Modeling the IR Spectra of Excited Quadrupole Molecules with Broken Symmetry in Polar Solvents

An approach is developed that allows estimation of excited quadrupole molecule parameters responsible for changes in vibration frequencies in states with symmetry breaking by charge transfer from time-resolved IR spectra. The approach is tested on a molecule like A-π-D-π-A composed of an electron-accepting group A coupled with electron-donating group D by means of π-conjugated bonds, a D pyrrolopyrrole core, and two cyanophenyl acceptors. The expression derived for the IR spectrum of a molecule with symmetry breaking is shown to perfectly describe experimental data. The numerical values of the parameters of asymmetry in a series of solvents with different polarities and the solvent-independent parameters of the molecule itself are determined.