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

Abstract

A multistable spin crossover (SCO) molecular alloy system [Fe_1−xM_x(nBu-im)₃(tren)](P_1−yAs_yF₆)₂ (M = Zn^II, Ni^II; (nBu-im)₃(tren) = tris(n-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 (T_c) and hysteresis widths (ΔT_c, memory) can be selected at will. The pristine derivative 100As (x = 0, y = 1) displays a strong cooperative two-step SCO and two reversible structural phase transitions (PTs). The low temperature PT^LT and the SCO occur synchronously involving conformational changes of the ligand's n-butyl arms and two different arrangements of the AsF₆⁻ anions [T¹_c = 174 K (ΔT¹_c = 17 K), T²_c = 191 K (ΔT²_c = 23 K) (scan rate 2 K min⁻¹)]. The high-temperature PT^HT takes place in the high-spin state domain and essentially involves rearrangement of the AsF₆⁻ anions [T^PT_c = 275 K (ΔT^PT_c = 16 K)]. This behavior strongly contrasts with that of the homologous 100P [x = 0, y = 0] derivative where two separate cooperative one-step SCO can be selected by controlling the kinetics of the coupled PT^LT at ambient pressure: (i) one at low temperatures, T_c = 122 K (ΔT_c = 9 K), for temperature scan rates (>1 K min⁻¹) (memory channel A) where the structural modifications associated with PT^LS are inhibited; (ii) the other centered at T_c = 155 K (ΔT_c = 41 K) for slower temperature scan rates ≤0.1 K min⁻¹ (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 (ca. 0.1 GPa) denoting the subtle effect of internal chemical pressure on the SCO behavior. Precise control of AsF₆⁻ ↔ PF₆⁻ substitution, and hence of the PT^LT kinetics, selectively selects the memory channel B of 100P when x = 0 and y ≈ 0.7. Meanwhile, substitution of Fe^II with Zn^II or Ni^II [x ≈ 0.2, y = 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.