Unusually strong electrochemiluminescence from iridium-based redox

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Functional Inorganic Materials and Devices

Unusually strong electrochemiluminescence from iridium-based redox polymers immobilized as thin layers or polymer nanoparticles Serena Carrara, Bradley D Stringer, Alireza Shokouhi, Pria Ramkissoon, Johnny Agugiaro, David J. D. Wilson, Peter J. Barnard, and Conor F. Hogan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12995 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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ACS Applied Materials & Interfaces

Unusually strong electrochemiluminescence from iridium-based redox polymers immobilized as thin layers or polymer nanoparticles Serena Carrara,a Bradley Stringer,a Alireza Shokouhi,a Pria Ramkissoon,a Johnny Agugiaro,a David J. D. Wilson,a Peter J. Barnarda and Conor F. Hogana* a

Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria 3086, Australia.

ABSTRACT: A new class of redox metallopolymer

based on cyclometalated iridium(III) centers is described, with unusually intense luminescence properties in aqueous media. We report the facile synthesis, photophysical and electrochemical characterization, supported by DFT calculations and their electrochemiluminescence (ECL) properties which, under some circumstances, are significantly greater than the analogous ruthenium-based materials. The photoluminescence (PL) and ECL of these materials are further dramatically enhanced when dispersed or immobilized as polymeric nanoparticles (PNPs). This Aggregation-Induced Emission (AIE and AIECL) operates by providing important protection for the cyclometallated iridium(III) centers against the types of quenching processes which commonly afflict iridiumbased luminophores in aqueous media. The results suggest interesting new avenues of research for the application of such materials in and PL and ECL-based detection and imaging as well as light-emitting devices.

KEYWORDS: iridium complexes, metallopolymers, electrochemiluminescence, nanoprecipitation, polymeric nanoparticles, aggregation-induced electrochemiluminescence

1. INTRODUCTION Iridium complexes have been widely used in many applications such as OLEDs, catalysis, water splitting and solar cells.1–5 Principally, they are sought after for their remarkable luminescent properties; high quantum efficiency, long excited state lifetime, large Stokes shift, and tunable emission from blue to red through tailored ligand design. While research on blue emitting iridium(III) complexes has played crucial role in the development of new OLEDs,6 luminescence-based sensing and bioanalysis is now also emerging as an important application area for Ir(III) complexes.7 In particular, their use in electrogenerated chemiluminescence (ECL) has recently gained

significant momentum,8–10 driven by the need for a greater diversity of emitters beyond the established [Ru(bpy)3]2+ centred paradigm. However, issues such as oxygen quenching and poor solubility or diminished performance in aqueous media have posed significant challenges to the development of real world sensing applications for ECL-active iridium complexes.11 Exploiting ECL emitters in immobilized format is an attractive option because it can simplify sensing arrangement and sometimes enhance the response. Such solid state ECL has been mostly based on [Ru(bpy)3]2+ or its analogues, and immobilization of such materials on the electrode to develop regenerable ECL sensors, has attracted significant interest.12–14 This has been achieved in a number of ways, including direct adsorption of solid material,15 entrapment within a polymeric matrix such as Nafion12,16 or attachment to a polymeric backbone such as polyvinyl pyridine (PVP).17 Rusling et al. have demonstrated the utility of metallopolymers based on PVP with ruthenium(II) or osmium(II) centers for applications such as the detection of oxidized DNA18 and genotoxicity screening.19 Unfortunately, the direct attachment and polymeric entrapment approaches suffer from stability issues; and, while the PVP strategy solves this issue, the resulting metallopolymer is a somewhat weaker emitter than the parent complex, [Ru(bpy)3]2+, which has limited the scope of their sensing applications. Immobilization seems like an excellent strategy to harness the high quantum yields of iridium complexes while potentially offering protection from the ravages of oxygenated aqueous environments. Only a few studies have reported modifying the working electrode with iridium(III) complexes for ECL applications.20–23 Direct immobilization by adsorption of a complex20–22 or dendrimer24 onto the electrode surface, immobilisation after encapsulation in organosilica nanoparticles23 or as a thin polymer layers.20 However, these systems also suffered from either poor stability and/or low emission intensities compared to the solution phase.

