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Ultralong Persistent Room Temperature Phosphorescence of Metal Coordination Polymers Exhibiting Reversible pH–responsive Emission Yongsheng Yang, Kezhi Wang, and Dongpeng Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03956 • Publication Date (Web): 02 Jun 2016 Downloaded from http://pubs.acs.org on June 3, 2016

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ACS Applied Materials & Interfaces

Ultralong Persistent Room Temperature Phosphorescence of Metal Coordination Polymers Exhibiting Reversible pH–responsive Emission

Yongsheng Yang,[a] Ke-Zhi Wang*[a] and Dongpeng Yan*[a,b]

[a] Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875 (P. R. China), E-mail: [email protected] (Yan D); [email protected] (Wang KZ)

[b] State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China.

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ABSTRACT: Ultralong–persistent room temperature phosphorescence (RTP) materials have attracted much attention and present various applications in illumination, displays and bio–imaging fields; however, the persistent RTP is generally from the inorganic phosphor materials to date. Herein, we show that the metal coordination polymers (CPs) could be new types of emerging long–lived RTP materials for potential sensor application. Firstly, two kinds of Cd–based CPs, Cd (m-BDC) (H2O) (1) and Cd (m-BDC) (BIM) (2) (m-BDC = 1,3-benzenedicarboxylic acid, BIM = benzimidazole) were obtained through a hydrothermal process, and the samples were found to exhibit two–dimensional layered structures, which are stabilized by interlayer C–H···π interaction and π···π interaction, respectively. The CPs show unexpected second–timescale ultralong–persistent RTP after the removal of UV excitation and this persistent emission can be detected easily in the timescale 0–10 seconds. The CPs also feature tunable luminescence decay lifetime by adjusting their coordination situation and packing fashion of ligands. Theoretical calculation further indicates that the introduction of the second ligand could highly influence the electronic structure and intermolecular electron transfer towards tailoring the RTP of the CP materials. Moreover, the CP 2 exhibits well–defined pH and temperature dependent phosphorescence responses. Therefore, this work provides a facile way to develop new type of CPs with steady–state and dynamic tuning of RTP properties from both experimental and theoretical perspectives, which have potential applications in the areas of displays, pH/temperature sensors and phosphorescence logic gates. On account of suitable incorporation of inorganic and organic building blocks, it can be expected that the ultralong–persistent RTP CPs can be extended to other similar systems due to the highly tunable structures and facile synthesis routes.

KEYWORDS: room temperature phosphorescence, coordination polymers, tunable luminescence, pH/temperature sensor


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1. INTRODUCTION In recent years, the chemistry and materials of the coordination polymers (CPs) have been advanced extensively, due to their facile construction from a variety of molecular building blocks with tunable interactions and functional groups.1 CPs have got a series of achievements in the fields of gas storage,2-7 catalysis,8-9 ion separation,10-11 sensor device,12-20 and so on. In addition, luminescent CPs came to prominence owing to their excellent performances in white–light illumination,21-22 tunable photoemission,23 and electroluminescence devices.24-25 As an important type of luminescent systems, phosphorescent materials are attracting extensive attention recently. In comparison to fluorescence systems, it usually involves triplet excited states and comparatively slower decay rates,26 which facilitate further use of biological systems and time–gated experiments, since the phosphorescence could exclude the effects of the cellular auto–fluorescence background and thus allows easy monitoring of cellular phenomena.27 At present, the long lasting phosphorescent materials are mainly from the hybrid inorganic materials (such as spinel oxides, CdSiO3:Dy3+, ZnS:Cu+, Sr4Al14O25:Eu2+, Sr4Al14O25:Dy3+).28-29 However, the harsh synthetic conditions and the lack of tunable structures and properties have impeded their applications. Compared with these traditional inorganic phosphors, the CPs materials feature ease of tunable photoactive units and photo–emissive properties (such as wavelength and luminescence lifetime) by adjusting their coordination situation and packing fashion based on moderate reaction conditions. In this regard, great efforts have been devoted to the exploration of new long–persistent phosphorescence materials based on CPs. Up to now, the exploration of CPs–based phosphors mainly focused on the lanthanide and precious metal complexes. However, the phosphorescence lifetimes of lanthanide complexes are usually at the millisecond timescale, and that of precious metal complexes are even at the microsecond or nanosecond timescale. To the best of our knowledge, there has been scarely reported on the phosphorescent CPs with emission decay at the second timescale. Recently, Sun et al. have reported that the CP–based phosphorescence can be detected by the naked eye for ca. 2 s at 77 K.30 In addition, the as–reported room temperature afterglow behavior has been focused on single–ligand Zn–based CP;31 however, the facile design of multiple–ligand phosphorescent CPs have not been explored to date. Therefore, it is essential to develop the



