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Temperature-Driven Crystal-to-Crystal Transformations and Luminescence Properties of Coordination Polymers Built with Diphenyldibenzofulvene Based Ligand Qiyang Li, Xiuju Wu, Xiaoli Huang, Xue Xiao, Shuping Jia, Zhihua Lin, and Yonggang Zhao Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01392 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 20, 2018

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Crystal Growth & Design

Temperature-Driven Crystal-to-Crystal Transformations and Luminescence Properties of Coordination Polymers Built With Diphenyldibenzofulvene Based Ligand Qiyang Li, a Xiuju Wu,a Xiaoli Huang,a Xue Xiao,a Shuping Jia,a Zhihua Lin*, b and Yonggang Zhao*, a a

Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 211816, PR China b

College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816, PR China

KEYWORDS: Coordination polymers, Temperature effect, Crystal-to-Crystal Transformations, Fluorescence

ACS Paragon Plus Environment

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT. A propeller-shaped ligand di(4-carboxyphenyl)dibenzofulvene (H2L) was used for the construction of coordination polymers. From a single starting materials with only varying the temperature, four coordination polymers, formulated as [Zn(L)(H2O)]·2DMF (CP-1), [Zn(L)(H2O)]·2DMF·H2O (CP-2), [Zn(L)(H2O)]·DMF (CP-3) and [Zn3(L)2(µ3-OH)2]·4H2O (CP-4) have been successfully synthesized in the relatively low temperature range of 25 to 115 °C. The structural diversity can be ascribed to the temperature effect and the conformational flexibility of the ligand. Upon heating, CP-1 or CP-2 undergoes a series of solvent-associated crystal-to-crystal transformations successively to CP-3, finally the thermodynamically stable CP-4. In addition, [Zn2(L)2(BPY)]·DMF·3.5H2O (CP-5) can be synthesized in a crystal-tocrystal transformation manner from CP-2 with addition of the auxiliary ligand 4,4’-bipyridine (Bpy). Owing to the H2L ligand possessing the feature of aggregation-induced emission (AIE), the solid-state fluorescence of CP-1 to CP-5 has also been studied in details.

INTRODUCTION In the past two decades, remarkable progress has been made in the area of coordination polymers (CPs) or metal-organic frameworks (MOFs) because of their fascinating structural related properties and multi-functionality,1-3 which make them highly promising for various applications,4-8 including but not limited to gas storage, separation science, catalysis, magnetism, drug delivery and luminescence based sensing.9-18 In this field, one major goal is the design and preparation of solid materials with desired structures, as the properties being directly related to the superstructure with the materials. Hence, general strategies for a more controlled synthesis of coordination polymer for specific applications need to be developed. Despite the vigorous growth in the library of CPs/MOFs, rational design and synthesis of target materials are still of great challenge, since structural uncertainty or diversity is an intrinsic


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characteristic in crystal engineering of coordination polymers.19-22 Normally, coordination polymers are prepared under harsh conditions (hydrothermal/solvothermal methods), and the product is directly controlled by many factors, including reactants, solvents, counterions, templates, concentration, pH, reaction time and reaction temperature et al.23-37 A subtle change of one of the above factors may lead to a drastic change in the structure with the inherent topologies and physical properties. Among the aforementioned factors, reaction temperature is the most used variable, since it can influence not only the conformation of organic ligand, but also the reaction energy barrier in reaction thermodynamics and reaction rate in reaction kinetics.38-41 On the other hand, upon the tuning of temperature, single-crystal to single-crystal (SCSC) transformations can be observed in some cases,42-46 mainly owing to the temperature induced cleavage or formation of bonds, removal/movement of guest, sliding of layers.42-43,47-51 Therefore, temperature-tuning is a very useful strategy for the synthesis of coordination polymer with different dimensionality, topology, as well as intriguing properties, Herein, we report a series of Zn(II) based coordination polymers with using a propeller-shaped diphenyldibenzofulvene based dicarboxylate ligand, di(4-carboxyphenyl)dibenzofulvene (H2L) as linker, and the investigation of the temperature effect on the structures. The H2L ligand exhibits a modicum of flexibility, from which different conformers can be achieved and utilized in accessing great structural diversity. Besides, the ligand also shows a typical aggregationinduced emission (AIE) phenomenon,52-55 which can feature the luminescence to the resulting materials. From a single starting materials with only varying the reaction temperature, four coordination polymers with diverse structures, formulated as [Zn(L)(H2O)]·2DMF (CP-1), [Zn(L)(H2O)]·2DMF·H2O (CP-2), [Zn(L)(H2O)]·DMF (CP-3) and [Zn3(L)2(µ3-OH)2]·4H2O (CP-4), are reported herein. The temperature-driven crystal-to-crystal structural transformations


