Stereochemically Active and Inactive Lone Pairs in Two Room

13 hours ago - Two new Pb2+-based coordination polymers (CPs), [Pb2(HBTC)2(DMF)] (1) and [Pb(HPTC)] (2), have been synthesized under solvothermal ...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Stereochemically Active and Inactive Lone Pairs in Two RoomTemperature Phosphorescence Coordination Polymers of Pb2+ with Different Tricarboxylic Acids Xu-Sheng Gao,*,† Hai-Jie Dai,† Mei-Juan Ding,† Wen-Bo Pei,† and Xiao-Ming Ren*,†,§ †

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State Key Laboratory of Materials−Oriented Chemical Engineering and College of Chemistry & Molecular Engineering, Nanjing Tech University, Nanjing 211816, PR China § State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, PR China S Supporting Information *

ABSTRACT: Two new Pb2+-based coordination polymers (CPs), [Pb2(HBTC)2(DMF)] (1) and [Pb(HPTC)] (2), have been synthesized under solvothermal condition; herein, H3BTC and H3PTC represent 1,3,5benzenetricarboxylic acid and 2,4,6-pyridine tricarboxylic acid, respectively. Both 1 and 2 were characterized by microanalysis, infrared spectra (IR), thermal gravimetric analyses (TGA), and powder X-ray diffraction (PXRD) techniques. Single-crystal structural analysis indicates that 1 and 2 show different coordination sphere around Pb2+ ions and distinct coordination frameworks. The I1O2 type three-dimensional (3D) nonporous metal− organic framework forms in 1, where the Pb2+ ion shows holo-directed coordination geometry, while the I0O2 type two-dimensional (2D) coordination polymeric layered structure forms in 2, where Pb2+ ion shows a hemidirected coordination sphere and the 6s2 lone electron pair in Pb2+ ion is stereochemically active. The two CPs emit intense and longlasting greenish phosphorescence in air at room temperature, with absolute quantum yields of 1.2% for 1 and 4.7% for 2 and decay lifetimes of 0.73 ms for 1 and 1.52 ms for 2.

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INTRODUCTION Luminescent materials have a variety of practical applications in optical devices, e.g., optical waveguide used as the transmission medium in local and long haul optical communication systems or the components in integrated optical circuits,1,2 light sources (fluorescent lamps and LEDs),3−8 disinfection,9 medical imaging,10−12 horticulture,13 temperature sensors,14−16 and chemical sensors,17−21 and so on; thus, lots of research attention has always been paid to exploring a new family of luminescent materials.22−26 Luminescent coordination polymers (LCPs), as a developing family of promising luminescent materials, have received a large amount of attention over the last two decades, and this is because compared with the traditional inorganic and organic luminescent materials the LCPs show several advantages, such as the designable and modifiable crystal structures and various potential emissive centers, including those in which the emission arises from the f → f electron transition within a rare earth metal ion,11,27 π* → π electron transition within a ligand,28,29 and MLCT or LMCT electron transition between metal center and ligand.1,30,31 In this regard, most of research has hitherto focused on the closed-shell d-block transition metal and lanthanide metal CPs,1,18,19,28,31−33 while the area of main group metal CPs remains less developed. In contrast to © XXXX American Chemical Society

well-developed closed-shell d-block transition metal ions (e.g., Zn2+ and Cd2+ ions), Pb2+ ion is an important main group heavy metal ion with a large radius (0.119 nm) which shows diverse coordination numbers (2−12) and good coordination ability; moreover, the lone electron pair of 6s2 can be either stereochemically active or inactive in Pb2+ complexes. These unique features allow Pb2+ ions to form complexes with abundant structures from the polynuclear cluster, 1D to 3D coordination polymers. In contrast, both the main group metal and the closed-shell d-block transition metal CPs generally emit ligand-based fluorescence, and the emission corresponds to the spin-allowed transition from S1 (the lowest singlet excited state) to S0 (the ground state) of ligands. Quite few of them emit ligand-based phosphorescence because the transition between T1 (the lowest triplet excited state) and S0 is spin-forbidden. It is well-known that the heavy atom has strong spin−orbital coupling effect, which could efficiently enhance intramolecular intersystem crossing; as a result, it is possible to achieve the ligand-based phosphorescent CPs by rational design of organic ligand structure and selection of heavy metal ion to assembling Received: January 22, 2019

