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, C-60-Co-TTF-Co-C-60, 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 C-60-Co-TTF-Co-C-60 array. In addition, the monomeric compounds, Co(C60Ar5)(S2C2R2) (R = CO2Me and CN) and Co(C60Ar5)(S2C2S2C = CS2C2R2) were synthesized as references. The C-60-Co-TTF-Co-C-60 array exhibits very strong transitions in the near-infrared region (lambda(max) = 1,100 nm, epsilon = 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 C-60-Co-TTF-Co-C-60 array. The third-order NLO measurement of the array gave the magnitude of the third-order nonlinear susceptibility, vertical bar chi((3))vertical bar, values to be 9.28 x 10(-12) esu, suggesting the pi-conjugation of donors and acceptors in the array.