| Second-order nonlinear optical (NLO) polymeric materials have attracted considerable attention due to their potential applications in broad bandwidth telecommunication, information processing, THZ imaging and radar systems, because they possess large and rapid NLO responses, low dielectric constant, high laser damage thresholds, low cost, superior chemical flexibility and ease of processing as compared to conventional inorganic crystalline materials. However, it is still a challenge to design and synthesize NLO polymers with large NLO effects, high temporal stability and low optical loss to fit the requirements of electro-optic (EO) apparatus, especially it is very difficult to translate large hyperpolarizability (β) of the organic chromophores into macroscopic optical nonlinearity of polymers, due to the strong intermolecular electrostatic interactions among high dipole moment chromophores. To introduce some bulky isolation spacers into chromophores or lock the chromophores inside the dendrimer shell has been demonstrated an effective approach for minimizing this interaction and enhancing the poling efficiency. However, the poling efficiencies and the macro-scale NLO properties of polymers containing dendronized chromophore as side groups, are expected to be heavily related to the subtle difference in architectural design. Thus, it is needed to study the structure-property relationship in detail. Especially, for a given chromophore, there is still very scarce information about how to control its shape to achieve optimized poling efficiency. In this thesis, various new series of NLO polymers were designed and synthesized, in which the isolation groups in different size were introduced into chromophore moieties, thus we could investigate the effect of the size of isolation spacer on the resultant NLO properties in detail. Moreover, by the usage of "click chemistry" or Sonogashira coupling reaction, a new series of NLO dendrimers, dendronized polymers and hyperbranched polymers with good comprehensive propeties were prepared successfully, while the relationship between dendritic structure and macroscopic properties was studied.Organic/polymeric light-emitting diodes (O/PLEDs) have been attracting great attention in the past decades due to their potential applications in flat display, white light source and telecommunication, where they exhibit advantages over existing technology, such as high efficiency, high brightness, wide angle viewing, low cost and consumption, low voltage drive, fast response, light weight, superior chemical flexibility, ease of processing, possibility of full-color and large-area flexible flat panel display, etc. However, organic/polymeric materials possessing high efficiency, low cost, high stability and long durability simultaneously has been still in the development of O/PLEDs. In comparison with well-known linear oligomers and polymers, dendritic conjugated macromolecules with large branches exhibit intrinsic two- or three-dimensional architectures, which can effectively depress the possible aggregation and excimer formation to enhance the thermal stability and improve the light emitting efficiency. Also such dendritic structure can make the materials forming good quality amorphous films. In this thesis, a new series ofπ-conjugated dendrimers and hyperbranched conjugated polymers used as emitting layer or hole-transporting layer have been developed by simple synthetic route.This thesis contains two parts. The first part concerns the synthesis, characterization and NLO properties of novel linear polymer, dendrimers, dendronized polymers and hyperbranched polymers (Chapter 2-20). The second part concerns the synthesis, characterization and electroluminescent or hole-transporting properties ofπ-conjugated dendrimers and hyperbranched conjugated polymers (Chapter 21-26). The main contents and results are described as following:Recent progress of NLO materials is reviewed in Chapter 1. Based on a brief introduction to EO modulators, the design and synthesis of novel dendritic EO materials was focused, mainly including EO dendrimers, dendronized EO polymers, supermolecular self-assembled EO polymers and hyperbranched EO polymers. Then the future directions of the EO polymers are discussed. Finally, the strategies and main contents of this thesis have also been outlined.Since there are still scarce information concerned on the relationship between the structure of isolation spacers and the resultant macroscopic NLO effects of the polymers, in Chapter 2, two series of polyurethanes P1-P10 containing dendronized NLO chromophores as side chains were prepared, in which the size of isolation groups in theπbridge near the donor side of NLO chromophore moieties changed from small atoms to much larger groups. Thus we could investigate the