Single-Molecule-Based Electroluminescent Device as Future White

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Functional Inorganic Materials and Devices

Single-Molecule Based Electroluminescent Device as Future White Light Source Muhammad Usman, Krishna Prasad Bera, Golam Haider, Batjargal Sainbileg, Michitoshi Hayashi, Gene-Hsiang Lee, Shie-Ming Peng, Yang-Fang Chen, and Kuang-Lieh Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17107 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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ACS Applied Materials & Interfaces

Single-Molecule Based Electroluminescent Device as Future White Light Source Muhammad Usman,∆,† Krishna Prasad Bera,∆,‡,§ Golam Haider,‡ Batjargal Sainbileg,┴ Michitoshi Hayashi,┴ Gene-Hsiang Lee,# Shie-Ming Peng,# Yang-Fang Chen*,‡ Kuang-Lieh Lu*,†

†

Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan Department of Physics, National Taiwan University, Taipei 106, Taiwan § Nano-Science and Technology Program, Taiwan International Graduate Program, Academia Sinica, Taipei 106, Taiwan ┴ Center for Condensed Matter Sciences, National Taiwan University & Center of Atomic Initiative for New Material, National Taiwan University, Taipei 106, Taiwan # Department of Chemistry, National Taiwan University, Taipei 106, Taiwan ‡

KEYWORDS. Electroluminescence • Photoluminescence • Strontium • Single Molecule • WLEDs

ABSTRACT. During last two decades spectacular development of light emitting diodes (LEDs) has been achieved owing to their widespread application possibilities. However, traditional LEDs suffer from unavoidable energy loss due to the down conversion of photons, toxicity due to the involvement of rare-earth materials in their production, higher manufacturing cost, and reduced thermal stability that prevent them from all-inclusive applications. To address the existing challenges associated with current commercially available white LEDs, herein, we report on a broadband emission originating from an intrinsic lanthanide-free single-molecule based LED. Self-assembly

of

a

butterfly-shaped

strontium-based

compound

ACS Paragon Plus Environment

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{[Sr(H2btc)2(MeOH)(H2O)2]·2H2O} (1) was achieved through the reaction of Sr(NO3)2 with a benzene-1,2,3-tricarboxylic acid hydrate (1,2,3-H3btc) under hydrothermal conditions. White LED based on this single molecule exhibited a remarkable broadband luminescent spectrum with Commission Internationale de l’Eclairage (CIE) coordinates at (0.33, 0.32) under 30 mA current injection. Such a broad luminescent spectrum can be attributed to the simultaneous existence of several emission lines originating from the intramolecular interactions within the structure. To further examine the nature of the observed transitions, density functional theory (DFT) calculations were carried out to explore the geometric and electronic properties of the complex. Our study thus paves the way toward a key step for developing a basic understanding and the development of high performance broadband light emitting devices with environment-friendly characteristics based on organic‒inorganic supramolecular materials.

1. Introduction Light emitting diodes (LEDs) are omnipresent in modern technology owing to their superior advantages over traditional lighting sources, such as lower power consumption and longer lifetimes.1‒3 It is one of the most rapidly developing fields in lighting technology because of its enormous potential for diverse applications.4 The recent Noble Prize in 2014, for the invention of the gallium nitride (GaN) based blue LED, highlights the importance of the light emitting diode as a replacement for current lighting sources.5 A blue emitting GaN with a yellow phosphor [Cerium-activated yttrium aluminum garnet (YAG:Ce3+)] is one of the most commonly used techniques for generating white light.6,7 The production of efficient visible LED, such as white LED (WLED) can also be achieved by the RGB approach using a combination of red (R), green (G) LEDs or phosphorescent emissive units and blue (B) LEDs.8-10 However, the practical


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application of these types of commercial WLEDs is still limited due to existing multiple shortcomings such as, colour quality, non-sustainable colour, higher production costs, poor thermal stability, environmental concerns related to rare-earth mining, and energy loss due to the down conversion of photons.2,11 In order to circumvent the existing challenges, non-rare-earth optical materials based on coordination polymer (one component) with highly enriched luminescence properties that exhibit a continuous broadband emission is reported very recently.2,11‒13 Several studies have focused along this guideline, however, white light emission from a single molecule without any doping is very rare and significant improvements are needed and will require much more effort (Scheme 1). Supramolecular chemistry pertains to the study of weak non-covalent interactions, which includes molecular systems with hydrogen bonding networks, metal coordination chemistry, electrostatic forces, hydrophobic effects, van der Waals forces and π−π interactions.14‒16 Owning to their superior features such as molecular recognition, folding, molecular self-assembly and host‒guest interactions etc., supramolecular networks have bright prospects for use in the design of efficient luminescent materials.17‒19 Considerable efforts have been made to investigate the optoelectronic properties of supramolecular arrays.20,21 Charge transfer pathways have been identified in supramolecular nano-structural molecules formed by self-organization of conjugated organic and metal‒organic molecules on a metallic surface.20‒23 Metal nanoclusters with thiolate outer shells have been reported to be highly luminescent materials through metal‒ligand charge transfer emission.24,25 The use of transition metal centers and conjugated organic molecules in the formation of nano-scale supramolecular arrays has evolved into one of the major strategies for controlling luminescent properties.26-30


