Photophysics of safranine-O and phenosafranine in reverse micelles of BHDC

Abstract

The photophysics of safranine-O (3,7-diamino-2,8-dimethyl-5 phenyl phenazinium chloride, SF) and phenosafranine (3,7-diamino-5-phenyl phenazinium chloride, PSF) was investigated in reverse micelles (RMs) of the cationic surfactant benzyl hexadecyl dimethylammonium chloride (BHDC). The excited singlet state properties were measured by absorption and fluorescence spectroscopy. All the measurements indicate that both dyes are localized in the interface, sensing a medium of lower polarity than water. Stokes? shift increases and fluorescence quantum yield decreases with increasing the water content, but never reach the values or pure water. The triplet state properties of the dyes in RMs were investigated by laser flash photolysis. The maximum of the T?T absorption spectra in RMs confirms that, in spite of their positive charge, the dyes remain at the interface co-micellizing with BHDC. The triplet lifetime is much longer in the RMs than in homogeneous organic solvents. The two dyes present a different dependence of their photophysical properties with the water content. The two methyl groups in the ring of SF produce a stronger preference of the dye for a hydrophobic environment, and consequently a deeper location in the interface closer to the organic phase. A remarkable difference is observed in the triplet quenching by aliphatic amines. While the quenching by hydrophobic tributylamine is much lower in BHDC/benzene RMs than in a homogeneous solvent, the hydro soluble triethanolamine is near two orders of magnitude more effective in the RMs than in homogeneous solution. This is explained by the different local concentration of the amines in the interface.