DOI: 10.1002/chem.201305006
Communication
& Supramolecular Flow Chemistry
Two-Dimensional Assembly Based on Flow Supramolecular Chemistry: Kinetic Control of Molecular Interactions Under Solvent Diffusion Munenori Numata* and Tomohiro Kozawa[a] processes; it can amplify the resultant nanostructures along the continuous flow, leading finally to the creation of novel supramolecular architectures that would never be accessible through conventional self-assembly. In our developed system, the final morphologies and dimensions of the self-assembled architectures can be regulated not only through molecular design but also through the hydrodynamic properties of the microfluidic system, which would be controllable in a topdown manner. Based on this concept, we have successfully created fibrous, tape, and rod-like architectures by combining designed molecules capable of one-dimensional (1D) assembly with appropriate flow conditions. In general, a microflow channel provides a 2D dynamic flowing liquid–liquid interface at which self-assembly of molecules can occur.[4] This interface is temporal in that a solvent gradation would appear across the stream, finally disappearing in the downstream region. In this present study, we applied such a temporal 2D template of miscible solvents (e.g., THF/water) as a self-assembling field for porphyrin derivatives having potential 2D assembling ability, expecting spontaneous creation of sheet-like porphyrin architectures on the micrometer scale. A feature of this system is that the solvent polarity gradually varies across the stream line. This gradation of the solvent, which can also be considered as a sort of temporal multilayered solvent structure, is basically controllable not only by changing the composition of the injected solvent but also by adjusting the diffusion length of solvent (i.e., width of solution layer) as well as flow rates. It is expected that the self-assemble rates of functional molecules, that is, self-assemble pathway, can be precisely tuned by regulating these factors. When the aggregation rate is comparable to the diffusion rates of solvents, self-assembly would occur on the diffusing solvent layer across the stream. In addition, when the concentration of the activated species surrounded by poor solvent is high enough under an appropriate flow rate, continuous intermolecular interaction would occur along the stream, leading to the creation of sheet-like self-assembled structures. To highlight these points, for this study we examined the self-assembly of tetraphenyl porphyrin (TPP) and its derivatives having n-butyl substituents (TPPBu) in the designed flowing field (Figure 1). These porphyrins would undergo different rates of self-assembly, thereby exerting different effects in the various solvent gradations in the microflow channel. In the following experiments, we mainly employed TPPBu because of its moderate rates of aggregation.
Abstract: Self-assembly of porphyrin molecules can be controlled kinetically to form structures with lengths extending from the nano- to the micrometer scale, through a programmed solvent-diffusion process in designed microflow spaces. Temporal solvent structures generated in the microflow were successfully transcribed into molecular architectures.
Porphyrins are functional molecules exhibiting photoconductivity and charge transport properties; they appear in connection with the natural light-harvesting complexes of chlorophylls used in photosynthesis.[1] Assembled structures of porphyrins, in particular two-dimensional (2D) architectures, with precisely regulated lengths, widths, and thicknesses have been attractive targets for many years because of their practical applications in organic photovoltaic devices based on bulk heterojunctions. In general, however, there are no versatile strategies at present for the creation of self-assembled molecular architectures with precisely tuned sizes and shapes, especially in the submicrometer regime. To this end, we would require a means of controlling intermolecular interactions over long distances. Herein, we report an approach for overcoming this limitation; we have used it to create 2D micrometer-sized architectures from porphyrin units through kinetically controlled self-assembly in a microflow channel.[2] We suspect that this developed strategy would also be applicable for other molecules and, therefore, overcome some of the fundamental problems encountered in supramolecular chemistry as well as in materials science. In previous studies, we exploited the possibility of using a microflow channel as a designable flowing field for the selfassembly of functional molecules[3c, d] and polymers.[3a, b] We have found that microfluidic systems enable us to tune intermolecular (or interpolymer) interactions in a way that is different form that encountered in thermodynamic self-assembly [a] Prof. Dr. M. Numata, T. Kozawa Department of Biomolecular Chemistry Graduate School of Life and Environmental Sciences Kyoto Prefectural University Shimogamo, Sakyo-ku, Kyoto 606-8522 (Japan) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201305006. Chem. Eur. J. 2014, 20, 1 – 7
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Communication stream and squeezed it between the THF/water mixed solvent at the first or second cross-point while varying the flow-rate ratio. At higher flow rates of side flows, the central flow stream was narrower, generating sharp gradation across the stream; in contrast, the gradation was flatter under slower flow rates of side flows (Figure S4 in the Supporting Information). This approach affected the length of time required for all of the TPPBu molecules to come into contact with the poor solvent (water), leading to kinetic control over the self-assembly process. When we injected an appropriate mixed solvent containing 56 % water from the first side legs and adjusted the flow-rate ratio between the center and side flows [(center flow)/(side flow)] to be 1.0:4.5, we obtained a final THF/water composition of 50:50 at the first cross-point, with no need to inject the second solvent. In this single-squeeze flow system, we estimated (from a microscopy image) the width of the central flow to be about 4 mm (Figure S4 in the Supporting Information). The UV/Vis spectra of the resultant solution of TPPBu immediately after it had eluted from the microflow channel are summarized Figure 2 a and b. Relative to the spectrum of the solution in the vial (THF/water, 50:50, v/v), we observed a hypochromic effect of the Soret band (green line), supporting the view that TPPBu self-assembled through p–p stacking interactions. The slight but significant blue-shift by 2.6 nm compared to the vial sample implies that TPPBu adopted H-type molecular packing, in contrast to the J-type aggregate prepared in the vial sample. The average full width at half maximum (FWHM) was 16.6 nm for the vial sample and this value was also changed after eluting from the channel (Figure 2). The absorption peak of solid-state aggregate, which was isolated from the eluted solution by centrifugation, was in good agreement with that of the eluted solution (Figure 2 e; green and light-green lines). This result clearly reveals that monomeric TPPBu would be quantitatively converted to its assembled structure under the present microflow condition. Next, we widened the central flow by changing the flowrate ratio at the first cross-point [(center flow)/(side flow)] to 1.0:2.0; here, we adjusted the final THF/water composition to 50:50 by injecting a second solvent from the second leg (for details, see the Supporting Information). In this doublesqueeze flow, we estimated the widths of the flow at the first and second cross-points to be about 8 and 20 mm, respectively. Accordingly, the width of the central flow was almost twotimes wider at the first cross-point compared to that in the single-squeeze flow.[5] The UV/Vis spectrum in Figure 2 (red line) reveals a slight blue-shift of 0.6 nm (FWHM; 17.6 nm) for the Soret band, suggesting that although TPPBu self-assembled in the microflow channel, its packing mode was slightly different from that under the single-squeeze flow conditions. In addition to the spectral shifts, microscopy images revealed that the TPPBu formed precipitated inside the channel under both single- and double-squeeze flow conditions. Together with the absence of any precipitate in the vial sample at the same solvent composition (THF/water, 50:50, v/v), this result suggests that injected TPPBu self-assembles to afford supramolecular assemblies under these-flow conditions.
