Improving Efficiency of Red Thermally Activated Delayed Fluorescence Emitter by Introducing Quasi‐Degenerate Orbital Distribution

Abstract

Comprehensive Summary Red thermally activated delayed fluorescence (TADF) emitters are essential for the development of organic light‐emitting diodes (OLEDs). To better understand and develop red TADF emitters, herein, the effect of quasi‐degenerate orbital distribution was investigated on their performance. Two red TADF emitters 10,10'‐(11,12‐difluorodibenzo[a,c]phenazine‐3,6‐diyl)bis(10H‐phenoxazine) (DPXZ‐BPF) and 10,10',10”‐(12‐fluorodibenzo[a,c]phenazine‐3,6,11‐triyl)tris(10H‐phenoxazine) (TPXZ‐BPF) were constructed by combining two or three phenoxazine (PXZ) donor units with a rigid coplanar acceptor core. As expected, a couple of PXZ units can induce quasi‐degenerate orbital distribution by virtue of the high steric hindrance between each donor unit and acceptor core, which enables multiple excited states engaged in the reverse intersystem crossing (RISC) process. Consequently, DPXZ‐BPF and TPXZ‐BPF exhibit fast RISC rates (kRISCs) of 1.07 × 106 s–1 and 1.29 × 106 s–1 and high PLQYs of 74.6% and 81.1%, respectively. And the higher kRISC and PLQY of TPXZ‐BPF are mainly attributed to the additional quasi‐degenerate orbitals, which further enhance the triplet RISC process and the singlet radiative process. As a result, TPXZ‐BPF‐based OLEDs achieved improved efficiencies with a red‐shifted emission compared with DPXZ‐BPF‐based OLEDs. These results demonstrated the tremendous potential of introducing quasi‐degenerate orbital distribution in developing red TADF emitters.

Balancing fluorescence and singlet oxygen formation in push–pull type near-infrared BODIPY photosensitizers

“…The quasi-degenerate HOMO−2, HOMO−1, and HOMO energies are within 0.05 eV from each other, allowing an inversion between the character of HOMO/HOMO−1 (both localized on the donor unit) and HOMO−2 (localized on the styryl-BODIPY moiety). 72 The smaller dihedral angles in BODIPYs 5d–f lead to increased HOMO delocalization toward the donor unit (Fig. S2, ESI†).…”

Realization of switching between TADF and HLCT emissions through modulation of the intramolecular charge transfer character

“…To study the effects of a pair of donors on the excited-state characters, four singlet and three triplet excited states are analyzed in detail. As can be seen from the energies of these states in D–A–D type molecules, the double donors create degenerate-like states in which two states are close in energy, 94 such as S 1 and S 2 , S 3 and S 4 , and T 2 and T 3 in molecules 2CZP-BZ , and T 1 and T 2 in molecules 2POZ-BZ , although some of them are not strictly degenerate.…”

Ultralong Organic Phosphorescence Modulation of Aromatic Carbonyls and Multi‐Component Systems

“…Based on thermally activated delayed fluorescence [ 103 ] (TADF)‐assisted energy transfer (ET) [ 104‐106 ] process strategy, Zhou and Yan designed tunable D–A two‐component crystals. [ 102 ] Melamine (ME) is chosen to be the energy donor for its triazine and amino moieties can favor the TADF process.…”