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hydrogen acceptor and donor must be rather short. Experimental data for 1AC give r” = 2.95 A, a fairly large value. It might be that after the excitation to S, the moieties of the dimer attract each other stronger than in So,due to the changes in the electronic distribution (pyridine-type nitrogen becomes more negative and that of pyrrole-type more positive). That may lead to the “contraction” of the dimer and the reduction of the N-N distance. There are two other processes, the influence of which on ESDPT in solid 1AC can apparently be neglected. These are (a) the geometry relaxation of the whole dimer in the excited state, and (b) the rearrangement of the environment. Both have been shown to play a major part in 7AI-alcohol complexes.’* Finally, it should be stressed that the results presented in this paper do not allow us to definitely reject a possibility of a tunneling
mechanism of ESPT observed, e.g., in 9-hydro~yphenalenone.1~ Two points, however, seem to act against such an interpretation. First is the above-discussed lack of fluorescence from initially excited dimers. The other one lies in the fact that ESPT in 1AC dimers occurs between two completely unequivalent structures. In such a case, the probability of tunneling should be greatly reduced with respect to a symmetrical doubleminimum p0tentia1.I~ Acknowledgment. Dr. J. Lipkowski and Dr. K. SuwiAska are kindly acknowledged for allowing us to use their drawings of the crystal structure of 1AC. We thank Dr. J. Dobkowski for his assistance in liquid helium investigations. ~
~~
(13) Bondybey, V. E.; Haddon, R. C.; English, J. H. J. Chem. Phys. 1984, 80, 5432. Bondybey, V. E.; Haddon, R. C. Rentzepis, P. M. J. Am. Chem. SOC.1984, 106, 5969. (14) de la Vega, J. R.; Busch, J. H.: Schauble, J. H.; Kunze, K. L.; Haggert, B. E. J. Am. Chem. Soc. 1982, 104, 3295.
(12) Herbich, J.; Sepiol, J.; Waluk, J. J. Mol. Struct. 1984, 114, 329.
Excited-State Double Proton Transfer in 1-Azacarbazole-Alcohol Complexes J. Waluk,* S. J. Komorowski, and J. Herbich Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01 -224 Warsaw, Poland (Received: January 24, 1986; In Final Form: April 28, 1986)
Dual fluorescence is observed in alcohol solutions of 1-azacarbazole, two different ground-state species being responsible for each band. The low-energy emission is due to structures that undergo a double proton transfer after excitation. These are assigned to 1:1 complexes with alcohol. The higher energy band is emitted by a species which is unable to tautomerize. They constitute a large majority in S, and are attributed to 1:2 complexes. Their decay from SIoccurs via internal conversion, contrary to the monomers of I-azacarbazole, in which intersystem crossing provides the dominant deactivation path.
Introduction It has recently become clear that in some cases for which the presence of two emissions was considered a manifestation of an excited-state reaction, another mechanism operates: each fluorescence has a different ground-state precursor. Such is the case in the “classical” structure undergoing excited-state proton transfer (ESPT)-methyl salicylate,’-4 in which high-energy emission comes from rotamers that are unable to tautomerize. The other type of rotamers does undergo ESPT, which leads to the tautomeric species emitting the fluorescence with a large Stokes shift. Another example is provided by 3-hydroxyflavone in nonpolar solvents. Here again the tautomeric luminescence occurs from structures prepared for the reaction already in the ground state by a strong internal hydrogen bond. The other fluorescence, previously ascribed to initially excited 3-hydroxyflavone, is in fact due to complexes with minute amounts of water or alcohol present in the solution and can be totally suppressed by careful purification of the ~ o l v e n t . ~ In the present work we have investigated excited-state double proton transfer in complexes formed by 1-azacarbazole (1AC) with alcohols. It is concluded that at least two different structures (1) Goodman, J.; Brus, L.E. J . Am. Chem. SOC.1978, 100, 24. (2) Ford, D.; Thistlethwaite, P. J.; Woolfe, G. J. Chem. Phys. Lett. 1980, 69,246. ( 3 ) Acufia, A. U.; Amat-Guerri, F.; Catalin, J.; Gonziles-Tablas, F. J. Phys. Chem. 1980.84, 629. (4) Heimbrook, L.A.; Kenny, J. E.; Kohler, B. E.; Scott, G. W. J. Phys. Chem. 1983,87,280. McMorrow, D.; Kasha, M. J. Phys. Chem. 1984,88, 2235. mom, S. R.; Barbara, P. F. J. Phys. Chem. 1985,89,4489. Strandjord, A. J. G.; Barbara, P. F. J. Phys. Chem. 1985, 89, 2355. (5) McMorrow, D.; Dzugan, T. P.; Aartsma, T. J. Chem. Phys. Lett. 1984, 103, 492. McMorrow, D.; Kasha, M. J. Phys. Chem. 1984, 88, 2235.
