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A Blue Light Emitting Cadmium Coordination Polymer with 75.4% External Quantum Efficiency José Antônio do Nascimento Neto, Ana Karoline Silva Medanha Valdo, Cameron Capeletti da Silva, Freddy Fernandes Guimarães, Luiz Henrique Keng Queiroz Júnior, Lauro June Queiroz Maia, Ricardo Costa De Santana, and Felipe Terra Martins J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Journal of the American Chemical Society

A Blue Light Emitting Cadmium Coordination Polymer with 75.4% External Quantum Efficiency José Antônio do Nascimento Neto,† Ana Karoline Silva Mendanha Valdo,† Cameron Capeletti da Silva,† Freddy Fernandes Guimarães,† Luiz Henrique Keng Queiroz Júnior,† Lauro June Queiroz Maia,‡ Ricardo Costa de Santana,‡ and Felipe Terra Martins*,† † Instituto ‡

de Química, Universidade Federal de Goiás, CP 131, 74001-970, Goiânia, GO, Brazil.

Instituto de Física, Universidade Federal de Goiás, CP 131, 74001-970, Goiânia, GO, Brazil.

Supporting Information ABSTRACT: We report a novel bright deep-blue emitting crystal form based on a simple cadmium coordination polymer with an impressive external quantum efficiency of 75.4(9)%, which means, at least thus far, the most efficient photoluminescent material in the solid state.

Organic light emitting devices (OLEDs) as solid state lighting sources have meant the enterprise of display technologies.1–3 In this context, the search for new lightemitting materials in the solid state with maximum external quantum efficiency (EQE) in the visible spectral range is crucial to improve the performance of electronic devices and has attracted interest of many chemists of materials and electronics companies.4–9 EQE is defined as the ratio of emitted photons to the total incident photons over the material, while the internal quantum efficiency (IQE) is the ratio of emitted photons to the absorbed photons by the material.10 Therefore, since high IQE can have low absorption associated, the external efficiency of a light-emitter dictates its applicability into OLEDs of practical uses. In this context, photoluminescence assessment is the first step in the screening of such new promising candidates.7,8,11–18 Lately, heavy-metal complexes have been extensively explored in order to obtain high IQE in the solid state, approaching 100%,7,8,15–17,19–23 even though EQE does not reach ca. 50%. In this study, we report a novel bright deep-blue emitting crystal form based on a simple cadmium coordination polymer with an impressive EQE of 75.4%, which means, at least up to now, the most efficient photoluminescent material in the solid state for the fabrication of blue OLED devices. Furthermore, its quantitative synthesis is achieved at room temperature in one solution preparation step needing only low cost reagents {cadmium acetate dihydrate [Cd(AcO)2.2H2O] and aminopyrazine (ampyz) in equimolar ratio} and vehicle (ethyl alcohol), followed by slow vehicle evaporation and isolation of the monophasic crystalline product, adding still more attractive advantages to

this material. Its synthesis and characterization is provided in the Supporting Information (SI). The complex [Cd(ampyz)(AcO)2(H2O)]n (1) is a infinite 1D coordination polymer made up of Cd2+ transition metal ions spaced by 7.5660(5) Å through an ampyz (Figure 1). Each Cd2+ center is seven-coordinated by two nitrogens from two bidentate ampyz molecules, four oxygens from two bidentate acetate anions and one oxygen atom from one water molecule. Its coordination geometry can be described as somewhat distorted pentagonal bipyramidal, with oxygens forming the distorted basal plane [root-mean square deviation (RMSD) from the least-square plane calculated through them of 0.272 Å] and the nitrogens occupying the axial positions. Therefore, acetates are almost perpendicular to ampyz. The mean plane calculated through non-hydrogen pyrazine atoms form angles of 82.9(2)° and 85.52(18)° with those of acetates. The bond angles O—Cd—O around the metal ion are ranging from 53.55(8)° (oxygens from same acetate) to 138.11(4)° (oxygens from different acetates) while the N—Cd—N angle measures 175.33(12)°. The Cd—Oacet coordination bonds range between 2.361(2) Å and 2.464(2) Å, while the Cd— Ow one measures 2.273(3) Å. The Cd—Nampyz coordination bonds measure 2.397(3) Å. As aforementioned, this complex can be described as a polymeric structure formed by [Cd(ampyz)(AcO)2(H2O)]n units, which can be expanded by mean of the Cd—N coordination bond (Figure 1). Furthermore, all hydrogen bonding functionalities are saturated in this structure. There are two classical intramolecular hydrogen bonds between the amino group and acetates. The amino group is also an intermolecular hydrogen bonding donor to another acetate anion from a neighboring chain packed perpendicular to [010] with a rise of 2.15 Å normal to [201] (Figure 1). Even though amino group is disordered over to occupancy site sets, both of them have the same contacts pattern with slight differences in their metrics which is reflected in their similar refined site occupancy factors [52.6(2)% and

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Figure 1. Crystal structure drawing of 1. Cyan dashed lines outline hydrogen bonds. CH hydrogens and NH2 group fraction with 47.4(2)% occupancy, which is bonded to carbon vicinal to that holding the shown 52.6(2)% NH2 sites, were omitted for clarity.

