In this section, some of the characteristics of different excited states, including their associated photophysical properties, will be discussed. Several possible transitions can occur in diimine rhenium(I) tricarbonyl complexes, including ligand field (LF), MLCT, ligand-to-ligand charge transfer (LLCT),-bond-to-ligand charge transfer (–*), and intraligand (IL) excited states. Due to the strong ligand field effect exerted from the third-row transition metal, the LF transitions are usually located at higher energies compared to other common transitions. Consequently, we will not concentrate in detail on the photophysical role of LF transitions in this case study. Also not discussed here in detail are –* transitions, because these give rise primarily to dissociation and not emission.
2.62.2.1 Complexes with Lowest MLCT Excited States
In general, diimine ReI tricarbonyl complexes with lowest MLCT states feature fairly intense absorption bands in the near-UV to visible spectral region. These bands show negative solvato- chromism (see Chapter 2.27), as revealed by band shifts to lower energy in less polar solvents.1,3,6The direction of the solvent dependence is associated with a reduced (and reversed) molecular dipole in their MLCT excited states. Emissions from these complexes are typically broad and structureless, and they also often exhibit a rigidochromic effect.3,4,6Tables 1and2summarize the luminescence characteristics and environmental effects on absorption and emission maxima for ClRe(CO)3L complexes.2 Both emission quantum yields and lifetimes are significantly increased when the solution is cooled to 77 K, implying that the radiative decay pathways are favored in the more rigid environment; the emission lifetimes are typically governed by the energy gap law.11,13,45,46
Table 3andFigure 1summarize the excited-state decay parameters for the MLCT excited states of fac-[LRe(CO)3(bpy)](PF6) complexes and their resulting energy gap plot, respectively. These complexes also usually exhibit substantial photostability under visible light irradiation and, due
Table 1 Luminescence characteristics for various ClRe(CO)3L complexes.a,b Emission max Lifetime
(103cm1) (ms)
e(15%)c e(15%)
L 298K 77K 298K 77K at 298K at 77K
phen 17.33 18.94 0.3 9.6 0.036 0.33
bpy 18.87 0.6 3.8
5-Me-phen 17.01 18.83 0.65 5.0 0.030 0.33
4,7-Ph2-phen 17.24 18.18 0.4 11.25
5-Cl-phen 17.12 18.69 0.65 6.25
5-Br-phen 17.12 18.69 0.65 7.6 0.020 0.20
5-NO2-phen d 18.28 11.8 0.033
phen-5,6-dione d 18.45 2.5
biquine d 14.58
a Data taken from ref.2. b Measurements in EPA at 77 K or CH2Cl2at 298 K. c Quantum yields determined in benzene at 298 K.
d Luminescence was not detectable from these complexes in solution at 298 K. e biquinẳ2,20-biquinoline.
Table 2 Environmental effects on absorption and emission maxima of several ClRe(CO)3L complexes.a
First Emission max
Environment absorption max (103cm1)
L (T, K) (103cm1) (,ms)
phen CH2Cl2(298) 26.53 17.33 (0.3)
polyester resin(298) 18.52 (3.67)
EPA(77) 18.94 (9.6)
5-Me-phen benzene(298) 25.65 17.00 (0.65)
CH2Cl2(298) 26.32 17.01
CH3OH(298) 27.05 17.00
pure solid(298) 18.42
polyester resin(298) 18.48 (3.5)
EPA(77) 18.83 (5.0)
5-Br-phen benzene(298) 25.32 17.15 (0.65)
CH2Cl2(298) 25.84 17.12
CH3OH(298) 26.88 17.04
pure solid(298) 17.83
polyester resin(298) 18.32 (2.2)
EPA(77) 18.69 (7.6)
5-Cl-phen CH2Cl2(298) 25.91 17.12
pure solid(298) 17.99
EPA(77) 18.69 (6.25)
a Data taken from ref.2.
