Rigid Medium Stabilization of Metal-to-Ligand Charge Transfer

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J. Phys. Chem. B 2007, 111, 6930-6941

Rigid Medium Stabilization of Metal-to-Ligand Charge Transfer Excited States† David W. Thompson,*,‡ Cavan N. Fleming,§ Brent D. Myron,‡ and Thomas J. Meyer*,§ Department of Chemistry, The UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290, and the Department of Chemistry, Memorial UniVersity of Newfoundland, St. John’s, Newfoundland A1B 3X7, Canada ReceiVed: December 18, 2006; In Final Form: March 12, 2007

The salts [Ru(bpy)3](PF6)2, cis-[Ru(bpy)2(py)2](PF6)2, trans-[Ru(bpy)2(4-Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, and [Re(bpy)(CO)3(4-Etpy)](PF6) (bpy ) 2,2′-bipyridine, py ) pyridine, 4-Etpy ) 4-ethylpyridine, and tpy ) 2,2′:6′,2-terpyridine) have been incorporated into poly(methyl methacrylate) (PMMA) films and their photophysical properties examined by both steady-state and time-resolved absorption and emission measurements. Excited-state lifetimes for the metal salts incorporated in PMMA are longer and emission energies enhanced due to a rigid medium effect when compared to fluid CH3CN solution. In PMMA part of the fluid medium reorganization energy, λoo, contributes to the energy gap with λoo ∼ 700 cm-1 for [Ru(bpy)3](PF6)2 from emission measurements. Enhanced lifetimes can be explained by the energy gap law and the influence of the excited-to-ground state energy gap, Eo, on nonradiative decay. From the results of emission spectral fitting on [Ru(bpy)3](PF6)2* in PMMA, Eo is temperature dependent above 200 K with ∂Eo/∂T ) 2.8 cm-1/deg. cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 are nonemissive in CH3CN and undergo photochemical ligand loss. Both emit in PMMA and are stable toward ligand loss even for extended photolysis periods. The lifetime of cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is temperature dependent, consistent with a contribution to excited-state decay from thermal population and decay through a low-lying dd state or states. At temperatures above 190 K, coinciding with the onset of the temperature dependence of Eo for [Ru(bpy)3](PF6)2*, lifetimes become significantly nonexponential. The nonexponential behavior is attributed to dynamic coupling between MLCT and dd states, with the lifetime of the latter greatly enhanced in PMMA with τ ∼ 3 ns. On the basis of these data and data in 4:1 (v/v) EtOH/MeOH, the energy gap between the MLCT and dd states is decreased by ∼700 cm-1 in PMMA with the dd state at higher energy by ∆H0 ∼ 1000 cm-1. The “rigid medium stabilization effect” for cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is attributed to inhibition of metal-ligand bond breaking and a photochemical cage effect.

Introduction The metal-to-ligand charge transfer (MLCT) excited states of Ru(II) polypyridyl complexes are commonly used in a variety of studies.1-3 The results of temperature-dependent lifetime (and quantum yield),4-11 resonance Raman,12,13 emission spectral fitting,14-16 and recent laser flash photolysis experiments17-21 have provided insight into the structure and reactivity of these excited states. A complicating feature arises because of lowlying dd states that often lie near in energy to the emitting MLCT states and are thermally accessible.21-33 The intervention of dd states is signaled by the appearance of ligand loss photochemistry4,5 and the onset of a significant temperature dependence in lifetimes (τ) and emission quantum yields (Φem).4,5 The dd states provide an additional pathway for nonradiative decay and are the origin of ligand-loss photochemistry.5,20-33 The resulting photoinstability can severely limit use of Ru polypyridyl complexes for studies of photoinduced electron and energy transfer in molecular assemblies in solution.33 Some progress has been made in suppressing MLCT f dd surface crossing through judicious choice of acceptor and nonchromophoric ligands.25,32,35 In complexes of Re(I) or Os(II), where 10Dq is 20-30% higher, the dd states are usually

thermally inaccessible.5 The intervention of dd states is minimized at lower temperatures where population by thermally activated barrier crossing is suppressed. Another approach to stabilizing otherwise unstable complexes is to imbed them in rigid media. In an early observation by Allsop and Kemp, a slight temperature dependence of τ for [Ru(bpy)3]2+* was observed in cellulose acetate pointing to little or no intervention by dd states in that medium.36-38 In retrospect, this observation pointed to the possibility of using rigid media at room temperature to stabilize MLCT excited states toward ligand loss and photodecomposition. The use of poly(methyl methacrylate) (PMMA) in low temperature (4-77 K) emission studies on [Ru(bpy)3]2+* was first described by Crosby.39 Hecker, Fanwick, and McMillan have noted that excited-state lifetimes for [Ru(tpy)2]2+* and [Ru(tpy)(bpy)(CH3CN)]2+* are significantly enhanced in PMMA.40

†

Part of the special issue “Norman Sutin Festschrift”. Memorial University of Newfoundland. § The University of North Carolina. ‡

