Luminescence and positron spectroscopies studies of tris(2,2,6,6-tetramethyl-3,5-heptanedionate) europium(III) and terbium(III) complexes containing 2-pyrrolidone as coligand

Author’s Accepted Manuscript Luminescence and positron spectroscopies studies o f tris(2,2,6,6-tetramethyl-3,5-heptanedionate) Europium(III) and Terbium(III) complexes containing 2-pyrrolidone as coligand Alex S. Borges, Fernando Fulgêncio, Jeferson G. Da Silva, Tatiana A. Ribeiro-Santos, Renata Diniz, Dario Windmöller, Welington F. Magalhães, Maria Helena Araujo

PII: DOI: Reference:

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S0022-2313(18)30399-5 https://doi.org/10.1016/j.jlumin.2018.07.031 LUMIN15778

To appear in: Journal of Luminescence Received date: 4 March 2018 Revised date: 19 July 2018 Accepted date: 20 July 2018 Cite this article as: Alex S. Borges, Fernando Fulgêncio, Jeferson G. Da Silva, Tatiana A. Ribeiro-Santos, Renata Diniz, Dario Windmöller, Welington F. Magalhães and Maria Helena Araujo, Luminescence and positron spectroscopies studies of tris(2,2,6,6-tetramethyl-3,5-heptanedionate) Europium(III) and Terbium(III) complexes containing 2-pyrrolidone as coligand, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.07.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Luminescence and positron spectroscopies studies of tris(2,2,6,6-tetramethyl-3,5-heptanedionate) Europium(III) and Terbium(III) complexes containing 2-pyrrolidone as coligand Alex S. Borges1,*, Fernando Fulgêncio2, Jeferson G. Da Silva3, Tatiana A. Ribeiro-Santos2, Renata Diniz2, Dario Windmöller2, Welington F. Magalhães2,*, Maria Helena Araujo2,* 1

Coordenadoria de Química e Biologia, IFES, Campus Vitória-ES, 29040-780, Brazil. Departamento de Química, UFMG, Belo Horizonte-MG, 31270-901, Brazil. 3 Departamento de Farmácia, UFJF, Campus Governador Valadares-MG, 35010-177, Brazil. 2

*

Corresponding authors: Tel.: 55-27-3331-2198, 55-31-3409-7557 e-mail addresses: [email protected]; [email protected] and [email protected]

Abstract In this work is reported the synthesis, characterization, luminescent properties and positronium formation yields of europium(III) and terbium(III) complexes of stoichiometric formula Ln(dpm)3(2-pyr), where: Ln = Eu and Tb, dpm = 2,2,6,6tetramethyl-3,5-heptanedionato (dipivaloylmethanate) ion, a -diketonate ligand, and 2-pyr = 2-pyrrodilidone, a γ-lactam. The crystal structures were determined by singlecrystal X-ray diffraction. The complexes crystallize in the space group Pī with one complex in the asymmetric unit. The Eu(III) and Tb(III) ions are seven-coordinated by six O atoms of three -diketonate ligands, and one O atom of γ-lactam molecule. We present and discuss experimental intensity parameters of 4f-4f transitions in the Eu(III) complexes under UV excitation. The photoluminescent properties of the complexes depend on the energy positions of the ligand-to-metal charge transfer (LMCT) states. The temperature dependence of the Eu(III) 5D0 relaxation rate of the Eu(III) complexes are presented. Positronium formation in Ln(dpm)3 and Ln(dpm)3(2-pyr) (Ln = Eu and Tb) complexes were investigated. A correlation between the parameters of luminescence and positron annihilation spectroscopies has been observed. The results, which strongly evidence the participation of molecular excited states in the positronium formation, were then discussed in terms of the recently proposed Ps formation mechanism, named cybotactic correlated system kinetic mechanism (CCSKM).

Keywords: Eu(III) and Tb(III) complexes; -diketonate; Lactam; Luminescence properties; Positronium formation.

