# Tuning the Electroluminescence of Binuclear Copper(I) Emitters Into the NIR-I Region

Abdelaziz Jouaiti,* Lavinia Ballerini, Wei-Min Zhang, Federico Polo, Christophe Gourlaouen, Hai-Ching Su,* and Matteo Mauro*

Optoelectronic devices that emit into the near infrared (NIR) region are appealing technologies with applications in optical communication and as disposable, lightweight and cheaper photodynamic biomedical devices. The development of efficient devices is currently hampered by the lack of suitable NIR-emissive materials. Achieving this goal is even more challenging when the use of non-toxic, earth-abundant metal complexes as electroactive materials is targeted. Herein, an enlarge family of binuclear Cu(I) emitters (D1-D4) bearing the bridging thiazolo[5,4-d]thiazole scaffold is described along with their mononuclear counterparts (M3-M4), which are synthetized and characterized comprehensively by a combination of chemical, spectroscopical, electrochemical and computational techniques. By means of an efficient dinuclearisation strategy in combination with the selective tuning of the *π*-accepting feature of the lateral N-heterocyclic rings enabled modulation of photo- and electro-luminescence spectra into the NIR-I region. Light-emitting electrochemical cells (LECs) fabricated by employing the binuclear copper complexes displayed electroluminescence into the deep-red to NIR-I region. Remarkably, derivative D2 shows the combination of $\text{EL}_{\text{max}}$ up to 782 with a spectral profile squarely falling into the NIR region, excellent carrier balance and good EL performances among all types of emissive materials used for NIR LECs.

## 1. Introduction

Light-emitting devices active in the near-infrared (NIR) spectral region between 700-1100 nm, namely NIR-I, are very appealing with great potential application in bio-imaging, telecommunication, and night-vision displays. NIR organic light-emitting diodes (OLEDs) have shown promising emission properties, especially in flexible display applications.[1] However, sophisticated multilayer device structures require high-vacuum deposition processes. Further-more, thorough encapsulation is generally needed for OLEDs to prevent the degradation of air-sensitive electrodes. Alternatively, NIR light-emitting electrochemical cells (LECs), which feature the unique capability of in situ electrochemical doping, facilitate ohmic contact at the electrodes.[2,3] This enables the creation of a straightforward single-layer device structure sandwiched between two air-stable electrodes. Doped layers result in low-voltage

operation, balanced carrier injection, and thus high-power efficiency of LECs. These latter are well-suited for cost-effective NIR light-emitting applications due to their compatibility with solution-based fabrication methods, including roll-to-roll,[4] inkjet printing,[5] and spray deposition,[6] on flexible substrates.[7] A broad variety of materials have been used in NIR LECs, including conjugated polymers,[8–11] ionic transition metal com plexes (iTMCs),[12–29] small molecules,[30–35] and perovskites.[36] Since iTMCs are inherently ionic, no additional ion-conducting materials or salts are required for this type of LECs. Using a single-component emissive layer simplifies device fabrication and minimizes the risk of phase separation. Furthermore, iTMCs typically exhibit higher electroluminescence (EL) efficiencies owing to their phosphorescent properties. As such, iTMCs are highly
promising candidates to be used as emissive materials in NIR LECs.

In particular, platinum-group metal complexes, such as those based on cyclometalated Pt(II) and Ir(III), are leading emitters for efficiently harvesting triplet excited states in light emitting devices due to the combination of excellent redox and optical properties with the enhanced (photo-)chemical robustness.[37] On the other hand, earth-abundant metals as those based on the first row of the *d*-block group are currently under intense investigation as potential alternatives.[38-40] Cu¹-based emitters being considered the most attractive candidates to date.[41-44] This is also due to the fact that their closed-shell *d*¹⁰ electronic configuration rules out the possibility of having low-lying metal-centered (MC) states that may efficiently quench the radiative process. Different families of Cu¹ complexes with various nuclearities have been investigated yielding luminescent compounds with emission ranging from the blue[45,46] to the orange region to date.[41-42] Nevertheless, pushing the emission profile further into the deep-red or even to NIR region is very challenging and requires careful molecular design strategies.[28,47]

Three concomitant factors have to be accounted as the reasons: i) red to NIR emitters possess intrinsically smaller $k_r$ due to the third-power dependency with emission energy, as by Einstein's theory of spontaneous emission; ii) non-radiative vibronic coupling between $T_1$ and ground state is very efficient due to the negative Napierian logarithmic dependency with the $T_1-S_0$ energy difference (energy gap law);[48] and, iii) the smaller spin orbit coupling (SOC) constant associated with the copper ion compared to iridium and platinum ($\zeta_{Cu} = 857$ cm⁻¹; $\zeta_{Ir} = 3909$ cm⁻¹; $\zeta = 4481$ cm⁻¹) yields complexes with slower radiative rate constant ($k_r$).

The design of luminescent molecular compounds that possess the combination of high $k_r$ and low $k_{nr}$ plays therefore a pivotal role in achieving efficient photo- and electro-luminescence into the NIR spectral region.

Recently, dinuclearisation strategies based on different ligand scaffold have appeared in the literature as potential design approaches for efficient longer-wavelength emitters.[49,50] In this framework, we have very recently described a novel family of binuclear Cu(I) complexes as electroluminescent materials that allowed to achieve a stable NIR electroluminescence (EL) in LEC devices with emission maximum, $\lambda_{EL,max}$ at $\approx 750$ nm, for the first time.[28]

Prompted by these promising results and by means of a fine tuning of the electronic properties of the chromophoric ligands, we herein aim at enlarging such family of Cu(I) emitters to modulate the luminescence spectra further into the NIR-I region. Selective tuning of the π-accepting feature of the bridging thiazolo[5,4-*d*]thiazole (TzTz) scaffold yielded a series binuclear copper complexes that displayed electroluminescence in LEC with $\lambda_{EL,max}$ up to 782 nm and squarely falling into the NIR region, unprecedentedly.

## 2. Results and Discussion

### 2.1. Synthesis and X-ray Characterization

To tune the optical properties into the deep red to NIR-I region of this class of Cu(I) complexes selective modulation of the elec-

tronic features of the chromophoric thiazolo[5,4-*d*]thiazole (TzTz) scaffold was carried out by introducing substituents with either electron donating (methyl), π-extended (quinoline) and electron-accepting (pyrazine) groups on the lateral N-based coordinating site, yielding ligand L1, L2 and L3, respectively (see supporting information for chemical structures). **Figure 1** displays the chemical structure of resulting binuclear Cu¹ complexes D1–D4. The synthetic strategy follows the one recently reported by us[28] after suitable modification. Briefly, the chelating phosphine P^P, being either DPEPhos or Xantphos, was reacted with [Cu(CH₃CN)₄]PF₆ as the copper(I) precursor in CH₂Cl₂ at room temperature, followed by the addition to the chromphoric ligand L1–L4 to the reaction mixture. After purification by filtration and washing, the target complexes were obtained straightforwardly as red solid in high reaction yields and purity. On the other hand, the synthesis of the corresponding mononuclear derivatives M3 and M4 bearing the 2-(pyrazin-2-yl)benzo[*d*]thiazole ligand was carried out as well for comparison purposes. The two compounds differ on the diphosphine chelate employed. Related mononuclear derivatives bearing the 2-(quinolin-2-yl)benzo[*d*]thiazole and the 2-(pyridin-2-yl)benzo[*d*]thiazole chromophoric ligands were reported previously.[51] The experimental details about the synthetic procedures and full chemical characterization of the novel compounds can be found in the supporting information Figures S1–S30 (Supporting Information), whereas Schemes S1 and S2 (Supporting Information) display the synthetic strategy employed.

To confirm the chemical connectivity and investigate about the presence of intramolecular interligand as well as intermolecular interactions, single-crystal X-ray diffractometric analysis was carried out for all the complexes and the corresponding ORTEP diagrams are displayed in **Figure 2** for complexes D1 and D2, and Figures S31–S34 (Supporting Information) of the Supporting Information for derivatives D3–D4 and M3–M4. Tables S1–S6 (Supporting Information) summarize the crystallographic refinement parameters.

For all the derivatives and as expected for a Cu(I) complex, the *d*¹⁰ metal ion adopts a pseudo-tetrahedral geometry with [Cu(N^N)P(P^P)]⁺ motif, each being coordinated by the π-extended ligand in a N^N fashion involving one N atom of the TzTz scaffold and one on the lateral N-containing ring. The coordination sphere is completed by the chelation of the bidendate rigid diphosphine (P^P).

