Aliphatic Ligand‐Mediated Cu–Cu Distance Tuning Enables Near‐Unity Efficient and Color‐Tunable Circularly Polarized Luminescent Copper(I) Iodide Clusters

ABSTRACT

Copper(I) iodide clusters are emerging as a new generation of multi‐color circularly polarized luminescence (CPL) materials due to their exceptional luminescent properties and tunable electronic characteristics. However, the substantial decrease in photoluminescence quantum yield (PLQY) during emission color tuning has remained a major challenge for developing high‐performance CPL materials. Herein, diverging from conventional ligand‐involved charge‐transfer mechanisms for color tuning, we employ structurally related aliphatic ligands to reduce ligand‐involved contributions and promote cluster‐centered emission mainly associated with the inorganic [Cu₄I₄] core. Subtle ligand modifications preserve the rigid [Cu₄I₄] cluster framework while reinforcing hydrogen‐bonding networks, inducing variable compression of Cu–Cu distances, enabling continuous emission tuning from green (550 nm) to red (633 nm) while maintaining ultrahigh, near‐unity PLQY. Moreover, the introduction of chiral ligands induces CPL activity with a luminescence dissymmetry factor (g _lum) reaching 2.3 × 10^{− 3}. These remarkable performances render these Cu–I clusters promising for anticounterfeiting and encryption applications. Overall, this work demonstrates the integration of near‐unity PLQY with tunable CPL in metal hybrid clusters, providing a new strategy for high‐performance multicolor CPL materials.

Chiral aliphatic ligands enable precise modulation of the luminescence properties of [Cu₄I₄] clusters. By regulating Cu‐Cu distances, emission color tuning from green to red is achieved together with near‐unity photoluminescence quantum yields and circularly polarized luminescence. These features highlight their potential for advanced optical anticounterfeiting and information encryption applications.

1. Introduction

Copper(I) iodide clusters have garnered significant attention as promising luminescent materials due to their exceptional photophysical properties and highly tunable electronic characteristics [1, 2, 3, 4, 5]. Benefiting from the structural diversity of the inorganic core (Cu _n I _n ) and the versatility of ligand (L = N‐, P‐, S‐, Sb‐, or Se‐donor ligands) [6, 7, 8, 9, 10], these clusters offer a flexible platform for regulating emission behaviors [10, 11, 12]. For instance, pyridine‐based ligands bearing different electron‐donating or electron‐withdrawing substituents allow [Cu₄I₄] clusters to achieve emission modulation from blue to red regions. Similarly, diphosphine ligand systems shift [Cu₂I₂] clusters from green to red (Scheme 1) [13, 14]. However, such tunability is generally accompanied by a significant decrease in photoluminescence quantum yield (PLQY; e.g., from 92.4% to 5.7% in the former example and 99% to 2% in the latter), underscoring the persistent trade‐off between emission adjustability and quantum efficiency [15, 16, 17, 18, 19, 20]. This pronounced decline is generally attributed to conventional ligand‐dominated complex charge‐transfer processes [21, 22], such as metal‐to‐ligand charge transfer (MLCT), halide‐to‐ligand charge transfer (XLCT), and intra‐ligand charge transfer (ILCT). These processes often introduce additional nonradiative decay pathways, perturbing the excited‐state landscape and significantly reducing emission efficiency [23, 24]. In contrast, the triplet cluster‐centered (³CC) emission mechanism, driven by strong cuprophilic (Cu–Cu) interactions within the inorganic core, effectively circumvents the complexities of ligand‐involved charge transfer processes, decoupling the luminescent behavior from the nature of the organic ligands. Although traditionally regarded as an energy‐quenching site, ³CC has recently been demonstrated to yield ultrabright luminescence, challenging previous assumptions and highlighting its untapped potential [25, 26, 27, 28]. Nevertheless, rationally tuning the ³CC energy level through structural design to achieve color‐tunable emission while maintaining high PLQY has remained largely unexplored.

SCHEME 1.

Illustration of the design strategy for Cu–I clusters with color‐tunable, near‐unity PLQY, and CPL activity in previous work and this work.

