時間解析光譜於有機材料三重態激子之動力學探討

Time-resolved Spectroscopic Studies of Triplet-exciton Kinetics in Organic Materials

黃俊穎(Chun-Ying Huang)

指導教授 ： 周必泰

國立臺灣大學/理學院/化學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202100924

摘要

三重態激子的使用在發光材料的開發中為舉足輕重的角色，因為於電致發光 中，材料所產生的激子中於統計上將有百分之七十五為三重態。有效率地使用三重 態激子於是普遍地使科學家獲得表現優異的發光元件—特別是有機發光二極體。 然而有機材料中，三重態激子的產生為一複雜的動力學過程，並且存在許多因素將 使得三重態激子以非發光途徑流失，造成低發光效率的元件。因此，在系統性且有 序地改善有機發光元件的過程中，從根本上了解獲取三重態激子過程的動力學一 直被認為是重要的議題。本研究以時間解析光譜、穩態光譜及數值模擬軟體，系統 性地研究有機材料中三重態激子的動力學。文章的第一部分討論了手性分子排列 如何影響固有的三重態激子動力學，從而導致外消旋晶體中相對增強的深紅色室 溫磷光(Nat. Commun. 11, 2145 (2020))。於第二部分，文章探討於一以熱活化延 遲螢光(TADF)分子為主體及近紅外螢光分子為客體的發光系統中，主體分子在 透過反向系間交換以及螢光共振能量轉移將三重態能量傳至客體分子時的相關動 力學(J. Mater. Chem. C 8, 5704-5714 (2020))。本論文的結果將為三重態動力學提 供新的了解，並有助於推進實現高效率有機發光材料的相關進展。
第一部分
有機分子堆積結構與其室溫磷光之間的相關性，以及如何有序地提高其發光 強度，學界一直以來缺乏清楚的解釋。於本研究中，我們通過消旋的 2,2'-雙二苯膦 基-1,1'-聯萘(rac-BINAP)的晶體樣品，呈現一手性發光分子將在消旋晶體排列下 展現了相較於其單一手性晶體更佳的室溫磷光強度—即消旋增強室溫磷光。除了與長久存在的 Wallach 規則一致，rac-BINAP 具有較高晶體密度，我們的結果更顯 示其在波長 680 奈米處具有相較於其單一手性晶體更加深紅的室溫磷光。透過時 間解析光譜及晶體結構分析，我們發現在 rac-BINAP 晶體中，R 與 S 形式之間的 交叉排列堆積顯著地抑制雙分子淬熄行為—三重態-三重態煙滅(triplet-triplet annihilation)，因此抑制了其非輻射衰退途徑，從而提高室溫磷光強度。我們因此 成功地將 Wallach 規則拓展到手性分子在光物理上的差異。
第二部分
儘管激發錯合體主體與螢光分子客體之間的能量轉移是獲得高效率有機發光 二極體(OLED)中非常重要的過程，特別是在近紅外光區域，至今對其能量轉移 動力學的深入探討仍舊相當不足。在本研究中，我們首先開發了有著電子予體–受 體–予體(D–A–D)構造的新型深紅至近紅外光螢光分子，NOz-TPA 和 NOz-t-TPA。 在以 1 wt%之濃度摻雜進激發錯合共主體材料 Tris-PCz:CN-T2T(莫爾比 1:1) 後，NOz-TPA 和 NOz-t-TPA 的摻雜薄膜顯現相似的深紅至近紅外光光譜，並且其 光致量子產率分別高達 42%(光譜峰值為 680 奈米)和 28%(709 奈米)。全面 的時間解析光譜量測和動力學分析揭示兩種樣品在能量傳遞途徑上的顯著差異， 即激發錯合物主體與螢光分子客體之間的 Förster 與 Dexter 型能量傳遞。在 NOz- t-TPA 的參雜薄膜中，其所引入的叔丁基大幅度地抑制了 Dexter 能量轉移途徑， 這使得儘管相較於 NOz-TPA，NOz-t-TPA 薄膜的光致量子產率較低，數值模擬仍 預測 NOz-t-TPA 將是更好實現高效電致發光的候選者。而摻有 NOz-t-TPA 的 OLED 性能確實證實這份預測，其光譜展現峰值波長為 710 奈米的近紅外光，並且 其外部量子產率(EQE)高達 6.6%，創下該波段純有機近紅外光 OLED 的最佳記錄之一。而更重要的是，我們透過本研究深入探討了能量傳遞途徑在實現此類型的 高性能 OLED 中所起的關鍵作用。

