Optical Materials 35 (2013) 2357–2365

Contents lists available at SciVerse ScienceDirect

Optical Materials

journal homepage: www.elsevier .com/locate /optmat

Synthesis, structural characterization and photophysical propertiesof highly photoluminescent crystals of Eu(III), Tb(III) and Dy(III)with 2,5-thiophenedicarboxylate

0925-3467/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.optmat.2013.06.034

⇑ Corresponding author. Tel.: +55 32 21023310; fax: +55 32 21023314.E-mail address: flavia.machado@ufjf.edu.br (F.C. Machado).

Lippy F. Marques a, Anderson A.B. Cantaruti Júnior a, Sidney J.L. Ribeiro b, Felipe M. Scaldini a,Flávia C. Machado a,⇑a Departamento de Química-ICE, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-330, Brazilb Institute of Chemistry, São Paulo State Univ., UNESP, CP 355, Araraquara, SP 14801-970, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 February 2013Received in revised form 15 March 2013Accepted 18 June 2013Available online 11 July 2013

Keywords:Photoluminescence2,5-Thiophenedicarboxylic acidLanthanides

Lanthanide compounds of general formula [Ln2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O (Ln = Eu(III) (1), Tb(III)(2), Gd(III) (3) and Dy(III) (4), dmf = N,N0-dimethylformamide and 2,5-tdc2� = 2,5-thiophedicarboxylateanion) were synthesized and characterized by elemental analysis, X-ray powder diffraction patterns,thermogravimetric analysis and infrared spectroscopy. Phosphorescence data of Gd(III) complex showedthat the triplet states (T1) of 2,5-tdc2� ligand have higher energy than the main emitting states of Eu(III),Tb(III) and Dy(III), indicating that 2,5-tdc2� ligand can act as intramolecular energy donor for these metalions. An energy level diagram was used to establish the most relevant channels involved in the ligand-to-metal energy transfer. The high value of experimental intensity parameter X2 for the Eu(III) complexindicate that the europium ion is in a highly polarizable chemical environment. The emission quantumefficiency (g) of the 5D0 emitting level of Eu(III) was also determined. The complexes act as possible lightconversion molecular devices (LCMDs).

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Lanthanide coordination compounds with organic ligands areattractive systems to study. They can be used as active centers ofluminescent materials [1–3] or as structural probes for a varietyof chemical and biological studies [4,5]. Moreover, lanthanideslong excited-state lifetimes and their high chromaticity are alsopertinent to applications in the domain of solid-state photonicmaterials. For instance, Eu(III), Tb(III), Dy(III) and Tm(III) cationsare used as red, green, yellow and blue emitters, respectively, inmulticolor displays and organic light-emitting diodes (OLEDs)[6,7]. However, the use of such lanthanide ions systems with directabsorption of the f excited states is very inefficient because the f–ftransitions are parity forbidden, resulting in very low absorptioncoefficients. In order to overcome this drawback, suitable chro-mophores have been employed as antennas (or sensitizers) thathave the capability of transferring energy indirectly to lanthanideions [8]. Therefore, over the past few years, efforts have been madeto augment the absorption coefficients and thereby obtain signifi-cantly more intense lanthanide ion emissions. Fortunately, thisobjective can be accomplished by prudent selection of organic

ligands that can serve as ‘‘antenna’’ and the list includesb-diketones [9–12] and aromatic carboxylic acids [13–15] amongothers. In particular, when aromatic carboxylic acids are employedas antenna ligands, the coordinated lanthanide ions exhibit higherluminescent stabilities than those ligated with other organic li-gands [16–18]. 2,5-thiophenedicarboxylic acid (H2tdc), a multiden-tate ligand that belongs to heterocyclic acids class, presents highconnectivity that can establish bridges among several metal cen-ters [19–22], adopts several coordination modes, and producesmultidimensional networks, including 2D and 3D structures. Inaddition, 2,5-tdc2� ligand also draw attention to their inherentchemical features: (a) carboxyl group can readily coordinate to lan-thanide ions because of the strong affinity between oxygen atomsand lanthanide ions; (b) the p ? p� transitions of the organic li-gand cause strong absorption in the UV region, which is efficientto sensitize lanthanide emissions through energy transfer in termsof the ligand (S1) ? Ligand (T1) ? Ln�; (c) the robustness and cer-tain degree of rigidity to the framework provided by the aromatic-ity in the organic backbone hinder the nonradiative deactivationprocess. It is important to emphasize that previous reports of Me-tal-Organic Frameworks (MOFs) with this ligand, discuss brieflythe photophysical investigations [23–25]. Hence, to date, a moredetailed study of photoluminescent properties of functional com-pounds with 2,5-tdc2� will be presented. Given the important po-