Circularly polarized luminescence (CPL) materials have attracted increasing interest due to their unique applications in fields such as 3D displays, encrypted data storage, and quantum communications [29, 30, 31, 32, 33]. High‐performance CPL materials ideally combine high PLQY, large luminescence dissymmetry factors (g _lum), and tunable emission wavelengths. In this regard, Cu–I clusters are ideal candidates for advanced CPL materials as they combine efficient luminescence with the facile incorporation of chiral ligands [17, 34, 35, 36, 37]. In 2020, Tang et al. made a breakthrough by introducing chiral R/S‐MBA ligands into [Cu₄I₄] core, resulting in strong orange (630 nm) CPL with PLQY = 60% and |g _lum| = 1 × 10^{− 2} [38]. Subsequent studies reported green (511 nm) (R/S‐Cu₂I₂(BINAP)₂, |g _lum| = 9.5 × 10^{− 3}) and red (625 nm) ((D/L‐Cys)₂Cu₄I₄, |g _lum| = 1.5 × 10^{− 2}) CPL, albeit with relatively modest PLQYs (4.7% and 14.03%, respectively) [35, 39]. Most recently, Mao et al. achieved record performance by designing a 1D chiral cluster Cu₄I₄(R/S‐3‐quinuclidinol)₃, achieving 100% PLQY and |g _lum| = 3.0 × 10^{− 3} at yellow emission (575 nm) [27]. Despite these advances, chiral copper‐based clusters with high PLQY are still rare, and their emission wavelengths are usually fixed. The synergistic optimization of tunable emission wavelengths with preserved high PLQYs and large g _lum in CPL‐active Cu–I clusters remains a formidable challenge.

In this study, we present a chiral aliphatic ligand‐regulated approach to achieve a synergistic optimization of broad wavelength‐tunable emission, near‐unity PLQY, and CPL in chiral Cu–I clusters (Scheme 1). By integrating four pairs of chiral aliphatic ligands with the [Cu₄I₄] cubane tetramer, the Cu–I clusters with desired properties R/S‐Cu₄I₄(3‐Methylmorpholine)₄ ( R/S‐1), R/S‐Cu₄I₄(2‐Methylpiperidine)₄ ( R/S‐2), R/S‐Cu₄I₄(2‐Methyl‐pyrrolidine)₄ ( R/S‐3), and R/S‐Cu₄I₄(3‐Piperidinol)₄ ( R/S‐4) were constructed. The non‐conjugated nature of these aliphatic ligands is expected to suppress ligand‐related charge‐transfer contributions, thereby favoring cluster‐centered (³CC)‐dominated emission mainly associated with the [Cu₄I₄] inorganic core. This enables all clusters to maintain ultra‐high PLQY (∼100%) throughout the color‐tuning process. Ligand design enables the modulation of intermolecular interactions, leading to varying compression of Cu–Cu bonds and consequently regulating the ³CC energy levels. Accordingly, ligand‐induced hydrogen bonding modulates Cu–Cu distances from 2.8061 to 2.6804 Å, tuning emission wavelengths from 550 to 633 nm. The successful transfer of ligand chirality endows these clusters with CPL activity of a dissymmetry factor g _lum reaching 2.3 × 10⁻³. This study demonstrates the simultaneous realization of emission tunability, near‐unity PLQY, and CPL‐active emission in metal hybrid clusters, offering new insights into the ³CC emission mechanism and providing valuable design strategies for future high‐performance CPL materials.

