Light-induced Spin Transition Research Articles

Kinetics of photo-induced phase transition and relaxation in the spin-crossover complexes [FexZn1-x(phen)2(NCS)2], influence of metal dilution

The kinetics of light-induced spin transition and relaxation in [FexZn1-x(phen)2(NCS)2] (phen=1,10-phenanthroline) has been investigated from time/temperature dependent x-ray powder diffraction with in situ optical excitation. We show that the phase transformation is driven by a heterogeneous nucleation and growth mechanism with phase separation, in both the thermally induced and light-induced regimes. The high spin to low spin isothermal relaxation curves strongly differ from first-order kinetics, and are interpreted using the Kolmogorov-Johnson-Mehl-Avrami model of phase transformation, from which the activation energy to domain growth is derived. Dilution tends to increase the activation energy, and slows down the whole relaxation process. Non-linear photo-induced kinetics as a function of laser power is evidenced, resulting from a light-induced phase separation process; a laser power threshold effect is pointed out. It is found that dilution speeds up the photo-conversion kinetics and reduces the laser power threshold.

The light-induced spin transition of tetranuclear spin crossover complex [Fe4(CN)4(bpy)4(tpa)2](PF6)4

We report on the light induced spin transition in the tetranuclear spin crossover complex [Fe4(CN)4(bpy)4(tpa)2](PF6)4. The photo-conversion occurs at the specific site (Fe2) of four FeII ions. The red light irradiation (1.79 eV) gives rise to full conversion of Fe2 into the high spin state from the low spin state. The green light irradiation (2.33 eV) can convert only the half of Fe2 into the high spin state, though the photo-conversion rate in the beginning is much higher than that with the red light. We present a simple model in which the photo-conversion kinetics is controlled by a large background absorption due to remaining three FeII ions (Fe1, Fe3 and Fe4).

Complete temperature study of the relaxation from HS to LS state in the mixed [FexZn1-x(Phen)2(NCS)2] systems (with x = 1, 0.73, 0.5, 0.32, 0.19 and 0.04)

The thermal and light-induced spin transitions in [FexZn1-x(Phen)2(NCS)2] (Phen = 1,10-phenantholine) have been investigated by magnetic susceptibility, photomagnetism and diffuse reflectivity measurements. These complexes display a thermal spin transition and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. For each compound, the thermal spin transition temperature, T1/2, and the relaxation temperature of the photo-induced high-spin state, T(LIESST), have been systematically determined. It appears that T1/2 decreases with the metal dilution while T(LIESST) remains unchanged. This behaviour is discussed on the basis of the kinetic study performed by combining the use of SQUID magnetometer, up to the T(LIESST) value, and the dynamic diffuse reflectivity setup, from the T(LIESST) temperature up to the thermal spin transition.

Thermal and light-induced spin-transitions in iron(ii) complexes of 2,6-bis(4-halopyrazolyl)pyridines: the influence of polymorphism on a spin-crossover compound

The syntheses of 2,6-bis(4-chloropyrazol-1-yl)pyridine (L1), 2,6-bis(4-bromopyrazol-1-yl)pyridine (L2) and 2,6-bis(4-iodopyrazol-1-yl)pyridine (L3) by electrophilic halogenation of 2,6-bis(pyrazol-1-yl)pyridine are reported. The complex [Fe(L1)2][BF4]2 crystallises in two different solvent-free polymorphs. The tetragonal (alpha) form crystallises in a known version of the "terpyridine embrace" structure, and undergoes an abrupt spin-transition at 202 K. The orthorhombic (beta) form exhibits a modified form of the same packing motif, containing two unique iron sites in a 2 : 1 ratio. One-third of the complex molecules in that material undergo a very gradual thermal spin-crossover centred at 137 K. Comparison of the two structures implies that spin-crossover cooperativity in the alpha-polymorph is transmitted in two dimensions within the extended lattice. [Fe(L2)2][BF4]2 is isostructural with alpha-[Fe(L1)2][BF4]2 and exhibits a similarly abrupt spin-transition at 253 K. In contrast, [Fe(L3)2][BF4]2 is low-spin as a powder at 360 K and below and can be crystallised as two different solvates from acetone solution. All three compounds exhibit the LIESST effect at 10 K, with photoconversions of 40-100%. Their LIESST relaxation temperatures obey the empirical T(LIESST) = T0- 0.3T(1/2) (T0 = 150 K) law that we have previously proposed for this class of compound.

