Second harmonic generation on chiral cyanido-bridged FeII–NbIV spin-crossover complexes

Abstract

Incorporating chiral organic ligands into cyanido-bridged Fe^II–Nb^IV assemblies synthesized chiral spin-crossover complexes, Fe^II₂[Nb^IV(CN)₈](L)₈·6H₂O (L = R-, S-, or rac-1-(3-pyridyl)ethanol: R-FeNb, S-FeNb, or rac-FeNb). Rietveld analyses based on a racemic complex of rac-FeNb indicate that the chiral complexes have a cubic crystal structure in the I2₁3 space group with a three-dimensional cyanido-bridged Fe^II–Nb^IV coordination network. All the complexes exhibit spin crossover between the high-spin (HS) and the low-spin (LS) Fe^II states without thermal hysteresis. Chiral complexes of R-FeNb and S-FeNb show second harmonic generation (SHG) due to their non-centrosymmetric structure. The I2₁3 space group provides second-order susceptibility tensor elements of χ_xyz, χ_yzx, and χ_zxy, which contribute to SHG. The temperature-dependent second harmonic light intensity change is due to spin crossover between Fe^II_HS and Fe^II_LS.