Pentacyclic coumarin-based blue emitters – the case of bifunctional nucleophilic behavior of amidines

An unexpected discovery of the novel cyclocondensation reaction of 1,8-diazabicyclo[5.4.0]undec-8-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) with alkyl coumarin-3-carboxylates is described. First, an unknown skeleton is generated through the concomitant formation of new nitrogen–carbon and carbon–carbon bonds followed by the oxidation of the intermediate product by the second equivalent of the starting coumarin. Pentacyclic DBN-derivatives exhibit strong fluorescence both in solutions (Φ_fl ∼ 0.6–0.8) and in the solid state, while non-planar DBU-derivatives exhibit strong fluorescence in the solid state only.