Spectral Signatures of Ultrafast Spin Crossover in Single Crystal [FeII(bpy)3](PF6)2

Solvated iron(II)‐tris(bipyridine) ([Fe^II(bpy)₃]²⁺) has been extensively studied with regard to the spin crossover (SCO) phenomenon. Herein, the ultrafast spin transition dynamics of single crystal [Fe^II(bpy)₃](PF₆)₂ was characterized for the first time using femtosecond transient absorption (TA) spectroscopy. The single crystal environment is of interest for experiments that probe the nuclear motions involved in the SCO transition, such as femtosecond X‐ray and electron diffraction. We found that the TA at early times is very similar to what has been reported in solvated [Fe^II(bpy)₃]²⁺, whereas the later dynamics are perturbed in the crystal environment. The lifetime of the high‐spin state is found to be much shorter (100 ps) than in solution due to chemical pressure exerted by the lattice. Oscillatory behavior was observed on both time scales. Our results show that single crystal [Fe^II(bpy)₃](PF₆)₂ serves as an excellent model system for localized molecular spin transitions.     

Photo‐ and Electronically Switchable Spin‐Crossover Iron(II) Metal–Organic Frameworks Based on a Tetrathiafulvalene Ligand

A major challenge is the development of multifunctional metal–organic frameworks (MOFs), wherein magnetic and electronic functionality can be controlled simultaneously. Herein, we rationally construct two 3D MOFs by introducing the redox active ligand tetra(4‐pyridyl)tetrathiafulvalene (TTF(py)₄) and spin‐crossover Fe^II centers. The materials exhibit redox activity, in addition to thermally and photo‐induced spin crossover (SCO). A crystal‐to‐crystal transformation induced by I₂ doping has also been observed and the resulting intercalated structure determined. The conductivity could be significantly enhanced (up to 3 orders of magnitude) by modulating the electronic state of the framework via oxidative doping; SCO behavior was also modified and the photo‐magnetic behavior was switched off. This work provides a new strategy to tune the spin state and conductivity of framework materials through guest‐induced redox‐state switching.     

Coupled crystallographic order-disorder and spin state in a bistable molecule: multiple transition dynamics

A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H₄L, has led to an Fe^II spin‐crossover (SCO) complex, [Fe(H₄L)₂][ClO₄]₂ ? H₂O ? 2 (CH₃)₂CO ( 1 ), with an intricate network of intermolecular interactions. It exhibits a 40 K wide hysteresis of magnetization as a result of the spin transition (with T_0.5 of 133 and 173 K) and features an unsymmetrical and very rich structure. The latter is a consequence of the coupling between the SCO and the crystallographic transformations. The high‐spin state may also be thermally trapped, exhibiting a very large T_TIESST (≈104 K). The structure of 1 has been determined at various temperatures after submitting the crystal to different processes to recreate the key points of the hysteresis cycle and thermal trapping; 200 K, cooled to 150 K and trapped at 100 K (high spin, HS), slowly cooled to 100 K and warmed to 150 K (low spin, LS). In the HS state, the system always exhibits disorder for some components (one ClO₄^? and two acetone molecules) whereas the LS phases show a relative ≈9 % reduction in the Fe? N bond lengths and anisotropic contraction of the unit cell. Most importantly, in the LS state all the species are always found to be ordered. Therefore, the bistability of crystallographic order–disorder coupled to SCO is demonstrated here experimentally for the first time. The variation in the cell parameters in 1 also exhibits hysteresis. The structural and magnetic thermal variations in this compound are paralleled by changes in the heat capacity as measured by differential scanning calorimetry. Attempts to simulate the asymmetric SCO behaviour of 1 by using an Ising‐like model underscore the paramount role of dynamics in the coupling between the SCO and the crystallographic transitions.     