Light-Induced Excited Spin State Trapping: Ab Initio Study of the Physics at the Molecular Level

This paper provides a qualitative analysis of the physical content of the low-energy states of a spin-transition compound presenting a light-induced excited spin state trapping (LIESST) phenomenon, namely, [Fe(dipyrazolpyridine)2](BF4)2, which has been studied using the wave function-based CASPT2 method. Both the nature of the low-energy states and the relative position of their potential energy wells as a function of the geometry are rationalized from the analysis of the different wave functions. It is shown that the light-induced spin transition occurring in such systems could follow several pathways involving different excited spin states. In an ideal octahedral geometry, the interconversion from the excited singlet state to the triplet of lower energy, which is usually seen as an intermediate state in the LIESST mechanism, is quite unlikely since there is no crossing between the potential energy curves of these two states. On the contrary, in lower-symmetry complexes, the geometrical distortion of the coordination sphere due to ligand constraints is responsible for the occurrence of a crossing between these two states in the Franck-Condon region, leading to a possible participation of this triplet state in the LIESST mechanism. In the reverse LIESST process, a crossing between the potential energy curves of another triplet state and the excited quintet state occurs in the Franck-Condon region as well.

Effect of the metal dilution on the thermal and light-induced spin transition in [Fe xMn 1− x(bpp) 2](NCSe) 2: When T(LIESST) reaches T1/2

The thermal and light-induced spin transition in [Fe x Mn 1− x (bpp) 2](NCSe) 2 (bpp = 2,6-bis(pyrazol-3-yl)pyridine) has been investigated by magnetic susceptibility, photomagnetism and diffuse reflectivity measurements. These complexes display a thermal spin transition and exhibit the light-induced excited spin state trapping (LIESST) effect at low temperature. For each mixed-crystal system, the thermal spin transition temperature, T 1/2, and the relaxation temperature of the photo-induced high-spin state, T(LIESST), have been systematically determined. It appears that T 1/2 decreases with the metal dilution while T(LIESST) remains unchanged, suggesting that the two interconversion processes are controlled by different factors; i.e. the photomagnetic properties are governed at the molecular scale and the thermal spin crossover regime is affected by both the ligand field and crystal packing effects. For highly metal-diluted complex with x < 0.2, it is found that when T 1/2 reaches the T(LIESST), the complex remains HS on the whole range of temperature.

The interaction between the spin transition and a crystallographic phase transition in the spin-crossover compound [Fe(bbtr) 3](ClO 4) 2: Nucleation, formation of domains and fluctuations

The thermal and the light-induced spin transition in [Fe(bbtr) 3](ClO 4) 2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)) as well as the high-spin → low-spin relaxation following the light-induced population of the high-spin state below the thermal transition temperature are discussed in relation to the accompanying crystallographic phase transition. The experimental data have exclusively been obtained using optical single crystal absorption spectroscopy.

Effect of metal dilution on the light-induced spin transition in [FexZn1−x(phen)2(NCS)2] (phen = 1,10-phenanthroline)

The thermal and light-induced spin transitions in [Fe(x)Zn(1-x)(phen)2(NCS)2] (phen = 1,10-phenantholine) have been investigated by magnetic susceptibility, photomagnetism and diffuse reflectivity measurements. These complexes display a thermal spin transition and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. For each compound, the thermal spin transition temperature, T1/2, and the relaxation temperature of the photo-induced high-spin state, T(LIESST), have been systematically determined. It appears that T1/2 decreases with the metal dilution while T(LIESST) remains unchanged. This behaviour is discussed on the basis of the kinetic study governing the photo-induced back conversion.

Kinetics of light-induced first-order phase transformation in molecular solids:Fe(btr)2(NCS)2∙H2O

The mechanism and kinetics of the thermally and light-induced spin transition of the highly cooperative spin crossover material $\mathrm{Fe}(\mathrm{btr}{)}_{2}(\mathrm{NCS}{)}_{2}∙{\mathrm{H}}_{2}\mathrm{O}$ ($\mathrm{btr}=4,4\mathrm{\ensuremath{'}}$--bis--1,2,4--triazole) have been investigated by single crystal x-ray diffraction techniques. The key role of like-spin domains is quantitatively analyzed through a nucleation, growing, and coarsening mechanism whose kinetics follow the Kolmogorov-Johnson-Mehl-Avrami model with low dimensional characteristics.

A novel Fe(II) framework structure: correlations to its thermal and light induced spin transitions

A novel Fe(II) framework structure: correlations to its thermal and light induced spin transitions

Direct observation of light induced spin transitions in new 3,5-bis(2-pyridyl)pyrazolato bridged thiocyanato diiron(II) complexes by monitoring variable temperature laser Raman spectra.

