Abstract
| - A new series of donor−acceptor fused dyads consisting of a C60 doubly tethered to a substitutedTTF moiety has been synthesized. Cyclic voltammetry of the new fullerene derivatives in solutionshows a modulation in the difference between the first reduction potential of the fullerene moietyand the first oxidation potential of the TTF moiety with the substituents of the TTF addend. Alongwith the neutral bichromophoric compounds, we also report the generation and characterizationby EPR and absorption spectroscopies of the corresponding persistent open-shell species obtainedelectrochemically, namely their radical cations and radical anions. Spin density distributions ofradical cations and radical anions derived from dyads 1a−c are mainly located on the TTF andfullerene moieties, respectively, as ascertained from the g values and 33S hyperfine couplingconstants. Interestingly, the EPR of the radical anion derived from the bisadduct 1d exhibits astructured signal (g = 2.0005) arising from the coupling of the unpaired electron with the hydrogenatoms of the addends. The modification of the donor strength of the TTF moiety allows the tuningof the HOMO−LUMO gap of dyads, permitting a study of the interaction between the twoelectroactive centers of the molecules as a function of the donor strength. Nanosecond-resolvedflash photolysis in the UV−vis region of dyads 1a−c in degassed benzonitrile shows a rapidquenching of the corresponding excited triplet states, which indicates different lifetimes dependingon the donor ability of their TTF addends. Excited triplet states of 1b and 1c evolve toward transientcharge-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 theTTF moiety. These biradical species are also observed by LESR, having in frozen solution spectraconsistent with strong exchange coupling between both electrons.
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