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Supratim Basak and Rajadurai Chandrasekar *

Multiluminescent Hybrid Organic/Inorganic Nanotubular Structures: One-Pot Fabrication of Tricolor (Blue–Red–Purple) Luminescent Parallepipedic Organic Superstructure Grafted with Europium Complexes

This article focuses on an innovative one-pot fabrication of organic/inorganic hybrid parallepipedic tubes with rectangular cavities displaying multicolor luminescence. Firstly, using a novel back-to-back coupled 2,6-di-pyrazol-1-ylpyridine ligand, blue-emitting several-micrometer-long (ca. 50 μ m) paralle-pipedic organic nanotubes with rectangular cavities were fabricated in THF/water via supramolecular (H-bonding and ππ – ππ stacking) and solvent-assisted self-assembly. Secondly, in the same pot, the ligand molecules available on the surface of the ligand nanotubes were reacted with Eu(tta) 3 molecules at the solid/liquid interface to form a layer of red-emitting Eu(III) complex coating on the inner and outer surface of the tubes. The resultant organic/inorganic hybrid parallepipedic nanotubes fabricated using this novel bottom-up one-pot technique display tricolor (blue–red–purple) lumines-cence, i.e., blue and red dual emission from the organic ligand and the Eu(III) complex, respectively, and a purple color due to the mixing of the two colors. This simple technique signifi es an innovative and important method in the development of bottom-up nanotechnology of multiluminescent organic/inorganic hybrid nanotubes.

1. Introduction

Luminescent organic nanotubes are of great importance as a building blocks in future miniaturized devices. [ 1 , 9b ] With the rapid advancement of nanoelectronics, one-dimensional lumi-nescent nanotubular structures have attracted particular interest because of their unique optoelectronic properties due to a two-dimensional quantum confi nement effect. [ 1 ] Untill now, most of the known luminescent nanotubular structures are purely organic in nature and display only single-color emission. Nano-fabrication of multicolor-emitting hybrid nanotubular mate-rials from two different soft matters (organic/inorganic) is an unexplored and sophisticated research area of nanoscience and technology. Successful groundwork in this area could lead to new hybrid nanotubular luminogens, capable of performing

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DOI: 10.1002/adfm.201002270

Prof. R. Chandrasekar, S. BasakSchool of ChemistryUniversity of HyderabadProf. C. R. Rao Road, Gachi Bowli, Hyderabad – 500 046, India E-mail: rcsc@uohyd.ernet.in; chandrasekar100@yahoo.com

a sensing, photonic, biological, or trans-port function. However, the controlled and selective fabrication of hybrid nano-tubes with multicolor emission requires a rational design and step-wise preparation strategy at the nanoscale for a successful integration into nanofunctional devices.

Controlling and understanding the growth [ 2 ] of an organic soft matter into a shape-defi ned nanotubular confi guration (cylindrical, hexagonal, rectangular, and parallelogrammatic) is itself a challenging area. Untill now, most of the organic nano/submicrotubes fabricated from the self-assembly of disclike molecules, [ 3 ] den-drimers, [ 4 ] amphiphiles, [ 5 ] small organic molecules, [ 6 ] peptides, [ 7 ] and polymers [ 8 ] are all of single-component nature, and shape control at the nanoscale has been achieved by using crystallization, template synthesis, vaporization, and amphiphilic assembly techniques. It has been recently recognized that, apart from supramo-

lecular interactions [ 2 – 9 ] (van der Waals interactions, hydrogen bonding, coordination, and π – π stacking), the solubility of molecular building blocks in organic solvents and their con-centration also play a crucial role in controlling the shape, size, and texture of the organic nanostructures. [ 6 , 9b ] To fabricate multi-functional organic/inorganic hybrid nanotubular structures, the use of small organic ligand molecules as nanobricks is a logical approach, since it provides easy structural tunability and diversity, which are important to control their solubility as well as their molecular function at the nanoscale. Additionally the ligand molecules available on the surface of the shape-defi ned primary nano-superstructure can also be effectively utilized to perform coordination chemistry with different metal ions (surface modifi cation) at the solid/liquid interface. This meth-odology may form pre-shape-instructed secondary inorganic nanostructures on the primary organic nano-superstructure with multifunctional properties. Based on this idea, we have fab-ricated a tricolor luminescence parallepipedic organic/inorganic nanostructure via a step-wise self-assembly approach starting from ligand 1 ( Figure 1 ).

