The optical and conductive properties of the liquid-crystalline organic semiconductor 5,5' '-bis (5-hexyl-2-thienylethynyl)-2,2':5',2' '-terthiophene (TR5-C6) spin-coated on rubbed polyimide (PI) were studied. The absorption of light linearly polarized parallel to the rubbing direction largely exceeds that in the perpendicular direction. In contrast, a small anisotropy in the photoconductance was found from time-resolved microwave conductivity (TRMC) measurements. From analysis of the optical data, it is inferred that the terthiophene chains in the TR5-C6 molecules are oriented along the PI rubbing direction with a tilt angle of 53 degrees with respect to the plane of the substrate. The absence of a strong anisotropy in the mobility of charge carriers can be understood on the basis of calculated charge transfer integrals, which were found to be comparable for charge-transfer steps in different directions. This is due to arrangement of the molecules in a herringbone structure. Due to the relatively small values of the charge transfer integrals, it is likely that charge transport in TR5-C6 occurs via polaron hopping between localized states.
Grzegorczyk, W. J., Savenije, T. J., Valeton, J. J. P., Fratiloiu, S., Grozema, F. C., Leeuw, de, D. M., & Siebbeles, L. D. A. (2007). Optical and conductive properties of large-area liquid crystalline monodomains of terthiophene derivatives. Journal of Physical Chemistry C, 111(49), 18411-18416. https://doi.org/10.1021/jp076180h