FULL PAPER DOI: 10.1002/asia.201402153
Self-Assembly of Five 8-Hydroxyquinolinate-Based Complexes: Tunable Core, Supramolecular Structure, and Photoluminescence Properties Guozan Yuan,*[a, b] Weilong Shan,[a] Xuelong Qiao,[a] Li Ma,[a] and Yanping Huo*[c] Abstract: Five new ZnII complexes, namely [Zn3(L)6] (1), [Zn2(Cl)2(L)2(py)2] (2), [Zn2(Br)2(L)2(py)2] (3), [Zn(L)2(py)] (4), and [Zn2ACHTUNGRE(OAc)2(L)2(py)2] (5), were prepared by the solvothermal reaction of ZnX2 (X = Cl, Br, F, and OAc) salts with a 8-hydroxyquinolinate ligand (HL) that contained a trifluorophenyl group. All of the complexes were characterized by elemental analysis, IR spectroscopy, and powder and
single-crystal X-ray crystallography. The building blocks exhibited unprecedented structural diversification and their self-assembly afforded one mononuclear, three binuclear, and one trinuclear ZnII structures in response to difKeywords: 8-hydroxyquinolinates · photoluminescence · self-assembly · supramolecular chemistry · zinc
Introduction
obtained with yellow-light-emitting Znq2 as an electrontransport material, thus suggesting enhanced electron injection and transport in Znq2. These results can be explained by the formation of the energetically favored tetramer (Znq2)4 in vacuum-deposited thin films and by the stronger p···p overlap between the ligands and the extended electronic states in tetrameric (Znq2)4.[3a] In general, the photophysical properties of 8-quinolinolate-based complexes in the solid state are dependent on the character of the metal ion, the degree of aggregation, and the molecular and supramolecular structures, as determined by the intermolecular non-covalent interactions involved.[4] Nevertheless, Mqn-type mononuclear complexes will limit and even hamper the synthesis of new 8-quinolinolate-based materials with controllable structures and luminescence properties. Hence, the development of new synthetic methods and strategies for the fabrication of well-defined heteroleptic molecular complexes or multinuclear clusters that are supported by organic 8-hydroxyquinoline derivatives, with unique self-assembled structures and properties, and, hence, improved and tunable fluorescence features, remains a big challenge for chemists.[5] Over the past two decades, it has been demonstrated that the self-assembly of complexes from polytopic organic linkers[6–9] could be affected by many structure-directing factors, such as the metal, the coordinating ability of the anion, the pH value, and the reaction temperature.[10–14] Of these factors, the anion plays a crucial role in influencing the formation of the coordination compound and leads to their structural and dimensional variation, supramolecular isomerism, and new topologies.[10] With a few notable exceptions, the systematic investigation of the effect of the anion on 8-hydroxyquinolinate-based complexes remains unexplored.[15a] In recent years, we have reported the synthesis and characterization of a series of transition-metal complexes that in-
Organometalliccomplexes have frequently been used as emissive and electron-transport materials or host materials for fluorescent dyes in organic light-emitting diodes (OLEDs) and as buffer layers in organic solar cells (OSCs).[1] In particular, tris(8-quinolinolate)aluminumACHTUNGRE(III) (Alq3) is one of the most widely studied complexes for OLEDs and OSCs, owing to its superior properties, such as high electron affinity (about 3.0 eV), high ionization potential (about 5.95 eV), low-lying HOMO (about 5.7 eV), good thermal stability, and ease of thin-film fabrication through a vacuum-evaporation technique.[1b, 2] Inspired by the success of Alq3, 8-quinolinolate chelates of other metals, such as Be2+ and Zn2+, have also been used in OLEDs.[3] Compared to Alq3 devices, lower operating voltages were
[a] Prof. Dr. G. Yuan, W. Shan, X. Qiao, L. Ma School of Chemistry and Chemical Engineering Anhui University of Technology Maanshan 243002 (P. R. China) Fax: (+ 86) 555-2311552 E-mail: [email protected] [b] Prof. Dr. G. Yuan State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Institution Chinese Academy of Sciences Fuzhou, Fujian 350002 (P. R. China) [c] Prof. Dr. Y. Huo School of Chemical Engineering and Light Industry Guangdong University of Technology Guangzhou 510006 (P. R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402153.
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ferent anions and solvent systems. Complexes 1–5 featured four types of supramolecular network controlled by non-covalent interactions, such as p···pstacking, CH···p, hydrogen-bonding, and halogen-related interactions. Investigation of their photoluminescence properties exhibited disparate emission wavelengths, lifetimes, and quantum yields in the solid state.
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volve 2-substituted 8-quinolinolate ligands with a NO donor set.[15] In this context, one 8-quinolinolate-based ligand (HL), which contained a trifluorophenyl group, was chosen for fabricating ZnII complexes in different solvent systems. The introduction of a trifluorophenyl group not only modified the energies of the excited states of the corresponding complexes, but also facilitated the formation of a diverse range of halogen bonds (such as CF···HC, CF···p, and CF···FC interactions). These halogen-related interactions may affect the supramolecular structures of ZnII complexes by extending the low-dimensional entities into higher-dimensional supramolecular networks, thereby generating fascinating properties in the solid state.[16] By using a solvothermal method, five complexes (1–5) were constructed by the self-assembly of ZnX2 (X = Cl, Br, F, and OAc) with the HL ligand (Scheme 1). Structural diversification with
Guozan Yuan, Yanping Huo et al.
within the temperature range 50–100 8C. Single-crystal X-ray diffraction analysis revealed that the binuclear building block of compound 2 consisted of two coordinated chloride anions. Thus, three other ZnII salts were used in DMF/ MeOH/water/pyridine systems to investigate the influence of the counterion on the formation and structures of the complexes. Three other mono- or binuclear complexes, [Zn2(Br)2(L)2(py)2] (3), [Zn(L)2(py)] (4), and [Zn2 ACHTUNGRE(OAc)2(L)2(py)2] (5), were obtained under solvothermal conditions. The structural assignment of complexes 1–5 was supported by elemental analysis, IR spectroscopy, and single-crystal X-ray diffraction. Structural Characterization Complex 1 crystallized in the monoclinic space group P21/c, with Z = 4. The asymmetric unit contained one formula unit, that is, three crystallographically independent ZnII cations and six L ligands. As shown in Figure 1, three pairs of li-
Scheme 1. Synthesis of complexes 1–5 from ligand HL.
regard to the building units was observed in this series of ZnII complexes. One mononuclear, three binuclear, and one trinuclear ZnII core structures were fabricated in response to different anions and solvent systems. In addition, their fluorescent properties showed disparate emission wavelengths, lifetimes, and quantum yields in the solid state. This unique capability may provide a useful strategy for tuning the optical properties of 8-hydroxyquinoline materials, which could be exploited as important components of optoelectronic devices.
