Determination of the Quantum Yield of Intersystem Crossing of Rose Bengal

The quantum yield of intersystem crossing (Φisc) of a sensitizer is related to the quantum yield of singlet‐oxygen production (Φ(1O2)) by the efficiency of the energy transfer (φet) and is an important parameter in the evaluation of potential applications of sensitized photo‐oxidations. Using two different laser photolysis techniques, the energy‐transfer method and the partial saturation method, Φisc of rose bengal has been determined in MeOH and in aqueous solutions. The results confirm that with Φisc(H2O) = 1.05(± 0.06) and Φisc(MeOH)=0.90(±0.08), the generally assumed relation Φisc · φet = Φ(1O2), with φet = 1, cannot be maintained any longer (Φ(1O2, H2O) = 0.75 and Φ(1O2, MeOH) = 0.76). During these experiments, a second intermediate has been observed which is produced from the triplet state of rose bengal and, stabilized in a anionic micellar solution, has been shown to be the radical cation of the sensitizer. The efficiency of the electron transfer has been evaluated from transient absorption and bleaching recordings, and it seems conclusive to attribute the results to the difference between Φisc and Φ(1O2).