Interplay of Structure and Dynamics in Functional Macromolecular and

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Interplay of Structure and Dynamics in Functional Macromolecular and Supramolecular Systems As Revealed by Magnetic Resonance Spectroscopy Michael Ryan Hansen,† Robert Graf, and Hans Wolfgang Spiess* Max Planck Institute for Polymer Research, P.O. Box 3148, 55021 Mainz, Germany 3.8. Graphene-Related Systems 3.8.1. Exfoliated Graphene 3.8.2. Graphene Oxide 3.8.3. Graphene Nanoribbons 4. Conclusions Author Information Corresponding Author Present Address Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. NMR and EPR Techniques 2.1. Anisotropic Spin Interactions 2.2. Structural Information from Chemical Shifts and Dipole−Dipole Couplings 2.3. Dynamics from Motional Averaging 3. Applications 3.1. Poly(ethylene), Prototype of Synthetic Polymers and Their Mechanical Properties 3.2. Polypeptides: A Special Class of Biocompatible Copolymers 3.2.1. Copolymers Composed of Amino Acids 3.2.2. Supramolecular Structures with Peptide Side Chains 3.3. Natural and Synthetic Functional Objects 3.3.1. Human Serum Albumin, a Versatile Transport Protein 3.3.2. Linear Polymers with Dendritic Side Groups 3.3.3. Thermoresponsive Polymers 3.4. Polymers on Surfaces and in Confinement 3.5. Polymers for Organic Electronics 3.5.1. π-Conjugated Polymers 3.5.2. Donor−Acceptor-Based π-Conjugated Polymers 3.5.3. Organic Solar Cells 3.6. Polymeric Proton and Ion Conductors 3.7. Supramolecular Systems 3.7.1. Columnar Stacks and Charge Carrier Mobility 3.7.2. Interplay of Competing Interactions Controlling Supramolecular Organization

© 2015 American Chemical Society

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1. INTRODUCTION The characterization of structure and dynamics of macromolecular and supramolecular systems represents an ongoing challenge. This is due to the fact that soft matter systems composed of synthetic as well as natural moieties are often not uniform but include regions with different degrees of order. As dictated by thermodynamics, phase separation is found even in polymers with only a single type of repeat unit. This leads to the well-known phenomenon of semicrystallinity. In polymer blends, mixtures of different macromolecules or copolymers, containing different moieties in their backbones or side groups, phase separation is often particularly strong.1,2 Moreover, incompatibility of repeat units, or structure-directing interactions, such as hydrogen bonds, ionic forces, or π−π interactions, will favor self-organization.3 Likewise, attachment of organic functional groups to surfaces provides another way to organize soft matter.4 In macromolecular or supramolecular systems, the different spatially separated moieties often show significantly different molecular dynamics. Even if highly ordered on a local scale, such systems often do not display three-dimensional, long-range periodicity and are for this reason not crystalline in the traditional sense. As a consequence, their structures cannot be unraveled by conventional X-ray or neutron scattering with atomic resolution.5 Instead, different complementary methods should be combined.6 This calls for improved powerful characterization techniques, which can probe both structure and dynamics at the same time. Moreover, the determination of structure and mobility on the molecular level requires site selectivity. Last, but

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Special Issue: Frontiers in Macromolecular and Supramolecular Science Received: April 28, 2015 Published: August 27, 2015

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DOI: 10.1021/acs.chemrev.5b00258 Chem. Rev. 2016, 116, 1272−1308

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Figure 1. Sketch of different soft matter species, phenomena, and methods for studying structure and dynamics in nanostructured systems. Reproduced with permission from ref 16. Copyright 2014 Royal Society of Chemistry.

material. This is reflected in the division of the Review into subsections based on molecular structure, selected for different architectures and functionalities. Special emphasis is given to how the information obtained from EPR and NMR relates to that obtained by other techniques.

not least, the macroscopic organization of the material induced by thermal annealing may be used as a handle to control the material properties. This includes, in particular, the enhanced mechanical strength of polymer fibers and the improved transport of ions, defect electrons, or holes. However, these kinds of materials are often highly viscous or may even be characterized as a solid soft matter material, which intrinsically may include “slow” molecular dynamics, happening on time scales ranging from microseconds to seconds or even minutes. Obviously, many techniques fulfill one or the other of these requirements for studying the molecular structure and dynamics. The unique power of nuclear magnetic resonance (NMR) spectroscopy, however, is due to the fact that it simultaneously meets all of the requirements.7,8 Remarkably, even 60 years after its discovery, acknowledged by awarding the Nobel Prize in Physics, 1952, to Felix Bloch and Edward M. Purcell, NMR methodology developments are as vivid as ever.9 Moreover, NMR spectroscopy can be combined with computer simulation, increasing the applicability of the technique even further.10,11 The older sister of NMR spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, also often called Electron Spin Resonance (ESR) spectroscopy of stable spin labels and spin probes,12,13 today enjoys a remarkable revival in synthetic polymers and, in particular, in studies of structure and dynamics in proteins and nucleic acids. This is made possible by advanced microwave (MW) technology making pulse techniques almost routine today.14,15 These remarkable developments now allow applications of pulsed and high-field EPR/ESR methods to study complex polymers, biopolymers, and supramolecular soft matter.16 In this Review, we present recent advances in NMR and EPR spectroscopy and their application to functional polymers and supramolecular systems, as sketched in Figure 1, mostly based on results from our group. Naturally, a complete coverage of all macromolecular and supramolecular systems is beyond the scope of our Review. Thus, the specific systems described should be seen as examples rather than a comprehensive coverage of the field. Functionality of such polymeric and supramolecular systems depends on their molecular structure (repeat unit) and the organization (architecture) of the

2. NMR AND EPR TECHNIQUES Nuclear magnetic resonance spectroscopy today is a powerful tool in the fields of analytical chemistry, solid-state physics, and various aspects of structural biology. A particularly important step that made this development possible was the introduction of the concept of Fourier transformation to NMR spectroscopy, honored by awarding the Nobel Prize in Chemistry 1991 to Richard R. Ernst. This allowed the routine observation of isotopes, like 13C or 15N, although they show particularly low sensitivity due to their inherent low magnetogyric ratios and low natural abundance of 1% and 0.36%, respectively.17,18 Obviously, NMR spectroscopy of these isotopes is particularly informative because of the important role of the associated elements in organic chemistry and biochemistry. The increase of sensitivity and spectral resolution by the use of high-field superconducting magnets, and the extension of Fourier spectroscopy to two- and higher dimensions originally proposed by Jean Jeener,19 made studies of proteins and nucleic acids possible, which was honored by awarding the Nobel Prize in Chemistry 2002 to Kurt Wüthrich. Similar advances of NMR methodology applied to inorganic materials containing nuclei with low magnetogyric ratios and quadrupole moments have also seen remarkably increased attention in recent years.20−25 Such multidimensional NMR techniques are now routinely applied to investigate structure and dynamics of proteins in solution26 but also are increasingly applied to unravel, e.g., structure and dynamics of membrane proteins in the solid state.27−35 In fact, multidimensional solid-state NMR spectroscopy was developed in the field of polymer science first and provided, e.g., unforeseen details on the complex packing of soft matter materials in terms of dynamic heterogeneity.7 Further advances in multidimensional NMR spectroscopy in solids and highly viscous soft matter are based on the 1273


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Figure 2. Dependence of the 1H chemical shift on internuclear distance (a) in hydrogen bonds and (b) in the neighborhood of aromatic rings visualized via a nuclear independent chemical shift (NICS) map. Figure courtesy of Daniel Sebastiani.

handle by quantum mechanics. In actual calculations, however, we have to deal with different orientations of molecular entities in a solid sample under study. This requires introducing the polar angles Θλ and Φλ, specifying the orientation of the external magnetic field in the PAS for each interaction. In general, these angles will be different for different interactions, but in special cases they can also be identical. Because the anisotropic spin interaction results from the geometry and electronic structure of molecules, the PASs are linked to the molecular geometry. Examples of particular relevance for the systems reviewed here are the unique axis (λzz) of the 13C CSA for carbons in aromatic moieties, which is typically close to perpendicular to the aromatic ring plane. In contrast, both the 13 C−1H DDC and the 2H QC are centered along C−H bond directions, i.e., in the plane.7 In solids, the orientations of molecular moieties of which the sample is composed have equal probability for all orientations with respect to the external magnetic field. This results in broad, yet characteristic NMR spectra, designated as powder patterns, which for complex materials often cannot be uniquely analyzed, unless isotope labeling by 2H, 13C, or 15N is applied.7 Magic angle spinning (MAS) splits up the spectrum and distributes the total intensity into discrete sidebands at multiples of the rotor spinning frequency ωR. The resulting spinning sideband patterns offer high spectral resolution but preserve the information about the anisotropic spin interactions. In the case of very high ωR only the center band is retained, which is comparable to what happens in solution, where the angular-dependent terms of the anisotropic nuclear spin interactions are averaged by rapid molecular rotations. Due to the similarity of NMR and EPR, the latter provides similar information about structure and dynamics of polymers and supramolecular systems.14,15 Nitroxide spin probes and spin labels are particularly versatile and are employed extensively in soft matter science.12 In solution, spin probe as well as spin label EPR spectra are determined by the g-factor (equivalent to the chemical shift in NMR) and the hyperfine splitting (denoted as a) due to the interaction of the unpaired electron with the 14N nucleus of the NO group. Under circumstances of rapid molecular tumbling, the g-factor determines the location of the center of the triplet spectrum, which reflects the hyperfine coupling with 14N with its spin I = 1. For applications in soft matter science, we exploit that these EPR parameters are sensitive to the electronic environment, for

application of magic angle spinning (MAS) NMR combined with sophisticated pulse sequences. These new spectroscopic techniques are able to unravel the molecular packing and local dynamics of the different moieties in supramolecular assemblies. These developments as well as early experimental examples have been reviewed earlier in this series.8 2.1. Anisotropic Spin Interactions

Solid-state NMR spectroscopy systematically exploits the sensitivity of anisotropic nuclear spin interactions, which are sensitive to the surroundings of the moiety under investigation. The nuclear shielding, commonly known as chemical shift, is the basis of site selectivity. Geometric features, such as the orientation of aromatic rings and internuclear vectors, can be elucidated by the anisotropy of the chemical shifts, dipole− dipole couplings, and electric field gradient tensors probed by the quadrupole interaction of nuclei with spin greater than 1/2, e.g., the deuteron 2H. Provided that these couplings are much weaker than the Zeeman interaction with the applied external magnetic field, the spin Hamiltonians describing these interactions can be treated by first-order perturbation theory of quantum mechanics to yield the orientation-dependent NMR frequency as7 ωλ(Θλ , Φλ) − ωL Δ = ωiso + λ (3 cos 2 Θλ − 1 − η λsin 2 Θλ cos 2Φλ) 2

(1)

The different terms in this fundamental equation are ωL, the Larmor frequency; ωiso, the isotropic chemical shift; and the angular-dependent terms Δλ and ηλ. The former specifies the strength of the anisotropy, and ηλ is called the asymmetry parameter, describing the deviation of the interaction from axial symmetry. The three nuclear spin interactions introduced above, namely, the chemical shift anisotropy (CSA), the dipole−dipole coupling (DDC), and the quadrupole coupling (QC), are encoded by the subscript λ. In high-field NMR spectroscopy as utilized today, with 1H NMR frequencies above 500 MHz, these couplings are in the range of tens of kHz for CSA and DDC and up to 250 kHz for QC of 2H, often employed in macromolecular or supramolecular science.7 Another indispensable concept to describe the anisotropic interactions is the so-called principal-axes system (PAS). In this Cartesian coordinate system, the interaction tensor contains diagonal elements only and, therefore, is particularly easy to 1274


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strong dipole−dipole couplings between two electron spins. This is illustrated by the scheme in Figure 3. The

instance, whether the spin probe is located in a hydrophilic or hydrophobic environment.36 In the solid state, similar to the anisotropic nuclear spin interactions in NMR, the anisotropy of the g-tensor results in broad EPR powder patterns. Such patterns can be analyzed, and the resulting EPR parameters can be used as fingerprints of the local structure and dynamics. 2.2. Structural Information from Chemical Shifts and Dipole−Dipole Couplings

The simplest source of structural information in NMR is provided by the isotropic chemical shift. This is well-established for hydrogen bonds,10 where the 1H chemical shift strongly depends on the length of the hydrogen bond as shown in Figure 2a. Therefore, hydrogen-bonded protons can easily be identified in 1H MAS NMR spectra, typically observed in the spectral range between 8 and 20 ppm,37 and encode the strength of hydrogen bonds.8,38 Moreover, if aromatic moieties are present, the 1H chemical shift also probes the so-called aromatic ring currents. These lead to low- or high-field shifts, depending on whether the aromatic group has an aromatic or antiaromatic character, in comparison to the equivalent NMR signals of the same moiety in solution. Such 1H chemical shifts can serve as convenient indicators of π−π stacking. Quantitative nuclear independent chemical shift (NICS) maps allow us to relate the 1H shifts observed to the crystal packing as these sensitively depend on the positioning of the moieties relative to each other.39 Last, but not least, the well-known sensitivity of 13C NMR chemical shifts to local chain conformation through the γgauche effect40 can be exploited not only in solution but also in bulk samples. Here, the packing often changes the conformation of side groups or the side groups are important elements in determining the structure of macromolecular or supramolecular systems, vide infra. Highly specific and quantitative packing information is accessible via measuring the distance between specific proton positions at building moieties in close proximity, applying highresolution, two-dimensional (2D) 1H−1H double quantum (DQ) solid-state NMR under fast MAS. These techniques have been reviewed elsewhere in detail, and here it suffices to say that the DQ coherences are typically detected in homonuclear 2D 1H−1H DQ-single quantum (SQ) correlation NMR spectra. In such 2D NMR spectra, protons with the same isotropic chemical shift lead to diagonal peaks, whereas protons with different isotropic chemical shifts manifest themselves in socalled off-diagonal or cross-peaks. In both cases the protons involved in the DQ coherence must be at distances below 0.5 nm to be detected under standard experimental conditions. Such information is invaluable to determine the structure of supramolecular systems with aromatic groups and functional macromolecules for organic electronics (see section 3.5).41,42 Intermolecular proximities between different functional groups are also conveniently probed via 1H−13C heteronuclear 2D NMR spectra,43 where 1H−13C proximities within 0.5 nm manifest themselves in corresponding correlation signals. To achieve the necessary spectral resolution, MAS is often combined with 1H−1H homonuclear multiple pulse decoupling.44,45 In systems containing nitroxide spin probes or spin labels, intermolecular distances below 1 nm are also accessible via EPR exploiting the dipole−dipole couplings between the unpaired electrons and the nuclear spins of nearby moieties. Significantly longer distances up to about 8−10 nm are accessible via the

Figure 3. Dipole−dipole coupling between electron spins at different distances. Figure courtesy of Dariush Hinderberger.

corresponding pulsed double resonance techniques are known as electron−nuclear (ENDOR) or double electron− electron resonance (DEER), respectively.46,47 For recent reviews of applications in material sciences, see ref 48. 2.3. Dynamics from Motional Averaging

