J. Phys. Chem. 1995, 99, 2241
Reply to “Comment on Excited-State Acid-Base Kinetics and Equilibria in Norharmane” Sonja Draxler and Max E. Lippitsch” Znstitut f i r Experimentalphysik, Karl-Franzens Universitat, Universitatplatz 5, A-8010 Graz, Austria Received: July 28, 1994 When writing our paper on norharman,’ we were aware of the work published by the group from Portugal. This is reflected in the citation (ref 16 in our paper). However, we do not agree with their results. For norharmane in methanol they claim a triple-exponential decay2 with 2.8, 5.4, and 23 ns components (single exponential with 2.8 ns as stated in the Comment only at 365 nm). Since 23 ns is the decay time of the cation, this would imply that in methanol the cationic form is present in a considerable amount. Clearly this is not true for the ground state, as is proven by the absorption spectrum (Figure 2, 1 in ref 2). So one would be forced to assume that a protonation reaction occurs in the excited state, with the solvent as the proton donor. Acidity constants are very small (of the order for alcohols, however, so that the reaction could only occur if excited neutral norharmane were a unreasonably strong base (pKa = 16 or higher). The fluorescence spectrum of norharmane in methanol shows a considerable long-wavelength part, but this clearly does not reflect the presence of the cation, since the cation emission has its maximum at 450 nm, while the longwavelength emission in methanol has two maxima (430 and 490 nm). We suspect that the quality of the decay curves used in that paper did not allow a clear distinction between single and multiple exponentials and that the authors decided in favor of the three-exponential fit because of the erroneous assumption of the presence of a cation emission. This supposition is corroborated by the data given in the Comment. We feel that a decay curve with 20 000 counts in the maximum is insufficient to postulate decay components with amplitudes 0.009 and 0.002, as done in Figure 1 (370 nm) in the Comment. Assuming only Gaussian noise would give a noise level of 0.007. In fact, the curves show a noise level of that magnitude, as can be easily verified from the figures. In addition, the results presented in Figures 1 and 3 are not easily understood. Figure 3b gives an amplitude ratioA3lA1 of 500 at pH 10. A comparison with Figure 1 (370 nm) shows that A3 must be the shortest component and A1 the longest one. This would imply, however, that the amplitude of the long component, which clearly is due to the cation, goes up with increasing
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pH. To have a higher cation concentration at higher pH is quite unreasonable. The figures and the text report the decay to be independent of pH, in decay times as well as amplitudes, between pH 6 and 10, which is clearly at variance with the fact that the fluorescence emission of the cation decreases strongly within that range of pH. The reasons why Varela et al. obtained so different results are not known to us. We only have experienced that norharmane solutions are not as stable as one would expect and that aged solutions differ in the absorption as well as the decay properties. The second part of the Comment criticizes our evaluation. It has to be pointed out that the approach is not new (see ref 20 in ref 1) and that we are well aware of the uncertainties envolved, as is explicitly stated in our paper. Even taking into account the error propagation, however, we feel that the results have some real significance. In our set of rate equations, we have four unimolecular rate constants (which are not identical, as is supposed in the Comment, with the reciprocal measured fluorescence lifetimes, since there are competing nonradiative processes) and 12 pH-dependent rate parameters. These parameters can be expressed, as is done in our eq 3, by two (one pseudounimolecular and one bimolecular) rate constants. So we have 28 parameters to be determined. The four rate equations were applied to results obtained at eight different pH values; hence we had 32 equations. Thus, the problem is, in fact, “overdefined”. The reference to the paper of Seixas de Melo and Macanita3 in the Comment is irrelevant, since it deals with 7-hydroxy-4-methylcoumarin, where the situation is different than in norharmane (as is pointed out in our paper’ when mentioning ref 20, which in essence has the same content as ref 3 but appeared 14 years earlier). Nevertheless, we agree that the rate constants obtained are of very uncertain character. It is the more satisfactory that the pKa* values extracted from them are in good agreement with Forster-Weller calculations (as far as they are applicable, that is, for the equilibria between cation and neutral molecule as well as between neutral molecule and anion).
References and Notes (1) Draxler, S.; Lippitsch, M. E. J. Phys. Chem. 1993, 97, 1149311496. (2) Dias, A,; Varela, A. P.; da G. Miguel, M.; Macanita, A. L.; Becker, R, s. J , phys, Chem. 1992, 96, 10290-10296~ (3) Seixas de Melo, J.; Macanita, A. L. Chem. Phys. Lett. 1993, 204, 556-562.
0 1995 American Chemical Society
