The synthesis, absorption, fluorescence and photoisomerisation of 2-aryl-4-arylmethylidene-pyrroline-5-ones

Base catalyzed Claisen–Schmidt type condensation of ethyl-4,5-dihydro-5-oxo-2-phenyl(1H)pyrrole-3-carboxylate and its 2-biphenylyl analogue with various aldehydes was studied. The absorption and fluorescence spectra of ethyl-4,5-dihydro-5-oxo-2-aryl-4-(arylmethylidene)-(1H)pyrrole-3-carboxylate at both room (300 K) and low (77 K) temperatures were analyzed. E–Z photoisomerisation was observed for all derivatives; the Z-isomer was confirmed as being thermodynamically stable by NMR spectroscopy. PM3 quantum chemical method was used for ground state geometry calculations of the Z-isomers, while INDO/S calculations enabled interpretation of the absorption spectral shifts. Photoisomerisation forms the main deactivation channel after excitation in fluid solution, in contrast to rigid frozen glass, where fluorescence is preferred.