Solvent Effect on the Stokes Shift and on the Nonfluorescent
Decay of the Daidzein Molecular System
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Abstract
The flavonoids have been the target
of several experimental works
due to its influence in the human health as antioxidant elements.
The fluorescence properties of these compounds have been widely studied
due to the large Stokes shifts experimentally observed and the variety
of processes that lead to the fluorescence. In the present work the
role of the solvent in the large Stokes shift experimentally observed
in the daidzein molecular system in water is theoretically studied.
Also studied is the nonfluorescent decay mechanism in a polar aprotic
solvent like acetonitrile. The solvent effect in the ground and in
the low-lying excited electronic states is taken into account by using
the sequential-QM/MM methodology. Excited state properties like equilibrium
geometries and transition energies were studied by using multiconfigurational
calculations, CASSCF and CASPT2. The excited electronic state responsible
for the fluorescence spectrum in water was identified, and the large
Stokes shift seems to be the result of the large interaction of the
system in this electronic state with the solvent. On the other hand,
spin–orbit coupling calculations, between the singlet and triplet
electronic states, indicate favorable conditions for intersystem crossing,
in agreement with the experimental result of nonfluorescence observation