structure-properties relationship in detail. The experimental results demonstrated that for nitro-based chromophores, the best isolation groups were carbazole (Cz) moieties, and the d33 value of its corresponding polymer P4 was up to 82.3 pm/V,1.46 times that of P1; for sulfonyl-based chromophores, P6 with phenyl rings (Ph) as the isolation groups exhibited the highest d33 value,1.76 times that of P5. The obtained experimental results were summarized as the concept of "suitable isolation groups":for a given chromophore moiety and given linkage position in a given polymeric system, there should be a suitable isolation group present to boost its microscopicβvalue to possible higher macroscopic NLO property efficiently.In Chapter 3 and 4, to further investigate the relationship between the structure of isolation spacers and the resultant macroscopic NLO effects of the polymers, we changed the bonded position and species of isolation groups, and introduced different isolation groups into chromophore moieties in theπbridge near the acceptor side to prepare polymers P11-P19 through Sharpless’s "click chemistry" or Sonogashira coupling reaction. The obtained experimental results demonstrated that the NLO effects of polymers can be enhanced effectively by the introduction of suitable isolation groups, while the d33 value of P17 was up to 89.7 pm/V,1.62 times of that of Pll without any isolation groups, further confirming the concept of "suitable isolation groups".In Chapter 5 and 6, to solve the mainly problem encountered in the development of main-chain polymers:the low poling efficiency, we designed and synthesized a series of main-chain polyurethanes P20-P27 with the structure well adjusted by introducing different isolation groups into chromophore moieties.In addition, for comparison, the corresponding side-chain polyurethanes P28-P31 were also prepared. The SHG test results further confirmed the concept of "suitable isolation groups", while the processability and temporal stabilities of polymers were also improved.In Chapter 7, to develop more NLO polymers with encouraging performance, and broaden the applyication area of "suitable isolation group", we choosed PVK as polymeric backbone, synthesized a series of PVK-based NLO polymers P33-P39 with different isolation groups introduced through post-Knoevenagel condensation reaction. The obtained results indicated that by introducting suitable isolation groups into PVK-based NLO polymeric system, the existed problem of this system (bad processability and poling efficiency) could be solved effectively, as well as the temporal stabilities were still relatively good, which demonstrated that the "nonlinearity-stability trade off" might be alleviated in some degree by applying the concept of "suitable isolation groups". In addition, a new side-on PVK-based NLO polymer P40, with different structure from P33-P39, was prepared, in which the linking position of spacer on the chromophore was at its center bridge. P40 exhibited a relatively high d33 value (56.0 pm/V), in comparison with that of P35, containing the same active chromophores and isolation groups.For the first time, the "click" chemistry reaction was applied to synthesize a series of conjugated polymers P41-P46 containing NLO azobenzene chromophores as side chains through polymer reactions in Chapter 8. The synthetic route was very simple and high efficient, due to the perfect quality of click reaction. All the polymers demonstrated good solubility, processability, thermal stability, improved optical transparency, and the SHG test demonstrated the NLO effect of polymers could be enhanced by introducing suitable isolation groups, while P46 exhibited a better d33 value (39.6 pm/V), nearly two times that of P44 without any isolation groups. In order to decrease synthetic procedure and improve the synthetic efficiency, the isolation groups in different size were introduced into the chromophore moieties in polymers to yield two series of NLO polyurethanes through polymer reactions by using the "click chemistry" in Chapter 9, making the subtle structure of the polymers being modified conveniently. And in Chapter 10, a series of polyurethanes (P59-P61) containing sulfonyl-based NLO chromophores, which could not be obtained from the direct synthetic route, were prepared through a two-step polymer reaction, consisting of the post azo coupling and esterification reactions conveniently with high yields. In comparison with those of the polymers through directly copolymerization reaction, the designed route of these two chapters was very simple, and the purification was very easy, making their preparation much effective. In addition, because the polymers with different isolation groups were derived from the same mother polymers, we could analyze their NLO properties nearly on