Photoresponse within dye-incorporated metal-organic architectures

Phenazinium- and Malachite Green-Based Pd(II) Cages: Chiroptical Discrimination of Nucleoside Triphosphates

Organic chromophores have been successfully implemented into supramolecular systems to bestow them with distinct photophysical properties for various applications, ranging from solar energy conversion, photochemical reactions or as receptors for guest molecules with optical readout. We had previously introduced first members of the large family of coal-tar dyes (methylene blue, crystal violet and rhodamine) as integral components of coordination cages. Here, we add two new chromophores, malachite green (MGP) and a purple phenazinium dye (PHP), serving as backbones of bis-monodentate banana-shaped ligands with pyridine donors. We show the formation of corresponding green and purple coloured Pd2L4 coordination cages and investigate their interaction with chiral guest molecules via UV-Vis and CD spectroscopy. The PHP cage can be used to recognize nucleoside triphosphates, based on chirality transfer from the guests to the structurally flexible helicate. In combination with the already known methylene blue cage MBP we could further differentiate between all four canonical NTPs through characteristic changes in the observed CD signatures.

Photocatalytic activity of a 2D copper porphyrin metal-organic framework for visible light overall water splitting

A 2D copper tetrakis(4-carboxyphenyl)porphyrin metal-organic framework has been prepared and used as a photocatalyst for overall water splitting, measuring under visible light irradiation (λ > 450 nm) under one sun power conditions a H2 production rate of 120 μmolH2 gcatalyst -1 h-1 that is among the highest ever reported. While the 2D Cu porphyrin MOF undergoes substantial degradation in 3 h upon UV irradiation (320-380 nm) in the presence of air, it appears to be photostable under the conditions of the overall water splitting and visible light exposure, exhibiting similar temporal profiles for H2 and O2 evolution. Photocurrent experiments and band energy measurements indicate that the photocatalytic efficiency derives from an efficient charge separation in the visible region (apparent electron charge extraction efficiency at 540 nm of 0.1%) and adequate alignment of the redox potential of the conduction (-0.59 V vs. NHE) and valence (+1.48 V vs. NHE) bands for water splitting.

Encapsulation engineering of porous crystalline frameworks for delayed luminescence and circularly polarized luminescence

Delayed luminescence (DF), including phosphorescence and thermally activated delayed fluorescence (TADF), and circularly polarized luminescence (CPL) exhibit common and broad application prospects in optoelectronic displays, biological imaging, and encryption. Thus, the combination of delayed luminescence and circularly polarized luminescence is attracting increasing attention. The encapsulation of guest emitters in various host matrices to form host-guest systems has been demonstrated to be an appealing strategy to further enhance and/or modulate their delayed luminescence and circularly polarized luminescence. Compared with conventional liquid crystals, polymers, and supramolecular matrices, porous crystalline frameworks (PCFs) including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), zeolites and hydrogen-bonded organic frameworks (HOFs) can not only overcome shortcomings such as flexibility and disorder but also achieve the ordered encapsulation of guests and long-term stability of chiral structures, providing new promising host platforms for the development of DF and CPL. In this review, we provide a comprehensive and critical summary of the recent progress in host-guest photochemistry via the encapsulation engineering of guest emitters in PCFs, particularly focusing on delayed luminescence and circularly polarized luminescence. Initially, the general principle of phosphorescence, TADF and CPL, the combination of DF and CPL, and energy transfer processes between host and guests are introduced. Subsequently, we comprehensively discuss the critical factors affecting the encapsulation engineering of guest emitters in PCFs, such as pore structures, the confinement effect, charge and energy transfer between the host and guest, conformational dynamics, and aggregation model of guest emitters. Thereafter, we summarize the effective methods for the preparation of host-guest systems, especially single-crystal-to-single-crystal (SC-SC) transformation and epitaxial growth, which are distinct from conventional methods based on amorphous materials. Then, the recent advancements in host-guest systems based on PCFs for delayed luminescence and circularly polarized luminescence are highlighted. Finally, we present our personal insights into the challenges and future opportunities in this promising field.

Guest-Responsive Near-Infrared-Luminescent Metal-Organic Cage Organized by Porphyrin Dyes and Yb(III) Complexes

Metal-organic cages (MOCs) with luminophores have significant advantages for the facile detection of specific molecules based on turn-on or turn-off luminescence changes induced by host-guest complexation. One important challenge is the development of turn-on-type near-infrared (NIR)-luminescent MOCs. In this study, we synthesized a novel MOC consisting of two porphyrin dyes linked by four Yb(III) complexes, which exhibit bimodal red and NIR fluorescence signals upon photoexcitation of the porphyrin π system. Single-crystal X-ray structural analysis and computational molecular modeling revealed that planar aromatic perfluorocarbons were intercalated into the MOC. The tight packing between the MOC and guests enhanced the NIR fluorescence of Yb(III) by suppressing energy transfer from the photoexcited porphyrin to oxygen molecules. Guest-responsive turn-on NIR fluorescence changes in an MOC were successfully demonstrated.

