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A series of novel indium complexes derived from aromatic carboxylic acids: Hydrothermal syntheses, crystal structures, supramolecular networks and fluorescent properties
Inorganic Chemistry Communications 55 (2015) 73–76
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A series of novel indium complexes derived from aromatic carboxylic acids: Hydrothermal syntheses, crystal structures, supramolecular networks and fluorescent properties Xin-Ming Wang a, Rui-Qing Fan a,⁎, Liang-Sheng Qiang a, Ping Wang a, Yu-Lin Yang a,⁎, Yu-Lei Wang b a b
Department of Chemistry, Harbin Institute of Technology, Harbin 150001, PR China National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 4 February 2015 Received in revised form 8 March 2015 Accepted 9 March 2015 Available online 12 March 2015 Keywords: Hydrothermal condition Fluorescence Solvent-dependent photoluminescence Thermal stability
a b s t r a c t Five new compounds with mixed-ligand formulated as (H4bptc)(phen) (1), [In(phen)2Cl2](H4bptc)(NO3)(H2O) (2), [In(Hbptc)(phen)(H2O)]2 (3), In(2,6-pydc)(phen)(H2O)Cl (4), and {[In(2,6-pydc)(Ox)0.5(H2O)2](H2O)}2 (5) have been synthesized under hydrothermal conditions. Compounds 1–5 display white, green and blue fluorescence at 298 K in the solid state, respectively. It is shown that 1 assumes solvent-dependent photoluminescence. By contrast, the different polarities of solvents do not alter the luminescence position of 3 and 5. The thermogravimetric curves show that binuclear compounds 3 and 5 have excellent thermal stability, whose structures are stable up to 190 and 272 °C, respectively. © 2015 Published by Elsevier B.V.
Since the first synthesis of a metal–organic complex, transition and lanthanide metal organic complexes have dominated the field of metal– organic materials. After tris(8-hydroxyquinolinolato) aluminium(III) (Alq3) discovered as electro-luminescent material was applied in organic light-emitting diodes (OLEDs) [1], there have been growing interests to design and develop group 13 metal chelates emitting different colors [2]. We considered indium to be a good candidate for constructing complex due to the larger ion radii, more fascinating coordination properties and hypotoxicity among Group 13 elements. However, most challenges from synthesis process are the recognized bottleneck of In(III) salt hydrolysis [3]. As an important multidentate O and N-donor ligand, 2,6-H2pydc (2,6-pyridine dicarboxylic acid), with a rigid 120° angle between the central pyridine ring and the two carboxylate groups, is a versatile ligand, and could provide various coordination modes [4]. H4bptc (3,3 ,4,4 benzophenonetetracarboxylate), a flexible tetracarboxylic ligand is also worth to be introduced based on the following considerations: First, there are multiple bridging carboxylic groups, which may provide a variety of connection modes with metal centers [5]. Second, the relative flexibility around the ketone group can also generate different torsion angles between the two phenyl planes and the carboxylic groups, resulting in formation of different structures. Finally, deprotonation of H4bptc may be affected by pH values, which will again have a significant influence on structure [6].
⁎ Corresponding authors. E-mail addresses: [email protected] (R.-Q. Fan), [email protected] (Y.-L. Yang).
http://dx.doi.org/10.1016/j.inoche.2015.03.016 1387-7003/© 2015 Published by Elsevier B.V.
