Hydrogen-bonded Supramolecular Ï-Functional Materials - Chemistry

310 Chem. Mater. 2011, 23, 310–325 DOI:10.1021/cm101817h

Hydrogen-bonded Supramolecular π-Functional Materials† David Gonz alez-Rodrı´ guez*,‡ and Albertus P. H. J. Schenning*,§ ‡

Contribution from the Departamento de Quı´mica Org anica, Facultad de Ciencias, Universidad Aut onoma de Madrid, E-28049 Madrid, Spain, and §The Laboratory of Functional Organic Materials and Devices, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands Received June 29, 2010. Revised Manuscript Received September 24, 2010

Recent developments in the area of H-bonded supramolecular assemblies of π-conjugated systems, that is, oligomers and polymers, are described. The state-of-the-art summary of the recent developments in the design of discrete systems and functional materials is presented. 1. Introduction π-Conjugated systems have been extensively used in advanced applications such as sensors and in electronics.1 For these applications, π-functional materials are required that are able to form organized supramolecular assemblies of which the properties can be controlled as a function of the self-assembly process and the chemical structure. Such a control is important for the improved performance of existing materials and to create new materials with tunable optical and electronic properties. Furthermore, there is a clear interest for organic materials that combine the ease of processing of π-conjugated polymers with the structural definition and monodispersity of small synthetic molecules. The rational strategy in that direction is to exploit the self-assembly of small functional molecules into supramolecular polymers in solution or in the solid state.2 With these views, considerable efforts are being focused toward the modification of the structure of π-conjugated systems to program the self-organization. These studies have generated a wealth of knowledge to the design of a variety of π-functional materials with intriguing properties. Hydrogen bonds (H-bonds) are ideal noncovalent interactions to construct supramolecular architectures since they are highly selective and directional.3,4 H-bonds are formed when a donor (D) with an available acidic hydrogen atom is interacting with an acceptor (A) carrying available nonbonding electron lone pairs. The strength depends mainly on the solvent and number and sequence of the H-bond donors and acceptors.5 In order to construct a significant amount of desired H-bonded assemblies, high association constants are required. In many cases, however, relatively weak hydrogen bond interactions are used so that additional supramolecular interactions are required to obtain nanosized assemblies. In addition, a detailed knowledge of the assembly mechanism is necessary. Two mechanisms for supramolecular polymerization have been envisaged, that is, isodesmic and cooperative self-assembly.6 In isodesmic Accepted as part of the “Special Issue on π-Functional Materials”. *Corresponding authors. E-mail: [email protected]; [email protected]. †

pubs.acs.org/cm

assembly, there is a gradual increase in the number and length of the aggregated species, and it is only at high concentrations or for high association constants that long, nanometer-sized objects are formed. In the case of cooperative self-assembly, there is a bimodal distribution of monomers and elongated objects. This review will focus on the most significant developments in the last ca. five years in the field of H-bonded supramolecular π-functional materials. Most of the supramolecular assemblies of organic functional materials are constituted by polymeric arrays of π-conjugated, typically aromatic, molecules associated in one or more dimensions. In many of these nanostructures, H-bonds have been regularly used to increase the molecule-molecule binding strength and provide enhanced stability to the ensembles and/or to position different semiconducting molecules in a certain arrangement within the nanostructures. In order to divide this review, we have taken into account the purpose of the H-bonding arrays and the type and dimensionality of the assemblies obtained. We will discuss the construction of discrete nanosystems and functional polymeric materials.7 Particular emphasis will be given to the specific properties and the potential use of these assemblies for sensing, imaging, or optoelectronic applications. 2. H-bonded Polymeric π-Functional Systems 2.1. π-Functional Systems H-Bonded along the Stacking Polymer Axis. The self-assembly of organic semiconductors into wire-like monodimensional structures, in which the molecules strongly interact through π-π stacking along the wire axis, is considered an appealing and promising strategy to facilitate electron and hole transport in organic electronic devices.8 This is in fact a very frequent mode of aggregation of planar, π-conjugated dyes in poor solvents leading to the formation of long, polymeric fibers.9 The exact way in which organic semiconducting materials self-assemble into nano- or microscopic materials is sometimes difficult to predict since aromatic interactions that promote stacking do not have a very specific directionality. The additional use of H-bonding interactions can be

Published on Web 10/22/2010

r 2010 American Chemical Society

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Figure 1. (a) Model of a stack of π-conjugated molecules stabilized by H-bonding (urea) interactions along the polymer axis. (b) Structure of triphenylene trimer 1. (c) Self-sorted stacks formed by OT 2 and PBI 3 with entanglement regions that act as heterojunctions. (d) Structure of PBIs 4a and 4b and absorption changes shown upon increasing the methylcyclohexane/chloroform solvent ratio. (e) Structure of Pps 5a and 5b, whose aggregates can be stabilized by covalent (photo)polymerization. AFM images of the decalin gel of 5a before (top) and after (bottom) UV irradiation and CHCl3 rinsing. Adapted from: (d) ref 17g with permission from Wiley-VCH and (e) ref 15b with the permission of the American Chemical Society.

interesting if they cooperate with π-π stacking and solvophobic interactions and form polymeric arrays along the fiber axis (Figure 1a). In this way, the kinetic and thermodynamic stability of the wires will be reinforced significantly and the position of the π-conjugated cores within the fibers can be fixed. That is the case of some H-bonding functions, classically amides or ureas, when properly attached to the dye cores. Since the first article by Schoonbeek et al. in 1999, where mono- and bithiophene bearing pendant alkylureas were organized into fibers that displayed improved charge transport features,10 the self-assembling properties of several organic semiconductors functionalized with amide or urea groups have been studied. Recent examples include aromatic amines like phenothiazine or carbazol,11 electronically rich entities like tetrathiofulvalene (TTF),12 discotic molecules such as hexabenzocoronene (HBC),13 phthalocyanine (Pcs)14 or porphyrin (Pp),15 fused aromatics like pyrene,16 perylene bisimide (PBI),17 triphenylenes,18 or azatriphenylenes,19 and conjugated oligomers like oligophenylenevinylene (OPV),20 oligofluorene (OF)21,20c or oligothiophene (OT),22,20c among others. The principal aim is to increase the stability of the stacks and to enhance the degree of order of the stacking molecules within a single column, since it has been demonstrated that the charge-carrier mobility and hence the performance of optoelectronic devices, such as field-effect transistors (FETs), light-emitting diodes (LEDs), and photovoltaic cells, depends to a large extent

on this parameter.8 In addition, it has been demonstrated that, due to their dipolar nature, amide H-bonding motifs can assist in the alignment of the supramolecular fibers when an electric field is applied.23 A representative example is the self-assembly of three discotic triphenylene molecules connected to a central 1,3,5 aromatic trisamide unit (1; Figure 1b).18 The presence of the latter H-bonding unit guides triphenylene stacking into long wires in which the adjacent discs strongly interact through their π-surfaces with a very short twist angle. This organization led to a charge carrier mobility that is about five times higher compared to materials based on individual triphenylene molecules. The same aromatic trisamide building block was employed to organize Pp15e or OPV20a trimers into columnar stacks that exhibit a strong tendency to aggregate in solution, giving rise to long and stable fibers. Remarkably, the topology of the amide bond determined the stability and helicity of the fibers in solution and the length of the fibrils at a surface. Electroactive materials like TTF that exhibit high electron conductivities in crystalline structures8b are also relevant candidates for the construction of ordered conducting wires through cooperative π-π stacking and H-bonding interactions. With this idea in mind, several groups have prepared amide-12a-d,f or urea-appended12e TTFs and studied their fibrilar assemblies. Remarkably, when these wires are subjected to annealing and/or doping processes, mixed-valence states are formed creating materials that display absorbance

