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Research (Wash D C)
Research (Wash D C)
RESEARCH
Research
2639-5274
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10.34133/research.0005
0005
Research Article
Sky Blue and Yellow Cluster Light-Emitting Diodes Based on Asymmetric Cu4I4 Nanocubes
Zhang Nan 1 †
Qu Lei 1 †
Hu Huan 1
Huo Ran 1
Meng Yushan 1
Duan Chunbo 1
Zhang Jing 1 *
Han Chunmiao 1
Xie Guohua 2 *
Xu Hui 1 *
1 Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education) and School of Chemistry and Material Science, Heilongjiang University, 74 Xuefu Road, Harbin 150080, P. R. China.
2 Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, P. R. China.
* Address correspondence to: zhangjing@hlju.edu.cn (J.Z.); guohua.xie@whu.edu.cn (G.X.); hxu@hlju.edu.cn (H.X.)
† These authors contributed equally to this work.

15 12 2022
2022
2022 000512 6 2022
19 10 2022
Copyright © 2022 Nan Zhang et al.
2022
Nan Zhang et al.
https://creativecommons.org/licenses/by/4.0/ Exclusive licensee Science and Technology Review Publishing House. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution License 4.0 (CC BY 4.0).

Controllably optimizing excited-state characteristics is crucial for luminescent nanoclusters but remains a formidable challenge. Herein, we report an effective “ligand-induced asymmetrization” strategy for constructing thermally activated delayed fluorescence-featured cubic Cu4I4 nanoclusters with asymmetric configurations, named [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4. Through changing 3,6-di-tert-butyl-carbazole (tBCz) to phenothiazine (PTZ) with a stronger electron-donating effect, emission color is tuned from greenish blue of [tBCzDBFDP]2Cu4I4 to yellow of [PTZDBFDP]2Cu4I4, as well as the triplet locally excited state of the former to the triplet charge transfer state of the latter. Temperature-correlated spectroscopic investigation indicates that in terms of triplet quenching suppression, [tBCzDBFDP]2Cu4I4 is superior to [PTZDBFDP]2Cu4I4, in accord with the stabilities of their triplet locally excited state and triplet charge transfer state. As a consequence, these asymmetric Cu4I4 nanocubes endowed their cluster light-emitting diodes with the external quantum efficiencies beyond 12% for sky blue and 8% for yellow. These results suggest the significance and effectiveness of ligand engineering for optoelectronic nanoclusters.
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pmcIntroduction

Luminescent nanoclusters emerge rapidly in recent decades [1–3]. This kind of material features inorganic nano-skeletons stabilized with organic ligands, thus combining the advantages of organic molecules with high luminescence efficiencies and processibility, and inorganic motifs with high rigidity, unique electronic structures, and excellent thermo- and photostability [1,2,4]. The structural diversity of nanoclusters provides flexibility in exactly modulating shape, size, organic–inorganic ratio, and so on [5–7]. Therefore, luminescent nanoclusters are widely used in bioimaging [8], sensing/detection [9–11], light conversion [12–14], etc. For such organic–inorganic hybrids, functions of ligands are already beyond stabilizing cluster cores, since it is demonstrated that radiative transitions of clusters are predominantly attributed to ligand-centered excited-state components, e.g., intraligand charge transfer(ILCT), counterion-to-ligand charge transfer(XLCT), and ligand locally excited state [15]. Furthermore, our recent study on cluster semiconductors indicated the significant contributions of aromatic ligands to electrical properties [16]. Therefore, ligand engineering provides an effective approach to modulate the comprehensive properties of nanoclusters.

