Photomagnetic properties of iron(ii) spin crossover complexes of 2,6-dipyrazolylpyridine and 2,6-dipyrazolylpyrazine ligands

Abstract

The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2, [Fe(L2)2][BF4]2, [Fe(L2)2][ClO4]2, [Fe(L3)2][BF4]2, [Fe(L3)2][ClO4]2 and [Fe(L4)2][ClO4]2 (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds display a complete thermal spin transition centred between 200-300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS --> LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T(1/2): T(LIESST) = T(0)- 0.3T(1/2). T(0) for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms, the light-induced high-spin state, and the low-spin complex [Fe(L4)2][BF4]2, are described.

Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex

“…While it is difficult to draw further comparison between 1-3 and 4, this emphasizes the structural complexity of the [Fe (bpp)2] 2+ system in this particular lattice type. The T(LIESST) values from 1-3 are in line with previous expectation, [20,28] but the relaxation kinetics of their metastable high-spin states unexpectedly involve a mixture of weakly (exponential) and strongly (sigmoidal) cooperative regimes. The explanation for this behavior is uncertain, given that 1, and 2 and 3, exhibit different phase behavior in the temperature range that could be examined (above 100 K).…”

“…For all complexes, a drastic increase in the magnetic signal under green light irradiation was observed at 10 K. However, unusually for [Fe(bpp)2] 2+ derivatives, [7,20,[28][29][30] the photoconversion efficiency was not quantitative. The maximum MT values indicate photoconversion efficiencies of 70 % (for 1), 57 % (2) and 60 % (3).…”

“…[20,28] T(LIESST) = T0 -0.3T½ (1) However, although the differences are small, it is noteworthy that within this series 1 has both the lowest T1/2 and the lowest T(LIESST) values, in apparent violation of eq 1 and Hauser's inverse energy gap law.…”

“…[7,20,[28][29][30] The low spin high spin photoconversion was investigated on 1-3 in bulk condition using a SQUID magnetometer coupled to a CW optical source. The samples were irradiated at the following wavelengths: 405, 510, 640, 830 and 980 nm.…”

Effect of Central Metal Ion, Co<SUP>II</SUP> in the Fe<SUP>II</SUP> Spin-Crossover Complex in Emulsion Polymerization of Trifluoroethylmethacrylate Using Poly(Vinyl Alcohol)

“…2) Gütlich 3) and Carbonera 4) have described the design of molecular switches made of spin transition (ST) molecules displaying the light-induced excited spin state trapping (LIESST) effect.…”