Weak Temperature Dependence of Electron Transfer Rates in Fixed

J. Am. Chem. SOC.1994, I 1 6, 9700-9709

9700

Weak Temperature Dependence of Electron Transfer Rates in Fixed-Distance Porphyrin-Quinone Model Systems Lutfur R. Khundkar,+*$Joseph W. Perry,'*+James E. Hanson,Ll and Peter B. Dervan'vs Contribution from the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91 109, and Contribution No. 8939 from the Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91 125 Received April 13, 1994'

Abstract: Electron transfer rate constants of several derivatives of [5-(4'-(4"-(2'",5'"-benzoquinonyl)bicyclo[2.2.2]octyl)phenyl)-2,3,7,8,12,13,17,18-octamethylporphyrinato]zinc(II) have been measured as a function of temperature in 2-methyltetrahydrofuran, toluene, and toluene-d8. The observed temperature dependencies of the electron transfer rate constants are relatively weak in both the polar and nonpolar solvents. Nonexponential ET dynamics are observed at low temperatures and described in terms of an initial (k& and an average ET rate constant (kav). The ~ E T O values for the molecules with different driving forces, spanning a range of 0.2 eV, show parallel trends over the range of temperatures studied. The trends in kETo are described in terms of the effects of temperature-dependent changes in solvent dielectric constants on the barrier height. Good agreement is observed for the case of toluene solvent, using a semiclassical model, but poorer quantitative agreement is found for the 2-methyltetrahydrofuran data. The temperature dependence of k, is described using a model incorporating an angle-dependent electronic coupling and interconversion of rotational conformers. A temperature-dependent solvent isotope effect is observed on going from toluene to tolueneda, with k~~O(toluene)/k~~~(toluene-d8) being as large at 1.5 over the range of temperatures studied.

1. Introduction Photoinduced electron transfer (ET) in molecular systems has long been of interest in chemistry and biology.ld In recent years, much effort has been directed toward a better understanding of the influence of variables such as driving force, donor-acceptor distance and relative orientation, and solvent properties on ET rates. One impetus for such studies has been the desire to gain greater insight into the mechanisms which lead to efficient charge separation as achieved by photosynthetic reaction centers (RC). Such insights promise to be useful in the design of synthetic biomimetic catalytic systems,' photochromic materials,* and molecular electronic devices9 based on photoinduced ET. Jet Propulsion Laboratory. address: Department of Chemistry, Northeastern University, Boston, MA 02115. Division of Chemistry and Chemical Engineering. * Present address: Department of Chemistry, Seton Hall University,South Orange, NJ 07079. Abstract published in Advance ACS Abstracts, September 1, 1994. (1) (a) Gust, D.; Moore, T. A. Science 1989,244,35. (b) Connolly, J. S.; Bolton, J. R. In Photoinduced Electron Transfer;Fox, M. A., Chanon, M., Eds.; Elsevier: New York, 1988.(c) Wasielewski, M. R. Chem. Rev. 1992, 92,435. (2)(a) Marcus, R. A. J. Chem. Phys. 1965,43,679.(b) Marcus, R. A.; Sutin, N. Blochim. Biophys. Acra 1985,811,265. (c) Hopfield, J. J. Proc. Narl. Acad. Sci. U.S.A. 1974,71,3640.(d) Jortner, J. J. Chem. Phys. 1976, 64,4860. (e) Sutin, N. Acc. Chem. Res. 1982,15, 275. (3)(a) Miller, J. R.; Calcaterra, L. T.; Closs, G. C. J. Am. Chem. SOC. 1984,106, 3047-3049. (b) Gunner, M.R.; Robertson, D. E.; Dutton, P. L. J. Phys. Chem. 1986,90,3783-3795.(c) Wasielewski, M. R.; Niemczyk, M. P.; Svec, E. B.; Pewitt, E. B. J. Am. Chem. SOC.1985,107,1080. (d) Fox, L. S.;Kozik, M.; Winkler, J. R.; Gray, H.B. Science 1990,247, 1069. (4)(a) Oevering, H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.; Cotsaris, E.; Verhoeven, J.; Hush, N. S.J . Am. Chem. SOC.1987,109,3258. (b) Closs, G.L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J. Phys. Chem. 1986,90,3673. (5) (a) McLendon, G. Acc. Chem. Res. 1988,21, 160. (b) Closs, G.L.; Miller, J. R. Science 1988,240,440.(c) Siders, P.; Cave, R. J.; Marcus, R. A. J . Chem. Phys. 1984,81, 5613. (d) Marcus, R. A.; Siders, P. J. Phys. Chem. 1982,86,622. (6)(a) Nielson, R. M.; McManis, G. E.;Golovin, M. N.; Weaver, M. J. J. Phys. Chem. 1988,92,3441.(b) McManis, G.E.; Weaver, M. J. J. Chem. Phys. 1988,90,912. (7)See, e.g.: Proceedings of 131h DOE Solar Photochemistry Research Conference; Silver Creek, CO, June 1989. (8) Beratan, D. N.; Perry, J. W. U S . Patent 5,062,693,Nov 1991. (9)Hopfield, J. J.; Onuchic, J. N.; Beratan, D. N. Science 1988,241,817. t Present

