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Pyrene-Based Asymmetric Supramolecule: Kinetically Controlled Polymorphic Superstructures by Molecular Self-Assembly Minwook Park, Won-Jin Yoon, Dae-Yoon Kim, Yu-Jin Choi, Jahyeon Koo, Seok-In Lim, Dong-Gue Kang, Chih-Hao Hsu, and Kwang-Un Jeong Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01688 • Publication Date (Web): 13 Feb 2017 Downloaded from http://pubs.acs.org on February 15, 2017

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Crystal Growth & Design

Pyrene-Based Asymmetric Supramolecule: Kinetically Controlled Polymorphic Superstructures by Molecular Self-Assembly Minwook Park,† Won-Jin Yoon,† Dae-Yoon Kim,† Yu-Jin Choi,† Jahyeon Koo,† Seok-In Lim,† Dong-Gue Kang,† Chih-Hao Hsu‡ and Kwang-Un Jeong†* †

BK21 Plus Haptic Polymer Composite Research Team & Department of Polymer-Nano Science and Technology, Chonbuk National University, Jeonju, Jeonbuk 54896, Korea ‡

Department of Polymer Science, The University of Akron, Akron, OH 44325, USA

KEYWORDS: polymorphism, metastable phase, self-assembly, supramolecule, monotropic phase transition.

ABSTRACT

To understand the kinetically controlled polymorphic superstructures of asymmetric supramolecules, a pyrene-based asymmetric supramolecule (abbreviated as Py3M) was newly synthesized by connecting two pyrene head groups (Py) to a biphenyl-based dendritic tail (3M) with an isophthalamide connecter. Based on thermal, microscopic, spectroscopic and scattering results, it was realized that Py3M exhibited the monotropic phase transition between a stable crystalline phase (K1) and a metastable crystalline phase (K2). This monotropic phase transition behavior was mainly originated from the competitions of intra- and inter-molecular interactions

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(π-π interactions and hydrogen-bonds) as well as from the nanophase separations. From the twodimensional (2D) wide-angle X-ray diffraction (WAXD) patterns and transmission electron microscopy (TEM) images of the self-assembled Py3M superstructures, it was found that Py3M formed two synclinically tilted crystalline superstructures: the 6.75 nm and 4.4 nm periodicities of layered structures for K1 and K2 phases, respectively. The stable K1 phase was predominantly induced by the π-π interactions between pyrenes, while the inter-molecular hydrogen-bonds between isophthalamides were the main driving forces for the formation of metastable K2 phase. Ultraviolet-visible (UV-Vis) and photoluminescence (PL) experiments indicated that the photophysical properties of Py3M were directly related with their molecular packing superstructures.


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INTRODUCTION Material scientists and engineers in supramolecular chemistry have tried to find the solid relationships between targeted physical properties and molecular packing superstructures by manipulating noncovalent interactions between molecular building blocks. For the construction of well-defined hierarchical superstructures on the different length scales, molecular building blocks should be smartly programmed and precisely synthesized by utilizing physical interactions including hydrogen (H)-bond, van der Waals force, π-π interaction, and electrostatic interaction. Among various physical interactions, inter- and intra-molecular H-bonds often serve as useful tools to construct the designed superstructures due to mild physical bonding energy, good

selectivity,

directionality

and

reversibility.1-5

Additionally,

the

self-assembled

superstructures can be finely tuned with the help of nanophase separations and inter-molecular interactions. The well-defined hierarchical superstructures have been applied in organic sensors, optical devices, and micro-reactors.9-14 Polyaromatic molecules, such as pyrene,18-20 perylene21 and anthracene,22 have been intensively studied as potential fluorescent materials because of their high quantum yield and high photoluminescence (PL) efficiency. Especially, pyrene-based molecules exhibit remarkable photo-physical properties, such as long fluorescence lifetime, high PL efficiency and efficient excimer formation. Note that pyrene excimers are short-lived dimeric or heterodimeric molecules which are associated in excited electronic states but dissociated in their ground electronic states. Its monomeric fluorescence is characterized as distinguishable spectra at 375, 379, 385, 395, and 410 nm, whereas a broad and unstructured band ranging from 430 nm to 550 nm corresponds to the excited state excimer fluorescence. Quenched fluorescence of monomer originated from the favorable π-π stackings between pyrene molecules is inversely proportional to the excimer


