Reversible switching in molecular electronic devices.

This manuscript compares the conductance when molecules connecting single-walled carbon nanotubes (SWNTs) change their states of conjugation. We use electrodes made by excising a nanoscale section from an individual SWNT, having shown previously that this is a general method to wire a small number (<10) of molecules into electrical circuits (Figure 1A).1-3 These SWNT molecular devices can be made sensitive to pH,1 electron deficient aromatics,2 and protein/substrate binding.3 Here we incorporate diarylethenes that are well-known to switch between open (nonconjugated) and closed (conjugated) states.4 We show that the thiophene-based device can be switched from the insulating open form to the conductive closed form but not back again, while the pyrrole-based device does cycle between the open and closed states. The design of the molecular switches follows previous studies5-7 in solution, where the thiopheneand pyrrole-based molecules are known to switch from an open, non-conjugated form to a closed, conjugated form (Figure 1B). Previously these molecules have been investigated in self-assembled monolayers (SAMs) on gold surfaces,8 SAMs on particles,9 and switched ex situ and inserted into devices,10 but there are no reVersibly switchable, single molecule devices.11 It has been hypothesized that when they are wired to gold electrodes the thiophene-based devices do not switch because of the overlap of the excited-state of the open form of the molecule with the plasmon bands of the gold electrodes.11,12 Since we have developed SWNTs as easily and generally derivatizable electrodes, we were eager to see whether the fundamentally different band structure of the SWNT electrodes and/or the covalent attachment of the organic conductor to the carbon electrode would yield a switchable system. We fabricated devices by a method that has been described previously, in which we grow individual SWNTs,13 place metal electrodes on the tubes through a stencil, and then oxidatively cut the nanotubes through a lithographic mask defined with an electron beam.1-3 Using this method we can create gaps that are of molecular size. The oxidative cutting functionalizes the ends of these carbonbased electrodes with carboxylic acids. This functionality allows a smooth introduction of the molecular bridge through formation of amide bonds (Figure 1A). We synthesized the diamines 1 and 2.14 In solution, as expected, the photocyclization requires UV light (365 nm), and the photoreversion occurs with visible light (>500 nm). The UV-vis spectra for 1 and 2 are in the Supporting Information (Figure S1). Amide bond formation between the amines of 1 and 2 and the carboxylic acids that terminate the electrodes provides the chemical attachment to span the gap and forms a molecular switch. In all cases, the rejoined SWNTs recover their original general electrical behavior (either metallic or semiconducting), albeit at reduced current values relative to the starting SWNT. The yield and fidelity of this connection reaction were determined by a method that has been previously described.1,3 Similar to these previous studies with molecular wires,1,3 the yield is ∼3% for the connection of 1 (from 140 devices) and for the connection of 2 (from 96 devices). Figure 2A displays the electrical characteristics for a metallic SWNT connected with the molecular bridge open-1; these are similar to data obtained previously except the current levels for open-1 are generally lower than for other molecular wires.1-3 This is expected because the molecules in their open state have the pathway of conjugation broken. When this cut tube that had been reconnected with open-1 was irradiated with a low-intensity (23 W) handheld UV source (365 nm), there was a 25-fold increase in the conductance of the device (Figure 2B). The UV-sensitivity of the open state of these devices is remarkable. The reason for this may be that when an open molecule bridges the gap it will be in a conformation that is primed to cyclize.4 When the light is extinguished, the current through these devices remains at the higher level (Figure 2C). We saw no change in the conductance over several weeks. We observe the same phenomenon when we employ a semiconducting SWNT instead of a metallic one. Table S1 summarizes similar behavior for several different devices using both metallic and semiconducting devices. We are confident that the increase in conductance was a result of the behavior of the molecular bridge. We used two different molecular bridges that cannot switch and found that UV irradiation had a negligible effect (Figure S2). To rule out artifacts from the association of the amine-rich compounds on the surface of the carbon nanotubes, we have incorporated 1 and 2 into SWNTs that were only partially cut and found little effect when the devices were irradiated with UV or visible light (Figure S3). We have tested Figure 1. (A) Molecular bridges between the ends of an individual SWNT electrode. (B) Switching between conjugated and non-conjugated molecular structures.