Credit: © 2010 Wiley

Long-distance charge transport is an important characteristic for the components of organic electronic devices. Research has recently turned to DNA as a scaffolding material because it can hold molecules in helical arrangements, for example through the substitution or modifications of its base pairs. Perylene-based polyaromatic molecules have previously been attached to phosphate groups and incorporated into oligonucleotides. On photoexcitation, two adjacent perylene diimide (PDI) units were found to be coupled due to co-facial ππ stacking. Now, Michael Wasielewski, Frederick Lewis and co-workers at Northwestern University have investigated the charge transport across such PDI moieties1.

They prepared a series of DNA hairpins in which two to four adjacent bases in the double helix were substituted for a PDI moiety in a zipper-like manner — adjacent to each other but on opposite strands. UV–visible spectra of the species showed that adjacent, ππ stacked PDI moieties were exciton-coupled.

In each hairpin, one of the PDI units was reduced into its radical ion. The resulting singly reduced duplexes were then investigated by electron paramagnetic resonance spectroscopy. These revealed that an unpaired electron hopped between the two PDI units of the dimer, but only between two units of the trimer, and three units of the tetramer. The fact that not all PDI moieties participated in electron sharing could be due to a difference between energy levels across the moieties, or to imperfect stacking. The rapid electron hopping rate observed (above 107 s−1) suggests that such DNA–PDI systems hold promise for charge transport materials.