Universidade de São Paulo

2014-01

A polymeric europium complex with the ligand

thiophene-2-carboxylic acid: synthesis,

structural and spectroscopic characterization Polyhedron,Amsterdam : Elsevier,v. 67, p. 65-72, Jan. 2014http://www.producao.usp.br/handle/BDPI/51300

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Departamento de Física e Ciências Materiais - IFSC/FCM Artigos e Materiais de Revistas Científicas - IFSC/FCM

A polymeric europium complex with the ligand thiophene-2-carboxylicacid: Synthesis, structural and spectroscopic characterization

Flavia Cagnin a, Marian R. Davolos a,⇑, Eduardo E. Castellano b

a Univ. Estadual Paulista, Campus de Araraquara, Instituto de Química, Departamento de Química Geral e Inorgânica, Laboratório de Materiais Luminescentes, 4800-900Araraquara-SP, Brazilb USP – Universidade de São Paulo, Campos de São Carlos, Instituto de Física, Grupo de Cristalografia, 13560-970 São Carlos-SP, Brazil

a r t i c l e i n f o

Article history:Received 2 May 2013Accepted 28 August 2013Available online 5 September 2013

Keywords:Photoluminescence spectroscopyFTIR spectroscopyThermal analysisX-ray diffractometry

a b s t r a c t

A polymeric complex [Eu(a-tpc)3(a-Htpc)2]n and its characterization by single crystal X-ray and thermalanalysis, infrared and photoluminescence spectroscopies are described. The compound crystallizes in themonoclinic Cc space group. The asymmetric unit is formed from a europium ion bonded to one carboxyloxygen of five different thiophene carboxylic moieties. Three of these moieties are deprotonated andbridge between neighboring europium ions giving rise to an infinite polymer along the c axis. Besidesthe europium characteristic emission lines, the emission spectra show unambiguously the crystal sizeeffect on the 5D0 ?

7F0 transition. The complex thermal decomposition at 220 �C leads to a stable lumi-nescent complex in which the 5D0 ?

7F4 transition reveals a monomeric characteristic. The Judd–Ofeltintensity parameters to the polymeric and the monomeric compound with the same ligand and coordi-nation number were compared.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The lanthanide ions have high coordination number and showparticular magnetic and luminescent properties. Lanthanide me-tal–organic framework (MOFs) have attracted much attention [1]due potential usefulness in photo [2] and thermal sensing [3,4],biomedical imaging [5], drug delivery [6] and magnet devices [7–9] among other applications [10–13].

The thiophene-based molecules show some interesting applica-tions like in OLED [14,15], electrochromic agents [16–18] solarcells [19,20], linear and nonlinear optical properties [21], photovol-taic devices [22–24], electronics in general [25–28], cancer treating[29–32] and antimicrobial agent [33,34]. The presence of a sulfuratom has major consequences in that it enhances the aromaticcharacter of thiophene. Thiophene stands as a unique five-mem-bered heterocycle as it maintains steadfastly its aromaticity char-acter through all manners of transformations, including fusionwith other aromatic rings, resulting in thousands of other aromaticderivatives. The main point is that the thiophene ring is rich inelectrons as sulphur increases the electronegativity of the groupand so rises the reactivity for any kind of electrophilic reagent[35,36]. This property makes the thiophene-based molecules use-ful as complexes building blocks [37–41]. Luminescent polymericcomplex with trivalent lanthanide ions have been frequently re-

ported [42–45]. In particular, the thiophene ligands havesuccessfully being used to obtain molecular organic frameworks[46–52].

We report here the luminescent properties of the polymeric[Eu(a-tpc)3(a-Htpc)2]n and its thermally decomposed stable com-plexes. The polymeric synthesis method is cleaner and consider-ably softer than that one described in the literature by Yuan [53],for which the synthesis involves ZnO addition.

2. Experimental

2.1. Preparation of the complexes

Thiophene-2-carboxylic acid, C5H4O2S, europium(III) and gado-linium(III) oxides, and zinc acetate dihydrate, were Fluka and Al-drich products respectively, with analytical-grade purities. Theeuropium(III) and gadolinium(III) with thiophene-2-carboxylicacid (a-tpc) complexes were prepared by addition of 1.6 � 10�4 -mol of LnCl3 (previously prepared from Ln2O3) to 1.0 mmol of li-gand, both in aqueous solution (T = 60 �C) with pH solutionadjusted to 5.0. The mixture was left in stirring for 24 h. Afterfew days, colorless crystals appeared in the form of needles. Be-cause the Gd(III) ion does not present emitting levels in the visibleregion, in the Gd(III) homologue complex the triplet ligand levelwas determined, thus enabling the triplet state determination bythe ligand emission.

