Persistent and Transient Open-Shell Species Derived from C₆₀−TTF Cyclohexene-Fused Dyads

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Abstract

A new series of donor−acceptor fused dyads consisting of a C60 doubly tethered to a substituted TTF moiety has been synthesized. Cyclic voltammetry of the new fullerene derivatives in solution shows a modulation in the difference between the first reduction potential of the fullerene moiety and the first oxidation potential of the TTF moiety with the substituents of the TTF addend. Along with the neutral bichromophoric compounds, we also report the generation and characterization by EPR and absorption spectroscopies of the corresponding persistent open-shell species obtained electrochemically, namely their radical cations and radical anions. Spin density distributions of radical cations and radical anions derived from dyads 1a−c are mainly located on the TTF and fullerene moieties, respectively, as ascertained from the g values and 33S hyperfine coupling constants. Interestingly, the EPR of the radical anion derived from the bisadduct 1d exhibits a structured signal (g = 2.0005) arising from the coupling of the unpaired electron with the hydrogen atoms of the addends. The modification of the donor strength of the TTF moiety allows the tuning of the HOMO−LUMO gap of dyads, permitting a study of the interaction between the two electroactive centers of the molecules as a function of the donor strength. Nanosecond-resolved flash photolysis in the UV−vis region of dyads 1a−c in degassed benzonitrile shows a rapid quenching of the corresponding excited triplet states, which indicates different lifetimes depending on the donor ability of their TTF addends. Excited triplet states of 1b and 1c evolve toward transient charge-separated open-shell species that have remarkably long lifetimes (1b, τ = 75 × 10-6 s; 1c, τ = 79 × 10-6 s) and show absorptions around 460 and 620 nm due to the radical cation on the TTF moiety. These biradical species are also observed by LESR, having in frozen solution spectra consistent with strong exchange coupling between both electrons.

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