Structure And Properties Of Flexible Crystal Materials Based On Anthracene Derivatives

As one of the functional materials,organic single crystals have a wide range of applications in light-emitting diodes,optical waveguides,and materials that respond to pressure,temperature,and light.However,organic crystals are usually brittle and unprocessable,which limits their applications to a large extent.Therefore,in the past15 years,the exploration of organic crystals with mechanical properties(such as plasticity and elasticity)has become a frontier field.The discovery of flexible organic crystals compatible with optical and electronic functions in recent years has promoted the application of these flexible materials in the field of optoelectronics.In this paper,we have studied two simple organic small molecules that have been reported and grown their crystals.Through variable temperature XRD,spectroscopy,three-point bending and other test methods,the influence of crystal structure on its mechanical properties is deeply studied,and a new strategy is provided for temperature-controlled phase change to adjust the mechanical properties of organic crystals.In addition,the optical waveguide properties of the crystal were also studied,and its application in flexible optical devices was explored.In Chapter 2,we studied the organic molecule9,10-diphenylethynylanthracene,and obtained the orange needle-like crystal phase A through solvent diffusion recrystallization.Phase A crystals can be bent elastically at room temperature.In addition,we also found that the elasticity of phase A crystals can be maintained from-196°C to 150°C.XRD analysis shows that the crystal stacks with isotropic intermolecular interaction and lateral structure can meet the requirements of crystal elasticity.At the same time,the insensitivity of crystal axis and crystal volume to temperature makes the molecular conformation and packing mode almost the same at different temperatures.Therefore,the crystal can still maintain its original elasticity at different temperatures.When the crystal of phase A is heated above 160°C,a single crystal to single crystal phase transition will occur to obtain another orange-yellow crystal B.This transition is irreversible.The crystal of phase B exhibits unique thermoplasticity: that is,it cannot be bent at room temperature,and its shape can be changed arbitrarily once it is heated to above 120°C.After cooling to below 120°C,the newly formed shape can be retained.The whole process can be repeated many times.This unique property allows the crystal to be heat-treated into the desired shape.The single crystal data shows that the weak interaction of the crystal of phase B causes the molecular layer to slip and plastically bend when the crystal is stressed.At the same time,the crystal axis and crystal volume are more sensitive to temperature.The anisotropic expansion of the crystal axis and the increase in the distance between molecular layers caused by heating are the core reasons for the realization of the thermoplasticity of the B-phase crystal.In addition,the crystal of phase A also has lower optical loss,and good flexibility and optical waveguide ability indicate that the crystal has potential application value in flexible optical devices.2.In Chapter 3,we purified and grown the organic molecule9-Anthracenecarbonitrile crystals.The crystal can be plastically bent on multiple sides.Through single crystal X-ray diffraction,theoretical calculations and scanning electron microscopy tests,it is determined that the two-directional sliding plane,uniform intermolecular force and positive cross-sectional shape are important reasons for its ability to achieve multiple surface bending.The crystal can also restore its linear shape after twisting.Through detailed crystallographic analysis and comparative experiments,we have proposed its self-repairing mechanism: centered on the molecular chain at the neutral axis position without relative sliding,from the outside to the inside,the layer is restored to its original position.As the relative displacement of the molecular chain decreases,the fracture interaction caused by the relative sliding is restored.The molecular chain that has been restored to the original position will promote the partial chain in the adjacent layer to return to its original position,and finally complete the complete restoration of the entire crystal at the molecular level.The crystal also has flexible optical waveguide properties,and its optical loss coefficient is a relatively low value among organic crystals that have been reported so far.In summary,we have studied two simple organic molecules and prepared their crystals,and thoroughly studied their elastic and plastic properties and their causes.Through the research and characterization of crystals,not only the flexibility of organic crystals is extended to thermoplasticity,but also an idea for adjusting the mechanical properties of organic crystals through temperature-controlled phase change is provided.