Comprehensively Understanding Isomorphism and Photoluminescent

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Comprehensively Understanding Isomorphism and Photoluminescent Nature of Two-Dimensional Coordination Polymers of Cd(II) and Mn(II) with 1,1′-Ethynebenzene-3,3′,5,5′tetracarboxylic Ligand Lu Zhai,†,‡ Zhu-Xi Yang,† Wen-Wei Zhang,*,‡ Jing-Lin Zuo,‡ and Xiao-Ming Ren*,†,‡,§ †

State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 210009, P. R. China ‡ State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023 P. R. China § College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P. R. China S Supporting Information *

ABSTRACT: Four isomorphic two-dimensional (2D) homoand heterometallic coordination polymers (CPs), [(CdxMn1−x)3(HEBTC)2(DMSO)6] with x = 1 (1), 1/3 (2), 0.5 (3), and 2/3 (4) were prepared by conventional one-pot self-assembly approach, using Cd2+ or mixtures of Cd2+ and Mn 2+ with 1,1′-ethynebenzene-3,3′,5,5′-tetracarboxylic (H4EBTC) under solvothermal conditions. The crystal structures of four isomorphic CPs are composed of centersymmetric trinuclear metal clusters building units linked by HEBTC3− ligands, extending into (3, 6)-connected topological 2D nets. Four CPs are isomorphic to the Mn−CP, [Mn3(HEBTC)2(DMSO)6], we recently reported. The solidstate photoluminescence of 1−4 shows dual emissions at ambient condition, where the emission bands centered at ca. 390 and 562 nm in 1 are assigned to the fluorescence and phosphorescence within HEBTC3− ligand, respectively; however, the emission bands centered at around 397 and 470 nm in 2−4 are attributed to fluorescence, corresponding to electron transition within HEBTC3− ligand and MLCT transition between HEBTC3− ligand and Mn2+ ion. In addition, the origin of isomorphism between 1 and Mn−CP is also discussed.

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INTRODUCTION

not only the desirable crystal structure of MOFs/CPs is achievable and controllable,9−11 but also the luminescent performance of MOFs/CPs is tunable.12−15 On the other hand, the luminescent performance of a MOF/CP is also possible modulated by exchange of guest molecules or ions.16−20 These unique advantages endow luminescent MOFs/CPs with promising applications in the materials science areas for advanced flat-panel displays,21 solid-state lighting sources,22 sensors,23−26 photovoltaics,27 reverse saturable absorption,28 high-resolution bioimaging,29 photocatalysis,30 and photodynamic therapy (PDT).31 In the context of luminescent MOFs/CPs with closed-shell electron configuration metal ions, the ligand-based fluorescence, originating mainly from the π−π* electron transition within a conjugated organic ligand, has been often observed, whereas the ligand-based phosphorescence seldom reported. With regard to the fluorescent MOFs/CPs, the phosphorescent

Metal−organic frameworks (MOFs)/coordination polymers (CPs) are an emerging class of regular crystalline solids and possess intrinsically well-organized host structures formed by the coordination of metal ions or metal−oxygen cluster ions with organic ligands. In the last three decades, MOFs/CPs have attracted much research attention,1−3 and many efforts have been devoted to the controllable preparation of MOFs/CPs with desirable functionality using the rational design concepts and crystal engineering strategies.4,5 The luminescent MOFs/CPs, as a subclass of functional MOFs/CPs, have unique advantages regarding well-developed organic or conventional inorganic luminescence materials. The emission arises from various possible electron transitions in the luminescent MOFs/CPs, for example, π−π* electron transition in the conjugated organic ligands,6 f−f electron transition within the metal ions of lanthanide-MOFs/CPs,7 and chargetransfer electron transition between the metal centers and the conjugated organic ligands,8 etc. By judicious incorporation of metal centers and rational design of conjugated organic ligands, © XXXX American Chemical Society

Received: February 9, 2018

A

DOI: 10.1021/acs.inorgchem.8b00374 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data and Structural Refinements for 1−4

a

compound

1

2

3

4

formula formula weight CCDC no. temp. (K) wavelength (Å) crystal size/mm crystal system space group i/Å b/Å c/Å α/Å β/Å γ/ Å V/Å3 Z F(000) θmin,max/◦ GOF R1, wR2 [I > 2σ(I)]a

