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Combining Polycarboxylate and Bipyridyl-like Ligands in the Design of Luminescent Zinc and Cadmium based Metal-Organic Frameworks Jose M. Seco, Sonia Pérez-Yáñez, David Briones, Jose Angel Garcia, Javier Cepeda, and Antonio Rodriguez-Dieguez Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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Crystal Growth & Design

Combining Polycarboxylate and Bipyridyl-like Ligands in the Design of Luminescent Zinc and Cadmium Based Metal-Organic Frameworks Jose M. Seco,a Sonia Pérez-Yáñez,b David Briones,c José Ángel García,d Javier Cepeda,*,a Antonio Rodríguez-Diéguez*,e a

Departamento de Química Aplicada, Facultad de Química, Universidad del País Vasco/Euskal

Herriko Unibertsitatea, UPV/EHU, 20018, San Sebastián, Spain. bDepartamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea, UPV/EHU, 01006 Vitoria-Gasteiz, Spain. cChemical and Energy Technology Department, Chemical and Environmental Technology, Mechanical Technology and Analytical Chemistry, Universidad Rey Juan Carlos, 28933 Móstoles, Spain. dDepartamento de Física Aplicada, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea, UPV/EHU, Apdo. 644 48080 Bilbao, Spain. eDepartamento de Química Inorgánica, Universidad de Granada, 18071, Granada, Spain. KEYWORDS: X-ray Structures, Metal-Organic Frameworks, N2 Adsorption, Luminescence Properties, Long-Lasting Phosphorescence.

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Abstract

Detailed structural characterization and photoluminescence properties of four new metalorganic frameworks (MOFs) based on zinc(II) or cadmium(II) metal ions, di- or tricarboxylic aromatic ligands and bipyridyl-like elongated ancillary linkers, namely {[Zn2(µ4-bdc)2(µpbptz)]·2DMF·3H2O}n

(1),

{[Cd(µ3-bdc)(µ-pbptz)]·3DMF}n

(2),

{[Cd3(µ5-btc)2(µ-

pbptz)]·2DMF}n (3), and {[Zn2(µ-dhbdc)2(µ-pbptz)(DMF)4]·2DMF·H2O}n.(4) (where bdc = benzene-1,4-dicarboxylato, btc = benzene-1,3,5-tricarboxylato, dhbdc = 2,5-dihydroxobenzene1,4-dicarboxylato, pbptz = 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine, DMF = N,N-dimethylformamide) are reported. The occurrence of large accessible volumes and structural and topological diversity are a constant for crystal structures of these compounds, which is a result of the connectivity established among the metal-carboxylato building units formed in each case. 3D pcu frameworks of compounds 1 and 2 are built from the linkage of dimeric cores (established by the coordination of dicarboxylato bdc ligands) into two-dimensional networks that are further joined together by ancillary ligands, whereas the novel jcr7 topological 3D framework is achieved in 3 owing to the presence of the tricarboxylic btc ligand. 2D layers are generated in 4 given the bidentate coordination of both dhbdc and pbptz ligands. Interestingly, most crystal structures (3D frameworks of 1, 2 and 3) exhibit open architectures containing large solvent-occupied void systems that account for high relative void volumes. A deep analysis of the photophysical properties has been also accomplished for all compounds, confirming an overall blue emission under UV excitation in the steady state. Compound 3 characterizes for a strong phosphorescent emission consisting of that lasts few seconds and is observed by the naked eye, which constitutes an infrequent photoluminescent (PL) behavior for metal-organic materials.


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Introduction

The design and construction of coordination polymers (CPs), and especially the subclass of metal-organic frameworks (MOFs), can be regarded nowadays as a main and probably one of the most actives research areas in the field of porous materials.[1-5] The latter derives from the fact that permanent porosity, record surface areas, large pore volumes, and even adjustable pore sizes with uniform (or not) distribution and shapes are all well known features that are often used to describe the amazing diversity of structures and topologies available for MOFs,[6-12] a fact that constitutes an important advantage as well as a major drawback of these materials at the same time. In this sense, controlling the resulting architecture is a still challenging task that is limited to some few cases, though some indications for obtaining pre-designed MOFs presenting a particular topology have been already identified by deconstructing their crystal structures into nodes and spacers (which obey the classical concepts of reticular chemistry).[13-16] In any case, the key point in achieving such control involves the knowledge on the adequate synthetic conditions (temperature, solvent, stoichiometry, pH and so on) that give access to the desired secondary building units (SBUs), which, arising from the junction between inorganic units (metal ions or clusters) and organic linkers, eventually govern the growth of the structure giving rise to a particular topology.[17-21] As a consequence of their impressive porous architectures, MOFs have been extensively tested for storage and separation of small gas molecules such as H2 and CO2, in such a way that highly desired goals encompassing purification of H2 from CO2 or CO in the syngas mixture (an intermediate of the synthetic natural gas) or efficient CO2 capture and sequestration technologies can be implemented at an industrial level.[22-25] To that end, the quest for optimal metal-organic adsorbents requires pore sizes and shapes to adapt to the


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aforementioned molecules, which may be achieved by modulating the linkers for a particular topology or even exploring new topologies.[26-30] The large family of highly porous frameworks synthesized from the (3,24)-connected network possessing the rht topology,[31-33] that consisting of 4,8-connected networks with the scu topology,[34-37] or the IRMOF isoreticular pcu series[38] are excellent examples for precise pore design and control, which permits tuning the adsorption capacity almost at will. On another level, the hybrid inorganic-organic nature of MOFs may endorse them with other equally interesting properties such as luminescence, magnetism, drug delivery, catalytic activity or ion exchange.[39-44] Among them, solid-state light-emitting metal-organic materials are receiving large attention since the first compounds exhibiting persistent luminescence, longlasting phosphorescence (LLP) and afterglow are starting to be reported.[45-51] These phenomena have been largely explored for lanthanide-doped inorganic matrixes,[52-55] although the easily tunable luminescence shown by these kinds of MOFs makes of them ideal platforms to boost the development of improved devices for applications in organic light-emitting diodes (OLEDs), or flat panel displays (PDP) at an industrial level.[56-58] In fact, novel exciting applications may emerge from coupling the adsorption capacity and phosphorescence in these materials. A good strategy to achieve MOFs with LLP behavior focuses on the use of metal ions with closed shell d10 configuration such as ZnII and CdII, since, as inferred from the few phosphorescent compounds reported,[45-51] these ions afford ligand-to-metal charge (LMCT) transfers and enable intersystem crossings driven by their spin-orbit coupling effects that permit the population of the triplet states of the complexes, thus giving strong phosphorescent emissions.[59-62] Moreover, these ions are also excellent candidates for the construction of MOFs because they exhibit flexible coordination environments associated with the closed-shell configuration, allowing them