Table 3 Excited-state decay parameters for the MLCT excited states offac-[LRe(CO)3(bpy)](PF6) complexes in deoxygenated methylene chloride at 296 K.a
Eem kr knr
L (103cm1) e (ns) (s1) (s1)
Cl 16.08 0.005 51 9.79104 1.95107
4-(N,N-dimethylamino)pyridine 16.39 0.017 95 1.78105 1.03107
4-aminopyridine 16.75 0.052 129 4.06105 7.34106
N-methylimidazole 16.98 0.058 161 3.59105 5.85106
4-ethylpyridine 17.64 0.18 604 2.96105 1.36106
pyridine 17.92 0.16 669 2.36105 1.26106
P(CH3)3 18.38 0.27 1,169 2.32105 6.23105
CH3CN 18.66 0.41 1,201 3.43105 4.90105
a Data taken from ref.13.
Non-biological Photochemistry Multiemission 733
to their relatively long-lived triplet characteristics, the emission lifetimes are easily quenched by bimolecular electron- and/or energy-transfer processes in solution.3,47
The electronic structures of MLCT excited molecules of diimine ReItricarbonyl complexes can be viewed as a charge-separated species, [LReII(CO)3(diimine)]*. With an essentially oxidized metal center and reduced diimine ligand, several spectroscopic techniques can be employed to detect the electronic and structural parameters of the excited states. The MLCT excited state experiences a decrease in the extent of Re–CO -back bonding, and this effect can be easily monitored by time-resolved IR spectroscopy and time-resolved resonance Raman spectro- scopy.48,49 Thus, the nanosecond time-resolved IR spectrum of ClRe(CO)3(bpy) shows an average shift tohigher energy by 55 cm1 in the three (CO) bands and the transient infrared spectrum of [(4-Me-py)Re(phen)(CO)3)]þshows an average shift to higher energy by 46 cm1in the three (CO) bands.50,51 Indeed, time-resolved IR spectroscopy has been able to differentiate the lowest excited state between MLCT or IL levels in ClRe(bpy)(CO)3containing phenyleneethyny- lene oligomers.52Transient resonance Raman spectroscopy also provides evidence, based on the resonance enhancement of the (CO) Raman peaks, for identifying the lowest excited states and possible excited-state intermediates.53,54 In such cases, intense excited-state Raman lines have been observed that are associated with the radical anion of the diimine ligand.
2.62.2.2 Complexes with Lowest LLCT Excited States
In coordination complexes with both reducing- and oxidizing-type ligands, excited states can arise that are the result of charge transfer from one ligand to another. Several rhenium tricarbonyl-based chromophore-quencher complexes are known to have lowest excited states featuring LLCT character.10,55Owing to the very weak electronic interaction between the donor and the acceptor components, the extinction coefficients for such LLCT bands are usually very low. For example, the extinction coefficient of the LLCT band for complex [(py-PTZ)ReI(CO)3(bpy)]þ is only 2.4 M1cm1.56 Nevertheless, the LLCT state can be indirectly populated by MLCT excitation followed by an intramolecular electron-transfer process. A representative case is the chromophore- quencher complex, [(py-PTZ)ReI(CO)3(bpy)]þ. Optical excitation into the d(Re) to * (bpy) MLCT transition generates the excited-state species, [(py-PTZ)ReII(CO)3(bpy)]þ.57 Thereafter, rapid electron transfer from py-PTZ to ReII takes place, with a determined rate constant higher than 4.8109s1. The species subsequently formed is [(py-PTZþ)ReI(CO)3(bpy)]þ, which can be considered as a py-PTZ to bpy charge transfer (LLCT) excited state. Direct evidence for the formation of this charge-separated species has been provided from time-resolved resonance Raman and UV–vis spectroscopies, revealing that the complex has both the characteristics of the reduced bpyand oxidized PTZþmoieties.55,58,59The LLCT excited state decays tothe ground
15 16 17 18 19
13.5 15.5 17.5
Innrk
Emission energy (kK)
Figure 1 Plot of lnknrvs. Eemfor the MLCT excited states of a series offac-LRe(CO)3(bpy) complexes in CH2Cl2at 296 K (see Table 3 for complexes). Adapted from ref. 13.
