TD-DFT assessment of the excited state intramolecular proton transfer in hydroxyphenylbenzimidazole (HBI) dyes.

Dyes undergoing excited state intramolecular proton transfer (ESIPT) received increasing attention during the last decades. If their unusual large Stokes shifts and sometimes dual-fluorescence signatures have paved the way toward new applications, the rapidity of ESIPT often prevents its investigation with sole experimental approaches, and theoretical simulations are often welcome, if necessary, to obtain a full rationalization of the observations. In the present paper, we evaluate both the absorption and the fluorescence spectra of, respectively, the enol and keto forms of a series of hydroxyphenylbenzimidazole (HBI) using a robust protocol based on Time-Dependent Density Functional Theory (TD-DFT). Optical spectra were obtained accounting for both vibronic and environmental effects. The aim of this work is therefore not to evaluate the radiationless pathway going through the twisted ESIPT structures, though excited-state reaction paths between enol and keto forms have been rationalized. First we have compared three dyes differing by the strength of the donor groups, and we have quantified the impact of the flexible butyl chain substituting the imidazole side. In accordance with experiments, we show that the presence of a dialkylamino auxochrome allows to tune the excited-state potential energy surface leading to a weaker tendency to ESIPT. This trend is rationalized in terms of both structural and electronic effects. Next, larger hydroxyphenyl-phenanthroimidazole (HPI) were considered to assess the impact of a stronger π-delocalization. 0-0 energies and vibrationally resolved spectra of the corresponding fluoroborate derivatives were studied as well. The dialkylamino auxochrome significantly decreases the 0-0 energies due to the presence of an important charge transfer character, while the addition of a BODIPY moiety induces a change of the emission signature now localized on the BODIPY side rather than on the NBO core.