J . Phys. Chem. 1988, 92, 6545-6547
6545
Solvent Relaxation and Excited-State Proton Transfer: 7-Azaindole in Ethanol Richard S. Moog,* Scott C. Bovine,? Department of Chemistry, Franklin and Marshall College, Lancaster, Pennsylvania 17604
and John D. Simon*’$ Department of Chemistry and Institute for Nonlinear Studies, University of California San Diego, La Jolla, California 92093 (Received: March 9, 1988)
Picosecond emission spectroscopy is used to investigate the excited-state intramolecular proton-transfer reaction of 7-azaindole in ethanol solution. Wavelength-dependentrise times and decays are observed for the emission spectrum of the excited-state tautomer. These data clearly show that following excited-state intramolecular proton transfer, the emission spectrum of the tautomer undergoes a time-dependent spectral shift to lower energy. The data are discussed in terms of solvent relaxation following the proton-transfer reaction.
Introduction The influence of the solvent on intramolecular proton-transfer reactions in solution has been the subject of several recent theoretical and experimental studies.’ During this same period, direct measurements on solvent relaxation dynamics in response to photoinduced changes in solute properties have been reported.* These results provide information on solvent motion, which is necessary for the development of a full description of the role of the solvent in chemical reaction dynamics in solution. Despite these advances, there have been few investigations of the role of solvent relaxation in intermolecular chemical reactions. Virtually all examinations have involved systems in which the solvent is responding to intramolecular electronic rearrangements. Recently, one of us has examined solvent dynamics following photodissociation reactiom3 Such studies not only provide insight into how the solvent responds to chemical changes in the solute, but can also examine how solvent relaxation affects the chemical reaction dynamics. In this article, we examine another chemical system that is particularly well suited for an investigation of the effect of solvent dynamics on an intermolecular process. Picosecond techniques are used to examine the excited-state intermolecular proton-transfer reaction of 7-azaindole (7-A1):ethanol complexes. These systems have been extensively ~ t u d i e d and ,~ the presence of an excited-state intermolecular double-protontransfer reaction from cyclically hydrogen-bonded states in both 7-AI dimers and 7-A1:alcohol complexes has been well established through a number of steady-state and time-resolved spectroscopic measurements. The general reaction scheme for the intermolecular proton transfer between 7-AI and ethanol is shown in Figure 1. In a related study, McMorrow and A a r t ~ m examined a~~ the time-dependent emission from a solution of 7-azaindole in methanol. They examined the emission at both 395 and 5 10 nm, corresponding to fluorescence from the “normal” 7-AI species and the proton-transfer-produced tautomer, respectively. The “normal”s emission was characterized by an essentially instantaneous rise followed by an exponential decay with a time constant of 165 ps. The time dependence of the tautomer fluorescence was observed to have a much more complicated form, and the authors were able to fit the observed decay by using a functional form containing two different rise times. Their interpretation of this result was that there are two distinct excited-state populations of solute-solvent complexes giving rise to tautomer production. One of these is a cyclically hydrogen-bonded conformation, which undergoes instantaneous proton transfer. The other hydrogenbonded species is not a cyclic conformation and requires some solvent reorganization to achieve the necessary arrangement for proton transfer to occur. This second population gives rise to the Hackman Scholar, 1987. *NationalScience Foundation Presidential Young Investigator 1985-1990; Alfred P. Sloan Fellow 1988-1990.
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slower component of the tautomer rise time (see Results and Discussion). However, fluorescence studies of systems that undergo a time-dependent Stokes shift also show similarly complicated fluorescence decays at multiple wavelengths. For example, Maroncelli and Fleming2b report that the single wavelength fluorescence from coumarin 153 was generally found to be well fit by a sum of three exponentials, but that “no physical meaning is ascribed to the derived exponential parameters”. In these cases, the multiexponential behavior is indicative of changes in the emission spectrum as a function of time, resulting from solvent reorganization induced by the new charge distribution of the excited state. Similar results have been reported for excited-state emission from charge-transfer states created by rapid intramolecular electron-transfer reactions.k6 These studies demonstrate that solvent relaxation can occur following a chemical reaction. Thus, the multiexponential character of the tautomer fluorescence of 7-AI in alcohol solution may not indicate the presence of solvent motion before proton transfer, but rather could reflect solvent reorganization dynamics following this photoinduced chemical reaction. This can be tested by examining the emission behavior at several wavelengths in the tautomer band.
