Optical emission studies of new europium and terbium dinuclear complexes with trifluoroacetylacetone and bridging bipyrimidine. Fast radiation and high emission quantum yield

Polyhedron 102 (2015) 16–26

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Optical emission studies of new europium and terbium dinuclear complexes with trifluoroacetylacetone and bridging bipyrimidine. Fast radiation and high emission quantum yield Rashid Ilmi, Khalid Iftikhar ⇑ Lanthanide Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi 110 025, India

a r t i c l e

i n f o

Article history: Received 28 April 2015 Accepted 7 July 2015 Available online 26 July 2015 Keywords: Dinuclear Lanthanides 1,1,1-Trifluoro-2, 4-pentanedione Bpm Luminescence

a b s t r a c t New homodinuclear lanthanide complexes of the type [Ln(tfaa)3]2bpm (Ln = La, Eu and Tb; tfaa = 1,1,1trifluoro-2,4-pentanedione) were synthesized by a one pot-one step method and characterized by elemental analysis, FT-IR, thermogarvimetry and 1H NMR spectroscopy. In these complexes the planar 2,20 -bipyrimidine (bpm) ligand affords a tetradentate coordination mode. The crystal structure of [Tb(tfaa)3]2bpm was determined by single-crystal X-ray diffraction. The intramolecular Tb–Tb distance across the bpm bridging ligand is 6.760(1) Å. The dinuclear complexes are thermally stable up to 180 °C, as shown by thermal analysis. The Eu(III) and Tb(III) dinuclear complexes exhibit intense red and green emissions with luminescence lifetimes of 810 and 490 ls, respectively. The quantum yields, Uoverall, for the two complexes [Eu(tfaa)3]2bpm and [Tb(tfaa)3]2bpm are 34% and 48%, respectively. Substitution of a water molecule from the coordination sphere of [Ln(tfaa)3H2O] by bpm and the joining of two [Ln(tfaa)3] units through bpm leads to 10- and 2-fold increases, respectively, in the overall quantum yield for the dinuclear Eu and Tb complexes. This enhanced improvement originates mainly due to the (i) better sensitization efficiency of the ancillary ligand (bpm) and (ii) elimination of non-radiative deactivation pathways through harmonics of O–H vibrations. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The pursuit to design stable luminescent lanthanide complexes with conjugated organic ligands continues to stimulate ever increasing technological applications, such as light emitting diodes and display devices [1], optoelectronic devices [2], lasers [3], biological imaging applications [4], light-emitting sensors for heteroand homogeneous fluoroimmunoassays [5], which are based on the exploitation of their magnetic or optical properties. In the case of trivalent lanthanides, the emission is due to transitions inside the 4f shell, thus intraconfigurational f–f transitions. Because the partially filled 4f shell is well shielded from its environment by the closed 5s2 and 5p6 shells, ligands in the first and second coordination spheres perturb the electronic configurations of the trivalent lanthanide ions (Ln3+) only to a very limited extent. This shielding is responsible for the specific properties of lanthanide luminescence, more particularly for the narrow band emission and the long lifetimes of the excited states. Although photoluminescence of lanthanide ions can be an efficient process, all ⇑ Corresponding author. E-mail address: [email protected] (K. Iftikhar). http://dx.doi.org/10.1016/j.poly.2015.07.046 0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

lanthanide ions suffer from weak light absorption. Because the molar absorption coefficient e of most of the transitions in the absorption spectra of trivalent lanthanide ions is smaller than 10 L mol1 cm1, only a very limited amount of radiation is absorbed by direct excitation in the 4f levels. Since the luminescence intensity is not only proportional to the luminescence quantum yield but also to the amount of light absorbed, weak light absorption results in weak luminescence. However, the problem of weak light absorption can be circumvented by the so-called antenna effect [6] (or sensitization), appearing as a magic potion. The coordination chemistry of the lanthanides is driven by electrostatic interactions between the ligands and generally trivalent lanthanide ions. The lanthanides typically adopt high coordination numbers and the coordination geometries are heavily influenced by steric effects [7]. The use of chelating ligands and hard donor atoms is important for generating stable complexes of the trivalent lanthanide ions. 2,20 -Bipyrimidine (bpm) is a planar and electronically delocalized heterocyclic ligand that has been shown to be a suitable ligand for generating dinuclear complexes by offering the two equivalent NN binding sites for the lanthanide ion [8]. Apart from acting as a neutral subordinate ligand, it can also link two Ln(b-diketone)3 units to form a facile lanthanide(III) dinuclear

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complex. This laboratory has recently become interested in the synthesis, NMR, 4f–4f absorption and luminescent properties of homo as well as hetero-dinuclear lanthanide(III) complexes bridged by 2,20 -bipyrimidine (bpm) using different b-diketones [8a–e]. There are recent reports of structurally characterized dinuclear lanthanide complexes of bpm in which bpm acts as bridging unit to connect two lanthanide b-diketonate units [8f–k,8m]. Other homo- and hetero-dinuclear lanthanide complexes are also known in which ligands other than bpm are used to connect two lanthanide units [8n]. However, there are only few reports on structurally characterized Tb(III) dinuclear complexes of b-diketone bridged by bpm [8i,8m]. Recently, we have reported the synthesis and 4f–4f absorption and luminescence properties of nine-coordinated mononuclear and seven-coordinated dinuclear lanthanide complexes of 1,1, 1-trifluoro-2,4-pentanedione with tptz and pyz (where tptz = 2,4,6-tris(2-pyridyl)-1,3,5-triazine and pyz = pyrazine) [9a,b]. Two most interesting things that require attention are: (i) as far as we know, most of the studies on 1,1,1-trifluoro-2,4pentanedione are devoted to hybrid materials, i.e. incorporated into polymer matrices [10], and report their photoluminescence properties and (ii) the majority of the complexes of this particular 1,1,1-trifluoro-2,4-pentanedione ligand are mononuclear [9b,c,11] and reports on polynuclear or dinuclear complexes are very limited for 1,1,1-trifluoro-2,4-pentanedione. As far as we know there is only one report on dinuclear complexes of the type [Ln(tfaa)3]2pyz [9a] (where Ln = La, Nd, Eu and Tb) by us and a one-dimensional polynuclear species reported by Zhu et al. of the general type [La(tfaa)3(bpyN2O2)]n [12] (where bpyN2O2 = 4, 40 -bipyrdyl-N,N0 -oxide). Therefore, it is very motivating to study the synthesis of dilanthanide complexes of the 1,1,1-trifluoro2,4-pentanedione ligand with the bridging bpm ligand and subsequent to investigate the effect of this bridging ligand on the resulting properties of the complexes. In this paper, we report on a deliberate facile one step-one pot synthesis for complexes of the type [Ln(tfaa)3]2bpm (Ln stands for La, Eu and Tb).

