Browsing by Author "Wu, Chung-Wen"

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H-bonded effects on novel supramolecular dendrimers containing electron-transporting donor dendrons and single/double H-bonded acceptor emitters
(2006-11-14)
Wu, Chung-Wen
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Lin, Hong-Cheu
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材料科學與工程學系
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Department of Materials Science and Engineering
Novel asymmetric/symmetric supramolecular dendrimers were constructed by two kinds of (single/double) H-bonded acceptor chromophores, i.e., pyridyl/bispyridyl acceptor emitters, encapsulated with (one or two) 1,3,4-oxadiazole (OXD) donor dendrons in proper acceptor/donor molar ratios. These acceptor emitters were also complexed with another type of dendritic donor cores to form exterior emitting shells reverse to the previous system (with emitting cores) via self-assembly of complementary H- bonded moieties. Because of the shielding effect of bulky OXD dendritic shells in H- bonded donors, the supramolecular dendrimers are able to prevent acceptor emitters from spatial aggregation, and thus to induce glass-forming properties (with T-g) and show stronger emission intensities via H-bonds. Besides, the dendritic donors act as efficient light-harvesting antennae capable of transferring light energy from their peripheral OXD arms to their emitting acceptors, where the chromophore luminance induced by energy transfer is more efficient than that by direct excitation of the emitting cores. Compared with analogous dendritic mixtures without H-bonds, higher quantum yields were observed in the supramolecular dendrimers because of better energy transfer from OXD units. Therefore, compared with acceptor emitters, not only can the emission wavelength be tuned (up to 100 nm of red-shift) by H- bonds, but also much higher emission efficiencies of the H- bonded complexes were induced by reduced aggregation and energy transfer from the OXD donor dendrons.
Study of Supramolecular Side-Chain Copolymers Containing Light-Emitting H-Acceptors and Electron-Transporting Dendritic H-Donors
(2008-12-23)
Yang, Po-Jen
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Wu, Chung-Wen
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Sahu, Duryodhan
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Lin, Hong-Cheu
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材料科學與工程學系
A novel light-emitting hydrogen-bonded (H-) acceptor PBB (M1) containing three conjugated aromatic rings, including one pyridyl terminus and two lateral methoxyl groups (on the middle ring), was successfully synthesized via Horner-Wadsworth-Emmons (HWE) olefination and Sonogashira coupling reaction. Moreover, different molar ratios of light-emitting H-acceptor monomer PBB (MI) and hole-transporting monomer CAZ (M2) bearing a carbazole unit were copolymerized through free radical polymerization to obtain light-emitting and hole-transporting H-acceptor copolymers (P1-P5). H-acceptor copolymers P3 and P4 were complexed with different generations of dendritic H-donors (G1COOH-G3COOH) bearing 1,3,4-oxadiazole (OXD) dendrons and terminal benzoic acids via H-bonded self-assembly to form supramolecular side-chain copolymers (i.e., H-bonded dendritic complexes). In contrast to H-acceptor homopolymer P1(HPBB), H-acceptor copolymers P2-P4 incorporated with carbazole (CAZ) moieties effectively enhance the glass transition temperatures (T(g)s) and minimize the interchain interations of the light-emitting H-acceptor units, and similar effects occur in their H-bonded dendritic complexes. In addition, red shifts of photoluminescence (PL) emissions in H-bonded dendritic complexes can be tuned up to 61 nm. Furthermore, H-bonded dendritic complexes excited at 305 nm of OXD absorption can create a stronger fluorescence than that excited at 397 nm of PBB absorption, indicating that the intensity of the sensitized emission of PBB (by energy transfer from OXD absorption at 305 nm) is even stronger than that of a direct emission of PBB (by merely PBB absorption at 397 nm). The OXD dendritic wedges in H-bonded dendritic complexes can lower the LUMO energy levels and provide a better electron injection property. H-acceptor polymer P4 and its H-bonded dendritic complexes showed electroluminescence (EL) emissions in the range of 464-519 nm from blue to green. In addition, a PLED device containing H-bonded dendritic complex P4/G1COOH showed an EL emission of 519 nm under a turn-on voltage of 6.5 V. with a maximum luminance of 408 cd/m(2) at 18 V and a luminance efficiency of 0.39 cd/A at 100 mA/cm(2), respectively.
