5398
J. Phys. Chem. 1996, 100, 5398-5407
Photoinduced Inter- and Intramolecular Proton Transfer in Aqueous and Ethanolic Solutions of 2-(2′-Hydroxyphenyl)benzimidazole: Evidence for Tautomeric and Conformational Equilibria in the Ground State† Manuel Mosquera,* J. Carlos Penedo, M. Carmen Rı´os Rodrı´guez, and Flor Rodrı´guez-Prieto* Departamento de Quı´mica Fı´sica, Facultade de Quı´mica, UniVersidade de Santiago de Compostela, E-15706 Santiago de Compostela, Spain ReceiVed: NoVember 15, 1995; In Final Form: January 16, 1996X
Excited-state proton transfer in aqueous and ethanolic solutions of 2-(2′-hydroxyphenyl)benzimidazole (HBI) was investigated by means of UV-vis absorption and fluorescence spectroscopy. The behavior of HBI in water differed from its behavior in ethanol, and in both solvents fluorescence behavior depended on acidity. In both neutral water and neutral ethanol, ground-state HBI exhibits conformational equilibrium between a cis-enol form with an intramolecular hydrogen bond and a trans-enol form that is hydrogen-bonded to the solvent; the ground-state keto tautomer is also present in water but was not detected in ethanol. The excited cis-enol conformer always undergoes ultrafast intramolecular proton transfer to afford the excited keto tautomer. The excited trans-enol form fluoresces in both solvents, and in water it also loses its hydroxyl proton to the solvent, leaving the excited anion. In both acidic aqueous solution and acidic ethanol, excited protonated HBI loses its hydroxyl proton to give the excited keto form, but this process is faster in water than in ethanol, in which fluorescence by the cation is also observed.
Introduction Organic molecules with acidic and basic functional groups often undergo proton transfer processes when photoexcitation changes their charge density distribution and hence the acidbase properties of the groups in question. Among such molecules are 2-(2′-hydroxyphenyl)benzimidazole (HBI), 2-(3′hydroxy-2′-pyridyl)benzimidazole (HPyBI), 2-(2′-hydroxyphenyl)benzoxazole (HBO), and 2-(2′-hydroxyphenyl)benzothiazole (HBT) (Figure 1). In the ground state, the most stable form of these molecules is usually the normal or enolic form (E), in which they feature an acidic hydroxyl group and a basic nitrogen atom (N(3)). In the first excited singlet state, in which the hydroxyl group is much more acidic and the nitrogen much more basic than in the ground state, the most stable structure is usually the keto tautomer K (which also has zwitterionic resonance forms). As a result, excitation of the enol form tends to be followed by transformation to the keto form in the excited state. This phototautomerization process may course via excited-state intramolecular proton transfer (ESIPT) if the groups involved are suitably arranged in space (in which case they will probably already be linked by a hydrogen bond). Such processes, which are usually ultrafast, are the object of considerable current interest.1-5 Molecules featuring intramolecular hydrogen bonds in nonpolar solvents may lack such bonds in hydroxylic solvents that are themselves capable of forming hydrogen bonds with the relevant groups of the molecule in question. As a result, ESIPT processes that take place along intramolecular hydrogen bonds in nonpolar solvents may have to compete with intermolecular proton transfer to or from the solvent in hydroxylic solvents. In particular, the special properties of water (high polarity, capacity for hydrogen bond formation, and capacity to act as both acid and base), which lend it great influence over protontransfer processes of all kinds, mean that knowledge of ESIPT † Dedicated to the Royal University of Santiago de Compostela on the occasion of its Fifth Centenary. X Abstract published in AdVance ACS Abstracts, March 1, 1996.
