Photochemistry of organoborates: Intra-ion pair electron transfer to cvanines

Abstract

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. © 1990 IUPAC