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low–cost and long–lasting room temperature phosphorescence (RTP) CPs, since the cryogenic phosphorescence is still difficult for application in the daily life. In this work, we have developed new Cd–based CPs with high–efficiency RTP. The selection of Cd as metal unit is based on the fact that several doped Cd salts (such as CdSiO332 and CdWO433) have exhibited preferable afterglow RTP behaviors. Moreover, we have chosen the 1,3-benzenedicarboxylic acid (m-BDC) and benzimidazole (BIM) as coordination ligands based on the following expectations: firstly, carbonyl group can effectively enhance spin–orbit coupling and related intersystem crossing (ISC) process, and the strong coordination bonds in the orderly condensed states can significantly impede the nonradiative depopulation of triplet excitons.34 Secondly, BIM can serve as an electron–buffering component owing to its high electron mobility, which has been widely used in the design of RuII, OsII and IrIII CPs.35-37 The as–fabricated CPs materials feature ease of tunable luminescence lifetime, which also exhibit well–defined temperature– and pH–dependent phosphorescence responses (Scheme 1). Therefore, this work provides a new platform to construct CPs–based RTP materials with potential pH sensor application, which also take the advantages of low cost, moderate reaction conditions, facile structural modifications and tunable phosphorescent properties.

Scheme 1. Schematic representations of the two Cd–based CPs with persistent RTP and their pH–dependent phosphorescence responses.


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2. RESULTS AND DISCUSSION 2.1 Structural Description of CPs: Cd (m-BDC) (H2O) (1) and Cd (m-BDC) (BIM) (2). The single–crystal X–ray analysis of CP 1 reveals that its crystal structure belongs to the orthorhombic system with space group Pbcm (No. 57). In the CP 1, each building unit consists of one Cd cation, one m–BDC2- ligand and one terminal water molecules (Figure S1a and Table S1). The CdO7 polyhedron is formed by a coordinated water molecule and six oxygens from tridentate carboxylate groups of the four different molecules with (2121) connectivity (Figure 1a). The edge–sharing polyhedrons are connected by the carboxylate to form

one–dimensional chains,

which further get interconnected to

extend into

two–dimensional layered structure. The structure is stabilized by interlayer C–H···π interaction (2.780 Å, 118.75°) between the benzene rings of m-BDC (Figure 1b). Topologically, each m-BDC or Cd cation is considered as a 4–connected quadrangle node, and a two–dimension sheet of (4, 4) topology is formed by m-BDC and Cd cation. This type of net consists of four connected nodes shared by four quadrilateral units. The quadrilateral unit forms a four–member ring, containing two m-BDC ligands and two Cd cations. This CP can be simplified as a two–dimensional mesh (4, 4) topology (Figure 1c). As a mixed–ligand complex, the CP 2 belongs to the triclinic system with space group of Pī (No. 2). In the framework 2, each building unit consists of two Cd cations, two m-BDC2- and two BIM ligands (Figure S1b and Table S1). The CdO4N polyhedron is formed by a coordinated BIM ligand and four oxygens from one tridentate carboxylate group and two bidentate carboxylate groups of the three different molecules with (211) connectivity, and the CdO5N polyhedron is formed by a coordinated BIM ligand and five oxygens from one tridentate carboxylate group and three bidentate carboxylate groups of the four different molecules with (1211) connectivity (Figure 1d). The edge sharing polyhedrons are connected by the carboxylate to form one–dimensional chains and the chains get interconnected by another carboxylate anion to form an extended two–dimensional layered structure. The structure is stabilized by interlayer π···π interaction (3.592 Å) between the BIM rings (Figure 1e). Topologically, each Cd1 atom is considered as a 4–connected node, each Cd2 atom is considered as a 3–connected node and one part of m-BDC ligands are considered as a 4–connected node, the other part of m-BDC ligands are considered as a 3–connected node,



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respectively. This CP can be simplified as a two–dimensional mesh (43)(4.82)(42.84)(43.82.10) topology (Figure 1f).

