# Revisiting Cu(I)-Dppm Chemistry: Bright Thermally Activated Delayed Fluorescence Luminescence, Anion Dependance, Mechanochromism, and Singlet Oxygen Generation of a Dinuclear Cu(I) Homoleptic Complex

Chendong Xu, Adrien Schlachter, Jana Schiller, Vincent Delmas, Marsel

Shafikov, Alex Man-hei Yip, Justin Shum, Guillaume Calvez, Kenneth

Kam-wing Lo, Harmut Yersin, et al.

► To cite this version:

Chendong Xu, Adrien Schlachter, Jana Schiller, Vincent Delmas, Marsel Shafikov, et al. Revisiting Cu(I)-Dppm Chemistry: Bright Thermally Activated Delayed Fluorescence Luminescence, Anion Dependance, Mechanochromism, and Singlet Oxygen Generation of a Dinuclear Cu(I) Homoleptic Complex. ChemistryEurope, 2026, 4 (1), pp.e202500454. 10.1002/ceur.202500454. [hal-05448885](https://hal.science/hal-05448885v1)

HAL Id: hal-05448885

https://hal.science/hal-05448885v1

Submitted on 8 Jan 2026

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# Revisiting Cu(I)-Dppm Chemistry: Bright Thermally Activated Delayed Fluorescence Luminescence, Anion Dependance, Mechanochromism, and Singlet Oxygen Generation of a Dinuclear Cu(I) Homoleptic Complex

Chendong Xu, Adrien Schlachter, Jana Schiller, Vincent Delmas, Marsel Z. Shafikov, Alex Man-Hei Yip, Justin Shum, Guillaume Calvez, Kenneth Kam-Wing Lo, Harmut Yersin, Karine Costuas,* Manfred Scheer,* and Christophe Lescop*

The one-step syntheses of highly luminescent Cu(I) dimetallic complexes are reported using Cu(I) salts and the commercially available dppm ligand. Three crystalline phases A–C, obtained either with PF₆⁻ (A and B: [Cu₂(µ₂-dppm)₃](PF₆)₂) or TEF⁻ (C: [Cu₂(µ₂-dppm)₃](TEF)₂) anions, display distinct conformations and crystal packings that strongly influence photophysical properties. All phases show intense solid-state thermally activated delayed fluorescence with room temperature quantum yields of nearly 100%. Emission energies and decay times are

modulated by packing effects. Phases A and B exhibit pronounced mechanochromic luminescence upon gentle grinding, while phase C is mechanically insensitive, presumably due to its bulky and flexible counter-anions. Density functional calculation/time-dependent density functional calculations suggest that crystal packing impacts the structural molecular relaxation, in agreement with mechanochromic behaviors. In aerated CH₂Cl₂, these complexes generate singlet oxygen with quantum yield of up to 30%, highlighting their potential as photosensitizers.

## 1. Introduction

Sustainable economy requires the urgent development of innovative and cost-effective “low-tech” syntheses of multifunctional

photoactive materials adaptable for large-scale production. In this context, focus is laid on the development of luminescent complexes using readily available nonstrategic metal ions in place of rare and costly heavy metal ions,[¹⁻⁷] boosting cost-effective synthesis conducive to large-scale applications. For these purposes, luminescent molecular assemblies based on d¹⁰ Cu(I) ions gain considerable attention not only due to the metal's wide abundance and worldwide availability of the copper raw resources, but also due to the multifaced photophysical properties these derivatives manifest.[⁸⁻²⁷] In this field, significant attention is focused on the design of specific ligands that can both stabilize the Cu(I) ions and confer to the resulting assemblies molecular and electronic structures promoting the emergence of a variety of emission properties. Importantly, achieving preparation of such materials using simple commercially available ligands and a straightforward synthesis has decisive impact on large-scale access to such materials. Herein, the one-step synthesis of highly luminescent solid-state phases of a readily accessible homoleptic dicationic Cu(I) complex is reported based on commercially available and affordable bis(diphenylphosphino)methane (dppm) ligand. In spite of the high simplicity of the molecular structures and of the synthetic approach conducted, these novel derivatives carry rich diversity of structure/photophysical relationship that is notably impacted by the nature of the counter anion used and of the molecular packing observed. In addition, these homoleptic Cu(I) complexes display mechanochromic luminescence properties as well as attractive singlet oxygen photogeneration capability.

C. Xu, A. Schlachter, J. Schiller, V. Delmas, G. Calvez, K. Costuas, C. Lescop
University of Rennes

INSA Rennes

CNRS

ISCR (Institut des Sciences Chimiques de Rennes)

UMR 6226

F-35000 Rennes, France

E-mail: karine costuas@univ-rennes1.fr
christophe.lescop@insa-rennes.fr

J. Schiller, M. Scheer
Department of Inorganic Chemistry

University of Regensburg
D-93040 Regensburg, Germany

E-mail: Manfred.Scheer@chemie.uni-regensburg.de

M. Z. Shafikov, H. Yersin
Institut für Physikalische Chemie

Universität Regensburg
D-93053 Regensburg, Germany

A. M.-H. Yip, J. Shum, K. K.-W. Lo
Department of Chemistry
City University of Hong Kong

Kowloon, Tat Chee Avenue, Hong Kong

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Supporting information for this article is available on the WWW under https://doi.org/10.1002/ceur.202500454

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© 2026 The Author(s). ChemistryEurope published by Chemistry Europe and 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.
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## 2. Results and Discussion

