10518
J . Phys. Chem. 1991, 95, 10518-10524
"Photochemie mit Lasern"). We thank Prof. J. Troe for support and Mrs. A. Heinrich for technical assistance. M.N. thanks the DGICYT for a scholarship. Registry No. D2,7782-39-0;HBO (keto), 64758-55-0;HBO (enol), 835-64-3;DBO (kcto), 136863-25-7;DBO (enol), 30616-49-0 m-MeHBO (enol), 39720-17-7; m-MeDBO (keto), 136863-26-8;m-MeDBO (enol), 136863-27-9.
with the values obtained from transient-absorption experiments. The rate constants for the nonradiative decays of 3K*and 3E* to the respective ground states are strongly decreased by deuteration. Acknowledgment. This work has been supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 93,
Site Selectivity in Excited-State Reactions in Solutions Alexander P. Demchenko**+and Alexander I. Sytnik A . V. Palladin Institute of Biochemistry, Ukrainian Academy of Sciences, Kiev 252030, Ukraine (Received: August 16, 1991)
The distributions in solutesolvent interaction energies may produce inhomogeneity in reaction kinetics in solutions. We applied the principle of site selectivity to the studies of intramolecular excited-state reactions in solid (vitrified) and liquid solvents. The principle is realized by producing excitation at the red edge of the absorption band and observing the difference between reaction rates at midband and red-edge excitations as a function of the rate of solvent dipolar relaxations (variation of the temperature). Studies of two excited-state reactions in bichromophoric solutes (excited-state energy homotransfer between indole rings in tryptophanyltryptophan dipeptide and electron transfer in 9,9'-bianthryl) have demonstrated that the distribution in solutesolvent interaction energies is important in both cases. While smaller interactions are favorable for energy transfer, the electron transfer can be effective only when these interactions are very strong. Site selectivity is lost with an increase in solvent relaxation rate.
tional motions15 can also contribute. No doubt, solvent control and inhomogeneous kinetics should be interrelated directly, because
I. Introduction Solvent plays an important role in many physical and chemical phenomena in the condensed phase. In the cases when the solvent coordinate is important for the reaction, the reaction rate may be a function of not only the averaged properties of the solvent, but also of fluctuations of these properties and their distributions over the reaction sites in solution. The heterogeneity of reaction sites caused by distribution in interactions with solvent environment and its dynamics is thought to be especially important when the reaction occurs at interfaces' or in microheterogeneous media such as micelles,2 microemulsions, or liposome^.^ It was clearly observed in experiments on the reaction of ligand binding to heme protein^.^ In biological charge-transfer reactions and enzyme catalysis the optimal local interactions with the reactant and the pattern of their fluctuations were probably developed in evolution and designed for optimal f~nctioning.~ The reason why inhomogeneity in reaction kinetics is not a subject of common interest and of intensive investigations derives from the fact that being actually a mixture of differently solvated subspecies, the reactant nevertheless usually behaves as a single chemical species? This is true, however, only for singlecomponent liquid solvents, where the equilibria among the solvated subspecies are rapid, much faster than the rate-limiting reaction step. Even in liquid solutions the inhomogeneity can be observed if the solvent is two-component and affinity to the solvent components differs in the reactant and product states or if the solvent can undergo conformational change in the course of solvation change during the reaction.' In solid and viscous solutions where the solvent motions are retarded, the solvent-dependent effects due to inhomogeneity of reaction sites can become most pronounced.* Many reactions such as i~omerization,~ proton transfer,I0 and electron transfer" are solvent dynamics controlled in a sense that their rates are limited by the rate of solvent reorganization. The reorganization itself consists mainly in the reorientational motions of the dipolar solvent molecules,I* but their concerted vibrational motions,I3 rearrangement of hydrogen bonds,I4 change in the packing of the first solvate shell, and the corresponding transla-
(1) Leffler, J. E.; Zupancic, J. J. J . Am. Chem. SOC.1980, 102, 259. Leffler, J. E.; Miller, D. W. J . Am. Chem. SOC.1977, 99, 480. Lindley, S. M.; Flowers, G. C.; Leffler, J. E. J . Org. Chem. 1985, 50, 607. (2) Kuzmin, M. G.; Zaitsev, H. K. In The Interface Structure and Electrochemical Processes at the Boundary Between Two Immiscible Liquids; Kazarinov, V. E., Ed.; Springer-Verlag: Berlin, 1987; pp 207-244. (3) Schomacker, R. J . Phys. Chem. 1991,95,451. Ilyichev, J. V.; Demjashkevlch, A. B.; Kuzmin, M. G. Chem. High Energies (USSR) 1989, 23, 435. (4) (a) Austin, R. H.; Beeson, K. W.; Eisenstein, L.; Frauenfelder, H. Biochemistry 1975, 14, 5355. (b) Frauenfelder, H.; Young, R. D. Comments Mol. Cell. Biophys. 1986, 3, 347. (c) Agmon, N. Biochemistry 1988, 27,
3507.