A particularly attractive feature of magnetic resonance spectroscopy, both NMR and EPR/ESR, is the fact that both techniques can quantitatively elucidate the time scales and the amplitudes (geometries) of rotational motions of the structural units under study.7,49 For illustration, let us consider a moiety that performs an anisotropic rotation with a rate Ω exceeding the anisotropy Δλ. Under such circumstances, the anisotropic spin interactions involving nuclear or unpaired electron spins are reduced. As the anisotropic interactions that we exploit here are described by second-rank tensors, according to group theory as applied to quantum mechanics, this rotation leads to an averaged second-rank coupling tensor.7 As a consequence, the general equation describing the orientation-dependent NMR frequency is equivalent to eq 1, ωλ(ϑλ, ϕλ) − ωL δλ (3 cos 2 ϑλ − 1 − ηλ sin 2 ϑλ cos 2ϕλ) (2) 2 yet with reduced strength δλ and different asymmetry parameter η̅λ. The PASs of the averaged coupling tensors, however, will typically differ from the PASs of the moiety at rest. Therefore, different polar angles, ϑλ and φλ, are needed to define the orientation of the external magnetic field in these new coordinate systems. It is important to realize that an average tensor with these properties will result for each site even for exceedingly complex rotational motions, provided they are uniform and happen on sufficiently short time scales. Particularly interesting is the fact that η̅λ may be unequal to 0 even if ηλ itself vanishes.49 Group theory also dictates that rotations about a fixed axis with 3-fold or higher symmetry will result in axially symmetric averaged tensors with η̅λ = 0. A particularly important case leading to ηλ̅ = 0 is the axial rotation of planar aromatics around the column axis in columnar stacks. This places the unique axis (λzz) for all anisotropic spin interactions along the column axis, reducing the coupling for 2 H QC and 13C−1H DDC by a factor of 2, but only slightly for 13 C CSA. = ωiso +

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Group theory, of course, also applies to the anisotropic interactions in EPR/ESR spectroscopy. Consequently, EPR/ ESR spectra are also subject to averaging by rotational motions. As the anisotropic spin interactions involving unpaired electron spins are 3−6 orders of magnitude larger than those involving nuclear spins, motional averaging of EPR/ESR spectra requires motions in the range of 100 MHz rather than the 100 kHz sufficient to average NMR spectra. In macromolecular science, the understanding of motional averaging in EPR/ESR has benefitted tremendously from the pioneering work of Jack Freed, who provided extensive computer programs to simulate such spectra for different types of motions.50 Returning to NMR, we note that not all experiments will provide full information about the averaged coupling tensor. In particular, often the reduced coupling strength δλ is only accessible through MAS NMR.49 The ratio of δλ and Δλ is well established as the Maier−Saupe order parameter S in liquid crystals (LCs), where the mesogens perform axial motions around the director.51 Extending this concept to soft matter in general leads to defining a dynamic order parameter,52 Sλ =

Δλ , δλ

with 0 ≤ Sλ ≤ 1

Figure 4. Schematic representation of the macromolecular organization and chain diffusion in semicrystalline poly(ethylene). Adapted with permission from ref 54. Copyright 2008 American Chemical Society.

(3)

This allows us to quantify the amplitude of rotational motions in general. It should be appreciated that Sλ is an intrinsic quantity of the system under study; it neither implies local ordering nor implies a macroscopically ordered system. However, it is important to realize that the principal axes systems for different interactions λ can differ in their relation to the molecular moiety under study. Thus, Sλ can be vastly different for different anisotropic spin interactions. Therefore, it is particularly informative to measure Sλ for different spin interactions simultaneously. This helps to differentiate between different motional models, as Sλ predicted by different models will typically not be identical for all λ.

3. APPLICATIONS 3.1. Poly(ethylene), Prototype of Synthetic Polymers and Their Mechanical Properties

The solid-state NMR interactions and their sensitivity with respect to different motions also provide new insight into one of the basic issues of polymer physics, namely, the influence of morphology on chain dynamics.53 In particular, the conformation and the local motions of the chain residues at the interface connecting crystalline and noncrystalline regions in semicrystalline polymers is still under debate. However, as sketched in Figure 4, such conformations and local motions are crucial for the dynamics on different time and length scales and, even more important, for their macroscopic behavior in general and their drawability in particular. This important issue was unraveled in well-defined samples of ultrahigh molar mass linear poly(ethylene) (PE) in the solid state. In particular, the dynamic behaviors of solutioncrystallized (SC) and melt-crystallized (MC) samples of the same material were compared.54,55 As expected, the higher conformational order in the noncrystalline regions of the SC samples compared with the MC samples leads to a significantly reduced local mobility of the former as probed via 1H−13C dipole−dipole couplings. The corresponding heteronuclear 1 H−13C dipole−dipole MAS NMR sideband patterns for different temperatures are plotted in Figure 5. The strength of the 1H−13C dipole−dipole couplings is directly encoded in

Figure 5. Temperature-dependent MAS NMR sideband patterns due to heteronuclear 1H−13C dipole−dipole couplings in the noncrystalline regions of solution-crystallized (SC) and melt-crystallized (MC) ultrahigh molecular weight poly(ethylene). Adapted with permission from ref 54. Copyright 2008 American Chemical Society.

the width of the sideband patterns. They are rather broad in the solution-crystallized sample, exhibiting an almost temperatureindependent dynamic order parameter as high as SSC ≈ 0.4. In contrast, the dynamic order parameter of the melt-crystallized sample SMC reduces from 0.33 at 300 K to 0.24 at 360 K. Concerning the mechanical properties of the PE materials, chain diffusion between the parts of the stems in all-transconformation in the crystalline regions and parts of the stems containing gauche-conformations in the noncrystalline regions (see Figure 4) is most important.56 The difference in 13C chemical shift of the two types of stems due to the γ-gauche effect40 provides a simple and direct way to follow the chain diffusion, as the spin−lattice relaxation times for carbons in the 1276


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Figure 6. (a) Chain diffusion in comparison to local motions of linear poly(ethylene) crystallized from the melt. Arrhenius-type behavior is found for the chain transport (diffusion), while pronounced curvature is displayed by the local dynamics. (b) Comparison of temperature-dependent chain diffusion in MC and SC samples of the same ultrahigh molar mass poly(ethylene) material. Adapted with permission from ref 55. Copyright 2007 American Physical Society.

parameters determined from the heteronuclear 1H−13C dipole−dipole MAS NMR sideband patterns (Figure 5) specifying the conformational degrees of freedom. Our findings are further validated by a recent study of chain transport in monodisperse long n-alkanes.64 There, the authors concluded that the alkane chains slide around specific folds. In contrast, the methyl groups forming the chain-ends are located at the lamella surface. The observations summarized above clearly demonstrate that the restricted local chain mobility present in the noncrystalline region of solution-crystallized PE plays a crucial role for the chain diffusion between noncrystalline and crystalline regions. This also has important implications for the understanding of crystal thickening.65 For example, the solution-crystallized sample shows enhanced chain mobility along the crystallographic c-axis, which when combined with the regular stacking of crystals ultimately leads to doubling of the initial crystal thickness.66 In contrast, crystals in melt-crystallized samples of the same material hardly thicken at all. Remarkably the lamellar-doubling reduces the exchange rate between stems in the crystalline and noncrystalline regions by a factor of 2, indicating that the chain diffusion coefficient is largely independent of the lamellar thickness.67 Remarkably, chain diffusion in poly(ethylene) remains an active and still controversial issue. In two recent papers, local flips and chain motion in poly(ethylene) crystallites in meltcrystallized samples, reactor powders, and nanocrystals68 have been studied by solid-state NMR. It was concluded that the crystalline-to-amorphous long-range diffusions are at partial variance with previous findings and that the higher chain mobility in the amorphous domain of melt-crystallized samples has an accelerating effect on intracrystalline chain dynamics at high temperatures but is accompanied by a more progressive slowdown at low temperatures due to cooperativity effects.69 In another study, comparing local and long-range mobility of solution- and melt-crystallized samples, it was concluded that, contrary to the earlier findings55 the fold surface, which presumably influences the effective chain transport, does not have a strong effect on the time scale of the local chain flip process.70 The authors later conceded that their conclusions were unfortunately partly due to incorrect data analysis,69,71,72 which make their claims questionable. We would like to point out, however, that contrary to our studies54,55,67 the samples

two regions are vastly different. This was demonstrated in a pioneering study of Schmidt-Rohr and Spiess back in 1991 using 1D relaxation and 2D exchange NMR techniques.57 Applying advanced versions of the former techniques to study chain diffusion in a well-defined ultrahigh molar mass material allowed a comparison of SC and MC samples,54,55 as shown in Figure 6. The local dynamics of the stems in the crystalline region, observed by dielectric and mechanical spectroscopy58 and solidstate NMR,59,60 is non-Arrhenius, where the mobility increases with temperature beyond thermal activation. This is ascribed to the lattice expansion, which leads to increased defect concentration. In contrast, chain diffusion is strictly Arrhenius. Thus, contrary to textbook knowledge,53 the defects in the crystalline regions are not responsible for the transport of chains out of the crystalline regions, although their dynamics shows up in mechanical relaxation. Rather, twist modes of the entire stem, introduced as a mechanism of chain transport by Mansfield and Boyd in 1978,61 are apparently much more effective to generate chain diffusion. Indeed, such collective twist motions have recently been identified by advanced solidstate NMR techniques62 in poly(ethylene) samples prepared via acyclic diene metathesis (ADMET) synthesis63 with structural defects equally spaced along the polymer chain. In Figure 6b chain diffusions of SC and MC samples are compared, showing that chain diffusion in the SC sample is significantly faster than in the MC specimen. From the temperature dependence of the chain diffusion coefficient, the activation enthalpy (AE) for this process can be determined. Remarkably, both samples show the same AE of 50 ± 5 kJ/mol. The AE, however, reflects only the energetic aspect of a transition state. The differences in the observed chain diffusion coefficients are then attributed to differences in the entropic barrier the chains have to overcome to diffuse between the crystalline regions and the noncrystalline regions of the SC and MC samples. In our samples, this difference is estimated to be ΔS = 27 J/K as determined from the y-axis intercept in Figure 6b. Note that the entropic barrier is lower for the SC sample. This is consistent with the fact that the entropy difference between the all-trans chains in the crystalline regions and the γgauche-containing chains in the noncrystalline regions is lower in the SC sample compared to the MC sample. The entropy difference is directly reflected in the higher dynamic order 1277


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dynamics for polymers in mixed systems relative to their pure states83−86 and the role of configurational entropy.87 In this Review we focus on polypeptides, which are macromolecules with amino acids as repeat units. They are of academic interest, as they offer a welcome opportunity to compare polymers composed of synthetic moieties with biomacromolecules. Moreover, as polypeptides are intrinsically biocompatible, they are candidates for drug delivery or even gene therapy. Therefore, their synthesis, physicochemical properties, and biomedical applications have been extensively studied, and comprehensive reviews of the field are available.88−92 To be specific, synthetic random copolymers of L-tyrosine, L-glutamic acid, L-alanine, and L-lysine are considered in treating special aspects of multiple sclerosis and consequently are already produced commercially.93 Moreover it is now well-established that the superb performance of biological polypeptide-based materials such as hair or spiders’ silk results from a hierarchical superstructure on different length scales where the structure is controlled on each level of hierarchy.94,95 Local conformations play a decisive role in the organization of both natural and biological macromolecules. The polypeptides composed of amino acids exhibit two most characteristic conformations, also designated as secondary structures. The α-helix is formed through intramolecular hydrogen bonds, whereas the β-sheet is governed by intermolecular bonds. These conformations exhibit characteristic chemical shifts in 13C solidstate MAS NMR spectra96 and, therefore, are easy to identify, and their molecular packing can be unraveled by X-ray scattering.88 3.2.1. Copolymers Composed of Amino Acids. The field of copolymers with amino acids as building blocks has benefitted largely by the progress in synthetic polymer chemistry in recent years.97−100 In parallel, knowledge of their self-assembly and dynamics has progressed. These studies concentrated on the secondary structures of polypeptides introduced above. Examples are the determination of the persistence length of α-helices and the effect of chain topology on the type and persistence of secondary structures. Unravelling the complex hierarchical self-assembly and the multitude of dynamic processes possible in polypeptides requires a multitechnique approach, combining X-ray scattering and dielectric spectroscopy (DS)101 with advanced MAS NMR techniques, as described in detail in a readily available review.102 The different length scales of interest and the different experimental methods that deliver information about the different aspects of these interesting materials are collected in Figure 7. Specifically, the scheme considers a lamellar forming coil polypeptide diblock copolymer. From small-angle X-ray scattering (SAXS) the nanodomain spacing d is determined. The α-helical peptide secondary structure is determined by solid-state 13C MAS NMR and wide-angle X-ray scattering (WAXS). The molecular dynamics is probed by dielectric spectroscopy and site-specific solid-state NMR. Furthermore, α-helical polypeptides are type-A polymers103 with special properties in dielectric spectroscopy. This allows one to determine the persistence length, ξ, of the helical segments within the polypeptide. Clearly, the concerted use of the different techniques provides considerable more information as either technique would alone. As in the synthetic equivalent, copolypeptides containing more than one amino acid residue will in general display phase separation. In the case of copolypeptides, however, this offers

investigated in ref 70 differed in several respects and were not taken from the same batch. Obviously, molar mass, nature and concentration of defects, preparation protocols, etc. can also affect both local and long-range mobility.73 Recently, 13C−13C DQ NMR on isotope-labeled polymers has unraveled chain folds directly:74 such knowledge is expected to provide even more detailed information on the parameters governing chain diffusion than the dynamic order parameters used so far. In a related study, employing solid-state high-resolution 13C MAS NMR, it was found that helical jump motions of crystalline poly(ethylene oxide) (PEO) segments in PEO3/ LiF3SO3 complexes could only be observed for molecular weights of PEO above 2000 g/mol. Moreover, the helical jump rate increased with increasing molecular weight of PEO. This suggests that the helical jump rate of crystalline PEO segments depends on the relative content and chain mobility of the noncrystalline regions in PEO−alkali metal salt complexes. A sufficient amount of the noncrystalline phase is needed for the helical jumps to occur, and the chain mobility in the noncrystalline phase might be the driving force for the helical jump motion of the crystalline PEO segments. On the basis of these observations, it is likely that the helical jump motion corresponds to the movement of an entire PEO chain embedded in the crystallites.75 As pointed out earlier, on the basis of extended solid-state NMR studies, Hu and Schmidt-Rohr identified chain diffusion as a crucial condition for cold drawing of semicrystalline polymer.59 Another important limitation of the drawability is due to entanglements in the amorphous regions of such semicrystalline polymers.76 Entanglements, however, can be avoided by advanced synthetic means. Surprisingly, even crystals composed of single macromolecules can be generated, where the chains are spatially separated. Such unique specimens were investigated by a combination of rheology and NMR spectroscopy.77 Remarkably, by special melt protocols the authors were able to maintain the separation of the macromolecules even in the melt. This led to the concept of a “heterogeneous” melt, which contains regions where the macromolecules are intimately mixed and are entangled, and less entangled regions, which consist of spatially separated chains. These subtle differences in melt structure could be attributed to exceedingly small differences in chain dynamics detected via advanced NMR spectroscopic techniques. These findings are important for understanding the pronounced differences in the mechanical properties of these systems as the long-lived heterogeneous melt exhibits decreased melt viscosity and dramatically enhanced drawability on crystallization. Different chain organization in the melt is also discussed in recent theoretical considerations of polymer crystallization,78 ring polymers in the melt,79 and comparison of ring polymers with genome.80 The examples discussed here show that even poly(ethylene), the prototype of synthetic polymers, is still the object of intensive research and many questions concerning the interplay of structure and dynamics are not yet fully understood. 3.2. Polypeptides: A Special Class of Biocompatible Copolymers

Polymer blends and block copolymers are important classes of materials,1,2 both in academia and industry, and recent reviews are readily available.81,82 One- and two-dimensional NMR techniques allow selective determination of the glass transition in the different blocks via the time scales of slow chain 1278


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Figure 7. Scheme of the multitechnique approach for investigating the polypeptide−coil diblock copolymer forming lamellae. Small-angle Xray scattering (SAXS) is employed to determine the domain spacing, d. The peptide secondary structure is obtained from solid-state 13C MAS NMR and wide-angle X-ray scattering (WAXS). WAXS is also used to determine the lateral self-assembly of α-helices within hexagonal lattice within the polypeptide domain. The dynamics is probed by sitespecific solid-state NMR and dielectric spectroscopy (DS). Adapted with permission from ref 102. Copyright 2009 Wiley.

