Thesis (Ph. D.)--University of Rochester. Dept. of Physics and Astronomy, 2000.
We have studied the photophysics and photochemistry of the higher-lying triplet states of rose bengal (tetraiodo-tetrachlorofluorescein) in water.
Triplet-triplet absorption spectroscopy is used to identify a previously undescribed triplet state of rose bengal with an absorption cross section of (1.1 ± 0.1) × 10−16 cm2 at 1064 nm. This state is the least energetic triplet state, excluding T1, to be identified, so it will be denoted as T2.
Two-step excitation techniques for measuring reverse intersystem crossing, an intersystem crossing from a higher-lying triplet state to a singlet state, are described. The method of two-step laser-induced fluorescence is used to determine the quantum yield of reverse intersystem crossing for three higher-lying triplet states of rose bengal. Reverse intersystem crossing yields of 0.0076 ± 0.0002, <0.06, and 0.12 ± 0.02 are found for the triplet states excited by 1064-, 632-, 532-nm light, denoted respectively as T2, T3 and T4. Bond cleavage may also follow excitation of a higher-lying state. Other workers have found evidence for this photochemical process from T3 but not T4. The method of two-step laser-induced bleaching was used to determine whether bond cleavage occurs from T2. No evidence was found for a such a process. It had been hypothesized that reverse intersystem crossing was responsible for the lack of bond cleavage from T4. A theoretical investigation based on kinetic models including reverse intersystem crossing shows that variations in the yield of this process would produce only small changes in the yield of photoproducts formed by bond cleavage.
A two-photon absorption cross section of 0.028 ± 0.001 GM for rose bengal excited by 1064-nm light is determined using a fluorescence technique. The impact of higher-lying states on multiphoton excitation is discussed, and it is shown that the neglect of reverse intersystem crossing can lead to an apparent enhancement of the two-photon absorption cross section in the analysis of fluorescence data.
