Hydrazone-linked luminescent covalent organic frameworks based on AIE-active unit for acid vapour sensing

Abstract

Many two-dimensional (2D) covalent organic frameworks (COFs) exhibit weak fluorescence in the solid state or relatively strong fluorescence only in their suspensions. So it is significant to develop a COF with high luminescence in its as-synthesized solid-state for the better practical applications such as fluorescence sensor. We constructed a 2D COF of TPE-DHZ-COF with the hydrazone bond as the linkage and the monomers of triphenylbenzene (TPE) units with aggregation-induced emission (AIE) characteristics, and also synthesized an analogue of TPB-DHZ-COF without AIE-active unit as a comparison. Compared with TPB-DHZ-COF, the COF with TPE unit show excellent solid-state luminescence properties, demonstrating that introducing of AIE-active unit in COF can significantly improve the solid-state luminescence performance of hydrazone-linked COFs. In addition, all of these COFs could weaken or quench their fluorescence when exposure to strong-acid vapour. We regarded the response of TPE-DHZ-COF to trifluoroacetic acid (TFA) vapour as the research object, and found that TPE-DHZ-COF exhibited a fast response speed, strong adsorption capacity, excellent stability and recycling performance in the detection of TFA vapour. These results indicated that COFs would be a good candidate in solid-state fluorescence sensing.