In this paper, we report five compounds (H4bptc)(phen) (1), [In(phen)2Cl2](H4bptc)(NO3)(H2O) (2), [In(Hbptc)(phen)(H2O)]2 (3), In(2,6-pydc)(phen)(H2O)Cl (4), and {[In(2,6-pydc)(Ox)0.5(H2O)2] (H2O)}2 (5) (phen = phenanthroline, H2Ox = oxalic acid) prepared under hydrothermal synthesis. To the best of our knowledge, complexes 2 and 3 are the first examples of the In(III) complexes based on the H4bptc ligand. Compounds 1–5 are characterized by single-crystal X-ray diffraction (Table S1–S6), 1H NMR and infrared (IR) analysis (Fig. S1– S6). The PXRD pattern (Fig. S7–S11) closely match the simulated patterns generated from the results of single crystal diffraction data. In compounds 1–5, the discrete monomers are held together by hydrogen bonding and π–π interactions to form two-dimensional (2D) or three-dimensional (3D) topological structures (Table S7). The result of single-crystal X-ray diffraction analysis reveals that the asymmetric unit of 1 contains one H4bptc and one phen molecule (Fig. 1a). Two adjacent H4bptc units link with each other through O6\H6\O4 forming the centrosymmetric dimeric [graph set: R22(26)]. With 28-membered hydrogen bonded loop [graph set: R44(28)], the wave chains are stitched together resulting in the wavelike 2D layer network. Meanwhile, there is also the face to face π–π stacking interaction (3.79 Å). If considering the H4bptc and phen as 8-connected and 4connected nodes respectively, and the O\H\O and C\H\O hydrogen bonds as linkers, the resulted supramolecular structure could be simplified into a 2D topology structure (Fig. S12). In the asymmetric unit of 2 (Fig. 1b), In(III) cation is six-coordinated by four nitrogen atoms (N1, N2, N3 and N4) from two phen molecules and two terminal chloridions, showing a distorted [InN4Cl2] octahedral geometry (Fig. S13). The
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Fig. 1. (a) The crystal structure of 1; (b) the coordination environment of the In(III) cation in 2; (c) perspective view of the crystal unit in 3.
H4bptc ligand is fully protonated and does not participate in the coordination with In(III) cation. The 2D and 3D supermolecular structures are linked together via C\H\O resulting in the 2D + 3D → 3D supermolecular structure of 2 (Fig. S14). Compound 3 is a Hbptc3 − -bridged InIII 2 system derived from the partially deprotonated ligand H4 bptc (Fig. 1c). In(III) cation center adopts slightly distorted [InN2O4] octahedral geometry. The dinuclear moieties are connected by O\H\O together with π–π interaction (3.44 Å) leading to a 3D structure (Fig. S15). In compound 4 (Fig. 2), the central In(III) cation is sevencoordinated by three nitrogen atoms (N1, N2 and N3) from one 2,6pydc2 − and two phen ligands, three oxygen atoms (O1, O3 and O5) from 2,6-pydc2 − and water molecule, and one Cl− anion (Fig. S16). The self-assembly molecules in 4 are connected via O5\H5B\O2 synthon to form the 1D wave chain (Fig. S17). The adjacent wave chains are extended by C4\H4A\O4, C10\H10A\Cl1 and π–stacking interaction (3.59 Å) to complete the 2D layer structure. As shown in Fig. 3, In(III) cation is coordinated by six carboxylate O atoms from two water molecules, one Ox2−, one 2,6-pydc2−, and one N atom from 2,6-pydc2− anion in a centrosymmetric binuclear structure of 5, showing [InO6N] pentagonal bipyramid sphere as 4. It is clear that the binuclear formation is further extended to 1D chain and 2D layer structure by O\H\O. Moreover, the π–π stacking interaction (3.55 Å) combining with the above-mentioned O\H\O hydrogen bonds expand the discrete In2(2,6-pydc)2(Ox)(H2O)4 into a 3D hydrogen-bonded supramolecular net. The solid-state photoluminescence (PL) studies were carried out for H4bptc, 2,6-H2pydc, phen, H2Ox ligands and compounds 1–5 at 298 K and 77 K (Fig. S18–S22 and Table S8). On the basis of the emission
