COMMENT pubs.acs.org/JPCA
Comment on “Dual Fluorescence of Ellipticine: Excited State Proton Transfer from Solvent versus Solvent Mediated Intramolecular Proton Transfer” Zsombor Miskolczy, Laszlo Biczok,* and Istvan Jablonkai Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary
B
Scheme 1
anerjee et al. have recently reported in this journal the fluorescent behavior of ellipticine in various solvents.1 They confirmed our previous findings2 that dual fluorescence is emitted in methanol and ethylene glycol, whereas protonated ellipticine is produced in the ground state in hexafluoro2-propanol. However, our result on the ellipticine absorption spectrum in methanol is incorrectly represented in ref 1. We did not claim that ellipticine protonation occurs in the ground state in neat methanol.2 We emphasized that “Despite the negligible ground-state protonation of E (ellipticine) and ME (6-methylellipticine) in methanol, two fluorescence bands were detected, suggesting that the basicity increase upon light absorption facilitates the protonation in the excited state.” Moreover, Figure 1a in ref 2 demonstrates that no band appears above 430 nm in the absorption spectrum of ellipticine in methanol. This is in accordance with the spectrum presented by Banerjee et al. It is obvious that the absorption maxima in methanol given in Table 1 in ref 2 correspond to the two lowest-energy absorption maxima of the spectra shown in Figure 3a therein. The text and Figure 3a in ref 2 clearly show that no absorption is detected above 430 nm in neat methanol and the longwavelength absorption band emerges only upon addition of trifluoroacetic acid (TFA). In ref 1, the long-wavelength fluorescence in methanol was attributed to a tautomer, which is formed by solvent-mediated excited-state intramolecular proton transfer from the pyrrole nitrogen to the pyridine nitrogen. The long-distance process was proposed to occur via a hydrogen-bonded methanol chain connecting the two nitrogen atoms of ellipticine. This hypothesis is inconsistent with the most important result reported in ref 2. Namely, we found that the photophysical behavior of ellipticine and its 6-methyl derivative (Scheme 1) are analogous, and both compounds emit dual fluorescence in methanol and ethylene glycol. This is convincing evidence against the reaction mechanism proposed by Banerjee et al.1 The N-methyl substitution of the pyrrole ring removes the sole dissociable hydrogen from the molecule, precluding thereby the possibility of the photoinduced intramolecular proton transfer to the pyridyl nitrogen. Therefore, we ascribed the dual fluorescence to excited-state protonation by the solvent.2 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 the protonated ellipticine (τf = 8.5 ns) formed in the presence of 11 μM TFA. Figure 1A displays the resolution of the fluorescence spectrum of ellipticine in methanol. As we have shown,2 the long-wavelength band gradually vanishes upon r 2011 American Chemical Society
addition of tetrabutylammonium hydroxide (Bu4NOH). In the presence of 0.12 M Bu4NOH, only short-wavelength fluorescence is observed. The subtraction of this emission from the fluorescence spectrum of ellipticine in neat methanol provides the spectrum of the long-wavelength fluorescence, which matches the spectrum of the protonated ellipticine in the presence of 11 μM TFA (Figure 1B). The difference of the two spectra is displayed as a function of the wavelength in the lowest panel of Figure 1. The negligible deviation is strong evidence that the long-wavelength emission is due to the protonated ellipticine instead of the tautomer proposed in ref 1. It is very unlikely that both the spectrum and the decay time of the fluorescence are identical for the ellipticine tautomer and protonated ellipticine. Therefore, the long-wavelength emission cannot be assigned to the tautomer. All papers on the fluorescence of β-carbolines, the alkaloids structurally related to ellipticine, agree that the fluorescence lifetimes of the tautomer and the protonated form are markedly different. Moreover, it is generally accepted that the fluorescence band of the tautomer is significantly red-shifted compared to the band of the protonated species for all β-carbolines. The faster rise of the long-wavelength emission in ethylene glycol compared to that in methanol1 corroborates that photoinduced intermolecular proton transfer occurs from the solvent to the singlet excited ellipticine. Because ethylene glycol is a stronger acid than methanol, more rapid excited-state protonation is expected and indeed found in the former solvent. For the relative acidity of ethylene glycol compared to that of methanol, about 6-fold and 10-fold larger values were reported in water3 and isopropanol,4 respectively. Due to the significantly larger acidity, the long-wavelength absorption band emerges above 420 nm in ethylene glycol, indicating the partial protonation of ellipticine in the ground state. Such an effect is not observed in the less acidic methanol. Received: October 5, 2011 Revised: November 30, 2011 Published: December 15, 2011 899
dx.doi.org/10.1021/jp209762w | J. Phys. Chem. A 2012, 116, 899–900
The Journal of Physical Chemistry A
COMMENT
bond of the 1:1 complex enhances the acidity of the HO moiety, thereby facilitating the proton transfer in the excited state.5,6 The slow buildup of the emission at long wavelength was attributed to the formation of the excited hydrogen-bonded protonated alkaloid via the reaction of the excited 1:2 alkaloid (HFIP)2 complex with HFIP.5,6 The results on the time-resolved fluorescence behavior of MBC and MHN in the presence of HFIP closely resemble those described in ref 1 for ellipticine in methanol. This probably indicates that the main features of the reaction mechanism proposed by Carmona et al. are also applicable to ellipticine. The 1:1 binding of ellipticine with methanol is not enough to bring about proton transfer in the excited state. The hydrogen bonding of methanol molecules to the oxygen of the N 3 3 3 H O bond of the 1:1 complex is needed to induce excited-state proton transfer. The stepwise formation of such an excited complex, in which the number and the orientation of the methanol molecules allow proton transfer, has relatively low probability, and consequently, slow proton transfer occurs. In summary, all experimental results published in refs 1 and 2 suggest that the dual fluorescence of ellipticine in methanol originates from the excited-state intermolecular proton transfer from the solvent to the nitrogen of the six-membered heterocyclic ring of the fluorophore. The spectrum and decay time of the long-wavelength fluorescence in methanol are identical to those of protonated ellipticine. This fact and the analogous fluorescent behavior of ellipticine and its 6-methyl derivative are unambiguous evidence against the reaction mechanism proposed by Banerjee et al. in ref 1.
