1160
J. Phys. Chem. 1984, 88, 1160-1 162
Viscosity vs. Temperature Effects in Excited-State Double Proton Transfer. Comparison of I-Azacarbazole with 7-Azaindole J. Waluk,* A. Grabowska, B. Pakula, and J. Sepiol Institute of Physical Chemistry, Polish Academy of Sciences, 01 -224 Kasprzaka 44, Warsaw, Poland (Received: January 7, 1983; In-Final Form: July 7, 1983)
Excited-state double proton transfer in 1-azacarbazole dimers is controlled by viscosity rather than by temperature. The activation energy is related to internal rotation of monomeric moieties. The possibility of another energy barrier, along the coordinate of proton motion between two nitrogen atoms, is also discussed.
Introduction Owing to its great importance for both chemistry and biology, excited-state proton transfer continues to be a subject of intense research.' Of special interest is the case of 7-azaindole (7AI) dimersZ and alcohol complexes3 in which two protons are simultaneously shifted after electronic excitation from the pyrrolic toward the pyridine-type nitrogen atoms. The reaction is very fast, m u r s within picoseconds: and cannot be stopped by lowering the temperature even down to 4 K. To account for this, various mechanisms have been postulated: (a) tunneling,z (b) excitation above the energy barrier," and (c) negligibility of activation energy along the coordinate of proton transfer.s While suggesting mechanism c, we hint that the apparent activation energy may be due to the torsional motion of 7AI moieties with respect to each other. The above hypothesis is corroborated by the results presented in this work. We have investigated double proton transfer previously shown to occur in excited dimers of l-azacarbazole ( 1AC),6trying to separate temperature and viscosity contributions (see Scheme I). It is shown that the tautomerization process is governed by viscosity, not by temperature. This implies that the activation energy is related to some conformational changes (most probably flattening of the dimers). In this context, the differences between 1AC and 7AI are discussed. Experimental Section 1-Azacarbazole was kindly given to us by Prof. M. Zander (Laboratorium der Rutgerswerke AG, Castrop-Rauxel). It was purified by vacuum sublimations. Spectral-grade 3-methylpentane (Fluka), methylcyclohexane (Eastman), and isopentane (Merck) were passed through silica gel prior to measurements. Spectroscopic-grade liquid paraffin (Merck, Uvasol), mixed with hexane in a 3:l ratio, was column-chromatographed over Alz03 and SiO,; hexane was then evaporated under vacuum. All solvents were checked for luminescence and showed no emission. Luminescence spectra were measured on a Jasny spectrofluorimeter.' A sampling technique was used for determination of fluorescence lifetimes, with an IGT 50 nitrogen laser as the excitation source. A phase-plane method8 was applied for deconvolution. (1) D. Huppert, M. Gutman, and K. J. Kaufmann in "Photoselective Chemistry", Part 11, J. Jortner, R. D. Levine, and S . A. Rice, Eds., Wiley, New York, 1981, p 643; P. M. Rentzepis and P. F. Barbara, ibid., p 627. (2) K.C. Ingham and M. A. El-Bayoumi, J . Am. Chem. Soc., 96, 1674 (1974); C. A. Taylor, M. A. El-Bayoumi, and M. Kasha, Proc. Natl. Acad. Sci. U.S.A., 63, 253 (1969); B. Delinger and M. Kasha, Chem. Phys. Lett., 38, 9 (1976). (3) P. Avouris, L. L. Yang, and M. A. El-Bayoumi, Photochem. Photobiol., 24, 211 (1976). (4) W. M. Hetherington 111, R. H. Micheels, and K. B. Eisenthal, Chem. Phys. Lett., 66, 730 (1979). ( 5 ) H. Bulska, A. Grabowska, B. Pakula, J. Sepiol, J. Waluk, and U. P. Wild, J . Lumin., in press. (6) C. Chang, N. Shabestary, and M. A. El-Bayoumi, Chem. Phys. Lett., 75, 107, (1980). (7) J. Jasny, J. Lumin., 17, 149 (1978). (8) J. N. Demas and A. W. Adamson, J. Phys. Chem., 75, 2463 (1971).