peaks, the O-donor H4bptc ligand shows contribution to the fluorescent emissions of 1–3. The emission peaks of 4 and 5 can be ascribed to π* → π transitions of phen and 2,6-H2pydc respectively, because the similar peak appears for the these ligands [7]. For compound 1, it can be observed that a broad emission occurs from higher-energy (HE) emission 420 nm to lower-energy (LE) emission at 566 nm, generating light with Commission Internacionale d'Eclairage (CIE) coordinates of 0.30, 0.33, which is generally considered white (Fig. 4). Compared with 1, 2 exhibits better monochromatic, which generates blue light (λmax = 420, 492sh nm) with CIE coordinates of 0.16, 0.12. Binuclear compound 3 shows emission band at 517 nm, generating green light with CIE coordinates of 0.28, 0.54. Compared with the mononuclear complex 2, its emission peak is largely red-shifted by 97 nm. It is due to the chelating of the Hbptc3− ligand to In(III) cation, which could increase rigidity of the ligands and decrease the HOMO–LUMO energy gap [8]. Compounds 4 and 5 exhibit luminescence in the blue region at 394 nm and 441 nm, respectively. These bands can be assigned to ligand-centered fluorescent emissions of phen and 2,6-H2pydc, respectively [9]. Compared with 4, 5 displays lower emission energy with the maximum emission spectrum red shifted 47 nm. This phenomenon could be attributed to the much larger conjugated system of 5 [10]. At 77 K (Fig. S23), the crystalline solids of 2 undergoes an obvious red shift (ca. 158 nm) chromic process (420, 492sh nm → 578 nm), and gives light with CIE coordinates of 0.44, 0.47, extending down into the yellow region of the visible spectrum (λmax = 578 nm). The emission spectra of 3–5 are much broader and more structured than those at 298 K. On the contrary, the PL spectrum of 1 (λem = 494sh, 531 nm) becomes more narrow than it at 298 K, with CIE coordinates of 0.27, 0.51 in the green region. It is showed that the lifetimes of 1–5 at 77 K increase
Fig. 2. (a) Coordination environment of In(III) cation center in 4; (b) the 2D supramolecular framework generated through O\H\O, C\H\Cl and π-stacking interactions.
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Fig. 3. (a) Coordination environment for In(III) cations in compound 5; (b) the 2D supramolecular framework in 5 generated through C\H\O hydrogen bonding contacts; (c) the 3D supramolecular structure of 5 with π-stacking interaction; (d) view of 3D topology structure generated through noncovalent interactions.
conspicuously to those at 298 K (Fig. S24), since cold conditions would be favorable to reduce the non-radiation decay and collisional quenching [11]. We further examined the emission response sensitivity of 1–5 to the polar solvents as DMSO, CH3CN, CH3OH and CHCl3 at 298 K and 77 K (Table S9). It is observed that 1–5 display blue–violet fluorescence emissions in these polar aprotic or polar protic solvents at 298 K. The solid state emission spectrum emits broader peak profile in comparison with those in solutions. This phenomenon could be attributed to the hydrogen bond interaction and π-stacking of the aromatic rings in these molecules in the solid state [12]. What's certain is that PL of 1 is more susceptible to different polar solvents than 2–5 (polarity: CHCl3 b CH3OH ≈ CH3CN b DMSO). The energy trends of the same band for 1 in solvents are found to follow the order CHCl 3 N CH3 CN N CH3 OH N DMSO (λ em = 306, 324, 365 and 424 nm) (Fig. S25). The observed red-shift (118 nm) phenomenon might be attributed to the presence of a high polarized excited state [13]. Although CH 3OH possesses a similar polarity to CH3CN, the fluorescence spectrum is red shift ca. 41 nm in CH3OH compared with CH3CN, due to the donor ability of CH3OH as polar protic solvent