17

10 °C min1 on Exstar 6000 TGA/DTA and DSC 6220 instruments from SII Nano Technology Inc., Japan. The electronic spectra of the complexes were recorded on a Perkin–Elmer Lambda-40 spectrophotometer equipped with a deuterium lamp (ultraviolet region) and a tungsten lamp (visible region), with the samples contained in a 1 cm3 stoppered quartz cell of 1 cm path length, in the range 200–1100 nm. The slit width was 2 nm. Steady state luminescence and excitation spectra were recorded on an Horiba  Jobin Vyon Fluorolog 3–22 spectrofluorimeter with a 450 W xenon lamp as the excitation source and an R-928P Himamatsu photomultiplier tube as the detector and they were detected at an angle of 90° for diluted solution measurements. All the spectra were corrected for instrumental functions. The excitation and emission slit width was 2 nm. In order to determine the peak center maximum, full width at half-maximum (fwhm or peak width) and peak area, Origin Pro 8 was used. The luminescence lifetimes were recorded on a single photon counting spectrometer from Edinburgh Instruments (FLS920), with a microsecond pulse lamp as the excitation source. The data were analyzed by software supplied by Edinburgh Instruments. Relative quantum yields (Uoverall) of the sensitized Eu(III) and Tb(III) emissions of the complexes were measured in chloroform at room temperature and are cited relative to a reference solution of [Eu(hfaa)3phen] (Ur = 46%) [14] and [Tb(hfaa)3phen] (Ur = 32%) [14], with an experimental error of 10%. The relative quantum yields were calculated using Eq. (1) [15]:

US ¼

ðAr Þðg2S ÞIS Ur ðAS Þðg2r ÞIr

ð1Þ

2. Experimental

where r stands for the reference and s for the sample. A is the absorbance at the excitation wavelength, g is the index of refraction of the solvent, and I is the integrated luminescence intensity. The refractive index is assumed to be equivalent to that of the pure solvent (g = 1.45 for chloroform). The concentration of the samples was measured as 6  104 M, to avoid complex dissociation. For the determination of the quantum yield, the excitation wavelength was chosen so that A < 0.05.

2.1. Chemical and general procedures

2.2. X-ray structure determination

The commercially available chemicals that were used without further purification are: Ln2O3 (Ln = La, Eu and Tb; 99.9%) from Aldrich, 1,1,1-trifluoro-2,4-pentanedione (trifluoroacetylacetone) from Lancaster and 2,20 -bipyrimidine (bpm) from Merck. The solvents used in this study were either AR or spectroscopic grade. Oxides were converted to the corresponding chlorides, LnCl3nH2O (n = 6–7), by dissolving the oxides in minimum conc. HCl, then diluting with water and evaporating to near dryness on a water bath. This process of adding water and then evaporating to near dryness was repeated several times until the pH of the solution was between 4 and 6. The chloride solution was finally evaporated to dryness and kept in a desiccator [13]. Infrared spectra were recorded on a Perkin–Elmer spectrum RX 1 FT-IR spectrophotometer as KBr discs operating between 4000 and 400 cm1. Elemental analyses were carried out at the University of Delhi. A Bruker Avance III 500 MHz NMR spectrometer was used to record the 1H NMR spectra of the new compounds in a chloroform-d solution at 300 K, equipped with 5 mm PABBO probe. The chemical shifts are reported in parts per million relative to tetramethylsilane (SiMe4). The melting points of the complexes were recorded by the conventional capillary method as well as on a DSC instrument (6220 Exstar 6000) in aluminum pans at a heating rate of 10 °C min1. Thermal analyses of the complexes were carried out under a dinitrogen atmosphere at a heating rate of

A single crystal suitable for X-ray analysis was obtained by slow evaporation of an ethanolic solution of the [Tb(tfaa)3]2bpm complex. A single crystal X-ray diffraction study of a crystal mounted on a capillary was carried out on a BRUKER AXS SMART APEX diffractometer with a CCD area detector (KR, 0.71073 Å, monochromator: graphite) [16]. Frames were collected at T = 293 K by x, u and 2h-rotation at 10 s per frame with SAINT software [17]. The measured intensities were reduced to F2 and corrected for absorption with SADABS [16]. The structure solution, refinement and data output were carried out with the SHELXTL program [18]. Non-hydrogen atoms were refined anisotropically. C–H hydrogen atoms were placed in geometrically calculated positions using a riding model. Images were created with the Diamond program [19]. 2.3. Synthesis of [Ln(tfaa)3H2O] The synthesis of the [Eu(tfaa)3H2O] and [Tb(tfaa)3H2O] complexes is reported elsewhere [9a]. 2.4. Synthesis of complexes of the type [Ln(tfaa)3]2bpm All the complexes of the type [Ln(tfaa)3]2bpm (where Ln = La, Eu and Tb) have been synthesized by a similar one step-one pot

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Scheme 1. One step-one pot synthetic pathway for the synthesis of dinuclear complexes of the type [Ln(tfaa)3]2bpm (Ln = La, Eu and Tb).

method (Scheme 1). The synthesis of [Eu(tfaa)3]2bpm described here is representative. A solution of Htfaa (0.25 g, 1.62 mmol) in ethanol (5 mL) was added to 0.12 mL (0.027 g, 1.62 mmol) 25% ammonia solution. This mixture, contained in a 50 mL beaker, was covered until the smell of ammonia disappeared. To this NH4tfaa solution were added 5 mL ethanol solutions of both bpm (0.0427 g, 0.27 mmol) and EuCl36H2O (0.1981 g, 0.54 mmol). The reaction mixture was stirred at room temperature for 5 h. A white precipitate of NH4Cl (which did not melt up to 300 °C) appeared during stirring and this was filtered off repeatedly. The filtrate was covered and left for slow solvent evaporation at room temperature. A colourless powder appeared after three days, which was filtered off and washed with CHCl3 several times. The product thus obtained was recrystallized twice from ethanol to get the pure compound and dried under vaccum over P4O10. A similar procedure was employed to synthesize the other complexes. [La(tfaa)3]2bpm: La2C38H30O12F18N4; Colour: white; Yield: 70%; Elemental analysis, Found: C, 33.85; H, 2.47; N, 3.92; Requires: C, 33.70; H, 2.23; N, 4.13%; Mp: 195 °C. [Eu(tfaa)3]2bpm: Eu2C38H30O12F18N4; Colour: pale; Yield: 71%; Elemental analysis, Found: C, 33.85; H, 2.30; N, 4.15; Requires: C, 33.06; H, 2.19; N, 4.06%; Mp: 183 °C. [Tb(tfaa)3]2bpm: Tb2C38H30O12F18N4; Colour: white; Yield: 65%; Elemental analysis, Found: C, 33.15; H, 2.25; N, 4.20; Requires: C, 32.73; H, 2.16; N, 4.02%; Mp: 186 °C. 3. Results and discussion 3.1. Synthesis, characterization and structure The syntheses of stable dinuclear complexes of the type [Ln(tfaa)3]2bpm in high yield are outlined in Scheme 1. The [Ln(tfaa)3]2bpm complexes are synthesized by a one step-one pot

method, where trifluoroacetylacetone, ammonium hydroxide (25% ammonia), 2,20 -bipyrimidine and LnCl3nH2O in a 6:6:1:2 M ratio in ethanol were allowed to react in one pot at ambient temperature. The stoichiometry used was based on the assumption that both the chelating sides of bpm (i.e. both NN sides) would bind to the two Ln(b-diket)3 units and dinuclear complexes of the type [Ln(b-diket)3]2bpm would be formed; this presumption was found to be correct (Scheme 1), as clearly shown by the elemental analyses, 1H NMR spectra and the yields of the complexes. Air and moisture stable eight-coordinated dinuclear complexes of the type [Ln(b-diket)3]2bpm were formed in which each Ln(III) ion is coordinated to six O atoms from the tfaa units and two nitrogen atoms from the bridging bpm unit. The complexes are readily soluble in common organic solvents, except hexane and carbon tetrachloride. The pure products were obtained by repeated crystallization from absolute ethanol and characterized by elemental analyses, IR spectra and thermal analyses. An X-ray-quality crystal was grown through slow evaporation of an ethanolic solution of the Tb complex. The structural analysis of [Tb(tfaa)3]2bpm (Fig. 1) indicates that it crystallizes without solvent in the lattice in the monoclinic space group P21/n. Its elemental analysis and IR data support the structure determined by single-crystal X-ray crystallography. Crystal refinement data are presented in Table 1. The coordination geometry around the Tb(III) ion is best described as a distorted square antiprism (Inset of Fig. 1). The coordination sphere around each Tb(III) ion consists of six O atoms from three tfaa ligands and two N atoms from the bpm ligand (the Tb1–O distances range from 2.308 to 2.344 Å, the Tb1–N1 distance is 2.578(7) Å and the Tb1–N2 distance is 2.603(7) Å). The intramolecular Tb–Tb distance across the bpm bridging ligand is 6.760(1) Å. The average distances (Tb–O)avg and (Tb–N)avg are 2.328 and 2.590 Å, respectively. These values are similar to those reported for [Tb(acac)3]2bpm [8m]; (Tb–O)avg = 2.333, (Tb–N)avg = 2.623 and Tb–Tb = 6.887 Å. These

