8-Aminoquinoline-containing squaric acid congeners as polarity and viscosity probes

Squaraine-type dyes are privileged motifs and scaffolds with valuable and tunable photophysical properties, and have found numerous applications in various areas of chemical and biological sciences. Yet, environment-sensing capabilities of squaraine dyes have not been widely explored. This study illustrates how selective incorporation of a readily available 8-aminoquinoline moiety onto squaric acid or squarate scaffolds fine-tunes environment-sensing capabilities of these dyes. The squaraine motif is required for achieving meaningful correlations between probe’s photophysical properties and media’s properties, whereas semi-squaraine and squaramide analogues exhibit inferior environment-sensing capabilities. The 1,3-bis(8-aminoquinolinyl)squaraine dye shows potential as a multiplexing environment-sensitive probe that could sense media’s polarity variations via absorption changes, and environmental viscosity via amplitude-weighted fluorescence lifetime changes.