b College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266510, China;
c Institute of Marine Materials Science and Engineering, Shanghai Maritime University, Shanghai 201306, China
Self-assembly is a natural and spontaneous process which occurs mainly through non-covalent interactions,such as hydrogen bonding,electrostatic,π-π stacking,van der Waals,metal- ligand coordination bonding and hydrophobic interactions. The self-assembly approach has been widely adopted in the effort for the fabrication of organic functional molecular nanostructures . Based on different non-covalent interactions,various nanostructures, such as cubes,fibers,sheets,rods,and tubes,have been fabricated from different functional molecular materials [2, 3, 4, 5]. Porphyrins are particularly attractive building blocks for selfassembly because the intimate packing of these aromatic macrocycles can result in enhanced photophysical and photochemical performances that are important in many technological applications,such as catalysis,optical device,storage,field-effect transistor and photovoltaic . Vesicles of meso-tetrakis[(bixinylamino)- o-phenyl]porphyrin formed in water at pH 9 can essentially remain intact even on dry solid surfaces . Hao and coworkers reported the formation of vesicles of an amphiphilic manganese complex of Mn(Ⅲ)TPP(COOH) with four stearic acid groups on one side of the ring plane in the non-aqueous solution . Nanotubes with photocatalytic activity were fabricated by electrostatic interaction between two oppositely charged porphyrins . Twisted supramolecular nanoscrews of a simple porphyrin with tetraheptyl functional groups were fabricated in acetonitrile solvent . Li and coworkers reported on fabricated 2D or 3D structured nanopatterns and arrays via the colloidal selfassemble process of a novel porphyrin molecule 5,10,15,20-[1,4- benzodioxane-6-carboxalde]porphyrin (TEOP) . Investigation over the organogel formation properties of a series of porphyrins with amide groups as peripheral hydrogen bonding sites indicated that the aggregation mode of porphyrin stacks can be tuned by hydrogen bonding interaction . Jiang and coworkers found the meso-substituted phenyl groups of porphyrin ligand modified by different numbers of hydroxyl groups led to the formation of various nanostructures,clearly indicating the effect of hydrogen bonding interaction in controlling the intermolecular interaction between conjugated molecules in the self-assembly process. Furthermore,studies on conjugated molecules modified by different substitutes,self-assembled at the liquid-solid interface revealed,by scanning tunneling microscope (STM),the ordered molecular packing of porphyrin derivatives . However,to the best of our knowledge,there is still no report on the novel morphologies (nanorods and nanothorns) with highly ordered arrangements of porphyrins (TAPP) in nanostructures whichwere directly imaged by high resolution transmission electron microscopy (HRTEM).
In this paper,porphyrin (TAPP) nanostructures with various morphologies (nanospheres,nanorods and nanothorns) were fabricated. Interestingly,highly ordered arrays of porphyrin molecules in the aggregates of nanorods and nanothorns were directly imaged by HRTEM and detailed investigations revealed that the size of each quadrate supramolecular array agrees well with that of TAPP molecule. 2. Experimental
meso-Tetrakis(4-aminophenyl)porphyrin (TAPP) was synthesized according to published methods . All the solvents were of analytical grade and used without further purification. The molecular structure of TAPP is shown in Fig. 1.
|Fig. 1.The molecular structure of meso-tetrakis(4-aminophenyl)porphyrin (TAPP).|
The nanoaggregates of TAPP formed in different concentrations of chloroform solution were fabricated by a phase transfer method according to the following procedure. CH3OH (1 mL) was injected rapidly into 4 mL chloroform solution of TAPP with different concentrations. After being aged for three days at room temperature, these aggregates were transferred by pipetting to the surface of the carbon-coated grid for the TEM and SEM observations and to a quartz substrate for XRD as well as UV-vis analysis.