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Fig. 1. (a) ORTEP view of [Tb(tfaa)3]2bpm with the numbering scheme. Hydrogen atoms are omitted and fluorine (green) and carbon (grey) are not labelled for the sake of clarity. Ellipsoids represent 50% probability. (b) Coordination geometry of the Tb(III) ion. (Color online.)

Table 1 Crystal structure [Tb(tfaa)3]2bpm

3.2. IR and thermal analysis data,

collection

and

Empirical formula Formula weight T (K) k (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g/m3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) h range for data collection Index ranges Reflections collected Independent reflections Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit (GOF) on F2 Final R indices [I > 2r(I)] R indices (all data) Largest difference peak and hole (e Å3)

structure

refinement

parameters

of

Tb2C38H30O12F18N4 1394.52 298(2) 0.71073 monoclinic P21/n 12.653(3) 13.239(3) 16.523(4) 90 111.446(4) 90 2576.3(11) 2 1.717 2.845 1348.0 0.30  0.20  0.10 2.31–22.42 15 6 h 6 15, 15 6 k 6 15, 19 6 l 6 19 23774 4529 0.510 and 0.752 Full-matrix least-squares on F2 4518/0/337 1.289 R1 = 0.0666, wR2 = 0.1406 R1 = 0.0759, wR2 = 0.1451 2.027 and 1.912

w = 1/[r2(F2o) + (0.0617P)2 + 7.1535P], where P = (F2o + 2F2c )/3

average bond lengths are shorter than those reported for the Eu analogue [Eu(tmhd)3]2bpm (where tmhd is the anion of 2,2,6,6tetramethyl-2,4-heptanedione) [8l]. The shorter bond lengths are consistent with the well known decrease in ionic radii across the lanthanide row.

The IR spectra of the [Eu(tfaa)3]2bpm and [Tb(tfaa)3]2bpm complexes (Fig. S1 in the Supplementary information) are similar in shape and exhibit bands at 1617 and 1540 cm1. These have been assigned to C@O and C@C stretching vibrations, respectively, characteristic of lanthanide b-diketonates [9,13]. Two strong absorption bands appear at 1190 and 1136 cm1, which are assigned to the C–F stretching mode of the CF3 group and is an important feature common to both complexes. The IR spectra of the complexes display an asymmetric doublet around 1578 (strong) and 1537 cm1 (weak). The asymmetric doublet in this region is considered as diagnostic of the bis-chelating mode of the bipyrimidine ligand (bridging bipyrimidine) [8a–e]. A strong band appearing at 1403 cm1 in free bipyrimidine has been shifted in the complexes and appears as a medium intensity band at around 1411 ± 1 cm1. The absence of any absorption between 3600 and 3200 cm1 implies that the complexes do not contain any kind of water (coordinated or lattice held). The thermograms of the complexes were recorded under a dinitrogen atmosphere at a heating rate of 10 °C min1 (Fig. 2). The thermograms are similar in shape and show a two step weight loss. The thermal analysis of [Tb(tfaa)3]2bpm discussed here is representative. The complex does not display any weight loss up to 180 °C, indicating that it does not contain any kind of water (coordinated or lattice held). The first inflexion point for the [Tb(tfaa)3]2bpm complex appears in the temperature range 250– 325 °C, with a weight loss of 66.8% which corresponds well with the loss of six tfaa moieties (theoretical weight loss for six tfaa is 66.29%). The next inflexion point for [Tb(tfaa)3]2bpm is observed at 450 °C with a weight loss of 13.8%, representing expulsion of the bpm unit (theoretical weight loss for bpm is 11.34%). The DTA of the complex shows two exothermic peaks at these temperatures. The DTA curve of the complex displays one endothermic peak at the lower temperature (186 °C for [Tb(tfaa)3]2bpm]) representing the melting of the complex. A comparison of the thermal stability of the present [Ln(tfaa)3]2bpm] complexes with the analogous [Ln(acac)3]2bpm [8c] complexes reveals that the tfaa

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80.00

100.0

[Tb(tfaa)3 ]2bpm

90.0

70.00

80.0

66.8%

[Eu(tfaa)3 ]2bpm

DTA uV

50.00

70.0

[Eu(tfaa)3]2bpm

40.00

60.0

TG %

60.00

50.0 30.00 40.0

184.4 Cel 0.89 u V

20.00

.

10.00

.

30.0

13.9%

185.0 Cel 11.47 u V 0.00

50.0

100.0

150.0

200.0

250.0

300.0

20.0

350.0

400.0

450.0

500.0

550.0

600.0

Temp Cel Fig. 2. TGA/DTA of the [Eu(tfaa)3]2bpm (magenta) and [Tb(tfaa)3]2bpm (brown) complexes under a dinitrogen atmosphere. (Color online.)

complexes are thermally less stable than the acetylacetone complexes. The lower thermal stability of the tfaa complexes is due to the presence of highly electronegative fluorine atoms at one terminal resulting in weakening of the Ln–O bonds. 3.3. Nuclear magnetic resonance The NMR spectra of the [La(tfaa)3]2bpm and [Eu(tfaa)3]2bpm complexes were recorded in CDCl3 and are consistent with the dinuclear nature of the complexes (Fig. 3). The NMR spectrum of the La complex displays four signals, two due to the bpm ligand and two due to the tfaa ligands, in an intensity ratio of 2:4:18:6. These signals appear at d 9.32, 7.70, 5.66 and 1.90 ppm for the H2, H-3, –CH and –CH3 protons, respectively. The intensity ratio of the signals substantiate presence of two La(tfaa)3 units connected through one bpm moiety. The spectra of the paramagnetic Eu and Tb complexes have been interpreted as first order spectra. In the case of the Eu complex, the signals of bpm are shifted substantially to the downfield side and appear at d 11.60 and 11.90 ppm for the H-2 and H-3 protons, respectively. The downfield paramagnetic shift is larger [d 4.02 ppm] for H-3 as compared to the H-2 protons [d 2.28 ppm]. The signals at d 1.95 and 3.50 ppm, which integrate to 6 and 18 protons, are assigned to the methine (–CH) and methyl (–CH3) groups of the coordinated tfaa moieties. The methine protons have suffered a high field shift and appear at d 1.95 ppm. It is an important observation that out of the two signals of tfaa, the –CH3 resonance has been shifted downfield while the –CH signal has moved upfield. It is an exception. Examples of this type are very rare, however, a corresponding sign variation has been found for the a- and b-protons in tris(tropolonato)thulium(III) [20] and a high field shift has been observed for the a-protons in [Tm(OP(OC5H11)3]2 [21]. A similar sign reversal has been observed for the H-2 and H-4 protons of bpy in [Ln(bpy)2(SCN)3(H2O)2] (Ln = Er and Yb) [22] and the H-2 and H-4 protons of phen in [Tm(hfaa)3phen] [14]. Since the dipolar shift is limited by the geometry of the complex species, the dependence of the shift on the geometric factor, (3 cos2h  1), could

