Uniform approach to bacteriochlorophyll-based monolayers on conducting, semiconducting, and insulating substrates

A general approach is demonstrated for the formation of monolayers comprised of free-base and metalated Bacteriochlorophyll-based derivatives providing a new vehicle for studying photosynthetic motifs and chromophore thin-film interactions. Accessibility to covalent and self-assembled systems on conducting, semiconducting, and insulating substrates is realized utilizing identical molecular building blocks. The monolayers retain the optical features typical for the new systems in solution. Molecular organization of chromophore interaction motifs can be sequentially designed using preassembled building blocks in solution and expressed in the thin film optical properties. For instance, intramolecular pi-pi stacking is conserved for the dimeric Ni-based chromophores as deduced from the spectroscopic measurements of the monolayers and in solution.     

Robust multilayer thin films containing cationic thiol-functionalized gold nanorods for tunable plasmonic properties

Gold nanorods have great potential in a variety of applications because of their unique physical properties. In this article, we present the layer-by-layer (LbL) assembly of thin films containing positively charged gold nanorods that are covalently functionalized by cationic thiol molecules. The cationic gold nanorods are uniformly distributed in ultrathin nanocomposite LbL thin films. We studied the collective surface plasmon resonance coupling in the LbL films via UV-visible spectroscopy and evaluated their application in the surface-enhanced Raman scattering detection of rhodamine 6G probe molecules. Furthermore, we successfully manufactured freestanding nanoscale thin films containing multilayers of gold nanorods with a total thickness of less than 50 nm. The surface morphology and their optical and mechanical properties were systematically investigated, and the polycationic gold nanorods were found to play an important role in manipulating the properties of the nanocomposite thin films. Our findings reveal that such nanorods are excellent building blocks for constructing functional LbL films with tunable plasmonic behavior and robust mechanical properties.     

Experimental and computational evidence of the intermolecular motifs in the crystal packing of luminescent pentacoordinated gallium(III) complexes

This paper reports the synthesis, characterization, photophysical and structural properties of the homologous series of good emitting pentacoordinated GaQ'2L complexes 1-3, where Q' is 2-methyl-quinolin-8-olate and L is a phenolate substituted in para position with respect to the oxygen donor atom. A combined approach between the experimental structural analysis (i.e. the molecular fragments involved in intermolecular pi-pi interactions) and the computational study (i.e. the nature of the molecular orbitals residing therein) is discussed in order to compare the charge transport aptitude of complexes 1-3, in relation to the facing of their LUMO/LUMO, HOMO/HOMO and HOMO/LUMO arising from the molecular packing. The different phenolate ligands significantly change the packing characteristics of 1-3, with indirect effects on the electron hopping kinetics responsible for conduction in amorphous thin films. The observed preference for pyridyl-pyridyl stacking proves a faster electron conduction in comparison to hole conduction in the three complexes studied.     

Direct measurements of the interaction between pyrene and graphite in aqueous media by single molecule force spectroscopy: understanding the pi-pi interactions

Pyrene derivatives can absorb onto the surface of carbon nanotubes and graphite particles through pi-pi interactions to functionalize these inorganic building blocks with organic surface moieties. Using single molecule force spectroscopy, we have demonstrated the first direct measurement of the interaction between pyrene and a graphite surface. In particular, we have connected a pyrene molecule onto an AFM tip via a flexible poly(ethylene glycol) (PEG) chain to ensure the formation of a molecular bridge. The pi-pi interaction between pyrene and graphite is thus indicated to be approximately 55 pN with no hysteresis between the desorption and adhesion forces.     

