Photophysical Properties of Soluble Polypyrrole−Polypyridyl

J. Phys. Chem. B 2001, 105, 4801-4809

4801

Photophysical Properties of Soluble Polypyrrole-Polypyridyl-Ruthenium(II) Complexes He´ le` ne Laguitton-Pasquier,*,† Agne` s Martre, and Alain Deronzier‡ Laboratoire d’Electrochimie Organique et de Photochimie Redox, UMR CNRS 5630, UniVersite´ Joseph Fourier Grenoble 1, BP 53, 38041 Grenoble CEDEX 9, France ReceiVed: June 16, 2000; In Final Form: NoVember 15, 2000

The photophysical properties of polypyridyl ruthenium(II) complexes substituted by pyrrole groups and of their corresponding soluble polymers synthesized either by electrooxidative or photoredox techniques are investigated. While their spectral properties and the temperature dependence of their lifetime are similar and close to that found for the parent [Ru(bpy)3]2+ complex, the emission quantum yields (ΦL) and the timeresolved emission decays are strongly affected by the polymer structure. Most of the polymers are characterized by a nonexponential emission decay that can be satisfactorily analyzed globally over different temperature in the framework of a distribution of decay rates. An important decrease in ΦL along with the increase of the width of the distribution of decay rates occurs as the degree of cross-linking is increased. The 3MLCT excited states features are also substantially dependent on the length of the alkyl linkage between the pyrrole group and the Ru(II) complex as well as on the synthetic method of polymerization used. Furthermore, the accessibility of the excited states to quenchers as methyl-viologen (electron withdrawer) or N-methylphenothiazine (electron donor) is not significantly affected by the polymer structure indicating that the un-cross-linked polymers seem to adopt an extended coil-like structure. Using the photophysical data, attempts were made to obtain insights into the polymer architecture.

Introduction One of the various approaches explored to create artificial photosynthesis molecular assemblies1-4 is based on soluble polymers. Polymer-bound metal complexes seem particularly appropriate for the fabrication of a device combining several photoredox sites in the same molecular system. Due to the high local concentration of active sites, such molecular assemblies are expected to achieve multielectronic photoredox reactions in limited space. Meyer and co-workers1,5-9 have shown the occurrence of intrastrand photochemical electron and energy transfer in soluble polymers containing Ru(II) or Os(II) polypyridyl complexes. They developed a synthetical strategy based on the use of preformed polymeric backbones which can be further derivatized. Functional molecules as transition metal chromophores associated with electron donor and electron acceptor groups (diad or triad structure) can be then attached to a soluble polymer by such a way. An alternative synthetical approach can be considered involving the direct polymerization (e.g., electropolymerization) of monomeric structures containing a combination of chromophores, electron-transfer functions, and polymerizable groups. Despite the enormous interest of this technique for making thin films on electrode surfaces,10a only a few examples relate to supramolecular assemblies.10b On the other hand, no application for synthesis of soluble polymers has been yet reported. To provide a sound basis on which to form an unambiguous

interpretation of the effects of this kind of polymer structure upon the photoredox properties of the diad or triad unit, it is important to establish, first, to what extent the excited states properties of the metal complex chromophores are affected by this polymeric environment. In this context, we have recently shown that the photopolymerization and, in a less extent, the electropolymerization of pyrrole-substituted ruthenium tris-bipyridyl complexes (Scheme 1) yield to soluble polymers11,12 exhibiting redox properties closely related to those of the corresponding monomers. Photopolymerization of those monomers was accomplished in the presence of an irreversible oxidative quencher (O2 or diazonium salt). In the present work, the photophysical properties of the resulting polymers are reported and compared with those found for monomers. As there are no appropriate experimental methods suitable for the separation of such cationic polymers containing both organic and inorganic parts, no direct evidences for the polymer structure can be obtained. Hence, using the luminescence approaches (steady-state and time-resolved spectroscopies) and the Ru(II) complexes as probes, attempts are made to gain valuable information regarding the polymer structure (compactness, heterogeneity of the microenvironments, ...). Indeed, the chemical structure of the monomeric units (number of pyrrole groups per monomer determining the cross-linking degree of the polymer, length of the pyrrole-complex link) has a strong influence on the polymeric architecture. Experimental Section

* Corresponding author. † Current address: Laboratoire de Chimie Physique, UMR 8000, Bat. 350, Universite´ Paris-Sud XI, 91405 Orsay, Cedex, France. Tel: + 33 1 69 15 4204. Fax: + 33 169 156188. E-mail: [email protected]•psud.fr. ‡ Tel: + 33 476 514861. Fax: + 33 476 514267. E-mail: [email protected]

Materials. The ligands L1, L2, and L3 and the monomer complexes [Ru(bpy)2(L1)](PF6)2, [Ru(bpy)(L1)2](PF6)2, [Ru(L1)3](PF6)2, [Ru(bpy)2(L2)](PF6)2, and [Ru(bpy)2(L3)](PF6)2 (Scheme 1) were synthesized as previously described.11,12 [Ru(bpy)3](PF6)2 was prepared from its chloride salt (Strem) by anion

10.1021/jp002188e CCC: $20.00 © 2001 American Chemical Society Published on Web 05/04/2001

4802 J. Phys. Chem. B, Vol. 105, No. 21, 2001

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SCHEME 1: Structure of the Pyrrole-Substituted Ruthenium(II) Trisbipyridyl Monomers Investigated

metathesis using Amberlite resin IRA-93. Tetra-n-butylammonium perchlorate (TBAP) obtained from Fluka was dried under vacuum at 80 °C for 72 h. Methyl viologen hexafluorophosphate (MV2+) was obtained by converting methyl viologen dichloride (Fluka) to the PF6- salt using an ion exchange column containing Amberlite IRA-93 ion exchanger in the hexafluorophosphate form. N-Methylphenothiazine (N-MPTZ) (Aldrich) was used as received. Spectroquality acetonitrile (SDS) was used without further purification. Polymer Synthesis. Photopolymers were synthesized from millimolar solutions of pyrrole-substituted complexes in aerated acetonitrile with the light of a 250 W Hg lamp filtered through UV and IR cutoff filters as previously described.11 Electropolymers were prepared by electrooxidative polymerization of a millimolar solution of pyrrole-substituted complexes in deoxygenated acetonitrile containing 0.1 M TBAP.12,13 The complete polymerization was controlled by NMR and electrochemistry.11,12 Especially, the NMR spectra are characterized by a broadening of all the signals as it is generally observed for polymer proton NMR14 and a total disappearance of the proton resonances of the pyrrole groups due to the polymerization.11,12 As previously demonstrated,11-13 whatever the polymerization way used, the resulting soluble functionalized polypyrroles were obtained in their nonconductive overoxidized form. This has been evidenced by IR spectroscopy of the polymers which exhibit carbonyl bands typical of the presence of some pyrrolidone moeties15 and by the lack of the polypyrrole electroactivity on cyclic voltammogram.11,12 Furthermore, Any absorption of the polypyrrole backbone have been detected in the UV-vis part. On the orther hand, the solubility of the various photo- and electropolymers in acetonitrile have been previously estimated.12 It was shown that an increase in the number of pyrrole units per complex or in the hydrophobic character of the ligand lowered the solubility of the polymer. Furthermore, the electropolymers are less soluble than the corresponding photopolymer. As a matter of fact, the photoredox process produces shorter-chain polymers and/or oligomers than the oxidative electropolymerization process.

