Fluorescent Coordination Polymeric Gel from Tartaric Acid-Assisted

Publication Date (Web): February 17, 2010. Copyright © 2010 American Chemical Society. *Corresponding author: e-mail [email protected]; Fax ...
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Fluorescent Coordination Polymeric Gel from Tartaric Acid-Assisted Self-Assembly Sudip K. Batabyal, Wei Lee Leong, and Jagadese J. Vittal* Department of Chemistry, National University of Singapore, Singapore 117543 Received November 3, 2009. Revised Manuscript Received February 6, 2010 A fluorescent organogel is obtained from the reaction of Zn(OAc)2 3 2H2O, 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (bpd), and tartaric acid (H4tar) in methanol. The gel is proposed to have formed by the cross-linking of linear 1D coordination polymers [Zn(bpd)]n with tartarate coligand in a highly random fashion which entrapped the solvent molecules through hydrogen-bonding interactions between the tar coligand and solvent molecules. Higher dimensional coordination polymeric structure is proposed for this gel based on the corresponding complexes formed by oxalic and succinic acids. The presence of three components is essential for the gelation. Interestingly, organogelation of the coordination polymer has induced remarkable fluorescence properties in the weakly emissive bpd. Such fluorescence enhancement is attributed to the reduction in nonradiative decay in the aggregated state. The organogel exhibits viscoelastic behavior as evidenced from the rheological studies.

Introduction Self-assembly of small molecules to functional materials is a powerful tool for the development of novel materials for the futuristic device technology. Supramolecular self-assembly of low-molecular-mass organic gelators leading to gel formation has attracted emerging research interest. Supramolecular gels have been regarded as novel soft materials in numerous applications such as drug-delivery systems, tissue engineering, sensing devices, separation, and optoelectronic devices.1-8 Incorporation of metal ions into organic gelators to form metallogels and coordination polymeric gels has attained great success in recent years.9 Metallogels and coordination polymeric gels have been reported to show unusual functional properties such as *Corresponding author: e-mail [email protected]; Fax þ65-6779-1691.

(1) Lloyd, G. O.; Steed, J. W. Nat. Chem. 2009, 1, 437–442. (2) Weiss, R. G.; Terech, P. Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Springer: Dordrecht, 2006. (3) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3160. (4) Ishi-i, T.; Shinkai, S. Top. Curr. Chem. 2005, 258, 119–160. (5) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201–1218. (6) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821–836. (7) Hirst, A. R.; Escuder, B.; Miravet, J. F.; Smith, D. K. Angew. Chem., Int. Ed. 2008, 47, 8002–8018. (8) Dastidar, P. Chem. Soc. Rev. 2008, 37, 2699–2715. (9) Fages, F. Angew. Chem., Int. Ed. 2006, 45, 1680–1682. (10) Kawano, S.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2004, 126, 8592–8593. (11) Tsuchiya, K.; Orihara, Y.; Kondo, Y.’; Yoshino, N.; Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2004, 126, 12282–12283. (12) Tu, T.; Assenmacher, W.; Peterlik, H.; Weisbarth, R.; Nieger, M.; D€otz, K. H. Angew. Chem., Int. Ed. 2007, 46, 6368–6371. (13) Xing, B.; Choi, M.-F.; Xu, B. Chem.;Eur. J. 2002, 8, 5028–5032. (14) Kuroiwa, K.; Shibata, T.; Takada, A.; Nemoto, N.; Kimizuka, N. J. Am. Chem. Soc. 2004, 126, 2016–2021. (15) Roubeau, O.; Colin, A.; Schmitt, V.; Clerac, R. Angew. Chem., Int. Ed. 2004, 43, 3283–3286. (16) Shirakawa, M.; Fujita, N.; Tani, T.; Kaneko, K.; Ojima, M.; Fujii, A.; Ozaki, M.; Shinkai, S. Chem.;Eur. J. 2007, 13, 4155–4162. (17) Weng, W.; Beck, J. B.; Jamieson, A. M.; Rowan, S. J. J. Am. Chem. Soc. 2006, 128, 11663–11672. (18) Kuroiwa, K.; Shibata, T.; Takada, A.; Nemoto, N.; Kimizuka, N. J. Am. Chem. Soc. 2004, 126, 2016–2021. (19) Kim, H.-J.; Lee, J.-H.; Lee, M. Angew. Chem., Int. Ed. 2005, 44, 5810–5814. (20) Kishimura, A.; Yamashita, T.; Aida, T. J. Am. Chem. Soc. 2005, 127, 179– 183. (21) Tam, A. Y.-Y.; Wong, K. M.-C.; Yam, V. W.-W. J. Am. Chem. Soc. 2009, 131, 6253–6260. (22) Cardolaccia, T.; Li, Y.; Schanze, K. S. J. Am. Chem. Soc. 2008, 130, 2535– 2545.