Figure 1. (a) Structure of CP 1, viewed along the a–axis. (b) Interlayer C–H···π interaction (2.780 Å, 118.75°) between the benzene rings of m-BDC. (c) Schematic representation of the two–dimensional mesh (4, 4) topology. (d) Structure of CP 2, viewed along the c–axis. (e) Interlayer π···π interaction (3.592 Å) between the BIM rings. (f) Schematic representation of the two–dimensional mesh (43)(4.82)(42.84)(43.82.10) topology.

2.2 Photophysical Properties of CPs: Cd (m-BDC) (H2O) (1) and Cd (m-BDC) (BIM) (2). The solid–state fluorescent and phosphorescent properties of CP 1 and CP 2 were investigated under room temperature. The colorless CP crystals exhibit blue–fluorescence, with single broad emission bands centered at 392 and 396 nm (λex = 322 nm for CP 1 and λex = 317 nm for CP 2, Figure S2a), corresponding to the dark blue (color coordination: (0.1553, 0.1324)) for CP 1 and light blue (color coordination: (0.1563, 0.1507)) for CP 2 respectively (Figure 2a, 2b). Interestingly, after the removal of UV excitation, the two CPs present dark green (color coordination: (0.2093, 0.5520)) and light green (color coordination: (0.2016, 0.5275)) RTP afterglow, respectively (Figure 2a, 2b). The CPs 1 and 2 emitted green light with the emission bands centered at 514 nm (λex = 317 nm for CPs 1 and 2, Figure S2b) and the persistence time could be traced in the time range 0‒10 s by the naked eye (Figure 3a 3b). Considering the similarity of long‒lived phosphorescent emissions for CPs 1 and 2, the phosphorescence of


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CPs 1 and 2 should come from a Cd–m-BDC–centered triplet excited state.38 The fluorescence and phosphorescence quantum yields for CP 2 are estimated as 4.31% and 3.09%, which are higher than that of the CP 1 (with quantum yields of 4.07% for fluorescence and 2.12% for phosphorescence). The difference in photoemission properties can be ascribed to the different stacking interactions (C–H···π interaction for CP 1 and π···π interaction for CP 2) between the two CPs. The phosphorescent decay lifetimes of CPs 1 and 2 in the solid state were determined to be 698 and 755 ms respectively (Figure 3c), which are clearly higher than that of m-BDC34 and BIM (Figure S2c). The longer phosphorescent decay lifetimes of CPs 1 and 2 can be ascribed to the restriction of intramolecular motions based on effective coordination interactions between Cd(II) and m-BDC in the CPs, which help to lock and rigidify the molecular conformations, thus minimizing the nonradiative loss of triplet excitons and reducing the phosphorescence decay rate. Based on the photoluminescence spectra (Figure S2c, S2d), the energy diagrams of m-BDC, BIM ligands and two CPs can be deduced as shown in Figure S2e, S2f. For m-BDC and BIM, the energy gap between S1 and T1 (∆ES1T1) are 0.98 and 1.02 eV, respectively, which are higher than that of CP 1 (0.75 eV) and CP 2 (0.72 eV). The small energy gap is favorable for the intersystem crossing process,34 and this fact indicates that the coordination of Cd(II) boosts the phosphorescence emission. Comparing with CP 1, the lifetime of CP 2 is relative longer, and this result can be related to the fact that the incorporation of BIM ligand increased the space steric hindrance and brought about more free single–electrons. The large steric hindrance suppressed the motion of m-BDC ligand or even the whole CP movement to promote phosphorescence activation.39-40 This result confirms that the RTP properties of CP–based materials can be highly optimized by changing the coordination situation of center metal, and these two CPs are promising candidates for solid–state phosphorescence materials.



