EFFECT OF RESONANCE POLARITY ON THE RATE OF ISOMERIZATION OF MEROCYANINE DYES

The photophysical properties of a moderately polar derivative of Meroc yanine 540 have been recorded in ethanol solution. For such molecules, internal polarity arises because of the occurrence of zwitterionic re sonance forms in which the barbiturate unit acts as an electron accept or. Upon replacing the (usual) terminal sulfur atom with oxygen there is a slight increase in dipole moment for the ground-state molecule wh ile 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 iso merization is increased, in part because there is more potential energ y available to the excited singlet state. Since isomerization competes with intersystem crossing there is a concomitant decrease in the trip let 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 s inglet molecular oxygen. The overall effect is to provide a fluorescen t derivative of Merocyanine 540 that is more stable when assimilated i nto intact leukemia cells.