( 5 ) Careri, G.; Gratton, E. In The Fluctuating Enzyme; Welch, G. R., Ed.; Wiley: New York, 1985; pp 227-262. Somogyi, B.; Welch, G. R.; Damjanovich, S. Biochim. Biophys. Acta 1984, 768, 81. Demchenko, A. P. Luminescence and Dynamics of Protein Structures; Naukova Dumka: Kiev, 1988; 280 pp. (6) Leffler, J. E.; Gmnvald, E. Rates and Equilibria of Organic Reactiotw; Wiley: New York, 1963; 458 pp. (7) Gorodyski, V. A. Vestnik Leningrad Univ. Ser. Phys. Chem. (USSR) 1985,11,64. Gorodyski,V. A,; Stepanova, I. A. Kinet. Katal. (USSR) 1980,
21., 880. --
(8) Wright, B. B.; Platz, M. S. J . Am. Chem. S O ~1984,106,4175. . Platz, M. S.; hthilnathan, V. P.; Wright, B. B.; McCurdy, Jr., C. W. J. Am. Chem. SOC.1982, 104, 6494. Tomioka, H.; Griffin, G. W.; Nishiyama, K. J . Am. Chem. SOC.1979, 101, 6009. Neiss, M. A.; Willard, J. E. J . Phys. Chem.
1975, 79, 783. (9) Barbara, P. F.; Jarzeba, W. Acc. Chem. Res. 1988, 21, 195. Abrash, S.; Repinec, S.; Hochstrasser, R. M. J . Chem. Phys. 1990,93,1041. Bowman, R. M.; Eisenthal, K. 8.; Millar, D. P. J . Chem. Phys. 1988, 89, 762. (10) Lee, M.; Yardley, J. T.; Hochstrasser, R. M. J. Phys. Chem. 1987, 91,4621. Cukier, R. I.; Morillo, M. J. Chem. Phys. 1989,91,857. Barbara, P. F.; Walsh, P. K.; Brus, L. E. J . Phys. Chem. 1989, 93, 29. (11) Weaver, M. J.; Phelps, D. K.; Nielson, R. M.; Golovin, M. N.; McManis, G. E. J . Phys. Chem. 1990,94, 2949. Weaver, M. J.; McManis, G. E. Acc. Chem. Res. 1990, 23, 294. Weaver, M. J.; McManis, G. E.; Jarzeba, W.; Barbara, P. F. J . Phys. Chem. 1990, 94, 1715. Su,S.-G.;Simon, J. D. J. Chem. Phys. 1988, 89, 908. (12) (a) Bakhshiev, N. G. In Solvatochromy. Problems and Methods; Bakhshiev, N. G., Ed.; Leningrad University Press: Leningrad, 1989; pp 11-54. (b) Simon, J. D. Acc. Chem. Res. 1988, 21, 128. (13) Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1988, 89, 5044. Karim, 0. A,; Haymet, A. D. J.; Banet, M. J.: Simon, 1. D. J . Phys. Chem. 1988, 92, 3391.
'Address for correspondence: Dr. A. P. Demchenko, A. V. Palladin Institute of Biochemistry, 9 Leontovich Street, Kiev 252030, Ukraine.
0022-3654191 , ,12095-10518%02.50/0 0 1991 American Chemical Societv I
-
Site Selectivity in Excited-State Reactions the same solvent motions should be responsible both for averaging of local interactions and for dynamic stabilization during the reaction act. This matter, however, has not been investigated yet, and the reason for this is the lack of adequate experimental approaches and of conceptual ideas on their application. Promising possibilities, however, are offered by fluorescence spectroscopy in the studies of excited-state reactions for the following reasons: (i) The reaction can be started by a very fast light pulse and its kinetics observed at picosecond and nanosecond times by recording the time-dependent disappearance of the reactant and appearance of the product emission bands.I6 (ii) Essentially the same technique can be applied for the studies of the rate of solvent relaxation in the environment of the electronically excited chromophore molecule. It results in gradual shifts of emission spectra in time toward longer wavelength^.'^^^'^ (iii) The selection in the ensemble of reactant-solvent molecular configurations can be easily produced by site-selective excitation within the inhomogeneously broadened reactant absorption band.'* The inhomogeneous broadening pattern is usually hidden under the envelope of diffuse electronic absorption bands and can be resolved effectively only in fluorescence line-narrowing and hole-burning experiments at very low (liquid helium) temperat u r e ~ . ' ~Sufficient site selectivity, however, can be achieved at more common conditions by excitation at the long-wavelength edge of the spectra. In polar environments it results in a number of spectroscopic phenomena which are known under the common name "red-edge effects". Static red-edge effects can be observed in the steady-state spectra in rigid and viscous chromophore environments. They consist in the shifts of fluorescenceZoand phmphorescence20b*21 spectra to longer wavelengths on the increase of excitation wavelength. The long-range excitation energy transfer fails at the red-edge excitations.22 The excitation spectra display dependence on emission wavelength at the short-wavelength
emission^.^^ In liquid solutions the inhomogeneous broadening becomes dynamic. It can be detected if the observation time is faster than the relaxation time of the solvent. It can manifest itself in time-resolved spectra at early observation timesz4and as a result of influence of factors that decrease the fluorescence lifetime, fF (application of collisional fluorescence quenchers) or increase in solvent relaxation time, Q, which can be easily achieved by decrease in temperatures2' Thus, the solvent relaxation can be considered to be not only the process of attaining the equilibrium solvation of the excited state, but also the process of redistribution and averaging of substates with different solvation energies. The red-edge phenomena suggest a tool for studying the effect of initial solvation energy of the reactant on the reaction kinetics. The aim of the present investigation is to approach the analysis of effects of polar (14) Castner, Jr., E. W.; Maroncelli, M.; Fleming, G. R. J . Chem. Phys. 1987, 86, 1090. (15) Hubbard, J. B.; Kayser, R. F.; Stiles, P. J. Chem. Phys. Lerr. 1983, 95.399. Van der Zwan. G.: Hvnes, J. T. Chem. Phvs. Leu. 1983.101.367. Bagchi, B.; Chandra, A,; Fleming, G. R. J. Phys.