title: Electron transfer within self-assembling cyclic tetramers using chlorophyll-based donor-acceptor building blocks.
abstract: 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 (Chl) 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-NDI(2). 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(+•)-PI(-•)-NDI and Chl(+•)-PI(-•)-NDI(2). This initial charge separation is followed by a rapid charge shift from PI(-•) to NDI and subsequent charge recombination of Chl(+•)-PI-NDI(-•) and Chl(+•)-PI-(NDI)NDI(-•) on a 5-30 ns time scale. Charge recombination in the Chl-PI-NDI(2) cyclic tetramer (τ(CR) = 30 ± 1 ns in toluene) is slower by a factor of 3 relative to the monomeric building blocks (τ(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.
authors.affiliation: Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Victoria L
authors.initials: VL
authors.lastname: Gunderson
authors.forename: Amanda L
authors.initials: AL
authors.lastname: Smeigh
authors.forename: Chul Hoon
authors.initials: CH
authors.lastname: Kim
authors.forename: Dick T
authors.initials: DT
authors.lastname: Co
authors.forename: Michael R
authors.initials: MR
authors.lastname: Wasielewski
mesh_terms: D002734:Chlorophyll / Q000737:chemistry*; D003500:Cyclization; D004563:Electrochemistry; D004579:Electron Transport; D008958:Models, Molecular; D015394:Molecular Structure; D009281:Naphthalenes / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D009281:Naphthalenes; D009942:Organometallic Compounds; C547163:naphthalene-1,8:4,5-bis(dicarboximide); D002734:Chlorophyll