Spin-crossover modulation via single-crystal to single-crystal photochemical [2 + 2] reaction in Hofmann-type frameworks

[2 + 2] Photocyclization converts thermally induced spin crossover effect into "hidden hysteresis" one

The light induced [2 + 2] cyclization of the flexible coumarin-based ligand (L) converts the spin crossover active HS1 ⇆ LS1 mononuclear system [Fe(L)6](BF4)2·4CH3CN (1) into the high spin 1D coordination polymer (2). The contribution of the resulting high spin form HS2 is directly related to the degree of photoconversion and, at the same time, practically does not affect the properties of the remaining thermally active spin crossover centers (HS1). The origin of such a fundamental change in properties is an appearance of strain caused by ligand dimerization, which acts directly on the metal chromophores and is transmitted to the crystal lattice. The spin state of 2 can be changed by applying pressure as well as by light irradiation revealing a "hidden hysteresis" phenomenon (Appl. Phys. Lett., 2008, 93, 21906), referring to the appearance of the low spin state not accessible through thermal activation but through reversed-LIESST. A unique feature of 2 is the feasibility to attain any steady state within the hidden hysteresis region by combination of perturbations triggered by changes in temperature and light (808 nm HS2 → LS2 and 532 nm LS2 → HS2). Such states are stable within a time scale of several hours.

Design of crystalline layered coordination polymers that respond to light and heat stimuli

Layered coordination polymers have attracted significant attention as a class of crystsalline materials characterized by the layer-by-layer stacking of rigid two-dimensional (2D) coordination networks. One of their remarkable features is the flexibility of their crystal structures, which allows for interlayer displacement, swelling, and exfoliation. Incorporating stimuli-responsive moieties into their structures is a promising strategy for the rational design of layered coordination polymers with targeted flexible properties and functions. Despite the challenges associated with crystal design, a variety of stimuli-responsive layered coordination polymers have been developed over the past two decades. This article provides an overview of representative examples of layered coordination polymers whose properties and functions can be modulated by photo- and thermal stimuli.

Switching the Multiple Function Channels in 2D Hofmann-Type Coordination Polymers

The modulation of multifunctional molecular materials by utilizing the stimuli-responsive spin crossover (SCO) has attracted considerable research interest due to its potential applications in information storage and smart switching devices. However, the complexity of achieving the integration and interconnection of multiple functions constitutes a formidable challenge. Herein, we present a pair of 2D FeII-based Hofmann-type coordination polymers (HTCPs), namely {FeII(aep)2[AgI(CN)2]2}·0.3DMF (1) and {FeII(avp)2[AgI(CN)2]2} (2), using fluorescence ligands 4-[2-(9-anthracenyl)ethynyl]pyridine (aep) and 4-[2-(9-anthryl)vinyl]pyridine (avp), respectively. Both complexes exhibit one-step SCO, with transition temperatures of 216 K for complex 1 and 255 K for complex 2. Their dielectric properties align well with the observed magnetic behaviors, demonstrating the dielectric transition process caused by the change of spin state. A variable-temperature fluorescence study reveals the coexistence of SCO and luminescent properties in both complexes, with a remarkable synergistic coupling observed in complex 2 due to the shorter distance between the SCO centers and the fluorophore. These findings underscore the potential of HTCPs as a promising platform for modulating multiple functions. By manipulating their spin states through external stimuli, SCO materials will promisingly advance the development of next-generation molecule-based sensors and devices.

Photodimerization-Triggered Photopolymerization of Triene Coordination Polymers Enables Macroscopic Photomechanical Movements