C48H50Cd3O22S6 1508.44 1822028 295(2) 0.71073 0.21 × 0.20 × 0.19 triclinic P-1 9.9088(10) 12.6624(13) 13.1080(14) 89.612(2) 71.529(2) 69.514(2) 1451.5(3) 1 754 3.18−25.01 1.093 0.0807, 0.1940

C48H50Cd2Mn1O22S6 1450.98 1821754 296(2) 0.71073 0.20 × 0.17 × 0.15 triclinic P-1 9.834(2) 12.510(3) 13.032(3) 89.903(5) 72.837(5) 69.644(5) 1427.0(6) 1 731 2.33−27.61 1.026 0.0926, 0.2467

C48H50Cd1.5Mn1.5O22S6 1422.25 1821757 296(2) 0.71073 0.20 × 0.18 × 0.15 triclinic P-1 9.765(2) 12.615(2) 13.002(2) 90.653(5) 107.683(5) 110.519(5) 1416.7(5) 1 720 3.18−27.66 1.074 0.1080, 0.2879

C48H50Cd1Mn2O22S6 1393.52 1821759 296(2) 0.71073 0.20 × 0.17 × 0.15 triclinic P-1 9.766(3) 12.489(4) 12.951(4) 90.498(4) 107.055(4) 110.553(4) 1403.0(8) 1 708 1.66−27.61 1.001 0.1320, 0.3158

R1=∑||F0| − |Fc|/∑|F0| and wR2={∑[w(F02 − Fc2)2]/∑[w(F02)2]}1/2.

Figure 1. (a) An asymmetric unit of 1 with thermal ellipsoids drawn at 50% probability level. (b) Representation of the trinuclear cluster as SBU with cadmium as assigned in ball model. (c) Top view of a single layer in crystal structure of 1 with strong π···π interaction (all H atoms as well as DMSO, except O atoms, were omitted for clarity). (d) Two types of face-to-face π···π stacks.

energy transfer occurs from the first excited singlet state (S1) to the first excited triplet state (T1). The intersystem process between different spin states can be assisted by spin−orbit coupling or vibration couplings. It is an efficient strategy to achieve phosphorescent MOFs/CPs by assembly of heavy

MOFs/CPs are of particularly increasing interest because of their practical applications in the fields of chemosensors,32 oxygen sensor,33 and light-emitting electrochemical cells.34 A phosphorescent emission concerns the intersystem crossing process in a conjugated organic molecule, namely, non-radiative B

DOI: 10.1021/acs.inorgchem.8b00374 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

