| Elastic electronic devices have rapidly developed in recent years and relevant products gradually prevailed in our daily life.Organic crystal materials have the advantages of less defects,long-range ordered structure,high carrier mobility,superior photostability and thermostability.Therefore,organic crystal materials have become an ideal candidate for producing organic electronic devices.However,conventional crystal materials are commonly fragile and they will rupture under external stress,which considerably hampers their application in organic optoelectronic field,especially elastic electronic devices.Hence,it is of great significance to construct the system of elastic luminescent crystal materials for the development of material science.In order to solve this problem,the thesis aims at the design and preparation of organic elastic luminescent crystal materials.Crystals obtain excellent material mechanic properties by the synergy of regulating molecular and crystal structures.Moreover,organic elastic crystals are deeply exploited for their potential application in the optoelectronic field based on their mechanic and luminescent properties.Schiff Base,which includes manifold compounds possessing?-conjugated structure,are selected as our objects.We successfully prepared a series of crystal materials that exhibit different material mechanic properties then systematically characterized their material mechanic properties and profoundly explored the relationship between crystal elasticity and crystal structure,explicitly revealing the mechanism of crystal elasticity and exploring the application of elastic crystal materials in the optoelectronic field.In chapter two,we selected a dimethylamino-substituted hydroxy-1-naphthaldehyde Schiff Base derivative.Its ultra-long needle-like crystals which displays red emission are attained by recrystallization.The crystal can bend under external applied force and automatically return to the initial state after withdrawing force.The whole process could be repeated without the rupture of crystals,which shows excellent elastic property.The mechanic property of crystals is characterized by three-point bending test.The result verifies that the flexural modulus of the crystal is about1.4–2.1 GPa.The analysis of crystal structure indicates that molecules pack in a chain structure by?···?interaction in the crystals.Crystals could slightly expand or narrow the distance of?···?interaction to adjust to its own expansion or compaction.Meanwhile,molecules in the crystal can also subtly rotate to accustom the alteration of curvature generating in the crystallographic plane bending.The C–H···O hydrogen bond can affect at a long distance to facilitate crystals to recover to the initial position,which ensure the elasticity of crystals.Additionally,the crystal possesses a well optical property.Its quantum efficiency is 0.43 and the low optical loss coefficients of crystals in the natural and bent state are experimented by optical waveguide tests,the values are0.270 d Bmm-1and 0.274 d Bmm-1,respectively.These results indicate that the crystal is enormously potential in the flexible optoelectronics field.In chapter three,we selected a two-methoxy substituted hydroxy-1-naphthaldehyde Schiff Base derivative.Two different crystals are obtained by regulation of the humidity of solvent evaporation circumstance:green emission crystal G and yellow emission crystal Y.Crystal G exhibits good flexibility,whose flexural modulus is tested to be 5.99–6.43 GPa.In addition,crystal G can also plastically twist and the twisted crystals can still carry out elastic bending.Crystal Y shows good elasticity but crystals will rupture during twisting,indicating the loss of plasticity.Single crystal structure of G shows that there are only compound molecules in the crystals.However,single crystal structure of crystal Y indicates that crystal Y is a cocrystal of compound molecules with ground state proton transfer structure and water molecules.In contrast with two crystal structures,two crystals possess the same flexible bending mechanisms:the bending ability derives from the alteration of?···?interaction in the crystals,which adapts the strain that generates from bending process.The flexibility originates from the change of the length of hydrogen bonds.The difference of hydrogen bonds between two crystals mainly results in two different material properties.The shear force will directly apply to the glide lamella during twisting process.The existence of water molecules in crystal Y generates stronger hydrogen bonds that restrict the slide of molecular layers,which does not exist in crystal G.We notice that the direction of polarized light can be controlled by changing the twisting angle of crystal,realizing the function of polarization rotator based on crystals.This work simplifies the structure of polarization and will expand the application of organic elastic crystal materials in more other fields.In chapter four,we select the salicylaldehyde Schiff Base derivative that introduces cyanostyrene structure.Two polymorphs,needle and block,are obtained by solvent diffusion and vacuum sublimation.The transformation of two different polymorphs is realized by heating.Block crystals will break under external applied force,which indicates that block crystals are fragile.On the contrary,needle crystals can be bent and recover after the withdrawal of force,which is reversible and could repeat for many times at room temperature(298K).Surprisingly,needle crystals can also exhibit the same excellent elasticity under ultralow temperature(77K),displaying particular ultralow temperature elastic property.Single crystal structure indicates that molecules in the crystal pack in a chain structure by strong?···?interaction whose distance will change for the strain generated during bending process.Moreover,molecules will also rotate slightly for the curvature alteration of crystal face.Abundant hydrogen bond networks will restrict the long-range slide of molecules,avoiding the generation of plasticity.We compared the crystal structure under different temperatures.The strong?···?interaction,abundant hydrogen bonds and crisscrossing packing mode of molecules are essential to the ultralow temperature elastic property.These factors guarantee the stability of molecular configuration and packing structure in the crystal structure,which reduces the impact on the flexibility of crystals.Furthermore,the material mechanic properties of crystals will also change during the transformation of needle crystals to block crystals.Based on this property,we design and accomplish the method of manufacturing thermoforming crystals.This research not only solves the problem that polymer materials become brittle at low temperature and provides a new path for the elastic crystal materials but also facilitates the exploration of ultralow temperature circumstance such as polar region or extraterrestrial space in the future. |
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