Gonzalo Guirado

Gated photochromism and acidity photomodulation of a diacid dithienylethene dye.

The present study quantitatively analyses the gated photochromism and the acidity photomodulation properties of a diacid dithienylethene compound. Photoisomerisation between the open and closed isomers was investigated by UV/visible and (1)H NMR spectroscopy. It was found that the photocyclisation quantum yield of the diacid form was remarkably high (around 90%). Partial neutralisation of the open isomer revealed a gated photochromism as the photocyclisation quantum yield of the mono- and dianion were 50 and 67%, respectively. A considerable photomodulation of the acidity was observed: the closed isomer is more acid than the open one by more than one pK(a) unit. This effect has been shown to be exploitable for a reversible photo-acid generation. This is the first time that a complete quantitative investigation that allows for the determination of the main photochromic, spectral and thermodynamic parameters of a base-sensitive photochromic diarylethene has been carried out.

Fabrication of one-dimensional organic nanostructures using anodic aluminum oxide templates

Organic nanostructures are new comers to the fields of nanoscience and nanotechnology. In recent years novel methods for controlling the growth and uniformity of one-dimensional (1D) organic nanostructures (nanowires and nanotubes) have been developing. The use of hard templates as molds for the formation of organic nanowires or nanotubes seems to be a reliable and convenient method. In this review we will discuss the use of anodic aluminum oxide (AAO) templates as the inorganic hard template of choice. We will briefly survey advances in the fabrication of 1D polymer nanostructures using AAO templates, while the bulk of the review will focus on the synthesis of small molecule nanowires, nanotubes, and nanorods. We will also discuss unique properties of some highly crystalline small molecule nanorods fabricated using AAO templates.

Nucleophilic Aromatic Substitution of Hydrogen: A Novel Electrochemical Approach to the Cyanation of Nitroarenes

The nucleophilic aromatic substitution of hydrogen through electrochemical oxidation of the intermediate sigma complexes (Meisenheimer complexes) in simple nitroaromatic compounds is reported for the first time. The studies have been carried out with hydride and cyanide anions as the nucleophiles using cyclic voltammetry (CV) and preparative electrolysis. The cyclic voltammetry experiments allow for the detection and characterization of the sigma complexes and led us to a proposal for the mechanism of the oxidation step. Furthermore, the power of the CV technique in the analysis of the reaction mixture throughout the whole chemical and electrochemical process is described.

Stable Spirocyclic Meisenheimer Complexes

Meisenheimer complexes are important intermediates in Nucleophilic Aromatic Substitution Reactions (SNAr). They are formed by the addition of electron rich species to polynitro aromatic compounds or aromatic compounds with strong electron withdrawing groups. It is possible to distinguish two types of Meisenheimer or σ-complexes, the σH-complex or σX-complex (also named ipso), depending on the aromatic ring position attacked by the nucleophile (a non-substituted or substituted one, respectively). Special examples of σX- or ipso-complexes are formed through intermediate spiro adducts, via intramolecular SNAr. Some of these spirocyclic Meisenheimer complexes, a type of σX-complex, are exceptionally stable in solution and/or as solids. They can be isolated and characterized using X-ray, and various spectroscopic techniques such as NMR, UV-Vis, IR, and fluorescence. A few of these stable spirocyclic Meisenheimer complexes are zwitterionic and exhibit interesting photophysical and redox properties. We will review recent advances, synthesis and potential applications of these stable spirocyclic Meisenheimer complexes.

Mechanistic studies on the reactivity of halodinitrobenzene radical-anion

Abstract The electrochemical behaviour of 1-F-2,4-dinitrobenzene, 1-Cl-2,4-dinitrobenzene and 1-Br-2,4-dinitrobenzene in DMF is described. The 1-F-2,4-dinitrobenzene radical anion dimerises before cleaving, whereas 1-Cl-2,4-dinitrobenzene and 1-Br-2,4-dinitrobenzene radical anions dimerise after cleavage. This change in mechanism allows the obtention of 2,2′,4,4′-tetranitrobiphenyl in a selective way, and with good efficiency. The electrochemical oxidation of σ-complexes is shown to be an alternative means of obtaining products that are difficult to obtain through traditional procedures.

