# Rational Design of Single-Component White-Light Emissive Molecular Copper Clusters

Alexia Rocheteau, Lanig Lemeur, Marie Cordier, Michaël Paris, Jean-yves

Mevellec, Camille Latouche, Hélène Serier-Brault, Sandrine Perruchas

► To cite this version:

Alexia Rocheteau, Lanig Lemeur, Marie Cordier, Michaël Paris, Jean-yves Mevellec, et al.. Rational Design of Single-Component White-Light Emissive Molecular Copper Clusters. Advanced Optical Materials, 2025, 13 (2), 10.1002/adom.202402040. [https://doi.org/10.1002/adom.202402040](https://doi.org/10.1002/adom.202402040)

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# Rational Design of Single-Component White-Light Emissive Molecular Copper Clusters

Alexia Rocheteau, Lanig Lemeur, Marie Cordier, Michaël Paris, Jean-Yves Mevellec, Camille Latouche, Hélène Serier-Brault, and Sandrine Perruchas*

The development of low-cost and earth-abundant phosphors for substituting expensive noble metals and toxic lead materials is currently of major importance for sustainable solid-state lighting technologies. Hybrid copper halide materials are currently receiving considerable attention due to their unique and diverse structural and photophysical properties combined to their low cost and toxicity. This article reports the rational design of copper iodide compounds for developing single-component white light emitters. By a ligand engineering strategy, the energy layout of the molecular clusters can be adjusted and the variation in the relative intensity of two emission bands can be obtained, enabling white light to be emitted. Temperature dependence of this dual-emission feature also allows high contrast luminescence thermochromism. Thanks to their good solubility properties, the studied copper iodide clusters have been successfully incorporated within organic polymer leading to polymer materials with processability suitable for application development. Therefore, the potential applicability of these composite materials has been demonstrated in various fields as luminescence ratiometric thermometry, single-component emitters for phosphor-converted white-light emitting diodes, and as ink for anticounterfeiting properties. This study highlights the power of the ligand design strategy to develop high potential multifunctional materials based on light-emitting copper materials.

## 1. Introduction

In today's world of digital technologies, it is essential to reduce one of the most important electricity consumption coming from light-emitting devices. Although luminescent materials with enhanced performance are required for high-quality lighting and displays technologies, they are first and foremost highly demanded for energy savings. Besides, there is an urgent need to consider sustainable and ecological aspects in solid-state lighting technologies. For this reason, the development of low-cost, earth-abundant phosphors for substituting expensive noble-metal-based materials is currently of major importance.[1–6] In regards to phosphor-converted white-light emitting diodes (pc-WLEDs), lead halide perovskites have been recognized as most promising phosphors due to their excellent physical properties. However, the toxicity of lead and structural instability are significant obstacles to commercial developments.[7,8] In this context, copper-based materials and in particular hybrid copper(I) halides,[9–11] are currently receiving considerable attention due to their unique and diverse photophysical properties combined with their modest cost and low toxicity.[12,13] Their potential applications in scintillators,[14–18] light-emitting diodes,[19–24] and advanced stimuli-responsive materials,[25–28] have been indeed reported. Because of their chemical and thermal stabilities, copper iodide materials in particular, constitute solid alternatives for pc-WLEDs applications. Although several white emitting copper iodide compounds have recently been reported by combination with organic cations,[29–34] the study of single-component white phosphors based on neutral compounds with broad-band emission appears of great interest to avoid the difficulties of controlling the mixing of different luminophores. The only few reported examples of such single-molecular emitters, prompts the exploration of new white phosphors based on copper iodide materials.[35–37]

Among the remarkable structural diversity of copper iodide compounds that originates from the versatile coordination chemistry of Cu(I) able to accommodate any types of ligands, the polymetallic complexes exhibit particularly rich photophysical properties.[38,39] The intriguing cubane cluster of [Cu₄I₄L₄] (L = organic ligand) formula, is emblematic of this. This compact molecular structure is built on four copper and four

A. Rocheteau, L. Lemeur, M. Paris, J.-Y. Mevellec, C. Latouche,

H. Serier-Brault, S. Perruchas

Nantes Université

Institut des Matériaux de Nantes Jean Rouxel

IMN

CNRS

Nantes F-44000, France

E-mail: sandrine.perruchas@cnrs-imn.fr

M. Cordier

Univ. Rennes

ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226

CNRS

Rennes F-35000, France

C. Latouche

Institut Universitaire de France (IUF)

Paris F-75005, France

<figure><img src="image_3.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.202402040

© 2024 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.202402040
<figure><img src="image_4.png" /><figcaption>Figure 1. General representation of the studied [Cu₄I₄L₄] clusters C-CF₃ (L = phosphine ligand).</figcaption></figure>

iodide atoms which alternatively occupy the vertices of a distorted cube with ligands completing the fourfold coordination sphere of Cu(I).[40,41] These clusters usually present high photoluminescence quantum yield and chemical photostability,[42–47] making them promising for a wide range of applications but especially giving them relevant characteristics as single-component white-light-emitting phosphors. Examples of [Cu₄I₄L₄] clusters displaying white emission are extremely rare[35,37] and the rational design of such single component white emitters along with the comprehension of the underlying photophysical processes continue to pose challenges. Indeed, two main limitations of these [Cu₄I₄L₄] clusters emerge being their emission energy range and their poor processability mainly due to low solubility. Indeed, for the fabrication of efficient luminescent devices, solution-based processes are preferred because of the low stability during vacuum deposition of these compounds, but these processes require good solubility in conventional solvents that is not easily encountered for these compounds. Additionally, polymer materials are more easily handled and more suitable for practical applications and the homogeneous dispersion of the phosphors within polymeric matrix requires also good solubility properties. The other limitation is their emission that usually lies in the visible range mainly from green to yellow color. The obtaining of white emission with these clusters is usually reached by mixing with additional commercial phosphors but the assembly can be complex. Getting white emission from these single component clusters would require to enlarge their panel of emission colors by modification of their emissive states’ layout. Red-emitting [Cu₄I₄L₄] clusters exist,[43] but only very few blue emissive clusters at room temperature are known.[37,48,49] One of them was reported by our group and was obtained by coordination of the copper atoms with phosphine ligands bearing trifluoromethyl (CF₃) groups.[48] The stabilization of the blue emission at room temperature is render possible by the strong electron withdrawing character of the ligands. Based on this result, we thought that thanks to a ligand engineering strategy, by adjusting the electron withdrawing character of the ligand, we could obtain both blue and yellow emission at room temperature, leading to the targeted white emission.

