Aggregate nanostructures of organic molecular materials

Conjugated organic molecules are interesting materials because of their structures and their electronic, electrical, magnetic, optical, biological, and chemical properties. However, researchers continue to face great challenges in the construction of well-defined organic compounds that aggregate into larger molecular materials such as nanowires, tubes, rods, particles, walls, films, and other structural arrays. Such nanoscale materials could serve as direct device components. In this Account, we describe our recent progress in the construction of nanostructures formed through the aggregation of organic conjugated molecules and in the investigation of the optical, electrical, and electronic properties that depend on the size or morphology of these nanostructures. We have designed and synthesized functional conjugated organic molecules with structural features that favor assembly into aggregate nanostructures via weak intermolecular interactions. These large-area ordered molecular aggregate nanostructures are based on a variety of simpler structures such as fullerenes, perylenes, anthracenes, porphyrins, polydiacetylenes, and their derivatives. We have developed new methods to construct these larger structures including organic vapor-solid phase reaction, natural growth, association via self-polymerization and self-organization, and a combination of self-assembly and electrochemical growth. These methods are both facile and reliable, allowing us to produce ordered and aligned aggregate nanostructures, such as large-area arrays of nanowires, nanorods, and nanotubes. In addition, we can synthesize nanoscale materials with controlled properties. Large-area ordered aggregate nanostructures exhibit interesting electrical, optical, and optoelectronic properties. We also describe the preparation of large-area aggregate nanostructures of charge transfer (CT) complexes using an organic solid-phase reaction technique. By this process, we can finely control the morphologies and sizes of the organic nanostructures on wires, tubes, and rods. Through field emission studies, we demonstrate that the films made from arrays of CT complexes are a new kind of cathode materials, and we systematically investigate the effects of size and morphology on electrical properties. Low-dimension organic/inorganic hybrid nanostructures can be used to produce new classes of organic/inorganic solid materials with properties that are not observed in either the individual nanosize components or the larger bulk materials. We developed the combined self-assembly and templating technique to construct various nanostructured arrays of organic and inorganic semiconductors. The combination of hybrid aggregate nanostructures displays distinct optical and electrical properties compared with their individual components. Such hybrid structures show promise for applications in electronics, optics, photovoltaic cells, and biology. In this Account, we aim to provide an intuition for understanding the structure-function relationships in organic molecular materials. Such principles could lead to new design concepts for the development of new nonhazardous, high-performance molecular materials on aggregate nanostructures.     

Nanogap structures for molecular nanoelectronics

This study is focused on the realization of nanodevices for nano and molecular electronics, based on molecular interactions in a metal-molecule-metal (M-M-M) structure. In an M-M-M system, the electronic function is a property of the structure and can be characterized through I/V measurements. The contact between the metals and the molecule was obtained by gold nanogaps (with a dimension of less than 10 nm), produced with the electromigration technique. The nanogap fabrication was controlled by a custom hardware and the related software system. The studies were carried out through experiments and simulations of organic molecules, in particular oligothiophenes.     

Single molecular switch based on thiol tethered iron(II)clathrochelate on gold

Molecular electronics has been associated with high density nano-electronic devices. Developments of molecular electronic devices were based on reversible switching of molecules between the two conductive states. In this paper, self-assembled monolayers of dodecanethiol (DDT) and thiol tethered iron(II)clathrochelate (IC) have been prepared on gold film. The electrochemical and electronic properties of IC molecules inserted into the dodecanethiol monolayer (IC-DDT SAM) were investigated using voltammetric, electrochemical impedance spectrpscopy (EIS), scanning tunneling microscopy (STM) and cross-wire tunneling measurements. The voltage triggered switching behaviour of IC molecules on mixed SAM was demonstrated. Deposition of polyaniline on the redox sites of IC-DDT SAM using electrochemical polymerization of aniline was performed in order to confirm that this monolayer acts as nano-patterned semiconducting electrode surface.     

