On-Off Switching of a Photocatalytic Overall Reaction through Dynamic Spin-State Transition in a Hofmann Clathrate System

Polyoxometalate Anion-Induced On-Off Spin-Crossover Property in Two Isomeric Cobalt(II) Complexes with Proton Conductivity

The integration of polyoxometalate (POM) anions with functionalized cations within a single molecule remains a challenging task, even though POMs have garnered significant interest over an extended period. Here, we constructed two cobalt(II) complexes by incorporating isomeric POM anions with [Co(pyterpy)2]2+. Intriguingly, a locked high-spin (HS) state and a spin-crossover (SCO) behavior of the 3d7-CoII ion in the identical cation [Co(pyterpy)2]2+ were governed by α-type and β-type [Mo8O26]4- anions, and the HS-Co(II) ion showed field-induced slow magnetic relaxation. Furthermore, both complexes exhibited potential as solid-state proton conductors. This study revealed that POM anions possessed the dual capability of modulating magnetism and synthesizing multifunctional molecular materials.

Spin State Regulation for Peroxide Activation: Fundamental Insights and Regulation Mechanisms

Peroxides are widely used in environmental applications due to their strong oxidizing properties, however, traditional activation methods often face challenges such as uncontrolled reactive oxygen species (ROS) generation and high energy barriers. Recent advancements in spin state regulation provide a promising alternative to enhance the efficiency of peroxide activation. This review provides an overview of spin fundamentals and discusses the key factors affecting spin state in catalytic materials, including crystal field configuration, ligand environment, and valence changes. Subsequently, the role of electron spin state in peroxide activation is comprehensively analyzed, with a focus on how spin state regulation can tune adsorption energy, lower energy barriers, facilitate electron transfer between transition metals and peroxides, and promote selective ROS generation. Finally, this review briefly outlines the practical applications of peroxide activation in water treatment and concludes with a summary and perspectives on future research directions. This review aims to provide a comprehensive perspective on the role of spin state regulation in advancing peroxide activation strategies.

Spin Crossover Composite Film as Recyclable Catalyst for Acetalization Reaction

Polymer-based composite films represent a promising approach for developing recyclable catalysts. In this study, a [Fe(NH2trz)3](NO3)2 @TPU composite film with spin crossover properties was fabricated using the blade casting method. Various characterisation techniques confirmed the uniform distribution of [Fe(NH2trz)3](NO3)2 particles within the TPU matrix, while maintaining the spin-crossover properties of the embedded complex. The composite film exhibited excellent catalytic efficiency and reusability in acetalization reaction, enabling efficient catalysis for up to 11 cycles under batch conditions and sustained activity for 7 hours under flow conditions. In addition, the color of the film served as a convenient visual indicator of its suitability for reuse in subsequent catalytic reactions. This study demonstrates that [Fe(NH2trz)3](NO3)2 @TPU composite film can serve as an effective and recyclable catalysts for acetalization reactions.

Precise manipulation of iron spin states in single-atom catalytic membranes for singlet oxygen selective production

Heterogeneous single-atom catalysts are attracting substantial attention for selectively generating singlet oxygen (1O2). However, precise manipulation of atom coordination structures remains challenging. Here, the fine coordination structure of iron single-atom carbon-nitride catalysts (Fe-CNs) was manipulated by precisely tuning the heating rate with 1 °C min-1 difference. Multiple techniques in combination with density functional theory (DFT) calculations reveal that FeN6 coordination sites with high Fe spin states promote the adsorption, electron transfer, and dissociation of peroxymonosulfate (PMS), resulting in nearly 100% selection of 1O2 generation. A lamellar single atom catalytic membrane is constructed, exhibiting high permeance, high degradation, high-salinity resistance and sustained operation stability. This work provides ideas for regulating spin states of the metal site to fabricate catalysts with selective 1O2 generation for membrane separation and environment catalysis applications.

Optimizing photocatalysis via electron spin control

Solar-driven photocatalytic technology holds significant potential for addressing energy crisis and mitigating global warming, yet is limited by light absorption, charge separation, and surface reaction kinetics. The past several years has witnessed remarkable progress in optimizing photocatalysis via electron spin control. This approach enhances light absorption through energy band tuning, promotes charge separation by spin polarization, and improves surface reaction kinetics via strengthening surface interaction and increasing product selectivity. Nevertheless, the lack of a comprehensive and critical review on this topic is noteworthy. Herein, we provide a summary of the fundamentals of electron spin control and the techniques employed to scrutinize the electron spin state of active sites in photocatalysts. Subsequently, we highlight advanced strategies for manipulating electron spin, including doping design, defect engineering, magnetic field regulation, metal coordination modulation, chiral-induced spin selectivity, and combined strategies. Additionally, we review electron spin control-optimized photocatalytic processes, including photocatalytic water splitting, CO2 reduction, pollutant degradation, and N2 fixation, providing specific examples and detailed discussion on underlying mechanisms. Finally, we outline perspectives on further enhancing photocatalytic activity through electron spin manipulation. This review seeks to offer valuable insights to guide future research on electron spin control for improving photocatalytic applications.

