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

• Published in: The journal of physical chemistry. B Vol. 106; no. 16; pp. 4276 - 4283
• Main Authors: Moliner, Nicolás, Salmon, Lionel, Capes, Laurence, Muñoz, M. Carmen, Létard, Jean-François, Bousseksou, Azzedine, Tuchagues, Jean-Pierre, McGarvey, John J, Dennis, Andrew C, Castro, Miguel, Burriel, Ramón, Real, José Antonio
• Format: Journal Article
• Language: English
• Published: American Chemical Society 25.04.2002
• Subjects: Chemical Sciences; Material chemistry; Mössbaeur spectroscopy; Spin-crossover compounds; Calorimetry; Raman spectroscopy; LIESST effect
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp013872b

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, 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 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 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 [FeN6] pseudo-octahedron has been estimated from the measured spectra as ΔS vib = 36 J K-1 mol-1, and the total entropy change is ΔS = ΔS vib + ΔS 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.