Scheme 1. Schematic representation of a chemosensor: (a ) Turn-on of the luminescence, (b) Turn-off of the luminescence.
In this sense, luminescent chemical sensors based on MOFs have emerged as a successful alternative for sensing of NACs as they show intense luminescence, low limits of detection (LODs) (up to single-molecule level), specificity, tunable porosity, chemical functionality and the ability to use them as powdered materials directly without further treatments. Specifically, luminescent MOFs (LMOFs), based on d10 metal ions, have been of great interest for designing and synthesizing chemosensors selective to NACs. Regarding to this, Allendorf et al. in 2007 21⁠ pointed out the importance of the design of LMOFs based on closed-shell metal ions, and also taking into account that it is possible to tune the linker-centered emission by modifying the structure of the linker. This fact is mostly studied since this type of MOFs are unlikely metal light emisor, as the Zn-based and Cd-based LMOFs reported by W. Liu and co-workers.22 Based on these premises, Lustig et al. (2016)23⁠ developed a series of LMOFs with ligand-centered emission. They performed the modification of the linkers (tetrakis(4-carboxyphenyl) ethylene) by both fluorination and elongation. Thus, the quantum yield of luminescence and thermal stability were improved in these systems, concluding that the adequate selection of the linker, whose emission can be increased or tuned, is a versatile strategy to design LMOFs based on closed-shell transition metal ions. In recent years, vast published literature has been reported about LMOFs in the area of chemical sensors selective to explosive aromatic compounds. In most cases, the most efficient reported mechanism of transduction, is through a luminescence quenching response (turn-off mechanism). This fact is due to the excellent electron-donating capability of MOFs, the strong electron-accepting ability of NACs and probability of photoinduced electron transfer (PET) between both systems.24-25 In this mechanism, the detection pathway of the analyte involves a PET from the LMOFs to the guest deactivating the electronic emissive state. Here, the extinction of the luminescence requires an electron structure where the analyte orbitals have adequate energy to produce the PET. 26⁠-27 Regarding this, several studies of density functional theory (DFT) calculations have been reported, providing a more detailed description of the proposed sensing mechanism. From these results, researchers have proposed that the most probable PET mechanism is due to electron transfer from the conduction band (CB) of the MOF to lowest unoccupied molecular orbital (LUMO) of NACs.24-28 Nevertheless, since most of the theoretical reports deal the computations of the MOF and analyte separately, which have limited the analysis to an orbital energy comparison between both structures, it becomes imperative to address the study of host-guest systems to design MOFs as sensors. This last is a remarkable topic recalled in literature due to their role in changes on the photophysical properties that govern the recognition mechanisms of analytes.29⁠⁠-30 That is why we consider that MOF-analyte interaction study is a key to understand the path of the activation or deactivation of the luminescence in a sensing process in a more accurately protocol. The current status of computational methods and theoretical chemistry opens the possibility of investigating photo-physical processes that modulate the luminescent properties of LMOFs chemosensors. In this work, we investigated and assessed the turn-off fluorescence mechanism of the Zn-based LMOF [Zn2(OBA)2(BYP)]DMA, where H2OBA: 4,4’-oxybis (benzoic acid); BYP: 4,4’-bipyridine; DMA = N, N’-dimethylacetamide, through quantum mechanics calculations, synthesized and reported by Jing Li et al. in 2011,⁠31 as a selective chemical sensor to high explosive NACs. A methodology including molecular and electronic properties of the systems, for evaluating the relationship between the structure of the LMOF and analyte-induced luminescence change was successfully established. In terms of MOF-analyte interaction, we are interested in the contribution of each term to the total interaction energy, i.e , electrostatic interaction, orbital interaction, a dispersive interaction as well as repulsive Pauli interaction. In this sense, it was demonstrated that the energy decomposition scheme proposed by Morokuma-Ziegler provides valuable information to this regard as well as NOCV (Natural Orbitals for Chemical Valence) calculations to characterize the charge transfer channels.32⁠-33