Pentacene analogues containing nitrogen and sulfur heteroatoms were synthesized using N,N'-diphenylbenzene- 1,4-diamine as a starting material. Ring-closing reaction of N,N'-diphenylbenzene-1,4-diamine with sulfur powder resulted in formation of [1,4]benzothiazino[2,3-b]phenothiazine (compound 1). Catalytic amounts of iodine and reflux temperature of o-dichlorobenzene were necessary. 3H,10aH-[1,4]Benzothiazino[2,3-b]phenothiazine (compound 2) were obtained by oxidization and dehydrogenation of compound 1. The compounds 1 and 2 were purified by vacuum sublimation and characterized by UV-vis absorption spectra and cyclic voltammetry. Their optical energy gaps, energy levels of highest occupied molecular orbitals (HOMOs), and the lowest unoccupied ones were calculated. The HOMO energy levels of the compounds 1 and 2 are -5.85 and -5.42 eV, respectively. The absorption spectra of the compound 2 in thin film shows an obvious red-shift compared with that in CH₂Cl₂ solution, which indicates strong molecular interactions in thin film. Moreover, the crystal structure of the compound 2 was obtained and it has a planar structure. Strong π…π and N…S interactions between adjacent molecules were observed. However, there is no S…S, CH…π interactions, or hydrogen bond, which leads to no OFET performance of the compound 2. Organic field-effect transistors (OFETs) were fabricated using compound 1 as a semiconducting layer. Bottom-gate and top-contact OFET devices were fabricated by vacuum deposition on polystyrene (PS) or octadecyltrichlorosilane (OTS)-treated SiO₂/Si substrates. The OFET devices fabricated on OTS-treated SiO₂/Si substrates showed a good field-effect performance with a mobility of 0.01 cm²·V^-1·s^-1 and a current on/off ratio of 6.6×10³. However, the OFET devices fabricated on the PS modified SiO₂/Si substrates exhibited a lower mobility of 2.5×10^-3 cm²·V^-1·s^-1. AFM images were used to understand field-effect performance of the OFET devices fabricated on PS or OTS-treated SiO₂/Si substrates. All OFET devices were tested under air. A higher field-effect performance of the pentacene analogues semiconductor could likely be achieved through proper control in functional group and device fabrication.

Key words: organic thin film transistors, pentacene, organic semiconductors, linear molecule