Verification and Temperature-Dependent Rectification by HBQ, the Smallest Unimolecular Donor-Acceptor Rectifier.

Five years ago, rectification of electrical current was found in 4'-bromo-3,4-dicyano-2',5'-dimethoxy-[1,1'-biphenyl]-2,5-dione (1), a hemibiquinone (which we will call either 1 or HBQ) that has a very small working length (1.1 nm). Monolayers of HBQ on AuTS were detected by "nanodozing" atomic force microscopy (AFM) and were contacted with two types of top electrodes: either cold Au or eutectic Ga-In. Here, we describe cyclic voltammetry of a self-assembled monolayer (SAM) of HBQ and its orientation on a gold substrate with angle-resolved X-ray photoelectron spectroscopy. New measurements of its rectification as a monolayer as a function of bias range and temperature confirm and prove that HBQ is truly the smallest donor-acceptor rectifier and provide some insight into the mechanism of rectification.

Observation of current rectification by a new asymmetric iron(iii) surfactant in a eutectic GaIn|LB monolayer|Au sandwich.

In this paper we expand on the search for molecular rectifiers of electrical current and report on a hexacoordinate metallosurfactant [FeIII(LN3O)(OMe)2], where (LN3O)- is the deprotonated form of the new asymmetric ligand 2-((E)-((4,5-bis(2-methoxyethoxy)-2-(((E)-pyridin-2-ylmethylene)amino)phenyl)imino)methyl)-4,6-di-tert-butyl-phenol. This species rectifies current when deposited as a Langmuir-Blodget film in a "EGaIn/Ga2O3|LB|Au" sandwich with rectification ratios ranging from 25 to 300 at 1 Volt.

Janus Reversal and Coulomb Blockade in Ferrocene-Perylenebisimide and N,N,N',N'-Tetramethyl-para-phenylenediamine-Perylenebisimide D-σ-A Rectifiers.

Sandwiches "EGaIn|Ga2O3|LB monolayer of 2|Au" and "EGaIn|Ga2O3|LB monolayer of 3|Au" rectify. They are formed from a Langmuir-Blodgett (LB) monolayer of 2 or 3 transferred onto thermally evaporated gold. Molecules 2 and 3 are of the donor-sigma-acceptor (D-σ-A) type and have the same perylenebisimide (PBI) acceptor as previously studied molecule 1. Molecule 1 has the weak donor pyrene, 2 has the good donor ferrocene, and 3 has the very strong donor N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). All three molecules have a long swallowtail ending in a thioacetyl group, which ensures slow chemisorption onto the Au electrode. These molecules were contacted directly by a gallium indium eutectic (EGaIn) drop, covered by a defective oxide Ga2O3 layer. As before for 1, the direction of rectification for 2 is bias-dependent. In the ±1.0 V range, the rectification is at positive V, with a rectification ratio (RR) that is initially greater than 5 and then decreases on successive scans to 2, while the currents decrease by as much as 2 orders of magnitude. In the ±2.5 V range, the rectification direction for 2 reverses, while upon repeated scanning the rectification ratio (in the negative direction) increases and the currents decrease. For molecule 3, both directions have a charge-trapped state (Coulomb blockade) leading to Voffset in both biases, but at high potentials rectification set is, with large RR (up to 2,800) at ±2.5 V.

Surprisingly Big Rectification Ratios for a Very Small Unimolecular Rectifier.

To shrink electrical circuits beyond the looming practical limits of Moore's "Law", it has been a common goal to create carbon-based components. Here is the first example of a new class of donor-σ-acceptor rectifiers: hemibiquinones (HBQs). HBQs possess donor and acceptor moieties that are electronically isolated by inter-ring torsion. A HBQ-dinitrile self-assembles on a template-stripped gold surface through directed chemisorption, forming a 1.1 nm-thick monolayer. Rectification is measured through the monolayer or single molecules by using three different top electrode arrangements. Even though the HBQs are composed of relatively weak electrophores, rectification ratios ranging from 5 to 160 were observed at 2.5 V.

