10493
J . Phys. Chem. 1991, 95, 10493-10495
Mechanism and Kinetics of Proton-Transfer Reaction in Excited Internally Hydrogen Bonded Benzoxazole Derivatives As Studied by Picosecond Transient Absorption and Stimulated Emission Pumping Anna Grabowska,* Jerzy Sepiol, Institute of Physical Chemistry, Polish Academy of Sciences, 44 Kasprzaka, 01 -224, Warsaw, Poland
and Claude Rullihe Centre de Physique Moleculaire Optique et HerziPnne, U.A. du CNRS No. 283, UniversitP de Bordeaux I , 351, Cours de la LibZration. 33405 Talence, France (Received: May 6,1991)
Dually fluorescent molecules 2,5-bis(benzoxazolyl)hydroquinone and 3,6-bis(benzoxazolyl)pyrocatechol undergo the excited-state intramolecular single and double proton transfer, respectively. Both together with a model compound, 2,5-bis(benzoxazolyl)-4-methoxyphenol,reveal transient absorption and gain bands detected with a picosecond temporal resolution. Gain bands found in the spectral range of the phototautomeric fluorescence served as a guide for the application of the stimulated emission pumping (SEP): radiation resonant with the phototautomeric emission depopulates the corresponding energy level. This effect is followed exactly by the primary (blue) fluorescence. The result confirms the existence of the equilibria in S, states of both molecules and shows the new application of SEP technique.
Introduction Since the excellent inspiring papers by Michael Kasha and co-workers, problems of inter- and intramolecular excited-state proton-transfer (ESIPT) reactions got the renewed interest.'-4 Large aromatic molecules undergoing internal proton transfer in excited state (phototautomerisations) are particularly interesting when they bear the direct information about the primary excited form, and the reaction product as well. This direct information-in the form of a dual fluorescence and phosphorescence-is found The large majority of reactions of this rather e~ceptionally.~.~ type represent the irreversible kinetics of extremely fast processes, where no time is left for the emission from the primary structure, the only fluorescence being the strongly Stokes shifted, phototautomeric band.'** In this paper we report on particular systems: a group of dually fluorescent molecules, derivatives of benzoxazole. These are 2,5-bis(benzoxazolyl)hydroquinone (BBHQ), its monomethylated derivative 2,5-bis(benzoxazolyl)-4-methoxyphenol(BBMP), and 3,6-bis(benzoxazolyl)pyrocatechol (BBPC). They are known already from a series of our earlier p a p e r ~ ~ ~as ~ -well l * as from continuation and criticism by other author^.'^-'^ Previous results most important for the present study are the following: all three molecules reveal the dual fluorescence, blue and red, emitted by ~ _ _ _ _ _ _
(1) Taylor, C. A.; El-Bayoumi, M. A.; Kasha, M. Proc. Natl. Acad. Sei. U.S.A. 1969, 63, 253. (2) Kasha, M.; Horowitz, P.; El-Bayoumi, M. A. In Molecular Spectroscopy, Modern Research; Academic Press: New York, 1972; p 287. (3) McMorrow, D. C.; Kasha, M. Proc. Narl. Acad. Sci. U.S.A. 1984,81, 3375; J . Phys. Chem. 1984, 88, 2235. (4) Chou, P.; McMorrow, D. C.; Aartsma, T. J.; Kasha, M. J . Phys. Chem. 1984,88, 4596. ( 5 ) Mordziiiski, A.; Kiihnle, W. J . Phys. Chem. 1986, 90, 1455. (6) Rodriguez-Prieto, M. F.; Nickel, 8.; Grellmann, K.-H.; Mordziiiski, A. Chem. Phys. Lett. 1988, 146, 387. (7) Flom, S.R.; Barbara, P. F. Chem. Phys. Lett. 1983, 94, 488. (8) Chem. Phys. 1989, 136; special issue on proton-transfer reactions. (9) Mordziiiski, A.; Grabowska, A,,; Kiihnle, W:; Krbwczyiiski, A. Chem. Phys. Lett. 1983, 101, 291. Mordziiiski, A.: Grabowska, A.; Teuchner, K. Chem. Phys. Letr. 1984, 111, 384. (10) Grabowska, A.; Mordziiiski, A.; Tamai, N.; Yoshihara, K. Chem. Phys. Lett. 1988, 153, 389; Chem. Phys. Lett. 1990, 169,450. (1 1) Grabowska, A.; Mordziiiski, A.; Kownacki, K.; Gilabert, E.; RulliCre, C . Chem. Phys. Letr. 1991, 177, 17. (1 2) Sepiok, J. Chem. Phys. Lett. 1990, 175, 41 9. (13) Emsting, N. P. J . Phys. Chem. 1985, 89, 4932. (14) Dick, B. Chem. Phys. Lett. 1989, 158, 37. (19) Brackmann, U.; Emsting, N. P.; Ouw, D.; Schmitt, K. Chem. Phys. Lett. 1984, 110, 319.
