Self-Assembled Spheres, Flowers, and Fibers from the Same

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Self-Assembled Spheres, Flowers, and Fibers from Same Backbone and Similar Side chains Anup Kumar Singh, and Kothandam Krishnamoorthy Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02728 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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Self-Assembled Spheres, Flowers, and Fibers from Same Backbone and Similar Side chains Anup Kumar Singh and Kothandam Krishnamoorthy†* CSIR-National Chemical Laboratory, CSIR-Networks of Institutes for Solar Energy, Dr Homi Bhabha Road, Pune 411008, India Key words: iso-indigo, Self-Assembly, Spheres, Fibers, Flowers

Rylene imides (RIs) self-assemble into various nanostructures. Often, synthesis of unsymmetrical RIs (URIs) is required to achieve nanostructures. However, synthesis of URIs is non-trivial. Thus, a structurally similar alternative is desirable. iso-indigo (i-indigo) has a π core and lactam rings that is structurally similar to the RIs. Unsymmetrical isoindigo (i-indigo) can be easily synthesized by condensing oxindole and isatin. We have synthesized a series of unsymmetrical i-inidgo molecules. In these molecules π-π interaction, hydrogen bonding, and vanderwaal interactions are in operation. Due to these, the molecules self-assemble into spheres, fibers, and dahlia flower morphologies. If the hydrogen bonding interaction is disrupted, all of them form fibers. Control experiments indicate that the complete absence of hydrogen bonding is deleterious to self-assembly. We also show that the lower analogs of i-indigo are not suffice to form self-assembled nanostructures.

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INTRODUCTION Molecules with π core and hydrogen bonding moieties have been widely explored as building blocks for preparing self-assembled nanostructures.1-4 For example, thiophene and its dimer were connected with bisurea to form self-assembled fibers. It was found that the charge transport was efficient within these fibers.5,6 Terthiophenes were connected with cholesterol to prepare materials with helical features.7 Oligo (phenylene vinylene) (OPV) was substituted with ureidotriazine to obtain liquid crystalline materials.8 OPV has been embedded in peptide backbone to achieve nanofibers that are useful in bioelectronic applications.9 OPV has been unsymmetrically substituted with bismelamines to prepare short nanorods and elongated nanotapes.10 Tetrathiafulvalene moiety was connected with a C3 symmetric core and hydrogen bonding motif was used to prepare helical fibers.11 Among them, rylene imides (RIs) have been attractive candidates due to the presence of multiple substitution possibilities.12,13 Often, unsymmetrical substitution is required to achieve various morphologies in RIs. For example, phenyleneethynylene was connected with unsymmetrical perylenebisimdes (UPBIs) to prepare foldamers.14,15 UPBIs were connected with polyisocyanide strands to prepare large fibers.16 An amphiphilic perylenebisimide was synthesized to resemble a wedge using UPBIs. They assembled into spheres and rods.17 These are some examples, and a detailed review on various structures and self-assembly of perylenebisimides (PBIs) is available in the literature.18 Naphthalenebisimides (NBIs), which is a lower analogue of PBIs is also widely explored for its self-assembly properties. Unsymmetrical naphthalenebisimide (UNBIs) was connected with tyrosine and alkyl chain to form fibers.19 A chiral gelator synthesized using UNBIs formed fibers with left handed helicity.20 Recently, UNBIs were used as a structure directing unit in a polymer that formed spherical and cylindrical micelles.21 The readers are directed to a review on various


