Characterization of the triplet charge-transfer state of 4-amino-N-methylphthalimide in aprotic and protic media by laser flash photolysis

Abstract

The intermediate charge-transfer (CT) transients of 4-amino-N-methylphthalimide (4-AMP) have been attempted to be characterized mainly by laser flash photolysis and other steady-state and time-resolved fluorescence studies. Previously, the singlet intramolecular-charge-transfer (ICT) states have been investigated for various phthalimides using fluorescence techniques. This is probably the first attempt to identify the triplet state of CT transients of 4-AMP by laser flash photolysis studies. The various photophysical parameters have been estimated in different aprotic, protic and heterogeneous media to differentiate the types of CT character in the different media. In aprotic solvents ICT predominates but in protic media the formation of a twisted intramolecular-charge-transfer (TICT) state from the ICT state is facilitated owing to reduction of the torsional barrier. Hence, in polar protic solvents some of the non-radiative deactivation channels, especially internal conversion (IC) from the excited singlet state to the ground state, become effective, which reduces the fluorescence quantum yield and lifetime of the singlet ICT species. Moreover, in protic and micellar media, formation of semiquinone radical takes place through hydrogen abstraction from the media by triplet 4-AMP as further confirmed by application of an external magnetic field.