Theoretical Study On The Luminescence Behavior Of Metal-organic Frameworks

Metal-organic framework material is a kind of crystalline materials with net work structure,uniform pore diameter,adjust-able and a huge surface area. Because of it has rich space topology structure, and unique light, electric and magnetic properties has been more widely recognized by scholars. Especially in the luminescence aspects has a great advantage compared to traditional luminescence materials MOFs offer a unique platform for the development of solid-state luminescent materials as they have a degree of structural Predictability. Luminescence MOFs can arise from direct organic ligand emission, metal-centered emission (widely observed in lanthanide MOFs through the so-called antenna effect), and charge-transfer emission, included ligand-to-metal charge transfer (LMCT), metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT). Furthermore, the guest molecules can also result in luminescence onto MOFs. The optical properties of MOFs can use as sensor, photosensitive materials, photovoltaic cells, and so on. In this paper, we used the relativistic density functional theory(DFT) and relativistic time-dependent density functional theory (TDDFT) to study the luminescent properties of MOFs. The main content of the study are as follows:(1) We chooses the multi-functional metal-organic framework MOF-5, which is a very good luminescent materials. We calculate the electronically excited state and luminescence properties of MOF-5. In the past, there are two views for the luminescent mechanism of MOF-5. One point thought the luminescence mechanism is ZnO-like quantum dot luminescence; another view though that of ligand-based luminescence. We demonstrated that the luminescence mechanism of MOFs is the ligand-to-ligand charge transfer (LLCT) by the frontier molecular orbitals and electronic configuration. We further observed Zn4O13quantum dot is a rigid, and the ligands BDC2-are non-rigid in electronically excited state. The reason for the MOF-5luminescence was that the benzene ring and the Zn, O, C six-membered ring in ground-state and electronic excited-state have a excellent coplanarity.(2) The rich hydrogen bonds, coordination bond and π-π stacking interaction were included in the metal-organic framework [Ag(4,4’-bipy)n[Ag(HBTC)]n. Hydrogen bond in the electronic excited state changes are closely related to its luminescence properties and may also lead to fluorescence quenching. We calculate the electronically excited state and luminescence properties of [Ag(4,4’-bipy)n[Ag(HBTC)]n. We demonstrated that the emission mechanism of [Ag(4,4’-bipy)n[Ag(HBTC)]n is the ligand-to-ligand charge transfer (LLCT). The calculated result does not agree with experiment result (LMCT). The hydrogen bonds C1-01…H-05and coordination bond Agl-Ol as the channel of charge transfer ware lengthened in the electronic excited state is not conducive to from ligand to ligand charge transfer leading to the [Ag(HBTC)]4light enhancement..(3) The electronically excited state and luminescence property of the metal-organic framework [ZnsCl4BBTA)3] also was studied in this paper. The frontier molecular orbitals and electronic configuration of [Zn5Cl4BBTA)3] have been calculated. The result shows that the luminescent mechanism of [Zn5Cl4(BBTA)3] is the ligand-to-ligand charge transfer (LLCT). The Zn-N bond are strengthened in the electronic excited state, and resulting in the N-N bond are lengthened, which makes the luminescence charge transfer more difficult because of the channel of the luminescence charger transfer lengthened. So these changes can enhanced the luminescence of the...