Intramolecular charge transfer with 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) and other aminobenzonitriles. A comparison of experimental vapor phase spectra and crystal structures with calculations.

Calculations of molecular structures in the electronic ground state S(0) and of excited state and fluorescence energies generally refer to the gas phase. This complicates a comparison with experimental data, which often are only available for molecules in solution. Therefore, experimental absorption and fluorescence spectra in the vapor phase are presented for 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6), 1-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMC6), 4-(dimethylamino)benzonitrile (DMABN), and 4-(diisopropylamino)benzonitrile (DIABN). NTC6 and DIABN show a dual fluorescence in the gas phase, with emissions from an intramolecular charge transfer (ICT) and a locally excited (LE) state, whereas with NMC6 and DMABN only LE emission is observed. For a comparison of the experimental molecular structure in S(0) with the results of recent computations, X-ray crystal structures of NTC6, NMC6, and several analogues are presented. For DMABN, NMC6, and NTC6, LE/ICT energy diagrams are constructed, in which the experimental energies of the Franck-Condon singlet excited states S(1) and S(2), and the LE and ICT states together with their emissions, are compared with the calculations. The LE and ICT dipole moments are also discussed. This comparison reveals substantial differences, in particular for the ICT energies, but even for the structure of the S(0) ground states. It is concluded that the computed ICT states of NTC6 and DMABN, in which the full conjugation of the phenyl ring is interrupted, is different from the ICT states measured in the experiments.