Paper Luminescence and resonance energy transfer in the colloids of cyanine dyes and saponite modified with cationic surfactants

The luminescence and resonance energy transfer (RET) between two cyanine dyes, 3,3'-diethyl oxacyanine and 3,3'-diethyl thiacyanine, in organically-modified saponite colloids were investigated. It is known that the formation of molecular assemblies usually reduces the luminescence and photoactivity of organic dyes. Therefore, one of the objectives of this work was optimizing the properties of the colloids by using organic surfactants to prevent dye molecular aggregation. The organically-modified saponite colloids were prepared by an ion-exchange with n-alkyl(trimethyl)ammonium surfactants with alkyl chains having an even number of carbon atoms (from C6H13 to C18H37). The modification with the surfactants was adjusted to keep colloidal dispersions stable. Dye cations were added to the organically-modified saponite dispersions, and luminescence properties and the efficiency of RET were studied directly in these colloids using fluorescence spectroscopy. The properties of luminescent colloids were interpreted in terms of the structure of an organic phase present on the surface of saponite particles. The presence of the organophilic phase affecting dye aggregation in most cases led to the enhancement of luminescent properties and RET yields. The effect of the type of surfactant used for the modification on RET yields was observed. The formation of luminescent J-aggregates likely played a significant role.