一、激發態質子與電子轉移偶和反應的光譜和飛秒動力學 二、近紅外光區的螢光抑制劑-Azulenylocyanine 染料的光物理現象 三、II-VI族半導體量子點的載子鬆弛動力學和雙光子吸收截面積與大小的關係

Abstract

Part I: 利用飛秒螢光和頻技術來研究ESIPT的分子是2-(2’-hydroxy-4’- dietheylaminophenyl) benzothiazole (HABT)和相關的衍生物，同樣的也是利用飛秒螢光和頻技術；而在環己烷中，2-(2’-hydroxyphenyl) benzothiazole的ESIPT速率大於35 fs-1，而HABT則相對較慢可以清楚的解析出速率為1.8 ps-1；至於在極性非質子性溶劑中，在初期動態學清楚的顯示，ESIPT正好匹配溶劑鬆弛的速率；另外當到達極性平衡後，此時發生的ESIPT會受到溶劑所造成的能壁限制，這是因為產物和生成物有著不同的極性，另外支持的證據就是2-(2’-methoxy-4’–dietheylamino -phenyl) benzothiazole (MABT)，這個分子由於被甲基化，所以不會發生ESIPT。基本上，N*的狀態在能量上是有利於ESIPT，在速率上，則跟溶劑的鬆弛速率競爭，在CH3CN這種極性很高的溶劑中，N*和T*的平衡分佈相對於低度極性的溶劑會受到很大的改變，而從實驗結果得知，不同溶劑下平衡比例，分別是Cyclohexane：CH2Cl2：CH3CN為23.5：4.71：0.57，而從變溫的實驗得知，在CH2Cl2溶劑下，溶劑引發的能量障壁大約為1.88 kcal/mol，而室溫下的ESIPT速率為6.8 ps-1，藉此讓我們對ESICT/ESIPT的偶和反應有更進一步的認識。 Part II: 有關AC的系統從合成以來，就一直有許多的科學家在研究其光物理的機制，但是AC似乎跟azulene不太相同，在溶劑中azulene違背了Kasha規則，除了S1→S0放光以外，還有直接從S2→S0放光。而AC在任何溶劑中，雖然有很高的吸收係數，但不放光(Φf < 10-6)；在飛秒螢光和頻與瞬態吸收光譜的實驗中，以及結合TDDFT的理論計算下，發現最低的S0→S1躍遷狀態基本上被禁止的，因此當760奈米波長的吸收，應該是S0→Sn的躍遷！從實驗中，發現到從Sn→S1能態的內部轉換約小於130 飛秒，而輻射衰退的速率大於233 ns，因為這是有一個約710 ± 70 fs的非輻射的鬆弛機制！ Part III: 主要是藉由飛秒螢光和頻技術來研究這兩種不同電子/電洞鬆弛方式的type-II量子點。首先是CdSe/ZnTe量子點，從光譜和飛秒動態測量的結果得知，光導致的電子核電洞在空間分離的速率，正好跟核大小的增加成反比，而跟殼的厚度無關，這個結果正符合CdSe的核心裡面電子與電洞的結合強度。至於另外一種形式的type-II量子點CdTe/CdSe，同樣的放光主要也是在IR區域，從飛秒螢光和頻測量的結果得知，當CdTe/CdSe的直徑從6.0、6.7增加到7.4 nm時，由光引發的電子轉移速率從510、690減少到930 fs-1，而且也跟殼層的厚度無關，在量測螢光時也發現到6.7 nm (核)/2.2 nm (殼)的CdTe/CdSe的量子點，聲子振動的頻率大約是12 cm-1，這種有限速率的電子分離機制，說明是電子與聲子的偶和效應相當的低。從上面這些研究成果，可以讓我們知道這種量子點的電子與電洞結合不同大小，以及載子在空間上分離速率的解析，正好可以配合太陽能電池中電荷在基質上的轉移，這一方面的研究。 還有就是由於量子點的雙光子吸收數值非常的大，遠大過目前所有的有機分子，按照理論預測，可能會高達50000GM，因此一直受到許多研究人員的注目，並希望能應用到生物醫學研究的雙光子顯微技術上，但是有關這一方面理論研究非常的缺乏，而我們研究的主題是集中在是否量子點會隨著體積大小，對於雙光子的吸收截面積有顯著的改變。從實驗結果清楚的得知，隨著量子點的體積增加，的確雙光子吸收截面積也隨之增加，似乎跟體積成正比（半徑的三次方）。