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DOI: 10.1021/acs.inorgchem.9b00215 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(cm−1): 3446 (m), 3069 (w), 1606 (s), 1587 (s), 1544 (s), 1425 (m), 1354 (m), 1264 (m), 1211 (w), 1090 (w), 1014 (w), 936 (w), 779 (m), 742 (s), 714 (s). Crystallography. Suitable crystals of 1 and 2 were selected and mounted on a Siemens SMART-CCD diffractometer equipped with graphite monochromatic Mo Kα radiation (λ = 0.71073 Å). Data reductions and empirical absorption corrections were applied to the data using the SAINT39 and SADABS240 software packages, respectively. The crystal structures of 1 and 2 were solved by direct methods and refined by the full-matrix least squares on F2 using the SHELXTL package.41 All non-hydrogen atoms were found using Fourier difference maps and ultimately refined anisotropically, and hydrogen atoms were located at geometrically calculated positions. CCDC 1834060 and 1834061 contain the supplementary crystallographic data for this paper. Crystal data and structural refinement results of 1 and 2 are given in Table 1. Selected bond lengths and bond angles are tabulated in Tables S1.

CPs. In previous studies, we used such a functionality-directed crystal engineering strategy to achieve successfully a series of Pb2+- and Cd2+-based phosphorescent CPs which emit phosphorescence with high quantum yield under ambient condition.34−37 To extend further the study of CPs emitting ligand-based phosphorescence, herein, we present two highthermal-resistance Pb2+-based CPs, [Pb2(HBTC)2(DMF)] (1) and [Pb(HPTC)] (2), which are comprised of Pb2+ ions with rigid aromatic tricarboxylic acids ligands, 1,3,5-benzenetricarboxylic acid (H3BTC) and 2,4,6-pyridine tricarboxylic acid (H3PTC), respectively, and the two Pb2+-based CPs emit intense greenish phosphorescence under ambient condition.

