Adsorption properties of M-UiO-66 (M = Zr(IV); Hf(IV) or Ce(IV)) with BDC or PDC linker.

The increasing CO2 emissions and their direct impact on climate change due to the greenhouse effect are environmental issues that must be solved as soon as possible. Metal-organic frameworks (MOFs) are one class of crystalline adsorbent materials that are thought to have enormous potential in CO2 capture applications. In this research, the effect of changing the metal center between Zr(IV), Ce(IV), and Hf(IV), and the linker between BDC and PDC has been fully studied. Thus, the six UiO-66 isoreticular derivatives have been synthesized and characterized by FTIR, PXRD, TGA, and N2 adsorption. We also report the BET surface area, CO2 adsorption capacities, kinetics, and the adsorption isosteric heat (Qst) of the UiO-66 derivatives mentioned family. The CO2 adsorption kinetics were evaluated using pseudo-first order, pseudo-second order, Avrami's kinetic models, and the rate-limiting step with Boyd's film diffusion, interparticle diffusion, and intraparticle diffusion models. The isosteric heats of CO2 adsorption using various MOFs are in the range 20-65 kJ mol-1 observing differences in adsorption capacities between 1.15 and 4.72 mmol g-1 at different temperatures due to the electrostatic interactions between CO2 and extra-framework metal ions. The isosteric heat of adsorption calculation in this report, which accounts for the unexpectedly high heat released from Zr-UiO-66-PDC, is finally represented as an increase in the interaction of CO2 with the PDC linker and an increase in Qst with defects.

Tunable optical properties of isoreticular UiO-67 MOFs for photocatalysis: a theoretical study.

A theoretical study of the reported photocatalytic systems based on Zr-based MOF (UiO-67) with biphenyl-4,4'-dicarboxylic acid (bpdc) and 2,2'-bipyridine-5,5'-dicarboxylic acid (bpydc) as linkers was performed. Quantum chemical calculations were carried out to understand the optical properties of the materials and to facilitate the rational design of new UiO-67 derivatives with potentially improved features as photocatalysts under ambient conditions. Hence, the effect of the structural modifications on the optical properties was studied considering different designs based on the nature of the linkers: in 1 only the bpdc linker was considered, or the mixture 1 : 1 between bpdc and bpydc linkers (labeled as 1A). Also, substituents R, -NH2, and -SH, were included in the 1A MOF only over the bpdc linker (labeled as 1A-bpdc-R) and on both bpdc and bpydc linkers (labeled as 1A-R). Thus a family of six isoreticular UiO-67 derivatives was theoretically characterized using Density Functional Theory (DFT) calculations on the ground singlet (S0) and first excited states (singlet and triplet) using Time-Dependent Density Functional Theory (TD-DFT), multiconfigurational post-Hartree-Fock method via Complete Active Space Self-Consistent Field (CASSCF). In addition, the use of periodic DFT calculations suggest that the energy transfer (ET) channel between bpdc and bpydc linkers might generate more luminescence quenching of 1A when compare to 1. Besides, the results suggest that the 1A-R (R: -SH and NH2) can be used under ambient conditions; however, the ET exhibited by 1A, cannot take place in the same magnitude in these systems. These ET can favor the photocatalytic reduction of a potential metal ion, that can coordinate with the bpydc ligand, via LMCT transition. Consequently, the MOF might be photocatalytically active against molecules of interest (such as H2, N2, CO2, among others) with photo-reduced metal ions. These theoretical results serve as a useful tool to guide experimental efforts in the design of new photocatalytic MOF-based systems.

UiO-66(Zr) as drug delivery system for non-steroidal anti-inflammatory drugs.

The toxicity for the human body of non-steroidal anti-inflammatory drugs (NSAIDs) overdoses is a consequence of their low water solubility, high doses, and facile accessibility to the population. New drug delivery systems (DDS) are necessary to overcome the bioavailability and toxicity related to NSAIDs. In this context, UiO-66(Zr) metal-organic framework (MOF) shows high porosity, stability, and load capacity, thus being a promising DDS. However, the adsorption and release capability for different NSAIDs is scarcely described. In this work, the biocompatible UiO-66(Zr) MOF was used to study the adsorption and release conditions of ibuprofen, naproxen, and diclofenac using a theoretical and experimental approximation. DFT results showed that the MOF-drug interaction was due to an intermolecular hydrogen bond between protons of the groups in the defect sites, (µ3 - OH, and - OH2) and a lone pair of oxygen carboxyl functional group of the NSAIDs. Also, the experimental results suggest that the solvent where the drug is dissolved affects the adsorption process. The adsorption kinetics are similar between the drugs, but the maximum load capacity differs for each drug. The release kinetics assay showed a solvent dependence kinetics whose maximum liberation capacity is affected by the interaction between the drug and the material. Finally, the biological assays show that none of the systems studied are cytotoxic for HMVEC. Additionally, the wound healing assay suggests that the UiO-66(Zr) material has potential application on the wound healing process. However, further studies should be done.

