Photocontrolled Supramolecular Assembling of Azobenzene-Based

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Photo-controlled supramolecular assembling of azobenzenebased biscalix[4]arenes upon starting and stopping laser trapping Ken-ichi Yuyama, Lionel Marcélis, Pei-Mei Su, Wen-Sheng Chung, and Hiroshi Masuhara Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03780 • Publication Date (Web): 29 Dec 2016 Downloaded from http://pubs.acs.org on January 1, 2017

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Photo-controlled

supramolecular

assembling

of

azobenzene-based biscalix[4]arenes upon starting and stopping laser trapping Ken-ichi Yuyama#, Lionel Marcelis, Pei-Mei Su, Wen-Sheng Chung*, and Hiroshi Masuhara* Department of Applied Chemistry and Institute of Molecular Science, National Chiao Tung University, Hsinchu 30010, Taiwan.

Keywords: Laser trapping – Calixarene – Supramolecular chemistry – Gelator – Azobenzene.

Abstract: Laser trapping in chemistry covers various studies from single molecules, nanoparticles, and quantum dots to crystallization and liquid-liquid phase separation of amino acids. In this work, a supramolecular assembly of azobenzene-based biscalix[4]arene is generated in ethyl acetate by laser trapping; its nucleation and growth are elucidated. No trapping behavior was observed when a 1064 nm laser beam was focused inside the solution; however, interesting assembling phenomena were induced when it was shined at the air/solution interface. A single disk having two-layers was first prepared at the focal point of ~1 µm and then expanded to the size of a few tens µm, although no optical force is exerted outside the focal volume. Upon

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switching the trapping laser off, needles were generated at the outer-layer of the assembly, giving a stable sea urchin-like morphology to the generated assembly. At a 30–50% dilution of the initial solution in ethyl acetate, a mushroom-like morphology was also observed. Laser trappinginduced assembly of azobenzene-based biscalix[4]arene was quite different from the sharpellipsoidal aggregates obtained by spontaneous evaporation of the solution. These trapping phenomena were specifically observed for the biscalix[4]arene in the trans conformation of azobenzene moiety but not for the cis-form, suggesting that the laser trapping of this azobenzenebased biscalix[4]arene is photo-controllable. Dynamics and mechanism of the supramolecular assembling is considered, referring to laser trapping-induced nucleation and liquid-liquid phase separation of amino acids.


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INTRODUCTION Ashkin et al. developed laser trapping using a tightly focused single laser beam in 1986.1 After that, it has played innovative roles not only in physics, optics, and biology but also in chemistry and materials science by trapping various small objects of the size ranging from micrometer to nanometer in solution.2–11 Laser trapping in dilute colloidal solution has been demonstrated for single nanometer-sized objects like polymeric nanosphere, plasmonic nanoparticle, nanowire, nanorod, and quantum dot.12–15 In molecular and colloidal solutions of relatively high concentration, numerous objects are confined in the focal volume of the trapping laser; in this process, which we have called Just Trapping,16 one sphere-like assembly is formed at the focus. This optical assembly formation has been demonstrated for polymer chain, micelle, vesicle, amino acid, protein, as well as above targets.17–25 When the molecules associated at the focus evolve to a nucleus for liquid-liquid phase separation or crystallization, the resultant liquid droplet or crystal grows outwardly from the focus, and their size is increased up to millimeter scale in some cases. We have classified these nucleation phenomena as Trapping Nucleation & Growth.16 It should be noted that these nucleation phenomena were achieved by laser trapping of amino acid not inside the solution but only at solution surface and glass/solution interface. Through laser trapping experiments of various molecular systems, we found also an intermediate case in which molecular orientations, intermolecular interactions, heating, and convection extend the molecular assembly outwardly from the focus. The assembly propagates up to a few tens micrometers distance, which is recognized as Extended Trapping.16 In a liquid crystal film, anisotropic orientation expands up to 20 µm, showing a domain around the focus.26 An assembly of the similar size was also formed for poly(N-isopropylacrylamide) (PNIPAM) through the coupling of laser trapping and local temperature elevation inducing its sol-gel transition.27


