DipoleâDipole Interaction Driven Self-Assembly of Merocyanine Dyes

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Dipole−Dipole Interaction Driven Self-Assembly of Merocyanine Dyes: From Dimers to Nanoscale Objects and Supramolecular Materials Frank Würthner* Universität Würzburg, Center for Nanosystems Chemistry & Institut für Organische Chemie, Am Hubland, 97074 Würzburg, Germany CONSPECTUS: π-Conjugation between heterocyclic donor (D) and acceptor (A) groups via a polymethine chain leads to dyes with dipole moments greater than 10 D. These dipole moments direct the selfassembly of the dyes into antiparallel dimer aggregates, even in dilute solution, with binding strengths that are far beyond those observed for other π-scaffolds whose self-assembly is driven primarily by dispersion forces. The combination of directionality and exceptional binding strength of dipolar interactions between D-π-A dyes indeed resembles the situation of the hydrogen bond. Thus, similar to the latter, dipolar interactions between merocyanine dyes, a unique class of D-π-A chromophores, can be utilized to construct sophisticated supramolecular architectures of predictable geometry, particularly in low polarity environments. For bis(merocyanine) dyes it has been demonstrated that the self-assembly pathway is encoded in the tether between the two constituent merocyanine chromophores. If the tether enables the antiparallel stacking of the two appended dyes, folding takes place, which may be followed by further self-assembly into extended H-aggregate π-stacks at higher concentrations in solvents of low polarity. For tethers that do not support folding, the formation of bimolecular complexes of four merocyanine units, cyclic oligomers, and supramolecular polymers has been observed. For the former case, that is, formation of a bimolecular stack of four merocyanine units from tweezer-type molecules, association constants >109 M−1 were measured in chloroform. On the other hand, because only one π-face is utilized in the formation of supramolecular polymers from bis(merocyanine) dyes, higher hierarchical structures typically originate in which the other π-face is surrounded by an antiparallel π-stacked neighbor molecule. Among the observed self-assembled structures, nanorods in particular have attracted considerable attention because their selfassembly into well-defined H-aggregates falls under kinetic control and is slowed tremendously with decreasing solvent polarity. Co-assembly of achiral and chiral merocyanine building blocks or two enantiomers of a chiral merocyanine in different ratios provided insight into “majority rules” and “sergeant-and-soldiers” effects as well as the autocatalytic fiber growth process. With regard to materials applications, it is important to note that the high propensity for dipolar aggregation was disadvantageous for many envisioned applications of these dyes in the area of nonlinear optics. However, this aggregation behavior proved to be advantageous for the recently demonstrated applications of D-π-A dyes, in particular, merocyanines as p-type organic semiconductors in organic electronics and photovoltaics. Thus, organic transistors with hole mobilities >0.5 cm2/(V s) and organic solar cells with power conversion efficiencies >6% could be achieved with merocyanine-based organic semiconductor molecules.

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by this concept.3 Furthermore, the tunable polarization of the π-scaffolds by donor and acceptor end groups proved to be the key to providing suitable electronic structures for second or third harmonic generation nonlinear optics,4 as well as for the design of photorefractive materials.5 Because many of these nonlinear optical applications rely on a noncentrosymmetric arrangement of the dyes, extensive investigations in the 1990s elaborated the alignment of polymer-dissolved dipolar dyes by strong electric fields (“poling”).6 Quite disappointingly, however, while dyes with large dipole moments appeared to be particularly promising for such electric field-induced orientation, in many cases the macroscopic nonlinear

INTRODUCTION Donor−acceptor substituted π-scaffolds constitute a special class of dyes whose optical and dipolar properties are strongly governed by the push−pull effect exerted by the +M and −M effects of the respective donor and acceptor end groups.1 As exemplified in Figure 1, the π-electron distribution in these dyes arises from the resonance between nonpolar polyene-type valence structures (left) and polar zwitterionic valence structures (right). While for pure polymethine chains (first example in Figure 1) the participation of the zwitterionic structure drops quickly with increasing chain length, quinonoid heterocyclic units support charge separation due to the gain of aromaticity in the zwitterionic ground state electron configuration.2 Accordingly, molecules with the largest known ground state dipole moments (μg higher than 20 D) become accessible © XXXX American Chemical Society

Received: January 23, 2016

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DOI: 10.1021/acs.accounts.6b00042 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

merocyanine dimer aggregate demonstrating its nonpolar centrosymmetric character (lacking orientation in electric fields) and pronounced binding strength (existence even in dilute solutions).8 In this initial study, it became apparent that the observed supramolecular structure was indeed quite unique and should be highly versatile toward the design of various more sophisticated supramolecular architectures. The reason is given by the fact that the self-assembly of such dipolar dyes is not energetically dominated by dispersion interactions as is the case for aggregates of other π-conjugated molecules.9 Instead, electrostatic dipole−dipole interactions provide the largest contribution to the noncovalent interaction enthalpy leading to several unique features.10 First, much higher binding constants are obtained for dipolar merocyanines dyes than for other πscaffolds of comparable size.11 Second, while for other chromophores, whose π−π-stacking is governed by dispersion forces, isodesmic or cooperative self-assembly processes often lead to the growth of large (and often polydisperse) oligomeric aggregates, for merocyanines dimerization is strongly favored with regard to further growth into larger aggregates. For this reason, very defined aggregates of limited size become easily accessible. Third, and maybe most important, for very dipolar push−pull chromophores as given by merocyanine dyes, a

