MEDIUM EFFECTS ON PHOTOINDUCED ELECTRON-TRANSFER IN LANGMUIR-BLODGETT-FILMS

The Langmuir-Blodgett technique has been used to prepare films in which electron donor and acceptor layers are separated by a spacer layer one molecule thick in which both the length and the electronic nature of the molecules were changed without altering donor or acceptor layers. A thiacyanine dye located at the hydrophilic interface acted as an excited-state electron acceptor. The donor layer incorporated a long-chain carboxylic acid derivatized with a ferrocene chromophore at the hydrophobic interface as a ground-state electron donor. By monitoring the quenching of the thiacyanine fluorescence by the ferrocene as a function of the composition of the spacer layer, the influence of the medium on the electron-transfer process was determined. For both series of saturated fatty acids and trans-stilbene-derivatized fatty acids the rate of electron transfer decreased with increasing separation distance of donor and acceptor, but the rate of attenuation was greater for the saturated fatty acids. Although the data do not conform to a simple exponential attenuation of electron transfer with separation distance, it is possible to fit it to this form by assuming that about half the excited thiacyanine dye is inactive in the electron-transfer process, or by assuming two populations which quench differently due to their different lifetimes. Both treatments lead to very low attenuation coefficients (beta) for the fatty acid and trans-stilbene spacers; within the context of this analysis the attenuation coefficient is about 50% greater for the saturated fatty acids than the trans-stilbene spacers.