Elucidating the effect of simultaneous disubstitution on the fluorescence of ESIPT dyes through experimental observation and theoretical calculation.

No abstract

The significant progress recently achieved in designing smart acid-responsive materials based on intramolecular charge transfer inspired us to utilize excited-state intramolecular proton transfer (ESIPT) for developing a turn-on acid-responsive fluorescent system with an exceedingly large Stokes shift. Two ESIPT-active fluorophores, 2-(2-hydroxyphenyl)pyridine (HPP) and 2-(2-hydroxyphenyl)benzothiazole (HBT), were fused into a novel dye (HBT-HPP) fluorescent only in the protonated state. Moreover, we also synthesized three structurally relevant control compounds to compare their steady-state fluorescence spectra and optimized geometric structures in neutral and acidic media. The results suggest that the fluorescence turn-on was caused by the acid-induced shift of the ESIPT-responsible intramolecular hydrogen bond from the HPP to HBT moiety. This work presents a systematic comparison of the emission efficiencies and basicity of HBT and HPP for the first time, thereby utilizing their differences to construct an acid-responsive smart organic fluorescent material. As a practical application, red fluorescent letters can be written using the acid as an ink on polymer film.

Excited-state intramolecular charge transfer (ESICT) and excited-state intramolecular proton transfer (ESIPT) are two competitive reactions that occur in the excited states of organic dyes that contain intramolecular hydrogen-bonds and electron acceptors and donors. Determining the mechanisms of these processes is key to understanding their multiple emission features, as the manner in which these processes interact can be modulated by modifying the dye structure. In addition, donor-π-acceptor (D-π-A) molecules often suffer from aggregation-induced quenching. Herein, we report the synthesis of three nitrophenyl-modified 2-(2-hydroxyphenyl)benzothiazole (HBT) derivatives, HBT-s-NO , HBT-d-NO , and HBT-t-NO , which have C-C, C=C, and C≡C bonds between their HBT and nitrophenyl moieties, respectively. Compared with the enol emissions from HBT-s-NO and HBT-t-NO , that from HBT-d-NO exhibits outstanding solvatochromism owing to consecutive ESICT-ESIPT. In addition, X-ray diffraction reveals that despite the highly planar and polar nature of HBT-d-NO , which is strongly H-aggregated, it exhibits highly efficient fluorescence. Hence, this study provides new insight into the design of ESICT/ESIPT-coupled systems and for engendering planar dipolar molecules with excellent emission properties in the solid state.

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.