  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 which could
be used to interpret a future time-resolved x-ray absorption experiment.
