Cornelia van der Pol

Unambiguous one-molecule conductance measurements under ambient conditions.

One of the challenging goals of molecular electronics is to wire exactly one molecule between two electrodes. This is generally nontrivial under ambient conditions. We describe a new and straightforward protocol for unambiguously isolating a single organic molecule on a metal surface and wiring it inside a nanojunction under ambient conditions. Our strategy employs C(60) terminal groups which act as molecular beacons allowing molecules to be visualized and individually targeted on a gold surface using an scanning tunneling microscope. After isolating one molecule, we then use the C(60) groups as alligator clips to wire it between the tip and surface. Once wired, we can monitor how the conductance of a purely one molecule junction evolves with time, stretch the molecule in the junction, observing characteristic current plateaus upon elongation, and also perform direct I-V spectroscopy. By characterizing and controlling the junction, we can draw stronger conclusions about the observed variation in molecular conductance than was previously possible.

Control over charge transfer through molecular wires by temperature and chemical structure modifications.

A series of electron donor-acceptor arrays containing π-conjugated oligofluorenes (oFL) of variable length between a zinc porphyrin (ZnP) as electron donor and fullerene (C(60)) as electron acceptor have been prepared by following a convergent synthesis. The electronic interactions between the electroactive species were determined by cyclic voltammetry, UV-visible, fluorescence, and femto/nanosecond transient absorption spectroscopy. Our studies clearly confirm that, although the C(60) units are connected to the ZnP donor through π-conjugated oFL frameworks, no significant electronic interactions prevail in the ground state. Theoretical calculations predict that a long-range electron transfer occurs primarily due to a maximized π-conjugated pathway from the donor to the acceptor. Photoexcitation of ZnP-oFL(n)-C(60) results in transient absorption maxima at 715 and 1010 nm, which are unambiguously attributed to the photolytically generated radical ion pair state, [ZnP(•+)-oFL(n)-C(60)(•-)], with lifetimes in the microsecond time regime. Temperature-dependent photophysical experiments have shown that the charge-transfer mechanism is controllable by temperature. Both charge separation and charge recombination processes give rise to a molecular wire behavior of the oFL moiety with an attenuation factor (β) of 0.097 Å(-1). The correlation β to the connection pattern between the ZnP donor and the oFL linker revealed that even small alterations of the linker π-electron system break the homogeneous π-conjugation pattern, leading to higher values of β.

Precise control of intramolecular charge-transport: the interplay of distance and conformational effects

A new series of donor-bridge-acceptor (D-B-A) compounds consisting of π-conjugated oligofluorene (oFL) bridges between a ferrocene (Fc) electron-donor and a fullerene (C60 ) electron-acceptor have been synthesized. In addition to varying the length of the bridge (i.e., mono- and bi-fluorene derivatives), four different ways of linking ferrocene to the bridge have been examined. The Fc moiety is linked to oFL: 1) directly without any spacer, 2) by an ethynyl linkage, 3) by a vinylene linkage, and 4) by a p-phenylene unit. The electronic interactions between the electroactive species have been characterized by cyclic voltammetry, absorption, fluorescence, and transient absorption spectroscopy in combination with quantum chemical calculations. The calculations reveal exceptionally close energy-matching between the Fc and the oFL units, which results in strong electronic-coupling. Hence, intramolecular charge-transfer may easily occur upon exciting either the oFLs or Fcs. Photoexcitation of Fc-oFL-C60 conjugates results in transient radical-ion-pair states. The mode of linkage of the Fc and FL bridge has a profound effect on the photophysical properties. Whereas intramolecular charge-separation is found to occur rather independently of the distance, the linker between Fc and oFL acts (at least in oFL) as a bottleneck and significantly impacts the intramolecular charge-separation rates, resulting in beta values between βCS 0.08 and 0.19 Å(-1). In contrast, charge recombination depends strongly on the electron-donor-acceptor distance, but not at all on the linker. A value of βCR (0.35±0.01 Å(-1)) was found for all the systems studied. Oligofluorenes prove, therefore, to be excellent bridges for probing how small structural variations affect charge transport in D-B-A systems.

Determination of the attenuation factor in fluorene-based molecular wires

Fluorene-based bridges exhibit a molecular wire-like behaviour in C(60)-wire-exTTF systems with a very low attenuation factor (beta = 0.09 A(-1)).