2-(2-Hydroxyphenyl) Benzothiazole Derivatives:Effects Of Substituents On Intramolecular Proton Transfer In Excited States

Excited intramolecular proton transfer(ESIPT)is one of the basic reactions studied by many researchers in photochemical and photobiological processes.Compounds based on this mechanism have large Stokes shifts,and well-designed fluorescence sensors,fluorescent probes,etc.Although the organic dyes based on excited state intramolecular proton transfer mechanism have attracted significant attention,the structure-property relationship of ESIPT dyes needs to be further exploited.Based on the ESIPT mechanism,three series of ethynyl-extended regioisomers of 2-(2-hydroxyphenyl)benzothiazole(HBT),at the 3'-,4'-and 6-positions,respectively,have been synthesized.And we study the effects of position of substituents and electronic on its luminescence.Changes in the absorption and emission spectra were correlated with the position and electronic nature of the substituent groups.Although 4'-and 6-substituted HBT derivatives exhibited absorption bands at longer wavelengths,the keto-emission of 3'-substituted HBT derivatives was found at a substantially longer wavelength.The gradual red-shifted fluorescence emission was found for 3'-substituted HBT derivatives where the electron-donating nature of substituent group increased,which was opposite to what was observed for 4'-and 6-substituted HBT derivatives.Our study could potentially provide experimental and theoretical basis for designing novel ESIPT dyes that possess unique fluorescent properties.Secondly a new HBT derivative(HBT-HPP)has also been synthesized.Two ESIPT-active fluorophores,2-(2-hydroxyphenyl)pyridine(HPP)and HBT,were fused into a novel dye(HBT-HPP)fluorescent only in the protonated state.HBT-HPP bearing an electron-withdrawing pyridin-2-yl moiety and an electron-donating phenolic hydroxyl at 4'-and 5'-positions,respectively,with the simultaneous formation of another intramolecular H-bond between the planar conformation of HBT-HPP.Study the fluorescence changes of this compound in neutral and acidic media,suggesting that the fluorescence turn-on in acidic medium was caused by the acid-induced shift of the ESIPT-responsible intramolecular hydrogen bond from HPP moiety to HBT moiety.This work presents a first-time systematic comparison of the emission efficiencies and basicity of HBT and HPP,utilizing their differences to construct an acid-responsive smart organic fluorescent material.As a practical application,red fluorescent letters can be written using acid as ink in polymer film.