Design, synthesis, and optoelectronic properties of benzothiadiazole-fused sulfur and nitrogen-containing polycyclic heteroaromatics†

The optoelectronic properties of sulfur and nitrogen-containing polycyclic heteroaromatics are still understudied and structure–property relationships are not well understood as compared to acenes and sulfur-heterocyclic aromatic hydrocarbons; this is primarily due to the limited variety of polycyclic structures available. By fusing the rigid, electron-deficient benzothiadiazole into the linear polycyclic heterocyclic skeleton, a series of novel sulfur–nitrogen polycyclic heteroaromatic compounds, designated as BzPTT-Cn, containing different alkyl side chains with benzothiadiazole at both ends have been designed and synthesized. It is found that the introduction of benzothiadiazole exerts an important effect on the physicochemical properties of the molecules due to the intramolecular charge transfer effect. Moreover, by regulating the alkyl group on the N-site of the pyrrole ring in BzPTT, we were able to efficiently adjust the molecular stacking, physicochemical properties, and morphology, which, in turn, influenced the optoelectronic properties. Among the BzPTT-Cn series, BzPTT-C4C8-based organic field-effect transistor (OFETs) devices exhibited the highest hole mobility (up to 0.24 cm² V⁻¹ s⁻¹) and the best photo-detection characteristics with photosensitivity (P) and specific detectivity (D*) up to 10⁵ and 10¹¹ Jones, respectively, which is scarcely reported for donor–acceptor structured polycyclic aromatics. In addition, distinctive single excitatory postsynaptic current (EPSC) behaviors were also observed for BzPTTC4C8-based phototransistors, suggesting potential applications in bionic artificial synapses. This work has implications for further design of sulfur–nitrogen-containing donor–acceptor structured polycyclic heterocyclic aromatics.