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between these four coordination polymers are investigated. Also, another compound [Zn2(L)2(Bpy)]·DMF·3.5H2O (CP-5) containing an auxiliary ligand Bpy is reported. In addition, luminescent properties of these coordination polymers have also been explored. EXPERIMENTAL SECTION Materials and Measurements. All commercially available starting materials, reagents and solvents were used as supplied, unless otherwise stated, and purchased from Energy Chemical and Alfa Aesar. Standard column chromatography methods were employed for a chromatographic separation purpose. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8-ADVANCE with Cu Kα radiation (λ = 1.54 Å). (VT)-PXRD measurements were performed on a SmartLab X-Ray diffractometer with Cu Kα (λ= 1.54 Å), equipped with a VT stage. The sample was held at the designated temperatures 20 min between each scan. Infrared spectra of the CP-1 to CP-5 were measured on a Bruker ALPHA spectrometer between 4000 and 400 cm−1, equipped with a diamond attenuated total reflectance (ATR) accessory. Thermogravimetric analysis (TGA) was measured under a nitrogen stream by using with a heating rate of 10 °C/min on a Mettler Toledo equipment. 1HNMR spectra and

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C NMR were

recorded on a Varian mercury-plus 400 spectrometer. Fluorescence spectra were obtained by using a F-4600 fluorescence spectrophotometer. Absolute quantum yield data were obtained using an Edinburgh Instruments F920 analytical spectrometer equipped with integrating sphere using BaSO4 as white standard. Elemental analysis of C, H and N were carried out on a Vario EL Cube elemental analyzer. Synthesis of [Zn(L)(H2O)]·2DMF (CP-1). A mixture of Zn(NO3)2·6H2O (29.7mg, 0.1mmol), H2L (20.9mg, 0.05mmol), H2O (3 mL), and DMF (8 mL) was sealed in a 20 mL Pyrex vial. The mixture was first stirred under sonication for 10 min, then the mixture was put at the room


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temperature for two weeks, colorless block crystals were filtrated and dried in air (yield: 62% based on H2L). Elemental analysis of CP-1, calculated (%): C 63.21, N 4.34, H 4.99; found: C 62.91, N 4.24, H 4.87. IR: 3352 (br), 3053 (w), 2958 (w), 2930 (w), 1652 (m), 1583 (S), 1536(s), 1504 (m), 1444 (m), 1399 (s), 1377(s), 1175 (m), 1107 (m), 1020 (m), 835 (m), 777 (s), 730 (s), 690 (s), 604 (m), 496 (m), 839 (m). Synthesis of [Zn(L)(H2O)]·2DMF·H2O (CP-2). A mixture of Zn(NO3)2·6H2O (29.7mg, 0.1mmol), H2L (20.9mg, 0.05mmol), H2O (3mL), and DMF (8 mL) was sealed in a 20 mL Pyrex vial. The mixture was first stirred under sonication for 10 min, then heated at 40°C for 3 days and then gradually cooled to room temperature. Colorless block crystals of CP-2 were collected by filtration and washed with DMF, drying in air (yield: 50% based on H2L). Elemental analysis of CP-2, calculated (%): C 61.50, N 4.22, H 5.16; found: C 61.32, N 3.96, H 5.01. IR: 3354 (br), 3053 (w), 2960 (w), 2927 (w), 1652 (m), 15843 (S), 1538(s), 1505 (m), 1445 (m), 1400 (s), 1373(s), 1172(m), 1104 (m), 1017 (m), 835 (m), 780 (s), 729 (s), 695 (s), 670 (m), 604 (m), 495 (m), 437 (m). Synthesis of [Zn(L)(H2O)]·DMF (CP-3). The same synthetic method as that for CP-2 was used except that the temperature was changed to 70°C. The colorless block crystals of CP-3 were isolated by washing with DMF and drying in air (yield: 70% based on H2L). Elemental analysis of CP-3, calculated (%): C 64.99, N 4.40, H 2.44; found: C 64.85, N 3.94, H 2.23. IR: 3337 (br), 3059 (w), 2960 (w), 2933 (w), 2873(w), 1652 (m), 1575(s), 1530(s), 1504 (m), 1446 (m), 1434 (m), 1400(s), 1378(s), 1177 (m), 1106 (m), 1017 (m), 830 (m), 784 (s), 730 (s), 631 (m), 557(m), 441 (m). Synthesis of [Zn3(L)2(µ3-OH)2]·4H2O (CP-4). The same synthetic method as that for CP-2 was used except that the temperature was changed to 115°C, Colorless tabular crystals of CP-4


