jp505036d_si_002.avi (5.27 MB)
TD-DFT Assessment of the Excited State Intramolecular Proton Transfer in Hydroxyphenylbenzimidazole (HBI) Dyes
media
posted on 2015-02-12, 00:00 authored by Ymène Houari, Siwar Chibani, Denis Jacquemin, Adèle D. LaurentDyes
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.