1

1. INTRODUCTION There is a great interest in the study of the luminescence properties of trivalent lanthanide complexes with organic ligands. Special interest lies in the possibility of designing light-conversion molecular devices (LCMD) based on such complexes, and their applications as optical signal amplifiers, electroluminescent devices and luminescent probes in biological systems, etc [1–4]. To obtain an efficient LCMD it is necessary to optimize the luminescence process. An efficient strategy to control the luminescence intensity of lanthanides complexes is to synthesize mixed-ligand complexes containing, along with β-diketonate anions, organic ligands which can act as fluorophores. The main role of the organic ligands is to collect the photons provided by the light source in order to allow an efficient energy transfer to the emitting levels of the Ln(III) ion. It is generally accepted that the energy transfer from ligand to Ln(III) ion in complexes occurs by on three steps: i) strong absorption from the ground state to the excited singlet state (S0→S1) of the ligand; ii) singlet state decays nonradiatively to the triplet state (S1→T1) through intersystem crossing; and iii) non-radiative energy transfer pathway from the T1 state of the ligand to excited

2S+1

LJ states of the

Ln(III) ion [5]. However, competitive nonradiative pathways such as multiphonon relaxation assigned to high-energy vibrations (e.g., C–H and O–H stretching modes) contribute to luminescence quenching of the Ln(III) complexes [6]. It is noteworthy that the luminescence quenching through multiphonon relaxation is more efficient when water and alcohol molecules act as ligands. This optical behavior may be significantly minimized by replacing these types of luminescence self-quenching molecules by secondary ligands [5]. Another significant luminescence self-quenching channel for Ln(III) complexes is through low energy ligand-to-metal charge-transfer (LMCT) states, present in many Eu(III) complexes. Low energy LMCT states occur when the metallic ion present high electronic affinity and the ligand present low ionization potential. The Eu(III) ion have a considerably higher electronic affinity among all Ln(III), so the low emission quantum yield of many Eu(III) complexes is due to a luminescence self-quenching through LMCT states. Thus, a great deal of attention has been devoted to Eu(III) complexes because their typical strong red luminescence may be significantly affected by the presence of low energy LMCT states [7–11]. According to Faustino et al [10,12], luminescence selfquenching via LMCT states in Eu(III) complexes is more efficient when these states have 2

close or lower energy than the T1 ligand state. Among the Eu(III) complexes that present LMCT at lower energy, those containing the 2,2,6,6-tetramethyl-3,5-heptanedione (dpm) ligand have received considerable attention from both experimental and theoretical points of view, since it forms anhydrous Ln(III) complexes due to the bulky tert-butyl groups present in this β–diketone ligand [10,13,14]. It was observed that, while the Eu(dpm)3 complex present LMCT states in the visible region, some Eu(dpm)3L complexes, where L = monodentate [15] or bidentate heteroaromatic [16] ligands, present LMCT states at higher energies (UV region), leading to more efficient luminescence [10,12–14]. The interaction of positrons with material medium has been used as an important probe to access microscopic information [17–19]. The bound state between a positron (e+) and an electron (e-), called positronium (Ps), can be formed with two different spin states: para-positronium (p-Ps, singlet state) and ortho-positronium (o-Ps, triplet state), with intrinsic vacuum lifetimes of 0.125 and 142 ns, respectively. The long o-Ps lifetime decreases to a few nanoseconds in a material medium due to pick-off annihilation of the positron by surrounding electrons. The positron chemistry for Ps formation in solid coordination compounds has been studied since the 1980s in the Laboratory of Positron Annihilation Spectroscopy (LEAP) at the Federal University of Minas Gerais – UFMG [20–31]. These studies have evidenced the importance of the chemical properties of both ligand and central metal ion in Ps formation [20–29]. Regarding lanthanide complexes, Ps formation has been observed in all Ln(III) complexes, except in many of those containing Eu(III) ions [23,25-31]. Ps formation in Eu(III) complexes was first observed by Faustino et al [28] in 2006, where a correlation between o-Ps formation intensities (Io-Ps) and luminescence quantum efficiency (η) was first detected. According to this work, highly luminescent Eu(III) complexes, which present high η and high energy LMCT states (UV region), show relatively high Io-Ps (Io-Ps between 8% and 18%), while complexes with low η, i.e. with low energy LMCT states (visible region), practically do not form Ps (Io-Ps < 5%) [28]. Following this line of research, our group studied other Ln(III) complexes by positron annihilation lifetime spectroscopy (PALS) and optical spectroscopies, where further correlations between the energy of the LMCT states, luminescence intensity and Io-Ps were observed [23,29-31]. Since luminescence is a phenomenon related to electronic excited states, a kinetic mechanism was proposed involving the participation of the 3