Controlling the packing of olefinic molecules in crystals is essential for triggering solid-state [2 + 2] photocycloaddition reactions and the synthesis of photocontrolled smart materials. Herein, we report the stepwise photodimerization-triggered photopolymerization of two triene coordination polymers (CPs), {[Zn(2-BBA)2(tpeb)]·0.5CH3CN}n (1, 2-HBBA = 2-bromobenzoic acid, tpeb = 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene) and {[Zn(3-BBA)2(tpeb)]·CH3CN)}n (2, 3-HBBA = 3-bromobenzoic acid). Upon irradiation with 420 nm light, each pair of closely packed and parallel olefinic bonds in 1 undergoes a [2 + 2] cycloaddition reaction, which connects two adjacent Z-shaped chains into a ladder-like coordination chain [Zn(2-BBA)2(bpbdpvpcb)0.5]n (1a, bpbdpvpcb = 1,3-bis(4-pyridyl)-2,4-bis(3,5-di(2-(4-pyridyl)vinyl)phenyl]cyclobutene) through single-crystal to single-crystal (SCSC) transformation. After photodimerization from 1 to 1a has occurred, the olefinic bonds that were initially distant are brought in close enough proximity to meet the requirements for a subsequent [2 + 2] cycloaddition reaction. Upon further light irradiation, the neighboring bpbdpvpcb ligands in 1a experience a SCSC photopolymerization based on [2 + 2] photocycloaddition and transform into poly-3b,4,5,5a,8b,9,10a-octahydro-4,5,9,10-tetrapyridyl-2,7-di(2-(4-pyridyl)vinyl)dicyclobuta[e,l]-pyren (poly-otpdpvdcbp). 2 showed similar structural changes under UV light illumination. Under light exposure, single crystals of 1 and 2 with different morphologies exhibit bending, cracking, and jumping photomechanical motions. The composite film (1-PVA) engineered by dispersing crystalline particles of 1 in poly(vinyl alcohol) (PVA) displays interesting light-wavelength-dependent photomechanical motions and can perform photodriven swimming on a liquid surface. This work provides a useful and promising approach to enable photodimerization of those photoinactive olefin pairs embedded in CPs and opens a new route to synthesize organic polymers by using olefinic CP platforms.

Above room temperature spin crossover in mononuclear iron(II) complexes featuring pyridyl-benzimidazole bidentate ligands adorned with aliphatic chains

Two bidentate ligands (L1 = 1-pentyl-2-(pyridin-2-yl)-1H-benzimidazole and L2 = 1-heptyl-2-(pyridin-2-yl)-1H-benzimidazole) were employed for the synthesis of five mononuclear Fe(II) coordination compounds 1-5 containing perchlorate, tetrafluoroborate and triflate counterions. Single-crystal X-ray diffraction analysis confirmed the expected molecular structures of all the reported compounds, revealing a moderately distorted octahedral geometry of {FeN6} coordination chromophores. All five compounds exhibit thermal spin crossover with T1/2 temperatures allocated above 400 K. The theoretical calculations supported the experimental magnetic investigation and helped to explain the electronic structures of the reported complexes with respect to the occurrence of thermal spin state switching. In addition, compound 4 was employed for the preparation of Langmuir-Blodgett films and fabrication of molecular films using the method of spontaneous evaporation of the subphase. While the formation of Langmuir-Blodgett films was unsuccessful due to the instability of the compound at the water/air interface, the latter technique allowed the formation of molecular films of 4 with well-defined thickness and homogeneity.

Mix and match - controlling the functionality of spin-crossover materials through solid solutions and molecular alloys

The influence of dopant molecules on the structure and functionality of spin-crossover (SCO) materials is surveyed. Two aspects of the topic are well established. Firstly, isomorphous inert metal ion dopants in SCO lattices are a useful probe of the energetics of SCO processes. Secondly, molecular alloys of iron(II)/triazole coordination polymers containing mixtures of ligands were used to tune their spin-transitions towards room temperature. More recent examples of these and related materials are discussed that reveal new insights into these questions. Complexes which are not isomorphous can also be co-crystallised, either as solid solutions of the precursor molecules or as a random distribution of homo- and hetero-leptic centres in a molecular alloy. This could be a powerful method to manipulate SCO functionality. Published molecular alloys show different SCO behaviours, which may or may not include allosteric switching of their chemically distinct metal sites.

