JSSC 14

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Synthesis, structure and photoluminescence propertiesof amine-templated open-framework bismuth sulfates

Subba R. Marri, J.N. Behera n

School of Chemical Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar 751005, India

a r t i c l e i n f o

Article history:Received 19 July 2013Received in revised form14 October 2013Accepted 20 October 2013Available online 30 October 2013

Keywords:Bismuth sulphatesOpen-frameworkThree-dimensional structurePhotoluminescenceLayered compound

a b s t r a c t

Two organically-templated bismuth sulfates of the compositions, [C6N2H14] [Bi(SO4)2(NO3)], (1) and[C4N2H12]4[Bi4(SO4)10(H2O)4], (2), with open architecture have been synthesized and their structuresdetermined by single crystal X-ray diffraction. 1 has a corrugated layered structure with 8-memberedaperture wherein the SO4 tetrahedra and the BiO8 polyhedra join together to form (4, 4) net sheets of themetal centers while 2 has a three-dimensional structure possessing 8- and 12-membered channels. Boththe compounds show good fluorescence properties exhibiting blue luminescence. Time-resolvedfluorescence behavior of 1 and 2 shows mean fluorescence life time of 0.9 and 1.0 ns, respectively.

& 2013 Elsevier Inc. All rights reserved.

1. Introduction

The synthesis of complex inorganic materials with open archi-tecture is one of the major activities in contemporary solid statechemistry for their applications in the areas of catalysis, ion-exchange, sorption and separation process. During the last twodecades or so large number of new materials with novel architec-tures have been synthesized and characterized. Accordingly, inor-ganic open-framework structures involving silicates [1], phosphates[2] and carboxylates [3,4] have been widely investigated in the pastfew years. It has been demonstrated recently that oxyanions such asselenite [5], selenate [6] and sulfate [7] can also be made use of tobuild up open architectures. Though many transitional metal sulfateshave one- or-two dimensional structures, three-dimensional organi-cally-templated sulfates of uranium [8], lanthanum [9], scandium[10], and nickel [11], however, have been reported. Organically-templated metal sulfates are mainly focused on transition (both d-and f-block) metals. However, main group (p-block) metal sulfatesare very rarely reported [12] and no three-dimensional bismuthsulfate appears to be known.

Metal oxides accomodating lone pair cations such as Pb2þ andBi3þ have been of great interest due to their rich structuralchemistry [13]. Bi3þ with its ionic radius of 1.16 Å has one inert6s2 electron pair and forms non-transition metal center complexeswith higher coordination-number atoms. Therefore, bismuth is

expected to form higher dimensional frameworks. Bismuth con-taining compounds with their low-toxicity, low-cost, and goodchemical stability [13d–13f] are promising for potential applicationin medicine treatment, catalyst and luminescence [14]. Also,bismuth can exist in a wide variety of coordination environmentthat can lead structures of different dimensionalities. We areinterested in exploring the use of SO4

2� tetrahedra to buildopen-framework materials of heavy main group metals. Open-framework materials are usually synthesized by employing hydro/solvothermal techniques that require the reaction mixture to beheated at a particular temperature in a sealed autoclave [15]. Wehave been able to prepare both layered and three-dimensionalbismuth sulfates employing simple aqueous mediated reaction atroom temperature. In this article, we report the synthesis, struc-ture, and luminescence properties of [C6N2H14] [Bi(SO4)2(NO3)], 1,and [C4N2H12]4[Bi4(SO4)10(H2O)4], 2.

2. Experimental

2.1. Materials and methods

All materials were commercially available and used as received.Infrared spectra were recorded on a Perkin Elmer FTIR spectrometerequipped with an attenuated total reflectance accessory. UV-visiblespectra were collected using a Perkin Elmer UV/Vis/NIR spectro-photometer (Lambda 750). Excited and emission spectra of groundmixture of polycrystalline powders of all three compounds wererecorded using Perkin–Elmer LS 55 Fluorescence Spectrometer. In all

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0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.jssc.2013.10.032

n Corresponding author. Tel.: þ91 674 230 4094; fax: þ91 674 230 4070.E-mail address: [email protected] (J.N. Behera).