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fluorescence at longer wavelengths.25-27 Even though a pyrene-based molecule has a higher charge-carrier mobility and a lower ionization potential than other chromophores, it is commonly unfit for use as a light emitting material because of its high tendency of crystallization.28 However, pyrene-based molecules have been often utilized as a fluorescence probe to obtain structural and dynamic information of synthetic polymers, DNA, and multi-molecular assemblies.29-31 To understand the photo-physical properties of hierarchical superstructures selfassembled by competitive noncovalent interactions between H-bonding interactions and π-π interactions, we systematically investigated the newly synthesized pyrene-based asymmetric supramolecule (abbreviated as Py3M). The Py3M molecule was prepared by chemically connecting two pyrene head groups (Py) to a biphenyl-based dendritic tail (3M) with an isophthalamide connecter. The mesogenic moieties in the 3M dendron were located between hydrophilic ethylene oxide chains and hydrophobic alkyl tails to enhance the nanophase separations. Two different kinetic packing processes driven by distinct secondary interactions make the asymmetrical Py3M molecules to form symmetrical dimeric building blocks for the formation of metastable and stable bilayered superstructures, as shown in Scheme 1. On the basis of combined techniques of differential scanning calorimetry (DSC), cross-polarized optical microscopy (POM), Fourier transform infrared (FT IR) spectroscopy and wide-angle X-ray diffractions (WAXD), phase transition behaviors and hierarchical superstructures of Py3M were systematically investigated. Utilizing ultraviolet-visible (UV-Vis) and photoluminescence (PL) experiments, photo-physical properties of Py3M were further studied by relating to its molecular packing superstructures.


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RESULTS AND DISCUSSIONS Chemical structure and energy-minimized molecular conformation of Py3M A pyrene-based asymmetric supramolecule (Py3M) containing two pyrene head groups (Py) and a biphenyl-based dendron (3M) is newly synthesized as illustrated in Figure 1. The competition of physical interactions between π-π interaction and H-bonding is introduced by chemically attaching two pyrene groups to the central isophtalate core with three amide functions. Detail synthetic route of Py3M is also described in Figure S1 and S2. Chemical structure and purity of Py3M are confirmed by 1H-NMR spectroscopy (Figure S3 - S10) in addition to MALDI-ToF mass spectroscopy (Figure S11). Expected molecular weight of Py3M is 1997.49 g/mol, and the measured molecular weight of Py3M is 2019.57 g/mol [M+Cu+]. Utilizing a Cerius2 computer simulation program from Accelrys, the energy-minimized geometric dimension of Py3M in the isolated gas phase is estimated (Figure S12). In the side view, the length of energy-minimized Py3M is around 5 nm along the long axis.

Monotropic phase transition behaviors of Py3M Since Py3M reveals a 5% weight loss at 404 °C during thermogravimetric analysis (TGA) under N2 atmosphere (Figure. S13), Py3M is expected to be thermally stable in the experimental temperature window (25 to 250 °C) of this research without any thermal degradations. As represented in Figure 2, the DSC experiment is first conducted to determine the thermal transition temperatures and associated enthalpy changes. During cooling at a faster rate than -5 °C/min, two obvious exothermic thermal transitions are detected. Upon increasing the cooling


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rate, the onset temperatures of high-temperature broad peak and low-temperature sharp peak shift to lower temperatures and their released heats (∆H) are reduced. The high-temperature transition temperature shifts more rapidly than the low-temperature one, so that two transitions start to merge at -20 °C/min, as shown in Figure 2. The transition-temperature dependence of the cooling rate often indicates that the exothermic transition is related with a crystallization process.32,33 Upon cooling at -5 °C/min, the onset temperatures of two exothermic transitions are 169.4 °C and 200 °C. However, only a single exothermic peak is observed at -2.5 °C/min due to the sufficient time of Py3M to be organized in a stable crystalline phase (K1). During heating at 20 °C/min, the onset temperature of the endothermic transitions and the exothermic transition are 187.6 °C (2.93 J/g), 199.6 °C (32.23 J/g) and 196.7 °C (-2.85 J/g) by turns. The first endothermic transition and subsequent exothermic transition could be attributable to the development of less stable structure during a fast cooling process at -20 °C/min. This explanation can be supported by considering the fact that the heats of transitions during the organization process (-31.4 J/g + 2.85 J/g = -34.25 J/g) is almost identical to the summation of heats during the melting process (2.93 J/g + 32.23 J/g = 35.16 J/g). Therefore, there are two ordered phases (K1 and K2) for the Py3M compound, and it apparently exhibits a monotropic phase transition behavior.34-37 Although DSC technique can provide quantitative thermodynamic properties, it does not afford direct information regarding to the molecular packing structures. The formations of ordered phases are monitored under polarized optical microscopy (POM), which can provide morphological information on the micrometer length scale depending on phases. As shown in Figure 3, a fissured crystal texture with strong birefringence is observed at 180 and 170 °C (Figure 3a, b) after cooling from the isotropic (Iso) state at -2.5 °C/min, whereas two different crystals coexisted at 150 °C (Figure 3d) from partially isotropic state at 180 °C (Figure 3c)


