Straightforward synthesis of photoactive chalcogen functionalized benzimidazo[1,2-a]quinolines†
A series of new organochalcogen derivatives of benzimidazo[1,2-a]quinolines were synthesized in moderate to excellent yields and in short reaction times from chalcogen benzimidazoles, in a straightforward synthetic procedure, through transition-metal-free cascade reactions involving a sequential intermolecular aromatic nucleophilic substitution (SNAr), followed by an intramolecular Knoevenagel condensation. Both the sulfur and selenium derivatives presented similar photophysical properties, with absorption maxima located in the UV region (∼355 nm) related to spin and symmetry allowed electronic p–p* transitions, and fluorescence emission located in the violet-blue region (∼440 nm) with a relatively large Stokes shift (∼90 nm). The fluorescence quantum yields were slightly influenced by the chalcogen, with the sulfur derivatives presenting higher values than the selenium analogues, probably due to the intersystem crossing allowed by the selenium atom. Moreover no clear evidence for charge transfer in either compound in the ground and excited states was observed.