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in the near-infrared12a and a considerable increase in the conductivity.12c Very recently, the incorporation of amide H-bonding functions at the peripheral chains of Pcs has been profited to direct their columnar aggregation perpendicular to a variety of conducting substrates like highly oriented pyrolitic graphite (HOPG) or gold surfaces modified with alkanethiols having amide H-bonding functions.14 AFM and FT-IR data indicated a layer-by-layer growth of Pc films from dilute solutions and confirmed that the Pc planes lie parallel to the solid support. Conductive-tip AFM measurements served to evaluate the conductance of these ordered films, which decreased with the layer height. Systems containing p- and n-type nanofibers with entanglement regions that act as heterojunctions have been proposed as a promising architecture for photocurrent generation. Sugiyasu et al. reported on the formation of long fibers from OT and PBI molecules (2 and 3 in Figure 1c) functionalized with chiral cholesterol moieties having four or two H-bonding sites, respectively.24 Spectroscopic techniques suggest that the two compounds form separate stacks in the mixed system due to a selfsorting process induced by the different number and position of H-bonding sites at each dye. The fluorescence of a cast film of the self-sorting material on an ITO electrode shows significantly quenched fluorescence, indicating photoinduced electron transfer from the OT to the PBI fibers. Furthermore, upon irradiation of the film, a photocurrent can be generated, demonstrating an efficient contact between the two fibers despite the self-sorted organization. Nevertheless, the authors state that the current results are likely not optimal as a consequence of the faster aggregation of the OT chromophore, which has a higher number of H-bonding units, thereby limiting the number of p-n heterojunctions. Very recently, Sakai et al. introduced the supramolecular zipper approach to prepare coaxial p-n heterojunctions. The authors studied a material composed of rigid p-oligophenyl (OP) or p-oligophenylethynyl (OPE) rods and pending naphthalenediimide (NDI) molecules bearing either positively or negatively charged ionic groups.25 The ability of NDIs to form interdigitated stacks via π-π stacking, H-bonding between amide moieties, and ionic interactions allows organizing the building blocks in vertically aligned structures with respect to a gold substrate. The controlled layer-by-layer incorporation of multiple NDI derivatives of different electronic character led to the absorption of light at various wavelengths and, remarkably, to the creation of a redox gradient. Using this approach, the authors managed to structure functional thin films at the molecular level so that the redox gradient along both n- and p-type materials mediates the antiparallel directional flow of electrons and holes to their respective electrodes. A rather general consequence of introducing secondary intermolecular H-bonding interactions along the stacks is the formation and/or stabilization of organogels in a variety of solvents, especially nonpolar ones.

Gonz alez-Rodrı´guez and Schenning

Low molecular weight gelators26 have attracted considerable interest during the last years as supramolecular materials with applications as templates for mineralization27 and ion-selective membranes28 or for thermo-and mechanoresponsive sensors.29 The incorporation of dyes into these materials can afford organogels with appealing luminescence, sensing and switching properties.30 Regarding fluorescence properties, both the intensity and the wavelength of the emission intensity may vary dramatically between the gel and solution states, depending on the polarity of the gelated solvent11c or the stacking geometry of the molecules. The latter effect has been shown by the group of W€ urthner in a series PBI functional gels in which the chromophores can stack in either parallel (H-type)17b,c,g or tilted (J-type)17d,e,g conformation as a function of the nature of the peripheral H-bonding substituents (Figure 1d). For example, the stacking of PBI 4a, substituted with long alkyl chains, led to blueshifted absorption bands, whereas PBI 4b exhibited redshifted absorption and emission features in the aggregate, as a consequence of the tilted packing imposed by the bulkier side tails.17g A different, more versatile strategy to vary the emission color of dye-based organogels is by proper election of a combination of fluorophores that are able to mix in columnar gel phases. Very recently, Abbel et al. reported on light emitting gels constituted by oligofluorene derivatives in which the central aromatic unit was changed (fluorene, naphthalene, quinoxaline, benzothiadiazole, or thienopyrazine), giving rise to different emission wavelengths.21 All of them were equipped with long alkyl side chains and amide groups at both ends, which promoted the formation of gels of modulable emission colors. Interestingly, by mixing the appropriate oligomers in the correct ratio, white light emission from the gel state was observed. In some cases, when equipped with the appropriate functions, the resulting columnar nanostructures can be covalently fixed once assembled. The formation of directional H-bonds between neighboring stacked molecules may leave (photo)polymerizable groups at the correct distances and angles for postpolymerization under specific conditions. That is the case of diacetylene-substituted hexaazatrinaphthylene19b or porphyrin (5a; Figure 1e)15b molecules, which can be photo-cross-linked in the aggregate state under irradiation with UV light. The resulting fibers display a high stability and can be rinsed with solvents to selectively remove unreacted monomers. Covalent immobilization of H-bonded wires has also been performed via in situ sol-gel polycondensation of dyes with covalently linked, peripheral triethoxysilyl groups, as in compound 5b (Figure 1e).15c,d,16 The organic/inorganic hybrid superstructures formed in this way enjoy very high thermal and mechanical stabilities, which are a requisite for material applications. 2.2. π-Functional Systems H-Bonded Perpendicular to the Stacking Polymer Axis. The use of H-bonding in the previous examples, aside from reinforcing stack stability and directing the internal structure, do not fully profit from the extraordinary attributes of this selective,

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Figure 2. (a) Model of a stack of π-conjugated molecules where the components are H-bonded in the direction perpendicular to the polymer axis. (b) Structure of OPV oligomers 6 and models of the H-bonded homodimer and the helical fibers. (c) Structure of the trimer formed by triple H-bond interactions between OPV 7 and PBI 8. Below, a tapping mode AFM image obtained from the 7-8-7 complex upon spin-coating from methylcyclohexane on a glass slide. (d) Structure of melamine derivatives 9a (substituted with two azobenzene groups) and 9b (substituted with two OPEs). Both compounds, having two H-bonding sites available, form stable rosettes in the presence of 1 equiv of cyanurate derivatives like 10, which further aggregate via π-π stacking. In the case of compound 9b, the aggregates formed have a closed toroidal structure, as shown below in the AFM phase image of a sample spincoated from decane solution on HOPG. Adapted from: (c) ref 37a with permission from the American Chemical Society and (d) ref 43 with the permission of Wiley-VCH.

