# Bright and Stable Electrochemiluminescence by Heterobimetallic IrIII-MI (MI = CuI, AuI) Complexes

Mattia Reato, Beatrice Polido, Anna Bonfiglio, Giulia Moro, Vincent César, Stéphane Bellemin-laponnaz, Alessandro Alla, Neso Sojic, Matteo Mauro, Federico Polo

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Mattia Reato, Beatrice Polido, Anna Bonfiglio, Giulia Moro, Vincent César, et al.. Bright and Stable Electrochemiluminescence by Heterobimetallic IrIII-MI (MI = CuI, AuI) Complexes. Advanced Optical Materials, 2025, 13 (14), pp.2403430. 10.1002/adom.202403430. hal-05141864

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# Bright and Stable Electrochemiluminescence by Heterobimetallic Ir¹¹¹-M¹ (M¹ = Cu¹, Au¹) Complexes

Mattia Reato, Beatrice Polido, Anna Bonfiglio, Giulia Moro, Vincent César, Stéphane Bellemin-Laponnaz, Alessandro Alla, Neso Sojic, Matteo Mauro,* and Federico Polo*

Ionic transition metal complexes (iTMCs) often suffer from low photoluminescence quantum yield, especially in the red to near-infrared spectral region. Rational molecular design strategies unlock recovering the emission features efficiently. A potential solution is offered by bimetallic complexes, which are rapidly emerging as a valid and attractive class of emitters. In this work, the yet unexplored electrochemiluminescence (ECL) properties of a series of heterobimetallic complexes of general structure Ir¹¹¹-M¹ are investigated, where the formally neutral cyclometalated iridium complex provides efficient photo- and electro-active properties to the molecular emitters. Whereas bulky M¹ (gold or copper) metalloligand increases structural rigidity and chemical stability of the architecture while enabling larger values for the radiative rate constant. The Ir¹¹¹-M¹ shows highly efficient red ECL at $λ_{em,max}$ = 600 nm, which is stable in a wider range of applied potential. The activation is triggered at 0.45–0.65 V, ca. 0.3–0.5 V lower than that of tris-(2,2'-bipyridyl) ruthenium(II) complex, [Ru(bpy)₃]²⁺. Remarkably, the ECL quantum yield relative to [Ru(bpy)₃]²⁺ is up to six-fold higher, thus positioning this new series of iTMCs amongst the brightest ECL-active emitters to date and possibly offering a platform to develop novel and even more efficient ECL probes for biosensing and bioimaging applications.

## 1. Introduction

Electrochemiluminescence (ECL) is a radiative process that is triggered upon a highly energetic electron transfer (eT) reaction. This process occurs in solution close to the electrode surface between two electro-generated radical species, among which at least one originated from an ECL-active luminothore species, L.[1,2] The activation of ECL can be attained through one of the two following pathways: i) direct annihilation of the radical cations and anions of L or ii) annihilation of one radical of L, either cation or anion, with one radical originated from a suitable sacrificial coreactant, AB.

The direct annihilation mechanism can be summarized by considering the following reaction mechanism described by Equations (1–4):

$$L + e \rightleftharpoons L^{-} \qquad E_{L/L^{-}}^{0} \qquad (1)$$

$$L \rightleftharpoons L^{+} + e \qquad E_{L/L^{+}}^{0} \qquad (2)$$

M. Reato
Department of Chemical Sciences
University of Padova
Via Marzolo 1, Padova 35121, Italy

B. Polido, G. Moro, F. Polo
Department of Molecular Sciences and Nanosystems
Ca' Foscari University of Venice
Via Torino 155, Venezia 30172, Italy
E-mail: federico.polo@unive.it

A. Bonfiglio, S. Bellemin-Laponnaz, M. Mauro
Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS)
UMR7504
Université de Strasbourg & CNRS
23 rue du Loess, Strasbourg 67083, France
E-mail: mauro@unistra.fr

V. César
LCC-CNRS
Université de Toulouse, CNRS
205 route de Narbonne, Toulouse cedex 4, 31077, France

A. Alla
Department of Mathematics
Sapienza University of Rome
Piazzale Aldo Moro 5, Rome 00185, Italy

N. Sojic
Université de Bordeaux, CNRS
Bordeaux INP, ISM, UMR 5255
Talence F-33400, France

F. Polo
European Centre for Living Technology (ECLT)
Ca' Bottacin, Venice 30124, Italy

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.202403430

© 2025 The Author(s). Advanced Optical Materials published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adom.202403430
$$L^{-} + L^{+} \rightarrow {}^{1 \text{ or } 3}L^{*} + L \qquad -\Delta G^{0} \qquad (3)$$

$$^{1 \text{ or } 3}L^{*} \rightarrow L + h\nu \qquad (4)$$

As described in Equation (4), the excited state species in their either singlet or triplet state, depending on the features of L and the energetics of the eT reaction involved (Equation 3), can then emit a photon with a wavelength typically falling into the visible light region.[1,2]

