A feasibility study on obtaining <Emphasis Type="Italic">d</Emphasis>-orbital populations from aspherical-atom refinements on three spin crossover compounds

Although compounds undergoing thermally induced spin crossover have been widely studied, their experimental d-orbital populations from single-crystal X-ray diffraction have rarely been reported. Three pairs of structures of iron/manganese coordination compounds were re-evaluated. Least-squares refinements relied on aspherical scattering factors obtained from molecular quantum-mechanical DFT single-point computations of the respective solid-state conformation, initiated by accurate starting structures from preliminary invariom refinements of the ligand environment. Further evaluation concerned d-orbital populations of metal ions from (a) single-point computations projected onto the Hansen-Coppens multipole model and from (b) experimental refinements of the metal atoms only. The latter were successful for good-quality data, independent of temperature, and provided only one spin state was exclusively present in the crystal. Crystals that underwent light-induced excited spin state trapping were not showing the expected d-orbital populations.     

Ground- and excited-state electronic structure of an iron-containing molecular spin photoswitch

The electronic structure of the cation of [Fe(ptz)(6)](BF(4))(2), a prototype of a class of complexes that display light-induced excited-state spin trapping (LIESST), has been investigated by time-independent and time-dependent density-functional theories. The density of states of the singlet ground state reveals that the highest occupied orbitals are metal centered and give rise to a low spin configuration Fe(2+)(3d(xy) ( upward arrow downward arrow)3d(xz) ( upward arrow downward arrow)3d(yz) ( upward arrow downward arrow)) in agreement with experiment. Upon excitation with light in the 2.3-3.3 eV range, metal-centered spin-allowed but parity-forbidden ligand field (LF) antibonding states are populated which, in conjunction with electron-phonon coupling, explain the experimental absorption intensities. The computed excitation energies are in excellent agreement with experiment. Contrary to simpler models we show that the LF absorption bands, which are important for LIESST, do not originate in transitions from the ground to a single excited state but from transitions to manifolds of nearly degenerate excited singlets. Consistent with crystallography, population of the LF states promotes a drastic dilation of the ligand cage surrounding the iron.     

Theoretical study of low-spin, high-spin, and intermediate-spin states of [Fe(III)(pap)2](+) (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato). Mechanism of light-induced excited spin state trapping

The mechanism of light-induced excited spin state trapping (LIESST) of [FeIII(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) was discussed on the basis of potential energy surfaces (PESs) of several important spin states, where the PESs were evaluated with the DFT(B3LYP) method. The PES of the quartet spin state crosses those of the doublet and sextet spin states around its minimum. This means that the spin transition occurs from the quartet spin state to either the doublet spin state or the sextet spin state around the PES minimum of the quartet spin state. The PES minimum of the sextet spin state is slightly less stable than that of the doublet spin state by 0.18 eV (4.2 kcal/mol). This small energy difference is favorable for the LIESST. The doublet-sextet spin crossover point is 0.41 eV (9.6 kcal/mol) above the PES minimum of the sextet spin state. Because of this considerably large activation barrier, the thermal spin transition and the tunneling process do not occur easily. In the doublet spin state, the ligand to ligand charge transfer (LLCT) transition is calculated to be 2.16 eV with the TD-DFT(B3LYP) method, in which the pi orbital of the phenoxy moiety and the pi* orbital of the imine moiety in the pap ligand participate. This transition energy is moderately smaller than the visible light of 550 nm used experimentally. In the sextet spin state, the ligand to metal charge transfer (LMCT) transition is calculated to be at 2.36 eV, which is moderately higher than the visible light (550 nm). These results indicate that the irradiation of the visible light induces the LIESST to generate the sextet spin state but the reverse-LIESST is also somewhat induced by the visible light, indicating that the complete spin conversion from the doublet spin state to the sextet one does not occur, as reported experimentally.     

