Zachary Dance
Northwestern University
2190 Campus Drive, Room 2025
Evanston, IL 60208
E-mail: zexd@northwestern.edu
Ph: (847)491-4409
Joint student with Mark Ratner (theoretical chemistry group)
A necessary requirement in designing and creating functional materials for use in molecular electronics or photonic devices is the ability to achieve efficient long-distance charge transport. Currently, relatively few synthetic systems exhibit this characteristic, although a variety of natural systems, such as the photosynthetic reaction center, are proficient at it.[1] This capability has been the subject of several recent studies in systems classified as molecular wires.[2-5] Controlling the operation of these device-oriented materials depends critically on a comprehensive understanding of the behavior of the components which comprise the system.
I use time-resolved electron paramagnetic resonance (TREPR) spectroscopy to explore the dynamics of spin transport in a series of donor-bridge-acceptor (D-B-A) molecular wires. The D-B-A system employs a series of p-phenylene (Ph n) oligomers, where n = 1-5, to link a phenothiazine (PTZ) electron donor to a perylene-3,4:9,10-bis(dicarboximide) (PDI) electron acceptor, Figure 1. Previous work has shown that selective photoexcitation of PDI within PTZ-Ph n-PDI results in charge separation to produce a spin-coherent singlet radical ion pair (RP), 1(PTZ +•-Ph n-PDI -•).[2.6] The sequence of events following the initial charge separation is found to be dependent on both temperature and the magnitude of 2J, the intrinsic singlet-triplet energy splitting of the RP corresponding to the spin-spin exchange interaction. The energy of 2J correlates strongly with the distance separating the RPs, r DA, which relates directly to the electronic coupling matrix element for charge recombination (CR), V CR.
At early times the sublevels of the localized recombinant triplet state have unique non-Boltzmann spin populations, which result in spin-polarized EPR spectra that can be detected using TREPR spectroscopy. The spin dynamics that accompany the charge transfer events within these systems provide an important probe of both the mechanisms of spin transport and the electronic coupling matrix elements, V CR, for the various charge transfer processes.
[1] Parson, W. W. Electron donors and acceptors in the initial steps of photosynthesis in purple bacteria: a personal account Photosynthesis Research2003, 76, 81-92.
[2] Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.; Wasielewski, M. R. Making a Molecular Wire: Charge and Spin Transport through para-Phenylene Oligomers J. Am. Chem. Soc.2004, 126, 5577-5584.
[3] Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Molecular-wire behavior in p-phenylenevinylene oligomers Nature (London)1998, 396, 60-63.
[4] Davis, W. B.; Wasielewski, M. R.; Ratner, M. A.; Mujica, V.; Nitzan, A. Electron transfer rates in bridged molecular systems: A phenomenological approach to relaxation J. Phys. Chem. A1997, 101, 6158-6164.
[5] Goldsmith, R. H.; Sinks, L. E.; Kelley, R. F.; Betzen, L. J.; Liu, W.; Weiss, E. A.; Ratner, M. A.; Wasielewski, M. R. Wire-Like Charge Transport at Near Constant Bridge Energy Through Fluorene Oligomers Proc. Natl. Acad. Sci. U. S. A.2005, 102, 3540-3545.
[6] Weiss, E. A.; Tauber, M. J.; Kelley, R. F.; Ahrens, M. J.; Ratner, M. A.; Wasielewski, M. R. J. Am. Chem. Soc.2005, in press.
Education
2002 - Present Northwestern University
Ph.D. Candidate – Chemistry
1997 - 2002 Colorado State University – Honors Graduate
Major: Chemistry
Minors: Mathematics, Music Performance
Awards
- CSU Honors Graduate (Spring 2002)
- CSU Eddy Scholar (Spring 2002)
- Phi Beta Kappa (Spring 2000)
- Sigma Xi Science Honor Society (Spring 2001)
- Rueben G. Gustavson Senior Award (Spring 2001)
- Golden Key National Honor Society (Fall 1999)
- College of Liberal Arts Dean's List (Fall 1998)
- CSU Creative and Performing Arts Award (1998-2001)
- CSU Distinguished Scholar Scholarship (1997-2001)
|