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Here, we describe two novel iridium(III)-based metallopolymers (chart 1), where the metal centers are directly bonded to a polyvinylpyridine/polystyrene, (2:1 PVP:PS), backbone. We report their synthesis, photophysical, electrochemical and ECL properties in solution and after deposition onto a glassy carbon (GC) surface.

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photoluminescence spectra of each polymer dissolved in THF. The absorption bands at 220 nm and 257 nm for [Ir(dfppy)2(PVP/S)2]+ and [Ir(ppy)2(PVP/S)2]+ respectively, can be assigned to ligand-centered (1LC) 1π-π* transition of the ppy ligands, while the broad band at 300-400 nm can be ascribed to singlet-manifold ligandto-ligand charge transfer (1LLCT) and metal-to-ligand charge transfer (1MLCT) transitions. The less intense bands centered at ca 400 - 450 nm are attributed to 1 MLCT and 3MLCT processes.

Chart 1. Structures of the investigated metallopolymers a) [Ir(dfppy)2(PVP/S)2]+ and b) [Ir(ppy)2(PVP/S)2]+. In both cases the metal loading was 1:5, (i.e. 1 metal center per 5 monomeric units); and the ratio of pyridine to styrene units in the polymer backbone was 2:1.

Significantly, we also show that it is possible by nanoprecipitation25,26 to form polymeric nanoparticles (PNPs)27,28 with a narrow size distribution and remarkably enhanced luminescence properties. Previous studies on PNPs, have largely focused on conjugated organic polymers,28–30 and rarely on metallopolymers,31 but so far there are no report of iridium-based PNPs for ECL applications. The Aggregation-Induced Emission (AIE) and Aggregation-Induced ECL (AIECL)32,33 resulting from the formation of these bright and stable soft nanoparticles represents an excellent strategy to overcome the sub-optimal performance or solubility issues sometimes encountered with iridium(III) complexes in aqueous media. Such aggregates show great promise for application in bioanalysis, bioimaging and optic devices.34 2. RESULTS AND DISCUSSION The synthesis of the polymers is remarkably facile. The metallopolymers in Chart 1 were synthesised by mixing THF solutions of the respective iridium(III) dichlorobridged dimer starting materials, [Ir(ppy)2Cl]2 and [Ir(dfppy)2Cl]2, with a 2:1 co-polymer of 4-vinylpyridine and styrene (PVP/S), (MW approximately 105 gmol-1), prepared as described previously.35 Styrene was incorporated in the backbone in order to more readily facilitate the formation of PNPs (see later). Using this method, luminescent metallopolymers were formed within seconds at room temperature (see Supporting Information section 1.2 for details). The relative ease of this approach also had the advantage that polymers with different metal loadings could be readily prepared. The photophysical properties of the two novel metallopolymers, (summarized in Table 1), were first evaluated in solution phase. Figure 1 shows the absorption and

Figure 1. Absorbance (dotted traces) and photoluminescent emission (solid traces) of a) metallopolymers [Ir(dfppy)2(PVP/S)2]+ (dotted blue trace) and [Ir(ppy)2(PVP/S)2]+ (dark green trace) in aerated THF. The iridium concentration was 4 × 10-6 M, λex was 350 nm; b) model complexes [Ir(dfppy)2(pic)2]+ (violet trace) and [Ir(ppy)2(pic)2]+ (light green trace) in THF. The concentration was 4 x 10-6 M, λex was 350 nm.

The photoluminescence spectra for the same solution phase polymer samples are shown also in Figure 1a. These exhibit structured emission profiles, suggestive of a LC excited state,36–39 with λmax at 478 nm and a shoulder at 502 nm for [Ir(dfppy)2(PVP/S)2]+ and λmax at 504 nm with a shoulder at 534 nm for [Ir(ppy)2(PVP/S)2]+. The emission was moderately intense in both cases with photoluminescence quantum yields (ΦPL) of 3% and