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redox responsiveness,10,11 catalytic action,12,13 absorption,10,14,15 emission,16-22 magnetism,15 and electron emission.16,23 It also established that the gelation of coordination polymer make it more processable for direct device applications.23 To date, most of the coordination polymeric gels reported contain long alkyl chains appended to the organic ligands to form cross-linking network.9 Nonetheless, tetrazole ligand, for example, has been reported to form hydrogel with LaCl3 though the ligand is not specifically designed as gelator.24 Recently, we have shown that Zn2þ and Mg2þ can form hydrogels with N-(7-hydroxy-4methyl-8-coumarinyl)amino acids in the absence of long chain hydrophobic groups. The flexibility in the amino acid backbone allows hydrogen-bonding interactions that entrapped water molecules.25,26 Research interest in this area is to further explore the study on coordination polymeric gels by utilizing more rigid ligands without long chain gelators. Rigid ligands have more constraint and defined geometries and therefore lead to more predictable coordination polymeric structures. For instance, the bent shape rigid bridging achiral imidazole ligands and linear coordination geometry of Agþ have been designed to form Agþ helical coordination polymer that leads to gelation.27 Zn2þ coordination polymeric microparticles containing 4,40 -bis(1-imidazolyl)biphenyl also have been recently reported to form gel upon sonication.28 Furthermore, bis(pyridylurea) derivatives have been shown to form metallogels with Agþ and Cu2þ, and their rheological properties can be fine-tuned by changing the anions.29,30 (23) Batabyal, S. K.; Peedikakkal, A. M. P.; Ramakrishna, S.; Sow, C. H.; Vittal, J. J. Macromol. Rapid Commun. 2009, 15, 1356–1361. (24) Andrews, P. C.; Junk, P. C.; Massi, M.; Silberstein, M. Chem. Commun. 2006, 3317–3319. (25) Leong, W. L.; Tam, A. Y.-Y.; Batabyal, S. K.; Koh, L. W.; Kasapis, S.; Yam, V. W.-W.; Vittal, J. J. Chem. Commun. 2008, 3628–3630. (26) Leong, W. L.; Batabyal, S. K.; Kasapis, S.; Vittal, J. J. Chem.;Eur. J. 2008, 14, 8822–8829. (27) Zhang, S.; Yang, S.; Lan, J.; Yang, S.; You, J. Chem. Commun. 2008, 6170– 6172. (28) Zhang, S.; Yang, S.; Lan, J.; Tang, Y.; Xue, Y.; You, J. J. Am. Chem. Soc. 2009, 131, 1689–1691. (29) Applegarth, L.; Clark, N.; Richardson, A. C.; Parker, A. D. M.; Radosavljevic-Evans, I.; Goeta, A. E.; Howard, J. A. K.; Steed, J. W. Chem. Commun. 2005, 5423–5425. (30) Piepenbrock, M.-O. M.; Clarke, N.; Steed, J. W. Langmuir 2009, 25, 8451– 8456.

