Studies On Flexible Organic Emissive Crystalline Materials Based On Amino Intersubstituent/dimethyl Terephthalate Derivatives

Organic crystals with long-range ordered molecular stacking structure and anisotropic properties have attracted much attention in the past few decades.At present,many organic crystals with high emission efficiency and high carrier mobility have been designed and synthesized,and have been successfully applied to organic light-emitting diodes,organic solid-state lasers,active optical waveguides,sensors,and other optoelectronics fields.However,although a large number of high-performance organic crystals have been reported one after another,their inherent brittleness severely restricts their applications in soft optoelectronics and wearable devices.According to the traditional crystallography theory,crystals and flexibility are incompatible.that is,molecular crystal is always fragile and easy to be cracked under external forces.Therefore,the development of organic crystals with adjustable flexibility and high luminescence performance is of great significance to expand the application of organic crystals.In recent years,the study of organic flexible crystals with elastic or plastic deformation has become a hot topic in crystal engineering.Although the deformation mechanism of organic crystals is still controversial,it is indisputable that the mechanical properties of organic crystals are directly related to the arrangement and interaction of molecules.At the same time,the arrangement of molecules in the crystal and the interaction between molecules also determine the two key properties of organic luminous crystal materials:luminous color and luminous efficiency.And thus,crystal engineering is considered a great way to construct organic crystals with elastic/plastic deformation ability and to obtain a variety of organic crystals with different mechanical behavior.In this paper,a series of organic single-benzene flexible luminescent crystal materials with excellent mechanical compliance and high luminescence performance were prepared by crystal engineering strategy with amino substituted dimethyl isophthalate or dimethyl terephthalate as the core.Through systematic testing,the intrinsic factors of the excellent properties of these materials were explored,and the application potential of these crystals in the field of flexible optoelectronics was studied.The details are as follows:In chapter 2,we prepared two kinds of organic crystals DMAI-B and DMAI-G by precise control of crystallization conditions based on Dimethyl 5-amino-m-phthalate.Crystal DMAI-B emits blue light under ultraviolet light,and the fluorescence quantum efficiency is 0.14.Crystal DMAI-G emits green light under UV irradiation,and the fluorescence quantum efficiency is up to 0.38.Although the luminescence properties of the two crystals are different,both of two can bend to a semicircle under the external force,and then recover after the removal of external force.that is,they show good elastic properties.The optical waveguide experimental results show that the elastic modulus of DMAI-B and DMAI-G are about 1.5 GPa and 2.1 GPa respectively.The crystal structure analysis shows that there is an obvious?×××?interaction in the crystal DMAI-G,and the?conjugation degree is larger,the luminescence is more redshifted.The synergistic effect of van der Waals forces and intermolecular hydrogen bonds can effectively prevent the dislocation between layers during crystal bending,which is the main reason for the elastic properties of crystals.The optical loss coefficients of crystal DMAI-B are calculated to be 0.374 and 0.388 d B mm^-1 respectively in straight and bending states,the crystal DMAI-G are 0.233 and 0.242 d B mm^-1 respectively.It shows that the two kinds of organic flexible small molecule crystals have infinite application potential in the new flexible optical propagation field in the future.In Chapter 3,two kinds of organic luminescent crystals DMAT-B and DMAT-G with different emission and mechanical properties were prepared by crystal engineering strategy based on 2-amino dimethyl terephthalate.Blue crystal DMAT-B exhibits elastic deformation under external force,while green crystal DMAT-G exhibits brittleness under external force.The elastic modulus of DMAT-B is about 5.4 GPa.The crystal structure analysis shows that the adjacent molecules in DMAT-G crystal have about 1/3 of the overlap area of benzene rings,but there is no obvious?×××?interactions in DMAT-G crystal,so the luminescence of DMAT-G crystal is redshifted.The single-triplet energy level difference of DMAT-B and DMAT-G is4.31 and 4.30 e V respectively.The crystal structure shows that the abundant intermolecular hydrogen bond interactions between the(010)plane limit the long-distance molecular movement,resulting in macroscopic elastic deformation of the crystal.Because molecules can move in both directions[010]and[001]at the same time,DMAT-B exhibits macroscopic plastic torsion when torsional force is applied to both ends of the crystal.Significantly,the crystal still exhibits good elasticity after plastic torsion.In addition,crystal DMAT-G can be transformed into crystal DMAT-B by heating and grinding,indicating that crystal DMAT-B is a stable state of the compound.The optical loss coefficients of the flexible crystal DMAT-B in the straight,elastic bending and plastic torsion states are 0.162,0.164 and 0.181 d B