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to be adapted to a wide range of geometries often acquired in the most common SBUs.[63,64] Bearing these ideas in mind, we have employed various ligands featuring carboxylate and bipyridyl-like chemical functions and combined them with zinc and cadmium metal ions in order to obtain new MOFs that could deal with the above mentioned goal. This constitutes a well established synthetic strategy followed by our research group, which takes advantage of the coordination flexibility provided by carboxylic ligands, able to adopt different coordination modes, and the rigidity exerted by the bipyridyl-like linkers, which affords robustness to the crystal builiding.[26,65,66] All in all, we report herein the preparation, and structural, chemical and photoluminescent characterization of four new MOFs combining various aromatic carboxylic (benzene-1,4dicarboxylic, benzene-1,3,5-tricarboxylic or 2,5-dihydroxybenzene-1,4-dicarboxylic) and 3,6bis(4-pyridyl)-1,2,4,5-tetrazine ligands and zinc/cadmium metal ions. These compounds contain potentially porous frameworks with high void volume relative percentages, one of which, in turn, shows intense and long-lived emissions. Thus, coupling of open structures with photoluminescent emissions results in porous/luminescent materials that could bring in further functionalities in the drive towards still undeveloped applications.

Experimental Procedures

Chemicals. All the chemicals were of reagent grade and were used as commercially obtained. Synthesis of {[Zn2(µ4-bdc)2(µ-pbptz)]·2DMF·3H2O}n (1). 0.1 mmol (29.7 mg) of Zn(NO3)2·6H2O dissolved in 1 mL of DMF are placed in a glass vessel of 8 mL of capacity. 1 mL of a DMF solution containing 0.1 mmol (17 mg) of 1,4-benzenedicarboxylic acid (H2bdc) was added over the previous solution. Then, 2 mL of a DMF solution of 3,6-bis(4-pyridyl)-


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1,2,4,5-tetrazine (pbptz) (0.07 mmol,16.6 mg) was added to the previous mixture, which is mixed until homogeneity. The closed vessel is placed in an oven at 95 ºC. Single crystals of 1 were grown after 3 days. They were filtered off, washed with water and ethanol and dried. Yield: 45% based on metal. Anal. calc. for C34H36N8O13Zn2 (%): C, 45.60; H, 4.05; N, 12.51; Zn, 14.61. Found: C, 46.75; H, 3.94; N, 12.65; Zn, 14.33. Synthesis of {[Cd(µ3-bdc)(µ-pbptz)]·3DMF}n (2). 0.1mL of a DMF solution containing 0.1 mmol (30.8 mg) of Cd(NO3)2·4H2O, placed in a glass vessel of 8 mL of capacity, are mixed with 1 mL of the solution containing the H2bdc (0.1 mmol, 16.6 mg). Upon it, 0.05 mmol of pbptz (11.8 mg) dissolved in 2 mL of DMF were added over the solution containing the metal and carboxylic ligand. The resulting solution was closed in the vessel and left at 95 ºC in an oven for 3 days and single crystals of 2 were obtained. The same washing procedure was applied. Yield: 35% based on metal. Anal. calc. for C29H33CdN9O7 (%): C, 47.58; H, 4.54; Cd, 15.36; N, 17.22. Found: C, 47.71; H, 4.69; Cd, 15.15; N, 17.31. Synthesis of {[Cd3(µ5-btc)2(µ-pbptz)]·2DMF}n (3). 0.1 mmol (21 mg) of 1,3,5benzenetricarboxylic acid (H2btc) dissolved in 1 mL of DMF are added over 1mL of the DMF solution containing 0.1 mmol (30.8 mg) of Cd(NO3)2·4H2O in a glass vessel of 8 ml of capacity. Then, 0.05 mmol of pbptz (11.8 mg) dissolved in 2 mL of DMF are added to the previous solution and, after being completely homogenized. The vessel is closed and placed in an oven at 95 ºC for 2 days. Colourless single crystals are filtered and washed. Yield: 35% based on metal. Anal. calc. for C48H36Cd3N14O14 (%): C, 42.08; H, 2.65; Cd, 24.61; N, 14.31. Found: C, 41.92; H, 2.68; Cd, 24.43; N, 14.55. Synthesis of {[Zn2(µ-dhbdc)2(µ-pbptz)(DMF)4]·2DMF·H2O}n.(4). 0.1 mmol (29.7 mg) of Zn(NO3)2·6H2O dissolved in 1mL of DMF are mixed with a DMF solution (1 mL) of 0.1 mmol


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(19.8 mg) of 2,5-dihydroxyterephthalic acid (H2dhbdc) and a DMF solution (2 mL) containing 0.05 mmol of pbptz. The resulting mixture is placed in a glass vessel of 8 mL of capacity, which is closed and left in an oven at 95 ºC for 3 days. Colorless single crystals of 4 are filtered and washed. Yield: 75% based on metal. Anal. calc. for C46H60N12O19Zn2 (%): C, 45.44; H, 4.97; N, 13.82; Zn, 10.76. Found: C, 45.30; H, 4.80; N, 13.94; Zn, 10.68. Physical Measurements. Elemental analyses (C, H, N) were performed on an Euro EA Elemental Analyzer, whereas the metal content, determined by inductively coupled plasma (ICPAES) was performed on a Horiba Yobin Yvon Activa spectrometer. IR spectra (KBr pellets) were recorded on a ThermoNicolet IR 200 spectrometer in the 4000−400 cm−1 spectral region. Thermal analyses (TG/DTA) were performed on a TA Instruments SDT 2960 thermal analyzer in a synthetic air atmosphere (79% N2 / 21% O2) with a heating rate of 5 ºC·min–1. Nitrogen physisorption data for compounds 1, 2 and 3 were recorded with an Autosorb iQ Quantachrome Instruments analyzer at 77 K, after activating the samples in vacuum for 12 h at 150 °C (for 1, 2 and 3) and 180 ºC (for 3). The specific surface area was calculated from the adsorption branch in the relative pressure interval using the Brunauer−Emmett−Teller (BET) method[67] and the consistency criteria proposed by Walton and Snurr that is commonly applied for MOFs,[68] while the micropore volume was estimated by fitting the measured N2 isotherms with the t-plot method.[69] A closed cycle helium cryostat enclosed in an Edinburgh Instruments FLS920 spectrometer was employed for steady state photoluminescence (PL) and lifetime measurements in the 10–300 K range. All samples are fist placed under high vacuum (of ca. 10–7 mbar) to avoid the presence of oxygen or water in the sample holder. For steady state measurements a MüllerElektronik- Optik SVX1450 Xe lamp or an IK3552R-G HeCd continuous laser (325 nm) were used as excitation source, whereas microsecond pulsed lamp were employed for recording the


