Dibenzothiophene Sulfone-Based Phosphine Oxide Electron

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Dibenzothiophene Sulfone-Based Phosphine Oxide Electron Transporters with Unique Asymmetry for High-Efficiency Blue Thermally Activated Delayed Fluorescence Diodes Chaochao Fan, Chunbo Duan, Chunmiao Han, Bin Han, and Hui Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10020 • Publication Date (Web): 23 Sep 2016 Downloaded from http://pubs.acs.org on September 29, 2016

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ACS Applied Materials & Interfaces

Dibenzothiophene Sulfone-Based Phosphine Oxide Electron Transporters with Unique Asymmetry for High-Efficiency Blue Thermally Activated Delayed Fluorescence Diodes Chaochao Fan,‡ Chunbo Duan,‡ Chunmiao Han*, Bin Han, and Hui Xu* Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Material Science, Heilongjiang University, 74 Xuefu Road, Harbin (150080), China KEYWORDS. phosphoine oxide, electron transporting material, thermally activated delayed fluorescence, asymmetrical configuration, interfacial effect. ABSTRACT. Three asymmetrical electron transporters as dibenzothiophene sulfone (DBSO)-diphenylphosphine oxide (DPPO) hybrids, collectively named mnDBSODPO, were designed and prepared. All of these materials achieve the high triplet energy of ~3.0 eV to restrain the exciton linkage from emissive layers (EML). Taking use of the dependence of inductive and steric effects for DPPO groups on substitution position, the intermolecular interaction suppression, the encapsulations of high-polar DBSO cores and the favorable electrical performance are successfully integrated on 36DBSODPO, which can simultaneously suppress the exciton 1 ACS Paragon Plus Environment


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quenching by formation of interfacial dipole and enhance the charge flux balance. As the result, 36DBSODPO endowed its tetralayer blue thermally activated delayed fluorescence (TADF) devices with the impressive performance, including the maximum external quantum efficiency around 19% and reduced efficiency roll-offs, which verifies the great potential of asymmetrical electron transporting materials for highly efficient TADF devices.

1. Introduction Thermally activated delayed fluorescence (TADF) materials and their organic light-emitting diodes (OLED) emerge in recent years.1-4 This kind of devices can harvest all of electrogenerated excitons, realizing the 100% internal quantum efficiency (IQE).5 Most of TADF emitters are pure-organic donor-acceptor (D-A) systems with strong molecular polarity and intramolecular charge transfer (ICT).6-7 Their separated frontier molecular orbitals (FMO) and CT featured excited states can give rise to the small singlet-triplet splitting (EST), thereby facilitate the reverse intersystem crossing (RISC) from the first triplet (T1) to the first singlet (S1) excited states, making triplet exciton utilization feasible during electroluminescence (EL) process.8-13 Nevertheless, the ICT character and high polarity of TADF dyes make their emission performance highly sensitive to environment and simultaneously worsen the intermolecular interaction-induced quenching,14-15 especially for blue emitters, which not only reduces EL efficiencies but also accelerate efficiency roll-offs.16-25 In this case, most of blue 2 ACS Paragon Plus Environment

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TADF diodes employ emissive layers (EML) as host matrixes doped with emitters.26 Furthermore, complicated device structures with multiple carrier transporting and interfacial modification layers are also adopted for both charge flux balance and interfacial interaction suppression.27 On the basis of a seven-layer device configuration, bis[4-(9,9-dimethyl-9,10-dihydroacridine)-phenyl]sulfone (DMAC-DPS) as one of the most popular blue TADF dyes, achieved the state-of-the-art external quantum efficiency (EQE) of 19.5% and reduced EQE roll-off less than 18% at 1000 cd m-2.27 In recent years, amount of high-efficiency blue TADF dyes are developed with EQE beyond 15%, however, still accompanied by remarkable efficiency roll-offs, which is known as the main bottleneck of blue TADF diodes for practical applications.16-23, 27-32 Concerning this issue, the development of high-performance host materials and carrier transporting materials (CTM), especially electron transporting materials (ETM), is crucial with comprehensive considerations in not only high T1 energy for exciton confinement and good electrical performance for charge carrier recombination as common requirements on host and ETM in blue-emitting devices,33 but also reduced interlayer and intralayer interactions for quenching suppression specific to blue TADF dyes with high environmental sensitivity and dependency.14,

34

Consequently, host materials with spiro-cyclic cores,35 asymmetrical configurations26, 36-37

and bulky groups38-42 for high efficient green and blue TADF devices were

constructed. Our group reported a series of electron-transporting dominant phosphine oxide (PO) host materials with asymmetric43-45 and twisted46 configurations, rendering the recombination zones approaching to hole-transporting layers (HTL) and restrained 3 ACS Paragon Plus Environment


