K. The associated excited state was proposed to arise from a triple intraligand (3ππ*) translation from the (C≡C-C≡CPh) unit and a 3MLCT [Pt→ π*(diimine)] transition from excimer emission. In the UV spectrum, the 1MLCT (5d(Pt)- π*) overlapped with the 1IL for the low lying HOMO in the 250-350 nm region. 79 Figure 3.4 N N Pt C C C C 13 3.1.2 Platinum Complexes Linked by a Bridge When two platinum(II) units are in close proximity so as to allow a metal-metal and a ligand-ligand (π-π) contact, a low-energy photoluminescence which was red shifted from the MLCT emission of the mononuclear species was typically observed. The electronic excited-state associated with this emission was denoted as 3[dσ*, π*] as discussed in the literature.3, 4, 15-17, 20-22 The binuclear Pt-Pt complexes can result in an increase in the metal-metal and ligand-ligand (π-π) interaction. They also can be spectroscopically characterized by model intermolecular interactions in Pt(II) polypyridine species and to investigate the photophysics and solid-state structures of cyclometalated Pt(II) oligomers. A series of binuclear cyclometalated Pt(II) complexes,15 Pt2(L)2(µ-dppm)(ClO4)2 14-19, Pt2(L1)(pz)(ClO4)2 20, Pt2(L1)(dppC3/C5)(ClO4)2 21 and 22 were synthesized in order to examine solution and solid-state oligomeric d8-d8 and ligand-ligand interactions (Figure 3.5). The intramolecular Pt-Pt bond was 3.245 Å and 3.612 Å for complexes 17 and 20 respectively as determined by X-ray crystal structures. The UV-absorption spectra for complexes 14-20 exhibited λmax > 400 nm were due to 1[dσ*-π*] transition. Complexes 14-19 also exhibited structureless emissions with peak maxima at 654 - 662 nm in CH3CN at room temperature which were blue shifted to 633 - 644 nm at 77 K. The solid state also showed emission at 298 K but at 77 K the energy was red-shifted. This may be attributed to the shortening of the Pt-Pt distances due to lattice contraction. Though a bridge existed in 21 and 22, the Pt-Pt distance was too long for the length of the bridge. The absorption tail at λ > 500 nm represented the transition between 80 MLCT and 1[dσ*-π*]. Complexes 21 and 22 had an emission band at 570 nm due to a 3MLCT. However, at 77 K in CH3CN, three peaks at 555, 590, 651 nm appeared in the spectrum of 22 and were indicative of weak π-π interaction, assigned to 3MLCT and 3[dσ*-π*]. Figure 3.5 R R N N N Pt Pt Ph2P 2+ N 14: R=H 15: R=Ph 16: R=4-ClPh 17: R=4-MePh 18: R=MeoPh 19: R=3,4,5-(MeO)3Ph PPh2 2+ N N N Pt Pt N N N 20 2+ N N N Pt Pt N PPh2 Ph2P n n=3, 21; n=4, 22 81 Figure 3.6 2+ N N N Pt N Pt C C N N 23 The complex [(CNN)Pt]2(C≡N(CH2)3N≡C)](PF6)2 23 20 (Figure 3.6) also, has a bridge that brings the two Pt centers closer. The 340 nm (ε = 104 M-1cm-1) absorption (in CH2Cl2) was assigned to the intraligand 1IL (π-π*) transition and intense low-energy bands with λmax in the range 390-430 nm (ε = 102 M-1cm-1) were assigned to 1MLCT [(5d)Pt→π*(CNN)] transitions. Though compound 23 had absorption at 511 nm, it was not similar to the 1[5dσ*→6pσ*] transition in Pt2(µ-P2O5H2)4.2a In solution, the emission spectrum of 23 had two peaks at 555 nm for a MLCT and 630 nm for 3IL. However when reducing the temperature to 77 K, the 600 nm was observed which was assigned to 3MMLCT in the glass matrix. In the solid state, a band at 711 nm was observed at 298 K and had red shift to 744 nm at 77 K. From these data, the nature of the exited state was determined by the metal-metal bond and the distance between the two ligands. If the bridge is quite long, the emission and absorption spectra are similar to those of the mononuclear system; if the alkyl