Emerging solid-state cycloaddition chemistry for molecular solar thermal energy storage

Recently discovered designs of solid-state molecular solar thermal energy storage systems are illustrated, including alkenes, imines, and anthracenes that undergo reversible [2 + 2] and [4 + 4] photocycloadditions for photon energy storage and release. The energy storage densities of various molecular designs, from 6 kJ mol-1 to 146 kJ mol-1 (or up to 318 J g-1), are compared and summarized, along with effective strategies for engineering their crystal packing structures that facilitate solid-state reactions. Many promising molecular scaffolds introduced here highlight the potential for achieving successful solid-state solar energy storage, guiding further discoveries and the development of new molecular systems for applications in solid-state solar thermal batteries.

Template-Directed Selective Photodimerization Reactions of 5-Arylpenta-2,4-dienoic Acids

We developed an efficient method that enables selective photodimerization of 5-arylpenta-2,4-dienoic acids (i.e., vinylogous cinnamic acids). The use of 1,8-dihydroxynaphthalene as a template ensures proximity of the two reacting olefins so that irradiation of template-bound dienoic acids gives mono [2 + 2] cycloaddition products in good to excellent yields (up to 99%), as single regioisomers, and with high diastereoselectivities (dr = 3:1 to 13:1). The geometrical and stereochemical features of compounds 12a, 16a, and 22a were analyzed by X-ray crystallography.

Construction of olefinic coordination polymer single crystal platforms: precise organic synthesis, in situ exploration of reaction mechanisms and beyond

Olefin [2+2] photocycloaddition reactions based on coordination-bond templates provide numerous advantages for the selective synthesis of cyclobutane compounds. This review outlines the recent advances in the design and construction of single crystal platforms of olefinic coordination polymers for precise organic synthesis, in situ exploration of reaction mechanisms, and possible developments as comprehensively as possible. Numerous examples are presented to illustrate how the arrangements of the olefin pairs influence the solid-state photoreactivity and examine the types of cyclobutane products. Furthermore, the photocycloaddition reaction mechanisms are investigated by combining advanced techniques such as single crystal X-ray diffraction, powder X-ray diffraction, nuclear magnetic resonance, infrared spectroscopy, fluorescence spectroscopy, laser scanning confocal microscopy and theoretical calculations. Finally, potential applications resulting from promising physicochemical properties before and after photoreactions are discussed, and existing challenges and possible solutions are also proposed.

In situ observation of a stepwise [2 + 2] photocycloaddition process using fluorescence spectroscopy

Using highly sensitive and selective in situ techniques to investigate the dynamics of intermediates formation is key to better understand reaction mechanisms. However, investigating the early stages of solid-state reactions/transformations is still challenging. Here we introduce in situ fluorescence spectroscopy to observe the evolution of intermediates during a two-step [2 + 2] photocycloaddition process in a coordination polymer platform. The structural changes and kinetics of each step under ultraviolet light irradiation versus time are accompanied by the gradual increase-decrease of intensity and blue-shift of the fluorescence spectra from the crystals. Monitoring the fluorescence behavior using a laser scanning confocal microscope can directly visualize the inhomogeneity of the photocycloaddition reaction in a single crystal. Theoretical calculations allow us to rationalize the fluorescence behavior of these compounds. We provide a convenient strategy for visualizing the solid-state photocycloaddition dynamics using fluorescence spectroscopy and open an avenue for kinetic studies of a variety of fast reactions.

A recyclable thermoset with built-in thermocleavable group developed from a cis-cyclobutane-1,2-dicarboxylic acid

A novel class of recyclable thermoset has been developed from cis-3,4-diphenylcyclobutane-1,2-dicarboxylic acid (CBDA-4) due to its thermocleavability at high temperature. This key CBDA-4 building block was synthesized from β-trans-cinnamic acid using a [2+2] photocycloaddition reaction. CBDA-4 was subsequently linked with glycerol via esterification to give a thermoset with T_g of 68 °C. The thermoset was heated to 300 °C to analyze its degradation. A key intermediate was successfully obtained after purification of the degraded polymer. NMR, FT-IR, HRMS, and single crystal X-ray diffraction confirmed the intermediate was glycerol cinnamate, which was the result of splitting cyclobutane in the polymer backbone at high temperature. Glycerol cinnamate was readily hydrolyzed reforming the starting materials glycerol and trans-cinnamic acid to complete the recycling loop.

