Novel Hydrogen-Bonding Cross-Linking Aggregation-Induced Emission

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Novel Hydrogen-Bonding Crosslinking Aggregation-Induced Emission: Water as a Fluorescent ‘‘Ribbon’’ Detected in a Wide Range Ani Wang, Ruiqing Fan, Yuwei Dong, Yang Song, Yuze Zhou, Jianzong Zheng, Xi Du, Kai Xing, and Yulin Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01254 • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 21, 2017

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ACS Applied Materials & Interfaces

Novel Hydrogen-Bonding Crosslinking Aggregation-Induced Emission: Water as a Fluorescent ‘‘Ribbon’’ Detected in a Wide Range Ani Wang, Ruiqing Fan,* Yuwei Dong, Yang Song, Yuze Zhou, Jianzong Zheng, Xi Du, Kai Xing and Yulin Yang*

KEYWORDS: Schiff base, detect trace water, water-activated hydrogen-bonding crosslinking AIE, theoretical calculations, wide detection range.

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. of China Corresponding Author: * Ruiqing Fan and Yulin Yang E-mail: [email protected] and [email protected]

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ABSTRACT: The development of efficient sensors for detection of the water content in a wide detection range is highly desirable for balance many industrial processes and products. Presented herein are six novel different substituted of Schiff base Zn(II) complexes, which exhibit the remarkable capability to detect traces of water in wide linear range (the most can reach to 0–94%, v/v), low detection limit of 0.2% (v/v) and rapidly response time of 8s in various organic solvents by virtue of an unusual water-activated hydrogen-bonding crosslinking AIE (WHCAIE) mechanism. As a proof-of-concept, WHCAIE mechanism is well explained by Single X-ray diffraction, absorption spectra, fluorescence spectra, dynamic light scattering, the 1HNMR spectra and theoretical calculations. In addition, the molecules demonstrated their application for the detection of humidity (42–80%). These Schiff base Zn(II) complexes become one of the most powerful water sensors known due to their extraordinary sensitivity, fast response and their wide detection range of for water.

INTRODUCTION It is well known fluorescence properties will be influenced by the intermolecular

interaction, molecular conformation and packing arrangement of the same molecules. Aggregation-induced emission (AIE)1-3 is a phenomenon which can lead to the enhancement of fluorescence emission intensity through the formation of aggregates. To illustrate the AIE behaviors elaborately, a handful of mechanisms have been put forward,

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for instance, restriction of intramolecular vibration (RIV),9 restriction of

intramolecular rotation (RIR),10 formation of J-aggregates11, restriction of intramolecular motion (RIM),12 and hydrogen bonding-assisted formation of aggregates.13 Among the known AIE-active complexes, one interesting system is the Schiff base complexes,14-15 which integrates the molecular aggregation with intramolecular hydrogen bonding. The enhancement of fluorescence emission in the aggregate state was first reported by Tang and his coworkers.5-6 The AIE phenomenon has been used to develop fluorescence sensors for gases, pH, explosives and hazardous materials.16 17-18 But reports on AIE-active complexes detecting water are rare. Water as an impurity in many common organic solvents, which will lead to lowering of the yields of products or quenching of reactions or may cause catastrophic dangers and failures, such as fires and explosions.19 Therefore, detection of water is an urgent problems needed to be solved in organic synthesis and in the industrial 2


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applications alike. The most popular method for detection of water in solvents is the standard Karl Fischer method,20 The most popular method for the detection of water in solvents is the standard Karl Fischer method, as a traditional and classic method, it features universal detection range of the method (0.001–100% H2O) and high sensitivity, but it has some short-comings, for instance, instability, time-consuming procedure and costly instrumentation. So far a large number of sensors possessing outstanding advantages including in situ monitoring, fast response and low cost

