Supramolecular Orientational Memory - American Chemical Society

Dec 30, 2016 - Bisimide Columns via Supramolecular. Orientational Memory. Dipankar Sahoo,. †. Mihai Peterca,. †,‡. Emad Aqad,. †. Benjamin E. ...
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Tetrahedral Arrangements of Perylene Bisimide Columns via Supramolecular Orientational Memory Dipankar Sahoo,† Mihai Peterca,†,‡ Emad Aqad,† Benjamin E. Partridge,† Paul A. Heiney,‡ Robert Graf,§ Hans W. Spiess,§ Xiangbing Zeng,∥ and Virgil Percec*,† †

Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States ‡ Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6396, United States § Max-Planck Institute for Polymer Research, Mainz 55128, Germany ∥ Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom ABSTRACT: Chiral, shape, and liquid crystalline memory effects are well-known to produce commercial macroscopic materials with important applications as springs, sensors, displays, and memory devices. A supramolecular orientational memory effect that provides complex nanoscale arrangements was only recently reported. This supramolecular orientational memory was demonstrated to preserve the molecular orientation and packing within supramolecular units of a selfassembling cyclotriveratrylene crown at the nanoscale upon transition between its columnar hexagonal and Pm3̅n cubic periodic arrays. Here we report the discovery of supramolecular orientational memory in a dendronized perylene bisimide (G2-PBI) that self-assembles into tetrameric crowns and subsequently self-organizes into supramolecular columns and spheres. This supramolecular orientation memory upon transition between columnar hexagonal and body-centered cubic (BCC) mesophases preserves the 3-fold cubic [111] orientations rather than the 4-fold [100] axes, generating an unusual tetrahedral arrangement of supramolecular columns. These results indicate that the supramolecular orientational memory concept may be general for periodic arrays of self-assembling dendrons and dendrimers as well as for other periodic and quasiperiodic nanoscale organizations comprising supramolecular spheres, generated from other organized complex soft matter including block copolymers and surfactants. KEYWORDS: supramolecular orientational memory, tetrahedral, oriented columns, perylene bisimide spheres, body-centered cubic, self-assembly

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the cubic phase generated an orthogonal arrangement of columnar hexagonal domains. The supramolecular columns in these domains were oriented according to the 4-fold symmetric [100]cub directions of the preceding cubic phase, parallel to the edges of the cube. This supramolecular orientational memory effect was driven by the formation of supramolecular spheres and the preservation of their orientation and molecular packing upon transition between the columnar hexagonal and cubic phases. The columnar character of the tetrahedrally distorted spheres from the faces of the Pm3̅n cubic phase was most probably responsible for this supramolecular orientational memory effect. Additional examples of supramolecular orientational

emory effects correlate a past state or event with a current state.1 Their application in chemical systems has provided functional materials, including thermally responsive ceramics,2,3 alloys,4−8 actuators,9,10 sensors,11 information carriers,12 and biomedically relevant plastics13−17 via shape memory18,19 as well as ubiquitous liquid crystal displays20,21 via orientational memory.22 The utility of these materials relies on the preservation of macroscopic structural information from one periodic array or state to another. A supramolecular orientational memory effect, in which structural information was preserved from one nanoscale periodic array to another, was recently discovered23 in a selfassembling dendronized cyclotriveratrylene crown (CTV). Heating an aligned columnar hexagonal phase self-organized from dendronized CTV formed a preferentially oriented cubic Pm3̅n phase, known also as Frank−Kasper A15. Cooling from © 2016 American Chemical Society

Received: November 10, 2016 Accepted: December 30, 2016 Published: December 30, 2016 983

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copolymers,30−34 surfactants,35−37 and other soft matter,38,39 including Im3̅m body-centered cubic (BCC),40,41 P42/mnm tetragonal,42 and 12-fold liquid quasicrystalline (LQC).43,44 Here it is demonstrated that a perylene bisimide functionalized with second-generation self-assembling dendrons (G2-PBI) able to self-organize into supramolecular spheres exhibits a supramolecular orientational memory effect at the transition from its oriented columnar hexagonal to its BCC phase and back to its columnar hexagonal periodic arrays. Like the supramolecular orientational memory effect observed at the transition between the columnar hexagonal and the Pm3n̅ lattice of CTV, cubic axes are preserved upon cooling to the columnar hexagonal phase. However, a different epitaxial relationship operates in the BCC lattice of G2-PBI compared to the Pm3n̅ lattice of CTV. The resultant nanoscale architecture represents an unusual tetrahedral morphology of nanoscale columns accessible via this supramolecular orientational memory.

