Theoretical studies on how excited state hydrogen and coordination bonds affect luminescent properties of Metal Organic Framework Cu4(L)4•2EtOH

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) were employed to study the solvent dependent luminescence of metal–organic framework (MOF) Cu4(L)4•2EtOH (L = 5-(4-pyridyl) tetrazole). The truncated representative fragment of Cu4(L)4•2EtOH 1 and its solvated form 2 in ethanol were employed for the computation. The structures of 1 and 2 in S0, S1 and T1 states were fully optimized. Based on our frontier molecular orbital and the electronic configuration analysis of the optimized ground state structures of 1 and 2, the ligand-to-metal charge transfer (LMCT) luminescence was confirmed and the charge transfer pathway during the luminescent process was proved to be elongated by the hydrogen bond which led to the luminescence enhancing effect. Besides, the strengthening of hydrogen bond in S1 and T1 states, proved by our TDDFT calculation, enlarged the HOMO-LUMO energy gap which inhibited non-radiative decay thus in favor of luminescence. Particularly, the behaviors of coordination bonds in S1 and T1 states were investigated. The weakening of hydrogen bond greatly strengthened the coordination bond Cu1―N1 in S1 and T1 states. The strengthening of this coordination bond confined the charge transfer from Cu1 to Cu2 which further enhanced the luminescence.