Solid-state fluorescence properties of small iminocoumarin derivatives and their analogues in the coumarin series

Considering the many potential applications of fluorescent materials, dyes that exhibit emission in the solid state are in high demand, especially if they are easy to prepare and inexpensive. In this respect, exploring the behaviour of basic iminocoumarins seemed relevant. The spectroscopic properties of 3-cyano-7-diethylamino-2-iminocoumarin and a derivative bearing an ethoxycarbonyl group were therefore studied in the solid state, and a comparison was made with the corresponding coumarin. The three compounds had very close fluorescence properties in solution. In contrast, the coumarin derivative was much more emissive in the solid state than the iminocoumarins. This discrepancy was tentatively related to the crystal packing modes. A comparison was made with 7-diethylaminocoumarin to clarify the role of the cyano group, which may not be advantageous from a spectroscopic viewpoint. Using a solvent-exchange method, the three cyano compounds generated elongated microparticles, the substituted iminocoumarin derivative even led to flat microfibers. This work shows that small coumarins are valuable solid-state fluorescent dyes while small iminocoumarins should be used as building blocks to access more elaborated compounds if solid-state applications are envisaged.