Perylene Diimide Bearing Different Trialkyl Silyl Ethers: Impact of

Apr 30, 2018 - (1−7) PDI exhibits favorable properties such as high photochemical stability, .... 458 nm, attributed to the 0–0, 0–1, and 0–2 ...
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Cite This: Chem. Mater. 2018, 30, 3571−3577

Perylene Diimide Bearing Different Trialkyl Silyl Ethers: Impact of Asymmetric Functionalization on Self-Assembly into Nanostructures Rachael Matthews,† Jordan Swisher,‡ Kristin M. Hutchins,§ and Emily B. Pentzer*,† †

Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, United States Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States § Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, Texas 79409, United States ‡

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S Supporting Information *

ABSTRACT: For over a decade, a great amount of research effort has focused on controlling the size and shape of organic small molecule crystals, as these parameters impact physical and optoelectronic properties. A thorough understanding of how functionalization impacts assembly as well as guiding principles to control aggregation and self-assembly are vital to producing novel organic nanostructures for electronic applications such as organic photovoltaics (OPVs). Herein, we study the influence of unsymmetrical functionalization of perylene diimide (PDI) on self-assembly. The guiding hypothesis of this work is that the identity of the pendant functionalities will impact the size, aspect ratio, and surface properties of the resulting assemblies. Twelve asymmetrically functionalized PDI molecules are reported, in which the length of the alkyl substituents at the imide position is varied, and include alcohol and silylated alcohol functionalities at the end of the alky chain. Morphologies of these self-assembled structures were characterized by scanning and transmission electronic microscopy; crystallinity was verified by powder X-ray diffraction, and the optoelectronic and thermal properties are also reported. On the basis of the functionality of the PDI molecules, different shaped assemblies are prepared, including high aspect ratio structures with widths ranging from 0.1 to 2.5 μm and lengths 1−800 μm.

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INTRODUCTION Perylene diimide (PDI) is a rylene dye used in industrial paints and is also a well-known organic n-type semiconductor.1−7 PDI exhibits favorable properties such as high photochemical stability, high molar absorption coefficient, low cost, nearunity fluorescence quantum yields, and charge carrier mobility as high as 0.1 cm2/(V s) in the liquid crystalline form.8 As such, PDI and its functionalized derivatives are attractive for use as active materials in a variety of (opto)electronic devices, including organic field-effect transistors (OFETs), dye lasers, and organic photovoltaics (OPVs).1,2,7−12 Typically, PDI derivatives with symmetric substitution at the imide positions have been studied; specifically, the imide groups have been functionalized with a variety of alkyl groups. Such functionalization controls both the solubility and aggregation/selfassembly of the molecules and can impact device performance.2,8,11,13−15 Compared to functionalization of the aromatic rings of PDI at the bay positions, functionalization of the imides does not impact the electronic properties of the molecule, as both the HOMO and LUMO have nodes at the imide position.10,11,14,16 Controlling the self-assembly of organic molecules into ordered structures is a challenging feat, but will have positive impacts on diverse fields, including biochemistry, engineering, and green chemistry, among others.8,17,18 © 2018 American Chemical Society

The morphology of self-assembled PDI depends on the alkyl functionalization of the imide positions, as these substituents dictate the balance of molecule−molecule interactions by π−π stacking of aromatic systems and hydrophobic interactions of alkyl chains.8,13−16,19−21 Longer alkyl chains provide solubility and thus processability in common organic solvents; however, the insulating nature of these alkyl substituents can lower device efficiency.10,14,22 Typically, self-assembly of PDI into nanostructures is achieved by one of two methods: (1) a solvent evaporation method, in which solvent slowly evaporates from a concentrated solution of the molecule; or (2) a solvent diffusion process7 in which a bad solvent is added on top of a solution of PDI in a good solvent and self-assembly occurs at the interface of the two liquids. Symmetrically functionalized PDI molecules and their assemblies include: n-alkyl and branched substituents that result in nanowires,9 ethoxy substituents resulting in nanobelts,14,21,23 and swallowtail substituents that give spherical nanostructures.14 More exotic substituents have been used to access nanostructures of intricate design, including: glucopyranoside, polyhedral oligosilsesquioxane (POSS), and aniline that result in helical Received: April 13, 2018 Revised: April 30, 2018 Published: April 30, 2018 3571

