Synthesis, structure, absorption and fluorescence of Pechmann dye heteroanalogues

•Oxidative dimerization of 5-aryl-pyrrolinone esters produced N-analogues of Pechmann dye.•X-ray diffraction structures confirmed E isomers of phenyl and 2-thienyl derivatives.•No E/Z photoisomerization was observed by NMR spectroscopy.•Fluorescence was studied in solution, low temperature solvent glass and solid-state.•(TD) DFT computations were able to describe all observed structural and spectral phenomena.

Three N-analogues of Pechmann dye with identical aryl groups in the 5, 5′-positions (aryl = phenyl, 2-naphthyl, 2-thienyl) and carboxy ester groups in the 3,3′-positions were synthesized by oxidative dimerization of the corresponding 5-aryl-pyrrolinone esters. Derivatives with improved solubility in most organic solvents were prepared by subsequent N-methylation. The dyes were confirmed as E isomers, i.e. holding an all-trans 1,6-diaryl-1,3,5-hexatriene backbone, with slight torsion of central CC bond by X-ray diffraction and density functional theory (DFT) calculations. No E/Z photoisomerization was observed. Full assignment of 1H and 13C NMR signals was performed. The absorption spectra are quite similar with the maxima in the range 572 nm (aryl = phenyl)–643 nm (aryl = 2-thienyl) in accordance with time dependent DFT calculations of excitation energies. No fluorescence was observed in solution, while all compounds fluoresce in low temperature solvent glass (77 K) and a weak solid-state fluorescence of phenyl and 2-naphthyl derivatives in red/IR region was detected.

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