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lifetime measurements. Photographs of irradiated single-crystal and polycrystalline samples were taken at room temperature in a micro-PL system included in an Olympus optical microscope illuminated with a Hg lamp. X-ray Diffraction. X-ray data collection of suitable single crystals were done at 100(2) K on a Bruker VENTURE area detector equipped with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) by applying the ω-scan method. The data reduction was performed with the APEX2[70] software and corrected for absorption using SADABS.[71] Crystal structures were solved by direct methods using the SIR97 program[72] and refined by full-matrix least-squares on F2 including all reflections using anisotropic displacement parameters by means of the WINGX crystallographic package.[73,74] All hydrogen atoms were included as fixed contributions riding on attached atoms with isotropic thermal displacement parameters 1.2 times or 1.5 times those of their parent atoms for the organic ligands. Lattice solvent molecules could not be refined owing to their disordered disposition in the voids of the structures, so the electron density at the voids was subtracted from the reflection data by the SQUEEZE procedure as implemented in PLATON program[75] during the refinement. Moreover, some soft constraints concerning the coordinated DMF molecules had to be employed for the final refinement of the structure of compound 4. Details of the structure determination and refinement of compounds 1–4 are summarized in Table 1. Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary publication nos. CCDC 1543458-1543461. Copies of the data can be obtained free of charge on application to the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, U.K. (Fax: +44-1223-335033; e-mail: [email protected] or http://www.ccdc.cam.ac.uk). The X-ray powder diffraction (PXRD) patterns were collected on a Phillips X'PERT powder diffractometer with Cu-Kα radiation (λ =


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1.5418 Å) over the range 5 < 2θ < 50º with a step size of 0.026º and an acquisition time of 2.5 s per step at 25 ºC. Indexation of the diffraction profiles were made by means of the FULLPROF program (pattern-matching analysis)[76,77] on the basis of the space group and the cell parameters found for single crystal X-ray diffraction.

Table 1

Computational details. TD-DFT theoretical calculations were performed on ligand molecules taken from X-ray crystal structures using the Gaussian 09 package.[78] The Becke three parameter hybrid functional with the non-local correlation functional of Lee-Yang-Parr (B3LYP)[79-81] was employed for all atoms with 6-311G++(d,p)[82-84] basis set. The latter has been proven to be an adequate method to describe the absorption and emission spectra.[85,86] The 40 lowest excitation and emission energies were calculated on model 3 by the TD-DFT method. Gaussian results were analyzed using the GaussSum program package[87] and molecular orbitals plotted using GaussView 5.[88]

Results and Discussion

Structural description of {[Zn2(µ4-bdc)2(µ-pbptz)]·2DMF·3H2O}n (1). The X-ray diffraction analysis revealed that the 3D crystal structure of compound 1 builds up from paddlewheel like SBUs established by the coordination of zinc(II) atoms to the carboxylato oxygen atoms of four bdc ligands. The asymmetric unit is comprised by two crystallographically independent zinc atoms, two bdc anions (A and B) and one pbptz ligand. Zn1 and Zn2 atoms


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show almost identical square pyramidal coordination environments (SSPY of 0.29 and 0.41 for Zn1 and Zn2), in which carboxylato oxygen atoms define the basal plane of pyramid. The apical positions of the NO4 donor sets are completed by the coordination of the pyridine nitrogen atoms of the µ-pbptz linkers (Table 2). Within the units, the four carboxylato moieties impose an intradimeric Zn···Zn distance of ca. 3.0 Å. Each paddle-wheel unit joins to four neighbouring ones by means of the µ4-bdc-κO:κO':κO'':κO'' linkers, thus giving rise to a 2D neutral [Zn2(µ4bdc)2] layer that shows a distorted squared grid of approximate dimensions of 10.9 x 10.8 Å2 assuming the centroids of the SBUs. Bdc linkers get almost perpendicularly arranged with respect to the crystallographic ab plane, over which the mean plane of the layer is spread out, which demands A ligand to be remarkably folded, compared to the essentially planar B ligand, in order to adapt to the bi-dimensional arrangement (as inferred from the high twist found for both carboxylato groups regarding the aromatic ring, of ca. 15.3 and 18.6º).

Figure 1

Table 2

The layers are then assembled together by means of µ-pbptz pillaring spacers that emerge almost perpendicularly from the building units. These ligands are substantially twisted owing to the rotation freedom between their three aromatic rings (establishing an angle of ca. 36º between pyridine rings) and impose an intermetallic distance of about 15 Å (between the units of adjacent layers), leading to a highly open 3D pcu network with the (412.63) point symbol.[89,90] Despite the


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extremely large void left by such a cubic framework, or probably as a consequence thereof, compound 1 crystallizes as a twofold interpenetrated structure. In the overall entangled building, the second subnet is placed closed to the first net in order to maximize the face-to-face π-π interactions existing between the aromatic rings of bdc and pbptz ligands when passing through the rings of the architecture. Even so, this compound still exhibits extremely large cavities that are occupied by DMF molecules and stand for the 40.8% of the total unit cell volume as confirmed by PLATON[75] that results in 1D channels running along the (1 1 0) direction that contain very wide (ca. 18 Å) and narrow (ca. 3 Å) sections (Figure 2). In fact, considering the flexible nature of the open network as well as the occurrence of entanglement, the void system could be bound, a priori, to a dynamic behaviour that is likely to promote unexpected adsorption behaviour. However, when a probe molecule of the kinetic size of N2 is employed, it can be viewed that the voids are not eventually connected with each other, meaning that these kinds of adsorbate molecules are not able to diffuse along the framework (Figure S1). Accordingly, the measure of N2 adsorption isotherm at 77 K on a polycrystalline outgassed sample of 1 (12 h at 150 ºC) reveals a curve characteristic of a non-porous material. A priori, it may assumed that pores get occluded probably aided by a subnetwork displacement phenomenon (commonly reported for entangled architectures)[91] taking place once the solvent molecules are lost based on TG/DTA results (see Supp. Info.), which is confirmed by PXRD and TG data on the outgassed sample. Therefore, this compound constitutes a clear example bearing an experimental porosity that differs from the crystallographic porosity.

