Mutual influence of photochromic and magnetic sublattices in crystals of erbium(III) tetrakis(hexafluoroacetylacetonate) salt with 1-[(1′,3′,3′-trimethylspiro[2і/-1-benzopyran-2,2′-indoline]-8-yl)methyl]pyridinium

We report the synthesis, structural characterization, and physicochemical properties of two erbium(III) tetrakis(hexafluoroacetylacetonate) salts: the known sodium complex Na⁺[Er(hfac)₄]⁻ (1) and a novel compound featuring a photochromic cation, (C₂₅H₂₅N₂O)⁺[Er(hfac)₄]⁻ (2), where the cation is 1-[(1′,3′,3′-trimethylspiro[2H-1-benzopyran-2,2′-indoline]-8-yl)methyl]pyridinium.

Single crystal X-ray diffraction analysis reveals that complex 2 crystallizes in the monoclinic space group P2₁/c, with alternating layers of magnetic anions and photochromic cations held together by ionic and van der Waals interactions. Notably, the replacement of the small Na⁺ cation in 1 with a bulky organic cation in 2 leads to a distortion of the [ErO₈] coordination geometry from a square antiprism to a distorted dodecahedron, suppressing the single-ion magnet (SIM) behavior observed in 1. Conversely, the magnetic anion in 2 activates solid-state photochromism in the cation, which is inactive in its bromide form. This effect could be attributed to the highly polar environment provided by the [Er(hfac)₄]⁻ anion, which stabilizes the zwitterionic merocyanine form of the cation, the effect being analogous to solvatochromism. These findings demonstrate a rare and mutually responsive interaction between magnetic and photochromic sublattices in the crystalline material, offering valuable insights into the design of multifunctional hybrid systems with opto-magnetic switching capabilities.