Spin-orbital coupling effects on the photophysical properties and photocytotoxicity of merocyanine dyes

A series of merocyanine dyes possessing different heteroatoms in the terminal aromatic and barbiturate units has been synthesized. The nature of the heteroatom does not affect the all-trans alignment of the central polyenic bridge, as demonstrated by H-1 NMR spectroscopy. There are pronounced changes, however, in the photophysical properties of the individual molecules due to structural, electronic and spin-orbital coupling effects induced by the heteroatoms. In particular, it has been possible to vary the quantum yield for formation of the lowest-energy triplet excited state by more than two orders of magnitude. Quantum yields and lifetimes for fluourescence and photoisomerization are also affected by the choice of heteroatom. Despite the major changes in triplet yield that take glace upon insertion of heteroatoms displaying strong spin-orbital coupling effects (e.g., Se) the efficacy for light-induced cytotoxicity towards cancer cells shows but a modest dependence on the structure of the dye. There is a reasonably good correlation between triplet yield and the rate of light-induced cell killing as might be expected for a mechanism based on intermediate generation of singlet molecular oxygen. However, equally good correlations exist between cell-killing performance and both relative lipophilicity and the rate at which the dye penetrates into intact biological cells.