Synthesis, photochemistry and photophysics of stilbene-derivatized fullerenes

Abstract

The photochemistry and photophysics of two sets of stilbene-derivatized fullerene isomers, in which stilbene is covalently linked to C₆₀, are described. Synthesis and characterization of cis- and trans-stilbene substituted methanofullerenes 1 and 2, and cis- and trans-stilbene substituted fulleropyrrolidines 7 and 8 are described. While UV irradiation of the stilbene-substituted ketal precursors to 1 and 2 lacking the fullerene moiety afforded a photostationary state with 90 : 10 cis : trans ratio, similar to that of other model stilbene systems, direct and fluorenone-sensitized irradiation of 1 and 2 led to complete conversion to the trans isomer 2, as determined by HPLC analysis. The same results were obtained using cis–trans isomers 7 and 8, namely, the photostationary state on excitation below 350 nm is essentially 100% trans. No isomerisation in either system was obtained on excitation above 400 nm, where all the light is absorbed by the fullerene moiety. By analogy to previous studies of quenching of stilbene excited states, these results suggest that both singlet and triplet excited states of the trans-stilbene moiety in 2 and 8 are being quenched by intramolecular energy transfer to the attached C₆₀, while the much shorter lived cis-stilbene excited states are not similarly quenched. Fluorescence studies on compound 8 support this hypothesis, since the characteristic fluorescence emission of trans-stilbene and trans-stilbene derivatives is not observed in the case of adduct 8. Because of the well established fact that trans-stilbene S₁ states are longer lived than the S₁ states of the corresponding cis isomers, rapid intramolecular singlet–singlet energy transfer to the appended C₆₀ moiety, k_et ∼10¹² s⁻¹, is able to compete effectively with radiative and radiationless deactivation of the trans-stilbene S₁ states in 2 and 8, but not in the corresponding cis isomers 1 and 7.