Self-assembled viologens on HOPG: solid-state NMR and AFM unravel the location of the anions.

The controlled growth of self-assembled networks on surfaces based on viologen salts is a major scientific challenge due to their unique electronic properties. The combination of solid-state NMR spectroscopy and atomic force microscopy at ambient conditions can unravel the fine organization of the supramolecular network on a graphitic surface by positioning the counter-ions relative to the viologen cation.

Remote template effect in the synthesis of bipyridine-strapped porphyrins.

A bipyridine-strapped porphyrin was prepared using a remote template effect of alkali or transition metal cations in the bipyridine subunit to enhance the yield 10-fold. The flexibility of the bipyridine-strap also allowed the synthesis of a doubly strapped porphyrin.

Going Up the Ladder: Stacking Four 4,4'-Bipyridine Moieties within a Ti(IV)-Based Tetranuclear Architecture.

Biphenol-based ligands have proven their ability to bind titanium(IV) centers and generate sophisticated self-assembled structures in which auxiliary nitrogen ligands often complete the coordination sphere of the metal and improve stability. Here, a central 4,4'-bipyridine, which acts as both a spacer and a source of monodentate nitrogen to complete the coordination sphere of the Ti(IV) complex, was incorporated within two bis-2,2'-biphenol strands, 3H4 and 4H4. Both proligands possess structural features that are well adapted to form self-assembled structures built from titanium-oxygen-nitrogen units; however, their different degrees of torsional freedom strongly influenced the nuclearity of the complexes formed. The presence of a phenyl spacer between the bipyridine and the biphenol moieties of 3H4 provided enough flexibility for the ligand to wrap around one titanium(IV) center to form a mononuclear complex Ti(3)(DMF)2 in the presence of dimethylformamide (DMF). Assembly of the more rigid ligand 4H4 with Ti(OiPr)4 afforded a tetranuclear complex Ti4(4)2(4H)2(OEt)2 containing four stacked 4,4'-bipyridine units as shown by the X-ray structure of the complex. Density functional theory studies suggested that the assembly of this tetrametallic complex involves a dimetallic intermediate with TiO6 nodes that is converted to the thermodynamically stable tetranuclear complex with two TiO6 nodes and two TiO5N units with enhanced covalent character.

Functionalized 4,4'-Bipyridines: Synthesis and 2D Organization on Highly Oriented Pyrolytic Graphite.

Commercial 4,4'-bipyridine is a popular scaffold that is primarily employed as a linker in 3D self-assembled architectures such as metallo-organic frameworks or as a connector in 2D networks. The introduction of alkyl substituents on the bipyridine skeleton is instrumental when 4,4'-bipyridines are used as linkers to form 2D self-assembled patterns on surfaces. Here, several synthetic strategies to access 4,4'-bipyridines functionalized at various positions are described. These easily scalable reactions have been used to introduce a range of alkyl substituents at positions 2 and 2' or 3 and 3' and at positions 2,2' and 6,6' in the case of tetra-functionalization. Scanning tunneling microscopy studies of molecular monolayers physisorbed at the graphite-solution interface revealed different supramolecular patterns whose motifs are primarily dictated by the nature and position of the alkyl chains.

Modelling haemoproteins: porphyrins and cyclodextrins as sources of inspiration.

The association of hydrophobic cavities with porphyrin derivatives has been used to mimic haemoprotein structures. The most employed cavity in this field is ß-cyclodextrin (ß-CD), and scaffolds combining ß-CDs and porphyrins are expected to inspire the combination of porphyrins and cucurbiturils in the near future. Aside from providing water solubility to various porphyrinic structures, the ß-CD framework can also modulate and control the reactivity of the metal core of the porphyrin. After a general introduction of the challenges faced in the field of haemoprotein models and the binding behavior of ß-CDs, this article will discuss covalent and non-covalent association of porphyrins with ß-CDs. In each approach, the role of the CD differs according to the relative position of the concave CD host, either directly controlling the binding and transformation of a substrate on the metalloporphyrin or playing a dual role of controlling the water solubility and selecting the axial ligand of the metal core. The discussion will be of interest to the cucurbituril community as well as to the cavitand community, as the information provided should be useful for the design of haemoprotein mimics using cucurbiturils.