ligand’s excited states in the Ps formation, called correlated cybotactic system kinetic mechanism (CCSKM). We present herein the synthesis and characterization of a new Eu(III) and Tb(III) complexes containing dpm as main ligand, and 2-pyr as secondary ligand. The structures of the complexes were determined by single crystal X-ray diffraction and the photoluminescent properties of the Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr) complexes and its precursors Eu(dpm)3 and Tb(dpm)3 were compared and discussed. Furthermore, PALS and Doppler broadening annihilation radiation lineshape (DBARL) measurements were performed, where a correlation between positron annihilation parameters and the photoluminescence of the complexes was observed. The results, which evidenced that the Ps formation is kinetically controlled and involves participation of molecular excited states, were discussed in terms of the recently proposed kinetic Ps formation mechanism, CCSKM [23,30,31].

2. Experimental 2.1. Materials and measurements Europium(III) and terbium(III) oxides (Eu2O3, Tb4O7), 2,2,6,6-tetramethyl-3,5heptanedione (dpm) and 2-pyrrolidone (2-pyr) were purchased from Aldrich Co. and used as received. The oxides were converted to their chloride by treatment with concentrated hydrochloric acid. The pH of the mixture was raised to 6 by repeated evaporation and addition of water, yielding an aqueous solution of EuCl3.6H2O. The Ln(dpm)3 (Ln = Eu, Tb) complexes were then synthesized and purified as described in the literature [32]. The chemical characterization was made by elemental analysis, infrared spectroscopy and by single crystal X-ray diffraction. Microanalyses were performed on a PerkinElmer 2400 series 2 Elemental Analysis Instrument. The lanthanides concentrations were determined by EDTA titration using xylenol orange as indicator. All samples were obtained as powder and were measured without further preparation. The vibrational analysis was obtained in an Agilent Cary 630 FTIR spectrophotometer scanning between 650 and 4000 cm–1 fitted with KBr optics and complimentary diamond ATR accessory. Thermogravimetric analyses (TGA) were performed by using a thermobalance TG 60-H Shimadzu under a uniform air flow rate of 50 mLmin–1 from room temperature to 800 °C, with a heating rate of 10 °Cmin–1. Single crystal diffraction data for Eu(dpm)3(2-pyr) were collected using an Oxford4

Diffraction GEMINI-A-Ultra diffractometer (LabCri-UFMG) using graphite-Enhance Source MoKα radiation (λ = 0.71073 Å) at 150(2) K, for Tb(dpm)3(2-pyr) was used an Agilent Supernova diffractometer (UFJF) using graphite-Enhance Source MoKα radiation at 298(2) K. The data collection, cell refinements, and data reduction were performed using the CRYSALISPRO software [33]. The structures were solved and refined using the SHELX-14 program package [34], and the figures shown were produced using the programs ORTEP 3 for Windows [35], MERCURY [36] and VESTA [37]. Photoluminescence spectra and lifetime measurements were collected using an Agilent Cary Eclipse Fluorescence Spectrophotometer computer controlled rationing fluorescence spectrophotometer with measurement modes for fluorescence and phosphorescence. Czerny-Turner 0.125 m monochromators, 190–1100 nm wavelength range, fixed selectable SBW from 1.5–20 nm, full spectrum Xe pulse lamp single source with exceptionally long life, horizontal beam geometry, dual R928 PM tubes. All reflective optical system with quartz over coated optics, Schwarzschild source optics for increased energy throughput and precise imaging and focusing, scan rates from 0.01–24000 nm/min, 80 data points per second maximum measurement rate in fluorescence mode, non-measurement phase stepping wavelength drive, room light immunity in fluorescence mode, centrally controlled by computer. PALS (Positron Annihilation Lifetime Spectroscopy) measurements were performed at 294 K using a conventional fast–fast coincidence system (Ortec), with time resolution of 230 ps given by the