2358 L.F. Marques et al. / Optical Materials 35 (2013) 2357–2365

tential applications of lanthanide carboxylates , we describe thesynthesis, structural characterization and the photophysical studyof four new lanthanide 2,5-thiophenedicarboxylate complexes[Ln2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O (Ln = Eu(III) (1), Tb(III) (2),Gd(III) (3) and Dy(III) (4), dmf = N,N0-dimethylformamide and2,5-tdc2� = 2,5-thiophedicarboxylate anion). Furthermore, for Eu(-III) ion, experimental intensity parameters (Xk) , quantum emis-sion efficiency (g), lifetime of emitting 5D0 level, radiative (ARAD)and nonradiative (ANRAD) coefficients are also reported. The CIE col-or coordinates (x,y) were determined from the emission spectra.

2. Experimental

2.1. Materials and measurements

All synthetic work was performed in air and at room tempera-ture. LnCl3�6H2O (Ln = Tb, Dy and Gd) and 2,5-thiophenedicarboxy-lic acid (2,5-H2tdc) were obtained either from Aldrich� or Fluka�

and used as received. EuCl3�6H2O were prepared by dissolvingeuropium oxide in hydrochloric acid and then dried. Elementalanalyses for C, H and N were carried out using a Perkin–Elmer2400CHN analyzer. X-ray diffraction patterns (PXRD) of the singlecrystals were recorded on a Rigaku RINT2000 diffractometer usingCu Ka radiation (30 kV and 15 mA) from 5� to 70� and 0.02� of passtime. FTIR spectra were recorded with a Bomem Michelson 102FTIR spectrophotometer using KBr pellets in the wavenumberrange of 4000–400 cm�1 with an average of 128 scans and4 cm�1 of spectral resolution. Thermal analysis (TGA) was obtainedon a Shimadzu TG-60 equipment. 6–10 mg sample were heated at10 �C/min from room temperature to 800 �C in a dynamic nitrogenatmosphere (flow rate = 100 mL/min). The luminescence excitationand emission spectra at room temperature (303 K) and liquidnitrogen temperature (77 K) were obtained at an angle of 22.5�(front face) with a Jobin–Yvon Model Fluorolog FL3-22 spectropho-tometer equipped with a R928 Hamamatsu photomultiplier and450 W xenon lamp as excitation source, the spectra were correctedwith respect to the Xe lamp intensity and spectrometer response.Measurements of emission decay were performed with the sameequipment by using a pulsed Xe (3 ls bandwidth) source.

2.1.1. Synthesis of [Eu2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O (1)To a suspension of 2,5-H2tdc (34 mg; 0.20 mmol) in 10 mL of

water, 0.40 mL of an aqueous solution of NaOH (1 mol L�1) wasadded. After 30 min of stirring, the resulting solution was trans-ferred to a tube. 10 mL of N,N0-dimethylformamide solution con-taining 0.13 mmol (50 mg) of EuCl3�6H2O were added. After animmediate precipitation, the solid was filtered off and the resultingsolution was set aside. After 2 weeks, colorless single crystals wereobtained. Yield: 38%. Anal. Calcd. for Eu2C30H40O19N4S3: C, 31.04; H3.47; N, 4.83%. Found: C, 31.01; H, 3.38; N, 4.81%.

2.1.2. Synthesis of [Tb2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O (2)Compound 2 was obtained by applying the same synthetic

procedure described for 1, except that TbCl3�6H2O (50 mg;0.13 mmol) was used instead of EuCl3�6H2O. Yield: 35%. Anal.Calcd. for Tb2C30H40O19N4S3: C, 30.67; H, 3.43; N, 4.78%. Found:C, 31.11; H, 3.31; N, 4.78%.

Fig. 1. X-ray powder diffraction patterns of the [Ln2(2,5-tdc)3(dmf)2(H2O)2]-�2dmf�H2O complexes, with Ln = Eu (1), Tb (2), Gd (3) and Dy (4).

2.1.3. Synthesis of [Gd2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O (3)Compound 3 was obtained by applying the same synthetic pro-

cedure described for 1, except that GdCl3�6H2O (50 mg; 0.13 mmol)was used instead of EuCl3�6H2O. Yield: 31%. Anal. Calcd. for Gd2C30-

H40O19N4S3: C, 30.76; H, 3.44; N, 4.78%. Found: C, 31.04; H, 3.39; N,4.76%.

2.1.4. Synthesis of [Dy2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O (4)Compound 4 was obtained by applying the same synthetic pro-

cedure described for 1, except that DyCl3�6H2O (50 mg; 0.13 mmol)was used instead of EuCl3�6H2O. Yield: 41%. Anal. Calcd. for Dy2C30-

H40O19N4S3: C, 30.49; H, 3.41; N, 4.74%. Found: C, 31.07; H, 3.38; N,4.71%.