Experimental Section A complete description of the picosecond emission apparatus has recently been published.6 A frequency doubled CW Nd3+:YAG laser is used to synchronously pump a rhodamine-6G dye laser. At 600 nm, an output power of 100 m W is obtained. Autocorrelation using a noncollinear optical arrangement indicates (1) (a) Lee,M.; Yardley, J. T.; Hochstrasser, R. M. J. Phys. Chem. 1987, 91, 4621. (b) Bruker, G. A.; Kelley, D. F. J . Phys. Chem. 1987, 91, 2856. (c) Bruker, G. A.; Kelley, D. F. J . Phys. Chem. 1987, 91, 2862. (d) Flom, S.R.; Barbara, P. F. J . Phys. Chem. 1985, 89, 4489. (e) Strandjord, A. J. G.; Barbara, P. F. J . Phys. Chem. 1985,89, 2355. (0 Strandjord, A. J. G.; Barbara, P. F. J . Phys. Chem. 1985,89, 2362. (g) Catalan, J.; Perez, P. J . Theor. Biol. 1979, 81, 213. (h) Waluk, J.; Bulska, H.; Grabowska, A.; Mordzinski, A. N o w . J . Chim. 1986, 10,413. (i) Kasha, M. J . Chem. SOC., Faraday Trans. 2 1986,82, 2379. 0) Banacky, P. Chem. Phys. 1986, 109, 307. (2) (a) Su, S-G.; Simon, J. D. J . Phys. Chem. 1987, 91, 2693. (b) Maroncelli, M.; Fleming, G. R. J. Chem. Phys. 1987.86, 6221. (c) Nagaragan, V.; Brearley, A. M.; Kang, T-J.; Barbara, P. F. J. Chem. Phys. 1987,86, 3183. ( 3 ) (a) Xie, X.; Simon, J. D. J. Phys. Chem. 1986, 90, 6751. (b) Xie, X.; Simon, J. D. J. Phys. Chem. 1987, 91, 5 5 3 8 . (4) (a) Taylor, C. A,; El-Bayoumi,M. A,; Kasha, M. Proc. Nat. Acad. Sci. (USA) 1969,63,253. (b) Ingham, K. C.; El-Bayoumi, M. A. J . Am. Chem. SOC.1974, 96, 1674. (c) Hetherington, W. M.; Michaels, R. M.; Eisenthal, K. B. Chem. Phys. Lett. 1979, 66, 230. (d) Collins, S.T. Doctoral Th’esis, 1981, Florida State University. (e) Collins, S. T. J . Phys. Chem. 1983,87, 3202. (f) McMorrow, D.; Aartsma, T. J. Chem. Phys. Lett. 1986, 125, 581. ( 5 ) In alcohol solutions, the “normal” emission is considered to arise from a 7-A1:alcohol exciplex that does not undergo proton transfer. See ref 4d,e. (6) Simon, J. D.; Su,S-G.J . Chem. Phys. 1987,87, 7016.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 23, 1988
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Proton Transfer 6
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Wavelength (nm) Figure 2. Steady-state emission spectrum of 7-AI in ethanol at 2 O C . Two distinct emission bands are observed, corresponding to emission from the "normal" and tautomer excited states.
