Extending Photoinduced Charge Separation Lifetimes by Using Supramolecular Design: Guanine-Perylenediimide G-Quadruplex

We report here a potassium-induced guanine quadruplex as a supramolecular platform for controlled assembly of electron donor-acceptor systems. A monodisperse, C-4-symmetric octamer of a guanine-perylene-3,4,9,10-bis(dicarboximide) conjugate (GPDI) was prepared in tetrahydrofuran. The two layers of cyclic guanine tetramers have the same direction of rotation, and the PDI moiety between the layers adopts a nearly eclipsed relationship (H-aggregation), as revealed by small- and wide-angle X-ray scattering, NMR spectroscopy, and steady-state UV/vis absorption. Following photoexcitation of the PDI moiety in the quadruplex, charge separation occurs in tau(CS) = 98 +/- 12 ps to give G(+center dot)-PDI-center dot that recombines in tau(CR) = 1.2 +/- 0.2 ns, which is >100 times longer than that in the monomeric GPDI dyad. The transient absorption spectrum of G(+center dot)-PDI-center dot within the GPDI quadruplex suggests the formation of a radical anion delocalized over the neighboring PDI units, and this result is consistent with the more favorable electrochemical reduction potential for PDIs in the quadruplex relative to the monomer.