Binuclear derivative D1 displays two identical intramolecular interligand π-interactions (d = 3.62 Å) established between one of the phenyls of each of the phosphines and the coordinating π-deficient ligand L1, whereas two couples of interligand π-interactions are present in the X-ray structure of D2 involving one phenyl ring for each phosphine group. The shorter one (d = ≈3.30 Å) is established between the phenyl of the phosphine and the electron poor TzTz core, the longer one (d = ≈4.14 Å) is a π-π staking between a phenyl bound to the second chelating P-atom and the quinoline scaffold. These intramolecular interligand interactions are of high importance for preserving the pseudo-tetrahedral geometry and reduce the excited-state quenching vibrational coupling associated with the large pseudo Jahn-Teller flattening distortion.[39-41]

No interligand close interactions are present for complex D3 and D4 most likely due to the highly distorted geometry adopted
<figure><img src="image_1.png" /><figcaption>**Figure 1.** Chemical structure of the investigated complexes D1–D4 and their mononuclear counterparts M3–M4. All the complexes were prepared as PF₆⁻ salt.</figcaption></figure>

by the π-extended ligand L3 upon coordination to Cu(I) centers. On the other hand, while no π-interactions are shown by the X-ray structure of mononuclear complex M3, a weak intraligand π-π staking (d ≈3.93 Å) involving two phenyl rings of the co-ordinating Xantphos ligand is present for derivative M4. In addition, for all the complexes the supramolecular arrangement is developed based on C–H···F hydrogen bonding interactions, besides π-π interactions. The distances between the six fluorine atoms and the closest hydrogen atom involved in the PF₆···C–H···C(phenyl) interactions are in the range of 2.412–2.644 Å, 2.463–2.619 Å, 2.177–2.663 Å, 2.372–2.662 Å, 2.426–2.665 Å and 2.426–2.665 Å for M3, M4, D1, D2, D3 and D4, respectively. Moreover, intermolecular π-π interactions are present between two adjacent pyrazines (M3–M4), and quinolines (D2) along with the phenyls

of P³P ligands of neighbor molecules. The thermal properties of all the compounds were investigated by means of thermogravimetric analysis (TGA) under air and the thermograms are shown in Figures S35–S40 (Supporting Information). In this condition, they display excellent thermal stability, being T<sub>5%</sub> = 282–345 °C, with the lower value associated to the mononuclear species (T<sub>5%</sub> is the temperature at which 5% of the initial weight loss has occurred).

## 2.2. Electrochemistry

The electrochemical properties of all the investigated complexes were assessed by means of cyclic voltammetry in CH₂Cl₂/0.1 M TBAPF₆ solution. The positive- and negative-going

<figure><img src="image_2.png" /><figcaption>**Figure 2.** ORTEP diagram obtained for the complexes D1 (**left**) and D2 (**right**) by means of single crystal X-ray diffractometric analysis. Thermal ellipsoids are shown at a 50% probability level. PF₆⁻ anions and solvent molecules are omitted for clarity.</figcaption></figure>
<table><caption>**Table 1.** Electrochemical data recorded for binuclear complexes **D1–D4** as well as mononuclear counterparts **M3** and **M4** in CH₂Cl₂/0.1 M TBAPF₆ solution. Potential values are given against Fc⁺ | Fc couple.</caption><thead><tr><th>Complex <i>i</i></th><th><i>E</i><sub>p,O2,i</sub> [V]<sup>a)</sup></th><th><i>E</i><sub>p,O1,i</sub> [V]<sup>a)</sup></th><th><i>E</i><sup>0</sup><sub>R<sub>1,i</sub></sub> [V]<sup>b)</sup></th><th><i>E</i><sup>0</sup><sub>R<sub>2,i</sub></sub> [V]<sup>b)</sup></th><th><i>E</i><sup>0</sup><sub>R<sub>3,i</sub></sub> [V]<sup>b)</sup></th><th><i>e</i>Δ<i>E</i><sub>H-L</sub> [eV]<sup>c)</sup></th></tr></thead><tbody><tr><td><b>D1</b></td><td>0.86</td><td>-1.29</td><td>-1.61<sup>d)</sup></td><td>-1.82</td><td>-1.79</td><td>2.15</td></tr><tr><td><b>D2</b></td><td>0.94</td><td>-1.28</td><td>-1.59<sup>d)</sup></td><td>-1.82</td><td>-1.79</td><td>2.22</td></tr><tr><td><b>D3</b></td><td>0.86</td><td>-1.29</td><td>-1.61<sup>d)</sup></td><td>-1.82</td><td>-1.79</td><td>2.15</td></tr><tr><td><b>D4</b></td><td>0.79</td><td>-1.04</td><td>-1.40<sup>d)</sup></td><td>-1.59</td><td>-1.79</td><td>1.83</td></tr><tr><td><b>M3</b></td><td>1.18</td><td>1.05<sup>e)</sup></td><td>-1.38<sup>a),e)</sup></td><td>-1.54</td><td>-1.79</td><td>2.56</td></tr><tr><td><b>M4</b></td><td>1.30<sup>a)</sup></td><td>0.99</td><td>-1.55<sup>a),e)</sup></td><td>-1.65</td><td>-1.79</td><td>2.54</td></tr></tbody></table>

scans were recorded separately to better appreciate the re-
dox processes, while covering the whole potential window of
interest. The data are listed in **Table 1** and the correspond-
ing voltammograms are shown in **Figure 3**, where indica-
tion of the electrochemical HOMO–LUMO band gap is also
provided (dashed lines). More details concerning the elec-
trochemical characterization are provided in the Supporting
Information.

In the positive-going scan, as reported in Figure 3, all com-
pounds showed an irreversible oxidation process, *O*<sub>1,i</sub> (with *i* de-

noting the investigated compound), whose peak potentials *E*<sub>p</sub> fall
within the range +0.79 – +1.18 V versus Ferrocene/Ferricenium
(Fc<sup>0</sup>|Fc<sup>+</sup>) redox couple, used as the internal standard. *E*<sub>p,i</sub> slightly
depends on the nature of the ancillary N'N or phosphine lig-
ands. This redox process can be confidently ascribed to the ox-
idation that is mainly centered onto the metal and the phospho-
rous atoms. Moreover, once the potential scan is reverted a ca-
thodic peak appeared in the range 0–0.5 V, which might be re-
lated to the reduction of the of species forming during the irre-
versible oxidation process, more likely followed by a chemical re-
action (EC process). The nature of this process was not further
investigated as it lies beyond the scope of this work. The two
mononuclear complexes **M3** and **M4** also showed a second oxi-
dation process, more likely due to the oxidation of the ligand. In-
stead, in the negative-going scan, up to three reversible reduction
processes *R*<sub>n,i</sub> (where *n* denotes the process number and *i* the in-
vestigated compound) occurred, whose standard potential values
*E*<sup>0</sup><sub>R</sub> fall within the range from –1.04 to –1.82 V versus Fc<sup>0</sup>|Fc<sup>+</sup>
(se<sup>e</sup> in Figure 3). Clear differences are observed in the negative go-
ing scan between the different complexes. Mononuclear deriva-
tives **M3** and **M4** show one main electrochemical wave, which is
preceded by a weak redox process referred to as *E*<sup>0</sup><sub>R<sub>M3–M4</sub></sub>, whose
*E*<sup>0</sup><sub>R<sub>2,M3–M4</sub></sub> are –1.54 and –1.65 V, respectively, and that can be
ascribed to a reversible, one-electron, reduction process located
onto the π-electron poor 2-(5-(3,5-di-*tert*-butylphenyl)pyrazin-2-
yl)benzo[*d*]thiazole ligand. In fact, the first reduction process for
all the binuclear copper complexes occurs at a more positive po-
tential than for the mononuclear counterpart, thus witnessing a
higher delocalization and stabilization of the electronic structure
in the corresponding radical anion. As a result, the lowest unoc-
cupied molecular orbital (LUMO) lies at a lower energy for the
bimetallic compounds, thus narrowing the electrochemical band
gap. The following processes are attributed to the second reduc-
tion process of the ditopic thiazolo[5,4-*d*]thiazole-based ligand.
Overall, a steady narrowing of the HOMO-LUMO gap for com-
pound **D1–D4** is observed in comparison of the mononuclear
counterparts, as determined by the electrochemical energy band
gap (e-Δ*E*<sub>H-L</sub>). Furthermore, as previously observed for other
bimetallic complexes,<sup>[28]</sup> by taking into account the diffusion co-
efficient of the binuclear complexes, when compared with those
of the mononuclear counterpart, and by comparing the current
intensities of their *R*<sub>n,i</sub> and *O*<sub>1,i</sub> processes with the oxidation of

