Organic semi-conducting architectures for supramolecular electronics

The properties of organic electronic materials in the solid-state are determined not only by those of individual molecules but also by those of ensembles of molecules. The ability to control the architectures of these ensembles is thus essential for optimizing the properties of conjugated materials for use in electronic devices (light emitting diodes, field effect transistors, solar cells, …) and is primordial for potential technological applications in nanoelectronics.Here, we report on the observation by atomic force microscopy (AFM) of 1D and 2D nanoscale architectures obtained in the solid-state from solutions of molecularly-dissolved conjugated block copolymers or oligomers, and demonstrate that the conjugated molecules can organize onto a surface over lengthscales from nanometers to several microns, forming semiconducting fibrils or bi-dimensional organizations (monolayers) by π-stacking processes (by changing the sample preparation conditions).     

Push-pull macrocycles: donor-acceptor compounds with paired linearly conjugated or cross-conjugated pathways

Two-dimensional π-systems are of current interest in the design of functional organic molecules, exhibiting unique behavior for applications in organic electronics, single-molecule devices, and sensing. Here we describe the synthesis and characterization of "push-pull macrocycles": electron-rich and electron-poor moieties linked by a pair of (matched) conjugated bridges. We have developed a two-component macrocyclization strategy that allows these structures to be synthesized with efficiencies comparable to acyclic donor-bridge-acceptor systems. Compounds with both cross-conjugated (m-phenylene) and linearly conjugated (2,5-thiophene) bridges have been prepared. As expected, the compounds undergo excitation to locally excited states followed by fluorescence from charge-transfer states. The m-phenylene-based systems exhibit slower charge-recombination rates presumably due to reduced electronic coupling through the cross-conjugated bridges. Interestingly, pairing the linearly conjugated 2,5-thiophene bridges also slows charge recombination. DFT calculations of frontier molecular orbitals show that the direct HOMO-LUMO transition is polarized orthogonal to the axis of charge transfer for these symmetrical macrocyclic architectures, reducing the electronic coupling. We believe the push-pull macrocycle design may be useful in engineering functional frontier molecular orbital symmetries.     

Calix[4]phyrin based redox architectures: towards new molecular tools for electrochemical sensing

New redox active molecular macrocyclic architectures characterized by a direct connection between dipyrrin, tripyrrin and ferrocenyl fragments have been synthesized and characterized. Contrarily to fully conjugated porphyrins, in which four pyrrole moieties contribute to the overall aromatic pi-electronic system and behave as a unique electroactive species, calixphyrins can be regarded as an assembly of independent redox active pyrrole and conjugated oligopyrrole fragments linked through sp(3) hybridized meso carbon atoms. The disruption of the conjugation pathway not only multiplies the number of redox centres throughout the molecule but also leads to a large variety of molecular architectures with specific physico-chemical properties. These novel ferrocene containing hybrid macrocycles exhibit especially attractive electronic and structural features suited for use as molecular sensing tools. An efficient voltammetric sensing of exogenic electron rich anionic species could especially be performed using a metallo-calix[4]phyrin-(1.1.1.1) through the displacement of the labile axial binding site, the perturbation of the Fc(0/+) redox couple being directly related to complexed species features.     

Synthesis and photophysical properties of monodisperse oligo(9,9-di-n-octylfluorene-2,7-vinylene)s functionalized anthracenes

Four new well-defined monodisperse linear oligofluorenes with anthracene core An-OFVn (n = 1-4) have been synthesized through Heck and Wittig reaction. These conjugated oligomers exhibit good solubility in common organic solvents. It is found that the conjugation length of the oligomers can be extended and the formation of excimers can be suppressed with increasing the number of fluorene-vinylene units linked in the 9,10-positions of anthracene.     