Figure 1. Examples for push−pull chromophores whose electronic structures change from top to bottom (arrow) from polyene-like structures via equilibrated cyanine-type structures (middle) to zwitterionic (bottom) structures.

susceptibilities did not meet expectations. The latter was in particular true for highly dipolar dyes and at high dye concentrations, wherein the disappointing macroscopic nonlinear susceptibilities were attributed to the formation of centrosymmetric dye aggregates.7 Despite being considered as a reason for reduced performance of electric-field poled electrooptical materials, there was no experimental evidence for the formation of dipolar dye aggregates in solution prior to our initial study on a

Figure 2. Series of merocyanine dyes 1−8 (a) whose solvent-dependent self-assembly into antiparallel dimer aggregates (b) has been studied by concentration-dependent UV/vis spectroscopy.10 (c) Concentration-dependent UV/vis spectra of dye 1d in 1,4-dioxane at room temperature from 1.0 × 10−6 to 3.0 × 10−5 M (solid lines) and calculated spectra for pure monomer and dimer species (dashed lines). The arrows indicate absorption changes with increasing concentration. (d) Correlation between experimentally determined dipole moments, μg, and Gibbs free association energies into antiparallel dimers ΔdimG° according to eq 1. B


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constant of 2.8 × 105 M−1 was obtained in 1,4-dioxane at room temperature for dye 1j, followed by derivatives 1a, 1c, and 1d bearing the least bulky hydrogen and n-alkyl substituents (KD = (1.1−1.4) × 105 M−1). In contrast, dyes 1h,i bearing bulky isopropyl substituents at position R4 and in addition sec-pentyl at position R1 in the case of 1i showed the smallest binding constants of 1900 M−1 (1h) and 900 M−1 (1i). A second important aspect of this study10 was provided by the structural evidence for the antiparallel alignment of the merocyanine dyes as suggested in Figure 2b. Thus, for dyes of the series 1a−j as well as some other dyes of the given series, single-crystal X-ray data and 2D NMR data are available. Further concentration-dependent permittivity studies supported the antiparallel arrangement leading to a vanishing of the dipole moments. The most general and least timeconsuming assessment of the structure of the dimer aggregate is, however, possible by UV/vis spectroscopy where a characteristic displacement of the absorption band of the monomer to the dimer aggregate is observed. For dye 1d, this hypsochromic shift is from λmax = 568 nm for the monomer to λmax = 492 nm for the dimer in 1,4-dioxane (Figure 2c). The shift to smaller wavelengths (blue shift) suggests the presence of face-to-face stacked dyes (H-aggregates) and the magnitude of this shift (as well as those for the other dyes) is in good compliance with a calculation based on Kasha’s exciton model.14 It is indeed striking that dyes with dipole moments of up to 20 D can be dissolved in low polarity solvents at all. This property is directly related to the vanishing dipole moment (μg ≈ 0 D) for centrosymmetrically stacked dimer molecules (Figure 2b).

limiting situation is reached that resembles the one given for hydrogen bonds.12,13 Thus, the strong electrostatic contribution leads to a unique directionality that makes dipolar dyes particularly suitable to produce supramolecular architectures not only of predictable size but also of predictable geometry. In this Account, our research on the self-assembly of merocyanine dyes during the past decade will be described.

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DIMER AGGREGATES OF MEROCYANINES To elucidate in more detail the relationship between the structural and dipolar properties of the dyes and their aggregation strength, a comprehensive series of merocyanine dyes was synthesized and investigated by a broad variety of experimental techniques (Figure 2). The majority of the investigated dyes were composed of two heterocyclic units connected by a mono- or dimethine bridge. Through tuning of the donor and acceptor strength by appropriate substituents and employing heterocycles with different aromatization energies, the series of dyes 1−8 covers the range from more polyene-like dyes such as dye 7 (μg = 12 D) via intermediate polar dyes such as dye 2a (μg = 14 D) up to more zwitterionic dyes such as dye 8 (μg = 19 D). For a given length of a donor−π−acceptor system, the experimentally determined dipole moments, μg, are indeed a good measure for the zwitterionic character of a merocyanine dye. Concentrationdependent UV/vis studies for the given series of dyes in 1,4dioxane as a solvent of intermediate polarity revealed monomer to dimer equilibria for all dyes 1−8 (for an example, see Figure 2c) with the exception of dyes 4a,b whose self-association is hampered by sterically demanding groups (see later). The binding constants for dimerization, KD, showed, however, an amazingly large variation from 75 M−1 (dye 2a) to 8 × 106 M−1 (dye 8) in 1,4-dioxane at room temperature. If these binding constants are transformed into respective Gibbs free binding energies, there is a good correlation to the square of the dipole moments (i.e., ΔGdim ≈ μg2) if we only consider those dyes that exhibit a linear D-π-A scaffold and that lack bulky substituents (solid squares in Figure 2d). Astonishingly, despite ignoring dispersion forces,9 these data points could be properly fitted by eq 1 taking into account only electrostatic interactions arising from the interaction of the dyes’ dipole moment with the reaction field of the solvent (Kirkwood−Onsager function, first term) and the electrostatic interaction between two interacting dipole moments (second term, for the definition of the physical quantities, see inset of Figure 2d) within a dielectric with permittivity εr.10 A variation of the solvent permittivity from εr = 2.24 (CCl4) to 10.4 (1,2-dichloroethane) for dye 1d provided further evidence for the suitability of eq 1 to describe the thermodynamic driving force for the self-assembly of dipolar merocyanine dyes in compliance with the model illustrated in Figure 2b, that is, treatment of solvent molecules as point dipoles. The deviations originating for some of the investigated dyes (open squares in Figure 2d) were rationalized by a bent chromophore (dyes 2a−d) or the presence of sterically bulky groups (4a,b and several dyes of series 1) that prohibit the otherwise typical close proximity of about 3.4 Å (π−π-distance R in inset of Figure 2d). Owing to the r3 term in eq 1, there is indeed a strong impact of distance on the interaction energy between the two antiparallel dipoles, which was in particular elucidated for a series of dyes 1a−j (substituents are defined in Figure 2a) bearing groups of varying bulkiness at five different locations of the π-scaffold. Here, the largest dimerization