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Considering the climate change that is now occurring and related environmental concerns, choosing environment-friendly alkaline earth metals as inorganic nodes for designing the inorganic‒organic compounds for white light emission would be more suitable as compared to the commonly used lanthanides. In addition, inorganic materials containing strontium have been utilized in fireworks due to the strong color emission of strontium that is produced upon heating.31 A recent report related to the area of designing electrically driven natural white light LED from an intrinsic non-rare-earth, a Sr-based MOF is a prominent illustration.11-13 In order to create charge transfer pathways via intermolecular interactions with alkaline earth metals, a polycarboxylic acid is the right choice as a linker, since such compounds are capable of creating a variety of coordination architectures.32,33 However, to date, the coordination of Sr ions with 1,2,3benzenetricarboxylic acid has not been reported. Herein, we report on the successful formation of a supramolecular assembly of Sr-based single molecule through the reaction of Sr metal ions with a 1,2,3-tricarboxylate ligand under hydrothermal conditions (Equation 1). This single molecule exhibits a broadband light emission, thus representing an innovation and rare case of a nonlanthanide doped emitter with a well resolved broad emission band which could have a great impact on advancements in the design of organic‒inorganic single-molecule based WLEDs and represents a timely, key step for producing the next generation of solid-state lighting.

2. Results and Discussion Self-assembly of Sr-based Supramolecular Compound (1) A butterfly-like Sr-based supramolecular complex {[Sr(H2btc)2(MeOH)(H2O)2]·2H2O} (1) was assembled under hydrothermal conditions by reacting Sr(NO3)2 and 1,2,3-


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benzenetricarboxylic acid hydrate (1,2,3-H3btc) in a solution of H2O/MeOH/KOH at 120 °C for 72 hours through a single-step self-assembly process (Equation 1). Crystal Structure of Sr-based Supramolecular Compound (1) The structure of the product was characterized by IR (Figure S1, ESI) and EA data which was further verified by a single-crystal X-ray diffraction analysis (Table S1). A single-crystal Xray diffraction technique has been used to analyze the crystal structure which reveals that compound 1 crystallizes in the monoclinic P21/m space group and its single-crystal refinement data is listed in Table S1. The asymmetric unit of 1 consists of a single crystallographic strontium (Sr(II)) site, two H2btc– ligands, one coordinated methanol molecule, two coordinated water molecules and two guest water molecules as illustrated in Figure 1. The Sr(II) center adopts a {SrO9} tricapped trigonal prismatic coordination geometry. Two H2btc– ligands are mutually coordinated to the metal center through monodentate or chelate carboxylate groups (O1, O3, O4, O1', O3', O4'). The Sr center is further coordinated by two water molecules (O8, O9) and one methanol moiety (O7) (Figure 1). Based on the synthetic design, the inclusion of a small amount of a base (KOH) in the reaction solution results in the formation of a partially deprotonated H2btc ligand. Because of the strong steric hindrance between the carboxylate groups, the H2btc ligand adopts a unique coordination mode (Figure S4), in which one carboxylate group and one OH group of the other carboxylic moiety remain uncoordinated. This allows them to interact with the coordinated water as well as guest water molecules. As a consequence, the mononuclear species are further extended to form a 2D array with an ABAB arrangement (Figure 1a) through hydrogen-bonding interactions. A strong hydrogen bonded network is therefore formed. The hydrogen bond interactions are drawn in Figure 1c and the bond distances are also listed in table S2. Weak parallel-displaced π-stacking interactions are also observed between two benzene rings


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in neighboring supramolecular units with a distance of ~3.5 Å (Figure 1b). From the surface morphology of 1, it can be seen that the crystals of supramolecules have rod-shape nano architectures with a width of 200~300 nm and a length of a few micrometers, as shown in Figure S6. Interestingly, the coordination of 1,2,3-benzenetricarboxylate with Sr ions resulted in the formation a supramolecular assembly, which offers a completely different symmetry for coordination due to the strong steric hindrance of its carboxylate groups.32,33 Herein, the coordination of Sr ions with 1,2,3-benzenetricarboxylic acid resulted in the formation of a supramolecular assembly and its structure has been fully characterized for application as light emitting material. Photoluminescence Properties of Sr-based Supramolecular Compound (1) The photoluminescence emission of compound 1 were examined under excitation by a 266 nm pulsed laser with a pulse frequency 10 Hz, and at an excitation power of 35 to 42 µW. Compound 1 exhibited an intense broadband photoluminescence spectrum at wavelength ranging from 350 to 600 nm with line width ~150 nm as shown in Figure 2a. The Commission Internationale de l’Eclairage (CIE) diagram of the emitted light is shown in Figure 2b. The corresponding chromaticity coordinates were found to be (0.19, 0.25). This broadband spectrum of 1 is the result of the recombination of photo-excited carriers at different blocks of the constituted material.11,13,28 It should be noted that the emitted luminescence spectrum is composed of three broad major peaks that can be de-convoluted into three emission lines as shown in supplementary Figure S7a. To further examine the emission process in the supramolecular crystals, a photoluminescence spectrum of the H3btc ligand was recorded under the same pumping conditions that are shown in Figure 2c. The emission peak of the ligand-based emission is centered on 510 nm. Thus, the low energy line in the emission from compound 1 is associated with carrier


