Thermal and Optical Switching of Molecular Spin States in the {[FeL[H2B(pz)2]2} Spin-Crossover System (L = bpy, phen)

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)[H 2 B(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, n HS, obtained from the Mössbauer spectra has been used to calibrate the thermal dependence of n HS deduced from the magnetic susceptibility measurements. The thermodynamic Slichter and Drickamer model was used to fit the thermal variation of n HS . The estimated thermodynamic parameters are Δ H = 7.7 kJ mol -1, Δ S = 47.4 J mol -1 K -1 and interaction parameter Γ = 2.1 kJ mol -1 for 1 and Δ H = 7.9 kJ mol -1, Δ S = 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: Δ H = 7.7 ± 0.4 kJ mol -1 and Δ S = 48 ± 3 J mol -1 K -1 for 1, and Δ H = 8.0 ± 0.4 kJ mol -1 and Δ S = 49 ± 3 J mol -1 K -1 for 2 . The transition temperatures, estimated from the heat capacity peaks, were T c = 159.5 K for 1, and T c (up) = 165.0 K, T c (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 [FeN 6 ] pseudo-octahedron has been estimated from the measured spectra as Δ S vib = 36 J K -1 mol -1, and the total entropy change is Δ S = Δ S vib + Δ S el = 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, E a, 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.