Light-induced excited spin state trapping in iron(iii) complexes

Abstract

This review discusses the correlation of the local and whole molecular structure of iron(iii) complexes with the magnetic properties including the light-induced excited spin-state trapping (LIESST) effect.

Structural studies into the spin crossover behaviour of Fe(abpt)2(NCS)2 polymorphs B and D

“…The phenomenon is relatively common for 3d 4 -3d 7 metal centres. 5 While many of the spin crossover complexes that have been studied are octahedral Fe(II) complexes with nitrogen [5][6][7][8][9][10] ligands, other examples include Fe(III) 4,[11][12][13][14] , Co(II) [15][16][17] and Ni(II) 18 . The change in electronic structure, as a result of the spin crossover, can result in changes in properties including molecular structure, colour and magnetism.…”

“…In the 1930's, the first observation of spin crossover was reported. 1 Spin crossover compounds display a reversible high spin (HS) to low spin (LS) transition at a metal centre, which can be induced by external stimuli such as temperature, pressure 2 or light irradiation 3,4 and can occur in both solution and the solid state. The phenomenon is relatively common for 3d 4 -3d 7 metal centres.…”

“…1 Spin crossover compounds display a reversible high spin (HS) to low spin (LS) transition at a metal centre, which can be induced by external stimuli such as temperature, pressure 2 or light irradiation 3,4 and can occur in both solution and the solid state. The phenomenon is relatively common for 3d 4 -3d 7 metal centres. 5 While many of the spin crossover complexes that have been studied are octahedral Fe(II) complexes with nitrogen [5][6][7][8][9][10] ligands, other examples include Fe(III) 4,[11][12][13][14] , Co(II) [15][16][17] and Ni(II) 18 .…”

Halogen-Substituted Spin-Crossover Fe(III) Compounds with Photoresponsive Properties

“…On the other hand, the relatively smaller changes in the average Fe–N/O bond distances between the high-spin (HS) and low-spin (LS) states in SCO Fe­(III) compounds, Δ r HL (= | r HS – r LS |), are thought to prohibit the light-induced excited spin-state trapping (LIESST) effect because a small Δ r HL value (∼0.13 Å for Fe III versus ∼0.20 Å for Fe II ) , promotes fast spin relaxation from the metastable HS state to the LS state of the Fe­(III) ion. − So, the LIESST effects in SCO Fe­(III) compounds previously did not attract much attention. But since the discovery of the first LIESST effect in the SCO Fe­(III) compound [Fe­(pap) 2 ]­ClO 4 ·H 2 O, it is suggested that the structural distortion parameters of the Fe­(III) coordination sphere, Σ (the sum of the deviation from 90° of the 12 cis -(N/O–Fe–N/O) angles about the iron atom) and Δ∠ HL,total (the sum of the bond angle differences between the different spin states; this parameter is calculated by using 15 cis and trans (N/O–Fe–N/O) bond angles), instead of the Δ r HL value determine the activation energy for metastable HS-to-LS relaxation, so this discovery has stimulated researchers’ enthusiasm in the study of LIESST effects in SCO Fe­(III) compounds. − Strong intermolecular interactions favoring cooperative SCO might contribute to large structural distortion responsible for achieving the LIESST effect but are not necessary. , Reversible LIESST effects in Fe­(II) SCO compounds have been examined systematically since 1986, which have opened up another way for accurate and reversible regulation of the spin states and therefore physical properties of SCO compounds. ,− Despite these achievements, the design of a bidirectional photocontrolled magnetic switching in the Fe­(III) SCO system remains a daunting challenge. ,,− …”

Vapor-triggered reversible crystal transformation of a nickel-based magnetic molecular switch

“…[1][2][3][4] Among the versatile stateof-the-art proposals, spin-crossover (SCO)-active first-row transition metal complexes with 3d n (n = 4-7) electronic configuration represent the most promising candidates, which are capable of undergoing a reversibly and controllably switchable characteristic between high spin (HS) and low spin (LS) states in response to external multiple stimuli. [5][6][7][8][9][10][11][12][13] Of particular interest are iron(II) SCO systems featuring an abrupt and stepwise transition with thermal hysteresis (DT 1/2 ), [14][15][16][17][18] a laser pulse-induced spin state switching inside thermal hysteresis loop 19,20 as well as the synergy with functionality, [21][22][23][24][25][26] which have rendered themselves suitable for the realization of molecular electronic and spintronic devices. 22 In contrast to the overwhelming majority of SCO complexes containing classical 3d 4 -3d 7 magnetic centres, equal attention to systems involved with the ''anomalous magnetism'' of d 8 ions is far behind.…”