Instead, when a suitable coreactant is employed, a chemical reaction occurs once the latter one is electro-oxidized or reduced, thus providing a powerful reducing or oxidizing agent, respectively (e.g., B• in Equation (6) as the reducing agent). The reaction scheme, when considering only the oxidation of AB in the well-known “oxidative-reductive” mechanism with the radical cation of L,[1,2] can be therefore summarized as follows (Equations 5–8):

$$AB \rightleftharpoons AB^{+} + e \qquad E_{AB/AB^{+}}^{0} \qquad (5)$$

$$AB^{+} \rightarrow A^{+} + B \qquad (6)$$

$$B^{\cdot} \rightarrow B^{+} + e \qquad E_{B^{+}/B}^{0} \qquad (7)$$

$$L^{+} + B^{\cdot} \rightarrow {}^{1 \text{ or } 3}L^{*} + B^{+} \qquad -\Delta G^{0} \qquad (8)$$

A mirror-like scheme can be easily derived when considering the reduction of AB and the radical anion of L. ECL can be therefore regarded as a powerful tool to generate luminescence from a suitable molecule without the need for sophisticated and expensive optical instrumentation, nor the requirement of an optical excitation source. Compared to photoluminescence (PL), ECL shares high sensitivity, as very low concentrations of L can be easily detected. Yet ECL holds several advantages, as it is not affected by phenomena such as light scattering, thus providing an “on”/“off” optical readout with high spatiotemporal resolution and high signal-to-noise ratio. Therefore, since the early ‘80s of the last century, it was successfully employed to devise electroanalytical methods to detect very low concentrations of analytes of interest, such as protein biomarkers, DNA fragments, etc.[3]

The most investigated, widely used, and commercially available ECL-active luminophore is the archetypal tris-(2,2'-bipyridyl) ruthenium(II) complex, [Ru(bpy)₃]²⁺, which features a fair PL quantum efficiency, Φ<sub>PL</sub>, in the order of 7%,[4,5] and ECL quantum efficiency, Φ<sub>ECL</sub>, which is ≈73% of the Φ<sub>PL</sub> (≈5%).[6,7] Its orange-red emission is centered at λ<sub>em,max</sub> ≈ 600 nm and arises from an emissive excited state with triplet metal-to-ligand charge transfer (³MLCT). Nowadays, [Ru(bpy)₃]²⁺ is undoubtedly regarded as the benchmark in the ECL field, and the scientific community worldwide, including us, has paid several efforts in designing the next generation of luminophores capable of outperforming [Ru(bpy)₃]²⁺ in terms of both Φ<sub>PL</sub> and Φ<sub>ECL</sub>, tunability of the emission wavelength, and redox states. Hence, since its discovery and characterization, several related derivatives have been intensively investigated. To date, several ECL-active luminophores appeared in literature,[8] from novel transition-metal complexes (TMCs)[9–11] to organic dyes.[12–15] and more recently nanostructured materials.[16–18]

In this quest to design and develop new luminophores that can offer a viable, more efficient alternative to [Ru(bpy)₃]²⁺, cationic cyclometalated iridium(III) complexes emerged as very promising and attractive emitters, due to their high Φ<sub>PL</sub> and radical stability, and the possibility of an easy tuning of their emission properties. Indeed, some of the cyclometalated iridium(III) luminophores recently developed achieved very high Φ<sub>PL</sub> (up to 100%) and Φ<sub>ECL</sub>, which in a few cases exceed that of [Ru(bpy)₃]²⁺,[19–23] with emission wavelengths covering the whole visible spectrum from red to blue. Φ<sub>PL</sub> and Φ<sub>ECL</sub> are related and the former surely is a prerequisite to achieve high values for the latter. However, inefficient ECL from some complexes showing high Φ<sub>PL</sub> were also observed. This phenomenon can be primarily understood in terms of radical stability.[20] Therefore, there are several requirements that must be fulfilled when designing a new ECL luminophore.