Journal of Solid State Chemistry 210 (2014) 15–21


cases the maximum in the excitation spectrum was used to generatethe emission spectrum. EDAX was performed by field emissionscanning electron microscopy (FE-SEM, Zeiss, Oxford). Optical imageof the sample was taken using a Zeiss LSM-780 confocal microscope.Time-resolved fluorescence measurement was carried out using atime-correlated single-photon counting (TCSPC) spectrometer (Edin-burgh, OB920). Thermogravimetric analyses were carried out at aramp rate of 10 1C/min under a flow of nitrogen using a NETZSCH TG209F1. Powder X-ray diffraction data were collected on a Bruker D8Advance X-ray powder diffractometer using CuKα radiation(λ¼1.5418 Å). Carbon, hydrogen and nitrogen analyses were obtainedfrom the Sophisticated Analytical Instrument at Central Drug ResearchInstitute, Lucknow, India.

2.2. Synthesis and characterization

Compounds 1 and 2 were synthesized by slow evaporation atroom temperature. In a typical synthesis of 1, 0.485 g of Bi(NO3)3 �4H2O was dissolved in 10 mL of distilled water in a 20 mLscintillation vial under constant stirring. To this mixture, 0.5 mL ofH2SO4 followed by 0.162 g of piperazine (PIP) were added and themixture was stirred for 30 min to obtain the clear solution. The finalmixture with the molar composition of Bi(NO3)3 �4H2O/H2SO4/PIP/H2O (1:9:2:55) was kept at room temperature in open air. After onemonth needle-shaped white crystals were collected after filtration,washed with water and dried in air (yield 80% with respect to Bi).Compound 2 was synthesized in a similar way to that of 1 by using1,4-diazabicyclo[2,2,2]octane (DABCO) as the structure directingamine. 0.485 g of Bi(NO3)3 �4H2O was dissolved in 10 mL of distilledwater in a 20 mL scintillation vial under constant stirring. To thismixture, 0.5 mL of H2SO4 followed by 0.228 g of DABCO were addedand the mixture was stirred for 30 min to obtain the clear solution.The scintillation vial containing the reaction mixture with the molarcomposition of Bi(NO3)3 �4H2O/H2SO4/DABCO/H2O (1:9:2:55) waskept at room temperature in open air. After one month needle-shaped crystals were collected with 73% yield.

Initial characterization of 1 and 2 was carried out by powder X-ray diffraction (PXRD), energy dispersive analysis of X-rays (EDAX),chemical analysis, thermogravimetric analysis (TGA) and IR spec-troscopy. The PXRD pattern exclusively exhibited reflections of ahitherto unknown material; and was consistent with the structuredetermined by single crystal XRD, which gave the compositions of1 and 2 as [C6N2H14][Bi(SO4)2(NO3)], and [C4N2H12]4[Bi4(SO4)10(H2O)4], respectively. All the compounds gave satisfactory elemen-tal analysis. The experimental and calculated (in wt%) values of C,H, N were as follows. 1: C, 12.5; N, 7.3; H, 2.5 (calc.: C, 12.6; N, 7.4;H, 2.3). 2: C, 8.5; N, 5.2; H, 2.2 (calc.: C, 8.7; N, 5.1; H, 2.6). EDAX,indicated the ratios of Bi and S to be 1:2, and 1:2.5 in 1 and 2,respectively, in agreement with the molecular formulae.

The IR spectra for the compounds were recorded as KBr pellets(Perkin–Elmer FT-IR SPECTROMETER—RX1). The IR spectroscopicstudies exhibit typical peaks corresponding to the hydroxyl group,the amino groups, etc. (Supporting Information, Fig. S1). The mainIR bands are (KBr): ν(H2O)¼3230–3450 cm�1, ν(N–H)¼3070–3110 cm�1, ν(C–H)¼2670–2780 cm�1, ν(C–H)¼1300–1500 cm�1,ν1(SO4)¼950–1000 cm�1, ν3(SO4)¼1100–1150 cm�1, δ(SO4)¼500–700 cm�1. In general, the free sulfate ions exhibit two bands at 1105and 615 cm�1, which are assigned to the ν3(F2) stretching [νd(SO)]and ν4(F2) stretching [δd(OSO)] modes, respectively. The coordinationof the free sulfate group to the metal centers would lower the overallsymmetry of the sulfate group and lead to the splitting of the ν3 andν4 modes. The spectra show multiple strong bands in the 900–1040 cm�1 region due to ν1 and in the 1050–1250 cm�1 region dueto ν3 vibrationmodes of the sulfate group [16]. The bending modes ofSO4

2� are found in the 500–550 and 600–795 cm�1 regions.

