- 學位論文
分子聚集體中相干性的光物理和元件應用
Photophysics and Device Applications of Coherence in Molecular Aggregates
摘要
在分子固體中，激發態電子能量流動的控制對光電器件的性能有很大影響。近年來，利用量子相干性來調控器件性能引起了極大的興趣。在本論文中，對分子聚集體的激發態相干性進行了全面研究，涵蓋了光物理性質和器件應用的基礎研究。內容分為三個主題： 1. 鑑定分子聚集體中激子離域的條件； 2. 激子離域對放光效率的影響； 3. 激子離域影響放光效率的機制，這些研究對於探討分子聚集體的量子干涉性應用提供嶄新的方向。 在第一部分，我們分析了分子晶體 2,2'-(thiazolo[5,4-d]thiazole-2,5-diyl)bis(4-methylphenol) (m-MTTM) 的激子離域，透過計算的激子耦合常數得知其晶體對應到 HJ 聚集體。線性偏振穩態吸收光譜證實紅移的電子躍遷主要來自與a軸不平行的聚集體。此外，變溫放光光譜顯示隨著溫度在 140-200 K 時降低，0-0 對 0-1 電子振動放光比的增加，其對應到 J 聚集體的特徵行為。為了解釋這些實驗結果，我們使用荷斯坦型哈密頓量進行模擬，考慮在兩粒子近似下的短程電荷轉移介導耦合（微擾機制），表明 ac 中的 3 × 3 層狀聚集體平面和 3 × 3 × 2 三維聚集體與 140 K 的發射光譜非常吻合。在 3 × 3 聚集體，ac 平面中的相干函數顯示沿 (1,0,-1) 的同相相關性，闡明了 J 聚集體如何在具有顯性正耦合的 HJ 聚集體中形成。這些發現為進一步控制三維分子固體中的激子離域提供初步的原理探討。 在第二部分中，我們研究了一種新興的近紅外 發光材料：聚集的Pt(II)配合物，它在室溫下表現出顯著的激子離域。我們從理論出發，指出顯著的激子離域可以將電子躍遷與基態的高能階振動去耦合，從而繞過能隙定律且顯著提高光致發光量子產率。通過設計和合成一系列新的 Pt(II) 配合物提供了實驗證明，這些配合物的離域長度為5-9 個分子，發光波長在 866-960 nm，固體中的光致發光量子產率為 5-12 %。理論和實驗結果都證實了激子離域可以使激子-振動去耦合，該現象應廣泛適用於其他排列良好的分子固體。 在第三部分中，為了詳細闡述激子離域和激子-振動解耦的原理細節，基於新型超快激發-偵測系統 (具有 3.2 fs解析度)，我們研究了自組裝 Pt(II)配合物4H在固體薄膜中的超快光譜動力學。結果顯示在激發態會出現頻率為 32 cm-1 (0.96 THz) 的顯著單模振動相干性。這種單模振動相干性 和分子間金屬-金屬-配體電荷轉移躍遷引起的集體面外運動有關，並且透過半衰期為 150 fs的系間竄越從單重態傳遞到三重態。類似的單模振動相干性特性也在其他高效率近紅外光發光體的Pt(II) 配合物中觀察到，並且其震動週期與電子離域長度有負相關性。從理論角度，我們發現單模振動相干性的守恆使激發態衰減能夠沿著低頻坐標進行，這有助於抑制非輻射衰減率並在 Pt(II)配合物的聚集體中造成高效率的近紅外光。這些新穎的結果突出了單模振動相干性在非輻射過程中的重要性，闡明了振動相干性及激子離域在分子固體中的基礎，作為改進器件性能的基準。 在最後的未來展望，我們提出用飛秒雷射退火的技術來做有機固態樣品的改質，進而控制分子相干性性質及發光效率，根據初步的結果，我們發現雷射退火後的樣品的分子相干性性質及激發態動力學沒有顯著改變，而相對放光強度最多增強大約2~3倍，這些結果如果再現性夠高的話，將會是在近紅外光的發光元件製程的重大突破。
並列摘要
Manipulating excited-state energy flows in molecular solids has a great influence on the devices developments. Among different approaches, utilizing quantum coherence to manipulate device performance has drawn great interest. In this thesis, a comprehensive study of excited-state coherence in molecular aggregates was presented, which covers fundamental investigations of photophysical properties and devices applications. The content is divided into three subjects: 1. Through-Space Exciton Delocalization in Segregated HJ-Crystalline Molecular Aggregates; 2. Overcoming the energy gap law in near-infrared OLEDs by exciton–vibration decoupling; 3. Excited-State THz Vibrations in Aggregates of Pt(II) Complexes Contribute to the Enhancement of Near-Infrared Emission Efficiencies. These studies pave ways for investigating novel application of quantum coherence in molecular aggregates. In the first section, the exciton delocalization of the molecular crystal was analyzed, which is the segregated HJ-aggregate characterized by the calculated exciton coupling of the single-crystal packing alignments. Linearly polarized steady-state absorption spectra confirm that the red-shifted transition primarily originates from aggregates oriented misaligned to the a-axis. Moreover, the temperature-dependent emission spectra reveal an increase in the ratio of 0−0 to 0−1 vibronic emission as the temperature decreases within the range of 140−200 K, characteristic of J-aggregates. To provide a more detailed explanation of these observations, we performed simulations using a Holstein-type Hamiltonian that