結合分子動力學與量子力學探討含有共軛側鏈之共軛高分子其分子鏈構形與紫外/可見光吸收光譜之關聯性

Abstract

在本研究中，我們利用全原子分子動力學與量子力學模擬方法來研究聚噻吩系列共軛高分子在有序排列與溶液無序狀態下分子鏈構形與紫外/可見光吸收光譜之間的關聯性。我們從共軛效應僅存在於主鏈方向上的 100% 頭接尾與頭頭/尾尾交替之 P3HT 與 PDBT 出發，並且將其烷基側鏈以己基-噻吩取代為 PD3QT 與 PD4QT ，以此四個材料做為研究對象，以了解位置選擇性與共軛側鏈所帶來的效應。為了說明分子之共軛主鏈構形如何影響吸收光譜，我們人為調整 3HT單體之二硫環間扭轉角進行分析，隨著偏離共平面角度的增加，導致吸收光譜產生藍位移現象；同樣地，將 D4QT 單體之主鏈上二硫環固定於反式構形(trans, 180o)，並且人為調整側鏈與主鏈二硫環間之扭轉角，由共平面偏轉至互相垂直時，吸收光譜不僅產生紅位移現象且由原先單一吸收峰分離出主鏈與側鏈各自貢獻的兩個吸收峰，增加整體吸收範圍，指出共軛主側鏈間存在侷限電子的競爭效應。在有序排列狀態，由於 P3HT 與 PDBT 分子鏈能夠藉由 pi-pi作用力形成緊密規整的排列，因此隨著分子量的增加，吸收光譜呈現出明顯的紅位移現象。對於共軛效應同時存在於主鏈與側鏈方向的 PD3QT 與 PD4QT 而言，PD3QT 有較大的比例為主鏈與側鏈維持在180o的共平面，導致主鏈有較大的扭曲，使得有效共軛長度不易維持，競爭效應無法抵消；而 PD4QT 其共軛側鏈呈現出成對偏離共平面60o的偏轉，不僅能削弱競爭效應同時能夠保持主鏈上共平面性，因此在吸收光譜中無論是整體吸收範圍或是紅位移的幅度均較 PD3QT 來的顯著。在溶液無序狀態，由於分子鏈間缺乏 pi-pi作用力，分子鏈運動較為自由，均呈現出蜷曲的構形，能夠維持的有效共軛長度大約為3∼5個重複單元，因此無論是 P3HT 與 PDBT 亦或是 PD3QT 與 PD4QT之吸收光譜皆相當類似。此外， P3HT 在溶液無序狀態下模擬的結果，最大吸收峰值455nm，吸收範圍200~620nm，與實驗值456nm與200~607nm相比有著高度的一致性，顯示我們的模擬方法是相當可靠的。

We employed all-atom molecular dynamics simulation and quantum mechanical methods to explore the relationship between molecular conformation and UV absorption spectra of polythiophene-based conducting polymers. The UV spectrum of 3HT monomer was observed in blue-shift, because of the deviation from coplanar of main chain; the D4QT was found in red-shift, due to the torsional deviation and competitive effect between conjugated main and side chain. In the ordered state, the P3HT and PDBT chains experience the pi-pi interaction and result in more coplanarity for main chain. Thus, they show significantly red-shift with increasing repeated unit. The absorption spectrum of PD3QT was shown in single peak, because the competitive effect of restricting electron could not be offseted and the coplanarity between side and main chain could be maintained. The torsional angle of PD4QT were shown in symmetry and located in 115o and 245o. Hence, the red-shift of PD4QT is more significant than PD3QT and shown in two peaks of spectrum. In the disordered state, the chain conformation is shown in coil conformation within dilute solution and the persistence length in disordered state is shorter than in ordered state. The persistence length of P3HT, PDBT, PD3QT and PD4QT are located in 3 to 5 repeated units by theoretical calculation. Therefore, the UV spectra for P3HT and PDBT or PD3QT and PD4QT are similar. Besides, our simulated spectrum for P3HT in disordered state showed the excellent agreement with experimental data. These results illustrate our methods is reliable.