DOI: 10.1021/acs.chemmater.8b01543 Chem. Mater. 2018, 30, 3571−3577

Article

Chemistry of Materials nanofibers,17 single crystalline nanobelts,24 and micellular shapes,25 respectively. Thus, for symmetrically substituted PDI molecules, the chemical nature of the imide functionalization plays a vital role in the structures formed.8,18 Alternatively, unsymmetrically functionalized PDI derivatives, in which each imide position is functionalized with a distinct functionality, have gained attention, though they are significantly more difficult to synthesize than their symmetrically functionalized counterparts.8,24,26 Most commonly, for asymmetrically functionalized PDI molecules one imide contains a solubilizing branched alkyl group27 and the other imide position contains a polar functionality. For example, PDI with one imide functionalized with a nonpolar alkyl chain and the other with a propylene oxide−ethylene oxide copolymer self-assembled into hollow nanotubes;24 other polar substituents and nanostructures include alkoxy substituents yielding nanocoils,28,29 polyoxyethylene giving nanobelts,27 and fluoroalkylated groups forming nanoribbons.26 Expanding the chemical functionalities that can be used to prepare asymmetrically functionalized PDIs and understanding their impact on self-assembly will facilitate the development of next generation materials for optoelectronic devices.13,30 Herein, we present the synthesis and characterization of 12 different asymmetrically functionalized PDI small molecules and study their self-assembly using the solvent diffusion method. One imide position of the PDI is functionalized with a short alkyl chain for solubility, and the other imide position is functionalized with an amino alcohol; the hydroxyl group of the amino alcohol is subsequently transformed to a trialkyl silyl ether using standard conditions to give different shaped molecular “building blocks” (Figure 1). We demonstrate that

Regardless of the functionality of the imide substituents and size and shape of the self-assembled structures, all are crystalline; however, they have distinct size and shape based on differences in the functionalities.

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RESULTS AND DISCUSSION Synthesis and Self-Assembly. An overview of the synthesis of the asymmetrically functionalized PDI derivatives is shown in Figure 2A, along with the amino alcohol, trialkyl silyl groups, and the naming scheme used (Figure 2B). Briefly, perylene dianhydride was converted to the diimide derivative using 3-aminopentane; then, one of the imides underwent saponification by tandem treatment with strong base and strong acid to yield perylene monoimide monoanhydride (PMIMA). The anhydride of PMIMA was functionalized with an amino alcohol to give three different asymmetrically functionalized PDI molecules: PDI 1-OH, PDI 2-OH, and PDI 3-OH. In these molecules, the branched alkyl chain provides solubility in organic solvents as well as alkyl−alkyl interactions during selfassembly, and the pendant alcohol functionality on the other imide position provides a handle for functionalization to control the overall shape of the molecules. The alcohols of these molecules were functionalized with different trialkylsilyl chlorides to give the corresponding silyl ethers: trimethylsilyl (TMS), tertbutyldimethylsilyl (TBDMS), and triisopropylsilyl (TIPS). All PDI derivatives were characterized by 1H and 13C NMR (Figures S1−S15), Fourier transform infrared (FTIR) spectroscopy (Figure S16), and MALDI-TOF (Figure S17). To determine the impact of the molecular functionality on self-assembly, a slow diffusion process was used. The PDI compounds were dissolved in a good solvent, chloroform, then a bad solvent, methanol, was gently added on top of the chloroform solution, such that methanol slowly diffused into chloroform.9,24,18,31 This diffusion resulted in aggregation and/ or crystallization of the PDI molecules into nanostructures with the size and shape dependent on the identity of the substituents. Optoelectronic Properties. The absorption spectrum of solvated PDI A, shown in Figure 3A (black spectrum), reveals the well-established absorption maxima at 526, 489, and 458 nm, attributed to the 0−0, 0−1, and 0−2 vibronic transitions, respectively.1,7,9,14,21,23,30,32 As mentioned above, the imide positions of PDI are nodes in the π orbital wave functions and therefore do not impact the electronic structure of the perylene core. Thus, all PDI molecules prepared have essentially the same absorption profile when fully solvated.10,11,14,16,22,29,32 As expected, the absorption spectra of alkylated PDI molecules changes upon self-assembly. Side chain morphology can play a crucial role in π−π stacking distance, ultimately dictating intermolecular interactions and thus controlling the self-assembly process.14,22,23 For example, Würthner et al. found that substitution with linear alkyl chain (least steric demand) formed H-aggregates that are identified by a shift in absorption to ∼550 nm.7 In compliment, when the imide positions are substituted with branched alkyl groups (higher steric demand), J-aggregates are formed, as observed by a shift in absorption to ∼620 nm.7 The absorption spectrum of PDI A aggregates in suspension is similar to the solvated form with three vibronic transitions, but with a slight increase in absorbance at longer wavelengths (Figure 3A, red spectrum). Similar trends were observed in the absorption spectra of suspensions of aggregated PDI B, PDI 1s, PDI 2s, and PDI 3s (Figure S18). The absorption bands emerging at longer

