Ultrafast charge transfer studied by femtosecond IR-spectroscopy and ab initio calculations

Femtosecond vibrational spectroscopy is a versatile method to monitor excited state dynamics evolving in photochemical reactions. Applying high-level calculations to analyze infrared absorption spectra allows to elucidate molecular structures of the transient compounds and to assign normal modes to the absorption lines. Striking differences in the experimental vibrational pattern of the locally excited states of 4-(dimethyl-amino)benzonitrile (DMABN) and 4-aminobenzonitrile (ABN) (dissolved in acetonitrile) are explained on the basis of planar and pyramidal structures obtained from ab initio complete-active-space self-consistent-field (CAS SCF) calculations, giving evidence for a strong sensitivity of the molecular structure on modest changes in the substituents. Different models for the charge transfer state of DMABN are evaluated.