The 1:1 reaction of $[Cu(CH_3CN)_4]^+$ (associated with $PF_6^-$ as counter anions) with dppm allows the preparation of the complex $[Cu_2(\mu_2-dppm)_2(CH_3CN)_4]^{2+}$[28] bearing labile acetonitrile moieties that can subsequently be substituted by a variety of donor ligands affording discrete polymetallic or polymeric heteroleptic assemblies embedded with a manifold of luminescence properties.[29-39] Regarding the structural variety of these ancillary ligands, highlighting a great tolerance toward sterical and electronic variations, it is surprising that no studies have been reported so far in which the $[Cu_2(\mu_2-dppm)_2(CH_3CN)_4]^{2+}$ fragment was reacted with an additional equivalent of dppm ligand, finally affording a homoleptic dimeric Cu(I) complex stabilized solely by the dppm ligands. This has triggered us to reinvestigate the coordination chemistry of Cu(I) ions with dppm ligands by studying the reaction of three equivalents of the dppm ligand with two equivalents of $[Cu(CH_3CN)_4]WCA$ (WCA = weakly coordinating anion), in $CH_2Cl_2$ at room temperature (RT) (Scheme 1). $PF_6^-$ and $TEF^-$ ($TEF^- = \text{tetrakis(perfluoro-tert-butoxy)aluminate}$) anions were selected as WCAs having significantly different size to explore the impact of the counter-anion on structural and photophysical properties of the solid-state phases obtained. These reactions yielded clear solutions, from which crystals were reproducibly obtained in nearly quantitative amounts after a few days of pentane vapor diffusion (Scheme 1). Specifically, two distinct types of air-stable, colorless crystals corresponding to phases A and B were isolated with $PF_6^-$ as WCA,[40] while a homogeneous batch of air-stable, colorless crystals of phase C was obtained with $TEF^-$ as the WCA. Solid state IR spectra of the species A–C[41] support the presence of the dppm ligand and WCA counter-anions in these derivatives, together with the absence of bands typical for acetonitrile ligands. This indicates their substitution upon coordination of the dppm ligands. Derivatives A and B can readily be dissolved in $CD_2Cl_2$ affording in both cases strictly identical ¹H and ³¹P{¹H} NMR spectra,[41] highlighting that both crystalline phases exhibit the same molecular structure in $CD_2Cl_2$ solution. RT ³¹P{¹H} NMR spectra in $CD_2Cl_2$ show a very sharp signal centered at -6.1 ppm, suggesting highly symmetrical and nonfluxional coordination of the dppm ligand on the Cu(I) metal centers. RT ¹H NMR spectra in $CD_2Cl_2$ exhibit very symmetrical patterns including a unique singlet at +3.71 ppm,

<figure><img src="image_6.png" /><figcaption>**Scheme 1.** a) Syntheses of the derivatives A–C and $A_g–B_g$; b) General molecular structure of the $[Cu_2(\mu_2-dppm)_3]^{2+}$ units.</figcaption></figure>

corresponding to the methylene group of the dppm ligand. Similarly, the RT ³¹P{¹H} NMR spectrum of derivative C in $CD_2Cl_2$ displays a sharp signal at -5.2 ppm and a singlet at +3.44 ppm in the RT ¹H NMR spectrum corresponding to the dppm ligand. Single-crystal X-ray diffraction studies[41] performed at 150 K confirmed these features, highlighting the formation of homoleptic $[Cu_2(\mu_2-dppm)_3](WCA)_2$ dinuclear complexes (Scheme 1b, **Figure 1**) in all A, B, and C species. They crystallize in the F23 space group of the cubic system, the Fdd2 space group of the orthorhombic system and the P̅-1 space group of the triclinic system, respectively. In the phases A and B, the dimetallic complexes show similar molecular configurations but different crystal organizations. In both species, the $[Cu_2(\mu_2-dppm)_3]^{2+}$ units (Scheme 1b) contain two Cu(I) ions coordinated with three $\mu_2$-bridging dppm ligands, forming a propeller-like structure (Figure 1a).[42,43] Two $PF_6^-$ counter anions and included $CH_2Cl_2$ solvent molecules complete the unit cells being located in the cavities lying between the dimetallic complexes. The Cu(I) atoms present a trigonal planar "P₃" coordination sphere (the sum of the P–Cu–P angles is 360.00° in A and 359.89° in B). The Cu–Cu distance is 2.9873(16) Å in A and is shorter in B (2.9175(9) Å). These values lie in the longer range of distances usually regarded for cuprophilic interactions.[44] The main differences between phases A and B occur in the molecular packing observed in these crystalline phases. In the cubic phase A each dimetallic complex is surrounded by six neighbors (**Figure 2a**) forming adamantate-like extended organizations, while in the