Figure 1. Overview of the work reported herein: asymmetrically functionalized PDI derivatives bearing different functionalities at the two imide positions to give different sized building blocks that selfassemble into nanostructures.

the identity of the amino alcohol and size of the trialkyl silyl group impact the shape and size of the crystals formed, but that the interplay of these two factors cannot be disentangled. The thermal properties of the materials are characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and the self-assembled structures are characterized by UV−vis and fluorescence spectroscopies, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and powder X-ray diffraction (XRD). 3572

DOI: 10.1021/acs.chemmater.8b01543 Chem. Mater. 2018, 30, 3571−3577

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Chemistry of Materials

Figure 2. (A) Synthetic procedure for the preparation of asymmetrically functionalized PDI molecules. (B) Asymmetric PDI molecules and naming scheme used herein. i = imidazole, 125 °C, 2-amino pentane; ii = (1) KOH, tBuOH, 90 °C, (2) AcOH, 2 N HCl; iii = imidazole, 125 °C, aminoalcohol; iv = imidazole, 125 °C, R3Si−Cl.

than those of PDI A (Figure 4C, purple spectrum). As both PDI A and PDI B bear only alkyl substituents, the difference in absorption spectra and thus assembly are attributed only to symmetric versus asymmetric functionalization and not to other van der Waals interactions (e.g., H-bonding). Pasaogullari et al. similarly observed different absorption spectra for symmetric and unsymmetric functionalized PDI aggregates and attributed this to different intermolecular interactions.35 The absorption spectra of PDI 1s, PDI 2s, and PDI 3s are impacted not just by differences in intermolecular interactions brought about by asymmetric functionalization, but also by other interactions, such as H-bonding. The solid state absorption spectra of aggregates of PDI 1-OH and PDI 2OH have broadened absorption and increased absorbance in the far red region (>700 nm), indicating stronger intermolecular coupling, and both H- and J- aggregation.30 Of note, absorption at the longest wavelengths can be attributed to hydrogen bonding.7,30,35,37 In comparison, the solid state absorption spectrum of aggregates of PDI 3-OH is less redshifted (>600 nm), and may indicate that the longer alkyl chain prevents strong H-bonding. All PDI 1-OSiR3, PDI 2-OSiR3, and PDI 3-OSiR3 (Figures 4A−C) show absorbance from ∼450− 600 nm, with a tailing into the red. The spectral shifts observed in the solid state are clearly influenced by the spacing between molecules particularly determined by the size/bulk of the substituents. Although, PDI A contains a short, branched alkyl chain, strong coupling interactions were observed that were not present in the spectra of PDI B, PDI 1-OSiR3, PDI 2-OSiR3, PDI 3-OSiR3, and PDI3OH. Moreover, length of the alkyl chain significantly impacts the electronic spectra and influences hydrogen bonding interactions. The emission spectra of solvated and aggregated PDI can also be used to understand intermolecular interactions; the black trace in Figure 3B shows the emission spectrum of solvated PDI A and reveals the expected three vibronic transitions: 0−0, 0−1, and 0−2 bands, a mirrored image of the