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were filtrated and washed with DMF, drying in air (yield: 67% based on H2L). Elemental analysis of CP-4, calculated (%): C 59.26, N 0, H 3.73; found: C 59.01, N 0.09, H 3.41. IR: 3639(w), 3055 (w), 2827 (w), 2856 (w), 1652 (m), 1584(s), 1545(s), 1506 (m), 1444 (m), 1398(s), 1384(s), 1173 (m), 1089(m), 1014 (m), 830 (m), 782 (s), 773 (s), 731 (s), 707(m), 678(m), 605(s), 497(m), 458 (m), 418(m). Synthesis of [Zn2(L)2(Bpy)]·DMF·3.5H2O (CP-5). The same synthetic method as that for CP3 was used except that the Bpy (3.9mg, 0.025mmol) was added. The colorless block crystals of CP-5 were isolated by washing with DMF and drying in air (yield: 75% based on H2L). Elemental analysis of CP-5, calculated: C 65.99, N 3.35, H 4.33; found: C 65.74, N 3.36, H 4.21. IR: 3064 (w), 2927 (w), 2850 (w), 1665 (m), 1632(s), 1613(s), 1552 (m), 1502 (w), 1486(m), 1445(m), 1398(s), 1220 (m), 1168(m), 1096 (m), 1012 (m), 890 (m), 830 (m), 783 (s), 773(s), 723(s), 705(m), 643(m), 528(m), 443(m). Experiments of structural transformations: Conversions between CP-1, CP-2, CP-3 and CP-4: Taking the conversion from CP-1 to CP-3 as an example. A mixture of CP-1 (40mg), H2O (3 mL) and DMF (8 mL) was sealed in a 20 mL Pyrex vial and heated at 70°C for 3 days. Colorless microcrystal of the product (CP-3) were filtrated and washed with DMF, then dried in air for further characterization. The same synthetic method was used for other structural transformations except that the temperature was changed to that target product favored. Conversion of CP-2 to CP-5: A mixture of CP-2 (40mg), Bpy (9.4mg, 0.06mmol), H2O (3 mL), and DMF (8 mL) was sealed in a 20 mL Pyrex vial and heated at 70°C for 3 days. Colorless microcrystal of product (CP-5) were filtrated and washed with DMF, then dried in air for further characterization.


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Crystal Structure Determination. The single crystal X-ray diffraction measurements were experimented on a Bruker Apex2 CCD diffractometer, operating at 50 kV and 30 mA and sealed tube X-ray source with Mo-Kα radiation (λ =0.71073 Å). The structures of CP-1 to CP-5 were solved by direct methods and a full-matrix least-squares refinement was performed with the SHELXTL program.56 The reflection data were corrected by using SADABS program. For all nonhydrogen atoms anisotropic thermal parameters were applied, and all hydrogen atoms of organic ligands were calculated and added at ideal positions. The crystallographic data of CP-1 to CP-5 are given in Table 1, and selected bond lengths and angles are given in Table S1 to Table S5 (Supporting Information). Table 1. Crystallographic Data for CP-1 to CP-5 compound

CP-1

CP-2

CP-3

CP-4

CP-5

C34 H32 N2 O7 Zn 645.98

C34 H34 N2 O8 Zn 664

C31 H25 N O6 Zn 572.89

C56 H42 O14 Zn3 1135

C69 H54 N3 O12.5 Zn2 1255.89

Monoclinic

Triclinic

Triclinic

Orthorhombic

Triclinic

P 21/n

Pī

Pī

Cmcm

Pī

11.3277(19)

9.304(1)

9.965(18)

38.074(14)

