Tuning Intramolecular Conformation and Packing Mode of Host

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Article Cite This: ACS Omega 2019, 4, 9129−9134

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Tuning Intramolecular Conformation and Packing Mode of Host Materials through Noncovalent Interactions for High-Efficiency Blue Electrophosphorescence

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Zhicai Chen, Huanhuan Li, Ye Tao,* Lingfeng Chen, Cailin Chen, He Jiang, Shen Xu, Xinhui Zhou,* Runfeng Chen,* and Wei Huang Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China S Supporting Information *

ABSTRACT: Molecular conformation plays an important role in tuning the packing modes of organic optoelectronic materials to achieve enhanced and/or balanced charge transport. Here, we introduce the noncovalent intramolecular interactions to the host materials of phosphorescent organic light-emitting diodes (PhOLEDs). Different numbers and/or positions of intramolecular CH···N noncovalent interactions were constructed by using different N-heterocycles of pyridine, pyrimidine, and pyrazine as acceptor units and carbazole as the donor unit in a donor-acceptor-donor (D-A-D) motif. Thus, designed D-A-D molecules were synthesized facilely through a one-step Ullmann reaction in high yields, showing varied intramolecular interactions to regulate the molecular conformation significantly. Impressively, owing to the quasi-parallel molecular conformation, which is beneficial for forming facile transporting channels of both holes and electrons, the newly designed host material of 9,9′-(pyridine-2,5-diyl)bis(9Hcarbazole) exhibits good device performance of blue PhOLEDs with current, power, and external quantum efficiencies up to 33.0 cd A−1, 32.1 lm W−1, and 16.3%, respectively. This work highlights the significant importance of the noncovalent interactions in designing advanced organic semiconductors for high-performance optoelectronic devices.

1. INTRODUCTION Phosphorescent organic light-emitting diodes (PhOLEDs), which can approach a 100% internal quantum efficiency through harvesting both singlet and triplet excitons for electroluminescence, have attracted considerable attentions in the academic research and commercial industry owing to their fundamental scientific importance and wide application potentials especially in the flat-panel display and solid-state lighting.1−6 However, the phosphorescent heavy-metal complexes with long-lived triplet excited states always suffer from serious triplet excitons induced quenching effects, such as triplet−polaron quenching, triplet−triplet annihilation, and singlet−triplet annihilation.7 Therefore, host−guest systems are widely used to disperse the emissive phosphors to suppress these quenching effects.8−12 Several essential demands of the host materials should be considered for the development of high-performance PhOLEDs. First, the host materials should have a higher triplet energy (ET) level than that of emissive phosphors to allow for efficient forward energy transfer from the host to dopant and guarantee triplet excitons in the emissive layer (EML). Second, the frontier molecular orbital (FMO) energy levels of host materials should well match with their neighboring active layers for enhancing the carrier injections and decreasing the driving voltages. Third, the © 2019 American Chemical Society

hosts should have excellent and balanced hole and electron transportation properties to broaden the recombination area in the EML.1,13 To meet these requirements simultaneously, bipolar molecules capable of both efficient hole and electron transports are currently drawn tremendous attentions among various host materials for high-performance PhOLEDs.14,15 The incorporation of donor (D) and acceptor (A) unit into a molecular skeleton through various linkages has been proved to be one of the most successful strategies to design bipolar host materials. Nevertheless, the host molecules in D-A structures not always exhibit efficient and balanced bipolar carrier flux ability, mainly due to their inferior molecular packing modes in the aggregated film states.1 Recently, several groups have demonstrated that the molecular conformation and intermolecular interactions can be rationally modulated through the utilization of conformation locking via intramolecular noncovalent interactions (NCIs), such as H···N, O···S, F···S, N···S, and so on; thus, effectively tuned the packing mode in film Received: March 15, 2019 Accepted: May 7, 2019 Published: May 23, 2019 9129