In this article, we report the successful achievement of this strategy for designing single-component white-light copper iodide emitters. Indeed, the energy layout of the system could be tuned by ligand engineering, allowing the relative intensity of the two emissions to be varied and white light emission properties to be achieved. Temperature dependence of this dual-emission characteristic also leads to high contrast lu-

minescence thermochromism. Thanks to their solubility in a wide range of solvents, the studied clusters have been successfully incorporated into organic polymer leading to polymer materials that are easily processed and suitable for applications development. The potential applicability of these composite materials has thus been demonstrated for luminescence ratioimetric thermometry and for white-light emitting diodes as single-component phosphors. Anti-counterfeiting properties were additionally evaluated. This study highlights the relevance of the ligand design strategy to achieve specific target properties that enable the development of high potential multifunctional materials based on cost-effective light-emitting copper materials.

## 2. Results and Discussion

### 2.1. Synthesis and Structural Characterizations

To coordinate the [Cu₄I₄L₄] clusters, the chosen ligands are phenyl-phosphine derivatives functionalized with trifluoromethyl CF₃ groups. The ligands differ by their number of CF₃ groups at the para position of the phenyl rings, from one to three per ligand. Accordingly, the electron withdrawing character increases with the number of CF₃ groups, as illustrated in Figure 1.

The clusters C-CF₃ (Figure 2) were synthesized in solution from CuI and the corresponding ligand (details in Experimental Section in Supporting Information). While the ligand of 3-CF₃ is commercially available, we have synthesized the mono- and di-substituted ligands of 1-CF₃ and 2-CF₃ respectively, according to literature reports.[50] The clusters were obtained as white crystalline powders in good yields (>80%), precipitated by adding a nonsolvent to the reaction medium. Single-crystals were obtained by slow solvent diffusion methods (details in Experimental Section). Note that the three complexes are readily soluble in most of the common organic solvents, given them good processability properties. Elemental analysis results confirm the [Cu₄I₄L₄] formula of the clusters with presence of solvent molecules for 2-CF₃

<table><caption>Figure 2. Designation of the studied clusters C-CF₃.</caption><tr><td>[Cu₄I₄(PPh₂(C₆H₄-CF₃))₄]</td><td>1-CF₃</td></tr><tr><td>[Cu₄I₄(PPh(C₆H₄-CF₃)₂)₄]</td><td>2-CF₃</td></tr><tr><td>[Cu₄I₄(P(C₆H₄-CF₃)₃)₄]</td><td>3-CF₃</td></tr></table>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** SCXRD molecular structures of **C-CF<sub>3</sub>** clusters. The Cu-Cu interactions are represented in dotted pink lines. The disordered CF<sub>3</sub> groups are not shown for clarity.</figcaption></figure>

from recrystallization (C<sub>6</sub>H<sub>14</sub>). Liquid <sup>31</sup>P and <sup>1</sup>H and <sup>19</sup>F NMR analyses agree with the ligand formula and its coordination to the copper atom (details in Supporting Information). Thermogravimetry analyses indicate thermal stability of the compounds up to 200 °C (Figure S17, Supporting Information).

Single crystal X-Ray diffraction (SCXRD) analysis at 150 K indicates that **1-CF<sub>3</sub>** crystallizes in the triclinic *P*-1 space group and **2-CF<sub>3</sub>** in the trigonal *P*-3 one (experimental details in Supporting Information and Table S1, Supporting Information). The crystal structure of **3-CF<sub>3</sub>** was already reported in the *P*-1 space group.[48] The molecular structures of the clusters are depicted in **Figure 3**. **C-CF<sub>3</sub>** compounds present the cubane geometry with four copper and four iodide atoms forming a distorted cube. The phosphine ligands are coordinated to the copper atoms leading to the pseudo-tetrahedral PCuI<sub>3</sub> coordination environment. The CF<sub>3</sub> groups present disorder in the three structures. In **1-CF<sub>3</sub>**, two ligands are disordered with two positions for the F atoms and also two positions for the CF<sub>3</sub> groups on two different phenyl groups of the same ligand. In **2-CF<sub>3</sub>**, one ligand is disordered with the CF<sub>3</sub> group positioned on the three phenyl groups, in accordance with the threefold axis passing through the P–Cu bond of this ligand. Solvent molecules were detected in **2-CF<sub>3</sub>** and could be identified as hexane from elemental analyses. In **3-CF<sub>3</sub>**, two CF<sub>3</sub> groups are disordered with two positions for one of the three F atom. The unit cell contents are shown in Figure S1 (Supporting Information). The structures can be described as assemblies of columns of clusters running along the *a* axis. In all three structures, multiple short H···F and F···F intermolecular contacts (< 3 Å) take place between the clusters.

Selected bond lengths and angles of the clusters are reported in **Table 1**. The Cu–P and Cu–I bond distances and the I-Cu-I angle values are all comparable and within the range of reported values for cubane-type copper iodide clusters coordinated by phosphine ligands.[42,45,51] The six Cu-Cu distances are in the common 2.8–3.2 Å range and are comparable with the sum of the van der Waals radii of copper of 2.80 Å. This indicates occurrence of cuprophilic interactions (d<sup>10</sup>–d<sup>10</sup> bonding) in the **C-CF<sub>3</sub>** compounds.[52,53] The Cu<sub>4</sub> tetrahedron volume are all comparable but is slightly larger for **1-CF<sub>3</sub>** with value of 3.18(1) Å<sup>3</sup> compared with 3.10(1) and 3.02(1) Å<sup>3</sup> of **2-CF<sub>3</sub>** and **3-CF<sub>3</sub>**, respectively. The larger standard deviation in the I-Cu-I angle values for **1-CF<sub>3</sub>** is also indicative of a more distorted cubane geometry compared with that of **2-CF<sub>3</sub>** and **3-CF<sub>3</sub>** (Table 1).

PXRD (Powder X-ray diffraction) analyses are in accordance to SCXRD data and confirm the purity and homogeneity of the powder samples (Figure S2, Supporting Information).

The three clusters were analyzed by solid-state NMR. The <sup>13</sup>C CPMAS NMR spectra are all similar with signals corresponding to the phenyl groups of the ligands (Figure S3, Supporting Information). The solvent (hexane) in the crystalline structure of **2-CF<sub>3</sub>** is also observed. The <sup>31</sup>P MAS spectra of the three compounds are shown in **Figure 4**. Because of the one-bond J-couplings between the <sup>31</sup>P nucleus and the copper isotopes of both nuclear spin value of 3/2 (<sup>65</sup>Cu and <sup>63</sup>Cu with 30.9% and 69.1% natural abundance, respectively), the signal of each phosphorus site consists in a multiplet composed of two overlapping quartets.[54] Therefore, the spectra of **C-CF<sub>3</sub>** are all multiplets lying in the same chemical shift range. Comparatively, the lines of the **1-CF<sub>3</sub>** and **2-CF<sub>3</sub>** spectra are much broader than those of **3-CF<sub>3</sub>** that reflects the presence of disorder of the ligands in the structure of two formers, as identified by SCXRD analysis.

