Artificial π-conjugated aromatic systems are promising candidates to serve as active materials in a number of advanced optoelectronic and electronic applications e.g. sensors, field effect transistors, photovoltaic devices or logical gates.50,99,165,176–178
In this bulk implementations not only the properties of the isolated π-conjugated systems but of the whole ensemble is crucial, which makes it important to precisely orient and position the chromophores to each other.179,180 Supramolecular chromophoric building blocks are perfectly suited to meet this task, yielding new materials with tunable optical and electronic properties.181,182 Furthermore, they combine the advantages of controlled structural definition and monodispersity with the ease of processing known from polymers.183 In this context the self-assembly is driven either exclusively by π-stacking184,185 of the chromophores or in combination with second intermolecular interactions.186
The group of Klaus Müllen exploits the strong π-stacking of hexabenzocoronenes as discotic aromatic systems to obtain self-assembled helical columnar structures.187,188 In the group of Takuzo Aida the π-stacking of hexabenzocoronenes was further supported by weak and non-directional hydrophilic-hydrophobic interactions. As a result, this amphiphilic system self-assembles into defined nanotubes with a homogenous diameter of only a couple of nanometers and an aspect ratio of greater than 1000 (Figure 13). Other derivatives of this supramolecular building block show promising results as organic semiconductor189,190 and energy transport material191.
Figure 13. Self-assembly driven mainly by π-stacking. Self-assembly of an amphiphilic hexa-peri-hexabenzocoronene into double-walled nanotubes, mainly driven by strong π-stacking.
The lateral electron poor trinitrofluorenones enable spatial charge separation accompanied by a quick photoconductive response with large on/off ratio.190
Introduction
In many cases, the supramolecular assemblies of π-conjugated aromatic systems are supported by additional H-bonding moieties due to the favorable properties of this second intermolecular interaction i.e. high selectivity and directionality. Those supramolecular assemblies can be further distinguished, depending on whether the H-bonds point along or perpendicular to the π-stacking axis.70,111,192 In the first case H-bonds can help to stabilize the resulting supramolecular structure and precisely control the stacking position of the chromophoric moieties. This was illustrated by the group of Würthner using perylene bisimides. Depending on the fashion of the H-bonding substituents the chromophores form either H-type or J-type aggregates and consequently exhibit quite different optical properties.193 In other examples, the supramolecular motif of 1,3,5-benzenetrisamides, equipped with lateral triphenylenes, was used to obtain one-dimensional aggregates with improved charge carrier mobility.194,195 In a joint attempt of the groups of Würthner, Schenning and Meijer the strong recognition of imides and diaminotriazines was used to couple oligophenylenevinylenes as p-type moiety with perylenebisimides as n-type moiety.
The obtained p-n-p type junction was formed by triple H-bonding perpendicular to the π-stacking axis of the involved chromophores (Figure 14).196,197 This orthogonal self-assembly resulted in supramolecular nano-fibers envisioned for antiparallel transport of charges and holes.
Figure 14. Self-assembly driven by π-stacking in combination with H-bonding. Aggregation of an oligo(p-phenylenevinylene) derivative and a perylene bisimide into a p-n-p type junction.111
A different approach, which recently got increasing attention, employs “mixed stack charge transfer” assemblies to obtain supramolecular architectures with increased conducting properties due to inherent, uniform doping.67,198 This is achieved by alternating assembly of π-conjugated donor and acceptor compounds.199,200 In order to ensure an alternate stacking of donor and acceptor moieties many design strategies are pursued involving the use of H-bonds, supramolecular amphiphiles and bolaamphiphiles.201–204 The group of Subi J. George used a non-covalent, amphiphilic design employing a coronene salt in combination with a viologene derivative yielding cylindrical nano-fibers with good p-type conductor properties and record field effect mobility values (Figure 15).205
Figure 15. Self-assembly driven by donor acceptor π-stacking. Alternated self-assembly of electron rich coronene salts (CS, donor) with electron poor viologene derivatives (DMV, acceptor) into cylindrical micelles and nanofibers with remarkable field effect mobility.205 An ambitious goal is pursued by the group of Stefan Matile, who wants to achieve self-assembled organic solar cells with precise architecture and functionality.206,207 In this concept, a layer-by-layer assembly is used to generate solar cells with fine-tuned morphology exhibiting a broad absorption range, large n/p heterojunction interfaces in combination with continuous pathways for electrons and holes along a directing
redox-Introduction
gradient. These self-assembled heterojunction devices with “oriented multichromophoric antiparallel redox gradients” represent an ideal morphology for organic solar cells. To achieve this goal p-oligophenyls or p-oligophenylethynyls acting as electron-donor backbone were equipped with electron accepting naphthalenediimides of suitable HOMO and LUMO levels and band gaps. The naphthalimides were, furthermore, functionalized either with cationic or anionic side chains allowing for layer-by-layer zipper self-assembly as shown in figure 16. The complex design of these supramolecular building blocks makes use of π,π-stacking, intrastack hydrogen bonding and interstack ion pairing. Although currently the organic solar cell characteristics of these systems are not among the best performing devices, they achieved promising results and illustrate the power of intermolecular interactions to precisely control the architecture of supramolecular assemblies.
Figure 16. A: Molecular structures and suggested architecture of a oriented multichromophoric antiparallel redox gradients supramolecular n/p heterojunction of p-oligophenyl POP-N initiator (1) and p-p-oligophenyl POP-Y (2, 3) and p-p-oligophenylethynyl POE-R (4, 5) propagators on gold. B: HOMO and LUMO levels show photo-induced (dashed arrows) e- (gray) and h+ (black) injection into e- (n)- and h+ (p)-transportation pathways (bold).207
Introduction