When considering the applications in a biological environment, charged compounds are expected to be the most suitable candidates due to their increased solubility in a polar (e.g., aqueous) medium. Nevertheless, ionic TMCs (iTMCs) often suffer from low Φ<sub>PL</sub>, especially in the red to NIR spectral region, which instead can be of particular interest for applications such as sensing, and the most recent one concerning ECL imaging.[24–26] The following two main causes can be invoked to explain this issue: i) the increase in the nonradiative rate constants, k<sub>nr</sub> due to the increase in the vibronic coupling between the potentially emitting excited state and the electronic ground state (energy gap law);[27–30] ii) the decrease of the radiative rate constant, k<sub>r</sub>, that scales with the third power of the emission energy, as described by the Einstein quantum theory of radiation.[31] Both effects cause detrimental consequences on Φ<sub>PL</sub> values when the emission profile approaches the deep-red to NIR spectral region, resulting often in compounds with poorer properties. Specific design strategies are required to recover emission features efficiently.[32–34]

In this framework, a potential alternative to overcome the abovementioned drawbacks is offered by bimetallic complexes, which are rapidly emerging as a valid and attractive design strategy.[35–39] In fact, this molecular design may lead to i) fine modulation of the excited states mixing, ii) enhanced spin-orbit coupling (SOC) and singlet (S)-triplet (T) mixing providing higher k<sub>r</sub>, iii) a more rigid scaffold that helps to suppress radiationless deactivation channels.

With the aim of developing iTMCs with efficient emission into the longer-wavelength region, very recently, we have investigated a series of heterobimetallic complexes of general structure Ir<sup>III</sup>-M¹, where the formally neutral cyclometalated iridium complex provides efficient photo- and electro-active properties to the overall structure.[40,41] The M¹ ancillary metalloligand brings the positive charge, being M¹ either a copper(I) or gold(I) compound grafted onto the octahedral complex via the bridging 1,3-dimesityl-5-acetylimidazol-2-ylidene-4-olate (IMesacac) ligand. The coordination sphere of the Ir<sup>III</sup> and the M¹ complex, the latter bearing two N-heterocyclic carbene (NHC) coordinated ligands with high-energy lying excited states, is judiciously chosen to avoid low-lying quenching states. This design strategy led to iTMCs IrCu-1, IrAu-1, IrCu-2 and IrAu-2, amongst others, bearing cyclometalating ligand C⁻N = 1-phenyl-isoquinoline (1-pq) and 2-(3,5-dimethylphenyl)-quinoline (dm-2-pq) with high
<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Chemical structure of the investigated heterobimetallic Ir-M complexes IrCu-1, IrAu-1, IrCu-2 and IrAu-2. The compounds were prepared as the PF₆⁻ salts.</figcaption></figure>

$\Phi_{PL}$ and good redox features that were successfully employed in the fabrication of light-emitting electrochemical cells (LEC) displaying efficient red electrophosphorescence.[41] The corresponding chemical structures of the complexes are displayed in **Scheme 1**.

In this work, we investigated the yet unexplored ECL properties of this class of heterobimetallic iTMCs, Ir<sup>III</sup>-M<sup>I</sup>, in organic aprotic solvent and compare the results and performances with the benchmark [Ru(bpy)<sub>3</sub>]<sup>2+</sup> and other highly efficient molecular emitters, which are based on transition metal complexes and represent the state-of-the-art. Moreover, we compare the molecular emitters with other cutting-edge ECL materials (e.g. metal nanoclusters, carbon, and quantum dots), which were described in the literature in recent years, to provide additional context and a conscious understanding of the field for the readers. The idea behind this work was to take advantage of the above-mentioned key features of the heterobimetallic iTMCs and explore their performances and their possible application as ECL probes. Remarkably, the heterobimetallic complexes showed highly efficient ECL that was stable in a wider range of applied potential when compared to the standard reference [Ru(bpy)<sub>3</sub>]<sup>2+</sup> and proved to be among the most efficient ECL-active molecular emitters to date.

## 2. Result and Discussions

The photophysical properties of the complexes have been investigated in detail and reported elsewhere.[41] Nevertheless, they will be recalled briefly for the sake of comparison with the ECL characterization described below. Both couples of compounds display intense, vibrant red PL whose properties little depend on the nature of the ancillary M<sup>I</sup> moiety (see **Figure 1**). In particular, **IrCu-1** and **IrAu-1** derivatives possess slightly structured emission spectra centered at $\lambda_{em} = 610$, 645 nm, whereas **IrCu-2** and **IrAu-2** display a featureless profile with $\lambda_{em} = 614$ nm, with $\Phi_{PL}$ values as high as 40–45% and 77–73%, respectively. For both couples, the radiative process is ascribed to a long-lived excited state with mainly <sup>3</sup>MLCT character, whereas a slightly higher admixed nature <sup>3</sup>MLCT/<sup>1</sup>LC is ascribed to derivatives bearing the 1-pq ligand. These photophysical properties, along with their redox feature (see below), are excellent for red-emissive iTMC and paved the way for further investigation as ECL-active species.