The <sup>1</sup>H-<sup>31</sup>P CP-MAS spectra were also recorded and are shown in Figure 4. When comparing <sup>31</sup>P MAS and <sup>1</sup>H-<sup>31</sup>P CP-MAS spectra, changes in the relative intensity of the peaks are observed for each of the three clusters. This effect is due to differences in the <sup>1</sup>H environments of the phosphorus sites in the ligands bearing the different C<sub>6</sub>H<sub>5</sub> and C<sub>6</sub>H<sub>4</sub>-CF<sub>3</sub> groups. Linear combinations of the MAS and CP-MAS spectra allow the construction of two sub-spectra, as reported in Figure S4 (Supporting Information). For **1-CF<sub>3</sub>**, each subspectrum accounts for half of the full signal corresponding to two of the four inequivalent phosphorus sites in the crystalline structure. The two associated isotropic chemical shifts can be estimated to -19 and -28 ppm. For **2-CF<sub>3</sub>**, the first subspectrum accounts for one quarter of the full signal while the second corresponds to three quarters, in full agreement with the trigonal symmetry with two inequivalent P sites of 1:3 ratio. The estimated two associated isotropic chemical shifts are -28 and -34 ppm. Finally, for **3-CF<sub>3</sub>**, each subspectrum accounts for half of the full signal, i.e., two of the four non-equivalent phosphorus sites of the crystalline structure, as in **1-CF<sub>3</sub>**. The isotropic chemical shifts of the two phosphorus sites associated with the first subspectrum are similar and close to -26 ppm. In opposite, for the second subspectrum, the isotropic chemical shifts of the two phosphorus sites are found different, around -18 and
<table><caption>Table 1. Selected bonds lengths and angles in C-CF₃ at 150 K.</caption><thead><tr><th>Compund</th><th>1-CF<sub>3</sub></th><th>2-CF<sub>3</sub></th><th>3-CF<sub>3</sub></th></tr></thead><tbody><tr><td rowspan="6">Cu-Cu [Å]</td><td>2.8801(14)</td><td>2.9030(12)*3</td><td>2.8849(8)</td></tr><tr><td>2.9193(14)</td><td>3.0540(13)*3</td><td>2.9053(9)</td></tr><tr><td>2.9388(14)</td><td></td><td>2.9267(9)</td></tr><tr><td>3.0249(13)</td><td></td><td>2.9605(9)</td></tr><tr><td>3.1535(14)</td><td></td><td>3.0137(8)</td></tr><tr><td>3.1719(14)</td><td></td><td>3.0267(9)</td></tr><tr><td rowspan="3">Mean standard-deviation<br>Cu<sub>4</sub> [Å<sup>3</sup>]</td><td>3.0148(14)</td><td>2.9785(13)</td><td>2.9530(9)</td></tr><tr><td>0.124</td><td>0.083</td><td>0.058</td></tr><tr><td>3.18(1)</td><td>3.10(1)</td><td>3.02(1)</td></tr><tr><td rowspan="10">Cu-I [Å]</td><td>2.6532(11)</td><td>2.6515(8)*3</td><td>2.6244(7)</td></tr><tr><td>2.6545(11)</td><td>2.6737(9)*3</td><td>2.6478(7)</td></tr><tr><td>2.6597(11)</td><td>2.6860(6)*3</td><td>2.7608(7)</td></tr><tr><td>2.6623(11)</td><td>2.6953(9)*3</td><td>2.6636(7)</td></tr><tr><td>2.6667(11)</td><td></td><td>2.6678(7)</td></tr><tr><td>2.6689(11)</td><td></td><td>2.7108(7)</td></tr><tr><td>2.6689(12)</td><td></td><td>2.6436(7)</td></tr><tr><td>2.6884(10)</td><td></td><td>2.6776(7)</td></tr><tr><td>2.6892(11)</td><td></td><td>2.7186(7)</td></tr><tr><td>2.6896(11)</td><td></td><td>2.6499(7)</td></tr><tr><td>2.6959(12)</td><td></td><td>2.6567(7)</td></tr><tr><td>2.6999(11)</td><td></td><td>2.7112(7)</td></tr><tr><td rowspan="2">Mean standard-deviation</td><td>2.6748(12)</td><td>2.6766(9)</td><td>2.6777(7)</td></tr><tr><td>0.017</td><td>0.017</td><td>0.040</td></tr><tr><td rowspan="4">Cu-P[Å]</td><td>2.244(2)</td><td>2.250(2)*3</td><td>2.255(1)</td></tr><tr><td>2.246(2)</td><td>2.264(3)</td><td>2.254(1)</td></tr><tr><td>2.249(2)</td><td></td><td>2.261(1)</td></tr><tr><td>2.255(2)</td><td></td><td>2.258(1)</td></tr><tr><td rowspan="2">mean standard-deviation</td><td>2.249(2)</td><td>2.254(3)</td><td>2.257(1)</td></tr><tr><td>0.005</td><td>0.007</td><td>0.003</td></tr><tr><td rowspan="14">I-Cu-I[°]</td><td>102.81(4)</td><td>106.70(3)*3</td><td>112.65(2)</td></tr><tr><td>102.91(4)</td><td>106.78(3)*3</td><td>108.44(2)</td></tr><tr><td>103.67(4)</td><td>109.74(3)*3</td><td>106.49(2)</td></tr><tr><td>104.05(4)</td><td>110.41(3)*3</td><td>111.22(2)</td></tr><tr><td>107.73(4)</td><td></td><td>107.08(2)</td></tr><tr><td>108.40(4)</td><td></td><td>105.79(2)</td></tr><tr><td>108.48(4)</td><td></td><td>108.82(2)</td></tr><tr><td>108.81(4)</td><td></td><td>108.62(2)</td></tr><tr><td>108.98(4)</td><td></td><td>106.37(2)</td></tr><tr><td>110.93(4)</td><td></td><td>109.83(2)</td></tr><tr><td>112.39(4)</td><td></td><td>108.97(2)</td></tr><tr><td>112.92(4)</td><td></td><td>111.64(2)</td></tr><tr><td rowspan="2">Mean standard-deviation</td><td>107.67(4)</td><td>108.41(3)</td><td>108.83(2)</td></tr><tr><td>3.568</td><td>1.759</td><td>2.200</td></tr></tbody></table>