Thermally and Photoinduced Spin-Crossover Behavior in Iron(II)-Silver(I) Cyanido-Bridged Coordination Polymers Bearing Acetylpyridine Ligands

We report two new cyanido-bridged Fe(II)-Ag(I) coordination polymers using different acetylpyridine isomers, {Fe(4acpy)2[Ag(CN)2]2} 1 and {Fe(3acpy)[Ag(CN)2]2} 2 (4acpy = 4-acetylpyridine; 3acpy = 3-acetylpyridine) displaying thermally and photoinduced spin crossover (SCO). In both cases, the acetylpyridine ligand directs the coordination polymer structure and the SCO of the materials. Using 4-acetylpyridine, a two-dimensional (2D) structure is observed in 1 made of layers stacked on each other by silver-ketone interactions leading to a complete SCO and reversible thermally and photoswitching of the magnetic and optical properties. Changing the acetyl group to a 3-position, a completely different structure is obtained for 2. The unexpected coordination of the carbonyl group to the Fe(II) centers induces a three-dimensional (3D) structure, leading to statistical disorder around the Fe(II) with three different coordination spheres, [N6], [N4O2], and [N5O]. This disorder gives rise to an incomplete thermally induced SCO with a poor photoswitchability. These results demonstrate that the choice of the acetyl position on the pyridine dictates the structural characteristics of the compounds with a direct impact on the SCO behavior. Remarkably, this work opens interesting perspectives for the future design of Fe-Ag cyanido coordination polymers with judiciously substituted pyridine ligands to tune the thermally and photoinduced SCO properties.

Phase Transition Control in Molecular Solids via Complementarity of Hydrogen-Bond Strength

Phase transitions in molecular solids involve synergistic changes in chemical and electronic structures, leading to diversification in physical and chemical properties. Despite the pivotal role of hydrogen bonds (H-bonds) in many phase-transition materials, it is rare and challenging to chemically regulate the dynamics and to elucidate the structure-property relationship. Here, four high-spin CoII compounds were isolated and systematically investigated by modifying the ligand terminal groups (X=S, Se) and substituents (Y=Cl, Br). S-Cl and Se-Br undergo a reversible structural phase transition near room temperature, triggering the rotation of 15-crown-5 guests and the swing between syn- and anti-conformation of NCX- ligands, accompanied by switchable magnetism. Conversely, S-Br and Se-Cl retain stability in ordered and disordered phases, respectively. H-bonds geometric analysis and ab initio calculations reveal that the electronegativity of X and Y affects the strength of NY-ap-H⋅⋅⋅X interactions. Entropy-driven structural phase transitions occur when the H-bond strength is appropriate; otherwise, the phase stays unchanged if it is too strong or weak. This work highlights a phase transition driven by H-bond strength complementarity - pairing strong acceptor with weak donor and vice versa, which offers a straightforward and effective approach for designing phase-transition molecular solids from a chemical perspective.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Single-Crystal Transformation Engineering the Spin Change of Metal-Organic Frameworks via Cluster Deconstruction

Spin crossover (SCO) materials with new architectures will expand and enrich the research in the SCO field. Here, we report two metal-organic frameworks (MOFs) containing tetradentate organic ligands and hexatopic linkers [Ag8 X8 (CN)6 ]6- (X=Br and I), which represents the first SCO MOF with clusters as building blocks. The silver halide cluster can be further removed after reacting with lithium tetracyanoquinodimethan (LiTCNQ). Such post-synthetic modification (PSM) is realized via single-crystal to single-crystal (SCSC) transformation from urk to nbo topology. Accordingly, the spin state and fluorescence properties are greatly modified by cluster deconstruction. Therefore, these achievements will provide new ideas for the design of new SCO systems and the development of PSM methods.

Nanoscale designing of metal organic framework moieties as efficient tools for environmental decontamination

Environmental pollutants, being a major and detrimental component of the ecological imbalance, need to be controlled. Serious health issues can get intensified due to contaminants present in the air, water, and soil. Accurate and rapid monitoring of environmental pollutants is imperative for the detoxification of the environment and hence living beings. Metal-organic frameworks (MOFs) are a class of porous and highly diverse adsorbent materials with tunable surface area and diverse functionality. Similarly, the conversion of MOFs into nanoscale regime leads to the formation of nanometal-organic frameworks (NMOFs) with increased selectivity, sensitivity, detection ability, and portability. The present review majorly focuses on a variety of synthetic methods including the ex situ and in situ synthesis of MOF nanocomposites and direct synthesis of NMOFs. Furthermore, a variety of applications such as nanoabsorbent, nanocatalysts, and nanosensors for different dyes, antibiotics, toxic ions, gases, pesticides, etc., are described along with illustrations. An initiative is depicted hereby using nanostructures of MOFs to decontaminate hazardous environmental toxicants.

