Insertion-Coupling-Cycloisomerization Domino Synthesis and Cation-Induced Halochromic Fluorescence of 2,4-Diarylpyrano[2,3-b]indoles

2,4-Diarylpyrano[2,3-b]indoles are formed via a Pd-Cu-catalyzed insertion coupling cycloisomerization domino reaction in moderate yields. Although the tricyclic systems are nonfluorescent in solution, protonation, quaternation, or complexation with metal ions induces intense green luminescence. Most strikingly, selective halochromic fluorescence of zinc and magnesium over calcium ions classify the title compounds as metal-selective luminescence sensors.