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In laser trapping of molecular systems, a single assembly prepared around and pinned at the focus is usually visible under a microscope as Just Trapping and Extended Trapping, while we also reported invisible trapping phenomena by studying crystallization of amino acid and crystal growth of protein in their aqueous solutions.16,28 A large sized, highly concentrated domain consisting of clusters of amino acid and protein is formed through liquid nucleation. In this case, many solvent molecules are associated with solute amino acid or protein, which is often called as liquid-like clusters in the field of crystallization and crystal growth.29,30 We considered that specific association and orientation at the focus propagate to a large distant area through hydrogen-bonding interactions among the liquid-like clusters. The large domain which is considered as a precursor state of their crystals is stable as long as the trapping laser is irradiated and it disappears upon switching off the laser.16,28 In addition to crystallization, liquid-liquid phase separation, and domain formation of liquid-like clusters, it was reported that laser trapping of supramolecules in solution resulted in gel formation. We studied wire-type dendrimers consisting of a rigid and fully conjugated poly(phenylene ethynylene) (PPE) backbone wrapped with flexible poly(benzyl ether) dendritic wedges.31 The dendrimer solution underwent phase separation during the spontaneous drying process of its THF solution, giving micrometer-sized sphere gels. These spheres included THF and were mechanically soft, so that they could be manipulated by laser trapping. A single spherelike gel showed contraction and swelling upon switching on and off the laser, respectively. Simultaneously their fluorescence spectra were measured and interpreted as the change in the conjugation length of PPE. Namely, laser trapping controlled not only gel morphology but also molecular conformation occurring inside the gel. Thus, it was demonstrated possible to form and manipulate unique molecular assemblies in which non-covalent interactions efficiently work.


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Supramolecular gels derived from low molecular mass compounds are formed through self-assembly of molecules by multiple non-covalent interactions such as hydrogen bonding, π-π stacking, dipole-dipole interactions, metal-ligand coordination, van der Walls force, and solutesolvent interactions.32,33 Transition from solution to gel is dependent on the concentration and characterized by a concentration above which gel formation occurs. Therefore, we anticipated that laser trapping in solutions of a known gelator at a concentration below its critical-gel formation concentration (CGC) could result in a spatio-temporally controlled gel formation, similarly to the domain formation of amino acid liquid-like clusters. Laser trapping in these solutions would operate as a new perturbation for assembling gelator molecules, favoring the formation of their cooperative network of non-covalent interactions. Interestingly, it is reported that supramolecular gels using photoisomerizable groups are sensitive to photo-irradiation, and that the gel-sol transition and morphology transformation can thus be controlled by light.34–37 In this context, we recently synthesized supramolecular gelators based on an azobenzene-based biscalix[4]arene scaffold and other aromatic bisisoxazole biscalix[4]arene derivatives.38–40 In this paper, we used such an azobenzene-based biscalix[4]arene (Figure 1) for laser trapping experiments. We demonstrated that laser trapping of this biscalixarene can be achieved at solution surface and leads to the formation of a large supramolecular assembly. Furthermore, additional UV irradiation causing photoisomerization of the central azobenzene part significantly alters the behavior and dynamics of the assembly formation by laser trapping. This result will be an important milestone for unique molecular assembly formation not only triggered by the use of classic laser trapping but also controlled by additional UV-visible irradiation.


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Figure 1: Chemical structure of azobenzene-based biscalix[4]arene.

EXPERIMENTAL SECTION The synthesis of the azobenzene-based biscalix[4]arene (Figure 1) and its conventional gelation behavior in bulk solution will be reported separately. The sample solutions for laser trapping experiments were prepared by dissolving the calixarene into 6 different organic solvents; acetonitrile, ethanol, toluene, chloroform, dimethyl sulfoxide, and ethyl acetate. Acetonitrile and ethanol are gel-inducer solvents, and their CGCs are 14 and 1.6 mg/mL, respectively.40 For these solvents, the solutions of the calixarene were prepared to be at a concentration equal to half of the CGCs. For the other solvents, the sample solutions were prepared at a concentration of 25 mg/mL. Each sample solution was first heated up to become homogeneous and then allowed to cool down to room temperature. A thin film of each solution was prepared by injecting a 35 µL of solution into a hand-made container, constructed with a glass tube (outer dimeter, 9 mm; inner dimeter, 7 mm; length, 1 cm) and a cover glass (Matsunami NEO; thickness, 0.12–0.17 mm). The height of the solution thin film was estimated to be 150 µm. Another cover glass was put on the top of the glass tube to suppress solvent evaporation, and then the sample container was placed on the stage of an inverted microscope (Olympus, IX71) for laser trapping experiments.