Light and redox switchable molecular components for molecular electronics

The field of molecular and organic electronics has seen rapid progress in recent years, developing from concept and design to actual demonstration devices in which both single molecules and self-assembled monolayers are employed as light-responsive components. Research in this field has seen numerous unexpected challenges that have slowed progress and the initial promise of complex molecular-based computers has not yet been realised. Primarily this has been due to the realisation at an early stage that molecular-based nano-electronics brings with it the interface between the hard (semiconductor) and soft (molecular) worlds and the challenges which accompany working in such an environment. Issues such as addressability, cross-talk, molecular stability and perturbation of molecular properties (e.g., inhibition of photochemistry) have nevertheless driven development in molecular design and synthesis as well as our ability to interface molecular components with bulk metal contacts to a very high level of sophistication. Numerous groups have played key roles in progressing this field not least teams such as those led by Whitesides, Aviram, Ratner, Stoddart and Heath. In this short review we will however focus on the contributions from our own group and those of our collaborators, in employing diarylethene based molecular components.     

Developing a molecular platform for potential carbon dioxide fixing

This paper presents an attempt to develop a new system for fixing carbon dioxide from the atmosphere. The proposed molecular system has been designed to have the capacity to spontaneously bind CO₂ from the atmosphere with high affinity. The molecular system is furthermore designed to have the ability to liberate CO₂ at a later stage in the process, i.e., in a separate compartment. The liberated CO₂ presents a carbon neutral way of obtaining pure CO₂. The proposed molecular system is based on a small stable organic molecule that potentially have two forms: one without bound CO₂ and one with bound CO₂. One class of molecules that undergo a reaction compatible with our purposal is the merocyanine dyes that exhibit photochromic properties. Based on this structural class of molecules, a system for the potential fixing of CO₂ has been developed.     

Organic Small Molecules for Photonics and Electronics from the [2.2]Paracyclophane Scaffold

In the search for molecules for organic photonics and electronics, several strategies have been used to modulate physical and chemical properties of [2.2]paracyclophane derivatives by sequential functionalization of their three-dimensional cores. This review summarizes the major design and synthetic strategies for tuning paracyclophane-containing small molecules by introducing various moieties featuring (hetero)aromatic rings directly connected to each other, as well as alternating (hetero)aryl and ethylene or ethynylene units. Several examples are presented that elucidate the structural, optical, and electronic consequences of incorporating these fragments in the aromatic decks, particularly relating to applications in organic optoelectronics.     

Molecular wire of urea in carbon nanotube: a molecular dynamics study

We perform molecular dynamics simulations of narrow single-walled carbon nanotubes (SWNTs) in aqueous urea to investigate the structure and dynamical behavior of urea molecules inside the SWNT. Even at low urea concentrations (e.g., 0.5 M), we have observed spontaneous and continuous filling of SWNT with a one-dimensional urea wire (leaving very few water molecules inside the SWNT). The urea wire is structurally ordered, both translationally and orientationally, with a contiguous hydrogen-bonded network and concerted urea's dipole orientations. Interestingly, despite the symmetric nature of the whole system, the potential energy profile of urea along the SWNT is asymmetric, arising from the ordering of asymmetric urea partial charge distribution (or dipole moment) in confined environment. Furthermore, we study the kinetics of confined urea and find that the permeation of urea molecules through the SWNT decreases significantly (by a factor of ～20) compared to that of water molecules, due to the stronger dispersion interaction of urea with SWNT than water, and a maximum in urea permeation happens around a concentration of 5 M. These findings might shed some light on the better understanding of unique properties of molecular wires (particularly the wires formed by polar organic small molecules) confined within both artificial and biological nanochannels, and are expected to have practical applications such as the electronic devices for signal transduction and multiplication at the nanoscale.     