Figure 2


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Structural description of {[Cd(µ3-bdc)(µ-pbptz)]·3DMF}n (2). The crystal structure of compound 2 also consists of an open 3D framework that is built up from planar carboxylato based [Cd2(CO2)2] dimeric SBUs instead of the above mentioned paddle-wheels. Within these centrosymmetric units, Cd1 shows a seven-coordinated environment established by the carboxylato oxygen atoms of three bdc ligands and two nitrogen atoms of two pbptz linkers, which resembles a pentagonal bipyramid geometry (SPBPY = 1.57) (Figure 3, Table 3). The dimeric core is bridged by two specular carboxylato moieties that show a local mixed chelatingbridging κ2O3A,O4:κO3A pattern, which imposes a Cd···Cd distance of 3.82 Å. Hereafter, bdc linkers, which are completely planar, join these cores one another by adopting the µ3-bdcκ2O3A,O4:κO3A:κ2O1A,O2A coordination mode, in such a way that each core is linked to four surrounding cores. The resulting 2D layer generated thereof displays a tetragonal grid of approximate dimensions of 21.5 x 12.6 Å2 (considering the centroids of the dimeric SBUs occupying the vertices). Two µ-pbptz linkers emerge from each cadmium atom of the dimeric core with an almost perpendicular arrangement with respect to mean plane of the layer and keeping a relative parallel disposition between their aromatic rings, given the occurrence of strong π-π interactions (see Figure 3 and Table S2). These linkers serve as pillars to connect adjacent layers with each other, thus yielding an open 3D architecture that preserves the same connectivity and pcu topology of that of compound 1 given the 6-connected nature of the dimeric cores. As far as we are aware, this kind of carboxylato mediated dimeric core is an infrequent motif for terephthalato based metal-organic architectures, being only present in the 2D layers of {[Cd4(µ4-bdc)4(3-(2-pyridyl)pyrazole)4](tetramethylbenzene)7/2}n compound.[92] In any case, compared to 1, this framework leaves a higher accessible volume owing to the lack of


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interpenetration, making the voids be interconnected into a two-dimensional channel system that stands for the 63.1 % unit cell volume (Figure S2). In this sense, it is worth mentioning that although these sorts of backbones consisting of dicarboxylato and pbptz linkers have been largely reported, most of them do not exhibit such a high pore volume/cell volume ratio.[93-96] Nonetheless, the thermal evacuation of the pores may not be carried out without causing the structural collapse as inferred from TG/DTA analysis and PXRD analyses (see Supporting Information). Despite the fact that lattice solvent molecules have tried to be exchanged by methanol (an easily removable molecule), which should avoid the structural collapse at outgassing conditions, all efforts gave rise to negligible adsorption isotherms. Nonetheless, the structure remains crystalline and almost unchanged as inferred from PXRD data on the outgassed sample, which also confirms its robustness. A similar behavior has been previously observed for other paddle-wheel based related materials, which is explained in terms of the discrepancies existing between the crystallographic porosity and experimental gas uptake.[97-100] In particular, as confirmed by positron annihilation lifetime spectroscopy on the porous Zn-HKUST-1 material, the lack of gas uptake may be caused by inherent surface instability after solvent removal, which creates a diffusion barrier that makes the material permeable only to strongly interacting molecular guests.[101] Therefore, given the structural resemblance between both materials, this effect must not to be discarded for compound 2.

Figure 4

Structural description of {[Cd3(µ5-btc)2(µ-pbptz)]·2DMF}n (3). Compound 3 crystallizes in the centrosymmetric P2/c space group and presents a three-dimensional framework that is


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closely related to that of compound 2 despite the replacement of the dicarboxylato bdc by tricarboxylato btc ligand. The asymmetric unit contains two crystallographically independent Cd atoms, one btc anion and one pbptz connector. Compared to compound 2, Cd1 atom is coordinated to an identical N2O5 donor set (which gives a more distorted pentagonal bipyramid, SPBPY = 2.39) established by two chelating and one bridging carboxylato oxygen atoms of three btc ligands, which gives rise to the afore described planar [Cd2(CO2)2] dimeric SBU. Each SBU completes the apical positions by the nitrogen atoms of symmetry related pbptz linkers, between which π-π interactions are likewise maintained. However, the third carboxylato group and their relative disposition at 120º in the aromatic ring of the btc ligand do not allow acquiring the preceding connectivity between dimeric entities, preventing the growth of a 2D layer with a tetragonal grid. In contrast, btc ligand demands the occurrence of a second cadmium atom, Cd2, which is set into the special position delimited by the crossing between the two-fold axis and c mirror plane in order to meet the charge balance (Table 4). Cd2 exhibits a six-coordinated environment by coordinating to two chelating carboxylato groups and two bridging carboxylato oxygen atoms (STPR = 7.55, TPR stands for trigonal prism), in such a way that it constitutes an additional SBU of the structure. It is noticeable that Cd2 are arranged out of the mean plane established by the dimeric SBUs in order to complete their coordination environment, which forces btc ligands to be somewhat folded (owing to the large rotation of carboxylato groups with respect to mean plane of the ring: 27.8º and 14.8º, and out of plane arrangement of 13.8º) while they adopt the µ5-κ2O1A,O2A:κ2O3A,O4A:κ2O5A,O6A:κO2A:κO3A coordination mode (Figure 5).