Crystal structure of 4,4'-di-bromo-2',5'-dimeth-oxy-[1,1'-biphen-yl]-2,5-dione (BrHBQBr).

In the title compound, C14H10Br2O4, the dihedral angle between the aromatic rings is 67.29 (19)°. Both meth-oxy-group C atoms lie close to the plane of their attached ring [deviations = -0.130 (4) and 0.005 (5) Å]. In the crystal, mol-ecules pack in a centrosymmetric fashion and inter-act via a mixture of weak π-π stacking inter-actions [centroid-centoid separations = 4.044 (2) and 4.063 (3) Å], weak C-H⋯O hydrogen bonding, and Br⋯Br halogen bonding. This induces a geometry quite different than that predicted by theory.

Unimolecular amplifier: principles of a three-terminal device with power gain.

A single molecule composed of three linked moieties can function as an amplifier of electrical current, when certain conditions are met by the molecular orbitals of the three component parts. This device should exhibit power gain at appropriate voltages. In this work, we will explain a plausible mechanism by which this device should work, and present its operating characteristics. In particular, we find that a fundamental requirement for current amplification is to have the LUMO of the central moiety more strongly coupled to a control electrode than it is to the other orbitals in the molecule, while the HOMO of this moiety should be more strongly coupled to the orbitals in the other moieties than it is to the control electrode.

Unimolecular electronic devices.

The first active electronic components used vacuum tubes with appropriately-shaped electrodes, then junctions of appropriately-doped Ge, Si, or GaAs semiconductors. Electronic components can now be made with appropriately-designed organic molecules. As the commercial drive to make ever-smaller and faster circuits approaches the 3-nm limit, these unimolecular organic devices may become more useful than doped semiconductors. Here we discuss the electrical contacts between metallic electrodes and organic molecular components, and survey representative organic wires composed of conducting groups and organic rectifiers composed of electron-donor and -acceptor groups, and the Aviram-Ratner proposal for unimolecular rectification. Molecular capacitors and amplifiers are discussed briefly. Molecular electronic devices are not only ultimately small (<3 nm in all directions) and fast, but their excited states may be able to decay by photons, avoiding the enormous heat dissipation endured by Si-based components that decay by phonons. An all-organic computer is an ultimate, but more distant, goal.

Unimolecular rectification of monolayers of CH3C(O)S-C14H28Q(+)-3CNQ(-) and CH3C(O)S-C16H32Q(+)-3CNQ(-) organized by self-assembly, Langmuir-Blodgett, and Langmuir-Schaefer techniques.

The advantage of "self-assembly" (strong covalent binding to substrates) was combined with the advantage of Langmuir-Blodgett (LB) or Langmuir-Schaefer (LS) transfer to a solid substrate (quantitative transfer of monolayers to the substrate). The electrical rectification (asymmetric conduction) by a monolayer of thioacetylalkylquinolinium tricyanoquinodimethanide was critically compared when these molecules had been transferred, by such competing techniques, onto gold electrodes, and then covered by a "cold gold" pad electrode. Unimolecular rectification was observed in the expected directions in the LB and LS monolayers. The Self-Assembled Monolayers (SAMs) were disordered; macroscopic measurements of rectification were unsuccessful for the SAMs, but successful for the down-stroke LB and LS monolayers, whose orientation and potential bonding to the Au surface should be identical to that of an ideal SAM.

Elastic and inelastic electron tunneling spectroscopy of a new rectifying monolayer.

Rectification of electrical current was observed in a Langmuir-Schaefer monolayer of fullerene-bis[ethylthio-tetrakis(3,4-dibutyl-2-thiophene-5-ethenyl)-5-bromo-3,4-dibutyl-2-thiophene] malonate, Au electrodes at room temperature (there are two regimes of asymmetry, at lower bias, i.e., between 0 and +/-2 V, and at higher bias), and also between Pb and Al electrodes at 4.2 K. The latter experiment was coupled with second harmonic detection of the second derivative of the current with respect to voltage (d2I/dV2). The d2I/dV2 spectrum shows intramolecular vibrations, and also two antisymmetric broad bands, centered at +/-0.65 V, due to resonant electron tunneling between the Fermi level(s) of the electrodes and the lowest unoccupied molecular orbital of the molecule.

