Research On Solution-state Photophysical Property Together With Solid-state Mechanofluorochromism And Mechanoluminescence Of Tetraarylimidazoles

Mechanoluminescence(ML)is a luminescence behavior emitted by mechanical stimuli on crystals such as grinding,rubbing,crushing,cutting or cleaving force.Mechanofluorochromism(MFC)is a kind of reversible and dynamic response to external force stimuli,emitting different wavelengths or strengths.Given their diversified and important applications in many high-tech fields,these two kinds of optical phenomena have attracted considerable attention.In this thesis,we have studied the MFC and ML properties of some organic crystals.The following is the main research content.1.A series of 4-cyanophenyl and phenothiazinyl substituted benzimidazole homologues were prepared,and the side chains on the phenothiazinyl group were regulated.The results show that the MFC performance of the crystal is closely related to the molecular conformation in the crystal phase: the more twisted the conformation is,the shorter the emission wavelength of the crystal is.And after grinding,the smaller the MFC activity of the sample is,the lower the cold crystallization temperature is during thermal annealing.The emission wavelength of the sample after grinding is not related to the side chain’s length and the degree of branching.A longer chain or a higher degree of branching will result in a looser accumulation of molecules,promoting a more distorted conformation of the molecules.When the side chain is phenyl,the extra π-πinteraction makes the crystal have a longer emission wavelength,and the grind sample also has a longer emission wavelength than the alkyl homologue.In this series of crystals,the crystals of molecules with n-hexyl side chains exhibit a strong ML effect.Although the crystals contain asymmetric space groups,the easy-slip characteristics caused by the accumulation of molecular chains make the crystals very brittle.Under the force of stimulation,many fracture surfaces will be formed,which greatly enhances the discharge intensity on the new crystal surface.2.Molecules IMTPA-1,IMTPA-2 and IMTPA-3 with 1,2 and 3 imidazole groups on the core of triphenylamine(TPA)were synthesized,and the derivatization density of molecular groups and its photophysical properties in solution and solid state is explored.It is found that different group derivatization densities have profound effects on the solvent discoloration behavior,emission wavelength and intensity,and the activities of MFC and ML.As the derivatization density increases,the solubility of the molecules decreases and the solvent discoloration performance decreases.Compared with the high emission intensity of IMTA-1 and IMTPA-2,IMTPA-3 crystal has fluorescence quenching.All three crystals exhibit ML activity,but the difficulty of luminescence becomes easier as the derivatization density increases.With the X-ray crystallographic analysis and theoretical calculation,we put forward our own new theory on the crystal ml phenomenon from the perspective of molecular stacking: the existence of high net dipole moment molecular pairs rather than space group properties is probably the real reason for the crystal ML activity.With the increase of derivatization density,the dipole moment of molecules gradually increases,leading to the formation of high net dipole moment of molecular pairs.3.Three imidazole compounds IMDMA,IMVDMA,IMVVDMA with different numbers of alkenyl groups in the electron-donating group were prepared.It is found that the increase in the number of alkenyl groups will cause the appearance of fine structures on the absorption and emission spectra in the solution and increase the ICT effect.In solid state,the strong intramolecular C-H in IMDMA crystal π-π-dimer is formed by the local planarization of molecules due to π interaction.The decomposition of the dimer after stress damage results in the short wavelength single molecule emission of the sample after grinding,thus emitting the blue shift MFC behavior.Both IMVDMA and IMVVDMA samples showed red-shifted MFC activity after grinding,and the red-shift amplitude of the latter(containing a larger number of alkenyl groups)was significantly smaller than the former.Since the crystal structures of the two compounds have not been obtained,the reason for the difference in their MFC activity is only speculated that the conformation of the latter in the crystal may be more planar.All three crystals have ML activity,and with the increase in the number of alkenyl groups,the ML emission wavelength has a significant red shift,and we have successfully obtained blue,green and yellow wavelengths of ML emission.This method of molecular structure regulation can easily realize the pleochroism of ML luminescence,which enriches the methodology for constructing ML systems.