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interaction-induced quenching in their blue and full-colour TADF diodes for high efficiencies and reduced roll-offs. Obviously, when employing hole-transporting predominant host materials, the interface between EML and electron transporting layers (ETL) should be modified to reduce the interfacial quenching effects. In our recent works, the negative influence of interfacial dipole on efficiencies of blue TADF devices was demonstrated, which can be reduced by separating the charged moieties of ETM molecules from interface through embedment or encapsulation with low-polar electron-transporting groups, e.g. diphenylphosphine oxide (DPPO).47-48 To further suppress interfacial quenching, it is rational to design ETMs referring to host development strategy, viz. reducing interfacial interaction through asymmetrical structures with locally irregular molecular packing.49 However, it is known that ordered molecular alignment is beneficial to realize high charge mobility. Consequently, there are only few asymmetrical ETMs reported to date.50 The main challenge of developing asymmetrical ETMs is how to harmonize the contradiction between electron transporting ability and interfacial interactions.

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Figure 1. (a) Synthetic procedure of mnDBSODPO: i. n-BuLi, Ph2PCl, THF, -78 oC, 1 h, 30% H2O2, CH2Cl2, 2 h; ii. 3-chloroperoxybenzoic acid, CH2Cl2, 8h; iii. HAc, H2SO4, NBS, 50 oC, 10 h; iv. NaOAc, Pd(OAc)2, Ph2PH, DMF, 130 oC, 24 h; Single crystal structures and packing diagrams of 26DBSODPO (b) and 36DBSODPO (c). Herein, to rationally figure out the primary and secondary determinants of device performance for ETM in simply structured blue TADF diodes, a series of 5 ACS Paragon Plus Environment


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dibenzothiophene sulfone (DBSO) derivatives 27DBSODPO, 26DBSODPO and 36DBSODPO, collectively named mnDBSODPO, were constructed by two DPPO substituents at 2,7-, 2,6- and 3,6-positions with different inductive and steric effects, respectively, to form asymmetrical configurations and various exposure degrees of their DBSO cores (Figure 1a). Owing to the insulating effect of PO groups, the T1 energy of mnDBSODPO is as high as ≈3.0 eV, effectively blocking the exciton leakage. Significantly, it is showed that the efficient interfacial exciplex suppression and high electron mobility (e) in the order of 10-4 cm2 V-1 s-1 are simultaneously achieved by 36DBSODPO in virtue of its ortho-DPPO with the largest encapsulation of DBSO and meta-DPPO with remarkable inductive effect at conjugation extension direction,

respectively.

In

consequence,

by

adopting

4,6-bis(diphenylphosphoryl)-dibenzothiophene (DBTDPO) and DMAC-DPS as host and dopant, respectively, 36DBSODPO endowed its simplified tetralayer blue TADF diodes with the state-of-the-art EQE of about 19%, accompanied with a reduced roll-off of 24% at 1000 cd m-2, which is comparable to the multilayer analogues. This work breaks common principle of ETM molecular design and demonstrates the potential of asymmetric ETMs for high-performance TADF devices.

2. Experimental Section

2.1. Reagents and Materials All the reagents and solvents were purchased from commercial companies and used directly. 2DBSOSPO and DBTSPO were synthesized on the basis of the previous 6 ACS Paragon Plus Environment

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references.48, 51 The materials used for device fabrication were purchased from Xi'An p-OLED company. The details of measurements were shown in Supporting Information.

2.2. Synthesis

2.2.1. General Procedure of Bromination Reaction At room temperature, phosphine oxide derivative (1 mmol) was dissolved in acetic acid (20 mL) and concentrated sulfuric acid (5 mL). Then the mixture was added in 0.178 g of NBS (1 mmol), and stirred at 50 oC for 10 h. The reaction was cooled to room temperature, and quenched with 4 mL of water. The system was extracted with dichloromethane. The dichloromethane layer was collected and dried with anhydride Na2SO4. Solvent was removed on a rotavapor. The residue was purified by silica gel chromatography. 2-Diphenylphosphoryl-7-bromo-dibenzothiophene

Sulfone

(7Br2DBSOSPO):

0.15 g of 7Br2DBSOSPO as white powder, yield of 30%. 1H NMR (TMS, CDCl3, 400 M Hz): δ = 8.400 (d, J = 11.2 Hz, 1H ), 7.978 (s, 1H ), 7.876 (d, J = 7.6 Hz, 1H ), 7.812 (d, J = 8.4 Hz, 1H ), 7.696 (q, J1 = 7.8, J2 = 10.6 Hz, 5H), 7.640 (t, J = 5.8 Hz, 3H ), 7.545 ppm (t, J = 7.8 Hz, 4H ); LDI-TOF: m/z (%): 494 (100) [M+]; elemental analysis (%) for C24H16BrO3PS: C 58.20, H 3.26, S 6.47; found: C 58.22, H 3.27, S 6.50. 6-Diphenylphosphoryl-3-bromo-dibenzothiophene

