Structural Transformation in Metal-Organic Frameworks for Reversible Binding of Oxygen

A singlet oxygen-storing covalent organic framework for "Afterglow" photodynamic therapy

Photodynamic therapy (PDT) is an emerging treatment but often restricted by the availability of oxygen. Enhancing the lifespan of singlet oxygen (1O2) by fractionated generation is an effective approach to improve the efficacy of PDT. Herein, an imine-based nanoscale COF (TpDa-COF) has been synthesized and functionalized with a pyridone-derived structure (Py) to create a 1O2-storing nanoplatform TpDa-COF@Py, which can reversibly capture and release 1O2. Under 660 nm laser exposure, Py interacts with 1O2 produced by the porphyrin motif in COF backbones to generate 1O2-enriched COF (TpDa-COF@Py + hv), followed by the release of 1O2 through retro-Diels-Alder reactions at physiological temperatures. The continuous producing and releasing of 1O2 upon laser exposure leads to an "afterglow" effect and a prolonged 1O2 lifespan. In vitro cytotoxicity assays demonstrates that TpDa-COF@Py + hv exhibits an extremely low half-maximal inhibitory concentration (IC50) of 0.54 µg/mL on 4T1 cells. Remarkably, the Py-mediated TpDa-COF@Py nanoplatform demonstrates enhanced cell-killing capability under laser exposure, attributed to the sustained 1O2 cycling, compared to TpDa-COF alone. Further in vivo assessment highlights the potential of TpDa-COF@Py + hv as a promising strategy to enhance phototheronostics and achieve effective tumor regression. Accordingly, the study supplies a generalized 1O2 "afterglow" nanoplatform to improve the effectiveness of PDT.

Bioinspired Photoactive Cu-Halide Coordination Polymers for Reduction Activation and Oxygen Conversion

Natural copper oxygenases provide fundamental principles for catalytic oxidation with kinetically inert molecular oxygen, but it remains a marked challenge to mimic both their structure and function in an entity. Inspired by the CuA enzymatic sites, herein we report two new photoactive binuclear copper-iodine- and bisbenzimidazole-comodified coordination polymers to reproduce the natural oxo-functionalization repertoire in a unique photocatalytic pathway. Under light irradiation, the Cu-halide coordination polymers effectively reduce NHP esters and complete oxygen reduction activation via photoinduced electron transfer for the aerobic oxidative coupling of hydroquinone with terminal alkynes, affording hydroxyl-functionalized ketones with high efficiency and selectivity. This supramolecular approach to developing bioinspired artificial oxygenases that merge transition metal- and photocatalysis supplies a new way to fabricate distinctive photocatalysts with desirable catalytic performances and controllable precise active sites.

Enhancing Fractionated Cancer Therapy: A Triple-Anthracene Photosensitizer Unleashes Long-Persistent Photodynamic and Luminous Efficacy

Conventional photodynamic therapy (PDT) is often limited in treating solid tumors due to hypoxic conditions that impede the generation of reactive oxygen species (ROS), which are critical for therapeutic efficacy. To address this issue, a fractionated PDT protocol has been suggested, wherein light irradiation is administered in stages separated by dark intervals to permit oxygen recovery during these breaks. However, the current photosensitizers used in fractionated PDT are incapable of sustaining ROS production during the dark intervals, leading to suboptimal therapeutic outcomes (Table S1). To circumvent this drawback, we have synthesized a novel photosensitizer based on a triple-anthracene derivative that is designed for prolonged ROS generation, even after the cessation of light exposure. Our study reveals a unique photodynamic action of these derivatives, facilitating the direct and effective disruption of biomolecules and significantly improving the efficacy of fractionated PDT (Table S2). Moreover, the existing photosensitizers lack imaging capabilities for monitoring, which constraints the fine-tuning of irradiation parameters (Table S1). Our triple-anthracene derivative also serves as an afterglow imaging agent, emitting sustained luminescence postirradiation. This imaging function allows for the precise optimization of intervals between PDT sessions and aids in determining the timing for subsequent irradiation, thus enabling meticulous control over therapy parameters. Utilizing our novel triple-anthracene photosensitizer, we have formulated a fractionated PDT regimen that effectively eliminates orthotopic pancreatic tumors. This investigation highlights the promise of employing long-persistent photodynamic activity in advanced fractionated PDT approaches to overcome the current limitations of PDT in solid tumor treatment.