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structures and assemblies of NBIs, which was published recently.22 Thus, unsymmetrical RIs form exotic structures, but synthesis of unsymmetrical RIs is non-trivial.23 Thus, a structurally similar molecule with a simple synthesis is desirable. i-indigo has a π core and lactam rings that can either be used as hydrogen bonding motif or as a position for substitution.24,25 Thus, i-Indigo is an attractive alternate to rylene imides. i-indigo has been widely used as building block to synthesize organic semiconductors.26-38 However, i-indigo’s self-assembly properties have not been explored despite comprising attractive structural motif. The i-indigo is synthesized by condensing oxindole and isatin.39 By substitution of either oxindole or isatin with alkyl chain and condensing those in the presence of acid would result in the synthesis of unsymmetrical i-indigo (Chart 1). The unsymmetrical i-indigo has -NH-CO- moiety that facilitates hydrogen bonding. The alkyl chain could facilitate interdigitation between other molecules and π-core facilitates stacking of the molecules. Thus, the unsymmetrical i-Indigo has three different types of interactions. We have found that the molecules could assemble into various morphologies if all three interactions are in operation. However, by addition of hydrogen bonding competing solvent, self-assembled fibers could be formed. In order to ascertain the role of hydrogen bonding, we have synthesized symmetrical i-indigo that does not have hydrogen bonding motifs. They do not self- assemble into nanostructures. Thus, complete absence of hydrogen bonding would be deleterious to self-assembly of i-indigo. In this report, series of i-indigo molecules have been synthesized and studied to identify the essential interactions required to prepare assemblies of i-indigo.


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Chart 1 Structure of N-alkylated isatin (i-R), unsymmetric i-indigo (U-iI-R) and symmetric iindigo (S-iI-R) molecules respectively. RESULTS AND DISCUSSION Unsymmetrical i-indigo was synthesized by condensing alkyl chain substituted 1H-indole-2,3dione with indolin-2-one (Scheme S1, ESI). The molecules will be mentioned as U-iI-6, U-iI-8, U-iI-10, U-iI-12, U-iI-14, and U-iI-16, wherein the numbers depict the number of carbon atoms in the alkyl chain (Chart 1). For example, in U-iI-6 the i-indigo is substituted with hexyl chain. Together, the unsymmetrical i-indigo will be mentioned as U-iI-R. We have also synthesized control molecules by condensing indolin-2-one and 1H-indole-2,3-dione followed by reacting it with bromoalkanes (Scheme S2, ESI). The chemical structures of the molecules (S-iI-6, S-iI-8, S-iI-10, and S-iI-12) are shown in Chart 1. Together the symmetrical i-indigo molecules will be mentioned as S-iI-R. The UV-vis absorption spectra of U-iI-R were recorded using 0.1 mM solution in chloroform. The absorption spectra showed two peaks at 394 nm and 479 nm. There was also a hump at lower wavelength (362 nm) (Figure S1, ESI). To understand the presence of


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Figure 1 Absorption (a) and emission spectra (b) of U-iI-6 as a function of concentration. Absorption spectra of U-iI-8 as a function of temperature (c). For the prupose of clarity, the absorbance scale has been shown between 1 and 1.3. assemblies in the solution, the concentration of U-iI-R was varied between 5 x 10-4 mM and 0.01 mM. The intensity of all the peaks (362, 394 and 479 nm) decreased as a function of decrease in concentration (Figure 1a). This indicates that the concentration has little effect on the assemblies in the solution.40 Emission spectra provide better insight on the assembly properties of molecules in solution. Thus, various concentrations (from 5 x 10-3 mM to 1 x 10-1 mM) of U-iI-R were excited at 393 nm, and the emission spectra were recorded between 400 and 600 nm. Three emission peaks were observed at 415, 436 and 459 nm. The intensity of the peaks was found to decrease upon increase in the concentration of U-iI-R (Figure 1b). This indicates that the molecules do assemble at higher concentrations, which lead to self-quenching of the emission.41,42 As mentioned earlier, all three interactions are likely to be in operation in chloroform solution. We intend to disrupt one of the interactions to study its impact on the assembly. Assemblies formed due to hydrogen bonding can be disrupted by increasing the temperature.18 Thus, the UV-Vis spectra of U-iI-R were recorded as a function of temperature. The temperature of 0.1 mM solution was varied between