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Figure 2: A schematic illustration of the optical setup. HWP: half-wave plate, PBS: polarizing beam splitter, DM: dichroic mirror, OL: objective lens.

Figure 2 shows a schematic illustration of the optical setup used in this study. A linearly polarized near-infrared continuous-wave laser beam from an Nd3+:YVO4 laser (Coherent Inc., MATRIX 1064-10-CW, λ = 1064 nm) was used as a trapping light source. The laser beam was introduced to the inverted microscope and focused into a sample solution through an objective lens (60 magnification; numerical aperture, 0.90). The laser power throughout the objective lens was tuned to 300 mW by rotating a half-wave plate positioned in front of a polarizing beam splitter. A He-Ne laser (Thorlab, λ = 632.8 nm) was also introduced to the microscope in the same optical path as that of the trapping laser in order to adjust the focal position. Firstly, the focus of the He-Ne laser was set at air/solution interface. Its power is adjusted to a low value so that no trapping event is induced. Then the He-Ne laser was switched off, and the trapping laser


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was turned on to start the laser trapping experiment at the solution surface. In the experiment coupled with photoisomerization, a laser beam of 325 nm (Kimmon Koha, IK3401R-F) was introduced to the sample from its above. A 7 mm2 area of the sample was homogeneously illuminated by the UV laser at the fluence of 2.3 mW/mm2. The trapping behavior was observed with a charge-coupled device (CCD) video camera (WATEC, WAT-231S2) under halogen lamp illumination. A transmission image in the area of 80 × 60 µm2 was sequentially captured at a video rate with a computer. Three-dimensional (3D) scattering imaging was carried out with a confocal scanning system (Olympus, FV300) for an assembly prepared by laser trapping. As a scattering light source, the green laser (Laser 2000 (UK) Ltd., 532 nm DPSS Laser) was introduced to the objective lens through the confocal scanning system. After the assembly formation, the trapping laser was switched off, and the green laser was turned on. The green laser was scanned threedimensionally, and the scattered light was detected through a pinhole with a photomultiplier tube incorporated in the confocal scanning system. The three-dimensional imaging during laser trapping was difficult, because the objective lens was common to the trapping laser and the green laser. Electron microscopy observation was carried out with a scanning electron microscopy (SEM) (JEOL, JSM-7401) for spontaneously formed aggregates. After the deposition of the sample solution, the solvent was naturally evaporated, and supramolecular aggregates were formed on the glass substrate. The aggregates were coated with gold thin layer to prevent surface charge and were observed by SEM.


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RESULTS AND DISCUSSION 1. Solvent dependent laser trapping behavior The synthesized biscalix[4]arene (Figure 1) is an excellent gelator for specific organic solvents; this result was confirmed by investigating gelation behavior of a bulk solution with more than 20 solvents.40 For comparative study on spontaneous gelation and optical assembly formation, here we selected 6 solvents having different physical properties, such as refractive index, extinction coefficient at 1064 nm, thermal conductivity, density, and viscosity. These parameters are important to the generation of optical trapping force and should be critical for optical assembly formation of the biscalix[4]arene. The laser trapping phenomenon for nanometer-sized objects is conventionally interpreted by the Maxell Boltzmann electromagnetic theory.41 The trapping force toward the focal spot, in other words, the gradient force is exerted on the object, which is given as follows;

= ∇

(1)

Here denotes the electric field, ∇ represents a gradient with respect to the spatial coordinate, and the parameter of represents the permittivity of the surrounding medium. The polarizability of the nanoparticle, , under the dipole approximation, is given by

= 4π

(2)

Notations of is the radius of the nanoparticle. and represent the refractive indices of the nanoparticle and the surrounding medium, respectively. Only when the target object has the refractive index higher than that of the surrounding medium, the gradient force toward the focal spot is exerted on the object, and its magnitude becomes larger with the increase in the difference between the refractive indices.