-Chem. 1990, 94, 5197. Fried, L. E.; Mukamel, S. J. Chem. Phys. 1990, 93, 932. (16) Maroncelli, M.; MacInnis, J.; Fleming, G. R. Science 1989, 243, 1674. Marcus, R. A,; Siders, P. J. Phys. Chem. 1982,86,622. Marcus, R. A. Int. J. Chem. Kinet. 1981, 13, 872. Angel, S. A.; Petets, K.S. J. Phys. Chem. 1989, 93, 713; 1991, 95, 3606. (17) Ware, W. R.;Lee, S . K.; Brant, G. J.; Chow, P. P. J . Chem. Phys. 1971, 54, 4729. Maroncelli, M.; Fleming, G. R. J. Chem. Phys. 1987, 86, 6221. Nagarajan, V.; Brearley, A. M.; Kang, T.-J.; Barbara, P. F. J . Chem. Phys. 1987,86, 3183. (18) (a) Dem'chenko, A. P. Ultravioler Spectroscopy ofProteins; Springer-Verlag. Berlin, 1986; 320 pp. (b) Madregor, R. B.; Weber, G. Ann. N.Y. Acad. Sci. 1981, 366, 140. (19) Friedrich, J.; Harer, D. Angew. Chem., rnt. Ed. Engl. 1984,23,113. (20) (a) Galley, W. C.; F'urkey, R. M. Proc. Narl. Acad. Sci. U.S.A. 1970, 67, 11 16. (b) Rubinov, A. N.; Tomin, V. I. Opr. Spekrrosk. (USSR)1970, 29, 1082. (21) Milton, J. G.; Purkey, R. M.; Galley, W. C. J. Chem. Phys. 1978,68, 5396. (22) Weber, G.; Shinitzky, M. Proc. Narl. Acad. Sci. U27.A. 1970,65,823. (23) Pavlovich, V. S. J . Appl. Specrrosk. (USSR)1976, 25, 480. (24) Rubinov, A. N.; Tomin, V. 1.; Bushuk, V. A. J. Lumin. 1982,26,377. (25) Demchenko, A. P.; Ladokhin, A. S. Eur. Biophys. J . 1988,15,369.
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 10519
t
UNRELAXED
L
A
L
LREACTION RATE
SUB-STATE PROBABILITY
Figure 1. Scheme illustrating site selectivity within the excited-state distribution on solvation energy a( Wd$) by excitation by low-energy quanta (shaded area), the time-dependent formation of the distribution of the relaxed state Q'(Wddc),and the solvation energy dependence of the reaction rates for the cases of unfavorable (A) and favorable (B) photoselection.
environments and their dynamics on the excited-state reactions by applying site-selective excitation at the red edge of the reactant absorption band and studying their temperature dependence. We concentrate on two excited-state reactions in bichromophoric solutes: excited-state energy transfer in tryptophanyltryptophan dipeptide and electron transfer in 9,9'-bianthryl. 11. Theory
Static Inhomogeneous Broadening. Consider a population of chromophores immersed in rigid polar and structureless environment. The interactions with the environment reduce the energy levels in the ground and excited states of every chromophore. But the extent of this reduction is different depending on relative configurationsof chromophore and solvent dipoles. Let the mean solvation energy (Wdd) refer to most probable solute-solvent configurations. These configurations for the two limiting cases of the best and worst solvations in terms of energy are of much lower probability. Consider, therefore, instead of discrete energy levels, a dispersed population of them which can be characterized by dipoldipole interaction energy distribution function n(wdd) (Figure 1). Since usually chromophore electronic structure changes dramatically on excitation, the chromophore-environment interactions also change, and this change will be different for various chromophore species in the population, which results from different interactions in the ground state. Thus, the distributions in the ground and the excited states, Q( Wdds) and Q( Wddc), are essentially identical, but the intermolecular force field would be different. Since excitation usually causes the redistribution in electronic density, which results in the changes in value and direction of the dipole moment, the advantageous dipole-dipole interactions with environment may become disadvantageous, and vice versa. Absorption of light quanta of sufficiently high energy, hv,, may result in excitation of the entire population of fluorophores, having dipole-dipole interaction energy distributions Q( Wddg) and Q( WdC). The spectra of individual species in different local environments are shifted to a different extent. When averaged, they form an inhomogeneously broadened fluorescence spectrum. If, however, fluorescence is excited by a light quantum whose energy is lower than the average energy of the electronic transition, hv,cdge,there is a possibility for only a selective excitation of certain chromophores
10520 The Journal of Physical Chemistry, Vol. 95, No. 25, 1991