RESULTS AND DISCUSSION Thermal, Structural, and Retrostructural Analysis by Differential Scanning Calorimetry (DSC) and X-ray Diffraction (XRD). The synthesis and structural and retrostructural analysis45,46 of G2-PBI were recently reported.47 As demonstrated previously, G2-PBI (Figure 1) self-organizes into four periodic arrays: a 3D crystalline columnar hexagonal phase (Φhk), two 2D columnar hexagonal arrays with intracolumnar order (Φhio1 and Φhio2), and a body-centered cubic (BCC) phase. It was previously shown that G2-PBI self-organizes into its four periodic arrays via three different tetrameric crowns (TCs)

Figure 1. Structure (a) and DSC traces (b) of G2-PBI. DSC traces were recorded with heating and cooling rates of 10 °C/min. Phases (defined in main text) determined by XRD, transition temperatures (in °C), and associated enthalpy changes (in parentheses in kcal/mol) are indicated.

memory were expected in other 3D phases generated from supramolecular spheres of self-assembling dendrimers,24−29 block

Figure 2. Schematic models of the four phases of G2-PBI. Phase notation: Φhk, 3D crystalline columnar hexagonal (P6mm) phase, comprising columns of asymmetric TCs; Φhio1 , low-temperature 2D liquid crystalline columnar hexagonal (P6mm) phase with intracolumnar order, comprising columns of symmetric TCs; Φhio2, high-temperature 2D liquid crystalline columnar hexagonal (P6mm) phase with intracolumnar order, comprising columns of supramolecular spheres assembled from symmetric and inverted TCs; BCC, body-centered cubic (Im3m ̅ ) periodic array generated from supramolecular spheres assembled from symmetric and inverted TCs. Transition temperatures determined by DSC23 upon first heating and first cooling at 10 °C/min are shown at the top. 984

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Figure 3. XRD patterns showing orientational epitaxy in ordered hexagonal phases upon cooling from the BCC phase. (a−d) Experimental IAXS patterns of the (a) Φhk, (b) Φhio1, (c) Φhio2, and (d) BCC phases collected from an oriented fiber upon first heating. (e−g) Experimental patterns of the (e) Φhk, (f) Φhio1, and (g) Φhio2 phases collected from the same fiber upon first cooling. Note the 6-fold symmetry of the patterns collected upon cooling. In (f), weak (110) and (210) features are highlighted by broken white lines. Temperature, phase, lattice parameters, and fiber axes are indicated. (h) Simulation of scattering from a columnar hexagonal liquid crystalline lattice with column c-axes ([001]hex directions) aligned along the body diagonals ([111]cub directions) of the BCC phase.

fiber (Figure 4a).50 The d-spacings observed in the XRD patterns upon cooling are identical to those observed upon heating, indicating that the same structural phase is present under both conditions. However, the 6-fold symmetry of the patterns in Figure 3e−g indicates that the supramolecular columns in the Φhk, Φhio1, and Φhio2 phases obtained upon cooling are not all aligned with the original fiber axis. The observation of well-defined orientations of the Bragg spots, including the original orientation of the hexagonal reflections, indicates that the system retains a memory of the original orientations, preserved via the BCC phase. This orientational relationship, known also as “epitaxy”, involves “the preservation of certain crystallographic directions upon an indicated phase transition”, in this case from the BCC to the Φh phase.23,51−55 The 6-fold symmetry in Figure 3e−g indicates that the Φhk, Φhio1, and Φhio2 phases preserve the orientational arrangement of the preceding BCC phase (Figure 3d). An orientational relationship between two phases may be dictated by one of two features: preservation of an aspect of the molecular arrangement or preservation of some feature of the lattice symmetry. The BCC phase can be considered a special case of the columnar hexagonal lattice,47 via a superlattice in which there are three types of columns comprising supramolecular spheres displaced along the column c-axis, or [001]hex direction, by 0, c/3, and 2c/3 respectively (as for Φhio2, Figure 4a).47 The BCC unit cell is then superimposed upon the Φh lattice such that the body diagonal of the cubic unit cell (the [111]cub direction) is aligned along the column axis ([001]hex, Figure 4b). Mechanism of Supramolecular Orientational Memory. Figure 5 illustrates the axes in the hexagonal and cubic phases

