Charge-transfer excited state dynamics in DNA hairpins substituted with an ethylenylpyrenyl-dU electron source and halo-dU traps.

In this work nine DNA hairpins (HPs) are studied at room temperature to observe their pyrene(*+)/dU(*-) CT excited-state dynamics following photoexcitation at 355 nm with a 25 ps laser pulse. The HPs are 18-24 bases long, have a central tetra-T loop, and have a single U(PE) (5-(2-pyren-1-yl-ethylenyl)-2'-deoxyuridine) substitution in the central region of their stems. Three of the HPs are also substituted with 5-XdU traps, where X = Br or F, to learn about the effects of these traps on CT excited-state lifetimes and emission quantum yields in U(PE) substituted HPs. The combination of lengthened average CT lifetime and enhanced CT emission quantum yield in HPs with excess electron traps compared to HPs lacking traps strongly suggests that excess electrons are injected into the DNA stem at pyrimidine sites external to U(PE) as well via charge separation within U(PE) itself. Furthermore, the increased CT emission quantum yield in HPs with traps compared to HPs without traps implies that externally injected electrons can migrate to uracil in U(PE) (i.e., Py(*+)dU) and thus indirectly form the emissive Py(*+)dU(*-) CT state of U(PE).