within the ensemble, Le., those having high interaction energy in the excited state (lower distribution edge of a( Wdde) shaded in Figure 1) and a low interaction energy in the ground state (upper distribution edge of a( Wddg)). Thus, it is possible to record not only averaged values of fluorescence parameters (such as spectra, polarization, and decay kinetics) but also those values which refer to chromophores excited selectively and representing the edge of the distributions. Such selective excitation may shift the fluorescence spectra toward the long-wavelength region (if the energy of an excitation quantum is low, that of an emitted quantum is likewise low). In polar solutions the estimated inhomogeneous component of the broadening is of the order of 100-500 cm-I.19 In many cases it is much smaller than the widths of diffuse vibronic bands in the spectra of large polyatomic molecules in liquids and disordered solids. The excitation at the steep long-wavelength absorption edge, however, can produce sufficient site s e l e c t i ~ i t y .Thus, ~~~~ for the observation of static inhomogeneous broadening, two requirements are important: substantial local disordering in solute-solvent interactions and slow rate of solvent motions. Dynamic Inhomogeneous Broadening. If the solvent motions are fast, the mean solvent relaxation time is shorter than the excited-state lifetime, and the light will be emitted from the relaxed state. The relaxation means not only an increase of the excited-state solvation (a decrease of the mean energy level), but also the loss of time correlation between the distributions a( Wdde) and a’( Wdde). The loss of correlation occurs with the decay rate of autocorrelation function of random fluctuations, which is the same (1 / T ~ as ) the relaxation rate. (This follows from the fluctuation-dissipation theorem.26) As a result, in the case of fast relaxation the chromophore excited at the red edge “forgets about this”: it has enough time to travel between a number of substates before the emission, and its spectroscopic and photophysical behavior will not be detected to be different from the mean. In the intermediate case, when T~ = T ~there , should be observed time-dependent changes both in spectra and in their site-selectivity effects. We base our considerationsisaon a “simple continuum” model of solvent relaxation27~z8 which considers a solvent to be a homogeneous medium whose only relevant property is its bulk frequency-dependent dielectric response.29 Site-Dependent Reaction Kinetics. Let the rate of photophysical or photochemical reaction be dependent on solvation energy. This dependence may be of different origins: the “cage effects” which modify the chromophore intramolecular potential^,^^ the stabilization of the reactant state with respect to the transition state,3’ or the existence of potential barrier to isomerization: to name a few. The chromophores distributed along the solvation energy profile a( Wdde) are microscopically in different conditions, and they could possess different reactivities. The two possible cases are illustrated in Figure 1. The higher edge of the distribution may be favorable for the reaction (see Figure lA), if the reaction requires excess of potential energy or short-wavelength position of the absorption band (the latter is the case of the excitation energy homotransfer which we consider below). The other extreme is the increased reaction rate for the species at the lower edge of the distribution (Figure lB), since it is populated by chromophores which interact most strongly with the environment. The increased rate is expected, for instance, if the environment produces the most (26) Landau, L. D.; Lifshits, E. M. Statistical Physics; Nauka: Moscow, 1964; 583 pp. (27) Mazurenko, Yu.T.; Bakhshiev, N. G. Opt. Spektrosk. (USSR)1970, 28, 905. Mazurenko, Yu. T.; Bakhshiev, N. G.; Piterskaya, 1. V. Opr. Spektrosk. (USSR)1968, 25, 92. (28) Bagchi, B.; Oxtoby, D. W.; Fleming, G.R. Chem. Phys. 1984, 86, 257. Castner, Jr., E. W.; Fleming, G. R.; Bagchi, B. Chem. Phys. Lett. 1988, 143, 270. Van der Zwan, G.; Hynes, J. T. J . Phys. Chem. 1985,89, 4181. (29) Limitations of the “simple continuum” model have been recently discussed by Maroncelli et al. in ref 16. (30) Dellinger, B.; Kasha, M. Chem. Phys. Lett. 1975, 36, 410; 1976, 38, 9. Kasha, M.; Dellinger, B.; Rrown, C. W. Inrernational Conference on Bioluminescence and Chemiluminescence;DeLuca, M., McElroy, W. D., Eds.; Academic Press: New York, 1981; pp 3-16. (31) Warhsel, A.; Russell, S. T. Q. Reo. Eiophys. 1984, 17, 283. Warshel, A. J. Phys. Chem. 1982, 86, 2218.
Demchenko and Sytnik significant stabilization of the product state, thus decreasing the activation energy barrier.3i This may also be the case for the activationless reaction if the strong reactant solvation is requires as its preliminary step. The observation of both the reaction kinetics and solvent relaxations depends upon the emission lifetime which determines a time window to record the reaction. Within this time window the correlation between the relaxation and the reaction rates is complex. At t = 0 the reaction rate is determined by the initial distribution a( Wdde). If there is a relaxation at longer times, it results in the temporal increase in solvation of the excited state (the new distribution a’( W,de) is shifted to lower energies), and also in the loss of time correlation and thus the decrease of the dependence of reaction rate on initial distribution. The interconversion and averaging of substates occur prior to the reaction.