bridge between the Pt centers is reduced, the [CNN]Pt groups would exhibit dinulcear properties. (Figure 3.7) Figure 3.7 N N Pt N N N Pt Pt N N Pt N 82 The electronic absorption spectrum of [Pt2(µ-dppm)2(C≡CC5H4N)4] (24) in ethanoldichoromethane (1: v/v) showed a low-energy absorption at ca. 368 nm with low-energy tails that extended to ca. 500 nm.21 According to the previous study,22 the absorption band were assigned to spin-allowed and spin-forbidden metal-metal-to-ligand-transfer (MMLCT) [dσ*(Pt2)→pσ(Pt2)/π*(C≡CR)] transitions. Complexes M2(dcpm)2(CN)4 (M=Pt, 25; Pd, 26) in CH2Cl2 showed absorption bands at 337 nm (ε = 2.41x104 M-1cm-1) and 328 nm (ε = 2.43x104 dm3mol-1cm-1) which were assigned to (5dσ*→6pσ) electrons transitions originating from a Pt(II)-Pt(II) interaction. There was also a weak, long-wavelength shoulder at 388 nm (ε = 300 M-1cm-1) and 375 nm (ε = 50 M-1cm-1) which was assigned to spin-forbidden analogues of the intense bands.23 3.2 Photochemistry of Pt(II) Complexes. The previous study of Pt(II) complexes showed that there were rich photophysical and photochemical properties. Some of them even had emission in solution at room temperature. There were have different excited states achieved by tuning different ligand coordinated with Pt(II). This type of complexes has potential use as chromophores for the conversion of light-tochemical energy. Efforts are now focused on the use of the Pt(diimine)X2 chromophores in dyads and triads with the goal of constructing a molecular photochemical device for light-to-chemical energy conversion.24 Connection of the Pt diimine chromophores to both a donor or reductive quencher and an acceptor is envisioned through new ligand bridges currently being synthesized using Pd-catalyzed coupling reactions and carbonyl condensations. Three fields are discussed next for Pt(II) complexes . 3.2.1 Induced Electron Transfer in Pt(II) Complexes Photoluminescent transition metal complexes are playing an increasingly useful role as probes of electron and energy transfer involving DNA and proteins. 25, 26 Photoinduced electron transfer can be regarded as a process where absorbed light energy is transformed into chemical energy. After formation of the equilibrated excited data, the subsequent event is the actual transfer of the electron. The property of some excited states acts is similar to such process as photosynthesis. Multicomponent molecules have been designed and constructed containing an 83 electron donor, an electron acceptor, and a metal complex-based charge transfer chromophore. Most commonly examined transition metal chromophores are octahedral d6 diimine complexes such as Ru(diimine)32+ and Re(diimine)(CO)3(py)+ 27 which have a 3MLCT excited state. Recently a relatively new transition metal chromophore, containing platinum complexes, also possesses a long-lived 3MLCT excited state. The nature of this excited state is similar to the charge transfer excited states of d6 diimine chromophores. The Eisenberg28 group used it to understand the factors influencing the longevity of charge separation in a multicomponent system containing a novel platinum diimine chromophore. The luminescent complex [Pt(terpy)OH]BF4 (2,2’:6’,2”-terpyridine) 29 underwent photoinduced electron transfer reactions with phenyl amine electron donors and nitrophenyl electron acceptors. Stern-Volmer analysis of the quenching of metal-to-ligand charge transfer phosphorescence (3MLCT) was used to calculate bimolecular rate constants for electron transfer. Rate constants varied from 108 to 1010 M-1s-1, depending on the thermodynamic driving force of the electron transfer reaction. The rate constants indicated that [Pt(terpy)OH]BF4 is a powerful photo-oxidant. Aromatic triplet energy acceptors can also quench the 3MLCT emission. When this complex bound to a guanine residue, the 3MLCT emission was completely quenched; an effect attributed to photoinduced electron transferred from guanine to the platinum complex excited state. 