Achieving Narrowed Bandgaps and Blue‐Light Excitability in Zero‐Dimensional Hybrid Metal Halide Phosphors via Introducing Cation–Cation Bonding

Zero‐dimensional (0D) hybrid metal halides, which consist of organic cations and isolated inorganic metal halide anions, have emerged as phosphors with efficient broadband emissions. However, these materials generally have too wide bandgaps and thus cannot be excited by blue light, which hinders their applications for efficient white light‐emitting diodes (WLEDs). The key to achieving a blue‐light‐excitable 0D hybrid metal halide phosphor is to reduce the fundamental bandgap by rational chemical design. In this work, we report two designed hybrid copper(I) iodides, (Ph3MeP)2Cu4I6 and (Cy3MeP)2Cu4I6, as blue‐light‐excitable yellow phosphors with ultrabroadband emission. In these compounds, the [Cu4I6]2− anion forms an I6 octahedron centered on a cationic Cu4 tetrahedron. The strong cation–cation bonding within the unique cationic Cu4 tetrahedra enables significantly lowered conduction band minimums and thus narrowed bandgaps, as compared to other reported hybrid copper(I) iodides. The ultrabroadband emission is attributed to the coexistence of free and self‐trapped excitons. The WLED using the [Cu4I6]2− anion‐based single phosphor shows warm white light emission, with a high luminous efficiency of 65 lm W−1 and a high color rendering index of 88. This work provides strategies to design narrow‐bandgap 0D hybrid metal halides and presents two first examples of blue‐light‐excitable 0D hybrid metal halide phosphors for efficient WLEDs.


Introduction
Recently, zero-dimensional (0D) hybrid metal halides, which consist of organic cations and isolated inorganic metal halide anions, [1][2][3] have aroused great attention for applications such as solid-state lighting [4][5][6] and scintillators, [7,8] due to their superior photoluminescence (PL) properties and low-cost solution processability.These materials exhibit localized band edges that are generally derived from the isolated metal halide anions and thus large bandgaps. [9,10]The optical excitation of such compounds results in an abrupt change in charge distribution, thereby destroying the balance of electron-mediated interatomic forces that determine the ground electronic state and causing the atoms to relax until the interatomic forces are rebalanced.36][37][38][39] The key to achieving a blue-light-excitable 0D hybrid metal halide phosphor is to reduce the fundamental bandgap by raising the valance band maximum (VBM) and/or lowering the conduction band minimum (CBM).[42][43][44] Thus, cations such as Sb 3+ and Cu + are often highly desired.On the other hand, low-lying unoccupied orbitals of a cation generally lead to a low-lying CBM, [45][46][47][48] and the formation of cationcation bonding states [49,50] can further lower the CBM.In this context, isolated inorganic metal halide anions centered on a cationic cluster are of specific interest.It is known that Cu cations can form cationic clusters under certain conditions. [51]Therefore, hybrid copper(I) halides, which exhibit componential and structural richnesses, [52,53] are expected to hold the promise to serve as blue-light-excitable phosphors with relatively narrow bandgaps. [54,55]n this work, we report two hybrid copper(I) iodides, (Ph 3 MeP) 2- Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 , as blue-light-excitable yellow phosphors with ultrabroadband emissions.The [Cu 4 I 6 ]2− anion in these compounds forms an I 6 octahedron centered on a cationic Cu 4 tetrahedron.The [Cu 4 I 6 ] 2− anions and the organic cation pairs adopt either a NaCltype or a CsCl-type arrangement, depending on the effective radius ratio.Density functional theory (DFT) calculations reveal that the strong cation-cation bonding states within the unique cationic Cu 4 tetrahedra enable these two compounds to have ultralow CBMs and thus bandgaps that are smaller than other reported hybrid copper(I) iodides.As a result, under the blue light excitation, these two compounds exhibit intense ultrabroadband yellow emissions ranging from green to nearinfrared regions.Temperature-dependent PL spectra and DFT calculations suggest that the ultrabroadband emission is attributed to the coexistence of the free exciton (FE) emission (around 2.3 eV) and the selftrapped exciton (STE) emission (around 2.0 eV) within the inorganic [Cu 4 I 6 ] 2− anions.The WLEDs using a commercial blue-GaN chip (450 nm) and the [Cu 4 I 6 ] 2− anion-based single phosphor show warm white light emission, with high luminous efficiency of 65 lm W −1 and a high color rendering index (CRI) of 88 due to the sufficient red light component.

