Ab initio calculations and supersonic jet studies on the geometry of 4-dimethylaminobenzonitrile (DMABN) and related compounds in the ground and excited state

Ab initio calculations at different levels of theory were performed to elucidate the geometry of 4-dimethylaminobenzonitrile (DMABN), 4-aminobenzonitrile (ABN) and 1-ethyl-2,3-dihydro-indole-5-carbonitrile (EIN) in the ground and first excited singlet state. The quantum chemical results are compared with experimental vibrational frequencies observed in the resonant two-photon ionization and dispersed emission spectra of the jet-cooled molecules and by simulating rotational band contours of vibronic transitions. The theoretical and experimental results are in good overall agreement. ABN is planar in the first electronically excited state, while the dimethylamino group of DMABN is twisted by 22 degrees out of the phenyl ring plane. In the ground state the phenyl ring of EIN is distorted due to the five-membered ring. In the excited singlet state a further increase of this distortion of the molecular geometry takes place. The different geometries and the accompanying different electronic structures of these molecules may reflect the different photophysical behaviour in solution, which has been studied over the past three decades. (C) 1998 Elsevier Science B.V. All rights reserved.