26a, d A bridge connecting both the electron donor and the electron acceptor is also useful to transferring an electron. Mixed valence compounds have attracted considerable attention because of their capability for photoinduced electron transfer, which has potential applications in energy conversion and photocatalysis. In such applications, the ability to transfer multiple electrons with a single photon is much desirable. Platinum seems to be suited ideally as the basis for such a complex capable of carrying photoinduced multielectron charge transfer because of the stability of Pt(II) and Pt(IV) complexes and the short-lived and unstable nature of Pt(III). Pt(II) and Pt(IV) prefer different geometry, thus charge transfer can also induce the formation and breaking of coordinate covalent bonds. Thus, it is possible to design species that would be capable of twoelectron charge transfer upon excitation with a single photon.31 Based on this theory, [L(NC)4Fe(II)-CN-Pt(IV)(NH3)4-NC-Fe(II)(CN)4L]4- (L is a CN- or a S-donor ligand) was 84 designed and provided such photoinduced multielectron charge transfer processes. These complexes exhibited intense metal–metal charge transfer (MMCT) bands in the blue portion of the spectrum (350–450 nm). Irradiation the MMCT band centered at 425 nm produced a new two electron charge transfer with a quantum yield of 0.01. Well defined oligomers and polymers of the iron based system which can be synthesized either as soluble materials or as adherent films on electrode surfaces. The photochemical reactivity and photophysics of these species were found to be a function of molecular geometry. In the case of the polymeric systems, onedimensional, two-dimensional, and network materials can be synthesized, using electrochemical techniques, to control the polymer reactivity sites. Polymer modified electrodes exhibit a photocurrent response which is diagnostic for the photochemistry occurring within the film. Correctly selected polymer morphologies lead to primary photoproducts on the electrode surface which is capable of oxidizing chloride to chlorine. This chemistry can be used to produce a photochemical energy conversion cycle in which visible light induces the oxidation of halides to energy rich halogens. The ion pair complex, {A2+[ML2]2-}, A2+ is a bipyridinium acceptor and [ML2]2- , M=Ni, Pd, Pt, contains a planar dithiolene ligand, is redox active and light-sensitive. They possess the same mean reorganization energy for electron transfer from the dianion to the dication. Proper selection of the acceptor reduction potential and the central metal allows the tailoring of optical electron transfer induced activation of dioxygen in solution. The primary electron transfer step affords the oxidized donor [ML2]- and the reduced acceptor, A•+ , which reduces dioxygen to superoxide. The latter reaction can compete with back-electron transfer step when the reduction potential of A2+ is more negative than -0.6 V. The overall reaction proceeds only in the case of M = Pt, suggesting that the photoreactive state has ion pair charge transfer character. 