聚噻吩衍生物在不同凝態環境下之光物理與光化學

Abstract

本論文利用時間相關單光子技術（Time-Correlated Single Photon Counting；TCSPC）來研究聚噻吩衍生物( POTT、POTTOT、POTPy及POTPyOT )在三種不同凝態環境下( 溶液、薄膜、混PMMA薄膜 )的光譜與動態學。我們以超短脈衝雷射光源分別激發這些高分子到第一單重態激發態(S1)上，並觀測其緩解過程以了解它們在激發態的動力行為。POTPy及POTPyOT主鏈的結構內具有吡啶(Pyridine)，導致共軛鏈的長度縮短，因此在吸收及螢光光譜的位置都比POTT、POTTOT還要藍位移。在動力學量測方面，POTT、POTTOT及POTPyOT在溶液的結果是相似的，系統間轉換過程的衰減時間常數都是 ~ 0.6 ns，然而在POTPy溶液中 ~ 2.0 ns，我們推測噻吩與噻吩之間單鍵的旋轉運動加速了系統間轉換的過程，而這個假設將由PMMA薄膜及理論計算的結果來加以證實。另外，我們也研究在薄膜及PMMA薄膜中分子聚集對光譜及動力學造成的影響。 理論計算的結果顯示當轉動噻吩間的共平面角度，由原本基態最穩定的角度-26°往兩個相反方向轉動，發現當轉至共平面角度0°時， 和 之間的能量間距是最小的。由能隙理論的觀念來看可以說明噻吩間單鍵的旋轉會加速系統間轉換的過程。另外我們計算1-3個數量OTPy的激發態，結果顯示隨著增加OTPy的數目， 和 之間的能量間距越來越大，這也可以解釋為什麼我們觀測到OTPy單體的 卻短至0.3ns，而POTPy的 為2.0 ns。

Picosecond time-resolved studies using time-correlated single photon counting technique（TCSPC） were performed for four Polythiophene derivatives ( POTT、POTTOT、POTPy and POTPyOT ) under three different conditions ( Solution、Film and PMMA Film). After excitation to the S1 state, subsequent possible electron relaxation processes were examined. Because POTPy and POTPyOT contain pyridine in the main chain, their conjugation length is shorter than that of POTT and POTTOT, leading to the UV and fluorescence spectra of the former are blue-shifted with respect to those of POTT and POTTOT. We found that the time coefficients of POTT 、POTTOT and POTPyOT in solution are the same (~0.6ns) due to Intersystem crossing (ISC).However, the decay ISC time coefficient is 2.0ns for POTPy. We propose that rotation of the single bond between adjacent thiophenes enhances the rate of ISC. The results of these compounds in PMMA films and theoretical calculations confirm this mechanism. Moreover, we studied the effect of aggregation affecting the spectra and dynamics of those molecules in film and PMMMA films. Theoretical calculations for DOTPyOT dimmer have shown that energy gap between S1 and T2 states decreased gradually when the dihedral angle of adjacent thiophenes changing from -26° to 0°. On the other hand, the energy gap between S1 and T2 states increased gradually when the number of OTPy monomer increases from 1 to 3. This result is consistent with our observation that the lifetime of POTPy is 2.0ns but the lifetime of OTPy monomer is only 0.3ns.