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have been reported,22-24 but improving the detection limit is most of them focused on, while ignoring the broadening of the detection range. For instance, Citterio et al. reported a water sensor in a very low detection limit of 0.001% (v/v), but the detection linear range was below 1% (v/v) in different solvent media.25 In fact, developing a water sensor with wide range detection is a requisite for many products and industrial processes. For instance, water, the most impurity in crude acetonitrile, ethanol, methanol and common organic solvents obtained from chemical reactions, which is most important parameter to balance productivity and economic performance. Generally speaking, the volume percentage of water is not less than 15%. Under the circumstances, fluorescent sensors with narrow detection range cannot accomplish this monitoring. Therefore, exploring water fluorescent sensors with broad response range is essential to our researchers. Herein, we presented a series of different substituted of Zn(II) AIE organic molecules, Zn1–Zn6, [ZnL1Cl2] (Zn1), [Zn(L2)2(NO3)2] (Zn2), [ZnL3Cl2] (Zn3), [ZnL4Cl2] (Zn4), [ZnL5Cl2] (Zn5), [ZnL6Cl2] (Zn6),which are synthesized based on the corresponding six ligands (E)-R-N-((quinoline-2-yl)ethylidene)aniline, where R = 4–NO2, L1; H, L2; 2–CH3, L3; 2,6–(CH3)2, L4; 2,6–(C2H5)2, L5; 2,6–(i-C3H7)2, L6) to detect

water in

different organic

solvents,

including dimethyl

sulfoxide,

dichloromethane and methanol with broad detection ranges. Interestingly, the molecule of Zn(II) complexes demonstrated an aggregation-induced enhancement emission(AIE) response to water. In spite of the fluorescence enhancement of Zn(II) complexes being attributed to the AIE phenomenon, the mechanism is speculated to the novel water-activated hydrogen-bonding crosslinking AIE (WHCAIE), not the familiar formation of J-aggregates or RIR. The chlorine atoms and oxygen atoms of Zn(II) complexes can interact with water through intermolecular hydrogen bonding to form large crosslinking networks between water molecules and Zn(II) complexes, resulting in the water-activated aggregates of Zn(II) complexes (Scheme 1). These 3



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phenomena indicate that Zn(II) complexes can act as a fluorescent ‘‘ribbon’’ of water.

Scheme 1 Structure of Zn1 and the plausible crosslinking network in the presence of water.

RESULTS AND DISCUSSION The six Schiff base (E)-R-N-((quinoline-2-yl)ethylidene)aniline ligands (where R

= 4–NO2, L1; H, L2; 2–CH3, L3; 2,6–(CH3)2, L4; 2,6–(C2H5)2, L5; 2,6–(i-C3H7)2, L6) were

readily

prepared

in

a

one-step

procedure

by

condensation

of

2-quinolinecarboxaldehyde with their corresponding Schiff base ligand in a 1:1 molar ratio in methanol/formic acid (catalyst) mixture solution (see Scheme 2). Yields for the six Schiff base ligands varied from 74 to 92%. Using the self-assembled method, these ligands were reacted with zinc salt to obtain a series of complexes, namely, Zn1–Zn6. X-ray quality single crystals of six Zn(II) complexes grown from sluggish evaporation of acetonitrile solutions and readily obtained in good yields within the range of 55−77%. Detailed synthesis process is given in the experimental section. All the complexes are stable at room temperature in the solid state when exposure them to air. The complexes are soluble in the majority of organic solvents, such as dichloromethane (CH2Cl2), dimethylsulfoxide (DMSO), Dimethylformamide (DMF), acetonitrile (CH3CN), ethanol (CH3CH2OH) and methanol (CH3OH). The 1H NMR spectra of the ligands L1– L6 and the six Zn(II) complexes were recorded in d6–DMSO at 298K (Figure S1−12, Supporting Information). In the 1H NMR spectra, the chemical locations of the protons in the complexes are different from that of non-coordinated ligands. The infrared spectra of these six Zn(II) complexes (Figure S13−14, Supporting Information) are similar to that of the corresponding ligands and the main IR affiliations of bands are given in the experimental section. For the IR spectra of L1–L6 ligands, C=N bonds is further demonstrated by the existence C=N peaks in a range of 1623–1643 cm–1. These 4


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bands undergo negative shifts of 2–11 cm–1 in the complexes, which may be due to the nitrogen of the imine atom coordinate with to the metal atoms.26-28 Four bands near 1479 (ν1), 1295 (ν4), 1000 (ν2), 838 (ν3) cm–1 in the IR spectra of the complex 2 could be attributed to vibrations of the coordinated nitrate group (respectively the vibrations ν1, ν4, ν2, ν3). The difference between the 1479 (ν1) and the 1295 (ν4) peak positions is about 190 cm–1 (typical for bidentate nitrate groups).29 To affirm the structures of ligands and six complexes, single crystals were obtained and analyzed by Single Crystal X-ray Diffraction. The crystallographic data for ligands L2, L4, L6 and complexes Zn1–Zn6 are listed in detail in Table 1. Description of the structures Structural analysis of ligands L2, L4 and L6 Yellow bulk crystals of the ligands L2, L4 and L6 are recrystallized from methanol. The crystal structure of ligands L2, L4 and L6 are shown in Figure S15 (Supporting Information). The double-bond distance of the imino linkage N2–C10 [C=N distance are 1.251, 1.257 and 1.255 Å, respectively] are in agreement with the values of the Schiff base which have been published previously.30 The dihedral angle between the quinoline and benzene rings are 22.9°, 57.7° and 56.1°, respectively.

