INTRAMOLECULAR ELECTRON-TRANSFER AND DUAL FLUORESCENCE - SOLVENT-INDUCED 2-LEVEL COUPLING

The magnitude of the energy gap Delta E(S-1,S-2) determines the occurr ence of dual fluorescence with aminobenzonitriles. It is shown that du al emission does not take place with molecules such as 4-aminobenzonit rile (ABN) and 6-cyanobenzoquinuclidine (CBQ), for which Delta E(S-1,S -2) is too large for efficient vibronic coupling. For 4-aminobenzonitr iles in which the amino nitrogen atom is part of an eight- to three-me mbered methylene ring, P8C to P3C, intramolecular charge transfer (ICT ) strongly decreases with decreasing ring size. With P3C in acetonitri le ICT does not occur. This is attributed to the increase in the serie s P8C-P3C of the energy barrier for the change of the amino nitrogen c onfiguration from pyramidal towards planar, which is considered to be an important reaction coordinate in the ICT process. It is therefore c oncluded that for the occurrence of ICT and dual fluorescence in amino benzonitriles two requirements hold: (1) a sufficiently small energy g ap between two interacting excited states and (2) the presence of a pr omoting mode such as the change in configuration of the amino nitrogen from pyramidal towards planar. The general validity of the requiremen ts (1) and (2) is supported by the finding that dual fluorescence is o bserved for 1-dimethylamino-4-cyano-naphthalene, but is absent for 2-d imethylamino-6-cyano-naphthalene, for which Delta E(S-1,S-2) is consid erably larger than For the former molecule.