SITE SELECTIVITY IN EXCITED-STATE REACTIONS IN SOLUTIONS

The distributions in solute-solvent interaction energies may produce inhomogeneity in reaction kinetics in solutions. We applied the principle of site selectivity to the studies of intramolecular excited-state reactions in solid (vitrified) and liquid solvents. The principle is realized by producing excitation at the red edge of the absorption band and observing the difference between reaction rates at midband and red-edge excitations as a function of the rate of solvent dipolar relaxations (variation of the temperature). Studies of two excited-state reactions in bichromophoric solutes (excited-state energy homotransfer between indole rings in tryptophanyltryptophan dipeptide and electron transfer in 9,9'-bianthryl) have demonstrated that the distribution in solute-solvent interaction energies is important in both cases. While smaller interactions are favorable for energy transfer, the electron transfer can be effective only when these interactions are very strong. Site selectivity is lost with an increase in solvent relaxation rate.