Porphyrin-Sensitizers and Anthracene-Annihilators Built in Isostructural Frameworks for Investigating Triplet-Triplet Annihilation Upconversion.

In this study, four isostructural pillar-layered frameworks were constructed using a porphyrin layer and an anthracene pillar, which served as the sensitizer and annihilator, respectively, in the triplet-triplet annihilation upconversion (TTA-UC) system. Framework 1 demonstrated the highest upconversion quantum yield of 1.01%. Additionally, 1 and 2 also exhibited down-conversion fluorescence resulting from the porphyrin component. A twist intramolecular charge transfer (TICT) state was observed in the bianthracene chromophore of 2, resulting in transient rotation of two anthracene rings and red-shifted emission. Both computational studies and experiments confirmed the transition from a locally excited state to a TICT state upon the inclusion of polar guest molecules into the framework.

Polypyridyl Ru(II) or cyclometalated Ir(III) functionalized architectures for photocatalysis.

The chemistry of polypyridyl Ru(II) and cyclometalated Ir(III) derivatives provides long-lasting interest to researchers due to the inherent advantage of their triplet states in a variety of photoactivities. The introduction of Ru(N^N)3 and Ir(C^N)2(X^N) modules into well-defined architectures extends the research areas of both photoactive metal complexes and network chemistry, generating a lot of new opportunities with interesting structural aesthetics and profound functional possibilities. The rapid development of research in integrating Ru(II) or Ir(III) metallotecons into the architectures has been apparent in recent years which makes this a fascinating subject for reviewing. This review focuses on the design and syntheses of Ru(N^N)3 and Ir(C^N)2(X^N) functionalized architectures of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), metallasupramolecules, organic supramolecules and supramolecular organic frameworks (SOFs). Furthermore, the photocatalytic applications including the hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO2RR), photocatalytic oxidation and photoredox catalysis of organic transformation are also presented.

Photoreactive Crystals Exhibiting [2 + 2] Photocycloaddition Reaction and Dynamic Effects.

ConspectusConducting a reaction in the solid state eliminates the usage of solvents. If such reactions are conducted in a single-crystal to single-crystal (SCSC) fashion, then structural characterization by single-crystal X-ray crystallography (SCXRD) techniques provides unequivocal structural details. Although topochemical principles govern, getting single crystals at the end of a SCSC reaction purely depends on the experimental skills of the researchers. SCSC reactions are common among solid-state [2 + 2] cycloaddition reactions (hereafter "photoreaction") after the classical work of Schmidt and co-workers in 1960s. Synthons and tectons in the crystal engineering box can be exploited to bring the functional groups into the required alignment and packing to achieve the desired chemical reactivities and physical properties, respectively. Bringing a pair of alkenes closer together in the organic molecules provides an effective starting point to achieve the goal of crystal engineering.Further, understanding and controlling photoreactivity in the solid state provide a gateway to designing new advanced materials, for example, making cycloreversible optical storage materials, photosalient and photomechanical materials, highly crystalline or even single-crystalline organic polymers, covalent organic framework structures, and organic polymers incorporated inside metal-organic frameworks (MOFs). Photoreactions often proceed in a SCSC manner due to the limited movements of the closely disposed reactive functional groups in the crystals. Thus, these photoreactions yield not only quantitative photoproducts but also regio- and stereospecificity, which are otherwise inaccessible by solution syntheses.The traditional definition of crystals being hard, rigid, and brittle is no longer valid ever since the mechanically responsive crystals were discovered. These dynamic crystals undergo various movements like curling, jumping, hopping, popping, splitting, and wiggling, when exposed to light (called "photosalient effect") or heat (called "thermosalient" effect). These crystals generate new methods of transforming light and heat energy into mechanical work. Recently, photosalient behavior during the [2 + 2] cycloaddition reaction under UV light has been frequently observed. With the emergence of the field of "crystal adaptronics", dynamic photoreactive crystals have emerged as smart actuating materials.This Account aims to provide an overview of the development in this area, since it has garnered much attention among solid state chemists. While presenting selected examples of important strategies, we try to illustrate the intentions and concepts behind the methods developed, which will help in a rational approach for the fabrication of advanced solid state materials. Apart from topochemical transformations, the important roles played by weak interactions, guest solvents, and mechanical grinding have been highlighted in several classes of compounds to show structural transformations that defy the expected outcomes. Overall, the progress of [2 + 2] cycloaddition reaction in solid state materials has been discussed from UV induced structural transformations to the development of smart actuating materials.