Figure 1. a) Chemical structures of TPP and TPPBu. b) Schematic representation of the single- and double-squeeze flow systems and the 2D assembly of porphyrins at the liquid–liquid interface featuring a gradation of solvent polarity.
First, in a reference experiment, we investigated the fundamental self-assembly behavior of TPPBu in THF when mixed with water in a vial. UV/Vis spectra recorded upon changing the THF/water composition from 100:0 to 20:80 (v/v) revealed that the absorption at 410 nm decreased gradually and redshifted upon increasing the water content (Figure S1 in the Supporting Information). From this experiment, we determined the critical THF/water solvent composition leading to aggregation to be 50:50 (v/v). Accordingly, we adjusted the final THF/ water composition in our system to be 50:50 (v/v) such that TPPBu would exert its inherent self-assembling abilities. We designed a hydrodynamic flow focusing channel having a width of 100 mm, a depth of 45 mm, and two cross-points with an interval of 10 mm; the total flow length from the first cross-point was 95 mm (for details, see the Supporting Information). At low Reynolds numbers, the central solution is squeezed into a narrow stream between the two adjacent streams; solvent diffusion occurs rapidly across the stream line because of the very short diffusion distance. The width of the central flow determines the diffusion time of solvent; it can be regulated by changing the flow-rate ratio. In our experiments, we introduced a THF solution of TPPBu (50 mm) from a central &
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Communication portantly, the sizes and the thicknesses of the sheets were affected by the applied flow conditions. For the sample prepared under the single-squeeze flow, Figure 3 a reveals that the sheet length extended to several tens of micrometers and, therefore, it could be recognized under an optical microscope. Furthermore, tape-like structures having lengths and widths of approximately 10 and 2 mm, respectively, were also evident in the same SEM image (Figure 3 a). In the magnified image in Figure 3 d, multilayer structures can be recognized at the edges of the sheet and the tape structures. The average thickness of each layer was approximately 10 nm; the total thickness was approximately 50 nm. These layered structures would induce the significant peak broadening observed in the UV/Vis spectra Figure 2 a and b (green line). The spontaneous creation of these extended sheet structures in the microflow indicates that all of the injected TPPBu molecules would be activated at the same time toward self-assembly. In this case, narrow central flow would provide the uniform chemical environment for all TPPBu molecules because of short diffusion time of water, thus increasing in the local concentration of activated TPPBu that would be surrounded by water toward self-assembly. The structural similarity of the sheet and tape structures on the nanometer scale suggests that they shared a common mechanism Figure 2. UV/Vis spectra of: a), b) TPPBu, and c), d) TPP recorded under different flow conditions; [TPPBu] = [TPP] = 5.0 mm; 3 mm cell; RT. Green line (FWHM; 25.8 nm): single-squeeze flow; blue (FWHM; 17.6 nm) for their self-assembly in the miand red (FWHM; 17.6 nm) lines: double-squeeze flow. Each number represents the composition of water at first croflow. The formation of multiand second cross-point after complete mixing. TPPBu tended to precipitate in the microchannels; formation of layer sheet structures of conlarge aggregates in the channels resulted in variations in absorbance. e) Solid-state UV/Vis spectra of TPPBu under stant thickness supports our different flow conditions. f) Comparison UV/Vis spectra of samples prepared from TPPBu/TPP mixed solutions under the single-squeeze flow condition (for the magnified spectra, see Figure S12 in the Supporting Informaview that the gradation of the tion). layered solvent across the stream line, generated through diffusion of water and THF, formed a dynamic liquid–liquid inSEM images of the structures self-assembled from TPPBu terface that acted as a temporal template for the 2D self-asunder the single-squeeze (Figure 3 a–d; Figure S5 in the Supsembly of TPPBu. In other words, injected TPPBu assembled on porting Information) and double-squeeze (Figure 3 e and f) the flowing liquid template in a continuous manner, where flows revealed the creation of sheet- or tape-like structures. ImChem. Eur. J. 2014, 20, 1 – 7
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Communication neously during the process of rapid solvent diffusion across the stream, thereby affording well-extended sheet structure (vide infra). Matching the aggregation rate with the diffusion rate and the flow rate would be necessary for the successful creation of extended sheet structures. The aggregation rate would be controllable by the width of the central flow and the solvent polarity. In this system, the rate of aggregation of TPPBu at the cross-point had a crucial effect on the continuous self-assembly of TPPBu along the flow. To further clarify the mechanism of formation of the sheet structures in the double-squeeze flow, we changed the solvent composition introduced at the first crosspoint to affect the rate of aggregation of TPPBu. Accordingly, we adjusted the injected solvent polarity (water composition) to 25 % at the first cross-point and increased it drastically to 80 % at the second cross-point.[6] Here, the width of the central flow was the same as that in the previous double-squeeze flow system. Although the gradation in solvent polarity across the stream line became flatter at the first cross-point, it increased dramatically at the second cross-point; therefore, we expected that the self-assembly of TPPBu would occur gradually at the first Figure 3. SEM images of micrometer-sized sheet-like architectures of TPPBu formed cross-point, and then complete at the second crossunder: a)–d) single-, and e), f) double-squeeze flow conditions. b), d) Magnified image of point. The UV/Vis spectrum in Figure 2 (blue line) re(a) and (c), respectively. veals a slight blue-shift for the Soret band (FWHM; 17.6 nm), suggesting that its packing mode was slightly different from that under the single-squeeze flow condition. Again, the SEM images reveal the creation of concentration of monomeric TPPBu would be constant under sheet-like structures, but they were rather small (several microappropriate flow-rate ranges. Under the double-squeeze flow, meters) relative to the extended sheet structures produced in the thickness of the sheet became thinner compared to that the tightly squeezed central flow (Figure 4 a and b). These prepared under the single-squeeze flow condition. The correlation between the sheet thickness and the flow conditions were clearly revealed from the height profiles of the AFM images (Figure S6 in the Supporting Information). The creation of such a thin-sheet structure would be explained by the increased local solvent polarity; that is, in the double-squeeze flow, a mixed THF/water of 37:63 (v/v) was injected from the side legs, which would cause faster aggregation of TPPBu at the liquid–liquid interface even under wider diffusion length (vide infra).