Picosecond Infrared Spectra of Isotope-Substituted 4-(Dimethylamino)benzonitriles and Molecular Structure of the Charge-Transfer Singlet Excited State

Picosecond transient infrared spectra of 4-(dimethylamino)benzonitrile (DMABN) and several isotope-substituted samples have been recorded in the fingerprint region in acetonitrile solutions. Among several strong transient infrared bands of the intramolecular charge transfer (ICT) excited state, the band at 1276 cm-1 (for normal species) shifts to lower frequencies on 15N, 13C, and deuterium substitution of the dimethylamino group, and is assigned to the ring C−NMe2 single-bond stretch. Other bands observed are attributable to vibrational modes of the benzonitrile moiety. The band frequencies obtained, together with those found in the literature, are compared with results of vibrational analyses based on CIS/6-31G level ab initio molecular orbital calculations. This treatment has made possible assignments of the bands due to the benzonitrile moiety. On the basis of the assignments, electronic structure of the ICT state is discussed. The electronic structure of the ICT state is suggested to be basically of benzenoid nature, with a significant contribution from quinoidal structure. An advanced theoretical treatment may be needed to obtain final conclusion on the structure (planar or twisted) of the ICT state.