Sulfone

(3Br6DBSOSPO):

0.15 g of 3Br6DBSOSPO as white powder, yield of 30%. 1H NMR (TMS, CDCl3, 400 7 ACS Paragon Plus Environment


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M Hz): δ = 7.938 (d, J = 7.6 Hz, 1H ), 7.836 (s, 1H), 7.773 (q, J1 = 7.0, J2 = 12.6 Hz, 4H), 7.651 (d, J = 8.0 Hz, 3H ), 7.584 (t, J = 7.6 Hz, 2H ), 7.506 ppm (t, J = 7.4 Hz, 4H ); LDI-TOF: m/z (%): 494 (100) [M+]; elemental analysis (%) for C24H16BrO3PS: C 58.20, H 3.26, S 6.47; found: C 58.21, H 3.26, S 6.53. 6-Diphenylphosphoryl-2-bromo-dibenzothiophene (2BrDBTSPO): 0.17 g of 2BrDBTSPO as white powder, yield of 36%. 1H NMR (TMS, CDCl3, 400 M Hz): δ = 8.311 (d, J = 7.2 Hz, 1H ), 8.165 （s, 1H）, 7.765 (q, J1 = 11.2, J2 = 20 Hz, 5H ), 7.567 (t, J = 7.0 Hz, 2H ), 7.454 (m, 8H); LDI-TOF: m/z (%): 462 (100) [M+]; elemental analysis (%) for C24H16BrOPS: C 62.21, H 3.48, S 6.92; found: C 62.25, H 3.49, S 6.97.

2.2.2. General Procedure of Phosphorylation Reaction In Ar, bromide (1mmol), 0.09 g of NaOAc (1.1 mmol), 0.011 g of Pd(OAc)2 (0.05 mmol) and 0.2 mL of Ph2PH (1.1 mmol) were dissolved in DMF (25 mL). The system was stirred at 130 oC for 24h. The mixture was cooled to room temperature, and extracted with dichloromethane and water. The organic layer was dried with anhydrous Na2SO4, and removed in vacuo. The residue was dissolved in dichloromethane (6 mL). The mixture was added in 1 mL of H2O2 (30%), and stirring for 2h. The reaction was extracted with dichloromethane. The dichloromethane layer was collected and removed in vacuo, and the crude product was purified by flash column chromatography.


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2,6-Bis(diphenylphosphoryl)-dibenzothiophene (26DBTDPO): white powder (0.28 g), 45% yield. 1H NMR (TMS, CDCl3, 400 M Hz): δ = 8.711 (d, J = 12.4 Hz, 1H ), 8.333 (d, J = 8.0 Hz, 1H ), 7.880 (d, J = 8.0 Hz, 1H ), 7.728 (q, J1 = 9.6, J2 = 20.6 Hz, 8H), 7.585 (q, J1 = 7.0, J2 = 14.2 Hz, 5H), 7.500 ppm (m, 10H ); LDI-TOF: m/z (%): 484 (100) [M+]; elemental analysis (%) for C36H26O2P2S: C 73.96, H 4.48, S 5.48; found: C 74.01, H 4.49, S 5.52. 2,7-Bis(diphenylphosphoryl)-dibenzothiophene Sulfone (27DBSODPO): white powder (0.20 g), 38% yield. 1H NMR (TMS, CDCl3, 400 M Hz): δ = 8.667 (d, J = 12.8 Hz, 1H ), 8.295 (d, J = 12.4 Hz, 1H ), 8.200 (d, J = 8.4 Hz, 1H ), 7.955 (d, J = 8.0 Hz, 1H ), 7.700 (m, 10H), 7.589 (t, J = 7.2 Hz, 4H ), 7.501 ppm (t, J = 6.6 Hz, 8H ); LDI-TOF: m/z (%): 616 (100) [M+]; elemental analysis (%) for C36H26O4P2S: C 70.12, H 4.25, S 5.20; found: C 70.11, H 4.23, S 5.24. 3,6-Bis(diphenylphosphoryl)-dibenzothiophene Sulfone (36DBSODPO): white powder (0.18 g), 35% yield. 1H NMR (TMS, CDCl3, 400 M Hz): δ = 8.222 (t, J = 9.6 Hz, 1H ), 8.047 (d, J = 7.6 Hz, 1H ), 7.947 (d, J = 8.0 Hz, 1H ), 7.787 (m, 1H ), 7.629 (m, 8H), 7.496 ppm (m, 7H ); LDI-TOF: m/z (%): 616 (100) [M+]; elemental analysis (%) for C36H26O4P2S: C 70.12, H 4.25, S 5.20; found: C 70.13, H 4.26, S 5.27. 2,6-Bis(diphenylphosphoryl)-dibenzothiophene Sulfone (26DBSODPO): At room temperature, 26DBTDPO (0.584 g, 1mmol) in dichloromethane (15 mL) was added in 3-chloroperoxybenzoic acid (0.52 g, 3.0 mmol), and stirred for 8h. The reaction was extracted with dichloromethane and water. After removing solvent, the residue was 9 ACS Paragon Plus Environment


