The Conductance Isotope Effect in Oligophenylene Imine Molecular Wires Depends on the Number and Spacing of ¹³C-Labeled Phenylene Rings

We report a strong and structurally sensitive ¹³C intramolecular conductance isotope effect (CIE) for oligophenyleneimine (OPI) molecular wires connected to Au electrodes. Wires were built from Au surfaces beginning with the formation of 4-aminothiophenol self-assembled monolayers (SAMs) followed by subsequent condensation reactions with ¹³C-labeled terephthalaldehyde and phenylenediamine; in these monomers the phenylene rings were either completely ¹³C-labeled or the naturally abundant ¹²C isotopologues. Alternatively, perdeuterated versions of terephthalaldehyde and phenylenediamine were employed to make ²H(D)-labeled OPI wires. For ¹³C-isotopologues of short OPI wires (<4 nm) in length where the charge transport mechanism is tunneling, there was no measurable effect, i.e., ¹³C CIE ≈ 1, where CIE is defined as the ratio of labeled and unlabeled wire resistances, i.e., CIE = R_heavy/R_light. However, for long OPI wires >4 nm, in which the transport mechanism is polaron hopping, a strong ¹³C CIE = 4-5 was observed. A much weaker inverse CIE < 1 was evident for the longest D-labeled wires. Importantly, the magnitude of the ¹³C CIE was sensitive to the number and spacing of ¹³C-labeled rings, i.e., the CIE was structurally sensitive. The structural sensitivity is intriguing because it may be employed to understand polaron hopping mechanisms and charge localization/delocalization in molecular wires. A preliminary theoretical analysis explored several possible explanations for the CIE, but so far a fully satisfactory explanation has not been identified. Nevertheless, the latest results unambiguously demonstrate structural sensitivity of the heavy atom CIE, offering directions for further utilization of this interesting effect.