Thermal and Light‐Induced Spin Transition in [Fe(bpen)X2] (bpen = 1,6‐Bis(2‐pyridyl)‐2,5‐diazahexane, X = NCS‐, NCSe‐)

Abstract

AbstractThe thermal and light‐induced spin transitions in [Fe(bpen)(NCS)2] and [Fe(bpen)(NCSe)2] have been investigated by means of magnetic susceptibility and Mössbauer measurements. The HS → LS transition in [Fe(bpen)(NCS)2] is detected at temperatures below 120 K (T1/2 [when the fraction of high‐spin species γHS = 0.5] = 68 K) with a residual HS fraction of 8% at 20 K. The transition is fairly abrupt and no hysteresis is observed, in contrast to earlier studies (ref. [8]). The generation of metastable HS states in [Fe(bpen)(NCS)2] is achieved by rapid cooling of the sample and by irradiating the compound at low temperatures with light of appropriate wavelengths (LIESST). The HS → LS relaxation of the metastable spin states after rapid cooling is observed at temperatures between 35 and 42.5 K and shows strong deviations from single exponential behavior. The spin‐transition behavior changes drastically on substitution of the thiocyanate ions by NCSe‐. In [Fe(bpen)(NCSe)2] a gradual and complete spin transition between 140 and 250 K is observed (T1/2 = 181 K). Irradiation of the compound with green light at low temperatures does not result in formation of long‐lived metastable HS states. The spintransition curves derived from magnetic susceptibility and Mössbauer studies show good agreement for each compound, and no hints of significantly different Lamb–Mössbauer factors for the HS and LS states are observed.