However, the applications of cluster materials in optoelectronic devices still face a big challenge in multifunctional integrity. For example, until now, the electroluminescence (EL) performance of cluster light-emitting diodes (CLEDs) is still far behind that of other kinds of light-emitting devices. In particular, the external quantum efficiencies (EQE, ηEQE) of CLEDs were mostly less than 10% [17], in contrast to the state-of-the-art value of ~20% for organic light-emitting diodes [18,19], quantum-dot light-emitting diodes [20], and perovskite light-emitting diodes [21,22]. On one hand, the existence of triplet quenching states, especially the triplet cluster-centered state [23–26], makes photoluminescence quantum yield (ϕPL) commonly less than 50% [7,27–30]. On the other hand, despite improving stabilities, rigid cluster structures render aggregation-induced quenching and poor film formability [31]. Therefore, most CLEDs adopted low doping concentrations within 10%, which not only reduces optimization space but also increases fabrication difficulty [32,33].

Copper clusters hold a promise for large-scale luminescent applications, owing to their high ϕPL, low cost, and environmental friendliness [34]. More importantly, phosphine-chelated copper nanoclusters reveal dominant EL performance among cluster emitters [17,35]. Our group demonstrated the first Cu4I4-based white CLED, despite a low ηEQE of 0.7% [36], which already exceeded the values of gold, silver [33], and molybdenum [32] cluster-based devices. Through introducing donor groups in bidentate phosphine ligand, we recently maximized ligand-centered excited-state components to achieve the highest ηEQE of 7.9% among Cu4I4 clusters [37]. Olaru et al. [38] reported another cationic green Cu4 cluster with a record ηEQE of 11.0% for copper CLEDs. Nonetheless, copper CLEDs still face the same challenges in multifunction balance and quenching suppression, as well as full-color emission.

Herein, an effective “ligand-induced asymmetrization” strategy is demonstrated with 2 diphosphine chelated Cu4I4 clusters, namely, [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4, whose ligands are respectively monofunctionalized with 3,6-di-tert-butyl-carbazole (tBCz) and phenothiazine (PTZ) (Schemes S1 and S2). Both clusters reveal the ligand-centered excited states with negligible single-triplet splitting energies (ΔEST), rendering their typical thermally activated delayed fluorescence (TADF) features. Using PTZ with a stronger donating effect instead of tBCz makes the emission peak wavelength of [PTZDBFDP]2Cu4I4 shift to ~530 nm, corresponding to yellow color, compared to the greenish blue emission of [tBCzDBFDP]2Cu4I4 that peaked at ~495 nm. Enhanced ILCT further induces stronger concentration dependence of the emission properties for [PTZDBFDP]2Cu4I4. Nonetheless, dispersing the clusters in a host matrix (bis-4-(N-carbazolyl)phenyl)phenylphosphine oxide [BCPO]), [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 achieved state-of-the-art ϕPL values of 68% and 45%, and state-of-the-art ηEQE values of 12.2% and 8.6% for copper CLEDs, respectively, at extremely high doping concentrations of 40% and 30%.

Results

Molecular design, structures, and theoretical simulation

Dibenzofuran (DBF) is chosen as the skeleton and electron-withdrawing group, which is disubstituted with diphenylphosphine (DP) at 4,6-positions to form bidentate coordination mode, which is further used to chelate Cu4I4 nanocubes (Fig. 1A). This orthogonal structure renders extremely high rigidity and stability for the clusters. As a consequence, [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 reveal the temperatures of decomposition (Td) beyond 420 oC (Fig. S1 and Table S1). Furthermore, single tBCz or PTZ is introduced at the 2-position of DBF. Despite plane symmetries of tBCzDBFDP and PTZDBFDP, monofunctionalization of 2 orthogonal ligands leads to asymmetric configurations of the clusters, which effectively suppress intermolecular regular packing. Therefore, no melting points (Tm) are observed, but morphological stabilities of solid-state clusters are verified by their extremely high temperatures of glass transition (Tg) over 290 oC. The clusters have good solubility in common solvents and therefore can be processed with wet approaches, e.g., spin coating, ink printing, and so on, making them competent for large-scale production. The spin-coated films show uniform and smooth morphology with roughness <1 nm (Fig. S2).

