含氰基丙烯酸基共軛分子的合成與鑑定及其做為界面改質劑在聚己基噻吩/二氧化鈦層狀異質界面的應用

Abstract

本研究中，為了改善有機/無機界面的相容性，合成出一系列全共軛分子做為界面改質劑，應用於聚己基噻吩/二氧化鈦層狀異質界面中。而為了避免改質劑對於界面的電荷分離與傳輸產生阻礙，界面改質劑的LUMO能階應位於聚己基噻吩與二氧化鈦之間。為達成此目標，在合成設計中，我們利用增加噻吩環數及導入donor-acceptor系統的概念，降低共軛分子的能隙值，並導入氰基丙烯酸基來達到降低LUMO的目的。合成部份以Kumada coupling與Stille coupling進行噻吩寡聚物與並苯二唑噻吩寡聚物的合成，再以Vilsmeier-Haack reaction在單邊導入醛基，最後以Knoevenagel condensation將氰基丙烯酸基導入分子中。 經核磁共振光譜與質譜的鑑定，證實界面改質劑確實被成功的合成及純化。由紫外/可見光吸收光譜也觀察到最大吸收波長隨環數增加有明顯紅位移，顯示能隙值逐漸下降的趨勢。利用循環伏安法測量界面改質劑之HOMO，搭配能隙值計算後，發現LUMO確實明顯降低，使改質劑之LUMO低於聚己基噻吩。此外，我們製備緻密二氧化鈦薄膜，作為層狀異質接面之電子受體，此緻密二氧化鈦薄膜採用噴霧裂解法製備，於450oC下燒結，二氧化鈦薄膜之晶型以XRD鑑定為anatse，表面型態則以掃描式電子顯微鏡觀察，發現其表面極為平坦。 接著將界面改質劑以自組裝單層膜的方式鍵結於二氧化鈦基材上，自組裝單層膜以EDX鑑定，顯示表面確實有改質劑分子的存在。並以接觸角量測觀察表面改質情形，發現改質後，二氧化鈦基材表面接觸角大於55o，呈現疏水性質。 利用聚己基噻吩塗佈於改質過之二氧化鈦基材表面，製備聚己基噻吩/二氧化鈦雙層異質接面結構，經由螢光光譜的測量顯示，隨改質劑分子能隙縮減與LUMO的降低，螢光焠滅效率漸增，顯示此一改變，確實能增加高分子與二氧化鈦之間的photo-induced charge transfer效率。

In this study, a series of fully-conjugated molecules were synthesized and used as interface modifier (IM) to enhance the compatibility of PTs and TiO2. In order to prevent the charge separation and transport between the interface was blocked by IM, the LUMO level of IMs should between the LUMO of PTs and the conduction band of TiO2. For this reason, the bandgap of IMs were decreased by either increasing the number of thiophene rings or establishing a donor-acceptor structure. In addition, cyanoacrylic carboxyl group was introduced to the molecule structure of IMs. The electron-withdrawing group effectively decreases the LUMO and carboxyl moiety provides anchoring sites to binding with TiO2. The synthesis of IMs involves the synthesis of thiophene oligomers and thiophene derivate of bezothiadiazole by Kumada and Suzuki coupling. Then, Vilsmeier-Haack reaction was adopted to perform one-side formylation of conjugated molecules. Finally, a cyanoacrylic carboxyl group was introduced into the molecular structure by Knoevenagel condensation. The as-synthesized IMs were characterized by NMR and mass spectrometry. The UV/Vis absorption of IMs showed a visible red-shift with increasing thiophene rings, implying the decrease of bandgap. The HOMO level of IMs were determined by cyclic voltammetry and the LUMO level of these IMs were then calculated from the summation of HOMO and bandgap. The results indicated the LUMO of W-2, W-3 and W-4 were lower than that of P3HT. Then, the IMs were used as functional molecules to form self-assembled monolayers on a flat titania thin film. The flat dense-TiO2 thin film was made using spray-pyrolysis method of titanium precursor, followed by calcination at 450oC. Crystallization and morphology of the dense-TiO2 thin film was examined by XRD and scanning electron microscopy. The properties of SAMs were characterized using EDX and contact angle measurements. After being modified by IMs, the contact angle increased from approximate 0o to 55o, indicating the TiO2 surface became hydrophobic. Subsequently, poly(3-hexylthiophene) (P3HT) was spin-coated on top of the IM-covered TiO2 substrate to form a bilayer heterojunction. The PL spectra and quenching efficiency measurements of the thus-prepared heterojunction indicated the decrease of bandgap and LUMO level of cyanoacrylic carboxyl group-containing molecules enhances the photo-induced charge transfer efficiency between polymer and titania.