Thermal and optical switching of molecular spin states in the {[FeL[H2B(pz)2]2} spin-crossover system (L = bpy, phen)
Résumé
Mössbauer and Raman spectroscopies, calorimetric measurements, the LIESST effect (light induced excited spin state trapping) and dynamics of the high-spin (HS) to low-spin (LS) relaxation have been investigated in order to thoroughly study the spin-crossover compounds of formula {Fe(L)[H2B(pz)2]2}, where L = 2,2'-bipyridine (1) and 1,10-phenanthroline (2). The Mössbauer studies have revealed the occurrence of an almost complete spin conversion at low temperature, while 9.5% (1) and 15% (2) of molecules remain LS at room temperature. The thermal variation of the proportion of HS molecules, nHS, obtained from the Mössbauer spectra has been used to calibrate the thermal dependence of nHS deduced from the magnetic susceptibility measurements. The thermodynamic Slichter and Drickamer model was used to fit the thermal variation of nHS. The estimated thermodynamic parameters are ΔH = 7.7 kJ mol-1, ΔS = 47.4 J mol-1 K-1 and interaction parameter Γ = 2.1 kJ mol-1 for 1 and ΔH = 7.9 kJ mol-1, ΔS = 48.3 J mol-1 K-1 and Γ = 3 kJ mol-1 for 2. These data agree with those obtained from heat capacity experiments: ΔH = 7.7 ± 0.4 kJ mol-1 and ΔS = 48 ± 3 J mol-1 K-1 for 1, and ΔH = 8.0 ± 0.4 kJ mol-1 and ΔS = 49 ± 3 J mol-1 K-1 for 2. The transition temperatures, estimated from the heat capacity peaks, were Tc = 159.5 K for 1, and Tc(up) = 165.0 K, Tc(down) = 162.7 K for 2, in heating and cooling processes. Raman spectra have been recorded for 2 both in the HS (300 K) and LS (110 K) states. Significant changes occur in the position of several low-frequency Raman modes in the low-temperature spectrum. The entropy change associated with the 15 normal vibrational modes of the [FeN6] pseudo-octahedron has been estimated from the measured spectra as ΔSvib = 36 J K-1 mol-1, and the total entropy change is ΔS = ΔSvib + ΔSel = 49 J K-1 mol-1. This value is comparable to that obtained from the calorimetric measurements. Photomagnetic studies revealed a complete conversion from the LS into the HS state upon irradiation at 10 K in the 647.1−676.4 nm wavelength region. The critical LIESST temperatures are 52 K (1) and 44 K (2). The dynamics of the LS to HS conversion was studied for the two compounds in the 6−50 K temperature range. In the thermally activated region the apparent activation energy, Ea, was estimated to be 240 cm-1 for 1 and 170 cm-1 for 2. In the tunneling region the rate constants were estimated to lie in the 9 × 10-7 to 1 × 10-6 s-1 range.