2. Results and Discussion

Using four pairs of enantiomeric chiral aliphatic ligands, we successfully obtained eight enantiomeric Cu–I clusters ( R/S‐1 to R/S‐4) via a triple‐layer slow diffusion method (Figure 1a), which is R/S‐Cu₄I₄(3‐Methylmorpholine)₄ ( R/S‐1), R/S‐Cu₄I₄(2‐Methylpiperidine)₄ ( R/S‐2), R/S‐Cu₄I₄(2‐Methyl‐pyrrolidine)₄ ( R/S‐3), and R/S‐Cu₄I₄(3‐Piperidinol)₄ ( R/S‐4). Single‐crystal x‐ray diffraction (SCXRD) analysis at 300 K (Tables S1–S4) reveals that all clusters adopt a classical [Cu₄I₄] cubane configuration, with copper and iodine atoms alternately occupying the vertices of a distorted cube and four ligands coordinated to the [Cu₄I₄] cubane tetramer through Cu–N bonds (Figure 1b). The use of chiral ligands directs all clusters to crystallize in chiral space groups. The structural variations of the ligands do not alter the overall framework of the clusters. However, the intramolecular of Cu–Cu within the [Cu₄I₄] are significantly influenced by the ligand engineering. Calculation of the average Cu–Cu distance ($\overline{d}$) reveals a progressive decrease with the ligand variation, which is 2.8061, 2.7804, 2.6992, and 2.6804 Å, respectively (Figure 1c). The Cu–Cu distances in these clusters are shorter than twice the van der Waals radius of Cu (2.80 Å), indicating the presence of strong cuprophilic interactions [40]. Besides, the Cu–I distances within the [Cu₄I₄] exhibit a slight increase, and Cu–N distance also varies with the ligand modification, highlighting the impact of ligand modification on the [Cu₄I₄] core geometry (Figure 1d, Tables S5 and S6). Concomitantly, as shown in Figure 1e, the volumes of Cu₄ tetrahedron core shrinkage from 2.5794 ų ( R‐1) to 2.2616 ų ( R‐4), mirroring the shortening of the average Cu–Cu distance and further verifying the significant regulatory effect of the ligands on the inorganic core [41, 42]. The powder x‐ray diffraction (PXRD) patterns are in excellent agreement with the simulated single‐crystal results, confirming the high phase purity of the obtained compounds (Figure S1).

FIGURE 1.

(a) Schematic illustrations for the synthesis processes of R/S‐1∼ R/S‐4 clusters. (b) Structure of the R‐1, R‐2, R‐3, and R‐4 clusters, where Cu, I, C, N, and O atoms are colored in orange, purple, grey, blue, and red, respectively. All hydrogen atoms are omitted for clarity. (c) The Cu–Cu distance in the Cu₄ unit in clusters R‐1 to R‐4. (d) The plots of the average Cu–Cu distance, average Cu–I distance, and Cu–N distance across these clusters. (e) The volume of the Cu₄ tetrahedron core.

To further elucidate the ligand‐mediated modulation of Cu–Cu distances, we performed detailed analyses of the packing mode of cluster R‐1 to R‐4 (Figure 2a). As shown in Figure 2b, clusters R‐1 to R‐3 exhibit similar parallel columnar stacking arrangements, adopting the tetragonal space group P4₂2₁2. In R‐1, the hydrogen atom belonging to the nitrogen of the ligand forms N–H⋯I hydrogen bonds (2.947 Å) with iodine atoms from adjacent clusters. Each [Cu₄I₄] unit is stabilized by eight hydrogen bonds (four on each side), resulting in a “sandwich”‐like zero‐dimensional (0D) structure. The same packing pattern was also observed in R‐2 and R‐3. Differently, the subtle ligand modifications in clusters R‐2 and R‐3 induce not only the conserved N–H⋯I hydrogen bonds (3.051 Å in R‐2 and 2.958 Å in R‐3), but additional H⋯H van der Waals contacts (2.285 Å in R‐2 and 2.393 Å in R‐3). To further visualize and quantify the proportions of intermolecular interactions, Hirshfeld surface (HS) and two‐dimensional (2D) fingerprint plots with percent relative contribution of various interaction sites were presented in Figure 2d,e. The fingerprint plot reveals that H···H contacts are prevalent across all clusters, contributing 68.3%, 85.1%, and 83.0% to the total HS interactions, respectively. Furthermore, the presence of oxygen atoms in R‐1 introduce additional O···H contact (14.3%) and I···I contact (0.3%). The obvious red spots on the HS surface indicate that the intermolecular distance is shorter than the sum of the van der Waals radii, which corresponding to the presence of N–H⋯I contact in cluster R‐1 to R‐3. These intermolecular interactions play a significant role in stabilizing the crystal structures of these clusters, which also accounts for the observed gradual shortening of the Cu–Cu distances. It should be noted that ligand electronic effects may also contribute to the modulation of Cu–Cu distances, as the varying electron‐donating ability of ligand nitrogen atoms can affect Cu–N coordination interactions through lone‐pair donation to the copper centers.