關鍵字

有機發光二極體 ； 有機室溫磷光 ； 熱激發延遲螢光 ； 三重態激子 ； 時間解析放光光譜 ； 分子間能量傳遞

並列摘要

Utilizing triplet excitons has played a pivotal role in lighting applications because naturally, electroexcitation of lighting materials statistically generates 75% excitons in triplet manifolds. Efficiently harvesting triplet excitons thus generally leads to successful lighting devices, especially for organic light-emitting diodes (OLEDs). However, harvesting triplet excitons in organic materials has been regarded as complicated kinetics among which the triplet excitons are significantly subject to plenty of quenching factors, resulting in non-radiative decay and limited device performance. Therefore, for organic materials, fundamentally understanding the kinetics of triplet excitons has been an important issue in the progress of rationally improving the relevant lighting performance. In this thesis, we use time-resolved emission spectroscopy, in combination with steady- state spectroscopy and simulation tools, to systematically study the kinetics of triplet excitons in organic materials. The first part discusses how arrangement of an organic chiral-chromophore influences inherent triplet-exciton kinetics which results in enhanced deep-red room-temperature phosphorescence (RTP) in a racemic crystal (Nat. Commun. 11, 2145 (2020)). The second part analyzes energy-transfer kinetics in a host-guest system where a thermally-activated-delayed-fluorescence (TADF) host transfers triplet energy to near-infrared (NIR) fluorescent guests via reverse intersystem crossing followed by Förster energy transfer (J. Mater. Chem. C 8, 5704-5714 (2020)). The results of this thesis afford new understandings of the triplet kinetics, advancing the progress to achieve high- performance organic lighting application.
Part I
The correlation between molecular packing structure and its RTP, hence rational promotion of the intensity, remains unclear. We herein present racemism-enhanced RTP chiral chromophores by 2,2-bis-(diphenylphosphino)-1,1-napthalene (rac-BINAP) in comparison to its chiral counterparts. The result shows that rac-BINAP in crystal with denser density, consistent with a long-standing Wallach’s rule, exhibits deeper-red RTP at 680 nm than that of the chiral counterparts. The combination of population-decay measurements, time-resolved emission spectra, and crystal analyses reveals that the cross packing between alternate R (R-BINAP) and S (S-BINAP) forms in rac-BINAP crystal significantly retards the bimolecular quenching pathway, triplet-triplet annihilation (TTA), and hence suppresses the non-radiative pathway, boosting the RTP intensity. The result extends the Wallach’s rule to the fundamental difference in chiral-photophysics.
Part II
Energy transfer between an exciplex host and a fluorescent guest is a demanding process for attaining high-performance organic light-emitting diodes (OLEDs), particularly in NIR region; yet, insight into the dynamics of energy transfer has been elusive. In this study, new deep-red/NIR chromophores, NOz-TPA and NOz-t-TPA where NOz and TPA denote naphthobisoxadiazole and triphenylamine, respectively, have been developed with an electron donor–acceptor–donor (D–A–D) configuration. The optimized 1 wt% doped films for NOz-TPA and NOz-t-TPA blended with the Tris- PCz:CN-T2T (1:1 in molar ratio) exciplex host showed similar deep-red/NIR emissions with photoluminescence quantum yields (QY) of 42% (680 nm) and 28% (709 nm), respectively. Comprehensive time-resolved measurements and dynamics analyses revealed significant difference in the energy-transfer pathways, i.e. Förster versus Dexter- type energy transfer between the exciplex host and the fluorescent guest, in which the introduction of bulky tert-butyl groups in the NOz-t-TPA doped film greatly suppressed the Dexter-type energy-transfer pathway. Despite the lower QY, the analytical simulation predicted NOz-t-TPA to be a better candidate for realizing highly efficient electroluminescence. Confirmation was provided by the performance of the NOz-t-TPA- doped OLED, showing an external quantum efficiency (EQE) of 6.6% with peak wavelength at 710 nm, which is among the best records for metal-free NIR OLEDs around 710 nm. Insight into the energy transfer pathways thus play a pivotal role in achieving the high-performance OLEDs that incorporate the exciplex host and fluorescent guest.

並列關鍵字

organic light-emitting diodes ； organic room-temperature phosphorescence ； thermally activated delayed fluorescence ； triplet exciton ； time-resolved emission spectra ； intermolecular energy transfer

參考文獻

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