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during the 5 °C/min cooling process. The dark crystal morphology with weak birefringence shows smooth boundaries of micro-domains. This result indicates that the dark crystal phase is less stable than bright one. The slow cooling and annealing process at high temperature (above 175 °C during cooling, as shown in Figure S14) generates spherulites (stable K1 phase), while the fast cooling forms the metastable K2 phase. From the kinetic viewpoint, the kinetically controlled polymorphic superstructures of Py3M should be related with the competitions between inter-molecular physical interactions during the molecular self-assembly processes.

Molecular origins for the monotropic phase transition of Py3M Possible molecular origins for the monotropic phase transition of Py3M are investigated using FT IR spectroscopy in the range of 40 - 220 °C. Figure 4 presents a set of FT IR spectra of Py3M at different temperatures. The N-H stretching vibration of amide groups exhibits a broad absorption band with the maximum intensity at 3282 cm-1 at 40 °C, and the amide I, II, and III motions generates absorption bands at 1641, 1555, and 1233 cm-1, respectively.38,39 By detecting the N-H stretching vibration band at 3282 cm-1 and the C=O stretching vibration at 1641 cm-1, the H-bonding between the N-H and the C=O groups can be monitored. The temperature-varied FT IR spectra in Figure 4 indicate that the N-H···O=C H-bond is formed at 40 °C. Two different spectral contribution bands are related to the N-H stretching mode between 3200 and 3500 cm-1. The relatively weak and narrow band at 3445 cm-1 is originated from the free N-H groups, while the other broadband between 3200 and 3400 cm-1 is assigned to the Hbonded N-H groups. The I mode is sensitive to the H-bonds as well as to the local conformational changes. The strong absorption peak at 1650 cm-1 is mainly due to the carbonyl


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band. A narrow band between 1620 and 1650 cm-1 with a width at half-height of 22 cm-1 (at 40 °C) is originated from the H-bonded C=O groups in ordered domains, while an undistinguishable broad band between 1640 and 1670 cm-1 comes from the H-bonded C=O groups in disordered domains. An absorption band between 1660 and 1690 cm-1 with a width at half-height of 28 cm-1 (at 220 °C) is due to the free C=O groups. During heating, the absorption intensity of free N-H groups at 3445 cm-1 becomes stronger and the area of H-bonded N-H groups is concomitantly decreased with the gradual shift of maximum intensity from 3282 to 3344 cm-1. Similarly, the intensity and area of free C=O band in the amide I mode grow with the gradational shift of maximum intensity from 1641 to 1672 cm-1. According to Figure 4, more than 90% of amide groups form the disordered H-bonds even in the Iso phase. When the temperature is increased to the isotropic transition temperature at 216 °C, the amount of free C=O groups and N-H groups is distinguishable and increased, whereas the intensity and area of bands for H-bonded N-H and Hbonded C=O groups in ordered domains is dropped below 10%. This result coincides well with the endothermic thermal transitions detected in DSC (Figure 2). It is notable that the intensity of absorption bands for H-bonded N-H and C=O groups in ordered domains is abruptly dropped at the isotropization temperature with the absorption intensity increases of free N-H and C=O groups and H-bonded C=O groups in disordered domains. From FT IR data, it is realized that the H-bond is closely attributed to the formation of K1 and K2 crystalline superstructures all over the temperature region. Additionally, three H-bondable amide units in the Py3M core can play a key role to form the macroscopic organogels. Reversible gelation in the organic solvents (cyclohexane) is shown in Figure S15a. The macroscopic gelation indicates the formation of nano-porous morphology, which are confirmed by SEM (Figure S15b).


