Competition of Major Forces Dominating the Structures of Porphyrin

Article pubs.acs.org/crystal

Competition of Major Forces Dominating the Structures of Porphyrin Assembly Juqing Ma,†,‡ Wenlong Zhang,† Zhan Li,‡ Qiang Lin,‡ Ji Xu,*,‡ and Yongsheng Han*,‡ †

School of Material Science & Engineering, Harbin University of Science and Technology, Harbin 150080, China State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

‡

ABSTRACT: Here we take the self-assembly of porphyrin molecules as an example to discover the reason for the diversity of assembled structures. The self-assembly is conducted at a range of pH conditions. Irregular, tubular, and rodlike particles are formed at pH 1.5, 6.5 and 12.5, respectively. X-ray diffraction spectra and UV absorption show that these particles have the same crystalline structures, while the packing types of molecules are different. At low pH, the molecules assemble together by end to end attachment with a J-type aggregation, forming irregular particles. This assembly is dominated by the competition of π−π interaction and repulsion force. The latter force is induced by the protonation of molecules under acid conditions. With the increase of pH, the repulsion force is attenuated and finally disappears under basic conditions. The π−π interaction becomes the major force under basic conditions, leading to a H-type aggregation via face to face attachment and forming rodlike particles. At the middle pH, the competition disturbs the symmetry of molecules assembly, forming tubular particles. Together with previous studies, we expect that the compromise between competing interactions is probably the governing rule for the diversity of assembled structures.

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INTRODUCTION Thousands of snow morphologies have been found in nature, and a crowd of material structures has been synthesized in laboratories.1−3 The diversity is a natural feature of material structures. Although well-defined assembled structures, such as nanodiscs,4 nanorods,5,6 nanowires,7,8 nanotubes,9 have been prepared by various approaches, the mechanisms governing these diverse structures have been insufficiently investigated. Most previous studies ascribed the diversity to the change of reaction parameters, such as ionic strength, pH, concentration, and surfactant.10−12 Few efforts go one step deeper to discover the nature behind the change of these parameters. Since the assembly is built up by molecules, the interaction of molecules should play a key role in the structure development of the assembly. If all the forces acting on the molecules are attractive, the molecules would simply aggregate into a stable structure; if they are repulsive, the product would disintegrate.13 Therefore, the competition of different forces should be one of the reasons for the formation of hierarchical assembled structures, which is the assumption we want to evaluate in this paper. We choose the self-assembly of molecules as an example to discover the competition effect of major forces on the structure development of assembles. The meso-tetraphenylporphyrin (TPP) is adopted as the initial monomer for its simple molecular framework and structure-dependent properties.14 For TPP molecules, the π−π interaction is the major attractive force guiding their assembly.15 To setup a competition to the attractive © XXXX American Chemical Society

force, we introduce an electrostatic repulsive force by keeping the assembly of molecules in acid solutions in which the TPP molecules are protonated.16 The electrostatic force is variable by pH values. Therefore, a competition between electrostatic forces and π−π interaction could be setup and adjusted by regulating the pH of the solution. With this strategy, we study the assembly of porphyrin molecules at a range of pH conditions. The structures of porphyrin assembly are characterized by optical and electron microscopies, and the mechanisms underlying the diversity of these structures are discussed in this paper.

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EXPERIMENTAL SECTION

Materials. Meso-tetraphenylporphyrin, dichloromethane, and ethanol were purchased from Aldrich. Chlorhydric acid and ammonium hydroxide, purchased from Beijing Chemical Reagent Corp., were used to regulate pH of the solutions in which the assembly occurs. Preparations and Characterizations of Samples. The selfassembly of meso-tetraphenylporphyrin was conducted in ethanol. The acidity of ethanol was regulated by chlorhydric acid and ammonium hydroxide to the designed values before the self-assembly. The meso-tetraphenylporphyrin was first dissolved in dichloromethane forming a solution with the concentration of 0.5 mM. Then 1.0 mL such solution was injected into 10 mL of ethanol in a 20 mL glass vial. The glass was shaken vigorously for 5 min and then left in the dark at Received: October 22, 2015 Revised: February 13, 2016

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DOI: 10.1021/acs.cgd.5b01502 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Figure 1. Atomic model of porphyrin (A), which is represented with ball-and-stick model. The charge (Q) is evenly distributed among the hydrogen atoms in the center of the porphyrin ring. The snapshot of the initial state of the simulation (B). The porphyrin molecules and chloride ions are represented by space-filling model; the waters are drawn by the surface filling model.

