Synthesis of 3H‐Pyrrolo‐(1,2‐a) Indole‐based Fluorophore Macrocycles and their Stable Cation Radicals

Abstract Small molecular fluorophores with high quantum yields are of particular interest because of their role in several applications that range from ion sensing to bioimaging. Indole based biomolecules are well known for their fluorescence properties. Pyrrolo[1,2‐ a ]indoles have gained tremendous interest in recent times due to their applicability in various fields such as biomedicine and dye‐sensitized solar cells (DSSC). In this article, we report the synthesis of new types of crowned 3 H ‐pyrrolo[1,2‐ a ]indole fluorophore macrocycles 1 – 3 which were prepared in 30–35% yields using commercially available compounds under simple reaction conditions. These fluorophore macrocycles 1 – 3 were characterized by HR‐MS, 1D & 2D NMR, X‐ray crystallography, absorption, fluorescence, electrochemical, and DFT/TD‐DFT studies. The fluorophores 1 – 3 absorb in the region of 465–480 nm and emit in the region of 615–645 nm with large Stokes shift (∼150 nm), 10–20% quantum yields, and singlet state lifetimes of 1.70–2.50 ns. The electrochemical studies revealed that macrocycles 1 – 3 are electron‐rich and undergo easier oxidations. The macrocycles 1 – 3 readily form cation radicals by chemical and electrochemical oxidation which are highly stable at low temperatures as well as at room temperature.