DOI: 10.1021/acsomega.9b00724 ACS Omega 2019, 4, 9129−9134

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states by NCIs leads to significant enhancement of the electrical performance of organic semiconductors.16−19 Here, we introduce NCIs into the host materials of blue PhOLEDs in D-A-D molecular architectures to explore the effect of the NCIs on the electrical performance of organic semiconductors. Carbazole with sufficiently high triplet energy and good hole-transporting ability was selected as the donor unit,20 and the strong electron-deficient nitrogen sixmembered heterocyclic rings of pyridine, pyrimidine, and pyrazine were used as the accepters to construct three new DA-D host molecules with noncovalent intramolecular CH···N interactions.19,21−23 These host molecules with high ET were facilely prepared through a one-step C−N coupling reaction in high yields.23 Combined computational and experimental investigations showed that the intramolecular CH···N interactions can successfully tune the molecular conformation and modulate their corresponding packing modes in solid states. Importantly, with the aid of NCIs, excellent charge transportation properties were achieved successfully and when used as host materials of blue PhOLEDs based on bis[2-(4,6difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) (FIrpic), high device performance with maximum external quantum efficiencies (EQEs) up to 16.3%, current efficiencies (CEs) of 33.0 cd A−1, and power efficiencies (PEs) of 32.1 lm W−1 were observed. These findings in purposefully modulating the molecular packing mode of host materials via intramolecular NCIs provide an effective strategy for the noncovalent molecular design and development of high-performance organic semiconductors.

Figure 1. (a) Synthetic procedure of D-A-D host molecules with noncovalent CH···N interactions. Conditions: (i) potassium carbonate (K2CO3), copper powder, nitrobenzene, 2,5-dibromopyridine, 2,5-dibromopyrimidine, or 2,5-dibromopyrazine, 180 °C, 48 h. (b) Single crystal structures of PyDCz, PymDCz, and PyaDCz.

also illustrated in Figure S7. Compared to PymDCz with a large difference torsion angle between α and β, the crystals of PyDCz and PyaDCz show more tightly and orderly packing mode, which may be due to their quasi-parallel arrangement of carbazole endowed by conformation locking through noncovalent CH···N interactions, thus leading to the greatly improved intermolecular charge transportation. (Figure S7). Thermal properties of these D-A-D molecules were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Figure S9). These molecules exhibit excellent thermal stabilities with decomposition temperatures (Td), corresponding to 5% weight loss of 320, 331, and 328 °C for PyDCz, PymDCz, and PyaDCz, respectively. The melting point (Tm) of PyDCz and PymDCz is as high as 229 and 258 °C (Table 1), respectively. These results indicate that the thermal properties are enhanced by the introduction of additional nonbonding CH···N interactions, suggesting their promising potential in the thermally evaporated device fabrications.21 2.2. Optical Properties. The UV−vis absorption and photoluminescence (PL) spectra of these D-A-D molecules with noncovalent CH···N interactions were studied in both dichloromethane (DCM) solution and aggregated film states (Figure 2). The absorption spectra of PyDCz, PymDCz, and PyaDCz are quite similar in both solution and film states, revealing two strong absorption band peaked at ∼290 and ∼340 nm, which could be ascribed to the π−π* and n−π* transitions of carbazole moiety, respectively.24,25 For the PL spectra, PyDCz, PymDCz, and PyaDCz show broad and structureless emission peaks (λmax) at 382, 431, and 445 nm in DCM solution, respectively, indicating their intramolecular charge transfer (ICT) emission features due to the CT from the electron donating carbazole unit to highly electrondeficient nitrogen heterocyclic ring.26 The characteristics of ICT emissions were further verified by the emission profiles in the different solvents with variated polarities. With increasing