@

Numerous experimental studies have been performed on synthetic model compounds where some of the molecular parameters can be systematically varied in order to elucidate their relative i m p o r t a n ~ e . l ~In- ~particular, much attention has been focussed on the driving force (-AGO) dependence of ET rates, which has led to the validation' of Marcus' prediction of an inverted region.28 The temperature dependence of ET in bacterial RCslO has been difficult to reconcile with the Marcus ET model which treats the relevant vibrational degrees of freedom classically. These results have been interpreted'O using a semiclassical model which accounts for quantum vibrational effects. While the classical and semiclassicalmodels both predict an inverted region at high driving forces, they differ in the predicted shape of the ET rate vs driving force function. In the classical model, the predicted dependencefor log kET is parabolic, centered at anoptimumvalueof-AGO, and the reaction is strongly activated in both the normal and inverted regions. The semiclassical model predicts an asymmetric dependence, with a more gradual decline in rates in the inverted region, and a broad range of -AGO where the reaction is essentially activationless. This has inspired recent interest in measurements of the temperature dependence of ET rates in model systems." In our studies of synthetic porphyrin-quinone molecules, we have examined a model system containing a zinc(I1) mesophenyloctamethylporphyrinlinked tovarious substituted quinones (generally denoted as ZnPLQ). The porphyrin (donor) and quinone (acceptor) are covalently linked by a rigid, saturated (10)(a) DeVault, D.; Chance, B. Biophys. J. 1966,6,825.(b) Kirmaier, C.; Holten, D.; Parson, W. W. Blochim. Biophys. Acra 1985,810, 33. (c) Kirmaier, C.; Holten, D.; DeBus, R. J.; Feher, G.; Okamura, M. Y . Proc. Natl. Acad. Sci. U.S.A. 1986,83,957. (d) Gunner, M. R.; Dutton, P. L. J. Am. Chem. SOC.1988,111, 3400. (1 1) (a) Harrison, R. J.; Pearce, B.; Beddard, G.S.;Cowan, J. A.; Sanders, J. K. M. Chem. Phys. 1987,116,429. (b) Heitele, H.; Michel-Beyerle, M. E.; Finckh, P. Chem. Phys. Lett 1987,138,237.(c) Delaney, J. K.; Mauzerall, D. C.; Lindsey, J. S.J. Am. Chem. Soc. 1990,112,957.(d) Liang, N.;Miller, J. R.; Closs, G.L. J. Am. Chem. SOC.1989,111,8740. (e) Liang, N.; Miller, J. R.; Closs, G. L. J . Am. Chem. SOC.1990,112,5353. (f) Rodriquez, J.; Kirmaier, C.; Johnson, M. R.; Freisner, R. A.; Holten, D.; Sessler, J. L. J. Am. Chem. SOC.1991,113, 1652.(8) Liu, J.; Bolton, J. R. J. Phys. Chem. 1992,96,1718.(h) Zeng, Y.; Zimmt, M. B. J. Phys. Chem. 1992,96,8395. (i) Kroon, J.; Oevering, H. 0.;Verhoeven, J. W.; Warman, J. M.; Oliver, A. M.; Paddon-Row, M. N. J . Phys. Chem. 1993, 97, 5065. (i)Chen, P.; Mecklenberg, S.L.; Meyer, T. J. J. Phys. Chem. 1993,97, 13126.

