Manipulating Constitutional Isomerism of Imine Linkages in Interfacially Confined Nanofilms toward Enhanced Fluorescence Sensing

][1,2,5]thiadiazole as the building blocks were facilely prepared via an interfacially confined condensation strategy. The distinct dipole moment orientations of imine linkages are involved in the π-delocalization of conjugated donor-acceptor systems diversely. They also played critical roles in affecting the fluorescence turn-on sensing of the obtained nanofilms for gaseous trifluoroacetic acid (TFA). The joint donor-C═N-acceptor sequence in nanofilm #2 resulted in relatively stronger fluorescence originally than that of nanofilm #1, featuring the disturbed donor-N═C-acceptor sequence. While after blowing trace TFA, the latter nanofilm #1 possessed prominent fluorescence enhancement and obvious color visualization. Comparative transient absorption observations and theoretical calculations elucidated the effective manipulation of the intramolecular charge transfer (ICT) efficiencies among the imine-linked functional skeletons. With the help of a laminated fluorescent sensor, a compact sensing platform was further integrated using optimized nanofilm #1. It exhibited good selectivity, excellent reversibility (≥50 recycles), an extraordinary detection limit (∼0.1 ppt), and a rapid recovery process to gaseous TFA. Our findings provide valuable optimizations of π-linkages in COFs and reliable fluorescent film sensors for monitoring toxic and hazardous gases.