Solution-Processed Thin Films of Thiophene Mesogens with Single-Crystalline Alignment†

Highly ordered thin films of organic materials are receiving a great deal of attention as active layers in a new generation of plastic electronic devices, particularly in organic field-effect transistors (OFETs). [1] Numerous organic-film preparation technologies have been tested over the past few decades. For commercial organic electronics, solution-processed, low-cost technologies such as spin-coating, inkjet printing, and screenprinting are preferred. [2] Mostly, these methods produce thin films with moderate structural order from soluble polymers and oligomers. The relevant physical parameters (e.g., chargecarrier mobilities) of thin organic films are strongly affected by the orientation, distribution, and size of the domains and domain boundaries, [3] and, more generally, by the degree of structural order. The endeavor to improve molecular alignment in active organic layers has led to the implementation of high-quality organic single crystals in OFETs with optimized performance. [4] However, recent methods for the fabrication of single-crystal devices are rather complex and inappropriate for practical applications. [5] Therefore, suitable organic compounds and preparation methods that yield highly ordered thin films are quite essential for progress in this field. In this paper, we present highly ordered thin films of 5,5″bis(5-hexyl-2-thienylethynyl)-2,2′:5′,2″-terthiophene (TR5-C6) prepared from solution by spin-coating and subsequent thermal treatment, and studied using X-ray diffraction (XRD) techniques. The calamitic TR5-C6 molecule consists of a central terthiophene unit with acetylene linkages connecting two terminal thiophene rings with hexyl end groups (Fig. 1a). The thermotropic liquid crystalline behavior of TR5-C6 has been studied by differential scanning calorimetry (DSC), and the mesophases have been assigned by polarized optical microscopy. During cooling, exothermic phase transitions from the nematic to the smectic C phase at 115°C, from the smectic C to the smectic B phase at 109°C, and from the smectic B