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Table 4

Figure 5

Hence, btc ligands serve as connector of two dimeric and two monomeric SBUs that are not coplanar, in such a way that the resulting 2D is remarkably corrugated. Hereafter, the parallel µpbptz ligands emerging from the dimeric SBUs bring those layers together giving rise to an open 3D framework. This framework can be simplified as a three-nodal network with the (4·62)2(4·64·8)(46·619·83) point symbol from the topological viewpoint, since the dimeric and monomeric SBUs behave as eight- and four-connected nodes whereas the btc ligand should be as well considered as a three-connected node owing to the disposition of its carboxylato groups, which are included within the latter cores. A careful inspection of this network with the topological databases implemented in TOPOS reveals that it consists of a new topology that has been named as jcr7.[89,90]

Figure 6

This framework contains a large accessible volume that is enclosed as wide and wavy infinite channels (employing the same probe radius of 1.4 Å) running along the crystallographic c axis, which show rectangular shaped windows generated by the voids left among the layers and pbptz pillars (Figure S3). These channels are occupied by disordered lattice DMF molecules and represent the 34.8% of the unit cell volume, which is a remarkably lower value compared to that


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of compound 2 that may be explained according to shape of the 2D layer in both compounds (corrugated shape in 3 vs planar shape in 2). While it is true that the void system still contains open and wide microchannels as to take up small adsorbates in within, they are interconnected by some narrower windows with sections that could prevent the complete release of DMF molecules, thus yielding partially occluded regions in the pore system. As a consequence, although different outgassing conditions (at 150 ºC and 180 ºC) and solvent exchange methodologies were checked, an almost negligible adsorption could only be observed for this compound. Moreover, CO2 isotherm was also measured at 273 K to check whether this smaller adsorbate (kinetic diameter is 3.30 Å for CO2 vs. 3.64 Å for N2) is able to diffuse through the microchannel system of 3, observing no adsorption. As expected, FTIR measurements on the outgassed sample confirm the presence of DMF molecules (see Supp. Info.). Structural description of {[Zn2(µ-dhbdc)2(µ-pbptz)(DMF)4]·2DMF·H2O}n (4). The crystal building of compound 4 is built up from the packing of 2D neutral layers grown by the coordination of bidentate dhbdc and pbptz ligands to zinc atoms. There are two zinc atoms in the asymmetric unit in addition to two dhbdc dianions, one pbptz ligand and four coordinated DMF molecules. In spite of their crystallographic independence, Zn1 and Zn2 atoms contain an almost indistinguishable NO4 coordination shell established by a nitrogen atom of a pbptz ligand, two carboxylato oxygen atoms of two dhbdc ligands (A and B) and two oxygen atoms of DMF molecules, all of which resembles a trigonal bipyramid (STBPY are 1.32 and 1.90 for Zn1 and Zn2) (Table 5).

Table 5


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The basal plane of the bipyramidal environments is occupied by dicarboxylato dhbdc ligands, adopting the µ-κO1:κO4 coordination mode, and pbptz ligands, both of which act as linear connectors and join Zn1 and Zn2 atoms one another into the planar 2D layer. As a result of the relative arrangement of the three connectors within the coordination shell (every 120º) in addition to their different length (imposed Zn1···Zn2 distances are of about 11.1 and 15.1 Å for dhbdc and pbptz bridges), the Shubnikov layers exhibit a distorted hexagonal grid that concords with the hcb topology and (63) point group (Figure 7).

Figure 7

A remarkable aspect to be noticed within the layers is that dhbdc ligands remain essentially planar (the highest out of plane rotation of carboxylato groups with respect to the aromatic ring is of ca. 8.2 º) owing to the intramolecular hydrogen bonding interactions established between the hydroxyl and non-bonding carboxylato oxygen atoms (see Table S4), whereas pbptz ligands are somewhat twisted around their main axis (angle between mean planes of pyridine rings of 31º). Coordinated DMF molecules get arranged almost perpendicular to the mean plane of the layer (1 1 –1). Therefore, the packing of the layers takes place in a very efficient way dictated by strong π-π interactions established among dhbdc and pbptz ligands belonging to adjacent layers while DMF molecules are allowed to pierce the rings of the neighboring hexagonal grids (Figure 8). Lattice water and DMF molecules get entrapped into the isolated voids generated by the packing of the layers, which stand for the 24.6% of the unit cell volume (Figure S4).


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Figure 8

Comments on the structural rationalization and synthetic details. The crystallization of compounds 1–4 is conducted by a series of solvothermal syntheses in which di- or tricarboxylic ligands and the elongated bipyridyl like linker act as connectors of zinc(II) or cadmium(II) based building units. X-ray diffraction analyses reveal that crystal structures, consisting of 3D frameworks (1–3) or 2D laminar packing (4), are quite related to each other and show a close linkage with the synthetic conditions. Firstly and most importantly, herein employed carboxylato ligands, as usually found for these kinds of linkers, may adopt a large number of binding patterns given the great coordination capacity of their carboxylato groups. Hence, as observed for compounds 1–3, the different metal-carboxylato cores achieved act as SBUs and direct the growth of a bi-dimensional network following the bonds imposed by the geometry of the ligands, i.e. the topology of the connectors. In contrast, the large pbptz linkers are strictly restrained to act as linear connectors between the first established 2D networks, provided that a strong metalcarboxylato SBU is initially formed. For instance, carboxylato groups of bdc ligands yield two different dimeric SBUs when coordinating to zinc and cadmium atoms: a paddle-wheel shaped entity in compound 1 and a planar four-connected entity in 2. This fact derives from the larger ion size of cadmium(II) compared to zinc(II), which exceeds the limit distance imposed by the M–O–C–O–M bridge, thus preventing the formation of the five-member metal-carboxylato ring that constitutes the paddle-wheel unit and forcing the carboxylato group to acquire the chelatingbridging mode. In fact, paddle-wheel entities are less frequently found for cadmium-carboxylato based structures, the few examples being almost an exception especially for polycarboxylato ligands.[102,103] Hereafter, pbptz linkers give rise to the 3D crystal buildings of 1 and 2 that


