Red-Shift (2-Hydroxyphenyl)-Benzothiazole Emission by Mimicking the Excited-State Intramolecular Proton Transfer Effect

Abstract

Novel strategies to optimize the photophysical properties of organic fluorophores are of great significance to the design of imaging probes to interrogate biology. While the 2-(2-hydroxyphenyl)-benzothiazole (HBT) fluorophore has attracted considerable attention in the field of fluorescence imaging, its short emission in the blue region and low quantum yield restrict its wide application. Herein, by mimicking the excited-state intramolecular proton transfer (ESIPT) effect, we designed a series of 2-(2-hydroxyphenyl)-benzothiazole (HBT) derivatives by complexing the heteroatoms therein with a boron atom to enhance the chance of the tautomerized keto-like resonance form. This strategy significantly red-shifted the emission wavelengths of HBT, greatly enhanced its quantum yields, and caused little effect on molecular size. Typically, compounds 12B and 13B were observed to emit in the near-infrared region, making them among the smallest organic structures with emission above 650 nm.

Keywords: benzothiazole; excited-state intramolecular proton transfer; fluorescence imaging; fluorophore; quantum yield; tautomerization; wavelength.

FIGURE 1

Design rationale for HBT-BF₂ fluorophores. (A) Enol and keto forms of HBT structures and their emissions. (B) The HBT-BF₂ structure and its resonance to a keto-like structure. (C) Structures of HBT-BF₂ series of fluorophores with various substituents.

FIGURE 2

Emission spectra of 2-10, 2B-10B in CH₃CN. (A) Normalized emission spectra of 2-5, 2B-5B in CH₃CN; (B) Plot of the maximum emission wavelength of 2-5, 2B-5B against the Hammet’s constant of the R substituent. (C) Normalized emission spectra of 7-10, 7B-10B in CH₃CN; (D) Plot of the maximum emission wavelength of 7-10, 7B-10B against the Hammett’s constant of the R substituent.

FIGURE 3

Structures of 11B-13B (A) and their emission spectra in comparison to those before BF₂-complexation in acetonitrile (B).

FIGURE 4

DFT optimized structures and molecular orbital plots (LUMO and HOMO) of selected fluorophores based on the optimized ground-state geometry (S₀). In the ball-and-stick representation, carbon, nitrogen, and oxygen atoms are colored gray, blue, and red, respectively.

FIGURE 5

Confocal fluorescence imaging of HeLa cells cultured with 13 [Ex: 405 nm; Em: 480–600 nm; (A–D)] and 13B [Ex: 405 nm; Em: 550–700 nm; (E–H)] for 30 min. Each compound was administrated at 20 μM. The 2nd row of pictures represented the images taken under fluorescence field, with partially expanded view shown in the 1st row. The 3rd row of pictures represented the images taken under bright field. The 4th row of pictures were merged ones. Scale bar: 25 μM.