<figure><img src="image_3.png" /><figcaption>**Figure 3.** Cyclic voltammograms of samples of complexes **D1** (—), **D2** (—), **D3** (—), **D4** (—), **M3** (—), **M4** (—) at concentration of 1 mM. Data recorded in CH₂Cl₂/0.1 M TBAPF₆ as supporting electrolyte using a three-electrode electrochemical cell. Working electrode: glassy carbon; counter electrode: Pt wire; quasi-reference electrode: Ag wire. Scan rate of 0.2 V s⁻¹.</figcaption></figure>

<footnote><sup>a)</sup>Irreversible process. Only the peak potential, *E*<sub>p</sub>, can be reported; <sup>b)</sup>Reversible process. The formal potential, *E*<sup>0</sup>, was calculated as the average of the cathodic and anodic peak of the process; <sup>c)</sup>The electrochemical band gap *e*Δ*E*<sub>H-L</sub>, was calculated as the difference between of the formal potential of *R*<sub>1,i</sub> process and the peak potential of *O*<sub>1,i</sub> process; <sup>d)</sup>The anodic counter-part is partially hidden by the following redox process or is weak: the *E*<sup>0</sup> value is just an estimation; <sup>e)</sup>Pre-peak.</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 4.** Electronic absorption (solid traces) and emission spectra (solid traces with symbols) of binuclear complex D1 (green), D2 (orange), D3 (black), and D4 (red) recorded in CH₂Cl₂ at concentration of 3 × 10⁻⁵ M at room temperature upon excitation at λexc = 480–500 nm. The spectra of complex Cu-NIR1 (grey) are reported for comparison purposes.</figcaption></figure>

Fc, one can confidently state that the redox processes are all monoelectronic. Overall, these findings agree well with the optical properties (see below) and support the idea that these complexes are potentially suitable candidates in the field of solid-state NIR electroluminescence devices.

## 2.3. Photophysical Investigation

The optical properties of the complexes were first investigated in dilute CH₂Cl₂ solution. The absorption and emission spectra are displayed in Figure 4 for the binuclear complexes D1–D4. Spectral comparison with the mononuclear derivatives M3–M4 is shown in Figure S41 (Supporting Information) of the Supporting Information. All the photophysical data are summarized in Table 2. Benchmark complexes Cu-NIR1 reported by us as well as mononuclear compound [Cu(DPEPhos)(2-(pyridin-2-yl)benzo[d]thiazole)] and [Cu(DPEPhos)(2-(quinolin-2-yl)benzo[d]thiazole)] described previously[51] are reported for comparison purposes.

As far as the binuclear complexes are concerned, the UV-vis spectra display three main absorption bands. On the higher energy side at λ<sub>abs</sub> = 265–300 nm, an intense band is present attributed to ¹π–π* (ligand centered, LC) transitions involving the aromatic groups of the phosphine ligands and the N-heterocyclic rings. The most intense (ε = 3.7–6.5 × 10⁴ M⁻¹ cm⁻¹) band is displayed at λ<sub>abs</sub> ≈ 370–430 nm and is mainly attributable to the ¹π–π* localized onto the π-extended chromophoric thiazolo[5,4-d]thiazole scaffold. On the lower energy side, the broader and less intense band (ε = 1.8–4.8 × 10³ M⁻¹ cm⁻¹) is ascribed to a mixed transition with singlet-manifold metal-to-ligand charge transfer (¹MLCT, ¹dπ(Cu)→π<sub>TzTz</sub>*) and ligand-to-ligand charge transfer (¹LLCT, ¹πₚ→π<sub>TzTz</sub>* character). This finding is in well agreement with the concomitant steady increase of the π-accepting ability of the substituted TzTz ligand along the series 6-methylpyridine (D1) → 5-phenylpyridine (Cu-NIR1) → quinoline (D2) → 5-phenylpyrazine (D3–D4). Whereas, the electronic properties of

the Cu(P'P) moiety would remain unaffected mainly, thus supporting this overall attribution of the spectral features.

Upon excitation in the lower-lying ¹MLCT band, all the complexes in CH₂Cl₂ display a broad and featureless photoluminescence with maximum centered into the deep-red to NIR region, yet with low PLQY. The emission profile spans over ≈100 nm, from λ<sub>em</sub> = 701–791 nm, when moving from complex D1 to D4. As a result, an overall narrowing of the optical bandgap is observed, which is accompanied by a lowering of the photoluminescence quantum yield and shortening of the excited state lifetime (τ = 193–73 ns), owing to the increased vibronic coupling between T₁ and S₀ state, as stated by the energy gap law.

These data allow us to estimate the radiative (k<sub>r</sub>) and non-radiative (k<sub>nr</sub>) rate constants characterizing the emissive excited state by using the following Equation (1) and –(2):

$$k_t = \text{PLQY}/\tau \quad (1)$$

$$k_{nr} = (1 - \text{PLQY})/\tau \quad (2)$$

The estimated k<sub>t</sub> values falls in the order of a few 10²–10³ s⁻¹. These data along with the shortening of the excited state lifetimes in presence of quenching dioxygen molecules in agreement with a bimolecular diffusion-controlled quenching process (data not shown) are indicative of a partially spin-forbidden transition arising from an excited state with mixed triplet ligand centered and metal-ligand-to-ligand charge transfer (³LC/³ML<sub>TzTz</sub>LₚCT) character. On the other hand, the estimated values of the k<sub>nr</sub> are three to four order of magnitude higher compared to k<sub>t</sub>, most likely due to the large geometrical flexibility and distortions occurring upon population of electronic excited states, as typical of other mononuclear [Cu(N'N)₂]<sub>+</sub> and [Cu(N'N)(P'P)]<sub>+</sub> derivatives.

Emission spectra of samples in the glassy matrix are shown in Figures S42 and S43 (Supporting Information) (see Table 2 for photophysical data). Upon lowering the temperature down to 77 K, CH₂Cl₂ samples of binuclear complexes display a more structured emission profile with a clear vibronic progression that is hypsochromically shifted by ≈2400–3000 cm⁻¹; whereas a broader spectrum is observed for the mononuclear counterparts with a smaller shift of ≈1250 cm⁻¹. These spectral changes are accompanied by a sizeable prolongation of the excited state lifetime that reaches values as long as τ = 151–541 µs. These findings are in agreement with the large triplet nature of the emissive excited state with mixed ³L<sub>TzTz</sub>C/³ML<sub>TzTz</sub>LₚCT character and rule out any thermally-activated delayed fluorescence (TADF) process taking place in solution as the origin of the emission.

Solid state samples as neat powder display broad and featureless photoluminescence spectra with profiles that shifted to shorter wavelengths and concomitant increase of the PLQY and prolongation of the excited state lifetimes. This effect might be due to the local molecular arrangement of the emitters already observed for red-emitting copper(I) complexes (see Figure S44 and Table S7 (Supporting Information) for emission spectra and photophysical data).[43]

