Investigation into dual emission of a cyclometalated iridium complex: The role of ion-pairing

Abstract

Time-resolved emission of [Ir(ppy)2dcbpy]Cl (1a, ppy = 2-phenylpyridine, dcbpy = 2,2′-bipyridine-5,5′-dicarboxylic acid) dissolved in DMF revealed a biexponential decay of the excited state. Prior studies on cyclometalated iridium complexes indicated that such dual emission behavior can be attributed to an impurity, compounds that have two LUMO states that are close in energy, or the presence of ion-pairing. To probe each of these effects, a series of homoleptic, Ir(ppy)3 (2), and heteroleptic complexes, [Ir(ppy)2dcbpy]X [X = PF6−(1b), BPh4−(1c)] and [Ir(ppy)2bpy]X [bpy = 2,2′-bipyridine, X = Cl− (3a) PF6−(3b)], were synthesized and photophysically characterized. Among these complexes, only 1a exhibited dual emission with the emission lifetimes of 430 ± 5 ns (80%) and 125 ± 6 ns (20%) in DMF. These lifetimes were further confirmed with the nanosecond transient absorption kinetics, indicating that the dual emission likely did not originate from an impurity. When probing the emission and absorption kinetics of the salts 1b and 1c only one lifetime of 550 ± 10 ns was observed, indicating that the short lifetime of the Cl− salt (1a) comes from a putative ion-paired complex. Density-Functional Theory (DFT) calculations on 1 and 2 fragments in DMF helped uncover the reason behind the ion-pair formation, with carboxyl substituents in 1a causing a loss in electron density on the 3,3′-position of dcbpy. Additionally, the loss in electron density was experimentally validated with 1H NMR, which showed a noticeable downfield shift of the 3,3′-protons in relative to the shifts in 1b and 1c. The data supports that the dual emission observed in 1a is not from two separately emissive states, rather from the dissociated and ion-paired complexes.

Keywords

• Dual emission
• Ion-pairing
• Iridium
• Transient spectroscopy
• Photosensitizer