Fig. 1. Molecular structure and electronic characteristics of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4. (A) Asymmetric molecular structures of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4. (B) Time-dependent density functional theory analysis of the singlet and triplet excitations of the clusters. “Hole” and “particle” are the highest occupied and lowest unoccupied natural transition orbitals, respectively. S0, S1, and T1 refer to the ground and the first singlet and triplet excited states, respectively. LE and CT refer to locally excited and charge transfer states, respectively. E, δ, f, and ΔEST refer to energy level, contribution weight, oscillator strength, and singlet-triplet splitting energy, respectively. dH-L, 〈ΨH|ΨL〉, and 〈ΨH2|ΨL2〉 are distance, wave function integral, and electron-cloud-density integral of the “hole” and “particle”, respectively. Superscripts “1” and “3” and subscripts “S” and “T” refer to singlet and triplet states, respectively. Superscript “*” refers to excited state.

Density functional theory simulation shows that for [tBCzDBFDP]2Cu4I4, its first 3 highest occupied molecular orbitals (HOMOs) are localized on Cu4I4, whose 80% are contributed by iodine atoms (Fig. S3). [PTZDBFDP]2Cu4I4 reveals the similar locations of its HOMO and HOMO-2, except for minor but sustainable dispersions on PTZ groups. Its HOMO-1 is even completely localized on PTZ. Meanwhile, the first 3 lowest unoccupied molecular orbitals (LUMOs) of [PTZDBFDP]2Cu4I4 symmetrically distribute on DBF groups of both ligands. On the contrary, the LUMO and LUMO+1 of [tBCzDBFDP]2Cu4I4 are located on single DBF groups, while its LUMO+2 is instead localized on tBCz. In accord with calculated data, the HOMO and LUMO energy levels of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 are respectively estimated as −5.59/−3.15 eV and −5.42/−3.34 eV with cyclic voltammetry (Fig. S4 and Table S1).

Nature transition orbital (NTO) analyses on the S0→S1 and S0→T1 excitations show that the “hole” and “particle” of the singlet excitations for [tBCzDBFDP]2Cu4I4 are respectively localized on iodine atoms and the DBF group, corresponding to the XLCT-predominant first singlet excited state (S1) (Fig. 1B). In comparison, despite the same “particle” location on DBF, the “hole” of the singlet excitation for [PTZDBFDP]2Cu4I4 is dispersed on iodine atoms and PTZ groups with ratios of 70% and 30%, respectively, corresponding to the XLCT/ILCT hybrid S1 state. Therefore, although the more uniform “hole” distribution of [PTZDBFDP]2Cu4I4 renders its smaller distance between “hole” and “particle” (dH-L) and larger wave function integral (〈ΨH|ΨL〉) and electron-cloud-density integral (〈ΨH2|ΨL2〉) of “hole” and “particle”, its singlet oscillator strength (fS) is still smaller than that of [tBCzDBFDP]2Cu4I4. The first triplet excited state (T1) of [PTZDBFDP]2Cu4I4 also features XLCT/ILCT hybrid, nearly identical to its S1 state. On the contrary, the T1 state of [tBCzDBFDP]2Cu4I4 is a locally excited state with overlapped “hole” and “particle” on tBCz. Therefore, intramolecular charge transfer (CT) interaction in [PTZDBFDP]2Cu4I4 is stronger, leading to the reduced S1 and T1 energy levels and ΔEST.

