Elastically driven cooperative response of a molecular material impacted by a laser pulse

Abstract

International audiencePhotoinduced phase transformations [1,2] occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently directing the functionalities [3,4,5,6,7]. The transient nature of photoinduced states has thus far severely limited the application scope. It is of paramount importance to explore whether structural feedback during the solid deformation has capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons long under scrutiny [8,9,10] , coherently propagating cell deformations over acoustic timescale [11,12,13,14] have not been explored to similar degree, particularly in light of cooperative elastic interactions. Herein we demonstrate experimentally and theoretically a self-amplified responsiveness in a spin-crossover material [15] during its delayed volume expansion. The cooperative response at material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically-driven cooperativity triggered by a light pulse offers a new efficient route to the generation and stabilization of photoinduced phases in many volume-changing materials

Cooperative elastic switching vs. laser heating in [Fe(phen)2(NCS)2] spin-crossover crystals excited by a laser pulse

“…2). 4,9,12,17 This can be seen in the discontinuous evolution of the lattice unit cell volume. Figure 3 shows the evolution of the intermolecular distance due to molecular swelling between LS and HS states.…”

“…The experiment is conducted 40 K below the phase transition to avoid any residual thermal effect and to allow the complete recovery of the LS phase within 1 ms. Our experimental data clearly show a sequence of three steps, similar to the ones already reported in Fe(III) spin-crossover crystals and summarized in Figure 1. 12-16 First, the absorption of light at the molecular level locally photo-switches a small fraction X HS hν of molecules (typically 2% here) from LS to HS states. We have thoroughly investigated this process in [Fe(Phen) 2 (NCS) 2 ] crystals with femtosecond optical and x-ray spectroscopies 4,21 and established that it occurs via ultrafast intersystem crossing.…”

“…We have recently demonstrated the possibility of taking advantage of this effect to generate through inter-molecular elastic coupling an elastically-driven cooperative response. 12 …”

The disentangling of hysteretic spin transition, polymorphism and metastability in bistable thin films formed by sublimation of bis(scorpionate) Fe(ii) molecules

“…The spin--state switching produces a change of molecular volume and elastic strains that influence the course of the cooperative process. 44 One consequence of that is a possible feedback exerted in--between the SCO particles (strain, friction) in close contact within the films. This effect, first suggested by some of us, 8 is hardly prevented in powders of nanoparticles as it was demonstrated that any particle coating 8,20 or matrix 45 may alter the SCO characteristics or promote the opening of a hysteresis.…”

Back to the Structural and Dynamical Properties of Neutral-Ionic Phase Transitions

“…Since the penetration depth at this wavelength is estimated to be about 2.8 µm [128], the process takes place on the slow acoustic time scale defined by the ratio between this length and the speed of sound. The recent observation of elastically driven cooperative response in a spin-crossover system is instructive [144]. It will be important to explore this point and to particularly the role of cooperative inter-stack elastic interactions.…”