Herein, we report our one-pot and two-step nanofabrication approach ( Scheme 1 ): i) fabrication of several-micrometer-long

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Figure 1 . A) ORTEP plot (50% probability) of a building block molecule 1 (front-view). B) Side-view of 1 . In both A and B H-atoms are omitted for clarity. C) intermolecular π – π stacking inter-actions ( d = 3.260 Å) of the pyridine rings forming 3D multilayered supramolecular aggregates. D) formation of a 1D chainlike structure through intermolecular hydrogen bonding interactions of the pyrazol-ring proton and the pyrazol nitrogen.

(ca. 50 μ m) blue-emitting primary organic rectangular nano-tubular structures with well-defi ned rectangular cavities (par-allepipedic nano-superstructures) from ligand molecule 1 in THF/H 2 O solvents; ii) formation of red-emitting Eu(III) com-plexes ( 2 ) on the surface of the preformed superstructure via ligand surface-selective reactivity; and iii) generation of a spec-tacular tricolor (blue–red–purple) luminescence from organic/inorganic hybrid parallepipedic nanostructures. We also present the syntheses of a directly back-to-back coupled 2,6-di-pyrazol-1-ylpyridine 1 and its Eu(III) complex 2 , [(Eu(tta) 3 ) 2 ( 1 )] (tta = thenoyltrifl uoroacetonate). The single-crystal X-ray structure of 1 , a comprehensive characterization of the organic and hybrid parallepipedic nanostructures using fi eld-emission scanning electron microscopy (FESEM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and confocal Raman microspectroscopy and the examination of tricolor luminescence properties of the hybrid nanostructures using confocal fl uorescence microscopy will also be presented.

2. Result and Discussion

The building-block ligand 1 was synthesized from 2,6-di-pyrazol-1-yl-pyridin-4-yl-iodide [ 9b − e] under Suzuki reaction conditions in a modest yield as white powder. The THF solution of 1 showed a

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fl uorescence maximum at 390 nm ( Φ f = 0.16) for the excitation ( λ ex ) performed at 331 nm (Figure S3 in the Supporting Information). Single crystals suitable for X-ray analysis and nanolevel studies were obtained from toluene via slow solvent evaporation. Exami-nation of the single-crystal X-ray structure of 1 ( P -1 space group) showed unique struc-tural features for hierarchical supramolecular self-assembly (Figure 1 ). [ 2 k, 10 ] The molecule is near planar [C(8)–C(9)–C(9)–C(10) tor-sion angle = 1.49 ° ] and the N py ⋅ ⋅ ⋅ N py distance between the two pyridine segment is ca. 0.71 nm. Both intermolecular hydrogen bonding and π – π stacking play a vital role in the supramolecular ordering and self-assembly. Each pyrazole-ring nitrogen (N1) participates in a head-to-tail intermolecular hydrogen bonding interaction with the pyrazole-ring proton (H6) of the neighboring molecule [N(1) ⋅ ⋅ ⋅ H(6) Pz = 2.699 Å] forming a 1D chain-like structure (Figure 1D ). Each 1D chain is closely packed and forms a pseudo sheetlike structure. These sheetlike structures form 3D multilayered supramolecular aggregates via intermolecular π – π stacking interactions ( d = 3.260 Å) of the pyridine rings (Figure 1C ).

The ultimate self-assembly of supramo-lecular aggregates [ 2 ] of 1 into shape-defi ned parallepipedic nanostructures is a solvent mediated process. In a typical procedure, 1 mL water was rapidly injected into a solution containing 2 mg of 1 dissolved in 2 mL THF.

The solution was kept for 4 h undisturbed to stabilize growth of nanostructure. Two drops of this solution were slowly evapo-rated on a clean glass substrate at room temperature or directly deposited onto a carbon-coated TEM grid. SEM revealed the for-mation of ca. 50- μ m-long parallepipedic nanotubular structures with rectangular open-ended features ( Figure 2 a–c ). The pres-ence of elongated hollow structures along the axis of the tubes is undoubtedly evident from a broken tube shown in Figure 2c . A closure look at the open-ended feature of each tube, shown in Figure 2j–l , shows the front (Figure 2j ), back (Figure 2k ), and side (Figure 2l ) of the tubes. These diverse open-ended features demonstrated the growth of the tube along the tube axis.