The electrochemical behavior of the bimetallic complexes was assessed by cyclic voltammetry (CV) in acetonitrile (ACN)/0.1 M tetra-*n*-butylammonium perchlorate (TBAP). All the electrochemical data are referenced against ferrocene/ferricenium couple (Fc/Fc<sup>+</sup>) as the internal standard and are reported in **Table 1**.

The CVs in the investigated potential range are shown in **Figure 2** and agree well with our previous report.[41] The redox processes in the positive- and negative-going scans are referred to as *O*<sub>n,i</sub> and *R*<sub>n,i</sub>, respectively, where *n* denotes the process number and *i* the investigated compound. In the positive-going scan, all compounds showed the main reversible oxidation processes, *O*<sub>1,i</sub>, which are related to the oxidation of the iridium metal center, formally from Ir<sup>III</sup> to Ir<sup>IV</sup>, and whose formal potential $E_{O_1}^0$ reflects the electron-withdrawing/donating features of the ligands. The first series of Ir<sup>III</sup>-M<sup>I</sup> (IrM-1) complexes bearing the electron-withdrawing ligand 1-pq show a formal potential $E_{O_1}^0 \approx +0.63$ V, which decreases to $\approx +0.46$ V in the second series IrM-2 bearing the more electron-donating ligand dm-2-pq. A similar trend, although the shift is weaker, can be observed in the negative-going scan, where the main reduction process $R_{1,i}$ exhibits a formal potential, $E_{R_{1,i}}^0$, $\approx -2.03$ V for the series IrM-1 and $-2.07$ V for IrM-2. $R_{1,i}$ is mainly centered on the cyclometalating ligand, being either 1-pq or dm-2-pq, with weak or no involvement of the second metal center. The main difference between the two series of complexes is that IrM-1 shows three consecutive reversible processes in reduction, namely $R_{1,3,i}$, whereas IrM-2 shows the first reduction process as a reversible wave and then one or more irreversible eT processes.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** UV–vis absorption and photoluminescence spectra of Ir<sup>III</sup>-M<sup>I</sup> compounds in dilute CH<sub>2</sub>Cl<sub>2</sub> solution at room temperature. Emission spectra were recorded upon $\lambda_{exc} = 420–480$ nm. Experimental data are taken from ref. [41].</figcaption></figure>
<table><caption>**Table 1.** Electrochemical data for the heterobimetallic Ir$^{III}$-M$^{I}$ complexes obtained by cyclic voltammetry (CV) carried out in ACN and 0.1 M TBAP as the supporting electrolyte. All the potential values are referenced to the ferrocene/ferricenium (Fc/Fc$^{+}$) couple.</caption><thead><tr><th>Complex</th><th>E<sup>0</sup><sub>O<sub>1,i</sub></sub> [V]<sup>a)</sup></th><th>ΔE<sub>p₂,O<sub>1,i</sub></sub> [mV]<sup>b)</sup></th><th>E<sup>0</sup><sub>R<sub>1,i</sub></sub> [V]<sup>a)</sup></th><th>ΔE<sub>p₂,R<sub>1,i</sub></sub> [mV]<sup>b)</sup></th><th>e|ΔE<sub>H-L</sub>[eV]<sup>c)</sup></th></tr></thead><tbody><tr><td>IrCu-1</td><td>+0.64</td><td>54</td><td>-2.02</td><td>59</td><td>2.66</td></tr><tr><td>IrAu-1</td><td>+0.62</td><td>63</td><td>-2.03</td><td>63</td><td>2.65</td></tr><tr><td>IrCu-2</td><td>+0.45</td><td>60</td><td>-2.07</td><td>43</td><td>2.52</td></tr><tr><td>IrAu-2</td><td>+0.47</td><td>60</td><td>-2.07</td><td>43</td><td>2.54</td></tr></tbody></table>

Moreover, all complexes show a second oxidation process, O<sub>2,i</sub>, in the potential range from + 0.5 to -0.15 V. It is interesting to note that the intensity of O<sub>2,i</sub> for IrCu-1, IrAu-1, and IrAu-2 increases if the potential in the negative bias is made progressively more negative before reverting the scan direction toward positive potential (data not shown here). As previously observed,[41] when the potential is scanned in the negative-going scan past R<sub>2,i</sub>, the ligand capping M<sup>I</sup> is made labile and undergoes a chemical process providing a species that exposes the second metal center, which is immediately re-oxidized when the scan is reverted. O<sub>2,i</sub> for IrCu-1 and IrAu-1 have similar features and potential values that differ only ≈40 mV. However, O<sub>2,i</sub> for IrAu-2 shows a very sharp anodic peak at ≈-0.14 V that, together with its cathodic counterpart (a weak peak at ≈-0.42 V), very much re-

sembles the behavior of bare gold in solution. Whereas for IrCu-2 the same process shows a very weak current increase above the blank. In Figure 2, both the electrochemical HOMO-LUMO bandgap, e|ΔE<sub>H-L</sub> as the potential difference -ΔE<sup>0</sup><sub>R<sub>1,i</sub>-O<sub>1,i</sub></sub> (eV) = -(E<sup>0</sup><sub>R<sub>1,i</sub></sub> - E<sup>0</sup><sub>O<sub>1,i</sub></sub>)(eV), and the blank-subtracted CVs of the related reversible O<sub>1,i</sub>, and R<sub>1,i</sub> processes are shown in the dashed line to be easily visualized.

