以理論計算探討含硫系統之光譜動力學效應與能源應用

本篇論文主旨為探討嶄新含硫系統在應用於有機發光二極體 (Organic Light-Emitting Diode, OLED) 或電池等能源相關材料時之獨特性質。本篇論文共分為兩個部分。在第一部分（第三、四、五章）中，我們以第一原理計算探討含硫之有機小分子系統的光譜動力學效應。第二部分（第六章）是以理論計算探討將一維TiS2(en)奈米結構應用於鋰離子電池電極之可行性。在第一部份中，我們探討在有機小分子系統中引入硫羰基之光物理現象。含硫羰基之有機小分子的最低單重激發態(S1)之躍遷為nπ*，同時硫原子的重原子效應可促使S1(nπ*)-Tn(ππ*)間自旋-軌道耦合的進行，進而導致室溫磷光的產生。在第三章中，我們以人工合成之DNA鹼基對dTPT3為雛形，精心設計了一系列的純有機小分子，並由共同合作之台大周必泰教授實驗室進行合成與光譜測量。其中擁有硫羰基物種之S1(1nπ*)與T1(3ππ*)能階間SOC積分值遠大於擁有羰基之物種，此現象歸因於前者適當的激發態能階排列與硫的重原子效應，進而導致在室溫下的溶液或固態中皆能夠同時產生螢光與磷光。在第四章中，我們與台大周必泰教授實驗室合作並共同發表了O-H----S氫鍵的形成以及其激發態分子內氫鍵開關反應所導致的室溫磷光。在此篇具開創性的文獻中，此現象由擁有強極性(C-O-H)----分散性(S=C)型氫鍵之DM-7HIT所展示。經激發後，DM- 7HIT異常地在550與685 nm處出現了雙重室溫磷光。我們發現DM- 7HIT最低激發態(S1)之躍遷為硫之非鍵結軌域(n)至π*，造成O-H鍵從有氫鍵之S1(nπ*)結構翻轉為無氫鍵之S’1(nπ*)結構，再透過系統間跨越與內轉換使T’1(nπ*)之電子分布提升。快速的氫鍵開/關行為在T’1(nπ*)與T1(nπ*)間發生，並在平衡達成後放出T1(nπ*，550 nm)與T’1(nπ*，685 nm)雙重磷光。這些結果證實含硫氫鍵開關機制的普遍性，也對氫鍵的化學開啟了一個新的篇章。在第五章中，我們使用第一原理計算探索硫酮衍生物中S2能階不同的去活化路徑。本研究所使用之模擬模型包含先前已成功合成之硫酮化合物與人工設計之分子，以找出可能可以展現S2激子分裂現象之基本共振單元。我們將不同的分子骨架以及取代基進行交互配對以調控較低能量能階之相對排列。透過合理且精細的分子設計，我們發現被改造之硫酮衍生物擁有接近2 eV之大的S2-S1能差同時也擁有進行稀有S2激子分裂的可能性，其三重態激子之放光波段被預測在紅光與近紅外光的範圍。在第二部份 (第六章) 中，我們擴大了所研究的含硫系統大小至一維鍊狀固態晶體結構。TiS2(en)為對層狀結構之TiS2使用維度縮減方法所切割出之一維鏈狀產物。我們使用第一原理計算，發現鋰原子嵌入LixTiS2(en)的過程遵循Rüdorff模型，且被預測將沿著一維TiS2(en)結構之軸方向進行擴散。LixTiS2(en)僅約0.27 eV的擴散能障近似於已商業化之LixCoO2(約0.21 eV)與LixFePO4(約0.16 eV)，表示鋰離子可順暢地在LixTiS2(en)中進行移動。LixTiS2(en)(0≦x≦1)之開路電壓為1.6 V至1.04 V，介於如1M LiPF6 in EC/DEC (1:1)之常見電解液的穩定工作電壓範圍(1.0 V – 4.7 V)內。有鑑於上述性質，TiS2(en)有機會被設計為一高安全性、擁有足夠輸出電壓，且能快速充放電的電池。此研究可激發未來對維度縮減之奈米結構在鋰離子電池之應用，相信可對尚未被發掘出的優勢進行更深入的探索。