Ratiometric Electrochemiluminescence of Zirconium Metal-Organic Framework as a Single Luminophore for Sensitive Detection of HPV-16 DNA

This study developed a new zirconium metal-organic framework (MOF) luminophore named Zr-DPA@TCPP with dual-emission electrochemiluminescence (ECL) characteristics at a resolved potential. First, Zr-DPA@TCPP with a core-shell structure was effectively synthesized through the self-assembly of 9,10-di(p-carboxyphenyl)anthracene (DPA) and 5,10,15,20-tetra(4-carboxyphenyl)porphyrin (TCPP) as the respective organic ligands and the Zr cluster as the metal node. The reasonable integration of the two organic ligands DPA and TCPP with ECL properties into a single monomer, Zr-DPA@TCPP, successfully exhibited synchronous anodic and cathodic ECL signals. Besides, due to the impressively unique property of ferrocene (Fc), which can quench the anodic ECL but cannot affect the cathodic ECL signal, the ratiometric ECL biosensor was cleverly designed by using the cathode signal as an internal reference. Thus, combined with DNA recycle amplification reactions, the ECL biosensor realized sensitive ratiometric detection of HPV-16 DNA with the linear range of 1 fM-100 pM and the limit of detection (LOD) of 596 aM. The distinctive dual-emission properties of Zr-DPA@TCPP provided a new idea for the development of ECL luminophores and opened up an innovative avenue of fabricating the ratiometric ECL platform.

Porphyrin-anthracene covalent organic frameworks for sustainable photosterilization

Covalent organic frameworks (COFs) have promising applications in enhanced phototherapy. However, COFs that can sustainably play a role in phototherapy without continuous irradiation are extremely scarce. Herein, we report the fabrication of porphyrin-anthracene multifunctional COFs (Por-DPA) for sustainable photosterilization and bacterial-infected wound healing. A porphyrin photosensitizer, as one of the monomers, was used to provide photothermal and photodynamic activities under irradiation. An anthracene derivative, a good chemical source of singlet oxygen (1O2), was selected as another monomer to capture 1O2 and release it continuously via cycloreversion in the dark. The prepared Por-DPA COF prevents the self-aggregation quenching of the photosensitizer and thermal damage caused by continuous exposure to external light sources. Besides, Por-DPA exhibits good photothermal conversion performance and efficient 1O2 production capacity through dual pathways of photosensitization and cycloreversion. The developed sustainable photosterilization platform not only has good bactericidal effects on Escherichia coli and Staphylococcus aureus, but also promotes wound healing without obvious side effects, and is expected to be a novel efficient bactericide.

Covalent organic frameworks: linkage types, synthetic methods and bio-related applications

Covalent organic frameworks (COFs) are composed of small organic molecules linked via covalent bonds, which have tunable mesoporous structure, good biocompatibility and functional diversities. These excellent properties make COFs a promising candidate for constructing biomedical nanoplatforms and provide ample opportunities for nanomedicine development. A systematic review of the linkage types and synthesis methods of COFs is of indispensable value for their biomedical applications. In this review, we first summarize the types of various linkages of COFs and their corresponding properties. Then, we highlight the reaction temperature, solvent and reaction time required by different synthesis methods and show the most suitable synthesis method by comparing the merits and demerits of various methods. To appreciate the cutting-edge research on COFs in bioscience technology, we also summarize the bio-related applications of COFs, including drug delivery, tumor therapy, bioimaging, biosensing and antimicrobial applications. We hope to provide insight into the interdisciplinary research on COFs and promote the development of COF nanomaterials for biomedical applications and their future clinical translations.