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purified by silica gel chromatography to afford 0.26 g of 26DBSODPO as white powder, 42% yield. 1H NMR (TMS, CDCl3, 400 M Hz): δ = 8.399 (d, J = 11.6 Hz, 1H ), 8.004 (d, J = 7.2 Hz, 1H ),7.768 (q, J1 = 7.2, J2 = 12.4 Hz, 5H), 7.669 (q, J1 = 5, J2 = 14.2 Hz, 6H), 7.589 (t, J = 7.8 Hz, 5H), 7.491 ppm (m, 8H); ; LDI-TOF: m/z (%): 616 (100) [M+]; elemental analysis (%) for C36H26O4P2S: C 70.12, H 4.25, S 5.20; found: C 70.10, H 4.25, S 5.21.

3. Results and Discussion

3.1. Design and Structures On account of the symmetry for two phenyls in DBSO core, only three unsymmetrical disubstituted constitutional isomers can be constructed through permutation of ortho-, meta- and para-position substitutions. Besides of serving as electron-transfer bridge, bulky DPPO groups in mnDBSODPO should have two functions, viz. enhancing steric hindrance to reduce intermolecular interactions and embedding high-polar sulfone moieties to suppress interfacial effects. Regarding to the former, the hindrance of ortho-DPPO at short axis of DBSO is the biggest, followed by para- and meta- DPPOs, successively; while, the encapsulation effect of DPPO on sulfone is directly proportional with the distance between them, i.e., ortho-DPPO > meta-DPPO >> para-DPPO. In this case, comprehensively considering the spatial effects of two different DPPO groups in mnDBSODPO, the intermolecular interactions of 27DBSODPO, 36DBSODPO and 26DBSODPO are successively decreased. Simultaneously, 36DBSODPO has the most encapsulated DBSO core, followed by 10 ACS Paragon Plus Environment

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26DBSODPO; while, DBSO in 27DBSODPO is almost naked. Obviously, 27DBSODPO and 26DBSODPO would be relatively inferior in either quenching suppression or electron transportation. In contrast, the two functions of DPPO groups are rationally integrated in 36DBSODPO with the appropriately reduced intermolecular interactions and the most separated high-polar DBSO core among mnDBSODPO, which can preserve the electron transporting ability to the greatest extent, at the same time of effectively suppressing DBSO-involved quenching. In this sense, it can be expected that the EML|ETL interfacial effect can be dramatically reduced by adopting 36DBSODPO as ETM without sacrificing electrical performance.

mnDBSODPO

can

be

conveniently

prepared

through

bromination,

phosphorylation and sulfoxidation with moderate yields of about 30% (Figure 1a). The structure characteristics were established on the basis of 1H nuclear magnetic resonance (NMR) spectra, mass spectra (MS) and elemental analysis (EA). The single-crystal structures of 26DBSODPO and 36DBSODPO are further confirmed with X-ray diffraction analysis (Figure 1b and 1c). Two DPPO groups of 26DBSODPO substituted at opposite ortho- and para-positions partially break molecular asymmetry, giving rise to the ordered molecular packing in its single crystal with only one kind of molecular orientation. Along c axis, the distance between adjacent 26DBSODPO moelcules is as large as 25.049 Å. In the a,b-plane, although the - interaction between two phenyls of adjacent DPPO groups with edge-to-centroid distance of 3.706 Å would support a channel for intermolecular 11 ACS Paragon Plus Environment