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.

When the Study of the Post-Synthetic Modification Method on a 1D Spin Crossover Coordination Polymer Highlights its Catalytic Activity

We are interested in studying the catalytic activity of the spin crossover (SCO) complex ([Fe(NH2trz)3](NO3)2). In this work, we demonstrate that, by adapting the experimental conditions, we can switch from a quantitative post-synthetic modification (PSM) reaction to the use of this complex as a catalyst for the formation of imine from 4-amino-1,2,4-triazole. During the catalytic reaction, the iron complex undergoes two different PSM reactions: the first is the action of the aldehyde on the NH2 groups present on the complex, whereas the second PSM reaction occurs between the imine complex and aminotriazole, leading back to the starting complex. These two PSM reactions are at least partially involved in the catalytic mechanism. Furthermore, the combination of these two PSM reactions enables us to modulate the particle size and shape of the final amine complex without altering its excellent SCO properties. This result is of interest in the field of heterogeneous catalysis, where particle size has a strong influence on the catalytic activity, and for the proper integration in devices for different applications.

Spin-state effect on the efficiency of a post-synthetic modification reaction on a spin crossover complex

The spin state of a metal center significantly influences the catalytic activity of its complex, a phenomenon so crucial that it has led to the dedicated field of spin catalysis. Here we investigate the effect of the spin state of an iron-based metal complex on the organic reactivity of its ligands. Specifically, we examined the post-synthetic modification of the spin crossover (SCO) complex [Fe(NH2trz)3](NO3)2 with p-anisaldehyde. A series of experiments were performed to study the transformation of the amino groups depending on the spin state of the metal. Owing to the wide thermal hysteresis loop of the SCO complex, both spin states were compared under identical conditions. The results revealed that the high-spin state led to the formation of 1.34 times more imine functional groups than the low-spin state, we propose that this arises from the different interactions between the solvent and the SCO at the different spin states.

Nitric Oxide-Releasing Nanoscale Metal-Organic Layer Overcomes Hypoxia and Reactive Oxygen Species Diffusion Barriers to Enhance Cancer Radiotherapy

Hafnium (Hf)-based nanoscale metal-organic layers (MOLs) enhance radiotherapeutic effects of tissue-penetrating X-rays via a unique radiotherapy-radiodynamic therapy (RT-RDT) process through efficient generation of hydroxy radical (RT) and singlet oxygen (RDT). However, their radiotherapeutic efficacy is limited by hypoxia in deep-seated tumors and short half-lives of reactive oxygen species (ROS). Herein the conjugation of a nitric oxide (NO) donor, S-nitroso-N-acetyl-DL-penicillamine (SNAP), to the Hf12 secondary building units (SBUs) of Hf-5,5'-di-p-benzoatoporphyrin MOL is reported to afford SNAP/MOL for enhanced cancer radiotherapy. Under X-ray irradiation, SNAP/MOL efficiently generates superoxide anion (O2 -.) and releases nitric oxide (NO) in a spatio-temporally synchronized fashion. The released NO rapidly reacts with O2 -. to form long-lived and highly cytotoxic peroxynitrite which diffuses freely to the cell nucleus and efficiently causes DNA double-strand breaks. Meanwhile, the sustained release of NO from SNAP/MOL in the tumor microenvironment relieves tumor hypoxia to reduce radioresistance of tumor cells. Consequently, SNAP/MOL plus low-dose X-ray irradiation efficiently inhibits tumor growth and reduces metastasis in colorectal and triple-negative breast cancer models.

Reversibly Modulating the Selectivity of Carbon Dioxide Reduction via Ligand-Driven Spin Crossover

Selectivity is an essential aspect in catalysis. At present, the improvement of the selectivity for complex reactions with multiple pathways/products, for example the carbon dioxide reduction reaction (CO2RR), can usually be achieved for only one pathway/product. It is still a challenge to reversibly modulate the selectivity between two reaction pathways or products of the CO2RR by one catalyst. Here, we propose the reversible modulation of selectivity between two products via spin crossover. By employing first-principles calculations, six spin crossover molecular catalysts are found among 17 kinds of transition metal embedded porphyrin derivatives (ppy_TM), where the changes in axial ligand configurations can reversibly switch the spin state of catalysts between high spin and low spin. For ppy_Os and ppy_Ru, the alteration in spin state can effectively influence the reduction of CO2 into either formic acid or carbon monoxide by changing the relative stability of the key intermediates *COOH and *HCOO.

