Electronic structural transition. Reaction of organophosphate terminal ligand (bpp) with CuI under a solvothermal condition yielded colorless block crystals of bppCu4I4P4 (Supplementary Fig. 2). The single-crystal sample was phase-pure, as evidenced by powder XRD pattern and element microanalysis. Given that the two phosphine atoms on the terminals of bpp can link the neighboring clusters through strong Cu − P bonds, the thermal stability of the crystalline bppCu4I4P4 sample was significantly enhanced (Supplementary Fig. 3), as compared to those of the discrete structures.43,44 Single-crystal X-ray diffraction at 100 K revealed that bppCu4I4P4 crystallizes in the monoclinic P2/c space group and the asymmetric unit contains one and two halves of crystallographically distinct Cu4I4 clusters and four bpp ligands, respectively (Fig. 1 and Supplementary Table 1). In the structure of bppCu4I4P4, the cubic Cu4I4 clusters are interlinked via terminal Cu − P bonds by a pair of bpp ligands, giving rise to a one-dimensional (1D) cluster chain. The 1D chains are further extended into two-dimensional framework by π…π stacking interactions with the shortest interatomic distance of 3.465 Å. These intermolecular interactions synergistically rigidify the metal‒organic framework, which favors the additional structural stability at high temperatures.45–51
Both single-crystal and powder XRD patterns of bppCu4I4P4 revealed a reversible structural transformation with changing temperature (Fig. 2a and Supplementary Fig. 4, 5). At 298 K, the diffraction at ∼10o shows a distinct doublet peak, while they began to merge with increasing temperature. The structural change at high temperatures was confirmed by the shifts in the diffraction peaks of ∼14.8, 23.2 and 30.8o to the lower Bragg angles, as compared to the structure at room temperature (R.T.). Electronic absorption spectra at different temperatures indicated that the ligand’s electronic structure was undisturbed upon heating (Supplementary Fig. 6). The structural transition is related to the elongation of intracluster Cu − Cu distance in the crystal lattice. As the temperature increases, the excess thermal energy drives the weakening of intermetallic bonding interaction and the elongation of Cu − Cu distance. Raman spectra in the region of 500 − 3500 cm− 1 turned out to be similar at different temperatures (Supplementary Fig. 7), which correspond to the phosphine ligand, in accordance with IR analyses. In the low-frequency region, the bands below 100 cm− 1 are attributed to normal vibrational modes of Cu − I stretching. The additional band at 130 cm− 1 is responsible for Cu − Cu vibrations (breathing). The weak peak at 220 cm− 1 is assigned to the Cu − P stretching. These observations are consistent with both the experimental and calculated results on these cubane clusters.52 With increasing temperature above 300 K, the band below 100 cm− 1 continues to grow in its strength, especially, a new peak appears at ca. 35 cm− 1, alongside with the reduction of the band at 130 cm− 1. These results indicated that thermal stimuli essentially impact the intracluster bonding interactions and give rise to the stronger Cu − I ionic bonds and weaker coprophilic Cu − Cu interactions.
To probe the temperature dependence of coprophilic Cu − Cu interactions, the X-ray diffraction analyses were performed on an individual single crystal in the temperature sequence of 100→200→250→300→325→350→375→400→100 K. The space group changed from P2/c at 100 K to C2/c at temperatures higher than 200 K. The shortest Cu‒Cu bond distances are provided in Fig. 2b and the results are completely reversible in heating/cooling cycles (Supplementary Table 2). While the Cu − P and Cu − I bond distances are quite similar at different temperatures, the Cu − Cu interactions are clearly temperature-dependent. At 100 K, the Cu − Cu distances, such as 2.7418(7) Å for Cu1 − Cu3 and 2.7570(6) Å for Cu5 − Cu6, in the bppCu4I4P4 cluster are shorter than the sum of the van der Waals radii of copper(I) (2.80 Å), suggesting significant cuprophilic bonding interaction (d10 − d10) at this temperature. Upon heating the crystal, the Cu − Cu distance shows a quasi-linear increase and approaches beyond the cuprophilic bonding interaction. The results suggested that the heating can gradually induce the weakening of Cu − Cu bonding interaction (Fig. 2c).