<figure><img src="image_7.png" /><figcaption>**Figure 1.** "Top," "side" and schematic view of the molecular structures of the $[Cu_2(\mu_2-dppm)_3]^{2+}$ dimetallic compounds as observed in the crystal structures of the phases a) A/B and b) C.</figcaption></figure>
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 2.** Views of the intermolecular arrangements of the dimetallic complexes in a) the cubic phase A, b) the orthorhombic phase B, and c) the triclinic unit cell of derivative C for A and B: the different neighbors are numbered from 1 to 6 for phase A and 1–10 for phase B; the central complexes are shown in green in the CPK model while the surrounding neighbors are depicted either in a stick and ball model or represented as node (located at centers of the intermetallic segments of the bimetallic neighboring complexes) of the polyhedra displaying the geometry of the surrounding neighbor complexes; in addition the long range “adamantate”-like organization observed in phase A is shown in Figure 2a; for C a central complex and its two neighbors with the intermolecular interactions highlighted with black dotted lines (left) and same view with these complexes in CPK modes and the twelve TEF anions sharing close contact interactions with the central complex (right).</figcaption></figure>

orthorhombic phase B ten neighbors are surrounding each Cu(I) dimer (Figure 2b). This defines a different network of short π-CH intermolecular environment for each phase. In the case of derivative C, the asymmetric unit consists of two symmetrically independent $[Cu_2(\mu_2-dppm)_3]^{2+}$ units bearing both the same molecular conformation with four highly disordered TEF anions[41] and three included $CH_2Cl_2$ solvent molecules. Importantly, while the $[Cu_2(\mu_2-dppm)_3]^{2+}$ chemical composition observed in C is similar to the ones characterized in A and B, a different molecular conformation is identified in C (Figure 1b) since the “propeller” like symmetry (Figure 1a) of dimetallic complexes found for the phases A and B is replaced in C by a “C4”; like symmetry, due to a different relative orientation of one in three of the $\mu_2$-dppm chelating ligands (Figure 1b). The Cu(I) metal centers in C present also distorted trigonal planar “P$_3$” geometries (sum of the P–Cu–P angles for each symmetrically independent complex: 359.55° and 360.00°; 359.68° and 360.00°) and the intermetallic Cu–Cu distance amounts to 2.9145(4) and 2.9208(4) Å comparing those observed in phase B (Table 1). Importantly, while phases A and B present a dense network of intermolecular contacts between the $[Cu_2(\mu_2-dppm)_3]^{2+}$ units, only two π-CH interactions between two neighboring Cu(I) dimers are observed in C. The other short contact interactions occurring for the dimetallic units in C involve twelve TEF anions

(Figure 2c). Very likely, the larger dimensions of the TEF anions[45,46] compared to the PF$_6$ anion prohibit the establishment of a dense network of short contact interactions between the Cu(I) complexes as observed in phases A and B. Consequently, a molecular solid is obtained that displays intermolecular contacts involving almost exclusively the counter-anions (Figure 2c).

<table><caption>**Table 1.** Structural (at T = 150 K) and photophysical data for the neat materials of phases A–C and A$_g$–B$_g$ at 300 and 80 K, respectively. Values at 80 K are given in parentheses; $k_r = \Phi_{em}/\tau_{obs}$, $k_{nr} = (1-\Phi_{em}/\tau_{obs}$.</caption><thead><tr><th></th><th>d(Cu-Cu) [Å]</th><th>λ<sub>em</sub> [nm]</th><th>EQY [%]</th><th>τ<sub>obs</sub> [μs]</th><th>k<sub>r</sub> [s<sup>-1</sup>]</th><th>k<sub>nr</sub> [s<sup>-1</sup>]</th></tr></thead><tbody><tr><td>A</td><td>2.9873(16)</td><td>495<br>(520)</td><td>72<br>(99)</td><td>68<br>(469)</td><td>1.1*10<sup>4</sup><br>(2*10<sup>3</sup>)</td><td>4*10<sup>3</sup><br>(21)</td></tr><tr><td>A<sub>g</sub></td><td>–</td><td>510<br>(522)</td><td>30</td><td>71<br>(405)</td><td>4*10<sup>3</sup></td><td>1*10<sup>4</sup></td></tr><tr><td>B</td><td>2.9175(9)</td><td>460<br>(485)</td><td>–<sup>[47]</sup></td><td>2<br>(308)</td><td>–</td><td>–</td></tr><tr><td>B<sub>g</sub></td><td>–</td><td>505<br>(537)</td><td>52</td><td>93<br>(530)</td><td>6*10<sup>3</sup></td><td>5*10<sup>3</sup></td></tr><tr><td>C<sup>a</sup></td><td>2.9145(4) and 2.9208(4)</td><td>478<br>(490)</td><td>99<br>(99)</td><td>120<br>(433)</td><td>8*10<sup>3</sup><br>(2*10<sup>3</sup>)</td><td>8*10<sup>2</sup><br>(23)</td></tr></tbody></table>

<footnote>a) two crystallographically independent molecules are found in the asymmetric unit of C.</footnote>
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 3.** Temperature variable solid state normalized emission spectra and plot of emission decay lifetime versus temperature upon excitation at 370 nm (red line represents the fit according to Equation (1) in Supporting Information) of the $[Cu_2(\mu_2-dppm)_3]^{2+}$ dicationic dimetallic complexes in the species A, B, and C.</figcaption></figure>

The solid-state RT UV-visible absorption spectrum of A and B are similar displaying a broad band centered at 315 and 320 nm, respectively, while this band is slightly redshifted at 330 nm for derivative C (Figure S10–S12, Supporting Information).[41] Phases A and B exhibit markedly different eye-perceived colors of their solid-state RT luminescence (A: cyan, B: deep blue) while the derivative C displays an intense blue colored light ($\lambda_{ex}$ = 365 nm). Overall, the variable-temperature (VT) photophysical properties of these three species display similar trends (Figure 3).[41] At RT, solid-state emission spectra display broad, featureless bands that gradually redshift upon cooling. These emission bands are associated with monoexponential excited-state decay profiles with RT times constants (with a particularly low value of 2 µs for phase B, Table 1), increasing steadily as the temperature decreases. The decay profiles remain monoexponential down to 80 K, displaying a sigmoidal profile for the VT trace of the excited-state decay times. Finally, species A and C present remarkably high emission quantum yields (EQY) at RT of 72% and 100%, respectively, and nearly 100% at 80 K in both cases.[47] All these VT photophysical properties are typical of Cu(I) derivatives displaying thermally activated delayed fluorescence (TADF), in agreement with the values of the RT radiative constant rates $k_r$ (Table 1).[5-7] Nevertheless, the compounds present also noticeable differences in terms of luminescence energies and decay kinetics. In particular, in spite of being based on the same $[Cu_2(\mu_2-dppm)_3]^{2+}$ unit having very similar molecular conformations, a blueshift of the spectra recorded for the phase B is observed all along the cooling process compared to the corresponding spectra of phase A (Figure 3, Table 1). In addition, the decay times of the excited state (Table 1) differ also substantially.

