Very bright mechanoluminescence and remarkable mechanochromism using a tetraphenylethene derivative with aggregation-induced emission

Chiral amplification in liquid crystals (LCs) is a well-known strategy. However, current knowledge about the underlying mechanism was still lacking; in particular, how it was realized at the nano scale still remained to be revealed. Here, we provide systematical exploration of chiral amplification of chiral aggregation induced emission (AIE) molecules in LCs from direct visualization of their co-assemblies at the nano scale to theoretical calculation of the molecular packing modes on a single molecular level. Using AFM imaging,we directly visualized the co-assembly formed by chiral AIE molecules/LCs at the nano scale: the chiral AIE molecules self-assembled into helical fibers to serve as the helical template for LCs to bind, while the LCs helically bound to the helical fibers to form the co-assembly, giving the morphology of pearled necklaces or thick rods. Theoretical calculation suggested that chiral AIE molecules were packed into left-handed helical fibers with a large volume of empty space between neighboring molecules, which provided the binding cites for LCs. Structural analysis showed that the π-π stacking between aromatic groups from LCs and TPE groups and the σ-π hyperconjugation between LC aromatic groups and cholesterol aliphatic groups play an important role in stabilizing the binding of LCs in the confined space on the surface of the helical assemblies.

Direct Visualization of Chiral Amplification of Chiral Aggregation Induced Emission Molecules in Nematic Liquid Crystals.

Organic chromophores with large Stokes shifts and dual emissions are fascinating because of their fundamental and applied interest. Vibration-induced emission (VIE) refers to a tunable multiple fluorescence exhibited by saddle-shaped N,N'-disubstituted-dihydribenzo[a,c]phenazines (DHPs), which involves photo-induced configuration vibrations from bent to planar form along the N-N axis. VIE-active molecules show intrinsic long-wavelength emissions in the unconstrained state (planar state) but bright short-wavelength emissions in the constrained state (bent state). The emission response for VIE-active luminogens is highly sensitive to steric hindrance encountered during the planarization process such that a tiny structural variation can induce an evident change in fluorescence. This can often be achieved by tuning the intensity ratio of short- and long-wavelength bands. In some special cases, the alterations in the emission wavelength of VIE fluorophores can be achieved step by step by harnessing the degree of bending angle motion in the excited state. In this perspective, we summarize the latest progress in the field of VIE research. New bent heterocyclic structures, as novel types of VIE molecules, are being developed, and the general features of the chemical structures are also being proposed. Technologically, novel emission color-tuning approaches and VIE-based probes for visualizing biological activity are presented to demonstrate how the dynamic VIE effect can be exploited for cutting-edge applications.

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The endeavor of vibration-induced emission (VIE) for dynamic emissions

Thermally activated delayed fluorescence (TADF) has been explored actively in luminescent organic materials. Yet, realizing such TADF-active, multifunctional emitters with high emission efficiency still remains hugely challenging. In this context, a series of twist-conjugated organic molecules bearing diphenylsulfone and 9,9-dimethylacridine moieties are designed and prepared, and are found to show, in one molecule, TADF, room-temperature phosphorescence, triboluminescence, and aggregation-induced emission enhancement. In addition, remarkably high photoluminescence quantum efficiency, up to ≈100%, is achieved for these novel molecules. Single-crystal analysis and theoretical calculations reveal that the through-space charge transfer (TSCT) effect in these molecules is responsible for both the multifunctional emission and high emission efficiency. A maximum external quantum efficiency of 20.1% is achieved, which is among the highest recorded in a solution-processable device containing TSCT-based TADF materials. These results illustrate a new approach to achieving highly efficient TADF-active, multifunctional emitters.

Intramolecular Through-Space Charge Transfer Based TADF-Active Multifunctional Emitters for High Efficiency Solution-Processed OLED

Mechanofluorochromic (MFC) materials are a class of luminescent materials that have ability to show changes in their emission when subjected to various mechanical stimuli. A wide range of MFC materials...

Mechanochromic luminogens with hypsochromically shifted emission switching property: Recent advances and perspectives

Herein, we investigated the mechanochromism (MC) and mechanoluminescence (ML) properties of an “old” tetraphenylethylene derivative p-NH2 and its isomer m-NH2 and provided important insights into the mechanism of these two properties. Both compounds possess aggregation induced emission (AIE) properties inherited from their TPE skeleton. However, their fluorescence differs in the aggregation state due to their different conjugation degrees. Molecule p-NH2 is both MC- and ML-active, while m-NH2 is only ML-active. The single-crystal structures and related experimental results suggest that the ML/MC properties are closely related to intermolecular interactions and molecular conformation. There are abundant intermolecular interactions in their single crystals, endowing both isomers with ML properties, while too strong intermolecular interactions and planar conformation on the other hand inhibit a reversible transition between different molecular packing modes and disable the MC activity of m-NH2.

Jiaqiang Wang

Papers

Insight from the old: mechanochromism and mechanoluminescence of two amine-containing tetraphenylethylene isomers