Ac. Benniston et al., EFFECT OF RESONANCE POLARITY ON THE RATE OF ISOMERIZATION OF MEROCYANINE DYES, Journal of the Chemical Society. Faraday transactions, 94(4), 1998, pp. 519-525
Citations number
30
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
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.