Near-UV photo-induced modification in isoreticular metal-organic frameworks

A series of isoreticular metal-organic frameworks (IRMOFs) have been prepared using two different ligands protected with photolabile groups: 2-((2-nitrobenzyl)oxy)terephthalic acid (L1) and 2-((4,5-dimethoxy- 2- nitrobenzyl) oxy) terephthalic acid (L2). Irradiation at either 365 or 400 nm results in postsynthetic deprotection (PSD), removing the nitrobenzyl protecting groups from these ligands and generating phenolic groups in the pores of the MOF (55-83% yield). The photochemical behaviour of the ligands in the IRMOFs was not wholly predicted by their reactivity in solution (i.e. free ligand). A mixed ligand approach was used, by combining L1 or L2 with NH2-BDC to produce mixed ligand IRMOFs with 30-40% incorporation of the NH2-BDC. These mixed-ligand IRMOFs were then subjected to both postsynthetic modification (PSM) and PSD. Two routes to achieve both PSM and PSD were explored: PSM followed by PSD (route 1) and PSD followed by PSM (route 2). When using 365 nm light for the PSD reaction, route 1 was superior due to absorption of NH2-BDC at 365 nm. Irradiation at 400 nm gave fewer differences between route 1 and route 2, but the PSD reaction was less efficient (30-40%) for all systems. By combining PSD and PSM, MOFs with highly functionalized pores can be obtained through a combination of pre- and postsynthetic methods.