Cyclic Tautomer Complex Figure 1. Schematic representation of the potential energy surface for the intermolecular proton-transfer reaction of 7-azaindole in ethanol solution. Proton transfer to form the excited-statetautomer occurs from a cyclically bonded complex between 7-AI and ethanol. a pulse width of approximately 1.0 ps (fwhm). The dye laser output is amplified with a three-stage longitudinally pumped pulsed dye amplifier. The driver is a nanosecond Nd3+:YAG laser, providing 130 mJ at 532 nm at a repetition rate of 20 Hz. Rhodamine-640 in methanol is used as the amplifying medium. Saturable absorber jets of crystal violet in ethylene glycol are used to isolate the stages of the amplifier. The output pulse is approximately 1 ps (fwhm), 1-2 mJ/pulse, f10% pulse-pulse fluctuation, at a repetition rate of 20 Hz. The pulse energy to amplified spontaneous emission (ASE) is better than 1OO:l. The output of the amplifier is frequency doubled by using a 0.2 cm angle tuned KDP doubler. A weak reflection of the remaining red light is focused onto an FND-100 photodiode, the output of which triggers the streak camera. Both the UV and remaining red beam are sent down optical delays of approximately 30 ft. This is made necessary by the internal delay in the streak camera. The UV is passed through a polarizer and focused onto the sample. The cell is masked so that emission over a 0.2-cm region is imaged onto the detector. The red light travels a slightly shorter delay than the UV and is used as a timing marker for signal averaging (prepulse). Fluorescence is collected 90" from excitation. The light is collimated and passed through a second polarizer, which is oriented at magic angle with respect to the excitation light. The light is then focused on the input slit of a Hamamatsu C979 streak camera. The streak camera output is recorded by a Reticon detector connected to a high-speed parallel computer interface. The data is transferred to an LSI-11/23+ computer system via a parallel DMA interface. The sample was housed in a temperature-controlled brass block. The sample temperature could be varied over the range from -213 to +313 K. Static fluorescence spectra were recorded by using a Spex Fluorolog 11 1 with a temperature-controlled sample holder. All spectra were obtained at an excitation of 300 nm, matching the excitation wavelength used in the picosecond experiments, and were corrected for instrument response as a function of wavelength. Results and Discussion The absorption spectrum of 7-AI in ethanol (1 X M) shows a peak at 289.5 nm, with shoulders at 284.5 and 296.5 nm. This spectrum is significantly red-shifted and blurred relative to the spectrum in nonpolar solvents,&Sbpresumably due to the presence of strong hydrogen-bonding interactions. Figure 2 shows the
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Time (ps) Figure 3. Emission dynamics of 7-AI in ethanol at 2 O C monitored at 380 nm. This data corresponds to the population decay of the "normal"
state, which does not undergo excited-state proton transfer (see text). The decay is well fit by a single exponential with a time constant of ~ 2 0 0 PS.
steady-state emission spectrum of such a solution at 2 OC. There are two distinct bands in the emission spectrum. These have been previously identified as "normaln5fluorescence (Amm = 355 nm) and tautomer fluorescence (Arnm = 500 nm).4d,e Although separation of the two emission bands is not complete, fluorescence at wavelengths greater than 460 nm is clearly dominated by emission from the proton-transfer-induced tautomer. The time dependence of emission at 380 nm is shown in Figure 3. As was reported by McMorrow and Aartsma for 7-AI in methanol solution, the "normal" emission is characterized by an extremely rapid rise time ( > l o ps) and an exponential decay. The time constant for the decay in ethanol (200 ps) is slightly longer than that reported for 7-AI in methanol (165 ps49. McMorrow and Aartsma4' interpreted the lifetime of the "normal" emission as indicative of the rate of formation of cyclically hydrogen-bonded complexes in the excited state, which then undergo extremely rapid proton transfer. The assumption of essentially instantaneous reaction after complex formation is also supported by the work of Hetherington et al.," who showed that the intrinsic rate of proton transfer in 7-azaindole dimers in nonpolar solvents is extremely rapid (>5 ps-'). McMorrow and Aartsma further proposed a ground-state equilibrium between two different 7-AI: alcohol c~mplexes.~'Those complexes that are appropriately prepared in the ground state for the excited-state transfer are assumed to tautomerize immediately upon excitation. The other complexes are assumed to undergo solvent rearrangement after excitation into the cyclically hydrogen-bonded form, which then also tautomerizes rapidly. This model of excited-state conversion from noncyclically to cyclically hydrogen-bonded complexes predicts that the tautomer emission rise time in ethanol must have two components-an extremely rapid rise due to instantaneous proton transfer and a slower (ca. 200 ps) component from the
7-Azaindole in Ethanol
The Journal of Physical Chemistry, Vol. 92, No. 23, 1988 6541
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at 470 nm also confirms that the intrinsic rate of proton transfer in the 7-A1:alcohol complex must be large (>5 ps-I), consistent with the previously mentioned results for 7-AI dimers in nonpolar solvents. The three curves shown in Figure 4 have all been normalized to the same maximum intensity so that differences in their shapes can be readily seen. These decays clearly do not have the same time dependence, again in conflict with the predictions based on the ground-state equilibrium model. Note that the three curves have very different rise times and that the rise time increases as the monitoring wavelength is increased. These features are characteristic of the time-dependent Stokes shift observed in many molecules following electronic excitation and excited-state charge-transfer These observations suggest that there is solvent relaxation following the formation of the tautomer. In many recent studies on the time-resolved Stokes shift, the dynamics are quantified by measuring the correlation function, C(t),defined by7
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Time (ps) Figure 4. Time-resolved fluorescence data for the tautomer band of 7-AI in ethanol at 2 OC. The wavelengths monitored are 470 (-), 510 (---), and 570 nm (- --). All three curves are normalized to the same maximum intensity for display purposes. The dynamics are dependent on the emission wavelength examined. The blue edge of the tautomer band reveals an instantaneous rise. For lower emission energies, the rise time is longer. The decay at 470 nm cannot be described by a single exponential. The long time decay constants for the three wavelengths shown are identical within experimental error.
