Intermolecular Interaction-Induced Thermally Activated Delayed Fluorescence Based on a Thiochromone Derivative

Torsion Angles between Donor and Acceptor Moieties as a Descriptor for Designing Nonlinear Optics and Thermally Activated Delayed Fluorescence Materials

The performances of nonlinear optics (NLO) and thermally activated delayed fluorescence (TADF) materials are strongly related to the torsion angles (θ) between donor (D) and acceptor (A) moieties in D-A architecture molecules. However, the underlying relationships connecting θ to the performances of NLO/TADF materials remain unclear. Herein, we present a comprehensive theoretical study on NLO/TADF materials composed of a series of D-A backbone molecules (TPAAP/TPAAQ series and AQ-DMAC/AQ-MeFAC series) to shed light on these relationships. It is found that changing θ via the intramolecular locking strategy can greatly influence values of the first hyperpolarizability (β) and singlet-triplet energy gap (ΔEST), further leading to better/worse performances of NLO/TADF materials, respectively. Intriguingly, a more detailed analysis indicates that the variation trends between θ and β/ΔEST are changeable in low θ regions, exhibiting volcano-like relationships. The large coefficients of determination (R2, ranging from 0.76 to 0.93) suggest that this experimentally measurable parameter (θ) can be used as a promising descriptor to evaluate the performances of related materials. Following the revealed θ-β/θ-ΔEST correlations, the optimal/worst torsion angles for different materials are identified. These findings highlight the importance of the intrinsic structure-performance relationships, thus providing novel design strategies for high-performance NLO/TADF materials.

Visualization of Frontier Molecular Orbital Separation of a Single Thermally Activated Delayed Fluorescence Emitter by STM

Because the spatial distribution of frontier molecular orbitals (FMOs) regulates the thermally activated delayed fluorescence (TADF) property, researchers synthesize TADF emitters by designing their FMO distribution. However, it remains challenging to clarify how the FMO distribution is altered at molecular interfaces. Thus, visualizing the FMOs at molecular interfaces helps us to understand the working behavior of TADF emitters. Using scanning tunneling microscopy (STM), we investigated the electronic structure of a single TADF emitter, hexamethylazatriangulene-triazine, at molecule-metal and molecule-insulating film interfaces. FMOs at the molecule-metal interface were not spatially confined to the donor-acceptor moieties because of hybridization. Meanwhile, FMOs at the molecule-insulator interface exhibited spatially separated filled and empty states confined to each moiety, similar to the calculated gas-phase FMOs. These observations illustrate that the molecule-environment interaction alters the spatial distribution of FMOs, proving that STM is a powerful tool for studying TADF molecules.

Paper Information Recording and Security Protection Using Invisible Ink and Artificial Intelligence

Conventional paper information protection mainly relies on stimuli-responsive functional materials that can display color or luminescence under external stimuli; however, this method is rather predictable and can be easily cracked. In this work, a paper information protection scheme combining fluorescent invisible ink and artificial intelligence was proposed. The ink was prepared by dissolving carbon nanoparticles in water, which has a high quantum yield and outstanding light stability and salt stability, thus ensuring the integrity of information in complex environments. A five-layer convolutional neural network (one of the two mainstream architectures in today's artificial intelligence fields) was specially trained based on ultraviolet light excited symbols printed by invisible ink. Using this scheme, the correct information could only be read with the specially trained neural network after ultraviolet (UV) irradiation. Without this trained neural network or UV irradiation, misleading messages will be presented. Moreover, it was possible to design unpredictable and highly complex password books to further increase information security. This smart strategy provides new opportunities for high-level paper information encryption and also proposes new ideas for the applications of carbon nanoparticles and artificial intelligence.

Sky-Blue Thermally Activated Delayed Fluorescence with Intramolecular Spatial Charge Transfer Based on a Dibenzothiophene Sulfone Emitter

Intramolecular spatial charge transfer (ISCT) plays a critical role in determining the optical and charge transport properties of thermally activated delayed fluorescence (TADF) materials. Herein, a new donor/acceptor-type TADF compound based on rigid dibenzothiophene sulfone (DBTS) moiety, STF-DBTS, was designed and synthesized. Fluorene unit was used as a rigid linker to position the rigid acceptor and donor subunit in close vicinity with control over their spacing and molecular structure and to achieve high photoluminescence quantum yield (∼53%) and TADF property. For comparison purposes, we constructed the more flexible STF-DPS with a less rotationally constrained diphenylsulphone (DPS) acceptor instead of the rigid DBTS units, and STF-DPS showed no TADF properties and lower PLQY (16.0%). Organic light-emitting diodes (OLEDs) based on STF-DBTS achieve an external quantum efficiency (EQE) of 10.3% at 488 nm, which is a fivefold improvement in EQE with respect to STF-DPS.

Cu(I)-Catalyzed Enantioselective Alkynylation of Thiochromones

A highly efficient asymmetric synthesis of chiral thiochromanones is developed via Cu(I)/phosphoramidite catalyzed asymmetric alkynylation of thiochromones under mild reaction conditions. The catalyst system is tolerant of various thiochromone precursors and terminal alkynes. The established asymmetric transformation provides different enatiomeric-enriched thiochromanones with more molecular complexity and enables access to chiral thioflavanones, a subgroup of flavonoid by further functionalization.

A Simple Organic Molecule Realizing Simultaneous TADF, RTP, AIE, and Mechanoluminescence: Understanding the Mechanism Behind the Multifunctional Emitter

Aggregation-induced emission (AIE), thermally activated delayed fluorescence (TADF), room-temperature phosphorescence (RTP), and mechanoluminescence (ML) have attracted widespread interest. However, a multifunctional organic emitter exhibiting simultaneous AIE, TADF, RTP, and ML has not been reported. Now, two multifunctional blue emitters with very simple structures, mono-DMACDPS and Me-DMACDPS, exhibit typical AIE, TADF, and RTP properties but different behavior in mechanoluminescence. Crystal structure analysis reveals that large dipole moment and multiple intermolecular interactions with tight packing mode endow mono-DMACDPS with strong ML. Combined with the data of crystal analysis and theoretical calculation, the separated monomer and dimer in the crystal lead to the typical TADF and RTP properties, respectively. Simple-structure mono-DMACDPS is the first example realizing TADF, RTP, AIE, and ML simultaneously.