| Summary: | Nitrophenols are important nitroaromatic compounds, both important environmental pollutants and dangerous explosives, posing a devastating danger and pollution threat to humans. It is vital to detect efficiently trace nitrophenols in the environment. In this contribution, a series of fully flexible cyclotriphosphazene-based COFs (FFCP COFs: HDADE, HBAPB, and HBPDA), prepared with both a flexible knot and flexible linkers of different lengths, were used for sensing 2,4,6-trinitrophenol (TNP) and <i>p</i>-nitrophenol (<i>p</i>-NP) in real time with excellent sensitivity and selectivity. The quenching constants of HDADE by TNP, HBAPB, and HBPDA by <i>p</i>-NP are 6.29 × 10<sup>4</sup>, 2.17 × 10<sup>5</sup>, and 2.48 × 10<sup>5</sup> L·mol<sup>–1</sup>, respectively. The LODs of TNP and <i>p</i>-NP are 1.19 × 10<sup>−11</sup>, 6.91 × 10<sup>−12</sup>, and 6.05 × 10<sup>−12</sup> mol·L<sup>−1</sup>. Their sensitivities increase with the linker length, which is better than the corresponding COFs composed of rigid linkers. There is only a photoinduced electron transfer mechanism in the fluorescence quenching of HBPDA by <i>p</i>-NP. Meanwhile, the mechanisms of photoinduced charge transfer and resonance energy transfer exist in the fluorescence quenching of HDADE by TNP and the fluorescence quenching of HBAPB by <i>p</i>-NP.
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