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Molecular Magnetic switches
Molecular magnetism is a fruitful source of functional complexes. Indeed, it has provided a broad spectrum of magnetic compounds exhibiting switchable properties. A non-exhaustive list, containing spin-crossover complexes, valence tautomerism complexes or even metal-to-metal charge transfer (MMCT) complexes (e.g.Prussian Blue Analogues), is illustrated figure 1.4
This chapter will mainly focus on switchable compounds around which this work was built: Iron(II) spin crossover complexes and photomagnetic polymetallic complexes. The objective is to provide a general description of their properties, proving their ability to accommodate to a wide collection of functionalities. Then, we will discuss the processing methods developed to include these molecular switches into materials. Finally, we will outline the main scientific challenges encountered in this research area underway.
Iron(II) Spin-crossover complexes
Phenomenon and techniques
The spin-crossover phenomenon was first discovered in an Fe(III) dithiocarbamate mononuclear complex by Cambi and coworkers5 in 1931. This observation was then extended to other octahedral transition metal complexes with dn (4 ≤ n ≤7) electronic configuration, mainly in the first row with d4 Cr(II), d5 Fe(III), d6 Fe(II) and d7 Co(II) ions. Spin transition was described in solid, liquid state as well as soft matter.
Octahedral 3d complexes, with d4 to d7 electronic configuration, may exist either in high-spin state (HS) or low-spin state (LS). The example of an octahedral Fe(II) d6 is given figure 2. Depending on the ligand field strength, Δ, relative to the mean pairing energy, Π, several cases arise. For strong ligand field (Δ > Π), Δ the ground state is the one with the minimum spin multiplicity (S = 0) as all d electrons are paired in t2g orbitals (t2g)6. The Fe(II) complex is thus in a diamagnetic low-spin configuration. On the opposite, weak ligand fields (Δ < Π), stabilize the high-spin state of maximum multiplicity (S = 2).
For intermediate cases, when the ligand field and the mean pairing energy are of similar magnitude (Δ ≈ Π), the zero-point energy difference ΔE°HL can become close to thermal energy (ΔE°HL ≈ kBT). It is then possible to thermally induce the transition from one spin state to another. This molecular spin-state switching is called spin crossover (SCO). Spin-crossover complexes may also respond to other stimuli such as pressure, magnetic/electric fields or even light. The so-called Light-Induced Excited Spin-State Trapping (LIESST) effect will be discussed in more details section I.1.3.
Most of the reported SCO complexes are iron complexes, and the most prominent family being represented by the Fe(II) complexes in N6 donor environment. These complexes are particularly interesting because they exhibit drastic changes in their magnetic and optical signature, going from a diamagnetic LS state (S= 0) showing intense colour to a pale paramagnetic one (S= 2). The spin-state change of the compound is then easily detectable by SQUID magnetometry or optical measurements. These techniques are the most commonly used to measure the thermally-induced transition.
The spin transition results in other significant changes of the structural, physical and (less frequently) chemical properties of the complex. Any technique sensitive for instance to a change of electronic, vibrational, structural parameters during the transition can thus be relevant to follow the transition (e.g. IR, Raman, NMR, Mossbauer spectroscopies).
Single crystal X-ray Diffraction (or X-ray Powder Diffraction) is particularly used to probe the structural variations. Generally, FeII-N bond lengths in LS state ( 1̴.8–2.0 Å) are about 10% shorter than in the HS state ( 2̴.0– 2.2 Å), due to absence of electrons in antibonding eg orbitals.6 The LS-HS transition is also easily revealed by significant distortions in the Fe(II) coordination sphere.
More rarely, Differential Scanning Calorimetry (DSC) is used to provide thermodynamic parameters (ΔH°; ΔS°) associated to the spin transition.
Spin crossover profiles
The general way to report a thermally induced spin transition is to plot the high spin fraction γHS as a function of temperature. The temperature for which LS and HS fractions are equal to 50% is called the transition temperatures, T1/2. Different nature of spin-state changes have been identified (fig. 3a-e). The spin state change can be gradual/continuous (fig. 3a) over a broad range of temperature. It can also be abrupt (fig. 3b-c) and arises within a narrow range of temperature (few Kelvin). In some more complex cases, the transition may show several steps (fig. 3d) or be incomplete (fig. 3e).