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2.4% for the fluorinated and non-fluorinated materials respectively in aerated media, rising to 8.5% and 4.5% on de-oxygenation of the solutions. Similarly, the excited state lifetimes increased from 0.044 µs to 0.366 µs; and from 0.262 µs to 0.716 µs when the solution was deaerated. These observations are in accord with the well-known sensitivity of the triplet excited state of iridium complexes to oxygen quenching.40 The blue/green structured emission of the polymers was somewhat unexpected as it differs from what is generally observed for bis-cyclometallated iridium complexes with a similar (N^C)2(N^N) coordination sphere. For example, [Ir(ppy)2(bpy)]+ (where bpy is 2,2'-bipyridine), emits in the red with an un-structured emission profile characteristic of an MLCT excited state. This is in contrast with the analogous ruthenium-based metallopolymers, where the emission properties of [Ru(bpy)3]2+ are largely conserved (at least qualitatively) in the polymeric analogue [Ru(bpy)2(PVP)]2+.41 The similarity of the polymer emission profiles to those of the corresponding tris-cyclometallated (N^C)3 monomeric complexes, Ir(ppy)3 and Ir(dfppy)3, raised the possibility of an unusual bonding of the metal to a carbon atom in the polymer. Therefore, we sought to gain insight into the coordination spheres and properties of the iridium centers when bound to these polymers by synthesizing two model complexes, [Ir(dfppy)2(pic)2]+ and [Ir(ppy)2(pic)2]+, where pic is 4-picoline, (Chart S1, Crystal structure S1, Table S2), for comparison. The emission spectra of the model complexes, shown in Figure 1b, both exhibit a vibronic structures similar to that observed for the polymer complexes, but are blue-shifted by approximately 20 nm in each case.42 The similarity of the emission profiles confirms that the polymeric materials have the same coordination sphere as the model complexes. It seems therefore, that the coordinated alkyl pyridine fragments in the polymers, result in a very different electronic environment compared to the case with rigid bidentate ligands, such as [Ir(ppy)2(bpy)]+. To understand this further, theoretical studies were undertaken using density functional theory (DFT) on eight complexes of the general formula [Ir(C^N)2(N^N)]+, where C^N is either ppy or dfppy and N^N is bpy, pic, bpp, or PVP/S, where bpp is 2,4-bis(4-pyridyl)pentane, (see Figure S1 for structures). Molecular orbitals (MO) were calculated at the BP86/def2-TZVP level of theory inclusive of solvent effects. While there is a DFT functional dependence on the calculated MO energies and consequently the HOMO-LUMO (H-L) gaps, the relative trends produced by each DFT functional are consistent and enable a meaningful comparison with the experimental results. A Kohn Sham diagram of MO energies is presented in Figures S2-S3, with the H-L gaps for each complex given in Table S1, including a comparison

with those obtained from voltammetric experiments (Figure S6). Analysis of MO energies reveals several important trends. Firstly, comparing the MO energies in Figure S2 with those in Figure S3, the well-known effect of adding electron-withdrawing groups, such as fluorine, to the phenyl rings of the ppy ligand can be seen. The effect is to increase the H-L gap of the complex by stabilizing the HOMO.43 This is consistent with the fluorinated analogues displaying bluer emission than the nonfluorinated complexes. There is a good correlation between the calculated H-L gaps and the experimental gaps from electrochemical responses, as well as the measured emission energies of the metal complexes. Table 1. Photophysical properties of [Ir(dfppy)2(PVP/S)2]+ and [Ir(ppy)2(PVP/S)2]+ metallopolymers in THF and water, and Ir model complexes Ir(F2ppy)2(pic)2 and Ir(ppy)2(pic)2 in THF at room temperature.

a

λabs (nm)

λem (nm)a

ΦPL (%)

τ (µs)b

[Ir(dfppy)2(PVP/S)2]+ in THF

220, 350, 400450

478, 502

3.0

0.044

[Ir(dfppy)2(PVP/S)2]+ NPs in H2O

-

478, 502

39.0

1.000

[Ir(ppy)2(PVP/S)2]+ in THF

257, 350, 450

504, 534

2.4

0.262

[Ir(ppy)2(PVP/S)2]+ NPs in H2O

-

504, 534

26.0

1.226

[Ir(dfppy)2(pic)2]+ in THF

262, 350

458, 488, 519

5.7

0.036

[Ir(ppy)2(pic)2]+ in THF

268, 350

482, 514, 548

1.6

0.103

λex=350 nm; b λex=344 nm nano-led light source.

