Influence of a GC Base Pair on Excitation Energy Transfer in DNA-Assembled Phenanthrene π-Stacks

The effect of a GC base pair on the excitation energy transfer in a DNA-based, light harvesting assembly of phenanthrene and pyrene chromophores is described. After absorption of light at 320 nm by the stacked phenanthrene building blocks, the excitation energy is transferred to the pyrene and leads to the formation of a phenanthrene-pyrene exciplex. The fluorescence intensity depends on the number of light absorbing phenanthrenes, as well as on the type of DNA base pair flanking the phenanthrene stack. In comparison to an AT base pair, a GC base pair located next to the stacked aromatic residues results in a reduction of fluorescence. The degree of quenching is dependent on the length of the phenanthrene stack that separates the GC base pair from the exciplex. Overall, a large number of stacked phenanthrenes positively affects exciplex fluorescence by increasing the quantity of absorbed light and, at the same time, reducing the effect of quenching by GC base pairs.