Competition between triplet energy transfer and electron transfer of photoexcited C60in the presence of [beta ]-carotene by changing solvent polarity

Abstract

The electron transfer and triplet energy transfer from β-carotene to photo-excited C60 in polar and nonpolar solvents have been investigated by 532 nm laser flash photolysis with observation of the transient absorption bands in the visible and near-IR regions. The transient absorption band of the triplet state of C60, which appeared immediately after nanosecond laser exposure, was effectively quenched by β-carotene. With the decay of the triplet state of C60, the intense absorption band of the cation radical of β-carotene appeared at 980 nm with a weak absorption shoulder of the anion radical of C60 at 1070 nm in addition to the triplet state of β-carotene at 550 nm. This indicates that electron transfer takes place from β-carotene to the triplet state of C60 in addition to energy transfer from the triplet state of C60 to β-carotene. In nonpolar solvents, triplet energy transfer predominantly takes place. The relative fraction of the two routes was varied by changing the solvent polarity. After electron transfer, the cation radical of β-carotene disappeared by back electron transfer.