become important enough to change the sign of the shift. The NMR spectrum of the Tb complex is more interesting and displays huge paramagnetic shifts for the tfaa and bpm moieties (Fig. 4). The shifts noted for the bpm protons are hugely upfield, where the H-2 and H-3 protons resonances are observed at d 193.00 and 84.00 ppm, respectively, to the high field side of TMS, while the methine and methyl signals are moved significantly downfield and appear at d 132.00 and 11.54 ppm, respectively (Fig. 4). The spectrum covers a chemical shift range of about 325 ppm (from 132 to 193 ppm) and consists of four resonances. It is worth mentioning that the paramagnetic shifts of the bpm protons induced by both the metal ions in these complexes are uniformly directed. We have compared the direction and magnitude of the paramagnetic shifts noted for the present dinuclear complexes of Eu and Tb with those of their mononuclear analogues [Eu(tfaa)3(bpy)] and [Tb(tfaa)3(bpy)] (bpy is 2,20 -bipyridyl) [9c]. In the case of [Eu(tfaa)3(bpy)], the H-2 and H-3 proton resonances of bpy appear at d 13.18 and 8.55 ppm, respectively, while the methyl and methine protons resonate at d 2.54 and 1.93 ppm, respectively. Similarly for the Tb(tfaa)3(bpy)] complex, these protons resonate at d 208.6 (H-2), 38.32 (H-3), 124.20 (methine) and 14.25 (methyl) ppm, respectively. The paramagnetic shifts of the mono and dinuclear complexes are comparable which implies that the two Ln sites in the dinuclear complexes are independent of each other and behave as isolated centers and that the interaction between the two paramagnetic Ln centers is negligible for the interpretation of the 1H NMR spectra. It is in agreement with our earlier observation that paramagnetic shifts in Ln(III)–Ln(III) complexes are independent of the presence of two Ln centers due the negligible interaction between them [8d] and the observed paramagnetic shifts in these dinuclear complexes have a magnetic-dipolar nature [8n]. 3.4. Absorption spectra and photoluminescence properties The absorption spectra of the ligands (Htfaa and bpm) and the [Eu(tfaa)3H2O], [Tb(tfaa)3H2O] [Eu(tfaa)3]2bpm and

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Fig. 3. 1H NMR spectra of [Eu(tfaa)3]2bpm and (a) [La(tfaa)3]2bpm in CDCl3. (b) Shows higher resolution of the d 9.5–7.0 ppm region of (a).

Fig. 4. 1H NMR spectrum of [Tb(tfaa)3]2bpm in CDCl3.

[Tb(tfaa)3]2bpm complexes were recorded in CHCl3 solution (Figs. S2 and S3, in the Supplementary information). The absorption spectrum is dominated by spin allowed p–p⁄ transitions of tfaa and bpm in the ultraviolet region (200–400 nm). Bpm shows a strong band at 39,370 cm1 (254 nm), while Htfaa displays a strong absorption band at 38,461 cm1 (260 nm). The absorption bands of the b-diketonate and bipyrimidine ligands are shifted to longer wavelengths in the complexes and show stabilization of the ligand orbitals after complex formation (nephelauxetic effect). The spectral shape of the complexes are similar to that of Htfaa, indicating that the coordination of the Eu(III) and Tb(III) ions does not significantly influence the energy of the singlet state of the bdiketone ligand. The band shape and the oscillator strength of the 4f–4f absorption transitions can be used as a tool to find out the number of lanthanide ions present in a given complex. We are

the first to use this correlation, which is very useful in deciding the number of lanthanide ions in a multinuclear complex.[8a– 7 c,9] The oscillator strength (P) of the 5D2 F0 absorption 1 transition (21,505 cm ; 465 nm) of [Eu(tfaa)3]2bpm (1.45  10– 6 ) is twofold higher than the oscillator strength of this transition for [Eu(tfaa)3H2O] (0.72  10–6). The values given in the parentheses are the oscillator strengths of the transitions of the complexes in chloroform, determined from the spectra and procedure outlined in our previous papers [8a–c]. This is a manifestation of the presence of two europium centers in the dinuclear complex, both of which contribute to the oscillator strength. The photoluminescence spectra of the Eu and Tb complexes were recorded in chloroform at room temperature (300 K) and are shown in Figs. 5 and 6, respectively. The excitation spectrum

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of the [Eu(tfaa)3]2bpm complex was obtained by monitoring the most intense transition (16,368 cm1; 611 nm) and the result is displayed in Fig. 5. The excitation spectrum of the complex contains a broad band that corresponds to the excitation of the organic chromophores (S0 ? S1). The excitation spectrum also features narrow bands that arise from intraconfigurational transitions from the 7F0 ground state to the following levels: 5L6 (25,474 cm1; 392.55 nm), 5D2 (21,545 cm1; 464.14 nm), 5D1 (18,763 cm1; 532.96 nm) and 7F1 ? 5D0 (19,022 cm1; 525.70 nm). The relative intensity of the broad band is much higher than the intraconfigurational transitions, which proves that luminescence sensitization via excitation of the ligand is much more efficient than the direct excitation of the Eu(III) absorption level. The emission spectra of [Eu(tfaa)3]2bpm was obtained by exciting the complex at 372 nm (Fig. 5). The five expected peaks are observed for the 5D0 ? 7F0–4 transitions and are well resolved; these are assigned to 5D0 ? 7F0 (17,297 cm1; 578.03 nm), 5 D0 ? 7F1 (16,906 cm1; 591.69 nm), 5D0 ? 7F2 (16,368 cm1; 610.64 nm), 5D0 ? 7F3 (15,368 cm1; 651.00 nm) and 5D0 ? 7F4 (14,302 cm1; 699.27 nm). The first emission transition, 5 D0 ? 7F0, is in principle forbidden, but when allowed by symmetry, borrows intensity mainly from the 5D0 ? 7F2 transition through the J-mixing effect [23]. The presence of only one peak [full width at half height (fwhm) = 2.1 nm] corresponding to this non-degenerate 5D0 ? 7F0 transition around 17,297 cm1 suggests the existence of one local site symmetry for the dinuclear [Eu(tfaa)3]2bpm complex and only one type of Eu(III) species present in solution. The 5D0 ? 7F2 transition is typical an electricdipole transition and strongly varies with the local symmetry around the Eu(III) ion. The hypersensitive 5D0 ? 7F2 (red) transition is very intense. The high intensity of this transition suggests a low symmetry chemical environment around the Eu(III) ion [24]. Furthermore, the absence of a ligand emission indicates that the ligand triplet state plays an important role in the luminescence sensitization of the Eu(III) ion in the [Eu(tfaa)3]2bpm complex. It

7

6

1.6x10

7

1.40x10

F0

5x106 7

F5

4x10

D4

S1

S0

3x106

7

F6

2x106

[Tb(tfaa)3H2O] 7

F3

1x106

7

F4

0

325 350 375 400 425 450 475 500 525 550 575 600 625

Wavelength (nm) Fig. 6. (a) Excitation and (b) emission spectra of [Tb(tfaa)3H2O] (broken line) and [Tb(tfaa)3]2bpm (solid line) in chloroform at 300 K.

