Effect of resonance polarity on the rate of isomerization of merocyanine dyes

The photophysical properties of a moderately polar derivative of Merocyanine 540 have been recorded in ethanol solution. For such molecules, internal polarity arises because of the occurrence of zwitterionic resonance forms in which the barbiturate unit acts as an electron acceptor. Upon replacing the (usual) terminal sulfur atom with oxygen there is a slight increase in dipole moment for the ground-state molecule while the first allowed absorption transition is blue shifted by ca. 20 nm. The excited singlet and triplet states are of comparable polarity to the ground state. More significantly, the rate of light-induced isomerization is increased, in part because there is more potential energy available to the excited singlet state. Since isomerization competes with intersystem crossing there is a concomitant decrease in the triplet yield. The triplet can also be produced via sensitization and, by varying the energy of the donor, it has been possible to estimate the triplet energy. A further consequence of exchanging the heteroatom is that the ground-state dye is less susceptible to oxidative attack by singlet molecular oxygen. The overall effect is to provide a fluorescent derivative of Merocyanine 540 that is more stable when assimilated into intact leukemia cells.