SPIN-ORBITAL COUPLING EFFECTS ON THE PHOTOPHYSICAL PROPERTIES AND PHOTOCYTOTOXICITY OF MEROCYANINE DYES

A series of merocyanine dyes possessing different heteroatoms in the t erminal aromatic and barbiturate units has been synthesized. The natur e of the heteroatom does not affect the all-trans alignment of the cen tral polyenic bridge, as demonstrated by H-1 NMR spectroscopy. There a re pronounced changes, however, in the photophysical properties of the individual molecules due to structural, electronic and spin-orbital c oupling 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 y ields and lifetimes for fluourescence and photoisomerization are also affected by the choice of heteroatom. Despite the major changes in tri plet yield that take glace upon insertion of heteroatoms displaying st rong spin-orbital coupling effects (e.g., Se) the efficacy for light-i nduced cytotoxicity towards cancer cells shows but a modest dependence on the structure of the dye. There is a reasonably good correlation b etween triplet yield and the rate of light-induced cell killing as mig ht be expected for a mechanism based on intermediate generation of sin glet molecular oxygen. However, equally good correlations exist betwee n cell-killing performance and both relative lipophilicity and the rat e at which the dye penetrates into intact biological cells.