To better evaluate the luminescence properties of this novel family of longer-wavelength Cu(I) complexes in view of their
<table><caption>**Table 2.** Photophysical data recorded for complexes D1–D4, Cu-NIR1, and M3–M4 in dilute CH₂Cl₂ for degassed solution at both room temperature and in frozen matrix at 77 K. *sh* denotes a shoulder.</caption><thead><tr><th rowspan="2">Compound</th><th colspan="5">Room temperature</th><th colspan="2">77 K</th></tr><tr><th>λ<sub>abs</sub> (e) [nm,<br>{10<sup>3</sup> M<sup>-1</sup> cm<sup>-1</sup>)]</th><th>λ<sub>em</sub> [nm]</th><th>PLQY [%]</th><th>ε [ns]</th><th>k<sub>r</sub> [×10<sup>3</sup> s<sup>-1</sup>]</th><th>k<sub>ir</sub> [×10<sup>6</sup> s<sup>-1</sup>]</th><th>λ<sub>em</sub> [nm]</th><th>ε [μs]</th></tr></thead><tbody><tr><td rowspan="4">D1</td><td>275 (sh), (36.3)</td><td rowspan="4">701</td><td rowspan="4">0.50</td><td rowspan="4">193</td><td rowspan="4">26</td><td rowspan="4">5.2</td><td rowspan="4">597, 653, 715, 791</td><td>253 (33%)</td></tr><tr><td>375, (37.0)</td><td>600 (67%)</td></tr><tr><td>390 (sh), (30.6)</td><td>106 (50%)</td></tr><tr><td>452, (3.8)</td><td>106 (50%)</td></tr><tr><td rowspan="4">D2</td><td>280 (sh), (39.9) 400, (51.2)</td><td rowspan="4">765</td><td rowspan="4">0.12</td><td rowspan="4">35 (87%)<br>224 (13%)</td><td rowspan="4">9.2</td><td rowspan="4">7.7</td><td rowspan="4">620, 680, 751</td><td>213 (50%)</td></tr><tr><td>420, (42.6)</td><td>56.7 (36%)</td></tr><tr><td>500, (4.8)</td><td>56.7 (36%)</td></tr><tr><td>265 (sh), (46.2)</td><td>790</td><td>0.04</td><td>972</td><td>1.0</td><td>665, 731 812</td></tr><tr><td rowspan="4">D3</td><td>430, (64.6)</td><td rowspan="4">790</td><td rowspan="4">0.04</td><td rowspan="4">90</td><td rowspan="4">2.2</td><td rowspan="4">11</td><td rowspan="4">658 724 800</td><td>168 (64%)</td></tr><tr><td>520 (sh), (3.6)</td><td>184 (57%)</td></tr><tr><td>427, (61.2)</td><td>70 (43%)</td></tr><tr><td>528 (sh) (1.8)</td><td>70 (43%)</td></tr><tr><td rowspan="4">Cu-NIR1</td><td>279sh, (41.5)</td><td rowspan="4">746</td><td rowspan="4">0.13</td><td rowspan="4">39 (36%)<br>82 (64%)</td><td rowspan="4">1.8</td><td rowspan="4">14</td><td rowspan="4">627, 686, 758, 842 (sh)</td><td>321 (72%)</td></tr><tr><td>407, (57.7)</td><td>82 (28%)</td></tr><tr><td>504sh, (3.0)</td><td>82 (28%)</td></tr><tr><td>261 (sh), (26.9) 285 (sh),<br>(23.1) 376, (33.1)</td><td>749</td><td>0.08</td><td>112.2</td><td>8.9</td><td>684</td></tr><tr><td rowspan="4">M3</td><td>458, (4.1)</td><td rowspan="4">745</td><td rowspan="4">0.04</td><td rowspan="4">26.7</td><td rowspan="4">15</td><td rowspan="4">37</td><td rowspan="4">683</td><td>29 (52%)</td></tr><tr><td>267, (32)</td><td>98 (41%)</td></tr><tr><td>375, (30.1)</td><td>88 (35%)</td></tr><tr><td>455 (sh), (4.0)</td><td>88 (35%)</td></tr><tr><td rowspan="4">M4</td><td>293 (21), 310 (19), 327 (16),<br>345 (8.8),<br>415 (2.4)</td><td rowspan="4">735</td><td rowspan="4">0.06</td><td rowspan="4">1-9 μs</td><td rowspan="4">–</td><td rowspan="4">–</td><td rowspan="4">639</td><td>35 (62), 99 (32), 274<br>(6)</td></tr><tr><td>287 (24), 353 (20), 368 (19),<br>457 (3.2)</td><td>730</td><td>–</td><td>1-9 μs</td><td>–</td><td>700</td><td>36 (80), 135 (20)</td></tr></tbody></table>

<footnote>a) data from ref. [51].</footnote>
<table><caption>**Table 3.** Photophysical data recorded for doped film with complexes **D1–D4**, **Cu-NIR1**, and **M3–M4** in PMMA 10 wt.% and in [BMIMPF<sub>6</sub>] at 20 wt.%.</caption><thead><tr><th rowspan="2">Compound</th><th colspan="3">PMMA 10 wt.%</th><th colspan="3">[BMIMPF<sub>6</sub>] 20 wt.%</th></tr><tr><th>λ<sub>em</sub> [nm]</th><th>PLQY [%]</th><th>τ<sup>a</sup> [μs]</th><th>λ<sub>em</sub> [nm]</th><th>PLQY [%]</th><th>τ<sup>a</sup> [μs]</th></tr></thead><tbody><tr><td>D1</td><td>645</td><td>4</td><td>6.1 (58%)<br>28.8 (35%)<sup>[23]</sup></td><td>665</td><td>3.1</td><td>7.1 (38%)<br>1.9 (55%)<br>[5.6]</td></tr><tr><td>D2</td><td>706</td><td>1.7</td><td>0.7 (56%)<br>1.8 (41%)<br>[1.4]</td><td>720</td><td>1.1</td><td>0.3 (56%)<br>0.9 (41%)<br>[0.7]</td></tr><tr><td>D3</td><td>717</td><td>1</td><td>0.9 (65%)<br>2.3 (33%)<br>[1.7]</td><td>736</td><td>0.6</td><td>3.1 (37%)<br>0.8 (57%)<br>[2.4]</td></tr><tr><td>D4</td><td>746</td><td>0.7</td><td>0.7 (52%)<br>1.8 (44%)<br>[1.5]</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Cu-NIR1</td><td>692</td><td>0.5</td><td>3.4 (55%)<br>14.7 (36%)<br>[11.8]</td><td>702</td><td>1.4</td><td>1.4 (55%)<br>5.8 (38%)<br>[4.7]</td></tr><tr><td>M3</td><td>642</td><td>1.4</td><td>1.6 (51%)<br>5.6 (42%)<br>[4.6]</td><td>675</td><td>0.9</td><td>0.5 (42.4%)<br>1.3 (51.7%)<br>[1.1]</td></tr><tr><td>M4</td><td>644</td><td>1.2</td><td>1.2 (54%)<br>4.5 (40%)<br>[3.6]</td><td>663</td><td>0.9</td><td>0.5 (36.4%)<br>1.3 (54.4%)<br>[1.1]</td></tr></tbody></table>

potential use as solid state emitters in NIR LEC devices, the optical properties were investigated in thin-film samples fabricated by spin coating from a CH<sub>2</sub>Cl<sub>2</sub> solution of poly-(methylmetacrylate) (PMMA) at 10 wt.% doping level as well as in BMIMPF<sub>6</sub> ionic liquid matrix. The data are listed in **Table 3** and the photoluminescence spectra are displayed in **Figures 5** and S45 (Supporting Information). Rigidification of the environ-

<figure><img src="image_5.png" /><figcaption>**Figure 5.** Emission spectra of mononuclear copper(I) complexes **M3** (blue trace) and **M4** (violet trace) and binuclear counterparts **D1** (green trace) **D2** (orange trace) **D3** (black trace) and recorded in BMIMPF<sub>6</sub> thin films at doping level of 20 wt.% upon excitation at λ<sub>exc</sub> = 455 nm (**D1**), 520 nm (**D2**), 475 nm (**D3**), 470 nm (**M3**) and 465 nm (**M4**).</figcaption></figure>

ment yields a sizeable rigidochromic effect with hypochromic shift of the emission that retains an unstructured profile indicative of a large CT nature of the emissive excited state. This spectral shift is much higher for mononuclear complexes compared to the binuclear derivatives and, for the latter emission profile display maxima in the NIR region even in the thin film state, being λ<sub>em</sub> = 720 and 736 nm for derivative **D2** and **D3**, respectively. On the other hand, mononuclear derivatives **M3** and **M4** as well as complex **D1** display the least extended π-conjugated system amongst the series of binuclear complexes shown an emission shifted into the red and deep-red region only. Interestingly, the binuclear complex **D1** possesses a PLQY value that is three-fold higher compared to the mononuclear **M3–M4** despite the very similar emission spectra. Remarkably, complex **D2** and **D3** display a PLQY value that is still ≈1% and an excited state lifetime in the order of 0.7–2.4 µs in doped ionic liquid thin films. Overall, these findings support the fact that dinuclearisation strategy is a superior design approach for enhancing the optical properties of NIR emitters. To further elucidate the mechanism of the emission process at play, steady-state and time-resolved variable temperature measurements were carried out on the doped thin films in the temperature range 293–77 K. Spectral profiles recorded for samples of complex **D1–D2** and **M2** show a hypochromic shift with the appearance of a vibronic structure upon lowering the temperature (data not shown), ruling out the occurrence of TADF process in this condition. On the other hand, the IL:complex **M3** thin film shows a small bathochromic shift that allowed to estimate the S<sub>1</sub>-T<sub>1</sub> energy difference, ΔE(S<sub>1</sub>-T<sub>1</sub>), of 40 meV; a value that is small enough to enable thermal equilibration between the two states via a TADF mechanism (see Figure S46, Supporting Information). Variable temperature

