Nanostructured silica thin films self-assembled with electron donors and acceptors to measure electron tunneling.

Surfactant-templated, mesostructured thin films are synthesized such that photoelectron donors and electron acceptors are separated spatially in the different regions of the thin film. A photoelectron donor is placed within the silica framework by using a silylated derivative of the well-known tris(bipyridine)ruthenium(II) cation. Selective placement of the electron acceptor is achieved by using a surfactant derivative of methyl viologen. Luminescence decay traces and luminescence spectra are collected for the electron donor in the presence of varying amounts of the electron acceptor. Because of the spatial separation of the donor and acceptor noncontact electron transfer occurs and the electron-transfer rate decreases exponentially with the distance separating the donor and acceptor. Luminescence decay traces are calculated and fit to the experimental data in order to extract a value for the contact quenching rate, k0 (s(-1)), as well as the exponential decay constant beta (A(-1)) which governs how fast the electron-transfer rate decreases as a function of the donor-acceptor distance. The value beta = 2.5 +/- 0.4 A(-1) shows that the mesostructured material is an excellent insulator, better than frozen organic glasses or proteins and approaching that of vacuum. Combining deliberate placement methods, spectroscopy, and calculations has made possible the first measurement of beta for the silica region of mesoporous thin films.