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To investigate the temperature-dependent structural development of Py3M, 1D WAXD patterns are collected at various temperatures. Figure 5a shows a set of 1D WAXD powder patterns of Py3M during cooling and subsequent heating at the rate of 2.5 °C/min. Molecular packing arrangements can be identified on two different length scales: structures observed on the nanometer length scale can be identified from the diffractions detected between 1.5° and 8°, while the diffractions above 8° provide the information of sub-nanometer structures. As shown in Figure 5a, 1D WAXD patterns show a single-phase transition, which is well matched with the result of DSC (Figure 2). Above 216 °C, Py3M is in the Iso state and exhibits two amorphous halos at 2θ = 2.53° (d = 3.48 nm) and 2θ = 18.92° (d = 0.47 nm). The low-angle halo indicates the average periodicity of electron density fluctuations between nanophase separated rigid parts (i.e. pyrene and isophtalate moieties) and mesogenic side chains, whereas the high-angle halo represents the average distance between the mesogens.40 Below 204 °C, two overlapped low-angle reflections start to emerge and their 2θ angles are identified at 2θ = 2.66° and 4.46° (d = 3.32 and 1.98 nm), respectively. At the same time, the broad diffraction halo involving several peaks between 2θ = 15° and 22° is observed with distinct two diffractions corresponding to the stacked pyrenes from π-π interactions at 2θ = 13.29° and 24.28° (d = 0.66 and 0.37 nm, respectively). As the temperature continuously decreases, a new diffraction peak develops at 2θ = 3.66° (d = 2.41 nm), and then it diminishes in the subsequent heating process. This indicates the facts that the diffraction peak with the dspacing of 2.41 nm is related with the phase transition from the Iso phase to the K2 phase and that the slower cooling (below 2.5 °C/min) and enough annealing processes can form the highly ordered K1 phase. Note that the formation of highly ordered crystalline K1 phase should be originated from the π-π interactions.


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To confirm the role of π-π interaction in the formation of K1 phase and the driving force for developing the K2 phase, 1D WAXD experiments are additionally conducted for the specifically prepared samples. Py3M sample is cooled to room temperature via a quenching process or a slow cooling process, as shown in Figure 5b. The quenched Py3M powder has the diffraction peaks in the low-angle region, exhibiting a long-range ordered structure at d = 4.0 nm, 2.6 nm and 2.0 nm, but it has no diffraction peaks at around 2θ = 13° and 25° corresponding to the peaks from the π-π interactions. At the slow-cooled Py3M powder, whereas, two apparent low-angle diffractions are detected at d = 3.3 nm and 1.8 nm with 2θ = 13.9° and 26.2°. These diffractions confirm again that the K1 phase is formed by the intermolecular π-π stacking between pyrene groups. However, here is still an unanswered question: what is the origin of driving force for the formation of K2 phase? To answer this questions, FT IR spectroscopy is employed again. Figure 6 shows the FT IR spectra of Py3M samples at room temperature, which are prepared by slow cooling at 2.5 °C/min and by quenching from the Iso state, respectively. Two samples show similar spectra which include the absorption bands related with H-bondings: the N-H stretching vibration band at 3315 cm-1, the C=O stretching vibration at 1654 cm-1 and the amide II band at 1557 cm-1. The quenched sample shows the broad H-bonded absorption band and the undistinguishable free N-H groups. Note that the K2 phase comes from a rapid quenching process with the increase of H-bonding. This means that the intermolecular H-bonds strongly affect even for the formation of K2 phase. Figure S16 is the photographs of two Py3M samples fabricated from the different cooling pathways. The quenched sample has a yellowish color and smooth surface, while the slow cooling sample has a constricted glossless surface with a darker color than the quenched one.


