Determination of the Quantum Yield of Intersystem Crossing of Rose Bengal

Abstract

Abstract The quantum yield of intersystem crossing (Φ isc ) of a sensitizer is related to the quantum yield of singlet‐oxygen production (Φ( 1 O 2 )) by the efficiency of the energy transfer (φ et ) 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, Φ isc of rose bengal has been determined in MeOH and in aqueous solutions. The results confirm that with Φ isc (H 2 O) = 1.05(± 0.06) and Φ isc (MeOH)=0.90(±0.08), the generally assumed relation Φ isc · φ et = Φ( 1 O 2 ), with φ et = 1, cannot be maintained any longer (Φ( 1 O 2 , H 2 O) = 0.75 and Φ( 1 O 2 , 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 Φ isc and Φ( 1 O 2 ).