Design, synthesis, and properties of polystyrene‐based through‐space charge transfer polymers: Effect of triplet energy level of electron donor moiety on delayed fluorescence and electroluminescence performance

Abstract

Two kinds of polystyrene‐based through‐space charge transfer (TSCT) polymers consisting of spatially‐separated acridan donor moieties bearing phenyl or naphthyl substituents and triazine acceptor moieties are designed and synthesized. It is found that TSCT polymers containing phenyl‐substituted acridan donors exhibit high‐lying singlet (S1) and triplet (T1) states with small singlet‐triplet energy splitting (∆EST) of 0.04–0.05 eV, resulting in thermally activated delayed fluorescence (TADF) with reverse intersystem crossing rate constants of 1.1–1.2 × 106 s−1. In contrast, polymers bearing naphthyl‐substituted acridan donors, although still having TSCT emission, exhibit no TADF effect because of the large ∆EST of 0.30–0.33 eV induced by low‐lying locally excited T1 state of naphthyl donor moiety. Solution‐processed organic light‐emitting diodes using TSCT polymers containing phenyl‐substituted acridan donors reveal sky‐blue emission at 483 nm together with maximum external quantum efficiency (EQE) of 11.3%, which is about 30 times that of naphthyl‐substituted counterpart with maximum EQE of 0.38%, shedding light on the importance of high triplet energy level of donor moiety on realizing TADF effect and high device efficiency for through‐space charge transfer polymer.

Recent Advances in Fluorescent Polymers with Color-Tunable Aggregate Emission

“…Nonetheless, it is important to note that TADF does not consistently correlate with TSCT interactions. For instance, Wang’s group synthesized a naphthyl triazine acceptor with extended π-conjugation, resulting in a polymer displaying a TSCT emission peak but lacking delayed fluorescence. Hudson’s group systematically studied the combinations of common TADF donors and acceptors in an ATRP-base system .…”

“…Nonetheless, it is important to note that TADF does not consistently correlate with TSCT interactions. For instance, Wang's group 113 system. 114 Notably, not all of the resulting polymers exhibited TADF; in some cases (e.g., the copolymer of phenoxazine and triazine), phosphorescence was identified.…”

Through-Space Charge-Transfer Thermally Activated Delayed Fluorescence Alternating Donor–Acceptor Copolymers for Nondoped Solution-Processable OLEDs

“…Although OLEDs with deep-blue to yellow emission with good EQE (from 3 to 20%) were successfully fabricated by solution methods, these approaches suffer from a complicated multistep synthesis of TADF monomers. ,,,,− Therefore, much attention during recent years has been focused on side-chain TADF polymers with through-space charge-transfer (TSCT), which consist of a nonconjugated polymer chain with spatial donor–acceptor architecture. The emission colors of OLEDs based on TSCT polymers cover the whole visible range reaching EQE values as high as 19%. − Despite the promising results, the simplification of the synthetic methods to TSCT polymers is strongly required for cost-efficient fabrication of OLEDs. On the other hand, with rare exceptions, these polymers were synthesized using a conventional radical polymerization technique that results in high polydispersity and significant structural heterogeneity from one polymer chain to another or by ring-opening metathesis polymerization leading to the contamination of polymers by heavy metal. − Aiming at overcoming the above-mentioned limitations of TADF polymers, we proposed a new synthetic strategy toward TSCT polymers consisting of the preparation of alternating donor–acceptor copolymers with a nonconjugated structure, where donor ( D ) and acceptor ( A ) side groups will be located close enough to enable through-space π–π interaction between them.…”