Correlating Molecular Structure with Electronic Properties

The achievement to contact small assemblies of molecules or even single molecules enables investigating correlations on electronic properties with molecular structure. Factors that influence the current-voltage characteristics of molecules include intrinsic properties such as molecular length and conformation. In addition, the type of molecule-electrode contact affects the electronic properties.

The transport through individual molecules connected between electrodes is expected to be different from bulk transport because of the inherently small size of molecules. The conductance G through a molecule with a distance d is described in the tunneling physics theory by the Landauer formula as:

d e A G_{= ⋅} −β

The decay constant ß is determined by electronic parameters of the molecule backbone. A is the prefactor and depends on the electron density-of-states at the point where the molecule contacts the electrodes.

Increasing the length of a molecule and therefore the electronic pathway electrons have to tunnel through, the conductance decreases exponentially.[66-68] Alkanethiols served as ideal model compounds to investigate the dependence of current on molecule length. The length can be systematically varied without affecting the molecule’s backbone and therefore keeping the decay constant ß constant. Independent of the metal contacts (gold, mercury), the integration setups (cp-AFM, mercury droplet junction, STM, crossed-wire junction) and the number of contacted molecules, the conductance decreases exponentially with increasing molecular length d.[68]

Considering the Landauer formula, it is obvious that the anchor groups of a molecule are not only for fixing the molecule to the electrodes, they also provide electronic coupling between the molecule and electrodes.[69] It is important to design proper molecule-electrode contacts because the contacts play a critical role in the electron-transport properties of molecules.[70-75] _{To date, the most widely used}

anchoring groups to adsorb on gold surfaces are thiols,[76, 77] _{although pyridine,}[78]

isocyanides,[71, 79] _selenium[80-82] _{and amines}[83] _{have also been studied. Anchoring}

group effects on single-molecule conductance were systematically investigated in an STM break junction.[76] n-Alkanes were chosen as model compounds, because by varying the number of CH2 groups it was possible to isolate the molecule-electrode

contact effects from other factors. Three anchor groups, thiols, amines and carboxylic acids were compared.

Figure 15: Logarithmic plots of single-molecule conductance vs. molecular length for dithiol- (orange), diamine- (blue), and dicarboxylic-acid-terminated (purple) alkanes. The conductance decreases exponentially with increasing length.[70]

For each anchor group the conductance decays exponentially with molecular length. The prefactor A is highly sensitive to the type of anchoring group, and varies in the order of Au-S>Au-NH2>Au-COOH. This large dependence is attributed to different

electronic coupling efficiencies provided by the different anchor groups. Binding strength information was obtained by measuring the average length over which one can stretch each molecular junction until it breaks. The binding strength varies in the order of Au-S>Au-NH2>Au-COOH.

Considering compounds comprising more than one aromatic ring in the electron pathway, the effect of the angle between the rings was investigated.[84-86] Venkataraman and co-workers reported on the interdependence between the calculated molecular conformation and the single-molecule conductance of a series of various substituted biphenyls bearing terminal amino groups.[84] Despite several varying parameters such as electron density on the phenyl rings and steric repulsion of the substituents, a linear correlation was found between the conductance and the square of the cosine of the calculated torsion angle Ф between the planes of the two rings (Figure 16 C). Vonlanthen et al. systematically investigated the effect of torsion angle on thiol anchor groups comprising biphenyl systems.[86] The torsion angle was

chemically controlled while bridging the two phenyl units with different length alkyl chains and was experimentally obtained from the X-ray structures (D in Figure 16). The conductivities of the thiol terminated biphenyls were deduced from single- molecule junctions by STM break junction measurements and linearly correlated with

cos2 (Ф) (E in Figure 16).

Figure 16: The effect of the torsion angle of a biphenyl system on the conductance was investigated in amine terminated biphenyl systems[84] _{(A, B, C) and in thiol terminated biphenyl structures with a}

fixed torsion angle in STM break junctions.[86] _{While increasing the torsion angle, the conjugation is}

reduced (A) which leads to a decrease in conductance (B). The conductance linearly correlates with the cosine of the square of the torsion angle.

The electric current through a molecular rod was investigated in relation to the position of the anchor group. Two rods consisting of a conjugated π-system comprising two thiol anchor groups either in meta- or para-position were investigated in an MCBJ setup (Figure 17).[87]

C2 Au

Au S S AuAu AuAu S S AuAu

C4

Figure 17: The conductance through a molecule was investigated in relation to the position of the anchor groups. C2 bears the thiol anchor groups the in para-positions and is better conducting than C4 with the thiol anchor groups the in meta-positions.

The para-substituted compound C2 was found to be better conducting than the meta- functionalized compound C4, which is attributed to decoupling the molecule from the electrodes while breaking its conjugation.

These selected examples indicate that electronic properties can be tuned by carefully designing molecular structures. The very rich palette of functional groups provides an infinite variety of molecular structures which are accessible by synthetic organic chemistry. Molecules are very promising objects for future applications in nanotechnology because not only their structure, but also their properties can be tailored.