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electron hopping, DBSO cores in adjacent 26DBSODPO molecules are completely separated by DPPO groups. The largest hindrance of DPPO groups in 26DBSODPO renders the centroid-to-centroid distance between adjacent DBSO cores as long as 8.765 Å, reflecting its weakest DBSO-involved interactions among mnDBSODPO as designed (Figure 1b). The stagger meta- and ortho-DPPO groups render the more unsymmetrical structure for 36DBSODPO, giving rise to locally disordered molecular packing with four different kinds of molecular orientations. The situation of molecular packing in the a,b-plane of 36DBSODPO single crystal is similar to that of 26DBSODPO. The DPPO groups in adjacent 36DBSODPO can also form an intraplanar electron transport network through edge-to-face - interactions between phenyls on the basis of short distance as 3.774 Å (Figure 1c). Despite the smaller hindrance of DPPO groups in 36DBSODPO, the centroid-to-centroid distance between adjacent DBSO cores is still remained as 8.156 Å. Nevertheless, along c axis, the strong edge-to-edge - interactions between DBSO cores of adjacent 36DBSODPO molecules is recognized with a significantly reduced distance of 3.013 Å to afford efficient interplanar electron hopping. In consequence, the complementation of DBSO core and peripheral DPPO groups in 36DBSODPO exactly establishes a three-dimensional network for rapid and efficient electron transportation. It is noticed that for disubstituted DBSO derivatives, a single para- or ortho-DPPO can restrain the face-to-face - interactions between adjacent DBSO cores with large centroid-to-centroid distances around 8-12 Å, therefore giving rise to the similar DPPO-assistant charge transfer process in the a,b-planes of 12 ACS Paragon Plus Environment

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mnDBSODPO; while, the other DPPO groups in mnDBSODPO determine the edge-to-face interactions between interplanar DBSO cores, following the order of meta-DPPO >> ortho-DPPO ≈ para-DPPO, which would be in accord with the order of the electron transporting ability for mnDBSODPO, viz. 36DBSODPO ≈ 27DBSODPO > 26DBSODPO.

Figure 2. Energy levels and contours of frontier molecular orbitals at ground states and spin density distribution of triplet states for mnDBSODPO. 13 ACS Paragon Plus Environment


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3.2. DFT Simulation The influence of PO substitution on electronic characteristics of mnDBSODPO was investigated with density function theory (DFT) simulation. Their ground states and T1 states were optimized at the level of B3LYP/6-31g* to figure out the distribution of the frontier molecular orbitals (FMO) and triplet spin density, indicating the relative optoelectrical properties (Figure 2). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of mnDBSODPO are mainly localized on their DBSO cores. Nevertheless, the LUMO energy level of 27DBSODPO is -2.13 eV, which is the deepest among mnDBSODPO and comparable to those of the symmetrical isomers 28DBSODPO (-2.16 eV) and 37DBSODPO (-2.22 eV)47 (Figure 2 and Table 1). 26DBSODPO and 36DBSODPO show the shallower LUMO energy levels of -1.901 and -1.980 eV, which are close to those of 2DBSOSPO and 3DBSOSPO,48 respectively. It is noticeable that 2,8-C and 3,7-C of DBSO core are involved in the LUMO locations, while its 4,6-C are excluded. Therefore, the LUMO energy levels of mnDBSODPO are mainly determined by their 2,3-substituted DPPO groups. Furthermore, the resonance effect of 3-DPPO at the meso-position and long axis of DBSO core can remarkably decrease the LUMO energy, rendering their LUMO energy in the order of 27DBSODPO > 36DBSODPO > 26DBSODPO. On the other hand, in contrast to 26DBSODPO, the HOMOs of 27DBSODPO and 36DBSODPO are appreciably dispersed to their 3,7-substituted DPPO groups, reflecting the slight 14 ACS Paragon Plus Environment

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conjugation extension by P=O at the long axis of their DBSO cores, which give rise to their HOMO energy order of 26DBSODPO > 36DBSODPO > 27DBSODPO. In this case, the difference between the HOMO-LUMO energy gaps of mnDBSODPO is reduced, ascribed to not only the insulating characteristic of PO linkages but also the consistent inductive effect of P=O on the HOMO and the LUMO.

The dominant contribution of DBSO core to electron injection and transportation is further validated by the electron cloud distributions of the LUMO+1 orbitals for mnDBSODPO mainly localized on DBSO core, accompanied by minor dispersion on DPPO groups. Furthermore, for the LUMO+2, the electron cloud distributions of 27DBSODPO and 36DBSODPO are dispersed on whole molecules with significant contributions from their two DPPO groups; while, 26DBSODPO shows the relatively concentrated LUMO locations on its DBSO and 2-DPPO groups, which would halve bridge function of DPPO groups during electron transportation. In this case, the negligible influence of DPPO groups at 4,6-positions of DBSO cores on FMO energy levels reflects that the main functions of ortho-substituted P=O groups are encapsulating charged DBSO core and facilitating intermolecular charge hopping. On contrary, DPPO groups at 2,8 and 3,7-positions of DBSO core can simultaneously enhance the electron affinity of the compounds and also form the assistant electron transporting channels. Nevertheless, through comparison on single-crystal packing diagram of 26DBSODPO and 36DBSODPO (Figure 1b and 1c), the hindrances of 2,8- and 4,6-DPPO groups would weaken the DBSO-DBSO interactions at corresponding directions; while, the steric effect of 3,7-DPPO groups can be ignored. 15 ACS Paragon Plus Environment


