Re-examining the Photomediated Dissociation and Recombination Kinetics of Hexaarylbiimidazoles

The recombination of lophyl radicals generated by the photodissociation of hexaarylbiimidazole (HABI) compounds has been previously reported to proceed as either first-, 3/2-, or second-order reactions. Here, we re-examine the recombination of HABI-derived lophyl radicals to resolve these disparate reported recombination reaction orders. EPR spectroscopy was used to monitor the radical concentration for two HABI-based compounds in solution both during irradiation until steady state was achieved, then in the dark, where only the radical recombination reaction proceeded. Over short dark periods, lophyl radical recombination could be adequately described by second-order reaction kinetics. To better evaluate these reactions, UV–vis spectrophotometry measurements were performed over longer dark recombination periods. The molar absorptivities of the lophyl radical species were determined and used to express UV–vis absorbance data as radical concentrations. Analysis of these radical concentration curves over extended dark periods revealed that the recombination reactions at low initial HABI concentrations and incident irradiation intensities were well fit as 3/2- and second-order reactions for the two respective parent HABI compounds; however, raised initial HABI concentrations and irradiation intensities progressively increased deviation from the reaction order fits. The fitted recombination and corresponding photodissociation rate constants were validated by predicting radical concentration curves using stepped irradiation intensity profiles, which were compared against experimentally determined radical concentrations obtained under identical reaction conditions