13.042(4)

18.625(3)

12.874(2)

10.552(19)

6.146(2)

14.041(5)

14.984(3)

15.045(2)

13.73(3)

26.564(10)

17.697(6)

90

72.692(2)

85.91(3)

90

78.271(4)

102.842(4)

85.774(2)

69.62(2)

90

88.366(5)

90

69.098(2)

82.41(2)

90

81.156(5)

3082.2(9)

1606.1(4)

1341(4)

6216(4)

3135.2(18)

296(2)

296(2)

296(2)

296(2)

296(2)

4

2

2

4

2

1.392

1.373

1.419

1.213

1.33

0.848

0.818

1.04

1.204

0.83

1344

692

2320

1298

θ [º]

2.144-25.010

1.419-25.000

592 1.58225.000

1.870- 24.994

1.580-24.998

Reflections collected/unique

20005/5427

11434/5597

20980/ 2842

22480/10925

chem formula fw crystsyst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) temp (K) Z Dcalcd (g·cm−3) µ [mm-1] F [000]

9464/4664


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Goodness-of-fit 1.04 1.024 0.946 on F2 R1a, wR2b [I > 0.0533, 0.0359, 0.0502, 0.1097 0.0955 0.0998 2σ(I)] 0.0743 0.0259 0.1264 Rint a b 2 2 2 R1 = Σ||F0| − |Fc||/Σ|F0|. wR2 = [Σw(F0 − Fc ) /Σw(F02)2]1/2.

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0.983

0.947

0.0844, 0.2924

0.0658, 0.1665

0.056

0.068

RESULTS AND DISCUSSION Structure description of CP-1. CP-1 can be prepared at ambient condition. Single-crystal Xray diffraction analysis reveals that CP-1 crystallizes in monoclinic crystal system, space group P21/c, with a 2D layer structure constructed from a di-nuclear Zn(II) secondary building unit (SBU) (Figure 1). Of the di-nuclear SBU, each Zn(II) is square-pyramidally coordinated by four O atoms from four carboxylate groups and one O atom from water at the apical position, and two crystallographically equivalent Zn(II) ions share eight O atoms of four different L2- ligands to generate a paddle-wheel type Zn2(COO)4 cluster, seen commonly in many reported MOF structures.57-60 Each di-nuclear SBU is joined to four neighbouring SBUs through four L2- ligands in a gammadion cross fashion, to form a 2D layer with sql topology. From the topological viewpoint, this 2D layer structure can be simplified as a 4-connected network. The L2- ligands adopt bis-bidentate coordination mode in structure of CP-1, and each bridges two neighbouring SBUs. Due to the bent nature and containing rigid fluorene moieties, the ligands are set in upper or lower positions in each layer, to bond with the di-nuclear cluster, and give rise to a “quasibilayer” structure，other than a planar layer structure. It is noteworthy that close C−H…π interactions (distance of 2.67 Å) between two neighbouring L2- ligands are found in each layer. Furthermore, these layers are connected through the π−π stacking interactions between aromatic rings of fluorene (distance between two planes, 3.53 Å) from two adjacent layers, which lead to close layer-by-layer stacking and the formation of a 3D architecture.


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Figure 1. (a) and (b) 2D layer structure in CP-1 viewed along the a- and b-axis, respectively. Color code: Zn, turquoise; O, red, C, pale gray. (c) Topological 4-connected 2D net of CP-1. Hydrogen atoms and guest molecules have been omitted for clarity. Structure description of CP-2. CP-2 crystallizes in the triclinic crystal system of Pī space group, and exhibits 1D ∞–like chains (Figure 2). The basic SBU in CP-2 is also paddle-wheel type core structure. In the dizinc(II) unit, Zn(II) atoms exhibit similar coordination geometry to those in CP-1: each Zn(II) atom has a distorted square pyramidal coordination geometry, surrounded by four O atoms from four L2- ligands and one O atom from water at the axial position. Two L2- ligands bridge two adjacent dizinc(II) units to form an infinite looped doublechain structure. Moreover, the adjacent 1D double-chains are further bridged through π−π interactions between aromatic cycles of fluorene (distance between two planes, 3.66 Å), as well as C−H…π interactions (distance of 2.71 Å), to give a 3D structure.


