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.