Intramolecular Electron Transfer through the 20-Position of a Chlorophyll a Derivative:An Unexpectedly Efficient Conduit for Charge Transport

摘要

Suzuki cross-coupling reactions have afforded 20-phenyl-substituted Chlorophyll a derivatives (ZCPh) in good yields and significant quantities from readily available Chi a.A series of donor-acceptor dyads was synthesized in which naphthalene-1,8:4,5-bis(dicarboximide) or either of two perylene-3,4:9,-10-bis(dicarboximide) electron acceptors is attached to the para position of the 20-phenyl group.Comparisons with the analogous dyads based on a zinc 5,10,15-tri(n-pentyl)-20-phenylporphyrin donor show that,for a given acceptor and solvent,the rates of photoinduced charge separation and recombination as well as the calculated electronic coupling matrix elements,V,for these reactions differ by less than a factor of 2.However,EPR and ENDOR spectroscopy corroborated by DFT calculations show that the highest occupied MO of ZCPh~(+centre dot) has little spin (charge) density at the 20-carbon atom,whereas Z3PnPh~(+centre dot) has significant spin (charge) density there,implying that V,and therefore the electron-transfer rates,should differ significantly for these two macrocyclic donors.DFT calculations on ZCPh~(+centre dot) and Z3PnPh~(+ centre dot),with two -0.5 charges located where the nearest carbonyl oxygen atoms of the acceptor would reside in the donor-acceptor dyads,show that the presence of the negative charges significantly shifts the charge density of both ZCPh~(+ centre dot) and Z3PnPh~(+ centre dot) from the macrocycle onto the phenyl rings.Thus,the presence of adjacent covalently linked radical anions at a fixed location relative to each of these radical cations results in nearly identical electronic coupling matrix elements for electron transfer and therefore very similar rates.