Heteroleptic FeII Complexes of 2,2′‐Biimidazole and Its Alkylated Derivatives: Spin‐Crossover and Photomagnetic Behavior

Three iron(II) complexes, [Fe(TPMA)(BIM)](ClO₄)₂?0.5H₂O ( 1 ), [Fe(TPMA)(XBIM)](ClO₄)₂ ( 2 ), and [Fe(TPMA)(XBBIM)](ClO₄)₂ ?0.75CH₃OH ( 3 ), were prepared by reactions of Fe^II perchlorate and the corresponding ligands (TPMA=tris(2‐pyridylmethyl)amine, BIM=2,2′‐biimidazole, XBIM=1,1′‐(α,α′‐o‐xylyl)‐2,2′‐biimidazole, XBBIM=1,1′‐(α,α′‐o‐xylyl)‐2,2′‐bibenzimidazole). The compounds were investigated by a combination of X‐ray crystallography, magnetic and photomagnetic measurements, and Mössbauer and optical absorption spectroscopy. Complex 1 exhibits a gradual spin crossover (SCO) with T_1/2=190 K, whereas 2 exhibits an abrupt SCO with approximately 7 K thermal hysteresis (T_1/2=196 K on cooling and 203 K on heating). Complex 3 is in the high‐spin state in the 2–300 K range. The difference in the magnetic behavior was traced to differences between the inter‐ and intramolecular interactions in 1 and 2 . The crystal packing of 2 features a hierarchy of intermolecular interactions that result in increased cooperativity and abruptness of the spin transition. In 3 , steric repulsion between H atoms of one of the pyridyl substituents of TPMA and one of the benzene rings of XBBIM results in a strong distortion of the Fe^II coordination environment, which stabilizes the high‐spin state of the complex. Both 1 and 2 exhibit a photoinduced low‐spin to high‐spin transition (LIESST effect) at 5 K. The difference in the character of intermolecular interactions of 1 and 2 also manifests in the kinetics of the decay of the photoinduced high‐spin state. For 1 , the decay rate constant follows the single‐exponential law, whereas for 2 it is a stretched exponential, reflecting the hierarchical nature of intermolecular contacts. The structural parameters of the photoinduced high‐spin state at 50 K are similar to those determined for the high‐spin state at 295 K. This study shows that N‐alkylation of BIM has a negligible effect on the ligand field strength. Therefore, the combination of TPMA and BIM offers a promising ligand platform for the design of functionalized SCO complexes.     

Hysteretic Four‐Step Spin Crossover within a Three‐Dimensional Porous Hofmann‐like Material

Materials that display multiple stepped spin crossover (SCO) transitions with accompanying hysteresis present the opportunity for ternary, quaternary, and quinary electronic switching and data storage but are rare in existence. Herein, we present the first report of a four‐step hysteretic SCO framework. Single‐crystal structure analysis of a porous 3D Hofmann‐like material showed long‐range ordering of spin states: HS, HS_0.67LS_0.33, HS_0.5LS_0.5, HS_0.33LS_0.67, and LS. These detailed structural studies provide insight into how multistep SCO materials can be rationally designed through control of host–host and host–guest interactions.     

100 Picosecond Diffraction Catches Structural Transients of Laser‐Pulse Triggered Switching in a Spin‐Crossover Crystal

We study by 100 picosecond X‐ray diffraction the photo‐switching dynamics of single crystal of the orthorhombic polymorph of the spin‐crossover complex [(TPA)Fe(TCC)]PF₆, in which TPA=tris(2‐pyridyl methyl)amine, TCC^2?=3,4,5,6‐Cl₄‐Catecholate^2?. In the frame of the emerging field of dynamical structural science, this is made possible by using optical pump/X‐ray probe techniques, which allow following in real time structural reorganization at intra‐ and intermolecular levels associated with the change of spin state in the crystal. We use here the time structure of the synchrotron radiation generating 100 picosecond X‐ray pulses, coupled to 100 fs laser excitation. This study has revealed a rich variety of structural reorganizations, associated with the different steps of the dynamical process. Three consecutive regimes are evidenced in the time domain: 1) local molecular photo‐switching with structural reorganization at constant volume, 2) volume relaxation with inhomogeneous distribution of local temperatures, 3) homogenization of the crystal in the transient state 100 µs after laser excitation. These findings are fundamentally different from those of conventional diffraction studies of long‐lived photoinduced high spin states. The time‐resolution used here with picosecond X‐ray diffraction probes different physical quantities on their intrinsic time‐scale, shedding new light on the successive processes driving macroscopic switching in a functionalized material. These results pave the way for structural studies away from equilibrium and represent a first step toward femtosecond crystallography.     

