Cubane Copper(I) Iodide Clusters with Remotely Functionalized Phosphine Ligands: Synthesis, Structural Characterization and Optical Properties

Abstract

We present here the synthesis, chemical, and photophysical study of a series of three new copper halide derivatives, namely 2a–c. They are all tetranuclear copper-iodide clusters of general formula [Cu(μ₃-I)P]₄ consisting of a cubane-like {Cu₄I₄} motif and P = phosphine. They differ in the type of the phosphines used as ligands: a monophosphine with a single pendant ester unit (complex 2a), two pendant ester units (2b), and a diphosphine containing two esters in the linker (2c). The molecular structure of the complexes was determined by single-crystal X-ray diffraction analysis. All the investigated derivatives were found to be photo- and thermally-stable luminescent species. In the solid state, the complexes display intense and long-lived photoluminescence in the orange region with PLQY values of 0.43–0.84 at room temperature associated mainly with a ³CC excited state with mixed ³XMCT character.

1. Introduction

Earth-abundant photo-active transition metal complexes are currently the subject of intensive research in inorganic photochemistry because of their potential applications in photocatalysis, solar energy conversion, and light emitting devices [1,2,3,4,5]. Among the metals studied, copper(I)-based emitters are certainly the ones that attract the most attention because of their many advantages. They are considered an attractive alternative to emitters containing platinum group metals for the development of (electro)luminescent materials [6,7,8,9,10] since copper is less toxic and cheaper than the latter.

However, the photophysical properties of Cu(I)-based emitters remain inferior to those of platinum group based complexes mainly due to the smaller spin-orbit coupling exerted by the lighter metal elements, which scales roughly with the fourth power of the atomic number. Hence, increasing efforts are being made to improve the emission properties of copper-based complexes, primarily through the design and synthesis of copper-specific ligands that can control and enhance their photophysical properties. Thus, depending on the geometry, nuclearity, and electronic properties of the ligand surrounding the metal, copper complexes can exhibit emissions derived from different excited states, namely metal-ligand charge transfer (MLCT), halide-to-ligand charge transfer (XLCT) and cluster-centered (CC) [11]. Among all the recent developments, some Cu(I) complexes possess close-lying singlet and triplet manifolds with MLCT character (^1,3MLCT) that enables thermally activated delayed fluorescence (TADF) mechanism with interesting application in organic light-emitting diodes (OLEDs) [12,13,14,15,16,17]. This prompts the study of the coordination chemistry of Cu(I) with the design of new ligands that can provide enhanced photophysical properties.

Multinuclear copper compounds bearing nitrogen or phosphorus ligands are also widely studied [11]. They present a polymeric or discrete structure typically formed by the assembling of {Cu₂X₂} or {Cu₄X₄} subunits. Copper-iodide tetranuclear clusters of the general formula [Cu(μ₃-I)P]₄, consisting of a {Cu₄X₄} core and apically located ligands, have attracted major attention due to their novel photophysics, including double emission from two electronically decoupled excited states, namely a high-energy (HE) ³XLCT state and a low-energy ³CC state (LE) [18].

In the search for new copper complexes of interest, we have recently shown that phosphine ligands, which differ only in minor chemical modifications can induce profound effects on solid-state emission properties, highlighting the importance of the microenvironment for the emitters in the aggregated phase [19,20]. In continuity, we studied the coordination chemistry of three phosphine ligands. Herein, the synthesis of a phenylphosphine- and a diphenylphosphine ligand functionalized by ester moieties are reported along with their coordination on Cu(I) yielding photoactive cubane clusters. The ligands were easily obtained in one step through hydrophosphination reaction, and the corresponding copper-iodide tetranuclear clusters featuring {Cu₄X₄} core were obtained by direct reaction with CuI and characterized by single crystal X-ray diffraction. The photophysical properties in the solid state were also studied and hereafter presented.

2. Materials and Methods

2.1. Synthesis of Ethyl 3-(Diphenylphosphanyl)propanoate 1a

A mixture of 2.0 g (10.7 mmol) of diphenyl phosphine Ph₂PH, 1.15 g (11.5 mmol) of ethyl acrylate H₂C=CHCO₂Et and 2-MeTHF (4.3 mL) were placed under argon in a closed vessel. The mixture was stirred for 4 h at 90 °C. Excess of ethyl acrylate and Me-THF was then removed under reduced pressure and the remaining residue was purified by flash chromatography on silica gel (gradient from cyclohexane to cyclohexane/ethyl acetate 8:2) giving 2.9 g of colorless viscous liquid (95% yield).