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Figure 2. (a) 1D ∞–like chain structure in CP-2 viewed along the a-axis, and the paddle-wheel dizinc(II) SBU. (b) Packed 1D chains viewed along the b-axis. Color code: Zn, turquoise; O, red, C, pale gray. (c) Depiction of the 2D structure containing 1D channels in CP-2 along the a-axis. Hydrogen atoms and guest molecules have been omitted for clarity. Structure description of CP-3. Single-crystal X-ray diffraction analyses revealed that the CP3 crystallizes in the same crystal system as CP-2 with the same space group of Pī. CP-3 also features a 1D looped double-chain structure, and contains only one crystallographically independent Zn(II) center (Figure 3). Different with the paddle-wheel type di-nuclear core, in the dizinc(II) Zn2(COO)4 SBU of CP-3, each Zn(II) atom adopts a distorted tetrahedral coordination geometry, and is coordinated by three O atoms from three different carboxyl groups and one O atom from water. For the L2- ligand in CP-3, one carboxylate group adopts monodentate coordination mode, and the other one with bidentate mode, which makes it a µ3-bridge to link three Zn(II) atoms of two dizinc(II) units. Two L2- ligand link two neighbouring [Zn2(COO)4] units to form a M2L2 loop, and each [Zn2(COO)4] unit binds to two neighbouring units through


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four L2- ligands to form a 1D double-chains. The adjacent 1D chains are further stacked through C−H…π (distance of 2.90 Å) interactions into an 2D layer network in an interdigitated way, then adjacent 2D layers are closely packed through C−H…π interactions (distance of 2.80 Å) and OH⋯O H-bonding interactions (H⋯O distance of 1.88 Å) with the formation of 3D structure.

Figure 3. (a) 1D ∞–like chain structure in CP-3 viewed along the a-axis, and the dizinc(II) SBU. Color code: Zn, turquoise; O, red, C, pale gray. (b) and (c) Packed 1D chains viewed along the baxis and a-axis, respectively. Hydrogen atoms and guest molecules have been omitted for clarity. Structure description of CP-4. CP-4 crystallizes in the orthorhombic space group of Cmcm. The asymmetric unit is composed of two crystallographically independent Zn(II) atoms (with half and one-fourth occupancy, respectively), one ligand with half occupancy and one OH with half occupancy. CP-4 contains trizinc(II) units formed through the bridging carboxylate group of L2- ligand and µ3-OH group. In the trizinc(II) unit, Zn1 adopts a octahedral coordination geometry, and is bridged by four carboxylate O atoms of different ligands in the equatorial positions and two µ3–OH group occupying the axial positions. Zn2 adopts a tetrahedral


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coordination geometry, and is bridged by two carboxylate O atoms of different L2- ligands and two µ3–OH groups. Notably, the O atoms of both carboxylate and µ3-OH group bridge the trizinc(II) units to form a 1D Zn cluster chain running along the b-direction. The L2− ligand in CP-4 adopts a µ4-η1:η1:η1:η1 coordination mode and bonds to four Zn atoms (two Zn1 and two Zn2) from two adjacent 1D metal chains. Thus, via bridging the carboxylate groups and the µ3OH groups, the adjacent 1D cluster chains are further interconnected to form a 2D layer containing 1D rhombic-shaped channel along the b-direction (Figure 4). Besides, with the help of C-H…π interactions (distance of 2.80 Å) between the fluorene moieties of adjacent layers, the 2D layers are further stacked into a 3D architecture in an ABAB fashion, with rectangularshaped 1D channels along the b-direction.

Figure 4. (a) The 2D structure of CP-4 viewed along the a-axis, and (b) the 1D metal chains viewed along the c-axis. (c) Parallel packing 2D structure of CP-4 containing two types of 1D channel along the a-axis. Color code: Zn, turquoise; O, red, C, pale gray. Hydrogen atoms and guest molecules have been omitted for clarity.