Stabilization of the Low‐Spin State in a Mononuclear Iron(II) Complex and High‐Temperature Cooperative Spin Crossover Mediated by Hydrogen Bonding

The tetrapyridyl ligand bbpya (bbpya=N,N‐bis(2,2′‐bipyrid‐6‐yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)₂] ( 1 ) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai’s domain model, and powder X‐ray diffraction measurements, 1 is low‐spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T_1/2=418 K. Although the SCO of compound 1 spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound 1 shows strong N?H???S intermolecular H‐bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex 1 . DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N′‐di(pyrid‐2‐yl)‐2,2′‐bipyridine‐6,6′‐diamine); this stabilizes the LS state and destabilizes the HS state in 1 compared with [Fe(bapbpy)(NCS)₂] ( 2 ). Periodic DFT calculations suggest that crystal‐packing effects are significant for compound 2 , in which they destabilize the HS state by about 1500 cm^?1. The much lower transition temperature found for the SCO of 2 compared to 1 appears to be due to the combined effects of the different ligand field strengths and crystal packing.     

Femtosecond Polarization Switching in the Crystal of a [CrCo] Dinuclear Complex

Capability to control macroscopic molecular properties with external stimuli offers the possibility to exploit molecules as switching devices of various types. However, application of such molecular‐level switching has often been limited by its speed and thus efficiency. Herein, we demonstrate ultrafast, photoinduced polarization switching in the crystal of a [CrCo] dinuclear complex by ultrafast pump–probe spectroscopy in the visible and mid‐infrared regions. The photoinduced polarization switching was found to have a time constant of 280 fs, which makes the [CrCo] complex crystal the fastest polarization‐switching material realized using the metastable state. Moreover, the pump–probe data in the visible region reveal the pronounced appearance of coherent nuclear wavepacket motion with a frequency as low as 22 cm^?1, which we attribute to a lattice vibrational mode. The pronounced non‐Condon effect for its resonance Raman enhancement implies that this mode couples the relevant electronic states, thereby facilitating the ultrafast polarization switching.     

A New Family of Anionic FeIII Spin Crossover Complexes Featuring a Weak‐Field N2O4 Coordination Octahedron

Unprecedented anionic Fe^III spin crossover (SCO) complexes involving a weak‐field O,N,O‐tridentate ligand were discovered. The SCO transition was evidenced by the temperature variations in magnetic susceptibility, Mössbauer spectrum, and coordination structure. The DFT calculations suggested that larger coefficients on the azo group in the HOMO?1 of a ligand might contribute to the enhancement of a ligand‐field splitting energy. The present anionic SCO complex also exhibited the light‐ induced excited‐spin‐state trapping effect.     

Different Spin‐State Behaviors in Isostructural Solvates of a Molecular Iron(II) Complex

The complex [FeL₂][BF₄]₂ ( 1 ; L=4‐(isopropylsulfanyl)‐2,6‐di(pyrazol‐1‐yl)pyridine) forms solvate crystals 1 ?solv (solv=MeNO₂, MeCN, EtCN, or Me₂CO). Most of these materials lose their solvent sluggishly on heating. However, heating 1 ?MeNO₂ at 450 K, or storing 1 ?EtCN under ambient conditions, leads to single‐crystal to single‐crystal exchange of the organic solvent for atmospheric moisture, forming 1 ?H₂O. Solvent‐free 1 ( 1 ?sf) can be generated in situ by annealing 1 ?H₂O at 370 K in the diffractometer or magnetometer. The different forms of 1 are isostructural (P2₁/c, Z=4) and most of them exhibit spin‐crossover (SCO) at 141≤T ≤212 K, depending on their solvent content. The exception is the EtCN solvate, whose pristine crystals remain high‐spin between 3–300 K. The cooperativity of the spin‐transitions depends on the solvent, ranging from gradual and incomplete when solv=acetone to abrupt with 17 K hysteresis when solv=MeCN. Our previously proposed relationship between molecular structure and SCO explains some of these observations, but there is no single structural feature that correlates with SCO in all the 1 ?solv materials. However, changes to the unit cell dimensions during SCO differ significantly between the solvates, and correlate with the SCO cooperativity. In particular, the percentage change in unit cell volume during SCO for the most cooperative material, 1 ?MeCN, is 10 times smaller than for the other 1 ?solv crystals.     