All attempts to characterize the average molecular weight of the polymers using gel permeation chromatography or light scattering experiments were unsuccessful. Sample Preparation. The complex concentration was typically 5 × 10-6 M for stationary experiments (absorption and emission spectra, emission quantum yield determination). Due to the weak luminescence of the Ru complexes, the decay curves have been recorded on samples of 5 × 10-5 M to obtain the best signal-to-noise ratio. In all the oxidative and reductive quenching experiments, the concentration of the complexes was 6 × 10-6 M and constant ionic strength was held using TBAP 0.1 M. The concentration of MV2+ varied from 1.25 × 10-3 M to 1.25 × 10-4 M and that of N-MPTZ from 1.75 × 10-3 M to 1.25 × 10-4 M. All solutions were freshly prepared prior to use. All the photophysical experiments were performed in CH3CN on deoxygenated solutions (Ar bubbling for 1h) and at room temperature except where otherwise stated. Apparatus. UV-vis spectra were obtained using a Cary 1 absorption spectrophotometer on 1-cm path length quartz cells. The steady-state emission spectra were recorded on a Photon Technology International (PTI) SE-900M spectrofluorimeter. Emission quantum yield φL were determined at 25 °C in deoxygenated acetonitrile solutions with a CH3CN solution of 16 at 25 °C) [Ru(bpy)3](PF6)2 as a standard (φref L ) 0.062 according to eq 1:

ISL (1 - 10-OD ) ref φL -OD Iref ) L (1 - 10 ref

φSL )

(1)

where IL, the emission intensity, was calculated from the spectrum area ∫I(νj)dνj, OD represents the optical density at the excitation wavelength (454 nm). The superscripts “S” and “ref” refer, respectively, to the sample and to the standard. Time-resolved studies were performed on a PTI QuantaMaster luminescence lifetime spectrometer. It consists of a nanosecond flash lamp (fwhm of 5 ns) coupled to a lens-based T-format sample compartment that has a stroboscopic detector.

Soluble Polypyrrole-Polypyridyl-Ru(II) Complexes

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4803

TABLE 1: MLCT Absorption Band Maxima (λmax abs ), Luminescence Maxima (λmax em ), and Emission Quantum Yield (OL) for Monomer Complexes at 298 K, in Deoxygenated Acetonitrile Solutiona complexes 2+

[Ru(bpy)3] [Ru(bpy)2(L1)]2+ [Ru(bpy)(L1)2]2+ [Ru(L1)3]2+ [Ru(bpy)2(L2)]2+ [Ru(bpy)2(L3)]2+ a

λmax abs (nm)

λmax em (nm)

φL

450 453 457 460 453 453

604 612 618 617 614 615

0.06216 0.060 0.049 0.052 0.060 0.060

max Error limits are: λmax abs , λem ) (1 nm; φL ) (0.002.

The detailed description of the instrument has been previously reported.17 To obtain the best signal-to-noise ratio, the decay curves have been performed under excitation over the wavelength window 300-450 nm and using a 475 nm cutoff filter which screened scattered lamp light. We checked that the decay curves did not depend on the excitation wavelengths (300450 nm) in agreement with literature.18,19 Decay curves were obtained by monitoring at 610 nm. No systematic variation in the characteristics of the decay curves was found in the 550800 nm range. Flash pulses scattered from a nonfluorescing CH3CN solution in a 1-cm path length quartz cell were used to generate the instrument response function. The decay curves were analyzed by using PTI software for single- and biexponential fits or Igor Software (Wavemetrics) for fits based on the function of distribution of decay rates. Both are based on iterative reconvolution using a Marquardt algorithm.20 Accuracy of the fit was evaluated by inspecting residuals. Results Monomers. The methyl and alkyl-pyrrole substitutions on the 4,4′ positions of the bipyridyl ligand induces only slight modification of the absorption and emission spectra versus those of the parent [Ru(bpy)3]2+ complex (Table 1). The MLCT absorption and the emission bands have been red-shifted from their positions in the parent complex. This shift is of the same order of magnitude as that observed for an unpolymerizable analogous complex, [Ru(dmbpy)3]2+ (λMLCT ) 460 nm and abs 21,22 dmbpy ) 4,4′-dimethyl-2,2′-bipyridine). λmax em ) 628 nm; The five complexes give a value of the emission quantum yield (φL) close to that of [Ru(bpy)3]2+ (Table 1). φL is independent of the length of the alkyl chain and decreases by 10% for complexes bearing more than one substituted bipyridyl ligand ([Ru(bpy)(L1)2]2+ and [Ru(L1)3]2+). The decay curves of all the monomers are single-exponential although a long lifetime component is observed, in addition to the monoexponential component, which percentage does not exceed 2%. The fact that it constitutes only a small fraction of

the total emission allows us to assign the slow-decaying component to a luminescent impurity. On the other hand, since the 3d-d excited state gives rise to photosubstitution reactions,19,23-26 we check that at high temperature, the irradiation of solutions containing each complex does not lead to ligand photodissociation. The 1H NMR spectrum of [Ru(bpy)2(L1)]2+ remains the same (intensity and chemical shift) upon visible irradiation (λ ) 454 nm) of the solution for more than 4 h at 70 °C. This means that no ligand loss occurs and the photopolymerization of the pyrrole-substituted complexes cannot be achieved without oxidative quencher in the solution,11 even at high temperature. The temperature dependence of the lifetime τ(T) could be satisfactorily accounted for by assuming the relation in eq 2 suggested by Van Houten and Watts.27-29