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recombination due to a ligand to ligand transition (Figure S7c), which is assisted by the -stacking interactions. The energy level diagram and energy band structure for 1 illustrate the emission mechanism between ligand and metal and is shown in the Supporting Information (Figure S7(c & d)). The transition centered at around 480 nm can be interpreted as a metal-centered transition of the strontium-based compound, based on the previous report.11,12 Thus, the high energy emission centered at around 440 nm is associated with ligand to metal charge transfer,27,28 which was further confirmed by electroluminescence measurement as shown below. The emission of 1 from the organic linker is assisted by the π-interactions and hydrogen bonding within the molecular array as shown in Figure 1. Such π-conjugated organic linkers functionalized with multicarboxylate groups usually exhibit an emission corresponding to the transition from the lowest excited singlet state to the singlet ground state. Through charge transfer, the coordination of the ligand to the metal reduces the loss of energy by non-radiation decay, thus increasing the overall luminescence. Thus, the emission of 1 is much stronger than only the ligand based emission. In order to further unveil the emission process, we recorded time resolved photoluminescence (TRPL) spectra of compound 1 at the maxima of different emission lines, as shown in Figure 2d. The measured lifetime of the free ligand was found to be less than 1 ns (Figure S7b, ESI). The carrier lifetime of metal-based emission is reported to be ~1 ns.11 Thus, the obtained lifetimes are less than 1 ns for the peaks centered at 510 nm and 480 nm originating from ligand and metal centered emission, respectively. On the other hand, the peak centered at 440 nm possesses a longer lifetime of ~2 ns which is due to an intersystem charge transfer from the ligand to the metal.11, 28 The experimentally observed life time of different emissions are tabulated in supplementary Table S3. These results also support the de-convoluted PL spectra of compound 1 (Figure S7a). The observed PL lifetime of the organic linker and the Sr-based complex are in good agreement with previously published


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reports on inter-ligand, intermetallic and metal to ligand charge transfer emission from MOF complex.28 Photo- and Electrochemical Stability of Material In order to investigate the photo-stability of 1, we recorded the PL under the excitation of a laser over an extended period of time, and the results indicate that the emission properties of the material remain essentially unchanged. Moreover, we measured the PL emission spectra of a thin film composed of grinded nanoparticles of 1, as shown in Figure S8. Interestingly, no significant change was observed in the PL spectra before grinding and after grinding, which confirms that the compound 1 has a high photo-stability. In addition, the electrochemical stability of the luminescent supramolecule was also examined. Figure 3a shows a schematic of the device used in the electrochemical study. The electrochemical stability was evaluated through IV measurements by exposing compound 1 to a constant electrical bias of 20 V during 8 hours at ambient conditions, as shown in Figure 3b. A very stable IV curve indicates the electrochemical stability of the material under ambient conditions. Moreover, the optical properties were confirmed to be stable by systematically recording PL and Raman scattering spectra, as shown in Figure 3(c) & (d), respectively. Electronic Band Structures of 1 with and without Solvent Molecules In order to further evaluate the role of solvent molecules on electronic property of the inorganic‒organic material, we carried out the first-principles density function theory (DFT) calculations for compound 1 with and without solvent molecules by using Vienna Ab-initio Simulation Package (VASP) (Figure 4).34‒36 For both cases, the conduction band minimum (CBM) and valence band maximum (VBM) occur at Γ-point, which reveals that 1 can be classified


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as a direct bandgap semiconducting inorganic‒organic material. The calculated value for the band gap energy is 2.52 eV for 1 with solvent molecules, which is in good agreement with experimental photoluminescence measurements of 1, considering the maxima of emission peak (475 nm) (Figure 4a). While, the simulated results for the supramolecule without solvent molecules indicate that the bandgap had decreased to 2.0 eV (Figure 4b), demonstrating that solvent molecules with intermolecular interactions play a role in the electronic properties of 1. This change in bandgap with the removal of solvent molecules will clearly effect the excitation and emission mechanisms involved during photoluminescence due to alterations in the electronic structure via changes in the coordination sphere.27,28 This is a rare case of a broadband blue emission spectrum generated by a solvent-assisted Sr-based supramolecular entity for efficiently generating blue light. Incorporating solvent molecules and π-conjugated organic ligands along with Sr metal will increase intermolecular interactions and charge transfer in the crystal structure and hence broaden the spectrum to result in broadband light emitting materials. However, the major contributions in emission spectra are from charge transfer between ligand and metal. This photoluminescent Srbased supramolecular array can be mixed with a yellow phosphor to produce an efficient white emitting device.37,38 Electroluminescent Device based on Single-Molecular Crystalline Assembly of 1 The schematic of the device is illustrated in Figure 5a. In order to design the device, the nanorod shaped single crystal of 1 (Sr-based molecule) was ground into to nanoparticles, and the resulting powder was suspended in ethanol. A 199.6 nm Ag film was prepared on a Si/SiO2 substrate as the bottom electrode by thermal evaporation. We then used a RF sputtered unit to sputter the ZnO layer as an electron transport layer. The nanoparticles of compound were then spin coated on top of the ZnO layer (Figure 5d). Indium-tin-oxide (ITO) coated glass was used as the