Scheme 2 Syntheses routes of ligands and complexes. 5



Structural analysis of Zn1 The N atoms of ligand L1 are coordinated with ZnCl2 to form mononuclear [ZnL1Cl2] (Zn1). Complex Zn1 crystallizes (yellow rectangular block crystals) in the monoclinic space group P21/c. As shown in Figure 1a, the asymmetric unit of complex Zn1 includes one metal Zn(II) atom, one L1 ligand and two chlorine anions. It is noteworthy that introducing the Zn(II) ion come into being a five-membered ring after coordinated, resulingt in coplanar with intrinsic quinolone ring and indirectly brings about the extended conjugate. Using Addison’s model,31-32 the coordination geometry of the center zinc atom in Zn1 (τ4 = 0.745 for Zn1) can be better described as a tetrahedral (Figure S16a, Supporting Information). In complex Zn1, the quinoline ring and the phenyl rings of L1 ligand are twisty and the dihedral angle is about 20.4° (Figure S16b, Supporting Information). The average bond lengths around the center zinc atom of Zn–N and Zn–Cl are 2.080(2) and 2.201(7) Å, respectively. Simultaneously, the bond angles are in the range of 80.1–119.1°. The selected (just related to the center metal) bond lengths and bond angles of complex Zn1 are shown a clear statement of account in Table S1 (Supporting Information). Single crystal analysis shows that the H···Cl distances (2.797 Å and 2.865 Å) between the chlorine atoms and the CH atom is shorter than the summation of the van der Waals radii for H atom and Cl atom (ca. 1.2 Å for H, 1.75 Å for Cl), and the C–H···Cl angles are 151.0° and 123.3°, which indicates a satisfactory intermolecular hydrogen bonding.33-37 First, the independent units are linked through C10–H10A···Cl1 and C10–H10A···Cl2 hydrogen bonding interactions to generate a 2D layer (Figure 1b). The 2D layer combining with the hydrogen bonding interactions C15–H15A···O1 further extends into three-dimensional supramolecular network (SMOFs) (Figure 1c). Intermolecular π⋯π stacking interactions exist with distances of 3.615 Å in head-to-tail manner (J-type) between the two quinoline rings, which provides further stabilization in complex Zn1 (Figure S17, Supporting Information). The detailed data of π···π interactions and C–H···O/Cl hydrogen bonds for Zn1 are listed in Table 2. Furthermore, regarding the C–H···Cl/O interactions as linkers, and the mononuclear unit in Zn1 as 4-connected node, so that complex Zn1 exhibits 3D cds topology with the point symbol of {65·8} (Figure 1d).

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Figure 1 (a) Asymmetric unit cell of Zn1 with the crystallographic numbering scheme. Some H atoms omitted for clarity. (b) 2D layer and (c) 3D network structure in Zn1. Red dotted lines represents the C–H···O/Cl interactions. (d) Topological structure of the 3D network for Zn1.

Structural analysis of Zn2 _

Complex [Zn(L2)2(NO3)2] (Zn2) crystallized in the triclinic P1 space group. As shown in Figure 2a, for a complete molecule, two equivalent [L2] moieties are bridged to the central Zn2+ ion by one µ1-ƞO2 NO3− anions, resulting in the formation of a mononuclear structure. The central Zn2+ ions is six-coordinated with four N atoms (N1, N2, N3, N4) from Schiff base ligand L2 and two O atoms from the coordinated NO3− anion (O1 and O2) at the axial position, resulting in a distorted octahedral geometry. In addition, there existing an uncoordinated NO3− anion in the system. Zn2 have a nonplanar configuration and the zinc atom stuck up out of the ligand L2.The dihedral angle between quinoline and benzene rings is 60.1/67.4°. The independent units are linked through C5–H5A···O2, C10–H10A···O4, C16– H16A···O5, C21–H21A···O6 and C23–H23A···O4 hydrogen bonding interactions to generate a 3D network (Figure 2b). There also exist intermolecular π···π interactions with separations of about 3.693 Å between two neighbouring quinoline rings (Figure 2c). Regarding the C–H···O hydrogen bonding interactions as linkers, and the mononuclear unit in Zn2 as 6-connected node, Zn2 exhibits 3D pcu topology with the point symbol of {412·63} (Figure 2d). 7