32 3.2.2 Reaction with Oxygen In photooxidation, singlet oxygen is a reactive intermediate, produced from molecular oxygen. The molecular orbital diagram shows that the electronic configuration of oxygen is as follows: (1σg)2(1σu)2(2σg)2(2σu)2(3σg)2(3σg)2(1πu)4(1πg)2 , so the ground state of the oxygen is thus a triplet state, denoted 3Σg- utilizing following spectroscopic state notation 1∆g. ( Figure 3.8) 85 E [kJ•mol ] -1 156.9 94.2 Σg+ (1πg) ∆g (1πg) Σg- (1πg) Figure 3.8 Simplified representation of the electron occupation of the 1πg(πpx,y*) orbital in the ground and in the first two excited states of molecular oxygen33 If the triplet ground state of oxygen absorbs energy two kinds of excited state 1∆g and Σg+ may form. However the electronic transitions between them are spin-forbidden transitions. Singlet oxygen can be produced by photosensitization or by thermal processes (Equation 3.1). Sens(S0) Sens*(T1) ISI hv + Sens*(S1) O2 Sens*(T1) Sens(S0) + Equation 3.1 O2 Platinum(II) and Palladium(II) diimine complexes have strong absorptions assigned as MLCT, 3LC, etc.34 They can sensitize the formation of singlet oxygen (1O2) forming ground- state oxygen as a chromophore. Several mixed ligand Pt(II) and Pd(II) generate 1O2 on irradiation at the LLCT band.34a Their ability to photosensitize depends on the metal ion, the number of metal ions in the complex, the nature of the diimine ligand and distortion from planar geometry of the metal complex. There are two mechanisms proposed for the photooxidation of metal complexes. One is through electron transfer to form a radical cation complex and superoxide (O2-) and the other involves a ground state complex and a singlet state oxygen, 1O2 (exited state) through energy transfer. 86 Figure 3.9 t-Bu t-Bu S N + Pt N S hv O2 S N Ph Ph Pt N Ph + S H2O2 Ph t-Bu t-Bu (dbbpy)PtII(edt) t-Bu O N O S H S H Pt N t-Bu t-Bu t-Bu S N + O2 Pt N S O O S N H hv Pt N H S O t-Bu H H O t-Bu (dbbpy)PtII(dpdt) t-Bu O N S H S H Pt N O t-Bu Complexes (dbbpy)PtII(edt) and (dbbpy)Pt(dpdt) (dbbpy = 4,4’-di-tert-butyl- 2,2’bipyridine; dpdt = meso-1,2-diphenyl-1,2-ethanedithiolate; edt = 1,2-ethanedithiolate) were photostable in deoxygenated solution. However, photolysis in the visible charge transfer band in air-saturated solutions induced moderately efficient photooxidation. Photooxidation of (dbbpy)PtII(dpdt) produced the dehydrogenation product (dbbpy)PtII, In contrast, photooxidation of (dbbpy)Pt(dpdt) produced S-oxygenated complexes in which one or two thiolated ligands were converted to a sulfinated(-SO2R) ligand. Mechanistic photochemical studies and transient absorption spectroscopy revealed that photooxidation occured: 1) energy transfer from the charge 87 transfer from a diimine excited state of (dbbpy)PtII(dpdt) to 3O2 to produce 1O2; and 2) reaction between 1O2 and the ground state of (dbbpy)PtII(dpdt). Kinetic data indicated that the excited state (dbbpy)PtII(dpdt) produced 1O2 efficiently and that the reaction between the ground state (dbbpy)PtII(dpdt) and 1O2 occured with k = 3x108 M-1s-1. 35 (Figure 3.9) The violet color of Pt(bpy)(bdt)(bpy=2,2’-bipyridine; bdt=1,2-benzenedithiolate) was due to a Pt/S→diimine charge-tranfer transition; the emission originated from the corresponding triplet state (τ = 460 ns). Photochemical oxidation of Pt(bpy)(bdt) occurred in the presence of oxygen in N,N-dimethylformamide, acetonitrile, or dimethyl sulfoxide solution; the reaction has been investigated by 1H NMR and UV-visible absorption spectroscopy. Singlet oxygen produced by energy transfer of the excited complex was implicated as the active oxygen species. There is sequential formation of sulfinate, Pt(bpy)(bdtO2), and disulfinate, Pt(bpy)(bdtO4) products, both of which have been characterized by X-ray crystallography, The rate of photooxygenation was strongly dependent on water concentration and transient absorption spectra were consistent with the formation of at least one intermediate. On the whole, the data suggest that the photooxidation chemistry of platinum(II) diimine dithiolates was similar to that of organic sulfides. 