¹H NMR (400 MHz, CDCl₃): δ 1.26 (t, J = 7.1 Hz, 3H, CH₃), 2.28 (m, 4H, CH₂CH₂P), 4.01 (q, J = 7.1 Hz, 2H, CH₂O), 7.33–7.43 (m, 6H, CH_arom), 7.43–7.50 (m, 4H, CH_arom) ppm; ¹³C NMR (CDCl₃, 101 MHz): δ 14.25, 23.04 (d, J = 12 Hz), 31.87 (d, J = 19 Hz), 60.64, 128.54 (d, J = 6.5Hz), 128.84, 132.86 (d, J = 19 Hz), 137.84 (d, J = 12.5 Hz), 173.23 (d, J = 15 Hz) ppm; ³¹P NMR (162 MHz, CDCl₃) δ: −15.64 ppm. MS (positive ESI) 287.12 m/z (%): [M+H]⁺; FTIR: ν = 3072, 2980, 2872, 1731 (CO), 1480, 1431, 1371, 1348, 1216, 1165, 1026, 732, 691 cm⁻¹.

2.2. Synthesis of Diethyl 3,3′-(Phenylphosphanediyl)dipropionate 1b

A mixture of 2.00 g (18.2 mmol) of phenyl phosphine PhPH₂, 4.00 g (39.9 mmol) of ethyl acrylate H₂C=CHCO₂Et and 2-MeTHF (7.3 mL) was placed under argon in a closed vessel. The mixture was stirred for 6 h at 90 °C. Excess of ethyl acrylate and Me-THF is then removed under reduced pressure and the remaining residue was purified by flash chromatography on silica gel (gradient from cyclohexane to cyclohexane/ethyl acetate 8:2) giving 3.45 g of colorless viscous liquid (61% yield) [21].

¹H NMR (500 MHz, CDCl₃): δ 1.13 (t, J = 7.0 Hz, 6H), 1.94 (m, 4H), 2.25 (m, 4H), 4.01 (q, J = 7.4 Hz, 4H), 7.27 (m, 3H), 7.44 (m, 2H) ppm; ¹³C NMR (CDCl₃, 125 MHz): δ 14.19, 22.8 (d, J = 11.0 Hz), 30.69 (d, J = 17.0 Hz), 60.59, 128.63 (d, J = 6.5 Hz), 129.48, 132.63 (d, J = 19 Hz), 136.06 (d, J = 12.5 Hz), 173.17 (d, J = 13.6 Hz) ppm; ³¹P NMR (202 MHz, CDCl₃) δ: −22.71 ppm; MS (positive ESI) 317.14 m/z (%): [M+Li]⁺; FTIR: ν: 3067, 2978, 2905, 1730 (C=O), 1431, 1344, 1371, 1215 (C-O), 1159 (C-O), 1040, 737, 696 cm⁻¹.

2.3. Synthesis of 2,2-Dimethylpropane-1,3-diyl Bis(3-(diphenylphosphanyl)propanoate) 1c

A mixture of 2.00 g (10.7 mmol) of diphenyl phosphine Ph₂PH, 1.1 g (5.2 mmol) of diacrylate 2,2-dimethyl-1,3-propanediol and 2-MeTHF (4.3 mL) were placed under argon in a closed vessel. The mixture was stirred for 8 h at 90 °C. Excess of ethyl acrylate and Me-THF is then removed under reduced pressure and the remaining residue was purified by flash chromatography on silica gel (cyclohexane/ethyl acetate 8:2) giving 2.6 g of white solid (84% yield).

¹H NMR (400 MHz, CDCl₃): δ 0.85 (s, 6H), 2.23–2.37 (m, 8H), 3.77 (s, 4H), 7.20–7.30 (m, 12H), 7.30–7.38 (m, 8H) ppm; ¹³C NMR (CDCl₃, 101 MHz): δ 21.90. 23.04 (d, J = 12.1 Hz), 30.76 (d, J = 19.4 Hz), 34.78, 69.45, 128.69 (d, J = 6.7 Hz), 128.96, 132.84 (d, J = 18.7 Hz), 137.81 (d, J = 12.5 Hz), 173.15 (d, J = 15.0 Hz) ppm; ³¹P NMR (162 MHz, CDCl₃) δ: −15.8 ppm HRMS (positive ESI): m/z calcd. for C₃₅H₃₈O₄P₂ 584.2200, found 584.2207; FTIR: ν = 3052, 2967, 2860, 1731 (CO), 1476, 1433, 1373, 1214, 1148, 1047, 735, 693 cm⁻¹.

