Photoproduction of Proton Gradients with π-Stacked Fluorophore Scaffolds in Lipid Bilayers

Recent progress of core-substituted naphthalenediimides: highlights from 2010

Core-substituted naphthalenediimides (cNDIs) are rapidly emerging as a powerful strategy to create functional nanomaterials and their implications in biological and supramolecular chemistry are significant. Recent developments in the synthesis of cNDIs have allowed several groups to probe the function of this interesting class of dye molecules in a molecular and supramolecular sense. Core-substitution of the NDI can be seen as an opportunity to extend the planar, rigid core and could be used to prepare novel structures for applications in organic, biosupramolecular chemistry, biomedicine, materials science and organic solar cells. In this Emerging Area, we provide up-to-date recent progress in the field of cNDIs. We begin with a general discussion and the applications of cNDIs in the field of supramolecular chemistry i.e. generation of nanostructures such as vesicles and nanotubes etc., and we also discuss advances in artificial photosynthesis. Following this is a section on their implications in the field of sensors, particularly DNA intercalation, anion sensing and NDI based pH sensors. Finally, we explore the recent development of cNDIs in organic solar cell applications. We conclude with our views on the prospects of cNDIs in future research.

Amino acid derivatized arylenediimides: a versatile modular approach for functional molecular materials

Nature's elegant molecular designs and their assemblies with specific structure-property correlations have inspired researchers to design and develop bio-mimics for advanced functional applications. To realize such advanced molecular materials, naturally evolved amino acids are arguably the ideal auxiliaries due to their remarkable molecular/chiral recognition and distinctive sequence specific self-assembling properties. Over the years, this modular approach of derivatizing naphthalenediimides (NDIs) and perylenediimides (PDIs) with amino acids and peptides have resulted in several hitherto unknown molecular assemblies with phenomenal impact on their performance. Derivatization with versatile arylenediimides is especially interesting due to their wide spread applications in fields ranging from biomedicine to electronics. Herein some of these seminal reports of this rapidly emerging field and the design principles embraced are discussed.

Self-assembly of supra-amphiphiles based on dual charge-transfer interactions: from nanosheets to nanofibers

With the elaborate engineering of supra-amphiphiles based on dual charge-transfer interactions, the rational design and programmable transformation of well-defined 1D and 2D nanostructures have been demonstrated. First, H-shaped supra-amphiphiles are successfully obtained on the basis of the directional charge-transfer interactions of naphthalene diimide and naphthalene, which self-assemble in water to form 2D nanosheets. Second, by complexation of the H-shaped supra-amphiphiles with pyrene derivatives, the 2D nanosheets transform into ultralong 1D nanofibers. Therefore, this line of research represents a successful example of supramolecular engineering and has enriched its realm.

Biosupramolecular nanowires from chlorophyll dyes with exceptional charge-transport properties

Conductive tubes: Self-assembled nanotubes of a bacteriochlorophyll derivative are reminiscent of natural chlorosomal light-harvesting assemblies. After deposition on a substrate that consists of a non-conductive silicon oxide surface (see picture, brown) and contacting the chlorin nanowires to a conductive polymer (yellow), they show exceptional charge-transport properties.

Concave versus planar geometries for the hierarchical organization of mesoscopic 3D helical fibers

Chromophore-peptide systems: a study on a series of pentapeptides covalently connected to planar π systems (1 a and 1 b) or to a curved π system (1 c) showed the influence of the concave shape on the efficient chiral transmission at nano- and mesoscales. Control over the hierarchical growth by H bonding, π-π, and solvophobic interactions made possible the efficient generation of electroactive 3D helical fibers.

Electron transport through single π-conjugated molecules bridging between metal electrodes

Understanding electron transport through a single molecule bridging between metal electrodes is a central issue in the field of molecular electronics. This review covers the fabrication and electron-transport properties of single π-conjugated molecule junctions, which include benzene, fullerene, and π-stacked molecules. The metal/molecule interface plays a decisive role in determining the stability and conductivity of single-molecule junctions. The effect of the metal-molecule contact on the conductance of the single π-conjugated molecule junction is reviewed. The characterization of the single benzene molecule junction is also discussed using inelastic electron tunneling spectroscopy and shot noise. Finally, electron transport through the π-stacked system using π-stacked aromatic molecules enclosed within self-assembled coordination cages is reviewed. The electron transport in the π-stacked systems is found to be efficient at the single-molecule level, thus providing insight into the design of conductive materials.

Light-induced electron transfer over distances of 5, 10, and 15 Å within water-filled yoctowells

A small series of variable-depth yoctowell cavities with 'functional' walls on aminated silica particles and gold electrodes has been established. The dimensions of the gaps formed were 2.2 nm in diameter with varying 'functional' depths of 5, 10, and 15 Å, depending on the length of bolaphiles applied and the position of the positive rim; these gaps were prepared through a Michael addition of the incorporated ene-amide groups. Using this construct and electrostatic interactions between the positive rim and anionic quinones as a means of immobilization, a porphyrin-quinone dyad system has been prepared. The distance between the donor and acceptor was changed systematically in aqueous solution, whilst maintaining a similar environment in each case. Upon photoexcitation of the porphyrin, efficient electron transfer occurs between the porphyrin and quinone units in a distance-dependent manner on the nanosecond timescale.

A two-color, self-controlled molecular beacon

Control yourself! A two-color molecular beacon with non-nucleosidic chromophores in a triplex stem is presented. Pyrene and PDI fluorophores act as mutual quenchers by formation of a donor-acceptor complex in the closed form. Hybridization with the target results in two independent fluorescence signals. The two-color read-out provides a "self-control" feature, which helps to eliminate false positive signals in imaging and screening applications.

Adaptive supramolecular nanomaterials based on strong noncovalent interactions

Noncovalent systems are adaptive and allow facile processing and recycling. Can they be at the same time robust? How can one rationally design such systems? Can they compete with high-performance covalent materials? The recent literature reveals that noncovalent systems can be robust yet adaptive, self-healing, and recyclable, featuring complex nanoscale structures and unique functions. We review such systems, focusing on the rational design of strong noncovalent interactions, kinetically controlled pathway-dependent processes, complexity, and function. The overview of the recent examples points at the emergent field of noncovalent nanomaterials that can represent a versatile, multifunctional, and environmentally friendly alternative to conventional covalent systems.

Naphthalene- and perylenediimides with hydroquinones, catechols, boronic esters and imines in the core

The green-fluorescent protein of the jellyfish operates with the most powerful phenolate donors in the push-pull fluorophore. To nevertheless achieve red fluorescence with the same architecture, sea anemone and corals apply oxidative imination, a process that accounts for the chemistry of vision as well. The objective of this study was to apply these lessons from nature to one of the most compact family of panchromatic fluorophores, i.e. core-substituted naphthalenediimides (cNDIs). We report straightforward synthetic access to hydroxylated cNDI and cPDI cores by palladium-catalyzed cleavage of allyloxy substituents. With hydroxylated cNDIs but not cPDIs in water-containing media, excited-state intramolecular proton transfer yields a second bathochromic emission. Deprotonation of hydroquinone, catechol and boronic ester cores provides access to an impressive panchromism up to the NIR frontier at 640 nm. With cNDIs, oxidative imination gives red shifts up to 638 nm, whereas the expanded cPDIs already absorb at 754 nm upon deprotonation of hydroquinone cores. The practical usefulness of hydroquinone cNDIs is exemplified by ratiometric sensing of the purity of DMF with the "naked eye" at a sensitivity far beyond the "naked nose". We conclude that the panchromatic hypersensitivity toward the environment of the new cNDIs is ideal for pattern generation in differential sensing arrays.

