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

Journal Article · Mon Jan 28 23:00:00 EST 2013 · Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry

DOI:https://doi.org/10.1021/jp311067q· OSTI ID:1064463

Conron, Sarah M. Mickley; Shoer, Leah E; Smeigh, Amanda L; Ricks, Annie Butler; Wasielewski, Michael R ^[1]

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1. NWU

Two X-shaped, cruciform electron donor₂–acceptor–acceptor'₂ (D₂-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^•–.

• Research Organization: Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, IL (US)
• Sponsoring Organization: DOE - BASIC ENERGY SCIENCES
• OSTI ID: 1064463
• Journal Information: Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry, Journal Name: Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry Journal Issue: 7 Vol. 117; ISSN 1520-6106
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: ENGLISH