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To understand the detail molecular packing structures and symmetry information on the ordered K1 and K2 phases, SAXS and 2D WAXD patterns of uniaxially oriented sample are obtained by radiating X-ray normal to the mechanically oriented Py3M film. Macroscopically oriented Py3M samples are prepared by mechanical shear at 190 °C and then quenched to below 30 °C. The oriented and quenched Py3M sample is further annealed at 100 °C to reach the highly ordered state. Layered superstructures on the nanometer scale are observed between 0° and 8°, while the lateral molecular packing structures of biphenyl mesogens are detected on the subnanometer scale above 10°.41 Figure 7 shows the 2D WAXD pattern of K1 phase mixed with that of K2 phase and its 1D WAXD pattern is also represented in Figure 5b. As shown in Figure 7a, there are three distinct diffractions on the equator below 8° close to the X-ray beam stop, i.e. 2θ = 1.99, 2.64 and 4.69°, with the corresponding d = 4.4, 3.4 and 1.8 nm. From Figure 5b, it is realized that the diffractions with d = 3.4 and 1.8 nm come from the K1 crystalline structure, while the diffraction with d = 4.4 nm is due to the K2 crystalline structure. On the basis of SAXS data (Figure S17), the layered superstructure is confirmed from the observation of diffraction peaks with d = 6.75 and 3.39 nm which can be expressed by q-value ratios of 1:2. In other words, the diffractions with d = 6.75 (Figure S17) and 3.39 nm (its higher order diffraction, at Figure 7a) should be assigned as (001) and (002). On the meridian at Figure 7a, a pair of symmetric strong diffraction arcs related with the π-π interactions between pyrene moieties are detected at 2θ = 25.35° (d = 0.35 nm) in the wide-angle region. Additionally, the diffraction arcs on meridian and equator directions at 2θ = 13.41° (d = 0.65 nm) is identified as regular lateral packing of pyrenes via π-π interactions. Four pairs of relatively diffused diffraction arcs on the quadrants are observed at 2θ = 20.15° (d = 0.44 nm), which are ± 45° and ± 38° away from the meridian. These arcs are generated by the lateral molecular packings of biphenyl groups as well as the


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nanophase separation between ethylene oxide chains and alkyl chains. This result means that biphenyl mesogens are anticlinically or synclinically tilted with respect to the layer normal and it directly affects molecular packing structure such as the layer d-spacing, density and order parameter. However, it is unclear that which arcs is relevant to K1 or K2 phase. This problem will be referred with the correspondence between 2D XRD and TEM results. Note that the 2D WAXD pattern of smectic crystal phase is often obtained from the tilted molecular arrangements in the layered structures. Considering the phase formation, the molecular shape, the molecular length (along the long axis at Figure 1) and the analysis of 2D WAXD, the building block of layered superstructure should be tilted and interdigitated with overlapping of two Py3M molecules along the layer normal direction. This analysis can be further supported by comparing the calculated length of the Py3M (5 nm) with the measured layer spacing (L = 6.75 nm). However, because the arcs with d = 4.4 nm which is considered as the expanded diffraction of the diffraction with d = 4 nm from the quenched sample (K2 phase) at Figure 5b have extinct or no higher order diffractions, it is uncertain if the arcs with d = 4.4 nm indicate the layered d-spacing of second layered structure or not. To resolve this ambiguous explanation, TEM morphological observations of self-assembled Py3M superstructures are conducted in this research. As shown in Figure 7b, the layer direction coincides with the shear direction and is perpendicular to c-axis. In addition, the diffractions at 2θ = 25.35° on the wideangle region, which is rotated 90 degree compared with Figure 7a, are observed. The ring patterns obtained along the c-axis (normal to in-plane layer) in Figure 7c provide an evidence to prove that Figure 7a and 7b are mixed fiber patterns of the K1 and K2 phases. In the core packing, the layer axis is parallel to the SD (along the meridian), while the a- and b-axes are rotated within the equator plane. Based on the combined results, two Py3M molecules form a


