Intramolecular [2 + 2] Photodimerization Achieved in the Solid State via Coordination-Driven Self-Assembly

Metal-Free C-X Functionalization in Solid-State via Photochemistry in Ball Mills

We report the first metal- and catalyst-free protocol for the facile cross-coupling of aryl halides towards C-B, C-P and C-S bonds under solid-state ball milling conditions via UV light irradiation. The reactions can be performed in the absence of bulk solvents at room temperature in a mixer mill, yielding up to 99 % and being tolerant towards various functionalized aryl halides (X=I or Br). Furthermore, we developed a novel photoreactor design increasing the light intensity. With this we could demonstrate that our protocol surpasses classical solvent based as well as purely mechanochemical approaches in terms of green metrics and energy efficiency.

High-Temperature Solid-State Post-Synthetic Modification of Highly Luminescent Cu(I) Metallacycles toward New Luminescent Thermic Tracers

A new luminescent Cu(I) tetrametallic metallacycle B is reported that features very rare semi-bridging aqua ligands. When heated markedly above room temperature, this compound undergoes a post-synthetic transformation in the solid-state, affording the new luminescent metallacycle C. Thermogravimetric analysis, IR spectroscopy and single-crystal X-ray diffraction reveal that this alteration preserves the gross tetrametallic macrocycle structure, but is caused by the release of the coordinated water molecules with the concomitant formation of cuprophilic interactions. This transition induces a shift from eye-perceived green (B) to blue (C) room-temperature luminescence for these molecular solids. Photophysical measurements and time-dependent density-functional theory calculations have been conducted to identify the origins of the emission properties lying in these structurally related assemblies, and suggest that thermally activated delayed fluorescence dominates the radiative relaxation pathways. This study highlights the innovative feature of Cu(I) derivatives, offering access to stimuli-sensitive materials that can witness, a posteriori, the exceeding of critical temperatures in their environment.

Reversing the stereoselectivity of intramolecular [2+2] photocycloaddition utilizing cucurbit[8]uril as a molecular flask

Macrocyclic hosts, such as cucurbit[8]uril (CB8), can significantly influence the outcomes of chemical reactions involving encapsulated reactive guests. In this study, we demonstrate that CB8 completely reverses the stereoselectivity of intramolecular [2+2] photo-cycloaddition reactions. Notably, it was also found that CB8 can trigger the unreactive diene to be reactive.

Isolation of elusive cyclobutane ligands via template assisted photochemical [2+2] cycloaddition reaction and their utility in engineering crystalline solids

Solid-state photochemical [2+2] cycloaddition reactionis one of the promising routes to isolate highly strained cyclobutane derivatives whichmay otherwise be very difficult to obtain via conventional synthetic methodology. Various methods have...

Template-Directed Photochemical Homodimerization and Heterodimerization Reactions of Cinnamic Acids

We developed a general method for the selective photochemical homo- and heterodimerization of cinnamic acid derivatives with the use of commercially available 1,8-dihydroxynaphthalene as a covalent template. A variety of symmetrical and unsymmetrical β-truxinic acids were obtained in high yields and as single diastereomers. The use of a template not only provides the alignment of the two olefins with suitable proximity (<4.2 Å) but also allows the heterodimerization of two different cinnamic acids, leading to unsymmetrical β-truxinic acid products.

On-surface synthesis of 2D COFs via molecular assembly directed photocycloadditions: a first-principles investigation

Over the past decades, the rational synthesis of two-dimensional covalent organic framework (2D COFs) monolayer via on-surface chemistry has been widely explored. Herein, we propose the [2 + 2] photocycloaddition as a novel strategy for large-scale fabrication of COFs from theoretical perspective. Thanks to the symmetry forbidden of thermal [2 + 2] cycloaddition, the molecular precursors carrying vinyl groups will not chemically interact with each other during thermal annealing, which is essential to achieve molecular assembly. The subsequent photocycloaddition of these precursors may produce large-scale 2D COFs at low temperatures, in which the symmetry of molecular assembly remains unchanged. Our results show that 2D COFs can be produced via [2 + 2] photocycloadditions directed from self-assembled precursors, in which alkylbenzene molecules with vinyl groups on side chains exhibit appropriate intermolecular distances. By performing high-throughput calculations, several promising molecular precursors are proposed to achieve large-scale 2D COFs. This work provides an applicable strategy for the large-scale synthesis of 2D carbon materials.

