Photodissociation and geminate dynamics in solution phase: Picosecond transient absorption studies of tetraphenylhydrazines and diphenyl disulfides

Abstract

Photodissociation of aromatic molecules and the following geminate dynamics of the photochemically produced fragment radical pair in liquid phase are discussed in relation to the results of ultrafast transient absorption measurements. Photodissociation of tetraphenylhydrazine occurs from the unequilibrated excited singlet state competing with the relaxation to the fluorescence state. In addition to the rapid fragmentation, the N−N bond rupture is clearly demonstrated to take place in the fluorescence state of tetraphenyl-and tetratolyl-hydrazine in tens of picoseconds time scale. In the case of di-p-aminophenyl disulfide in nonpolar solvents, geminate recombination of the p-aminophenylthiyl radical and the formation of the radical dimer were observed in tens of picoseconds. The observed slow geminate recombination can be explained by the repulsion between the large dipole of the fragment radical preventing the reformation of the S−S bond and by the restricted conformation required for the dimer formation at the encounter of radicals.