Excited State Intramolecular Proton Transfer in Ethynyl‐Extended Regioisomers of 2‐(2′‐Hydroxyphenyl)benzothiazole: Effects of the Position and Electronic Nature of Substituent Groups

Abstract

Although the organic dyes based on excited state intramolecular proton transfer (ESIPT) mechanism have attracted significant attention, the structure-property relationship of ESIPT dyes needs to be further exploited. In this paper, three series of ethynyl-extended regioisomers of 2-(2'-hydroxyphenyl)benzothiazole (HBT), at the 3'-, 4'- and 6-positions, respectively, have been synthesized. 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. The results derived from the theoretical calculations were in conformity with the experimental observations. Our study could potentially provide experimental and theoretical basis for designing novel ESIPT dyes that possess unique fluorescent properties.

Highly Miscible Hybrid Liquid-Crystal Systems Containing Fluorescent Excited-State Intramolecular Proton Transfer Molecules

“…It has been proposed that the twisting of the central carbon–carbon bond, which links the phenyl and benzothiazole rings, consumes the excited-state energy. − We aimed to suppress the twisting by increasing the double-bond character in the C–C bond in the photoexcited state; this is the reason for our previous findings that C 6 Ph-HBT and Ph-HBT (Scheme ) showed a larger fluorescence quantum yield (Φ FL ) than pristine HBT in solution as well as in nematic LCs . Similar molecular structures were reported recently, where one or two alkyne groups were introduced into HBT or HBO: (2-(2-hydroxyphenyl)­benzoxazole). − The effect of alkyne groups was not clear despite that these compounds recorded large Φ FL values in toluene (0.38 for the HBO and 0.08 for the HBT above).…”

Ethynyl‐Tolyl Extended 2‐(2′‐Hydroxyphenyl)benzoxazole Dyes: Solution and Solid‐state Excited‐State Intramolecular Proton Transfer (ESIPT) Emitters

“…We wish to underline that these purely organic solution‐state emissive ESIPT emitters can be obtained in only two or three synthetic steps from commercially available starting materials, allowing a facile synthesis on a multigram scale in the laboratory. Moreover, it is worth mentioning that these results highlight the interest of using oxygen‐based rather than sulfur‐based heterocycles, the latter more efficiently leading to intersystem crossing and, consequently smaller Φ F . Triple bond extended conjugation on the phenol ring, along with the presence of a benzoxazole scaffold seem to be of paramount importance in order to reach improved luminescence intensity in solution.…”

Catalytic‐Type Excited‐State N−H Proton‐Transfer Reaction in 7‐Aminoquinoline and Its Derivatives

“…In this study, for generalization, we lift the boundary of the unimolecular‐type ESIPT and extend the research to systems in which the proton donor (‐NHR) and acceptor are greatly separated, precluding the formation of an intramolecular H‐bond. Therefore, the occurrence of excited‐state proton transfer (ESPT), if available, requires the assistance of surrounding solvent molecules .…”