Solvent and linker influences on AQ^.--/dA^.+ charge-transfer state energetics and dynamics in anthraquinonyl-linker- deoxyadenosine conjugates

The goal of this work is to produce high yields of long-lived AQ ^.--/dA^.+ charge transfer (CT) excited states (or photoproducts). This goal fits within a larger context of trying generally to produce high yields of long-lived CT excited states within DNA nucleoside conjugates that can be incorporated into DNA duplexes. Depending upon the energetics of the anthraquinonyl (AQ) ³(π,π*) state as well as the reduction potentials of the subunits in particular anthraquinonyl-adenine conjugates, CT quenching of the AQ ³(π, π*) state may or may not occur in polar organic solvents. In MeOH, bis(3′,5′-O-acetyl)-N⁶-(anthraquinone-2-carbonyl)- 2′-deoxyadenosine (AQCOdA) behaves as intended and forms a ³(AQ^.--/dA^.+) CT state with a lifetime of 3 ns. However, in nonpolar THF the AQ^.--/dA^.- CT states of AQCOdA are too high in energy to be formed, and in DMSO a ¹(AQ ^.--/dA^.+) CT state is formed but lives only 6 ps. Although the lowest energy excited state for AQCOdA in MeOH is a ³(AQ ^.--/dA^.+ CT state, for N⁶-(anthraquinone-2- methylenyl)-2′-deoxyadenosine (AQMedA) in the same solvent it is a ³(π,π*) state. Changing the linking carbonyl in AQCOdA to methylene in AQMedA makes the anthraquinonyl subunit harder to reduce by 166 mV. This raises the energy of the ³(AQ^.--/dA^.+) CT state above that of the ³(π,π*) in AQMedA. The conclusion is that anthraquinonyl-dA conjugates will not have lowest energy AQ^.--/dA^.+ CT states in polar organic solvents unless the anthraquinonyl subunit is also substituted with an electron-withdrawing group that raises the AQ-subunit's reduction potential above that of AQ. A key finding in this work is that the lifetime of the ³(AQ^.--/dA ^.+) CT excited state (ca. 3 ns) is ca. 500-times longer than that of the corresponding ¹(AQ^.--/dA^.+) CT excited state (ca. 6 ps).′