M. Sirish et al., INTRAMOLECULAR EXCITATION-ENERGY TRANSFER IN ISOMERIC PORPHYRIN ANTHRACENE DYADS, Journal of photochemistry and photobiology. A, Chemistry, 93(2-3), 1996, pp. 129-136
Isomeric donor-acceptor (D-A) dyads in which an anthracene donor moiet
y is covalently linked, via a short ether bridge, to either ortho, met
a or para position of one of the aryl groups of 5, 10, 15, 20-tetraphe
nylporphyrin have been synthesized and characterized by spectral and e
lectrochemical methods. UV-visible and H-1 nuclear magnetic resonance
data of these D-A systems suggest the presence of weak intramolecular
pi-pi interaction between the porphyrin and the anthracene. Fluorescen
ce from the anthracene subunit in each dyad is found to be quenched in
comparison with the fluorescence of free anthracene. Excitation spect
ral data provide evidence for an intramolecular excitation energy tran
sfer (EET) from the singlet anthracene to the porphyrin and the energy
transfer efficiency is found to be dependent on the site of attachmen
t (i.e. ortho > meta > para) of the donor to the acceptor. Detailed an
alysis of the data suggests that Forster's dipole-dipole mechanism doe
s not adequately explain this energy transfer and that an electron-exc
hange-mediated mechanism can, in principle, contribute to the intramol
ecular EET in these short ether bridged dyads. Furthermore, arguments
based on the thermodynamic considerations and also the solvent-depende
nt fluorescence data reveal that, while quenching of the fluorescence
in the ortho isomer could be explained solely on the basis of EET, inv
oking an intramolecular electron transfer can rationalize the observed
quenching in the meta and para isomers. Finally, a comparison is made
of the EET reactions in these isomeric dyads with those observed for
the previously reported porphyrin-based energy transfer systems which
include a supramolecular, D-4-A pentad porphyrin bearing four anthrace
ne donor subunits.