Figure 1. View of the coordination geometries of the ZnII atoms in compound 1; H atoms are omitted for clarity.
gands with offset face-to-face p–p-stacking interactions are arranged around the trimeric ZnII core in a propeller-like form. The zinc atoms, which are located at either end of the trimeric unit, are five-coordinated by three phenolate oxygen atoms and two nitrogen atoms from three L ligands, thereby displaying a distorted-trigonal-bipyramidal geometry. The coordination environment around the central ZnII atom is a distorted octahedron and the equatorial plane is occupied by the NO3 donor atoms of three ligands, whilst the apical position by one phenolate oxygen atom and a pyridine nitrogen atom. For compound 1, the bond lengths [] and angles [8] around the ZnII atom are 2.114(4)–2.232(3) (ZnN), 1.960(3)–2.203(3) (ZnO); 75.24(10)–170.99(11) (O-Zn-O), 78.49(12)–154.19(11) (O-Zn-N), 93.66(12)– 112.22(14) (N-Zn-N). The three ZnII ions in compound 1 are bridged by the phenolato oxygen atoms of four ligands, with Zn···Zn distances of 3.2584(4) and 3.2865(4) . There are numerous CH···O intramolecular hydrogenbonding interactions in compound 1 between the phenolato oxygen atoms and the CH groups of the quinoline ring or
Results and Discussion Synthetic Considerations Trinuclear complex [Zn3(L)6] (1) was prepared by a solvothermal reaction in a mixture of DMF, MeOH, and water. The reaction was originally performed with a 1:2 molar ratio of ZnII and HL, but the products were not significantly affected by a change in the molar ratio. To fabricate other multinuclear ZnII complexes based on the L ligand, other ZnII salts of different anions (such as Br, F, and OAc) were used instead of ZnCl2 ; however, crystalline compound 1 was obtained in each case. Pleasingly, when an appropriate amount of pyridine was added to the reaction solution, binuclear ZnII complex [Zn2(Cl)2(L)2(py)2] (2) was synthesized
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the ethenyl group (C···O 3.079(6)–3.317(5) , CH···O 128.0–155.08), which play a significant role in the construction of trimeric ZnII units (see the Supporting Information, Figure S1). The trimeric Zn3L6 units are linked into a 2D supramolecular structure in the bc plane through C50 H50···p (between CH dichlorophenyl groups and adjacent pyridyl rings, 3.476(8) ) and CH···F intermolecular interactions that involve quinoline CH groups and the fluorine atoms of adjacent 2,4,6-trifluorophenyl units (C···F 3.307(6), 3.332(7) ; CH···F 156, 1698; Figure 2 and the Supporting
Figure 3. a) 2D supramolecular structure of compound 1, mediated by C89H89···F16 (dashed red lines) and p···p stacking interactions (dashed yellow lines) in the ac plane. b) 1D intertwined supramolecular helical polymeric chains in compound 1 along the b axis.
Figure 2. 2D supramolecular structure of compound 1, mediated by C50 H50···p (dashed yellow lines) and CH···F intermolecular interactions (dashed pink lines).
Information, Figure S2). Owing to abundant p···p-stacking and intermolecular C89H89···F16 interactions (C···F 3.171(5) ; Figure 3 a), the supramolecular structure extends into a 3D network, as shown in the Supporting Information, Figure S3. In particular, the trimeric ZnII units in compound 1 are linked into a helical chain through C50H50···p and C89H89···F16 intermolecular interactions along the b axis. Thus, two intertwined helical chains (one is right-handed, the other is left-handed) are generated around the crystallographic 2(1) axis, as shown in Figure 3 b. The two helices have an identical pitch of 27.479 (5) (equal to the length of the b axis). On treatment of 8-hydroxyquinoline ligand HL with ZnCl2 in a mixture of DMF/MeOH/water/pyridine, one binuclear complex, [Zn2(Cl)2(L)2(py)2] (2), was obtained. Single-crystal structural analysis revealed that compound 2 crystallized in the monoclinic space group P21/n, with one half of a formula unit in the asymmetric unit, that is, one ZnII atom, one ligand L, one coordinated chlorine atom, and one coordinated pyridine moiety. The structure of compound 2 is built around a binuclear ZnII structure (Figure 4). The two zinc atoms in the building unit are pentacoordinate and adopt a trigonal-bipyramidal geometry, with the equatorial plane occupied by the NO2 donors of two L ligands and the apical position by one chlorine atom and one nitrogen atom of a pyridine molecule. However, complex 2 appears
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Figure 4. Coordination structure of compound 2.
to be neutral, because the hydroxy groups of the coordinated L ligands are fully deprotonated. Interestingly, in the solid-state structure of this complex, a type of meso-helical chain (P+M) along the b axis is constructed through CH···p interactions between the pyridine CH groups and the phenolate ring of the neighboring unit. As shown in Figure 5 a, the two 21 helices have an identical pitch of 10.7957(8) (equal to the length of the b axis). Furthermore, CH···F intramolecular hydrogen bonds between the fluorine atom and the ethenyl CH group (see the Supporting Information, Figure S4; C···F 2.897, 2.739 ; C H···F 124.0, 1048), as well as intermolecular CH···F hydrogen bonds between the pyridine CH groups and the fluorine atoms of adjacent binuclear units (C···F 3.382(3) ; C H···F 1508), also play a vital role in the consolidation of the solid-state structure (Figure 5 b). Owing to the cooperative action of these intermolecular non-covalent interactions (C H···F and CH···p) and p···p-stacking interactions (between the phenolate and trifluorophenyl rings of adjacent binuclear units, 3.781(3) ), the structure extends into a 3D network (Figure 6). The structure of complex 3 is identical to
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diffraction analysis revealed that compound 4 crystallized in a triclinic system with the P1 space group. As shown in Figure 7 a, the asymmetric unit of compound 4 is composed of one Zn center, two L ligands, and one coordinated pyridine molecule. The Zn center is five-coordinated by the O1, N1,
Figure 5. a) View of 1D intertwined supramolecular helical chains in compound 2 along the b axis. b) 2D supramolecular structure of compound 2, mediated CH···F intermolecular interactions.
Figure 7. a) Coordination structure of compound 4. b) Three types of intermolecular interaction in compound 4.
O2, and N2 atoms from two distinct L ligands and by the N3 atom from a coordinated pyridine molecule to form a trigonal-bipyramidal geometry. The ZnN distances are 2.0616(19), 2.1936(18), and 2.2583(19) and the ZnO distances are 1.9468(18) and 1.9578(17) . For the L ligand, the dihedral angles between the trifluorophenyl and quinoline rings are 9.58 and 13.38, respectively. Intermolecular interactions play a significant role in the crystal packing. As shown in Figure 7 b, CH···F, CH···p, and p···p intermolecular interactions link the neutral mononuclear ZnII units in compound 4 into 2D layers in the ac plane (Figure 8 a). Each trifluorophenyl ring lies parallel to the ring on the reverse side of the neighboring unit in the bc plane (Figure 8 b) and p···p interactions exist between them, with a face-to-face distance of 3.573(3) . The 2D layers are further assembled into a 3D supramolecular network (see the Supporting Information, Figure S9) through p···p interactions. Binuclear complex 5 was readily obtained in good yield by heating ZnACHTUNGRE(OAc)2 and ligand HL in a DMF/MeOH/
Figure 6. The 2D supramolecular structure of compound 2 extends into a 3D network, mediated by CH···p (dashed pink lines) and p···p interactions (dashed red lines).
that of complex 2. The main distinction is that the coordinated chloride ions were replaced with bromide anions, so there were small differences between them. To further investigate the influence of the counterion on the formation and structures of the complexes, mononuclear complex [Zn(L)2(py)] (4) was obtained when the 8-hydroxyquinoline ligand HL was reacted with ZnF2 in the DMF/ MeOH/water/pyridine reaction system. Single-crystal X-ray
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Figure 9. Coordination structure of compound 5.
