PHOTOPHYSICAL PROPERTIES OF 3,3'-DIALKYLTHIACARBOCYANINE DYES IN ORGANIZED MEDIA - UNILAMELLAR LIPOSOMES AND THIN POLYMER-FILMS

All symmetrical dialkylthiacarbocyanine dyes, with the exception of th e diethyl derivatives, are incorporated into liposomes. Absorption and fluorescence data indicate a solubilization site close to the bilayer surface with the alkyl chains penetrating into the lipid bilayer. Inc orporation into organized assemblies affects the photophysical paramet ers of these dyes. Photoisomerization occurring from the first excited state becomes more difficult as the restrictive effect of the solubil ization site increases. As a consequence, competing deactivation proce sses, such as fluorescence and triplet formation, become more efficien t with the result that fluorescence quantum yields, triplet yields and singlet oxygen quantum yields are larger in liposomes than in homogen eous solution. Dihexylthiacarbocyanine iodide has a fluorescence quant um yield of 0.27 and 0.10 (25-degrees-C) in dimyristoylphosphatidylcho line liposomes and ethanol, respectively, and the singlet oxygen yield increases by a factor three to 0.006 on going from ethanol to liposom es. The effect of a highly organized environment is even more pronounc ed in thin polymer films. In these systems, photoisomerization is comp letely inhibited and only triplet formation is observed in the transie nt absorption spectrum.