2.4. Synthesis of Cluster [Cu(μ₃-I)(1a)]₄ 2a

Copper iodide CuI (100 mg, 0.52 mmol) and ethyl 3-(diphenylphosphanyl)-propanoate 1a (141 mg, 0.55 mmol, 1.05 eq.) were placed in a flame-dried Schlenk tube under argon. A total of 5 mL of dry toluene was added and the solution was heated at 110 °C for 24 h. Then the mixture was cooled down to room temperature and the solvent was removed under vacuum. The solid residue was dissolved in a minimum of CH₂Cl₂, filtered through a pad of silica, and the solution was poured into Et₂O. The complex precipitates directly and was filtered and washed several times with Et₂O and n-hexane. The product was dried under vacuum giving a colorless solid (173 mg, 70%).

¹H NMR (500 MHz, CD₃CN) δ: 7.68–7.56 (m, 16H), 7.53–7.17 (m, 24H), 4.01 (q, ³J_H-H = 7.1 Hz, 8H), 2.61–2.39 (m, 16H), 1.15 (t, ³J_H-H = 7.1 Hz, 12H). ¹³C NMR (126 MHz, CD₃CN) δ 173.2 (d, 4C, ³J_C-P = 18.3 Hz), 134.2 (d, 8C, ¹J_C-P = 27.8 Hz), 133.9 (d, 16C, ²J_C-P = 13.8 Hz), 131.0 (s, 8C), 129.6 (d, 16C, ³J_C-P = 9.0 Hz), 61.4 (4C), 30.5 (d, 4C, ²J_C-P = 9.4 Hz), 23.0 (d, 4C, ¹J_C-P = 19.5 Hz), 14.5 (4C). ³¹P NMR (202 MHz, CD₃CN) δ: −29.74 (br. s). HRMS (ESI+, m/z) [M+Na]⁺calculated 1928.77338 found 1928.7934. TGA: 5% weight loss at T_5% = 253 °C. CCDC Deposition Number 2256486.

2.5. Synthesis of Cluster [Cu(μ₃-I)(1b)]₄ 2b

Copper iodide CuI (100 mg, 0.52 mmol) and diethyl 3,3′-(phenylphosphanediyl)-dipropionate 1 (171 mg, 0.55 mmol, 1.05 eq.) were placed in a flame-dried Schlenk tube under argon. A total of 5 mL of dry toluene was added and the solution was heated at 110 °C for 24 h. Then the mixture was cooled down to the room temperature and the solvent was removed under vacuum. The solid residue was dissolved in a minimum of CH₂Cl₂, filtered through a bad of silica and the solution was poured into Et₂O. The complex precipitates directly and was filtered and washed several times with Et₂O and n-hexane. The product was dried under vacuum giving a colorless solid (177 mg, 68%).

¹H NMR (500 MHz, CD₃CN) δ: 7.90–7.73 (m, 8H), 7.63–7.23 (m, 12H), 4.03 (q, ³J_H-H = 7.1 Hz, 16H), 2.72–2.44 (m, 8H), 2.48–2.13 (m, 24H), 1.18 (t, ³J_H-H = 7.1 Hz, 24H). ¹³C NMR (126 MHz, CD₃CN) δ: 173.3 (d, 8C, ³J_C-P = 15.9 Hz), 133.8 (d, 8C, ²J_C-P = 13.7 Hz), 132.4 (d, 4C, ¹J_C-P = 25.3 Hz), 131.4 (s, 4C), 129.7 (d, 8C, ³J_C-P = 9.0 Hz), 61.3 (s, 8C), 30.3 (d, 8C, ²J_C-P = 6.8 Hz), 22.5 (d, 8C, ¹J_C-P = 18.4 Hz), 14.48 (s, 8C). ³¹P NMR (202 MHz, CD₃CN) δ: −35.06 (br.s). HRMS (ESI+, m/z) [(M-Ph)+Na]⁺calculated 1947.81882 found 1947.8169. TGA: 5% weight loss at T_5% = 252 °C. CCDC Deposition Number 2256487.

