The DFT calculations were conducted to better understand the different fluorescence emission behavior of the COF in the presence of selected analytes, including Fe 3+ , I 2 and TNT, which can quantitatively evaluate the interactions between JUC-557-nanosheet and small molecules. As shown in Supplementary Table and Fig. , the adsorption energy of -CN group with Fe 3+ , I 2 , and TNT was -0.28 eV, 2.31 eV, and -1.06 eV, respectively. In addition, the adsorption energy of carbazole group with Fe 3+ , I 2 , and TNT was -0.29 eV, -0.18 eV, and -0.97 eV, respectively. Obviously, these results indicated that one or more Fe 3+ , I 2 or TNT could be located inside an interaction sphere around the luminescent species to form a dark complex, leading to its instantly decreased fluorescence based on a static quenching mechanism. In particular, the adsorption energy of functional groups in JUC-557-nanosheet with TNT was higher than those of the nanosheet with Fe 3+ or I 2 , which well explains that among these small molecules, JUC-557-nanosheet with TNT has the highest K a and much lower LOD compared with Fe 3+ and I 2 (Supplementary Table ). Furthermore, the frontier molecular orbital distributions of JUC-555 fragment calculated by DFT calculations based on Gaussian 16 (Supplementary Fig. ). Obviously, the LUMO energy level of JUC-557 fragment (-2.08 eV) was higher than those of nitro-compounds, such as TNT (-3.81 eV), p-DNB (-3.55 eV), o-DNB (-3.11 eV), and m-DNB (-3.19 eV), and the difference of LUMO energy (ΔE LUMO ) between JUC-557-nanosheet and TNT was highest (up to 1.73 eV), which contributes to superior fluorescent quenching effect of JUC-557nanosheet for TNT (Supplementary Table ).