Furthermore the bright-fi eld TEM images of nanostruc-tures obtained from 1 evidently supported the formation of a parallepipedic nanotubular structure with rectangular cavity by providing a 3D perspective of the nanotubes (Figure 2d–g ). The selected area electron diffraction (SAED) pattern revealed a twinned diffraction pattern originating from the front and back faces of the nanotubes, which demonstrated the crystalline nature of the tubes (Figure 2g and inset). In order to determine an accurate width (W) and height (H) profi le of the nanotubes, semi-contact-mode AFM was performed (Figure 2h and i ). Figure 2i showes a typical H × W profi le of two parallepipedic nanotubular structures, which is 275 nm × 400 nm. The H × W distribution pattern for thirty nanotubular structures that was acquired using AFM revealed that the aspect ratio (H/W)

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Scheme 1 . Schematic elucidation of the possible formation mechanism of a tricolor-luminescence organic/inorganic parallepipedic nanotubular structures in THF/water via supramolecular self-assembly and ligand surface-selective coordination chemistry.

varies from 0.29–0.8. In this distribution range the majority of the tubes have an aspect ratios below 0.51 (Figure S5). Addi-tionally, results from SEM and TEM supported a several-step

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possible formation mechanism [ 11 ] of organic nanotubes from the supramolecular aggregates of 1 (Scheme 1 , and Figure S4): i) formation of sheetlike structures of micrometer dimension

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Figure 2 . SEM microscopy images of parallepipedic organic nanostructures obtained from 1 : a) nearly monodispersed rectangular tubes (scale bar: 50 μ m); b) front-view of a tube exhibiting a rectangular open-ended cavity [white dotted circle: X , Y , Z coordinates mark the rectangular cross-section of the tube (scale bar: 1 μ m)]; c) a broken tube displaying the hollow cavity (scale bar: 2 μ m). Bright-fi eld TEM microscopy imags: d,e) parallepipedic nanotubular structure with irregular open-ended features [white double headed arrows: width (W) and height (H); scale bar: 200 nm]; f,g) a tube sitting on top of another tube demonstrating the crystalline nature of the tubes; the inset of (g) shows a SAED pattern displaying twinned diffraction pattern originating from the four faces of the two tubes (scale bar: 200 nm); h) non-contact mode AFM image of the parallepipedic nanotubular structure; i) height and length profi le of selected tubes shown in Figure h; j) front-view of a tubes; k) back-view of a tube; l) side-view of a tube.

through supramolecular interactions (H-bonding and π – π stacking); ii) a threefold 90 ° folding of the sheets along the sheet axis forming rectangularly curled structures; and iii) seaming of the curled sheets to form rectangular tubes with a rectangular cavities. The main driving force behind the sheet folding and curling process is probably the minimization of surface energy. Once the low-surface-energy tubular morphology has formed, further growth proceeds only along the tube axis (concentra-tion dependent) with constant width and height dimension. The growth of the tube along the tube axis ( Z -axis) is steered by the rapid growth of the tube walls ( XZ- plane) along the Z -axis (Figure 2d–e ).

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Since the parallepipedic nanostructures exhibited blue emis-sion, by utilizing the metal-coordination ability of the building-block ligand molecules 1 available on the tube surface, syn-thesis of a thin layer of red-light emitting Eu(III) coordination complexes [ 9a , 12 ] on the inner and outer surface of the tubes was envisaged. We anticipated that this selective coordination chem-istry at the solid/liquid interface on the surface of the preformed ligand superstructure may form a inorganic/organic hybrid nanotubular structure that could display three colors upon exci-tation with UV light: i.e., a sensitized [ 13 ] red emission from the inner and outer surface of the tube (from the hypersensitive transitions 5 D 0 → 7 F J ( J = 0–4) of the Eu III ion), blue emission