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Figure 2. (a) Normalized photoluminescence spectra of CP 1 and CP 2 (blue and green line were measured under fluorescence and phosphorescence modes, respectively). (b) The corresponding position in the color coordinates diagram of CP 1 and CP 2.

Figure 3. Photographs of the long–lived RTP CPs 1 (a) and 2 (b) materials taken at different time intervals before and after turning off the UV excitation (365 nm) under ambient conditions. (c) Time–resolved emission decay curves (at 514 nm) for CPs 1 and 2 under ambient conditions. 2.3 Thermal Properties and Heat–Responsive Phosphorescence of CPs: Cd (m-BDC) (H2O) (1) and Cd (m-BDC) (BIM) (2). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed to investigate the thermal properties of CPs 1 and 2. The two CPs show high thermal stability with the decomposition temperatures at 383 ºC and 395 ºC (Figure 4a). The thermal stability of CP 2 is better than that of CP 1, which can be attributed to the large steric hindrance by coordinating with BIM ligand that suppressed the motion of the whole CP movement.41-42 It can be observed that, for CP 1, the initial weight loss in the range from room temperature to 240 ºC accompanying the weight loss of 7% (calculated theoretical value: 6%) is due to the removal of coordinating water molecule. The following weight loss of ~49%


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in the temperature range from 383 to 800 ºC can be attributed to the decomposition of the m-BDC ligand. Similarly, the weight loss of ~67% for CP 2 in the temperature range from 395 to 800 ºC is related to the decomposition of the m-BDC and BIM. The remaining weight for CPs 1 and 2 are 44% and 33% respectively, which can be assigned to cadmium oxide (calculated theoretical values: 44% and 33%). To detect the heat–related persistent phosphorescence, temperature–dependent spectra of CP 2 were further measured. It was observed that the bands at 514 nm (λex = 317 nm) were reduced obviously upon the increase of the temperature from ambient condition (293 K) to 453 K (Figure 4b), while the bands at 514 nm (λex = 317 nm) were increased obviously upon the decrease of the temperature from 293 to 77 K (Figure 4c). It can be found that the dependence of the phosphorescence intensity on the temperature can be divided into two main zones: the intensity decreases sharply from 77 to 293 K, but it decreases slightly from 293 to 453 K, which shows a nearly linear relationship with the temperature (Figure 4d). Such behavior can be attributed to that the molecular vibrations and nonradiative loss have increased under high temperature. Similar behavior also occurs for several other organic phosphor systems.43

Figure 4. (a) TGA curves of CP 1 and CP 2. (Inset: the corresponding DSC traces). The phosphorescence spectra of CP 2 measured at temperatures from 293 to 453 K (b), and from 293 to 77 K (c). (d) The phosphorescence intensity along with the change of temperature.



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2.4 Theoretical Calculations on the CPs: Cd (m-BDC) (H2O) (1) and Cd (m-BDC) (BIM) (2). To obtain energy level and electronic structural information of the CP 1 and CP 2, periodic density functional theoretical (PDFT) calculations were performed on their crystal models. For the CP 1 systems (Figure 5a), frontier orbital analysis shows that the highest occupied molecular orbitals (HOMO), the lowest unoccupied molecular orbitals (LUMO) as well as HOMO–1 and LUMO+1 are mainly populated on the benzene ring and the carboxyl group of m-BDC. This suggests that the photoemission including fluorescence and phosphorescence is mainly derived from the m-BDC units within the framework. There is a lack of electronic transfer process between CdO7 clusters and the ligands, suggesting that no metal–to–ligand or ligand–to–metal charge transfer (MLCT or LMCT) occurs. For the CP 2 system (Figure 5b), HOMO–1 and HOMO are distributed on the BIM ligand, while the LOMO and LOMO+1 are mainly dominated by the m-BDC ligand, which can be attributed to the typical ligand–to–ligand charge transfer mechanism. Such process usually present prolonged excitation electron/hole separation, corresponding to the enhanced phosphorescence lifetime as observed in experiment.

