The deuterium isotope effect on the fluorescence lifetime of contact ions pairs (CIP) composed of 9,10-dicyanoanthracene and weak aromatic electron donors in acetonitrile has been investigated. For all pairs studied, an increase of the emission lifetime with increasing deuterium substitution on both the electron acceptor and the electron donor was observed. The free ion yield shows the same variation upon deuteration. It is concluded that the efficiency of free ions formation is essentially determined by the competition, within the CIP, between charge recombination and dissociation into free ions and that loose ion pairs do not play a significant role.

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^EX_dia , and of back ET, k^EX_BET , 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^EX_BET 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^EX_BET exhibits a logarithmic free energy dependence with a slope of about −2 eV^-1. Moreover, k^EX_dia is not constant but increases continuously with diminishing donor's oxidation potential.

The competition between electron transfer (ET) and triplet energy transfer (TT) in the quenching of benzophenone, xanthone, and anthraquinone in the triplet state by molecules with both a sufficiently small oxidation potential and low triplet state was investigated in the picosecond to microsecond time scales. In the longer time scale, the product distribution depends strongly on the relative exergonicity of ET and TT processes, the yield of the lower energy product being at least four times larger than that of the other product. Picosecond transient grating measurements reveal that if TT is more exergonic than ET, the TT product is predominantly formed by two sequential ET reactions, i.e., by spin-allowed back ET within the triplet geminate ion pair formed upon ET quenching. However, if ET is more exergonic than TT, no conversion from the TT product to the ET product could be detected. In this case, the product distribution in the microsecond time scale seems to reflect the competition between the two processes. When both processes are exergonic, ET appeared to be always faster than TT. This is in agreement with the severe orbital overlap requirement for TT via the Dexter exchange mechanism.

A ps transient grating setup using white light continuum for probing is presented. Measurements on an aromatic molecule in solution have been carried out with this system. The diffracted spectrum is analyzed using Kogelnik’s coupled wave theory. At short time delay after excitation, the diffracted spectrum is strongly dominated by absorption and in this case transient grating spectroscopy is equivalent but more sensitive to transient absorption spectroscopy. If some of the excitation energy is dissipated as heat, the diffracted spectrum is essentially the same as the dispersion spectrum of the transient species at time delays approaching half the acoustic period. The performances of this technique and of transient absorption spectroscopy are compared.