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

Abstract

Mo ¨ssbauer 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 Mo ¨ssbauer 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 Mo ¨ssbauer 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 ∆H ) 7.7 kJ mol -1 , ∆S ) 47.4 J mol -1 K -1 and interaction parameter Γ ) 2.1 kJ mol -1 for 1 and ∆H ) 7.9 kJ mol -1 , ∆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: ∆H ) 7.7 ( 0.4 kJ mol -1 and ∆S ) 48 ( 3 J mol -1 K -1 for 1, and ∆H ) 8.0 ( 0.4 kJ mol -1 and ∆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 ∆S vib ) 36 J K -1 mol -1 , and the total entropy change is ∆S ) ∆S vib + ∆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.

Spin Crossover Behavior in the Iron(II)−2-pyridyl[1,2,3]triazolo[1,5-a]pyridine System: X-ray Structure, Calorimetric, Magnetic, and Photomagnetic Studies

“…The remainder entropy variation, 21 J K -1 mol -1 , is mainly due to the intramolecular vibrational changes. 5,[20][21][22] It should be noted that the calculated Γ values account for the poor cooperativity observed for the three derivatives as Γ < 2RT 1/2 . 12 Observation of important amounts of trapped HS molecules at low temperatures is usually ascribed to the occurrence of two different sites in the crystal.…”

The Role of Molecular Vibrations in the Spin Crossover Phenomenon

“…The role of molecular vibrations as a primary contributor to the entropy change accompanying SCO, initially suggested by Sorai and Seki from the results of heat capacity measurements [50], has been fully confirmed by all subsequent approaches including vibrational spectroscopy [7][8][9][10][11], SCO models accounting for the role of molecular vibrations [36,48,54,55,58], and DFT calculations [8,65]. While most results point to the prominent contribution of metal-ligand vibrations [7][8][9][10], some recent data indicate that other intra-and intermolecular (lattice) vibrations can make significant contributions as well [11].…”

“…While most results point to the prominent contribution of metal-ligand vibrations [7][8][9][10], some recent data indicate that other intra-and intermolecular (lattice) vibrations can make significant contributions as well [11]. However, their relative contributions are not yet clearly evaluated.…”

“…In most cases variable temperature mid-IR or far-IR techniques have been employed [5,6] while Raman spectroscopy has become only recently a well established technique for the characterization of the SCO phenomenon [7][8][9][10][11]. It is worth noting that IR spectroscopy of SCO complexes is complicated by the fact that the SCO is very sensitive to mechanical treatments (grinding, milling, pressure, etc.…”

“…The inherent limitations of the average frequency approach have been addressed by summing the entropy terms associated with the distinct vibrational modes of the [Fe(phen) 2 (NCS) 2 ] [8] and [Fe(phen)(H 2 B(pz) 2 ) 2 ] [10] complexes. For the latter complex, the entropy change associated with the 15 normal vibrational modes of the FeN 6 pseudo-octahedron has been estimated from the measured spectra as DS vib =36 JK 1 mol 1 , and the total entropy 5 Thermal variation of the equilibrium temperature T 1/2 as function of the averaged intra-molecular vibrations of the LS state with the ratio a=w LS /w HS =3 [60] change is DS=DS vib +DS el =49 JK 1 mol 1 .…”