Electronic And Steric Effect On The Fluorescent Properties Of 2-(2-Hydroxyphenyl)Benzothiazole Derivatives

Due to the dual wavelength ratio detection,optical nonlinear and large Stokes shift features,the fluorescence sensors based on excited state intramolecular proton transfer(ESIPT)have attracted significant attention in fundamental investigation and have been applied in luminescent materials,fluorescent probes,laser dyes and UV filtering,etc.However,for ESIPT fluorophores,the intramolecular hydrogen bond strength has decisive influence on their photophysical properties,which is susceptible to the external factors.In addition,the general organic fluorescent dyes have the characteristics of aggregation-induced quenching,which limits their applications in many aspects.Therefore,solid fluorescent organic materials with high fluorescence quantum yield(?f)have been widely concerned in recent years.Herein,a series of 2-(2-hydroxyphenyl)benzothiazole(HBT)-based ESIPT dyes with ethyl bond were synthesized by Sonogashira-coupling reaction.Various substitutions with different Hammett substituent constants(?)were anchored in the 5'-position of the phenol ring of HBT.Through absorption and fluorescence spectra,the electronic effects of substituts on the fluorescence properties were extensively studied.Experimental data show that the HBT derivative with larger ? contains the stronger intramolecular hydrogen bond,which is less likely to be damaged by external factors,and that it has relatively stronger ketone emission,blue-shifted fluorescent spectrum with improved fluorescence quantum yield.More importantly,unlike other HBT derivatives and parent HBT displaying anion emission in PBS medium,the two HBT derivatives bearing methyl ester and cyano substitutions,respectively,emit as keto tautomer,which are expected to be applied in biomedical aspects.Additionally,two HBT derivatives using ethenyl bond as linking unit: phenylethylene-modified HBT(HBT-s-Ph)and diphenylethylene-modified HBT(HBT-d-Ph),were synthesized by facile Suzuki cross-coupling reactions.In toluene solutions,the fluorescence quantum yield of HBT-s-Ph is slightly larger than that of HBTd-Ph.But in crystalline forms,both of their ?f has substantial improvement,and the ?f of HBT-d-Ph reaches 78%,which is twice that of HBT-s-Ph.The crystal structure revealed that the striking emission enhancement of HBT-d-Ph was attributable to its distinct molecular packing model and the resultant relatively weak intermolecular interactions induced by the single phenyl group cis to the HBT moiety.Therefore,This strategy could be applied for fabricating high-performance luminescent and sensory materials for optical displays,biomedical imaging,and molecular electronics...