Synthesis, Optical Property And Structure Of Poly(Thiophene)s Studied By Synchrotron Radiation Technique

In recent years, poly(thiophene)s have attracted considerable attention because of their great potential in light-emitting diodes, photovoltaic cells and field-effect transistors, etc.. In order to realize the practical application, their photoelectric properties still need to be further improved. Besides the chemical structure of materials, photoelectric properties have a close relationship with physical morphology and behavior of the molecular. Up to now, most of the studies about the effects of physical morphology on the photoelectric properties of poly(thiophene)s have been focused on the effect on conductivity and only a few researches were centered on the fluorescence. In this dissertation, poly[3-(2-methoxyphenyl)thiophene](PMP-Th) power and PMP-Th nanowire arrays were synthesized and fabricated via chemical oxide polymerization and electropolymerization, respectively. Fluorescence properties of the PMP-Th power and nanowire arrays as well as the corresponding mechanism were investigated in detail. Using the near edge X-ray absorption fine structure spectroscopy (NEXAFS) technique based on synchrotron radiation, the effects of different electronic properties of lateral chains on the electron structure and molecular orientation have been studied.The main results of this dissertation are described as follows:1. 3-(2-methoxyphenyl)thiophene was successfully synthesized via Grignard reaction and then polymerized to PMP-Th by means of the catalyst of FeCl₃ TGA analysis demonstrated that PMP-Th has good thermal stability because its critical temperatures of weight loss in air are 330℃. The polymer exhibited red photoluminescence with a main peak at approximately 687 nm and narrow spectral bandwidth. These results demonstrate PMP-Th is a good near-infrared emission material. XRD analysis indicated that there are smaller crystal domains in the sample which would be possibly attributed to the molecule regioregularity.2. Highly ordered porous anodic aluminum oxide (AAO) templates were obtained in oxalic solution and sulfuric solution by a two-step anodization process. respectively using high purity (99.999%) aluminum foils as the starting materials. The pore diameter of the AAO was tuned from 30 nm to 80 nm by changing the condition for the fabrication. The pore density is about 10¹⁰ pore/cm². Using the home-made AAOs with various pore diameters as templates, the PMP-Th nanowire arrays were fabricated by cyclic voltammetry (CV). The diameters of the nanowires are close to the pore diameters of AAO. The length of the nanowires could be tuned by controlling the total charges consumed. It has been demonstrated that CV method combined with AAO template is a simple and effective rout to fabricate ordered polymer nanostructures, which is easy to tune the nanostructure growth and control the dimention.3. The photoluminescence (PL) properties of PMP-Th nanowires in AAO with different pore diameters fabricated from oxalic solution were studied. Compared to the PL of PMP-Th film, PMP-Th nanowires in AAO exhibit obvious blue-shifts and great intensity enhancement. Interestingly, the PMP-Th nanowires in AAO with various pore diameters showed evident size dependent emissions. The maximum peak position of the emission spectra gradually shifted from 580 nm to 560 nm as the diameter decreased from 80 nm to 60 nm, while when the diameter continued to decrease to 40 nm, the emission peak gradually shifted back to 580 nm. According the results of FT-IR and considering the different behavior of the polymeric molecules in the nanowires and the thin film, it is reasonable to attribute the blue-shift to the nanosize confined effect. The red shift in small pore diameters comes from the alignment of the polymeric chains in AAO which results in the increase of conjugated length and the band gap decreased. The intensity enhancement mechanism was discussed according to the absorption spectra and PL excitation spectra of AAO and PMP-Th film. It was proposed that the efficient Forster energy transfer from AAO to the polymer molecules is responsible for the PL enhancement. The PL enhancement decrease with the decreasing of the pore diameter is caused by the decreasing of the spectral superposition of donor and acceptor together with low PL efficiency which results from the molecule alignment.4. The PL comparison of PMP-Th nanowires in AAO fabricated from oxalic solution (C-AAO) with that from sulfuric solution (S-AAO) showed that the optical properties of these two composites have the same nano-size confinement effect. This demonstrated that the different chemical surroundings of the pores have no effect on the PL properties of the nanowires and further illuminated that there isn't chemical interaction between the polymer and the AAO. Unlike PMP-Th, the PL intensity of poly(3-Brthiophene) (PBr-Th) nanowires in C-AAO and S-AAO remarkably decrease compared to the PBr-Th film. This may be attributed to the high alignment of PBr-Th chains in AAO and complex interactions between the polymer and AAO. However, PL peak shifts of PBr-Th nanowires in S-AAO and C-AAO have the same nano-size confinement as PMP-Th nanowires. So, systematic studies on this system are needed.5. NEXAFS based on the synchrotron radiation was employed to investigate the electronic structure of PMP-Th and PBr-Th. The angle dependent NEXAFS evidenced the preferential orientation of PMP-Th and PBr-Th molecules on Pt electrode, respectively. PBr-Th chains tilt away from the Pt surface, while the orientation of PMP-Th molecules at the surface is nearly random owing to the double effect of steric hindrance and electronic property of methoxypheny lateral.