1.香豆素衍生物之合成及其在有機電致發光二極體及熱致變色材料物理性質之探討2.激發態電荷/質子轉移競爭平衡反應與分子偶極矩的關係

Abstract

本研究目的為開發以香豆素衍生物為主要分子架構的新穎有機電致發光二極體材料，總共合成出三個系列的具有二苯基集團的香豆素衍生物，分別會放射出紅、綠、藍三種不同螢光波長，如圖 (一) 所示。為了克服有機分子聚集化的現象，在分子設計上引入二苯基、並且在二苯基胺的鄰位及對位上修飾不同的推拉電子基，如：CN、CF3、Cl、H、Me、OMe，不僅可以有效的調控分子的共軛長度，和螢光的放射波長，進而控制分子的氧化電位，也就是游離電位，更進一步調控了香豆素衍生物的量子產率，因為這些拉電子基會造成電子與體的游離電位提高，而提高了TICT的能階，使得ICT的能階會低於TICT，所以螢光會由能量較穩定的ICT路徑放射出來，而抑制了TICT的路徑，呈現出取代基效應、游離電位、及量子產率有一良好的線性關係。這些香豆素衍生物具有良好的電化學特性，而三苯胺基的非共平面性，再加上香豆素內酯環的螢光強度，創造出三個系列具有潛力的非結晶性、傳輸電洞的發光材料，可以應用在有機電致發光二極體上。 另外，為開發以香豆素衍生物為主要分子架構的新穎可逆式熱致變色材料，如圖(二)中 3Br 與 3Br-2 所示。在鄰位的位置引入立體阻礙大的溴原子，進而控制質子化過程的酸鹼平衡只發生在 benzothiazole 上的氮原子上，企圖達到只有中性 (neutral) 分子，與單陽離子 (monocation) 兩種穩定的形式存在，經由外界的刺激下，例如溫度或酸的量的改變，促使逆反應的發生，而達到熱致變色的效果。 由變溫紫外光可見光光譜 (VT-UV-vis) 的測定，在不同極性的有機溶劑下，加入酸進行質子化反應，顏色會由黃色變為橘色。3Br (或3Br-2) 在二氯甲烷或氯仿中，由室溫冷卻到零度，顏色會由橘色變回黃色 (在氰甲烷中，則由室溫加熱到迴流)。變溫螢光光譜 (VT-Fluorescence) 與氫核磁共振光譜 (1H NMR) 同樣觀察到質子態的單陽離子，在加熱或降溫的過程中，發現具有明顯逆反應的發生，且具有相當好的熱穩定性。 我們並且合成出其他不同的電子與體及電子受體的香豆素衍生物，來說明取代基的立體效應與電子效應對整個熱致變色現象的影響。並結合中研院趙奕姼實驗室，共同探討在電子與體及受體中間不同長度及種類的連接架橋，利用理論計算的方法進一步加以快速篩選有效的連接架橋，並加以合理化並佐證熱致變色的現象。 最後，在激發態電荷轉移 (ESICT) 與激發態質子轉移 (ESIPT) 會有能階差，所以在不同溶劑極化環境下會導致明顯的ESICT/ESIPT競爭的平衡反應。不同於化合物3-hydroxyflavone只觀察到異構物的質子轉移螢光放射峰的結果，在極性、非質子酸溶劑 (aprotic solvents) 裡，我們會觀察到3-hydroxyflavone衍合物有兩個螢光放射峰，是分別經由ESICT與ESIPT兩種不同路徑個別的貢獻。 基於化學合成上的設計，可以透過巧妙地修飾分子結構上的官能基位置及推拉電子基強度，會改變了偶極矩的方向，可以調整 ESICT/ ESIPT 的平衡反應。在3-hydroxyflavone的4’及7號位置分別修飾擔任電子與體的二烷基胺類，在光譜上會觀察到兩種不同螢光波長，分別屬於ESICT和ESIPT，但若同時修飾二烷基胺類，在不同極性溶劑下，會只有一個 ESIPT 的螢光。證明改變偶極矩方向，可以微調ESICT/ESIPT的競爭平衡反應，這使得透過改變總偶極矩向量，進一步的合理設計ESICT/ESIPT系統變得具有可行性。