Kincaid and co-workers have demonstrated significantly enhanced excited-state lifetimes for [Ru(bpy)3,]2+*,

10.1021/jp068682l CCC: $37.00 © 2007 American Chemical Society Published on Web 05/03/2007

Rigid Medium Stabilization Effect [Ru(tpy)2]2+*, [Ru(bpy)2(daf),]2+*, and other RuII chromophores incorporated in zeolites and attributed the effect to matrix inhibition of MLCT-dd interconversion by destabilizing ligand field or dd states, thereby increasing the MLCT-dd energy gap.41 Similar effects have been observed for other RuII polypyridyl complexes incorporated in poly(ethylene oxide),42 Sephadex SP C-25,43 and sol-gel matrices.44

In earlier communications we have reported on photoinduced electron and energy transfer in molecular assemblies and the inhibition of photochemistry in PMMA and sol-gel monoliths.45,46 In this paper we report the results of a series of photophysical studies in PMMA which were designed to reveal the microscopic origins of the “rigid medium stabilization effect” which lead to stabilization of MLCT excited states. Experimental Section Materials. The salts [Ru(bpy)3](PF6)2, cis-[Ru(bpy)2(py)2](PF6)2, trans-[Ru(bpy)2(4-Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, and fac-[Re(bpy)(CO)3(4-Etpy)](PF6) were available from previous studies.47-49 The salts [Ru(bpy)3](PF6)2, trans-[Ru(bpy)2(4Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, [Re(bpy)(CO)3(4-Etpy)](PF6), and [Os(bpy)3](PF6)2 were purified by cation exchange HPLC with an Aquapore CX-300 column (1.0 × 10.0 cm) of poly(DL-Asp)-silica (Brownlee) with an elution gradient of 0-400 mM KBr in 2:3 (v/v) CH3CN/0.6 mM phosphate buffer (pH ) 7.2). Acetonitrile and chloroform were obtained from Burdick and Jackson and used without further purification. Poly(methyl methacrylate) (PMMA: average Mw ) 350000, Tg (midpoint) ) 122 °C) and tetra-n-butylammonium chloride, [N(n-C4H9)4]Cl, were purchased from Aldrich and used as received. Aluminum foil coated with Teflon was purchased from Cole Parmer. Sample Preparation. Poly(methyl methacrylate) (0.6 g) was dissolved in CHCl3 (5 mL) with constant stirring. Dissolution occurred over a period of 30 min with the application of mild heat (40 °C). The complex was dissolved in CHCl3 or CH3CN depending on the solubility and the concentration was adjusted to an absorbance value of 0.6 at the MLCT maximum. One milliliter of the complex solution was added to the polymer solution, and the mixture was stirred and poured into a Teflon mold. The molds were placed in a vacuum desiccator partially open to the atmosphere in the dark. Care was taken to do all sample manipulations in the presence of a minimal amount of light. The solutions were allowed to evaporate slowly (3-4 days) following which the molds were placed in vacuo for 2-3 days. Typical weight percents of metal complex salts to polymer were 0.05-0.2. At >0.5 wt %, phase separation of the complex salts from the polymer matrix occurred. Phase separation was also observed when attempts were made to incorporate more highly charged ions (>3+) into PMMA by this method. Once formed, the films typically weighed 0.7-1.0 g. In our hands this method of solvent casting gave nonuniform film thicknesses of 0.05-1 mm. Measurements UV-Visible Spectra. UV-visible spectra were recorded on a Hewlett-Packard model 8451A diode array spectrophotometer

