Figure 3. Frontier molecular orbitals of Zn-MOF and Zn-MOF-analyte systems.
Furthermore, TD-DFT calculations allowed us to confirm that the absorption spectra only implies electronic transitions involving active orbitals of the OBA linkers. This result was observed regardless of whether the systems do or do not have an analyte, see Table 2.
The absorption spectrum of the Zn-MOF-nitrobenzene showed that the molecular orbitals involved in the absorption bands are localized on the OBA linker. These molecular orbitals are presented in Figure 4, which evidences the excitations of intra-ligand charge-transfer (ILCT) character. Similar results were obtained for Zn-MOF-toluene computed at the same level of theory, see Figure S2 in the Supporting Information.
Table 2. Singlet - Singlet electronic transitions, absorption data, for Zn-MOF-analyte at B3LYP/ def2-TZVPP theoretical level.