3 Investigations

7 ABSTRACT: The electronically excited state and luminescence 8 property of metal−organic framework MOF-5 were investigated 9 using relativistic density functional theory (DFT) and time-10 dependent DFT (TDDFT). The geometry, IR spectra, and UV− 11 vis spectra of MOF-5 in the ground state were calculated using 12 relativistic DFT, leading to good agreement between the 13 experimental and theoretical results. The frontier molecular orbitals 14 and electronic configuration indicated that the luminescence 15 mechanism in MOF-5 follows ligand-to-ligand charge transfer 16 (LLCT), namely, π * → π, rather than emission with the ZnO 17 quantum dot (QD) proposed by Bordiga et al. The geometry and IR 18 spectra of MOF-5 in the electronically excited state have been 19 calculated using the relativistic TDDFT and compared with those for 20 the ground state. The comparison reveals that the Zn4O13 QD is rigid, whereas the ligands BDC 2 − are nonrigid. In addition, the 21 calculated emission band of MOF-5 is in good agreement with the experimental result and is similar to that of the ligand H2BDC. 22 The combined results confirmed that the luminescence mechanism for MOF-5 should be LLCT with little mixing of the ligand-23 to-metal charge transfer. The reason for the MOF-5 luminescence is explained by the excellent coplanarity between the six-24 membered ring consisting of zinc, oxygen, carbon, and the benzene ring. 2