J. Phys. Chem. B, Vol. 111, No. 24, 2007 6931 or an Olis-modified Cary 14 spectrophotometer interfaced to an IBM microcomputer. Visible spectra were recorded on all samples before and after excited-state measurements to ensure that samples did not undergo photodecomposition. Visible spectra were recorded on films that were optically transparent. Emission Lifetimes. Emission lifetimes were obtained with a PRA LN1000 pulsed nitrogen laser as an excitation source at 337 nm coupled to a PRA grating LN102/1000 tunable dye head laser. Variable temperature measurements were made by using a Janis model NDT-6 cryostat coupled to a Lakeshore DRC84C temperature controller. All lifetimes were obtained following 460 nm excitation except for the film containing fac[Re(bpy)(CO)3(4-Etpy)](PF6) for which 420 nm excitation was used. The excitation beam was passed through a collection of lenses and defocused onto the film. For the films, the incident excitation beam was directed 90° to the film surface and emission was monitored from the front face of the film at an angle 45° to the excitation beam. The emitted light was passed through a PRA B204-3 2.5 monochromator and to a Hamamatsu R-928 water-cooled photomultiplier tube. Scattered light was removed by a dichromate filter solution. A LeCroy 9400 digital oscilloscope or a LeCroy 7200A digitizer was interfaced with an IBM PC microcomputer to collect the kinetic data. Reported lifetimes are the average of 150-200 decay traces and were independent of monitoring wavelength. Wavelength-dependent, time-resolved emission data were collected at 10-20 nm increments with 1 mm slits. The observed intensities were not corrected for the instrument response. The emission lifetime for [Ru(bpy)3](PF6)2 in PMMA at room temperature was independent of concentration in the range 0.05-0.5 wt %. Excited-state measurements were obtained under ambient conditions without concern for O2 quenching due to the slow rate of O2 diffusion in PMMA (D ) 1 × 10-8 cm2 s-1).50 The effect of O2 quenching in polymeric media has been described previously.51 Samples, which were kept in the dark under vacuum, did not exhibit any changes in excited-state properties over a period of several months.52 Transient Absorption Measurements. Nanosecond transient absorption spectra and kinetic decays were measured by means of previously described methods and instrumentation.33 The third harmonic (354.7 nm) of a Quanta Ray DCR-2A Nd:Yag laser was used to pump a Quanta Ray PDL-2 dye laser to produce 420 nm, 4 ns excitation pulses with energies less than 5 mJ/ pulse which were used to excite the samples. The excitation beam was collinear with the monitoring beam. The probe beam utilized a 300 W pulsed Xe arc lamp; detection was obtained by using an f/3.4 grating monochromator and a five-stage PMT. The resulting output was collected with the use of a LeCroy 9400 transient digitizer. Electronic control and synchronization of the laser, probe, and transient digitizer were provided by electronics of our own design. Time-resolved emission spectra were measured on the picosecond time scale by time-correlated single photon counting using the instrument described in detail elsewhere.53 Spectra were corrected for the response of the emission grating and detector. Kinetics. The procedures and protocols used for nonlinear least-squares analysis of the emission decay traces have been described previously.33,35 Global analysis of time-resolved emission data from [Ru(bpy)3](PF6)2 was performed by the procedure of Maeder and Zuberbuler.54 This treatment of large, multivariant data sets including both kinetic and equilibrium data, provides the spectra of components as well as rate and/or equilibrium constants.55 The program Specfit56 as modified for

6932 J. Phys. Chem. B, Vol. 111, No. 24, 2007

Thompson et al.

TABLE 1: Spectroscopic and Photophysical Data in CH3CN and PMMA at 298 ( 3 K λmax (abs), nma complexb PMMA [Ru(bpy)3]2+ cis-[Ru(bpy)2(py)2]2+ trans-[Ru(bpy)2(4-Etpy)2]2+ [Ru(typ)2]2+ [Os(bpy)3]2+ [Re(CO)3(bpy)(4-Etpy)]+

c

φp

0.068d 0.2d 0.49g

CH3CN

PMMA

454 468 478 478 478 360

300 (sh) 454 470 478 478 480 356

λmax (em), nm CH3CN

PMMA

622 n.o.f n.o.f n.o.f 743 590

482 590 630 620 (br)h 620g 704 524

τem, ns CH3CN 918d 2.7 n.o.f 0.25i 60 192

PMMA (β) b µGS ≈ 0 and b µES and b µGS, the dipole moments of the excited and ground states. For a dd excited state there is no change in radial electronic distribution, (dπ)6 f (dπ)5(dσ*)1, and b µES ≈ b µGS ) 0.9 It follows from eqs 17 and 19 that the free energy difference between states 0 , compared to the difference in a frozen medium, ∆Gfr,dd-MLCT in a fluid with comparable dielectric properties, is given by eqs 25-27. 0 0 ∆Gfr,dd-MLCT ) (∆G0dd,fr - ∆GMLCT,fr )) 0 MLCT ) + (λdd ) (25) (∆G0dd,fl - ∆GMLCT,fl 00 - λ00 0 ∆Gfr,dd-MLCT ) ∆G0fl + ∆λ00

(26)

MLCT ∆λ00 ) λdd 00 - λ00

(27)

MLCT On the basis of this analysis with λdd ≈ 00 and λ00 720 cm-1 from emission measurements and eq 16, the MLCT state is destabilized in PMMA relative to the dd state by ∼700 cm-1. MLCT-dd Barrier Crossing Dynamics. ActiVation Free Energy. The MLCT to dd interconversion through kdd is a

vibronically induced transition between different eigenstates of the same molecule. In the classical limit kdd a is related to a free energy of activation, ∆G*, as shown in eqs 28 and 29.9 In these expressions λMLCTfdd is the total reorganization energy between the dd and MLCT states in PMMA and is the sum of medium (λMLCTfdd ) and vibrational reorganization energies (λMLCTfdd ) 0,i i with the latter treated classically.

kdd a

∝ exp-

∆G* )

(λ

[

] ( )

0 (λMLCTfdd + ∆Gdd-MLCT )2

4λ

MLCTfdd

kBT

) exp -

∆G* kBT

(28)

0 + ∆Gdd-MLCT )2 MLCTfdd

MLCTfdd

)

4λ

0

(

0

)

∆Gdd-MLCT λMLCTfdd ∆Gdd 1 + MLCTfdd (29) + 4 2 2λ It follows from eq 29 that ∆G* in a frozen medium, ∆G/fr, and a comparable fluid, ∆G/fl, are related as in eq 30. In eq 30, the ∆λ are differences in λ between the excited MLCT MLCT states with ∆λi ) λdd , ∆λ0i ) λdd , and ∆λ00 i - λi 0i - λ0i dd MLCT ) λ00 - λ00 . The λdd and λMLCT are reorganization energies between the respective excited states and the ground state.

