Through-Space Charge-Transfer Emitters Developed by Fixing the Acceptor for High-Efficiency Thermally Activated Delayed Fluorescence

Abstract

Through-space charge-transfer (TSCT) emitters have been extensively explored for thermally activated delayed fluorescence (TADF), but arranging various donors and acceptors into rigid cofacial conformations for various efficient TSCT TADF emitters has remained challenging. Here, we report a "fixing acceptor" design to reach various efficient TSCT TADF emitters. By chemically fixing the acceptor (benzophenone) with a rigid spiro-structure and cofacially aligning various donors with the fixed acceptor, a series of efficient TSCT TADF emitters have been developed. Single-crystal structures and theoretical calculations have verified closely packed cofacial donor/acceptor conformations and favorable TSCT in the emitters. In doped films, the emitters afford sky blue to yellow TADF emission, with high photoluminescence efficiencies up to 0.92 and reverse intersystem crossing rates up to 1.0 × 10 6 s −1 . Organic light-emitting diodes using the emitters afford sky blue to yellow electroluminescence with high external quantum efficiencies up to 20.9%. The work opens a new avenue toward a wide variety of efficient TSCT TADF emitters.

Vibronic effects accelerate the intersystem crossing processes of the through-space charge transfer states in the triptycene bridged acridine–triazine donor–acceptor molecule TpAT-tFFO

“…Because ISC and rISC between singlet and triplet chargetransfer (CT) states of equal electronic structure are orbitally forbidden, spin-vibronic coupling with a nearby locally excited (LE) triplet state has been argued to be essential for enhancing the radiationless 1 CT + 3 CT transitions in both, through-bond and through-space donor-acceptor complexes. 1,[3][4][5][6][7][8][9][10][11][12] Quantum chemical calculations on TpAT-tFFO in the framework of timedependent density functional theory (TDDFT) 1 suggest the presence of a 3 LE state on the T acceptor in close energetic proximity of the lowest-lying 1 CT and 3 CT states. The results of this study further indicate that a tilted face-to-face (tFF) alignment of A and T gives rise to stronger spin-orbit coupling (SOC) between 1 CT and 3 LE (0.61 cm À1 ) than the parallel face-to-face (pFF) alignment (0.05 cm À1 ) of the donor and acceptor moieties while the SOC between 1 CT and 3 CT vanishes (0.00 cm À1 ) in both conformations.…”

Rare Earth Complexes with 5d–4f Transition: New Emitters in Organic Light-Emitting Diodes

“…But they have limited maximum EUE up to only 25% because of the forbidden transition between the triplet-singlet states . Therefore, luminescent materials that can utilize triplet excitons to reach 100% EUE are developed, such as phosphorescence materials − and TADF materials. − However, the emitting process involving triplet excitons makes their excited-state lifetimes (τ) longer than that of fluorescence materials, which is detrimental to device stability . In recent years, many efforts have been invested in the development of luminescent materials with co-instantaneous high EUE and short excited-state lifetime.…”

Benefits of the Exciplex-like Framework in Reducing the Singlet-Triplet Energy Difference: A Theoretical Perspective on the Role of the Exciton Binding Energy

“…Regarding the molecular topology effect on the TADF efficiency, the exciplex-like emitters have been consistently reported in the literature; − since Adachi and his group utilized the TADF from an exciplex in the OLEDs, , there has been growing interest in exciplex systems as TADF emitters. Because of the inconsistent relative conformation between the D and A units, however, these exciplex systems hardly outperformed the conventional D-A-type TADF emitters.…”

“…Controlling these conformations in exciplex systems is always a challenging task, and thus, a rigid L unit was again introduced into exciplex systems (U-type emitters; see Figure ). Indeed, a few of these materials showed excellent performance. − Their enhanced TADF is often ascribed to through-space charge transfer (TSCT) character against through-bond charge transfer (TBCT) nature of the corresponding conventional D-L-A (S-type) emitters (Figure ). − Then, what are the fundamental distinctions between TSCT and TBCT that cause performance differences in the TADF phenomenon?…”

“…Indeed, a few of these materials showed excellent performance. − Their enhanced TADF is often ascribed to through-space charge transfer (TSCT) character against through-bond charge transfer (TBCT) nature of the corresponding conventional D-L-A (S-type) emitters (Figure ). − Then, what are the fundamental distinctions between TSCT and TBCT that cause performance differences in the TADF phenomenon? However, such questions regarding better performances of U-type emitters remain elusive.…”