Structure and Luminescence Colour Change of Iridium(III) Bis-Terpyridine Complex Crystal Triggered by Water Sorption/Desorption

Abstract

A cationic complex crystal of [Ir(tpy)₂]Br₃·7H₂O (tpy = 2,2’;6’,2”-terpyridine) shows a luminescence vapochromism, a reversible conversion of yellow and orange emission in response to heating/cooling. An X-ray crystallography reveals that [Ir(tpy)₂]Br₃·7H₂O is in a triclinic crystal system and constructed by Ir(III) complex layers and 2D hydrophilic sheets consisting of H₂O and Br^–. The orange emissive crystal is determined as [Ir(tpy)₂]Br₃·2H₂O in orthorhombic, where 1D hydrophilic channels consisting of H₂O and Br^– and a close approach of Br^– to Ir(III) complex are found. The yellow emission of [Ir(tpy)₂]Br₃·7H₂O is due to the dual emission from the ligand centered π-π* state and the Br^–-to-ligand charge-transfer (XLCT) state. The π-π* emission is quenched by the switching to the XLCT emission by the close approach of Br^– to Ir(III) complex resulting from the H₂O desorption. The reversible H₂O sorption/desorption results in not only the luminescence colour change but also the drastic structural change between triclinic and orthorhombic crystal systems. The weak stimulus of H₂O sorption/desorption triggers the phase transition of Ir(III) complex crystal; 5H₂O squeeze into the 1D channel in [Ir(tpy)₂]Br₃·2H₂O crystal to change Ir–Br^– framework from orthorhombic to triclinic, and inversely the elimination of 5H₂O from the 2D hydrophilic sheet in [Ir(tpy)₂]Br₃·7H₂O contributes to reconstruction of the hydrophilic channel along with the conversion of triclinic to orthorhombic in crystal system.