Nanographenes as active components of single-molecule electronics and how a scanning tunneling microscope puts them to work

Single-molecule electronics, that is, realizing novel electronic functionalities from single (or very few) molecules, holds promise for application in various technologies, including signal processing and sensing. Nanographenes, which are extended polycyclic aromatic hydrocarbons (PAHs), are highly attractive subjects for studies of single-molecule electronics because the electronic properties of their flat conjugated systems can be varied dramatically through synthetic modification of their sizes and topologies. Single nanographenes provide high tunneling currents when adsorbed flat onto conducting substrates, such as graphite. Because of their chemical inertness, nanographenes interact only weakly with these substrates, thereby preventing the need for special epitaxial structure matching. Instead, self-assembly at the interface between a conducting solid, such as the basal plane of graphite, and a nanographene solution generally leads to highly ordered monolayers. Scanning tunneling spectroscopy (STS) allows the current-voltage characteristics to be measured through a single molecule positioned between two electrodes; the key to the success of STS is the ability to position the scanning tunneling microscopy (STM) tip freely with respect to the molecule in all dimensions, that is, both parallel and perpendicular to the surface. In this Account, we report the properties of nanographenes having sizes ranging from 0.7 to 3.1 nm and exhibiting various symmetry, periphery, and substitution types. The size of the aromatic system and the nature of its perimeter are two essential features affecting its HOMO-LUMO gap and charge carrier mobility in the condensed phase. Moreover, the extended pi area of larger substituted PAHs improves the degree of self-ordering, another key requirement for high-performance electronic devices. Self-assembly at the interface between an organic solution and the basal plane of graphite allows deposition of single molecules within the well-defined environment of a molecular monolayer. We have used STM and STS to investigate both the structures and electronic properties of these single molecules in situ. Indeed, we have observed key electronic functions, rectification and current control through single molecules, within a prototypical chemical field-effect transistor at ambient temperature. The combination of nanographenes and STM/STS, with the PAHs self-assembled in oriented molecular mono- or bilayers at the interface between an organic solution and the basal plane of graphite and contacted by the STM tip, is a simple, reliable, and versatile system for developing the fundamental concepts of molecular electronics. Our future targets include fast reversible molecular switches and complex molecular electronic devices coupled together from several single-molecule systems.     

Developing a molecular platform for potential carbon dioxide fixing

This paper presents an attempt to develop a new system for fixing carbon dioxide from the atmosphere. The proposed molecular system has been designed to have the capacity to spontaneously bind CO₂ from the atmosphere with high affinity. The molecular system is furthermore designed to have the ability to liberate CO₂ at a later stage in the process, i.e., in a separate compartment. The liberated CO₂ presents a carbon neutral way of obtaining pure CO₂. The proposed molecular system is based on a small stable organic molecule that potentially have two forms: one without bound CO₂ and one with bound CO₂. One class of molecules that undergo a reaction compatible with our purposal is the merocyanine dyes that exhibit photochromic properties. Based on this structural class of molecules, a system for the potential fixing of CO₂ has been developed.     

Interfacial phenomena between conjugated organic molecules and noble metals

Understanding interfacial interaction between conjugated organic molecules and noble metals is important not only for surface science, but also in relation to organic epitaxy, the architecture of intermolecular networks or nanostructures, and organic electronics. Particularly, properties of interfacial geometric and electronic structures and their related phenomena have attracted much interest for their potential in various electronic and optoelectronic applications, and thus extensive efforts have been devoted to understand and control organic/metal interfaces. We provide an overview of interfacial phenomena between conjugated organic molecules and noble metals via various interactions at the organic/metal interfaces such as surface-molecule and intermolecular interactions, as well as recent progress achieved in this area.     

NDDO-Based CI Methods for the Prediction of Electronic Spectra and Sum-Over-States Molecular Hyperpolarization

NDDO-based (AM1) configuration interaction (CI) calculations have been used to calculate the wavelength and oscillator strengths of electronic absorptions in organic molecules and the results used in a sum-over-states treatment to calculate second-order hyperpolarizabilities. The results for both spectra and hyperpolarizabilities are of acceptable quality as long as a suitable CI-expansion is used. We have found that using an active space of eight electrons in eight orbitals and including all single and pair-double excitations in the CI leads to results that agree well with experiment and that do not change significantly with increasing active space for most organic molecules. Calculated second-order hyperpolarizabilities using this type of CI within a sum-over-states calculation appear to be of useful accuracy.     