(Figure 2).23 These TCs are formed from two dimers of coplanar G2-PBI molecules stacked with an interdimer rotation of 90° and an interdimer distance of 4.8 Å (Figure 2). Below 7 °C, the two dimers of the TC are displaced such that their C2-axes of symmetry are not colinear. These asymmetric TCs stack to form a 3D crystalline columnar hexagonal phase (Φhk). Above 7 °C, the molecules reorganize to form symmetric TCs that stack into supramolecular columns arranged in a 2D columnar hexagonal array with intracolumnar order, denoted Φhio1. Heating above 125 °C allows inversion of some of the TCs to generate approximately spherical supramolecular units (Figure 2). These spherical units self-organize into both a columnar phase, in which the spheres are orientationally disordered but remain stacked into supramolecular columns, denoted Φhio2, and a body-centered cubic BCC phase above 158 °C, in which the spheres are orientationally disordered and adopt cubic order. The core of the supramolecular spheres of G2-PBI is harder than that of the previously reported dendronized CTV23 which may explain why packing into an Im3̅m BCC phase rather than a Pm3̅n phase is favored.48 BCC phases are rarely observed for self-assembling dendrons40 but are commonly encountered in block copolymers.33,49 Orientational Relationship Observed in XRD Patterns of Oriented Fibers. X-ray diffraction (XRD) patterns of oriented fibers of the Φhk, Φhio1, Φhio2, and BCC phases of G2-PBI were recorded during first heating47 (Figure 3a−d) and first cooling (Figure 3e−g). The direction of the column c-axis, or [001]hex direction, in the hexagonal Φhk, Φhio1, and Φhio2 phases observed during heating is determined by the direction in which the fiber is extruded: the [001]hex axis is aligned with the long axis of the 985

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When the BCC phase reverts to a columnar hexagonal phase upon cooling, each of these [111]cub directions, the one along the fiber axis and also those oriented tetrahedrally about it, forms the possible basis for a new columnar c-axis ([001]hex). Hence in the reformed Φhio2 array one of the [001]hex directions is aligned with the old fiber axis direction, and the other new [001]hex directions lie in a cone of randomly oriented new orientations, with the opening angle of the cone equal to the 109.5° angle found in a tetrahedron (Figure 5a, right). There are therefore a large number of directions for the (100) and (110) diffractions. However, numerical simulation (Figure 3h) shows that the intensity of the primary diffraction is concentrated at 60° intervals around the azimuth, resulting in a 6-fold symmetric pattern as observed experimentally (Figure 3e−g). The numerical simulation also reproduces the diffuse scattering due to the increased positional disorder of the spheres in the Φhio2 phase. This phenomenon, in which the orientation of axes is preserved during a phase transition between supramolecular assemblies, was recently discovered in a CTV derivative and termed supramolecular orientational memory.23 Persistence of Preferred Orientations upon Subsequent Thermal Treatment. This supramolecular orientational memory effect occurs upon every phase transition between the cubic BCC and columnar hexagonal Φhio2 phases. Hence the [001]hex direction of each columnar hexagonal monodomain generated upon first cooling from the BCC to the Φhio2 phase can become the [111]cub direction of a cubic monodomain upon subsequent heating. Upon cooling, each of these [111]cub directions can become a new [001]hex direction. Thus, the number of possible orientations of the columnar hexagonal monodomains increases exponentially as a function of the number of heating and cooling cycles, manifested in XRD patterns as a subtle broadening of the diffraction features recorded during second and third heating and cooling (Figure 6). However, even after three heating and cooling cycles, XRD patterns clearly show distinct columnar hexagonal and cubic phases. Similarly, fibers of a CTV derivative exhibiting supramolecular orientational memory maintained their preferred orientation after cycling six times between the columnar hexagonal and cubic phases.23 The preferred orientations of the columnar hexagonal and cubic phases can be erased by melting the oriented fiber into the isotropic state. In an isotropic melt, all preferred orientations are eliminated, and the sample generates an isotropic distribution of cubic monodomains upon cooling to the BCC phase. Comparison of Supramolecular Orientational Memory in Dendronized PBI and CTV. G2-PBI provides the second example of a system which exhibits a supramolecular orientational memory effect, the other being a CTV derivative reported recently.23 In both systems, supramolecular orientational memory is observed upon transition between a columnar hexagonal phase and a cubic phase generated from spheres, and hence G2-PBI and the CTV derivative must be able to assemble into both columnar and spherical supramolecular objects. This is achieved in both systems via crown-like molecular conformations with strongly interacting cores. For G2-PBI, the second-generation dendron attached to the bisimide is sufficiently flexible to allow reorganization into a sphere,47 whereas bowl-to-bowl inversion is utilized by CTV to generate a spherical structure.23,28 Supramolecular orientational memory has been observed in two different cubic phases: BCC (Im3̅m) for G2-PBI,47 and