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dimeric building block and further self-assembles synclinically tilted crystalline structures with different layer d-spacings via thermal and time pathways. Thin films of Py3M are prepared for TEM experiments via the solution drop casting from the 0.05 % (w/v) chloroform solution. Bright-field TEM images of K1 and K2 phases are shown in Figure 8a and 8b, respectively. A multi-layered lamellar pattern with the periodic width of 6.6 nm are observed in Figure 8a. It is worthwhile to note that the layer d-spacing (L = 6.75 nm) of K1 phase matches to the periodic width of this TEM image. Compared with the bright stripe, it is reasonable to speculate that the dark stripe consists of pyrene moieties with high electron density. Additionally, note that the intermolecular interactions between pyrene moieties are predominant driving forces for the formation of K1 phase. The inset image in Figure 8a shows the fast Fourier transform (FFT) image demonstrating the fact that the assembled Py3M molecules form the ordered 1D striped pattern with a well-defined periodicity in the direction parallel to the layer normal direction (LN). From the analysis of 2D WAXD and TEM, the molecular packing structure in the K1 phase is simulated in Figure 8c by utilizing a Materials Studio computer simulation program from Accelrys. When the tilted angle of K1 structure is 45°, the analysis between 2D XRD, TEM and simulation is consistent and reasonable. On the simulation, two dimensional (2D) unit cell is calculated with 2D lattice parameters of a = 9.1 nm, b = 1.9 nm, and α = 136.1°. The simulated d-spacing (L = 6.3 nm) is relatively matched with the experimental layer d-spacing (L = 6.75 nm) of K1 phase. In addition, the calculated distance of π-π interaction between their centroids is 0.38 nm (Figure S18a) and this value accords closely with the experimental result (0.37 nm in Figure 5a). As mentioned above, the layered superstructure appears with the layer d-spacing of 6.75 nm in K1 phase. The other multi-layered lamellar pattern shown in Figure 8b has the periodic


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width of 4.3 nm. Note that the layer d-spacing in K2 phase can be expanded to about 4.4 nm. In this aspect, the layer d-spacing (L = 4.4 nm) of K2 phase corresponds to the periodic width (4.3 nm) in Figure 8b. In K2 phase, the smaller layer d-spacing comes from the more interdigitated and overlapped molecular structure due to the H-bonding interactions. FFT inset image in Figure 8b indicates that the K2 phase also shows the well-ordered pattern along the LN. From the experimental results, in Figure 8d, the molecular packing structure (tilted angle = 38°) of K2 phase is also simulated with 2D lattice parameters of a = 7.1 nm, b = 2.9 nm, and α = 143.1°. The simulated d-spacing (L = 4.3 nm) is well fitted with the experimental layer d-spacing (L = 4.4 nm) of K2 phase. Additionally, the calculated distance of H-bond is 0.27 nm (Figure S18b). We can also crystallize Py3M in a solution. Figure S19 shows the AFM image of Py3M crystals grown in solution. The crystal domain thickness along the c-axis is 4.5 nm, which is well matched with the layer d-spacing of K2 phase.

Photo-physical properties according to the molecular packing structures of Py3M Photo-physical properties of the organic π-conjugated molecules is dependent on the molecular packing structure originated from intermolecular interactions in solid and solution state. To investigate the correlation between photo-physical properties and molecular packing, the absorbance and steady state fluorescence emission spectra of Py3M are first recorded in chloroform at concentrations of ca. 25 ppm (part per million). As can be seen from an inspection of Figure 9a, four characteristic bands are detected in the absorption spectrum (black solid line): 289, 314, 329 and 345 nm, respectively. In order to avoid excimer, photoluminescence (PL) experiment in Figure 9a is conducted in diluted conditions, because the excimer formation highly


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depends on the concentration of solution. PL data serve to highlight the fact that the Py3M molecule gives rise to only monomer emission features (λmax = 376 and 396 nm) when irradiated at 329 nm in chloroform. When the concentration increases, PL intensity is noticeably diminished not only with the disintegration of the vibronic fine structure but also with the decline of the intensity of the excimer band (λ = 446 nm) (Figure 9b). The PL intensity at 376 nm (monomer emission) sharply decreases with the growth of the concentration (the inset graph in Figure 9b). This can be taken as an evidence that the aggregated structure causes the quenching effect at high concentrations.42 When considering the decrease of excimer band, the aggregation should be based on the H-bonds rather than the pyrene stackings and induce non-radiative energy transfer under the collision quenching from S1 to S0 state. The emission spectra are strongly dependent of the solvent polarity.43 As represented in Figure S20, when the mole fraction of methanol solvent in mixture of chloroform-methanol goes up, the PL intensity at 396 nm and the ratio of monomer/excimer (396/467 nm) are decreased. It means that the Py3M molecules is not soluble in methanol and the excimer is respectively increased (= pyrene stacking). In the case of other solvent (dimethyl sulfoxide; DMSO), the PL intensity and ratio of monomer/excimer are finally increased. Note that the Py3M molecule which is well dissolved in DMSO can stay in monomer compared with methanol. The emission spectra of Py3M powder samples in different states at room temperature is shown in Figure 10 with an excitation wavelength of 329 nm. These emission spectra are redshifted and broadened when compared with those of free monomers. In general, it is well known that the emission spectrum shifts to a longer wavelength called bathochromic shift or red shift by increasing the face-to-face (π-stacking) arrangements of chromophores with planar aromatic cores and the structure changes, such as H-aggregation, J-aggregation and Herringbone


