Halogen‐Containing TPA‐Based Luminogens: Different Molecular Packing and Different Mechanoluminescence

Abstract

Organic mechanoluminescence (ML) materials have attracted much attention for their various application prospects in luminescent devices. However, the pace of their development has been greatly limited by the unclear inherent mechanism. Herein, halogen atoms are systematically introduced into ML for the first time to deeply explore the light‐emitting process and internal mechanism. Thus, a series of triphenylamine (TPA) derivatives is obtained with diverse ML properties, including TPA‐oF, TPA‐mF, TPA‐pF, TPA‐oCl, TPA‐mCl, TPA‐pCl, TPA‐oBr, TPA‐mBr, TPA‐pBr, and TPA‐ph. Among them, with the introduced chlorine and bromine atoms in the para‐position of the phenyl ring in TPA‐ph, the resultant TPA‐pCl and TPA‐pBr demonstrate bright ML; whereas the introduced fluorine atom in TPA‐pF drastically reduces the ML performance. Careful analysis of their optical properties and crystal structures coupled with theoretical calculations confirms that different halogen atoms in the TPA derivatives can really lead to diverse intermolecular interactions, and the strong intermolecular interactions are actually beneficial for the ML effect by reducing energy loss through nonradiative relaxation channels under a stimulus of mechanical force; however, unsatisfying ML performance is observed in the crystal with an overtight molecular packing, on account of the unbreakable crystals with high rigidity.

Miracles of molecular uniting

“…According to the intensive investigation on organic ML materials, the stuck packing mode is the key factor, in addition to the piezoelectric effect, since the possible molecular slippage under the mechanical force is the main non-radiative process (Fig. 5a) [42][43][44]. Similar to photoluminescence (PL), ML can be fluorescence or phosphorescence under different conditions, mainly re-lated to the electronic property of the excited state generated by mechanical stimuli.…”

Shedding Light on the Origin of Solid‐State Luminescence Enhancement in Butterfly Molecules

“…and some externalp hysical stimuli (mechano-luminescence) are also criticala spects to be considered in the designo fl ight-emitting solid materials. [14][15][16] An umber of bulky chemical groups with sterically congested structures, such as tetraphenylethylene (TPE), cis,cis-1,2,3,4tetraphenylbutadiene,t etraphenylpyrazine (TPP) and hexaphenylsilole (HPS), are commonly used to obtain SLE luminogens. [8-10, 14, 17-19] In general,t he incorporation of these moieties has ah uge impact on the fluorescencee mission wavelength and quantum yield.…”

“…), the procedure used to obtain it (drop casting, spin coating, etc.) and some external physical stimuli (mechano‐luminescence) are also critical aspects to be considered in the design of light‐emitting solid materials [14–16] …”

Heartbeat-Sensing Mechanoluminescent Device Based on a Quantitative Relationship between Pressure and Emissive Intensity

“…The corresponding flexible and wearable devices represent convenient practical applications for communications, information storage, and health care, with the possibility of improving our quality of life. After deeply exploring the mechanism of this kind of efficient ML luminogen through different theoretical models, the main factors to be taken into consideration when designing efficient ML materials are: (1) a twisted conformation could be a good choice to effectively restrict p-p interactions for the realization of bright ML emission in aggregated state; 19,21,29 (2) the introduction of a self-assembly group is beneficial for the adjustment of suitable molecular packing due to the more suitable and controllable intra-/intermolecular interactions; 24,28,34,40 (3) the low potential energy barrier of the molecule should receive special attention, as it may help the packing mode to remain flexible and controllable in crystal and make the ML signal more sensitive and stable. These findings could shed light on the understanding of the ML processes in mechanisms and applications, and provide more possibilities to further design excellent ML materials.…”

“…The main reason for this might be that upon force stimulus, the orderly molecular arrangement of ML luminogens is destroyed during conversion from the crystalline state to the amorphous state (without the ML property). [24][25][26][27]…”