Efficient Deep-Blue Fluorescent OLEDs with a High Exciton Utilization Efficiency from a Fully Twisted Phenanthroimidazole–Anthracene Emitter

Abstract

A novel, efficient, deep-blue fluorescent emitter mPAC, with a meta-connected donor–acceptor structure containing phenanthroimidazole (PPI) as the donor and phenylcarbazole-substituted anthracene (An-CzP) as the acceptor, was designed and synthesized. The meta-linkage provided a highly twisted molecular conformation, which efficiently interrupts the intramolecular π-conjugation, resulting in a deep-blue emission. The optimized nondoped device based on mPAC displayed a deep-blue emission with a narrow full width at half-maximum of 56 nm and Commission Internationale de L’Eclairage coordinates of (0.16, 0.09). The maximum external quantum efficiency (EQEmax) is 6.76%, corresponding to a high exciton utilization efficiency (EUE) of 59.3–88.9%. Experimental results and theoretical analysis indicated that the high EUE is mainly ascribed to the reverse intersystem crossing (RISC) from T2 to S1, a “hot exciton” path in which the large T2–T1 energy gap (1.45 eV) and small T2–S1 energy difference (0.18 eV, T2 > S1) hamper the internal crossing from T2 to T1 and facilitate the RISC process. For the hot exciton path, the T2 state can be feasibly arranged to a high energy level, forming a thermal equilibrium with S1, even slightly higher than the deep-blue S1 to realize an exergonic RISC process, which is usually difficult for the thermally activated delayed fluorescence emitters.

Electron‐Rich Twistacene‐Modified Arylboron Donor–Acceptor Systems: Synthesis, Photophysics, and Electroluminescence with Hot Exciton Response

“…However, the S 2 , T 2 , and T 3 states may generally be converted into S 1 and T 2 states by internal conversion (IC). Thus, compared with a high probability of RISC T 1 →S 1 for other systems, the TADF through T 3 →S 2 and T 2 →S 1 for 4 and T 2 →S 2 and T 2 →S 1 for 5 barely occurs, which can explain the limited EQE in this work.…”

“…Consequently,t he smalle nergy difference (0.05 eV for 4 and 0.20 eV for 5)b etween S 1 and the hot excited state T 2 and other small energy differences among variouse xcited states offer potentialR ISC T 3 or T 2 !S 2 or S 1 processes by TADF.H owever,t he S 2 ,T 2 ,a nd T 3 states mayg enerally be convertedi nto S 1 and T 2 states by internal conversion (IC). Thus, compared with ah igh probability of RISC T 1 !S 1 for other systems, [47][48][49] the TADF through T 3 !S 2 and T 2 !S 1 for 4 and T 2 !S 2 and T 2 !S 1 for 5 barely occurs, which can explain the limited EQE in this work.…”

Imidazole derivatives for efficient organic light-emitting diodes

“…Three novel TPE derivatives with different alkoxy chain lengths, TPEO1-PPI, TPEO4-PPI, and TPEO6-PPI, were synthesized and fully characterized [77]. All of them showed obvious AIE properties, with high PLQY values in the film state.…”

Imidazo[1,2‐a]pyridine as an Electron Acceptor to Construct High‐Performance Deep‐Blue Organic Light‐Emitting Diodes with Negligible Efficiency Roll‐Off

“…Consequently,t he exciton annihilation causedb y T 1 excitons could be efficiently weakenedt hrough fast RISC from T m to S 1 and high efficiency in the devices could be achieved. [22,23] To adjust the conjugation lengths and alter ICT effect of molecules for deeper emission, it is imperative to explore somes uitable Da nd Asegments.…”

“…In addition, Ma's group proposed an efficient reverse intersystem crossing (RISC) process that occurs from T m to S 1 by a “hot exciton mechanism” when the ICT effect was suppressed. Consequently, the exciton annihilation caused by T 1 excitons could be efficiently weakened through fast RISC from T m to S 1 and high efficiency in the devices could be achieved . To adjust the conjugation lengths and alter ICT effect of molecules for deeper emission, it is imperative to explore some suitable D and A segments.…”