A ring-fluorinated heptamethine cyanine dye: synthesis, photophysical properties, and vapochromic properties in response to ammonia†

Abstract

Heptamethine cyanine dyes (HMCDs) have attracted considerable attention in biological and energy applications owing to their unique near-infrared (NIR) photophysical properties. Therefore, the development of molecules that change both visible and fluorescent colours in a stimulus-responsive manner by exploiting the NIR optical properties of HMCDs has been a subject of increasing interest. Most research results are based on a highly nucleophilic anion addition or reversible intramolecular addition reaction of a weakly nucleophilic neutral nucleophile with the CN bond of the terminal indol-1-ium moiety. Examples of intermolecular addition of weakly neutral nucleophiles and the use of solid or polymer materials are not available. Here, we report the synthesis of a NIR-absorbing ring-perfluorinated HMCD. The HMCD's unique properties in various solvents and rapid and reversible vapochromic response to various amines, including NH₃, based on the noteworthy structural modification induced by fluorine atoms on the aromatic ring are also presented. The ring-fluorinated HMCD adsorbed on neutral filter paper responds quickly to even low-nucleophilicity NH₃ vapour. Repeatability tests on filter paper adsorbed with the ring-fluorinated HMCD and NH₃ and HCl vapours show excellent reproducibility in 13 blue-green and yellow colour transitions. These results are the first examples of intermolecular addition of weakly neutral nucleophiles into HMCDs and stimulus responsiveness not in solutions.