Light triggers molecular shuttling in rotaxanes: control over proximity and charge recombination.

We present the synthesis of novel rotaxanes based on mechanically interlocked porphyrins and fullerene and their advanced investigations by means of photophysical measurements. To this end, a fullerene-capped dumbbell-type axle containing a central triazole was threaded through strapped (metallo)porphyrins-either a free-base or a zinc porphyrin. Femtosecond-resolved transient absorption measurements revealed charge-separation between the porphyrin and fullerene upon light excitation. Solvent polarity and solvent coordination effects induced molecular motion of the rotaxanes upon charge separation and enabled, for the first time, subtle control over the charge recombination by enabling and controlling the directionality of shuttling.

A Highly Stable Organic Radical Cation.

Functionalization of a methylviologen with four methyl ester substituents significantly facilitates the first two reduction steps. The easily generated radical cation shows markedly improved air stability compared to the parent methylviologen, making this derivative of interest in organic electronic applications.

Pairing-up viologen cations and dications: a microscopic investigation of van der Waals interactions.

The microscopic origin of van der Waals- and magnetic-interactions in 4,4' methyl viologen cation-based units (MV+˙ and MV2+) was inspected using wave function (variational DDCI and perturbative MP2, CASPT2) and density functional theory (DFT) calculations. The analysis deepens the comprehension of the magnetic behavior of experimental bis-viologen cyclophanes ([CYC]2(+˙)), in which the MV+˙ units are connected through alkyl linkers of different lengths. The formation of the so-called long-multicenter bonds in such radical dimers, responsible for the quenching of the magnetic response, was analyzed in [MV2]2(+˙). Dynamical correlation effects, accessible from second-order perturbation corrections, were decisive in observing a bonding regime characterized by an equilibrium distance of 3.3 Šand a 45 kJ mol-1 dissociation energy. At larger intermolecular distances, our calculations on [MV2]2(+˙) indicate that the singlet and triplet states are energetically competing (i.e. weak exchange interactions, JAB). Despite the absence of any clear bonding regime at the MP2 level, the puzzling association of two di-cations into [MV2]4+ is anticipated at 3.3 Šusing weakly screened point charges (ε = 1.5) to account for the Coulomb interactions between the solvated subunits. The main conclusion is that both dispersion interactions and environment effects are required to overcome the Coulomb repulsion associated with doubly-charged species. All these data provide some complementary insights into the nature and amplitude of interactions between cation and dication units, and their relevance in various experimental manifestations.

Ring-Oxidized Zinc(II) Phthalocyanine Cations: Structure, Spectroscopy, and Decomposition Behavior.

A bromonium oxidizing agent was used to produce a ring-oxidized zinc phthalocyanine (PcZn), [PcZn(solvent)]•2[BArF4]2 (1·solvent), in good yield. This material is dimeric in the solid state with one axially coordinated solvent [tetrahydrofuran (THF) or 1,2-dimethoxyethane (DME)] and close intradimer ring-ring distances of 3.18 and 3.136 Å (THF and DME respectively); this proximity facilitates strong antiferromagnetic coupling to yield diamagnetic dimers. 1·THF is present in solution as a monomer and a dimer. In CH2Cl2, the dimer is favored above 0.1 mM, and it is almost exclusively present in solvents with a high dielectric constant such as acetonitrile. The material 1·THF/DME decomposes in DME to a meso-nitrogen-protonated species, [HPcZn(DME)2][BArF4] (2), which was isolated and represents the first example of such a structurally characterized, protonated, unsubstituted PcM complex. A partially oxidized dimer or "pimer" [(PcZn(DME))2]•[BArF4] (3) was also structurally characterized and has a intradimer ring-ring distance of 3.192 Å, similar to 1·THF/DME. Dimer 3 also represents the first isolated PcM-based pimer. Electron paramagnetic resonance analysis of a 1.0 mM solution of 1·DME in DME showed the production of 3 over hours by the combination of 1·DME and 2 in solution.

Peripherally Metalated Porphyrins with Applications in Catalysis, Molecular Electronics and Biomedicine.

