UV/vis absorption and fluorescence spectroscopic study of some new 4-hydroxy-7-methoxycoumarin derivatives. Part I: Effect of substitution by a benzo-1,4-dioxanyl or an ethyl furoate group in the 3-position

The UV/vis spectroscopic behaviour of some new 4-hydroxy-7-methoxycoumarins was investigated with the aim of analysing the influence of the substituent in the 3-position. In comparison with the unsubstituted analogue, the ethyl furoate and benzo-1,4-dioxanyl derivatives exhibited UV/vis absorption and emission spectra strongly shifted to the red. The highest fluorescence quantum yields and longest lifetimes were encountered for the ethyl furoate derivatives, which also showed little sensitivity to environmental factors. In contrast, substitution by a benzo-1,4-dioxanyl group induced the appearance of positive solvatochromism together with a drastic decrease of the fluorescence efficiency. Emission properties were particularly sensitive to viscosity and temperature variations. The involvement of a non-radiative rotatory decay mechanism was discussed, based on the analysis of the photophysical constants. AM1 MOPAC calculations pointed out some differences in the molecular geometry and energy levels, which allowed the substituent effect to be better identified.