ligand with three carboxylate atoms links six Cd2+ ions through bidentate (μ2-η1, η1 and μ2-η2, η1) coordination fashions and serves as a μ2-bridge linker. As shown in Figure S5, two phenyl rings in HEBTC3− make a dihedral angle of 84.78°, indicating a strong spatial-distortion effect from coordination with the Cd2+ ions, presumably to accommodate the steric demands of the dense SBU. Each trinuclear SBU links six neighbors by six HEBTC3− ligands, and each HEBTC3− ligand connects three adjacent trinuclear SUBs, such type of connectivity extends into a 2D grid-like coordination polymer layer. The 2D grid-like layer is parallel to the crystallographic ac-plane and depicted in Figure 1c. As depicted in Figure 1d, there exist two types of face-toface π···π stacking interactions between phenyl rings, with the centroid-to-centroid distances of 3.521 and 4.105 Å. Analysis of the crystal packing of 1 reveals that the neighboring 2D layers are densely stacked in an offset ···ABAB··· stacking way with the DMSO molecules reside between the layers and fill in the void space of 2D grid-like, preventing the interpenetrating framework formation (Figure S6). By comparison of two crystal structures of [M3(HEBTC)2(DMSO)6] (M = Cd (1) and Mn8), it is found that two CPs not only are isomorphic to each other but also possess fairly analogous cell parameters. The isomorphism is an appealing phenomenon and commonly observed in the rare-earth metal MOFs/CPs,7 and this is because that the lanthanide ions have fairly close ionic radii, the same outermost electron configuration and positive charge, etc.38,39 The isomorphism phenomenon has been also found in the transition-metal MOFs/CPs, although it is not as common as in the rare-earth metal MOFs/CPs.40−44 Most recently, Ren et al. investigated the crystal structures of a series of threedimensional open-framework hybrid crystals of M(en)3Ag2I4 (M2+ = Mg2+, Mn2+, Ni2+, Zn2+, and Cd2+; en = ethylenediamine),40,45 and discovered that all members in this family are isomorphic and crystallize in the hexagonal non-centrosymmetric space group P6322 with fairly analogous cell parameters at room temperature, and the metal coordination cation, M(en)32+ (where M2+ = Mg2+, Mn2+, Ni2+, Zn2+ and Cd2+) with symmetry of D3 point group, plays an important role in the process of directing the formation of three-dimensional hexagonal non-centrosymmetric framework of {Ag2I42−}∞.40 Moreover, all metal centers in the M(en)32+ in this family of M(en)3Ag2I4 (M2+ = Mg2+, Mn2+, Ni2+, Zn2+ and Cd2+) show approximately spherical electron cloud distribution, however, the metal ion with non-spherical electron cloud distribution, for example, M2+ = Cu2+ or Co2+ ion, directs to give differently structural inorganic silver-iodide framework or failed to achieve the crystals even if the self-assembly undergoes under the same condition.40,46 These observations suggest that the electron configuration of the metal centers in the coordination compounds, in some cases, is also one of the critical factors to affect the formation isomorphs. In the isomorphic [M3(HEBTC)2(DMSO)6] (M = Cd (1) and Mn8), the Mn2+ and Cd2+ centers have distorted octahedral coordination environment formation by the oxygen atoms from carboxyl groups or DMSO molecules. In this situation, the Mn2+ ions adopt the high-spin state owing to the weak ligand-field effect from both the carboxyl groups and DMSO molecules. The semifilled shell Mn2+ centers should have roughly spherical electron cloud distribution in [Mn3(HEBTC)2(DMSO)6], and this is similar to that in the close shell Cd2+ centers in [Cd3(HEBTC)2(DMSO)6]. On the other hand, the ionic radii

metal ions (showing strong spin−orbit coupling effect) with fluorophore ligands. In a previous study, we observed that a CP of light metal magnesium ions with π-electron-rich H2EBTC2− ligands emits bright dual emissions of the ligand-based fluorescence and phosphorescence at room temperature,35 and this finding indicates that H4EBTC is an excellent phosphor. We further achieved another CP of EBTC4− ligands with Pb2+ ions, which, as anticipated, emits intense greenish phosphorescence under ambient conditions owing to the heavy atom effect of Pb2+ ions.36 In our continuous efforts to study MOFs/CPs with H4EBTC ligand, we chose heavy metal ion Cd2+ with H4EBTC ligand to build new luminescent MOFs/CPs using solvothermal reaction and successfully achieved CP [Cd3(HEBTC)2(DMSO)6] (1). CP 1 shows dual emissions of fluorescence and phosphorescence at ambient condition. In addition, it is discovered that 1 is isomorphic to [Mn3(HEBTC)2(DMSO)6],8 a previously reported CP, with similar cell parameters. Then, three Cd/Mn heterometallic CPs, [(CdxMn1−x)3(HEBTC)2(DMSO)6] with x = 1/3 (2), 0.5 (3), and 2/3 (4) were further prepared. In this paper, we describe synthesis, characterization, and photoluminescence properties of 1−4 and discuss the electron transition assignment of dual emissions and the possible origin of isomorphism between [Mn3(HEBTC)2(DMSO)6] and 1.

