Study On Structural Transformation And Piezochromism Of Multi-Benzene Ring Carbon-Based Molecules Under High Pressure

As an indispensable element of organic matter and living organisms,carbon,with its unique bonding ability,can form covalent bonds with various elements to form carbon-based,structurally rich multi-benzene ring carbon-based molecules,which play an irreplaceable role in human production and life.Because of its variable structure and adjustable properties,it is the focus of attention to search for piezochromism materials in multi-benzene ring carbon-based molecules.In a narrow sense,piezochromism refers to the change of absorption band gap of materials under pressure,which shows the change of material color.In a broad sense,it refers to the change of absorption spectrum of materials under pressure.Because of the strong covalent bonds and uniqueπ-electrons between the carbon-based atoms,easily adjustable intermolecular interactions and geometric configurations,multi-benzene ring carbon-based piezochromism materials are an ideal source for designing and constructing materials with excellent piezochromism property,and have great prospects for applications in piezochromism property,anti-counterfeiting,pressure sensing,etc.,which have received wide attention from researchers.They have great application prospects in the fields of piezochromism coatings,anti-counterfeiting,pressure sensing,etc.,and have attracted wide attention from researchers.Under high pressure,the interaction between atoms and molecules in the material is significantly changed,causing changes in the structure of the electronic energy band,which makes the electron jumping mode change,especially to regulate the absorption of light,which provides a new way to achieve piezochromic design.It is an important scientific issue in the current research direction to design and construct multi-benzene ring carbon-based materials with excellent piezochromism property,further study their high-pressure structure transformation and reveal their piezochromism mechanism.In order to solve these important problems,this paper focuses on the influence of intermolecular interaction on the piezochromism properties of multi-benzene ring carbon-based molecules.Two precursors of multi-benzene ring carbon-based materials,namely,macrocyclic co-crystal material and D-π-A type azo-phenyl crystal material,are selected to explore their structural transformation under high-pressure,using pressure to change the intermolecular interaction.The change of molecular stacking mode and molecular conformation can be achieved to construct the response materials with different piezochromism properties.Specific research contents are as follows:1.The piezochromism response of macrocyclic co-crystal P5-PDI·2THF under high-pressure was investigated by using the high-pressure technique of diamond anvil cell,and the physical mechanism of its structural evolution driving the color change was explored.Under atmospheric pressure,P5PDI·2THF showed red color,but no significant change was found in its color when the pressure was increased to 18.82GPa.Interestingly,in the process of pressure relief,its color changes from red to yellow,showing the piezochromism property.Through the characterization of in-situ infrared spectra and XRD diffraction spectra at high-pressure,it was found that the piezochromism exhibited by the P5-PDI·2THF co-crystal is most likely due to the effective regulation of the THF adsorption site by pressure,thus changing the charge transfer state of the co-crystals.Further,the crystal structure of THF on the new adsorption site of P5-PDI·2THF was obtained by combining with theoretical simulation analysis.It was also found that the appearance of new adsorption sites in the released samples increased the amount of THF adsorbed,and the color of the co-crystals changed from yellow to orange when THF was dropped into the released samples,indicating that the original adsorption sites in the macrocyclic co-crystals may still remain active during the compression/uncompression.In addition,when THF is replaced by CH₂Cl₂,the color compression transformation from red to orange can still be realized in the process of pressure relief,which indicates that the pressure-regulated interaction between the co-crystal adsorbed molecular guest and host can realize the construction of piechromatic materials,and provides a new idea and selection strategy for the design and construction of multi-benzene ring carbon-based materials.2.The piezochromism response of AN-Azo-PTZ crystal under high-pressure was investigated by using the high-pressure technique of diamond anvil cell,and the internal mechanism of color change driven by molecular conformation was revealed.It was found that the color of AN-Azo-PTZ crystals at 0-7.92 GPa exhibited a piezochromic shift from yellow to orange to black with increasing pressure under near-violet visible light irradiation,and the redshift range of its absorption spectrum reached 250 nm,achieving a wide range of redshift modulation.In situ spectroscopic characterization and theoretical simulations revealed that the overall conformation of AN-azo-PTZ molecule was planarized at 0-2.95 GPa,and the intramolecular charge transfer was promoted by the bridging effect of azobenzene（azo）molecule,which contributed to the red-shift of the band gap and exhibited a yellow-to-orange piechromatic transition;while when the pressure exceeded 2.95 GPa,a The dihedral angle between PTZ and Azo is reversed and the distortion of molecules connected to both sides of the N=N bond increases,and the local conformational change slows down the charge transfer between molecules,but due to the overall molecular conformational planarization of AN-Azo-PTZ crystals and the enhancedπ-πinteractions between molecules,the absorption edge of the crystal still exhibits a red-shifted behavior,further leading to its orange to black piechromatic transition.The results reveal the novel piechromatic response of AN-azo-PTZ crystals and reveal the intrinsic mechanism of its molecular conformation-driven color change,which provides new insights into the mechanism of the influence of the molecular conformation of D-π-A-type azobenzene on the color change of the material.