Fluorene-benzene Polymer Synthesis, Photophysical Properties, Aggregation Structure And Mutual Relations

Organic polymer materials have attracted much attention in the past decade because of their potential applications in light emitting diodes, field-effect transistors, photovoltaic cells, non-linear optical materials, and electrochromic devices. Among these polymers, fiuorene-based polymers are a kind of most promising blue-light-emitting materials for commercial applications due to their excellent properties such as high photoluminescence (PL) efficiency, good processability and thermal stability. However, much research about polyfluorenes and fluorene-based derivatives is mainly focused on the synthesis of polymers with novel chemical structures, and the investigations on condensed state structure, photophysical properties and their relationship are surprisingly lacking. In fact, it is of great significance to in-depth understand the relationship between condensed state structure and photophysical properties, which can provide more insights into the design of novel optoelectronic polymers as well as the fabrication of high performance devices. In this work, a series of fluorene-benzene based polymers were thus designed and successfully synthesized, and the relationship of molecular structure-condensed state structure-photophysical properties of these polymers were investigated in detail by using various characterization techniques.1. A series of so-called hairy-rod fluorene-alt-benzene based polymers with different substituted groups on phenylene were synthesized through Suzuki route. The optical properties, fluorescence quantum yields (QY) and electrochemical properties of these polymers were investigated. It was found that the optical properties of the polymers did not show obvious dependence on the alkyl length on phenylene. However, the spectral stability was increased, and the 0-1 transition and the full width at the half-maximum (fwhm) of photoluminescence (PL) spectra was decreased with the increase of the alkoxy length on phenylene. The nature (e.g. electron-donating and electron-withdrawing) of the substituted side groups on phenylene has obvious effect on the optical properties, QY and electrochemical properties of the polymers. Thus, the photophysical properties of the polymers can be tuned by attaching different substituted side groups onto the phenylene. The spectra of the polymers in solutions exhibit dependence on dielectric constants of the solvents used, and the spectra of the polymer solutions shift to longer wavelength with increasing the dielectric constants of the solvents.2. The thermal stability of the spectra of the polymers was investigated by annealing experiments. A new emission peak located at long wavelength (at about 515 nm) appeared in PL spectra of the unsubstituted polymer (PF6P) films annealed in air, and the intensity of the long wavelength emission increased with the increase of annealing temperature or annealing time. However, the long wavelength emission did not appear in PL spectra of the polymers substituted with alkyl and alkoxy. By comparing the PL spectra of PF6P and the substituted polymers, Fourier-transform infrared (FTIR) Spectroscopy, and PL lifetime measurements have revealed that the long wavelength emission could be attributed to the formation of fluorenone-based excimers. Compared with PF6P, the attachment of alkyl or alkoxyl groups on phenylene effectively inhibits the formation of fluorenone-based excimers and thus remarkably improves the thermal stability of the spectra.3. Morphology and phase behavior of the polymers substituted with alkyl and alkoxyl groups on phenylene were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and other techniques. These polymers exhibit typical fibrillar morphology in the films. During heating in DSC, all the polymers substituted with alkoxy on phenylene show a nematic phase. The polymers substituted with hexyl and octyl on phenylene also show a nematic phase, however, the nematic phase disappears for the polymer substituted with decyl. The polymers substituted with alkoxy on phenylene self-organize into a lamellar structure, and their phase behavior shows obvious dependence on the alkoxy length. PF6OC6 shows two crystalline phases; further increasing the length of alkoxy, PF6OC8, PF6OC10 and PF6OC12 only show one crystalline phase. During heating in DSC, PF6OC8 exhibits a melting-recrystallization phenomenon, which is steadily inhibited as the length of alkoxy on phenylene increases. In addition, for the polymers substituted with alkoxy on phenylene, the 0-1 transition in PL spectra of the films decreases with the increase of ordered structure.4. The nonisothermal crystallization kinetics of PF6OC10 and PF6OC12 was investigated by using differential scanning calorimetry (DSC) under different cooling rates from the melt. It was found that the Ozawa method failed to describe the nonisothermal crystallization behavior of the two polymers. Although the Avrami method did not effectively describe the nonisothermal crystallization kinetics of the two polymers for overall process, it was valid for describing the early stage of crystallization. The combined Avrami-Ozawa method proposed by Liu was able to satisfactorily describe the nonisothermal crystallization behavior of the two polymers. The crystallization activation energies determined by Kissinger, Takhor and Augis-Bennett models were comparable. The melting temperature of PF6OC10 increased with increasing molecular weight. For low molecular weight sample, PF6OC10 showed the characteristic of double melting phenomenon. The interval between the two melting peaks decreased with increasing molecular weight, and only one melting peak was observed for the high molecular weight sample.5. A series of so-called rod-coil fluorene-benzene based polymers were synthesized through Suzuki coupled polymerization, and the photophysical properties and phase behavior of these polymers were investigated. The photophysical properties of the polymers did not show obvious dependence on the fraction of coil segments in backbone. However, with increasing the fraction of rod segments in backbone, the spectra of the polymers shift to long wavelength and the HOMO-LUMO energy gap increases. The glass transition temperature of the polymers steadily decreases with the increase of the fraction of coil segments in backbone. The polymer (P1(PF6P4P)) shows complex phase behavior, and a strip liquid crystalline texture is observed at low temperature and the typical double melting behavior can be observed at high temperature.