Stimuli-responsive spin crossover nanoparticles for drug delivery and DNA-binding studies

Aminated silica hybrid, spin-crossover (SCO) nanoparticles (AmNPs) coupled with (S)-naproxen (NAP) were proposed for potential drug nanocarriers through drug release experiments at various pH values. DNA- and albumin-binding studies were also carried out using diverse techniques in order to investigate the interaction of the nanoparticles with calf-thymus DNA and serum albumins and to determine the corresponding binding constants.

Redox-Programmable Spin-Crossover Behaviors in a Cationic Framework

Metal-organic frameworks (MOFs) provide versatile platforms to construct multi-responsive materials. Herein, by introducing the neutral tetradentate ligand and the linear dicyanoaurate(I) anion, we reported a rare cationic MOF [Fe^II(TPB){Au^I(CN)₂}]I·4H₂O·4DMF (TPB = 1,2,4,5-tetra(pyridin-4-yl)benzene) with hysteretic spin-crossover (SCO) behavior near room temperature. This hybrid framework with an open metal site (Au^I) exhibits redox-programmable capability toward dihalogen molecules. By means of post-synthetic modification, all the linear [Au^I(CN)₂]^- linkers can be oxidized to square planar [Au^III(CN)₂X₂]^- units, which results in the hysteretic SCO behaviors switching from one-step to two-step for Br₂ and three-step for I₂. More importantly, the stepwise SCO behaviors can go back to one-step via the reduction by l-ascorbic acid (AA). Periodic DFT calculations using various SCAN-type exchange-correlation functionals have been employed to rationalize the experimental data. Hence, these results demonstrate for the first time that switchable one-/two-/three-stepped SCO dynamics can be manipulated by chemical redox reactions, which opens a new perspective for multi-responsive molecular switches.

Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand

A detailed study of the two-dimensional (2-D) Hofmann-like framework [Fe(furpy)₂Pd(CN)₄]·nG (furpy: N-(pyridin-4-yl)furan-2-carboxamide, G = H₂O,EtOH (A·H₂O,Et), and H₂O (A·H₂O)) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environments─this is a strategic approach that we achieve by the inclusion of a ligand with multiple host-host and host-guest interaction sites. Variable-temperature magnetic susceptibility studies reveal an asymmetric multistep SCO for A·H₂O,Et and an abrupt single-step SCO for A·H₂O with an upshift in transition temperature of ∼75 K. Single-crystal analyses show a primitive orthorhombic symmetry for A·H₂O,Et characterized by a unique Fe^II center─the multistep SCO character is attributed to local ligand orientation. Counterintuitively, A·H₂O shows a triclinic symmetry with two inequivalent Fe^II centers that undergo a cooperative single-step high-spin (HS)-to-low-spin (LS) transition. We conduct detailed structure-function analyses to understand how the guest ethanol influences the delicate balance between framework communication and, therefore, the local structure and spin-state transition mechanism.

Spin-State Modulation in FeII -Based Hofmann-Type Coordination Polymers: From Molecules to Materials

Spin crossover complexes that reversibly interconvert between two stable states imitate a binary state of 0 and 1, delivering a promising possibility to address the data processing concept in smart materials. Thus, a comprehensive understanding of the modulation of magnetic transition between high spin and low spin and the factors responsible for stabilizing the spin states is an essential theme in modern materials design. In this context, the present review attempts to provide a concise outline of the design strategy employed at the molecular level for fine-tuning the spin-state switching in Fe^II -based Hofmann-type coordination polymers and their effects on the optical and magnetic response. In addition, development towards the nanoscale architectures of HCPs, i. e., in terms of nanoparticles and thin films, are emphasized to bridge the gap between the laboratory and reality.