Figure 8. Sketch of the diblock copolymer self-assembly of PBLG and PLP α-helices, determined from NMR and WAXS, which pack in hexagonal lattices of different size. The respective unit cells are indicated in red and black. Adapted with permission from ref 105. Copyright 2012 American Chemical Society.

the opportunity to alter and control both the type and persistence of peptide secondary structures. For instance, chain stretching can cause partial annihilation of α-helical structural defects. Moreover, copolypeptides with incommensurate dimensions may induce chain folding of β-sheets. Likewise, blocks with both secondary motifs can lead to destabilization of β-sheets.99,104 Such effects should be taken into account when such peptides are considered for drug delivery. Of particular interest are synthetic polypeptides with unusual amino acids such as proline, the only amino acid without amide hydrogens and thus not capable of hydrogen bonding. This makes proline unique in protein conformation and protein folding. Moreover, the available conformations are limited by the bulkiness of the pyrrolidine moiety. Therefore, synthetic polypeptides containing proline residues provide an almost unique way of studying the interplay between hydrogen bonding and geometric packing effects. Even more interesting are copolymers, in which one block is composed of proline residues. Having this in mind, the hierarchical self-assembly of a diblock copolymer of poly(L-proline) (PLP) and poly(gammabenzyl-L-glutamate) (PBLG) was studied.105 Interestingly, both blocks consist of helices, yet stabilized by different mechanisms, namely, hydrogen bonds in PBLG and steric hindrance in PLP. This results in two hexagonal cells with different dimensions. To compensate for this mismatch in available unit cell volume, PLP displays an intriguing trans/cis conformational change that remarkably mimics the isomerization of isolated proline residues in proteins.106 These cis-PLP conformations reside primarily in the PLP/PBLG interface in order to circumvent the packing frustration and allow PBLG and PLP helices to pack in bulk; see Figure 8. As stated earlier, solid-state NMR not only can probe the structure but also is able to separately unravel the local chain dynamics in the two building blocks. An intriguing question in the current system resulted from the fact that thermal analysis by differential scanning calorimetry (DSC) detected only the PBLG glass temperature in the copolymers and none to be assigned to PLP.102,105 Because of the site selectivity of NMR spectroscopy, the dynamics of both polypeptides can be probed simultaneously via 13C MAS NMR spectra recorded as a function of temperature; see Figure 9.102,104 Indeed, the PLP Cδ and PBLG amide CO resonances show distinctly different

Figure 9. (Left) 13C MAS NMR spectra of PBLG73-block-PLP as a function of temperature. For details, see ref 105. Adapted with permission from ref 105. Copyright 2012 American Chemical Society.

temperature dependences. The former resonance loses ∼40% of its integrated intensity in the range from 300 to 400 K, whereas the latter is reduced to a level that cannot be distinguished from the background noise. These intensity losses occur because the local peptide dynamics interfere with frequencies related to MAS, cross-polarization (CP), and dipole−dipole decoupling, which are all in the range of 20 kHz. This means that the motional rates displayed by these units in the temperature range where the intensity losses are observed must also be ∼20 kHz. Specifically, the loss of signal intensity for PBLG indicates low amplitude backbone motions of α-helical segments. However, within the same temperature range, the PLP resonances are less affected. These findings indicate a mobile PBLG backbone and a comparatively more rigid PLP chain. As PBLG has a lower glass temperature, this independently suggests a pronounced phase separation of the 1279


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Figure 10. Diffusion of (a) poly-Z-L-lysine-functionalized polyphenylene dendrimers bearing the fluorescent perylenediimide core (b) from FCS and (c) peptide secondary structures from 13C NMR. Adapted with permission from ref 115. Copyright 2007 American Chemical Society.

Figure 11. Scheme of the melt self-assembly of functionalized polyphenylene dendrimers with (a) short oligopeptides (n < 16) and (b) longer polypeptides (n > 20). Notice the absence of a well-defined secondary peptide structure in the former. The polyphenylene cores form a hexagonal lattice as deduced from SAXS. In the systems with longer peptide side chains, the α-helical secondary structure prevails (with persistence length ξ). In addition, WAXS shows that the helices display packing in a hexagonal lattice (with a distance d). Notice the absence of correlations between the cores. For details, see ref 116. Copyright 2006 American Chemical Society.

synthetic molecules of high immunological impact can be generated. For instance, branched oligolysine carriers for attaching multiple copies of antigens have already been explored. The spatial conformations of theses carriers, however, are ill-defined. This could have unfavorable effects on presenting antigens to the immune system. Thus, MAPs based on dendrimers with polypeptides of defined secondary structures are considered. Indeed, the synthesis of improved MAPs of peptidefunctionalized polyphenylene cores have recently been reported.113 Moreover, for optical detection strongly fluorescent perylenediimide cores have been prepared, where fluorescence correlation spectroscopy (FCS)114 can be employed to unravel the dynamics of molecules in solution and to study translational diffusion in these systems. The method is based on following fluctuations of the fluorescent light intensity in a small observation volume, typically defined by the focus of a confocal microscope. By characterizing the size and type of the peptide secondary structures by 13C MAS

two polypeptides and further points to a much higher glass temperature for PLP. 3.2.2. Supramolecular Structures with Peptide Side Chains. Polypeptides not only can form block copolymers but also can be building blocks of hybrid materials with species capable of self-assembly. In these systems, the structure of the peptides, namely, coiled versus elongated conformations, can even control the supramolecular organization. In view of the importance of dendrimers107 in drug and gene delivery108,109 as noted above, let us consider hybrids of polypeptides and dendrimers. Rigid polyaromatic dendrimers stand out because of their high structural precision including end-group functionality.110,111 Indeed, such systems have already been applied as diagnostic tools and for therapeutic purposes. Dendrimers are new tools in controlled drug delivery, gene transfection, boron neutron-capture therapy, and novel antimicrobial systems.112 Central for such applications is the design of multiple antigen peptides (MAPs), by attaching different polypeptides to these nanoparticles. In this way, 1280


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organization, from completely disordered random coils via molten globules and linked folded domains to mostly folded crystallizable proteins in the case of biopolymers and amorphous via self-organized structures to semicrystalline polymers in the synthetic case. A reason for the attention being paid to disordered regions of proteins today is that techniques have recently been developed to analyze their structural propensities in solution by multidimensional NMR and pulsed EPR spectroscopy.124−128 These studies of intrinsically disordered proteins (IDPs) or disordered protein regions indicate that proteins in general have a conformational ensemble of varying breadth. 3.3.1. Human Serum Albumin, a Versatile Transport Protein. As a specific example from our group showing that well-ordered proteins also can gain significant flexibility, let us consider the functional structure of human serum albumin (HSA). It is the most abundant protein in human blood plasma because it serves as a transporter for various endogenous compounds and drug molecules.129 In particular, HSA is able to bind and transport multiple fatty acids (FAs). Research on HSA is difficult because of the complex protein structure.130 Even more important for understanding the binding properties of the protein, however, are the structures of complexes of HSA and transported molecules, such as fatty acids.131 In particular, it was found that fatty acids are distributed highly asymmetrically in the protein crystal despite the fact that HSA itself exhibits a symmetric primary and secondary structure. In the context of partially disordered proteins, we note that the surface-exposed parts of HSA show a high degree of flexibility, which constitutes a key to the proteins binding versatility toward various molecules. Already in the 1950s, Karush developed a concept that accounted for conformational adaptability of the binding sites,132 which was later refined.133 As noted in section 2, distances and distance distribution between spin labels on the nanometer scale can now be determined with pulsed double electron−electron resonance (DEER) spectroscopy.47,48 By using spin-labeled fatty acids, it is then possible to unravel the functional structure of HSA with respect to its binding of fatty acids directly from the fatty acids’ point of view.134 In this way the distribution of the fatty acid binding sites can be detected without signals arising from the complex protein itself. This represents an enormous simplification. The structural information on the binding sites is obtained by determining the distance distributions between the fatty acids in frozen solution. To sample distances between different binding sites, fatty acids with different labeling positions were applied. In 5-doxylstearic acid (5-DSA) the unpaired electron resides near the anchoring carboxylic acid group, whereas in 16-DSA it is located near the end of the methylene chain. Thus, information on the anchor positions in the protein and the entry points into the fatty acid channel formed of the protein can be obtained separately. The complex experimental distance distribution of 5-DSA probing the anchoring points nicely fits that of the crystal structure.131 In contrast, the distance distribution of the entry points (16-DSA) strongly deviates significantly from that of the crystal structure and indicates that the entry points are distributed much more symmetrically and homogeneously over the protein surface than expected from the crystal structure.131 As depicted in Figure 12, this leads to a picture of the functional protein structure containing a more rigid, asymmetric inner part of the protein, while the surface of the protein shows much larger structural flexibility. These findings

NMR spectroscopy, it is then possible to relate the size of such hybrids to the translational diffusion in solution measured via FCS. As a specific example, we have studied poly-Z-L-lysine functionalized fluorescent perylenediimide cores in bulk and in dimethyl sulfoxide (DMSO) solution.115 Moreover, the systems investigated consist of polyphenylene cores of the first and second generation substituted with a series of poly-Z-L-lysines with different degrees of polymerization, ranging from 14 to 84. They are designated as GgFf Nn, where G, F, and N stand for generation, lysine functionality, and peptide degree of polymerization, respectively, and 1 ≤ g ≤ 2, 4 ≤ f ≤ 16, and 14 ≤ n ≤ 84. The resulting starlike polylysines contained different fractions of poly-ε-benzyloxycarbonyl-L-lysine chains with different lengths as depicted in Figure 10. The peptide secondary structure is of special interest, if such systems are considered as MAPs. Despite the fairly low concentrations, typically 10−3 M, 13C NMR is able to clearly unravel the peptide secondary structure.115 For peptide chains with more than n = 16 residues, the characteristic 13C chemical shifts for the elongated α-helices are found, whereas for shorter peptides one observes those characteristic of β-sheet secondary structures. This nicely confirms the results of FCS, where a distinct increase in hydrodynamic radius RH is found for residues above n = 16. Thus, the increase in the hydrodynamic radii can directly be attributed to the change of the peptide secondary structure from coil/β-sheet to elongated α-helical conformations.115 It should be appreciated that such a change in secondary structure and the consequences on the particles’ diffusion should be taken into account for efficient design of MAPs. Hybrids of dendritic moieties and polypeptides also exhibit interesting features in bulk. There the dendrons can selfassemble to form columnar structures.116 The peptide conformations, however, have pronounced effects on the selfassembly as illustrated in Figure 11. For short oligopeptide side chains, the columns dominate the structure and form a welldefined hexagonal lattice with disordered polypeptide chains in between. On the other hand, longer polypeptides form elongated α-helices, which are not compatible with the lattice formed by the columns and thus lead to less supramolecular organization of the dendrons.116 The examples reviewed here show that including amino acids as moieties into polymer chemistry adds significantly to the breadth of functional materials that can be generated and is particularly attractive in order to make systems biocompatible. 3.3. Natural and Synthetic Functional Objects

The functions of natural macromolecules such as proteins are manifold.117 The wealth of structural data available today118 is mainly from X-ray studies of protein single crystals. However, solid-state NMR of proteins has now reached a mature state due to substantial advancements of NMR methodologies in recent years. This is evidenced by the large number of articles on protein solid-state NMR in a recent special issue on “Frontiers in Solid-State NMR” in Accounts of Chemical Research,27,30,31,35,119−123 which we recommend for the interested reader in relation to this topic. Moreover, as stated in an extended review124 on this subject, the occurrence of unstructured regions of significant size (>50 residues) is surprisingly common in functional proteins. These disordered regions are characterized by great structural flexibility and plasticity. Obvious similarities between proteins and synthetic polymers are that both classes span a wide range of 1281


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sites. Moreover, their interaction with solvents, in particular water, should be controlled and exploited in their selforganization. It is evident that successful projects in this direction will have considerable impact on both biological and materials sciences. One approach along these lines is to incorporate building blocks such as amino acids, generating bioinspired polymers.89 A full synthetic approach makes use of the enormous variety of dendrons and dendritic groups;107 for recent reviews, see refs 138 and 139. The structure of dendrititic groups can be varied in different ways, e.g., by controlling their size by their generation, by generating amphiphilic character by incorporating hydrophobic and hydrophilic building blocks, or by varying the conformational freedom from completely rigid (polyphenylene dendrimers)140 to highly flexible as in hyperbranched polymers.141 Linear polymers jacketed with dendrons attached via their apex provide a conceptually simple class of dendronized polymers. For such polymers with conventional backbone, poly(styrene) or poly(methacrylate), the polymer shape can be controlled through the self-assembly of flexible dendritic side groups and the degree of polymerization (DP).142 For low DP spheres are observed, whereas for high DP cylinders are obtained.142 Experimental 1H and 13C solidstate NMR characterization on the latter revealed details of the organization of the dendritic groups within the supramolecular polymer.143 The dendrons contain aromatic moieties and flexible ethylene oxide linkers (Figure 13). In the supramolecular assembly, however, they largely lose their flexibility and exhibit dynamic order parameters S as high as 0.4−0.8, displaying a gradient of mobility that decreases from inside to out. This significant immobilization nicely demonstrates their role as structure-directing moieties displaying “edge-on” and “face-on” contacts between the ethylene units and the aromatic rings facilitating the formation of helices; see Figure 13. 3.3.3. Thermoresponsive Polymers. The shape of macromolecular objects can also be changed by external stimuli.144 For instance, thermoresponsive polymeric materials are of great interest due to their potential use in fields such as actuation, drug delivery, and surface modification.145 Ever since the discovery by Wu and Zhou of the coil−globule transition of single poly(N-isopropylacrylamide) (PNiPAAm) chains near the lower critical solution temperature (LCST),146 the collapse mechanism and the formation of stable mesoglobules have been

Figure 12. Illustration of the flexibility of the fatty acid binding site entry points yielding a much more homogeneous and symmetric distribution over the protein surface than expected from the crystal structure. Only one binding site is shown for clarity. Adapted with permission from ref 134. Copyright 2010 Wiley.

