Guest-responsive elastic frustration 'on-off' switching in flexible 2-D spin crossover frameworks

In this study we exploit the flexible nature of porous coordination polymers (PCPs) with integrated spin crossover (SCO) properties to manipulate the multi-stability of spin-state switching profiles. We previously reported the 2-D Hofmann-type framework [Fe(thtrz)2Pd(CN)4].EtOH,H2O (1.EtOH,H2O), N-thiophenylidene- 4H-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 step-wise guest removal from the parent phase (1.EtOH,H2O → 1.H2O → 1..). 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..) phases show abrupt and hysteretic one-step SCO behaviors with differing transition temperatures (1.H2O: T.↓: 215 T.↑: 235 K; 1..: T.↓: 170 T.↑: 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.., with non-quantitative (ca. 50 %, i.e. LS ↔ 1:1 HS:LS) and quantitative (ca. 100 %, LS ↔ HS) photo-induced 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 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 flexible in these porous framework architectures.