111. Experimental Section Crystalline L-tryptophan and L-tryptophanyl-L-tryptophan(see formula) were obtained from Sigma. Their concentrations I QcHz-I;
I NH
I COOH
corresponded to optical densities 0.2 at 280 nm. The 9,9’-bianthryl was a gift from Dr. G. Walker (University of Minnesota, Minneapolis, MN). The bianthryl concentrations corresponded to optical density 0.05 at 365 nm. Glycerol, triacetin (glycerol triacetate), propylene glycol, and ethanol (Aldrich) were distilled and checked for the absence of fluorescent impurities before use. The fluorescence spectra were recorded using a Hitachi MPF-4 spectrofluorimeter operated in the signal ratio mode. The light sources were a 150-W xenon lamp and, for excitations at 365 and 403 nm, a high-pressure mercury lamp. The spectral bandwidths were 1 nm for excitation and 5 nm for emission monochromators. Experiments were performed in quartz tubes with a diameter of 3 mm located in a quartz dewar. The temperature was regulated by flow of nitrogen vapor and measured by thermocouple. IV. Results and Discussion Excited-State Energy Homotransfer (EEHT) between Two Indole Rings in Tryptophan Dipeptide. The energy transfer rate according to dipole-dipole inductive resonance mechanism32can be expressed as a function of the distance R between donor and acceptor and overlap integral, J, between the emission spectrum of the donor and the absorption spectrum of the acceptor.
kT = 8.7
X
10z3~Zn-4JkFR“
(1)
Here kF is the intrinsic fluorescence rate of the donor, x is the orientation factor, and n is the refraction index. The inductive resonance transfer for like-like molecules will be inefficient because of the intrinsically small value of J. In the tryptophan dipeptide, the chromophores are connected by a flexible chain and are situated within a distance sufficient for effective transfer. The characteristic distance Ro for which a 50% transfer efficiency is expected is estimated to be 10-12 A.33 Computer-assisted conformational analysis demonstrates that the relative distances between the rings are within this range. EEHT cannot be studied by means of separate determinations of donor and acceptor spectra and their kinetics because, on average, the donor and the acceptor possess the same spectral and kinetic properties. Since we are interested in solvent dynamics (32) Forster, T. Discuss. Faraday Soc. 1959, 27, 7. (33) Eisinger, J.; Feuer, B.; Lamola, A. A. Biochemistry 1969, 8, 3908.
Site Selectivity in Excited-State Reactions
The Journal of Physical Chemistry, Vol, 95, No. 25, 1991 10521
320
P 8 330 E
&E 0;
340
I
- 120
I
I
-80 -40 TEMPERATURE ("C)
'A 0
+ 40
Figure 3. Temperature dependence of the positions of the maxima of fluorescence spectra of tryptophan monomer and the dimer tryptophanyltryptophan in glycerol: (0)monomer and ( 0 )dimer at kX= 280 nm; (0) monomer and (m) dimer at A,, = 305 nm.
effect on the EEHT, and the dynamics may be associated with rotations of chromophores themselves, we prefer to avoid fluorescence polarization studies which are commonly applied for this p ~ r p o s e . ~The ~ , following ~~ approach is used: since in the systems with inhomogeneous broadening EEHT results in the shifts of fluorescence spectra to longer wavelength^,^^ we compare the monomer and the dimer spectra at the mean and the red-edge excitations as a function of temperature. The fluorescence spectra of tryptophan and of tryptophanyltryptophan are presented in Figure 2. At the midband excitation at 280 nm a significant shift is observed between monomer and dimer spectra in the rigid environment (glycerol at -60 to -100 "C).This difference almost disappears in liquid environment at 26 OC,when the spectra of both solutes are substantially shifted to longer wavelengths due to solvent relaxation. If the chromophores are excited at the red edge (at 305 nm) where the intensity is at about 8% of its value at the band maximum, the difference between the monomer and dimer spectra disappears both in solid and in liquid environments, and the solvent-relaxational shift of their spectra becomes much smaller. The dependence of positions of maxima of fluorescence spectra on the temperature (Figure 3) demonstrates that both the relaxational shift of the spectra and the decrease in spectral differences between the monomer and the dimer occur in the same temperature region, which corresponds to nanosecond rates of dielectric relaxations in glycerol.36 We now consider in more detail the mechanism of EEHTassociated long-wavelength shift in a rigid environment. The two indole rings in the dimer are surrounded by randomly oriented polar solvent molecules. In general, they are stabilized by different interaction energies with the solvent; i.e., they belong to different substates. The species that belong to the higher part of the distribution (Figure 1A) possess the absorption and fluorescence spectra at shorter wavelengths than the mean, which increases
