Structural and charge-transport properties of a liquid-crystalline α,ω-disubstituted thiophene derivative: A joint experimental and theoretical study

We present a joint experimental and theoretical study of the structural and charge-transport properties of a liquid-crystalline α,ω- disubstituted oligothiophene derivative for application in organic field-effect transistors. The structural properties of the crystalline and smectic phases are investigated by atomic force microscopy, X-ray reflectometry, and X-ray diffraction. To complement these data, molecular mechanics calculations together with the simulation of X-ray diffraction spectra were performed to determine the relative positions of the molecules in the unit cell. The electrical characteristics of field-effect transistors based on the oligothiophene derivative were measured and compared in the crystalline and smectic phases. Although the silanation of the SiO₂ gate dielectric promoted a marked improvement in the charge-carrier mobilities in the crystalline phase, the expected suppression of grain boundaries in the liquid-crystalline phase was not unambiguously evidenced. The experimental results were further complemented by a detailed theoretical analysis of the electronic couplings governing the charge-transport properties on the molecular scale.