(

∆G/fr ) ∆G/fl 1 +

MLCT Since ∆λ00 ) λdd , and λdd 00 - λ00 00 ≈ 0

∆G/fr

)

∆G/fl

(

)

∆λ00 ∆λi + ∆λ0i

)

λMLCT 00 1∆λi + ∆λ0i

(30)

(31)

Rigid Medium Stabilization Effect

J. Phys. Chem. B, Vol. 111, No. 24, 2007 6939

This is an important result in showing that the classical activation free energy for MLCT-dd barrier crossing is generally lower in a rigid medium compared to a comparable fluid. MLCT-dd Reorganization Energy. It is also possible to estimate λi for the MLCT-dd transition, λMLCTfdd . It follows i from eq 29 that in the limit, ∆G , 2λ

∆G* ≈

λMLCTfdd ∆G0 + 4 2

(32)

and

λMLCTfdd ≈ 2(2∆G* - ∆G0)

(33)

From the data in Table 4, ∆G0 ∼ 250 cm-1 in PMMA at 298 K. The quantity ∆G* can be calculated from eq 34 with ν, the frequency factor for barrier crossing.

(

k ) ν exp -

∆G* RT

)

(34)

Assuming ν ) 1013 s-1, ∆G* ) 2180 cm-1 at 298 K. With this value and ∆G0 ) 250 cm-1, λMLCTfdd ∼ 8200 cm-1. This t quantity includes both λi and λ0i with, λMLCTfdd ) ∆λi + ∆λ0i. Since there is no charge-transfer character in the dd excited state, MLCT λdd . A value for this quantity is 0i ≈ 0 and ∆λ0i ≈ -λ0i available from the data for [Ru(bpy)3]2+* in PMMA in Table 2 with ∆λ0i ≈ -λMLCT ∼ -360 cm-1. 0i ∆λi includes bond distance changes from both MLCT f ground state and dd f ground state transitions with ∆λi ) λdd i - λMLCT . By using λMLCT ∼ 1540 cm-1 from emission spectral i i fitting, Table 2, λdd ) ∆λi + λMLCT ∼ 10000 cm-1. This i i estimate is an upper limit since it neglects low-frequency . vibrational contributions to λMLCT i To the best of our knowledge, this is the first estimate of λdd t from kinetic data. Ohno et al. have reported λi ) 5400 cm-1 for a MLCT-dd crossing in single crystals.79 Values of λdd i ranging from 3400 to 8600 cm-1 have been reported for dπ6 [M(NH3)6]n+ and [M(en)3]n+ (en is ethylenediamine) complexes of Ru(II) and Ir(III).80 Comparison can also be made to high to low spin relaxation (Fe(II)HS[(dπ)4(dσ*)2] f Fe(II)LS[(dπ)6]) in Fe(II) polypyridyl spin crossover complexes, as described below. In spin crossover transitions in Fe(II) polypyridyl complexes extremely large Huang-Rhys factors, eq 35, are observed for low-frequency metal-ligand stretching modes.81 In the average mode approximation S is related to the force constant (f), quantum spacing (hν ) pω), and change in equilibrium displacement, ∆Qe, of the average coupled mode as shown in eq 35.

S)

f (∆q)2 2pω j

(35)

Values of S as high as 50 have been reported based on analysis of kinetics data for spin interconversion.81 Assuming pω j ) 400 cm-1 for the MLCT-dd transition in cis-[Ru(bpy)2(py)2]2+, S ∼ 12.5. Pre-exponential Factor. As noted above there is an extraordinary decrease of >106 in the pre-exponential factor for MLCT f dd barrier crossing in PMMA compared to 4:1 EtOHMeOH. A reduced pre-exponential factor of the same magnitude has been observed for electron transfer in betaine-30 in polystyrene films.82 In order to discuss this phenomenon, it is useful to rewrite the expression for ddd a in eq 34 as in eq 36 with the pre-

Figure 5. Energy-coordinate diagram in PMMA in the average mode approximation illustrating the energy and reorganization energy relationships for the MLCT and dd states and the MLCT-dd barrier. See text.

exponential term shown as the product of a barrier crossing frequency and an entropic term, ∆S*. In this formulation the entropic term includes the density of states and number of reaction channels available at the classical barrier crossing.23

(

dd kdd a ) νa exp -

∆S* ∆H* exp R RT

) (

)

(36)