Solvents regulate the packing porosity of a bilayer metal-organic cage

Metal-organic cages (MOCs) are an emerging class of porous materials with promising applications. However, controlling the configuration of the cage packing, which can influence the overall porosity of the materials, remains a difficulty, as many factors can influence the cage assembly and stacking. Herein, we report a solvent strategy to fine-tune the packing configuration of a bilayer MOC, a small triangular prism cage (six Cu ions act as vertices, three nitrate ions act as pillars, and six nitrate ions act as caps) incorporated into a large triangular prism cage (another six Cu ions act as vertices, a couple of oxygen atoms act as pillars and six ligands (L1: 3,5-bis(pyridine-3-yl)-4H-1,2,4-triazole) act as a jointed cap) by the coordination between the triazole nitrogen from L1 and the inner vertex Cu ions. The involved solvents water, acetonitrile (MeCN) and N,N'-dimethylformamide (DMF) form hydrogen bonds with this bilayer MOC, resulting in three different types of packing associated with systemically tuned porosity (NTU-93: 12.2%, NTU-94: 19.3%, and NTU-95: 42.1%). Gas adsorption and breakthrough tests demonstrate that NTU-95 has potential ability for C2H2/C2H4 separation. This work not only shows a case of finely tuned packing of coordination cages, but also provides a powerful tool that may be extended to other cage families.

Photoswitchable coordination cages

Stimuli-responsive behaviour is key to the design of smart materials, surfaces, nano-systems and effector molecules, allowing their application as switchable catalysts, molecular transporters, bioimaging probes or caged drugs. Supramolecular chemistry has embraced the widespread integration of photoswitches because of their precise spatiotemporal addressability and waste-free nature. In the vibrant area of discrete metal-mediated self-assembly, however, photoswitches are still rarely employed. Only recently has it been shown that embedding photoswitches into the organic backbones of coordination cages enables control of their host and material properties and thus unlocks the hitherto unexploited dynamic adaptivity of such systems. Here we discuss four cases where triggering ligand-integrated photoswitches leads to (1) control over disassembly/reassembly, (2) bi-stable switching between defined states, (3) interplay with thermal processes in metastable systems and (4) light-fuelled dissipative self-assembly. We highlight first clues concerning the relationship between fundamental photophysics and dynamic assembly equilibria and propose directions for future development.

Electrochemical and Biocatalytic Signal-Controlled Payload Release from a Metal-Organic Framework

A metal-organic framework (MOF), ZIF-8, which is stable at neutral and slightly basic pH values in aqueous solutions and destabilized/dissolved under acidic conditions, is loaded with a pH-insensitive fluorescent dye, rhodamine-B isothiocyanate, as a model payload species. Then, the MOF species are immobilized at an electrode surface. The local (interfacial) pH value is rapidly decreased by means of an electrochemically stimulated ascorbate oxidation at +0.4 V (Ag/AgCl/KCl). Oxygen reduction upon switching the applied potential to -0.8 V allows to return the local pH to the neutral/basic pH, then stopping rapidly the release process. The developed method allows electrochemical control over stimulated or inhibited payload release processes from the MOF. The pH variation proceeds in a thin film of the solution near the electrode surface. The switchable release process is realized in a buffer solution and undiluted human serum. As the second option, the pH decrease stimulating the release process is achieved upon an enzymatic reaction using esterase and ester substrate. This approach potentially allows the release activation controlled by numerous enzymes assembled in complex biocatalytic cascades. It is expected that related electrochemical or biocatalytic systems can represent novel signal-responding materials with switchable features for delivering (bio)molecules within biomedical applications.

Probing the influence of substrate binding on photocatalytic dehalogenation with a heteroleptic supramolecular [M4La2Lb2] square containing PDI photosensitizers as ligands

Photoredox catalysis is a valuable tool in a large variety of chemical reactions. Main challenges still to be overcome are photodegradation of photocatalysts and substrates, short lifetimes of reactive intermediates, and selectivity issues due to unwanted side reactions. A potential solution to these challenges is the pre-organization of the photosensitizer, substrate and (co)-catalyst in supramolecular self-assembled structures. In such architectures, (organic) dyes can be stabilized, and higher selectivity could potentially be achieved through pre-organizing desired reaction partners via non-covalent interactions. Perylene diimide (PDI) is an organic dye, which can be readily reduced to its mono- and dianion. Excitation of both anions leads to highly reducing excited states, which are able to reduce a variety of substrates via single electron transfer. The incorporation of PDI into a heteroleptic [M4La2Lb2] supramolecular square has been recently demonstrated. Herein we investigate its photophysical properties and demonstrate that incorporated PDI indeed features photocatalytic activity. Initial results suggest that the pre-organisation by binding positively affects the outcome.

