Iron(III) spin-crossover compounds [Fe(pap)(2)]ClO(4) (1), [Fe(pap)(2)]BF(4) (2), [Fe(pap)(2)]PF(6) (3), [Fe(qsal)(2)]NCS (4), and [Fe(qsal)(2)]NCSe (5) (Hpap=2-(2-pyridylmethyleneamino)phenol and Hqsal=2-[(8-quinolinylimino)methyl]phenol) were prepared and their spin-transition properties investigated by magnetic susceptibility and Mössbauer spectroscopy measurements. The iron(III) compounds exhibited spin transition with thermal hysteresis. Single crystals of the iron(III) compounds were obtained as suitable solvent adducts for X-ray analysis, and structures in high-spin (HS) and low-spin (LS) states were revealed. Light-induced excited-spin-state trapping (LIESST) effects of the iron(III) compounds were induced by light irradiation at 532 nm for 1-3 and at 800 nm for 4 and 5. The activation energy E(a) and the low-temperature tunneling rate k(HL)(T-->0) of iron(III) LIESST compound 1 were estimated to be 1079 cm(-1) and 2.4x10(-8) s(-1), respectively, by HS-->LS relaxation experiments. The Huang-Rhys factor S of 1 was also estimated to be 50, which was similar to that expected for iron(II) complexes. It is thought that the slow relaxation in iron(III) systems is achieved by the large structural distortion between HS and LS states. Introduction of strong intermolecular interactions, such as pi-pi stacking, can also play an important role in the relaxation behavior, because it can enhance the structural distortion of the LIESST complex.

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The thermal spin transitions of iron(II) spin-crossover compounds [Fe(PM-iPA)2(NCS)2] (1) and [Fe(PM-iPA)2(NCSe)2] (2) (PM-iPA = N-(2′-pyridylmethyl)-isopropylamine) have occurred at T1/2 = 267 K and 376 K without thermal hysteresis. No light-induced excited spin state trapping (LIESST) effect was observed for compounds 1 and 2 even at 5 K. The iron(II) spin-crossover compounds [Fe(PM-L)2(NCX)2] (PM-A = N-(2′-pyridylmethyl)-aniline, PM-BiA = N-(2′-pyridylmethyl)-4-aminobiphenyl, PM-TeA = N-(2′-pyridylmethyl)-4-aminoterphenyl, PM-PEA = N-(2′-pyridylmethyl)-4-(phenylethynyl)aniline, and PM-AzA = N-(2′-pyridylmethyl)-4-(phenylazo)aniline; X = S and Se) with ligands having π-systems have exhibited the LIESST effect, and the critical LIESST temperature, Tc(LIESST), has been observed. The compounds 1 and 2 are crystallized at Pnna and C2/c at 298 K, respectively, and the space groups of the compounds remained unchanged until 100 K, although the compounds show the spin transition. The molecular packing structure and thermodynamic parameters of the spin transitions calculated from the magnetic susceptibility curves suggest that compounds 1 and 2 have no strong intermolecular interactions between the complexes, while the compounds with π-system ligands form π–π intra- and intermolecular interactions between the ligands. Our conclusion is that the intermolecular interactions play an important role in trapping a light-induced metastatble state.

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Formation of organogels from europium(III) complexes consisting of ligands having an amide or a urea unit has been achieved; the organogels are characterized by strong red-emissions.