Charge separation and energy transfer in a caroteno–C₆₀ dyad: Photoinduced electron transfer from the carotenoid excited states

We have designed and synthesized a molecular dyad comprising a carotenoid pigment linked to a fullerene derivative (C–C₆₀) in which the carotenoid acts both as an antenna for the fullerene and as an electron transfer partner. Ultrafast transient absorption spectroscopy was carried out on the dyad in order to investigate energy transfer and charge separation pathways and efficiencies upon excitation of the carotenoid moiety. When the dyad is dissolved in hexane energy transfer from the carotenoid S₂ state to the fullerene takes place on an ultrafast (sub 100 fs) timescale and no intramolecular electron transfer was detected. When the dyad is dissolved in toluene, the excited carotenoid decays from its excited states both by transferring energy to the fullerene and by forming a charge-separated C˙⁺–C₆₀˙-. The charge-separated state is also formed from the excited fullerene following energy transfer from the carotenoid. These pathways lead to charge separation on the subpicosecond time scale (possibly from the S₂ state and the vibrationally excited S₁ state of the carotenoid), on the ps time scale (5.5 ps) from the relaxed S₁ state of the carotenoid, and from the excited state of C₆₀ in 23.5 ps. The charge-separated state lives for 1.3 ns and recombines to populate both the low-lying carotenoid triplet state and the dyad ground state.