has been well documented for lanthanide ions that the intensity ratio of the electric-dipole to the magnetic-dipole transition measures the symmetry and coordination sphere of the lanthanide complexes. The intensity ratio of the hypersensitive 5D0 ? 7F2 transition and the magnetic dipole 5D0 ? 7F1 transition for the complex under study is 14.74, signifying that this complex has a structure with no imposed symmetry. In fact, complexes with a centrosymmetric coordination sphere have 5D0 ? 7F2/5D0 ? 7F1 intensity ratios lower than 0.7, whereas an intensity ratio higher than 8 is indicative of a low symmetry environment around the Eu(III) ion [25]. The 5D0 ? 7F1 transition of Eu(III) also presents interesting features. It is a parity allowed magnetic-dipole

5

D

2

(c)

6

1.4x10

7

F2

6

Intensity (au)

1.2x10

7

1.05x10

(a)

6

1.0x10

5

8.0x10

5

6.0x10

D

1

D

4.0x10

Intensity (au)

(b)

5

5

5

S0

S1

(b)

5

(a)

6

Intensity (au)

22

1

7

F

0

5

2.0x10

7

F

1

6

7.00x10

5

0.0

L6

450 460 470 480 490 500 510 520 530 540 550

Wavelength (nm)

5

D0

7

F0

6

3.50x10

7

F0 7F1 7

F3

7

F4

0.00 350

400

450

500

550

600

650

700

Wavelength (nm) Fig. 5. (a) Excitation and (b) emission spectra of [Eu(tfaa)3H2O] (broken line) and [Eu(tfaa)3]2bpm (solid line) in chloroform at 300 K. Inset (c) higher resolution of the excitation spectra.

23

R. Ilmi, K. Iftikhar / Polyhedron 102 (2015) 16–26

transition and its moderate intensity is almost independent of the environment (host matrix). The stark splittings gives information about the symmetry around the europium in the complex. This transition of the complex has three stark splittings at 17,008, 16,889 and 16,778 cm1 which represents that the complex has low symmetry. The high intensity ratio together with the stark splittings in the 5D0 ? 7F1 transition reflects that the environment around the Eu(III) ion is substantially asymmetric. The room temperature excitation spectrum of [Tb(tfaa)3]2bpm was obtained by monitoring the most intense 18,356 cm1 (544.78 nm) line of the 5D4 ? 7F5 emission. It contains a broad band that corresponds to the excitation of the organic chromophores (S0 ? S1). The characteristic Tb(III) energy level structure, which is attributable to transitions between the 7F5 and 5L6, 5G6, 5L10 and 5L9 levels, was absent in the spectrum, thus proving that luminescence sensitization proceeds via ligand excitation rather than by direct excitation of the Tb(III) ion absorption levels. The luminescence spectrum of [Tb(tfaa)3]2bpm (Fig. 6) displays transitions emanating from the 5D4 emitting state of Tb(III) to 7Fj (j = 6, 5, 4, 3) manifolds. The spectrum is dominated by the 5D4 ? 7F5 transition located at 18,356 cm1 and is responsible for the green luminescence of this complex. A moderate intensity transition, 5D4 ? 7F6 appearing at 20,460 cm1 (488.75 nm), shows the second strong green luminescence. The transitions 5D4 ? 7F4 (17,186 cm1; 581.86 nm) and 5D4 ? 7F3 (16,176 cm1; 618.19 nm) are comparable in intensity. Both the emission transitions 5D4 ? 7F4 and 5 D4 ? 7F3 are sensitive to the coordination and stark splitting is indicative of an asymmetric environment around the Tb–Tb centers in the complex [23]. It is worth noting that in the present complex there is no apparent residual ligand-based emission in the 350– 450 nm region, thus implying an efficient energy transfer from the ligand excited states to the terbium f-excited states. The emission spectra of the [Eu(tfaa)3H2O] and [Tb(tfaa)3H2O] complexes were also recorded under the same experimental conditions for a comparison with the dinuclear complexes. In fact, the emission spectra of [Eu(tfaa)3H2O] [9b] has been reported by us elsewhere. However, comparative spectra of the mononuclear and dinuclear complexes are shown in Figs. 5 and 6. The excitation bands of the [Eu(tfaa)3]2bpm and [Tb(tfaa)3]2bpm complexes are 26 and 11 nm red shifted as compared to the [Eu(tfaa)3H2O] and [Tb(tfaa)3H2O] complexes, respectively, because of the larger p-conjugation of newly synthesized dinuclear complexes. Thus, less photodecomposition is expected for the excitation of the luminescent compounds by electromagnetic radiation of lower energy. The emission spectra of the [Eu(tfaa)3H2O] and [Tb(tfaa)3H2O] complexes are similar to their respective dinuclear complexes, except for the intensity and stark splitting which is well resolved in the case of the dinuclear complexes. The symmetry of the coordination sphere [23,24] of the lanthanide complexes can be measure by the intensity ratio of the electric-dipole transition to the magnetic-dipole transition and this ratio increases with the increasing number and mass of the ligand coordinated to the Ln(III) ion [25a]. For the hydrated complexes the intensity ratio of the 5D0 ?7F2 and 5D0 ?7F1 transitions of [Eu(tfaa)3H2O] and the 5D4 ? 7F6 and 5D4 ? 7F5 transitions of [Tb(tfaa)3H2O] are 6.07 and 0.35, respectively. The substitution of H2O by bpm in the dinuclear complexes enhances the luminescence intensities and the intensity ratio for [Eu(tfaa)3]2bpm and [Tb(tfaa)3]2bpm are 14.74 and 0.39, respectively, for the same set of transitions. In conclusion, the presence of bipyrimidine enhances the luminescence intensity of the hypersensitive transitions of the Eu(III) and Tb(III) ions. For the hydrated complexes, the surrounding environment is less disturbed and the intensity of the electric dipole transition 5D0 ?7F2 for Eu(III) and 5D4 ? 7F6 for Tb(III) is relatively weak. However, in the case of dinuclear complexes coordination of bpm

exhibit disorder of a certain magnitude. Under the influence of the electric field of the surrounding ligands, the distortion of the symmetry around the lanthanide ion by the bridging bpm results in the polarization of the Eu(III) and Tb(III) ions, which increases the probability for the electric-dipole allowed transition. The influence of bpm on the coordination environment of the Eu(III) and Tb(III) ions changes the energy-transfer probabilities of the electric dipole transitions, accounting for the increases in luminescent intensity of the 610 and 488 nm peaks.

3.4.1. Lifetime (sobs) and quantum yield analyses of the Eu(III) and Tb(III) complexes The 5D0 and 5D4 lifetimes (sobs) of the red and green emissions of the Eu(III) and Tb(III) ions (Figs. S4–S6, in the Supplementary information), respectively, were determined from the mono-exponential fitting of the decay curve and they are consistent with the presence of one major luminescent species. It is in agreement with the results of the Eu(III) emission spectrum where only one peak corresponding to the 5D0 ? 7F0 transition is observed and the NMR spectrum where only one set of signals is observed for the b-diketonate and bpm protons, suggesting that only one species is present in solution and the two Eu(III) sites are equivalent. The luminescence lifetimes and quantum yield values of the dinuclear complexes, together with those of the hydrated complexes, are collected in Tables 2 and 3. The lifetimes of the hydrated complexes [Eu(tfaa)3H2O] and [Tb(tfaa)3H2O] are shorter as compared to those for [Eu(tfaa)3]2bpm and [Tb(tfaa)3]2bpm. The shorter lifetimes of the hydrated complexes are due to the presence of the water molecule in the coordination sphere which effectively quenches the [Eu(tfaa)3H2O] luminescence, while [Tb(tfaa)3H2O] (220 ls) is less quenched as compared to [Tb(tfaa)3]2bpm (490 ls). Relatively efficient coupling of the Eu(III) excited states occurs to the third vibrational overtone of the proximate OH oscillators moH  3,300–3,500 cm1) and to the fourth harmonic in the case of Tb(III), which is consistent with the less efficient quenching observed for Tb(III) where the Franck–Condon overlap factor is less favorable [26]. The luminescence quantum yield, Uoverall, of the [Eu(tfaa)3]2bpm and [Tb(tfaa)3]2bpm complexes in CHCl3 solution is 34% and 48%. The quantum yield of the present Eu complex is