36 (Figure 3.10) O Figure 3.10 S N Pt S N O S N hv,O2 Pt N S O O S N Pt Pt(bpy)(bdt) S N O O 88 3.2.3 Reaction with Halocarbon Reactant. The lowest exited state of Pt(II) is readily to be oxidized to Pt(IV). Irridating a mixture of the Pt(II) complex and a halocarbon reactant leads to ligand substitution or an oxidation reaction. The complexes, Pt(CO)(PR3)X2 (X=Cl, Br, I; PR3=PEt3, PMePh2 and PPh3) exhibited photoinduced cis-trans isomerization in chlorocarbon solvent such as CHCl3 and CH2Cl2.37 When (TBA)2[Pt(Ecda)2] in dilute chloroform solution was photolyzed with λ < 360 nm, changes in the absorption spectrum occurred. Four isobestic points between 240 nm and 640 nm were observed during the photoreaction. Similar changes were also observed for the (TBA)2[Pt(i-mnt)2] and (PPN)2[Pt(Ecda)2] in CHCl3 (TBA=tetra-n-butylamomnium, Ecda=1(ethoxycarbonyl)-1-cyanoethylene-2-dithiolate, i-mnt=1,1-dicyanoethylene-2,2-dithiolate, and PPN=bis(triphenylhosphoranylidene)ammonium). The rate of photolysis was different in CHCl3, and CH2Cl2, PhCl and PhBr, and followed the rate order: CHCl3 > CH2Cl2 and PhBr > PhCl. These results indicated that the process involves photoreduction of the halocarbon solvent and the rate was dependent on the reduction potential of the halocarbon. If excited by λ > 360 nm, no photoreaction occurred from out of the lowest-energy excited state. 38 The tetrakis(µ-pyrophosphito)diplatinum(II) tetraanion, Pt2(µ-P2O2H2)44- has a long-lived phosphorescence at ambient temperature in aqueous solution. This triplet excited state is both a strong reductant and oxidant. Under photochemical conditions ( λmax > 350 nm), the exited state reacted with alkyl and aryl bromides. The first detectable photo product is Pt2(µ-P2O2H2)4Br4- . The proposed mechanism followed a SRN1 pathway (Equation 3.2). 39 Equation 3.2 Pt2(µ-P2O2H2)4 4-* 2Pt2(µ-P2O2H2)4Br 4- + RBr k2 2hv Pt2(µ-P2O2H2)4Br 4- Pt2(µ-P2O2H2)4 4- + + R. Pt2(µ-P2O2H2)4Br2 4- Another case is Pt(thpy)2. thpy- is the ortho-C-deprotonated form of 2-(2thienyl)pyridine.40 In CH2Cl2, CHCl3 or CH3CN/CH2Cl2 solvents, the complex maintained its luminescent properties and exhibited a photo-oxidative addition reaction with formation of 89 Pt(thpy)2(Cl)(R) (R=CH2Cl or CHCl2) as the sole observed product. In the mixed solvent, the quantum yield of the photoreaction increased with increasing CH2Cl2 concentration. In neat CH2Cl2 the quantum yield of the photoreaction was 0.30 and 0.10 for 313 nm and 430 nm excitation respectively. In CH2Cl2, complete quenching of the luminescent 3MLCT excited state by anthracene via an energy-transfer mechanism was accompanied by only partial quenching of the photoreaction. On contrast, oxygen was a better quencher for the photoreaction than for the luminescent emission. In both cases the fraction of quenched reaction depended on the excitation wavelength. These and other results were interpreted on the basis of a mechanism involving generation of Pt(thpy)2Cl and CH2Cl radicals via 1) a charge transfer to the solvent (CTTS) exited state populated from the intraligand (IL) and metal–to-ligand charge-transfer (MLCT) state obtained by light absorption, and 2) the thermally relaxed 3MLCT luminescent level. Through conversion to CTTS or a bimolecular reaction with CH2Cl2 the primary