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arrangement.44,45 The less broad excimer emission (solid line) centered at 470 nm is recorded from the slowly cooled sample (K1 phase). It is found that H-aggregated emission spectrum appears in the lower wavelength region with superimposing of pyrene moieties in the K1 phase, while the quenched sample (K2 phase) which is J-aggregated or randomly aggregated emits a lower light energy in a higher wavelength. In other word, the K1 phase has arranged and superimposed structure based on the π-π interaction and the K2 phase has less superimposed structure due to the circumjacent various environment originated from H-bonding interactions.

CONCLUSIONS A pyrene-based asymmetric supramolecule (abbreviated as Py3M) consisted of two pyrene head groups (Py), an isophthalamide connector and a biphenyl-based dendritic tail (3M) was newly designed and successfully synthesized for understanding the kinetically controlled polymorphic superstructures of asymmetric supramolecules. Owing to the competitions of intra- and intermolecular interactions (π-π interactions and H-bonds) and the nanophase separations, two synclinically tilted crystalline superstructures were constructed by exhibiting the monotropic phase transition behaviors. The stable K1 phase formed by the slow cooling and annealing process had the 6.75 nm periodicity of layered superstructure, while the metastable K2 phase formed by the quenching process from the isotropic phase had the 4.4 nm periodicity of layered superstructure. Based on thermal, microscopic, spectroscopic and scattering results, it was realized that the stable K1 phase was predominantly induced by the π-π interaction between pyrenes, while the main driving force for the formation of metastable K2 phase was the intermolecular H-bonding among isophthalamides. Additionally, from the UV-Vis and PL


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experiments, we realized that the photo-physical properties of Py3M were directly related with their molecular packing superstructures.

EXPERIMENTAL SECTION Materials and Synthesis The Py3M molecule was synthesized on the basis of references.46,47 The more details of synthetic procedures and the information of materials were described in the supporting information.

Characterization Chemical structure of Py3M was confirmed by proton (1H) nuclear magnetic resonance (NMR, JNM-EX400, JEOL) in deuterated chloroform. Chemical shifts were quoted in part per million (ppm) with a reference of solvent peak. Molecular weight of Py3M was identified by matrixassisted laser desorption ionization-time of flight MS (MALDI-ToF MS, CJ105). Chemical analyses of the Py3M samples were carried out using Fourier transform infrared (FT IR) spectrometer (IRTracer-100, SHIMADZU) in the range between 100 and 4000 cm-1. The ultraviolet-visible (UV-vis) absorption spectra were obtained in a chloroform solution by the UV-Vis spectrometer (Jasco, ARSN-733). The photoluminescence (PL) property of Py3M was evaluated by PL spectrometer (RMS-1000, Olis, Inc.) with a light source of 75W xenon arc lamp & 150W xenon arc lamp.