The light induced excited spin state trapping LIESST and the reverse LIESST-like phenomena are observed below and above the spin transition temperature, respectively, for new dinuclear diiron(II) complexes and familiar Fe(II) complexes with thiocyanate ligands by monitoring the Raman spectra where only excitation light of various wavelengths for the spectroscopy was used without extra excitation light sources.

On the nature of metastable high-spin states formed during spin transitions in iron(II) complexes

The metastable high-spin (HS) state of iron is formed and fixed in iron(II) complexes during irradiation of a low-spin (LS) sample at low temperatures (~15 K) with green laser light at a wavelength of 514.5 nm [1]. This phenomenon was studied further in [2-4] (see also references in the above papers). In particular, it was shown that the metastable HS form is transformed back to the LS form by irradiation of crystals with red laser light at a wavelength of 752.7 nm. This effect may be of interest for molecular electronics, because molecular systems reversibly changing their state during irradiation with laser light may be used as optical keys in devices for processing and storage of optical data. M/Sssbauer spectroscopy is an essential tool for investigating light-induced spin transitions. The main results obtained by this method are the ratios between the LS and HS forms of iron, determined in various experimental conditions. The aim of this work is to draw attention to a new possibility offered by this method in studying the nature of the metastable HS state of Fe(II) formed in light-irradiated samples. Formulation of the problem will ftrst demand several explanations concerning the nature of the HS and LS forms of iron that are simultaneously observed in a particular temperature range during sharp spin transitions (ST) caused by temperature variations. In some papers (e.g., review [5]), it was shown that the HS form, which appears at increased temperatures, as well as the LS form, which appears during cooling, exist in crystals in the form of domains, i.e., certain regions in which all iron atoms are in the same spin state. An alternative form is chaotic distribution of iron atoms, which do not create regular regions. The domain structure of the LS and HS forms during ST is proved by hysteresis on the curves of the temperature dependences of the effective magnetic moment of the complex [5]. Let us consider the crystal in which the HS form appears during irradiation with light. It would be reasonable to ask whether this state exists in the crystal as dom~in~ or individual paramagnetic centers. The idea of using MiSssbauer spectroscopy for solving this problem is based oft the following fact. The HS state appears in the matrix of the LS state, in which iron atoms participate in cooperative interactions; hence the reaction of the atoms of the LS state to an admixture of the metastable HS state will depend on whether the atoms of the HS state form domains or individual paramagnefic centers in the low-spin matrix. We can assume that in the case of domains their influence on the structure of the LS state will be negligible due to the relatively low content of the neighboring pairs of atoms with different values of spins. The case is different when the HS centers are chaotically distribfited in the LS matrix. The changes in the spin states of iron atoms as a result of irradiation with light can lead to changes in the electronic states of the neighboring iron atoms of the LS matrix because of considerable differences in the metal-ligand distances of the LS and HS forms [7] and due to cooperative interactions in the LS matrix. The effect of the "large" paramagnetic admixture on the structure of the LS matrix will be more simaiflcant for crystals with strong cooperative interactions and for crystals with higher concentrations of admixtures. The "large" paramagnetic admixture loosens the structure of the LS matrix and decreases the covalence of the metal-ligand bond of the LS form, thus increasing the chemical shift c3LS of the M/Sssbauer line for the I~ form. Recently, we reported on a similar effect in [6], in which we studied the influence of iron substitution by zinc on the ST in the polynuclear Fe(ATr)3(NO3) 2 complex (ATr = 4-amino-l,2,4triazole). We found that the chemical shift of the LS form of the complex measured at 78 K monotonically increases with the concentration of zinc atoms. Tiffs effect was explained in terms of the model of steric strai-~ arising in crystals

Thermal and Light-Induced Spin Transition in the High- and Low-Temperature Structure of [Fe(0.35)Ni(0.65)(mtz)(6)](ClO(4))(2).