To better appreciate the redox processes and assess the stability of the radical cations (L<sup>+</sup>) / anions (L<sup>-</sup>), CVs for both the positive- and negative-going scans were acquired separately. The effect of the scan rate was investigated over the range of 50–500 mV s<sup>-1</sup>, and the peak current was found to depend linearly on the square root of the scan rate for all compounds, thus witnessing that the heterogeneous electron transfer process is diffusion-controlled. On the other hand, the difference between the peak potential and the halfway peak potential, E<sub>p</sub> - E<sub>p/2</sub>, varied in the range of 50–60 for O<sub>1,i</sub> and 40–63 mV for R<sub>1,i</sub> regardless of the scan rate, whereas peak potentials did not shift (see Table S1, Supporting Information). These data well agree with the occurrence of a diffusion-controlled heterogeneous eT reaction. Moreover, as displayed in Figure S1 (Supporting Information), the radical cations and anions generated in the O<sub>1,i</sub>, and R<sub>1,i</sub> processes, respectively, showed reversibility at scan rates as low as 50 mV s<sup>-1</sup> for all compounds, with the exception of the radical anions of IrCu-2 and IrAu-2, whose reversibility is attained for scan rate higher than 100 mV s<sup>-1</sup>.

With respect to the redox processes O<sub>1,i</sub> and R<sub>1,i</sub>, investigating the stability of the radical cations/anions of a luminophore, is the first step to assess whether efficient and stable ECL can be triggered or not, as those radicals are involved in the generation of the excited state species, thus leading to the emission process. All the electrogenerated radicals of the luminophores investigated in this work feature a relatively high stability, enabling ECL by means of a direct annihilation mechanism, as shown in Figure S2 (Supporting Information). The normalized spectra closely resemble those obtained by PL (Figure 1), showing a maximum wavelength emission at λ<sub>em</sub> ≈ 600 nm, indicating that the same emitting excited state is generated by both photo- and electrochemical processes in solution.

Concerning the ECL transient, displayed in Figure S3 (Supporting Information) for all the complexes, one can observe the stability of the emission during repeated pulsed cycles and the higher intensity of the photocurrent for the IrM-2 series

<figure><img src="image_4.png" /><figcaption>**Figure 2.** CVs of 1 mM of the investigated compounds in ACN/0.1 M TBAP showing O<sub>1,2,n</sub> and R<sub>1,3,n</sub> redox processes. Vertical dashed lines indicate the electrochemical HOMO-LUMO bandgap, e|ΔE<sub>H-L</sub>, whereas the dashed-lined blank-subtracted CVs show the reversible O<sub>1,i</sub> and R<sub>1,i</sub> process. The starting potential and direction of the scan are indicated with horizontal arrows. Scan rate: 0.1 V s<sup>-1</sup>. Experimental data are partially taken from ref. [41].</figcaption></figure>

