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

[Display omitted] •Progressively adjusting the probe’s structural complexity generates an environment-sensitive squaraine-based probe, with superior sensing capabilities as compared to the corresponding semisquaraine, squarate and squaramide.•Squaraine-based probe is a polarity probe based on its absorption characteristics, and a viscosity probe based on fluorescence characteristics.•Amplitude-weighted lifetime (amp) is a photophysical parameter that should be used for establishing viscosity-based correlations for this probe. 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.