Tuning Growth of Low-Dimensional Organic Nanostructures for

Jun 11, 2012 - Tuning Growth of Low-Dimensional Organic Nanostructures for Efficient Optical Waveguide Applications. Taifeng Liu†, Yongjun ... Citat...
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Tuning Growth of Low-Dimensional Organic Nanostructures for Efficient Optical Waveguide Applications Taifeng Liu,† Yongjun Li,† Yongli Yan,‡ Yuliang Li,*,† Yanwen Yu,† Nan Chen,† Songhua Chen,† Chao Liu,† Yongsheng Zhao,*,‡ and Huibiao Liu† †

CAS Key Laboratory of Organic Solids and ‡CAS Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: A novel amphiphilic porphyrin derivative DOCP-Zn has been synthesized and utilized to construct tunable nanostructures from 0D to 1D by controlling the axial coordination of zinc porphyrin with 4,4′-bipyridine. SEM images indicated that patterns between 0D nanospheres and 1D nanorods or nanoslices can be reversibly converted by the construction or destroy of the axial coordination interaction. This facile approach provided a new method for controlling the nanostructure morphologies and dimension by patterns conversion. The photoluminescence microscopy images indicated the 1D nanorods material has the potential application as nanoscale photonic elements.

1. INTRODUCTION In the past decade, the molecular aggregation chemistry has attracted considerable attention. The rapid development of this chemistry field has promoted the understanding of the concepts and strategies of design of molecular aggregations. 1−8 Importantly, the aggregate structures produced by molecular building block have exhibited enormous advantages on organic electrons, optical devices, chemical and biosensors and catalyst, and so on.9−18 Porphyrins are convenient building blocks for the design of many supramolecules possessing various architectures.19−22 The design and structure of supramolecular systems based on various numbers of porphyrin units with various aggregate nanostructures and unique properties are described.23,24 However, achieving fine-tunability in the dimension, shape, and properties of organic nanostructures is still a challenge in aggregate chemistry.25−37 Here we report the reversible tuning on dimension of a novel amphiphilic porphyrin molecule for producing new types of highly stable functional materials on nanoscales and optical waveguide property of the independent nanowires. A novel dioctaoxacylo-porphyrin zinc complex (DOCP-Zn) 7 was synthesized by olefin metathesis reaction (ESI). Computer simulations indicated the lowest energy structure of the DOCP-Zn molecule. The DOCP-Zn molecule is nearly a planar butterfly-like structure with tortuous glycol side chains (Figure 2). The central porphyrin ring is hydrophobic, and both ends of glycol side chains are hydrophilic for the formation of a typical amphiphilic molecule. The zinc atom was introduced to axial coordinate with 4,4′-bipyridine for tuning the assembly process. This strategy has been perfectly practiced by nature in the self-assembly, leading to various aggregate structures by the novel amphiphilic porphyrin molecules. © 2012 American Chemical Society

2. EXPERIMENTAL DETAILS 2.1. Synthesis of DOCP-Zn 7 and DOCP-Zn-bpy 8. The compound (DOCP-Zn) 7 was synthesized by olefin metathesis and characterized by NMR, MALDI-TOF MS, and element analyses. (See the Supporting Information.) When 0.5 mol equiv of 4,4′-bipyridine was added to the DOCP-Zn solution in CHCl3, the zinc atom is able to coordinate with 4,4′-bipyridine to form the DOCP-Zn-bpy complex 8, which can be characterized by 1H NMR titration experiments (Figure 1).

Figure 1. 1H NMR-titration of 4,4′-bipyridine with DOCP-Zn in CDCl3.

Received: February 29, 2012 Revised: May 18, 2012 Published: June 11, 2012 14134

dx.doi.org/10.1021/jp301998d | J. Phys. Chem. C 2012, 116, 14134−14138

The Journal of Physical Chemistry C

Article

The equilibrium constant of 1H NMR titration indicated that two DOCP-Zn molecules interact with one 4,4′-bipyridine by zinc nitrogen atoms coordination and formed the DOCP-Znbpy complex 8. (Figure S1 the Supporting Information.) 2.2. Characterization of the Morphology. The aggregate nanostructures of the DOCP-Zn and DOCP-Zn-bpy were prepared in silicon slide and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and confocal laser scanning microscopy (CLSM).

volatility, leading to the DOCP-Zn molecule to form different aggregate structures, for example, 1D nanorods in methanol and 1D bandage-like nanoslices in isopropanol. Figure 3e,f shows the CLSM of these nanostructures, which show the typical red fluorescence emission of porphyrins.