Results and Discussion
Firstly, we synthesized (Ph 3 MeP) 2 Cu 4 I 6 crystals using a simple antisolvent diffusion method.The crystals are thick yellow needles with a hexagonal cross-section.The single-crystal X-ray diffraction (SCXRD) measurement revealed that (Ph 3 MeP) 2 Cu 4 I 6 crystallizes in the rhombohedral space group R3̅ c, with Z = 2 in unit cell of dimensions a = b = c = 15.499Å, α = β = γ = 53.269°(see Figure S1 and Table S1, Supporting Information), as reported in literature. [56]The complex has an overall C 3v symmetry, with the threefold axis (i.e., the rhombohedral [111] direction) coinciding with the needle elongation.As shown in Figure 1a, the [Ph 3 MeP] + cation adopts an overall geometry that resembles a propeller, where the three twisted phenyl rings and the methyl serve as the "blades" and "shaft", respectively.Each pair of [Ph 3 MeP] + cations is aligned along the threefold axis, forming a sextuple phenyl embrace (SPE). [57,58]As shown in Figure 1b, the [Cu 4 I 6 ] 2− anion consists of a tetrahedron of Cu atoms in which one Cu atom lies on the threefold axis and three Cu atoms are positioned symmetrically about the axis, with each pair of Cu atoms being bridged by an I atom lying out from the tetrahedron edge.Every alternate [Cu 4 I 6 ] 2− anion has its inverted Cu 4 tetrahedron but still accommodates the same I atoms lying out from its tetrahedron edges.Any individual tetrahedron has its full complement of atoms, but the time-averaged superimposition of two Cu 4 tetrahedra causes the Cu atoms to appear halfweighted.Alternatively, the [Cu 4 I 6 ] 2− anion can be viewed as an I 6 octahedron that elongates slightly along its threefold axis, with four Cu atoms lying out from the centers of four non-edge-sharing octahedron faces toward the center of the Cu 4 tetrahedron.Thus, each Cu atom has approximately trigonal planar coordination, while the Cu 4 tetrahedron as a whole exhibits octahedral coordination.The two sets of non-edgesharing octahedron faces generate two Cu 4 tetrahedra, one of which is inverted with respect to the other, leading to the half-weighted Cu atoms.The effective ionic radii of the pair of [Ph 3 MeP] + cations (larger ion) and the [Cu 4 I 6 ] 2− anion (smaller ion) were calculated to be 6.02 and 4.43 Å, respectively, by the box-model approach. [59]The ionic radius ratio of the central [Cu  1c.We expected that other similar "propellerlike" organic cations may also template the [Cu 4 I 6 ] 2− anions to form analogous hybrids and that if the ionic radius ratio of the central [Cu 4 I 6 ] 2− anion and the peripheral "propellerlike" organic cation pairs is slightly smaller than the minimum radius ratio of 0.732 for the body-centered cubic coordination, the principal topology of the NaCl-type structure may form instead. Therefore, we chose [Cy 3 MeP] + , which is larger than [Ph 3 MeP] + , as the templating organic cation and synthesized (Cy 3 MeP) 2 Cu 4 I 6 crystals, which are yellowish and have the shape of truncated octahedra.The SCXRD measurement revealed that (Cy 3 MeP) 2 Cu 4 I 6 crystallizes in the cubic  S2, Supporting Information).The [Cy 3 MeP] + cation adopts a "propeller-like" configuration and each pair of [Cy 3 MeP] + cations form a SPE (Figure 1d).The [Cu 4 I 6 ] 2− anion consists of a slightly elongated I 6 octahedron centered on a Cu 4 tetrahedron (Figure 1e).Both the [Cy 3 MeP] + cation pairs and the [Cu 4 I 6 ] 2− anion have the C 3v symmetry.These features for (Cy 3 M-eP) 2 Cu 4 I 6 are basically the same as those for (Ph 3 MeP) 2 Cu 4 I 6 .The four [Cy 3 MeP] + cation pairs and the four [Cu 4 I 6 ] 2− anions in the cubic cell are aligned with their threefold axes parallel to the four threefold axes of the cubic cell, respectively.The effective ionic radius of the [Cy 3 MeP] + cation pair was calculated to be 6.41 Å, larger than that of the [Ph 3 MeP] + cation pair.The ionic radius ratio was determined to be 0.702, smaller than the minimum value of 0.732 for the bodycentered cubic coordination.As a result, the pairs of [Ph 3 MeP] + cations form a face-centered cubic packing while the [Cu 4 I 6 ] 2− anions fill all of the octahedral voids, thus generating the principal topology of the expected NaCl-type structure, as shown in Figure 1f.