2. RESULTS AND DISCUSSION 2.1. Design, Synthesis, and Thermal Stability. To take advantage of NCIs for the controlling of molecular conformation of host materials, three molecules composed of different CH···N interactions, namely, 9,9′-(pyridine-2,5-diyl)bis(9H-carbazole) (PyDCz), 9,9′-(pyrimidine-2,5-diyl)bis(9Hcarbazole) (PymDCz), and 2,5-di(9H-carbazol-9-yl)pyrazine (PyaDCz), were designed in a directly linked D-A-D structure and readily synthesized through a one-step C−N coupling of a series of electron-deficient nitrogen heterocyclic rings and carbazole using a classic Ullmann reaction in high yields up to 92% (Figure 1a).22 These compounds were fully characterized by 1H and 13C nuclear magnetic resonance (NMR), matrixassisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), and elemental analyses (Figures S1−S6, Supporting Information). The molecular conformation and intra/intermolecular interactions of PyDCz, PymDCz, and PyaDCz were further investigated by the single-crystal X-ray diffraction (XRD) analysis (Figure 1b and Figure S7). With the increase of the number of noncovalent CH···N interactions, these D-A-D molecules exhibit enhanced intramolecular interactions with a distance of 2.700 Å for PyDCz, 2.625 Å for PyaDCz, and 2.400 Å for PymDCz and significantly decreased torsion angle (α) between carbazole and nitrogen heterocyclic ring with an angle of 46.6° for PyDCz, 43.2° for PyaDCz, and 26.0° for PymDCz. However, the remarkably increased torsion angle (β) is observed in PymDCz, which should be attributed to its stronger H−H repulsion compared to that of PyDCz and PyaDCz as revealed by the reduced density gradient (RDG) (Figure S8). These results indicate that NCIs can successfully regulate the molecular conformation at the single-molecule state. The detailed packing modes of these host molecules are 9130

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Table 1. Photophysical and Electrochemical Properties of PyDCz, PymDCz, and PyaDCz and their Device Performance of Blue PhOLEDs λabs (nm)

λem (nm)

comp.

Td/Tm(°C)

DCM

film

PyDCz PymDCz PyaDCz

320/229 331/258 328c

290, 340 293, 338 288, 328

292, 342 298, 338 291, 329, 403

opt

Eg (eV)

CV (eV)

device performance

DCM

film

Eexp T(eV)

HOMO

LUMO

Von (V)

max efficiencya

382 431 445

393 421 465

3.03 2.98 2.66

−5.71 −5.82 −5.73

−2.31 −2.64 −2.86

3.2 3.9 3.7

33.0, 32.1, 16.3 23.7, 17.3, 11.3 14.6, 11.7, 6.5

3.40 3.18 2.87

b

In the order of CE (cd A−1), PE (lm W−1), and EQE (%). bCalculated according to the equation of ELUMO = Eg + EHOMO. cTm is not determined.

a

of commonly used blue phosphors of FIrpic, which can support exothermic energy transfer from the host to FIrpic.11 All of the optical properties data are summarized in Table 1. 2.3. Electrochemical and Theoretical Investigations. The electrical properties of these D-A-D molecules were investigated by both experimental cyclic voltammetry (CV) measurements and density functional theory (DFT) calculations.28 The highest occupied molecular orbital (HOMO) energy levels of PyDCz, PymDCz, and PyaDCz were determined to be −5.71, −5.82, and −5.73 eV according to their onset oxidation potentials at 0.95, 1.06, and 0.97 V (Figure S12), respectively. The lowest occupied molecular orbital (LUMO) energy levels were calculated on the basis of opt Egs and HOMOs.13 The experimental values are listed in Table 1. The theoretical calculations demonstrate that their HOMOs are distributed through the whole molecular skeleton for these three compounds, endowing them with very similar HOMO energy levels either for theoretical or experimental values (Figure 3).31 The LUMOs are mainly dispersed on the

Figure 2. UV−vis absorption and PL spectra of PyDCz, PymDCz, and PyaDCz in (a) DCM (∼10−5 M) solution and (b) thin film state (solid and open symbols are for absorption and PL spectra, respectively).

solvent polarities from hexane to methanol, the significantly red-shifted luminescent peaks in the PL spectra of these three compounds are observed (Figure S10), testifying again the typical ICT emissions of these compounds in solution.26,27 In film states, PyDCz and PyaDCz show slightly red-shifted emission compared to their emission profile in the solution, with a λmax at around 393 and 465 nm, respectively. However, PymDCz exhibits a blue-shifted emission with a λmax at 421 nm, which may be due to its slightly reduced ICT emission that caused by the twist molecular conformation in the film state (Figure S7). Notably, these PL spectra demonstrate additional emission peaks in the range of 550−650 nm, which may be originated from the intermolecular aggregation and π−π stacking.28 The optical band gap (optEg) of these molecules was estimated from the film absorption edge.29 The calculated optEgs of PyDCz, PymDCz, and PyaDCz were 3.40, 3.18, and 2.87 eV, respectively. The phosphorescence spectra of these molecules were measured in methyltetrahydrofuran (Me-THF) solution at 77 K with a 5 ms delay time (Figure S11).30 The triplet energy levels (ETs) of PyDCz, PymDCz, and PymDCz were identified to be 3.03, 2.98, and 2.66 eV according to their highest energy vibronic peaks at ∼410, ∼416, and 466 nm, respectively. The lower ET of PyaDCz may be attributed to its relatively stronger intermolecular donor−acceptor interactions. To our delight, the ETs of PyDCz, PymDCz, and PyaDCz are higher than that