<footnote><sup>a</sup> average lifetimes are reported in brackets. See Supporting Information for the definition.</footnote>
exited state lifetimes were measured over the same temperature range and the obtained data are shown in Figure S46 (Supporting Information), which were fitted by using a Boltzmann-type process involving a two-state model as described by the following Equation (3):

$$ \bar{\tau} = \frac{3 + e^{\frac{-\Delta E(S_1 - T_1)}{k_B T}}}{3k(T_1 + kS_1) \times e^{\frac{\Delta E(S_1 - T_1)}{k_B T}}} \quad (3) $$

where $\Delta E(S_1 - (T_1))$ is the estimated energy separation between the $S_1$ and $T_1$ manifolds involved in the TADF process, $k_B$ is the Boltzmann constant, T the absolute temperature, $k(T_1)$ and $k(S_1)$ are the radiative rate constants of the phosphorescence and the prompt fluorescence arising from the $T_1$ and $S_1$ state, respectively. The fitting provided an estimated $\Delta E(S_1-T_1)$ of 58 meV and intrinsic excited state lifetime of the $T_1$ level $\tau_1^0(T_1)$ of 54.9 µs. This latter well compares with the average lifetime value of 43.0 µs recorded at 77 K.

## 2.4. Computational Study

The structures of complexes D1, D2, and D3 were optimized by means of Density Functional Theory (DFT), and the computed geometries are in good agreement with the experimental ones. For a list of selected geometrical parameters, a comparison is provided in Tables S8–S11 (Supporting Information), and the corresponding atom numbering can be found in Scheme S3 (Supporting Information). In particular, geometry optimization of D1 and D2 yielded a minimum-energy structure that features interligand interactions similar to what was observed experimentally. For derivative D3 several close-energy conformations were computed, one of which presents π-interactions between ligand L3 and the diphosphine ligand. This structure lies 1.7 kcal mol⁻¹ above the lowest-energy conformation that has been optimized directly from the X-ray data. For derivative M3, some distortions are observed due to the formation of π-stacking interactions between one aryl of each phosphine group with ligand 4 in agreement with the significant distortions of the coordination sphere around the copper centers with both bending (α = 157.6° for D1) and flattening (β = 101.4° for D2) parameters as observed experimentally.

As far as the series of binuclear complexes is concerned, the molecular orbitals (MOs) close to the frontier region display very similar features, expectantly. The MOs energies are listed in Table 4 and the corresponding isodensity surfaces are displayed in Figures S47 and S48 (Supporting Information). The HOMO and the degenerated HOMO-1 consist in the P-Cu bonding orbital. The LUMO is a π* orbital localized on the π-extended electron-accepting ligand, namely L1, L2 and L3 for D1, D2 and D3-D4, respectively. Hence, a selective energetic stabilization of the LUMO is observed along the series yielding an overall decrease of the HOMO-LUMO gap. This agrees nicely with the narrowing of the optical and electrochemical bandgap experimentally observed going from D1 to D3 (see above).

The frontier orbitals of M3 are similar. The LUMO is also a π* orbital localized on ligand 4 and is significantly higher

<table><caption>**Table 4.** Energies of the MOs closer to the frontier region and HOMO-LUMO gap. The values highlighted in bold correspond to the energy of the highest occupied orbital localized on the ligands L. Energy values are provided in [eV].</caption><thead><tr><th></th><th>M3</th><th>D1</th><th>D2</th><th>D3</th></tr></thead><tbody><tr><td>LUMO+1</td><td>-2.329</td><td>-2.809</td><td>-3.153</td><td>-3.312</td></tr><tr><td>LUMO</td><td>-3.222</td><td>-3.754</td><td>-3.841</td><td>-4.129</td></tr><tr><td>HOMO</td><td>-6.327</td><td>-6.697</td><td>-6.572</td><td>-6.752</td></tr><tr><td>HOMO-1</td><td>-6.906</td><td>-6.724</td><td>-6.614</td><td>-6.753</td></tr><tr><td>HOMO-2</td><td><b>-6.968</b></td><td>-7.058</td><td>-7.214</td><td><b>-7.156</b></td></tr><tr><td>HOMO-3</td><td>-7.150</td><td>-7.240</td><td>-7.218</td><td>-7.218</td></tr><tr><td>HOMO-4</td><td><b>-7.313</b></td><td><b>-7.329</b></td><td><b>-7.231</b></td><td><b>-7.222</b></td></tr><tr><td>HOMO-LUMO gap</td><td><b>3.105</b></td><td><b>2.943</b></td><td><b>2.731</b></td><td><b>2.623</b></td></tr></tbody></table>

in energy than the dinuclear complexes LUMO (Table 4). As in the other complexes, the HOMO is a P-Cu bonding orbital also destabilized compared to the other complex but to a lesser extent. This results in an increased HOMO-LUMO gap as expected.

The computed absorption spectra of the different complexes along with the ligands L1–L3 are shown in Figures S49–S52 (Supporting Information) and the corresponding absorption transitions are listed in Tables S12–S14 (Supporting Information). The spectra display similar features and are in good agreement with experimental data, and the electron density difference maps (EDDMs) associated with the three main electronic processes are displayed in Figure 6 for derivative D1 as an example (see Figures S53–S55, Supporting Information for the other complexes). They all present a first weak absorbing band at low energy ranging from 499 to 570 nm, with a sizeable bathochromic shift when moving from mononuclear complex M3 to binuclear derivatives D1–D3. This lowest energy band arises from the HOMO→LUMO ($S_0→S_1$) electronic transition and it can be described as of mixed $^1MLCT/^1L_pL_{TzTz}CT$ nature, where TzTz is ligand L1–L3. On the higher energy side, an intense band is computed in the near-UV domain (350–400 nm) for all complexes, resulting from the convolution of several electronic transitions of mixed nature, being mainly $^1L_{TzTz}C/^1L_pL_{TzTz}CT$ ($S_0→S_7$, the most intense) and a higher-energy $^1M_{TzTz}CT$ ($S_0→S_{11}$) for derivative D1 (see Figure 6). For the four computed binuclear complexes this band is close to the intense $^1L_{TzTz}C$ transitions calculated for the corresponding proligand L1–L3, yet slightly shifted bathochromically by ≈230 (D2) and 3650 (M3) cm⁻¹ in the complexes due to the metal coordination and mixing with the diphosphine ligand. However, the theoretical spectra do not reproduce the structure of this band observed experimentally (cf. Figure 4 and Figures S49–S52, Supporting Information). Considering the nature of the most intense transition which is mainly of $^1L_{TzTz}C/^1L_pL_{TzTz}CT$ character, the structure of this band can be attributed to vibronic progression associated to the heteroaromatic TzTz ligand. This is further supported by the vibronic spectra computed for the first singlet state in both absorption and emission for both a simplified version of L1 where the methyl groups were removed, namely L1-Me, and L2 (Figures S56 and S57, Supporting Information).
<figure><img src="image_6.png" /><figcaption>**Figure 6.** Electron Density Difference Maps (EDDMs) between ground and excited state computed at Franck-Condon geometry for complex **D1** for selected electronic transitions. Electronically enriched and depleted areas are colored green and red, respectively. Hydrogen atoms are omitted for clarity.</figcaption></figure>

