Lin Group Research
Research Group & Research Interests
RESEARCH INTERESTS:
We have employed both cw and pulsed EPR techniques to study the structure and dynamics of paramagnetic species: reactive intermediates, such as free radicals; photo-induced paramagnetism, such as photo-excited triplet state; transition metal containing compounds and enzymes. Below are two areas of our current interests.
(1) Applications of zero-field (ZF) and near zero-field (NZF) EPR spectroscopy
Recently, we developed a field and frequency agile pulsed EPR spectrometer to measure the properties and dynamics of photo-excited triplet state of organic molecules in ZF and near zero magnetic fields. Three projects are encompassed in this area:
- Near zero-field voltage scanning spectrometers. We designed a new device to map the orientational anisotropy of paramagnetic systems without physically changing the crystal orientations in NZF pulsed EPR experiments (see Fig. 1). By implementing three sets of orthogonal coils around the sample, we are able to create a magnetic vector up to 2 mT in any three dimensional (3D) orientation in space. In NZF region, the hyperfine tensor elements are comparable to the electronic Zeeman interaction energy, thus very rich spectral patterns can be obtained by “dialing” in a magnetic field vector without moving the sample. The technique further allows us to examine the site symmetry of organic crystals and powdered solids doped with chromophores which can be photo-excited to the triplet state by laser light.
- Molecular motion at phase transition temperature. Pulsed zero-field electron paramagnetic resonance free induction decay (ZF EPR FID) techniques can be applied to study the molecular motion at the phase transition temperature of organic crystals by measuring selectively populated photo-excited triplet ZF transitions of a probe molecule in an organic crystal. The sensitivity of ZF spectroscopy to small shifts in local magnetic fields enables studies of guest-host configuration changes over a wide temperature range. Previously, we reported observing guest pentacene (-h14 and -d14 ) triplet ZF EPR FID spectra disappearing abruptly at Tc and spectral broadening and shift below Tc. We interpret these spectral changes as evidence for guest couplings to host phenyl rings. Further, these data allow us to assign spectroscopic sites to particular crystallographic sites occurring in the phase transition (see Fig. 1 given in the front page of our web-page).
- Nuclear quantum oscillation and quantum computer (in collaboration with Prof. Gerd Kothe, Freiberg University, Germany). When the nuclear spins are actively involved in the intersystem crossing process during the photo-excitation of organic triplet states, we expect to observe oscillatory nuclear spin polarization which gives rise to large signal enhancement factors in NMR. A fast field switching technique would further allow us to study the nuclear coherence at a level-crossing region of the near-by triplet spin substates. Quantum oscillations and polarization of the nuclear spin can then be utilized to build quantum computing (Fig. 2).
(2) Chemistry in the confined space of mesoporous silica (MPS) materials (in collaboration with Prof. Chung-Yuan Mou, National Taiwan University, Taiwan)
We have studied the physical chemistry of molecules and catalytic centers confined in the nano channels of MPS materials. MPS materials come in different pore size and dimension: MAS-9 (pore size: 9 -11 nm), SBA-15 (5 - 9 nm), MCM-48 (3 - 6 nm), and MCM-41 (2 - 5 nm, hexagonal channels) (see Fig. 3). These materials also possess the following unique properties to meet our applications: huge surface area (e.g., ~1000 m2/g in MCM-41), modifiable surface, and restricted pore nanospaces. Specifically, the surfaces of nanospace have controllable geometric parameters and well-defined physical and chemical properties, such as pore size, hydrophilic/ hydrophobic balance, mechanical and chemical stability, and surface chemistry. The molecules and catalytic centers of our interests are transition metal complexes, and enzymes and their biomimetics. The heterogenous catalysts provide stability for the catalytic centers, site isolation for good product selectivity, and easy product separation. These features are especially important for the application of enzymes as biocatalysts, as enzymes are sensitive to environmental conditions, such as temperature, pH value and solvent. We are interested in the following two different areas:
- Enzyme confinement. The project is to examine the physical chemistry of enzyme confinement, methods of immobilization, catalytic activity and advantages of protein confinements. The enzyme confinements in MPS materials could generate synergistic effects that enhance enzyme stability, improve product selectivity, and facilitate separation and reuse of enzymes (see Fig. 4). The goal is to show that immobilized enzymes in the nanospaces of MPS can be applied as viable biocatalysts for chemical and pharmaceutical industries. Initially, we studied a relatively small protein cytochrome c (25x25x37
), we will go on to larger enzymes. See our recent review article: C.-H. Lee, T.-S. Lin, and C.-Y. Mou, "Mesoporous Materials for Encapsulating Enzymes", NanoToday 4, 165-179 (2009). - Biomimetics confinement. Model complexes containing transition metals will be synthesized to mimic native enzymes. The goals of this project are to examine the stability and reactivity of biomimetic model complexes immobilized in the nanochannels of MPSs, to study the catalytic reaction mechanisms (see Fig. 5) in nanospace, and to exploit nanospace for single-site catalysis and enhancing product selectivity in nanoreactors. We have synthesized dinuclear copper complexes (hydroxo-bridged phenanthroline (or bipyridine) - copper) to mimic catechol oxidase, and copper zinc complexes to mimic superoxide dismutase. We then immobilized these biomimetics in the nanochannels of MSP materials. The studies enable us to gain insights into the configuration of the catalytic active site and the corresponding reaction pathways. See our recent reports on this subject matter: J. Phys. Chem. -B. 109, 775-784 (2005) and J. Phys. Chem. - C 113, 16058-16069 (2009).