[5] In contrast to these results, we did not observe any such self-assembled structures in the sample prepared in the Figure 4. a) SEM image of a TPPBu structure formed in the double-squeeze vial (water/THF, 50:50, v/v; Figure S2 in the Supporting Inforflow system. b) Magnified view of the structure in (a). mation), supporting the notion that TPPBu has limited self-assembly ability under conventional self-assembly conditions, consistent with the UV/Vis spectral data. sheets were, however, remarkably thick (10–20 mm). In this The flowing THF/water interface appeared to act as a tempocase, the gradation of the solvent polarity was very flat at the ral template in which the TPPBu units aggregated spontanefirst cross-point, so that, as an initial stage, nuclei would graduously to afford nuclei (oligomeric aggregates) in the upper ally generate in the wider central flow. As a second stage, the stream region; these nuclei then interacted with other nuclei central flow containing nuclei was further squeezed by side or monomeric TPPBu units on the template. The creation of flows having higher solvent polarity, where the liquid–liquid innuclei would occur continuously at the cross-point, thus leadterface no longer act as an effective template for self-asseming to the creation of extended 2D structures. Under both bling TPPBu. Consequently, the monomeric TPPBu units might single- and double-squeeze flow conditions, the nucleation of have organized onto the nuclei predominantly across the all TPPBu molecules would presumably occur almost simulta&
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Communication stream line (i.e., not along the flow), thereby affording thicker sheet structures compared to the previous single- and doublesqueeze system. These results suggest that the rate of aggregation of TPPBu can be controlled precisely by changing either the diffusion length (i.e., the width of the central flow) or the solvent gradation across the stream, or both. In our systems, we fixed the total flow rate at 20 mL min 1. Accordingly, the single-squeeze flow system featured short diffusion time and a moderate solvent polarity; as a result, the rate of nucleation would be comparable with the flow rate and the efficient concentration of nuclei would be generated in the squeezed central flow, so that intermolecular interactions occurred continuously along the stream. The double-squeeze flow system featured a wider diffusion length (longer diffusion time), rendering a flatter gradation and resulting in a slower rate of nucleation relative to the flow rate. Even for a wide diffusion length, an extremely sharp solvent gradation (e.g., injected water composition of 63 %) at the first cross-point would induce local aggregation of TPPBu within very thin solvent layer, thus affording thinner sheet structures as shown in Figure 3 e and f. In this case, the self-assembly process would be mainly governed by the solvent polarity, and not be affected by the width of the central flow. Similarly, a flatter solvent gradation (e.g., injected water composition of 20 %) at the first cross-point would induce local aggregation of TPPBu moieties along the direction of solvent diffusion (across the flow direction), thereby affording thicker sheet structures. Notably, such a wide range of 2D structures could never be created through conventional self-assembly in a vial. Precise tuning of the nucleation process of TPPBu through the controlled solvent diffusion enabled us to regulate the self-assembly of TPPBu to form structures having dimensions ranging from the nano- to the micrometer scale. Next, to further support our notion that the self-assembly of TPPBu could be regulated by balancing the rate of aggregation with the diffusion rate and the flow rate, we tested the self-assembly behavior of TPP, which we expected to aggregate faster than TPPBu because of its lack of n-butyl substituents. Accordingly, we injected a solution of TPP in THF into the central leg and squeezed it between side flows under the single-squeeze flow conditions. The SEM images reveal, in sharp contrast to the well-extended sheet structures formed from TPPBu, polyhedral aggregates of TPP, presumably because of its more highly crystalline nature (Figure 5). Even
under the sharp gradation conditions, we could not observe any extended 2D structures similar to those displayed in Figure 3 a.[7] This finding reveals that the rate of aggregation of TPP was fast relative to the flow rate. In such a situation, the molecular design would mainly govern the nucleation process; in other words, the liquid–liquid interface would no longer act as an effective template for the self-assembly process. The rate of self-assembly of TPP could be controlled by mixing it with an appropriate composition of TPPBu. To further investigate the effect of the local concentration of TPP on the self-assembly behavior, we injected a solution containing both TPP and TPPBu into the central leg and squeezed it between the two side solutions. First, we injected a THF solution containing TPPBu (12.5 mm) and TPP (37.5 mm) ([TPPBu]/[TPP] = 1:3) from the central leg using the single-squeeze flow system (final total concentration was always 5 mm). The SEM images reveal the creation of tape- or rod-like structures having lengths of several tens of micrometers, in contrast to the polyhedral crystalline structures created from TPP alone (Figure 6 a
Figure 6. SEM images of microstructures formed from solutions containing mixtures of TPP and TPPBu under the single-squeeze flow condition. a) Rodlike structures obtained from the solution of [TPPBu]/[TPP] = 1:3; b) magnified image of (a). c) Sheet structures obtained from [TPPBu]/[TPP] = 3:1; d) magnified image of (c). For further images; see Figure S11 in the Supporting Information.
