Synthesis of isocoumarins and PAHs with electron-withdrawing substituents: Impact of the substituent nature on the photophysical behavior

The efficient conditions for rhodium catalyzed oxidative coupling of electron-poor carboxylic acids with diphenylacetylene have been optimized. The best protocol uses Ag2CO3 as an oxidant and DMF as a solvent. The reactions smoothly proceed with the formation of isocoumarins and polycyclic aromatic hydrocarbons (PAHs). The isonicotinic, coumalic, coumarin-3-carboxylic, 4-nitrobenzoic, 4-cyanobenzoic, 4-(ethoxycarbonyl)benzoic, and terephthalic acids were successfully involved in the annulation. In contrast, in the case of picolinic acid, the formation of the stable chelate complex Cp*RhCl(O,N-pic) prohibits the catalytic reaction. The photophysical properties of isocoumarins and PAHs in solutions, such as fluorescence emission and solvatochromic effects, have been studied. In particular, the isocoumarin dyes demonstrate a phenomenon of aggregation-induced emission, while PAHs with electron-withdrawing substituents exhibit strong fluorescence emission in the violet region (370-390 nm) with quantum yields up to 55%.