Figure 3. Theoretical (black) and experimental (red) energy levels of PyDCz, PymDCz, and PyaDCz, as well as the isosurfaces of the frontier orbitals and spin density distributions (SDDs).

pyridine for PyDCz, pyrimidine for PymDCz, and pyrazine for PyaDCz; therefore, the lowest LUMO energy level of PyaDCz may be due to its stronger electron deficient ability of pyrazine than that of pyridine and pyrimidine.32,33 The spin density distribution (SDD) was also calculated to further understand the properties of triplet excited state; the delocalized triplet 9131

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Figure 4. (a) Device configuration and energy level diagram of the materials used in the blue PhOLEDs hosted by PyDCz, PymDCz, and PyaDCz. (b) Current density−voltage and luminance−voltage curves and electroluminescent spectra at 8.0 V (insert). (c) Efficiency−current density curves of the blue PhOLEDs and (d) current−voltage (J−V) characteristics of the single-carrier transporting devices based on PyDCz, PymDCz, and PyaDCz (solid symbols represent the electron-only device, while open symbols are for the hole-only device).

excited state with a high ET would be helpful for the energy transfer from the host to the guest. The natural transition orbital (NTO) analysis of three molecules was performed (Figure S13) to reveal the feature of their excited state. These three molecules exhibited quite similar hybrid localized and charge transfer state, demonstrating that the triplet energy should be mainly dominated by the whole molecule. Thus, the lowest ET of PyaDCz may be attributed to the lowest triplet energy level of its acceptor unit of pyrazine. 2.4. Electroluminescence Performance of Blue PhOLEDs. In light of the high thermal stability, acceptable ET, and suitable FMO energy levels of these D-A-D molecules of PyDCz, PymDCz, and PyaDCz, the thermally evaporated blue PhOLEDs with the conventional structure of ITO/poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, 30 nm)/1,1-bis(di-4-tolylaminophenyl)cyclohexane (TAPC, 20 nm)/1,3-bis(N-carbazolyl)benzene (mCP, 8 nm)/10 wt % FIrpic:host (PyDCz, PymDCz, or PyaDCz, 20 nm)/1,3,5tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB, 35 nm)/LiF (1 nm)/Al (100 nm) were prepared to evaluate their host properties (see in Figure 4a).10 In these devices, PEDOT:PSS and LiF were used as the hole- and electron-injection layers, respectively. FIrpic was served as the blue emissive phosphorescent dopant, TAPC and TmPyPB acted as the hole- and electron-transporting layers, respectively. Efficient forward energy transfer from the host to FIrpic was observed in the blue PhOLEDs hosted by PyDCz and PymDCz, which can be well verified by their pure electroluminescence (EL) spectra of FIrpic (Figure 4b). However, the EL spectra of PyaDCz endowed blue PhOLED not only exhibited a FIrpic emission with a band peaked at 476 and 499 nm, respectively, but also showed a small shoulder emission band at around 450 nm. In order to figure out the emission band, the PL spectra of the composite films with 10 wt %

