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: 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, e ~ 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.