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COMPARISON WITH HYDROGEN-BONDED COMPLEXES It is interesting to compare the dipolar aggregation of merocyanine dyes with self-association by other noncovalent forces with regard to binding strength and directionality. That metal−ligand coordinative bonds offer both high binding strength and high directionality was assuredly a factor in the impressive development of the study of metal−ligand supramolecular structures during the last two decades. These structures include the largest supramolecular nanoobjects both with regard to size as well as with regard to the number of constituent building blocks.15 Although of weaker binding strength, the high directionality of the hydrogen bond was of particular relevance to advancements in the field of hydrogenbonded supramolecular architectures toward most impressive three-dimensional capsules,16 supramolecular polymers,13,17 and other fascinating multicomponent architectures.12 In contrast, other intermolecular interactions such as those based on dispersion forces11 (which are of high relevance in the field of π-conjugated materials) or interactions between ions and coordinating ligands such as crown ethers are more challenging to utilize for the design of larger supramolecular architectures18 due to their lower degree of directionality. In this regard, it is important that the pronounced electrostatic contribution to the dipolar stacking of merocyanine dyes distinguishes their π−π-stacking from the less directional one of other π-conjugated scaffolds.11,19 Accordingly, for nonpolar (μg = 0 D) discoid molecules like triphenylenes and hexabenzocoronenes20 or quadrupolar dyes such as phthalocyanines and perylene bisimides21 (μg = 0 D), supramolecular arrangements with various translational or rotational offsets afford quite similar π−π-stacking energies. However, a distinct energy C


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Accounts of Chemical Research minimum is observed for the antiparallel cofacial dimeric πstacks of dipolar merocyanine dyes owing to electrostatic interactions. Because both hydrogen-bonding and dipole− dipole interactions among merocyanines exhibit a major electrostatic contribution to the intermolecular interaction energy (for hydrogen bonds22 as well as for dimer aggregates between zwitterionic dyes 19 about 50% of the interaction energy is attributed to electrostatic interactions), it is of interest to compare these two interactions in more detail. It is indeed a common perception that dipole−dipole interactions are weak and accordingly cannot organize molecules in solution but rather only in the crystal lattices of the solid state. This is corroborated by torsion balance experiments for the quantification of dipole−dipole interactions among dipolar C−F and CO groups.23 Accordingly, the attractive interactions between dipolar molecules at room temperature are ensemble averaged over different rotational orientations of the dipoles because the dipole−dipole interaction strength cannot afford spatially fixed dipolar molecules (Figure 3a, Keesom interaction). However, hydrogen

sufficiently large intermolecular interaction energies to afford the antiparallel orientation in solution. This is made possible by the much larger dipole moments of these dyes compared with their hydrogen-bonded counterparts. This issue is nicely exemplified by the μg2 terms in eqs 2 and 4 for hydrogen fluoride with μg2 = 3.5 D2 and dyes 1 with μg2 = 290 D2. The large μg2 term of dye 1 compensates for the larger distance for π-π-stacking (∼3.4 Å) compared with that of hydrogen bonding (∼1.8 Å). Therefore, in addition to hydrogen-bonded complexes, dipolar aggregation of D-π-A molecules also constitutes a situation where the position and orientation of the molecular building blocks in the supramolecular ensembles become highly predictable. In addition to the discussed structural aspects, the Gibbs binding energies are also quite comparable for hydrogenbonded complexes and complexes formed between dipolar dyes as illustrated in Figure 4. Thus, it is well-known that quite high

Figure 3. Energy considerations for the interactions between two dipoles (neglecting solvent effects) with the limiting situations for (a) small dipole moments/large molecular size leading to random orientations (Keesom interaction) and large dipole moments or small distances r leading to the formation of supramolecular complexes with (b) parallel or (c) antiparallel dipole−dipole arrangements. Examples for the parallel case are hydrogen-bonded complexes such as those between hydrogen fluoride and nitriles, while those for the antiparallel case are antiparallel stacked dipolar dyes where the electrostatic potential map for dye 1 (a simplified model) dimer is shown.

Figure 4. Examples for the Gibbs binding energies for hydrogenbonded complexes in chloroform and those for dipolar merocyanine dyes in 1,4-dioxane at room temperature and definition and application of the size-related parameter, −ΔG°/A.

