Efficient and Tunable Thermally Activated Delayed Fluorescence Emitters Having Orientation‐Adjustable CN‐Substituted Pyridine and Pyrimidine Acceptor Units

Abstract

A series of twisted D–π–A type emitters based on the acridine donor unit and CN‐substituted pyridine, pyrimidine, and benzene acceptor units are studied. They not only allow one to systematically probe the influence of different acceptor strengths, but also permit one to intriguingly probe the influence of tunable conformations (twist angles) within the acceptor moieties through controlling the orientation of asymmetric heteroaromatic ring relative to the donor component. Intramolecular charge‐transfer transitions are observed in all these compounds and emission wavelengths are widely tunable from deep blue to yellow not only by the general acceptor strength due to the characters of heteroarene and CN‐substitution pattern but also by the subtle control of in‐acceptor conformation (twist angles). Small triplet‐to‐singlet energy gaps (ΔEST) and significant thermally activated delayed fluorescence (TADF) characteristics are obtained in a series of D–π–A compounds with sufficient acceptor strengths and tunable in‐acceptor conformation, yielding a series of efficient blue‐green to yellow TADF emitters with promisingly high photoluminescence quantum yields of 90%–100%. Highly efficient blue‐green to yellow TADF organic light‐emitting diodes (OLEDs) having external quantum efficiencies of up to 23.1%–31.3% are achieved using these efficient TADF emitters, which are among the most efficient TADF OLEDs ever reported.

Thermally Activated Delayed Fluorescence Emitters for Deep Blue Organic Light Emitting Diodes: A Review of Recent Advances

“…As expected, deep blue TADF OLEDs with a FWHM of 48 nm, a high EQE of 14.0%, and chromaticity coordinates of (0.14, 0.12) were obtained using the newly designed deep blue TADF emitter (22). Pan et al also used cyano acceptor to design TADF materials (23, Figure 5) [87]. OLEDs comprising 23 as the emitter gave deep blue devices emitting a light with colour coordinates of (0.16, 0.06).…”

Development of Materials for Blue Organic Light Emitting Devices

“…on the photophysical properties as well as device performance, a series of twisted DÀA type emitters based on acridine donor and CN-substituted pyridine (Py), pyrimidine (Pm), and benzene (Bz) acceptors were examined. [69] All the compounds were found to display sufficient thermal stability with decomposition temperatures (Td 5 ), in the range of 230-290 8C. Though similar oxidation potentials were observed for these molecules, the reduction potentials varied greatly with the acceptor units following the magnitude of reduction potential as the order: PhÀBz-cyano > PhÀPycyano > PhÀPm-cyano > PhÀPy-di-cyano.…”

Recent progress of pyrimidine derivatives for high-performance organic light-emitting devices

“…6(b), the reported molecular design strategy of pyrimidine derivatives as TADF emitters is shown. Pyrimidinebased TADF emitters are mainly categorized into donor-acceptor-donor (D-A-D) [68][69][70][71][72] and donor-acceptor (D-A) 29,[73][74][75][76] types, where the pyrimidine backbone is used as the acceptor unit, and phenoxazine, acridine, and carbazole derivatives are frequently used as donor units. Among D-A-D types, the donor unit is attached to the 4,6-position of pyrimidine, whereas among D-A types, the donor unit is attached to the 2-position or the 4-position of pyrimidine.…”

“…In particular, the Ac-PM based device showed deep-blue emission with a CIE of (0.15, 0.15) and an EQE of 11%, whereas the MFAc-PPM based device showed a high EQE of 20% and pure blue emission with a CIE of (0.16, 0.23). Pan et al 29 group developed TADF emitters with CN-substituted pyrimidines called Pm2 and Pm5. Due to the stronger acceptor character induced by the CN group, the enhanced CT emission showed very large PLQE values up to 100%.…”

“…[14][15][16][17][18][19][20][21][22][23][24][25] Until recently, several TADF OLEDs have achieved a high EQE of over 30% at maximum. [26][27][28][29][30][31][32] The efficiency of the TADF-based OLEDs is expected to exceed that of OLEDs based on phosphorescent emitters due to the unlimited molecular design of pyrimidine derivative-based TADF emitters.…”