2.3. Single-crystal structure determination

A suitable single crystal of each compound was carefully selectedunder a polarizing microscope and mounted at the tip of the thinglass fiber using cyanoacrylate (super glue) adhesive. Single crystalX-ray data were collected on a Bruker Smart Apex II diffractometerequipped with an Oxford Cryostream low-temperature device and afine-focus sealed-tube X-ray source (MoKα radiation, λ¼0.71073 Å,graphite monochromated) operating at 50 kV and 30 mA. Raw datacollection and refinement were done using SMART. Data reductionwas performed using SAINT [17] and corrected for Lorentz andpolarization effects. An empirical absorption correction based onsymmetry equivalent reflections was applied using SADABS [18].The structure was solved by direct methods using SHELXS-97 [19],which readily revealed all the heavy atom positions (Bi, S) andenabled us to locate the other non-hydrogen (N, O and C) positionsfrom the difference Fourier maps. For the final refinement, hydro-gen atom of both the framework as well the ammonium ion in 1and 2 were placed geometrically and held in the riding mode. Thelast cycles of refinement included atomic positions, anisotropicthermal parameters for all the non-hydrogen atoms, isotropicthermal parameters for all the hydrogen atoms. Full-matrix least-squares structure refinement against |F2| was carried out using theSHELXTL-PLUS [20] package of programs. Details of the structuredetermination and final refinements for 1 and 2 are listed in Table 1.X-ray diffraction patterns of 1 and 2 were in good agreement withthe simulated patterns based on single-crystal data, indicative oftheir phase purity (Fig. S2 in the Supporting information).

3. Results and discussion

3.1. Layered [C6N2H14][Bi(SO4)2(NO3)], 1

The asymmetric unit of 1 contains 23 non-hydrogen atoms outof which 15 belong to the inorganic framework and 8 belong to theextra framework amine molecule (Fig. 1a). It has one crystal-lographically independent Bi3þ cation, two sulfate anions, onenitrate anion and one diprotonated DABCO cation. The Bi3þ cationin 1 is eight-coordinated to two sulfate and one nitrate ions in thebidentate fashion and two sulfate ions in the monodentatefashion. The Bi–O bond distances are in the range of 2.344(18)–2.740(2) Å [(Bi–O)av¼2.490 Å)]. The O–Bi–O bond angles are in the

Table 1Crystal data and structure refinement parameters for 1 and 2.

Parameters 1 2

Empirical formula C6H14BiN3O11S2 C16H48Bi4N8O44S10a (Å) 9.70(4) 19.84(9)b (Å) 13.36(6) 19.35(9)c (Å) 20.74(8) 13.21(6)α (1) 90 90β (1) 90 91.93(10)γ (1) 90 90V (Å3) 2691(19) 5071.3(4)Z 8 4Formula weight 577.30 2213.14Space group Pbca (61) P21/c (14)T (1C) 23 23λ(MoKα) Å 0.71073 0.71073ρcalc(g cm�3) 2.850 2.899μ (mm�1) 13.484 14.381R[I42s(I)] R1¼0.0215 R1¼0.0385

wR2¼0.0531 wR2¼0.0761R (all data) R1¼0.0273, R1¼0.0628,

wR2¼0.0555 wR2¼0.0837

aR1¼∑|F0|� |Fc|; bwR2¼{[w(F02–Fc2)2]/[w(F02)2]}1/2, w¼1⧸[s2(F0)2þ(aP)2þbP]P¼[F02þ2Fc2]⧸3; where a¼0.0300; b¼0.30 for 1, a¼0.0350; b¼2.50 for 2.

