Femtosecond optical pump-probe reflectivity studies of spin-state photo-switching in the spin-crossover molecular crystals [Fe(PM-AzA)2(NCS)2]

International audienceWe report here on the ultrafast photo-switching dynamics of a Fe(II) molecular material [Fe(PM-AzA)2(NCS)2]. It undergoes a thermal spin-crossover which can be detected by magnetic measurements or by optical reflectivity. We use here femtosecond optical reflectivity to study the ultrafast photo-switching dynamics. Our results indicate that the HS state is reached from the LS state within less than 100 fs, through an intermediate MLCT state. This ultrafast relaxation from the electronic excited state towards the structurally relaxed HS state is followed by a vibrational cooling of the hot HS molecules within ≈1 ps timescale

Nanoseconde study of spin state transition induced by a single nanosecond laser shot on [Fe(NH₂trz)₃] compounds inside and outside their thermal hysteresis loops

We show that the combination of a free running 80 MHz femtosecond Ti–sapphire oscillator with a nanosecond Nd:YAG laser makes it possible to record in real time, with a nanosecond time resolution, reversible or irreversible phase transitions photo-induced by a single laser pulse. This set-up is used to study the spin state transition photo-induced inside and outside the thermal hysteresis loops of a series of [Fe(NH2trz)3]X2·nH2O compounds (with X = NO3 and Br). Original dynamics are presented and discussed. The results indicate that at room temperature, it is the laser induced heating that drives the recorded photo-induced phase transition

Time-domain terahertz spectroscopy of spin state transition in [<em class="emphitalic">Fe</em>(<em class="emphitalic">NH</em><sub class="emphinferior">2</sub> − <em class="emphitalic">trz</em>)<sub class="emphinferior">3</sub>]<sup class="emphsuperior">2+</sup> spin crossover compounds

We have measured the evolution of the THz spectrum of iron(II) spin crossover compounds within the low-spin/high-spin thermal hysteresis loop in the 0.6-6 THz frequency range. This study enabled to follow both the variations of the refractive optical index and absorption during the spin state transition. Marked absorptions centered ∼2 - 3 THz and ∼5 THz shifting with the spin state are revealed. Our work provides a means to store optically information and to read it out in the THz domain and also offers indications about the structural evolution occurring during the spin state transition

Transient absorption spectroscopy of the [Fe(2 CH3-phen)3]2+ complex: Study of the high spin ↔ low spin relaxation of an isolated iron(II) complex

We report on the light induced spin crossover dynamics of [Fe(2 CH3-phen)3]2+ complex in solution. The excitation of the complex by pulses centered at λ = 500 nm along the metal to ligand charge transfer absorption band indicates a selective excitation of the LS state. The excitation of the complex by pulses centered at λ = 330 nm or λ = 335 nm reveals an increase of the LS state population and a photo-bleaching of the HS state population that is associated to a transient photo-dissociation. A method that makes it possible to deduce the kinetics of both the HS → LS state and LS → HS state is proposed.

Nanoseconde study of spin state transition induced by a single nanosecond laser shot on [Fe(NH₂trz)₃] compounds inside and outside their thermal hysteresis loops

We show that the combination of a free running 80 MHz femtosecond Ti–sapphire oscillator with a nanosecond Nd:YAG laser makes it possible to record in real time, with a nanosecond time resolution, reversible or irreversible phase transitions photo-induced by a single laser pulse. This set-up is used to study the spin state transition photo-induced inside and outside the thermal hysteresis loops of a series of [Fe(NH2trz)3]X2·nH2O compounds (with X = NO3 and Br). Original dynamics are presented and discussed. The results indicate that at room temperature, it is the laser induced heating that drives the recorded photo-induced phase transition

Femtosecond pump-probe reflectivity studies of spin-state photo-switching in the Fe(II) molecular crystals Fe(PM-AZA)2(NCS)2

International audienc

Mosaicity and structural fatigability of a gradual spin-crossover single crystal

The present Letter introduces a novel approach to test the efficiency of spin crossover materials with regard to structural fatigability. By measuring single-crystal mosaicity, structural fatigability is evidenced in a gradual SCO iron compound. The non fatigability of the analogue non-SCO zinc compound demonstrates the role of SCO in such observation. The mosaicity strongly increases during the first cycles. It is therefore clearly shown that fatigability can affect non-cooperative SCO systems. Magnetic properties appear however not altered by the observed structural fatigability which is thus related to mechanical aspects of SCO

Investigating the roles of ligand-field states in femtosecond LIESST and REVERSE LIESST dynamics for spin crossover molecular crystals

International audienc

Coherent spin state photoswitching dynamics in the [FeII(PM-AZA)2(NCS)2] molecular crystals investigated by femtosecond optical and XANES pump-probe techniques

International audienc

Sequential Activation of Molecular Breathing and Bending during Spin-Crossover Photoswitching Revealed by Femtosecond Optical and X-Ray Absorption Spectroscopy

We study the basic mechanisms allowing light to photoswitch at molecular scale a spin-crossover material from low-to high-spin state. Combined femtosecond x-ray absorption performed at LCLS X-FEL and optical spectroscopy reveal that the structural stabilization of the photoinduced HS state results from a two step structural trapping. Molecular breathing vibrations are first activated and rapidly damped as part of the energy is sequentially transferred to molecular bending vibrations. During the photoswitching, the system follows a curved trajectory on the potential energy surface.Etude femtoseconde rayons X et optique de la dynamique ultrarapide de photocommutation de matériaux moléculaires magnétique