Achieving Enhanced Thermally Activated Delayed Fluorescence Rates and Shortened Exciton Lifetimes by Constructing Intramolecular Hydrogen Bonding Channels

A fast radiative rate, highly suppressed nonradiation,
and a short
exciton lifetime are key elements for achieving efficient thermally
activated delayed fluorescence (TADF) organic light-emitting diodes
(OLEDs) with reduced efficiency roll-off at a high current density.
Herein, four representative TADF emitters are designed and synthesized
based on the combination of benzophenone (BP) or 3-benzoylpyridine
(BPy3) acceptors, with dendritic 3,3″,6,6″-tetra-tert-butyl-9′H-9,3′:6′,9″-tercarbazole
(CDTC) or 10H-spiro­(acridine-9,9′-thioxanthene)
(TXDMAc) donors, respectively. Density functional theory simulation
and X-ray diffraction analysis validated the formation of CH···N
intramolecular hydrogen bonds regarding the BPy3-CDTC and BPy3-TXDMAc
compounds. Notably, the construction of intramolecular hydrogen bonding
within TADF emitters significantly enhances the intramolecular charge
transfer (ICT) strength while reducing the donor–acceptor (D-A)
dihedral angle, resulting in accelerated radiative and suppressed
nonradiative processes. With short TADF exciton lifetimes (τTADF) and high photoluminescence quantum yields (ϕPL), OLEDs employing BPy3-CDTC and BPy3-TXDMAc dopants realized
maximum external quantum efficiencies (EQEs) up to 18.9 and 25.6%,
respectively. Moreover, the nondoped device based on BPy3-TXDMAc exhibited
a maximum EQE of 18.7%, accompanied by an extremely small efficiency
loss of only 4.1% at the luminance of 1000 cd m–2. In particular, the operational lifetime of the sky-blue BPy3-CDTC-based
device was greatly extended by 10 times in contrast to the BP-CDTC-based
counterpart, verifying the idea that the in-built intramolecular hydrogen
bonding strategy was promising for the realization of efficient and
stable TADF-OLEDs.