Design, Synthesis And Applications Of New Mechanochromic Materials

Mechanochromic materials are a unique family of smart materials, whose optical behavior(such as absorption, reflection, fluorescence and phosphorescence) can undergo reversible changes under external force stimuli. Because mechanochromic materials have wide potential applications in mechanosensors, memory chips and security ink, they have attracted more and more attention in recent years. However, up to now, the families of mechanochromic materials are still limited and the applications of mechanochromic materials are mostly conceptual demonstrations, which cannot satisfy the requirement of practical applications. Therefore, present studies on mechanochromic materials are mostly focused on developing new mechanochromic materials and extending their practical applications. Starting from different mechanochromic mechanisms, we have developed three kinds of new mechanochromic materials based on intramolecular conformation changes, the heterolytic cleavage of intramolecular weak bond and intermolecular proton transfer, respectively. In addition, we have demonstrated their potential applications in anti-counterfeiting, ink-free writing and pressure-sensor.In the second chapter, we find that the inertness of tetraphenylethylene(TPE) to mechanical force comes from its high symmetrical molecular conformation by analyzing its molecular geometrical conformation. Thus, we have proposed a single-arm extended strategy to fabricate tetraphenylethylene-based mechanochromic materials. Based on this strategy, we have successfully developed a new series of full-color(from ~450 nm to ~740 nm) tunable mechanofluorochromic materials. These materials exhibit efficient solid-state emission(quantum yield Φf >10 %) and high mechanofluorochromic contrast(wavelength shift from ~50 nm to ~100 nm). More importantly, we have discovered an unexpected excitation-dependent emission phenomenon of mechanofluorochromic materials and proposed to utilize this new excitation-dependent emission behavior of materials to evaluate their mechanical-responsive performances more comprehensively. Finally, the unique feature of abundant emissions of mechanofluorochromic materials by changing the excitation lights has shown the application potentials in dual channel anti-counterfeiting.In the third chapter, we are the first to observe the dynamic tautomerization equilibrium(around 8 % ring-open isomers under ambient pressure) of benzo[1,3]oxazine OX-1 in constraint crystal environment. Interestingly, the dynamic tautomerization equilibrium can be quantitatively controlled by hydrostatic pressure. When pressure is increased to 10 GPa from ambient pressure, the proportion of ring-open isomers of OX-1 can reach around 83 % from 8 % under ambient pressure. In the meanwhile, the absorption spectra of OX-1 are red-shift to around 700 nm from around 450 nm, which almost cover the whole visible light region. For a small organic crystal, such a continuous and large-scale absorption modulation is the first to be reported. After detailed studies, we think that the pressure-controllable molecular tautomerization should come from the low tautomeric energy barrier of tautomerization and the different compressibility of two isomers. The large-scale absorption modulation of OX-1 crystal mainly comes from the tautomerization equilibrium shifting. Finally, OX-1 has been used as a visual pressure-sensor due to the continuous and naked-eye distinguished color changes under pressure.In the fourth chapter, we have designed and synthesized a new family of twisted and conjugated acid-base amphoteric molecules. This kind of molecules have dynamic intermolecular proton transfer behavior with quite small amount(around 1 ‰) under ambient condition. However, when exerted on external forces, their intermolecular proton transfer processes can be significantly accelerated; anisotropic force(namely mechanical grinding) and isotropic force(namely hydrostatic pressure) can increase the proportions of intermolecular proton transfer to around 10 % and more than 60 %, respectively. Because there are obvious color changes involved in the intermolecular proton transfer processes of twisted and conjugated acid-base amphoteric molecules, they are good candidates for mechanochromic materials with the potential applications in ink-free writing and pressure-sensor.In conclusion, we have developed three kinds of new mechanochromic materials based on different mechanochromic mechanisms, which not only enriches mechanochromic materials, but also provides some references for designing new mechanochromic materials.