Marie-Laure Boillot

Spin crossover in transition metal compounds

C.N.R. Rao, M.M. Seikh, C. Narayana: Spin-State Transition in LaCoO3 and Related Materials .- H.A. Goodwin: Spin Crossover in Cobalt(II) Systems .- Y. Garcia, P.Gutlich: Thermal Spin Crossover in Mn(II), Mn(III) Cr(II) and Co(III) Coordination Compounds .- D.N. Hendrickson, C.G. Pierpont: Valence Tautomeric Transition Metal Complexes .- P. Guionneau, M. Marchivie, G.Bravic, J.-F. Letard, D. Chasseau: Structural Aspects of Spin Crossover. Example of the [Fe(II)Ln(NCS)2] Complexes .- J. Kusz, P. Gutlich, H. Spiering: Structural Investigations of Tetrazole Complexes of Iron(II) .- A. Hauser: Light-Induced Spin Crossover and the High-Spin Low-Spin Relaxation .- F. Varret, K. Boukheddaden, E. Codjovi, C. Enachescu, J. Linares: On the Competition Between Relaxation and Photoexcitations in Spin Crossover Solids under Continuous Irradiation .- P. Gutlich: Nuclear Decay Induced Excited Spin State Trapping (NIESST) .- M.-L. Boillot, J. Zarembowitch, A. Sour: Ligand-Driven Light-Induced Spin Change (LD-LISC): A Promising Photomagnetic Effect

Femtosecond Spin-State Photoswitching of Molecular Nanocrystals Evidenced by Optical Spectroscopy†

In the field of control science, which aims at switching the physical properties of materials, photoinduced phase transitions open fascinating perspectives for driving a material towards a new state, far from thermal equilibrium. Such photoswitching will impact future technologies as it provides doorways to the light-control of various photoswitchable functions (for example, magnetic, optical, conducting, and ferroelectric). In control science, tailored laser pulses are widely regarded as the most likely source for achieving that goal. In that respect, the great challenge for molecularbased materials is directing the functionality, both at the relevant size and time scales. If we attempt simple parallels here, the goal is to achieve at the level of a material what femtochemistry has accomplished at the level of a molecule. In observing and understanding how materials work during elementary dynamical processes, several challenging basic questions are confronted. For instance, ultrafast information processing based on the control of light-driven switching of the physical properties of materials requires that such systems be directed through a complex pathway from atomic to material scales, and that the fundamental limits of transformation speed be overcome, or circumvented. Molecular magnets, and especially the spin-crossover compounds (SCO), are ideal candidates for photo-active prototypes, which show photomagnetic and photochromic properties driven by the switching of the constituent molecules between their electronic low spin (LS) and high spin (HS) states. Herein we report the ultrafast spin state photoswitching of a spin-crossover nanocrystal of an Fe complex, [Fe(3-MeOSalEen)2]PF6 (Figure 1), as studied through femtosecond optical spectroscopy (H-3-MeO-SalEen being the condensation product of 3-methoxy-substituted salicylaldehyde and N-ethyl-ethylenediamine). This result provides proof-of-principle for femtosecond switching at the nanoscale in SCO materials showing photomagnetic and photochromic responses. SCO materials are bistable systems, for which nanosecond laser excitation within the range of thermal hysteresis can generate LS to HS transition. Ultrafast investigations of similar photo-transformations have been mainly limited to single molecules in solution, and only recently also carried out on crystals. Despite formidable progress in the chemistry and engineering of spin-crossover nanoparticles, as well as their nano-patterning and nanoscale assembling 11] while preserving their switchable properties, the ultrafast switching of such materials has not yet been observed. Herein we study the ultrafast LS-to-HS spin-state photoswitching pathway of nanocrystals (Figure 1), taking advantage of growing knowledge in the field of ultrafast chemical physics. [*] R. Bertoni, Dr. M. Lorenc, Dr. M. Servol, Prof. E. Collet Institut de Physique de Rennes, UMR CNRS 6251 Universit Rennes 1, 35042 Rennes cedex (France) E-mail: maciej.lorenc@univ-rennes1.fr eric.collet@univ-rennes1.fr

