Abstract
A novel diiridium complex [(N^C^N)2Ir(bis-N^C)Ir(N^C^N)2Cl]PF6 (N^C^N = 2-[3-tert-butyl-5-(pyridin-2-yl)phenyl]pyridine; bis-N^C = 3,6-bis(4-tert-butylphenyl)pyridazine) was designed, synthesised and characterised. The key feature of the complex is the bridging pyridazine ligand which brings two cyclometallated Ir(iii) metal centres close together so that Cl also acts as a bridging ligand leading to a cationic complex. The ionic nature of the complex offers a possibility of improving solubility in water. The complex displays broad emission in the red region (λem = 520–720 nm, τ = 1.89 μs, Φem = 62% in degassed acetonitrile). Cellular assays by multiphoton (λex = 800 nm) and confocal (λex = 405 nm) microscopy demonstrate that the complex enters cells and localises to the mitochondria, demonstrating cell permeability. Further, an appreciable yield of singlet oxygen generation (ΦΔ = 0.45, direct method, by 1O2 NIR emission in air equilibrated acetonitrile) suggests a possible future use in photodynamic therapy. However, the complex has relatively high dark toxicity (LD50 = 4.46 μM), which will likely hinder its clinical application. Despite this toxicity, the broad emission spectrum of the complex and high emission yield observed suggest a possible future use of this class of compound in emission bioimaging. The presence of two heavy atoms also increases the scattering of electrons, supporting potential future applications as a dual fluorescence and electron microscopy probe.
Highlights
In comparison to traditional bio-imaging and photodynamic therapy (PDT) agents, transition metal complexes combine longer emission lifetimes and higher photo-stability, with relative ease of chemical modi cation.[1]
These complexes were the easiest to synthesise because the iridium centres are achiral and the formation of diastereomers is not possible; the strong trans in uence of the metallated rings of both the N^C^N and bis-N^C ligands directs the relative positions of these ligands around the iridium centres so that only one product is formed.[35]
We have demonstrated that a novel pyridazine-bridged cationic di-iridium complex can enter live cells, has photo-physical properties favourable for imaging applications, and generates singlet oxygen in considerable yield
Summary
In comparison to traditional bio-imaging and photodynamic therapy (PDT) agents, transition metal complexes combine longer emission lifetimes and higher photo-stability, with relative ease of chemical modi cation.[1]. Iridium(III) complexes formed by ditopic bis-N^C ligands in conjunction with symmetrically-substituted terminal N^C^N ligands.[35] These complexes were the easiest to synthesise because the iridium centres are achiral and the formation of diastereomers is not possible; the strong trans in uence of the metallated rings of both the N^C^N and bis-N^C ligands directs the relative positions of these ligands around the iridium centres so that only one product is formed.[35] The presence of two closely positioned heavy atoms might increase the scattering of electrons suggesting the use of di-iridium complexes as contrast reagents and combined with the high quantum yields as dual-modality agents. Five chelate rings form as a result, providing the driving force for the reaction and leading to a rigid architecture Another consequence of the bridging Cl atom is that the complex becomes ionic, which somewhat improves solubility in water. It's use as a contrasting reagent in transmission electron microscopy (TEM) was assessed
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