The porphyrin aggregates formed in the mixed solvent of CHCl3/CH3OH (v/v,4/1) are characterized by scanning electron microscopy (SEM,JEOL),high-resolution transmission electron microscopy (HRTEM,JEM-2100,JEOL) with an accelerating voltage of 200 kV. The sphere-like nanoaggregates formed at the lower concentration of TAPP were characterized by dynamic light scattering (DLS) (Nano S90 (Red badge) ZEN1960,Malvern). Small-angle X-ray diffraction (SAXRD) measurement was carried out on a Bruker D8 Advance diffractometer using Cu Kα (λ = 1.5418Å ). UV-vis absorption spectra were recorded on a (UV-3200PC) UV-vis spectrophotometer. 3. Results and discussion 3.1. Morphologies of porphyrin nanoaggregates
Typically,samples were prepared by different dilutions of the original CHCl3 solutions of porphyrin with the mixed solvent of CHCl3/CH3OH (5 × 10-4,7.5 × 10-4,1× 10-3 and 1.5 × 10-3 mol L-1 respectively). The morphologies of the formed aggregates were examined by transmission electronic microscopy (TEM) and scanning electron microscopy (SEM). It can be observed that various morphologies of nanoaggregates were fabricated at different concentrations of porphyrin solutions in chloroform by the phase transfer method. As shown in Fig. 2A,a large amount of nano-scale solid spheres with a diameter from ca. 75 nm to 110 nm were formed in the mixed solvent of CHCl3/CH3OH (v/v,4/1) with the concentration (5 × 10-4 mol L-1) of chloroform. Moreover, some nanospheres of several nanometers in diameter can also be found. Further evidence to confirm the formation of solid nanospheres was provided by SEM,as shown in Fig. 2B. Owing to the regular spheres of TAPP nanoaggregates obtained in a mixed solvent of CHCl3/CH3OH (v/v,4/1) with the lower concentration (5 × 10-4 mol L-1) of chloroform,DLS measurements were carried out to investigate the distribution of these regular nanospheres. This is attributed to the spherical nanoparticles with a variety of different diameters,but well-defined shapes . As shown in Fig. 3, two particle size distributions of nanospheres appeared at ca. 15 nm and 110 nm,respectively,which is in good accordance with that of TEM and SEM images.
|Fig. 2.TEM (A) and SEM (B) images of TAPP nanospheres self-assembled in the mixed solvent of CHCl3/CH3OH (v/v,4/1) with the original concentration (5 × 10-4 mol L-1) of CHCl3 solution.|
|Fig. 3.DLS of TAPP nanospheres formed in the mixed solvent of CHCl3/CH3OH (v/v, 4/1) with the original concentration (5 × 10-4 mol L-1) of CHCl3 solution.|
When the original concentration of TAPP chloroform solution increased to 7.5 × 10-4 mol L-1,nanorods grown from the irregular nanoparticles were found in a mixed solvent of CHCl3/CH3OH (v/v,4/1),as shown in Fig. 4A. However,when the concentration of TAPP chloroform solution was further increased to 1 × 10-3- 1.5 × 10-3 mol L-1,nanothorns appeared in a mixed solvent of CHCl3/CH3OH (v/v,4/1) as shown in Fig. 5A.
|Fig. 4.TEM (A) and HRTEM (B) images of TAPP nanoaggregates self-assembled in the mixed solvent of CHCl3/CH3OH (v/v, 4/1) with the original concentration (7.5 × 10-4 mol L-1) of CHCl3 solution.|
|Fig. 5.TEM (A) and HRTEM (B) images of TAPP nanothorns self-assembled in the mixed solvent of CHCl3/CH3OH (v/v, 4/1) with the original concentration (1.5 × 10-3 mol L-1) of CHCl3 solution,respectively.|
The different morphologies of the formed aggregates of TAPP may be explained with the help of the growth mechanism of nanoparticles. As is known,there is a balance between the rate of nucleation and the growth rate of crystal nuclei. If the rate of nucleation is faster than the growth rate of crystal nuclei,the number of crystal nuclei is large,resulting in a relatively smaller size of nanoparticles [15, 16]. When CH3OH solvent was added to the CHCl3 solution of TAPP,TAPP nanoaggregates with sphere-like morphology were formed owing to the decreasing of solubility of TAPP in CH3OH solvent. After aging for three days,stable nanospheres of TAPP were obtained due to the balance between the rate of nucleation and the growth rate of nanoaggregates. Upon increasing the concentration of the TAPP,the growth rate of aggregate nuclei was faster than that of nucleation as well as the growth anisotropy of TAPP aggregates,which result in different morphologies of nanoaggregates,such as nanorods and nanothorns .