[14]. For mononuclear compounds 2 and 4, there are some blue shift in emission spectra as solvent polarity increases (λem = 434 → 398 nm for 2, 373 → 352 for 4) (Fig. S26 and Fig. S28). However, the different polarities of solvents slightly alter the PL peak position of 3 and 5 (λem = 425 → 435 nm for 3, 446 → 453 nm for 5) (Fig. S27 and Fig. S29). The quantum yields for 1–5 in DMSO, CH 3CN, CH3OH, and CHCl3 are 0.111–0.085, 0.174–0.086, 0.207–0.106, 0.155–0.089 and 0.193–0.155. The remarkable increase of quantum yield in polar aprotic solvent DMSO is observed. The emissions of 1–5 are red shift obviously at 77 K in these solutions than those at 298 K and the largest shift is 163 nm for 1 in CH3CN. Luminescent lifetimes for 1–5 in solutions at 77 K are mostly longer than that at 298 K (Fig. S30–S34). These features might be ascribed to that cold conditions would be favorable to reduce the non-radiation decay and collisional quenching [11]. Comparing the thermal gravimetric curves of 1–5, 3 and 5 (190 and 272 °C) show better thermal stability than 1, 2 and 4. It indicates that the binuclear structures for 3 and 5 benefit the thermal stability (Fig. S35). Conclusion Five novel compounds, (H4bptc)(phen) (1), [In(phen)2Cl2](H4bptc) (NO3)(H2O) (2), [In(Hbptc)(phen)(H2O)]2 (3), In(2,6-pydc)(phen) (H2O)Cl (4), and {[In(2,6-pydc)(Ox)0.5(H2O)2](H2O)}2 (5) were constructed from H4bptc and 2,6-H2pydc ligand in the presence of auxiliary phen and H2Ox ligand. Structural determinations of 1–5 have demonstrated that the strong hydrogen bonding and π–π interactions determine the formation of the 2D or 3D supermolecular structures. 1–5 display green, white and blue fluorescent at 298 K in the solid state respectively. The emission peak positions of 1 are sensitive to different polar solvents. By comparison, the PL spectra of 3 and 5 do not change largely in different solvents. Binuclear compounds 3 and 5 have excellent thermal stability, whose major structures are stable up to 190 and 272 °C respectively. Acknowledgment
Fig. 4. Solid state emission spectra of compounds 1–5 at 298 K.
This work was supported by the National Natural Science Foundation of China (Grant 21371040 and 21171044), the National Key Basic Research Program of China (973 Program, No. 2013CB632900), the
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Fundamental Research Funds for the Central Universities (Grant No. HIT. IBRSEM. A. 201409), and the Program for Innovation Research of Science in Harbin Institute of Technology (PIRS of HIT No. A201416 and B201414). Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version, at http://dx.doi.org/10.1016/j.inoche.2015.03.016. These data include MOL file and InChiKey of the most important compounds described in this article. References [1] C.W. Tang, S.A. VanSlyke, Organic electroluminescent diodes, Appl. Phys. Lett. 51 (1987) 913–920. [2] W.J. Ji, Q.G. Zhai, S.N. Li, Y.C. Jiang, M.C. Hu, An anionic metal–organic framework based on infinite [In3(μ3-OH)2]n inorganic chains synthesized in ionic liquid, Inorg. Chem. Commun. 28 (2013) 16–19. [3] N. Stock, S. Biswas, Synthesis of metal–organic frameworks (MOFs): routes to various MOF topologies, morphologies, and composites, Chem. Rev. 112 (2012) 933–969. [4] E.G. Moore, J. Grilj, E. Vautheyd, P. Ceronib, A comparison of sensitized Ln(III) emission using pyridine- and pyrazine-2,6-dicarboxylates—part, Dalton Trans. 42 (2013) 2075–2083. [5] J. Zhang, Z.J. Li, Y. Kang, J.K. Cheng, Y.G. Yao, Hydrothermal syntheses, crystal structures, and properties of a novel class of 3,3 ,4,4 -benzophenone-tetracarboxylate (BPTC) polymers, Inorg. Chem. 43 (2004) 8085–8089. [6] J.Q. Liu, Y.N. Zhang, Y.Y. Wang, J.C. Jin, E.K. Lermontovaa, Q.Z. Shi, Interplay of coordinative and supramolecular interactions in formation of a series of metal–organic complexes bearing diverse dimensionalities, Dalton Trans. (2009) 5365–5378.