directional, and strength-tunable noncovalent interaction. Some organic fragments display an array of H-bonding donor and acceptor functions that can be designed to modulate the strength and specificity to recognize complementary H-bonding patterns. In this way, the homo- or heteroassociation of two or more π-functional molecules in solution or in the dry state can be achieved by formation of one or more H-bonds. The binding strength increases with the number of H-bonds formed, as long as they display the required complementarity (Figure 2a).4,5 Using this approach, there have been numerous examples during the last years of, for instance, electron-donor and acceptor molecules that bind in solution into homo- or heterodimers, trimers, and, in general, linear or cyclic oligomers (quartets, rosettes; vide infra).31 We will only deal in this section with those examples of π-conjugated molecules equipped with H-bonding motives that form discrete H-bonded systems which subsequently polymerize via π-π stacking into fibrilar materials. The final outcome is a hierarchically organized material where H-bonding serves to select and define the monomeric building block, comprised of one or several different π-conjugated molecules, which then polymerize in the perpendicular direction with respect to the H-bond interactions.

Within this context, p-phenylenevinylene oligomer 6 (Figure 2b), functionalized at one terminus with an ureidotriazine moiety, constitutes a highly versatile model system whose self-assembly has been thoroughly studied within the group of Meijer and Schenning et al.32 In first instance, homodimerization takes place by formation of four complementary H-bonds between the ureidotriazines. H-bonding assists here in providing dimers with a larger π-conjugated surface that, in a second step, polymerize by π-π stacking into helical fibers of about 100 nm long. It was demonstrated that the stability of the stacks increases with the size of the π-conjugated homodimers.32c When chiral side chains are attached to the OPVs, either covalently or via H-bonding,32h helical stacks are formed with a preferred handedness. These systems have served as models for multiple studies conducted in order to weigh the possibilities of using ordered individual fibers in nanosized optoelectronic devices. For instance, these onedimensional OPV stacks provide a useful system for studying and modeling the mechanism of excitonic migration along π-π stacked molecules.33 The fast exciton dynamics observed, as demonstrated by femtosecond-transient absorption measurements,33a-c are reminiscent of those found for thin films of poly(phenylenevinylene), which underlines the

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expediency of the supramolecular approach toward organic materials. Moreover, when short oligomers are mixed with a small amount of long oligomers (i.e., having a lower HOMO-LUMO bandgap), excitation of the short OPVs results in a fast intrastack energy transfer process to the longer oligomers.33d-f An additional important conclusion of these studies was that the degree of ordering within the stacks is a highly determining factor in the dynamics of exciton transfer; ordered helical stacks show much faster photoexcitation mobility processes.33g-j Charge transport along the columnar stacks has also been modeled and studied in solution by time-resolved microwave conductivity measurements.34 However, the mobility values obtained, 3 10-3 and 9 10-3 cm2/(V s) for holes and electrons, respectively, are not very high, and this was ascribed to the relatively large twist angle between neighboring chiral molecules. The fibers of OPV 6 can be transferred to solid supports or deposited over electrodes, in order to assess their conductive properties, without significant dissociation or restructuring. AFM studies showed that the transfer of the OPV fibers from solution to a solid substrate is possible provided the right concentration and suitable substrates are employed.32c The electrical conduction of the fibers deposited on Au-Pd electrodes is, however, quite poor, which was attributed either to the presence of defects within the fibers or to an intrinsically poor conductance of the π-π stacks, but not to the contacts between the wire and the electrons.35 It is interesting to note that related OPVs substituted at one end with a diaminotriazine unit self-assemble into hexameric rosettes by formation of pairs of H-bonds (see Figure 7 below).36 This is a clear example of how a small chemical transformation of the H-bonding motif leads to a change in the self-recognition mode which, at the same time, is translated in a complete readjustment of the internal chromophore arrangement within the nanostructure. The planar cyclic hexamers display a large driving force for π-π stacking into tubular chiral objects, as demonstrated in solution by UV-vis, fluorescence, and CD spectroscopy. On the other hand, AFM and small angle neutron scattering (SANS) studies indicated the formation of fibers with a diameter which matches that of the stacked rosettes. H-bonding between self-complementary motifs thus constitutes a valuable strategy to preorganize a discrete number of well-defined semiconducting oligomers into larger planar π-conjugated structures that then stack into polymeric cylinders. Within this line, an additional step in complexity would be the heteroassociation of π-conjugated molecules with different electronic properties (i.e., electron-donor and acceptor molecules, dyes absorbing at different wavelengths, etc.; Figure 2a) to construct functional materials. A well-known complementary motif for this purpose is the heteroassociation of imides and diaminotriazines by triple H-bonding, which has been widely utilized in the formation of supramolecular chromophore systems.37,38 The groups of W€ urthner, Schenning, and Meijer studied the coassembly of OPV-diaminotriazine (7)


and PBI (8) in order to form p-n-p type junctions that, upon stacking, could serve as efficient wires for the antiparallel transport of both electrons and holes (Figure 2c).37 Two OPVs H-bond to both termini of the perylene dye and the resulting triad forms helical columnar stacks of about 7 nm in diameter, that then coil into larger, chiral rod superstructures, as determined by AFM studies (see Figure 2c). Time-resolved transient absorption measurements confirmed the occurrence of a fast and efficient electron transfer process from the electron donor (OPV) to the acceptor (PBI) within the stacks. Absorption and CD studies suggest that this charge-transfer process occurs via an intermolecular pathway in J-type stacks of the hydrogen-bonded OPV-PBI arrays.37c This and other39 studies carried out with the electron donor OPV and electron acceptor PBI couple shed light on the importance of the chromophore supramolecular organization in thin films for application in efficient organic field-effect transistors or photovoltaic devices. For instance, ambipolar field-effect transistors could be constructed from a dyad complex showing two independent pathways for charge transport.39b Other H-bonding self-complementary residues have been widely used for the formation of multichromophoric π-π stacked assemblies. On the basis of the same triple H-bonding association pattern between diaminopyridine and imide motifs, the classical interaction between melamine and barbiturate/cyanurate derivatives can yield multiple supramolecular structures around which multiple dyes can be assembled. Depending on the number of available triple H-bonding sites in each molecule, the melamine-barbiturate/cyanurate association can lead to dimers (one site each), trimers (two sites in one of them and one site in the other), tetramers (three sites in one of them and one site in the other), cyclic hexamers or rosettes (two sites in each residue), or 2D networks40 (all three sites available for both molecules).31 In recent years, the group of Yagai, among others, has investigated the versatility of this complementary heteroassociation and demonstrated how changes in the structure of the selfassembling monomers can promote the formation of diverse supramolecular architectures.41-44 For instance, a melamine derivative substituted with two azobenzene groups (9a; Figure 2d) was shown to form stable rosettes in the presence of cyanurate derivatives like 10, but only when the azo-dyes presented an E stereochemistry.42a The planar rosettes formed by combination of melamine 9a and cyanurate 10 can then polymerize to form discotic columns that induce the gelation of apolar solvents like cyclohexane. Irradiation of the gel with UV light promotes the E f Z photoisomerization of the azobenzene groups which results in more sterically demanding species that tend to dissociate.42b In contrast, a related melamine compound substituted with OPE oligomers (9b; Figure 2d) led, outstandingly, to the formation of toroidal objects of nanometer dimensions in the presence of cyanurate 10.43 The authors proposed that the cause for this unusual organization was the ∼45° tilting of the OPE segments with respect to the H-bonded rosette plane, which produces a curvature