<footnote>a) The formal potential is defined as the average value between the cathodic and anodic peak potentials of the investigated redox process, E<sub>p,c</sub> and E<sub>p,a</sub>, respectively: E° = 0.5(E<sub>p,c</sub> - E<sub>p,a</sub>). The values provided are the average values obtained when varying scan rates in the range 0.05–0.5 Vs<sup>-1</sup>; b) ΔE<sub>p/2</sub> is defined as E<sub>p</sub> - E<sub>p/2</sub>, where E<sub>p/2</sub> is the potential value when the current i is at half its peak value, ip, for the processes under investigation (O<sub>1,i</sub> or R<sub>1,i</sub>, respectively); c) The electrochemical bandgap, e|ΔE<sub>H-L</sub> (eV), expressed in eV as -ΔE<sup>0</sup><sub>R<sub>1,i</sub>-O<sub>1,i</sub></sub> = -(E<sup>0</sup><sub>R<sub>1,i</sub></sub> - E<sup>0</sup><sub>O<sub>1,i</sub></sub>), consider the O<sub>1,i</sub> and R<sub>1,i</sub> processes.</footnote>
<table><caption>**Table 2.** ECL data, obtained by direct annihilation or by employing TPrA as coreactant, for the heterobimetallic Ir<sup>III</sup>-M<sup>I</sup> complexes obtained in ACN and 0.1 M TBAP as the supporting electrolyte. All the potential values are referenced to the ferrocene/ferricenium (Fc/Fc<sup>+</sup>) couple.</caption><thead><tr><th colspan="2">Annihilation</th><th colspan="3">Coreactant</th></tr><tr><th>Complex</th><th>λ<sub>max</sub> [nm]</th><th>E<sub>p,ECL</sub> [V]<sup>a)</sup></th><th>λ<sub>max</sub> [nm]</th><th>Φ<sub>ECLr</sub><sup>c)</sup></th></tr></thead><tbody><tr><td>IrCu-1</td><td>604</td><td>+1.21</td><td>597<sup>b)</sup></td><td>2.01</td></tr><tr><td>IrAu-1</td><td>610</td><td>+1.23</td><td>595<sup>b)</sup></td><td>1.52</td></tr><tr><td>IrCu-2</td><td>596</td><td>+0.70</td><td>595</td><td>4.13</td></tr><tr><td>IrAu-2</td><td>607</td><td>+0.67</td><td>595</td><td>6.07</td></tr></tbody></table>

compared to the IrM-1 series. The data concerning the ECL are collected in **Table 2**.

Since the anodic eT reaction is reversible for all the complexes, the ECL performances were also investigated by employing 25 mM TPrA as the sacrificial coreactant to enable the “oxidative-reductive” mechanism described in the introduction (see Equations 5–8). The potential was swapped only in the positive-going scan at 50 mV s<sup>-1</sup> to oxidize both the coreactant and the luminophores.

In **Figure 3a**, the characteristic voltammetric curves of ECL intensity versus applied potential are shown for all Ir<sup>III</sup>-M<sup>I</sup> complexes along with that of [Ru(bpy)<sub>3</sub>]<sup>2+</sup>, used as the ECL reference standard in the same experimental conditions.

The ECL occurs for all compounds as soon as the oxidation of the luminophore is reached. In a coherent manner with the oxidation process, the onset potential for ECL generation is lower for both IrM-2 complexes in comparison to both IrM-1.

The normalized ECL spectra, obtained by using the sacrificial coreactant, also resemble those obtained by photoluminescence (see Figures 3b and 1) with a shoulder at ≈640 nm for the IrM-1 series, slightly anticipated with respect to the PL spectra. As expected, the former is much more intense and better defined than those obtained by the direct annihilation mechanism (Figure S2, Supporting Information). In fact, upon oxidation of TPrA, a high concentration of the powerful reducing agent (radical type TPrA*) as in Equation 6) is made available to continuously populate the LUMO level of L via eT.

It is worth noting that all the Ir<sup>III</sup>-M<sup>I</sup> complexes showed an activation potential for ECL that is ≈0.3–0.5 V lower than that of [Ru(bpy)<sub>3</sub>]<sup>2+</sup>. Then ECL increased to reach a maximum at potential values in the range 0.79–1.23 V versus Fc/Fc<sup>+</sup> couple (see Table 2), between O<sub>1,i</sub> and O<sub>3,i</sub>. The latter is an irreversible oxidation process occurring at very positive potential values, which are close to the solvent/electrolyte discharge (see Figure S4 (Supporting information)). Notably, the ECL of all Ir<sup>III</sup>-M<sup>I</sup> complexes outperforms that of [Ru(bpy)<sub>3</sub>]<sup>2+</sup>. In particular, the performances of IrCu-2 and IrAu-2 showed a remarkable relative quantum efficiency, Φ<sub>ECLr</sub>, of 4.13 and 6.07 compared to that of [Ru(bpy)<sub>3</sub>]<sup>2+</sup>, respectively (see Table 2).

Moreover, the ECL intensity appeared to be stable in a relatively wide potential range (≈250–500 mV), especially for IrCu-2 and IrAu-2, as the best-performing compounds. Complex IrAu-2

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Figure 3.** a) ECL intensity versus the applied potential for 1 mM Ir<sup>III</sup>-M<sup>I</sup> complexes and 25 mM TPrA in ACN/0.1 TBAP. Scan rate = 50 mVs<sup>-1</sup>. b) Normalized ECL spectra of 1 mM Ir<sup>III</sup>-M<sup>I</sup> complexes, obtained by employing 25 mM TPrA, in ACN/0.1 M TBAP. Scan rate: 0.5 Vs<sup>-1</sup>. Integration time 0.5 s.</figcaption></figure>

was therefore taken as an example due to its excellent performances and further characterized as displayed in **Figure 4a**. The evolution of ECL spectra with the applied potential, also known as “spooling,”<sup>[42]</sup> clearly shows the stability of ECL in a relatively wide potential range. Further details concerning the evolution of ECL spectra with the applied potential for all Ir<sup>III</sup>-M<sup>I</sup> complexes can be found in Figure S4 (Supporting Information).

