Intramolecular charge transfer in the excited state. Kinetics and configurational changes

The fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4-aminobenzonitriles is discussed. The magnitude of the energy gap between the two lowest excited states is shown to determine the occurrence or absence of ICT. The excited state behaviour of a series of six 4-aminobenzonitriles in which the amino nitrogen atom is part of a three- to eight-membered heterocyclic ring, P3C to P8C, is studied by using photostationary and time-resolved fluorescence measurements. The ICT rate constant strongly decreases with decreasing ring size. ICT does not occur with P3C and P4C in diethyl ether. This is attributed to the increase of the amino nitrogen inversion barrier with decreasing ring size. The configurational change of the amino nitrogen from pyramidal to planar is considered to be an important reaction coordinate in the ICT process. The photophysics of the 4-aminobenzonitriles is different from that of other systems such as donor/acceptor-substituted stilbenes and 9,9'-bianthryl, which are governed by the charge distribution and macroscopic Coulombic interaction in their CT states.