Electron Transfer through Surface-Grown, Ferrocene-Capped Oligophenylene Molecular Wires (5-50 Å) on n-Si(111) Photoelectrodes.

We report the surface growth of oligophenylene molecular wires on Si(111) substrates and their electron-transfer (ET) properties. Iterative wire growth of biphenylene was achieved via Pd-catalyzed Negishi reactions for lengths of nphenyl = 1, 2, 4, 6, 8, and 12 (d ≈ 5-50 Å). The triflato-capped wires were functionalized with vinylferrocene for potentiometric studies. For the oligophenylenes of nphenyl = 1, 2, and 4 (wire length d ≈ 5-20 Å), there was a strong distance dependence (kapp = 22.6, 16.0, 8.40 s(-1), respectively), correlated to β = 0.07 Å(-1). In contrast, longer oligophenylenes for nphenyl = 4-12 (d ≈ 20-50 Å) displayed a negligible distance dependence with an ET rate of kapp ≈ 10.0 ± 1.6 s(-1). These data suggest a distance-dependent tunneling mechanism at short lengths (d < 20 Å) and a distance-independent ET at longer lengths (d > 20 Å).