Application of the flexible polyethylene polyamine-based covalent organic frameworks for fluorescence sensing nitrophenols and iodide ions

Recently, flexible covalent organic frameworks (COFs) prepared with nitrogen-rich building units have become targets for selective detection of pollutants to serve a clean environment. To meet the requirements of the practical applications, a series of novel flexible covalent organic frameworks containing polyethylene polyamine units (the flexible PEPA-based COFs, remember as TEDA, TDETA, TTETA, TTEPA, and TPEHA) were designed and developed to create fluorescence sensors. The flexible PEPA-based COFs were found to be excellently fluorescent properties in presence of some solvents under irradiation with UV light and can sense some nitrophenols with good sensitivity for DNP, o-NP, or TNP by fluorescence quenching with high S-V constants (K_SV) of 5.01 × 10⁴, 6.92 × 10³, 7.42 × 10⁴, 7.37 × 10³, and 1.59 × 10⁴ L mol⁻¹, respectively. Theoretical calculations and experimental studies show that the fluorescence quenching in the presence of nitrophenols is put down to photoinduced electron transfer, energy resonance transfer, and the difference between nitrophenols binding sites to the flexible PEPA-based COFs. In addition, TEDA, TDETA, TTETA, TTEPA, and TPEHA can fluorescently sense iodide ions with high K_SV of 2.28 × 10⁴, 1.09 × 10⁴, 6.88 × 10³, 8.81 × 10³, and 1.82 × 10⁴ L mol⁻¹, which are the most sensitive among POP materials.