Supramolecular Coordination Cages for Artificial Photosynthesis and Synthetic Photocatalysis

Because sunlight is the most abundant energy source on earth, it has huge potential for practical applications ranging from sustainable energy supply to light driven chemistry. From a chemical perspective, excited states generated by light make thermodynamically uphill reactions possible, which forms the basis for energy storage into fuels. In addition, with light, open-shell species can be generated which open up new reaction pathways in organic synthesis. Crucial are photosensitizers, which absorb light and transfer energy to substrates by various mechanisms, processes that highly depend on the distance between the molecules involved. Supramolecular coordination cages are well studied and synthetically accessible reaction vessels with single cavities for guest binding, ensuring close proximity of different components. Due to high modularity of their size, shape, and the nature of metal centers and ligands, cages are ideal platforms to exploit preorganization in photocatalysis. Herein we focus on the application of supramolecular cages for photocatalysis in artificial photosynthesis and in organic photo(redox) catalysis. Finally, a brief overview of immobilization strategies for supramolecular cages provides tools for implementing cages into devices. This review provides inspiration for future design of photocatalytic supramolecular host-guest systems and their application in producing solar fuels and complex organic molecules.

Guest-Modulated Circularly Polarized Luminescence by Ligand-to-Ligand Chirality Transfer in Heteroleptic PdII Coordination Cages

Multicomponent metallo-supramolecular assembly allows the rational combination of different building blocks. Discrete multifunctional hosts with an accessible cavity can be prepared in a non-statistical fashion. We employ our shape-complementary assembly (SCA) method to achieve for the first time integrative self-sorting of heteroleptic PdII cages showing guest-tunable circularly polarized luminescence (CPL). An enantiopure helicene-based ligand (M or P configuration) is coupled with a non-chiral emissive fluorenone-based ligand (A or B) to form a series of Pd2 L2 L'2 assemblies. The modular strategy allows to impart the chiral information of the helicenes to the overall supramolecular system, resulting in CPL from the non-chiral component. Guest binding results in a 4-fold increase of CPL intensity. The principle offers potential to generate libraries of multifunctional materials with applications in molecular recognition, enantioselective photo-redox catalysis and information processing.

Advances in the Structural Strategies of the Self-Assembly of Photoresponsive Supramolecular Systems

Photosensitive supramolecular systems have garnered attention due to their potential to catalyze highly specific tasks through structural changes triggered by a light stimulus. The tunability of their chemical structure and charge transfer properties provides opportunities for designing and developing smart materials for multidisciplinary applications. This review focuses on the approaches reported in the literature for tailoring properties of the photosensitive supramolecular systems, including MOFs, MOPs, and HOFs. We discuss relevant aspects regarding their chemical structure, action mechanisms, design principles, applications, and future perspectives.

Modifying electron injection kinetics for selective photoreduction of nitroarenes into cyclic and asymmetric azo compounds

Modifying the reactivity of substrates by encapsulation is essential for microenvironment catalysts. Herein, we report an alternative strategy that modifies the entry behaviour of reactants into the microenvironment and substrate inclusion thermodynamics related to the capsule to control the electron injection kinetics and the selectivity of products from the nitroarenes photoreduction. The strategy includes the orchestration of capsule openings to control the electron injection kinetics of electron donors, and the capsule’s pocket to encapsulate more than one nitroarene molecules, facilitating a condensation reaction between the in situ formed azanol and nitroso species to produce azo product. The conceptual microenvironment catalyst endows selective conversion of asymmetric azo products from different nitroarenes, wherein, the estimated diameter and inclusion Gibbs free energy of substrates are used to control and predict the selectivity of products. Inhibition experiments confirm a typical enzymatic conversion, paving a new avenue for rational design of photocatalysts toward green chemistry.

Two isomeric metal-organic frameworks bearing stilbene moieties for high volatile iodine uptake

The efficient, green, and economical removal of radioactive iodine (I2) has drawn worldwide attention in the safe development of nuclear energy. Metal-organic frameworks (MOFs) have been demonstrated to be a...

Photoinduced host-to-guest electron transfer in a self-assembled coordination cage

Light–powered host–guest charge transfer (HGCT) is shown for a coordination cage based on electron-rich phenothiazines, containing an anthraquinone acceptor as guest. Transient absorption spectroscopy and spectroelectrochemistry data is presented.

Structural Chemistry of Giant Metal Based Supramolecules

The review presents a bird-eye view on the state of research in the field of giant nonbiological discrete metal complexes and ions of nanometer size, which are structurally characterized by means of single-crystal X-ray diffraction, using the crystal structure as a common key feature. The discussion is focused on the main structural features of the metal clusters, the clusters containing compact metal oxide/hydroxide/chalcogenide core, ligand-based metal-organic cages, and supramolecules as well as on the aspects related to the packing of the molecules or ions in the crystal and the methodological aspects of the single-crystal neutron and X-ray diffraction of these compounds.