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Dongho Kim

Publications and source records attributed to Dongho Kim.

At least 19 recordsLinked to original sources
PubMed2006

Nonlinear optical properties and excited-state dynamics of highly symmetric expanded porphyrins.

A strong correlation among calculated Nucleus-Independent Chemical Shift (NICS) values, molecular planarity, and the observed two-photon absorption (TPA) values was found for a series of closely matched expanded porphyrins. The expanded porphyrins in question consisted of [26]hexaphyrin, [28]hexaphyrin, rubyrin, amethyrin, cyclo[6]pyrrole, cyclo[7]pyrrole, and cyclo[8]pyrrole containing 22, 24, 26, 28, and 30 pi-electrons. Two of the systems, [28]hexaphyrin and amethyrin, were considered to be antiaromatic as judged from a simple application of Hückel's [4n + 2] rule. These systems displayed positive NICS(0) values (+43.5 and +17.1 ppm, respectively) and gave rise to TPA values of 2600 and 3100 GM, respectively. By contrast, a set of congeners containing 22, 26, and 30 pi-electrons (cyclo[n]pyrrole, n = 6, 7, and 8, respectively) were characterized by a linear correlation between the NICS and TPA values. In the case of the oligopyrrolic macrocycles containing 26 pi-electron systems, a further correlation between the molecular structure and various markers associated with aromaticity was seen. In particular, a decrease in the excited state lifetimes and an increase in the TPA values were seen as the flexibility of the systems increased. Based on the findings presented here, it is proposed that various readily measurable optical properties, including the two-photon absorption cross-section, can provide a quantitative experimental measure of aromaticity in macrocyclic pi-conjugated systems.

Molecular Structure↗
PubMed2006

Relationship between incoherent excitation energy migration processes and molecular structures in zinc(II) porphyrin dendrimers.

Multiporphyrin dendrimers are among the most promising architectures to mimic the oxygenic light-harvesting complex because of their structural similarities and synthetic convenience. The overall geometries of dendrimers are determined by the core structure, the type of dendron, and the number of generations of interior repeating units. The rigid core and bulky volume of exterior porphyrin units in multiporphyrin dendrimers give rise to well-ordered three-dimensional structures. As the number of generations of interior repeating units increases, however, the overall structures of dendrimers become disordered and randomized due to the flexibility of the repeating units. To reveal the relationship between molecular structure and processes of excitation-energy migration in multiporphyrin dendrimers, we calculated the molecular structure and measured the time-resolved transient absorption and fluorescence anisotropy decays for various hexaarylbenzene-anchored polyester zinc(II) porphyrin dendrimers along with three types of porphyrin dendrons as references. We found that the congested two-branched type dendrimers exhibit more efficient energy migration processes than one- or three-branched type dendrimers because of multiple energy migration pathways, and the three-dimensional packing efficiency of dendrimers strongly depends on the type of dendrons.

Dendrimers↗
PubMed2006

Inverted sapphyrin: a new family of doubly N-confused expanded porphyrins.

The synthesis of an inverted, methoxylated sapphyrin derivative is described. This system, wherein inversion of pyrrolic nitrogen atoms is configurationally enforced via the use of a 3,3'-bipyrrolic precursor, displays what is best described as "weak aromaticity" as judged from its spectroscopic features and supporting theoretical calculations.

Crystallography, X-Ray↗
PubMed2006

Synthesis of brominated directly fused diporphyrins through gold(III)-mediated oxidation.

Highly efficient synthesis of meso,meso-dibromo doubly and triply fused diporphyrins has been achieved through a powerful oxidative coupling mediated by AuCl3-AgOTf combination. In addition, palladium-catalyzed debromination of meso-bromoporphyrins has been developed. This debromination protocol enables employment of bromine as a protecting group for the reactive meso-position of porphyrins.

Bromine↗
PubMed2006

Interaction of muscleblind, CUG-BP1 and hnRNP H proteins in DM1-associated aberrant IR splicing.

In myotonic dystrophy (DM1), both inactivation of muscleblind proteins and increased levels of CUG-BP1 are reported. These events have been shown to contribute independently to aberrant splicing of a subset RNAs. We demonstrate that steady-state levels of the splice regulator, hnRNP H, are elevated in DM1 myoblasts and that increased hnRNP H levels in normal myoblasts results in the inhibition of insulin receptor (IR) exon 11 splicing in a manner similar to that observed in DM1. In normal myoblasts, overexpression of either hnRNP H or CUG-BP1 results in the formation of an RNA-dependent suppressor complex consisting of both hnRNP H and CUG-BP1, which is required to maximally inhibit IR exon 11 inclusion. Elevated levels of MBNL1 show RNA-independent interaction with hnRNP H and dampen the inhibitory activity of increased hnRNP H levels on IR splicing in normal myoblasts. In DM1 myoblasts, overexpression of MBNL1 in conjunction with si-RNA mediated depletion of hnRNP H contributes to partial rescue of the IR splicing defect. These data demonstrate that coordinated physical and functional interactions between hnRNP H, CUG-BP1 and MBNL1 dictate IR splicing in normal and DM1 myoblasts.

