A Unique Independent One-Dimensional Green Luminescent

Jul 19, 2011 - CPNT-1 shows a fascinating 1D → 2D interdigitated architecture, and the final three-dimensional supramolecular columnar network was f...
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A Unique Independent One-Dimensional Green Luminescent Coordination Polymer Nanotube Based on an Unsymmetric Tricarboxylate Linker Lina Li, Junhua Luo,* Shuyun Wang, Zhihua Sun, Tianliang Chen, and Maochun Hong Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China

bS Supporting Information ABSTRACT: A novel independent one-dimensional (1D) coordination polymer nanotube [Cd3(BPT)2(phen)3(H2O)] 3 4.5H2O (CPNT-1, H3BPT = biphenyl-3,40 ,5-tricarboxylic acid; phen = 1,10-phenanthroline) has been synthesized and structurally characterized. CPNT-1 shows a fascinating 1D f 2D interdigitated architecture, and the final three-dimensional supramolecular columnar network was formed via ππ stack interactions between the terminal phen ligands of the coordination polymer nanotubes. Luminescent studies show that CPNT1 displays strong green emission at room temperature, and the guest water molecules affect its luminescent properties.

ince the first discovery of carbon nanotubes (CNTs),1 nanotubular structures including inorganic nanotubes (e.g., C, B, N, S, Si),2 organic nanotubes (e.g., cyclic peptides, lipids, oligocyclodextrins),3 and coordination polymer nanotubes, have been intensely studied due to their novel structures and, more importantly, their potential applications in ion exchange, molecular sieves, catalysis, gas storage, sensors,4 and they even may possess new applications in nanotechnology such as molecular electronics and devices, photoelectronics, etc. Though abundant porous coordination polymers (CPs) which can be readily selfassembled by the coordination of metal cations/clusters with organic linkers have been reported,5 only a few coordination polymer nanotubes (CPNs) have been obtained in the past decades. For example, a nanosized tube constructed by cadmium ion and mixed organic ligands was synthesized by Hong and coworkers.6 Three novel lanthanide-transitional-metal coordination polymers containing nanotubes have been reported by Cheng and Liao's group,7 and an exceptionally stable hollow tubular Zn-organic architecture has been successfully acquired by zur Loye and co-workers.8 However, the above-mentioned nanotubulars were normally assembled into highly three-dimensional frameworks through further interconnection or interweaving of the nanotubular units by the multidentate ligands, and only a few metal coordination polymers with isolated nanotubular structures have been reported.9 Therefore, it is a particularly interesting and great challenging subject to design and construct independent onedimensional (1D) coordination polymer nanotubes. It is well-known that the coordination frameworks can be controlled and modulated by selecting the coordination geometry of metal ions, and the chemical nature of terminal and bridging ligands. Such coordination approach has been proven

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to be very effective to construct nanotubular structures, such as in the assembly of cylindrical helix and a 3.5 nm Pd coordination nanotube.10 Utilization of unsymmetrical linkers may allow the formation of novel coordination polymers with structures and properties previously undiscovered.11,12 In order to build nanosized-tubular molecular architectures through molecular assembly, we chose a long bridging unsymmetric ligand, biphenyl3,40 ,5-tricarboxylate (H3BPT),13 as the linker which contains only chemically inequivalent carboxylates with distinct coordination environments, and in addition, for the purpose of constructing an independent coordination polymer nanotube, 1,10phenanthroline (phen) was chosen as terminal ligand to prevent the further interconnection of the independent coordination nanotubes. Here we report a unique coordination polymer [Cd3(BPT)2(phen)3(H2O)] 3 4.5H2O with nanosized tubular structure. To our best knowledge, it is a rare example of a 1D independent coordination polymer nanotube showing 1D f 2D interdigitated architecture14 and the final three-dimensional (3D) supramolecular columnar structure through ππ stack interactions. Hydrothermal reaction of H3BPT, phen, and Cd(II) metal salt at 140 °C yielded CPNT-1 with the formula of [Cd3(BPT)2(phen)3(H2O)] 3 4.5H2O, which was determined by crystallographic studies, elemental analysis, and thermal gravimetric analysis.15 Single crystal X-ray diffraction analysis shows that CPNT-1 crystallizes in the centrosymmetric P1 space group.16 The asymmetric unit consists of three independent Cd(II) atoms, as shown in Figure 1a,b. All the three Cd atoms have Received: June 23, 2011 Revised: July 14, 2011 Published: July 19, 2011 3744

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Figure 1. (a, b) Coordination environments of the Cd(II) ions in CPNT-1; (c) the coordination modes of BPT ligands (the H atoms and the C atoms of phen ligand are not shown for clarity).