concentrations are needed to assemble molecules by means of single hydrogen bonds. Accordingly, inspired by natural Watson−Crick base pairs such as thymine−adenine (two Hbonds) and cytosine−guanine (three H-bonds), the concept of multiple hydrogen bonds has been applied to increase the binding strength toward complexes of high stability even in dilute solution.13 However, with each additional hydrogen bond donor−acceptor pair, the molecules grow in size, and accordingly it appeared to us that a meaningful comparison of binding strength of these hydrogen-bonded molecules with merocyanine dimerization should include size considerations. A simple parameter in this regard is the ratio between the Gibbs binding energy and the required area size to bring the molecules in contact, that is, eq 5 (Figure 4). With the parameters given in Figure 4 for hydrogen-bonded and π−πstacked surfaces, it becomes apparent that similar surface sizerelated binding strengths σ = −ΔG°/A are given for the most dipolar merocyanine 8 and triple or quadruple hydrogenbonded base pairs. Hydrogen-bonded complexes based on more distant hydrogen bonds26 are clearly inferior in this regard. Thus, the still underexplored dipole−dipole interactiondriven π-stacking exhibits all prerequisites (i.e., binding strength and directionality) required to accomplish sophisticated

bonding and merocyanine dipolar aggregation exemplify the two major exceptions of preferential parallel (Figure 3b) and antiparallel (Figure 3c) organization. In the simple point dipole approximation (where geometrical considerations are neglected and a spherical molecular shape is assumed), the linear parallel geometry is energetically the most favored. This situation is given for hydrogen bonding and accordingly the X−H···Y (here X and Y denote electronegative elements like N, O, or F) geometry is typically collinear.24 The reason for this special situation is given by the negligible size of the hydrogen bond (small distance r in eq 2) combined with the high dipolarity of X−H (e.g., μg = 1.86 D for HF). Most molecules are, however, not spherical but anisotropic, which holds in particular true for dipolar D-π-A scaffolds as given in merocyanine dyes. Here the other energy minimum with antiparallel orientation of the two dyes constitutes the lower energy situation. For such π-stacks, distances between 3.2 and 3.5 Å become possible,25 leading to D


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Figure 5. Structures of bis(merocyanines) 9−16 and schematic illustration of their self-assembly into (a) supramolecular polymers, (b) bimolecular stacks of four merocyanine units, (c) trimolecular cycles, and (d) foldamers with subsequent folding-driven polymerization.

supramolecular architectures, which we will discuss in the following sections of this Account.

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LARGER SIZED MEROCYANINE AGGREGATES For both hydrogen-bonded complexes and dipolar merocyanines, binding strength and structural complexity can be increased by the multivalency principle.27 Because the individual binding unit of a merocyanine is already of large molecular size, formation of superstructures with nanodimensions is already accomplished for simple bis(merocyanine) scaffolds where two merocyanine dyes are covalently linked by a tether molecule. Figure 5 illustrates the structures reported so far for bis(merocyanines) bearing various spacer units. The high binding strength provided by merocyanine dye 1 is also expressed by the respective bis(merocyanine) dyes 9−13. Accordingly, tweezer-type compounds 12b,c and 13b,c selfassemble into their bimolecular complexes comprising four merocyanine units with three dipolar π−π-interactions even at submillimolar concentrations in chloroform (i.e., at typical NMR conditions, see Figure 6).28 For the most perfectly

Figure 6. Fraction of monomer (αM) calculated from UV/vis data and results of the nonlinear regression analysis based on the dimerization model for dyes 1a, 12b,c, and 13b,c and a trimerization model for dye 11 in CHCl3.

tailored bis(merocyanine) 13c, the self-association constant in chloroform is >109 M−1 and accordingly about 7 orders of magnitude larger than the one for the monomeric reference 1a in the same solvent (5.9 × 102 M−1). This binding constant compares with the highest known ones for hydrogen-bonded E


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Figure 7. (a) UV/vis spectra of 10−5 M solutions of dye 9a in solvents of different polarity at 293 K. (b) AFM height image of 9a after spin-coating from MCH solution on HOPG, scale bar 50 nm. (c) Structural models for the different levels of organization observed for bis(merocyanine) dye 9a and their dependence on the solvent polarity and dye concentration.30

complexes in the same solvent.13 Accordingly, even in the micromolar concentration regime utilized for UV/vis studies (Figure 6), only a small fraction of the bimolecular complexes of 13c dissociates. The pronounced stability of this bimolecular complex enabled detailed 2D ROESY NMR studies in CDCl3 for (13c)2, which corroborated the existence of a centrosymmetric bimolecular complex bearing two different kinds of merocyanine units located either on the outside or in the interior. The high binding strength provided by the antiparallel stacking of merocyanines 1 is also responsible for the selfassembly of calixarene-tethered bis(merocyanine) 11 into cyclic trimers, which likewise takes place at lower concentrations than observed for the monomeric reference 1a (Figure 6). The stronger bias for the self-assembled state and the sharper transition in the concentration-dependent UV/vis studies are both attributable to the excellent preorganization provided by this particular spacer unit with regard to trimerization (∼60° in the cone conformation).29 An effective molarity of 0.145 M has been calculated for this trimer macrocycle in chloroform, which means that only above this high concentration (which is not experimentally accessible for solubility reasons) the thermodynamic equilibrium would favor the transformation into supramolecular polymers. Supramolecular polymers are, however, formed whenever the preorganization of the two merocyanine dipoles does not favor a particular arrangement where quadruple π-stacking, cyclic πstacking or folding (see later) is enabled by the utilized tether unit. This is the case for bis(merocyanines) 9a,b, which constitute an interesting series of building blocks for the elucidation of supramolecular polymerization processes. Already in 2003, it was recognized that bis(merocyanine) 9a follows a remarkable concentration- and solvent-dependent self-assembly pathway (Figure 7).30 Thus, while at typical concentrations of UV/vis studies (∼10−5 M), dichloromethane and THF are sufficiently polar to solvate the monomeric dyes (Figure 7c, structure I), their self-assembly into oligomers (Figure 7c, structure II) becomes unavoidable at increasing concentration in THF or in less polar solvents such as

