Dual emission from an iridium(iii) complex/counter anion ion pair

Abstract

[Ir(tpy)₂](PF₆)₃ (tpy = 2,2′:6′,2′′-terpyridine) dissolved in CH₃CN was found to exhibit dual color luminescent emission depending on the excitation wavelength. Specifically, blue and green emissions were obtained with excitation at 350 and 410 nm, respectively. Because the associated emission spectra were consistent with those of [Ir(tpy)₂]Cl₃ in water and [Ir(tpy)₂](PF₆)₃ in the crystalline state, respectively, this dual emission is attributed to emissions from the [Ir(tpy)₂]³⁺ cation and its ion pair [Ir(tpy)₂]³⁺·PF₆⁻. The emission is assigned to the ³π–π* transition of the ligands based on time-dependent density functional theory (TD-DFT) calculations. Conversely, [Ir(tpy)₂]I₃ in CH₃CN shows emission due to [Ir(tpy)₂]³⁺ but not [Ir(tpy)₂]³⁺·I⁻, while crystalline [Ir(tpy)₂]I₃ emits red luminescence at 77 K that is inconsistent with that from [Ir(tpy)₂]³⁺. Since the emission energies of crystalline [Ir(tpy)₂]X₃ (X⁻ = Cl⁻, Br⁻ or I⁻) show a good correlation with the electron affinity of X, the emissions are assigned to a counter anion to complex ion charge-transfer transition. This hypothesis is supported by TD-DFT calculations regarding [Ir(tpy)₂]³⁺·X⁻.