Photophysical properties

In dilute solutions, the clusters display similar electronic absorption bands that peaked around 230, 290, and 350 nm, corresponding to π→π*, n→π*, and ILCT transitions, respectively (Fig. S5). The absorption bands of ligands and clusters are similar, verifying the antenna effect of ligands in energy absorption of clusters (Fig. S6a). Nonetheless, in contrast to the CT-predominant excitation of PTZDBFDP, excitation bands of tBCzDBFDP are composed of locally excited and CT components, due to the weaker electron-donating effect of carbazole. The ligand-originated excitation bands are secondary in the spectra of clusters, in which a long-wavelength band attributed to MLCT, XLCT, and coordination-enhanced ILCT becomes primary. Compared to tBCzDBFDP, the stronger intramolecular CT in PTZDBFDP is verified by its emission red-shifted by 80 nm (Fig. S6b). Coordination with Cu4I4 also enhances CT interactions, resulting in the similar red shifts by 64 and 37 nm for [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4, respectively. In neat films, all the absorption peaks are enhanced due to intermolecular interactions (Fig. 2a). In particular, weak long absorption tails are observed in the range of 450 to 600 nm, ascribed to XLCT transitions. The steady-state photoluminescence (PL) spectrum of the neat [tBCzDBFDP]2Cu4I4 film nearly overlapped with that of BCPO:40% [tBCzDBFDP]2Cu4I4, whose peak wavelengths are at ~495 nm (Table). In contrast, the neat [PTZDBFDP]2Cu4I4 film reveals orange emission that peaked at 547 nm, but the peak wavelength of BCPO:30% [PTZDBFDP]2Cu4I4 shifts to 523 nm, corresponding to yellow emission. After doping in BCPO matrix, [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 realize high ϕPL values of 68% and 45%, respectively, which are outstanding among Cu4I4 clusters.

Fig. 2. Photophysical properties of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4. (A) Electronic absorption and steady-state photoluminescence (PL) spectra of BCPO:x% [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 films. BCPO is bis-4-(N-carbazolyl)phenyl)phenylphosphine oxide used as matrix. x = 100 for neat films (hollow symbols), 40 for the [tBCzDBFDP]2Cu4I4-doped film (blue dashed line), and 30 for the [tBCzDBFDP]2Cu4I4-doped film (orange dashed line). a.u., arbitrary units. (B) Time decays (left) of BCPO:x% [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 films in atmosphere (lines) and vacuum (hollow symbols), and prompt fluorescence (PF), delayed fluorescence (DF), and phosphorescence (PH) spectra. (C) TRES of BCPO:x% [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 films recorded at 50, 100, 200, and 300 K, respectively. (D) Contours of temperature-dependent steady-state PL spectra for BCPO:x% [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 films recorded at the temperature range of 20 to 300 K with an interval of 10 K.

Table. Physical properties of the Cu4I4 clusters

Cluster	λPL (nm)	S1 (eV)	T1 (eV)	ΔESTa (eV)	ϕPLb (%)	ηEQEc (%)	ηEUEd (%)	
[tBCzDBFDP]2Cu4I4	495e	2.77f	2.70f	0.07f	68eg	12.2g	81	
498eg	2.55h	2.50i	0.05hi					
[PTZDBFDP]2Cu4I4	547e	2.72f	2.65f	0.07f	45eg	8.6g	76	
523eg	2.38h	2.34i	0.04hi					
aSinglet-triplet splitting. bPhotoluminescence quantum yields of BCPO:40% [tBCzDBFDP]2Cu4I4 and BCPO:30% [PTZDBFDP]2Cu4I4 films. cExternal quantum efficiencies of CLEDs based on BCPO:40% [tBCzDBFDP]2Cu4I4 and BCPO:30% [PTZDBFDP]2Cu4I4. dExciton utilization efficiencies of CLEDs based on BCPO:40% [tBCzDBFDP]2Cu4I4 and BCPO:30% [PTZDBFDP]2Cu4I4. eIn film. fGaussian simulation results of single molecules. gFor BCPO:40% [tBCzDBFDP]2Cu4I4 and BCPO:30% [PTZDBFDP]2Cu4I4. hEstimated according to 0–0 transition of prompt fluorescence. iEstimated with 0–0 transition of phosphorescence.