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並列摘要

This thesis focuses on the physical properties of novel sulfur-containing systems aiming for practical applications on organic light-emitting diodes, batteries, and other energy materials. This thesis includes two parts. In the first part (the 3rd to the 5th chapters), we explore some of the important photophysical properties that sulfur atom exhibits in small and pure organic systems. In the second part (the 6th chapter), we evaluate the feasibility of the application of the 1D TiS2(en) nanostructure on lithium-ion battery electrode. In the first part (Chapter 3 to Chapter 5), we design and simulate a series of small organic molecules based on the unnatural DNA base pair, dTPT3. Through the collaboration with Prof. Pi-Tai Chou’s group in Natinoal Taiwan University (NTU), we find that for organic molecules bearing the thiocarbonyl group, a much larger SOC integral between S1(1nπ*) and T1(3ππ*) states can be achieved compared with their carbonyl counterparts. This is due to the appropriate energy level alignment and the heavy sulfur atom effect, resulting in the appearance of both fluorescence and phosphorescence in solution and solid state at room temperature. In Chapter 4, we collaborate with Prof. Pi-Tai Chou’s group in NTU and report on the uncommon O-H----S hydrogen-bond (H-bond) formation and its excited-state intramolecular H-bond on/off reaction unveiled by room-temperature phosphorescence (RTP). In this seminal work, this phenomenon is demonstrated with 7-hydroxy-2,2-dimethyl-2,3-dihydro- 1H-indene-1-thione (DM-7HIT), which possesses a strong polar (hydroxy)-dispersive (thione) type H-bond. Upon excitation, DM-7HIT exhibits anomalous dual RTP with maxima at 550 and 685 nm. This study finds that the lowest lying excited state (S1) of DM-7HIT is a sulfur nonbonding (n) to π* transition, which undergoes O-H bond flipping from S1(nπ*) to the non-H-bonded S’1(nπ*) state, followed by intersystem crossing and internal conversion to populate the T’1(nπ*) state. Fast H-bond on/off switching then takes place between T’1(nπ*) and T1(nπ*), forming a pre-equilibrium that affords both the T’1(nπ*, 685 nm) and T1(nπ*, 550 nm) RTP. The generality of the sulfur H-bond on/off switching mechanism, dubbed a molecule wiper, is rigorously evaluated with a variety of other H-bonded thiones. These results open a new chapter in the chemistry of hydrogen bonds. In Chapter 5, we explore the possibilities of different S2 deactivating pathways in thiones through first-principles calculations. Several theoretical models including the previously synthesized thiones and the artificially designed molecules are investigated to find the basic conjugation unit that exhibits the prospect of S2 fission. Various molecular motifs and different substituents are combined to maneuver the relative alignment of the relevant low excited energy states. Through rational and delicate molecular designs, we find that the thione derivatives may be engineered to possess a large S2-S1 energy gap as high as 2 eV and that these systems may exhibit a rare S2 fission to triplet excitons in the red to near infrared region. In the second part (Chapter 6), first-principles investigations on 1D TiS2(en) are performed to evaluate its potential as the electrode of lithium ion batteries. The intercalation of lithium ions into LixTiS2(en) follows the Rüdorff model and the lithium ions are predicted to diffuse along the one-dimensional axis of the TiS2(en) nanostructure. The small diffusion barrier of 0.27 eV of LixTiS2(en) is comparable to the commercialized LixCoO2 (≈0.21 eV) and LixFePO4(≈0.16 eV) electrodes, implying a facile transportation of lithium ions in LixTiS2(en). The voltage range of LixTiS2(en) is 1.6 V to 1.04 V for 0≦x≦1, which matches the wide stability window of an approximate 1.0 V - 4.7 V in the commonly-used 1 M LiPF6 in EC/DEC (1:1) electrolyte. In view of the above characters, TiS2(en) may be utilized to construct a safe LIB with an adequate voltage output and fast charging/discharging rate.