Synthesis, Self-assembly Nanostructure, and Optoelectricroperties of Oligo-PPV with Alkyl Side Chains
Date Issued
2008
Date
2008
Author(s)
Wu, Yuan-Yu
Abstract
In this research, the oligo(2,5-dipropoxy-p-phenylene vinylene) (OPV3-C3) was successfully synthesized by Wittig reaction, and characterized by NMR and IR spectra. A series of experiments including thermal properties, optical properties, and observation of self-assembly nanostructure had been done on OPV3-C3, oligo(2,5-bis(hexyloxy)-p-henylene vinylene) (OPV3-C6), and oligo(2,5-bis(octyloxy)-p-phenylene vinylene) (OPV3-C8) to study the alkyl side chain effect on the basic properties. The thermal analysis showed the different degradation temperature(Td), isotropic temperature(Tm), and liquid crystal temperature(TLC). In the cooling process, the OPV3-C3 was observed two different smetic liquid crystal phase. However, OPV3-C6 and OPV33-C8 was observed only one. These phenomena were also verified by POM. The purified and solvent-recrytallized compound was treated with heating to its melting temperature and slowly cooling down to the room temperature. The structure of the compounds with and without heat-treatment were investigated at the same time by SAXS andWAXD. In the SAXS, we discovered the original compounds were turned to pack in 3-dimension, and the parameter “a” which symbolized the packing structure also increased with increasing the length of the alkyl side chain. However, after the compounds were heat-treated, the SAXS of them were analyzed its parameter “a”. The OPV3-C3 existed the 3-D packing due to two different smetic liquid crystal phase, but OPV3-C6 and OPV3-C8 still maintained the 2-D packing owing to only one smetic liquid crystal phase. So “a” of OPV3-C3 was close to 4, but “a” of other two were close to 2. Moreover, in the WAXD results, we found the compounds without heat-treatment have some peaks not observed on the heat-treated compounds at 2θ> 35o, which stands for the systematic arrangement of intramolecular structure. We used UV and PL to study the optical properties of OPV3 compounds dissolving in the chloroform, hexane, and decane and in thin films by spinning coating. From the UV spectra, compounds dissolving in different solvents showed absorption peaks would be blue-shifted with increasing the solvent polarity. Nevertheless, solid thin films showed red-shifted absorption peaks compared to the compounds in solutions. The self-assembly nanostructure of compounds was also comfirmed by AFM. The OPV3-C3 deposited on mica from chloroform solution would self-assembled as long stripes; but forming the disc about 20 nm in diameter from THF and hexane solution. The OPV3-C6 and OPV3-C8 would also have long stripes structure from chloroform solution as like as the OPV3-C3. But tubular structure would occur in OPV3-C6 from hexane and THF solution. It is believed that the driving forces for all above self-assembly processes were the combination of dipole-dipole interaction, π-π interaction, and Van der Waals force. In addition, the compounds would be evaporated on mica by vacuum evaporation, and self-assembly nanostructure of thin film was observed by AFM. In thin films of three compounds , confused long stripes would be observed, and the length of stripes would be increased with increasing the length of side chain. At last, we used cyclic voltammetry and space-charge limited current (SCLC) method to calculate the values of HOMO, LUMO, and hole mobilities of OPV3 compounds. With increasing the length of side chain, the values of onset anodic potential, band gap, and HOMO were small. The hole mobility of OPV3-C3 obtained from calculation was 1.8×10-10 (cm2/Vs).
Subjects
self-assembly
supramolecule
π-conjugated system
smetic liquid crystal
space-charge limited current method
hole mobility
cyclic voltammetry
OTFTs
Type
thesis
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