Langmuir-Blodgett Films of Potential Unidimensional Organic Rectifiers

The progress of the Organic Rectifier Project (ORP) is reviewed. Several molecules of the type D-σ-A (D=organic one-electron donor, σ = covalent saturated bridge, A = organic one-electron acceptor) have been synthesized, to test the Aviram-Ratner Ansatz that they could be one-molecule thick rectifiers of electrical current. Many of these molecules (σ= urethane) self-assemble as Langmuir-Blodgett films; the cyclic voltammetry peaks confirm that these molecules preserve the good D and A characteristics. Calculations confirm that the zwitterionic state D+-σ-A- is much below the state D-σ-A+, as expected. Preliminary tests of electrical rectification have failed. With the installation of a new scanning electron microscope, the Aviram-Ratner Ansatz will soon be put to a final and definitive test

Charge Recombination via Intercolumnar Electron Tunneling through the Lipid-like Mantle of Discotic Hexa-alkyl-hexa-peri-hexabenzocoronenes

The recombination of the mobile charge carriers formed in pulse-ionized hexa-alkyl-substituted hexa-peri-hexabenzocoronenes occurs mainly via intercolumnar electron tunneling through the intervening hydrocarbon mantle. This is evidenced as a dramatic increase in the time scale of the decay of the radiation-induced conductivity from a few hundred nanoseconds to close to a millisecond as the peripheral alkyl substituents increase in size from 8 to 24 carbon atoms with corresponding disk diameters, D, from 23.4 to 36.6 A. The decay kinetics are a function only of the total number of peripheral carbon atoms with no evidence for specific effects of chain branching. The 1/e decay time, tau(e), increases exponentially with D according to tau(e) = tau(e)(0) exp(betaD) with tau(e)(0) = 48 fs and beta = 0.63 A(-1). Taking into account the tilted columnar configuration of the molecules in the solid phase leads to a beta value of ca. 0.8 A(-1) for the distance dependence of intercolumnar electron tunneling. In contrast to the orders of magnitude changes in the time scale for intercolumnar charge recombination, the intracolumnar charge hopping times vary by only a factor of 4, between 40 and 160 fs, with no systematic dependence on the nature of the alkyl substituents. On the basis of the results, the time scale estimated for electron tunneling across a 40 A thick lipid membrane is estimated to be close to 1 ms.

The Study of Solid/Liquid Interfaces with X-ray Standing Waves

The principles and applications of the x-ray standing wave technique are described. Emphasis is placed on its utility for the study of structure, composition, and distribution of interfacial species especially at solid/liquid interfaces.