The most probable single-molecule conductance of each member of a series of 12 conjugated molecular wires, 6 of which contain either a ruthenium or platinum center centrally placed within the backbone, has been determined. The measurement of a small, positive Seebeck coefficient has established that transmission through these molecules takes place by tunneling through the tail of the HOMO resonance near the middle of the HOMO−LUMO gap in each case. Despite the general similarities in the molecular lengths and frontier-orbital compositions, experimental and computationally determined trends in molecular conductance values across this series cannot be satisfactorily explained in terms of commonly discussed "single-parameter" models of junction conductance. Rather, the trends in molecular conductance are better rationalized from consideration of the complete molecular junction, with conductance values well described by transport calculations carried out at the DFT level of theory, on the basis of the Landauer−Buẗtiker model.
The past decade has seen a remarkable surge in studies of thin-film and single-molecule electronics, due in no small part to the development and advancement of experimental methods for the construction and measurement of metal|molecule|metal junctions. Within the plethora of molecular structures that have been investigated, metal complexes of general form trans-M(C[triple bond, length as m-dash]CR)2(Ln) have attracted attention from the inorganic and organometallic chemistry community in the search for efficient molecular wires due to the potential π-d-π orbital mixing along the molecular backbone. In this article progress towards this goal will be summarised, and design strategies for future molecular components discussed.
This
paper describes the syntheses of several functionalized dihydropyrene
(DHP) molecular switches with different substitution patterns. Regioselective
nucleophilic alkylation of a 5-substituted dimethyl isophthalate allowed
the development of a workable synthetic protocol for the preparation
of 2,7-alkyne-functionalized DHPs. Synthesis of DHPs with surface-anchoring
groups in the 2,7- and 4,9-positions is described. The molecular structures
of several intermediates and DHPs were elucidated by X-ray single-crystal
diffraction. Molecular properties and switching capabilities of both
types of DHPs were assessed by light irradiation experiments, spectroelectrochemistry,
and cyclic voltammetry. Spectroelectrochemistry, in combination with
density functional theory (DFT) calculations, shows reversible electrochemical
switching from the DHP forms to the cyclophanediene (CPD) forms. Charge-transport
behavior was assessed in single-molecule scanning tunneling microscope
(STM) break junctions, combined with density functional theory-based
quantum transport calculations. All DHPs with surface-contacting groups
form stable molecular junctions. Experiments show that the molecular
conductance depends on the substitution pattern of the DHP motif.
The conductance was found to decrease with increasing applied bias.
Indium
tin oxide (ITO) is an attractive substrate for single-molecule
electronics since it is transparent while maintaining electrical conductivity.
Although it has been used before as a contacting electrode in single-molecule
electrical studies, these studies have been limited to the use of
carboxylic acid terminal groups for binding molecular wires to the
ITO substrates. There is thus the need to investigate other anchoring
groups with potential for binding effectively to ITO. With this aim,
we have investigated the single-molecule conductance of a series of
eight tolane or “tolane-like” molecular wires with a
variety of surface binding groups. We first used gold–molecule–gold
junctions to identify promising targets for ITO selectivity. We then
assessed the propensity and selectivity of carboxylic acid, cyanoacrylic
acid, and pyridinium-squarate to bind to ITO and promote the formation
of molecular heterojunctions. We found that pyridinium squarate zwitterions
display excellent selectivity for binding to ITO over gold surfaces,
with contact resistivity comparable to that of carboxylic acids. These
single-molecule experiments are complemented by surface chemical characterization
with X-ray photoelectron spectroscopy, quartz crystal microbalance,
contact angle determination, and nanolithography using an atomic force
miscroscope. Finally, we report the first density-functional theory
calculations involving ITO electrodes to model charge transport through
ITO–molecule–gold heterojunctions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.