Figure 4. Schematic depiction of a BCC lattice treated as a √3 × √3 hexagonal lattice (Φhio phase). The “spheres” are the 12-molecule subunits shown in Figure 2. The distance between spheres in each column is the hexagonal c-parameter. The cubic lattice a-parameter is the distance between identical columns, a = c·√(8/3). Spheres within columns 1, 2, and 3 are displaced along the column axis by z = 0, z = c/3, and z = 2c/3, respectively.

and their persistence through thermal transitions. The direction of the [001]hex axis in the hexagonal Φhk, Φhio1, and Φhio2 phases observed upon heating is determined by the direction in which the fiber is extruded: The [001]hex axis is aligned with the long axis of the fiber (Figure 5a).50 This axis has 6-fold symmetry in a hexagonal lattice. As discussed (Figure 4), the BCC phase can be considered as a special case of a columnar hexagonal lattice, with the [001]hex axis corresponding to a [111]cub axis. Thus, when the BCC phase is formed upon heating, one of its [111]cub axes is aligned along the fiber axis, and the heights of the spheres within the columns develop intercolumnar order such that the hexagonal superlattice is formed. A BCC phase has four different [111]cub directions at 109.5° angles56 to each other, similar to the tetrahedral arrangement of C−H bonds in methane. One of these is oriented along the fiber axis, while the others are randomly pivoted about that axis (Figure 5a, center). 986

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Figure 5. Orientation epitaxy of the assemblies generated by G2-PBI. (a) Schematic representation of crystallographic axes in the hexagonal Φhio2 and cubic BCC phases. (b) Crystallographic axes from panel (a) overlaid on supramolecular structure. (c) Supramolecular columns packed into hexagonal domains (left) are defined by supramolecular spheres (center). Upon cooling, hexagonal domains are formed with their c-axes ([001]hex) aligned with [111]cub. The schematics in (c) represent idealized domains and do not account for disorder present in the physical system.

Pm3n̅ for recently reported CTV.23 In the supramolecular assembly from that CTV derivative, there were two possible orientational relationships between the columnar hexagonal Φhio and cubic Pm3̅n phases: [001]hex to [111]cub, which would maintain symmetry features of the lattice, and the observed [001]hex to [200]cub, which preserves the structure of supramolecular spheres. Previous reports of Pm3̅n phases were dominated by lattice symmetry and exhibited [001]hex to [111]cub epitaxy,51−55 whereas the CTV derivative maintained molecular packing within the supramolecular sphere via a [001]hex to [200]cub epitaxial relationship.23 This epitaxial relationship exploits the continuous columnar character of the supramolecular spheres from the faces of the Pm3̅n unit cell.25,27 In a Φhio to BCC system, a [001]hex to [111]cub relationship maintains both lattice symmetry and local structural

arrangements and hence is observed for the assemblies of G2-PBI. Furthermore, it has been shown that the supramolecular spheres of a BCC lattice are distorted such that continuous columnar character exists along the body diagonals ([111]cub directions).41 The resultant architecture comprising columnar hexagonal domains oriented in tetrahedral directions (Figure 5c, right) is an unusual morphology, distinct from the orthogonal orientation observed in the CTV derivatives.23 Neither of these two complex nanoscale periodic arrangements can be generated by any other means. Additional experiments are required to ascertain whether the nature of the continuous columnar character of some selected spheres from the cubic lattice is the key molecular determinant of the orientational relationship observed in systems exhibiting supramolecular orientational memory. 987

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Figure 6. Experimental IAXS patterns of the Φhk, Φhio1, Φhio2, and BCC phases collected from an oriented fiber upon (a) second heating, (b) second cooling, (c) third heating, and (d) third cooling. Temperature, phase, and fiber axes are indicated.