1 ) k0 + kd with kd ) k1 e-∆E/kBT τ(T)

(2)

The temperature-dependent term has been interpreted as involving thermally activated population of the near-lying 3d-d state from the emitting 3MLCT excited states.24,27-29 The values of the fit parameters k0, k1, and ∆E are summarized in Table 2 as well as the estimated lifetime at 25 °C calculated from eq 2. They are of the same order of magnitude as those exhibited by [Ru(bpy)3]2+ in CH3CN solution.16,30 Nevertheless, from the data of Table 2, it appears that there is no correlation between the values of parameters ∆E and k1 and the number of substituted bipyridine ligands per complex or the length of the alkyl chain. Unfortunately, the curve fitting used here is not sensitive enough for the temperature range to provide reliable measure for both the energy gap (∆E) between the 3MLCT and 3d-d excited states and the rate constant k1. In contrast, the rate constant kd appears to be more reliable and the value of kd obtained in the present work and in the literature16 for [Ru(bpy)3]2+ are identical. It can be clearly seen in Figure 1 and Table 2 that the variation of the lifetime with the temperature is weaker for all the five complexes with respect to that of [Ru(bpy)3]2+. This trend suggests that the substitution on the 4,4′ positions of the 2,2′bipyridine ligand by methyl or alkyl groups leads to a decrease of the 3MLCT f 3d-d surface crossing rate (kd), preventing slightly the populating of the thermally activated 3d-d excited state. It should be noted that the presence of electrolyte (TBAP) in the solution does not affect the photophysical properties of the complexes. The absorption and emission spectra of solution of [Ru(bpy)3]2+ or [Ru(bpy)2(L1)]2+ containing 0.1 mol L-1 TBAP are identical to those recorded on solution free of TBAP. Furthermore, no significant changes on the values of the emission quantum yield and of the parameters k0, k1, and ∆E can be noted.

TABLE 2: Kinetic Parameters for the Decay of the MLCT Excited States of Monomers in Deoxygenated Acetonitrile Solutiona,b

[Ru(bpy)3]2+ literature16 [Ru(bpy)2(L1)]2+ [Ru(bpy)(L1)2]2+ [Ru(L1)3]2+ [Ru(bpy)2(L2)]2+ [Ru(bpy)2(L3)]2+

τ (298K) (ns)

k0 (s-1 × 10-5)

k1 (s-1 × 10-14)

∆E (cm-1)

kd (s-1 × 10-5)

kr (s-1 × 10-4)

knr (s-1 × 10-5)

819 855 911 935 912 924 948

5.95 5.60 6.52 7.15 7.17 6.99 7.33

5.18 0.58 1.05 93.9 7.87 2.96 7.11

4250 3800 3990 5065 4440 4240 4460

6.25 6.22 4.49 2.23 3.80 3.98 3.15

7.5 7.7 6.4 5.3 6.7 6.5 6.2

5.2 4.8 5.9 6.6 6.6 6.3 6.7

a The lifetime τ and the rate constant k are calculated at 25 °C from the parameters k , k and ∆E by using eq 2. The radiative (k ) and nonradiative d 0 1 r (knr) rate constants have been determined from the experimental value of φL at 298 K and the calculated value of τ at 25 °C, by using eq 5 and eq 6. b Error limits are: k0 ) ((0.13-0.56) × 105 s-1; k1 ) ((0.13-2.99) × 1014 s-1; ∆E ) ((30-216) cm-1.

4804 J. Phys. Chem. B, Vol. 105, No. 21, 2001

Figure 1. Temperature dependence of the luminescence lifetime for [Ru(bpy)3]2+, [Ru(bpy)2(L1)]2+, [Ru(L1)3]2+, and [Ru(bpy)2(L2)]2+ in deoxygenated acetonitrile solution. The solid line is the theoretical curve fit in the framework of eq 2. The values of the fit parameters k0, k1, and ∆E are given in Table 2.

Polymers. For all the studied polymers (photo- and electropolymers), there are no significant change on the MLCT absorption and emission maxima with respect to those of the corresponding monomers. Nevertheless, a broadening of the absorption spectra of the polymers is observed increasing from the photopolymers to the electropolymers (Figure 2) while the full width at half-maximum of the emission spectrum is not affected by the polymer matrix. From the data in Table 3, it can be shown that the emission quantum yields (φL) are highly sensitive to the degree of cross-linking and the mode of polymerization used. Except for the photopolymers poly-[Ru(bpy)2(L1)]2+, poly-[Ru(bpy)2(L2)]2+ and poly-[Ru(bpy)2(L3)]2+ where the decrease in φL is rather marginal and close to the experimental error, the values of φL exhibited by the other

Laguitton-Pasquier et al. polymers are smaller than those obtained for the corresponding monomers. The decrease in φL is significant since it reaches 60% in the case of the photopolymer poly-[Ru(L1)3]2+. Furthermore, increasing of the degree of cross-linking decreases φL. Especially, a decrease by 70% in φL from the photopolymer poly-[Ru(bpy)2(L1)]2+ to the highly cross-linked photopolymer poly-[Ru(L1)3]2+ is observed. It has to be noted that the length of the alkyl pyrrole chain does not affect the value of φL. Indeed, identical values of φL are obtained for the photo- or electropolymers poly-[Ru(bpy)2(L1)]2+ (short alkyl chain) and poly-[Ru(bpy)2(L2)]2+ (long alkyl chain). Furthermore, at variance with complexes bearing short chains, an increase of the degree of cross-linking leads to an insignificant change on the value of φL in the case of complexes with long chain. The decay curves of the polymers are clearly not singleexponential except those of the photopolymers poly-[Ru(bpy)2(L1)]2+, poly-[Ru(bpy)2(L2)]2+, and poly-[Ru(bpy)2(L3)]2+ which remain single-exponential. Since the electropolymers poly-[Ru(bpy)(L1)2]2+ and poly-[Ru(L1)3]2+ are poorly or not soluble in CH3CN solution, no decay curves with a good signal-to-noise ratio can be obtained. Attempts are made to fit the decay curves using a biexponential function. Despite rather good residuals (Figure 3), the behavior with temperature of the two lifetime components does not follow eq 2, thus appears to be not satisfying. Furthermore, such a model bears no physical meaning in our systems; the existence of two components could mean, for example, that the polymers consist of two distinct emissive species or excited states. The success of the fit is likely a consequence of the introduction of four variable parameters into the fit. As the decay time of the polypyridyl Ru(II) complexes are highly sensitive to their microenvironment,16,26,30 the nonexponential behavior of the decay curves may be ascribed to some local heterogeneities in the polymer backbone creating multiple environments. Furthermore, the polarization of the surrounding medium by an excited state could also contribute to the nonexponentiality. The excited states created under that polarization field would exhibit different decay times. Hence, one can expect to analyze the decay curves in polymers by a function

Figure 2. Absorption and emission (λex ) 454 nm) spectra of [Ru(L1)3]2+ as monomer, photopolymer, and electropolymer in deoxygenated acetonitrile solution at 25 °C.

