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.
Keywords: CHEF; Fluorescence sensor; Friedel-Crafts; dipyrromethene; titration spectroscopy.
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