High-Spin → Low-Spin Relaxation in [Fe(bpp)2](CF3SO3)2 H2O after LIESST and Thermal Spin-State Trapping-Dynamics of Spin Transition Versus Dynamics of Phase Transition

The iron(II) complex [Fe(bpp)2]‐(CF3SO3)2 H2O (bpp = 2,6‐bis(pyrazolyl‐3‐yl)pyridine) shows a thermal spin transition associated with a hysteresis of approximately 140 K width. The transition temperatures T1/2 (where the fraction of HS species γHS = 0.5) are 147 K and ≈︁285 K in the cooling and heating directions, respectively. The compound shows the LIESST and reverse‐LIESST effects at low temperatures. The relaxation of the metastable HS states generated by LIESST was observed quantitatively at temperatures between 77.5 and 85 K by Mössbauer spectroscopy. Metastable HS states can also be generated by rapid cooling of the sample. The relaxation of the metastable HS states formed by thermal spin‐state trapping was monitored at temperatures between 104 and 118 K by magnetic susceptibility measurements. The relaxation mechanisms of the HS states generated by LIESST and thermal spinstate trapping are completely different. We suggest that the HS → LS relaxation after thermal spin‐state trapping is triggered by an additional structural phase transition of the system. Different high‐spin → low‐spin relaxation mechanisms were revealed by a study of the spin‐crossover compound [Fe(bPP)2](CF3SO2)2·H2O by means of Mossbauer spectroscopy and magnetic susceptibility measurements. Long‐lived metastable high‐spin states can be generated by rapid cooling as well as by use of the LIESST (light‐induced excited spin state trapping) effect; the choice of method affects the relaxation behavior. The Figure shows relaxation curves after thermal spin‐state trapping at temperatures from 104 (+) to 118 K (+).