Efficient synthesis and characterization of meso-substituted bis(benzothiazolium) pentamethine cyanine dyes and investigation of the impact of meso-substitution on water solubility and the structural rigidity to improve photophysical properties and fluorescent brightness

To achieve structurally rigid pentamethine cyanine dyes with worthful photophysical properties for biological applications, five series of symmetrical meso-substituted bis(benzothiazolium) pentamethine cyanine dyes were synthesized via an efficient and simple triethylamine-promoted approach involved condensation of various N-substituted benzothiazolium salts with preformed β-substituted vinamidinium salts in high yield and purity without the nontrivial chromatographic separation. The chemical structures of the synthesized dyes were confirmed by HPLC-MS, 1H NMR, 13C NMR, and FTIR techniques. The photophysical properties of the new dyes were investigated in methanol, recognizing that the dyes demonstrate excellent fluorescent brightness. The solutions of the dyes exhibited maximum absorption and fluorescence emission in the range of 600–650 nm with high molar absorption coefficients (around 1-3 × 105 dm3 mol−1 cm−1) and 610–670 nm with remarkable fluorescence quantum yield, respectively. In addition, the photostabilities and photoluminescent stabilities of the synthetic dyes were studied in methanol, revealing their outstanding stability and resistance to photodecomposition. Examining the photochemical behavior (UV light) of the dyes in organic solvents with different polarities verified the main role of various meso substitutions on the rigidity of molecular structures and cis-trans isomerization to improve the photophysical properties. The presence of the pyridinium substituent rings into the polymethine chain makes the dyes rigid and more water-soluble because of their bulky form and positive charge, respectively.