Domino synthesis of protochromic “ON–OFF–ON” luminescent 2-styryl quinolines

The microwave-assisted coupling-isomerization reaction (MACIR) opens a straightforward domino access to 2-styryl quinolines in good to excellent yields. The push-pull character of these lumophores can be enhanced by placing a dimethyl amino group as an auxochrome in the para-position of the styryl moiety whereas anti-auxochromes are located in the 6-position of the quinoline part. The optical absorption and emission properties of the compounds were studied in solvents of different polarity and at various pH. The pronounced proto- and solvochromicity of the absorption and emission properties qualify them as luminescent pH sensors with "ON-OFF-ON" emission profiles triggered by pH variation over a broad pH range. The electronic structure of the chromophores is rationalized by DFT calculations.