Solid-State Ligand-Driven Light-Induced Spin Change at Ambient Temperatures in Bis(dipyrazolylstyrylpyridine)iron(II) Complexes

We previously reported that an Fe-II complex ligated by two (Z)-2,6-di(1H-pyrazol-1-yl)-4-styrylpyridine ligands (Z-H) presented a solid state ligand-driven light-induced spin change (LD-LISC) upon one-way Z-to-E photoisomerization, although modulation of the magnetism was trivial at ambient temperatures (Chem. Commun. 2011, 47, 6846). Here, we report the synthesis of new derivatives of Z-H, Z-CN and Z-NO2, in which electron-withdrawing cyano and nitro substituents are introduced at the 4-position of the styryl group to attain a more profound photomagnetism at ambient temperatures. Z-CN and Z-NO2 undergo quantitative one-way Z-to-E photochromism upon excitation of the charge transfer band both in acetonitrile and in the solid state, similar to the behavior observed for Z-H. In solution, these substituents stabilized the low-spin (LS) states of Z-CN and Z-NO2, and the behavior was quantitatively analyzed according to the Evans equation. The photomagnetic properties in the solid state, on the other hand, cannot be explained in terms of the substituent effect alone. Z-CN displayed photomagnetic properties almost identical to those of Z-H. Z-CN preferred the high-spin (HS) state at all temperatures tested, whereas photoirradiated Z-CN yielded a lower chi T-M at ambient temperatures. The behavior of Z-NO2 was counterintuitive, and the material displayed surprising photomagnetic properties in the solid state. Z-NO2 occupied the LS state at low temperatures and underwent thermal spin crossover (SCO) with a T-1/2 of about 270 K. The photoirradiated Z-NO2 displayed a higher value of chi T-M and the modulation of chi T-M exceeded that of Z-H or Z-CN. Z-NO2 center dot acetone, in which acetone molecules were incorporated into the crystal lattice, further stabilized the LS state (T-1/2 > 300 K), thereby promoting large modulations of the chi T-M values (87% at 273 K and 64% at 300 K) upon Z-to-E photoisomerization. Single crystal X-ray structure analysis revealed that structural factors played a vital role in the photomagnetic properties in the solid state. Z-H and Z-CN favored intermolecular pi-pi stacking among the ligand molecules. The coordination sphere around the Fe-II nucleus was distorted, which stabilized the HS state. In contrast, Z-NO2 center dot acetone was liberated from such intermolecular pi-pi stacking and coordination distortion, resulting in the stabilization of the LS state.