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top electrode and hole injection layer. As shown in Figure S9, the I−V characteristics of the device indicate a nonlinear increase in the injection current under forward bias, which is due to the formation of a p‒n junction with an ideality factor of 1.68. The turn on voltage for the heterojunction was found to be ~1.5 V, which is much lower than the all previously reported organic and metal‒organic materials based LED devices.39-43 We further investigated device stability and the conductivity of the Sr-based supramolecule. The estimated conductivity is ~10‒3 S/cm, which is relatively high compared to previously-reported luminescent metal‒organic compounds.27,28,39 The high conductivity is achieved through the use of a suitable semiconducting organic ligand in the butterfly-like structure of the mononuclear compound. The organic linker not only forms a bridge between two metallic ions but also plays a crucial role in the luminescent mechanism. Figure 5c shows the electroluminescence (EL) spectra of our fabricated LED device under a driving current of 30 mA. The device possesses a broadband bright electroluminescence spectrum covering the visible range. A strong blue light emission band centered at 448 nm associated with a broad yellow light emission band centered at 544 nm is observed under different current injections. Figure 5b shows the EL emission from the device under a current injection of 30 mA, as captured by a mobile camera. The existing major difference in the PL and EL spectrum can be understood from the emission process of the device. As discussed above, the emission process of 1 is quite different compared to conventional semiconducting materials and consists of a metal centered emission, a ligand centered emission and an emission due to metal to ligand charge transfer or vice-versa, where -stacking and hydrogen-bonding interactions in the supramolecular assembly play an important role.27,28,39 In the present crystal, while it is excited by optical pumping, every emissive block becomes activated because of the presence of sufficient


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carriers from optical pumping. As a result, we observed a broad continuous emission consisting three broad peaks. Upon electrical pumping, the ligand centered emission, along with the ligand to metal charge transfer dominate over the metal centered emission. This is because the ligand forms a conducting bridge for the electrons in the system. Thus, the injected carriers in the ligand recombine with the injected holes at the metal and ligand HOMO level.11,12,33 Moreover, the deconvoluted emission spectrum, as shown in supplementary Figure S7a, the intensity of the metal centered emission is much lower compared to the ligand centered and ligand to metal charge transfer emission. Hence, low level pumping by current injection cannot produce sufficient carriers at the LUMO energy level of the metal. As a result, a discrete EL emission is observed. The energy band diagram corresponding to the emission process is shown in Figure 5e. The calculated chromaticity of the EL emission has a value close to the point (0.33, 0.32), showing a visible broadband emission, which is much closer to white light as shown in Figure S10. To further investigate the difference between the PL and EL spectra of 1, we collected driving voltage dependent electroluminescence spectra by applying different forward injection currents into the device. The results revealed the interesting voltage-dependent EL emission (Figure 6a). The EL spectra under different forward bias currents exhibited a progressive enhancement in the EL intensity when the potential is increased. It is evident that the hump between the blue emission band and the yellow emission still exists with the enhancement in pumping power in the electronically driven LED device. Interestingly, the intensity of the blue emission band increases as compared to the yellow emission band with increasing excitation energy. This is expected, because at higher potential, the ligand to metal charge transfer responsible for the blue emission dominates the light emission as compared to an inter ligand transition, which is the source of the yellow emission. When the bias voltage is increased, the rate


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of charge transfer from ligand to metal is also increased. This is due to the accumulation of more electrons in the ligand and holes in the metal, as shown in Figure 5e. According to the band diagram, more excitation energy is necessary for the charge transfer from the ligand to the metal as compared to a charge transfer process in the inter ligand system. Thus, the CIE diagram reveals a shift in the color coordinate toward a blue color space from the yellow color space with increasing excitation potential, as shown in Figure S10. All of the above findings reveal that our fabricated device demonstrates a voltage dependent EL emission. Hence, there is flexibility to adjust the correlated color temperature (CCT) of preference in our design without the need for any doping or lithographic processes. White color tuning can be set up by controlling the voltage while providing additional flexibility in the colors of OLEDs for inner decoration and lighting. Device Performance, Quantum Efficiency and Stability To further evaluate the optoelectronic performance of the device, by estimating the quantum efficiency (QE), which was found to be ~1.0%. The photoluminescence quantum efficiency of the intrinsic compound is ~14%. We have estimated that the CCT for CIE color coordinate (0.33, 0.32) of our fabricated broadband LED device is 5620 K, and the corresponding CRI is ~ 96 using blue LED light source. The result reveals an important fact that the device is capable of producing a white light spectrum which has a close resemblance to that for commercial white light emitters. It is noteworthy that commercial white light emitters are composed of multiple materials and function with the down conversion of photons, which induces a loss of energy in the process. In contrast, our design is composed of a singular material which can emit a broadband spectrum without the down conversion of photon energy. Thus, our device can potentially reduce the design complexity of current commercial devices. In addition, considering the importance of alkaline earth metal complexes as environmentally-friendly materials, herein, we demonstrate a


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proof-of-concept for producing a white light device that can potentially reduce the environmental footprints of current commercially available devices. We also estimated the stability of the device performance by repeatedly measuring the luminescence efficiency over a long period of time. The stability data is shown in Figure 6b. The device performance was consistent with a negligible change under ambient conditions for a period of more than 70 days. This indicates that our fabricated device has a high stability in term of light emission. The high stability of the device efficiency can be attributed to several factors including a) the fact that 1 is electrochemically and optically data, b) an excellent ZnO film layer is formed, since the ZnO film is deposited by an RF sputtering method instead of spin coating, and c) the film is protected by the top ITO/glass layer. Conclusions We have illustrated a judicious design of next generation broadband white light emitting device based on nano-rod shaped Sr-based single molecule. Such a non-rare-earth broadband emission behavior from a single molecule without any doping is rarely seen. We further used the supramolecule to design an electrically pumped broadband white light emitter. The LED device is highly stable and capable of emitting light with a CIE coordinate (0.33, 0.32) which is very close to that of white light. In addition, the photo- and electrochemical stability data of the supramolecular assembly and device indicated that our LED device is stable at ambient conditions. This work demonstrates that the significant concept “a single molecule can produce a broadband white light emission without any doping of lanthanides” can be verified and this goal can be achieved. Furthermore, simple fabrication process permits production costs to be dramatically reduced. Henceforth, in order to address the ever-growing demand for a suitable technology to meet global challenges, our study thus paves the way toward the design of high performance broadband LEDs.


