Abstract
Second-order nonlinear optical (NLO) materials have attracted interest because of their potential applications in optoelectronic technology. Organic molecular materials and polymer systems have emerged in recent years as a new class of promising nonlinear optical material because they offer the flexibility, both molecular and bulk levels, to optimized the nonlinearity and other required properties for device applications. However, some crucial issues such as thermal stability, temporal stability of dipole orientation, and optical loss are important challenges to material scientists. In this work, a series of new chromophores based on two-dimensional (2-D) charge transfer architectures have been developed. The intrinsic and the macroscopic properties of the two-dimensional charge transfer molecules have been understood via studying the intrinsic properties of the two-dimensional charge transfer molecules and guest host systems.Intrinsic characteristics of the 2-D carbazole chromophores: Taking advantage of the multifunctional characteristics of carbazole along with rational molecular design, a series of two-dimensional (2-D) carbazole chromophores were synthesized by a facile synthetic route. In this work, a systematic investigation of correlation between chromophoric chemical structures and intrinsic properties has been investigated. Thermal properties of the 2-D chromophores were characterized via DSC and TGA thermograms. The hyperpolarizabilities of the chromophores were acquired via solvatochromism. The conjugation length and coplanarity of the chromophores affect the thermal properties and the first hyperpolarizabilities. Melting point (Tm) and hyperpolarizabilities of the chromophores increase as the conjugation length. Strong acceptor group (NO2) and excellent coplanarity, the chromophore Cz2PhNO2 possesses largest first hyperpolaribility. Less coplanarity leads the chromophore Cz2PhSO2 possessing smaller first hyperpolaribility owing to the steric structure of the sulfonyl groups.Guest-Host systems Based on 2-D Carbazole chromophores: To observe the macroscopic NLO properties, an organosoluble polyetherimide was synthesized. By doping the 2-D chromophores into the polyetherimide, a series of guest-host NLO polymers were acquired. High doping levels, i.e. Cz2PhSO2 with 38-wt%, were obtained without observing aggregation of NLO chromophores. The compatibility between polyetherimide and chromophore was investigated by SEM and extraction experiment. In the aspect of compatibility, the molecular weight distribution of the polyetherimide plays an important role. The second harmonic coefficients (d33) for the guest-host system are ranged from 2 to 23 pm/V dependent on the chromophore and doping level. The effect of two-dimensional structure on the NLO temporal stability was investigated by tracing the second harmonic coefficient as functions of temperature and time, respectively. Moreover, the relaxation behavior of the NLO systems was further examined by dielectric analysis. Large rotational corn volumes make the two-dimensional chromophore possessing excellent orientational stability when the temperature approaches to the glass transition temperature observed from DEA. The chromophore size effect dominates temporal stability of the oriented dipole, when operation temperature is low (T<T0). As the temperature elevates, the rigidity of the chromophore dominate the stability of the oriented dipole. Flexible and large sulfonyl group enlarge the free volume of the polymer PI2Cs. This leads its possess lower T0 and less temporal stability at higher operation temperature.Λ-shaped NLO polyurethane based on 2-D chromophore: A series of new NLO-active polyurethanes based on a two-dimensional NLO chromophore have been investigated. Two ends of this lambda-shaped chromophore can be directly bound to the main chain of polyurethanes. These polyurethanes possess excellent solubility. The polymer thin films easily aligned under applied field. After poling, large second harmonic coefficients of 33.8 pm/V was found for MDI-PU. This improves the solubility and poling problem for most of “head-to-tail” main-chain polymers. The effect of Λ-shaped architecture on the NLO temporal stability was investigated by tracing the second harmonic coefficient as functions of temperature and time, respectively. Fast relaxation of the effective SH coefficient was observed at temperatures higher than 122 ℃ for MDI-PU. Moreover, excellent temporal stability at 100 ℃ was observed despite that the operating temperature is very close to the fast relaxation temperature. Embedding the lambda-shaped chromophores onto the polymer backbone decrease the plasticizer effect of the chromophore. Moreover, enlarging the chromophore size via two-dimensional structure the rotational corn volumes were increased. These factors effectively restrict molecular motion at temperatures close to Tg.All organic NLO sol-gel systems: Sole-gel materials have potentially application for nonlinear optics because of its low-temperature processing capability, excellent optical quality, high temporal stability and ease of device fabrication. In this work, an all-organic sol-gel material based on (hydroxymethyl) benzoguanamine (HMBG) was developed. To correlate the structure of the chromophore and the NLO properties, a series of NLO active chromophores based on amino azobenzene dyes and stilbenzoid carbazole have been synthesized. All organic NLO-active materials were obtained by introducing the chromophores into (hydroxymethyl) benzoguanamine (HMBG) systems by sol-gel process. All the sol-gel materials exhibit excellent optical transparency. Morphology, thermal and optical properties of these polymers have been characterized. These NLO-active polymers can be classified into reactive systems and guest-host systems. In the reactive systems, the chromophores were condensed with HMBG in the sol-gel process, respectively. In the guest-host systems, the nonreactive chromophores were simply doped into HMBG matrix, respectively. Optimized poling/curing conditions were performed through different process. A conveniently optimized method was performed in the sol-gel materials via tracing the characteristic absorbance as a function of temperature during a certainly period. A second harmonic coefficient, d33, of 23.2 pm/V has been obtained. Moreover, a systematic investigation of variation of chromophoric chemical structures on NLO properties has been investigated. Covalent bonding effect between chromophore and benzoguanamine as well as steric effect are shown to greatly enhance the temporal stability for these NLO-active sol-gel systems.