J . Phys. Chem. 1989, 93, 5 173-5 179
5173
Solvent Polarity Effects on Intramolecular Electron Transfer. 1. Energetic Aspects H. Heitele,* P. Finckh, S. Weeren, F. Pollinger, and M. E. Michel-Beyerle Institut fur Physikalische und Theoretische Chemie, Technische Universitat Munchen, Lichtenbergstrasse 4, 8046 Garching, FRG (Received: December 6 , 1988) We present time-resolved fluorescence measurements on intramolecular electron donorlacceptor compounds in different solvents as a function of temperature. In moderately polar solvents, biexponential fluorescence decay is observed, which is interpreted as evidence for intramolecular charge separation from an excited state to an ion-pair state and subsequent,thermally activated repopulation of the excited initial state. The free energy of the charge-separationreaction in these solvents, charge-separation and repopulation rates, and the lifetime of the ion-pair state are estimated from an analysis of the biexponential decay kinetics. Temperature and solvents effects on these quantities are shown to be consistent with current electron-transfer theories.
1. Introduction Since the classical papers by Marcus' and Hush2 on the theory of electron-transfer reactions in condensed media, the role of the solvent has been a main focus of interest. A considerable amount of work3-' has been devoted to the determination of solvent reorganization energies and their dependence on the dielectric constant, the refractive index, and molecular size effects. A very widely used method in this context proved to be the determination of electron-exchange rates of symmetric reactions by ESR measurements.* Among the more recent measurements, pulse radiolysis experiments on intramolecular electron-transfer systems9 have established the close interrelation between solvent polarity, solvent reorganization, and the dependence of the electron-transfer rate on the reaction energy, especially in the so-called inverted regime. Whereas most of the work has been aimed at the activation term of the transfer rate, experimental evidence concerning an unexpected dependence of the preexponential factor on the solvent was published recently,I0which was interpreted as a manifestation of electronic coupling between donor and accepotor mediated by superexchange via solvent molecules. A very active field of research is focused also on the influence of solvent dielectric relaxation on electron transfer and other radiationless transitions. Theory" predicts a complicated and ( I ) Marcus, R A. J . Chem. Phys. 1956,24,988; J . Chem. Phys. 1965,43, 679. (2) Hush, N. S. Z . Elektrochem. 1957,61, 734; J . Chem. Phys. 1958,28, 962; Trans. Faraday SOC.1961, 57, 557. (3) Extensive data and references on organic electron-transfer reactions can be found in: Eberson, L. Adu. Phys. Org. Chem. 1982, 18, 79. (4) For inorganic and metal organic reactions, the following review articles are recommended: Newton, M. D.; Sutin, N . Annu. Rev. Phys. Chem. 1984, 35, 437. Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265,
and references therein. (5) Oevering, H.; Paddon-Row, M. N.; Heppener, M.; Oliver, A. M.; Cotsaris, E.; Verhoeven, J. W.; Hush, N . S. J . A m . Chem. SOC.1987, 109, 3258. (6) Harrison, R. J.; Pearce, B.; Beddard, G . S.; Cowan, J. A,; Sanders, J. K. M. Chem. Phys. 1987, 116, 429. Joran, A. D.; Leland, B. A,; Felker, P. M.; Zewail, A. H.; Hopfield, J. J.; Dervan, P. B. Nature 1987, 327, 508. (7) Wasielewski, M. R.; Niemczyk, M. D.; Svec, W. A,; Pewitt, E. R. J . A m . Chem. SOC.1985, 107, 1080. (8) Ward, R. L.; Weissman, S. I. J . Am. Chem. Soc. 1954. 76,3612; 1957, 79, 2086. Kowert, B. A.; Marcoux, L.; Bard, A. J. J . Am. Chem. SOC.1972, 94, 5538. Grampp, G.; Jaenicke, W. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 325, 335. (9) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J . Am. Chem. SOC.1984, 106, 5057. Closs, G. L.; Calcaterra, L. T.; Green, N. J.; Penfield, K. W.; Miller, J. R. J . Phys. Chem. 1986, 90, 3673. See also: Gould, I. R.; Moody, R.; Farid, S. J . A m . Chem. SOC.1988, 110, 7242.
( I O ) Oliver, A. M.; Craig, D. C.; Paddon-Row, M. N.; Kroon, J.; Verhoeven, J. W. Cheni. Phys. Lett. 1988, 150, 366. (11) Zusman, L . D. Chem. Phys. 1980, 49, 295; Chem. Phys. 1983, 80, 29; Chem. Phys. 1988, 119, 51. Helman, A. B. Chem. Phys. 1982,65,271; Chem. Phys. 1983, 79,235. Rips, I.; Jortner, J. Chem. Phys. Lett. 1987,133, 41 1. Sumi, H.; Marcus, R. A. J . Chem. Phys. 1986,84,4272,4894. Nadler, W.; Marcus, R. A. J . Chem. Phys. 1987,86, 3906; Chem. Phys. Letr 1988, 144.24. Sparpaglione. M.; Mukamel, S. J . Chem. Phys. 1988,88,4300,3263. Garg, A.; Onuchic, J. N.; Ambekaokar, V. J . Chem. Phys. 1985, 83, 4491. Onuchic, J. N. J . Chem. Phys. 1987, 86, 3925. Calef, D. F.; Wolynes, P.G. J . Phys. Chem. 1983, 87, 3387; J . Chem. Phys. 1983, 78, 470. Jortner, J.; Bixon, M. J . Chem. Hys. 1988, 88, 167.
