Figure 6. Frontier molecular orbitals of Zn-MOF-nitrobenzene.
Based on these results, the Franck-Condon principle and selection rules, we support the hypothesis that upon photoexcitation the electron relaxes until reaching the first excited electronic state, presumably located in de nitrobenzene, from which the dissipates of energy through the non-radiative transition. For that reason, the electron relaxation from PBY linker of the OBA linker is blocked resulting in the turn-off of the fluorescence. Besides, this electronic configuration of the coupled system Zn-MOF-nitrobenzene would explain the experimental interpretation of the quenching fluorescence is due to the LUMO of nitrobenzene appear below the emissive state of the MOF.31
Considering these results, this theoretical insight was carried out from another perspective based on a theoretical approach most commonly reported in the literature. This method consists of separately treating the electro-donor species, MOF, and electro-acceptor species, NACs, at the same level of theory. 58-59 The FMO analysis showed that the LUMO of the nitrobenzene is located lower energy level than the LUMO of Zn-MOF, see Figure 7, thus the photoinduced charge-transfer (PET) between Zn-MOF and nitrobenzene is possible.