π-Expanded benzothiazole dyes with excited-state intramolecular proton-transfer process: Synthesis, photophysical properties, imaging in cells and zebrafish
Graphical abstract
Introduction
Organic fluorescent dyes have sparked tremendous research interest on account of their promising use in biomedical, materials science and environmental science, such as medical diagnosis, bio-labeling, photodynamic therapy, etc[1]. Many diverse types of fluorescent dyes, such as squaraine[2], coumarin[3], rhodamine[4], BODIPY[5], tetraphenylethene[6], benzothiazole derivatives[7] and other dyes[8], have greatly facilitated the progress of fluorescence in practical applications. Among these dyes, 2-(2-hydroxyphenyl)benzothiazole (HBT) as a classic fluorescent dyes with the excited state intramolecular proton-transfer (ESIPT) property, and has been widely used in the construction of fluorescent chemsensors[9]. However, excitation and emission wavelengths of HBT are too short for application in biological imaging as there are autofluorescence in this region. Thus, to overcome the autofluorescence interference for imaging, it is of great interest in building new dyes or modifying traditional dyes based on HBT.
The fluorescent emission property of organic dyes, mainly depends on effective conjugate (π-expanded system), molecular rigid structure, molecular planarity, substitution heteroatoms [10]. In recent years, many of traditional dyes have been modified to change their fluorescence properties, such as excitation and emission wavelengths, Stokes shift, fluorescence quantum yield, etc[11]. It remains a challenge to develop new fluorescent dyes with large Stokes shift, long excitation and emission wavelengths for fluorescence imaging in the biological systems.
In this work, we designed and synthesized four HBT-fused aryl-imidazole derivatives HBTA, HBTB, HBTC and HBTD through π-expanded system (plane/nonplanar or nitrogen heterocyclic rings), which were consisted of 1H-phenanthro[9,10-d]imidazole, 4,5-diphenyl-1H-imidazole, 1H-imidazo[4,5-f][1], [10]phenanthroline and 3,3′-(1H-imidazole-4,5-diyl)dipyridine moieties and HBT units, respectively. We speculated that four new fluorescent dyes will still retain the ESIPT property (E* and K* emission) because of a proton transfer process in the excited state (Fig. 1).
Section snippets
Synthesis and characterization of HBTA, HBTB, HBTC and HBTD
The synthetic routes of dyes HBTA, HBTB, HBTC and HBTD are shown in Scheme 1. The HBTA, HBTB, HBTC and HBTD were synthesized by a simple “one-pot” reaction of the compound 1 with ammonium acetate and 1,2-diaryl ethylenedione in good yields. The overall chemical structures of dyes were characterized with 1H NMR and 13C NMR and HRMS spectrometry (Figs S1-S12).
Optical properties of HBTA, HBTB, HBTC and HBTD
Firstly, we investigated their UV-vis absorption properties in different solvents (EtOH, CH3CN, DMSO, Toluene). HBTA exhibited quite
Conclusions
In conclusion, we have designed and synthesized four HBT-fused aryl-imidazole derivatives HBTA, HBTB, HBTC and HBTD through microchemical structure changes. The dyes have large absorption extinction coefficients, relatively high quantum yields and large Stokes shifts. HBTA, HBTB, HBTC and HBTD still retained ESIPT property (E* and K* emission). The HBTA showed a single E* emission band in EtOH, CH3CN, DMSO, Toluene. However, HBTB, HBTC and HBTD showed E* or K* emission in EtOH, CH3CN, DMSO. HBTB
Synthetic procedures
The compound 1 was prepared based on the reported methods[15]. 1H NMR (400 MHz, CDCl3) δ 12.72 (s, 1H), 10.02 (s, 1H), 8.06 (d, J = 8.2 Hz, 1H), 7.96 (d, J = 8.1 Hz, 1H), 7.86 (d, J = 8.0 Hz, 1H), 7.64–7.53 (m, 2H), 7.50–7.46 (m, 2H).
Compound 1 (63.1 mg, 0.25 mmol), phenanthrenequinone (63.1 mg, 0.30 mmol) and ammonium acetate (95.3 mg, 1.24 mmol) in 5 mL acetic acid were refluxed. After the reaction was complete (TLC), the reaction mixtures poured into water (20 mL) and the precipitate was
CRediT authorship contribution statement
Lihong Wang: Data curation, Formal analysis, Visualization. Haixia Zheng: Data curation, Formal analysis, Visualization. Kaibo Zheng: Conceptualization, Methodology, Writing–original draft, Project administration, Funding acquisition. Jiaying Yan: Software, Funding acquisition. Nuonuo Zhang: Writing–review & editing, Supervision. Wei Yu: Formal analysis.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the 111 Project (No. D20015) and Foundation of Science and Technology Bureau of Yichang City (A21-3-012)
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Lihong Wang and Haixia Zheng contributed equally to this work