Nonadiabatic Molecular Dynamics Simulations Provide Evidence for Coexistence of Planar and Nonplanar Intramolecular Charge Transfer Structures in Fluorazene

Fluorazene is a model compound for photoinduced intramolecular
charge transfer (ICT) between aromatic moieties. Despite intensive
studies, both spectroscopic and theoretical, a complete model of its
photophysics is still lacking. Especially controversial is the geometry
of its ICT structure, or structures. In order to fill in the gaps
in the state of knowledge on this important model system, in the present
study I report the results of nonadiabatic molecular dynamics (NAMD)
simulations of its photorelaxation process in acetonitrile solution.
To afford a direct comparison to spectroscopic data, I use the simulation
results as the basis for the calculation of the transient absorption
(TA) spectrum. The NAMD simulations provide detailed information on
the sequence of events during the excited-state relaxation of the
title compound. Following initial photoexcitation into the bright
S2 state, the molecule undergoes rapid internal conversion
into the S1 state, leading to the locally excited (LE)
structure. The LE structure, in turn, undergoes isomerization into
a population of ICT structures, with geometries ranging from near-planar
to markedly nonplanar. The LE → ICT isomerization reaction
is accompanied by the decay of the characteristic excited-state absorption
band of the LE structure near 2 eV. The anomalous fluorescence emission
band of fluorazene is found to originate mainly from the near-planar
ICT structures, in part because they dominate the overall population
of ICT structures. Thus, the planar ICT (PICT) model appears to be
the most appropriate description of the geometry of the ICT structure
of fluorazene.