The curve shape is an indicator of the cooperativity. The cooperativity is due to elastic intermolecular interactions between the spin-crossover neighbouring molecules. A gradual transition accounts for the absence of cooperativity between SCO-centres. In this case, the transition can be described as a simple thermal equilibrium based on a Boltzmann distribution.
This behaviour is typical of spin transition in solution or solid solutions.
On the opposite, sharp transitions are the signature of cooperative effects. In fact the structural changes occurring on one complex (i.e. the increase or decrease of the volume of the complex) can trigger the spin-state change on its neighbouring molecules. (fig. 3c) When cooperativity becomes strong enough, thermal hysteresis loop can be observed. The latter are thus the extreme illustration of cooperativity and are the most coveted spin transition profile. Two transition temperatures border the loop width, T1/2↓ and T1/2↑. Within this loop, the compound is bistable, i.e. it can exist in two different spin states in for an identical value of the temperature. Hysteresis confers a memory effect as the compound retains a given spin state (memory) which depends of its entry pathway in the loop. The two spin states can be encoded with a binary code 0/1 (or ON/OFF) and the SCO material appears as a molecular memory component.
The design of SCO complexes with a complete, reproducible hysteresis around room temperature opens perspective for optical or magnetic data storage. This behaviour can be observed when intermolecular contacts (such as H-bonds, π-π interactions and so on) efficiently promote elastic interaction. A more rational way to achieve such cooperative behaviour is to design coordination polymers where SCO centres are linked by coordination bonds.7,8
At the scale of an isolated molecule, it is very challenging to obtain bistability. A smart strategy was developed by Zarembovitch et al. in the nineties. It consisted in designing potential SCO complexes containing photo-isomerizable ligands. During isomerization, the ligand field is modified, triggering a spin-state change as shown figure 4.9 The effect is called Ligand-Driven Light Induced Spin Change, LD-LISC. It has been evidenced in solution but it is difficult to realize in the solid state where dense molecular assembly can prevent the structural reorganisation accompanying the ligand isomerization.
In the present work, we mainly focus on molecular systems whose properties can be switched by a light stimulus. The next sections will thus provide a description of photo-switchable SCO and MMCT complexes.
Photomagnetism in spin crossover systems
As mentioned in section 1.1., in some cases spin-state switching can be triggered in SCO complexes by a light irradiation at low temperatures. This phenomenon is known as the Light-Induced Excited Spin-State Trapping (LIESST) effect. As for the thermal stimulus, the spin-state change is accompanied by strong changes in the magnetic properties (photomagnetic) and strong changes in the optical properties (photochromism).
This effect was first discovered in an Fe(II) SCO molecular system in solution by McGarvey et al.10,11Then, Decurtins et al.12 reported the light sensitive electronic changes of [FeII(ptz)6](BF4) (ptz = 1-propyl-tetrazole), in solid state at low temperature (20 K). An explanation of the LIESST effect will be given through this example, (figure 5).13
Upon irradiation at 514nm, the [FeII(ptz)6](BF4) complex is excited from the initial singlet ground state, 1A1 , to the excited singlet 1T1. The excited singlet can relax back to the ground state 1A1 or to a metastable quintuplet state 5T2, through intersystem crossing.
At low temperatures, the system remains trapped in the paramagnetic metastable state, which can exhibit very long lifetime.
wells of the metastable spin state 5T2 and the ground state 1A1 can be overcome if the temperature is raised. Quantum relaxation can also occur at low temperatures. The limit temperature, above which the metastable magnetic information is lost, is called the TLIESST.
Graphically, it corresponds to the inflexion point of this relaxation curve and is determined by the minimum of the ∂χMT/∂T curve.14
Interestingly, Hauser showed that it was also possible to convert the system back to LS state by irradiating the metastable state.15 This effect was called reverse-LIESST. In principle, it is possible to obtain SCO complexes where one wavelength triggers the LS to HS conversion whereas another one triggers the HS to LS conversion. The described light induced bistability opens perspectives for using photomagnetic materials in technological applications. However, the main deficiency of LIESST effect lies in the low relaxation temperature (ca TLIESST < 150 K).