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enclose huge void percentages. On another level, when the tricarboxylato btc linker is employed instead of bdc under similar solvothermal conditions, although the afore mentioned planar [Cd2(COO)2] SBUs of 2 are also established, the resulting anionic 2D layer of 3 is forced to get somewhat corrugated as to accommodate Cd2+ cations in order to balance the overall charge. Hence, despite the fact that pbptz pillars join the layers in the same way, the 3D framework presents a novel topology and the porosity of the framework, in terms of accessible void per unit cell volume, is almost halved. Finally, the architecture of 4 may be considered as an exception to the remaining compounds since it lacks a dimeric SBU. This structure, indeed, does not fit the structural design raised for compounds 1–3 although the same metal:carboxylato ligand:pbptz ratio of 2:2:1 and solvothermal temperature are employed in the synthesis. A possible explanation for that structural dissimilarity seems to be originated at the intraligand hydrogen bonding interaction formed between hydroxide and carboxylato groups in dhbdc, which somehow decreases coordination capacity of the carboxylato groups and precludes establishing a dimeric SBU. Therefore, the three dhbdc and pbptz linkers that coordinate the zinc atoms act linear connectors by tailoring 2D layers. Photoluminescence Properties. Solid state photoluminescence (PL) was studied for compounds 1–4 in order to gain access into their emission characteristics and to study the influence of the assembling of the ligands into these open d10 metal-organic frameworks. As stated elsewhere, all employed carboxylato based bdc, btc and dhbdc as well as pbptz ligands characterize for exhibiting wide emission bands with maxima peaking at 380, 385, 510 and 430/460 nm, respectively, owing to their strong capacity to absorb UV light through π → π* or σ → π* transitions (as confirmed by TD-DFT calculations, see S9 section in the Supp. Info.).[104-


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107]

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When excited under UV light at 10 K, a clear distinction can be made between emission

characteristics of compounds 1–3 and that of 4 (Figure 9). Polycrystalline samples of 1–3 show a quite similar emission profile consisting of a main band centered at 445–455 nm, an intermediate one peaking in the 380–395 nm range (which represents a shoulder to main band), and less intense and isolated band at 605–620 nm range. Among them, it must be clearly stated that compound 3 presents a much more intense emission compared to 1 and 2, as inferred from the comparison of their raw emission spectra shown in Figure S16. In fact, the emission of compound 1 is so weak that its spectrum could only be recorded when excited under laser monochromated radiation at 325nm, in contrast to compounds 2 and 3 that were excited at 305 nm (which corresponds to their excitation maxima, see Figure S17). With the aim of getting deeper insights, TD-DFT calculations were carried out on suitable models of ligand molecules in order to reproduce the experimental main bands, which showed a good concordance and allowed to interpret the emission mechanism of the compounds (Scheme 1). As a result, it may be stated that these emissions are ascribed to π ← π* transitions originated at the aromatic rings of the ligands given the similar shape and small blue shifts of the bands corresponding to all compounds 1–4 with respect to those of corresponding free molecules (of ca. 10 nm for bdc, btc and pbptz ligands in compounds 1–3). On the other hand, the bands sited in the 605–620 nm range may be tentatively assigned to a ligand-to-ligand charge transfer (LLCT) mechanism occurring between the excited energy levels of the carboxylato ligands and the ground levels of pbptz ligands within the complexes, in good agreement with the observed energy levels diagram. Upon monochromated laser excitation at 325 nm, the emission spectrum of compound 4 shows a very broad band covering the 400–700 nm in which two main peaks at 460 and 500 nm may be distinguished. It should be noted that 4 gives the poorest emission among all herein studied


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compounds. These two bands are undoubtedly attributed to the blue shifted ligand-centered charge transfer (LCCT) emissions arising from pbptz and dhbdc ligands. Overall, the small hypsochromic shifts observed for all compounds correspond to the coordination of the ligands to zinc or cadmium atoms into their crystal structures. This also brings a notorious emission enhancement given that most of vibrational modes of the ligands, which usually represent the main non-radiative deactivation in metal-organic compounds, are substantially quenched Figure S19.[108,109] Polycrystalline samples were slowly heated up to room temperature revealing that the emission capacity is progressively decreased and bands are slowly broadened according to the increase of the kinetic (thermal) energy of the bond electrons (see Figures S20-22).

Figure 9

Scheme 1

Micro-PL images collected at room temperature for single crystals of all compounds (Figure 10) under excitation at 435 nm confirm that the emission capacity decreases following the order: 3 > 2 > 1 > 4 (see Figure S16). As observed, compounds 1 and 2 exhibit dominant red emissions under excitation at 365 nm given that the selected wavelength is not enough as to excite the strongest emitting molecular levels responsible for the main blue emissions shown in Figure 9. Instead, a bright yellowish green emission is noticed for compound 3 according to its high emission strength in the 530–580 nm range. In fact, the high PL capacity of compound 3 provides a strong emission over the whole visible spectrum that endows it with a multicolored


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luminescence, as inferred from the images taken under illumination of sample at different excitation beams (Figure S23). Finally, almost no emission was observed for the sample of compound 4.

Figure 10

Interestingly, during the PL characterization of the bulk sample of 3, it was observed that after the removal of the UV source, the bright blue emission is not immediately turned off but a bluish green afterglow that lasts at least one second is perceived by the naked eye. Therefore, an LLP behavior may be claimed for compound 3 at low temperature. In order to get deeper insights into the delayed emission of this material, decay curves were recorded for some representative emission wavelengths (each 20 nm) over the whole spectrum and exciting the sample at 305 nm (Figure S25). At first glance, there are two clearly distinctive regions according to the shape of the decay curves. On the one hand, curves included within the 350–400 nm and 580–700 nm ranges, show an abrupt slope and are characteristic of very short lifetimes (in the order of ns since they are shorter than the pulse of the lamp). On the other hand, slow and gradual decays resembling multi-exponential profiles are observed for the 420–560 nm range, which in fact corresponds to the main emission band, meaning that different processes are involved in the PL performance of this compound. Therefore, these curves were fitted with the [It = A0 + A1 exp(– t/τ1) + A2 exp(–t/τ2) + A3 exp(–t/τ3)] expression in which two or three parameters were employed according to their slope (Table 6). According to the best fittings, long-lived components of 0.580.76 s and intermediate lifetimes of 0.12-0.42 s, in addition to the shortest components (below


