Molecular Design Investigations On Two-photon Absorption Materials

Recently, two-photon absorption of organic materials has received more and more attention due to the continuous improvement of the laser technique. Materials with large two-photon absorption response can be applied in the fields of chemistry, physics and biology. Two-photon absorption as one of nonlinear optical responses also plays a significant role in the national defense (such as laser protection). Design and synthesis of materials with large two-photon absorption cross section is the important basis of its development. In this thesis, in order to design and select organic materials with large two-photon absorption response, we have performed systematic theoretical research of various organic materials. The quantum chemical methods-density functional theory and AM1 have 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 combined SOS equation and self-compiled program. We have designed some unknown compounds using the same method so as to provide a theoretical basis of synthesizing novel materials with large two-photon absorption cross section. The following is the main results: 1. One-and two-photon absorption (TPA) properties of a series of novel multi-branched molecules including two different two-photon absorption chromphores have been investigated. The one-photon absorption (OPA) of compounds containing two different stilbene ( or a stilbene and an azobenzene) derivatives offers an unprecedented combination over that of the individual parts. These molecules exhibit comparatively large TPA cross sections and the TPA cross sections of combinations are much larger than the simple sum of that of the individual parts. 2. The one-photon absorption and two-photon absorption properties of a series of paracyclophane derivatives have been investigated. The results show that not only on the OPA spectra but also on the TPA spectra, the conjugated length in molecules has the most important effect. The three-dimensional dimers formed by the connection of paracyclophane bridge have much larger TPA cross sections than the one-dimensional parent monometic molecules do by four times. Our calculated results have confirmed the experimental finding on the importance of the molecular symmetry. Paracyclophane based molecules can be classified as a completely new and promising kind of nonlinear optical materials from the standpoint of the high transparency and relatively large NLO response. The paracyclophane molecular template as a novel strategy for "through-space"charge-transfer is worth being investigated further. 3. Conventional and novel octupolar compounds and their dipolar counterparts have been studied. The calculated results suggest that the increase of the electron-donating ability of substituents, the improvement of electron-accepting ability of molecular center and the increase of conjugation length have positive effects on the increase of TPA cross section. In the octupolar oligofluorenes, an increase of in the transition dipole moment (M0k) between the ground state and the lowest excited state, an increase of in the transition dipole moment (Mkn) between the lowest excited state and the TPA final state and a decrease in the detuning term (E0k-E0n/2) contribute importantly to the increase of TPA cross section. Deviations from the factor 3 increase in the TPAcross section of octupolar molecules with respect to that of the corresponding dipolar ones can be accounted for the weak coupling among the three arms of the C3h or C3 structure. In comparison to the corresponding dipolar molecules, the octupolar compounds preserve the high transparency and have larger TPA cross section, which are promising TPA materials. 4. Two series of tetrahedral octupolar molecules have been investigated theoretically. The electronic structure, OPA and TPA properties were studied in detail. The calculated results show that the alteration of the conjugated bridge has the most essential effect on the position of OPA peak, and the molecular length is the most important parameter of determining the OPA intensity. For two series of molecules, increasing the conjugation length enhances the TPA cross sections drastically. Selecting P+ as the center is favorable for the efficient increase of TPA cross section in the higher energy region in comparison to C atom as the center. While the molecules centered by C atom all exhibit fairly large TPA cross section at the region of 400 nm to 600nm in a wider area than those centered by P+, and this property may lead tetrahedral octupolar molecules with the C atom as the center to more practical applications, for instance, optical limiting. 5. The geometries, electronic structures, one-and two-photon absorption spectra for a series of dendritic molecules have been studied. The calculated results show that the studied molecules possess high transparency and the range of the maximal TPA cross section values is from 2615.36 ×10-50 cm4·s/ photon to 102222 ×10-50 cm4·s/ photon. They are ones of the largest TPA cross section reported so far in references. The maximum TPA cross section almost linearly increases as the number of stilbenes in molecules increases. This type of novel dendritic chromphores possessing high transparency is worth being further investigated. 6. The theoretical research on the TPA properties of C60 and C70 has been...