(Photo)magnetic polymetallic complexes
Photomagnetism in Prussian Blue Analogues (PBAs)
The photomagnetic effect can also be achieved in some mixed-valence polymetallic species. In these systems, a light irradiation induces an electron transfer from one metallic centre M to another M’. Interestingly, photomagnetism based on electron transfer has only been observed in cyanide-bridged compounds.
The first example was reported in a Prussian Blue Analogue (PBA), of formula K0.4Co1.3[Fe(CN)6]∙5H2O, by Hashimoto and coworkers in 1996.16 In this material, an increase of the magnetization and the magnetic ordering temperature Tc is observed upon red light irradiation at 5 K. This result was explained by the conversion of diamagnetic {FeIILS-CN-CoIIILS} pairs (Fe t2g6eg0, S = 0; Co t2g6eg0, S = 0) to paramagnetic {FeIIILS -CN-CoIIHS} ones (Fe t2g5eg0, S = 1/2; Co t2g5eg2, S = 3/2) as illustrated figure 6.
This process is named Electron Transfer Coupled to a Spin Transition (ETCST) as a spin transition occurs on the Co ion. Similarly, to the LIESST effect, it is possible to thermally relax the photo-induced metastable state and extract a relaxation temperature, Trelax. In this family of compounds, relaxation temperatures can be usually higher than those of pure SCO systems but they remain under 200 K.
The discovery of Hashimoto et al. gave rise to many studies aiming at improving the photomagnetic properties. However, the task is challenging as Prussian Blue analogues are non-stoichiometric inorganic polymers that exhibit a complex local structure. This point plays a critical role in the rationalization of their physical properties.
Indeed, the PBAs, of general formula, CxMy[M’(CN)6]z □(1-z) ∙nH2O, (where M and M’ transition metals; C+, alkali ion; □, [M’(CN)6] vacancies), can contain various amount of inserted cations and {Fe(CN)6} vacancies. This leads to the coexistence of non-equivalent {Fe-CN-Co} pairs in the material and various Co environments, exhibiting different ligand fields, redox potentials, etc.
Overall, the occurrence of photomagnetic properties in those systems was shown to depend on various parameters such as: (i) the coordination environment of the cobalt ion; (ii) the nature and the amount of inserted cations; (iii) the number of [Fe(CN)6] vacancies; (iv) structural parameters such as the cyanide bridges geometry. As these parameters are interdependent, the photo-induced electron transfer in PBAs is difficult to control.
To deepen the knowledge of the ETCST phenomenon, a strategy has consisted in studying lower (soluble) dimensional models of PBAs. Indeed, the use of model compounds where the environment of the Fe and Co ions is well-controlled enable researchers to better control structural and electronic parameters influencing ETCST. Besides the use of soluble complexes allow the access of accurate electronic information through various solution techniques (electrochemistry, NMR etc) not accessible in PBAs due to their poor solubility.
From 3D PBAs networks to their 0D molecular models
In the past decade, molecular models scaling down to the smallest constitutive unit (M-CN-M’ pair) of Fe/Co PBAs skeleton have been synthesized. Some examples exhibiting attractive ETCST are illustrated figure 7.17, 18, 19, 20
The conception of these well-defined soluble and photo responsive molecular models is based on an insightful choice of the building blocks. Octametallic cubic models are particularly interesting as they are the only ones for which the role of the alkali ion could be taken into account. We will focus the discussion on the two published examples (figure 7c-7d).
The example 7c corresponds to the molecular cube of formula {[Co(Tpe)]4[(pzTp)Fe(CN)3]4(ClO4)4 (where Tpe = 2,2,2,-tris(pyrazolyl)-ethanol) reported by Holmes and co-workers in 2008.18 The {Fe4Co4} cube exhibits outstanding magnetic properties. Indeed, the thermally induced ETCST is abrupt and occurs closed to room temperature (T1/2 = 250 K). The conversion is complete as the maximum χMT value at high temperature is consistent with four isolated LS Fe(III) and four HS Co(II).
Concerning its photomagnetic properties, in addition to the remarkable efficiency of the photo-conversion, the measured Trelax is equal to 180 K. The measured lifetime of the metastable state at 180 K is around 10 years. It must be highlighted that commercial applications require to hold information more than 10 years at room temperature (stability ratio C-1 = ΔE/kBT > 50).21 The achievement of such photo-responsive molecules is thus motivating for the design of molecular-based materials.
