Spacer-directed comparable and contrasting optical properties in D–A molecules enabling the fabrication of efficient novel thin-film phototransistors

Abstract

Intuitive and insightful reasoning based on experimental evidence can be employed for devising a hypothesis aimed at developing functional molecules equipped with the desired optical properties. Using this rationale, the present study details a bimodal strategy for creating contrasting and comparable optical properties using novel donor–acceptor type molecules. The study relied on the distinct architecture of curcumin-BF₂-based D–A molecules, which was achieved by varying the spacers from rigid (conjugated; D–π–A) to flexible (conjugation forbidden; D–σ–A) as well as varying the strength of the donor species from anthracene to carbazole. Four molecules, AnBr/CbBr (D–π–A) and AnBf/CbBf (D–σ–A), were designed and synthesized. The combined synthetic, steady-state and time-resolved photophysical and theoretical approaches established the contrasting emissive behaviors of the D–π–A and D–σ–A systems, indicating that the former was more emissive in solution (aggregation-caused quenching) and the later was more emissive in the solid state (aggregation-induced emission). The comparative optical behaviors observed were achieved by varying the strength of the donor, which led to strong dual emission in AnBf that contrasted with that of CbBf. However, the higher-emission quantum yield of CbBr relative to AnBr demonstrated the strong electron-donating ability of the carbazole. Owing to their specific absorption in the solid state, AnBf and CbBf were further employed for the fabrication of blue-sensitive photodetectors. These photodetectors were fabricated in a phototransistor geometry, which showed a large variation in the threshold voltage and off-current of the devices. The devices with the CbBf and AnBf molecules showed excellent photoresponsivities of 3 A W⁻¹ and 1.4 A W⁻¹, respectively, for blue light with an intensity of 15 W m⁻².