Three-Component Synthesis and Photophysical Properties of Novel Coumarin-Based Merocyanines

Novel E-configurated coumarin-based merocyanines were efficiently synthesized by a one-pot three-component Sonogashira coupling-Michael addition starting from triflyl coumarins, terminal alkynes and secondary amines. The photophysical properties of the synthesized yellow to orange merocyanines were studied by UV/Vis and fluorescence spectroscopy and rationalized by Hammett-Taft correlations and DFT and TD DFT calculations. Most compounds were only weakly fluorescent in solution; however, two compounds were investigated in more detail with respect to their aggregation behavior. The system for R2 = H and NR23 = pyrrolidinyl shows aggregation induced emission at a water content of 40-60% in methanol, while the chromophore with R2 = p-Me2NC6H4 and NR23 = morpholinyl displays considerable aggregation induced emission enhancement with a concomitant red shift at increasing water contents in THF.