Electron tunneling through phenylene bridges

Citation

Villahermosa, Randy (2002) Electron tunneling through phenylene bridges. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-05232005-084837

Abstract

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A series of donor-bridge-acceptor (D-B-A) complexes, [...] (where n = 2 to 5, bpy = 2,2'-bipyridine, XY = 2,5-xylene, and G is 3-ethynyl-4-methoxy-N,N-dimethylaniline; abbreviated [...]), were designed, synthesized, and characterized to study electron tunneling through phenylene bridges. [...] is a versatile D-B-A scaffold that exhibits a strong correlation between xylene conformation and electron transfer properties.

[...] was assembled from three components in a convergent process. Stepwise oligomer growth produced the well-defined bridging ligand, [...]; bipyridine was regioselectively functionalized with a xylene oligomer through multiple cycles of palladium-catalyzed cross-couplings. The donor, synthesized separately, was joined to [...] using an alkyne linkage. Metalation with a [...] fragment finished assembly of the D-B-A complex. The D-B-A series was analyzed with mass spectrometry and NMR.

Spectroscopic, electrochemical, and spectroelectrochemical characterizations of [...] indicate no significant electronic or chemical difference among the members of the series. UV-visible absorption spectra, with a metal to ligand charge transfer (MLCT) band maximum of 460 nm, resemble the model complexes [...] and [...] (where TMS = trimethylsilyl). Representative cyclic voltammograms of [...] contain reversible redox couples for Ru[...] and G[...], with potentials of 1.24 and 0.59 V (vs. Ag/AgCl). Spectroelectrochemical traces, displaying loss of MLCT intensity and increased absorption centered at 520 nm, indicate the formation of [...].

The flash-quench technique was used to measure the electron transfer rates for [...]. The rates, 9.0 ± 0.3 x 10[...], 2 ± 1 x 10[...], and 6 ± 1 10[...] for n = 3 through 5, have a strong dependence on donor-acceptor distance. Estimates of the donor-acceptor distance were used to determine a distance decay constant, [...], of 0.84 [...]. The typical [Beta]-value for electron tunneling through phenylenes is 0.4[...]. The unusually high [Beta]-value for [...] is attributed to near-orthogonal dihedral angles between adjacent xylene rings. UV-visible spectra, cyclic voltammograms, and structural information, from molecular modeling calculations and a crystal structure, all support a near-orthogonal twist angle.

The versatility of [...] as a molecular scaffold was demonstrated in studies on electron transfer reactions in nanocrystalline TiO2 solar cells. Modified to incorporate a terminal carboxyl group, [...] (where BA = 4-ethynyl-benzoic acid) was synthesized and used as a TiO2 solar cell dye. Utilizing the flash-quench method, the second-order rate constant for dye regeneration with [...], [...] in homogenous fluid solution was 1.5 x [...] for all three dyes in the series n=0 to 2.

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