Unique Metal Cation Recognition via Crown Ether-Derivatized Oligo(phenyleneethynylene) Molecular Junction

The formation of a 2:1 sandwich-type complex of 15-crown-5 with K+ has been previously used for the development of various smart materials and devices through detecting K+-induced changes in absorbance, electrochemical impedance, or fluorescence. However, K+-induced changes in the junction conductance of crown ethers have not been fully addressed at the molecular level. An understanding of such properties would not only advance our fundamental knowledge of electronic transport in crown ethers but also lead to practical conductance-based sensing applications. Here, we synthesized a rigid and structurally well-defined oligo(phenyleneethynylene) (OPE) molecular wire functionalized with a 15-crown-5 ether moiety (1), to measure conductance in the presence of various metal cations using the STM-BJ technique. The conductance of 1 with either Li+, Na+, or Rb+ was only slightly greater than that of 1, whereas the conductance of 1 with K+ exhibited a significant 4-fold increase over that of 1. Quantum transport calculations confirmed that the K+-induced increase in conductance was due to the formation of a 2:1 sandwich-type supramolecular junction, with a "4-anchor" binding mode, which is the optimal configuration to harness these unique recognition capabilities. Our findings provide a solid foundation for the design of practical molecular electronic components that can be incorporated into novel sensing devices.