Study On Piezochromic Behavior Of Several Typical Organic Luminescent Crystals

Organic piezochromic materials,as a new type of intelligent functional material,can change its luminescent performance under external force stimulation such as grinding or hydrostatic pressure,which have advantages and broad application prospects in the fields of pressure-sensitive devices,information storage and display,and pressure-induced sensing et al.The essence of the piezochromic effect is that high-pressure induces the changes of molecular aggregated structure or molecular conformation,which cause the disturbance of electronic energy levels,the phase changes,the appearance of lattice defects or the isomerization of molecular structure,and finally make effects on the band shape and position of electron absorption and the emission spectra of luminescent materials.In recent years,luminescent materials with charge transfer(CT)characteristics have become a type of hot materials in the field of organic solid-state materials,which are extremely sensitive to the slight changes of surrounding microenvironment.In this thesis,we designed and selected several simple and typical organic luminescent crystals for investigations through high-pressure(hydrostatic pressure)spectroscopy combined with a diamond anvil cell(DAC)device.The intra-/ inter-molecular structural deformation is monitored to regulate and control the properties of excited states,which deeply understands how high-pressure influences molecular CT properties and the relationship among the molecular conformation,the aggregated state structure and the photo-physical properties in order to reveal the piezochromic mechanism.It also realizes the continuous multi-color regulation of the luminescent materials,and provides effective guidance for the design of novel luminescent functional materials.The specific research results were shown as following:(1)4-(anthracen-9-yl)-N,N-diphenylaniline(TPA-AN)is a typical molecule with hybridized localized excited and charge-transfer(HLCT).We explored the differences in spectral responsive behaviors and related piezochromic mechanisms of TPA-AN crystal under different force stimulation(mechanical grinding or hydrostatic pressure).Upon ground,the long-range ordered structure of TPA-AN crystal was destroyed with a transformation from crystalline phase to amorphous phase casuing that the fluorescent spectrum showed a weak blue-shift and enhanced intensity.The ground sample was allowed to stand for a period of time,and the spectrum was restored to the original state,which indicates that TPA-AN has good recrystallized and self-recovered properties.Under hydrostatic pressure,the TPA-AN crystal exhibited a clear red-shift,and a new fluorescent band appeared at the longer wavelength,which showed a larger red-shift with pressure increasing,that is,it is more sensitive to pressure.Therefore,we believe that the new emission band belongs to intramolecular CT,which is derived from the separation of HLCT state caused by pressure-induced molecular structural changes.Meanwhile,on the basis of the piezochromic behavior of TPA-AN powder and the results of high-pressure Raman,we speculated that intermolecular interactions are of significance in the piezochromic mechanism of TPA-AN crystal.Compared with the piezochromic materials reported in previous literatures,the pressure-induced red-shift of TPA-AN crystal was up to 124 nm,which achieved a wide range of spectral regulation at a relatively low pressure(pressure range: 0-3.92 GPa).(2)We designed and synthesized two compounds with D-A-D structure,TPA-Ph-CN and TPA-Py-CN,and studied on the differences in CT characteristics and pressure-responsive behaviors between them.As the polarity of the solvent increased,their absorption spectra were unchanged and the emission bands showed significant red-shift,indicating that the polarization of molecular excited state is larger than ground state,and there exist twisted intramolecular charge transfer(TICT)characteristic.Under the same conditions,TPA-Py-CN exhibited more obvious red-shift due to the stronger electron-withdrawing ability of cyano-substituted pyridine,suggesting that it has stronger intramolecular CT characteristics.Their aggregated structures are different in crystalline state.TPA-Py-CN crystal owned a dimer structure so that it showed three emission peaks(located at 449,477 and 538 nm),and the band at 538 nm belongs to the excimer.In addition,upon grinding,TPA-Ph-CN only showed a slight blue-shift,while TPA-Py-CN displayed obvious size-dependent emission characteristic,and the emission bands became a single fluorescent peak when fully ground.Under hydrostatic pressure,TPA-Ph-CN only showed a monotonous red-shift and broadened FWHM without the obvious changes in fluorescent photos.As a comparison,the luminescence of TPA-Py-CN was changed from green to yellow,and the fluorescent intensity at 538 nm was relatively enhanced mainly due to the effective enhancement of pressure-induced molecular interactions.High-pressure Raman spectra and theoretical analysis provided further evidences.