Cinnamoyl Dipyrromethenes as Fluorescence Zinc(II) Ion Sensor

Friedel‐Crafts acylation of dipyrromethane with cinnamoyl chloride was conducted to obtain dicinnamoyl dipyrromethane compounds 3 and 4. Both compounds were subsequently oxidized by DDQ to produce dicinnamoyl dipyrromethene ligands (DC‐1 and DC‐2). A large bathochromic shift compared to dipyromethene (D) was observed at 95 nm for DC‐1 and 67 nm for DC‐2. Both compounds showed remarkable chelation‐enhanced fluorescence (CHEF) upon addition of zinc(II) ions. Similar to the quadrupolar system, DC‐1 exhibited absorption and emission near the optical windows of the tissue. However, asymmetrical DC‐2 had a better “turn‐on” CHEF, with a fluorescence intensity that was 22 times higher than that of compound D. The DC‐2 ligand also showed a limit of detection (LOD) of up to 3.0×10−8 M and selectivity toward zinc(II) ions compared to alkali and alkaline earth metal ions. Dipyrromethenes with α,α′ and α,β′‐dicinnamoyl substituents were successfully synthesized via Friedel‐Crafts acylation. The ligand could give remarkable chelation‐enhanced fluorescence (CHEF) upon the addition of zinc(II) ions and showing an LOD of up to 3.0×10−8 M. The resulting dyes have absorption and emission spectra close to optical windows for tissue that would benefit biological applications.