Distance dependence of the charge transfer rate for peptide nucleic acid monolayers.

Charge transfer studies have been performed for self-assembled monolayers of single-stranded and double-stranded peptide nucleic acids (PNAs) having a C-terminus cysteine and an N-terminus ferrocene group as a redox reporter. The decay of the charge transfer rate with distance was strong for short single-stranded PNA molecules and weak for long single-stranded and double-stranded PNAs. Possible mechanisms for this "softening" of the distance dependence are discussed. The nature of the mechanism change can be explained by a transition of the charge transport mechanism from superexchange-mediated tunneling for short PNAs to a "hopping" mechanism for long PNAs.