Light-induced electron transfer is investigated in a ferrocene-aluminum(III) porphyrin-fullerene supramolecular triad (FcAlPorC(60)) and the constituent dyads (AlPorC(60) and FcAlPorPh). The fullerene unit (C(60)) is bound axially to the aluminum(III) porphyrin (AlPor) via a benzoate spacer, and ferrocene (Fc) is attached via an amide linkage to one of the four phenyl groups in the meso positions of the porphyrin ring. The absorption spectra and voltammetry data of the complexes suggest that the ground state electronic structures of the Fc, AlPor, and C(60) entities are not significantly perturbed in the dyads and triad. Time-resolved optical and transient electron paramagnetic resonance (EPR) data show that photoexcitation of the AlPorC(60) dyad results in efficient electron transfer from the excited singlet state of the porphyrin to fullerene, producing the charge-separated state AlPor(•+)-C(60)(•-). The fluorescence and transient EPR data also suggest that some energy transfer from the porphyrin to fullerene may occur. The lifetime of the radical pair AlPor(•+)-C(60)(•-) measured by transient absorbance spectroscopy is found to be 39 ns in o-dichlorobenzene at room temperature. At 200 K, transient EPR experiments place a lower limit of 5 μs on the radical pair lifetime. In the triad, the data suggest that excitation of the porphyrin gives rise to the charge-separated state Fc(•+)-AlPor-C(60)(•-) in two electron transfer steps. Photocurrent measurements demonstrate that both dyads and the triad have good photovoltaic performance. However, when Fc is appended to AlPorC(60), the expected improvement of the radical pair lifetime and the photovoltaic characteristics is not observed.

The distance dependence of sequential electron transfer has been studied in six, vertical, linear supramolecular triads, (TTF-Ph(n)-py → AlPor-Ph(m)-C60, n = 0, 1 and m = 1, 2, 3), constructed using tetrathiafulvalene (TTF), aluminum(III) porphyrin (AlPor) and fullerene (C60) entities. The C60 and TTF units are bound to the Al center on opposite faces of the porphyrin; the C60 through a covalent axial bond using a benzoate spacer, and the TTF through a coordination bond via an appended pyridine. Time-resolved optical and EPR spectroscopic methods and computational studies are used to demonstrate that excitation of the porphyrin leads to step-wise, sequential electron transfer (ET) between TTF and C60, and to study the electron transfer rates and exchange coupling between the components of the triads as a function of the bridge lengths. Femtosecond transient absorption studies show that the rates of charge separation, k(CS) are in the range of 10(9)-10(11) s(-1), depending on the length of the bridges. The lifetimes of the charge-separated state TTF˙(+)-C₆₀˙⁻ obtained from transient absorbance experiments and the singlet lifetimes of the radical pairs obtained by time-resolved EPR are in good agreement with each other and range from 60-130 ns in the triads. The time-resolved EPR data also show that population of the triplet sublevels of the charge-separated state in the presence of a magnetic field leads to much longer lifetimes of >1 μs. The data show that a modest stabilization of the charge separation lifetime occurs in the triads. The attenuation factor β = 0.36 Å(-1) obtained from the exchange coupling values between TTF˙(+) and C₆₀˙⁻ is consistent with values reported in the literature for oligophenylene bridged TTF-C60 conjugates. The singlet charge recombination lifetime shows a much weaker dependence on the distance between the donor and acceptor, suggesting that a simple superexchange model is not sufficient to describe the back reaction.

Two self-assembled supramolecular donor-acceptor triads consisting of Al(III) porphyrin (AlPor) with axially bound naphthalenediimide (NDI) as an acceptor and tetrathiafulvalene (TTF) as a secondary donor are reported. In the triads, the NDI and TTF units are attached to Al(III) on opposite faces of the porphyrin, through covalent and coordination bonds, respectively. Fluorescence studies show that the lowest excited singlet state of the porphyrin is quenched through electron transfer to NDI and hole transfer to TTF. In dichloromethane hole transfer to TTF dominates, whereas in benzonitrile (BN) electron transfer to NDI is the main quenching pathway. In the nematic phase of the liquid crystalline solvent 4-(n-pentyl)-4'-cyanobiphenyl (5CB), a spin-polarized transient EPR spectrum that is readily assigned to the weakly coupled radical pair TTF(.+)NDI(.-) is obtained. The initial polarization pattern indicates that the charge separation occurs through the singlet channel and that singlet-triplet mixing occurs in the primary radical pair. At later time the polarization pattern inverts as a result of depopulation of the states with singlet character by recombination to the ground state. The singlet lifetime of TTF(.+)NDI(.-) is estimated to be 200-300 ns, whereas the triplet lifetime in the approximately 350 mT magnetic field of the X-band EPR spectrometer is about 10 μs. In contrast, in dichloromethane and BN the lifetime of the charge separation is <10 ns.

The ability of PPors to photo-oxidize Ir(III)Cp* to Ir(IV)Cp* indicates that they are promising photoanode components.