Figure 8. a) 2D supramolecular structure of compound 4 in the ac plane, mediated by three types of intermolecular interaction. b) 2D supramolecular structure of compound 4 in the bc plane, mediated by p···p-stacking interactions.
water/pyridine mixture. Complex 5 crystallized in a triclinic system and the P 1 (no. 2) space group. The binuclear building block contained two ZnII atoms, two L ligands, two coordinated pyridine molecules, and two coordinated acetate anions. Both of the ZnII centers adopted a distorted octahedral geometry; the equatorial plane was occupied by the NO3 donor atoms of one L ligand, one pyridine molecule, and an acetate anion and the apical position was occupied by quinoline nitrogen and oxygen atoms (Figure 9). The two ZnII ions are bridged by phenolate oxygen atoms of two ligands, with Zn···Zn distances of 3.2494(4) . The binuclear ZnII units are linked into a supramolecular chain along the b axis through intramolecular CH···O hydrogen-bonding interactions between the pyridine CH group and the acetate oxygen atom (Figure 10 c). Furthermore, p···p interactions between the phenolate and trifluorophenyl rings of adjacent chains (Figure 10 a) expand the parallel 1D chains into a 2D network in the bc plane (Figure 10 b). Moreover, there is one type of CF···FC interaction in compound 5 (Figure 11 b), which also play an important role in its assembly in the solid state. The intermolecular distance between fluorine atoms is 2.93 , shorter than the sum of their van der Waals radii (2.94 ). Recently, halogen-related interactions, including halogen-bonding interactions and related halogen···halogen and halogen···p intermolecular interactions, have been another attractive type of
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Figure 10. a) 1D infinite chain of compound 5, fabricated by p···p-stacking interactions along the c axis. b) 2D supramolecular structure of compound 5 in the bc plane. c) The binuclear building units are linked into a 1D chain through CH···O intermolecular hydrogen-bonding interactions.
interaction in crystal engineering. Such halogen–halogen interactions have been termed as either type-I (cis and trans geometry) and type-II (electrophile–nucleophile model), depending on the angular approach of the halogen atoms toward each other (Figure 11 a). Thus, the intermolecular
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Figure 12. PXRD patterns of complexes 1–5.
Figure 11. a) Geometrical classification of the halogen···halogen (X···X) interactions as either type-I or type-II. b) Two binuclear building units are connected by a trans-CF···FC interaction.
The luminescence behavior of five ZnII complexes and the corresponding free HL ligand were investigated in the solid state at room temperature. When excited with 340 nm light, the free HL ligand exhibited a strong emission peak at 457 nm (see the Supporting Information, Figure S13), which originated from internal charge transfer. Upon excitation at 360 nm, complexes 1–5 exhibited intense photoluminescence, with emission maxima at 568, 547, 547, 562, and 550 nm, respectively (Figure 13). Compared with the free HL ligand, the bright and extensive yellow emissions of complexes 1–5 predominantly originated from a ligand-tometal charge-transfer (LMCT) transition and exhibited a remarkable red-shift,[15b, 18] which may be attributed to: 1) the incorporation of ZnII effectively increased the conformational rigidity of the ligand and 2) decreased the loss of energy through vibrational motion.[18] On the other hand, the formation of five-membered (ZnONC2) rings increase the conjugation of the L ligand and, accordingly, narrow the energy gap between the p and p* molecular orbitals of the ligand.[19] However, differences between the luminescence properties of compounds 1–5 in the solid state were observed: Complexes 1 and 4 were red-shifted compared with the other three complexes, owing to their close-packed supramolecular structures (more intra- or intermolecular p···p-stacking interactions).
F···F interaction with a trans geometry (q1 = q2 = 133.38) is a type-I halogen–halogen interaction. To the best of our knowledge, the presence of halogen-bonding interactions in metal-coordination compounds remains largely unexplored and needs more-elaborate and systematic studies.[16] Based on this structure description, the role of the anion in determining the structures of the five zinc complexes was unambiguously determined. Compared to Cl and Br, the binding ability of F was the worst in the fabrication of a zinc complex. Thus, the F ion could not coordinate to the zinc center in the self-assembly of mononuclear complex 4. The OAc ion induced the six-coordinate ZnII center in compound 5 to adopt a distorted octahedral geometry. Conversely, the ZnII atoms in compounds 2 and 3 were five-coordinated by one halogen atom and four N2O2 atoms and adopted a distorted-trigonal-bipyramidal geometry. Moreover, the OAc ions facilitated the formation of intermolecular CH···O hydrogen-bonding interactions (Figure 10 b), which linked the binuclear units into a 1D chain along the b axis. Different sizes, binding abilities, and geometries of the anion may affect the coordination geometries of zinc and conformation of the L ligands to minimize steric repulsion and maximize secondary interactions, thus leading to structural diversification.[17] In addition, non-covalent forces, such as hydrogen-bonding, p···p-stacking, CH···p, and halogen-related interactions may also influence the final structures. Photophysical Properties To confirm that the crystal structures of complexes 1–5 were truly representative of their bulk materials as employed in the photochemical studies, powder X-ray diffraction (PXRD) experiments were performed on the as-synthesized samples. As shown in Figure 12, the peak positions of the experimental and simulated PXRD patterns were in good agreement with one other. The differences in intensity may be due to the preferred orientation of the crystal samples.
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Figure 13. Emission spectra of complexes 1–5 in the solid state.
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To further understand the fluorescence properties of complexes 1–5, their fluorescence lifetimes and quantum yields were investigated in the solid state (Table 1). The average lifetimes (see the Supporting Information, Figures S14–S18) were determined by allowing ai and ti to vary and then convoluting Equation (1) with the instrument-response function.
which were controlled by intermolecular non-covalent interactions. This unique capability may provide a useful synthetic strategy to tune the structures and properties of new photoluminescent materials.
Experimental Section Table 1. Photoluminescence properties of compounds 1–5: average fluorescence lifetime (tavg), fluorescence quantum yield (F), maximum excitation wavelength (lex), and emission wavelength (lem). Compound
lex [nm]
lem [nm]
tavg [ns]
F
1 2 3 4 5
360 360 360 360 360
568 547 547 562 550
1.6 8.7 7.6 0.9 9.8
0.11 0.12 0.16 0.26 0.24
Experimental All of the chemicals were commercially available and used without further purification. The HL ligand was synthesized according to a literature procedure.[15b] Elemental analysis was performed on an EA1110 CHNS-0 CE elemental analyzer. IR spectra (KBr pellets) were recorded on a Nicolet Magna 750 FTIR spectrometer within the range 400–4000 cm1. Powder X-ray diffraction (PXRD) data were collected on a DMAX2500 diffractometer by using CuKa radiation. The calculated PXRD patterns were produced by using the SHELXTL-XPOW program and single-crystal reflection data. Fluorescence measurements were performed on a LS 50B Luminescence Spectrometer (PerkinElmer, Inc., USA). Room-temperature lifetime measurements were determined on a FLS920 time-resolved and steady-state fluorescence spectrometer (Edinburgh Instruments).