Review

in the aggregate that eventually leads to a closed ring superstructure comprised of about 160 monomeric rosettes. Hoeben et al. also employed this melamine-cyanurate couple to construct trimers constituted by two OPV-diaminotriazine with only one H-bonding site available and one porphyrin-cyanurate conjugate with two H-bonding sites.45 These ensembles polymerized by π-π stacking to yield helically ordered wires that, interestingly, exhibited efficient energy transfer from the OPV units to the porphyrin dye. 2.3. Main-Chain H-Bonded π-Functional Polymers. The previous sections dealt with supramolecular π-functional materials in which H-bonding was combined with π-π stacking interactions to yield wire-like polymers in which H-bonds are formed either along the stacking axis or in the perpendicular direction. This section covers the most recent advances made toward supramolecular π-conjugated polymers in which H-bonding interactions are formed along the polymer chain and are the main or only noncovalent force that guides monomer association (Figure 3a).6 This approach, when compared with the most common covalent π-conjugated polymers, holds great promise in terms of producing π-functional materials with a high degree of modularity. In fact, it permits the fine-tuning of the electronic properties, dynamic features, morphology, and mechanical and processing characteristics of the polymers formed by proper choice of the nature of the π-conjugated monomers and the H-bonding units. This kind of supramolecular polymers constitutes therefore an illustration of how to combine the synthetic simplicity and well-defined structure of small molecules with the processability of π-conjugated polymers, at the same time avoiding phase segregation problems. N-unsubstituted PBIs already constitute an example of main-chain H-bonded π-conjugated polymers. Polymerization is driven by formation of pairs of H-bonds between the antiparallel imide functions while, in order to limit stacking and improve the processability of the material, bulky solubilizing groups are often grafted at the bay positions.46 Kaiser et al. observed that this type of PBI compounds polymerized into J-type aggregates which, remarkably, can exhibit fluorescence quantum yields near unity46a,c or, when substituted with amine groups at the bay positions, absorption maxima in the near-infrared (i.e., up to 900 nm).46b Moreover, octachloroPBI molecules have been shown to self-assemble in H-bonded n-type stacks in the crystalline state that can operate under air atmosphere with high electron mobilities.46e The aggregation mechanism of these N-unsubstituted PBI dyes have been studied recently in more detail.46d On the other hand, the supramolecular versatility of the imide functions of PBIs for the construction of H-bonded supramolecular polymers is noteworthy. In combination with melamine derivatives with two available H-bonding sites, the structural and physical properties of the materials can be modulated47a,b and other chromophores or functional groups can be introduced within the polymer backbone.47c The widely used melamine-barbituric acid triple H-bonded recognition motif has been studied by Huang et al. for the construction of all-organic photovoltaic devices.48 Electron-donor OTs symmetrically functionalized


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Figure 3. (a) Model of a polymer comprised of π-conjugated molecules where the monomers are associated via H-bonding interactions along the polymer main chain. (b) Structure and association mode of OT-bismelamine 11 and fullerene C60 barbiturate derivative 12. (c) Structure of OF 13, OPV 14, and PBI 15 functionalized at the terminal positions by quadruple H-bonding Upy groups. Below on the left a titration experiment is shown (blue: pure 13, green: successive addition of 14, red: further addition of 15). The solid arrows indicate spectral changes upon addition of 14 to 13 and the dotted arrow upon addition of 15 to a mixture of 13 and 14. The inset shows the PL spectrum corresponding to a solution containing 13:14:15 in a ratio of 59:33:8. On the right, the solutions of pure di-UPy chromophores 13, 14, 15, and a white emitting mixture in chloroform under UV irradiation is displayed. Adapted from: (c) ref 55d with permission from the American Chemical Society.

at both ends with melamine functions (11) were codeposited with an electron-acceptor fullerene-barbiturate compound (12) (Figure 3b). The mixture yielded homogeneous films in which the two components form alternate supramolecular polymers, thus minimizing phase segregation.48a Photovoltaic devices made of these films led to a 2.5-fold enhancement in light energy to electrical energy conversion. Furthermore, these self-recognizing molecules were organized into molecular-level heterojunctions on gold supports by means of melamine- or barbiturate-grafted alkanethiol self-assembled monolayers.48b The same melaminebarbituric acid H-bonding motif has also been used by the

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groups of W€ urthner and Yagai to generate diverse supramolecular architectures, such as nanoribbons and nanoropes, based on merocyanine,49 azobenzene,49c OT,50 or PBI51 π-conjugated molecules. Other H-bonding motifs are also good candidates for the generation of main-chain self-assembled polymers. The organization of perylene dyes, appended in this case at both ends with thymine or uracil moieties combined with melamine, has been studied in the solid state.52 Crystal X-ray diffraction results confirmed the formation of long chains constituted by H-bonded arrays that alternate the melamine and the PBI-thymine/uracil chromophores. Barbiturate derivatives can form different modes of two-dimensional H-bonded arrays, which have been employed recently to organize OPV chromophores into nanosystems that show a reversible transformation between rings and coils.53 On the other hand, 4,6-diaminopyrimidin-2(1H)-one is a very interesting supramolecular motif that combines the ADD H-bonding pattern of guanine on one side and the DAA H-bonding pattern of cytosine on the other. The formation of photoresponsive azobenzene supramolecular tapes using this appealing residue has been studied.54 H-bonded supramolecular polymers are also a promising alternative to π-conjugated covalent polymers as far as light-emitting properties are concerned. The group of Schenning and Meijer has studied the fluorescence emission and energy transfer characteristics of H-bonded polymers of oligofluorenes.55 For such a purpose, these fluorophores were functionalized at both ends with a 2-ureido-4[1H]-pyrimidinone (Upy) residue,5,56 which is able to strongly dimerize via four H-bonds with an association constant of 6 10-7 in CHCl3. The resulting bis-UPy-terminated oligomers self-assemble into supramolecular polymers57 that can be end-capped by a variety of functional chain-stoppers (i.e., dyes with a single UPy group).55a It was demonstrated that these end-capping fluorophores, namely, OPV and PBI dyes, can accept energy from the excited OF main chain both in solution and in the solid state, and that H-bonding actually enhances the efficiency of the energy transfer process toward the polymer ends. Femtosecond-resolved photoluminescence spectroscopy was employed to study in detail this excitation energy transfer process, which was compared with Monte Carlo simulation models.55c Since the dynamics of energy migration was found to be dependent on the chain length, the output of these simulations served as a powerful means to determine the molecular weight distribution in these end-capped supramolecular polymers.55b The results fit a Flory distribution nicely, which is based on the assumption of equal reactivity of all functional groups. Moreover, the use of three different fluorophores (blueemitting OF (13), green-emitting OPV (14), and redemitting PBI (15)), all of them end-capped with UPy groups at both termini (Figure 3c), allowed the preparation of supramolecular polymers that cover a wide region of the UV-vis absorption spectrum and that, using appropriate mixing ratios, display white-light luminescence in solution (Figure 3d).55d In thin films, these supramolecular


