White Electroluminescent Phosphine-Chelated Copper Iodide

Aug 1, 2017 - Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Mat...
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White Electroluminescent Phosphine-Chelated Copper Iodide Nanoclusters Mingchen Xie,†,‡ Chunmiao Han,†,‡ Jing Zhang,† Guohua Xie,*,§ and Hui Xu*,† †

Key Laboratory of Functional Inorganic Material Chemistry (Ministry of Education) & School of Chemistry and Material Science, Heilongjiang University, Harbin 150080, China § Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, China S Supporting Information *

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opper(I) complexes have been extensively studied as luminescent materials for decades, with the advantages in low cost, high brightness and facile optical modulation.1 According to the number of Cu+ ions (n) in coordination center, this kind of materials can be broadly divided into nuclear/dinuclear complexes (n = 1, 2)2−9 and nanoclusters (n ≥ 3),10−13 accompanied by different physical properties. In recent years, along with the deeper understanding on the excited-state nature of mononuclear/dinuclear Cu+ complexes, especially their dual radiative singlet and triplet transitions for thermally activated delayed fluorescence (TADF) and phosphorescence (PH), respectively,14,15 their applications in organic light-emitting diodes (OLED) emerges.16,17 It is a common sense that this kind of Cu+ complexes can realize favorable electroluminescent (EL) performance when they were rigid enough to suppress excited-state structural relaxation.16,18 As an example, we constructed an efficient EL Cu+ complex TTPPCuI on the basis of a tridentate phosphine ligand, which dramatically enhanced not only structural rigidity but also spin−orbital coupling (SOC) to facilitate PH and realize the almost ideal dual emission.19 Actually, in comparison to mononuclear/dinuclear complexes, copper nanoclusters are superior in rigidity and therefore thermo- and photostability,12 which are intensively studied for diverse optical applications, e.g., surface coatings for full-color inorganic light-emitting devices (LED).12,20,21 However, no copper iodide clusters can be used as emitters in OLEDs to date. The main challenge is ascribed to their poor processability and weak electroactivity, which can be only overcome through ligand engineering.22 As a proof of concept, herein we report the first EL copper iodide cluster named [DBFDP]2Cu4I4 with OLED application (Figure 1a). Its chelate phosphine ligand 2,9-di(diphenylphosphine)-dibenzofuran (DBFDP) with the host feature not only improves solution processability and electroactivity of the cluster but also enhances dual emission characteristics under field, rendering the single-molecule white EL with CIE 1931 coordinates of (0.37, 0.45) and the excellent exciton utilization efficiency (EUE) up to ∼80% from its bilayer spin-coated OLEDs. DBFDP was designed with considerations of (i) its two P atoms with a P−P distance of 5.8 Å can provide strong coordination ability and appropriate space to chelate Cu4I4 cluster; (ii) its electron-deficient dibenzofuran (DBF) ring is widely employed as high-energy-gap chromophore in electrophosphorescent hosts with the functions of energy level © 2017 American Chemical Society

Figure 1. (a) Synthetic procedure of [DBFDP]2Cu4X4; (b) singlecrystal structure and packing diagram of [DBFDP]2Cu4I4.

modulation, electron transport enhancement and solubility improvement.23,24 Therefore, DBFDP can readily coordinate with CuI to generate desired Cu4I4 cluster with the yield beyond 90% (Figure 1a and experimental detail in Supporting Information). DBFDP endows the cluster with good solubility (>10 mg mL−1) in common solvents, e.g. chloroform, THF and chlorobenzene, for thin film preparation with solution processing. Single-crystal X-ray diffraction analysis shows the precise rigid structure of [DBFDP]2Cu4I4 as a Cu4I4 core chelated and embedded by two almost orthogonal DBFDP ligands with dihedral angle of ∼73° between their DBF rings (Figure 1b and Figure S1). Cu−P distances are ∼2.24 Å less than the common Cu−P bond length of 2.26 Å, reflecting the enhanced coordination strength. Furthermore, [DBFDP]2Cu4I4 has an elliptical configuration with a mean Received: April 8, 2017 Revised: August 1, 2017 Published: August 1, 2017 6606

