Abraham Colin-Molina

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 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.

Fluorescent compounds 2 and 3 have been prepared by using Ullmann-type coupling reactions. Solid-state NMR ( ¹³ C CPMAS and ² H) and periodic DFT computations were used to determine that intramolecular motion coexists with emission in these crystals.