Interaction of the excited singlet state of neutral red with aromatic amines

Quenching of neutral red (NR; neutral form of the dye) fluorescence by a number of aromatic amines has been investigated in acetonitrile solutions. The bimolecular quenching constants (kq) obtained from steady-state and time-resolved measurements for a particular donor-acceptor pair are seen to be the same within experimental error. Correlation of the changes in the kq values with the oxidation potentials of the donors (amines) indicates that electron transfer (ET) is the mechanism operative in the present systems. Direct evidence for ET has been obtained from picosecond transient absorption studies on a suitable amine-NR pair. Experimentally determined kq values are seen to correlate well with the free energy changes (delta G0) for the ET reactions, within the frame-work of the Marcus outer sphere ET theroy. From the correlation between the experimentally determined and theoretically calculated kq values, it appears that solvent reorganization plays a major role in governing ET dynamics in the systems investigated.