considers short-range charge transfer-mediated couplings in the perturbative regime, employing the two-particle approximation. The simulation results demonstrate a consistent fit between the emission spectrum at 140 K and the presence of 3×3 laminar-like aggregates in the ac-plane and 3×3×2 three-dimensional aggregates. These discoveries open up possibilities for further controlling exciton delocalization in three-dimensional molecular solids. The second section of this study focuses on the investigation of aggregated Pt(II) complexes, an emerging near-infrared (NIR) luminescent material that exhibits remarkable exciton delocalization even at room temperature. We demonstrate that this significant exciton delocalization leads to notable enhancements in the photoluminescence quantum yield by decoupling the electronic transition from highly vibrational ladders in the singlet ground state, effectively bypassing the energy gap law. To provide experimental evidence, we designed and synthesized a series of new Pt(II) complexes with a delocalization length ranging from 5 to 9 molecules. These complexes exhibit emission in the range of 866-960 nm with photoluminescence quantum yields ranging from 5 to 12 % in solid films. In the third section, to elaborate the details of exciton delocalization and exciton–vibration decoupling, the ultrafast excited-state dynamics of the self-assembled Pt(II) complex 4H in a vapour deposited film was studied based on a home-build ultrafast pump-probe system with tunable time resolution. The results reveal the presence of a distinct single-mode vibrational coherence (VC) with a frequency of 32 cm-1 (~ 0.96 THz) in the excited state of the system. This single-mode VC is associated with collective out-of-plane motions induced by intermolecular metal-to-metal charge transfer transitions, facilitated through ultrafast intersystem crossings with lifetimes of 150 fs. Remarkably, similar single-mode VC characteristics were observed in analogues of 4H and other Pt(II) complexes exhibiting intense near-infrared (NIR) emission. The persistence of this single-mode VC enables the excited-state deactivation to predominantly occur along low-frequency vibrational coordinates. This phenomenon contributes to the suppression of nonradiative decay rates and results in highly intense NIR emission in aggregated Pt(II) complexes. These novel findings underscore the significance of VC in comprehending nonradiative processes and shed light on the fundamental role of VC in molecular solids. Furthermore, the observed VC serves as a benchmark for advancing device performance in optoelectronic applications. At last, we propose to use femtosecond laser annealing to modify organic solid-state samples to control the properties of coherent properties and emission efficiency. According to the preliminary results, we found that after laser annealing, the vibronic coherences and excited state dynamics are not significantly altered. By contrast, the relative emission intensity is increased by up to 4.3 times. If the reproducibility of these results is high enough, it will be a breakthrough in the device developments of NIR OLED.