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Phase transitions and their thermodynamic properties were investigated by differential scanning calorimetry (DSC) using a TA Instruments DSC 2010. For DSC experiments, the sample weight (about 4 mg) and the pan weight were kept constant at a precision of ± 0.001 mg. Additionally, the heating and cooling rates were kept identical. The heating experiments always preceded the cooling experiments for eliminating the prior thermal histories. The onset temperature of phase transition was determined during the cooling process. To figure out the thermal stability of Py3M, thermogravimetric analysis (TGA, TA instrument, US/TA5000) was employed. The scanning rate was kept at 10 °C/min from 25 to 700 °C under nitrogen atmosphere. Optical textures at different temperatures were observed with cross-polarized optical microscope (POM, Nikon ECLIPSE E600POL) coupled with a LINKAM LTS 350 heating stage. The phase transition was monitored during the heating process at 2.5 °C/min. To study the molecular packing structures, 1D WAXD experiments were conducted in the reflection mode of a Rigaku 12 kW rotating-anode X-ray (Cu Kα radiation) generator coupled with a diffractometer. To monitor the structural evolutions with temperature changes, a hot stage calibrated to be within ±1 °C error was coupled to the diffractometer. The oriented 2D WAXD patterns were obtained using a Rigaku X-ray imaging system with an 18 kW rotatinganode X-ray generator. The diffraction peak positions and widths were also calibrated with silicon crystals. At least 30 min exposure time was required for a high-quality pattern. In 2D WAXD experiments, the background scattering was subtracted from the sample scans. TEM (JEM-ARM-200F, JEOL) images were obtained to examine crystal morphology on a nanometer scale using an accelerating voltage of 200 kV. Materials Studio (version 8.0 using COMPASS II forcefield) computer program from Accelrys was utilized for simulating the crystalline structures.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

Chemical materials, synthetic methods, 1H NMR spectra, MALDI-ToF MS spectra, computer energy-minimized models, TGA, POM, photographs, SAXD patterns, computer simulations, AFM, and PL.

AUTHOR INFORMATION Corresponding Author [email protected] (K.-U. Jeong)

ACKNOWLEDGMENT This work was mainly supported by Mid-Career Researcher Program (2016R1A2B2011041), MOTIE-KDRC (10051334) and BRL (2015042417) of Korea.

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(47) Varughese, S. J. Mater. Chem. C 2014, 2, 3499.


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Scheme 1. Illustration of the kinetically controlled bilayerd superstructures of Py3M via different thermal pathways.


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Figure 1. Synthetic route and chemical structure of Py3M. Conditions: (i) DIPC, DPTS, CHCl3, 25 °C for 1day; (ii) NaOH, THF/MeOH, 60 °C for 2 h; (iii) EDC, HOAt, DMF, 25 °C for 1 day.


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Figure 2. Sets of DSC cooling and subsequent heating thermal diagrams for Py3M at different scanning rates from 2.5 to 20 °C/min.


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Figure 3. POM images of Py3M observed at (a) 180 °C and (b) 170 °C during the 2.5 °C/min cooling process and at (c) 180 °C and (d) 150 °C during the 5 °C/min cooling process, respectively.


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Figure 4. FT IR spectra of Py3M at different temperatures during the 2.5 °C/min heating process.


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Figure 5. (a) A set of 1D WAXD powder patterns of Py3M at different temperatures during the cooling and subsequent heating processes at 2.5 °C/min, and (b) 1D WAXD powder patterns of Py3M at room temperature prepared by different thermal processes.


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Figure 6. FT IR spectra of the Py3M solids prepared by different thermal processes.


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Figure 7. 2D WAXD patterns of the sheared crystalline phase of Py3M obtained at room temperature with the X-ray beam parallel to (a) the b-axis (SD, shear direction), (b) the a-axis and (c) the c-axis, respectively.


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Figure 8. Bright-field TEM images of self-assembled Py3M superstructures prepared (a) by the slow cooling and annealing process and (b) by the quenching process, and (c, d) their corresponding simulated molecular packing structures from Materials Studio, respectively. Inset images in (a) and (b) are the corresponding fast Fourier transform (FFT) patterns, respectively. .


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Figure 9. (a) UV-vis and PL spectra of Py3M in chloroform at 25 ppm and (b) PL spectral changes at different concentrations. Inset in (b) represents the PL intensity changes at 376 nm with respect to concentration.


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Figure 10. Normalized PL spectra of Py3M crystals prepared by different thermal pathways. .


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For Table of Contents Use Only Pyrene-Based Asymmetric Supramolecule: Kinetically Controlled Polymorphic Superstructures by Molecular Self-Assembly Minwook Park,† Won-Jin Yoon,† Dae-Yoon Kim,† Yu-Jin Choi,† Jahyeon Koo,† Seok-In Lim,† Dong-Gue Kang,† Chih-Hao Hsu‡ and Kwang-Un Jeong†*

To understand the kinetically controlled polymorphic superstructures, the self-assembled superstructures of a pyrene-based asymmetric molecule were studied by the combination of scattering, morphological and thermal analyses.


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Pyrene-Based Asymmetric Supramolecule ... - ACS Publications

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