Figure 2 (a) Asymmetric unit cell of Zn2 with the crystallographic numbering scheme. (b) 3D network structure, Some H atoms have been omitted for clarity. Blue dotted lines represent the C– H···O interactions. (c) Intermolecular π···π interactions in Zn2. (d) Topological structure of the 3D network for Zn2.

Structural analysis of Zn3 The N atoms of ligand L3 are coordinated with ZnCl2 to form mononuclear [ZnL3Cl2] (Zn3), as shown in Figure 3a. The coordination geometry of the center zinc atom in Zn3 (τ4 = 0.745) can be better described as a tetrahedral. Likewise, the M–N distances of imino are relatively shorter than that of quinoline. The average distances of the M–N bonds are 2.142 Å. The dihedral angle between the quinoline and benzene rings is 80.3°, which is serious twisty compared with that of the ligand L3. Intermolecular π···π interactions with separations of about 3.715 Å and 3.655 Å form a 2D layers (Figure 3b). Single crystal analysis show that complexes Zn3 are three-dimensional network structures (Figure 3c) constructed by the C–H···Cl hydrogen bonding and π···π interactions. The resulting 3D nets may be simplified into a 6-connected pcu topology (the point symbol is {412·63}) for Zn3, as shown in Figure 3d.

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Figure 3 (a) Asymmetric unit cell of Zn3 with the crystallographic numbering scheme. (b) Intermolecular π···π interactions in Zn3. (c) 3D network structure, Some H atoms have been omitted for clarity of Zn3. Dotted lines represent the C–H···Cl interactions and π···π interactions. (d) Topological structure of the 3D network for Zn3.

Structural analysis of Zn4–Zn6 Single crystal X-ray diffraction analysis reveals that complexes Zn4–Zn6 have analogous comparable structure unit as shown in Figure 4. In complexes Zn4–Zn6, each asymmetric unit consists of one independent center metal Zn2+ ion, one coordinated L4, L5 or L6 ligand and two chloride ions. The Zn (II) center all adopts a distorted tetrahedral geometry (τ4 = 0.736 for Zn4, 0.754 for Zn5, 0.743 for Zn6, respectively). Complex Zn4–Zn6 have dihedral angles of 73.7°, 70.1° and 76.3° between the quinoline ring and the benzene ring, which indicates that the ligand L4 displays serious twisty structure. For Zn4, π···π interactions link the neighbouring molecule to form a 2D layer (Figure 4b), which may be show in a different way simplified as a 2D 3-connected hcb topology with the point symbol of {63}, as shown in Figure S18 (Supporting Information). For Zn5，regarding the π···π stacking interactions and the C–H···Cl hydrogen bonding as linkers, the mononuclear unit in Zn5 as 4-connected node, Zn5 exhibit 2D (Figure 4d) sql topology with the point symbol of {44.62} (Figure S19, Supporting Information). Two adjacent molecules are 9



linked by the hydrogen bonding interactions to form a 1D chain for Zn6 (Figure 4f).

Figure 4 Asymmetric unit cell of (a) Zn4 (c) Zn5 (e) Zn6 with the crystallographic numbering scheme. The π···π stacking interactions and intermolecular C–H···Cl hydrogen bonds in (b) Zn4 (d) Zn5 (f) Zn6.

Effect of different substitution on supramolecular architecture Taking the structures into account, dramatic influence of substitutions on the steric hindrance of the complexes was occurring: with changing different substituents on ligands, the steric hindrance of the complexes gradually transforming. The complexes display better coplanarity with the nitryl substitutions and no substitutions result in 3D supramolecular architectures. [dihedral angles between quinoline and benzene rings in Zn1 and Zn2 are 20.4° and 60.1°, respectively]. But complexes with aryl substitutions (Zn3–Zn6) display twisted conformation with dihedral angles are more than 69.7° result in 3D →1D supramolecular architectures. That is to say, methyl substituents [2–CH3, 2,6–(CH3)2, 2,6–(C2H5)2, and 2,6–(i-C3H7)2] could hinder molecular assemblies result in larger steric hindrance of ligands, and influence the final supramolecular architectures. On the other hand, X-ray diffraction analyses revealed that in complexes Zn1– Zn6 are all exsiting C−H···Cl/O hydrogen bonds generated by chlorine atoms and oxygen atoms to construct 3D→1D supramolecular frameworks, which indicate that 10