0022-3654/84/2088-1160$01.50/0
Scheme I
Dimer
t Tautomer
Standard INDO/S9 served for calculation of excited-state parameters, with carbazole geometry as input,1° All singly excited configurations lying below 10 eV were used in the CI procedure. Results and Discussion Figure 1 presents room-temperature spectra of 1AC in 3methylpentane. The emission is dominated by high-energy monomer fluorescence. At the low-energy side another luminescence appears (F2),exhibiting a large Stokes shift, and with the excitation spectrum shifted 1500 cm-' to the red from that of the monomer. It was assigned by El-Bayoumi et ale6as the fluorescence of phototautomers formed from excited 1AC dimers. The excitation spectrum of F2would thus correspond to the absorption of dimeric species. To eliminate monomer fluorescence, we excite a concentrated solution of 1AC at its absorption red edge: Only Fz is then observed, but no emission of primarily excited dimers. This indicates that tautomerization at 293 K is so fast that the excited dimers do not live long enough to emit photons. Absorption and luminescence of neutral and ionic species of 1AC are shown in Figure 2. The application of the Forster cycle" to the above spectra gave ApK, = +7.5 A 0.5 for the equilibrium cation/molecule and ApK, = -10.8 f 1.0 for the equilibrium molecule/anion. Ground-state pKa values are 4.2 f 0.2 and 14.3 f 1.0 for the first and second equilibria, respectively (determined by the spectrophotometric method). The change of pK for deprotonation is so large that one could even expect to see anion fluorescence in alcoholic solutions of 1AC. It is not observed, apparently due to kinetic limitations. The pKa values show that both the acidity of pyrrolic and the basicity of pyridine-type nitrogen atoms are significantly enhanced in S1. This would greatly facilitate excited-state tautomerization. Similar acidity changes upon excitation were observed in 7ALS When the temperature is lowered, a decrease of monomer fluorescence intensity occurs. This is caused by dimerization, which seems to be complete at around 200 K. Only Fzis emitted at 183 K in methylcyclohexane-isopentane mixture (see Figure
-
(9) J. E. Ridley and M. C. Zerner, Theor. Chim. Acta, 32, 11 1 (1973). (10) B. S . Basak and B. N. Lahiri, Indian J . Pure Appl. Phys., 7, 234 (1969). (11) Z. R. Grabowski and A. Grabowska, Z. Phys. Chem. (Frankfurt am Main), 101, 197 (1976).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 6,1984 1161
Excited-State Double Proton Transfer
't
16
18
20
22
24
26
28
30
32
34
16
18
20
22
24
26
28
'5[io3 ern-']
Figure 1. 1AC in 3-methylpentane,293 K: monomer absorption (-),
fluorescence (. and fluorescence excitation tautomer emission and 5 X lo4 and excitation (---) spectra. The concentrations were M for measuring monomer and tautomer spectra, respectively. Monomeric luminescence was excited at 31 000 cm-l, and the tautomeric one at 28 000 cm-'. a),
(--e);
!
log E
......
:
30
32
IO' cm"] Figure 3. 1AC in methylcyclohexane-isopentane (1:3); c = lo4 M. Fluorescence spectra excited at 28 000 cm-': 183 (---), 143 (-), 113 (...) K; F, (-) and F2 excitation spectra at 143 K. (-.a)
Scheme I1
D"
kPT
> : : : z T"
I
D
t
\ I
L
i
12.5
;I ,
,
,
I
,
,
I
I
I
1
,A
30 35 P [10~crn-'1 Figure 2. Absorption and emission of neutral and ionic species of 1AC: cation, BH* (0.1 NH2SOl + 10% EtOH, -); neutral form, B (HzO + 10% EtOH, ---); anion, B- (8 N KOH + 10% EtOH, (0,O) energies were calculated as averages of absorption and fluorescence maxima. 20
25
on the experimental decay curve of F2 occurs earlier than that of F,. The reverse should be expected in the case where excited dimers were the precursors of tautomers. These results deserve special attention: they show that the apparently evident scheme of tautomerization from the excited dimers is not quite precise and should be reconsidered more carefully. If the phototautomers were really formed from dimers that found themselves in S1, their lifetime, i.e., F2 decay, could not be shorter than that of F,.This conclusion is readily arrived at if one considers the scheme of tautomerization from initially excited dimers (see Scheme 11). kDoand kToare the sums of rate constants for all processes of S1 depopulation in dimer and tautomer, respectively, except the reactions of forward and back proton transfer, the rates of which are denoted by kpT and k,. In general, the decays of dimer and tautomer are given by12
..e).