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Structure description of CP-5. The crystal structure analysis reveals that CP-5 crystallizes in the triclinic space group of Pī. The asymmetric unit of CP-5 contains one Zn(II) ion, one L2− ligand and half of a Bpy molecule. In the paddle-wheel type dizinc(II) unit of CP-5, each Zn(II) atom has a distorted square pyramidal geometry, coordinated by four carboxylate oxygen atoms in the equatorial plane and one nitrogen atom from Bpy ligand in the axial positions. The adjacent dizinc(II) units are bridged through four L2− ligands to form a 1D loop double-chain structure. These 1D chains containing paddlewheel units are further pillared by Bpy linkers with forming a 2D network (Figure 5). Adjacent 2D networks are interlocked with each other to give a 4-fold interpenetrating structure, which is further stabilized by C-H…π interactions (distance of 2.94 Å) and π···π interactions between the fluorene rings of different 2D networks (distance between two planes, 3.29 Å). Notably, the CP-2 can be considered as the precursor of CP-5, and Bpy replace the coordinated water and link adjacent chains to form the 2D network in CP-5.


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Figure 5. (a) and (b) The 2D structure of CP-5 viewed along the b- and c-axis, respectively. Color code: Zn, turquoise; O, red, C, pale gray. (c) Representation of the 4-fold interpenetrating structure of CP-5 viewed along the a-axis. Hydrogen atoms and guest molecules have been omitted for clarity. Structural Transformations As has been noted, CP-1 to CP-5 can be synthesized at relatively low temperature (< 115°C) with small reaction temperature difference (e.g. 15°C between CP-1 and CP-2, 30 °C between CP-2 and CP-3). In addition, structural analysis shows some relevance of these coordination polymers, for instance, containing the same building unit for CP-1 and CP-2, featuring the similar 1D ∞-like double-chain structures for CP-2 and CP-3. It occurred to us that we could start the structural transformation studies (i.e., crystal-to-crystal), to provide vital clues toward the temperature effect on the structural diversity. After being dispersed in the mixture of DMF and water (v:v, 8:3)，heating at 70 °C for three days, CP-1 can experience a crystal-to-crystal phase transition to CP-3. Under similar conditions, CP-2 can be also completely converted to CP-3. Likewise, by simply increasing the temperature to 115 °C, CP-1, CP-2 and CP-3 in their mother liquor can all be finally converted to CP-4 (Scheme1). PXRD investigations confirmed these structural transformations and good crystallinity for all products. Therefore, the structures of CP-1 to CP-3 may all represent the stepwise evolution of CP-4, since that CP-1 or CP-2 undergoes a series of crystal-to-crystal isomerization successively to CP-3, finally the stable CP4 upon heating. It is noteworthy that, despite the paddle-wheel dizinc(II) units presented in CP-1 and CP-2 while an antisymmetric zinc-carboxylate unit in CP-3, CPs 1-3 are basically three supramolecular isomers, which may be critical to their structural transformations. Notably, the transformations from CP-1 and CP-2 to CP-3 involving of the breakage of robust [Zn2(COO)4]


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units are still rare.61-65 Structural analysis shows that the (Zn…Zn) –O(H2O) angles in CP-1 and CP-2 are considerably bent (166°in CP-1 and 168°in CP-2), indicating them as the kinetic products which can transform to more stable thermodynamic products upon heating. CP-2 and CP-3 are all 1D structures with the similar connectivity, thus we speculate that the partial of Zncarboxylate coordination bonds in the paddle-wheel unit breaking, then with rotation of the released carboxylate groups during the transformation of CP-2 to CP-3 (Figure S23). However, for the transformation from 2D structure of CP-1 to 1D structure of CP-3, it involves changes not only in the complete breakage of [Zn2(COO)4] unit and reconstruction of Zn-carboxylate bonds, but also in the lattice parameters and molecular stacking fashions, which is much complicated and still unclear. It is noteworthy that CP-1 cannot be converted to CP-2 with using the same condition but heating at a lower temperature (40 °C). Generally, low temperature favors the kinetic controlled product, increasing reaction time may favor the thermodynamic product. So we designed a control experiment with prolonging the reaction time at 40 °C to two weeks, but the result turned out to be ineffective. And if we raise the temperature, CP-3 will be formed. This result suggests that the both temperature and reaction time cannot drive the structure variation between CP-1 and CP-2, which may be due to their intrinsic structures. Moreover, our experiments showed that all the transformations between CPs are not reversible, suggesting the thermodynamically stable product formed after each transformation upon heating. Besides, since CP-2 can be considered as the precursor of the CP-5, it is also possible for converting CP-2 to CP-5 in the crystal-to-crystal manner. Our experiment clearly shows that with introduction of the pillar Bpy and keeping all other reaction conditions, highly crystalline CP-5 can be conveniently synthesized from CP-2.