0277-5387/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.poly.2013.08.063

⇑ Corresponding author. Tel.: +55 16 33019634.E-mail address: davolos@iq.unesp.br (M.R. Davolos).

Polyhedron 67 (2014) 65–72

Contents lists available at ScienceDirect

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

2.2. Characterization methods

Thermal analysis was performed on a Thermoanalyzer TGA/DSCsimultaneous STA 409 PC Luxx, Netzsch, under the following con-ditions: synthetic air, 50 mL/min, and heating rate 10 K/min, from25 to 1300 �C. Infrared spectra were recorded on an FT-IR SpectrumPerkin Elmer 2000 with samples prepared as KBr pellets (10 kbar)using about of 1:10 sample:KBr ratio. The luminescence spectrawere recorded at �298 K and �77 K in a Spectrofluorimeter Fluo-rolog Horiba Jobin Yvon FL3–222 model with a 450 W Xenon lamp.The emission lifetime was obtained using a Phosphorimeter JobimYvon, model FL-1040 equipped with a xenon arc pulsed lamp(25 Hz). The monomeric complex structure was calculated usingsemi empirical method which has advantages due the low com-puter time for calculus and high accuracy in the prediction of bondlengths and bond angles [54–56]. The Sparkle/AM1 methodology[57] was developed and implemented in the MOPAC2009 package[58].

2.3. X-ray structural determination

A colorless prismatic crystal of dimensions 0.26 � 0.16 � 0.07mm3 was selected and mounted on an Enraf–Nonius Kappa-CCDdifractometer with graphite monochromated Mo Ka (k = 0.71073Å) radiation. The final unit cell parameters were based on all reflec-tions. Data collections were made using the COLLECT program [59];integration and scaling of the reflections were performed withthe HKL Denzo–Scalepack system of programs [60]. Absorptioncorrections were carried out using the GAUSSIAN method [61]. Thestructure was solved by direct methods with SHELXS-97 and themodel was refined by full-matrix least squares on F2 by meansof SHELXL-97 [62].

3. Results and discussion

3.1. Crystal structure

As shown by the X-ray analysis the formula [Eu(a-tpc)3(a-Htpc)2]n, results. The structure is disordered in that all thiophenolicrings occupy alternatively two positions related to one another byan 180o rotation about the C–C sigma bond. In this situation thesulfur and one carbon atom of the ring occupy the same position.Therefore sulfur and carbon atoms were assign to that site withoccupancies constrained to add up to one. For the five differentthiophenes the occupancy factors for the shared sites convergedto minimum and maximum values of 0.5004 and 0.8280. Thisstrategy produced a much lower R1 factor but, of course, it intro-duced bias in the geometry and displacement parameters of therings. For this reason, hydrogen atoms were not included in themodel. The asymmetric unit is formed from a europium ionbonded to one carboxyl oxygen of five different thiophenecarboxy-lic moieties. Three of these moieties are deprotonated and bridgebetween neighboring europium ions giving rise to an infinite poly-mer along the c axis. The other two moieties have their second oxy-gen atom protonated [as indicated by their longer interatomicdistances, O(32)–C(31) = 1.338(7) and O(52)–C(51) = 1.323(8)]and are therefore uncharged, as required to balance the overallelectric charge. The compound crystallizes in the monoclinic sys-tem, space group Cc (No. 9). Data collection and experimental de-tails are summarized in Table 1. An ORTEP [63] projection of theasymmetric unit is shown in Fig. 1.

According to the Fig. 1, the following formula, [Eu(a-tpc)3(a-Htpc)2]n, was proposed. The view along the b and c axis is showedin Fig. 2a and b, respectively. In the Table 2 are the selected bondlengths [Å] and angles [�] for the polymeric [Eu(a-tpc)3(a-Htpc)2]n

complex. Structure data have been deposited with the CambridgeCrystallographic Data Centre (CCDC 922603).