3. RESULTS AND DISCUSSION 3.1. Aggregate Nanostructures of DOCP-Zn 7. Solvent evaporation-driven force played a key role in controlling morphologies of the nanostructures and dimension of the nanostructures. Figure 3 shows typical SEM images of DOCPZn molecule 7. When the complex was prepared in THF/ MeOH (v/v 1:1) at 25 °C for 2 min, the nanorods with 1D structure were obtained (Figure 3a). The length of the rods was from 2 to 20 μm. The diameter of the nanorods was 20−700 nm. The corresponding TEM images indicated that the interior of the nanorods was solid, whereas in THF/i-PrOH (v/v 1:1), the growth of DOCP-Zn molecule is still along the 1D direction, forming the long bandage-like slices (Figure 3b). The length of the bangage-like slices was from 5 to 15 μm, and the width was from ∼2 to 3 μm. According to the similar assembly mechanism,35,36 the drive force of self-assembly comes from the interaction among porphyrins rings, π−π interaction, and hydrogen bonds interaction. The formation of microstructures could be attributed to “head-to-tail” assembly.34,35 Chloroform was a good solvent and alcohol was a poor solvent for DOCPZn, and they possess different volatility, which can lead to the change of the ratio of the mixed solvent and result in reduced solubility of DOCP-Zn in the system. This phenomenon could influence the π−π interaction and hydrogen bonding interaction between the porphyrin rings and thus produce easily the multilayers films, as shown in Figure 2. The alcohol molecules are able to expand from the interspace between multilayers films to result in the reorganization on aggregate structures for the formation of new nanostructures of nanorods or nanoslices. According to the difference of the volatility of the solvents such as methanol and isopropanol, we could find that the self-assembly process elongated with the decrease in the

Figure 3. SEM images in silicon slices of 7 prepared in (a) THF/ MeOH (v/v 1:1), nanorods, and (b) THF/iPrOH (v/v 1:1), long thin slices at the temperature of 25 °C. (The insets of panels a and b show the corresponding TEM images.) The microscope and CLSM (λex = 415 nm) images of the DOCP-Zn 7 on slide glass in THF/MeOH (v/ v 1:1) (c,e) and THF/i-PrOH (v/v 1:1) (d,f).

3.2. Aggregate Nanostructures of DOCP-Zn-bpy 8. Moreover, to understand the relationship between aggregate structures and molecular structures, we performed the control experiments on DOCP-Zn molecules through the axial coordination with 4,4′-bipyridine. This formed biporphyrinzinc-bpy complex (BDOCP-Zn-bpy) 8 was confirmed by 1H NMR-titration spectra (Figure S1 of the Supporting Information, ESI). The BDOCP-Zn-bpy complex 8 can aggregate to produce 0D nanospheres in the mixed solvent of CHCl3/cyclohexane (v/v 1:1) at 35 °C (Figure 4a). The diameters of nanospheres were in the range 300−500 nm. The corresponding TEM images indicated that the interior of the nanostructure was solid (Figure 4b). This termolecular complex DOCP-Zn-bpy could easily aggregate into multilayer films through the π−π stacking interactions between DOCP-Zn-bpy complexes by the evaporation-driven process in chloroform/ cyclohexane system. The coordination interaction enlarged the distance between two glycol side chains in the termolecular complex. The lipophilic solvent of CHCl3/cyclohexane also reduced the hydrogen bands interaction between two ether chains in the termolecular complex. According to these results, the glycol side chains in the DOCP-Zn-bpy complex multilayer films have bigger space for movement and become more flexible compared with the ether chains in DOCP-Zn multilayer films in the mixed solvent of CHCl3/cyclohexane. Ultimately,

Figure 2. Schematic outline shows the aggregation procedure of nanostructures and the nanostructures conversion between spheres and slices or nanorods. 14135

