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.