13 -1 -6 cm-1 K-1 in Assuming that νdd a ) 10 s , ∆S* ) 1 × 10 PMMA compared to ∆S* ) 0.2 cm-1 K-1 in solution. ∆S* is dominated by solvent effects and, in this interpretation, there is a significant prohibition to barrier crossing in the rigid medium due to the limited number of surrounding polarization configurations accessible for barrier crossing with energy conservation.23 dd Excited State Lifetime. Photochemical Cage Effect. A proposed mechanism for ligand loss from cis-[Ru(bpy)2(py)2]2+in PMMA is shown in Scheme 2. It is an elaboration of Scheme 1 and includes ligand loss and recoordination steps after the dd state is formed. In Scheme 1, kdd is the sum of kdd nr dd and k-L in Scheme 2. Photodissociation of py in the first step is presumably accompanied by coordination of weakly coordinating PMMA, which is not shown in Scheme 2. In solution, cis-[Ru(bpy)2(py)2]2+* is highly photoactive toward ligand loss, with quantum yields as high as 0.3. When the efficiency of reaching the dd state by MLCT-dd barrier crossing is included, the efficiency of ligand loss is ∼1.6 Since there is no kinetic evidence for back-crossing in solution, the dd ) rate constant for ligand loss from the dd excited state (k-L dd must be rapid with k-L > 1 × 1010 s-1 at 298 K.

6940 J. Phys. Chem. B, Vol. 111, No. 24, 2007 By contrast, the rate constant for ligand loss is greatly dd attenuated in PMMA with k-L < 3 × 108 s-1. The primary role of the rigid matrix in this case is presumably to inhibit the large amplitude displacements associated with ligand loss.38,83 Watts and Missimer have discussed environmentally hindered radiationless transitions where the rigid matrix distorts the potential energy surfaces for excited states that undergo JahnTeller distortions.83 Given the appearance of photochemistry in the presence of added Cl-, photochemical ligand loss from cis-[Ru(bpy)2(py)2](PF6)2 in PMMA does continue to occur but with greatly diminished efficiency. In the absence of added Cl-, diffusion of released py away from the reaction site is inhibited perhaps creating a “photochemical cage effect” which promotes redd coordination of py, k+L in Scheme 2. In the presence of Clthere is a competition at the reaction site between recoordination of py and Cl-. Both are held in the second coordination sphere by restricted diffusion.

Thompson et al. Appendix The rate constants appearing in Scheme 1 are related to experimentally derived rate constants from biexponential fits to the emission decay data for cis-[Ru(bpy)2(py)2](PF6)2*, k1 ) (τ1)-1, k2 ) (τ2)-1, and k ) (τ0)-1, and normalized amplitudes I1 and I2 ) (1 - I1) as shown in eqs A-1 to A-4.77

Acknowledgment. Work was supported at the University of North Carolina at Chapel Hill by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, U.S. Department of Energy under Grant DE-FG0206ER15788. D.W.T. thanks the Natural Science and Engineering Research Council of Canada (NSERC) for a postdoctoral fellowship for portions of this work completed at UNC and a NSERC discovery grant, and Memorial University for work completed at Memorial University of Newfoundland. The authors thank Earl Danielson and Dr. James K. McCusker for valuable discussions and Dr. Wayne Jones, Jr. for providing preliminary experimental data in the early portions of this study.

(A-1)

1 dd k2 ) {(x + y) - x[x - y]2 + 4kdd a k-a} 2

(A-2)

where

I1 )

x - k2 k1 - k2

(A-3)

and

x ) (τ0)-1 + kdd a

Conclusions and Summary Our results and a kinetic analysis of the temperaturedependent lifetime of cis-[Ru(bpy)2(py)2](PF6)2 in PMMA point to the existence of a low-lying dd state or states dynamically coupled to the lowest MLCT manifold of states. Ligand loss through the dd state is inhibited by a rigid medium effect. Energy relationships between the two excited states and between the excited states and the ground state are illustrated in Figure 5. Differences in MLCT absorption and emission energies between fluid and rigid media can be understood by application of dielectric continuum theory. The MLCT energy gap increases in rigid media because part of the medium reorganization energy is frozen and becomes part of the energy gap. Temperaturedependent emission spectral fitting for [Ru(bpy)3]2+ reveals that the medium reorganization energy for the 3MLCT to ground state transition is dependent on both medium and temperature. On the basis of a kinetic analysis of nonexponential emission decay kinetics for cis-[Ru(bpy)2(py)2](PF6)2* in PMMA, the free energy difference between the 3MLCT and 3dd states is ∆G ∼ 270 cm-1 at 298 K. From these data the lifetime of the dd excited state in cis-[RuII*(bpy)2(py)2](PF6)2 at room temperature in PMMA is ∼3 ns compared to τ ∼ 25 ns for [RuII*(bpy)3]2+ in H2O at 298 K found in other work.21 An estimate has been obtained for the intramolecular reorganization energy between 4 -1 the MLCT and dd states with λdd i ∼ 1 × 10 cm . The energy gap between MLCT and dd states is decreased in PMMA compared to 4:1 (v/v) EtOH/MeOH with ∆H0 ∼ 1000 cm-1 in the former. Rigid medium stabilization of cis-[Ru(bpy)2(py)2](PF6)2* is attributed to inhibition of metal-ligand bond breaking and a photochemical cage effect.