and b; Figure 5). We attribute this dramatic morphological transition to the inherent 2D self-assembly behavior of TPPBu. When we changed the [TPPBu]/[TPP] molar ratio to 3:1, the resulting microstructures were extended sheets (Figure 6 c and d) having average lengths of several micrometers (ca. 3 mm)— significantly shorter than the original sheet structures created from TPPBu alone. In addition, unlike the rectangular shapes of the original sheets (Figure 3 a–d), here the sheet structures had complex shapes with curved edges, presumably originating from the crystalline nature of TPP units. Furthermore, the thickness of each sheet was approximately 80 nm; we could not recognize any multilayer structures, as we had in the TPPBuonly sheets, for these sheet structures. These variations in mor-
Figure 5. a) SEM image of the microstructures formed from TPP in the single-squeeze flow system. b) Magnified view of the structure in (a). For further images; see Figure S10 in the Supporting Information. Chem. Eur. J. 2014, 20, 1 – 7
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Communication the solvent was THF/water (50:50, v/v). For further details, see the Supporting Information.
phology relative to the original self-assembled structures presumably arose from the decreased local concentrations of TPPBu and TPP. UV/Vis spectra clearly support this view; that is, absorption peak of the aggregated TPPBu and TPP in the microflow appeared at 417.8 (FWHM; 25.8) and 416.2 nm (FWHM; 13.0), respectively, whereas they were slightly red-shifted to 418.2 nm (FWHM; 20.0 nm) when the [TPPBu]/[TPP] molar ratio was set to 3:1 (Figure 2 f and Figure S12 in the Supporting Information). This means that self-assembly of TPPBu (or TPP) would be affected by TPP (or TPPBu) under the present flow conditions, thus affording unique architectures featured by their original assembled structures, without phase-separation. When the [TPPBu]/[TPP] molar ratio was at an appropriate value (e.g., 3:1), the rate of aggregation of TPP was comparable with the diffusion rate of solvent (the local concentration of the activated species would be constant under the flow rate) as a result of the dilution effect of excess TPPBu, thereby leading to the formation of extended sheets. In conclusion, we have demonstrated that a variety of micrometer-sized porphyrin architectures can be prepared using a combination of conventional supramolecular assembly (based on molecular design) and self-assembly mediated by a flow field generated within a microflow system. Our results suggest that the kinetic process of self-assembly can be controlled in terms of the diffusion length, flow rates and the gradation of solvent polarity across the stream in the designed flowing solvent. In the initial stages, the intermolecular interactions of the porphyrin units were regulated under such kinetic conditions; they were then amplified continuously along the flow in the second stage. This strategy would presumably also be applicable for the self-assembly of other functional molecules and polymers into various microstructures, thereby extending the frontiers of supramolecular chemistry and materials science.
Acknowledgements TEM and SEM observations were supported by the ZE Research Program, IAE (ZE25B-17). This study was supported financially by Grants-in-Aid for Scientific Research on Innovative Areas “Reaction Integration”. Keywords: microfluidics · supramolecular chemistry
A THF solution containing TPPBu and/or TPP (total concentration: 50 mm) was introduced as the central flow and squeezed between flows of THF/water of an appropriate composition. The flow rates of the central and lateral solutions were fixed at 2 and 9 mL min 1, respectively, providing a total flow rate of 20 mL min 1. Under the flow conditions, the central solution was always diluted by 10 times. The final concentration of the porphyrin(s) was 5 mm and
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[1] a) M. R. Wasielewski, Chem. Rev. 1992, 92, 435; b) A. Harriman, J.-P. Sauvage, Chem. Soc. Rev. 1996, 25, 41; c) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 1993, 26, 198; d) M.-S. Choi, T. Yamazaki, I. Yamazaki, T. Aida, Angew. Chem. 2004, 116, 152; Angew. Chem. Int. Ed. 2004, 43, 150. [2] For self-assembly under kinetic conditions, see: a) R. M. Capito, H. S. Azevedo, Y. S. Velichko, A. Mara, S. I. Stupp, Science 2008, 319, 1812; b) T. Soejima, M. Morikawa, N. Kimizuka, Small 2009, 5, 2043; c) B. A. Grzybowski, Chemistry in Motion, Wiley, Hoboken, 2009; d) P. A. Korevaar, S. J. George, A. J. Markvoort, M. M. J. Smulders, P. A. J. Hibers, A. P. H. J. Schenning, T. F. A. De Greef, E. W. Meijer, Nature 2012, 481, 492; e) J. Boekhoven, J. M. Poolman, C. Maity, F. Li, L. van der Mee, C. B. Minkenberg, E. Mendes, J. H. van Esch, R. Elkema, Nat. Chem. 2013, 5, 433. [3] a) M. Numata, Y. Takigami, M. Takayama, Chem. Lett. 2011, 40, 102; b) M. Numata, Y. Takigami, M. Takayama, T. Kozawa, N. Hirose, Chem. Eur. J. 2012, 18, 13008; c) M. Numata, M. Takayama, S. Shoji, H. Tamiaki, Chem. Lett. 2012, 41, 1689; d) M. Numata, T. Kozawa, Chem. Eur. J. 2013, 19, 12629. [4] a) P. J. A. Kenis, R. F. Ismagilov, G. M. Whitesides, Science 1999, 285, 83; b) Y. Song, J. Hormes, C. S. S. R. Kumar, Small 2008, 4, 698; c) M. Pumera, Chem. Commun. 2011, 47, 5671, and references therein. [5] The THF/water composition injected from the first leg was 63 %, more polar than the solvent employed in the single-squeeze flow system (56 %). [6] The final water composition at the first cross-point was 20 % (THF/water, 80:20, v/v). By injecting a mixed second solvent having an appropriate composition (THF/water, 20:80, v/v), the final water composition could be always adjusted to 50 % (THF/water, 50:50, v/v). [7] The self-assembly behavior of TPP was enhanced in the microflow; upon mixing a THF solution of TPP with water in a vial, we could not observe any of these self-assembled structure (Figure S3 in the Supporting Information).
Experimental Section
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COMMUNICATION & Supramolecular Flow Chemistry M. Numata,* T. Kozawa && – &&
Go with the flow: Self-assembly of porphyrin molecules can be controlled kinetically through a programmed solvent diffusion process in designed microflow
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spaces (see figure). Temporal solvent structures generated in the microflow were successfully transcribed into molecular architectures.