Published on Web 02/17/2010

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Bipyridyl-based spacer ligands have been widely employed for the construction of a large group of novel networks with various shapes and sizes of pores and channel.31-33 In contrast to extensive studies on the coordination polymer of 4,40 -bipyridine, literature on the relatively rigid and long π-conjugated analogues, 1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene (bpd), is rather scarce. To date, only few coordination polymers are built from this ligand.34-36 However, it is anticipated that bpd can act as an excellent rigid spacer in the construction of coordination polymers. Apart from bpd, we intend to employ dicarboxylic acids as coligand in this context. Dicarboxylic acid such as tartaric acid (H4tar) is expected to bridge the metal centers and form an extensive hydrogen bonding owing to the presence of two hydroxyl groups. It has been shown that L-tar formed binary organogel with alkoxy-substituted stilbazols via multiple hydrogen bonds, and the organogel showed strongly enhanced fluorescence emission in the gel state.37 In this article, we report the gelation of Zn2þ coordination polymer containing bpd and tar. Their gelation, morphology, fluorescence, and rheological properties have been discussed in detail.

Experimental Section All the starting materials were obtained commercially and used as received. The elemental analyses were performed in the microanalytical laboratory, Department of Chemistry, National University of Singapore. IR spectra were recorded using an FTS165 Bio-Rad FTIR spectrophotometer in the range of 4000400 cm-1. ESI mass spectra were recorded on a Finnigan MAT LCQ mass spectrometer using the syringe pump method. Solvent present in the compounds was determined using an SDT 2960 TGA thermal analyzer with a heating rate of 10 °C min-1 from room temperature to 600 °C. X-ray powder diffraction patterns were obtained using a D5005 Bruker X-ray diffractometer equipped with Cu KR radiation. The accelerating voltage and current were 40 kV and 40 mA, respectively. The ligand bpd was synthesized according to the previously reported procedure.34 The UV-vis absorption spectra were obtained from a Shimadzu UV2501-PC. The fluorescence spectra were recorded on a PerkinElmer LS 55 luminescence spectrometer. Scanning electron microscopy images were taken using a Jeol JSM-6700F field emission scanning electron microscope (FESEM) operated at 5 kV and 10 μA. Transmission electron microscopy (TEM) images were obtained from a JEOL JSM-3010 instrument. Rheological measurements were carried out on freshly prepared gels using a controlled stress rheometer (AR-1000N, TA Instruments Ltd., New Castle, DE). Parallel plate geometry of 40 mm diameter and 1.5 mm gap was employed throughout. Following loading, the exposed edges of samples were covered with a silicone fluid from BDH (100 cSt) to prevent water loss. Dynamic oscillatory work kept a frequency of 1 rad s-1.

Synthesis of Organogel [Zn(bpd)(H2tar)] 3 nMeOH (1).

To a solution of Zn(OAc)2 3 2H2O (22 mg, 0.1 mmol, 1 mL MeOH) and bpd (21 mg, 0.1 mmol, 1 mL MeOH), L-tartaric acid (15 mg, 0.1 mmol, 1 mL MeOH) was added. A yellow opaque organogel was obtained after standing at room temperature for about 10 min. The minimum gelator concentration for the formation of organogel 1 calculated based on the initial total (31) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629–1658. (32) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334– 2375. (33) Biradha, K.; Sarkar, M.; Rajput, L. Chem. Commun. 2006, 4169–4179. (34) Ciurtin, D. M.; Dong, Y.-B.; Smith, M. D.; Barclay, T.; zur Loye, H.-C. Inorg. Chem. 2001, 40, 2825–2834. (35) Patra, G. K.; Goldberg, I. Cryst. Growth Des. 2003, 3, 321–329. (36) Mahmoudi, G.; Morsali, A.; Hunter, A. D.; Zeller, M. CrystEngComm 2007, 9, 704–714. (37) Bao, C.; Lu, R.; Jin, M.; Xue, P.; Tan, C.; Liu, G.; Zhao, Y. Org. Biomol. Chem. 2005, 3, 2508–2512.