Substituent effects control the self-association of molecular clips in the crystalline state

We report the X-ray crystal structure of 11 molecular clips and analyze the influence of substituents (e.g., OMe, Me, and NO2) and their location on the observed crystal packing. Molecular clips 3a and 3b form tapelike structures in the crystal due to pi-pi interactions between the aromatic walls. Compounds 3d, 3eC, and 3fC form dimers driven by critical C-H...O interactions and then form tapes driven by pi-pi interactions in the crystal. These two building motifs, pi-pi and C-H...O interactions, can be used to rationalize the enantio- and diastereoselectivity observed in the X-ray crystal structures of the remaining five molecular clips. For example, the C-H...O interactions are found to dictate the formation of homochiral dimers in the structures of (+/-)-3eT and (+/-)-3fT and to control the diastereoselective formation of 6a2-6c2 dimeric motifs with internal p-dimethoxy-o-xylylene walls. Overall, the results suggest that substituent effects that induce even weak intermolecular interactions (e.g., C-H...O) can be used to reliably control crystal packing within glycoluril-based systems.     

Layer-by-layer assembly: from conventional to unconventional methods

Layer-by-layer (LbL) assembly is a powerful means for fabricating multilayer thin films with controlled architecture and composition. This feature article discusses different types of methods for LbL assembly. On the one hand, some of the conventional LbL methods are introduced, which are driven by electrostatic interactions, hydrogen bonds, step-by-step reactions, sol-gel processes, molecular recognition, charge-transfer, stepwise stereocomplex assembly, and electrochemistry. On the other hand, some of the unconventional methods for fabricating of the building blocks which can not be assembled by conventional methods are also summarized. These unconventional methods usually involve the formation of supramolecular structures via one type of self-assembly. These structures can subsequently be used as building blocks in another type of self-assembly. To take advantage of these conventional and unconventional methods, a great number of building blocks can be fabricated into multilayer thin films with a defined sequence structure in a designed way. It has been demonstrated that LbL methods provide new horizons for surface molecular engineering.     

Fullerenes: multitask components in molecular machinery

Molecular machines are molecular-scale devices that carry out predetermined tasks derived from molecular motion. This Minireview illustrates how fullerenes can be used as multitask building blocks in molecular machinery, providing new perspectives for fullerenes. Indeed, C(60) can be applied as a photo- and electroactive stopper owing to its size, as a probe for molecular motion as a result of its well-defined physicochemical properties, and to induce motion through pi-pi interactions. Such molecular motion can be employed to modulate light-driven electron-transfer events, extending the potential applications of molecular machines to the typical fields of application of fullerenes.     

Solid-state packing of conjugated oligomers: from pi-stacks to the herringbone structure

The solid-state structures of a series of bithiazole and thiophene oligomers, as well as a series of substituted pentacenes, are rationalized in terms of "pitch and roll" inclinations from an "ideal" cofacial pi-stack. Pitch inclinations translate adjacent molecules relative to one another in the direction of the long molecular axis, whereas roll inclinations translate the molecules along the short molecular axis. Thus, moderately large pitch distortions preserve pi-pi interactions between adjacent molecules, whereas roll translations greater than 2.5 A essentially destroy pi-pi overlap between adjacent molecules. The familiar herringbone packing is characterized by large roll distortions. It is shown that thiophenes tend to exhibit large roll translations, whereas thiazoles have small roll but large pitch translations. Substituted pentacenes tend to have both moderate pitch and roll distances. The relationship of molecular packing to transport properties is discussed.     

Interactions modes and physical properties in transition metal chalcogenolene-based molecular materials

Molecule-based materials are extremely versatile materials as they can be built from specifically designed building blocks with the desired size, shape, charge and electronic properties which determine their intermolecular interactions and, thus, their organization in the solid. The intermolecular interactions, therefore, in particular van der Waals interactions, π–π and π–d interactions, H-bonding, etc., play a crucial role in self-assembling these pre-designed molecular units and may provide a powerful way to afford layered mono- and multifunctional molecular materials with new or unknown physical properties. In this review the relationship between interaction modes and physical properties of organic/inorganic hybrids based on transition metal complexes with chalcogenolene ligands will be examined and an outlook will be proposed. With this goal, magnetic materials, highly conducting and metallic single-component materials containing dithiolene complex building blocks, multifunctional materials where the dithiolene complex is the magnetic or conducting component in addition to more complex systems involving other types of building block such as the metal oxalate complexes, will be discussed.     