Figure 5. Frontier orbitals for CP 1 (a) and CP 2 (b). The blue/yellow colors denote ±wave functions. Moreover, total/partial electronic densities of states (TDOS/PDOS) analyses (Figure 6a, 6b) reveal that, around the Fermi level, the electronic density is mainly dominated by the p


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orbitals from C/O atoms of carboxylate and benzene in m-BDC in the CP 1 system. While for the CP 2, electronic density is dominated by the p orbitals from C/N atoms of imidazole and benzene in BIM as well as those from C/O atoms of carboxylate and benzene in m-BDC. The calculated band gap of CP 1 is 3.11 eV (399 nm), which is higher than the CP 2 (3.07 eV, 404 nm) (Figure 6c, 6d). This is consistent well with the difference of their fluorescence spectra in experiment. Therefore, the introduction of the second ligand could change the stacking fashions as well as the interaction modes (from C–H···π to π···π interaction), and this makes the alternation of CP structure. The electronic structures of CPs can also be highly modified due to the change of the CP structure, which may further influence the optical properties as shown in both experimental and computational results.

Figure 6. Total/partial electronic density of state (TDOS/PDOS) for CP 1 (a) and CP 2 (b). Fermi energy level EF was set to zero. (c) Calculated energy band structures of CP 1. G (0, 0, 0); T (-0.5, 0, 0.5); S (-0.5, 0.5, 0); U (0, 0.5, 0.5); R (-0.5, 0.5, 0.5) are the selected reciprocal points in the first Brillouin zone (BZ). (d) Calculated energy band structures of CP 2. G (0, 0, 0); F (0, 0.5, 0); Q (0, 0.5, 0.5); Z (0, 0, 0.5); G (0, 0, 0) are the selected reciprocal points in the first BZ.

2.5 The Reversible pH–Responsive Phosphorescence Emission Stable solid–state persistent RTP is an important criterion if the material is to have practical applications in phosphorescent afterglow. To study the RTP stability of the CPs 1 and 2 under



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solution condition, we detected the persistent RTP under the water firstly. It was observed that the RTP afterglow of these two CPs can maintain under the water (pH = 7). The phosphorescent decay lifetimes of CPs 1 and 2 under the water were determined to be 404 and 554 ms, respectively (Figure S3a). Then, the phosphorescence spectra of CP 1 and CP 2 have been detected under different pH conditions (Figure 7a, 7b). With the pH value varying from 3 to 11, it was observed that the phosphorescence intensity for both of two CPs has a decreasing trend. Particularly, the phosphorescence intensity presents a nearly linear relationship with the pH value for CP 2, but not for CP 1. To explain such result, the powder X–ray diffraction (XRD) was measured after treated at different pH in solution and gas phases (Figure S3b, S3c and Figure 7c), which shows an alternation of the XRD profiles for CP 1 under high pH value and reveals the phase transformation, indicating the structure is not really reversible because of the structural changes. This confirms that the skeleton of CP 1 has been decomposed under high pH value due to the competitive coordination substitution of metal with OH– ion. Moreover, the variation in luminescence intensity with pH was further explored by alternate treatment on CP 2 by NH3 and HCl gas (Figure 7d, 7e), and the sample of CP 2 showed well stability in this process (Figure S3d). It can be observed that the CP 2 showed a reversible photoemission response, and the change of luminescence can be readily repeated at least for 4 times during cycles upon NH3–HCl treatment, indicating the high RTP stability and reproducibility. This also ensures that the CP 2 can be potentially as pH–sensitive RTP sensor. In our opinion, compared with the CP 1, the second ligand (BIM) in CP 2 could work as a proton buffering unit to enhance the stability under both acid and alkali environments.


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Figure 7. Phosphorescence spectra (λex = 317 nm) of CP 1 (a) and CP 2 (b) detected at different pH values and the phosphorescence intensity along with the change of pH value (inset). (c) The PXRD pattern for CP 1 after treated by NH3 and HCl gas. (d) Reversible variation of the phosphorescence intensity for CP 2 under alternate NH3 and HCl. (e) Photographs of the phosphorescence intensity for CP 2 under alternate treatment of NH3 and HCl. 3. CONCLUSION In conclusion, two CPs with ultralong persistent RTP and easily recognized afterglow (0–10 seconds) were fabricated. New CPs–based RTP materials take several advantages (such as the low cost, high yield, the mild and controllable synthesis condition, and the long lasting RTP with high thermostability), relative to the traditional inorganic phosphor materials. Moreover, the tunable RTP properties can be achieved by incorporating the second ligand. The dynamic RTP transformations were further obtained after treatments under high temperature, acid and alkali conditions. Moreover, the reversible pH–dependent phosphorescence ensures that the CP 2 can serve as a pH–responsive luminescent switch. In addition, this work also highlights the effect of the second ligand (BIM) as stabilizer under the acid and alkali environment: BIM