In dilute toluene, [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 reveal the microsecond time decays, in comparison to nanosecond decays of the corresponding ligands (Fig. S7). [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 neat films reveal similar time decay curves in the microsecond range (Fig. 2B). It is noted that in vacuum, lifetimes of the films are markedly elongated, indicating that the emissions originated from triplet states. Moreover, emission of the [PTZDBFDP]2Cu4I4 film is significantly more sensitive to oxygen than the [tBCzDBFDP]2Cu4I4 film. It is shown that lifetimes of the BCPO:x% cluster are basically in reverse proportion to x, since BCPO matrix can also alleviate collision-induced concentration quenching, e.g., triplet–triplet annihilation (Figs. S8 and S9). BCPO matrix effectively restrains oxygen exposure-induced quenching. Therefore, time decays of the BCPO:40% [tBCzDBFDP]2Cu4I4 film in air and vacuum nearly overlapped. However, BCPO:30% [PTZDBFDP]2Cu4I4 still reveals considerably elongated decay in vacuum. It means that the triplet locally excited state of [tBCzDBFDP]2Cu4I4 is more stable than the triplet CT state of [PTZDBFDP]2Cu4I4. With the time-resolved method, prompt fluorescence, delayed fluorescence, and phosphorescence spectra of the clusters almost overlapped, corresponding to near-zero ΔEST within 0.05 eV (Table S1), which are in accord with NTO results. Compared to tBCzDBFDP with vibrational shoulder peak, the prompt fluorescence profile of [tBCzDBFDP]2Cu4I4 is markedly narrowed, owing to enhanced molecular rigidity.

Time-resolved emission spectra (TRES) of [tBCzDBFDP]2Cu4I4 in dilute toluene solutions further demonstrate the rigid enhancement by cluster formation markedly narrow profile (Fig. S11). Temperature-dependent TRES (TDTRES) of BCPO:x% [tBCzDBFDP]2Cu4I4 films show that, at x = 100, increasing temperature from 50 to 300 K makes emission contours gradually shortened from 100 to 50 μs, while at x = 40, TDTRES is nearly unchanged from 50 to 200 K, but shortened at 300 K, indicating the balance between triplet quenching and radiation acceleration and the effect of BCPO on quenching suppression (Fig. 2C and Figs. S8 and S9). In contrast, for BCPO:x% [PTZDBFDP]2Cu4I4 films, besides alleviated triplet quenching at x = 30, increasing temperature induces markedly shortened contours from 10 ms to hundreds of microseconds, revealing radiation shift from the forbidden triplet state to allow singlet state at 300 K for [PTZDBFDP]2Cu4I4. Temperature-correlated PL spectra further show that compared to neat films, PL intensities of BCPO:40% [tBCzDBFDP]2Cu4I4 at 200 to 300 K largely increase, manifesting the effect of BCPO host on quenching suppression and radiation facilitation (Fig. 2D and Fig. S10). The influence of BCPO on [PTZDBFDP]2Cu4I4-based films is more remarkable. Compared to largely decreased PL intensities of neat films at temperature >50 K, BCPO:30% [PTZDBFDP]2Cu4I4 achieves the bigger PL intensities in the range of 200 to 300 K, further manifesting its TADF-predominant radiative feature. As a consequence, ϕPL values of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 are more than twice of those of their ligands.

CLED performance

CLEDs based on [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 were fabricated through spin coating with a conventional trilayer structure of indium tin oxide|poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (40 nm)|BCPO:x% cluster (40 nm)|bis(2-(diphenylphosphino)phenyl) ether oxides (10 nm)|1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (50 nm)|LiF (1 nm)|Al (100 nm) (Fig. 3A). The doping concentration x% was tuned to achieve the optimal device performance (Figs. S11 and S12). It is shown that increasing x% induced EL emission red-shifted by 4 to 12 nm (Table S2). At the optimal x of 40 and 30 for [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4, respectively, their devices achieved the highest performance. The [tBCzDBFDP]2Cu4I4-based device displayed greenish blue emission that peaked at 490 to 500 nm with Commission Internationale de lEclairage coordinates of (0.21 ± 0.1, 0.43 ± 0.1), while the yellow EL emission from [PTZDBFDP]2Cu4I4 peaked at 532 nm, corresponding to Commission Internationale de lEclairage coordinates of (0.34, 0.56) (Fig. 3b). Despite its deeper LUMO and shallower HOMO, the driving voltages of [PTZDBFDP]2Cu4I4-based devices were still higher than those of [tBCzDBFDP]2Cu4I4-based analogs, due to relative low radiative efficiency (Fig. 3c). It indicates that host–dopant energy transfer and exciton migration are the predominant EL mechanisms. The CLEDs revealed the duration similar to previously reported spin-coated devices of copper-based materials [39], due to the intrinsic disadvantages of the spin-coating technique such as simple stacks and encapsulation.

