TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: Preparing COFs further involves: (d) adding an acid solution to the resulting mixture of step (c); and (e) filtrating the product obtained in step (d). The combination of the first organic solvent and the second organic solvent of steps (a) and (b) gives a factor F higher than 50. Each of the first organic solvent and the second organic solvent of steps (a) and (b) are independently selected from the group of m-cresol, dimethyl sulfoxide and mixtures. The combination of the first organic solvent and the second organic solvent of steps (a) and (b) gives a factor F lower than 50. Each of the first organic solvent and the second organic solvent of steps (a) and (b) are independently selected from the group of dichloromethane, chloroform, methanol, ethanol, acetone, tetrahydrofuran and mixtures. Preferred Framework: The COFs are in the form of a gel, or in the form of a solid with sphere-like microstructure. Alternatively, the COFs comprises: the set of amine subunits, and the set of aldehyde subunits, where the COFs are in the form of a gel or a solid with sphere-like microstructure. The COFs in the form of a gel comprise layers with an average thickness between 300 and 0.3 nm. The COFs in the form of a solid with sphere-like microstructure comprise spheres with an average diameter between 10 and 1000 nm. Moreover, (a) the amine subunits are selected from substituted and un-substituted 1,3,5-triaminobenzene, substituted and un-substituted 1,3,5-tris(4-aminophenyl)benzene, substituted and un-substituted 1,3,6,8-tetrakis(4-aminophenyl)pyrene, substituted and un-substituted 5,10,15,20-tetrakis(4-aminophenyl)phorphyrin and substituted and un-substituted tetrakis((4-aminophenyl)dioxaborolane)phtalocyanine, and (b) the aldehyde subunits are selected from substituted and un-substituted 1,3,5-benzenetricarbaldehyde, substituted and un-substituted 1,3,5-tris(4-formylphenyl)benzene, substituted and unsubstituted 1,3,6,8-tetrakis(4-formylphenyl)pyrene, substituted and un-substituted 5,10,15,20-tetrakis(4-formylphenyl)phorphyrin and substituted and un-substituted tetrakis((4-formylphenyl)dioxaborolane)phtalocyanine. The COFs further contains a metal atom coordinated to iminic nitrogens, in particular the metal atom is selected from manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), ruthenium (Ru), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), gold (Au), neodymium (Nd), europium (Eu) and mixtures. EXAMPLE - No suitable example given.