0022-3654/96/20100-5398$12.00/0
Figure 1. Molecular structures of the species studied in this work and related molecules.
processes occurring in other solvents is unlikely to be extrapolatable to water. In the research carried out in our laboratory on the influence of hydroxylic solvents on ESIPT processes, the first species studied was HPyBI,6 the ground-state enol and keto forms of which proved to be coexistent and almost isoenergetic in water (whereas in other solvents the keto form was not detected in the ground state). Moreover, the excited enol form surprisingly underwent ESIPT with almost 100% efficiency in both hydroxylic and nonhydroxylic solvents (whereas, for other species, ESIPT processes that occur in nonpolar solvents are almost totally inhibited in hydroxylic solvents).7 In order to throw light on the unexpected behavior observed in both the ground and excited states, we have now investigated the behavior of HBI in water and ethanol. HBI was chosen because it is structurally similar not only to HPyBI but also to HBO and HBT, whose intramolecular proton-transfer processes have been intensively studied in singlet and triplet states.8-23 HBI nevertheless differs from HBO and HBT in its much greater fluorescence quantum yield at room temperature, which facilitates study of proton transfer in the first excited singlet state (for the same reason, HBI has been proposed as a suitable proton-transfer laser dye).24-26 © 1996 American Chemical Society
2-(2′-Hydroxyphenyl)benzimidazole Proton Transfer
J. Phys. Chem., Vol. 100, No. 13, 1996 5399
ESIPT in HBI has been the subject of several recent papers.27-31 The study described here was aimed specifically at establishing (a) the nature of the species present in the ground state in aqueous and ethanolic solutions of HBI and (b) the characteristics of the photoinduced proton transfer processes undergone by these species. To this end, we investigated the UV-vis absorption spectra and steady-state and time-resolved fluorescence of HBI in aqueous and ethanolic solutions of various acidities. In order to corroborate certain interpretations, we also investigated the fluorescence behavior of the methoxy derivative 2-(2′-methoxyphenyl)benzimidazole (MBI; Figure 1). Our results show that in neutral ethanol and neutral water ground-state HBI exhibits a conformational equilibrium between two enol forms that has not been detected in nonpolar media and that in neutral water there are also tautomeric equilibria between the enol forms and the keto tautomer. Identification of the various tautomers and conformers was facilitated by the differences among their fluorescence characteristics, which derive from their undergoing different ESIPT processes under photoexcitation. In addition, excited-state transfer of protons to the solvent ocurrs more readily in water than in ethanol because of the greater basicity and ion-stabilizing capacity of water. Experimental Section HBI was synthesized by condensation of 2-hydroxybenzoic acid with 1,2-phenylenediamine (both from Aldrich) at high temperature;32 the crude product was purified by recrystallization from methanol and column chromatography. MBI was synthesized by condensation of 2-methoxybenzaldehyde with 1,2phenylenediamine in acidified ethanol;32 the precipitate was purified by column chromatography. Solutions were made up as described elsewhere6 for HPyBI in spectroscopy grade ethanol and double distilled water; all aqueous solutions contained 2% of ethanol (by volume) because HBI and MBI, which are poorly soluble in water, were initially dissolved in the alcohol. No solutions were deoxygenated. Depending on the pH required, acidity was controlled with HClO4, NaOH, or acetic acid/sodium acetate, NaH2PO4/Na2HPO4 or ammonium perchlorate/ammonia buffers (all made up with Merck p.a. products); buffer concentrations were of the order of 10-3 mol dm-3, preliminary experiments having shown that the results were not affected by the buffer at these concentrations but were slightly affected at higher concentrations. Ionic strength was maintained at 0.1 mol dm-3 with Merck p.a. NaClO4. All experiments were carried out at 25 °C. pH was measured with a Radiometer PHM-82 pH meter. UV-vis absorption spectra were recorded in Kontron Uvikon 810 and 930 spectrophotometers. Fluorescence excitation and emission spectra were recorded in a Spex Fluorolog-2 FL340 E1 T1 spectrofluorometer, with correction for instrumental factors by means of a Rhodamine B quantum counter and correction files supplied by the manufacturer. Fluorescence lifetimes were determined by single photon timing in an Edinburgh Instruments CD-900 spectrometer equipped with a hydrogen-filled nanosecond flash lamp and the analysis software supplied by the manufacturer. The emission band pass for the lifetime measurements was usually 15 nm. Fluorescence quantum yields were determined using quinine sulfate (
