Thermo- and photo-stable symmetrical benzo[cd]indolenyl-substituted heptamethine cyanine dye carrying a tetrakis(pentafluorophenyl)borate that absorbs only near-infrared light over 1000 nm

Abstract

Anion exchange of the symmetrical benzo[cd]indolenyl-substituted heptamethine cyanine dyes from a small hydrophilic perchlorate (ClO₄⁻) anion to a large hydrophobic tetrakis(pentafluorophenyl)borate ((C₆F₅)₄B⁻) anion resulted in the reduction of the hydrophilic molecular interaction. Consequently, the prepared benzo[cd]indolenyl-substituted heptamethine cyanine dyes carrying the (C₆F₅)₄B⁻ anion can be purified by column chromatography, even with the use of a less polar solvent, such as dichloromethane, because the R_f values of the symmetrical benzo[cd]indolenyl-substituted heptamethine cyanine dyes carrying the (C₆F₅)₄B⁻ anion are much greater than that of the dye carrying the ClO₄⁻ anion. Significantly, the symmetrical benzo[cd]indolenyl-substituted heptamethine cyanine dye 5 carrying a cyclopentene ring shows no absorption in the visible light region in CH₂Cl₂. Among them, the prepared benzo[cd]indolenyl-substituted heptamethine cyanine dye 5c carrying the (C₆F₅)₄B⁻ anion and 4-methylphenoxy group at the meso-position is most thermostable. Furthermore, the prepared benzo[cd]indolenyl-substituted heptamethine cyanine dye 5c carrying the (C₆F₅)₄B⁻ anion and 4-methylphenoxy group at the meso-position is also photostable, as is the dye carrying the ClO₄⁻ anion in dichloromethane under irradiation by a white LED at 25 °C, with the exception of the dye carrying a 4-tolylthio substituent at the meso-position.