Thermodynamical aspects of the spin crossover phenomenon

Fundamental aspects of spin crossover (SCO) mechanisms are reviewed through considerations of ligand/crystal field theory, thermodynamics, and modeling of the thermoinduced spin transition in the solid state based on macroscopic–mesoscopic approaches . In particular, we highlight success of thermodynamic models in the simulation of first-order spin transitions with hysteretic behaviors (bistability) and multistep conversions. Bistability properties originate from elastic interactions, the so-called cooperativity between SCO molecules in the crystal packing. Although physical and chemical properties and thermodynamical quantities of noninteracting SCO compounds can be readily injected in macroscopic models, taking cooperativity into account remains problematic. The relationship between phenomenological numerical parameters and experimentally accessible quantities can only be most of the time indirectly established. Recent extensions of these thermodynamical models to grasp SCO properties at the nanoscale and combinations with ab initio numerical methods show that macroscopic models still constitute useful theoretical tools to investigate SCO phenomena. The necessity to further probe the thermomechanical properties of SCO materials is also emphasized.     

Hexakis-adducts of [60]fullerene as molecular scaffolds of polynuclear spin-crossover molecules

A family of hexakis-substituted [60]fullerene adducts endowed with the well-known tridentate 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligand for spin-crossover (SCO) systems has been designed and synthesized. It has been experimentally and theoretically demonstrated that these molecular scaffolds are able to form polynuclear SCO complexes in solution. UV-vis and fluorescence spectroscopy studies have allowed monitoring of the formation of up to six Fe(ii)–bpp SCO complexes. In addition, DFT calculations have been performed to model the different complexation environments and simulate their electronic properties. The complexes retain SCO properties in the solid state exhibiting both thermal- and photoinduced spin transitions, as confirmed by temperature-dependent magnetic susceptibility and Raman spectroscopy measurements. The synthesis of these complexes demonstrates that [60]fullerene hexakis-adducts are excellent and versatile platforms to develop polynuclear SCO systems in which a fullerene core is surrounded by a SCO molecular shell.

Polynuclear spin-crossover molecules showing both thermal and photoinduced spin transitions have been prepared using a [60]fullerene hexakis-adduct endowed with Fe(ii) complexes of tridentate 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligand.     

Thermal‐ and Light‐Induced Spin Crossover in a Guest‐Dependent Dinuclear Iron(II) System

We previously reported the dinuclear material [Fe^II₂(ddpp)₂(NCS)₄] ? 4 CH₂Cl₂ ( 1? 4 CH₂Cl₂; ddpp=2,5‐di(2′,2′′‐dipyridylamino)pyridine) and its partially desolvated analogue ( 1? CH₂Cl₂), which undergo two‐ and one‐step spin‐crossover (SCO) transitions, respectively. Here, we manipulate the type and degree of solvation in this system and find that either a one‐ or two‐step spin transition can be specifically targeted. The chloroform clathrate 1? 4 CHCl₃ undergoes a relatively abrupt one‐step SCO, in which the two equivalent Fe^II sites within the dinuclear molecule crossover simultaneously. Partial desolvation of 1? 4 CHCl₃ to form 1? 3 CHCl₃ and 1? CHCl₃ occurs through single‐crystal‐to‐single‐crystal processes (monoclinic C2/c to P2₁/n to P2₁/n) in which the two equivalent Fe^II sites become inequivalent sites within the dinuclear molecule of each phase. Both 1? 3 CHCl₃ and 1? CHCl₃ undergo one‐step spin transitions, with the former having a significantly higher SCO temperature than 1? 4 CHCl₃ and the latter, and each has a broader SCO transition than 1? 4 CHCl₃, attributable to the overlap of two SCO steps in each case. Further magnetic manipulation can be carried out on these materials through reversibly resolvating the partially desolvated material with chloroform to produce the original one‐step SCO, or with dichloromethane to produce a two‐step SCO reminiscent of that seen for 1? 4 CH₂Cl₂. Furthermore, we investigate the light‐induced excited spin state trapping (LIESST) effect on 1? 4 CH₂Cl₂ and 1? CH₂Cl₂ and observe partial LIESST activity for the former and no activity for the latter.     