All complexes are stable in air, non-hygroscopic and insolublein common organic solvents such as ethanol, dimethylsulfoxide,acetone, chloroform and dimethylformamide.

3. Results and discussion

3.1. General characterization

According to the analytical data, all four compounds present thesame minimum molecular formula, with stoichiometric ratio of(2:3)/(Ln(III):2,5-tdc2�). These compounds, together with congen-ers reported in recent literature [21,23–25], tend to be highly crys-talline species, with consequent structural determination throughsingle crystal X-ray diffraction analysis. However, in the presentcase, a great structural disorder was observed and it was not pos-sible to build a crystallographic model for refinement. For this rea-son, polycrystalline X-ray diffraction patterns (PXRD) wereobtained. From PXRD patterns exhibited in Fig. 1, high crystallinitycan be noticed for the four complexes. These data show that com-plexes 1, 2 and 3 are isomorphous, while compound 4 pattern, pre-sents two peaks at 18.45� and 21.72� (marked in asterisks) that areabsent in the other patterns. In addition, in compound 4 diffractionpattern, some peaks are shifted relatively to the others. For in-stance, the peaks at 16.53� and 19.36� in compound 2 pattern areshifted to 16.17� and 20.17� in compound 4 pattern, respectively,suggesting different crystal lattices. Although the proposed com-pounds formulation is the same, the crystal packing of compound4 is not, and this fact can be attributed to the lanthanide contrac-tion phenomenon [26]. In addition, the difference in reflectionintensities among the PXRD patterns was due to the variation inpolycrystalline samples preferred orientation during the experi-mental data collection.

In order to examine the complexes thermal stability, thermo-gravimetric analyses were carried out using 1–4 polycrystallinesamples in the temperature range of 25–800 �C under N2 atmo-sphere. As can be seen in Fig. 2, TGA curves for all compoundsare very similar, as expected. Therefore, only complex 2 thermalbehavior will be discussed in detail as a representative compoundfor this group. Three well defined weight losses can be noticed in

Fig. 2. TGA curves for complexes 1–4.

1 For interpretation of color in Fig. 4, the reader is referred to the web version ofthis article.

L.F. Marques et al. / Optical Materials 35 (2013) 2357–2365 2359

TGA traces. The first one, between 50 and 77 �C, corresponds to therelease of one lattice water molecule per formula unit (obsd. 1.63%;calcd. 1.58%). The second, observed from 80 to 189 �C, is consistentwith the release of two dimethylformamide molecules and onecoordination water molecule (obsd. 14.69%; calcd. 14.19%). After-wards, the weight loss of 13.05%, is presumably due to two coordi-nated dimethylformamide molecules removal (calcd. 12.83%).From 419 to 650 �C the decomposition of organic moiety (2,5-tdc2�) takes place (obsd. 37.57%; calcd. 38.34%). At 800 �C, theresidual percentage weight is consistent with ½ mol of Tb4O7

(obsd. 33.04%; calcd. 32.33%).The similarity of lanthanide complexes IR spectra confirms the

results obtained from the PXRD method. The spectra of 2,5-H2tdc,Na2(2,5-tdc) sodium salt and [Ln2(2,5-tdc)3(dmf)2(H2O)]�2dmf�H2Ocomplexes are shown in Fig. 3a. In all complexes spectra a broadband centered at 3425 cm�1 may be assigned to m(OAH) stretchingvibrations, suggesting the presence of water molecules in the struc-ture in accordance to the thermal analysis results discussed above.The weak bands near 2930 and 2867 cm�1 were assigned to m(CAH)stretching modes of methyl groups from dimethylformamide. Inaddition, the band at 1664 cm�1 corresponds to m(C@O) stretchingmode, confirming the presence of dimethylformamide moleculesin the complexes structures [27]. The absence of m(CAOH) absorp-tion bands of 2,5-H2tdc at 1271–1292 cm�1 range in the complexesspectra, indicates the complete deprotonation of this ligand uponcoordination to lanthanide centers. The most relevant aspect of FTIRspectra of the studied compounds is concerned with theasymmetric and symmetric stretching frequency values of COO�

groups. In this class of compounds, the difference betweenmasym(COO�) and msym(COO�) (Dm) in comparison to the correspond-ing values in ionic species is currently employed to propose thecarboxylate group coordination mode [28]. The FTIR spectrum of2,5-tdc2� ligand in ionic form presents strong bands at 1572 and1382 cm�1, which are attributed to masym(COO�) and msym(COO�)stretching modes, respectively, providing Dm = 190 cm�1. The FTIRspectra of the complexes show a splitting of the band attributedto masym(COO�) vibrational mode at 1568 and 1545 cm�1, which isbest viewed in an enlarged portion (1620–1450 cm�1) in Fig. 3b(where the dashed lines are the respective bands obtained bydeconvolution), while the msym(COO�) remains the same(1383 cm�1). These data provide two Dm values of 185 and162 cm�1, suggesting two different coordination modes of carbox-ylate groups; bidentate bridging and chelate modes, respectively.Fig. 3c shows the possible coordination modes of carboxylategroups to lanthanide ions.

