Photoinduced Hole Injection into a Self-Assembled π-Extended G-Quadruplex

We have prepared a G-quadruplex (GQ:1) that incorporates an 8-(4'-aminophenylethynyl)guanine (GEAn) electron donor covalently attached to a 4-aminonaphthalene-1,8-imide (ANT) chromophore and a naphthalene-1,8:4,5-bis-(dicarboximide) (NDI) electron acceptor (GEAn-ANI-NDI, 1). In the presence of KPF6 in tetrahydrofuran (THE), 1 self-assembles into a monodisperse, C-4-symmetric GQ:1 with small spatial intraquadruplex overlap between the ANI-NDI units. Photoexcitation of monomeric 1 induces the two-step charge transfer GEAn-1*ANI-NDI -> GEAn(+center dot)-ANI(-center dot)-NDI -> GEAn(+center dot)-ANI-NDI-center dot that occurs in tau(CSI) = 5 ps and tau(CS2) = 330 ps, respectively, while charge recombination in ca. 300 ns. Sharpening of the GEAn(+center dot) transient absorption and a shift of the ethynyl vibrational frequency in 1 were observed, concomitant with the stepwise electron transfer from ANI(-center dot) to NDI. Formation of GQ:1 from 1 in THE increases the secondary charge-shifting rate (tau(CS2) = 110 ps) and results in no change in ethynyl vibrational frequency. Charge recombination in GQ:1 is slowed by enhanced radical-pair intersystem crossing driven by the greater number of hyperfine couplings in the assembly. Moreover, time-resolved EPR spectroscopy shows that the spin spin-exchange interaction (J) between the radicals of GEAn(+center dot)-ANI-NDI-center dot within GQ-1 is smaller than that of 1, suggesting that the spin (charge) density in GEAn(+center dot) is more dispersed in GQ-1. The spectroscopic results are consistent with hole sharing among the guanines within the G-quadruplex that is kinetically competitive with the formation of GEAn(+center dot). This suggests that G-quadruplexes can serve as effective hole conduits in ordered donor-acceptor assemblies.