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Two-Dimensional Assembly Based on Flow Supramolecular Chemistry: Kinetic Control of Molecular Interactions Under Solvent Diffusion
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Two-Dimensional Assembly Based on Flow Supramolecular Chemistry: Kinetic Control of Molecular Interactions Under Solvent Diffusion Munenori Numata* and Tomohiro Kozawa[a] processes; it can amplify the resultant nanostructures along the continuous flow, leading finally to the creation of novel supramolecular architectures that would never be accessible through conventional self-assembly. In our developed system, the final morphologies and dimensions of the self-assembled architectures can be regulated not only through molecular design but also through the hydrodynamic properties of the microfluidic system, which would be controllable in a topdown manner. Based on this concept, we have successfully created fibrous, tape, and rod-like architectures by combining designed molecules capable of one-dimensional (1D) assembly with appropriate flow conditions. In general, a microflow channel provides a 2D dynamic flowing liquid–liquid interface at which self-assembly of molecules can occur.[4] This interface is temporal in that a solvent gradation would appear across the stream, finally disappearing in the downstream region. In this present study, we applied such a temporal 2D template of miscible solvents (e.g., THF/water) as a self-assembling field for porphyrin derivatives having potential 2D assembling ability, expecting spontaneous creation of sheet-like porphyrin architectures on the micrometer scale. A feature of this system is that the solvent polarity gradually varies across the stream line. This gradation of the solvent, which can also be considered as a sort of temporal multilayered solvent structure, is basically controllable not only by changing the composition of the injected solvent but also by adjusting the diffusion length of solvent (i.e., width of solution layer) as well as flow rates. It is expected that the self-assemble rates of functional molecules, that is, self-assemble pathway, can be precisely tuned by regulating these factors. When the aggregation rate is comparable to the diffusion rates of solvents, self-assembly would occur on the diffusing solvent layer across the stream. In addition, when the concentration of the activated species surrounded by poor solvent is high enough under an appropriate flow rate, continuous intermolecular interaction would occur along the stream, leading to the creation of sheet-like self-assembled structures. To highlight these points, for this study we examined the self-assembly of tetraphenyl porphyrin (TPP) and its derivatives having n-butyl substituents (TPPBu) in the designed flowing field (Figure 1). These porphyrins would undergo different rates of self-assembly, thereby exerting different effects in the various solvent gradations in the microflow channel. In the following experiments, we mainly employed TPPBu because of its moderate rates of aggregation.
Abstract: Self-assembly of porphyrin molecules can be controlled kinetically to form structures with lengths extending from the nano- to the micrometer scale, through a programmed solvent-diffusion process in designed microflow spaces. Temporal solvent structures generated in the microflow were successfully transcribed into molecular architectures.
Porphyrins are functional molecules exhibiting photoconductivity and charge transport properties; they appear in connection with the natural light-harvesting complexes of chlorophylls used in photosynthesis.[1] Assembled structures of porphyrins, in particular two-dimensional (2D) architectures, with precisely regulated lengths, widths, and thicknesses have been attractive targets for many years because of their practical applications in organic photovoltaic devices based on bulk heterojunctions. In general, however, there are no versatile strategies at present for the creation of self-assembled molecular architectures with precisely tuned sizes and shapes, especially in the submicrometer regime. To this end, we would require a means of controlling intermolecular interactions over long distances. Herein, we report an approach for overcoming this limitation; we have used it to create 2D micrometer-sized architectures from porphyrin units through kinetically controlled self-assembly in a microflow channel.[2] We suspect that this developed strategy would also be applicable for other molecules and, therefore, overcome some of the fundamental problems encountered in supramolecular chemistry as well as in materials science. In previous studies, we exploited the possibility of using a microflow channel as a designable flowing field for the selfassembly of functional molecules[3c, d] and polymers.[3a, b] We have found that microfluidic systems enable us to tune intermolecular (or interpolymer) interactions in a way that is different form that encountered in thermodynamic self-assembly [a] Prof. Dr. M. Numata, T. Kozawa Department of Biomolecular Chemistry Graduate School of Life and Environmental Sciences Kyoto Prefectural University Shimogamo, Sakyo-ku, Kyoto 606-8522 (Japan) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201305006. Chem. Eur. J. 2014, 20, 1 – 7
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Communication stream and squeezed it between the THF/water mixed solvent at the first or second cross-point while varying the flow-rate ratio. At higher flow rates of side flows, the central flow stream was narrower, generating sharp gradation across the stream; in contrast, the gradation was flatter under slower flow rates of side flows (Figure S4 in the Supporting Information). This approach affected the length of time required for all of the TPPBu molecules to come into contact with the poor solvent (water), leading to kinetic control over the self-assembly process. When we injected an appropriate mixed solvent containing 56 % water from the first side legs and adjusted the flow-rate ratio between the center and side flows [(center flow)/(side flow)] to be 1.0:4.5, we obtained a final THF/water composition of 50:50 at the first cross-point, with no need to inject the second solvent. In this single-squeeze flow system, we estimated (from a microscopy image) the width of the central flow to be about 4 mm (Figure S4 in the Supporting Information). The UV/Vis spectra of the resultant solution of TPPBu immediately after it had eluted from the microflow channel are summarized Figure 2 a and b. Relative to the spectrum of the solution in the vial (THF/water, 50:50, v/v), we observed a hypochromic effect of the Soret band (green line), supporting the view that TPPBu self-assembled through p–p stacking interactions. The slight but significant blue-shift by 2.6 nm compared to the vial sample implies that TPPBu adopted H-type molecular packing, in contrast to the J-type aggregate prepared in the vial sample. The average full width at half maximum (FWHM) was 16.6 nm for the vial sample and this value was also changed after eluting from the channel (Figure 2). The absorption peak of solid-state aggregate, which was isolated from the eluted solution by centrifugation, was in good agreement with that of the eluted solution (Figure 2 e; green and light-green lines). This result clearly reveals that monomeric TPPBu would be quantitatively converted to its assembled structure under the present microflow condition. Next, we widened the central flow by changing the flowrate ratio at the first cross-point [(center flow)/(side flow)] to 1.0:2.0; here, we adjusted the final THF/water composition to 50:50 by injecting a second solvent from the second leg (for details, see the Supporting Information). In this doublesqueeze flow, we estimated the widths of the flow at the first and second cross-points to be about 8 and 20 mm, respectively. Accordingly, the width of the central flow was almost twotimes wider at the first cross-point compared to that in the single-squeeze flow.[5] The UV/Vis spectrum in Figure 2 (red line) reveals a slight blue-shift of 0.6 nm (FWHM; 17.6 nm) for the Soret band, suggesting that although TPPBu self-assembled in the microflow channel, its packing mode was slightly different from that under the single-squeeze flow conditions. In addition to the spectral shifts, microscopy images revealed that the TPPBu formed precipitated inside the channel under both single- and double-squeeze flow conditions. Together with the absence of any precipitate in the vial sample at the same solvent composition (THF/water, 50:50, v/v), this result suggests that injected TPPBu self-assembles to afford supramolecular assemblies under these-flow conditions.