–32 ppm. Although the increase of the electron withdrawing effect along the 1-3-CF₃ series is not clearly observed in the ³¹P signal chemical shift values, these last are all in the range of values reported for copper iodide cubane clusters coordinated by phosphine ligands.[55–57]

The FTIR and Raman spectra of the C-CF₃ clusters are reported in Figures 5 and 6. Simulations of the spectra were carried

out by DFT calculations and the relatively good match with the experimental data permits to suggest an assignment of the vibrational bands (details in Supporting Information and Figures S5 and S6, Supporting Information). FTIR spectra in the far-IR (50–650 cm⁻¹) and mid-IR (400–4000 cm⁻¹) regions are presented in Figure 5. Above 200 cm⁻¹, the three spectra are very similar and the bands can be all attributed to ligand vibrations. In particular, the C-H of the phenyl groups are found ≈3050 cm⁻¹. Vibrations involving the CF₃ groups are also identified. The intense vibration at 1320 cm⁻¹ can be attributed to the CF₃ antisymmetric stretching. In the 1200–1000 cm⁻¹ range, the multiple peaks are from C-F stretching. The bands at 597 and 524 cm⁻¹ can be attributed to CF₃ deformation modes. The vibrations of the [Cu₄I₄] inorganic core are found in the low frequencies region. In particular, two bands at ≈90 and 130 cm⁻¹ can be identified. While those of 1-CF₃ and 2-CF₃ have almost same position, those of 3-CF₃ are broader and shifted. These bands can be attributed to Cu-I vibrations (wagging and stretching) together with some [Cu₄I₄] core breathing motion, in accordance with the DFT simulations and previous studies.[58–61] These two low frequency bands appear to be a specific signature of the [Cu₄I₄] cubane moiety.

Raman spectra from 65 to 3300 cm⁻¹ are shown in Figure 6. The three spectra present similar signature and as for the FTIR analysis, vibrations above 200 cm⁻¹ are from the ligands. The intense bands centered ≈3060 cm⁻¹ correspond to the C-H bond stretching of the phenyl groups. In accordance to the FTIR analysis, vibrations involving the CF₃ groups are observed. The band at 1327 cm⁻¹ can be attributed to the carbon bearing the CF₃ moieties in a stretching motion with the carbon of the phenyl group. Between 1000 and 1102 cm⁻¹, four intense bands are observed for 1-CF₃ and 2-CF₃, whereas two are only present for 3-CF₃ which is also nicely reproduced by the simulations (Figure S6, Supporting Information). The one at 1063 cm⁻¹ is assigned to C-F stretching and the three others are from the phenyl groups. In particular, the two bands at 1000 and 1030 cm⁻¹ can be attributed to the unsubstituted phenyl group (C₆H₅) only present in 1-CF₃ and 2-CF₃, in opposite to the substituted one centered at 1102 cm⁻¹ (C₆H₄-CF₃) present in the three clusters. Similarly, the band at 1610 cm⁻¹ correspond to C-C stretching of the substituted phenyl group whereas that at 1588 cm⁻¹ is from the unsubstituted one. The band at 525 cm⁻¹ is attributed to CF₃ deformation modes and the one at 781 cm⁻¹ to C-C stretching modes of the phenyls. Broad bands are observed at low frequency which can be assigned to vibrations of the [Cu₄I₄] cluster core. In particular, those found ≈80, 115, and 150 cm⁻¹ correspond to Cu-Cu and Cu-I movements, in agreement with previous studies[58–61] and simulations.

## 2.2. Photoluminescence Properties

The crystalline powders of C-CF₃ are white under ambient light and display photoluminescence properties under UV light. While 3-CF₃ presents a blue emission, 1-CF₃ emits a white color whereas 2-CF₃ displays an intermediate whitish emission, at room temperature (Figure 7). When the powders are cooled to liquid nitrogen temperature, their emission all change to blue with a deeper blue color for 3-CF₃. Upon warming up to room temperature, the pristine emissions are recovered, indicating a
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Solid-state ³¹P MAS and ¹H·³¹P CP-MAS NMR spectra of C-CF₃.</figcaption></figure>

completely reversible thermochromic luminescence properties for the C-CF₃ clusters series.

Solid-state emission and excitation spectra of the clusters from 295 to 77 K are shown in **Figure 8**. The corresponding data are reported in **Table 2**. At 295 K, the emission spectra present two unstructured emission bands, one in the blue and the other in the yellow region, namely HE (High Energy) and LE (Low Energy), respectively. These bands are centered at 450 and 560 nm for **1-CF₃**, at 445 and 550 nm for **2-CF₃** and at 445 and 520 nm for **3-CF₃** ($\lambda_{ex}$ = 350 nm). The intensity of the yellow LE band increases comparatively to that of the blue HE one as of the number of CF₃ groups decreases, from **3** to **1-CF₃** cluster. The different relative intensities of these two bands are therefore in agreement with the emission color observed by the naked eyes with the bluer emission displayed by **3-CF₃** and the whitish and white emission by increasing the yellow contribution, for **2-CF₃** and **1-CF₃**, respectively. The CIE chromaticity coordinates clearly illustrate the effect of the number of CF₃ groups with values of

(0.34; 0.40), (0.31; 0.37), and (0.23; 0.28) for **1-3-CF₃** respectively, with the first two close to pure white emission (0.33; 0.33). The PLQY (absolute internal quantum yields) measured at room temperature are 41%, 33%, and 6% for **1-3-CF₃** respectively, ($\lambda_{ex}$ = 350 nm, Table 2). The lower value of **3-CF₃** may be attributed to more important non-radiation processes because of its more flexible structure due to its higher number of CF₃ groups. Emission lifetimes were recorded for the two HE and LE emission bands of each cluster, at 295 K. The emissions all present biexponential decay with values in the microsecond range (Table 2), as commonly observed for these phosphorescent clusters.[42,45,51,55,62] A shorter component is systematically observed for the HE emission compared to the LE one, in accordance to previous reports.[42,45,51]

Upon lowering the temperature, the relative intensity of the two emission bands varies with a similar behavior for the three clusters. In the 295–200 K range, the intensity of the LE band increases comparatively to the HE one. Below 200 K, the intensity of the LE band decreases progressively. At 77 K, the LE

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Far- and mid-FTIR spectra of C-CF₃.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 6. Raman spectra of C-CF₃.</figcaption></figure>

band has disappeared and the HE band becomes the only one observed, leading to the blue emission observed by the naked eyes (Figure 7). Accordingly, the CIE coordinates at 77 K are (0.16; 0.12), (0.16; 0.14), and (0.15; 0.12) for 1-3-CF₃, respectively. The intensity increase of the LE band followed by its decrease upon cooling, can be explained by first, the classical enhancement of radiative processes and then, by excitation competition with the HE related excited state. The excitation profiles of the two emission bands are almost similar for the three clusters. From the excitation versus emission cartographies (Figure 8), both the HE and LE bands can be excited in a large 250–370 nm wavelength range at 77 and 295 K. Excitation-dependent emission is also displayed by the C-CF₃ clusters and in particular at room temperature for which the relative intensity of the LE and HE bands can vary. The Stokes shift between excitation and emission maxima of the LE bands of 1.33, 1.29, and 1.16 eV for 1-3-CF₃ respectively, are as expected, much larger compared to those of the HE bands of 0.79, 0.76, and 0.76 eV, respectively.