Self-assembled nanostructures of oligopyridine molecules

The high potential of self-assembly processes of molecular building blocks is reflected in the vast variety of different functional nanostructures reported in the literature. The constituting units must fulfill several requirements like synthetic accessibility, presence of functional groups for appropriate intermolecular interactions and depending on the type of self-assembly processsignificant chemical and thermal stability. It is shown that oligopyridines are versatile building blocks for two- and three-dimensional (2D and 3D) self-assembly. They can be employed for building up different architectures like gridlike metal complexes in solution. By the appropriate tailoring of the heterocycles, further metal coordinating and/or hydrogen bonding capabilities to the heteroaromatic molecules can be added. Thus, the above-mentioned architectures can be extended in one-step processes to larger entities, or in a hierarchical fashion to infinite assemblies in the solid state, respectively. Besides the organizational properties of small molecules in solution, 2D assemblies on surfaces offer certain advantages over 3D arrays. By precise tailoring of the molecular structures, the intermolecular interactions can be fine-tuned expressed by a large variety of resulting 2D patterns. Oligopyridines prove to be ideal candidates for 2D assemblies on graphite and metal sufaces, respectively, expressing highly ordered structures. A slight structural variation in the periphery of the molecules leads to strongly changed 2D packing motifs based on weak hydrogen bonding interactions. Such 2D assemblies can be exploited for building up host-guest networks which are attractive candidates for manipulation experiments on the single-molecule level. Thus, "erasing" and "writing" processes by the scanning tunneling microscopy (STM) tip at the liquid/solid interface are shown. The 2D networks are also employed for performing coordination chemistry experiments at surfaces.     

Assembly of one dimensional inorganic nanostructures into functional 2D and 3D architectures. Synthesis, arrangement and functionality

This review will focus on the synthesis, arrangement, structural assembly, for current and future applications, of 1D nanomaterials (tubes, wires, rods) in 2D and 3D ordered arrangements. The ability to synthesize and arrange one dimensional nanomaterials into ordered 2D or 3D micro or macro sized structures is of utmost importance in developing new devices and applications of these materials. Micro and macro sized architectures based on such 1D nanomaterials (e.g. tubes, wires, rods) provide a platform to integrate nanostructures at a larger and thus manageable scale into high performance electronic devices like field effect transistors, as chemo- and biosensors, catalysts, or in energy material applications. Carbon based, metal oxide and metal based 1D arranged materials as well as hybrid or composite 1D materials of the latter provide a broad materials platform, offering a perspective for new entries into fascinating structures and future applications of such assembled architectures. These architectures allow bridging the gap between 1D nanostructures and the micro and macro world and are the basis for an assembly of 1D materials into higher hierarchy domains. This critical review is intended to provide an interesting starting point to view the current state of the art and show perspectives for future developments in this field. The emphasis is on selected nanomaterials and the possibilities for building three dimensional arrays starting from one dimensional building blocks. Carbon nanotubes, metal oxide nanotubes and nanowires (e.g. ZnO, TiO(2), V(2)O(5), Cu(2)O, NiO, Fe(2)O(3)), silicon and germanium nanowires, and group III-V or II-VI based 1D semiconductor nanostructures like GaS and GaN, pure metals as well as 1D hybrid materials and their higher organized architectures (foremost in 3D) will be focussed. These materials have been the most intensively studied within the last 5-10 years with respect to nano-micro integration aspects and their functional and application oriented properties. The critical review should be interesting for a broader scientific community (chemists, physicists, material scientists) interested in synthetic and functional material aspects of 1D materials as well as their integration into next higher organized architectures.     

From Interaction to Function in DNA-Templated Supramolecular Self-Assemblies

DNA-templated self-assembly represents a rich and growing subset of supramolecular chemistry where functional self-assemblies are programmed in a versatile manner using nucleic acids as readily-available and readily-tunable templates. In this review, we summarize the different DNA recognition modes and the basic supramolecular interactions at play in this context. We discuss the recent results that report the DNA-templated self-assembly of small molecules into complex yet precise nanoarrays, going from 1D to 3D architectures. Finally, we show their emerging functions as photonic/electronic nanowires, sensors, gene delivery vectors, and supramolecular catalysts, and their growing applications in a wide range of area from materials to biological sciences.     

Investigation of the Electronic Structure of Alkyl Allyl Radicals

The electronic structure of the homologous series of CH₃(CH₂)_nCHCHCH₂ (n = 0÷5) allyl all radicals is studied. The obtained spin density distribution is used to determine the fragment serving as the radical center. The delocalization of spin density over the basin of the radical center is shown to be responsible for two free valencies associated with two classical canonical structures (the conjugated fragment). The conjugation phenomenon is studied and electronic parameters are determined for the “standard” conjugated fragment CHCHCH₂.     