Table 2 Radiative (ARAD) and non-radiative (ANR) decay rates, 5D0 lifetime (sobs), intrinsic quantum yield (UEu), energy transfer efficiency (gsen) and overall quantum yield (Uoverall) for the complexes [Eu(tfaa)3H2O] and [Eu(tfaa)3]2bpm in chloroform at room temperature. [Eu(tfaa)3H2O]

[Eu(tfaa)3]2bpm

5

D0 ? 7F0 (nm; cm1) 5 D0 ? 7F1 (nm; cm1)

578.13; 17,297 590.76; 16,927

5

611.56; 16,351 650.11; 15,382 695.67; 14,374 6.50 9.01 330 1.06 631 3576 3 20.83 15.36

578.13; 17,297 587.95; 17,008 592.10; 16,889 596.01; 16,778 610.94;16,368 650.70; 15,368 699.20; 14,302 14.74 4.27 810 1.15 448 797 34 35 97

D0 ? 7F2 (nm; cm1) D0 ? 7F3 (nm; cm1) 5 D0 ? 7F4 (nm; cm1) Intensity ratioa Fwhm (nm)b sobs (ls)c 5

v2 ARAD (s1) ANR (s1) Uov erall (%)d UEu (%)e gsen (%)f a b c d e f

Ratio of the area under the peaks of 5D0 ? 7F2 and 5D0 ? 7F1. Full width at half height (fwhm) of 5D0 ? 7F2 (error = ± 5). sobs = error = ± 2. Overall quantum yield calculated using eq 1 (error = ± 10). Intrinsic quantum yield calculated by using Eq. (2). gsen = Uoverall/UEu.

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R. Ilmi, K. Iftikhar / Polyhedron 102 (2015) 16–26

Table 3 D4 lifetime (sobs), intrinsic quantum yield (UTb) and overall quantum yield (Uoverall) for the complexes [Tb(tfaa)3H2O] and [Tb(tfaa)3]2bpm in chloroform at room temperature. 5

5

D4 ? 7F6 (nm; cm1) 5 D4 ? 7F5 (nm; cm1) 5 D4 ? 7F4 (nm; cm1) 5 D4 ? 7F3 (nm; cm1) Intensity ratioa Fwhm (nm)b sobs (ls) Uov erall (%)c UTb ð%Þd a b c d

[Tb(tfaa)3H2O]

[Tb(tfaa)3]2bpm

(488.73; (544.18; (582.24; (617.74; 0.35 9.38 220 21 7.3

488.75; 545.78; 581.86; 618.19; 0.39 9.70 490 48 16.33

20,461) 18,376) 17,175) 16,188)

20,460 18,356 17,186 16,176

Ratio of the area under the peaks of 5D4 ? 7F6 and 5D4 ? 7F5. Full width at half height (fwhm) of 5D4 ? 7F5. overall quantum yield (Uoverall) calculated using the Eq. (1). Intrinsic quantum yield (UTb) calculated by using the Eq. (2).

[Eu(tdh)3]2bpm (342 ls) [8l] and [Tb(tdh)3]2bpm (39.00 ls) [8l]. However, it is noteworthy that the triplet energy level of tdh [32] is 22,000 cm1, which is similar in magnitude to the triplet energy level of tfaa. Therefore, the enhanced lifetimes of the present dinuclear complexes over the tdh complexes is due to (i) structural differences between the two diketonates ligands (in the case of tfaa one terminal is occupied by a –CH3 group and in the case of tdh one terminal is occupied by a t-butyl group, while the other terminal is a –CF3 group in both cases), (ii) the solvent used; in our case we used chloroform as the solvent [Gutmann Donor number (G. N.) = 4] [33] while in the case of the tdh complex it was methanol [Gutmann Donor number (G. N.) = 19] [33]. 3.4.3. Intrinsic quantum yield (ULn) of the Eu(III) and Tb(III) complexes The intrinsic quantum yield (ULn) can be calculated from the ratio of the observed emission and radiative lifetimes, which in turn is easily calculated from Eq. (2):

ULn ¼ higher than the closely related dinuclear complex [Eu(HTH)3]2bpm (Uoverall = 28.40%) [8k] and the well-known [Eu(tta)3phen] complex (Uoverall = 30.00%) [27] (where HTH and tta are the anions of 4,4,5,5,6,6,6-heptafluroro-1-(2-thienyl)hexane-1,3-dione and thenoyltrifluoroacetone). It is very interesting to mention that the quantum yield of [Tb(tfaa)3]2bpm (Uoverall = 48%) is significantly higher than those reported for the Tb(III)-hexafluoroacetylacetone and dipivaloylmethanato complexes (Uoverall = 27 and 40%, respectively) [28] despite the fact that the magnitude of the 5 D4 lifetime of the [Tb(tfaa)3]2bpm complex is not very high as compared to some of the recently reported highly luminescent complexes of Tb(III) (Uoverall = 56%; sobs = 2.63 ms) [29]. However, there are reports of Uoverall = 40% and sobs = 0.46 ms for Tb(III)dipivaloylmethanato complexes [28]. It is not surprising that a highly luminescent complex has a short lifetime since the lifetime is the inverse of the total de-activation rate, and is the sum of the radiative and non-radiative rates. It simply means that the radiative rate is fast, as observed in the present [Tb(tfaa)3]2bpm complex. Due to its electronic structure, Tb(III) has many levels which can mix with the ligand wave functions, including a relatively low-lying 4f 5d state, which may explain why the lifetime is relatively short. It is a well established fact that the luminescence efficiency of lanthanide complexes depends upon an effective match between the emitting level of the Ln(III) ion and the ligand centered triplet state [30]. In the present dinuclear complexes there are two types of ligands attached to the Ln(III) ion which have different triplet states, (i) tfaa 22,720 cm1 [10b] and (ii) bpm 24,100 cm1 [8f]. The triplet energy level of the tfaa ligand is found to be energetically compatible for an efficient energy transfer process between both the 5D0 17,280 cm1 and 5 D1 18,880 cm1 levels, while the energy difference between the emitting 5D4 (20,500 cm1) level of Tb(III) and the triplet states of tfaa and bpm are approximately 2220 cm1 and 3600 cm1, respectively. It has been reported that an optimal ligand-to-metal energy transfer process for Tb(III) needs DE (Tr–5D4) = 2400 ± 300 cm1 [31]. Therefore, these investigations suggests that tfaa together with the bridging bpm efficiently sensitize the Eu(III) and Tb(III) luminescence. 3.4.2. Comparison of the luminescence lifetime with similar Eu(III) and Tb(III) complexes The lifetimes (sobs) of the present dinuclear complexes of europium ([Eu(tfaa)3]2bpm) and terbium ([Tb(tfaa)3]2bpm) in chloroform were compared with the similar [Eu(tdh)3]2bpm and [Tb(tdh)3]2bpm complexes (where tdh is the anion of 1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedione). The lifetimes of the present dinuclear complexes are higher than those reported for