3.2. Thermal analysis

The thermal analysis of the [Eu(a-tpc)3(a-Htpc)2]n shows themass loss at about 100 �C, which was attributed to 1.5 H2O mole-cules (3.16% Anal./3.06% Calc.) that were probably adsorbed inthe complex surface. There is also a loss of one ligand moleculestarting close to 200 �C (14.6% Anal./14.5% Calc.) and subsequentlytwo more ligands molecules are observed to be lost at 390 �C(30.1% Anal./29.1% Calc.). The residual mass is attributed to aEu2(SO4)3 [64] molecule (33.6% Anal./33.5% Calc.), Fig. 3.

It is important to note that there is a stable product in the re-gion around 180–380 �C that was attributed to a product resultingfrom one ligand molecule lost. This stable intermediate product,denominated [Eu(a-tpc)3(a-Htpc)], will be described in the follow-ing sections.

3.3. FT-IR analysis and theoretical calculations

The ligands containing the carboxylic groups can be coordi-nated to the metal by monodentate or bidentate modes. Whenthe coordination is bidentate, it can occur by chelate or bridgemode and analyzing the infrared spectra, specifically themasym(COO–) and msym(COO–) bands, it is possible to make acoordination mode prediction. When the difference between themasym(COO–) and msym(COO–) bands is larger than in the ligand salt,the coordination mode is monodentate. If this value is significantlyless than the anionic value the ligand mode is chelate and if thisvalue is close to that of the isolated ligand, the coordination isbridging [64].

The vibrational frequencies related to masym (COO�) andmsym (COO�) and the difference between them (Dasym–sym) for

Table 1Crystal data and structure refinement.

Empirical formula C25H17EuO10S5

Formula weight 789.65Temperature (K) 293(2)Wavelength (Å) 0.71073Crystal system monoclinicSpace group CcUnit cell dimensionsa (Å) 20.5494(5)b (Å) 14.1831(4)c (Å) 9.7897(3)a (�) 90b (�) 92.021(2)c (�) 90Volume (Å3) 2851.47(14)Z 4Density (calculated) (mg/m3) 1.839Absorption coefficient (mm�1) 2.621F(000) 1560Crystal size (mm) 0.260 � 0.162 � 0.070Theta range for data collection (�) 2.69–26.00Index ranges �24 < h < 25, �17 < k < 16,

�12 < l < 11Reflections collected 9604Independent reflections (Rint) 5333 (0.0652)Completeness to theta = 26.00� 99.7%Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 5333/2/380Goodness-of-fit on F2 1.034Final R indices [I > 2r(I)] Rl = 0.0332, wR2 = 0.0869R indices (all data) Rl = 0.0345, wR2 = 0.0881Absolute structure parameter �0.029(11)Largest difference in peak and hole

(e �3)0.551 and �2.094

66 F. Cagnin et al. / Polyhedron 67 (2014) 65–72

thiophene-2-caboxylic ammonium salt is 251 cm�1 and for thecomplexes are 241 cm�1 {[Gd(a-tpc)3(a-Htpc)2]n}, 261 cm�1

{[Eu(a-tpc)3(a-Htpc)2]n}and 113 cm�1 for the monomeric com-plex. These values indicate that both oxygen atoms of the

COO� group are involved in coordination to the metal ions mainlyby bridge mode [65], resulting in a polymeric metal complexeswith thiophene-2-caboxylic acid, in accordance with the crystallo-graphic results (Fig. 1).

Fig. 1. Ortep projection of the complex.

Fig. 2. 3D network representation to the complex along the b axis (a) and c axis (b).

F. Cagnin et al. / Polyhedron 67 (2014) 65–72 67

To study the stable decomposition product, [Eu(a-tpc)3(a-Htpc)], the polymeric complex was heated for 4 h at a temperatureof 220 �C. The vibrational frequencies, related to masym (COO�) andmsym (COO�), confirm a chelate coordination mode. The groundstate geometry of [Eu(a-tpc)3(a-Htpc)] was calculated using Spar-kle/AM1 [57] implemented in MOPAC2009 package [58] and isshown in Fig. 4. The structure obtained from Sparkle/AM1 showsa dodecahedral symmetry around the Eu3+ ion. The average Eu–Odistance calculated from Sparkle/AM1 is 2.3271 Å.

3.4. Luminescence spectroscopy

In order to determine the energy position of the triplet (T) statearising from the a-tpc ligand, the emission spectra to the Gd(III)complex and to the pure ligand were recorded at 77 K (kexc = 380 -nm). The pure ligand (Fig. 5) emission spectrum shows a broadband around 440 nm and the gadolinium complex shows a similarbut enlarged band. The triplet level determined is 19833 cm�1,Fig. 6.

