帶有烷基側鏈oligo-PPV之合成，自組裝奈米結構，和光電性質之研究

Abstract

在本研究中，我們成功地利用Wittig Reaction合成出oligo(2,5-dipropoxy-p-phenylene vinylene) (OPV3-C3)，利用NMR與IR的圖譜去鑑定合成出來的產物，並且與之前實驗室的學姐利用Wittig-Horner Reaction合成出來的oligo(2,5-bis(hexyloxy)-p-phenylene vinylene) (OPV3-C6)與oligo(2,5-bis(octyloxy)-p -phenylene vinylene) (OPV3-C8)共同做一系列熱性質、光學性質和自組裝奈米結構的探討與比較。利用TGA及DSC的熱分析結果，我們可得到三種化合物的熱裂解溫度(Td)、熔點(isotropic temperature)、與形成液晶的溫度，而且在DSC圖譜中，我們發現OPV3-C3在降溫的過程中出現了兩種不同的層列型(smetic)液晶相，然而在OPV3-C6與OPV3-C8的圖譜中只有發現一種層列型液晶相，這個結果亦可藉由POM的觀察得到印證。我們將再結晶所得到的OPV3系列化合物(簡稱原始試片)與加熱至熔點並緩慢冷卻後的OPV3系列化合物(簡稱加熱後試片)去測量SAXS及WAXD。在原始試片的SAXS圖譜中，代表著化合物排列形狀的a值隨著化合物側鏈增加而增加，並愈趨向於三度空間的排列，然而在加熱後試片的SAXS圖譜中，OPV3-C3的a值則因為具有兩種液晶態而趨向於三度空間的排列，但是OPV3-C6與OPV3-C8仍維持二度空間的排列。另外，在原始試片與加熱後試片的WAXD圖譜中可發現，前者可看到分子內結構所造成的peak，而後者在2θ=35o以上幾乎沒有peak產生。我們利用UV-Vis與PL光譜來研究OPV3系列化合物溶在chloroform、hexane與decane下以及藉由旋轉塗佈所形成固態薄膜的光學性質，當溶劑由極性改為非極性溶劑時，UV圖譜會有藍位移的現象，且固態的UV與PL圖譜會較溶液的UV與PL圖譜紅位移。另外，我們將OPV3系列的化合物溶在不同的溶劑下，以AFM觀察其沈降在雲母片上的自組裝奈米結構。當OPV3-C3溶在chloroform時，會自組裝排列成條狀結構，然而當溶在hexane及THF時僅會形成直徑約為20 nm的圓盤狀結構。而OPV3-C6溶在hexane與THF時易形成中空管狀結構，在chloroform當中亦會形成條狀結構。OPV3-C8則會在chloroform與THF中形成條狀結構，在hexane中僅形成片狀結構。藉由化合物的分子結構，我們推測其自組裝結構的驅動力為偶極-偶極作用力、π-π堆疊交互作用力與凡得瓦耳力。除此之外，我們還利用熱蒸鍍方式將化合物蒸鍍至雲母片上，同樣以AFM觀察其固態的自組裝奈米結構。在蒸鍍量較多的區域中可發現，OPV3系列的三種化合物皆會形成密密麻麻的條狀結構，而且條狀結構的長度隨著側鏈長度的增加而增加。最後，我們利用了循環伏安法與空間電荷限制電流法(space-charge limited current，SCLC)求出了OPV3系列分子的HOMO、LUMO與電洞位移率(hole mobility)。隨著化合物的側鏈長度愈長，則陽極起始電位、能帶間隙與HOMO則愈小。而OPV3-C3的電洞位移率經由計算可得為1.8×10-10 (cm2/Vs)。

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- phenylene 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).