Switching photochromism in coordination polymers by forcing lanthanide contraction

Developing photochromic materials and acquiring fundamental knowledge of their underlying photophysics is significant in diverse fields, including optoelectronics, sensing, catalysis, data storage, and more. Herein, we demonstrate how the photochromism of coordination polymers can be realized by creating lanthanide contraction, whose role in directing photophysics has previously remained elusive. Integrating different lanthanide cations and a π-electron conjugated terpyridine-based ligand (tpbz–) affords nine isomorphous one-dimensional lanthanide coordination polymers of the formula Ln(tpbz)3 (where the lanthanide [Ln] ranges from europium to lutetium). The decreasing ionic radii of the Ln centers across the 4f block result in increasingly condensed packing of the metal-ligand assembly. This trend affords a spectrally selective photochromic response in the lutetium polymer Lu(tpbz)3, contrasting sharply with the nonphotochromic earlier analogs. Experimental and theoretical investigations reveal that the chromism can be attributed to the progressively enhanced intermolecular charge transfer of adjacent tpbz– moieties induced by either Ln contraction or external pressure.