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these molecules have a good capability to form C–H···O/Cl hydrogen bonding interactions based on the O atoms and Cl atoms. As aforementioned, structure of these complexes can form C–H···O/Cl hydrogen bonding interactions, so what property the good capability to form C–H···O/Cl hydrogen bonding interactions based on the O atoms and Cl atoms can produce? It inspired us to further exploration.”

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Table 1 Crystallographic and structural data for three ligands L2, L4, L6 and complexes Zn1–Zn6.

CCDC No. formula Mr crystal system space group a [Å] b [Å] c [Å] α [˚] β [˚] γ [˚] Volume [Å3] Z Dc [g·cm–3] µ [mm–1] F (000) Θ range [˚] h range k range l range data/restraints/params GOF R1, wR2[I>2σ(I)]a R1, wR2[all data]a ∆ρmax, ∆ρmin [e·Å–3] [a]

L2 1527512 C16H13N2 233.28 Triclinic

L6 1527510 C22H26N2 318.45 Monoclinic P21/c

Zn1 1527515 C16H14N3Cl2O2Zn 414.56 Monoclinic P21/c

Zn2 1527509 C32H24N2O6Zn 653.96 Triclinic

P1

L4 1527511 C18H16N2 260.33 Monoclinic P21/n

11.808(2) 5.3450(11) 19.780(4) 90 90 90 1244.5 4 1.245 0.074 492 3.46 – 24.99 –14 ≤ h ≤14 –6 ≤ k ≤ 6 –23 ≤ l ≤ 23 2127 / 0 / 163 1.098 0.0445, 0.1327 0.0620, 0.1414 0.353, –0.119

13.1018 (18) 8.2767(11) 14.528(2) 90 113.434(2) 90 1445.5(3) 4 1.196 0.071 552 2.99 – 25.00 –15 ≤ h ≤15 –9 ≤ k ≤9 –17 ≤ l ≤ 17 2455 / 1 / 181 1.034 0.0448, 0.1324 0.0626, 0.1440 0.168, –0.174

13.25(3) 11.094(3) 12.227(3) 90 93.562(3) 90 1885.3(8) 4 1.122 0.065 688 1.47 – 24.99 –15 ≤ h ≤16 –13 ≤ k ≤13 –14 ≤ l ≤14 3197 / 0 / 210 1.063 0.0490, 0.1205 0.1100, 0.1461 0.300, –0.154

12.951(3) 10.193(2) 13.757(3) 90 114.13(3) 90 1657.5(6) 4 1.661 1.818 836 3.25 – 27.54 –16 ≤ h ≤16 –13 ≤ k ≤12 –17 ≤ l ≤17 3806 / 0 / 217 0.858 0.0361, 0.1049 0.0601, 0.1231 0.353, –0.398

_

P1

Zn3 1527514 C17H14N2Cl2Zn 382.57 Monoclinic P21/n


Zn5 1527513 C20H19Cl2N2Zn 423.64 Monoclinic P21/n


10.7791(6) 11.2397(7) 13.7090(8) 102.360(5) 107.945(5) 103.080 (5) 1465.16(15) 2 1.482 0.895 672 3.16 – 27.56 –14 ≤ h ≤12 –14 ≤ k ≤11 –17 ≤ l ≤17 6687 / 2 / 406 1.049 0.0667, 0.1680 0.1006, 0.1905 0.628, –0.548

7.7156(3) 15.3428(6) 13.7305(53) 90 94.3010(10) 90 1620.82(11) 4 1.568 1.841 776 2.96 – 27.54 –10 ≤ h ≤10 –19 ≤ k ≤19 –17 ≤ l ≤17 3581 / 0 / 199 1.045 0.0408, 0.0921 0.0671, 0.1115 0.403, –0.288

8.1659(16) 15.945(3) 13.549(3) 90 100.63(3) 90 1733.8(6) 4 1.519 1.724 808 3.32 – 27.58 –100 ≤ h ≤10 –20 ≤ k ≤20 –16 ≤ l ≤17 3824 / 0 / 208 1.009 0.0333, 0.0751 0.0650, 0.0854 0.303, –0.236

