Ab Initio Investigation of Intramolecular Charge Transfer in Dmabn by Calculation of Transient X-Ray Absorption Features

Abstract

Dual fluorescence in dimethylaminobenzonitrile (DMABN) and its derivatives in polar solvents, has been studied extensively for the past several years. Intramolecular charge transfer (ICT), in addition to the localized low-energy (LE) valence 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 coupledcluster singles and doubles (EOM-CCSD) and time-dependent density functional (TD-DFT) methods to investigate the landscape of excited state potential energy surfaces across the several geometric conformations proposed as ICT structures, including Franck-Condon (FC), twisted (TICT), partially twisted (PTICT), wagged (WICT), and rehybridized (RICT) structures. Initial geometries for each type of structure were selected for excited state PES optimizations, as well as a systematic exploration of the low-lying PESs starting from the FC geometry. We find a number of minimum-energy (near)crossing points among the lowest three excited states leading to the eventual ICT minima. Finally, we have calculated the carbon and nitrogen K-edge transient absorption spectra for all important "signpost" structures in order to investigate the differential pump-probe features along the LE and ICT pathways.