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Scheme 1. Structural Diagrams of the Ligands Used in This Study

weight of the reactants was 1.9 wt %. The organogel was dried under vacuum, and the resulting powder was washed with excess of water, methanol, and diethyl ether and dried under vacuum for analytical and microscopic analysis. Anal. Calcd for [Zn(bpd)(H2tar)] C16H14N4O6Zn: C = 45.36, H = 3.33, N = 13.22. Found: C = 45.35, H = 3.71, N = 13.56. Selected IR peaks (KBr disk, cm-1): 3424 [ν(OH)], 1627 [νas(CO2)], and 1595 [νs(CO2-)]. ESIMS (m/z (%)): 458.9(24) [Zn(bpd)(H2tar)(H2O)2Hþ]. Synthesis of [Zn(bpd)(suc)] 3 2.5H2O (2). To a solution containing Zn(OAc)2 3 2H2O (22 mg, 0.1 mmol, 1 mL MeOH) and bpd (21 mg, 0.1 mmol, 1 mL MeOH), succinic acid (11.8 mg, 0.1 mmol, 1 mL MeOH) was added. A white precipitate was obtained immediately. The resulting precipitate was washed with excess of methanol and diethyl ether and dried under vacuum. Anal. Calcd for C16H21N4O6.5Zn: C = 43.8, H = 4.82, N = 12.77. Found: C = 42.74, H = 4.08, N = 11.26. Selected IR peaks (KBr disk, cm-1): 3442 [ν(OH)], 1636 [νas(CO2-)], and 1614 [νs(CO2-)]. ESIMS (m/z (%)): 422.7(20) [Zn(bpd)(suc)(CH3OH)Hþ], 489.4 (26) [Zn(bpd)(suc)(CH3OH)3Hþ]. TGA calcd for 2.5 H2O: 10.3%. Found: 9.0%. Synthesis of [Zn(bpd)(ox)] 3 H2O (3). To a solution mixture containing Zn(OAc)2 3 2H2O (22 mg, 0.1 mmol, 1 mL MeOH) and bpd (21 mg, 0.1 mmol, 1 mL MeOH), oxalic acid (15 mg, 0.1 mmol, 1 mL MeOH) was added. A white precipitate was obtained immediately. The resulting precipitate was filtered, washed with excess of methanol and diethyl ether, and dried under vacuum. Anal. Calcd for C14H14N4O5Zn: C = 43.83, H = 3.68, N = 14.60. Found: C = 44.37, H = 3.06, N = 14.71. Selected IR peaks (KBr disk, cm-1): 3443 [ν(OH)], 1678 [νas(CO2-)], and 1610 [νs(CO2-)]. ESIMS (m/z (%)): 437.3 (100) [Zn(bpd)(ox)(H2O)4Hþ], 489.4 (72) [Zn(bpd)(ox)(CH3OH)4Hþ]. TGA calcd for 1H2O: 4.7%. Found: 4.8%.

Results and Discussion Gelation of Coordination Polymer. The ligand bpd was synthesized according to the literature method.34 Three dicarboxylic acids, namely, L-tartaric acid (H4tar), succinic acid (H2suc), and oxalic acid (H2ox) (Scheme 1), have been tested as cross-linking coligands in the formation of coordination polymeric gel. In a typical reaction, a yellow clear methanolic solution mixture of Zn(OAc)2 3 2H2O and bpd in molar ratio 1:1 were prepared to form coordination polymer. Similar complexes of [Zn2(bpd)2(OAc)4] 3 2MeOH and [Mn2(bpd)2(OAc)4] 3 2MeOH have been reported to have 2D coordination polymeric structures in the solid-state with bpd and acetate ligands.38 We postulate that the acetate anions can be replaced by dicarboxylic acid functional group by the addition of H4tar, H2suc, or H2ox. Interestingly, Zn2þ coordination polymer transformed to different assembled structures with different dicarboxylic acids. Addition of H4tar probably generates higher dimensional coordination polymeric structure [Zn(bpd)(H2tar)] 3 nMeOH (1) by randomly cross-linking the linear 1D coordination polymers formed by [Zn(bpd)]n (38) Zhang, G.; Yang, G.; Ma, J. S. Cryst. Growth Des. 2006, 6, 1897–1902.