Under excitation, the (Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 crystals exhibited yellow-orange and yellow-green emissions, respectively (Figure S3, Supporting Information).To reveal the PL mechanism of these compounds, their photophysical properties were measured and summarized in Table 1.(Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 exhibited strong and broad excitonic at room temperature (see the black dashed lines in Figure 2a,b, respectively), as the cases of other low-dimensional photoluminescent materials such as Cs 2 TeCl 6 . [60]The edges of the excitonic absorptions were located at around 500 nm (i.e., 2.5 eV), which agrees well with the yellow and yellowish appearances of the synthesized crystals.The existence of the strong and broad excitonic absorption hinders the determination of the fundamental bandgap at room temperature to some extent. [18,61]The PL excitation (PLE) and PL spectra for (Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 are also shown in Figure 2a,b, respectively.For both the two compounds, the PLE band edges (the blue solid lines) resemble the excitonic absorption edges (the black dashed lines), respectively, with the PLE peaks at around 470 nm (i.e., 2.64 eV).Their PL spectra showed asymmetric broadband yellow emissions ranging from green to near-infrared region (i.e., 500-800 nm), from which two Gaussian-broadened peaks can be well resolved.As far as we know, the broadband emissions in 0D metal halide phosphors mainly arise from the formation of STEs.According to the theory of Toyozawa, [62] the temperature-dependent emission broadening can determine the Huang-Rhys factor (S), which quantitively evaluates structural deformation induced by electron-phonon coupling. [12,13]To verify the origins of the dual-excitonic emission behavior, temperature-dependent PL spectra were measured by taking (Ph 3 MeP) 2 Cu 4 I 6 as an example.As shown in Figure S4, Supporting Information, the full-width-at-half-maximum (FWHM) of the lowerenergy emission peak was highly dependent on the temperature, with a large S value of 51.22, which indicates the STEs were formed due to the strong electron-phonon couplings. [12,63,64]In contrast, the FWHM of higher-energy emission peak was only weakly dependent on the temperature, suggesting recombinations of FEs that are confined within the undistorted octahedral [Cu 4 I 6 ] 2− anions.To determine the lifetimes of the STE and FE emissions, time-resolved PL (TRPL) spectra were measured.For the STE emissions, the PL signals were monitored at 650 nm where the FE emissions were negligible.The average lifetimes for the STE emissions of (Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 were calculated to be 2.20 and 2.52 μs, respectively, as shown in Figure 2c,d, respectively.For the FE emissions, the PL signals were monitored at 540 nm at a low temperature of 80 K for wiping out the STE emissions (see Figure S5, Supporting Information).The average lifetimes of FE emissions of (Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 were determined to be 3.14 and 2.20 μs, respectively (see Table S3, Supporting Information), as shown in Figure 2c,d, respectively.These long lifetimes suggest that both the STE and FE emissions arose from the recombinations of triplet states. [12,65]The overall PL quantum yields (PLQYs) of (Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 are measured to be 69.0%and 59.1% under 450 nm excitation, respectively.
To gain insights into the unique PL excitation behavior of the title compounds with [Cu 4 ]-centered octahedral [Cu 4 I 6 ] 2− anions, we performed DFT calculations for the ground and excited states.[68][69][70] To better understand the origin of the relatively small bandgaps of the title compounds, we calculated band structures and performed chemical bonding analyses.As shown in Figure 3b, for (Ph 3 MeP) 2 Cu 4 I 6 , the calculated band structure shows a direct bandgap at the Γ point.The valence band maximum (VBM) consists of the antibonding states of Cu 3d and I 5p orbitals, while the conduction band minimum (CBM) is derived mainly from the bonding states of Cu 4s orbitals within the Cu 4 tetrahedra.The [Ph 3 MeP] + cations contribute little to the band edges, indicating a type-I band alignment (between the [Cu 4 I 6 ] 2− anions and the organic matrix) favored  for efficient light emission. [71]The band structure of (Cy 3 MeP) 2 Cu 4 I 6 (Figure S6, Supporting Information) is similar to that of (Ph 3 MeP) 2- Cu 4 I 6 but with a bit larger bandgap due to the stronger ionicity of the [Cy 3 MeP] + cation and the larger spacing between the [Cu 4 I 6 ] 2− anions.