2.6. Synthesis of Cluster [Cu(μ₃-I)]₄(1c)₂ 2c

Copper iodide CuI (100 mg, 0.52 mmol) and 2,2-dimethylpropane−1,3-diyl bis(3-(diphenylphosphanyl)propanoate) 1c (161 mg, 0.275 mmol, 0.5 eq.) were placed in a flame-dried Schlenk tube under argon. A total of 5 mL of dry toluene was added and the solution was heated at 110 °C for 24 h. Then the mixture was cooled down to the room temperature and the solvent was removed under vacuum. The solid residue was dissolved in a minimum of CH₂Cl₂, filtered through a bad of silica and the solution was poured into Et₂O. The complex precipitates directly and was filtered and washed several times with Et₂O and n-hexane. The product was dried under vacuum giving a colorless solid (163 mg, 65%).

¹H NMR (500 MHz, CDCl₃) δ: 7.73–7.47 (m, 16H), 7.47–7.35 (m, 8H), 7.36–7.04 (m, 16H), 3.90 (s, 8H), 2.79–2.40 (m, 16H), 0.91 (s, 12H).¹³C NMR (126 MHz, CDCl₃) δ: 173.1 (d, 4C, ³J_C-P = 20.7 Hz), 133.4 (d, 16C, ²J_C-P = 12.8 Hz), 133.1 (d, 8C, ¹J_C-P = 27.5 Hz), 129.7 (s, 8C), 128.6 (d, 16C, ³J_C-P = 9.0 Hz), 72.3 (s, 4C), 35.1 (s, 2C), 29.6 (d, 4C, ²J_C-P = 9.4 Hz), 22.8 (d, 4C, ¹J_C-P = 18.8 Hz), 22.2 (s, 4C). ³¹P NMR (202 MHz, CDCl₃) δ: −31.68 (br.s). HRMS (ESI+, m/z) [M+H]⁺ calculated 1930.79144 found 1930.7937. TGA: 5% weight loss at T_5% = 307 °C. CCDC Deposition Number 2256488.

2.7. X-ray Analyses

For complexes 2a and 2b, X-ray diffraction data collection were collected on a Nonius Kappa-CCD diffractometer, using Mo-Kα radiation (λ = 0.71073 Å). The apparatus was equipped with an Oxford Cryosystem liquid N₂ device. The crystal-detector distance was 36 mm and the cell parameters were determined from reflections taken from one set of 10 frames (1.0° steps in phi angle), each at 20 s exposure (Denzo software) [22]. The structures have been solved by direct methods using the program SHELXS-2013 [23] and the refinement and all further calculations were carried out using SHELXL-2013 [24]. The H-atoms were included in the calculated positions. They were treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F². A semi-empirical absorption correction has been applied by means of the MULscanABS routine in the PLATON [25].

For complexes 2c, X-ray diffraction data collection was carried out on a Bruker APEX II DUO Kappa-CCD diffractometer equipped with an Oxford Cryosystem liquid N₂ device, using Mo-Kα radiation (λ = 0.71073 Å). The crystal-detector distance was 38 mm. The cell parameters were determined from reflections taken from three sets of 12 frames, each at 10 s exposure (APEX2 software). The structure has been solved by direct methods using the program SHELXS-97 [26] and the refinement and all further calculations have been carried out using SHELXL-97. The H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F2. SADABS in APEX2 was used to apply a semi-empirical absorption correction.

3. Results

3.1. Synthesis of the Phosphine Ligands 1a–c

Phosphines 1a–c were easily synthesized in a one-step procedure starting from diphenylphosphine (Ph₂PH) or phenylphosphine (PhPH₂) and the corresponding alkene derivative (i.e., ethyl acrylate or ethylene glycol diacrylate), according to our previously reported procedure [27]. They were prepared by hydrophosphination of the corresponding alkene compounds over the phosphine in the presence of 2-MeTHF (4 eq.) under an argon atmosphere in a closed vessel. The product was then purified by flash chromatography on silica gel using cyclohexane/ethyl acetate as eluent, with reaction yield ranging from 61% to 95% (Figure 1).

