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 π-stacked donors and acceptors. PDI secondary electron acceptors were chosen because of their propensity to form self-ordered, π-stacked assemblies in solution, while the ZnTPnP and ZnTPP donors were selected to test the effect of peripheral substituent steric interactions on the π-stacking characteristics of the cruciforms. Small- and wide-angle X-ray scattering measurements in toluene solution reveal that 1 assembles into a π-stacked structure having an average of 5 ± 1 molecules, when [1] =/~ 10(-5) M, while 2 remains monomeric. Photoexcitation of the π-stacked structure of 1 results in formation of ZnTPnP(•+)-PI-PDI(•-) in τ(CS1) = 0.3 ps, which is nearly 100-fold faster than the formation of ZnTPnP(•+)-PI(•-) 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 π-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(•+)-PI(•-)-PDI → ZnTPP(•+)-PI-PDI(•-), where τ(CS1) = 33 ps and τ(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(•+)-PI-PDI(•-) 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 τ(CR) = 24 ns for ZnTPP(•+)-PI-PDI(•-).
Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
Sarah M Mickley
SM
Conron
Leah E
LE
Shoer
Amanda L
AL
Smeigh
Annie Butler
AB
Ricks
Michael R
MR
Wasielewski
D004579:Electron Transport; D004583:Electrons; D007094:Imides / Q000737:chemistry*; D008665:Metalloporphyrins / Q000737:chemistry*; D010569:Perylene / Q000031:analogs & derivatives* / Q000737:chemistry; D011166:Porphyrins / Q000737:chemistry; D053838:Scattering, Small Angle; D014961:X-Ray Diffraction
C509964:5,10,15,20-tetraphenylporphyrin; D007094:Imides; D008665:Metalloporphyrins; D011166:Porphyrins; C521332:perylenediimide; C017802:zinc hematoporphyrin; D010569:Perylene