Discrimination between two memory channels by molecular alloying in a doubly bistable spin crossover material

• Published in: Chemical science (Cambridge) 2019-04, Vol.10 (13), p.3807-3816
• Main Authors: Valverde-Muñoz, Francisco Javier, Seredyuk, Maksym, Meneses-Sánchez, Manuel, Muñoz, M Carmen, Bartual-Murgui, Carlos, Real, José A
• Format: Article
• Language: English
• Subjects: Alloy systems; Anions; Chemistry; Crossovers; Crystallography; Electron spin; High temperature; Homology; Hydrostatic pressure; Organic chemistry; Phase transitions; Pressure effects; Single crystals; Substitutes; Temperature
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c8sc05256e

A multistable spin crossover (SCO) molecular alloy system [Fe M ( Bu-im) (tren)](P As F ) (M = Zn , Ni ; ( Bu-im) (tren) = tris( -butyl-imidazol(2-ethylamino))amine) has been synthesized and characterized. By controlling the composition of this isomorphous series, two cooperative thermally induced SCO events featuring distinct critical temperatures ( ) and hysteresis widths (Δ , memory) can be selected at will. The pristine derivative 100As ( = 0, = 1) displays a strong cooperative two-step SCO and two reversible structural phase transitions (PTs). The low temperature PT and the SCO occur synchronously involving conformational changes of the ligand's -butyl arms and two different arrangements of the AsF anions [ 1c = 174 K (Δ 1c = 17 K), 2c = 191 K (Δ 2c = 23 K) (scan rate 2 K min )]. The high-temperature PT takes place in the high-spin state domain and essentially involves rearrangement of the AsF anions [ PTc = 275 K (Δ PTc = 16 K)]. This behavior strongly contrasts with that of the homologous 100P [ = 0, = 0] derivative where two separate cooperative one-step SCO can be selected by controlling the kinetics of the coupled PT at ambient pressure: (i) one at low temperatures, = 122 K (Δ = 9 K), for temperature scan rates (>1 K min ) (memory channel A) where the structural modifications associated with PT are inhibited; (ii) the other centered at = 155 K (Δ = 41 K) for slower temperature scan rates ≤0.1 K min (memory channel B). These two SCO regimes of the 100P derivative transform reversibly into the two-step SCO of 100As upon application of hydrostatic pressure ( 0.1 GPa) denoting the subtle effect of internal chemical pressure on the SCO behavior. Precise control of AsF ↔ PF substitution, and hence of the PT kinetics, selectively selects the memory channel B of 100P when = 0 and ≈ 0.7. Meanwhile, substitution of Fe with Zn or Ni [ ≈ 0.2, = 0] favors the low temperature memory channel A at any scan rate. This intriguing interplay between PT, SCO and isomorphous substitution was monitored by single crystal and powder X-ray diffractometries, and magnetic and calorimetric measurements.