Anomalous magnetic behaviour induced by a structural phase transition with anisotropic thermal expansion in a spirodiradical

Abstract

The compound 2,2′,7,7′-tetra(tert-butyl)-9,9′(10H,10′H)spirobigermaacridine-10,10′-dioxyl (1-Ge) exhibits a decrease in the magnetic moment at 252 K; its magnetic moment decreases upon heating, contrary to the expected entropy gain. Structural analyses revealed anisotropic thermal expansion, with negative thermal expansion along the b-axis, and a structural phase transition occurring at the same temperature as that for the observed specific magnetic behaviour. This structural phase transition enables a dimer, comprising two acridine moieties, to shift toward the overlap of the two N–O sites. Although their slight misalignment suggests ferromagnetic coupling, contrary to the theoretical results, 1-Ge exhibited an abrupt magnetic decrease across a low- to high-temperature boundary. This finding indicates the presence of a neatly overlapping configuration with strong antiferromagnetic coupling as a metastable phase in the crystal.