1 dd k1 ) {(x + y) + x[x - y]2 + 4kdd a k-a} 2

(A-4)

with dd y ) kdd + k-a

(27)

In order to simplify the analysis it was assumed that (τ0)-1 ) 2.6 × 105 s-1 (τ0 ) 3.8 µs) is independent of temperature based on lifetime data at 150-170 K where the emission decay was exponential. Lifetimes in this temperature range are relatively independent of temperature, Supplementary Table 1. Supporting Information Available: Figures of visible spectrum of cis-[Ru(bpy)2(py)2](PF6)2, time-resolved emission spectra of [Ru(bpy)3]2+* in PMMA following 460 nm excitation, transient absorption difference spectrum of [Re(CO)3(bpy)(4Etpy)](PF6)2 in PMMA obtained immediately following 460 nm excitation, plots of (∆V1/2)2 vs temperature for [Ru(bpy)3](PF6)2* in PMMA and CH3CN, visible changes with time for photolysis, and plots of kadd vs temperature and table of parameters derived from kinetic fits to temperature-dependent emission decay traces. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Durr, H.; Bossman, S. Acc. Chem. Res. 2001, 34, 905. (2) Vlcek, A., Jr. In Electron Transfer in Chemistry Vol 2; Balzani, V., Ed.; Wiley-VCH: New York, 2001; pp 804-877 and references therein. (3) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (4) Juris, A.; Balzani, V.; Barrigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85. (5) Meyer, T. J. Pure Appl. Chem. 1986, 14, 1293. (6) Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 4803. (7) Van Houten, J.; Watts, R. J. Inorg. Chem. 1978, 17, 3381. (8) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853. (9) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583. (10) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444. (11) Barrigelletti, F.; Juris, A.; Balzani, V.; Belser, P.; von Zelewsky, A. J. Phys. Chem. 1987, 91, 1095. (12) Thompson, D. G.; Schoonover, J. R.; Timpson, C. J. Meyer, T. J. J. Phys. Chem. A 2003, 107, 10250. (13) Caspar, J. V.; Westmoreland, T. D.; Allan, G. H.; Bradley, P. G.; Meyer, T. J.; Woodruff, W. H. J. Am. Chem. Soc. 1984, 106, 3492. (14) Kestell, J. D.; Williams, Z. L.; Stultz, L. K. and Claude, J. P. J. Phys. Chem. 2002, 106, 5768. (15) Kober, E. M.; Caspar, J. V. Lumpkin, R. S.; Meyer, T. J. J. Phys. Chem. 1986, 90, 3722. (16) Claude J. P. Ph.D. Dissertation University of North Carolina at Chapel Hill, 1995.