Advances in two-dimensional heterostructures by mono-element intercalation underneath epitaxial graphene

Two-dimensional (2D) materials have displayed many remarkable physical properties, including 2D superconductivity, magnetism, and layer-dependent bandgaps. However, it is difficult for a single 2D material to meet complex practical requirements. Heterostructures obtained by vertically stacking different kinds of 2D materials have extensively attracted researchers’ attention because of their rich electronic features. With heterostructures, the constraints of lattice matching can be overcome. Meanwhile, high application potential has been explored for electronic and optoelectronic devices, including tunneling transistors, flexible electronics, and photodetectors. Specifically, graphene-based van der Waals heterostructures (vdWHs) by intercalation are emerging to realize various functional heterostructures-based electronic devices. Intercalating atoms under epitaxial graphene can efficiently decouple graphene from the substrate, and is expected to realize rich novel electronic properties in graphene. In this study, we systematically review the progress of the mono-element intercalation in graphene-based vdWHs, including the intercalation mechanism, intercalation-modified electronic properties, and the practical applications of 2D intercalated heterostructures. This work would inspire edge-cutting ideas in the scientific frontiers of 2D materials.     

Highly selective addition of cyclosilanes to alkynes enabling new conjugated materials

Main group organometallic compounds can exhibit unusual optical properties arising from hybrid σ,π-conjugation. While linear silanes are extensively studied, the shortage of methods for the controlled synthesis of well-defined cyclic materials has precluded the study of cyclic conjugation. Herein we report that Ru-catalyzed addition of cyclosilanes to aryl acetylenes (hydrosilylation) proceeds with high chemoselectivity, regioselectivity, and diastereoselectivity, affording complex organosilanes that absorb visible light. We further show that the hydrosilylation products are useful building blocks towards novel conjugated polymers.

Hybrid σ,π-conjugated cyclosilanes were synthesized via highly selective hydrosilylation and have shown great potentials as building blocks to construct novel conjugated polymers with control of tacticity.     

A Thiophene Backbone Enables Two-Dimensional Poly(arylene vinylene)s with High Charge Carrier Mobility

Linear conjugated polymers have attracted significant attention in organic electronics in recent decades. However, despite intrachain π-delocalization, interchain hopping is their transport bottleneck. In contrast, two-dimensional (2D) conjugated polymers, as represented by 2D π-conjugated covalent organic frameworks (2D c-COFs), can provide multiple conjugated strands to enhance the delocalization of charge carriers in space. Herein, we demonstrate the first example of thiophene-based 2D poly(arylene vinylene)s (PAVs, 2DPAV-BDT-BT and 2DPAV-BDT-BP , BDT=benzodithiophene, BT=bithiophene, BP=biphenyl) via Knoevenagel polycondensation. Compared with 2DPAV-BDT-BP , the fully thiophene-based 2DPAV - BDT - BT exhibits enhanced planarity and π-delocalization with a small band gap (1.62 eV) and large electronic band dispersion, as revealed by the optical absorption and density functional calculations. Remarkably, temperature-dependent terahertz spectroscopy discloses a unique band-like transport and outstanding room-temperature charge mobility for 2DPAV-BDT-BT (65 cm² V⁻¹ s⁻¹), which far exceeds that of the linear PAVs, 2DPAV-BDT-BP , and the reported 2D c-COFs in the powder form. This work highlights the great potential of thiophene-based 2D PAVs as candidates for high-performance opto-electronics.     

Extrusion 3D printing of conjugated polymers

Conjugated polymers combine electronic charge transport properties with the ability to transport ions, enabling transduction between ionic and electronic currents. Many applications of conjugated polymers, such as biointerfaces, actuators, and energy storage, benefit from 3D structures. Among different methods for 3D fabrication, extrusion-based 3D printing is a versatile approach that is compatible with multimaterial fabrication processes. This review summarizes progress in the emerging field of 3D printed conjugated polymers using three extrusion printing processes: direct ink write, meniscus-guided printing, and electrohydrodynamic printing. Ink designs for direct in write are described in depth, including strategies for modifying the rheology and conductivity of the inks.     

