From Absorption to Fluorescence: Case of 3,6-Substituted Coumarin Derivatives

A theoretical study was carried out on the coumarin derivatives substituted at positions 3 and 6 by means of the two quantum methods: DFT and time-dependent density functional theory (TDDFT). The compound 3-fluorophenyl 2-oxo-2H-1-benzopyran-3-carboxylate is specific. The mesomeric donor effect of fluorine creates a strong delocalization of charge generating a twisted structure with a very high dipole moment. This compound show fluorescence at 446 nm with Stokes shift of 90 nm in ethanol. The presence of two transitions of the σ–σ* type in this compound leads to the breakdown of the carbon–oxygen bond and generates the phenolate ion, a fluorescence quenching agent. Whatever the nature and position of the substituent, all the excited states of the 3,6-substituted derivatives have a high value of the dipole moment and HOMO/LUMO electronic transitions of charge transfer nature .