Raanan Carmielli

Northwestern University
2190 Campus Drive
Ryan Hall 1027
Evanston, IL 60208

E-mail: r-carmielli@northwestern.edu

Time resolved EPR (TREPR) is a well established method for measuring radical pair (RP) and triplet states in a donor – acceptor systems. The experiment constitute the application of a laser pulse and then the measurement is carried out using continuous microwave (CW) or through a series of microwave pulses for the detection of the polarized EPR signal. While optical spectroscopy has a clear advantage concerning the obtainable time resolution down to the femtosecond regime, the coupling of the reduction and oxidation is generally only visible in parallel kinetics of the component in the absorbance – difference spectra belonging to the two different redox partners. In contrast, TREPR suffers from a comparatively low time resolution of presently 10 ns but has a clear advantage of a direct observation of the coupling between the redox partners. This is due to the sensitivity of the magnetic resonance method to weak interactions that cannot be resolved by optical techniques on large molecules.

In the present research we use TREPR spectroscopy to study charge transport in a Donor – Bridge – Acceptor (D-B-A) systems such as DNA hairpins (Figure 1) and Pyrene – PDI aggregates. In addition we try to apply modern Pulse EPR techniques such as ESEEM, out of phased ESEEM and ENDOR on those systems.

Figure 1. Chemical structure of PDI (a) and of the DNA hairpins with PDI (b).

Figure 2. Triplet EPR spectra of the DNA hairpins recorded using the TrCW mode (a) and with the Two – pulse echo sequence (b).