The excitation wavelength dependence of time-resolved EPR is used to demonstrate the pathway of
intramolecular energy transfer in a covalently linked copper(II)−free base porphyrin dimer. Spin polarized
spectra are presented for selective excitation of both the copper(II) porphyrin donor (at 540 nm) and the free
base porphyrin acceptor (at 640 nm) at 50 and 80 K. In all cases the observed spectra are assigned to the
triplet state of the free base which is coupled weakly to the copper ground state doublet. The polarization
pattern generated by selective excitation of the free base half is indicative of intersystem crossing (ISC),
whereas excitation of the copper(II) half gives an eaa/eea polarization pattern. The latter is rationalized in
terms of energy transfer via the lowest excited trip−quartet state of the copper(II) moiety, followed by selective
depopulation from the spin states with doublet character in the weakly coupled free base triplet-copper doublet
system. This leads to a spectrum which resembles that of the free base triplet state with overpopulation of the
T+1 and T-
1 sublevels. The spin-selective electronic relaxation is supported by the fact that the rise time of
the polarization is consistent with the decay rate of the triplet signal generated via ISC following direct
excitation of the free base. Superimposed on these triplet spectra is a narrow emissive feature at g = 2.02 and
a very broad a/e pattern, both of which decay with the same rate. In addition, a short-lived absorptive feature
at g = 2.00 is observed at temperatures below 50 K. From their g-values and temperature dependence these
features are tentatively assigned to quartet and doublet states in conformations of the complex in which the
coupling between the free base triplet and Cu(II) ground state is strong.
Triplet−triplet intramolecular energy transfer in a covalently
linked copper(II) porphyrin−free base porphyrin
hybrid dimer was examined by time-resolved (TR) ESR measurements of the
lowest excited triplet (T1) state
of the free base porphyrin component after laser pulse excitation at
532 nm. The TRESR spectra of the free
base moiety in the hybrid dimer in toluene glass at 77 K exhibit a spin
polarization pattern different from that
for the free base porphyrin monomer. The observed pattern for the
dimer cannot be explained by any
intersystem crossing (ISC) process, and it is ascribed to
intramolecular energy transfer that takes place from
the copper porphyrin part to the free base counterpart between the
triplet manifolds. On the other hand, the
TRESR spectrum in 2-methyltetrahydrofuran (2-MTHF) glass, where the
energy transfer is prohibited by the
fast deactivation of the excited copper porphyrin, shows the same spin
polarization pattern as that of the free
base porphyrin monomer. Therefore, the copper porphyrin does not
affect spin selectivity of ISC in the free
base counterpart. The analysis of the TRESR spectrum in toluene
suggests that the energy transfer produces
the spin population dominantly into T+1 and
T-
1 high-field spin sublevels of the
T1 state of the free base
porphyrin. In addition to the ESR signals of the T1
state showing the same fine structure as that for the free
base monomer, a moderately intense and narrow emissive band with a
larger decay rate was observed at
around 324 mT in toluene while in 2-MTHF this band is absent. This
emission band may arise from a dimer
having a different conformation in which the two porphyrin halves are
closer to each other. This kind of
conformation gives rise to the strong interaction between the electron
spins in the ground state of the copper(II)
porphyrin and in the T1 state of the free base
porphyrin.
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