Guest-Responsive Elastic Frustration “On–Off” Switching in Flexible, Two-Dimensional Spin Crossover Frameworks

Abstract

In this study we exploit the flexible nature of porous coordination polymers (PCPs) with integrated spin crossover (SCO) properties to manipulate the multistability of spin-state switching profiles. We previously reported the two-dimensional Hofmann-type framework [Fe(thtrz)₂Pd(CN)₄]·EtOH,H₂O (1·EtOH,H₂O), N-thiophenylidene-4 H-1,2,4-triazol-4-amine), displaying a distinctive two-step SCO profile driven by extreme elastic frustration. Here, we reveal a reversible release mechanism for this elastic frustration via stepwise guest removal from the parent phase (1·EtOH,H₂O → 1·H₂O → 1·Ø). Parallel variable temperature structural and magnetic susceptibility measurements reveal a synergistic framework flexing and "on-off" switching of multistep SCO character concomitant with the onset of guest evacuation. In particular, the two-step SCO properties in 1·EtOH,H₂O are deactivated such that both the partially solvated (1·H₂O) and desolvated (1·Ø) phases show abrupt and hysteretic one-step SCO behaviors with differing transition temperatures (1·H₂O: T_1/2↓: 215 T_1/2↑: 235 K; 1·Ø: T_1/2↓: 170 T_1/2↑: 182 K). This "on-off" elastic frustration switching is also reflected in the light-induced excited spin state trapping (LIESST) properties of 1·EtOH,H₂O and 1·Ø, with nonquantitative (ca. 50%, i.e., LS ↔ 1:1 HS:LS) and quantitative (ca. 100%, LS ↔ HS) photoinduced spin state conversion achieved under light irradiation (510 nm at 10 K), respectively. Conversely, the two-step SCO properties are retained in the water saturated phase 1·3H₂O but with a subtle shift in transition temperatures. Comparative analysis of this and related materials reveals the distinct roles that indirect and direct guest interactions play in inducing, stabilizing, and quantifying elastic frustration and the importance of lattice flexibility in these porous framework architectures.