Bis(5-alkylthiophen-2-yl)arene liquid crystals as molecular semiconductors
Mesogens with a general structure bis(alkylthiophene)arene, in which the central arene moiety is 1,4-benzene, 2,5-pyridine, 3,6-pyrimidine, 2,6-naphthalene and 2,6-dithieno[3,2-b;2′,3′-d]thiophene, were synthesized, and the effect of the central unit and the length of the alkyl chains on the liquid crystalline behavior was studied by differential scanning calorimetry and polarized optical microscopy, and in the case of 1,4-bis(5-tridecylthiophen-2-yl)benzene (2a) and 2,6-bis(5-tridecylthiophen-2-yl)DTT by synchrotron X-ray analysis. Several 2,5-bis(5-alkylthiophen-2-yl)pyridines and 1,4-bis(5-tridecylthiophen-2-yl)benzene 2a exhibit a rare transition – formation of the SmF phase directly from the isotropic liquid; in the case of 2a, formation of higher ordered smectic phases was observed on further cooling. Hole-transporting properties of 2a in different mesophases were studied by the time-of-flight technique. The hole mobility, μh, in SmF reached 6 × 10−3 cm2 V−1 s−1 and increased with an increase of the order of the mesophase up to 0.02 cm2 V−1 s−1 in the S1 phase (tentatively assigned as the crystal G phase), ∼0.09 cm2 V−1 s−1 in the S2 phase (tentatively assigned as the crystal H phase), and 0.11 cm2 V−1 s−1 in the crystalline phase at 70 °C.