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0.1 s) are detected along the selected wavelengths covering the main emission band. In view of these results, time resolved emission spectra (TRES) were measured upon this material in order to get a more precise analysis of the evolution of the emitted color (Figure 11). As inferred from the plot, the emission spectrum collected after 5 ms after removal of the excitation source resembles the steady state one, so it can be taken as a reference for the experiment. It is worth noticing that the emission intensity drops relatively rapid over time and, besides, the shoulders peaking at ca. 380 and 610 nm in the steady state are almost disappeared after 25 ms, signifying that the phosphorescent emission composes mainly of the main band. Hereafter, the band is maintained almost unchanged (with the maximum peaking at 447 nm) while it progressively losses intensity, although a significant signal is still appreciated after 1 s after the initial excitation (see Figures S27-28). As far as we are aware, this MOF based material constitutes the first example showing long-lived blue emission, in contrast to the green or orange phosphorescence exhibited by most of compounds reported so far.[45-48,50,51] According to the dominant thought that considers that the phosphorescent emission proceeds through a radiative spin forbidden T1 → S0 transition centered on the d10 metal atom,[40,51,110,111] i.e. following Kasha’s rule,[112] it seems reasonable that the occurrence of phosphorescent emission observed for this compound and not for compounds 1, 2 and 4 obeys two major factors: i) a more accessible triplet state of the complex in order to be populated (intersystem crossing driven the spin-orbit coupling facilitated by the metal atom) and ii) the absence of efficient non-radiative processes that are coupled with the T1 → S0 transition. Therefore, it may be assumed that the electronic structure in compound 3 suits well both requirements. Additionally, the phosphorescent emission of the material was checked according to the temperature, recording the lifetimes at the longest-lived emission wavelengths from 10 K up to 250 K. As shown in Figure


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S26, the phosphorescent emission is quite stable up to 100 K, whereas at higher temperatures it practically disappears and only the fluorescent emission is observed in good agreement with the increased molecular vibrations and subsequent nonradiative quenching.

Table 6

Figure 11

Conclusions

Four new compounds consisting of di- or tricarboxylic aromatic anions (i.e. benzene-1,4dicarboxylic, benzene-1,3,5-tricarboxylic or 2,5-dihydroxybenzene-1,4-dicarboxylic) and 3,6bis(4-pyridyl)-1,2,4,5-tetrazine ligands assembled with zinc or cadmium metal ions have been obtained under solvothermal conditions. All compounds characterize for open architectures of variable dimensionality (2D and 3D) and topology as a result from the combination of the flexibility provided by carboxylic ligands and the rigidity imposed by the pbptz pillaring linkers when coordinating to the selected closed-shell metal ions. In this regard, it deserves to be noticed the architecture of compound 3 since it exhibits an unprecedented topological class that has been named as jcr7. Despite the fact that compounds 1–3 present open 3D frameworks with large and accessible void systems as to be considered potentially porous materials, the adsorption capacity of the materials is probably prevented by the occurrence of dynamic behavior derived from entanglement (in 1) or incomplete evacuation of solvent DMF molecules from the pores at the


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studied conditions. This fact does not necessarily imply that the porous frameworks, particularly for the frameworks of 1 and 2, could adsorb other sorts of molecules under different outgassing conditions. On another level, under UV steady state excitation, all compounds show blue emission ascribed to π ← π* transitions originated at the ligands, although the intensity of the signal varies deeply from one another as inferred from micro-PL photographs taken on solid samples. In particular, LLCT mechanism seems only to be enabled for compounds 1–3 in view of the relative energy of the excited levels centered on bdc and btc and ground state levels of pbptz computed by TD-DFT. Among them, compound 3 shows the strongest emission that is retained after the removal of the excitation source and can be perceived by the naked eye for about one second when the sample is cooled below 100 K. A more accurate description of the phosphorescent emission is provided by the emission decay curves at different wavelengths and TRES experiments, which reveal that the bright blue emission with a radiative lifetime (0.76 s) that is close to the best results found for ZnII- or CdII-based MOFs or CPs, meaning that this material constitutes the first example emitting blue phosphorescence. Thus, the open framework and interesting PL behavior of compound 3 could endorse this material with a promising dual functional character (adsorptive-luminescent performances) to take a step forward in the development of smart materials with enhanced sensor capacity.

ASSOCIATED CONTENT Supporting Information. Additional figures of crystal structures and data, FTIR spectra, thermogravimetric analysis, powder X-ray diffraction analyses, photoluminescence spectra and lifetimes, time-resolved emission spectra, TD-DFT computational results, and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. C.) *E-mail: [email protected] (A. R. D.) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Red Guipuzcoana de Ciencia, Tecnología e Innovación (OF215/2016) and University of the Basque Country (GIU14/01, EHUA16/32), Junta de Andalucía (FQM-1484) and Ministerio de Economía, Industria y Competitividad of Spain (Projects CTQ2014-56312-P and MAT201675883-C2-1-P). The authors thank for technical and human support provided by SGIker of UPV/EHU and European funding (ERDF and ESF). REFERENCES [1] Tan, J.-C.; Civalleri, B. Metal–Organic Frameworks and Hybrid Materials: From Fundamentals to Applications. CrystEngComm 2015, 17, 197-198. [2] Zhou, H.-C.; Kitagawa, S. Metal–Organic Frameworks (MOFs). Chem. Soc. Rev. 2014, 43, 5415-5418. [3] Dey, C.; Kundu, T.; Biswal, B. P.; Mallick, A.; Banerjee, R. Crystalline metal-organic frameworks (MOFs): synthesis, structure and function. Acta Cryst. 2014, B70, 3-10.


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For Table of Contents Use Only Combining Polycarboxylate and Bipyridyl-like Ligands in the Design of Luminescent Zinc and Cadmium Based Metal-Organic Frameworks Jose M. Seco, Sonia Pérez-Yáñez, David Briones, José Ángel García, Javier Cepeda, Antonio Rodríguez-Diéguez TOC graphic:

Synopsis: Detailed structural and photoluminescent characterization of new MOFs consisting of zinc or cadmium metal ions, di-/tricarboxylate and 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine linkers is reported. The combination of blue luminescence and open architectures with huge voids in these materials provides them with a dual functional character to perform as potential sensors. One of them stands out for its long-lasting blue phosphorescence (unprecedented for MOFs).


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FIGURES AND SCHEMES

Figure 1. Excerpt of the 3D backbone of a single subnet of compound 1 showing the SBU and numbered atoms forming the coordination environment. Note that A and B labels of atoms stand for A and B bdc ligands.


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Figure 2. View of the packing of 1 along the (1 1 0) direction showing the two subnets (blue and red) and the one-dimensional channels (yellow solid).


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Figure 3. Excerpt of the structure of compound 2 showing the dimeric SBU, its simplified topological connectivity and the pyramidal bipyramid polyhedron of the metal coordination environment. Dashed orange double lines stand for π–π interactions between aromatic rings of pbptz ligands.


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Figure 4. View of the 3D crystal building of compound 2 generated from the junction of layers consisting of a rhombic grid.