FIGURE 2.

(a) The subtle modifications of the ligands. (b, c) The packing mode of cluster R‐1 to R‐4. Inset the hydrogen bonds interaction within the crystal structure. (d) 2D fingerprint plots of clusters R‐1 to R‐4.  (e) The main contributions of the intermolecular contacts to the Hirshfeld surface area.

Specifically, the introduction of the R‐3‐piperidinol ligand in R‐4 leads to a more intricate hydrogen‐bonding network (Figure 2c). This network features both N–H⋯I (2.836∼2.858 Å) hydrogen bonds, and strong O–H⋯O (1.974 ∼2.048 Å) hydrogen bonds, along with weaker interactions including C–H⋯I van der Waals contacts and O–H⋯C weak hydrogen bonds (Figure S2). This robust hydrogen‐bonding network further compresses the Cu₄ core, resulting in the shortest Cu–Cu distance in these clusters. 2D fingerprint plots and the red spots of HS confirmed the presence of O–H···O and N–H⋯I hydrogen bonds. The strong O···H interactions appeared as forceps‐like spikes, contributing 14.3% to the total intermolecular interactions. These multiple interactions significantly enhance the structural rigidity of the R‐4. In addition, the use of chiral aliphatic ligands directs the formation of chiral crystal packings and well‐defined intermolecular interaction networks, which contribute to the structural rigidity of the clusters and are beneficial for suppressing nonradiative relaxation. To further distinguish the nature of O···H interactions in R‐1 and R‐4 with identical relative contributions, the decomposed 2D fingerprint plots that separate individual H···H, I···H, O···H, and I···I contacts were presented in Figure S3. For R‐1, the O···H region displays broad and diffuse signals attributed to weak C–H···O interactions. In contrast, R‐4 shows distinct sharp spikes in the O···H region. Combined with the bright red signals around ligand oxygen atoms in the HS (Figure 2e), these features are characteristic of O–H···O hydrogen bonds. Despite the identical 14.3% contribution of O···H contacts in the two clusters, their interaction strength, interatomic distances, and spatial distribution differ remarkably. Additionally, the good thermal stability of these clusters was confirmed by Thermogravimetric analysis (Figure S4).

Photoluminescence spectroscopy was conducted to investigate the PL properties of these clusters. Under UV light irradiation, the optical images of cluster R‐1 to R‐4 exhibit green to red emission (Figures 3a and S5). The corresponding excitation and steady‐state fluorescence spectra show the emission peaks redshift successively from 550, 595, 615 to 633 nm, evidencing broadband luminescence across most of the visible spectrum (Figure 3b). UV‐vis absorption spectra further verify a progressive narrowing of the optical bandgap from R‐1 to R‐4 (Figure S6). This redshift is associated with the shortening of Cu–Cu distances, supporting the assignment of the emission to a ³CC‐dominated excited state (ES). In addition, the saturated aliphatic ligands lack extended π‐conjugation, which reduces the possibility of ligand‐related charge‐transfer contributions and suggests that the photoluminescence is mainly governed by the intrinsic structural characteristics of the Cu–I cluster core. Besides, all clusters exhibit large Stokes shifts (>150 nm) and broad full width at half maximum (FWHM) values (110–136 nm) (Table S7), which consistent with the characteristics of previously reported Cu(I)‐based halides [43, 44] The mono‐exponential PL decay profiles, with lifetimes ranging from 10.76 to 19.61 µs (Figures 3c and S7), confirm the phosphorescent nature of the emission and further support its assignment to the ³CC ESs. Most importantly, these clusters exhibit exceptionally intense emission with near‐unity PLQYs of 98.07%, 97.94%, 96.24%, and 98.15%, respectively (Figures 3d and S8–S15). Based on the measured PLQYs and lifetimes, we further calculated the radiative decay rates (k_r ) and nonradiative decay rates (k_nr ) constants of R‐1 to R‐4. As summarized in Table S8, all four clusters exhibit radiative rate constants of 5.1∼9.1 ×  10⁴ s ⁻¹ and much lower nonradiative rate constants of 0.96∼2.5 × 10³ s ⁻¹. It suggests that the compact crystal packing effectively suppresses nonradiative relaxation. Consequently, radiative decay dominates the excited‐state deactivation process, leading to the near‐unity PLQYs despite the substantial variation in Cu–Cu distance and emission wavelength. To the best of our knowledge, such a combination of near‐unity quantum yield and broad color tunability remains rarely achieved in metal halides. Compared with other systems, in which emission color tuning is achieved by alkali doping or modifying the electron accepting/donating abilities of aromatic ligands (the details are given in Table S9), our work suppresses ligand‐related charge‐transfer pathways. The luminescence modulation is mainly ascribed to the variation of Cu–Cu distances, which strongly influences the ³CC ESs. In this way, excellent emission tunability and near‑unity quantum yields are achieved simultaneously. These merits endow the present clusters with superior photophysical properties (Figure 3e).

