INFLUENCE OF CATIONIC SURFACTANT ON THE PHOTOPROCESSES OF EOSIN AND ROSE-BENGAL IN AQUEOUS-SOLUTION

The influence of cetylpyridinium chloride (CPC) on the photoprocesses of rose bengal (RB) and eosine has been investigated in aqueous solutions. When the molar ratio of dye to CPC is 1:2, hydrophobic ion pairs, dye(CPC)2, are formed. In water the ion pairs easily aggregate and can be extracted into benzene. Upon further addition of CPC to an aqueous solution, the ion pairs appear to be hosted individually in the hydrophobic interior of positively charged micelles formed at or above the critical micelle concentration of the surfactant. The aprotic interior of the micelles is responsible for a red shift in the absorption and fluorescence spectra of the dyes and longer singlet lifetimes, which greatly enhances fluorescence. The photochemical properties of the different forms of RB and eosine are affected by bimolecular deactivation processes between the dye molecules in the excited state and those in the ground state. When the dye molecules are separated from each other in the micelles, the dye triplet lifetime becomes longer because the contribution to triplet decay from self-quenching is diminished. As a result, copious singlet oxygen production is observed in micelles, while dye photobleaching is considerably reduced. The quantum yields of singlet oxygen formation for the micellar forms of RB and eosine in D2O are 0.75 and 0.24, respectively. The dyes in cationic micelles are also less sensitive to acidic pH and to the presence of a fluorescence quencher in the aqueous phase. On the other hand, the fluorescence of the ion-pair aggregates is decreased and their triplet state is effectively self-quenched, resulting in poor singlet oxygen formation and much faster photobleaching. Singlet oxygen does not appear to play a significant role in the photobleaching processes.