the overlap integral J (eq 1) and favors the transfer when they serve as donors, and decreases the probability of transfer when they are acceptors. The species at the lower part of the distribution (shaded in Figure 1A) with long-wavelength-shifted absorption and fluorescence spectra are in contrast better acceptors than donors. Thus instead of random exchange of energy in the dimer there is a direct flow of energy to those chromophores which emit at longer wavelengths. This is probably the same effect which is observed in concentrated solutions of indole3' and other aromatic compound^**^^^ in solid and viscous solvents. Based on these considerations the mechanism of failure of energy transfer from the red-edge excited chromophore to its partner is the following. The excitation of a chromophore at the edge of the distribution, which can absorb low energy quanta, is of low probability. The transfer requires the partner (the acceptor) to belong to the same edge of the distribution; otherwise the overlap integral J is small. The probability of selecting such a pair is very low; the more probable case is the localization of excitation energy with the donor and its subsequent emission. Thus the EEHT rate is dependent upon the donor solvation energy (schematically presented in Figure 1A). This is probably the mechanism of an increase in polarization due to failure of EEHT in the concentrated dye solutions; i.e., the effect which was originally observed by Weber.22 In rigid environments the transfer-associated shifts of fluorescence spectra and their decrease of red-edge excitation can be observed by time-resolved spectros~opy.~~ An increase of the solvent relaxation rate results in a decrease of the spectral selectivity of the transfer. This is expected because the fluctuations in the solvent produce rapid interconversion between substates. In liquid environments the transfer should exist but it does not produce significant influence upon the fluorescence spectra. The Site-Selective Effectiveness of Intramolecular Electron Transfer. Intramolecular electron transfer in bianthryl-the excited-state reaction between two identical covalently linked chromophores-has been most extensively The nonpolar locally excited (LE) form possesses the anthracene-like spectrum which exhibits a small Stokes shift and substantial vibronic structure. The polar charge-transfer (CT) form is structureless and shifted to longer wavelengths. The latter is characterized by a significant charge separation: one of the
(34) Kim, Y.R.; Share, P.;Pereira, M.; Sarisky, M.; Hochstrasser, R. M. J . Chem. Phys. 1989, 91,1557. (35) Gulis, 1. M.; Komjak, A. I. J . Appl. Spectrosc. (USSR) 1977, 27, 841. Gulis, I. M.; Komjak, A. 1.; Demchuk, M. I.; Dmitriev, C. M. J . Appl. Spectrosc. (USSR)1978, 29, 817. Koyava, V. T.; Popechitz, V. I. J . Appl. Spectrosc. (USSR)1979, 31, 982. (36) Davidson, D. W.; Cole, R. H. J. Chem. Phys. 1951, 19, 1484. McDuffie, G. E.; Litovitz, T. A. J . Chem. Phys. 1962, 37, 1699.
(37) Reference 18a, p 192. (38) Nemkovich, N. A.; Rubinov, A. N.; Tomin, V. I. J . Lumin. 1981, 23, 349. (39) Schneider, F.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 1155; 1970, 74, 624. (40) Lueck, H.; Windsor, M. W.; Rertig, W. J . Phys. Chem. 1990, 94, 4550. (41) Rettig, W. Angew. Chem., Int. Ed. Engl. 1986, 25, 971.
300
34 0
380
Aem (nm)
Figure 2. Fluorescence spectra of tryptophan monomer (1, 3) and
tryptophanyltryptophan dimer (2,4) in glycerol at the excitation wavelengths 280 nm (A) and 305 nm (B). Temperature, -100 OC (1,2) and 26 "C (3, 4).
10522 The Journal of Physical Chemistry, Vol. 95, No. 25, 1991
400
Demchenko and Sytnik
F
5
402, 404
0 300
320
340 &x
360
380
(nm
Figure 4. Influence of the excitation wavelength on fluorescence spectra of anthracene in propylene glycol at the temperatures -1 10 ( I ) and 18 "C (2). Curves 1 and 2 and the right scale represent the positions of the fluorescence maxima. Curve 3 and the left scale represent the excitation spectrum.
01'
1
I
I
400
440
Aem ( n m )
480
520
Figure 6. Fluorescence spectra of bianthryl in the higher temperature range. The data for excitation wavelengths 365 and 403 nm reveal no differences. ( I ) In propylene glycol at 6 OC; (2) in glycerol triacetate at -7 OC; (3) in ethanol at -67 "C.
A
I
1 8
-200
I
-160
1
-120
-80
TEMPERATURE ('C)
I
I
-40
0
Figure 7. Temperature dependence of the relative contribution of the C T form (A) and of relaxational shift of the spectra of this form (B): ( I ) in propylene glycol; (2) in glycerol triacetate; (3) in ethanol. (A) Exis the ratio of fluorescence incitation wavelength 365 nm, IF480/IF4i5 tensities a t 480 and 415 nm. (B) Excitation wavelength 403 nm.
Figure 5. Fluorescence spectra of bianthryl in the lower temperature range at excitation wavelengths 365 (A) and 403 nm (B): (1) in propylene glycol at -103 OC; (2) in glycerol triacetate at -92 OC; (3) in ethanol at -196 OC.
anthracene rings attains a negative charge and the other a positive charge. The electron-transfer rate in bianthryl occurs in relatively polar solvents and is known to correlate with the dynamic prop erties of the solvent; its rate is probably limited by the rate of rotations of solvent dipoles.42 Therefore, it is interesting to look for the possibility of site-photoselection effects on this reaction. The nonpolar anthracene molecule is known to possess substantial electronic p o l a r i ~ a b i l i t y . ~Therefore, ~ inhomogeneous broadening of its spectra in a polar environment may be due to both the distribution in the order of the first solvent-shell packing, and also to inhomogeneity in the interaction of solvent dipoles with the induced dipole of the solute. Figure 4 demonstrates the existence of the excitation wavelength dependent red-edge effect (42) Kang, T.-J.; Kahlow, M. A.; Giser, D.; Swallen, S.;Nagarajan, V.; Jarzeba, W.; Barbara, P. F. J. Phys. Chem. 1988, 92, 6800. Kang, T.-J.; Jarzeba, W.; Barbara, P. F.; Fonscca, T. Chem. Phys. 1990,149,81. Hara, K . ; Arase, T.; Osugi, J. J . Am. Chem. Soc. 1984,106,1968. Anthon, D. W.; Clark, J. H. J . Phys. Chem. 1987, 91, 3530. Mataga, N.; Yao, H.; Okada, T.; Rettig, W. J . Phys. Chem. 1989, 93, 3383. (43) Liptay, W.; Walz, G.;Baumann, W.; Schlosser, H.-J.; Deckers, H.; Detzer, N. 2.Naturforsch. 1971, 26A, 2020.
