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