<figure><img src="image_9.png" /><figcaption>Figure 7. Photos of the crystalline powders of C-CF₃ under ambient light and under UV (365 nm lamp) at room temperature (295 K) and in liquid nitrogen (77 K).</figcaption></figure>

DFT calculations were conducted to rationalize the luminescence properties of the C-CF₃ clusters series. The geometries of the singlet ground states (S₀) were optimized (see computational details in Supporting Information). Unfortunately, all our attempts to relax any of the lowest triplet states failed. Relevant computed data of the cluster's geometry at S₀ are reported in Table 3 along with corresponding SCXRD data (in brackets). The computed values in D₂ and T symmetries for 1-2-CF₃ and 3-CF₃ respectively, are in a satisfying agreement with the experimental ones, as illustrated for the Cu-Cu distances (Figure S7, Supporting Information).

The ground state electronic structures of the C-CF₃ clusters were calculated and their Kohn-Sham orbital diagrams are presented in Figure 9. The HOMO-LUMO bandgaps values (4.24, 4.18, 4.22 eV for 1-3-CF₃, respectively) are close in energy, fully explaining that the observed experimental optical properties remain in the same energies range. The HOMOs (Highest Occupied Molecular Orbitals) are mainly composed of copper (3d) and iodine (5p) orbitals (34%, 35%, and 43% respectively, for all clusters). The 24 lowest unoccupied orbitals (LUMO-LUMO + 23) form a block of orbitals which are combinations of the π* orbitals of the phenyl groups of the phosphine ligands. Just above this block of 24 orbitals lies an in-phase orbital of a symmetry (LUMO + 24) of large Cu 4s/4p character (80%), for the three clusters.

The electronic transitions of lowest energy have been calculated by TD-DFT (Time Dependent-DFT). The calculated absorption transitions correspond to electron promotion from the HOMO to one of the lowest π* combination localized on the ligands. Such transitions are thus of mixed ¹(X + M)LCT character (X and M for I and Cu, respectively). The calculated spectra show broad bands in the 310–320 nm range and centered ≈350 nm (Figure S8, Supporting Information). This result is in fair agreement with the experimental UV-visible spectra of C-CF₃ exhibiting broad absorption bands ≈280 and 340 nm (Figure S9, Supporting Information).

Based on previous studies on C3-CF3 and other cubane copper iodide clusters, the emission properties of the clusters can be rationalized as follows.[48,59,63] The HE emission band has for origin the promotion of an electron from the HOMO into one of the LUMO-LUMO + 23 orbitals which generates the T₂ excited state corresponding to a ³(X + M)LCT transition. Regarding the LE emission, the electron goes to the LUMO + 24 orbital having a strong Cu character. Due to its nature, this transition is named ³CC for Cluster Centered (XMCT + MM), and causes a substantial shortening of the Cu-Cu distances which significantly stabilizes this other T₁ triplet state compared with the T₂ one. The luminescence thermochromism can be thus interpreted by the thermal equilibrium between these two T₂ and T₁ states. T₂ is populated at low temperature leading to the HE emission. As temperature rises, the energy barrier is overpassed and T₁ is populated from T₂ and the LE emission occurs, as schematized in the simplified energy diagram in Figure 10. Temperature variation of the relative intensity of the HE and LE emission bands thus leads to the displayed thermochromism.

By analyzing the C-CF₃ series, the effect of the electron withdrawing properties of the CF₃ groups of the ligands appears as energy stabilization of the whole orbitals' layout (Figure 9). The energy of the orbitals thus decreases from C1-CF₃ to C3-CF₃ by
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Temperature-dependent solid-state luminescence spectra of C-CF₃ with emission in solid and excitation spectra in dashed or dotted lines, and excitation (250–370 nm) versus emission (380–735 nm) cartographies ($\lambda_{ex}$ vs $\lambda_{em}$) at 77 and 295 K.</figcaption></figure>

roughly 0.7 eV, while the bandgaps values are almost constant for the three clusters. This agrees with the similar energy of the absorption bands and of the HE emission (450 nm at 295 K) of the three clusters. Interestingly, the energy difference between the LUMO and the LUMO + 24 increases significantly from 1-CF₃ to 3-CF₃ with values of 1.534, 1.504, and 1.471 eV, respectively. This energy difference can be related to the energetic barrier to overcome to excite the T₁ state from the T₂ one (Figure 10). Therefore, as the LUMO + 24 is getting less accessible, the excitation of the T₁ state is less probable and so the LE emission. This agrees with the different relative intensity of the LE and HE emission bands of the three clusters. Indeed, as the number of CF₃ groups decreases, the intensity of the LE band increases going from 3-CF₃ to 1-CF₃, that is fully in accordance to our expectations. While 1-CF₃ and 2-CF₃ present similar LE emission wavelengths (560 and 550 nm at 295 K, respectively), that of 3-CF₃ is blue shifted (520 nm at 295 K). The energy of this transition

is commonly reported to be dependent on the Cu-Cu interactions due to the copper bonding nature of the ³CC excited state.[59,63–65] Here, no correlation can be established because the Cu-Cu distances are almost similar in the three clusters (SCXRD Table 1). Other phenomena must be involved in this de-excitation pathway, typically in the non-radiative molecular distortion relaxation.

