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This is the peer-reviewed version of the following article: The Journal of Physical Chemistry B 2013, 117, 884–896, DOI:10.1021/jp311709c, which has been published in final form at https://pubs.acs.org/doi/abs/10.1021/jp311709c. This article may be used for non-commercial purposes only ; 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 ; This work has been supported by the Spanish Government and the European Regional Development Fund (Grant CTQ2010-17835) and the Xunta de Galicia (Grant CN 2012/314). A.B., M.V., and J.L.P.L. are thankful for a "Fundación ...

This is the peer-reviewed version of the following article: The Journal of Physical Chemistry Letters 2014, 5, 989–994, DOI:10.1021/jz5001648, which has been published in final form at https://pubs.acs.org/doi/abs/10.1021/jz5001648. This article may be used for non-commercial purposes only ; Proton transfer from strong photoacids to hydroxylic solvents is much under debate. Experimentally, the main issue stems from relaxation and diffusion processes that are concomitant with ultrafast proton transfer and blur population dynamics. To overcome this, we propose a fast photodissociation reaction that, however, proceeds slower than solvent relaxation. Fluorescence spectroscopy of the cationic photoacid 2-(1′-hydroxy-2′-naphtyl)benzimidazolium reveals a two-stage mechanism: (a) reversible elementary proton transfer inside the solvent shell and (b) irreversible contact-pair splitting. The time evolution of the fluorescence signal is complex, yet this is explained quantitatively by simultaneous, spectrally overlapping emission of the acid, the conjugate base, and the contact proton-transfer pair. The latter attains high transient concentration in linear alcohols. Microscopic rate constants of dissociation are determined ; We thank the Spanish Government and the European Regional Development Fund (Grants CTQ2010-17835 and CTQ2010-17026) and the Xunta de Galicia (Grants CN 2012/314,EM2012/091, and GPC2013/052) for financial support of our work. J.L.P.L. thanks the Spanish Ministry of Economy and Competitiveness for funding through the Ramón y Cajal Programm 2009. M.V.G. and A.B. thank the Spanish Government and the "Segundo Gil-Dávila" Foundation, respectively, for financial support ; SI