A peculiar quenching concentration dependence of photoinduced fragmentation in dithiane–carbonyl adducts: A mechanistic experimental and theoretical study

Photoinduced fragmentation in methyldithiane–benzaldehyde adduct, which is externally sensitized by electron transfer sensitizers such as benzophenone, shows a peculiar Stern–Volmer quenching dependence when quenched by diethyl sulfide or DABCO. At lower diethyl sulfide concentration not only negative curvature but also negative slope is observed. At higher concentration the slope of the plot is conventionally positive. This atypical behavior is rationalized in terms of co-sensitization by the quencher at low concentrations. Ab initio and DFT computations were carried out to support the suggested mechanistic rationale. Also the partitioning of the dithianyl radical and benzophenone ketyl radical between the productive disproportionation channel and the recombination channel is determined to be 1:1 at low conversion. These findings provide critical experimental and theoretical insights into the mechanism of this photoinduced fragmentation.