The excitation spectrum of the polymeric [Eu(a-tpc)3(a-Htpc)2]n complex (recorded with kem = 613 nm), shows a broadband at 300 nm, Fig. 7. This maximum is in agreement with a pos-sible charge transfer (LMCT). This band is attributed either to ap ? p⁄ transition in the aromatic ring or else to a n ? p⁄ that oc-curs in carbonyl compounds [66]. The [Eu(a-tpc)3(a-Htpc)] excita-tion spectrum is broader than in the polymeric complex andshifted to lower energy.

The [Eu(a-tpc)3(a-Htpc)] emission spectra Fig. 8, shows thecharacteristic Eu3+ transition 5D0 ? 7FJ (J = 0, 1, 2, 3, 4) and infor-mation related to the Eu3+ environment symmetry. The bandrelated to the 5D0 ? 7F0 transition (around 578.5 nm) in the

[Eu(a-tpc)3(a-Htpc)2]n complex is composed of at least two bands,Fig. 9, due to at least two sites without inversion center and the ionsymmetry must belong to a group of low symmetry, Cn, Cs, or Cnv,as in the similar complex [Eu(a-tpc)3(H2O)2] [67]. To verify thispossibility, the emission profile of the polymeric complex wasinvestigated by obtaining another spectrum from a powder sampleof the crystal. In this 77 K spectrum little variation is observed,indicating that there is no important change around the emissionion. However after careful analysis in the 5D0 ? 7F0 transition be-tween 576 and 578.5 nm, it was concluded that there were smallchanges in the emission site. Before the sample pulverization, therewere at least two symmetry sites without inversion center and thelow energy site is favored, Fig. 9(b). After the sample pulverizationthe high energy site is favored (Fig. 9(c)) and there is a general dis-placement of the 5D0 ? 7F0 transition to higher energy after thepulverization. A close relationship between 5D0 ? 7F0 transitionposition and the system covalence has been reported in the litera-ture [68–70]. The covalence decreases due to the increase of super-ficial sites in the pulverized sample. In the [Eu(a-tpc)3(a-Htpc)]

Table 2Selected bond lengths [Å] and angles [�] of the polymeric [Eu(a-tpc)3(a-Htpc)2]n

complex.

Distances [Å] Angles [�]

0(11)–Eu 2.309(4) C(ll)–0(11)–Eu 141.6(4)0(12)–Eu 2.330(4) C(ll)#l–0(12)–Eu 142.8(4)0(21)–Eu 2.362(5) C(21)–0(21)–Eu 143.7(4)0(42)–Eu#2 2.458(3) C(21)#2–0(22)–Eu 133.9(4)0(41)–Eu 2.409(4) C(41)–0(41)–Eu 146.3(3)0(22)–Eu 2.427(4) C(41)–0(42)–Eu#2 139.5(3)0(31)–Eu 2.476(3) C(31)–0(31)–Eu 143.4(4)0(51)–Eu 2.495(4) C(51)–0(51)–Eu 137.7(4)Eu–0(42)#1 2.458(3)

Symmetry transformations used to generate equivalent atoms: #1 x, �y + 2, z � l/2,#2x, �y + 2, z + l/2.

Fig. 3. Thermogravimetric analysis of the [Eu(a-tpc)3(a-Htpc)2] complex.

Fig. 4. Calculated structure to the [Eu(a-tpc)3(a-Htpc)] complex using Sparkle/AM1model.

Fig. 5. Ligand emission spectrum (T77 K, kem = 320 nm).

68 F. Cagnin et al. / Polyhedron 67 (2014) 65–72

complex, the site of lowest energy is favored and there is a nephel-auxetic red shift indicating the larger covalence in [Eu(a-tpc)3(a-Htpc)] as compared with the polymeric complex. The higher cova-lence is confirmed by the average Eu�O bond length in which onthe polymeric complex is 2.41 Å. The lowest average bond lengthin monomeric complex indicates the greater overlap, consequentlyhigher covalence.

The 5D0 ? 7F2 transition is hypersensitive to the ligand fieldstrength since this transition is of electric dipole nature, whilethe intensity of the 5D0 ? 7F1 transition (585–605 nm) which canunfold into three bands, is to an allowed transitions by magneticdipole, whose intensity is independent of the Eu3+ ion chemicalenvironment [71]. The shape, position and intensity ratios of the5D0 ? 7F0,1,2 transitions, indicate the degree of covalence andasymmetry of the Eu3+ ions local environment.

