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

Abstract

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.

A Biomimetic Model of the Electron Transfer between P680 and the TyrZ–His190 Pair of PSII

“…This supports the fact that the two aromatic rings adopt a coplanar orientation in solution through a strong intramolecular hydrogen bond between the phenolic OH group and the imidazole nitrogen atom. [6] This is confirmed by the 1 H NMR spectrum of 1 in [D 6 ]DMSO, in which a dissymmetry of the phenanthroline protons is apparent. In the solid state, the presence of intramolecular hydrogen bonding is supported by the absence of OH group vibration characteristics in the IR spectrum of 1; indeed these features are present in the IR spectroscopy data for 2 (Supporting Information).…”

Advanced Organic Optoelectronic Materials: Harnessing Excited‐State Intramolecular Proton Transfer (ESIPT) Process

“…[ 42 ] Among various ESIPT molecules, HBI was studied in aqueous solution as a prototypical model to investigate prototropic equilibria and photophysical properties of the three protomers. (protonated C , neutral, deprotonated A , Scheme 4 ) [ 43 ] In neutral aqueous solution, HBI exhibited typical ESIPT behavior with a π -π * transition peak at 320 nm in the absorption spectrum and dual bands in the emission spectrum which comprise a weak E * emission at 350 nm and a large Stokes' shifted ( > 100 nm) K * emission at 450 nm, respectively ( Figure 3 a,e).…”

Phototransformation of 4-phenoxyphenol sensitised by 4-carboxybenzophenone: Evidence of new photochemical pathways in the bulk aqueous phase and on the surface of aerosol deliquescent particles

“…In the case of 4PP, the presence of two Em bands (∼320 and ∼380 nm) instead of only one could be explained by a process of photoinduced intra-or intermolecular proton transfer. In that case, a part of the molecule in the excited state acts as proton donor and another part (or another molecule) as proton acceptor, with formation of two different species with differing emission spectra (Mosquera et al, 1996). Another possible issue is that 4PP is a dimeric species (phenol dimer) that could have two fluorophores, one for each of the phenolic moieties.…”