Biomedical subjects

Lin X Chen

Publications and source records attributed to Lin X Chen.

11 recordsLinked to original sources

PubMed2006

Charge separation and surface reconstruction: a Mn2+ doping study.

Hydrothermal synthesis of Mn doped anatase (TiO2) nanoparticles using scrolled nanotubes of TiO2 and MnCl2 as the starting materials is described. Incorporation of Mn2+ ions on the substitutional sites was confirmed using X-ray absorption fine structure (FT-XAFS) while the oxidation state Mn(II) and coordination environment were determined using both electron paramagnetic resonance (EPR) and X-ray absorption near edge spectroscopy (XANES). Two different hyperfine couplings of 96 and 86 G were found using high-field (130 GHz) EPR reporting that Mn atoms occupy two distinct sites: one undercoordinated (reconstructed surface) and the other octahedral crystalline geometry (nanoparticle core), respectively. It was found that Mn atoms that occupy surface layers are weakly bound to the anatase lattice and can be easily leached using simple dialysis, while those incorporated in the nanoparticle core are bound more strongly and cannot be removed by dialysis. Light excitation EPR reveals that Mn ions incorporated in the surface layers participate in the charge separation, while those trapped deeply in the nanoparticle core do not show any photoactivity. Doping of the core of nanoparticles with Mn2+ ions, on the other hand, enables synthesis of optically transparent films having superparamagnetic behavior at room temperatures with a saturation magnetic moment of 1.23 microB per Mn atom.

Journal Article↗

PubMed2006

Dynamics of photoinduced electron transfer in a molecular donor-acceptor quartet.

The electronic structures and dynamics of photoinduced charge separation and recombination in a new donor/acceptor quartet molecule with bis-oligothiophene (BOTH) and bis-perylenediimide (BPDI) blocks attached to a benzene ring were described. Detailed transient spectroscopic studies were carried out on this compound and reference compounds at isolated molecular levels in solution. Two different dynamics of charge separation and recombination associated with two types of donor/acceptor pair conformations in solution were observed. These results were discussed based on Marcus theory and ascribed to both through-bond and through-space electron-transfer processes associated with two different orientations of the acceptors relative to the donor group. This molecular system exhibits a more efficient charge separation than charge recombination processes in both polar and nonpolar organic solvents, indicating that the material is an interesting candidate for photovoltaic studies in solid state.

Electron Transport↗

PubMed2006

The influence of bridging ligand electronic structure on the photophysical properties of noble metal diimine and triimine light harvesting systems.

This manuscript discusses the photophysical behavior of transition metal complexes of Ru(II) and Os(II) employed in development of light harvesting arrays of chromophores. Particular emphasis is placed on the relationship between the photophysical behavior of complexes having metal-to-ligand charge transfer (MLCT) excited states and the electronic characteristics of bridging ligands used in preparing oligometallic complexes. Examples are presented that discuss intramolecular energy migration in complexes having two distinct MLCT chromophores with bridging ligands that only very weakly couple the two chromophores. In addition, systems having bridging ligands with localized triplet excited states lower in energy than the MLCT state of the metal center to which they are attached are discussed. These systems very often have excited states localized on the bridging ligand with excited state lifetimes on the order of tens of microseconds. Finally, systems having Fe(II) metal centers, with very low energy MLCT states, are discussed. In complexes also containing bridging ligands with low energy triplet states, energy partitioning between the Fe center MLCT state (or Fe localized ligand field states) and the ligand triplet state is observed; the two states relax to the ground state via parallel pathways, but the Fe(II) center does not serve as an absolute excitation energy sink.

Electrons↗

PubMed2005

Excited state dynamics and structures of functionalized phthalocyanines. 1. Self-regulated assembly of zinc helicenocyanine.

Recently synthesized zinc helicenocyanine (ZnHc), where four helicene groups are fused with a phthalocyanine (Pc) core through all-carbon linkages, exhibits an unusually strong tendency of forming soluble molecular aggregates in organic solvents. The aggregation results in a strong optical absorption across most of the visible region, which is drastically different from that of its monomer. The aggregation is suppressed by dissolving ZnHc in a liquid crystal, octylbiphenylcarbonitrile (OBCN), where the monomer ZnHc dominates and exhibits a typical optical absorption spectrum of monomeric zinc phthalocyanine, except red shift in both Q- and B- bands due to pi-conjugation expansion. This study correlates optical properties and excited state dynamics of ZnHc with intra- and intermolecular electronic interactions, using quantum mechanical calculations and ultrafast transient absorption spectroscopy. Structural details of the aggregates are revealed by small-angle X-ray scattering (SAXS) to be uniformly dimers with alkoxy chains wrapped around the core of a face-to-face dimer. The results suggest that while the peripheral helicene moieties in ZnHc are electronically coupled to the Pc core via expansion of the pi-conjugation of the macrocycle, the coupling is attenuated by the "lock washer" conformation of the nonplanar peripheral helicenes which prevents pi-conjugation throughout the entire macrocycle. The interplay between pi-conjugation expansion in the macrocyle plane and the pi-pi stacking out of the macrocyle plane produces a structure that facilitates the unique optical properties and self-regulated assembly into nanoscale structures in solution. These novel optical properties are explored for potential applications in various areas.