3.2. Optical properties of 1, 2 and 4

Owing to the excellent luminescent properties of Eu(III), Tb(III)and Dy(III) ions, the photoluminescence of compounds 1, 2 and 4were investigated. The single crystals of 1 and 2 exhibit intenseluminescence in the primary colors red1 and green, respectively,when exposed to UV light, as can be seen in Fig. 4.

Fig. 3. (a) Vibrational FTIR spectra of 2,5-H2tdc, Na22,5-tdc and complexes 1–4. (b) Enlarged portion (1620–1450 cm�1) of infrared spectrum for the complexes. (c) Possiblecoordination modes of 2,5-tdc2� ligand in the complexes 1–4.

Fig. 4. Luminescence of single crystals of complexes 1 and 2 when exposed to UVlight at room temperature.

2360 L.F. Marques et al. / Optical Materials 35 (2013) 2357–2365

The excitation spectra for all three complexes exhibit a broadband between 270 and 325 nm for 1 and 2, and 270–315 nm for4 which is attributed to the ligand (2,5-tdc2�) centered S0 ? S1

(p, p�) transition of the aromatic thiophene moiety. The normal-ized excitation spectrum for complex 1, which was recorded at303 K and monitored around the intense 5D0 ?

7F2 transition for

Eu(III) ion, is shown in Fig. 5a. The excitation spectrum exhibits aseries of bands arising from 4f–4f transitions from the ground state7F0 level to 5G6 (360 nm), 5H4 (374 nm), 5L7 (383 nm), 5L6 (393 nm),5D3 (415 nm), 5D2 (464 nm) and 5D1 (534 nm) excited states. How-ever, these transitions are less intense than that attributable to theligand levels, which proves that luminescence sensitization is moreefficient than the direct excitation of the Eu(III) ion absorption lev-els. Complex 1 exhibits several characteristic emission bands5D0 ?

7FJ (J = 0–4) upon excitation in the ligand absorption band,at 310 nm (Fig. 5b). The 5D0 ?

7F0 transition (580 nm) consists ofone peak, which gives a strong indication that all Eu(III) ions occu-py sites of same symmetry and that Eu(III) ions experience thesame crystal – field strength. To confirm these results, an emissionspectrum at 5D0 ?

7F0 region was obtained at 77 K (inset inFig. 5b), and no splitting of this band was observed. From the com-parison between the spectra obtained at 303 and 77 K, it can be no-ticed a better resolution of the spectral lines at low temperaturebut no significant difference due to Stark levels were observed(see the whole spectrum in Supplementary Material). A band witha full width at half maximum (FWHM) of approximately 22 cm�1,suggests again the presence of a single Eu(III) chemical environ-ment [29]. The relationship between the magnetic (5D0 ?

7F1)and electric (5D0 ?

7F2) transitions give information about theion surrounding symmetry. The ratio I(5D0 ?

7F2)/I(5D0 ?7F1) is

about 5.4, which suggests that the symmetry surrounding Eu(III)ion is noncentrosymmetric. Additionally, the presence of intense

Fig. 5. Photoluminescence excitation and emission spectra for 1 (a and b), 2 (c and d) and 4 (e and f) at 303 K. Excitation spectra were obtained at emission maximum, whilethe emission spectra were obtained upon excitation at 310 nm.

L.F. Marques et al. / Optical Materials 35 (2013) 2357–2365 2361

2362 L.F. Marques et al. / Optical Materials 35 (2013) 2357–2365

5D0 ?7F4 transition may be caused by the system rigidity, suggest-

ing the formation of a MOF network. The solid state excitation(Fig. 5c) and emission (Fig. 5d) for Tb(III) complex were studiedat room temperature. Several Laporte – forbidden f–f transitionbands at 333–500 nm range, corresponding to characteristic tran-sitions of Tb(III) ion can be noticed. These bands were assignedto 7F6 ? 5L6 (340 nm), 7F6 ?

5L9 (350 nm), 7F6 ? 5L10 (368 nm),7F6 ?

5G6 (376 nm) and 7F6 ?5D4 (486 nm) transitions. Complex

2 exhibits several characteristic emission bands upon excitationin the ligand absorption band, at 310 nm (Fig. 5d). The emissionspectrum is composed of the typical Tb(III) green emission, corre-sponding to 5D4 ? 7F6 (490 nm), 5D4 ?