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15 and 50 °C, and the UV-Vis absorption spectra were recorded between 300 and 700 nm. The intensity of the peak at 394 nm decreased as a function of increase in temperature. We also observed a blueshift by 2 nm to 392 nm (Figure 1c). These two changes indicate that this peak is due to the assembly of U-iI-R in the solution. Upon increase in temperature, the intensity of the hump at 362 nm increases, which reveal the formation of monomers. The isosbestic point between 362 and 394 nm indicate the equilibrium between the monomers and assembled structures.43 However, all the peaks do exist in the temperature window of 15 to 50 °C indicating that the assemblies do not disassemble completely. From the available data, we conclude that assemblies of U-iI-R are present in the solution. To identify the type of assemblies, scanning electron microscopy (SEM) imaging was employed. Spherical particles with an average diameter of 120 nm were found for U-iI-6 (Figure 2a). The particle size distribution was performed by using the ImageJ 1.51k software and the graph was plotted as a function of particle size per count (Figure S2, ESI). We attempted to measure the size of the spherical assemblies in solution using dynamic light scattering (DLS). However, large particles were observed with very high polydispersity (Figure S3, ESI). Furthermore, the size, as well as the polydispersity, varied from sample to sample. Thus, we have concluded that DLS is not a suitable technique to study U-iI-R. In the SEM images of U-iI-6, we did observe formation of small rods. Are these rods converted from the spheres upon aging? To address this question, we monitored the UV-vis spectra of the solution as a function of time. The absorption profile did not vary as a function of time indicating the absence of morphology change upon aging (Figure S4). In the SEM images, occasionally we did observe spheres with extremely narrow particle distribution. However, we would like to reemphasize that often images similar to Figure 2a was obtained. Large fibers and spheres were observed for U-iI-10 (Figure S5, ESI). Randomly


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oriented fibers were found for U-iI-12, and U-iI-14 (Figure S5, ESI). Large dahlia flower type assemblies were found for U-iI-16 (Figure 2b). These features can be confirmed by transmission electron microscope (TEM) images as well (Figure S5 , ESI).

Figure 2 SEM image showing the particle morphology of U-iI-6 (a). SEM image showing the dahlia flower morphology of U-iI-16 (b). TEM image showing spherical morphology of U-iI6 (c) and dahlia flower morphology of U-iI-16 (d).


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The TEM images of spheres of U-iI-6 and dahlia flowers of U-iI-16 are shown in Figure 2c and Figure 2d, respectively. To understand the morphology, we looked for clues in the crystal structure of U-iI-R. Single crystals of U-iI-8, U-iI-10, and U-iI-12 could be grown, and the XRD patterns were collected. Upon solving the crystal structures, we found that a twist is required to fulfill hydrogen bonding between the molecules (Figure 3a). This distortion leads to assemblies such as dahlia flowers and spheres. In the crystal structure, we could also find interdigitation of alkyl chains that lead to extended assemblies of the U-iI-R (Figure 3b). In order to obtain welldefined assemblies, the hydrogen bonding must be minimized. If the hydrogen bonding is minimized, the π stacking will be efficient. We presumed that the improved π-stacking, as well as interdigitation of alkyl chains, will lead to well-defined assemblies. Thus, we decided to disrupt the hydrogen bonding and study the morphology of the U-iI-R. Disruption of intermolecular hydrogen bonding can be achieved by addition of solvent such as methanol.43,18 The UV-Vis absorption spectra of U-iI-R were recorded as a function of various quantity of methanol. The quantity of methanol was varied between 0% and 3% by addition of methanol to a binary solution of methylcyclohexane (MCH)-chloroform (95:5). The concentration of the solution was maintained at 0.1 mM. In the absorption spectra, three peaks were observed at 369, 389 and 481 nm (Figure S6, ESI). Upon addition of methanol, the peak at 389 nm increased, and the peak at 369 nm decreased up to 1% methanol addition (Figure 3c). However, both the peaks decreased upon further addition of methanol. The initial addition of methanol facilitates better π stacking between the U-iI-R by disrupting the intermolecular hydrogen bonding. This is further corroborated by the slight bathochromic shift of the peak at 389 nm. The decrease in the intensity of monomer peak at 369 nm is an added confirmation to this conclusion. The decrease in the intensity of all the peaks upon addition of methanol above 1% is due to decreased solubility of


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U-iI-R in the solvent mixture. From these experiments, it is clear that the methanol addition disrupts hydrogen bonding and increase π stacking, which may lead to well-defined assemblies of U-iI-R. SEM imaging of U-iI-R was carried out using methanol solution (1.0 mM). Well defined fibers were observed for U-iI-6 (Figure 3d), which is different from that of spherical morphology obtained for the same molecule in chloroform. The interdigitation of alkyl chain and absence of twist between U-iI-6 molecules lead to this assembly. We did observe similar morphology for other U-iI-R while the samples were prepared from a methanol added solution (Figure S7, ESI). Thus, addition of hydrogen bond competing methanol is essential to obtain well-defined fibers. We were interested in finding out the effect of solvent on the morphology of U-iI-R. For