## 2.5. Electroluminescent Characteristics of LECs

The EL properties of the LECs utilizing the proposed mononuclear complexes **M3–M4** and binuclear complexes **D1–D3** were evaluated through device fabrication and EL measurement processes. The device configuration has the following architecture: glass/indium-tin oxide (ITO) (70 nm)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (40 nm)/emissive layer (complex 80 wt.% and [BMIM⁺(PF₆)⁻] 20 wt.%)/Ag (100 nm). The device efficiency of the LEC based on each complex was optimized by testing three different thicknesses. The measured EL data of these LECs are summarized in **Table 5**. The EL spectra of LECs changed with time due to the altered microcavity effect induced by the moving emission zone. The EL spectra finally stabilized when the emission zone stopped moving, i.e., the doping processes reached a steady state. The time-dependent EL spectra of the LECs based on monomer com-

plexes **M3** and **M4** with different device thicknesses are shown in Figures S58 and S59 (Supporting Information), respectively. The stabilized EL spectra of the LECs with different device thicknesses in comparison with the PL spectrum for complex **M3** and **M4** are shown in **Figure 7a,b**, respectively. Both complexes showed deep-red thin-film PL spectra ($\lambda_{PL, max}$ = 654 and 662 nm for complex **M3** and **M4**, respectively) and they also exhibited predominantly deep-red EL emission. However, their EL spectra were influenced by the altered microcavity effect depending on the device thickness.[25,52] At certain device thicknesses, the EL emission peaks of complexes **M3** and **M4** red-shifted to 726 and 695 nm, respectively. In addition, a dual-emission spectrum was observed due to the optical destructive interference (Figure 7b). Compared with the EL spectrum of the thinner LEC based on complex **M4** (146 nm) (black line, Figure 7b), the EL spectrum of the thicker LEC (271 nm) (green line, Figure 7b) suffered a significant reduction in the emission intensity ≈800 nm, which

<table><caption>**Table 5.** Summary of the EL characteristics of the LECs based on mononuclear complex **M3–M4**, and binuclear **D1–D3** (80 wt.%) and [BMIM⁺(PF₆)⁻] at 20 wt.% doping level.</caption><thead><tr><th>Complex</th><th>Concentration [mg mL<sup>-1</sup>]<sup>a)</sup></th><th>Thickness [nm]</th><th>Bias [V]</th><th>EL<sub>max</sub> [nm]<sup>b)</sup></th><th>L<sub>max</sub> [µW cm<sup>-2</sup>]<sup>c)</sup></th><th>η<sub>ext, max</sub> [%]<sup>d)</sup></th><th>η<sub>p, max</sub> [mW W<sup>-1</sup>]<sup>e)</sup></th></tr></thead><tbody><tr><td rowspan="3">M3</td><td>10</td><td>82</td><td>5.5</td><td>676</td><td>12.50</td><td>0.02</td><td>0.07</td></tr><tr><td>20</td><td>148</td><td>6</td><td>726</td><td>2.08</td><td>0.12</td><td>0.32</td></tr><tr><td>40</td><td>286</td><td>7</td><td>649</td><td>3.59</td><td>0.09</td><td>0.24</td></tr><tr><td rowspan="3">M4</td><td>20</td><td>146</td><td>9</td><td>695</td><td>5.43</td><td>0.12</td><td>0.23</td></tr><tr><td>40</td><td>236</td><td>11</td><td>664</td><td>2.87</td><td>0.13</td><td>0.23</td></tr><tr><td>60</td><td>271</td><td>12</td><td>653</td><td>1.29</td><td>0.09</td><td>0.14</td></tr><tr><td rowspan="3">D1</td><td>20</td><td>137</td><td>3</td><td>679</td><td>50.40</td><td>0.65</td><td>3.77</td></tr><tr><td>40</td><td>202</td><td>3.2</td><td>739</td><td>34.08</td><td>0.89</td><td>4.51</td></tr><tr><td>60</td><td>235</td><td>5</td><td>737</td><td>37.87</td><td>0.83</td><td>2.74</td></tr><tr><td rowspan="3">D2</td><td>20</td><td>125</td><td>2.3</td><td>738</td><td>4.41</td><td>0.08</td><td>0.56</td></tr><tr><td>40</td><td>168</td><td>2.7</td><td>782</td><td>28.88</td><td>0.26</td><td>1.48</td></tr><tr><td>60</td><td>276</td><td>2.5</td><td>683</td><td>11.38</td><td>0.09</td><td>0.57</td></tr><tr><td rowspan="3">D3</td><td>10</td><td>77</td><td>4</td><td>743</td><td>3.24</td><td>0.02</td><td>0.09</td></tr><tr><td>20</td><td>153</td><td>5</td><td>773</td><td>6.45</td><td>0.10</td><td>0.30</td></tr><tr><td>40</td><td>271</td><td>6</td><td>711</td><td>1.41</td><td>0.04</td><td>0.10</td></tr></tbody></table>

<footnote>a) Solution concentration for spin coating; b) Stabilized EL emission peak wavelength; c) Maximal light output power; d) Maximal external quantum efficiency; e) Power efficiency.</footnote>
<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 7.** Stabilized EL spectra of the LECs based on complexes a) M3, b) M4, c) D1, d) D2, and e) D3, each with varying thicknesses. For comparison, the PL spectra of the emissive layers are also displayed.</figcaption></figure>

resulted in a narrowed emission peak at ≈650 nm and a small emission peak ≈800–1000 nm. Compared with the microcavity effect, the effect of employing DPEPhos or Xantphos ligand in complexes M3 and M4 played a minor role in adjusting the EL spectra. Although the microcavity effect is effective in spectral tuning, the spectral tuning range is still limited, i.e., in the emission band of the complex. Complexes with emission bands covering NIR spectral region are still necessary to take advantage of the microcavity effect and harvest NIR emission peaks.

Binuclear complexes D1 and D2 differed in two lateral substituents on the bridging ligand, i.e., methyl-pyridine for D1 and quinoline for D2. Compared with complex D1 with methyl substituents, complex D2 with fused phenyl substituents showed significantly red-shifted PL emission (λ<sub>PL,max</sub> from 650 to 764 nm, Figure 7c,d). The time-dependent EL spectra of the LECs based on binuclear complexes D1 and D2 with different device thicknesses are shown in Figures S60 and S61 (Supporting Information), respectively. The microcavity effect also affected the EL spectra of these devices. In thicker LECs based on complex D1 (202 and 235 nm), the stabilized EL emission peak red-shifted to ≈740 nm by microcavity effect (Figure 7c). With a proper device thickness (168 nm), complex D2 exhibited an even more red-shifted stabilized EL emission peak at 782 nm (Figure 7d). To the best of our knowledge, the NIR LEC based on complex D2 showed the longest EL peak wavelength among the reported copper-based LECs.[3,28] The binuclear complex with another type of bridging ligand, D3, showed comparable PL emission (λ<sub>PL,max</sub> = 745 nm) (Figure 7e) with that of binuclear complex D2. The time-dependent EL spectra of the LECs based on binuclear complex D3 with different device thicknesses are shown in Figure S62 (Supporting Information). The EL emission peak from the LEC based on complex D3 with thickness of 153 nm approached 773 nm. These data confirmed that the proposed ligand
<figure><img src="image_12.png" /><figcaption>Figure 8. Time-dependent current density, light output, and EQE of the optimized LECs based on complexes a) M3, b) M4, c) D1, d) D2, and e) D3. The optimized device thickness is shown in each subfigure.</figcaption></figure>

structures were effective in moving the EL emissions of copper iTMCs deeper into the NIR spectral region ($\lambda_{EL, max}$ >750 nm).

To optimize the device efficiency, three device thicknesses were tested for each complex. The bias voltage for each device of varying thickness was selected to achieve optimal device efficiency. The time-dependent current density, light output, and external quantum efficiency (EQE) of the LECs based on complexes M3–M4 and D1–D3 with optimized device thicknesses are depicted in Figure 8. The corresponding time-dependent EL characteristics of the unoptimized, i.e., thinner or thicker LECs are shown in Figures S63–S67 (Supporting Information). All LECs showed similar time-dependent EL characteristics. Following the application of bias, the mobile ions within the emissive layer of the LEC migrated toward the electrodes, resulting in the gradual formation of electrochemically doped p- and n-type layers. The doped layers aided in carrier injection, leading to a progressive increase in device current and light output over time. Following the attainment of peak values, both device current and light output exhibited a decline over time due to material degradation. Nevertheless, the light output exhibited a more rapid decline compared to the device current. This observation suggests that an additional mechanism, such as exciton quenching resulting from the extension of doped layers toward the emission zone, played a role in diminishing the light output prior to the decrease in device current.[53] Moreover, higher bias voltages (Figure 8a,b,e) lead to a much earlier decline in the light output than the decline in device current. A higher bias voltage resulted in a faster formation of broader doped layers in the LEC. As such, the light

output underwent a much earlier and more significant decline due to faster and severe exciton quenching. The improved carrier balance within the device, brought about by the expanding doped layers, led to a rapid increase in EQE following the application of bias. Note that the EQE started to decrease earlier than the light output. This further confirms that exciton quenching, stemming from the expanding doped layers, began to diminish the light output in the early stages of operation. Consequently, the EQE deteriorated even as the light output continued to increase.

