Thermally Sensitive Self-Assembly of Glucose-Functionalized

Article pubs.acs.org/Langmuir

Thermally Sensitive Self-Assembly of Glucose-Functionalized Tetrachloro-Perylene Bisimides: From Twisted Ribbons to Microplates Kai Sun, Chengyi Xiao, Chunming Liu, Wenxin Fu, Zhaohui Wang,* and Zhibo Li* Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: Chiral supramolecular structures are becoming increasingly attractive for their specific molecular arrangements, exceptional properties, and promising applications in chiral sensing and separation. However, constructing responsive chiral supramolecular structures remains a great challenge. Here, glucosefunctionalized tetrachloro-perylene bisimides (GTPBIs) with thermally sensitive self-assembly behaviors are designed and synthesized. In a methanol/water mixture, GTPBIs self-assembled into twisted ribbons and microplates at 4 and 25 °C, respectively. Furthermore, the ribbon structure was metastable and could transform into microplates when the temperature was increased from 4 to 25 °C. Transmission electron microscopy (TEM) was used to track the evolution of morphology and study the assembly mechanisms of correponding nanostructures at different time intervals. The supramolecular structures were characterized with various techniques, including circular dichroism, TEM, scanning electron microscopy, atomic force microscopy, ultraviolet−visible absorption, and fluorescence spectra. This study provides insight into controlling molecular parameters and assembly conditions to construct chiral supramolecular structures.

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gas,28 switchable interfaces,9 and carbohydrate−lectin interactions.29,30 Herein, we report a thermally sensitive selfassembly of glucose-functionalized tetrachloro-perylene bisimdes (GTPBIs). Temperature has a significant effect on the assembled structures. GTPBI assembled into twisted ribbons and microplates in a methanol/water mixture at 4 and 25 °C, respectively. The ribbon structure was metastable and could be transformed into microplates when the temperature was increased from 4 to 25 °C. In particular, we carefully explored the time-dependent assembly processes and mechanisms of these two supramolecular structures using transmission electron microscopy (TEM) at different time intervals during the selfassembly process together with circular dichroism (CD) spectra. The devices based on microplates revealed that molecular arrangements were quite important for charge transport performance.

INTRODUCTION Self-assembly offers a versatile stratergy for creating wellorganized structures.1−3 In general, researchers could take advantage of multiple intermolecular interactions and experimental conditions to tune and optimize the assembled nanostructures.4−11 Perylene bisimides (PBIs) are promising building blocks for n-type functional supramolecular architectures.12−14 The self-assembly of PBIs has been extensively studied as the assembled supramolecular structures exhibit unique properties and have promising applications in electronic and optoelectronic nanodevices, such as organic field effect transistors (OFETs) and organic photovoltaics (OPVs).15−17 Chirality is a universal phenomenon in biological systems and is associated with numerous bioactivities. On one hand, chiral molecules can assemble into chiral supramolecular structures;18,19 on the other hand, chiral supramolecular structures can be formed via chiral species-induced selfassembly of achiral molecules.20−24 Because of specific molecular arrangements, many chiral supramolecular structures possess exceptional properties and have potential applications in chiral sensing and separation.25−27 Therefore, it is an urgent challenge to construct chiral supramolecular structures with unique quality via controlled self-assembly.18 Saccharides make up an important class of naturally available chiral molecules that can induce chiral assembly because of their inherent chiral centers and hydrogen bonding. Recently, carbohydrate-based perylene bisimides have attracted considerable research interest because of their potentials in sensing of © 2014 American Chemical Society

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EXPERIMENTAL SECTION

Materials and Methods. 1H nuclear magnetic resonance (NMR) and 13C NMR spectra were recorded in deuterated solvents on a Bruker AVANCE 400NMR spectrometer. 1H NMR chemical shifts are reported in parts per million downfield from a tetramethylsilane (TMS) reference using the residual protonated solvent as an internal standard. Mass spectra (matrix-assisted laser desorption ionization Received: June 27, 2014 Revised: August 27, 2014 Published: August 28, 2014 11040

