Thermally Activated Delayed Fluorescence in an Organic Cocrystal: Narrowing the Singlet–Triplet Energy Gap via Charge Transfer

Smart molecular crystals with light‐driven mechanical responses have received interest owing to their potential uses in molecular machines, artificial muscles, and biomimetics. However, challenges remain in control over both the dynamic photo‐mechanical behaviors and static photonic properties of molecular crystals based on the same molecule. Herein, we show the construction of isostructural co‐crystals allows their light‐induced cracking and jumping behaviors (photosalient effect) to be controlled. Hydrogen‐bonded co‐crystals from 4‐(1‐naphthylvinyl)pyridine (NVP) with co‐formers (tetrafluoro‐4‐hydroxybenzoic acid (THA) and tetrafluorobenzoic acid (TA)) crystallize as isostructural crystals, but have different static and dynamic photo‐mechanical behaviors. These differences are due to alternations in the orientation of NVP and hydrogen‐bonding modes of the co‐formers. After light activation, the 1D NVP‐TA crystal splits and shears off within 1 s. For NVP‐THA, its photostability and high quantum yield give novel photonic properties, including low optical waveguide loss, highly polarized anisotropy, and efficient up‐conversion fluorescence.

Manipulating Light‐Induced Dynamic Macro‐Movement and Static Photonic Properties within 1D Isostructural Hydrogen‐Bonded Molecular Cocrystals

Fang

et al. 2020

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

Liu

et al. 2020

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.

A Calix[3]acridan‐Based Host–Guest Cocrystal Exhibiting Efficient Thermally Activated Delayed Fluorescence

Zhou

Zhang

et al. 2022

A supramolecular strategy to construct thermally activated delayed fluorescence (TADF) materials through host–guest charge transfer interactions was proposed. Consequently, a new class of macrocycle namely calix[3]acridan was conveniently synthesized in 90 % yield. The host–guest cocrystal formed by calix[3]acridan and 1,2‐dicyanobenzene exhibited efficient TADF properties due to intense intermolecular charge transfer interactions. Moreover, the spatially separated highest occupied molecular orbital and lowest unoccupied molecular orbital resulted in a very small singlet–triplet energy gap of 0.014 eV and hence guaranteed an efficient reverse intersystem crossing for TADF. Especially, a high photoluminescence quantum yield of 70 % was achieved, and it represents the highest value among the reported intermolecular donor–acceptor TADF materials.