Tunable Raman Scattering of Fe(II) Coordination Networks With Magnetic Fields.

Controlling terahertz (THz) waves is important for next-generation communications, sensing, and imaging technologies. However, it is still unclear precisely how THz signals can be manipulated by various materials. Herein, we report the design and synthesis of four coordination networks, {[Fe2(Xpz)2][Hg(SCN)4]2[Hg(SCN)2]}n (Xpz = fluo140ropyrazine, 1-F; Xpz = chloropyrazine, 1-Cl; Xpz = bromopyrazine, 1-Br) and {[Fe(Ipz)][Hg(SCN)4]}n (Ipz = iodopyrazine, 1-I) as tunable THz absorbers. 1-Cl and 1-Br form isostructural three-dimensional frameworks closely related to 1-F, differentiated in the Xpz ligand orientation. In contrast, 1-I adopts a distinct two-dimensional framework due to the more sterically demanding halogen. Clear differences are observed in the sub-THz (< 1 THz) absorption features between the four materials, with the lowest features shifting to lower frequencies from 1-F to 1-Br. As these materials contain open-shell high-spin Fe(II) ions, we show that the application of a magnetic field can precisely modulate low-frequency (LF) Raman bands by ca. 0.06 THz/T at 4 K in the THz range. We show using Raman magnetospectroscopy (RAMS) that this tunability arises from the zero-field splitting (ZFS) of Fe(II).