Furthermore, it can also be observed that the cyclometalating ligand dm-2-pq indeed plays a role in lowering the HOMO level of the series IrAu-2 and IrCu-2, more likely facilitating the

<footnote><sup>a)</sup> E<sub>p,ECL</sub> is the peak potential at which the maximum ECL intensity is attained; <sup>b)</sup> A shoulder at 628 nm is also visible; <sup>c)</sup> Relative ECL quantum efficiency Φ<sub>ECLr</sub> referenced to that of [Ru(bpy)<sub>3</sub>]Cl<sub>2</sub> in the same experimental conditions.</footnote>
<figure><img src="image_7.png" /><figcaption>**Figure 4.** a) CVs of 1 mM IrAu-2 alone in the full potential width in oxidation (—), and with 25 mM TPrA (—) showing the ECL intensity (—). b) Waterfall plot showing the evolution of ECL spectra as a function of the imposed potential, obtained for 1 mM IrAu-2 and 25 mM TPrA in ACN/0.1 M TBAP. Scan rate: 0.05 Vs⁻¹. Integration time for the kinetic mode: 0.5 s. c) Comparison of the Φ<sub>ECL</sub> of the two series of Ir<sup>III</sup>-M<sup>I</sup> while considering the cyclometalating ligand. d) ECL-wall plot showing the coordinates (in terms of energy requirements) of several mono- and multimetallic complexes reported in the literature (see Table S2, Supporting Information for references), including the ones investigated in this work.</figcaption></figure>

eT from the TPrA• radical, thus enhancing the ECL quantum yield. Finally, for the sake of fairness and scientific completeness, we must mention that it was not possible to compare the ECL emission of the related mononuclear parental complexes Ir(C*N*)₂(IMes-acacH)]PF₆, as the latter ones are not stable in polar solvent. However, their quantum efficiency is expected to be very low, as shown by the PL investigation previously reported.[40]

The performances of both annihilation and oxidative-reductive methods to trigger ECL witness the high stability of the electro-generated radicals that we mainly attribute to the rigidity and robustness of the structures due to the presence of the second heavy metal center, leading also to a decrease of radiationless deactivation channels as previously observed in the solid state. In fact, according to previous DFT calculations,[41] the presence of a second heavy metal center in close proximity to the first one does not seem to contribute to the orbitals closer to the frontier region nor in the lower-energy lying electronic transitions. Nevertheless, it affects both ECL and PL properties by increasing structural rigidity and chemical stability, which render less accessible nonradiative channels and yields to better MLCT/LC excited state mixing, enabling larger values for the *k*<sub>r</sub>. In fact, we observed a highly ef-

ficient and stable ECL signal over a range of applied potentials that is remarkably wider than that of [Ru(bpy)₃]²⁺.

When considering the emission wavelength and the standard potential for the oxidation process of the luminophores (Equation 1), we can undoubtedly position our heterobimetallic Ir<sup>III</sup>-M<sup>I</sup> complexes in the region of the energy-sufficient systems in the “ECL-wall” plot proposed by Hogan and co-workers.[10] This concept was introduced when considering the energetic requirements a luminophore should fulfill to provide efficient ECL. Briefly, a series of five heteroleptic Ir<sup>III</sup> complexes were considered, whose ECL is triggered by employing TPrA as the sacrificial coreactant (i.e., AB species in Equation 6) in the oxidative-reductive mechanism pathway (Equations 6–8). They compared the energy provided by the direct annihilation reaction of electro-generated radical cation and anion, described by a difference of the standard potentials (Equations 1 and 2), expressed as ΔE<sub>1/2</sub>, and the energy released by the light emission from the excited state E<sub>MLCT</sub> (the energy of the <sup>3</sup>MLCT band), while taking into account the HOMO level of the powerful reducing agent TPrA• (i.e., B• in Equation 6) generated upon oxidation of TPrA. Then, for a given emission wavelength and E<sub>MLCT</sub> value, the oxidation
(and the reduction) potential of *L* must be sufficiently positive for the eT from the HOMO of TPrA* to the LUMO of the complex to be energetically favorable. With this in mind, the authors could develop a plot featuring a critical value above which the system is energy-sufficient and thus exhibits ECL. Otherwise, ECL does not occur.

