The desire to develop new data storage and information processing devices has led to an interest in materials that exhibit bistability. This has rejuvenated efforts to generate spin crossover (SCO) compounds.91,94
Spin crossover is a phenomenon of transition metal complexes with an electronic configuration between d4 and d7.91,94 The metal ions within these complexes can be in a high spin (HS) or low spin (LS) state depending on the field strength of the ligand that binds to the metal centre (Figure 1.10).94 For an intermediate ligand field the energy gap (∆E0HL) between the lowest vibronic level of the two states (HS and
LS) can be sufficiently small that an external perturbation such as the application of light, heat or pressure can instigate a change in state.94 This process is known as a spin transition (ST) or spin crossover (SCO) and is usually accompanied by a change in the physical properties (colour, magnetism and bond lengths) of the material. If there is a high degree of cooperativity between the metal ions of SCO materials, the transition can occur abruptly or via a hysteresis loop.94 SCO compounds are therefore ideal
16 Within the literature, FeII complexes have shown the greatest propensity to display SCO properties. The use of nitrogen donor ligands to generate complexes with a {FeN6} coordination sphere has proved particularly successful in the production of
SCO materials.91,94 Predominantly, work in this area focuses on the formation of supramolecular arrays which contain SCO centres linked via weaker interactions, such
as π-π stacking, hydrogen bonding or coordination bonds.95,96 These intermolecular
forces result in a high degree of cooperativity between the metal centres enabling the SCO to occur through a hysteresis loop.
Kahn and co-workers have developed a synthetic approach for the formation of FeII coordination polymers (Figure 1.11) that display fascinating SCO properties.97 These compounds consist of FeII ions linked in 1D polymeric chains composed of [Fe(Rtrz)3]2+ sub-units (Rtrz = 4-R-1,2,4-triazole where R = H or NH2)97,98 The
covalent linking of the FeII ions within the polymeric chains gives rise to strong
Figure 1.10: a) Electronic configuration for a d6 FeII ion in the LS and HS states. Equilibrium
between these two states exists in the case of spin crossover. b) Representation of the potential well
for 1A
1 (LS)and 5T2 (HS) states in a FeII SCO system.94 b) a)
Figure 1.11: a) The structure of [Fe(Htrz)3]2+ (H atoms have been removed for clarity).97 b) The
temperature dependent magnetic susceptibility measurement for [Fe(Htz)3]2+ revealing a broad
hysteresis loop.98
b) a)
17 cooperativity.90 These cooperative effects result in abrupt spin crossover transitions which occur in the room temperature range.97 These polymeric species also reveal broad thermal hysteresis loops of between 5 K and 20 K in width.97 For example, Figure 1.11 shows the structure of the [Fe(Htrz)3]2+ complex which has two
characteristic spin crossover temperatures at 285 K and 305 K.97
Recently there has also been an endeavour to utilise SCO behaviour for molecular recognition purposes.40,99,100 These approaches involve the recognition of a guest molecule within a host through a change in the magnetic and/or optical properties of the material. It has previously been shown that the magnetic properties of metal-organic frameworks can be alerted dramatically through the presence of guest molecules such as solvent molecules.24 This work led researchers to investigate
if SCO centres could be incorporated within framework structures. Examples of such systems have been reported by Kepert et al. and are known as spin crossover metal-
organic frameworks (SCOFs).99,100,101,102 Within these frameworks, the FeII ions
centres are linked by rigid pyridyl ligands.101 These frameworks were shown to contain large 1D channels which can incorporate guest solvent molecules.100,101 The presence of the guest molecules was recognised by a change in the spin crossover temperature of the material.100,101
A relevant compound is the highly cited SCOF, [Fe2(azpy)4(NCS)4]·EtOH
(azpy = trans-4,4′-azopyridine), which undergoes a spin crossover transition at 150 K
Figure 1.12:.a) The X-ray crystal structure of the spin crossover metal-organic framework (SCOF),
Fe2(azpy)4(NCS)4·(EtOH) viewed approximately down the 1-D channels (c axis). Framework atoms
are represented as sticks and atoms of the ethanol guests as spheres. In the framework the ethanol guests occupy every second 1-D channel in a "chessboard" arrangement.99 b) Temperature dependent
magnetic susceptibility measurements showing the change in SCO properties upon the absorption of guest solvent molecules.99
b) a)
18 (Figure 1.12).99 However, when the ethanol molecules are removed the compound no longer displays a SCO transition and it remains in the HS state at all measured temperatures.99 It was shown that [Fe2(azpy)4(NCS)4]·EtOH could absorb other guest
molecules such as methanol and propanol molecules.99 The presence of these guest molecules were recognised by a change in the SCO behaviour of the compound. When methanol molecules are absorbed by the compound it exhibits a one step SCO transition at 140 K, however the propanol containing analogue displays a two step SCO transition with a plateau centered at 120 K.99