10.0862(3) 17.0415(7) 11.4066(4) 90 107.945(5) 100.033 (3) 1930.63(12) 4 1.458 1.553 868 3.81 – 27.56 –13 ≤ h ≤8 –19 ≤ k ≤22 –11 ≤ l ≤14 6291 / 415 / 208 1.161 0.0656, 0.1450 0.0959, 0.1596 1.086, –2.040

9.8409(10) 18.3984(19) 12.2578(13) 90 100.2890(10) 90 1620.82(11) 4 1.377 1.378 936 2.69 – 27.61 –12 ≤ h ≤12 –23 ≤ k ≤23 –15 ≤ l ≤15 5031 / 0 / 244 0.958 0.0361, 0.0856 0.0643, 0.0985 0.275, –0.228

_

R1 = ∑||Fo | – |Fc||/∑|Fo|; wR2 = [∑[w (Fo2 – Fc2)2]/∑[ w (Fo2)2]]1/2.

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Table 2 The geometrical parameters of π···π stacking interactions and C–H···O/Cl hydrogen bonding for complexes Zn1–Zn6.

Zn1

Zn2

Zn3

Zn4

Zn5

substitute

coordination geometry

dihedral angle (deg)a

X–H···Y

X–H/Å

H···Y/Å

X···Y/Å

X–H···A/°

dimension

4–NO2

tetrahedral

20.1°

C10–H10A···Cl1

0.930

2.864

3.344

123.395(2)

3D

{65·8}

C10–H10A···Cl2

0.930

2.933

3.845

166.732(1)

C15–H15A···O1

0.930

2.418

2.699

157.236

C5–H5A···O2

0.930

2.505

3.342

149.953(5)

3D

{412·63}

C10–H10A···O4

0.930

2.413

3.241

148.331(4)

C16–H16A···O5

0.930

2.445

3.128

130.299(7)

C21–H21A···O6

0.930

2.427

3.284

153.121(6)

C23–H23A···O4

0.930

2.448

3.288

150.273(1)

C2–H2A···Cl2

0.9305

2.916

3.787

156.452(3)

3D

{412·63}

2D

{63}

173.427

2D

{44.62}

155.164

1D

–

H

2–CH3

2,6–(CH3)2

2,6–(C2H5)2

octahedron

tetrahedral

tetrahedral

tetrahedral

60.1/67.4°

80.3°

73.7°

70.1°

πCg1···πCg1

3.715

πCg2···πCg2

3.655

πCg1···πCg2

3.695

πCg2···πCg2

3.586

C10–H10A···Cl2

0.9300

2.909

πCg1···πCg1 Zn6 a

2,6–(i-C3H7)2

tetrahedral

76.3°

C19–H19A···Cl1

3.834

point symbol

3.619 0.9300

Between the quinoline ring and benzene ring; Cg1 = quinoline ring; Cg2 = benzene ring;

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2.922

3.814


Luminescent Properties The d10 metal complexes have been reported in recent years leaning to exhibit solid state emissions with modulated wavelengths and intensities.38-42 Hence, the luminescence properties of the ligands L1–L6 and the corresponding Zn1–Zn6 complexes were examined in the solid state at 298K. The excitation spectra of complexes Zn1–Zn6 in the solid state were also recorded (Figure S20, Supporting information). For the excitation spectra, there existing two obvious absorption band located at about 320nm and 350nm. Simultaneously, the free ligands L1–L6 exhibit emission bands centered at 493, 496, 518, 521, 511, 500 nm (Figure S21a, Supporting Information), respectively, which may be attributed to the π*–π transitions. For the complexes Zn1–Zn6, the emission bands are observed at 498, 501, 528, 533, 521 and 515 nm (Figure S21b, Supporting Information), respectively. Due to the emission bands are similar to that of the corresponding ligand, the emissions of these six Zn(II) complexes may be attributed to the intraligand transitions.44-45 Notably, the maxima emission wavelengths of the ligands and the complexes should be on the base of both the electron-donating ability of the R group and the conjugative extent of the ligand (free or in the complexes). The conjugative extent of the ligands in the order: L6 < L5 < L4 < L3 < L2 < L1, and the electron-donating ability of the R group in the order: (– NO2 < H

Novel Hydrogen-Bonding Cross-Linking Aggregation-Induced Emission

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