Two-Dimensional Metal-Organic Framework Materials: Synthesis, Structures, Properties and Applications.

Among the recent developments in metal-organic frameworks (MOFs), porous layered coordination polymers (CPs) have garnered attention due to their modular nature and tunable structures. These factors enable a number of properties and applications, including gas and guest sorption, storage and separation of gases and small molecules, catalysis, luminescence, sensing, magnetism, and energy storage and conversion. Among MOFs, two-dimensional (2D) compounds are also known as 2D CPs or 2D MOFs. Since the discovery of graphene in 2004, 2D materials have also been widely studied. Several 2D MOFs are suitable for exfoliation as ultrathin nanosheets similar to graphene and other 2D materials, making these layered structures useful and unique for various technological applications. Furthermore, these layered structures have fascinating topological networks and entanglements. This review provides an overview of different aspects of 2D MOF layered architectures such as topology, interpenetration, structural transformations, properties, and applications.

Highly Efficient Multiphoton Absorption of Zinc-AIEgen Metal-Organic Frameworks.

A series of luminescent frameworks was synthesized from the selective combination of aggregation induced emission (AIE)-linker tetra-(4-carboxylphenyl)ethylene (H4 TCPE) and Zn2+ . Complex 1 was formed by the close packing of Zn-TCPE hinge, and isostructural complexes 2-5 were constructed by the linkage of Zn-TCPE layer and pillar ligands. These complexes exhibit highly efficient multiphoton excited photoluminescence (MEPL) and concomitant third-harmonic generation (THG). The multiphoton absorption (MPA) parameters of 1 are superior to other multiphoton emission materials including the perovskite nanocrystals. The incorporation of pillar linkers slows down the charge transfer between layers of Zn-TCPE, and the aromatic core of pillar linkers has a great influence on the MPA performance of the corresponding frameworks.

Reversible Thermosalience in a One-Dimensional Coordination Polymer Preceded by Anisotropic Thermal Expansion and the Shape Memory Effect.

Thermally responsive crystals hold great potential for their use as actuating materials by acting as energy transducers to convert heat energy to mechanical work. Control over defined phase transition temperature with rapid reconfiguration is of great advantage for actuation. The thermosalient (TS) effect is a rarely observed phenomenon in coordination polymers (CPs), let alone the reversibility of thermosalience in CPs. Herein, we report the reversible TS effect in a one-dimensional CP due to the martensitic phase transition during both heating and cooling cycles. The TS effect was preceded by anisotropic thermal expansion showing high expansion coefficients. In addition, the nonmolecular crystals show reversible contraction and recovery during multiple heating-cooling cycles due to the self-restorative shape memory effect. The reversible actuation of the CP could be repeated for 20 heating-cooling cycles in differential scanning calorimetry experiments, suggesting its great potential as a multicyclic actuator. Such thermal responsive behavior is unique in metal-organic materials.

Construction of 2D Interdigitated Polyrotaxane Layers and their Transformation to a 3D Polyrotaxane by a Photocycloaddition Reaction between Wheels.

