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.

The synthesis of three 2D interdigitated Zn^II coordination polymers (CPs), by using three monotopic ligands containing CC 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.

The extremely rare examples of dynamic single crystals where excitation by light or heat induces macroscopic motility present not only a visually appealing demonstration of the utility of molecular materials for conversion of energy to work, but they also provide a unique opportunity to explore the mechanistic link between collective molecular processes and their consequences at a macroscopic level. Here, we report the first example of a photosalient effect (photoinduced leaping) observed with crystals of three coordination complexes which is induced by a [2+2] photocycloaddition reaction. Unlike a plethora of other dimerization reactions, when exposed to even weak UV light, single crystals of these materials burst violently, whereby they are propelled to travel several millimeters. The results point to a multistep mechanism where the strain energy that has been accumulated during the dimerization triggers a rapid structure transformation which ultimately results in crystal disintegration.

Organic polymers are usually amorphous or possess very low crystallinity. The metal complexes of organic polymeric ligands are also difficult to crystallize by traditional methods because of their poor solubilities and their 3D structures can not be determined by single-crystal X-ray crystallography owing to a lack of single crystals. Herein, we report the crystal structure of a 1D Zn^II coordination polymer fused with an organic polymer ligand made in situ by a [2+2] cycloaddition reaction of a six-fold interpenetrated metal–organic framework. It is also shown that this organic polymer ligand can be depolymerized in a single-crystal-to-single-crystal (SCSC) fashion by heating. This strategy could potentially be extended to make a range of monocrystalline metal organopolymeric complexes and metal–organic organopolymeric hybrid materials. Such monocrystalline metal complexes of organic polymers have hitherto been inaccessible for materials researchers.

A [2+2] cycloaddition reaction has been observed in a number of solids. The cyclobutane ring in a photodimerized material can be cleaved into olefins by UV light and heat. The high thermal stability of the metal–organic salt K₂SDC (H₂SDC=4,4’-stilbenedicarboxylic acid) has been successfully utilized to investigate the reversible cleavage of a cyclobutane ring. The two polymorphs of K₂SDC undergo reversible cyclobutane formation by UV light and cleavage by heat in cycles. Of these, one polymorph retains its single-crystal nature during the reversible processes. Polymorphs are known to show different physical properties and chemical reactivities. This work reveals that the retention of single-crystal nature is strongly associated with the packing of molecules, which is controlled by kinetics and thermodynamics. The photoemissive nature of the products makes this as a promising material for photoswitches and optical data storage devices.

Distortional isomers, or bond-stretch isomers, differ only in the length of one or more bonds, which is due to crystallographic disorder in most cases. The term distortional isomerism is introduced to describe the structures of polyrotaxane 2D coordination polymers (CPs) that differ only by the relative positions in the neighboring entangled axles. A large ring and a long spacer ligand in 2D CPs yielded four different supramolecular isomers, of which two have an entangled polyrotaxane structure. One pair of CC bonds in the spacer ligand is well-aligned in one isomer and undergoes [2+2] cycloaddition reaction, whereas the other isomer is photoinert. They also have different sensing efficiency for several aromatic nitro compounds. However, both isomers show selective PL quenching for the Brady’s reagent. Structurally similar supramolecular isomers with different photochemical reactivity and sensing abilities appear to be unprecedented.

An attempt has been made to design double-stranded ladder-like coordination polymers (CPs) of hemidirected Pb^II. Four CPs, [Pb(μ-bpe)(O₂C-C₆H₅)₂]⋅2H₂O (1), [Pb₂(μ-bpe)₂(μ-O₂C-C₆H₅)₂(O₂C-C₆H₅)₂] (2), [Pb₂(μ-bpe)₂(μ-O₂C-p-Tol)₂(O₂C-p-Tol)₂]⋅ 1.5 H₂O (3) and [Pb₂(μ-bpe)₂(μ-O₂C-m-Tol)₂(O₂C-m-Tol)₂] (4) (bpe=1,2-bis(4′-pyridyl)ethylene), have been synthesised and investigated for their solid-state photoreactivity. CPs 2–4, having a parallel orientation of bpe molecules in their ladder structures and being bridged by carboxylates, were found to be photoreactive, whereas CP 1 is a linear one-dimensional (1D) CP with guest water molecules aggregating to form a hydrogen-bonded 1D structure. The linear strands of 1 were found to pair up upon eliminating lattice water molecules by heating, which led to the solid-state structural transformation of photostable linear 1D CP 1 into photoreactive ladder CP 2. In the construction of the double-stranded ladder-like structures, the parallel alignment of CC bonds in 2–4 is dictated by the chelating and μ₂-η²:η¹ bridging modes of the benzoate and toluate ligands. The role of solvents in the formation of such double-stranded ladder-like structures has also been investigated. A single-crystal-to-single-crystal transformation occurred when 4 was irradiated under UV light to form [Pb₂(rctt-tpcb)(μ-O₂C-m-Tol)₂(O₂C-m-Tol)₂] (5).

