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 nu mber of aromatic amines has been investigated in acetonitrile solutions. Th e bimolecular quenching constants (k(q)) obtained from steady-state and tim e-resolved measurements for a particular donor-acceptor pair are seen to be the same within experimental error. Correlation of the changes in the k(q) 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 absorpti on studies on a suitable amine-NR pair. Experimentally determined kq values are seen to correlate well with the free energy changes (Delta G(0)) for t he ET reactions, within the framework of the Marcus outer sphere ET theory. From the correlation between the experimentally determined and theoretical ly calculated k(q) values, it appears that solvent reorganization plays a m ajor role in governing ET dynamics In the systems investigated.