## 2.3. Luminescence Thermometry

In order to get samples suitable for applications, composite materials were synthesized by incorporating the C-CF₃ clusters within a polymer matrix. PMMA (polymethylmethacrylate) polymer was chosen for its good chemical stability and transparency in the UV-visible region. Clusters and PMMA were mixed in solution, dichloromethane was used but other solvents could also be
<table><caption>**Table 2.** Photoluminescence data of C-CF₃ clusters recorded at λ<sub>ex</sub> = 350 nm. Lifetimes (τ) recorded at λ<sub>ex</sub> = 340 nm and 295 K with amplitude percentages in brackets. Photoluminescence quantum yields (PLQY) recorded at 295 K.</caption><thead><tr><th rowspan="2"></th><th colspan="2">λ<sup>max</sup><sub>em</sub> 77 K</th><th colspan="2">λ<sup>max</sup><sub>em</sub> 200 K</th><th colspan="2">λ<sup>max</sup><sub>em</sub> 295 K</th><th rowspan="2">τ [μs] [λ<sub>ex</sub> = 340 nm]</th><th rowspan="2">PLQY [%] [λ<sub>ex</sub> = 350 nm]</th></tr><tr><th>HE</th><th>LE</th><th>HE</th><th>LE</th><th>HE</th><th>LE</th></tr></thead><tbody><tr><td>1</td><td>450</td><td>–</td><td>445</td><td>565</td><td>450</td><td>560</td><td>2.0 (36%) 5.7 (64%) [HE]<br>4.3 (55%) 6.6 (45%) [LE]</td><td>41</td></tr><tr><td>2</td><td>450</td><td>–</td><td>450</td><td>550</td><td>445</td><td>550</td><td>1.6 (39%) 4.0 (61%) [HE]<br>7.7 (3%) 22.9 (97%) [LE]</td><td>33</td></tr><tr><td>3</td><td>445</td><td>–</td><td>450</td><td>520</td><td>445</td><td>520</td><td>1.6 (97%) 6.0 (3%) [HE]<br>1.7 (97%) 8.5 (3%) [LE]</td><td>6</td></tr></tbody></table>

<table><caption>**Table 3.** Geometrical parameters (mean values) of the optimized DFT structures at the ground state (S₀) of C-CF₃ compared with the SCXRD data in brackets.</caption><thead><tr><th rowspan="2"></th><th>1-CF<sub>3</sub></th><th>2-CF<sub>3</sub></th><th>3-CF<sub>3</sub></th></tr><tr><th>S<sub>0</sub> (D<sub>2</sub>)</th><th>S<sub>0</sub> (D<sub>2</sub>)</th><th>S<sub>0</sub> (T)</th></tr></thead><tbody><tr><td>HOMO-LUMO gap [eV]</td><td>4.24</td><td>4.18</td><td>4.22</td></tr><tr><td>Cu-Cu [Å]</td><td>3.088 [3.015(1)]</td><td>3.008 [2.979(1)]</td><td>3.002 [2.953(1)]</td></tr><tr><td>Cu-I [Å]</td><td>2.715 [2.675(1)]</td><td>2.711 [2.677(1)]</td><td>2.715 [2.678(1)]</td></tr><tr><td>Cu-P [Å]</td><td>2.347 [2.249(2)]</td><td>2.308 [2.254(3)]</td><td>2.346 [2.257(1)]</td></tr></tbody></table>

employed. After evaporation of the solvent, bulk and film samples were obtained whose shape can be easily varied depending on the mold used. The composite materials thus obtained, namely P1-3-CF₃ (P for polymer), were colorless and crack-free (Figure 11). Due to the very good dispersion of the cluster within the polymer matrix, the composites are transparent up to 40 wt.% of cluster. Beyond this value, the composites become white and diffusive. The preservation of the clusters within the composites was attested by Raman spectroscopy. The main vibrational bands of the clusters are indeed observed in the spectra of the composites which are absent in the pure PMMA polymer (Figure S10, Supporting Information). As shown in Figure 11, all the three

P-CF₃ composites are photoluminescent under UV irradiation with emission color ranging from white to blue, in accordance to the cluster's emission properties.

Solid-state emission and excitation spectra of the P-CF₃ composites from 77 to 315 K are shown in Figure 12a and the corresponding data are reported in Table S2 (Supporting Information). Similarly to the corresponding clusters, the emission spectra present two unstructured emission bands, one blue (HE) and the other yellow (LE), whose relative intensities vary for the different C-CF₃ clusters. The two bands are centered at 460 and 545 nm for P1-CF₃ (λ<sub>ex</sub> = 350 nm), at 460 and 550 nm for P2-CF₃ (λ<sub>ex</sub> = 345 nm) and at 445 and 540 nm for P3-CF₃ (λ<sub>ex</sub> = 350 nm). The different relative intensities of the two bands lead to white to blue emission color of the composites whose CIE coordinates are (0.27; 0.31), (0.28; 0.31) and (0.22; 0.23) for P1-3-CF₃, respectively (Figure 12b). From the excitation versus emission cartographies (Figure S11, Supporting Information), the HE presents a larger excitation wavelength range compared to the LE one at 295 K. This indicates a more pronounced excitation-dependent emission for the P-CF₃ materials compared with the powders, leading to possible emission color tuning. The Stokes shifts for the LE bands of 1.27, 1.29, and 1.25 eV, are again larger compared to those of the HE bands of 0.85, 0.85, and 0.76 eV, for P1-3-CF₃, respectively. The PLQY measured at 295 K of 27%, 29%, and 23% for P1-3-CF₃ respectively, (λ<sub>ex</sub> = 350 nm, Table S2, Supporting Information) are relatively high values, relevant for application perspectives. Emission lifetimes for the two HE and LE emission bands all present triexponential decay with values in the microsecond range (Table S2, Supporting Information), as for the powders. However, a longer component is observed ≈10 µs for the composites. This result and the relatively similar PLQY values may imply a somehow protective effect of the polymer matrix with less nonradiative phenomena of the incorporated clusters.

<figure><img src="image_11.png" /><figcaption>**Figure 9.** Kohn-Sham orbital diagram of C-CF₃ at the ground state (S₀). The MO localizations (in %) are given in the following order: Cu/I/P. Orbital plots of the HOMO, LUMO, and LUMO + 24 of 2-CF₃ with Cu atoms in orange, I in purple, F in blue, and the others in gray.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 10.** Simplified energy diagram illustrating the thermal equilibrium between the two excited states of C-CF₃.</figcaption></figure>

When lowering the temperature to 77 K, the P-CF₃ exhibit a similar thermochromic behavior than the clusters with a decrease of the LE emission and concomitant increase of the HE one (Figure 12a; Figure S11, Supporting Information). The opposite intensity variation of the HE and LE emission bands with temperature, as illustrated by the temperature-dependent integrated intensity (A) of the two bands (A<sub>HE</sub> and A<sub>LE</sub> in Figure 12c), renders the composites appealing as ratiometric luminescent thermometers.