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3. Experimental Section Materials and methods Chemical reagents were obtained from commercially available sources and used during experiments without further purification. Perkin-Elmer PARAGON 1000 FT-IR spectrometer has been used to measure the infrared spectra in the 4000–400 cm–1 range for compound 1 and the ligand using potassium bromide (KBr) pellet method. Perkin-Elmer 2400 CHN elemental analyzer was operated for the detailed elemental analyses of 1. Thermogravimetric analyses (TGA) were carried out using a Perkin-Elmer TGA-7 TG analyzer under nitrogen flow from 25 to 680 °C. Powder X-rays diffraction data were obtained by operating a Siemens D-5000 diffractometer at 40 kV, 30 mA for Cu Kα (λ = 1.5406 Å), with the scan speed of one second and a step size of 0.02° per step. The electronic characterizations of the device were performed using a Keithley 2400 electrometer. The optical measurements were performed using a Horiba Jobin Yvon iHR 550 spectrometer and 266 nm pulsed laser source. The spectral information was collected by iHR 550 spectrometer which was coupled with highly sensitive array detector “Synapse CCD by Horiba Jobin Yvon”. The carrier lifetime was recorded using a pulsed laser system of pulse width 20 ps and frequency 40 MHz, a Keithley 2520 high-speed electrometer, and a Horiba Jobin Yvon TRIAX 320. Synthetic Procedures for Compound 1 A mixture of Sr(NO3)2 (42.4 mg, 0.2 mmol), benzene-1,2,3-tricarboxylic acid hydrate (1,2,3-H3btc; 21.0 mg, 0.1 mmol), with KOH (0.1 M)/MeOH (5 drops) and H2O (4 mL) solutions was sealed in a Teflon-lined stainless steel Parr acid digestion bomb and treated at 120 °C for 3 days (72 hours) and then cooled down to ambient temperature for 2 days (48 hours). The transparent crystals were separated by filtration followed by aqueous washing and drying at


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ambient conditions. Good quality transparent crystals of compound 1 were synthesized in 40.7% yield with optimized reaction conditions. While synthesizing the compound, we were able to obtain the same product using different molar ratios of reactants (Sr(NO3)2:H3btc = 1:1 or 2:1), but the

yields

were

different.

Elementary

anal.

for

C19H22O17Sr

=

{[Sr(H2btc)2(MeOH)(H2O)2]·2H2O}. Calcd. (%): C, 37.37; H, 3.60; Exp. (%): C, 37.36, H, 3.51. IR data (KBr, cm−1): 1732(s), 1686(m), 1643(w), 1580(m), 1522(s), 1470(m), 1457(w), 1396(s), 1306(s), 1246(s), 1211(w), 1156(w), 1131(m), 1088(w), 995(m), 980(w), 901(s), 850(s), 799(w), 766(s), 690(m), 670(s), 570(w), 534(m), 465(m) (Figure S1). Crystal Structure Determination A Brucker-Nonius Kappa CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.7107 Å) was utilized to collect all the parameters for the internal lattice of crystalline structure of compound 1 at 200 K which are listed in Table S1 (ESI). CCDC = 1820038 contains the supplementary crystallographic data for this paper. These data can be found at the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Crystal structure refinement has been performed using the SHELXL-97 program package. Accurate cell dimensions and the crystal structure of 1 was obtained via direct methods and refined by fullmatrix least squares on F2.44 All the hydrogen atoms were clearly located and subsequently placed in their ideal, defined positions, with isotropic thermal parameters riding on their respective carbon atoms, whereas non-hydrogen atoms were defined anisotropically. Simulations Details Optimized molecular structure and electronic properties such as the density of states and band gap energy were calculated using DFT at the same level as that used for the periodic system.


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Its electronic structure was determined by using experimentally measured values for the unit cells of the supramolecular array, the atomic positions of elements and the lattice parameters of 1. The energy cutoff value for the plane-wave basis was set at 520 eV and 8×4×8 k-point meshes were used. The experimentally determined periodic structure of 1 were optimized until the maximum forces acting on every atom were lower than 1 meV/Å as well as the convergence threshold for energy was set to 10−5 eV. Electrical Measurements and Device Designing Procedure For conductivity measurements, we spin-coated the nanoparticles of the compound on top of a SiO2/silicon substrate. Ag electrodes were then deposited on the top of material, as shown in Figure 3 (a). Current-voltage characteristic of the compound at ambient conditions before and after application of a constant voltage (20 V) for 8 hours is shown in Figure 3b. From current density data, we calculated the conductivity by using formula, J = σE, where J is the current density, σ is the conductivity and E is the electric field. For the LED device fabrication, a silicon wafer with a 150 nm thick SiO2 dielectric layer was used as the substrate. The SiO2/Si substrate had been ultrasonically cleaned for 10 min in ethanol, acetone and deionized (DI) water, respectively, to eliminate any unwanted contaminants. The substrate was then heated on a hotplate at 80 °C for 10 min to remove the absorbed moisture, which resulted in the formation of a high quality film. A 199.6 nm thick silver layer was deposited on the pre-cleaned SiO2/Si substrate using thermal evaporation under high vacuum conditions (< 5 × 10‒7 torr). The substrate with the silver layer was loaded into a magneto sputtering system to deposit an n-ZnO thin film. A thin ZnO film with a thickness of 155.5 nm was deposited by radiofrequency (RF) magnetron sputtering. Cylindrical ceramic zinc with a 99.95% purity was used as the ~ 8 cm diameter and 2‒5 mm thick target. The distance between target and substrate was fixed