Part I: Detailed insights into the excited state intramolecular proton transfer (ESIPT) reaction in 2-(2’-hydroxy-4’-dietheylaminophenyl) benzothiazole (HABT) have been investigated via steady state and femtosecond fluorescence up-conversion approaches. In cyclohexane, in contrast to the ultrafast rate of ESIPT for the parent 2-(2’-hydroxyphenyl) benzothiazole (> 35 fs-1), HABT undergoes a resolvable, relatively slow rate (~1.8 ps-1) of ESIPT. In polar, aprotic solvents competitive rate of proton transfer and rate of solvent relaxation was resolved in the early dynamics. After reaching the equilibrium polarization in the normal state (N*), ESIPT takes place, associated with a solvent induced barrier due to different polarization equilibrium between normal (N*) and tautomer (T*) states. Supplementary support was also rendered via the study of 2-(2’-methoxy-4’-dietheylaminophenyl) benzothiazole (MABT), in which ESIPT is prohibited due to the lack of hydroxyl proton. The results are rationalized by a similar dipolar character between N and T* species, whereas due to the charge transfer effect N* possesses an appreciable dipolar change with respect to both N and T*. ESIPT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to the solvation relaxation process. In CH3CN, due to the strong solvent stabilization there exists an equilibrium between N* and T* states in e.g. CH2Cl2, and both forward and reversed ESIPT dynamics are associated with a solvent induced barrier due to different polarization equilibrium between N* and T*. The N* ↔ T* equilibrium constant was sdeduced to be 24.5, 4.71 and 0.57 in cyclohexane, CH2Cl2 and CH3CN, respectively. Temperature dependent relaxation dynamics further resolved a solvent induced barrier of 1.88 kcal/mol with a rate of 6.8 ps-1 at 298 K for the forward reaction in CH2Cl2. Part II: A Azulenylocyanine dye (AC) has been synthesized to investigate its associated photophysical properties. AC is essentially nonluminescent (Φf < 10-6) in any solvents despite its very high absorption extinction coefficient (760 nm, ε ~ 8.2×104 M-1cm-1 in methanol). Femtosecond fluorescence upconversion, anisotropy kinetics and transient absorption experiments, in combination with the theoretical TDDFT approach, lead us to conclude that the lowest S0 → S1 transition is partial optically forbidden in character, while the 760 nm absorption is ascribed to the fully allowed S0 → Sn (n ≥ 2) transition. The observed <130 fs decay component is attributed to the Sn → S1 internal conversion, while the S1 → S0, with a much slower radiative decay time (> 233 ns) undergoes a dominant radiationless deactivation 7 process (710 ± 70 fs) possibly governed by strong interaction between S1 and S0 potential energy surfaces. Part III: CdSe/ZnTe and CdTe/CdSe type-II quantum dots (QDs) are characterized in near-IR interband emission. Spectroscopic and femtosecond dynamic measurements reveal that the rate of photoinduced electron/hole spatial separation decreases with increases in the size of the core, and is independent of the thickness of the shell in the CdSe/ZnTe QDs. The results are consistent with the binding strength of the electron and hole confined at the center of CdSe. So far as CdTe/CdSe is concerned, the femtosecond fluorescence upconversion measurements on the relaxation dynamics of the CdTe core emission and CdTe/CdSe interband emission reveal that as the size of the core increases from 5.3, 6.1 to 6.9 nm, the rate of photoinduced electron separation decreases from 510, 690 to 930 fs. The finite rates of the initial charge separation are tentatively rationalized by the low electron-phonon coupling, causing small coupling between the initial and charge-separated states. The correlation between the core/shell size and the electron/hole spatial separation rate resolved in this study may provide valuable information for applications where rapid photoinduced carrier separation followed by charge transfer into a matrix or electrode is crucial, such as in photovoltaic devices. Tuning CdSe quantum dots (QDs) sizes and consequently their corresponding two-photon absorption (TPA) cross section have been systematically investigated. As increasing the size (diameter) of the quantum dots, the TPA cross section was found to be dependent on a 3.5 ± 0.5 and 5.6 ± 0.7 and 5.4 power of CdSe and CdTe QDs diameters, respectively. TPA cross section was measured to be as high as 1.0 × 10-46 cm4•s photon-1(104 GM) for CdSe QDs with a diameter of 4.8 nm. The results are rationalized on theoretical levels incorporating both one-photon and two-photon excitation properties on an exciton system.