Host-Guest Interactions in Metal-Organic Frameworks Doped with Acceptor Molecules as Revealed by Resonance Raman Spectroscopy

Host-guest charge transfer for scalable single crystal epitaxy of a metal-organic framework

Methods to grow large crystals provide the foundation for material science and technology. Here we demonstrate single crystal homoepitaxy of a metal-organic framework (MOF) built of zinc, acetate and terephthalate ions, that encapsulate arrays of octahedral zinc dimethyl sulfoxide (DMSO) complex cations within its one-dimensional (1D) channels. The three-dimensional framework is built of two-dimensional Zn-terephthalate square lattices interconnected by anionic acetate pillars through diatomic zinc nodes. The charge of the anionic framework is neutralized by the 1D arrays of Zn ( DMSO ) 6 2 + cations that fill every second 1D channel of the framework. It is demonstrated that the repeatable and scalable epitaxy allows square cuboids of this charge-transfer MOF to grow stepwise to sizes in the centimeter range. The continuous growth with no size limits can be attributed to the ionic nature of the anionic framework with cationic 1D molecular fillers. These findings pave the way for epitaxial growth of bulk crystals of MOFs.

Computational and Machine Learning Methods for CO2 Capture Using Metal-Organic Frameworks

Machine learning (ML) using data sets of atomic and molecular force fields (FFs) has made significant progress and provided benefits in the fields of chemistry and material science. This work examines the interactions between chemistry and materials computational science at the atomic and molecular scales for metal-organic framework (MOF) adsorbent development toward carbon dioxide (CO2) capture. Herein, a connection will be drawn between atomic forces predicted by ML algorithms and the structures of MOFs for CO2 adsorption. Our study also takes into account the successes of atomic computational screening in the field of materials science, especially quantum ML, and its relationship to ML algorithms that clarify advancements in the area of CO2 adsorption by MOFs. Additionally, we reviewed the processes for supplying data to ML algorithms for algorithm training, including text mining from scientific articles, and MOF's formula processing linked to the chemical properties of MOFs. To create ML algorithms for future research, we recommend that the digitization of scientific records can help efficiently synthesize advanced MOFs. Finally, a future vision for developing pioneer MOF synthesis routes for CO2 capture is presented in this review article.

Surface-enhanced Raman scattering nanotag of tunable ZIF-8 shell-encapsulated magnetic core-plasmonic satellites for disentangling chemical enhancement from electromagnetic enhancement

To enhance the stability of Raman reporters, these reporters were trapped in a metal organic framework (MOF) exoskeleton that was grown and compressed on Fe3O4@Au core-satellites, producing recyclable surface-enhanced Raman scattering (SERS) nanotags. Furthermore, encapsulation of Raman reporters in the assembled MOF-based nanocomposites was divided into two types of patterns, pre-enrichment and post-enrichment, in order to disentangle chemical enhancement of charge transfer (CT) from electromagnetic enhancement (EM) in SERS. Hence, to demonstrate the effect of encapsulation, a typical non-thiolated Raman reporter, for example crystal violet (CV) trapped in a core-satellite nanoassembly-based zeolitic imidazolate framework (ZIF-8) shell, was selected. The results suggest that stability and Raman intensity are remarkably improved. Moreover, the pattern of incorporation of CV into the ZIF-8 shell with tunable shell thickness can contribute to the disentangling of CT effects from EM effects.

Highly Luminescent TCNQ in Melamine

Optical properties of molecules change drastically as a result of interactions with surrounding environments as observed in solutions, clusters, and aggregates. Here, we make 7,7,8,8-tetracyanoquinodimethane (TCNQ) highly luminescent by encapsulating it in crystalline melamine. Colored single crystals are synthesized by slow evaporation of aqueous tetrahydrofuran solutions of melamine and TCNQ. Single-crystal X-ray diffraction reveals the lattice structure of pure melamine, meaning that the color is of impurities. Both mass spectrometry and UV-vis spectroscopy combined with density-functional theory calculations elucidate that the impurity species are neutral TCNQ and its oxidation product, dicyano-p-toluoyl cyanide anion (DCTC-), whose concentrations in a melamine crystal can be controlled by adjusting the molar ratio between melamine and TCNQ in the precursor solution. Fluorescence excitation-emission wavelength mappings on the precursor solutions illustrate dominant emissions from DCTC- while the emission from TCNQ is quenched by the resonance energy transfer to DCTC-. On the contrary, TCNQ in crystalline melamine is a bright fluorophore whose emission wavelength centered at 450 nm with internal quantum yields as high as 19% and slow fluorescence lifetimes of about 2 ns. The method of encapsulating molecules into transparent melamine would make many other molecules fluorescent in solids.

