2 research outputs found
Solvent Effect on the Stokes Shift and on the Nonfluorescent Decay of the Daidzein Molecular System
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
Impact of Electronic State Mixing on the Photoisomerization Time Scale of the Retinal Chromophore
Spectral data show
that the photoisomerization of retinal protonated
Schiff base (rPSB) chromophores occurs on a 100 fs time scale or less
in vertebrate rhodopsins, it is several times slower in microbial
rhodopsins and it is between one and 2 orders of magnitude slower
in solution. These time scale variations have been attributed to specific
modifications of the topography of the first excited state potential
energy surface of the chromophore. However, it is presently not clear
which specific environment effects (e.g., electrostatic, electronic,
or steric) are responsible for changing the surface topography. Here,
we use QM/MM models and excited state trajectory computations to provide
evidence for an increase in electronic mixing between the first and
the second excited state of the chromophore when going from vertebrate
rhodopsin to the solution environments. Ultimately, we argue that
a correlation between the lifetime of the first excited state and
electronic mixing between such state and its higher neighbor, may
have been exploited to evolve rhodopsins toward faster isomerization
and, possibly, light-sensitivity