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at 15 cm. The sputtering system was evacuated to a pressure lower than 10‒5 torr using a diffusion and a primary pump. RF sputtering was performed in an argon and oxygen mixed gas system by supplying a 200 W RF power at 13.56 MHz frequency. The concentrations of Ar and O2 were regulated using mass flow controllers. The single-crystal nanorods of Sr-based supramolecule having cubic shape were ground to form nanoparticles. A solution of nanoparticles was prepared by dissolving them in ethanol. The solution was ultrasonicated for 4‒5 hours to completely disperse of the nanoparticles in the ethanol. This solution was spin-coated up to thickness of 341.9 nm on the top of the previous RF sputtered ZnO film. Finally, ITO glass was used as top electrode. ASSOCIATED CONTENT Supporting Information. Crystal Data, FTIR spectra, thermogravimetric analysis, powder Xray diffraction studies, crystal structure, hydrogen bonding distances, deconvoluted PL spectrum of the compound 1, TRPL spectrum, photoluminescence carrier lifetime PL spectra (before and after grinding), IV characteristics, CIE diagram of the EL emissions, total density of state (DOS) and partial density of states (PDOS) of 1. The following files are available free of charge. brief description (file type, i.e., PDF) AUTHOR INFORMATION Corresponding Author Kuang-Lieh Lu: [email protected] Yang-Fang Chen: [email protected] Author Contributions


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YFC and KLL supervised the overall project. MU, GH and KPB conceived the project. MU and KLL synthesized the crystal, performed the structural characterizations. GHL and SMP performed the crystallographic measurements. KPB and GH designed the device and measured all the optoelectronic properties. BS and HM performed the theoretical analysis. MU, KPB, GH, YFC, and KLL wrote the manuscript with the input from all of the authors. These authors contributed equally.

∆

Notes There are no conflicts to declare. ACKNOWLEDGMENT We gratefully acknowledge Taiwan International Graduate Program, Academia Sinica, and the Ministry of Science and Technology, Taiwan for their financial support. BS and MH thank the financial support provided by the Center of Atomic Initiative for New Materials (AI-Mat), National Taiwan University, Taiwan from the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan.

REFERENCES (1)

Pattison, P. M.; Tsao, J. Y.; Brainard, G. C.; Bugbee, B. LEDs for Photons, Physiology and Food. Nature 2018, 563, 493-500.

(2)

Oh, N.; Kim, B. H.; Cho, S.-Y.; Nam, S.; Rogers, S. P.; Jiang, Y.; Flanagan, J. C.; Zhai, Y.; Kim, J.-H.; Lee, J.; Yu, Y.; Cho, Y. K.; Hur, G.; Zhang, J.; Trefonas, P.; Rogers, J. A.; Shim, M. Double-Heterojunction Nanorod Light-Responsive LEDs for Display Applications. Science 2017, 355, 616.

(3)

Murawski, C.; Leo, K.; Gather, M. C. Efficiency Roll-Off in Organic Light-Emitting Diodes. Adv. Mater. 2013, 25, 6801-6827.

(4)

Pust, P.; Schmidt, P. J.; Schnick, W. A Revolution in Lighting. Nat. Mater. 2015, 14, 454.


18

Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5)

Nanishi, Y. The Birth of the Blue LED. Nat. Photonics 2014, 8, 884.

(6)

Xia, Z.; Liu, Q. Progress in Discovery and Structural Design of Color Conversion Phosphors for LEDs. Prog. Mater. Sci. 2016, 84, 59–117.

(7)

Xia, Z.; Meijerink, A. Ce3+-Doped Garnet Phosphors: Composition Modification, Luminescence Properties and Applications. Chem. Soc. Rev. 2017, 46, 275–299.

(8)

Wang, L.; Xie, R.-J.; Suehiro, T.; Takeda, T.; Hirosaki, N. Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives. Chem. Rev. 2018, 118, 1951-2009.

(9)

Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lüssem, B.; Leo, K. White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature 2009, 459, 234.