An Investigation of Photo‐ and Pressure‐Induced Effects in a Pair of Isostructural Two‐Dimensional Spin‐Crossover Framework Materials

Two new isostructural iron(II) spin‐crossover (SCO) framework (SCOF) materials of the type [Fe(dpms)₂(NCX)₂] (dpms=4,4′‐dipyridylmethyl sulfide; X=S ( SCOF‐6(S) ), X=Se ( SCOF‐6(Se) )) have been synthesized. The 2D framework materials consist of undulating and interpenetrated rhomboid (4,4) nets. SCOF‐6(S) displays an incomplete SCO transition with only approximately 30 % conversion of high‐spin (HS) to low‐spin iron(II) sites over the temperature range 300–4 K (T_1/2=75 K). In contrast, the NCSe^? analogue, SCOF‐6(Se) , displays a complete SCO transition (T_1/2=135 K). Photomagnetic characterizations reveal quantitative light‐ induced excited spin‐state trapping (LIESST) of metastable HS iron(II) sites at 10 K. The temperature at which the photoinduced stored information is erased is 58 and 50 K for SCOF‐6(S) and SCOF‐6(Se) , respectively. Variable‐pressure magnetic measurements were performed on SCOF‐6(S) , revealing that with increasing pressure both the T_1/2 value and the extent of spin conversion are increased; with pressures exceeding 5.2 kbar a complete thermal transition is achieved. This study confirms that kinetic trapping effects are responsible for hindering a complete thermally induced spin transition in SCOF‐6(S) at ambient pressure due to an interplay between close T_1/2 and T(LIESST) values.     

100 picosecond diffraction catches structural transients of laser-pulse triggered switching in a spin-crossover crystal

We study by 100?picosecond X-ray diffraction the photo-switching dynamics of single crystal of the orthorhombic polymorph of the spin-crossover complex [(TPA)Fe(TCC)]PF(6), in which TPA = tris(2-pyridyl methyl)amine, TCC(2-) = 3,4,5,6-Cl(4)-Catecholate(2-). In the frame of the emerging field of dynamical structural science, this is made possible by using optical pump/X-ray probe techniques, which allow following in real time structural reorganization at intra- and intermolecular levels associated with the change of spin state in the crystal. We use here the time structure of the synchrotron radiation generating 100?picosecond X-ray pulses, coupled to 100?fs laser excitation. This study has revealed a rich variety of structural reorganizations, associated with the different steps of the dynamical process. Three consecutive regimes are evidenced in the time domain: 1)?local molecular photo-switching with structural reorganization at constant volume, 2)?volume relaxation with inhomogeneous distribution of local temperatures, 3)?homogenization of the crystal in the transient state 100?μs after laser excitation. These findings are fundamentally different from those of conventional diffraction studies of long-lived photoinduced high spin states. The time-resolution used here with picosecond X-ray diffraction probes different physical quantities on their intrinsic time-scale, shedding new light on the successive processes driving macroscopic switching in a functionalized material. These results pave the way for structural studies away from equilibrium and represent a first step toward femtosecond crystallography.     

Ultrafast structural rearrangements in the MLCT excited state for copper(I) bis-phenanthrolines in solution

Ultrafast excited-state structural dynamics of [Cu(I)(dmp)(2)](+) (dmp = 2,9-dimethyl-1,10-phenanthroline) have been studied to identify structural origins of transient spectroscopic changes during the photoinduced metal-to-ligand charge-transfer (MLCT) transition that induces an electronic configuration change from Cu(I) (3d(10)) to Cu(II) (3d(9)). This study has important connections with the flattening of the Franck-Condon state tetrahedral geometry and the ligation of Cu(II)* with the solvent observed in the thermally equilibrated MLCT state by our previous laser-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS) results. To better understand the structural photodynamics of Cu(I) complexes, we have studied both [Cu(I)(dmp)(2)](+) and [Cu(I)(dpp)(2)](+) (dpp = 2,9-diphenyl-1,10-phenanthroline) in solvents with different dielectric constants, viscosities, and thermal diffusivities by transient absorption spectroscopy. The observed spectral dynamics suggest that a solvent-independent inner-sphere relaxation process is occurring despite the large amplitude motions due to the flattening of the tetrahedral coordinated geometry. The singlet fluorescence dynamics of photoexcited [Cu(I)(dmp)(2)](+) were measured in the coordinating solvent acetonitrile, using the fluorescence upconversion method at different emission wavelengths. At the bluest emission wavelengths, a prompt fluorescence lifetime of 77 fs is attributed to the excited-state deactivation processes due to the internal conversion and intersystem crossing at the Franck-Condon state geometry. The differentiation between the prompt fluorescence lifetime with the tetrahedral Franck-Condon geometry and that with the flattened tetrahedral geometry uncovers an unexpected ultrafast flattening process in the MLCT state of [Cu(I)(dmp)(2)](+). These results provide guidance for future X-ray structural studies on ultrafast time scale, as well as for synthesis toward its applications in solar energy conversion.     

