Research On Design,synthesis And Photophysical Properties Of Some Mechanofluorochromic Fluorophores With Novel V-shaped Structures

In recent years,the research on stimuli-responsive solid fluorescent materials has been increasingly attracted.Because they can recognize,record and feedback external stimuli then output their unique optical signals,they can be widely used in information sensing,display,and storage.In various fields,they have the advantages of high sensitivity,strong anti-interference,and good repeatability.Among them,mechanofluorochromic materials have become one of the widely studied types of stimuli-responsive solid fluorescent materials due to their operability and ease of molecular design.In this paper,some research work has been carried out that obtaining the parent compound of 2'-(2-benzo[d]thiazolyl)-N,N-dimethyl-(1,1'-biphenyl)-4-amine of V-shaped molecular structure,and the vinyl and benzene rings are inserted into different parts of the parent molecule to obtain two types of isomerized derivatives.We also study the structure-property relationship in both solution and solid state.The followings are our results:1.A series of vinyl isomerized derivatives has been synthesized for which vinyl groups are inserted separately or simultaneously between the electron-donating or electronwithdrawing groups and the centered phenyl ring.It is shown that solution-state emission wavelengths only rely on the number rather than the location of vinyl.At the same time,the more vinyls a V-shaped compound have,the longer emission wavelength it displays and the stronger ICT effect it has.All of derivatives have significant MFC activities.As the number of vinyl groups is increased,the solid-state emission wavelength of the corresponding compound is red-shifted and the time consumed for its solid-state sample to be restored from amorphous to crystalline phase is shortened.The emission wavelength is also related to the position of the vinyl group.However,emission wavelength shifts before and after force stimuli seem to be dependent on neither the number nor the position of vinyl.By the subtle structure modification,a largely wide and approximately successive range of solid-state emission wavelengths can be achieved on these V-shaped fluorophores in exposure to force stimuli.In addition,polymorphism and nano self-assembling occur on one of these molecules with its multi-color adjustable fluorescence,while the resistance of a polymorph against force stimuli is confirmed by X-ray crystallography to be attributed to a special molecule packing modeinvolved by molecular pairs.Only one-way transformation can be achieved between the two polymorphs from the thermodynamically stable one to the kinetic one by vapour fuming or annealing.The reason for this rarely converse transformation is still unknown.2.A series of phenyl-isomerized derivatives have been synthesized,phenyl group is respectively inserted into the electron-donoring or electron-withdrawing groups in ortho-,meta-and para-pattern to form the position-and region-isomers.The fluorescence of the ortho-and meta-isomers is very weak or even completely quenched in various solvents,while the para-ones have bright fluorescence and more significant ICT effects than that of the parent.In addition,both types of isomers exhibit competitively dual emission,but the parent does not has this phenomenon.We suggested that such difference may be induced by the asynchronous rotation between the inserted phenyl and dimethylaminophenyl group or2-benzo[d]thiazole group when molecules are excited,resulting in competitive emission between the LE state and the ICT state.This series of compounds have the classical two-color fluorescence and some isomers also have rarely emission intensity enhancement under force stimuli.The red shift of emission wavelengths for the derivatives is smaller than of the parent in response to external force,which may also be ascribed to the abovementioned asynchronous rotation.X-ray crystallography confirms that the unexpected small red shift of emission wavelength under force stimuli should be attributed to the packing mode participated by molecular pairs.We assume that both the conformational planarization and the electronic effect caused by positional isomerism may account for the unusually force-induced emission enhancement.The hypothesis still needs to be confirmed by theoretic calculations in due research.