The data were successfully modeled by using double exponentials and the average lifetime was determined by using Equation (2). In both equations, t is time, t is lifetime, and a is the pre-exponential factor.[20] The shorter fluorescence lifetimes of complexes 1 and 4 (1 = 1.6 ns, 4 = 0.9 ns) may arise from a combination of contributions from a competitive non-radiative-decay process in the metal complex (such as the incompletely hindered electron transfer) and distinct supramolecular structures compared with complexes 2, 3, and 5. However, the different fluorescence properties of complexes 1–5 can be explained by various molecular-packing characteristics (molecular and supramolecular structures). These observations are in line with the recently reported correlation between the molecular density of the packing and the length of interACHTUNGREliACHTUNGREgand interactions between neighboring clusters in the crystal structures and the photophysical properties of mer-Alq3 polymorphs (owing to different dispersive and dipolar interactions, as well as different p–p orbital overlaps).[5b] IðtÞ ¼
n X
ai expðt=ti Þ
Synthesis of [Zn3(L)6] (1) A mixture of ZnCl2 (2.7 mg, 0.02 mmol), HL (3.0 mg, 0.01 mmol), water (0.2 mL), DMF (0.1 mL), and MeOH (2.0 mL) in a capped vial was heated at 80 8C for 1 day. Yellow block-like crystals of compound 1 that were suitable for single-crystal X-ray diffraction were collected, washed with Et2O, and dried in air. Light-yellow crystals were obtained in 82 % yield based on the HL ligand. IR (KBr): n˜ = 3437.21 (s), 2927.02 (w), 2360.39 (w), 1625.17 (m), 1590.08 (w), 1554.98 (m), 1506.06 (m), 1437.05 (s), 1377.32 (m), 1338.07 (m), 1307.44 (w), 1274.30 (w), 1208.95 (w), 1169.15 (w), 1120.63 (m), 1105.15 (w), 1033.05 (m), 1000.31 (w), 965.29 (w), 830.70 (m), 753.62 (m), 614.21 (w), 567.75 (w), 515.41 (w), 474.33 (w); elemental analysis calcd (%) for (w), 449.53 cm1 C102H54F18N6O6Zn3 : C 61.33, H 2.72, N 4.21; found: C 61.10, H 2.95, N 3.98. Synthesis of Complexes 2–5 ZnX2 (X = Cl, Br, F, OAc ; 0.02 mmol), the HL ligand (3.0 mg, 0.01 mmol), and phthalic acid (16.6 mg, 0.1 mmol) were dissolved in a mixture (15 mL) of water (0.2 mL), DMF (0.1 mL), MeOH (2.0 mL), and pyridine (py, 0.1 mL). The mixture was placed in a capped vial (10 mL) under autogenous pressure and heated at 80 8C for 1 day. Yellow crystals were collected, washed with Et2O, and dried in air.
ð1Þ
ACHTUNGRE[Zn2(Cl)2(L)2(py)2] (2): IR (KBr): n˜ = 3748.04 (w), 3445.69 (s), 2922.98 (w), 2360.34 (w), 1624.17 (w), 1594.30 (w), 1555.51 (m), 1505.87 (w), 1434.60 (m), 1376.94 (w), 1334.55 (m), 1273.51 (w), 1167.26 (w), 1118.88 (m), 1102.34 (m), 1032.10 (w), 1001.69 (w), 832.55 (m), 754.09 (w), 705.45 (w), 605.29 (w), 507.11 cm1 (w); elemental analysis calcd (%) for C44H28Cl2F6N4O2Zn2 : C 55.03, H 2.94, N 5.83; found: C 54.83, H 3.25, N 5.61.
i¼1
tavg ¼
a1 t21 þ a2 t22 a1 t1 þ a2 t2
ð2Þ
Conclusions
ACHTUNGRE[Zn2(Br)2(L)2(py)2] (3): IR (KBr): n˜ = 3437.93 (s), 2920.00 (w), 2360.49 (m), 2341.51 (w), 1622.06 (w), 1607.28 (w), 1593.75 (m), 1551.26 (m), 1505.41 (m), 1489.20 (w), 1432.88 (s), 1375.44 (m), 1358.70 (w), 1165.75 (m), 1117.43 (m), 1101.90 (m), 1069.48 (w), 1056.29 (w), 1043.52 (w), 1030.94 (m), 1001.52 (m), 832.06 (s), 762.62 (m), 734.79 (w), 704.54 (m), 678.80 (w), 655.22 (w), 637.54 (w), 614.28 (m), 553.90 (w), 514.24 (m), 471.38 (m), 451.20 cm1 (m); elemental analysis calcd (%) for C44H28Br2F6N4O2Zn2 : C 50.36, H 2.69, N 5.34; found: C 50.03, H 2.80, N 5.21.
Five ZnII complexes with different structures were prepared under solvothermal conditions, by using ZnX2 salts (with different anions, X = Cl, Br, F, and OAc) and a 2-substituted 8-quinolinonate ligand that was synthesized from the cheap, commercially available 8-hydroxyquinaldine. In the solid state, the building blocks of compounds 1–5 exhibited remarkable dependence on the counteranion and solvent system, thereby affording one mononuclear, three binuclear, and one trinuclear core structures. In addition, these ZnII complexes exhibited disparate photophysical properties, owing to their different core and supramolecular structures,
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[Zn(L)2(py)] (4): IR (KBr): n˜ = 3444.37 (s), 2926.03 (w), 2851.57 (w), 2360.62 (m), 2341.76 (w), 1624.89 (m), 1594.09 (m), 1550.98 (m), 1505.20 (w), 1491.96 (w), 1439.46 (s), 1373.85 (m), 1342.51 (m), 1306.86 (w), 1270.37 (w), 1208.99 (w), 1167.49 (m), 1119.97 (m), 1102.86 (m), 1031.54 (m), 1000.32 (m), 882,42 (w), 827.09 (m), 799.47 (w), 754.11 (m), 735.81
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(w), 707.70 (w), 669.30 (w), 611.25 (w), 513.68 (m), 467.73 (m), 445.21 cm1 (w); elemental analysis calcd (%) for C39H23F6N3O2Zn: C 62.87, H 3.11, N 5.64; found: C 62.98, H 3.46, N 5.37. [7]
ACHTUNGRE[Zn2ACHTUNGRE(OAc)2(L)2(py)2] (5): IR (KBr): n˜ = 3441.74 (s), 3098.33 (w), 3044.78 (w), 3000.16 (w), 2626.72 (w), 2848.45 (w), 2360.55 (m), 2341.42 (w), 1623.23 (w), 1595.11 (m), 1554.21 (m), 1504.40 (m), 1490.53 (w), 1434.55 (s), 1400.96 (m), 1377.96 (m), 1352.03 (w), 1335.65 (m), 1309.25 (w), 1277.77 (m), 1221.83 (w), 1208.84 (w), 1169.62 (m), 1220.05 (m), 1102.61 (m), 1069.98 (w), 1029.49 (m), 1000.63 (m), 970.76 (m), 882.48 (w), 830.49 (m), 802.20 (w), 752.82 (m), 733.97 (w), 702.37 (m), 671.87 (m), 634.68 (w), 616.13 (w), 589.63 (w), 556.51 (w), 514.80 (m), 473.54 (w), 451.56 cm1 (m); elemental analysis calcd (%) for C48H34F6N4O6Zn2 : C 57.22, H 3.40, N 5.56; found: C 57.43, H 3.70, N 5.23.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21201002, 21172047, and 21372051), the Anhui Provincial Natural Science Foundation (1308085QB22), the Provincial Natural Science Research Program of Higher Education Institutions of Anhui Province (KJ2013Z028), and the Foundation of State Key Laboratory of Structural Chemistry (201210360087).
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Received: March 2, 2014 Revised: March 31, 2014 Published online: && &&, 0000
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FULL PAPER Famous five: Five 8-hydroxyquinolinate-based complexes were prepared and characterized. In the solid state, the building blocks of complexes 1–5 exhibited remarkable dependence on the counteranions and solvent system and self-assembled into one mononuclear, three binuclear, and one trinuclear core structures. Varying the stoichiometry, shape, and supramolecular structure of the resulting complexes led to changes in their fluorescent properties.