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i-PrOH was added, the nanospheres dissolved and formed regular thin slices with the length of ∼2 μm and thickness of about 30 to 50 nm (Figure 5c), The surface of the nanospheres was dissolved fast by i-PrOH and continually reaggregated into rods or slices. Whereas the mixed solvent of CHCl3/ cyclohexane (v/v 1:1) was dropped to the rods or slices in silicon substitute, the rods or slices on the surface gradually dissolved and converted into the initial nanospheres. The pattern conversion between nanospheres and nanoslices was reversible and could be easily controlled by tuning the axial coordination interaction between zinc porphyrin and 4,4′bipyridine (Figure S8 of the Supporting Information, ESI). This is a novel strategy to control the morphologies and dimensions. The resulting controlled organic nanoaggregate structures show a high efficiency for producing defined shape and dimensional architecture. The controlling of the patterns conversion from nanospheres to nanoslices in isopropanol at different temperature and time was employed to study the self-assembly process (Figures S4− S7 of the Supporting Information, ESI). The conversion process of patterns was easily understood from the SEM at low temperature (Figures S6 and S7 of the Supporting Information, ESI). With the degression of growth temperature, the size of nanoslices became smaller; the length of the nanoslices decreased from about tens of micrometers to 1 μm. (Figure S9a−h of the Supporting Information, ESI). The area of the slice in Figure S9h of the Supporting Information (ESI) was nearly the superficial area of the initial nanosphere with the diameter about 300 to 700 nm. This was strong evidence that nanoslices were converted from nanospheres in situ. Figure S2a of the Supporting Information (ESI) shows the UV/vis absorption spectra of the DOCP-Zn-bpy solution in CH2Cl2, the nanospheres film of BDOCP-Zn-bpy complex from CHCl3/cyclohexane (v/v 1:1), the nanoslices film, and nanorods film converted from the above nanospheres. The UV/vis spectra of the aggregated nanostructures are significantly different from that of the corresponding DOCP-Zn-bpy solution. For DOCP-Zn-bpy in CH2Cl2 at the concentration of 5 × 10−6 M, the Soret band is ∼422 nm and Q bands are in the range of 530−605 nm (Figure S2a of the Supporting Information, ESI). The absorption spectra of the nanospheres film, the nanoslices film, and nanorods film are similar, without red shift or blue shift compared with each other. The Soret band is at 435 nm, and the Q bands are in the range of 550− 630 nm. Both the Soret and Q absorption bands of the films are broadened and red-shifted when compared with the organic solution phase. These observations indicated J-type (edge-toedge) interaction in the nanospheres. These absorption spectra suggest the linear array of porphyrin units by π−π stacking and axial coordination in the nanospheres. Figure S2b of the Supporting Information (ESI) shows the fluorescence spectra of the DOCP-Zn-bpy solution in CH2Cl2 at an excitation wavelength of 422 nm and of nanospheres film, nanoslices film, and nanorods film obtained at an excitation wavelength of 435 nm on quartz sheets. The emission bands of the nanospheres were not observed clearly because of the diffuse reflection. The emission bands of nanoslices film and nanorods film were redshifted ∼10 nm compared with the DOCP-Zn-bpy solution in CH2Cl2. These observations suggest J-type (edge-to-edge) interaction in the nanostructures. The internal structure of the self-assembled nanostructures obtained from DOCP-Zn was further investigated by XRD analysis (Figure S10 of the Supporting Information, ESI).

Figure 4. (a) SEM images in silicon slices of the BDOCP-Zn-bpy complex 8 prepared in CHCl3/cyclohexane (v/v 1:1) at the temperature of 35 °C and (b) the corresponding transmission electron microscopy (TEM) images. The microscope (c) and CLSM (d) (λex = 415 nm) images of the BDOCP-Zn-bpy 8 on slide glass in CHCl3/cyclohexane (v/v 1:1).

the DOCP-Zn-bpy complex formed arcuate multilayers films with the lowest energy in lipophilic solvent and self-assembled into the nanospheres structure (Figure 2). Obviously, the DOCP-Zn in CHCl3/cyclohexane aggregated into irregular slices (Figure S3 of the Supporting Information, ESI). 3.3. Reversible Nanostructures Conversion. Interestingly, with the destruction of the axial coordination interaction by the competition of solvents, the nanospheres (Figure 5a)

Figure 5. SEM images of the nanostructures of DOCP-Zn-bpy complex 8 in CHCl3/cyclohexane (v/v 1:1) (a) and pattern conversion from nanospheres to nanorods in CHCl3/MeOH (v/v 1:1) (b) or nanoslices in i-PrOH (c) at the temperature of 30 °C, scale bar: 5 μm. (d) Experiment procedure.