trichloroethylene (TCE) or chloroform (Figure 7a). If the capacity of the solvent to solvate dipolar molecules is further decreased (i.e., for tetrachloromethane or methylcyclohexane (MCH)), these oligomers self-assemble into highly defined nanorods for which the structure (III) shown in Figure 7c has been suggested based on AFM (Figure 7b), SEM, and X-ray diffraction studies.31 The highly defined and almost crystal-like packing of the merocyanine dyes with two closely π-stacked antiparallel neighbors is the reason for the substantial hypsochromic shift (i.e., H-aggregation) of the absorption band from ∼570 nm (monomers) to 447 nm, and the absorption band sharpening is due to exciton delocalization. The bundling of these H-aggregated nanorods (Figure 7b) at increasing concentration leads to pronounced viscosity increases and finally gelation of the solvent. Subsequent studies with chiral derivatives 9b revealed the expected impact of the chiral side chains on the helicity of the nanorod fibers and “majority rules” and “sergeant-and-soldiers” effects32 for mixtures of (R,R)- and (S,S)-9b and mixtures of one of the enantiomerically pure 9b derivatives with achiral 9a, respectively. In addition, under conditions of bad solvation, that is, at higher MCH contents (ratio MCH/THF > 70:30), a severe slow-down of the self-assembly kinetics was observed. Thus, the hierarchical growth from simple oligomer strands II into H-aggregate nanorods III took about 1 h. Even more remarkably, these initial nanorods transformed into more compact ones over the course of several days.33 Because the CD signals of these two structurally similar nanorods are almost mirror images, implying their inverse helicity, while their UV/ vis spectra suggest almost identical H-aggregates, this process may be called supramolecular stereomutation. Subsequent CD studies for mixtures of the (S,S)-9b and (R,R)-9b enantiomers and those of (S,S)-9b or (R,R)-9b with 9a provided details into the equilibration kinetics by reshuffling of monomers based on time-dependent majority rules34 and sergeant-and-soldier effects, respectively.35 Because these studies have been highlighted in a previous feature article,36 they will not be discussed in further detail here. When the structural (para instead of meta connectivity) and steric situation (fewer alkyl F


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yield and lifetime both increase upon formation of antiparallel dimer aggregates due to the concomitant rigidification of the πscaffolds.42 This was an astonishing result because these dye aggregates are clearly classified as H-aggregates. However, due to a slight rotational offset, the J-band is not fully forbidden and accordingly enables fluorescence if other relaxation pathways are sufficiently impeded. Because this is obviously the case for merocyanine dye aggregates tightly bound by dipolar interactions, it is not surprising that a significant number of fluorescent materials, including gels and liquid crystals, have been reported for this class of dyes.43 Another very favorable outcome of merocyanine dye aggregation is given in the area of organic electronics and photovoltaics. For these applications, dipolar disorder is a major obstacle that is unavoidable for randomly ordered molecules in amorphous solids. Accordingly, dipolar molecules and even dipolar substituents were vastly avoided in organic semiconductor research for decades. However, as shown in our recent work,44 there is such a driving force for the formation of dye aggregates of highly dipolar merocyanines with antiparallel orientation of the dipole moments of neighboring dyes that dipolar disorder is greatly reduced. As a consequence, dipolar merocyanines became quite useful materials for both organic transistors and solar cells.25,45 This is a direct consequence of their inherent self-assembly behavior into antiparallel dimer aggregates. For the very same reason, however, applications of merocyanine dyes in nonlinear optics remain problematic because the aggregation of these dipolar dyes is difficult to overcome even after attachment of bulky substituents in real world applications, that is, for polymer-based materials with high dye loading.46 This has been elucidated in a model system based on centrosymmetric dimer aggregates of dye 1d whose degree of aggregation could hardly be influenced by strong external fields.47 Therefore, even the largest possible electric field strengths for a field-induced poling into noncentrosymmetric materials are not sufficient to overcome the internal electric field of a dipolar dye created at a π−π-stacking distance of about 3.5 Å. The utilization of opposing intermolecular forces such as hydrogen bonds might be an approach to this problem but would involve elaborate synthetic work.48

substituents) is changed in bis(merocyanine) 10, no nanorods are formed. Instead, a nucleation−elongation mechanism drives the self-assembly under thermodynamic control into lamellar structures.37 While the bis(merocyanine) dyes 9−13 based on the highly dipolar merocyanine dye 1 were designed in a way that prohibited intramolecular folding of the two dipolar merocyanines into antiparallel π-stacking arrangements, such folding became possible for the majority of investigated bis(merocyanines) 14a−i based on the bent chromophore 2.38 The major outcome of this study was that the utilization of flexible linkers leads inevitably to the formation of intramolecular pleated structures for bis(merocyanines) 14b−i as already suggested for structurally related bis(merocyanines) 16a,b.39 These pleated structures are in a solvent-dependent chemical equilibrium with disordered, unfolded species. Accordingly, even for quite long tether units consisting of 12 methylene groups (i.e., bis(merocyanine) 14h), folding was observed in low polarity solvents such as MCH. When the concentration was increased, further self-assembly into supramolecular polymers consisting of π-stacks with a characteristic hypsochromically shifted H-aggregate absorption band was observed for 14i. This folding-driven self-assembly into supramolecular polymers could be applied for the solution processing of organic solar cells with improved photocurrents that originated from the ordered H-type stacking of the dyes.40 Thus, if a suitable solvent polarity gradient was exerted during the spin-coating procedure to a solution of meta-xylylene bridged bis(merocyanine) 14i and PCBM fullerene, bulk heterojunction solar cells with improved performance could be obtained. While folding prevailed for the meta-xylylene tether, the respective para isomer 14a constituted the only example among the series of bis(merocyanines) 14a−i for which no folding was observed but rather self-assembly into a bimolecular stack of four merocyanine units.38 Another special situation is provided by bis(merocyanine) 15 where a rigid 1,8-naphthylene spacer enforces the stacking of the two dyes either in parallel or antiparallel arrangement.41 The observed syn/anti ratio was indeed strongly solvent dependent from 5:95 in CDCl3 to 90:10 in CD3CN/CD3OD. For CH2Cl2/CH3OH mixtures, the polarity effect on the equilibrium was further elucidated by UV/ vis studies, which revealed an increasing fraction of Haggregated dyes upon increasing content of the more polar solvent methanol. It is noteworthy that in this particular case the cofacial π−π-stacking with H-type exciton coupling encompasses a parallel orientation of the dipole moments, which is unfavorable from the electrostatic point of view.

