Poly Miscellaneous Fluorene Synthesis, Structure, And Optical And Electrical Properties

The constant research interests on conjugated polymers in recent years are due to the requirements of the rapid development of new emerging high technologies such as large-area flat panel displays. In the molecular design of conjugated polymers for organic light emitting diodes (OLEDs), a novel strategy is to make use of heteroatoms. One the one hand, the particular interactions between the heteroatom and the Ï€-conjugated system is a highly effective way to modify the electronic structure so as to modify the optoelectronic properties of the material. On the other hand, the organic-inorganic hybrid molecular structure of the heteroatom-containing material is of great scientific importance. The introduction of heteroatoms into Ï€-conjugated molecules will significantly renovate the molecular design concept and enlarge the room for the development of Ï€-conjugated materials.In this dissertation, we introduced heteroatoms to substitute the sp³ hybridized carbons at position 9 of fluorene in polyfluorenes, which is a class of widely investigated Ï€-conjugated polymer for OLEDs, resulting in a new concept of polyheterofluorenes. Then the molecular structures and optoelectronic properties of the heterofluorenes and polyheterofluorenes were studied theoretically for the first time by means of DFT calculations. In addition, we successfully addressed the key issue in polyheterofluorene preparation, i.e. the monomer synthesis of dibromoheterofluorene. With such an approach, seven new 2,7-dibromo-9-heterofluorenes have been synthesized according to the general synthetic strategy outlined for the preparation of 2,7-dibromo-9-heterafluorenes. Furthermore, the copolymers of the obtained 2,7-dibromo-9-heterofluorene were prepared by Suzuki polymerization and their properties were investigated practically.The dissertation is divided into six chapters.In Chapter 1, the basic principles of OLEDs and molecular design of conjugated polymers are introduced briefly. The features of heteroatoms and their applications in OLEDs are discussed in detail. Finally, the research interests and main work of this dissertation are put forward accordingly at last.In Chapter 2, DFT studies were applied to investigate the structural, electronic, and optical properties of 9-heterofluorenes achieved by substituting the carbon at 9position of fluorene with silicon, germanium, nitrogen, phosphor, oxygen, sulfur, selenium, or boron. These heterofluorenes are highly aromatic and electrooptically active. The alkyl and aryl substituents of the substituting atom have limited influence, but the oxidation of this atom has significant influence on their molecular structures and optoelectronic properties. The HOMO-LUMO interaction theory was successfully applied to analyze the energy levels of these heterofluorenes. Carbazole and selenafluorene have type C of interaction, enabling them to be good hole-transporting material with high-lying HOMO. Most heterofluorenes belong to type B, enabling them to be good electron-transporting material with low-lying LUMO. Types C and D of heterofluorenes, such as carbazole, oxygafluorene, sulfurafluorene, and selenafluorene, have high triplet state energy, showing their potential as host materials. The HOMO-LUMO interaction theory has also been successfully utilized to explain the origin of four kinds of wave functions of these heterofluorenes. The extrapolated HOMO and LUMO indicate that polyselenafluorene dioxide has the lowest LUMO; polycarbazole has the highest HOMO; polyselenafluorene has the highest band gap (Eg); and polyborafluorene has the lowest E_g. All the heterofluorenes and polyheterofluorenes studied are electronically and optically active, and their coplanar nature, i.e. the Ï€-conjugation, is the origin of the electronic and optical properties of heterofluorenes. The introduction of heteroatoms into fluorene is a highly efficient way to modify the optoelectronic properties. Heterofluorenes and their oligomers and polymers are worthy of experimental exploration, especially those having extraordinary properties revealed in this chapter.In Chapter 3, a facile, highly efficient, and economical procedure for the preparation of 6,6'-diiodo-4,4'-dibromo-3,3'-dimethoxylbiphenyl has been invented. From this compound, a general synthetic strategy for the preparation of 2,7-dibromo-9-heterafiuorenes has been developed. Seven novel 2,7-dibromo-9-heterofluorenes have been facilely synthesized for the first time according to this synthetic procedure. All the compounds related have been fully characterized by ¹³C NMR, ¹H NMR, GC-MS, MALDI-TOF, and element analysis.In Chapter 4, two novel series of soluble blue light-emitting conjugated random and alternating copolymers derived from 9,9'-dioctylfluorene (FO) and 3,6-dimethoxyl-9,9'-dimethyl-9-silafluorene (DMSiF) as well as from FO and 3,6-dimethoxyl-9,9' -diphenyl-9-silafluorene (DPhSiF) were successfullysynthesized by Suzuki coupling copolymerization. The random copolymers of 3,6-dimethoxyl-9-methyl-9- vinylsilafluorene (VSiF), methyl methacrylate, and vinylcarbazole were obtained by free radical copolymerization. The Yamamoto homopolymerization of 3,6-dimethoxyl-9,9'-methyloctadecyl-9-silafluorene was also investigated. Chemical structures and optoelectronic properties of the polymers were characterized by NMR, UV absorption, photoluminescence (PL), cyclic voltammetry (CV), and electroluminescence (EL). From the structure probe of methoxyl in silafluorene, it was concluded that the silafluorene structure remained in the copolymer after the Suzuki and free radical polymerization, while it broke in Yamamoto polymerization due to Si-C breakage. Also from the methoxyl probe, the actual content of silafluorene in polymers can be calculated. It revealed that the two silafluorenes both have lower content in polymer than in the feed and DPhSiF have lower actual content than DMSiF at the same feed ratio, indicating that the reactivity of 2,7-dibromid of DMSiF is lower than 2,7-dibromofluorene but is higher than that of DPhSiF, which is mainly due to the spatial hindrance of methoxyl and diphenyl substituents. Silafluorene was an attractive novel building unit for optoelectronic materials, and its copolymers with fluorene had considerably modified the properties of poly(9,9'-dioctylfluorene) (PFO), showing that the small amount of silafluorene incorporation is an efficient and economical way to modify the properties of the polymer. The modifying ability of DPhSiF is stronger than that of DMSiF. The polymer of VSiF is a kind of high band gap light-emitting material with better luminescence spectrum than polyvinylcarbazole as a host for blue electrophosphorescent emitters.In Chapter 5, a soluble blue light-emitting conjugated copolymer derived from FO and 3,6-dimethoxyl-9,9-dibutyl-9-germafluorene (DBuGeF) was successfully prepared by Suzuki coupling polymerization. Chemical structures and optoelectronic properties of the copolymer were characterized by NMR, UV absorption, photoluminescence (PL), and cyclic voltammetry (CV). From the structure probe of methoxyl in germafluorene, it was concluded that the germafluorene structure remained in the copolymer after the Suzuki polymerization and the post treatment. Also from the methoxyl probe, the actual content of germafluorene in copolymer was found to be slightly higher than its feed ratio, suggesting that the reactivity of 2,7-dibromid of DBuGeF is higher than 2,7-dibromofluorene, in spite of the spatial hindrance of methoxyl around the polymerization cites. Germafluorene was found tobe another attractive building unit for optoelectronic materials for the first time, and its copolymer with fluorene had considerably impacts on the properties of PFO, showing that the small amount of germafluorene incorporation is an efficient and economical way to modify the properties of the polymer. The modifying ability of DBuGeF is stronger than that of DMSiF and DPhSiF. The solution-processable germafluorene-fluorene conjugated copolymer is a new class of high stable and high energy gap conjugated polymer that can be used not only as efficient blue light emitting materials but also as host materials.In Chapter 6, two soluble blue light-emitting conjugated copolymers derived from FO and 3,6-dimethoxyl-9-phenyl-9-phosphafluorene (PhPF) and from FO and 3,6-dimethoxyl-9- butyll-9-phosphafluorene oxide (BuPFO) were successfully prepared by Suzuki coupling copolymerization. Chemical structures and optoelectronic properties of the copolymers were characterized by NMR, UV absorption, photoluminescence (PL), cyclic voltammetry (CV), and electroluminescence (EL). From the structure probe of methoxyl in phosphafluorene, it was concluded that the phosphafluorene structure remained in the copolymer after the Suzuki polymerization and the post treatment. Also from the methoxyl probe, the actual content of phosphafluorene in copolymer can be calculated. It was found that BuPFO had higher content in the copolymer than in the feed, while PhPF had lower content in the copolymer than in the feed, suggesting that the reactivity of 2,7-dibromid of BuPFO is higher than that of 2,7-dibromofluorene, while 2,7-dibromid of PhPF is lower than 2,7-dibromofluorene. Phosphafluorene was an attractive novel building unit for optoelectronic materials and its copolymer with fluorene had considerably impacts on the properties of PFO especially on the electroluminescence, showing that the small amount of phosphafluorene incorporation is an efficient and economical way to modify the properties of the polymer. The copolymer of PhPF with wide EL spectrum has great potential for white light-emitting diodes.