(3)4-methyl-2-(5-[4-dimethylaminophenyl]-1H-pyrazole-3-yl)phenol(4MPP)is a typical molecule with excited-state intramolecular proton transfer(ESIPT)feature.We obtained two crystalline phases GC(green emission)and BVC(blue-violet emission),and studied the polycrystal-dependent stimuli-responsive behaviors under pressure or temperature,which achieved the conversion between alcoholic emission(E*)and ketone luminescence(K*)in crystal.GC has a single near-plane molecular conformation,which is mainly assigned to K*.However,BVC contained two conformations(twisted and near-planar molecular conformations),which is dominated by E*.The comparison indicates that the luminescent properties are related to molecular conformation and aggregated structure.In a certain range of hydrostatic pressure(0-1.01 GPa),the fluorescent intensity of GC decreased obviously with a slight blue-shift.In contrast,BVC can stand for higher pressure with two-stage changes during the compression process.At first stage(0-4.79 GPa),the intensity of the fluorescent band at 430 nm decreased,while fluorescent band at 616 nm gradually enhanced,mainly due to the pressure-induced molecular planarization and the enhancement of intramolecular hydrogen bonds promoting the occurrence of ESIPT process.At second stage(4.79-11.19 GPa),the molecules accumulated excessively with the unchanged conversion between two bands under high pressure and thereby the energy loss of the nonradiative transition increased,which significantly reduced the fluorescent intensity of BVC emission band.During the total pressurization process,the emission peak at 616 nm of BVC showed continuous blue-shift trend,which is mainly related to the degree of molecular aggregation.The high-pressure Raman and theoretical simulation results further confirmed that high pressure can effectively induce shortened distance of H···N,lengthened O-H bond and enhanced intramolecular hydrogen bonding to form a more stable six-membered ring structure,thereby achieving the transformation from E* to K*.Additionally,as the temperature decreased,GC achieved a conversion from K* to E*.Meanwhile,as the temperature rised to 190 ?,BVC suffered an irreversible phase transition.Raman spectra,PXRD and single crystal diffraction results adequately proved that high-temperature induced irreversible phase transformation from BVC to GC.(4)A series of co-crystals based on pyridine derivatives were designed and synthesized by halogen bonds as the driving force,and we carried out high-pressure spectroscopy experiments combined with diamond anvil cell(DAC)to study their high-pressure behaviors.Firstly,BIPY was used as the host molecule with different co-formers(IPFB,DITFB and IFB),and we obtained three co-crystals.The analysis and comparison of three single crystal structures demonstrate that with the increased electron-withdrawing ability of the guest molecule(the number of F atoms),the distance of I···N gradually decreased,and the interaction of C-I···N was enhanced.Among them,BIPY-DITFB exhibited obvious pressure-induced emission enhancement(PIEE).The high-pressure Raman results showed that the C-I stretching vibration peak was significantly changed with pressure increasing.The theoretical results also showed that the distance of I···N gradually decreased and the twist angle of C-I···N increased firstly and then decreased during the compression process,which caused the changes of C-I···N intermolecular interaction.These results above proved that the enhancement of the PIEE of BIPY-DITFB originates from the pressure-induced C-I···N to form a new intermolecular CT state.In other words,the intermolecular CT state can be regulated by high pressure.Then,based on the design strategy and piezochromic mechanism above,TMDP was used as the host molecule with different co-formers(DITFB,IFB and PFBA)to form three other co-crystals.TMDP-DITFB also showed obvious PIEE behavior under high pressure.It should be noted that TMDP-DITFB,TMDP-IFB and TMDP-PFBA exhibited high-pressure irreversible piezochromic behavior and can maintain stable red emission when the pressure was released.A kind of stable high-pressure irreversible co-crystals was first obtained.