Pimentel Ashley E, Pham Lan D, Carta Veronica, Su Timothy A
Department of Chemistry, University of California, 92521, Riverside, California, USA.
Materials Science and Engineering Program, University of California, 92521, Riverside, California, USA.
Angew Chem Int Ed Engl. 2025 Jan 21;64(4):e202415978. doi: 10.1002/anie.202415978. Epub 2024 Nov 6.
We report the first conductance measurements of [n]staffane (bicyclopentane) oligomers in single-molecule junctions. Our studies reveal two quantum transport characteristics unique to staffanes that emerge from their strained bicyclic structure. First, though staffanes are composed of weakly conjugated C-C σ-bonds, staffanes carry a shallower conductance decay value (β=0.84±0.02 n) than alkane chain analogs (β=0.96±0.03 n) when measured with the scanning tunneling microscopy break junction (STM-BJ) technique. Staffanes are thus more conductive than other σ-bonded organic backbones reported in the literature on a per atom basis. Density functional theory (DFT) calculations suggest staffane backbones are more effective conduits for charge transport because their significant bicyclic ring strain destabilizes the HOMO-2 energy, aligning it more closely with the Fermi energy of gold electrodes as oligomer order increases. Second, the monostaffane is significantly lower conducting than expected. DFT calculations suggest that short monostaffanes sterically enforce insulating gauche interelectrode orientations over syn orientations; these steric effects are alleviated in longer staffanes. Moreover, we find that [2-5]staffane wires may accommodate axial mechanical strain by "rod-bending". These findings show for the first time how bicyclic ring strain can enhance charge transmission in saturated molecular wires. These studies showcase the STM-BJ technique as a valuable tool for uncovering the stereoelectronic proclivities of molecules at material interfaces.
我们报道了在单分子结中对[n]金刚烷（双环戊烷）低聚物的首次电导测量。我们的研究揭示了金刚烷因其双环结构而具有的两种独特的量子传输特性。首先，尽管金刚烷由弱共轭的C-C σ键组成，但当用扫描隧道显微镜断结（STM-BJ）技术测量时，金刚烷的电导衰减值（β = 0.84±0.02 n）比烷烃链类似物（β = 0.96±0.03 n）更浅。因此，按每个原子计算，金刚烷比文献中报道的其他σ键合有机主链更具导电性。密度泛函理论（DFT）计算表明，金刚烷主链是更有效的电荷传输通道，因为其显著的双环环应变使HOMO-2能量不稳定，随着低聚物阶数的增加，使其与金电极的费米能量更紧密地对齐。其次，单金刚烷的导电性明显低于预期。DFT计算表明，短的单金刚烷在空间上强制绝缘的gauche电极间取向优于syn取向；在较长的金刚烷中，这些空间效应会得到缓解。此外，我们发现[2-5]金刚烷线可以通过“杆弯曲”来适应轴向机械应变。这些发现首次展示了双环环应变如何增强饱和分子线中的电荷传输。这些研究展示了STM-BJ技术作为一种在材料界面揭示分子立体电子倾向的有价值工具。