7F5 (545 nm), 5D4 ? 7F4

(583, 586 and 590 nm), 5D4 ? 7F3 (621 nm), 5D4 ?7F2 (652 nm)

and 5D4 ?7F1 (696 and 701 nm). The compound was also excited

at 350 nm (7F6 ? 5L9 transition), but no difference in 4f–4f transi-tions profile was observed, indicating that the same emissionmechanism takes place. The Dy(III) complex excitation spectrum(Fig. 5e) shows from 325 to 400 nm the narrow bands assignedto 4f9 – intraconfigurational transitions from the ground state6H15/2 to 4G9/2 (323 nm), 4F5/2 + 4D5/2 (336 nm), 4M15/2 + 6P7/2

(349 nm), 4I11/2 (363 nm), 4K17/2 (377 nm), 4M19/2 + 4M21/2

(386 nm) and 4G11/2 (449 nm) excited states. The emissionspectrum of solid sample of 4 at room temperature upon photo –excitation at 310 nm, is shown in Fig. 5f. The complex had sharpemission bands characteristic of Dy(III) ion corresponding to4F9/2 ?

6H15/2 (480 nm), 4F9/2 ?6H13/2 (572 nm) and 4F9/2 ?

6H11/2 (663 nm) transitions. This complex presents the strongestline with DJ = 2 and connected to 4F9/2 ?

6H13/2 transition. Thishypersensitive emission peak at 572 nm is affected by the Dy(III)environment [30]. The yellow emission which is stronger thanthe blue one was observed in the solid state.

Fig. 6. (a) Luminescence spectra of the Gd(III) complex in solid state, at 77 K: (a) Excitat525 nm. (b) Emission spectrum monitored on the S0 ? S1 transition at 310 nm and (c) e

The photophysical properties of the 2,5-tdc2� donor states inEu(III), Tb(III) and Dy(III) complexes have been investigated onthe basis of Gd(III) complex phosphorescence spectrum. Becausethere is a large energy gap (ca. 32,000 cm�1) between the groundstate 8S7/2 and the first excited state 6P7/2 of Gd(III) ion, it cannotaccept any energy from 2,5-tdc2� ligand first excited triplet statevia intramolecular ligand – to – metal energy transfer. Differentlyfrom Lu(III) (4f14) and La(III) (4f0) ions that are, respectively, smal-ler and larger than Eu(III) ion, Gd(III) ion is very similar in size toEu(III) ion and consequently, the effect of Gd(III) and Eu(III) ionson 2,5-tdc2� ligand states should also be similar [31]. Fig. 6a showsthe excitation spectrum of the gadolinium complex, recorded at250–400 nm spectral range at 77 K, that displays a band with max-imum at 310 nm that can be attributed to ligand centered S0 ? S1

(p, p�) transition. The emission spectrum of this complex (Fig. 6b)recorded from 400 to 700 nm, by monitoring the excitation at310 nm (S0 ? S1 transition), shows a strong band with maximumaround 525 nm, assigned to the phosphorescence (T1 ? S0) transi-tion. The lifetime of Gd(III) complex recorded with emissionmonitored at 525 nm showed long values (s = 2.13 ms) of the emit-ting state lifetime at room temperature, 298 K. This result suggeststhe triplet character of the emitting states that contribute to thatband around 525 nm. Thus, on the basis of the phosphorescencespectra of complex 3 (Fig. 6b), the triplet energy level (T1) of 2,5-tdc2� corresponds to the zero phonon transition and appears atapproximately 22123 cm�1 (452 nm). The triplet energy level of2,5-tdc2� appears at appreciably higher energy than 5D0 for Eu(III),5D4 for Tb(III) and 4F9/2 for Dy(III), thus indicating that this ligandcan act as an antenna for the photosensitization of trivalent lantha-nide ions, collecting UV energy and transferring it in a nonradiativeprocess through T1 state to excited states of these lanthanide [32].

ion spectrum under emission on the T ? S0 transition centered on the 2,5-tdc2� , atnergy transfer scheme for 1, 2 and 4 complexes.

Table 1Photoluminescence data of the [Eu2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O (1) complex inthe solid state. The parameters’ errors are estimated within 5–10%.

Compound X2

(10�20 cm2)X4

(10�20 cm2)ARAD

(s�1)ANRAD

(s�1)Atot

(s�1)g(%)

(1) 13.60 11.80 512 1146 1658 32

L.F. Marques et al. / Optical Materials 35 (2013) 2357–2365 2363

A schematic energy level diagram based on the foregoing is pre-sented in Fig. 6c. Latva’s empirical rule states [33] that an optimalligand – to – metal energy transfer process for Ln(III) needsDE(3pp� – 5DJ) >2000 cm�1 for Tb(III). In complex 2, the energygap, DE(3pp� – 5D4) is lower (1715 cm�1) than the optimal value, however the ligand – to – metal energy transfer process is stilleffective, despite the back energy transfer between 5D4 levels ofTb(III) ion and 3pp�.