An Insight into molecular structure and properties of flexible amorphous polymers: A molecular dynamics simulation approach

Visualization of polymer molecules by molecular dynamics simulation remains a challenging area in molecular modeling, as it involves a number of factors like type of force field, simulation time, simulation steps, and so forth. In our present study, we have used the condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field, which is specific for polymers and organic molecules, in order to visualize the macromolecular chains of various flexible amorphous polymers: natural rubber, polybutadiene rubber, styrene–butadiene rubber, nitrile rubber, polychloroprene rubber, and fluoroelastomer. The study covered a wide range from nonpolar rubbers to polar rubbers, and the COMPASS force field was promising to understand the local structure and the packing of the chains inside the simulation box. The distance between different adjacent pairs of carbon atoms within the polymer chains was discussed in detail from intramolecular radial distribution function and represented pictorially in the polymer chains. Various bond angles were also examined for further details. In addition, interchain contacts, glass-transition temperature, and solubility parameter were predicted and compared with the experimental values. The diffusive characteristic of the chains was assessed by mean square displacement which in turn described the polymer chain mobility. In essence, the present study is expected to aid in the vivid conceptualization of molecular orientation of various polymers and would help in predicting various physical properties. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47457     

Phosphorus and phosphorus-nitrogen doped carbon nanotubes for ultrasensitive and selective molecular detection

A first-principles approach is used to establish that substitutional phosphorus atoms within carbon nanotubes strongly modify the chemical properties of the surface, thus creating highly localized sites with specific affinity towards acceptor molecules. Phosphorus-nitrogen co-dopants within the tubes have a similar effect for acceptor molecules, but the P-N bond can also accept charge, resulting in affinity towards donor molecules. This molecular selectivity is illustrated in CO and NH3 adsorbed on PN-doped nanotubes, O2 on P-doped nanotubes, and NO2 and SO2 on both P- and PN-doped nanotubes. The adsorption of different chemical species onto the doped nanotubes modifies the dopant-induced localized states, which subsequently alter the electronic conductance. Although SO2 and CO adsorptions cause minor shifts in electronic conductance, NH3, NO2, and O2 adsorptions induce the suppression of a conductance dip. Conversely, the adsorption of NO2 on PN-doped nanotubes is accompanied with the appearance of an additional dip in conductance, correlated with a shift of the existing ones. Overall these changes in electric conductance provide an efficient way to detect selectively the presence of specific molecules. Additionally, the high oxidation potential of the P-doped nanotubes makes them good candidates for electrode materials in hydrogen fuel cells.     

π-Conjugated cyanostilbene derivatives: a unique self-assembly motif for molecular nanostructures with enhanced emission and transport

π-Conjugated organic molecules represent an attractive platform for the design and fabrication of a wide range of nano- and microstructures for use in organic optoelectronics. The desirable optical and electrical properties of π-conjugated molecules for these applications depend on their primary molecular structure and their intermolecular interactions such as molecular packing or ordering in the condensed states. Because of the difficulty in satisfying these rigorous structural requirements for photoluminescence and charge transport, the development of novel high-performance π-conjugated systems for nano-optoelectronics has remained a challenge. This Account describes our recent discovery of a novel class of self-assembling π-conjugated organic molecules with a built-in molecular elastic twist. These molecules consist of a cyano-substituted stilbenic π-conjugated backbone and various terminal functional groups, and they offer excellent optical, electrical, and self-assembly properties for use in various nano-optoelectronic devices. The characteristic "twist elasticity" behavior of these molecules occurs in response to molecular interactions. These large torsional or conformational changes in the cyanostilbene backbone play an important role in achieving favorable intermolecular interactions that lead to both high photoluminescence and good charge carrier mobility in self-assembled nanostructures. Conventional π-conjugated molecules in the solid state typically show concentration (aggregation) fluorescence quenching. Initially, we describe the unique photoluminescence properties, aggregation-induced enhanced emission (AIEE), of these new cyanostilbene derivatives that elegantly circumvent these problems. These elastic twist π-conjugated backbones serve as versatile scaffolds for the preparation of well-defined patterned nanosized architectures through facile self-assembly processes. We discuss in particular detail the preparation of 1D nanowire structures through programmed self-assembly. This Account describes the importance of utilizing AIEE effects to explore optical device applications, such as organic semiconducting lasers (OSLs), optical memory, and sensors. We demonstrate the rich electronic properties, including the electrical conductivity, field-effect carrier mobility, and electroluminescence of highly crystalline 1D nanowire and coaxial donor-acceptor nanocable structures composed of elastic twist π-conjugated molecules. The electronic properties were measured using various techniques, including current-voltage (I-V), conducting-probe atomic force microscopy (CP-AFM), and space-charge-limited-current (SCLC) measurements. We prepared and characterized several electronic device structures, including organic field-effect transistors (OFETs) and organic light-emitting field-effect transistors (OLETs).     