Tunable photosalient behaviours within coordination polymers via functional molecular prearrangements

Four Cd(II)/diene coordination polymers (CPs) with similar 1D chain motifs exhibit different photosalient (PS) behaviours in response to UV light. The [2+2] photoreaction between the CC groups within these CPs results in diverse PS behaviours of their crystals with different CC pair arrangements. The interesting PS behaviours of these CPs can be applied in design and fabrication of advanced photoactuating materials.

Porous carbon nanoarchitectonics for the environment: detection and adsorption

As a post-nanotechnology concept, nanoarchitectonics has emerged from the 20th century to the 21st century. This review summarizes the recent progress in the field of metal-free porous carbon nanoarchitectonics.

A metal-carbene template approach enables efficient synthesis of a functionalized cage-annulated crown ether

A facile synthetic method to form cage-annulated crown ether with anchored imidazolium units was developed. The present work verified the potential application of the metal-carbene template approach (MCTA) in the preparation of novel flexible polyimidazolium cages by photochemical [2+2] cycloaddition reactions and may provide a new method for the preparation of flexible pure organic cages with incorporation of a variety of functional sites.

Cubane-forming cyclic dienes that exhibit orthogonal reactivities in the solid state

Photoirradiation of a binary cocrystal composed of two different cyclic dienes generates a highly-symmetric cubane-like tetraacid cage regioselectively and in quantitative yield. The cage forms by a double [2+2] photodimerization of one of the diene cocrystal components. The second diene while photostable in the cocrystal reacts in a double [2+2] photodimerization as a pure form quantitatively to form a tetramethyl cubane-like cage. The stereochemistry of the cage is structurally authenticated.

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 Å between 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.

Topochemical polymerizations for the solid-state synthesis of organic polymers

Topochemical polymerizations are solid-state reactions driven by the alignment of monomers in the crystalline state. The molecular confinement in the monomer crystal lattice offers precise control over the tacticity, packing and crystallinity of the polymer formed in the topochemical reaction. As topochemical reactions occur under solvent- and catalyst-free conditions, giving products in high yield and selectivity/specificity that do not require tedious chromatographic purification, topochemical polymerizations are highly attractive over traditional solution-phase polymer synthesis. By this method, polymers having sophisticated structures and desired topologies can be availed. Often, such ordered packing confers attractive properties to the topochemically-synthesized polymers. Diverse categories of topochemical polymerizations are known, such as polymerizations via [2+2], [4+4], [4+2], and [3+2] cycloadditions, and polymerization of diynes, triynes, dienes, trienes, and quinodimethanes, each of which proceed under suitable stimuli like heat, light or pressure. Each class of these reactions requires a unique packing arrangement of the corresponding monomers for the smooth reaction and produces polymers with distinct properties. This review is penned with the intent of bringing all the types of topochemical polymerizations into a single platform and communicating the versatility of these lattice-controlled polymerizations. We present a brief history of the development of each category and comprehensively review the topochemical synthesis of fully-organic polymers reported in the last twenty years, particularly in crystals. We mainly focus on the various molecular designs and crystal engineering strategies adopted to align monomers in a suitable orientation for polymerization. Finally, we analyze the current challenges and future perspectives in this research field.

Ladderane Natural Products: From the Ground Up

The ladderane family of natural products are well known for their linearly concatenated cyclobutane skeletal structure. Owing to their unique carbocyclic framework, several chemical syntheses have been reported since their discovery in 2002. The focus of this review is to showcase the novel tactics that have been used to generate the ladderane core and the challenges that are associated with the synthesis of these unusual and complex natural products.

Photochemical [2 + 2] polymerization of metal-organic gels of a rigid and angular diene with silver-salts of diverse anions: selective dye-sorption and luminescence by xerogels

Two similar types of dienes, one rigid and the other flexible, were explored for their gel formation abilities with Ag(i) salts. The rigid and angular dienes have shown an exceptional ability for gel formation with silver salts of nitrate, triflate, tetrafluoro borate and hexafluorophosphate. These metal-organic gels (MOGs) and their xerogels are found to have an excellent ability to undergo the photochemical [2 + 2] polymerization reaction upon irradiation. The reactions were monitored, and the products were characterized via1H NMR and MALDI-TOF analyses. Further, the solid-state luminescence behaviour and the selective dye-sorption of the gels have been explored before and after the photo-polymerization reaction.

