Manipulation Of Aggregation Structures And Optical Properties Of Organic Crystals

Organic luminescent materials have attracted widespread attention due to their potential application in many fields such as organic solid-state lasers,anti-counterfeiting,and organic light-emitting diodes?OLED?.Generally,organic luminescent materials are used in optical applications in solid-state.It is found that the luminescent performance of organic solid-state materials depends not only on its chemical structure,but also on the aggregation structure and intermolecular interactions.The organic crystals have definite and highly ordered molecular packing structure,which provide a model for exploring the"structure-property"relationship.The purpose of this thesis is to design and cultivate organic luminescent monomolecular polymorphs,organic cocrystal and cocrystal polycrystalline materials,and further study their"structure-property"relationship to guide the design of more optical functional materials.In this thesis,a series of divinyl derivatives with highly efficient solid-state luminescence are taken as the study object,and by changing the crystallization conditions and introducing coformer,the aggregation structures can be effectively controlled to obtain a series of new light-emitting crystal materials.The intrinsic relationship between the fluorescent properties and the aggregation structures of these materials are explored,and new functions of light-emitting crystal materials are expanded.The main contents are as follows:1.A fluorophenyl substituted divinylbenzene derivative,named TFVB,is designed and synthesized,and four kinds of polymorphs?C1-C4?are prepared through slow solvent-evaporation method.These four polymorphs have similar emission spectra with maximum emission peaks ranging from 461 nm to 477 nm.However,they show distinct photoluminescence quantum yields?PLQY?.The PLQY of C2 is the highest,up to 86.1%,and the PLQY of C4 is the lowest,40.1%.The molecular conformations of TFVB in the four polymorphs are highly distorted.C1 and C4 adopt cross dipole stacking structure.There are several weakp-pinteractions in C1 and C4,and the long molecular axis of TFVB in the C4 is arranged in herringbone packing structure,which reduces the radiation transition rate?k_r?and leads to low PLQY.In addition,C2 and C3 adopt a quasi-isolated stacking method without face-to-facep-pstack,and have abundant intermolecular interaction forces,so that they exhibit not only a high k_r,but also a low non-radiation transition rate(k_nr),making their fluorescence efficiency significantly higher than those of C1 and C4.The results show that the quasi-isolated stacking method has obvious advantages in obtaining efficient luminescent materials.2.Two kinds of hydrogen bonding cocrystals,CNDSB-HQ?CHQ?and CNDSB-TFHQ?CFHQ?,are designed and prepared.It is found that the cocrystallization effectively controlls the material's amplified spontaneous emission?ASE?properties.Compared with single-molecule CNDSB crystals,the two cocrystal exhibit significant blue-shifted,narrow,and fine-structured fluorescence spectra,as well as higher k_r.Comparative analyses of the crystal structure reveal that the CNDSB molecules in CHQ and CFHQ adopt J-aggregation and herringbone packing modes,respectively,both of which effectively weaken the intrinsicp-pinteractions in the cocrystal lattice.Furthermore,in the cocrystals,along the direction of long crystal axis,the CNDSB molecules adopt uniaxially oriented stacking mode,which is conducive to the generation of polarized emission and stimulated emission.Further research finds that both cocrystals exhibit ASE properties,while CNDSB crystals do not show ASE properties.Therefore,we believe that cocrystallization is an effective strategy to obtain ASE properties.3.A donor-acceptor cocrystal,CT-R,is designed and prepared.The research finds that under anisotropic grinding force and isotropic hydrostatic pressure,CT-R exhibits two distinct luminescent responses:upon grinding,the luminescence color changes from red to orange,and the fluorescence intensity is significantly enhanced;on the contrary,under the hydrostatic pressure,its luminescent color changes from red to near-infrared red.In addition,the fluorescence peak position is red-shifted to744 nm,and the fluorescence intensity is obvious weakened.By further controlling the crystallization conditions and changing the ratio of the two components,orange-emitting cocrystal CT-O is obtained.By comparative analysis of powder X-ray diffraction?XRD?,Raman spectroscopy,and crystal structure,it is found that under grinding,molecules in the CT-R crystals rearrange from segregated-stack to mixed-stack.Simultaneously,thep-pand charge transfer?CT?interactions are weakened,resulting in enhanced and blue-shifted emission.while,under hydrostatic pressure,the crystals adopt tighter mixed-stack,leading to enhancedp-pand CT interaction,which further results in the weakened and red-shifted emission.The study of piezochromic properties of organic donor-acceptor cocrystals not only enriches piezochromic materials,but also provides a better experimental basis for a deeper understanding of the effects of external stimuli on the characteristics of organic light-emitting materials.4.Two types of cocrystal polymorphs?I and II?of the BP4VA-FIB system are obtained by the solution method.The crystal-to-crystal phase transition from I to II occurs under multiple external stimuli.Crystal I and II show yellow and green fluorescence,respectively.Crystal I is transformed into green-emitting solids?I_f/I_h?by the tetrahydrofuran?THF?fumigation or heating.Comparative experimental analyses reveal that crystal I is transformed into crystal II.According to the crystal structure,it is found that the similar molecular arrangement sequence and distinct crystalline driving forces in the two crystals lay the structural foundation for the phase transition from crystal I to II.In addition,upon grinding,I_f/I_h and crystal II show yellow fluorescence.Due to their similar fluorescent properties with crystal I,we speculate that crystal II can be reversibly transformed into crystal I.In addition,hydrostatic pressure and acid-base stimulation also induce fluorescence changes in the two cocrystals.We present a rare example of reversible luminescent switching of cocrystal based on crystal-to-crystal phase transition,and provide an alternative strategy to develop multi-stimuli responsive materials.