Design, Synthesis And Characterization Of Macromolecules With Large Second-Order Nonlinear Optical Effects

The21st century is an age of global information. Second-order nonlinear optical (NLO) materials have attracted considerable interest because of their huge potential applications in optical telecommunication, data storage, information processing, THz imaging and so on. In comparison with traditional inorganic NLO crystals, organic/polymeric NLO materials have been put forth as promising candidates for the future NLO applications, due to their numerous advantages such as ultrafast response time, large nonlinearity, good processability and so on. Large optical nonearity, low optical loss, and excellent temporal stability of the dipole orientation are basic requirements of practical applications. Of these, the efficient translation of the large ?? values of the organic chromophores into high macroscopic NLO activities of the polymers is the major problem, because the strong dipole-dipole interactions between the chromophore moieties make the pole-induced noncentrosymmetric alignment of chromophore moieties a daunting task during the poling process under an electric field.In this thesis, we focused on how to design and synthesize second-order NLO macromolecules with large NLO coefficients. Based on this subject, we prepared104second-order NLO macromolecules (including28linear polymers,29hyperbranched polymers and47dendrimers) and15model molecules, explorered the relationship between their structures and properties. The main contents and discoveries are as follows:1. Chapter1reviewes the literartures in this research field. Based on a brief introduction to second-order NLO materials, the design and synthesis of organic/polymeric materials was focused, and the future directions of this area were also discussed. Finally, the strategies and main contents of this thesis were outlined.2. The first part of this thesis is from Chapter2to Chapter8, which focused on isolation chromophore. According to "site-isolation principle" and "suitable isolation group", it is an effective approach for minimizing this interaction and enhancing the poling efficiency to introduce some bulky isolation spacers with suitable size onto chromophores. However, the introduction of isolation group could also decrease the effective concentration of the NLO chromophore moieties. Considering this, in Chapter2and3, we tried to use one chromophore with a lower ? value as an isolation group for another chromophore group with a higher ? value. The result showed that the introduction of chromophore moieties with lower ? values could decrease the strong electronic interactions between chromophore moieties and enhance the poling efficiency. Also, the chromophores with lower ? values can directly contribute to the macroscopic NLO effect; meanwhile, due to the presence of the chromophore with lower ? value, the optical transparency of NLO polymers could be also improved. Based on these results, we proposed the concept of "isolation chromophore". In Chapter4and5, we designed and synthesized a new type of "half main chain" NLO polymers, in which main chromophore was in the main chain and isolation chromophore was on the side chain. In this type of NLO polymer, the introduction of "isolation chromophore" could improve the poling efficiency of NLO polymers, and the "half main chain" structure could further improve the stability of the dipole orientation after poling. In Chapter6-8,"isolation chromophore" was introduced into dendritic macromolecules with good isolation effect. The result showed that the hyperbranched polymers and dendrimers, with two types of chromophore regular alternating arrangement and the main chromophores at the periphery, demonstrated large NLO coefficients. For example, the NLO coefficient of D4, the fifth generation dendrimer, were tested as253.0pm/V.3. The second part of this thesis is from Chapter9to Chapter14, which focused on Ar-ArF interactions. Different from normal aromatic rings, pentafluoroaromatic rings were electropositive, which could lead to strong electrostatic interactions between the pentafluoroaromatic rings and the normal benzene ones. In Chapter9and10, pentafluoroaromatic ring was used as isolation groups of linear NLO polymers instead of normal phenyl rings. The result showed that there were strong electrostatic interactions between pentafluoroaromatic rings and polymer main chain, which could lead to better poling efficiency and stability of the dipole orientation after poling. In Chapter11and12, this Ar-ArF interaction was further used in hyperbranched polymers. The results showed that both co-polymer unit and position of pentafluoroaromatic could affect the NLO coefficient of NLO hyperbranched polymers. For the AB2-type hyperbranched polymers, the pentafluorophenyl in the periphery produced higher NLO coefficient than the normal phenyl in the periphery, whereas the perfiuoroaromatic rings in the interior architecture produced slightly lower NLO coefficient. By utilizing this point into dendrimers, in Chapter13and14, we prepared a series of NLO dendrimers with different chromophores and different types of isolation groups. The result showed that the different strong electrostatic interactions between pentafluoroaromatic rings in the periphery and the donor of chromphore in the interior could improve the topological structure of the dendrimers, and further enhance their poling efficiency. Furthermore, the concept of "isolation chromophore" still works in the dendrimrs with pentafluorophenyl in the periphery. By introducing isolation chromophore into the high generation dendrimers with pentafluorophenyl in the periphery, their NLO coefficients were further improved, and the NLO coefficient of fifth generation dendrimer D22were tested as257pm/V, which was the highest value so far for simple azo chormophore.4. The third part of this thesis is from Chapter15to Chapter19, which focused on star-type chromophore and global-like dendrimers. In Chapter15, by linking three chromophores into a branched core, we designed and synthesized a new type of chromophore, which could be named as star-type chromophore. The shape of the star-type chromophore was close to the spherical shape, the ideal configuration proposed by Dalton, leading to relative high NLO coefficient. In Chapter16-18, by using this star-type chromophore as core, low generation AB2-type dendrimers as branched unit, some new NLO dendrimers were prepared. In comparison with normal AB2-type dendrimers, the shape of these new prepared ones was more like spherical shape, and thus, we called them "global-like dendrimers" to differentiate the normal ones. Due to their perfect site-isolation effect, all of these global-like dendrimers showed large NLO coefficients (13of them were higher than200pm/V). In Chapter19, considering that the synthesis of higher generation dendrimers was usually tedious and costly, which was unfavorable for the real application, we tried to simplify the synthetic route to the high generation dendrimers by using "orthogonal" approach and "one-pot" method. Through the simplified route, the fourth generation global-like dendrimer could be prepared with the yield of49.2%by two "one-pot" steps.5. The fourth part of this thesis is from Chapter20to Chapter23, which focused on the topological structure of NLO polymers. In Chapter20and21, the topological structure of NLO hyperbranched polymers was improved by enhancing the degree of branching. Through the AB4or AB2+AC2approach, two hyperbranched polymers were prepared. From their NMR spectra, their degree of branching was determined as0.691and0.791, respectively. The relative high degree of branching meant better site-isolation effect of hyperbranched polymers, which could further enhance the NLO coefficients of hyperbranched polymers. In Chapter22and23, by combining dendrimers with linear polymers (or hyperbranched polymers), five new types of polymers were prepared, including two types of dendronized polymers, one type of hyperbranched polymer with low generation dendrimers as side chain, two types of hyperbranched polymers with low generation dendrimers as monomers. Among them, the hyperbranched polymers with low generation global-like dendrimers as monomers demonstrated the best comprehensive performance, and the NLO coefficient of HP28was tested as166pm/V. Furthermore, its onset temperature for d33value was also up to177?.