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During laser trapping with the 1064-nm laser, local temperature elevation should also be considered. The laser light is partially absorbed by solvent as well as solute molecules, and temperature around the focal spot is elevated. Ito et al. reported that the local temperature elevation in pure solvent is linearly increased as a function of ⁄ , where is extinction coefficient of solvent at the wavelength of the trapping laser and is thermal conductivity of the medium.42 The thermal energy is diffused outwardly, and the resultant temperature distribution leads to convection flow and mass transfer that are correlated with density and viscosity of solvent. It is reported that the convection flow and mass transfer occasionally play an important role for the molecular assembly under laser trapping conditions.43–45 Table 1 shows physical properties of solvents used, the gelation behavior of the conventional bulk solution, and the result of optical assembly formation at the solution surface. The values of refractive index, thermal conductivity, density, and viscosity were cited from the reference,46 whereas extinction coefficient at 1064 nm was measured by ourselves (see Supporting Information). When the laser was irradiated inside the solution, no trapping event was observed for all solvents including acetonitrile and ethanol which are good solvents for gelation in bulk.40 We focused the laser at the air/solution interface based on the consideration that laser trapping-induced gel formation would be similar to the domain formation of amino acid liquid-like clusters which is induced at solution interfaces. However, once again optical assembly formation was observed neither in acetonitrile nor ethanol. Ethanol has higher extinction coefficient at 1064 nm due to the overtone band of OH vibrational mode. Based on the report of Ito et al.,42 the local temperature elevation in ethanol under the present trapping condition was calculated to be 14.5 K. This temperature elevation may suppress the trapping of the biscalix[4]arene and destabilize the assembly even when it is formed. In toluene, chloroform,


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and dimethyl sulfoxide, we could not observe optical assembly formation as well as gelation of bulk solution. Since refractive indices of these solvents are relatively high, the trapping force acting on the biscalix[4]arene may be too weak to induce optical assembly formation.

Table 1: Physical properties of 6 solvents used and the gelation and laser trapping behaviors in respective solutions.

Thermal Extinction Density Viscosity Gelation coefficient conductivity (g/cm3) (mPa.s) behavior (W/m.K) (m-1)#

Laser trapping behavior

Solvent

refractive index

Acetonitrile

1.3442

1.32

0.188

0.7857

0.369

gel (14 mg/mL)

-

Ethanol

1.3611

12.1

0.167

0.7893

1.074

gel (1.6 mg/mL)

-

Toluene

1.4941

2.20

0.131

0.8623

0.560

-

-

Chloroform

1.4459

1.69

0.117

1.4788

0.537

-

-

Dimethyl sulfoxide

1.4793

4.15

0.186

1.1010

1.987

-

-

Ethyl acetate

1.3723

1.70

0.144

0.9003

0.423

-

Assembly formation

#

Details of the measurement of extinction coefficients at 1064 nm are described in Supporting Information.

In contrast, laser trapping in ethyl acetate led to the formation of a novel molecular assembly having a shape of sea urchin or mushroom when irradiated at the air/solution interface. It is interesting to note that ethyl acetate was not a gel-inducer solvent for this biscalix[4]arene in conventional experiments.40 The optical assembly formation seemed to be less related to density


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and viscosity. In the following sections, we describe the details of the unique assembly formation induced by laser trapping at air/solution interface of the ethyl acetate solution.

