Electron Tunneling Through Phenylene Bridges

Citation

Villahermosa, Randy Michael (2002) Electron Tunneling Through Phenylene Bridges. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qxxp-m840. https://resolver.caltech.edu/CaltechETD:etd-05232005-084837

Abstract

A series of donor-bridge-acceptor (D-B-A) complexes, [(bpy)2Ru(bpy-(XY)n-G)](PF6)2 (where n = 2 to 5, bpy = 2,2'-bipyridine, XY = 2,5-xylene, and G is 3-ethynyl-4-methoxyN, N-dimethylaniline; abbreviated [Ru-(XY)n-G]2+), were designed, synthesized, and characterized to study electron tunneling through phenylene bridges. [Ru-(XY)n -G]2+ is a versatile D-8-A scaffold that exhibits a strong correlation between xylene conformation and electron transfer properties.

[Ru-(XY)n -G](PF6)2 was assembled from three components in a convergent process. Stepwise oligomer growth produced the well-defined bridging ligand, bpy-(XY)n; bipyridine was regioselectively functionalized with a xylene oligomer through multiple cycles of palladium-catalyzed cross-couplings. The donor, synthesized separately, was joined to bpy-(XY)n using an alkyne linkage. Metalation with a (bpy)2Ru fragment finished assembly of the D-8-A complex. The D-B-A series was analyzed with mass spectrometry- and NMR.

Spectroscopic, electrochemical, and spectroelectrochemical characterizations of [Ru-(XY)n -G]2+ 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 [Ru(bpy)3]2+ and [Ru-XY-TMS]2+ (where TMS = trimethylsilyl). Representative cyclic voltammograms of [Ru-(XY)3-G]2+ contain reversible redox couples for Ru111m and Gw+, 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 [Ru111-(XY)n -G+]4+.

The flash-quench technique was used to measure the electron transfer rates for [Rum·(XY)11-G] 3+-+ [Ruu-(XY)n-G+]3+. The rates, 9.0 ± 0.3 x 106, 2 ± 1x105, and · 6 ::: 1 103 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, J), of 0.84 A-1 • The typical 13-value for electron tunneling through phenylenes is 0.4 A-1 • The unusually high ~-value for [Ru-(XY)n -G]2+ 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 [Ru-(XY)n-G]2+ as a molecular scaffold was demonstrated in studies on electron transfer reactions in nanocrystalline Ti02 solar cells. Modified to incorporate a terminal carboxyl group, [Ru-(XY)n -BA]2+ (where BA : 4wethynyl-benzoic acid) was synthesized and used as a Ti02 solar cell dye. Utilizing the flash-quench method, the second-order rate constant for dye regeneration with r- ([Rum·(XY)n-BA] 3+-+ [Ru-(XY)11-BA]2+) in homogenous fluid solution was 1.5 x 1010 M'1s-1 for all three dyes in the series n=O to 2.

Thesis Files