ACS PRF | ACS
All e-Annual Reports
43082-AEF
Elucidating Structural Dynamics Coupled to Wire-Like Charge Transport in Donor-Bridge-Acceptor Molecules Using Time-Resolved Stimulated Raman Spectroscopy
My research has been geared toward investigating charge transport in solar energy relevant molecules. Specifically, through separate projects, I have focused on two major factors influencing the charge separation and recombination processes of organic donor-acceptor molecules: the effects of solvent and the role of intramolecular structural dynamics.
Intramolecular photoinduced charge separation and recombination within the donor-acceptor molecule 4-(N-pyrrolidino)naphthalene-1,8-imide-pyromellitimide, 5ANI-PI, were studied the room-temperature ionic liquid solvent, 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [EMIM][Tf2N]. Effects of this unconventional solvent on the charge transfer processes were studied using ultrafast transient absorption spectroscopy. Surprisingly, we found that the rate constants of both photoinduced charge separation and charge recombination at room temperature for 5ANI-PI in [EMIM][Tf2N] were comparable to those observed in pyridine, which has a static dielectric constant similar to that of [EMIM][Tf2N] but a viscosity that is nearly 2 orders of magnitude lower than that of [EMIM][Tf2N]. The electron-transfer dynamics of 5ANI-PI in [EMIM][Tf2N] were also compared to those in pyridine as a function of temperature. We found that in the ionic liquid only the charge recombination rate was significantly temperature dependent and attributed this result to a combination of the greater molecular structural changes of 5ANI-PI associated with the charge recombination process and the increased ionic liquid viscosity at lower temperatures. These results were published in the Journal of Physical Chemistry B.
The structural dynamics coupled to charge transport in electron donor-bridge-acceptor (D-B-A) molecules are the least well understood part of this fundamental process. A time resolved Raman spectroscopy technique was employed to help elucidate the role of structural change. To access the ultrafast time regime of the electron transfer events, a new technique, recently developed by Mathies et al., femtosecond time-resolved stimulated Raman spectroscopy (FSRS), was built in our lab and used to study the changes in molecular structure especially of the bridge that occur upon electron transfer in a series of D-B-A systems. The molecules under investigation contain a donor unit consisting of a 3,5-dimethyljulolidine molecule attached to anthracene (DMJ-A). Phenylene ethynyl units (PE) serve as the bridge to a naphthalenediimide (NI) acceptor. FSRS spectra of the DMJ-A-PE-NI molecules and the individual components were obtained and used to determine the important vibrational modes with the charge transfer process. The preliminary results of this project were presented at the Spring 2007 National ACS meeting. This is an ongoing project that is expected to yield important structural dynamics information associated with molecular charge transport.
I intend to continue research in the field of alternative energy. I will be starting another postdoctoral position at Argonne National Lab soon and will be investigating other solar energy related molecular systems using X-ray transient absorption spectroscopy.
