Activation parameters for the recombination reaction of intramolecular radical pairs generated from the radical diffusion-inhibited HABI derivative

Activation parameters were determined for the recombination of radical pairs arising from newly designed, photochromic, radical diffusion-restricted hexaarylbiimidazole (HABI) derivative. We have developed a new type of radical diffusion-inhibited HABI derivative, which contains two equivalent HABI units and yields a tetraradical with four equivalent 2,4,5-triphenylimidazolyl radical (TPIR) units by photoirradiation. This radical dimerization proceeds by a successive first-order reaction from the tetraradical to the parent molecule via a diradical. The rate constants of each reaction were determined from the decay profile of EPR signal intensities. The entropies of activation (AS') for the first and the successive dimerization steps were estimated to be -178.5 and -205.5 J K-1 mol(-1), respectively. Within the experimental temperature range, the radical dimerizations are entropy-controlled (-T Delta S-double dagger > Delta H-double dagger). The large negative Delta S-double dagger values imply a highly ordered transition state, indicating that the radical dimerizations occur when the TPIR units interact at a specific orientation. The present study demonstrates the availability of radical diffusion-inihibited HABI for the kinetic study of radical-radical reaction.