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In this sense, with two-dimensional separation, the electron transporting ability of 26DBSODPO would be inferior to those of 27DBSODPO and 36DBSODPO. The T1 locations of mnDBSODPO are mainly on their DBSO cores as shown by the spin density distributions (SDD) of T1 states, giving rise to their similar T1 energy of ~2.8 eV (Figure 2 and Table 1). Nevertheless, the T1 values of 27DBSODPO and 36DBSODPO are slightly lower than that of 26DBSODPO, in accord with the minor but considerable contributions of 3,7-substituted DPPO groups to the SDDs of the former two, which should be ascribed to the involvement of 3,7-DPPO in conjugation extension. Even so, the influence of DPPO groups on HOMO-LUMO energy gaps and T1 energy of mnDBSODPO is still negligible. The functions of DPPO groups are embodied in tuning FMO energy levels by resonance effect and intermolecular interactions by steric hindrance, which are strongly dependent on their substitution positions.


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Figure 3. (a) Cyclic voltammograms of mnDBSODPO measured in CH2Cl2 at room temperature with tetrabutylammonium hexafluorophosphate (0.1 M) as electrolyte under Ar with scanning rate of 100 mV s-1; (b) volt-ampere characteristics of mnDBSODPO-based single-layer electron-only devices (symbols), fitted with space charge limited current (SCLC) model for mobility evaluation (lines).



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3.3. Electrical Properties The electrochemical properties of mnDBSODPO were measured with the cyclic voltammetry (CV) analysis to determine their redox characteristics (Figure 3a). mnDBSODPO revealed the similar irreversible anodic peaks, corresponding to the oxidation of their DBSO cores. Evaluated by the onset voltages, the HOMO energy levels of 27DBSODPO, 26DBSODPO and 36DBSODPO are -6.18, -6.34 and -6.36 eV, respectively (Table 1). Simultaneously, the reduction curves of mnDBSODPO are similar, containing one reversible cathodic peak characteristic of DBSO cores and one irreversible peak originated from DPPO groups. It is noticeable that the reductive onset voltage of 27DBSODPO was the biggest as -1.31 eV, corresponding to the LUMO energy level of -3.47 eV, which was 0.15 eV deeper than those of 26DBSODPO and 36DBSODPO. In this sense, the inductive effects of 2,8- and 3,7-substituted DPPO groups on electron affinity enhancement are actually comparable. Therefore, according to CV analysis, the electron injecting abilities of mnDBSODPO are in the order of 27DBSODPO > 36DBSODPO ≈ 26DBSODPO. With asymmetrical configurations, the competence of mnDBSODPO as ETM is one of the most concerns. The single-layer nominal electron-only devices of mnDBSODPO were fabricated with configuration of ITO|LiF (1nm)|mnDBSODPO (100nm)|LiF (1nm)|Al to experimentally evaluate their intrinsic electron mobility with the I-V characteristics, in which LiF served as electron-injecting layers (Figure 3b). It is showed that the current densities (J) of 27DBSODPO and 36DBSODPO-based 18 ACS Paragon Plus Environment

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devices were comparable at the same voltages, which were one order of magnitude larger than that of 26DBSODPO-based analogues. Although the electron affinity of 27DBSODPO is higher than that of 36DBSODPO, they have the same spatial distributions of DPPO groups, namely each DPPO group along long and short axis of DBSO core, respectively, rendering their similar intermolecular interactions and charge transporting modes, which support the comparable electron transporting abilities to 27DBSODPO and 36DBSODPO. On contrary, the two DPPO groups at short axis of DBSO core of 26DBSODPO would result in the different electron transporting mode in its devices. The electron transporting abilities of mnDBSODPO can be quantitatively evaluated by their electron mobility (e), according to the model of field-dependent space charge limited current (FD-SCLC). e of 27DBSODPO and 36DBSODPO were comparable as 2~3×10-4 cm2 V-1 s-1, about one order of magnitude larger than that of 26DBSODPO (Table 1). It can be noticed that e of 27DBSODPO and 36DBSODPO were between those of 37DBSODPO and 28DBSODPO; while e of 26DBSODPO and 46DBSODPO were comparable.47 In this sense, the mobility of these DBSO-based analogues is mainly in direct proportion to their intermolecular interactions rather than electron affinities. The interplanar interactions preserved in solid states of asymmetrical 27DBSODPO and 36DBSODPO just give rise to their favorable electron transporting abilities. In consequence, through integrating the inductive and steric effects of DPPO groups at different substitution positions, even on the basis of asymmetrical configuration, mnDBSODPO, especially for 27DBSODPO and 36DBSODPO, can 19 ACS Paragon Plus Environment


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still achieve the high electron affinities and favorable electron mobility to balance charge flux in their devices.