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Scheme 1. Schematic illustration of the structural transformations between CPs. To find more details on the crystal-to-crystal transformation between resultant products, in-situ temperature dependent PXRD were performed (Figure S12). Starting from CP-1 at room temperature, the CP-1 phase is stable up to 70 °C upon heating, after which a new phase appears together with CP-1 and structural transformation is completed at 100 °C. Then no structural transformation can be observed till the temperature run up to 200 °C. Surprisingly, by comparing to the simulated patterns of CP-2, CP-3 and CP-4, the new phase doesn’t match with any of them, indicating that CP-1 transforms to a stable unknown phase. The variable temperature PXRD study suggests that CPs 1-3 all undergo a solvent-associated crystal-to-crystal transformation to form the more thermodynamically stable CP-4. Fourier transform infrared (FTIR) spectroscopy analysis of CPs was performed before and after the structural transformations, to correlate the structural changes (Figure S14-S20). The IR spectra of CP-3 converted from CP1 or CP-2 display the antisymmetric stretching vibration of carboxylate groups in the range of


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1609-1553 cm-1. Different from that of CP-1 or CP-2, the splitting of this band in CP-3 can be explained by the different coordination environment of the carboxylate groups in the dizinc(II) units. The new peak at 3639 cm-1 corresponding to the stretching band of µ3–OH groups in CP-4, can be the indicative of structural transformations from CPs 1-3 to CP-4.

Scheme 2. Coordination modes of L2−(a) and metal (b) in CPs 1−5. So far, we have reported four coordination polymers constructed from the H2L ligand and d10 Zn(II) at the same reaction conditions by varying only the synthesis temperature. It is very clear that temperature plays a pivotal role on the structural variation of coordination polymers. As shown in Scheme 2a, the dicarboxylate ligand exhibits only two different coordination modes in a V-shaped fashion to bond two adjacent metal clusters: both carboxylate groups act as bidentate mode in CP-1, CP-2 and CP-4; one carboxylate group acts as bidentate mode while the other acts as monodentate mode in CP-3. Besides, the ligand in a V-shaped coordination fashion has slightly different angles (θ1) in four CPs, 111.03°for CP-1, 111.49°for CP-2, 112.74°for CP-3 and 113.93°for CP-4, suggesting the flexibility of the linker (Figure 6). More importantly,


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the torsion angles of the carboxylate group with respect to the aromatic ring (θ2 and θ3) in CP-1 to CP-4 are quite different, and deviate significantly from zero. The torsion angles of two phenyl rings (θ4 and θ5) are also different. Clearly, the carboxylate groups adopt different orientations to meet the requirement of coordination geometries of neighbouring building blocks for the minimization of energy in the crystallization. It is well known that increasing temperature will be propitious to make the ligand in a thermodynamically favored conformation with large activation barrier. Therefore, the structural diversity of CP-1 to CP-4 may arise from the variable conformation of the ligand at different reaction temperatures.

Figure 6. The selected bond and torsion angles in CP-1, CP-2, CP-3 and CP-4. Due to the break or reorganization of Zn-O bonds, the coordination mode of metal center and the formed building unit will certainly be affected upon heating. As can be seen in Scheme 2b, three different inorganic building units were employed in four coordination polymers: two of which are dizinc(II) unit and the other is trizinc(II) unit within an infinite Zn-O cluster chain. At higher temperature, with the removal of coordinated water molecule by competitive metal complexation from carboxylate groups, infinite Zn-O-Zn cluster chain is formed, with the network dimensionality being increased from 1D to 2D. It should be noted that increasing temperature is always accompanied with the increase of pressure, especially under the hydro/solvothermal conditions. Therefore, along with the effects of


