Syntheses of the pt-TEG and pt-TOxT-Sq ligands, the [Ir(C^N)2(dm-bpy)](PF6), [Ir(C^N)2(ptb)](PF6) and [Ir(C^N)2(pt-TEG)]Cl complexes, and the [Ir(C^N)2(pt-TOxT-Sq)]+ ECL labels (where C^N = piq, bt, ppy, or df-ppy) are described in the ESI.† The solubility of the [Ir(C^N)2(pt-TEG)]+ complexes was approximately 1 mM [Ir(bt)2(pt-TEG)]+ and [Ir(ppy)2(pt-TEG)]+ and 0.5 mM [Ir(df-ppy)2(pt-TEG)]+ in water, and 0.1 mM [Ir(piq)2(pt-TEG)]+ in water with 10% acetonitrile. Stock [Ir(C^N)2(pt-TEG)]+ solutions were subsequently prepared at 0.1 mM. Acetonitrile (Scharlau, Spain) was distilled over calcium hydride under a nitrogen atmosphere and collected as needed. All other chemicals were obtained from commercial sources and used as received.

¶Commercial sources of chemicals: tris(2,2′-bipyridine)ruthenium(ii) hexafluorophosphate ([Ru(bpy)3](PF6)2) and tetrabutylammonium hexafluorophosphate (TBAPF6; electrochemical grade) were purchased from Sigma-Aldrich (NSW, Australia). Tris(2,2′-bipyridine)ruthenium(ii) chloride hexahydrate ([Ru(bpy)3]Cl2·6H2O) and bis(cyclopentadienyl)iron (ferrocene; Fc) were purchased from Strem Chemicals (MA, USA). The four Ir(C^N)(acac) complexes and five ECL labels (bis(2,2′-bipyridyl)(4-methyl-4′-carboxypropyl-2,2′-bipyridyl)ruthenium(ii) hexafluorophosphate ([Ru(bpy)2(mbpy-COOH)](PF6)2), bis(4,6-difluoro-2-(2-pyridyl)phenyl-C2,N)(4-methyl-4′-carboxypropyl-2,2′-bipyridyl)iridium(iii) chloride ([Ir(df-ppy)2(mbpy-COOH)]Cl), bis(2-phenylpyridine-C2,N)(4-carboxypropyl-2,2′-bipyridyl)iridium(iii) hexafluorophosphate ([Ir(ppy)2(mbpy-COOH)](PF6)), bis(2-phenylbenzothiazole-C2,N)(4-methyl-4′-carboxy-2,2′-bipyridyl)iridium(iii) chloride ([Ir(bt)2(mbpy-COOH)]Cl), bis(1-phenylisoquinoline)(4-methyl-4′-carboxypropyl-2,2′-bipyridyl)iridium(iii) chloride ([Ir(piq)2(bpy-COOH)]Cl)) were purchased from SunaTech (Jiangsu, China).