PHOTOPROCESSES OF EOSIN AND ROSE-BENGAL ION-PAIRS WITH CATIONIC SURFACTANT IN NONPOLAR-SOLVENTS - APPLICATION IN PHOTOSENSITIZATION STUDIES

The anionic dyes (D) eosine (Eo) and rose bengal (RB) form hydrophobic neutral ion pairs (DS2) with cationic surfactants (S) which are solub le in non-polar solvents (Bilski et al. J. Phys. Chem., 95 (1991) 5784 ). The solubility of DS2 in non-polar solvents increases with increasi ng size of the surfactant cation. We have studied the photoproperties of ion pairs formed between RB or Eo and the dimethyldioctadecylammoni um cation (RBS2 and EoS2) in isooctane, CCl4, toluene and CH2Cl2. No s ignificant concentration-dependent aggregation occurred in the dye sol utions at room temperature. However, some features of initially formed ''loose'' aggregates developed in isooctane, CCl4 and toluene, while in CH2Cl2 all DS2 spectra were characteristic of the monomeric dyes. D S2 showed a red shift in absorption/fluorescence and low-temperature p hosphorescence which is characteristic of xanthene chromophores placed in an aprotic environment. The fluorescence intensity was also high i n CH2Cl2 owing to its aproticity and the lack of aggregation. By contr ast, aggregation lowered the fluorescence of the dyes in isooctane. Tr ansient absorption measurements in degassed toluene indicated the form ation of a triplet state (lambda(max)=630 nm for RB and 600 nm for Eo) which was self-quenched giving rise to transient absorbances at 380 a nd 460 nm, probably due to the formation of the dye radical anion and cation respectively. We found no evidence for photoredox reactions bet ween the ion pair constituents, S+ and D2-. In aerated solutions, stro ng O-1(2) phosphorescence at 1268 nm was detected; we have compared th e efficiency of O-1(2) phosphorescence emission by the DS2 ion pair wi th that of tetraphenylporphine zinc complex. The quantum yield of phot odepletion of 1,3-diphenylisobenzofuran (DPBF) sensitized by RBS2 (7 m uM) in toluene was 0.89. The quantum yield of oxygen consumption durin g this reaction was 0.75; Eo was about three times less efficient as a sensitizer. The photostability of hydrophobic EoS2 and RBS2 ion pairs , as well as the photosensitization in non-polar solvents, are similar to those in H2O for both dyes. DS2 may be the photosensitizer of choi ce in some non-polar solvents.