Donor/Acceptor Mixed Self-Assembled Monolayers for Realising a Multi-Redox-State Surface

Mixed molecular self-assembled monolayers (SAMs) on gold, based on two types of electroactive molecules, that is, electron-donor (ferrocene) and electron-acceptor (anthraquinone) molecules, are prepared as an approach to realise surfaces exhibiting multiple accessible redox states. The SAMs are investigated in different electrolyte media. The nature of these media has a strong impact on the types of redox processes that take place and on the redox potentials. Under optimised conditions, surfaces with three redox states are achieved. Such states are accessible in a relatively narrow potential window in which the SAMs on gold are stable. This communication elucidates the key challenges in fabricating bicomponent SAMs as electrochemical switches.We acknowledge the financial support of the EU projects ERC StG 2012-306826 e-GAMES, ITN iSwitch (GA no. 642196) CIG (PCIG10- GA-2011-303989), ACMOL (GA no. 618082), the Networking Research Center of Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), the DGI (Spain) with project BE-WELL CTQ2013- 40480-R and the Generalitat de Catalunya with project 2014- SGR-17. The authors also acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496). N.C acknowledges the RyC Program. J.C-M. and E.M. are enrolled in the Materials Science PhD program of UAB.Peer reviewe

An electrically driven and readable molecular monolayer switch using a solid electrolyte

The potential application of molecular switches as active elements in information storage has been demonstrated through numerous reported works. Importantly for devices, such switching capability has also been observed on self-assembled monolayers (SAMs). SAMs of electro-active molecules have been exploited in the last few years as electrochemical switches. Typically, the state of these switches could be read out through their optical and/or magnetic responses. These output reading processes are difficult to integrate into devices and in addition there is a need to use liquid environments to switch the redox-active molecular systems. Here, we overcome both these challenges using an ion-gel as electrolyte medium achieving an unprecedented solid state device based on a single molecular layer. Further, electrochemical impedance has been successfully exploited as the output of the system.We thank R. Pfattner for his help and F. J. del Campo for providing the silicon mold. We acknowledge the financial support of the EU projects ERC StG 2012-306826 e-GAMES, ITN iSwitch (GA no. 642196) and CIG (PCIG10-GA-2011-303989), the Networking Research Center of Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), the DGI (Spain) with project BE-WELL CTQ2013-40480-R and the Generalitat de Catalunya with project 2014-SGR-17. E. M. acknowledges the Materials Science PhD program of UAB and N.C the JdC program.Peer reviewe

Exchange Reactions between Alkanethiolates and Alkaneselenols on Au{111}

When alkanethiolate self-assembled monolayers on Au{111} are exchanged with alkaneselenols from solution, replacement of thiolates by selenols is rapid and complete, and is well described by perimeter-dependent island growth kinetics. The monolayer structures change as selenolate coverage increases, from being epitaxial and consistent with the initial thiolate structure to being characteristic of selenolate monolayer structures. At room temperature and at positive sample bias in scanning tunneling microscopy, the selenolate-gold attachment is labile, and molecules exchange positions with neighboring thiolates. The scanning tunneling microscope probe can be used to induce these place-exchange reactions

Design, Control, and Measurement of Molecular and Supramolecular Assemblies

Molecular switches and motors respond structurally, electronically, optically, and/or mechanically to external stimuli, testing and potentially enabling extreme miniaturization of optoelectronic devices, nanoelectromechanical systems, and medical devices. Assemblingmotors and switches on surfaces makes it possible both to measure the properties of individual molecules as they relate to their environment, and to couple function between assembled molecules. We designed molecules with precise functionality and assembled them on solid substrates either as isolated single molecules, linear one-dimensional chains, or as two dimensional islands in order to measure and to test the fundamental limits and cooperative function of the assemblies. We established that proximate functional molecules interact with each other to drive unprecedented cooperative motion at the nanoscale. In conjunction with theory, we establish the mechanism by which the molecules perform this nanoscale action. We modify the environments in which these assemblies are adsorbed to tune their dipole-dipole interactions via self- and directed assembly. We establish the role of the tethers that decouple the functional molecules from the underlying conductive substrates. By varying the composition of the tether we understood that the molecular function varies inversely with the conductance of the tether. In order to circumvent problems with steric hindrance in molecular assemblies, novel functional molecules were designed and tested at single-molecule and at ensemble scales. This thesis details the effects of parameters such as the molecular environment, intermolecular interactions, and internal functional groups of molecular switches on their nanoscale actuation

Manipulating double-decker molecules at the liquid-solid interface

We have used a scanning tunneling microscope to manipulate heteroleptic phthalocyaninato, naphthalocyaninato, porphyrinato double-decker molecules at the liquid/solid interface between 1-phenyloctane solvent and graphite. We employed nano-grafting of phthalocyanines with eight octyl chains to place these molecules into a matrix of heteroleptic double-decker molecules; the overlayer structure is epitaxial on graphite. We have also used nano-grafting to place double-decker molecules in matrices of single-layer phthalocyanines with octyl chains. Rectangular scans with a scanning tunneling microscope at low bias voltage resulted in the removal of the adsorbed doubledecker molecular layer and substituted the double-decker molecules with bilayer-stacked phthalocyanines from phenyloctane solution. Single heteroleptic double-decker molecules with lutetium sandwiched between naphthalocyanine and octaethylporphyrin were decomposed with voltage pulses from the probe tip; the top octaethylporphyrin ligand was removed and the bottom naphthalocyanine ligand remained on the surface. A domain of decomposed molecules was formed within the double-decker molecular domain, and the boundary of the decomposed molecular domain self-cured to become rectangular. We demonstrated a molecular “sliding block puzzle” with cascades of double-decker molecules on the graphite surface

Charge Transport and Conductance Switching of Redox-Active Azulene Derivatives

Azulene (Az) is a non-alternating, aromatic hydrocarbon composed of a five-membered, electron-rich and a seven-membered, electron-poor ring; an electron distribution that provides intrinsic redox activity. By varying the attachment points of the two electrode-bridging substituents to the Az center, the influence of the redox functionality on charge transport is evaluated. The conductance of the 1,3 Az derivative is at least one order of magnitude lower than those of the 2,6 Az and 4,7 Az derivatives, in agreement with density functional theory (DFT) calculations. In addition, only 1,3 Az exhibits pronounced nonlinear current–voltage characteristics with hysteresis, indicating a bias-dependent conductance switching. DFT identifies the LUMO to be nearest to the Fermi energy of the electrodes, but to be an active transport channel only in the case of the 2,6 and the 4,7 Az derivatives, whereas the 1,3 Az derivative uses the HOMO at low and the LUMO+1 at high bias. In return, the localized, weakly coupled LUMO of 1,3 Az creates a slow electron-hopping channel responsible for the voltage-induced switching due to the occupation of a single molecular orbital (MO)