Single-Crystal to Single-Crystal Transformation of a Spin-Crossover Hybrid Perovskite via Thermal-Induced Cyanide Linkage Isomerization

Linkage isomers involving changes in the bonding mode of ambidentate ligands have potential applications in data storage, molecular machines, and motors. However, the observation of the cyanide-linkage-isomerism-induced spin change (CLIISC) effect characterized by single-crystal X-ray diffraction remains a considerable challenge. Meanwhile, the high-spin and low-spin states can be reversibly switched in spin-crossover (SCO) compounds, which provide the potential for applications to data storage, switches, and sensors. Here, a new perovskite-type SCO framework (PPN)[Fe{Ag(CN)₂}₃] (PPN⁺ = bis(trisphenylphosphine)iminium cation) is synthesized, which displays the unprecedented aging and temperature dependences of hysteretic multistep SCO behaviors near room temperature. Moreover, the thermal-induced cyanide linkage isomerization from Fe^II-N≡C-Ag^I to Fe^II-C≡N-Ag^I is revealed by single-crystal X-ray diffraction, Raman, and Mössbauer spectra, which is associated with a transition from the mixed spin state to the low-spin state and a dramatic volume shrinkage. Considering the wide use of cyanogen in magnetic systems, the association of CLIISC and SCO opens a new dimension to modulate the spin state and realize a colossal negative thermal expansion.

[AuI(CN)2]-Armed [FeIII2FeII2] Square Complex Showing Unusual Spin-Crossover Behavior Due to a Symmetry-Breaking Phase Transition

The incorporation of two different cyanide building blocks of [(Tp^R)Fe^III(CN)₃]^- and [Au^I(CN)₂]^- into one molecule afforded a novel hexanuclear [Fe^III₂Fe^II₂Au^I₂] complex (1·2Et₂O), in which the cyanide-bridged [Fe^III₂Fe^II₂] square was further grafted by two [Au^I(CN)₂]^- fragments as long arms in syn orientations. Complex 1·2Et₂O undergoes a gradual spin crossover (SCO) ffrom low-spin (LS) to high-spin (HS) state for the Fe(II) centers upon desolvation. Remarkably, its desolvated phase (1) exhibits a reversible but atypical two-step (sharp-gradual) SCO behavior with considerable hysteresis (21 K). Variable-temperature single-crystal X-ray structural studies reveal that the hysteretic spin transition takes place synchronously with the concerted displacive motions of the molecules, representing another rare example including multistep and hysteretic spin transitions due to the synergetic SCO and structural phase transition.

Discovery of Kinetic Effect in a Valence Tautomeric Cobalt-Dioxolene Complex

Two isostructural valence tautomeric (VT) complexes with different critical temperatures were prepared and fully investigated through a series of magnetic, structural, spectral, and differential scanning calorimetry evidence. The kinetic effect in the VT complex was observed for the first time through scan-rate-dependent studies and further validated by annealing tests.

A Mononuclear Iron(II) Spin-Crossover Molecule Decorated by Photochromic Azobenzene Group

Aiming at constructing photoresponsive spin crossover (SCO) behavior, herein we designed a new ligand Abtz (Abtz = (E)-N-(4-((E)-phenyldiazenyl)phenyl)-1-(thiazol-4-yl)methanimine) which was decorated by a photochromic azobenzene group. Based on this photochromic ligand, a mononuclear Fe(II) SCO molecule [Fe(Abtz)₃](BF₄)₂·(EAC)₂ (1, EAC = ethyl acetate) was successfully synthesized and showed a complete one-step SCO behavior. Under continuous UV light and blue-light exposure, the cis–trans photoisomerization of both ligand Abtz and compound 1 in the liquid phase was confirmed through UV–Vis spectra. Moreover, the ¹H-NMR spectra of Abtz reveal a trans–cis conversion ratio of 37%. Although the UV–Vis spectra reveal the photochromic behavior for 1 in the solution phase, the SCO behavior in the liquid state is absent according to the variable-temperature Evans method, suggesting the possible decomposition. Moreover, in the solid state, the cis–trans photoisomerization of both Abtz and 1 was not observed, due to the steric hindrance.

Single-Crystal-to-Single-Crystal Transformation of Two Copper(II) Metal-Organic Frameworks Modulated by Auxiliary Ligands

