Design and synthesis of hexaphenylbenzene based artificial photosynthetic antenna-reaction centers

Natural photosynthetic systems are composed of a variety of antenna chromophores which absorb light and transfer this excitation energy to a reaction center where charge separation occurs. Artificial photosynthesis utilizes the basic principles behind such remarkable machinery. In natural systems, chromophores are well arranged in a protein matrix for efficient energy and electron transfer processes. This dissertation describes the synthesis, characterization and photophysical processes of a number of artificial photosynthetic antenna-reaction center complexes organized on a hexaphenylbenzene framework. The hexaphenylbenzene scaffold is ideally suited for arranging various chromophores due to its rigidity, and to allow electronic interactions between the attached chromophores that lead to rapid energy and electron transfer. A synthetic molecular hexad is described that features four coumarin 343 antennas linked to a hexaphenylbenzene core that also bears two porphyrins. Excitation of antennas is followed by energy transfer from the antennas to the porphyrin on the picosecond time scale with a quantum yield of essentially unity. Self-assembly has been utilized here for linking reaction-center function to antenna function. A dipyridyl fullerene acceptor self-assembles to both porphyrins of the zinc hexad via dative bonds to form a heptad. Singlet-singlet energy transfer from coumarin 343 antennas to the zinc porphyrins is followed by electron transfer from the porphyrin excited states to the fullerene acceptor to generate a long-lived charge-separated state with a quantum yield of unity.;In another system, the hexaphenylbenzene core bearing two zinc porphyrins serves as an excellent tether for regioselective bis-addition on fullerene to give a trans-2 macrocyclic triad. Excitation of its porphyrins is followed by a photoinduced electron transfer to the fullerene moiety with a time constant of 1.1 ps to generate a charge-separated state with a lifetime of 2.7 ns. Also described here is a donor-acceptor molecular dyad that features a porphyrin and a naphthalene diimide organized on a hexaphenylbenzene framework. These studies demonstrate the versatile nature of the hexaphenylbenzene core for organization of arrays of chromophores which successfully mimic the processes occurring in natural photosynthetic systems.