Direct investigation of the dynamics of charge recombination following the fluorescence quenching of 9,10-dicyanoanthracene by various electron donors in acetonitrile

The dynamics of the intermediate generated upon diffusional electron transfer (ET) quenching of 9,10-dicyanoanthracene by electron donors of varying oxidation potential in acetonitrile has been investigated using several transient grating techniques. With most of the donor/acceptor pairs studied, the transient grating spectrum cannot be differentiated from those of the free ions. Exciplex fluorescence, with the same lifetime as that of the ion pair, is observed with all donors. To extract from the measured kinetics the rate constant of exciplex dissociation, k(dis)(EX), and of back ET, k(BET)(EX), within these exciplexes, three different schemes have been considered. The best agreement is obtained by assuming that charge recombination predominantly takes place within the exciplex. The obtained k(BET)(EX) values are substantially different from the BET rate constants deduced indirectly from the free-ion yields and with a donor-independent rate constant of separation. For each class of donors, k(BET)(EX),exhibits a logarithmic free energy dependence with a slope of about -2 eV(-1). Moreover, k(dis)(EX) is not constant but increases continuously with diminishing donor's oxidation potential.