Figure 1. a) Chemical structures of TPP and TPPBu. b) Schematic representation of the single- and double-squeeze flow systems and the 2D assembly of porphyrins at the liquid–liquid interface featuring a gradation of solvent polarity.
First, in a reference experiment, we investigated the fundamental self-assembly behavior of TPPBu in THF when mixed with water in a vial. UV/Vis spectra recorded upon changing the THF/water composition from 100:0 to 20:80 (v/v) revealed that the absorption at 410 nm decreased gradually and redshifted upon increasing the water content (Figure S1 in the Supporting Information). From this experiment, we determined the critical THF/water solvent composition leading to aggregation to be 50:50 (v/v). Accordingly, we adjusted the final THF/ water composition in our system to be 50:50 (v/v) such that TPPBu would exert its inherent self-assembling abilities. We designed a hydrodynamic flow focusing channel having a width of 100 mm, a depth of 45 mm, and two cross-points with an interval of 10 mm; the total flow length from the first cross-point was 95 mm (for details, see the Supporting Information). At low Reynolds numbers, the central solution is squeezed into a narrow stream between the two adjacent streams; solvent diffusion occurs rapidly across the stream line because of the very short diffusion distance. The width of the central flow determines the diffusion time of solvent; it can be regulated by changing the flow-rate ratio. In our experiments, we introduced a THF solution of TPPBu (50 mm) from a central &
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Communication portantly, the sizes and the thicknesses of the sheets were affected by the applied flow conditions. For the sample prepared under the single-squeeze flow, Figure 3 a reveals that the sheet length extended to several tens of micrometers and, therefore, it could be recognized under an optical microscope. Furthermore, tape-like structures having lengths and widths of approximately 10 and 2 mm, respectively, were also evident in the same SEM image (Figure 3 a). In the magnified image in Figure 3 d, multilayer structures can be recognized at the edges of the sheet and the tape structures. The average thickness of each layer was approximately 10 nm; the total thickness was approximately 50 nm. These layered structures would induce the significant peak broadening observed in the UV/Vis spectra Figure 2 a and b (green line). The spontaneous creation of these extended sheet structures in the microflow indicates that all of the injected TPPBu molecules would be activated at the same time toward self-assembly. In this case, narrow central flow would provide the uniform chemical environment for all TPPBu molecules because of short diffusion time of water, thus increasing in the local concentration of activated TPPBu that would be surrounded by water toward self-assembly. The structural similarity of the sheet and tape structures on the nanometer scale suggests that they shared a common mechanism Figure 2. UV/Vis spectra of: a), b) TPPBu, and c), d) TPP recorded under different flow conditions; [TPPBu] = [TPP] = 5.0 mm; 3 mm cell; RT. Green line (FWHM; 25.8 nm): single-squeeze flow; blue (FWHM; 17.6 nm) for their self-assembly in the miand red (FWHM; 17.6 nm) lines: double-squeeze flow. Each number represents the composition of water at first croflow. The formation of multiand second cross-point after complete mixing. TPPBu tended to precipitate in the microchannels; formation of layer sheet structures of conlarge aggregates in the channels resulted in variations in absorbance. e) Solid-state UV/Vis spectra of TPPBu under stant thickness supports our different flow conditions. f) Comparison UV/Vis spectra of samples prepared from TPPBu/TPP mixed solutions under the single-squeeze flow condition (for the magnified spectra, see Figure S12 in the Supporting Informaview that the gradation of the tion). layered solvent across the stream line, generated through diffusion of water and THF, formed a dynamic liquid–liquid inSEM images of the structures self-assembled from TPPBu terface that acted as a temporal template for the 2D self-asunder the single-squeeze (Figure 3 a–d; Figure S5 in the Supsembly of TPPBu. In other words, injected TPPBu assembled on porting Information) and double-squeeze (Figure 3 e and f) the flowing liquid template in a continuous manner, where flows revealed the creation of sheet- or tape-like structures. ImChem. Eur. J. 2014, 20, 1 – 7
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Communication neously during the process of rapid solvent diffusion across the stream, thereby affording well-extended sheet structure (vide infra). Matching the aggregation rate with the diffusion rate and the flow rate would be necessary for the successful creation of extended sheet structures. The aggregation rate would be controllable by the width of the central flow and the solvent polarity. In this system, the rate of aggregation of TPPBu at the cross-point had a crucial effect on the continuous self-assembly of TPPBu along the flow. To further clarify the mechanism of formation of the sheet structures in the double-squeeze flow, we changed the solvent composition introduced at the first crosspoint to affect the rate of aggregation of TPPBu. Accordingly, we adjusted the injected solvent polarity (water composition) to 25 % at the first cross-point and increased it drastically to 80 % at the second cross-point.[6] Here, the width of the central flow was the same as that in the previous double-squeeze flow system. Although the gradation in solvent polarity across the stream line became flatter at the first cross-point, it increased dramatically at the second cross-point; therefore, we expected that the self-assembly of TPPBu would occur gradually at the first Figure 3. SEM images of micrometer-sized sheet-like architectures of TPPBu formed cross-point, and then complete at the second crossunder: a)–d) single-, and e), f) double-squeeze flow conditions. b), d) Magnified image of point. The UV/Vis spectrum in Figure 2 (blue line) re(a) and (c), respectively. veals a slight blue-shift for the Soret band (FWHM; 17.6 nm), suggesting that its packing mode was slightly different from that under the single-squeeze flow condition. Again, the SEM images reveal the creation of concentration of monomeric TPPBu would be constant under sheet-like structures, but they were rather small (several microappropriate flow-rate ranges. Under the double-squeeze flow, meters) relative to the extended sheet structures produced in the thickness of the sheet became thinner compared to that the tightly squeezed central flow (Figure 4 a and b). These prepared under the single-squeeze flow condition. The correlation between the sheet thickness and the flow conditions were clearly revealed from the height profiles of the AFM images (Figure S6 in the Supporting Information). The creation of such a thin-sheet structure would be explained by the increased local solvent polarity; that is, in the double-squeeze flow, a mixed THF/water of 37:63 (v/v) was injected from the side legs, which would cause faster aggregation of TPPBu at the liquid–liquid interface even under wider diffusion length (vide infra).