VOLUME 101, NUMBER 44, OCTOBER 30, 1997
© Copyright 1997 by the American Chemical Society
LETTERS Temperature and Isotope Effects on the Rates of Photoinduced Iridium(I)-Pyridinium Electron-Transfer Reactions David Wiedenfeld, Max Bachrach, T. Mark McCleskey, Michael G. Hill, Harry B. Gray,* and Jay R. Winkler* Beckman Institute, California Institute of Technology, Pasadena, California 91125 ReceiVed: December 4, 1996; In Final Form: September 3, 1997X
Intramolecular electron-transfer (ET) rates have been measured as functions of reaction driving force (0.02.0 eV) and temperature (200-280 K) for a series of iridium dimers (Ir2) covalently bound to substituted pyridinium (py+) groups. We found that the rates of recombination reactions (py• f Ir2+) at high driving forces are strongly dependent on temperature and that Ir2* f py+ and py• f Ir2+ rates in both the normal and inverted regions are slower on deuteration of the pyridinium acceptors. Analysis of the rates in terms of semiclassical theory gives the following parameter values: HAB ) 52(9) cm-1, λ ) 1.05(2) eV (280 K); HAB ) 22(2) cm-1, λ ) 1.04(1) eV (210 K). 4-Phenyl-N-ethylpyridinium iodide, chosen as a model for the acceptors in the Ir2* f py+ reactions, exhibited resonance Raman scattering that implicated a 1565-cm-1 mode as dominant for inner-sphere reorganization.
Introduction According to semiclassical ET theory,1 electron-transfer (ET) reaction rates (kET) are governed by three parameters (eq 1): (1) the electronic coupling (HAB) between reactants and products, (2) the free-energy change for the reaction (∆G°), and (3) the total reorganization energy (λ), the latter including both innersphere (λI) and solvent (λS) contributions.
kET )
[
]
-(∆G° + λ)2 4π2 2 1 HAB exp h 4λRT (4πλRT)1/2
(1)
As predicted by eq 1, the rates of intramolecular ET in [Ir(µ-pz′)(CO)(Ph2PO(CH2)2py+)]2(BPh4)2 (pz′ ) 3,5-dimethylpyrazolyl, Ph ) phenyl, py+ ) pyridinium or substituted pyridinium) in acetonitrile solution exhibit a Gaussian freeenergy dependence.2 Contrary to conclusions derived from results of spectroscopic3 and kinetics experiments4 on organic X
Abstract published in AdVance ACS Abstracts, October 1, 1997.
S1089-5647(96)03984-3 CCC: $14.00
ET systems, however, our [Ir2/py+] ET data suggest a minor role for nuclear tunneling. Since we would like to understand this apparent difference in mechanism, we have measured the temperature dependence of the [Ir2/py+]2 ET rates in the inverted region.5 We also have examined isotope effects on the rates6 and made a spectroscopic evaluation of the nuclear reorganization parameters for the reactions. Temperature Effects. We have investigated photoinduced ET from the singlet (1Ir2* to py+: 1ET) and triplet (3Ir2* to py+: 3ET) excited states of Ir2, as well as thermal back ET (py• to Ir2+: bET) (Figure 1) as a function of temperature in butyronitrile.7,8 The results are shown in Figure 2. As in the previous study at room temperature, data at lower temperatures consistently yielded good fits to the semiclassical theory (eq 1) when the different ET reaction rates (kET) were put on the same driving-force curve for a given temperature; the λ of 1.04 eV at 200 K did not change markedly with temperature.9 Furthermore, plots of ln(kETT1/2) versus 1/T yielded straight lines with © 1997 American Chemical Society
8824 J. Phys. Chem. B, Vol. 101, No. 44, 1997
Letters
Figure 1. [Ir2/py+] (R ) H, 4-Me, 3,4-Me2, 2,4,6-Me3, and 4-Ph) photoprocesses: kF and kP are radiative decay rates; 1kNR and 3kNR are nonradiative decay rates; kisc is the intersystem crossing rate; 1kET, 3kET, and bkET are ET reaction rates.
