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 C6C6, C6C8, C6C10, C8C8, C8C10, and C10C10 (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., C4C6, C6C6, C6C8, C6C10)—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.