The thermal and light induced spin transition in [Fe(0.35)Ni(0.65)(mtz)(6)](ClO(4))(2) (mtz = 1-methyl-1H-tetrazole) was studied by (57)Fe Mössbauer spectroscopy and magnetic susceptibility measurements. In addition to the spin transition of the iron(II) complexes the compound undergoes a structural phase transition. The high-temperature structure could be determined by X-ray crystallography of the isomorphous [Fe(0.25)Ni(0.75)(mtz)(6)](ClO(4))(2) complex at room temperature. The X-ray structural analysis shows this complex to be rhombohedric, space group R&thremacr;, with a = 10.865(2) Å and c = 23.65(1) Å with three molecules in the unit cell. The transition to the low-temperature structure occurs at approximately 60 K without changing the spin state of the molecules. By subsequent heating of the complex the high-temperature structure is reached again between ca. 170 and 200 K. The spin transition behavior is strongly influenced by the structural changes, and the observed spin transition curves are completely different for the high- and low-temperature phases. In the high-temperature structure a complete and gradual spin transition between 220 and 120 K (T(1/2)(gamma(HS) = 0.5) = 185 K) is detected; the high-spin (HS) state is represented by one HS doublet in the Mössbauer spectra. In the low-temperature structure a two-step transition curve is detected in the heating mode. About 36% of the molecules show a LS (low-spin) --> HS transition between ca 50 and 75 K. Then the HS fraction stays constant up to 150 K. A further increase in the high-spin fraction is observed at temperatures above 150 K. In this structural phase the HS state is represented by two different HS doublets in the Mössbauer spectra. The formation of metastable HS states by making use of the LIESST effect is only possible in the low-temperature structure. By excitation of the LS molecules with green light, two different HS states are populated which show very different relaxation behavior. One HS state shows a relaxation to the LS state even at 10 K; the other HS state shows a very slow HS --> LS relaxation at 60 K (within days), leading to the HS fraction corresponding to the thermal equilibrium value.

Effect of metal dilution on the thermal and light-induced spin transition in [FexZn1 –x(mtz)6][ClO4]2(mtz = 1-methyl-1H-tetrazole)

Download options Please wait... Article information DOI https://doi.org/10.1039/DT9940002285 Article type Paper Download Citation J. Chem. Soc., Dalton Trans., 1994, 2285-2288 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Effect of metal dilution on the thermal and light-induced spin transition in [FexZn1 –x(mtz)6][ClO4]2(mtz = 1-methyl-1H-tetrazole) T. Buchen, P. Poganiuch and P. Gütlich, J. Chem. Soc., Dalton Trans., 1994, 2285 DOI: 10.1039/DT9940002285 To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Search articles by author Thomas Buchen Peter Poganiuch Philipp Gütlich

Structures of hexakis(1-propyltetrazole)iron(II) bis(tetrafluoroborate), [Fe(CHN4C3H7)6](BF4)2, hexakis(1-methyltetrazole)iron(II) bis(tetrafluoroborate), [Fe(CHN4CH3)6](BF4)2, and the analogous perchlorates. Their relation to spin crossover behaviour and comparison of Debye–Waller factors from structure determination and Mössbauer spectroscopy

Single-crystal structure determinations of the spin crossover compounds [Fe(ptz) 6 ]X 2 , and [Fe(mtz) 6 ]X 2 (ptz=C 3 H 7 N 4 CH, mtz=CH 3 N 4 CH, X=BF 4 , ClO 4 ) have been performed at different temperatures. The temperature dependence of the lattice constants was measured for the two mtz compounds between 300 and 110 K. The two different structure types for the ptz and mtz compounds are compared and discussed in relation to the physical properties associated with the temperature-induced and light-induced spin transitions. In particular, the atomic displacement parameters for the two inequivalent Fe complexes in the mtz structure were found to be significantly different

Calculation of the lattice deformation at the phase transitions of [Fe(ptz)6](BF4)2from powder diffraction patterns

The thermal and light-induced high-spin (HS: 5T2g) low-spin (LS: 1A1g) transitions of [Fe(ptz)6](BF4)2 (ptz = 1-propyltetrazole) are investigated by X-ray powder diffractometry in the range between ambient temperature and ~15 K. The anisotropic lattice deformation caused by variation of the HS fraction and by temperature change was calculated directly from the differences in peak positions of the powder diagrams measured at different temperatures. An analytical expression for the dependence of peak shifts and splittings on the deformation tensor elements was derived. The differences of the lattice deformations accompanying the thermal and light-induced spin transitions are discussed.

Thermal- and light-induced spin transition in [Fe(mtz)6](BF4)2: first successful formation of a metastable low-spin state by irradiation with light at low temperatures

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTThermal- and light-induced spin transition in [Fe(mtz)6](BF4)2: first successful formation of a metastable low-spin state by irradiation with light at low temperaturesP. Poganiuch, S. Decurtins, and P. GuetlichCite this: J. Am. Chem. Soc. 1990, 112, 9, 3270–3278Publication Date (Print):April 1, 1990Publication History Published online1 May 2002Published inissue 1 April 1990https://doi.org/10.1021/ja00165a003RIGHTS & PERMISSIONSArticle Views511Altmetric-Citations122LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (979 KB) Get e-Alerts Get e-Alerts