A new colorimetric and fluorescent ‘on–off’ chemosensor, 1O, based on a photochromic diarylethene with a quinoline unit was designed and synthesized. The chemosensor 1O demonstrated selective and sensitive detection of Hg²⁺ ions in the presence of other competitive metal ions in acetonitrile. The stoichiometric ratio of the sensor 1O for Hg²⁺ was determined to be 1 : 1, and the limit of detection of the probe 1O was calculated to be 56.3 nM for Hg²⁺. In addition, a molecular logic circuit with four inputs and one output was successfully constructed with UV/vis light and metal-responsive behavior. ESI-MS spectroscopy, Job's plot analysis, and ¹H NMR titration experiments confirm the binding behavior between 1O and Hg²⁺.