Indoles↗

PubMed2005

Probing transient molecular structures in photochemical processes using laser-initiated time-resolved X-ray absorption spectroscopy.

Molecular structures during chemical processes are crucial for predicting molecular reactivity and reaction mechanisms. Using a laser pulse as an internal clock for starting fundamental chemical processes, molecular structural dynamics can be characterized by coherent vibrational motions and by incoherent transitions between different intermediate states. Recent developments in pulsed X-ray facilities allow structural determination of discrete excited states and reaction intermediates using laser-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS). Moreover, femtosecond X-ray sources have begun making significant contributions in monitoring coherent molecular motions. This review summarizes recent developments in the field, including technical and scientific challenges as well as several examples involving excited state molecular structure and electronic configuration determinations. Future applications of this technique with high time resolution will enable visualization of fundamental chemical events in many systems and further our understanding in photochemistry.

Absorptiometry, Photon↗

PubMed2004

Preparation and characterization of the native iron(II)-containing DNA repair AlkB protein directly from Escherichia coli.

The Escherichia coli AlkB protein was recently found to repair cytotoxic DNA lesions 1-methyladenine and 3-methylcytosine by using a novel iron-catalyzed oxidative demethylation mechanism. This protein belongs to a family of 2-ketoglutarate-Fe(II)-dependent dioxygenase proteins that utilize iron and 2-ketoglutarate to activate dioxygen for oxidation reactions. We report here the overexpression and isolation of the native Fe(II)-AlkB with a bound cofactor, 2-ketoglutarate, directly from E. coli. UV-vis measurements showed an absorption peak at 560 nm, which is characteristic of a bidentate 2-ketoglutarate bound to an iron(II) ion. Addition of excess amounts of single-stranded DNA to this isolated Fe(II)-AlkB protein caused a 9 nm shift of the 560 nm band to a higher energy, indicating a DNA-binding-induced geometry change of the active site. X-ray absorption spectra of the active site iron(II) in AlkB suggest a five-coordinate iron(II) center in the protein itself and a centrosymmetric six-coordinate iron(II) site upon addition of single-stranded DNA. This geometry change may play important roles in the DNA damage-searching and damage-repair functions of AlkB. These results provide direct evidence for DNA binding to AlkB which modulates the active site iron(II) geometry. The isolation of the native Fe(II)-AlkB also allows for further investigation of the iron(II) center and detailed mechanistic studies of the dioxygen-activation and damage-repair reactions performed by AlkB.

Binding Sites↗

PubMed2004

Structural characterization of modular supramolecular architectures in solution.

Structures of modular supramolecular architectures consisting of a hexameric, diphenylethyne-linked porphyrin macrocyclic array and the corresponding host-guest complex formed by inclusion of a tripyridyl guest molecule were characterized in solution using high-angle X-ray scattering. Scattering measurements made to 6 A resolution coupled with pair distance function (PDF) analyses demonstrated that (1) the porphyrin architectures are not rigid but are distributed across a conformational ensemble with a mean diameter that is 1.5 A shorter than the diameter of a symmetric, energy-minimized model structure, (2) the conformational envelope has limits of 3 A positional dispersion and full rotational freedom for all six porphyrin groups, and (3) insertion of the tripyridyl guest molecule expands the diameter of the host conformer by 0.6 A and decreases the configurational dispersion by approximately 2-fold. These results validate the molecular design, provide a new measure of conformational ensembles in solution that cannot be obtained by other techniques, and establish a structural basis for understanding the photophysical and guest-hosting functions of the hexameric porphyrin architectures in liquids.

Journal Article↗

PubMed2004

Taking snapshots of photoexcited molecules in disordered media by using pulsed synchrotron X-rays.

Photoexcited molecules are quintessential reactants in photochemistry. Structures of these photoexcited molecules in disordered media in which a majority of photochemical reactions take place remained elusive for decades owing to a lack of suitable X-ray sources, despite their importance in understanding fundamental aspects in photochemistry. As new pulsed X-ray sources become available, short-lived excited-state molecular structures in disordered media can now be captured by using laser-pulse pump, X-ray pulse-probe techniques of third-generation synchrotron sources with time resolutions of 30-100 ps, as demonstrated by examples in this review. These studies provide unprecedented information on structural origins of molecular properties in the excited states. By using other ultrafast X-ray facilities that will be completed in the near future, time-resolution for the excited-state structure measurements should reach the femtosecond time scales, which will make "molecular movies" of bond breaking or formation, and vibrational relaxation, a reality.

