Jason B. Harper

Wet chemical routes to the assembly of organic monolayers on silicon surfaces via the formation of Si–C bonds: surface preparation, passivation and functionalization

Organic functionalization of non-oxidized silicon surfaces, while allowing for robust chemical passivation of the inorganic substrate, is intended and expected to broaden the chemical, physical and electronic properties of the currently most relevant technological material. Numerous protocols are now available for the preparation of Si-C, Si-O and Si-N bound layers. In particular, the covalent attachment of 1-alkenes and 1-alkynes onto hydride-terminated Si(100) and Si(111) has seen a wealth of research activity starting from the pioneering work of Linford and Chidsey (Alkyl monolayers covalently bonded to silicon surfaces, J. Am. Chem. Soc., 1993, 115(26), 12631-12632). This critical review aims to bring together the available wet-chemical routes toward the formation of silicon-organic monolayers under ambient conditions. Particular emphasis is placed on discussing the reasons behind the need for novel chemical approaches that are straightforward, modular and of wide scope so as to allow the application of silicon electrodes in aqueous electrolytes. A general introduction to biomolecular recognition events at functionalized silicon surfaces is also presented (281 references).

Why are aromatic compounds more soluble than aliphatic compounds in dimethylimidazolium ionic liquids? A simulation study

Abstract Molecular dynamics simulations of solutions of benzene in dimethylimidazolium chloride and dimethylimidazolium hexafluorophosphate have been performed with a view to answering the question posed in the title. The difference between the chemical potential of a normal model of benzene and one with no charges was found to depend on the solvent but is at least 4 kBT. This difference is sufficient to account for the observed solubility differences. There are substantial changes in the local structure around benzene with and without charges.

Using an Electrical Potential to Reversibly Switch Surfaces between Two States for Dynamically Controlling Cell Adhesion

Smart surfaces presenting both antifouling molecules with a charged functional group at their distal end, and molecules that are terminated by RGD peptides for cell adhesion, were fabricated and characterized (see picture). By applying potentials of +300 or -300 mV, the surfaces could be dynamically switched to make the peptide accessible or inaccessible to cells.

Macroscopic and microscopic properties of solutions of aromatic compounds in an ionic liquid

Molecular dynamics simulations of binary mixtures of benzene, 1,3,5-trifluorobenzene and hexafluorobenzene with dimethylimidazolium hexafluorophosphate were carried out to examine their macroscopic and microscopic properties. The energies and volumes of mixing of these mixtures correlate well with observed microscopic properties including coordination number about the aromatic compound. The local ordering of the ions about an aromatic molecule was found to depend on the quadrupole moment of the aromatic species and to remain qualitatively the same on varying the mole fraction of the aromatic species. Interaction energies showed the most significant interactions to be between the aromatic molecule and the ions located about its equator. These findings have implications for the practical use of ionic liquids as solvents for chemical processes.

The effect of changing the components of an ionic liquid upon the solubility of lignin

A facile technique for the quantification of lignin solubility in ionic liquids has been established. This was used to examine the effect of the anionic and cationic components of ionic liquids on lignin solubility. Changing the cation was shown to have a significant effect on lignin solubility. Interaction of aromatic cations with the solute was significant in most, but not all, ionic liquids. The anion was required to have a minimum hydrogen bonding basicity value for lignin to dissolve, but after this point the anion effect on the overall lignin solubility was negligible relative to the cation.

Ionic liquids: just Molten salts after all?

While there has been much effort in recent years to characterise ionic liquids in terms of parameters that are well described for molecular solvents, using these to explain reaction outcomes remains problematic. Herein we propose that many reaction outcomes in ionic liquids may be explained by considering the electrostatic interactions present in the solution; that is, by recognising that ionic liquids are salts. This is supported by evidence in the literature, along with studies presented here.

Investigating the origin of entropy-derived rate accelerations in ionic liquids

The effects on the rate and activation parameters of a series of Menschutkin processes on changing from a molecular solvent to an ionic liquid were investigated. The removal of delocalised pi-systems from the reagents does not affect the change in activation parameters on changing solvent. In each of the cases investigated, rate accelerations observed on moving to the ionic liquid could be attributed to an increase in reaction entropy. This suggests a specific interaction of the ionic liquid with the nucleophilic centre, rather than the delocalised pi-systems of either the electrophile or the nucleophile.

Does the cation really matter? The effect of modifying an ionic liquid cation on an SN2 process

The rate of reaction of a Menschutkin process in a range of ionic liquids with different cations was investigated, with temperature-dependent kinetic data giving access to activation parameters for the process in each solvent. These data, along with molecular dynamics simulations, demonstrate the importance of accessibility of the charged centre on the cation and that the key interactions are of a generalised electrostatic nature.

Towards reaction control using an ionic liquid: biasing outcomes of reactions of benzyl halides

The effect of the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide on substitution and elimination reactions of benzyl halides is examined and solvent control of the reaction outcome demonstrated. In competing reactions, the ionic liquid is shown to favour unimolecular processes over bimolecular processes and substitution over elimination, irrespective of the mole fraction of ionic liquid used. Temperature dependent analyses, where possible, are used to determine the microscopic origins of these effects.

Effect of Modifying the Anion of an Ionic Liquid on the Outcome of an SN2 Process

The effect of a series of ionic liquids containing different anions (bis(trifluoromethanesulfonyl)imide, dicyanimide, hexafluorophosphate, tetrafluoroborate, and bromide) on the rate constant of a bimolecular substitution process was investigated. A general ionic liquid effect was noted, with increases in the rate constant observed in all ionic liquids used when compared with that in acetonitrile. Temperature-dependent kinetic data allowed calculation of activation parameters in each of the reaction mixtures considered; these parameters showed that the microscopic origins of the rate enhancements observed were not the same for all of the ionic liquids used, demonstrating the importance of the nature of the anion.