The optimized LECs employing complexes M3 and M4 exhibited comparable peak EQEs of 0.12 and 0.13%, respectively (Table 5). These comparable EQEs were consistent with the similar PLQYs of their emissive layers (0.9% for both complexes) (cf. Table 3). It is noted that the thin films (20 wt.% BMIMPF$_6$) used for PLQY measurements and the emissive layers used in LECs have the same composition but only differ in thickness. The PLQY of the emissive layer is thickness-independent since no cavity structure is present in the sample for PLQY measurement. With increasing PLQYs (3.1 and 1.1% for complexes D1 and D2, respectively), the peak EQEs of the optimized LECs employing complexes D1 and D2 reached 0.89 and 0.26%, respectively. The optimized LEC based on complex D3 exhibited the lowest peak EQE of 0.1% due to the lowest PLQY of its emissive layer (0.6%). The equation (Equation 3) below considers the parameters that determine the EQE of LECs based on these complexes for evaluating device performance.

$$ \eta_{EQE} = \eta_{out} \times \gamma \times \eta_{S,T} \times \eta_{QY} \quad (4) $$
In this equation, $\eta_{EQE}$ is the measured device EQE, $\eta_{out}$ is the optical outcoupling efficiency, $\gamma$ is the factor of carrier balance in the LEC, $\eta_{S,T}$ is the emissive exciton generation efficiency of the complex and $\eta_{QY}$ is the thin-film PLQY of the complex. For phosphorescent materials, both singlet and triplet excitons can be harvested ($\eta_{S,T} = 100\%$). If the optical outcoupling efficiency is similar, the highest device efficiency can be obtained only when the best carrier balance is achieved. As such, the best carrier balance of each LEC can be compared in spite of different electrochemical doping levels under a different bias. When estimating the optical outcoupling efficiency ($\approx 20-30\%$) of a LEC device[54,55] and the PLQY of the emissive layer (Table 3), the LECs based on complexes D1 and D2 showed better factors of carrier balance, i.e., $\gamma = 0.96$ and $0.79$ for complexes D1 and D2, respectively, even when $\eta_{out}$ reached $30\%$. These results confirm a good carrier balance of the LECs based on complexes D1 and D2. As such, the NIR LEC employing complex D2 not only exhibited the longest EL peak wavelength among the reported NIR LECs based on copper complexes but also showed promising device efficiency due to good carrier balance.

To compare the device performance of the proposed NIR LECs with the reported NIR LECs to date, the EL characteristics of the NIR LECs exhibiting EL peak wavelength >750 nm are summarized in Table S15 (Supporting Information). The NIR LECs based on complexes D2 and D3 delivered light output power comparable with those obtained from the reported NIR LECs showing similar EL spectra. It is noted that the peak EQE, especially from the NIR LEC based on complex D2, is rather high among these reported NIR LECs. Therefore, the EL efficiency achieved for complex D2 is not only good among the copper complexes but also among all types of emissive materials used for NIR LECs. However, the EL stability of iTMCs is still worse than small molecules and conjugated polymers. Further improvement in the EL stability of iTMCs is essential to achieve efficient and stable NIR LECs.

### 3. Conclusion

A series of cationic binuclear copper(I) NIR emitters is presented and thoroughly characterized jointly by means of chemical, spectroscopical, electrochemical, and computational techniques and used as electroluminescent materials in LEC devices. The complexes feature the thiazolo[5,4-d]thiazole as the bridging scaffold and feature multiple intramolecular π-stacking interactions between the phenyl rings of the chelating diphosphines and the π-electron poor TzTz scaffold that disfavor both ground and excited state geometrical distortions notably, as shown by the single-crystal X-ray diffractometric analysis. All the binuclear complexes display photoluminescence in CH₂Cl₂ solution with an emission profile that spans from the deep-red to NIR-I region, namely $\lambda_{em} = 701-791$ nm, that is modulated selectively by π-accepting properties of the lateral N-based coordinating rings. NIR emission is preserved in doped thin-film condition for the binuclear derivatives that possess more extended π-systems, whereas hypochromic shift of the emission for the mononuclear counterparts lets the emission fall into the deep-red region. These findings prompted the use of these novel emitters as electroactive materials in single-layer NIR LEC devices, which displayed EL peak wavelength up to $\lambda_{EL} = 782$ nm, the longest one among the reported copper-based LECs, remarkably. In addition, by comparing

with other devices emitting at $\lambda_{EL} > 750$ nm, the presented design strategy provided emitters with excellent carrier balance and appealing EL performances among all types of emissive materials used for NIR LECs. These results corroborate the superior properties of binuclear complexes compared to the more classical mononuclear complex design for achieving EL into the longer wavelength region.

### 4. Experimental Section

CCDC 2352706 (D1), 2352707 (D2), 2352708 (D3), 2352709 (D4), 2352713 (M3) and 2352714 (M4) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.

### Acknowledgements

This work was supported by the Université de Strasbourg and the CNRS. The French Agence Nationale de Recherche (ANR) is kindly acknowledged for financial support through the grants ANR-21-CE29-0015 "ChirON" (M.M and C.G.) and ANR-22-CE07-0049-02 "BoostOLED" (M.M. and A.J.). This work was supported by the Higher Education Sprout Project of the National Yang Ming Chiao Tung University and Ministry of Education (MOE), Taiwan. Dr. Loïc Mager (IPCMS) is gratefully acknowledged for the help with the variable temperature setup. The authors kindly acknowledge the Optics characterisation platform of the IPCMS.

### Conflict of Interest

The authors declare no conflict of interest.

### Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

### Keywords

copper complexes, light emitting electrochemical cells, near-infrared emission, optoelectronics, phosphorescence

[1] G. Qian, Z. Y. Wang, *Chem. Asian. J.* **2010**, *5*, 1006.

[2] B. Pashaei, S. Karimi, H. Shahroosvand, M. Pilkington, *Adv. Funct. Mater.* **2020**, *30*, 1908103.

[3] Y.-D. Lin, C.-W. Lu, H.-C. Su, *Chem. – Eur. J.* **2023**, *29*, e202202985.

[4] A. Sandström, H. F. Dam, F. C. Krebs, L. Edman, *Nat. Commun.* **2012**, *3*, 1002.
[5] G. Mauthner, K. Landfester, A. Köck, H. Brückl, M. Kast, C. Stepper, E. J. W. List, *Org. Electron.* **2008**, *9*, 164.

[6] A. Sandström, A. Asadpoordarvish, J. Enevold, L. Edman, *Adv. Mater.* **2014**, *26*, 4975.

[7] D. Luo, Y.-R. Li, T.-H. Hsiao, Y.-C. Chen, R.-H. Yi, C.-W. Lu, C.-H. Chang, H.-C. Su, *Org. Electron.* **2021**, *96*, 106218.

[8] P. Murto, S. Tang, C. Larsen, X. Xu, A. Sandström, J. Pietarinen, B. Bagemihl, B. A. Abdulahi, W. Mammo, M. R. Andersson, E. Wang, L. Edman, *ACS Appl. Energy Mater.* **2018**, *1*, 1753.

[9] S. Tang, P. Murto, J. Wang, C. Larsen, M. R. Andersson, E. Wang, L. Edman, *Adv. Opt. Mater.* **2019**, *7*, 201900451.

[10] W. Xiong, S. Tang, P. Murto, W. Zhu, L. Edman, E. Wang, *Adv. Opt. Mater.* **2019**, *7*, 201900280.

[11] T. T. Filate, S. Tang, Z. Genene, L. Edman, W. Mammo, E. Wang, *Adv. Mater. Technol.* **2024**, *9*, 2301696.

[12] S. Xun, J. Zhang, X. Li, D. Ma, Z. Y. Wang, *Synth. Met.* **2008**, *158*, 484.

[13] H. J. Bolink, E. Coronado, R. D. Costa, P. Gaviña, E. Ortí, S. Tatay, *Inorg. Chem.* **2009**, *48*, 3907.

[14] A. Breivogel, M. Park, D. Lee, S. Klassen, A. Kühnle, C. Lee, K. Char, K. Heinze, *Eur. J. Inorg. Chem.* **2014**, *2*, 288.