To evaluate the thermometric performances of P-CF₃ composites, the Δ = A<sub>HE</sub>/A<sub>LE</sub> intensity ratio was determined and due to the overlap of the two emission bands two different approaches were studied and compared. The first approach we used is the most common way consisting in integrating the area of the emission band limited by two arbitrary chosen zones: 390-512 nm for A<sub>HE</sub> and 512-690 nm for A<sub>LE</sub> (Figure S12a, Supporting Information). The second approach is the fit of the emission spectrum using Gaussian curves to get the whole integrated area of each band (Figure S12b, Supporting Information). Although this latter way was highlighted by Brites et al.[66]

<figure><img src="image_13.png" /><figcaption>**Figure 11.** Photos of the composite materials P1-3-CF₃ (10 wt.%) as pellets under ambient light (top) and under UV (bottom, 365 nm lamp) at room temperature (295 K).</figcaption></figure>

on upconverting Y₂O₃:Yb³⁺/Ho³⁺/Tm³⁺ nanocrystals, it is not widespread in the community, and has never been applied to copper(I) compounds.[67-72] For both approaches, the thermal evolution of the thermometric parameter Δ = A<sub>HE</sub>/A<sub>LE</sub> was plotted, and fit with the Boltzmann function from Origin software (Figure S13, Supporting Information), all fitting parameters being reported in Table S3 (Supporting Information).

In order to evaluate the thermal performances of P-CF₃, we then calculated the relative thermal sensitivity defined as S<sub>r</sub> = |∂Δ/∂T|/Δ which is a usual figure of merit to compare the performances of distinct ratiometric systems (Figure 13). First, by comparing the two approaches to determine the integrated emission intensity, we can see that integrating two different zones of emission spectrum underestimates the relative thermal sensitivity (Figure 13 and Table 4), as also reported by Brites et al.[66] Thus, with this approach, the most performant composite is P2-CF₃ with a maximum of relative thermal sensitivity (S<sub>m</sub>) of 2.27%K⁻¹ while P1-CF₃ reaches a S<sub>m</sub> value of 3.73%K⁻¹ with the Gaussian fit. Consequently, the use of Gaussian decomposition seems more appropriate when an important overlap between emission bands exists. If the thermal sensitivities are similar for P1-CF₃ and P3-CF₃, P2-CF₃ appears less sensitive (S<sub>m</sub> = 2.83%K⁻¹) probably due to the presence of solvent molecules in the structure which can act as quenchers. The thermal evolution of A<sub>HE</sub> and A<sub>LE</sub> (Figure 12c) shows that the thermal quenching of the HE emission band is superimposable for all composites while the increase of the LE emission depends on the number of CF₃ groups of the ligand. Thus, the largest variation appears for P1-CF₃ while the lowest one concerns P2-CF₃. We can therefore establish a correlation between the highest thermal sensitivity and the most pronounced opposite trend in the temperature dependency of the emissions, i.e., for P1-CF₃, in line with the report by Cheng et al.[73]

For comparison, analog investigation was conducted for C-CF₃ clusters, and similarly higher relative thermal sensitivities with the Gaussian decomposition were obtained (Figures S14 and S15, and Table S4, Supporting Information). Noticeably, the C-CF₃ clusters are more sensitive than the P-CF₃ composites for which lower intensity of the LE band is probably due to less rigid environment in the polymer reducing radiative phenomena. Furthermore, comparison along the C-CF₃ series reveals that, this time, the 3-CF₃ cluster is the most performant material with a S<sub>m</sub> value at 4.87%K⁻¹. Finally, all materials present relatively high values of thermal sensitivities compared to other reported systems,[66-72] which lie in the medium temperature range, i.e., between 100 and 300 K. Note that the T<sub>m</sub> value, i.e., the temperature where the relative sensitivity is maximum, shifts from 140 to 190 K when the number of CF₃ groups of the ligand decreases (Table 4). This may be related to the energy gap between the HE and LE excited states, which depends on the withdrawing effect of the CF₃ groups (Figure 10).

To evaluate whether the material could be used as a reliable thermometer, cycle tests were additionally performed on P1-CF₃, and the repeatability was calculated according the Equation (1):

$$R = 1 - \frac{\max |\Delta_c - \Delta_i|}{\Delta_c} \qquad (1)$$
<figure><img src="image_14.png" /><figcaption>**Figure 12.** a) Temperature-dependent solid-state luminescence spectra of P-CF₃ with emission in solid and excitation spectra in dotted lines between 77 and 315 K. b) CIE chromaticity coordinates diagram (1931). c) Thermal evolution of the integrated intensity of the HE and LE bands, A<sub>HE</sub> and A<sub>LE</sub> (determined by Gaussian decomposition).</figcaption></figure>

where Δ<sub>C</sub> is the mean thermometric parameter, and Δ<sub>1</sub> is the value of each measurement of the thermometric parameter. Then, four cycles between 150 and 200 K were carried out (Figure S16, Supporting Information) to get a repeatability R > 97%, which confirms the robustness of the P-CF<sub>3</sub> materials for an application as luminescent thermometers.

**2.4. Solid-State Lightening: pc-WLED**

Owing to the relevant properties of the C-CF₃ clusters as white emitters and their large Stokes shift, they were further studied

as single-component phosphors for energy-efficient solid-state
lightening. The composite materials were then employed as con-
verting layers to fabricate pc-WLEDs. Taking advantage of the
processability of the P-CF₃ composites, they were coated onto
commercially available 365 nm GaN chips. Photos of the operat-
ing LEDs devices, of the luminescence spectra and correspond-
ing CIE diagrams, are gathered in **Figure 14**.

The emission spectra present the broad emission band of the
compounds ranging from 400 to 750 nm. Differences with the
P-CF₃ properties, in particular the intensity of the LE emission
band, is ascribed to the different excitation wavelength of the

<figure><img src="image_15.png" /><figcaption>**Figure 13.** Relative thermal sensitivity (S<sub>r</sub>) in the 77–315 K temperature range for P-CF<sub>3</sub> composites a) for integrated areas A<sub>HE</sub> and A<sub>LE</sub> obtained from Gaussian decomposition, and b) for integrated areas A<sub>HE</sub> and A<sub>LE</sub> obtained with arbitrary interval integration.</figcaption></figure>
<table><caption>**Table 4.** Maximum relative thermal sensitivity ($S_m$) and temperature at maximum sensitivity ($T_m$) of C-CF$_3$ and P-CF$_3$ obtained from both approaches. Gauss is for integrated intensities obtained by Gaussian decomposition and int by arbitrary interval integration.</caption><thead><tr><th rowspan="2"></th><th colspan="3">C-CF<sub>3</sub></th><th colspan="3">P-CF<sub>3</sub></th></tr><tr><th>1</th><th>2</th><th>3</th><th>P1</th><th>P2</th><th>P3</th></tr></thead><tbody><tr><td>S<sub>m</sub> gauss [%K<sup>-1</sup>]</td><td>4.15</td><td>3.73</td><td>4.87</td><td>3.73</td><td>2.83</td><td>3.57</td></tr><tr><td>S<sub>m</sub> int [%K<sup>-1</sup>]</td><td>3.80</td><td>3.33</td><td>3.63</td><td>2.09</td><td>2.27</td><td>2.05</td></tr><tr><td>T<sub>m</sub> gauss [K]</td><td>210</td><td>190</td><td>170</td><td>190</td><td>165</td><td>140</td></tr><tr><td>T<sub>m</sub> int [K]</td><td>190</td><td>180</td><td>175</td><td>160</td><td>155</td><td>145</td></tr></tbody></table>