Base Sequence↗
PubMed2006

Intramolecular and intermolecular energy transfers in donor-acceptor linear porphyrin arrays.

We present highly time-resolved spontaneous fluorescence spectra of a porphyrin array system that consists of an energy donor and an acceptor linked by a phenyl group. The donors are meso-meso directly linked zinc(II) porphyrin arrays and the acceptor is a zinc(II) 5,15-di(phenylethynyl)porphyrin. The spectra over the entire Q (S1) emission band following the excitation of the donor B (S2) state have been measured directly without the conventional spectral reconstruction method. The time-resolved fluorescence spectra revealed detailed energy relaxation processes within the donor and subsequent energy transfer to the acceptor. The observed energy transfer rates to the acceptor are consistent with the Forster energy transfer rates calculated on the assumption that the energy is localized in the Q state of each porphyrin unit of the donor prior to the energy transfer. The passage of the energy deposited initially on one porphyrin unit of the donor to the acceptor illustrates a sequence of energy delocalization and localization processes before it finally reaches the acceptor.

Journal Article↗
PubMed2006

Construction of segregated arrays of multiple donor and acceptor units using a dendritic scaffold: remarkable dendrimer effects on photoinduced charge separation.

Dendritic molecules appended with multiple zinc porphyrin units (DPm, m [number of zinc porphyrin units] = 6, 12, and 24) trap bipyridine compounds carrying multiple fullerene units (Py2Fn, n [number of C60 units] = 1-3), affording coordination complexes DPm superset Py2Fn having a photoactive layer consisting of spatially segregated donor and acceptor arrays on their surface. Complexes DPm superset Py2Fn are stable enough (K [average binding affinity] = 1.1 x 10(6)-4.4 x 10(6) M(-1) in CHCl3 at 25 degrees C) to be isolated by gel permeation chromatography. UHV-STM microscopy enables clear visualization of a petal-like structure of DP12 superset Py2F3. Photoexcitation of the zinc porphyrin units in DPm superset Py2Fn results in a zinc porphyrin-to-fullerene electron transfer to generate a charge separation. The charge-separation rate constant (kCS) in CH2Cl2 at 20 degrees C increases from 0.26 x 10(10) to 2.3 x 10(10) s(-1) upon increment of m and n, whereas the charge-recombination rate constant (kCR) remains almost unchanged at 4.5 x 10(6)-6.7 x 10(6) s(-1). Consequently, DP24 supersetPy2F3 furnishes the largest ratio of kCS/kCR (3400) among the family.

Dendrimers↗
PubMed2006

Comparative photophysical properties of free-base, bis-Zn(II), bis-Cu(II), and bis-Co(II) doubly N-confused hexaphyrins(1.1.1.1.1.1).

We have comparatively investigated the photophysics of a series of bis-metal doubly N-confused hexaphyrins(1.1.1.1.1.1) using time-resolved fluorescence, femtosecond transient absorption, two-photon absorption measurements, and geometry-optimized ab initio calculations. Bis-Zn(II) and free-base doubly N-confused hexaphyrins exhibit well-resolved and red-shifted B- and Q-like absorption bands compared with porphyrins. Their allowed transitions are (pi,pi) transitions of the hexaphyrin ring, as confirmed by the HOMO and LUMO frontier orbitals based on ab initio calculations at the B3LYP/6-31G level. On the other hand, the absorption spectra of bis-Cu(II) and bis-Co(II) doubly N-confused hexaphyrins are relatively broad, presumably due to large couplings between the metal d-orbitals and pi-electrons of the hexaphyrin ring. Owing to these couplings, bis-Cu(II) and bis-Co(II) doubly N-confused hexaphyrins have much shorter excited-state lifetimes of 9.4 +/- 0.3 ps and 670 fs, respectively, than those (267 +/- 16 and 62.4 +/- 1.2 ps, respectively) of bis-Zn(II) and free-base doubly N-confused hexaphyrins. The two-photon absorption cross section (sigma(2)) values, which are believed to depend strongly on the ring planarity (pi-conjugation), are in line with the excited-state lifetime trends.

Journal Article↗
PubMed2006

High fidelity self-sorting assembling of meso-cinchomeronimide appended meso-meso linked Zn(II) diporphyrins.