Figure 3c  (a 1 ) and high-lying triply-degenerated antibonding states (t 2 *). [51]hen, the low-lying singlet bonding state (a 1 *) further interacts with the I 5p orbitals, and the antibonding state becomes the LUMO, which is thus lower than that of the common [CuI 4 ] 3− anion.Therefore, the "bandgap" of the [Cu 4 ]-centered [Cu 4 I 6 ] 2− anion is much smaller than that of the common [CuI 4 ] 3− anion.These results reasonably explain the origin of the narrowed bandgaps and thus, more importantly, the blue-light excitability for the compounds with the [Cu 4 I 6 ] 2− anions.[74][75] The calculated energies of the excitonic absorption and the triplet STE emission of (Ph 3 MeP) 2 Cu 4 I 6 are 2.71 and 2.11 eV, respectively, which are close to the corresponding experimental values (i.e., 2.64 and 1.96 eV). Figure 4b shows the spin-polarized density of states (DOS) for the ground state structure with an average Cu-Cu bond length of 2.72 Å and an average longitudinal I. ..I distance of 6.3 Å, in which the spin-up and spin-down components are identical.Upon excitation, FEs and free carriers are generated.As shown in Fig- ure 4c, for the FE state (i.e., without ion relaxation based on the Franck-Condon principle), the calculated DOS shows a triplet electron-hole pair in the bandgap.The FE overcomes an energy barrier by coupling with acoustic phonons and gives rise to a local structural distortion of the [Cu 4 I 6 ] 2− anions to form STEs. [12,65] The STE structure exhibits obvious shortened Cu-Cu bond lengths (with an average value of 2.53 Å) and slightly lengthened longitudinal I. ..I distances (with an average value of 6.40 Å).The shortened Cu-Cu bonds enhance the bonding interaction of Cu 4s orbitals in the [Cu 4 ] tetrahedron, while the lengthened I. ..I distances weaken the antibonding interaction between [Cu 4 ] a 1 state and I 5p orbitals.Therefore, the structural distortion as a whole occurs to lower the excited electron level and raise the excited hole level, forming a tighter electron-hole pair, as shown in Figure 4d.These results explain well the origin of the dual-excitonic emission in the title compounds.
To the best of our knowledge, (Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 reported in this work are the first examples of 0D hybrid metal halides that exhibit ultrabroadband emission under the blue-light excitation, [1][2][3]   as summarized in Table 2.7][38][39]76,77] Therefore, a prototype of WLED was fabricated using a commercial blue-GaN chip (450 nm) as the excitation source and the (Ph 3 MeP) 2 Cu 4 I 6 powders as the single-component phosphor.The measured electroluminescent spectrum is shown in Figure 5a.The Commission Internationale de I'Eclairage (CIE) chromaticity coordinate is (0.41, 0.39) (Figure 5b), with the correlated color temperature (CCT) of 3500 K, indicating a warm white emission.The luminous efficiency of the device is up to 65 lm W −1 , which is much higher than those of the common broadband phosphors-based WLEDs that have to be excited by ultraviolet chips.Besides, the (Ph 3 MeP) 2 Cu 4 I 6 phosphor-based WLED also exhibits a high CRI of 88, due to the ultrabroadband emission (with sufficient red light) of the (Ph 3 MeP) 2 Cu 4 I 6 phosphor.

Conclusions
In summary, we have developed two hybrid copper(I) iodides, (Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 , as blue-light-excitable yellow phosphors with ultrabroadband emissions.The [Cu 4 I 6 ] 2− anion in these compounds adopts a configuration of an I 6 octahedron centered on a cationic Cu 4 tetrahedron.DFT calculations reveal that the strong cationcation bonding states within the cationic Cu 4 tetrahedra enable these two compounds to have ultralow CBMs and thus bandgaps that are smaller than other reported hybrid copper(I) iodides.Under the blue light excitation, these two compounds exhibit intense ultrabroadband yellow emissions ranging from green to near-infrared regions.The temperature-dependent PL spectra and DFT calculations suggest that the ultrabroadband emission is attributed to the coexistence of the FE emission (around 2.3 eV) and the STE emission (around 2.0 eV) within the inorganic [Cu 4 I 6 ] 2− anions.The WLEDs using a commercial blue-GaN chip (450 nm) and the [Cu 4 I 6 ] 2− anion-based single phosphor show warm white light emission, with high luminous efficiency of 65 lm W −1 and a high color rendering index of 88 due to the sufficient red light component.This work provides the strategies to design narrower-bandgap 0D hybrid metal halides and presents two first examples of blue-light-excitable 0D hybrid metal halide phosphors for efficient WLEDs.More interesting photophysical properties may be found for 0D hybrid metal halides with unique cation-cation interactions.

