Preparation And Structure-Property Research Of Cyanostilbene-Based Solid-State Luminescent Materials

With the development of society,the status of material science is increasing.Among them,luminescent materials have become the focus of attention due to their great influences to our lives,ranging from communication,health,and environment.However,most conventional luminescent molecules suffer from luminescence quenching in aggregation（solid）state,which is usually the existing form of material.In 2001,an intriguing phenomenon named aggregation induced emission（AIE）came out.The beakthrough not only solves the problem of emission quenching fundamentally but also put forward a novel design concept of solid-state luminescent materials.At present,AIE materials show a thriving development trend.AIE effect has shifted the attention from the study of molecular properties to the exploration of aggregate properties.However,due to the existence of non-covalent interactions,aggregate properties are not only determined by molecules themselves but also related to their molecular packing.More than emission enhancement,more complex excited-state characteristics should be seriously reconsidered.At present,although some original achievements have been made in the exploration of supramolecular luminescence mechanism,its relationship with molecular structure is not clear.Functional AIE materials still have a broad space for development.Based on the above facts,cyanostilbene molecular model is taken as the core and functional application is served as the guidance in this paper.We design and synthesize novel luminescent molecules.We prepare solid-state luminescent materials.We explore the functional application of materials.The relationship of“molecular structure-aggregated structure-material properties”is committed to the establishment.The main research contents are as follows:In Chapter 2,we modify the cyanostilbene skeleton with rotatable benzamide and synthesize a luminescent molecule PBA.PBA can form polymorphic crystals with blue（1-B）and green emission（1-G）.In 1-G,the molecules connected through continuousπ-πinteractions,leading to a red-shifted emission.However,noπ-πinteractions are found in 1-B,leading to single molecule emission characteristics.Under anisotropic shearing,1-G shows a unique emission blue shift.Experimental investigations show that the shearing causes a crystal-to-crystal transition from 1-G to 1-B.The destruction of molecularπ-stacking accounts for the blue-shifted emission.In sharp contrast,upon isotropic hydrostatic pressure,1-G exhibits an obviously red-shifted emission owing to the formation of a tighter packing structure,where theπ-πinteractions are reinforced.This discovery reflects the universal law ofπ-πinteractions regulating luminescence and enrich the scope of mechanochromic materials.In Chapter 3,based on the molecular structure of the previous chapter,the benzene is replaced by naphthalene.We synthesize another luminescent molecule NPBA.A mechanical grinding assisted photochromic emission is achieved.The pristine powder of NPBA emits blue emission,and has high crystallinity and good photostability.Upon grinding,on the one hand,force makes the molecular conformation planar,resulting in a red-shifted emission（green emission）.On the other hand,force reduces the crystallinity and expands the intermolecular space without any molecular packing change.Such grinding caused lattice damage promotes the molecular motion,which makes the photostable powder change into a photosensitive one.Therefore,upon UV irradiation,the ground powder undergoes[2+2]cycloaddition photochemical reaction,which makes its emission color returns to blue again.The light induced blue-shifted emission can be completed in only 2 seconds.The system is further applied to information security and anti-counterfeiting areas.This work proves the important role of molecular motion in regulating the properties of solid materials.In Chapter 4,a two-component system is adopted.Functional pyridine group is introduced to cyanostilbene skeleton to obtain a luminescent molecule BPPa.Three cocrystals（BPPa-2I,BPPa-3I,and BPPa-TCNB）based on BPPa,2I（diiodo-tetrafluorobenzene）,3I（triiodo-trifluorobenzene）,and TCNB（1,2,4,5-tetracyanobenzene）are fabricated through halogen bonding and charge transfer（CT）interactions.With the enhanced CT degrees in excited state,three cocrystals exhibit red-shifted emissions,covering blue via green to yellow.According to the crystal structure analysis,the effective distance between the two components in molecular packing is responsible for the different CT degrees;that is,a fully face-to-face stacking between donor and acceptor can improve the CT degree more effectively.This finding is helpful to understand the relationship of“packing-CT degree-emission”,and paves an easy way to build multi-color luminescent materials.In Chapter 5,polymorphs and cocrystals are combined.Another pyridine functionalized molecule（CF₃-CN-Py）is selected.CF₃-CN-Py can form polymorphic2D plate crystal and 3D microhelix by changing the molecular growth environment.When a halogen bond donor（diiodo-tetrafluorobenzene）is inrtoduced to CF₃-CN-Py,1D rod cocrystal is fabricated.Among them,2D crystal emits blue emission and 3D microhelix emits green emission,while 1D cocrystal emits sky-blue emission.Crystal structure analysis reveals that 2D morphology is attributed to the orthogonal direction of intermolecular interactions,and 1D morphology is attributed to the continuousπ-stacking of molecules.Both 2D crystal and 1D cocrystal are applied to optical waveguide area.Owing to the difference in the stacking mode of molecular transition dipole moment between them,2D crystal exhibits asymmetric optical waveguide characteristics.While in 1D cocrystal,the emitted light propagates along the primary growth direction.The optical loss coefficient is 1.561 d B/mm.Such materials show great potential in modulating the morphology of optoelectronic devices.