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RESULTS AND DISCUSSION Crystal Structure and Isomorphism. Single-crystal X-ray diffraction analyses revealed that 1−4 are isomorphic to the analogue [Mn3(HEBTC)2(DMSO)6]8 with fairly similar cell parameters to each other (Table 1) and have almost the same packing structures. As a result, herein only the crystal structure of 1 is described in detail. CP 1 crystallizes in triclinic space group P-1 with the formula of C48H50Cd3O22S6. As shown in Figure 1a, the asymmetric unit of 1 consists of two crystallographically unique six-coordinated Cd2+ ions, one HEBTC3− ligand together with two coordinated and one uncoordinated DMSO molecule as well. Both two crystallographically independent Cd2+ ions (labeled as Cd1 and Cd2, respectively) are coordinated with six oxygen atoms, however, show significantly different coordination environments. The Cd1 coordination sphere displays regular octahedral geometry and six coordinated oxygen atoms coming from six carboxylates in six different HEBTC3− ligands, where the Cd1 lies in the center of coordination octahedron and the Cd−O distances range from 2.259(6) to 2.292(6) Å. The Cd2 coordination sphere can be viewed as a distorted octahedral geometry, where four coordinated oxygen atoms offered by three carboxylates in three HEBTC3− ligands and the other two coordinated oxygen atoms provided by two terminal coordinated DMSO molecules (Figure S4). The Cd−O distances in 1, ranged from 2.203(7) to 2.668(6) Å with the averaged value of 2.485 Å, are comparable to the values in other Cd2+ coordination compounds.37 In the crystal structure of 1, as shown in Figure 1b, each Cd1 ion is bridged to two neighboring Cd2 ions by four −O−C− O− bridges and two μ2-Ocarboxyl atoms to afford a trinuclear {Cd3} cluster as secondary building unit (SBU), where the Cd1, Cd2, and Cd2#1 are strictly linear owing to the Cd1 located at an inversion center. The Cd1···Cd2 distance is 3.589 Å within a trinuclear cluster, which is a little longer than the Mn···Mn distance (3.536 Å) in the isomorphic [Mn3(HEBTC)2(DMSO)6]8 due to the ionic radii of Cd2+ ion being slightly bigger than that of Mn2+ ion. Each HEBTC3− C

DOI: 10.1021/acs.inorgchem.8b00374 Inorg. Chem. XXXX, XXX, XXX−XXX

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intraligand π → π* electron transitions within the aromatic rings and the carboxyl groups as well as the n → π* electron transitions in the carboxyl groups. With respect to the UV−vis absorption spectrum of H4EBTC, the absorption bands show a slight blue-shift in 1−4, suggesting that the prominent ligandbased electronic energy levels are little affected by its coordination to Cd2+ and/or Mn2+ metal ion.49 The solid-state photoluminescent properties of 1−4 with H4EBTC were investigated at room temperature (Figures 3 and 4, Figure S11−S13), including the excitation, emission spectra, the photoluminescence decay time and the absolute quantum yields. The corresponding spectroscopic and photophysical parameters are listed in Table 3. Among 1−4, the photoluminescent property of 1 has been systematically investigated in solid state. Besides the photoluminescent properties studied at room temperature, its variable-temperature solid state emission spectra were also measured in the ranges of 10−300 K. The solid-state emission spectra of 1 and H4EBTC are depicted in Figure 3a and Figure S11, respectively, for the powdered samples in open air at room temperature. The emission spectrum of H4EBTC displays only one band with a maximum at 388 nm upon excitation at 278 nm, attributed to intraligand π−π* electron transition. However, the emission spectrum of 1 exhibits well-resolved dual emissions under the ultraviolet light (λex = 286 nm) excitation at room temperature and such emissions lead to colorless rectangle crystals of 1 under daylight displaying blue color under 330−380 nm ultraviolet irradiation at room temperature (Figure 3b,c). In Figure 3a, the high-energy (HE) emission band centers at 390 nm and the lower-energy (LE) emission band shows a maximum located at 562 nm, and the intensity of LE emission band is slightly lower than that of HE emission band. Though the dual emissions were observed in the solid-state PL spectrum of the isomorphic [Mn3(HEBTC)2(DMSO)6] at room temperature, the theoretical calculation demonstrated that the HE emission band arises from electron transition within HEBTC3− and the LE emission band is contributed to the MLCT transition between HEBTC3− and Mn2+ centers as well.8 By comparison of the emission spectra of 1 and H4EBTC, it is found that the maximum of the HE emission band in 1 is close to that of the emission band in H4EBTC. The further investigation disclosed that the HE and LE bands in 1 possess the emission lifetimes of 2.19 ns and 520 μs versus the quantum yields of 1.14% and 0.65%, respectively. In general, the conjugated organic molecules, such as typical dyes, commonly show the emission lifetime in the time scales of 1−10 ns,50,51 nevertheless, there are a small amount of longer lived exceptions, for example, the conjugated organic molecule, pyrene, shows a lifetime of 400 ns in degassed solvents or 100 ns in lipids and the coronene has a lifetime of 200 ns. If the electron transition undergoes between the electron states with different spin multiplicity, the fluorophores show much longer lifetimes, for instance, the f−f electronic transitions in the lanthanide ions show a lifetimes of 0.5−3 ms in the complexes of lanthanide. In addition, the MLCT transitions have the lifetimes from 10 ns to 10 μs in some heavy transition-metal ion complexes, and the transitions between T1 and S0 states in the organic phosphors show the range of lifetimes from microseconds to milliseconds as well.52 Obviously, the emission lifetimes of HE and LE emission bands fall within the nanosecond and submicrosecond time scales (Figure S13 and Table 3), indicating the typical fluorescence and phosphor-

