Magnetism and Molecular Nonlinear Optical Second-Order Response Meet in a Spin Crossover Complex

The quadratic hyperpolarizability of two inorganic Schiff base metal complexes which differ from each other by the nature of the central metal ion (Fe-II or Zn-II) is estimated using hyper-Rayleigh light-scattering (HRS) measurements. The investigated Fe-II microcrystals exhibit a thermal spin-crossover (SCO) from a diamagnetic to a paramagnetic state centered at T-1/2 = 233 K that can be reproduced by the HRS signal whose modest intensity is mainly due to their centrosymmetric packing structure. Diamagnetic Zn-II microcrystals even lead to much weaker (similar to 400 times) HRS intensities which are in addition temperature-independent. These observations allow us to ascribe the change in HRS of the Fe-II complex to two contributions, namely, the molecular SCO phenomenon and the crystal orientation with respect to the light polarization. A connection between the SCO and a nonlinear optical property has thus been demonstrated for the first time, with potential future applications in photonics.