Reverse intersystem crossing from a triplet state of rose bengal populated by sequential 532- +1064-nm laser excitation Original Research Article

A previously unstudied triplet state of rose bengal has been identified by near-infrared laser flash photolysis measurements. Its absorption peaks between 1050 and 1075 nm with a cross-section of (1.1±0.1)×10−16 cm2 in phosphate-buffered saline. This state, denoted as T2, was further characterized through two-step laser-induced fluorescence measurements. The photophysical parameters describing the higher-lying triplet populated by the two-step, two-color excitation were determined from fits of a model of the kinetics to the experimental data. This model allows reverse intersystem crossing from T2 to S1 followed by S1→S0 fluorescence. On the basis of this analysis T2 has a quantum yield of reverse intersystem crossing of 0.0142±0.0003, a lifetime of 5.8±1.6 ps, and a thermalization rate constant of 1.30±0.18 ps−1.