Following the pioneering work of Sauvage and Stoddart on rotaxanes, construction of higher dimensional polyrotaxanes in metal-organic frameworks (MOFs) via a modified protocol is challenging. We present the formation of a two-dimensional (2D) polyrotaxane and its conversion to a three-dimensional (3D) polyrotaxane MOF via a photoreaction between interdigitated "olefin wheels". For this purpose, a 2-fold entangled 2D MOF [Pb2(bpp)(sdc)2] (1), showing a 2D + 2D → 2D polyrotaxane motif, has been synthesized from the solvothermal reaction of lead(II) nitrate, 3,3'-stilbenedicarboxylic acid (H2sdc) containing an olefin group, and 1,4-bis(4-pyridyl)piperazine (bpp). The single-crystal X-ray diffraction analysis of 1 revealed that the adjacent entangled 2D layers are interdigitated, with the separation of 3.72 Šbetween C═C bond pairs in adjacent layers satisfying Schmidt's criteria for the occurrence of a [2 + 2] photocycloaddition reaction. Irradiation of the single crystals of 1 under UV light resulted in formation of a 3D polyrotaxane, [Pb2(bpp)(rctt-tccb)]n (2), due to a [2 + 2] photocycloaddition reaction between two wheels via a single-crystal to single-crystal transformation. The photocycloaddition and partial thermal cleavage reaction between 1 and 2 were confirmed by 1H NMR and powder X-ray diffraction (PXRD) in solution and the solid state, respectively. The present approach could contribute to the understanding of the construction of higher dimensional polyrotaxanes which are not accessible by the traditional routes.

Single-Crystal-to-Single-Crystal [2 + 2] Photocycloaddition Reaction in a Photosalient One-Dimensional Coordination Polymer of Pb(II).

In a remarkable example, we report a one-dimensional coordination polymer (CP) of Pb(II) showing photosalient (PS) properties triggered by [2 + 2] cycloaddition of olefinic ligands, which is seldom observed in CPs. Macroscopic rod-shaped crystals show various photomechanical effects such as jumping, splitting, rolling, and breaking upon UV illumination. In this rare example, we could determine the solid-state structure of the 100% dimerized product and three intermediate structures, even after the shattering of crystals into small pieces. Detailed mechanistic investigation from the single-crystal data indicates that the strain generated in the unit cell due to anisotropic expansion played a bigger role for the PS effects. Nucleated growth of the photoproduct crystal created different domains inside the single crystal, which multiplied the already developed stress leading to the photomechanical movements. This example falls in the gray area of a clean single-crystal-to-single-crystal (SCSC) transformation and violent PS effect. Such photochemical behavior has never been reported before.

Water Stable Zn(II) Metal-Organic Framework as a Selective and Sensitive Luminescent Probe for Fe(III) and Chromate Ions.

Sensing and monitoring toxic contaminants like Fe3+, CrO42-, and Cr2O72- ions in water is very important due to their harmful effects on biological and environmental systems. Enhanced hydrolytic stability, sensitivity, and selectivity, in addition to their excellent luminescence properties, are important attributes of metal-organic framework (MOF)-based sensors for sensing applications. In this work, the water stable Zn-MOF [Zn2(tpeb)(bpdc)2] (where tpeb = 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene and bpdc = biphenyl-4,4'-dicarboxylic acid) was synthesized and characterized. The framework retains its crystallinity and structural integrity in harsh acidic and basic conditions (pH 4-11). Most interestingly, the Zn-MOF demonstrates a strong blue luminescence in water that can be quenched selectively only by contaminants like Fe3+, CrO42-, and Cr2O72- ions. Higher Ksv values and low detection limits in selective luminescence quenching confirm the superior sensing performance, which is comparable to those of contemporary materials. Furthermore, in all cases, quenching efficiency remains unaltered in the presence of interfering ions, even after the compound is used in multiple cycles, which makes this MOF an attractive, reliable, and recyclable luminescent sensor material. The luminescence quenching mechanism is based on the competitive absorption and weak interactions. It is worth noting that most of the reported MOF-based sensors used for the separate sensing of Fe(III) and chromate ions are used in organic media due to their poor hydrolytic stabilities. Reports on the dual sensing of Fe(III) and chromate ions, which are also in aqueous media, are rare. Based on these results, Zn-MOF can be considered as a suitable candidate for advanced practical applications for the efficient sensing of Fe(III) and chromate ions in water.

Giant Enhancement of Second Harmonic Generation Accompanied by the Structural Transformation of 7-Fold to 8-Fold Interpenetrated Metal-Organic Frameworks (MOFs).