Electrochemically promoted nucleophilic aromatic substitution in room temperature ionic liquids—an environmentally benign way to functionalize nitroaromatic compounds

The current manuscript shows the electrochemical studies performed to rationalize the mechanism and develop new green synthetic routes for the synthesis of substituted nitroaromatics based on the advantages of the electrochemical approach to the nucleophilic aromatic substitution reaction (such as (a) low cost and ready availability of reagents, (b) atom economy, (c) high yields, approaching 100%) and the use of Room Temperature Ionic Liquids (RTILs) as green alternative solvents to organic aprotic solvents. Four of the most popular RTILs (1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6), 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)-imide ([BMIM]TFSI) and 1-butyl-3-methylimidazolium acetate ([BMIM]AcO) have been chosen since they have different properties in terms of solvation effects that can increase the regioselectivity of the reaction. The nucleophiles used to study the feasibility and viability of the reaction were the classical hydride, methoxide, ketones, cyanides and amines, whereas the nitroarenes selected were 4-nitrotoluene, 1,3-dinitrobenzene, 2,4-dinitroaniline, 1,3,5-trinitrobenzene, 1,3-dinitronaphthalene, 1-chloro-2,4,6-trinitrobenzene and 2,4,6-trinitroanisole. The electrocatalysis and regioselectivity effects of using RTILs are also investigated. The article concludes by analyzing the economic cost of performing this electrosynthesis in RTILs and organic solvent electrolyte systems, which contain 0.1 M of supporting electrolyte.

Electrochemical Synthesis of Nitroanilines

Alkylamines and amides are readily prepared by nucleophilic aromatic substitution of hydrogen in nitroarenes by electrochemical oxidation. Useful yields (15−85%) are achieved in a simple direct and regioselective amination process. The synthetic method has been examined in the absence and presence of external bases, used to promote the first step of the nucleophilic aromatic substitution reaction, i.e. the nucleophilic attack. In both cases, good results were obtained. The unreacted starting material can easily be recovered at the end of the electrochemical oxidation process. This new method represents an environmentally favourable route to amino- and amido-substituted nitroaromatic compounds.

Electrochemical Synthesis of Nitroaromatic Ketones

Nitroaromatic ketones are readily prepared by nucleophilic aromatic substitution of hydrogen in nitroarenes by electrochemical oxidation. Carbanions of various ketones were added to selected nitroarenes in DMF/ketone mixtures leading to formation of the σH complexes. The reaction was promoted using potassium tert-butoxide as a base. Useful yields were achieved (80−100%) in the C-arylation of ketones. In most cases, the process proceeded with high selectivity. This new method represents an environmentally favourable route for obtaining nitroaromatic ketones.

Accurate oxidation potentials of 40 benzene and biphenyl derivatives with heteroatom substituents.

The redox equilibrium method was used to determine accurate oxidation potentials in acetonitrile for 40 heteroatom-substituted compounds. These include methoxy-substituted benzenes and biphenyls, aromatic amines, and substituted acetanilides. The redox equilibrium method allowed oxidation potentials to be determined with high precision (≤ ±6 mV). Whereas most of the relative oxidation potentials follow well-established chemical trends, interestingly, the oxidation potentials of substituted N-methylacetanilides were found to be higher than those of the corresponding acetanilides. Density functional theory calculations provided insight into the origin of these surprising results in terms of the preferred conformations of the amides versus their cation radicals.

The role of cations in the reduction of 9-fluorenone in bis(trifluoromethylsulfonyl)imide room temperature ionic liquids

A report is presented on some fundamental electrochemical studies based on the effects of cations on the determination of the standard potential values for five bis(trifluoromethylsulfonyl)imide Room Temperature Ionic Liquids (RTILs) in the electrochemical reduction of 9-fluorenone, a well-known redox probe. The standard potential values obtained can be related to the solvation process of the electrogenerated fluorenone radical anion. The accurate determination of the above-mentioned values for RTILs, which are composed of unsymmetrical organic cations and a weakly-coordinating inorganic anion, can be used as a good starting point for establishing the basis for the design of new more environmentally friendly and energetically efficient electrosynthetic routes, as well as new optoelectronic devices based on fluorenone chemistry in those ionic solvents.