state via back electron transfer from bpytoPTZþwith a rate constant of 1.1107s1.58Typically, the nonradiative decay parameters of the LLCT excited states in such complexes with analogous bpy derivatives follows the energy gap law. Scheme 1 summarizes the electron-transfer processes taking place in this system.58
Due to the generally nonemissive nature of LLCT states, their excited-state properties can be studied only by transient spectroscopy, or indirectly analyzed by their effect on the MLCT excited-state lifetimes of the emissive chromophores. However, if the electron-donor ligand is not stable toward oxidation, then subsequent photochemical reactions may occur. These irreversible photochemical reactions can, thus, be monitored to quantitatively determine the photophysical parameters of LLCT states.60–65Scheme 2depicts the excited-state processes of a typical example, involving the rhenium chromophore coordinated to a 1,2-diamine donor ligand.62 Initial excitation into the MLCT excited state is followed by rapid forward electron transfer from the 1,2-diamine donor ligand with a rate constant of 9.9106s1. The sub- sequently formed 1,2-diamine radical cation is unstable and undergoes rapid CC bond fragmentation to form an-aminoradical and an iminium ion.66,67 The calculated lower limits for the rate constants of back electron transfer and bond fragmentation are 1.5108 and 1.0108s1, respectively.
Another important type of LLCT state arising in diimine rhenium(I) tricarbonyl complexes is found in IReI(CO)3(diimine) complexes. When I replaces Cl or Br, the lowest excited states changes from being MLCT in nature to that of XLCT (halide-to-ligand charge transfer) in character.68 Figure 2 clearly shows that the lowest energy band of the transient absorption
[(4,4′-(x) -2bpyReI(CO) (py-PTZ)]+
[(4,4′-(x) -bpy Re (CO) (py-PTZ )]2 ã– I 3 ã+ + [(4,4′-(x) -bp2 ã–)Re (CO) (py-PTZ)]II
3
+*
kq kq hν 1/τ
) 3
y
)
Scheme 1
(reproduced by permission of the American Chemical Society fromJ. Phys. Chem.1991,95, 5850–5858.)
II N –N N
(CO)3 MLCT
CH2 Ph
H Ph
+hν kd kBET
I N
H Ph
Ph N
N N
CH2
CH2 (CO)3
(CO)3
(CO)3
(CO)3
CH2
CH2
CH CH
kFET
LLCT
N H
N
H NH
N (bpy)ReIN
(2) + 2 PhCHO + Ph
Ph
Ph Ph
t-1 e-1 (4)
N
CH Ph N
(bpy)ReI N
+ +
(5) + H O2
argon (recombination and back electron transfer) +ã
ã kBF
(3) (4)
+
O2, –H+
O2, –H+
bpy Re ( ) bpy Re
( )
bpy ReI
( )
Scheme 2
(reproduced by permission of the American Chemical Society fromJ. Phys. Chem.1995,99, 1961–1968.) Non-biological Photochemistry Multiemission 735
(a)
Cl/bpy
Br/bpy
I/bpy
400 500 600 700 800
400 500 600 700 800
400 500 600 700 800
Wavelength (nm)
Wavelength (nm)
Absorbance changeAbsorbance change Absorbance change
0.15
0.10
0.05
0.00 0.00 0.05
0.10
0.05
0.00 (b)
(c)
Wavelength (nm)
Figure 2 Ground state (– – – –) and transient absorption (———) spectra of XRe(CO)3(bpy) in THF measured 10 ns after laser excitation at 460 nm. Xẳ(a) Cl; (b) Br; (c) I. (reproduced by permission of the
American Chemical Society from ref. 69.).
spectrum is red shifted on progressing from CltoI.69For the case of IReI(CO)3(bpy), a broad but distinct low-energy band appears around 780 nm. In contrast to the above-mentioned LLCT (L to diimine) transitions, which are weak due to the very small electronic coupling between the donor and acceptor, the halide py and * (diimine) orbitals are now directly coupled by either sharing the metal dyzorbital or by through-space py-* interactions (see Scheme 3). Thus, XLCT transitions exhibit comparable or slightly weaker intensities compared to MLCT transitions, as shown inFigure 3.69
2.62.2.3 Complexes with Lowest IL Excited States
Lowest IL states usually occur in complexes containing extended conjugation of the ligands, where the electron is excited predominantly from the ligand-based n- or -orbitals.