radicals were involved in a chain mechanism of the type discussed for other oxidative addition reactions, with an average chain length of about 40.41 Photo-oxidizing [(bpy)Pt(tdt)] in CHCl3 led to unstable [(bpy)Pt(tdt)]+ upon irradiation with 577 nm light. The dominant photoinduced reaction of Pt(acac)2 in weakly coordinating solvents was redox decomposition ( other products arise from the oxidation of a ligand and/or a molecule of solvent).42 (Equation 3.3) Equation 3.3 Pt(acac)2 hv CH2Cl2 Pt + Hacac + In CHCl3, the [PtII(bpy)Cl2] was smoothly oxidized to other products [PtIV(bpy)Cl2] when short wavelength light (280 nm < λ < 300 nm) was used for irradiation. The mechanism proposed was shown in Equation 3.4.43 90 Equation 3.4 [PtII(bpy)Cl2] + [PtIII(bpy)Cl2] + [PtIV(bpy)Cl3]+ + hv [PtIII(bpy)Cl3] + . CHCl2 CHCl2 [PtIV(bpy)Cl3]+ + - CHCl2 - [PtIV(bpy)Cl4] + CHCl3 . CHCl2 :CHCl Photolysis of a chloroform solution of [Pt(bpy)(dmt)] ( bpy = 2,2’-bipyridine; dmt = dianion of 3,4-toluenedithiol) at 577 nm in the ligand-to-ligand charge-transfer (LLCT) transition region resulted in electron transfer from the complex to chloroform. A ligand-centered radical (probably the easily oxidized dmt) [Pt(bpy)(dmt)]+ decomposed to unidentified products. A prominent characteristic of square-planar platinum(II) complexes is their ability to undergo oxidative addition reactions which are markedly dependent on the nature of ancillary ligands. A photochemical reaction was observed when a degassed acetonitrile solution containing 24 and iodomethane was exposed to UV radiation for 12h at r.t., and a new Pt(IV) cyanide complex trans-[(CNN)Pt(CN)I2] (25) was obtained, the structure of which was determined by Xray crystallography. Hence oxidation of the metal center and fragmentation of the diaminocarbene ligand was apparent. 44 (Figure 3.11) Figure 3.11 + ClO4 N I N Pt hv H But N H 24 N N Pt CH3I I N 25 91 3.3.3 Objective of Study DAP (1,8-bis(diphenylphosphino)anthracene) (Figure 3.12) has two PPh2 groups connected at the 1,8 position of anthracene and the intramolecular phosphorus distance is about Å. 45 It has several merits as a ligand: 1) The DAP can act as a tridentate PCP ligand; the metal ion can insert into a C-H aryl bond facilitated by the adjacent phosphines, thus providing structurally well-defined metal complexes with many interesting properties; 2) Cyclometalated complexes are known to be strongly emissive and have long luminescent lifetimes (microseconds) in solution indicative of emission from the triplet excited state. 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Photochem. 1992, 17, 1. 96 [...]... absorption at ca 36 8 nm with low-energy tails that extended to ca 500 nm.21 According to the previous study,22 the absorption band were assigned to spin-allowed and spin-forbidden metal- metal-to-ligand-transfer (MMLCT) [dσ*(Pt2)→pσ(Pt2)/π*(C≡CR)] transitions Complexes M2(dcpm)2(CN)4 (M=Pt, 25; Pd, 26) in CH2Cl2 showed absorption bands at 33 7 nm (ε = 2.41x104 M-1cm-1) and 32 8 nm (ε = 2.43x104 dm3mol-1cm-1)... been designed and constructed containing an 83 electron donor, an electron acceptor, and a metal complex-based charge transfer chromophore Most commonly examined transition metal chromophores are octahedral d6 diimine complexes such as Ru(diimine )32 + and Re(diimine)(CO )3( py)+ 27 which have a 3MLCT excited state Recently a relatively new transition metal chromophore, containing platinum complexes, also... sulfides 36 (Figure 3. 10) O Figure 3. 10 S N Pt S N O S N hv,O2 Pt N S O O S N Pt Pt(bpy)(bdt) S N O O 88 3. 