Bidirectional photoswitchability in an iron(iii) spin crossover complex: symmetry-breaking and solvent effects

Abstract

The impact of solvent on spin crossover (SCO) behaviour is reported in two solvates [Fe(qsal-I)₂]NO₃·2ROH (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate; R = Me 1 or Et 2) which undergo abrupt and gradual SCO, respectively. A symmetry-breaking phase transition due to spin-state ordering from a [HS] to [HS-LS] state occurs at 210 K in 1, while T_1/2 = 250 K for the EtOH solvate, where complete SCO occurs. The MeOH solvate exhibits LIESST and reverse-LIESST from the [HS-LS] state, revealing a hidden [LS] state. Moreover, photocrystallographic studies on 1 at 10 K reveal re-entrant photoinduced phase transitions to a high symmetry [HS] phase when irradiated at 980 nm or a high symmetry [LS] phase after irradiation at 660 nm. This study represents the first example of bidirectional photoswitchability and subsequent symmetry-breaking from a [HS-LS] state in an iron(iii) SCO material.

Fig. 1. Visual representation of (a) LIESST and reverse-LIESST and (b) light activation to access hidden (not easily accessible) spin states.

Fig. 2. Unit cell components of 1 at 150 K (left) and 300 K (right). The a, b and c axis at 300 K and 150 K correspond to different directions.

Fig. 3. Schematic illustration of the packing from the asymmetric unit to the final supramolecular 3D structure for 1 at 150 K (blue Fe1 centres, red Fe2 centres). This figure is intended as a guide to the discussion only.

Fig. 4. Plots of the variation of the d_chain (top) and d_plane (bottom) for 1, 2, and 3 at different temperatures. The changes in these values are indicated by the grey bars. *indicates irradiated samples.

Fig. 5. χ _M T versus T plot (recorded at 0.5 K min⁻¹) of 1 showing the thermal behaviour in the dark without any previous light irradiation (blue), after LIESST effect at 980 nm (red) and after reverse-LIESST effect at 650 nm (black) and thermal behaviour of 2 (green).

Fig. 6. Plot of the Fe-N_av distance for 1 at 300 K (orange), 240 K (yellow), 150 K (green), 100 K (light blue), 10 K (blue), 10 K (980 nm) (purple) and 10 K (660 nm) (red). Circles represent the Fe1 centre and squares the Fe2 centre.