Slow magnetic relaxation in a complex of photochromic spiropyran in a merocyanine form and cobalt(ii) hexafluoroacetylacetonate

Abstract

The interaction between photochromic 8-methoxy-1′,3′,3′-trimethyl-6-nitro-spiro[chromene-2,2′-indole] (MNSP) and cobalt(II) hexafluoroacetylacetonate yields a deep red-violet solution due to the coordination-induced transition of MNSP from a colorless closed state to a colored open merocyanine (MC) form. The resulting complex {Co^II(hfac)₂·MNSP} (1) is obtained as crystals, and its structure at 250 K shows the coordination of two oxygen atoms from MNSP with Co^II, forming a distorted octahedral surrounding around Co^II. The unit cell of 1 contracts below 170 K by 5.5%, which is accompanied by shortening O–Co bonds and altering O–Co–O angles in the structure solved at 150 K. The χ_MT value of 3.06 emu K mol⁻¹ at 300 K indicates the formation of high-spin Co^II (S = 3/2) with a large orbital contribution characteristic of the octahedral surrounding. The contraction of the unit cell of 1 below 170 K provides a reversible 2.2% change of χ_MT in the 170–160 K range during heating and cooling regimes without hysteresis. The electron paramagnetic resonance signal of 1 was simulated with g-values of g_x = 2.342, g_y = 2.364, and g_z = 2.084, and an isotropic g-factor of 2.267. The temperature-dependent χ_MT and field-dependent magnetization of 1 allow us to determine a positive zero-field splitting parameter of +20.1 cm⁻¹. A study of the dynamic properties of 1 shows slow magnetic relaxation for Co^II in a static field of 1000 Oe. Magnetic hysteresis loops were observed for 1 at 0.5 and 2 K; these loops are closed in the zero field region but opened at fields higher than ±100 and 700 Oe, respectively. The collapse of the loop is observed at 5 K. Excitation of the solution of 1 with green light decreases partially the intensity of the bands of the complex, whereas subsequent exposure of this solution to UV light partially restores these intensities.