Figure 2. Plot of ln(kET) versus -∆G° for 3ET (-∆G° < 0.5 eV), 1ET (0.5 < ∆G° < 1.0 eV), and bET (-∆G° > 1.0 eV) reactions. Each of the solid lines is the best fit (280 K: HAB ) 52 cm-1; λ ) 1.05 eV. 210 K: HAB ) 22 cm -1; λ ) 1.04 eV) of each data set to eq 1.
slopes whose absolute values decrease as (∆G° + λ) approaches zero (Figure 3). However, there are several ways in which the observed temperature dependence is not in agreement with the predictions of eq 1. First, the apparent HAB is temperature dependent, increasing from 20 cm-1 at 200 K to 52 cm-1 at 280 K. Although the reaction rates depend on temperature in both the normal and inverted regions, the experimental ∆H* values1 are 0.05-0.1 eV greater than the theory predicts for the excitedstate ET reactions (Table 1). The activation energies found for the recombination reactions also deviate from the predicted values, but not in such a systematic manner. The phosphonite spacer in the [Ir2/py+] system is not rigid and so it is unlikely that a single conformation is adopted in solution. In related studies with polypeptide spacers,10 multiexponential kinetics were attributed to a thermal distribution of conformers. The monoexponential nature of charge separation and recombination kinetics in the [Ir2/py+] complexes can be accounted for by a model (see scheme below) in which [Ir2/ py+] h [Ir2/py+]′ conformational changes are faster than the ET reaction of any of the conformers; the dominant rate is kET′. Formation of the more reactive conformation(s) requires 0.05-
Figure 3. Arrhenius plots: (top) 1Ir2* f 3,4-Me2py+ (-∆G° ) 0.08 eV); (middle) 4-Phpy• f Ir2+ (-∆G° ) 1.52 eV); (bottom) py• f Ir2+ (-∆G° ) 1.66 eV) reactions.
0.1 eV, which is consistent with the energy requirements of bending motions. Ir2*/py+
Ir2*/py+′
kET
kET′
Ir2+/py•
Ir2+/py•′
To evaluate the likelihood of rapid [Ir2/py+] h [Ir2/py+]′ dynamics at low temperature in butyronitrile, we examined the excited-state decay of tetraphenylethylene (TPE).7 In the singlet excited state of TPE, bonding in the olefin is disrupted and the forces retaining planar (Ph)2CC(Ph)2 geometry are greatly diminished. The deactivation pathway of singlet-excited TPE is believed to involve rotation about the olefinic bond, and the solvent-viscosity-dependent barriers to this process are likely to be similar to those associated with [Ir2/py+] h [Ir2/py+]′ dynamics. Consistent with literature reports, TPE luminescencedecay kinetics cannot be resolved at room temperature using time-correlated single-photon counting (tcspc, system response: fwhm ca. 70 ps).11 When the sample temperature is
Letters
J. Phys. Chem. B, Vol. 101, No. 44, 1997 8825
TABLE 1: Driving Forces, Activation Energies, and Isotope Effects for [Ir2/py+] ET Reactions
donor 3
Ir2* 3Ir * 2 3 Ir2* 1 Ir2* 1 Ir2* 1 Ir2* 1 Ir2* 1 Ir2* 4-Phpy• py• 4-Mepy• 3,4-Me2py• 2,4,6-Me3py•
acceptor
-∆G° (eV)a
exptl ∆H* (eV)b
calcd ∆H* (eV)c
2,4,6-Me3py+ 3,4-Me2py+ 4-Mepy+ 2,4,6-Me3py+ 3,4-Me2py+ 4-Mepy+ py+ 4-Phpy+ Ir2+ Ir2+ Ir2+ Ir2+ Ir2+
0.00(6) 0.08(6) 0.18 0.50(6) 0.58(6) 0.68 0.84 0.98 1.52 1.66 1.82 1.92(6) 2.03(6)
0.32(1) 0.29 0.24 0.15(1) 0.15(1) 0.14(1) 0.12 ca. 0d 0.09(1) 0.16(1) f 0.13(6) g
0.26 0.22 0.18 0.07 0.05 0.03 0.01 ca. 0 0.07 0.09 0.15 0.19 0.24
kH/kD at 200 K
1.43(8) 1.18(9) 1.28(16) 1.20(14)e f
