Sequential Activation of Molecular and Macroscopic Spin‐State Switching within the Hysteretic Region Following Pulsed Light Excitation

Abstract

Molecular spin‐crossover (SCO) compounds constitute a promising class of photoactive materials exhibiting efficient photoinduced phase transitions (PIPTs). Taking advantage of the unique, picture‐perfect reproducibility of the spin‐transition properties in the compound [Fe(HB(1,2,4‐triazol‐1‐yl)3)2], the spatiotemporal dynamics of the PIPT within the thermodynamic metastability (hysteretic) region of a single crystal is dissected, using pump–probe optical microscopy. Beyond a threshold laser‐excitation density, complete PIPTs are evidenced, with conversion rates up to 200 switched molecules per absorbed photon. It is shown that the PIPT takes place through the sequential activation of two (molecular and macroscopic) switching mechanisms, occurring on sub‐microsecond and millisecond timescales, governed by the intramolecular and free energy barriers of the system, respectively. The main finding here is that the thermodynamic metastability has strictly no influence on the sub‐millisecond switching dynamics. Indeed, before this millisecond timescale, the response of the crystal to the laser excitation involves a gradual, molecular conversion process, as if there were no hysteresis loop. Consequently, in this regime, even a 100% photoinduced conversion may not give rise to a PIPT. These results provide new insight on the intrinsic dynamical limits of the PIPT, which is an important issue from a technological perspective.

Memory effect in ferroelectric polyvinylidene fluoride (PVDF) films via spin crossover probes

“…[3][4][5][6][7] Thus, SCOs have potential applications for data storage, sensing, displays and actuators. [8][9][10][11][12] Moreover, the tunability and molecular design of SCO compounds has allowed their incorporation into multifunctional composite materials, looking to exploit the switchability of SCO systems to deliver bistable multifunctional materials. 13 This has resulted in hybrid materials exhibiting switchable conductivity, 14,15 magnetism, 16 luminescense, 17,18 dielectricity, 19,20 ferroelasticity, 21 ion conductivity 22 or photomagnetism.…”

Hysteretic Spin Crossover with High Transition Temperatures in Two Cobalt(II) Complexes

“…As one of the most important molecular magnetic materials with switchable magnetic properties, spin crossover (SCO) materials have been attracting intense attention. , The rearrangement of the electrons within the nonbonding t 2g and antibonding e g orbitals of the SCO materials can not only switch the spin state between the low spin (LS) and high spin (HS) states but also lead to modifications of physical properties through the application of external perturbations (such as pressure, magnetic and electric field, light irradiation, and guest molecules) and applications in areas such as display and memory, electrical, luminescent, and mechanical actuators. − …”

Optical characterization of isothermal spin state switching in an Fe(II) spin crossover molecular and polymer ferroelectric bilayer

“…Functional spin crossover (SCO) molecular complexes exhibiting a spin state transition that can be mediated by external stimulation such as light, temperature, electric field, and magnetic field [1][2][3][4][5][6][7][8][9], have significant potential for new molecularbased devices [4,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. In SCO molecular compounds, depending on the ligand field strength, the transition metal ion can adopt two different stable spin states called the low spin (LS) state and the high spin (HS) state and can be switched between these two states [1,2,[8][9][10].…”