Charl F. J. Faul

Ionic Self-Assembly for Functional Hierarchical Nanostructured Materials

CONSPECTUS: The challenge of constructing soft functional materials over multiple length scales can be addressed by a number of different routes based on the principles of self-assembly, with the judicious use of various noncovalent interactions providing the tools to control such self-assembly processes. It is within the context of this challenge that we have extensively explored the use of an important approach for materials construction over the past decade: exploiting electrostatic interactions in our ionic self-assembly (ISA) method. In this approach, cooperative assembly of carefully chosen charged surfactants and oppositely charged building blocks (or tectons) provides a facile noncovalent route for the rational design and production of functional nanostructured materials. Generally, our research efforts have developed with an initial focus on establishing rules for the construction of novel noncovalent liquid-crystalline (LC) materials. We found that the use of double-tailed surfactant species (especially branched double-tailed surfactants) led to the facile formation of thermotropic (and, in certain cases, lyotropic) phases, as demonstrated by extensive temperature-dependent X-ray and light microscopy investigations. From this core area of activity, research expanded to cover issues beyond simple construction of anisotropic materials, turning to the challenge of inclusion and exploitation of switchable functionality. The use of photoactive azobenzene-containing ISA materials afforded opportunities to exploit both photo-orientation and surface relief grating formation. The preparation of these anisotropic LC materials was of interest, as the aim was the facile production of disposable and low-cost optical components for display applications and data storage. However, the prohibitive cost of the photo-orientation processes hampered further exploitation of these materials. We also expanded our activities to explore ISA of biologically relevant tectons, specifically deoxyguanosine monophosphate. This approach proved, in combination with block copolymer (BCP) self-assembly, very fruitful for the construction of complex and hierarchical functional materials across multiple length scales. Molecular frustration and incommensurability, which played a major role in structure formation in combination with nucleotide assembly, have now become important tools to tune supramolecular structure formation. These concepts, that is, the use of BCP assembly and incommensurability, in combination with metal-containing polymeric materials, have provided access to novel supramolecular morphologies and, more importantly, design rules to prepare such constructs. These design rules are now also being applied to the assembly of electroactive oligo(aniline)-based materials for the preparation of highly ordered functional soft materials, and present an opportunity for materials development for applications in energy storage. In this Account, we therefore discuss investigations into (i) the inclusion and preparation of supramolecular photoactive and electroactive materials; (ii) the exploitation and control over multiple noncovalent interactions to fine-tune function, internal structure, and long-range order and (iii) exploration of construction over multiple length scales by combination of ISA with well-known BCP self-assembly. Combination of ISA with tuning of volume fractions, mutual compatibility, and molecular frustration now provides a versatile tool kit to construct complex and hierarchical functional materials in a facile noncovalent way. A direct challenge for future ISA activities would certainly be the construction of functional mesoscale objects. However, within a broader scientific context, the challenge would be to exploit this powerful assembly tool for application in areas of research with societal impact, for example, energy storage and generation. The hope is that this Account will provide a platform for such future research activities and opportunities.

Modulating helicity through amphiphilicity - Tuning supramolecular interactions for the controlled assembly of perylenes

Here we show that it is possible to modulate the supramolecular assembly of designed H-bonding amphiphilic perylene-based materials through simple solvent interactions. These modulated supramolecular interactions have been translated to and observed in macroscopic properties, and provide new pathways to the preparation of switchable interfaces based on designed supramolecular interactions.

Helical supramolecular aggregates, mesoscopic organisation and nanofibers of a perylenebisimide–chiral surfactant complex via ionic self-assembly

A chiral supramolecular liquid-crystalline material was produced by the complexation of a cationic perylene bisimide dye (PTCDI) with an anionic chiral phosphate surfactant (BDP) via ionic self-assembly. The aggregation behaviour of the PTCDI–BDP complex was investigated in detail in solution and in the bulk state. The UV-Vis and CD results show that helical supramolecular aggregates of the complex were formed with the enhancement of π–π stacking between perylene molecules in solution (due to low solubility in poor solvents) as well as in the bulk. Thin films of the complex exhibit fan-shaped textures in polarized optical microscopy (POM) investigations, which corresponds to a columnar mesophase. X-ray diffraction analyses confirmed this phase identification. Moreover, the complex formed a nanofibrillar structure, as imaged by TEM, when cast from dilute ethanol solutions.

Facile synthesis of optically functional, highly organized nanostructures: Dye-surfactant complexes

Multiply charged dye molecules can be precipitated from water by complexation with oppositely charged surfactants. It is shown that this complex formation occurs with 1:1 stoichiometry in a highly cooperative fashion. The resulting solids show either a gel-like or a supramolecular fibrillar morphology with a very high degree of order on the nanoscale, as evidenced by small-angle X-ray scattering and pleochroic behavior under plane-polarized light.