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Scheme 2. Schematic Representation of the Products Obtained When Different Coligands Are Added to the Methanolic Solution of Zn(OAc)2 3 2H2O and bpd

Figure 1. FTIR spectra of 1-3.

repeating units, which entrapped the solvents to form the organogel. Standard inverted test tube experiment can easily demonstrate the formation of organogel 1 (Figure S1). In order to investigate the role of hydroxyl groups and chain length in dicarboxylic acid, controlled experiments were carried out with H2suc and H2ox. When H2suc or H2ox was added to a methanolic solution containing Zn2þ and bpd in 1:1:1 molar ratio, cloudy floppy dispersions of white precipitates were obtained which are presumed to be the coordination polymers, [Zn(bpd)(suc)] 3 2.5H2O (2) and [Zn(bpd)(ox)] 3 H2O (3). Scheme 2 summarizes the results and microscopic images of the products obtained when different coligands were mixed with the methanolic solution of Zn(OAc)2 3 2H2O and bpd. Structural Aspects. The bpd serves as rigid spacer to form a linear 1D coordination polymer for the metal-to-ligand ratio of 1:1. The dicarboxylic acids are likely to act as coligand to crosslink the 1D coordination polymeric strands. X-ray powder diffraction pattern of dried gel 1 shows mostly amorphous nature while 2 and 3 exhibit better crystallinity (Figure S2). The crystallinity in 2 and 3 may be due to the formation of highly ordered 2D or even 3D coordination polymeric network structures. On the other hand, the absence of long-range order can easily be understood in line with the nature of the xerogel 1. The loss of crystallinity in the gel structure may be understood as follows. We believe that in gel 1 there are no two 1D polymer chains perfectly aligned by tartarate bridging to form ladder or 2D coordination polymeric grid structure. Instead, the tartarate coligands bridge the 1D polymer strands in a highly random fashion. Such random cross-linking by tartarate is likely to yield a 3D entangled network structure without long-range order and thus account for the amorphous nature of 1. Methanol molecules can be firmly trapped within the network by hydrogen-bonding interactions with the hydroxyl groups of tartarate ligands. It is likely that the presence of hydroxyl groups of the tartarate coligands is the key component responsible for the organogel formation.37 Self-assemblies of Zn2þ, bpd, and suc/ox allow the formation of 2D polymeric structures in 2 and 3. It has been reported that a structurally similar complex, [Ni(suc)(4-bpo)(H2O)2] 3 5H2O (bpo = 2,5-bis(4-pyridyl)-1,3,4-oxadiazole), displayed a layered 2D coordination polymeric network with 4-bpo and succinate as spacer ligands. The interlayer hydrogen-bonding interactions has facilitated a 3D network. It has also been found that some lattice water molecules were occupying the channels.39 Since 2 and 3 are very much related to the above structure, they are also expected to have similar network structure. Controlled studies on 2 and 3 reveal that the presence of hydroxyl groups in dicarboxylic acid is essential for gelation while the chain length has no influence. In 2 and 3, hydroxyl groups are absent, and hence they are unable to assist the gel formation;

