The photo-peroxidation of unsaturated organic molecules. O 21Δg-acceptor re-encounter probabilities

Abstract

A previous report to the effect that the self-sensitised photo-peroxidation of 1,3-diphenylisobenzofuran in solution proceeds at a rate which is independent of acceptor concentration when this is very low, has been confirmed and interpreted in terms of a re-encounter of O 2 1 Δ g and ground state acceptor molecules generated in the same triplet-triplet annihilation act. The overall re-encounter probability (with or without reaction) is found to be 0.61 in agreement with theoretical predictions and with values obtained from a similar treatment of the excimer/molecular intensity ratio in the delayed fluorescence spectrum of pyrene and other organic fluors. This interpretation also affords a quantitative explanation of the failure of tetramethylethylene to inhibit the reaction completely and eliminates the necessity to invoke the spin-forbidden direct addition of O 2 3 Σ g − to the excited singlet state of the acceptor.