A critical review of the T(LIESST) temperature in spin crossover materials − What it is and what it is not

Abstract

Light-Induced Excited Spin-State Trapping has been studied since 1982 in solution and 1984 in solid state as it offers a reversible way of photoswitching the electronic configuration of spin crossover systems. Since then, the lifetime of the photo-induced state was deeply investigated through kinetics measurements. In 1998, a fast and easy way to record the limit temperature above which the photo-induced state is erased, denoted T(LIESST), was introduced. This procedure has been widely used in the spin crossover community due to its easiness and its efficiency to provide detailed information on the photo-induced state. Correlations between T(LIESST) and structural parameters have been proposed for instance. However, it intrinsically contains drawbacks that can lead to misinterpretation of behaviours and can lead to an over-estimation of its scope. This review aims to present and discuss not only the correct way to measure T(LIESST) but also the essential contributions it has brought and the limits not to be exceeded in its interpretation.

Spatio-temporal Investigations of the Incomplete Spin Transition in a Single Crystal of [Fe(2-pytrz)2{Pt(CN)4}]·3H2O: Experiment and Theory

“…Indeed, the distortion experienced by one of the iron sites (see Figure 1b), originating from an elastic frustration occurring inside the lattice, induces the trapping of Fe1 in the HS state, thus precluding its transition at low-temperature. In addition to the thermal hysteresis and the incomplete HS to HS-LS spin transition, Figure 1 reports the photomagnetic response of this compound under green light (λ =510 nm) showing a clear evidence of a LIESST (Light-Excited-Spin-State-Trapping) [45][46][47][48] effect allowing to access metastable photo-induced HS state at low-temperature. The resultant irradiated HS state relaxes back to the intermediate HS-LS state around ∼52 K. On the other hand, the excitation of the intermediate HS-LS low-temperature phase using a red wavelength (λ =830 nm) produced the reverse-LIESST effect [45,[47][48][49], leading to the fully "stable" LS state, whose thermal relaxation to the intermediate HS-LS phase takes place at T 90 K. The investigations of the dynamical properties of this LS state allowed in a previous study [45] to demonstrate its stability at very low-temperature.…”

Thermal and Light‐Activated Spin Crossover in Iron(III) qnal Complexes

“…The photomagnetic properties were recorded using an optical fibre connected on one side to a set a photodiodes and irradiating the sample inside the magnetometer cavity on the other side. The irradiation intensity was set to prevent any consequent warming effect . The surface reflectivity measurements were performed with a home‐built system at the Centre de Recherche Paul Pascal, operating between 10 and 300 K and in a spectrometric range from 400–1000 nm.…”

“…80 K { T (LIESST) being the relaxation temperature above which the photo‐induced state is erased upon warming at ca. 0.4 K min –1 } , , . Recent work by some of us has shown how light reveals a large hidden hysteresis in [Fe(naphBzen) 2 ]I (naphBzen = 1‐{[2‐(benzylamino)ethylimino]methyl}‐2‐naphtholate) …”

“…In these complexes, there are two possible spin states: high spin (HS, maximum number of unpaired spins) and low spin (LS, minimum number of unpaired spins). Provided that the ligand field is suitable so both spin states can be accessed, they can be switched by changes in external stimuli such as temperature,, pressure, light, or application of a magnetic field. The switching may occur through a Boltzmann equilibrium over a broad temperature range or over just a few Kelvin in the case of an entropically driven transition.…”

Solvent Effects on the Spin Crossover Properties of Iron(II) Imidazolylimine Complexes

“…SCO in Fe(II) complexes with an octahedral geometry is dominant as it transforms a diamagnetic LS state (S = 0) to a paramagnetic HS state (S = 2) with a clear change in colour and lengthening of the Fe(II)-ligands bond distances [3][4][5][6][7]. Materials displaying SCO behaviour continue to be intensively studied due to their potential use as active components in memory, display and sensing devices [8][9][10] particularly photo-induced SCO complexes, or light-induced spin state trapping (LIESST) [11,12] which has been reported in many Fe(II) SCO systems.…”