Thermal and light-induced spin transition in [Fe(bpen)X(2)] (bpen equals 1,6-bis(2-pyridyl)-2,5-diazahexane, X=NCS-, NCSe-)

The 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 Mossbauer measurements. The HS --> LS transition in [Fe(bpen)(NCS)(2)] is detected at temperatures below 120 K (T-1/2 [when the fraction of high-spin species gamma(HS) = 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 (T-1/2 = 181 K). Irradiation of the compound with green light at low temperatures does not result in formation of long-lived metastable HS states. The spin-transition curves derived from magnetic susceptibility and Mossbauer studies show good agreement for each compound, and no hints of significantly different Lamb-Mossbauer factors for the HS and LS states are observed.