| This thesis presents the results of an investigation of electron transfer between gold electrodes and ferrocene bridged by oligo(phenylenevinylene)s (OPVs), a class of π-conjugated organic molecules. Chapter 1 explains the experimental techniques used in this study. Chapter 2 describes the synthesis of ferrocene OPV thiols and their characterization in mixed self-assembled monolayers (SAMs). Cyclic voltammetry and elipsometry provide evidence that the SAMs are well-packed and that ferrocene exhibits the reversible electrochemistry expected for isolated redox sites. Chapter 3 reports the measurement of rate constants for electron transfer through these bridges using the Indirect Laser-Induced Temperature Jump (ILIT) technique. The distance dependence of the rate constants is unusual in that the rate constants do not decrease exponentially, as expected for a tunneling process, for bridge lengths out to 28 Å. Instead, the rate constants for bridges containing two, three and four repeat units are nearly identical. Analysis of the temperature dependence of the rate constants suggests an activation energy of 0.25 eV. Chapters 4 and 5 evaluate the possibility of multi-step tunneling, or hopping. This requires that an OPV frontier orbital is within 0.25 eV of the ferrocene highest occupied molecular orbital (HOMO). Chapter 4 describes experimental determination of HOMO levels from valence band photoelectron spectra. Chapter 5 contains calculations of OPV HOMOs using density functional theory. On the basis of these results, I find a minimum difference of 0.6 eV between the ferrocene HOMO and the OPV HOMO. Because this result is large compared with 0.25 eV, a hopping mechanism is ruled out.; Chapter 6 is a discussion of the work of a collaborator, Marshall Newton, who has calculated electron transfer matrix elements for electron transfer through OPVs. His calculations predict that for a single-step tunneling process in which tunneling is rate limiting, an exponential distance dependence with a decay constant of 0.4 Å−1 is expected. This mechanism can not explain the experimental results. We conclude that tunneling in this system is sufficiently fast so that it does not limit the rate, and we propose a structural reorganization as a rate-limiting process and experiments to test that hypothesis. |