3.2. Synthesis and Characterization of the Cubane-like Copper Cluster 2a–c

3.2.1. Synthesis

By reacting copper iodide with a stochiometric amount of phosphine (1a or 1b) or half an equivalent of diphosphine (1c), the tetrameric copper cluster was formed (Equation (1)). In all cases, the reaction was performed in dry toluene at 110 °C under controlled atmosphere (N₂) for 24 h. All products were isolated as air stable off-white solids. All compounds were characterized using classical analytical methods (Figures S1–S9 in Supplementary Materials). ³¹P{¹H} NMR spectra showed a broad signal slightly shifted to downfield with respect to the free ligand confirming the successful formation of the copper cluster.

(1)

3.2.2. Crystal Structure of the Cubane Clusters

• Complex 2a

Crystals of 2a suitable for X-ray crystallography were obtained from CH₂Cl₂/n-pentane. The molecular structure of the cubane is depicted in Figure 2. Complex 2a was crystallized in the triclinic P-1 space group and has the general formula [Cu₄I₄L₄] with L being 1a: the compound presents the classical cubane-like structure formed by 4 copper atoms and 4 iodine atoms and the phosphine ligands are coordinated to each copper atom. Therefore, all copper atoms present a pseudo-tetrahedral PCuI₃ geometric environment. In the structure, the Cu-I bond distances and I-Cu-I angle values are within the range of reported values for such types of compounds containing phosphorus-based ligands [28,29,30,31,32,33,34]. The copper cluster has a mean Cu^…Cu distance of 2.95 Å, which is much greater than the sum of the van der Waals radii of 2.80 Å, implying weak or no cuprophilic interaction in such system [35]. We note the presence of a non-covalent inter-ligand interaction of type C=O^…H-C between C(16) and O(7) of 2.89 Å and on the other side between C(50) and O(3) of 3.36 Å. Therefore, the ester chains interact with each other, two by two.

• Complex 2b

Crystals of 2b suitable for X-ray crystallography were obtained from CH₂Cl₂/diethyl ether. The molecular structure of the cluster is depicted in Figure 3, as well as selected bond lengths and angles. Complex 2a was crystallized in the tetragonal P-4c2 space group and there is ¼ molecule in the asymmetric unit. The four phosphine ligands form a highly symmetric intermolecular network forming a square in which the copper cluster is embedded. The alternating arrangement of the phosphine ligands confers an overall D_2d symmetry to the tetramer. Figure 4a,b provides a better visualization of the alternative entanglement of the 4 ligands with two 90° offset views. We also note weak inter-ligand interactions that should enhance the molecular rigidity of the system: an O(3)-C(11) atom distance of 2.904 Å is indicative of a C=O^…H₃C- interaction.

Regarding the metal cluster, the Cu−I bond distances (2.70 Å) and the I−Cu−I angle mean values are comparable and within the range of reported values for this type of compound. The Cu-Cu distances are at 2.97 Å, which is also identical to the previous copper cubane 2a and the P-Cu bond distance is 2.244 Å.

• Complex 2c

Single crystals of complex 2c were obtained from CH₂Cl₂/n-pentane. The molecular structure of the cubane is depicted in Figure 5. Complex 2c was crystallized in the triclinic P-1 space group and shows characteristics close to the previous compounds. The diphosphine is coordinated on two adjacent copper atoms, thus forming a 16-membered cycle. The metal cluster contains an average distance between Cu-I atoms of 2.69 Å and the I−Cu−I angle mean values are comparable and within the range of reported values for this type of compound. The Cu-Cu distances are at ca. 2.95 Å, and the P-Cu mean bond distance is 2.254 Å.

3.3. Photophysical Characterization

No emission was detected for solution samples at room temperature, as expected, owing to the flexible nature of the overall cubane Cu-I scaffold and dynamic ligand dissociation in such conditions [8,36].

In the solid state, all three complexes show one intense and structureless yellow-orange photoluminescence with emission maximum centered at λ_em = 569, 575, and 578 nm for 2a, 2b, and 2c, respectively. Complex 2a is the most emissive amongst the series with photoluminescence quantum yield (PLQY) as high as 84% in neat powder (see

Table 1. Photophysical data of compounds 2a–c in the solid state at room temperature.

