Photostability and Halochromic Properties of Near‐Infrared Absorbing Anionic Heptamethine Cyanine Dyes

We synthesized near-infrared-absorbing anionic heptamethine cyanine (HMC) dyes and investigated their photostability and halochromic properties in detail. Upon changing the methyl groups of the anionic HMC dye to phenyl or perfluorophenyl groups, the maximum absorption wavelengths of the dye solutions were red-shifted and dye photostability in solution under white LED irradiation at 25 degrees C was improved. The anionic HMC dye featuring methyl groups presented halochromic properties in solution, and the solution color changed from green to yellow in the presence of acids. This is the first reported anionic HMC dye with halochromic properties. Furthermore, we produced a stimuli-responsive filter paper adsorbed with anionic HMC dyes that changed color from green to yellow upon exposure to HCl gas and from yellow to green upon subsequent exposure to NH3 gas.