[7] Z.Y. Zhang, Z.P. Deng, L.H. Huo, H. Zhao, S. Gao, Well-designed strategy to construct helical silver(I) coordination polymers from flexible unsymmetrical bis(pyridyl) ligands: syntheses, structures, and properties, Inorg. Chem. 52 (2013) 5914–5923. [8] J.Y. Gao, N. Wang, X.H. Xiong, C.J. Chen, W.P. Xie, X.R. Ran, Y. Long, S.T. Yue, Y.L. Liu, Syntheses, structures, and photoluminescent properties of a series of zinc(II)-3amino-1,2,4-triazolate coordination polymers constructed by varying carboxylate anions, CrystEngComm 15 (2013) 3261–3270. [9] P. Cucos, F. Tuna, L. Sorace, I. Matei, C. Maxim, S. Shova, R. Gheorghe, A. Caneschi, M. Hillebrand, M. Andruh, Magnetic and luminescent binuclear double-stranded helicates, Inorg. Chem. 53 (2014) 7738–7747. [10] (a) P. Ivanov, S. Stanimirov, S. Kaloyanova, I. Petkov, Synthesis and photophysical properties of yellow-emitting Iridium complexes, effect of the temperature on the character of triplet emission, J. Fluoresc. 22 (2012) 1501–1507; (b) H.B. Zhang, P. Lin, X.C. Shan, F.L. Du, Q.P. Li, S.W. Du, An inorganic–organic composite framework with an unprecedented 3D heterometallic inorganic connectivity and white-light emission, Chem. Commun. 49 (2013) 2231–2233. [11] R.R. Hu, C.F.A. Gómez-Durán, J.W.Y. Lam, J.L. Belmonte-Vázquez, C.M. Deng, S.J. Chen, R.Q. Ye, E. Peña-Cabrera, Y.C. Zhong, K.S. Wong, B.Z. Tang, Synthesis, solvatochromism, aggregation-induced emission and cell imaging of tetraphenylethene-containing BODIPY derivatives with large Stokes shifts, Chem. Commun. 48 (2012) 10099–10101. [12] X.M. Liu, H. Xia, W. Gao, Q.L. Wu, X. Fan, Y. Mu, C.S. Ma, New rhenium(I) complexes with substituted diimine ligands for highly efficient phosphorescent devices fabricated by a solution process, J. Mater. Chem. 22 (2012) 3485–3492. [13] K. Senthil Kumar, A. Patnaik, Solvent-polarity-tunable dimeric association of a fullerene (C60)-N, N-dimethylaminoazobenzene dyad: modulated electronic coupling of the azo chromophore with a substituted 3D fullerene, Chem. Eur. J. 17 (2011) 5327–5343. [14] (a) P.V. Komarov, V.G. Plotnikov, The effect of protic solvents on luminescent properties of dye molecules, J. Lumin. 132 (2012) 1139–1143; (b) J.A. Mondal, V. Samant, M. Varne, A.K. Singh, T.K. Ghanty, H.N. Ghosh, D.K. Palit, The role of hydrogen-bonding interactions in the ultrafast relaxation dynamics of the excited states of 3- and 4-aminofluoren-9-ones, ChemPhysChem 10 (2009) 2995–3012.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
A series of novel indium complexes derived from aromatic carboxylic acids: Hydrothermal syntheses, crystal structures, supramolecular networks and fluorescent properties Xin-Ming Wang a, Rui-Qing Fan a,⁎, Liang-Sheng Qiang a, Ping Wang a, Yu-Lin Yang a,⁎, Yu-Lei Wang b a b
Department of Chemistry, Harbin Institute of Technology, Harbin 150001, PR China National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, PR China
a r t i c l e
i n f o
Article history: Received 4 February 2015 Received in revised form 8 March 2015 Accepted 9 March 2015 Available online 12 March 2015 Keywords: Hydrothermal condition Fluorescence Solvent-dependent photoluminescence Thermal stability
a b s t r a c t Five new compounds with mixed-ligand formulated as (H4bptc)(phen) (1), [In(phen)2Cl2](H4bptc)(NO3)(H2O) (2), [In(Hbptc)(phen)(H2O)]2 (3), In(2,6-pydc)(phen)(H2O)Cl (4), and {[In(2,6-pydc)(Ox)0.5(H2O)2](H2O)}2 (5) have been synthesized under hydrothermal conditions. Compounds 1–5 display white, green and blue fluorescence at 298 K in the solid state, respectively. It is shown that 1 assumes solvent-dependent photoluminescence. By contrast, the different polarities of solvents do not alter the luminescence position of 3 and 5. The thermogravimetric curves show that binuclear compounds 3 and 5 have excellent thermal stability, whose structures are stable up to 190 and 272 °C, respectively. © 2015 Published by Elsevier B.V.
