Elsevier

Coordination Chemistry Reviews

Volume 419, 15 September 2020, 213396
Coordination Chemistry Reviews

Review
Spin crossover polymer composites, polymers and related soft materials

https://doi.org/10.1016/j.ccr.2020.213396Get rights and content

Highlights

  • The article reviews spin crossover polymer composite materials.

  • Recent development in spin crossover polymers.

  • Elaboration of soft materials based on spin crossover compounds.

Abstract

We review the synthesis, properties and applications of spin crossover polymer composites, polymers and some related ‘soft’ materials. These materials have received recently much attention because they provide an efficient way for the processing of spin crossover complexes in various shapes at various size scales and can give rise also to unique physical properties. First, we discuss in detail the state of the art of the elaboration of spin crossover polymer composites, using either inorganic complex precursors in solution or preformed spin crossover powder. A particular attention is paid on the influence of the polymer matrix on the spin crossover properties and on the use of ‘active’ polymers for development of synergies between the properties of the matrix and the load. Polymer composite devices for applications in the fields of artificial muscles, energy harvesting and thermochromic sensors are also highlighted. Then, more recent works, in which organic polymeric chains are used as ligands for the transition metal ions are presented. Finally, we overview various related ‘soft’ spin crossover compounds including spin crossover dendrimers, gels, liquid crystals and Langmuir Blodgett films with particular emphasis on compounds with supramolecular interactions of alkyl chains.

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:

  • -

    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.

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