FIGURE 3.

Optical properties of the synthesized Cu–I clusters. (a) Fluorescence microscopy images of R‐1, R‐2, R‐3, and R‐4. (b) Excitation and emission spectra of R‐1, R‐2, R‐3, and R‐4. Inset shows the corresponding CIE chromaticity coordinate diagram. (c) Time‐resolved photoluminescence (PL) decay spectra at room temperature. (d) PLQY of the Cu–I clusters. (e) Comparison of wavelength and PLQY in other Cu–I color‐tunable systems. (f) CPL and the direct current (DC) voltage spectra of clusters R‐1, R‐2, R‐3, and R‐4 upon excitation at 300 nm.

To validate the chiroptical activity properties of these clusters, circular dichroism (CD) and CPL characterizations were performed. The CD spectra of these clusters exhibit opposite mirror‐symmetric signals, corresponding to their respective absorption spectra. The peaks around 400 nm are assigned to the inorganic components, which are consistent with the reported chiral hybrid copper(I) iodides built by Cu₄I₄ cubane tetramer, indicating effective chirality transfer from the ligands to the inorganic framework (Figures S16 and S17) [27, 36]. Notably, R/S‐4 displays a significantly stronger CD signal than the other clusters, likely due to its lower symmetry orthorhombic space group P2₁2₁2 compared to the tetragonal space group P4₂2₁2. Additionally, the hydrogen‐bonding network in R/S‐4 may enhance chiral transfer [48]. CPL measurements further elucidated the excited‐state chirality of the clusters. The luminescence anisotropy factor (g _lum) defined by the formula g _lum = 2(I _L − I _R)/ (I _L + I _R) was employed to evaluate CPL performance, which reflect the relative difference in left and right polarized emission intensities [45, 46]. As shown in Figures 3f and S18, all enantiomeric cluster display clear mirror‐image CPL responses, with peak positions of DC aligning with their PL emission peaks. The CPL spectra collected at different rotation angles show consistent mirror‐image signals and no obvious periodic variation or sign inversion. The g _lum values at their emission peak for these chiral cluster R/S‐1, R/S‐2, R/S‐3 and R/S‐4 are 2.0 × 10⁻³, −1.5 × 10⁻³, 1.6 × 10⁻³, −1.8 × 10⁻³, 9.6 × 10⁻⁴, −9.2 × 10⁻⁴, 2.3 × 10⁻³, and −2.2 × 10⁻³, respectively (Figure S19). To better contextualize these results, representative CPL‐active metal halides are summarized in Table S9, including their emission ranges, PLQY values, g _lum values, and proposed emission mechanisms. Considering that efficient CPL output is generally associated with both chiroptical activity and emission efficiency, the coexistence of measurable CPL activity and near‐unity PLQY suggests a favorable balance between the photophysical and chiroptical properties of this system [47, 48, 49].