Polarization of charge-transfer bands and rectification in hexadecylquinolinium 7,7,8-tricyanoquinodimethanide and its tetrafluoro analog.

A Langmuir-Blodgett (LB) multilayer film of the unimolecular rectifier hexadecyl-gamma-quinolinium-7,7,8-tricyanoquinodimethanide (C(16)H(33)gammaQ-3CNQ) has two distinct polarized charge-transfer bands, one at lower film pressures (28 mN m(-1)) with a peak at 530 nm, due to an intramolecular charge transfer or intervalence transfer (IVT); past the collapse point (32 to 35 mN m(-1)), this band disappears, and a new intermolecular charge-transfer band appears with peak at 570 nm. An LB multilayer film of the tetrafluoro analogue, hexadecyl-gamma-quinolinium-2,3,5,6-tetrafluoro-7,7,8-tricyanoquinodimethanide (C(16)H(33)gammaQ-3CNQF(4)) shows, for all film pressures, only one IVT band with a peak at 504 nm; when sandwiched between gold electrodes, (C(16)H(33)gammaQ-3CNQF(4) is also an LB monolayer electrical rectifier.

Spectroscopy and rectification of three donor-sigma-acceptor compounds, consisting of a one-electron donor (pyrene or ferrocene), a one-electron acceptor (perylenebisimide), and a C19 swallowtail.

We report spectroscopic characterization and unimolecular rectification (asymmetric electrical conduction) measurements of three donor-sigma-acceptor (D-sigma-A) compounds N-(10-nonadecyl)-N-(1-pyrenylmethyl)perylene-3,4,9,10-bis(dicarboximide) (1), N-(10-nonadecyl)-N-(4-[1-pyrenyl]butyl)perylene-3,4,9,10-bis(dicarboximide) (2), and N-(10-nonadecyl)-N-(2-ferrocenylethyl)perylene-3,4,9,10-bis(dicarboximide) (3). These molecules were arranged as one-molecule thick Langmuir-Blodgett monolayers between Au electrodes. In such an "Au | D-sigma-A | Au" sandwich, molecule 1 is a unimolecular rectifier, with rather small rectification ratios (between 2 and 3 at +/-1 V) that decrease upon cycling. Molecule 2 does not rectify. Molecule 3 rectifies, with a rectification ratio of between 14 and 28 at +/-1 V; the through-film rectification and currents persist, even with scans of +/-2 V, for up to 40 cycles of measurement. Qualitative arguments, based on a two-level rectification mechanism, are consistent with the current asymmetries observed in the monolayers of 1 and 3.

Unimolecular rectifiers: methods and challenges.

Six unimolecular rectifiers have been studied at the University of Alabama: Langmuir-Blodgett (LB) or Langmuir-Schaefer (LS), or self-assembled monolayers of these molecules show asymmetric electrical conductivity between Au or Al electrodes. These molecules are gamma-hexadecylquinolinium tricyanoquinodimethanide (Fig. 1, 2), 2,6-di[dibutylamino-phenylvinyl]-l-butylpyridinium iodide, 3, dimethylanilino-aza[C60]-fullerene, 4, fullerene-bis-[4-diphenylamino-4''-(N-ethyl-N-2-ethyl)-amino-1,4-diphenyl-1,3 -butadiene] malonate, 5, N-(10-nonadecyl)-N-(2-ferrocenyl-ethyl)-pyrenyl-methyl)pery-lene-3,4,9,10-bis(dicarboxyimide), 6, and 4,5-dipentyl-5'-methyltetrathiaful-valen-4'-methyl-oxy-2,4,5-trinitro-9-dicyanomethylenefluorene-7-(3-sulfonylpropionate), 7. Many ancillary experiments must be performed before unimolecular rectification can be fully understood. This review will focus on the fabrication techniques and the analytical tools that can help understand the asymmetric current-voltage (IV) curves. These tools include molecular orbital calculations, cyclic voltammetry, ultraviolet photoelectron spectroscopy, scanning tunneling microscopy, contact angle goniometry, ultraviolet-visible-near-infrared spectroscopy, grazing-angle Fourier transform infrared spectroscopy, surface plasmon resonance, spectroscopic ellipsometry, grazing-incidence X-ray reflectometry, core-level and valence-band X-ray photoelectron spectroscopy.