doped FIrpic were investigated. As shown in Figure S14a, the PL spectra of PyDCz and PymDCz hosted films revealed the only emission bands of FIrpic without apparent luminescent peaks of the host materials, which are in good agreement with the EL spectra of FIrpic doped emitting layer; as similar as the EL spectra of PhOLED hosted by PyaDCz, the PL emission of the composite film also demonstrated an additional emission inherited from the PL emission of PyaDCz film (Figure S14a). Notably, with the increasing of doping concentrations of FIrpic, the emission peak from PyaDCz gradually became weaker, reflecting the enhanced energy transfer between from PyaDCz to FIrpic at higher doping concentrations. (Figure S14b).34 The appearance of host emission reveals that there is a reverse energy transfer from FIrpic to PyaDCz due to its relatively low ET of 2.66 eV. The blue PhOLEDs hosted by the D-A-D molecules obtained high device performance with turn-on voltages (Von) of 3.2, 3.9, and 3.7 V, current efficiencies (CE) of 33.0, 23.7, and 14.6 cd A−1, power efficiencies (PE) of 32.1, 17.3, and 11.7 lm W−1, and EQEs of 16.3, 11.3, and 6.5% for PyDCz, PymDCz, and PyaDCz, respectively (Figure 4b,c). The device based on PyaDCz showed the lowest performance, which should attribute to its inefficient exothermic energy transfer. Notably, PyDCz hosted blue PhOLEDs exhibited a higher device performance than that of PymDCz, although they have quite similar frontier orbital energy levels and ETs. To understand the exact reasons of the good performance of PyDCz-based devices, the single-carrier transporting devices of these three molecules were evaluated through a space charge limited currents (SCLC) model (Figure 4d).15 The hole- and electron-only devices were fabricated using the structure of ITO/PEDOT:PSS (35 nm)/host (100 nm)/MoO3 (10 nm)/ Al and ITO/LiF (1 nm)/host (100 nm)/LiF (1 nm)/Al, respectively. The carrier transportation ability of these 9132

DOI: 10.1021/acsomega.9b00724 ACS Omega 2019, 4, 9129−9134

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91833306, 21674049, 21604039, and 21704042), 1311 Talents Program of Nanjing University of Posts and Telecommunications (Dingshan), the Six Talent Plan (2016XCL050), the Natural Science Fund for Colleges and Universities in Jiangsu Province (grant no. 17KJB150017), the China Postdoctoral Science Foundation funded project (2018M642284), and the Scientific Starting Fund from Nanjing University of Posts and Telecommunications (NUPTSF) (NY217140).

molecules is in the order of PymCz < PyaDCz < PyDCz, which should be the main reason for the highest PhOLEDs performance endowed by PyDCz. It should be also noted that although PyDCz showed the largest electron injection barrier owing to its highest LUMO energy level among these three hosts, PyDCz-based electron-only devices obtained the highest electron currents. The highest and most balanced hole and electron transporting ability of PyDCz would be ascribed to the combined results of its good molecular packing mode with multiple hole and electron transporting channels at aggregated states facilitated by the noncovalent CH···N interactions and small hole and electron reorganization energies (Table S2).35

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3. CONCLUSIONS In summary, we have designed and synthesized three D-A-D hosts with apparent NCIs to tune their molecular conformation and packing models for high-performance blue PhOLEDs. By changing the number and/or position of intramolecular CH···N NCIs, carbazole groups in PyDCz exhibit quasi-parallel arrangement for a close packing mode in their aggregated states. As indicated by the single-carrier devices and computational simulations, the well stacked D-A-D host of PyDCz possesses the facile transport channels of hole and electron simultaneously. Consequently, by using PyDCz as the host, the high-efficiency blue PhOLED based on FIrpic was successfully fabricated, showing a maximum CE of 33.0 cd A−1, a PE of 32.1 lm W−1, and an EQE of 16.3%. These results are comparable to the efficient blue PhOLEDs based on other types of bipolar hosts. Given the ease of synthesis and improved electrical performance through utilization of NCIs, this study provides an efficient design map for the construction of bipolar host materials capable of finely modulated molecular packing mode, opening up a new avenue for the design of highperformance organic semiconductor for advanced optoelectronic applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00724. Synthesis and characterization, crystallography, thermal properties, electrochemical properties, DFT calculations, and device fabrication and characterization (PDF) Crystallographic information of PyDCz, PymDCz, and PyaDCz (ZIP)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.T.). *E-mail: [email protected] (X.Z.). *E-mail: [email protected] (R.C.). ORCID

Xinhui Zhou: 0000-0001-6849-6979 Runfeng Chen: 0000-0003-0222-0296 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (61875090, 9133

DOI: 10.1021/acsomega.9b00724 ACS Omega 2019, 4, 9129−9134

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DOI: 10.1021/acsomega.9b00724 ACS Omega 2019, 4, 9129−9134