for anthracene in propylene glycol low-temperature glass and the absence of this effect in the liquid solvent. Since the electronic polarizability in bianthryl is substantially greater than that in anthracene the inhomogeneous broadening and the effects of photoselection for the former are expected to be more significant. For bianthryl in rigid environments, usually the LE form is observed, which possesses anthracene-like spectra.39 However, at the red-edge excitation, instead of shifts of the spectra of this form, we observe the generation of a new form, the spectrum of which is structureless and shifted to longer wavelengths (Figure 5 ) . These properties allow us to recognize the CT form, which is very similar to that observed in liquid polar solvents (Figure 6), for which no excitation-wavelength dependence is observed. Its spectrum is located at shorter wavelengths, however, and this suggests inequilibrium solvation of this form at low temperatures. The data on bianthryl in propylene glycol were reported earlier." The excitation wavelength dependence in this solvent reflects the site-selective generation of the CT form. The possibility of photoselection of this form existing already in the ground state or any impurity is unlikely, since the excitation spectra do not reveal any emission wavelength dependence. The results on direct excitation of CT form are in accord with earlier observations on bianthryl absorbed on porous glass$s in polymer films$6 and in (44) Demchenko, A. P.; Sytnik, A. I. Proc. Natl. Acad. Sci. U S A . , in
press.
(45) Nakashima, N.; Phillips, D. Chem. Phys. Lett. 1983, 97, 337. (46) AI-Hassan, K. A.; Azumi, T. Chem. Phys. Lett. 1988, 150, 344. AI-Hassan, K.A. Chem. Phys. Lett. 1991, 179, 195.
Site Selectivity in Excited-State Reactions bianthryl complexes with polar molecules in a free jet.47 In all these cases, similarly to solid polar solvent solutions, stabilization by intermolecular polar interactions is probably the cause for generation of the CT state. The temperature dependence of the transition between LE and C T forms in three different solvents (ethanol, propylene glycol, and glycerol triacetate), as shown in Figure 7, demonstrates that the transition range between these forms corresponds to the range of nanosecond dielectric relaxations of the solvents.48 Such similarity should be observed if T ~ - I == kEr, the rate of electron transfer. This is in accord with the results of time-resolved spectroscopy which also demonstrates the correlation between kET and 7R-1.42 The transition range and the forms of the observed temperature functions for the positions of fluorescence spectra of the CT form (generated by the red-edge excitation a t 403 nm) are the same as that for the relative concentration of this form (Figure 7). The sigmoid functions are usual for the effect of solvent relaxation. The substantial relaxational shift of the spectra demonstrates the polar character of the C T form and its high dipole moment. Comparison between protonic solvent, propylene glycol, and nonprotonic glycerol triacetate, which have similar solvent relaxation ranges, do not reveal any differences in the observed effects. Thus, the experiments on bianthryl demonstrate the possibility of direct photoselection between different chromophore-environment interactions in rigid media. Photoselected are those configurations which are ‘ready” for the transfer and which correspond to lower (shaded) part of the distribution schematically described in Figure 1B. It is those configurations which can be formed also dynamically by fluctuations in fast-relaxing liquid solvents. If it is so, then the solvent relaxation should control the rate of electron-transfer reaction not by a direct involvement in the electron-transfer act, but rather by dynamic formation of solvent-environment configurations which are optimal for this reaction.
V. Concluding Remarks Application of the same approach which is based on site-selective excitation for the initiation of the reaction and modulation of the relaxation rate by varying the temperature for two different excited-state reactions allowed us to obtain several nontrivial results which are discussed briefly below. Inhomogeneous Kinetics. The results presented here demonstrate that solute molecules which are excited at different wavelengths due to different interaction energies with the solvent may have dramatically different reaction rates. This conclusion is supported by the results of recent studies of C O rebinding to myoglobin at low temperature^."^^ Following CO photodissociation a temporal blue shift in the near-UV absorption spectrum is observed, which results from inhomogeneous distribution of activation barriers to recombination. The correlation between the position of the absorption band and the reaction rate is such that the molecules absorbing a t the red edge recombine faster than those absorbing at the blue edge. Of a simi!ar origin is the inhomogeneous kinetics in photosynthetic reaction centemso Solvent Dynamics Control. The coupling between reaction rates and solvent relaxation rates in the excited-state reaction^^-^^ is usually discussed on the basis of one side of solvent relaxation mechanism: attaining equilibrium from an initially nonequilibrium state. Our results demonstrate that the other side of relaxation is of no less importance: the relaxation is the dynamic averaging of the initially excited substates and loss of time correlation. (47) Kajimoto, 0.; Yamasaki, K.; Arita, K.; Hara, K. Chem. Phys. Left. 1986, 125, 184. Honma, K.; Arita, K.; Yamasaki, K.; Kajimoto, 0.J . Chem. Phys. 1991, 94, 3496. (48) Akhadov, Ya. Yu. Dielectric Properties of Pure Liquids; Nauka: Moscow, 1972; 4 1 I pp. (49) Agmon, N.; Hopfield, J. J. J . Chem. Phys. 1983, 79, 2042. Campbell, B. F.; Chance, M. R.; Friedman, J. M. Science 1987, 238, 373. (50) Kleinfeld, D.; Okamura, M. Y.; Feher, G. Biochemistry 1984, 23, 5780.
