Research On Synthesis And Photophysical Properties Of Imidazole Derivatives With Mechanofluorochromism And Mechanoluminescence

In recent years,organic luminescent materials with mechanofluorochromism（MFC）and mechanoluminescence（ML）activities have received increasing attention as smart materials that respond to external stimuli,they have been gradually applied in many emerging high-tech fields such as pressure detection,biosensors,information record carriers,encrypted ink or writing materials,internal light sources for kinetic therapy or photochemical reactions,solar energy converters and so on.In this thesis,we have studied the MFC and ML properties of crystals of some imidazole derivatives.The main research content isthe following:1、By changing the introduction positions of N-ethylcarbazole and 4-cyanophenyl,two positional isomers benzo[d]imidazole compounds are obtained.The results show that the isomers of N-ethylcarbazole and 4-cyanophenyl at N₁ and C₂ positions have good MFC/ML activity.However,the isomers of N-ethylcarbazole and 4-cyanophenyl respectively at the C₂ and N₁ positions do not have MFC/ML activity.The MFC activity of the former results from the maintenance of the intramolecular ICT effect before and after the stress,while the apparent weakening of the intramolecular ICT effect under force stimuli should be responsible for the MFC inertia of the latter.Although the former crystal has a non-centrosymmetric space group and the latter crystal has a centrosymmetric space group,the crystal of the former N-n-octyl homologue has a centrosymmetric space group and still exhibits strong ML activity.Crystallographic analysis combined with quantitative calculations show that ML activity is not related to space groups but depends on the existence of couples with large net dipole moments in the crystal.2、The 4-diphenylaminophenyl group and 4-cyanophenyl group were introduced into the C₂ and N₁ positions of benzo[d]imidazole respectively.After the solvent was volatilized,two polymorphs were obtained,showing blue and green solid-state fluorescence respectively.Under force stimuli,the crystals with blue and green fluorescence respectively show red-shifted and blue-shifted emission wavelength,and the crystalline structure of the ground samplecan be fully restored by fumingor annealing treatmenton the former but cannot be done so on the latter,which is transformed into that of the former.Based on the crystallographic analysis,we speculate that blue fluorescence comes from the emission of themolecules in local excited state,while green fluorescence comes from the emission of themolecules inexcited state with twisted intramolecular charge transfer.The irreversible MFC of the crystal with green fluorescence may well be attributed to the higher activation energy which restricts the transfomation from amorphous to crystalline phase.Both crystals hold centrosymmetric space group and do not contain polar molecular couples.Crystallographic analysis shows that cleavage planes exist in both crystals.Under force stimuli,the endogenous friction discharge caused by relative movements between cleavage planes would excite molecules on these planes to generate mechanoluminescence.The mechanoluminescence emission wavelength is consistent with the fluorescence emission wavelength in the crystal phase,which provides one of the significant evidences for the speculation of triboelectric excitation.3、In order to further verify that polar molecular couples rather than the crystalline space groups are the essential factor for mechanoluminescence of crystals,we introduced triphenylamine and 4-cyanophenyl respectively at N₁ and C₂ positions to obtain imidazole,benzimidazole and phenanthro[9,10-d]imidazole.The former two crystals show remarkable MFC effect,while the latter one is MFC-inert.We assume that the MFC inertness of the latter is due to the strong intramolecular C-H…π interaction which locks the rotation of triphenyamine group around the imidazole core and makes the conformational planarization hindered.In addition,crystallographic analysis together with calculations once again confirm that the ML activity of the three crystals should be ascribed to the contribution of polar molecular couples.Meanwhile,the high molecular dipole moments in the three imidazolesare also helpful for the crystals to display good ML performance.Hence,large net dipole moments of couples along with highmolecular dipole moments will make intense discharge released to excite molecules on freshly cracked surfaces when molecules inside couples are separated under force stimuli.