Effect of temperature on energy transfer in liquid toluene and cumene

Picosecond pulse radiolysis has been used to study the rate of energy transfer from irradiated liquid arenes to solutes at low concentration. At room temperature the rates of energy transfer to solutes such as biphenyl and diphenylanthracene are rapid and rate constants are in the range from 5 to 7 x 1010 M-1 s-1, in agreement with values measured via low energy photochemical excitation. With solutes such as CHCl3 and CCl4 the yield of singlet excited toluene was reduced via reaction of these solutes with a precursor presumably the electron which neutralizes the solvent cation producing excited states. Decreasing the temperature of the irradiated toluene systems to -700C leads to a marked increase in the rate of energy transfer from toluene to a solute. The nature of this effect and the effect of added e- scavengers suggest that lowering the temperature leads to a change in the mechanism of energy transfer from direct energy transfer to the formation of solute ions followed by ion neutralization leading to solute excited states. The onset of this latter process coincides with increased light scatter by the toluene which may be due to the formation of large aggregates in the liquid