(
)
504
A. Sen, V. KrishnanrChemical Physics Letters 294 1998 499–506
decay pathway for the excited singlet state of the
unprotonated porphyrin unit. By excitation spec-
troscopy the dependence of the intensity of the por-
phyrin emission on the excitation wavelength is de-
tected. The emission was monitored at a wavelength
700 nm where the majority of the fluorescence arises
from the diprotonated tetraphenylporphyrin. The ex-
citation spectra of the free-base porphyrin dimer, its
tron transfer reactions in these dimers which com-
pete with the excitation energy transfer. Brookfield
w
x
et al. 20 has reported an excitation energy transfer
from Zn-porphyrin to the free-base porphyrin for a
series of covalently linked Zn–H2 porphyrin dimers
Ž
.
with different chain length ns2 . . . 6 . In these
systems an appreciable amount of excitation light
Ž
.
30% at lex 550 nm is directly absorbed by the
Ž .
Ž
.
zinc II derivatives, intermolecular mixture of the
free-base porphyrin H2 P unit in the dimer and
hence not available for energy transfer. In the present
study, the excitation wavelengths used are at 505 and
540 nm depending on the nature of the protonated
dimer, H2 F5–Hq4 2 H5 DM or ZnF5–Hq4 2 H5 DM. At
these lex values, the protonated porphyrin has least
absorption and hence all the excitation energy is
efficiently transferred to the emission of protonated
porphyrin unit of the dimer. Photoexcitation of an
intermolecular mixture of monomers in acid solution
at the distinct absorption maxima of the unproto-
nated species results in the emission spectrum of the
excited species. The emission spectrum observed for
the porphyrin dimers is found to be independent of
excitation wavelength. The highly efficient singlet–
singlet energy transfer in the partially protonated
dimer is confirmed by the corrected excitation spec-
Ž .
monomeric porphyrin units and their zinc II deriva-
tives are recorded and representative spectra are
shown in the inset of Fig. 3. It is clear that the
absorption of light by unprotonated fluoroarylpor-
phyrin unit in the dimer contributes significantly to
the emission of the protonated unit. Interestingly, the
ratio of relative fluorescence peak intensities of
equimolar mixture of free-base monomer porphyrins
Ž .
and its zinc II derivatives in acidic solution is found
to be dependent on the excitation wavelength. This is
in contrast to that found for the emission of partially
Ž .
protonated free-base porphyrin dimer and its zinc II
derivatives. These observations is indicative of an
excitation energy transfer from the excited singlet
state of neutral fluoroarylporphyrin to the diproto-
nated tetraphenylporphyrin unit.
Ž
tra of the dimers. The excitation spectrum after
normalisation at lmax of 660 nm of diprotonated
4. Discussion
.
porphyrin coincides with the absorption spectrum of
The covalently linked dimers exhibit marginal
shift in absorption bands accompanied by reduced ´
values relative to the admixture of monomeric con-
stituents. This indicates a weak electronic interaction
between porphyrin rings of the dimer caused by the
steric distortion of the covalent linkage. Electro-
chemical redox data of the dimers are characteristic
with the appearance of one electron ring oxidation
and reductions of the constituent monomer porphyrin
units. The magnitude of redox potentials of dimers
are marginally shifted relative to that observed for
the individual porphyrins indicating weak interaction
the partially protonated dimers. This demonstrates
that the energy transfer is almost quantitative. The
photophysical investigations reveal efficient energy
transfer in the partially protonated dimers and this
essentially proceeds through intramolecular mecha-
nism. The Forster mechanism describes the energy
transfer between the donor and acceptor substituents
through space and hence direct contact between the
donor and acceptor is not required. This mechanism
is applicable to the dimers reported here considering
that the individual donorracceptor porphyrin units
retains their spectroscopic identity owing to weak
electronic coupling between the molecular subunits.
Ž
.
between the two porphyrin rings Table 1 . The
Ž
.
Ž
.
charge transfer energies ECT of the dimers,
The average donor–acceptor distance R0 at which
H2 F5yØ–H2 Hq5 Ø DM, ZnF5yØ–H2 Hq5 Ø DM, ZnF5yØ
–
Ž
.
the rate constant for energy transfer keet equals the
ZnHq5 Ø DM calculated from electrochemical redox
data lie in the range of 1850–2000 MeV as com-
pared to the energies of the first singlet excited state
of porphyrins in the range of 1900–2100 MeV. This
suggests weak exergonicity for photoinduced elec-
rate constant for non-radiative decay of the donor is
w
x
determined using Forster equation 21 .
9 ln 10 k 2F J
Ž
.
D
R60 s
1
Ž .
128p5Nh4
Sen, Avijit