Table 1. Configuration of the Simulations case

Q

no. porphyrin

no. water

no. Cl−

I II III

0.0 1.0 2.0

20 20 20

3477 3457 3437

0 20 40

Figure 2. An illustration of a singular porphyrin molecule.

Figure 3. Microscopic images of the porphyrin particles assembled at different pHs. A1−A3 showing irregular particles assembled at pH 1.5; B1−B3 showing tubular particles assembled at pH 6.5; C1−C3 showing rodlike particles assembled at pH 12.5. The images of A3, B3, and C3 were taken by confocal laser scanning microscopy, while others were taken by scanning and transmission electron microscopy.

room temperature for 12 h. The products assembled in a vial were collected and characterized by scanning electron microscopy (JSM-7001F, JEOL, Japan) and transmission electron microscopy (JEM-2100(UHR), JEOL, Japan). The samples for SEM were collected by pipetting the suspension onto aluminum foil. UV−vis spectroscopy (Evolution 600, Thermo Scientific, America) was used to measure the optical absorbance of samples. A ZEISS LSM 710 confocal microscope was used to characterize the morphology of samples. X-ray diffractometer (Empyrean, PANalytical, Netherlands), with CuKα radiation, was used to characterize the crystalline structures of samples which were prepared by depositing a suspension on a quartz plate. Simulation on Porphyrin Assembly. The Gromos43a1 force field17 is adopted to build the porphyrin model, and the SPC model18 is used for water molecules. To study the effect of protonation due to variation of pH, three models of different charges of the central porphyrin ring, as indicated by the Q value in Figure 1A, are constructed. To construct the initial state, the porphyrin molecules are

randomly added into the simulation region; then the waters are added; if the system is not neutral, the chloride ions are added randomly. The configurations of the three porphyrin models are listed in Table 1, and the snapshot of the initial state of Case III is shown in Figure 1B. All the systems are first energy-minimized for 1000 steps with the steepest descent method.19 The cutoff radius of short-range van der Waals and electrostatic interactions is set to 0.9 nm. The particle mesh Ewald (PME) method20,21 is adopted to compute the long-range electrostatic interaction, with the Fourier grid set to 0.12 nm. All the bonds are constrained with the LINCS method.22,23 The thermostat-barostat (NPT) ensemble is adopted, with the temperature kept at 298.15 K by the velocity-rescaling method24 and the pressure kept at 1.0 atm by the Berendsen method.25 Periodic boundary condition is applied in the x, y, and z directions. After energy minimization, the systems are equilibrated for 20 ps with positions restraints applied on the heavy atoms of porphyrin molecules; in this process the time step is set as 1.0 fs. The production simulation is run to 100 ns with the time step B



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tubular particles, as shown in B1−B3 in Figure 3. These particles have a length of more than 10 μm and a diameter of approximately 5 μm, with a smooth surface. At the end of tubular particles, the shapes of openings are various. Most openings have tetragonal shape, while triangular and irregular openings are also observed, as shown in B1. The TEM image of B2 and the confocal image of B3 demonstrate that the tubular particle has hollow structure inside. Further increasing pH to 12.5 leads to the formation of rodlike particles, as shown in C1−C3 in Figure 3. The rodlike particles have similar appearances as the tubular particles, but there are no openings at the end of these particles, which is confirmed by the TEM image of C2 and the confocal image of C3. The crystallinity of the porphyrin products were characterized by X-ray diffraction. These three samples have similar diffraction patterns, as shown in Figure 4. The peaks at 2θ of 7.30°, 8.70°, and 10.60° are ascribed to the facets of (001), (010), (100) planes, respectively, which indicates that the samples belong to the same orthorhombic crystal family 15. The differences of intensity at (001), (010), and (100) peaks are remarkable, especially the ratio of the intensities in (001) and (100), which may be caused by the preferred assembly of molecules. To discover the formation mechanism of the porphyrin structures, the samples were characterized by UV−vis spectroscopy, as shown in Figure 5. The typical absorption of nonaggregated TPP has the Soret band at 418 nm and Q bands at 515, 548, 590, and 647 nm, respectively.29 Figure 5A compares the absorption spectra of the sample prepared at pH 1.5 (red line) and the porphyrin monomers (black line). The assembled structures have a similar profile as their monomers except for two differences. First, the Soret band at 418 nm is shifted to 436 nm with a pronounced broadening, as shown in the inset of Figure 5A. Second, the new peaks at 356 and 656 nm are remarkable. The red shift of the Soret band suggests that the assembly of porphyrin molecules follows the J-type aggregation in which the head to tail arrangement of monomers is normally present.30 The enhancements of the peaks at 356 and 656 nm indicate that the inner nitrogen of the porphyrin ring is protonated.31 The absorption spectra of the samples assembled in pH 6.5 have similar profile as the monomer of TPP except that a shoulder from 412 to 418 nm is observed, as shown in Figure 5B. The samples assembled at pH 12.5 have a slight blue shift on the Soret band, as shown in the inset of Figure 5C. The blue shift of the Soret band indicates that the assembly of porphyrin molecules follows the H-type aggregation in which face to face arrangement of monomers is predominated.32