excited-state solvent rearrangement prior to reaction. In addition, this model assumes that the only solvation dynamics are occurring prior to proton transfer, thus, the time dependence of the tautomer emission is expected to be independent of emission wavelength probed. If the “normal” emission lifetime is representative of the rate of formation of proton-transfer complexes, then the rise time for tautomer fluorescence must include this slower component at all wavelengths of tautomer emission. However, this is not observed experimentally. Time-dependent decays for emission from the tautomer species at three different wavelengths are shown in Figure 4. Note that the rise of fluorescence at 470 nm is instantaneous within the experimental response. Furthermore, there is no evidence of any slow component to this rise. This is contrary to the prediction that all tautomer emission should show a rise corresponding to the rate of solvent rearrangement to form complexes that then undergo proton transfer. In addition, the rise time of the fluorescence on the red side of the tautomer emission band (570 nm) is faster than the 200-ps decay time of the “normal” fluorescence. These results demonstrate that the kinetics of the “normal” emission are not controlled by solvent relaxation forming cyclically hydrogen-bonded complexes, which then undergo proton transfer. A possible explanation for the short lifetime of the “normal” emission is quenching due to electron ejection from an exciplex, as suggested previously by Collins.4dpe Thus, contrary to the model proposed by McMorrow and Aartsma, there is no evidence for the formation of cyclically hydrogen-bonded complexes in the excited state, assuming that tautomer production would follow. Only those complexes originally prepared in the cyclically hydrogen-bonded conformation are observed to undergo excited-state double-proton transfer. The rapid rise in fluorescence
where u(O), v(t), and I ( - ) are the emission maxima at time zero, t and infinity (fully relaxed). Attempts to calculate this Stokes shift correlation function for the 7-AI tautomer emission were hindered by the small overall shift of the fluorescence spectrum (estimated to be ;=lo nm). However, an estimate of the relaxation time can be made by examining the time-resolved data to see when the emission spectrum stops evolving. N o evidence of spectral evolution is observed for times greater than ~ 2 5 ps.’ 0 This rate of solvent relaxation is consistent with the ~ 5 0 - relaxation p~ time for C(t) reported for ethanol solution at 0 oC.1a95Consistent with this analysis, as the temperature is lowered, the differences in the emission curves as a function of wavelength are enhanced, and the apparent Stokes shift correlation time is also increased. These data provide clear evidence for the presence of solvent relaxation dynamics following the proton-transfer process in 7azaindo1e:ethanol complexes. The multicomponent profile of emission a t a particular wavelength is seen to be indicative not of multiple pathways to tautomer production, but of solvation dynamics. Further studies on the effects of solvent relaxation on proton-transfer reactions are in progress.
Acknowledgment. This work is supported by a Whitaker Foundation Grant of Research Corporation ( R S M ) , the Hackman Scholars program of Franklin and Marshall College (R. S.M.), and the National Science Foundation (J.D.S). We thank S.-G. Su for technical assistance. Registry No. 7-AI, 271-63-6; 7-AI tautomer, 89267-19-6; ethanol, 64- 17-5.
(7) (a) Bagchi, D.; Oxtoby, D. W.; Fleming, G. R. Chem. Phys. 1984,86, 257. (b) van der Zwan, G.; Hynes, J. T. J. Phys. Chem. 1985, 89, 4181. (8) For times greater than 250 ps, all wavelengths in the tautomer emission are found to decay with the same time constant, characteristic of the lifetime of the excited tautomeric state ( ~ 1 . 0ns).