Rationally Designing High-Performance Versatile Organic Memristors through Molecule-Mediated Ion Movements

Organic memory has attracted tremendous attention for next-generation electronic elements for the molecules' striking ease of structural design. However, due to them being hardly controllable and their low ion transport, it is always essential and challenge to effectively control their random migration, pathway, and duration. There are very few effective strategies, and specific platforms with a view to molecules with specific coordination-groups-regulating ions have been rarely reported. In this work, as a generalized rational design strategy, the well-known tetracyanoquinodimethane (TCNQ) is introduced with multiple coordination groups and small plane structure into a stable polymers framework to modulate Ag migration and then achieve high-performance devices with ideal productivity, low operation voltage and power, stable switching cycles, and state retention. Raman mapping demonstrates that the migrated Ag can specially coordinate with the embedded TCNQ molecules. Notably, the TCNQ molecule distribution can be modulated inside the polymer framework and regulate the memristive behaviors through controlling the formed Ag conductive filaments (CFs) as demonstrated by Raman mapping, in situ conductive atomic force microscopy (C-AFM), X-ray diffraction (XRD) and depth-profiling X-ray photoelectron spectroscopy (XPS). Thus the controllable molecule-mediated Ag movements show its potential in rationally designing high-performance devices and versatile functions and is enlightening in constructing memristors with molecule-mediated ion movements.

Electronic modulation and structural engineering of tetracyanoquinodimethane with enhanced reaction kinetics for aqueous NH4+ storage

Lithium-ion batteries (LIBs) have received much attention because of their environmental, financial, and safety concerns. The advantages of aqueous electrochemical energy storage include environmental friendliness and safety, and the development of prepared electrode materials is predicted to alleviate these issues. A redox-active organic compound, 7,7,8,8‑tetracyanoquinodimethane (TCNQ), is a suitable electrode for aqueous batteries. In this work, the porous and electronic interconnected structure of TCNQ is designed by electronic modulation and structure engineering. With the reduced graphene oxide (rGO) in situ homogeneous loading TCNQ by a one-step facile approach, the exquisite architecture has enhanced conductivity and connected conductive networks, favoring the storage and transportation of NH₄⁺ or electrons in aqueous electrolytes. As a cathode, the obtained TCNQ-rGO exhibits superior performance for NH₄⁺ batteries with an improved reversible capacity of 92.7 mAh/g at 1 A/g of quadruple capacity boosting to pure TCNQ and stable cycle life (5000 cycles at 10 A/g). The adjustment of the loading ratio of TCNQ and rGO for the cycling performance has been studied in detail. Furthermore, the superior ammonium storage mechanism of the TCNQ-rGO hybrid is thoroughly discussed by in situ Raman or ex situ measurements, which also determine the redox activity center groups of the TCNQ-rGO hybrid. Energy level calculations are conducted to help illustrate its potential as an electrode material. Our work demonstrates that electronic modulation and structural engineering of TCNQ can improve the electrochemical performance of molecular organic compound-based electrodes for aqueous rechargeable batteries in a simple and effective way.

Enhancing Third-Order Nonlinear Optical Property by Regulating Interaction between Zr4(embonate)6 Cage and N, N-Chelated Transition-Metal Cation

Herein, the combination of anionic Zr₄L₆ (L = embonate) cages and N, N-chelated transition-metal cations leads to a series of new cage-based architectures, including ion pair structures (PTC-355 and PTC-356), dimer (PTC-357), and 3D frameworks (PTC-358 and PTC-359). Structural analyses show that PTC-358 exhibits a 2-fold interpenetrating framework with a 3,4-connected topology, and PTC-359 shows a 2-fold interpenetrating framework with a 4-connected dia network. Both PTC-358 and PTC-359 can be stable in air and other common solvents at room temperature. The investigations of third-order nonlinear optical (NLO) properties indicate that these materials show different degrees of optical limiting effects. It is surprising that increasing coordination interactions between anion and cation moieties can effectively enhance their third-order NLO properties, which can be attributed to the formation of coordination bonds that facilitate charge transfer. In addition, the phase purity, UV-vis spectra, and photocurrent properties of these materials were also studied. This work provides new ideas for the construction of third-order NLO materials.