Compound	${\mathit{\lambda }}_{\mathit{e}\mathit{m}}$	$\mathit{P}\mathit{L}\mathit{Q}\mathit{Y}$	$\mathit{C}\mathit{I}\mathit{E} \mathit{C}\mathit{h}\mathit{r}\mathit{o}\mathit{m}\mathit{a}\mathit{t}\mathit{i}\mathit{c}\mathit{i}\mathit{t}\mathit{y}$	${\mathit{\tau }}_{\mathit{o}\mathit{b}\mathit{s}}$	${\mathit{k}}_{\mathit{r}}$[a]	${\mathit{k}}_{\mathit{n}\mathit{r}}$[a]
	[nm]		(x, y)	[μs]	[104s−1]
2a	569	0.84	0.40, 0.51	5.17	16.24	3.09
2b	575	0.43	0.39, 0.44	5.11	8.42	11.16
2c	578	0.50	0.42, 0.47	5.89	8.49	8.49

[a] k_r and k_nr were estimated by using the following equations: k_r = PLQY/τ and k_nr = (1 − PLQY)/τ.

and Figure 6). Compounds 2b and 2c have slightly lower PLQY values of 43% and 50%, respectively. Despite the absence of cuprophilic interaction, no higher energy (HE) band is observed in the region at ca. 400–480 nm surprisingly, as often observed for related cubane [Cu₄I₄L₄] derivatives, where L is a either N- or P-based coordinated ligand, such as for instance substituted pyridine or phosphine [37,38]. Interestingly, time-resolved emission traces can be nicely fitted with mono-exponential decays providing an excited state lifetime in the range τ = 5.11–5.89 μs. Closer analysis of the excited-state kinetic parameters yields an estimated radiative kinetic constant, k_r, of 8.4–16.2 × 10⁴ s⁻¹. The smaller value of the non-radiative rate constant, k_nr, was observed for derivative 2a and was 3.1 × 10⁴ s⁻¹, giving rise to the compound with the highest PLQY value amongst the series. The linear nature of the phosphine ligand in 2a seems to provide a less flexible and better packing motif in the solid state reducing the radiationless deexcitation pathways compared, for instance, to the compound bearing the bidentate diphosphine ligand, complex 2c. Overall, these findings allow us to ascribe the radiative process as originating from a single excited state with main triplet cluster-centered (³CC) with mixed copper-centered and triplet halide-to-metal charge transfer (³XMCT) character in accordance with previously reported derivatives [8,36,37,38].

4. Discussion

We describe here the synthesis of three copper cubanes containing the subunit {Cu₄I₄} and stabilized by phosphine ligands. The phosphines used are either monodentate or bidentate ligands and are characterized by the presence of ester functions [39]. The three complexes were fully characterized, including the determination of their molecular structure by single-crystal X-ray diffraction. Of the three molecular structures, that of complex 2b is particularly remarkable, with the ligands arranged in high symmetry to give an overall D_2d symmetry to the tetramer. Interestingly, compounds 2a and 2c crystallize in the low-symmetry space group P-1 (with apparently no relation to the molecular symmetry), while compound 2b crystallizes in the highly symmetric tetragonal space group P-4c2. In all three cases, the Cu-Cu intramolecular distance is greater than 2.95 Å, which is greater than the sum of the van der Waals radii of 2.80 Å. Thermogravimetric analyses (TGA) revealed that they are highly robust (Figures S10–S12 in Supplementary Materials). The monodentate phosphine-stabilized complexes 2a and 2b show a weight loss of 5% at 253 and 252 °C, respectively, while complex 2c requires a temperature of 307 °C for the same weight loss, which could be correlated with the chelate effect of the diphosphine.

All three cubanes are highly luminescent in the solid state but not in solution, which is typical for this class of Cu(I) compounds, with emission that is found to be relatively independent of the nature of the phosphine capping ligand (λ_em = ca. 570–580 nm) and with high PLQY values ranging from 43% (2b) to 84% for complex 2a (see Figure S1 in Supplementary Materials for CIE coordinates of compound 2a–2c). Our studies also allow us to attribute the radiative process as originating from a single excited state with a mixed ³CC/³XLCT character. The presence of ester functions on the ligands could allow good formulation compatibility in organic polymers such as polyacrylates or polyurethanes, which can be a relevant strategy to design materials with enhanced functionalities [40,41,42].

5. Patents

A patent application has been filed on these results: Bissessar, D.; Bellemin-Laponnaz, S.; Steffanut, P. Tetra-nuclear copper (I) complexes with diarylphosphine ligands. 2021, US2021253611.