Characterization of self-assembled monolayers of oligo(phenyleneethynylene) derivatives of varying shapes on gold: effect of laterally extended pi-systems

Fully conjugated organic molecules, such as the oligo(phenyleneethynylene) (OPE) systems, are of growing interest within the field of molecular electronics, as is the self-assembly of well-defined molecular thin films with predefined functions. The structure and function of such films are intimately related and governed by the structures of their molecular constituents, through the intermolecular interactions and the interactions between the molecules and the substrate, onto which the film is assembled. Here we report on the synthesis of a series of three OPE derivatives, with the general structure phenylethynylene-aryl-ethynylenephenylene-headgroup, and the structural investigation of the self-assembled monolayers (SAMs) formed from them on Au(111) surfaces. The SAMs were characterized by infrared reflection-absorption spectroscopy, spectroscopic ellipsometry, high-resolution X-ray photoemission spectroscopy, and near-edge X-ray absorption fine structure spectroscopy. The effective thickness of the SAMs was observed to decrease as the pi-system of the aryl moiety of the OPE adsorbate was extended perpendicular to its molecular long axis. Changing the aryl moiety from benzene to naphthalene to anthracene resulted in lower molecular surface densities and larger molecular inclination. The average tilt angles for the benzene, naphthalene, and anthracene SAMs were found to be about 30 degrees , 40 degrees , and 42 degrees from the surface normal, respectively. For the largest adsorbate, the anthracene derivative, there is spectroscopic evidence suggesting the existence of nonequivalent binding sites. The differences observed between the SAMs are rationalized in terms of the shape of the adsorbates and the strength of the pi-pi interactions between them.     

Interfaces between Molecular and Polymeric “Metals”: Electrically Conductive,Structure-Enforced Assemblies of Metallomacrocycles

The design, synthesis, characterization, and understanding of new molecular and macro-molecular substances with “metal-like” electrical properties represents an active research area at the interface of chemistry, physics, and materials science. An important, long-range goal in this field of “materials by design” is to construct supermolecular assemblies which exhibit preordained collective phenomena by virtue of “engineered” interactions between molecular building blocks. In this review, such a class of designed materials is discussed which, in addition, bridges the gap between molecular and polymeric conductors: assemblies of electrically conductive metallomacrocycles. It is seen that efforts to rationally construct stacked metal-like molecular arrays lead logically to structure-enforced macromolecular assemblies of covalently linked molecular subunits. Typical building blocks are robust, chemically versatile metallophthalocyanines. The electrical optical, and magnetic properties of these metallomacrocyclic assemblies and the fragments thereof, provide fundamental information on the connections between local atomic-scale architecture, electronic structure, and the macroscopic collective properties of the bulk solid.     

Molecular Links and Knots from Naphthalenediimide: A Balance of Weak Interactions

Owing to its unique structural, electronic, spectroscopic, and redox properties, naphthalenediimide (NDI) is a versatile building block for the construction of a wide variety of supramolecular assemblies with diverse structures, properties, and functions. In this Minireview, the synthesis of topologically nontrivial molecular links and knots based on naphthalenediimide‐derived building blocks will be discussed. In particular, the supramolecular forces that contribute to the formation of these molecular links and knots and their interactions will be discussed.     

Molecular and Crystal Structure Diversity,and Physical Properties of Tetrathiafulvalene Derivatives Substituted with Various Aryl Groups through Sulfur Bridges