suggest that the conformational flexibility at the periphery of HSA is a prerequisite for its function as a carrier for many different compounds. When comparing these EPR-derived results with similar measurements on bovine serum albumin (BSA), one finds that in BSA the structural (peripheral) flexibility is by far not as advanced as in HSA.135 By modifying the shell of HSA by the addition of poly(amidoamine) (PAMAM) dendrimers, significantly higher loading of the drug doxorubicin was achieved compared with the native protein. This is ascribed to the availability of binding pockets of the HSA core and interaction with the dendritic shell, demonstrating its great potential as a transporter for drug molecules.136 Another important aspect that influences the protein’s conformation is the distribution of charges, which has to be taken into account in the interaction with polyelectrolytes.137 3.3.2. Linear Polymers with Dendritic Side Groups. Mimicking the size and eventually the function of biomacromolecules has been a dream of polymer chemists for decades.138 This requires not only giant molecular structures to be generated, whose dimensions are on the order of tens and even hundreds of nanometers, but also that these man-made objects should have a useful, predetermined shape. Last, but not least, they should contain functionalities at both the periphery and the interior for molecular recognition or catalytically active

Figure 13. Structure and dynamics of directing dendrons in cylindrical supramolecular macromolecules. (a) Local packing allowing the formation of helices and (b) restricted motion as indicated by high dynamic order parameters with a mobility gradient inside−out. Adapted with permission from ref 143. Copyright 2003 American Chemical Society. 1282


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intense topics of research.147 Despite these efforts, a molecularscale picture of what happens when thermoresponsive polymers start to dehydrate at a certain temperature, subsequently collapsing, and then assemble into mesoglobules did not exist. This absence severely hampers rational materials design. Dendronized polymers with amphiphilic dendritic groups based on oligoethylene glycol (OEG) helped to shed more light on the phase separation, as they exhibit fast and fully reversible as well as particularly sharp transitions as observed in turbidity measurements.148 These dendronized polymers with terminal ethoxy groups are soluble in water. Their LCSTs lie in a physiologically interesting temperature range between 30 and 36 °C and mainly depend on the periphery of the dendrons. However, there are indications that such thermal responses proceed by the formation of structural inhomogeneities of variable lifetimes on the nanometer scale that are still poorly understood. Indeed, this topic has been identified as one of the major challenges of current research in the macromolecular sciences.149 Structure and lifetimes of these local inhomogeneities will obviously influence the aspired function, for instance drug delivery. Magnetic resonance techniques as intrinsically local methods are particularly suited to probe structural inhomogeneities of functional macromolecules in general.7,150 For instance, with multidimensional NMR the lifetime of dynamic heterogeneities in amorphous soft matter in the vicinity of the glass transition was already detected by multidimensional NMR spectroscopy in 1991.151 Conventional continuous wave (CW) EPR/ESR spectroscopy utilizing nitroxide radicals as paramagnetic tracer molecules offers a convenient way of studying the molecular environment of thermoresponsive dendronized polymers in general and in particular when they undergo a thermal transition.152 Such spin probes are sensitive to the local viscosity, which will give rise to changes in the rotational correlation time and to the local polarity/hydrophilicity. The latter affects the electronic structure of the radical and changes the spectral parameters, specifically the g-factor and the hyperfine coupling constant to 14 N. The amphiphilic radical 2,2,6,6-tetramethylpiperidine-1oxyl (TEMPO) is especially suited to sample both hydrophobic and hydrophilic regions and also mimics a small molecule being delivered by the dendronized polymer. The results of such a study153 are depicted in Figure 14a.153 When the temperature is raised above the transition temperature TC, the aggregation of the complete polymer sample is triggered by dynamic structural inhomogeneities of a few nanometers. At these temperatures spin probes move in and out of large hydrophilic and small hydrophobic regions. Macroscopic turbidity observed by light scattering makes one believe that the thermoresponsive polymer undergoes a sharp phase transition. However, EPR spectroscopy clearly shows that the dehydration of the polymer chains occurs over a temperature range of at least 30 K. Such a complex process cannot be considered as a single deswelling process corresponding to a thermodynamic phase transition. Instead, the dehydration in such systems is an example of a molecularly controlled nonequilibrium process. Local heterogeneities develop, and polymer chain fluctuations slow down. Within a few degrees above TC, the majority of the dehydration is completed and percolation of the hydrophobic regions is achieved. At even higher temperatures, residual water is released. While the aggregation temperature depends on the periphery of the dendrons, the efficiency of the dehydration process is governed by the hydrophobicity of the inner core.

Figure 14. (a) Sketch of the thermal collapse of the dendronized polymers as deduced from EPR spectroscopy of admixed spin probes. For details, see ref 153. Copyright 2010 Wiley. (b) Illustration of the skin barrier effect in mesoglobules of different sizes. For details, see ref 154. Copyright 2011 American Chemical Society.

In a subsequent study154 the spectral differences observed in the EPR spectra in dependence of the heating rate, the chemical nature of the dendritic substructure of the polymer, and the concentration point to formation of a dense macromolecular layer at the periphery of the mesoglobule (Figure 14b). This is called a skin barrier,145 which is formed within a remarkably narrow temperature range of only ∼4 K above TC. The skin barrier is important in applications of such systems as it prohibits release of molecules that are trapped in the aggregate. Large mesoglobules can be formed at low heating rates and at high polymer concentrations. Under these conditions a significant amount of water is entrapped and is microphase separated from the collapsed polymer chains at high temperatures. Thus, aggregates are formed that consist of an aqueous core and corona-collapsed macromolecules. On the other hand, inclusion of water is largely circumvented at fast heating rates, low macromolecule concentrations, and hydrophobic subunits in the polymer. These conditions result in higher degrees of vitrification. These findings should be appreciated when designing thermoresponsive polymers in the rapidly growing field of drug delivery.155,156 3.4. Polymers on Surfaces and in Confinement

Polymers on surfaces will form thin films, which makes studies using solid-state NMR particularly difficult due to the low content and therefore low signal intensity. Thus, more information on the effects that the proximity to a surface has on the structure and dynamics of polymers is available from studies of polymers in confinement 157,158 and porous media.159,160 Moreover, surface and confinement effects are particularly important in organic electronics, covered in section 3.5. In flexible polymers the chains tend to form random coils and the local conformations allow isotropic rotational motions of the repeat units/residues by a combination of angular fluctuations and conformational transitions.53,161 In so-called hairy rod macromolecules162 with a rigid backbone and flexible side groups, the conformational freedom along the chain is largely reduced. This favors the formation of layered structures 1283


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Figure 15. (a) Extended chain conformation of syndiotactic poly(n-alkyl methacrylates). (b) Anisotropic chain motion occurring during vitrification. (c) Static 13C NMR spectra showing the signature of anisotropic motion above Tg as described in the text. Adapted with permission from ref 165. Copyright 2003 American Physical Society.

conformations assumed lead to locally structured polymer melt similar to the hairy rigid-rod macromolecules mentioned earlier, in which the polar and less flexible polymethacrylate backbones form disordered layers. Indeed, this conjecture could be confirmed through temperature-dependent wide-angle X-ray scattering (WAXS).166 By systematically varying the molar mass of PEMA, a minimum chain length of 5−10 repeat units was identified for the separation of time scales to occur.167 In the vicinity of the glass transition, the time scales of the two processes in PEMA differ by more than an order of magnitude, where the anisotropic motions follows a simple Arrhenius law, whereas the isotropization process follows the Williams− Landel−Ferry (WLF) equation. In view of the increasing importance of surface science,168 the chain dynamics of polymers grafted, or attached to, surfaces and the characterization of the interface between the organic and inorganic components of “core−shell” colloidal nanocomposite particles are of particular interest.169,170 With this in mind, the chain dynamics was studied in PEMA, grafted onto nanoparticles.171 Through selective 13C labeling, different parts of the PEMA brush were labeled, leading to a brush labeled at the particle surface (brush A), in the middle (brush B), and at the chain end (brush C). In both brush A and brush B, the isotropization is significantly slowed down, in particular at elevated temperatures; see Figure 16a,b. The increased curvature of the data indicates a significant increase of Tg by ∼20 K as well as significant changes in WLF parameters. Remarkably, the part of the chain directly bound to the surface, brush A, consisting of ∼40 repeat units, displays virtually identical reduction in isotropization mobility as does the part in the middle of the brush, brush B, where the labeled part is separated from the core by ∼60 repeat units. This is remarkable, as the nanostructures of PEMA mentioned earlier involve 5−10 repeat units only. Thus, these structures, which are the reason for the clear separation of the time scales of the local chain motion and the isotropization in PEMA, are significantly affected by the presence of the nanoparticle. One can compare this effect with the significant reduction of the chain reptation in star

in the solid and even in the molten state and results in highly anisotropic motions.163 This leads to questions about whether in more conventional macromolecules extended conformations over several repeat can be formed. Moreover, such systems could develop conformational memory resulting in collective anisotropic motions. This would manifest itself in locally anisotropic chain mobility. On a longer time scale, loss of conformational memory would then lead to randomization of conformation and isotropic chain motion in a separate process. Poly(n-alkyl methacrylates), RnCnH2n+1, which consist of a polar backbone and flexible nonpolar side groups, exhibit unusual relaxation behavior.164 Therefore, they were the subject of extended experimental investigations by solid-state NMR and X-ray scattering techniques. The backbone of these macromolecules shows extended syndiotactic sequences resulting in extended chain conformations; see Figure 15a. Two processes should be distinguished in the dynamics of a polymer chain at the molecular level: conformational exchange and rotational dynamics. With this in mind, the backbone dynamics of poly(n-alkyl methacrylates) has been investigated by advanced solid-state NMR spectroscopy. As should be obvious from the introduction to NMR in section 2, this powerful technique can probe conformational exchange and rotational motions separately.165 The former process is detected through changes in the isotropic 13C chemical shift. The latter can be followed by measuring the anisotropic 13C chemical shift of the carboxyl group, because its unique principal axis points along the local chain. It was found that randomization of conformations and isotropization of backbone orientation indeed occur on the same time scale, but remarkably, they are both much slower than the slowest relaxation process of the macromolecule identified previously by conventional techniques.164 The separation of the two dynamic processes was found to be most pronounced for poly(ethyl methacrylate) (PEMA). As introduced earlier, special dynamics is attributed to extended backbone conformations, which exhibit conformational memory and only allow axial local chain motions (Figure 15b,c). Going a step further, the question was raised as to whether the extended 1284


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As noted above, the study of molecules attached to surfaces by NMR is severely hampered by the low sensitivity of NMR. This can in part be overcome by studying systems with high surface area, such as zeolites,175 or nanoparticles. In these systems, the peculiar organization in self-assembled monolayers has been elucidated by 2D NMR as long as a dozen years ago.176 Today, the NMR signals can be drastically enhanced by dynamic nuclear polarization (DNP).177−180 This has recently been exploited for studying the surface coverage and organosiloxane polymerization on silica nanoparticles, functionalized by silanization.181,182 Thus, in the future, solid-state NMR will be able to contribute significantly to the important field of polymers attached to or grafted to surfaces, in particular nanoparticles. 3.5. Polymers for Organic Electronics

Polymers with extended π-conjugation are of interest for flexible organic electronic devices.183 This includes organic solar cells (OSCs), organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs), where the π-conjugated polymer has to be optimized with respect to light harvesting, charge-carrier mobility, and light emission, respectively.184−187 The main advantage of using such macromolecules as active materials is that the resulting devices can be fabricated via solution processing.188 This process takes advantage of the spontaneous self-assembly of the π-conjugated polymer and results in an active material that contains regions with high and low order, as sketched in Figure 17, typically referred to as

Figure 17. Sketch of a semicrystalline macromolecular assembly with high (black) and low (gray) ordered regions. The regions with low order are often referred to as being amorphous, whereas those with high order are often termed crystalline. The regions with high order may contain lamellar packed polymer chains, polymer chain folds, and possibly polymer chains connecting two high-ordered regions, providing a pathway for charge transport.202 Adapted with permission from ref 203. Copyright 2012 Wiley.

Figure 16. Arrhenius plots of the two dynamic processes of (a) brush A labeled at the particle surface (magenta), (b) brush B labeled in the middle of the brush shell (orange), and (c) brush C labeled at the chain ends (red). For details, see ref 171. Copyright 2013 American Institute of Physics.

semicrystallinity.53 However, as mentioned in the Introduction, the lack of three-dimensional long-range order prevents direct access to details about the polymer organization at the molecular level from conventional X-ray diffraction (XRD) techniques. These techniques are well established for materials with high three-dimensional order, in particular single crystals, where atomic-resolution information about the specific structure can be obtained.189,190 For bulk polymer-based materials with low order, the structural information is typically inferred from wide-angle and small-angle X-ray scattering methods (WAXS and SAXS, respectively), whereas studies of polymer thin films as used for organic electronic applications

polymers, where the star point does not move and chain motion can only occur via arm-retraction.172 In fact, from 2H NMR on selectively deuterated 4-arm star poly(butadiene), Brereton and co-workers173 found a similar behavior, namely, almost uniform dynamics for the middle part of the arm yet significantly shorter correlation times for the chain ends. Our work also motivated computer simulation of chain dynamics of grafted chains, where it was also found that the repeat units at the end relax faster than units further inside along the chain as previously observed for planar brushes but not expected theoretically.174 1285


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Figure 18. (a) 2D 1H−1H DQ−SQ NMR and (b) 2D 13C{1H} FSLG-HETCOR NMR correlation spectra of P3HT (Sepiolid P200 from BASF, Ludwigshafen, Germany; MW = 25.0 kg mol−1, PDI = 1.6, regioregularity >98%) after annealing at 150 °C. Additional experimental details can be found in ref 203. The labels A, Cr, and HS refer to amorphous, crystalline, and hexyl side chain, respectively. (c−e) Packing models for P3HT and their corresponding NICS maps, where the red and green arrows in (c−e) illustrate 1H−1H distances above and below 4.0 Å, respectively. Adapted with permission from ref 203. Copyright 2012 Wiley.