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EXPERIMENTAL SECTION

Materials and Measurements. H3PTC was synthesized by oxidization of 2, 4, 6-trimethylpyridine with potassium permanganate according to a published method.38 All other chemicals and solvents, obtained commercially, were used without further purification. C, H, and N microanalyses were measured with a PerkinElmer 240 elemental analyzer. The Fourier Transform infrared (FT-IR) spectra were recorded on an AVATAR-360 spectrophotometer in the wavenumber range of 4000−400 cm−1, and the sample was prepared as a pellet with KBr. Ultraviolet−visible diffuse reflectance spectra were recorded in the wavelength range of 200−900 nm at room temperature using a PerkinElmer Lambda 950 UV/vis spectrometer, which is equipped with an integrating sphere, and BaSO4 was used as a reflectance standard. The phase purity of CPs was examined by powder X-ray diffraction (PXRD) on a Bruker D8 Discover diffractometer, using Cu Kα (λ = 1.54056 Å) rays at the radiation source, and the scan speed is of 5° min−1 and 0.02° per step in 2θ angles. The simulated PXRD patterns were obtained using the Mercury1.4.1 software program from single-crystal structure data. Thermal gravimetric analyses (TGA) were performed on a DTA-TGA 2960 thermogravimetric analyzer in nitrogen stream from 30 to 800 °C, and the heating rate was set as 10 °C min−1. The measurements of steady-state emission and excitation spectra were performed on an Edinburgh Instruments FLS920P fluorescence spectrometer for the solid samples at room temperature, and such an instrument is equipped with a 150 W xenon lamp as an excitation source. The emission decay lifetime was measured using a single photon counting Edinburgh FLS980 spectrometer equipped with a continuous Xe900 xenon lamp. Quantum yields of CPs 1 and 2 were recorded on a FM4P-TCSPC spectrofluorometer (Horiba Jobin Yvon). Preparation and Characterization of 1 and 2. Preparation of [Pb2(HBTC)2(DMF)] (1). A mixture of Pb(NO3)2 (0.50 g, 1.5 mmol), H3BTC (0.38 g, 1.8 mmol), and N,N-dimethylformamide (DMF) (10 mL) were stirred for 1 h at room temperature and then transferred to a Teflon-lined stainless-steel autoclave (volume: 23 mL). This Teflonlined stainless-steel autoclave was sealed and heated at 120 °C for 4 days followed by cooling to ambient temperature. The light yellow block crystals of 1 were separated by suction, washed with 1 mL of ethanol, and dried in the air. Yield: 56% (based on the reactant Pb(NO3)2). Microanal. calcd for C21H15NO13Pb2 (%): C, 27.90; H, 1.67; N, 1.54. Found (%): C, 28.66; H, 1.83; N, 1.82. Main bands in IR spectrum (cm−1): 3450 (m), 3083 (w), 1695 (s), 1643 (s), 1605 (m), 1513 (s), 1434 (m), 1376 (s), 1250 (s), 751 (m), 723 (m), 680 (m). Preparation of [Pb(HPTC)] (2). The preparation procedure of 2 is similar to that of 1. A mixture of Pb(NO3)2 (0.17 g, 0.5 mmol), H3PTC (0.14 g, 0.67 mmol), and deionized water (10 mL) were stirred for about 1 h at ambient temperature and then transferred to a Teflon-lined stainless-steel autoclave (volume: 23 mL). This Teflonlined stainless-steel autoclave was sealed and heated at 175 °C for 2 days followed by cooling to ambient temperature. The colorless crystals of 2 were separated by suction, washed with 1 mL of ethanol, and dried in the air. Yield: 48% (based on the reactant Pb(NO3)2). Microanal. calcd for C8H3NO6Pb (%): C, 23.07; H, 0.73; N, 3.36. Found (%): C, 22.41; H, 0.74; N, 2.93. Main bands in IR spectrum

Table 1. Crystallographic Data and Structure Refinement Parameters for 1 and 2 at Room Temperature compound empirical formula CCDC deposit no. Fw (g mol−1) crystal system space group crystal color crystal size temperature (K) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm−3) F(000) reflections collected/ unique (Rint) observed reflections refinement method data/restraints/ parameters goodness-of-fit on F2 final R factor wR2 final residual electron density

1 C21H15NO13Pb2

2 C8H3NO6Pb

1834060

1834061

903.74 monoclinic C2/c light yellow 0.30 × 0.20 × 0.10 mm3 296(2) 17.276(6) 6.928(2) 19.226(6) 90 101.008 (5) 90 2258.8(12) 4 2.658 1664 7351/2634 [Rint = 0.0421]

416.30 monoclinic P21/n colorless 0.17 × 0.16 × 0.05 mm3 294(2) 9.5247(4) 5.5333(2) 15.7382(7) 90 91.488(2) 90 829.17(6) 4 3.335 752 4868/1666 [Rint = 0.0457]

2297

1451

full-matrix least-squares on F2 2612/27/189

full-matrix least-squares on F2 1666/0/149

1.030

1.070

0.0279 0.0693 −1.821 < ρ < 2.100 e/Å3

0.0634 0.0672 −1.411 < ρ < 1.446e/Å3

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RESULTS AND DISCUSSION Crystal Structures. CP 1 crystallizes in the monoclinic space group C2/c with the asymmetric unit containing one Pb2+ ion and one partially deprotonated HBTC2− ligand together with one-half of a disordered DMF molecule (Figure 1a). The Pb2+ ion is surrounded by eight oxygen atoms from three different carboxylate groups and one disordered DMF molecule, displaying the coordination geometry with a B

DOI: 10.1021/acs.inorgchem.9b00215 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Asymmetric unit and atom labeling in 1 (H3 is retained, and other H atoms are deleted for clarity); the thermal ellipsoids are set at the 50% probability level. (b) Distorted monocapped pentagonal bipyramid coordination atmosphere of Pb1. (c) Connecting modes of HBTC2− ligand. (d) H-bonds formed between the carboxylic acid group and the carboxylate group in an adjacent ligand. (e) {PbO6}∞ chain running along b-axis. (f) In 1, 3D I1O2 type of network constructed by {PbO6}∞ chains and HBTC2− ligands viewed along the b-axis. All H atoms are deleted for clarity.