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 10523 Site-selective excitation spectroscopy proves to be a valuable tool in the study of relaxation phenomena. Symmetry. The charge transfer in bianthryl is associated with symmetry breaking one monomer component gains a positive, and the other a negative chargees’ The situation is similar to primary charge separation in photosynthetic reaction centers: despite the existence of a peudo-C2 symmetry axis which relates the monomers belonging to two branches, L and M, the charge separation occurs almost exclusively along the L branch.52 Our studies of excitation energy homotransfer in bichromophoric molecules demonstrate that the symmetry in terms of energy does not exist in polar solution. One of the monomers has stronger solvation that the other, and this determines the direction of the reaction. In the case of bianthryl this may result in a gradient of the excited-state potential between monomers which guides the direction of the electron transfer. In protein molecules this gradient can be prearranged by protein conformation and this may determine the pathway of electron transfer in photosynthetic reaction centers. Dimensionality of Solvent Coordinates. The common feature of the two reactions we have studied is their dependence upon a single solvent parameter, the solute-solvent interaction energy. Thus, the isoenergetic solvent configurations are thought to have the same reaction probabilities. The real situation may be more complex, but it cannot be resolved using our approach based on photoselection by the energy of absorbed light quanta. The singlecoordinate approach may apply to reactions with small or zero activation energies, whereas in the cases of high activational barriers one may have to consider the reaction pathway in multidimensional solvent coordinate space.53 It has to be investigated whether the description of the solvent as a “heat bathHs4 or by a single “reduced solvent variablenssis effective in different specific cases. Nanequilibrium Solvation. Site selectivity produced by red-edge excitation is expected to be a valuable tool for the study of equilibrium or nonequilibrium solvation of the solute in the course of the reaction. In the former, the solvent polarization is in equilibrium with the electronic charge distribution in the course of a reaction, while in the latter it is not. Equilibrium solvation was postulated in many theories and their validity is under question now in view of recent experimental data.56 Therefore, it is important to know whether interconversion between local substates and averaging their interactions with the solvent are faster or slower than the rate-limiting step of the reaction. If the solvent motions are fast, then no site selectivity should be observed. Resolution of Reaction Mechanisms. The solvent-dependent inhomogeneity of reaction kinetics and its dependence upon the solvent dynamics may be of different origins. In the cases of diffusional transition over the activation barrier the solvent role may be reduced to “fricti~n”.~’In other cases such as electron transfer in bianthryl, the rate is controlled by solvent relaxations. An important criterion for this mechanism is the possibility of site photoselection in rigid (unrelaxed) media of the species with the chromophore-environment interactions close to the relaxed state and observation of their increased reaction rates. It is possible that this is a peculiarity of reactions with the low activation barriers (E, < kBT). If E, > kBT,the situation may be reversed, the species excited at the “blue edge” will use their excess energy in the reaction (less energy will be required for thermal activation) (51) Nakashima, N.; Murakawa, M.; Mataga, N . Bull. Chem. Soc. Jpn. 1976, 49, 854. (52) Bundil, D. E.; Cast, P.; Chang, C. H.; Schiffer, M.; Worris, J . Annu. Rev. Phys. Chem. 1987, 38, 561-583. (53) For recent discussion of this problem, see: Harris, J. G.; Stillingcr, F. H. Chem. Phys. 1990, 149.63. Gonzalez-Lafont, A.; Lluch, J. M.; Oliva, A.; Bertran, J. J . Phys. Chem. 1989, 93, 4677. (54) Berezhkovskii, A. M.; Zitserman, V. Yu. Chem. Phys. Len. 1990,172, 235. (55) Gertner, B. J.; Bergsma, J . P.; Wilson, K . R.; Lee, S.; Hynes, J. T. J . Chem. Phys. 1987,86, 1377. (56) Kim, H. J.; Hynes, J. T. J . Chem. Phys. 1990, 93, 5194; 1990,93, 521 1. (57) Morillo, M.; Yang, D. Y.;Cukier, R. I. J . Chem. Phys. 1989, 90, 5711.
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and those excited at the "red edge" will be trapped in a deep potential energy well, which will hamper the reaction. There is a notion, however, that only the activationless reactions can depend upon initial solvent condition^.^*^^^ More effort should be made to resolve these problems. (58) Bagchi, B.; Fleming, G. R. J . Phys. Chem. 1990, 94, 9. (59) Rips, I.; Jortner, J. J . Chem. Phys. 1988, 88, 818.
Demchenko and Sytnik Acknowledgment. This paper is dedicated to Professor Michael Kasha on the occasion of his 70th birthday. We are grateful to Professor Kasha for stimulating discussion of the results and to Dr. G. Walker for the sample of 9,9'-bianthryl. Registry No. Trp, 73-22-3; Trp-Trp, 20696-60-0; 9,9'-bianthryl, 1055-23-8; propylene glycol, 57-55-6; glycerol triacetate, 102-76-1; ethanol, 64-17-5.