Figure 3. Optoelectronic data. (A) UV−vis absorption spectra of PDI A in chloroform (black), a suspension of the aggregates (red), and aggregates in the solid state deposited on glass (purple). (B) Fluorescence emission spectra of PDI A solvated in chloroform (black) and aggregates in suspension (purple).

wavelengths indicate strong π−π interactions and cofacial molecular stacking.7,11,14,33,34 The UV−vis spectra of aggregated PDI molecules drop cast onto a glass slide were distinctly different from the suspended aggregates, suggesting that solvated PDI molecules dominate the spectrum of suspended aggregates. The absorption profiles of many PDI derivatives in the solid state have been associated with an overall decrease in absorption intensity, broadening of vibronic bands, and formation of new bands at longer and shorter wavelengths, indicating the formation of both H- and J-aggregates.4,14,35 Likewise, the absorption spectrum of PDI A aggregates in the solid state (Figure 3A, purple spectrum) indicates formation of both H- and J-aggregates (absorption at 561 and 606 nm, respectively). Whereas branched alkyl chains can increase the distance between aromatic cores of neighboring PDI molecules,14,30,36 linear alkyl chains should facilitate selfassembly.9,14,23 The solid state UV−vis spectrum of aggregates of PDI B, which bears one branched alkyl substituent and one linear alkyl substituent, shows absorbance over the entire region, 400−550 nm, indicating coupling interactions weaker 3573

DOI: 10.1021/acs.chemmater.8b01543 Chem. Mater. 2018, 30, 3571−3577

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Figure 4. UV−vis spectra of aggregated PDI molecules drop cast on a microscope slide. (A) PDI A, PDI 1s; (B) PDI 2s; (C) PDI B, PDI 3s. Fluorescence spectra (λex = 450 nm) of suspended PDI crystals (D) PDI A, PDI 1s; (E) PDI 2s; and (F) PDI B, PDI 3s. Refer to Figure 2B for color code key.

absorption spectrum.4,14,15,18,23,24 The purple trace in Figure 3B shows the emission spectrum of a suspension of aggregates of PDI A, showing similar transitions to the solvated form, but with a different relative intensity. Specifically, emission intensity at longer wavelengths is observed and can be attributed to Jaggregate formation. The emission maxima of the longer wavelength bands blue shifts ∼2 nm and is likely due to solvent effects.11 The emission spectra of all PDI 1s, PDI 2s, and PDI 3s resemble that of the PDI A, both in the solvated form, and for the suspensions of the aggregates (Figures 4D−F). Emission spectra in the solid state of assembled PDIs were featureless with no observable signal. Thermal Properties. To evaluate the impact of functionalization on the thermal properties of the asymmetrically functionalized PDI molecules, TGA and DSC were used. Figures 5A−D show the weight loss profiles of the asymmetrically functionalized PDI molecules, organized by the amino alcohol used (Figure 5B = PDI 1s, Figure 5C = PDI 2s, and Figure 5D = PDI 3s). All silylated PDI molecules are more thermally stable than the hydroxyl containing molecules, showing slight differences before rapid weight loss above 400 °C due to loss of alkyl chains (see Figure S19 for details), followed by another degradation step above 600 °C for the perylene core.38,39 All hydroxyl containing PDI compounds show a distinct, though slight weight loss transition at ∼200 °C, which may indicate reactivity of hydroxyl groups or changes in