Since the first synthesis of a metal–organic complex, transition and lanthanide metal organic complexes have dominated the field of metal– organic materials. After tris(8-hydroxyquinolinolato) aluminium(III) (Alq3) discovered as electro-luminescent material was applied in organic light-emitting diodes (OLEDs) [1], there have been growing interests to design and develop group 13 metal chelates emitting different colors [2]. We considered indium to be a good candidate for constructing complex due to the larger ion radii, more fascinating coordination properties and hypotoxicity among Group 13 elements. However, most challenges from synthesis process are the recognized bottleneck of In(III) salt hydrolysis [3]. As an important multidentate O and N-donor ligand, 2,6-H2pydc (2,6-pyridine dicarboxylic acid), with a rigid 120° angle between the central pyridine ring and the two carboxylate groups, is a versatile ligand, and could provide various coordination modes [4]. H4bptc (3,3 ,4,4 benzophenonetetracarboxylate), a flexible tetracarboxylic ligand is also worth to be introduced based on the following considerations: First, there are multiple bridging carboxylic groups, which may provide a variety of connection modes with metal centers [5]. Second, the relative flexibility around the ketone group can also generate different torsion angles between the two phenyl planes and the carboxylic groups, resulting in formation of different structures. Finally, deprotonation of H4bptc may be affected by pH values, which will again have a significant influence on structure [6].
⁎ Corresponding authors. E-mail addresses: [email protected] (R.-Q. Fan), [email protected] (Y.-L. Yang).
http://dx.doi.org/10.1016/j.inoche.2015.03.016 1387-7003/© 2015 Published by Elsevier B.V.
In this paper, we report five compounds (H4bptc)(phen) (1), [In(phen)2Cl2](H4bptc)(NO3)(H2O) (2), [In(Hbptc)(phen)(H2O)]2 (3), In(2,6-pydc)(phen)(H2O)Cl (4), and {[In(2,6-pydc)(Ox)0.5(H2O)2] (H2O)}2 (5) (phen = phenanthroline, H2Ox = oxalic acid) prepared under hydrothermal synthesis. To the best of our knowledge, complexes 2 and 3 are the first examples of the In(III) complexes based on the H4bptc ligand. Compounds 1–5 are characterized by single-crystal X-ray diffraction (Table S1–S6), 1H NMR and infrared (IR) analysis (Fig. S1– S6). The PXRD pattern (Fig. S7–S11) closely match the simulated patterns generated from the results of single crystal diffraction data. In compounds 1–5, the discrete monomers are held together by hydrogen bonding and π–π interactions to form two-dimensional (2D) or three-dimensional (3D) topological structures (Table S7). The result of single-crystal X-ray diffraction analysis reveals that the asymmetric unit of 1 contains one H4bptc and one phen molecule (Fig. 1a). Two adjacent H4bptc units link with each other through O6\H6\O4 forming the centrosymmetric dimeric [graph set: R22(26)]. With 28-membered hydrogen bonded loop [graph set: R44(28)], the wave chains are stitched together resulting in the wavelike 2D layer network. Meanwhile, there is also the face to face π–π stacking interaction (3.79 Å). If considering the H4bptc and phen as 8-connected and 4connected nodes respectively, and the O\H\O and C\H\O hydrogen bonds as linkers, the resulted supramolecular structure could be simplified into a 2D topology structure (Fig. S12). In the asymmetric unit of 2 (Fig. 1b), In(III) cation is six-coordinated by four nitrogen atoms (N1, N2, N3 and N4) from two phen molecules and two terminal chloridions, showing a distorted [InN4Cl2] octahedral geometry (Fig. S13). The
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Fig. 1. (a) The crystal structure of 1; (b) the coordination environment of the In(III) cation in 2; (c) perspective view of the crystal unit in 3.
