Enhancement of the curie temperature by isomerization of diarylethene (DAE) for an organic-inorganic hybrid system: Co₄(OH)₇(DAE) _0.5·3H₂O

Intercalation of an organic photochromic molecule into layered magnetic systems may provide multifunctional properties such as photomagnetism. To build up a photosensitive multifunctional magnet, an organic-inorganic hybrid system coupled with a photochromic diarylethene anion, 2,2′-dimethyl-3,3′- (perfluorocyclopentene-1,2-diyl)bis(benzo[b]-thiophene-6-sulfonate) (DAE), and cobalt LDHs (layered double hydroxides), Co₄(OH)₇(DAE) _0.5·3H₂O, was synthesized by the anion exchange reaction between Co₂(OH)₃(CH₃COO)·H ₂O and DAE. In the dark and under UV-irradiated (313 nm) conditions, Co₄(OH)₇(DAE)_0.5·3H₂O with open and closed forms of DAE were obtained, respectively. The magnetic susceptibility measurements elucidated ferromagnetic intra- and interlayer interactions and Curie temperatures of Tc = 9 and 20 K for cobalt LDHs with the open and closed forms of DAE, respectively. The enhancement of the Curie temperature from 9 to 20 K by substitution of the open form of DAE with the closed form of DAE as an intercalated molecule is attributed to the delocalization of the π-electrons in the closed form of DAE, which enhances the interlayer magnetic interaction. The enhancement of the interlayer magnetic interaction induced by the delocalization of π-electrons in intercalated molecules is strongly supported by the fact that the Curie temperature (26.0 K) of cobalt LDHs with (E,E)-2,4-hexadienedioate having a conjugated π-electron system is enormously higher than that (7.0 K) of the cobalt LDHs with hexanedioate. By UV irradiation at 313 nm, Co₄(OH) ₇(DAE)_0.5·3H₂O shows the photoisomerization of DAE from the open form to the closed one in the solid state, which leads to the enhancement of Curie temperature.