Solvothermal reaction of Zn(NO₃)₂⋅4 H₂O, 1,4-bis[2-(4-pyridyl)ethenyl]benzene (bpeb) and 4,4′-oxybisbenzoic acid (H₂obc) in the presence of dimethylacetamide (DMA) as one of the solvents yielded a threefold interpenetrated pillared-layer porous coordination polymer with pcu topology, [Zn₂(bpeb)(obc)₂]⋅5 H₂O (1), which comprised an unusual isomer of the well-known paddle-wheel building block and the trans–trans–trans isomer of the bpeb pillar ligand. When dimethylformamide (DMF) was used instead of DMA, a supramolecular isomer [Zn₂(bpeb)(obc)₂]⋅2 DMF⋅H₂O (2), with the trans–cis–trans isomer of the bpeb ligand with a slightly different variation of the paddle-wheel repeating unit, was isolated. In MeOH, single crystals of 2 were transformed by solvent exchange in a single-crystal-to-single-crystal (SCSC) manner to yield [Zn₂(bpeb)(obc)₂]⋅2 H₂O (3), which is a polymorph of 1. SCSC conversion of 3 to 2 was achieved by soaking 3 in DMF. Compounds 1 and 2 as well as 2 and 3 are supramolecular isomers.

Three coordination polymers, [Cd₂(pvba)₂(tbdc)(dmf)₂] (1), [Co₂(pvba)₂(tbdc)(dmf)₂(H₂O)₂] (2), and [Ni₂(pvba)₂(tbdc)(dmf)₂(H₂O)₂] (3) (H₂tbdc=2,3,5,6-tetrabromobenzenedicarboxylic acid, Hpvba=trans-2-(4′-pyridyl)vinylbenzoic acid), were synthesized by solvothermal methods. The solid-state structures of compounds 1 and 2 were determined by X-ray crystallography. In compounds 1 and 2, the bimetallic cores acted as secondary building units that connected the tbdc ligands in one direction and a pair of pvba ligands, which were aligned in a head-to-tail parallel manner, in the orthogonal direction to form sheet structures. The CC bonds in these pvba ligand pairs in all three compounds were well-aligned to undergo quantitative [2+2] cycloaddition reactions in the solid state under UV irradiation, thereby yielding their cyclobutane derivatives. This photochemical reaction appeared to facilitate structural transformations from one 2D structure into another in the solid state. The photoreactive Co^II- and Ni^II coordination polymers exhibited a reversible dehydration–rehydration reaction that was accompanied by color changes from pink to purple and green to yellow, respectively, owing to a change in coordination number from six to five. Magnetic studies showed that compound 2 was an antiferromagnet, which displayed a field-dependent transition with a critical field (H_c) of 40 kOe at 2 K; the antiferromagnetic interaction between the Co₂ units was strengthened and weakened by dehydration and UV irradiation, respectively. The cyclobutane ligand in the photodimerized products was cleaved on heating to yield a mixture of trans- and cis-isomers of pvba, as monitored by ¹H NMR spectroscopy. The Cd^II coordination polymer underwent quantitative cleavage of the cyclobutane ring whilst the other two underwent partial cleavage.

Reaction of the Co^II ion and the bpe [trans-1,2-bis(4-pyridyl)ethene] ligand in the presence of the CH₃CO₂^–, CF₃CO₂^–, C₆H₅CO₂^– and NO₃^– anions affords six coordinationpolymers, namely [Co₂(μ-bpe)₂(μ-O₂CCH₃)₂(O₂CCH₃)₂]·H₂O (1), [Co₂(μ-bpe)₂(μ-O₂CCF₃)₂(O₂CCH₃)₂] (2), [Co(μ-bpe)₂(O₂CCF₃)₂] (3), [Co(μ-bpe)(O₂CC₆H₅)₂(HO{O}CC₆H₅)₂] (4), [Co(μ-bpe)(O₂CC₆H₅)₂(CH₃OH)₂] (5) and [Co₂(μ-bpe)₃(NO₃)₃(CH₃OH)](NO₃) (6). All the compounds have been characterized by X-ray crystallography, IR spectroscopy and thermogravimetry. Variable-temperature magnetic measurements were also performed on 3 and 6. Compounds 1 and 2 form 1D molecular ladder coordination polymers bridged by acetate ligands. Replacing the acetate ligands completely by trifluoroacetate ligands produced interpenetrated square-grid sheets in 3. Replacing acetate and trifluoroacetate ligands by benzoate ligands resulted in two 1D coordination polymers, 4 and 5, having zigzag and linear structures, respectively. When the nitrate anion was used, a 3D network, 6, was afforded with inclined interpenetration of a 1D molecular ladder structure with threefold polycatenation. The variation of anions, reaction conditions and the ratio of reactants profoundly affect the final topological structure of the coordination polymers synthesized. The trifluoroacetate and nitrate anions influence the topology and the degree of interpenetration in the coordination polymers 3 and 6, respectively.

One-dimensional hydrogen-bonded complex [Zn(bpe)₂(H₂O)₄](NO₃)₂⋅8/3 H₂O⋅2/3 bpe (1, bpe=4,4′-bipyridylethylene) containing coordination complex cations [Zn(bpe)₂(H₂O)₄]²⁺ with parallel and crisscross double bonds undergoes photochemical [2+2] cycloaddition in the solid state and produces tetrakis(4-pyridyl)cyclobutane (tpcb) in up to 100 % yield with rctt-tpcb (2 a) as major and rtct-tpcb (2 b) as minor product. The bpe ligands with crisscross conformation of CC bonds appear to undergo pedal-like motion prior to photodimerization. Grinding single crystals to powder accelerates the pedal motion of crisscrossed olefins in the bpe ligands to parallel alignment and provides the rctt-cyclobutane stereoisomer 2 a quantitatively. In addition, 100 % photodimerization of ground 1 indicates that the free bpe ligands, which are remote from each other in a pool of water, and NO₃⁻ ions moved toward each other to give a mixture of rctt- and rtct-tpcb isomers.