Why Ligand Doping Increases the Fluorescence of an Anthracene-Based Metal-Organic Framework

Metal-organic frameworks (MOFs) built from fluorescent ligands frequently exhibit enhanced fluorescence when doped with inert ligands. This study focuses on a MOF of the UiO-68 structure, which is built from a fluorescent dibenzoate-anthracene ligand doped with a dibenzoate-benzene ligand. Our investigation aims to understand the mechanism behind the doping-enhanced emission of this MOF. We rule out several possible mechanisms, including exciton coupling, electron transfer between ligand and metal center, and ligand intersystem crossing induced by the metal center. Inhibition of the interligand charge transfer is considered a possible way to enhance emission. Furthermore, we propose that the conformational change of the anthracene-based ligand in the MOF cavity is also a way for enhancement. Our molecular dynamics simulations of the MOF structure filled with solvents reveal that the steric crowding in the cavity induces a conformational change at different doping levels, affecting the rate of intersystem crossing of the ligand.

BODIPY as a Multifunctional Theranostic Reagent in Biomedicine: Self-Assembly, Properties, and Applications

The use of boron dipyrromethene (BODIPY) in biomedicine is reviewed. To open, its synthesis and regulatory strategies are summarized, and inspiring cutting-edge work in post-functionalization strategies is highlighted. A brief overview of assembly model of BODIPY is then provided: BODIPY is introduced as a promising building block for the formation of single- and multicomponent self-assembled systems, including nanostructures suitable for aqueous environments, thereby showing the great development potential of supramolecular assembly in biomedicine applications. The frontier progress of BODIPY in biomedical application is thereafter described, supported by examples of the frontiers of biomedical applications of BODIPY-containing smart materials: it mainly involves the application of materials based on BODIPY building blocks and their assemblies in fluorescence bioimaging, photoacoustic imaging, disease treatment including photodynamic therapy, photothermal therapy, and immunotherapy. Lastly, not only the current status of the BODIPY family in the biomedical field but also the challenges worth considering are summarized. At the same time, insights into the future development prospects of biomedically applicable BODIPY are provided.

Ratiometric singlet oxygen self-detecting and oxygen self-supplying nanosensor for real-time photodynamic therapy feedback and therapeutic effect enhancement

Integration of singlet oxygen (¹O₂) detection that provides necessary therapeutic feedback into nanotheranostics for hypoxic tumor photodynamic therapy (PDT) is desirable but still challenging. Herein, we report a nanosensor (denominated PAPD) by combining dual-channel ratiometric sensing and oxygen-augmenting strategies, which synergistically realizes real-time ¹O₂ self-detection, O₂ self-supply and enhanced phototherapy. PAPD nanosensor is constructed by encapsulating anthracene-based ¹O₂ sensitive fluorophore (DPA) into porphyrin metal-organic frameworks (PCN-224), decorating gold nanoparticles (AuNPs) as nanoenzymes, and coating polyethylene glycol thiol (PEG-SH) by the Au-S bond. PCN-224 serves as ¹O₂ reference fluorescence (FL) agent and photosensitizer. Once PCN-224-induced ¹O₂ is synthesized, the dual-channel ratiometric FL signal of PAPD actualizes sensitive, accurate and dynamic ¹O₂ visualization and gives real-time therapeutic information correlated with the therapeutic progression. Additionally, the catalase-like activity of PAPD possesses in situ O₂ production via intracellular H₂O₂ decomposition and accelerates ¹O₂ yields for amplifying the tumor cell killing efficiency. Moreover, the ratiometric ¹O₂ self-detection affords the capacity to evaluate the O₂ self-supplying effect in tumor 4T1 cells. Consequently, the rational-designed nanosensor PAPD provides a paradigm for real-time therapeutic evaluation and precise hypoxic tumor treatment clinically.

Enzyme-Inspired Coordination Polymers for Selective Oxidization of C(sp3)-H Bonds via Multiphoton Excitation

Nature's blueprint provides the fundamental principles for expanding the use of abundant metals in catalysis; however, mimicking both the structure and function of copper enzymes simultaneously in one artificial system for selective C-H bond oxidation faces marked challenges. Herein, we report a new approach to the assembly of artificial monooxygenases utilizing a binuclear Cu₂S₂Cl₂ cluster to duplicate the identical structure and catalysis of the Cu_A enzyme. The designed monooxygenase Cu-Cl-bpyc facilitates well-defined redox potential that initially activated O₂via photoinduced electron transfer, and generated an active chlorine radical via a ligand-to-metal charge transfer (LMCT) process from the consecutive excitation of the in situ formed copper(II) center. The chlorine radical abstracts a hydrogen atom selectively from C(sp³)-H bonds to generate the radical intermediate; meanwhile, the O₂^•- species interacted with the mimic to form mixed-valence species, giving the desired oxidization products with inherent product selectivity of copper monooxygenases and recovering the catalyst directly. This enzymatic protocol exhibits excellent recyclability, good functional group tolerance, and broad substrate scope, including some biological and pharmacologically relevant targets. Mechanistic studies indicate that the C-H bond cleavage was the rate-determining step and the cuprous interactions were essential to stabilize the active oxygen species. The well-defined structural characters and the fine-modified catalytic properties open a new avenue to develop robust artificial enzymes with uniform and precise active sites and high catalytic performances.

