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

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) x 10(-16) cm(2) in phosphate-buffered saline. This state, denoted as T-2, 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 T-2 to S-1 followed by S-1 --> S-0 fluorescence. On the basis of this analysis T-2 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). (C) 1999 Elsevier Science B.V. All rights reserved.