3). At lower temperatures another fluorescence ( F , ) begins to appear, slightly red shifted with respect to that of the monomer. The excitation spectra of F , and F2 match one another, strongly suggesting that F,is emitted from the initially excited 1AC dimers. At 113 K F, disappears completely; only F, and a weak phosphorescence-a low-energy shoulder in Figure 3-are observed, contrary to the case of 7AI dimers in which F2 persists even at 4 K. The solution of 1AC in paraffin exhibits basically the same luminescence pattern as that found for much less viscous solvents. However, the region of F1 and F2 coexistence (123-173 K in methylcyclohexane-isopentane) is shifted to much higher temperatures (223-273 K). No F, can be detected already below 213
[D*Io being the initial concentration after excitation with a Dirac &shape pulse. If the equilibrium is not established within the lifetime of the excited state, the product kPTkm-is small and can be neglected (which is the case in this work, since we obtain different decay parameters of F1and F2). Then [D*] = [D*]oe-kD'
K. Again, a drastic difference between 1AC and 7AI is observed. The luminescence of 1AC and 7AI in paraffin at 77 K is presented in Figure 4. One can see the intense F2 and much weaker F1 fluorescence of 7A1, whereas 1AC emits only F , and a weak phosphorescence. The measurements of fluorescence lifetimes at the temperature region where both F, and F2 are observed reveal that F1decays always much more slowly than F2: e.g., in paraffin at 223 K T(F,) = 12.4 ns, T ( F ~=) 3.3 ns, and in methylcyclohexane-isopentane a t 123 K 7(F1)= 14.4 ns, 7(F2) = 4.8 ns. Also, the maximum
It is seen that tautomers cannot decay faster than their precursors, excited dimers. As the experiment does give shorter lifetimes of F2 than F,,the above scheme is not valid in the present case. In other words, dimeric species which emit F,fluorescence do not participate in tautomerization. We postulateds for 7AI the existence of two different dimeric species: (a) cyclic, doubly hydrogen bonded, "prepared" for rapid, (12) J. B. Birks, N o w . J . Chim., 1, 453 (1977).
Waluk et al.
1162 The Journal of Physical Chemistry, Vol. 88, No. 6,1984
‘I
16
18
20
22
24
26
28
30
32 P[103cm-’]
M) and 7AI M) in paraffin Figure 4. Luminescence of 1AC at 77 K. Low-energy part in 1AC spectrum is due to phosphorescence. Weak phosphorescence of 7AI is hidden under the low-energy fluorescence band. The excitation energy was 32000 cm-’ for both compounds.
picosecond excited-state proton transfer, and (b) singly bonded, nonplanar structures for which some activation energy is necessary to attain the cyclic conformation. From the kinetic measurements it is seen that also in 1AC two dimeric forms exist. By analogy, with 7A1, we assign them to cyclic and nonplanar species, based on the already-mentioned experimental observations: (1) Excitation spectra of Fl and F2 are nearly identical, implying similar absorption of species responsible for these two emissions. One can thus exclude the possibility of direct excitation of tautomers. The latter, if present in the ground state, would absorb at much lower energies than the dimers. On the other hand, singly and doubly bonded species can be safely expected not to differ much in absorption. (2) The temperature region in which both F1and Fz can be observed depends crucially on solvent viscosity. F2 disappears in paraffin at the temperature which is 100 K higher than that needed to suppress this emission in 3-methylpentane. It seems natural to associate the viscosity-dependent energy barrier with torsional motion, leading from nonplanar to cyclic structures. At room temperature, only F2 is observed, no matter which nonpolar solvent we use, indicating that the excited-state barrier for tautomerization must not be very high in nonplanar dimers, as it is effectively crossed at 293 K. (3) The lifetime of Fz is shorter than the decay of F1,indicating that we in fact excite two different species, of which only one is capable of tautomerization (cyclic dimers). The other one emits F, (singly bonded structures). From the results obtained at 293 K (lack of F,)it can be inferred that at sufficiently high temperatures also the singly bonded species can undergo tautomerization by thermal crossing of the barrier (attainment of a planar, cyclic conformation). The kinetic scheme described above would then be applicable. Unfortunately, we could not obtain such an experimental regime, as at higher temperatures considerable amounts of monomeric 1AC are present, whose fluorescence dominates much weaker F1and F2. The concept of two differently hydrogen-bonded dimeric species of 1AC is indeed very similar to the scheme proposed for 7AI. There are, however, two important differences: (a) F2fluorescence does not vanish in 7AI even at 4 K, contrary to lAC, in which no tautomerization with zero (or a negligible) activation energy can be detected. (b) Tautomerization by way of crossing the energy barrier is still observed in 7AI a t 77 K$ whereas it is suppressed in 1AC even at more elevated temperatures. Higher energy barrier for tautomerization in 1AC than in 7AI can be understood if one associates it with internal rotation of the monomer moieties, which can lead from nonplanar to cyclic structures. In larger 1AC dimers, rotational diffusion will be hindered more effectively than in 7AI. The time resolution of our equipment did not allow us to monitor the rise time of F2, which in principle should be composed of two components, corresponding to tautomerization from both types of dimers. Thus, we were not able to compare viscosity effects
