Polarized-light spectroscopic study of indocarbocyanine dyes solubilized in amphiphile aggregates
The solubilization and the fluorescence properties of indocarbocyanines in ionic and non-ionic micelles and cubic liquid crystals have been studied. The indocarbocyanines are two dialkyl derivates (C2H5— and C14H29—), commonly denoted as DiIC2(3) and DiIC14(3). The steady-state anisotropy and the quantum yield of fluorescence have been measured, and the radiative lifetime of both dyes determined (2.2 ns). In the aggregates of ionic amphiphiles such as sodium octanoate, octylammonium chloride and dodecyltrimethylammonium chloride the rotational correlation time of both dyes was found to increase with increasing concentration of the detergent. In micelles of the non-ionic pentaethyleneglycol mono-n-dodecylether no such dependence on the concentration is found. A reasonable explanation is that the large indocarbocyanine molecules perturb the relatively small ionic aggregates while the large non-ionic ones are much less affected.
From linear-dichroism studies of the dyes in lamellar liquid crystals it has been concluded that both these chromophores are oriented with their long axis preferentially parallel to the plane of a bilayer.
The reciprocal rate of the non-radiative processes is found to increase linearly with increasing rotational correlation time of the fluorophore. Very similar behaviour has been found for ordinary solutions of alcohols, which suggests that the orientational distribution of the dye molecules in the amphiphile aggregates has a negligible effect on the non-radiative processes of the indocarbocyanines.