Stimuli-Responsive Crystalline Smart Materials: From Rational Design and Fabrication to Applications

Stimuli-responsive smart materials that can undergo reversible chemical/physical changes under external stimuli such as mechanical stress, heat, light, gas, electricity, and pH, are currently attracting increasing attention in the fields of sensors, actuators, optoelectronic devices, information storage, medical applications, and so forth. The current smart materials mostly concentrate on polymers, carbon materials, crystalline liquids, and hydrogels, which have no or low structural order (i.e., the responsive groups/moieties are disorderly in the structures), inevitably introducing deficiencies such as a relatively low response speeds, energy transformation inefficiencies, and unclear structure-property relationships. Consequently, crystalline materials with well-defined and regular molecular arrays can offer a new opportunity to create novel smart materials with improved stimuli-responsive performance. Crystalline materials include framework materials (e.g., metal-organic frameworks, MOFs; covalent organic frameworks, COFs) and molecular crystals (e.g., organic molecules and molecular cages), which have obvious advantages as smart materials compared to amorphous materials. For example, responsive groups/moieties can be uniformly installed in the skeleton of the crystal materials to form ordered molecular arrays, making energy transfer between external-stimulus signals and responsive sites much faster and more efficiently. Besides that, the well-defined structures facilitate in situ characterization of their structural transformation at the molecular level by means of various techniques and high-tech equipment such as in situ spectra and single-crystal/powder X-ray diffraction, thus benefiting the investigation and understanding of the mechanism behind the stimuli-responsive behaviors and structure-property relationships. Nevertheless, some unsolved challenges remain for crystalline smart materials (CSMs), hampering the fabrication of smart material systems for practical applications. For instance, as the materials' crystallinity increases, their processability and mechanical properties usually decrease, unavoidably hindering their practical application. Moreover, crystalline smart materials mostly exist as micro/nanosized powders, which are difficult to make stimuli-responsive on the macroscale. Thus, developing strategies that can balance the materials' crystallinity and processability and establishing macroscale smart material systems are of great significance for practical applications.In this Account, we mainly summarize the recent research progress achieved by our groups, including (i) the rational design and fabrication of new stimuli-responsive crystalline smart materials, including molecular crystals and framework materials, and an in-depth investigation of their response mechanism and structure-property relationship and (ii) creating chemical/physical modification strategies to improve the processability and mechanical properties for crystalline materials and establishing macroscale smart systems for practical applications. Overall, this Account summarizes the state-of-the-art progress of stimuli-responsive crystalline smart materials and points out the existing challenges and future development directions in the field.

Photodecaging of a Mitochondria-Localized Iridium(III) Endoperoxide Complex for Two-Photon Photoactivated Therapy under Hypoxia

Despite the clinical success of photodynamic therapy (PDT), the application of this medical technique is intrinsically limited by the low oxygen concentrations found in cancer tumors, hampering the production of therapeutically necessary singlet oxygen (¹O₂). To overcome this limitation, we report on a novel mitochondria-localized iridium(III) endoperoxide prodrug (2-O-IrAn), which, upon two-photon irradiation in NIR, synergistically releases a highly cytotoxic iridium(III) complex (2-IrAn), singlet oxygen, and an alkoxy radical. 2-O-IrAn was found to be highly (photo-)toxic in hypoxic tumor cells and multicellular tumor spheroids (MCTS) in the nanomolar range. To provide cancer selectivity and improve the pharmacological properties of 2-O-IrAn, it was encapsulated into a biotin-functionalized polymer. The generated nanoparticles were found to nearly fully eradicate the tumor inside a mouse model within a single treatment. This study presents, to the best of our knowledge, the first example of an iridium(III)-based endoperoxide prodrug for synergistic photodynamic therapy/photoactivated chemotherapy, opening up new avenues for the treatment of hypoxic tumors.

