Femtosecond Pulse Shaping Enables Rapid Two-Dimensional Infrared Spectroscopy

Jul 01, 2013
By Spectroscopy Editors
Volume 28, Issue 7

What conclusions or insights are you gleaning from these results?

Zanni: One satisfying result from our work was a collaboration that was formed with James Nowick, who is an organic chemist at the University of California-Irvine. Nowick designs amyloid inhibitors. He saw my talk, and based on the mechanism, he and I designed a series of inhibitors together. They worked as we intended and we are now writing a manuscript on the topic. It illustrates that, for amyloid formation, the final fiber structure is less important for designing inhibitors than are intermediates.

Another field to which you have applied this method is solar cell research — specifically the study of charge injection, which is a key step in the conversion of solar to electrical energy in dye-sensitized nanocrystalline thin films. Why was this a good problem to tackle using your method?

Zanni: This problem is a good one for our method for similar reasons as for amyloid aggregation described above: NMR and most other standard structural tools cannot be applied to semiconductor interfaces like dyes on semiconductors. Thousands of dyes have been studied as potential next-generation solar cell materials, but the structure and orientation for nearly all of these dyes on the TiO2 is unknown. Two-dimensional IR spectroscopy has the added benefit that it is also an ultrafast technique, and so we also used it to time-resolve the injection of electrons from the dyes into the TiO2. For a model compound that we started with, we discovered that the molecule adopted two different conformations on the TiO2 surface and that one of those conformations had an electron injection time that was at least 10 times faster than the other. To my knowledge, no one had so definitively shown multiple conformations and certainly no one had resolved different injection rates on the same sample. The general thinking is that fast electron injection leads to higher efficiency. Thus, if one were trying to optimize a solar cell, one would presumably try to maximize the number of molecules that bind in the preferential conformation.

Your pulse-shaping approach has been extended to 2D visible spectroscopy. What is the status of that work?


Martin Zanni, PhD, is the Meloche-Bascom Professor of chemistry at the University of Wisconsin-Madison. Direct correspondence to: [email protected]
Zanni: Soon after collecting our first 2D IR spectrum using a mid-IR pulse shaper we realized that a similar approach could be used to collect 2D visible spectra as well. My research group did not own a visible pulse shaper, so I called a good friend of mine, Niels Damrauer at the University of Colorado. Niels is an expert in pulse shaping and so within a couple of months he had written the computer code to generate the visible pulse sequences and we got our spectra. As I stated above, visible pulse shaping has been around for 20 years so there are many researchers worldwide with the equipment already in their labs to perform experiments like these. There are quite a few groups that have now mimicked our work in this regard and are looking at very interesting materials and biological systems.

What are the next steps in your work on this method?

Zanni: We have just performed experiments that I think are a new technological and intellectual milestone in 2D visible spectroscopy. I think that it will enable many new insights into polymer and molecular photovoltaics. But it isn't published yet, so you'll just have to wait a few months.

References

(1) P. Hamm, M.H. Lim, and R.H. Hochstrasser, J. Phys. Chem. B 102(31), 6123–6138, DOI: 10.1021/jp9813286 (1998).

(2) S.-H. Shim, R. Gupta, Y.L. Ling, et al. Proc. Natl. Acad. Sci. U. S. A. 106(16), 6614–6619, DOI: 10.1073/pnas.0805957106 (2009).

(3) C.T. Middleton, P. Marek, P. Cao, et al., Nat. Chem. 4(5), 355–360, DOI: 10.1038/NCHEM.1293 (2012).


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