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 (ANI) chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NDI) electron acceptor (GEAn-ANI-NDI, 1). In the presence of KPF₆ in tetrahydrofuran (THF), 1 self-assembles into a monodisperse, C₄-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^+•-ANI^-•-NDI → GEAn^+•-ANI-NDI^-• that occurs in τ_CS1 = 5 ps and τ_CS2 = 330 ps, respectively, while charge recombination in ca. 300 ns. Sharpening of the GEAn^+• transient absorption and a shift of the ethynyl vibrational frequency in 1 were observed, concomitant with the stepwise electron transfer from ANI^-• to NDI. Formation of GQ-1 from 1 in THF increases the secondary charge-shifting rate (τ_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^+•-ANI-NDI^-• within GQ-1 is smaller than that of 1, suggesting that the spin (charge) density in GEAn^+• 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^+•. This suggests that G-quadruplexes can serve as effective hole conduits in ordered donor-acceptor assemblies. (Graph Presented).