Photoinitiated Electron Transfer in Zinc Porphyrin-Perylenediimide Cruciforms and Their Self-Assembled Oligomers

Two X-shaped, cruciform electron donor(2)-acceptor-acceptor'(2) (D-2-A-A'(2)) molecules, 1 and 2, in which D = zinc 5-phenyl-10,15,20-tripentylporphyrin (ZnTPnP) or zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP), respectively, A = pyromellitimide (PI), and A' = perylene-3,4:9,10-bis-(dicarboximide) (PDI), were prepared to study self-assembly motifs that promote photoinitiated charge separation followed by electron and hole transport through it-stacked donors and acceptors. PDI secondary electron acceptors were chosen because of their propensity to form self-ordered, it-stacked assemblies in solution, while the ZnTPnP and ZnTPP donors were selected to test the effect of peripheral substituent steric interactions on the it-stacking characteristics of the cruciforms. Small- and wide-angle X-ray scattering measurements in toluene solution reveal that 1 assembles into air-stacked structure having an average of 5 +/- 1 molecules, when [1] congruent to 10(-5) M, while 2 remains monomeric. Photoexcitation of the it-stacked structure of 1 results in formation of ZnTPnO(center dot+)-PI-PDI center dot- in tau(CS1) = 0.3 ps, which is nearly 100-fold faster than the formation of ZnTPnP(center dot+)-PI center dot- in a model system lacking the PDI acceptor. The data are consistent with a self-assembled structure for 1 in which the majority of the intermolecular interactions have the ZnTPnP donor of one monomer cofacially pi-stacked with the PDI acceptor of a neighboring monomer in a crisscrossed fashion. In contrast, 2 remains monomeric in toluene, so that photoexcitation of ZnTPP results in the charge separation reaction sequence: (1)*ZnTPP-PI-PDI -> ZnTPP center dot+-PI center dot- -> PDI ZnTPP center dot+-PI-PDI center dot-, where tau(CS1) = 33 ps and tau(CS2) 239 ps. The perpendicular orientation of ZnTPnP and ZnTPP relative to PDI in 1 and 2 is designed to decrease the porphyrin PDI distance without greatly decreasing the overall number of bonds linking them. This serves to decrease the Coulomb energy penalty required to produce D center dot+-PI-PDI center dot- relative to the corresponding linear D-PI-PDI array, while retaining the weak electronic coupling necessary to achieve long-lived charge separation, as evidenced by tau(CR) = 24 ns for ZnTPP center dot+-PI-PDI center dot-.