Nam Seob Baek

Lanthanide-cored supramolecular systems with highly efficient light-harvesting dendritic arrays towards tomorrow’s information technology

We have developed novel lanthanide-cored supramolecular systems with highly efficient light-harvesting dendritic arrays for integrated planar waveguide-typed amplifiers. Er3+ ions were encapsulated by the supramolecular ligands, such as porphyrins and macrobicyclics. The supramolecular ligands have been designed and synthesized to provide enough coordination sites for the formation of stable Er(III)-chelated complexes. For getting a higher optical amplification gain, also, the energy levels of the supramolecular ligands were tailored to maintain the effective energy transfer process from supramolecular ligands to erbium(III) ions. Furthermore, to maximize the light-harvesting effect, new aryl ether-functionalized dendrons as photon antennas have been incorporated into lanthanide-cored supramolecular systems. In this paper, molecular design, synthesis and luminescent properties of novel lanthanidecored integrated supramolecular systems with highly efficient light-harvesting dendritic arrays will be discussed.

Synthesis and electroluminescence properties of novel silicon-based copolymers containing oxadiazole and fluorene units for PLED

Abstract Novel silicon-based copolymers containing an electron-deficient oxadiazole unit and a fluorine unit have been successfully synthesized through the Heck reaction. They are soluble in common organic solvents such as THF, CHCl3, etc. Their UV–visible absorption spectra exhibit a strong maximum band at the range of 355–381 nm in thin film. Upon a photoexcitation of 350 nm, their photoluminescence spectra show a strong maximum band around 455–475 nm in thin film. The multi-layered light-emitting diodes (LEDs) of Al(200 nm)/Ca(50 nm)/EL polymer(80 nm)/PEDOT(50 nm)/ITO were fabricated. J–V curves show the turn-on voltage in the range of 4.4–7 V. These LEDs emit the white emissive color, due to the combination of a blue electroluminescent (EL) color and a red EL color arising from the formation of a certain charge complex.

Synthesis and luminescent properties of novel silicon-based poly(p-phenylene) related polymers containing oxadiazole units for PLED

Two silicon-based alternating copolymers, namely SiHMOXD-1 and SiHMOXD-2, functionalized with an electron-deficient oxadiazole units, have been successfully synthesized through Heck reaction for blue light-emitting materials. The chemical structure and purity of these polymers have been characterized by FT-IR, H-NMR and C-NMR, gel permeation chromatography, 11 3 UV–Vis, photoluminescence (PL) and excitation spectroscopies, etc. The SiHMOXD-1 exhibits one strong UV absorption band at 356 nm. With an excitation wavelength of 366 nm, its PL spectrum shows a strong band at 425 nm in the blue region. The SiHMOXD-2 shows one strong UV absorption band at 342 nm. With an excitation wavelength of 352 nm, its PL spectra gives two strong bands at 425 and 456 nm in the blue region. 2002 Elsevier Science B.V. All rights reserved.

Molecular Design and Photophysical Criteria for Lanthanide Emission Enhancement in Erbium(III)-Cored Complexes Based on Dendritic Ligands for Information Technology