Polymerization in MOF-Confined Nanospaces: Tailored Architectures, Functions, and Applications

This feature article describes recent trends and advances in structuring network polymers using a coordination-driven metal-organic framework (MOF)-based template approach to demonstrate the concept of crystal-controlled polymerization in confined nanospaces, forming tailored architectures ranging from simple linear one-dimensional macromolecules to tunable three-dimensional cross-linked network polymers and interwoven molecular architectures. MOF-templated network polymers combine the characteristics and advantages of crystalline MOFs (high porosity, structural regularity, and designability) with the intrinsic behaviors of soft polymers (flexibility, processability, stability, or biocompatibility) with widespread application possibilities and tunable properties. The article ends with a summary of the remaining challenges to be addressed, and future research opportunities in this field are discussed.

Modulating the regioselectivity of solid-state photodimerization in coordination polymer crystals

Coordination polymers [Cd(1,4-bpeb)(L1)] (1), [Zn2(1,4-bpeb)2(L2)2(SO42-)2] (2) and [Cd(1,4-bpeb)(L3)] (H2O) (3) (H2L1, 3-[2-(3-hydroxy-phenoxymethyl)-benzyloxy]-benzoic acid; HL2, 1H-Indazole-3-carboxylic acid; H3L3, benzene-1,2,3-tricarboxylic acid; 1,4-bpeb, 1,4-bis[2-(4-pyridyl)vinyl]benzene have been synthesized under solvothermal conditions. Complexes 1-3 underwent photodimerization in the solid-state to give quantitative yields of single isomeric products. The choice of carboxyl ligands L and metal center determined the arrangement of 1,4-bpeb ligands, which in turn directed the regiochemistry of the final photoproducts. The solid-state network structures of cadmium based 1 and 3 had 1,4-bpeb pairs aligned face-to-face with both C[double bond, length as m-dash]C centres in each ligand at an appropriate distance and alignment for photodimerization to give the corresponding para-[2.2]cyclophane (pCP) exclusively. By contrast, compound 2 possessed dinuclear (ZnSO4)2 metallocycles that positioned the 1,4-bpeb "arms" face-to-face, but with C[double bond, length as m-dash]C centres offset at an appropriate distance for only one pair to undergo [2 + 2] cycloaddition to yield a single stereoisomer of the monocyclobutane photo-product bpbpvpcb. This work highlights crystal engineering design principles that can be used to facilitate regio- and stereospecificity in solid-state transformations.

Photoinduced Bending of Single Crystals of a Linear Bis-Olefin via Water-Templated Solid-State [2+2] Photopolymerization Reaction

The single crystals of two structural isomers of bis-olefinic molecules were shown to have contrasting properties in terms of their photoreactivity: one exhibits an excellent ability to form polymers, accompanied with bending of crystals upon irradiation, while the other is photostable. The photoreactive crystal is a first example in which [2+2] polymerization leads to bending of the crystals, with implications for the design of photoactuators. The hydrate formation ability of one of these molecular isomers promotes the solid-state reactivity in its crystal, as the H₂ O molecules act as a template to bring the olefin molecules into the required arrangement for [2+2] polymerization. Further, the crystals of the polymer exhibited better flexibility and smoothed surfaces compared to those of the monomers. In addition, under UV-light the diene emits bluish violet light while the polymer emits green light, indicating that the luminescence property can be tuned through photoirradiation.

The effect of coordinated solvent molecules on metal coordination environments in single-crystal-to-single-crystal transformations

We summarized the effect of coordinated solvent molecules on the metal coordination environments in single-crystal to single-crystal transformations.

Solid or gel? Which one works better for [2 + 2] photochemical polymerization in pyridine appended flexible phenylene 1, 4-bis-olefins by Ag(i) templation?

Two diene molecules were shown to undergo photopolymerization reactions in their metal-organic gels and xerogels, while their respective crystalline CPs are photostable. These reactions reveal the advantages of the gels and xerogels compared to their crystalline counterparts and also the utility of AgAg interactions in the gels to promote topochemical polymerizations.