Multifunctional π‑Expanded Macrocyclic Oligothiophene 6‑Mers and Related Macrocyclic Oligomers

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