60

Co prompt curve. The

22

Na positron source, with

approximately 20 Ci activity, was sandwiched between two 7.5 m thick kapton foils, and the source correction was approximately 20%. The lifetime spectra (minimum of three spectra per sample) were resolved with four components using Positron fitExtended program [38,39], except for Eu(dpm)3, which was treated with three components due to its low o-Ps intensity. The obtained lifetimes, τi, and intensities, Ii, where i = 1, 2, 3 and 4 refer, respectively, to p-Ps, free positron, o-Ps trapped into small (until 0.3 nm radius) free volumes in the bulk material and o-Ps trapped in larger free volumes, which are considered as any dynamic or static spaces with low or zero electron density, and may vary from larger molecular interstices, with radius grater then nearly 0.3 nm, to mesopores. Typical values for τ1, τ2, and τ3 are 0.120 ns, 0.150 – 0.430 ns, and 0.900 – 3.000 ns, respectively, while the maximum value for τ4 is 142 ns, corresponding to o-Ps annihilation in vacuum. In this work, the PALS spectra were analyzed fixing τ1 at 5

0.120 ns, which reduces the correlation among the fitted parameters, leading also to similar fitted parameters values of the unrestrained fitting procedure [23,29,30]. In solids presenting large free volumes (> 5 – 7 nm) and the PALS spectra are better fitted with four components, which is associate with the o-Ps trapped inside these low electron density spaces. The DBARLS spectroscopy measurements were performed using a high pure Ge detector (ORTEC, model GEM-F5930) with a resolution of 1.23 keV at the 511 keV annihilation gamma energy, as determined by linear interpolation of the full width at halfmaximum (FWHM) of the nuclear rays of 356.005 and 383.851 keV of 569.670 keV of

207

133

Ba and

Bi, simultaneously measured with the annihilation line for calibration

purposes (55.7 eV/channel). DBARL result is given in terms of the full width at halfmaximum (FWHM) of the annihilation line, which were determined by the Annpeak program [40] with ± 0.02 keV of experimental repeatability uncertainty as an average of six spectra per sample. For gamma high pure Ge (HPGe) detector with energy resolution around 1.25 keV at the 514 keV gamma ray of the Sr-85, compounds presenting FWHM larger than 2.83 keV do not form Ps or has very low Ps formation probability. Significantly narrower FWHM (between 2.44 and 2.66 keV) is characteristic of high Ps formation probabilities [25,31,41]. 2.2. Synthesis of the Ln(dpm)3(2-pyr) (Ln=Eu, Tb) complexes 2-pyrrolidone (0.12 g, 1.41 mmol) was added dropwise to stirred solutions of Ln(dpm)3 (Ln = Eu, Tb) (0.80 g, 1.13 mmol) in methanol (30 mL). The reaction mixture was left overnight at room temperature yielding a white solid for each complex, which was washed with water to remove excess of 2-pyrrolidone, filtered and then vacuumdried. Eu(dpm)3 (1) C33H57EuO6: calcd. C 56.48%, H 8.19%, Eu 21.65%; found: C 57.09%, H 8.82%, Eu 22.39%. IR (KBr/cm-1): 1574 (m), 1551 (s), 1536 (s), 1497 (s), 1449 (s), 1398 (m), 1276 (s), 1225 (m), 1177 (s), 1130 (w), 868 (w), 793 (m), 761 (m), 736 (m). Molar conductivity in nitromethane: 5.09 cm2 Ω−1 mol−1. Eu(dpm)3(2-pyr) (2) Yield: 0.81 g (90%). C37H64EuNO7: calcd. C 56.48%, H 8.20%, N 1.78%, Eu 19.31%; found: C 57.90%, H 8.02%, N 1.73%, Eu 18.95%. IR (KBr/cm-1): 6

