J. Phys. Chem. 1990, 94, 1224-1226
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Solvent-Tuned Intramolecular Charge-Recombination Rates in a Conjugated Donor-Acceptor Molecule Lutfur R. Khundkar,t A. E. Stiegman,* and Joseph W. Perry* Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 (Receiued: December 13, 1989) The nonradiative charge-recombination rates from the charge-transfer state of a new conjugated donor-acceptor molecule (p-cyano-p'methylthiodiphenylacetylene) can be tuned over almost an order of magnitude by varying the polarity of the solvent. These measurements of intramolecular recombination show a turnover of rates as a function of emission energy, consistent with the "normal" and "inverted" behavior of Marcus theory. Steady-state spectra and time-resolved measurements allow us to quantitatively compare thermal and optical electron-transfer rates as a function of driving force and demonstrate their correspondence. Electron-transfer (ET) reactions play a central role in a broad range of chemical and biological processes and have been the subject of extensive investigation.' The influence of factors such as donor-acceptor distance, driving force (-AGO), and medium dielectric and dynamical properties on the rates of such reactions are of critical importance in understanding how biological systems attain high efficienciesof charge separation as well as in the design of systems for molecular electronics.2 The AGO dependence of ET rates is of particular interest because several theories3 predict a maximum rate at some system-dependent optimum value and lower rates at lower and higher driving forces. These regimes are commonly referred to as the "normal" and "inverted" regimes of ET, respectively. Although Miller et aL4 have presented results showing normal and inverted behavior in charge-shift reactions (CSH), convincing evidence for the existence of both regimes in intramolecular charge separation (CS)5 and charge recombination (CR)6 is lacking. Some recent work6bon bimolecular CR in ion pairs has provided evidence for rates in both regimes. For the most part, CS rates increase as AGO becomes more n e g a t i ~ ein , ~some instances approaching a maximum value asymptotically. C R rates generally show inverted behavior only;6 Le., rates decrease with increasing driving force. In some cases,' the rates of both CS and CR processes in the same system have been measured and superimposed on a common AGO scale in attempts at demonstrating the theoretically predicted Gaussian dependence on driving force. Recent theoretical works suggests that these three categories of ET reactions may exhibit different AGO dependences if the dielectric unsaturation approximation breaks down. Therefore, it is important to explore the full AGO dependence of all three types of reactions individually. In this paper, we report the experimental observation of a bell-shaped energy gap dependence of the rate of a nonradiative C R process in a conjugated donor-acceptor molecule p-cyanop'-methylthi~diphenylacetylene~(I).
Unlike systems studied previously, these recombination rates show an initial increase with increasing driving force (-AGO), followed by a decrease at higher driving forces. These observations are consistent with the classical Marcus theory of electron transfer as well as with the formally equivalent theory of nonradiative decaylo of large molecules in the strong-coupling limit. We are able to access a significant range of driving forces simply by using solvents of varying polarity, which afford different degrees of stabilization to the dipolar excited state populated by optical excitation. While this approach unavoidably involves a change in driving force and reorganization energy (A), both parameters can be estimated directly from the absorption and emission 'NRC-NASA Resident Research Associate.
