Solution‐Processed Thin Films of Thiophene Mesogens with Single‐Crystalline Alignment

Abstract

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 COMMUNICATIONS 896

Symmetrical and Nonsymmetrical Liquid Crystalline Oligothiophenes: Convenient Synthesis and Transition‐Temperature Engineering

“…We paid particular attention to this compound because of its very low enthalpy of transition between the LC and crystalline phases, which could enable us to obtain also single-crystalline aligned films for OFET applications. [17] At lower temperatures, two different crystalline phases exist, and the transition between them is at 32°C (Figure 1). The mosaic texture (POM) of the phase between 41°C and 79°C (clearing point) suggests its smectic liquid crystalline nature.…”

Temperature induced structural evolution of a fluorene‐thiophene copolymer on rubbed surfaces

“…On the one hand, the alignment can be achieved by mechanical load like film rubbing or film drawn. On the other hand, the alignment can be obtained by using an auxiliary layer like a photoalignment layer or rubbed surfaces which induce a preferred orientation of molecules during deposition7 or heat treatment 8…”

Syntheses, Phase Behavior, Supramolecular Chirality, and Field‐Effect Carrier Mobility of Asymmetrically End‐Capped Mesogenic Oligothiophenes

“…Such a desirable phase sequence is reminiscent of the similar phase behavior exhibited by 5,5′′‐bis(5‐alkyl‐2‐thienylethynyl)‐2,2′:5′,2′′‐terthiophene. The gradual increase in order and the slow transitions between the phases were demonstrated to be a useful characteristic for obtaining a large‐area liquid‐crystal monodomain 9b. c…”

“…Then upon heating to the mesophase temperature of 160 °C (Figure 2, curve b), sharp higher‐order reflections were observed in the SAR and several marked reflection peaks superposed on the broad band in the WAR consistently indicated the formation of a highly ordered smectic phase 3f. 9b, c, 24, 25 All the peaks could be indexed with a centered rectangular smectic E (SmE) phase as 39.4 (001), 19.72 (002), 9.90 (004), 4.67 (110), 3.99 (200), and 3.29 Å (210) with a layer spacing of 39.4 Å. Upon further increasing the temperature to 190 °C, it displayed no SAR reflections, but only a diffuse peak in the WAR that confirmed nematic phase formation (Figure 2, curve c).…”

“…i On the other hand, with an extension of π conjugation, the hybridized co‐oligomers of thiophene and phenylene have been developed as a new class of light‐emitting materials12 and easily processable OFETs 11. 13 In addition, compared with their polymer analogues, well‐defined oligomeric liquid crystalline semiconducting materials with high purity, precise chemical structure, and conjugation length are of higher order and are solution processable; thus, they are considered to be realistic candidates for industrial applications as OFETs for cheap, large area, smart electronic devices 9b. c, 11, 14…”