Fig. 3. EL performance of CLEDs. (A) Device structure and energetic diagram of spin-coated CLEDs and molecular structures of employed BCPO host, bis(2-(diphenylphosphino)phenyl) ether oxide (DPEPO) as exciton-blocking layer, and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) as electron-transporting layer. Carrier injection, transportation, and recombination routines were marked with arrows. ITO, indium tin oxide. PEDOT:PSS, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). (B) Luminance–current density (J)–voltage curves and EL spectra (inset) of the devices with optimal doping concentrations as 40% for [tBCzDBFDP]2Cu4I4 and 30% for [PTZDBFDP]2Cu4I4, respectively. (C) Efficiency–luminance curves of [tBCzDBFDP]2Cu4I4-based (square) and [PTZDBFDP]2Cu4I4-based (circle) CLEDs.

More importantly, [tBCzDBFDP]2Cu4I4 endowed its CLEDs with state-of-the-art efficiencies up to 29.3 cd A−1 for current efficiency (CE, ηCE), 26.3 lm W−1 for power efficiency (CE, ηPE), and 12.2% for ηEQE, which are the state-of-the-art values of cyan CLEDs reported so far (Table S3). The maximum efficiencies of [PTZDBFDP]2Cu4I4-based CLEDs also reached 28.1 cd A−1, 22.0 lm W−1, and 8.6%, respectively, which are markedly higher than those of yellow mononuclear copper complexes [3]. It is noted that compared to the maximum ηEQE of ~1% for the parent cluster [DBFDP]2Cu4I4 [36], functionalization with tBCz and PTZ and asymmetric configurations result in efficiency improvements of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 by 12- and 8-fold. Furthermore, compared to its bifuncitonalized congener [DtBCzDBF]2Cu4I4 [37], the ϕPL of [tBCzDBFDP]2Cu4I4 is similar, but its ηEQE was improved by 50%. It indicates that asymmetrical configuration can further reduce the involvement of quenching states in the EL process. In contrast to our recently reported acridine-modified green Cu4I4 clusters [40], the cyan and yellow PL and EL emissions from [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 manifest the significant influence of the electron-donating effect on excited energy levels, which provides a facile approach for constructing full-color CLEDs. Taking the light out-coupling efficiency of indium tin oxide glass as 25%, the exciton utilization efficiency (EUE, ηEUE) of [tBCzDBFDP]2Cu4I4- and [PTZDBFDP]2Cu4I4-based CLEDs was 81% and 76%, respectively, which were higher than 70% for [DBFDP]2Cu4I4-based analogs [36]. Nonetheless, compared to ηEUE, the differences between ηEQE values of the CLEDs are markedly larger, which is attributed to the big ϕPL difference of the clusters. Therefore, it is convincing that excited-state optimization for simultaneous radiation acceleration and quenching suppression is the key factor determining both PL and EL performances of copper nanoclusters.