Experimental Section
Chemicals: Copper(I) iodide (AR, 99.5%) and Methyltriphenylphosphonium iodide (98%), cyclohexylphosphine (96%), and methyl p-toluenesulfonate (99%)  were purchased from INNOCHEM.Acetonitrile (AR, >99%), hydroiodic acid (AR, 45-50 wt.% in water, stabilized by 1.5% H 3 PO 2 ), and absolute ethanol (AR, 99.5%) were purchased from Aladdin.All reagents were used as received without further purification.Synthesis: The crystals of (Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 were prepared according to the previous paper with some optimization: [56] (Ph 3 MeP) 2 Cu 4 I 6 -To a 25-ml vial, copper(I) iodide (0.60 g, 3.15 mmol) and methyl triphenylphosphonium iodide (0.60 g, 1.48 mmol) were added in mixed solvents of 4 ml acetonitrile and 1 ml hydroiodic acid.The mixture was heated under 80 °C in an oil bath for 30 min to fully dissolved.The solution was then filtered by a 0.22 μm filter membrane to remove impurities.The clear orange solution was added to 15 ml absolute ethanol dropwise and become stratified finally.The vial was sealed and let stand at room temperature for 24 h, and the (Ph 3 MeP) 2 Cu 4 I 6 precipitated out as needle-like or block yellow crystals.The crystalline samples were then washed with absolute ethanol and dried in air for further characterization (0.96 g, yield, 79%).
(Cy 3 MeP) 2 Cu 4 I 6 -In a 25 ml vial, cyclohexylphosphine (0.46 g, 4.00 mmol) and methyl p-toluenesulfonate (0.75 g, 4.03 mmol) were dissolved in 4 ml acetonitrile and stirred under 90 °C for 12 h.The resulting solution was acidified with 1 ml hydroiodic acid and then added copper(I) iodide solution (1.52 g, 8.00 mmol in 8 ml acetonitrile).The (Cy 3 MeP) 2 Cu 4 I 6 precipitated out as block yellowish crystals (1.86 g, yield: 73%).Structure determination: SCXRD measurements were conducted on a Rigaku XtaLAB PRO diffractometer with a Cu K α radiation (λ = 1.54178Å).The collected data were integrated and applied with empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm.The crystal structures were solved by the SHLEXT (Intrinsic Phasing method) program and further refined by the SHLEXL (full-matrix leastsquares on F 2 ) program.The resultant two crystallographic information files (CIFs) for (Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 can be obtained from the Cambridge Crystallographic Data Centre (CCDC) by quoting the deposition numbers 2183093 and 2183094, respectively.Powder X-ray diffraction (PXRD) measurements were conducted on a Rigaku SmartLab-SE diffractometer with a Cu K α radiation (λ = 1.54178Å).Simulated powder diffraction patterns were obtained using the CIFs by VESTA.Optical spectroscopy: Optical absorption spectroscopy measurements were performed on a PerkinElmer Lambda 35 UV-vis-NIR spectrometer.Optical diffuse reflectance spectra were measured in an integrated sphere using BaSO 4 powder as the reference.The PL excitation spectra were measured on an Edinburgh FLS1000 spectrofluorometer equipped with a 450 W Xe lamp.The final spectra were recorded signals subtracted by the source spectra in the range of 300-500 nm.A 460 nm PhosLED was used as the excitation source for TRPL measurements.The emission counts were monitored at 540 and 650 nm for both samples.The corresponding lifetimes were obtained by double-exponential fittings, except the self-trapped excitonic emission of (Cy 3 MeP) 2 Cu 4 I 6 , which was well fitted with a single-exponential fitting.The absolute PLQYs were determined by double-curve methods in an integrated sphere (SM4): two spectra (with and without a sample in the integrated sphere) were measured, the excitation signal intensity of blank (EX blank ) and sample (EX sample ), as well as their emission signal intensity (EM blank and EM sample ) can be calculated.The PLQY of each sample was calculated by the following equation: Computational details: DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP) 6.1 code [80] with the projection-augmented wave (PAW) method.The plane-wave cutoff energy was set to 400 eV.The generalized gradient approximation (GGA) Perdew-Burke-Ernzerhof (PBE) [81] was used as the exchange-correlation functional for structural relaxation.Γ-centered k-meshes with k-spacing of 0.2 Å−1 were employed for sampling the Brillouin zones.The crystal structures were fully relaxed until the total force on each atom was <0.01 eV Å−1 .For the calculation of excited states, the structural optimizations and energy calculations were performed using the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. [82,83]The total energies of excited states were calculated by fixing the occupation numbers of the electron-and hole-occupied eigenlevels (Δ self-consistent field (ΔSCF) method).