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Scheme 1. Chemical Structure of GTPBI

Figure 1. UV−vis absorption spectra of GTPBI in a methanol/water mixture (a) with the water content varied from 0 to 80 vol % at room temperature (C = 0.05 mg/mL) and (b) at different temperatures in a 0.05 mg/mL methanol/water mixture [50/50 (v/v)]. time of flight) were determined on a Bruker BIFLEX III mass spectrometer. Ultraviolet−visible (UV−vis) absorption spectra were measured with a TU-1901 spectrophotometer in a 1 cm quartz cell. Fluorescence spectra were recorded on a Hitachi F-4500 spectrofluorometer. CD spectra were recorded on a JASCO J-815 spectrometer. Specific rotation was obtained on a PerkinElmer model 343 polarimeter. Dynamic light scattering (DLS) was performed with a Malvern Zetasizer Nano ZS instrument. X-ray diffraction (XRD) analysis was performed on a Rigaku D/MAX 2500 diffractometer. TEM images were obtained using a JEM-2200FS instrument. Selected area electron diffraction (SAED) images were obtained with a JEM-2100FS instrument. Scanning electron microscopy (SEM) was performed with a JEOL-6700 field-emission scanning electron microscope. Atomic force microscopy (AFM) was performed in tapping mode (Nanoscope IIIa, Digital Instruments, Inc.). The devices based on microplates formed from GTPBI were fabricated on a Micromanipulator 6150 probe station through an organic ribbon mask technique. All chemicals were purchased from commercial suppliers and used without further purification unless otherwise specified. Synthesis of GTPBI. The synthetic route and characterization of compounds are shown in the Supporting Information.

behaviors of GTPBI, UV−vis absorption and fluorescence spectra were recorded for various methanol/water volume ratios and concentrations (Figure 1 and Figures S2 and S3 of the Supporting Information). When water was gradually added into the GTPBI/methanol solution, the absorption spectra displayed a bathochromic shift and the intensity decreased. Also note that as the water content increased, the A0→0/A0→1 ratio (Frank Condon principle) decreased, which was an indication of PBI aggregation.31 In addition, a noticeable signal appeared at longer wavelengths with an increase in water content. All these characteristics suggested that the GTPBI molecules underwent self-assembly. The corresponding fluorescence spectra showed similar self-assembly characteristic in good aggrement with UV−vis absorption spectra. As the water content increased, the fluorescence intensity decreased dramatically because of the quenching of aggregation. Next, we used TEM to observe the aggregation nanostructure and morphology transition. Figure S4 of the Supporting Information shows the coexistence of twisted fibers and microplates. Considering the effects of hydrogen bonding originating from hydroxyl groups of glucose,32−34 we assumed that temperature might have a significant influence on the resultant morphologies. The self-assembly of GTPBI was performed at different temperatures. Systemic TEM measurements found that GTPBI formed twisted ribbons, microplates, and irregular spheres at 4, 25, and 50 °C, respectively (Figure 2). In particular, left-handed twisted ribbons are clearly observed in panels a and b of Figure 2 and Figure S5 of the Supporting Information, indicating that the chiral groups played a crucial role in the formation of chiral nanostructures. The width of twisted ribbons is approximately tens of nanometers, and the length ranges from hundreds of nanometers to several micrometers. The thickness of twisted ribbons estimated from TEM is around 13 nm (Figure S5 of the Supporting Information). SEM images of microplates are shown in Figure S6 of the Supporting Information. The

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RESULTS AND DISCUSSION Scheme 1 shows the chemical structure of GTPBI. Glucose was successfully grafted to the perylene chromophore via click chemistry (Scheme S1 of the Supporting Information). As a result of tetrachlorine substitution at the bay regions, GTPBI exhibits solubility in common polar organic solvents (e.g., methanol and ethanol) that is better than those of other reported carbohydrate-based perylene bisimides.28 The self-assembly of GTPBI proceeded via the selective solvent addition method by the addition of water to the GTPBI/methanol solution. Upon addition of water, an obvious change in color from orange to red was observed immediately. Then, noticeable aggregates were observed within 2 h, and their amount grew with time, which indicated a transformation from solution to suspension. To understand the aggregation 11041

