New Class of Molecular Conductance Switches Based on the [1,3]-Silyl Migration from Silanes to Silenes

Abstract

On the basis of first-principles density functional theory calculations, we propose a new molecular photoswitch which exploits a photochemical [1,3]-silyl(germyl) shift leading from a silane to a silene (a Si=C double bonded compound). The silanes investigated herein act as the OFF state, with tetrahedral saturated silicon atoms disrupting the conjugation through the molecules. The silenes, on the other hand, have conjugated paths spanning over the complete molecules and thus act as the ON state. We calculate ON/OFF conductance ratios in the range of 10-50 at a voltage of +1 V. In the low bias regime, the ON/OFF ratio increases to a range of 200-1150. The reverse reaction could be triggered thermally or photolytically, with the silene being slightly higher in relative energy than the silane. The calculated activation barriers for the thermal back-rearrangement of the migrating group can be tuned and are in the range 108-171 kJ/mol for the switches examined herein. The first-principles calculations together with a simple one-level model show that the high ON/OFF ratio in the molecule assembled in a solid state device is due to changes in the energy position of the frontier molecular orbitals compared to the Fermi energy of the electrodes, in combination with an increased effective coupling between the molecule and the electrodes for the ON state.

Scheme 1. Photochemical [1,3]-Silyl Shift of Acyloligosilanes 1-OFF to Their Isomeric Brook-Silenes 1-ON and the Thermal Back-Rearrangement (1a R = t-Bu, 1b R = Adamantyl,^, and 1c R = Me)

Reaction and activation free energies for the thermal processes of 1a computed at the B3LYP/6-31G(d) level as reported herein (vide infra). Transition state abbreviated as TS.

Scheme 2. Potential Acyloligosilane/Brook-Silene Switches and Their Carbon Analogues, Where R = −CC–C₆H₄– for a and R = −C₆H₄– for b

Figure 1

Calculated current–voltage (I–V) characteristics for OFF and ON state structures of compounds 2a and 3a, respectively. Note the different scales for the calculated currents in the ON and OFF states.

Figure 2

Space resolved normalized density of states at zero bias for the junctions with compound 2a. The OFF state is shown in the top panel and the ON state in the bottom panel. The space resolved density of states have the transport coordinate on the x-axis and the energy of the state on the y-axis. The color scale goes from blue (low density of states) to red (high density of states).

Figure 3

Visualization of the most transmitting eigenchannel wave function (incoming from the left electrode) at the Fermi energy for the junctions with compound 2a. OFF state shown in the top panel and ON state in the bottom panel. The isosurfaces of the eigenchannel wave function are colored according to phase and sign, the positive/negative real part being colored in deep-purple/light-blue (the imaginary part is below the cutoff value and not visible), and both junctions plotted with the same isovalue to be comparable.

Figure 4

Suggested cyclic molecular conductance switches with different acceptor moieties and migrating groups. A = NMe or O, and R = SiMe₃, Si^tBu₂Me, GeMe₃, or Ge^tBu₂Me.

Figure 5

Relative reaction energies for the extended cyclic silane/silene switches. Blue bars denote N acceptor groups (A = NMe), while red bars denote the oxygen acceptors (A = O).

Figure 6

Current–voltage of cyclic switches and model calculations. Markers denote values obtained from the DFT-NEGF calculations, while the full lines are obtained from the model calculations. Note the different scales for the calculated currents in the ON and OFF states.

Figure 7

Transmission spectra, at zero bias (V = 0), for the OFF compounds (top panel) and the ON compounds (bottom panel).

Figure 8

Local currents in compound 16-OFF (top panel) and 16-ON (bottom panel), where the cross-sectional area of the cylinder is proportional to the current density. Red currents represent positive transport direction, and blue currents represent negative direction. The currents are calculated at the Fermi energy ((μ_L + μ_R)/2), but a small positive bias voltage is assumed for calculation purposes. For a full derivation see Okabayashi et al. and Paulsson and Brandbyge.

Figure 9

Magnitudes of the switching ratio (SR) as a function of the change of the closest MO (ΔE₀) and the coupling between the MO and the electrodes (ΔΓ).