Here, we utilize the NICS approach to quantify π−π stacking effects using experimental 1H NMR chemical shifts as fingerprints of the lamellae formed by π-conjugated polymers. Specifically, we evaluated how the local magnetic fields are modified by neighboring polymer chains within a stack composed of five layers, taking into account the unit cell dimensions determined from powder XRD. The specific NICS maps are calculated under periodic boundary conditions, and the chemical shift differences resulting from the packing are evaluated for the innermost layer (third layer), thereby accounting for π−π stacking effects from five layers above and below. A so-prepared NICS map is then used to evaluate the differences between solution- and solid-state 1H NMR chemical shifts, observed experimentally in terms of packing. Specifically, we construct and analyze different packing models in silico. These models have to be in accord with both the longand short-range constraints as deduced from XRD and solidstate NMR. In the two examples below, we have utilized this strategy to determine the specific molecular packing of two πconjugated polymers, namely, poly(3-hexyl thiophene) (P3HT) and a donor−acceptor polymer based on cyclopentadithiophene (CDT) as the donor and benzothiadiazole (BTZ) as the acceptor units. The final example of this section focuses on the application of 2D 13C{1H} heteronuclear solidstate NMR to reveal details about the interfacial structures in a bulk heterojunction OSC based on a benzo[1,2-b:4,5-b′]dithiophene−thieno[3,4-c]pyrrole-4,6-dione (PBDTTPD) polymer with branched and linear side chains and phenyl-C61butyric acid methyl ester (PC61BM), a fullerene derivative. P3HT is a semiconducting polymer with widespread applications in organic electronics devices.204,205 This comes as a result of its favorable processability, superb charge-carrier mobility reaching 0.1 cm2 V−1 s−1, and stability.206,207 P3HT is a semicrystalline polymer with a π-conjugated backbone. It

rely on grazing-incident X-ray scattering (GIXS) techniques and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.191 These techniques provide valuable and important information about how the polymers self-assemble on a defined lattice in addition to chain-to-chain and π−π stacking, as well as the molecular orientation with respect to the substrate when addressing polymer thin films.53,191 Similarly, scanning-probe techniques, like atomic force microscopy (AFM) and transmission electron microscopy (TEM), offer down to nanometer-scale images, revealing details about domain sizes and sample homogeneity.192,193 Beyond these basic understandings of the self-assembled polymer structures, it is difficult to get detailed information about the precise molecular packing structure. Such information is obviously of great importance for the further understanding and potential improvement of the optoelectronic properties for a semiconducting polymer-based material.191,194 Indeed, previous studies by deAzevedo and co-workers195−198 and Yazawa et al.199−201 have shown that solid-state NMR is a unique experimental method capable of revealing these important structural details, concerning both molecular structure and molecular dynamics for π-conjugated polymers. 3.5.1. π-Conjugated Polymers. With the above consideration in mind, we have recently developed a systematic strategy based on a combined approach using XRD, NMR, and quantum-chemical calculations.203 This strategy employs XRD to assess the long-range order of the polymer structure and high-resolution 1H as well as 13C solid-state NMR spectroscopy to determine specific molecular constraints. 1H and 13C NMR chemical shifts and 1H−1H dipole−dipole couplings (DDCs) provide the information on packing and conformation. Specific and quantitative information about the packing is then obtained from calculations of NMR chemical shifts with the aid of NICS maps as introduced earlier in this Review (see section 2.2). 1286


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can occur even in noncrystalline regions, where the slow main chain dynamics and that of the side chains influence the 13C NMR peak widths at lower temperatures (0 °C), where narrower thiophene 13C resonances are observed, the thiophene main chain performs conformation rearrangements that occur on fast time scales. This example indicates a general problem, namely, that different methods yield different fractions of different order in the sample, and rather than reporting values of “crystallinity”, one should also mention the method on which it is based, e.g., X-ray crystallinity, NMR crystallinity, etc. For a more quantitative analysis, we have calculated NICS maps under periodic boundary conditions on the basis of the dimensions of the monoclinic unit cell parameters determined from XRD for P3HT. Such calculations were performed for the three packing models summarized in Figure 18c−e, i.e., the simple stacking of P3HT polymer chains (Figure 18c), displacement of adjacent P3HT layers by one thiophene unit (Figure 18d), and flipping of every other layer (Figure 18e). The models of Figure 18c and e are in accord with the distance constraint of 4.0 Å. The model in Figure 18e shows better space-filling for the hexyl side chains and is for this reason favored. A better picture of the P3HT packing is obtained from the NICS maps (Figure 18c−e), indicating that the 1H NMR chemical shifts for the thiophene protons are shielded by 1.1− 1.6 ppm, which further demonstrates the sensitivity of the thiophene 1H chemical shift toward π−π stacking in P3HT. Notably, this also favors the molecular packing in Figure 18e, which also both fulfills the distance constraint and is consistent with the space group P21/c determined from XRD. Thus, taking the information derived from the different techniques together, the structure of the bulk phase I for P3HT as shown in Figure 19 is obtained. We note that this structural model for bulk P3HT deviates from the structure derived from electron diffraction of epitaxially grown P3HT thin films, where the π−π stacking distances differ significantly, i.e., 3.4 versus 3.9 Å determined for bulk P3HT. These differences suggest that the molecular packing of P3HT in epitaxially grown thin films and in the bulk might be quite different. In fact, recent molecular dynamic simulations suggest that it is plausible that both epitaxial growth and slow evaporation rates favor the crystallization into form I′ for P3HT.224 3.5.2. Donor−Acceptor-Based π-Conjugated Polymers. One possibility for achieving high charge-carrier mobility is to incorporate donor and acceptor groups into the polymer structure in an alternating fashion.184 For the copolymer based on BTZ-CDT, this gave exceptionally high FET hole mobilities exceeding 3 cm2 V−1 s−1 in films225 and as high as 5.6 cm2 V−1 s−1 in single fibers.226 From solid-state NMR studies of the BTZ-CDT copolymer, the 2D 1H−1H DQ−SQ NMR spectrum displayed in Figure 20 clearly show the relevant packing contacts, confirming the expected π−π stacking for the polymer backbone. The packing of the donor and acceptor groups, however, was found to be more delicate. In our initial study, we concluded that the acceptor groups are π−π stacked in a lamellar fashion and that these groups are ordered in an alternating way.225 Thereby, the acceptor groups in one layer are located on top of the acceptor groups in adjacent layers; however, they are not always in the exact same position, leading to a heterogeneous packing of the polymer chains as manifested by the spread of 1H chemical shifts along the diagonal in Figure 20. The origin of the spread of 1H chemical shifts to high field and the assignments in Figure 20 were verified by comparing

displays a head-to-tail arrangement of the polythiophene units to which alkyl side groups are attached.208−210 The overall crystallinity for P3HT depends strongly on the specific tacticity controllable by synthesis.211,212 P3HT exhibits three different phases, designated as I, II, and III. These phases show threedimensional crystallinity, 2D crystalline order with disordered side chains, and a smectic layered phase, respectively.213 Figure 18a,b summarizes the 2D solid-state NMR spectra of P3HT after annealing at 150 °C. The thiophene protons display two partially resolved resonances at 6.9 and 6.0 ppm, as deduced from the 2D 1H−1H DQ−SQ NMR correlation spectrum in Figure 18a. Moreover, the resonance at 6.0 ppm shows a characteristic, albeit weak, autocorrelation peak on the diagonal. The signal at 6.9 ppm in the solid is assigned to the amorphous regions with low order, as it is close to the NMR spectrum in solution where the thiophene protons resonate at δiso = 6.96 ppm.203,214−216 The second thiophene signal in the solid at 6.0 ppm is shifted by 0.96 ppm to high field, indicating π−π stacking.217 Therefore, it is assigned to P3HT polymer chains in the crystalline regions that include π−π stacking at distances below 4.0 Å as deduced from the autocorrelation peak at 6.0 ppm.60 Note that the shortest intramolecular distance between thiophene protons in regioregular P3HT is ∼6.0 Å.203 In solid-state NMR, the semicrystalline nature of P3HT shows up clearly in the 2D 13C{1H} frequency-switched Lee− Goldburg heteronuclear correlation (FSLG-HETCOR) spectrum in Figure 18b. Here, the 13C NMR signals in the crystalline regions are sharp, while those from the amorphous fraction show up as high-frequency shoulders, in particular, for the carbon sites of the thiophene protons (labeled orange). Thus, it is possible to determine the relative crystallinity of P3HT from fully relaxed, single-pulse 1H MAS NMR experiments performed at high spinning frequencies (>20.0 kHz) by simple spectral deconvolution, assuming overlapping mixed Gauss−Lorentzian line shapes as described in ref 203. This gives a relative crystallinity of 0.37 for the P3HT sample studied in Figure 18a,b from NMR, whereas crystallinities of 0.62 and 0.13 are reported from XRD and DSC for the same sample, respectively.203 These values can be seen to differ significantly, where the crystallinity from DSC likely is overestimated due to the value used for the melting enthalpy of ΔHm100 = 99 J g−1,218 as pointed out by Pascui et al. and recently refined by Balko et al.219,220 The differences between the relative crystallinities from NMR and XRD, however, reflect the fact that it is much easier to quantify X-ray reflections from the crystalline regions compared to the diffuse scattering arising from the disordered amorphous parts of the sample.221 This suggests that the crystallinity determined from XRD should be taken as the 3D crystallinity index, comprising the crystallinity of both P3HT polymer main chains and hexyl side chains, whereas those obtained from NMR reflect the amount of π−π stacking of P3HT polymer main chains only. It should be appreciated that the degree of chain stacking is highly relevant for the charge-transport properties for P3HT.202 Recent work by Nieuwendaal and co-workers shows that crystallinities can also be deduced from 13C solid-state NMR.222,223 However, the crystallinities determined from 13C NMR do not necessarily correlate directly with the crystallinity obtained from DSC, although very recent results from Snyder et al. suggest that it is possible to correlate deconvoluted, nonquantitative 13C{1H} CP/MAS NMR results with the normalized melt enthalpy determined from DSC.223 Nevertheless, the results from Nieuwendaal and co-workers further showed that local order 1287


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Figure 20. 2D rotor-synchronized 1H−1H DQ−SQ correlation NMR spectrum of the conjugated CDT-BTZ copolymer with long, linear side chains (R = C16H33) recorded at 20.0 T. The assignments of the two packing arrangements in green and red correspond to the molecular packing shown in parts a and c of Figure 21, respectively. The label A+P refers to overlapping 1H resonances from the amorphous (A) and parallel (P) acceptor domains. Adapted with permission from ref 227. Copyright 2013 Wiley.

ments of the BTZ acceptor moieties and three with antiparallel arrangements), illustrating that the chemical shifts of the BTZ and CDT groups vary considerably over a broad range, in line with the 2D 1H−1H DQ−SQ NMR spectrum in Figure 20. Thus, on the basis of these results, it is possible to refine the molecular packing structure of the donor−acceptor BTZ-CDT copolymer down to the subnanometer scale as summarized by Figure 21a,c and the (color) assignments in Figure 20. Moreover, this study also concluded that the unprecedented hole carrier mobilities observed in films and fibers of the CDTBTZ copolymers arise from a close packing of the polymer chains into a close-to-registry assembly, providing optimal wave function overlap, together with the intrinsically higher electronic bandwidth for charge motion along the chains. This arrangement is primarily triggered by van der Waals interactions between the long, linear alkyl chains and not by electrostatic donor−acceptor interactions. 3.5.3. Organic Solar Cells. A potential source of renewable and sustainable energy is OSCs based on the bulk heterojunction (BHJ) principle.228,229 Here, an electrondonating π-conjugated polymer or oligomer with strong absorption of solar illumination is cast together with an electron-accepting molecule, such as a fullerene derivative, into a film in a common organic solvent, forming a polymer nanocomposite material or blend.230 This casted film constitutes the photoactive layer in an OSC device, and only a few studies have been reported in the literature so far using solid-state NMR to reveal molecular-scale properties.231−238 Most of these studies have focused on BHJ samples prepared by drop-casting to ensure that enough material was available for studies using solid-state NMR (10−20 mg using a 2.5 mm rotor), while only a few, including those reported by Mens et al.

Figure 19. Side and top views of the P3HT structure in bulk samples after annealing, showing the alternating packing of P3HT polymer chains and the spatial overlap between thiophene groups. The green arrows indicate thiophene 1H−1H distances below 4.0 Å (see Figure 18a−e). Note that not all hexyl side chains are shown. Adapted with permission from ref 203. Copyright 2012 Wiley.

liquid- and solid-state 1H chemical shifts in addition to explicit quantum-chemical calculations of 1H chemical shifts. Note that the 1H resonance located at 8.0 ppm does not show any autocorrelation signals, implying that this signal originates from donor−acceptor groups in anti configurations (cf. Figures 20, 21a, and 21c). The model for heterogeneous packing of the polymer described above further allows for optimal packing of the side chains, which in the case of linear alkyl chains (C16H33) is advantageous in order to avoid steric clash. Thus, solid-state NMR conclusively does not reveal any donor−acceptor overlap within 4 Å. To further characterize the molecular packing structure, we have recently performed a combined study using grazing-incidence wide-angle X-ray scattering (GIWAXS), solid-state NMR, molecular dynamics, and quantum-chemical calculations.227 These in depth studies showed that the longitudinal displacement of the conjugated BTZ-CDT backbones by 1−2 Å changes the electronic coupling mediating hole hopping by over 1 order of magnitude. Interestingly, these subtle structural changes have clear fingerprints in both X-ray diffraction patterns and 1H NMR chemical shifts as can be identified from Figure 21. The NICS maps in Figure 21 display six representative model stacks (three with parallel arrange1288


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Figure 21. NICS maps of the BTZ-CDT packing models corresponding to a longitudinal displacement of (a, b) −2.0 Å (‘Minus2A’) and (c, d) −1.0 Å (‘Minus1A’) away from the cofacial organization shown in (e, f). Intrachain 1H−1H dipole−dipole couplings and magnetic shielding areas for identification of acceptor (BTZ) stacking are marked by black dashed ellipses. Top and bottom rows represent parallel and antiparallel organization of BTZ groups, respectively. Adapted with permission from ref 227. Copyright 2013 Wiley.

and Nieuwendaal et al., have employed spin-casting to prepare a presumably large number of BHJ thin films.232,239,240 Moreover, these studies focused on poly(2-methoxy-5-(30,70dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV) and P3HT in BHJ blends with [6,6]-phenyl-C61butyric acid methyl ester (PC61BM), respectively. Both MDMO-PPV and P3HT are readily available from commercial sources in large amounts (>500 mg), enabling the application of spin-casting techniques, where large quantities of the blended material is often lost. Despite these differences in sample preparation (drop- versus spin-casting), it remains an open question whether the morphologies obtained from the two casting techniques differ significantly. However, recent developments in both microcoil detection241−244 and ultrafast-spinning MAS NMR rotors245 give hope that it may even be possible to study single BHJ thin films, which in cases of limited material availability is particular beneficial. One of the most critical points of an OSC based on the BHJ principle is the intermolecular arrangements at the donor/ fullerene interfaces, where the charges are separated.246 Very limited information about these kind of interfaces is available due to their complex three-dimensional arrangement and the lack of techniques to measure local order on the subnanometer scale.247−249 In fact, other leading scientists in the field of OPV have recognized the potential of solid-state NMR in this area:250 “The nano-scale interface between polymer and fullerene is probably the most critical structural feature of a BHJ, but the structure details of this interface are beyond the reach of most measurement methods with the possible exception of nuclear magnetic resonance.” A recent study performed by Graham et al. has reported on these interfacial structures, focusing on a BHJ OPV material based on a donor−acceptor PBDTTPD polymer with different side chains and PC61BM as the electron-accepting material.251 The authors hypothesized that the performance of an OPV material strongly depends on the choice of side chains, either linear or branched, and whether these are attached to the donor or acceptor group of the π-conjugated polymer. To test their hypothesis, the authors performed 2D 13C{1H} HETCOR NMR correlation experiments using a long cross-polarization (CP) time of 8.0 ms, and Figure 22 summarizes these results for an 8 wt % PBDTTPD-EH/C8 in a PC61BM blend. From the 2D spectrum in Figure 22, the 13C signals associated with the fullerene ball (C60) resonate at 140−148 ppm and show strong correlations with the 1H signals resonating at ∼1.2 and ∼8.3 ppm (Figure 22c, red arrows). These 1H signals belong to the alkyl and aromatic 1H moieties of the polymer, respectively. It is noteworthy that the only protons present at and a part of

