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

Multifunctional pi-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(center dot+) and 1(2+), 2(center dot+) and 2(2+), and 4(center dot+) and 4(2+), respectively. Although oligothiophene radical cations containing beta,beta-disubstituted thiophenes usually do not form pi-dimers, 4(center dot+) clearly formed a pi-dimer owing to its planar, round shape. As for the dications of 1, 2, and 4, 12+, which has 34 pi-electrons, exhibited a large diatropic ring current effect, whereas 34 pi dication 4(2+) only showed a medium diatropic ring current effect. In contrast to 1(2+) and 4(2+), 52 pi dication 2(2+) had biradical cationic character instead of Huckel-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 degrees C, whereas an amorphous film of 1 crystallized in the temperature range of 80-83 degrees C to form a lamellarly stacked microcrystalline film, which melted at 139 degrees C. The polymorphism of 1 was applied to either fluorescence switching or switching of field effect transistor (FET) activity and electrical conductivity.