mm^-1,respectively.The amplified spontaneous emission experiments of the crystal DMAT-B in three kinds of conditions preliminary proved its application potential in the field of laser gain medium.And half peak width measured linear state most hours the y value of CIE coordinates of only 0.026,shows that it as deep blue laser medium,is expected to be wearable photoelectric technology breakthroughs in the future.In chapter 4,based on the 2,5-dithiophenylamine-1,4-terephthalate compound,we prepared two kinds of Cry-R and Cry-O with different emission and mechanical properties by crystal engineering strategy.Red crystal Cry-R is brittle in nature and is prone to be cracked under external forces,but by regulating the crystal growth conditions we can occasionally obtain naturally curved crescent red crystal.Orange crystal Cry-O shows good elastic bending ability,and Y-type crystal can be obtained by solvent growth or post-processing.The elastic modulus of Cry-O needle crystal is about 0.74 GPa by three-point bending experiment.Crystal structure analysis shows that the molecules in crystal Cry-R have a planar structure,while the molecules in crystal Cry-O have a relatively distorted conformation.Therefore,the?conjugation degree of Cry-R crystal is larger,and the emission peak position is relatively red shifted.The bent(001)plane in crystal Cry-O is formed by an intermolecular hydrogen bond network,which greatly reduces the risk of crystal cracking when bending.At the same time,in the process of bending,the lateral stacking of molecules can effectively prevent the long-distance slippage of the layered structure,endowing the crystal with good elasticity.In the crystal Cry-R,the(101)planes with hydrogen bonding networks are stacked in a layered manner,without interlocking structures that prevent the molecules from sliding over long distances,so the crystal is not elastic.The optical loss coefficients of Cry-R crystal in straight and naturally bent are 0.196 and 0.275 d B mm^-1 respectively.The optical loss coefficients of Cry-O crystal in straight and elastic bent are 0.220 and 0.224 d B mm^-1 respectively.Finally,the dual-output optical waveguide properties of organic crystals are demonstrated based on naturally grown Y-type crystals.Elastic orange light-emitting Cry-O crystals exhibit low threshold amplification and spontaneous emission characteristics in both straight and elastic bending states,demonstrating their multifunctional applications in flexible optical devices and improving the application value of organic flexible light-emitting crystals.In chapter 5,we synthesized red emitting crystal 1-4 with four different side chain substituents based on dimethyl terephthalate single-benzene(luminescence peak position:598-617 nm,fluorescence quantum efficiency:0.21-0.32)through a simple one-step reaction,and realized three different mechanical behaviors:plastic,elastic and brittle in these crystals,respectively.When external stress is applied to the(001)or(010)plane of crystal 1,permanent deformation occurs.That is the crystal exhibits two-dimensional plasticity.Crystal 2 and crystal 3 can undergo elastic deformation under the action of external force,showing elastic behavior.While blocky crystal 4 shows brittle behavior under the action of external force.The elastic moduli of crystal 2 and crystal 3 are 1.6 and 1.1 GPa by three-point bending experiments.Crystal structure analysis shows that the molecules in crystal 1 are assembled in a stacked manner,with no horizontal or longitudinal arrangement of molecules.Therefore,when the(010)or(001)planes of crystal are subjected to external forces,crystal 1 exhibits macroscopic two-dimensional plastic deformation due to the destruction and reorganization of weak intermolecular interactions.Crystals 2 and 3 are ideal stacking structures for elastic crystals:the molecules first form a columnar structure and then form a wavy structure by intermolecular hydrogen bonding.However,the molecules in crystal 4 have fewer intermolecular hydrogen bonds,and their morphology and molecular arrangement are not conducive to elastic or plastic deformation.Functional tests show that crystals 1-3not only show low loss waveguide characteristics in straight state,but also have good light conduction ability in curved states.At the same time,crystals 1-4 showed low threshold ASE properties in both straight and bent states,which provided a research basis for the multifunctional application of side chain engineering in flexible organic light-emitting crystals.To sum up,based on the amino-substituted dimethyl isophthalate or dimethyl terephthalate structure,we designed and synthesized a series of organic flexible light-emitting crystal materials with excellent mechanical compliance and high luminescence performance by adjusting the relative position of amino and ester groups on the benzene ring or modifying different side chain groups on the amino group through crystal engineering strategy.Through the analysis of single crystal and the testing of mechanical and optical properties,the relationship between the mechanical and photophysical properties of these compounds and the crystal arrangement and stacking structure was revealed.Finally,the optical waveguide,amplified spontaneous emission and polarization properties of these flexible organic crystals are tested,proving their potential applications in optical transmission,organic laser media and polarization rotators.