Thermochromic Solid-State Emission of Dipyridyl Sulfoxide Cu(I) Complexes

Copper(I) complexes (Cu-DPSO and Cu-Me-DPSO) utilizing sulfoxide-bridged dipyridyl ligands are reported. Cu-DPSO demonstrates photophysical properties typical with [Cu(disphosphine)(diimine)]+ complexes, however with additional steric bulk in the 6- and 6′-positions of the diimine ligand to give complex Cu-Me-DPSO; the photophysics are greatly altered. This species is isolated as an amorphous powder (a-Cu-Me-DPSO) which emits yellow light; upon heating to 180 °C, a crystalline powder is formed (c-Cu-Me-DPSO) which shows a large bathochromic shift (>100 nm) in emission, and shows orange luminescence attributed to a flattening distortion of the complex away from a tetrahedral geometry. On cooling to −196 °C, c-Cu-Me-DPSO displays a reversible thermochromic solid-state emission, from orange at room temperature to yellow at low temperatures. Using solid-state variable temperature excitation and absorption data, this phenomenon is attributed to a change in coordination geometry about the copper atom in the excited state. At low temperatures, a pseudo-tetrahedral geometry is preferred, giving higher energy emission, whereas, at higher temperatures, a flattened geometry dominates, giving a lower energy emission under UV irradiation. This is a unique example of a monometallic copper(I) complex capable of reversible thermochromic emission in the solid state, and highlights the impact that subtle ligand tuning plays on the photophysical properties. Previous copper(I) thermochromic materials were limited to copper halide clusters, and this study opens new avenues toward highly functionalized, thermal stimuli-responsive materials.