One of the objectives of this dissertation is to develop coumarin-based derivatives suitable for the application of OLED. Accordingly, three series of coumarin-based derivatives bearing diarylamino groups have been synthesized and their photophysical and electrical properties are evaluated. Their structures are showed in Fig.1 and they should emit red, green, and blue fluorescenct light, respectively. To address the structure-photophysical properties relationship, substituents with e--donating (i.e. methyl (-Me), (-OMe)) or e--withdrawing groups (i.e. cyano (-CN), trifluoromethyl (-CF3), chloro (-Cl)) are introduced at either para- or meta-positions. These substituents can tune the conjugation length, fluorescent wavelength, quantum yield, as well as oxidative potentials (ionization potential) of the molecules. As a result, the e--withdrawing groups exert effects on the raising ionization potential of e- donor and the energy level of TICT state. Thus, ICT radiation pathway becomes the dominant one. Good linear correlations were evidenced between substituents’ electronic effects and ionization potentials/quantum yields. Furthermore, these coumarin-based derivatives exhibit amorphous behaviors and hole-transporting materials, which are critical for effective emitting properties in the application of OLED. Development of novel reversible thermochromic materials is also the aim of my research endeavors. The coumarin-based molecules and the mechanism of reversible thermochromic behaviors are shown in Fig. 2. The bulky ortho-brominated aryl groups are implanted to create nonplanarity around amino center. Thus, the effective conjugation length to the protonation site is shortened and the energy gap between neutral and protonated forms is narrowed. The protonation can only occur at the nitrogen atom of the benzothiazole and the temperature variation disturbs the equilibrium between the neutral and monocation and the reverible thermochromic effect can thus be achieved. As evidenced by VT-UV-vis spectroscopy, unique “yellow-to-orange” colorimetric transitions of 3Br in various solvents were observed in the presence of 6N HCl (1 μL). In CH2Cl2 or CHCl3, the color of solution gradually changed from orange to yellow between r.t. and 0 oC The backward “yellow-to-orange” transition did occur when the stimulus is removed afterwards from the system. In CH3CN, it requires heating to 80 oC to observe the color changes. Both of VT-Fluorescence and VT-1H NMR spectra confirm the acid-base equilibrium in the heating-and-cooling cycles. The system showed excellent thermo-stability. Different e--donors and e--acceptors are incorporated at the phenyl ring of the benzothiazole to decipher the relationship between basicity of nitrogen atom and thermochromic phenomenon. In collaboration with Dr. Ito Chao et al. at Academic Sinica, different bridges with distinct conjugation features and lengths are evaluated for their roles on the thermochromic phenomenon. The last part of the dissertation is to study excited state intramolecular charge transfer (ESICT) and proton transfer (ESIPT) coupled reactions exemplified by a series of 3-hydroxyflavone derivatives. The difference in solvent polarity environment between excited state intramolecular charge transfer (ESICT) and proton transfer (ESIPT) states leads to a remarkable ESICT/ ESIPT coupled reaction. In polar, aprotic solvents, in contrast to a unique PT tautomer emission observed in 3-hydroxyflavone (3HF), dual emissions are resolved, consisting of ESICT and ESIPT bands. Based on the chemical synthetic design, we demonstrate an approach to modify the positions of functional groups and e-donating (or withdrawing) abilities, which can fine-tune the ESICT/ ESIPT coupled reaction via the dipolar orientation of the molecular framework. Dialkylamine, which is an e-donor, was introduced at the C(4’) or C(7) positions in 3HF resulting in similar ESICT/ESIPT dual emission. Even the net dipole moment, the orientation of which is affected by the CT effect, is expected different. Dialkylamine attached at both C(4’) and C(7) positions in 3HF, of which the net dipole moment may be associated with the interplay between two charge-transfer systems, displayed different photo behaviors. As a consequence, switching the dipolar property of the CT state can greatly tune the ESICT/ESIPT coupling behavior among them. This makes feasible further rational design of ESICT/ESIPT systems by altering the net dipolar vector.