PICOSECOND AND NANOSECOND STUDIES OF THE PHOTOREDUCTION OF BENZOPHENONE BY 1,4-DIAZABICYCLO[2.2.2]OCTANE - CHARACTERIZATION OF THE TRANSIENT

The transient that is produced in the quenching of triplet benzophenone by 1,4-diazabicyclo[2.2.2]octane (DABCO) has been examined by use of nano- and picosecond laser photolysis. The initial step in all solvents, both polar and nonpolar, is electron transfer to form a triplet contact ion pair. In nonpolar solvents, the ion pair remains in this form until it decays. For polar solvents, the spectra change somewhat over the first 100 ps showing that the solvation changes and the ion pair becomes solvent separated. The lifetime of the ion pair varies greatly with the solvent. In saturated hydrocarbons it is about 80 ps. Nonpolar solvents with either π electrons or a lone pair of electrons stabilize the ion pair on the nanosecond to microsecond time scale. A small amount of alcohol in benzene also stabilizes the ion pair by hydrogen bonding. A shift in the peak position with time toward the blue accompanies the formation of hydrogen bonds in this case. Quenching of the ion pair by oxygen and the dependence of the ion-pair lifetime on temperature show different behavior for the two types of solvent and correlate with the form of the ion pair. The lifetime of the triplet ion pair is controlled alternatively by the rates of intersystem crossing, intrapair proton transfer, or the energetics of back electron transfer. © 1990 American Chemical Society.