title: Photoinduced proton and charge transfer in 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine.
abstract: This paper deals with the interplay between solvent properties and isomerism of 2-(2'-hydroxyphenyl)imidazo[4,5-b]pyridine (1), and the proton and charge-transfer processes that the different isomers undergo in the first-excited singlet state. We demonstrate the strong influence of these processes on the fluorescence properties of 1. We studied the behavior of 1 in several neutral and acidified solvents, by UV-vis absorption spectroscopy and by steady-state and time-resolved fluorescence spectroscopy. The fluorescence of 1 showed a strong sensitivity to the environment. This behavior is the result of conformational and isomeric equilibria and the completely different excited-state behavior of the isomers. For both neutral and cationic 1, isomers with intramolecular hydrogen bond between the hydroxyl group and the benzimidazole N undergo an ultrafast excited-state intramolecular proton transfer (ESIPT), yielding tautomeric species with very large Stokes shift. For both neutral and cationic 1, isomers with the OH group hydrogen-bonded to the solvent behave as strong photoacids, dissociating in the excited state in solvents with basic character. The pyridine nitrogen exhibits photobase character, protonating in the excited state even in some neutral solvents. An efficient radiationless deactivation channel of several species was detected, which we attributed to a twisted intramolecular charge-transfer (TICT) process, facilitated by deprotonation of the hydroxyl group and protonation of the pyridine nitrogen.
authors.affiliation: Departamento de Química Física and Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CIQUS), Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain.
authors.forename: Alfonso
authors.initials: A
authors.lastname: Brenlla
authors.forename: Manoel
authors.initials: M
authors.lastname: Veiga
authors.forename: J Luis
authors.initials: JL
authors.lastname: Pérez Lustres
authors.forename: M Carmen
authors.initials: MC
authors.lastname: Ríos Rodríguez
authors.forename: Flor
authors.initials: F
authors.lastname: Rodríguez-Prieto
authors.forename: Manuel
authors.initials: M
authors.lastname: Mosquera
mesh_terms: D001562:Benzimidazoles / Q000737:chemistry; D006860:Hydrogen Bonding; D007536:Isomerism; D011522:Protons; D011725:Pyridines / Q000737:chemistry*; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry; D013050:Spectrometry, Fluorescence
chemical_list: D001562:Benzimidazoles; D011522:Protons; D011725:Pyridines; D012997:Solvents; C031000:benzimidazole; C023666:pyridine