Near-Infrared Squaraine Dyes as Bright Fluorescent Probes: A Structure–Activity Photophysical Investigation in Liposomes

The efficient investigation of biological membranes is a constant demand in chemical biology. Several families of Near Infra-Red (NIR) dyes have been already studied for the selective staining of a plethora of biological structures, macro and supramolecular assemblies. In this context, a small library of symmetrical and unsymmetrical NIR squaraine dyes have been synthetized and investigated as fluorescent probes to stain the lipid bilayer membrane in large unilamellar vesicles (LUVs) as cell membrane model. The hydrophobic-hydrophilic character of the NIR dyes has been modulated by introducing carboxylic moieties on the aromatic core, as well as by varying the length of the alkyl chains used in the quaternarization of the indolenine nitrogen. The photophysical properties and the kinetic of the insertion into different LUVs models have been analysed with respect to the probe structures. The discussed results represent versatile key data to support the design of new NIR sensors for bioimaging.