Charge separation in acridine- and phenothiazine-modified DNA.

The formation of the long-lived, charge-separated state in DNA upon visible light irradiation is of particular interest in molecular-scale optoelectronics, sensor design, and other areas of nanotechnology. However, the efficient generation of the charge-separated state is hampered by fast charge recombination within a contact ion pair, which limits the application of DNA for photoelectrochemical sensors and devices. In this study, a series of protonated 9-alkylamino-6-chloro-2-methoxyacridine (Acr+)- and phenothiazine (Ptz)-modified DNAs were synthesized for the further understanding of the mechanism of charge separation in DNA to generate a long-lived, charge-separated state with a high quantum yield (Phi). The Acr+ serves as a photosensitizer to produce a hole on guanine (G), and the G-C base pairs were used as a hole-transporting pathway to separate a hole from Acr* (the one-electron-reduced form of Acr+) to be trapped at Ptz. Since Acr+ oxides only G upon photoexcitation, the A-T base pair can be used as a spacer between Acr+ and the G-C base pair to avoid the formation of a contact ion pair. The charge injection dynamics was investigated by steady-state fluorescence spectra and fluorescence lifetime measurements, and the Phi and the lifetime of the charge-separated state produced upon photoirradiation were assessed by nanosecond laser flash photolysis of the Acr+- and Ptz-modified DNA. A long-lived, charge-separated state was successfully formed upon visible-light irradiation, and the Phi was the highest for the DNA having a single intervening A-T base pair between Acr+ and the G-C base pair. These results clearly demonstrated that the charge separation process in DNA can be refined by putting a redox-inactive intervening base pair as a spacer between a photosensitizer and the nucleobase to be oxidized to slow down the charge recombination rate.