The single-crystal-to-single-crystal (SCSC) transformations of metal-organic frameworks (MOFs) are fascinating because we can directly observe the change of the crystal structure during the transformation process. It also greatly helps to understand the delicate interaction between the guest molecules and the skeleton framework and therefore fosters a deep understanding of gas storage and separation within the frameworks. Herein, we report two novel MOFs, [Cu₈(BCB)₄(μ₃-OH)₂(μ₃-O)(H₂O)₈(Py)₅]·16DMF·52H₂O (1) and [Cu₃(BCB)₂(Py)₆]·DMF·11H₂O (2) (Py = pyridine; DMF = N,N'-dimethylformamide), which were constructed through the self-assembly of Cu²⁺ and 4,4',4″-benzenetricarbonyltribenzoic acid (H₃BCB) by a solvothermal reaction. Although the structure and coordination patterns of compound 1 are pretty different from those of 2, the two Cu-MOFs were prepared from identical ligands and similar reaction conditions. Interestingly, compound 1 will change to 2 wholly and gradually after the addition of a certain amount of Py with a small amount of dilute hydrochloric acid. This conversion represents a scarce example of SCSC transformation involving transition-metal-based MOFs. Moreover, with its microporous nature, compound 2 shows large carbon dioxide (CO₂) uptake capability and good selectivity for CO₂/N₂ separation. Furthermore, both compounds 1 and 2 could be used as excellent heterogeneous catalysts toward the cyanosilylation reaction under solvent-free conditions.

Topological structural transformation of a two-dimensional coordination polymer via single-crystal to single-crystal photoreaction

A two-dimensional coordination polymer can carry out photoinduced C–C coupling reaction through single-crystal to single-crystal transformation and exhibit photocontrolled fluorescence.

Post-synthetic modification mechanism for 1D spin crossover coordination polymers

Suitable solvent os crucial to achieve a quantitative PSM reaction. Then, this method is not restricted to porous materials.

Crystallographic Visualization of a Guest-Induced Solar-Driven Cycloaddition Reaction Based on a Recyclable Nonporous Coordination Polymer

Stimuli-responsive solids with adjustable photophysical properties are particularly attractive because they can be used as smart materials in anticounterfeiting, information storage, holographic imaging, and other fields. Herein, we report a unique nonporous coordination polymer, {[Ag(3,3'-dpe)](2,2'-Hbpdc)}_n (1; 3,3'-dpe = 1,2-dipyridin-3-ylethene and 2,2'-H₂bpdc = 2,2'-biphenyldicarboxylic acid), that can convert to an extremely photoreactive compound, 1·H₂O·MeCN (MeCN = acetonitrile), through guest capture. Upon irradiation of sunlight, 1·H₂O·MeCN can transform to {[Ag(3,3'-tpcb)_0.5](2,2'-Hbpdc)(H₂O)(MeCN)}_n (2·H₂O·MeCN; 3,3'-tpcb = 1,2,3,4-tetrapyridin-3-ylcyclobutane). 2·H₂O·MeCN can lose its solvent molecules to form 2 and further return to 1 at high temperature. Accompanied by direct visualization based on multistep single-crystal-to-single-crystal conversions, the recyclable crystalline solid exhibits remarkable fluorescence changes, which makes it a supramolecular switch for application in multiple anticounterfeiting.

On the nature of bonding in the photochemical addition of two ethylenes: C-C bond formation in the excited state?

In this work, the 2s + 2s (face-to-face) prototypical example of a photochemical reaction has been re-examined to characterize the evolution of chemical bonding. The analysis of the electron localization function (as an indirect measure of the Pauli principle) along the minimum energy path provides strong evidence supporting that CC bond formation occurs not in the excited state but in the ground electronic state after crossing the rhombohedral S₁/S₀ conical intersection.

Bistable Hofmann-Type FeII Spin-Crossover Two-Dimensional Polymers of 4-Alkyldisulfanylpyridine for Prospective Grafting of Monolayers on Metallic Surfaces

Aiming at investigating the suitability of Hofmann-type two-dimensional (2D) coordination polymers {FeII(Lax)2[MII(CN)4]} to be processed as single monolayers and probed as spin crossover (SCO) junctions in spintronic devices, the synthesis and characterization of the MII derivatives (MII = Pd and Pt) with sulfur-rich axial ligands (Lax = 4-methyl- and 4-ethyl-disulfanylpyridine) have been conducted. The thermal dependence of the magnetic and calorimetric properties confirmed the occurrence of strong cooperative SCO behavior in the temperature interval of 100-225 K, featuring hysteresis loops 44 and 32.5 K/21 K wide for PtII-methyl and PtII/PdII-ethyl derivatives, while the PdII-methyl derivative undergoes a much less cooperative multistep SCO. Excluding PtII-methyl, the remaining compounds display light-induced excited spin-state trapping at 10 K with TLIESST temperatures in the range of 50-70 K. Single-crystal studies performed in the temperature interval 100-250 K confirmed the layered structure and the occurrence of complete transformation between the high- and low-spin states of the FeII center for the four compounds. Strong positional disorder seems to be the source of elastic frustration driving the multistep SCO observed for the PdII-methyl derivative. It is expected that the peripheral disulfanyl groups will favor anchoring and growing of the monolayer on gold substrates and optimal electron transport in the device.