A Modified Oxazolone Dye Dedicated to Spectroscopy and Optoelectronics

Here we present a newly synthesized bifunctional organic chromophore with appealing spectroscopic and nonlinear optical features. The positions of absorption and emission maxima of the dye vary with increasing solvent polarity and exhibit positive solvatochromism. The determined change in the dipole moment upon excitation based on the Bilot and Kawski theory is 5.94 D, which corresponds to the intermolecular displacement of a charge equal to 1.24 Å. An investigated organic-based system represents a significant, repeatable, and stable over time optical signal modulation in the manner of the refractive index value. Its magnitude is varied both by optical pumping intensity as well as by external frequency modulation, which indicates that such system is an alluring and alternative core unit for optoelectronic devices and complex networks. Then, the same active system, due to the nonresonant mechanism of higher harmonics of light inducement, can provide second and third harmonic signals. According to the introduced laser line spatial modifications (parallel or perpendicular polarization directions), it is resulted in output SHG signal with magnitude varied about 100%. Its magnitude is noticeably small; however, to construct sensitive optical sensors or infrared indicators, such feature may guarantee satisfying circumstances.

Utilizing Raman Spectroscopy as a Tool for Solid- and Solution-Phase Analysis of Metalloorganic Cage Host-Guest Complexes

The host-guest chemistry of coordination cages continues to promote significant interest, not least because confinement effects can be exploited for a range of applications, such as drug delivery, sensing, and catalysis. Often a fundamental analysis of noncovalent encapsulation is required to provide the necessary insight into the design of better functional systems. In this paper, we demonstrate the use of various techniques to probe the host-guest chemistry of a novel Pd₂L₄ cage, which we show is preorganized to selectively bind dicyanoarene guests with high affinity through hydrogen-bonding and other weak interactions. In addition, we exemplify the use of Raman spectroscopy as a tool for analyzing coordination cages, exploiting alkyne and nitrile reporter functional groups that are contained within the host and guest, respectively.

Donor–acceptor systems in metal–organic frameworks: design, construction, and properties

In this highlight, the development of donor acceptor (D–A) MOF was briefly reviewed and summarized in the aspects of design, construction, and properties. Also, an outlook about the research and potential application of D–A MOF has been presented.

Threefold reactivity of a COF-embedded rhenium catalyst: reductive etherification, oxidative esterification or transfer hydrogenation

The reactivity of the novel Re(i) catalyst [Re(C12Anth-py2)(CO)3Br] is modulated by its interactions with the covalent organic framework (COF) TFB-BD.

Quantum Capacitance through Molecular Infiltration of 7,7,8,8-Tetracyanoquinodimethane in Metal-Organic Framework/Covalent Organic Framework Hybrids

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been extensively investigated during the last two decades. More recently, a family of hybrid materials (i.e., MOF@COF) has emerged as particularly appealing for gas separation and storage, catalysis, sensing, and drug delivery. MOF@COF hybrids combine the unique characteristics of both MOF and COF components and exhibit peculiar properties including high porosity and large surface area. In this work, we show that the infiltration of redox-active 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules into the pores of MOF@COF greatly improves the characteristics of the latter, thereby attaining high-performance energy storage devices. Density functional theory (DFT) calculations were employed to guide the design of a MOF@COF-TCNQ hybrid with the TCNQ functional units incorporated in the pores of MOF@COF. To demonstrate potential application of our hybrids, the as-synthesized MOF@COF-TCNQ hybrid has been employed as an active material in supercapacitors. Electrochemical energy storage analysis revealed outstanding supercapacitor performance, as evidenced by a specific areal capacitance of 78.36 mF cm^-2 and a high stack volumetric energy density of 4.46 F cm^-3, with a capacitance retention of 86.4% after 2000 cycles completed at 0.2 A cm^-2. DFT calculation results strongly indicate that the high capacitance of MOF@COF-TCNQ has a quantum capacitance origin. Our liquid-phase infiltration protocol of MOF@COF hybrids with redox-active molecules represents a efficacious approach to design functional porous hybrids.

