Solution-Processible Organic Semiconductors Based on Selenophene-Containing Heteroarenes, 2,7-Dialkyl[1]benzoselenopheno[3,2-b][1]benzoselenophenes (Cn-BSBSs): Syntheses, Properties, Molecular Arrangements, and Field-Effect Transistor Characteristics

A series of 2,7-dialkyl[1]benzoselenopheno[3,2-b][1]benzoselenophenes (C-n-BSBSs) were synthesized as novel soluble organic semiconductors. Electrochemical and photochemical studies on C-n-BSBSs have revealed that their molecular properties are very similar to those of their sulfur counterparts, 2,7-dialkyl[1]benzothieno[3,2-b][1]benzothiophenes (C-n-BTBTs), whose solution-processed organic field-effect transistors (OFETs) show superior FET characteristics with field-effect mobility (mu(FET)) higher than 1.0 cm(2) V-1 s(-1). Thin film deposition of C-n-BSBSs on Si/SiO2 substrates was easily accomplished by physical vapor deposition or spin-coating of their solutions in chloroform. Atomic force microscopy (AFM) showed that the thin films, regardless of the alkyl chain length and the deposition method, consist of crystalline grains. X-ray diffraction (XRD) measurements, on the other hand, indicated that all the thin films on the substrate have a well-ordered "molecular lamella" structure where C-n-BSBS molecules have an edge-on orientation on the substrate, as observed in the thin films of C-n-BTBTs. However, the short intermolecular distances (d-spacings) of C-n-BSBS thin films compared to those of C-n-BTBT thin films with the same alkyl chain length correspond to the large inclination of the molecular long axis from the substrate normal, indicating that the intermolecular overlap between the BSBS cores is less effective than that for the two-dimensional interactive structure observed for C-n-BTBT thin films. In accordance with the molecular arrangement and the resulting less interactive electronic structure of C-n-BSBS thin films, the characteristics of C-n-BSBS-based OFETs were less remarkable than those of C-n-BTBT-based ones, although the maximum mu(FET) of 0.23 cm(2) V-1 s(-1) with I-on/off of 10(5) was achieved. The results indicate that even subtle molecular modifications without significant changes of the molecular electronic structure could alter the molecular arrangement in the solid state, which would result in a large difference in device characteristics. For the further development of superior organic semiconductors, not only the molecular electronic structure but also the electronic structure in the solid state must be taken into account.