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could be a proton and electron buffer to balance the electron–rich or electron–poor conditions, which guarantees the reversible pH–dependent phosphorescence of the CP materials. It can be anticipated that, the CPs could be idealized models to develop a broad of RTP materials, due to the moderate reaction, the facile structural modification and highly tunable phosphorescent properties.

EXPERIMENTAL SECTION Materials and Characterization Method Materials: Analytically pure Cd(NO3)2·4H2O, 1,3-benzenedicarboxylic acid (m-BDC) and benzimidazole (BIM) were purchased from Sigma Chemical. Co. Ltd. and used without further purification. Characterization: Single–crystal X–ray diffraction data of all compounds were collected on a Bruker SMART APEX CCD diffractometer44 equipped with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å) at room temperature using the ω–scan technique. Empirical absorption corrections were applied to the intensities using the SADABS program.45 The structures were solved using the program SHELXS–9746-47 and refined with the program SHELXL–97.48 All non–hydrogen atoms were refined anisotropically. The crystallographic data for CPs 1 and 2 are listed in Tables S1 (Supporting Information). Crystallographic data for the CP structures in this work have also been deposited with the CCDC as deposition no. CCDC 1454719–1454720 (available free of charge, on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; e–mail: [email protected]). Powder XRD patterns of all compounds were collected on a Rigaku Ultima–IV automated diffraction system with Cu Kα radiation (λ = 1.5406 Å). Measurements were made in a 2θ range of 5–50° at room temperature with a step of 0.02° (2θ) and a counting time of 0.2 s/step. The operating power was 40 kV, 50 mA. IR spectra were recorded in the range of 4000–400 cm–1 on a Tensor 27 OPUS (Bruker) FT–IR spectrometer. Thermogravimetric analysis (TGA) experiments were carried out on a Perkin–Elmer Diamond SII thermal analyzer from room temperature to 800 ºC with a heating rate of 10 ºC min−1. Room temperature time–resolved photoluminescence (PL) experiments were conducted on an Edinburgh FLS980 fluorescence spectrometer. The temperature dependence of the


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phosphorescence intensity, and phosphorescence lifetime were measured on a temperature controller attached to a cryostat (Oxford Ltd. OptistatDN2) in FLS980 fluorescence spectrometer. The PL quantum yield (PLQY) values at room temperature were estimated using a Teflon–lined integrating sphere (F–M101, Edinburgh, diameter: 150 mm and weight: 2 kg) in a FLS980 fluorescence spectrometer. Elemental analyses (C, H, O and N) were performed on a Vario EL elemental analyzer. Electronic Structure Calculations of Cd (m-BDC) (H2O) (1) and Cd (m-BDC) (BIM) (2): All calculations were performed with the periodic density functional theory (DFT) method using Dmol349-50 module in Material Studio software package.51 The initial configuration was fully optimized by Perdew–Wang (PW91)52 generalized gradient approximation method with the double numerical basis setsplus polarization function (DNP). The core electrons for metals were treated by effective core potentials. The self–consistent field converged criterion was within 1.0 × 10−5 hartree atom−1 and the converging criterion of the structure optimization was 1.0 × 10−3 hartree bohr−1. The Brillouin zone is sampled by 1 × 1 × 1 k–points, and test calculations reveal that the increase of k–points does not affect the results. Synthesis of Cd(m-BDC)(H2O) (1): A mixture of Cd(NO3)2·4H2O (0.5 mmol, 0.160 g), BIM (0.2 mmol, 0.026 g), m-BDC (0.5 mmol, 0.083 g), CH3CN (2 mL) and water (8 ml) was sealed in a 23 mL Teflon reactor and was kept under autogenous pressure at 150 ºC for 72 hours, then cooled with the speed of 10 ºC per minute to room temperature. Colourless bulk crystals were filtered off, washed with distilled water, enthanol in turn and dried in air. Yield: 80% (based on Cd). The as–synthesized crystal is insoluble in water and common organic solvents. IR spectrum (KBr, cm-1): 3391.00 s, 3266.59 s, 3063.09 w, 2975.33 w, 2936.01 w, 1883.57 w, 1776.52 w, 1599.06 vs, 1546.01 vs, 1479.47 s, 1445.71 vs, 1397.49 vs, 1383.02 vs, 1315.51 m, 1273.07 m, 1163.13 m, 1099.47 m, 1082.11 m, 1045.46 m, 945.16 w, 915.26 m, 878.61 w, 835.21 s, 744.55 vs, 723.34 s, 659.68 m, 553.59 w, 503.44 m, 425.32 s. Elemental Anal. Calc. (Found %) for C8H6CdO5 (1): C, 32.59 (32.41); H, 2.04 (1.96); O, 27.16 (26.88). Synthesis of Cd(m-BDC)(BIM) (2): A mixture of Cd(NO3)2·4H2O (0.5 mmol, 0.160 g), BIM (0.2 mmol, 0.026 g), m-BDC (0.5 mmol, 0.083 g), NaOH (1 mmol, 0.040 g) and water (10 ml) was sealed in a 23 mL Teflon reactor and was kept under autogenous pressure at 150 ºC for 72 hours, then cooled with the speed of 10 ºC per minute to room temperature. Colourless bulk



