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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 polymers and their behavior when doped and when used in optoelectronic devices. 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 Schrödinger equation for electrons that reside largely between molecules, and calculating 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 and diatomic molecules 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,
thermoelectrics and photovoltaics 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. For
thermoelectrics, the polymers are doped with strong
oxidizing agents, and the reduction potential of the
dopant and the way its counterion interacts with the
holes on the polymer are the main factors determining
carrier mobility. Thus, much of our work focuses
on understanding the relationship between the
structure and function of polymer-based active layers
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 quantum chemistry
calculations, and the device physics of working
polymer solar cells and thermoelectrics. Projects
include: spectroscopic studies of the electronic
structure of conjugated polymer films and doped films;
manipulating the interactions between polymer chains
and fullerenes or dopants using using sequential
processing and/or self-assembly techniques; studying
the physics of carriers in working polymer-based
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. D. A.
Stanfield, Z. Mehmedović and B. J. Schwartz, "Vibrational
Stark Effect Mapping of Polaron Delocalization in
Chemically-Doped Conjugated Polymers," Chem.
Mater. (2021); DOI:
10.1021/acs.chemmater.1c02934. M.
G. Voss, J. R. Challa, D. T. Scholes, P. Y. Yee,
E. C. Wu, X. Liu, S. J. Park, O. Leon Ruiz, S.
Subramaniyan, M. Chen, S. A. Jenekhe, X. Wang, S.
H. Tolbert and B. J. Schwartz, "Driving
Force and Optical Signatures of Bipolaron
Formation in Chemically-Doped Conjugated
Polymers," Adv. Mater. 2000228,
1-7 (2020); DOI: 10.1002/adma.202000228. A. Vong, D. R. Widmer and B. J.
Schwartz, "Nonequilibrium
Solvent Effects during Photodissociation in
Liquids: Dynamical Energy Surfaces,
Caging and Chemical Identity," J.
Phys. Chem. Lett. 11, 9230-8
(2020); DOI: 10.1021/acs.jpclett.0c02515. T. J. Aubry, J. C. Axtell, V. M.
Basile, K. J. Winchell, J. R. Lindemuth, T. M.
Porter, J-Y. Liu, A. N. Alexandrova, C. P. Kubiak,
S. H. Tolbert, A. M. Spokoyny and B. J. Schwartz, "Dodecaborane-Based
Dopants Designed to Shield Anion Electrostatics Lead
to Increased Carrier Mobility in a Doped Conjugated
Polymer," Adv. Mater. 1805647, 1-8
(2019); DOI: 10.1002/adma.201805647. D. R. Widmer and B. J.
Schwartz, "Solvents
Can Control Solute Molecular Identity," Nature
Chemistry 10, 910-6 (2018); DOI:
10.1038/s41557-018-066-z. D. T. Scholes, P. Y. Yee, J.
R. Lindemuth, H. Kang, J. Onorato, R. Ghosh, C. K.
Luscombe, F. C. Spano, S. H. Tolbert and B. J.
Schwartz, "The Effects of
Crystallinity on Charge Transport and the Structure of
Sequentially-Processed F4TCNQ-Doped
Conjugated Polymer Films," Adv. Funct. Mater. 1702654, 1-13
(2017); DOI: 10.1002/adfm.201702654. 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. Complete Publication List
(including links to full text of papers) last updated
5/11/22. 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 |
