Solid-state red-emissive (cyano)vinylene heteroaromatics via Pd-catalysed C–H homocoupling

Abstract

Thiophene-based π-conjugated systems are important materials for organic electronics; thus, their synthesis is of topical interest. We report fluorescent thiophene/furan-based vinylene and cyanovinylene systems via Pd-catalysed homocoupling [Pd(OAc)₂, pivalic acid, KOAc, DMAc, 140 °C]. The methodology is versatile and allows the development of a variety of π-conjugated systems without the need for pre-functionalized building units. The reaction tolerates electron-rich, electron-deficient and large π-conjugated substrates. The developed compounds absorb in the visible region (400–515 nm) and emit green to orange fluorescence in the solution state (510–600 nm). Most importantly, the compounds exhibit strong aggregation-induced emission (AIE) in the NIR region (λ_em = 650 nm), with quantum yields reaching up to 10%. Steric hindrance imparted by vinylene/cyanovinylene units is responsible for the strong solid-state luminescence. DFT-optimized structures reveal an apparent twist of 20–40° in the molecular backbone of the compounds, supporting the AIE behaviour of the compounds.