The metal-mediated self-assembly process allows the well-directed and controlled construction of supramolecular architectures, and the assembled metal−ligand complexes display diverse functionalities depending on the composition of the complex template. Through the deliberate introduction of a metal-binding nucleobase, cytosine, to the macrocyclic diacetylene (MCDA), a macrocyclic ligand, CytMCDA, was synthesized. On account of the metal affinity of cytosine and the π−π interaction of the diacetylene template, a Hg-coordinated unidirectional columnar self-assembly of CytMCDA was generated that formed into organic nanotubes. The monomeric CytMCDA-Hg is covalently cross-linked to the blue-phase macrocyclic polydiacetylene nanotubes (CytMCPDA-Hg) via UV-induced topochemical polymerization. CytMCPDA-Hg displayed a naked-eye-detectable sensing response toward heat and solvents with a brilliant blue−red chromatic transition. Moreover, owing to the high affinity of the mercury complex toward sulfur, CytMCPDA-Hg displayed a high sensitivity against thiols.