ARAD sobs ¼ ARAD þ ANR srad

ð2Þ

where srad in the case of Tb(III) is 3000 ls [34]. The srad value for the Eu(III) complex could be very easily calculated using Eq. (3) [35]. Due to the presence of the purely magnetic-dipole allowed 5 D0 ? 7F1 transition, whose oscillator strength is virtually independent of the ligand field and symmetry of the complex,

ARAD ¼

1

srad

¼ AMD;0  n3 ðItot =IMD Þ

ð3Þ

where AMD,0 (14.65 s1) is the spontaneous emission probability of the 5D0 ? 7F1 transition in vaccuo, Itot/IMD is the ratio of the total area of the corrected Eu(III) emission spectrum to the area of the 5 D0 ? 7F1 band, and n is the refractive index of the medium [34]. The intrinsic quantum yield (ULn) values for the Eu(III) and Tb(III) complexes are listed in Tables 2 and 3. According to energy gap theory, radiationless transitions are prompted by ligands and solvents with high frequency vibrational modes. The synthesis of Eu(III) complexes with higher quantum yields is directly linked to the suppression of radiationless transitions caused by vibrational excitations in the surrounding media [36]. The lower quantum yield of the [Eu(tfaa)3H2O] complex is due to the presence of an O–H oscillator in the system, which effectively quenches the luminescence of the Eu(III) ion. On the other hand, the [Eu(tfaa)3]2bpm complex exhibits a higher quantum yield and a longer lifetime due to the substitution of H2O by bpm in the coordination sphere, resulting in the decrease of the level of non-radiative multiphonon relaxation by coupling to O–H vibrations and a non-radiative transition ratefrom 3576 s1 to 797 s1 (Table 2). A comparison of the non-radiative transition rate (ANR) of the present [Eu(tfaa)3]2bpm complex with a reported trinuclear complex of the type [Eu3(dbm)3(phen)6L] [37] (dbm = dibenzoylmethane, L = 1,3,5tris(2,2-dibenzoylethyl)benzene) shows a lower value of ANR  797 s1 as compared to ANR  2073.38 s1 [37]. The lower value of ANR eventually increases the luminescence lifetime of the present dinuclear complex. The substantial contribution of the ancillary ligand to the overall sensitization of the Eu(III)-centered luminescence in [Eu(tfaa)3]2bpm is confirmed by (i) an increase in the intrinsic quantum yield from 20% in [Eu(tfaa)3H2O] to 35% in [Eu(tfaa)3]2bpm and (ii) the significant enhancement of gsen from 15 to 97%. The intrinsic quantum yield (ULn) of the present [Eu(tfaa)3]2bpm (35%) complex is higher than those of [Eu3(dbm)3(phen)6L] (27.95%) [37], [Eu(tdh)3]2bpm (11.4%) [8l], [Eu(tdh)3bpy] (9.6%) [8l], [Eu(dbm)3phen] (10%) and [Eu(dbm-carb)3phen] (18%) [38] [dbm-carb = 1-{[6-(9H-carbazol9-yl)hexoxy]phenyl}-3-{[6 (9H-carbazol-9-yl)-hexoxy]phenyl}propane-1,3-dione)]. Similarly the intrinsic quantum yield of the

R. Ilmi, K. Iftikhar / Polyhedron 102 (2015) 16–26

of the [Tb(tfaa)3]2bpm complex (16.33%) is higher than those of [Tb(tdh)3]2bpm (1.3%) [8l], [Tb(tdh)3bpy] (2.0%) [8l] and [Tb(hfaa)3]bpm (4.6%) [39]. 4. Conclusion In conclusion, red and green luminescent dinuclear complexes of Eu(III) and Tb(III) with tfaa ligands and bridged by bpm have been synthesized by a facile one-pot synthetic path in good yield. Discrete dinuclear tfaa complexes bridged by bpm have not been reported previously. The X-ray crystal structure of [Tb(tfaa)3]2bpm reveals a distorted square antiprismatic arrangement around the Tb(III) atom. The sensitization mechanism for the luminesencent Eu(III) and Tb(III) complexes involves an usual triplet pathway, in which the ligands absorb energy and which is transferred to the Eu(III) and Tb(III) ions via the ligand-centered triplet excited state. The characteristic emission spectrum of the [Eu(tfaa)3]2bpm complex shows a very high intensity for the hypersensitive 5D0 ? 7F2 transition, pointing to a highly polarizable chemical environment around the Eu(III) ion. The intensity ratio of the electric- dipole transition to magnetic-dipole transition together with the stark splittings in the 5D0 ? 7F1 emission transition of Eu suggest that the europium ion is in a position of low symmetry. The 5D0 and 5D4 lifetimes (sobs) of the red and green emissions of the Eu(III) and Tb(III) ions were found to be 810 and 490 ls, respectively and the intrinsic quantum yields (ULn) were 36.23 and 16.33%, respectively. It could be inferred that the process of energy transfer from the organic moieties (tfaa and bpm) is more efficient in the case of the Eu(III) ion. The high thermal stability, longer luminescence lifetime and highly monochromatic clear emission of the 5D0 ? 7F2 transition of the dinuclear Eu(III)-1,1, 1-trifluoro-2,4-pentanedione complex involving bridging bipyrimidine, with a full width at half height (fwhm) of 4.27 nm, may find potential application in emitting materials in organic light-emitting diodes. Most importantly, the bpm ligand could serve as an effective ancillary ligand in conjunction with 1,1,1-trifluoro-2, 4-pentanedione to afford high luminescence performance in Eu(III) and Tb(III)-tris-b-diketonate complexes, which could result in outstanding candidates for the design of red- and green-emitting electroluminescent materials. Acknowledgments Part of this research is supported by the UGC Special Assistance Program (DRS-II) of the Department of Chemistry, Jamia Millia Islamia (No.F.540/8/DRS/2013/SAP-I) which is gratefully acknowledged. The authors are thankful to Dr. N. Sood (for time resolved emission data) and A. Kumar (for 1H NMR spectra of the complexes) of AIRF, JNU, New Delhi. Appendix A. Supplementary data CCDC 866669 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2015.07.046. References [1] H. Yersin, Highly Efficient OLEDs with Phosphorescent Materials, Wiley, 2008. [2] T. Jüstel, H. Nikol, C. Ronda, Angew. Chem., Int. Ed. 37 (1998) 3084.