3.4. Optical Properties The electronic absorption spectra of mnDBSODPO in dilute solution (10-6 mol L-1 in CH2Cl2) consist of three main bands peaked around 330, 280 and 230 nm, corresponding to n→* transition of DBSO, and →* transitions of DBSO and DPPO groups, respectively (Figure 4a and Table 1). According to the absorption edges, their optical band-gaps (approximate to S1 level) are about 3.5 eV, which are similar to those of mono-substituted analogues (mDBSOSPO),48 but 0.1 eV higher than those of symmetrically disubstituted congeners (mDBSODPO).47 The absorption properties of mnDBSODPO are almost identical to those of monosubstituted analogues and DBSO, revealing the limited influences of asymmetrically disubstituted DPPO groups on electronic behaviours of DBSO core. Nevertheless, similar to mDBSOSPO and mDBSODPO, fluorescence (FL) spectra in dilute solution of mnDBSODPO show a slight bathochromic shift of about 10 nm in contrast to that of DBSO core. Furthermore, compared with 27DBSODPO and 36DBSODPO, the FL emission peak wavelength of 26DBSODPO is 5 nm longer, which should be ascribed to its relatively symmetrical disubstitution configuration. On contrary, phosphorescence (PH) spectra of mnDBSODPO are plenty of overlap in profile and range. Evaluated by the 0-0 transition-attributed first peaks, their T1 levels as 2.97 eV are high enough to effectively suppress triplet exciton leakage from 20 ACS Paragon Plus Environment

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adjacent layers in the devices. Therefore, owing to the feature of DPPO as insulating linkage and its polarity lower than that of DBSO core, the variation of DPPO substitution positions hardly influences the optical properties, which establishes the basis to selectively optimize interfacial effect and electrical performance.

Figure 4. (a) Electronic absorption and emission spectra of mnDBSODPO in dilute solution (10-6 M in CH2Cl2) and the time-resolved phosphorescence spectra (inset) measured at 77K with a delay of 300 s after 300 nm excitation; (b) emission spectra 21 ACS Paragon Plus Environment


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of bilayer thin films with the structures of DBTDPO│mnDBSODPO as shown in inset figure. According to the HOMO energy level of DBTDPO as -6.05 eV, the energy gaps between the LUMOs of mnDBSODPO and the HOMO of DBTDPO are only 2.5~2.7 eV, significantly smaller than excited energy of DMAC-DPS (~2.9 eV). In this case, the formation of interfacial dipoles between DBTDPO and mnDBSODPO through interfacial charge transfer (ICT) from dibenzothiophen (DBT) to DBSO as low-energy

traps

would

remarkably

quench

the

excitons

close

to

DBTDPO|mnDBSODPO interface in their devices. With this consideration, the interfacial interactions between DBTDPO and mnDBSODPO were investigated with emission variation of their bilayer thin films, which is sensitive to the ICT processes (Figure 4b). The bilayer thin films of DBTDPO (30 nm)|mnDBSODPO (30 nm) were deposited on quartz substrates through vacuum evaporation under the same consideration of device fabrication. In contrast to the solid-state emission from neat 27DBSODPO films, the emission from its bilayer films is relatively remarkably broadened in the long wavelength range with an increase of full width at half maximum (FWHM) by 10 nm. The emissions from the bilayer films would be the combination of the main peaks from mnDBSODPO, DBTDPO and interfacial dipole-featured shoulder peaks in the long-wavelength ranges (inset in Figure 4b). On account of the similar narrow solid-state emissions of mnDBSODPO and DBTDPO peaked around 375 nm with FWHM less than 50 nm,51 Rationally, the widened profiles in the long-wavelength ranges would be ascribed to the exciplex emissions 22 ACS Paragon Plus Environment

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from interfacial dipole, which is almost negligible in the emissions of 26DBSODPO and 36DBSODPO-based bilayer films. Since the embedment effect of DPPO on high-polar sulfone is in the order of 4,6-DPPO > 2,7-DPPO >> 2,8-DPPO, the encapsulation of sulfone moiety in 27DBSODPO should be inferior to those in 26DBSODPO and 36DBSODPO, rendering the relatively serious interfacial dipolar interactions for the former. Nevertheless, it can be noteworthy that all of mnDBSODPO revealed the interfacial exciplex emissions significantly weaker than those of their symmetrical analogues47,

reflecting the superiority of asymmetrical

systems in intermolecular interaction suppression.

