COMMENT pubs.acs.org/JPCA
Reply to “Comment on ‘Dual Fluorescence of Ellipticine: Excited State Proton Transfer from Solvent versus Solvent Mediated Intramolecular Proton Transfer’” Sanghamitra Banerjee, Ashok Pabbathi, Chandra Sekhar M, and Anunay Samanta* School of Chemistry, University of Hyderabad, Hyderabad 500 046, India n their Comments1 on our recently published article, 2 Miskolczy et al. state that the absorption spectrum of ellipticine in methanol, as reported in their earlier article,3 is incorrectly represented in our paper. However, this is not true. Contrary to their statement “We did not claim that ellipticine protonation occurs in the ground state in neat methanol”,1 the 424 nm absorption maximum of ellipticine in neat methanol, as indicated in Table 1 of their work,3 clearly suggests ground-state protonation of ellipticine in this medium. Hence, the first paragraph of this Comment is inappropriate. Considering the fact that while postulating an alternative mechanism in our article we explicitly wrote2 “We take note of the argument used by Miskolczy et al. in support of their mechanism. However, we are left with no choice but to reconsider an alternative mechanism to account for the new results presented in this work. At this moment, we have no idea how to explain the dual fluorescence of 6-methylellipticine using our mechanism. This is an issue that needs to be looked into”, the second paragraph of the Comment is completely unwarranted. In the third paragraph of their Comment, Miskolczy et al. state “the long-wavelength fluorescence band in methanol cannot be assigned to a tautomer because (i) it matches the fluorescence spectrum of the protonated ellipticine (Figure 1) and (ii) the fluorescence decay time of the long-wavelength emission corresponds to that of protonated ellipticine...”. That these arguments do not unequivocally establish the protonation mechanism is in fact indirectly admitted by Miskolczy et al. in a later statement, “It is very unlikely that both the spectrum and the decay time of the fluorescence are identical for the ellipticine tautomer and protonated ellipticine”. Their subsequent statement “All papers on the fluorescence... β-carbolines” is not supported by the literature. In contrast, it is well-known that both the tautomer and cation can exhibit very similar fluorescence properties.4,5 In their earlier work,3 Miskolczy et al. reported single exponential decay for the short-wavelength fluorescence band. However, our observation was very different.2 We not only found that both bands display biexponential kinetics but also observed a rise component for the long-wavelength emission band. The latter offered the first direct evidence of the excited-state process and allowed estimation of its kinetics in neat methanol.2 As the observed decay behavior was inconsistent with the mechanism of Miskolczy et al.,3 we had to propose an alternative mechanism. In their Comment,1 Miskolczy et al. attempt to explain our findings by proposing two different solvated species. These two species are speculated as 1:1 and 1:2 ellipticine methanol complexes, where only one of them can undergo proton transfer due to correct orientation of the methanol molecule around ellipticine. To substantiate their explanation, they cite literature on the
excited-state proton-transfer reaction in two different β-carbolines in hexafluoro-2-propanol (HFP)6,7 but not in methanol, even though the hydrogen-bond-donation ability of HFP is very different from that of methanol. In our opinion, this explanation of Miskolczy et al. is far less realistic than the one that we proposed. In short, we do not find any additional data in this Comment1 as evidence that establishes the excited-state protonation of ellipticine unequivocally.
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r 2012 American Chemical Society
’ REFERENCES (1) Miskolczy, Z.; Biczok, L.; Jablonkai, I. J. Phys. Chem. 2012, DOI: 10.1021/jp209762w. (2) Banerjee, S.; Pabbathi, A.; Sekhar, M. C.; Samanta, A. J. Phys. Chem. A 2011, 115, 9217. (3) Miskolczy, Z.; Bicz ok, L.; Jablonkai, I. Chem. Phys. Lett. 2006, 427, 76. (4) Dias, A.; Varela, A. P.; Miguel, M. d. G.; Macanita, A. L.; Becker, R. S. J. Phys. Chem. 1992, 96, 10296. (5) Penedo, J. C.; Lema, I. G. a.; Lustres, J. L. P. r; Mosquera, M.; Rodríguez-Prieto, F. J. Phys. Chem. A 2005, 109, 10189. (6) Carmona, C.; Balon, M.; Galan, M.; Angulo, G.; Guardado, P.; Mun~oz, M. A. J. Phys. Chem. A 2001, 105, 10334. (7) Carmona, C.; Balon, M.; Coronilla, A. S. n.; Mun~oz, M.A. J. Phys. Chem. A 2004, 108, 1910.
Received: December 9, 2011 Published: January 10, 2012 901
dx.doi.org/10.1021/jp211859q | J. Phys. Chem. A 2012, 116, 901–901