DOI: 10.1021/acs.chemmater.7b01443 Chem. Mater. 2017, 29, 6606−6610

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Chemistry of Materials

cycle. The frontier molecular orbital energy levels of [DBFDP]2Cu4I4 was further experimentally evaluated as −5.28 and −2.98 eV, respectively, with cyclic voltammetry (CV) analysis (Figure S3). The latter is even comparable with conventional electron transporting complexes, e.g., aluminum tris(8-hydroxyquinolinate) (Alq 3 , LUMO = −3.0 eV). Obviously, the employment of DBFDP as ligand can remarkably balance the carrier injection in the clusters and facilitates the direct charge recombination and exciton capture on [DBFDP]2Cu4I4, which is beneficial to reduce energy loss during host-dopant energy transfer in emissive layers (EML) of OLEDs and thereby improve exciton utilization efficiency (EUE).25 Natural transition orbital (NTO) analysis on the S0 → S1 and S0 → T1 excitations of [DBFDP]2Cu4I4 manifests its excited states characteristic of major charge transfer (CT) and minor locally excited states (LE) (Figure 2). It is shown that for both singlet and triplet transitions, “hole” mainly locates on Cu4I4 and two P atoms, as well as Cu−P σ orbitals, which further slightly extend to P-linked phenyls and DBF; whereas, “particle” is contributed by one DBF and the corresponding C−P σ orbitals. Considering the contributions of P, Cu+ and I− to “hole” with proportions of about 15%, 30% and 45%, respectively, the singlet and triplet transitions should mainly originate from Cu+ and I− involved 1dπ* and 3dπ* with ratios beyond 70%. Thus, the multiple triplet metal/halogen-ligand charge transfer (3M/XLCT)-enhanced SOC in the cluster enlarges the allowedness of triplet transitions for PH emission. Simultaneously, the 1MLCT-featured S1 state results in the negligible singlet−triplet splitting (ΔEST) of 0.044 eV, promoting reverse intersystem crossing (RISC) for TADF. Furthermore, the considerable contribution of LE state as ∼10% increases the oscillator strength ( f) to 0.0086. In consequence, [DBFDP]2Cu4I4 is potential to achieve dual emission of TADF and PH on the basis of the balance between intersystem crossing (ISC) and RISC. The absorptions of [DBFDP]2Cu4I4 in dilute solution (10−6 M in CH2Cl2) peaked at 236 and 281 nm correspond to π→π* and n→π* transition of DBFDP (Figure 3a). The M/XLCTattributed absorption tail is recognized from 300 to 400 nm, in accord with TDDFT simulation result. The bluish green emission peaked at 491 nm is observed with a full width at halfmaximum (fwhm) of 95 nm. The as-prepared crystalline-state solid of [DBFDP]2Cu4I4 is nonluminous, but becomes cyan emissive after grinding with peak at 492 nm, corresponding to the typical mechanochromic behavior of copper iodide clusters, which is attributed to the reduction of intermolecular interaction-induced quenching through the transformation from crystalline to amorphous state as evidenced by powder X-ray diffraction analysis (Figure S4).26 Furthermore, the spincoated films of [DBFDP]2Cu4I4 showed the favorable thermoand photostabilities with the almost unchanged photoluminescence quantum yield (PLQY) after heating at 200 °C for 1 h and only ∼30% PLQY reduction after UV exposure for 1 month (Figure S5). In this sense, [DBFDP]2Cu4I4 can be competent as dopant in EML of OLEDs. The time-resolved PH spectrum was measured at 77 K after a delay of 300 μs, which consists of three bands around ∼450, ∼530 and ∼610 nm, respectively, corresponding to hybrid 3LE and 3MLCT, 3XLCT and cluster centered (3CC) transitions, respectively (Figure 3a).27 The S1 and T1 energy levels of 3.13 and 2.97 eV are estimated according to the fluorescence (FL) and PH onsets.

