| Two-photon-absorption materials are playing an important role in many fields such as photoelectricity, medical treatment, biology and so on. Organic materials with large two-photon absorption cross section have attracted considerable attention for their potential applications such as two-photon upconverted lasing, two-photon optical power limiting, three-dimensional optical storage and photodynamic therapy. The development of two-photon technology relies much on the success of synthesis of new molecules with very large TPA cross sections at desirable wavelengths. Many factors can affect the two-photon absorption properties in organic materials. So it is important to investigate the structure and the two-photon absorption property relationship for organic materials.In this thesis, the effects of the conjugated length, the strength of subsitituents and the type of π conjugated bridge on the two-photon absorption properties were systematically studied by the ZINDO/SOS (Sum-Over-States) method. And in order to study the structure—property relationship for the muti-branched molecules , the influences of the number of branches, the structure of the branches and the coupled center on the muti-branched molecules were systematically investigated as well. All the investigated molecules were designed based on the stilbene, dithienothiophene and pyridinium chromophores. In this thesis, the following conclusions can be deduced:1. The two-photon absorption cross section of organic materials depends almost linearly on the conjugation length but the two-photon absorption cross section for the molecules beyond some chain length would be saturated.2. Increasing the strength of donor or acceptor moieties attached symmetrically or asymmetrically to a conjugated linker can result in the enhancement of the TPA cross section. However the enhancement depends on the type of the π conjugated bridge. The donor and acceptor can enhance the TPA cross section at different wavelength...
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