Self-Assembly of Diboronic Esters with U-Shaped Bipyridines: "Plug-in-Socket" Assemblies

Self-assembled complexes utilizing the ditopic dative bond acceptor 1,3-diboronic acid with catechol and complementary U-shaped donors in the form of 1,8-dipyridylnaphthalenes (1,8-bis(4-pyridyl)naphthalene (DPN), 1,8-bis(4-ethylenylpyridyl)naphthalene (DEPN), and 1,8-bis(4-ethynylpyridyl)naphthalene (DAPN)) yielded discrete two-component structures. The assemblies exhibit "plug-in-socket" geometries. DFT calculations are consistent with the donor pyridyl and acceptor catecholate being electron poor and rich, respectively. The assemblies pack via π-π interactions and support the inclusion of a solvent (i.e., DPN, DAPN). The materials may form a basis for the design of complex B-based structures (e.g., supramolecular dyads).

Superstructural diversity in salt-cocrystals: higher-order hydrogen-bonded assemblies formed using U-shaped dications and with assistance of π--π stacking

Salt cocrystals with components that assemble by hydrogen bonds and aromatic anion-molecule stacks (π--π stacks) are reported. U-shaped bipyridines and an isocoumarin carboxylic acid self-assemble to form 5-, 6-, and 10-component aggregates with components in double and quadruple face-to-face stacks. DFT calculations support the π--π stacks to help stabilize the salt cocrystals.

Regioselective Photochemical Cycloaddition Reactions of Diolefinic Ligands in Coordination Polymers

The pure diolefinic ligand 1,4-bis(pyridin-4-yl)-1,3-butadiene (bpbde) is photostable in the crystalline state. With the assistance of coordination-driven metal-organic assemblies, the photoreactivity of this diolefinic ligand can be significantly enhanced. A hydrothermal reaction of bpbde with Cd(NO₃ )₂ ⋅4 H₂ O and the auxiliary ligand adipic acid resulted in the formation of a two-dimensional photoreactive coordination polymer (CP), [Cd(adipate)(bpbde)]_n (1). When the aliphatic carboxylic acid was replaced by pimelic acid, another photoreactive CP [Cd(pimelate)(bpbde)]_n (2) with a three-dimensional framework was obtained. With irradiation of 365 nm UV light, the bpbde ligands in crystalline 1 and 2 underwent a regioselective photochemical [2+2] cycloaddition reaction and converted to 3,4,7,8-tetra(pyridin-4-yl)tricyclo[4.2.0.0^2,5 ]octane (tptco) and 1,3-bis(pyridin-4-yl)-2,4-bis(2-(pyridin-4-yl)vinyl)cyclobutane (bpbpvcb), respectively. The results provide an interesting insight into the rational design of highly regio- or stereoselective photocatalytic reactions for the formation of special organic molecules.

Physicochemical Properties and Photochemical Reactions in Organic Crystals

Background:

Molecular organic photochemistry is concerned with the description of physical and chemical processes generated upon the absorption of photons by organic molecules. Recently, it has become an important part of many areas of science: chemistry, biology, biochemistry, medicine, biophysics, material science, analytical chemistry, among others. Many synthetic chemists are using photochemical reactions in crystals to generate different types of organic compounds since this methodology represents a green chemistry approach.

