Enhancing the Conjugation of Cyclotriphosphazene Covalent Organic Frameworks to Improve the Fluorescence Detection Performance of Hydrazine.

Hydrazine (N2H4) is widely employed in industrial processes; however, its high toxicity poses significant environmental and occupational health risks due to potential leakage or unintended exposure. Consequently, the development of a simple, sensitive and selective method for N2H4 detection in environmental matrices is essential. In this work, covalent organic framework (PTCOF) was synthesized via condensation of hexachlorocyclotriphosphazene (HCCP) and 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT). Leveraging the high reactivity of the P-Cl bonds in PTCOF, aniline was site-selectively grafted onto the framework to yield PTCOF-AN. This functionalization strategically optimized the intrinsic donor-π-acceptor (D-π-A) architecture and enhanced intramolecular charge transfer (ICT) efficiency within PTCOF-AN. PTCOF-AN exhibited fluorescence "turn-on" response toward N2H4, with low limit of detection (LOD) of 153 nM, representing improvements in both sensitivity and selectivity over PTCOF. The fluorescence enhancement is attributed to the reducing capability of N2H4 and the inhibition of ICT process in PTCOF-AN. This work demonstrates a rational design strategy for tailoring the photophysical properties of functionalized COF, offering a promising avenue for N2H4 monitoring.