To investigate the dynamic electronic structure induced by the weakening of bonding interactions, solid-state 63Cu WURST-QCPMG and CPMAS 31P NMR spectra were recorded at different temperatures. The compound of bppCu4I4P4 exhibited a particularly wide span of 63Cu static NMR spectra at 298 K, as compared to the similar [Cu4I4] cubane structure,52,53 and hence the individual components were poorly resolved (Fig. 2d). As temperature increased, the 63Cu NMR patterns began to be weak. At 373 K, the 63Cu NMR signals cannot be detected, which is likely induced by the paramagnetic Cu(II) or free electrons. The assumption was further supported by the 31P NMR spectra. Considering the J couplings between one 31P atom and two copper isotopes (65Cu and 63Cu of I = 3/2 with natural abundance of 30.8% and 69.2%, respectively), four quartets can be expected with two crystallographically independent phosphorus sites in bppCu4I4P4. As shown in Supplementary Fig. 8, bppCu4I4P4 exhibited highly overlapped 31P NMR resonances at 298 K due to quite similar phosphorus environment, which originates from the J couplings of 31P−63Cu and 31P−65Cu. We attempted to deconvolute the 31P NMR spectrum using a dmfit software, but the software algorithm was limited in the presence of paramagnetic species. The 31P NMR spectrum at 323 K showed an electronic structure similar to that of 298 K with a slight shift and broadening of the peaks. However, upon further heating the sample to 373 K, the increasing amount of paramagnetic species resulted in a significant broadening of its half peak width and a decrease of J coupling. This result highlighted the sensitivity of the MAS 31P NMR technique to detect the temperature dependence of electronic structures, thereby allowing us to state that the heating treatment can effectively induce electronic structural transition. As the Cu − Cu interactions in the excited states are of bonding character, the PL emission should be correlated to the Cu − Cu distances in the discrete metallic clusters.54
Zero-TQ property and mechanism. The PL spectra of bppCu4I4P4 at 298 K revealed a single broad yellow emission band with a peak at ca. 600 nm arising from the cluster-centered (3CC) low-energy (LE) band common for [Cu4I4L4]-type clusters,55–59 with a full-width at half-maximum (FWHM) of ∼84 nm under 355 nm excitation and an absolute PL quantum yield (PLQY) of ~ 80% (Fig. 3a and Supplementary Fig. 9). While some spectral parameters (including FWHM and PLQY) of bppCu4I4P4 are close to those of commercial yellow phosphors (such as YAG:Ce3+ and (Sr,Ba)2SiO4:Eu2+), the emission spectra of bppCu4I4P4 showed a sufficient red component. Thus, the bppCu4I4P4-based wLED device is expected to have high color rendering index (CRI) (> 80) and low correlated color temperature (CCT) (< 4500 K). The strong electronic absorption in the near-ultraviolet (NUV) as well as its intense yellow emission make it potential as an NUV white LED.60–64 Upon heating from R.T., bppCu4I4P4 exhibited a thermally stable emission (i.e., zero-TQ PL), without obvious emission loss up to a temperature of 378 K under λex = 355 nm (Fig. 3c,d). The variation of the Cu–Cu bond distances seemed to be insufficient to induce modification of the corresponding emissive state. The zero-TQ PL was also reflected in the temperature-dependent PL excitation spectrum (Supplementary Fig. 10).
To reveal the nature of excited states, the time-resolved PL spectroscopy was conducted at 298 K (Supplementary Fig. 11). The LE-band decay curve of bppCu4I4P4 can be well fitted by a single-exponential function, yielding a lifetime (τ) of 5.0 µs, which suggests that an efficient phosphorescence occurs at R.T. To further unveil the nature of excited states, temperature-dependent femtosecond time-resolved transient absorption (fs-TA) spectra were recorded (Supplementary Fig. 12). The fs-TA kinetic decays of bppCu4I4P4 turned out to hold a nearly identical constant of ca. 5.0 µs in the range of 78 − 350 K (Fig. 3e). Such a temperature independence precludes the possibility of thermally activated delayed fluorescence and confirms that a 3CC excited state is operative in the detected temperature region.