display the clear characters of single (C−O) and double (C O) bonds. Each HBTC2− ligand links five Pb2+ ions with three carboxylates through monodentate (η1-COOH), bidentate (η1:η1-COO−), and bidentate μ3-(η2:η2-COO−) binding modes and acts as a μ5−bridge linker, which is illustrated in Figure 1c. The carboxylic acid group of the HBTC2− ligand and the oxygen atom in a carboxylate group of the adjacent HBTC2− ligand form H-bond (Figure 1c and Figure 1d), with the H-bond parameters of dO3···O6#5 = 2.668 Å, dH3···O6#5 = 1.857 Å, and ∠O3−H3···O6#5 = 169.6° (symmetric code: #5 = −0.5 + x, −0.5 + y, z). The neighboring coordination monocapped pentagonal bipyramid are connected together through sharing the edges (O1−O2) in their equatorial planes into a {PbO6}∞ chain which runs along the direction of b-axis (Figure 1e), and such types of chains are further extended into nonporous 3D I1O2 type coordination network45−47 by connection of HBTC2− ligands (Figure 1f). The notation of InOm for a hybrid inorganic−organic framework material was first suggested by Cheetham and Rao,45 where the symbols (n, m) represent the dimensionality of the inorganic (I) and organic (O) connectivity.

distorted monocapped pentagonal bipyramid and holodirected coordination sphere. As illustrated in Figure 1b, the O1, O2, O1#1, O2#2, and O6#4 atoms form the deformed equatorial plane of the bipyramid. The O5#4 and O7 atoms occupy the two vertices of the bipyramid, and the O4#3 caps the trigonal face, which is comprised of the O1#1, O2, and O6#4 atoms, with the symmetric codes #1 = 0.5 − x, 0.5 + y, 0.5 − z; #2 = 0.5 − x, −0.5 + y, 0.5 − z; #3 = 0.5 + x, 1.5 − y, 0.5 + z; and #4 = x, 2 − y, 0.5 + z. The Pb−O bond lengths range from 2.422 to 2.824 Å. The O−Pb−O bond angles fall in the range of 48.3−163.0°. These bond data are comparable to those for other reported CPs of Pb2+ with carboxylate ligands.42−44 The DMF molecule shows disorder, in which the nitrogen atom (labeled as N1) locates at Wyckoff position 4e, and the other atoms are in Wyckoff position 8f (Figure 1a). The tricarboxylic acid is partially deprotonated to give HBTC2− ligand during the process of 1 crystal growth, which is further supported by means of analyzing the C−O bond lengths of three carboxylates. As shown in Figure 1a,c, the bond lengths dC7−O1 = 1.258(6) and dC7−O2 = 1.268(6) Å as well as dC9−O5 = 1.274(6) and dC9−O6 = 1.253(6) Å show the averaged bond lengths of single (C−O) and double (CO) bonds, whereas the bond lengths dC8−O3 = 1.320(6) and dC8−O4 = 1.212(6) Å C

DOI: 10.1021/acs.inorgchem.9b00215 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) Asymmetric unit of 2 and atom labeling (H4A remains, and other H atoms are deleted for clarity). The thermal ellipsoids are set at the 50% probability level. (b) Hemidirected coordination environment of Pb1 with symmetric codes: #1 = 1 − x, 1 − y, − z; #2 = x, 1 + y, z; #3 = 0.5 − x, −0.5 + y, 0.5 − z. (c) Connectivity of HPTC2− ligand. (d) Layer of coordination polymer. (e) H-bonds formed between the neighboring coordination polymer layers with symmetric codes: #4 = 1 − x, 1 − y, 1 − z. (f) In 2, packing structure viewed along approximate b-axis direction, exhibiting that the CPs are connected by the H-bonding interactions.

