Structure and luminescence colour change of an 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 luminescence vapochromism as a reversible conversion between yellow and orange emissions in response to heating/cooling. X-ray crystallography reveals that [Ir(tpy)₂]Br₃·7H₂O exists as a triclinic crystal system, comprising Ir(III) complex layers and 2D hydrophilic sheets of H₂O and Br⁻. The orange emissive crystal is determined to be the orthorhombic [Ir(tpy)₂]Br₃·2H₂O, comprising 1D hydrophilic channels of H₂O and Br⁻, with Br⁻ in close proximity to the Ir(III) complex. The yellow emission of [Ir(tpy)₂]Br₃·7H₂O is due to the dual emission from the ligand-centered π–π* transition and the Br⁻-to-ligand charge transfer (XLCT). 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 the Ir(III) complex crystal; 5 H₂O molecules are incorporated into the 1D channel of the [Ir(tpy)₂]Br₃·2H₂O crystal to change the Ir–Br⁻ framework from orthorhombic to triclinic, and inversely, the elimination of 5 H₂O molecules from the 2D hydrophilic sheet in [Ir(tpy)₂]Br₃·7H₂O contributes to the reconstruction of the hydrophilic channel along with the reversion of the framework to orthorhombic.