Objective & Method:

Chemical reactions in crystals are quite different from reactions in solution. The range of organic solid state reactions and the degree of control which could be achieved under these conditions are quite wider and subtle. Therefore, for a large number of molecular crystals, the photochemical outcome is not the expected product based on topochemical principles. To explain these experimental results, several physicochemical factors in crystal structure have been proposed such as defects, reaction cavity, dynamic preformation or photoinduced lattice instability and steric compression control. In addition, several crystal engineering strategies have been developed to bring molecules into adequate orientations with reactive groups in good proximity to synthesize complex molecules that in many cases are not available by conventional methods. Some strategies involve structural modifications like intramolecular substitution with different functional groups to modify intermolecular interactions. Other strategies involve chemical techniques such as mixed crystal formation, charge transfer complexes, ionic and organometallic interactions. Furthermore, some examples of the single crystal to single crystal transformations have also been developed showing an elegant method to achieve regio and stereoselectivity in a photochemical reaction.

Conclusion:

The several examples given in this review paper have shown the wide scope of photochemical reactions in organic molecular crystals. There are several advantages of carrying photochemical reaction in the solid state. Production of materials unobtainable by the traditional solution phase reactions, improved specificity, reduction of impurities, and enhancement in the yields by the reduction of side reactions. These advantages and the multidisciplinary nature of solid-state photochemistry make this discipline quite likely to develop a lot in the future.

Photochemical reactions of metal complexes in the solid state

This perspective focusses on the solid-state reactivity and structural transformation driven by photochemical methods in discrete metal complexes, organometallic compounds, metallo-macrocycles and cages. Changes in the metal-metal bond distances, racemization of chiral centres, fusion of cages, formation of coordination polymers, expected [2 + 2] and [4 + 4] cycloaddition products, unusual phenyl-olefin dimerization, and linkage isomerization of -SO₂, -NO & -NO₂ ligands cause the structural transformations. Of these, [2 + 2] photo-cycloaddition reactions have been widely studied and the photoreactions are made possible by various supramolecular interactions including hydrogen bonds, metallophilic, ππ and C-Hπ interactions, ligand design and metallic clips to bring the reactive functional groups closely into correct orientation close to the transition state. These photoreactions are often accompanied by crystal bending, mechanical motion, and changes in the magnetic and photoluminescence properties. In several cases, the single crystals have been preserved at the end of the reactions, which are known as single-crystal-to-single-crystal (SCSC) reactions.

Solid state [2 + 2] photocycloaddition for constructing dimers of N,N′-diacyl-1,4-dihydropyrazines based on thiourea-induced assembly

Unprecedented photoadducts were obtained through solid state [2 + 2] photocycloaddition of 1,4-dihydropyrazines, assembled using thiourea as a template.

A new tecton with parallel halogen-bond donors: a path to supramolecular rectangles

A new tecton, 1,8-diiodoethynylanthracene, with two halogen-bond donor sites was synthesized and characterized. This tecton is capable of forming two parallel halogen bonds at once, which makes it a useful building block for the construction of a variety of supramolecular squares and rectangles.

Single-crystal-to-single-crystal transformation of a coordination polymer from 2D to 3D by [2 + 2] photodimerization assisted by a coexisting flexible ligand

A 2D interdigitated [Ni₂(adipate)₂(spy)₄(H₂O)₂] was transformed to a 3D coordination polymer through [2 + 2] photodimerization with conformational changes of adipate ligands.

Single-crystal-to-single-crystal conversions of two metal-mediated photoreactive coordination polymers based on stereoselective [2 + 2] photocycloaddition reactions

Two stereoselective [2 + 2] photocycloaddition reactions occur in two metal-mediated coordination polymers via single-crystal-to-single-crystal transformations.

Metal-Organic Coordination versus Hydrogen Bonding: Highly Efficient Templated Photocycloadditions of Trisubstituted Isomeric Olefins in the Solid State

Trisubstituted olefins are reactants in template-directed, solid-state [2+2] photocycloaddition reactions. Whereas hydrogen-bond-based templates afford crystalline assemblies with olefins that are photostable, Ag^I coordination results in carbon-carbon double bonds that react stereoselectively and result in quantitative yield.