2. Local supramolecular assembly by starting and stopping laser trapping at air/solution interface Figure 3 shows a series of optical micrographs upon laser irradiation into air/solution interface of the ethyl acetate solution. When the irradiation was started, the biscalix[4]arenes were gathered and assembled at and around the focal spot, and a molecular assembly of 17 µm in diameter was formed at 2 sec (panels (1)–(2) in Figure 3). Further irradiation into the assembly caused its continuous growth (panel (3) in Figure 3). The resultant assembly had a two-layer structure in which the outer layer seemed darker. The assembly was extended to the outside of the focal spot while keeping the two-layer structure and reached the size of 50 µm in dimeter after 12 sec of the irradiation (panel (4) in Figure 3). This molecular assembly formation by laser trapping looked similar to the case of PNIPAM, known as thermoresponsive polymer.27 In both cases of the present biscalix[4]arene and PNIPAM, a small molecular assembly formed at the focal point is extended to the outside of the focal spot in a few tens micrometers scale. It was explained that the large-sized molecular assembly of PNIPAM in H2O results from cooperative work of laser trapping and local temperature elevation as a typical example of Extended Trapping, because the trapping laser elevates local temperature over the coil-to-globule transition point of PNIPAM. In contrast, the present biscalix[4]arene is dissolved well at higher temperature due to the increase in its solubility. This consideration allows us to conclude that the formation of a supramolecular assembly extending outward from the focal spot is, in this case,


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not due to thermal effect but probably due to nucleation and subsequent growth induced by laser trapping.

Figure 3: A series of optical micrographs captured during laser trapping of the biscalix[4]arene at air/solution interface of the ethyl acetate solution. Red indicators are shown in images in order to identify the assembly. Elapsed time from the beginning of the irradiation is included in each panel. The size of images is 80 µm in width and 60 µm in height.

A representative example showing laser trapping-induced nucleation and growth is crystallization of amino acid at the solution surface. We demonstrated that single crystal of glycine or L-phenylalanine is formed not only in their supersaturated solution but also in unsaturated one.16,47 The crystallization is observed from the focal point, and the formed crystal grows to its outside, reaching to a few tens micrometers. In addition, we proposed that a highly concentrated domain of amino acid liquid-like clusters is formed around the focal point and grows up even to millimeter size.47 This domain formation is also postulated for explaining


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dynamics of laser trapping-induced crystal growth of lysozyme.28 Furthermore, polystyrene colloidal nanoparticles of 200 nm in diameter form a disk-like assembly upon laser trapping at the solution surface.48 All these trapping behaviors showing an expansion from the focal point to its outside were observed for laser trapping at solution surface and interface. A similar large assembly formation at the solution surface is now realized for our supramolecule of azobenzenebased biscalix[4]arene. After the trapping laser was turned off, the supramolecular assembly growth was stopped, and the resultant assembly diffused away from the original position where the assembly formation was initiated (panel (5) in Figure 3). We manually operated the objective lens and the sample stage of the microscope and chased the assembly diffusing in the solution. The assembly seemed to sink down from the solution surface without dissolution. As described above, laser trapping induces the large domain/assembly formation for amino acid and protein clusters, and nanoparticles. It is quite possible that the association structure in such a large domain/assembly undergoes relaxation and dissolution and the component molecules reorient upon switching off the trapping laser. Interestingly, in the case of the present biscalix[4]arene, needle-like structure was observed at a surface of the assembly just after turning off the trapping laser, as shown in panel (6) in Figure 3. Namely, a sea urchin-like assembly was formed by stopping laser trapping. The needles at the assembly surface seemed to have length of several micrometers in the obtained optical micrograph. Their width cannot be precisely determined but is possibly about a few hundred nanometers, which is in the same level of the spatial resolution of the present microscope system. This needle-like structure was never confirmed at a surface of the growing molecular assembly during laser irradiation; the growing assembly was usually observed as a


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round shape in the focal plane of the microscope. Therefore, it is reasonable to consider that the morphological change in the assembly was triggered by turning off the trapping laser.

3. Concentration dependent supramolecular assembly The supramolecular assembly formation by laser trapping was extended to different solution concentrations. The standard sample solution used above was diluted with ethyl acetate in the range of 10–75%. In the solutions diluted to 60–75% (i.e. 15–18.75 mg/mL), the assembly formation dynamics was identical to that of the standard solution described above. That is, a round-shaped assembly was initially formed during laser irradiation. Subsequently, needle-like structure was generated at the assembly surface after switching off the laser. In the solutions diluted to 30–50% (i.e. 7.5–12.5 mg/mL), laser trapping led to the formation of a molecular assembly having a mushroom-like shape, as shown in Figure 4a. During laser irradiation, a round-shaped molecular assembly in the focal plane was grown to a larger size continuously. When the laser was turned off, the resultant assembly diffused away while rotating. The mushroom-like shape of the assembly was clearly confirmed during this diffusion process, because we could accidentally observe the assembly from different angles. In the solutions diluted to 10–20% (i.e. 2.5–5 mg/mL), the formed assembly gradually dissolved after turning off the laser (Figure 4b). The dissolution of the resultant assembly should be ascribed to unsaturation of the remaining solution.


