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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 semiconducting polymer-based solar cells. 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. Our group develops new techniques for deriving the pseudopotentials needed in mixed quantum/classical simulations, solving the Schr�dinger equation for electrons that reside largely between molecules, and calculating the free energies of quantum-mechanical objects in condensed phases. Our 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 electrons 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. For polymer-based
photovoltaics, the polymers are blended with an
electron acceptor, usually a fullerene derivative, and
the nanometer-scale morphology of the blend is
critical to device performance. Thus, much of
our work focuses on understanding the relationship
between the structure and function of polymer-based
solar cells so that the morphology can be controlled
and optimized for different applications. We
work to achieve this goal using information from
femtosecond and steady-state spectroscopies,
scanning probe microscopies, simulations and the
device physics of working polymer solar cells.
Projects include: spectroscopic studies of the
electronic structure of conjugated polymer films;
manipulating the interactions between polymer chains
and fullerenes using using sequential processing
and/or self-assembly techniques; studying the physics
of carriers in working photovoltaic devices using a
variety of transient current techniques; and studying
the effects of different types of disorder using
Drift-Diffusion simulations. 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. J. R.
Casey, B. J. Schwartz and W. J. Glover, "Free
Energies of Cavity and Noncavity Hydrated Electrons at
the Air/Water Interface," J. Phys. Chem. Lett.
7, 3192-8 (2016); DOI:
10.1021/acs.jpclett.6b01150. R. C.
Huber, A. S. Ferreira, R. Thompson, D. Kilbride, N. S.
Knutson, L. Sudha Devi, D. B. Toso, J. Reddy Challa, Z.
Hong Zhou, Y. Rubin, B. J. Schwartz and S. H. Tolbert, "Long-lived
Photoinduced Polaron Formation in Conjugated
Polyelectrolyte-Fullerene Assemblies," Science 348, 1340-3 (2015);
DOI: 10.1126/science.aaa6850. J. C.
Aguirre, S. A. Hawks, A. S. Ferreira, P. Yee, S.
Subramanyian, S. A. Jenekhe, S. H. Tolbert and B. J.
Schwartz, "Sequential
Processing
for Organic Photovoltaics: Design Rules for
Morphology Control by Tailored Semi-Orthogonal Solvent
Blends," Adv.
Energy Mater. 1402020, 1-11 (2015); DOI:
10.1002/aenm.201402020. W. J. Glover, J. R. Casey and B. J. Schwartz, "Free Energies of Quantum Particles: The Coupled-Perturbed Quantum Umbrella Sampling Method," J. Chem. Theor. Comput. 10, 4661-71 (2014); DOI: 10.1021/ct500661t. J. C.
Aguirre, C. Arnsten, S. Hernandez, R. Huber, A. M.
Nardes, M. Halim, D. Kilbride, Y. Rubin, S. H. Tolbert,
N. Kopidakis, B. J. Schwartz and D. Neuhauser, "Understanding
Local and Macroscopic Electron Mobilities in the
Fullerene Network of Conjugated Polymer-Based Solar
Cells: Time-Resolved Microwave Conductivity and
Theory," Adv.
Funct. Mater. 24(6), 784-92 (2014); DOI:
10.1002/adfm.201301757. J. R.
Casey, R. E. Larsen and B. J. Schwartz, "Resonance
Raman and Temperature-Dependent Electronic Absorption
Spectra of Cavity and Non-Cavity Models of the
Hydrated Electron," Proc. Nat. Acad. Sci. USA 110(8), 2712-7
(2013). S. C.
Doan and B. J. Schwartz, "Nature
of Excess Electrons in Polar Fluids:
Anion-Solvated Electron Equilibrium and Polarized
Hole-Burning in Liquid Acetonitrile," J. Phys. Chem. Lett. 4, 1471-6
(2013). A. L.
Ayzner, S. C. Doan, B. Tremolet de Villers and B. J.
Schwartz, "Ultrafast
Studies
of Exciton Migration and Polaron Production in
Sequentially Solution-Processed Conjugated
Polymer/Fullerene Quasi-Bilayer Photovoltaics," J. Phys. Chem. Lett.
3, 2281-7
(2012). A. E. Bragg, W. J. Glover and B. J. Schwartz, "Watching the Solvation of Atoms in Liquids One Solvent Molecule at a Time," Phys. Rev. Lett. 104, 233005, 1-4 (2010) Complete Publication List
(including links to full text of papers) last updated
08/03/16. 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 May, 2016. Back row,
from left to right: Matt Fontana; Chen-Chen Zho;
Ben Taggart; Ben Schwartz; Matthew Voss; Zongwu Fan;
Godwin Kanu; Zach Gvildys Not
pictured: Chien-Fu Huang (for the Southpark version of the Schwartz
group, click here).
Evolution
of
the Schwartz Group Please send comments on this page, notice of missing links, etc. to schwartz at chem.ucla.edu
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