Hydrogen bonding as the origin of the switching behavior in dithiolated phenylene-vinylene oligomers

We investigate theoretically the switching behavior of a dithiolated phenylene-vinylene oligomer sandwiched between Au(111) electrodes using self-interaction corrected density-functional theory combined with the nonequilibrium Green's-function method for quantum transport. The molecule presents a configurational bistability, which can be exploited in constructing molecular memories, switches, and sensors. We find that protonation of the terminating thiol groups is at the origin of the change in conductance. H bonding at the thiol group weakens the S-Au bond and reduces by about one order of magnitude the transmission coefficient at the Fermi level, and thus the linear response conductance. Furthermore, protonation downshifts in energy the position of the highest occupied molecular orbital, so that the current of the protonated species is lower than that of the unprotonated one along the entire bias range investigated, from $-1.5$ to 1.5 V. A second protonation at the opposite thiol group has only minor effects and no further drastic reduction in transmission takes place. Our results allow us to re-interpret the experimental data originally attributing the conductance reduction to H dissociation.

Figure 1

Schematic diagram of the devices investigated. These comprise various oligophenylenevinylene derivatives sandwiched between Au electrodes. Here we label as (a) opv3, (b) opv3H, and (c) opv32H, respectively, a oligophenylenevinylene molecule including either no, one, or two thiolate groups. Color code: blue $=$ H, green $=$ S, purple $=$ C, and yellow $=$ Au.

Figure 2

Schematic diagram of the two isomers of opv3 studied: (a) cis-trans and (b) trans-trans. Color code: blue $=$ H, green $=$ S, and purple $=$ C.

Figure 3

Transmission coefficient for hollow (energetically most favorable), bridge, and top site contact geometries of opv32H on Au(111).

Figure 4

Zero-bias transmission coefficients $T\left(E\right)$ for the two isomers of the oligophenylenevinylene derivatives. Here panels (a${}_1$), (a${}_2$), and (a${}_3$) refer to the trans-trans isomers of opv3, opv3H, and opv32H, respectively, while panels (b${}_1$), (b${}_2$), and (b${}_3$) refer to their corresponding cis-trans isomers.

Figure 5

DOS projected on the C (blue shadowed area) and S (red shadowed area) atoms closest to the electrodes. For a better understanding the transmission coefficient at zero bias is also plotted as thin black line. Panels (a${}_1$), (a${}_2$), and (a${}_3$), respectively, refer to opv3, opv3H, and opv32H calculated using the LDA. Panels (b${}_1$), (b${}_2$), and (b${}_3$) are corresponding results calculated using the SIC.

Figure 6

LDA isosurfaces of the transmission eigenchannels, showing that the first two occupied molecular orbitals contribute to the transmission. Panel (a) is for the HOMO$-1$ level and (b) for the HOMO level.

Figure 7

Current and differential conductance (calculated numerically) for the oligophenylenevinylene derivatives as a function of the bias voltage. The left panels, (a${}_1$) and (a${}_2$), refer to the LDA and the right panels, (b${}_1$) and (b${}_2$), to the SIC.

Figure 8

Transmission coefficient as a function of energy, $T(E;V)$, for different bias voltages. We show results for opv3 and opv3H as obtained with the LDA (left) and SIC (right). Panels from top to bottom correspond to the following bias voltages (in V): $-1.5$, $-1.0$, $-0.5$, 0, 0.5, 1.0, 1.5. The red boxes mark the bias window. The integral of $T(E;V)$ over the bias window determines the current at that particular voltage.

Figure 9

Same data as shown in Fig. 4, with the transmission plotted on a logarithmic scale.

Figure 10

Evidence that the spurious peak of Fig. 4(a${}_3$) does not contribute to the transmission.