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Figure 4: A series of optical micrographs captured during laser trapping of the biscalix[4]arene at air/solution interface of the ethyl acetate solution diluted to (a) 40% and (b) 20%. Elapsed time from the beginning of the irradiation is included in each panel. The size of images is 80 µm in width and 60 µm in height.

In order to examine the detailed structure of the sea urchin-like and the mushroom-like assemblies, we carried out 3D light scattering imaging for these assemblies prepared in two solutions diluted to 75% (18.75 mg/mL) and 40% (10 mg/mL). Figure 5 highlights the results of the scattering imaging. In the 3D images, green background corresponds to the bottom glass. At higher concentration (75%), the cross-sectional image in Figure 5a showed a round-shaped assembly of which surface was covered with needles. This image was well consistent with the transmission optical micrograph shown above for the undiluted solution. Based on the 3D image, the assembly seemed to be a spheroid-shape covered with needles. At lower concentration (40%), a mushroom-like assembly was observed where the surface of the assembly was not covered with needles. This mushroom-like assembly could be tilted on the bottom glass after turning off


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the trapping laser. The cross-sectional and 3D images in Figure 5b clearly showed a mushroomlike molecular assembly tilted on the glass. In order to confirm the morphology of sea urchin- and mushroom-like supramolecular assemblies by electron microscopies, we tried to pick up the assembly of a few tens micrometers; however it was not succeeded to take out the formed assembly from the solution. In another way, we prepared several assemblies by laser trapping and subsequently evaporated the remaining solvent in order to deposit the prepared assemblies on the glass substrate. Spontaneously generated aggregates covered up the laser trapping formed-assemblies on the glass substrate, and it was impossible to find the assemblies after the solvent evaporation. Electron microscopies of the assemblies still remain as the issue to be solved at the present stage.

Figure 5: A schematic illustration, a cross-sectional image, and a 3D scattering image are shown for a supramolecular assembly prepared by laser trapping in the solution diluted to (a) 75% and (b) 40% of the initial ethyl acetate solution. The size of cross-sectional images is 160 µm in width and 160 µm in height. The dimension of 3D images is shown in respective images.


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The 3D light scattering imaging made clear that, in both of the sea urchin- and mushroomlike assemblies, their structure in the central part is different from that of the outer part. This result is well consistent with the two-layered appearance observed in the transmission imaging. We consider that the difference in the structure is ascribed to strong trapping force. The dynamics and mechanism is discussed in section 5.

4. Photochemically controlled supramolecular assembly The role of the azo-benzene core is that trans to cis isomerization can be induced under UV irradiation.33–37,49,50 It was demonstrated that UV irradiation to bulk solutions of our biscalix[4]arene prevents its gel formation during conventional experiments.40 Interestingly, UV irradiation also prevented the optical assembly formation in the undiluted ethyl acetate solution. Indeed, nothing was observed during laser trapping when the UV laser illuminated the sample together with the trapping laser; namely, the supramolecular assembly is formed only for the trans form. After switching the UV laser off, a lag time was necessary to effectively generate a new assembly by the trapping laser, as summarized in Figure 6. At 150 sec after the stop of the UV laser irradiation, the assembly was formed and subsequently extended to the outside of the focal point. Needle-like structure was similarly observed at the surface of the assembly after turning off the trapping laser. The lag time for the assembly formation was shortened with the decrease in the UV irradiation time. This tendency should be ascribed to the time necessary for the diffusion of trans forms from the surrounding solution toward the focal point as well as the spontaneous back-isomerization from cis to trans form. Although, at the present stage, we cannot explain how the structural change accompanying photoisomerization from trans to cis suppresses the optical assembly formation, this experiment clearly shows that supramolecular assembly


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formation by laser trapping is controllable through photochemical process; this experiment demonstrates the feasibility of performing a combination of photochemical and photomechanical processes on a suitable target molecule.

