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 (DMA BN) and several isotope-substituted samples have been recorded in the finge rprint region in acetonitrile solutions. Among several strong transient inf rared bands of the intramolecular charge transfer (ICT) excited state, the band at 1276 cm(-1) (for normal species) shifts to lower frequencies on N-1 5,C-13,and deuterium substitution of the dimethylamino group, and is assign ed to the ring C-NMe2 single-bond stretch. Other bands observed are attribu table to vibrational modes of the benzonitrile moiety, The band frequencies obtained, together with those found in the literature, are compared with r esults 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, elec tronic structure of the ICT state is discussed. The electronic structure of the ICT state is suggested to be basically of benzenoid nature, with a sig nificant contribution from quinoidal structure. An advanced theoretical tre atment may be needed to obtain final conclusion on the structure (planar or twisted) of the ICT state.