Construction Of Solid-State Photo-Responsive Fluorescent Materials Based On Cyanostilbene Derivatives

Fluorescent solid-state photo-responsive materials exhibit obvious phototrigger changes in luminescence color,and are ideal materials for manufacturing smart soft matter and advanced photonic devices,which combine sensitivity to light stimulation and sensitivity of fluorescence signal output.The performance change of photoresponsive materials stems from the photochemical reaction of photochromic molecules,and the occurrence of the reaction is affected by the dual influence of molecular structure and aggregate structure.In order to explore more diverse photoresponsive structures,achieve more controllable photo-responsive regulation,and create richer fluorescence changes,this thesis takes photo-responsive cyanostilbene molecules as the core to construct the primitives,adjusts the molecular arrangement in the crystal through crystal engineering strategies,and then regulates the solid-state photodimerization reaction,so that the material switches between the “photoactive state”of the photochemical reaction and the “photostable state” that does not react,showing a variety of fluorescence transformation effects.The detailed results of the research are as follows:Firstly,for crystals that cannot undergo photodimerization reaction,we have found that simple mechanical grinding can make them transition from “photostable state” to“photoactive state”.The chlorine-substituted cyanostilbene molecules emit blue fluorescence after crystallization,which have good crystallinity and photostability.The shear force will promote the planarization of molecules and shorten the distances between molecules,so that the crystals become amorphous with green fluorescence after grinding.Amorphous powder is photosensitive and can undergo photodimerization under UV irradiation,and fluorescence will blueshift after the reaction.With the transition from “photostable” crystal to “photoactive” amorphous powder to photodimerization product,the dual adjustments of mechanofluorochromism and photofluorochromism are realized.Secondly,for crystals that can undergo photodimerization reaction,we have found that the stacking orientation and the intensity of intermolecular interaction will affect the photo-response effect.The difluorine-substituted cyanostilbene molecule has the property of polymorphism,and produces three crystals with different shapes under controlled crystallization conditions.Under UV light,the crystals will undergo photodimerization reaction,but the photo-responsive effects are completely different.The block-shape crystals are “photosalient active”,showing movement behaviors such as crystal cracking and jumping under light,while turning on blue fluorescence.Stickshaped crystals are also “photosalient active”,exhibiting deformation of bending toward light and blue fluorescence enhancement with irradiation.The rod-shape crystals,on the other hand,are “photosalient stable” and do not cause macroscopic photoinduced motion,but UV light attenuates the initial blue fluorescence.Further,by using the thermal transformation characteristics between polymorphs,the transition from “photosalient stable” crystal to “photosalient active” crystal can be completed,and the controllable adjustment of photo-responsive behavior can be realized.Thirdly,for crystals that can undergo photodimerization but do not emit fluorescence,we use cocrystal strategy to regulate molecular aggregation,and construct a highcontrast,reversible fluorescence switch between the crystal and the cocrystal.The meta-pyridine-substituted cyanostilbene molecule exhibits fluorescent off state and“photoactive state” prone to photodimerization reactions.The introduction of halogenbonded donors will change the molecular stacking arrangement,so that the molecules are no longer prone to reaction,and the resulting cocrystals exhibit “photostable state”and turn on cyan fluorescence.In particular,the lower sublimation point of the donor makes the cocrystal turn back to non-fluorescent crystal after heating at 130 °C.After adding and grinding with an equal amount of donor,fluorescent cocrystal can be generated again.With the reversible uptake and escape of the donors,the transition between the “photoactive” crystal and the “photostable” cocrystal can be realized,and the construction of in-situ,reversible and fluorescent switches can be achieved.Fourthly,based on a single molecule,we use the above crystal engineering strategies to achieve multi-level and multi-color fluorescence regulation between polymorphs,cocrystals and photodimerization products.The hydroxyl-substituted cyanostilbene molecule has the property of polymorphism,forming two crystals that exhibit sky-blue and yellow fluorescence,respectively.However,the molecular aggregation in the crystal does not meet the critical rules for photodimerization reaction,and both crystals are in a “photostable state”.After the introduction of three hydrogen-bonded acceptors with different electron-donating capabilities,the three cocrystals show non-fluorescent,sandybrown and orange-red emissions,respectively.In particular,the arrangement of double bonds in cocrystals meets the conditions of photodimerization reaction,and the three cocrystals are in "photoactive states".Under blue light irradiation,the fluorescence color of hydrogen-bonded cocrystals will blueshift to varying degrees after the photodimerization reactions.With the transition from “photostable”polymorphs to “photoactive” hydrogen-bonded cocrystals to photodimerization products,multi-level and multi-color fluorescence transitions are realized in one system.Furthermore,by means of fuming,grinding and irradiation,a fluorescent color code system for information storage and compilation is designed.In summary,this paper systematically regulates the photodimerization reaction of cyanostilbene.At the molecular level,the types of photo-responsive molecules are broadened by mechanical grinding.At the supramolecular level,the effect of photoresponse changes is enriched by polymorphism strategy.Based on this,high-contrast fluorescent switches and multi-color fluorescence patterns for information encryption storage were prepared in photo-responsive fluorescent materials by cocrystal and crystal engineering strategies.The relationship of "molecular structure-stacking arrangement-photoresponsiveness-fluorescence properties" established in this way can also provide reference for the design of photo-responsive molecules and the development of fluorescent materials in the future.