LED. The corresponding CIE chromaticity coordinates of the LEDs are (0.29; 0.38), (0.29; 0.35), (0.25; 0.32) for 1-3-CF$_3$ respectively. While P3-CF$_3$ LED lies in the blue region, the first two are close to pure white light (0.3, 0.3), in accordance to the emitting white light observed (Figure 14). The light emitting devices exhibit color temperature (CCT) of 7154, 7667, and 11 319 K and color rendering index (CRI) of 73, 78, and 75 for 1-3-CF$_3$ respectively, which are relevant for quality white light for indoor lighting applications. While most examples of white emissive phosphor-converted LED based on copper iodide complexes are reported by employing two or more additional emitters (commercial), the present results demonstrate that the studied copper iodide clusters are relevant as a single-component emitting phosphors for pc-WLED lighting applications.

## 2.5. Multifunctional Luminescent Inks: Anticounterfeiting

The distinguishable emission features of the P-CF$_3$ composite materials exhibiting temperature and excitation-dependent emission and their excellent ink-forming ability, prompt us to investigate their anticounterfeiting applications. Inks of the three clusters were synthesized by dissolving C-CF$_3$ (10 wt.%) in a solution of PMMA in ethyl acetate. As displayed in Figure 15, a landscape could be drawn on filter paper, by filling a pen with the different inks. To make the landscape sunnier, a yellow emissive ink was additionally used by employing a yellow phosphor. The latter is also a cubane copper iodide cluster formulated [Cu$_4$I$_4$L$_4$] L = PPh$_2$(CH$_2$CH$_2$CH$_3$), known for its intense yellow emission and thermochromic properties.[74]

While invisible in natural light due to the colorless inks, the landscape displays under UV irradiation, the four-emission color pattern of the clusters of blue, green white to yellow light. The temperature- and excitation-dependent emission of the compounds offer additional protection for multi-mode anticounterfeiting applications. Therefore, at low temperature a change of the color of the drawing in the blue region, is clearly visualized, in accordance to the luminescent thermochromic properties of the clusters (77 K in Figure 15). Similarly, by changing the excitation wavelength (365 nm in Figure 15), the emission color become greener and disappearance of the sun even occurs. This result demonstrates the anti-counterfeiting application prospects of the studied copper iodide clusters thanks to their specific luminescence and processability of characteristics, which can also be relevant for information encryption and transmission.

## 3. Conclusion

The rational design of copper iodide clusters for developing single-component white-light emitters by ligand engineering, is described in this study. Original single-component white-emissive copper iodide clusters are thus reported that expand the library of cost-effective copper-based white emitters. By synthetic adjusting the electron withdrawing character of the phosphine ligands, the energy layout of the molecular clusters could be tuned to get dual emission feature in the visible range, appropriate for white emission. The compounds were structurally characterized by X-ray diffraction, solid-state NMR and vibrational spectroscopies. DFT calculations permit to rationalize the effect of the ligand on the photophysical properties of the clusters. Modification of the interplay between the two triplet states appears to be responsible for the different relative intensity of the two emission bands. The white emissive clusters present intense photoluminescence and high-contrast thermochromic properties with temperature-dependent intensity variation of the dual emission feature. The good solubility of the studied compounds in a wide range of solvents, permit the synthesis of composite materials by incorporation into a polymer matrix. The processability of these polymer materials allowed us to study of their potential applications in diverse fields.

The luminescence thermochromism displayed by the clusters appears perfectly adapted for luminescence thermometry applications. The study of their ratio metric thermometry properties

<figure><img src="image_16.png" /><figcaption>**Figure 14.** Photos of the 365 nm-LEDs coated with P-CF$_3$ materials (cluster content 30 wt.%), corresponding emission spectra and CIE diagram coordinates, at room temperature (295 K).</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Figure 15.** Photos of a landscape painted by using C-CF₃ as inks, at different temperature (295 and 77 K) and under different excitation lamp (302 and 365 nm).</figcaption></figure>

revealed high relative thermal sensitivity values in the medium temperature range. These findings show that these compounds are highly promising candidates for luminescence thermometry applications based on ratiometric detection. Their use as single-component emitting phosphors for white LED lighting is also reported. The coating of a blue-LED with the composite materials permit to get phosphor-converted white-light emitting diodes without additional commercial phosphor. The measured color temperature and color rendering indexes are relevant for indoor lighting applications. The facile preparation, excellent solubility and ink-forming ability of the studied compounds combined with their switchable emission in response to excitation and temperature, also allowed us to evaluate their anticounterfeiting properties. Multicolor drawings could thus be realized that emission colors can be modified upon temperature and the emission wavelength changes. These results demonstrated the great potential of these phosphors in multi-mode anticounterfeiting applications.

Finally, aside significant perspectives of applications of the reported copper halide materials, this study emphasizes the power of ligand design strategy to achieve specific target properties that enable the development of advanced multifunctional materials based on cost-effective and eco-friendly light-emitting copper materials.

**Supporting Information**

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

**Acknowledgements**

The "Région Pays de la Loire" and the Defense Innovation Agency are thanked for financial support of the Ph.D. fellowship of A.R. This work

received financial support under the EUR LUMOMAT project. E. Ishow is gratefully acknowledged for lifetimes measurements. T. Roisnel is thanked for help in structures analyses. Luminescence, Raman, XRD and solid-state NMR were performed on IMN's equipment platform, PLASSMAT, Nantes, France. C. Latouche is grateful for financial support from the Institut Universitaire de France (IUF). The CCIPL (Centre de Calculs Intensifs des Pays de Loire) is acknowledged for computational resources.

**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**

anticounterfeiting, copper cluster, light-emitting diode, phosphine ligand, ratiometric thermometry, thermochromism, white luminescence

Received: July 31, 2024

Revised: October 18, 2024

Published online: November 21, 2024

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