materials showed no phase separation, and the electroluminescence color could be modulated as a function of the dye molecule: blue, green, red, or, combining the three molecules, white emission. This strategy presents utmost interest in view of its high tunability and possibilities: choosing the right fluorophores, at the right composition, a wide range of emission wavelengths can be obtained in light-emitting diodes. The group of Ajayaghosh has recently reported a similar approach toward white-light-emitting organogels.58 2.4. Two-Dimensional π-Functional H-Bonded Networks. The study of the electronic properties and the self-assembly of organic semiconductors arranged in two dimensions on solid substrates is an intriguing field with potential applications in the field of fabrication of molecular nanoelectronic devices.59 Scanning tunneling microscopy (STM) is the method of choice to characterize the molecular packing on solid supports and study dynamic processes such as monolayer formation, domain boundary evolution,60 and molecular movements.61 While molecule-substrate interactions (adsorption) normally rule monolayer formation and stability and define domain orientation, molecule-molecule interactions are also a main parameter, since the adsorbed molecules usually have sufficient mobility to pack in the most thermodynamically favorable arrangement. In the specific case of self-assembly at the solid-liquid interface, the interactions of the solvent with the substrate and molecules must also be taken into account.62 In short, noncovalent interactions acting between molecules in the parallel direction to the substrate can guide the nanostructural features of the final two-dimensional assemblies. Among these, H-bonding unquestionably plays a very significant role. The incorporation of H-bonding motifs in π-conjugated molecules constitutes a successful strategy to drive the self-assembly of one or multiple organic semiconductors onto a surface, since, being a highly directional force, the geometry of the assemblies and the positioning of the dyes can be frequently predicted by chemical intuition. Typical H-bonding motifs employed in two-dimensional self-assembly are the dimerization of carboxylic acids or amides,60,63 the association between hydroxyl functions,64 the interaction between amines or pyridines and carboxylic acid derivatives, or the association between 2,6-diaminopyridine and imide functions,65 among others. In such a way, choosing the proper H-bonding motifs and molecular geometries, one can, in principle, rationally design the formation of monolayers comprised either of discrete linear or cyclic ensembles, or of polymeric linear arrays or 2D networks on a variety of solid substrates. The former case will be discussed in Section 3.3. In this section, we will only deal with the formation of H-bonded polymeric assemblies limited to two dimensions, where no control is reached over the size of the assemblies or the number of molecules involved. 1D H-bonded polymeric assemblies are typically observed by STM when: (i) the rigid π-conjugated molecules are furnished with two H-bonding entities disposed in a linear, rigid arrangement or (ii) when the H-bonding unit is able to promote growth into ribbon or tape structures. An example of the first case is the self-assembly of naphthalene or perylene bisimides on a variety of substrates.66,46d

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Figure 4. (a) Structure of the OPV-peptide conjugate 16 and its STM images at the 1-octanoic acid-HOPG interface. (b) STM images showing domains of the monolayers formed from mixtures of perylene dicarboxylic anhydride and 4,400 -diamino-p-terphenyl (17), 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (18), or 1,2,4,5-tetraaminobenzene (19). (c) STM image and model of a monolayer of dicarboxylic acid-substituted PBI 20 where C60 has been codeposited within the voids left by the H-bonded hexagonal arrays. Adapted from: (a) ref 20b and (b) refs 73 and 74 with permission from the American Chemical Society and (c) ref 80 with the permission of the Royal Chemical Society.

Most commonly, row-like structures develop as a consequence of the formation of H-bonds between amide pairs at both ends of the molecule. Depending on the balance between intermolecular and molecule-substrate interactions or, in other words, the ability of the molecules to diffuse freely on the surface, further 1D or 2D growth of these linear assemblies is observed. That is as well the case for linear π-conjugated oligomers, such as stilbene,67 azobenzene,67 or oligothiophene,68 substituted at both termini with carboxylic acid functions, which can dimerize leading to 1D polymers on the surface. The second case is general of many π-conjugated units to which a H-bonding residue that promotes the formation of linear ribbons is grafted. For instance, bithiophenes have been self-assembled into ribbons at the liquid-HOPG surface by formation of linear arrays of H-bonds between appended urea units.69 Interestingly, the bithiophene molecules do not lie flat on the surface but are slightly tilted giving rise to π-π stacking interactions along the ribbon direction. More recently, the groups of Spada and Samori observed the organization of OT-guanosine molecules into ribbon B 1D structures on HOPG where the H-bonding pattern dictates the packing of the dye segments (see Figure 5).70 Some peptide sequences that lead to β-sheets, the natural motif for ribbon organization, have been employed for the self-assembly of OPV-peptide hybrid amphiphiles, like 16 (Figure 4a).20b STM studies at the 1-octanoic acidHOPG interface showed that these π-conjugated oligomers form H-bonded bilayers in which the molecules are arranged in an antiparallel β-sheet conformation. Very often,

the arrangement of molecules having chiral groups leads to the expression of chirality in the ribbons formed within the monolayer. The origin of this chiral arrangement in porphyrin rows that are stabilized by H-bonding between amide groups has been studied at the graphite-heptanol interface by Linares et al.71 Different is the situation when one or several of the selfassembling dyes are preprogrammed with two or more H-bonding units in a nonlinear arrangement and/or when the H-bonding motif itself forces binding in a nonlinear fashion. In those cases, 2D H-bonded networks can be obtained in which the nanostructure of the monolayer is defined by the angle imposed by the substitution of the rigid molecules or the H-bonding motif. This case is nicely represented by the organization of 1,3,5-tricarboxylic (trimesic) acid aromatic derivatives, in which the dimerization of the acid functions (forming an angle of 60°) leads to hexagonal networks on surfaces60 or the self-assembly of perylene bisimides through their A-D-A H-bonding array, which can be recognized by, for instance, D-A-D melamine derivatives. The geometry imposed by this H-bonding association is the basis for the formation of monolayers with hexagonal patterns.72 Other patterned 2D networks are also possible. Treier et al. studied the mixed self-assembly of perylene dicarboxylic anhydride and aromatic di- or triamines (17-19), in which each amine binds two bisanhydride molecules via the carbonyl groups (Figure 4b).73 The authors managed to characterize a collection of monolayers whose nanostructure can be varied as a function of the geometry and the number of amino groups of the aniline

