Excited singlet state reaction of phenosafranine with electron donors Role of the heavy-atom effect in triplet induction

Abstract

Phenosafranine fluorescence is quenched effectively by halogenated benzene derivatives in the negative and positive regions of the Gibbs energy change, ΔG_et. The observed quenching constants are compared with the k_q values calculated using ΔG from the Rehm–Weller expression and k_d=2.8×10¹⁰ d mol^-1 s^-1 from the Smoluchowski expression. Good correlation between observed and calculated k_q values in the exothermic region of ΔG_et reveals that the reaction proceeds here via electron transfer. For positive ΔG_et, the difference between experimental and calculated k_q values shows that quenching occurs via the heavy-atom effect and not via electron transfer. The induced intersystem crossing efficiency in the presence of heavy-atom-substituted quenchers is evaluated using laser flash photolysis.