Anisotropy in structural and physical properties in tetrathiafulvalene derivatives-based zone-cast layers as seen by Raman spectroscopy, UV-visible spectroscopy, and field effect measurements

We have studied anisotropy of thin layers of amphiphilic tetrathiafulvalene derivatives (TTF-4SCn, with n=12, 18, and 22) obtained by zone-casting technique. All the films show optical anisotropy, as seen by polarized optical microscopy and polarized UV-visible spectroscopy. By using polarized Raman spectroscopy an angular dependence of intensity of different vibrational modes in respect to the zone-casting direction was determined. It was found that intensities of the modes related to central and ring C=C vibrations in the TTF core depend very strongly on the angle between the zone-cast direction and polarization plane of incident laser light. Comparison of the deduced orientation of the molecules in one of the films (TTF-4SC18) with its crystal structure shows that the polarized Raman spectroscopy can be useful for controlling orientation of molecules in thin films (e.g., for online monitoring). Organic field effect transistors (OFETs), with channels oriented in parallel and perpendicularly to the zone-casting direction, were built using the oriented TTF-4SCn films. In all cases a strong anisotropy of the charge carrier mobility (mu) was found; the best results were obtained for OFETs with TTF-4SC18, for which mu(parallel to) = 0.25 cm(2)/V s, ON/OFF > 10(5), and mu(parallel to)/mu(perpendicular to) approximate to 170. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3311554]