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Figure 5. Fragment of the 3D framework of compound 3 showing details of the three nodes of the topological network: dimeric and monomeric SBUs, and btc ligands.


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Figure 6. Unit cell of 3 showing the molecular and topological views of the 3D framework and the connectivity of the nodes of the novel network.


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Figure 7. (a) Asymmetric unit of compound 4. (b) 2D hcb topological layer.


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Figure 8. Packing of the layers by means of π-π interactions in compound 4.


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Figure 9. Normalized emission spectra of compounds 1–4 with arbitrary emission intensity recorded at 10 K under selected excitation wavelengths. The main emission bands have been indicated.

Scheme 1. TD-DFT computed and experimentally measured main emissions of the ligands showing the molecular orbitals involved (H and L stand for HOMO and LUMO).


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Figure 10. Micro-PL images taken at room temperature on single crystals or polycrystalline aggregates of compounds 1–4 with a panchromatic bright field or under a 365 nm excitation line.


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Figure 11. TRES of compound 3 at 10 K at selected delays (λex = 305 nm). Excitation band pass = 5 nm, emission band pass = 2.5 nm.


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TABLES Table 1. Crystallographic data and structure refinement details of all compounds. Compound

1

2

3

4

Chem. form.

C34H36N8O13Zn2

C29H33CdN9O7

C48H36Cd3N14O14

C46H60N12O19Zn2

Form. weight

895.48

732.04

1370.11

1215.80

Cryst. system

Triclinic

Orthorhombic

Monoclinic

Triclinic

Space group

P-1

Pbam

P2/c

P-1

a (Å)

10.837(6)

21.51(2)

10.281(1)

11.228(5)

b (Å)

10.847(7)

12.561(11)

15.7562(1)

12.302(6)

c (Å)

17.986(10)

15.795(14)

17.4331(1)

21.501(10)

α (º)

94.75(2)

90

90

99.524(13)

β (º)

95.84(2)

90

102.317(4)

98.357(13)

γ (º)

108.61(2)

90

90

108.353(13)

V (Å3)

1979(2)

4268(7)

2759.1(4)

2717(2)

Z

2

4

2

2

0.937

0.959

1.007

1.180

GOF

a

Rint

0.0731

0.0968

0.0980

0.0855

2 c

R1 / wR [I>2σ(I)]

0.0761 / 0.0793

0.0424 / 0.0781

0.0641 / 0.1144

0.0657 / 0.0686

R1 b / wR2 c (all data)

0.1891 / 0.1920

0.0798 / 0.0875

0.1507 / 0.1668

0.2768 / 0.2796

b

[a] S = [∑w(F02 – Fc2)2 / (Nobs – Nparam)]1/2 [b] R1 = ∑||F0|–|Fc|| / ∑|F0| [c] wR2 = [∑w(F02 – Fc2)2 / ∑wF02]1/2; w = 1/[σ2(F02) + (aP)2 + bP] where P = (max(F02,0) + 2Fc2)/3 with a = 0.0966 (1), 0.0342 (2), 0.0861 (3), 0.0200 (4), and b = 14.0958 (1).

Table 2. Selected bond lengths for compound 1.a

Zn1–O2A

2.034(2)

Zn2–O1A

2.071(2)

Zn1–O3A(i)

1.996(3)

Zn2–O4A(i)

2.039(3)

Zn1–O1B

1.979(3)

Zn2–O2B

1.909(3)

Zn1–O3B(ii)

2.121(2)

Zn2–O4B(ii)

2.005(2)

Zn1–N1C

2.008(2)

Zn2–N16C

2.053(2)

[a] Symmetries: (i) x – 1, y, z; (ii) x, y, z – 1.


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Cd1–O1A

2.520(4)

Cd1–O4A(i)

2.356(4)

Cd1–O2A

2.263(4)

Cd1–N1B

2.302(4)

Cd1–O3A(i)

2.462(4)

Cd1–N1B(iii)

2.302(4)

Cd1–O3A(ii)

2.399(4)

[a] Symmetries: (i) –x – 1/2, –y + 1/2, z; (ii) –x + 1/2, y + 1/2, –z; (iii) –x, –y, –z.


Cd1–O1A

2.292(7)

Cd2–O3A(iv)

2.336(7)

Cd1–O2A

2.692(6)

Cd2–O3A(v)

2.336(7)

Cd1–O2A(i)

2.291(6)

Cd2–O5A

2.347(7)

Cd1–O3A(ii)

2.530(7)

Cd2–O5A(vi)

2.347(7)

Cd1–O4A(ii)

2.316(6)

Cd2–O6A

2.300(7)

Cd1–N1B

2.310(10)

Cd2–O6A(vi)

2.300(7)

Cd1–N16B(iii)

2.277(9)

[a] Symmetries: (i) –x, y, –z + 1/2; (ii) –x – 1, –y, –z; (iii) –x, –y + 1, –z; (iv) –x + 1, –y + 1, –z + 1; (v) x, –y + 1, z + 1/2; (vi) –x + 1, y, –z + 3/2. Table 5. Selected bond lengths for compound 4.a

Zn1–O1A

1.954(2)

Zn1–O4A

1.966(2)

Zn1–O1B

1.994(2)

Zn1–O4B(i)

1.977(2)

Zn1–N1C

2.045(3)

Zn1–N16C(ii)

2.065(10)

Zn1–O1D

2.050(2)

Zn1–O1F

2.087(2)

Zn1–O1E

2.065(2)

Zn1–O1G

2.114(2)

[a] Symmetries: (i) x – 1, y + 1, z; (ii) x, y + 1, z + 1.


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Table 6. Best fit results of decay curves measured at 10 K for compound 3 monitoring the emission wavelengths in the 420–560 nm range.

Wavelength (nm)

τ1 (ms)

τ2 (ms)

τ3 (ms)

Chi Sq.

420 440 447 460 470 480 500 520 540 560

41(7) 50(7) 31(3) 41(9) 49(4) 24(3) 42(14) 25(4) 65(4) 53(5)

170(27) 128(13) 168(17) 142(27) 247(33) 148(86) 115(36) 139(17) 427(18) 338(18)

678(97) 578(28) 759(73) 655(43) 705(55) 726(59) 523(32) 663(75) – –

1.528 1.517 1.461 1.417 1.489 1.512 1.461 1.509 1.576 1.513


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Combining Polycarboxylate and Bipyridyl-like ... - ACS Publications

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