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employed (i.e., 4,400 -diamino-p-terphenyl (17) or 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (18)). In contrast, the selfassembly of mixtures of perylene dicarboxylic anhydride with 1,2,4,5-tetraaminobenzene (19) gives rise to alternating rows that are stabilized by H-bonding.74 Interestingly, the size-modulable voids within these networks have been employed to encapsulate functional π-conjugated molecules such as C60,75 Pc,76 heterocirculenes,77 or coronene78 derivatives. This strategy, that is, the use of H-bonding molecules that can form 2D networks on surfaces leaving empty spaces which can then host another shape-complementary functional entity, has become widely utilized in recent years. The self-assembled network acts therefore as a template to which other molecules can be precisely positioned. For instance, 1,3,5-tris(10-carboxydecyloxy)benzene, a tris-carboxylic acid derivative having flexible spacers, has been used to pattern HOPG substrates in order to guide the deposition of fullerene, Pc, or HBC derivatives.79 Very recently, the group of Champness demonstrated the inclusion of C60 molecules within the triangular empty space left by H-bonded arrays of six perylene bisimides (20) onto Ag-Si(111) surfaces (Figure 4c).80 Furthermore, the hexagonal voids formed by the H-bonded perylene bisimide-melamine 1:1 mixture have been employed by Madueno et al. to build self-assembled monolayers of alkanethiols.81 This work represents a nice example of creation of ordered patterns with nanometer precision combining different concepts and methods of supramolecular chemistry. 3. H-Bonded Discrete π-Functional Systems Most of the supramolecular assemblies of π-functional materials are constituted by polymeric arrays of molecules associated in one or more dimensions, with limited control over the size or uniformity of the nanoobjects formed. However, one of the principal aims within the field of π-functional nanomaterials, as in many others basic scientific areas, is to correlate function with structure, which can only be achieved if an exquisite control over the assembly size and internal architecture is reached. The concept of rationally controlling the number of self-assembled molecules, as well as the size, monodispersity, and internal nanostructure of the supramolecular objects formed, is an appealing and challenging one that deserves scientific attention in the near the future.82 Within this section we highlight the most recent and relevant studies in which π-conjugated functional molecules have been rationally organized into discrete, uniform nanoobjects via, among other noncovalent interactions, H-bonding. By discrete, we mean any self-assembled architecture whose size has been limited in one or more dimensions. We therefore include here nanoparticles comprised of a few π-conjugated molecules (0D), nanofibers in which the degree of polymerization is controlled by means of, for instance, templates of a precise length (1D), and monolayers in which one or several organic semiconducting molecules have been organized into closed, well-defined objects on a given surface (2D).


3.1. H-Bonded Discrete Nanoparticles. The supramolecular synthesis of discrete, monodisperse particles comprised of a few π-conjugated chromophores arranged in a well-defined geometry is a very interesting way to modulate their optical properties, since these are susceptible to alterations as a function of the local environment around the dye. This approach is mastered by nature. The performance of photosynthetic centers and a wide variety of fruit and flower colors are originated from natural chromophores that interact in different ways within well-defined discrete noncovalent complexes.83 One way of preparing such assemblies is to employ rigid molecular units equipped with directional supramolecular motifs (i.e., H-bonding84 or metal-ligand interactions85) that are restrained at specific distances and angles, so that oligomerization toward closed, finite objects is favored over polymerization.86 Other intermolecular interactions that lead to supramolecular polymerizations in, at least, one dimension, such as π-π stacking or van der Waals interactions, are restricted within the well-defined assembly. Focusing on H-bonding interactions, the work carried out in the group of Reindhoudt on melamine-barbituric acid assemblies represents a magnificent example.84 The mixture of 3 equiv of calix[4]arene molecules (21) appended with two melamine groups each and 6 equiv of merocyanine dye (22) results in the exclusive formation of an assembly comprised of two coplanar chiral rosettes (Figure 5a). The whole structure is greatly stabilized via the cooperative action of 36 H-bonds formed between the melamine and the barbiturate residues.87 Remarkably, the defined geometrical arrangement of the incorporated chromophores within these nonameric assemblies, resembling that of the blue-colored commelinin architecture present in natural flowers, produced a hypsochromic shift of 14 nm in the dye absorption maximum and a chirooptical response. Recent studies by different research groups have disclosed the singular characteristics of guanine (G) selfassembly88 to organize different dye molecules,89 such as Pps,89a pyrenes,89b OTs,70 OPVs,89c and paramagnetic radical species like 4-carbonyl-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO).90 In the absence of any salt added, G derivatives typically self-assemble into linear oligomeric ribbons or cyclic tetrameric quartets, depending on the substitution of the G unit (Figure 5b).70,89c,91 In contrast, in the presence of alkaline salts in organic solvents, multiple noncovalent forces (i.e., H-bonding, π-π stacking, cation-dipole interactions, and cation-anion Coulombic attraction forces) work in concert to provide discrete92-94 and fairly stable94,95 G-quadruplexes. The G-quadruplex architecture thus constitutes a very interesting scaffold to which a small number of π-functional molecules can be covalently attached and stacked in a well-defined arrangement. For instance, the formation of octameric nanoparticles of OPV-G chromophores in the presence of potassium salts was studied recently by Gonzalez-Rodrı´ guez et al. (Figure 5b).89c The diskshaped particles obtained in nonpolar solvents, of about 1.8 nm high and 10 nm wide, exhibited an extraordinary

Review

Figure 5. (a) Schematic representation and molecular structure of bismelamine-calix[4]arene 21, merocyanine dye 22, and their nine-component H-bonded assembly (21)3-(22)6. (b) Schematic representation of the equilibrium between G-ribbons, G-quartets, and, in the presence of alkaline salts, G-octamers of guanosine-OT 23 or guanosine-OPV 24. In the case of 23, G-ribbons are observed by STM onto HOPG surfaces. For 24, empty G-quartets are instead formed, as shown in the STM image. In the presence of potassium salts, both compounds form octameric nanoparticles that have been visualized for 24 by AFM on mica or graphite substrates. Adapted from: (a) ref 84 with permission from the National Academy of Sciences and (b) ref 70 with the permission of WileyVCH and ref 89c with permission from the American Chemical Society.