Soluble Polypyrrole-Polypyridyl-Ru(II) Complexes

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4805

TABLE 3: Emission Quantum Yields of Monomers and Polymers at 298 K in Deoxygenated Acetonitrile Solutiona monomer photopolymer electropolymer a

[Ru(bpy)2(L1)]2+

[Ru(bpy)(L1)2]2+

[Ru(L1)3]2+

[Ru(bpy)2(L2)]2+

[Ru(bpy)2(L3)]2+

0.060 0.055 0.031

0.049 0.033 0.020

0.052 0.020 b

0.060 0.054 0.034

0.059 0.055 0.031

Error limits are: ΦL ) (0.002. b Insoluble polymer in CH3CN 0.1 M TBAP.

TABLE 4: Temperature Dependence of the Parameters β and k for Electropolymer Poly-[Ru(bpy)2(L2)]2+ in Deoxygenated Acetonitrile Containing 0.1 M TBAPa

a

temperature (°C)

β

k (s-1)

-9 8 17 24 32.5 44 52.5 75

0.81 0.81 0.80 0.79 0.79 0.80 0.81 0.79

7.47 × 105 9.11 × 105 1.08 × 106 1.28 × 106 1.62 × 106 2.57 × 106 3.44 × 106 9.73 × 106

Error limits are: β ) (0.01; k ) ((0.43-1.9) × 105 s-1.

TABLE 5: Kinetic Parameters for the Decay of the MLCT Excited States of Polymers in Deoxygenated Acetonitrile Solutiona τ (298 K) k0 kd ∆E (ns) (s-1 × 10-5) (s-1 × 10-5) (cm-1) [Ru(bpy)2(L1)]2+ photopolyelectropoly[Ru(bpy)(L1)2]2+ photopoly[Ru(L1)3]2+ photopoly[Ru(bpy)2(L2)]2+ photopolyelectropoly[Ru(bpy)2(L3)]2+ photopolyelectropoly-

Figure 3. Emission decay recorded at 610 nm for the electropolymer poly-[Ru(bpy)2(L3)]2+ in deoxygenated acetonitrile solution at 25 °C and best fits (a) using model from eq 2, function of distribution of decay rates or (b) for biexponential decay. Top panel: difference between the fitted curve (a) model from eq 2, distribution of decay rates or (b) biexponential function and the experimental data.

of distribution of decay rates. For this purpose, the decay curves have been fitted by the first derivative, with respect to time, of the Kohlrausch/Williams-Watts function31,32 which has been derived and applied to described relaxation in disordered media:31-37

I(t) ) B0 + B1 tβ-1 e-(kt)

β

(3)

β is related to the width of the distribution (0 e β e 1) and k designates the average rate constant of the 3MLCT excited-state decaying of the Ru complexes in polymers. The single curve fitting of the nonexponential decay curves using the distribution function (eq 3) gives also good residuals. The major effect of the temperature variation is to change the value of k, while the parameter β fluctuates and appears to be not dependent on temperature (Table 4). To obtain more accurate fit parameters,38 the global analysis of the nonexponential decay curves in the framework of eq 3 have been performed, linking the parameter β over different temperatures. For each decay curve, good residuals are obtained (Figure 3). The temperature dependence of the average rate constant k, deduced from the global analysis

899 1030 990 727 895 780 899 603

6.27 6.70 8.06 11.3 6.10 7.75 6.79 14.3

4.85 2.98 2.03 2.38 5.07 5.20 4.33 2.38

3420 4710 4430 4655 3550 4090 3865 5491

R τ and kd are calculated at 298 K using eq 2 from the temperaturedependent rate constant k. The values of k are obtained from the global analysis of the nonexponential decay curves over different temperature in the framework of eq 3 except for the photopolymers poly[Ru(bpy)2(L1)]2+, poly-[Ru(bpy)2(L2)]2+, and poly-[Ru(bpy)2(L3)]2+ of which decays have been analyzed according to a single-exponential. b Error limits are: k ) ((0.23-0.71) × 105 s-1; ∆E )((63-286) 0 cm-1.

of the time-resolved decays, can be satisfactorily analyzed using eq 2. The fit parameters k0, k1, and ∆E as well as the values of kd are of the same order of magnitude as those obtained for the corresponding monomers (Table 5). This indicates that in polymers the 3d-d excited state is populated in a similar way than in monomers. The polymer matrix does not provide a more stabilizing and encaging structure for the Ru complexes that would prevent the ligand photodissociation. As expected, the value of β (Table 6) decreases upon increasing the degree of cross-linking. Indeed, while in the atactic photopolymer poly-[Ru(bpy)2(L1)]2+ the value of β reaches unity, β amounts to 0.74 for the highly cross-linked photopolymer poly-[Ru(L1)3]2+. This suggests that an increase in the degree of cross-linking generates multiple local environments and/or raises the local density of Ru complex sites. On the other hand, unlike the emission quantum yield, the parameter β depends on the length of the alkyl chain, however, this effect is less pronounced for complexes bearing long pyrrole-complex link. Bimolecular Oxidative and Reductive Quenching. The oxidative and reductive quenching of the 3MLCT excited state of Ru complexes in photo- and electropolymer by the noncovalently linked methyl viologen (MV2+, electron acceptor) and N-methylphenothiazine (N-MPTZ, electron donor) have been investigated in acetonitrile solutions. As the electrochemical11,12

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TABLE 6: Values of Parameter β Deduced from the Global Analysis of the Emission Decays over Different Temperatures in the Framework of Eq 3a β

[Ru(bpy)2(L1)]2+

[Ru(bpy)(L1)2]2+

[Ru(L1)3]2+

[Ru(bpy)2(L2)]2+

[Ru(bpy)2(L3)]2+

photopolymer electropolymer

1b 0.87

0.87 c

0.74 c

1b 0.80

1b 0.75

a Error limits are: β ) ( 0.02. b Decay is single-exponential. The fit of the single-exponential decay using eq 3 leads to values of β equal to 1 ( 0.02 and values of k similar to that found using a single-exponential fit function. c Insoluble polymer in CH3CN 0.1 M TBAP.

