Synthesis, characterization and photophysics of new photoactive ESIPT lipophilic dyes. Partition experiments with different composed liposomes

• New lipophilic derivatives were synthesized from an ESIPT fluorescent precursor.
• The dyes present dual fluorescence emission in the violet–yellow regions.
• Photoactive liposomes and ethosomes were prepared from different phospholipids.
• The dyes experience different polarity environments in liposome and ethosome depending on the nature of the alkyl chain.
• Fluorescence titration experiments were performed to obtain the partition coefficient.

New lipophilic dyes were synthesized from a fluorescent precursor and used to produce photoactive liposomes and ethosomes made of different phospholipids. The lipophilic dyes absorbed in the ultraviolet region due to π–π* transitions and had a dual fluorescence emission in the violet–yellow regions, depending on solvent polarity. The long alkyl chain lipophilic dyes had a narrow emission band with a very regular shape, and their photophysical behaviour indicates that these dyes were inserted into the lipid bilayer. The dye with short alkyl chain appeared to be in a polar environment, which indicated this fluorophore experienced a more hydrophobic environment. Fluorescence titration experiments were also performed with this compound and the partition coefficient was determined, which showed higher values than those found in the literature for similar intramolecular proton transfer dyes, likely due to better interaction between this dye and the phospholipids, due to the alkyl chain present in the dyes.

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