Light-induced excited spin-state trapping in spin crossover model system

We computationally investigate the light-induced excited spin-state trapping (LIESST) in a spin crossover (SCO) model system, derived out of [Fe(abpt)₂(NCS)₂] consisting of Fe(II) SCO center coordinated by bidenate as well as monodentate ligands. For this purpose, we use two complementary techniques: (a) time-dependent density functional theory (TDDFT) with the choice of different exchange-correlation functional and (b) multireference approach of complete active space self-consistent field and complete active-space second-order perturbation (CASPT2) theory. We calculate the potential energy curves (PECs) of low-energy states, as well as spin-orbit couplings at crossing points of these PECs. Inputting these pieces of information, and the information related to nuclear degrees of freedom within the Franck-Condon theory, we compute the relaxation rates of possible LIESST mechanisms, as suggested by the two approaches. Based on our findings, we conclude that TDDFT may not be an unreasonable approach to estimate the relaxation rates of real complexes, consisting of several tens to several hundreds of atoms, given its computationally inexpensive nature compared with that of the multireference approaches.     

Light-induced excited-state spin trapping in tetrazole-based spin crossover systems

Ab initio calculations have been performed on Fe (II) (tz) 6 (tz = 1- H-tetrazole) to establish the variation of the energy of the electronic states relevant to (reverse) light-induced excited-state spin trapping (LIESST) as function of the Fe-ligand distance. Equilibrium distances and absorption energies are correctly reproduced. The deactivation of the excited singlet is proposed to occur in the Franck-Condon region through overlap of vibrational states with an intermediate triplet state or an intersystem crossing along an asymmetric vibrational mode. This is followed by an intersystem crossing with the quintet state. Reverse LIESST involves a quintet-triplet and a triplet-singlet intersystem crossing around the equilibrium distance of the high-spin state. The influence of the transition metal is studied by changing Fe (II) for Co (II), Co (III), and Fe (III). The calculated curves for Fe (III) show remarkable similarity with Fe (II), indicating that the LIESST mechanism is based on the same electronic conversions in both systems.     

Computational modeling of LD LISC and LIESST rearrangements of a Fe(II) complex with phenanthroline modified by photochromic chromene

Quantum-chemical study (B3LYP*/6-311++G(d,p)) has shown that the Fe(II) complex containing the phenanthroline ligand with annelated chromene and dihydrobis(pyrazol-1-yl)borate anions is able to exhibit thermally induced spin crossover. The calculations predict that the magnetic properties of this compound can be controlled by irradiation (LIESST and LD LISC effects).     

Deposition of the Spin Crossover FeII–Pyrazolylborate Complex on Au(111) Surface at the Molecular Level

The interaction at the molecular level of the spin-crossover (SCO) Fe^II((3,5-(CH₃)₂Pz)₃BH)₂ complex with the Au(111) surface is analyzed by means of rPBE periodic calculations. Our results show that the adsorption on the metallic surface enhances the transition energy, increasing the relative stability of the low spin (LS) state. The interaction indeed is spin-dependent, stronger for the low spin than the high spin (HS) state. The different strength of the Fe ligand field at low and high temperature manifests on the nature, spatial extension and relative energy of the states close to the Fermi level, with a larger metal–ligand hybridization in the LS state. This feature is of relevance for the differential adsorption of the LS and HS molecules, the spin-dependent conductance, and for the differences found in the corresponding STM images, correctly reproduced from the density of states provided by the rPBE calculations. It is expected that this spin dependence will be a general feature of the SCO molecule–substrate interaction, since it is rooted in the different ligand field of Fe site at low and high temperatures, a common hallmark of the Fe^II SCO complexes. Finally, the states involved in the LIESST phenomenon has been identified through NEVPT2 calculations on a model reaction path. A tentative pathway for the photoinduced LS→HS transition is proposed, that does not involve the intermediate triplet states, and nicely reproduces both the blue laser wavelength required for the activation, and the wavelength of the reverse HS → LS transition.     

Photomagnetic properties of iron(II) spin crossover complexes of 2,6-dipyrazolylpyridine and 2,6-dipyrazolylpyrazine ligands

The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2, [Fe(L2)2][BF4]2, [Fe(L2)2][ClO4]2, [Fe(L3)2][BF4]2, [Fe(L3)2][ClO4]2 and [Fe(L4)2][ClO4]2 (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds display a complete thermal spin transition centred between 200-300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS --> LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T(1/2): T(LIESST) = T(0)- 0.3T(1/2). T(0) for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms, the light-induced high-spin state, and the low-spin complex [Fe(L4)2][BF4]2, are described.     