[5] In contrast to these results, we did not observe any such self-assembled structures in the sample prepared in the Figure 4. a) SEM image of a TPPBu structure formed in the double-squeeze vial (water/THF, 50:50, v/v; Figure S2 in the Supporting Inforflow system. b) Magnified view of the structure in (a). mation), supporting the notion that TPPBu has limited self-assembly ability under conventional self-assembly conditions, consistent with the UV/Vis spectral data. sheets were, however, remarkably thick (10–20 mm). In this The flowing THF/water interface appeared to act as a tempocase, the gradation of the solvent polarity was very flat at the ral template in which the TPPBu units aggregated spontanefirst cross-point, so that, as an initial stage, nuclei would graduously to afford nuclei (oligomeric aggregates) in the upper ally generate in the wider central flow. As a second stage, the stream region; these nuclei then interacted with other nuclei central flow containing nuclei was further squeezed by side or monomeric TPPBu units on the template. The creation of flows having higher solvent polarity, where the liquid–liquid innuclei would occur continuously at the cross-point, thus leadterface no longer act as an effective template for self-asseming to the creation of extended 2D structures. Under both bling TPPBu. Consequently, the monomeric TPPBu units might single- and double-squeeze flow conditions, the nucleation of have organized onto the nuclei predominantly across the all TPPBu molecules would presumably occur almost simulta&
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Communication stream line (i.e., not along the flow), thereby affording thicker sheet structures compared to the previous single- and doublesqueeze system. These results suggest that the rate of aggregation of TPPBu can be controlled precisely by changing either the diffusion length (i.e., the width of the central flow) or the solvent gradation across the stream, or both. In our systems, we fixed the total flow rate at 20 mL min 1. Accordingly, the single-squeeze flow system featured short diffusion time and a moderate solvent polarity; as a result, the rate of nucleation would be comparable with the flow rate and the efficient concentration of nuclei would be generated in the squeezed central flow, so that intermolecular interactions occurred continuously along the stream. The double-squeeze flow system featured a wider diffusion length (longer diffusion time), rendering a flatter gradation and resulting in a slower rate of nucleation relative to the flow rate. Even for a wide diffusion length, an extremely sharp solvent gradation (e.g., injected water composition of 63 %) at the first cross-point would induce local aggregation of TPPBu within very thin solvent layer, thus affording thinner sheet structures as shown in Figure 3 e and f. In this case, the self-assembly process would be mainly governed by the solvent polarity, and not be affected by the width of the central flow. Similarly, a flatter solvent gradation (e.g., injected water composition of 20 %) at the first cross-point would induce local aggregation of TPPBu moieties along the direction of solvent diffusion (across the flow direction), thereby affording thicker sheet structures. Notably, such a wide range of 2D structures could never be created through conventional self-assembly in a vial. Precise tuning of the nucleation process of TPPBu through the controlled solvent diffusion enabled us to regulate the self-assembly of TPPBu to form structures having dimensions ranging from the nano- to the micrometer scale. Next, to further support our notion that the self-assembly of TPPBu could be regulated by balancing the rate of aggregation with the diffusion rate and the flow rate, we tested the self-assembly behavior of TPP, which we expected to aggregate faster than TPPBu because of its lack of n-butyl substituents. Accordingly, we injected a solution of TPP in THF into the central leg and squeezed it between side flows under the single-squeeze flow conditions. The SEM images reveal, in sharp contrast to the well-extended sheet structures formed from TPPBu, polyhedral aggregates of TPP, presumably because of its more highly crystalline nature (Figure 5). Even
under the sharp gradation conditions, we could not observe any extended 2D structures similar to those displayed in Figure 3 a.[7] This finding reveals that the rate of aggregation of TPP was fast relative to the flow rate. In such a situation, the molecular design would mainly govern the nucleation process; in other words, the liquid–liquid interface would no longer act as an effective template for the self-assembly process. The rate of self-assembly of TPP could be controlled by mixing it with an appropriate composition of TPPBu. To further investigate the effect of the local concentration of TPP on the self-assembly behavior, we injected a solution containing both TPP and TPPBu into the central leg and squeezed it between the two side solutions. First, we injected a THF solution containing TPPBu (12.5 mm) and TPP (37.5 mm) ([TPPBu]/[TPP] = 1:3) from the central leg using the single-squeeze flow system (final total concentration was always 5 mm). The SEM images reveal the creation of tape- or rod-like structures having lengths of several tens of micrometers, in contrast to the polyhedral crystalline structures created from TPP alone (Figure 6 a
Figure 6. SEM images of microstructures formed from solutions containing mixtures of TPP and TPPBu under the single-squeeze flow condition. a) Rodlike structures obtained from the solution of [TPPBu]/[TPP] = 1:3; b) magnified image of (a). c) Sheet structures obtained from [TPPBu]/[TPP] = 3:1; d) magnified image of (c). For further images; see Figure S11 in the Supporting Information.