diameter of 1.5 nm, making it competent as a nanoemitter for super-resolution display. Its symmetrical structure renders single molecular orientation in the packing diagram. π−π stacking interactions between adjacent diphenylphosphine (DPP) and DBF moieties are observed with centroid-to-edge distances less than 3.5 Å, giving rise to two-dimensional networks. The rigid and stable structure of [DBFDP]2Cu4I4 was further verified by its temperatures of glass transition (Tg) and decomposition (Td, at weight loss of 5%) as high as 209 and 361 °C, respectively, which are among the highest values for 0D Cu4I4 clusters reported so far (Figure S2).12 The outstanding morphological stability of [DBFDP]2Cu4I4 is beneficial to suppress joule heat-induced phase separation in its doping devices for long-time operation. Density function theory (DFT) and time-resolved DFT (TDDFT) simulations on ground state (S0) and the first singlet (S1) and triplet (T1) excited states of [DBFDP]2Cu4I4 was performed to figure out its configuration variation and transition characteristics (Figure 2). It is showed that the

Figure 2. Coordination geometries of [DBFDP]2Cu4I4 in the S0, S1 and T1 states optimized with DFT and TDDFT, the frontier molecular orbital (FMO) contours of S0 state and NTO of S1 and T1 states. ΔE is the HOMO−LUMO energy gap. The S1 and T1 energy is estimated by TDDFT. σ and f refer to associated weight and oscillator strength, respectively.

ground and excited state configurations are almost identical, accompanied by the negligible differences of bond angles within 5° between P atoms and Cu4I4 cluster, which verifies the extremely high structural rigidity of the bis-phosphine chelated cluster and the corresponding effective suppression of structural relaxation-induced excited-state energy loss. Cu4I4 makes the major contribution to the highest occupied molecular orbital (HOMO) of [DBFDP]2Cu4I4, giving rise to the shallow energy level of −5.07 eV due to the strong reducibility of Cu+ and I−; while, its lowest unoccupied molecular orbital (LUMO) is localized on DBF with the deep energy level of −1.56 eV ascribed to the electron-deficiency of aromatic oxygen hetero6607

DOI: 10.1021/acs.chemmater.7b01443 Chem. Mater. 2017, 29, 6606−6610

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Chemistry of Materials

configuration (Scheme S1). As expected, the EL spectrum was significantly broadened with a fwhm as large as 190 nm, which is centered at 550 nm with two shoulder peaks at 492 and 610 nm to cover the green, blue and red regions, respectively, thereby resulting in the single-molecular white EL with CIE1931 coordinates of (0.37, 0.45) (Figure 4a). The

Figure 3. (a) Electronic absorption, room-temperature photoluminescence (PL) and time-resolved phosphorescence (PH at 77 K after 300 μs, inset) spectra of [DBFDP]2Cu4X4 in CH2Cl2 (10−5 mol L−1). (b) Temperature-correlated fractional contributions of TADF and PH to total emissions of [DBFDP]2Cu4X4 and emission lifetime (τ) variation (inset) during temperature increasing simulated according to the time decay curves in Figure S6. Figure 4. (a) EL spectra, CIE coordinates and photographs (insets) of the OLEDs using [DBFDP]2Cu4I4 as dopant. LWPH refers to longwavelength phosphorescence; (b) luminance−current density (J)− voltage relationship and EQE/EUE−J curves (inset).

The resulted ΔEST of 0.16 eV is relatively modest to make RISC feasible but limited, thereby facilitating the dual emission. The emission decay characteristics of [DBFDP]2Cu4I4 is strongly temperature dependent, which was measured in the range of 77 to 300 K (Figure S6). The lifetime (τ) of the delayed portion sharply decreases from ∼5 ms at 77 K to ∼2 μs at 300 K, corresponding to the increase of radiative constant (kr) for thousand folds, which reflects the thermally activated population of the S1 state with spin-allowed radiative transition (inset in Figure 3b). Therefore, the fraction curves of PH and TADF also reveal the strong temperature dependency (Figure 3b). At room temperature, TADF is predominant with contribution beyond 80%, accompanied by a considerable PH proportion of ∼15%. Therefore, [DBFDP]2Cu4I4 shows a typical dual emissive characteristic, whose triplet state with wide-band radiative transitions would improve the triplet exciton harvesting and establish the basis for single-molecular white emission. It should be noted that the extremely short τ of 1.9 μs at room temperature is one of the smallest values reported so far for Cu+ complexes,12 which can dramatically reduce the collision-induced quenching effects during EL process, e.g., singlet/triplet−triplet annihilation (STA/TTA). Encouraged by the favorable processability and optoelectronic properties of [DBFDP]2Cu4I4, it was used as dopant to fabricate the spin-coated OLEDs with a simple bilayer

multipeak fitting showed that the EL spectrum was combined with two blue and red components, consistent with TADF and the 3CC-attributed long-wavelength PH (LWPH) band (Figure 3a). Furthermore, the CIE coordinates of EL were almost exactly in line between those of TADF and LWPH. It is rational that the high-energy triplet excitons corresponding to 3M/ XLCT states can better match 1 M/XLCT states in energy and electronic characteristics for triplet-to-singlet conversion through RISC; whereas, the low-energy triplet excitons at 3 CC state more tend to radiative transition for PH. In this sense, the single-molecular white EL was achieved in virtue of the dual emissive characteristics of [DBFDP]2Cu4I4, especially its wide-band PH originated from multiple triplet transitions of its cluster core. Furthermore, in accord with the small PL blue shift under heating restrained by rigid DBFDP, these devices revealed the favorable emission color stability with the slightly hypsochromic EL spectra after long-term operation (Figure S7). The luminance of the devices can reach up to ∼1500 cd m−2, which is comparable to those of the devices employing primary6608

DOI: 10.1021/acs.chemmater.7b01443 Chem. Mater. 2017, 29, 6606−6610

Communication

Chemistry of Materials structural mononuclear/dinuclear Cu+ complexes (Figure 4b).28 Despite PLQY of ∼5%, [DBFDP]2Cu4I4 still endowed its devices with the maximum external quantum efficiency (EQE) of 0.73%, corresponding to EUE beyond 70%, which robustly demonstrates the potential of copper iodide clusters in 100% harvesting of electrogenerated excitons (inset in Figure 4b). Furthermore, the devices revealed the reduced efficiency roll-off fitted with TTA model rather than mixed TTA and STA model, which should be ascribed to the decreased singlet exciton concentration because of the high triplet exciton percentage for PH emission. In summary, a bidentate phosphine ligand was utilized to construct a copper iodide cluster [DBFDP]2Cu4I4 with high stability and rigidity. The favorable processability, electroactivity and dual emissive characteristics support the impressive EL performance to its devices, especially the single-molecular white emission and EUE beyond 70%. This work successfully manifests the great potential and superiority of nanoclusters as optoelectronic integrated systems for diverse device applications, as well as the significance of ligand engineering. Obviously, the PLQY of [DBFDP]2Cu4I4 leaves a huge space for further improving the EL performance of copper iodide clusters, which is ongoing in our laboratory.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01443. Experimental details, thermal and mechanochromic properties and transient emission characteristics (PDF) Data for C72H52Cu4I4O2P4 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*G.X., email: [email protected]. *H.X., email: [email protected]. ORCID

Guohua Xie: 0000-0003-0764-7889 Hui Xu: 0000-0002-2687-5388 Author Contributions ‡

M.X. and C.H. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Young Cheung Kong Scholars Program of Ministry of Education (China), NSFC (21672056, 51373050, 61605042 and 21602048), Science and Technology Bureau of Heilongjiang Province (JC2015002) and Harbin Science and Technology Bureau (2015RAYXJ008).

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DOI: 10.1021/acs.chemmater.7b01443 Chem. Mater. 2017, 29, 6606−6610