Nitrogen and oxygen bridged calixaromatics: synthesis, structure, functionalization, and molecular recognition

Pedersen, Lehn, and Cram established supramolecular chemistry through their pioneering work with crown ethers, cryptands, and spherands. Since then, the hallmark of supramolecular science has been an increasing sophistication in the design and construction of macrocyclic molecules, as manifested in cyclodextrin derivatives, calixarenes, resorcinarenes, cyclotriveratrylenes, cucurbiturils, calixpyrroles, cyclophanes, and many other examples. Indeed, macrocyclic compounds provide unique models for the study of noncovalent molecular interactions. They also constitute building blocks for constructing high-level molecular and supramolecular architectures and fabricating molecular devices and advanced materials. As a postgraduate in the Huang laboratory in the late 1980s, I became interested in the calix[n]arenes because of their unique conformational structures and versatile complexation properties. The notion of introducing heteroatoms, and particularly nitrogen, into the bridging position of conventional calixarenes was particularly intriguing. Nitrogen, unlike methylene, can adopt either an sp(2) or sp(3) electronic configuration, providing different conjugation systems with adjacent aromatic rings. Consequently, depending on the configuration and conjugation, a range of C-N bond lengths and C-N-C bond angles is possible. The conformation and cavity size in heteroatom-bridged calixarenes might thus be tuned through the bridging heteroatoms and the number of aromatic rings. Furthermore, because heteroatom linkages significantly affect the electron density and distribution on aromatic rings, the electronic features of macrocyclic cavities might be regulated by heteroatoms. Given the essentially limitless combinations possible, only synthetic hurdles would prevent access to numerous diverse heteracalixaromatics. We began a systematic study on nitrogen- and oxygen-bridged calixarenes in 2000, years later than originally envisioned. Before this study, very few heteracalixaromatics had been reported, owing to the formidable synthetic challenges involved. Apart from thiacalixarene, the synthesis of nitrogen- and oxygen-bridged calixarenes appeared very difficult. But since our first publications in 2004, we have been delighted to see the rapid and tremendous development of the supramolecular chemistry of this new generation of macrocycles. In this Account, I summarize the synthesis of N- and O-bridged calixaromatics and their regiospecific functionalization on the rims and bridging positions, focusing on the fragment coupling approach and contributions from our laboratory. I describe the construction of molecular cages based on heteracalixaromatics and discuss the effect of both bridging heteroatoms and substituents on macrocyclic conformations and cavity sizes. Molecular recognition of neutral organic molecules and charged guest species is also demonstrated. The easy accessibility, rich molecular diversity, unique conformation, and cavity tunability of heteracalixaromatics make them invaluable macrocycles for research in supramolecular chemistry. New heteracalixaromatics, with well-defined conformations and cavity properties, will provide powerful tools for probing noncovalent interactions, leading to the development of new molecular sensing and imaging systems. Multicomponent molecular self-assembly of heteracalixaromatics as functional modules with metals, metal clusters, or charge-neutral species should result in multidimensional solid and soft materials with diverse applications. The profitable incorporation of heteracalixaromatics into molecular devices can also be anticipated in the future. Moreover, the construction of enantiopure, inherently chiral heteracalixaromatics should provide important applications in chiral recognition and asymmetric catalysis.     

Integration or segregation: how do molecules behave at oil/water interfaces?