For spin crossover materials to be applicable for future devices the spin crossover transition must occur within an appropriate temperature range.103 Therefore, there is an ongoing effort to understand the factors which induce and influence spin crossover behaviour.104 It has been demonstrated that molecular shape has an
important influence on spin crossover in FeII and FeIII complexes.104,105 A HS molecule will not undergo a spin transition if its shape differs too strongly from that preferred by its LS form.106 The molecular rearrangement that would be required in such a case can not be accommodated by a rigid solid lattice and the molecule remains trapped in its HS.106 This suggests that the structure of the organic ligand used in SCO complexes can have a dramatic impact on their SCO properties. This effect was exemplified by Halcrow and co-workers who have investigated the SCO behaviour of a series of FeIII saltrien complexes (Figure 1.13a).105 The spin-state behaviour (Figure 1.13b) of these complexes is influenced by two factors.105 Firstly, the HS and LS forms of the complexes may co-exist in solution at ambient temperatures. Therefore, the least soluble form of the complex is likely to crystallise.105 The second factor is the shape of the molecules, which is highly non-spherical with two phenoxy “arms” projecting from the same face of the molecule.105 The relative orientations of these arms, α differs substantially between the compounds (61.8° ≤ α ≤ 125.0°),
demonstrating a substantial degree of flexibility in the saltrien ligand framework.105 It was concluded that only HS [Fe(saltrien)]+ complexes whose ligand conformations resemble those adopted by the LS species, can undergo spin-crossover.105 Therefore, only species with α values less than 90° should display SCO behaviour.105 This was
demonstrated by the coordination compound [FeL5]PF
6 (L5 = 1,8-diamino-3,6-
diazaoctane) which has an α value of 100.94º and does not exhibit any SCO
19 c) d) PF6- CF3SO3- ClO4-
Figure 1.13:a) The ligands used in the formation of the FeIII saltrien complexes developed by Halcrow
and co-workers and b) the temperature dependent magnetic susceptibility measurements of these
complexes showing the influence of ligand conformation on the spin transition process.105 c) the
magnetic properties of the [Fe(btzx)3]2+ complexes demonstrating the effect the counterion has on the
spin crossover properties of these materials.107 d) The structure of the [Fe(btzx)3]2+complexes
synthesised by Quesada et. al.107
a)
20 The counterion employed in spin crossover systems also influences their SCO behaviour.104,108 Quesda et al. have investigated this effect for a series polymeric
[Fe(btzx)3]2+ complexes (where btzx = m-xylene(bis)tetrazole) (Figure 1.13d).107 In
this study PF6-, ClO4- and CF3SO3- were selected as the counterions.107 Figure 1.13c
shows the temperature dependent magnetic susceptibility for each compound and demonstrates the effect counterions have on the transition temperatures of these complexes. When PF6- is employed as the counterion the compound undergoes a one
step SCO transition with a transition temperature of 160 K but for the analogous ClO4-
and CF3SO3- compounds the transition temperatures are lowered to 130 K and 110 K,
respectively.107 Quesda et al. suggest that the variation in distances between the
counterion and the FeII centre within the compound is the reason for this change in
transition temperature.107 Short counterion to metal distance leads to a distortion of
the FeII octahedral coordination environment.107 This distortion limits the freedom of the compound’s tetrazole rings to move, thus stabilising the HS state.107 This effect
gives rise to the lower transition temperatures observed for [Fe(btzx)3](ClO4)2 and
[Fe(btzx)3](CF3SO2)2.107
Since 1984, when Decurtins et al. discovered the phenomenon of light-
induced 1A1 (LS) →5T2 (HS) transitions in FeII complexes109 a significant part of the
research in the field of FeII spin crossover has focused on the use of light as a means of instigating spin crossover.91,108 This photo-induced transition (Figure 1.14a) involves the excitation of the 1A1(LS) state to the 1T1 state.91 This is followed by two
successive inter-systems crossing (ISC) steps (1T1→3T1→5T2), resulting in the
population of the metastable 5T2 (HS) state.91 The energy barrier between the two
states (1A1 and 5T2) is large enough to ‘trap’ the system in the HS state at low
temperatures.108 This phenomenon is known as light induced excited spin state trapping or LIESST and has been investigated for a wide variety of FeII complexes.110,111,112 The LIESST behaviour is only observed at very low temperatures because above a certain critical temperature defined as T(LIESST), the system clears
the energy barrier between the two spin states and relaxes to the LS state.112
An example of a FeII compound that displays LIESST behaviour is the highly cited [Fe(PM-BiA)2(NCS)2] (PM-BiA = N-(2-pyridylmethylene)aminobiphenyl)
complex synthesised by Kahn and his co-workers (Figure 1.14b).113 This compound
has a T(LIESST) of 80 K and was the first to display a phenomenon known as light
21 dependence of the magnetic susceptibility is not the same in the cooling and warming modes when the compound is irradiated at temperatures below T(LIESST).113