the same distorted pentagonal bipyramid coordination geometry [CdN2O5]. Cd1 is coordinated by four oxygen atoms (O1, O2, O7, O8) from two carboxylate groups of two BPT ligands, one coordinated water molecule oxygen atom (O13), and two nitrogen atoms (N1, N2) from one phen terminal ligand, while the five oxygen atoms coordinated to Cd2 or Cd3 are provided by three carboxylate groups of three different BPT ligands. The CdN bond lengths are from 2.315(7) to 2.369(6) Å, and the CdO bond lengths fall in the range 2.263(5)2.619(6) Å, which are well in agreement with those reported in other Cd(II) complexes with N,O-mixed ligands.17 The ligands completely deprotonated to connect with four cadmium ions, which can be confirmed by the lack of the characteristic peak at around 1700 cm1 in the IR spectra of CPNT-118 (Figure S1, Supporting Information). It is noted that three carboxylate groups in the ligand BPT exhibit two kinds of connection modes: one is in chelating mode and the other in chelating/bridging mode (Figure 1c). The framework of CPNT-1 contains two types of secondary building units (SBUs). One is the binuclear SBU [Cd2 (CO2)4(phen)4] with Cd2 and Cd3 bridged through two carboxyl groups, and the separation of Cd 3 3 3 Cd is 3.8811 (11) Å. The other is the mononuclear SBU [Cd(CO2)2(phen)2]. As depicted in Figure 2a, the two kinds of SBUs are linked together by four BPT ligands to generate a rhombus with dimensions of 14.06  15.38 Å2 parallel to the ac plane, and along the b axis the neighboring rhombi are linked by upward and downward carboxylic groups of the BPT ligands to form a unique 1D independent coordination nanotube, with a distance between the neighboring

rhombi of 13.8659 Å (Cd1 3 3 3 Cd1) (Figure 2b,c). As mentioned above, the cadmium ions have an affinity for a seven-coordinate environment, five of which are occupied by BPT ligands and H2O in the coordination nanotube, and the remaining two leave binding sites for the terminal phen ligands. Interestingly, the terminal phen ligands which coordinate with Cd1 act as side arms to insert into adjacent nanotubes, resulting in a fascinating 1D f 2D interdigitated architecture. The neighbor phen planes are almost parallel with a dihedral angle of 0.126° and distance of 3.3 Å, which is the typical ππ interaction as shown in Figure 2d. We propose that the strong ππ interaction is one potential driving force for the 1D f 2D interdigitated architecture. Furthermore, the phen terminal ligands from Cd2 and Cd3 ions stack together to generate a 3D columnar supramolecular structure of CPNT-1 (Figure 2e). The disordered H2O molecules lie in the nanotubes, and calculation using PLATON19 based on the crystal structure shows that the total solvent-accessible volume is 745.2 Å3 per unit cell, comprising 23.6% of the crystal volume, which may be the result of the existence of obstructive phen entities in the nanotubes. The thermogravimetric analysis CPNT-1 was performed with a heating rate of 10 °C/min under N2 atmosphere from 30 to 800 °C. The results show a gradual weight loss of 6.60% in the temperature range of 30200 °C, which is attributed to the loss of all 4.5 guest water molecules and one coordinated water molecules (calcd 6.41%). No further weight loss was observed until 330 °C at which the compound starts decomposing (Figure S2, Supporting Information). Powder XRD pattern of CPNT-1 after 3745

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Figure 3. The solid-state emission spectra of CPNT-1 (blue), dehydrated CPNT-1 (green), phen (red).

It is said that increasing molecular rigidity will weaken the intramolecular vibrations, so the molecule excitation energy is not easy released as heat due to vibration. Additionally, the increase of molecular rigidity will enhance the coplanarity, which is in favor of the fluorescent production. After removal of H2O molecules, the emission of dehydrated CPNT-1 is slightly blueshifted and becomes weaker compared with that of CPNT-1, which is possibly attributed to the less rigid framework architecture of CPNT-1 after water molecules are removed.20 The strong green emission of CPNT-1 makes this complex an excellent candidate for solid green luminescent material. In summary, a unique independent 1D coordination polymer nanotube has been synthesized based on an unsymmetric tricarboxylate linker and terminal phen ligand. It presents a rare 1D f 2D interdigitated structure and a 3D columnar supramolecular framework. We also investigated the luminescent properties of CPNT-1 and found that it displayed intense green photoluminescence and the guest H2O molecules affected its luminescence properties.

’ ASSOCIATED CONTENT

bS

Supporting Information. Complete data for TGA, IR, and PXRD patterns. This information is available free of charge via the Internet at http://pubs.acs.org/.

Figure 2. (a) The rhombus generated by two kinds SBUs; (b) perspective view of the nanotube along the b axis; (c) nanotube with a stick inside; (d) view of ππ interaction of phen between nanotubes and 1D f 2D interdigital structure; (e) the 3D columnar supramolecular structure.

activation at 200 °C confirms the stability of the host framework upon solvent removal (Figure S3, Supporting Information). The photoluminescent spectra of the free H3BPT ligand, phen ligand, compounds CPNT-1 and dehydrated CPNT-1 (removed H2O molecules by heating at 200 °C for 6 h) have been investigated in the solid state at room temperature (Figure 3). The emission band for free phen ligand is at 435 and 410 nm excited at 350 nm, while CPNT-1 exhibits an intense emission at 451 nm upon excitation at 370 nm, which is red-shifted compared with that of the free phen ligand. This intense emission is probably assigned to the intraligand charge transfer of phen.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +86-591-83730955. Fax: +86591-83730955.

’ ACKNOWLEDGMENT We gratefully acknowledge the One Hundred Talent Program of the Chinese Academy of Sciences, the 973 key programs of the MOST (2010CB933501, 2011CB935904) for financial support. ’ REFERENCES (1) Iijima, S. Nature 1991, 354, 56–58. (2) (a) Chen, J.; Liu, H. Y.; Weimer, W. A.; Halls, M. D.; Waldeck, D. H.; Walker, G. C. B. J. Am. Chem. Soc. 2002, 124, 9034–9035. (b) Tremel, W. Angew. Chem., Int. Ed. 1999, 38, 2175–2179. 3746

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