Journal Article↗

PubMed2003

MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations.

The molecular structure and dynamics of the photoexcited metal-to-ligand-charge-transfer (MLCT) state of [Cu(I)(dmp)(2)](+), where dmp is 2,9-dimethyl-1,10-phenanthroline, in acetonitrile have been investigated by time-domain pump-probe X-ray absorption spectroscopy, femtosecond optical transient spectroscopy, and density functional theory (DFT). The time resolution for the excited state structural determination was 100 ps, provided by single X-ray pulses from a third generation synchrotron source. The copper ion in the thermally equilibrated MLCT state has the same oxidation state as the corresponding copper(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu-N distance 0.04 A shorter than that of the ground state [Cu(I)(dmp)(2)](+). The results confirm the previously proposed "exciplex" structure of the MLCT state in Lewis basic solvents. The evolution from the photoexcited Franck-Condon MLCT state to the thermally equilibrated MLCT state was followed by femtosecond optical transient spectroscopy, revealing three time constants of 500-700 fs, 10-20 ps, and 1.6-1.7 ns, likely related to the kinetics for the formation of the triplet MLCT state, structural relaxation, and the MLCT excited-state decay to the ground state, respectively. DFT calculations are used to interpret the spectral shift on structural relaxation and to predict the geometries of the ground state, the tetracoordinate excited state, and the exciplex. The DFT calculations also indicate that the amount of charge transferred from copper to the dmp ligand upon photoexcitation is similar to the charge difference at the copper center between the ground-state copper(I) and copper(II) complexes.

Journal Article↗

PubMed2003

Excited state molecular structure determination in disordered media using laser pump/X-ray probe time-domain X-ray absorption spectroscopy.

Advances in X-ray technologies provide opportunities for solving structures of photoexcited state molecules with short lifetimes. Using X-ray pulses from a modern synchrotron source, the structure of a metal-to-ligand-charge-transfer (MLCT) excited state of Cu1(dmp)2+ (dmp = 2,9-dimethyl-1,10-phenanthroline) was investigated by laser pump/X-ray probe X-ray absorption fine structure (LPXP-XAFS) in fluid solution at room temperature on a nanosecond time scale. The experimental requirements for such pump probe XAFS are described in terms of technical challenges: (1) conversion of optimal excited state population, (2) synchronization of the pump laser pulse and probe X-ray pulse, and (3) timing of the detection. Using a laser pump pulse for the photoexcitation, a photoluminescent MLCT excited state of Cu1(dmp)2(BArF), (dmp = 2,9-dimethyl-1,10-phenanthroline, BArF = tetrakis(3,5-bis(trifluoromethylphenyl)borate) with a lifetime of 98 +/- 5 ns was created. Probing the structure of this state at its optimal concentration using an X-ray pulse cluster with a total duration of 14.2 ns revealed that (1) a CuII center was generated via a whole charge transfer; (2) the copper in the MLCT state bound an additional ligand to form a penta-coordinate complex with a likely trigonal bipyramidal geometry; and (3) the average Cu-N bond length increases in the MLCT excited state by 0.07 A. In contrast to previously reported literature, the photoluminescence of this pentacoordinate MLCT state was not quenched upon ligation with the fifth ligand. On the basis of experimental results, we propose that the absorptive and emissive states have distinct geometries. The results represent X-ray characterization of a molecular excited state in fluid solution on a nanosecond time scale.

Journal Article↗

PubMed2002

Rapid excited-state structural reorganization captured by pulsed X-rays.

Visible light excitation of [Cu(I)(dmp)(2)](BArF), where dmp is 2,9-dimethyl-1,10-phenanthroline and BArF is tetrakis(3,5-bis(trifluoromethylphenyl))borate, in toluene produces a photoluminescent, metal-to-ligand charge-transfer (MLCT) excited state with a lifetime of 98 +/- 5 ns. Probing this state within 14 ns after photoexcitation with pulsed X-rays establishes that a Cu(II) center, borne in a Cu(I) geometry, binds an additional ligand to form a five-coordinate complex with increased bond lengths and a coordination geometry of distorted trigonal bipyramid. The average Cu-N bond length increases in the excited state by 0.07 A. The transiently formed five-coordinate MLCT state is photoluminescent under the condition studied, indicating that the absorptive and emissive states have distinct geometries. The data represent the first X-ray characterization of a molecular excited state in fluid solution on a nanosecond time scale.

Journal Article↗