Recent progress with functional biosupramolecular systems

The objective of this account is to summarize our recent progress with functional biosupramolecular systems concisely. The functions covered are artificial photosynthesis, anion transport, and sensing in lipid bilayer membranes. With artificial photosynthesis, the current emphasis is on the construction of ordered and oriented architectures on solid surfaces. Recent examples include the zipper assembly of photosystems with supramolecular n/p-heterojunctions and oriented antiparallel redox gradients. Current transport systems in lipid bilayers reveal new interactions at work. Examples include anion-macrodipole or anion-π interactions. Current attention with membrane-based sensing systems shifts from biosensor approaches with enzymatic signal generation to aptamers (i.e., the DNA version of immunosensing) and differential sensing with dynamic polyion-counterion transporters. The functional diversity accessible with biosupramolecular systems is highlighted, as is the critical importance of cross-fertilization at intertopical convergence zones.

Unsymmetrical diimine chelation to M(II) (M=Zn, Cd, Pd): atropisomerism, pi-pi stacking and photoluminescence

Three types of atropisomeric unsymmetrical diimine complexes, tetrahedral (L(R)(Φ))MX(2) (M = Zn, Cd; X = Cl, Br; R = Me, CMe(3), OH, OMe, Cl; 1a-k, type-I), tetrahedral (L(Me2)(Φ))ZnBr(2) (2, type-II) and square planar (L(OH)(ϕ))PdCl(2) (3, type-III) with different photoluminescence properties, have been reported (L(R)(Φ) = (E)-4-R-N-(pyridine-2-ylmethylene)aniline; Φ = dihedral angle between the diimine unit including the pyridine ring and the phenyl ring planes). In crystals, Φ = 0° for type-I, 90° for type-II and 63° for type-III atropisomers have been confirmed by single crystal X-ray structure determinations of 1c, 1e, 2 and 3·H(2)O isomers. Optimizations of geometries in methanol have established Φ = 28-32° for type-I, 90.83° for type-II and 43.44° for type-III isomers. In solids, type-I atropisomers with Φ = 0, behave as conjugated 14πe systems facilitating π-π stacking and are brightly luminescent at room temperature while type-II and type-III isomers in solid and type-I isomers in solutions are more like non-conjugated 8πe + 6πe systems and non-emissive. Frozen glasses of acetonitrile, methanol and dichloromethane-toluene mixture at 77 K of type-I isomers are emissive and display structured excitation and emission spectra for R = Me, CMe(3), OMe species. Excitation and emission maxima of frozen glasses (λ(ex) = 320-380 nm; λ(em) = 440-485 nm) are red shifted in the solid (λ(ex) = 390-455 nm; λ(em) = 470-550 nm). TD-DFT calculations on 1b, 1d, 1f and stacked (1b)(2) isomers and luminescence lifetime measurements have elucidated that an excited (1)ILCT state has been the origin of emission of the type-I isomers and delocalizations of the photoactive π(diimine) and π(diimine)(*) orbitals of the L(R)(Φ) over the stacked layers shift the λ(ext) and λ(em) of solids to lower energies than those in frozen glasses. The trends of diimine ligand based electron transfer events of the complexes in DMF have been investigated by cyclic voltammetry at 298 K.

Zipper and layer-by-layer assemblies of artificial photosystems analyzed by combining optical and piezoelectric surface techniques

Quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) were used to study zipper and layer-by-layer multilayer assemblies of artificial photosystems based on naphthalenediimides (NDIs) attached to an oligophenylethynyl (OPE-NDI) or p-oligophenyl (POP-NDI) backbone in dry and wet state. For the most interesting OPE-NDI zipper, one obtains for the dry film a monolayer thickness of 1.85 nm and a density of 1.58 g/cm(3), while the wet film has a larger monolayer thickness of 3.6 nm with a water content of 36%. The dry thickness of a monolayer in OPE-NDI zippers corresponds to about one-half of the length of the OPE scaffold in agreement with the proposed structure of the zipper. The low water content of the OPE-NDI films confirms their compact structure. The dry monolayer thickness of the POP-NDI films of 1.45 nm is smaller than that for the OPE-NDI films, which is probably related to a tilt of the POP scaffolds within the adsorbed layer. The POP-NDI films swell in water much more substantially, suggesting a much more open structure. These features are in excellent agreement with the better photophysical performance of the OPE-NDI assemblies when compared to the POP-NDI films.