In their Research Article, Lionel Salmon, Azzedine Bousseksou, and co-workers report the implementation of spin crossover composite filaments for the rational design of 4D-printed mechanical metamaterials. The obtained bi-material structures guided by a theoretical analysis based on analytical calculations and finite element analysis simulations exhibit giant programmable positive or negative deformation with coefficients of thermal expansion up to 14000 ppm/°C.

In the past decade, 3D-printed cellular materials have witnessed an impressive advancement affording a wealth of remarkable mechanical properties, such as negative Poisson’s ratio, negative compressibility, and negative coefficient of thermal expansion (CTE). Recent efforts in this field have been increasingly considered 4D-printed metastructures, which leverage shape-morphing properties of stimuli-responsive materials. Here, we introduce a new class of 4D-printed metamaterials based on bistable spin crossover (SCO) molecular materials. These systems synergistically couple dissimilar materials at different size scales to harness mismatched thermomechanical properties—specifically differential thermal expansion and stiffness—to generate large directional deformations upon heating or cooling. Through a combination of theoretical modeling and experimental validation, we demonstrate that our SCO-based 4D-printed structures can achieve programmable motions, including positive and negative expansion. The associated CTE reaches peak values of ca. +14400 and −11400 ppm/◦C, respectively, more than 10 times greater than those reported in the literature for 3D-printed analogues. This work establishes a versatile and generalizable conceptual strategy for engineering multilevel, hierarchical architectures with programmable functionalities, advancing the design of energy-efficient soft actuators and reconfigurable/adaptive material systems.

Reference

4D-Printed Spin Crossover Metamaterials with Giant Programmable Positive or Negative Thermal Expansion
Adelais Trapali, Yuteng Zhang, Seyed Ehsan Alavi, Nagham Mawassy, Raja Zulkarnain Gábor, Molnár, Lionel Salmon & Azzedine Bousseksou
Advanced Materials 2026
https://doi.org/10.1002/adma.202522073

Funding: European project ERC (Horizon 2020).

Hofmann-type clathrates constitute one of the most versatile families of coordination frameworks displaying spin-crossover (SCO) behaviour, offering unparalleled opportunities for tuning bistability from bulk materials to nanoparticles and thin films. Over the past two decades, significant advances in synthetic methodologies have been achieved, particularly through the use of tetra- and di-cyanometallate building blocks ([M^II(CN)4]^{2 –}, [MI(CN)2]^–) combined with mono- or bidentate bridging organic ligands. These strategies have enabled the construction of structurally diverse FeII-based frameworks exhibiting cooperative and often abrupt spin transitions. This review highlights recent progress in bulk synthesis, structural engineering, and property modulation in Hofmann-type SCO complexes. It emphasizes how dimensionality, ligand design, and metal-metal connectivity govern transition temperatures, hysteresis, and multistep behavior. In this bulk regime, particular attention is devoted to the interplay between spin transition and electron transfer in mixed-valence or redox-active clathrates, which emerges as a powerful handle to encode and expand multifunctional responses. A considerable research effort has also focused on downsizing these materials into nanoparticles while maintaining, or even enhancing, cooperativity. This approach unlocks new prospects for solution processability, surface functionalization, and device integration.Parallel advances in host-guest chemistry demonstrate that these flexible frameworks can encapsulate neutral or charged species, leading to pronounced modulation of SCO behavior, guest-triggered phase transitions, and stimuli-responsive properties. Particular attention is devoted to 2D and 3D host-guest systems and to the emergence of “active” guest molecules capable of mechanical, electronic, or photonic coupling with the SCO matrix. Moreover, advances in the fabrication of thin films, micro-structured architectures, and nanostructured surfaces are increasingly positioning Hofmann-type clathrates at the forefront of applications in molecular electronics, memory devices, spintronics, sensing, and photo-switching. Yet, despite their remarkable functional potential, their practical implementation in devices remains limited. This is likely owing to their intrinsic fragility and the challenges associated with their manipulation, processing, and integration into innovative device architectures. Additional efforts are therefore required, particularly in the development of advanced nanotechnologies, to ultimately translate these materials into genuine societal applications. This review provides a comprehensive account of the field, underscoring key structure-property relationships across multiple length scales. It also outlines current challenges and opportunities for developing high-performance, multifunctional SCO Hofmann frameworks for emerging technologies.

(New J. Chem., 2024, 48, 8343)

A hydrogel is a gel in which the swelling agent is water. Hydrogels derived from Pluronic F127 (PL) are capable of spontaneously forming micelles in water. Their organization leads to the formation of a gel in the 20-40°C range, which is compatible with medical applications. Although perfectly biocompatible and non-toxic, they suffer from poor mechanical strength. This can be greatly improved by adding chitosan (CS), which promotes cross-linking. We used these PL/CS hydrogels as a support for incorporating a RuNO complex.

We incorporated the water-soluble trans(Cl,Cl)-[RuFTCl₂NO]Cl complex into PL/CS hydrogels at RuNO concentrations ranging from 13 to 520 µg.g^‒1 without observing any demixing (Figure 2). The hydrogels show a sol-gel transition between 35 and 40°C depending on the RuNO complex content (35°C for the hydrogel without RuNO). It is therefore little affected by the incorporation and concentration of the complex, which makes it suitable for cutaneous applications. Under irradiation at 400 nm and at physiological temperature (37°C), PL/CS/RuNO hybrid hydrogels are capable of releasing nitric oxide, as demonstrated by electron paramagnetic resonance (Figure 3) and electrochemical detection (NO- sensor). The quantum yield of of photorelease (Φ_NO) is 0.014 (at 37°C for irradiation at 400 nm).

Since the 2000’s a lot of regulations for the chemical and pharmaceutical industries have appeared especially in terms of efficiency, waste management and energy input. All these issues are now addressed and termed « Green Chemistry » a multifaceted field dealing with what we call the twelve principles of P. T. Anastas and J. C. Warner. Most important of them are: atom economy, preventing the use of solvents volatile and/or toxic, minimize chemical waste and minimize energy. By focusing on the Green Chemistry reactions, the alternative energy sources that appeared and developed are: photochemistry through light excitation, microwave, sonochemistry irradiation, and mechanochemistry.

According to IUPAC, a mechanochemical reaction is a “Chemical reaction that is induced by the direct absorption of mechanical energy”. Wilhelm Ostwald (Nobel Prize in 1909), was the first who mentioned the term “Mechanochemistry” and defined it as a “branch of chemistry which is concerned with chemical and physico-chemical changes of substances of all states of aggregation due to the influence of mechanical energy”. In the two last decades, mechanochemistry has been developed considerably in a multitude of areas such as:  inorganic compounds and metal complexes synthesis, catalysis, polymers, nanomaterials, and organic synthesis used for creating carbon-carbon, carbon-heteroatom, metal-ligand coordination bonds. It is important to point out also the many efforts developed towards mechanistic level understanding of mechanochemical processes and the possible links between the mechanical effect and the action of the forces generated at the molecular level.

The dipolar approach leads to chromophores with limited σ_TPA. Therefore, other systems with more sophisticated electronic properties have been studied. In particular, bimetallic species consisting of two conjugated monometallic subunits, such as complex 3 below:

Bimetallic compounds of this type can all be considered electronically pseudo-centrosymmetric. A remarkable point is that their associated σ_TPA greatly exceeds twice that of basic monometallic complexes. Thus, σ_TPA = 1523 GM for 3, under irradiation at 700 nm, which is a record for our bimetallic [Ru-NO] complexes. (Chem. Eur. J. 2022, 28, e202201692, 1-14).