a Calculated using the energies of the 1Ir * and 3Ir * excited states 2 2 and the Ir2+/0 and py+/0 reduction potentials measured by fast-scanrate voltammetry. Potentials were calculated from irreversible peak potentials for the 3,4-Me2py+ and 2,4,6-Me3py+ derivatives assuming pyridinyl radical dimerization rate constants of 107 < k < 1010 M-1 s-1 (Andrieux, C. P.; Nadjo, L.; Save´ant, J. M. J. Electroanal. Chem. 1970, 26, 147-186). b Unless otherwise indicated, errors from leastsquares fits to the Arrhenius-type plots are less than 0.01 eV. c Calculated for 4-Phpy• f Ir + from ∆H* ∼ λ/4 + ∆H° [1 + ∆G°/ 2 λ]/2 - (∆G°)2/4λ (ref 1). In this case, the calculated values of ∆G* (0.06 eV) and ∆H* (0.07 eV) are nearly the same. Other ∆H* values were assumed to be equal to ∆G* values (eq 1, λ ) 1.04 eV). d The 1Ir * state is completely quenched by the 4-phenylpyridinium acceptor 2 at all temperatures studied (200-300 K). e This effect was constant at all temperatures studied (200-280 K). f Low solubility prevented measurements of recombination rates. g At temperatures 1 × 1011 s-1;12 these data indicate that [Ir2/py+] h [Ir2/py+]′ equilibration can occur very rapidly over the entire temperature range used to study ET kinetics. That energetically taxing bending motions occur prior to ET in [Ir2/py+] complexes accounts for the constant deviation for the predicted and observed activation energies for reactions in the normal region and for the increase in the apparent HAB with increasing temperature. However, it does not account for the nonsystematic deviations in activation energies for the inverted reactions. Isotope Effects. Deuterated pyridine, 4-methylpyridine, and triethylamine were used to prepare isotopically labeled analogues of the previously studied natural abundance systems. The rates of both normal and inverted ET reactions were found to be slower in the deuterated [Ir2/py+] derivatives at 200 K (Table 1). For 1ET and 3ET from the Ir2 core to the 4-methylpyridinium acceptors, the values of kH/kD are 1.18(9) and 1.43(8), respectively. Similar values were obtained for 1ET and bET with pyridinium acceptors. The bET isotope effect was found not to depend on temperature. Iridium(I) dimers with deuterated triethylammonium groups exhibited triplet and singlet lifetimes that were not significantly different from those of the natural abundance analogue; for example, kH/kD ) 1.02(2) at 200 K for triplet emission from the triethylammonium complex pair. The isotope effects in the [Ir2/py+] ET reactions unequivocally demonstrate that the semiclassical model (eq 1) cannot fully account for the observed kinetics. Nuclear Reorganization Parameters. Charge-transfer absorption spectra and the associated resonance Raman scattering
Figure 4. Raman spectra of 4-phenyl-N-ethylpyridinium iodide and chloride salts (354.7 nm excitation). Solvent (CD3CN) peaks are indicated by +. Resonance enhanced peaks are indicated by *.
spectra provide independent information about the distortions involved in ET processes.3,13 Guided by previous studies,13 the 4-phenyl-N-ethylpyridinium iodide complex was chosen as a model for the pyridinium acceptors in the [Ir2/py+] excitedstate ET reactions. The Raman spectra of the iodide and chloride salts of 4-phenyl-N-ethylpyridinium are shown in Figure 4. It is apparent that at least five modes are resonance enhanced in the iodide spectrum; the greatest enhancement is found for a 1565(18)-cm-1 vibration. The enhanced Raman scattering from excitation in resonance with the charge-transfer absorption band is indicative of inner-sphere distortions associated with reduction of the pyridinium group. A single-mode quantum rate expression (eq 2;14 hν ) 1565 cm -1; Sf ) λI/hν) was used to estimate the inner-sphere and outer-sphere contributions to the reorganization energy.
