Browsing by Author "Chou, PT"

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Applications of semiconductor nanoparticles in electron transfer, energy transfer and metal ion recognition.
(2004-08-22)
Chou, YH
;
Chiu, HT
;
Chen, CY
;
Cheng, CT
;
Chou, PT
Heteroleptic cyclometalated iridium(III) complexes displaying blue phosphorescence in solution and solid state at room temperature
(2005-10-31)
Yang, CH
;
Li, SW
;
Chi, Y
;
Cheng, YM
;
Yeh, YS
A series of heteroleptic Ir(III) metal complexes 1-3 bearing two N-phenyl-substituted pyrazoles and one 2-pyridyl pyrazole (or triazole) ligands were synthesized and characterized to attain highly efficient, room-temperature blue phosphorescence. The N-phenylpyrazole ligands, dfpzH = 1-(2,4-difluorophenyl)pyrazole, fpzH = 1-(4-fluorophenyl)-pyrazole, dfmpzH = 1-(2,4-difluorophenyl)-3,5-dimethylpyrazole, and fmpzH = 1-(4-fluorophenyl)-3,5-dimethylpyrazole, show a similar reaction pattern with respect to the typical cyclometalated (C boolean AND N) chelate, which utilizes its orthosubstituted phenyl segment to link with the central Ir(III) atom, while the second 2-pyridylpyrazole (or triazole) ligand, namely, fppzH = 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole, fptzH = 3-(trifluoromethyl)-5-(2-pyridyl)triazole, and hptzH = 3-(heptafluoropropyl)-5-(2-pyridyl)triazole, undergoes typical anionic (N boolean AND N) chelation to complete the octahedral framework. X-ray structural analyses on complexes [(dfPZ)(2)Ir(fppz)] (1a) and [(fMPZ)(2)Ir(hptz)] (3d) were established to confirm their molecular structures. Increases of the pi pi* energy gaps of the Ir(III) metal complexes were systematically achieved with two tuning strategies. One involves the substitution for one or two fluorine atoms at the N-phenyl segment or the introduction of two electron-releasing methyl substituents at the pyrazole segment of the H(CAN) ligands. Alternatively, we have applied the more electron-accepting triazolate in place of the pyrazolate segment for the third (N boolean AND N)H ligand, Our results, on the basis of steady-state, relaxation dynamics, and theoretical approaches, lead to a conclusion that, for complexes 1-3, the weakening of iridium metal-ligand bonding strength in the T-1 state plays a crucial role for the fast radiationless deactivation. For the case of [(fMPZ)(2)Ir(hptz)] (3d), a thermal deactivation barrier of 4.8 kcal/mol was further deduced via temperature-dependent studies. The results provide a theoretical basis for future design and synthesis of the corresponding analogues suited to blue phosphorescent emitters.
Highly efficient red phosphorescent osmium(II) complexes for OLED applications
(2004-07-19)
Tung, YL
;
Wu, PC
;
Liu, CS
;
Chi, Y
;
Yu, JK
The nonionic, red-emitting complexes [Os(fppz)(2)L-2] (L = PPh2Me (1), PPhMe2 (2)) and [Os(bptz)(2)L-2] (L = PPh2Me (3)) were synthesized, showing highly intense red phosphorescent emission in CH2Cl2 solution at lambda(max) 617, 632, and 649 nm, respectively. The electroluminescent properties of these compounds on OLEDs showed promising device efficiencies required for future OLED applications.
