過去實驗室的研究中透過時間解析光譜發現共軛高分子MEH-PPV薄膜在光激發後短時間內會經歷迅速的能量耗散造成光譜紅移，這種現象歸因於分子鏈受激發快速扭轉所引起的電荷自縛現象(self-trapping)，且在2皮秒後光譜會因熱逸散以較慢的速率持續紅移。本研究同樣透過時間解析光譜來探討共軛高分子溶液在皮秒時間尺度下的能量衰減行為，發現MEH-PPV溶液在短時間內也會有較快速的光譜紅移，然而在2皮秒內的能量耗損速率(7~8 meV/ps)遠低於薄膜(30meV/ps)，當溶液濃度大幅提高時此階段的紅移速率亦有些微上升。除了MEH-PPV外，我們也測量了其他共軛高分子溶液例如P3HT和聚二苯胺-咔唑類環狀共聚物等，並同樣觀察到不同程度的紅位移，即便是一般認為在溶液中具有柔軟性且可自由轉動的高分子鏈仍可能因受光激發所引起的迅速扭轉運動而造成激發電荷的self-trapping，我們推測self-trapping的效應與分子鏈的柔軟程度以及solvated state有關。此外我們也在MEH-PPV溶液中加進了小分子量寡聚物進行超快光譜研究，有光惰性的PS以及前述的2種共軛高分子，從混摻PS及P3HT的溶液中我們似乎沒有觀察到特別的interaction存在，然而，在與環狀共軛高分子的混摻實驗中，我們觀察到MEH-PPV和這個環狀高分子之間可能存在某些coupling以及interchain energy transfer的現象，我們推測在光激發過程此環狀高分子可能與MEH-PPV形成某種激發態複合物，並且此環狀高分子可能具有足夠大的體積與MEH-PPV分子鏈靠近以及高度重疊的光譜能夠在溶液中與MEH-PPV分子鏈產生鏈間能量傳遞。透過此研究我們對共軛高分子在超快時間尺度下的放光行為以及能量弛豫機制有了更深入的了解。

Our previous research through time-resolved spectroscopy revealed that conjugated polymer MEH-PPV films undergo rapid energy dissipation shortly after photoexcitation, leading to a spectral redshift. This phenomenon has been attributed to the rapid torsional motion of the polymer chains upon excitation, which induces a charge self-trapping effect. Following this first stage, the redshift continues slower after 2 picoseconds driven by thermal dissipation.
In this study, we employ time-resolved spectroscopy to investigate the energy relaxation dynamics of conjugated polymer solutions on a picosecond timescale. Our results show that the energy dissipation rate within the first 2 picoseconds in MEH-PPV solution (7–8 meV/ps) is much lower than in thin films (30 meV/ps). When the solution concentration is substantially increased, the redshift rate in this early stage also slightly increases. Beyond MEH-PPV, we also investigated other conjugated polymer solutions, such as P3HT and a conjugated polymer poly (diphenylamine-co-carbazole) cyclic block copolymer, and observed varying degrees of spectral redshift. These findings indicate that, even in solution, where polymer chains are generally assumed to be flexible and capable of free rotational motion, rapid torsional movement upon photoexcitation may still induce self-trapping of excited charges. We suggest that the extent of self-trapping is influenced by the flexibility of polymer chains and the solvated state.
Moreover, we performed ultrafast spectroscopic studies by blending some oligomers into the MEH-PPV solution, including PS and the two aforementioned conjugated polymers. No significant interaction was detected in the MEH-PPV/PS and MEH-PPV/P3HT blended solutions. However, in the blended solutions with the conjugated cyclic polymer, we observed possible electronic coupling and interchain energy transfer between MEH-PPV and the cyclic conjugated polymer. We speculate that this cyclic conjugated polymer may form an excited-state complex during the photoexcitation process with MEH-PPV, and the large volume of the cyclic polymer allows it to come closer to MEH-PPV chains, and the significant spectral overlap between the two enhances interchain energy transfer in solution.
This study provides deeper insights into the photoluminescence behavior and energy relaxation mechanisms of conjugated polymers on ultrafast timescales.

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