Lipophilic derivative of rhodamine 19: characterization and spectroscopic properties Original Research Article

A new octadecylamide derivative of rhodamine 19 was synthesized by reaction of rhodamine 19 ethyl ester (rhodamine 6 G) with octadecylamine. This lipophilic dye exists in two forms: as a cationic species containing a secondary amide group and as an electrically neutral molecule containing an additional ring closed between the oxygen of the amide group and the central carbon of the xanthene part. Both forms were isolated and characterized in detail by means of elemental analysis, mass spectrometry, IR, NMR and UV–VIS spectroscopy. Depending on the hydrogen ion activity in solution and the availability of solvent hydrogen for hydrogen bond formation, the cationic form easily converted into a neutral one and vice versa. On the basis of absorption and emission UV–VIS spectroscopic data, the equilibrium of the two forms was studied in various solvents. An increase in dye concentration in acidified methanol/water mixture (volume fraction of water, ϕ=80%) led to aggregation of the cationic molecules. Using the UV–VIS absorption data, a corresponding dimerization constant was evaluated.