tively.48−56 The 6s2 lone pair of electrons is either stereochemically active or inactive in a Pb2+ coordination sphere depending strongly on the nature of ligands. The formation of hemidirected coordination sphere of Pb2+ in 2 is relevant to the strong chelating ability of HPTC2− using a tridentate (O1, N1, O6; see Figure 2b). Similar to the observation in 1, the tricarboxylic acid is partially deprotonated to give HPTC2− ligand during the process of 2 crystal growth. The carboxylate with C7, with dC7−O3 = 1.205(10) and dC7−O4 = 1.326(11) Å, shows typical characters of single (C−O) and double (CO) bonds. The connectivity of HPTC2− ligand is depicted in Figure 2c; the HBTC2− ligand serves as a μ4-bridge linker to connect four Pb2+ ions with pyridyl N atom and two of three carboxylates through bidentate μ2-(η1:η1−COO−) and bidentate μ3(η1:η2−COO−) binding modes. Two PbO6 coordination polyhedra are connected into a Pb2O10 dimer (Figure 2c) by means of edge-sharing mode, and the Pb2O10 dimers are bridged by HPTC2− ligands to extend into a 2D coordination polymeric layer, which is parallel to the (101) plane (Figure 2d). The carboxylic groups in the neighboring coordination polymeric layers form H-bonds, as shown in Figure 2e, with the typical H-bond parameters of

CP 2 crystallizes in the monoclinic P21/n space group, with the asymmetric unit comprising of one Pb2+ ion and one HPTC2− ligand (Figure 2a). In contrast with the holo-directed coordination sphere of Pb2+ ion in 1, the Pb2+ ion shows the hemidirected coordination sphere in 2, as displayed in Figure 2b, which is built from five oxygen atoms from carboxylate groups of three different HPTC2− ligands as well as one nitrogen atom of pyridyl ring. The bond lengths of Pb−O/N are within the range of 2.427−2.793 Å, falling in the ranges of Pb−O/N coordination bonds (2.46−2.96 Å). The O−Pb−O/ N bond angles range from 66.2 to 150.9°; however, the exterior ∠O−Pb−O angles are in excess of 200°, indicating that the Pb2+ lone pair orbital is predominantly 6s with a small amount of 6p contribution.48 The Pb2+ ion displays hemidirected conformations in the crystal structure of 2, in which the HPTC2− ligands being not distributed through the entire sphere surround the Pb2+ ion. Moreover, there are clearly gaps in the Pb2+ coordination spheres (Figure 2c), suggesting that the Pb2+ ion contains a stereochemically active lone electron pair in 2. Interestingly, the electron configuration of Pb2+ ion is [Xe]4f145d106s2, and the lone electron pair of 6s2 is either stereochemically active or inactive in Pb2+ complexes, where the terms hemi- and holo-directed were coined, respecD

DOI: 10.1021/acs.inorgchem.9b00215 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. TG curves of (a) 1 and (b) 2.

Figure 4. Solid-state UV−vis diffuse reflectance spectra of (a) 1 and H3BTC and (b) 2 and H3PTC.

dO4···O5#4 = 2.623 Å, dH4A···O5#4 = 1.842 Å, and ∠O4−H4A··· O5#4 = 161.2° (symmetric code: #4 = 1 − x, 1 − y, 1 − z). The 3D coordination polymeric framework is built from 2D coordination polymeric jointed via strong H-bonding interactions between carboxylic groups, which is illustrated in Figure 2f. IR Spectra, PXRD Patterns, and TG. IR spectra of CPs 1 and 2 are displayed in Figures S1 and S2, respectively. The broad bands at 3450 and 3446 cm−1 are attributed to the O−H stretching vibration in −COOH group. The intense bands centered at 1643 and 1434 cm−1 in 1, while those at 1633 cm−1 (a shoulder) and 1425 cm−1 in 2 are related to the asymmetrical and symmetrical stretching vibrations of carboxylate groups, respectively.57,58 The CO stretching vibration in DMF appears at 1695 cm−1 in 1. In the IR spectra of 1 and 2, the weak bands corresponding to the C−H stretching vibrations are observed near to 3000 cm−1. The experimental and simulated PXRD patterns for 1 and 2 are shown in Figure S3a,b, respectively. The well-matched experimental and simulated PXRD patterns indicate the high phase purity of the crystalline samples. TG curves of the two CPs are displayed in Figure 3. CP 1 starts to lose weight at ca. 183 °C. Two consecutive steps of weight loss appear in the range of 183−395 °C, with 8.4% weight loss, which corresponds to the liberation of coordinated DMF molecules (8.1% calculated from the formula). The temperature range of DFM liberation (183−395 °C) in 1 is much higher than the decomposition temperature of DMF (151 °C) owing to its being confined in the lattice of coordination polymer. A similar phenomenon has been also observed in other coordination polymers previously reported.59,60 There is no lattice solvent in the crystal of 2, and its framework is thermally stable up to ca. 310 °C. These