Figure 4. X-ray diffraction patterns of porphyrin samples assembled at different pH values. set to 2.0 fs, with the results saved every 0.1 ns for analysis. All the simulations in this study are completed with GROMACS 5.0 package,26 and the images are rendered with the VMD software.27

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RESULTS AND DISCUSSION The meso-tetraphenylporphyrin (TPP) molecule has square shape with four phenyl groups perpendicular to the central ring, as shown in the Figure 2. There are two free nitrogen atoms in the center of the ring. Under an acid condition, the protonation occurs on these two nitrogen atoms. The protonation gives molecules positive charge in the center. The main interactions involved in the molecules include van der Waals forces, hydrogen bonds, π−π interactions, and electronic repulsions if the center is protonated.28 Among these interactions, the latter two forces are the major forces controlling the assembly of porphyrin molecules under an acid condition. The π−π interaction tends to arrange the molecules in a way of superposition, while the electronic repulsion force does not favor the superposition. The interplay of the repulsion and π−π interaction dominates the assembly of molecules, leading to the formation of diverse structures, which are going to be evaluated in the following. The assembly of TPP molecules was conducted at different pH values, and the structures assembled were characterized by electron microscopy and confocal laser scanning microscopy, as shown in Figure 3. A1−A3 in Figure 3 shows that under an acid condition, irregular particles with size 1−3 μm were largely formed. These particles show a red color under UV excitation (405 nm). Increasing the pH to 6.5 results in the formation of

Figure 5. UV−visible absorption spectra of porphyrin assembly (red line) and the prophyrin monomers (black line) at pH 1.5 (a), pH 6.5 (b), and pH 12.5 (c). The inset of each figure is the magnified profile from 380 to 490 nm. C



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Figure 6. A proposed mechanism on porphyrin assembly at various pH, leading to the formation of diverse structured assembly.

The UV−vis results indicate that at different pH values the assembly of porphyrin molecules has distinct packing types, which should be caused by the competition of the repulsion and attraction forces. At a low pH, the electrostatic repulsion force is remarkable due to the complete protonation, which repels other molecules nearby. At the same time, the π−π interaction given by the big ring attracts the molecules, tending to pack molecules in a way of superposition. Under the competition of repulsion and attraction, the molecules assemble together slowly in a balance of the competition. Because of a strong repulsion at low pH, the TPP molecules assemble together following a head to tail arrangement via J-type aggregation. The weak interaction among the molecules as a result of the limited overlapping makes the connected molecules behave like a long chain polymer. They tend to curl and twine to form irregular particles with the growth of the chain to minimize their surface energy, as illustrated in Figure 6A. With the increase of pH to 6.5, the protonation is attenuated and the π−π interaction becomes dominant. Most molecules assemble following a face to face aggregation. The weak repulsion disturbs the perfect superposition to a certain content, which leads to the loss of symmetry in aggregation, creating accidental asymmetry in the assembly. Later growth magnifies this asymmetry, leading to the formation of tubular structures, as shown in Figure 6B. When the pH is increased to 12.5, the protonation disappears, and the π−π interaction dominates a face to face aggregation of molecules, forming perfect rodlike particles, as shown in Figure 6C. Therefore, the variation of pH changes the electrostatic interaction of the molecules, which shifts the balance between the electrostatic repulsion and π−π interaction, leading to the formation of diverse structures. To evaluate the proposed mechanism, we have conducted a molecular simulation on the assembly of porphyrin molecules at different charges which corresponds to different pH conditions. At the basic condition, the charge (Q) is set to 0 while it turns into 2 at the acid condition. Porphyrin and water molecules are added into the simulation regions, respectively. The simulation is run to 100 ns with the results saved every 0.1 ns for analysis. The details are described in the Experimental Section. Although the environment of the simulation, typically the constituents of the solvent, is relatively simpler than the experiments, the comparison of the assemblies at different charges is meaningful and inductive. The simulation results are