Porphyrins are conjugated, stable chromophores with a central core that binds a variety of metal ions and an easily functionalized peripheral framework. By combining the catalytic, electronic or cytotoxic properties of selected transition metal complexes with the binding and electronic properties of porphyrins, enhanced characteristics of the ensemble are generated. This review article focuses on porphyrins bearing one or more peripheral transition metal complexes and discusses their potential applications in catalysis or biomedicine. Modulation of the electronic properties and intramolecular communication through coordination bond linkages in bis-porphyrin scaffolds is also presented.

A water-soluble supramolecular complex that mimics the heme/copper hetero-binuclear site of cytochrome c oxidase.

In mitochondria, cytochrome c oxidase (CcO) catalyses the reduction of oxygen (O2) to water by using a heme/copper hetero-binuclear active site. Here we report a highly efficient supramolecular approach for the construction of a water-soluble biomimetic model for the active site of CcO. A tridentate copper(ii) complex was fixed onto 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinatoiron(iii) (FeIIITPPS) through supramolecular complexation between FeIIITPPS and a per-O-methylated ß-cyclodextrin dimer linked by a (2,2':6',2''-terpyridyl)copper(ii) complex (CuIITerpyCD2). The reduced FeIITPPS/CuITerpyCD2 complex reacted with O2 in an aqueous solution at pH 7 and 25 °C to form a superoxo-type FeIII-O2-/CuI complex in a manner similar to CcO. The pH-dependent autoxidation of the O2 complex suggests that water molecules gathered at the distal Cu site are possibly involved in the FeIII-O2-/CuI superoxo complex in an aqueous solution. Electrochemical analysis using a rotating disk electrode demonstrated the role of the FeTPPS/CuTerpyCD2 hetero-binuclear structure in the catalytic O2 reduction reaction.

Fluorescence Commutation and Surface Photopatterning with Porphyrin Tetradithienylethene Switches.

Four tetradithienylethene (DTE) substituted porphyrins, the free base 1H2 , and three metal derivatives (1Zn, 1Co, 1Ni), were synthesized and studied. These dyads, for which the DTE units are connected to the porphyrin's meso positions via a meta-phenyl spacer, exhibit reversible photochromic properties in all cases, with conversion to the photostationary state (PSS) up to 88 %, as confirmed by absorption and NMR spectroscopies. Compounds 1H2 and 1Zn are fluorescent in solution and display a red emission. Upon irradiation with UV light to trigger the closing of the DTEs, the fluorescence of both the free base and zinc porphyrin was very efficiently quenched in solution. The reversible, photo-switching of luminescence was retained in a tetra-DTE free-base porphyrin-doped polystyrene film, for which photo-patterning was demonstrated by confocal scanning microscopy. The tunable fluorescent properties of this multi-DTE framework render this compound of interest as a photo-rewritable fluorescent ink.

CuAAC in a Distal Pocket: Metal Active-Template Synthesis of Strapped-Porphyrin [2]Rotaxanes.

The synthesis of a porphyrin rotaxane by dipolar cycloaddition takes advantage of the ditopic character of a phenanthroline-strapped porphyrin. The success of the click reaction was conditioned by the presence of both a coordinatively unsaturated metal in the porphyrin and a copper(I) bound to the phenanthroline, pointing at a new "tandem active metal template" mechanism.

From Models of Hemoproteins to Self-Assembled Molecular Wires.

In nature, the various properties of tetrapyrrolic macrocycles are mostly due to their proteic environment and result from an evolutionary process that is difficult to reproduce during the lifetime of a synthetic chemist. Thus, the task of synthetic chemists attempting to reproduce the biological role of porphyrin architectures, which perform functions from catalysis to light harvesting, is complicated. This account describes how a phenanthroline-strapped porphyrin architecture initially designed to afford new hemoprotein models led, over the last two decades, to the preparation of highly linear self-assembled nano-objects inspired by light-harvesting architectures. The approach summarized herein mimics, in a very modest way, the polyvalence of tetrapyrrolic macrocycles found in nature.

Viologen cyclophanes: redox controlled host-guest interactions.

