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-<i>x</i> M _<i>x</i> (<i>n</i>Bu-im)₃(tren)](P_1-<i>y</i> As _<i>y</i> F₆)₂ (M = Zn^II, Ni^II; (<i>n</i>Bu-im)₃(tren) = tris(<i>n</i>-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 (<i>T</i> _c) and hysteresis widths (Δ<i>T</i> _c, memory) can be selected at will. The pristine derivative 100As (<i>x</i> = 0, <i>y</i> = 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 <i>n</i>-butyl arms and two different arrangements of the AsF₆ ^- anions [<i>T</i>1c = 174 K (Δ<i>T</i>1c = 17 K), <i>T</i>2c = 191 K (Δ<i>T</i>2c = 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 [<i>T</i>PTc = 275 K (Δ<i>T</i>PTc = 16 K)]. This behavior strongly contrasts with that of the homologous 100P [<i>x</i> = 0, <i>y</i> = 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, <i>T</i> _c = 122 K (Δ<i>T</i> _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 <i>T</i> _c = 155 K (Δ<i>T</i> _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 (<i>ca.</i> 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 <i>x</i> = 0 and <i>y</i> ≈ 0.7. Meanwhile, substitution of Fe^II with Zn^II or Ni^II [<i>x</i> ≈ 0.2, <i>y</i> = 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.