The Theoretical Study Of Molecules' Two-photon Absorption Properties

Because two-photon absorption (TPA) materials have good potential applications in medical, military and biology fields, they catch people's great attention. And as time as organic synthesis technology goes ahead, more and more organic compounds with great TPA properties appear. So, this thesis studies a series of organic molecules in gas and in solutions. It will be helpful for our finding and synthesizing strong TPA materials.This thesis used density functional theory (DFT) to optimized the organic molecules, then studied their TPA cross sections, solvent effect and structure-to-property by response theory and few states model. The molecules we studied are a series of trans-stilbene derivatives.First, we applied DFT method optimized the new synthesis molecule 4,4'-bis(dimethylamino) stilbene's structure, and used few states model calculated its TPA cross section. The results agree well with the experiment data. The total work is done on GAUSSIAN-98 program pack. We used density functional theory (B3LYP), which is Becke's three parameter hybrid function using the Lee, Yang and Parr (LYP) correlation function. The basis sets we used is 6-31G*.The excited state energies and electric dipole transition moments are calculated by time-dependent DFT. The results indicate that for one dimension π center conjugated organic molecules, the tree-state model can well calculate its low-excited states TPA cross sections.At the same time, the solvent effect of 4,4'-bis(dimethylamino) stilbene is carried out by the polarized continuum model (PCM), the PCM is selecting the solvent cavity's form to be the form of solute molecule's. And after that, we used self-consistent reaction field method calculate the molecule's TPA property. The result indicates that, with the increase of polarity of the solvent, the excited energies are red-shifted and the two-photon absorption cross sections augment. But if we are eager to receive more precise results, the molecule's vibration and the hydrogen bonds with solvent molecules must be considered.At last, the structure-to-property is researched, we design a series of trans-stilbenederivatives with different arrangements of the substitutions and study their two-photon absorption properties by using the response theory method at HF level. For push-pull compounds, many researches have indicate that, organic conjugated molecule TPA cross section relate to not only molecule length, π center property, substitutions strength but also substitutions' symmetry, molecule dimension and so on. And someone found that symmetry molecule's TPA cross section is larger than asymmetry molecule's. But, whether it is always true, we need to research continuously. In this thesis, we design a series of trans-stilbene derivatives and study their two-photon absorption properties by response and few states model. The work is done on DALTON and GAUSSIAN-98 program packs. The study result indicate that, the asymmetry substituted molecules display larger two-photon absorption cross sections than the symmetry substituted molecules, which illustrates that the effect of the different substitutions on molecule's two-photon absorption property is related to the property of molecule's π -center. The charger-transferred processes of the charge-transferred states are displayed in order to understand their maximal two-photon absorption cross sections.