Figure 3 Single crystal X-ray structure showing the twist due to hydrogen bonding (a) and interdigitation of alkyl chain in U-iI-6 (b). Absorbance spectra showing the variation in absorbance as a function of concentration of methanol in U-iI-10 (c). SEM image showing the fibers of U-iI-6 (d). ACS Paragon Plus Environment

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example, what is the morphology of U-iI-R in DMF? DMF dissolves U-iI-R and is a hydrogen bond competing solvent. Thus, U-iI-R may self-assemble into fibers. The morphology of U-iI-R was studied using SEM. All the U-iI-R formed fibers (Figure S9, ESI) due to the hydrogen bond competing nature of DMF. The fibers were found to be short in case of U-iI-8. In case of other U-iI-R, the fibers were found to present in clusters. Thus, the U-iI-R forms fibers that are different from that of the fibers obtained by controlled addition of methanol to a chloroform solution. However, it must be noted that the fibers are the dominating morphology in presence of hydrogen bond competing solvents such as methanol and DMF. Considering this result, the question arise, is hydrogen bonding essential to obtain well-defined assemblies? To answer this question, we synthesized S-iI-R by following the reported procedure.39 Unlike U-iI-R, the SEM images of S-iI-R did not show specific and well defined morphology. They formed a rough film. This clearly indicates that the π stacking and interdigitation of alkyl chains are not sufficient to form well-defined assemblies. Hydrogen bonding is required, but the twisting of i-indigo backbone must be minimized. The other question is, do we need i-indigo to form the assemblies? This question arises because isatin and N-alkylated isatin has carbonyl and either -N-H or -N-R moieties. Are they suffice to form self-assembled structures? To test the possibility of assembly formation of isatin and N-alkylated isatin, various alkylated isatins (i-6, i-8, i-10, i-12, i-14, and i-16) were synthesized (Scheme S3, ESI). Together, the isatin derivatives are mentioned as i-R (Chart 1). Absorption spectra of i-R showed two peaks at 302 nm and 430 nm. These peaks decrease upon decrease in the concentration of i-R. The emission spectra of the i-R were recorded by exciting the solution at 377 nm while the emission was monitored between 387 and 650 nm. The emission peak intensity increased upon increase in the concentration of i-R. This indicates that the molecules do not assemble in solution (Figure S14, ESI). The SEM images of i-


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R were recorded to identify the type of assemblies. We did not observe well-defined assemblies for i-R (Figure S8, ESI). Thus, i-indigo that has extended π core is required for self-assembly. Conclusions In summary, to understand the interactions required to form assemblies of i-indigo derivatives, we synthesized U-iI-R, S-iI-R, and i-R. The U-iI-R form assemblies in solution as well as in dry state. However, they are not well defined. From the single crystal XRD studies, the hydrogen bonding was found to twist the i-indigo unit leading to random growth of assemblies. To circumvent this undesired twist, hydrogen bonding competing solvent (methanol) was added. The addition of methanol lead to the formation of well-defined fibrous assemblies of U-iI-R. Control molecules S-iI-R (no hydrogen bonding) did not well defined and specific morphologies. This indicates that complete absence of hydrogen bonding is not desirable. The morphology studies of i-R indicate that the π backbone should be sufficiently extended up to i-indigo to achieve well-defined assemblies.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental details, spectra and microscopic images are provided. AUTHOR INFORMATION Corresponding Author * [email protected] Present Addresses


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†Phyisikalische Chemie, Carl von Ossietzky Universität Oldenburg, Germany. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Council of Scientific and Industrial Research (NWP 54) ACKNOWLEDGMENT KK thanks CSIR for financial support through TAPSUN (NWP 54). KK also thank Alexander von Humboldt foundation for experienced researcher fellowship. AKS thank CSIR-UGC for fellowship.

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