In order to get more insight into the energetics and emission properties in these dimers and to identify the origin of the different photophysical properties of these phases, density functional calculations (DFT) and Tamm-Dancoff time-dependent density functional calculations (TD-DFT) were performed.[41] In a first step, the ground state geometries of both molecular arrangements of $[Cu_2(\mu_2-dppm)_3]^{2+}$ observed in the crystal structures of the species A (B displays the same conformation as A) and C, were optimized without constraints, giving the $A_{opt}(S_0)$ and $C_{opt}(S_0)$ gas phase geometries, respectively (Figure S40, Supporting Information).[41] Interestingly after full geometry optimization, the observed

$Cu^{2+}$-Cu distances are in both case shorter than the experimental ones (2.7463 Å in $A_{opt}$ vs. 2.9873(16) Å in A (and 2.9169(8) Å in B), and 2.685 Å for $C_{opt}$ vs. 2.9145(4) and 2.9208(4) Å for C). This clearly indicates the impact of the crystal arrangement and intermolecular interactions on the geometrical parameters of the $[Cu_2(\mu_2-dppm)_3]^{2+}$ units, as already reported recently on related Cu(I) assemblies.[37,38] Since solid state calculations at the excited state are not feasible,[41] we chose to run a second batch of calculations (affording $A_{cons}(S_0)$ and $C_{cons}(S_0)$) in which an extreme theoretical case is considered, constraining the $Cu^{2+}$-Cu distances to be fixed at the X-ray crystallographic distances. The differences of the calculated total energies of the $A_{opt}$ and $A_{cons}$ as well as $C_{opt}$ and $C_{cons}$ isomers are quite moderate (0.169–0.19 eV, Table S5, Supporting Information), $C_{opt}$ being the most stable scaffold.[41] On the geometrical point-of-view, the shortening of the calculated $Cu^{2+}$-Cu distances observed between the fully relaxed and the constrained geometries affects weakly the electronic excitation properties of the different $[Cu_2(\mu_2-dppm)_3]^{2+}$ conformers (Table S3, Supporting Information). The absorption spectra of A and C in $CH_2Cl_2$ solutions (Figure S35 and S36, Supporting Information) are in good agreement with the calculations of $A_{opt}$ and $C_{opt}$ (Table S3, Supporting Information) with a first absorption at ≈320 nm and more intense absorption at ≈290 nm. The main computed data concerning the first relaxed excited states are given in (Table S3 and S4, Supporting Information). The constrained and nonconstrained optimized excited states structures show that the reorganization energy at the excited states are comparable (roughly 0.5 eV). In the case of compounds $A_{opt}$ and $A_{cons}$, there is a quasi-degeneracy of the energies $S_1$ and $S_2$ excited states. The nature of these two first electronic excitations corresponds to a moderate electronic transfer (mainly HOMO to LUMO Figure 4 and S41, Supporting Information) from a delocalized density lying over half of the compound's scaffold (with a noticeable contribution of the phenyl groups) to both Cu atoms, leading to the formation of a weak cuprophilic interaction with a $3p_{Cu1}-3p_{Cu2}$ character (Table S4, Figure 4, S41, Supporting Information). In the case of $C_{opt}$ and $C_{cons}$, the first excitation is of the same nature (Figure S41, Supporting Information) but the lowering of molecular symmetry compared to A scaffolds leads to a more delocalized electron density loss (see HOMOs, Figure 4). Thus, the optimized excited state geometries display a shortening of the Cu-Cu
<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 4.** Iso-contour plots of HOMO and LUMO at the geometry ground state of A<sub>opt</sub> and C<sub>opt</sub> (± 0.02 [e bohr<sup>-1</sup>]1/2) and atomic contributions.</figcaption></figure>

distance in the nonconstraint A<sub>opt</sub> and C<sub>opt</sub> structures together with a slight shortening of Cu–P bonds (roughly by 0.03 Å).

The calculated emission energies, both for fluorescence and phosphorescence for A, are lower in the optimized systems compared to the constrained one (Table S5, Supporting Information). This correlate with the alteration of the emission spectra of the phase A upon grinding (vide infra).

All in all, these phases based on the same very simple molecular structures present remarkably rich, different, and enhanced solid-state luminescence properties (Table 1). In these cases, the computational results indicate that intermolecular interactions (which govern how structural reorganizations occur after photoexcitation in the solid-state; A versus B) have a stronger influence on the photophysics than the molecular scaffold conformation (A/B vs. C).