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spectra.]' Steady-state and picosecond time-resolved fluorescence measurements were used to deduce nonradiative decay rates, which show a distinct maximum as a function of emission energy. A schematic of the states involved in the photophysical processes is given in Figure 1. Absorption and fluorescence spectra of I show mirror image symmetry, suggesting that the ground and excited potential energy surfaces (and thus the force constants) are similar. The intense, lowest lying absorption band (A,, = 325 nm, t = 3.35 X lo4 M-lan-' in acetonitrile) is thought to be due to a sulfur nonbonding (ns) to cyano T antibonding ( T * ~ ~ ) charge-transfer transition. The excited state ('CT) undergoes stabilization through fast solvent relaxation12and shows a strong, solvatochromic emission (A, = 4 4 2 nm in acetonitrile; A,, = 351 nm in n-hexane). The steady-state solvatcchromism can be qualitatively explained by the simple dipole in a dielectric continuum model. The Stokes' shift in emission (Figure 2) appears to follow the Lippert-Mataga equationI3
where BabS and Ffl denote energy (wavenumbers) of the absorption (1) Marcus, R. A.; Sutin, N . Eiochim. Eiophys. Acta 1985, 811, 265. Gust, D.; Moore, T. A. Science 1989, 244, 35. See, e.&: Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988. (2) Hopfield, J. J.; Onuchic, J. N.; Beratan, D. N. J . Phys. Chem. 1989, 93, 6350. (3) Marcus, R. A . J . Chem. Phys. 1965, 43, 679. Hopfield, J. J. Proc. Nail. Acad. Sci. U.S.A.1974, 71, 3640. Jortner, J. J . Chem. Phys. 1976, 64, 4860. (4) Miller, J. R.; Calcaterra, L. T.; Closs, G. C. J. Am. Chem. SOC.1984, 106, 3047. Penfield, K. W.; Miller, J. R.; Paddon-Row, M. N . ; Cotsaris, E.; Oliver, A. M.; Hush, N . S. J . Am. Chem. SOC.1987, 109, 5061. (5) See, e.&: Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A,; Pewitt, E. B. J . Am. Chem. SOC.1985,107, 1080. Joran, A. D.; Leland, B. A.; Felker, P. M.; Zewail, A. H.; Hopfield, J. J.; Dervan, P. B. Nature 1987, 327, 508. Marshall, J. L.; Stiegman, A. E.; Gray, H. B. In Excited States and Reaction Intermediates; ACS Symp. Ser. 307; Lever, A. B. P., Ed.;American Chemical Society: Washington, DC, 1986; p 167. (6) (a) See, e.g.: Gould, I. R.; Moser, J. E.; Ege, D.; Farid, S. J . Am. Chem. SOC.1988, 110, 1991. (b) Mataga, N.; et al. Chem. Phys. 1988,127, 249.
(7) See, e.&: Lindsey, J. S.; Delaney, J. K.; Mauzerall, D. C.; Linschitz, H. J . Am. Chem. SOC.1988, 110, 3610. Irvine, M. P.; Harrison, R. J.; Beddard, G. S . ; Leighton, P.; Sanders, J. K. M. Chem. Phys. 1986,104, 315. Fox, T. L. S. Ph.D. Thesis, California Institute of Technology, 1989. (8) Kakitani, T.; Mataga, N. J . Phys. Chem. 1987, 91, 6277. (9) 1 was synthesized by the method of Takahashi, S. et al. Synfhesis 1980,627. The compound was characterized by NMR and elemental analysis. A full report will be published later. (10) Englman, R.; Jortner, J. J . Mol. Phys. 1970, 18, 145. Freed, K. F.; Jortner, J. J . Chem. Phys. 1970, 52, 6272. (11) See, e.g.: Creutz, C. Prog. Inorg. Chem. 1983, 30, 1. ( 1 2) The equilibration of the polar excited state may involve intramolecular motion, Le., twisting. Since no dual emission or significant rise time of fluorescence is observed for I, we conclude that formation of the equilibrated 'CT state occurs in less than 20 ps. (13) (a) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Spectra; Dekker: New York, 1970. (b) Brunschwig, B. S.;Ehrenson, S.; Sutin, N. J . Phys. Chem. 1987, 91, 4714.
0 1990 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1225 increasing
solvent polarity
Figure 1. Schematic potential energy curves for the ground and low-lying excited electronic states involved in the various transitions described in the text. The dashed curve indicates the stabilized 'CT state in a polar solvent.
r
Acet o n I t r I I
2
0
6
4
0
Time (nanoseconds) Figure 3. Fluorescence decays of I in selected solvents spanning a range of polarities: (a) acetonitrile ( T = 1.97 ns), (b) tetrahydrofuran ( T = 1.12 ns), (c) n-hexane ( T = 0.72 ns), and (d) response function. The solid lines are least squares fits to the data (points).
8000~ I
1
N
I
I
I
I
n-Hexane
2000
,
,
,
24
26
zB
801
0.2 0.3 Reactlon Field Parameter 0.1
0.4
Figure 2. Steady-state Stokes' shift of I in various solvents as a function of the reaction field parameter (eq I ) . The solid line is the best fit line.