Another example going further in the mimicking of PBAs, were reported by D. Garnier et al. and J.R. Jimenez et al. in our group in 2016 and 2017 respectively.20 The mixed-valence {Fe4Co4} cage of formula A {[FeII(Tp)(CN)3]4[CoIII(pzTp)]3[CoII(pzTp)]} contains an inserted alkali ion, A. They thus offer the possibility to probe the influence of A on the ETCST and to confront these results to those obtained in PBAs. These cubes show an incomplete gradual thermally-induced ETCST at high temperature (T> 300 K). As the predecessor example of Clérac et al., an efficient increase of the magnetization is observed upon irradiation at 808 nm at 20 K.
However, the relaxation temperatures are lower (Trelax = 80 K).
An innovation of this work can be ascribed to the detailed analysis of the molecular structure and electronic properties in crystal state and solution. Indeed, the solubility and the stability of the molecular cube containing potassium allowed its study by a complete set of techniques such as EPR, NMR spectroscopy and mostly by cyclic voltammetry. This enabled to highlight some remarkable electronic properties: a stability over a wide range of potential with 9 accessible redox states; their electro-chromism; their photo-magnetism; a slow magnetic relaxation (“field-induced SMM behaviour”) at low temperature.
Following efforts in the group were devoted to the exploration of the influence of the inserted cation nature within a cube family of generic formula A {Fe4Co4} where A = K+, Rb+, Cs+, Tl+, NH4+ and NH3CH3+.
Table of contents :
GENERAL INTRODUCTION
CHAPTER 1 Context and state of art
I. Molecular Magnetic switches
1. Iron(II) Spin-crossover complexes
1.1. Phenomenon and techniques
1.2. Spin crossover profiles
1.3. Photomagnetism in spin crossover systems
2. (Photo)magnetic polymetallic complexes
2.1. Photomagnetism in Prussian Blue Analogues (PBAs)
2.2. From 3D PBAs networks to their 0D molecular models
II. Implementing magnetic molecular switches: toward responsive molecular-based materials
1. Some challenges to face.
2. Magnetic molecular switches on surfaces
3. Switchable complexes-containing organic polymers.
CHAPTER 2 Functionalized molecular model compounds
I. Iron(II) spin crossover complexes based on scorpionate ligands
1. Third-generation trispyrazol(-1-yl)borates : a useful platform for molecular switches functionalization
2. Syntheses of a third-generation tris(pyrazol-1-yl) borates serie
2.1. Syntheses of ligands (4A), (4C) and (4D)
2.2. Spectroscopic characterization of ligands (4A), (4C) and (4D)
2.3. Synthesis and characterization of ligand 4B
3. Syntheses of the homoleptic [FeII(RB(pz)3)2] complexes
4. Characterizations
4.1. NMR
4.2. Optical Studies
4.3. Magnetic properties of the complexes
4.4. X-rays crystal structures analyses
4.5. Discussion
5. The special case of the [FeII(C6F5Tp)2] complex
5.1. From a gradual transition to an hysteresis loop
5.2. Verification of the compound identity
5.3. Differential Scanning Calorimetry (DSC) measurements
5.4. Temperature dependence single-crystal X-ray diffraction
6. Conclusion
II. Functionalization of charge transfer cyanide-bridged {Fe4Co4} cubes
1. Background study : Role of the inserted cations in a A {Fe4Co4} cages family
2. Interests and objectives of this part
3. Investigating the effect of tbuPhTp ligand in a A {Fe(Tp)Co(tbuPhTp)} cubes series
3.1. Synthesis and NMR characterization of Cs {Fe(Tp)Co(tbuPhTp)}
3.2. Influence of the experimental conditions on paramagnetic: diamagnetic cubes ratio
3.3. Cs {Fe(Tp)Co(tbuPhTp)} stability in solution followed by NMR
3.4. FT-Infrared absorption spectroscopy