In order to explore further the influence of crystal packing on these different phases based on the $[Cu_2(\mu_2-dppm)_3]^{2+}$ unit, we have proceeded to a gentle mechanical grinding by applying simply a slight manual pressure on both solid-state phases A and B in a mortar using a pestle. This process afforded colorless powder of the new ground phases A<sub>g</sub> and B<sub>g</sub> (Scheme 1a) that do not display any eye-perceived color change compared to the pristine phases A and B. However, under uv light irradiation (365 nm) at RT, deep blue light was emitted from the original phase B, while green emission is observed at places where mechanical stress was applied (Figure 5a). In contrast, the eye-perceived emission color change of phase A to the phase A<sub>g</sub> is much more moderate (Figure 5a) but still slightly perceptible. <sup>31</sup>P{$^1$H} and <sup>1</sup>H NMR spectra of the phases A<sub>g</sub> and B<sub>g</sub> dissolved in CD<sub>2</sub>Cl<sub>2</sub>, as well as their solid-state IR spectra are similar to those observed

for the $[[Cu_2(\mu_2-dppm)_3](PF_6)_2]$ species. Recrystallization of A<sub>g</sub> and B<sub>g</sub> from n-pentane vapors diffusions in CH<sub>2</sub>Cl<sub>2</sub> solutions afforded almost quantitatively single crystals of the cubic phase A (confirming the predominance of this phase in the crystalline solid-state).<sup>[40]</sup> These characterizations indicates that no chemical transformation was induced by the mechanical grinding. The X-ray powder diffraction diagrams recorded for the A<sub>g</sub> and B<sub>g</sub> phases (Figure S39, Supporting Information) reveal a significant amorphization.<sup>[48]</sup> This suggests that the grinding process leads to a loss of crystalline homogeneity, allowing the local formation of new phases. The RT solid-state emission spectrum of the phase A<sub>g</sub> presents a featureless band centered at 510 nm ($\lambda_{ex}$ = 380 nm, Figure 5b) which is redshifted compared to the RT emission spectrum of derivative A (Table 1). Similarly, the RT solid-state emission spectrum of the phase B<sub>g</sub> is also characterized by a featureless band with a maximum at 505 nm ($\lambda_{ex}$ = 400 nm, Figure 5c), being also redshifted compared to the emission of B (Table 1). The RT average decay time of the excited state are 71 µs for phase A<sub>g</sub> and 93 µs for phase B<sub>g</sub> while the EQY are 30% for A<sub>g</sub> and 52% for B<sub>g</sub>. Upon cooling from 300 to 80 K, the emission profiles of A<sub>g</sub> and B<sub>g</sub> exhibit a gradual redshift of the featureless bands from 510 nm (300 K) to 522 nm (80 K) and from 505 to 537 nm, respectively. The average 80 K emission decay times are determined to 405 µs for A<sub>g</sub> and 530 µs for B<sub>g</sub>. Therefore, the emission spectra of the grinded phases are redshifted compared to those of the pristine phases.

The thermal dependance of the average decay time of both grinded phases exhibit very roughly a linear profile (Figure 5b,c) in contrast to the sigmoidal profiles found for the phases A and B. This change of profile can reasonably be assigned to overlapping contributions of different photoactive phases resulting from site inhomogeneity in the grinded phases.<sup>[49]</sup> However, in spite of a decrease compared to the pristine phases, the RT EQYs of both grinded phases remain high, which confirms the efficiency of the radiative relaxation processes taking place in the $[Cu_2(\mu_2-dppm)_3]^{2+}$ scaffold. Conversely to phases A and B, no mechano-chromic (grinding) luminescence change is observed for the samples of phase C. Such discrepancies can be related with different intermolecular contacts that are encountered in these different phases. In the case of phases A and B bearing the “small” PF<sub>6</sub> anion, dense networks of intermolecular interactions are observed between neighboring photoactive complexes (Figure 2a,b) while in the derivative C bearing the large TEF<sup>-</sup> anions, intermolecular contacts mostly involve the very disordered trifluoromethyl groups of the TEF<sup>-</sup> anions (Figure 2c).<sup>[41]</sup> It is very likely that application of a mechanical stress on C will impact predominantly the intermolecular interactions and structural parameters of the TEF<sup>-</sup> anions that are “innocent” in the optical energy range. Moreover, the large flexibility of the trifluoromethyl groups may strongly dampen the impact of local alterations induced by applying an external mechanical stress. As a consequence, ground state metrics of the dimetallic chromophores in phase C are very likely only moderately impacted by external mechanical stress, contrary to the situation found for phases A and B. This opens new perspectives for designing systems that should remain insensitive with respect to application of
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 5.** a) Photographs of solid state samples of A, Ag, B, and Bg at RT under 375 nm irradiation; b,c) normalized solid state emission spectra at different temperatures and plot of emission decay time versus temperature for the phases Ag (λ<sub>ex</sub> = 380 nm) and Bg (λ<sub>ex</sub> = 400 nm).</figcaption></figure>

external mechanical stress, by using large and flexible anions such as TEF⁻.