The temperature‐dependent PL spectra, complemented by SCXRD analyses from 100 to 300 K, were further conducted to reveal the relationship between temperature‐induced structural changes and luminescence behavior. Detailed crystallographic parameters are provided in Tables S10–S13. Figure 4a exhibits the Cu–Cu distance in the Cu₄ unit of R‐1 under different temperature from 300 to 100 K, it is demonstrated that the average Cu–Cu distance shortens from 2.8061 to 2.7507 Å (the details are given in Table S14). The normalized temperature‐dependent PL spectra for R‐1 exhibit a single emission peak without splitting, suggesting that luminescence originates from a single ES (Figure 4b). Besides, the phosphorescence emission exhibits a gradual redshift from 550 nm at 300 K to 558 nm at 100 K, while the FWHM increases from 70 nm (100 K) to 113 nm (300 K), indicating pronounced spectral broadening. To correlate these luminescent shifts with structural changes, we analyzed the temperature‐dependent variations in unit cell volume, average Cu–Cu distance, average Cu–I distance, and emission maximum wavelength for R‐1 (Figure 4c). The unit cell volume at 100 K decreases by 2.3% compared to that at 300 K, reflecting the thermal contraction of the crystal lattice. Concurrently, the average Cu–I distance and Cu–Cu distance exhibits a slight reduction. These results imply that decreasing temperature induces the global contraction of the Cu–I clusters. Driven by the shortened Cu–Cu distance, the s‐p orbital overlap between adjacent Cu centers enhanced, reducing the energy gap of the ³CC ES, resulting in longer‐wavelength emission. Similar temperature‐dependent spectral shifts accompanied by Cu–Cu distance variations are observed for R‐2 and R‐4, with their emission maxima red‐shifting from 595 to 606 nm and from 633 to 657 nm upon cooling, respectively (Figure S20, Tables S15 and S16). Such temperature‐induced structural evolution and emission redshift further confirm that the Cu–Cu distance plays a decisive role in controlling the emission color in our clusters.

FIGURE 4.

(a) The Cu–Cu distance in the Cu₄ unit of R‐1 under different temperature from 300 to 100 K. (b, e) Normalized temperature‐dependent PL spectra of R‐1 and R‐3. (c, f) The plots of R‐1 and R‐3 for the variation trends of volume of unit cell, average Cu–Cu distance, average Cu–I distance, emission maximum wavelength as function of temperature. (d, g) Temperature‐dependent CIE chromaticity coordinate diagrams of R‐1 and R‐3.

Unlike R‐1, R‐2 and R‐4, cluster R‐3 exhibit unique emission behavior under varying temperature conditions. As illustrated in Figure 4e,f, when the temperature decreases from 300 to 150 K, the emission peak initially redshifts from 615 to 655 nm. However, upon further cooling to 100 K, it exhibits an anomalous blueshift to 637 nm. SCXRD analysis reveals that at 100 K, R‐3 undergoes a space group transition from P4₂2₁2 to P2₁2₁2, accompanied by an unusual increase in the unit cell volume (Figure S21, Table S12). Notably, despite the low‐temperature phase transition, R‐3 still maintains a rigid and densely packed crystal structure stabilized by N–H···I interactions and short H···H contacts, which helps suppress lattice vibrations and nonradiative decay (Figures S22 and S23). Although the average Cu–Cu distance generally decreases with decreasing temperature (Figure 4f), an anomalous elongation of the Cu–Cu distance (2.724 Å) occurs in the R‐3 structure at 100 K (Table S17), leading to an increase in the emission energy level and consequently causing the blue‐shifted phenomenon. Thus, it might be the longest Cu–Cu distance within the [Cu₄I₄] motif that dictates the emission wavelength of the clusters. The corresponding temperature‐dependent CIE chromaticity coordinate diagrams for R‐1 to R‐4 are shown in Figures 4d, g, S24, and S25. The naked‐eye‐discernible thermochromic redshifts in R‐2, R‐3, and R‐4 underscore the pronounced temperature dependence of their luminescence. Nevertheless, compared to the substantial modulation of the Cu–Cu distances by the ligands, which can induce emission wavelength shifts of exceeding 80 nm, the temperature‐induced changes in Cu–Cu distances are relatively minor, resulting in wavelength shift of approximately 10–40 nm. Ligand modulation incorporating temperature responsiveness enables broad tuning of the luminescent properties in [Cu₄I₄] clusters. To further clarify the emissive‐state nature, temperature‐dependent phosphorescence lifetime measurements were conducted (Figure S26b). Upon cooling from 300 to 100 K, only marginal lifetime variations were observed for all clusters. Such weak temperature dependence indicates highly suppressed nonradiative decay pathways, consistent with the near‐unity PLQYs observed at room temperature. Moreover, no additional emissive component emerges throughout the investigated temperature range. These observations provide further evidence that the emission predominantly originates from a triplet cluster‐centered (³CC) ES localized on the [Cu₄I₄] core.