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[3] G. Blasse, B.C. Grabmaier, Luminescent materials, Springer-Verlag, 1994. [4] T.R. Krishna, M. Parent, M.H.V. Werts, L. Moreaux, S. Gmouh, S. Charpak, A.-M. Caminade, J.-P. Majoral, M. Blanchard-Desce, Angew. Chem., Int. Ed. 45 (2006) 4645. [5] D. Geibler, N. Hildebrandt, Curr. Inorg. Chem. 1 (2011) 17. [6] (a) J.C.G. Bunzli, S.V. Eliseeva, Chem. Sci. 4 (2013) 1939; (b) J.-C.G. Bunzli, S.V. Eliseeva, Springer Ser. Fluoresc. 7 (2011) 1; (c) E.G. Moore, A.P.S. Samuel, K.N. Raymond, Acc. Chem. Res. 42 (2009) 542. [7] L.C. Thompson, Chapter 25 Complexes, in: Karl A. Gschneidner Jr, E. LeRoy (Eds.), Handbook on the Physics and Chemistry of Rare Earths, Elsevier, 1979, p. 209. [8] (a) M. Irfanullah, K. Iftikhar, Inorg. Chem. Commun. 12 (2009) 296; (b) M. Irfanullah, K. Iftikhar, Inorg. Chem. Commun. 13 (2010) 694; (c) R. Ilmi, K. Iftikhar, Inorg. Chem. Commun. 13 (2010) 1552; (d) M. Irfanullah, K. Iftikhar, J. Photochem. Photobiol., A 224 (2011) 91; (e) M. Irfanullah, K. Iftikhar, Inorg. Chim. Acta 394 (2013) 373; (f) G. Zucchi, O. Maury, P. Thuéry, M. Ephritikhine, Inorg. Chem. 47 (2008) 10398; (g) M.H. Baker, J.D. Dorweiler, A.N. Ley, R.D. Pike, S.M. Berry, Polyhedron 28 (2009) 188; J.A. Fernandes, R.A. Sá Ferreira, M. Pillinger, L.D. Carlos, J. Jepsen, A. Hazell, P. Ribeiro-Claro, I.S. Gonçalves, J. Lumin. 113 (2005) 50; (i) G. Zucchi, X.F. Le Goff, Inorg. Chim. Acta 380 (2012) 354; (j) K.H.-Y. Li, J. Wu, W. Huang, Y.-H. Zhou, H.-R. Li, Y.-X. Zheng, J.-L. Zuo, J. Photochem. Photobiol., A 208 (2009) 110; (k) G. Richards, J. Osterwyk, J. Flikkema, K. Cobb, M. Sullivan, S. Swavey, Inorg. Chem. Commun. 11 (2008) 1385; (l) G. Zucchi, X.F. Le Goff, Polyhedron 52 (2013) 1262; (m) G. Zucchi, T. Jeon, D. Tondelier, D. Aldakov, P. Thuery, M. Ephritikhine, B. Geffroy, J. Mater. Chem. 20 (2010) 2114; (n) N. Ishikawa, T. Lino, Y. Kaizu, J. Phys. Chem. A 107 (2003) 7879. [9] (a) R. Ilmi, K. Iftikhar, Inorg. Chem. Commun. 20 (2012) 7; (b) R. Ilmi, K. Iftikhar, J. Coord. Chem. 65 (2012) 403; (c) R. Ilmi, K. Iftikhar, Unpublished results. [10] (a) Y. Zheng, Y. Liang, H. Zhang, Q. Lin, C. Gou, S. Wang, Mater. Lett. 53 (2002) 52; (b) Y. Zheng, J. Lin, Y. Liang, Y. Yu, Y. Zhou, C. Guo, S. Wang, H. Zhang, J. Alloys Compd. 336 (2002) 114; (c) E. Stathatos, L. Panayiotidou, P. Lianos, A.D. Keramidas, Thin Solid Films 496 (2006) 489; (d) S. Katagiri, Y. Tsukahara, Y. Hasegawa, Y. Wada, Bull. Chem. Soc. Jpn. 80 (2007) 1492; (e) Y.-J. Gu, B. Yan, Y.-Y. Li, J. Solid State Chem. 190 (2012) 36; (f) Y.-Y. Li, B. Yan, L. Guo, J. Fluoresc. 22 (2012) 729; (g) Q.-P. Li, B. Yan, Dalton Trans. 41 (2012) 8567; (h) X.-L. Mei, Y. Ma, L.-C. Li, D.-Z. Liao, Dalton Trans. 41 (2012) 505; (i) Y. Zheng, J. Lin, Y. Liang, Q. Lin, Y. Yu, Q. Meng, Y. Zhou, S. Wang, H. Wang, H. Zhang, J. Mater. Chem. 11 (2001) 2615; (j) J.-H. Olivier, F. Camerel, R. Ziessel, Chem - Eur. J. 17 (2011) 9113; (k) J. Guo, L. Fu, H. Li, Y. Zheng, Q. Meng, S. Wang, F. Liu, J. Wang, H. Zhang, Mater. Lett. 57 (2003) 3899. [11] M. Sayeed, N. Ahmad, J. Inorg. Nucl. Chem. 43 (1981) 3197. [12] W.X. Zhu, Y. He, J. Coord. Chem. 55 (2002) 251. [13] Z. Ahmed, W.A. Dar, K. Iftikhar, J. Coord. Chem. 65 (2012) 3932. [14] Z. Ahmed, K. Iftikhar, Inorg. Chim. Acta 392 (2012) 165. [15] W.H. Melhuish, J. Phys. Chem. 65 (1961) 229. [16] Bruker Analytical X-ray Systems, SMART: Bruker Molecular Analysis Research Tool, 2000. [17] Bruker Analytical X-ray Systems, SAINT-NT, 2001. [18] Bruker Analytical X-ray Systems, SHELXTL-NT, 2000. [19] B. Klaus, DIAMOND, 1999. [20] E.L. Muetterties, C.M. Wright, J. Am. Chem. Soc. 87 (1965) 4706. [21] T.H. Siddall Iii, W.E. Stewart, D.G. Karraker, Inorg. Nucl. Chem. Letts. 3 (1967) 479. [22] A.A. Khan, K. Iftikhar, Indian J. Chem. 39A (2000) 1286. [23] F.S. Richardson, Chem. Rev. 82 (1982) 541. [24] J. Kai, D.F. Parra, H.F. Brito, J. Mater. Chem. 18 (2008) 4549. [25] (a) A.F. Kirby, D. Foster, F.S. Richardson, Chem. Phys. Lett. 95 (1983) 507; (b) A.F. Kirby, F.S. Richardson, J. Phys. Chem. 87 (1983) 2557. [26] A. Døssing, Eur. J. Inorg. Chem. 2005 (2005) 1425. [27] C. Adachi, M.A. Baldo, S.R. Forrest, J. Appl. Phys. 87 (2000) 8049. [28] S.V. Eliseeva, O.V. Kotova, F. Gumy, S.N. Semenov, V.G. Kessler, L.S. Lepnev, J.C.G. Bunzli, N.P. Kuzmina, J. Phys. Chem. A 112 (2008) 3614. [29] A.P.S. Samuel, E.G. Moore, M. Melchior, J. Xu, K.N. Raymond, Inorg. Chem. 47 (2008) 7535. [30] M. Latva, H. Takalo, V.-M. Mukkala, C. Matachescu, J.C. Rodríguez-Ubis, J. Kankare, J. Lumin. 75 (1997) 149. [31] S. Sato, M. Wada, Bull. Chem. Soc. Jpn. 43 (1970) 1955. [32] S.B. Meshkova, A.V. Kiriak, Z.M. Topilova, A.M. Andrianov, Russ. J. Inorg. Chem. 52 (2007) 556. [33] M. Montalti, A. Credi, L. Prodi, T. Gandolfi, Handbook of Photochemistry, Third ed., Taylor & Francis, 2006. [34] M.H.V. Werts, R.T.F. Jukes, J.W. Verhoeven, PCCP 4 (2002) 1542.

26

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[35] S. Viswanathan, A. de Bettencourt-Dias, Inorg. Chem. 45 (2006) 10138. [36] (a) C. Peng, H. Zhang, J. Yu, Q. Meng, L. Fu, H. Li, L. Sun, X. Guo, J. Phys. Chem. B 109 (2005) 15278; (b) Y. Hasegawa, T. Ohkubo, K. Sogabe, Y. Kawamura, Y. Wada, N. Nakashima, S. Yanagida, Angew. Chem., Int. Ed. 39 (2000) 357.

[37] C. Yang, J. Luo, J. Ma, M. Lu, L. Liang, B. Tong, Dyes Pigm. 92 (2012) 696. [38] Y. Zheng, F. Cardinali, N. Armaroli, G. Accorsi, Eur. J. Inorg. Chem. 2008 (2008) 2075. [39] A. Fratini, G. Richards, E. Larder, S. Swavey, Inorg. Chem. 47 (2008) 1030.