Finally, an investigation of the photophysical properties of [Cu₂(µ₂-dppm)₃](WCA)₂ was initiated in aerobic and aerated CH₂Cl₂ solutions. The high resolution and symmetric patterns displayed by the RT multinuclear NMR spectra,[⁴¹] suggest indeed that these bimetallic complexes retain a stable molecular structure in solution, without undergoing dissociation/reassociation processes commonly reported for Cu(I) assemblies.[⁵⁰⁻⁵³] 20 µM aerobic and aerated solutions of these species display eye-perceived greenish colored emission upon photoirradiation (λ<sub>ex</sub> = 365 nm) characterized by unstructured bands centered at 530 nm (PF₆⁻) and 525 nm (TEF⁻) (Figure S37 and S38, Supporting Information).[⁴¹,⁵⁴] These emissions are redshifted compared with the solid-state values. The emission decay times are 13 µs (PF₆⁻) and 18 µs (TEF⁻) in aerated solution and 35 µs (PF₆⁻) and 42 µs (TEF⁻) in nonaerated solutions. These values suggest significant triplet state involvement in the radiative processes taking place in solution together with a clear impact of dissolved molecular oxygen on the emission decay times. Interestingly, photosensitized singlet oxygen quantum yields (Φ<sub>Δ</sub>) of 30% for the PF₆⁻ and 26% for the TEF⁻ counterion were found in aerated CH₂Cl₂ at 298 K under 337 nm irradiation. The corresponding quantum yield was determined via detection of the ¹O₂ phosphorescence at 1270 nm.[⁵⁵] These Φ<sub>Δ</sub> values fall within the medium range compared to those reported for other Cu(I) compounds.[⁵⁶⁻⁵⁸] This

efficiency may be attributed to the molecular rigidity of the
[Cu₂(µ₂-dppm)₃]²⁺ scaffold in solution and the occurrence of suit-
able triplet energy levels as well as appropriate triplet state decay
times. Obviously, this Cu(I) dimer offers favorable conditions for
promoting efficient singlet oxygen generation.

3. Conclusion

In conclusion, we report the straightforward and simple syntheses of highly luminescent [Cu₂(µ₂-dppm)₃](WCA)₂ complexes using readily available Cu(I) salts and dppm ligands. Distinctly different conformations and molecular arrangements across three distinct solid-state phases A, B, and C are identified. These phases/compounds show different photophysical properties that are strongly influenced by the nature of the counter-anion and the crystal packing. Remarkably high RT EQY (nearly 100%) thermally activated delayed fluorescence is observed. Moreover, the phase B (with PF₆⁻ as WCA counter-anion) displays significant mechanochromic luminescence color change via gentle grinding. In contrast, for phase C with bulky TEF⁻ anions, such mechanical responsiveness is lacking. This highlights the role of intermolecular interactions in the stimulus-responsive behavior of Cu(I) based luminophores. In particular, the nature of the counter-anion plays an important role for the design of such stimuli-sensitive photoluminescent materials. Indeed, use of different
<figure><img src="image_16.png" /></figure>

WCAs allows to control properties of the luminescent center, opening innovative perspectives for anions, such as TEF⁻, to design luminescent systems whose properties remain largely unaffected by the application of external mechanical stress. In addition, it is important to note that the grinded phases A_g and B_g are clearly different regarding their photophysical properties. This reveals the importance of the solid-state lumino-phores' preorganization on the way the application of external mechanical stimulus can alter photophysical properties. DFT and TD-DFT calculations show that the emission energies are modulated by structural relaxation processes highly impacted by crystal packing. Moreover, upon photoexcitation, aerated CH₂Cl₂ solution of [Cu₂(H₂-dppm)₃](WCA)₂ compounds show energy transfer to O₂ to produce ¹O₂ with singlet oxygen quantum yields up to 30%, highlighting the potential of these simple materials as photosensitizers for reactive oxygen generation.[⁵⁹,⁶⁰] This study showcases the versatility of Cu(I) complexes in combining solid-state enhanced luminescence, mechanochromism, and photoinduced ¹O₂ generation. All in all, this allow envision the reproduceable, low-budget, and large-scale preparation, from a single and simple Cu(I) photoactive derivative of several different photoluminescent phases having modulated multifunctional properties.

These results highlight the value of re-examining thoroughly the coordination chemistry of simple and well-known precursors, providing novel results relevant for the large-scale, “low-tech,” preparation of highly efficient photoactive species. This opens perspectives for preparation of innovative materials toward a more sustainable economy.

## Acknowledgements

This work was supported by the CNRS, the ANR (ANR PRC SMAC and ANR PRCI SUPRALUM), the French "Ministère de l'Enseignement Supérieur, de la Recherche et de l'Innovation" (Partenariat Hubert Curien Procope), and the French "Ministère des Affaires Etrangères" as well as by the DAAD and the Deutsche Forschungsgemeinschaft within the projects Sche 384/42-1 and Sche 284/44-1. C. L. thanks the Alexander von Humboldt Foundation for a fellowship for experienced researcher. K. C. acknowledge support by the "Grand équipement national de calcul intensif (GENCI)" through HPC resources of CINES, TGCC, and IDRIS (grants AD010800649R1). R. Maurice is acknowledged for his attempts to converge post-Hartree Fock SCF calculations.

## 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 • luminescence • singlet oxygen • solid-state transitions • stimuli-sensitive

[1] S. Dehnen, J. W. Steed, K. Lo, K. Heinze, O. S. Wenger, S. Singh, H.-B. Yang, S.-Q. Zang, *Inorg. Chem.* **2023**, *62*, 14823.

[2] X. Luo, B. Tian, Y. Zhai, H. Guo, S. Liu, J. Li, S. Li, T. D. James, Z. Chen, *Nat. Rev. Chem.* **2023**, *7*, 800.

[3] V. W.-W. Yam, V. K.-M. Au, S. Y.-L. Leung, *Chem. Rev.* **2015**, *115*, 7589.

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

[5] R. Czerwieniec, M. J. Leitl, H. H. H. Homeier, H. Yersin, *Coord. Chem. Rev.* **2016**, *325*, 2.

