Introducing Spiro‐locks into the Nitrogen/Carbonyl System towards Efficient Narrowband Deep‐blue Multi‐resonance TADF Emitters

Abstract

The current availability of multi‐resonance thermally activated delayed fluorescence (MR‐TADF) materials with excellent color purity and high device efficiency in the deep‐blue region is appealing. To address this issue in the emerged nitrogen/carbonyl MR‐TADF system, we propose a spiro‐lock strategy. By incorporating spiro functionalization into a concise molecular skeleton, a series of emitters (SFQ, SOQ, SSQ, and SSeQ) can enhance molecular rigidity, blue‐shift the emission peak, narrow the emission band, increase the photoluminescence quantum yield by over 92%, and suppress intermolecular interactions in the film state. The referent CZQ without spiro structure has a more planar skeleton, and its bluer emission in the solution state redshifts over 40 nm with serious spectrum broadening and a low PLQY in the film state. As a result, SSQ achieves an external quantum efficiency of 25.5% with a peak at 456 nm and a small full width at half maximum of 31 nm in a simple unsensitized device, significantly outperforming CZQ. This work discloses the importance of spiro‐junction in modulating deep‐blue MR‐TADF emitters.

Role of Bridging Groups in Regulating the Luminescence and Charge Transfer Properties of Thermally Activated Delayed Fluorescence Molecules: A Theoretical Perspective

“…The steric hindrance between donor and acceptor can effectively separate the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and promote the reduction of the energy gap of singlet–triplet (Δ E ST ). The triplet exciton (T 1 ) can undergo a thermally triggered reverse intersystem crossing (RISC) process, converting it back into a singlet exciton (S 1 ). − The rate of RISC ( k RISC ) primarily relies on Δ E ST and spin–orbit coupling (SOC) between these excited states. As a result, TADF emitters have the capability to harvest both S 1 and T 1 , theoretically achieving 100% internal quantum efficiencies.…”

Elevating the upconversion performance of a multiple resonance thermally activated delayed fluorescence emitter via an embedded azepine approach

“…, Δ E ST ≥ 0.12 eV) due to the inherent short-range charge transfer (SRCT) of the excited state. 13,14 Thus, reverse intersystem crossing (RISC) processes from triplet to singlet excited states in MR-TADF systems are slower than in conventional TADF emitters. And their rate constants are generally lower than 10 4 s −1 , resulting in serious device efficiency roll-offs at high luminance.…”