Discussion

In summary, 2 monofunctionalized diphosphine ligands are developed to demonstrate “ligand-induced asymmetrization” strategy for high-efficiency Cu4I4 clusters. Incorporating a single tBCz or PTZ group makes ligand-involved transitions predominant in excited states of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4, rendering near-zero ΔEST and typical TADF characteristics. Different intensities of electron-donating effects for tBCz and PTZ groups result in not only greenish blue and yellow emissions but also different triplet locally excited state and triplet CT state of the clusters. Nonetheless, both [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 achieve high ϕPL values of 68% and 45%, respectively. In comparison to the parent cluster [DBFDP]2Cu4I4, ηEQE values of [tBCzDBFDP]2Cu4I4- and [PTZDBFDP]2Cu4I4-based CLEDs were improved by 12- and 8-fold to new record values of 12.2% and 8.8%, respectively. These results suggest that excited-state modulation by ligand engineering is feasible and crucial for optimizing single-molecule and condensed-state properties of clusters. This work further solidly demonstrates great potentials of copper CLEDs in practical displaying and lighting applications.

Materials and Methods

Experimental details, thermal properties, density functional theory simulation, electrochemical and photophysical properties, and device performance are included in the Supplementary Materials.

Acknowledgments

Funding: This project was financially supported by the National Natural Science Foundation of China (92061205, 62175060, 51873056, 61905070, and 22005088), the Young Innovative Team Supporting Projects of Heilongjiang Province, Natural Science Foundation of Heilongjiang Province (YQ2020B006), the Postdoctoral Science Foundation of Heilongjiang Province (LBH-Q2116), and the Fund for Distinguished Young Scholars of Heilongjiang University (JCL202001). Author contributions: H.X., G.X., and J.Z. conceived the idea. N.Z., L.Q., H.H., R.H., Y.M., C.D., and C.H. performed the synthesis and device fabrication. N.Z., L.Q., C.D., J.Z., and C.H. performed measurement. All the authors discussed the data and wrote the paper together. Competing interests: The authors declare that they have no competing interests.

Data Availability

All other data are available from the authors upon reasonable request.

Supplementary Materials

Supplementary Materials Scheme S1. Synthetic procedures of [tBCzDBFDP]2Cu4I4.

Scheme S2. Synthetic procedures of [PTZDBFDP]2Cu4I4.

Fig. S1. TGA and DSC curves of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4.

Fig. S2. FMO energy levels and contours of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 simulated with the B3LYP/6-31G* method.

Fig. S3. CV curves of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 measured at room temperature with the scanning rate of 100 mV s−1.

Fig. S4. Electronic absorption spectra of [tBCzDBFDP]2Cu4I4 and [PTZDBFDP]2Cu4I4 in dilute dichloromethane (10−6 mol l−1).

Fig. S5. Doping concentration dependence of time decay curves for BCPO:x% [tBCzDBFDP]2Cu4I4 films (x = 20 to 50 and 100).

Fig. S6. Doping concentration dependence of time decay curves for BCPO:x% [PTZDBFDP]2Cu4I4 films (x = 20 to 50 and 100).

Fig. S7. Time decay curves of BCPO:x% [tBCzDBFDP]2Cu4I4 (x = 40 and 100 for neat film) in the temperature range from 20 to 300 K with an interval of 10 K.

Fig. S8. Time decay curves of BCPO:x% [PTZDBFDP]2Cu4I4 (x = 30 and 100 for neat film) in the temperature range from 20 to 300 K with an interval of 10 K.

Fig. S9. PL spectra of BCPO:x% [tBCzDBFDP]2Cu4I4 and BCPO:x% [PTZDBFDP]2Cu4I4 (x = 40 for the former, 30 for the latter, and 100 for neat films) in the temperature range from 20 to 300 K with an interval of 10 K.

Fig. S10. (a) EL spectra (inset) and current density (J)–voltage–luminance characteristics of BCPO:x% [tBCzDBFDP]2Cu4I4-based CLEDs at different x. (b) Efficiencies vs. luminance relationships.

Fig. S11. (a) EL spectra (inset) and current density (J)–voltage–luminance characteristics of BCPO:x% [PTZDBFDP]2Cu4I4-based CLEDs at different x. (b) Efficiencies vs. luminance relationships.

Table S1. Physical properties of the clusters.

Table S2. EL performance of CLEDs based on the clusters.
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