Design,Synthesis And Properties Of Novel Anthracene Mechanoluminescence Materials

Mechanoluminescence(ML)refers to the luminescence of a sample caused by external mechanical forces(scratching,grinding,compression,etc.).Although ML was discovered as early as in 1605,it has not attracted much attention of researchers until recent years due to the lack of relevant detection instruments.The fast excitation process of ML makes it difficult to study it detailedly,which leads to the slow progress in the mechanistic investigation of ML.The current explanation of the ML mechanism mainly relies on the piezoelectric effect,which is commonly found in non-centrosymmetric crystals.Two opposite charge distributions are formed on the fracture surface when the crystals were broken and the electric field formed will act as the excitation source to generate ML.Under continuous mechanical stimulation,the organic ML materials will change from crystalline to amorphous state,while the ML strength weakens or even disappears.Therefore,nearly all reported organic ML materials do not maintain ML properties all the time under prolonged mechanical stimulation.According to the literature,the introduction of groups with self-assembly effect can help the molecule can form an equilibrium of order molecular arrangement after mechanical stimuli,thus keeping the ML for a long time.Anthracene has a large π-conjugation plane and high luminescence efficiency and is widely used in various fields.However,few anthracene derivatives with ML properties have been reported.Here,anthracene was chosen as the basic unit of luminescence,and the borate ester group was introduced at the 9 site of anthracene to enhances strong intermolecular interactions in the crystal,which could suppress the non-radiative leap process under mechanical forces and facilitate the realization of ML.Then thiophene group was selected as the self-assembly unit to maintain ML for a long time.B2 T and B3 T were obtained by changing the linked sites of thiophene to investigate the effect of the linked sites of self-assembled units on ML.B2 T has no ML phenomenon,however,the bright blue ML can be observed when scraping B3 T crystals or powder under daylight,and it can maintain the bright ML under prolonged mechanical stimulation.After careful analysis of the crystals,it can be found that the linked sites of the thiophene groups influence the molecule packing modes and the intermolecular interactions,which then results in the different ML properties.Later,based on the structural basis of B2 T and B3 T,B2TM and B3 TM were synthesized by introducing methyl substituent groups to the thiophene moiety.B2 TM and B3 TM,which are isomers of each other,exhibit different ML by regulating the molecular packing modes and intermolecular interactions in the crystal through the introduction of methyl groups.When B3 TM crystals or powder were scratched,bright blue ML could be observed even under daylight,and the bright ML properties could be maintained under prolonged mechanical stimulation,this is similar to the ML properties of B3 T.B2T has no ML phenomenon,but the introduction of methyl group makes B2 TM exhibit a weak ML phenomenon.The analysis of the crystals revealed that the introduction of methyl groups changed the packing mode of B2 T molecules,and transformed the B2 T crystals with centrosymmetry into non-centrosymmetric B2 TM crystals with the piezoelectric effect.Therefore,the molecular packing mode plays a crucial role in ML materials.When designing ML materials to introduce self-assembly units,the linked sites and substituent groups of self-assembly units have a great impact on obtaining strong and long-lasting ML.