Photo-initiated electron transfer reactions in dye—borate ion pairs: energy, distance and solvent dependence

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Abstract

When dissolved in non-polar solvents, the borate salts (Ar4B) of cyanine dyes (Cy+) exist exclusively as tight ion pairs. The absorption spectra of the ion pairs are identical with that of the dye itself. Excitation of the dye with visible light generates its excited singlet state. Electron transfer from the associated borate anion to the excited singlet state of the dye competes with fluorescence. Electron transfer converts the ion pair to a radical pair. For the case of alkyltriarylborates [RB(Ar)3], the boranyl radical cleaves rapidly to give an alkyl radical that is capable of initiating the polymerization of acrylates. For the case of tetraaryl borates, bond cleavage from the boranyl radical is slow and back electron transfer to regenerate the ground-state ion pair predominates.

We have examined the distance, structure and energy dependence of the bond cleavage rate, and the rates for electron transfer to the excited dye and back electron transfer to re-form the ground-state ion pair.

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