Research Article www.acsami.org
Rigidity-Induced Delayed Fluorescence by Ortho Donor-Appended Triarylboron Compounds: Record-High Efficiency in Pure Blue Fluorescent Organic Light-Emitting Diodes Young Hoon Lee,†,¶ Sunghee Park,‡,¶ Jihun Oh,† Jong Won Shin,§ Jaehoon Jung,*,† Seunghyup Yoo,*,‡ and Min Hyung Lee*,† †
Department of Chemistry and Energy Harvest Storage Research Center, University of Ulsan, Ulsan 44610, Republic of Korea School of Electrical Engineering, Korea Advanced Institute of Science & Technology, Daejeon 34141, Republic of Korea § Korea Institute of Science and Technology Information, Daegu 41515, Republic of Korea ‡
S Supporting Information *
ABSTRACT: A synthetic approach to highly efficient thermally activated delayed fluorescence (TADF) is proposed that uses ortho donor (D)−acceptor (A) compounds (PXZoB, DPAoB, and CzoB), wherein the acceptor is based on triarylboron and the donor is phenoxazine (PXZ), diphenylamine (DPA), or carbazole (Cz). Combined with the ortho D−A connectivity, the bulky nature of the triarylboron endows the D−A dyads with inherent steric “locking” for a highly twisted arrangement, leading to a small energy difference between singlet and triplet excited states (ΔEST) and thus exhibiting very efficient TADF with microsecond-range lifetimes. In sharp contrast, the corresponding para D−A derivatives, DPApB and CzpB, only display short-lived, normal fluorescence. Organic light-emitting diodes (OLEDs) incorporating the proposed ortho D−A compounds as emitters display orange, greenish-blue, and pure blue emission and exhibit high external quantum efficiency (ηEQE). In particular, the pure blue OLEDs based on the proposed ortho D−A emitters with a carbazole donor (CzoB) show a record-high ηEQE of 22.6% with CIE color coordinates of (0.139, 0.150), well illustrating the validity of the proposed approach. Upon optical optimization, the ηEQE is further improved to 24.1%. KEYWORDS: TADF, OLEDs, pure blue, ortho donor−acceptor, triarylboron
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INTRODUCTION
However, the majority of TADF compounds adopt para donor−acceptor connectivity between the donor (D) and acceptor (A) units. Even though experimental and theoretical studies have shown twisted orientation between the donor and acceptor groups, these para D−A compounds could be prone to free rotation, as evidenced by their solution NMR spectroscopy, causing relaxation of the twisted structure. This tends to limit the choice of donor groups that can lead to efficient TADF; although there have been reports on efficient TADF emitters based on conventional para D−A compounds possessing small donor groups, such as unsubstituted diphenylamine and carbazole,15,16 examples of such systems are not yet widespread and, if any, display weak12,17 (i.e., a large ΔEST) or no18,19 delayed fluorescence despite the occurrence of a strong intramolecular charge-transfer (ICT) transition. This has prompted us to consider introducing ortho D−A connectivity,20,21 as the donor and acceptor groups in the ortho position can exert mutual steric hindrance due to the close proximity.
Thermally activated delayed fluorescence (TADF) materials have recently attracted great attention as efficient emitters in organic light-emitting diodes (OLEDs) because TADF emitters can theoretically harvest 100% of excitons through the upconversion of triplet excitons to emissive singlet excitons by thermally activated reverse intersystem crossing (RISC).1−14 To facilitate RISC within a practically reasonable temperature range, TADF emitters require a very small energy difference (ΔEST) between the excited singlet (S1) and triplet states (T1). This is usually achieved by constructing TADF emitters with a twisted donor−acceptor structure, allowing for effective reduction in the spatial overlap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).14 Accordingly, most TADF emitters possess sterically hindered donor groups, such as phenoxazine, 9,9dimethylacridane, or 3,6-disubstituted carbazole, to induce a perpendicular arrangement between the donor and acceptor groups. This strategy has been widely utilized to design molecular scaffolds of TADF emitters and has successfully resulted in highly efficient delayed fluorescence. © XXXX American Chemical Society
Received: April 21, 2017 Accepted: June 27, 2017 Published: June 27, 2017 A
DOI: 10.1021/acsami.7b05615 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Furthermore, combining the ortho D−A connectivity with a bulky acceptor could synergistically reinforce the potential benefit of inherent steric “locking” between donor and acceptor groups. To provide insight into this approach, we prepared a series of ortho D−A emitters based on triarylboron compounds in which different donor groups, such as phenoxazine (PXZ), diphenylamine (DPA), or carbazole (Cz), are appended to the ortho position of the phenyl ring of the dimesitylphenylboron (PhBMes2) acceptor (Chart 1). Triarylboron acceptor was Chart 1. Chemical Structures of Ortho and Para DonorAppended Triarylboron Compounds
Figure 1. X-ray crystal structures of PXZoB (left) and CzoB (right) (40% thermal ellipsoids). The H atoms are omitted for clarity. Dihedral angles: ∠PXZ−phenylene = 79.03(5)° and ∠Cz−phenylene = 76.59(4)°. ∑(C−B−C) = 360° for both PXZoB and CzoB.
