Fine Modulation of the Higher-Order Excitonic States toward More Efficient Conversion from Upper-Level Triplet to Singlet

Abstract

Hot exciton luminogens capable of harvesting nonemissive triplet excitons via reverse intersystem crossing from high-order triplet (hRISC) to singlet have great potential in high-efficiency fluorescent organic light-emitting diodes (OLEDs). Although spin–orbit coupling (SOC) is regarded as a key factor affecting the RISC process, its effects on hot exciton materials are poorly understood. Herein, we design and synthesize two blue-emitting hot exciton luminogens, PABP and PAIDO, to study this issue by modulating the excited-state properties. Theoretical and experimental research contributions demonstrate that a stronger SOC between energetically close S1 (π–π*) and T n (T3, n−π*) of PAIDO gives rise to faster and more efficient hRISC in comparison to that of PABP, leading to a higher external quantum efficiency and a higher exciton utilization efficiency. Crucially, the experimentally measured hRISC rate (k hRISC) of hot exciton materials is on the order of 107 s–1, which is much faster than that of the thermally activated delayed fluorescence materials.

Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes

“…It was found that all three compounds present low energy levels of T 1 and clearly large energy gaps between T 2 and T 1 (ΔE T2-T1 ), which can greatly suppress interconversion (IC) decay from T 2 to T 1 as the rate of IC is in an inverse relation with ΔE T2-T1 . 36 In contrast, the three compounds exhibit small energy gaps between T 2 and S 1 , which can trigger spin-flip at high-lying excited states and boost the high-lying reverse intersystem crossing (hRISC) process occurring from T 2 to S 1 , according to the Fermi golden rule. [37][38][39] Moreover, the three compounds exhibit large spin-orbital coupling (SOC) constants between T 2 and S 1 states (<S 1 |Ĥ SO |T 2 >, Supporting Information Table S1), which are in favor of fast hRISC.…”

Narrowband Emission from Organic Fluorescent Emitters with Dominant Low‐Frequency Vibronic Coupling

“…Hence, the RISC is likely owed to the T 1 state via an intermediary T n (n ≥ 2) with large SOC and small energy differences (Table S2, Supporting Information), as proposed for some other emitters. [ 41–43 ] Furthermore, based on Φ F and fluorescence lifetimes of the doped films, the main kinetic parameters are calculated and tabulated in Table S3, Supporting Information. The radiative rate constants ( k r ) of the QA‐Ns calculated from the prompt PL quantum efficiency (Φ PF ) and τ P are larger than those of other flexible emitters, which indicates the effective radiative transition process owing to the high oscillator strengths and large conjugated rigid molecular frameworks.…”

Phenothiazine–Quinoline Conjugates Realizing Intrinsic Thermally Activated Delayed Fluorescence and Room‐Temperature Phosphorescence: Understanding the Mechanism and Electroluminescence Devices

“…Recent experimental studies and quantum chemical calculations ensure that ISC and rISC between 1 CT singlet and 3 LE triplet states are mediated by vibronic coupling of the 3 CT to the locally excited triplet ( 3 LE) states to allow SOC to the 1 CT state, [ 6 ] which accelerates the rate constant of rISC ( k rISC ). [ 7 ]…”