0022-365418912093-5173$01.50/0
multifarious behavior dependent on intramolecular properties such as electronic coupling and intramolecular reorganization as well as on specific solvent relaxation properties, among them dielectric relaxation times, distribution characteristics thereof, and molecular features. experiment^'^-'^ are beginning to bear out key aspects of these theories, such as the Occurrence of nonexponential reaction kineticsI3 and the proportionality of the rate to the inverse of (a fractional power of) the longitudinal dielectric relaxation time.14 From a theoretical point of view, efforts are under way to gain a closer understanding of the microscopic picture of the role of the solvent. In this context, molecular dynamicsI5 and related simulations16have been shown to be promising tools. In the course of our own we have studied a series of molecules in which an electron donor is covalently bound to an electron acceptor via different molecular bridges with varying donor/acceptor distances. One of these molecules (AID) together with a reference substance ( A l ) is shown in Figure 1. In this compound, optically excited anthracene acts as an electron acceptor and dimethylaniline as a donor. Intramolecular electron-transfer rates were determined via temperature-dependent fluorescence lifetime measurements in highly polar solvents and separated into preexponential factors and activation energies.2',22 It was found18-20,22 that the dependence of the preexponential factor on the donor/acceptor distance and the structure of the bridging elements can be understood on the basis of an electronic through-bond coupling via the spacer between the donor and acceptor. Combined with transient absorption data, these experiments allowed the estimate of the solvent and intramolecular reorganization energies2* The magnitude and the dependence of the former on the donor/acceptor distance could be explained within conventional electron-transfer theory. ~~
(12) Huppert, D.; Kanety, H.; Kosower, E. M. Faraday Discuss. Chem. SOC.1981, No. 74, 161. McManis, G. E.; Weaver, M. J. Chem. Phys. Letr. 1988, 145, 55. Nielson, R. M.; McManis, G. E.; Golovin, M. N.; Weaver, M. J. J . Phys. Chem. 1988.92, 3441. Huppert, D.; Rentzepis, R. M. J . Phys. Chem. 1988, 92, 5466. (13) Huppert, D.; Ittah, V.; Kosower, E. M. Chem. Phys. Lett. 1988, 144, 15. (14) Kosower, E. M.; Huppert, D. Chem. Phys. Lett. 1983, 96, 433. McGuire, M.; McLendon, G. J . Pys. Chem. 1986, 90, 2549. (15) Hwang, J.-K.; Warshel, A. J . A m . Chem. SOC.1987, 109, 715.
Karim, 0. A.; Haymet, A. D. J.; Banet, M. J.; Simon, J. D. J . Phys. Chem. 1988, 92, 339 1. (16) Kuharski, R. A.; Bader, J. S.; Chandler, D.; Sprik, M.; Klein, M. L.; Impey, R. W. J . Chem. Phys. 1988,89, 3248. (17) Heitele, H.; Michel-Beyerle, M. E. J . Am. Chem. SOC.1985, 107, 8068.
(18) Heitele, H.; Michel-Beyerle, M. E. Finckh, P. In Antennas and Reaction Centers of Photosynthetic Bacteria; Michel-Beyerle, M. E., Ed.; Springer: Berlin, 1985; pp 250-255. (19) Heitele, H.; Michel-Beyerle, M. E.; Finckh, P.; Rettig, W. In Tunneling; Jortner, J., Pullman, B., Eds.; Reidel: Dordrecht, The Netherlands, 1986; pp 333-344. (20) Heitele, H.; Michel-Beyerle, M. E.; Finckh, P.Chem. Phys. Lett 1987, 134, 273. (21) Heitele, H.; Michel-Beyerle, M. E.; Finckh, P. Chem. Phys. Letr. 1987, 138, 231. (22) Finckh, P.; Heitele, H.; Volk, M.; Michel-Beyerle, M. E. J . Phys. Chem. 1988, 92, 6584.
0 1989 American Chemical Society
5174
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989
A1 D
A1
Figure 1. Structure formulas of donor/acceptor compound A I D and reference substance A 1.
Part 1 of this work contains a presentation of the phenomenology of temperature and solvent effects. It focuses on the thermodynamic aspects and their implications for the kinetics of the system. A more quantitative discussion of solvent influences on electron-transfer rates in terms of reorganization energies and preexponential factors will be published in Part 2.