0022-3654/91/2095- 10493$02.50/0
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the primary and phototautomeric form, respectively. For BBHQ and BBMP it was shown that during the lifetime of SIstate the equilibrium is established between both proton-exchanging reaction partners. Moreover, by comparison with monomethoxy derivatives of BBHQ and BBPC, it was tentatively proved that the first molecule (BBHQ) transfers only one proton, while the second (BBPC) retains the symmetry, undergoing the double proton transfer (DPT) reaction (see Scheme I). The thermodynamics of ESIPT reactions of BBHQ and BBMP in solution are also known: the enthalpy changes, AHo, in 3methylpentane5g9 are -0.5 and -1.5 kcal/moI, respectively. The corresponding activation energies for PT processes in methyltetrahydrofuran, A E ,are 0 . 3 9 and 1.1 kcal/mol.s The existence of the barrier on PT reaction pathway of excited BBHQ was confirmed independently by the supersonic beam technique.13 Finally we managed to determine directly the PT rate constants for the forward and back reactions of at1 three molecules discussed here, with the application of the time-correlated single-photoncounting technique of high temporal resolution. All rate constants are of the order of 1O'O s-' at room temperature.1° Two challenging points are left for the present work: (1) Do two symmetric molecules, BBHQ and BBPC, undergo SPT and DPT,respectively, as shown in Scheme I? Here the role of model compounds mimicking the structure of the expected reaction product will be discussed. (2)Are the dual fluorescences in these cases artifacts, 0 1991 American Chemical Society
Grabowska et al.
10494 The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 OD 1.0
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Figure 4. Transient absorption and gain spectra measured 150 ps after the excitation pulse: BBPC (curve 1); PhQ (curve 2); room temperature; solvent, tetrahydrofuran.
as has been suggested in the literature,14 or are they the intrinsic properties of the excited molecule? In connection with that, the usefulness of SEP method will be demonstrated. Experimental Section Technical details of the transient absorption spectroscopy and stimulated emission pumping were already described as well as the origin and purification of the investigated molecules and model compounds.",'* Results Scheme I shows why BBMP and phenanthrenequinone (PhQ) in H-bonding solvent were chosen as model compounds. Of course, PhQ mimics the expected diketo form of BBPC only approximately; our hope was however that the proximity of two keto groups in both structures will decide on the character of the lowest energy transitions. Figures 1 and 3 show ground-state absorption spectra, and Figures 2 and 4 show transient absorptions of both pairs, studied molecule and the model compound. Transient absorptions are measured with the temporal resolution of about 20 ps. It is clearly seen that in both cases the ground-state absorption spectra are totally different, while electronic excitation to the SI state makes the spectra of both compared partners closely similar. This similarity is particularly striking in the case of BBPC and PhQ, as seen in Figure 4. The transient spectra of Figures 2 and 4 reveal another important feature: three molecules undergoing the phototautomerization, BBHQ, BBMP, and BBPC, show gain bands (negative optical density) in spectral regions of keto tautomeric fluorescences, with maxima at 15 000, 16 200 and 18 300 cm-I, respectively. Since BBHQ and BBMP were reported as systems reaching the tautomeric equilibrium on the level of S,state, the experiment applying the radiation resonant with the phototautomeric fluorescence seemed to be particularly promising. Thus both molecules, each in heptane solution, were exposed to a typical SEP
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experiment. Two pulses, pump pulse, and the delayed dump pulse (this last was resonant with the radiation emitted by the monoketo tautomer), coaxially entered the cell. The frequency of the dump pulse was chosen according to the maximum of the gain band detected in transient spectra of Figure 2. The result is shown on the example of BBMP in Figure 5. The dump pulses were delivered by the broadband dye laser. For BBMP rhodamine 6G (585 nm) and for BBHQ rhodamine B (620 nm) were used. Figure 5 shows clearly that the depopulation of the phototautomer observed as the reduction of its fluorescence intensity by a factor of about two (the low-frequency maximum, curves 1 apd 2) is transmitted to the fluorescence of the primary excited form occuring in a very distant spectral region (high-frequency maximum, curves 1 and 2). In BBHQ where the gain was much weaker (Figure 2, curve l), a similar experiment gave the decrease of both fluorescence bands by about 20%. The meaning of the phenomenon will be discussed below. Here we note only that such a behavior has one explanation: in both molecules, both emitting tautomers are kinetically linked on the fluorescent level by the reaction that is fast in comparison to all rate constants of the processes depopu-
Internally Hydrogen Bonded Benzoxazole Derivatives
The Journal of Physical Chemistry, Vol. 95, No. 25, 1991 10495
lating both reaction partners toward the ground state.