[6] R. Czerwieniec, H. Yersin, *Inorg. Chem.* **2015**, *54*, 4322.

[7] H. Yersin, U. Monkowius, *Adv. Photonics Res.* **2024**, *6*, 2400111.

[8] B. Hupp, J. Nitsch, T. Schmitt, R. Bertermann, K. Edkins, F. Hirsch, I. Fischer, M. Auth, A. Sperlich, A. Steffen, *Angew. Chem., Int. Ed.* **2018**, *57*, 13671.

[9] B. Hupp, J. Nitsch, T. Schmitt, R. Bertermann, K. Edkins, F. Hirsch, I. Fischer, M. Auth, A. Sperlich, A. Steffen, *Angew. Chem.* **2018**, *130*, 13860.

[10] R. Hamze, J. L. Peltier, D. Sylvinson, M. Jung, J. Cardenas, R. Haiges, M. Soleilhavoup, R. Jazzar, P. I. Djurovich, G. Bertrand, M. E. Thompson, *Science* **2019**, *363*, 601.

[11] A. Schlachter, K. Tanner, P. D. Harvey, *Coord. Chem. Rev.* **2021**, *448*, 214176.

[12] A. Y. Baranov, E. A. Pritchina, A. S. Berezin, D. G. Samsonenko, V. P. Fedin, N. A. Belogorlova, N. P. Gritsan, A. V. Artem'ev, *Angew. Chem., Int. Ed.* **2021**, *60*, 12577.

[13] A. Y. Baranov, E. A. Pritchina, A. S. Berezin, D. G. Samsonenko, V. P. Fedin, N. A. Belogorlova, N. P. Gritsan, A. V. Artem'ev, *Angew. Chem.* **2020**, *133*, 12685.

[14] C. Sun, L. Llanos, P. Arce, A. Oliver, R. Wannemacher, J. Cabanillas-Gonzalez, L. Lemus, D. Aravena, *Chem. Mater.* **2021**, *33*, 6383.

[15] H. Yersin, R. Czerwieniec, U. Monkowius, R. Ramazanov, R. Valiev, M. Z. Shafikov, W.-M. Kwok, C. Ma, *Coord. Chem. Rev.* **2023**, *478*, 214975.

[16] Y. Jin, Q. Peng, J. Xie, K. Li, S. Zang, *Angew. Chem., Int. Ed.* **2023**, *62*, e202301000.

[17] Y. Jin, Q. Peng, J. Xie, K. Li, S. Zang, *Angew. Chem.* **2023**, *135*, e202301000.

[18] W.-T. Chen, C.-H. Li, Z.-Y. Liang, Z.-L. Zhang, D.-Q. Liu, J.-W. Ye, L. Chen, X.-M. Chen, *ACS Mater. Lett.* **2024**, *6*, 2077.

[19] A. Schlachter, F. Moutier, R. Utrera-Melero, J. Schiller, A. M. Khalil, G. Calvez, M. Scheer, K. Costuas, C. Lescop, *Adv. Opt. Mater.* **2024**, *12*, 2400347.

[20] A. Schlachter, C. Xu, J. Schiller, R. U. Melero, S. Kerneis, G. Calvez, K. Costuas, C. L. M.Scheer, *Angew. Chem. Int. Ed.* **2024**, *64*, e202413151.

[21] A. Schlachter, C. Xu, J. Schiller, R. U. Melero, S. Kerneis, G. Calvez, K. Costuas, C. L. M.Scheer, *Angew. Chem.* **2024**, *137*, e202413151.

[22] H.-J. Zhang, Z.-R. Chen, J.-T. Xu, J.-W. Ye, L. Chen, X.-M. Chen, *Chem. Sci.* **2025**, *16*, 6872.

[23] H. Zheng, Y. Li, W.-W. Zhan, J. Zhou, G. Wu, C. Zhang, D. Cun, Y. Yang, *Angew. Chem. Int. Ed.* **2025**, *64*, e202423787.

[24] H. Zheng, Y. Li, W.-W. Zhan, J. Zhou, G. Wu, C. Zhang, D. Cun, Y. Yang, *Angew. Chem.* **2025**, *137*, e202423787.

[25] K. C. Dwivedi, G. Sabharwal, A. Pandey, M. S. Balakrishna, *Inorg. Chem.* **2025**, *64*, 10989.

[26] J.-W. Liu, D. Heng, L.-H. He, S.-J. Liu, H.-R. Wen, J.-L. Chen, L.-P. Lin, *Inorg. Chem.* **2025**, *64*, 15932.

[27] H. Yersin, R. Czerwieniec, M. Z. Shafikov, A. F. Suleymanova, *ChemPhysChem* **17**, *18*, 3508.

[28] M. M. Wu, L. Y. Zhang, Y. H. Qin, Z. N. Chen, *Acta Cryst.* **2003**, *E59*, m195.

[29] P. D. Harvey, M. Drouin, T. Zhang, *Inorg. Chem.* **1997**, *36*, 4998.

[30] B. Nohra, Y. Yao, C. Lescop, R. Réau, *Angew. Chem. Int. Ed.* **2007**, *46*, 8242.

[31] B. Nohra, Y. Yao, C. Lescop, R. Réau, *Angew. Chem.* **2007**, *119*, 8390.

[32] S. Bassoli, G. A. Ardizzioa, B. Therrien, S. Brenna, *Dalton Trans.* **2019**, *48*, 9276.