It has been over 250 years since Benjamin Franklin, fascinated with the wave-stilling effect of oil on water, performed his famous oil-drop experiments; nevertheless, the behavior of water molecules adjacent to hydrophobic surfaces continues to fascinate today. In the 18th century, the calming of the seas seemed the most pertinent application of such knowledge; today, we understand that oil-on-water phenomena underlie a range of important chemical, physical, and biological processes, including micelle and membrane formation, protein folding, chemical separation, oil extraction, nanoparticle formation, and interfacial polymerization. Beyond classical experiments of the oil-water interface, recent interest has focused on deriving a molecular-level picture of this interface or, more generally, of water molecules positioned next to any hydrophobic surface. This Account summarizes more than a decade's work from our laboratories aimed at understanding the nature of the hydrogen bonding occurring between water and a series of organic liquids in contact. Although the common perception is that water molecules and oil molecules positioned at the interface between the immiscible liquids want nothing to do with one another, we have found that weak interactions between these hydrophilic and hydrophobic molecules lead to interesting interfacial behavior, including highly oriented water molecules and layering of the organic medium that extends several molecular layers deep into the bulk organic liquid. For some organic liquids, penetration of oriented water into the organic layer is also apparent, facilitated by molecular interactions established at the molecularly thin region of first contact between the two liquids. The studies involve a combined experimental and computational approach. The primary experimental tool that we have used is vibrational sum frequency spectroscopy (VSFS), a powerful surface-specific vibrational spectroscopic method for measuring the molecular structures of aqueous surfaces. We have compared the results of these spectroscopic studies with our calculated VSF spectra derived from population densities and orientational distributions determined through molecular dynamics (MD) simulations. This combination of experiment and theory provides a powerful opportunity to advance our understanding of molecular processes at aqueous interfaces while also allowing us to test the validity of various molecular models commonly used to describe molecular structure and interactions at such interfaces.     

电子电镀添加剂的分子设计

提出了电镀添加剂设计的概念,给出了锡基电子电镀添加剂设计的基本方法。由于电镀添加剂作用的复杂性,要得到性能优异的添加剂,必须从分子水平上对添加剂进行设计,而添加剂在镀层中的夹杂是通过化学夹杂和物理夹杂引起,从分子水平上设计的低吸附型镀锡添加剂可以减少有机分子的夹杂。     

A bio-inspired design strategy: Organization of tryptophan-appended naphthalenediimide into well-defined architectures induced by molecular interactions

The chemistry of molecular assemblies involves weak yet complex non-covalent interactions, and the molecular organization of the π-conjugated material is crucial in determining the performance of an organic electronic device. Herein we demonstrate a bioinspired design strategy to tune the self-assembly of naphthalenediimides (NDIs) by minute structural variations, π-π stacking, hydrophobic interactions and metal interactions. We address some of the limitations associated with current design strategies, such as restriction to a specific molecular interaction or the difficulty in controlling the assembly due to several complicated intermolecular interactions. Hydrophobic-effect-induced J-type aggregation and sodium-interaction-induced H-type aggregation of tryptophan-appended NDIs have been illustrated. (1)H NMR spectra further reveal sodium cation-π interactions in tryptophan-appended NDIs, while NMR and IR spectroscopic studies confirm the structural variations associated with the molecular assembly. In summary, the molecular organization has been successfully transformed from nanospheres to particles, nanobelts, fibers and fractals. Such drastic changes in the morphology are clear and striking evidence of the importance of non-trivial weak non-covalent forces.     

On the relationship between corrosion inhibiting effect and molecular structure

In order to relate the corrosion inhibition effect of organic compounds for steel in acid media to their molecular characteristics, SCF calculations have been performed for thiourea and nine derivatives, for two further sulphur-containing organic compounds and for urea and two derivatives. The structure of all compounds has been optimised, ionisation potential, dipole moments and the electron density distribution has been computed. It is seen that the dominant molecular parameter for the inhibition effect is the ionisation potential and that there is a clear relation between the increase in corrosion inhibition and the decrease of the ionisation potential.