S.R. Marri, J.N. Behera / Journal of Solid State Chemistry 210 (2014) 15–2116


range 59.03(7)–146.83(6)1. The selected bond distances are givenin Table 2 (details bond distances and bond angles are given inTable S1 in the Supporting Information). Of the two frameworksulfate anions, both bind to one Bi3þ cation in a bidentate fashion,and to another Bi3þ cation in a monodentate manner leaving aterminal S–O bond (Fig. 1b). Bond valence sum calculations for 1based on the method of Brown and Altermatt [21], using r0(Bi–O)¼2.094 Å, shown that the valence sums for Bi¼2.98 Å. Thecharges on Bi and S are clearly þ3 and þ6 respectively. These dataare consistent with the stated composition. The linkage of the BiO8

polyhedra and the SO4 tetrahedra by sharing vertices and edges,gives rise to anionic inorganic layers parallel to the ab-plane of theunit cell, containing eight membered rings (Fig. 1b). The threesulfate tetrahedra and one nitrate ion sharing edges with the BiO8

polyhedra bind to the Bi3þ cation in a cis fashion rendering thelayers corrugated, with the terminal N–O of the nitrate grouppointing out of the zig–zag layers. Protonated DABCO moleculesreside in the crest and trough regions which forms hydrogenbonds with the framework oxygen that gives extra stability to thestructure. The layer formed by the connectivity of the Bi centers

Fig. 1. (a) ORTEP plot of [C6N2H14][Bi(SO4)2(NO3)], 1. The asymmetric unit is labeled. Thermal ellipsoids are given at 50% probability. (b) Top view of the layer parallel to theab-plane in 1, formed by joining the BiO8 polyhedra and SO4 tetrahedra. (c) The (4, 4) net sheet formed by the joining of the metal centers.

S.R. Marri, J.N. Behera / Journal of Solid State Chemistry 210 (2014) 15–21 17


and with the sulfate units can be visualized as (4, 4) net sheetswith the Bi centers acting as nodes (Fig. 1c). The entire structure isformed by the stacking of the (4, 4) net sheets along the c-axis ofthe unit cell. The anionic inorganic layers in 1 are stacked over oneanother along the c-axis of the unit cell in AAA⋯ fashion and areseparated by a half unit cell length along the c-axis of the unit cell(Fig. 2).

The structure of 1 can be compared with the structures of[C6N2H14]2 [Ln2(SO4)4(H2O)4] [SO4][C2N2H8][H2O]3, (Ln¼La, Pr orNd) [9a], (A), La2(H2O)2(C2H10N2)3(SO4)6 �4H2O [9b], β-(NH4)La(SO4)2 [22], and (N2H5)Nd(H2O)(SO4) [23]. These compounds arebuilt from corrugated anionic layers of rare earth sulfates, with theNH4

þ , organic amine and water molecules residing in the inter-lamellar space and interacting with the layers through hydrogenbonds. In (A), the isolated sulfate ion in the interlayer space alsoform hydrogen bonds with the layers.

3.2. Three-dimensional [C4N2H12]4[Bi4(SO4)10(H2O)4], 2

The bismuth sulfate 2 is a three-dimensional network structuremade up of Bi(1)O9, Bi(2)O9, Bi(3)O8, Bi(4)O8 polyhedra and SO4

tetrahedra incorporating diprotonated piperazine molecules withinthe pores. The asymmetric unit of 2 contains 82 non-hydrogen

atoms of which 58 belong to the inorganic framework and 24belong to the extra framework guest molecule (Fig. S3 in theSupporting Information). There are four crystallographically distinctBi3þ cations and ten SO4

2� anions as shown in Fig. 3a. BothBi3þ(1) and Bi3þ(2) cations are nine-coordinated by the oxygensfrom five sulfate ions and one water molecule, with each onesurrounded by three bidentate sulfate anions and two monodentatesulfate ions. Bi3þ(3) and Bi3þ(4) cations are eight coordinated bythe oxygens from five sulfate ions and one water molecule, witheach metal ion surrounded by two bidentate sulfate anions andthree monodentate sulfate ions. Bi(1)O9 linked to the Bi(2)O9

polyhedra through sulfate tetrahedra by sharing edges and cornerswith S(10)O4 and S(7)O4, respectively, thereby forming an infinitechain along the [0 0 1] direction. Bi(2)O9 linked to Bi(3)O8 and Bi(4)O8 polyhedra through sulfate anions by sharing edges with S(4)O4