The transition 5D0 ?7F4 (around 697 nm) in the spectrum of

the europium polymeric complex, appears with higher intensityas compared to the one in the [Eu(a-tpc)3(a-Htpc)] complex. Thehigh intensity of the 5D0 ?

7F4 band was discussed in terms ofsymmetry around Eu3+ ion [69] and long-range interactions [72].In the present work the polymeric complex presenting a highintensity 5D0 ?

7F4 transition may be an indication of a decreasein non radiative process due to rigidity of the polymeric chainand the separation distances between europium ions, differentfrom the monomeric [Eu(a-tpc)3(a-Htpc)] complex obtained bythe thermal decomposition.

According to the theory model developed for the 4f-4f transi-tions, the calculus to obtain the intensity parameters Xk (k = 2,4), called Judd–Ofelt, are determined through the transition bandintensities of the 5D0 to the 7FJ (J = 2 and 4) levels. The lumines-cence intensity (I) of the Eu3+ ion transitions is given by Eq. (1)

I0J ¼ �h-0JAoJNo ð1Þ

A0?J is the Einstein spontaneous emission coefficient, No is the pop-ulations of the emitter level, 5D0, and �h- transition energy. By thecalculed ratio between the transitions (5D0 ? 7FJ, J = 2 and 4) andthe magnetic dipole allowed transition 5D0 ?

7F1, the spontaneousemission coefficient is determined [73]:

A0J ¼ A01S0J

S01

� �r01

r0J

� �ð2Þ

S0J and S01 represent the integrated emission curve area of thetransitions 5D0 ?

7FJ (J = 2 and 4), r01 and r0J is the energy(cm�1) referring to the barycenter of the 5D0 ?

7FJ band (J = 2and 4), A01 in Eq. (2) is 3.1 � 10�12�(g)3�(r01)3 and its value is esti-mated to be around 50 s�1 [74]. The experimental intensity param-eters XJ, can be determined by the following equation [75]:

XJ ¼4 � e2 �x3 � v � h7FJ j UðJÞj5D0i � AOJ

3 � �h � c3 ð3Þ

v is the local field Lorentz correction term, which is given byv = n(n2 + 2)2/9, n is the refractive index adopted to complexes(n = 1,5) and h7FJ UðJÞ

��� 5D0

�� i2 are the squares of the reduced elements,(3.2 � 10�3 and 2.3 � 10�3 for J = 2 and 4, respectively) and XJ arethe Judd–Ofelt intensity parameters [76,77].

The X2 parameter can be related to the hypersensibility aroundthe Eu3+ ion and it depends directly of the covalence degree in theligand field and the symmetry around the emitter ion. Higher X2

values are obtained in covalent and asymmetric chemical environ-ments. The X4 parameter is influenced by the ion–ligand and theion–ion bond distances. The smaller emitter ions distance, thegreater the X4 value indicates that there are long-range interac-tions between luminescent ions and these ions are in symmetrya site close to an inversion center [69,72]. Comparing the two X2

values for complexes (Table 3), was observed that the monomeric

Fig. 6. Energy level to the Eu3+, Gd3+ ions and the ligand.

Fig. 7. Excitation spectra obtained at �77 K to polymeric crystal and decomposedcomplexes, kem = 613 and 617 nm, respectively.

Fig. 8. Emission spectra obtained at �77 K to crystal (a) and pulverized (b)polymeric complex with kex = 300 nm and to decomposed complexes (c) withkex = 312 nm.

F. Cagnin et al. / Polyhedron 67 (2014) 65–72 69

complex show higher covalence than the polymeric one. This canbe explained based on the overlap polarizability into the ligandsand the Eu3+ ion in [Eu(a-tpc)3(a-Htpc)] complex [68]. The X4

comparison leads to propose that the polymeric complex exhibitsa symmetry site close to an inversion center and there is a long-range interaction between luminescent ions, in agreement withthe crystal structure.

The intensity of the 5D0 ?7F0 transition allowed by symmetry

can be explained by the J mixing effect. The predominant effectis the mixing in the ligand field Hamiltonian, between the 7F0 stateand the components of the 7F2 state. It is a very small effect due tothe weak interaction between the 4f orbitals with the chemicalenvironment. The R02 parameter, gives information about the Jmixing effect and it is associated with the 5D0 ?

