Joseph Bullock
Northwestern University
2190 Tech Drive
Ryan Hall 1030
Evanston, IL 60208
Phone: 847-467-6696
E-mail: j-bullock@northwestern.edu
Education:
2000 - 2004: University of Kentucky
Major: Chemistry
2004 – Present: Northwestern University,
Ph. D. Candidate – Chemistry
Publications:
Woody, K. B.; Bullock, J. E.; Parkin, S. R.; Watson, M. D. Alternating Arene-Perfluoroarene Poly (phenylene ethynylenes).Macromolecules, 2007, 40, 4470-4473.
Bullock, J. E. ; Kelley, R. F.; Wasielewski, M. R. Self-Assembled Nanostructures for Organic Photovoltaics.PMSE Preprints, 2007, 96, 805-806.
Bullock, J. E. ; Kelley, R. F.; Cohen, B.; Wasielewski, M. R. Pi-stacked Nanostructures Bearing Charge-Separating Triads with Long-Lived Radical Pair States. Abstracts of Papers, 233rd ACS National Meeting, Chicago, IL, United States, March 25-29, 2007 (2007)
Swartz, C. R.; Parkin, S. R.; Bullock, J. E.; Anthony, J. E.; Mayer, A. C.; Malliaras, G. G. Synthesis and Characterization of Electron-Deficient Pentacenes.Org. Lett.2005, 7, 3163-3166.
Hegmann, F. A.; Tykwinski, R. R.; Lui, K. P. H.; Bullock, J. E.; Anthony, J. E. Picosecond Transient Photoconductivity in Functionalized Pentacene Molecular Crystals Probed by Terahertz Pulse Spectroscopy. Phys. Rev. Lett.2002, 89, 227403/1-227403/4.
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Research
Utilizing unimolecular photoactive electron donor-acceptor (D-A) systems in electronic
devices requires a method for creating an extended regioregular array of molecules.
Covalent attachment of an extended solid would be costly and inefficient. My project in
the Wasielewski group focuses on the design and synthesis of D-A systems that selfassemble
into well-ordered stacks. These systems display strong pi- pi orbital overlap
which leads to high charge mobility for many organic semiconducting systems.
Application of these materials to photovoltaic applications is also being studied.
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Self-assembled structure of the above molecule when
dissolved in methylcyclohexane as determined by SAXS |
Characterization of the self-assembled structures is performed in solution via small- and
wide-angle x-ray scattering (SAXS/WAXS). These experiments are conducted at
Argonne National Laboratory’s Advanced Photon Source where we collaborate with
numerous faculty members as well as beamline scientists.
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The Advanced Photon Source at Argonne National Laboratory |
Optical characterization of these materials is performed in our advanced laser laboratory
located adjacent to our synthesis laboratory. Femtosecond and nanosecond pulsed laser
spectroscopy gives us excellent time resolution of the charge-transfer events.
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Femtosecond transient absorption (left) reveals that the charge-separation occurs in 1.4
ps. Nanosecond transient absorption (right) indicates that it occurs in 47.1 ns |
In addition to x-ray and optical characterization, I am currently in the process of
analyzing the electron-transfer via electron paramagnetic resonance (EPR). The
Wasielewski group has a Bruker X-band EPR spectrometer with which I can excite my
material with a laser pulse, then use microwave pulses to measure the activity of the
transferred electron. Through this experiment we can see the free charges delocalize
through the noncovalent pi-stacks, confirming that these materials are excellent
candidates for photovoltaic applications.
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Time-resolved continuous-wave EPR results in toluene (left), showing full hyperfine splittings. On
the right, EPR results in methylcyclohexane show little to none hyperfine interaction, indicating a
much longer radical-pair distance. |
The time-resolved EPR results from the disggregated molecule show numerous hyperfine
interactions, due to the proximity between the anion and the cation. The results from the
aggregated structure show all the interactions becoming nearly neglible. This is a clear indication
that the distance between the anion and the cation is greater in the aggregated structure. This is
due to delocalization of the anion between all the pi-stacked PDI chromophores.
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