Ultrafast spin-state photoswitching in a crystal and slower consecutive processes investigated by femtosecond optical spectroscopy and picosecond X-ray diffractionw

We report the spin state photo-switching dynamics in two polymorphs of a spin-crossover molecular complex triggered by a femtosecond laser flash, as determined by combining femtosecond optical pump-probe spectroscopy and picosecond X-ray diffraction techniques. The light-driven transformations in the two polymorphs are compared. Combining both techniques and tracking how the X-ray data correlate with optical signals allow understanding of how electronic and structural degrees of freedom couple and play their role when the switchable molecules interact in the active crystalline medium. The study sheds light on crossing the border between femtochemistry at the molecular scale and femtoswitching at the material scale.

Ligand-Driven Light-Induced Spin Change Activity and Bidirectional Photomagnetism of Styrylpyridine Iron(II) Complexes in Polymeric Media

Two pseudo-octahedral iron(II) complexes, Fe(stpy)(4)(NCSe)(2), containing photoresponsive ligands (cis <--> trans isomerization of -CHCH-) were prepared with trans- or cis-styrylpyridine (stpy) isomers. The magnetic behavior of the polycrystalline solids was previously shown to depend on the configuration of the stpy ligand. The crystal X-ray structures were determined at 293 and 104 K for both isomers. The all-trans and all-cis compounds crystallize in the orthorhombic (Pna2(1)) and the monoclinic space groups (C2/c), respectively. No symmetry change occurs upon cooling to 104 K. The Fe(II) centers lie in axially compressed octahedra with NCSe anions in the apical position and the four pyridinic nitrogens in the meridional plane. The variation of metal-ligand bond lengths as a function of temperature reflects the thermal S = 0 <--> S = 2 crossover of all-trans complexes and the S = 2 ground state of all-cis complexes. The unit-cell volumes per metal ion also change accordingly, and the relative variation due to the spin-crossover compares those associated with the formal change of configuration of the four stpy isomers. The photomagnetic responses were investigated at 130 K with doped polymer thin films containing all-cis (high-spin) or all-trans species (partly low-spin). The 130 K illumination of these doped poly(methyl methacrylate) (PMMA) films leads to the UV-vis absorption features typical for the cis <--> trans photoisomerization of the stilbenoid moiety. The direct magnetic measurements have unambiguously established the photomagnetic effect named ligand-driven light-induced spin change (LD-LISC). The 355 nm excitation of doped thin films produces very long lifetime states that are manifested by high-spin to low-spin (all-cis complex) and low-spin to high-spin (all-trans complex) changes of the Fe(II) magnetic behavior; the process is bidirectional. A structural analysis based on the single-crystal X-ray diffraction data has been proposed to rationalize the LD-LISC activity detected here for doped PMMA thin films.

First Evidence of a Photoinduced Spin Change in an FeIII Complex Using Visible Light at Room Temperature

An iron(III) complex [Fe(salten)(Mepepy)]BPh4 containing only one photoisomerizable ligand (Mepepy) has been synthesized, it exhibits a thermally-induced spin crossover in the solid state and in solution {H2salten = 4-azaheptamethylene-1,7-bis(salicylideneiminate); Mepepy = 1-(pyridin-4-yl)-2-(N-methylpyrrol-2-yl)ethene}. The photoisomerizations of both the free and coordinated Mepepy ligand have been observed at room temperature with visible-light irradiation and monitored by UV/Vis and 1H NMR spectrometries. trans-to-cis isomerization of only one photosensitive ligand in the iron(III) complex is sufficient to detect a partial spin change of the iron(III) ion. This photoinduced spin change is seen for the first time from a high-spin state to a low-spin state.

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.

Control of the thermal hysteresis of the prototypal spin-transition FeII(phen)2(NCS)2 compound via the microcrystallites environment: experiments and mechanoelastic model

Microcrystals of FeII(phen)2(NCS)2 (phen = 1,10-phenanthroline), isolated by solvent-induced precipitation, were dispersed in glassy or semicrystalline matrices and subjected to thermal or chemical treatments. Interactions occurring between crystals and their macromolecular (or molecular) surroundings were revealed by drastic alteration of the first-order spin-transition afforded by this material. Depending on matrices and experimental treatments, the cooperativity in particles dispersion can be dampened, resulting in a more gradual transition or enhanced, providing a large hysteresis. The hysteresis broadening was also demonstrated by the incorporation of another first-order transition molecular material (bulk) into similar matrices. The different features associated with the in-polymer dispersions are accounted for in the framework of a mechanoelastic model based on a Monte-Carlo standard procedure.