-
on two channels that can participate in proton transfer. It seems natural, however, to assume that the reaction starting from “twisted” dimers should be much more viscosity-dependent than the process in which the dimeric fragments are initially coplanar, although in the latter case the role of viscosity may also be nonnegligible. The results presented in this paper allow us to state that the crucial factor governing the kinetics of cooperative double proton transfer in 1AC dimer is the viscosity-dependent energy barrier, most probably associated with internal rotation. Another problem remains, concerning the shape of the potential energy curve along the coordinate of proton motion between nitrogen atoms in planar, cyclic structures. Semiempirical calculations of 7AI dimer13J4 suggested the existence of a double minimum in the excited state. However, the computed height of the energy barrier was drastically dependent on the N-N distance.14 We postulated5 for 7AI that, even if such a barrier exists, it is negligibly small. For 1AC we can say that this barrier is certainly lower than that imposed by the solvent viscosity. The assumption of negligible excited-state energy barrier for tautomerization in cyclic structures, joined with the fact that F2 is completely suppressed upon cooling, would lead to the conclusion that only singly bonded, twisted structures remain as energetically more favorable in the ground state than the cyclic ones. Such a situation, although not impossible (the role of steric factors) seems rather improbable. The more plausible way of explaining the experiment is to assume that a barrier for tautomerization exists also in planar species. It should be recalled here that no Fz is observed at low temperatures, also when the cooling rate is rapid, so that some fraction of cyclic dimers should presumably be present in the ground state. This fact suggests that excited dimers that found themselves in a favorable planar conformation still need some energy to tautomerize, contrary to the case of 7AI. One can try to account for this difference by looking at the results of INDO/S calculations of charge densities. In 7A1, a drastic redistribution of charge is obtainedS after excitation to SI and especially to Sz: pyridine-type nitrogen gains an excessive electronic charge, while that of the pyrrole type loses it. The So excitation is so big that an change in polarity after Sz increase of dipole moment from 2.3 to 8.6 D is calculated. It is exactly the Sz state of 7AI that can be correlated with the lowest excited singlet state of the tautomer. Although for 1AC the calculations give essentially the same trend as in 7A1, the computed changes in electron density redistribution upon excitation are definitely smaller. This is clearly illustrated by the obtained values of dipole moments in So (2.1 D), SI (2.9 D), and S2 (2.6 D). We can tentatively say that the driving force for tautomerization is weaker in 1AC than in 7A1, which results in the appearance of a barrier for proton motion in the former species, but not in the latter. Excited dimers of 7AI are most probably more tightly bound; ie., monomeric moieties are closer to each other than the species of 1AC. In view of the already-mentioned results of semiempirical calculations, the shorter N-H. .N distance in 7AI dimers would significantly reduce the barrier (or even make it disappear) with respect to 1AC. To separate the viscosity-dependent energy barrier for tautomerization from the purely intramolecular one we are left with several options such as the following: (a) isotope substitution of pyrrolic hydrogen; (b) use of deuterated solvents, especially alcohols, which form complexes with 1AC and 7AI; (c) investigation of “mixed dimers”, such as 1AC-7AI; lS (d) preparation of dimers which are definitely planar or nonplanar (substitution, bridging). We are presently working along these lines.
-
Acknowledgment. We thank Prof. Z. R. Grabowski for helpful discussion. A. Mordzibski’s help in INDO calculations is fully appreciated. The work was done under project PAN 03.10.5.2.2. Registry No. 1AC, 244-76-8; 7A1, 271-63-6. (13) V. I. Pechenaya and V. I. Danilov, Chem. Phys. Lett., 11,539 (1971), V. I. Danilov, L. G. Ilchenko, and V. I. Pechenaya, Dopou. Akad. Nauk Ukr. RSR, Ser. A: Fiz.-Mat. Tekh. Nauki, 3, 253 (1978). (14) J. Catalan and P. Perez, J. Theor. Bioi., 81, 213 (1979). (15) J. Sepio4 and U. P. Wild, Chem. Phys. Lett., 93, 204 (1982).
