A Near-Infrared Dye That Undergoes Multiple Interconversions through Acid-Base Equilibrium and Reversible Redox Processes

A near-infrared (NIR) polymethine dye (1), consisting of a cyclohepta[1,2-b;4,3-b']dithiophene and two phenol moieties, was synthesized. This dye exhibited pH-responsive changes in its photophysical properties due to a two-step acid-base equilibrium that produced a protonated cation (1H⁺ ) and an anion (1^- ). While 1H⁺ showed an intense fluorescence in the red region of the visible spectrum, 1^- exhibited a strong absorption in the NIR region. The tropylium ion character in 1H⁺ induces high pK_a1 and pK_a2 values for 1. Moreover, a stable radical (1^. ) was prepared, which showed a NIR absorption band with a maximum at circa 1600 nm. The cyclic voltammogram of 1^. revealed a two-step reversible redox process that produced 1^- and the cation 1⁺ , which is different from 1H⁺ . These redox processes accompany drastic electrochromic changes in the vis-NIR region. Overall, 1 is susceptible to multiple interconversions between five forms, due to the multifaceted character of the cycloheptadithiophene skeleton.