A library of tetrathiafulvalene (TTF) derivatives ( TTF‐1 – TTF‐47 ) bearing aryl groups attached through sulfur bridges has been created. The peripheral aryl groups exert a significant influence on both the electronic and crystallographic properties of the resulting TTFs. These TTFs display broad absorption bands at 400–500 nm caused by intramolecular charge‐transfer transitions between the aryl groups and central TTF core, and their first redox potentials increase with increasing electron‐withdrawing ability of the aryl groups. In their crystal structures (22 examples), the central TTF cores adopt various conformations, including chair, half‐chair, boat, and planar conformations. Moreover, the peripheral aryl groups exhibit multiple alignment modes with respect to the central TTF core, caused by their rotation about the two C? S bonds of the sulfur bridges. The packing motifs of these TTFs depend on both the nature of the aryl groups and their spatial alignment modes. Driven by intermolecular van der Waals forces and π–π interactions between the aryl groups and between the aryl groups and the TTF core, these TTFs adopt various packing structures. As a typical example, TTF‐14 , an achiral molecule, adopts a helical chain stack through intermolecular atomic close contacts. Moreover, the molecular geometries and packing motifs of these TTFs are sensitive to environmental variation, as exemplified by TTF‐28 , which adopts three distinct crystal modifications with diverse molecular geometries and stacking modes under different crystallization conditions. This work indicates that these TTFs are potential candidates as electronic materials, as well as functional building blocks for supramolecular assembly.     

First systematic investigation of C-H...Cl hydrogen bonding using inorganic supramolecular synthons: lamellar, stitched stair-case, linked-ladder, and helical structures

Using a group of six neutral M(II)Cl(2)-containing coordination compounds as building blocks, the first systematic investigation of C-H...Cl hydrogen-bonding interactions was performed. Single-crystal X-ray structural analyses of four new compounds (pseudo-tetrahedral Co(II) and Zn(II); distorted trigonal bipyramidal Zn(II)) authenticate the metal coordination geometry. To provide a unified view of the presence of noncovalent interactions in this class of compounds, we have re-examined the packing diagram of two previously reported compounds (a distorted square-pyramidal Cu(II) complex and a trans-octahedral Co(II) complex). The organic ligands of our choice comprise bidentate/tridentate pyrazolylmethylpyridines and an unsymmetrical tridentate pyridylalkylamine. This systematic investigation has allowed us to demonstrate the existence of versatile C-H...Cl(2)M interactions and to report the successful application of such units as inorganic supramolecular synthons. Additional noncovalent interactions such as C-H...O and O-H...Cl hydrogen bonding and pi-pi stacking interactions have also been identified. Formation of novel supramolecular architectures has been revealed: 2D lamellar (p-cyclophane) and 3D lamellar, 3D "stitched staircase" (due to additional hydrogen-bonding interactions by water tetramers, with an average O-O bond length in the tetramer unit of 2.926 A, acting as "molecular clips" between staircases), 3D linked ladder, and single-stranded 1D helix.     

DNA-templated photonic arrays and assemblies: design principles and future opportunities

Molecular photonics is a rapidly developing and multi-disciplinary field of research involving the construction of molecular assemblies comprising photoactive building blocks that are responsive to a light stimulus. A salient challenge in this field is the controlled assembly of these building blocks with nanoscale precision. DNA exhibits considerable promise as an architecture for the templated assembly of photoactive materials. In this Concept Article we describe the progress that has been made in the area of DNA photonics, in which DNA acts as a platform for the construction of optoelectronic assemblies, thin films and devices.     

Silver(I) coordination polymers containing heteroditopic ureidopyridine ligands: the role of ligand isomerism, hydrogen bonding, and stacking interactions

New silver (I) coordination polymers has been successfully designed and synthesized using heteroditopic ureidopyridine ligands 1 and 2 via a combination of coordinations bonds, hydrogen bonding, and pi-pi stacking interactions. This study shows an example of the orientation of the pyridine nitrogen relative to the urea moiety (4-substituted, 1, or 3-substituted, 2), used to control the packing of resulting crystalline coordination polymers. The ureidopyridine ligands present some flexibility because of the conformational rotation around the central urea moiety. The co-complexation of the silver(I) cation by two pyridine moieties and of the PF(6)(-) counteranion by the urea moiety results in the formation of discrete [1(2)Ag](+)PF(6)(-), (3) and [2(2)Ag](+)PF(6)(-), (4) complexes presenting restricted rotation around the central urea functionality. The geometrical information contained in the structures of ligands 1 and 2 and the heteroditopic complexation of silver hexafluorophosphate are fully exploited in an independent manner resulting in the emergence of quasi-rigidly preorganized linear and angular building blocks of 3 and 4, respectively. Additional pi-pi stacking contacts involving interactions between the pi-donor benzene and the pi-acceptor pyridine systems reinforce and direct the self-assembly of the above-described combined structural motifs in the solid state. Accordingly, linear and tubular arrays of pi-pi stacked architectures are generated in the solid state by synergistic and sequential metal ion complexation, hydrogen bonding, and pi-pi stacking interactions.     