TABLE 7: Oxidative and Reductive Quenching Results at 25 °C in Deoxygenated Acetonitrile 0.1 M TBAPa 2+

kMV (×109 L q mol-1 s-1)b complexes ]2+

[Ru(bpy)3 literature

[Ru(bpy)2(L1)]2+ [Ru(bpy)(L1)2]2+ [Ru(L1)3]2+ [Ru(bpy)2(L2)]2+ [Ru(bpy)2(L3)]2+

monomer photopolyelectropolymonomer photopolymonomer photopolymonomer photopolyelectropolymonomer photopolyelectropoly-

kN-MPTZ (×109 L q mol-1 s-1)b

I/IQc

τ/τQd

I/IQc

τ/τQd

2.2 2.360 2.461 1.8 1.9 1.8 1.8 1.5 2.0 2.0 2.3 2.7 1.6 2.1 1.5 1.6

2.3

1.05 1.3[62]

0.89

2.3 2.4 2.3 2.2 1.9 2.3 2.3 2.1 2.2 2.2 1.6 1.6 1.6

0.55 0.55 0.50 0.24 0.26 0.20 0.09 0.53 0.46 0.45 0.32 0.41 0.35

0.40 0.51 0.52 0.23 0.20 0.10 0.12 0.35 0.49 0.55 0.40 0.41 0.41

2+

Error limits are: kMV (c and d) ) ((0.1-0.3) × 109 L mol-1 s-1; q (c) ) ((0.2-0.7) × 108 L mol-1 s-1; kN-MPTZ (d) ) ((0.1q 1.1) × 108 L mol-1 s-1. b The values of the bimolecular quenching rate constant, kq, are calculated from the slope of the plots (a) I/IQ vs [Q] and (b) τ/τQ vs [Q]. a

kN-MPTZ q

and photophysical properties of the Ru(II) complexes studied here are similar to those of the parent [Ru(bpy)3]2+ complex, it is obvious that both the reduction of the excited Ru complexes by N-MPTZ39,40 and their oxidation by MV2+ 39 are thermodynamically allowed. The luminescence spectra as well as the decay curves of the monomers, photopolymers, and electropolymers are recorded in the absence and presence of MV2+ and N-MPTZ. Even at high quencher concentrations, the I/IQ vs [Q] or τ/τQ vs [Q] plots are linear for the quenching of the various monomers and polymers according to the Stern-Volmer relation:

τ I ) ) 1 + kq τ [Q] IQ τ Q

(4)

with I and IQ the integrated emission intensities in the abscence and presence of quencher Q, τ and τQ the decay time of the Ru complexes in the absence and presence of quencher, and [Q] the quencher concentration. kq designates the bimolecular quenching rate constant. The values of τ used are those estimated at 298 K using eq 2 from the temperature-dependent rate constant k41 (Table 2 and Table 5). From the slope of the straight lines, the value of kq can be calculated and is of the order of 2 × 109 L mol-1 s-1 for the quenching by MV2+ while kq is in the range of (5-1) × 108 L mol-1 s-1 for the quenching by N-MPTZ whatever the system is (monomer or polymer) (Table 7). For monomers, while the value of kq for the quenching by MV2+ is weakly affected neither by the length of the alkyl chain nor the number of substituted bipyridine ligand per complex, those for the quenching by N-MPTZ are significantly modified when the complex bears substituted ligand. Indeed, the alkyl

chain substitution of the bipyridine ligand slows down the quenching rate exhibited by N-MPTZ. This is supported by the value of kq which amounts to 1.05 × 109 L mol-1 s-1 for [Ru(bpy)3]2+ while kq is 5.5 × 108 L mol-1 s-1 for the complex bearing one substituted ligand, [Ru(bpy)2(L1)]2+. The phenomenom is enhanced as the number of alkyl chain per complex is increased, indicating that the alkyl chain encaged partly the Ru sites, reducing the accessibility for the quencher in agreement with the results of Rau et al.,42 for instance. Those observations reveal that the N-MPTZ quenching is somewhat sensitive to the accessibility of the complex core. On the other hand, comparing the values of kq for the monomer and polymer complexes, they are not much different which would indicate that the electron transfer from the excited state of the Ru complexes to MV2+ or N-MPTZ is not inhibited by the polymer backbone. It is consistent with the belief that most of the photopolymers and electropolymers investigated in the present paper behave as a relatively open coil. Upon addition of MV2+ to the solution of either the monomers or the polymers, a lifetime quenching is observed. This indicates that the bimolecular electron transfer occurs through a dynamic mechanism. This implies that MV2+ diffuses toward the Ru complexes in polymers. This diffusional quenching of MV2+ is consistent with the experimentally determined values of kq. On the other hand, except for the photo- poly[Ru(L1)3]2+ and electropolymer poly-[Ru(bpy)2(L3)]2+ where only a lifetime quenching is detected, the addition of N-MPTZ yields to a decrease of both the decay times and the initial emission intensity of the decay curves. This indicates that the bimolecular quenching by N-MPTZ occurs by both dynamic and static processes. The static process can be interpreted as the extent of the quencher N-MPTZ in contact with the Ru2+ sites.43 Since no static quenching by N-MPTZ has been detected in the photo- poly-[Ru(L1)3]2+ and the electropolymer poly[Ru(bpy)2(L3)]2+, this is presumably a result of a lower accessibility to the Ru complexes due to the polymer crosslinking. Discussion 1-Monomers. As expected,18 alkyl chain substitution on the 4,4′ positions of the bipyridine ligand produces only a minor change in the position and the feature of the absorption and emission spectra as well as in the values of the emission decay time. Hence, the alkyl substitution does not affect the electronic origin of the excited and ground states of the [Ru(bpy)3]2+ type complexes. The general model of the photophysics of polypyridyl Ru(II) complexes16,23,24,30,44-48 holds that the temperature dependence of the decay time τ is due to the activated surface crossing from 3MLCT to 3d-d which undergoes photochemistry and/or photophysic deactivations.19,23,26 As the 3d-d excited-state represents a major deactivation pathway for the 3MLCT excited states at room temperature, more than 50% of the 3MLCT excited states are deactivated via the 3d-d state (Table 2), it is important to establish accurently the partitions between the

