A new light-emitting polyacetylene bearing imidazole moieties in the side chain (P1), was conveniently prepared through a postfunctionalization strategy, as a sensory polymer to selectively report the presence of Cu²⁺ (with a detection limit of 1.48 ppm) based on the fluorescence “turn-off”. Interestingly, the quenched luminescence of P1 by Cu²⁺ could be turned on after the addition of CN⁻, making P1 a novel, sensitive, and selective cyanide probe.