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.