Soluble Polypyrrole-Polypyridyl-Ru(II) Complexes

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4807

TABLE 8: Experimental Test of the Energy Gap Law for Monomers Values of the Slope and Intercept (Eem ) 0) from the Plots of ln(knr) vs Eem slope (eV-1)

intercept (eV)

ref

-7.29 ( 0.9 28.08 ( 1.7 this work [Ru(bpy)3]2+ solvent dependence -7.45 28.02 14 [Ru(bpy)2L2]2+ compound seriesa -7.49 28.02 45 [Os(bpy)L4]2+ compound series -7.54 29.17 51 [Ru(bpy)3-x(L)x]

a

2+

In CH2Cl2 solution.

radiative, nonradiative and 3MLCT f 3d-d decay channels thanks to a study of the emission decay curves at various temperature. In eq 2, k0 is a rate constant which includes both radiative (kr) and nonradiative (knr) contributions to the rate of the 3MLCT excited states decay, assuming that the temperature dependence of kr and knr are negligible:44

k0 ) kr + knr

(5)

It follows that the quantum yield for emission at the temperature T, φL(T), is given by eq 6:

φL(T) ) ηisc kr τ(T)

(6)

where ηisc is the efficiency of population of the emitting 3MLCT states following excitation. As generally accepted for Ru(II) complexes in fluid solution at room temperature, ηisc is assumed to be unity.18,49-52 From the values of τ and φL, both estimated at 25 °C, the values of kr and knr have been determined using eq 5 and eq 6. From Table 2, the change in kr through the series of Ru(II) complexes is minor which supports the assumption of a common electronic origin of the emitting states. Earlier work on MLCT excited states based on [Os(II)(bpy)(L)4]2+,53 on [Ru(bpy)3]2+ 16, or on [Ru(bpy)2(L)2]2+ 44 (L ) pyridine or substituted pyridine) has shown that the rates of non radiative decay processes follow the energy gap law for radiationless transitions.54,55 The law predicts that knr ∝ exp(Eem) where Eem represents the emission energy. In agreement with the energy gap law, the plot of ln(knr) vs Eem for the series of Ru complexes studied here is linear. The estimated slopes (Table 8) are within experimental error of those found for a series of complexes of [Os(bpy)2(L)4]2+,53 of [Ru(bpy)2(L)2]2+, 44 and of [Ru(bpy)3]2+ where variations in Eem are achieved by variations in solvent.16 This indicates that the acceptor vibration remains the same for the various series meaning that the nonradiative deactivation is dominated by the bpy ring-streching vibrations.44,48 2-Polymers. The photophysical properties of the Ru complexes are largely retained in electro- and photopolymers: the absorption and emission spectra as well as the temperature dependence of the decay curves are nearly those of the monomers. Nevertheless, there are special effects arising from their multisite character. It has been seen that polymerization causes an important decrease in quantum yield. As the polypyrrole is overoxidized11,12 (see Experimental Section), quenching by electron transfer or energy transfer of the excited Ru complexes by the overoxidized polypyrrole backbone is ruled out. The decrease in ΦL can result from environmental effects or from self-quenching. Indeed, the photophysical characteristics (ΦL and τ) of MLCT excited states are known to be sensitive to their environment.16,26,30 The polymer matrix can create microenvironments whose polarity differs from the CH3CN one.56 This might lead

to a change in the values of the emission quantum yield and the decay times of the excited states. However, since no attempts were made to correlate the values of quantum yield of [Ru(bpy)3]2+ derived complexes with solvent dielectric properties, the effect of the local surrounding medium on the quantum yield is difficult to quantify. Even though the lower polarity of the polymer core with respect to that of the pure CH3CN solvent contributes to the low values of ΦL exhibited by the Ru complexes in the polymer, it cannot completely account for the general trend of ΦL in the polymer. On the other hand, Kelder et al.57 have previously shown that the addition of poly(vinyl) sulfate (PVS) in [Ru(bpy)3]2+ solution affects strongly the emission of [Ru(bpy)3]2+ as long as [3MLCT [Ru(bpy)3]2+]/ [PVS] > 4. They clearly showed that all the effects observed are due to the concentration of [Ru(bpy)3]2+ and of 3MLCT excited state in the polymer and interpreted the results as 3MLCT-3MLCT or 3MLCT-1A annihilation. Therefore, the 1g large decrease in ΦL can be also attributed to the self-quenching of the 3MLCT excited states of the Ru complexes which are in close proximity in the polymer as previously reported for soluble polymers with pendant [Ru(bpy)3]2+ groups.58,59 It is obvious that the local concentration of the Ru complexes, hence the distance between the Ru complexes, determines the extent of the quenching. A Fo¨rster-type energy transfer process between two environmental inequivalent Ru complexes cannot be ruled out. From the overlap of the absorption and emission spectra of the Ru complexes in the polymer, the Fo¨rster critical distance has been estimated to ∼20 Å. It is of the same order as the distance between two Ru complexes in the polymer which can be estimated to around 15 Å in poly-[Ru(L1)3]2+ by analogy with the polymer systems investigated by Friesen et al.7 Nevertheless, if energy transfer occurs, it is probably in a time scale shorter than the time resolution of our system (5 ns). Thus, no available information can be obtained. In the following part of the text, self-quenching must be understood as intrastrand energy transfer, 3MLCT-3MLCT or 3MLCT-1A1g annihilation. The photophysical parameters ΦL and β are highly dependent on the degree of cross-linking, on the length of the alkyl chain, as well as on the polymerization method used (electro- or photopolymerization). As pointed out above, the parameter β which is a measure of the width of the lifetimes distribution is related to the extent of both local heterogeneities and the average distance between the complexes in the polymer while the quantum yied ΦL seems to be mainly affected by the second factor. Therefore, the analysis of their values would provide us some insights on the macromolecular structure of the different polymers. For complexes bearing short alkyl chain, the large decrease in ΦL as the Ru complex functionality is increased indicates that self-quenching of the 3MLCT excited states is enhanced in cross-linked polymers. Despite the 2+ charges and the large molecular excluded volume of the Ru complexes, the network resulting from cross-linking should force them to be in close proximity. This should also account for the dependence of β on the degree of cross-linking. The effect of cross-linking on the photophysical parameters ΦL and β is strongly reduced in the case of complexes with long alkyl chain. When comparing the results for the photo- or electropolymers poly-[Ru(bpy)2(L2)]2+ and poly-[Ru(bpy)2(L3)]2+, no changes in ΦL are observed which would indicates that the extent of the self-quenching is not modified. Hence, when synthesized from monomers with long alkyl chains, the polymer network seems to be less dense and the individual Ru sites appear to be largely isolated from an electronic density

