Competition between triplet energy transfer and electron transfer of photoexcited C60 in the presence of β-carotene by changing solvent polarity

The electron transfer and triplet energy transfer from beta-carotene to photo-excited C-60 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 C-60, which appeared immediately after nanosecond laser exposure, was effectively quenched by beta-carotene. With the decay of the triplet state of C-60, the intense absorption band of the cation radical of beta-carotene appeared at 980 nm with a weak absorption shoulder of the anion radical of C-60 at 1070 nm in addition to the triplet state of beta-carotene at 550 nm. This indicates that electron transfer takes place from beta-carotene to the triplet state of C-60 in addition to energy transfer from the triplet state of C-60 to beta-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 beta-carotene disappeared by back electron transfer.