Recently, luminescent lanthanide complexes have considerable interest because of their academic interests and potential utility in a wide variety of photonic applications, such as planar waveguide amplifiers, plastic lasers, lightemitting diodes, and luminescent probes. The 4f electrons in lanthanide (Ln) ions are slightly perturbed by the effects of lattice phonons and static strain fields in the coordination environment of ions, since the f-electrons are shielded by the outer 5s and 5p electrons. It leads to the sharp spectral line-like emission bands. Also, the forbidden 4f -4f n electronic transitions renders the low absorption and emission cross-section of lanthanide ions, while luminescent lifetime is relatively long. To overcome these shortcomings, recently, luminescent ligands are being used to excite Ln ions via an energy transfer from the luminescent ligands to the Ln ions. In most cases, the luminescent Ln ions are usually coordinated to the organic luminescent ligands, acting as sensitizers or antenna chromophores, which efficiently absorb and transfer light to excite Ln ions via energy transfer process. This sensitization process is much more effective than the direct excitation of Ln ions, since the absorption coefficients of organic chromophores are many orders of magnitude larger than the intrinsically low molar absorption coefficients (typically 1-10 M cm) of Ln ions. Recently, several research groups have focused on developing the efficient artificial light-harvesting (LH) lanthanide complexes, in which the use of dendrimers for light harvesting systems has been widely demonstrated. The encapsulation of luminescent Ln ions into a luminescent dendrimer can lead to a system capable of shielding central Ln ion from nonradiative environment and efficiently transferring excited energy from the peripheral chromophores to the focal point of the dendrimer. For example, Frechet et al. have reported the site isolation and antenna effects on luminescent properties of spherical lanthanide(III)-cored dendrimer complexes. Although its spectral overlap integral (J) between the emission band of peripheral antenna and the absorption band of Ln ions was not satisfactorily large to obtain the effective energy transfer, the Frechet arylether typed dendrons were widely used as light-harvesting antenna. Moreover, very recently, to enhance the near-infrared (NIR) emission intensity and maintain the effective energy transfer process, our research efforts have been focused on developing stable and inert Er(III)-encapsulated complexes with artificial light-harvesting systems using dendritic luminescent ligands based on metalloporphyrins, naphthalenes, and anthracenes bearing the Frechet aryl-ether dendrons, namely, (Er3+-[Gn-Pt-Por]3(terpy), Er 3+-[Gn-Naph]3(terpy) and Er3+-[Gn-An]3(terpy)). We observed that the NIR emission intensity of the lanthanide complexes was dramatically enhanced with increasing the generation number (n) of the Frechet aryl-ether dendrons, due to the site-isolation and light-harvesting effects. Two possible energy transfer (ET) pathways for the sensitized emission in luminescent Ln(III) complexes have been suggested, as schematically illustrated in Scheme I. It is well-believed that in general only energy transfer from the

Temperature effect on photoluminescent properties of red light-emitting materials based on Ru(II)-chelated complexes

Abstract A new class of silicon-based copolymers containing Ru(II)-chelated complexes for new red light-emitting materials was developed by well-known Heck reaction between distyrylsilane monomer and the difunctionalized metal-chelated complexes. Ru(II)-chelated copolymers I and II showed one strong band approximately 265–289 nm for ligand units, one strong absorption band approximately 386–392 nm for π-conjugated backbones, and a broad shoulder metal-to-ligand charge transfer band approximately 460–465 nm. Ru(II)-chelated polymers exhibited a negligibly broad band approximately 495–500 nm in the greenish blue region and one/or two strong bands in the red region at room temperature. The photoluminescent (PL) properties of all materials as a function of temperatures were also investigated. With a photoexcitation wavelength of 325 nm at various temperatures, their PL intensity approximately 673–675 nm increased gradually with decreasing temperature, due to the restraint of the thermal relaxation decay.

Synthesis and Luminescent Properties of Novel Silicon-based Electroluminescent Copolymers with Ruthenium(II)-Chelated Complexes

Abstract A new class of silicon-based alternating copolymers having Ruthenium(II)-chelated complexes was synthesized to use as electroluminescent materials by Heck reaction between organosilicon divinyl monomers and Ru(II)-chelated monomers. The incorporation of organosilicon units with the aromatic or aliphatic groups on the silicon atoms into ®-conjugated systems improved their processability and interrupted the ®-conjugation length. The maximum absorption wavelength (λmax) of a non-chelated polymer (SiHMPhen) containing a phenanthroline (Phen) unit exhibited one strong bands at 305 nm for Phen units and a moderate peak around 350 nm for π-conjugated backbones. With an excitation wavelength of 360 nm, the PL spectrum of SiHMPhen exhibits a strong band at 420 nm in the blue region. Ru(II)-chelated copolymers showed strong absorption bands around 386–392 nm. Upon a photoexcitation with 400 nm, their PL spectra show a strong band at 430 nm in the blue region. The Ru(II)-chelated copolymers were thermally stable up to 300°C in air.