Functional Polyoxometalate Thin Films via Electrostatic Layer-by-Layer Self-Assembly

Polyoxometalates (POMs) comprise a structurally diverse class of inorganic transition metal oxygen clusters which—owing to their unique electronic properties—hold promise for a host of technological applications such as electrochromic windows, sensors, or heterogeneous catalysts, prototypic examples of which will be briefly exemplified. The integration of POMs into functional architectures and devices, however, necessitates the development of general methods that allow positioning these clusters in well-defined supramolecular architectures, thin films, or mesophases. This short review highlights recent advances in the preparation of composite multilayers fabricated by electrostatic layer-by-layer self-assembly (ELSA) of POMs and a variety of water-soluble cationic species, including transition metal complexes, cationic surfactants, polycations and bipolar pyridine.     

Conjugated macrocycles: concepts and applications

One of the most important objectives in materials, chemical, and physical sciences is the creation of large conjugated macrocycles with well-defined shapes, since such molecules are not only theoretically and experimentally interesting but also have potential applications in nanotechnology. Fully unsaturated macrocycles are regarded as models for infinitely conjugated π systems with inner cavities, and exhibit unusual optical and magnetic behavior. Macrocycles have interior and exterior sites, and site-specific substitution at both or either site can afford attractive structures, such as 1D, 2D, and 3D supramolecular nanostructures. These nanostructures could be controlled through the use of π-extended large macrocycles by a bottom-up strategy. Numerous shape-persistent π-conjugated macrocycles have been synthesized, but only a few are on the nanoscale. This Review focuses on nanosized π-conjugated macrocycles (>1 nm diameter) and giant macrocycles (>2 nm diameter), and summarizes their syntheses and properties.     

Organic-inorganic nanohybrids through the direct tailoring of semiconductor nanocrystals with conjugated polymers

Two main synthetic strategies are used to prepare conjugated oligomer- or polymer-nanocrystal nanohybrids. In the first strategy ligand exchange is invoked to either replace with a bifunctional ligand, which contains a second functional group for coupling with conjugated oligomers or polymers (COs or CPs), or exchange for the functionalized COs or CPs in a "grafting-onto" process. Alternatively, in the second strategy the nanocrystal (NC) is passivated with functional ligands from which COs or CPs are directly grown in the absence of ligand exchange. The well-defined interface between the COs or CPs and NCs facilitates an efficient charge-transfer between them.     

Electronic decay constant of carotenoid polyenes from single-molecule measurements

The conductance of carotenoid polyenes chemically bound at each end to gold contacts has been measured for single molecules containing 5, 7, 9, and 11 carbon-carbon double bonds in conjugation. The electronic decay constant, beta, is determined to be 0.22 +/- 0.04 A-1, in close agreement with the value obtained from first principles simulations (0.22 +/- 0.01 A-1). The absolute values of the molecular conductance are within a factor of 3 of those calculated from first principles. The small value of beta demonstrates that conductivity drops off only slowly with chain length, confirming that carotenoid conjugated chains are relatively good molecular "wires".     

Discrete DNA three-dimensional nanostructures: the synthesis and applications

Structural DNA nanotechnology, an emerging technique that utilizes the nucleic acid molecule as generic polymer to programmably assemble well-defined and nano-sized architectures, holds great promise for new material synthesis and constructing functional nanodevices for different purposes. In the past three decades, rapid development of this technique has enabled the syntheses of hundreds and thousands of DNA nanostructures with various morphologies at different scales and dimensions. Among them, discrete three-dimensional (3D) DNA nanostructures not only represent the most advances in new material design, but also can serve as an excellent platform for many important applications. With precise spatial addressability and capability of arbitrary control over size, shape, and function, these nanostructures have drawn particular interests to scientists in different research fields. In this review article, we will briefly summarize the development regarding the synthesis of discrete DNA 3D nanostructures with various size, shape, geometry, and topology, including our previous work and recent progress by other groups. In detail, three methods majorly used to synthesize the DNA 3D objects will be introduced accordingly. Additionally, the principle, design rule, as well as pros and cons of each method will be highlighted. As functions of these discrete 3D nanostructures have drawn great interests to researchers, we will further discuss their cutting-edge applications in different areas, ranging from novel material synthesis, new device fabrication, and biomedical applications, etc. Lastly, challenges and outlook of these promising nanostructures will be given based on our point of view.     

