Solid-State Photoconversion of a Discrete Mixed Iodine(I) System to a 1D Polymer

The first example of a mixed halogen(I) complex (2), containing three distinct iodine(I) moieties ([N-I-N]+, O-I-N, and [O-I-O]-) within the same structure, was synthesized with 4-styrylpyridine (4-stypy) and 3,4,5,6-tetrafluorophthalate as the stabilizing Lewis bases. This complex was observed to be in equilibrium with its respective bis(OIN) complex (1a), with isolated samples of 2 also being found to convert to 1a in solution. Upon UV irradiation of 2, a single-crystal-to-single-crystal [2 + 2] cycloaddition reaction was observed, converting the discrete salt 2 to the 1D polymer 5. Complex 5 retained all the iodine(I) moieties from prior to photoconversion and represents the first example of nondestructive photoconversion of a halogen(I) complex. To facilitate comparisons to 2 and 5, several additional closely related iodine(I) complexes were synthesized, with the iodine(I) complexes characterized by NMR (1H, 1H-15N HMBC) and SCXRD, as well as by Raman and IR spectroscopy for 2, 5, and their close structural analogue 1a.

Salts of trans-3-Styryl Pyridine: The Effect of N-Quaternization on Solid-State Photoreactivity, and Photochemical Crystal Melting

Investigating solid-state photoreactivity, driven by crystal packing, has been a major enduring research theme in Crystal Engineering. Trans-3-styryl pyridine (3-StPy), an unsymmetric olefin, is photo-stable. However, when converted to a series of salts, they exhibited solid-state photoreactivity under UV irradiation. Crystal structures of 3-StPy, its protonated salts, namely, (3-StPyH)(HSO4) ⋅ H2O (1), (3-StPyH)(CF3CO2) (2), and (3-StPyH)(p-Tol-SO3) (3), and methylated salt (3-StPyMe)I (4) were determined by X-ray diffraction. 3-StPy molecules were found not to align in the parallel arrangement required to undergo solid-state [2+2] photocycloaddition reaction; however, upon protonation and methylation, the unsymmetric 3-StPyH+ and [3-StPyMe]+ cations aligned in a head-to-tail parallel arrangement, predominantly governed by the cation⋅⋅⋅π interactions. Various structural features, e. g., the patterns of hydrogen bonding, etc. have been addressed, and established by Hirshfeld surface analysis. The salt with p-tolyl sulfonate anion (3) with Z'>1 represents a rare crystal class. It was also noted that the crystals of 1 and 2 melted into a liquid state upon photodimerization reaction. UV-vis absorption and fluorescence properties have been explored. The electronic structures of 3-StPy, 3-StPyH+, [3-StPyMe]+, and the dimer [3,3'-MPyPhCB]2+ cations have been elucidated by DFT calculations, and the effect of N-quaternization on crystal structures and photophysical properties has been discussed.

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.

Photomechanical effects based on a one-dimensional Zn coordination polymer crystal driven by [4 + 4] cycloaddition

Photomechanical crystals are promising candidates for photo actuators due to their ability to convert light energy into mechanical energy. We synthesized a coordination polymer crystal that can undergo [4 + 4] cycloaddition reactions with mechanical motion. The inclusion of {[ZnL2(4,4'-bipy)(CH3OH)2]}∞ in a polymer membrane significantly magnified the actuation behavior.

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.

Impact of solid-state photochemical [2+2] cycloaddition on coordination polymers for diverse applications

This Frontier article highlights the advancement of [2+2] photocycloaddition reactions within coordination polymers to fine tune their diverse physical and chemical properties.

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.

Varying the regiochemistry of a [2 + 2] cycloaddition reaction within isostructural hydrogen bonded cocrystals containing 4-stilbazole

Cocrystallization of 4-stilbazole with 4,6-diX resorcinol (X = Cl or Br) yields a pair of similar photoreactive solids that leads to different photoproducts.

Solid-state polymerisation via [2+2] cycloaddition reaction involving coordination polymers

Recent advancements in the construction of cyclobutane polymersviaphoto-dimerisation reaction in the monocrystalline solids of metal complexes, coordination polymers and metal–organic framework structures are reviewed.

Hemilabile silver(i) complexes containing pyridyl chalcogenolate (S, Se) ligands and their utility as molecular precursors for silver chalcogenides

A series of silver triphenylphosphine complexes containing pyridyl chacogenolate (S, Se) ligands has been synthesized by employing [AgCl(PPh₃)₃] and [Ag₂(μ-Cl)₂(PPh₃)₄].

An unusual single-crystal-to-single-crystal [2 + 2] photocyclisation reaction of a TTF-arylnitrile derivative

Irradiation of single crystals of a benzonitrile substituted tetrathiafulvalene with polychromatic light results in a [2 + 2] cycloaddition reaction.

Thermal and photochemical control of nitro–nitrito linkage isomerism in single-crystals of [Ni(medpt)(NO2)(η2-ONO)]

[Ni(medpt)(NO₂)(η²-ONO)] displays an equilibrium between the η¹-NO₂ and the η¹-ONO linkage isomers between 150–298 K; upon photoactivation at 100 K the percentage of the η¹-ONO isomer increases.