Reversible Humidity‐Induced Vapochromism of a Cu(II) Tetramine Complex Driven by Iodide Access
Masanori Wakizaka, Hisaaki Tanaka, Tetsu Sato, Shinya Takaishi, Masahiro YamashitaABSTRACT
Vapochromic metal complexes have attracted considerable interest as functional materials for sensing and chromic applications. Herein, we report a humidity‐responsive Cu(II) vapochromic complex based on reversible iodide coordination. A series of Cu(II) tetramine complexes with 1 R ,2 R ‐cyclohexanediamine (chxn), including [Cu II (chxn) 2 (H 2 O)]Br 2 ( 1 ) and [Cu II (chxn) 2 (H 2 O) 2 ]I 2 ( 2 ), were synthesized and characterized by single‐crystal x‐ray diffraction. Complex 2 adopts a discrete hydrated structure, unlike the previously reported chain‐type [Cu II (chxn) 2 I]I. Whereas the chloride and bromide analogues exhibited little or no color change upon humidity variation, complex 2 displayed rapid and reversible vapochromism, changing from bluish purple to bluish green under low humidity or vacuum. Powder x‐ray diffraction and IR spectroscopy revealed reversible structural changes associated with water adsorption and desorption. UV‐Vis spectroscopy showed a pronounced red shift of the d–d transition band upon dehydration. Electron spin resonance measurements combined with density functional theory calculations indicate that dehydration promotes axial coordination of iodide ions to Cu(II), yielding a [Cu II (chxn) 2 I 2 ] species in the solid state. These findings demonstrate a new vapochromic mechanism based on reversible iodide coordination at Cu(II) centers, providing a promising strategy for designing humidity‐responsive chromic materials.