Self-Assembly Guozan Yuan,* Weilong Shan, Xuelong Qiao, Li Ma, &&&&—&&&& Yanping Huo* Self-Assembly of Five 8-Hydroxyquinolinate-Based Complexes: Tunable Core, Supramolecular Structure, and Photoluminescence Properties
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Self-Assembly of Five 8-Hydroxyquinolinate-Based Complexes: Tunable Core, Supramolecular Structure, and Photoluminescence Properties Guozan Yuan,*[a, b] Weilong Shan,[a] Xuelong Qiao,[a] Li Ma,[a] and Yanping Huo*[c] Abstract: Five new ZnII complexes, namely [Zn3(L)6] (1), [Zn2(Cl)2(L)2(py)2] (2), [Zn2(Br)2(L)2(py)2] (3), [Zn(L)2(py)] (4), and [Zn2ACHTUNGRE(OAc)2(L)2(py)2] (5), were prepared by the solvothermal reaction of ZnX2 (X = Cl, Br, F, and OAc) salts with a 8-hydroxyquinolinate ligand (HL) that contained a trifluorophenyl group. All of the complexes were characterized by elemental analysis, IR spectroscopy, and powder and
single-crystal X-ray crystallography. The building blocks exhibited unprecedented structural diversification and their self-assembly afforded one mononuclear, three binuclear, and one trinuclear ZnII structures in response to difKeywords: 8-hydroxyquinolinates · photoluminescence · self-assembly · supramolecular chemistry · zinc
Introduction
obtained with yellow-light-emitting Znq2 as an electrontransport material, thus suggesting enhanced electron injection and transport in Znq2. These results can be explained by the formation of the energetically favored tetramer (Znq2)4 in vacuum-deposited thin films and by the stronger p···p overlap between the ligands and the extended electronic states in tetrameric (Znq2)4.[3a] In general, the photophysical properties of 8-quinolinolate-based complexes in the solid state are dependent on the character of the metal ion, the degree of aggregation, and the molecular and supramolecular structures, as determined by the intermolecular non-covalent interactions involved.[4] Nevertheless, Mqn-type mononuclear complexes will limit and even hamper the synthesis of new 8-quinolinolate-based materials with controllable structures and luminescence properties. Hence, the development of new synthetic methods and strategies for the fabrication of well-defined heteroleptic molecular complexes or multinuclear clusters that are supported by organic 8-hydroxyquinoline derivatives, with unique self-assembled structures and properties, and, hence, improved and tunable fluorescence features, remains a big challenge for chemists.[5] Over the past two decades, it has been demonstrated that the self-assembly of complexes from polytopic organic linkers[6–9] could be affected by many structure-directing factors, such as the metal, the coordinating ability of the anion, the pH value, and the reaction temperature.[10–14] Of these factors, the anion plays a crucial role in influencing the formation of the coordination compound and leads to their structural and dimensional variation, supramolecular isomerism, and new topologies.[10] With a few notable exceptions, the systematic investigation of the effect of the anion on 8-hydroxyquinolinate-based complexes remains unexplored.[15a] In recent years, we have reported the synthesis and characterization of a series of transition-metal complexes that in-
Organometalliccomplexes have frequently been used as emissive and electron-transport materials or host materials for fluorescent dyes in organic light-emitting diodes (OLEDs) and as buffer layers in organic solar cells (OSCs).[1] In particular, tris(8-quinolinolate)aluminumACHTUNGRE(III) (Alq3) is one of the most widely studied complexes for OLEDs and OSCs, owing to its superior properties, such as high electron affinity (about 3.0 eV), high ionization potential (about 5.95 eV), low-lying HOMO (about 5.7 eV), good thermal stability, and ease of thin-film fabrication through a vacuum-evaporation technique.[1b, 2] Inspired by the success of Alq3, 8-quinolinolate chelates of other metals, such as Be2+ and Zn2+, have also been used in OLEDs.[3] Compared to Alq3 devices, lower operating voltages were
[a] Prof. Dr. G. Yuan, W. Shan, X. Qiao, L. Ma School of Chemistry and Chemical Engineering Anhui University of Technology Maanshan 243002 (P. R. China) Fax: (+ 86) 555-2311552 E-mail: [email protected] [b] Prof. Dr. G. Yuan State Key Laboratory of Structural Chemistry Fujian Institute of Research on the Structure of Matter Institution Chinese Academy of Sciences Fuzhou, Fujian 350002 (P. R. China) [c] Prof. Dr. Y. Huo School of Chemical Engineering and Light Industry Guangdong University of Technology Guangzhou 510006 (P. R. China) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402153.
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ferent anions and solvent systems. Complexes 1–5 featured four types of supramolecular network controlled by non-covalent interactions, such as p···pstacking, CH···p, hydrogen-bonding, and halogen-related interactions. Investigation of their photoluminescence properties exhibited disparate emission wavelengths, lifetimes, and quantum yields in the solid state.
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volve 2-substituted 8-quinolinolate ligands with a NO donor set.[15] In this context, one 8-quinolinolate-based ligand (HL), which contained a trifluorophenyl group, was chosen for fabricating ZnII complexes in different solvent systems. The introduction of a trifluorophenyl group not only modified the energies of the excited states of the corresponding complexes, but also facilitated the formation of a diverse range of halogen bonds (such as CF···HC, CF···p, and CF···FC interactions). These halogen-related interactions may affect the supramolecular structures of ZnII complexes by extending the low-dimensional entities into higher-dimensional supramolecular networks, thereby generating fascinating properties in the solid state.[16] By using a solvothermal method, five complexes (1–5) were constructed by the self-assembly of ZnX2 (X = Cl, Br, F, and OAc) with the HL ligand (Scheme 1). Structural diversification with
Guozan Yuan, Yanping Huo et al.
within the temperature range 50–100 8C. Single-crystal X-ray diffraction analysis revealed that the binuclear building block of compound 2 consisted of two coordinated chloride anions. Thus, three other ZnII salts were used in DMF/ MeOH/water/pyridine systems to investigate the influence of the counterion on the formation and structures of the complexes. Three other mono- or binuclear complexes, [Zn2(Br)2(L)2(py)2] (3), [Zn(L)2(py)] (4), and [Zn2 ACHTUNGRE(OAc)2(L)2(py)2] (5), were obtained under solvothermal conditions. The structural assignment of complexes 1–5 was supported by elemental analysis, IR spectroscopy, and single-crystal X-ray diffraction. Structural Characterization Complex 1 crystallized in the monoclinic space group P21/c, with Z = 4. The asymmetric unit contained one formula unit, that is, three crystallographically independent ZnII cations and six L ligands. As shown in Figure 1, three pairs of li-
Scheme 1. Synthesis of complexes 1–5 from ligand HL.
regard to the building units was observed in this series of ZnII complexes. One mononuclear, three binuclear, and one trinuclear ZnII core structures were fabricated in response to different anions and solvent systems. In addition, their fluorescent properties showed disparate emission wavelengths, lifetimes, and quantum yields in the solid state. This unique capability may provide a useful strategy for tuning the optical properties of 8-hydroxyquinoline materials, which could be exploited as important components of optoelectronic devices.