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temperature, pressure may also play a role on the structural variation of coordination polymers, especially at higher temperature.66-68 Optical Properties of the CPs. As has been reported previously, diphenyldibenzofulvene and its derivatives are AIE active, and exhibit unusual aggregation-induced emission in the solid state. As expected, H2L ligand is almost nonemissive in a pure organic solution, such as DMF and acetonitrile. Upon the addition of water, the emission of the ligand is continuously intensified and reaches a maximum value at a water fraction of ∼90% (Figure S24), indicating the AIE characteristic of the ligand. This could be due to the formation of aggregates induced by the nonsolvent water, which can hinder the free rotation of the phenyl rings and block the nonradiative decay process. The emission spectra of H2L ligand and five CPs were measured in the solid state at ambient temperature, and are depicted in Figure 7. Upon the excitation at 350 nm, the H2L ligand emits strong fluorescence with the maximum emission peak being located at 475 nm. For the CPs, the maxima emission wavelength center at 468 nm for CP-1, 485 nm for CP-2, 482 nm for CP-3, and 489 nm for CP-4 upon excitation at 360 nm. In comparison to the free ligand, the emission peaks of CP-2, CP-3 and CP4, are red-shifted by 10, 7, 14 nm, respectively, and CP-1 exhibits a 7 nm blue-shift. The observed red- or blue-shift of the emission can be largely attributed to the ligand conformational alteration in the structures. Generally, a more twisted conformation can result in the breakdown of the conjugate system and consequently cause the widened energy gap, while a more planar conformation can narrow down the energy gap.69-70 As can be seen in Figure 6, the torsion angles of two phenyl rings (θ4 and θ5) in CP-1 and CP-3 are larger than those in CP-2 and CP-4, indicating that the ligand adopt a more twisted conformation in the structures， thus the lower degree of conjugation system in CP-1 and CP-3 induces the bluer emission than


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that of CP-2 and CP-4. In contrast, the ligand in CP-4 adopts the most planar conformation than the other three CPs with the highest degree of conjugation, leading to the largest emission redshift. This study clearly shows the crucial role of temperature not only on the structural variation of CPs, but also on their photophysical properties since that which are directly related to the variable conformation of the ligand in the CPs. In addition, CP-5 emits at 492 nm upon the excitation at 360 nm. The absolute fluorescence quantum yields of the ligand and CP-1 to CP-5 are 5%, 12%, 7%, 8%, 10%, and 4%, respectively. The high quantum yields can be ascribed to the increased conformational rigidity within the MOF matrixes via the formation of coordination bonds to the metal centers, as well as the closely packing or inter/intro-ligand interactions observed in the structures.

Figure 7. The emission spectra of H2L, CP-1, CP-2, CP-3, CP-4 and CP-5 obtained at room temperature (excitation at 350 nm for H2L, and 360 nm for CPs 1-5). CONCLUSION


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In conclusion, by using the temperature-tuning strategy, a series of 1D or 2D Zn(II) based coordination polymers built from a propeller-shaped ligand di(4-carboxyphenyl) dibenzofulvene, have been synthesized from a single starting mixture with only varying the reaction temperature. We also reveal the temperature-driven structural transformations between these CPs in a solventassociated crystal-to-crystal fashion. Upon heating, CP-1, CP-2 and CP-3 in their mother liquor can all be finally converted to the thermodynamically stable CP-4. Our studies clearly demonstrate that both the structural diversity and different photophysical properties of prepared CPs mainly originate from the temperature effect, and its impacts on the conformational variability of the ligand in the structures. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the Internet at http://pubs.acs.org.on. Synthetic routes to the ligand, the experimental and simulated PXRD, IR, TGA, Emission spectra and Crystal data etc. Accession Codes CCDC 1576309-1576313 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. AUTHOR INFORMATION Corresponding Author


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*Email: [email protected]. *Email: [email protected]. ORCID Yonggang Zhao: 0000-0002-0357-382X Funding Sources This work was financially supported by the National Natural Science Foundation of China (21471079 and 21501092), the NSF of Jiangsu Province for Youth (BK20140928), the NSF of Jiangsu Province for colleges and universities (13KJB150017), and Nanjing TechUniversity for their financial support to this work. Notes The authors declare no competing financial interest REFERENCES (1)

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For Table of Contents Use Only

Temperature-Driven Crystal-to-Crystal Transformations and Luminescence Properties of Coordination Polymers Built With Diphenyldibenzofulvene Based Ligand Qiyang Li, a Xiuju Wu,a Xiaoli Huang,a Xue Xiao,a Shuping Jia,a Zhihua Lin*, b and Yonggang Zhao*, a

With using temperature-tuning strategy, a series of coordination polymers (CPs) showing interesting crystal-to-crystal transformations and aggregation-induced emissions, have been constructed.


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Temperature-Driven Crystal-to-Crystal ... - ACS Publications

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