Multifunctional π-expanded macrocyclic oligothiophene 6-mers and related macrocyclic oligomers

Multifunctional π-expanded macrocyclic oligothiophene 6-mer 1, as well as 9- (2) and 12-mers (3), was synthesized using a McMurry coupling reaction as the key step. The 6-mer 1 was converted to cyclo[6](2,5-thienylene-ethynylene) (4) by using a bromination-dehydrobromination procedure. From X-ray analysis, the crystal structures of nonplanar 1 and round-shaped 2 and 4 were elucidated. STM showed that 4 formed a self-assembled monolayer at the liquid/solid interface to produce a hexagonal porous network. Chemical oxidation of 1, 2, and 4 with 1 and 2 equiv of Fe(ClO₄)₃ produced 1 ^•+ and 1²⁺, 2^•+ and 2²⁺, and 4^•+ and 4²⁺, respectively. Although oligothiophene radical cations containing β,β-disubstituted thiophenes usually do not form π-dimers, 4^•+ clearly formed a π-dimer owing to its planar, round shape. As for the dications of 1, 2, and 4, 1²⁺, which has 34π-electrons, exhibited a large diatropic ring current effect, whereas 34π dication 4²⁺ only showed a medium diatropic ring current effect. In contrast to 1²⁺ and 4²⁺, 52π dication 2²⁺ had biradical cationic character instead of Hückel-type cyclic conjugation. Interestingly, 6-mer 1 showed polymorphism and unusual melting point behavior due to the number of stable conformations in the solid state. Single crystals of 1 melted at 176 C, whereas an amorphous film of 1 crystallized in the temperature range of 80-83 C to form a lamellarly stacked microcrystalline film, which melted at 139 C. The polymorphism of 1 was applied to either fluorescence switching or switching of field effect transistor (FET) activity and electrical conductivity.