π-Extension of electron-accepting dithiarubicene with a cyano-substituted electron-withdrawing group and application in air-stable n-channel organic field effect transistors

Abstract

New π-extended dithiarubicene derivatives with multiply cyano-substituted ethenyl groups at the molecular termini (BisDCNE and BisTCNE) and with a dicyanomethylene-substituted quinoidal structure (TCNQE) were synthesized for application in n-type organic semiconductors. In their absorption spectra, it was observed that their π-conjugations are effectively extended in agreement with the quantum chemical calculation. Thanks to the existence of strong electron-withdrawing groups, BisTCNE and TCNQE exhibited low-lying LUMO energy levels at around −4.2 eV that allow easy electron injection and stable electron transfer under ambient conditions. BisTCNE exhibited the most excellent solubility probably due to the slightly twisted conformation. BisTCNE-based bottom-gate bottom-contact OFET devices exhibited the highest electron mobility (μ_e) up to 5.5 × 10⁻² cm² V⁻¹ s⁻¹ with an on–off current ratio (I_on/I_off) more than 10⁵ under vacuum, and can be operated under ambient conditions without the detrimental decrease of electron mobility (μ_e = 3.3 × 10⁻² cm² V⁻¹ s⁻¹). From GIXRD measurements, BisTCNE and TCNQE were found to show an edge-on molecular orientation on the HMDS-treated SiO₂ substrate. It is a remarkable result that devices based on BisTCNE exhibited a much better performance compared to devices fabricated from TCNQE with a quinoidal structure often applied to superior n-type OFETs.