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microplates are approximately several micrometers long and wide. AFM characterization revealed that the thickness of microplates was approximately tens of nanometers (Figure S7 of the Supporting Information). It is worth noting that both twisted ribbons and microplates were stable at 4 and 25 °C, respectively. Both can maintain their morphology for at least 1 month at the specified temperature. Considering the different morphologies at different temperatures, we investigated the absorption and fluorescence spectra to study the aggregation behaviors at different temperatures. From the absorption spectra (Figure 1b), the main absorption peaks did not move with an increase in temperature from 4 to 50 °C, indicating that the three different supramolecular structures had similar exciton coupling and intimate relations.35,36 In addition, the irregular spheres were least ordered while the microplates were most highly ordered, which was likely the reason for the observed nonmonotonous intensity trends. To gain a deep understanding of chirality, CD spectra were recorded for the assembled structures at different temperatures. Intense Cotton effects were observed for GTPBI in methanol/ water binary solvents at both 4 and 25 °C (Figure 3a). A bisignate signal is indicative of chiral excitonic coupling that arises when chromophores are aggregated in a helical fashion.37 The bisignate negative/positive signal with an increasing wavelength (within the absorption limits of perylene chromophores, from 375 to 575 nm) indicated a right-handed or clockwise (P) helical arrangement of the transition dipoles of

Figure 2. TEM images of assemblies formed from GTPBI in a 0.1 mg/ mL methanol/water mixture [50/50 (v/v)] at (a) 4, (c) 25, and (d) 50 °C. (b) High-resolution TEM image of twisted ribbons [the white arrows indicate the thickness of ribbons (see the Supporting Information)].

Figure 3. CD spectra of GTPBI in a 0.1 mg/mL methanol/water mixture [50/50 (v/v)] at different temperatures: (a) annealed at 4, 25, and 50 °C for 2 days, (b) assembled at 4 °C for 2 days followed by annealing at 25 and 50 °C for an additional 2 days, (c) assembled at 25 °C for 2 days followed by annealing at 4 and 50 °C for an additional 2 days, and (d) assembled at 50 °C for 2 days followed by annealing at 4 and 25 °C. 11042

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GTPBI molecules. Though microplates were apparently flat and achiral, the intense CD signal suggested that these microplates also had chiral molecular arrangements within the assembled structures. UV−vis absorption, fluorescence, and CD spectra clearly reveal that twisted ribbons and microplates are closely associated during the assembly process. To clarify this point, we varied temperature to track the evolution of structure using TEM. It was known that GTPBI assembled into microplates in a methanol/water mixture. Using this as a pristine solution, we did not observe a noticeable change in morphology when the sample solution temperature changed from 25 to 4 °C and from 25 to 50 °C. This suggested that preformed microplates were quite stable versus a mild change in temperature regardless of heating or cooling. In contrast, twisted ribbons can be easily converted into microplates when the sample solutions are heated from 4 to 25 °C or from 4 to 50 °C. From TEM characterization, the ratio of microplates to ribbons increased significantly with annealing time. Furthermore, if the sample solution was first annealed at 50 °C for 2 days before being cooled, a different final temperature resulted in the formation of distinct nanostructures. For example, twisted ribbons and microplates were obtained by cooling the solution from 50 to 4 °C and from 50 to 25 °C, respectively (Figure 4).