0002-7863/94/1516-9700$04.50/00 1994 American Chemical Society

Fixed- Distance Porphyrin-Quinone Model SystemJ

J . Am. Chem. SOC.,Vol. 116, No. 21, 1994 9701 lowered being observed for the molecules in ZMTHF and an increase in rates in toluene. We also find that the shape of the temperature dependence is quite similar for the different molecules, despite the differences in AGO. A semiclassical analysis, which accounts for temperature-dependent changes in solvent dielectric properties, successfully described the trends in toluene, but neither a classical nor a semiclassical treatment gave completely satisfactory results for 2MTHF. The slowing of the average ET rates in 2MTHF at temperatures below 200 K is consistent with an angle-dependent electronic coupling and slow conformational isomerization.

4

Figure 1. Molecular structures of the zinc(I1) mesophenylcctamethylporphyrin[ 2.2.21bicyclooctylquinone model compounds.

spacer of [2.2.2]-bicyclooctane units. In addition to its relevance as a model for certain reactions in the photosynthetic RCs, this system offers the advantage of maintaining the donor and acceptor at a fixed distance (14.8 A center-to-center for molecules with a single spacer) independent of conformational motion. The thermodynamic driving force can be varied by employing substituted quinones, with minimal perturbation of the electronic and nuclear factors involved in ET. Leland et al.12have previously reported studies of ET in ZnPLQ compounds with different donoracceptor distances (zero, one, or two spacers) which demonstrated a reduction of the ET rate by a factor of at least 500 for an increase in distance from 14.8 to 18.8 A. The one-spacer compound was also examined in 2-methyltetrahydrofuran glass at 77 K. The nonexponential decay observed was attributed to a distribution of rotational conformations and an angle-dependent ET rate; the optimal rate was suggested to vary only weakly with temperature. Joran et a l l 3 reported on the effect of varying driving force at a fixed distance (one spacer) in seven ZnPLQ compounds with substituted quinones. The ET rates were shown to be consistent with either the classical or semiclassical models over the range of driving forces explored (-0.6 eV). In this paper, we report on the temperature dependence of ET rates, as measured by time-resolved donor fluorescence quenching, in three homologous ZnPLQ compounds in toluene and 2-methyltetrahydrofuran (2MTHF). The ZnPLQ molecules studied, shown in Figure 1, are those where the acceptor is benzoquinone (l),methylbenzoquinone (2), and dimethylbenzoquinone (3), as well as a reference compound with no quinone acceptor, 4. We find that the rates in both solvents are weakly dependent on temperature, with a decrease in ET rate as the temperature is (12) Leland, B. A.; Joran, A. D.; Felker, P. M.; Hopfield, J. J.; Zewail, A. H.; Dervan, P.B. J. Phys. Chem. 1985,89, 5571. (13) Joran, A. D.;Leland, B. A.; Felker, P. M.;Hopfield, J. J.; Zewail, A. H.; Dervan, P.B. Nature 1987, 327, 508.