4808 J. Phys. Chem. B, Vol. 105, No. 21, 2001 point of view, at least in the photopolymers where the decay of the 3MLCT excited states is single-exponential. The slight decrease in the value of β from 0.80 to 0.75 exhibited by the electropolymers, respectively, poly-[Ru(bpy)2(L2)]2+ (one long alkyl chain per complex) and poly-[Ru(bpy)2(L3)]2+ (two long alkyl chains per complex) probably indicates that cross-linking diversifies the nature of the Ru complexes surrounding medium. In other words, the inhomogeneity of polymer grows when cross-linked. This conclusion can be reasonably stated whatever the length of the alkyl chain. Furthermore, for un-cross-linked polymers, the length of the alkyl chain also appears to affect the extent of the heterogeneity of the Ru complexes microenvironments (decrease in β) rather than the average distance between those sites (no changes in the value of ΦL). The invariance of ΦL is surprising. Indeed, an increase of the length of the alkyl chain link provides a growth in the degrees of freedom for the Ru complexes in the polymer structure. This allows the polymer to adopt a conformation where the assumed important repulsive interactions between cationic Ru centers as well as steric hindrances are substantially reduced. Thence, it should imply a polymer structure less dense (more free volume) in which the average distance between the Ru complexes is increased and, as a consequence, the value of ΦL should be higher. It has been previously shown in the case of polystyrene-bearing pendant [Ru(bpy)3]2+ groups that the increase of the length of the chemical link induces a more expanded polymer structure.7 Here, the invariance of ΦL would denote that a long alkyl chain yields a more coiled polymer in which long-range interactions between spatially close [Ru(bpy)3]2+ complexes, separated along the chain by several monomeric units, are promoted. Such interaction can induce self-quenching that can balance the effect of less tight polymer matrix. It has to be noted that those assumptions are also consistent with an increase of the microenvironments diversity for the Ru complexes. On the other hand, despite a similar general trend, a strong discrepancy in the values of the parameters ΦL and β between the photopolymers and the electropolymers has been observed, regardless of the degree of cross-linking and the length of the alkyl chain. The photopolymers always exhibit higher values of ΦL and β than the corresponding electropolymers. This indicates that self-quenching is enhanced and/or inhomogeneities increased in the electropolymer. A plausible explanation for such a difference is that the length of the polymer backbone is longer in the electropolymer than in the photopolymer in accordance with the lower solubility of the electropolymers.12 This comes from that photopolymerization is a homogeneous process while electropolymerization occurs at the electrode-electrolyte interface where pyrrolic radical cations (which are the key species of the polypyrrole chain formation) are produced at a high concentration. Furthermore, this matchs the results recorded on soluble polystyrene with pendant [Ru(bpy)3]2+ groups where a decrease in the parameters ΦL and β as the number of monomeric units per polymer is increased has been reported.7 Conclusion For most of the soluble polymers investigated in the present work, the luminescence decay of the Ru complexes is nonexponential. It reveals that the Ru sites in the polymers are perturbated by the polymer environment although the electronic origin of the excited states of the Ru complexes is not modified when polymerized. All the decay curves can be successfully analyzed in the framework of a distribution of decay rates. It can result from local inhomogeneities in the polymer backbone

Laguitton-Pasquier et al. and from a high local density of Ru complexes whose importance are strongly dependent on the degree of crosslinking, on the length of the alkyl-pyrrole chain, and on the way of polymerization used. It should be stressed that analysis of the two photophysical parameters, β, related to the width of the distribution of decay rates and ΦL, the emission quantum yield of the Ru complex, allows us to gain structural information on this kind of polymers. Hence, electropolymerization yields to a longer backbone than photopolymerization in agreement with solubility measurements.12 A long alkyl chain yields a more coiled (for un-cross-linked polymers) but less dense polymers than short alkyl chain. Increasing the degree of cross-linking or the length of the alkyl chain increases the microenvironment diversity for the Ru complexes. The structure of the polymers appears to be somewhat extended. The polypyrrole backbone does not provide an encaging structure for the Ru sites except when the degree of cross-linking is high. Under that condition, a high local density of Ru sites is obtained, which is a promising result in view to achieve multielectron photoredox reaction using similar polymer systems based on diad and triad units. Consequently, we are currently developing synthetic works to design this kind of polymers. Acknowledgment. We thank J.-C. Vial for preliminary photophysical experiments. References and Notes (1) Baxter, S. M.; Jones, W. E.; Danielson, E.; Worl, L.; Strouse, G. F.; Younathan, J.; Meyer, T. J. Coord. Chem. ReV. 1991, 111, 47-71. (2) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Kemp, T. J., Ed.; Ellis Horwood: London, 1991; pp 89-150. (3) Piotrowiak, P. Chem. Soc. ReV. 1999, 28, 143-150. (4) Connolly, J. S.; Bolton, J. R. Photoinduced electron transfer; Fox, M. A., Chanon, M., Eds.; Elsevier: New York 1988; Part D, Chapter 6.2, pp 303-393. (5) Jones, W. E.; Baxter, S. M.; Strouse, G. F.; Meyer, T. J. J. Am. Chem. Soc. 1993, 115, 7363-7373. (6) Dupray, L. M.; Devenney, M.; Striplin, D. R.; Meyer, T. J. J. Am. Chem. Soc. 1997, 119, 10243-10244. (7) Friesen, D. A.; Kajita, T.; Danielson, E.; Meyer, T. J. Inorg. Chem. 1998, 37, 2756-2762. (8) Strouse, G. F.; Worl, L. A.; Younathan, J. N.; Meyer, T. J. J. Am. Chem. Soc. 1989, 111, 9101-9102. (9) Devenney, M.; Worl, L. A.; Gould, S.; Guadalupe, A.; Sullivan, B. P.; Caspar, J. V.; Leasure, R. L.; Gardner, J. R.; Meyer, T. J. J. Phys. Chem. A 1997, 101, 4535-4540. (10) (a) The literature on this topic is too vast to be cited exhaustively: Deronzier, A.; Moutet, J.-C. Coord. Chem. ReV. 1996, 147, 339-371. (b) See the following reference for an example of supramolecular assembly: Deronzier, A.; Essakali El-Hocini, M. J. Chem. Soc., Chem. Commun. 1990, 242-244. (11) Deronzier, A.; Jardon, P.; Martre, A.; Moutet, J.-C.; Santato, C.; Balzani, V.; Credi, A.; Paolucci, F.; Roffia, S. New J. Chem. 1998, 33-37. (12) Deronzier, A.; Eloy, D.; Jardon, P.; Martre, A.; Moutet, J.-C. J. Electroanal. Chem. 1998, 103, 179-185. (13) Cosnier, S.; Deronzier, A.; Moutet, J.-C. J. Electroanal. Chem. 1985, 193, 193-204. (14) Chen, Z.-K.; Meng, H.; Lai, Y.-H.; Huang, W. Macromolecules 1999, 32, 4351-4358. (15) Christensen, P. A.; Hamnett, E. A. Electrochim. Acta 1991, 36, 1263-1286. (16) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 55835590. (17) Douglas, R. J.; Siemiarczuk, A.; Ware, W. R. ReV. Sci. Instrum. 1992, 63, 1710-1716. (18) Nakamaru, K. Bull. Chem. Soc. Jpn. 1982, 55, 2697-2705. (19) Strouse, G. F.; Schoonover, J. R.; Duesing, R.; Boyde, S.; Jones, W. E., Jr.; Meyer, T. J. Inorg. Chem. 1995, 34, 473-487. (20) Marquardt, D. W. J. Soc. Ind. Appl. Math. 1963, 11, 431-441. (21) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159-244. (22) Lin, C. T.; Bo¨ttcher, W.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1976, 98, 6536-6544. (23) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85-277.

