Synthesis And Optical Properties Of Several Azobenzene Derivatives

Under one or more external stimuli(e.g.light,electric field,magnetic field,temperature,etc.),stimuli-responsive compounds will change their specific physicochemical properties(such as conductivity,color,fluorescence and binding ability with some metal ions,etc.).Among these stimuli,light has many advantages as a non-invasive stimulus.In recent years,photoswitching molecules have attracted much attention due to their unique photoisomerization properties.Azobenzene and its derivatives are a class of stimuli-responsive molecules with excellent optical properties,which have very broad application prospects in many fields(such as biomedical memory sensor environmental management,etc.).In this paper,five azobenzene derivatives,including BNBC-t8,AOBC-t8,Azo-OH,Azo-I and Azo-SH were synthesized to characterize their photoresponse properties under visible and UV light irradiation,and to explore their aggregation-induced enhanceme of fluorescence emission(AIEE)performance under UV light irradiation.Moreover,the mechanism of fluorescence enhancement has been systematically explored.The details are as follows:1.We synthesized two azobenzene derivatives,BNBC-t8 and AOBC-t8,characterized their molecular structures by 1H NMR spectra and elemental analysis,and studied their gel properties in different solvents and photophysical properties in methanol.Both molecules have a high conversion rate of trans-cis isomers with the irradiation of 365 nm UV light,which can reach more than 90%.In addition,the methanol solutions of BNBC-t8 and AOBC-t8 are almost nonfluorescent before UV irradiation,while the quantum yield of BNBC-t8 and AOBC-t8 can reach 1.9 × 10-2 and 11.9 × 10-2 respectively after UV light irradiation,which are an enhancement of about 126 and 1227 times compared with nonirradiated solution.We speculate that the formation of the J-type aggregates of cis-isomers leads to further enhancement of the emission through ?-? interactions.2.Azo-OH,Azo-I and Azo-SH were synthesized and characterized their structures by 1H NMR,13 C NMR,DEPT NMR Spectra.Then,we explored the difference in photoisomerization rate and cis-trans isomer conversion efficiency of Azo-SH in different solvents using UV-Vis absorption spectroscopy.We find that the recovery efficiency of the molecule in toluene is the highest,close to 90%,and the isomerization rate in ethanol is the fastest.The steady-state fluorescence spectra show that the tetrahydrofuran solution of Azo-I has better fluorescence performance and its reversible photoisomerization process has good fatigue resistance.The ultrafast kinetic processes of Azo-OH,Azo-I and Azo-SH were studied by using femtosecond transient absorption technique.It is found that the reason why Azo-I shows strong fluorescence performance is due to the increase of long-life decay component on S1 state,which corresponds to trans-cis photoisomerization on the S1 potential energy surface through an inversion mechanism.