could be converted into nanorods or nanoslices in alcohol. When the nanospheres on silicon substrate were kept into the mixture of CHCl3/MeOH (v/v, 1:1), the nanospheres dissolved and reaggregated into long rods with 1D nanostructure. The length of the nanorods was about 20 to 30 μm, and the diameter was ∼500 nm to 1 μm (Figure 5b). However, the nanospheres were unable to convert into nanorods in pure methanol because it is a poor solvent for DOCP-Zn-bpy. When 14136

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resolved spectra of the waveguided emission that is outcoupled at a nanowire tip, and Figure 6c exhibits the output intensity as a function of propagation length, respectively. The optical loss coefficient (α) was calculated by a single exponential fitting [Itip/Ibody = A exp−αx, where x is the distance between the exciting site and the emitting tip and A is the ratio of the light escaping from the excitation spot and that of light propagating along the fiber]. The Ibody was adopted as reference to reduce the time fluctuation of input laser. Here α was determined to be 0.067 dB/μm. No obvious red shift was observed with the increase in propagation distances (Figure 6b). Hence, the overlap of the absorption and fluorescent spectra is negligible, which is a main factor for such a low-loss optical waveguide. Despite the reduced reabsorption, the smooth surface and distinctly flat end facets also minimized the optical loss caused by scattering, contributing to the excellent optical waveguide behaviors.

Figure S10A−C exhibits the diffraction patterns of the nanorods, nanoslices, and nanospheres, respectively. As shown in Figure S10A−C, the XRD pattern shows the same peak at 2θ = 21.2° (corresponding to 0.42 nm) that is ascribed to the stacking distance between neighbor porphyrin rings along the direction perpendicular to the porphyrin rings.38,39 In Figure S10B,C, the most intensive peak at 2θ = 5.95° (corresponding to 1.69 nm, nearly the width of the molecule DOCP-Zn, in Figure S10) indicates that there is the same direction of growth in nanorod and nanoslice,40,41 which is consistent of the aggregation model in the Figure 2.

4. OPTICAL WAVEGUIDE PROPERTIES OF NANORODS We used photoluminescence (PL) microscopy to study the optical waveguide properties of nanorods obtained from DOCP-Zn in THF/MeOH (v/v, 1:1). When we focused laser light onto the midst of the nanorod, the generated PL was strong enough to be guided to the end of the nanorod (Figure 6). This result indicates the nanorods material has the potential application as nanoscale photonic elements.42

5. CONCLUSIONS In summary, a novel amphiphilic porphyrin derivative with two octaoxa-aether rings (DOCP-Zn) has been synthesized and utilized to construct nanostructures by controlling the axial coordination of zinc porphyrin with 4,4′-bipyridine. The resulting controlling organic nanoaggregate structures show a highly efficient defined shape and dimension architecture. The controlling of the patterns conversion in isopropanol system in different temperature and time was also studied, indicating that patterns between 0D nanospheres and 1D nanorods or nanoslices can be reversibly converted. Another notable phenomenon is that these nanowires remain fairly high fluorescent in the solid state and exhibit high active waveguide property. These results indicate that the controlled growth of organic nanoaggregate structures combined with the active waveguide property offered these porphyrin aggregates the potential application as nanoscale photonic elements.

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ASSOCIATED CONTENT

* Supporting Information S

Figure 6. (a) Microscope images and the corresponding PL microscopy images, (b) the spatial resolved spectra of the waveguided emission that is outcoupled by excitation at a distance of 60, 50, 40, 30, 20, and 10 μm from the tip of a single nanorod (from 1 to 6 in panel a), and (c) the output intensity as a function of propagation length, respectively. The DOCP-Zn 7 was prepared in THF/MeOH (v/v 1:1) on slide glass, scale bar: 20 μm. The red area was excited by focused laser light (λex = 422 nm).

Full synthetic details and characterization, 1H NMR titration, the nanostructures conversion process, and results at different temperature and time. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+) 86-10-82616576. E-mail: [email protected] (Y.L.); [email protected] (Y.Z.).

To measure the microarea PL spectra of single nanowires, we excited the nanowires dispersed on a glass coverslip with a UV laser (λ = 422 nm, Beamlok, Spectra-physics). The excitation laser was filtered with a band-pass filter (400−450 nm), then focused to excite the nanowires with an objective (50×, N.A. = 0.80). The spot size was