Solution-processible organic semiconductors based on selenophene-containing heteroarenes, 2,7-dialkyl[1]benzoselenopheno[3,2-b][1]benzoselenophenes (C _n-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 (μ_fet) higher than 1,0 cm² V^-1 s ^-1. Thin film deposition of C_n-BSBSs on Si/SiO ₂substrates 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 μ_fet of 0.23 cm² V^-1 s ^-1 with 1_on/otf of 10⁵ 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.