Soluble Polypyrrole-Polypyridyl-Ru(II) Complexes (24) Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer,T. J. J. Am. Chem. Soc. 1982, 104, 4803-4810. (25) Durham, B.; Walsh, J. L.; Carter, C. L.; Meyer, T. J. Inorg. Chem. 1980, 19, 860-865. (26) Meyer, T. J. Pure Appl. Chem. 1986, 58, 1193-1206. (27) (a) Hager, G. D.; Crosby, G. A. J. Am. Chem. Soc. 1975, 97, 70317037. (b) Hager, G. D.; Watts, R. J.; Crosby, G. A. J. Am. Chem. Soc. 1975, 97, 7037-7042. (28) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 48534858. (29) Van Houten, J. C.; Watts, R. J. Inorg. Chem. 1978, 17, 33813385. (30) Sun, H.; Hoffman, M. Z. J. Phys. Chem. 1993, 97, 11956-11959. (31) Williams, G.; Watts, D. C.; Dev, S. B.; North, A. M. Trans. Faraday. Soc. 1971, 67, 1323-1335. (32) Williams, G.; Watts, D. C. Trans. Faraday Soc. 1970, 66, 80-85. (33) Castellano, F. N.; Heimer, T. A.; Tanhasetti, M. T.; Meyer, T. J. Chem. Mater. 1994, 6, 1041-1048. (34) (a) Scher, H.; Lax, M. Phys. ReV. B7 1973, 4491-4502. (b) Scher, H.; Lax, M. Phys. ReV. B7 1973, 4502-4519. (35) Blumen, A.; Klafter, J.; Silbey, R. J. Chem. Phys. 1980, 72, 53205332. (36) Palmer, R. J.; Stein, D. L.; Abrahams, E.; Anderson, P. W. Phys. ReV. Lett. 1984, 53, 958-961. (37) Lindsey, C. P.; Patterson, G. D. J. Chem. Phys. 1980, 73, 33483357. (38) Van der Auweraer, M.; Ballet, P.; De Schryver, F. C.; Kowalczyk, A. Chem. Phys. 1994, 187, 399-416. (39) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 48154824. (40) Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes; Plenum Press: New York, 1994; pp 165-215. (41) For monomers and photopolymers poly-[Ru(bpy)2(L1)]2+, poly-[Ru(bpy)2(L2)]2+, and poly-[Ru(bpy)2(L3)]2+, the values of k are obtained by fitting the decays by a single-exponential. For polymer exhibiting non exponential decays, k are deduced from the global analysis of the decay curves in the framework of eq 3.

J. Phys. Chem. B, Vol. 105, No. 21, 2001 4809 (42) Rau, H.; Frank, R.; Greiner, G. J. Phys. Chem. 1986, 90, 24762481. (43) Since no new emission bands appear upon addition of N-MPTZ to the monomer or polymer solutions, such a chromophore/quencher pair seems to be non emitting. (44) Kober, E. M.; Meyer, T. J. Inorg. Chem. 1982, 21, 3967-3977. (45) Creutz, C.; Chow, M.; Netzel, M.; Okumura, M.; Sutin, N. J. Am. Chem. Soc. 1980, 102, 1309-1319. (46) Morris, D. E.; Hanck, K. W.; De Armond, M. K. J. Am. Chem. Soc. 1983, 105, 3032-3038. (47) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444-2453. (48) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J. Am. Chem. Soc. 1984, 106, 2613-2620. (49) Klassen, D. M.; Crosby, G. A. J. Chem. Phys. 1968, 48, 18531858. (50) Demas, J. N.; Taylor, D. G. Inorg. Chem. 1979, 18, 3177-3179. (51) Demas, J. N.; Adamson, A. W. J. Am. Chem. Soc. 1971, 93, 18001801. (52) Damrauer, N. H.; Cerullo, G.; Yeh, A.; Boussie, T. R.; Shank, C. V.; McCusker, J. K. Science 1997, 275, 54-57. (53) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 630-632. (54) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145-164. (55) Freed, R. F.; Jortner, J. J. Chem. Phys. 1970, 52, 6272-6291. (56) Soutar, I.; Swanson, L. Applications of luminescence spectroscopy to the study of polymers, in Current trends in polymer photochemistry; Allen, N. S., Edge, M., Bellobono I. R., Selli, E., Eds.; Ellis Horwood: Chichester, U.K., 1995; pp 1-23. (57) Kelder, S.; Rabani, J. J. Phys. Chem. 1981, 85, 1637-1640. (58) Ennis, P. M.; Kelly, J. M.; O’Connel, C. M. J. Chem. Soc., Dalton Trans. 1986, 2485-2491. (59) Sumi, K.; Furue, M.; Nozakura, S. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 3779-3788. (60) Sun, H.; Yoshimura, A.; Hoffman, M. Z. J. Phys. Chem. 1994, 98, 5058-5064. (61) Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401-449. (62) Maestri, M.; Gra¨tzel, M. Ber. Bunsen-Ges. 1977, 81, 504-507.