Figure 22. Chemical structures of (a) PBDTTPD-EH/C8 with branched 2-ethylhexyl (EH, red/orange) and linear C8H17 (C8, blue), and (b) PC61BM. Numbers and Greek letters represent the assignment scheme used. The red and blue arrows in (b) indicate possible intermolecular interactions between the conjugated polymer and PC61BM. (c) 2D solid-state 2D 13C{1H} HETCOR NMR spectrum acquired at room temperature for an 8 wt % PBDTTPD-EH/C8 in PC61BM blend recorded under MAS conditions of 12.5 kHz and an 8 ms CP contact time. Note that the 1D spectra shown at the horizontal and vertical axes in (c) correspond to 13C{1H} CP/MAS and singlepulse 1H MAS NMR spectra, respectively. Adapted with permission from ref 251. Copyright 2014 American Chemical Society.

the π-conjugated polymer backbone are those associated with carbon site 2 of the benzo[1,2-b:4,5-b′]dithiophene (BDT) unit, each of which is adjacent to a thieno[3,4-c]pyrrole-4,61289


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Figure 23. (Left) Distribution of the CP−OH angle within a given phosphonic acid group of poly(vinyl phosphonic acid) (PVPA), predicted from first-principles molecular dynamics simulations. (Right) Experimental 2H NMR lineshapes as a function of temperature, illustrating motional narrowing due to the hydrogen-bond dynamics, exhibiting an effectively isotropic motion. Adapted with permission from ref 264. Copyright 2007 American Chemical Society.

polymer with high permeability for protons. This is generated by combining a hydrophobic perfluorinated polymer backbone with hydrophilic ionic side groups.258 In view of its utmost importance, it is truly remarkable that the structure of the Nafion ionomer, which is part of proton-exchange membranes in fuel cells, is still under debate. With the large number of Xray studies of this system, it seems even more remarkable that, only a few years ago, an advanced analysis of SAXS data of hydrated Nafion in combination with NMR spectroscopy provided new insight into the structure of this important system.259 This powerful combination showed that the characteristic “ionomer peak” in Nafion is due to long parallel water channels, which otherwise are randomly packed. Hydrophilic side branches surround these channels.260 This new model naturally explains important features of Nafion, in particular the rapid diffusion of protons and water at ambient temperatures. In a very recent study, dynamic nuclear polarization (DNP) was used to study the diffusivity in the channels, and it was concluded that both water and proton diffusivity are significantly higher near the fluorocarbon and acidic groups lining the water channels than within the water channels themselves. This suggests that surface chemistry at the (sub)nanometer scale should be considered as a new tool to control water and proton diffusivity in polymer electrolyte membranes.261 High-temperature applications of Nafion are limited by the boiling point of water. For technical reasons, however, operation of fuel cells at temperatures above 100 °C is desirable. This calls for water-free polymeric proton-exchange membranes, which do not operate by diffusion of small protoncarrying moieties, usually designated as a vehicle mechanism. In fact, such materials, which can operate at temperatures higher than 100 °C, are of key importance in future fuel cell technology with polymeric proton exchange membranes.255,256 Combining the functions of a protogenic group and the proton solvent in a single moiety is considered a promising approach to achieve this goal. Such systems should be amphoteric, i.e., they should at the same time act as proton donors (acids) and proton acceptors (bases) within a dynamical hydrogen-bond network. Candidates for such amphoteric liquids are, e.g., phosphoric acid and heterocycles such as imidazole, pyrazole, benzimida-

dione (TPD) moiety. Moreover, the 2D solid-state NMR results in Figure 22 further suggest that the type and placement of the alkyl groups influence the local configurations of the PC61BM moieties near the donor−acceptor polymer backbone. In particular, the 13C resonances located at 31 and 14 ppm, assigned to the carbon sites b−f and h (see Figure 22a), respectively, of the linear C8H17 alkyl chains attached to the TPD acceptor groups, show strong correlations (Figure 22c, blue arrows) with the 1H signals at 2−3 ppm arising from protons 2′−4′ of the PC61BM functional group (Figure 22c, blue band). While the 13C signal at 31 ppm contains overlapping correlations signals from carbon sites b−f of the linear C8H17 (C8) alkyl chains and δ of the branched 2ethylhexyl (EH) groups, most of the signal intensity appears to arise from the 13C atoms of the C8 chains (5/TPD moiety), as opposed to those of the EH groups (2/BDT moiety). These results combined may indicate that the PC61BM functional groups interact to greater extents with the linear C8 alkyl chains of the donor−acceptor-based PBDTTPD polymer, compared to the branched EH alkyl groups. However, most of the identified 13C−1H correlations in the 2D 13C{1H} HETCOR NMR correlation spectrum of Figure 22c are very weak, and although the choice of a long CP time of 8.0 ms does enhance intermolecular correlations,252 the directly bonded 1H−13C correlations will dominate and may hide or even obscure the more interesting intermolecular correlations. In such cases, more selective solid-state NMR techniques are needed, like 2D medium- and long-range HETCOR and 2D 13C-detected 1H spin diffusion experiments.253,254 On the basis of these and other more selective NMR experiments, it may be possible to obtain clear molecular-scale insights about the polymer/ fullerene arrangement and resulting intermolecular interactions, which may be key structural factors in not only determining, but also optimizing, the performance of organic photovoltaic (OPV) material systems. 3.6. Polymeric Proton and Ion Conductors

The interest in batteries and fuel cells has increased tremendously recently because of the worldwide energy problem. For polymer science the creation of protonconducting membranes, which can work at high temperatures and high pressures, represents an ongoing challenge.255−257 The “gold standard” in this field is Nafion, a perfluorinated 1290


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which depends on the choice of anion and cation pairs. With increasing temperature, rotation of the imidazole ring together with rotation of the phosphonate group is observed. This leads to intriguing cooperative ion conduction involving imidazole and methylphosphonate that was not realized in a previous study of the benzimidazole methylphosphonate analogue.267 As noted above, ion conductors are an important ingredient in developing new energy sources or energy storage possibilities, where prominent examples include electrochemical cells, batteries, supercapacitors, and fuel cells. For these systems, in situ studies of the different components are crucial.23,268Even discharge products occurring during use can now be identified by advanced solid-state 17O NMR experiments.269 A particularly close relation between ion mobility detected by NMR spectroscopy and ion conductivity can be unraveled by probing ion diffusion over longer length scales in the μm range by pulsed field gradient NMR.159,270,271 Thus, again solid-state NMR will be able to significantly contribute to the important field of “green energy”.

zole, and triazole. In the liquid state, they exhibit comparatively high proton conductivities. Moreover, this conductivity results from so-called structure diffusion. This means that hydrogen bonds are continuously broken and reformed, leading to intermolecular proton transfer of protonic defects, regardless of whether they are in excess or deficient in protons. This approach can then get rid of a liquid electrolyte by attaching protic groups to a polymer backbone, with a widely studied system along these lines being poly(vinyl phosphonic acid).262 To clarify the proton transport in this system, NMR spectroscopy of 1H, 2H, 13C, and 31P has been augmented by molecular dynamics (MD) computer simulations. This provides detailed information on the proton mobility, water content, and condensation of the phosphonic acid groups. The condensation is unwanted, limits the lifetime of these water-free proton conductors, and has prevented commercial use of these otherwise favorable systems. Indeed, high proton mobility is observed despite the fact that the phosphonic acid groups and the polymer backbone do not display mobility on the same time scale on which the protons move. The reason for this interesting dynamic behavior is unraveled by 1H solid-state NMR, where the chemical shifts of the P−OH protons indicate a complex hydrogen bond network; see Figure 23. This makes proton transport via a so-called Grotthus- or hopping mechanism possible.263 Most importantly, the proton transport can happen both along a given chain, i.e., intramolecular, as well as between adjacent chains, i.e., intermolecular. More important, the MD simulations can provide insights into the mechanism of the proton conduction as indicated in Figure 17. It is concluded that, in systems like poly(vinyl phosphonic acid), the high intrinsic proton mobility is due to the highly disordered hydrogen-bonded network,264 favoring a hopping mechanism for proton transport. In general, local mobility in polymer systems can be favored by separating the movable moiety from the polymer backbone via a so-called spacer. This concept is often found in biological functional systems and has successfully been applied in synthetic materials as well. Such systems are beautiful examples of the delicate interplay between local structure and dynamics. Site-selective NMR can provide unique information on the different dynamics of specific moieties in the material and their role in proton dynamics. As a specific case, let us look at a polymeric proton conductor with a heterocycle as the functional group. Specifically, in a system in which triazole groups are attached on a polysiloxane via an even more flexible spacer, thermodynamics and kinetics of formation and breaking of the hydrogen bonds could be unraveled by NMR.265 Interestingly, molecular reorientations of the triazole ring were found to be essential for proton transport to happen. However, even detailed information about the local dynamics is often not sufficient to unravel the long-range transport of small entities such as protons. In the present case, all the activation energies of the microscopic processes are lower than that of the proton conductivity itself. This means that there must be an additional energy barrier different from the local processes observed by NMR that controls the macroscopic proton transport. In fact, similar behavior is also observed in phosphonic acid-based systems.262 Hydrogen bonding and dynamics of a potential anhydrous polymer electrolyte have also been studied in systems where different protogenic groups were attached to the same or different polymer chains, e.g., imidazole methylphosphonate models.266 This salt exhibits ionic conductivity in the solid,

3.7. Supramolecular Systems

Supramolecular chemistry is a vivid science since the pioneering work of Jean-Marie Lehn.3,272 Different molecular, oligomer, and even polymeric building units have been assembled to generate different properties and functionalities, e.g., processability, recycling, and self-healing, or shape-persistent and highly ordered filaments. The use of strong and directional interactions among molecular subunits can achieve high degrees of internal order resembling the cell cytoskeleton and possess useful biological and electronic functions.273 Selfassembled nanoreactors have been prepared,274 and the directing role of dendritic side groups has been established.139 Here, we concentrate on structure and dynamics of columnar systems with different functionalities. 3.7.1. Columnar Stacks and Charge Carrier Mobility. Columnar stacks are the structure-determining feature of discotic liquid crystals.51 As noted in section 2.3 already, in the liquid crystalline phase the disc-shaped aromatic core units rotate around the column axis, which can be studied conveniently by NMR via 1H−13C dipole−dipole or 2H quadrupole coupling. Moreover, imperfections of the parallel packing within the column lead to a reduction of the respective dynamic order S to values below 0.5. Such disorder was indeed observed early on for the extended hexabenzocoronene (HBC) units with alkyl chains attached, whereas the smaller triphenylene moieties lead to much narrower discotic liquid crystalline (DLC) phase ranges but are much better packed.275 In fact, the high charge carrier mobility in a highly ordered helical columnar structure derived from a triphenylene derivative276 generated a remarkable interest in the semiconducting, photoconducting, and other electronic properties of columnar liquid crystal materials. By incorporating a phenylene ring between the HBC core and the alkyl chain, the order within the column of HBC could be highly improved277 and together with perylenediimide (PDI) was used to generate highly efficient self-organized thin films for organic photovoltaics.278 Indeed, PDI derivatives are attractive in all-organic photovoltaic solar cells and field-effect transistors.279 These applications rely on the high charge carrier mobilities that made PDIs the best n-type semiconductors available to date. PDIs have an elongated shape and can therefore display considerable dynamics even in the frozen, crystal-like state. 1291


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This was observed in a triethylene glycol (TEG)-substituted PDI.280 From X-ray scattering it was found that the PDI building blocks assemble into columns arranged in a hexagonal unit cell with a lattice parameter of 2.23 nm. The meridional reflections in the wide-angle region are assigned to the πstacking distance of 0.34 nm between individual molecules in the stacks. Additional weak and diffuse off-meridional reflections show a d-spacing of 0.70 nm, i.e., twice the simple π-stacking, indicating correlations of adjacent TEG-PDI molecules perpendicular to each other. The dynamics of these systems was studied by different solid-state NMR techniques. These show that TEG-PDI in its frozen state performs angular fluctuations with amplitudes up to ±40°, reflecting the rather fragile packing of the elongated PDI units perpendicular to each other. Such fragile packing of PDI molecules was not observed in the homologous system with aliphatic side chains of identical length and topology,281 which can be attributed to the difference in the tensors of gyration for these different side chains.282 In the LC phase of TEG-PDI, additional motional averaging in the NMR spectra is observed. The easiest motional process consistent with the observed averaging involves cooperative rotation of the PDI molecules by 90° around the column axis. Thus, whereas the restricted angular fluctuations in the solid phase can be considered as local processes, the increased dynamics in the LC phase must be highly cooperative in nature. Such cooperative dynamic modes are, of course, particularly important in processing such systems to align the columns on surfaces.283 Moreover, slow molecular dynamics284 and very slow phase transformation285 hamper the formation of the equilibrium phases of DLCs, and the different packings in equilibrium and nonequilibrium phases can have pronounced effects on the charge carrier mobilities. This was studied in detail in perylene bisdiimids (PBIs) functionalized with dendritic groups.286,287 These dendronized PBIs self-assemble into complex helical columns generated from tetramers containing a pair of two molecules arranged side-by-side and another pair in the next stratum of the column, turned upside-down and rotated around the column axis at an intratetramer angle that is different from that of the intertetramer angle; see Figure 24. In most cases, the intratetramer stacking distance in this column is 0.41 nm, while the intertetramer distance is 0.35 nm. However, the architecture of this complex helical column, the structure of its 3D periodic array, and its kinetically controlled self-organization with such a long intratetramer distance are not ideal for the design of supramolecular structures with high charge-carrier mobility. In fact, the mobility of electrons is only moderate. However, in some cases heating above 100 °C into the LC phase optimizes the packing, resulting in shorter intratetramer distances and much higher charge mobilities.286,287 This is accompanied by substantial narrowing of the corresponding 1H NMR resonances. Interestingly, computer simulations showed that this narrowing of the NMR spectra indicates a complex reorganization mechanism, where the PBI molecules leave the supramolecular column, flip over, and reenter a column at a later time as illustrated schematically in Figure 24 b,c. The obvious way of controlling supramolecular organization is the choice of the building moieties, where, in general, highly symmetric planar units are best suited to form columnar stacks. However, in a recent multitechnique study applying a variety of complementary techniques, including DSC, XRD, and solidstate NMR, it was found that there are cases where less

Figure 24. (a) Packing of dendronized PDI with equal intra- and interdimer stacking (left and right) of 0.35 nm, but larger intradimer packing of 0.41 nm due to nonequilibrium disorder (middle). (b) Tetramer motif stacking into columns. (c) Molecular reorganization: one PDI leaves the columns, flips over, and enters a column again. Adapted with permission from ref 286. Copyright 2011 American Chemical Society.

symmetric, twisted, and flexible molecules order far better into supramolecular structures than more ordered, planar, and rigid ones.288 This study was triggered by the observation that functionalization of the bay positions of PBI with halogens, including fluorine, chlorine, and bromine, has proven to address some of the limitations of nonhalogenated PBI building blocks.289 In particular, halogenation increases the electron acceptor ability of PBI and extends the lifetime of charge carriers.290−292 However, detailed comparative studies of the complex supramolecular structures formed by the self-assembly of planar and rigid nonchlorinated PBIs versus nonplanar twisted and flexible tetrachlorinated PBI (Cl4PBI) are not available. The higher order that can be achieved in these systems as found by the structural study,288 where the molecularly less symmetric Cl4PBI moieties self-organize into 1292


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Figure 25. Increasing supramolecular order by reducing molecular symmetry of the building block. For details, see ref 288. Copyright 2015 American Chemical Society.