the same level.In Chapter 11 and 12, based on control experiments, a new synthetic route was proposed to yield a series of nonlinear optical polymers P61-P69 and P71-P73, which contained the same isolation spacers, respectively. For the first time, the effect of the same isolation group on chromophore moieties with different acceptors, was studied. The obtained NLO properties showed that the same isolation group would result in different influence while the chromophore moieties were different. Or it is said, the same isolation group would lead to different effect in different NLO chromophores, which further confirmed the concept of "suitable isolation groups" on another side, but more powerful. It also should be pointed out that in P71-P73, pyrrole ring was firstly introduced into NLO polymers used as the bridge. P71 exhibited relatively good NLO properties, of which d33 value was up to 67.1 pm/V, and the onset temperature for decay was as high as 140℃.In Chapter 13, for the first time, a series of dendron-like main-chain polyurethanes P74-P77 embedded with "H"-type chromophores have been successfully designed and synthesized, and they all demonstrated very good NLO effects, while the d33 value of P75 was as high as 127.7 pm/V, indicating the superiority of "H"-type structure. The UV-vis spectra demonstrated that the effective isolation effect could be realized, when the chromophore moieties were well encapsulated by the bulky isolation spacers surrounded. By applying the concept of "suitable isolation groups", different size of isolation groups were introduced to modify the subtle structure of the used "H"-type chromophores, and theΦvalue of P75 with BOE as suitable isolation groups was 1.3 times that of P74, indicating the poling efficiency may be adjusted by controlling the shape of the "H"-type chromophores. However, because of the reduced loading density of chromophores, the d33 value of P75 was only enhanced about 10 pm/V. In Chapter 14, by changing the lingkage position of H"-type chromophores embedded into polymeric backbone, a new series of shoulder-to-shoulder polymers P78-P79 and P81-P84 were synthesized. The SHG test demonstrated that all the polymers exhibited very good NLO effects, and the highest d33 value was up to 94.7 pm/V. The results also indicated that the structure of polymers could be adjusted much better by introducing suitable isolation groups.In Chapter 15, to develop more comprehensive dendritic NLO materials with good performance and study their structure-properties relationship, the Sharpless "click" reaction, as well as azo coupling reaction were applied for the construction of NLO dendrimers G1-G3 easily. Also, through a "double-stage" method, in which the core (G2-8N3) was prepared through a divergent approach, while the end-capped dendrons (G1-≡and G2-≡) were synthesized through a convergent approach, high generation of dendrimers G4 and G5 were synthesized successfully. By the combination of "double-stage" method and click chemistry, the whole synthetic route was very convenient and with high-efficiently, which might give light on the syntheses of other functional dendrimers from more economic preparation routes. The resultant dendrimers demonstrated good solubility, processability, thermal stability, improved optical transparency, and the UV-vis spectra indicated that the effective isolation effect could be realized. The formed triazole rings were utilized as good isolation groups to minimize the strong dipole-dipole interactions among the chromophore moieties to enhance NLO effects. Accompanying with the increasing of the loading density of the chromophore moieties, the tested NLO effects are going higher, partially proving the prediction of Dalton et al and indicating that the frequently observed asymptotic dependence of electro-optic activity on chromophore number density may be overcome through rational design. The five generation of dendrimer G5 exhibited very excellent NLO properties, of which the d33 value was as high as 193.1 pm/V, as well as the onset temoeratture for decay was over 100℃, making it good candidate for optics applications.In Chapter 16, based on the work of Chapter 8 and Chapter 15, by the click reaction between P-N3 and Gl-≡or G2-≡, two new dendronized NLO polyfluorenes P85 and P86 were synthesized with high chromophore loading density by the introduction of high generation chromophore dendrons on the side chains. Thanks to the advantages of Sharpless’ click chemistry reaction, the dendrons were conveniently bonded to the backbone of polyfluorene with the conversion of 100%, although they were really very bulky. P85 and P86 exhibited relatively good NLO effects, their d33 values were up to 91 and 106 pm/V, respactively. In addition, similiar resluts to Chapter 15 were obtained in this chapter:accompanying