Synthesis, absorption and fluorescence of hydrazone colorants based on pyrrolinone esters

Ten azo dyes were prepared by diazotization of a series of electronically different para substituted anilines and subsequent azo coupling of these diazonium salts with ethyl 4,5-dihydro-5-oxo-2-aryl(1H)pyrrole-3-carboxylate as a the coupling component. All of the dyes were confirmed as keto-hydrazone tautomers and were found as a mixtures of E- and Z-isomers with respect to the exocyclic CN bond by 1H-NMR spectroscopy. The absorption spectra are all similar irrespective of substituent and solvent. By comparison the fluorescence is strongly dependent on the electronic character of the substituents. All compounds fluoresce in a low temperature solvent glass and in the solid state and only the 4-cyanophenyl and 4-nitrophenyl derivatives show fluorescence in solution at room temperature. The spectroscopic behavior is explained in terms of competition between E/Z isomerization and fluorescence after excitation.

► Coupling of diazotized anilines with pyrrolinone esters produces exclusively hydrazone dyes.
► Synthesized dyes arise as a mixture of E and Z isomers with respect to exocyclic CN bond.
► All ten dyes fluoresce in solid state and in low temperature solvent glass.
► Only p-nitro and p-cyano derivatives show fluorescence in solution.