Interpenetration in metal-organic frameworks (MOFs) is an intriguing phenomenon with significant impacts on their properties, and functional applications. Herein, we show that a 7-fold interpenetrated MOF (1) is transformed into an 8-fold interpenetrated MOF by the loss of DMF in a single-crystal-to-single-crystal manner. This is accompanied by a giant enhancement of the second harmonic generation (SHG ca. 125 times) and two-photon photoluminescence (ca. 14 times). The strengthened π-π interaction between the individual diamondoid networks and intensified oscillator strength of the molecules aid the augment of dipole moments and boost the nonlinear optical conversion efficiency. Large positive and negative thermal expansions of 1 occur at 30-150 °C before the loss of DMF. These results offer an avenue to manipulate the NLO properties of MOFs using interpenetration and provide access to tunable single-crystal NLO devices.

Solid-State Photochemical [2+2] Cycloaddition Reaction of MnII Complexes.

Three MnII complexes have been synthesized under similar experimental conditions. Of these [Mn2 (benzoate)4 (L)2 ] (where L=4-styrylpyridine or 4spy, 1 and 2-fluoro-4'-styrylpyridine or 2F-4spy, 3) are paddlewheel complexes, but crystallized in different space groups. Whereas [Mn2 (benzoate)4 (3F-4spy)4 ] (3F-4spy=3-fluoro-4'-styrylpyridine), 4 is a dinuclear complex having different stoichiometry from 1 and 3 with two pairs of 3F-4spy ligands aligned in face-to-face manner. An irreversible phase transition occurs from the space group P21 /c to C2/c when 1 was heated up to 125 °C to 2 in a single-crystal-to-single-crystal fashion or when ground 1 to powder. 2 is isomorphous and isostructural to 3. Complimentary π-π interactions in head-to-tail alignment of the styrylpyridine ligands furnishes 1D aggregates in 1-3 which are congenial to undergo [2+2] cycloaddition reaction under UV light. Whereas, face-to-face alignment of the 4spy pairs in 4 is expected to provide a head-to-head photoproduct. All the MnII complexes are indeed found to be photoreactive. To our surprise, contrary to their ZnII analogues, 2 and 3 were not found to be photosalient. The percentage volume expansion during the photoreaction as determined from the density measurements, was found to be too low (3.2 and 4.6 % respectively for 2 and 3) to have this behavior.

Chemical Patterning in Single Crystals of Metal-Organic Frameworks by [2+2] Cycloaddition Reaction.

Conventional chemical patterning involves films of polymeric materials. Herein, we demonstrate that the presence or absence of guest solvents in the crystal voids modulates the patterning of the cyclobutane rings in highly strained metal-organic frameworks (MOFs) under UV light. The olefin pairs of the spacer ligands, which resemble a ladder-like structure, in the MOF, undergo a [2+2] cycloaddition reaction in a single-crystal-to-single-crystal manner. For instance, a partial photoreaction in the MOFs in the absence of a guest solvent as well as with dimethylacetamide in the voids generated two different patterns of the cyclobutane rings. Surprisingly, the MOF with the lattice dimethylformamide undergoes 100 % photoreaction, but the photoproduct contains broken chains. Such chemical patterning at the molecular level represents a next step in crystal engineering.

Near-White Light Emission from Lead(II) Metal-Organic Frameworks.

Reaction of bpy (bpy = 4,4'-bipyridine) with Pb(OAc)2·3H2O in DMF (DMF = dimethylformamide) afforded a metal-organic framework (MOF), [Pb2(µ-bpy)(µ-O2CCH3)2(µ-O2CCH3)2]·H2O (1). Reaction of bpy with Pb(O2CCF3)2 in a methanol and chloroform mixture furnished another MOF, [Pb(µ-bpy)(µ-O2CCF3)2]·1/2CHCl3 (2). However, the reaction of bpy with Pb(OAc)2·3H2O in the presence of trifluoroacetic acid in a similar reaction condition yielded a hydrogen-bonded zwitter-ionic complex of Pb(II), [Pb(bpy-H)2(O2CCF3)4] (3). All compounds have been characterized by single crystal X-ray crystallography, FT-IR, and 1H NMR spectroscopies. Compound 1 forms four heptacoordinated Pb(II) joined by (OCCH3)-O- linkages, resulting in a 3D noninterpenetrated MOF net with a four-connected uninodal sra (SrAl2) topology. However, in 2, tetra-connected Pb4(O2CCF3)8 cluster units are linked further through eight bpy ligands to furnish a doubly interpenetrated MOF with a new topology but having the very similar connectivity of 1, whereas 3 forms a zigzag hydrogen-bonded chain structure. The variation of carboxylate anions, pH of the reaction medium, and the ratio of the reactants profoundly affected the final topological structure of the compounds synthesized. The solid-state photoluminescence of 1-3 was investigated at room temperature. Interestingly 1, 2, and 3 achieved close to white light emission when excited at 329, 376, and 330 nm, respectively. The systematic understanding of the photophysical properties of analogous Pb-based compounds may open new perspectives for developing single-phase white-light-emitting materials using Pb(II) based MOFs.

