A Series of Imidazo[1,2‐f]phenanthridine‐Based Sky‐Blue TADF Emitters Realizing EQE of over 20%

Abstract

platinum and iridium, and can convert 100% of the molecular excitons into light using heavy atom effect enlarging spinorbit coupling. The 3rd generation emitter is a thermally activated delayed fluorescent (TADF) emitter that can convert 100% of the molecular excitons into light, using upconversion from an excited triplet state (E T1 ) to an excited singlet state (E S1 ). [1][2][3] To realize efficient upconversion, small energy differences between E S1 and E T1 , where ΔE ST is less than 0.20 eV, is essential. A TADF emitter consists of an electrondonor unit and an electron-acceptor unit, and the emission color is roughly related to the energy difference between the highest occupied molecular orbital (HOMO) of the electron-donor unit and the lowest unoccupied molecular orbital (LUMO) of the electron-acceptor unit. Therefore, an electronacceptor unit with a shallow LUMO level is promising to realize blue emission, that needs a wide energy gap over 2.75 eV. In this regard, imidazole is very attractive as an electron-acceptor, because of its shallow LUMO and high E T1 level. [4] Furthermore, many imidazole-based blue fluorescent emitters exhibiting high photoluminescent quantum yields (η PL ) up to 100% and high efficiency fluorescent OLEDs with external quantum efficiencies (η ext ) over 7% have been reported so far. [5,6] Surprisingly, however, the corresponding TADF emitters are rarely explored, and only two green TADF emitters have been reported, very recently. [7] Wang and co-workers reported imidazo[4,5-f]-1,10-phenanthroline/diphenylphenazine conjugate molecules named PPZTPI and PPZPPI as TADF emitters. [7] These emitters exhibited very high external quantum efficiencies over 20% and high power efficiencies of nearly 70 lm W −1 in OLEDs. This report shows that imidazole is a promising acceptor unit of TADF emitters. However, until now, no blue to sky-blue TADF emitter has been realized using imidazole unit as an acceptor. In this study, we report highly efficient, imidazole-based, sky-blue TADF emitters for the first time. Among the state-of-the-art sky-blue TADF emitters, these EQE values are not best performances, however, we would like to enhance the scientific importance of the realization in the blue to sky-blue region imidazole-based TADF emitter. We developed a series of imidazo[1,2-f]phenanthridine-based TADF emitters showing blue to sky-blue emission. A sky-blue emitter based on benzimidazole exhibited an η ext,max over 21% and CIE of (0.22, 0.37). We also reveal an effective molecular design strategy to realize imidazole-based TADF emitters with a high η PL .Although imidazole derivatives are promising as blue fluorescent emitters in organic light-emitting devices (OLEDs), the corresponding blue thermally activated delayed fluorescence (TADF) emitter is rarely explored. In this study, a series of imidazo[1,2-f ]phenanthridine-based TADF materials in the blue to sky-blue region is developed. The resultant benzimidazole (BIP)-based TADF materials, with π-expanded acceptor and electron-donor phenoxaz...

Highly Efficient Deep‐Blue OLEDs using a TADF Emitter with a Narrow Emission Spectrum and High Horizontal Emitting Dipole Ratio

“…These k RISC are similar to that of the recently reported TADF emitters using oxygen‐bridged boron acceptor, but larger than previously reported blue TADF emitters. [ 12,20,36,37 ] k ISC of TDBA‐SAF was ten times larger than k RISC , resulting in longer decay lifetime of delayed emission than DBA‐SAB.…”

“…Various donor and acceptor moieties have been utilized to develop blue TADF emitters. Examples include carbazole, [1,[6][7][8] acridine, [9][10][11] phenoxazine, [3,12] azasiline, [13][14][15] and spiro-flourene [16] moieties for donor units and cyano, [17][18][19][20][21] nitrogen heterocycles, [4,16,22,23] oxygen heterocycles, [24,25] sulfone, [3,8,26] methanone, [27,28] and boron [29][30][31][32] moieties for acceptor units. Even after the large development and despite the importance of deep blue emitters with CIE y < 0.1 for full-color display and lighting, there have been few reports on highly efficient deep-blue-emitting TADF emitters due to broad emission spectra of TADF emitters with the full width at half maximum (FWHM) of 70-80 nm.…”

Efficient Orange–Red Delayed Fluorescence Organic Light‐Emitting Diodes with External Quantum Efficiency over 26%

“…[6][7][8][9] As an alternative, third-generation EL materials of thermally activated delayed fluorescence (TADF) with low-cost pure organic molecular structures have been attracting much attention in recent years since they can harvest both singlet and triplet excitons through reverse intersystem crossing (RISC) process to reach the theoretical IQE close to 100%. [9,10] To date, rapid progress of blue and green TADF materials has been made, [7,11] whereas there have been rare reports on high-efficiency orange-red TADF OLEDs. [12][13][14] Therefore, much space is still left for improvement in orange-red TADF emitters to make up for their backward.…”

“…[17] The most common molecular design strategy of TADF emitters is using a pre-twisted donor-acceptor (D-A) type structure with twisted intramolecular charge transfer (TICT) transition from the highest occupied molecule orbital (HOMO) to the lowest unoccupied molecule orbital (LUMO), which can significantly reduce the ΔE ST values. [10,[18][19][20] For the design of efficient TADF molecules, another important factor is the molecular rigidity. Molecular rigidity can be obtained by a large dihedral angle in a twisted donor-acceptor structure, which is favorable for the RISC process.…”

Molecular Design Strategy of Thermally Activated Delayed Fluorescent Emitters Using CN‐Substituted Imidazopyrazine as a New Electron‐Accepting Unit

“…[36] Until now,o nly af ew TADF emittersh ave been developedb ased on the imidazole unit due to itsw eak electron acceptorstrength. [26,37] In this work, we successfully designedT ADF OLEDsb ased on the 2-phenyl imidazopyrazine (ImPyr) as an acceptor unit. Due to the presenceo fo ne extra nitrogen atom in the acceptor unit, the ImPyr unit can be considered as as trongera cceptor than the well-known 2-phenyl imidazopyridine unit.…”