into microplates. Notably, some small irregular aggregates appeared when the twisted ribbon solution was heated from 4 to 50 °C. Therefore, the signal of the CD spectrum at 50 °C was less intense than that at 25 °C. The CD spectra in Figure 3b were in line with the changes in morphology. Moreover, twisted ribbons and microplates were obtained when the sample solution temperature was reduced from 50 to 4 °C and from 50 to 25 °C, respectively, which agreed well with the CD signal from silent to active shown in Figure 3d. Specifically, CD signals also appeared at longer wavelength, which could be assigned to chiral scattering of the light as a result of the interaction of the light with the chiral nanostructures.27 Given the results discussed above, we believed the twisted ribbons formed at 4 °C were at an upper state in comparison with the microplates formed at 25 °C. When the temperature was increased, the arranged molecules in the ribbons would absorb energy to clear the potential barrier to reach a lowerenergy state, accompanied by molecular rearrangements to form microplates. The crystalline structure of microplates was investigated by XRD. The XRD patterns (Figure S10 of the Supporting Information) displayed a series of diffraction peaks with 2θ = 5.0°, 10.0°, and 19.9°, implying the formation of a well-ordered crystalline structure. The d value corresponding to 2θ (5.0°) is 1.76 nm, which is in agreement with the molecular length. The SAED pattern of microplates in Figure S11 of the Supporting Information confirmed ordered arrangements within the crystals. To gain a better understanding of the formation mechanism, we traced the evolution of morphologies with TEM at different time intervals (Figure 5 and Figure S12 of the Supporting Information). The proposed mechanism is illustrated in Scheme 2. In the initial stage, irregular spheres formed when water was added to the GTPBI/methanol solution because of the sudden change in solubility. Then through synergistic effects of π−π stacking, hydrogen bonding, and solvent−solute interactions, the preformed spheres aggregated and fused into rough rods/ribbons. Gradually, twisted ribbons and microplates formed at 4 and 25 °C, respectively, after a long period of progression. For organic semiconductor materials, we have performed standard current−voltage (I−V) measurements of microplates formed from GTPBI at 25 °C using the so-called organic ribbon mask technique (Figure S13 of the Supporting Information). The conductivity of microplates was relatively lower compared to values published in other reports. We thought it was caused by the twisted molecular arrangements as shown in Scheme 2, which was disadvantageous for the charge carrier to transport along the π−π stacking direction.

Figure 4. TEM images of assemblies formed from GTPBI in a 0.1 mg/ mL methanol/water mixture [50/50 (v/v)] at different temperatures: (a) assembled at 4 °C for 2 days followed by annealing at 25 °C for an additional 2 days, (b) assembled at 4 °C for 2 days followed by annealing at 50 °C for an additional 2 days, (c) assembled at 50 °C for 2 days followed by annealing at 4 °C for an additional 2 days, and (d) assembled at 50 °C for 2 days followed by annealing at 25 °C for an additional 2 days.

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CONCLUSIONS In summary, we have successfully constructed chiral supramolecular structures via the thermally sensitive self-assembly of GTPBI, which forms twisted ribbons and microplates at different temperatures. We investigated the corresponding transformation of morphology with the change in temperature and demonstrated that temperature can affect the interaction of glucose moieties, which ultimately determined the assembly nanostructures in solution. This research provides insight into controlling molecular parameters and assembly conditions to create chiral supramolecular structures.

CD spectra clearly reveal the change in chirality when the temperature is varied (Figure 3). As microplates formed at 25 °C were stable, the CD spectra (Figure 3c) almost remained the same when the temperature changed, which was consistent with the TEM and SEM results. However, compared to microplates, the twisted ribbons formed at 4 °C were metastable with an increase in temperature and transformed 11043

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Figure 5. TEM images of the evolution of microplates formed from GTPBI in a 0.1 mg/mL methanol/water mixture (water content of 50 vol %) at 25 °C at different time intervals: (a) 0 min, (b) 5 min, (c) 15 min, (d) 30 min, (e) 45 min, (f) 60 min, (g) 2 h, (h) 6 h, (i) 8 h, (j) 10 h, (k) 12 h, and (l) 24 h.

Notes

Scheme 2. Illustration of the Self-Assembly Mechanism for Twisted Ribbons and Microplates of GTPBI

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21225209, 51225306, and 91027043).

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ASSOCIATED CONTENT

S Supporting Information *

Details of synthesis, specific rotations, optical spectra, additional microscopy images, XRD patterns, and DLS spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 11044

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