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Figure 3 . Formation of a complex 2 coating on the surface of the par-allepipedic nanotubular structure of 1 in THF/water via self-assembly. TEM images: a) a tube coated with complex 2 displaying dark contrast; b, c) a partially reacted tube surface with Eu(III) ions. The dark (coated) and light (uncoated) contrast areas are clearly marked; d) a fully Eu(III) coated area; e) FESEM image: EDAX data collected in a selected area are shown as a rectangle, the scale bar is 100 nm; f) EDAX analysis of the selected area shown in (e) confi rming the presence of Eu ions on the tubes (the actual KCnt of Si is ca. 3.3, which is not shown fully for the sake of picture clarity).

from remaining organic ligand part of the tube (the unreacted middle layer), and purple color due to the mixing of blue and red colors. Firstly, the shape-defi ned parallepipedic nanotubular superstructures were grown in a mixture of THF/water solvents as mentioned before. Secondly, by injecting a THF solution of Eu(tta) 3? (H 2 O) 3 (0.065 mg per 0.02 mL, 1 mol%) to the solution containing the tubes obtained from ligand 1 , followed by gentle shaking, a layer of [(Eu(tta) 3 ) 2 ( 1 )], coordination complex 2 , was formed on the surface of the nanotubes. Here, each 2,6-bispyra-zolylpyridine moiety present in ligand 1 acts as a tridentate ligand and expels the water molecules out of the fi rst coordina-tion sphere of the Eu(tta) 3 ⋅ (H 2 O) 3 complex [12b,c] forming a binu-clear complex 2 with a distorted tricapped trigonal prismatic geometry around the Eu(III) ion. Once all the exposed ligand molecules available on the tube surface have reacted (sub-stochiometric binding) [ 2h ] further reaction is not possible, thereby forming a thin layer of inorganic complex 2 on the organic superstructure. The coating of the complex 2 on the tube surfaces is probably stabilized by the noncovalent binding forces between the complex molecules and unreacted ligand mole-cules present in the lattice of the tube. TEM bright fi eld images obtained from the resultant organic/inorganic nanostructures showed a clear dark contrast for areas coated with complex 2 and light contrast for the unreacted or less-reacted part of the nanotubes ( Figure 3 a–c ). This observation clearly supported the growth of complex 2 on the tube surface. Furthermore, FESEM on the Eu(III)-coated tubes exhibited clear distinctive areas on the tube surfaces and edges (Figure 3e,f ) due to the growth of complex 2 . Energy-dispersive X-ray analysis (EDAX) performed on this areas formed on the tube surfaces confi rmed the presence of Eu ions (Figure 3f ).

Additionally, confocal Raman microspectroscopy on the tubes that self-assembled from 1 and the tubes coated with 2 have shown evidence for the formation of a layer of complex 2 on the tube surface by exhibiting characteristics Raman shifts (Figure 4 ). Particularly, the signal intensities of the tube (middle organic layer) of 1 is higher than the intensi-ties originating from the complex 2 layer due to the surface-selective reactivity and the resultant complex layer formation in and around the tube by leaving the majority of the organic nano-superstructure of 1 unreacted. In order to determine dual-color emission from the Eu(III)-coated nanotubes scan-ning confocal fl uorescence microscopy measurement was performed. The topography of the tubes ( XYT -mode) and the corresponding emission ( XY λ -mode; the λ step width is 10 nm) properties were recorded. The topographic structural data were directly coupled with the spectroscopic properties of the specimen. For the excitation Ar-ion (UV 356/365), laser light was used. The pure organic rectangular tubes obtained from 1 exhibit blue emission at 455 nm (Figure S7). Remark-ably, from the tubes coated with complex 2 , blue- ( λ em = 455 nm) and red-color ( λ em ≈ 540–720 nm region) dual emis-sion was detected at the blue and red channels of the photo-multiplier tube due to the formation of a thin layer of red-emissive Eu III complexes 2 on the surfaces of the blue-emissive nanotubular superstructure ( Figure 5 a,b ). The wavelengths of the observed red- emission peaks and their intensities are matching the bulk sample of 2 , supporting the formation of the same at the tube surface (Figure S8b). Additionally, the

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inclusive reactivity of the Eu(tta) 3 on the ligand nanotubular surface was clearly evident from the image shown in Figure 5b . As a result of the merging of these blue and red colors the hybrid tubes also displayed an additive purple color (Figure 5c ), even under normal UV light excitation, there by dis-playing three colors i.e., blue–red–purple. The signal inten-sity of the Eu(III)-centered emission was lower than the ligand-molecule emission due to the presence of a thin layer of complex 2 molecules on the ligand nanotubular surface, leaving the majority of the interior ligand molecules of the tubes unreacted. Interestingly, the intensity of the Eu(III) centered emission [ 5 D 0 → 7 F J ( J = 0–4)] increases progressively upon increasing the amount of Eu(tta) 3 ⋅ (H 2 O) 3 due to the increase of the reactivity of Eu(tta) 3 to the most of the acces-sible ligand molecules available on the ligand tube surfaces (Figure S6).

