Theoretical Study Of The Organic Nonlinear Optical Materials Using The TDDFT-SOS Method

With the rapid development of optical telecommunication, optical computing and signal-processing devices etc, materials with larger nonlinear optical coefficient are still in great demand due to the critical role that they are playing in photoelectric devices. The organic materials are of major interest in the nonlinear optical field, due to their large nonlinear optical coefficient, fast nonlinear optical response times, relatively low cost, ease of fabrication and integration into devices, tailorability which allows one to fine tune the chemical structure and properties for a given nonlinear optical process, high laser damage thresholds. Theoretical calculation can be a good tool for obtaining some insight into molecular property, each tensor component of hyperpolarizability can be assessed directly, unlike optical experiment in which the property is obtained as macroscopic quantity. Usually, a macroscopic quantity cannot be directly converted to a microscopic one due to intermolecular or collective effects. Therefore, calculation of molecular hyperpolarizabilities and comparison of the results with corresponding experimental results are of importance in establishing the structure-property relationship and estimating the amount of such effects on molecular optical nonlinearity.In this thesis, we have performed systematic theoretical research of novel organic nonlinear optical materials by using time-dependent density-functional theory combined with sum-over-states method (TDDFT-SOS). Gold-pentacene complex, phenolpyridyl boron complexes, organoimide derivatives of hexamolybdates and spirosilabifluorene derivatives were optimized by density functional theory. On the basis of the optimized molecular geometries, electronic spectrum, charge transport and nonlinear optical properties of these systems are obtained and structure-property relationships are also established. These results may provide a theoretical basis of designing novel materials with large nonlinear optical coefficeents. Our work has been focus on the following five aspects:1. The nonlinear optical properties of a series of organic molecules are calculated by using TDDFT-SOS method and compared with the literature theoretical and experimental values. The results show that our theoretical values are in agreement with the experimental and CCSD(T) ones. This indicates we can use TDDFT-SOS method to study the NLO properties of the organic systems. The effects of different functional/basis set on NLO properties were tested. Moreover, the convergent behaviors and efficiency of various functionals are also discussed.2. Four isomers formed by a gold atom attached to a pentacene molecule were investigated by density functional theory due to their potential applications in molecular electronics. When attaching a gold atom to a pentacene molecule, the gold atom attached to the center benzene ring is the most stable. The gold and carbon atoms can form a covalent bond, which hasσsingle-bond character. However, there are strong donor-acceptor interactions between the gold and carbon atoms of systems 3 and 4. Moreover, the influence of the number of benzene rings on the bonding nature or interactions is great. Increasing the number of benzene rings can shorten the Au-C distance and enhance the Au-C bond strength. However, when the number of benzene rings is 9, the Au-C distance does not change. Adding a gold atom to the pentacene obviously changes the transition nature of pentacene and results in the increase of possible transitions. These systems possess moderate molecular second-order polarizabilities compared with the organometallic and organic complexes. Theβvalue of system 4 is much larger than that of system 1. Thus, subtle variations in the molecular architecture result in substantial enhancement to the second-order NLO response.3. The natural, cation and anion structures of 1,6-bis(2-hydroxyphenol)pyridylboron bis(4-n-butylphenyl)phenyleneamine were optimized with the B3LYP functional. The charge transport properties were investigated within the framework of the charge hopping model. The results show that 1,6-bis(2-hydroxyphenyl)pyridineboron ((dppy)BF) functions acts as a electron transport group and triphenylamine as a hole transport group; the charge transport ability for the two types of carriers is not only high but also nearly balanced, which explains why it is an efficient single-layer electroluminescent device. On the basis of the large second-order polarizability value and high transparency, this compound has the possibility to be an excellent second-order nonlinear optical material. The main origin of this large second-order nonlinear optical response is charge transfer from the triphenylamine group to (dppy)BF.4. Electronic spectrum of organoimide derivatives of hexamolybdates have first been calculated within the time-dependent density-functional theory. The results show that organoimide acts as donor and hexamolybdates as acceptor. Those derivatives possess donor-acceptor-donor (D-A-D) configurations. The accurate electronic absorption spectrum of organoimide derivatives of hexamolybdates can be achieved using the GRAC potential. Adding the organoimide to [Mo₆O₁₉]^2- can increase the transition probability compared to [Mo₆O₁₉]^2-. The transition nature of the diagonal-substituted derivatives is different from that of the orthogonal-substituted derivatives. The organoimide derivatives of hexamolybdates are found to possess remarkably larger static third-order polarizabilities. For our studied systems, increasing the conjugation length and diagonal substituted are efficient ways to enhance the third-order polarizability.5. We have investigated the electronic structure and the second-order nonlinear optics properties of the asymmetric spirosilabifluorene derivatives and elucidated structure-property relationships from the micromechanism. The results show that these compounds possess many favorable features for application in the second-order nonlinear optical field. First, these compounds have remarkably larger molecular second-order polarizabilities compared with the typical organic compounds. Second, all the compounds generate large nonresonant optical nonlinearities over a wide frequency zone, which can be used for a frequency conversion optical material. Third, a small dipole moment can guarantee manifestation of the large macroscopic second-order susceptibility. Fourth, they have high transparency in the visible light area.