Optimizing luminescence sensitivity and moisture stability of porous coordination frameworks by varying ligand side groups

Hydrophobic ethyl, butyl or hexyl groups were introduced into the dicarboxylate ligand in the fluorescent porous coordination framework [Zn-2(fda)(2)(bpy)] (LMOF-202, H(2)fda=9H-fluorene-2,7-dicarboxylic acid, bpy=4,4-bipyridine) for improving water stability and tuning oxygen sensitivity. The long hexyl groups gave satisfactory water stability but its oxygen sensitivity is low (70.8% fluorescence quenched at 1 bar O-2 (1 bar=10(5) Pa)). In contrast, the shorter side groups gave high oxygen sensitivity (93.9% fluorescence quenched at 1 bar O-2) and low water stability. The derivation of the Stern-Volmer curves of the O-2 luminescence quenching data from the linear form can be used for detecting trace impurities in the luminescent framework, being much more sensitive than conventional methods such as powder X-ray diffraction. Mixing the ethyl and hexyl groups in the solid-solution manner brought high oxygen sensitivity (96.4% fluorescence quenched at 1 bar O-2) and high water stability simultaneously in the same coordination framework.