stability toward concentration or temperature changes and maintained their structural integrity when deposited on graphitic or mica substrates.87 In solution, the octameric complexes displayed negative Cotton effects, bathochromic shifts, and, interestingly, an enhancement of the fluorescence emission intensity. These features are also


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characteristic of larger organic nanoparticles (>25 nm), typically produced by reprecipitation methods,96 that show size-dependent emission quantum yields. The G-quadruplex-mediated self-assembly hence constitutes an efficient and, in principle, general strategy to organize chromophores into small monodisperse nanoparticles whose structure persists in different conditions, that is, in diluted solutions, dried on solid substrates, and so forth, without dissociation or further polymerization. 3.2. H-Bonded Templated Nanofibers. The search for a control over the length and the relative positioning97 of π-conjugated molecules in fibrilar architectures, in order to have monodisperse, well-defined objects whose properties relate to their finite size and internal sequence of chromophores, is a highly interesting area of research. This is a challenging task that cannot rely on π-π stacking or solvophobic interactions as the sole supramolecular forces but that also requires directional (i.e., H-bonding) interactions to guide the self-assembly of the π-conjugated dyes. Several approaches toward this goal have already been pursued, among which the use of well-defined templates is probably the most successful. In 2007, Sugimoto et al. reported on the templated selfassembly of porphyrin dyes (25) into chiral double stranded helices (Figure 6a).98 The template molecules, poly(dialkylammonium) strands of about 130 monomeric units (26),98b were able to recognize by double H-bonding the 2,6-bis(2oxazolyl)pyridine ligands connected to the porphyrin macrocycle at opposite sides. AFM studies confirmed the formation of linear helical assemblies with fairly reduced polydispersity whose height (ca. 2.3 nm) and average length (ca. 86 nm) was consistent with the templated structure expected. In terms of synthetic versatility and simplicity, DNA or PNA oligomers enjoy several attributes that make them the ideal candidates as templates for linear helical assemblies: (i) they are commercially available in a defined length and sequence of nucleobases, (ii) they can be made soluble in water (DNA) or in organic solvents (PNA), (iii) the nucleotides can be synthetically modified with π-conjugated molecules and incorporated to the tailored oligomeric strands using solid-phase automated methodologies, (iv) the array of nucleobases in each strand can specifically recognize molecules having complementary H-bonding patterns and bind them in a chiral, π-π stacked arrangement, and (v) DNA can be used to create predefined complex one-, two- and three-dimensional nanosized structures via sticky-end cohesion. The generation of covalently functionalized nucleic acids, in which the H-bonded duplex DNA is used as a supramolecular scaffold to arrange multiple chromophores, is a research topic of increasing interest.99 The general idea is to prepare dye-modified nucleotide analogues and incorporate them into longer oligomeric sequences so that, once the double helix is formed upon addition of the complementary sequence, the dyes interact with each other in a controllable and predetermined fashion. Azobenzene,100 pyrene,101 phenanthrene,102 PBI,103 OT,104 or Pp105 dyes have been employed for this purpose. Instead of the natural nucleosides, acyclic linkers can be

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Figure 6. (a) Schematic illustration of the construction of an artificial double-helix comprised of Pp 25 that H-bond to the low-polydisperse dialkylammonium polymer 26. (b) The alternating sequence of free-base Pp-dU (27) and Zn(II) Pp-dU (28) conjugates and dA in both complementary strands of DNA places the porphyrin units adjacent to each other in the DNA hybrid, as shown in the CPK model, and promotes energy transfer processes. (c) Schematic representation of the ssDNA-templated self-assembly and the molecular structures of the host oligothymidine template and the π-conjugated diaminotriazine guests 29 and 30. Adapted from: (a) ref 98b with permission from the American Chemical Society and (b) ref 99 with the permission of Wiley-VCH.

integrated between the phosphodiester bridges for the incorporation of a small number hydrophobic chromophores within the duplex structure, thereby interacting as natural base pairs do. For instance, Baumstark et al. managed to stack up to six PBI units inside the duplex, which resulted in a strong red-shifted excimer-type emission.103 If the dye fragments are grafted to the ribose or the nucleobase residue of natural nucleotides, the dye organization typically occurs around the duplex periphery. The group of Stulz has studied the self-assembly of Pps attaching them to the 5-position of 20 -deoxyuridine using an acetylene linker (see Figure 6b).105 By means of this approach, up to 11 tetraphenyl-substituted Pp units were consecutively incorporated into oligonucleotides.105a Duplex stabilization was improved by alternating sequences of Pp deoxyuridine (dU) and deoxyadenosine (dA) in both complementary DNA strands. In this way, the authors could prepare a DNA duplex composed of alternating zinc(II) (27) and free-base (28) Pps (Figure 6b), in which the fluorescence of the former is significantly quenched by the metal-free Pp due to resonance energy transfer.105c The organization of chromophores by incorporation of modified nucleotides in H-bonded duplex DNA structures is, however, a synthetically elaborate process that does not profit from all the supramolecular information and template capability of these natural polymers. It would be more versatile, but also more challenging, to

make use of the H-bonding arrays of the nucleobases to supramolecularly bind different chromophores. If these chromophores are functionalized with the complementary base, one can ideally template a fiber nanostructure comprised of π-π stacked molecules whose size and relative molecular positioning is determined by the length and sequence of the oligonucleotide strand. This approach was first successfully demonstrated by Iwaura et al. using OPV molecules that are substituted at both ends with thymidine bases and coassembled in the presence of the complementary oligoadenylic acid 20-mer in aqueous solutions.106 UV-vis and CD spectroscopic measurements, together with AFM observations, revealed the formation of helically stacked structures with widths corresponding to the binding of the oligoadenine template to one or both sites of the bis(thymidine)-substituted OPV. The length and polydispersity of the fibers was higher than that corresponding to the oligonucleotide strand, indicating that the template effect was not as successful as desired. More recently, a modified strategy was employed which relies on the H-bonding interaction between oligonucleotide single strands and π-conjugated molecules substituted with only one complementary H-bonding motif.107 The templated self-assembly process was studied by means of a wide collection of techniques, such as NMR, electrospray mass spectrometry,107d absorption, emission or CD spectroscopy, light-scattering, molecular