Following the Frank-Condon principle, the energy differences between excited states and ground states for both free excitons and self-trapped excitons are the excitation and emission energy, respectively.WLED fabrication: A WLED was fabricated using a blue-GaN chip (450 nm) as the excitation source and (Ph 3 MeP) 2 [Cu 4 I 6 ] as the yellow phosphor.The proper amounts of phosphor were mixed with resin, and the obtained mixture was coated on the chip.The PL spectra, CCT, and CIE color coordinates of the device were tested using an integrating sphere spectroradiometer system (ATA100, Everfine).

Figure 1 .
Figure 1.a) Molecular structure of the [Ph 3 MeP] + cation and the simple quasicubic packing of the cation pairs.b) Configuration of [Cu 4 I 6 ] 2− anion and the simple quasicubic packing of the [Cu 4 I 6 ] 2− anions.c) CsCl-type staking of the Ph 3 MeP + cation pairs and the [Cu 4 I 6 ] 2− anions for (Ph 3 MeP) 2 Cu 4 I 6 .d) Molecular structure of the [Cy 3 MeP] + cation and the face-centered cubic packing of the cation pairs.e) Configuration of [Cu 4 I 6 ] 2− anion and the face-centered cubic packing of the [Cu 4 I 6 ] 2− anions.f) NaCl-type staking of the Ph 3 MeP + cation pairs and the [Cu 4 I 6 ] 2− anions for (Cy 3 MeP) 2 Cu 4 I 6 .The blue, purple, brown, and red spheres indicate Cu, I, C, and P atoms, respectively.The half-filled blue spheres indicate that the Cu atoms are half-weighted.The hydrogen atoms are omitted for clarity.The blue octahedra represent [Cu 4 I 6 ] 2− clusters.

Figure 2 .
Figure 2. a, b) PLE spectra monitored at 650 nm emission (blue solid lines), PL spectra excited under 450 nm excitation (orange solid lines), and absorption spectra (black dashed lines) of a) (Ph 3 MeP) 2 Cu 4 I 6 and b) (Cy 3 MeP) 2 Cu 4 I 6 .The green and red dashed lines represent the contributions from the FE emissions and the STE emissions, respectively.c, d) TRPL spectra of c) (Ph 3 MeP) 2 Cu 4 I 6 and d) (Cy 3 MeP) 2 Cu 4 I 6 .The green and red dots represent FE emissions monitored at 540 nm at 80 K and the STE emissions monitored at 650 nm at room temperature, respectively.
shows the qualitative chemical bonding diagrams for the [Cu 4 ]-centered [Cu 4 I 6 ] 2− anions and the common Cu + -centered polyhedral anions represented a tetrahedral [CuI 4 ] 3− anion.For the common [CuI 4 ] 3− anion, the lowest unoccupied molecular orbital (LUMO) is directly derived from the antibonding state of the 4s orbital of the central single Cu + cation and the I 5p orbitals.For the [Cu 4 ]-centered [Cu 4 I 6 ] 2− anion, the LUMO is formed by two steps.Firstly, the 4s orbitals of the four Cu atoms produce a low-lying singlet bonding state

Figure 3 .
Figure 3. a) Calculated bandgaps of (Ph 3 MeP) 2 Cu 4 I 6 and (Cy 3 MeP) 2 Cu 4 I 6 , along with other hybrid copper(I) iodides with the common Cu-centered polyhedral anions.b) Calculated band structure of (Ph 3 MeP) 2 Cu 4 I 6 and isosurface plots of charge density corresponding to VBM and CBM.c) Qualitative molecular orbital diagram for [CuI 4 ] 3− and [Cu 4 I 6 ] 2− anions.The energy levels are qualitatively arranged on the energy scale.

Figure 4 .
Figure 4. a) Schematic diagram of the dual-emission mechanism.b-d) Spinpolarized DOSs of b) the ground state, c) triplet FE state, and d) triplet STE state in (C 19 H 18 P) 2 Cu 4 I 6 .Inset: calculated local structure of the [Cu 4 I 6 ] 2− anions, respectively.

Figure 5 .
Figure 5. a) Emission spectrum and b) CIE chromaticity coordinate of the WLED based on the (Ph 3 MeP) 2 Cu 4 I 6 phosphor.