The Judd–Ofelt theory is a useful tool for analyzing 4f–4f transi-tions [34]. Interactions parameters of ligand fields are given by theJudd–Ofelt parameters, Xk (where k = 2, 4 and 6). In particular, X2

is more sensitive to symmetry and sequence of ligands fields. In or-der to get further information on the Eu(III) ion chemical environ-ment for the [Eu2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O 1 complex, theexperimental X2 and X4 intensity parameters, radiative rates(A0?J) (for the 5D0 ?

7F2 and 5D0 ?7F4 electronic transitions) and

emission quantum efficiency (g) were determined according toRefs. [35,36]. These values are shown in Table 1. For A0?J calcula-tion, the bellow equation is considered:

A0!J ¼r0!1

S0!1� S0!J

r0!J� A0!1 ð1Þ

where r0?1 and r0?J correspond to 5D0 ?7F2 and 5D0 ?

7F4 transi-tions energy barycenters (in cm�1), respectively. Similarly, S0?1 andS0?J are the emission curve surface corresponding to 5D0 ? 7F1 and5D0 ?

7FJ transitions, respectively. As known, the magnetic dipoleallowed 5D0 ?

7F1 transition was taken as Ref. [37], since A0?1 rateis almost insensitive to chemical environment changes around theEu(III) ion with A0?1 ffi 50 s�1. Complexes 1 and 2 luminescencelifetimes in the solid state are determined at room temperature un-der excitation at 310 nm, with emission monitored at 5D0 ?

7F2 and5D4 ? 7F5 transitions for Eu(III) and Tb(III) complexes. Each of thedecay curves (Fig. 7) is well – reproduced by single exponentialfunctions, consistent also with only one symmetry site for bothcomplexes, as predicted by data from the Eu(III) complex emission

Fig. 7. Typical luminescence decay profiles observed for [Eu2(2,5-tdc)3(dmf)2(H2O)2]�2dmtemperature.

spectrum. The lifetime values (s), at 303 K (Fig. 7) and 77 K (Fig. S2in the Supplementary Material) for [Eu2(2,5-tdc)3(dmf)2(H2O)2]-�2dmf�H2O (1) were found to be 0.639 and 0.879 ms, and for[Tb2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O (2) 0.879 and 0.959 ms,respectively. The luminescence decay curves obtained at room tem-perature result in a slight decrease in the emission lifetime of bothcomplexes. This is caused by the vibrational contributions increas-ing that decrease the transmitter level population and the emissionintensities. In the same system, the s values of Tb(III) ion are higherthan those of Eu(III) ion, due to the larger energy gap between theexcited states of terbium ion (�14,800 cm�1) compared with theeuropium gap (�12,300 cm�1).

Based on the emission spectrum and lifetime measurements,and considering that only radiative and nonradiative processesare essentially involved in the depopulation of the 5D0 state, wecan estimate the emission quantum efficiency of 5D0 excited state(g) according to equation (2), where ARAD is obtained by summingthe radiative rates A0?J for each 5D0 ?

7FJ transition [34] (ARAD =rA0?J), and nonradiative rates, ANRAD, are calculated from theexperimental decay rates through Eq. (3).

g ¼ ARAD

ARAD þ ANRADð2Þ

1s¼ ARAD þ ANRAD ð3Þ

The emission quantum efficiency data show that g � 32% for[Eu2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O (1) is due to the contribu-tion of nonradiative (ANRAD) arising from the OAH oscillator oftwo water molecules coordinated to the Eu(III) ion. In addition,the presence of dimethylformamide molecules in the complexmay also promote the quenching of luminescence by CAH groupshigh frequencies vibrations. The experimental intensities parame-ters were calculated from the spontaneous emission coefficients(A0?J), according to the following expression [38,39]:

Xk ¼3�hc3A0!J

4e2x3vh7FJjjUðkÞjj5D0i2ð4Þ

where v is the Lorentz local field correction term, given by

v ¼ nðnþ2Þ29 and h7FJjjUðkÞjj5D0i2 is a squared reduced matrix element

with value of 0.0032 for the 5D0 ? 7F2 transition and 0.0023 forthe 5D0 ? 7F4 one. The refraction index (n) has been assumed equal

f�H2O (1) and [Tb2(2,5-tdc)3(dmf)2(H2O)2]�2dmf�H2O (2) in the solid state at room

Fig. 8. Chromaticity diagram showing luminescence coordinates for complexes 1, 2and 4.

