Theoretical Study On The NLO Properties Of New Types Of Conjugated Molecules (Including Rubrene And Open-Cage Fullerene Derivatives)

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 (NLO) field, due to their large nonlinear optical coefficient, fast nonlinear optical response times, relatively low cost, ease of fabrication and integration into devices, tailor ability which allows one to fine tune the chemical structure and properties for a given nonlinear optical process, high laser damage thresholds. In this thesis, in order to design and select organic materials with large two-photon absorption response, we have performed systematic theoretical research of novelÏ€-conjugated materials. The quantum chemical methods-density functional theory and AM1 have been applied to obtain the molecular equilibrium geometries. On the basis of the optimized molecular geometries, one- and two-photon absorption properties are obtained by ZINDO-SOS method. In addition, we also performed systematic investigations on the structure, stability and NLO properties of open-cage C₆₀ and C₅₀ derivatives. These results may provide a theoretical basis of designing novel fullerene derivatives with large NLO coefficients. Our work has been focus on the following five aspects:1. One- and two-photon absorption properties of a noteworthy perpendicularÏ€-electron system rubrene have been investigated. Calculation results show that relatively larger two-photon cross sections (δ_max) of rubrene than that of tetracene verify that there exits CT from phenyl groups to the tetracene backbone. So introducing symmetrical CT is also one of effective ways to enlarge theδ_max. By enlarging theÏ€-conjugation extension of rubrene, theδ_max show a distinct increase, However, attaching electron donating amino group or electron withdrawing nitro group to rubrene bring apparent decrease ofδ_max in contrast with rubrene. So from the molecular designing aspect, the extension ofÏ€-conjugation is the best choice for relatively largerδ_max.2. One- and two-photon absorption properties of a series of three-branched and two-branched as well as one-branched oligofluorenes with boron center have been theoretically investigated. Changing the number of branch brings significant enhancement on theδ_max values. CBTFn has the largestδ_max with respect to CBDFn and CBSFn. The comparisons ofδ_max with simplified exciton model show that the effective coupling between boron center and individual fluorene arm is relatively strong. Moreover, the coupling in CBTFn is larger than that in CBDFn, which also proves that the three-branched ologomer is a better choice for attaining largerδ_max than the two-branched counterpart when the coupling between the center and the individual arm is allowed. So CBTFn is a kind of promising TPA materials for optical power limiting. 3. One- and two-photon absorption properties of a novel PPV derivative poly 2-(9-phenylanthracen-10-yl)-1,4-phenylenevinylene (P1), which are highly soluble in common organic solvents, have been investigated. The pendent groups bring a significant decrease on theδmax value of P1 compared with PPV. This decrease can be explained by the influence from pendent groups on the effectiveÏ€-conjugation length of P1. In addition, the charge transfer contributing to TPA of P1 is through backbone to pendent group, so the pendent group of P1 can be regarded as a good two-photon absorber. From the application point of view, on one hand, it should introduce certain group to make PPV easy fabrication. On the other hand, it should keep goodÏ€-conjugation extension of PPV backbone. In general, for a promising TPA materials for application, the above two aspects should be taken into consideration.4. The structural and electronic as well as NLO properties of a series of open-cage oxo fullerene C₆₀ derivatives: C₅₅O₅ and C₅₄O₆ have been theoretically investigated. Calculation results indicate that C₅₅O₅ is a chemically stable structure with a larger band gap compared to C₆₀ calculated at the same theory level. The cohesive energy calculations reveal that the formation of C₅₅O₅ may be easier than that of C₅₄O₆, which may provide useful guidance for experiments. Therefore, it makes us to believe that the experimental synthesis of C₅₅O₅ will certainly be able to achieve in the near future. When the cage of C₆₀ is opened, it induces relatively larger second-order NLO response, thus it opens a novel route towards the research on searching new type of NLO materials based on C₆₀ derivatives. Analysis of the main contributions to the second-order NLO response of C₅₅O₅ and C₅₄O₆ reveals that charge transfer along the z-axis direction plays a key role.5. The structural and electronic properties as well as NLO properties of a series of open-cage oxo D5h C₅₀ derivatives: C45O5 and C44O6 have been theoretically investigated. Calculation results indicate that C₄₅O₅F₁₀, which is from D5h C₅₀ open-cage oxo derivative C45O5 and then saturated with ten fluorine atoms at pentagon-pentagon vertex fusions, is a chemically stable structure with a larger band gap compared to C₆₀ calculated at the same theory level. The cohesive energy calculations reveal that the formation of C₄₅O₅F₁₀ may be easier than the open-cage oxo D5h C₅₀ derivatives: C45O5 and C44O6 (including their corresponding halogen passivated derivatives). Therefore, it makes us to believe that the experimental synthesis of C₄₅O₅F₁₀ will certainly be able to achieve in the near future. The dynamic second-order NLO response of C₄₅O₅F₁₀ shows small dispersion behavior in a wide frequency range from 0 to 1.5 eV under second harmonic generation (SHG) process, showing that C₄₅O₅F₁₀ is available to be used for frequency conversion optical materials. Analysis of the main contributions to the second-order NLO response of C₄₅O₅F₁₀ reveals that charge transfer along the z-axis direction plays a key role.