## 3 Results and Discussion
In addition, the Cu-O BE at 533 eV in Figure 2c further confirms the coordination chelation of metallic copper with the porphyrin carboxyl group. On the basis of the aforementioned results, the two-dimensional Cu-ETTC MOF sheet was formed using coordination chelation between H\({}_{4}\)ETTC and Cu(II) paddlewheel (Cu\({}_{2}\)(COO)\({}_{4}\), secondary structure units) metal nodes [43]. Since the MOF sheet constructed using H\({}_{4}\)ETTC and copper ions causes the ACQ effect, the question of which molecules can restore the fluorescence of Cu-ETTC materials has become worth exploring. As shown in Figure 3a, an energy-level transition occurs after the MOF sheet is excited from the S\({}_{0}\) ground state to the high-energy S\({}_{\text{n}}\) excited state. Subsequently, the S\({}_{\text{n}}\) excitation energy will reach the S\({}_{1}\) excited state through vibrational relaxation and internal transitions, and finally return to the S\({}_{0}\) ground state by releasing the excitation energy in the form of fluorescence and nonradiative energy. However, there is an LMCT process in the Cu-ETTC MOF structure, which transfers the S\({}_{1}\) excitation energy  
Figure 2: **(a)** TEM images of the Cu-ETTC MOF sheet obtained at different reaction molar ratios. (b) Full XPS, (c) O 1s, and (d) Cu 2p spectrum of the Cu-ETTC MOF sheet. to the metal nodes, resulting in apparent fluorescence quenching. If an electron donor is added to compete with the LMCT process to produce a reduced or blocked effect, the S\({}_{1}\) excitation energy of the MOF sheet will still be released in the form of fluorescence, and the matrix coordination-induced emission effect will reappear (Figure 3b). In order to verify the hypothesis that electron donors can restore the fluorescence of the Cu-ETTC MOF sheet, we performed fluorescence recovery experiments using lipoic acid, 3,3\({}^{\prime}\)-dithiodipropionic acid, 2,2\({}^{\prime}\)-dithiodiethanol, DL-homocystine, glycine, L-lysine, L(+)-arginine, L-aspartic acid, L-tryptophan, L-kynureine, DL-dithiothreitol, 3-mercaptoethanolpropionic acid, succimer, N-acetyl-L-cysteine, and glutathione. As shown in Figure 4a, the addition of lipoic acid, 3,3\({}^{\prime}\)-dithiodipropionic acid, and 2,2\({}^{\prime}\)-dithiodiethanol failed to awaken the fluorescence of the Cu-ETTC MOF sheet. However, DL-homocystine with a similar structure makes the MOF sheet emit strong fluorescence, with the obvious difference being that this molecular structure contains two amino groups. As a strong electron-donating group, the amino group can provide electrons to the copper metal nodes in the Cu-ETTC structure, reduce the intermolecular LMCT process in the MOF sheet, and make part of the S\({}_{1}\) excitation energy express in the form of fluorescence. Therefore, it is logical that glycine can also cause the fluorescence recovery of the Cu-ETTC MOF sheet, but its fluorescence intensity is lower than that of DL-homocystine, which is due to the lower number of amino-donor groups. Subsequently, other types of amino acids were also analyzed. As shown in Figure 4b, these amino acids with different groups can give the MOF sheet a fluorescence recovery effect to a certain extent. The difference is only the degree of intensity, which mainly depends on whether the group attached contains electron donors or acceptors. In addition, some molecules containing sulfhydryl groups have been studied. As shown in Figure 4c, these sulfhydryl-containing molecules are all able to restore the fluorescence of the Cu-ETTC MOF sheet and have a strong effect, indicating that more S\({}_{1}\) excitation energy is converted into fluorescence. Notably, two molecules, N-acetyl-L-cysteine and glutathione, which contain both amino and sulfhydryl groups, have strong effects in awakening the fluorescence of the Cu-ETTC MOF sheet. Figure 3: Schematic representation of (**a**) the excited energy transfer process and (**b**) the matrix coordination-induced emission effect.