Figure 6: (a) The temporal change in a supramolecular assembly formed by the irradiation of the trapping laser and the UV laser. The UV laser was turned off at 180 sec, and the trapping laser was switched off at 375 sec. (b) A series of optical micrographs captured during the assembly formation.

5. Possible mechanism for supramolecular assembly Laser trapping of the present biscalix[4]arene at the surface layer of its ethyl acetate solution forms a unique single assembly, but such phenomenon is far different from the gelation that occurs in acetonitrile and ethanol. In ethyl acetate, gel formation cannot be observed. Nevertheless, a lot of sharply edged molecular aggregates with the size of a few tens micrometers are obtained by slow solvent evaporation from the concentrated solution (25 mg/mL); the spontaneously deposited solids were observed by SEM (Figure 7a). Many


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ellipsoidal aggregates of micrometer-size were also generated in the undiluted initial solution after a certain incubation time. These aggregates were not observed in the 60% diluted solution, confirming that these solids are likely to be generated by spontaneous precipitation in supersaturated solutions. We could generate sharp-ellipsoidal aggregates and sea urchin-like assemblies in the same sample. After the formation of several sea urchin-like assemblies by repeating the start and stop sequence of laser trapping in the undiluted solution, we waited for a while for the spontaneous formation of these sharp-ellipsoidal aggregates for comparison purpose of the two structures in the same solution. One obtained result given in Figure 7b shows a transmission image where two laser trapping-formed molecular assemblies and six spontaneously formed molecular aggregates can be seen. The large difference in morphology clearly supports that laser trapping-formed molecular assembly is from a solution state which is not accessible by spontaneous molecular association. In order to confirm the nature of sharp-ellipsoidal aggregates, we observed the aggregates and the sea urchin-like assemblies under the crossed polarization conditions (Figure 7c). The sharp-ellipsoidal aggregates were observed as a bright image depending on the angle of polarizers, which support that the aggregates are anisotropic like a crystal. On the other hand, the incident light was not modified for the sea urchin-like assemblies. Single crystal could be obtained in the slow diffusion process of methanol with chloroform, and its crystallographic analysis is being carried out. Based on these results at the present stage, we consider that sharpellipsoidal aggregates have an anisotropic property due to periodic molecular arrangement but are not single crystal.


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Figure 7: (a) An SEM image of spontaneously formed supramolecular aggregates. (b, c) The transmission images for spontaneously generated sharp-ellipsoidal aggregates and laser trappingformed sea urchin-like assemblies. The image in (b) was obtained without polarizers, whereas images in (c) were captured under the crossed polarization conditions. The image size is 240 µm in width and 180 µm in height.

On the basis of the above considerations, we suggest a possible mechanism of the laser trapping-induced supramolecular assembly formation (Figure 8). Firstly, the focused laser beam gathers and traps many biscalix[4]arenes in the focal volume, where a very high concentration is achieved (panel (1) in Figure 8). The packing and relative orientation of supramolecules are characteristic of intense laser field in the trapping potential and triggers nucleation. Namely, an energy barrier leading to a unique nucleation and giving a morphology different from spontaneous aggregation and gelation is overcome (panel (2) in Figure 8). The nucleus is then grown to a small supramolecular assembly in the highly concentrated area. Further it expands from the focal point to its outside and forms additionally the outer-layer association of the supramolecules (panel (3) in Figure 8). It is important to note that no optical force is exerted outside the focal point where inner and outer-layers are prepared only through solute-solute and solute-solvent interactions. This expansion behavior and mechanism of laser trapping at the