( )
2π 2 π kET ) HAB h λSRT
1/2
∞
exp(-Sf)
(
Sfm
∑ m! ×
m)0
exp -
)
(∆G° + λS + mhν)2 4λSRT
(2)
Fits to the ET data between 200 and 280 K consistently gave λI < 0.04 eV; λS ) 1.04 eV at 200 K and did not change markedly with temperature.9 As found in the fits to eq 1, the apparent HAB increases with increasing temperature (20 cm-1 at 200 K to 42 cm-1 at 280 K). The increase in apparent HAB with increasing temperature can be accounted for by [Ir2/py+] conformational dynamics (vide supra).15
8826 J. Phys. Chem. B, Vol. 101, No. 44, 1997 Raman measurements were made with deuterated derivatives of the 4-phenypyridinium iodide complex:16 on perdeuteration, the frequency of the most intense Raman peak shifts to 1527 cm-1. Using this frequency shift in eq 2 (with λS ) 1.0 eV and λI ) 0.04 eV) leads to a calculated kH/kD ratio of less than 1.01 over a wide driving force range (0 e -∆G° e 1.5 eV) at 200 K. Clearly, the single-mode rate expression (eq 2) cannot account for both the driving-force dependence and deuterium isotope effects of [Ir2/py+] ET. In agreement with our earlier interpretation of driving-force effects,2 the temperature dependence of [Ir2/py+] ET rates suggests a minor role for nuclear tunneling. However, we have found that tunneling does occur, as there are significant isotope effects upon deuteration of the pyridinium groups. Further, resonance Raman spectra of pyridinium iodide salts indicate that there is inner-sphere distortion upon electron transfer to a pyridinium derivative, although the isotopic shift of the dominant vibrational frequency is not consistent with the [Ir2/py+] kinetic isotope effects. Absolute resonance Raman cross sections for the 4-phenylpyridinium iodide complex will provide an independent evaluation of λI. We can then use multimode quantum expressions to try to reconcile the observed behavior of the [Ir2/ py+] ET reactions. Acknowledgment. We thank David Beratan for helpful discussions. This work was supported by the National Science Foundation (CHE-9311587 and CHE-9610164). A fellowship to D.W. from the National Institutes of Health is gratefully acknowledged. References and Notes (1) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265322. (2) Fox, L. S.; Kozik, M.; Winkler, J. R.; Gray, H. B. Science 1990, 247, 1069-1071. (3) Myers, A. B. Chem. ReV. (Washington, D.C.) 1996, 96, 911-926 and references therein. (4) (a) Closs, G. L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J. Phys. Chem. 1986, 90, 3673-3683. (b) Closs, G. L.; Miller, J. R. Science 1988, 110, 7242-7244. (c) Gould, I. R.; Ege, D.; Mattes, S. L.; Farid, S. J. Am. Chem. Soc. 1987, 109, 3794-3796. (d) Gould, I. R.; Moody, R.; Farid, S. J. J. Am. Chem. Soc. 1988, 110, 7242-7244. (e) Gould, I. R.; Moser, J. E.; Armitage, B.; Farid, S. J. Am. Chem. Soc. 1989, 111,