2. Experimental Section The synthesis of molecules 1-4 has been described previously.l6 Samples were purified by chromatography (CHzC12, silica gel) immediately prior to the lifetime measurements, and the dichloromethane solvent was removed inuacuo. 2MTHF was distilled fromcalciumhydride and stored over calcium hydride under vacuum. Toluene (HPLC grade, Aldrich) and toluene-d8 (99+ atom %, Aldrich) were used as received (in sealed bottles or ampules, under nitrogen). The appropriate solvent was vacuum transferred onto the sample immediately before the fluorescence measurements. Samples had concentrations between 10-5 and M and were filtered through 0.5 Gm filters to minimize light scattering. Absorption spectra were obtained before and after each set of lifetime measurements to verify that the sample had not degraded during the experiment. Special low-temperature cuvettes (1.0 cm path length) with fused edges, obtained from NSG Precision Cells, were used. Cells with glued faces were destroyed whenever the solution froze during a run. Sample temperatures were regulated to within 0.1 K with a liquid helium cooled continuous flow cryostat (Oxford Instruments).14 Porphyrin fluorescencedecayswere measured by time-correlated single photon counting,I5using a cavity-dumped synchronously mode-locked dye laser (Coherent 702-3) pumped by the frequency doubled output of a CW mode-lockedNd-YAG laser (Quantronix 416). The full width at half maximum of the autocorrelation of the dye laser pulses was typically 5 ps. The wavelength of the exciting light was 570 nm and the fluorescence was collected and passed through a dichroic sheet polarizer set for magic angle detection. Fluorescence at 630 nm was isolated using a cutoff filter (Schott RG610) and a 0.25 m monochromator (Instruments SA, model H-10, 10 nm bandpass) and detected with a MCP-PMT (Hamamatsu 1564U-01). Decays were fit to the sum of two or three exponential components with a nonlinear least-squaresmethod based on the Marquardt algorithm.I6 The instrument response function (typical full width at half maximum 70 ps) was measured using scattered laser light and was accounted for in the fits by iterative reconvolution. The goodness-of-fit was judged according to the reduced xz parameter,I6of which values in the range of 1.01 to 1.40 were typically obtained and the maximum accepted value was 1.5. In certain cases, as described below, we were unable to obtain good fits to a double exponential model function, in which case a triple exponential function was used. In a typical set of experiments, the deaerated sample was first equilibrated close to room temperature in the dewar with helium gas flowing continuously. The temperature was lowered slowly, in steps of 10 to 50 K, and the sample allowed to equilibrate for at least 15 min at each temperature beforedata were acquired. After data had been acquired at the minimum desired temperature for a given run, the sample was warmed slowly, using uniform increments, and data were collected as before. The specific temperatures at which decays were recorded during the warm-upphase were chosen between those used in the cool-down half of the run. The trends of the fitted lifetimes for decays obtained during cooling and warming showed no systematic differences, indicating that the samples were thermally equilibrated at a given temperature when the decays were recorded.

3. Results E T rates were determined, as described previously,~2J3by picosecond time-resolved measurements of fluorescence quenching (14) The quoted temperatures are accurate to within +1 K. (15) OConnor, D. V.; Phillips, D. In Time Correlated Single Photon Counting, Academic Press: New York, 1984. (16) (a) Joran A. D.; Leland, B. A.; Geller, G. G.; Hopfield, J. J.; Dervan, P. B. J. Am. Chem.Soc. 1984,206,6090. (b) Joran,A. D. Doctoral Dissertation, California Institute of Technology, Pasadena, CA, 1986. (c) Leland, B. A. Doctoral Dissertation, California Institutue of Technology, Pasadena, CA, 1987.

Khundkar et al.

9702 J. Am. Chem. SOC.,Vol. 116, No. 21, 1994

functions with one of them being dominant depended on the solvent and the particular molecule being studied. Over such a range, the ET rate constant (kET)is well-defined and was obtained from eq 1, using the decay constant of the fast component as the quenched lifetime. At lower temperatures, where the decays appear multiexponential, a single E T rate for the quenched component cannot be defined. The decays in this temperature range were represented as the sum of three exponentials (eq 2), with the longest component (73) having a lifetime comparable to TO. The time-dependent fluorescence intensity, Q(t), was thus taken as

a. 4 1

0 ' 0

2

5

4 I

6

8

1

Time (nsec) I

I

0

I

1

2

I

b.

0

2

I

I

I

4

6

8

I

'

1 0 1 2 1 4

Time (nsec) Figure 2. Semilogarithmic plots of typical fluorescence decays of 1 in (a) toluene and (b) ZMTHF at various temperatures. The points are the

experimental data, and the solid lines are fitted curves as described in the text. Note that the instrument response function has been accounted for in these fits. of the excited porphyrin donor. In the absence of an acceptor, the excited porphyrin emits from the SIstate (-2.15 eV relative to SO)with a characteristic lifetime TO. In the presence of an acceptor, E T competes with fluorescence and the observed lifetime T I is shortened. The rate constant of electron transfer, kET, can then be determined as follows:

