Richard F. Kelley

From Harrisburg, PA
Office Room #1031
Office Phone # 847-467-4944
r-kelley@northwestern.edu

Molecular photonic devices require well ordered chromophores through which energy and charge transfer is rapid and directional. The ideal chromophore should self-assemble and absorb light over a broad range of wavelengths. Additionally, for many applications efficient energy transport and quantitative charge separation among chromophores is highly desirable. A number of research groups have selected Chlorophyll a as a molecular component because it potentially satisfies all of these requirements. 1,2 Although efficient electron transfer was demonstrated in these systems, the choice of attachment of the various electron acceptors to the chlorophyll a macrocycle prevented any chance of self-assembly. Our work aims to recover the natural self-assembly properties of Chlorophyll a while maintaining desirable electron transfer characteristics.

My research is centered on cross-coupling to the d-meso position of Chlorophyll a. 3 This methodology allows for a variety of electron acceptors to be rigidly attached to the macrocycle core. Most importantly, the choice of attachment allows self-assembly to occur unhindered in all the donor-acceptor dyads we have synthesized. Perylene diimide acceptors serve as excellent antenna chromophores by absorbing strongly in the spectral region not covered by the chlorophyll donor and funneling that excitation energy back to the donor. Our spectroscopic results reveal that radical ion pairs result from excitation with virtually any visible wavelength. The lifetimes of these ion pairs vary with electron acceptor and range from 1 to 4 ns, which are comparable to lifetimes achieved in the previously studied chlorophyll-acceptor dyads. 1,2 Aggregation of the chlorophyll-based dyads is revealed by UV-Vis spectroscopy. As a whole, the results found to date set the stage for our future investigation of energy and charge migration through non-covalently bound systems.

(1) Helaja, J.; Tauber, A. Y.; Abel, Y.; Tkachenko, N. V.; Lemmetyinen, H.; Kilpelainen, I.; Hynninen, P. H., J. Chem. Soc., Perkin Trans. 11999, 2403-2408.

(2) Holtzwarth, A. R.; Katterle, M.; Muller, M. G.; Ma, Y.-Z.; Prokhorenko, V., Pure Appl. Chem.2001, 73, 469-474.

(3) Kelley, R. F.; Wasielewski, M. R., (in prep)2005.

Transient absorption spectra of ZC-Ph-PDIa in toluene following excitation with 655 nm, 120 fs laser pulses.