In noncoordinating solvents, meso-cinchomeronimide appended Zn(II) porphyrin 2 forms a cyclic trimer, while diporphyrins 7 exhibit high-fidelity self-sorting assembling to form discrete cyclic trimer, tetramer, and pentamer with large association constants from 7(in-in), 7(in)(-)(out), and 7(out-out), respectively, through almost perfect discrimination of enantiomeric and conformational differences of the meso-cinchomeronimide substituents. In the latter self-sorting processes, the dihedral angles dictated by the two pyridyl nitrogen atoms control the size of the aggregates; the trimer from 7(in-in), the tetarmer from 7(in)(-)(out), and the pentamer from 7(out-out). Cyclic structures of (2)(3) and (R-7(out-out))(5) have been determined by single-crystal X-ray diffraction analysis.

Biomimetic Materials↗
PubMed2006

Assemblies of supramolecular porphyrin dimers in pentagonal and hexagonal arrays exhibiting light-harvesting antenna function.

Porphyrin-based supramolecular macrocyclic arrays were synthesized as mimics of photosynthetic light-harvesting (LH) antennae. Pentameric and hexameric macrocyclic porphyrin arrays EP5 and EP6 were constructed by complementary coordination of m-bis(ethynylene)phenylene-linked zinc-imidazolylporphyrin Zn-EP-Zn. The proton NMR spectra of noncovalently linked N-EP5 and N-EP6 indicate fast rotation of the porphyrin moieties along the ethyne axis. These macrocycles were covalently linked and identified as C-EP5 (6832 Da) and C-EP6 (8199 Da) by mass spectrometry. Fluorescence quantum yields of C-EP2 (10.0%), C-EP5 (10.1%), and C-EP6 (11.0%), even larger than that of the unit coordination dimer C-EP1 (9.3%), were significantly increased from those of the series without the ethynylene linkage. The order of increasing fluorescence quantum yields was parallel to that of decreasing fluorescence lifetimes (C-EP1 (1.65 ns), C-EP2 (1.45 ns), C-EP5 (1.42 ns), and C-EP6 (1.38 ns)), indicating that the radiative decay rate kF increased relative to the other decay rates with an increase in the number of ring components. Based on the exciton-exciton annihilation and anisotropy depolarization times, the excitation energy hopping (EEH) times in these macrocyclic systems were obtained as 21 ps for C-EP5 and 12.8 ps for C-EP6. EEH times depend strongly on the orientation factor of the component transition dipoles in the macrocyclic arrays. The hexagonal macrocyclic array with an orientation of better transition dipole coupling resulted in faster EEH time compared to the pentagonal one.

Journal Article↗
PubMed2006

Photoluminescence of Cd1-xMnxS (x < or = 0.3) nanowires.

Cd1-xMnxS (x = 0.1-0.3) nanowires were synthesized by using the chemical vapor deposition method. They all consist of a single-crystalline wurtzite CdS structure with a [010] or [011] growth direction. The X-ray diffraction pattern reveals the contraction of the lattice constants due to the incorporation of Mn. The Mn2+ emission at approximately 2.15 eV, originating from the d-d (4T1 --> 6A1) transition, appears below 50-80 K. Its decay time is in the range of 0.55-1 ms, showing a decrease with increasing Mn content. The Mn doping reduces significantly the decay time of band-edge emission from 590 ps to 20-30 ps. Upon applying magnetic field (up to 7 T), the Mn2+ emission is suppressed and donor-acceptor pair emission becomes dominant, suggesting the energy transfer from the band electrons to the Mn2+ ions.

Journal Article↗
PubMed2006

A directly fused tetrameric porphyrin sheet and its anomalous electronic properties that arise from the planar cyclooctatetraene core.

Oxidation of a directly meso-meso linked cyclic porphyrin tetramer 2 gave a porphyrin sheet 3. The symmetric square structure of 3 is indicated by its simple 1H NMR spectrum that exhibits only two signals for the porphyrin beta-protons. The absorption spectrum of 3 displays characteristic Soret-like broad bands and weak Q-bands, and its magnetic circular dichroism (MCD) spectrum exhibits a negative Faraday A term at the 762 nm band as a rare case, indicating the absorption as a transition from a nondegenerate level to a degenerate level. A slightly longer S1-state (1.1 ps) and smaller TPA cross section (2750 GM) than a tetrameric linear porphyrin tape also indicate its unique electronic properties. The porphyrin sheet 3 forms stable 1:2 complexes with guest molecules G1 and G2, whose 1H NMR spectra exhibit remarkable downfield shifts for the guest protons that are located just above the cyclooctatetraene (COT) core of 3, whereas the imidazolyl protons bound to the zinc(II) porphyrin local cores are observed at slightly upfield positions. These results have been qualitatively accounted for in terms of the presence of a strong paratropic ring current around the COT core that propagates through the whole pi-electronic network of 3, hence competing with and cancelling the weak diatropic ring currents of the local zinc(II) porphyrins. This explanation was supported by DFT calculation performed at the GIAO-B3LYP/6-31G level, which indicated large positive NICS values within the COT core and small NICS values within the local zinc(II) porphyrins.