with the increasing of the loading density of the chromophore moieties, the tested NLO effects were going higher, further confirming the frequently observed asymptotic dependence of electro-optic activity on chromophore number density may be overcome through rational design.In Chapter 17, by using triphenylamine as the core, and the chromophores as branches, a new series of one-, two- and three-dimension NLO dendrimers T1-T3 were designed and synthesized succsessfully to investigate the effect of the shape of macromolecules on macroscopic NLO properties. To decrease the optical loss of the NLO dendrimers, 1,2,3,4,5-pentafluorobenzene moieties were choosed as isolation groups. The SHG test demonstrated that accompanying with the increasing of the dimension, the d33 values increased rapidly, and T3 exhibited a very high d33 value of 191.8 pm/V. This should be attributed to the size of macromolecules enlarged accompanying with the dimension increased, making their shape much nearly to sphericity.Hyperbranched polymers, a second class of branched polymers exhibiting similar unusual properties to dendrimers, are typically obtained in a one-pot reaction and can be easily prepared in larger quantities. In Chapter 18, we designed and synthesized one new AB2-type NLO hyperbranched polymers P87 via simple Sonogashira coupling reaction. Its corresponding linear analogue (P88) was also obtained from AB monomer for comparison. In spite of the lower effective chromophore density, P87 demonstrated enhanced second-harmonic coefficient (153.9 pm/V) and temporal stability (the onset temperature for decay was over 93℃) than those of P88. In addition, by modifying the synthetic procedure, the previous reported impossible approach was successfully utilized to construct new azo-chromophore-containing hyperbranched polymers (P89 and P90) from AB2 monomers through click chemistry reactions with the aid of copper(I) catalysis in Chapter 19. The NLO properties of P89 and P90 were relatively good, the d33 values were up to 77.9 and 124.4 pm/V, respectively, while the onset temoeratures for decay were 118℃and 93℃, respectively. However, both of these two polymers exhibited bad film-forming ability, which did not benefit to their applications. Thus in this chapter, according to the concept of "suitable isolation groups", the end-capped groups in different size were introduced into the periphery of the same hyperbranched polymer intermediate to afford six AB2-type polymers P91-P96 by a simple "one-to-six" process. The obtained results demonstrated P94-P96 exhibited much better solubility, processability, photo-physical properties than P90, which according with our original idea. The SHG test demonstrated that the change of the size of end-capped groups influenced the reslutant NLO properties in some degree, and Car moieties were the suitable end-capped groups.In Chapter 20, a facile route was designed to synthesize a new series of main-chain hyperbranched polymer P97 and P99-P104, and we expected these mid-type NLO polymers would have both of the merits of hyperbranched and main-chain NLO polymers:large optical nonlinearity and high stabilization of dipole moments. The obtained resluts accorded with our original idea, the d33 value and the temperature for decay of P97 were 143.8 pm/V and 153℃, much better than its correspongding linear main-chain polymer P98 (52.9 pm/V and 119℃). The same excellent results were obtained in P99-P104:the d33 value of P99 with nitro-based chromophores was up to 152.6 pm/V, and the d33 value of P101 with sulfonyl-based chromophores was up to 82.6 pm/V, and they also exhibited good temporal stabilies. In addition, our results may provide a new solution to solve the "nonlinearity-stability trade off’ existing in main-chain polymers to some degree.Dendritic conjugated macromolecules with large branches exhibit intrinsic two- or three-dimensional architectures, which can effectively depress the possible aggregation and excimer formation to make them exhibit good film-forming ability, enhance the thermal stability and improve the light emitting efficiency. Therefore, in Chapter 21, based on a brief introduction to OLEDs, we designed a simple route to synthesize a new series ofπ-congjugated dendrimers D1-D10 with mono-, bis, tri-covalent bond introduced. The dendrimers were well characterized by TGA, DSC, UV-vis and PL. Some dendrimers were used as emitting layer to construct double-layer devices:ITO/dendrimer/TPBI/LiF/Al, and the D3-based device exhibited a maximum brightness of 1325 cd/m2, and current efficiency of 1.03 cd/A, while D7-based device exhibited a maximum brightness of 1190 cd/m2, and current efficiency of 1.67 cd/A. In addition, we also investigated the hole-transporting ability of D3 and D8 through the fabrication of the device