Photochemistry of organoborates: intra-ion pair electron transfer to cyanines

Cyanine borate (R(Ph)3B-; R alkyl) salts dissolved in non-polar solvents exist predominantly as ion pairs. Irradiation of the ion pair with visible light absorbed by the cyanine leads to formation of a locally excited singlet state of the dye. The singlet excited cyanine in the ion pair is capable of oxidizing the borate anion to the boranyl radical (R(Ph)3B.) The rate constant for this electron transfer reactions (ket) depends on the free energy change (AGet). This latter value was estimated for a series of borates and the relationship between ket and AGet appears to give a maximum value consistent with the quadratic law predicted from Marcus theory. The boranyl radical undergoes carbon-boron bond cleavage to generate a free alkyl radical. The rate of bond cleavage depends directly on the stability of the alkyl radical formed. When stabilized alkyl radicals are formed, carbon-boron bond cleavage is faster than the back electron transfer reaction that regenerates the cyanine borate ion pair. Carbon-boron bond cleavage of the boranyl radical in the systems examined is irreversible and the bond dissociation energy of the parent hydrocarbon is a good predictor of the yield of radicals. The free alkyl radicals formed by the irradiation electron transfer bond cleavage sequence may be used in meaningful chemical processes such as the initiation of polymerization.