2364 L.F. Marques et al. / Optical Materials 35 (2013) 2357–2365

to 1.5. In this work, the 5D0 ? 7F6 transition was not observedexperimentally; consequently, the experimental X6 parametercould not be estimated. The large value found for X2 parameter tra-duces the high intensity of the hypersensitive transition 5D0 ?

7F2,suggesting that the dynamic coupling mechanism is operative andthe chemical environment around the Eu(III) ion is highly polariz-able, probably due to the strong delocalization of chelate rings p– electrons in 2,5-tdc2� ligand. X2 value is relatively low whencompared directly to those for Eu – b – diketonates, indicating a re-duced degree of covalence involving the M–L coordination bond[39,40]. The Commission Internationale L’Eclairage (CIE) coordi-nates obtained were also calculated as x = 0.682 and y = 0.316 for1, x = 0.313 and 0.519 for 2, x = 0.402 and y = 0.404 for 4 whichare located in red, green and yellow regions, respectively Fig. 8.As it can be seen, Eu(III) and Tb(III) complexes act as light conver-sion molecular devices (LCMDs) and produce intense monochro-matic emission colors.

4. Conclusions

In summary, we have successfully prepared highly luminescentlanthanide carboxylates of Eu(III), Tb(III) and Dy(III) with an easyone-step, where the single – crystals formation suggests high pur-ity of the complexes. The analytical results (elemental analysis,thermogravimetry) and FTIR spectroscopy suggest the formationof complexes with general formula [Ln2(2,5-tdc)3(dmf)2(H2O)2]-�2dmf�H2O. The PXRD patterns show that Eu(III), Tb(III) and Gd(III)compounds are isomorphous, and Dy(III) compound may have adifferent crystal packing. According to the photoluminescencestudy, the phosphorescence broad band from 2,5-tdc2� ligand isnot present in any spectra, which suggests that the intramolecularligand to metal energy transfer is efficient. This fact was reinforcedby measuring of energy triplet states (T1) of the 2,5-tdc2� ligandthat is above 5D0 Eu(III), 5D4 Tb(III) and 4F9/2 Dy(III) energy levels.The high value of experimental intensity parameter (X2) for Eu(III)complex indicates that europium ion is in a highly polarizablechemical environment. The emission quantum efficiency(g = 32%) for Eu(III) compound is related to two water moleculescoordinated to the metal ion. Photoluminescence data showed that

Eu(III), Tb(III) and Dy(III) complexes are potential candidates asemitters in photonic systems. Research for developing new inter-esting inorganic–organic hybrid and porous luminescent materialsusing carboxylate ligands is in progress in our laboratory.

Acknowledgments

The authors thank the Brazilian agencies CNPq, CAPES, FAPESPand FAPEMIG (CEX-APQ 01565-09) for financial support.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.optmat.2013.06.034.

References

[1] P.N. Remya, S. Biju, M.L.P. Reddy, A.H. Cowley, M. Findlater, Inorg. Chem. 47(2008) 7396.

[2] C.M.G. Santos, A.J. Harte, S.J. Quinn, T. Gunnlaugsson, Coord. Chem. Rev. 252(2008) 2512.

[3] F.T. Edelmann, Coord. Chem. Rev. 253 (2009) 343.[4] T.J. Mosmaun, Rapid colorimetric assay for cellular growth and survival:

application to proliferation and cytotoxicity assays, J. Immunol. Methods 65(1–2) (1983) 55.

[5] Y.B. Wang, J.H. Yang, X. Wu, L. Li, S. Sun, B.Y. Su, Z.S. Zhao, Progress of spectralprobes for nucleic acids, Anal. Lett. 36 (10) (2003) 2063.

[6] J. Hao, S.A. Studenikin, M. Cocivera, J. Lumin. 93 (2001) 313.[7] T. Justel, H. Nikol, C. Ronda, Angew. Chem. Int. Ed. 37 (1998) 3084.[8] N. Sabbatini, M. Guardigli, J.M. Lehn, Coord. Chem. Rev. 123 (1993) 201.[9] G.F. de Sá, O.L. Malta, C. de Mello Donega, A.M. Simas, R.L. Longo, P.A. Santa-

Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165.[10] L. Sun, J. Yu, G. Zheng, H. Zhang, Q. Meng, C. Peng, F. Liu, Y. Yu, Eur. J. Inorg.

Chem. 19 (2006) 3962.[11] A. Fratini, G. Richards, E. Lar-der, S. Swavey, Inorg. Chem. 47 (2008) 1030.[12] S. Biju, D.B.A. Raj, M.L.P. Reddy, B.M. Kariuki, Inorg. Chem. 45 (2006) 10651.[13] L. Charbonniere, S. Mameri, P. Kadjane, C.P. Iglesias, R. Ziessel, Inorg. Chem. 47

(2008) 3748.[14] J. Xia, B. Zhao, H.-S. Wang, W. Shi, Y. Ma, H.-B. Song, P. Cheng, D.-Z. Liao, S.-P.

