Instituto de Física, Universidade de São Paulo, CP 66318, 05314-970 São Paulo, SP, Brazil.
J Phys Chem A. 2013 May 30;117(21):4404-11. doi: 10.1021/jp4021646. Epub 2013 May 17.
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.
黄酮类化合物因其作为抗氧化元素对人体健康的影响而成为了许多实验研究的目标。这些化合物的荧光性质已经得到了广泛的研究，因为它们具有大的斯托克斯位移和多种导致荧光的过程。在本工作中，理论上研究了溶剂在大豆苷元分子体系在水中实验观测到大的斯托克斯位移中的作用。还研究了在极性非质子溶剂如乙腈中荧光非辐射衰减的机制。通过使用顺序量子力学/分子力学方法考虑了在基态和低激发电子态中的溶剂效应。通过多组态计算、CASSCF 和 CASPT2 研究了激发态性质，如平衡几何形状和跃迁能量。确定了导致水中荧光光谱的激发电子态，大的斯托克斯位移似乎是由于系统在该电子态与溶剂的相互作用较大所致。另一方面，单重态和三重态电子态之间的自旋轨道耦合计算表明，体系交叉有利于系间交叉，这与实验上观察到的非荧光结果一致。
