Photoinduced Radical Cleavage of Iodobenzophenones

Article abstract of DOI:10.1021/j100019a023

The triplet state cleavage of m-iodo-(mibp) and p-iodobenzophenone (pIBP) to benzoylphenyl and iodine radicals has been studied by both steady state and laser flash techniques.Phosphorescence spectra in methanol/ ethanol glasses at 77 K indicate triplet energies of 68.4 and 67.8 kcal/mol for mIBP and pIBP, respectively.In most organic solvents products are benzophenone and iodinated solvents; these are formed by hydrogen abstraction from solvent by the initially formed benzoylphenyl radicals.Quantum yields for product formation decrease with increasing solvent viscosity, ranging from 0.45 in acetonitrile to 0.28 in cyclopentane and 0.05 in cyclooctane; they appear to reflect significant in-cage recoupling of the initial radical pairs.Quenching of the reaction by naphthalene indicates 25 degC triplet lifetimes of 25 and 0.2 ns for mIBP and pIBP, respectively, which were confirmed by flash kinetics.Laser excitation produced transient signals assigned to the iodine atom-toluene complex (in toluene) and, for mIBP, the triplet ketone.Decay of the latter revealed activation parameters ΔH = 4.0 +/- 0.2 kcal/mol and log A = 10.6 +/- 0.2 for mIBP in both methanol and toluene.The strong phosphorescence of both ketones at 77 K indicates that ΔH for pIBP must be >= 3kcal/mol, such that log A is ca. 12, much higher than for mIBP.The different rates and A values reflect different ?^* spin densities at the meta and para positions in the lowest n,?^* triplets, which react either by mixing with dissociative ?,?^* states or by thermally converting into a dissociative ?,?^* or ?₁,?^* state.The dependence of ?^* spin density and the low A values strongly suggest an inefficient state interconversion at a surface crossing between n,?^* and ?,?^* states, because of their different symmetries.It is suggested that the low A values, which might indicate negative Δs values, are better understood as low transmission coefficients.