Theoretical Investigation On Two-photon Absorption Materials

Two-photon absorption is a third-order nonlinear optical process. Molecular two-photon absorption has attracted growing interest over recent years owing to its applications in advanced scientific fields. Design and synthesis of materials with large two-photon absorption cross-section is the important basis of its development. A series of novel organic two-photon absorption materials have been systematic theoretical investigated. The density functional theory has been applied to optimize the molecular equilibrium geometries. On the basis of the optimized structures, one- and two-photon absorption properties are obtained by ZINDO program combined SOS equation and self-compiled FTRNLO program. The investigation in this thesis will provide some useful information and valuable basis on the theoretical designing and experimental synthesizing new two-photon absorption materials with large two-photon absorption cross-section values.1. Two series of porphyrin-thiophene chromophores which were theoretically studied exhibit large two-photon absorption cross-section in the visible region. The results show that the number of thiophene units affects the properties of one-photon absorption and two-photon absorption. Porphyrin-thiophene chromophores featuring two or three thiophene units have wide two-photon absorption response ranges; they can be applied to many nonlinear optical areas, such as optical limiting. Intervening ethynyl unit is beneficial to extend the conjugated pathway, and increase the two-photon absorption cross-section. At the same time, the one-photon absorption and two-photon absorption wavelengths are bathochromially shifted. From viewpoint of the high transparency and large nonlinear optical response, porphyrin-thiophene chromophores will be promising two-photon absorption materials.2. The electronic structures, one-photon absorption and two-photon absorption properties of the azulenylporphyrins and azulene-fused porphyrins have been comparatively studied. With the number of azulenyl groups increasing, the one-photon absorption wavelengths of all molecules are red-shifted in range of 400~600 nm, at the same time, two-photon absorption cross-section are gradually enlarged. The azulene-fused structures facilitate expanding conjugated area and increasing two-photon absorption cross-section. The origin of two-photon absorption properties of studied compounds is studied with two-level model. In summary, the azulene-fused porphyrins exhibit strong two-photon absorption.3. Two types of donor-acceptor calix[4]arenes have been theoretically studied. The calculations show that the substitution of C≡C by the conjugation bridge C=C and N=N takes an important part in altering one- and two-photon absorption properties. The maximum one-photon absorption wavelengths of all studied compounds are less than 400 nm, which means high transparency. The geometry of the calixarenes strongly influences the two-photon absorption properties of the studied compounds. In addition, the nitro derivatives have wilder two-photon absorption response range than other non-nitro derivatives. The tetrasubsituted calix[4]arenes (type B calixarenes) exists larger two-photon absorption cross-section values than the bisubstituted calix[4]arenes (type A calixarenes).4. 3, 6- and 2, 7-carbazole derivatives have been investigated. The position of vinylpyridine substituents affects the properties of one-photon absorption and two-photon absorption. Compared with 3, 6-carbazo derivatives, the one-photon absorption wavelengths of 2, 7-carbazole derivatives are bathochromic shift and the oscillator strength increased; the TPA wavelengths are red-shifted and the two-photon absorption cross-section increased. 2, 7-carbazole derivatives will be more promising TPA materials.