Crystal Engineering of Supramolecular 1,4-Benzene Bisamides by Side-Chain Modification – Towards Tuneable Anisotropic Morphologies and Surfaces

Benzene bisamides are promising building blocks for supramolecular nano-objects. Their functionality depends on morphology and surface properties. However, a direct link between surface properties and molecular structure itself is missing for this material class. Here, we investigate this interplay for two series of 1,4-benzene bisamides with symmetric and asymmetric peripheral substitution. We elucidated the crystal structures, determined the nano-object morphologies and derived the wetting behaviour of the preferentially exposed surfaces. The crystal structures were solved by combining single-crystal and powder X-ray diffraction, solid-state NMR spectroscopy and computational modelling. Bulky side groups, here t-butyl groups, serve as a structure-directing motif into a packing pattern, which favours the formation of thin platelets. The use of slim peripheral groups on both sides, in our case linear perfluorinated, alkyl chains, self-assemble the benzene bisamides into a second packing pattern which leads to ribbon-like nano-objects. For both packing types, the preferentially exposed surfaces consist of the ends of the peripheral groups. Asymmetric substitution with bulky and slim groups leads to an ordered alternating arrangement of the groups exposed to the surface. This allows the hydrophobicity of the surfaces to be gradually altered. We thus identified two leitmotifs for molecular packings of benzene bisamides providing the missing link between the molecular structure, the anisotropic morphologies and adjustable surface properties of the supramolecular nano-objects.     

Enhancement of molecular properties in thin films by controlled orientation of molecular building blocks

We demonstrate the design and formation of thin films having divergent physicochemical properties by using two porphyrin building blocks with high chemical and optical resemblance. A predetermined variation in the molecular design is efficiently transferred and enhanced when constituting a two-dimensional film via control of molecular orientation. Variations of the peripheral substituents on the porphyrin ring resulted in control of the molecular orientation at the surface.     

Light-activated gating and permselectivity in interfacial architectures combining "caged" polymer brushes and mesoporous thin films

We demonstrate that "caged" macromolecular building blocks can endow mesoporous thin films with light responsiveness and lead to the creation of functional hybrid ensembles with phototriggered permselective properties.     

Noncovalent interactions in supramolecular complexes: a study on corannulene and the double concave buckycatcher

Stimulated by the recent observation of pi-pi interactions between C60 and corannulene subunits in a molecular tweezer arrangement (J Am Chem Soc 2007, 129, 3842), a density functional theory study was performed to analyze the electronic structure and properties of various noncovalent corannulene complexes. The theoretical approach is first applied to corannulene complexes with a series of benchmark molecules (CH4, NH3, and H2O) using several new-generation density functionals. The performance of nine density functionals, illustrated by computing binding energies of the corannulene complexes, demonstrates that Zhao and Truhlar's MPWB1K and M05-2X functionals provide energies similar to that obtained at the SCS-MP2 level. In contrast, most of the other popular density functionals fail to describe this noncovalent interaction or yield purely repulsive interactions. Further investigations with the M05-2X functional show that the binding energy of C60 with corannulene subunits in the relaxed molecular receptor clip geometry is -20.67 kcal/mol. The results of this calculation further support the experimental interpretation of pure pi-pi interactions between a convex fullerene and the concave surfaces of two corannulene subunits.