47.4(2)%]. Polymeric chains packed perpendicular to [010] are also strongly kept in contact through hydrogen bonds between water and acetate ligands, which propagate infinitely along [001] (Figure 1). These two patterns of intermolecular hydrogen bonds bearing either amino or water as donors to acetate ligands alternate perpendicularly to [010] and give rise to a 3D network of strongly hydrogen bonded polymeric chains.

fuse reflectance is higher than 80% for wavelengths above 410 nm. The strong PL emission spectrum upon excitation with a 365 nm UV light consists of a wide band with maximum centered at 402 nm, corresponding to deep-blue color [colorimetric coordinates x = 0.167 and y = 0.037, Figure 2(b)]. The colorimetric coordinates of our sample were included in the CIE 1931 chromaticity diagram of Figure 2(b).

Excitation and photoluminescence (PL) emission spectra of 1 are shown in Figure 2(a). The excitation spectrum was acquired monitoring the emission band at 402 nm and consists of a broad band in the UV region with two maxima at 275 nm and 365 nm, which is well correlated with diffuse reflectance spectrum (DRS) presenting two absorption bands centered at 270 and 370 nm [Figure 2(a)]. The dif

An impressive EQE of 75.4(9) % was observed with a corresponding IQE of 77.7(9) %. To the best of our knowledge, an EQE of approximately 64% has been achieved for green phosphorescent OLEDs,24 while this value attained 37% for blue thermally activated delayed fluorescence (TADF) OLEDs.17

100 80

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Emi = 402 nm

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Exc = 365 nm

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The decay of the excited state (ES) recorded at room temperature is shown in Figure 3, where the ES population exhibits a bi-exponential decay with lifetimes of 10.04(2) ns (92.96%) and 190(7) ns (7.04%). This observed decay behavior could be caused by different triplet sublevels within the same ES (due to spin-orbit coupling) or different emissive levels as found in our TD-DFT calculations. 70000

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50000 40000 30000 20000 10000 0

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Time (ns)

(b)

Figure 3. ES decay profile of 1.

Figure 2. (a) Excitation, emission and diffuse reflectance spectra of 1 (the photograph shows two penicillin-type vials with ca. 10 mg each, within a UV chamber). (b) CIE 1931 color coordinates of the PL emission, represented by the white circle.

In order to shed some light on the unexpected extremely high EQE of our material, TD-DFT spectra were computed with different basis sets (def2-SVP, def2-TZVP)25 and exchange correlation functionals (B3LYP, camB3LYP, M06-2X).26 A model consisting of one Cd atom and its ligands was used in the calculations, with optimiza-


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tion of hydrogens only, at all levels of theory. Likewise, a larger model with four above units, extracted out from crystal structure to have all possible intermolecular hydrogen bonds, as shown in Figure S1 (in SI), was also inputted in the calculations. All calculations show four electronic transitions for the experimental absorption band at 365 nm, except when using the B3LYP exchange correlation functional. It is well known that the M06-2X exchange correlation functional describe more accurately hydrogen bonds. Therefore, using this functional and cam-B3LYP in the TD-DFT, the transition with the largest oscillator strengths at 365 nm region correspond to HOMO/HOMO-1 to LUMO, followed by that from HOMO/HOMO-1 to LUMO+1 (Figure 4). The HOMO and HOMO-1 are almost degenerated with similar shape, being mainly distinguished from each other by phase difference. In both HOMO and HOMO-1, the wave function is spread over the two ampyz ligands, while LUMO and LUMO+1 are localized over just one of them. Therefore, the main transitions related to the highest EQE known thus far are due to a high-efficiency delocalization-to-localization charge transition phenomenon.27

ASSOCIATED CONTENT Supporting Information Crystallographic Information File (CCDC 1884658). PDF file with synthesis, crystallographic, optical and computational procedures and further characterization of 1 (PXRD, IR, 1H and 13C NMR). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank the Brazilian Research Council CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for the financial support. F.T.M. and L.J.Q.M. also thanks the CNPq for research fellowship.

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Figure 4. Experimental excitation and TD-DFT spectra calculated with different basis set and exchange correlation functional for one Cd2+ and its ligands (only hydrogens optimization; the model is shown on the top). ▬ • Experimental; ▬ TDDFT/B3LYP/def2-TZVP; ▬ TD-DFT/cam-B3LYP/def2-SVP; ▬ TD-DFT/cam-B3LYP/def2-TZVP; ▬ TD-DFT/M06-2X/def2SVP; ▬ TD-DFT/M06-2X/def2-TZVP. TD-DFT/M06-2X/def2SVP calculations for a larger model accounting inter-chain hydrogen bonds (Figure S1 in SI) were also performed with ▬ H optimization only or ▬ full optimization.

In summary, we have discovered a solid-state material with the highest energy conversion rate from UV to visible light known thus far. The next steps will consist of fabricating high-performance blue OLED devices with this crystal form and further exploration of its fascinating optical property to conceive other high performance light emitting devices.

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TOC graphic caption: A novel bright deep-blue emitting crystal form based on a simple cadmium coordination polymer with an impressive photoluminescence external quantum efficiency of 75.4%.

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