Figure 2. Molecular orbital energy diagram for Zn-MOF showing the vertical electronic transitions for the maxima absorption band at B3LYP/def2-TZVPP theoretical level.
In order to carry out a strong analysis of host-guest interactions, the Zn-MOF ground state was optimized with the incorporation of the analytes. The analytes were selected considering the reported largest and the lowest quenching efficiency of the Zn-MOF fluorescence;i.e. nitrobenzene (quenching, 84%) and toluene (non-quenching).31 All Zn-MOF-analyte systems were optimized at the same theoretical level as the free Zn-MOF. The proposed input geometries consisted on the pi-staking/aromatic interaction between the linkers and target molecules. Two interaction models were considered for each analyte, one where the interaction occurs with the aromatic ring of the OBA linker while the other displays the interaction with the BPY fragment. Both initial structures converged to the host-guest interaction between the analyte and BPY, with a distance between them within the range of 3.15–3.34 Å. Zn-MOF-nitrobenzene and Zn-MOF-toluene minima structures were selected for the calculations and the analyzes described below. The geometrical parameters computed for the Zn-MOF interacting with the analytes did not show significant differences in relation to the ground state of the free Zn-MOF, see Table S1, Supporting Information.