An anthracene based metal-organic framework showing efficient angle-dependent polarized emission, luminescence thermometry, and photoelectronic response

The development of luminescent metal-organic frameworks (MOFs) has attracted extensive attention due to their applications in photoelectric devices, organic light-emitting diodes (OLEDs), anti-counterfeiting, biological imaging and so on. In this work, a novel anthracene based metal-organic framework, [Cd(DCPA)(DMF)]·(H₂O) (1) (H₂DCPA = 9,10-di(p-carboxyphenyl)anthracene), has been successfully synthesized under solvothermal conditions. The highly ordered arrangement and special spatial conformation of the anthracene chromophore play a significant role in the photophysical properties of 1. The combination of theoretical calculations and experiments shows that the molecular orbitals have good separation for inhibiting the recombination of electrons and holes. Furthermore, the fluorescence emission of 1 can be instantaneously and reversibly tuned between blue and green at different polarizing angles. Temperature-dependent fluorescence measurements indicate a good linear relationship between the maximum emission intensity/wavelength and the temperature for efficient thermochromism and luminescence thermometry. Photoelectric measurements reveal that 1 shows high performance of photocurrent generation under light illumination. Therefore, our research affords a new perspective to extend the application of luminescent MOFs in the fields of polarized emission, thermometry and photoelectronic response.

Reversible [4 + 2] Photooxygenation in Anion-Coordination-Driven-Assembled A2L2-Type Complexes

Two bis-bis(urea) ligands (L¹ and L²) incorporating the photoactive 9,10-diphenylanthracene fragment were designed for the construction of anion-coordination-driven assemblies and subsequent oxygenation of anthracene moieties for singlet oxygen storage. The corresponding A₂L₂-type sulfate complexes [TEA]₄[(SO₄)₂(L¹)₂] (1) and [TEA]₄[(SO₄)₂(L²)₂] (2), where TEA = tetraethylammonium, were achieved by coordinating the ligands L¹ or L² with sulfate anions. Both 1 and 2 were able to undergo [4 + 2] photooxygenation to form endoperoxide photoproducts 1-EPO and 2-EPO, which can be partially converted back to the original anthracene compounds after heating. The structures of 1-EPO and 2-EPO were unambiguously confirmed by X-ray crystallography, NMR and UV-vis spectroscopy, and high-resolution electrospray ionization mass spectrometry.

Metal center regulation of the porphyrin unit in covalent organic polymers for boosting the photocatalytic CO2 reduction activity

Regulating the porphyrin's metal center of metalloporphyrin(MPor)/Ru(ii)-pincer complex(RuN3) covalent organic polymers (COPs) effectively boosted the CO2 photoreduction by promoting charge separation and sacrificial electron donor oxidation.

Cuprous Cluster-Based Coordination Sheets as Photocatalytic Regulators to Activate Oxygen, Benzoquinone, and Thianthrenium Salts

Cuprous clusters are well known for their important fluorescent properties and tunable redox behavior, but the coordinated protecting groups restrict their application in photocatalysis, in particular, the inner-sphere activation of substrates. By modifying fluorescent cuprous clusters with terminal iodides into two-dimensional coordination sheets, we report a photocatalytic regulator to synergistically combine electron transfer and energy transfer for the oxidative coupling of benzoquinone and terminal alkynes. Under visible light irradiation, the well-modified excited state of the cuprous clusters in the coordination sheets reduces benzoquinones to generate aoxy radicals through electron transfer and activates oxygen through energy transfer. The aoxy radicals interact with copper-coordinated phenylacetylene to form an active intermediate, which is further oxidized by the in situ formed active oxygen species and aryl ketones are obtained. The warranted potential of the excited coordination sheets enables the reductive activation of thianthrenium salts as radical precursors, facilitating radical capture and further C-N coupling via an inner-sphere activation mechanism. The new catalytic approach optimizes the redox properties and excited-state lifetime, shortens the electron transfer steps, and promotes the potential collision of a low concentration of active species in tandem catalytic cycles, thus paving a new way to develop ecologically benign, cost-effective, multipurpose, and flexible catalytic systems.