2. Experimental Details For fluorescence lifetime measurements, a single-photoncounting apparatus was used, which was equipped with a flash lamp (Edinburgh Instruments 199F) filled with hydrogen. For AID
n othyl
Heitele et al. excitation, a band with a width of 20 nm centered at 345 nm was selected with a double monochromator (Jobin-Yvon). Fluorescence light was passed through a cutoff filter (cutoff frequency 379 nm). The full width at half-maximum of the time response function of this setup was about 1.4 ns. Comparison with a laser-driven apparatus proved it to be reliable for lifetimes down to 400 ps. Typically, 4 X 103-2 X lo4 counts were sampled in the peak channel. For additional information, see Figure 2. Temperature was kept constant within fl "C. A1D was purified by column chromatography on silica gel and subsequent recrystallization from hexane. A1 was recrystallized and sublimated. In both compounds, no (volatile) impurities could be detected by gas chromatography. Apart from anthracene bands, no additional features were observed in fluorescencespectra at room temperature. All solvents were of spectroscopic grade and were used as purchased. Sample concentration were (1-2) X loe5 M. All solutions were degassed by several thaw-freezepump cycles. Some of the measured lifetimes are of the order of 200 ns, i.e., in 2 X 10" M solutions intermolecular processes could play a role. Because of the low intensity of the excitation source, we could not reduce the sample concentration further. A measurement in butyl acetate at a concentration of lo4 M gave the same long lifetimes, though, indicating that reactions between different A I D molecules can be neglected in these experiments. To improve the
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Figure 2. Fluorescence decay curves of A1 D in different solvents and temperatures (Exc = excitation pulse). All fluorescence curves in each solvent are normalized. The unbroken lines are the pertinent fit functions.
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5175
Polarity Effects on Intramolecular Electron Transfer TABLE I: Results of the Biexponential Fit to the Fluorescence Decay Curves in Moderately Polar Solvents as a Function of Temperature
T, K
a,
T , . ~
350 330 310 290 270 250 230 210
0.54 0.80 0.93 0.976 0.994 0.997 0.997 0.998
0.56' 0.73 0.82 0.86 0.95 1.1 1.4 2.0
300 290 280 270 260 250 230 210 190
1.0 0.18 0.35 0.59 0.81 0.92 0.985 0.995 0.996
5.1 2.2 1.2 1.7 2.0 2.3 2.6 3.0 3.6
310 290 270 250 240 230 220 210 200,
0.995 0.996 0.984 0.71 0.89 0.963 0.987 0.994 0.996
4.3 5.1 6.3 3.5 4.0 4.3 4.5 4.9 5.3
ns
ns
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m , O
ns
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lo6 s-'
Butyl Acetate 9.8 4.5 0.79b 22.7 5.1 0.91 56.4 5.9 0.97 6.9 0.99 150 0.92 200 8.0 202 9.1 0.80 0.61 180 10.1 10.9 0.41 150'
* * 4.5' 2.V 4.2 4.6 5.2 6.5
AEocs.d eV +0.004 -0.031 -0.061 -0.086 -0.1 13 -0.120 -0.109 -0.105 'VI
Diethyl Ether 6.5 8.3 12.9 24 49 203 282 276b
4.5 4.9 5.4 5.8 6.3 6.8 8.1 9.4 10.4
Y I
* 0.1Sb 0.21' 0.25 0.25 0.26 0.23 0.18
* * * * * 2.3' 2.8 3.1
* +0.029 +0.009 -0.015 -0.035 -0.068 -0.082 -0.077
C
2L
23
Diisopropyl Ether 29 32 19.4 16.5 38 82 120 111 9Sb
4.1 4.8 5.7 6.8 7.5 8.2 8.8 9.4 10.1
0.07b 0.09b 0.10 0.11 0.10 0.09
* * 3.5c 5.6 7.8 9.5
+0.014 -0.01 1 -0.036 -0.056 -0.067 -0.07 1
'
Estimated accuracy: f5-I 0%. Estimated accuracy: f20%. Estimated accuracy: f50%. dEstimated accuracy: see Figure 5. accuracy in cases when fluorescence components with widely differing lifetimes were present, two separate measurements in short and long time windows for each component were made.
3. Results We measured fluorescence decay curves of A1 and AID in the series of weakly and moderately polar solvents (dielectric constants at room temperature in parentheses), trichloroethylene (3.4), diisopropyl ether (3.9), diethyl ether (4.3), butyl acetate (5.0), and ethyl acetate (6.0), in the temperature range between 190 and 350 K. For illustrative purposes, a few characteristic decay curves of AI D in these solvents at different temperaures are shown in Figure 2. For simplicity, only measurements with a short time window were selected. The results of a biexponential fit to the experimental curves are summarized in Table I. With the most polar solvent ethyl acetate, a very strong quenching of the fluorescence is observed in A1D compared to AI at all temperatures. The decay in A1 is monoexponential whereas in A1 D it is biexponential with a predominant, short-lived and a weak long-lived component. The relative amplitude of