Yan, Inorg. Chem. 46 (2007) 3450.[15] M. Babji, A. Mondry, Opt. Mater. 34 (2012) 2061.[16] S. Raphael, M.L.P. Reddy, A.H. Cowley, M. Findlater, Eur. J. Inorg. Chem. 28

(2008) 4387.[17] R. Shyni, S. Biju, M.L.P. Reddy, A.H. Cowley, M. Findlater, Inorg. Chem. 46

(2007) 11025.[18] E.R. Souza, I.G.N. Silva, E.E.S. Teotonio, M.C.F.C. Felinto, H.F. Brito, J. Lumin. 130

(2010) 283.[19] O.Z. Yesilel, I. Ilker, M.S. Soylu, C. Darcan, Y. Suzen, Polyhedron 39 (2012) 14.[20] X.-Z. Sun, Z.-L. Huang, H.-Z. Wang, B.-H. Ye, X.-M. Chen, Z. Anorg, Allg. Chem.

631 (2005) 919.[21] L.F. Marques, M.V. dos Santos, S.J.L. Ribeiro, E.E. Castellano, F.C. Machado,

Polyhedron 38 (2012) 149.[22] W. Huang, D. Wu, P. Zhou, W. Yan, D. Guo, C. Duan, Q. Meng, Cryst. Growth

Des. 9 (2009) 1361.[23] C.-H. Zhan, F. Wang, J. Zhang, Inorg. Chem. 51 (2012) 523.[24] J.-G. Wang, C.-C. Huang, X.-H. Huang, D.-S. Liu, Cryst. Growth Des. 8 (2008)

795.[25] Y.-g. Sun, B. Jiang, T.-f. Cui, G. Xiong, P.F. Smet, F. Ding, E.-j. Gao, T.-y. Lv, K.V.

den Eeckhout, D. Poelman, F. Verpoort, Dalton Trans. 40 (2011) 11581.[26] J. Xu, J. Cheng, W. Su, M. Hong, Cryst. Growth Des. 11 (2011) 2294.[27] C.M. MacNeill, C.S. Day, S.A. Gamboa, A. Lachgar, R.E. Noftle, J. Chem.

Crystallogr. 40 (2010) 222.[28] G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33 (1980) 227.[29] L.D. Carlos, R.A.S. Ferreira, V. de Zea Bermudez, S.J.L. Ribeiro, Adv. Mater. 21

(2009) 509.[30] J. Kuang, Y. Liu, J. Zhang, J. Solid State Chem. 179 (2006) 266.[31] R.D. Archer, H. Chen, L.C. Thompson, Inorg. Chem. 37 (1998) 2089.[32] W.M. Faustino, L.A. Nunes, I.A.A. Terra, M.C.F.C. Felinto, H.F. Brito, O.L. Malta, J.

Lumin (2013) (in press).[33] M. Latva, H. Takalo, V.M. Mukkala, C. Matachescu, J.C. Rodriguez-Ubis, J.

Kanakare, J. Lumin. 75 (1997) 149.[34] M.H.V. Werts, R.T.F. Jukes, J.W. Verhoeven, Phys. Chem. Chem. Phys. 4 (2002)

1542.[35] G.F. de Sá, O.L. Malta, C.D. Donega, A.M. Simas, R.L. Longo, P.A. Santa-Cruz, E.F.

Silva, Coord. Chem. Rev. 196 (2000) 165.[36] H.F. Brito, O.L. Malta, M.C.F.C. Felinto, E.E.S. Teotonio, Luminescence

phenomena involving metal enolates, in: J. Zabicky (Ed.), Patai Series, TheChemistry of Metal Enolates, John Wiley & Sons Ltd., 2009, p. 131 (Chapter 3).

L.F. Marques et al. / Optical Materials 35 (2013) 2357–2365 2365

[37] L.D. Carlos, Y. Messadeq, H.F. Brito, R.A. Sá Ferreira, V. de Zea Bermudez, S.J.L.Ribeiro, Adv. Mater. 12 (2000) 594.

[38] W.T. Carnall, H.M. Crosswhite, Energy Structure and Transitions Probabilitiesin LnF3 of the Trivalent Lanthanides, Argonne National Laboratory Report,Argonne, IL, 1977.

[39] R. Pavithran, N.S.S. Kumar, S. Biju, M.L.P. Reddy, A. Severino Junior, R.O. Freire,Inorg. Chem. 45 (2006) 2184.

[40] O.L. Malta, H.F. Brito, J.F.S. Menezes, F.R. Gonçalves e Silva, A. Severino Júnior,F.S. Farias Jr., A.V.M. de Andrade, J. Lumin. 75 (1997) 255.