Comprehensive and High-throughput Electrolysis of Water and Urea by 3-5 nm Nickel and Copper Coordination Polymers

Metal-organic coordination polymers (CP) have attracted the scientific attention for electrochemical water oxidation as it has the similar coordination structure like natural photosynthetic coordinated complex. However, the harsh synthesis conditions and bulky nature pose a major challenge in the field of catalysis. Herein, 3-5 nm CP particles synthesized at room temperature using aqueous solutions of Ni²⁺ /Cu²⁺ and 2,5-dihydroxyterepthalic acid as precursor were applied for alkaline water and urea electrolysis. The overpotential required is only 300 mV at 10 mA cm^-2 by Nano-Ni CP for water oxidation, with turnover frequency (TOF) of 21.4 s^-1 which is around 8 times higher than its bulk-counterpart. Overall water and urea splitting were achieved with Nano-Cu (-) ∥ Nano-Ni (+) couple on Ni foam at 1.69 and 1.52 V to achieve 10 mA cm^-2 , respectively. High electrochemical surface area (ECSA), high TOF, and enhanced mass diffusion are found to be the key parameters responsible for the state-of-the-art water and urea splitting performances of nano-CPs as compared to their bulk counterparts.

High Loading of Air-Sensitive Guest Molecules into Polycrystalline Metal-Organic Framework Hosts

Loading air-sensitive guest molecules inside polycrystalline metal-organic framework (MOF) hosts is currently a challenging process. In this study, the air-sensitive guest molecule magnesocene (MgCp₂) was loaded into two porous MOF hosts, polycrystalline Ni-MOF-74 and NH₂-MIL-101(Al), using a gas-phase infiltration process. X-ray powder diffraction, Fourier transform infrared spectroscopy, scanning transmission electron microscopy, and scanning transmission electron microscopy-energy-dispersive X-ray mapping measurements demonstrated that MgCp₂ was successfully loaded inside the three-dimensional pores of NH₂-MIL-101(Al) with a maximum loading of 43.1 wt %. MgCp₂ was found to cover the outside of Ni-MOF-74 owing to the small one-dimensional channels.

Rational incorporation of defects within metal-organic frameworks generates highly active electrocatalytic sites

The allure of metal–organic frameworks (MOFs) in heterogeneous electrocatalysis is that catalytically active sites may be designed a priori with an unparalleled degree of control. An emerging strategy to generate coordinatively-unsaturated active sites is through the use of organic linkers that lack a functional group that would usually bind with the metal nodes. To execute this strategy, we synthesize a model MOF, Ni-MOF-74 and incorporate a fraction of 2-hydroxyterephthalic acid in place of 2,5-dihydroxyterephthalic acid. The defective MOF, Ni-MOF-74D, is evaluated vs. the nominally defect-free Ni-MOF-74 with a host of ex situ and in situ spectroscopic and electroanalytical techniques, using the oxidation of hydroxymethylfurtural (HMF) as a model reaction. The data indicates that Ni-MOF-74D features a set of 4-coordinate Ni–O₄ sites that exhibit unique vibrational signatures, redox potentials, binding motifs to HMF, and consequently superior electrocatalytic activity relative to the original Ni-MOF-74 MOF, being able to convert HMF to the desired 2,5-furandicarboxylic acid at 95% yield and 80% faradaic efficiency. Furthermore, having such rationally well-defined catalytic sites coupled with in situ Raman and infrared spectroelectrochemical measurements enabled the deduction of the reaction mechanism in which co-adsorbed *OH functions as a proton acceptor in the alcohol oxidation step and carries implications for catalyst design for heterogeneous electrosynthetic reactions en route to the electrification of the chemical industry.

The allure of metal–organic frameworks (MOFs) in heterogeneous electrocatalysis is that catalytically active sites may be designed a priori with an unparalleled degree of control.