Fast intramolecular electron transfer and dual fluorescence. Configurational change of the amino nitrogen (pyramidal→planar)

The fast excited state intramolecular charge transfer (ICT) and dual fluorescence observed with several 4‐aminobenzonitriles is discussed. It is shown that the magnitude of the energy gap between the two lowest excited states determines the occurrence or absence of ICT. The photophysical behavior of a series of four 4‐aminobenzonitriles in which the amino nitrogen atom is part of a four‐ to seven‐membered heterocyclic ring, P4C to P7C, is studied by using time‐resolved fluorescence measurements. The ICT rate constant strongly decreases with decreasing ring size. With P4C in diethyl ether ICT does not occur. This is attributed to the increase of the amino nitrogen inversion barrier with decreasing ring size. The change of the amino nitrogen from pyramidal to planar is considered to be an important reaction coordinate. The photophysics of the 4‐aminobenzonitriles is different from that of other ICT systems such as donor/acceptor‐substituted stilbenes and 9,9’‐bianthryl, which are governed by the charge dist...