different, as could be expected from the corresponding structures. In both discussed cases we believe to deliver additional convincing arguments in favor of the single proton transfer in BBHQ and double PT in BBPC on the level of evidence of electronic spectroscopy. The second point to be discussed is a new application of stimulated emission pumping technique that was already applied to proton-transferring systems.I7 In the present work a new type of experiment is demonstrated: any system in equilibrium established on the level of the excited state may be treated in the same way, if only one member of equilibrated reaction reveals the lasing abilities and the reaction reestablishing the equilibrium is fast in comparison to all processes depopulating the excited species. The case of BBMP was treated quantitatively in ref 12. Thus, according to so-called high-temperature approximation discussed by Birks in his classical paper,’* the excited system in equilibrium may be treated relatively simply. Taking the experimental values of depopulation rate constants of both partners of F T reaction obtained in our earlier works9J0 as equal 2 X lo8 s-I and the forward and back proton transfer rates as 2 X 1O1O s-I, the rate of stimulated emission (4)was obtained by a fitting procedure as = 2.8 X lo9, up to decrease of fluorescence intensity by a factor of about 2, as observed experimentally. This result is of course valid only for the fluence of our dump pulse (see Figure 5 ) . The demonstrated type of SEP experiment applied to equilibrated systems on the level of fluorescent state is a challenge for the information-transfer technique: the extremely specific perturbation can be transmitted with a very high velocity to a distant spectral region.
Discussion First the role of model compounds will be discussed: the drawbacks of electronic spectroscopy in evaluation of excited-state reaction mechanisms are its poor specificity and the lack of means of identification of the unstable species, e.g., excited reaction products. Until time-resolved vibrational spectroscopy techniques are widely available, model compounds are sometimes the only way of identification of short-lived transients existing only in the excited state. In the present case, both molecules used as models play a crucial role. They are, however, used in a different sense in each of two studied PT reactions: BBMP is an excited-state model “in potentia”. It is known that this molecule undergoes the ESIPT reaction, the only possible product of which is a monoketo tautomer (see Scheme I, 2). It is compared with the excited symmetric parent molecule, BBHQ, that in principle has two different ways of reacting: SPT or DPT. It was earlier observed that the overall photophysics of BBHQ follows that of BBMP. It was then concluded that the symmetric system undergoes a single proton transfer upon excitation, later explained by a qualitative theoretical treatment.16 Transient absorption spectra of Figure 2 strongly support this assignment: both curves 1 and 2 are very close (at least in the spectral range available for the experiment) showing that the steric hindrance introduced by the OH-OCH3 substitution, and the decreased conjugation of aromatic system clearly seen in the absorption spectra of So state (Figure 1) plays here a minor role. In the second pair of molecules (Scheme I, reactions 3, 4) phenanthrenequinone (PhQ), already in the ground state, represents a structure similar to the expected diketo tautomer, tentatively assigned previously as a product of the double proton transfer in BBPC. To make the model molecule as close as possible to the internally H-bonded BBPC, PhQ was studied in a protic solvent (butanol) providing the intermolecular hydrogen bonds. The transient absorption spectra of these two molecules show a remarkable similarity (Figure 4 curves 1, 2) in contrast to the ground-state absorpiion spectra (Figure 3); which are entirely (16) Nagaoka, S.;Nakashima, U. J . Phys. Chem. 1990, 94, 1425.
Acknowledgment. Our sincere thanks are due to A. Mordziiiski-a man whose work contributed mostly in “bisbenzoxazoles” photophysics-for providing us with samples of BBHQ and BBMP. The technical assistance of K. Kownacki, M.Sc., and P. Borowicz, M.Sc., is kindly acknowledged. Registry NO. BBHQ, 33450-1 1-2; BBPC, 88978-49-8; BBMP, 88978-50-1; PhQ, 84-1 1-7. (17) Dzugan, T. P.; Schmidt, J.; Aartsma, T. J. Chem. Phys. Leu. 1986, 127, 336 references therein. ( 1 8 ) Birks, J. Noun J. Chim. 1977, I, 453.