In this respect, to compare the performances of our het-
erobimetallic Ir<sup>III</sup>-M<sup>I</sup> complexes, we reconstructed the plot
(Figure 4d) and considered the ECL performances of several
other molecular emitters, featuring mono- and multimetal-
lic complexes reported in the literature (see Table S2, Sup-
porting Information for references).[43–64] These findings also
showed that the herein investigated complexes are among the
best-performing ECL luminophores as their Φ<sub>ECL</sub><sub>r</sub> is from
1.5 to 6 times higher than that of the already very efficient
[Ru(bpy)<sub>3</sub>]<sup>2+</sup> benchmark, while the potential necessary to trig-
ger ECL is 0.3–0.5 V lower. Although the focus of our re-
search work was on molecular emitters, we are aware that a
comparison with other cutting-edge ECL-active materials would
be beneficial for the readers while providing additional con-
text and a conscious understanding of the field. Recently devel-
oped ECL-active nanostructured materials mainly concern gold
nanoclusters (AuNCs) or gold nanoparticles (AuNPs),<sup>[18,65–67]</sup>
quantum dots (QDs),<sup>[68]</sup> and carbon dots (CDs).<sup>[69–71]</sup> Although
these materials are indeed very interesting and provide highly
efficient and intense PL and ECL, they cannot be strictly
regarded as molecular systems. The intrinsic advantage of
the latter one lies in the fine-tuning and precise control of
their physicochemical properties, such as optical and electronic
bandgap, and redox states, thus making molecular systems
a valuable tool to investigate the mechanisms underlying PL
and ECL phenomena and develop more complex systems and
applications.

**3. Conclusion**

In this work, we described the yet unexplored ECL properties of a
new class of heterobimetallic iTMCs, namely Ir<sup>III</sup>-M<sup>I</sup>, in organic
aprotic solvent and compare the results and performances with
the benchmark [Ru(bpy)<sub>3</sub>]<sup>2+</sup> complex under identical experimen-
tal conditions. Although the presence of a second heavy metal
center does not largely influence the energetics of lower-lying
electronic transitions in these bimetallic complexes, it seems to
positively affect both ECL and PL properties by increasing struc-
tural rigidity and chemical stability, also enabling larger values
of photoluminescence quantum yield when compared to the cor-
responding mononuclear counterparts. Remarkably, the investi-
gated heterobimetallic complexes showed highly efficient ECL,
whose emission wavelength maximum was centered at 600 nm,
and proved to be amongst the brightest ECL-active emitters de-
scribed to date. According to the ECL-wall plot previously pro-
posed by Hogan and co-workers, our luminophores lie in the
energy-sufficient region and show a stable emission requiring
a lower (0.3–0.5 V) activation potential than the benchmark
[Ru(bpy)<sub>3</sub>]<sup>2+</sup> while providing a Φ<sub>ECL</sub><sub>r</sub> which is six-fold higher. We
believe that a rational design at the molecular level along with
the key features and performances provided by this new class of
heterobimetallic iTMCs could offer a platform onto which it will
be possible to develop novel and even more efficient ECL probes,

opening the way to a wide range of applications encompassing
biosensing, imaging, and optoelectronics.

**Supporting Information**

Supporting Information is available from the Wiley Online Library or from
the author.

**Acknowledgements**

M.R. and B.P. contributed equally to this work. This work was supported by the Ca' Foscari University of Venice, the Université de Strasbourg and CNRS. F.P. kindly acknowledges Ca' Foscari University of Venice for financial support through the grant Supporting Principal Investigator (SPIN) 2021 for the project "TIDE". M.M kindly acknowledge the French Agence Nationale de Recherche (ANR) for financial support through the grants ANR-21-CE29-0015 "ChirON" (M. M.) and ANR-24-CE29-2108-01 "E-Polar" (M.M. and N.S.). F.P. and M.R. kindly acknowledges the financial support of the Italian Ministry of University and Research (MUR) under the National Recovery and Resilience Plan (NRRP), funded by the European Union - NextGenerationEU - Project Title "Engineering Functional Metal Nanocluster-Protein Architectures for Bio(sensing and catalytic) applications (ProNano4Bio)", Grant Assignment Decree No. 2022RSXC9X. The authors thank Dr. Julien Egly, Université de Strasbourg, for valuable chemical syntheses. The authors are grateful to Prof. Dr. Conor F. Hogan, La Trobe University, Australia, for sharing the original data concerning the ECL-wall and for the insightful discussion.

Open access publishing facilitated by Universita Ca' Foscari, as part of
the Wiley - CRUI-CARE agreement.

**Conflict of Interest**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available in the supplementary material of this article.

**Keywords**

electrochemiluminescence, electrochemistry, heterobimetallic ionic transition metal complexes, iridium complexes, quantum efficiency

Received: December 13, 2024

Revised: February 27, 2025

Published online: April 1, 2025

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