Tuning the visible-NIR absorption of azulenocyanine-based photosensitizers.

A new photosensitizer 1-WS55 (dyad) based on two dyes with excellent properties, azulenocyanine (1) and WS55, is proposed at the density functional theory level (M06/def2-SVP). 1 is a dye having a broad NIR absorption (~ 1000 nm), and WS55 is a metal-free organic dye that presents a huge photoelectric conversion efficiency (PCE) of 9.5%. The dyad presents a panchromatic absorption along the UV-Vis-NIR region. It exhibits two intense Q bands (880, 926 nm) in the NIR region, one strong band (672 nm) in the visible region, and several bands in 300-600 nm. Charge transfer bands in the dyad from 1 to WS55 were found in the visible region, which favors the adsorption on an anatase TiO2 surface. The interaction energies dyad (dye)-TiO2 were calculated as a periodic system and corrected by the basis set superposition error. These show better adsorption for the dyad than fragments 1 and WS55. The electron injection calculated from the dye (dyad) to TiO2 suggests an efficient solar energy conversion because of ΔGinj > 0.2 eV. Additionally, calculations performed for the reorganization energy of electrons and holes indicate that the dyad presents the highest charge mobility. In summary, the dyad proposed 1-WS55 constitutes an excellent candidate to be used as a potential photosensitizer for the DSSCs.

Expanding the Knowledge of the Selective-Sensing Mechanism of Nitro Compounds by Luminescent Terbium Metal-Organic Frameworks through Multiconfigurational ab Initio Calculations.

The current research shows that the excited-state dynamics of the antenna ligand, both in the interacting system sensor/analyte and in the sensor without analyte, is a safe tool for elucidating the detection principle of the luminescent lanthanide-based metal-organic framework sensors. In this report the detection principle of the luminescence quenching mechanism in two Tb-based MOFs sensors is elucidated. The first system is a luminescent Tb-MOF [Tb(BTTA)1.5(H2O)4.5]n (H2BTTA = 2,5-bis(1H-1,2,4-triazol-1-yl) terephthalic acid) selective to nitrobenzene (NB), labeled as Tb-1. The second system is {[Tb(DPYT)(BPDC)1/2(NO3)]·H2O}n (DPYT = 2,5-di(pyridin-4-yl) terephthalic acid, BPDC = biphenyl-4,4'-dicarboxylic acid), reported as a selective chemical sensor to nitromethane (NM) in situ, labeled as Tb-2. The luminescence quenching of the MOFs is promoted by intermolecular interactions with the analytes that induce destabilization of the T1 electronic state of the linker "antenna", altering thus the sensitization pathways of the Tb atoms. This study demonstrates the value of host-guest interaction simulations and the rate constants of the radiative and nonradiative processes in understanding and elucidating the sensing mechanism in Ln-MOF sensors.

Understanding the Deactivating/Activating Mechanisms in Three Optical Chemosensors Based on Crown Ether with Na+ /K+ Selectivity Using Quantum Chemical Tools.

The optical properties and transduction mechanisms in three reported optical chemosensors based on crown ether with selectivity turn-on luminescence toward Na+ over K+ , were investigated using Density Functional Theory/Time-Dependent Density Functional Theory (DFT/TD-DFT). The analysis of the structural stability of the conformers enables us to understand the optical properties of the sensors and their selectivity toward Na+ . The UV-Vis absorption and the radiative channels of the adiabatic S1 excited state were assessed. In these reported sensors, the Photoinduced Electron Transfer (PET) from the nitrogen and the oxygen (O-atoms of the substituted N-phenylaza group) lone pairs to fluorophore groups lead to a nonradiative deactivation process in the fluorophore to p-conjugated anilino-1,2,3-triazol ionophore. This Intramolecular Charge Transfer (ICT) deactivation produced the luminescence quenching in the free sensors and K+ /C1 complexes. The Na+ /sensor interaction produced a Chelation Enhanced Fluorescence (CHEF) due to the inhibition of the PET and ICT, which was confirmed via the calculated oscillator strength of the emission process. The K+ /sensor interaction displayed the possibility of PET in C3; however, this fact was inconclusive to affirm the quenching of luminescence, the CHEF in C2 and C3 and the selectivity toward Na+ over K+ in these systems. For this reason, simulation of the absorption and emissions spectra (calculated oscillator strength), calculation of the kinetic parameters (in charge transfers and radiative deactivations process), analysis of the metal-ligand interaction character, and the analysis of the structural stability of the conformers were determinant factors to understand the selectivity and the optical properties of these chemosensors. The results suggest that these theoretical tools can also be used to predict the optical properties and Na+ /K+ selectivity of optical chemosensors.

New Sensitive and Selective Chemical Sensors for Ni2+ and Cu2+ Ions: Insights into the Sensing Mechanism through DFT Methods.