Figure 1. View of the coordination geometries of the ZnII atoms in compound 1; H atoms are omitted for clarity.
gands with offset face-to-face p–p-stacking interactions are arranged around the trimeric ZnII core in a propeller-like form. The zinc atoms, which are located at either end of the trimeric unit, are five-coordinated by three phenolate oxygen atoms and two nitrogen atoms from three L ligands, thereby displaying a distorted-trigonal-bipyramidal geometry. The coordination environment around the central ZnII atom is a distorted octahedron and the equatorial plane is occupied by the NO3 donor atoms of three ligands, whilst the apical position by one phenolate oxygen atom and a pyridine nitrogen atom. For compound 1, the bond lengths [] and angles [8] around the ZnII atom are 2.114(4)–2.232(3) (ZnN), 1.960(3)–2.203(3) (ZnO); 75.24(10)–170.99(11) (O-Zn-O), 78.49(12)–154.19(11) (O-Zn-N), 93.66(12)– 112.22(14) (N-Zn-N). The three ZnII ions in compound 1 are bridged by the phenolato oxygen atoms of four ligands, with Zn···Zn distances of 3.2584(4) and 3.2865(4) . There are numerous CH···O intramolecular hydrogenbonding interactions in compound 1 between the phenolato oxygen atoms and the CH groups of the quinoline ring or
Results and Discussion Synthetic Considerations Trinuclear complex [Zn3(L)6] (1) was prepared by a solvothermal reaction in a mixture of DMF, MeOH, and water. The reaction was originally performed with a 1:2 molar ratio of ZnII and HL, but the products were not significantly affected by a change in the molar ratio. To fabricate other multinuclear ZnII complexes based on the L ligand, other ZnII salts of different anions (such as Br, F, and OAc) were used instead of ZnCl2 ; however, crystalline compound 1 was obtained in each case. Pleasingly, when an appropriate amount of pyridine was added to the reaction solution, binuclear ZnII complex [Zn2(Cl)2(L)2(py)2] (2) was synthesized
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the ethenyl group (C···O 3.079(6)–3.317(5) , CH···O 128.0–155.08), which play a significant role in the construction of trimeric ZnII units (see the Supporting Information, Figure S1). The trimeric Zn3L6 units are linked into a 2D supramolecular structure in the bc plane through C50 H50···p (between CH dichlorophenyl groups and adjacent pyridyl rings, 3.476(8) ) and CH···F intermolecular interactions that involve quinoline CH groups and the fluorine atoms of adjacent 2,4,6-trifluorophenyl units (C···F 3.307(6), 3.332(7) ; CH···F 156, 1698; Figure 2 and the Supporting
Figure 3. a) 2D supramolecular structure of compound 1, mediated by C89H89···F16 (dashed red lines) and p···p stacking interactions (dashed yellow lines) in the ac plane. b) 1D intertwined supramolecular helical polymeric chains in compound 1 along the b axis.
Figure 2. 2D supramolecular structure of compound 1, mediated by C50 H50···p (dashed yellow lines) and CH···F intermolecular interactions (dashed pink lines).
Information, Figure S2). Owing to abundant p···p-stacking and intermolecular C89H89···F16 interactions (C···F 3.171(5) ; Figure 3 a), the supramolecular structure extends into a 3D network, as shown in the Supporting Information, Figure S3. In particular, the trimeric ZnII units in compound 1 are linked into a helical chain through C50H50···p and C89H89···F16 intermolecular interactions along the b axis. Thus, two intertwined helical chains (one is right-handed, the other is left-handed) are generated around the crystallographic 2(1) axis, as shown in Figure 3 b. The two helices have an identical pitch of 27.479 (5) (equal to the length of the b axis). On treatment of 8-hydroxyquinoline ligand HL with ZnCl2 in a mixture of DMF/MeOH/water/pyridine, one binuclear complex, [Zn2(Cl)2(L)2(py)2] (2), was obtained. Single-crystal structural analysis revealed that compound 2 crystallized in the monoclinic space group P21/n, with one half of a formula unit in the asymmetric unit, that is, one ZnII atom, one ligand L, one coordinated chlorine atom, and one coordinated pyridine moiety. The structure of compound 2 is built around a binuclear ZnII structure (Figure 4). The two zinc atoms in the building unit are pentacoordinate and adopt a trigonal-bipyramidal geometry, with the equatorial plane occupied by the NO2 donors of two L ligands and the apical position by one chlorine atom and one nitrogen atom of a pyridine molecule. However, complex 2 appears
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Figure 4. Coordination structure of compound 2.
to be neutral, because the hydroxy groups of the coordinated L ligands are fully deprotonated. Interestingly, in the solid-state structure of this complex, a type of meso-helical chain (P+M) along the b axis is constructed through CH···p interactions between the pyridine CH groups and the phenolate ring of the neighboring unit. As shown in Figure 5 a, the two 21 helices have an identical pitch of 10.7957(8) (equal to the length of the b axis). Furthermore, CH···F intramolecular hydrogen bonds between the fluorine atom and the ethenyl CH group (see the Supporting Information, Figure S4; C···F 2.897, 2.739 ; C H···F 124.0, 1048), as well as intermolecular CH···F hydrogen bonds between the pyridine CH groups and the fluorine atoms of adjacent binuclear units (C···F 3.382(3) ; C H···F 1508), also play a vital role in the consolidation of the solid-state structure (Figure 5 b). Owing to the cooperative action of these intermolecular non-covalent interactions (C H···F and CH···p) and p···p-stacking interactions (between the phenolate and trifluorophenyl rings of adjacent binuclear units, 3.781(3) ), the structure extends into a 3D network (Figure 6). The structure of complex 3 is identical to
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diffraction analysis revealed that compound 4 crystallized in a triclinic system with the P1 space group. As shown in Figure 7 a, the asymmetric unit of compound 4 is composed of one Zn center, two L ligands, and one coordinated pyridine molecule. The Zn center is five-coordinated by the O1, N1,
Figure 5. a) View of 1D intertwined supramolecular helical chains in compound 2 along the b axis. b) 2D supramolecular structure of compound 2, mediated CH···F intermolecular interactions.
Figure 7. a) Coordination structure of compound 4. b) Three types of intermolecular interaction in compound 4.
O2, and N2 atoms from two distinct L ligands and by the N3 atom from a coordinated pyridine molecule to form a trigonal-bipyramidal geometry. The ZnN distances are 2.0616(19), 2.1936(18), and 2.2583(19) and the ZnO distances are 1.9468(18) and 1.9578(17) . For the L ligand, the dihedral angles between the trifluorophenyl and quinoline rings are 9.58 and 13.38, respectively. Intermolecular interactions play a significant role in the crystal packing. As shown in Figure 7 b, CH···F, CH···p, and p···p intermolecular interactions link the neutral mononuclear ZnII units in compound 4 into 2D layers in the ac plane (Figure 8 a). Each trifluorophenyl ring lies parallel to the ring on the reverse side of the neighboring unit in the bc plane (Figure 8 b) and p···p interactions exist between them, with a face-to-face distance of 3.573(3) . The 2D layers are further assembled into a 3D supramolecular network (see the Supporting Information, Figure S9) through p···p interactions. Binuclear complex 5 was readily obtained in good yield by heating ZnACHTUNGRE(OAc)2 and ligand HL in a DMF/MeOH/
Figure 6. The 2D supramolecular structure of compound 2 extends into a 3D network, mediated by CH···p (dashed pink lines) and p···p interactions (dashed red lines).
that of complex 2. The main distinction is that the coordinated chloride ions were replaced with bromide anions, so there were small differences between them. To further investigate the influence of the counterion on the formation and structures of the complexes, mononuclear complex [Zn(L)2(py)] (4) was obtained when the 8-hydroxyquinoline ligand HL was reacted with ZnF2 in the DMF/ MeOH/water/pyridine reaction system. Single-crystal X-ray
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Figure 9. Coordination structure of compound 5.