and b; Figure 5). We attribute this dramatic morphological transition to the inherent 2D self-assembly behavior of TPPBu. When we changed the [TPPBu]/[TPP] molar ratio to 3:1, the resulting microstructures were extended sheets (Figure 6 c and d) having average lengths of several micrometers (ca. 3 mm)— significantly shorter than the original sheet structures created from TPPBu alone. In addition, unlike the rectangular shapes of the original sheets (Figure 3 a–d), here the sheet structures had complex shapes with curved edges, presumably originating from the crystalline nature of TPP units. Furthermore, the thickness of each sheet was approximately 80 nm; we could not recognize any multilayer structures, as we had in the TPPBuonly sheets, for these sheet structures. These variations in mor-
Figure 5. a) SEM image of the microstructures formed from TPP in the single-squeeze flow system. b) Magnified view of the structure in (a). For further images; see Figure S10 in the Supporting Information. Chem. Eur. J. 2014, 20, 1 – 7
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Communication the solvent was THF/water (50:50, v/v). For further details, see the Supporting Information.
phology relative to the original self-assembled structures presumably arose from the decreased local concentrations of TPPBu and TPP. UV/Vis spectra clearly support this view; that is, absorption peak of the aggregated TPPBu and TPP in the microflow appeared at 417.8 (FWHM; 25.8) and 416.2 nm (FWHM; 13.0), respectively, whereas they were slightly red-shifted to 418.2 nm (FWHM; 20.0 nm) when the [TPPBu]/[TPP] molar ratio was set to 3:1 (Figure 2 f and Figure S12 in the Supporting Information). This means that self-assembly of TPPBu (or TPP) would be affected by TPP (or TPPBu) under the present flow conditions, thus affording unique architectures featured by their original assembled structures, without phase-separation. When the [TPPBu]/[TPP] molar ratio was at an appropriate value (e.g., 3:1), the rate of aggregation of TPP was comparable with the diffusion rate of solvent (the local concentration of the activated species would be constant under the flow rate) as a result of the dilution effect of excess TPPBu, thereby leading to the formation of extended sheets. In conclusion, we have demonstrated that a variety of micrometer-sized porphyrin architectures can be prepared using a combination of conventional supramolecular assembly (based on molecular design) and self-assembly mediated by a flow field generated within a microflow system. Our results suggest that the kinetic process of self-assembly can be controlled in terms of the diffusion length, flow rates and the gradation of solvent polarity across the stream in the designed flowing solvent. In the initial stages, the intermolecular interactions of the porphyrin units were regulated under such kinetic conditions; they were then amplified continuously along the flow in the second stage. This strategy would presumably also be applicable for the self-assembly of other functional molecules and polymers into various microstructures, thereby extending the frontiers of supramolecular chemistry and materials science.
Acknowledgements TEM and SEM observations were supported by the ZE Research Program, IAE (ZE25B-17). This study was supported financially by Grants-in-Aid for Scientific Research on Innovative Areas “Reaction Integration”. Keywords: microfluidics · supramolecular chemistry
A THF solution containing TPPBu and/or TPP (total concentration: 50 mm) was introduced as the central flow and squeezed between flows of THF/water of an appropriate composition. The flow rates of the central and lateral solutions were fixed at 2 and 9 mL min 1, respectively, providing a total flow rate of 20 mL min 1. Under the flow conditions, the central solution was always diluted by 10 times. The final concentration of the porphyrin(s) was 5 mm and
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[1] a) M. R. Wasielewski, Chem. Rev. 1992, 92, 435; b) A. Harriman, J.-P. Sauvage, Chem. Soc. Rev. 1996, 25, 41; c) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 1993, 26, 198; d) M.-S. Choi, T. Yamazaki, I. Yamazaki, T. Aida, Angew. Chem. 2004, 116, 152; Angew. Chem. Int. Ed. 2004, 43, 150. [2] For self-assembly under kinetic conditions, see: a) R. M. Capito, H. S. Azevedo, Y. S. Velichko, A. Mara, S. I. Stupp, Science 2008, 319, 1812; b) T. Soejima, M. Morikawa, N. Kimizuka, Small 2009, 5, 2043; c) B. A. Grzybowski, Chemistry in Motion, Wiley, Hoboken, 2009; d) P. A. Korevaar, S. J. George, A. J. Markvoort, M. M. J. Smulders, P. A. J. Hibers, A. P. H. J. Schenning, T. F. A. De Greef, E. W. Meijer, Nature 2012, 481, 492; e) J. Boekhoven, J. M. Poolman, C. Maity, F. Li, L. van der Mee, C. B. Minkenberg, E. Mendes, J. H. van Esch, R. Elkema, Nat. Chem. 2013, 5, 433. [3] a) M. Numata, Y. Takigami, M. Takayama, Chem. Lett. 2011, 40, 102; b) M. Numata, Y. Takigami, M. Takayama, T. Kozawa, N. Hirose, Chem. Eur. J. 2012, 18, 13008; c) M. Numata, M. Takayama, S. Shoji, H. Tamiaki, Chem. Lett. 2012, 41, 1689; d) M. Numata, T. Kozawa, Chem. Eur. J. 2013, 19, 12629. [4] a) P. J. A. Kenis, R. F. Ismagilov, G. M. Whitesides, Science 1999, 285, 83; b) Y. Song, J. Hormes, C. S. S. R. Kumar, Small 2008, 4, 698; c) M. Pumera, Chem. Commun. 2011, 47, 5671, and references therein. [5] The THF/water composition injected from the first leg was 63 %, more polar than the solvent employed in the single-squeeze flow system (56 %). [6] The final water composition at the first cross-point was 20 % (THF/water, 80:20, v/v). By injecting a mixed second solvent having an appropriate composition (THF/water, 20:80, v/v), the final water composition could be always adjusted to 50 % (THF/water, 50:50, v/v). [7] The self-assembly behavior of TPP was enhanced in the microflow; upon mixing a THF solution of TPP with water in a vial, we could not observe any of these self-assembled structure (Figure S3 in the Supporting Information).
Experimental Section
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COMMUNICATION & Supramolecular Flow Chemistry M. Numata,* T. Kozawa && – &&
Go with the flow: Self-assembly of porphyrin molecules can be controlled kinetically through a programmed solvent diffusion process in designed microflow
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spaces (see figure). Temporal solvent structures generated in the microflow were successfully transcribed into molecular architectures.
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Two-Dimensional Assembly Based on Flow Supramolecular Chemistry: Kinetic Control of Molecular Interactions Under Solvent Diffusion
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