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CONCLUDING REMARKS AND OUTLOOK Among the available π-scaffolds, those of merocyanine dyes offer the most intriguing possibilities with regard to the construction of supramolecular architectures ranging from small sized nanoobjects to extended supramolecular polymers and nanostructured materials. The reason for the usefulness of merocyanine dyes for the design of supramolecular structures is given by their unique combination of directionality and binding strength that both originate, as in hydrogen bonding, from the prevalence of electrostatic forces. Making use of this inherent feature of merocyanine dyes, so far realized merocyanine dye based supramolecular architectures are among the most appealing in the area of supramolecular dye chemistry. Future challenges in the field pertain to extending merocyanine-based supramolecular architectures toward larger sized objects. Conceptually this goal should be easily feasible by applying the multivalency principle, but this approach is very demanding due to the challenges involved in structural characterization. Another challenge is to direct merocyanine self-assembly into different arrangements than those obtained so far by cofacial antiparallel stacking, for example, via

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FUNCTIONAL PROPERTIES OF MEROCYANINE AGGREGATES The self-association of dyes into aggregates has considerable impact on their functional properties. Thus, while there are only moderate changes in the energy levels of the molecules’ (frontier) orbitals upon aggregation, there are profound changes in the absorption and fluorescence properties, which strongly depend on the mutual arrangements of the dyes in the aggregated state. Because merocyanine dyes typically suffer from low fluorescence quantum yields due to radiationless relaxation pathways via torsional motions (similar to stilbenes), there is indeed a good chance for an increase of the fluorescence quantum yield upon aggregation. This issue is exemplified by merocyanines 1 whose fluorescence quantum G


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(9) Grimme, S.; Antony, J.; Schwabe, T.; Mück-Lichtenfeld, C. Density functional theory with dispersion corrections for supramolecular structures, aggregates, and complexes of (bio)organic molecules. Org. Biomol. Chem. 2007, 5, 741−758. (10) Würthner, F.; Yao, S.; Debaerdemaeker, T.; Wortmann, R. Dimerization of Merocyanine Dyes. Structural and Energetic Characterization of Dipolar Dye Aggregates and Implications for Nonlinear Optical Materials. J. Am. Chem. Soc. 2002, 124, 9431−9447. (11) Chen, Z.; Lohr, A.; Saha-Möller, C. R.; Würthner, F. Selfassembled π-stacks of functional dyes in solution: structural and thermodynamic features. Chem. Soc. Rev. 2009, 38, 564−584. (12) Prins, L. J.; Reinhoudt, D. N.; Timmerman, P. Noncovalent Synthesis Using Hydrogen Bonding. Angew. Chem., Int. Ed. 2001, 40, 2382−2426. (13) Sijbesma, R. P.; Meijer, E. W. Quadruple hydrogen bonded systems. Chem. Commun. 2003, 1, 5−16. (14) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. The exciton model in molecular spectroscopy. Pure Appl. Chem. 1965, 11, 371−392. (15) Chakrabarty, R.; Mukherjee, P. S.; Stang, P. J. Supramolecular Coordination: Self-Assembly of Finite Two- and Three-Dimensional Ensembles. Chem. Rev. 2011, 111, 6810−6918. (16) Conn, M. M.; Rebek, J. Self-assembling capsules. Chem. Rev. 1997, 97, 1647−1668. (17) Yan, X.; Wang, F.; Zheng, B.; Huang, F. Stimuli-responsive supramolecular polymeric materials. Chem. Soc. Rev. 2012, 41, 6042− 6065. (18) Xue, M.; Yang, Y.; Chi, X.; Yan, X.; Huang, F. Development of Pseudorotaxanes and Rotaxanes: From Synthesis to StimuliResponsive Motions to Applications. Chem. Rev. 2015, 115, 7398− 7501. (19) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. Aromatic interactions. J. Chem. Soc., Perkin Trans. 2 2001, 5, 651−669. (20) Feng, X.; Marcon, V.; Pisula, W.; Hansen, M. R.; Kirkpatrick, J.; Grozema, F.; Andrienko, D.; Kremer, K.; Müllen, K. Towards high charge-carrier mobilities by rational design of the shape and periphery of discotics. Nat. Mater. 2009, 8, 421−426. (21) Fink, R. F.; Seibt, J.; Engel, V.; Renz, M.; Kaupp, M.; Lochbrunner, S.; Zhao, H.-M.; Pfister, J.; Würthner, F.; Engels, B. Exciton Trapping in π-Conjugated Materials: A Quantum-ChemistryBased Protocol Applied to Perylene Bisimide Dye Aggregates. J. Am. Chem. Soc. 2008, 130, 12858−12859. (22) Umeyama, H.; Morokuma, K. The Origin of Hydrogen Bonding. An Energy Decomposition Study. J. Am. Chem. Soc. 1977, 99, 1316−1332. (23) Kim, E.-i.; Paliwal, S.; Wilcox, C. S. Measurements of Molecular Electrostatic Field Effects in Edge-to-Face Aromatic Interactions and CH-π Interactions with Implications for Protein Folding and Molecular Recognition. J. Am. Chem. Soc. 1998, 120, 11192−11193. (24) Taylor, R.; Kennard, O. Hydrogen-bond geometry in organic crystals. Acc. Chem. Res. 1984, 17, 320−326. (25) Arjona-Esteban, A.; Krumrain, J.; Liess, A.; Stolte, M.; Huang, L.; Schmidt, D.; Stepanenko, V.; Gsänger, M.; Hertel, D.; Meerholz, K.; Würthner, F. Influence of Solid-State Packing of Dipolar Merocyanine Dyes on Transistor and Solar Cell Performances. J. Am. Chem. Soc. 2015, 137, 13524−13534. (26) Zeng, H.; Yang, X.; Brown, A. L.; Martinovic, S.; Smith, R. D.; Gong, B. An extremely stable, self-complementary hydrogen-bonded duplex. Chem. Commun. 2003, 13, 1556−1557. (27) Fasting, C.; Schalley, C. A.; Weber, M.; Seitz, O.; Hecht, S.; Koksch, B.; Dernedde, J.; Graf, C.; Knapp, E.-W.; Haag, R. Multivalency as a Chemical Organization and Action Principle. Angew. Chem., Int. Ed. 2012, 51, 10472−10498. (28) Lohr, A.; Grü ne, M.; Wü rthner, F. Self-Assembly of Bis(merocyanine) Tweezers into Discrete Bimolecular π-Stacks. Chem. - Eur. J. 2009, 15, 3691−3705. (29) Lohr, A.; Uemura, S.; Würthner, F. Trimeric Cyclic Assemblies of Calix[4]arene-Tethered Bismerocyanines. Angew. Chem., Int. Ed. 2009, 48, 6165−6168.

