Excitation of a triad artificial photosynthetic reaction center consisting of a porphyrin (P) covalentlylinked to a fullerene electron acceptor (C
60) and a carotenoid secondary donor (C) leads to the formation ofa long-lived C
+-P-C
60- charge-separated state via photoinduced electron transfer. This reaction occurs ina frozen organic glass down to at least 8 K. At 77 K, charge recombination of C
+-P-C
60- occurs on themicrosecond time scale, and yields solely the carotenoid triplet state. In the presence of a small (20 mT)static magnetic field, the lifetime of the charge-separated state is increased by 50%. This is ascribed to theeffect of the magnetic field on interconversion of the singlet and triplet biradicals. At zero field, the initiallyformed singlet biradical state is in equilibrium with the three triplet biradical sublevels, and all four stateshave comparable populations. Decay to the carotenoid triplet only occurs from the three triplet sublevels. Inthe presence of the field, the
S and
T0 states are still rapidly interconverting, but the
T+ and
T- states areisolated from the other two due to the electronic Zeeman interaction, and are not significantly populated.Under these conditions, recombination to the triplet occurs only from
T0, and the lifetime of the charge-separated state increases. This effect can be used as the basis for a magnetically controlled optical oroptoelectronic switch (AND gate).
