Recent progress in fundamental studies on multiporphyrin arrays has provided structural parameters for the molecular design of artificial light-harvesting antennae which mimic the wheel-like antenna complexes of photosynthetic purple bacteria. Covalent and noncovalent approaches have been employed for the construction of artificial light-harvesting multiporphyrin arrays. Such arrays are categorized into ring-shaped, windmill-shaped, star-shaped, and dendritic architectures. In particular, dendritic multiporphyrin arrays have been proven to be promising candidates for both providing a large absorption cross-section and enabling the vectorial transfer of energy over a long distance to a designated point. Such molecular and supramolecular systems are also expected to be potent components for molecular electronics and photonic devices.

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Hypericin (Hyp), the parent chromophore of the photoreceptor protein (stentorin) participates in photophobic movements in Stentor coeruleus. The photoreactivity in the lowest singlet excited state (SI) of hypericin has been studied with respect to the solvent effects on absorption and fluorescence spectra and fluorescence decay kinetics. The primary photoreaction of hypericin was probed through (1) hydrogen-bonding effect, (2) deuterium solvent isotope effect, (3) pH dependence and (4) fluorescence quenching by p-benzoquinone. From the pH dependence of the spectra, the following equilibrium reactions are established: ( H y p H y p ) + 20H-+ 2Hyp-+ 2Hz0, in aqueous solution; Hyp + OH-+ Hyp-+ HzO, in water/Triton X solution, with pKa = 11.3 in SO and pKa* = 12.2 in SI. The neutral species, Hyp, emits fluorescence (Amax = 600 nm) strongly, whereas the anionic species, Hyp-, emits fluorescence (Amax = 660-680 nm) very weakly. The dimer (Hyp-Hyp) shows virtually no emission. Fluorescence quenching occurs effectively in the presence of p-benzoquinone, which functions as an electron acceptor. Possible photoreaction pathways of hypericin (SI), Le., deprotonation and electron transfer, are discussed in relation to the primary photoreaction in stentorin.

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