To reveal the internal structure of the nanoaggregates,high resolution transmission electron microscopy (HRTEM) studies of the nanostructures were investigated. As shown in Figs. 4B and 5B, distinct lattice fringes,which distributed in the entire areas of the nanothorns,can be clearly observed in the HRTEM of nanorods and the nanothorns fabricated in different concentrations of CHCl3/ CH3OH mixed solvents of TAPP. Furthermore,from these figures, the interlattice spacing is determined as approximately 1.45 nm, which is in good agreement,according to the CPK model,with the distance between neighboring nitrogen atoms attached on the benzene ring of the porphyrin molecule (Fig. 1). Thus,four TAPP molecules formed a quadrate unit (Fig. 6B),which is in line with the results of highly ordered arrays of metal-free 5,10,15,20- tetrakis(4-chlorophenyl)porphyrin (TClPP) in nanotube aggregates formed by the coordination interaction between AAO templates and TClPP molecules according to our previous investigation . In addition,according to the previous reports,the nanoaggregates fabricated by the surfactant assisted self-assembly method can be found with parallel lattice fringes of nanorods of zinc 5,10,15,20- tetrakis(4-pyridyl)-21H,23H-porphine (ZnTPyP),while a quadrate unit of fringes cannot be imaged directly by HRTEM . Hence, our results indicated that H-bonds possibly existed in the intermolecules of TAPP and played an important role in the formation of the quadrate unit of fringes,indicating the formation of highly ordered arrays of TAPP in both nanorods and nanothorns. Moreover,Jiang and coworkers investigated fusiform morphology of the mixed (phthalocyaninato)(porphyrinato)europium molecule formed,due to the formation of H-N-H hydrogen bond between one octyl-substituted amidocyanogen group attached at the p-position of meso-attached phenyl group of the porphyrin ligand and one aza-nitrogen atom of the phthalocyanine ring of the neighboring double-decker molecule . Furthermore,they also found hollow spheres mainly formed in water depending on the intermolecular π-π interaction in cooperation with the hydrogen bonding interaction of metal-free tetrakis(4-hydroxyphenyl)porphyrin . In addition,an amphiphilic porphyrin bearing four β- D-galactopyranoside groups tended to aggregate in a onedimensional direction,resulting in very robust gels in DMF/alcohol mixed solvents,due to the stacking interaction and the hydrogen-bonding interaction among porphyrin moieties . Unfortunately,the highly ordered arrangements of tetrapyrrole derivatives,including porphyrins and analogs,were not directly imaged by HRTEM in these previous reports. However,our study reported the highly ordered arrangements of TAPP molecules in nanothorns and nanorods imaged directly by HRTEM,which is in accordance with that of porphyrins and analogs (phthalocyanines) on a HOPG,or an Au surface by STM .
|Fig. 6.XRD profile of nanothorns of TAPP (A) and the molecular structure of TAPP optimized by CPK model (B).|
The structure of our nanothorns was also characterized by the lower angle XRD analysis. As shown in Fig. 6A,the XRD pattern of the nanothorns showed a diffraction peak at 2θ = 6.15 (corresponding to 1.44 nm). This value (1.44 nm) was reasonably consistent with the size of each TAPP molecule (1.45 nm) (Fig. 6B) and well with the interlattice spacing (1.45 nm) obtained from HRTEM. 3.3. Electronic absorption spectra
Extensive studies have shown that porphyrin-porphyrin stacking modes can be easily revealed by electronic absorption spectroscopic examination. The electronic absorption spectra of both dilute chloroform solutions of the compound and nanoaggregates coated on the silica substrate are shown in Fig. 7. The electronic absorption spectra show clearly that the Soret band of TAPP nanoaggregates at 447 nm (Fig. 7B) redshifted 20 nm in comparison with that of dilute chloroform solution of TAPP at 427 nm (Fig. 7A). In addition to the broadening of the spectra,these data unambiguously indicated the intensive inter-molecular interaction in the nanoaggregates and suggested the formation of J aggregates of TAPP in the nanoaggregates [22, 23].
|Fig. 7.UV-vis absorption spectra of chloroform solution of (A) and aggregates selfassembled in the mixed solvent of CHCl3/CH3OH (v/v, 4/1) with original concentration of TAPP chloroform solution: 1.5 × 10-3 mol L-1 (B),respectively.|
On the basis of data described above,the molecular packing mode of TAPP molecules in the nanorods and nanothorns is illustrated in Fig. 8. As indicated by HRTEM results and XRD data,a tetrameric supramolecular structure of the self-assembled nanorods and nanothorns of TAPP possibly formed through intermolecular hydrogen bonding between four amino groups of porphyrin molecules together with π-π supramolecule interaction among TAPP molecules during the self-assembling process of TAPP (Fig. 8B). Simultaneously,this formed tetrameric supramolecular structure expanded,resulting in the formation of highly ordered arrangements of TAPP molecules on a large scale in aggregates of nanorods and nanothorns (Fig. 8C).
|Fig. 8.Schematic representation of packing mode of the highly ordered arrays of TAPP molecules in the nanorods and nanothorns.|
In summary,meso-tetrakis(4-aminophenyl)porphyrin (TAPP) can easily self-assemble into nanospheres,nanorods and nanothorns by a facile,phase-transfer method. Highly ordered supramolecular arrays of TAPP fabricated in the nanoaggregates were directly imaged by HRTEM. The formation of J-aggregates of TAPP in the nanoaggregates was verified by UV-vis absorption spectra with a red shift of the Soret band of TAPP. Because nonmetalloporphyrins are especially attractive for the radical tuning of their electrical and optical properties,they are believed to be helpful in opening new possibilities for the construction of molecular based nanoelectronics and nanooptoelectronics .Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Nos. 51042013,21271119),Innovation Fund of Shanghai (No. 10170502400),Science Foundation of Shandong Provincial Education Department (No. J08LC11),and Research Progect of "SUST Spring Bud" (No. 2008BWZ056).
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