[15] H. Shahroosvand, S. Abaspour, B. Pashaei, E. Radicchi, F. De Angelis, F. Bonaccorso, *Chem. Commun.* **2017**, *53*, 6211.

[16] B. Pashaei, H. Shahroosvand, M. Moharramnezhad, M. A. Kamyabi, H. Bakhshi, M. Pilkington, M. K. Nazeeruddin, *Inorg. Chem.* **2021**, *60*, 17040.

[17] S. Wang, X. Li, S. Xun, X. Wan, Z. Y. Wang, *Macromol. Sci.* **2006**, *39*, 7502.

[18] A. K. Pal, D. B. Cordes, A. M. Z. Slawin, C. Momblona, A. Pertegás, E. Ortí, H. J. Bolink, E. Zysman-Colman, *RSC Adv.* **2017**, *7*, 31833.

[19] G.-Y. Chen, B.-R. Chang, T.-A. Shih, C.-H. Lin, C.-L. Lo, Y.-Z. Chen, Y.-X. Liu, Y.-R. Li, J.-T. Guo, C.-W. Lu, Z.-P. Yang, H.-C. Su, *Chem. – Eur. J.* **2019**, *25*, 5489.

[20] A. R. Hosseini, C. Y. Koh, J. D. Slinker, S. Flores-Torres, H. D. Abruña, G. G. Malliaras, *Chem. Mat.* **2005**, *17*, 6114.

[21] D. A. W. Ross, P. A. Scattergood, A. Babaei, A. Pertegás, H. J. Bolink, P. I. P. Elliott, *Dalton Trans.* **2016**, *45*, 7748.

[22] D. Asil, J. A. Foster, A. Patra, X. de Hatten, J. del Barrio, O. A. Scherman, J. R. Nitschke, R. H. Friend, *Angew. Chem., Int. Ed.* **2014**, *53*, 8388.

[23] B. Pashaei, H. Shahroosvand, H. Douroudgari, S. Abaspour, M. Vahedpour, M. K. Nazeeruddin, *Inorg. Chem.* **2023**, *62*, 7622.

[24] Y.-X. Liu, R.-H. Yi, C.-H. Lin, Z.-P. Yang, C.-W. Lu, H.-C. Su, *J. Mater. Chem. C.* **2020**, *8*, 14378.

[25] J.-H. Hsu, H.-C. Su, *Phys. Chem. Chem. Phys.* **2016**, *18*, 5034.

[26] C.-L. Lee, C.-Y. Cheng, H.-C. Su, *Org. Electron.* **2014**, *15*, 711.

[27] Y.-D. Lin, P.-W. Hsiao, W.-Y. Chen, S.-Y. Wu, W.-M. Zhang, C.-W. Lu, H.-C. Su, *Chem. Eng. J.* **2023**, *469*, 144055.

[28] A. Jouaiti, L. Ballerini, H.-L. Shen, R. Viel, F. Polo, N. Kyritsakas, S. Haacke, Y. Huang, C.-W. Lu, C. Gourlaouen, H.-C. Su, M. Mauro, *Angew. Chem., Int. Ed.* **2023**, *62*, 202305569.

[29] L. Ballerini, W.-M. Zhang, T. Groizard, C. Gourlaouen, F. Polo, A. Jouaiti, H.-C. Su, M. Mauro, *J. Mater. Chem. C Mater.* **2024**, *12*, 12769.

[30] C.-C. Ho, H.-F. Chen, Y.-C. Ho, C.-T. Liao, H.-C. Su, K.-T. Wong, *Phys. Chem. Chem. Phys.* **2011**, *13*, 17729.

[31] M. Rémond, J. Hwang, J. Kim, S. Kim, D. Kim, C. Bucher, Y. Bretonnière, C. Andraud, E. Kim, *Adv. Funct. Mater.* **2020**, *30*, 202004831.

[32] M. Mone, S. Tang, P. Murto, B. A. Abdulahi, C. Larsen, J. Wang, W. Mammo, L. Edman, E. Wang, *Chem. Mater.* **2019**, *31*, 9721.

[33] E. Fresta, A. Charisiadis, L. M. Cavinato, N. Palandjian, K. Karikis, V. Nikolaou, G. Charalambidis, A. G. Coutsolelos, R. D. Costa, *Adv. Photonics Res.* **2021**, *2*, 202000188.

[34] Y. Chen, Y.-X. Wang, C.-W. Lu, H.-C. Su, *J. Mater. Chem. C Mater.* **2022**, *10*, 11211.

[35] M. Mone, S. Tang, Z. Genene, P. Murto, M. Jevric, X. Zou, J. Råfols-Ribé, B. A. Abdulahi, J. Wang, W. Mammo, M. R. Andersson, L. Edman, E. Wang, *Adv. Opt. Mater.* **2021**, *9*, 202001701.

[36] Y.-H. Su, Y.-C. Ji, Y.-T. Huang, D. Luo, S.-W. Liu, Z.-P. Yang, C.-W. Lu, C.-H. Chang, H.-C. Su, *J. Mater. Chem. C Mater.* **2022**, *10*, 18137.

[37] R. D. Costa, E. Ortí, H. J. Bolink, F. Monti, G. Accorsi, N. Armaroli, *Angew. Chem., Int. Ed.* **2012**, *51*, 8178.

[38] D. Volz, M. Wallesch, C. Fléchon, M. Danz, A. Verma, J. M. Navarro, D. M. Zink, S. Bräse, T. Baumann, *Green Chem.* **2015**, *17*, 1988.

[39] C. Förster, K. Heinze, *Chem. Soc. Rev.* **2020**, *49*, 1057.

[40] O. S. Wenger, *J. Am. Chem. Soc.* **2018**, *140*, 13522.

[41] C. E. Housecroft, E. C. Constable, *J. Mater. Chem. C Mater.* **2022**, *10*, 4456.

[42] G. U. Mahoro, J. Fernandez-Cestau, J. Renaud, P. B. Coto, R. D. Costa, S. Gaillard, *Adv. Opt. Mater.* **2020**, *8*, 2000260.

[43] V. Ferraro, C. Bizzarri, S. Bräse, *Adv. Sci.* **2024**, *11*, 2404866.

[44] A. Farokhi, S. Lipinski, L. M. Cavinato, H. Shahroosvand, B. Pashaei, S. Karimi, S. Bellani, F. Bonaccorso, R. D. Costa, *Chem. Soc. Rev.* **2025**, *54*, 266.

[45] L. M. Cavinato, S. Wolf, A. Pothig, E. Fresta, C. Garino, J. Fernandez-Cestau, C. Barolo, R. D. Costa, *Adv. Mater.* **2022**, *34*, 2109228.

[46] M. D. Weber, E. Fresta, M. Elie, M. E. Miehlich, J.-L. Renaud, K. Meyer, S. Gaillard, R. D. Costa, *Adv. Funct. Mater.* **2018**, *28*, 1707423.

[47] E. Fresta, M. D. Weber, J. Fernandez-Cestau, R. D. Costa, *Adv. Opt. Mater.* **2019**, *7*, 1900830.

[48] J. V. Caspar, T. J. Meyer, *J. Phys. Chem.* **1983**, *87*, 952.

[49] M. Mauro, *Chem. Com.* **2021**, *57*, 5857.

[50] G. Li, D. Zhu, X. Wang, Z. Su, M. R. Bryce, *Chem. Soc. Rev.* **2020**, *49*, 765.

[51] B. Hupp, C. Schiller, C. Lenczyk, M. Stanoppi, K. Edkins, A. Lorbach, A. Steffen, *Inorg. Chem.* **2017**, *56*, 8996.

[52] Z.-P. Yang, H.-C. Su, *Adv. Funct. Mater.* **2020**, *30*, 1906788.

[53] S. van Reenen, R. A. J. Janssen, M. Kemerink, *Adv. Funct. Mater.* **2015**, *25*, 3066.

[54] Y.-Z. Chen, D. Luo, C.-H. Hsiang, R.-H. Yi, C.-H. Lin, C.-W. Lu, S.-W. Liu, C.-H. Chang, H.-C. Su, *Org. Electron.* **2020**, *77*, 105515.

[55] R.-H. Yi, C.-L. Lo, D. Luo, C.-H. Lin, S.-W. Weng, C.-W. Lu, S.-W. Liu, C.-H. Chang, H.-C. Su, *ACS Appl. Mater. Interfaces.* **2020**, *12*, 14254.