Tunable optical responses in cyanoester-based organic π-conjugates for selective sensing of aliphatic diamine vapors

Abstract

Amine addition to α,β-unsaturated cyanoesters generally favors the competitive 1,4-addition over the 1,2-addition/elimination (nucleophilic acyl substitution). This work describes an easy, metal-free synthesis of a cyanoester-functionalized, conformationally twisted π-conjugate, CYE, which exhibits aggregation-induced emission (AIE) and decent solid-state emission. Such molecules exhibit visually detectable responses against diamine vapors. More importantly, vapor-induced fluorochromism at the solid–vapor interface was selective to the specific alkyl chain length, highlighting their sensitivity to subtle structural variations. Notably, the fluorophores show diverse, concentration-dependent fluorescence responses toward selective aliphatic biogenic amines (BAs) in the solid–vapor phase. Gradual exposure of the fluorophore to BA vapors induces a visible color change from orange through yellow to green, accompanied by an enhanced quantum yield. Mechanistic investigations reveal the gradual formation of mono- and diamides from the reactions between CYE and putrescine (PUT). The acid analog CYA formed a salt by reacting with ethylenediamine (EDA). Furthermore, these fluorophore-coated platforms are used for practical applications to monitor the freshness of Indian cottage cheese (paneer), during which significant fluorescence quenching was observed. These findings highlight the potential of CYE and CYA fluorophores as an efficient platform for detecting BAs and monitoring food quality.