Integration of Plasmonic Ag(I) Clusters and Fe(II) Porphyrinates into Metal-Organic Frameworks for Efficient Photocatalytic CO2 Reduction Coupling with Photosynthesis of Pure H2O2

Efficient photocatalytic CO2 reduction coupled with the photosynthesis of pure H2O2 is a challenging and significant task. Herein, using classical CO2 photoreduction site iron porphyrinate as the linker, Ag(I) clusters were spatially separated and evenly distributed within a new metal-organic framework (MOF), namely Ag27TPyP-Fe. With water as electron donors, Ag27TPyP-Fe exhibited remarkable performances in artificial photosynthetic overall reaction with CO yield of 36.5 μmol g-1 h-1 and ca. 100 % selectivity, as well as H2O2 evolution rate of 35.9 μmol g-1 h-1. Since H2O2 in the liquid phase can be more readily separated from the gaseous products of CO2 photoreduction, high-purity H2O2 with a concentration up to 0.1 mM was obtained. Confirmed by theoretical calculations and the established energy level diagram, the reductive iron(II) porphyrinates and oxidative Ag(I) clusters within an integrated framework functioned synergistically to achieve artificial photosynthesis. Furthermore, photoluminescence spectroscopy and photoelectrochemical measurements revealed that the robust connection of Ag(I) clusters and iron porphyrinate ligands facilitated efficient charge separation and rapid electron transfer, thereby enhancing the photocatalytic activity.

Manipulating Fe(II) spin states to achieve higher anti-tumor cell activities in multinuclear complexes

Exploring the different spin states of central metals in the complex to regulate the anti-tumor activity of cancer cells is of great significance in drug design and clinical use. However, it is a challenge to build a strong coupling between spin states and anti-tumor activities in one system. Herein, we present two complexes {FeII2L2[PdII(CN)4]2}·2H2O (L = Bztpen (1), Bztppn (2); Bztpen = N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethylenediamine, Bztppn = N-benzyl-N,N',N'-tris(2-pyridylmethyl)propylenediamine) showing different cytotoxic activities actuated by fine-tuning the structure with different spin states of Fe(II). Magnetic susceptibility measurements and X-ray diffraction revealed that the Fe(II) ion in complexes 1 and 2 remains in the LS and HS state, respectively, at room temperature. Cytotoxicity tests indicate that complex 1 is more biologically effective than complex 2. In complex 2, however, the high-spin Fe(II) played a key role in regulating its in vitro antitumor effects and seems to be associated with ROS-mediated apoptosis. These findings offer a new avenue for developing anti-cancer drugs by designing complexes with different spin states.

Four-step electron transfer coupled spin transition in a cyano-bridged [Fe2Co2] square complex

The design of molecular functional materials with multi-step magnetic transitions has attracted considerable attention. However, the development of such materials is still infrequent and challenging. Here, a cyano-bridged square Prussian blue complex that exhibits a thermally induced four-step electron transfer coupled spin transition (ETCST) is reported. The magnetic and spectroscopic analyses confirm this multi-step transition. Variable-temperature infrared spectrum suggested the electronic structures in each phase and a four-step transition model is proposed.

Spin-related excited-state phenomena in photochemistry

The spin of electrons plays a vital role in chemical reactions and processes, and the excited state generated by the absorption of photons shows abundant spin-related phenomena. However, the importance of electron spin in photochemistry studies has been rarely mentioned or summarized. In this review, we briefly introduce the concept of spin photochemistry based on the spin multiplicity of the excited state, which leads to the observation of various spin-related photophysical properties and photochemical reactivities. Then, we focus on the recent advances in terms of light-induced magnetic properties, excited-state magneto-optical effects and spin-dependent photochemical reactions. The review aims to provide a comprehensive overview to utilize the spin multiplicity of the excited state in manipulating the above photophysical and photochemical processes. Finally, we discuss the existing challenges in the emerging field of spin photochemistry and future opportunities such as smart magnetic materials, optical information technology and spin-enhanced photocatalysis.

Stepwise Protonation of Three-Dimensional Covalent Organic Frameworks for Enhancing Hydrogen Peroxide Photosynthesis

Three-dimensional covalent organic frameworks (3D COFs), recognized for their tailorable structures and accessible active sites, offer a promising platform for developing advanced photocatalysts. However, the difficulty in the synthesis and functionalization of 3D COFs hinders their further development. In this study, we present a series of 3D-bcu-COFs with 8 connected porphyrin units linked by linear linkers through imine bonds as a versatile platform for photocatalyst design. The photoresponse of 3D-bcu-COFs was initially modulated by functionalizing linear linkers with benzo-thiadiazole or benzo-selenadiazole groups. Furthermore, taking advantage of the well-exposed porphyrin and imine sites in 3D-bcu-COFs, their photocatalytic activity was optimized by stepwise protonation of imine bonds and porphyrin centers. The dual protonated COF with benzo-selenadiazole groups exhibited enhanced charge separation, leading to an increased photocatalytic H2O2 production under visible light. This enhancement demonstrates the combined benefits of linker functionalization and stepwise protonation on photocatalytic efficiency.