3.5. Crystallographic studies
3.6. Magnetic properties
4. Conclusion
III. Refining the {Fe4Co4} cage ETCST through a multistep rational design
1. Influence of the molecular precursors electrochemical potential
2. Design and study of the paramagnetic A {Fe(tbuPhTp)Co(Tp)} cubes
2.1. Bu4N[Fe(tbuPhTp)(CN)3] precursor synthesis and characterisations
2.2. Synthesis and crystallographic study of A {Fe(tbuPhTp)Co(Tp)} paramagnetic cube
2.3. A {Fe(tbuPhTp)Co(Tp)} (A = Cs+; Tl+) magnetic properties
IV. Cubes electrochemical studies in solution
V. Conclusion
CHAPTER 3 Synthetic approaches for the design of new metallopolymers
I. Synthetic strategies for metallopolymers design
II. Post-polymerization modification strategy with tris(pyrazolyl)borate ligand
1. Synthesis
2. Evaluation of the Tp-functionalized copolymer stability based on a model molecular compound
3. Conclusion
III. New functionalized 2,6-di(pyrazol-1-yl)pyridine ligands
1. Synthesis of ligand (A)
2. Synthesis of ligand (B) and spectroscopy
3. Synthesis of ligand (C) and spectroscopy
IV. Conclusion
CHAPTER 4 Photoswitching in electropolymerized ultra-thin films of {Fe4Co4} cages
I. Introduction
1. Hybrid thin films of poly(thiophene)-{Fe4Co4} cages: motivations and strategy
1. Thiophene electropolymerization: general aspects and mechanism
2. Preliminary studies on mononuclear cobalt complexes.
2.1. Electrochemistry in solution
2.2. Surface modification : electropolymerization of [Co(2-TPhTp)2]
2.3. Conclusion
II. Electropolymerization of thiophene-functionalized {FeII4CoIII4} diamagnetic building blocks
1.Syntheses of Cs{Fe(Tp)Co(2-TPhTp)}ClO4 and Cs {Fe(Tp)Co(3-TPhTp)}ClO4 precursors.
2. Electrochemistry in solution and surface modification
3. Electropolymerized thin-films chemical analyses
3.1. X-ray photoelectron spectroscopy (XPS): principle and specificity
3.2. Samples’ preparation and XPS method
3.3. XPS analysis of Cs {Fe(Tp)Co(2-TPhTp)}PF6 electropolymerized thin-film.
3.4. XPS analysis of Cs {Fe(Tp)Co(3-TPhTp)}PF6 electropolymerized thin-film.
3.5. Comparative summary
4. AFM characterization of Cs {Fe(Tp)Co(2-TPhTp)}PF6 thin-films of controlled thickness
4.1. Topography of Cs {Fe(Tp)Co(2-TPhTp)}PF6 thin-films
4.2. Thickness measurements : AFM scratching tests
5. Conclusion
III.Electropolymerization of thiophene-functionalized {Fe4Co4} paramagnetic building blocks
1. Synthesis of Tl {Fe(2-TPhTp)Co(Tp)} paramagnetic cube.
2. Electropolymerization and surface characterizations
2.1. Electropolymerization
2.2. XPS analyses
2.3. AFM characterizations
3. Photomagnetic properties of Tl {Fe(2-TPhTp)Co(Tp)} electropolymerized thin-films
IV. Conclusion
CHAPTER 5 Self-assembled layers of {Fe4Co4} cages on Au(111)
I. Introduction
1. Motivations and strategies
2. Foreword on thiophene self-assembled monolayers (SAMs)
II.Testing conditions for thiophene functionalized {Fe4Co4} cages SAMs formation from solution
1. PM-IRRAS technique as a screening method
2. Effect of concentration
3. Influence of immersion time
III. Testing the stability thiophene functionalized {Fe4Co4} cages SAMs.
1. Sample preparation
2. XPS characterizations of {Fe4Co4} cages adsorbed on Au substrates.
3. Stability of {Fe4Co4} adsorbed layers and comparative study using 3-hexylthiophene
3.1. Desorption tests and S 2p assignments
3.2. Evolution of oxidized sulfur amount in solution.
3.3. Electrospray deposition (ESI) of Cs {Fe(Tp)Co(3-TPhTp)}ClO4 on Au(111)
4. Cs {Fe(Tp)Co(3-TPhTp)}ClO4 stoichiometry after deposition on gold substrates
5. Topography of the Cs {Fe(Tp)Co(3-TPhTp)}ClO4 layers deposited on Au/mica
IV. Conclusion
CONCLUSION
EXPERIMENTAL PART
APPENDICES