A spin-crossover phenomenon in a 2D heterometallic coordination polymer with [Pd(SCN)4]2- building blocks

Two new two-dimensional (2D) coordination polymers, [Fe^II(L)₂{Pd^II(SCN)₄}] (L₁ = 3-(9-anthracenyl)-pyridine (1) and L₂ = 4-(9-anthracenyl)-pyridine (2)), were constructed by employing square-planar [Pd(SCN)₄]^2- building blocks. Compound 1 exhibits a complete spin-crossover (SCO) behaviour under normal atmospheric pressure, and represents the first SCO example in a 2D system containing [Pd(SCN)₄]^2- units. In contrast, compound 2 only shows paramagnetic behaviour at measured temperatures. It is clear that the fine-tuning of the monodentate ligand can modulate the ligand field and packing fashions, which sheds light on developing new SCO materials.

Photo-oligomerization by shifting the coordination site in a luminescent coordination polymer

A layered coordination polymer (CP) with the fine-tuned alignment of four diolefinic ligands has been designed by shifting the coordination site of the ligand. The trimeric and tetrameric cyclobutane derivatives were reversely achieved by the photoinitiated [2+2] cycloaddition of the CP due to the favorable Schmidt's distance. More interestingly, a dynamic fluorescence shift was observed during the photo-oligomerization and heat-cycloreversion of the CP system.

Spin crossover crystalline materials engineered via single-crystal-to-single-crystal transformations

This highlight illustrates the latest crystalline materials engineered via SCSC transformations, with emphasis on the onset and progress of spin crossover in a crystal control.

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.

Reversible Solid-State Phase Transitions between Au-P Complexes Accompanied by Switchable Fluorescence

Six complexes {(3-bdppmapy)(AuCl)₂}_n (1-6; 3-bdppmapy = N,N'-bis(diphenylphosphanylmethyl)-3-aminopyridine and tht = tetrahydrothiophene) were simultaneously formed by the reaction of Au(tht)Cl and 3-bdppmapy in CH₂Cl₂ followed by infusion with hexane. Complexes 4-6 could be produced independently by volatilizing solvent in air, solid-state heating, or solvothermal reaction. The PPh₂-Au-Cl moieties extended in different directions, forming Au-Au and Au-Au-Au interactions. Complex 4 could be converted to 5 by heating to 130 °C, with the cleavage of one Au-Au bond, while 5 reverted back to 4 upon exposure to CH₂Cl₂ vapor over 11 h. This solid-state phase transition could be recycled and was accompanied by a change in solid-state fluorescence, without obvious intensity decay over five cycles. The reason for both the phase transition and difference in photoluminescence is related to the different numbers and strengths of aurophilic interactions in each complex that could be modeled by density functional theory calculations.

Physical stimulus and chemical modulations of bistable molecular magnetic materials

In this Feature Article, we summarize the recent progress made in modulating the multifaceted magnetic behaviour of single-molecule magnets (SMMs) and spin-crossover (SCO) materials based on chemical modifications and external stimuli.

Temperature-dependent hysteretic two-step spin crossover in two-dimensional Hofmann-type compounds

Two 2D Hofmann-type compounds [Fe^II(ppe)₂M^II(CN)₄]·3H₂O [ppe = 1-(2-pyridyl)-2-(4-pyridyl)ethylene; M = Pd for 1 and Pt for 2] have been synthesized. Both of them show complete two-step hysteretic SCO transitions HS^1.0 ⇌ HS^0.6–0.5LS^0.4–0.5 ⇌ LS^1.0.

Functional metal–organic frameworks as effective sensors of gases and volatile compounds

This review summarizes the recent advances of metal organic framework (MOF) based sensing of gases and volatile compounds.