Subsequently, the detailed analysis of the temperature‐dependent photoluminescence was conducted to elucidate the underlying emission mechanism. R‐3 was excluded from this analysis due to its distinct phase transition behavior. Temperature‐dependent 2D pseudocolor maps reveal that the FWHM of all examined clusters broadens significantly with rising temperature (Figure 5a–c). This observation, coupled with large Stokes shifts, microsecond decay lifetime, and broad FWHM, is indicative of the radiative recombination of self‐trapped excitons (STEs) arising from strong electron‐phonon coupling [50]. To substantiate this hypothesis, Huang‐Rhys factor (S) was employed to quantify the electron‐phonon coupling interactions. The S values were determined by fitting the temperature‐dependent FWHM of photoluminescence peaks using the following Equation (1):

$$\mathrm{FWHM}\left(\mathrm{T}\right)=2.36\sqrt{S}\hslash {\omega }_{\mathrm{phonon}}\sqrt{coth\frac{\hslash {\omega }_{\mathrm{phonon}}}{2{\mathrm{K}}_{\mathrm{b}}\mathrm{T}}}$$ (1)

where ωphonon is the phonon frequency, ℏ is reduced Planck constant, K_b is the Boltzmann constant, and T is temperature. The fitted S values of R‐1, R‐2, and R‐4 is 56.31, 24.89, and 100.60, respectively (Figure 5d–f). These values are comparable to those reported Cu–I clusters [11, 25, 51], such as Cu₄I₄(3‐trifluoromethylpyridine)₄) (S = 29.2) and (DBA)₄Cu₄I₄ (S = 119.6), suggesting robust electron‐phonon coupling within the deformable crystal lattice of our clusters. To gain more insights into the electronic structures of the cluster, density functional theory (DFT) calculations were performed on R‐2 as a representative example. As shown in Figure 5g, the calculated band structures reveal a direct energy bandgap of 2.2 eV, which is smaller than the experimental value obtained from the UV‐Vis absorption spectrum. This discrepancy is attributed to systematic underestimation inherent in DFT methodologies. Additionally, the near‐flat conduction and valence bands suggest pronounced quantum confinement and localization, consistent with the 0D nature the crystal structures. Analysis of the density of states (DOS) and partial charge density reveal that the valence band maxima (VBM) is predominantly composed of a Cu‐d orbitals and I‐p orbitals, whereas the conduction band minimum (CBM) primarily comprises Cu‐s/d and I‐p orbitals. This supports a “cluster‐centered” ES, wherein excitation is localized within the [Cu₄I₄] core (Figures 5h and S27). Drawing on these results, we propose a mechanism for the photophysical processes (Figure 5i). Upon photo‐excitation, electrons transition from the ground state (GS) to ES and form excitons. These excitons, confined within the [Cu₄I₄] cluster, undergo lattice distortion (LD) to form STEs. Through a detrapping process, they subsequently return to the triplet state (³CC) energy level, culminating in the ultra‐bright luminescence. By adjusting ligands or temperature, the overlap of the s and p orbitals between adjacent Cu centers is modulated, thereby tuning the energy gap of ³CC state and achieving color‐tunable emission. This high efficiency emission dominated by ³CC mechanism is may benefit from the fast ISC process, which lead to a fast population of electrons in the triplet state [28, 52, 53]. In addition, the compact structure helped suppress nonradiative decay. Both the fast ISC rate and the suppressed nonradiative decay make the near‐unity emission possible.