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crystals were filtered off, washed with distilled water, enthanol in turn and dried in air. Yield: 70% (based on Cd). The as–synthesized crystal is insoluble in water and common organic solvents. IR spectrum (KBr, cm-1): 3600.29 m, 3416.08 s, 3264.66 s, 3112.28 m, 3978.22 w, 2916.49 w, 2848.02 w, 1602.91 vs, 1550.83 vs, 1480.43 m, 1437.03 s, 1384.95 m, 1271.14 m, 1253.78 m, 1162.16 m, 1086.93 m, 1046.43 w, 1003.99 w, 968.31 w, 922.98 w, 882.44 w, 834.25 m, 746.48 vs, 660.65 w, 626.89 w, 582.53 w, 558.42 w, 519.84 m, 438.82 m. Elemental Anal. Calc. (Found %) for C30H15Cd2N4O8 (2): C, 45.90 (46.03); H, 1.91 (1.92); O, 16.32 (16.63); N, 7.14 (6.94). Experiment of pH–responsive phosphorescence emission: The powdered CPs samples with the same amount were located into the solutions with different pH values for 10 minutes and luminescent spectra were then measured under the solution environment. The reversible pH–responsive experiments were performed by alternate treatment of CP 2 under the atmosphere conditions of ammonia (13 mol/L) and hydrochloric acid solution (10 mol/L), respectively.

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website. Figure S1a and Figure S1b show the coordination environments of the Cd(II) ions in two CPs. Figure S2a and Figure S2b show the fluorescence and phosphorescence excitation spectra of two CPs. Figure S2c shows the time–resolved emission decay curves for BIM under ambient conditions. Figure S2d and Figure S2e show the fluorescence and phosphorescence emission spectra of two ligands. Figure S2f and Figure S2g show the energy diagrams of ligands and CPs based on the photoluminescence spectra. Figure S3a shows the time–resolved emission decay curves for two CPs under water. Figure S3b and Figure S3c show the PXRD patterns for two CPs after treated with different pH solution. Figure S3d shows the PXRD pattern for CP 2 after been reversibly treated by NH3 and HCl gas for four times. Table S1 shows the crystal data and structure refinements for two CPs. Table S2 shows the fluorescent and phosphorescent color coordination for two CPs.

ACKNOWLEDGMENTS


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This work was supported by the 973 Program (Grant No.2014CB932103), the National Natural Science Foundation of China (NSFC), the Beijing Municipal Natural Science Foundation (Grant No. 2152016), the Fundamental Research Funds for the Central Universities, and Analytical and Measurements Fund of Beijing Normal University.

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