Oxygen ether chain containing covalent organic frameworks as efficient fluorescence-enhanced probe for water detection

Three covalent organic frameworks (COFs) named as TPPA-DTM-COF, TPPA-BMTP-COF and TPPA-BETP-COF with different lengths of oxygen ether chains were successfully synthesized by the condensation between 4,4′,4''-(1,3,5-triazine-2,4,6-triyl)trianiline (TPPA) with 2,5-dimethoxyterephthalaldehyde (DTM), 2,5-bis(2-methoxyethoxy)terephthalaldehyde (BMTP) and 2,5-bis(2-(2-methoxyethoxy)ethoxy)terephthalaldehyde (BETP), respectively, which have high crystallinity, large specific surface area and high thermochemical stability. The fluorescence performance of the as-prepared COFs in acetonitrile showed that the longer the oxygen ether chain, the stronger the fluorescence intensity, this may be related to the inhibition of aggregation-caused quenching (ACQ) effect with the increase of interlayer spacing. The resulting COFs also exhibited different emission in different solvents, and their fluorescence intensity has a significant and rapid response to water in organic solvents. Among them, TPPA-DTM-COF shows the most stable fluorescence emission and good dispersion in organic solvents. Its fluorescence intensity is linear with water content in acetonitrile ranging from 0% to 15% (V/V), and the detection limit was calculated to be 0.27%. In addition, the detection limits of COF in ethanol, THF and DMF are 0.15%, 0.43% and 0.39%, respectively. The mechanism of TPPA-DTM-COF detect water with fluorescence “on” may be that the water can form hydrogen bonds with N and O atoms on the skeleton of TPPA-DTM-COF, which enhanced the stability of the excited states of the COFs, thereby enhancing the intramolecular charge transfer (ICT) in the excitation state, leading to the enhancement of fluorescence. The superior properties of TPPA-DTM-COF inducing the as-prepared COF has a great prospect for trace amount water detection in organic solvents in the form of fluorescence enhancement.