Elastic Frustration Triggering Photoinduced Hidden Hysteresis and Multistability in a Two-Dimensional Photoswitchable Hofmann-Like Spin-Crossover Metal–Organic Framework

We
report a two-dimensional Hofmann-like spin-crossover (SCO) material,
[Fe­(trz-py)2{Pt­(CN)4}]·3H2O,
built from [FePt­(CN)4] layers separated by interdigitated
4-(2-pyridyl)-1,2,4,4H-triazole (trz-py) ligands
with two symmetrically inequivalent FeII sites. This compound
exhibits an incomplete first-order spin transition at 153 K between
fully high-spin (HS–HS) and intermediate high-spin low-spin
(HS–LS) ordered states. At low temperature, it undergoes a
bidirectional photoswitching to HS–HS and fully low-spin (LS–LS)
states with green and near-IR light irradiation, respectively, with
associated T­(LIESST = Light-Induced Excited Spin-State Trapping) and
T­(reverse-LIESST) values of 52 and 85 K, respectively. Photomagnetic
investigations show that the reverse-LIESST process, performed from
either HS-HS or HS-LS states, enables access to a hidden stable LS-LS
state, revealing the existence of a hidden thermal hysteresis. Crystallographic
investigations allowed to identify that the strong metastability of
the HS-LS state originates from the existence of a strong elastic
frustration causing antiferroelastic interactions within the [FePt­(CN)4] layers, through the rigid NC-Pt-CN bridges connecting the
inequivalent FeII sites. The existence of the stable LS-LS
state paves the way for a multidirectional photoswitching and allows
potential applications for electronic devices based on ternary digits.