Flash CIDNP measurement of triplet lifetimes and investigation of two-photon processes

Abstract

The photolysis of aliphatic ketones using laser pulse excitation (λ = 308 nm) was investigated by the time-resolved flash chemically induced dynamic nuclear polarization (CIDNP) method. The contributions of geminate and homogeneous processes to the CIDNP effects were separated. It was shown that the lifetimes of the triplet molecules can be determined from an analysis of CIDNP formation kinetics and from the Stern—Volmer dependence of the amplitude of geminate polarization in the presence of quencher (piperylene). Irradiation of the samples with high-intensity laser light revealed the presence of two-photon processes which are demonstrated by a change in the correlation of CIDNP line intensities of different products with variations in laser radiation energy and by the non-linear character of the Stern—Volmer dependence. Increased irradiation intensity leads to the appearance of radicals resulting from the decomposition of highly excited triplet molecules of aliphatic ketones. Model calculations of CIDNP kinetics and the Stern—Volmer dependence were performed in terms of a kinetic scheme involving short-lived triplet states of aliphatic ketones. The calculations enable an estimation to be made of the contribution of two-photon processes to the photolysis and the molar extinction coefficients of the triplet state for methylethylketone and cyclododecanone.