Approximately 50 species, including birds, mammals, reptiles, amphibians, fish, crustaceans and insects, are known to use the Earth's magnetic field for orientation and navigation. Birds in particular have been intensively studied, but the biophysical mechanisms that underlie the avian magnetic compass are still poorly understood. One proposal, based on magnetically sensitive free radical reactions, is gaining support despite the fact that no chemical reaction in vitro has been shown to respond to magnetic fields as weak as the Earth's ( approximately 50 muT) or to be sensitive to the direction of such a field. Here we use spectroscopic observation of a carotenoid-porphyrin-fullerene model system to demonstrate that the lifetime of a photochemically formed radical pair is changed by application of < or =50 microT magnetic fields, and to measure the anisotropic chemical response that is essential for its operation as a chemical compass sensor. These experiments establish the feasibility of chemical magnetoreception and give insight into the structural and dynamic design features required for optimal detection of the direction of the Earth's magnetic field.

A molecular triad consisting of a diarylporphyrin (P) covalently linked to a carotenoid polyene (C) and a fullerene (C60) has been prepared and studied using time-resolved spectroscopic methods. In 2-methyltetrahydrofuran solution, the triad undergoes photoinduced electron transfer to yield C−P•+−C60 •-, which evolves into C•+−P−C60 •- with an overall quantum yield of 0.14. This state decays by charge recombination to yield the carotenoid triplet state with a time constant of 170 ns. Even in a glass at 77 K, C•+−P−C60 •- is formed with a quantum yield of ∼0.10 and again decays mainly by charge recombination to give 3C−P−C60. The fullerene triplet, formed through normal intersystem crossing, is also observed at 77 K. The generation in the triad of a long-lived charge separated state by photoinduced electron transfer, the low-temperature electron transfer behavior, and the formation of a triplet state by charge recombination are phenomena previously observed mostly in photosynthetic reaction centers.

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A model photosynthetic antenna consisting of four covalently linked zinc tetraarylporphyrins, (PZP)3−PZC, has been joined to a free base porphyrin-fullerene artificial photosynthetic reaction center, P−C60, to form a (PZP)3−PZC−PC60 hexad. As revealed by time-resolved absorption and emission studies, excitation of any peripheral zinc porphyrin moiety (PZP) in 2-methyltetrahydrofuran solution is followed by singlet−singlet energy transfer to the central zinc porphyrin to give (PZP)3−1PZC−P−C60 with a time constant of ∼50 ps. The excitation is passed on to the free base porphyrin in 240 ps to produce (PZP)3−PZC−1P−C60, which decays by electron transfer to the fullerene with a time constant of 3 ps. The (PZP)3−PZC−P•+−C60 •- charge-separated state thus formed has a lifetime of 1330 ps, and is generated with a quantum yield of 0.70 based on light absorbed by the zinc porphyrin antenna. The complex thus mimics the basic functions of natural photosynthetic antenna systems and reaction center complexes.

A carotenoid (C) porphyrin (P) fullerene (C 60 ) molecular triad (C-P-C 60 ) has been synthesized and found to undergo photoinduced electron transfer from the porphyrin first excited singlet state or to the fullerene first excited singlet state to yield C-P •+ -C 60 •-. Electron transfer from the carotenoid then gives a C •+ -P-C 60 •final charge-separated state. This state is formed with quantum yields up to 0.88 and has a lifetime of up to 1 µs, depending upon the conditions. The various electron transfer rate constants are relatively insensitive to solvent and temperature. The quantum yield of C •+ -P-C 60 •is relatively constant under conditions ranging from fluid solutions at ambient temperatures to a rigid organic glass at 8 K. In most solvents, recombination of C •+ -P-C 60 •yields the carotenoid triplet state, rather than the ground state. The results suggest that the energies of the charge-separated states of fullerene-based systems are only about half as sensitive to changes in solvent dielectric constant as are those for similar molecules with quinone electron acceptors, and that total reorganization energies for electron transfer are also smaller.

Nature employs a TyrZ-His pair as a redox relay that couples proton transfer to the redox process between P680 and the water oxidizing catalyst in photosystem II. Artificial redox relays composed of different benzimidazole–phenol dyads (benzimidazole models His and phenol models Tyr) with substituents designed to simulate the hydrogen bond network surrounding the TyrZ-His pair have been prepared. When the benzimidazole substituents are strong proton acceptors such as primary or tertiary amines, theory predicts that a concerted two proton transfer process associated with the electrochemical oxidation of the phenol will take place. Also, theory predicts a decrease in the redox potential of the phenol by ∼300 mV and a small kinetic isotope effect (KIE). Indeed, electrochemical, spectroelectrochemical, and KIE experimental data are consistent with these predictions. Notably, these results were obtained by using theory to guide the rational design of artificial systems and have implications for managing proton activity to optimize efficiency at energy conversion sites involving water oxidation and reduction.

The photochemistry of a molecular triad consisting of a porphyrin (P) covalently linked to a carotenoid polyene (C) and a fullerene derivative (C 60 ) has been studied at 20 K by time-resolved EPR spectroscopy following laser excitation. Excitation of the porphyrin moiety yields C-1 P-C 60 , which decays by photoinduced electron transfer to yield C-P •+ -C 60 •-. This state rapidly evolves into a final charge-separated state C •+ -P-C 60 •-, whose spin-polarized EPR signal was observed and simulated. There is a weak exchange interaction between the electrons in the radical pair (J ) 1.2 G). The C •+ -P-C 60 •state decays to give the carotenoid triplet in high yield with a time constant of 1.2 µs. The spin polarization of 3 C-P-C 60 is characteristic of a triplet formed by charge recombination of a singlet-derived radical pair. The kinetics of the decay of 3 C-P-C 60 to the ground state were also determined. The photoinduced electron transfer from an excited singlet state at low temperature and the high yield of charge recombination to a spin-polarized triplet mimic similar processes observed in photosynthetic reaction centers.

A tetra-arylporphyrin dye was functionalized with three different anchoring groups used to attach molecules to metal oxide surfaces. The physical, photophysical and electrochemical properties of the derivatized porphyrins were studied, and the dyes were then linked to mesoporous TiO2. The anchoring groups were β-vinyl groups bearing either a carboxylate, a phosphonate or a siloxy moiety. The siloxy linkages were made by treatment of the metal oxide with a silatrane derivative of the porphyrin. The surface binding and lability of the anchored molecules were studied, and dye performance was compared in a dye-sensitized solar cell (DSSC). Transient absorption spectroscopy was used to study charge recombination processes. At comparable surface concentration, the porphyrin showed comparable performance in the DSSC, regardless of the linker. However, the total surface coverage achievable with the carboxylate was about twice that obtainable with the other two linkers, and this led to higher current densities for the carboxylate DSSC. On the other hand, the carboxylate-linked dyes were readily leached from the metal oxide surface under alkaline conditions. The phosphonates were considerably less labile, and the siloxy-linked porphyrins were most resistant to leaching from the surface. The use of silatrane proved to be a practical and convenient way to introduce the siloxy linkages, which can confer greatly increased stability on dye-sensitized electrodes with photoelectrochemical performance comparable to that of the other linkers.