of Mn2+ is 0.97 Å in the high-spin state,47 which is comparable to the ionic radii of Cd2+ (1.03 Å). These two factors probably result in [M3(HEBTC)2(DMSO)6] (M = Cd (1) and Mn) possessing isomorphism. In the crystal structures of heterometallic CPs 2−4, the partial Cd2+ ions are replaced by Mn2+ ions in the linear trinuclear cluster SBU, and the relative amount of Cd and Mn elements are determined by electron-dispersive spectroscopy (EDS). With the amount of Mn element increase, the averaged M−O bond length in the Cd/Mn coordination octahedra reduces a little owing to the ionic radii of Mn2+ being slightly shorter than that of Cd2+, which leads to the characteristic structure parameters (shown in Figure S5) in the trinuclear SBU and packing structure being a bit change. The characteristic structure parameters, such as the dihedral angle between the pair of phenyl rings of HEBTC3−, the M···M distance within a trinuclear SBU ,and the π···π distances between the phenyl rings in the neighboring HEBTC3− ligands are further summarized in Table 2. It is worth mentioning that the Table 2. Characteristic Structure Parameters in the Crystal Structures of 1−4 compound

n(Cd:Mn)

π···π distances (Å)

M1···M2 distance (Å)

dihedral angle (deg)

1 2 3 4 Mn8

3:0 2:1 1:1 1:2 0:3

3.521/4.105 3.480/4.117 3.532/4.076 3.514/4.067 3.505/4.060

3.589 3.566 3.538 3.545 3.536

84.78 84.34 86.70 86.77 85.20

trinuclear SBU geometries and crystal packing structures of 1− 4 together with [Mn3(HEBTC)2(DMSO)6] are rather similar to each other. Notably, although a large sum of heterolanthanide-transition frameworks with various motifs have been extensively reported, to the best of our knowledge, only very few researches have dealt with the heterometallic 3d−4d MOFs/CPs.48 Absorption and Luminescent Spectroscopes in Solid State. Figure 2 depicts the solid-state UV−vis absorption

Figure 2. UV−vis absorption spectra of 1−4 together with H4EBTC ligand (black line) in the range of 200−500 nm.

spectra of 1−4 together with the ligand H4EBTC at room temperature. Remarkably, four CPs show similar absorption spectra, where two peaks accompanied by two shoulders in the high-energy side appear in the ranges of 200−400 nm, no sizable absorption in the visible spectroscopy region. The high similarity in the UV−vis absorption spectra of 1−4 and H4EBTC indicates that all absorption bands arise from the D

DOI: 10.1021/acs.inorgchem.8b00374 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. (a) Solid-state emission spectrum of 1 at room temperature. (b) Photo of microcrystals of 1 under ambient light and (c) photo of crystals of 1 under 330−380 nm ultraviolet irradiation. (d) Temperature-dependent solid-state emission spectra of 1 in the range of 10−300 K excited under the UV light with λex = 286 nm.

Figure 4. Solid-state emission spectra of (a) 2, (b) 3, and (c) 4 at room temperature excited under the UV light with λex = 300 nm. (d) CIE 1931 chromaticity diagram of 1−4.

E

DOI: 10.1021/acs.inorgchem.8b00374 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 3. Photophysical Data of 1−4 Discussed in This Work abs

a

emission (300 K)

compound

λmax (nm)

λmax (nm)

1 2 3 4 Mn8 H4EBTC

279, 274, 272, 277, 276, 280,

390, 393, 386, 397, 397,

315 317 317 317 317 317

562a 471b 469b 472b 468b 392c

τHE (ns) 2.19 2.58 2.02 2.61 3.31 −

τLE 520 μs 3.51 5.13 5.18 5.89 −

ns ns ns ns

ϕHE (%)

ϕLE (%)

1.14 1.96 5.55 8.89 15.3 0.093

0.65 0.68 1.94 4.53 12.3 −

λex = 286 nm. bλex = 300 nm. cλex = 278 nm, ϕf, and ϕp; the respective quantum yields of fluorescence and phosphorescence.

