Frontispiece: Detection of the Dark States in Thermally Activated Delayed Fluorescence (TADF) Process of Electron Donor‐Acceptor Dyads: Insights from Optical Transient Absorption Spectroscopy

Abstract

Unraveling the complicated photophysics of thermally activated delayed fluorescence (TADF) compounds is not a trivial task, although it is critical to improve the performance of TADF emitters in OLED. In the Review by Zhao and colleagues (DOI: 10.1002/chem.202203737), recent developments of using the transient optical spectroscopic methods to study the photophysics of TADF emitters are summarized, e. g. the manifestation of the theoretically proposed spin‐vibronic coupling effect and the three‐state model describing the reverse intersystem crossing (RISC) in time‐resolved transient absorption spectra.

Observation of Long-Lived Charge-Separated States in Anthraquinone–Phenothiazine Electron Donor–Acceptor Dyads: Transient Optical and Electron Paramagnetic Resonance Spectroscopic Studies

“…These results show that a subtle variation of the molecular structure, even the linker, may impose a significant effect on the photophysical properties of electron donor–acceptor dyads. This is also solid experimental evidence that having 1 CS, 3 CS, and 3 LE states sharing similar energy is essential for TADF since the 3 CS → 1 CS process is too slow to produce any TADF . Moreover, it should be noted that the prompt fluorescence lifetime of PTZ-Ph-AQ , 62 ns, is exceptionally long for electron donor–acceptor type TADF emitters. ,, Note that the E T1 of the 2-phenylanthraquinone is 2.33 eV, and for the native AQ, the T 1 state energy is ca.…”

“…Under ideal circumstances, the molecular orbital occupancy is literally the same for both 1 CS and 3 CS states, with the radical anion localized on the acceptor and radical cation confined on the donor. The electronic absorption of the 1 CS state, i.e., the sum of the absorption bands of the D 0 + → D n + transition confined on the donor (radical cation) and the A 0 – → A n – transition confined on the acceptor (radical anion) are the same as compared to the electronic absorption of the 3 CS state, making it difficult to discriminate between these two states by transient optical absorption spectroscopic methods. , Although our recent study shows that this is not always the case, clearly more examples are needed to elucidate this issue.…”

“…However, in-depth investigation of the detailed photophysical processes involved in TADF is rare. − Initially, a two-state model was used to explain TADF, but recently, a three-state model has been proposed, , considering three states, 1 CS, 3 CS, and 3 LE as involved in the TADF process. It was further proposed that the 3 LE state is an essential intermediate state, by which the spin–vibronic coupling enhances the reverse ISC (rISC) for TADF. − However, direct experimental evidence for these theories is rare …”

Fine-tuning of the charge-separated state energy in compact orthogonal naphthalene–phenoxazine dyads and its effect on the thermally-activated delayed fluorescence

“…In the previously reported dyad, an alkylamine was attached to the imide N-atom. 32 It was shown that the redox potentials of the aromatic imide can be tuned by attaching electron-withdrawing moieties to the imide N-atom, 57 and thus we used aniline-containing electron-donating and electron-withdrawing moieties to prepare dyads (Scheme 1). We expected to tune the CT state energy, and thus the TADF property of the dyads, given that it is known that the TADF property of compact orthogonal dyads is strongly dependent on the energy of their 1 CT, 3 CT and 3 LE states (in terms of the energy gap and energy ordering).…”

“…Alternatively, SOCT-ISC is another fundamental principle employed for the development of functional materials, such as thermally activated delayed fluorescence (TADF). 27–32 Electron donor–acceptor dyad-based TADF emitters have attracted significant attention, because this type of compounds can be used as third-generation emitters in organic light-emitting diodes (OLEDs) to harvest both the singlet and the triplet excitons produced by CR in the light-emitting layer. 33–38 To date, although many electron donor–acceptor dyads have been developed as TADF emitters for the fabrication of OLED devices, the detailed photophysics of TADF emitters are still not fully understood.…”