Consideration of both sets of complexes (Figures S1 and S2) reveals another important trend. The LUMO energies of the complexes in each case increase in the order [Ir(C^N)2(bpy)]+ > [Ir(C^N)2(bpp)]+ > + [Ir(C^N)2(PVP/S)2] > [Ir(C^N)2(pic)2]+. The HOMO energies on the other hand remain relatively constant (Figure S4), resulting in a relative trend in H-L gaps that mirrors the trend in LUMO energies. Analysis of the character of the LUMO for each complex, (Figure S5), reveals the cause of this trend. For [Ir(C^N)2(bpy)]+, a majority of the electron density is localized on the ancillary bpy ligand (95%). The remaining complexes display a more shared distribution across the C^N and N^N ligands. For [Ir(C^N)2(bpp)]+, the LUMO is split between the two ligand types with about 65% on the N^N ligands. The remaining two complexes (with the pyridine ligands being either unlinked or linked through the polymer



backbone), [Ir(C^N)2(pic)2]+ and [Ir(C^N)2(PVP/S)2]+, also exhibit a shared distribution of LUMO electron density between the two ligand types, although they are now dominated by the C^N ligand with only about 20% of the LUMO associated with the pyridine units. It is apparent from this data that ring co-planarity and the resultant conjugation, as in bipyridine, yields the most stable LUMO. Uncoupling the two pyridine units with the 4-picoline and PVP/S ligands enables free rotation of the pyridine groups (to varying degrees) that disrupts conjugation. This results in a destabilization of the pyridine-based MO energies such that the main cyclometallating ligands (C^N) become the LUMO. Thus, the increasing freedom of rotation of the pyridine units is manifested as a progressively blue-shifted emission on moving from [Ir(C^N)2(bpy)]+ to [Ir(C^N)2(PVP/S)2]+ to + [Ir(C^N)2(pic)2] . Figure 2a shows the voltammetric behavior of the two Irpolymers dissolved in dimethylformamide. DMF was chosen as the solvent since it effectively solvates the polymers and has a larger electrochemical window than THF. The corresponding ruthenium-based polymer [Ru(bpy)2(PVP)2]2+ (Ru-PVP), which has been used effectively for ultra-sensitive bioanalytical applications,19,41,44 was used as a reference for the novel metallopolymers reported here. The electrochemical response is chemically reversible for [Ir(ppy)2(PVP/S)2]+ but is not for the fluorinated polymer. In this case a thin layer deposits on the electrode surface during the initial anodic scan, blocking it from further electron transfer reactions and therefore from ECL emission after the first cycle (see Figure S7). The oxidation peaks respectively at +1.36 V for [Ir(dfppy)2(PVP/S)2]+ and +1.16 V for [Ir(ppy)2(PVP/S)2]+ are formally assigned to the Ir3+/4+ couple, and they coincide with the onset of ECL emission in the presence of oxalate (Figure 2b). The voltammetric behavior of the two polymers in solution resembles the voltammogram of a thin layer of these species on a glassy carbon electrode at a scan rate of 0.1 V s-1 where the supporting electrolyte is 1 M H2SO4 (Figure S8). In common with the previously reported ruthenium polymer, it is possible to create an electrochemically active thin layer of these materials by drop casting the solution phase polymer onto a GC electrode surface. Although not as well behaved electrochemically as the rutheniumbased polymer, the iridium metallopolymers are capable of producing strong ECL in this format. In particular, despite its low ΦPL in THF solution, the ECL of the fluorinated polymer, [Ir(dfppy)2(PVP/S)2]+, is remarkably intense, giving an ECL intensity 36× higher than the Ru-PVP benchmark, as shown in Figure 2b. Unfortunately, due to a relatively poor physical stability of the thin layer on the electrode, the ECL response drops after the first cycle.