and corner with S(8)O4 tetrahedra along the a-axis. S(5)O4 and S(6)O4 tetrahedra share corners with Bi(3)O8 and Bi(4)O8 polyhedrarespectively through Bi(3)–O–S linkages along the c-axis parallel tothe Bi(1)–O–S–O–Bi(2) infinite chain. S(2)O4, S(9)O4, S(4)O4, S(3)O4

and S(8)O4 tetrahedra connect the infinite chains by sharing edgesand corners with the BiOn polyhedra of the adjacent chain along thea-axis to form a two-dimensional sheet in the ac-plane containing8-and 12-membered aperture in it (Fig. 3). A (4, 6) net sheet isformed by joining the metal centers of the four bismuth polyhedra(Fig. S4). The layers are cross linked by the S(1)O4 and S(3)O4

tetrahedra along the b-axis to form the three-dimensional structurepossessing channels along the b- and c-axes (Fig. 4). The channelsare filled by the diprotonated amine cations which are in twodifferent orientations, interact with the framework oxygen throughstrong N–H⋯O and C–H⋯O hydrogen bonds. The various hydrogenbond interactions for 1 and 2 are given in Table S3 (SupportingInformation).

The Bi–O distances fall in the range 2.292(4)–2.632(5) Å withan average distance of 2.483 Å (Table 3). The S atoms are atthe centers of tetrahedral sulfate ions as expected. The S atomsform (4-n) S–O–Bi bonds and n terminal S–O bonds, the actualnumber of the latter differing from one to another. The averagelength of the S–O bonds are 1.475 (S–O–Bi) and 1.456 Å (terminalS–O), with the O–S–O bond angles in the range 103.3(3)–113.0(3)1(details bond distances and bond angles are given in Table S2).Bond valence sum (BVS) calculations confirm the oxidation statesof Bi and S to be þ3 and þ6, respectively [21]. The frameworkstoichiometry of [Bi4(SO4)10]8� , with a net framework charge of

Table 2Selected bond distances for compound 1.

Moiety Distance (Å) Moiety Distance (Å)

Bi(1)–O(1) 2.3443(18) S(1)–O(4) 1.491(2)Bi(1)–O(2) 2.381(2) S(1)–O(1) 1.494(2)Bi(1)–O(3) 2.389(2) S(2)–O(11) 1.465(2)Bi(1)–O(4) 2.4107(19) S(2)–O(6) 1.481(2)Bi(1)–O(5)#1 2.448(2) S(2)–O(8) 1.483(2)Bi(1)–O(6)#2 2.603(2) S(2)–O(3) 1.492(2)Bi(1)–O(7) 2.611(2) O(2)–N(1) 1.302(3)Bi(1)–O(8)#2 2.740(2) O(7)–N(1) 1.239(3)S(1)–O(10) 1.456(2) N(1)–O(9) 1.222(3)S(1)–O(5) 1.486(2)

Symmetry transformations used to generate equivalent atoms:#1 xþ1/2, �yþ1/2, �zþ1 #2 �xþ1/2, y�1/2, z#3 x�1/2, �yþ1/2, �zþ1 #4 �xþ1/2, yþ1/2, z

Fig. 2. The stacking of the alternate inorganic layers along the c-axis of the unitcell. The protonated DABCO molecules reside in the crest and trough regions of theinorganic layer. Note the wavelike pattern of the inorganic layers.

Fig. 3. The layer, parallel to the bc-plane of the unit cell in [C4N2H12]4[Bi4(SO4)10(H2O)4], 2, formed by connecting BiOn polyhedra with SO4 tetrahedra,by sharing edges and vertices, and thereby forming 8-membered and12-membered rings.



�8, is balanced by the diprotonated four piperazine moleculepresent inside the channels.