instead, they are resulted in nanostructured crystalline products. Incorporation of hydroxyl groups in carboxylate backbone can hold the solvent molecules by hydrogen-bonding interactions. The polymeric chains formed by reacting Zn(II) with bpd are cross-linked randomly by hydroxyl groups functionalized tatarate coligands. So far, two main factors for the formation of coordination polymeric gels are based on cross-linking through metal ions17,18 and intermolecular interactions.19,25-27 Here we observed that the gelation is achieved by random cross-linking the 1D coordination polymers with tartarate coligands. Ordered cross-linking will only lead to the formation of highly crystalline 3D porous network structures. So the randomness of cross-linking has prominent role in creating the amorphous network structure to gelate the solvent. Physicochemical Characterization. The analysis of FTIR spectra reveals the coordination and connectivity in 1-3. In anhydrous compound 1, the absorption band at 3424 cm-1 corresponds to the stretching frequency of hydroxyl groups while the broad absorption band ca. 3440 cm-1 in complexes 2 and 3 indicates the presence of lattice water molecules. The difference in asymmetric (νas) and symmetric (νs) stretching frequencies of carboxylate group in complex 1 was found to be 32 cm-1, which suggests chelating mode of the carboxylate group.40 Similar differences are also observed for 2 and 3 with Δν of 22 and 68 cm-1, respectively (Figure 1). This differs from the previously reported Ni2þ complex in which only one of the oxygen atoms from the carboxylate group was involved in the coordination and the octahedral geometry of the metal center was completed by aqua ligands.39 Thermogravimetric analyses of 1 demonstrated the anhydrous nature of 1 as there is no weight loss below 160 °C. Compounds 2 and 3 showed the weight loss in the temperature range of 30-130 °C, suggesting the solvent molecules are in the crystal lattice and not coordinated to metal center (Figure S3). Furthermore, ESI-MS analyses of 1-3 confirm the existence of Zn2þ ternary complexes (Figure S4). Morphological Investigations. The morphology of the dried gel has been investigated by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Figure 2a shows the FESEM image of dried gel 1. The FESEM investigation reveals that the morphology of the gel 1 is sheetlike. These nanosheets are supposed to form by random cross-linking of 1D coordination polymeric chain by tartarate ligand. Polymeric chains form matlike structure and entrapped the solvent within the cross-link network. The TEM image of the dried gel 1 is shown Figure 2b, which clearly supports that the gel is formed by sheetlike structures. The self-assembly of Zn2þ, bpd, and suc/ox led to the formation of 2D polymeric structure in 2 and 3, and this is further confirmed by FESEM studies. Figure 2c

(39) Du, M.; Jiang, X.-J.; Zhao, X.-J.; Cai, H.; Ribas, J. Eur. J. Inorg. Chem. 2006, 2006, 1245–1254.

(40) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1986.

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shows the FESEM image of 2, which exhibits crystalline nanoplate morphology with thickness ∼100 nm and length ∼1 μm, whereas 3 consists of nanorod-like morphology as shown in Figure 2d. The length and diameter of these rods are 5-10 μm and 200-500 nm, respectively. Owing to the absence of hydroxyl groups in the side chain, compounds 2 and 3 are unable to randomly cross-link to generate the network; as a result, highly ordered structures are formed. Spectroscopic Studies. UV-vis absorption studies of the bpd free ligand and 1 show that they exhibit strong absorption around

Figure 2. (a) FESEM image of gel 1, (b) TEM image of gel 1, (c) FESEM image of 2, and (d) FESEM image of 3.

Figure 3. (a) Fluorescence spectra of free bpd (50 times magnified), bpd in the presence of 1 equiv of Zn(II) (50 times magnified), and organogels 1, 2, and 3 upon excitation at 300 nm. (b) Time-dependent fluorescence spectra during the formation of organogel 1 upon excitation at 300 nm. The inset shows the timedependent intensity (at 455 nm).

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300 nm due to the π-π* transition (Figure S5). Despite bpd having a high absorption coefficient, it fluoresces very weakly. There are some reports on the solid state emission properties of free bpd ligand with weak emission around 459, 487, and 530 nm.41 Yang et al. observed emission in [Zn2(bpd)2(OAc)4] 3 2MeOH in the solid state though the bpd ligand was found as nonemissive.38 Herein, 1 exhibits very interesting fluorescence properties when tartaric acid was employed. Figure 3a represents the emission spectra of free bpd, [Zn2(bpd)2(OAc)4], and organogel 1 upon excitation at 300 nm. The organogel emits strongly at 455 nm, which originates from the π-π* transition in the bpd. It is worthwhile to note that the fluorescence intensity of organogel is much higher than the free bpd and [Zn2(bpd)2(OAc)4]. Here, we observe remarkable fluorescence enhancement of upon addition of tartaric acid, i.e., the gel formation, as compared to free bpd and [Zn2(bpd)2(OAc)4] solution. Hence, a “nonemissive/ very weakly emissive” chromophore has generated highly emissive organogel in the absence of an excellent fluorophore. It is noticeable that suspension of 2 and 3 also shows emission in the same region of wavelength but with lesser intensity and broader band. The intense fluorescence property of organogel 1 is attributed to the rigidification of media upon gelation. Such aggregated structures have restricted the motion of the molecules and reduced the nonradiative decay paths of the excited state.42 We also noted such fluorescence enhancement in coordination polymeric gels containing coumarin groups.25,26 The time-dependent fluorescence studies of organogel 1 showed that fluorescence intensity is maximum when organogel 1 is completely formed. Figure 3b shows the time-dependent emission spectra of [Zn2(bpd)2(OAc)4] solution after the addition of tar, and the inset of Figure 3b displays the fluorescence intensity at 455 nm. It is also observed that the clear [Zn2(bpd)2(OAc)4] solution becomes gel completely in 10 min after the addition of tar, and the emission intenesity is also saturated at the same time, suggesting the fluorescent intensity enhancement is due to the formation of gel. Mechanical Properties. Rheological studies are essential to acquire valuable information regarding the structural correlation with viscoelasticity properties. In our previous studies, we have shown that coordination polymeric gels displayed both elastic and viscous behavior.25,26 In principle, the storage (or elastic) modulus G0 represents the solidlike character and energy stored while the loss (or viscous) modulus G00 reflects the liquidlike behavior and energy lost.43 The linear viscoelastic region (LVR) of organogel 1 was determined with increasing amplitude of deformation from 0.01% to 200% at 1 rad s-1, and storage modulus G0 and loss modulus G00 remained flat up to ≈1% (Figure 4a). Beyond this