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Figure S9 shows the ECL spectrum of a thin layer of [Ir(dfppy)2(PVP/S)2]+ immobilized on a GC electrode with oxalate as the co-reactant. It is clear that the excited state obtained via ECL is the same one populated by photoexcitation. In the presence of oxalate, the oxidation peak current is significantly increased (Figure S10), proving the existence of a mediated electrochemical reaction involving the oxidized iridium centers of the polymers.44 The oxidation of oxalate is accompanied by ECL emission from the layer, indicating that the oxidized metal center is reduced to give the excited state, which emits a photon on returning to the ground state. In fact, this is a typical case of an EC’ reaction,44 which has been extensively discussed by Bard et al.45 The mechanism for the iridium-based polymers is represented by eqs 1-5: [Ir(dfppy)2(PVP/S)2]+ - e− [Ir(dfppy)2(PVP/S)2]2+ (1) [Ir(dfppy)2(PVP/S)2]2+ + C2O42- [Ir(dfppy)2(PVP/ S)2]+ + C2O4•− (2) C2O4•− CO2 + CO2•− •−

(3) 2+

CO2 + [Ir(dfppy)2(PVP/S)2] CO2 + [Ir(dfppy)2(PVP/S)2]+* (4) + + [Ir(dfppy)2(PVP/S)2] * [Ir(dfppy)2(PVP/S)2] + hν (5)

Figure 2. a) CVs of [Ir(dfppy)2(PVP/S)2]+ (navy trace), [Ir(ppy)2(PVP/S)2]+ (green trace), Ru-PVP (red trace) in 0.1 M TBAPF6 dimethylformamide solution, using GC electrode as working electrode. Scan rate 0.1 V s-1; b) ECL emission recorded


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ACS Applied Materials & Interfaces during a cyclic voltammetry at scan rate 0.05 V s-1 in 0.1 M H2SO4 water solution of a thin layer of each metallopolymer drop cast on a GC electrode surface: [Ir(dfppy)2(PVP/S)2]+ (navy trace), [Ir(ppy)2(PVP/S)2]+ (green trace) and Ru-PVP (red trace) using 20 mM Na2C2O4 as co-reactant.

In this mechanism, the oxidation of oxalate is mainly through iridium mediated reaction, its direct oxidation being thermodynamically possible but kinetically slow.46 The concentration of metal centers in the layer is an important parameter to control because, from an electrochemical point of view, it can determine the rate of charge transport via electron hopping between adjacent sites; and also, from a photophysical aspect, it may influence quenching behaviors within the layer.17 The effect of iridium loading on the ECL intensity was investigated by synthesizing polymers with iridium:monomer ratios ranging from 1:50 to 1:5. The results of this study are presented in Figure S11 which shows a dramatic increase in ECL intensity at higher metal loadings. This is in contrast with results recently reported by O’Reilly et al.47 for ruthenium metallopolymers where the ECL efficiency was found to decrease as the concentration (or rate of generation) of Ru3+ increased. It may be surmised that, in the case of the iridium polymer, charge transport is the more important factor in determining the rate of photon emission. Despite the many favorable attributes of iridium-based luminophores, a significant issue, which has tended to limit their application in bioanalysis, is their often relatively poor performance in aqueous media. Stimulated by recent reports of enhanced emission resulting from aggregation induced ECL (AIECL),32,33 we have used these bright polymers for the synthesis of PNPs. A reprecipitation method was chosen to obtain small PNPs. [Ir(dfppy)2(PVP/S)2]+ and [Ir(ppy)2(PVP/S)2]+ PNPs were prepared starting from a small amount of THF solution injected under sonication into a water solution (for details see SI, section 1.2). TEM images (Figure 3a and S12-S13) reveal a spherical morphology of the particles and relatively narrow size distribution with an average diameter of 44 nm (see Figure S12-S13). Photophysical measurements reveal that the absorption profiles are relatively broad and featureless due to a scattering effect, while the emission profiles of the polymers are qualitatively unchanged in aqueous media as PNPs (Figures S14-S15). However, as illustrated by Figures 3b, 3c, S14 and S15, the ΦPL and lifetime (see Table 1), are greatly enhanced for both polymers showing that the LLCT/MLCT transitions are not affected by the aggregation but in fact the latter likely affords significant protection against quenching of the excited state by solvent and/or oxygen. Interestingly, the effects of particle formation on the radiative (kr) and nonradiative (knr) rate constants (see Table S3) differ for the fluorinated and non-fluorinated materials. This suggests a different non-radiative path for each of these polymers.