Fine-Tuning Window Apertures in ZIF-8/67 Frameworks by Metal Ions and Temperature for High-Efficiency Molecular Sieving of Xylenes

Separation of structurally similar components from their mixtures is one of the most promising applications of metal-organic frameworks (MOFs). A high efficiency of such molecular sieving requires fine tuning of the MOF structure. In this work, we investigate subtle metal- and temperature-induced changes in window dimensions of zeolitic imidazolate frameworks (ZIF-8(Zn) and ZIF-67(Co)) and apply such structural tuning for efficient separation of xylene isomers (p-, m-, and o-xylenes). The use of Co instead of Zn favorably modifies window geometry: it accelerates the diffusion of all components by a factor of 2-3 while maintaining closely the same separation efficiency as that of ZIF-8(Zn). Outstanding selectivity above 18:1 and faster isolation of demanded p-xylene from the ternary mixture using ZIF-67(Co) have been demonstrated at room temperature, opening new horizons for its energy-efficient xylene separation. More generally, our findings suggest the prospective ways to tune various MOFs for target liquid-state separations.

Trapping and Releasing of Oxygen in Liquid by Metal-Organic Framework with Light and Heat

Nanoporous materials can adsorb small molecules into their nanospaces. However, the trapping of light gas molecules dissolved in solvents suffers from low concentration and poor adsorption affinity. Here, the reversible trapping and releasing of dissolved oxygen are shown through integrating photosensitization and chemical capturing abilities into a metal-organic framework (MOF), MOMF-1. 9,10-Di(4-pyridyl)anthracene (dpa) ligands in MOMF-1 generates singlet oxygen from triplet oxygen under photoirradiation without additional photosensitizers, and successively reacts with it to produce anthracene endoperoxide, forming MOMF-2, which is proved crystallographically. The reverse reaction also proceeds quantitatively by heating MOMF-2. Moreover, MOMF-1 exhibits excellent water resistance, and completely removes oxygen of ppm order concentrations in water. The new material shown in this report allows controlling of the amount of dissolved oxygen, which can be applicable in various fields relating to numerous oxidation phenomena.

Tumor Microenvironment-Responsive Fe(III)-Porphyrin Nanotheranostics for Tumor Imaging and Targeted Chemodynamic-Photodynamic Therapy

The development of effective and safe tumor nanotheranostics remains a research imperative. Herein, tumor microenvironment (TME)-responsive Fe(III)-porphyrin (TCPP) coordination nanoparticles (FT@HA NPs) were prepared using a simple one-pot method followed by modification with hyaluronic acid (HA). FT@HA NPs specifically accumulated in CD44 receptor-overexpressed tumor tissues through the targeting property of HA and upon endocytosis by tumor cells. After cell internalization, intracellular acidic microenvironments and high levels of glutathione (GSH) triggered the rapid decomposition of FT@HA NPs to release free TCPP molecules and Fe(III) ions. The released Fe(III) ions could trigger GSH depletion and Fenton reaction, activating chemodynamic therapy (CDT). Meanwhile, the fluorescence and photodynamic effects of the TCPP could be also activated, achieving controlled reactive oxygen species (ROS) generation and avoiding side effects on normal tissues. Moreover, the rapid consumption of GSH further enhanced the efficacy of CDT and photodynamic therapy (PDT). The in vivo experiments further demonstrated that the antitumor effect of these nanotheranostics was significantly enhanced and that their toxicity and side effects against normal tissues were effectively suppressed. The FT@HA NPs can be applied for activated tumor combination therapy under the guidance of dual-mode imaging including fluorescence imaging and magnetic resonance imaging, providing an effective strategy for the design and preparation of TME-responsive multifunctional nanotheranostics for precise tumor imaging and combination therapy.