H4bptc ligand is fully protonated and does not participate in the coordination with In(III) cation. The 2D and 3D supermolecular structures are linked together via C\H\O resulting in the 2D + 3D → 3D supermolecular structure of 2 (Fig. S14). Compound 3 is a Hbptc3 − -bridged InIII 2 system derived from the partially deprotonated ligand H4 bptc (Fig. 1c). In(III) cation center adopts slightly distorted [InN2O4] octahedral geometry. The dinuclear moieties are connected by O\H\O together with π–π interaction (3.44 Å) leading to a 3D structure (Fig. S15). In compound 4 (Fig. 2), the central In(III) cation is sevencoordinated by three nitrogen atoms (N1, N2 and N3) from one 2,6pydc2 − and two phen ligands, three oxygen atoms (O1, O3 and O5) from 2,6-pydc2 − and water molecule, and one Cl− anion (Fig. S16). The self-assembly molecules in 4 are connected via O5\H5B\O2 synthon to form the 1D wave chain (Fig. S17). The adjacent wave chains are extended by C4\H4A\O4, C10\H10A\Cl1 and π–stacking interaction (3.59 Å) to complete the 2D layer structure. As shown in Fig. 3, In(III) cation is coordinated by six carboxylate O atoms from two water molecules, one Ox2−, one 2,6-pydc2−, and one N atom from 2,6-pydc2− anion in a centrosymmetric binuclear structure of 5, showing [InO6N] pentagonal bipyramid sphere as 4. It is clear that the binuclear formation is further extended to 1D chain and 2D layer structure by O\H\O. Moreover, the π–π stacking interaction (3.55 Å) combining with the above-mentioned O\H\O hydrogen bonds expand the discrete In2(2,6-pydc)2(Ox)(H2O)4 into a 3D hydrogen-bonded supramolecular net. The solid-state photoluminescence (PL) studies were carried out for H4bptc, 2,6-H2pydc, phen, H2Ox ligands and compounds 1–5 at 298 K and 77 K (Fig. S18–S22 and Table S8). On the basis of the emission
peaks, the O-donor H4bptc ligand shows contribution to the fluorescent emissions of 1–3. The emission peaks of 4 and 5 can be ascribed to π* → π transitions of phen and 2,6-H2pydc respectively, because the similar peak appears for the these ligands [7]. For compound 1, it can be observed that a broad emission occurs from higher-energy (HE) emission 420 nm to lower-energy (LE) emission at 566 nm, generating light with Commission Internacionale d'Eclairage (CIE) coordinates of 0.30, 0.33, which is generally considered white (Fig. 4). Compared with 1, 2 exhibits better monochromatic, which generates blue light (λmax = 420, 492sh nm) with CIE coordinates of 0.16, 0.12. Binuclear compound 3 shows emission band at 517 nm, generating green light with CIE coordinates of 0.28, 0.54. Compared with the mononuclear complex 2, its emission peak is largely red-shifted by 97 nm. It is due to the chelating of the Hbptc3− ligand to In(III) cation, which could increase rigidity of the ligands and decrease the HOMO–LUMO energy gap [8]. Compounds 4 and 5 exhibit luminescence in the blue region at 394 nm and 441 nm, respectively. These bands can be assigned to ligand-centered fluorescent emissions of phen and 2,6-H2pydc, respectively [9]. Compared with 4, 5 displays lower emission energy with the maximum emission spectrum red shifted 47 nm. This phenomenon could be attributed to the much larger conjugated system of 5 [10]. At 77 K (Fig. S23), the crystalline solids of 2 undergoes an obvious red shift (ca. 158 nm) chromic process (420, 492sh nm → 578 nm), and gives light with CIE coordinates of 0.44, 0.47, extending down into the yellow region of the visible spectrum (λmax = 578 nm). The emission spectra of 3–5 are much broader and more structured than those at 298 K. On the contrary, the PL spectrum of 1 (λem = 494sh, 531 nm) becomes more narrow than it at 298 K, with CIE coordinates of 0.27, 0.51 in the green region. It is showed that the lifetimes of 1–5 at 77 K increase
Fig. 2. (a) Coordination environment of In(III) cation center in 4; (b) the 2D supramolecular framework generated through O\H\O, C\H\Cl and π-stacking interactions.