Figure 7. Simulation results of porphyrin assembly at different charges (Q) with Q = 2.0 (A), 1.0 (B), and 0.0 (C), respectively. Left: the snapshots of the results at time 100 ns; Right: the typical packing pattern in the correspoonding models. The porphyrins are colored by their residue identities (resid) which are from 1 to 20 in the three cases.

shown in Figure 7. Going to the details of packing patterns of molecules, we found that at a low pH with a high charge of 2 D



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Figure 8. Assembly kinetics of porphyrin molecules at different pH values (C). Panel (A) shows the standard configuration of the absorbance of porphyrin molecules in dichloromethane at different concentrations. Panel (B) shows the decrease of porphyrin concentration with the running of assembly times.

the porphyrin molecules tend to interact weakly by an outside benzene group such as the ones of resid 4/20 and 15/17 in Figure 7A, which is caused by the repulsion force among molecules induced by protonation. If the charge (Q) is decreased to 1, the porphyrin molecules link to each other by mostly two benzenes of the side part, such as the ones of resid 9/11 and 17/19 in Figure 7B, which corresponds to the proposed assembly at a middle pH. Owing to the flexible connection of molecules, the asymmetric packing of molecules leads to the formation of curved patterns which later become tubular structures. If the charge is reduced to 0, the porphyrin molecules overlap with each other mostly by the plane, such as the ones of residue identity (resid) 12/20 and 4/6 in Figure 7C, which corresponds to the proposed mechanism at high pH. Therefore, the simulation results agree well with the proposed mechanism in Figure 5, which partly confirms our understanding of the porphyrin assembly at different pH conditions. To further evaluate the proposed mechanism, we quantify the assembly kinetics of porphyrin molecules at different pH values, as shown in Figure 8. The assembly kinetics was measured by quantifying the concentration of porphyrin molecules in dichloromethane with the running of assembly time, as shown in Figure 8B. At pH 1.5, the concentration of porphyrin molecules in solution does not change at the initial 4 h. After 2 days, the concentration decreases slightly, which indicates that the assembly of porphyrin molecules under an acid condition is slow. This result is confirmed by color observation. At low pH the solution in which assembly occurs shows a green color after 2 days, which indicates that a remarkable amount of individual or oligomer porphyrin molecules are still present in the solution. If the pH increases to 6.5, the concentration of porphyrin molecules in solution decreases remarkably, as shown in Figure 8B. At the same time, the solution turns colorless after 2 days, which indicates that most molecules have been assembled after 2 days. If the pH increases to 12.5, the solution becomes colorless after

initiating the assembly of 2 h, as a result of a high assembly rate under basic conditions, as shown in Figure 8C. Therefore, the interaction of molecules plays an important role in the assembly rate of molecules in solution. If the attraction force dominates the assembly, molecules assemble together quickly, while they assemble slowly when the repulsion force is remarkably. Furthermore, on the basis of the reaction limited aggregation model, compacted structures are easily formed in a reaction-limited condition.32−35 At pH 1.5, the assembly kinetics is the lowest, which leads to the formation of irregular particles with compacted structures, which agrees well with the reaction limited aggregation model (RLA).

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CONCLUSIONS

In this paper, we took the self-assembly of porphyrin molecules as an example to discover the reason for the diversity of assembled structures. Porphyrin particles with irregular, tubular, and rodlike morphologies were prepared at pH 1.5, 6.5, and 12.5, respectively. XRD spectra showed that these particles were crystalline and belonged to the same orthorhombic crystal family. The UV−vis measurement demonstrated that the molecular assembly follows different types of aggregation at various pH conditions. At low pH, the electrostatic repulsion given by the protonation of central rings took precedence over π−π interaction making molecules slip parallel to form J-type aggregation by sharing the edge of molecules. At high pH, the π−π interaction was dominant, which made the molecular assembly follow the H-type aggregation by sharing the center of molecules. The weak electrostatic force at medium pH disturbed the asymmetry of molecular assembly leading to a distorted H-type molecular aggregation in which tubular structures were formed. This study shows that the competition of major forces plays an important role in the structure development of molecular assembly, and the compromise between competing interactions dominates the structures and morphology of the assembly. E



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AUTHOR INFORMATION

Corresponding Authors

*(Y.H.) E-mail: [email protected]. *(J.X.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Hundreds Talent Program from the Chinese Academy of Sciences and the project from the State Key Laboratory of Multiphase Complex Systems (MPCS-2014-D-05). Financial support from National Natural Science Foundation of China (U1462130, 91534123, 21406232) is appreciated. Prof. Helmuth Moehwald from Max Planck Institute of Colloids and Interfaces is acknowledged for the fruitful discussions and revisions.

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Competition of Major Forces Dominating the Structures of Porphyrin

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