Face‐to‐Face Orientation of Quasiplanar Donor and Acceptor Enables Highly Efficient Intramolecular Exciplex Fluorescence

Abstract

Intramolecular through‐space charge‐transfer (TSCT) excited states have been exploited for developing thermally activated delayed fluorescence (TADF) emitters, but the tuning of excited state dynamics by conformational engineering remains sparse. Designed here is a series of TSCT emitters with precisely controlled alignment of the donor and acceptor segments. With increasing intramolecular π–π interactions, the radiative decay rate of the lowest singlet excited state (S1) progressively increased together with a suppression of nonradiative decay, leading to significantly enhanced photoluminescence quantum yields of up to 0.99 in doped thin films. A high‐efficiency electroluminescence device, with a maximum external quantum efficiency (EQE) of 23.96 %, was achieved and maintains >20 % at a brightness of 1000 cd m−2. This work sheds light on the importance of conformation control for achieving high‐efficiency intramolecular exciplex emitters.

Intramolecular Through-Space Charge-Transfer Emitters Featuring Thermally Activated Delayed Fluorescence for High-Efficiency Electroluminescent Devices

“…Figure S1 illustrates intramolecular π-π stacking interactions. For 1-5, the D and A units stack to each other with the centroid-to-centroid distances and dihedral angles of the two stacking rings at 3.3−3.8 Å and 10−29° (Figure S1 and Table S2), respectively, which indicates strong or considerable face-to-face π-π stacking interactions [35,39].…”

“…In the doped films, 1-5 afford sky-blue to yellow chargetransfer emission centered at 493, 520, 497, 504 and 550 nm, respectively, with high ΦPL values of 0.80−0.92 for 1-4 and a moderate ΦPL of 0.52 for 5 (Table 1). The remarkable enhancement of ΦPL in doped films in comparison with the solution results from further suppression of intramolecular vibrations/rotations in the solid state [35,39]. Figure S9 shows the low-temperature fluorescence and phosphorescence spectra of the doped films.…”

Donor/Acceptor Pairs Created by Electrostatic Interaction: Design, Synthesis, and Investigation on the Exciplex Formed Within the Pair

“…Table S7 summarizes the TDDFT calculation results.T he TDDFT calculations show that the first-excited singlet state (S 1 )i st he charge-transfer state (i.e., exciplex) formed between the donor and acceptor.I ti sn oted that the S 1 states of the D/A pairs exhibit very small oscillator strengths (around 0.0001), owing to the limited through-space overlap between the HOMO and LUMO orbitals,asalways observed for exciplex systems. [25][26][27][28][29][30][31][32][33]…”

“…[21,23,24] Stacking donor and acceptor in aface-to-face manner within amolecule has been performed, which has demonstrated effective formation of exciplex but usually requires judicious molecular design and complicated synthesis. [25][26][27][28][29][30][31][32][33] Apart from isolating the exciplex, for conventional exciplex systems,m anipulating the distance and relative orientation between the donor and acceptor, which essentially determines the property of exciplex, has remained another challenge. [18,34,35] Here we report D/A pairs created by electrostatic interaction, in which an isolated exciplex can be effectively formed and interactions between the donor and acceptor can be changed.…”

π‐Stacked Donor–Acceptor Dendrimers for Highly Efficient White Electroluminescence

“…p-Stacked molecules and assemblies formed by packing aromatic units in ordered arrangement have attracted much attention of researchers because they are able to exhibit novel properties different from their individual constituents by intra-/inter-molecular interactions. [1][2][3][4][5][6][7] Among them, pstacked emitters consisting of spatially-aligned donors and acceptors [1,2,4,5,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] have emerged as an appealing approach for developing luminescent materials owing to their spatial charge transfer (CT) nature and thermally activated delayed fluorescence (TADF) effect [23][24][25] that enables utilization of non-emissive triplet excitons.D ifferent from p-conjugated emitters based on covalently-bonded donors and acceptors, pstacked donor-acceptor emitters have spatially separated electron-donating and electron-accepting segments that are aligned in close proximity and (tilted) face-to-face pattern. This architecture not only reduces overlap of the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) to achieve small energy splitting between singlet and triplet states (DE ST )f or rapid reverse intersystem crossing (RISC), [4] but also leads to high photoluminescence quantum yield (PLQY) by spatial p-p interaction, [5] providing an attractive motif for the development of new TADF emitters.…”