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Fig. 3. (a) Coordination environment for In(III) cations in compound 5; (b) the 2D supramolecular framework in 5 generated through C\H\O hydrogen bonding contacts; (c) the 3D supramolecular structure of 5 with π-stacking interaction; (d) view of 3D topology structure generated through noncovalent interactions.
conspicuously to those at 298 K (Fig. S24), since cold conditions would be favorable to reduce the non-radiation decay and collisional quenching [11]. We further examined the emission response sensitivity of 1–5 to the polar solvents as DMSO, CH3CN, CH3OH and CHCl3 at 298 K and 77 K (Table S9). It is observed that 1–5 display blue–violet fluorescence emissions in these polar aprotic or polar protic solvents at 298 K. The solid state emission spectrum emits broader peak profile in comparison with those in solutions. This phenomenon could be attributed to the hydrogen bond interaction and π-stacking of the aromatic rings in these molecules in the solid state [12]. What's certain is that PL of 1 is more susceptible to different polar solvents than 2–5 (polarity: CHCl3 b CH3OH ≈ CH3CN b DMSO). The energy trends of the same band for 1 in solvents are found to follow the order CHCl 3 N CH3 CN N CH3 OH N DMSO (λ em = 306, 324, 365 and 424 nm) (Fig. S25). The observed red-shift (118 nm) phenomenon might be attributed to the presence of a high polarized excited state [13]. Although CH 3OH possesses a similar polarity to CH3CN, the fluorescence spectrum is red shift ca. 41 nm in CH3OH compared with CH3CN, due to the donor ability of CH3OH as polar protic solvent
[14]. For mononuclear compounds 2 and 4, there are some blue shift in emission spectra as solvent polarity increases (λem = 434 → 398 nm for 2, 373 → 352 for 4) (Fig. S26 and Fig. S28). However, the different polarities of solvents slightly alter the PL peak position of 3 and 5 (λem = 425 → 435 nm for 3, 446 → 453 nm for 5) (Fig. S27 and Fig. S29). The quantum yields for 1–5 in DMSO, CH 3CN, CH3OH, and CHCl3 are 0.111–0.085, 0.174–0.086, 0.207–0.106, 0.155–0.089 and 0.193–0.155. The remarkable increase of quantum yield in polar aprotic solvent DMSO is observed. The emissions of 1–5 are red shift obviously at 77 K in these solutions than those at 298 K and the largest shift is 163 nm for 1 in CH3CN. Luminescent lifetimes for 1–5 in solutions at 77 K are mostly longer than that at 298 K (Fig. S30–S34). These features might be ascribed to that cold conditions would be favorable to reduce the non-radiation decay and collisional quenching [11]. Comparing the thermal gravimetric curves of 1–5, 3 and 5 (190 and 272 °C) show better thermal stability than 1, 2 and 4. It indicates that the binuclear structures for 3 and 5 benefit the thermal stability (Fig. S35). Conclusion Five novel compounds, (H4bptc)(phen) (1), [In(phen)2Cl2](H4bptc) (NO3)(H2O) (2), [In(Hbptc)(phen)(H2O)]2 (3), In(2,6-pydc)(phen) (H2O)Cl (4), and {[In(2,6-pydc)(Ox)0.5(H2O)2](H2O)}2 (5) were constructed from H4bptc and 2,6-H2pydc ligand in the presence of auxiliary phen and H2Ox ligand. Structural determinations of 1–5 have demonstrated that the strong hydrogen bonding and π–π interactions determine the formation of the 2D or 3D supermolecular structures. 1–5 display green, white and blue fluorescent at 298 K in the solid state respectively. The emission peak positions of 1 are sensitive to different polar solvents. By comparison, the PL spectra of 3 and 5 do not change largely in different solvents. Binuclear compounds 3 and 5 have excellent thermal stability, whose major structures are stable up to 190 and 272 °C respectively. Acknowledgment
Fig. 4. Solid state emission spectra of compounds 1–5 at 298 K.
This work was supported by the National Natural Science Foundation of China (Grant 21371040 and 21171044), the National Key Basic Research Program of China (973 Program, No. 2013CB632900), the
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