Interplay between intra- and interligand charge transfer with variation of the axial N-heterocyclic ligand in osmium(II) pyridylpyrazolate complexes: Extensive color tuning by phosphorescent solvatochromism
(2005-10-21)
Li, SW
;
Cheng, YM
;
Yeh, YS
;
Hsu, CC
;
Chou, PT
The rational design and syntheses of a new series of Os-II complexes with formula [Os(fppz)(2)(CO)(L)] (1: L=4-dimethylaminopyridine; 2: L=pyridine; 3: L = 4,4'bipyridine; 4: L=pyridazine; 5: L = 4-cyanopyridine), bearing two (2-pyridyl)pyrazolate ligands (fppz) together with one carbonyl and one N-heterocyclic ligand at the axial positions are reported. Single-crystal X-ray diffraction studies of, for example, 2 reveal a distorted octahedral geometry in which both fppz ligands reside in the equatorial plane with a trans configuration and adopt a bent arrangement at the metal center with a dihedral angle of similar to 23 degrees, while the carbonyl and pyridine ligands are located at the axial positions. Variation of the axial N-heterocyclic ligand leads to remarkable changes in the photophysical properties as the energy gap and hence the phosphorescence peak wavelength can be tuned. For complexes 1 and 2 the solvent-polarity-independent phosphorescence originates from a combination of intraligand (3)pi-pi* ((ILCT)-I-3) and metal-to-ligand charge transfer transitions ((MLCT)-M-3). In sharp contrast, as supported by cyclic voltammetry measurements and theoretical calculations, complexes 3-5 exhibit mainly ligand-to-lilgand charge transfer (LLCT) transitions, resulting in a large dipolar change. The phosphorescence of complexes 3-5 thus exhibits a strong dependence on the polarity of the solvent, being shifted for example, from 560 (in C6H12) to 665 nm (in CH3CN) and from 603 (in C6H12) to 710 nm (in CH3CN) for complexes 3 and 5, respectively. The results clearly demonstrate that a simple, straightforward derivatization of the axial N-heterocyclic ligand drastically alters the excitation properties per se from intraligand charge transfer (ILCT) to LLCT transitions. The latter exhibit remarkable LLCT phosphorescence solvatochromism so that a broad range of color tunability can be achieved.
Iridium(III) complexes with orthometalated quinoxaline ligands: Subtle tuning of emission to the saturated red color
(2005-03-07)
Hwang, FM
;
Chen, HY
;
Chen, PS
;
Liu, CS
;
Chi, Y
Rational design and syntheses of four iridium complexes (1-4) bearing two substituted quinoxalines and an additional 5-(2-pyridyl) pyrazolate or triazolate as the third coordinating ligand are reported. Single-crystal X-ray diffraction studies of 1 reveal a distorted octahedral geometry, in which two dpqx ligands adopt an eclipse configuration, for which the quinoxaline N atoms and the C atoms of orthometalated phenyl groups are located at the mutual trans-and cis-positions, respectively. The lowest absorption band for all complexes consists of a mixture of heavy-atom Ir(III)-enhanced (MLCT)-M-3 and (3)pipi(*) transitions, and the phosphorescent peak wavelength can be fine-tuned to cover the spectral range of 622-649 nm with high quantum efficiencies. The cyclic voltarnmetry was measured, showing a reversible, metal-centered oxidation with potentials at 0.76-1.03 V, as well as two reversible reduction waves with potentials ranging from -1.61 to -2.06 V, attributed to the sequential addition of two electrons to the more electron-accepting heterocyclic portion of two distinctive cyclometalated CAN ligands. Complex 1 was used as the representative example to fabricate the red-emitting PLEDs by blending it into a PVK-PBD polymer mixture. The devices exhibited the characteristic emission profile of 1 with peak maxima located at 640 nm. The maximum external quantum efficiency was 3.15% ph/el with a brightness of 1751 cd/M-2 at a current density of 67.4 mA/cm(2) and the maximum brightness of 7750 cd/m(2) was achieved at the applied voltage of 21 V and with CIE coordinates of (0.64, 0.31).
Potassium ion recognition by 15-crown-5 functionalized CdSe/ZnS quantum dots in H2O
(2006-01-21)
Chen, CY
;
Cheng, CT
;
Lai, CW
;
Wu, PW
;
Wu, KC
Based on 15-crown-5 functionalized CdSe/ZnS quantum dots (QDs), we report a novel fluorogenic sensor to probe K+ ions in H2O; recognition of K+ can be achieved via the Forster type of energy transfer between two different color QDs, so that [K+] of the order of 10(-6) M can be promptly detected.
Spectroscopy and femtosecond dynamics of type-II CdSe/ZnTe core-shell semiconductor synthesized via the CdO precursor
(2004-07-29)
Chen, CY
;
Cheng, CT
;
Yu, JK
;
Pu, SC
;
Cheng, YM
CdSe/ZnTe type-II quantum dots (QDs) synthesized via the CdO precursor are reported. 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 it 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. 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.
