Electron spin resonance study of the photo-induced phase transition in the spin-crossover complex

The photo-induced phase transition (PIPT) in the spin-crossover complex, [FeII(2-pic)3]Cl2EtOH, has been investigated by monitoring the ESR signals of impurity Co2+ ions. The principal values of the g-tensor of Co2+ ions in the low temperature phase at 4 K were determined as gxx=2.1, gyy=2.6 and gzz=7.1. The light irradiation resonant to the d–d transition of Fe2+ ions (532 nm) causes the change of gzz from 7.1 to 7.35. The intensity of the photo-induced signal exhibits nonlinear dependence on the irradiation time. These results suggest that the crystal field on a Co2+ ion would approach to that of cubic symmetry by PIPT.