Bright long persistent luminescence in cluster-based MOFs derived from intra-ligand charge transfer characteristic

Abstract

The emerging metal–organic frameworks (MOFs) with long persistent luminescence (LPL) have raised particular attention among researchers due to their long-lived triplet states, while most LPL materials facing the problem of low luminescence efficiency. A semiflexible tripodal ligand (1′,1″-(2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene)tris(pyridin-4(1H)-one), namely TTP) with intra-ligand charge transfer (ILCT) characteristic is selected to form robust LPL MOFs for keeping high luminescence efficiency. Three LPL MOFs named LIFM-130, LIFM-131 and LIFM-132 exhibit highly emissive lime green LPL at ambient conditions with the highest luminescence efficiency of 36 %. It was found that the metal halide clusters in LIFM-130 have salient heavy atom effect (HAE), which can enhance inter- and intramolecular interactions, thus helping LPL emission prominently. Periodic density functional theory (DFT) calculations reveal the participation of heavy atoms affecting luminescence, in which the bright luminescence of MOFs arises from the ligand-center n-π* transition and metal-to-ligand charge transfer, while LIFM-130 has additional emission state of halogen-to-ligand charge transfer. This work illustrates that on the one hand, the construction of CT characterized MOFs is beneficial for the premise of LPL while maintaining high luminescence efficiency. On the other hand, the introduction of metal halide clusters into MOFs can further improve luminescence efficiency and phosphorescence lifetime synergistically by promoting ISC through HAE and reducing non-radiative transitions through the compact structure.