Self‐Enhanced Electrochemiluminescence of an Iridium(III) Complex: Mechanistic Insight

Heteroleptic cationic iridium complexes have been used as alternative luminophores to neutral ortho-metalated iridium complexes (e.g., fac-Ir(ppy)3, where ppyH is 2-phenylpyridine) for visual display applications. Single-layer devices known as light-emitting electrochemical cells (LEECs) can now be fabricated, which operate through ion diffusion to opposite electrodes thereby enhancing electronic charge injection at low operating voltages (the so-called electrodynamic model). We have synthesized several cationic iridium complexes of the form [(C^N)2Ir(N^N)], where C^N is a cyclometalating ligand and N^N is a neutral diimine ancillary ligand and explored their optoelectronic properties. We recently have discovered that iridium(III) complexes bearing aryltriazole C^N ligands exhibit bright electrochemiluminescence (or electrogenerated chemiluminescence, ECL), with which ECL efficiency is up to four times greater than that of [Ru(bpy)3] 2+ upon addition of benzoyl peroxide (BPO) as a co-reactant. Development of such highly efficient and stable ECL emitters over a broad spectrum of wavelengths, as illustrated by the Bard group for other luminophores, has been anticipated for many years, and might find wide applications such as for DNA determination or immunoassay development. With the goal of obtaining bright true blue-light emitters, we recently investigated the photophysical and ECL properties of ([(dFphtl)2Ir(dmabpy)] , 1, [dFphtl= 1-benzyl-4-(2,4difluoro-phenyl)-1H-1,2,3-triazole; dmabpy= 4,4’-(dimethylamino-2,2’-bipyridine); Scheme 1]. Though not apparent initially, during subsequent investigation of the ECL behavior of 1, we realized that we could exploit the redox chemistry of the two dimethylamino (dma) groups on the bpy ligand of 1 and thus enhance its ECL efficiency. This prediction was based on ECL studies using tri-n-propylamine (TPrA) as a coreactant to enhance the ECL and efficiently generate light in organic solvents though TPrA is the most efficient in generating light with luminophores in aqueous media. The classic form of ECL involves electron transfer between electrochemically generated radical ions in solution to produce excited species that emit light. In recent years, the use of TPrA as a co-reactant has been found to be a sensitive technique for biological detections. Herein, we report the electrochemistry and ECL of 1 in comparison with a structurally similar cationic Ir complex, [(dFphtl)2Ir(bpy)], 2, (bpy= 2,2’-bipyridine). For the first time, ECL auto-enhancement was observed, with three excited states in the ECL emission of 1 deconvoluted by means of our recently developed ECL spooling technique. By contrast, 2 emitted ECL only at one peak wavelength with no enhancement in the photocurrent. The ECL of 1 was discovered to be selfenhanced with the two dma groups acting as co-reactants. It is conceivable that 1 can bemodified with anchoring groups that form bonds with lipids, nucleic acids and proteins for simplified and enhanced ECL detection in biological applications. Complexes 1 and 2 (Scheme 1), similar in structure to other cationic iridium complexes that we have investigated, show a very good ECL efficiency in acetonitrile (ACN) in the annihilation path when the applied potential was scanned in the range between their first oxidation and first reduction. The pair of cyclic voltammogram (CV) and ECL–voltage curves (in red) of 1 in Figure 1a demonstrated quasireversible reduction at 1.82 V [Eq. (1)] and oxidation at 1.42 V [Eq. (2)] versus a saturated calomel electrode (SCE):