Scheme 1. Schematic Illustration of the Photophysical Processes of (a) CP 1 and (b) Energy Level Diagram of CP 2−4

Figure 5. Photos of microcrystals of 2−4 under (a, c, e) ambient light and 330−380 nm (b, d, f) ultraviolet irradiation, respectively, at room temperature.

F

DOI: 10.1021/acs.inorgchem.8b00374 Inorg. Chem. XXXX, XXX, XXX−XXX

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attributed to MLCT between HEBTC3− and partially substituted Mn2+ centers (Scheme 1b), and this situation is similar to the observation in CP of [Mn3(HEBTC)2(DMSO)6].8 The emission lifetimes were further measured for both HE and LE emission bands in 2− 4, as depicted in Table 3, and the emission lifetimes of both HE and LE bands in 2−4 fall within the nanoseconds time scale, which are comparable to the corresponding lifetimes of the emission band observed in [Mn3(HEBTC)2(DMSO)6],8 further supporting the assignment for the HE and LE emission bands in 2−4.

escence characters, respectively. As a consequence, the HE emission band in 1 is certainly assigned to the fluorescence process, arising from S1 → S0 electron transition within the ligand, and the LE emission band in 1 is attributed to the ligand-based phosphorescence process, namely, from the lowlying triplet state T1 to singlet ground state S1 (Scheme 1a). Theoretically, the MOFs or CPs, where the fluorescent ligands bind to the heavy metal ions, probably emit phosphorescence owing to the so-called heavy atom effect. In a recent study, we observed the intense and long-lived greenish phosphorescence in a Pb-based coordination polymer ([Pb2(EBTC)(DMSO)3]) at ambient condition, which arises from the electron transition within the p-type orbitals of EBTC4− ligand, and the theoretical calculation further demonstrated that the small amount of 6s orbitals of Pb2+ ions contributed to in the valence bands of the crystal orbitals, which promotes spin−orbit coupling interaction in the valence bands to enhance the phosphorescence efficiency of H4EBTC ligand.36 In 1, as described in the crystal structure section, each HEBTC3− ligand coordinates to six heavy Cd2+ ions through bidentate (μ2-η1, η1 and μ2-η2, η1) coordination fashions with three carboxyl groups, it is similar to the Pb-based CP ([Pb2(EBTC)(DMSO)3]),36 thus, the heavy atom effect of Cd2+ ions leads to 1 emitting phosphorescence in open-air at room temperature. The variable-temperature solid-state luminescence spectra are shown in Figure 3d for 1. Upon cooling, the profiles of emission spectra in the range of 165−300 K are comparable to that at room temperature, nevertheless the intensities of both HE and LE emission bands increase significantly with temperature decreasing. This situation is because the thermal activation of non-radiative relaxation process is more suppressed at low temperature, which results in increase of the radiative emission rate. From Figure 3d, it is clear that the lowtemperature environment favors the phosphorescence emission process of 1, and both the centers of HE and LE bands slightly blue-shift with temperature decreasing as well. Especially, a new broad emission band at ca. 440 nm together with a shoulder at ca. 590 nm comes into view at the temperatures below 140 K, and the new emission bands significantly enhance with temperature decreasing. The emission spectra change to lead to 1 showing thermochromic behavior, which is confirmed by the change of chromaticity coordinates from (0.2804, 0.3559) at 300 K to (0.3004, 0.4613) at 10 K (Figure S14). It is worth mentioning that the assignment has not been cleared for the new emission bands occurring at the temperatures below 140 K at present stage, and the further studies are needed. The solid-state photoluminescence spectra of 2−4 at room temperature, as shown in Figure 4a−c, also display dual emission bands, which give rise to colorless crystals of 2−4 emitting light blue to cyan color (Figure 5), respectively. The maximum of HE band locates at ca. 398, 392, and 396 nm for 2−4 successively, which are close to the maximum of HE band in 1. The maximums of LE band occur at ca. 470, 469, and 476 nm for 2−4 in turn; all of them deviate much from the center of LE emission band in 1 (562 nm), whereas are close to the center of LE emission band in [Mn3(HEBTC)2(DMSO)6] at room temperature (ca. 468 nm). It is also noted that the relative intensity of LE to HE emission bands increases with the relative amount of Mn element increasing in the emission spectra of 2−4. These observations suggest the HE emission band is assigned to the ligand-based fluorescence in the emission spectra of 2−4, whereas the LE emission band is

