Organic light-emitting diodes (OLEDs) still face a significant
challenge in finding blue thermally activated delayed fluorescence
(TADF) emitters that can achieve narrowband emission and high efficiency.
In this work, we successfully design and synthesize a novel kind of
TADF emitters based on rigid sulfur/oxygen-bridged triarylboron acceptor
for ultrapure blue with narrowband electroluminescence. Time-dependent
density functional theory (TD-DFT) calculations and photophysical
results indicate the different intramolecular charge-transfer (ICT)
character of two emitters. Benefiting from the rigid aromatic framework,
both emitters exhibited deep-blue emission at 444 and 447 nm with
a small full-width at half-maximum (fwhm) of about 33 nm, and a small
singlet (S1)–triplet (T1) energy gap
(ΔE
ST) of 0.23 and 0.36 eV. Consequently,
OLEDs based on PhCz-TOSBA and TPA-TOSBA exhibit
deep blue electroluminescence at 456 nm with fwhm of about 55 nm,
affording high external quantum efficiencies (EQEs) of 16.69% with
CIE coordinates of (0.14, 0.15) and 16.65% with CIE coordinates of
(0.14, 0.12), respectively. These findings show that PhCz-TOSBA and TPA-TOSBA are superior emitters in ultrapure blue
TADF devices.
Developing highly efficient blue thermally activated delayed fluorescence (TADF) emitters with a narrowband emission is still a challenge. Here, novel ultrapure blue TADF emitters of TSBA-Cz and TSBA-PhCz were designed and synthesized for organic light-emitting diodes (OLEDs). Photophysical and time-dependent density functional theory calculation results simultaneously show the similar intramolecular charge-transfer character of MR-type TADF emitters. Benefiting from the symmetrical and rigid molecular configuration, compounds TSBA-Cz and TSBA-PhCz emit a pure blue emission peak at 463 and 470 nm, a narrow full width at half-maximum (FWHM) of 30 and 36 nm, and a small singlet-triplet energy gap (ΔE ST ) of 0.21 and 0.18 eV, respectively, facilitating their excellent TADF behavior in doped films. Furthermore, highly efficient TADF-OLED devices using the TSBA-Cz and TSBA-PhCz with external quantum efficiencies of 23.4 and 21.3% emit ultrapure blue electroluminescence (EL) at 464 and 472 nm with a narrow FWHM of about 35 nm and CIE color coordinates of (0.14, 0.11) and (0.12, 0.18). This work provides novel TADF emitters for blue OLEDs with narrowband EL.
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