Effect Of Substitution Modes On Nitrated Aggregation-Induced Emission System

Luminescent materials have been widely used in various fields and are vital to the development of human society.Compared with inorganic luminescent materials,organic luminescent materials have the advantages of high efficiency,low cost,fast response,full color,high stability and easy film-formation on a large area,etc.However,conventional organic luminescent materials generally show aggregation-caused quenching（ACQ）effects,which severely limits their practical applications.Since 2001,Ben Zhong Tang firstly put forth the concept of aggregation-induced emission（AIE）,which intrinsically solved the ACQ problem and triggered the exploration of various new AIE systems.However,there are still no nitro AIE systems reported.On the one hand,nitro groups can induce intramolecular charge transfer and efficient intersystem crossing to quench the emission.On the other hand,the nitro compound has the advantages of easy purification,good crystallization ability and low cost.Here,we want to utilize the special property of nitro groups to design mechanochromic materials with AIE characteristics.We successfully introduced the nitro groups by the Suzuki coupling reaction to the archetypal AIE backbones.Six nitro AIE molecules were obtained with different numbers of subsituted nitro groups.The molecular structure and thermal stability were measured by nuclear magnetic resonance spectroscopy,thermogravimetric testing and differential scanning calorimetry.The twist intramolecular charge transfer（TICT）effect and the aggregation-induced emission（AIE）effect were simultaneously observed.Moreover,the number of nitro groups enhances the TCIT effect and the meta-position of the nitro groups have stronger quenching effect.The luminescence behavior of the crystal and the ground powders revealed that the fluorescence quantum yield of2NPMF increased from 0.08%to 0.68%after grinding,showing mechanic-induced fluorescence enhancement.2NPTPE has strong fluorescence before and after grinding and the fluorescence quantum yield is 2.70%and 2.13%respectively.4NP_p TPE exhibited mechanochromic performance,which fluorescence changed from green to yellow after grinding and the fluorescence quantum yields were7.00%and 4.92%respectively.The quantum yields of 4NP_m TPE were 0.14%and0.36%before and after grinding,showing mechanic-induced fluorescence enhancement.The fluorescence quantum yield of 8NP_mmm TPE increased from 0.08%to 0.24%after grinding while 8NP_pmm TPE from 0.06%to 0.51%,showed different degrees of mechanic-induced fluorescence enhancement and the film-forming performance was greatly improved.Through the analysis of their single crystal structures,it is further confirmed that the number and position of nitro groups greatly affect the molecular configuration,packing mode and the interactions between molecules to affect their photophysical properties.Through the research,we further enriched the AIE family,and proposed a new idea of using the characteristics of nitro group to construct the AIE mechanochromic molecules.Several mechanochromic candidates were found to possess potential applications in sensor or anti-counterfeiting.