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

Abstract

Spin-crossover crystals show multi-step responses to femtosecond light excitation. The local molecular photo-switching from low to high spin states occurs on sub-picosecond timescale. It is followed by additional conversion due to elastic (ns) and thermal (μs) effects. In [Fe(phen)2(NCS)2] crystals discussed herein, the thermal switching can be made unobtrusive for the investigation of cooperative elastic switching. We evidence a cooperative transformation induced by lattice expansion through elastic coupling between molecules in the crystal, where up to 3 molecules are transformed per photon.

Molecular Spin Crossover Materials: Review of the Lattice Dynamical Properties

“…Left panel: Schematic drawing of the out‐of‐equilibrium dynamics in SCO crystals displaying successive steps of photo (1), elastic (2), and thermal (3) spin‐state switching as well as the relaxation step (4) to the initial LS state following a fs single laser pulse (0). Reproduced with permission . Copyright 2016, The Royal Society of Chemistry.…”

“…The mechanical equilibrium with the environment is restored via the coherent propagation of collective strain waves throughout the crystal sample . The resulting volume expansion, limited by the speed of sound, occurs on the so‐called acoustic timescale (i.e., on the tens of nanoseconds) and drives a subsequent additional LS‐to‐HS switching (self‐amplification) . Indeed, when a significant fraction of molecules is photoconverted, the lattice expansion becomes sufficient to generate the development of regions where the local pressure (pulling forces) favors the HS state .…”

“…Indeed, when a significant fraction of molecules is photoconverted, the lattice expansion becomes sufficient to generate the development of regions where the local pressure (pulling forces) favors the HS state . Interestingly, while the elastically driven increase of the HS fraction was found linear with the excitation energy in [Fe(phen) 2 (NCS) 2 ], the elastic response was non‐linear and only appeared above a threshold light excitation in [Fe(3‐MeO‐SalEen) 2 ]PF 6 . This apparent discrepancy was attributed to the difference in the molecular volume change in Fe 2+ complexes compared to Fe 3+ systems.…”

Theoretical approach for elastically driven cooperative switching of spin-crossover compounds impacted by an ultrashort laser pulse

“…Spin crossover inorganic compounds are among the few systems showing bistability at a molecular scale with switchable properties by variations of temperature and pressure or by light irradiation 1,2 . Pulsed laser irradiation opens the possibility to control this bistability in an ultrafast way, through strongly out-of-equilibrium conditions [3][4][5][6][7][8] . Their molecular switching occurs between the paramagnetic low spin state (LS), stable at low temperatures and paramagnetic high spin (HS) state of higher entropy, stable at higher temperature.…”

Formation of local spin-state concentration waves during the relaxation from a photoinduced state in a spin-crossover polymer

“…The molecular structural trapping occurs at the molecular level on the 100 fs timescale (Cammarata et al, 2014;Bertoni et al, 2012;Bertoni, Cammarata et al, 2015;Lemke et al, 2015;Zhang et al, 2014;Tissot et al, 2011). It is followed on the ns timescale by lattice expansion, driven by molecular expansion (Bertoni, Lorenc, Cailleau et al, 2016; and on the ms timescale by lattice heating Bertoni, Lorenc, Graber et al, 2016). Philip Coppens helped us to explore deeper in the time-domain analysis of the structural changes involved, which is difficult to analyze when the molecular conversion rate is low (Collet, Moisan et al, 2012).…”