Rigid Medium Stabilization Effect (17) Dramrauer, N. H.; Cerulla, G.; Yeh, A.; Shank, C. V.; McCusker, J. K. Science 1997, 275, 54. (18) Dramrauer, N. H.; Weldon, B. T.; McCusker, J. K. J. Phys. Chem. A 1998, 102, 3382. (19) Yeh, A.; Shank, C. V.; McCusker, J. K. Science 2000, 289, 935. (20) Bhasikuttan, A. C.; Suzuki, M.; Nakashima, S.; Okada, T. J. Am. Chem. Soc. 2002, 124, 8398. (21) Thompson, D. W.; Wishart, J. F.; Brunschwig, B. S.; Sutin, N. J. Phys. Chem. A 2001, 105, 8117. (22) Tachiyashiki, S.; Ikezawa, H.; Mizumachi, K. Inorg. Chem. 1994, 33, 623. (23) Rillema, D. P.; Blanton, C. B.; Shaver, R. J.; Jackman, D. C.; Boldaji, M.; Bundy, S.; Worl, L. A.; Meyer, T. J. Inorg. Chem. 1992, 31, 1600. (24) Rillema, D. P.; Taghdiri, D. G.; Jones, D. S.; Keller, C. D.; Worl, L. A.; Meyer, T. J.; Levy, H. A. Inorg. Chem. 1987, 26, 578. (25) Walcholtz, W. F.; Auerbach, R. A.; Schmehl, R. H. Inorg. Chem. 1986, 25, 227. (26) Barigelletti, F.; Belser, P.; von Zelewsky, A.; Juris, A.; Balzani, V. J. Phys. Chem. 1986, 89, 3680. (27) Walcholtz, W. F.; Auerbach, R. A.; Schmehl, R. H.; Ollino, M.; Cherry, W. R. Inorg. Chem. 1985, 24, 1758. (28) Juris, A.; Barigelletti, F.; Balzani, V.; Belser, P.; von Zelewsky, A. Inorg. Chem. 1985, 24, 202. (29) Pinnick, D. V.; Durham, B. Inorg. Chem. 1984, 23, 3841. (30) Cherry, W. R.; Henderson, L. H. Inorg. Chem. 1984, 23, 983. (31) Henderson, L. H.; Fronczek, F.; Cherry, W. R. J. Am. Chem. Soc. 1984, 106, 5876. (32) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J. Am. Chem. Soc. 1984, 106, 2613. (33) Coe, B. J.; Friesen, D.; Thompson, D. W.; Meyer, T. J. Inorg. Chem. 1996, 34, 4575 and references therein. (34) Hager, G. D.; Crosby, G. A. J. Am. Chem. Soc. 1975, 97, 7031. (35) Barqawi, K. R.; Llobet, A.; Meyer, T. J. J. Am. Chem. Soc. 1988, 110, 7751. (36) Allsop, S. R.; Cox, A.; Kemp, T. J.; Reed, W. J. J. Chem. Soc., Faraday Trans. 1 1978, 74, 353. (37) Allsop, S. R.; Cox, A.; Kemp, T. J.; Reed, W. J.; Carassits, V.; Traverso, O. J. Chem. Soc., Faraday Trans. 1 1979, 75, 353. (38) Lumpkin, R. S.; Kober, E. M.; Worl, L. A.; Murtaza, Z.; Meyer, T. J. J. Phys. Chem. 1990, 94, 239. (39) Hager, G. D.; Crosby, G. A. J. Am. Chem. Soc. 1975, 97, 7031. (40) Hecker, C. R.; Fanwick, P. E.; McMillan, D. R. Inorg. Chem. 1991, 30, 659. (41) Kincaid, J. R. Chem. Eur. J. 2000, 6, 4055. (42) Campagna, S.; Bartolotta, A.; Di Marco, G. Chem. Phys. Lett. 1993, 206, 30. (43) Masschelein, A.; Kirsch-De Mesmaeker, A.; Willsher, C. J.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1991, 87, 259. (44) Mongey, K. F.; Vos, J. G.; MacCraith, B. D.; McDonagh, C. M. Coord. Chem. ReV. 1999, 185-186, 417 (45) Jones, W. E.; Jr.; Chen, P.; Meyer, T. J. J. Am. Chem. Soc. 1992, 114, 387. (46) Adelt, M.; Devenney, M.; Meyer, T. J.; Thompson, D. W.; Treadway, J. A. Inorg. Chem. 1998, 37, 2616. (47) Coe, B. J.; Meyer, T. J.; White, P. S. Inorg. Chem. 1993, 32, 4012. (48) Coe, B. J.; Thompson, D. W.; Culbertson, C. T.; Schoonover, J. R.; Meyer, T. J. Inorg. Chem. 1995, 34, 3385. (49) The fac-[(CO)3Re(bpy)(4-Etpy)](PF6) was synthesized according to ref 49b. The authors thank Dr. Pingyen Chen for the preparation of this complex. (b) Chen, P.; Westmoreland, T. D.; Danielson, E.; Schanze, K. S.; Anthon, D.; Neveux, P. E., Jr.; Meyer, T. J. Inorg. Chem. 1987, 26, 1116. (50) Handbook of Polymers; Brandrup, J., Immergut, E. H., Eds.; Wiley and Sons: New York, 1989. (51) (a) Victor, J. G.; Torkelson, J. M. Macromolecules, 1987, 20, 2241. (b) Naito, T.; Horie, K.; Mita, I. Macromolecules 1991, 24, 2907. (c) Tanabe, Y.; Muller, N.; Fischer, E. W. Polym. J. 1984, 16, 445. (52) (a) Samples kept in the dark were found to be stable from months to years. Other investigators have exploited PMMA films of [Ru(bpy)3](PF6)2 as laser actinometers.52b (b) Bergeron, B. V.; Kelly, C. A.; Meyer, G. J. Langmuir 2003, 19, 8389. (53) Fleming, C. F. Ph.D. Dissertation University of North Carolina at Chapel Hill, 2002. (54) Maeder, M.; Zuberbuhler, A. D. Anal. Chem. 1991, 95, 488. (55) Stultz, L. K.; Binstead, R. A.; Reynolds, M. S.; Meyer, T. J. J. Am. Chem. Soc. 1995, 117, 2520. (56) The program SPECFIT, described by Maeder, was modified by Dr. R. A. Binstead to perform kinetic analyses. The authors thank Dr.

