Tuning Nanoscale Conductance in Cyclic Molecules via Molecular Length and Anchoring Groups

This theoretical study investigates the electrical conductance of non-conjugated cyclic molecules featuring three terminal anchoring groups at the single-molecule level. Density Functional Theory (DFT) calculations demonstrate that the conductance of the symmetric and asymmetric cyclic structures <i>C</i>₆<i>C</i>₆, <i>C</i>₆<i>C</i>₈, <i>C</i>₆<i>C</i>₁₀, <i>C</i>₈<i>C</i>₈, <i>C</i>₈<i>C</i>₁₀, and <i>C</i>₁₀<i>C</i>₁₀ (where the numbers indicate the lengths of the upper and lower branches) decreases with increasing molecular length, independent of the anchor group chemistry. Distinct trends are observed across molecular series: the 6-unit branch-defined as molecules containing a common six-carbon saturated segment (e.g., <i>C</i>₄<i>C</i>₆, <i>C</i>₆<i>C</i>₆, <i>C</i>₆<i>C</i>₈, <i>C</i>₆<i>C</i>₁₀)-exhibits a non-conventional pattern, whereas the 8-unit and 10-unit branches display parabolic and conventional length-dependent behavior, respectively. A key finding is that cyclic molecules with identical total CH₂ units exhibit nearly identical conductance values, irrespective of structural symmetry. These theoretical predictions are strongly supported by previously reported scanning tunneling microscopy break-junction measurements, establishing a fundamental structure-property relationship for sigma-conjugated molecular systems. These findings provide critical design principles for developing advanced molecular-scale electronic devices.