Highly Efficient Red–Orange Delayed Fluorescence Emitters Based on Strong π‐Accepting Dibenzophenazine and Dibenzoquinoxaline Cores: toward a Rational Pure‐Red OLED Design

Abstract

Organic luminescent materials that exhibit thermally activated delayed fluorescence (TADF) can harvest both singlet and triplet excitons for light emission, leading to high electroluminescence (EL) quantum efficiencies in organic light‐emitting diodes (OLEDs). However, efficient red TADF materials are still very rare because of their restricted molecular design based on the energy gap law. To address this issue, elaborate π‐conjugated donor–acceptor (D–A) systems that can simultaneously achieve a large fluorescence radiative rate and small singlet–triplet energy splitting should be strategically designed. In this study, to produce high‐efficiency pure‐red TADF materials, a remarkably strong π‐accepting dicyanodibenzo[a,c]phenazine (CNBPz) unit has been introduced in a D–π–A molecular framework, and combined with a phenylene‐linked p‐ditolylamine or 9,9‐dimethylacridan moiety. The steady‐state and time‐resolved photophysical measurements revealed intense genuine red TADF emissions of these CNBPz‐based molecules in both solution and doped thin films. The OLEDs incorporating the CNBPz‐based TADF emitters achieve the desired high‐efficiency pure‐red EL, centered at 670 nm with color coordinates of (0.66, 0.34), accompanied by a high maximum external EL quantum efficiency of 15.0%. Therefore, it is concluded that CNBPz, with its expanded π‐conjugation and strong electron‐accepting characteristics, is a particularly useful building unit to design long‐wavelength TADF materials that can overcome the intrinsic energy gap law obstacle.

Saturated Red Electroluminescence From Thermally Activated Delayed Fluorescence Conjugated Polymers

“…Their ground-state (S 0 ) geometries were optimized at the BMK/6-31G(d) level in toluene. In the S 0 geometries, the dihedral angles between the donor and phenylene bridge are 36.5 • for 2FSO-TAQ and 38.0 • for 2CzSO-TAQ ( Figure S7), which are similar to those of TADF small molecules based on the diphenylamine donor (Zhang et al, 2014;Bin et al, 2017;Wang S. et al, 2017;Furue et al, 2018), indicating that the change of the backbone building segment has no significant effect on the twisted angle between the acceptor and donor of the TADF unit, and that the polymers could inherit the TADF characteristics of the TADF unit. As shown in Figure 3, the lowest unoccupied molecular orbitals (LUMOs) of 2FSO-TAQ and 2CzSO-TAQ are predominantly located on the anthraquinone acceptor, whereas the highest occupied molecular orbitals (HOMOs) are mainly distributed on the diphenylamine donor, the adjacent fluorene or carbazole rings and phenylene bridge.…”

“…However, as one of the three primary colors, the electroluminescent (EL) performances of red TADF emitters still lag far behind due to high non-radiative transition rates and serious concentration quenching effect (Kim et al, 2018a,b;Chen et al, 2019;Wang et al, 2019;Zeng et al, 2019). In addition, the currently reported red TADF materials are mainly based on organic small molecules with the twisted donor/acceptor structures (Furue et al, 2018;. In contrast, red TADF polymer has rarely been reported so far, partially due to the challenging material design and synthesis (Wang et al, 2018;Yang et al, 2018a).…”

Nanosecond-time-scale delayed fluorescence molecule for deep-blue OLEDs with small efficiency rolloff

“…Here, <S|Ĥ SOC |T> is the SOC matrix element between the excited singlet (S) and triplet (T) states, k B is the Boltzmann constant, and T is temperature. Using this relationship, minimization of ΔE ST is a widely adopted strategy to achieve efficient intramolecular charge-transfer (CT) of TADF molecules 8,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]32 . However, the spin-flip processes, that is, intersystem crossing ISC and RISC, between excited CT singlet ( 1 CT) and triplet ( 3 CT) states are very inefficient according to the El-Sayed rule because of the independent electric dipole moment with an electron spin, resulting from a weak SOC matrix element 37,38 .…”

Red/Near‐Infrared Thermally Activated Delayed Fluorescence OLEDs with Near 100 % Internal Quantum Efficiency

“…[6] With this strategy,wehave developed ared TADF emitter, DPXZ-BPPZ, which device gives an external quantum efficiency(EQE) over 20 %. [7a] Although this efficiency is already one of the highest among all red TADF emitters, [7] it is still lagged behind those of green and blue TADF emitters. [8] It is thus important to further improve performance of red TADF emitters.…”