1. Spectroscopy and Femtosecond Dynamics on the Excited-State Proton/Charge Transfer Coupled Reaction 2. The Photophysical Properties of the Azulenylocyanine Dye, a Near-infrared Nonfluorogenic Quencher 3. Carrier Relaxation Dynamic of the II-VI Semiconductor Quantum Dot and Size-dependent of the two-photon excitation Cross-Section relation
Date Issued
2006
Date
2006
Author(s)
Pu, Shih-Chieh
DOI
en-US
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, 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.
Subjects
奈米
飛秒
超快
染料
硒化鎘
碲化鎘
質子轉移
電荷轉移
nanometer
CdSe
CdTe
proton transfer
charge transfer
ultrafast
femtosecond
SDGs
Type
thesis
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