The spin polarization strategy regulates heterogeneous catalytic activity performance: from fundamentals to applications

In recent years, there has been significant attention towards the development of catalysts that exhibit superior performance and environmentally friendly attributes. This surge in interest is driven by the growing demands for energy utilization and storage as well as environmental preservation. Spin polarization plays a crucial role in catalyst design, comprehension of catalytic mechanisms, and reaction control, offering novel insights for the design of highly efficient catalysts. However, there are still some significant research gaps in the current study of spin catalysis. Therefore, it is urgent to understand how spin polarization impacts catalytic reactions to develop superior performance catalysts. Herein, we present a comprehensive summary of the application of spin polarization in catalysis. Firstly, we summarize the fundamental mechanism of spin polarization in catalytic reactions from two aspects of kinetics and thermodynamics. Additionally, we review the regulation mechanism of spin polarization in various catalytic applications and several approaches to modulate spin polarization. Moreover, we discuss the future development of spin polarization in catalysis and propose several potential avenues for further progress. We aim to improve current catalytic systems through implementing a novel and distinctive spin engineering strategy.

The Rational Design of Atomically Dispersed Catalysts via Spin Manipulation

The catalytic activity of transition-metal-based atomically dispersed catalysts is closely related to the spin states. Manipulating the spin state of metal active centers could directly adjust the d orbital occupancy and optimize the adsorption behavior and electron transfer of the intermediates and transition metals, which would enhance the catalytic activity. We summarize the means of manipulating spin states and the spin-related catalytic descriptors. In future work, we will build a quantifiable and accurate prediction intelligent model through artificial intelligence (AI) and machine learning tools. Furthermore, we will develop new spin regulation methods to carry out the directional regulation of atomically dispersed catalysts through this model, providing new insight into the rational design of transition-metal-based atomically dispersed catalysts through spin manipulation.

Structural Regulation of Covalent Organic Frameworks for Catalysis

ConspectusChemical reactions can be promoted at lower temperatures and pressures, thereby reducing the energy input, by introducing suitable catalysts. Despite its significance, the quest for efficient and stable catalysts remains a significant challenge. In this context, addressing the efficiency of catalysts stands out as a paramount concern. However, the challenges posed by the vague structure and limited tailorability of traditional catalysts would make it highly desirable to fabricate optimized catalysts based on the understanding of structure-activity relationships. Covalent organic frameworks (COFs), a subclass of fully designed crystalline materials formed by the polymerization of organic building blocks through covalent bonds have garnered widespread attention in catalysis. The precise and customizable structures of COFs, coupled with attributes such as high surface area and facile functional modification, make COFs attractive molecular platforms for catalytic applications. These inherent advantages position COFs as ideal catalysts, facilitating the elucidation of structure-performance relationships and thereby further improving the catalysis. Nevertheless, there is a lack of systematic emphasis on and summary of structural regulation at the atomic/molecular level for COF catalysis. Consequently, there is a growing need to summarize this research field and provide deep insights into COF-based catalysis to promote its further development.In this Account, we will summarize recent advances in structural regulation achieved in COF-based catalysts, placing an emphasis on the molecular design of the structures for enhanced catalysis. Considering the unique components and structural advantages of COFs, we present the fundamental principles for the rational design of structural regulation in COF-based catalysis. This Account starts by presenting an overview of catalysis and explaining why COFs are promising catalysts. Then, we introduce the molecular design principle for COF catalysis. Next, we present the following three aspects of the specific strategies for structural regulation of COF-based catalysts: (1) By designing different functional groups and integrating metal species into the organic unit, the activity and/or selectivity can be finely modulated. (2) Regulating the linkage facilitates charge transfer and/or modulates the electronic structure of catalytic metal sites, and accordingly, the intrinsic activity/selectivity can be further improved. (3) By means of pore wall/space engineering, the microenvironment surrounding catalytic metal sites can be modulated to optimize performance. Finally, the current challenges and future developments in the structural regulation of COF-based catalysts are discussed in detail. This Account provides insight into the structural regulation of COF-based catalysts at the atomic/molecular level toward improving their performance, which would provide significant inspiration for the design and structural regulation of other heterogeneous catalysts.