Ab Initio Investigation of Intramolecular Charge Transfer States in DMABN by Calculation of Excited State X‑ray Absorption Spectra

Dual fluorescence in 4-(dimethylamino)benzonitrile (DMABN)
and
its derivatives in polar solvents has been studied extensively for
the past several decades. An intramolecular charge transfer (ICT)
minimum on the excited state potential energy surface, in addition
to the localized low-energy (LE) minimum, has been proposed as a mechanism
for this dual fluorescence, with large geometric relaxation and molecular
orbital reorganization a key feature of the ICT pathway. Herein, we
have used both equation-of-motion coupled-cluster with single and
double excitations (EOM-CCSD) and time-dependent density functional
(TDDFT) methods to investigate the landscape of excited state potential
energy surfaces across a number of geometric conformations proposed
as ICT structures. In order to correlate these geometries and valence
excited states in terms of potential experimental observables, we
have calculated the nitrogen K-edge ground and excited state absorption
spectra for each of the predicted “signpost” structures
and identified several key spectral features that could be used to
interpret a future time-resolved X-ray absorption experiment.