J. Phys. Chem. 1986, 90, 3866-3868
3866
Exclte6State Double Proton Transfer In the Solld State: The Dimers of 1-Azacarbazole J. Waluk,* J. Herbich, Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01 -224 Warsaw, Poland
D. Oekrug, and S. Uhl Institute of Physical and Theoretical Chemistry, Tiibingen University, Auf der Morgenstelle 8, 7400 Tiibingen, FRG (Received: January 24, 1986; In Final Form: April 28, 1986)
Excited-state double proton transfer in solid 1-azacarbazole,consisting of planar, doubly hydrogen-bonded dimers, occurs without an apparent energy barrier. The radiative rate constant of tautomeric fluorescence seems to be temperature-dependent, which is consistent with the predicted g character of the lowest excited single state in the proton-transferred species.
Introduction From among many the chemical systems, molecules, complexes, and dimers, that undergo tautomerization after absorption of a photon, of particular interest is the case in which two protons are cooperatively transferred in the excited state. Such a reaction was initially observed’ for hydrogen-bonded dimers and alcohol complexes of 7-azaindole (7AI). Later, it was shown* to occur also in dimers of 1-azacarbazole (IAC) and “heterodimers”, composed of 7AI and lAC.3 One of the crucial problems in the investigation of excited-state proton transfer (FSFT) is the reconstruction of the potential energy hypersurface along which the reaction proceeds. For several compounds undergoing intramolecular ESPT it was found that tautomerization along a preexisting hydrogen bond occurs very rapidly, most probably without activation energy! We came to similar conclusions for the case of 7A15 and 1AC dimers6 in solution. For the latter system it was demonstrated that ESPT is mainly governed by viscosity, which strongly suggested that the main portion of energy required for tautomerization is used for flattening of initially twisted dimers. This would lead to a doubly hydrogen-bonded species with all the atoms directly involved in ESPT lying in one plane. The results presented in this paper fully support the above conclusion. We have investigated solid IAC, which is an ideal model for our purpose: it is composed of coplanar dimeric units.’ Thus, the main barrier for excited-state double proton transfer has been removed already in the ground state and the structure seems “prepared” for tautomerization. Indeed, it will be shown that in such a system the proton-transfer reaction cannot be stopped, even at 1.5 K. Furthermore, even at the lowest temperatures, the tautomerization seems to be very rapid and apparently proceeds without an energy barrier.
Experimental Section 1AC was recrystallized from ethanol and vacuum sublimed. Stationary and kinetic luminescence measurements were done with (1) Taylor, C. A.; El-Bayoumi, M. A.; Kasha, M. Proc. Nut. Acad. Sci.
US.1%9,63,253. Ingham, K. C.; El-Bayoumi, M. A. J . Am. Chem. Soc. 1974, 96, 167. (2) Chang, C.; Shabestary, N.; El-Bayoumi, M. A. Chem. Phys. Lett. 1980, 75, 107. (3) Sepid, J.; Wild, U. P. Chem. Phys. Lett. 1983, 93, 204. (4) Mc Morrow, D.; Dzugan, T. P.; Aartsma, T. J. Chem. Phys. Lett. 1984, 103, 492. Goodman, J.; Brus, L. E. J . Am. Chem. Soc. 1978, 100, 7472. Ding, K.; Courtney, S. J.; Strandjord, A. J.; mom, S.;Friedrich, D.; Barbara, P. F. J. Phys. Chem. 1983, 87, 1184. Woolfe, G. J.; Thistlethwaite, P. J. J . Am. Chem. SOC. 1980, 102, 6917. ( 5 ) Bulska, H.; Grabowska, A.; Pakuh, 9.;Sepiol, J.; Waluk, J.; Wild, U. P. J. Lumin. 1984, 29, 65. (6) Waluk, J.; Grabowska, A.; Pakula, B.; Sepid, J. J . Phys. Chem. 1984, 88, 1160. (7) Suwifiska, K. Acta Crystullogr., Sect. C: Cryst. Struct. Commun. 1985, C41, 973.
0022-36S4/86/2090-3866$01 S O / O
various modes of sample preparation (powders, thin films, small crystals). In order to measure absorption spectra at 293 and 77 K, a thin film of 1AC was deposited on a quartz plate by evaporating an ethyl ether or chloroform solution. The plate was then mounted into a home-built cryostat, composed of a copper block and a vacuum chamber with quartz windows. Luminescence measurements down to 12 K were performed in a continuous-flow helium cryostat (built in the Institute of Physics, Polish Academy of Sciences) mounted into a Jasny spectrofluorimeter.* The lowest temperatures (down to 1.5 K) were obtained with a cryostat originally designed for ODMR mea~urements.~The other part of experimental setup has been described e l ~ e w h e r e . ~ , ~ Another series of absorption and emission experiments was performed, which used a quite different instrumentation: small crystals of 1AC were placed on an aluminum block and pressed against a fused-silica plate. This sample holder was mounted into a cryocooler CTI 21 (Cryogenic Tech, Inc.) with a closed helium cycle. By this means, temperatures down to 20 K could be obtained. Fluorescence excitation and emission spectra have been recorded on a Spex fluorolog 222 double-beam spectrometer. The low-temperature reflectance absorption spectra were obtained by combining the cryocooler with an evacuated photometric integrating sphere that was adapted to the diffuse reflectance attachment of a Gary 14 spectrometer.
Results Crystallographic Structure. Two projections of 1AC dimer in the crystal are presented in Figure 1, according to ref 7. The structure consists of a doubly hydrogen-bonded dimer, with coplanar arrangement of all nitrogen atoms forming hydrogen bonds. The distance between donor and acceptor nitrogen atoms is rather long (2.95 A). Luminescence Properties. Figure 2 presents luminescence spectra taken at 293,77, and 13 K, respectively. Throughout this temperature region and still at 1.5 K an emission is observed, analogous to the tautomeric fluorescence, F2, which occurs in solution.z6 The experimental evidence shows that also in the solid state this fluorescence is due to the proton-transferred species: (a) absorption and excitation spectra agree pretty well; (b) both the shape and position of luminescence maxima in solution and solid are nearly identical; (c) decay times of F2 in the solid and in solution are also similar (Figure 3) (moreover, so are their temperature dependences). This last observation, in particular, allows us to reject a possibility of F2 in the solid being due to excimer emission, which could have arisen from closely lying, (8) Jasny, J. J . Lumin. 1978, 17, 149. (9) Herbich, J.; Dobkowski, J.; Jasny, J.; Tolbczko, M.; Disselhorst, J. A. J. M. Polish Chemical Society Annual Meeting, Lublin, Poland, 1982; Abstr. p 329.
0 1986 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol, 90, No. 17, 1986 3861 TF2 I n SI
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