Solvent templated luminescent metal-organic frameworks for specific detection of vitamin C in aqueous media

Two novel polymeric coordination compounds of same stoichiometry [Zn(2,6-ndc)(api)]n (where H2ndc = naphthalene-2,6-dicarboxylic acid and api = 1-(3-aminopropyl)-imidazole) have been synthesized. The two compounds PUC3 and PUC4 have been separated by a different mixture of solvents, namely DMA/MeOH/H2O and DMF/MeOH/H2O, respectively, leading to a different solid-state architecture. The single crystal structural analysis of PUC3 evidenced a four-fold interpenetrated 3D framework, while PUC4 is a porous framework formed by 2D corrugated layers. Thus, the two coordination polymers are an example of supramolecular isomerism induced by the template effect of solvent and flexibility of the aminopropyl-imidazole molecule. The synthesized PUC3 and PUC4 compounds have been successfully characterized with the help of single crystal X-ray diffraction (SCXRD), PXRD, FTIR, TGA, DSC and XPS. The two MOF's, applied as sensitive and selective probes for ascorbic acid (AA) detection in aqueous phase, appeared to react only with AA and not with any of the reactive oxygen/nitrogen species (ROS/RNS) tested. The calculated quenching constant (Ksv) values, of 0.4 × 105 M−1 (PUC3) and 0.5 × 105 M−1 (PUC4), indicate strong interaction between each metal organic framework (MOFs) and AA. They disclosed an excellent detection limit of 0.032 and 0.025 μM, respectively. Moreover, fluorescence resonance energy transfer (FRET) process is proposed as quenching mechanism and validated by DFT results.