Exciton Coupling and Conformational Changes Impacting the Excited State Properties of Metal Organic Frameworks

In recent years, the photophysical properties of crystalline metal-organic frameworks (MOFs) have become increasingly relevant for their potential application in light-emitting devices, photovoltaics, nonlinear optics and sensing. The availability of high-quality experimental data for such systems makes them ideally suited for a validation of quantum mechanical simulations, aiming at an in-depth atomistic understanding of photophysical phenomena. Here we present a computational DFT study of the absorption and emission characteristics of a Zn-based surface-anchored metal-organic framework (Zn-SURMOF-2) containing anthracenedibenzoic acid (ADB) as linker. Combining band-structure and cluster-based simulations on ADB chromophores in various conformations and aggregation states, we are able to provide a detailed explanation of the experimentally observed photophysical properties of Zn-ADB SURMOF-2: The unexpected (weak) red-shift of the absorption maxima upon incorporating ADB chromophores into SURMOF-2 can be explained by a combination of excitonic coupling effects with conformational changes of the chromophores already in their ground state. As far as the unusually large red-shift of the emission of Zn-ADB SURMOF-2 is concerned, based on our simulations, we attribute it to a modification of the exciton coupling compared to conventional H-aggregates, which results from a relative slip of the centers of neighboring chromophores upon incorporation in Zn-ADB SURMOF-2.

Improving the Stability and Visualizing the Structural Transformation of the Stimuli-Responsive Metal-Organic Frameworks (MOFs)

New metal-organic frameworks (MOFs) based on flexible tetra-carboxylate ligands and Cu(II) are designed to gain stimuli-responsive materials. Unstable MOFs can be more stable with unabated flexibility by replacing coordinated solvent molecules with auxiliary N-based ligands. Two of them are intensively studied by in situ single-crystal X-ray diffraction (SCXRD) analysis and the unit cell parameters during transformations have been observed in detail. They undergo exceptional structural transformations which can be divided into two processes: the thermal-responsive phase transition and the solvent-responsive phase transition. The thermal-responsive phase transition takes place in a narrow temperature interval reversibly. However, the solvent-responsive phase transition is a gradual and irreversible process. The stimuli-responsive mechanism has also been explored by comparing the parameters of the crystal structures under different temperatures. Fascinatingly, their exceptional structural transformations correlate with the flexibility of the ligand fragments and the [Cu₂(RCOO)₄] clusters.

From lamellar net to bilayered-lamella and to porous pillared-bilayer: reversible crystal-to-crystal transformation, CO2 adsorption, and fluorescence detection of Fe3+, Al3+, Cr3+, MnO4−, and Cr2O72− in water

Layered coordination polymers of different network topologies have showed reversible structural transformations and fluorescence detection toward Fe³⁺, Al³⁺, Cr³⁺, MnO₄^–, and Cr₂O₇^2– in water.

Water-Tolerant DUT-Series Metal-Organic Frameworks: A Theoretical-Experimental Study for the Chemical Fixation of CO2 and Catalytic Transfer Hydrogenation of Ethyl Levulinate to γ-Valerolactone

A series of highly thermally and hydrolytically stable porous solids with intriguing properties of zirconium- and hafnium-based metal-organic frameworks (MOFs) [Dresden University of Technology (DUT) series] was synthesized. The DUT MOFs were found to be effective catalysts for both epoxide-CO₂ cycloaddition reactions and the catalytic transfer hydrogenation (CTH) of ethyl levulinate (EL). In particular, 12-connected DUT-52(Zr) showed higher catalytic activity than eight- and six-connected catalysts in the synthesis of cyclic carbonates as well as in the production of γ-valerolactone (GVL). The secondary building unit connectivity, coexistence of a moderate number of acidic and basic sites, Brunauer-Emmett-Teller surface area, and combined effects of the pores of the MOFs seem to influence the catalytic activity. The reaction mechanism for the DUT-52(Zr)-mediated cycloaddition reaction of CO₂ and the CTH reactions were investigated in detail by using periodic density functional theory calculations. To the best of our knowledge, this is the first detailed computational study for the formation of GVL from EL by using MOF as the catalyst. In addition, grand canonical Monte Carlo simulations predicted the strong interaction of CO₂ molecules with the DUT-52(Zr) framework. Remarkably, the DUT-series catalysts possess extraordinary tolerance toward water. Further, DUT-52(Zr) is recyclable and is an efficient catalyst for cycloaddition and CTH reactions for at least five uses without obvious reductions in the activity or structural integrity.