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solution surface were already examined and proposed for laser trapping of amino acid and protein in aqueous solution as Trapping Nucleation and Growth.16 Upon switching the trapping laser off, the assembly growth is stopped and the two-layers of the assembly undergo relaxation. The packing and orientation of the outer layer are much modified giving different morphology, that is, needles in the sea urchin- like structure (panel (4) in Figure 8). Sea urchin- and mushroom-like supramolecular assemblies are formed in the supersaturated and unsaturated solutions by laser trapping, respectively. The mechanism of this concentration dependence is possibly ascribed to spontaneous supramolecular association in the respective initial solutions, which is considered based on laser trapping crystallization and crystal growth of amino acid and protein.16,28 In the case of the supersaturated supramolecular solution, supramolecules should exist as liquid-like clusters formed by linking solute and solvent molecules weakly. Laser trapping in the supersaturated solution assembles the spontaneously formed clusters of the supramolecule, while it suppresses their natural association for the generation of the sharp-ellipsoidal aggregates. Association structure of the laser trapping-formed assembly is possibly determined and stably pinned by the strong trapping force. Therefore, after turning off the laser, the assembly structure becomes unstable. Supramolecules in the assembly are rearranged toward a stable state, and the morphological change is induced. On the other hand, the formation of such clusters is hardly expected in the initial unsaturated solution, and supramolecules should exist as relatively individual molecules in the initial solution. They are directly assembled in accordance to the strong trapping force without competing with the natural association process. The prepared assembly is already stable, so that no structural change is induced after turning off the trapping laser.


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Novel three dimensional structures of sea-urchin or mushrooms suggest unconventional nucleation and growth are achieved in the optical potential. We could not find gelation or molecular aggregation into sharp-ellipsoidal structures upon switching the laser irradiation on and off as described above, so that the nucleation by laser trapping always leads to a unique molecular association which is never realized by conventional method.

Figure 8: A possible mechanism of the laser trapping-induced supramolecular assembly formation.

CONCLUSION Azobenzene-based biscalix[4]arene in ethyl acetate was studied by laser trapping with a 1064 nm continuous-wave laser beam, and interesting assembling behavior was observed at the air/solution interface. A single assembly consisting of two-layers was generated from the focal point of ~1 µm and expanded to the size of a few tens µm although no optical force was exerted outside the focal volume. Upon switching the trapping laser off, the needles were generated at the outer-layer giving sea urchin-like morphology. Decreasing the concentration of the initial


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solution to ca. 30–50%, a mushroom-like assembly was prepared. Laser trapping-induced assemblies were completely different from sharp-ellipsoidal aggregates formed by spontaneous evaporation of the solution. The azobenzene moiety in the biscalix[4]arene was demonstrated to have a determining role in the assembling; the trapping phenomena were shown for the biscalix[4]arene with the trans-form only but not for the cis-form, suggesting that the laser trapping of this biscalix[4]arene is photo-controllable. Dynamics and mechanism of the present laser trapping-induced supramolecular assembling was considered from the viewpoint of nucleation and growth comparing the results with those of amino acid in aqueous solution. Gelation and aggregation experiments are conventionally carried out in solution by changing solvent and tuning concentration and temperature, so that all the processes proceed randomly in parallel. The dynamic evolutions are analyzed as an ensemble of gels and aggregates formation, but no direct dynamic information on the gelation/aggregation process is accessible. In the present laser trapping experiment, however, the assembly formation dynamics is monitored from time to time around the focal point. Laser trapping is a very promising approach for preparing a single spherical assembly of supramolecules and analyzing its assembling dynamics. We demonstrated that the process can be further controlled using a photoresponsive molecule and an additional irradiation source. This is a very promising characteristic as a new experimental approach for preparing a single spherical assembly of supramolecules, analyzing its assembling dynamics, and fabricating micro-structures from single assemblies. Exploratory research along the present work will enable us to perform supramolecule assembling by watching always when and where individual assemblies of supramolecules are prepared, monitored and utilized.


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ASSOCIATED CONTENT Supporting Information are available free of charge.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] (W.-S.C.), [email protected] (H.M.). Tel: +886-3-5131517 (W.-S.C.), +886-3-571-2121 Ext. 56595 (H.M.). Corresponding Author # Research Institute for Electronic Science, Hokkaido Univer-sity, N20W10, Kita-Ward Sapporo 001-0020, Japan Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT The present work is supported by MOE-ATU Project (National Chiao Tung University) of the Ministry of Education, Taiwan to H.M., and by grants from Ministry of Science and Technology,


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Taiwan to H.M. (MOST 105-2811-M-009-022) and to W.-S.C. (MOST 104-2113-M-009-007 and MOST 105-2113-M-009-004).


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TABLE OF CONTENTS


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