Photoprocesses of eosine and rose bengal ion pairs with cationic surfactant in non-polar solvent: application in photosensitization studies

Abstract

The anionic dyes (D) eosine (Eo) and rose bengal (RB) form hydrophobic ion pairs (DS2) with cationic surfactants (S) which are soluble in non-polar solvents (Bilski et al., J. Phys. Chem., 95 (1991) 5784). The solubility of DS2 in non-polar solvents increases with increasing size of the surfactant cation. We have studied the photoproperties of ion pairs formed between RB or EO and the dimethyldioctadecylammonium cation (RBS2 and EoS2) in isooctane, CCI4, toluene and Ch2CI2. No significant concentration-dependent aggregation occurred in the dye solutions at room temperature. However, some features of initially formed “loose” aggregates developed in isooctane, CCl4 and toluene, which in CH2CI2 all DS2 spectra were characteristic of the monomeric dyes. DS2 showed a red shift in absorption/fluorescence and low-temperature phosphorescence which is characteristic of xanthene chromophores placed in an aprotic enviroment. The fluorescence intensity was also high in Ch2CI2 owing to its aproticity and the lack of aggregation. By contrast, aggregation lowered the fluorescence of the dyes in isooctane. Transient absorption measurements in degassed toluene indicated of a triplet state (λmax=630 nm for RB and 600 nm for Eo) which was self-quenched giving rise to transient absorbances at 380 and 460 nm, probably due to the formation of the dye radical anion and cation respectively. We found no evidence for photoredox reactions between the ion pair constituents, S+and D2-. In aerated solutions, strong 1O2 phosphorescence at 1268 nm was detected; we have compared the efficiency of 1O2 phosphorescence emission by the DS2 ion pair with that of tetraphenylporphine zinc complex. The quantum yield of photodepletion of 1,3- diphenylisobenzofuran (DPBF) sensitized by RBS2 (7 μM) in toluene was 0.89. The quantum yield of oxygen consumption during 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 choice in some non-polar solvents.