Figure 8. a) 2D supramolecular structure of compound 4 in the ac plane, mediated by three types of intermolecular interaction. b) 2D supramolecular structure of compound 4 in the bc plane, mediated by p···p-stacking interactions.
water/pyridine mixture. Complex 5 crystallized in a triclinic system and the P 1 (no. 2) space group. The binuclear building block contained two ZnII atoms, two L ligands, two coordinated pyridine molecules, and two coordinated acetate anions. Both of the ZnII centers adopted a distorted octahedral geometry; the equatorial plane was occupied by the NO3 donor atoms of one L ligand, one pyridine molecule, and an acetate anion and the apical position was occupied by quinoline nitrogen and oxygen atoms (Figure 9). The two ZnII ions are bridged by phenolate oxygen atoms of two ligands, with Zn···Zn distances of 3.2494(4) . The binuclear ZnII units are linked into a supramolecular chain along the b axis through intramolecular CH···O hydrogen-bonding interactions between the pyridine CH group and the acetate oxygen atom (Figure 10 c). Furthermore, p···p interactions between the phenolate and trifluorophenyl rings of adjacent chains (Figure 10 a) expand the parallel 1D chains into a 2D network in the bc plane (Figure 10 b). Moreover, there is one type of CF···FC interaction in compound 5 (Figure 11 b), which also play an important role in its assembly in the solid state. The intermolecular distance between fluorine atoms is 2.93 , shorter than the sum of their van der Waals radii (2.94 ). Recently, halogen-related interactions, including halogen-bonding interactions and related halogen···halogen and halogen···p intermolecular interactions, have been another attractive type of
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Figure 10. a) 1D infinite chain of compound 5, fabricated by p···p-stacking interactions along the c axis. b) 2D supramolecular structure of compound 5 in the bc plane. c) The binuclear building units are linked into a 1D chain through CH···O intermolecular hydrogen-bonding interactions.
interaction in crystal engineering. Such halogen–halogen interactions have been termed as either type-I (cis and trans geometry) and type-II (electrophile–nucleophile model), depending on the angular approach of the halogen atoms toward each other (Figure 11 a). Thus, the intermolecular
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Figure 12. PXRD patterns of complexes 1–5.
Figure 11. a) Geometrical classification of the halogen···halogen (X···X) interactions as either type-I or type-II. b) Two binuclear building units are connected by a trans-CF···FC interaction.
The luminescence behavior of five ZnII complexes and the corresponding free HL ligand were investigated in the solid state at room temperature. When excited with 340 nm light, the free HL ligand exhibited a strong emission peak at 457 nm (see the Supporting Information, Figure S13), which originated from internal charge transfer. Upon excitation at 360 nm, complexes 1–5 exhibited intense photoluminescence, with emission maxima at 568, 547, 547, 562, and 550 nm, respectively (Figure 13). Compared with the free HL ligand, the bright and extensive yellow emissions of complexes 1–5 predominantly originated from a ligand-tometal charge-transfer (LMCT) transition and exhibited a remarkable red-shift,[15b, 18] which may be attributed to: 1) the incorporation of ZnII effectively increased the conformational rigidity of the ligand and 2) decreased the loss of energy through vibrational motion.[18] On the other hand, the formation of five-membered (ZnONC2) rings increase the conjugation of the L ligand and, accordingly, narrow the energy gap between the p and p* molecular orbitals of the ligand.[19] However, differences between the luminescence properties of compounds 1–5 in the solid state were observed: Complexes 1 and 4 were red-shifted compared with the other three complexes, owing to their close-packed supramolecular structures (more intra- or intermolecular p···p-stacking interactions).
F···F interaction with a trans geometry (q1 = q2 = 133.38) is a type-I halogen–halogen interaction. To the best of our knowledge, the presence of halogen-bonding interactions in metal-coordination compounds remains largely unexplored and needs more-elaborate and systematic studies.[16] Based on this structure description, the role of the anion in determining the structures of the five zinc complexes was unambiguously determined. Compared to Cl and Br, the binding ability of F was the worst in the fabrication of a zinc complex. Thus, the F ion could not coordinate to the zinc center in the self-assembly of mononuclear complex 4. The OAc ion induced the six-coordinate ZnII center in compound 5 to adopt a distorted octahedral geometry. Conversely, the ZnII atoms in compounds 2 and 3 were five-coordinated by one halogen atom and four N2O2 atoms and adopted a distorted-trigonal-bipyramidal geometry. Moreover, the OAc ions facilitated the formation of intermolecular CH···O hydrogen-bonding interactions (Figure 10 b), which linked the binuclear units into a 1D chain along the b axis. Different sizes, binding abilities, and geometries of the anion may affect the coordination geometries of zinc and conformation of the L ligands to minimize steric repulsion and maximize secondary interactions, thus leading to structural diversification.[17] In addition, non-covalent forces, such as hydrogen-bonding, p···p-stacking, CH···p, and halogen-related interactions may also influence the final structures. Photophysical Properties To confirm that the crystal structures of complexes 1–5 were truly representative of their bulk materials as employed in the photochemical studies, powder X-ray diffraction (PXRD) experiments were performed on the as-synthesized samples. As shown in Figure 12, the peak positions of the experimental and simulated PXRD patterns were in good agreement with one other. The differences in intensity may be due to the preferred orientation of the crystal samples.
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Figure 13. Emission spectra of complexes 1–5 in the solid state.
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To further understand the fluorescence properties of complexes 1–5, their fluorescence lifetimes and quantum yields were investigated in the solid state (Table 1). The average lifetimes (see the Supporting Information, Figures S14–S18) were determined by allowing ai and ti to vary and then convoluting Equation (1) with the instrument-response function.
which were controlled by intermolecular non-covalent interactions. This unique capability may provide a useful synthetic strategy to tune the structures and properties of new photoluminescent materials.
Experimental Section Table 1. Photoluminescence properties of compounds 1–5: average fluorescence lifetime (tavg), fluorescence quantum yield (F), maximum excitation wavelength (lex), and emission wavelength (lem). Compound
lex [nm]
lem [nm]
tavg [ns]
F
1 2 3 4 5
360 360 360 360 360
568 547 547 562 550
1.6 8.7 7.6 0.9 9.8
0.11 0.12 0.16 0.26 0.24
Experimental All of the chemicals were commercially available and used without further purification. The HL ligand was synthesized according to a literature procedure.[15b] Elemental analysis was performed on an EA1110 CHNS-0 CE elemental analyzer. IR spectra (KBr pellets) were recorded on a Nicolet Magna 750 FTIR spectrometer within the range 400–4000 cm1. Powder X-ray diffraction (PXRD) data were collected on a DMAX2500 diffractometer by using CuKa radiation. The calculated PXRD patterns were produced by using the SHELXTL-XPOW program and single-crystal reflection data. Fluorescence measurements were performed on a LS 50B Luminescence Spectrometer (PerkinElmer, Inc., USA). Room-temperature lifetime measurements were determined on a FLS920 time-resolved and steady-state fluorescence spectrometer (Edinburgh Instruments).