ITO/dendrimer/Alq3/LiF/Al using D3 and D8 as hole-transporting layer, and their device exhibited a maximum luminance of 5262 and 6283 cd/m2, respectively, and a maximum current efficiency of 1.70 and 1.59 cd/A, respectively.Considering that the widely-used hole-transporting material TPD and NPB exhibited low glass transition temperature and bad film-forming ability, in Chapter 22, an effective synthetic route was designed to construct two newπ-congjugated dendrimers DTI and DT2 through a convergent approach. Both of these two dendrimers contained 22 triphenylamine units, and to the best of our knowledge, they are the largest size of dendrimers constructed only by triphenylamine moieties. DT1 and DT2 exhibited excellent thermal and morphological stabilities, and their glass transition temperatures were as high as 257 and 241℃, respectively, much higher than those of TPD and NPB. The film quality of DT1 was not good by spin-coating, while that of DT2 was much better, thus we fabricated the device using DT2 as hole-transporting layer to study its hole-transporting ability. The device exhibited a maximum current density of 400 mA/cm2, maximum luminance 5024 cd/m2, current efficiency of 2.37 cd/A and power efficiency of 2.40 lm/W.To develop more hyperbranched polyfluorenes and obtain excellent light-emitting materials, in Chapter 23, a new series of hyperbranched conjugated polymers P105-P107 containing fluorene moieties were synthesized from "A2+B3" approach based on Suzuki polycondensation reaction. Interestingly, P107 constructed only by fluorene moieties exhibited green luminescence, not like other PF derivatives with blue emissions, which might be due to the presence of theπ-stacked structure of poly(dibenzofulvene), based on several controlling experiments. And their film fluorescence spectra of their thin films were quite stable against heating, indicating the hyperbranched molecular structure really effectively hampers the aggregation formation of the polymer backbone or improves the resistance to the keto defect formation. The PLED device based on P107 as emitting layer exhibited a maximum luminance 142 cd/m2, and current efficiency of 0.15 cd/A.In Chapter 24, for the first time, hexaphenylbenzene moiety was used as the core to afford a new series of hyperbranched light-emitting polymers P110-P113 by introducing 1, 3,4-oxadiazole units in different molar ratios through a simple synthetic route. All the double-layer devices using P110-P113 as emitting layers exhibited deeply blue emission, and the spectra were very pure with a fwhm of about 40nm. P110-based device exhibited a maximum luminance 251 cd/m2, current efficiency of 0.46 cd/A, which is very good in comparion with "A3+B2"-type hyerbranched polymers reported in literatures, while the deficiency was that the turn-on voltage of device was much higher (12 V). The introduction of oxadiazole units could decrease the turn-on voltage, as well as enhance their luminance and current efficiency. When the molar ratios of three monomers (hexaphenylbenzene, oxadiazole and fluorene) was 1:1:2.5, the resultant polymer P113-based device exhibited better results, its maximum current density, luminance and current efficiency were up to 358 mA/m2,549 cd/m2 and 0.72 cd/A, respectively.For a good OLED device for practical applications, it is desired to control and achieve the charge balance, so that the recombination of the hole and electron occurred in the ETL away from electrodes to improve the emitting efficiency. In Chapter 25, we synthesized a novel hyperbranched polymer (P114) constructed only by carbazole moieties from "A4+B2" approach via Suzuki polycondensation reaction. The P114-based device yielded much better efficiency (3.05 cd/A), than that of PVK-based device (2.19 cd/A), under similar experimental conditions, which should be attributed to its low energy barrier, and enhanced hole-drifting ability in the P114 based device. In addition, for the first time, field effect transistors (FET) based on hyperbranched materials were fabricated, and the OFETs device displayed that P114 is a typical p-type FET operation with a saturation mobilityμ=1×10-5 cm2V-1s-1, the threshold voltage VT=-47.1 V and on-to-off current ratio Ion/Ioff=103. In Chapter 26, according to the excellent properties of P114, the core of P114 was utilized to develop a new series of hyperbranched light-emitting polymers P115-P117 via Sonogashira coupling reaction, and their electroluminescent properties were investigated. P116 demonstrated good performances, and the device of ITO/P116/TPBI/LiF/Al exhibited a maximum luminance of 138 cd/m2, and current efficiency of 0.26 cd/A; another device of TO/P116/TNS/Alq3/LiF/Al exhibited a maximum luminance of 638 cd/m2, and current efficiency of 1.09 cd/A. |
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