Synthesis and spectral properties of 6 `-triazolyl-dihydroxanthene-hemicyanine fused near-infrared dyes

We describe the synthesis of a range of 6 `-triazolyl-dihydroxanthene-hemicyanine (DHX-hemicyanine) fused dyes through an effective copper-catalyzed azide-alkyne cycloaddition (CuAAC) ``click'' reaction, with the aim of providing molecular diversity and evaluating the spectral properties of these near-infrared (NIR)-active materials. This was implemented by reacting 15 different aliphatic and aromatic azides with a terminal alkynyl-based DHX-hemicyanine hybrid scaffold prepared in four steps and 35% overall yield from 4-bromosalicylaldehyde. The resulting triazole derivatives have been fully characterized and their optical properties determined both in organic solvents and under simulated physiological conditions (phosphate buffered saline containing 5% of bovine serum albumin protein). This systematic study is a first important step towards the development of NIR-I fluorogenic ``click-on'' dyes or related photoactive agents for light-based diagnostic and/or therapeutic applications.