The First Use of a ReX5 Synthon to Modulate FeIII Spin Crossover via Supramolecular Halogen⋅⋅⋅Halogen Interactions

We have added the {Re^IVX₅}⁻ (X=Br, Cl) synthon to a pocket-based ligand to provide supramolecular design using halogen⋅⋅⋅halogen interactions within an Fe^III system that has the potential to undergo spin crossover (SCO). By removing the solvent from the crystal lattice, we “switch on” halogen⋅⋅⋅halogen interactions between neighboring molecules, providing a supramolecular cooperative pathway for SCO. Furthermore, changes to the halogen-based interaction allow us to modify the temperature and nature of the SCO event.     

Tuning the transition temperature and cooperativity of bapbpy-based mononuclear spin-crossover compounds: interplay between molecular and crystal engineering

In this study, we show that 1) different isomers of the same mononuclear iron(II) complex give materials with different spin‐crossover (hereafter SCO) properties, and 2) minor modifications of the bapbpy (bapbpy=N6,N6′‐di(pyridin‐2‐yl)‐2,2′‐bipyridine‐6,6′‐diamine) ligand allows SCO to be obtained near room temperature. We also provide a qualitative model to understand the link between the structure of bapbpy‐based ligands and the SCO properties of their iron(II) compounds. Thus, seven new trans‐[Fe{R₂(bapbpy)}(NCS)₂] compounds were prepared, in which the R₂bapbpy ligand bears picoline ( 9 – 12 ), quin‐2‐oline ( 13 ), isoquin‐3‐oline ( 14 ), or isoquin‐1‐oline ( 15 ) substituents. From this series, three compounds ( 12 , 14 , and 15 ) have SCO properties, one of which ( 15 ) occurs at 288 K. The crystal structures of compounds 11 , 12 , and 15 show that the intermolecular interactions in these materials are similar to those found in the parent compound [Fe(bapbpy)(NCS)₂] ( 1 ), in which each iron complex interacts with its neighbors through weak N? H ??? S hydrogen bonding and π–π stacking. For compounds 12 and 15 , hindering groups located near the N? H bridges weaken the N? S intermolecular interactions, which is correlated to non‐cooperative SCO. For compound 14 , the substitution is further away from the N? H bridges, and the SCO remains cooperative as in 1 with a hysteresis cycle. Optical microscopy photographs show the strikingly different spatio‐temporal evolution of the phase transition in the noncooperative SCO compound 12 relative to that found in 1 . Heat‐capacity measurements were made for compounds 1 , 12 , 14 , and 15 and fitted to the Sorai domain model. The number n of like‐spin SCO centers per interacting domain, which is related to the cooperativity of the spin transition, was found high for compounds 1 and 14 and low for compounds 12 and 15 . Finally, we found that although both pairs of compounds 11 / 12 and 14 / 15 are pairs of isomers their SCO properties are surprisingly different.     

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.     

2D Materials in Light: Excited‐State Dynamics and Applications

The development of two‐dimensional (2D) materials have attracted increasing interest due to their unique structure and various potential applications such as opto‐electronic devices and photocatalysis. Our group have contributed to this exciting field by creating novel preparation methods for a various of 2D materials including transition metal dichalcogenides (TMDs), carbon nitrides and single elemental 2D materials from Group 15. Particularly, employing powerful time‐resolved spectroscopic techniques such as femtosecond transient absorption spectroscopy, we elucidated the excited‐state dynamics of 2D materials behind their outstanding performance in photocatalytic and photonic devices. Therefore in this account, we focus on the effective fabrication methods of 2D materials and their photoinduced excited‐state dynamics. Following the introduction in Part 1 , we will summarize our novel strategies for fabricating 2D materials ( Part 2 ). Then in Part 3 we will introduce the instrumentation for exploring the photoinduced excited‐state dynamics of the 2D materials spanning a wide time scale from ultrafast fs to slow ms. Part 4 details the applications of the 2D materials in photocatalysis and nonlinear optics determined by their excited‐state physics and chemistry. Part 5 of perspectives summarizes a few future trends of 2D materials on a series of issues like fabrications, dynamic investigations and photonic optoelectronic applications. Collective efforts through researchers from interdisciplinary fields are expected to further push the exciting territory towards a new horizon.