[33] S. Evariste, A. M. Khalil, S. Kerneis, C. Xu, G. Calvez, K. Costuas, C. Lescop, *Inorg. Chem. Front.* **2020**, *7*, 3402.

[34] X. W. Chen, L. H. He, P. Ju, J. L. Chen, S. J. Liu, H. R. Wen, *J. Mater. Chem. C* **2020**, *8*, 16160.

[35] C. Lescop, *Chem. Rec.* **2021**, *21*, 544.

[36] A. M. Khalil, C. Xu, V. Delmas, G. Calvez, K. Costuas, M. Haouas, C. Lescop, *Inorg. Chem. Front.* **2021**, *8*, 4887.

[37] F. Moutier, J. Schiller, G. Calvez, C. Lescop, *Org. Chem. Front.* **2021**, *8*, 2893.

[38] F. Moutier, J. Schiller, C. Lecourt, A. M. Khalil, V. Delmas, G. Calvez, K. Costuas, C. Lescop, *Chem. - Eur. J.* **2022**, *28*, e202104497.
<figure><img src="image_17.png" /></figure>

[39] C. Lecourt, R. U. Melero, J. Schiller, F. Moutier, V. Dorcet, G. Calvez, C. Poidevin, K. Costuas, M. Scheer, C. Lescop, *Inorg. Chem. Front.* **2023**, *10*, 6976.

[40] Phase A was always obtained in majority as colorless prisms while only a few millimeter-size single crystals of phase B could be collected, allowing the manual separation of these species.

[41] For experimental details, photophysical measurements, crystallographic data and computational details, see ESI.

[42] A related propeller molecular scaffold was reported for the $[Cu_2(μ_2-dmpm)3]_2^{+}$ dimetallic cmplex (dpmp = bis(dimethylphosphanyl)methane], Z. Mao, H.-Y. Chao, Z. Hui, C.-M. Che, W.-F. Fu, K.-K. Cheung, N. Zhu, *Chem. Eur. J.* **2003**, *9*, 2885.

[43] E. Fournier, A. Decken, P. D. Harvey, *Eur. J. Inorg. Chem.* **2004**, *2004*, 4420.

[44] N. V. S. Harisomayajula, S. Makovetskyi, Y. C. Tsai, *Chem. - Eur. J.* **2019**, *25*, 8936.

[45] A. Kraft, J. Schaefer, I. Krossin, *Angew. Chem. Int. Ed.* **2018**, *57*, 13982.

[46] A. Barthelemy, P. Dabringhaus, E. Jacob, D. Röhner, M. Schmitt, M. Sellin, I. Krossing, *Compr. Inorg. Chem.* **2023**, 378.

[47] Due to the large dimensions of crystals of the phase B and their mecha-
nochromic behavior (affording phase Bₐ, vide infra), a reliable measure-
ment of the EQY of a pure phase of B could not be done due to technical
measurement constraints. However, phase B displays eye-perceived
intensity of light emitted upon UV-Vis light excitation that is comparable
to phase A.

[48] Due to the high sensitivity of the phases A and B upon application of external mechanical stress, it was not possible to record the powder X-ray diffraction patterns of the native phases A and B.

[49] S. Perruchas, *Dalton Trans.* **2021**, *50*, 12031.

[50] M. Mohankumar, M. Holler, M. Schmitt, J.-P. Sauvage, J.-F. Nierengarten, *Chem. Commun.* **2013**, *49*, 1261.

[51] J.-J. Cid, J. Mohanraj, M. Mohankumar, M. Holler, F. Monti, G. Accorsi, L. Karmazin-Brelot, I. Nierengarten, J. M. Malicka, M. Cocchi, B. Delavaux-Nicot, N. Armaroli, J.-F. Nierengarten, *Polyhedron* **2014**, *82*, 158.

[52] S. Keller, H. B. A.Prescimone, M. Sessolo, G. Longo, L. Martínez-Sarti, J. M. Junquera-Hernández, E. C. Constable, E. Ortí, C. E. Housecroft, *Dalton Trans.* **2018**, *47*, 14263.

[53] T. F. C. Cruz, P. S. Lopes, M. Amélia, N. D. A. Lemos, L. F. Veiros, P. T. Gomes, *Dalton Trans.* **2023**, *52*, 8003.

[54] 2 µM solutions of PF6- and TEF- species in CH2Cl2 display sim-
ilar photophysical properties, see (Figure S37-S38, Supporting
information).

[55] Phenalenone was used (ΦΔ = 0.98 in aerated CH3CN, λEx = 337 nm), R. Schmidt, C. Tanielian, R. Dunsbach, C. Wolff, *J. Photochem. Photobiol. A* **1994**, *79*, 11.

[56] C. Dong, W. Feng, W. Xu, L. Yu, H. Xiang, Y. Chen, J. Zhou, *Adv. Sci.* **2020**, *7*, 2001549.

[57] J. Beaudelot, S. Oger, S. o Peruško, T.-A. Phan, T. Teunens, C. Moucheron, G. Evano, *Chem. Rev.* **2022**, *122*, 16365.

[58] Y. Yang, F. Dötinger, C. Kleeberg, W. Frey, M. Karnahl, S. Tschierlei, *Front. Chem.* **2022**, *10*, 936863.

[59] L. C.-C. Lee, K. K.-W. Lo, ACS Cent. Sci. **2025**, *11*, 1289.

[60] L. C.-C. Lee, K. K.-W. Lo, *Chem. Rev.* **2024**, *124*, 8825.

Manuscript received: November 17, 2025

manuscriptRevised December 1, 2025

Version of record online: