Highly Efficient Self-Trapped Bluish White-Light Emission from [Pb ₄ Cl ₅ ] ³⁺ Nodes in a Moisture-Tolerant Metal–Organic Framework

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Feb 2022Highly Efficient Self-Trapped Bluish White-Light Emission from [Pb4Cl5]3+ Nodes in a Moisture-Tolerant Metal–Organic Framework Jinlin Yin, Yuan Yu, Xueling Song, Yilin Jiang and Honghan Fei Jinlin Yin School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, Shanghai 200092 , Yuan Yu School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, Shanghai 200092 , Xueling Song School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, Shanghai 200092 , Yilin Jiang School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, Shanghai 200092 and Honghan Fei *Corresponding author: E-mail Address: [email protected] School of Chemical Science and Engineering, Shanghai Key Laboratory of Chemical Assessment and Sustainability, Tongji University, Shanghai 200092 https://doi.org/10.31635/ccschem.021.202100795 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Highly luminescent zero-dimensional (0D) metal halide clusters attract widespread attention owing to strong exciton confinement and populated self-trapped states but often exhibit narrow emission and are susceptible to hydrolysis. Herein, we demonstrate a moisture-resistant metal–organic framework (MOF) consisting of cationic 0D [Pb4Cl5]3+ nodes bridged by adamantanetetracarboxylate. Upon near-UV excitation, the material emits intrinsic broadband bluish white-light emission with high external quantum efficiency of 35% and a color rendering index of 76. Unlike organoammonium cations in lead perovskites, the Pb-carboxylate coordination affords the MOF to be chemically stable and photostable in high humidity. The photoemitter exhibits undiminished photoemissions under ambient conditions [∼60% relative humidity (RH)] upon continuous UV irradiation (143 mW/cm2, 365 nm) for 7 days. The insertion of [Na4Cl]3+ moieties will connect 0D units into two-dimensional (2D) metal halide layers to limit structural strain and decrease the quantum efficiency from 35% to 15%, confirming the key importance of 0D units for efficient emission. Download figure Download PowerPoint Introduction A major challenge in solid-state lighting applications [e.g., white-light-emitting diodes (WLEDs)] is the search for white-light-emitting phosphors.1,2 In contrast to multichromatic emitters, single-component broadband white-light-emitting materials overcome self-absorption and long-term instability problems.3,4 Meanwhile, very few single-component broadband white-light emitters based on earth-abundant elements have both high quantum efficiency and high chemical robustness.5 Inorganic–organic metal halide crystalline materials are an emerging class of intrinsic broadband white-light emitters attracting widespread attention, based on the pioneering work of Karunadasa and co-workers in 2014.6,7 Their broad photoluminescence (PL) are amenable to crystal engineering, arising from self-trapped excitons (STEs) in the deformable lead halide building units (e.g., [PbX3]−, X: Cl−/Br−/I−).8–10 To facilitate the excitons self-trapping and improve the PL quantum yields (PLQYs), great efforts have been made to lower the inorganic dimensionality from two-dimension (2D) to one-dimension (1D), or even zero-dimension (0D).11,12 For example, the vast majority of 2D white-light-emitting perovskites have low PLQYs in the range of 0.5∼9%; however, 1D white-light-emitting perovskites, (C4N2H14)PbBr4 and (TDMP)PbBr4 (TDMP = trans-2,5-dimethylpiperazine), show increased PLQYs of 12∼28% and 45%, respectively.13–15 Indeed, intrinsic emission with near-unity PLQYs (75∼95%) have been achieved from 0D metal halide clusters, such as (C4N2H14Br)4SnBr3I3, (C9NH20)SbCl5, (C4N2H14I)4SnI6, and Cs3Cu2I5.16–21 However, PL from the vast majority of 0D structures does not cover the entire visible-light spectrum, thus requiring another phosphor to emit white-light by multichromatic strategies.14,22–26 More importantly, limited investigations have been performed to study the environmental stability (i.e., hydrolysis) of these 0D metal halide clusters, since the free organoammonium cations residing in the metal halide hybrids have high water affinity.27 To date, very few metal halide hybrids emitting self-trapped white-light demonstrate both high moisture stability and high PLQYs, which are two fundamental requirements for commercial utilization of white-light emitters.1 Very recently, (DAO)Sn2I6 (DAO = 1,8-octyldiammonium) was found to be water-stable for 15 h and showed a PLQY of 36%, while exhibiting orange-red emission centered at 634 nm.28 In contrast to free organoammonium cations templating conventional metal halide hybrids, our group has employed organocarboxylate anions as structural-directing agents to synthesize a class of ultrastable, cationic lead halide hybrids, including 3D [Pb4Br6]2+ frameworks, 2D [Pb2X2]2+ (X: Cl−/Br−) layered materials, and open frameworks containing 1D [Pb2X3]+ (X: Cl−/Br−/I−) chains.29–31 The strong Pb-carboxylate linkages largely contribute to the high environmental moisture stability, and some of the cationic materials are even stable in a wide range of pH (3∼10) as well as aqueous boiling conditions for 24 h.30,32,33 Moreover, the cationic haloplumbate units have strong structural strains over UV excitation, affording self-trapped, broadband white-light emissions despite low efficiencies (PLQY of 1∼15%).31,34,35 Thus, the PLQYs of these cationic lead halide materials must be increased for commercial lighting applications. Although unexplored in the literature, the formation of metal–organic frameworks (MOFs) with 0D cationic lead halide units bridged by symmetric, nonconjugated polycarboxylate ligands should be an excellent platform to enhance PLQYs of self-trapped white-light emission. Herein, we have successfully synthesized an extended framework with an ordered array of cationic 0D [Pb4Cl5]3+ nodes connected by 1,3,5,7-adamantanetetracarboxylate (adatc4−). The material exhibits broadband bluish "cold" white-light emission covering the visible-light spectrum (370∼700 nm), a full width at half-maximum (FWHM) of 158 nm, and a high PLQY of 35%. In addition, the single-component white-light emitter has high moisture stability and photostability, therefore attaining undiminished photoemission upon continuous irradiation under ambient conditions [∼60% relative humidity (RH)]. Meanwhile, Na+ insertion leads to the formation of [Na4Cl]3+ clusters linking the adjacent in-plane [Pb4Cl5]3+ clusters into [Na4Pb4Cl6]6+ inorganic layers. The increase in inorganic dimensionality from 0D to 2D largely hinders structural deformation and exciton self-trapping, resulting in a noticeable decrease in PLQY from 35% to 15%. Results and Discussion Solvothermal reactions between PbCl2, NH4Cl, concentrated HCl, and adatc4− in dimethylformamide (DMF)/EtOH afforded block-shaped colorless crystals of [Pb4Cl5](adatc)2Cl•guest [guest: (CH3)2NH2+/H3O+], which we denote as TJU-20 (TJU = Tongji University). NH4Cl is an important chloride source and acts as a stabilizer for the formation of cationic 0D units ( Supporting Information Figure S1).36 Single-crystal X-ray crystallography reveals that the MOF crystallizes in the tetragonal crystal system with the space group of I-42m ( Supporting Information Table S1). Each isolated [Pb4Cl5]3+ node consists of a square planar μ4-Cl in the center, which bridges four Pb2+ cations to form the square [Pb4Cl]7+ core (Figure 1a). Four corner Cl− anions coordinate to four different Pb2+ centers with bond angles of 145.73(4)°, completing the formation of the cationic [Pb4Cl5]3+ secondary building blocks (SBUs). In addition, an ordered array of the chloroplumbate SBUs are arranged in a square-grid fashion in the layers along the ab plane, and the layers are staggered-stacked with each other along the c-axis ( Supporting Information Figure S2). Indeed, each [Pb4Cl5]3+ unit has four in-plane neighboring clusters at a distance of ∼12.7 Å and eight out-of-plane neighboring clusters at a distance of ∼11.5 Å (Figures 1c and 1d and Supporting Information Figure S2). The adatc4− ligands serve as pillars in the interlamellar spacing, linking three 6-connected [Pb4Cl5]3+ clusters (Figure 1c and Supporting Information Figure S3). Figure 1 | (a) Top (top) and side views (bottom) of a single [Pb4Cl5]3+ SBU in TJU-20. (b) Photograph of TJU-20 crystals upon UV excitation (365 nm). (c and d) Crystallographic view of TJU-20 along the a-axis (c) and c-axis (d). (e and f) Crystallographic view of TJU-20(Na) along the a-axis (e) and c-axis (f). Guest molecules are omitted for clarity. Download figure Download PowerPoint Isoreticular synthesis was successful by adding NaCl during solvothermal conditions, affording an isostructural TJU-20(Na) with the insertion of the incoming [Na4Cl]3+ clusters into the porosity of TJU-20 (Figures 1e and 1f). The [Na4Cl]3+ clusters reside in the middle of the neighboring in-layer [Pb4Cl5]3+ nodes and extend the 0D clusters into 2D metal halide layers ( Supporting Information Figure S2). Overall, TJU-20 is considered as an open MOF containing [Pb4Cl5]3+ nodes; while TJU-20(Na) is an inorganic layered material (i.e., [Na4Pb4Cl6]6+ layers) pillared by adatc4− ligands. The good matching between the experimental powder X-ray diffraction (PXRD) pattern and the calculated pattern from single-crystal data evidences high-purity phases of as-synthesized TJU-20 and TJU-20(Na) (Figure 2). The chemical stability was examined by incubating TJU-20 materials (both single crystals and finely ground microcrystals) in polar organic solvents (e.g., DMF, MeOH, and CH3CN) as well as in a chamber of ∼90% RH for 24 h at room temperature (see details in Experimental Section of Supporting Information). High crystallinity was shown to be retained by PXRD patterns and scanning electron microscopy (SEM) (Figures 2a and 2b and Supporting Information Figure S4), and a negligible decrease (<5 wt %) in mass balance was observed after these chemical treatments. Thermogravimetric analysis indicates TJU-20 and TJU-20(Na) are stable up to 200 and 400 °C, respectively, under N2 flow ( Supporting Information Figures S5 and S6). The enhanced thermal stability of TJU-20(Na) is probably due to the densely packed structure as well as the formation of inorganic layers. Figure 2 | SEM images (left) and PXRD (right) of as-synthesized TJU-20 (a) and TJU-20(Na) (b) single crystals after incubating in 90% RH and polar organic solvents for 24 h at room temperature, showing the chemical stability of TJU-20 (a) and TJU-20 (b). Download figure Download PowerPoint The absorption spectra of TJU-20 indicate an optical band gap of 3.42 eV (Figure 3a and Supporting Information Figure S7), close to the band gap calculated by density functional theory (DFT) calculations (3.10 eV) (Figure 3b). The valence band maximum (VBM) and conduction band minimum (CBM) are dominated by the full Cl 3p orbitals and empty Pb 5p orbitals, respectively, analogous to lead halide perovskites.37,38 Meanwhile, O 2p and C 2p orbitals from adatc4− ligands partially contribute to the VBM and CBM, respectively, likely due to the Pb2+-carboxylate coordination in TJU-20 (Figure 3b). This is further confirmed by the density contour maps for lowest unoccupied molecular orbitals (LUMO) and highest occupied molecular orbitals (HOMO) ( Supporting Information Figure S8). Figure 3 | (a) Absorption and emission spectra of TJU-20. (b) Density of states of TJU-20. (c) CIE chromatic coordinates of TJU-20, TJU-20(Na), and pure white light. (d) Decay curves of cm-sized single crystals and μm-sized microcrystals of TJU-20 at 298 K. (e) PL decay curve (black) and fitting (red) of TJU-20 at 77 K. (f) Temperature-dependent emission spectra of TJU-20 from 297 to 177 K. Download figure Download PowerPoint Upon near-UV excitation at 350 nm, TJU-20 demonstrates large Stokes-shifted, broadband bluish white-light emission ranging from 370 to 700 nm with a FWHM of 158 nm (Figures 1b amd 3a). The coverage of visible-light spectrum along with a large Stokes shift of 120 nm overcomes the low color rendition and self-absorption problems in many conventional phosphors.39 The broad photoemission affords a high color rendering index (CRI) of 76 and a correlated color temperature (CCT) of 14,828 K, affording bluish "cold" white-light based on a single-component strategy. The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates are (0.23, 0.30) (Figure 3c). Importantly, the broadband emission from TJU-20 provides a remarkably high PLQY of 35% ( Supporting Information Figure S9), superior to the vast majority of broadband white-light emitters ( Supporting Information Table S2). Importantly, the nonconjugated H4adatc ligands have negligible emission upon 350 nm excitation, suggesting the broadband emission originating from [Pb4Cl5]3+ units ( Supporting Information Figure S10). Moreover, the moisture-resistant nature of TJU-20 results in ultrastable emission under ambient conditions (room temperature, ∼60% RH) with almost no change in intensity and position of broadband emission upon continuous UV irradiation (143 mW/cm2, 365 nm, 40 W) for 7 days ( Supporting Information Figure S11). Given that the PL from TJU-20 combines the advantages of high stability, high PLQY, and broadband emission, we sought to employ a variety of photophysical studies, as well as DFT calculations, to investigate the PL mechanism. First, identical emission and decay profiles are observed in cm-sized single crystals and μm-sized microcrystals, suggesting the intrinsic bulk nature of the photoemission rather than arising from the surface defects (Figures 3a and 3d). Moreover, TJU-20 shows nearly identical emission profiles upon excitation at different wavelengths from 330 to 350 nm ( Supporting Information Figure S12). The PL intensity linearly increases with excitation power density from 50 to 300 mW/cm2 at 298 K ( Supporting Information Figure S13), which confirms the emission does not arise from permanent defects. The triexponential fitting of the decay curve indicates two short PL lifetime of τ1 = 1.6 ns (free excitons) and τ2 = 8.4 ns (singlet STEs) and a long PL lifetime of τ3 = 158.9 ns (triplet STEs), affording an average lifetime of 66.2 ns ( Supporting Information Figure S14). Meanwhile, the decay curve fitting at 77 K gives an average PL lifetime of 0.43 μs (Figure 3e and Supporting Information Figure S15). The long lifetime at 77 K in μs-scale and the short lifetime at room temperature in ns-scale agree with the mechanism of intrinsic emission from STEs.19 To further confirm the self-trapped emission from electron–phonon coupling, the temperature-dependent PL of TJU-20 shows narrower and more intense emission when decreasing temperature from 297 to 77 K, consistent with electron–lattice coupling (Figure 3f). In addition, the temperature-dependent FWHM was fit using the following equation: Γ ( T ) = Γ 0 + Γ LO ( e E LO / k B T − 1 ) − 1 + Γ inh e − E b / k B T (1)Here, Γ0 represents the emission FHWM at T = 0 K, while the second and third terms represent the contribution of electron–phonon coupling and inhomogeneous broadening, respectively, to the overall PL broadening (see details in Supporting Information Figure S16). The best fit gives ELO (the longitudinal optical phonon energy) to be 14(2) meV. Importantly, this Raman-active phonon mode corresponds well with Pb–Cl stretching mode at 112 cm−1 ( Supporting Information Figure S17). In addition to a variety of photophysical studies, we have performed a control study to investigate the photochemical properties of isoreticular TJU-20(Na), which has the isostructural 0D chloroplumbate clusters connected into 2D layers by the [Na4Cl]3+ moieties. The absorption spectra and steady-state PL of TJU-20(Na) indicate a band gap of 3.38 eV and broadband emission centered at 510 nm, analogous to TJU-20 ( Supporting Information Figures S7, S18, and S21). However, a substantial decrease of PLQY from 35% (TJU-20) to 15% [TJU-20(Na)] was observed. This phenomenon suggests the mechanism of STEs, since increasing the dimensionality of metal halide building units will offer less populated STEs.9,40 In addition, the triexponential fitting to the decay curve of TJU-20(Na) gives an average lifetime of 28.2 ns at 298 K ( Supporting Information Figure S14). The radiative decay rates were calculated to be approximately 4.53 × 106 s−1 for TJU-20 and 5.32 × 106 s−1 for TJU-20(Na), while the nonradiative decay rates were approximately 1.06 × 107 s−1 for TJU-20 and 3.01 × 107 s−1 for TJU-20(Na), respectively. The higher nonradiative rate of TJU-20(Na) indicates the photocarriers in TJU-20(Na) are more susceptible to nonradiative recombination. We continue to use DFT calculations to evaluate the effects of increasing dimensionality