White light and long persistent luminescence from metal cluster-based metal-organic frameworks

Metal-organic frameworks (MOFs) with long persistent luminescence (LPL) have raised particular attention among researchers for their potential applications in optical recording devices, biological imaging, security systems, and so on. Compared with traditional MOFs materials constructed by the single metal as a node, MOFs materials with multinuclear cluster substructure units can effectually facilitate the fixation of molecular conformations through multiple coordination bonds, which would form a dense stacking and rigid environment to suppress the non-radiation transition of triplet excitons. At the same time, metal clusters can act as heavy atoms to increase the spin-orbit coupling to a certain extent, promote Sn-Tn energy conversion to effectively improve the ISC efficiency and enhance the LPL performance. However, MOFs-based LPL materials centred on multinuclear clusters are rarely studied. In this paper, two LPL MOFs (SUST-WJ-12 and SUST-WJ-13) based on the polydentate ligand 2-hydroxyniotinic acid (H2L) with multinuclear cluster are reported and their formulas are [CdL]n and [Zn3(L)2(HCOO)(OH)]n, respectively. In the solid state, they both exhibit excitation-dependent emission color-tuning from blue to green, in which SUST-WJ-12 obtains a white light emission under 280 nm excitation. Importantly, two MOFs both give the long-lived phosphorescence with the lifetimes of 69.92 and 63.22 ms, respectively, and emit the bright and visible LPL for 2-3 s after the removal of the excitation light source. Single-crystal X-ray diffraction analysis and theoretical calculations reveal that LPL of two MOFs arise from the ligand center which is enhanced by metal cluster. The thus obtained LPL provides potentials in lighting and security systems and so on.