Typical characteristics of IL emissions are structured profiles and longer emission lifetimes compared to MLCT transitions.70–78 The emission lifetimes are sometimes greatly influenced by temperature or medium effects, though, due to the presence of close-lying MLCT states.79
An early report by Wrighton and co-workers revealed that complex ClRe(CO)3(3-benzoylpyri- dine)2exhibits typical 3MLCT emission in room-temperature benzene solution. However, in a 77 K EPA glass, the rigidochromic effect shifts the 3MLCT state tohigher energy and, thus, multiple emissions from both 3MLCT and 3IL (n-*) excited states can be observed (seeFigure 4).
Here, the3MLCT and3n-* states are clearly not thermally equilibrated in this glassy environ- ment at low temperature.71
Many LRe(CO)3(X-phen) complexes, where L is a Lewis base and X-phen is phenanthroline or its derivatives, exhibit overlapping emissions from both3MLCT and3IL (–*) excited states at room temperature. By varying L, X-phen, and temperature, the emitting states can be tuned from
3MLCT to 3–* in nature.Figure 5 compares the emission spectra of a series of (py)Re(CO)3
(X-phen) complexes at 298 K and 77 K. More structured emissions were observed at 77 K, as well as in complexes with higher 3MLCT excited states.72 The excited-state decays are alsomore complicated at low temperature and feature bi- or multi-exponential kinetics.73
An intriguing example reported by Meyer and co-workers has revealed that the observed
3MLCT emission in a system with close-lying 3MLCT and 3IL states does not necessarily prove that the lowest excited state is 3MLCT in character. In the case of ClRe(CO)3(dppz) (dppz is dipyrido[3,2-a:20,30-c]phenazine), the lowest excited state was determined to be the
3–* excited state by time-resolved resonance Raman spectroscopy, although the emission apparently originates from the 3MLCT excited state. Replacing Cl by PPh3 yields emission originating from the 3–* state, which is also confirmed by time-resolved resonance Raman spectroscopy.80
pπ(I) dπ
pπ(Cl)
Re(CO)3 -diimine) x
X y
x OC
CO Re CO NN π*(α-diimine) z
(α
Scheme 3
Non-biological Photochemistry Multiemission 737
Complexes incorporating a ligand with a lowest non-emissive 3IL excited state that is energetically tunable by light-induced structural change (such as in stilbene or azobenzene derivatives) have potential applications as light-switching materials. When the olefin or azo groups are in trans-conformations, the complexes are weakly or non-emissive due to the presence of lowest non-emissive 3–* o r 3n–* excited states. Excitation into their 3MLCT excited state sensitizes the 3–* o r 3n–* excited states and results in trans-cis isomerization of the ligand. The 3–* o r 3n–* excited states in the cis-conformer are shifted to a higher energy position compared to the emissive 3MLCT state and, consequently, strong emission is observed.
Several reports have taken advantage of this unique property to design a variety of photo- switching systems in recent years.34–36,38,39,42–44 Moore and co-workers reported one particu- larly interesting example.36 The complexes in their study are fac-Re(CO)3(bpy) chromophores linked by a styryl pyridine that attaches an amine or an azacrown ether group (see Scheme 4 for structures). The absorption spectra of these complexes feature an intense intraligand charge-transfer (ILCT) band, which can be removed by protonation of the amine or azacrown
6,000
4,000
2,000
0
300 400 500 600 700
700 Wavelength (nm)
(a)
(b)
300 400 500 600
6,000
4,000
2,000
0
Wavelength (nm)
ε (M cm )–1–1ε (M cm )–1–1
Cl Br I
Cl Br I
Figure 3 Electronic absorption spectra of (a) XRe(CO)3(iPr-DAB); and (b) XRe(CO)3(iPr-PyCa) (XẳCl, Br, I) in THF at 293 K. Adapted from ref. 69.
ether (see Figure 6). Both the amine and azacrown complexes are only weakly emissive at room temperature. Prolonged irradiation of the complexes into either the ILCT or MLCT states results in no change of the absorption spectrum, indicative of negligible photochemistry.
However, under the same experimental conditions with the protonated complexes, the absorp- tion spectra changed profoundly, and this is consistent with efficient trans-cis photoisomeriza- tion at the olefin bond. Notably, the emission in these protonated species is also enhanced during the irradiation, reflecting the higher energy position of the 3–* states compared to the
3MLCT states in the cis-styryl pyridine complexes (see Figure 7).