(10) Ye, S.; Xiao, F.; Pan, Y. X.; Ma, Y. Y.; Zhang, Q. Y. Phosphors in Phosphor-Converted White Light-Emitting Diodes: Recent Advances in Materials, Techniques and Properties. Mater. Sci. Eng. R 2010, 71, 1– 34 (11) Haider, G.; Usman, M.; Chen, T.-P.; Perumal, P.; Lu, K.-L.; Chen, Y.-F. Electrically Driven White Light Emission from Intrinsic Metal–Organic Framework. ACS Nano 2016, 10, 83668375. (12) Usman, M.; Haider, G.; Mendiratta, S.; Luo, T.-T.; Chen, Y.-F.; Lu, K.-L. Continuous broadband emission from a metal–organic framework as a human-friendly white light source. J. Mater. Chem. C 2016, 4, 4728-4732. (13) Cornelio, J.; Zhou, T.-Y.; Alkaş, A.; Telfer, S. G. Systematic Tuning of the Luminescence Output of Multicomponent Metal–Organic Frameworks. J. Am. Chem. Soc. 2018, 140, 15470-15476. (14) Packwood, D. M.; Hitosugi, T. Materials Informatics for Self-Assembly of Functionalized Organic Precursors on Metal Surfaces. Nat. Commun. 2018, 9, 2469. (15) Lescop, C., Coordination-Driven Syntheses of Compact Supramolecular Metallacycles toward Extended Metallo-organic Stacked Supramolecular Assemblies. Acc. Chem. Res. 2017, 50, 885-894. (16) Holliday, B. J. & Mirkin, C. A. Strategies for the Construction of Supramolecular Compounds through Coordination Chemistry. Angew. Chem. Int. Ed. 2001, 40, 2022-2043. (17) Fan, C.; Wu, W.; Chruma, J. J.; Zhao, J.; Yang, C. Enhanced Triplet–Triplet Energy Transfer and Upconversion Fluorescence through Host–Guest Complexation. J. Am. Chem. Soc. 2016, 138, 15405-15412. (18) Hou, X.; Ke, C.; Bruns, C. J.; McGonigal, P. R.; Pettman, R. B.; Stoddart, J. F. Tunable Solid-State Fluorescent Materials for Supramolecular Encryption. Nat. Commun. 2015, 6, 6884. (19) Zhu, X.-H.; Peng, J.; Cao, Y.; Roncali, J. Solution-processable Single-Material Molecular Emitters for Organic Light-Emitting Devices. Chem. Soc. Rev. 2011, 40, 3509-3524.


19


Page 20 of 29

(20) Welte, L., Calzolari, A., Felice, R. D., Zamora, F. & Gómez-Herrero, J. Highly Conductive Self-Assembled Nanoribbons of Coordination Polymers. Nat. Nanotechnol. 2009, 5, 110. (21) Khalily, M. A.; Bakan, G.; Kucukoz, B.; Topal, A. E.; Karatay, A.; Yaglioglu, H. G.; Dana, A.; Guler, M. O. Fabrication of Supramolecular n/p-Nanowires via Coassembly of Oppositely Charged Peptide-Chromophore Systems in Aqueous Media. ACS Nano 2017, 11 , 6881-6892. (22) Lafferentz, L.; Ample, F.; Yu, H.; Hecht, S.; Joachim, C.; Grill, L. Conductance of a Single Conjugated Polymer as a Continuous Function of its Length. Science 2009, 323, 1193. (23) Faramarzi, V. et al. Light-Triggered Self-Construction of Supramolecular Organic Nanowires as Metallic Interconnects. Nat. Chem. 2012, 4, 485. (24) Goswami, N. et al. Luminescent Metal Nanoclusters with Aggregation-Induced Emission. J. Phys. Chem. Lett. 2016, 7, 962-975. (25) Klein, J. E. M. N.; Mandal, D.; Ching, W.-M.; Mallick, D.; Que, L.; Shaik, S., Privileged Role of Thiolate as the Axial Ligand in Hydrogen Atom Transfer Reactions by Oxoiron(IV) Complexes in Shaping the Potential Energy Surface and Inducing Significant H-Atom Tunneling. J. Am. Chem. Soc. 2017, 139, 18705-18713. (26) Basu Baul, T. S., Kundu, S., Ng, S. W., Guchhait, N. & Tiekink, E. R. T. Synthesis, Characterization, Photoluminescent Properties and Supramolecular Aggregations in Diimine Chelated Cadmium Dihalides. J. Coord. Chem. 2014, 67, 96-119. (27) Usman, M., Mendiratta, S. & Lu, K. L. Semiconductor Metal–Organic Frameworks: Future Low‐Bandgap Materials. Adv. Mater. 2017, 29, 1605071. (28) Allendorf, M. D., Bauer, C. A., Bhakta, R. K. & Houk, R. J. T. Luminescent Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1330-1352. (29) Rao, X.; Huang, Q.; Yang, X.; Cui, Y.; Yang, Y.; Wu, C.; Chen, B.; Qian, G. Color Tunable and White Light Emitting Tb3+ and Eu3+ Doped Lanthanide Metal–Organic Framework Materials. J. Mater. Chem. 2012, 22, 3210-3214. (30) Carlos, L. D., Ferreira, R. A. S., de Zea Bermudez, V., Julian-Lopez, B. & Escribano, P. Progress on Lanthanide-Based Organic–Inorganic Hybrid Phosphors. Chem. Soc. Rev. 2011, 40, 536-549. (31) Kan, D. et al. Blue-light Emission at Room Temperature From Ar+-Irradiated SrTiO3. Nat. Mater. 2005, 4, 816. (32) Dale, S. H., Elsegood, M. R. J. & Coombs, A. E. L. Hydrogen Bond Directed Supramolecular Arrays Utilising Hemimellitic Acid: Solvent Inclusion Clathrates. Cryst. Eng. Commun. 2004, 6, 328-335. (33) Usman, M.; Mendiratta, S.; Batjargal, S.; Haider, G.; Hayashi, M.; Rao Gade, N.; Chen, J.W.; Chen, Y.-F.; Lu, K.-L. Semiconductor Behavior of a Three-Dimensional StrontiumBased Metal–Organic Framework. ACS Appl. Mater. Interfaces 2015, 7, 22767-22774.