Representative fluorescence decays are shown in Figure 2 for 1 in toluene and 2MTHF at several different temperatures. The measured fluorescence decays are nonexponential. At room temperature, each decay can be cleanly resolved into two components, the major (>99%) component having a shortened lifetime relative to that of the reference compound 4 due to quenching by intramolecular ET, and the minor component having a lifetime similar to 4 and ascribed to trace amounts of molecules with chemically reduced quinone acceptors.'* The decays continue to be biexponential as the temperature is lowered until a phase transition (glass or liquid-solid) is approached, where they become more complex (multiexponential). The solvents were fluid over the range of temperatures studied. In toluene and toluene-da, the decays showed only a small change over the fulLrange of temperatures studied: room temperature to the freezing point of thesolvent (- 180 K). In ZMTHF, the fluorescence decaysvaried weakly as the temperature was lowered from room temperature to 180 K. On further cooling (to 110 K), the decays became progressively slower.17 The range of temperatures over which the decays could be adequately represented by a sum of two exponentially decaying

-

(17) In our studies, the solvent was liquid even at the lowest temperature at which results are reported. On further cooling, the solvent usually froze. Decays measured as thesolution equilibrated at these low temperatures showed significant lengthening of the decays in toluene and toluene-&

wherej = 2 or 3, ai is the amplitude, and Ti is the lifetime of the ith component, as obtained from fits of the model function to the data. We used the multiexponential model function above as a means of simulating the observed nonxponential decays. In this way, we approximated a possibly continuous distribution with a few discreet rate constants. Below we discuss the nonexponential decays in terms of the ETdynamics of a distribution of conformers that have a conformationally dependent ET rate constant. Such a dynamical system can be analytically described with a sum of exponentials, each corresponding to a particular conformer, when interconversion is slow. We approximate a possibly larger number of exponential components with just three, because of a lack of detailed knowledge of the number of conformers and their energetics, as discussed below. There could be other reasons for the observed nonexponential decays, such as the E T rate constants being controlled by solvent relaxation and there being a distribution of solvent relaxation times. We argue below that the time scales for solvent relaxation are too fast to be rate limiting for the systems studied. To characterize the complex E T dynamics, as revealed by multiexponential decays, we calculated an average E T rate parameter, k,,, from the reciprocal of the weighted mean of ET time constants, and an initial average E T rate constant, keTo, from the weighted mean of the ET rate constants. These quantities, which are different average measures of the E T dynamics when thereisa distributionof intrinsicETrateconstants, are defined in eqs 3a and 3b. The kETo parameter is an average

1- 1

kE: = $ai(:-:) (3b)

ki= 1a , measure of the rate constant for E T a t early times. The k,, parameter is a measure of the average rate constant for E T a t long times, as it is more heavily weighted by the slow decay components. The values of k,, and kETo are listed in Tables 1-3. These values represent the results from two or three independent measurements of the fluorescence decays as a function of temperature. In a few cases, at the lowest temperatures, the parameter a3 increased significantly above the 1% level observed a t higher temperatures, indicating that a significant fraction of the molecules has an E T rate constant smaller than ~ 0 - l . In these cases, kET0was calculated using all three exponential components.

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Fixed-Distance Porphyrin-Quinone Model Systems

J. Am. Chem. SOC., Vol. 116, No. 21, 1994 9103

Table 1. Temperature-Dependent ET Rate Constants' for 1-4 in Toluene T (K) 305 300 295 290 285 280 270 265 260 250 240 235 225 220 215 210 205 200 195 190 185 180

ZnPLQ (l) ZnPLQMe kPVb hoc (2) k E T o c 2.4 2.5 2.6 2.7 2.9 2.9 3.1 3.2 3.2 3.3 3.4 3.4 3.2 3.2

3.5 3.9

3.2

4.0

1.03 1.08 1.15 1.15 1.22 1.30 1.31 1.39 1.47 1.51 1.54 1.44 1.63 1.65 1.69 1.72 1.79 1.75 1.86 1.89

ZnPLQMe2 (3)

ET^^

ZnPtBu (4)s0-l

0.38 0.40 0.40 0.41

0.67

0.45 0.47 0.51 0.54 0.54 0.57 0.59 0.61 0.63

0.66

0.65

0.68 0.70 0.73 0.83

kETo = k., where only the ET' value is given. Uncertainty in rate constantsis 180 K, T > 140 K,