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

Abstract

Mossbauer 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 Mossbauer 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 Mossbauer 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 ...