Figure 1. (A) Fluorescence spectra of ellipticine in methanol (green), in the presence of 0.12 M tetrabutylammonium hydroxide (red), and the difference between the green and red spectra (blue). Excitation at 372 nm. (B) Fluorescence spectrum of protonated ellipticine recorded in the presence of 11 μM trifluoroacetic acid (black). The blue spectrum is the same as that in panel (A). The difference between the black and blue spectra is shown in the lowest panel.
’ AUTHOR INFORMATION It is well-known that the acidity strongly depends on the medium. As the proton-donating ability of alcohols diminishes in mixtures with less polar solvents, no long-wavelength fluorescence is emitted in tetrahydrofuran methanol and acetonitrile methanol mixtures. The more acidic ethylene glycol is able to protonate ellipticine both in the ground and the singlet excited states even in solvent mixtures. Banerjee et al. showed that the long-wavelength emission relative to the short-wavelength emission is more intense in ethylene glycol than that in methanol.1 This also confirms the excited ellipticine protonation by the solvent. The more acidic ethylene glycol donates a proton more efficiently, enhancing thereby the concentration of the protonated singlet excited ellipticine at the expense of the unprotonated species. The dual exponential character of the time-resolved fluorescence traces1 can also be rationalized on the basis of the photoinduced intermolecular proton-transfer mechanism. Two types of solvated ellipticine seem to exist in methanol. The first type emits the longer-lived fluorescence (τf = 3.5 ns) because the orientations of the methanol molecules in the solvate shell do not allow proton transfer upon excitation. The other type of solvated ellipticine has a 2.2 ns fluorescence lifetime and undergoes protonation in the excited state, which is indicated by the rise of the fluorescence intensity in the long-wavelength range. Previous studies on the structurally related alkaloids, 1,9-dimethyl9H-pyrido[3,4-b]indole (9-methylharmane, MHN) and 9-methyl9H-pyrido[3,4-b]indole (9-methy-β-carboline, MBC) have suggested a similar reaction mechanism.5,6 Carmona et al. demonstrated that photoinitiated proton transfer does not take place in the case of the 1:1 hydrogen-bonded complexes of MHN and MBC with HFIP. However, the hydrogen bonding of additional HFIP molecules to the oxygen of the N 3 3 3 H O
Corresponding Author
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[email protected].
’ ACKNOWLEDGMENT The authors very much appreciate the support of this work by the Hungarian Scientific Research Fund (OTKA, Grant K75015). ’ REFERENCES (1) Banerjee, S.; Pabbathi, A.; Sekhar, M. C.; Samanta, A. J. Phys. Chem. A 2011, 115, 9217–9225. (2) Miskolczy, Z.; Biczok, L.; Jablonkai, I. Chem. Phys. Lett. 2006, 427, 76–81. (3) Ballinger, P.; Long, F. A. J. Am. Chem. Soc. 1960, 82, 795–798. (4) Hine, J.; Hine, M. J. Am. Chem. Soc. 1952, 74, 5266–5271. (5) Carmona, C.; Balon, M.; Galan, M.; Angulo, G.; Guardado, P.; Mu~ noz, M. A. J. Phys. Chem. A 2001, 105, 10334–10338. (6) Carmona, C.; Balon, M.; Coronilla, A. S.; Mu~ noz, M. A. J. Phys. Chem. A 2004, 108, 1910–1918.
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dx.doi.org/10.1021/jp209762w |J. Phys. Chem. A 2012, 116, 899–900
