Cyanine dyes, such as Merocyanine 540 and Kryptocyanine, have become important photosensatizers for use in photodynamic therapy, especially for the potential eradication of the leukemia virus from blood products. These dyes, which possess unconstrained central double bonds, give low yields of triplet state upon excitation into the excited singlet state manifold. Instead, the predominant deactivation route for the first excited singlet state involves trans-cis isomerization of the central double bond. It is suggested that isomerization may be responsible for the observed photodynamic activity of the dye under in-vivo conditions and this proposal seeks to elaborate upon this hypothesis. Using single-photon-counting and laser flash photolysis techniques, it is proposed to determine rate constants, yields, activation barriers, and structures for a series of isomers produced by irradiation of synthesized merocyanine derivatives in a variety of environments. The importance of redox reactions and singlet oxygen generation will be monitored in related studies. Concurrent experiments will be undertaken to measure the efficiency with which the dye kills leukemia cells upon illumination. Evidence will be sought for any correlation between isomerization and cell-killing capability.
