Fluorescence properties of aza-helicenium derivatives for cell imaging

In this work the optical properties of three N-methyl-aza-[5]-helicenium salts having the same organic moiety but different counterions (I-, NO3 - and COOCF3 -) were investigated in different media. In particular, the absorption and fluorescence spectra of the compounds were strongly affected by the solvent properties. Fluorescence synchronous scans and time resolved fluorescence measurements indicated that in alcoholic solvents a dual emission behaviour occurred, likely due to aggregation phenomena which can be tuned by the counterion nature. The emission properties of the compounds were tested for cell labelling aims using Hude cells. The occurrence of aggregation processes controlled by the media properties resulted in a dual colour labelling in different cell compartments. In particular, the cell interior was visualized as a green fluorescence corresponding to a higher energy emission assigned to the dye molecules in monomeric form, while longer wavelength fluorescence was observed in cytoplasm, likely due to aggregation of the dye in this environment.