3D crystalline periodic arrays under thermodynamic control, is depicted schematically in Figure 25. Solid-state NMR can effectively unravel even complex packing of building moieties in such supramolecular structures on the molecular level. The results are most reliable when discussed on the basis of structures derived from X-ray scattering. As described earlier, the PBI molecules typically form tetramers built from dimers with two molecules packed side by side, where adjacent dimers are typically rotated by ∼90° (Figure 24). We note that the dimer formation of PBIs/ PDIs has also been reported in solution,293 consistent with the earlier reported formation of dimers from solid-state NMR.281 In sharp contrast, the Cl4PBI compound forms dimers of two Cl4PBI molecules perpendicular to each other. In these intertwined nonplanar molecules, angular fluctuations as in the TEG-PDI system described earlier are not possible. Nevertheless the dimers pack into columns in which neighboring moieties are rotated by ∼60°. The core protons can probe this structure, because they exhibit long intramolecular distances of 7 Å, but in the proposed structure they exhibit intradimer distances of 4.2−4.3 Å only. Such a structure can be conveniently probed by 1H−1H double-quantum singlequantum (DQ−SQ) correlation NMR. There, the core protons give rise to strong diagonal peaks, which is indeed observed; see Figure 26a. Also notice that the well-resolved spectrum of the chlorinated system differs remarkably from that of the nonchlorinated analogue. There the spectral features are broad and unresolved as shown in Figure 26b, due to the much lower supramolecular order. In the chlorinated system even the proton signals of the Cl4PBI core at 7.6 ppm are resolved from the signals of the proton in the outer phenyl rings at 7.2 ppm. Thus, the different packing leads to distinct 2D 1H−1H DQ−SQ correlation NMR spectra, and the unanticipated improved packing of the compound CL4PBI with lower symmetry can clearly be elucidated. 3.7.2. Interplay of Competing Interactions Controlling Supramolecular Organization. Another example of systems forming helical supramolecular assemblies with columnar mesophases, from flexible, nonplanar moieties, are the C3symmetric benzene-1,3,5-tricarboxamides (BTAs). The driving forces of the self-organization as well as the final supramolecular structure, however, can vary from system to system

Figure 26. 2D 1H−1H DQ−SQ correlation NMR spectra of chlorinated (a, c) and nonchlorinated (b, d) PBI supramolecular assemblies,288 recorded at 30 kHz MAS and 850 MHz 1H Larmor frequency and two DQ different excitation times given in multiples of the MAS rotor period tr. The signals of the Cl4PBI core and the phenyl rings of the dendron are marked by red dotted lines. (e) Schematic illustration of a dimer of CL4PBI indicating intermolecular correlations between Cl4PBI core protons (A), intramolecular correlations between outer phenyl protons and O−CH2 sites (B), and correlations between aliphatic sites of the alkyl chains and the Cl4PBI core protons (C). See ref 288. Copyright 2015 American Chemical Society.

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information mediated by a helical arrangement of building blocks. The complexity of these mechanisms has only begun to be elucidated.296,297 Crystallization of supramolecular homochiral and racemic systems as well as their deracemization entails the development of methodologies that are available for molecular but not supramolecular systems. Along these lines a library of hat-shaped dendronized cyclotriveratrylene (CTV) crowns substituted with chiral, racemic, or achiral peripheral alkyl chains has been synthesized and analyzed by a multitechnique approach.298 Remarkably, self-sorting of columns containing a mixture of enantiomers was observed as illustrated in Figure 28. The creation of homochiral columns in

despite equivalent, hydrogen-bonding central building blocks. Solid-state NMR combined with X-ray scattering, optical spectroscopy, and computer simulations provide insight into the subtleties of the organization from such moieties. When three bulky side groups, such as 3,3′-diamino-2,2′-bipyridines, are attached to BTAs, the self-assembly of the disclike molecules is mainly driven by steric interactions between neighboring molecules, where surprisingly hydrogen bonding along the columns has almost no influence on the final supramolecular structure. This results in a complicated helical arrangement with pitch angles of 13−16° only, in accord with quantum chemical calculations. Contrary to that, in BTAs with short chiral, aliphatic side chains, the supramolecular organization sensitively depends on the local hydrogen bonding. Unlike the common symmetric coplanar helical arrangement of carbonyl centered BTAs found in supramolecular polymers in gels (Figure 27a), NH centered BTAs

Figure 28. Formation of homochiral columns by chiral self-sorting during supramolecular helical organization of hat-shaped molecules.298 Note the extended time needed for self-sorting. Adapted with permission from ref 298. Copyright 2014 American Chemical Society.

this way is difficult to imagine for such hat-shaped moieties. In the relevant temperature range, however, solid-state NMR detected fluctuations of the flexible crown between “up” and “down” conformations as depicted in Figure 29. Thus, on average, the crown behaves like a disc, which allows rotation around the column axis and exchange between columns as known from discotic liquid crystals. This example shows again how solid-state NMR is able to elucidate complex local dynamics, providing a route to complex structural changes, which are mandatory to achieve the highest degree of organization. Natural channel-forming structures are mandatory for connecting different compartments within a living organism. For instance, transmembrane proteins function as ion channels, transporters, or antibiotics.299 Biomacromolecules that have been formed during evolution self-assemble into tubular structures with precisely defined positions of functional groups. The stimuli-responsive activity of these molecules has inspired the search for artificial channel-forming structures that can mimic the functionality of the natural systems.300−302 Artificial channel systems may even include new functionalities in advanced chemical applications.303,304 Self-assembly of shapepersistent low-symmetry arylene ethynylene butydiynylene macrocycles into molecular channels, taking advantage of dissipative forces of aromatic moieties with different electron affinities, offers a way of generating more flexible tubes.305 From solid-state NMR experiments combined with chemical shift calculations, it was possible to show that the channels are indeed empty. The decisive role of the length of the dendritic side chains, as well as the formation of a six-membered ring with weak intramolecular contact involving a proton site on an aromatic group of the ring and a flexible ethylene oxide linker

Figure 27. Helical C3-symmetric benzene-1,3,5-tricarboxamide stacks: (a, b) CO centered and (c, d) N centered. The tilted stacking leads to a splitting of the CH signals in 1H MAS NMR spectra as indicated by the NICS maps plotted below (e, f). Adapted with permission from ref 295. Copyright 2011 Royal Society of Chemistry.

adopt an asymmetric helical arrangement in the solid (Figure 27b). In the supramolecular stacks of BTAs, the aryl protons are drastically shifted to high-field due to the packing effect within columnar arrangement. The additional chemical shifts resulting from the packing are analyzed by NICS maps, introduced in section 2.2. It is rewarding that solid-state NMR combined with quantum chemical calculations can clearly distinguish these rather similar structures and clearly define the differences. Moreover, this approach was recently used to show that helical morphologies may even coexist in columnar stacks of star-shaped discotic hydrazones, having a NH centered BTA core and propeller-like side chains.294 Most natural compounds and biomacromolecules are homochiral, but creation of supramolecular chirality involves mechanisms of transfer and amplification of structural 1294


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Figure 29. Scheme of a sequence of events taking place during the deracemization process between enantiomerically rich columns (a), via a temporary disclike conformation moving from column to column during the CTV crown inversion (b, c, d). Adapted with permission from ref 298. Copyright 2014 American Chemical Society.

Figure 30. Sketch of empty helical stacks formed by shape-persistent macrocycles. A pitch angle of ∼60° between adjacent macrocycles was determined from 2D double quantum NMR correlation spectra in agreement with NICS maps. Adapted with permission from ref 306. Copyright 2011 Wiley.

and co-workers in 2004.307−309 This includes new, efficient ways of manufacturing graphene and graphene-related systems310−313 and its potential application in the next generation of electronic devices,314−317 composite materials,318−320 energy-storage devices,321−323 etc. due to its intriguing electrical, mechanical, and chemical properties. As far as magnetic resonance as a characterization tool is concerned, EPR is a well-suited characterization tool for studies of organic conductors,324 nanostructured graphite,325 and exfoliated graphene,326−329 in addition to graphene oxide and reduced graphene oxide.330 However, EPR mainly probes the electronic environments of the conduction electrons, typically via spin relaxation techniques, and can in some case also detect localized defects associated with unpaired electrons in, for example, graphene oxide and reduced graphene oxide flakes.331 Solid-state NMR, on the other hand, can be applied for characterization of the chemical structure for graphene-related

of the side group, was unravelled. The formation of a liquid crystalline phase then leads to the immediate and controllable formation of self-repairing nanochannels with either tighter or more permeable walls. These channels have an inner diameter well above 1 nm in the liquid crystalline phase. The channels are further stabilized by a helical arrangement with a pitch of ∼60° between individual macrocycles as identified from 2D 1 H−1H DQ−SQ correlation MAS NMR experiments and NICS calculations (Figure 30). For future applications it is envisaged that these artificial channels can function as sizeselective or even as molecular-selective pores for chemical sensing or directed transport of nanosized objects.306 3.8. Graphene-Related Systems

Graphene, a two-dimensional honeycomb sp2 carbon lattice, has been the target of extensive research by both physicists and chemists since the groundbreaking report by Geim, Novoselov, 1295


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systems, provided that only a limited number of unpaired electrons are present and that these are not localized.332,333 3.8.1. Exfoliated Graphene. Not surprisingly, the structural characterization of graphene using solid-state NMR reported in the literature is very rare, because graphene possesses a high intrinsic charge-carrier mobility.307 This makes the application of sophisticated 1D and 2D solid-state NMR techniques to elucidate coordination states and spatial connectivities (as illustrated throughout in this Review) impossible due to the very short nuclear relaxation time caused by the strong coupling between unpaired electrons and nuclei. Nevertheless, a recent study of nonlabeled graphene showed that it is possible to get a 1D 13C MAS NMR signal from unmodified graphene; see Figure 31.311 This spectrum was

Figure 32. (A) 1D 13C MAS and (B) 2D 13C−13C chemical-shift correlation solid-state NMR spectra of 13C-labeled graphite oxide with (C) slices selected from the 2D spectrum at the indicated positions (70, 101, 130, 169, and 193 ppm) in the ω1 dimension. All spectra were obtained at a 13C NMR frequency of 100.643 MHz with 90 kHz 1 H decoupling and 20 kHz MAS for 12 mg of the sample. The 2D experiment in (B) utilized the finite-pulse radio frequency-driven dipolar recoupling (fpRFDR) mixing sequence338 and a relatively short mixing time of 1.6 ms to identify 13C−13C pairs that are directly bonded or separated by two bonds. Adapted with permission from ref 334. Copyright 2008 American Association for the Advancement of Science.

129.3 ppm to epoxide, C−OH, and sp2 carbon environments, respectively. This assignment agrees well with what has previously been reported by Klinowski and co-workers.336 From the 2D solid-state 13C−13C finite-pulse radio frequencydriven dipolar recoupling (fpRFDR) NMR experiment in Figure 32B, it is possible to identify cross-peaks at the positions (ω1, ω2) = (133 ppm, 70 ppm) and (130 ppm, 59 ppm), corresponding to the green signals in Figure 32B. These crosspeaks represent spin polarization transfer from sp2 carbons observed at ∼130 ppm in ω1 to hydroxyl and C−OH and epoxide groups, appearing at 70 and 59 ppm in ω2, respectively. Thus, these cross-peaks directly demonstrate the connectivity between sp2 13C and 13C−OH, as well as that between sp2 and epoxide 13C. Moreover, the cross-peak intensities are ∼10% compared to the diagonal signals (ω1 = ω2), suggesting that a large fraction of the sp2 13C atoms are in fact directly bonded to 13 C−OH and/or epoxide 13C. Likewise, strong cross-peaks between 13C−OH and 13C-epoxide are also observed (red signals in Figure 32B). Again, the 2D NMR data suggest that a large fraction of C−OH and epoxide carbons are bonded to each other. The blue cross-peaks further indicate that the sp2 13 C species exist with slightly different chemical shifts and that these are in fact bonded to each other. Indeed, the sp2 13C chemical shifts for the cross-peaks (green) are slightly different for the cross-peaks to C−OH (133 ppm) and to epoxide (130 ppm). To summarize these results, Ishii and co-workers concluded that only two of the six previously published models for graphene oxide models are in agreement with the 2D NMR data derived from Figure 32B. These correspond to the Lerf− Klinowski and Dékány models.336,337 It is further noted that the model proposed by Dékány and co-workers may be correct for their more highly oxidized compound, because this model

13

Figure 31. C MAS NMR spectra of exfoliated graphene and graphene oxide. The measurements have been recorded at a 13C Larmor frequency of 176 MHz, 30 kHz MAS, a low flip angle of ∼20°, and a relaxation delay of 20 s. In the case of graphene, the MAS caused a substantial heating of the sample due to eddy currents and a partially filled rotor had to be used to achieve 30 kHz MAS. For graphene the overall measurement with poor signal-to-noise ratio took 3 days, while the same signal-to-noise ratio could be obtained in 8 h for graphene oxide. For details, see the Supporting Information of ref 311. Copyright 2014 American Chemical Society.

recorded in 3 days to ensure a reasonable signal-to-noise ratio, and it shows, not surprisingly, that the main 13C NMR signal from graphene is broad and from the sp2-hybridized carbons of the graphene sheets. 3.8.2. Graphene Oxide. A far more susceptible graphenerelated system for solid-state NMR is that of graphene oxide, the chemically reduced analogue of graphene, which is nonconducting. Such studies were recently performed by Ishii and co-workers, who characterized both reduced graphene oxide and nitrogen-doped graphene oxide.334,335 Both of these studies took advantage of isotope labeling by introducing 13C and 15N during the synthesis to overcome the low natural abundance of these nuclei, which is a prerequisite for performing homonuclear and heteronuclear 2D NMR experiments. In the case of reduced graphene oxide, this enables the possibility to explore the connectivities between the different types of carbons (or equivalently hybridization states) present in the graphene oxide layers, as illustrated by Figure 32. From the 1D 13C MAS NMR spectrum in Figures 31 and 32A, it is possible to assign the three major 13C signals at 59.7, 69.6, and 1296


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Figure 33. Solid-state NMR characterization of the precursor and graphene nanoribbon (GNR). The 2D 1H−1H DQ−SQ correlation spectra of (a) precursor and (b) GNR shown in (d) were recorded using a MAS frequency of 59 524 Hz and two rotor periods of DQ recoupling. (c) 1D 13C{1H} REPT-HSQC spectrum recorded with a short REDOR recoupling period of two rotor periods.352 (d) Assignment scheme. Adapted with permission from ref 347. Copyright 2014 American Chemical Society.