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CONCLUSION

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EXPERIMENTAL SECTION

In summary, we have achieved a two-dimensional CP of Cd2+ with fluorescent HEBTC3− ligand, [Cd3(HEBTC)2(DMSO)6], using solvothermal reaction, and this CP is isomorphic to twodimensional CP of [Mn3(HEBTC)2(DMSO)6]. We further obtained three isomorphic heterometallic CPs of [(CdxMn1−x)3(HEBTC)2(DMSO)6] with x = 1/3, 1, and 2/ 3. In the crystal structures of [M3(HEBTC)2(DMSO)6] (M = Cd and Mn), the Cd2+/Mn2+ centers adopt the distorted octahedral coordination geometry with six oxygen atoms from carboxyl groups/DMSO molecules. In such a weak ligand field, the Mn2+ ions are in the high-spin state, which leads to the semifilled shell Mn2+ ions showing spherical electron cloud distribution, and this situation is the same as the case of the closed-shell electron configuration Cd2+ ion. Additionally, the high-spin state Mn2+ ion and Cd2+ ion have a comparable ionic radius. These two structural factors respond to the isomorphism of [M3(HEBTC)2(DMSO)6] (M = Cd and Mn). In the solid-state photoluminescent spectra, both [M3(HEBTC)2(DMSO)6] (M = Cd and Mn) display dual emission bands, remarkably, the electron transition mechanisms responding to dual emissions are distinct from each other. The HE emission band in both [M3(HEBTC)2(DMSO)6] (M = Cd and Mn) is assigned to the HEBTC 3− ligand-based fluorescence, the LE emission band is attributed to the MLCT electron transition in heterometallic CPs, while the phosphorescence in homometallic Cd−CP is due to the heavy atom effect of Cd2+ ions. This study opens a way for better understanding of the isomorphism and photoluminescent nature often observed in many MOFs/CPs and gives a fresh impetus to achieve the MOFs/CPs-based long-lived phosphorescence materials at ambient condition.

Reagents and Materials. All reagents and materials are of analytical grade and used as received from commercial sources without further purification. H4EBTC was synthesized according to the method published before.53 Chemical Analysis and Physical Measurements. Elemental analyses (C, H) were carried out on a Perkin-Elmer 240 analyzer. The IR spectra were obtained on a NICOLET iS10 spectrometer in the 4000−400 cm−1 region. Thermal gravimetric analyses (TGA) were performed using a DTA-TGA 2960 thermogravimetric analyzer in nitrogen atmosphere with a heating rate of 10 °C/min from 25 to 800 °C. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 Discover diffractometer with Cu Kα radiation (λ = 1.54056 Å) with a scan speed of 5°/min and a step size of 0.02° in 2θ. The morphology and composition of the samples were inspected using a scanning electron microscope (SEM, S-3400N II, Hitachi) equipped with an energy-dispersive X-ray spectroscope (EDX, EX-250, HORIBA). UV−vis absorbance was collected in the solid state at room temperature on a Perkin-Elmer Lambda 950 UV−vis G