Redox‐Active, Organometallic Surface‐Relief Gratings from Azobenzene‐Containing Polyferrocenylsilane Block Copolymers

Organometallic gratings: the ionic self-assembly of metal-containing block-copolymer polyelectrolytes and azobenzene chromophores is exploited for the efficient production of stable photo-induced surface-relief gratings. We show that feature sizes can be tuned using simple redox chemistry, and that the chromophores can be removed during plasma treatment to yield ceramic-based optical materials.

Functional block-like structures from electroactive tetra(aniline) oligomers

Recent advances in the fast-growing area of block-like structures based on electroactive oligomers, with a particular focus on tetra(aniline)-based structures, are discussed in this review. We provide an overview of recent literature covering aspects of design of novel molecular architectures, synthetic strategies and theoretical investigations. Specifically, we discuss tetra(aniline)s (TANI), di- and tri-block architectures before providing details of recent computational studies. We highlight useful synthetic routes, advantages of utilising block-like structures, as well as opportunities for further exploration on both the synthetic and computational fronts.

Chiral Perylene Diimides: Building Blocks for Ionic Self‐Assembly

Abstract A chiral perylene diimide building block has been prepared based on an amine derivative of the amino acid l‐phenylalanine. Detailed studies were carried out into the self‐assembly behaviour of the material in solution and the solid state using UV/Vis, circular dichroism (CD) and fluorescence spectroscopy. For the charged building block BTPPP, the molecular chirality of the side chains is translated into the chiral supramolecular structure in the form of right‐handed helical aggregates in aqueous solution. Temperature‐dependent UV/Vis studies of BTPPP in aqueous solution showed that the self‐assembly behaviour of this dye can be well described by an isodesmic model in which aggregation occurs to generate short stacks in a reversible manner. Wide‐angle X‐ray diffraction studies (WXRD) revealed that this material self‐organises into aggregates with π–π stacking distances typical for π‐conjugated materials. TEM investigations revealed the formation of self‐assembled structures of low order and with no expression of chirality evident. Differential scanning calorimetry (DSC) and polarised optical microscopy (POM) were used to investigate the mesophase properties. Optical textures representative of columnar liquid–crystalline phases were observed for solvent‐annealed samples of BTPPP. The high solubility, tunable self‐assembly and chiral ordering of these materials demonstrate their potential as new molecular building blocks for use in the construction of chiro‐optical structures and devices.

Conjugated microporous polytriphenylamine networks

Conjugated microporous polytriphenylamine networks with surface areas of 530 m(2) g(-1) were synthesized via Buchwald-Hartwig coupling, resulting in high CO2 uptake (up to 6.5 wt%) and CO2-N2 selectivity (75) at 1 bar and 303 K.

Combination of ionic self-assembly and hydrogen bonding as a tool for the synthesis of liquid-crystalline materials and organogelators from a simple building blockElectronic supplementary information (ESI) available: IR, NMR, DSC and TGA data of the organic core and complexes. See http://www.rsc.org/suppdata/cc/b3/b303552b/

In this communication we report on the facile combination of hydrogen bonding and the ionic self-assembly (ISA) process to produce organized materials and fiber-containing organogel superstructures from functionalised oligoelectrolytic building blocks.

Delineating Poly(Aniline) Redox Chemistry by Using Tailored Oligo(Aryleneamine)s: Towards Oligo(Aniline)-Based Organic Semiconductors with Tunable Optoelectronic Properties

The simple and elegant Buchwald-Hartwig cross-coupling reaction has been used to synthesise a designed range of new aniline-based tetramers in one step, and without the need for protecting groups. Variation of the central aromatic ring has provided the opportunity to carefully tune the optoelectronic properties in this series, thus enabling a structure-activity relationship study by using a range of photophysical and electrochemical techniques. As a result, the long-proposed sequences of electron-electron (EE) and electron-chemical (EC) processes that support the complex redox and proton-transfer reactions involved in the well-known switching of redox states of poly- and oligo(aniline)s are revealed here for the first time. We also present the initial results from time-dependent DFT calculations to clarify the optoelectronic behaviour of these oligomers. The dc-conductivity measurements of conducting thin films of this series, doped with the prototypical poly(aniline) protonating agent D,L-camphor-10-sulfonic acid (CSA), externally plasticised with triphenyl phosphate (TPP), and processed from m-cresol (MC) solutions, are also presented.