ReviewSpin crossover polymer composites, polymers and related soft materials
Introduction
Polymer composites are multi-phase materials wherein at least one phase is a polymer [1]. The combination of these components results in original physical properties that differ from that of the constituents alone. In the majority of cases, they are composed of organic polymers as matrix and different fillers that act as the reinforcement [2]. Indeed, often the main objective that governs the development of such materials in various fields like construction [3], aerospace [4] and automotive [5] is the modification of their mechanical and thermal properties. Nevertheless, the scope of advanced polymer composite materials exceeds largely the thermomechanical aspects, providing opportunities to develop a large variety of original physical properties as well as material processing methods.
In the field of spin crossover (SCO) complexes of transition metal ions [6], [7], [8], [9], [10], [11], polymer composites have been developed for several reasons. In the early stages of SCO research, incorporation of SCO complexes into polymer matrixes was carried out in order to make possible some physical characterizations (e.g. photophysical measurements), which were not feasible (or meaningful) using microcrystalline powder samples or liquid solutions [12], [13]. To this aim, films or pellets of SCO-polymer composites were fabricated using simple methods such as spin coating or drop casting. Following the visionary ideas of Olivier Kahn in the 90s [14], [15], the past two decades the SCO research has moved to a considerable extent towards seeking potential technological applications of these smart, multifunctional molecular materials, exhibiting a spectacular change of their magnetic, optical, electrical, thermal and mechanical properties [16], [17]. As a result, the need for device integration and processing of SCO materials in different shapes and sizes (from the nanometric to the macroscopic scale) has also significantly increased. This conjuncture has motivated considerable research for the incorporation of SCO materials and nanomaterials [18] into malleable and processable polymer matrices using increasingly sophisticated methods, such as spray coating, matrix-assisted pulsed laser evaporation, electrospinning or 3D printing. However, impacts of the nature and the mechanical properties of the polymer on the spin crossover behaviours, and vice-versa, were clearly evidenced in many cases. These findings generated research for the theoretical modelling of SCO-matrix interactions [19] and, more recently, for the development of more sophisticated SCO polymer composite materials exhibiting synergies between the properties of the SCO particles and the polymer matrix. Notably, strain-coupled electroactive polymer-SCO composites have been developed with promising properties for the development of actuators, sensors and energy harvesters [16], [20], [21]. Alternative to multi-phase composite materials, several groups have also undertaken syntheses of ‘spin crossover organic polymers’, i.e. organic polymers functionalized by SCO entities. (N.B. We use the term ‘SCO organic polymers’ to avoid confusion with ‘SCO coordination polymers’, which refer to the well-known SCO coordination networks, such as Fe-triazole chains or Hofmann like clathrates.) The review is organized into three sections. The first section gathers a state of the art, which aims to be exhaustive on the synthesis and characterization of the physical properties of spin crossover polymer composites. The second chapter brings together the few reported examples of ‘spin crossover organic polymers’. The last section is a non-exhaustive overview of selected examples of conceptually related ‘soft’ SCO materials, including SCO dendrimers, gels, liquid crystals and Langmuir Blodgett films, which display uncommon properties and allow for easier material processing when compared to ‘conventional’ crystalline SCO materials.
Section snippets
Spin crossover in polymer composites
Embedding SCO complexes into polymer matrices is a straightforward, yet powerful and generic approach towards processable SCO materials, important for their different applications and integration into devices. From a conceptual point of view, it is interesting to divide these materials into two main categories: composites prepared from solutions (whether using a solubilized SCO complex or the corresponding precursors) and composites prepared from preformed SCO powder. In both cases, a variety
Spin crossover organic polymers
Fundamentally, we can divide ‘organic polymer SCO complexes’ into two basic categories based on the way the polymer is attached to the SCO complex: a) when the polymer is attached by supramolecular interactions to the SCO complex, for example representing the counter-anion of the complex (polymer backbone) such as in Nafion-SCO composites and b) when the polymer is covalently attached to the SCO ligands of the SCO complex. In this section, we focus exclusively on the second approach. (The
Related compounds
In this section, we review different classes of ‘soft’ SCO compounds, which are, strictly speaking, not polymeric and cannot be classified either as polymer composites, but are closely related to them. These include SCO dendrimers, gels, liquid crystals and Langmuir-Blodgett films. The common feature of these materials that they display properties, which are typical of ‘soft matter’, such as liquid crystal properties, viscoelasticity and large deformability. As such, they allow for
Concluding remarks and prospects
Driven to a large extent by the need for versatile methods for processing spin crossover complexes into films, micro/nano-structures and other technologically relevant objects, SCO-polymer composites and related soft materials have gained considerable interest in the past two decades. Following the achievements we reviewed in this paper, several important perspectives can be highlighted at the frontiers of coordination chemistry, polymer science and engineering:
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From the onset of this research
Conflict of interest
None.
Acknowledgments
Financial support from the ANR project 19-CE09-0008-01, from the CONACYT (AEC), Occitanie Région and the Federal University of Toulouse (MPB) is acknowledged.
References (136)
- et al.
Light-induced excited spin state trapping (LIESST) in [Fe(2-mephen)3]2+ embedded in polymer matrices
Chem. Phys. Lett.
(1988) - et al.
Emerging trends in spin crossover (SCO) based functional materials and devices
Coord. Chem. Rev.
(2017) - et al.
Spin-crossover nanoparticles and nanocomposite materials
Comptes Rendus Chim.
(2018) - et al.