Letters 1917-1919. (f) Chen, P.; Duesing, R.; Graff, D. K.; Meyer, T. J. J. Phys. Chem. 1991, 95, 5850-5858. (5) Previous studies on organic donor-acceptor systems have shown that there is a weak or nonexistent dependence of inverted ET reaction rates on temperature; nuclear tunneling between the nested potential energy surfaces of the reactants and products is believed to be responsible for this behavior. (a) Liang, N.; Miller, J. R.; Closs, G. L. J. Am. Chem. Soc. 1990, 112, 5353-5354. (b) Kroon, J.; Oevering, H.; Verhoeven, J. W.; Warman, J. M.; Oliver, A. M.; Paddon-Row: M. N. J. Phys. Chem. 1993, 97, 50655069. (c) A small temperature dependence of an inverted reaction has been reported (attributed to entropic effects and to changes in the distribution of solvent librations with temperature): Chen, P.; Mecklenburg, S. L.; Meyer, T. J. J. Phys. Chem. 1993, 97, 13126-13131. (6) Deuterium isotope effects in ET reactions have been interpreted in terms of nuclear tunneling. (a) Gould, I. R.; Farid, S. J. Am. Chem. Soc. 1988, 110, 7883-7885. (b) Doolen, R.; Simon, J. D. J. Am. Chem. Soc. 1994, 116, 1155-1156. (7) Bachrach, M. Ph.D. Thesis, California Institute of Technology, 1996. (8) Both the Ir2+/0 and py+/0 potentials were measured as a function of temperature: plots of ∆E° (py+/0 - Ir2+/0) vs T were linear with slopes equal to 0.16(3) mV/K for the [Ir2/4-Phpy+] case. (9) The data were fit by minimizing the function f ) ∑|log(kexperimental) - log(kcalculated)|. Confidence limits for the HAB and λ values were estimated at 210 and 280 K by examining which range of parameters gave f values that were within 10% of the minimum. Fits to eq 1: HAB ) 43-58 cm-1, λ ) 1.03-1.07 eV (280 K), HAB ) 20-24 cm-1 , λ ) 1.03-1.05 eV (210 K). Fits to eq 2: HAB ) 39-46 cm-1, λS ) 0.94-0.99 eV, λI < 0.04 eV (280 K); HAB ) 19-24 cm-1, λS ) 1.01-1.05 eV, λI < 0.01 eV (210 K). (10) (a) Schanze, K. S.; Sauer, K. J. Am. Chem. Soc. 1988, 110, 11801186. (b) Isied, S. S.; Ogawa, M. Y.; Wishart, J. F. Chem. ReV. (Washington, D.C.) 1992, 92, 381-394. (11) (a) Ma, J.; Dutt, G. B.; Waldeck, D. H.; Zimmt, M. B. J. Am. Chem. Soc. 1994, 116, 10619-10629. (b) Barbara, P. F.; Rand, S. D.; Rentzepis, P. M. J. Am. Chem. Soc. 1981, 103, 2156-2162. (12) Once the solvent becomes rigid, relaxation to the twisted state is suppressed; in this case, decay of the vertically excited state is dominated by conversion to and subsequent emission from the partially relaxed singlet excited state (see ref 7). (13) Blackbourn, R. L.; Johnson, C. S.; Hupp, J. T.; Bryant, M. A.; Sobocinski, R. L.; Pemberton, J. E. J. Phys. Chem. 1991, 95, 10535-10537. (14) Brunschwig, B. S.; Sutin, N. Comments Inorg. Chem. 1987, 6, 209235. (15) Conformational dynamics also apparently play an important role in the ET reactions of a related Ir2 dimer system (one with different linkers between the metal core and py+ acceptors): Kurnikov, I. V.; Zusman, L. D.; Kurnikova, M. G.; Farid, R. S.; Beratan, D. N. J. Am. Chem. Soc. 1997, 119, 5690-5700. (16) The reduction potentials of the natural abundance and perdeuterated hexafluorophosphate salts of this pyridinium derivative are the same within experimental error; thus it appears that the -∆G° values for the [Ir2/py+] ET reactions are not perturbed significantly by the isotopic substitutions.
