Quenching of aromatic ketone triplet excited states by polysilanes

Abstract

Polysilane oligomers quench the triplet excited states of m-chlorotrifluoroacetophenone, p-chlorotrifluoroacetophenone, p-cyanoacetophenone, acetophenone and benzophenone in benzene. The phosphorescence lifetimes τ were measured by laser flash photolysis for varying concentrations of polysilane quencher Q, where QMe 3Si(SiMe 2) n−2 SiMe 3 ( n = 2–4,6) and (MeSi) 6. From τ −1 vs. [Q], the bimolecular rate constants for quenching k q were obtained. Quenching by linear polysilanes Q exhibits a continuous “S-shaped” profile of log k q vs. ΔG et, the free energy for electron transfer. Limited data indicate a similar, although separate, profile for (Me 2Si) 6. In the exergonic region and for ΔG et of less than approximately 7–10 kcal mol −1, log k q follows the quenching theory via contact ion radical pairs. The regular trend observed in the endergonic region deviates from the theoretical curve and is attributed to a separate mechanism for quenching involving mainly hydrogen atom transfer to form pinacols. As electron transfer becomes favorable, silylated pinacols are observed as the major products of p-cyanoacetophenone and tetrasilane ( n = 4).