Quenching of singlet and triplet states of p-N,N-dimethylaminobenzonitrile by tertiary amines. Intramolecular charge-transfer state and excited-state three-electron bond

Abstract

The photolysis of p-N,N-dimethylaminobenzonitrile (DMABN) in a series of tertiary amines has been studied in detail. Interaction rates between the tertiary amines and three excited electronic states of DMABN, the intramolecular charge-transfer state, the twisted exciplex state and the triplet state, have been measured directly. These rates are strongly influenced by the electronic and geometric structures of the DMABN excited states. Triplet-state reaction is controlled by the energetics and is attributed to hydrogen-atom abstraction. Singlet-state reactions are controlled by the steric effects; the twisted state is more reactive than the relatively planar one. We attribute the singlet-state reaction to three-electron interaction with amine lone-pair electrons. These results demonstrate that the twisted single state (TICT state) can be satisfactorily viewed as a charge-localized biradical state, while the lowest triplet state cannot. These conclusions are further confirmed by experiments using planar analogue molecules which show no strong three-electron interaction. This study provides a novel example of how a single bond rotation can change the reactivity of a molecule by orders of magnitude.