Viologens can exist in three redox states varying from dicationic to neutral. This work emphasizes the control of the host-guest properties in bis-viologen cyclophanes. Two flexible cyclophanes were prepared by a cyclisation method sensitive to the odd/even number of carbons in the flexible chains linking two viologens. C5 and C7 cyclophanes were characterised by X-ray diffraction in their tetracationic state and their diradical dicationic state. In the presence of tetrathiafulvalene or methyl viologen as guests, inclusion complexes were obtained, including a mixed valence species.

Trapping Nanostructures on Surfaces through Weak Interactions.

The assembly of imidazole-functionalized phenanthroline-strapped zinc porphyrins (ZnPorphen) with alkyl or polyethylene glycol (PEG) side chains was studied in solution and by AFM after casting on highly oriented pyrolytic graphite (HOPG) or mica. The nature of the solvent and its evaporation time influenced the morphology of the objects observed. On HOPG, short rods of about 100 nm were observed after fast evaporation of solutions of the alkyl derivatives in CHCl3 , THF, or pyridine, whereas islands of aligned rows of longer wires were obtained from methylcyclohexane (MCH). Slow evaporation of MCH led to a three-dimensional assembly. The PEG porphyrin assembled into short wires on HOPG or fibers on mica after slow evaporation of solutions in THF. This study shows the role of surface-molecule interactions in the interfacial assembly of ZnPorphen derivatives and contributes to understanding the parameters that control their noncovalent assembly into molecular wires on a surface.

Dynamic assembly of porphyrin wires trapped on a highly oriented pyrolitic graphite surface.

Carefully designed porphyrin building blocks assemble through selective imidazole binding in various solvents to form linear multiporphyrin objects. From a dynamic mixture of monomers, dimers, and oligomers, linear objects were observed on a highly oriented pyrolitic graphite (HOPG) surface. On the surface, the objects' morphology clearly depended on the solvent used for deposition and was modified upon heating.

Highly linear self-assembled porphyrin wires.

An efficient noncovalent assembly process involving high geometrical control was applied to a linear bis(imidazolyl zinc porphyrin) 7Zn, bearing C(18) substitutents, to generate linear multiporphyrin wires. The association process is based on imidazole recognition within the cavity of the phenanthroline-strapped zinc porphyrin. In chlorinated solvents, discrete soluble oligomers were obtained after (7Zn)(n) was end-capped with a terminal single imidazolyl zinc porphyrin derivative 4Zn. These soluble species, as well as their destabilization in the presence of protic solvents, were studied by UV-visible and time-resolved luminescence. In the solid state, assemblies as long as 480 nm, which corresponds to 190 iterative units or a total of 380 porphyrins, were observed by atomic force microscopy measurements on mica. The length and linearity of the porphyrin wires obtained illustrate the potential of phenanthroline-strapped porphyrins for the directional control of self-assembly processes.

Toward generic models of hemoproteins.

The versatility of a resorcinol-substituted phenanthroline-strapped porphyrin was enhanced by appending allyl chains via a double Claisen rearrangement. Two pyridine arms were then appended to the resorcinol oxygens. This multifunctional porphyrin is a suitable generic building block for the preparation of liposoluble heme protein models. X-ray characterization of the pentacoordinated zinc complex and the binding of a sixth imidazole ligand to the ferric complex both suggest a facilitated access to an empty distal site.

Molecular tools for the self-assembly of bisporphyrin photodyads: a comprehensive physicochemical and photophysical study.

Accessible and hindered phenanthroline-strapped Zn(II) porphyrin receptors exhibited suitable topography tailored to strongly and selectively bind N(1)-unsubstituted imidazoles and imidazoles appended to free-base porphyrins. Distal binding was clearly driven by the formation of strong bifurcated hydrogen bonds with the phenanthroline unit of the receptors. An extensive physicochemical study emphasized the influence of bulkiness of the substrate and of the porphyrin receptor on the binding and self-assembly mechanism. Knowledge of the corresponding spectroscopic, thermodynamic, and kinetic data were of fundamental importance to elucidate and to model the photoinduced properties which occur within the self-assembled porphyrin dyads.