We report the synthesis and theoretical study of two new colorimetric chemosensors with special selectivity and sensitivity to Ni2+ and Cu2+ ions over other metal cations in the CH3CN/H2O solution. Compounds (E)-4-((2-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (A) and (E)-4-((3-nitrophenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)aniline (B) exhibited a drastic color change from yellow to colorless, which allows the detection of the mentioned metal cations through different techniques. The interaction of sensors with these metal ions induced a new absorption band with a hypsochromic shift to the characteristic signal of the free sensors. A theoretical study via time-dependent density functional theory (TD-DFT) was performed. This method has enabled us to reproduce the hypsochromic shift in the maximum UV-vis absorption band and explain the selective sensing of the ions. For all of the systems studied, the absorption band is characterized by a π → π* transition centered in the ligand. Instead of Ni2+ and Cu2+ ions, the transition is set toward the σ* molecular orbital with a strong contribution of the 3dx2-y2 transition (π → 3dx2-y2). These absorptions imply a ligand-to-metal charge transfer (LMCT) mechanism that results in the hypsochromic shift in the absorption band of these systems.

Kinetic study of removal heavy metal from aqueous solution using the synthetic aluminum silicate.

One of the problems that most affect humanity today is the wastewater discharge into different water bodies. It was estimated that more than 7 million tons of wastewater are generated worldwide and are discharged into rivers, lakes, and reservoirs. Among the most dangerous wastewaters are those from inorganic chemistry research laboratories, mainly due to heavy metals. These problems have become a highly relevant topic, and numerous researchers have tried to design wastewater treatment systems that will deal more efficiently with heavy metals elimination. In this work, the synthesis, characterization, and evaluation of hydrated aluminium silicate were performed as alternative wastewater treatment from chemistry research and teaching laboratories. The compound obtained was [Formula: see text], which was characterized by the determination of its physicochemical properties. These revealed a low density, very porous material, with low crystallinity, strong chemical resistance, a large surface area, and a high apparent ionic exchange capacity. Absorption kinetics studies of heavy metals in aqueous solutions, through more widespread models, have demonstrated that [Formula: see text] has excellent properties as absorbents of this material. The amorphous hydrated aluminium silicate achieves a decrease in the concentration of all the metal ions studied, reducing them to discharge levels permissible.

Sensing mechanism elucidation of a europium(III) metal-organic framework selective to aniline: A theoretical insight by means of multiconfigurational calculations.

A theoretical procedure, via quantum chemical computations, to elucidate the detection principle of the turn-off luminescence mechanism of an Eu-based Metal-Organic Framework sensor (Eu-MOF) selective to aniline, is accomplished. The energy transfer channels that take place in the Eu-MOF, as well as understanding the luminescence quenching by aniline, were investigated using the well-known and accurate multiconfigurational ab initio methods along with sTD-DFT. Based on multireference calculations, the sensitization pathway from the ligand (antenna) to the lanthanide was assessed in detail, that is, intersystem crossing (ISC) from the S1 to the T1 state of the ligand, with subsequent energy transfer to the 5 D0 state of Eu3+ . Finally, emission from the 5 D0 state to the 7 FJ state is clearly evidenced. Otherwise, the interaction of Eu-MOF with aniline produces a mixture of the electronic states of both systems, where molecular orbitals on aniline now appear in the active space. Consequently, a stabilization of the T1 state of the antenna is observed, blocking the energy transfer to the 5 D0 state of Eu3+ , leading to a non-emissive deactivation. Finally, in this paper, it was demonstrated that the host-guest interactions, which are not taken frequently into account by previous reports, and the employment of high-level theoretical approaches are imperative to raise new concepts that explain the sensing mechanism associated to chemical sensors.

Understanding the Selective-Sensing Mechanism of Al3+ Cation by a Chemical Sensor Based on Schiff Base: A Theoretical Approach.

A methodology that allows us to explain the experimental behavior of a turn-on luminescent chemosensor is proposed and verified in 1-[(1H-1,2,4-triazole-3-ylimino)-methyl]-naphthalene-2-ol] (L1), selective to Al3+ cations. This sensor increases its emission when interacting with ions upon excitation at 442 nm, which is denoted as the chelation-enhanced fluorescence effect. Photoinduced electron transfer is responsible for the fluorescence quenching in L1 at 335 nm, in Ni2+/L1 at 385 nm, and in Zn2+/L1 at 378 nm. In Ni2+/L, ligand-to-metal charge transfer (LMCT), from the molecular orbital of the ligand to the Ni 3dx2 - y2 orbital, can contribute to the quenching of fluorescence. Based on oscillator strength, the highest luminescence intensity of L1 at 401 nm and that of Al3+/L1 at 494 nm in relation to the others is evidenced. The consideration of the relative energies of the excited states and the calculation of the rate and lifetime of the electron transfer deactivation are necessary to get a good description of the sensor.