Journal of the American Chemical Society, Vol.134, No.9, 4363-4372, 2012
Electron Transfer within Self-Assembling Cyclic Tetramers Using Chlorophyll-Based Donor Acceptor Building Blocks
The synthesis and photoinduced charge transfer properties of a series of Chl-based donor acceptor triad building blocks that self-assemble into cyclic tetramers are reported. Chlorophyll a was converted into zinc methyl 3-ethylpyrochlorophyllide a (CM) and then further modified at its 20-position to covalently attach a pyromellitimide (PI) acceptor bearing a pyridine ligand and one or two naphthalene-1,8:4,5-bis-(dicarboximide) (NDI) secondary electron acceptors to give Chl-PI-NDI and Chl-PI-NDI2. The pyridine ligand within each ambident triad enables intermolecular Chl metal-ligand coordination in dry toluene, which results in the formation of cyclic tetramers in solution, as determined using small- and wide-angle X-ray scattering at a synchrotron source. Femtosecond and nanosecond transient absorption spectroscopy of the monomers in toluene-1% pyridine and the cyclic tetramers in toluene shows that the selective photoexcitation of Chl results in intramolecular electron transfer from (1)*Chl to PI to form Chl(+center dot)-PI-center dot- NDI and Chl(+center dot)-PI-center dot-NDI2. This initial charge separation is followed by a rapid charge shift from PI-center dot to NDI and subsequent charge recombination of Chr(+center dot)-PI-NDI-center dot and Chl(+center dot)-PI-(NDI)NDI-center dot on a 5-30 ns time scale. Charge recombination in the Chl-PI-NDI2 cyclic tetramer (tau(CR) = 30 1 ns in toluene) is slower by a factor of 3 relative to the monomeric building blocks (tau(CR) = 10 +/- 1 ns in toluene-1% pyridine). This indicates that the self-assembly of these building blocks into the cyclic tetramers alters their structures in a way that lengthens their charge separation lifetimes, which is an advantageous strategy for artificial photosynthetic systems.