additional hydrogen bonding or by sterical congestion. By these means, supramolecular structures showing novel functionality such as enhanced fluorescence and exciton transport are envisioned. Such structures are well-known for related cyanine dye aggregates but are so far unprecedented for merocyanines.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

I thank the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, the Alexander-von-Humboldt foundation, and the State of Bavaria for financial support of our research on merocyanine dye assemblies. Notes

The authors declare no competing financial interest. Biography Frank Würthner was born in 1964 in Villingen-Schwenningen (Germany), studied at the University of Stuttgart (Ph.D., 1993) and carried out postdoctoral research at MIT in Cambridge, MA, and at BASF central research laboratories in Ludwigshafen, followed by the Habilitation in Organic Chemistry at the University of Ulm (2001). Since 2002, he holds a chair at the University of Würzburg, where he has served as head of the Institute of Organic Chemistry, dean of the Chemistry Department, and founding director of the Center for Nanosystems Chemistry. His work encompasses the synthesis of πconjugated molecules and functional dyes, their application in organic electronics, photonics, and photovoltaics, the construction of complex supramolecular architectures composed of π-scaffolds, the mechanistic elucidation of self-assembly pathways, and the investigation of lightinduced processes in dye-based nanosystems.

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REFERENCES

(1) Dähne, S. The Ideal Polymethine State. Chimia 1991, 45, 288− 296. (2) Beverina, L.; Pagani, G. A. π-Conjugated Zwitterions as Paradigm of Donor-Acceptor Building Blocks in Organic-Based Materials. Acc. Chem. Res. 2014, 47, 319−329. (3) Parthasarathy, V.; Pandey, R.; Stolte, M.; Ghosh, S.; Castet, F.; Würthner, F.; Das, P. K.; Blanchard-Desce, M. Combination of Cyanine Behaviour and Giant Hyperpolarisability in Novel Merocyanine Dyes: Beyond the Bond Length Alternation (BLA) Paradigm. Chem. - Eur. J. 2015, 21, 14211−14217. (4) Marder, S. R.; Cheng, L.-T.; Tiemann, B. G.; Friedli, A. C.; Blanchard-Desce, M.; Perry, J. W.; Skindhøj, J. Large First Hyperpolarizabilities in Push-Pull Polyenes by Tuning of the Bond Length Alternation and Aromaticity. Science 1994, 263, 511−514. (5) Beckmann, S.; Etzbach, K.-H.; Krämer, P.; Lukaszuk, K.; Matschiner, R.; Schmidt, A. J.; Schuhmacher, P.; Sens, R.; Seybold, G.; Wortmann, R.; Würthner, F. Electrooptical Chromophores for Nonlinear Optical and Photorefractive Applications. Adv. Mater. 1999, 11, 536−541. (6) Ma, H.; Jen, A. K.-Y.; Dalton, L. R. Polymer-based optical waveguides: Materials, processing, and devices. Adv. Mater. 2002, 14, 1339−1365. (7) Dalton, L. R.; Harper, A. W.; Robinson, B. H. The role of London forces in defining noncentrosymmetric order of high dipole moment-high hyperpolarizability chromophores in electrically poled polymeric thin films. Proc. Natl. Acad. Sci. U. S. A. 1997, 94, 4842− 4847. (8) Würthner, F.; Yao, S. Dipolar Dye Aggregates: A Problem for Nonlinear Optics, but a Chance for Supramolecular Chemistry. Angew. Chem., Int. Ed. 2000, 39, 1978−1981. H