Table 1. Physical properties of mnDBSODPO. Compou nd

Absorption

Emissi on

(nm)

S1

T1

(eV)

(eV)

(nm) 27DBSO DPO

329, 286, 273, 255, 364a 228a 415b 339, 300, 259, 220, 200b

3.52c

3.00e

4.64

d

2.78

d

26DBSO DPO

333, 285, 251, 228a

368a

3.51c

2.97e

333, 285, 258, 220, 378b 200b

4.78d

2.85d

36DBSO DPO

329, 301, 287, 275, 362a 251, 228a 378b 335, 301, 255, 220, 200b

3.54c

3.00e

4.71d

2.81d

a

eh

(eV )

(cm-2 V-1 s-1)

d

LU MO

(oC)

(eV)

(eV)

-, -, 430

-6.18 -3.47 0.53 2.37×10f

f

4

-, 255, -6.34 -3.32 0.66 3.99×10f f 5 431

-, 313, -6.36 -3.32 0.64 2.85×10f f 4 448

In CH2Cl2 (10-6 mol L-1); b in film; c optical bandgap estimated according to the

absorption edges; d DFT calculated results; e calculated according to the 0-0 transitions 23 ACS Paragon Plus Environment

ERg

HO MO

Tg/Tm/T


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of the phosphorescence spectra; f calculated according to the equation HOMO/LUMO = 4.78 + onset voltage52; g reorganization energy of electron; h Electron mobility estimated by I-V characteristics of electron-only devices according filed-dependent SCLC model53. In consequence, through adjusting the DPPO-substitution position, the optimal interfacial interaction and the preserved electroactivity are successfully integrated on 36DBSODPO in virtue of the harmonized inductive and encapsulation effects of its two different DPPO groups.

3.5. EL performance of blue TADF diodes To validate the superiority of asymmetrical mnDBSODPO in optimizing EML-ETL interlayer interactions in blue TADF diodes, light-emitting devices were fabricated with a simple conventional configuration of ITO|MoO3 (6 nm)|NPB (70 nm)|mCP (5 nm)|DBTDPO:DMAC-DPS (20 nm, 10%wt.)|mnDBSODPO (50 nm)|LiF (1 nm)|Al, in which NPB and mCP refer to 4,4'-bis[N-(1-naphthyl)-N-phenylamino]biphenyl and N,N'-dicarbazole-3,5-benzene as hole-transporting and exciton-blocking layers, respectively (Figure 5a). On account of the high T1 energy for mCP and mnDBSODPO as ~3.0 eV, the excitons can be effectively confined in EML to support their devices with the identical EL spectra, originated from pure emissions of DMAC-DPS peaked at 472 nm (insets of Figure 5b). Despite the various electron mobility of mnDBSODPO, these devices revealed the comparable J-V characteristics (Figure 5b). Hole should be the majority carrier 24 ACS Paragon Plus Environment

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mainly contributed to J of these devices, in accordance to p-featured device structures on the basis of much higher hole mobility for the materials, e.g. 10-3 cm2 V-1 s-1 for NPB. Actually, the employment of mCP can not only retrain exciton linkage, but also render the barrier-free hole injection into EML through DMAC-DPS with the same HOMO energy level (Figure 5a). On contrary, the LUMO energy gap between mnDBSODPO and DMAC-DPS is ~0.5 eV, relatively restraining the electron injection. In this case, the charge carrier recombination zones of these devices should be close to their EML|ETL interfaces, making interfacial effect crucial to their EL performance. Therefore, the driving voltages of these devices were not inversely proportional to the electron mobility of their ETLs (Figure 5b and Table 2). 36DBSODPO endowed its devices with the lowest driving voltages of 3.5 V for onset and 6.0 and 9.5 V at 100 and 1000 cd m-2, respectively. 26DBSODPO-based devices achieved the same turn-on voltages, but their driving voltages at 100 and 1000 cd m-2 were 0.5 V higher. When using 27DBSODPO as ETL, the devices revealed the remarkably higher driving voltages as 4.0 V for onset and 8.0 and 11.5 V at 100 and 1000 cd m-2, respectively. It is rational that defined with respect to specific luminance, the driving voltage is dependent on not only carrier injection and transportation but also luminescence performance.54 In this sense, the tendency of operation voltages for mnDBSODPO-based devices can reflect from one point their EML|ETL interfacial interactions on exciton quenching, viz. 36DBSODPO < 26DBSODPO

Dibenzothiophene Sulfone-Based Phosphine Oxide Electron

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