0022-3654/86/2090-3868$0 1 .50/0
exist in the ground state, of which only one is able to undergo ESPT. The other species decays after excitation mainly via internal conversion, contrary to the behavior of monomeric 1AC in nonpolar solvents. 11. Experimental Section
1-Azacarbazole was purified by several sublimations and crystallizations from ethanol + heptane mixtures. All solvents were carefully checked prior to measurements and showed no residual emission. 3-Methylpentane (Fluka) was dried by passing through silica gel. Fluorescencegrade alcohols (Merck) were used. Absorption spectra were recorded on a Beckmann DU-8B spectrophotometer and stationary luminescence on a Jasny spectrofluorimeter.6 Fluorescence decays were measured by the sampling technique (an IGT-50 nitrogen laser as the excitation source, BCI 280 Boxcar averager, numerical reconvolution). Photoacoustic determination of triplet formation efficiency was performed with a pulsed-photoacoustic calorimeter.’ Results 1AC fonns complexes with alcohols. This can be deduced from the absorption spectra in nonpolar solvents to which small amounts of alcohol are gradually added. In Figure 1, an isosbestic point reveals the existence of two species-complexed and uncomplexed 1AC molecules. In alcohol solutions (ethanol, propanol, butanol, and ethylene glycol) of IAC, a double emission is observed (Figure 2). The more intense fluorescence, which we label Fl, has an excitation (6) Jasny, J. J. Lurnin. 1978, 17, 149. (7) Komorowski, S. J.; Grabowski, Z. R.; Zielenkiewicz, W. J. Phorochem. 1985, 30, 141.
0 1986 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3869
02 -
0 4 8 12 t[nsl Figure 3. Fluorescence decay curves of 1AC in propanol at 253 K, monitored at 26000 (F,, curve 1) and 17000 cm-' (F2, curve 2). Both curves are well described by a singb-exponential behavior: TF, = 0.55 0.17 ns, TF* = 2.12 f 0.18 ns. Curve 3: simulated F2decay, calculated under the assumption that both emissions have a common precursor (Le., a rise time of 1.55 ns was added).
1
1
1
1
350 310 n z Figure 1. Absorption spectra of 1AC solution (3 X
M) in 3methylpentane with added butanol (BuOH): solid line (a), no BuOH; curves b-g: 0.007, 0.015, 0.022, 0.036, 0.073, and 0.144 M BuOH, respectively.
Figure 2. Fluorescence spectra of 1AC a t 293 K in pure butanol and in 3-methylpentane with M butanol (-). For determination of the F2quantum yield, the low-energy tail of F1 was subtracted. (e..)
spectrum coinciding with the absorption. The other, green luminescence (FJ, with a large Stokes shift, was too weak to for its excitation spectrum to be scanned continuously. However, the identity of the Fl and F2excitation spectra may be deduced from the fact that excitation a t various energies gave the same ratio of F2/FI. Moreover, the Fz spectral position corresponds to that of a double-proton-transferred species observed in 1AC dimers m s o h t i ~ n * and . ~ in the solid.lO*ll Therefore, we ascribe F2 fluorescence to a structure in which two protons have been shifted in the excited state of the 1AC complex with alcohol: the hydrogen bond with the pyridine-type nitrogen atom becomes a chemical (8) Chang, C.; Shabestary, N.; El-Bayoumi, M. A. Chem. Phys. Lett. 1980, 75, 107. (9) Waluk, J.; Grabowska A.; P a M a , B.; Sepiol, J. J . Phys. Chem. 1984, 88, 1160. (10) Waluk, J.; Pakrtla, B. J. Mol. Strucr. 1984, 214, 359. (1 l! Waluk, J.; Herbich, J.; Oelkrug, D.; Uhl, S.J. Phys. Chem., preceding paper in this issue.
