Tuning The Emission Of A Water-soluble 3-hydroxyflavone Derivative By Host-guest Complexation

3-Hydroxyflavone derivatives were extensively studied in various organic solvents for the‘‘excited state intramolecular proton transfer’’process(ESIPT).They can provide information about the physicochemical properties of their microenvironment and were frequently used as fluorescent probes for solvent polarity,ions bonding,reverse micelles,host-guest complexes,lipid vesicles,cellular membranes and proteins.The molecular skeleton of 3-hydroxyflavone is so rigid that it was poorly soluble in water.This is a fatal defect for its further applications.Herein,an amphiphilic molecule bearing 3-hydroxyflavone and oligo(ethylene oxide)(denoted as 3HF-EO)was designed and synthesized.Different from the dual fluorescence in organic solvents,3HF-EO in aqueous solution showed a remarkable single fluorescence emission,which is ascribed to the fluorescence of its anionic species.When plotting the fluorescence intensity at 525 nm versus the corresponding concentration,3HF-EO showed an aggregation-caused quenching phenomenon.In order to tune the emission of 3HF-EO,we adopted the supramolecular encapsulation based on cyclodextrins.From the experimental results,α-CD has little effect on the emission,whileβ-CD andγ-CD lead to enhanced and reduced emissions of 3HF-EO,respectively.We observed the aggregation morphologies of 3HF-EO with or without different cyclodextrins(α-CD,β-CD,γ-CD)by atomic force microscope(AFM)and laser scanning confocal microscope(LSCM).The neat 3HF-EO could self-assemble into nanoparticles within 10 nm.The nanoparticles remained unchanged upon addition ofα-CD.The sheet structures with thickness of 1.3 nm and square structures with the thickness of 2.5 nm were observed upon addition ofβ-CD andγ-CD,respectively.These differences in morphologies suggest the differential interactions between 3HF-EO with three cyclodextrins.Moreover,we explored the possible binding mechanism by ~1H NMR.The chemical shifts of the H-3 and H-5 protons moved up-field,which indicates thatβ-CD and 3HF-EO formed a host–guest complex.Meanwhile,the up-field shift of the protons on the benzene ring B and the H-h,H-i,H-j and H-k protons in the vicinity are more obvious than that of the rest,suggesting thatβ-CD should mainly bond to the benzene ring B of 3HF-EO.2D NOESY spectra provide the direct proof to above conclusion.Job’s Plot indicate that the association ratio between 3HF-EO andβ-CD is 1:1,and between 3HF-EO andγ-CD is 2:1.β-CD can effectively prevent self-aggregation and alter the microenvironment of the guest molecules,and hence enhance the emission of 3HF-EO.3HF-EO was encapsulated withγ-CD,two molecules of 3HF-EO were confined in one cavity.This effect should be similar to the formation of aggregates of 3HF-EO,and theπ–πstacking interaction should be even stronger.This explains the decrease of the fluorescence intensity upon addition ofγ-CD.In addition,we found the fluorescent emission of 3HF-EO is solvent-responsive.In nonpolar solvent,3HF-EO exhibits due emission due to excited state intramolecular proton transfer(ESIPT).The fluorescent intensity of Normal(N*)form is higher than the tautomer(T*).In polar aprotic solvent,3HF-EO exhibits due emission as well.However,the fluorescent intensity of Normal(N*)form is higher than the tautomer(T*).In protic solvent,3HF-EO showed a single and enhanced emission.