Structures,stabilities, and IR and 13C-NMR spectra of dihedral fullerenes: A density functional theory study

Dihedral fullerenes are thermodynamically stable molecules with D nd or D nh symmetry.Based on experimental findings,two series of dihedral fullerenes with five-fold(C5) and six-fold(C6) symmetry have been studied using density functional theory(DFT).The DFT calculations showed that for both series the stabilities increased with increasing fullerene size.Structural analyses indicated that the stabilities are related to specific local geometries.In the case of the more abundant C5 series,the presence of approximately planar pentagons and hexagons on the top bowl favors their formation.That is to say,those fullerenes with small dihedral angles within the polygons are readily formed,because planar hexagons lead to strengthened conjugation which lowers average bonding energies(ABE) and increases thermodynamic stabilities.Non-planar hexagons at equatorial positions in tube-shaped fullerenes have an adverse effect on the conjugation and inhibit their formation.Calculations also demonstrated that fullerenes in the two series,including C 50(D 5h),C 60(D 6h),C 80(D 5d),C 96(D 6d),C 110(D 5h),and C 120(D 5d),have thermodynamically stable triplet structures with strong conjugation.The calculated IR and 13 C NMR spectra of the fullerenes show some similarities and regular trends due to their homogenous structures.The electronic structures indicate that short double bonds in hexagons with high electron occupancies are readily attacked by electrophilic agents and can also be coordinated by transition metals.Mechanistic discussions suggested that C 2 additions and C 2 losses constitute reversible processes at high temperature and C 2 additions in pentagonal fusions are crucial to the kinetics of the curvature of structures.C 3 additions lead to the formation of large fullerenes of other types.     

Organic electronic devices and their functional interfaces.

A most appealing feature of the development of (opto)electronic devices based on conjugated organic materials is the highly visible link between fundamental research and technological advances. Improved understanding of organic material properties can often instantly be implemented in novel device architectures, which results in rapid progress in the performance and functionality of devices. An essential ingredient for this success is the strong interdisciplinary nature of the field of organic electronics, which brings together experts in chemistry, physics, and engineering, thus softening or even removing traditional boundaries between the disciplines. Naturally, a thorough comprehension of all properties of organic insulators, semiconductors, and conductors is the goal of current efforts. Furthermore, interfaces between dissimilar materials-organic/organic and organic/inorganic-are inherent in organic electronic devices. It has been recognized that these interfaces are a key for device function and efficiency, and detailed investigations of interface physics and chemistry are at the focus of research. Ultimately, a comprehensive understanding of phenomena at interfaces with organic materials will improve the rational design of highly functional organic electronic devices.     

Nanoscale structural and electronic properties of ultrathin blends of two polyaromatic molecules: a Kelvin probe force microscopy investigation.

We describe a Kelvin Probe Force Microscopy (KPFM) study on the morphological and electronic properties of complex mono and bi-molecular ultrathin films self-assembled on mica. These architectures are made up from an electron-donor (D), a synthetic all-benzenoid polycyclic aromatic hydrocarbon, and an electron-acceptor (A), perylene-bis-dicarboximide. The former molecule self-assembles into fibers in single component films, while the latter molecule forms discontinuous layers. Taking advantage of the different solubility and self-organizing properties of the A and D molecules, multicomponent ultrathin films characterized by nanoscale phase segregated fibers of D embedded in a discontinuous layer of A are formed. The direct estimation of the surface potential, and consequently the local workfunction from KPFM images allow a comparison of the local electronic properties of the blend with those of the monocomponent films. A change in the average workfunction values of the A and D nanostructures in the blend occurs which is primarily caused by the intimate contact between the two components and the molecular order within the nanostructure self-assembled at the surface. Additional roles can be ascribed to the molecular packing density, to the presence of defects in the film, to the different conformation of the aliphatic peripheral chains that might cover the conjugated core and to the long-range nature of the electrostatic interactions employed to map the surface by KPFM limiting the spatial and potential resolution. The local workfunction studies of heterojunctions can be of help to tune the electronic properties of active multicomponent films, which is crucial for the fabrication of efficient organic electronic devices as solar cells.