2 .3 Reaction with Halocarbon Reactant The lowest exited state of Pt(II) is readily to be oxidized to Pt(IV) Irridating a mixture of the Pt(II) complex and a halocarbon reactant leads to ligand substitution or an oxidation reaction The complexes, Pt(CO)(PR3)X2 (X=Cl, Br, I; PR3=PEt3, PMePh2 and PPh3)... between the metal- centered, d-d states and the metal- to–ligand charge-transfer (MLCT) states The cyclometallated platinum(II) complexes are known to have low-lying MLCT states with useful photochemical and photophysical properties 46 Therefore a series of mononuclear cyclometalated platinum and palladium complexes were prepared They exhibited interesting spectroscopic and luminescent behavior and the results... r.t., and a new Pt(IV) cyanide complex trans-[(CNN)Pt(CN)I2] (25) was obtained, the structure of which was determined by Xray crystallography Hence oxidation of the metal center and fragmentation of the diaminocarbene ligand was apparent 44 (Figure 3. 11) Figure 3. 11 + ClO4 N I N Pt hv H But N H 24 N N Pt CH3I I N 25 91 3. 3 .3 Objective of Study DAP (1,8-bis(diphenylphosphino )anthracene) (Figure 3. 12)... quencher and an acceptor is envisioned through new ligand bridges currently being synthesized using Pd-catalyzed coupling reactions and carbonyl condensations Three fields are discussed next for Pt(II) complexes 3. 2.1 Induced Electron Transfer in Pt(II) Complexes Photoluminescent transition metal complexes are playing an increasingly useful role as probes of electron and energy transfer involving DNA and. .. < 30 0 nm) was used for irradiation The mechanism proposed was shown in Equation 3. 4. 43 90 Equation 3. 4 [PtII(bpy)Cl2] + [PtIII(bpy)Cl2] + [PtIV(bpy)Cl3]+ + hv [PtIII(bpy)Cl3] + CHCl2 CHCl2 [PtIV(bpy)Cl3]+ + - CHCl2 - [PtIV(bpy)Cl4] + CHCl3 CHCl2 :CHCl Photolysis of a chloroform solution of [Pt(bpy)(dmt)] ( bpy = 2,2’-bipyridine; dmt = dianion of 3, 4-toluenedithiol) at 577 nm in the ligand-to-ligand...Figure 3. 6 2+ N N N Pt N Pt C C N N 23 The complex [(CNN)Pt]2(C≡N(CH2)3N≡C)](PF6)2 23 20 (Figure 3. 6) also, has a bridge that brings the two Pt centers closer The 34 0 nm (ε = 104 M-1cm-1) absorption (in CH2Cl2) was assigned to the intraligand 1IL (π-π*) transition and intense low-energy bands with λmax in the range 39 0- 430 nm (ε = 102 M-1cm-1) were assigned to 1MLCT... oxygen as a chromophore Several mixed ligand Pt(II) and Pd(II) generate 1O2 on irradiation at the LLCT band .34 a Their ability to photosensitize depends on the metal ion, the number of metal ions in the complex, the nature of the diimine ligand and distortion from planar geometry of the metal complex There are two mechanisms proposed for the photooxidation of metal complexes One is through electron transfer... the quantum yield of the photoreaction was 0 .30 and 0.10 for 31 3 nm and 430 nm excitation respectively In CH2Cl2, complete quenching of the luminescent 3MLCT excited state by anthracene via an energy-transfer mechanism was accompanied by only partial quenching of the photoreaction On contrast, oxygen was a better quencher for the photoreaction than for the luminescent emission In both cases the fraction . Chapter 3 The Spectroscopy and Photochemistry of Pt(II) Complexes 73 3. 1 Spectroscopy of d 8 Metal Complexes Luminescent coordinatively unsaturated metal complexes. 74 Figure 3. 1 Simplified schematic molecular-orbital diagram of platinum(II) complexes of polypyridine ligands with metal- metal and ligand-ligand interaction The nature and energy of. 250 -35 0 nm region. 80 N N Pt C C C C F i g u r e 3 . 4 13 3. 1.2 Platinum Complexes Linked by a Bridge When two platinum(II) units are in close proximity so as to allow a metal- metal and