Conclusions
Quantum chemical approaches were successfully employed to assess the
relationship between the structure of the LMOF and analyte-induced
luminescence change.
First, we found that the molecular orbitals involved in the absorption
bands of the spectrum profile of the Zn-MOF, are localized on the
4,4’-oxybis (benzoic acid) (OBA) linker. Besides, the emission
phenomenon was characterized as fluorescence emission via LLCT
processes, which takes place from 4,4’-bipyridine (BYP) to the
4,4’-oxybis (benzoic acid) (OBA) linker. As second part of the work,
analyte (nitrobenzene) confinement was studied into the Zn-MOF, which
gave rise to a mixture of the molecular orbitals from both systems.
Interesting, the LUMO corresponds to orbitals of the nitrobenzene and
appears energetically close to the molecular orbital localized at the
BYP, which is involved in the emissive state of the free Zn-MOF. These
results suggest that the LUMO, located on nitrobenzene, blocked the LLCT
process between BPY and OBA, leading to the fluorescence quenching.
Therefore, it is demonstrated that the simulation of the host-guest
system is imperative to understand the luminescence changes that govern
the sensing mechanism in a chemosensor. Otherwise, the study of the
interaction via the Morokuma-Ziegler decomposition scheme and Natural
Orbital of Chemical Valence (NOCV) analysis, proposed by the Mitoraj,
were useful to propose possible channels of charge transfer between
Zn-MOF and nitrobenzene.
In summary, the intention of this study was to get theoretical insights
into the sensing mechanism MOF chemosensor selective to nitrobenzene
through analysis of host-guest interactions. Due to the large size of
this system, it was applied a fragmentation scheme of experimental X-ray
single-crystal data of the Zn-MOF, reaching a finite structural model
[Zn2(OBA)4(BYP)2] to
optimize time and computational resources. This structural model has
satisfactorily reproduced geometry parameters and photophysical
properties of the Zn-MOF with and without the analyte.