Synthesis and Photophysical Properties of Luminescent Erbium(III) Complexes Based on Coumarin Derivatives for Advanced Photonics Applications

The luminescent Er(III)-chelated complexes based on coumarin derivatives such as coumarin-3-carboxylic acid and coumarin 343 were prepared by ligand-exchange reaction in the presence of ErCl3and potassium coumarin derivative salts. The chemical structures of the present complexes were confirmed by elementary analysis, thermal gravimetric analysis. FT-IR, absorption and emission spectroscopies. The coumarin-based Er(III)complexes exhibited the near IR emission, corresponding to the characteristic4I13/2 → 4I15/2transition of trivalent erbium ions, taking place in the region of 1512 or 1530 nm. Their PL intensity also depends on the degree of the spectral overlap between the emission band of coumarin derivative and the absorption band of Er ion.

RECENT PROGRESS IN ERBIUM(III)-CORED COMPLEXES BASED ON DENDRITIC LIGANDS FOR ADVANCED PHOTONICS APPLICATIONS

We present that the stable and inert Er(III)-encapsulated complexes based on naphthalene and anthracene ligands bearing a Frechet aryl-ether dendron exhibit much stronger near-IR emission bands bands at 1530 nm, originated from the 4f–4f electronic transition of the first excited state (4I13/2) to the ground state (4I15/2) of the partially-filled 4f shell. A strong decrease in the fluorescence of Gn-aryl ether dendron (n = 0 or 2) is accompanied by strongly increasing the fluorescence intensity of the luminescent anthracene or naphthalene ligand with the generation number of the dendrons. The strong decrease of fluorescence intensity of luminescent ligand such as naphthalene and anthracene units is accompanied by strongly increasing the near infrared (IR) emission of the Er3+ ions in Er(III)-encapsulated complexes. It could be attributed to the efficient energy transfer process occurring between the aryl-ether dendron and anthracene moiety as well as between dendritic anthracene ligand and Er3+ ion. Thus, the emission intensity of the lanthanide complexes, upon photoexcitation of aryl-ether dendrons at 290 nm, was dramatically enhanced with an increase in the generation number n of dendrons, due to the site-isolation and light-harvesting effects. In addition, Er3+-[G2-An]3(terpy) exhibits the stronger PL intensity than Er3+-[G2-Na]3(terpy)) by 2.5 times, upon photoexcitation of aryl-ether dendrons at 290 nm. It may be due to the fact that the anthracene ligand in Er3+-[G2-An]3(terpy)) has higher spectral overlap integral (J) value than the naphthalene ligand in Er3+-[G2-Na]3(terpy) by 1.5 times. Surprisingly, all Er(III)-cored dendrimer complexes have excellent thermal- and photo-stability as well as good solubility.

LUMINESCENCE ENHANCEMENT OF EU(III)-CHELATED COMPLEXES BASED ON NAPHTHALENE DERIVATIVE THROUGH COORDINATION ENVIRONMENT

We have designed and synthesized novel luminescent lanthanide complexes based on a naphthalene derivative. The coordination number of complexed ligands with Eu3+ ions is an important parameter to enhance the photoluminescence (PL) intensity. The photoluminescence spectra of Eu(III)-cored complexes exhibit a sharp red emission band at 612 nm, corresponding to the 5D0→7F2 transition of the trivalent Eu ion. The saturated europium complexes (C-2 and C-3) have much stronger PL intensity than the unsaturated 6-coordinated europium complex (C-1), due to the instability of the C-1 complex. In this paper, the synthesis and photophysical properties of novel europium(III)-chelated complexes based on a naphthalene derivative will be discussed.

Direct synthesis and luminescent properties of new silicon-based alternating copolymers

A new class of silicon-based alternating copolymers having a thiophene or fused thiophene unit was synthesized to be used as electroluminescent materials using the Heck reaction between distyrylsilane monomers and appropriate thiophene dibromides. The incorporation of organosilicon units with the aromatic or aliphatic groups on the silicon atoms into π-conjugated systems improved their processability and interrupted the π-conjugation length. The resulting polymers were highly soluble in common organic solvents. They could be spin-cast onto various substrates to give highly transparent homogeneous thin films. Their glass transition temperatures were in the range of 94 to 116 °C. The UV-visible absorbance spectra of the resulting polymers appear at the wavelength range of 400 to 416 nm, due to the strong delocalization of the π-conjugated thiophene units. The silicon-based alternating copolymers showed a strong PL peak at around 512-526 nm in the green region.