2-((E)-2-((E)-4-Chloro-5-(2-((E)-5-methoxy-3,3-dimethyl-1-(3-phenylpropyl)indolin-2-ylidene) ethylidene)-1,1-dimethyl-1,2,5,6-tetrahydropyridin-1-ium-3-yl)vinyl)-5-methoxy-3,3-dimethyl-1-(3-phenylpropyl)-3H-indol-1-ium

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Abstract
A heptamethine fluorophore, ERB-60, has been synthesized efficiently in four steps in a good yield. The structure of this fluorophore consists of an electron-donating group (methoxy), a hydrophobic moiety (phenylpropyl) with a rotatable bond, a quaternary ammonium fragment, and indolium rings at the terminal ends connected via polymethine chain. All these inherent chemical features fine-tuned the optical properties of the fluorophore. This compound was characterized by both 1H NMR, 13C NMR and mass spectra. The optical properties, including molar absorptivity, fluorescence, Stokes’s shift, and quantum yield, were measured in different solvents such as DMSO, DMF, MeCN, i-PrOH, MeOH, and H2O. The wavelengths of maximum absorbance of ERB-60 were found to be in the range of 745–770 nm based on the solvents used. In decreasing order, the maximum wavelength of absorbance of ERB-60 in the tested solvents was DMSO > DMF > i-PrOH > MeOH > MeCN > H2O while the decreasing order of the extinction coefficient was found to be MeCN > MeOH > DMSO > i-PrOH > H2O > DMF. ERB-60 was found to be more photostable than IR-786 iodide, a commercially available dye, and brighter than the FDA-approved dye, indocyanine green (ICG).

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