The cooperative spin-state transition of an iron(III) compound [FeIII(3-MeO-SalEen)2]PF6: thermal- vs. ultra-fast photo-switching

The switching properties of the spin-transition solid [FeIII(3-MeO-SalEen)2]PF6 (H-3-MeO-SalEen resulting from the condensation of 3-methoxy-substituted salicylaldehyde and N-ethylethylenediamine), exhibiting a first-order transition, were investigated by using temperature and light as stimulation. The structural analysis reveals a first-order isostructural transition occurring with a 3 K width thermal hysteresis (T↓ = 162.5 K, T↑ = 165.5 K) coupled to the magnetic transition. The microscopic origin of this bistable behavior derives from strong coupling between pairs of complexes and the molecular packing. An extensive 3D network of intermolecular interactions, reinforced near the transition temperature, appears as a key feature for the cooperativity. The LMCT (ligand-to-metal charge transfer) transitions, detected by single-crystal transmission measurements, were selected for analyzing the photoswitching process of this cooperative solid. Only optical pump-probe experiments could detect photoinduced low-spin to high-spin conversion because of the too short lived excited state. The spin-state transformation induced by the femtosecond laser flash is very fast: electronic excitation corresponding to the low-spin LMCT transition relaxes toward the transient photoexcited high-spin state within 170 ± 50 fs.

Molecular-scale dynamics of light-induced spin cross-over in a two-dimensional layer

Spin cross-over molecules show the unique ability to switch between two spin states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensional spin cross-over layers.

FeII(pap-5NO2)2 and FeII(qsal-5NO2)2 Schiff-Base Spin-Crossover Complexes: A Rare Example with Photomagnetism and Room-Temperature Bistability

We focus here on the properties of Fe complexes formed with Schiff bases involved in the chemistry of Fe(III) spin-transition archetypes. The neutral Fe(pap-5NO2)2 (1) and Fe(qsal-5NO2)2·Solv (2 and 2·Solv) compounds (Solv = 2H2O) derive from the reaction of Fe(II) salts with the condensation products of pyridine-2-carbaldehyde with 2-hydroxy-5-nitroaniline (Hpap-5NO2) or 5-nitrosalicylaldehyde with quinolin-8-amine (Hqsal-5NO2), respectively. While the Fe(qsal-5NO2)2·Solv solid is essentially low spin (S = 0) and requires temperatures above 300 K to undergo a S = 0 ↔ S = 2 spin-state switching, the Fe(pap-5NO2)2 one presents a strongly cooperative first-order transition (T↓ = 291 K, T↑ = 308 K) centered at room temperature associated with a photomagnetic effect at 10 K (TLIESST = 58 K). The investigation of these magnetic behaviors was conducted with single-crystal X-ray diffraction (1, 100 and 320 K; 2, 100 K), Mössbauer, IR, UV-vis (1 and 2·Solv), and differential scanning calorimetry (1) measurements. The Mössbauer analysis supports a description of these compounds as Fe(II) Schiff-base complexes and the occurrence of a metal-centered spin crossover process. In comparison with Fe(III) analogues, it appears that an expanded coordination sphere stabilizes the valence 2+ state of the Fe ion in both complexes. Strong hydrogen-bonding interactions that implicate the phenolato group bound to Fe(II) promote the required extra-stabilization of the S = 2 state and thus determines the spin transition of 1 centered at room temperature. In the lattice, the hydrogen-bonded sites form infinite chains interconnected via a three-dimensional network of intermolecular van der Waals contacts and π-π interactions. Therefore, the spin transition of 1 involves the synergetic influence of electrostatic and elastic interactions, which cause the enhancement of cooperativity and result in the bistability at room temperature.