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Figure 4 . Confocal Raman microspectroscopic data of a parallepipedic nanotubular structure coated with complex 2 . Excitation wavelength λ ex = 633 nm (HeNe laser).

3. Conclusions

In summary, we demonstrated an effi cient one-pot technique for the fabrication of organic/inorganic hybrid parallepipedic nanotubular structures displaying tricolor luminescence by employing a step-wise solvent-assisted self-assembly technique.

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Figure 5 . Confocal fl uorescence microscopic images of parallepipedic nanotubular structure coated with a layer of complex 2 , formed using 1 mol% of (Eu(tta) 3 ) 2 ⋅ 3H 2 O. a) Before excitation; b) blue emission from the ligand part of the tubes (middle of the walls); c) red emission from the Eu(III) ion of the complex 2 formed on the surface of the tubes; d) overlay of the two images (b) and (c) displays purple color from the organic/inorganic hybrid tubes due to the mixing of the colors. Excitation source: Ar-ion (UV 356/365) laser.

Firstly, the parallepipedic nano-superstructures were grown from a novel blue-emitting π -conjugated ligand molecule 1 in THF/water. Secondly, for the fi rst time, a controlled sur-face-selective coordination chemistry was per-formed on the ligand superstructures by using Eu(tta) 3 at the solid/liquid interface to fabricate a thin coating of red-emitting [(Eu(tta) 3 ) 2 ( 1 )], complex 2 . Upon UV excitation the resultant organic/inorganic hybrid parallepipedic nanostructures display a blue emission from the unreacted ligand ( 1 ) of the nanotubes, a Eu(III)-centered red emission from the com-plex ( 2 ) coating, and the mixing of these two colors exhibit purple color ( 1 + 2 emissions). This simple and one-pot technique to fabricate tricolor emissive shape-defi ned parallepipedic nanotubular structures signifi es an innovative and important method in the bottom-up nano-technology of multi-luminescent organic/inorganic hybrid nano tubular structures.

[ 1 ] For a comprehensive overview on luminescent organic nanotubes: a) J. Hu , T. W. Odom , C. M. Lieber , Acc. Chem. Res. 1999 , 32 , 435 ; b) Y. S. Zhao , H. Fu , A. Peng , Y. Ma , Q. Liao , J. Yao , Acc. Chem. Res. 2010 , 43 , 409 ; c) Y. S. Zhao , J. Xu , A. Peng , H. Fu , Y. Ma , L. Jiang , J. Yao , Angew. Chem. Int. Ed. 2008 , 47 , 7301 ; d) Q. Liao , H. Fu , J. Yao , Adv. Mater. 2009 , 21 , 4153 ; e) Y. S. Zhao , A. Peng , H. Fu , Y. Ma , J. Yao , Adv. Mater. 2008 , 20 , 1661 ; f) L. Zhao , W. Yang , Y. Ma , J. Yao , Y. Li , H. Liu , Chem. Commun. 2003 , 2442 .

4. Experimental Section All the general procedures and additional experimental details are provided in the Supporting Information.

CCDC 776441 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was supported by DST (Fast Track Scheme No. SR/FTP/CS-115/2007), New Delhi. We thank the Centre for Nanotechnology (CFN), University of Hyderabad (UoH) for providing the TEM facility and Mr. M. Durga Prasad for recording the TEM images. We also thank CIL, UoH for SEM and confocal fl uorescence microscopy facilities, S.B. thanks CSIR-New Delhi for a SRF. Authors contributions: R.C. designed the Research; S.B., under the supervision of R.C., synthesized the molecule and carried out the experiments; R.C. analyzed the data and wrote the paper.

Received: October 13, 2010 Published online: December 13, 2010

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