toward the degree of structural deformation. Since self-trapped states in metal halides originate from the intersystem crossing between singlet state with triplet state, DFT calculations of spin-triplet excitons were applied to study the structural deformation upon UV excitation. The [Pb4Cl5]3+ clusters in both TJU-20 and TJU-20(Na) have structural deformation in both the [Pb4Cl]7+ core and the connectivity of corner Cl− anions (Figures 4a and 4b). First, the pronounced formation of Pb–Pb dimers was observed in the excited state of the square [Pb4Cl]7+ units. The calculated Pb–Pb distances in the excited state of TJU-20 was 3.93 Å, lower than the ground state of TJU-20 (4.42 Å) and the calculated excited state of TJU-20(Na) (4.25 Å) (Figures 4a and 4b). These calculations indicate TJU-20 has a higher tendency to form dimerized Pb23+ self-trapped electrons (STELs) than TJU-20(Na). STELs are known to have higher energy than self-trapped holes, which explains the larger Stokes shift from TJU-20(Na) than TJU-20 (Figure 4c).35 Meanwhile, the linear axial Pb–Cl–Pb bond angle in the [Pb4Cl]7+ center of excited-state TJU-20 is highly bent, and the calculated band angles change from 174.8° to 167° upon excitation. In addition, the calculation shows that the connectivity of corner Cl− is also more bent in TJU-20 than TJU-20(Na) clusters upon UV excitation. The calculated Cl–Pb–Cl angles in the excited-state TJU-20 change from 145.8° to 132.1°, and the closest Cl–Cl distance is decreased to 5.06 Å. Meanwhile, the calculated Cl–Pb–Cl angles in TJU-20(Na) remain at 150.7° with higher calculated Cl–Cl distances (5.76∼5.92 Å). In short, DFT calculations indicate the structural strain in [Pb4Cl5]3+ clusters of TJU-20 is stronger than TJU-20(Na), likely due to the latter occupying a 2D inorganic dimension. Figure 4 | (a and b) DFT calculations: structural deformation of [Pb4Cl5]+ units in TJU-20 (a) and TJU-20(Na) (b) upon UV excitation. (c) The configuration coordinate diagram for the structural deformation-induced broadband emission in TJU-20. Download figure Download PowerPoint Conclusions This is our first discovery of cationic 0D haloplumbate units with high structural strain, thus exhibiting substantially higher PLQY (35%) of white-light emission than our previous reported cationic 1D chains, 2D sheets, and 3D frameworks (PLQY of 1∼15%).28−30 Moreover, this is one of the few self-trapped white-light emitters demonstrating both high environmental stability and high PLQYs. Combining high PLQY, high CRI, broad photoemission, and long-term environmental stability, this single-component white-light emitter is a potential alternative to conventional phosphors. To overcome the toxicity of Pb2+, our ongoing studies will focus on extending to Sn2+/Bi3+ halide-based moisture-tolerant MOFs for commercial utilization. Supporting Information Supporting Information is available and includes experimental details, addition characterization, and crystallographic data. Conflicts of Interest The authors declare no competing financial interests. Acknowledgments This work was supported by grants from the National Natural Science Foundation of China (nos. 21971197 and 51772217), the Shanghai Rising-Star Program (no. 20QA1409500), the Recruitment of Global Youth Experts by China, the Fundamental Research Funds for the Central Universities, and the Science & Technology Commission of Shanghai Municipality (no. 19DZ2271500). References 1. Luo J.; Wang X.; Li S.; Liu J.; Guo Y.; Niu G.; Yao L.; Fu Y.; Gao L.; Dong Q.; Zhao C.; Leng M.; Ma F.; Liang W.; Wang L.; Jin S.; Han J.; Zhang L.; Etheridge J.; Wang J.; Yan Y.; Sargent E. 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K. of and in Google Scholar S.; S.; J.; of of in Google Scholar Zhang Liu and Metal–Organic , Information & Chemical work was supported by grants from the National Natural Science Foundation of China (nos. 21971197 and 51772217), the Shanghai Rising-Star Program (no. 20QA1409500), the Recruitment of Global Youth Experts by China, the Fundamental Research Funds for the Central Universities, and the Science & Technology Commission of Shanghai Municipality (no. 19DZ2271500).

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