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Our research is directed at building a molecular-level understanding of chemical reactivity in complex environments by studying the dynamics of condensed phase chemical reactions using both experimental and theoretical techniques: femtosecond lasers and molecular dynamics computer simulations. Our research efforts fall into two principle areas: studies of how solvent molecules control the choice of products or the rate of solution-phase chemical reactions, and investigations into the ultrafast photophysics and electronic structure of conducting polymers. We also have worked in the past at fabricating three-dimensional microstructures using multiphoton lithography. Our first main area of research centers around solvent effects on chemical reactivity. As a chemical reaction takes place in solution, chemical bonds are broken and formed and electrical charge is redistributed between reactants and products. The solvent environment responds (via molecular reorientation or translation) to these changes in charge distribution and in size and shape of the reacting species on a variety of time scales extending from femtoseconds to longer than microseconds. In our group, we use femtosecond pump-probe spectroscopies to experimentally monitor the course of solution phase chemical reactions in real time. With femtosecond time resolution, we are able to "watch" the motions of solvent molecules responding to chemical changes of reacting solutes, monitor the flow of energy between reacting species and the solvent, and identify the motion of electrons during charge transfer reactions. These experiments are accompanied by computer simulations, ranging from simple solutions of Newton's classical equations of motion to sophisticated algorithms allowing for quantum dynamics in the absence of the Born-Oppenheimer approximation. The simulations provide molecular detail that is unavailable from experiment; on the computer, for example, we can easily separate the roles of solvent rotational and translational motions in solvating a chemical species or in providing the energy needed to promote a charge transfer reaction. Projects range from studies of model systems such as solvated metal anions or solvated electrons and dielectrons to investigations of large organic molecules with complex photochemistry. Simulations are done in close connection to the femtosecond experiments, so that experimental results drive new simulations and vice-versa, providing students in the group with an opportunity to do both experimental and theoretical work. Our second main
area of research is focused on the electronic structure of conjugated
polymers. Conjugated polymers are remarkable materials that have the
electrical properties of semiconductors but the mechanical properties
and processing advantages of plastics. This gives these materials
enormous commercial potential for use in light-emitting diodes,
displays and photodetectors that have large areas and are flexible.
Upon photoexcitation, the electrons and holes created in semiconducting
polymers interact with their environment, leading to relaxation
processes on multiple time scales; many of the important dynamical
issues are qualitatively similar to the solution phase reactions
discussed above. One of the main focuses in our group is the study
of how excitations on neighboring conjugated polymer chains interact.
Changing the way in which a conjugated polymer film is made changes
the interactions between polymer chains, allowing electronic and
photophysical properties of the film to be optimized for desired
optoelectronic applications. By
combining information from femtosecond and steady-state spectroscopies,
scanning force microscopy, and the behavior of polymer light-emitting
or photovoltaic devices, we can identify and potentially eliminate
undesirable electronic properties. Projects include: studies
of energy transfer in inorganic/conjugated polymer composite materials;
studies of the electronic structure of conjugated polymer films;
manipulating the interactions between polymer chains using the
electrical charges present on conjugated ionomers and
polyelectrolytes; and characterization of polymer-metal electrode
interfaces using non-linear spectroscopies such as second harmonic and
sum-frequency generation. This work provides students the opportunity
to learn fundamental photophysics, polymer processing techniques, and
semiconductor device construction.
If you have problems viewing any of these papers (all in PDF format), try "Save Target As" instead of viewing directly through your browser. I. B. Martini, I. M. Craig, W. C. Molenkamp, H. Miyata, S. H. Tolbert and B. J. Schwartz, "Controlling Optical Gain in Semiconductor Polymers with Nanoscale Chain Positioning and Alignment," Nature Nanotechnology 2, 647-52 (2007). M. C.
Cavanagh, R. E. Larsen and B. J. Schwartz, "Watching Na atoms Solvate
into Na+:e¯
Contact Pairs: Untangling the Ultrafast
Charge-Transfer-to-Solvent Dynamics of Na¯ in Tetrahydrofuran
(THF)," J. Phys. Chem. A 111, 5144-57 (2007). M. J. Bedard-Hearn, R. E. Larsen and B. J. Schwartz, "Projections of Quantum Observables onto Classical Degrees of Freedom in Mixed Quantum/Classical Simulations: Understanding Linear Response Failure for the Hydrated Electron," Phys. Rev. Lett. 97, 130403 (2006). R. E. Larsen and B. J. Schwartz, "Nonadiabatic Molecular Dynamics Simulations of Correlated Electrons in Solution. I. Full Configuration Interaction (CI) Excited-State Relaxation Dynamics of Hydrated Dielectrons," J. Phys. Chem. B 110, 9681-91 (2006). M. J.
Bedard-Hearn, R. E. Larsen and B. J. Schwartz, "Mean-Field Dynamics with
Stochastic Decoherence (MF-SD): A New Algorithm for Nonadiabatic
Mixed Quantum/Classical Molecular Dynamics Simulations with
Nuclear-Induced Decoherence," J.
Chem. Phys. 123,
234106, 1-17 (2005). M. J.
Bedard-Hearn, R. E. Larsen and B. J. Schwartz, "The
Role of Solvent Structure in the Absorption Spectrum of Solvated
Electrons: Mixed Quantum/Classical Simulations in Tetrahydrofuran
(THF)," J. Chem. Phys.
122, 134506, 1-11 (2005). Evidence for the Direct Production of Interchain Species in Conjugated Polymer Films: The Ultrafast Stimulated Emission and Fluorescence Dynamics of MEH-PPV," Phys. Rev. B, 69(3) 035204, 1-12 (2004). B. J. Schwartz, "Conjugated
Polymers as Molecular Materials: How Chain Conformation and
Morphology Influence Energy Transfer and Interchain Interactions,"
Annu. Rev. Phys. Chem. 54, 141-72 (2003). I. B. Martini, E. R. Barthel and B. J. Schwartz, "Optical Control of Electrons During Electron Transfer," Science 293, 462-5 (2001). T.-Q. Nguyen,
J. Wu, V. Doan, B. J. Schwartz and S. H. Tolbert, "Control
of Energy Transfer in Oriented Conjugated Polymer/Mesoporous Silica
Composites," Science 288, 652-6 (2000). Complete Publication List (including links
to full text of papers) last updated 4/3/08. If you have problems viewing any of these papers (all in PDF format), try "Save Target As" instead of viewing directly through your browser.
Department
of Chemistry & Biochemistry
Student Office/Lab: (310) 206-6418
Current members of the Schwartz Group
The Schwartz group in October, 2005. Back row, from left
to
right: Mike
Bedard-Hearn, Ryan Young, Alex Ayzner, Cesar Mejia,
Stephanie Doan, Betsy Raymond, Molly Cavanagh, Gagik Labadzhyan Evolution
of the Schwartz Group Please send comments on this page, notice of missing links, etc. to schwartz@chem.ucla.edu
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