The PXZ and Cz ring planes are parallel with one mesityl ring plane to avoid mutual steric hindrance. To retain this conformation, the PXZ and Cz rings are nearly perpendicular to the central phenylene ring, as judged by the dihedral angles of 79.0° and 76.6°, respectively. The highly twisted connectivity between the PXZ and phenylene rings in PXZoB is similar to the features observed for other p-PXZ systems.15,18,19,23,26,27 To elucidate the difference in the dihedral angle of o- and p-Cz systems, we further obtained the crystal structure of CzpB (Figure S4). The measured dihedral angle in CzpB was 68.3°, which was smaller than that of CzoB (76.6°). In combination with the 1H NMR spectra, this finding may indicate relatively free rotation of the Cz moiety of CzpB in solution and its substantial conjugation with the acceptor unit. Photophysical Properties. UV−vis absorption and photoluminescence (PL) spectra of all compounds were obtained in toluene (Figure 2 and Table 1). PXZoB, DPAoB, and CzoB
chosen because it is well-known for its strong electronaccepting properties as well as for its steric bulkiness.22−24 The photophysical properties of these ortho compounds, including delayed fluorescence, were compared in detail to those of the corresponding para D−A derivatives. We find that ortho D−A structure is effective in attaining a delayed fluorescence and can be utilized as a highly efficient TADF emitter for pure blue (rather than sky blue) fluorescent OLEDs as well as for those with other colors. With the proposed ortho D−A emitters, we demonstrate blue fluorescent OLEDs that exhibit a record-high external quantum efficiency of 24.1% with Commission Internationale de l’Eclairage (CIE) color coordinates of (0.139, 0.198).
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RESULTS AND DISCUSSION Synthesis and Characterization. Ortho D−A compounds (PXZoB, DPAoB, and CzoB) were prepared in good yields from reactions between lithium salt derived from 1-Br-2-Xbenzene (X = PXZ, DPA, Cz) and dimesitylboron fluoride (Mes2BF) (see Supporting Information). The reference para D−A compounds (DPApB and CzpB) were analogously obtained from the reported procedures.25,26 The formation of ortho D−A compounds has been characterized by multinuclear NMR spectroscopy, elemental analysis, and X-ray diffraction. Interestingly, the 1H NMR spectra of the ortho D−A compounds showed very broad proton resonances for the BMes2 group at room temperature, indicating restricted motion of the Mes groups in solution due to steric hindrance (Figures S1−S3). In contrast, the 1H NMR spectra of para derivatives (DPApB and CzpB) exhibited sharp methyl and CAr−H proton resonances for the Mes groups. The broad 11B NMR signals at δ 74−79 ppm further confirmed the presence of a base-free trigonal planar boron center. X-ray diffraction studies on PXZoB and CzoB unambiguously revealed the sterically congested nature in the ortho D−A compounds (Figure 1).
Figure 2. (Left) UV−vis absorption and (right) PL spectra of PXZoB, DPAoB, and CzoB in toluene (5.0 × 10−5 M) at room temperature.
feature broad low-energy absorptions centered at 445, 415, and 387 nm, respectively, typical of intramolecular charge-transfer (ICT) transition. Because the acceptor moiety is identical for all three compounds, the observed absorption wavelength would be closely related to the HOMO level of the donor moieties. Indeed, electrochemical measurements showed that while the LUMO level (reduction potential) was slightly changed among the three compounds, an apparent difference in the HOMO level (oxidation potential) was observed, following the energy order PXZoB > DPAoB > CzoB (Figure S5). This finding was further evidenced by the computed HOMO and LUMO levels (see DFT results in Theoretical Calculations). The PL spectra B
DOI: 10.1021/acsami.7b05615 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Table 1. Photophysical Data of Ortho and Para D−A Compounds in Toluene compd PXZoB DPAoB CzoB DPApB CzpB
λabsa (nm) 320, 302, 324, 309, 291,
445 415 387 380 360
λPLa (nm)
ΦPLb N2/air (%)
585 498 463 437 401
23/6 73/38 79/19 91/80 75/65
τpc (ΦPF) [ns (%)] τdc (ΦDF) [μs (%)] 62 27 38 4.0 3.7
(8) (42) (24) (91) (75)
2.92 (15) 14.4 (31) 71.6 (55) g g
HOMO/LUMOd (eV)
ES/ETe (eV)
ΔESTf exp/calcd (eV)
−5.07/−2.35 −5.33/−2.41 −5.55/−2.32 h h
2.44/2.44 2.76/2.56 2.95/2.80 3.12/2.71 3.33/2.94