Synthesis and characterization of kinked and hyperbranched carbazole/fluorene-based copolymers
(2006-10-17)
Wu, Chung-Wen
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Lin, Hong-Cheu
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材料科學與工程學系
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Department of Materials Science and Engineering
To decrease the aggregation phenomena and improve the luminescent properties of polyfluorenes (PFs), a series of novel kinked and hyperbranched carbazole units were copolymerized by the Suzuki coupling polycondensation reaction to introduce disorder packing into the copolymer backbones. The thermal, photophysical, electrochemical, and electroluminescent properties of the copolymers were investigated. All of these polymers possess excellent thermal stability with onset decomposition temperatures at 407-466 degrees C and glass transition temperatures at 109-140 degrees C. Photoluminescent (PL) studies showed that these polymers were promising blue light-emitting materials, which exhibited high quantum efficiencies in solution and solid states. Long wavelength emissions at 500-600 nm, which were typical for PFs due to their self-aggregation in the solid state, were suppressed in these polymers. Annealing studies under air showed both improved thermal and photoluminescence stability of the polymers. It was proven that HOMO energy levels of the copolymers can be enlarged by increasing the carbazole content in the electrochemical measurements; hence, the hole injection was greatly enhanced. Pure-blue electroluminescence (EL) spectra with narrow fwhm (full width at the half-maximum) values (39-42 nm) and negligible low-energy excimer emission bands were successfully achieved, indicating that these copolymers could be good candidates for blue light-emitting materials.
Synthesis and characterization of poly(fluorene)-based copolymers containing various 1,3,4-oxadiazole dendritic pendants
(2006-06-27)
Wu, Chung-Wen
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Tsai, Chien-Min
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Lin, Hong-Cheu
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材料科學與工程學系
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Department of Materials Science and Engineering
A series of poly( fluorene)s (PFs) containing different generations of poly( benzyl ether) dendritic wedges with oxadiazole (OXD) peripheral functional groups, including copolymers bearing carbazole (CAZ) pendent groups, were synthesized. The dendritic side-chain polymers possessed excellent solubility in common solvents and good thermal stability with decomposition temperatures of 5% weight loss at more than 370 degrees C. Photophysical studies reveal that the photoluminescent ( PL) properties of the dendronized polymers were greatly affected by the size of the dendritic side chains. The G1- and G2-substituted PF derivatives greatly suppress the aggregation PF backbones and thus to induce pure blue PL emission ( with shorter wavelengths than PF). Excitation of the peripheral functional groups leads to substantial energy transfer from the dendrons to the poly( fluorene) backbones. In addition, the excitation of PF bearing G2 dendrons (G2-OXD) at 303 nm creates stronger fluorescence than that at 390 nm, indicating that the intensity of the sensitized emission ( by OXD absorption at 303 nm) is even stronger than that of a direct poly( fluorene) emission ( by PF backbone absorption at 390 nm). Double-layer light-emitting diode ( LED) devices with the configuration of ITO/PEDOT/polymer/LiF/Al were fabricated and reported. The device with PF bearing G0 dendrons and CAZ pendants (G0-OXDCAZ) as an emitter shows a turn-on voltage of 4.5 V and a bright luminescence of 2446 cd/m(2) at 12 V with a power efficiency of 0.24 cd/A at 100 mA/cm(2).
Synthesis and characterization of poly(fluorene-coalt-phenylene) containing 1,3,4-oxadiazole dendritic pendants
(2006-12-01)
Wu, Chung-Wen
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Sung, Hsiao-Hsien
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Lin, Hong-Cheu
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材料科學與工程學系
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Department of Materials Science and Engineering
A series of poly(fluorene-co-alt-phenylene)s containing various generations of dendritic oxadiazole (OXD) pendent wedges were synthesized by the Suzuki polycondensation of OY-D-functionalized 1,4-dibromophenylene with 9,9-dihexylfluorene-2,7-diboronic ester. The obtained polymers possessed excellent solubility in common solvents and good thermal stability. Photophysical studies showed that the dendronized polymers appended with higher generations of OXD dendrons exhibited enhanced photoluminescence efficiencies and narrower values of the full width at half-maximum. This was attributed to the shielding effect induced by the bulky dendiitic OXD side chains, which prevented self-quenching and suppressed the formation of aggregates/excimers. The energy transfer from the OXD dendrons to the polymer backbones was very efficient when excitation of the peripheral OXD dendrons resulted mainly in the polymer backbone emission alone. In particular, the photoluminescence emission intensities by the sensitized excitations of OXD dendrons in solid films of the polymers were all stronger than those by the direct excitations of their polymer conjugated backbones. (c) 2006 Wiley Periodicals, Inc.