FIGURE 5.

(a–c) Temperature‐dependent 2D pseudocolor map of R‐1, R‐2, and R‐4, respectively. (d–f) Experimental (dot) and fitted (line) temperature dependence of FWHM of R‐1, R‐2, and R‐4, respectively. (g) Calculated band structure and density of states and electron density contours of R‐2. (h) Partial charge density distribution for the CBM and VBM states. (i) Schematic of the photophysical mechanism processes.

Considering the tunable PL, chiral CPL properties, and near‐unity PLQY, these copper(I) iodide clusters offer promising opportunities for advanced anti‐counterfeiting and encryption applications. As illustrated in Figure 6a, a series of luminescent anti‐counterfeiting patterns was fabricated using the stencil printing method. These patterns display a green rose, yellow crescent moon, orange five‐petaled flower, and orange bear motif, whose distinct multicolor emissions under UV excitation demonstrate the material's versatility for visual information encoding. Furthermore, as a proof‐of‐concept, a stepwise decryption process involving the pattern “8888” stored in these chiral cluster‐based security systems was developed (Figure 6b). This system operates through three distinct signal channels: PL (colorimetric), CPL (chiroptical), and combined PL&CPL. Initially, under 365 nm UV irradiation, the information encryption template exists in an encrypted state, displaying a uniform “8888” pattern. This decoy appearance effectively conceals the underlying authentic data. Upon application of the first decryption key, distinct emission colors are distinguished, yielding the first‐level decryption information “8639.” Subsequently, a secondary response is triggered, enabling the detection of CPL signals and exposing the second‐level decryption information “0986.” Finally, the third decryption step is initiated, revealing the code “0535” with simultaneous colorimetric and chiroptical verification. This hierarchical encryption strategy ensures that the complete confidential data “863909860535” remains accessible only through the authorized decryption sequence. Unlike conventional color‐based encryption systems, the incorporation of CPL introduces an additional chiroptical dimension, enabling multi‐channel information encoding and substantially enhancing the security level of the encryption process. Consequently, these color‐tunable cluster pairings exhibit exceptional competitiveness in the fields of high‐security information encryption and optical information storage, establishing them as promising candidates for next‐generation anti‐counterfeiting technologies.

FIGURE 6.

(a) Different patterns prepared by stencil printing using R‐1, R‐2, R‐3, and R‐4, respectively. (b) The stepwise information encryption system constructed from R/S‐1 to R/S‐4.

3. Conclusion

In summary, we have successfully synthesized four pairs of chiral Cu–I clusters ( R/S‐1 to R/S‐4) that pioneer the synergistic optimization of high‐efficiency luminescence and tunable CPL. These clusters sustain near‐unity PLQY while allowing continuous modulation of emission wavelengths from green (550 nm) to red (633 nm), exhibiting a g _lum value on the order of 10⁻³. Unlike previously reported Cu–I clusters incorporating aromatic ligands, the use of non‐conjugated aliphatic ligands minimizes ligand‐involved charge‐transfer contributions and suppresses associated non‐radiative decay pathways, thereby favoring emission from the triplet cluster‐centered (³CC) ES of the [Cu₄I₄] core. Aliphatic ligand design enables precise modulation of Cu–Cu interactions, thereby tuning the energy gap of the ³CC state, not only overcoming the longstanding bottleneck of PLQY decline during wavelength tuning but also achieving the synergistic optimization with CPL performance. These outstanding properties make these Cu–I clusters promising candidates for anticounterfeiting and information encryption applications. Our work provides a new design strategy for color‐tunable high‐performance chiral luminescent materials, enriching the platform of Cu–I clusters for CPL applications.

Supporting Information

The Supporting Information is available free of charge. Experimental details, additional figures, tables, and x‐ray crystallographic data including Figures S1–S27 and Tables S1–S17 (PDF). CCDC 2466998–2467005 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk

Conflicts of Interest

The authors declare no conflict of interest.

Supporting information

Supporting File: smll74688‐sup‐0001‐SuppMat.docx.

Data Availability Statement

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