Theoretical elucidation of the turn-off mechanism of the luminescence of a chemosensor based on a metal-organic framework (MOF) [Zn2(OBA)4(BYP)2] (BYP: 4,4’-bipyridine; H2OBA: 4,4’-oxybis(benzoic acid)), selective to nitrobenzene via quantum chemical computations is presented. The electronic structure and optical properties of Zn-MOF were investigated through the combination of density functional theory (DFT) and time-dependent-DFT methods. Our results indicate that the fluorescence emission is governed by a linker (BPY) to linker (OBA) charge transfer (LLCT) involving orbitals π-type. Next, interaction with the analyte was analyzed, where very interesting results were obtained, i.e. the LUMO is now composed by orbitals from nitrobenzene, which changes the emissive state of the Zn-MOF. This fact suggests that the LLCT process is blocked, inducing then the fluorescence quenching. Otherwise, the Morokuma-Ziegler energy decomposition and NOCV (Natural Orbitals for Chemical Valence) on the Zn-MOF-nitrobenzene interactions were studied in detail, which illustrate the possible channels of charge transfer between Zn-MOF and nitrobenzene. Finally, we believe that this proposed methodology can be applied to different chemosensor-analyte systems to evidence the molecular and electronic factors that govern the sensing mechanisms.