Spin transitions of [Fe^II(phen)₂(NCS)₂] spin crossover complex in ethylene glycol matrix

Introduction. Spin crossover (SCO) materials, which are capable of spin-state switching between high spin and low spin states, are of significant focus in material science research for possible applications in spin-based devices. Here, we investigated spin transitions of Fe(II)-based spin crossover (SCO) complex [Fe^II(phen)₂(NCS)₂], which shows spin state bi-stability between its high spin (HS, S=2) and low spin (LS, S=0) embedded in ethylene glycol matrix.

Methods. The SCO complex is synthesized by the sol–gel method with a judicious choice of reacting materials and the materials are further characterized by X-ray powder diffraction, temperature dependent current–voltage (I-V) and current vs. time (I-t) measurements. Magnetic characterisations are also performed with electron paramagnetic resonance in the temperature range of 110-280K.

Results. Electrical measurement obtained by acquiring temperature-dependent I-V data shows thermal spin-state switching with prominent hysteresis loop. The low temperature (T=90 K) data show an LS state with higher conductivity, whereas at high temperatures, it shows lower conductivity in the HS state. The measured I-t data under dark and illumination at 300 K and 90 K, respectively, provide a better photo-induced response at lower temperature. The EPR data of 280 K show a prominent peak at ~2900 Gauss with g=2.1, along with an additional peak at~ 2300 Gauss corresponding to g=2.9. The additional peak is smeared out with a lowering in temperature, indicating HS to LS spin transition. Embedding polymer reduces the transition temperature by ~160 K, which is slightly lower than that of bulk (176 K), which may arise due to the reduction in strong co-operative interaction between the π-π or H bonding of the complex.

Conclusion. We demonstrated the spin transition characteristics of the SCO complex in the ethylene glycol matrix with features different from that of the bulk, which may be quite important in designing future applications.