Intramolecular charge-transfer interactions in pi-extended tetrathiafulvalene derivatives.

New electron-donor (D)-electron-acceptor (A) TTF architectures are presented in which two electron-donating 1,3-dithiole moieties are connected by a pi bridge to the weak electron-accepting quinoxaline moiety (D-pi-A compounds 9a and 9b and also two 1,3-dithiole-2-ylidene moieties are connected by a pi bridge to the electron-accepting thieno[3,4-b]quinoxaline bridge (D-pi-A-pi-D compounds 12a-c). There are through-bond intramolecular charge-transfer (ICT) interactions, predicted in theoretical calculations, and confirmed by UV-vis spectroscopy and cyclic voltammetry measurements. This work constitutes the first use of quinoxalines as electron-accepting moieties in D-pi-A compounds.

Current rectification in a Langmuir-Schaefer monolayer of fullerene-bis-[4-diphenylamino-4' '-(n-ethyl-n-2' ''-ethyl)amino-1,4-diphenyl-1,3-butadiene] malonate between Au electrodes.

Langmuir-Schaefer (LS) monolayer films of fullerene-bis-[4-diphenylamino-4' '-(N-ethyl-N-2' ''-ethyl)amino-1,4-diphenyl-1,3-butadiene] malonate, 1, sandwiched between two Au electrodes, exhibit pronounced current asymmetries (rectification) between positive and negative bias at room temperature, with no decay of the rectification after several cycles. The device shows symmetrical through-space tunneling for a bias up to +/-3 V, and asymmetrical, unimolecular, "U" type rectifier behavior in the voltage range from +/-3.0 to +/-5.4 V, with rectification ratios up to 16.5. The rectification is ascribed to the asymmetric placement of the relevant molecular orbitals, with respect to the metallic electrodes.

Unimolecular rectifiers and prospects for other unimolecular electronic devices.

We briefly review the progress towards useful one-molecule electronic devices, toward the ultimate reduction in integrated circuit sizes, and then describe five unimolecular rectifiers, or one-way conductors of electrical current: gamma-hexadecylquinolinium tricyanoquinodimethanide, an acetyl sulfoxide derivative of this compound, 2,6-di[dibutylamino-phenylvinyl]-1-butylpyridinium iodide, dimethylanilino-aza[C60]fullerene, and a new fullerene derivative.

Unimolecular rectifiers and proposed unimolecular amplifier.

The rectification by three molecules that form Langmuir-Blodgett monolayers between gold electrodes is reviewed, along with a proposal for the means to obtain gain in a unimolecular amplifier, the molecular analog of a bipolar junction transistor.

Condensed thiophenes and selenophenes: thionyl chloride and selenium oxychloride as sulfur and selenium transfer reagents.

3,4-Cyanomethyl substituted thiophenes reacted with thionyl chloride in the presence of base to give dicyano substituted thieno(3,4-c)thiophenes. The use of selenium oxychloride furnished the corresponding cyano substituted seleno(3,4-c)thiophene. 1,2-Phenylenediacetonitriles gave the corresponding cyano substituted benzo(c)thiophenes and benzo(c)selenenophenes, respectively, upon reaction with thionyl chloride and selenium oxychloride in the presence of base.