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.