Elastic models, lattice dynamics and finite size effects in molecular spin crossover systems
Comptes Rendus Chim.
(2018) - et al.
Processable magnetic plastics composites - spin crossover of PMMA/Fe(II)-complexes composites
Synth. Met.
(2004) - et al.
Structure of a spin-crossover Fe(II)-1,2,4-triazole polymer complex dispersed in an isotactic polystyrene matrix
Eur. Polym. J.
(2011) - et al.
Demonstration of the thermally induced high spin-low spin transition for a transparent spin crossover complex film [Fe(II)(H-trz)3]-Nafion (trz = triazole)
Polyhedron
(2005) - et al.
Spin transition and its photo-induced effect in spin crossover complex film based on [Fe(II)(trz)3]
Synth. Met.
(2003) Recent advances in perfluorinated ionomer membranes: structure, properties and applications
J. Memb. Sci.
(1996)- et al.
Development of hybrid diblock copolypeptide amphiphile/magnetic metal complexes and their spin crossover with lower-critical-solution-temperature (LCST)-type transition
Polymer
(2017)
Thermochromic sensor design based on Fe(II) spin crossover/polymers hybrid materials and artificial neural networks as a tool in modelling
Sensors Actuators B Chem.
Magnetic properties and vapochromism of a composite on the base of an iron(II) spin crossover complex
Inorg. Chem. Commun.
Cellulose fiber nanocomposites displaying spin-crossover properties
Colloids Surfaces A Physicochem. Eng. Asp.
Spin-crossover behaviour of iron(III) complexes with pendant type polymeric ligands
Inorganica Chim. Acta
Synthesis and characterization of novel coordination spin crossover poly(glycidyl methacrylate) with pendant iron(II)-4-amino-1,2,4-triazole groups
Inorg. Chem. Commun.
New spin crossover polymeric composite and another way to describe the result
Inorg. Chem. Commun.
Formation of hybrid spin crossover polymer microspheres
Synth. Met.
Spin crossover in soft matter
Coord. Chem. Rev.
Expand classical drug administration ways by emerging routes using dendrimer drug delivery systems: a concise overview
Adv. Drug Deliv. Rev.
Counterion effect on the spin-transition properties of the second generation iron(III) dendrimeric complexes
Inorg. Chim. Acta
Machining of Polymer Composites
A review on mechanical properties of natural fiber reinforced hybrid polymer composites
J. Miner. Mater. Charact. Eng.
Manufacturing aspects of advanced polymer composites for automotive applications
Appl. Compos. Mater.
Spin-Crossover Materials: Properties and Applications
Spin crossover phenomenon
Comptes Rendus Chim.
Molecular spin crossover phenomenon: recent achievements and prospects
Chem. Soc. Rev.
Mechanism of spin-state interconversion in ferrous spin-crossover complexes: direct evidence for quantum mechanical tunneling
J. Am. Chem. Soc.
Spin transition molecular materials for displays and data recording
Adv. Mater.
Spin-transition polymers: from molecular materials toward memory devices
Science
Coupling mechanical and electrical properties in spin crossover polymer composites
Adv. Mater.
Mechano-electric coupling in P(VDF-TrFE)/spin crossover composites
J. Mater. Chem. C
Stretchable energy-harvesting tactile electronic skin capable of differentiating multiple mechanical stimuli modes
Adv. Mater.
Storing spin-crossover and LC phase transitions information by hybridizing spin-crossover complexes with a thermotropic polymer matrix - a novel case of multiple switching
Mol. Cryst. Liq. Cryst.
Spin crossover-macromolecule composite nano film material
Chem. Commun.
Hybrid polystyrene based electrospun fibers with spin-crossover properties
J. Polym. Sci. Part B Polym. Phys.
Flexible and optically transparent polymer embedded nano/micro scale spin crossover Fe(II) complex patterns/arrays
Chem. Mater.
Robust spin crossover platforms with synchronized spin switch and polymer phase transition
Sci. Rep.
Search on multi-functional properties of spin-crossover system
Mol. Cryst. Liq. Cryst.
Spin crossover complex film, [FeII(H-trz)3]-Nafion, with a spin transition around room temperature
Chem. Lett.
Photoinduced phase transition and relaxation behavior in a spin-crossover Fe (II) complex Nafion-[Fe(Htrz)3] film
J. Phys. Soc. Japan
Study on the spin crossover transition and glass transition for Fe(II) complex film, [Fe(II)(H-triazole)3]@Nafion, by means of Mössbauer spectroscopy
Hyperfine Interact.
The morphology in nafion perfluorinated membrane products, as determined by wide- and small-angle x-ray studies
J. Polym. Sci. Polym. Phys. Ed.
Development of pH-sensitive spin-crossover iron(II) complex films, [FeII(diAMsar)] Nafion: manipulation of the spin state by proton concentration
Chem. Lett.
The spin-transition properties of Fe(III) complexes with hetarylformazan in an ion-exchange polymer: an EPR study
Russ. J. Phys. Chem. A
Room temperature bistability with wide thermal hysteresis in a spin crossover silica nanocomposite
J. Mater. Chem. C
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2023, Journal of Molecular StructureCitation Excerpt :Therefore, a considerable part of the current reports on SCO compounds are about how these factors modulate SCO behavior. Researchers have been committed to finding out their internal relations so as to better put SCO compounds into specific applications [42–44]. In previous work, we successfully prepared a series of SCO complexes containing MnⅢ and explored the factors that could affect the properties of SCO, such as counter anions, ligands and ligand substituents.