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Page 21 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. (35) Morales-García, Á.; Valero, R.; Illas, F. An Empirical, yet Practical Way To Predict the Band Gap in Solids by Using Density Functional Band Structure Calculations. J. Phys. Chem. C 2017, 121, 18862-18866. (36) Becke, A. D. Perspective: Fifty Years Of Density-Functional Theory In Chemical Physics. J. Chem. Phys. 2014, 140, 18A301. (37) Shen, C.; Chu, J.; Qian, F.; Zou, X.; Zhong, C.; Li, K.; Jin, S. High Color Rendering Index White LED Based on Nano-YAG:Ce3+ Phosphor Hybrid With Cdse/Cds/Zns Core/Shell/Shell Quantum Dots. J. Mod. Opt. 2012, 59, 1199-1203. (38) Chen, L., Lin, C. C., Yeh, C. W. & Liu, R. S. Light Converting Inorganic Phosphors for White Light-Emitting Diodes. Materials 2010, 3, 2172. (39) Goswami, S.; Ray, D.; Otake, K.-i.; Kung, C.-W.; Garibay, S. J.; Islamoglu, T.; Atilgan, A.; Cui, Y.; Cramer, C. J.; Farha, O. K.; Hupp, J. T. A Porous, Electrically Conductive HexaZirconium(Iv) Metal–Organic Framework. Chem. Sci. 2018, 9, 4477-4482. (40) Sun, L.; Campbell, M. G.; Dincă, M. Electrically Conductive Porous Metal–Organic Frameworks. Angew. Chem. Int. Ed. 2016, 55, 3566-3579. (41) Deng, H.; Grunder, S.; Cordova, K. E.; Valente, C.; Furukawa, H.; Hmadeh, M.; Gándara, F.; Whalley, A. C.; Liu, Z.; Asahina, S.; Kazumori, H.; O’Keeffe, M.; Terasaki, O.; Stoddart, J. F.; Yaghi, O. M. Large-Pore Apertures in a Series of Metal-Organic Frameworks. Science 2012, 336, 1018. (42) Ye, S.; Liu, Y.; Chen, J.; Lu, K.; Wu, W.; Du, C.; Liu, Y.; Wu, T.; Shuai, Z.; Yu, G. SolutionProcessed Solid Solution of a Novel Carbazole Derivative for High-Performance Blue Phosphorescent Organic Light-Emitting Diodes. Adv. Mater. 2010, 22, 4167-4171. (43) Chang, Z., Yang, D. H., Xu, J., Hu, T. L. & Bu, X. H. Flexible Metal–Organic Frameworks: Recent Advances and Potential Applications. Adv. Mater. 2015, 27, 5432-5441. (44) Sheldrick, G., A short history of SHELX. Acta Crystallogr. A 2008, 64, 112-122.


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Figures

Equation 1. Self-assembly of rod-shape Sr-based butterfly-like supramolecular compound 1.

Scheme 1. The innovation in designing a white light emitting device from three components to a single molecule over a period of time.


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Figure 1. (a) Compound 1 is connected in an ABAB fashion in the c-axis, which is extended into 2D sheets. (b) The weak parallel displaced -stacking interactions in between the neighbouring supramolecular arrays in compound 1. (c) Hydrogen-bonding interactions (O10‒H10E···O3 = 1.892 Å; O10‒H10D···O4 = 1.863 Å; (O2‒H2···O10 = 1.811 Å; O6‒H6···O10 = 1.850 Å; O9‒ H9B···O8 = 2.262 Å; O8‒H8···O4 = 2.215~2.616 Å) are shown in between the four neighbouring supramolecular units and listed in the table.


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Figure 2. (a) Photoluminescence spectra of 1 at variable laser power under illumination by a 266 nm pulsed laser. (b) CIE chromaticity diagram highlighting corresponding chromaticity coordinates of 1. The CIE coordinate corresponding to the emission was found to be (0.19, 0.25). The inset is a picture taken by a mobile camera while the material is exposed to 266 nm laser during the PL measurements. (c) Photoluminescence spectrum of organic molecule (benzene1,2,3-tricarboxylic acid hydrate). (d) Time Resolved Photo luminescence (TRPL) measurement: TRPL decay curve at different peak positions of PL spectra under 374 nm laser illumination.


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Figure 3. (a) A schematic of the device for electrochemical study, (b) current–voltage characteristic of compound 1 at ambient conditions before and after the application of a constant voltage (20 V) for 8 hours, (c) photoluminescent spectra and (d) Raman spectra of the compound under the same experimental conditions, respectively.


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Figure 4. (a) Optimized periodic structure of 1 with solvent molecules and energy band structure for HOMO‒LUMO states of 1 with solvent molecules (solvent molecules highlighted), (b) optimized periodic structure of 1 without solvent molecules and energy bands structure for HOMO‒LUMO states of 1 without solvent molecules. (Magenta: strontium; red: oxygen; dark grey: carbon; light grey: hydrogen).


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Figure 5. (a) Schematic illustration of visible broadband LED device, where the p-n junction is formed by 155.5 nm ZnO and Sr-compound on the top of the Ag 199.6 nm film. p-type Si/SiO2 and ITO were used as the substrate and the top electrode, respectively. (b) Optical photo of light emission from device taken by mobile camera. (c) Electroluminescence (EL) spectrum of Srcompound based LED device. (d) Scanning electron microscopic image of the solid-state light emitting device (cross sectional view). e) Energy band diagram for the constituent materials of the device showing different transitions during carrier injections.


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Figure 6. (a) EL spectra of the device under a forward injection current of 20 mA and 30 mA. (b) Stability of the device performance over time.


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Table of Contents


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Single-Molecule-Based Electroluminescent Device as Future White

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