outer (red circle) and the inner (blue circle) proton, respectively (see Figure 33b, d). Interestingly, the spectral features observed for this GNR are broader than those of a recently studied narrower GNR,347 most likely due to larger aromatic currents induced by the more extended aromatic cores. To study the carbons present at the edge, this study further utilized the 1D 13C{1H} recoupled polarization-transfer heteronuclear single-quantum correlation (REPT-HSQC) experiment as summarized in Figure 33c.347 By using a short rotational-echo double resonance (REDOR) recoupling time, only 13C signals from the carbons situated at and near the edges are observed. The spectrum in Figure 33c includes three relatively broad 13C signals, which can be assigned to aliphatic side chains (∼30 ppm) as well as aromatic CH (∼124 ppm) and quaternary aromatic carbons (∼139 ppm) at or close to the edges, based on their chemical shifts and intensities. The above examples show that solid-state NMR is still an emerging technique in the field of graphene-related systems. Clearly, the successful application of solid-state NMR to study the molecular structures and potential dynamics in detail will depend on future experimental developments, including higher spinning frequencies, higher magnetic fields, and possibly also the development of new EPR/DNP equipment. Nevertheless, recent progress in the area of solid-state NMR characterization of paramagnetic proteins and in cryogenic NMR under MAS gives hope that more advanced studies will be possible in the near future.122,348−351

corresponds to a considerably higher level of oxidization for complete modification of the sp2 network into a cyclohexaneslinked network. 3.8.3. Graphene Nanoribbons. Recently, graphene nanoribbons (GNRs) were introduced as the chemist’s way of making precise and well-defined graphene nanostrips via carefully designed bottom-up strategies.339−343 GNRs have an unique advantage compared to graphene sheets, which are semimetallic, in that they have a finite and tunable band gap.344,345 The magnitude of the band gap critically depends on the width and specific edge structures of the GNR, making it crucial to precisely control the GNR structures for both fundamental studies as well as for future nanoelectronic applications.346 The fact that GNRs have a nonzero band gap and thereby a much lower electron conductivity compared to that of graphene makes this kind of material accessible to solidstate NMR.347 Figure 33 summarizes the results from a recent study, which focuses on the edge structure as probes by 1H and 13 C MAS NMR techniques at a high spinning frequency. In agreement with the relatively flexible structure, the 2D 1H−1H DQ−SQ MAS NMR spectrum in Figure 33a of the tailor-made three-dimensional polyphenylene precursor that is dispersible in organic solvents (see Figure 33d) only includes narrow 1 H−1H correlation signals between the aromatic protons as well as between the aromatic and aliphatic protons. The resulting GNRs, obtained via oxidative cyclodehydrogenation with iron(III) chloride of the polyphenylene precursor, on the other hand, displays broad, stretched, and a split ridge of 1 H−1H correlation signals close to the spectrum diagonal (SQiso ≈ 7−15 ppm), in addition to a broad range of 1H−1H correlation signals between aromatic and aliphatic protons (SQiso ≈ 4−12 ppm) (Figure 33b). On the basis of the correlations to the aliphatic protons, the lower- and higherfrequency parts of the aromatic signals can be assigned to the

4. CONCLUSIONS In the last few decades, advances in synthesizing, characterizing, and understanding macromolecular and supramolecular systems have led to an enormous variety and complexity in the field of polymer science.149 The traditional separation in terms of structure versus dynamics, crystalline versus amorphous, or 1297


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experiment versus theory is increasingly overcome. As far as characterization and controlling the function of such materials is concerned, no experimental or theoretical/simulation approach alone can provide full information. Instead, a combination of techniques is called for, and conclusions should be supported by results provided by as many complementary methods as possible. As demonstrated in this Review, the information provided by NMR and EPR is indispensable and unique. Techniques of combining scattering and/or magnetic resonance spectroscopy with computer simulation are wellestablished today and should be applied more often instead of claiming structures and dynamic processes without experimental verification. In the case of crystalline and microcrystalline systems, such a combination led to the new research field termed NMR crystallography,353 with important applications in studies of pharmaceutical compounds354,355 and organic solids,356 where solid-state NMR has been particularly valuable in distinguishing different polymorphs formed under different processing conditions. Likewise, the structure and dynamics of macromolecular and supramolecular systems and, thus, their function can drastically change with different processing conditions, and a versatile tool such as solid-state NMR provides invaluable possibilities that should be better utilized in the future. Another aspect of supramolecular organization, which is sometimes not appreciated sufficiently, is the fact that the side chains, beyond their function as “lubricants”, often play a crucial role in the organization of the more rigid moieties, as observed in many of the examples presented in this Review. As far as structure and dynamics of biomacromolecules are concerned, magnetic resonance plays a prominent role in the emerging field of partially disordered protein.118,124 We hope that the examples described here show the power of the multitechnique approach in the supramolecular field involving the combination of spectroscopy, scattering, and computer simulation.

Michael Ryan Hansen was born in 1976 and received his Ph.D. from Aarhus University, Denmark, in 2006 for his work on quadrupolar nuclei in heterogeneous catalysis and inorganic materials characterized by solid-state NMR and DFT calculations. In 2007 he joined the group of Prof. Hans Wolfgang Spiess at the Max Planck Institute for Polymer Research (MPI-P) in Mainz, Germany, as a Carlsberg Foundation Postdoctoral Research Fellow. He was appointed research group leader at the MPI-P in 2010, with emphasis on organic supramolecular systems. From 2013−2015 he was an Assistant Professor at Aarhus University, Denmark, supported by a Young Investigator Grant from the Villum Foundation, Denmark. Since June 2015 he is a Professor of Physical Chemistry at the Westfälische Wilhelms-Universität Münster, Germany. His current scientific interests focus on soft matter systems and energy-related materials studied by solid-state NMR spectroscopy to elucidate their nanoscale assembly and molecular dynamics.

Robert Graf graduated from the Johann Wolfgang von Goethe University in Frankfurt/Main in the field of solid-state physics. The research for his Ph.D. studies on double-quantum NMR studies of amorphous polymers under fast magic-angle spinning conditions was done at the Max Planck Institute for Polymer Research under the supervision of Prof. Hans Wolfgang Spiess. In 1998 he became a staff member of the Max Planck Institute for Polymer Research, leading the solid-state NMR group. His main research interests are the elucidation of molecular mechanisms determining the local molecular packing and dynamic processes in partially disordered organic systems and novel functional materials using tailored NMR methods.

AUTHOR INFORMATION Corresponding Author

*[email protected]. Present Address †

Institute of Physical Chemistry, Westfälische WilhelmsUniversität Münster, Corrensstr. 28/30, D-48149 Münster, Germany.

Notes

The authors declare no competing financial interest. Biographies

Hans Wolfgang Spiess was born in the year 1942 and concluded his studies of Chemistry by being granted the title Dr. phil. nat. at the Johann Wolfgang von Goethe-Universität, Frankfurt am Main, in the summer of 1968. After that he spent two years as a postdoctoral fellow at the Department of Chemistry, Florida State University, Tallahassee, Florida. He then moved as a “Wissenschaftlicher Assistent” to the Max Planck Institut für Medizinische Forschung, Heidelberg, in 1970−1975 1298


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(15) Jeschke, G. DEER Distance Measurements on Proteins. Annu. Rev. Phys. Chem. 2012, 63, 419−446. (16) Graf, R.; Hansen, M. R.; Hinderberger, D.; Muennemann, K.; Spiess, H. W. Advanced magnetic resonance strategies for the elucidation of nanostructured soft matter. Phys. Chem. Chem. Phys. 2014, 16, 9700−9712. (17) Ernst, R. R.; Bodenhausen, G.; Wokaun, A. Principles of Nuclear Magnetic Resonance in One and Two Dimensions; Clarendon Press: Gloucestershire, U.K., 1990. (18) Waugh, J. S. Sixty Years of Nuclear Moments. Annu. Rev. Phys. Chem. 2009, 60, 1−19. (19) Jeener, J. Ampere International Summer School II, Basko Polje, Yugoslavia, 1971. (20) Massiot, D.; Messinger, R. J.; Cadars, S.; Deschamps, M.; Montouillout, V.; Pellerin, N.; Veron, E.; Allix, M.; Florian, P.; Fayon, F. Topological, Geometric, and Chemical Order in Materials: Insights from Solid-State NMR. Acc. Chem. Res. 2013, 46, 1975−1984. (21) Schurko, R. W. Ultra-Wideline Solid-State NMR Spectroscopy. Acc. Chem. Res. 2013, 46, 1985−1995. (22) Ashbrook, S. E.; Dawson, D. M. Exploiting periodic firstprinciples calculations in NMR spectroscopy of disordered solids. Acc. Chem. Res. 2013, 46, 1964−1974. (23) Blanc, F.; Leskes, M.; Grey, C. P. In Situ Solid-State NMR Spectroscopy of Electrochemical Cells: Batteries, Supercapacitors, and Fuel Cells. Acc. Chem. Res. 2013, 46, 1952−1963. (24) Ashbrook, S. E.; Sneddon, S. New Methods and Applications in Solid-State NMR Spectroscopy of Quadrupolar Nuclei. J. Am. Chem. Soc. 2014, 136, 15440−15456. (25) Ashbrook, S. E.; Dawson, D. M.; Seymour, V. R. Recent developments in solid-state NMR spectroscopy of crystalline microporous materials. Phys. Chem. Chem. Phys. 2014, 16, 8223−8242. (26) Shen, Y.; Lange, O.; Delaglio, F.; Rossi, P.; Aramini, J. M.; Liu, G.; Eletsky, A.; Wu, Y.; Singarapu, K. K.; Lemak, A.; et al. Consistent blind protein structure generation from NMR chemical shift data. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 4685−4690. (27) Hong, M.; Schmidt-Rohr, K. Magic-Angle-Spinning NMR Techniques for Measuring Long-Range Distances in Biological Macromolecules. Acc. Chem. Res. 2013, 46, 2154−2163. (28) Yan, S.; Suiter, C. L.; Hou, G.; Zhang, H.; Polenova, T. Probing Structure and Dynamics of Protein Assemblies by Magic Angle Spinning NMR Spectroscopy. Acc. Chem. Res. 2013, 46, 2047−2058. (29) Loquet, A.; Habenstein, B.; Lange, A. Structural Investigations of Molecular Machines by Solid-State NMR. Acc. Chem. Res. 2013, 46, 2070−2079. (30) Tang, M.; Comellas, G.; Rienstra, C. M. Advanced Solid-State NMR Approaches for Structure Determination of Membrane Proteins and Amyloid Fibrils. Acc. Chem. Res. 2013, 46, 2080−2088. (31) Asami, S.; Reif, B. Proton-detected solid-state NMR spectroscopy at aliphatic sites: application to crystalline systems. Acc. Chem. Res. 2013, 46, 2089−2097. (32) Opella, S. J. Structure Determination of Membrane Proteins in Their Native Phospholipid Bilayer Environment by Rotationally Aligned Solid-State NMR Spectroscopy. Acc. Chem. Res. 2013, 46, 2145−2153. (33) Murray, D. T.; Das, N.; Cross, T. A. Solid State NMR Strategy for Characterizing Native Membrane Protein Structures. Acc. Chem. Res. 2013, 46, 2172−2181. (34) Ding, Y.; Yao, Y.; Marassi, F. M. Membrane Protein Structure Determination in Membrana. Acc. Chem. Res. 2013, 46, 2182−2190. (35) Krushelnitsky, A.; Reichert, D.; Saalwächter, K. Solid-State NMR Approaches to Internal Dynamics of Proteins: From Picoseconds to Microseconds and Seconds. Acc. Chem. Res. 2013, 46, 2028−2036. (36) Knauer, B. R.; Napier, J. J. The nitrogen hyperfine splitting constant of the nitroxide functional group as a solvent polarity parameter. The relative importance for a solvent polarity parameter of its being a cybotactic probe vs. its being a model process. J. Am. Chem. Soc. 1976, 98, 4395−4400.

and the Johannes Gutenberg-Universität in Mainz in 1975−1978. He was Professor of Physical Chemistry at this institution in 1978−1980, the Westfälische Wilhelms-Universität Münster in 1981−1982, and a Professor of Macromolecular Chemistry at the Universität Bayreuth in 1983−1984. He was then subsequently appointed by the Max Planck Gesellschaft zur Förderung der Wissenschaften e. V. München as Director at the Max-Planck-Institut für Polymerforschung in Mainz in 1984, a year after this Institute was founded. He held this position until his retirement in the fall of 2012. He headed the Department for Polymer Spectroscopy with emphasis on solid-state nuclear magnetic resonance and pulsed electron paramagnetic (electron spin) resonance to study the structure and dynamics of macromolecular and supramolecular functional systems.

ACKNOWLEDGMENTS This Review is based on our recent work at the Max-PlanckInstitute for Polymer Research in Mainz, Germany. We would like to thank our students, colleagues, and co-workers for all their contributions. Special thanks are due to Dariush Hinderberger for his suggestions concerning the EPR studies. Our research was supported in particular by the Deutsche Forschungsgemeinschaft (SFB 625 and SPP 1369). M.R.H. acknowledges the Villum Foundation, Denmark, under the Young Investigator Program (VKR023122). REFERENCES (1) In Encyclopedia of Materials: Science and Technology; Buschow, K. H. J., Cahn, R. W., Flemings, M. C., Ilschner, B., Kramer, E. J., Mahajan, S., Eds.; Elsevier: Amsterdam, 2001. (2) In Macromolecular Engineering: Precise Synthesis, Materials Properties, Applications; Matyjaszewski, K., Gnanou, Y., Leibler, L., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2007. (3) Lehn, J.-M. Toward Self-Organization and Complex Matter. Science 2002, 295, 2400−2403. (4) Ulman, A. Formation and Structure of Self-Assembled Monolayers. Chem. Rev. 1996, 96, 1533−1554. (5) O’Keeffe, M.; Yaghi, O. M. Deconstructing the Crystal Structures of Metal−Organic Frameworks and Related Materials into Their Underlying Nets. Chem. Rev. 2012, 112, 675−702. (6) Spiess, H. W. Interplay of Structure and Dynamics in Macromolecular and Supramolecular Systems. Macromolecules 2010, 43, 5479−5491. (7) Schmidt-Rohr, K.; Spiess, H. W. Multidimensional Solid-State NMR and Polymers; Academic Press: Waltham, MA, 1994. (8) Brown, S. P.; Spiess, H. W. Advanced solid-state NMR methods for the elucidation of structure and dynamics of molecular, macromolecular, and supramolecular systems. Chem. Rev. 2001, 101, 4125−4155. (9) Emsley, L. Advances in Magnetic Resonance: From Stem Cells to Catalytic Surfaces. J. Am. Chem. Soc. 2013, 135, 8089−8091. (10) Sebastiani, D. Ab-initio calculations of NMR parameters in condensed phases. Mod. Phys. Lett. B 2003, 17, 1301−1319. (11) Bonhomme, C.; Gervais, C.; Babonneau, F.; Coelho, C.; Pourpoint, F.; Azais, T.; Ashbrook, S. E.; Griffin, J. M.; Yates, J. R.; Mauri, F.; et al. First-Principles Calculation of NMR Parameters Using the Gauge Including Projector Augmented Wave Method: A Chemist’s Point of View. Chem. Rev. 2012, 112, 5733−5779. (12) Berliner, L.; Reuben, J. Spin Labeling: Theory and Applications; Springer: New York, 2012. (13) Hubbell, W. L.; Cafiso, D. S.; Altenbach, C. Identifying conformational changes with site-directed spin labeling. Nat. Struct. Biol. 2000, 7, 735−739. (14) Schweiger, A.; Jeschke, G. Principles of Pulse Electron Paramagnetic Resonance; Oxford University Press: Oxford, U.K., 2001. 1299


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