Journal Article↗
PubMed2006

A new entry to doubly N-confused [26]hexaphyrins(1.1.1.1.1.1) from normal [26]hexaphyrins(1.1.1.1.1.1) through an unprecedented double pyrrolic rearrangement.

Treatment of hexakis(pentafluorophenyl)-substituted [26]hexaphyrin(1.1.1.1.1.1) (1) with CuCl in the presence of pyridine and molecular oxygen led to formation of doubly N-confused [26]hexaphyrin(1.1.1.1.1.1) 2 in a moderate yield through an unprecedented double pyrrolic rearrangement. Macrocycle 2 has been shown to serve as an effective bis-metal-coordinating ligand and exhibits attractive optical properties such as a sharp Soret-like band at 566 nm and low-energy fluorescence at 1058 nm.

Journal Article↗
PubMed2006

Relationship between two-photon absorption and the pi-conjugation pathway in porphyrin arrays through dihedral angle control.

Recently, covalently linked or self-assembled porphyrin array systems have attracted much attention for their enhanced two-photon absorption (TPA) behaviors. In this study, we have investigated the TPA properties of various dihedral angle controlled, directly linked porphyrin dimers and arrays to elucidate the relationship between the pi-conjugation pathway and TPA properties. We have demonstrated a strong correlation between pi-conjugation (aromaticity) and TPA properties in porphyrin assemblies.

Journal Article↗
PubMed2006

Giant porphyrin wheels with large electronic coupling as models of light-harvesting photosynthetic antenna.

Starting from a 1,3-phenylene-linked diporphyrin zinc(II) complex 2ZA, repeated stepwise Ag(I)-promoted coupling reactions provided linear oligomers 4ZA, 6ZA, 8ZA, and 12ZA. The intramolecular cyclization reaction of 12ZA under dilute conditions (1x10(-6) M) gave porphyrin ring C12ZA with a diameter of approximately 35 A in 60% yield. This synthetic strategy has been applied to a 1,3-phenylene-linked tetraporphyrin 4ZB to provide 8ZB, 12ZB, 16ZB, 24ZB, and 32ZB. The intramolecular coupling reaction of 24ZB gave a larger 24-mer porphyrin ring C24ZB with a diameter of approximately 70 A in 34% yield. These two large porphyrin rings were characterized by means of 1H NMR spectroscopy, matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectroscopy, UV-visible spectroscopy, gel permeation chromatography (GPC) analysis, and scanning tunneling microscopy (STM) techniques. The STM images of C12ZA reveal largely circular structures, whereas those of C24ZB exhibit mostly ellipsoidal shapes, indicating more conformational flexibility of C24ZB. Similar to the case of C12ZA, the efficient excitation energy transfer along the ring has been confirmed for C24ZB by using the time-correlated single-photon counting (TCSPC) and picosecond transient absorption anisotropy (TAA) measurements, and occurs with a rate of (35 ps)(-1) for energy hops between neighboring tetraporphyrin subunits. Collectively, the present work provides an important step for the construction of large cyclic-arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna.

Light-Harvesting Protein Complexes↗
PubMed2006

A comparative evaluation of radiation-induced DNA damage using real-time PCR: influence of base composition.

To study the radiosensitivity of DNA segments at the open reading frame (gene) level, real-time PCR was used to analyze DNA damages induced by ionizing radiation. After irradiation (1, 3 and 5 kGy) of genomic DNA purified from Salmonella typhimurium, real-time PCR based on SYBR Green fluorescence and melting temperature was performed using various primer sets targeting the rfbJ, rfaJ, rfaB, hilD, ssrB, pipB, sopD, pduQ, eutG, oadB, ccmB and ccmA genes. The ccmA and ccmB genes, which existed as two copies on the chromosome and had a high GC content ( approximately 70%), showed much lower radiosensitivities than the other genes tested, particularly at 5 kGy; this distinctive feature was seen only when the genes were located on the chromosome, regardless of copy number. Our results reinforce the concept that gene sensitivity to ionizing radiation depends on the base composition and/or the spatial localization of the gene on the chromosome.

Base Composition↗