Molecular Scissoring: Facile 3D to 2D Conversion of Lanthanide Metal Organic Frameworks Via Solvent Exfoliation.

Five lanthanide MOFs with pcu topology have been exfoliated into nanoplatelets of two-dimensional structures via sonication in the dimethylacetamide solvent. These nanosheets are fluorescent under two-photon excitation dominated by the ligand, indicating energy upconversion ability.

A Step-by-Step Assembly of a 3D Coordination Polymer in the Solid-State by Desolvation and [2+2] Cycloaddition Reactions.

Two solid-state structural transformations that occur in a stepwise and a controlled manner are described. A combination of desolvation and cycloaddition reactions has been employed to synthesise a 3D coordination polymer (CP) from 1D CP [Cd(bdc)(4-spy)2 (H2 O)]⋅2 H2 O⋅2 DMF (bdc=1,4-benzenedicarboxylate, 4-spy=4-styrylpyridine) presumably via a 2D layered structure, [Cd2 (bdc)2 (4-spy)4 ]. In the absence of single crystals to follow the course of the photocycloaddition reaction, thermogravimetry, XAFS and NOESY NMR experiments were used to propose the formation of layered and pillared layered structures. Further, the present strategy enables us to synthesise new multidimensional architectures that are otherwise inaccessible by the self-assembly process.

Formation of a Syndiotactic Organic Polymer Inside a MOF by a [2+2] Photo-Polymerization Reaction.

Getting suitable crystals for single-crystal X-ray crystallographic analysis still remains an art. Obtaining single crystals of metal-organic frameworks (MOFs) containing organic polymers poses even greater challenges. Here we demonstrate the formation of a syndiotactic organic polymer ligand inside a MOF by quantitative [2+2] photopolymerization reaction in a single-crystal-to-single-crystal manner. The spacer ligands with trans,trans,trans-conformation in the pillared-layer MOF with guest water molecules in the channels, undergo pedal motion to trans,cis,trans-conformation prior to [2+2] photo-cycloaddition reaction and yield single crystals of MOF containing two-dimensional coordination polymers fused with the organic polymer ligands. We also show that the organic polymer in the single crystals can be depolymerized reversibly by cleaving the cyclobutane rings upon heating. These MOFs also show interesting photoluminescent properties and sensing of small organic molecules.

Putting the squeeze on CH4 and CO2 through control over interpenetration in diamondoid nets.