The data were successfully modeled by using double exponentials and the average lifetime was determined by using Equation (2). In both equations, t is time, t is lifetime, and a is the pre-exponential factor.[20] The shorter fluorescence lifetimes of complexes 1 and 4 (1 = 1.6 ns, 4 = 0.9 ns) may arise from a combination of contributions from a competitive non-radiative-decay process in the metal complex (such as the incompletely hindered electron transfer) and distinct supramolecular structures compared with complexes 2, 3, and 5. However, the different fluorescence properties of complexes 1–5 can be explained by various molecular-packing characteristics (molecular and supramolecular structures). These observations are in line with the recently reported correlation between the molecular density of the packing and the length of interACHTUNGREliACHTUNGREgand interactions between neighboring clusters in the crystal structures and the photophysical properties of mer-Alq3 polymorphs (owing to different dispersive and dipolar interactions, as well as different p–p orbital overlaps).[5b] IðtÞ ¼
n X
ai expðt=ti Þ
Synthesis of [Zn3(L)6] (1) A mixture of ZnCl2 (2.7 mg, 0.02 mmol), HL (3.0 mg, 0.01 mmol), water (0.2 mL), DMF (0.1 mL), and MeOH (2.0 mL) in a capped vial was heated at 80 8C for 1 day. Yellow block-like crystals of compound 1 that were suitable for single-crystal X-ray diffraction were collected, washed with Et2O, and dried in air. Light-yellow crystals were obtained in 82 % yield based on the HL ligand. IR (KBr): n˜ = 3437.21 (s), 2927.02 (w), 2360.39 (w), 1625.17 (m), 1590.08 (w), 1554.98 (m), 1506.06 (m), 1437.05 (s), 1377.32 (m), 1338.07 (m), 1307.44 (w), 1274.30 (w), 1208.95 (w), 1169.15 (w), 1120.63 (m), 1105.15 (w), 1033.05 (m), 1000.31 (w), 965.29 (w), 830.70 (m), 753.62 (m), 614.21 (w), 567.75 (w), 515.41 (w), 474.33 (w); elemental analysis calcd (%) for (w), 449.53 cm1 C102H54F18N6O6Zn3 : C 61.33, H 2.72, N 4.21; found: C 61.10, H 2.95, N 3.98. Synthesis of Complexes 2–5 ZnX2 (X = Cl, Br, F, OAc ; 0.02 mmol), the HL ligand (3.0 mg, 0.01 mmol), and phthalic acid (16.6 mg, 0.1 mmol) were dissolved in a mixture (15 mL) of water (0.2 mL), DMF (0.1 mL), MeOH (2.0 mL), and pyridine (py, 0.1 mL). The mixture was placed in a capped vial (10 mL) under autogenous pressure and heated at 80 8C for 1 day. Yellow crystals were collected, washed with Et2O, and dried in air.
ð1Þ
ACHTUNGRE[Zn2(Cl)2(L)2(py)2] (2): IR (KBr): n˜ = 3748.04 (w), 3445.69 (s), 2922.98 (w), 2360.34 (w), 1624.17 (w), 1594.30 (w), 1555.51 (m), 1505.87 (w), 1434.60 (m), 1376.94 (w), 1334.55 (m), 1273.51 (w), 1167.26 (w), 1118.88 (m), 1102.34 (m), 1032.10 (w), 1001.69 (w), 832.55 (m), 754.09 (w), 705.45 (w), 605.29 (w), 507.11 cm1 (w); elemental analysis calcd (%) for C44H28Cl2F6N4O2Zn2 : C 55.03, H 2.94, N 5.83; found: C 54.83, H 3.25, N 5.61.
i¼1
tavg ¼
a1 t21 þ a2 t22 a1 t1 þ a2 t2
ð2Þ
Conclusions
ACHTUNGRE[Zn2(Br)2(L)2(py)2] (3): IR (KBr): n˜ = 3437.93 (s), 2920.00 (w), 2360.49 (m), 2341.51 (w), 1622.06 (w), 1607.28 (w), 1593.75 (m), 1551.26 (m), 1505.41 (m), 1489.20 (w), 1432.88 (s), 1375.44 (m), 1358.70 (w), 1165.75 (m), 1117.43 (m), 1101.90 (m), 1069.48 (w), 1056.29 (w), 1043.52 (w), 1030.94 (m), 1001.52 (m), 832.06 (s), 762.62 (m), 734.79 (w), 704.54 (m), 678.80 (w), 655.22 (w), 637.54 (w), 614.28 (m), 553.90 (w), 514.24 (m), 471.38 (m), 451.20 cm1 (m); elemental analysis calcd (%) for C44H28Br2F6N4O2Zn2 : C 50.36, H 2.69, N 5.34; found: C 50.03, H 2.80, N 5.21.
Five ZnII complexes with different structures were prepared under solvothermal conditions, by using ZnX2 salts (with different anions, X = Cl, Br, F, and OAc) and a 2-substituted 8-quinolinonate ligand that was synthesized from the cheap, commercially available 8-hydroxyquinaldine. In the solid state, the building blocks of compounds 1–5 exhibited remarkable dependence on the counteranion and solvent system, thereby affording one mononuclear, three binuclear, and one trinuclear core structures. In addition, these ZnII complexes exhibited disparate photophysical properties, owing to their different core and supramolecular structures,
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[Zn(L)2(py)] (4): IR (KBr): n˜ = 3444.37 (s), 2926.03 (w), 2851.57 (w), 2360.62 (m), 2341.76 (w), 1624.89 (m), 1594.09 (m), 1550.98 (m), 1505.20 (w), 1491.96 (w), 1439.46 (s), 1373.85 (m), 1342.51 (m), 1306.86 (w), 1270.37 (w), 1208.99 (w), 1167.49 (m), 1119.97 (m), 1102.86 (m), 1031.54 (m), 1000.32 (m), 882,42 (w), 827.09 (m), 799.47 (w), 754.11 (m), 735.81
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(w), 707.70 (w), 669.30 (w), 611.25 (w), 513.68 (m), 467.73 (m), 445.21 cm1 (w); elemental analysis calcd (%) for C39H23F6N3O2Zn: C 62.87, H 3.11, N 5.64; found: C 62.98, H 3.46, N 5.37. [7]
ACHTUNGRE[Zn2ACHTUNGRE(OAc)2(L)2(py)2] (5): IR (KBr): n˜ = 3441.74 (s), 3098.33 (w), 3044.78 (w), 3000.16 (w), 2626.72 (w), 2848.45 (w), 2360.55 (m), 2341.42 (w), 1623.23 (w), 1595.11 (m), 1554.21 (m), 1504.40 (m), 1490.53 (w), 1434.55 (s), 1400.96 (m), 1377.96 (m), 1352.03 (w), 1335.65 (m), 1309.25 (w), 1277.77 (m), 1221.83 (w), 1208.84 (w), 1169.62 (m), 1220.05 (m), 1102.61 (m), 1069.98 (w), 1029.49 (m), 1000.63 (m), 970.76 (m), 882.48 (w), 830.49 (m), 802.20 (w), 752.82 (m), 733.97 (w), 702.37 (m), 671.87 (m), 634.68 (w), 616.13 (w), 589.63 (w), 556.51 (w), 514.80 (m), 473.54 (w), 451.56 cm1 (m); elemental analysis calcd (%) for C48H34F6N4O6Zn2 : C 57.22, H 3.40, N 5.56; found: C 57.43, H 3.70, N 5.23.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21201002, 21172047, and 21372051), the Anhui Provincial Natural Science Foundation (1308085QB22), the Provincial Natural Science Research Program of Higher Education Institutions of Anhui Province (KJ2013Z028), and the Foundation of State Key Laboratory of Structural Chemistry (201210360087).
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Received: March 2, 2014 Revised: March 31, 2014 Published online: && &&, 0000
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FULL PAPER Famous five: Five 8-hydroxyquinolinate-based complexes were prepared and characterized. In the solid state, the building blocks of complexes 1–5 exhibited remarkable dependence on the counteranions and solvent system and self-assembled into one mononuclear, three binuclear, and one trinuclear core structures. Varying the stoichiometry, shape, and supramolecular structure of the resulting complexes led to changes in their fluorescent properties.
Self-Assembly Guozan Yuan,* Weilong Shan, Xuelong Qiao, Li Ma, &&&&—&&&& Yanping Huo* Self-Assembly of Five 8-Hydroxyquinolinate-Based Complexes: Tunable Core, Supramolecular Structure, and Photoluminescence Properties
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