And once again the quest for easily synthesizable fluorescent dyes was successful: Umbelliferone. (Notice the similarity of the molecule to Aesculine. It is just missing the glycosidically bound glucose.)This substance is a naturally occurring dye of the courmarin family, but it can also be manufactured by a simple reaction. Only two reactants are needed, Resorcinol and Malic Acid. Sulfuric Acid is added as a hygroscopic catalyst to produce Formylacetic Acid from the Malic Acid in situ.
The empirical reaction equation is as follows:
C4H6O5 + C6H6O2 ————> CO + 3 H2O + C9H6O3
Resorcinol has a molar mass of 110.10 g/mol, Malic Acid 134.09 g/mol.
This equates to a 1:0.82 weight ratio.
I used 1 gram of Malic Acid and 0.82 gram of Resorcinol and added a few drops of Sulfuric Acid. The whole mixture was heated with hot air until both educts were solved in the acid and bubbling started.
After a short while, the solution began to change color to a dark orange. Upon further heating, the color got darker until it was deep red and had the viscosity of honey.
A drop of the cooled product was added to a 600ml beaker of cold water and observed under ultraviolet light. Instantly, a bright green plume descending from the surface was visible, changing color to a bright blue while getting more diluted.
A small concentration in water fluoresces blue under UV light and green in sunlight. In ethanol the concentration shows no influence on the color, as it is always a bright yellowish green.
EDIT: On a second thought, the green color might come from fluorescein that was formed by a side reaction during synthesis of the umbelliferone. I will try to record absorption and emission spectra to figure out which substances are present.