Photomagnetism of a sym-cis-dithiocyanato iron(II) complex with a tetradentate N,N'-bis(2-pyridylmethyl)1,2-ethanediamine ligand

A comprehensive study of the magnetic and photomagnetic behaviors of cis-[Fe(picen)(NCS)(2) ] (picen = N,N'-bis(2-pyridylmethyl)1,2-ethanediamine) was carried out. The spin-equilibration was extremely slow in the vicinity of the thermal spin-transition. When the cooling speed was slower than 0.1?K min(-1), this complex was characterized by an abrupt thermal spin-transition at about 70?K. Measurement of the kinetics in the range 60-70?K was performed to approach the quasi-static hysteresis loop. At low temperatures, the metastable HS state was quenched by a rapid freezing process and the critical T(TIESST) temperature, which was associated with the thermally induced excited spin-state-trapping (TIESST) effect, was measured. At 10?K, this complex also exhibited the well-known light-induced excited spin-state-trapping (LIESST) effect and the T(LIESST) temperature was determined. The kinetics of the metastable HS states, which were generated from the freezing effect and from the light-induced excitation, was studied. Single-crystal X-ray diffraction as a function of speed-cooling and light conditions at 30?K revealed the mechanism of the spin-crossover in this complex as well as some direct relationships between its structural properties and its spin state. This spin-crossover (SCO) material represents a fascinating example in which the metastability of the HS state is in close vicinity to the thermal spin-transition region. Moreover, it is a beautiful example of a complex in which the metastable HS states can be generated, and then compared, either by the freezing effect or by the LIESST effect.     

Photomagnetic properties in a series of spin crossover compounds

The spin crossover (SC) compounds [Fe(PM-AzA)2(NCX)2] and [Fe(PM-FIA)2(NCX)2] (with PM-AzA = N-2'-pyridylmethylene-4-(phe-nylazo)aniline, PM-FIA = N-2'-pyridylmethylene-4-(2-amino)fluorene, and X = S, Se) have been prepared. The SC regimes have been deduced from variable-temperature magnetic susceptibility data. The enthalpy and entropy changes associated with the SC have been evaluated from DSC measurements. A cooperativity factor, C, has been defined, and its values for the different compounds have been deduced from the spin crossover curves. At 10 K, the light-induced excited spin state trapping (LIESST) effect has been observed within the cavity of the SQUID magnetometer. The critical temperatures Tc(LIESST) have been determined for [Fe(PM-AzA)2(NCS)2] and [Fe(PM-F1A)2(NCX)2], and the role of cooperativity has been analyzed. A linear correlation has been found between the Tc(LIESST) and C values. The kinetics of HS-->LS relaxation have been investigated; a thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures have been found. Finally, the magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed the occurrence of the light-induced thermal hysteresis (LITH) effect.     

The Role of Ligand‐Field States in the Ultrafast Photophysical Cycle of the Prototypical Iron(II) Spin‐Crossover Compound [Fe(ptz)6](BF4)2

Light‐induced excited spin‐state trapping (LIESST) in iron(II) spin‐crossover compounds, that is, the light‐induced population of the high‐spin (S=2) state below the thermal transition temperature, was discovered thirty years ago. For irradiation into metal–ligand charge transfer (MLCT) bands of the low‐spin (S=0) species the acknowledged sequence takes the system from the initially excited ¹MLCT to the high‐spin state via the ³MLCT state within ca. 150 fs, thereby bypassing low‐lying ligand‐field (LF) states. Nevertheless, these play a role, as borne out by the observation of LIESST and reverse‐LIESST on irradiation directly into the LF bands for systems with only high‐energy MLCT states. Herein we elucidate the ultrafast reverse‐LIESST pathway by identifying the lowest energy S=1 LF state as an intermediate state with a lifetime of 39 ps for the light‐induced high‐spin to low‐spin conversion on irradiation into the spin‐allowed LF transition of the high‐spin species in the NIR.     