Figure 4. (a) Dynamic strain sweep of organogel 1. (b) Dynamic frequency sweep of organogel 1 at a strain of 0.1% strain at 25 °C. Langmuir 2010, 26(10), 7464–7468

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level of deformation, drastic drop in both moduli suggested a catastrophic disruption of the networks. Hence, dynamic oscillation frequency sweeps were carried out with 0.1% strain which was within the LVR region. Figure 4b displays the frequency sweep response of the organogel 1. It can be seen that G0 is an order of magnitude higher than G00 , confirming that the gel have predominantly elastic rather than viscous character, and this may be understand from the proposed network structure of the coordination polymeric gel. Furthermore, the organogel exhibits minimal frequency dependence from 0.1 to 100 rad s-1 with the G0 around 4500 and 1800 Pa, respectively.

Conclusion In conclusion, the rigid bipyridyl-based linear spacer ligand bpd has been demonstrated to produce coordination polymeric gel assisted by tar. The rigid spacer ligand forms 1D coordination polymeric chains, and the tar coligand linked these chains randomly to yield a higher dimensional network structure. The tar ligand is not only involved in the formation of network structure but also holds the solvent molecules firmly to form (41) Zhou, X.-H.; Wu, T.; Li, D. Inorg. Chim. Acta 2006, 359, 1442–1448. (42) Li, S.; He, L.; Xiong, F.; Li, Y.; Yang, G. J. Phys. Chem. B 2004, 108, 10887–10892. (43) Malkin, A. Y.; Isayev, A. I. Rheology: Concepts, Methods and Applications; ChemTec Publishing: Toronto, 2006.

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organogel through hydrogen-bonding interactions. This appears to be the first example of gelation that is achieved by the crosslinking of 1D coordination polymer with dicarboxylate coligand. The presence of hydroxyl groups is essential for the gelation process. Such gelation only occurred when three components are involved. SEM and TEM studies on organogel reveal typical sheet-like network. More interestingly, the weakly emissive bpd becomes highly luminescent when all the three components assembled into coordination polymeric gel in which the methanol guests are held together by hydrogen bonding. The rigidity increases in aggregated structures which restrict the nonradiative decay. Such self-assembled structures with induced enhancement of emission properties may have potential application in optoelectronic devices. Furthermore, the coordination polymeric gel exhibits viscoelastic nature and, therefore, has better processability and applicability than conventional coordination polymers. Acknowledgment. We thank the Ministry of Education, Singapore, for funding this project through NUS FRC Grant R-143-000-371-112. Supporting Information Available: Photographs of the solution and organogel, powder XRD patterns, thermogravimetry, ESI-MS, and UV-vis data. This material is available free of charge via the Internet at http://pubs.acs.org.

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