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

Photoluminescence efficiencies of covalent organic frameworks (COFs) are significantly restricted by electron delocalization and charge transfer among the conjugated skeletons. Two nanofilms using tetraphenylethylene and benzo[c][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.