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Accounts of Chemical Research (30) Würthner, F.; Yao, S.; Beginn, U. Highly Ordered Merocyanine Dye Assemblies by Supramolecular Polymerization and Hierarchical Self-Organization. Angew. Chem., Int. Ed. 2003, 42, 3247−3250. (31) Yao, S.; Beginn, U.; Gress, T.; Lysetska, M.; Würthner, F. Supramolecular Polymerization and Gel Formation of Bis(Merocyanine) Dyes Driven by Dipolar Aggregation. J. Am. Chem. Soc. 2004, 126, 8336−8348. (32) Palmans, A. R. A.; Meijer, E. W. Amplification of Chirality in Dynamic Supramolecular Aggregates. Angew. Chem., Int. Ed. 2007, 46, 8948−8968. (33) Lohr, A.; Lysetska, M.; Würthner, F. Supramolecular Stereomutation in Kinetic and Thermodynamic Self-Assembly of Helical Merocyanine Dye Nanorods. Angew. Chem., Int. Ed. 2005, 44, 5071− 5074. (34) Lohr, A.; Würthner, F. Evolution of Homochiral Helical Dye Assemblies: Involvement of Autocatalysis in the “Majority-Rules” Effect. Angew. Chem., Int. Ed. 2008, 47, 1232−1236. (35) Lohr, A.; Würthner, F. Time-dependent amplification of helical bias in self-assembled dye nanorods directed by the sergeants-andsoldiers principle. Chem. Commun. 2008, 19, 2227−2229. (36) Lohr, A.; Würthner, F. Chiral Amplification, Kinetic Pathways, and Morphogenesis of Helical Nanorods upon Self-assembly of Dipolar Merocyanine Dyes. Isr. J. Chem. 2011, 51, 1052−1066. (37) Fernández, G.; Stolte, M.; Stepanenko, V.; Würthner, F. Cooperative Supramolecular Polymerization: Comparison of Different Models Applied on the Self-Assembly of Bis(merocyanine) Dyes. Chem. - Eur. J. 2013, 19, 206−217. (38) Zitzler-Kunkel, A.; Kirchner, E.; Bialas, D.; Simon, C.; Würthner, F. Spacer-Modulated Differentiation Between Self-Assembly and Folding Pathways for Bichromophoric Merocyanine Dyes. Chem. - Eur. J. 2015, 21, 14851−14861. (39) Lu, L.; Lachicotte, R. J.; Penner, T. L.; Perlstein, J.; Whitten, D. G. Exciton and Charge-Transfer Interactions in Nonconjugated Merocyanine Dye Dimers: Novel Solvatochromic Behavior for Tethered Bichromophores and Excimers. J. Am. Chem. Soc. 1999, 121, 8146−8156. (40) Zitzler-Kunkel, A.; Lenze, M. R.; Meerholz, K.; Würthner, F. Enhanced photocurrent generation by folding-driven H-aggregate formation. Chem. Sci. 2013, 4, 2071−2075. (41) Katoh, T.; Inagaki, Y.; Okazaki, R. Synthesis and Properties of Bismerocyanines Linked by a 1,8-Naphthylene Skeleton. Novel Solvatochromism Based on Change of Intramolecular Excitonic Coupling Mode. J. Am. Chem. Soc. 1998, 120, 3623−3628. (42) Rösch, U.; Yao, S.; Wortmann, R.; Würthner, F. Fluorescent HAggregates of Merocyanine Dyes. Angew. Chem., Int. Ed. 2006, 45, 7026−7030. (43) Yagai, S. Supramolecularly Engineered Functional π-Assemblies Based on Complementary Hydrogen-Bonding Interactions. Bull. Chem. Soc. Jpn. 2015, 88, 28−58. (44) Würthner, F.; Meerholz, K. Systems Chemistry Approach in Organic Photovoltaics. Chem. - Eur. J. 2010, 16, 9366−9373. (45) Lv, A.; Stolte, M.; Würthner, F. Head-to-Tail Zig-Zag Packing of Dipolar Merocyanine Dyes Affords High-Performance Organic ThinFilm Transistors. Angew. Chem., Int. Ed. 2015, 54, 10512−10515. (46) Xiong, Y.; Tang, H.; Zhang, J.; Wang, Z. Y.; Campo, J.; Wenseleers, W.; Goovaerts, E. Functionalized Picolinium Quinodimethane Chromophores for Electro-Optics: Synthesis, Aggregation Behavior, and Nonlinear Optical Properties. Chem. Mater. 2008, 20, 7465−7473. (47) Wortmann, R.; Rösch, U.; Redi-Abshiro, M.; Würthner, F. Large Electric-Field Effects on the Dipolar Aggregation of Merocyanine Dyes. Angew. Chem., Int. Ed. 2003, 42, 2080−2083. (48) Würthner, F.; Schmidt, J.; Stolte, M.; Wortmann, R. HydrogenBond-Directed Head-to-Tail Orientation of Dipolar Merocyanine Dyes: A Strategy for the Design of Electrooptical Materials. Angew. Chem., Int. Ed. 2006, 45, 3842−3846.

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DipoleâDipole Interaction Driven Self-Assembly of Merocyanine Dyes

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