Bisthienylethene Th2im and its complex (Th2imH)2[ReCl6]: crystalline-phase photochromism, and photochemical regulation of luminescence and magnetic properties

Abstract

Molecular assembly of bisthienylethene Th₂im (1) and [ReCl₆]²⁻ anions leads to the complex (Th₂imH)₂[ReCl₆] (2), in which a [ReCl₆]²⁻ anion connects two equivalent Th₂imH⁺ cations through Cl⋯N/C hydrogen bonds. Crystal structures of 1 and 2 indicate that two thiophene groups of each Th₂im/Th₂imH⁺ molecule adopt a photoactive antiparallel conformation. Thus, two compounds show crystalline-phase photochromism (CPP), i.e. reversible structural transformation between the open form and the closed form upon alternately irradiating the sample with UV light (365 nm) and visible light (574 nm for 1, 624 nm for 2). It was found that the CPP behaviors of 1 and 2 could regulate their luminescence and/or magnetic properties. Their solid-state emissions (433, 448, 482, 531 and 570 nm for 1, and 460, 489, 535 and 593 nm for 2) exhibited weaker intensities after UV irradiation with 365 nm light. Besides CPP and luminescence, compound 2 shows field-induced slow magnetic relaxation. Before and after UV irradiation, this compound revealed different magnetic behaviors, including the differences in the shape of the χ_MT vs. T plot, D parameter, and the values of the relaxation barrier U_eff and the preexponential factor τ₀.