3.3. Thermogravimetric studies.

Thermogravimetric analysis of 1 and 2 was carried out underconstant flow of N2 at a heating rate of 10 1C min�1 in thetemperature range 25–750 1C (Fig. S5 in the Supporting informa-tion). Compound 1 showed a two-step weight loss correspondingto the loss of DABCO and SO2 in the range 200–375 1C (obs¼52.4%,calcd¼50.8%), followed by the removal of rest of SO3 in the range380–600 1C. The total observed weight loss of 60.5% correspondsto the loss of the DABCO along with sulfates is in good agreementwith the calculated value of 59.5%. The PXRD pattern of the sample

heated to 750 1C corresponded to Bi2O3 (PDF: 00–029-0236). In 2,there was a three-step weight loss corresponding to the loss ofwater in the range 100–200 1C (obs¼3.4%, calcd¼3.3%), the aminemolecules at 320 1C followed by the decomposition of sulfate inthe range 350–500 1C (obs¼54%, calcd¼54.5%). The small weightloss below 80 1C could be the presence of small amount ofmoisture in the sample. The PXRD pattern of the sample heatedto 750 1C corresponded to Bi2O3 (PDF: 00–029-0236).

3.4. Photoluminescence studies

The optical properties of Bi3þ ions are governed, as a rule, to theelectron transitions between the states 6s2 and 6s6p. Its luminescenceproperties are usually explained with respect to the energy-levelscheme 1S0o3P0 o3P1o3P2o1P1 [24]. The excited levels correspond-ing to the configuration 6s6p are the triplet levels 3P0, 3P1, and 3P2 andthe singlet level 1P1 while the ground state is fully symmetric (1S0). Thetransition 1S0–

3P2 is forbidden by the selection rule for the totalmomentum of the atom J. The transition 1S0–

3P1 is allowed owing tothe spin–orbital interaction of the 3P1 and 1P1 states. The transitions1S0–

3P1 and 1S0–1P1, the first of which is longer-wave, are realized in

the bands of optical absorption and excitation of the photolumines-cence of bismuth-activated substances. The common property ofbismuth compounds is the presence of wide-band luminescence,which is a nonradiative transition, and is attributed to the lowest-frequency allowed transition of 3P1–

1S0. Also, the charge-transfertransition from the SO4

2� ion to the empty 6p level of the Bi3þ ionis possible. Generally the former is known to occur easily in manycases [25]. Furthermore, the luminescence of Bi ions usually appears inthe visible wavelength region and bismuth containing organic/inor-ganic hybrid materials has been reported to display blue luminescenceproperties [26].

The room-temperature photoluminescence properties 1 and 2were studied by exciting the sample at 280 nm. The emissionspectra of them are almost similar exhibiting blue photolumines-cence (Fig. 5). The optical image of the blue photolumiscence wastaken in a confocal microscope (inset of Fig. 5). On excitation at280 nm, 1 and 2 show two intense emission peaks around 410, and486 nm, respectively. The intense emission peak occurred at410 nm, may be assigned to the ligand-to-metal charge transferor the 1P1–

1S0 and 3P1–1S0 transition of the s2 electron of Bi3þ

cation as discussed above. The less intense peak for compound 1 islikely arises from a combination of factors involving the presenceof dinuclear Bi2O2 unit, absence of organic amine and the differentcoordination sphere with respect to Bi3þ cation. Bi3þ is nine-coordinated in 1, and both eight-and nine-coordinated in 2.

Fig. 4. View down the crystallographic c-axis showing the inorganic frameworkstructure of [C4N2H12]4[Bi4(SO4)10(H2O)4], 2, showing 8-membered and 12-mem-bered channels. Protonated amine molecules are sitting inside the channels.Hydrogen atoms are removed for clarity.

Table 3Selected bond distances for compound 2.

Moiety Distance (Å) Moiety Distance (Å)