We report the synthesis, structure, and sorption properties of a family of eight diamondoid (dia) metal-organic materials (MOMs) that are sustained by Co(II) or Zn(II) cations linked by one of three rigid ligands: 4-(2-(4-pyridyl)ethenyl)benzoate (1), 4-(pyridin-4-yl)benzoate (2), and 4-(pyridin-4-yl)acrylate (3). Pore size control in this family of dia nets was exerted by two approaches: changing the length of the linker ligand from 1 to 3, and using solvent as a template to control the level of interpenetration in nets based upon 1 and 3. The resulting MOMs, dia-8i-1, dia-5i-3, dia-7i-1-Zn, dia-7i-1-Co, dia-4i-3-a, dia-4i-3-b, dia-4i-2, and dia-4i-1, exhibit 1D channels with pore limiting diameters (PLDs) of 1.64, 2.90, 5.06, 5.28, 8.57, 8.83, 11.86, and 18.25 Å, respectively. We selected dia nets for this study for the following reasons: their 1D channels facilitate study of the impact of pore size on gas sorption parameters in situations where pore chemistry is similar (pyridyl benzoate-type linkers) or identical (in the case of polymorphs), and their saturated metal centers eliminate open metal sites from dominating sorbent-solvate interactions and possibly masking the effect of pore size. Our data reveal that smaller pore sizes offer stronger interactions, as determined by the isosteric heat of adsorption (Qst) and the steepness of the adsorption isotherm in the low-pressure region. The porous MOM with the smallest PLD suitable for physisorption, dia-7i-1-Co, was thereby found to exhibit the highest Qst values for CO2 and CH4. Indeed, dia-7i-1-Co exhibits a Qst for CH4 of 26.7 kJ/mol, which was validated through grand canonical Monte Carlo simulation studies of CH4 adsorption. This Qst value is considerably higher than those found in covalent organic frameworks and other MOMs with unsaturated metal centers. These results therefore further validate the critical role that PLD plays in gas adsorption by porous MOMs.

Assembly of 3D coordination polymers from 2D sheets by [2+2] cycloaddition reaction.

The synthesis of three 2D interdigitated Zn(II) coordination polymers (CPs), by using three monotopic ligands containing C=C bonds, is reported. Among these, two CPs with 4spy (4-styryl pyridine) and 2F-4spy (a 2'-fluoro derivative of 4spy) ligands showed quantitative formation of cyclobutane rings, thus demonstrating a unique synthetic procedure to synthesize metal-organic frameworks (MOFs) by using this photochemical reaction. Interestingly, these compounds can also be synthesized by mechanochemical grinding procedures by using Zn(OAc)2. In contrast, Zn(NO3)2 did not yield the required product, unlike in the solution route. In addition, compounds with 4vpy (4-vinylpyridine), 4spy and 2F-4spy ligands created different units in the CPs; 4vpy and 2F-4spy furnished paddle wheel units, whereas 4spy yielded tetrahedral Zn(II) repeating units. Furthermore, the change in coordination geometry manifests in the photoluminescence properties, attributed to the difference in charge-transfer and ligand-centered fluorescent phenomenon.

Oxygen-initiated stereoselective thermal isomerisation of a cyclobutane derivative in the solid state.

Solid-state [2+2] photochemical cycloaddition reactions have been extensively studied after the classical work of Schmidt in the 1960s. Of these, trans-1,2-bis(4'-pyridyl)ethylene (bpe) is one of the well-studied alkenes to synthesize tetrakis(4-pyridyl)cyclobutane (tpcb). However, almost all the solid-state [2+2] cycloaddition reactions of bpe yielded, almost exclusively, one of the four possible isomers, namely, the rctt-tpcb (r=regio c=cis and t=trans). Here we describe a stereoselective synthesis of the tetrahedrally disposed rtct-tpcb by the solid-state thermal isomerization of the rctt-isomer in atmospheric air. We propose that this isomerization occurs through a topochemical unimolecular mechanism by a radical chain pathway, initiated by molecular oxygen. This is supported by the nature of products formed in air and nitrogen, detection of a radical in ESR spectral studies, ESI-MS crossover experiments, VT PXRD studies along with QM, MD and docking calculations. The formation of a unique isomer by thermal isomerization may be a general phenomenon to quantitatively synthesize other useful stereoisomers from the existing isomers of cyclobutane derivatives.

Solid-state reactivity and structural transformations involving coordination polymers.

This tutorial review discusses recent literature on coordination polymers and metal-organic frameworks (MOFs) that exhibit solid-state reactivity and structural transformation under various experimental conditions. Removal or exchange of solvents and guest molecules, exposure to reactive vapours, and external stimuli such as heat, light or mechanochemical force cause such structural transformations and these are often manifested by various physical properties such as colour, magnetism, luminescence, chirality, porosity, etc. due to change in coordination number and geometry, dimensionality, interpenetration, etc. More drastic transformations related to the exchange of metal ions, pillar ligands and insertion of additional ligands between the layers have also been demonstrated. The vast area of dynamic behaviour of coordination polymers and their stimuli-responsive properties have also been addressed briefly.