The role of πσ* state in intramolecular electron-transfer dynamics of 4-dimethylaminobenzonitrile and related molecules

Evidence is presented which indicates that the photoinduced intramolecular charge transfer (ICT) in 4-dimethylaminobenzonitrile proceeds by a new mechanism in which pi sigma(C triple bond N) (*) state is the intermediate of a consecutive process that takes the initially excited pi pi(*) state to the fully charge-separated ICT state. The absence of the ICT-state formation in 4-aminobenzonitrile is attributed to the smaller electron-donor strength of the amino group relative to the dimethylamino group, which hinders the pi sigma(*)-->ICT charge-shift reaction.