To gain deeper insights into the mechanism behind the TQ effect, electron paramagnetic resonance (EPR) spectroscopy as a sensitive technique was undertaken in situ at varied temperatures (Fig. 4a). At 298 K, the complex of bppCu4I4P4 exhibited a weak and broad EPR absorption spectra. As the temperature increased, however, the absorption of EPR spectra became intense and narrow in linewidth, echoing well to the NMR result mentioned above. These observations demonstrated that Cu‒Cu bonding interaction at 298 K can lead to the localized bonding electron pairs on the 3d- and 4s-orbitals following orbital hybridization according to the classical valence-bonding theory, as illustrated in Fig. 4b. However, in the absence of appreciable d‒d interaction, the 3d-electrons are expected to be unpaired and hence poorly bonding (or with free electrons emerging). Accordingly, the spin state of the Cu ion is sensitive to the Cu‒Cu distance. Indeed, as the Cu‒Cu distance continued to increase upon heating, the EPR signal at 373 K began to show the characteristic of Cu2+ complex with an isotropic g ~ 2.1.65 The result is reminiscent of some Cu complexes in which spin flipping was observed both experimentally and computationally upon increasing (ca. 0.1 Å) in the Cu‒Cu distance.66,67
According to the energy level diagram for the lowest states, the Cu4I4P4 cluster exhibits two separate excited states of 3CC (T1) and 3MLCT/3XLCT (T2) below R.T. (Supplementary Fig. 13). As temperature increases from 78 K to R.T., the 3CC state is progressively populated at the expense of 3XLCT/3MLCT thanks to the increase of vibrational energies. The stabilized PL intensity from R.T. to 100 oC under 355 nm excitation was the consequence of weak intermetallic bonding interaction.68 The Cu–Cu elongation is not sufficient to induce changes in the energy of the T1 state and consequently of the LE emission band. However, the higher temperature (> 373 K) facilitates electronic structural transition from the bonding state to the non-bonding state, i.e., the release of bonding electrons, and a high concentration of Cu2+ species arises as a consequence of spin flipping. These paramagnetic species can be effectively transmitted between the neighboring clusters through covalent-linked cluster polymerization, giving rise to an emission reduction above 100 oC.
Performance of wLEDs. High-power LEDs lighting is a crucial challenge due to the significant TQ under a high flux operating current. Considering the unusual zero-TQ property in the wide temperature range, we investigated the high-power LEDs performance of the bppCu4I4P4 phosphor with an NUV LEDs chip (λmax = 365 nm). The results of single-component and white-light LEDs are shown in Fig. 5 and Supplementary Fig. 14. The electroluminescence (EL) intensities of the yellow-emitting bppCu4I4P4 phosphor and commercial yellow-emitting phosphors (such as YAG:Ce3+ and (Sr,Ba)2SiO4:Eu2+) increased with a low current below 300 mA (Fig. 5a,b and Supplementary Fig. 15). The bppCu4I4P4-based LED exhibited superior EL intensities above 300 mA, while the EL intensities of commercial (Sr,Ba)2SiO4:Eu2+ decreased above 300 mA, due to the serious TQ effect (Supplementary Table 3). For bppCu4I4P4-based white LED (BAM:Eu2+ as the blue component), the EL spectra covered the whole visible region from 400 to 750 nm (i.e., a white-light emission) and the EL intensities increased in the range of 100–1,000 mA. In contrast, the YAG:Ce3+-based wLED showed a reduced EL, especially at high currents (Fig. 5c).
To demonstrate the potential of practical applications as wLED devices, the colour stability during high-power LED operation were recorded.69 The CIE x (ca. 0.35) of bppCu4I4P4-based wLED remained basically unchanged, despite CIE y showed a slight shift from 0.31 to 0.35 in the range of high flux current of 100–1,000 mA (Fig. 5d and Supplementary Table 4 − 6). The CRI value of bppCu4I4P4-based wLED was as high as 90 with a CCT of 4809 K at a flux current of 1,000 mA. The low CCT and high CRI value showed the excellent optical properties of cluster-based phosphor at a high flux current. The fabricated bppCu4I4P4-based LED (Fig. 5e) exhibited a bright yellow emission at an applied current of 1,000 mA. It should be noted the wLED exhibited comparable white emission to the commercial YAG:Ce3+ (Fig. 5f,g). These results demonstrated that bppCu4I4P4 is a robust phosphor that can well function against the TQ effect, making it potentially ideal as a yellow-emitting phosphor for high-power white LEDs lighting.