observations suggest 1 and 2 having good thermal resistance. Noticeably, the slight mass loss of 4.6% in the temperature range of 310−373 °C is close to the value of 4.3% for liberation of one water molecule condensed from carboxyl groups (COOH). Obviously, the framework of 2 shows lower thermal stability than that of 1, and this difference between 1 and 2 is probably relevant to the dimensionality of the framework in 1 and 2. Even if there are carboxyl groups in the crystalline structures of 1 and 2, in contrast to the 2D flexibly framework in 2, the rigid 3D framework in 1 prevents two neighboring carboxyl groups from getting together, and it is hard for the carboxyl groups to condense and release a water molecule in 1. UV−Vis Reflectance Spectra and Photoluminescent Properties. The UV−vis diffuse reflectance spectra of solid samples for 1/H3BTC and 2/H3PTC under ambient condition are displayed in Figure 4a,b, respectively. Noticeably, the UV− vis diffuse reflectance spectra of H3BTC and H3PTC exhibit high similarity; the spectra of both compounds display four visible absorption bands in the ultraviolet region (200−400 nm), indicating that H3BTC and H3PTC have analogous electron structures. The maximum of the band locates at 285 nm together with one shoulder in the high wavelength side (at ca. 314 nm) as well as two shoulders in the low wavelength side (at ca. 264 and 217 nm) (Figure 4a). These absorption broads are attributed to the π → π* and n → π* electron transition in the aromatic rings and carboxyl groups of H3BTC ligand. As illustrated in Figure 4b, the absorption spectrum of H3PTC shows two maxima at 287 and 213 nm, together with two shoulders at ca. 309 and 247 nm, and these absorptions are attributed to the intraligand π → π* transitions in the pyridyl rings as well as the π → π* and n → π* electron transition in carboxyl groups of H3PTC. E

DOI: 10.1021/acs.inorgchem.9b00215 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Images of crystals of (a, b) 1 and (c, d) 2 under daylight (a, c) and UV light (b, d); photoluminescent spectra of (e) 1 and (f) 2 under ambient condition.

phosphorescence of CP Zn-TMA.33 Luminescence decay time for the H3BTC, H3PTC, and the two CPs was measured. The corresponding curves are shown in Figure S7a−d, respectively. The process of luminescence decay observed in ligands 1 and 2 follows a multiexponential decay law. Equation 1 was used for fitting the curves of luminescence decay of ligands, 1 and 2, where A is a pre-exponential factor and τ is the decay constant which represents the photoluminescence lifetime.

