Tuning the Electronic Properties of a Nitro- and Cyano-Functionalized Benzothienoquinolizinium Salt as Potential Organic N-Type Semiconductor: Experimental and Theoretical Study

Abstract

A new nitro- and cyano-functionalized, tetradecyl-substituted benzothienoquinolizinium tetrafluoroborate was synthesized through oxidative photocyclisation of a pyridinium precursor and evaluated as a potential small molecule N-type organic semiconductor. The precursor and the photocyclised salts were characterized by UV–Vis absorption spectroscopy, DSC, 1H and 13C NMR, and cyclic voltammetry. Their optoelectronic properties were further investigated by DFT and TD-DFT calculations. Photocyclisation induced a marked bathochromic extension of the absorption profile, decreasing the optical band gap from 3.19 eV for the pyridinium precursor to 2.61 eV for the fused benzothienoquinolizinium salt. Cyclic voltammetry revealed a stabilized electrochemical LUMO level of approximately −3.87 eV, supporting the electron-deficient character of the nitro/cyano-substituted cationic scaffold. Frontier molecular orbital analysis showed that the HOMO and LUMO are mainly localized on the fused π-conjugated core and electron-withdrawing aryl substituents, with negligible contribution from the tetradecyl chain and BF4− counterion. TD-DFT and electron excitation analyses indicated that the monomeric low-energy transition has mixed local/charge-transfer character, whereas π-stacked dimers, especially the face-to-face arrangement, enhance charge transfer character and reduce electron–hole Coulombic attraction. Marcus type charge transport calculations revealed packing dependent behavior, with the face-to-face dimer displaying nearly ambipolar transport with a slight electron preference. A preliminary theoretical donor–acceptor model with hexaphenyl-substituted hexabenzocoronene further suggested energetic compatibility and strong intermolecular charge-transfer character. Overall, these results identify this benzothienoquinolizinium tetrafluoroborate as a promising electron-deficient cationic π-scaffold for future organic optoelectronic materials.