one, whereas the opposite occurs on the other, pyrrolic nitrogen atom. An analogous reaction has been observed12J3in 7-azaindole complexes with alcohols. To account for the double luminescence, two different mechanisms might be postulated. First is the excited-state reaction occurring after the excitation of a single So species. In that case, F, and F2have the same precursor and thus are kinetically coupled: the decay time of F, should be equal to the rise time of F2. This model can be excluded on the basis of kinetic experiments. F, fluorescence decays with a lifetime of 1.O ns at 293 K (ethanol, propanol, butanol, and ethylene glycol), 1.5 ns at 253 K, and 2.0 ns at 243 K. F2emission, although weak, is still observed at those temperatures. However, no measurable rise time of F2could be found and its decay was monoexponential (Figure 3). These facts leave us with a second possibility: two species exist in the ground state; only one of them is "prepared" for a rapid excited-state tautomerization (due to the time resolution of our apparatus, rapid means here shorter than 0.5 ns). In order to get some insight into the nature of the two species, we repeated the experiments, using mixed solvents instead of pure alcohols. Solutions of 10-2-10-1 M alcohol in nonpolar solvents were used; the concentration of 1AC was around lo-' M. The amounts of alcohol were more than enough to complex all 1AC molecules. This was seen both in absorption and emission (complete disappearance of a strong monomeric 1AC fluorescence observed in a pure nonpolar solvent). On the other hand, it is known from the dielectric relaxation measurement^'^ that at these concentrations alcohol molecules exist in solution as monomers; only a t higher concentrations oligomers start appearing. It can be seen from Figure 2 that a large (more than threefold) increase of the F2intensity is observed in mixed solvents, whereas the quantum yield of F, remains the same as in pure alcohol. Also, decay times of F, and F2stay the same in the above media within our measuring accuracy, &lo%. Let us assume that a fraction a of 1AC complexes with alcohol are unable to tautomerize, whereas 1 - a are prepared for the reaction. If, moreover, the absorption spectra of both species are the same (a plausible assumption, because the F2/F, intensity ratio is independent of excitation wavelength), one obtains for the F, and F2 quantum yields 'PF, = a k l r T F , 'PF2 = (l
- a)kZrTF2
(12) Taylor, C. A.; El-Bayoumi, M. A.; Kasha, M. Proc. Not. Acad. Sci.
US.1969,63,253. Ingham, K.C.; El-Bayoumi, M. A. J. Am. Chem. SOC. 1974, 96, 1674. Avouris, P.; Yang, L. L.; El-Bayoumi, M. A. Photochem. Photobiol. 1976, 24, 21 1. (13) Bulska, H.; Grabowska, A.; Pakula, B.; Sepiol, J.; Waluk, J.; Wild, U. P. J . Lumin. 1984,29, 65. Herbich, J.; Sepiol, J.; Waluk, J. J. Mol. Srruct. 1984, 114, 329. (14) Glasser, I.; Crossley, J.; Smyth, C. P. J. Chem. Phys. 1972, 59, 3977.
Letters
3870 The Journal of Physical Chemistry, Vol. 90, NO. 17, 1986 TABLE I: SIDepopulation Parameters of 1AC at 293 K fluorescence emitting species quantum yield lifetime, ns monomer (n-heptane) 0.14 f 0.03 8.8 f 1.0 (1.5 f 0.4) X lo-* 1.0 f 0.2 alcohol complex' (BuOH) 1.4 f 0.2 tautomerb (BuOH) (5 2) x 10-4
triplet formation eff 0.64 f 0.07