hydrogen bonding. The thermotropic behavior of liquid crystal molecules such as PDI can be investigated by DSC;1,3,14,16,21,23,24,35 Figure 5E shows the thermal profile of PDI A, revealing an exothermic peak at 65.2 °C corresponding to crystallization (upon cooling) and an endothermic peak at 73.2 °C upon heating corresponding to melting. These data suggest that PDI A is crystalline. Figure 5E also shows the DSC trace of PDI B and reveals only weak signals for crystallization and melting, as reported for some PDI derivatives with weakened or distorted molecular stacking, and is in agreement with the electronic spectra discussed above.14,35 Figures 5F−H show the DSC thermal profiles of PDI 1s, PDI 2s, and PDI 3s, and all are nearly featureless. This does not support that the PDI molecules are crystalline. However, PDI 2-OTBDMS does show two distinct, though weak, transitions at 20.0 and −6.5 °C, indicating a crystalline nature (Figure 5G, pink), even though the absorption spectrum is similar to the other asymmetrically functionalized PDI molecules. For each sample, the third heating cycle is shown, so as to neglect any impact of thermal history, and heating was performed at 10 °C min−1 under nitrogen, as typical for PDI derivatives.1,6,16,18 Microscopy. A small drop of the suspended PDI aggregates was placed on copper grids covered with a thin carbon film for characterization by SEM and TEM. Figure 6 shows the SEM images of the self-assembled PDI molecules (other SEM images are included in Figures S21−S34). PDI A and PDI B are 3574

DOI: 10.1021/acs.chemmater.8b01543 Chem. Mater. 2018, 30, 3571−3577

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Chemistry of Materials

Figure 5. TGA weight loss profiles of: (A) PDI A and PDI B; (B) PDI 1s; (C) PDI 2s; and (D) PDI 3s; DSC thermograms of: (E) PDI A and PDI B, (F) PDI 1s; and (G) PDI 2s; and (H) PDI 3s. Refer to Figure 2B for color code key.

Figure 6. SEM images of symmetrically and asymmetrically functionalized PDI molecules after self-assembly using the solvent diffusion method. Expanded views of the samples and histograms are available in the Supporting Information (Figures S21−S34). Refer to Figure 2B for color code key.

which had relatively homogeneous surfaces. PDI 1-OTMS shows signs of nucleated assembly, indicating that aggregates of PDI may serve as nucleation sites for subsequent assembly of molecules, not observed for other compounds. In an attempt to prevent the nucleated growth, the chloroform solution was filtrated multiple times before addition of methanol, but no difference was observed. As seen in Figure 6, structures of assembled PDI 1-OTBDMS and PDI 1-OTIPS were splintered, which may be attributed to defects within the self-assembled

symmetrically and asymmetrically functionalized with only alkyl substituents; from the SEM images, the assemblies are homogeneous and thread-like with smooth surfaces. They are micrometers long, with the diameter dictated by the alkyl substituents. In contrast, for the PDI 1s, PDI 2s, and PDI 3s, both the identity of the amino alcohol and the identity of the trialkyl silyl group impact the shape of the structures formed. Within the family of PDI 1s, crystals were relatively heterogeneous with tapered ends, except for PDI 1-OH, 3575

DOI: 10.1021/acs.chemmater.8b01543 Chem. Mater. 2018, 30, 3571−3577

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Chemistry of Materials

size and shape of the assemblies are impacted by the length of the alkyl chain as well as the identity of the end group functionality (alcohol or silyl ether). Thus, while asymmetric functionalization of PDI can be used to dictate the structure formed upon self-assembly, no overarching trend is identified for the set of molecules provided herein. Ongoing work focuses on identifying the location of the hydroxyl and silylated groups within the assemblies as well as exploring other moieties to control the size, shape, and functionality of the structures.

structure due to the proximity of the bulky trialkyl silyl group to the PDI core. PDI 2-OH assembled into blocky and short crystals that were