7F0 and the5D0 ?

7F2 transition intensities. The lower the R02 value, the lowestthe interaction between the Eu3+ and the ligand field.

The R02 value is calculated by the relation Ið5D0!7F0ÞIð5D0!7F2Þ

[78]. The lowR02 value of the polymeric and monomeric complexes found is3.49 � 10�3 and 6.74 � 10�3.

The luminescence decay curves of the 5D0 emitting level of the[Eu(a-tpc)3(a-Htpc)2]n and [Eu(a-tpc)3(a-Htpc)] complexes re-corded under kex = 300/312 nm and kem = 613/617 nm, lifetimes

s = 1.1 and 0.85 ms, respectively, show the existence of one Eu cen-ter. The difference between lifetime may indicate that the morecovalent is the system, consequently faster is its lifetime, becausemore allowed is the transition. The emission quantum efficiencies(g) of the 5D0 emitting level in polymeric and monomeric com-plexes at room temperature were obtained based on the lumines-cence data (emission spectrum and emission decay curve of thecompound recorded at �298 K). Eq. (4) provides a means to deter-mine the g value from experimental spectroscopic data [74].

g ¼ Arad

Arad þ Anradð4Þ

where the coefficients Arad represent the radiative contribution ofthe forced electric dipole 5D0 ? 7F2 (A02) and 5D0 ? 7F4 (A04) transi-tions. The magnetic dipole 5D0 ? 7F1 transition (A01) was used asinternal standard owing to its almost insensitive behavior to thechemical environment around the Eu3+ ion. The denominator inEq. (4) is calculated from the lifetime of the emitting level (1/s = Arad + Anrad) and Arad is the sum of A01, A02 and A04. The valuesof radiative (Arad) and non-radiative (Anrad) rates of the emitting5D0 level to the complexes and the value of g are presented inTable 3. The high g and s values is due to absence of water

Fig. 9. 5D0 ?7F0 transitions in crystalline (a), pulverized (b) with kex = 300 nm and in [Eu(a-tpc)3(a-Htpc)] (c) with kex = 312 nm complexes.

Table 3Judd–Ofelt parameters (X2 and X4), intensity ratios R02, lifetimes (s), radiative rates (Arad), non-radiative rates (Anrad), total rates (Atotal) and quantum efficiencies (g) of thecomplexes.

X2(10�20 cm2) X4(10�20 cm2) R02 s (ms) Arad (S�1) Anrad (S�1) Atotal (S�1) g (%)

[Eu(a-tpc)3(a-Htpc)2]n 5.09 7.24 3.49 � 10�3 1.10 310.76 598.33 9.9.09 34.18[Eu(a-tpc)3(a-Htpc)] 11.4 5.43 6.74 � 10�3 0.85 469.76 706.71 1176.47 39.92

70 F. Cagnin et al. / Polyhedron 67 (2014) 65–72

molecules in the first coordination sphere, avoiding the quenchingof luminescence by O–H vibrations. Furthermore the polymericrigidity environment can contribute to these high values.

4. Conclusion

The polymeric complex [Eu(a-tpc)3(a-Htpc)2]n was synthesizedusing a cleaner obtaining method than the literature provides [53].The ligand has the potential to act as an antenna, since the tripletenergy is around 19833 cm�1 and thus can transfer energy to theEu3+ ion (17270 cm�1). The polymeric complex shows a high life-time due to the rigidity in which the emitter ion is inserted andthe absence of water molecules in the first coordination sphere,factors that decrease non-radiative lost through multiphononvibration. The [Eu(a-tpc)3(a-Htpc)] short lifetime is in agreementwith the lager values if X2 Judd–Ofelt parameters. The size de-crease in the crystal samples is related to the superficial emittingsites creation, decreasing the system covalence and resulting in a5D0 ?

7F0 transition displacement to high energy. In the mono-meric complex the 5D0 ? 7F0 transition position red shifted, con-firming the greater system covalence.

Acknowledgments

The authors thank FAPESP and CNPq and for funding this re-search. F.C. thanks FAPESP and CNPq to scholarships. We are verygrateful to Prof. A. M. Simas and Prof. O. L. Malta (CCEN-UFPE) bytransferring the knowledge about and helping with the computa-tional techniques and by transferring the knowledge about the4f–4f intensities theory and by the interpretation of the Judd–Ofeltintensity parameters.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.poly.2013.08.063.

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