A single-molecule diode

Abstract

We have designed and synthesized a molecular rod that consists of two weakly coupled electronic pi -systems with mutually shifted energy levels. The asymmetry thus implied manifests itself in a current-voltage characteristic with pronounced dependence on the sign of the bias voltage, which makes the molecule a prototype for a molecular diode. The individual molecules were immobilized by sulfur-gold bonds between both electrodes of a mechanically controlled break junction, and their electronic transport properties have been investigated. The results indeed show diode-like current-voltage characteristics. In contrast to that, control experiments with symmetric molecular rods consisting of two identical pi-systems did not show significant asymmetries in the transport properties. To investigate the underlying transport mechanism, phenomenological arguments are combined with calculations based on density functional theory. The theoretical analysis suggests that the bias dependence of the polarizability of the molecule feeds back into the current leading to an asymmetric shape of the current-voltage characteristics, similar to the phenomena in a semiconductor diode.

Scheme 1.

Synthesis of the molecular rods 1-3. a), tert.-BuLi, THF, –78°C; b), S₈, room temperature (RT); c), BrCH₂OCH₃, –78°C, RT, 77%; d), HCCSi(CH₃)₃, CuI, [Pd(PPh₃)₂Cl₂], Et₃N, 40°C, 69%; e), K₂CO₃,CH₃OH, RT, 98%; f), CuI, [Pd(PPh₃)₂Cl₂], Et₃N, 40°C, 72%; g), CuI, [Pd₂(dba)₃]·CHCl₃, PPh₃, Et(i-Pr)₂N, THF, RT, 67%; h), AgNO₃, CH₂Cl₂, EtOH, RT; i), AcCl, Ac₂O, CH₃C₆H₅, RT, 37%; j), CuI, [Pd₂(dba)₃], PPh₃, Et₃N, 40°C, 93%; k), AgNO₃, EtOH, RT; l), AcCl, C₅H₁₂, RT, 90%; m), CuI, [Pd₂(dba)₃], PPh₃, Et(i-Pr)₂N, THF, RT, 74%.

Fig. 1.

The rectifying device consisting of a single molecule 1′ immobilized between two Au electrodes and the corresponding control experiments with the immobilized symmetric molecular rods 2′ and 3′ between an Au electrode pair of a MCB.

Fig. 2.

Molecular structure of compound 11 having two different protection groups on the terminal sulfur (50% probability thermal ellipsoids). Selected bond lengths (pm) and bond angles (°): S(1)—C(2) 183.1(3), S(1)—C(3) 177.1(3), S(2)—C(30) 178.2(3), S(2)—C(33) 178.6(4), F—C 134.3(3)—135.4(3), O(1)—C(1) 142.6(5), O(1)—C(2) 139.9(4), O(2)—C(33) 118.7(4), C(9)—C(10) 120.0(4), C(14)—C(18) 150.1(3), C(25)—C(26) 120.1(5); C(2)—S(1)—C(3) 102.6(2), C(30)—S(2)—C(33) 103.8(2), C(1)—O(1)—C(2) 112.3(2), C(6)—C(9)—C(10) 179.0(3), C(9)—C(10)—C(11) 175.6(3), C(22)—C(25)—C(26) 178.6(4), C(25)—C(26)—C(27) 176.7(4).

Fig. 3.

UV-visible spectra of the molecular rods 1-3, 11, and 12 (1 × 10^–5 M in CH₃CN; T = 298 K). For clarity the spectra were shifted horizontally by steps of 0.4 units.

Fig. 4.

I–V reproducibly recorded for a stable Au–1′–Au junction in a MCB at T ∼ 30 K. Arrows point to step-like features of the I–V.

Fig. 5.

I–Vs reproducibly recorded for stable Au–2′–Au and Au–3′–Au junctions in a MCB at T ∼ 30 K.

Fig. 6.

Histograms evaluating the current ratio at ±1.5 V for all stable junctions with immobilized molecules 1′, 2′, and 3′. Whereas there is no symmetric contact (ratio = 1) with molecule 1′, molecules 2′ and 3′ always have ratios close to 1.

Fig. 7.

Lowest unoccupied molecular orbitals of the fluorized (F-dot, Left) and unfluorized (H-dot, Right) sections of molecule 1′ attached to gold leads. The positions of the Au atoms are indicated by the gold-colored lattice structure.

Fig. 8.

Flow of the molecular energy levels corresponding to the fluorized (F-dot, circles) and unfluorized (H-dot, squares) sections of molecule 1′ when applying a finite bias voltage. The dashed lines (dashed, H-dot; dotted, F-dot) show linear extrapolations based on the numerical data, indicated by the open symbols. The black line represents the experimental differential I–V based on the data shown in Fig. 4. Black vertical lines display the theoretical peak positions defined by the crossing (green dot) of two extrapolated lines, and red lines indicate experimental peaks not reproduced. The white region between dashed lines in magenta marks the voltage window for observable crossings. The orbitals corresponding to the LUMOs of F-dot and H-dot (blue, filled symbols) are depicted in Fig. 7 for the case of vanishing voltage.

Fig. 9.

Another set of I–Vs obtained for Au–1′–Au (red), its numerical derivative dI/dV (green), and, for comparison, the derivative and the I–V shown in Fig. 4 (1). Clear differences between the two spectra can be attributed to experimental uncertainties, most obviously a different orientation of the sample molecule. Some general properties, in particular the peak distance, are unchanged.