The absorption profile of 1 is different from that of H3BTC, and the broad absorption band of 1 falls in the wavelength range of 200−310 nm with two maxima at ca. 230 and ca. 308 nm, respectively, which are attributed to the electron transition within the intraligand. In regard to the solid-state UV−vis spectrum of H3BTC, the broad band centered at 343 nm in 1 is a new one (Figure 4a). The intensity of the new absorption band is less than the intensity of the bands at ca. 230 and 308 nm, and this band may be assigned to the charge transfer transition between HBTC2− ligand and Pb2+ ion.61 The diffuse reflectance UV−vis spectrum of 2 is also similar to that of 1 (Figure 4a,b): The intense absorption bands occur in the ultraviolet region, and a shoulder appears in the low energy side. Photoluminescent spectra of 1 and 2 and corresponding H3BTC and H3PTC spectra are depicted in Figures 5a,b and S5a,b, respectively. The H3BTC solid shows an emission maximum at 390 nm under irradiation with a wavelength of 340 nm. The H3PTC solid displays a photoluminescent emission centered at 405 nm upon λex = 352 nm. The emission of H3BTC and H3PTC solids arises from the π → π* transitions within the aromatic ring (phenyl ring in 1 and pyridyl ring in 2).62 Both 1 and 2 emit bright greenish light upon illumination with UV light at ambient temperature (Figure 5a−d). The corresponding luminescent spectra of 1 and 2 are shown in Figure 5e,f, respectively, which are similar to each other in spite of two CPs showing different coordination environments of Pb2+ ions, holo-coordination geometry around Pb2+ ions in 1 and the hemicoordination geometry around Pb2+ ions in 2. This observation demonstrates that both 1 and 2 emit ligandbased luminescence. The maximum of the emission band locates at 550 nm in 1 upon excitation at 340 and 560 nm in 2 upon irradiation at 349 nm. By comparison of the emission bands in H3BTC and H3PTC solids, the emission bands in corresponding CPs show pronounced redshift. The quantum yield was investigated for both CPs, revealing that 1 and 2, respectively, have 1.2 and 4.7% quantum yield under ambient condition, and these QY values are comparable to that in the

y = A exp( −t /τ )

(1)

The best fit of the time-dependent luminescence intensity to eq 1 led to the luminescence lifetimes τ of 0.73 ms for 1 and 1.52 ms for 2, and these values fall within the millisecond range, suggesting that 1 and 2 emit phosphorescence in nature. However, the luminescence lifetimes of ligands H3BTC and H3PTC are 0.17 and 0.53 ns, respectively, which fall within the nanosecond range and show the typical organic fluorescence character. This conclusion is also in agreement with the observation that larger Stokes shift occurs in 1 and 2 with respect to that in H3BTC and H3PTC solids.

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CONCLUSION Two coordination polymers of Pb2+ ion with tricarboxylic acid ligands HBTC2− (1) and HPTC2− (2) have been prepared using the solvothermal reaction. The Pb2+ ion shows a stereochemically inactive coordination sphere in 1, while it shows a stereochemically inactive coordination sphere in 2. Adopting the stereochemically active or inactive coordination sphere in a Pb2+ complex is strongly relevant to the influence of the ligands on lone electron pair of 6s2 in Pb2+ ion. The strong chelating ability of HPTC2− ligand responds to the formation of stereochemically active coordination sphere around the Pb2+ ion in 2. The distinct coordination spheres around the Pb2+ ions further lead to 1 and 2 showing different coordination frameworks. Both 1 and 2 emit intense and long-lasting greenish ligand-based phosphorescence in air at room temperature, which benefits from the Pb2+ ion heavy atom effect. This work, combined with our previous studies, F

DOI: 10.1021/acs.inorgchem.9b00215 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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demonstrates an efficient strategy for successful achievement of room-temperature phosphorescence coordination polymers via assembly of the heavy atom Pb2+ with aromatic multicarboxylic acid ligands.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00215. Selected bond lengths and bond angles, IR, XRD patterns for 1 and 2, packing diagram pattern of 1, excited and emission spectra of H3BTC, H3PTC, 1, and 2, emission decay of H3BTC, H3TC, 1, and 2 (PDF) Accession Codes

CCDC 1834060 and 1834061 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 25 58139527. Fax: +86 25 58139988. E-mail: [email protected] (X.S.G.). *E-mail: [email protected] (X.M.R.). ORCID

Xu-Sheng Gao: 0000-0001-7184-759X Xiao-Ming Ren: 0000-0003-0848-6503 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Nature Science Foundation of China (grant no. 21601084) and the Priority Academic Program Development of Jiangsu Higher Education Institutions for financial support.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00215 Inorg. Chem. XXXX, XXX, XXX−XXX