Abstract

Background: The reaction of 7-diethylamino 3-formyl coumarin with active methylene group afforded in good yields the corresponding new 3-(substituted ethenyl)coumarins. The structures of the synthesized compounds were confirmed by 1H NMR, 13C NMR spectral data, IR and elemental analysis. Optical properties were studied in dimethyl sulfoxide by UV/Vis absorption and fluorescence spectroscopy.

Methods: In a 250 mL, three-necked flask equipped with a condenser a mixture of 3-formyl-7- diethylaminocoumarin 1 (0.02 mol, 4.902 g), IRA 900 resin (0.02 mol, 2 g), and toluene (25mL) were introduced under a nitrogen atmosphere at 85°C and stirred for 3 h. 4-nitrobenzylcyanid a (0.02 mol, 3.243 g) was added then the mixture was refluxed for 27 h. Finally, the organic phase was separated from the solid catalyst and concentrated under reduced pressure. The solid obtained was recrystallised in ethanol.

Results: In this work we describe the synthesis of 3-(substituted ethenyl) coumarins via Knovenagel condensation of 3-formyl-7-diethylaminocoumarin with active methylene compounds. 3-formyl-7- diethylaminocoumarin was utilized as key starting material in the synthesis of 3-(substituted ethenyl) coumarins. 3-formyl-7-diethylaminocoumarin was reacted with various active methylene compounds in the presence of an equimolar amount of homogeneous basic catalysis (piperidine) or under heterogeneous conditions (Amberlite IRA 900) to give the corresponding 3-(substituted ethenyl) coumarins in moderate to good yields. Piperidine or Amberlite IRA 900 is highly selective, yielding products of inversion with nearly 100% selectivity. FTIR, 1H NMR, 13C NMR and Elemental analysis assigned their structures. The experimental UV/visible spectra of the 3-(substituted ethenyl) coumarins 1a-1b and 1e in DMSO were obtained and. The absorption spectra, for all compounds, showed only one intense band above 400 nm, without any fine vibronic structure. The absorption maxima of all the dyes were observed at 504 to 512 nm (coumarin 1 at 450 nm). This behaviour is associated with increasing conjugation length of the chromophoric system due to overlapping of π-orbitals of ethenyl group with π- orbitals of coumarin moiety. The fluorescence spectra 1a-1b and 1e with excitation at absorption wavelength of the conjugated backbone (504-512 nm) showed observable single bands at 646, 588 and 564 nm, respectively as show in Table 2. The Stokes' shift of the obtained compounds is larger than that of the precusseur coumarin 1 (Fig. 1). Fluorescence quantum yields (φ) of the compounds dyes in DMSO were estimated at ambient temperature (25 ± 1C) using a comparative method (Table 2). We were a little disappointed that the quantum yield of the compounds obtained was less than that of the starting coumarin.

Conclusion: We have presented the synthesis and the characterization of new heterocyclic compounds containing coumarin nuclei. In this investigation the process developed is particularly interesting for its simplicity and for the advantage of providing this family of coumarin compounds in good yield and high selectivity. The optical properties of two new coumarins dyes, differing by the substitution in the 3 position, were investigated in DMSO by UV/vis absorption and fluorescence spectroscopy.

Keywords: 7-diethylamino-3-formyl coumarin, active methylene group, ethenylcoumarin, fluorescence.

Graphical Abstract