Synthesis And Fluorescence Sensing Of Acetophenone Derivatives Substituted Viologen

Viologens(1,1’-disubstituted-4,4’-bipyridinium,V²⁺),acting as strong electron acceptors,can undergo two reversible reduction steps to generate an intensely colored radical cation(V^+·)or a neutral form（V⁰）under appropriate external stimuli,such as light,heat,electric potential,pressure or the electron donor molecules,and have received much attention in recent years.Because of the visible color change during the electron transfer（ET）process,viologens have been widely studied and applied in the fields of photochromic,thermochromism,electrochromic,solvatochromism,piezochromism and redox sensors,as well as host-guest complexes.However,the deep-colored viologen radical cation is very active in the air and faces a problem that viologens can be easily oxidized and faded by oxygen.Therefore,in most reports,anaerobic and non H₂O environments are used to avoid the strong radical quenching effect of H₂O and O₂,which will limit its applications.Some researchers have begun to pay attention to the fluorescence behavior of viologen derivatives to broaden the application of viologen recent years,but the research works have mostly focused on the fluorescence of viologen polymers and inorganic hybrid materials,and seldom pay attention to the fluorescence character of viologen itself.Based on these,this paper designed and synthesized four halogenated acetophenone substituted viologen compounds（VioF₂·2Cl,VioFCl·2Cl,VioCl₂·2Cl and VioCl·2Cl）,and regulated its color and fluorescence properties through different substituents.Acetophenone substituents were introduced to expand the conjugation degree of the viologen molecule,which can stabilize the radical structure and make the color change more lasting.Besides,The keto structure of acetyl group（-CH₂CO-）on the acetophenone substituent is prone to isomerize into enol structure（-CH=COH-）in organic solvents or external stimuli,resulting in a larger conjugated system to stabilize the molecular structure and enhance fluorescence at the same time.In the first part experiment,three fluorine/chloroacetophenone disubstituted viologens（VioF₂·2Cl,VioFCl·2Cl,VioCl₂·2Cl）were designed and synthesized.Their radical sensitivity and fluorescence intensity were studied under different pH values and the application of these three viologens in organic Lewis base fluorescence sensing was explored respectively.The results showed that the fluoroacetophenone substituted viologen was more sensitive to the discoloration of alkali due to the stronger electron attracting ability,and the chloroacetophenone substituted viologen with weaker electron attracting ability had stronger fluorescence.In addition,the photochromic mechanism of VioF₂·2Cl and VioCl₂·2Cl crystals is also analyzed through the perspective of their single crystal structures.In the second part experiment,based on VioCl₂·2Cl,which has the better fluorescence performance in the first part experiment,the character and mechanism of bi-（VioCl₂·2Cl）and mono-（VioCl·Cl）substituted viologen in solvatochromic/fluorescence recognition and temperature sensing were studied respectively.The results show that VioCl₂·2Cl displays sensitive solvatochromic in different solvents and shows a very strong fluorescence after fading.In solid state,it exhibits sensitive photochromic due to the formation of viologen radical after irradiation but whitout fluorescence which may caused by aggregation-caused quenching.Due to the solution fluorescence and solid state photochromic phenomenon of VioCl₂·2Cl,VioCl₂·2Cl@PMMA composite film with both photochromic and fluorescence behaviors was prepared by dispersing VioCl₂·2Cl in chloroform solution of PMMA.Different with VioCl₂·2Cl,VioCl·Cl was not sensitive to solvent and light,but showed water-induced luminescence character in solid state and showed different colors and fluorescence phenomena through the gain and loss of crystal water molecules.Besides,VioCl·Cl displays more sensitive ion-responsive chromism and photoluminescence behaviors,the fluorescence was enhanced/quenched after anion exchange with I^-and N₃^-,which can be used for the detection of I^-and N₃^-.In the third part of the experiment,the host-guest complex（VioCl₂·2Cl@Me-β-CD）waspreparedthroughcombining Methyl-β-cyclodextrin with VioCl₂·2Cl,and mainly investigated its formation mechanism and the discoloration/fluorescence properties of liquid/solid states.Its fluorescence intensity increased 165%than VioCl₂·2Cl in aqueous solution and the fluorescence intensity was stable in pH from 1 to 9.With Me-β-CD encapsulation,VioCl₂·2Cl enters into cells and aggregates in lysosomes.Cell imaging study shows that VioCl₂·2Cl@Me-β-CD had little cytotoxicity and good biocompatibility at low concentration（≦1.8μm）after formation of host-guest complex,which can be used in cell imaging,while under high concentration（>1.8μm）and long time culturing can inhibit the growth of tumor cells to a certain extent.In addition,the fluorescence intensity increased to 445%in solid state was after inclusion,and the sensitivity of photochromic,temperature sensing and ammonia detection was improved.Based on this,a multifunctional visual sensing paper with sensitive color/luminescence properties was prepared.