CONCLUSIONS Supramolecular orientational memory has been demonstrated in the Φhk, Φhio1, Φhio2, and cubic BCC nanoscale periodic arrays of G2-PBI generated via the hierarchical organization of TCs, columns, spheres, and columns from spheres. Preferentially oriented columns in three Φh phases transform via supramolecular spheres into an oriented cubic BCC phase. Upon cooling, the BCC phase generates a complex tetrahedral nanoscale architecture of columnar hexagonal domains oriented per the body diagonals of a cube (the [111]cub directions). G2-PBI exhibits a [001]hex to [111]cub epitaxy for a BCC phase, distinct from the [001]hex to [200]cub epitaxy observed for the cubic Pm3̅n phase of dendronized CTV.23 However, both systems comprise conformationally flexible molecular

building blocks that self-organize into supramolecular spheres which maintain their molecular packing during phase transition. This preservation of the supramolecular unit forms the basis for the supramolecular orientational memory observed for G2-PBI assemblies and for dendronized CTV23 and is expected to be discovered in related 3D phases self-organized from supramolecular spheres and columns of a diversity of complex soft matter.30−39

METHODS Synthesis of G2-PBI. The synthesis of G2-PBI was reported previously.47 The purity and the structural identity of the intermediary and final products were assessed by a combination of techniques including thin-layer chromatography, high-pressure liquid chromatography, 988

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H and 13C NMR, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Differential Scanning Calorimetry (DSC). Thermal transitions were measured on TA Instruments Q100 differential scanning calorimeter (DSC) integrated with a refrigerated cooling system (RCS). Heating and cooling rates are indicated in Figure 1. The transition temperatures were measured as the maxima and minima of their endothermic and exothermic peaks, respectively. Indium was used as the standard for the calibration. X-ray Diffraction (XRD). X-ray diffraction (XRD) measurements were performed using Cu−Kα1 radiation (λ = 1.542 Å) from a BrukerNonius FR-591 rotating anode X-ray source equipped with a 0.2 × 0.2 mm2 filament and operated at 3.4 kW. Osmic Max-Flux optics and triple pinhole collimation were used to obtain a highly collimated beam with a 0.3 × 0.3 mm2 spot on a Bruker-AXS Hi-Star multiwire area detector. To minimize attenuation and background scattering, an integral vacuum was maintained along the length of the flight tube and within the sample chamber. Samples were held in glass capillaries (1.0 mm in diameter), mounted in a temperature-controlled oven (temperature precision: ± 0.1 °C, temperature range from −10 to 210 °C). All XRD measurements were performed with the aligned sample axis perpendicular to the beam direction. Structural and Retrostructural Analysis. Structural and retrostructural analysis23 was performed via a methodology developed by our laboratory.45,46 This methodology involves reconstruction of electron density maps from XRD,24,25,27,57 generation of molecular models and simulation of their XRD patterns,50,58 and iterative improvement of the molecular model based on comparison of the simulated and experimental XRD patterns.58 Briefly, XRD patterns were measured of oriented fiber59 and powder samples. Diffraction features and lattice parameters were analyzed using Datasqueeze (version 3.0.5).60 Electron density maps were reconstructed from XRD diffraction features, and the phases of the diffraction peaks were assigned to maximize the segregation of high and low electron density regions within the supramolecular arrays.24,57,61 Initial molecular models and supramolecular models accounting for this microphase segregation were created using Accelrys Materials Studio (versions 3.1 and 5.0). Geometry optimization of the modeled molecule and subsequent supramolecular structure was performed using the VAMP module. The XRD pattern expected from this model was simulated using Accelrys Cerius.2 The simulated and experimental XRD patterns were compared, and the supramolecular model refined. This process of XRD simulation and model refinement was repeated iteratively until the level of agreement between simulated and experimental XRD patterns was acceptable.

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Mihai Peterca: 0000-0002-7247-4008 Benjamin E. Partridge: 0000-0003-2359-1280 Virgil Percec: 0000-0001-5926-0489 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Financial support by the National Science Foundation (DMR1066116 (V.P.), DMR-1120901 (V.P. and P.A.H.)), the Humboldt Foundation (V.P.), and the P. Roy Vagelos Chair at Penn (V.P.) is gratefully acknowledged. X.Z. acknowledges support from the joint NSF-EPSRC PIRE project “RENEW” (EPSRC grant EP-K034308). B.E.P. thanks the Howard Hughes Medical Institute for an International Student Research Fellowship. 989

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DOI: 10.1021/acsnano.6b07599 ACS Nano 2017, 11, 983−991

Article

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DOI: 10.1021/acsnano.6b07599 ACS Nano 2017, 11, 983−991