Bi(1)–O(1) 2.396(4) Bi(2)–O(18) 2.675(6)Bi(1)–O(2) 2.428(4) Bi(3)–O(19) 2.352(5)Bi(1)–O(3) 2.456(4) Bi(3)–O(20) 2.371(5)Bi(1)–O(4) 2.488(4) Bi(3)–O(21)#4 2.376(5)Bi(1)–O(5) 2.493(4) Bi(3)–O(22) 2.467(5)Bi(1)–O(6) 2.505(4) Bi(3)–O(23) 2.490(5)Bi(1)–O(7) 2.527(4) Bi(3)–O(24) 2.499(5)Bi(1)–O(8) 2.620(5) Bi(3)–O(25)#5 2.582(4)Bi(1)–O(9)#1 2.646(5) Bi(3)–O(26)#5 2.632(5)Bi(2)–O(10) 2.360(4) Bi(4)–O(27) 2.292(4)Bi(2)–O(11) 2.410(5) Bi(4)–O(28) 2.407(5)Bi(2)–O(12) 2.473(5) Bi(4)–O(29) 2.407(4)Bi(2)–O(13)#2 2.507(4) Bi(4)–O(30) 2.496(4)Bi(2)–O(14) 2.547(5) Bi(4)–O(31)#2 2.536(5)Bi(2)–O(15)#3 2.571(4) Bi(4)–O(32) 2.553(4)Bi(2)–O(16)#2 2.596(5) Bi(4)–O(33) 2.557(5)Bi(2)–O(17)#3 2.608(4) Bi(4)–O(34) 2.595(4)

Symmetry transformations used to generate equivalent atoms:#1�x,yþ1/2,�zþ1/2 #2 x,�yþ1/2,z�1/2 #3 �xþ1,y�1/2,�z�1/2#4 x,�yþ3/2,z�1/2 #5 �xþ1,�yþ1,�z #6�x,y�1/2,�zþ1/2#7 x,y,zþ1 #8 x,y,z�1 #9 x,�yþ1/2,zþ1/2 #10�xþ1,yþ1/2,�z�1/2#11 x, �yþ3/2, zþ1/2 #12 �xþ1, �yþ1, �zþ1 #13 �x, �yþ1, �zþ1 #14 �x,�y, �zþ1

Fig. 5. Excitation and emission spectra of compounds 1 and 2. Inset show anoptical image of the blue luminescent compound of 2 using confocal microscope.



Similar blue luminescence has been seen for bismuth-containingorganic–inorganic hybrid materials.

3.5. Lifetime studies

Time-resolved fluorescence measurement was carried outusing a time-correlated single-photon counting (TCSPC) spectro-meter (Edinburgh, OB920). The samples were excited at 375 nmusing a diode laser and the signals were collected at magic angle(54.71) using a Hamamatsu micro channel plate photomultipliertube (R3809U-50). The lamp profile was recorded by scatterer(dilute ludox solution in water) in place of the sample. Theinstrument response function of our setup is �75 ps for 375 nmdiode laser which was determined from full width half maxima(fwhm) of the lamp profile. The decay curve was analyzed bynonlinear least-squares iteration procedure using F900 decayanalysis software. The qualities of the fit were judged by the chisquare (χ2) value, and visual inspection of the residuals of thefitted function to the data. Mean fluorescence life time (τf) for thetri-exponential iterative fittings was calculated from the decaytimes (τi) and the relative amplitude (ai) using the followingequation

τf� �¼ a1τ1þa2τ2þa3τ3

The luminescence lifetimes of the excited states of both 1 and 2were investigated. The emission band for samples was monitoredfor the lifetime studies employing 375 nm excitation at roomtemperature. The fit of the curve for tri-exponential decay suggestsa mean lifetime value of 0.9 and 1.0 ns for 1 and 2, respectively(Fig. 6).

4. Conclusion

The synthesis, structure and characterization of two Bi(III)sulfates of layered and 3D channel structures have been accom-plished thereby establishing the versatility of the sulfate group inbuilding open architectures with the main group metals. Thepresence of nitrate ion in 1 is noteworthy. Both the compounds1 and 2 show blue luminescence possessing a mean life time of0.9 and 1.0 ns respectively. The results suggest that it may indeedbe possible to prepare other complex main group metal sulfateswith 2-D and 3-D networks with interesting photoluminescenceproperties. Compounds 1 and 2, with blue emission as one of the

three basic colors may be useful for developing new-blue–light-emitting devices.

CCDC-931288 (for 1), and 931289 (for 2) contain thesupplementary crystallographic data for this paper.

Acknowledgments

J.N.B thanks the Department of Science and Technology (DST),Government of India, for the award of a research grant. We thankDr. D. Pradhan of IIT Kharagpur, India for the help with the TGAmeasurements. Thanks are also due to Mr. Sudhir Kumar for timeresolved fluorescence measurement.

Appendix A. Supplementary materials

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.jssc.2013.10.032.

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