J. Phys. Chem. B, Vol. 111, No. 24, 2007 6941 Binstead for performing this analysis. The time-resolved emission spectrum for [Ru(bpy)3]2+* was subjected to the global analysis procedure described by Maeder and Zuberbuhler54 which showed that emission decay kinetics were independent of wavelength and the emission maximum invariant with time. The calculated spectra from the global analysis procedure by using the Williams-Watts equation were in excellent agreement with observed spectra. (57) Parker, C. A.; Rees, W. T. Analyst (London) 1960, 85, 857. (58) Silverstein, R. M.; Bassler, G. C.; Morril, T. C. Spectrophotometric Identification of Organic Compounds, 4th ed.; John Wiley and Sons: New York, 1981. (59) Chen, P. Y.; Meyer, T. J. Chem. ReV. 1998, 98, 1439. (60) As noted below, emission decay is nonexponential and there can be a contribution from an impurity, notably [Ru(bpy)3](PF6)2. The difficulty in avoiding this impurity has been discussed previously.31 The presence of a stable emitting impurity can be ruled out based on the following observations: (1) Decay kinetics were independent of exciting or monitoring wavelength, which argues against a mixture of two emitting species. (2) The excited state lifetimes obtained from the biexponential fits (eq 4) to the emission decay of cis-[Ru(bpy)2(py)2]2+* were significantly shorter than the lifetime of [Ru(bpy)3]2+* under similar conditions. (3) There was no detectable room-temperature emission from cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 salts in Ar-sparged 4:1 (v/v) EtOH/MeOH. (61) (a) Jones, W. E., Jr.; Chen, P. Y.; Meyer, T. J. J. Am. Chem. Soc. 1991, 114, 387. (b) Pfennig, B. W.; Chen, P. Y.; Meyer, T. J. Inorg. Chem. 1996, 35, 2898. (62) (a) Williams, G.; Watts, S. B.; North, A. M. Trans. Faraday Soc. 1977, 67, 1323. (b) Williams, G.; Watts, S. B. Trans. Faraday Soc. 1970, 66, 80. (c) Devenney, M.; Worl, L. A.; Gould, S.; Guadaloupe, A.; Sullivan, B. P.; Caspar, J. V.; Leasure, R. L.; Gardner, J. R.; Meyer, T. J. J. Phys. Chem. A 1997, 101, 4535. (63) (a) If I(t) is considered to arise from a superposition of exponentials, 〈τ〉 is defined as the average relaxation time.63b (b) Lindsey, C. P.; Patterson, G. D. J. Phys. Chem. 1980, 73, 3348. (64) (a) Treadway, J. A. Ph.D. Thesis, University of North Carolina, Chapel Hill, 1998. (b) Moss, J. A.; Leasure, R. M.; Meyer, T. J. Inorg. Chem. 2000, 39, 1052. (65) Wrighton, M. S.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998. (66) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. J. Phys. Chem. 1995, 99, 17311. (67) Kahlow, M. A.; Kang, T. J.; Barbara, P. F. J. Phys. Chem. 1987, 91, 6452. (68) Fleming, G. R.; Cho, M. Annu. ReV. Phys. Chem. 1996, 47, 109. (69) Marcus, R. A. J. Phys. Chem. 1990, 94, 4963. (70) Chen, P.; Meyer, T. J. Inorg. Chem. 1996, 35, 5520. (71) Kober, E. M.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23, 2098. (72) Worl, L. A.; Meyer, T. J. Chem. Phys. Lett. 1988, 143, 541. (73) Worl, L. A.; Duesing, R.; Chen, P.; Ciana, L. D.; Meyer, T. J. J. Chem. Soc., Dalton Trans., 1991, 849. (74) Local volumes in polymer films have been measured by several investigators.74b For PMMA, small-angle X-ray scattering (SAXS) measurements reveal an apparent lack of structural density fluctuations on the size scale of several tens of angstroms implying that the average size of the free volumes is small. Structural inhomogeneity of PMMA labeled with 3-perylenlylmethyl, methyleosin and 9,10-anthrylenemethyl dimethymethacrylate have also been investigated by using near field optical microscopy.74c (b) Tanabe, Y.; Muller, N.; Fischer, E. W. Polym. J. 1984, 16, 445. (c) Aoki, H.; Tanaka, S.; Shinzaburo, I.; Yamamoto, M. Macromolecules 2000, 33, 9650. (75) Nielson, L. E. Mechanical Properties of Polymers; Rheinhold: New York, 1962. (76) Fleming, C. N.; Dattelbaum, D. M.; Thompson, D. W.; Ershov, A. Y.; Meyer, T. J. Manuscript submitted. (77) (a) Benson, S. W. The Foundation of Chemical Kinetics; McGrawHill: New York, 1960. (b) Heitele, H.; Fincke, P.; Weeren, S.; Pollinger, F.; Michel-Beyerle, M. E. J. Phys. Chem. 1989, 93, 5173. (78) Claude, J. P.; Meyer, T. J. J. Phys. Chem. 1995, 99, 51. (79) Islam, A.; Ikeda, N.; Yoshimura, A.; Ohno, T. Inorg. Chem. 1998, 37, 3098. (80) Winkler, J. R.; Netzel, T. L.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1987, 109, 2381. (81) Gutlich, P.; Hauser, A.; Speiring, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 2024. (82) Walker, G. C.; Akesson, E.; Johnson, A. E.; Levinger, N. E.; Barbara, P. F. J. Phys. Chem. 1992, 96, 3728. (83) Watts, R. J.; Missimer, D. J. Am. Chem. Soc. 1978, 100, 5350.