π-Conjugated Multidonor/Acceptor Arrays of Fullerene−Cobaltadithiolene−Tetrathiafulvalene: From Synthesis and Structure to Electronic Interactions

The synthesis, structure, photoelectrochemical behavior, and nonlinear optical (NLO) properties of a symmetric acceptor−acceptor−donor−acceptor−acceptor array, C₆₀-Co−TTF−Co-C₆₀, have been described. The precursors, namely, cobalt dicarbonyl complexes Co(C₆₀Ar₅)(CO)₂ were synthesized from the penta(organo)[60]fullerenes, C₆₀Ar₅H, as starting materials. In the next step, two cobalt−fullerene complexes were connected to a tetrathiafulvalene (TTF) tetrathiolate bridge to obtain the C₆₀-Co−TTF−Co-C₆₀ array. In addition, the monomeric compounds, Co(C₆₀Ar₅)(S₂C₂R₂) (R = CO₂Me and CN) and Co(C₆₀Ar₅)(S₂C₂S₂ C = CS₂C₂R₂) were synthesized as references. The C₆₀-Co−TTF−Co-C₆₀ array exhibits very strong transitions in the near-infrared region (λ_max = 1,100 nm, ε = 30 000 M⁻¹·cm⁻¹) due to a ligand-to-metal-charge-transfer (LMCT) transition and six reversible electron transfer processes. In the crystal, a fullerene/TTF-layered packing structure is evident. Femtosecond flash photolysis revealed that photoexcitation of the array results in a charge separated state involving the strongly interacting cobaltadithiolene and TTF constituents which electronically relax via a resonance effect that extends all throughout the acceptor parts of the C₆₀-Co−TTF−Co-C₆₀ array. The third-order NLO measurement of the array gave the magnitude of the third-order nonlinear susceptibility, |χ⁽³⁾|, values to be 9.28 × 10⁻¹² esu, suggesting the π-conjugation of donors and acceptors in the array.