Review

dynamics, and TEM and AFM microscopy. Concretely, DNA- or PNA-based oligothymidines of different lengths were employed to guide the assembly of dyes in water or in organic solvents such as methylcyclohexane,107f respectively (Figure 6b). The guest π-conjugated molecules, either naphthalene (29) or OPV (30) chromophores, were equipped with a diaminotriazine or a diaminopurine moiety, both having the required D-A-D H-bonding pattern, complementary to the A-D-A pattern of the thymine base. Titration and variable temperature CD and absorption experiments gave a valuable insight into the mechanism of this templated supramolecular polymerization and the guest-guest and host-guest interaction parameters obtained provided evidence for a full coverage of DNA templates, which demonstrates the success of this templated approach to regulate stack size.107b Interestingly, some of the DNA host-guest complexes showed a dramatic sensitivity toward pH changes. It was found that decreasing the pH produced the protonation of the guest diaminopurinenaphthalene molecules, which resulted in an inversion of the helicity of the assembly and an increased binding strength to the oligothymidine template.107e If the H-bonded monomers are equipped with polymerizable groups, this DNA-templated strategy can also be applied to generate polymers with a very low degree of polydispersity and a length that matches that of the DNA template. This idea has very recently been explored by Lo et al.108 by the templated synthesis of nucleobase-grafted polyphenylene-ethynylene (PPE).109 The adenine- or thymine-appended monomers were subjected to a polymerization reaction: (i) in the absence of template, (ii) in the presence of H-bond-matching thymine- or adeninecontaining polymer (respectively) with an average degree of polymerization of 20, and (iii) in the presence of a nonmatching template that included H-bonding motifs which are not selective for the nucleobases in the monomer. The polymers thus obtained were analyzed by GPC in order to ascertain their polydispersity and average degree of polymerization. Only the polymerization reactions that were carried out in the presence of the correct template exhibited a low polydispersity and a degree of polymerization that is comparable to the template length. 3.3. π-Functional H-Bonded Discrete Systems on Surfaces. The sought control of size of H-bonded assemblies and the relative positioning of a small number of π-conjugated molecules can be more easily achieved providing supramolecular growth is only restricted to two dimensions. In this case, by choosing the appropriate H-bonding motifs and molecular geometry it is possible to limit the self-assembly to the formation of monolayers comprised of discrete dimers,110 trimers,111 or larger cyclic ensembles, such as quartets89c or rosettes.112 The hydrogen-bonding motifs and the association modes are in many cases the same as mentioned in Section 2.2, although the formation of tubular π-π stacked superstructures is here hampered by the strong moleculesubstrate interactions. Further 2D growth of the monolayers is typically observed by the action of secondary intermolecular forces like, for instance, the interdigitation of alkyl chains.62 Nonetheless, we are considering these systems


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Figure 7. (a) Structure of OPV-diaminotriazine compounds 31a-c. Both 31a and 31b self-assemble into chiral rosettes (the arrow shows the “rotation direction”) at the 1-phenyloctane-HOPG interface. An STM image and a magnification are shown for each compound. (b) Molecular structure of the rosette heterocomplex formed by H-bonding association between NBI 32 and bis(OPV)-melamine 33. Below, an STM image of the assembly formed at the 1-phenyloctane-HOPG interface is shown. Adapted from: (a) ref 36 with permission from the American Chemical Society and (b) ref 113b with the permission of the Royal Chemical Society.

as “discrete” and “uniform” since the same ensemble of H-bonded molecules is repeated throughout the different domains or islands observed within the monolayer. One of the most studied systems included within this group, mostly with regards to the expression of molecular chirality at surfaces, is the self-assembly of OPV-diaminotriazine 31 (Figure 7a).36,112 At the

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1-phenyloctane/graphite interface all molecules (31a, 31b, and 31c) self-organize into chiral hexameric rosettes as a result of the formation of pairs of H-bonds between aminotriazine fragments. Enantiomerically pure molecules 31a and 31b lead to the exclusive formation of one of the possible 2D mirror-type arrangements (i.e., clockwise or counterclockwise rotation) which, surprisingly, is different for 31a than for the longer OPV 31b. No rosettes of opposite chirality were found in those samples. On the contrary, the 2D crystals of achiral molecules 31c appear in an equal amount of mirror-image type domains in the absence of any discriminating influence. However, recent results have shown that the 2D crystallization of 31c in the presence of a chiral solvent can produce enantiomerically enriched and even homochiral monolayers of discrete hexameric assemblies.112b The formation of monolayers comprised of discrete assemblies obtained by heteroassociation of a mixture of π-conjugated chromophores is also possible providing the molecules are equipped with a proper H-bondingmotifs. Trimers formed between OPV 7 and PBI 8,39c as well as rosette-like assemblies formed by triple H-bonding between N-unsubstituted PBIs (8)113a or NBI (32)113b and melamine-OPV compounds having two available binding sites (33) have been visualized at the HOPG-solvent interface, as demonstrated by STM and scanning tunneling spectroscopy (STS) studies (Figure 7b). Interestingly, it was found that bias-dependent STM imaging is able to discern between the deposited molecules of different electronic nature. At high negative sample bias (electrons tunneling from the substrate to the tip), the electron rich OPV units appear brighter due to a more efficient tunneling process, whereas changing the bias voltage from negative to positive produces a brighter imaging of the electron poor PBI/NBI molecules.114 4. Conclusions and Outlook H-bonded self-assembly offers an attractive tool to construct supramolecular π-functional materials. These materials combine the well-defined electronic properties of the monodisperse oligomer and the processability of a polymer. H-bond interactions have been used to increase the molecule-molecule binding strength and to position different π-conjugated molecules in a certain arrangement within the nanostructures. Such a modular approach allowed simple mixing of different components instead of labor-intensive synthesis of new materials. The first optoelectronic devices based on these materials have been constructed demonstrating the proof of principle. However, there are still a number of very appealing targets. The lack of control to exactly position π-conjugated systems with nanometer precision and the difficulty in controlling the dimensions of supramolecular nanostructures will be a challenge. In this respect, the use of single stranded DNA as a template represents a promising field for obtaining monodisperse branched stacks in which the position of π-conjugated components is directed by the template via complementary H-bonding.


DNA templated H-bonded self-assembly, in combination with DNA structural nanotechnology, offers a precise positioning of functionalities in 1D, 2D, and 3D assemblies. In order to improve the performance of optoelectronic devices based on supramolecular materials most likely additional methods such as alignment layers, magnetic, or electric fields are required to align the nanometer-sized domains into highly order mesoscopic domains. The preparation of such monodomains will clearly increase the macroscopic properties of these materials. This review article shows that despite these challenges, hydrogen bonded π-conjugated assemblies with tailormade properties are appealing materials for future plastic and nanosized optoelectronic devices. Acknowledgment. The authors would like to acknowledge the many discussions and contributions with all of our former and current colleagues. Their names are given in the references cited. A special word of thanks is expressed to Bert Meijer for many inspiring discussions and collaborations. The research Eindhoven University of Technology has been supported by the Royal Netherlands Academy of Science (KNAW), The Netherlands Organization for Scientific Research (NWO) and the European Young Investigators Awards (EURYI). D.G.-R. would also like to acknowledge a Marie Curie Reintegration Grant (230964).

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