Effect of metal dilution on the light-induced spin transition in [Fe(x)Zn(1-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.     

Guidelines to design new spin crossover materials

This review focuses on new families of spin crossover (SCO) complexes based on polynitrile anions as new anionic ligands or on polyazamacrocycles as neutral macrocyclic ligands. We have shown that the structural and electronic characteristics (original coordination modes and high electronic delocalization) of the polynitrile anions can be tuned by slight chemical modifications such as substitution of functional groups or variation of the negative charge to design new discrete or polymeric SCO systems.In our ongoing work on the design of new molecular systems based on new ligands that can be fine-tuned via chemical modifications, another promising way which has been recently developed in our group concerns the use of new neutral polydentate ligands which are able to tune the ligand field energy around the metal centre. Here we report some recent original Fe(II) SCO complexes based on such polydentate ligands.     

Metastable states at low temperature in spin crossover compounds in the framework of the atom-phonon coupling model

In the current study we report on the unusual results obtained in the framework of the atom-phonon coupling model: two metastables states at low temperatures. These results can explain the experimental results reported by Ould Moussa et al. [9] and Matsumoto et al. [10]. This type of SCO behavior is due to the competition between the Light-Induced Excited Spin State Trapping (LIESST) effect, the elastic interaction between the molecules and the thermal activation. In order to determine the stability of the states founded at low temperature, the free energy F is presented for the case of the two step spin transition of a SCO compound.     

First observation of light-induced spin change in vacuum deposited thin films of iron spin crossover complexes

Thin films of [Fe(H(2)Bpz)(2)(phen)] (1) and [Fe(H(2)Bpz)(2)(bipy)] (2) are prepared by vacuum deposition and investigated with respect to their spin crossover behaviour. For the first time light-induced excited spin state trapping (LIESST) is observed in such systems. T(1/2) and T(LIESST) in the films are in agreement with the bulk values.     

Synthesis, magnetic and photomagnetic study of new iron(II) spin-crossover complexes with N₄O₂ coordination sphere

A new family of neutral mononuclear iron(II) spin crossover (SCO) compounds, Fe(L1??)? (L1?? = N'-((pyridin-2-yl)methylene)benzohydrazide (HL1), N'-(1-(pyridin-2-yl)ethylidene)-benzohydrazide (HL2), N'-(phenyl(pyridin-2-yl)methylene)benzohydrazide (HL3), 2-hydroxy-N'-((pyridin-2-yl)methylene)benzohydrazide (HL?), 2-hydroxy-N'-(1-(pyridin-2-yl)ethylidene)benzohydrazide (HL?), 2-hydroxy-N'-(phenyl(pyridin-2-yl)methylene)benzohydrazide (HL?)) with N?O? donor sets have been synthesized from series tridentate Schiff base ligands with N,N,O donor sets. The investigation of magnetic properties of these compounds reveal that in the measured temperature range, compound 1 is in the high-spin (HS) state, and compound 3 and 6 are mainly in the low-spin (LS) state, whereas the other compounds exhibit various SCO properties: compound 2 undergoes a gradual incomplete SCO with characteristic temperature T(1/2) higher than 350 K; compound 4 exhibits a special stepwise thermally induced SCO occurring at ~150 K (smooth) and 200 K (two-steps, with T(S1↑/↓) = 204/202 K and T(S2↑/↓) = 227/219 K) with a mixture of the HS and LS states yielded below 100 K; compound 5 shows a gradual and complete LS?HS SCO with characteristic temperature T(1/2) = 273 K. All the three SCO compounds show the LIESST (light induced exited spin state trapping) effect with different levels of photoconversion. To thoroughly analyze these behaviours, M?ssbauer spectra and DSC of 4 and 5, crystal structures of all the compounds at 290 K and 5 in the LS state at 110 K were carried out, which confirmed the structural changes accompanying the spin transition. In addition, alkyl substitution effect on the ligand field was suggested for this system.     

Quantum-chemical modeling of metal coordination compounds with photoswitchable magnetic properties controlled by ligand rearrangements

The complexation of chromenes with transition metal ions was studied using the B3LYP*/6-311++G(d,p) method. We are the first to show that transformation of the spin state of a transition metal complex through an LD LISC mechanism can be achieved by means of configurational isomerization of the metal-chelate coordination site induced by ligand rearrangement.     

Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex

Crystalline [Fe(bppSMe)₂][BF₄]₂ ( 1 ; bppSMe=4‐(methylsulfanyl)‐2,6‐di(pyrazol‐1‐yl)pyridine) undergoes an abrupt spin‐crossover (SCO) event at 265±5 K. The crystals also undergo a separate phase transition near 205 K, involving a contraction of the unit‐cell a axis to one‐third of its original value (high‐temperature phase 1; Pbcn, Z=12; low‐temperature phase 2; Pbcn, Z=4). The SCO‐active phase 1 contains two unique molecular environments, one of which appears to undergo SCO more gradually than the other. In contrast, powder samples of 1 retain phase 1 between 140–300 K, although their SCO behaviour is essentially identical to the single crystals. The compounds [Fe(bppBr)₂][BF₄]₂ ( 2 ; bppBr=4‐bromo‐2,6‐di(pyrazol‐1‐yl)pyridine) and [Fe(bppI)₂][BF₄]₂ ( 3 ; bppI=4‐iodo‐2,6‐di(pyrazol‐1‐yl)‐pyridine) exhibit more gradual SCO near room temperature, and adopt phase 2 in both spin states. Comparison of 1 – 3 reveals that the more cooperative spin transition in 1 , and its separate crystallographic phase transition, can both be attributed to an intermolecular steric interaction involving the methylsulfanyl substituents. All three compounds exhibit the light‐induced excited‐spin‐state trapping (LIESST) effect with T(LIESST=70–80 K), but show complicated LIESST relaxation kinetics involving both weakly cooperative (exponential) and strongly cooperative (sigmoidal) components.     

Photoexcitation and relaxation properties of a spin-crossover solid in the case of a stable high-spin state

The molecular solid [Fe(II)L(2)](ClO(4))(2).CH(3)CN where L is 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine provides a stable high-spin (HS) state at low temperature. Photoexcitation and subsequent relaxation have been studied using light-induced excited state spin trapping [LIESST(H --> L)] in the 700-850 nm range, determination of T(LIESST), relaxation curves at different temperatures, and temperature dependence of the light-induced spin equilibrium under constant irradiation. The measured photoinduced population of the metastable low-spin (LS) state (<30%) was drastically limited by the concomitant L --> H photoprocess. The absence of static light-induced thermal hysteresis and the stretched exponential shape of the relaxation curves respectively revealed the absence of sizable interactions and a large spreading of the activation energies attributed to the ligand flexibility. The whole data set has been simulated using a linear rate equation, with a simplified correction for the bulk extinction of light in the powder sample.     

Structural study of the thermal and photochemical spin states in the spin crossover complex [Fe(phen)2(NCSe)2

The first structural data for [Fe(phen)(2)(NCSe)(2)] (obtained using the extraction method of sample preparation) in its high-spin, low-spin and LIESST induced metastable high-spin states have been recorded using synchrotron radiation single crystal diffraction. The space group for all of the spin states was found to be Pbcn. On cooling from the high-spin state (HS-1) at 292 K through the spin crossover at about 235 K to the low-spin state at 100 K (LS-1) the iron coordination environment changed to a more regular octahedral geometry and the Fe-N bond lengths decreased by 0.216 and 0.196 A (Fe-N(phen)) and 0.147 A (Fe-N(CSe)). When the low-spin state was illuminated with visible light at about 26 K, the structure of this LIESST induced metastable high-spin state (HS-2) was very similar to that of HS-1 with regards to the Fe-phen bond lengths, but there were some differences in the bond lengths in the Fe-NCSe unit between HS-1 and HS-2. When HS-2 was warmed in the dark to 50 K, the resultant low-spin state (LS-2) had an essentially identical structure to LS-1. In all spin states, all of the shortest intermolecular contacts (in terms of van der Waals radii) involved the NCSe ligand, which may be important in describing the cooperativity in the solid state. The quality of the samples was confirmed by magnetic susceptibility and IR measurements.