The co-reactant ECL properties of [Ir(dfppy)2(PVP/S)2]+ PNPs were also investigated in aqueous media. Figure 3c shows the ECL intensity-potential profile of PNPs during the anodic scan in presence of oxalate. The emission rises at the same potential where the iridium centers are oxidized in the thin layer experiment, presumably following the same oxidative-reduction co-reactant mechanism (eqs 1-5). The intensity of the ECL signal is 12× higher than the unfolded polymer, in agreement with the higher ΦPL of the aggregated material and 250× more intense than a thin layer of the Ru-PVP benchmark under similar conditions. Also in this case, the stability of the PNPs layer on the electrode surface does not allow its multiple use. That is why we envisage its application for disposable sensing platforms, for instance immunoassays, which can be used only one time. The remarkable enhancement observed for the PNPs can be considered an example of AIE34 and AIECL32 where the polymer chains form nanoaggregates in water that preserve the emitting centers from external quenching agents.

Figure 3. a) TEM image of [Ir(dfppy)2(PVP/S)2]+ PNPs in water; b) Photographs of [Ir(dfppy)2(PVP/S)2]+ solution in THF and PNPs suspended in water under UV illumination; c) ECL emission recorded during a cyclic voltammetry of solid state [Ir(dfppy)2(PVP/S)2]+ polymer (light blue trace) and immobilized PNPs (dark blue trace) on GC electrode at scan rate 0.05 V s-1 in contact with 0.1 M H2SO4 water solution using 20 mM Na2C2O4 as co-reactant.

A significant advantage of these particles compared with previously reported encapsulated systems23 is that the



emitters cannot leach out from the particle since they are covalently bound to the polymer backbone.48 3. CONCLUSIONS In summary, two hydrophobic Ir(III)-based polymers have been synthesized using a rapid and facile approach which allows the functionalization of the polymer backbone with iridium redox centers. The photophysical and electrochemical properties have been elucidated, showing structured green and blue emission for the nonfluorinated and fluorinated materials respectively. DFT calculations and comparison with the properties of model complexes, clarified the coordination sphere of the metal centers in the polymers, and the results obtained are of fundamental importance in establishing that the coordination is N4C2, in which the iridium centers are bound to polymer chain through two pyridine moieties, rather than N3C3. Most importantly, we demonstrate that it is possible to produce intense ECL emission from these polymers by forming solid state thin layers on an electrode surface. Aggregation induced emission and ECL was used to further enhance the emission properties of the polymers, by forming polymeric nanoparticles using a reprecipitation method. In this format the iridium metal centers are protected from water and oxygen quenching. This new class of iridium polymer ought to offer significant advantages, particularly in PNP form, over the wellknown ruthenium-based materials, for solid state biosensing applications, not only due to its greater ECL efficiency but by virtue of ease of preparation.

ACKNOWLEDGMENTS We thank the Australian Research Council for financial support (DP150102741 and DP160103046). We thank Dr. Peter Lock and the LIMS Bioimaging facility for assistance in obtaining TEM images.

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. C.H. and S.C. designed the experiments. Synthesis of Ir polymers, nanoparticles and photophysical experiments were carried out by S.C. and B.S.; electrochemical experiments by S.C. and A.S.; P.R. synthesized the Ir model complexes, under the guidance of P.B.; Crystal structural studies were done by P.B. J.A. and D.W. performed all the theoretical calculations. S.C. wrote the manuscript under the guidance of C.H. All authors read and gave valuable suggestions on the manuscript.

Kiran, R. V.; Hogan, C. F.; James, B. D.; Wilson, D. J. D. Photophysical and Electrochemical Properties of Phenanthroline-Based Bis-Cyclometallated Iridium Complexes in Aqueous and Organic Media. Eur. J. Inorg. Chem. 2011, No. 31, 4816–4825.

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Bertoncello, P.; Dennany, L.; Forster, R. J.; Unwin, P. R. NafionTris(2-2’-Bipyridyl)Ruthenium(II) Ultrathin Langmuir-Schaefer Films: Redox Catalysis and Electrochemiluminescent Properties. Anal. Chem. 2007, 79 (19), 7549–7553.

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Dennany, L.; Forster, R. J.; Rusling, J. F. Simultaneous Direct Electrochemiluminescence and Catalytic Voltammetry

Supporting Information Supporting figures and synthesis and experimental methods. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author [email protected] (Conor F. Hogan)

Present Address Department of Chemistry and Physics, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria 3086, Australia

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Notes The authors declare no competing financial interests.


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Graphical Abstract


9

Unusually strong electrochemiluminescence from iridium-based redox

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