and emission maximum, respectively. IAPI is the magnitude of the difference in dipole moments between the ground and excited states, a is the radius of a spherical cavity containing the dipole; D,is the static bulk dielectric constant and n is the optical refractive index of the solvent. Taking a to be 6.4 A, which is half the estimated length of the long axis of the molecule, we find = 19 D. This suggests that a significant fraction ( - 2 5 % ) of an electron is being transferred, consistent with the assignment of the transition and the notion that the energy of the excited state can be tuned by varying the solvent polarity. The time-resolved fluorescence decaysi4of the emission from ICT in all solvents studied appear single exponential (Figure 3). The excited state has a much longer lifetime in polar solvents than in nonpolar solvents. The radiative decay rates,Is were found to be solvent-dependent and proportional to n2'vn3,in accord with the Strickler-Berg relationT6for states whose integrated absorption intensity is independent of the solvent, as is the case for I. Nonradiative rates (k,,) derived from our measurements are shown in Figure 4a as a function of the solvent-tuned ijfl. The rates show a strong dependence on the solvent and a distinct maximum and turnover at high Zfl. The observations indicate a (14) Fluorescence lifetimes were measured by time-correlated single photon
counting. The full width at half-maximum of the instrument response function was -70 ps. (15) Quantum yields, corrected for the refractive index of each solvent, were measured with a Perkin-Elmer fluorimeter (MPF-66) and referenced to 1.0 N quinine sulfate. The absorbance of each solution was matched to that of the reference at the excitation wavelength (340 nm). Parker, C. A,; Rees, W. T. Analyst (London) 1960, 85, 587. (16) Strickler, S.J.; Berg, R. A. J . Chem. Phys. 1962, 37, 814.
78
20
22
Energy
(cm".l)I1000
Idgal/ 3o
,
40
8?80
Scaled
Driving Force
,
80
Figure 4. (A) Nonradiative rates as a function of emission energy. The solid line is the best fit of a Gaussian function to the data. (B) Comparison of the driving force dependence of thermal and optical ET rates. The points are k,, scaled as described in the text. The parabolic curve is a fit of the data to eq 2. The other solid line is the normalized fluorescence intensity in acetonitrile plotted against (Zfl - ;/[A(CH,CN)]'/*).
solvent-dependent energy gap between 'CT and the final state in the nonradiative transition. This differential stabilization implies that the final state is substantially less polar than ICT. Thus, the nonradiative transition corresponds to a CR process. k,, may be compared to the predictions of ET theories6J7J8with the caveat that partial charge transfer may be involved. The Marcus expression for the rate of thermal ET can be written as
where Habis the electronic matrix element, k B is Boltzmann's constant, and T is the absolute temperature. When the force constants of the two states involved are equal, one can estimate the parameters AGO a n d X from the measured absorption and (17) It should be noted that the functional form of the energy gap dependence of k,, in the limit of large distortions (strong coupling) is the same as that of the AGO dependence of ket. While the matrix element that couples the two electronic states are different in the two theories, the exponentialpart of the dependence results from the Boltzmann-weighted Franck-Condon overlap factors which are common to both. Here, we emphasize the charge-transfer aspect of the problem. (18) See, e.g.: Meyer, T. Prog. Inorg. Chem. 1983, 30, 389. Caspar, J. V.; Sullivan, B. P.; Kober, E. M.; Meyer, T. J. Chem. Phys. Lett. 1982, 91,
91.
J. Phys. Chem. 1990, 94, 1226-1229
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emission spectra of 'CT using the expressions (3) hc X = $Sa&
- ij"]
(4)
where A G O , is the free energy of the ground state relative to the ICT state. The magnitudes of the changes in AGO, and X are comparable over the range of solvents used. The maximum rate shown in Figure 4a occurs at t n 26.3 X IO3 cm-I. According to eq 2 above, the maximum rate should occur when -AGO = A. If the CR process connects the ICT state directly to the ground state, the estimated value for X would be -2.8 eV, a value which is extremely large and inconsistent with the estimate (0.1-0.5 eV) from the spectra. This implies that the ground state is not responsible for the nonradiative ET process and a third, distinct, nonpolar state must be invoked as the final state in charge recombination. Candidates for this "dark" state are the na* and AT* triplet states, both of which lie at lower energy than the corresponding singlet states.19 An effective driving force for C R can be obtained from eq 3 and 4 after an additional constant (26.3 X lo3 cm-') corresponding to the energy of the dark state is subtracted out provided that the energy of this state is independent of solvent polarity.m For a comparison of the data with the Marcus equation, we plot In (k,,A1/2)vs (AGO + A ) / A 1 / * in Figure 4b, using the scaling factor to account for the variation in reorganization energy in the different solvents. A quadratic fit to these scaled rates is shown and the agreement indicates consistency between the experimental results and eq 2.