Spectroscopy and femtosecond dynamics of type-II CdTe/CdSe core-shell quantum dots
(2006-01-16)
Chou, PT
;
Chen, CY
;
Cheng, CT
;
Pu, SC
;
Wu, KC
Syntheses of CdTe/CdSe type-II quantum dots (QDs) using CdO and CdCl2, as precursors for core and shell, respectively, are reported. Characterization was made via near-IR interbond emission, transmission electron microscopy (TEM), energy dispersive spectroscopy (EDX), and X-ray diffraction (XRD). Femtosecond fluorescence upconversion measurements on the relaxation dynamics of the CdTe core (in CdTe/CdSe) emission and CdTe/CdSe inter-band 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 1.96, 1.44 to 1.07 X 10(12) s(-1). The finite rates of the initial charge separation ore tentatively rationalized by the small electron-phonon coupling, causing weak coupling between the initial and charge-separated states.
Syntheses and photophysical properties of type-II CdSe/ZnTe/ZnS (core/shell/shell) quantum dots
(2005)
Cheng, CT
;
Chen, CY
;
Lai, CW
;
Liu, WH
;
Pu, SC
Syntheses of CdSe/ ZnTe/ ZnS (core/shell/shell) type-II quantum dots (QDs) are reported. Structural characterization was made via TEM, EDX, XPS and XRD. Photophysical properties were investigated via the interband emission (CdSe -> ZnTe) and its associated quantum efficiency as well as relaxation dynamics. In comparison to the weak emission (Phi(f) similar to 4 x 10(-3) in toluene) of CdSe/ ZnTe (3.9/ 0.5 nm), capping ZnS (0.4 nm in thickness) enhances the CdSe -> ZnTe interband emission by similar to 30 fold (Phi(f) similar to 0.12), whereas the peak wavelength shifts only slightly to the red. By encapsulating dihydrolipoic acid ( DHLA), water-soluble CdSe/ ZnTe/ ZnS QDs were also prepared, and they exhibited an interband emission at y930 nm with an emission yield of similar to 0.1. Femtosecond pulse excitation (lambda(ex) similar to 1200 nm) measurement estimated a two- photon absorption cross section, sigma, of similar to 70 x 10(-50) cm(4) s photon(-1) for DHLA-capped CdSe/ ZnTe/ ZnS in water, supporting its suitability for the use as near-infrared (NIR) dyes in biomedical imaging.
Type-II CdSe/CdTe/ZnTe (core-shell-shell) quantum dots with cascade band edges: The separation of electron (at CdSe) and hole (at ZnTe) by the CdTe layer
(2005-12-01)
Chen, CY
;
Cheng, CT
;
Lai, CW
;
Hu, YH
;
Chou, PT
The rational design and synthesis of CdSe/CdTe/ZnTe (core-shell-shell) type-II quantum dots are reported. Their photophysical properties are investigated via the interband CdSe -> ZnTe emission and its associated relaxation dynamics. In comparison to the strong CdSe (core only) emission (lambda(max) approximate to 550 nm, Phi(f) approximate to 0.28), a moderate CdSe -> Cd Te emission (lambda(max) approximate to 1026 nm, Phi(f) approximate to 1.2 x 10(-3)) and rather weak CdSe -> Zn Te interband emission (lambda(max) approximate to 1415 nm, Phi(f) approximate to 1.1 x 10(-5)) are resolved for the CdSe/CdTe/ ZnTe structure (3.4/1.8/1.3 nm). Capping CdSe/CdTe with ZnTe results in a distant electron-hole separation between CdSe (electron) and ZnTe (hole) via an intermediate CdTe layer In the case of the CdSe/CdTe/ ZnTe structure, a lifetime as long as 150 ns is observed for the CdSe -> Zn Te (1415 nm) emission. This result further indicates an enormously long radiative lifetime of approximate to 10 ms. Upon excitation of the CdSe/CdTe/ Zn Te structure, the long-lived charge separation may further serve as an excellent hole carrier for catalyzing the redox oxidation reaction.