DOI: 10.1021/acs.inorgchem.8b00374 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry spectrometer equipped with Labsphere integrating over the spectral range 200−800 nm using BaSO4 as reflectance standards. Steady-state emission and excitation spectra were recorded for the solid samples on an F-7000 FL spectrophotometer equipped with a 150 W xenon lamp as an excitation source at room temperature. The photomultiplier tube (PMT) voltage was 700 V in all the measurements. The scan speed was 1200 nm/min. The variable-temperature photoluminescence study was performed on a Fluorolog-3-TAU fluorescence spectrophotometer. The phosphorescence lifetime and absolute photoluminescence quantum yields were measured on steady-state and timeresolved fluorescence spectrofluorometer. The photoluminescence decay time measurements were performed at room temperature for the emission bands with the maximum of peak at ca. 390 and ca. 562 nm for 1, 393 and 471 nm for 2, 386 and 469 nm for 3, and 397 and 472 nm for 4, respectively. The corresponding photoluminescence decay curves are shown in Figure S13. The emission decay lifetimes of both HE and LE bands at room temperature were characterized by single exponential curves. The absolute quantum yields of 1−4 were determined at room temperature by means of an integrating sphere, and the values obtained for the HE and the LE emissions are summarized in Table 3. Preparation and Characterization of CP 1−4. [Cd3(HEBTC)2(DMSO)6] (1). A solution of Cd(NO3)3·4H2O (6.5 mg, 0.021 mmol), H4EBTC (5 mg, 0.014 mmol), DMSO (0.4 mL), CH3OH (0.10 mL), HNO3 (0.02 mL, 1 M in DMF), and H2O (0.10 mL) were mixed and sealed in a 10 mL Teflon-lined autoclave, which was heated to 120 °C for 24 h. The colorless rectangle-shaped crystals were harvested after the Teflon-lined autoclave cooled to ambient temperature naturally (yield: 70% based on the reactant Cd(NO3)3· 4H2O). Anal. calcd for C48H50Cd3O22S6 (%): C, 38.22; H, 3.34. Found: C, 37.76; H, 3.43. Selected IR data (KBr pellet, cm−1): 3444 (b), 3074 (w), 3002 (w), 2915 (w), 1706 (s), 1616 (w), 1577 (w), 1355 (w), 1004 (s), 982 (w), 775 (s), 723 (s). The series of Cd/Mn mixed-metal CPs (2−4) were synthesized using exactly same synthetic conditions as preparation of 1, but the starting material, Cd(NO3)3·4H2O, was replaced by the mixture of Cd(NO3)3·4H2O and Mn(NO3)3·4H2O with a molar ratio of 2:1 for 2, 1:1 for 3, and 1:2 for 4. The yields are more than 70% for 2−4 based on the used H4EBTC in the reaction. The PXRD profiles of the microcrystalline materials of 1−4 are in good agreement with their simulated patterns (Figure S1) obtained from the single crystal structure data using Mercury 3.1 program, confirming that the as-synthesized products of 1−4 show high phase purity. Crystallographic Analyses. Suitable single crystals of 1−4 were carefully selected under an optical microscope and glued to thin glass fibers. Single crystal X-ray diffraction data were collected on a Bruker Smart Apex II CCD diffractometer at 296 K using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Data reductions and absorption corrections were performed with the SAINT54 and SADABS255 software packages, respectively. Structures were solved by a direct method using the SHELXL-2014 software package.56 The non-hydrogen atoms were anisotropically refined using the full-matrix least-squares method on F2. All hydrogen atoms were placed at the calculated positions and refined riding on the parent atoms. CCDC 1822028 (1), 1821754 (2), 1821757 (3), and 1821759 (4) 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. The selected bond distances and angles of 1 are listed in Table S1, respectively.

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coordination modes and the dihedral angle of the two phenyl rings of the ligand in 1−4, view of the crystal packing of 1 along a- and b-axes directions, SEM images of the heterometallic Cd/Mn-CPs of 2−4, EDS data of 2−4, solid-state PL spectra of H4EBTC (λex= 278 nm) at room temperature, solid-state excitation spectra of 1−4 at room temperature, emission decay of the HE emission and LE emission of 1−4, CIE 1931 chromaticity diagram of CP 1 from 10 to 300 K, and table of selected bond lengths (Å) and angles (o) in 1 (PDF) Accession Codes

CCDC 1821754, 1821757, 1821759, and 1822028 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]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Wen-Wei Zhang: 0000-0002-7482-3013 Jing-Lin Zuo: 0000-0003-1219-8926 Xiao-Ming Ren: 0000-0003-0848-6503 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors thank the Priority Academic Program Development of Jiangsu Higher Education Institutions and the National Nature Science Foundation of China (grant nos. 21671100 and 51173075) for financial support.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00374. Experimental and simulated PXRD patterns of 1−4 at room temperature, TG plots and IR spectra of 1−4, view of coordination environment of Cd1 and Cd2 atoms in 1, H

DOI: 10.1021/acs.inorgchem.8b00374 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00374 Inorg. Chem. XXXX, XXX, XXX−XXX