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

Cost-Effective Method for Free-Energy Minimization in Complex Systems with Elaborated Ab Initio Potentials

We describe a method to locate stationary points in the free-energy hypersurface of complex molecular systems using high-level correlated <i>ab initio</i> potentials. In this work, we assume a combined QM/MM description of the system although generalization to full <i>ab initio</i> potentials or other theoretical schemes is straightforward. The free-energy gradient (FEG) is obtained as the mean force acting on relevant nuclei using a dual level strategy. First, a statistical simulation is carried out using an appropriate, low-level quantum mechanical force-field. Free-energy perturbation (FEP) theory is then used to obtain the free-energy derivatives for the target, high-level quantum mechanical force-field. We show that this composite FEG-FEP approach is able to reproduce the results of a standard free-energy minimization procedure with high accuracy, while simultaneously allowing for a drastic reduction of both computational and wall-clock time. The method has been applied to study the structure of the water molecule in liquid water at the QCISD/aug-cc-pVTZ level of theory, using the sampling from QM/MM molecular dynamics simulations at the B3LYP/6-311+G­(d,p) level. The obtained values for the geometrical parameters and for the dipole moment of the water molecule are within the experimental error, and they also display an excellent agreement when compared to other theoretical estimations. The developed methodology represents therefore an important step toward the accurate determination of the mechanism, kinetics, and thermodynamic properties of processes in solution, in enzymes, and in other disordered chemical systems using state-of-the-art <i>ab initio</i> potentials