Excited States of 4-Aminobenzonitrile (ABN) and 4-Dimethylaminobenzonitrile (DMABN): Time-resolved Resonance Raman, Transient Absorption, Fluorescence, and ab Initio Calculations

Abstract

Structures of the locally excited (LE) states of 4-aminobenzonitrile (ABN) and 4-dimethylaminobenzonitrile (DMABN) have been studied by picosecond time-resolved resonance Raman and transient absorption spectroscopy. Contrary to reported time-resolved infrared experiments, our time-resolved resonance Raman spectra show characteristic frequency shifts of several modes from their ground-state values. Combined with ab initio geometric and vibrational analysis of the ground and LE states, our results indicate a similar planar structure for the LE states of both ABN and DMABN. In the LE state, the pyramidal conformation of the ground state flattens along the inversion coordinate, the phenyl ring expands, and the ph-N bond shortens. Partial charge transfer in the LE state, mainly from the amino nitrogen lone pair orbital to the ring π* orbital, is indicated by the observation of a frequency downshift of the ring CC stretching mode and an ∼30 cm-1 downshift of the C⋮N stretching mode. This is supported by calculated changes of electron distribution in molecular orbitals upon excitation.