-
(19) We were unable to detect phosphorescence from I in methylcyclohexane or 2-methyltetrahydrofuran at 77 K. This does not preclude the involvement of triplet states in the photophysics. (20) If the dark state was weakly polar, the inferred driving forces would be overestimated by a factor which is roughly linear in the emission energy. Thus, the exact form of k,,(AGe) shown in Figure 4b would not be correct, although it would still show the turnover. In addition, the analysis used assumes that the X for the dark state is the same as that for the other states.
The measured maximum rate (6.83 X 108 s-l) allows us to estimate a value of -1 cm-' for the matrix element Hab. This value is rather small for a donor and acceptor linked by a conjugated bridge, but consistent with a spin-forbidden transition which would be involved if the dark state were a triplet state. In a recent publication,21 Marcus has pointed out the correspondence between the driving force dependence of CS and C R rates and the band shape of the charge-transfer absorption and emission spectra. In Figure 4b, we also show a comparison of the fluorescence spectrum and recombination rates. We show the normalized and scaled fluorescence intensity (Z#) in acetonitrile as a function of tfl- t in Figure 4b. We note the agreement between the calculated k,,(AGo) curve, the normalized fluorescence intensity, and the experimental C R rates. This correspondence suggests that the potential energy surface of the final state in the nonradiative CR transition is not heavily distorted relative to the ground state. In summary, we have presented evidence for C R rates in a conjugated donor-acceptor molecule which span both normal and inverted regimes of electron transfer. The agreement between these results and the simple Marcus equation supports the validity of the dielectric unsaturation approximation for the system studied. Issues currently under investigation include verification of the mechanism proposed, the dependence of the dynamics on the length of the conjugated linker, and the role of solvent dynamics in the charge-transfer processes. Acknowledgment. This work was carried out at the Jet Propulsion Laboratory (JPL), California Institute of Technology, and was sponsored by the U.S. Department of Energy (Energy Conservation and Utilization Technologies Division) and the Strategic Defense Initiative Organization (Innovative Science and Technology Office) through an agreement with the National Aeronautics and Space Administration (NASA). L.R.K. thanks NASA and the National Research Council for a Resident Research Associateship at JPL. (21) Marcus, R. A. J. Phys. Chem. 1989, 93, 3078.
Photoinduced Formatlon of Spatial Patterns in the Belousov-Zhabotinskii Reaction M. Jinguji,* M. Ishihara, and T. Nakazawa Department of Chemistry, Medical University of Yamanashi, Tamaho, Nakakoma, Yamanashi 409-38, Japan (Received: September 15, 1989; In Final Form: November 27, 1989)
New patterns of chemical waves are induced on illumination of the Ru(bpy)32+-catalyzedBelousov-Zhabotinskii reaction system. Stable waves are successively initiated at the boundary between the illuminated and shadow regions of a thin layer of the reactant solution. The period of wave generation is about 47 s. These wave fronts propagate inward in the illuminated field at a nearly constant velocity of 3.4 mm/min and finally disappear at the center of their profiles. We have been able t o generate square and triangular waves also, which a r e the first examples of traveling chemical waves moving as straight lines.
Introduction The formation of spatial patterns in chemical systems resulting from the coupling of reaction with diffusion has been extensively studied, both theoretically and e~perimentally.'-~Little is known on the subject of the illumination effects on chemical waves, ( I ) Field, R. J.; Burger, M. Oscillations and Traveling Waves in Chemical Systems; Wiley-Interscience: New York, 1985. (2) Vidal, C . ; Hanusse, P. Int. Rev. Phys. Chem. 1986, 5 , I . (3) Epstein, 1. R . Chem. Eng. News 1987, 65, 24.
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although the influence of light on temporal oscillations has been in~estigated.~" Busse and Hess' reported that under suitable conditions a propagating wave was initiated at the ultravioletirradiated spot in a ferroin-catalyzed reaction system. Kuhnert (4) Vavilin, V. A.; Zhabotinskii, A. M.; Zaikin, A. N. Rum. J. Phys. Chem. 1968, 42, 1649. ( 5 ) Sharma, K. R.; Noyes, R. M. J. Am. Chem. SOC.1975, 97, 202. (6) GBspBr, V.; Bazsa, G.; Beck, M. T. Z . Phys. Chem. (Leipzig) 1983, 264, 43. ( 7 ) Busse, H.; Hess. B. Nature 1973, 244, 203.
0 1990 American Chemical Society