2956 (m) 1683 (m), 1572 (s), 1504 (s), 1450 (s), 1400 (s), 1356 (m), 1296 (s), 1224 (m), 1138 (s), 1024 (w), 960 (w), 868 (w), 792 (m), 792 (m), 602 (m). Molar conductivity in nitromethane: 5.91 cm2 Ω−1 mol−1. Tb(dpm)3 (3) C33H57TbO6: calcd. C 55.92%, H 8.11%, Tb 22.42%; found: C 56.79%, H 7.82%, Tb 21.91%. IR (KBr/cm-1): 1574 (m), 1551 (m), 1536 (s), 1494 (s), 1450 (s), 1400 (s), 1276 (s), 1224 (m), 1178 (s), 1124 (w), 868 (w), 792 (m), 762 (m), 634 (m). Molar conductivity in nitromethane: 6.59 cm2 Ω−1 mol−1. Tb(dpm)3(2-pyr) (4) Yield: 0.79 g (88%). C37H64TbNO7: calcd. C 55.98%, H 8.13%, N 1.78%, Tb 19.31%; found: C 57.25%, H 8.47%, N 1.89%, Tb 18.26%. IR (KBr/cm-1): 2955 (m) 1682 (m), 1570 (s), 1508 (s), 1454 (s), 1402 (s), 1354 (m), 1300 (s), 1222 (m), 1136 (s), 1024 (w), 960 (w), 869 (w), 790 (m), 790 (m), 602 (m). Molar conductivity in nitromethane: 7.87 cm2 Ω−1 mol−1. 2.3. Crystal structure determination Crystals suitable for X-ray diffraction studies were obtained by slow evaporation of solutions of Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr) in a mixture of water and methanol at 1:3 ratio in volume. The structure was solved by direct methods using SHELXS-2013/1 and Full-matrix least-squares refinement procedure on F2 with anisotropic thermal parameters was carried on using SHELXL-2014/7 [34]. Positional and anisotropic atomic displacement parameters were refined for all non-hydrogen atoms. Hydrogen atoms were placed geometrically and the positional parameters were refined using a riding model. Crystal, collection parameters and structure-refinement data are summarized in Table 1. Table 1. Crystal data and structure refinement for Ln(dpm)3(2-pyr) (Ln = Eu, Tb). Parameter Eu(dpm)3(2-pyr) Tb(dpm)3(2-pyr) Empirical Formula C37H64EuNO7 C37H64TbNO7 Formula Weight 786.85 793.81 Crystal System Triclinic Triclinic Space Group P-1 P-1 Unit cell dimensions a (Å) 10.293(5) 10.4678(2) b (Å) 13.326(5) 13.3974(3) c (Å) 15.649(5) 15.7401(3) 76.952(5) 76.815(2)  (º) 88.225(5) 87.721(2)  (º) 72.868(5) 74.270(2)  (º) 7

V (Å3) Z / Density calc. (Mg∙m–3) Absorption coefficient (mm–1) F(000) Crystal size (mm3)  range for data coll. (o)

1996.8(14) 2068.16(8) 2 / 1.309 2 / 1.275 1.614 1.752 824 828 0.457 x 0.112 x 0.060 0.325 x 0.225 x 0.168 1.867 to 26.372 3.214 to 29.748 -12≤h≤12,-16≤k≤16, - -14≤h≤14,-18≤k≤18, 19≤l≤19 21≤l≤21 48592 / 8152 [R(int) = 55515 / 10682 [Rint = 0.0405] 0.0487] 100 99.8 1.0000 and 0.71172 1.0000 and 0.4830 8152 / 0 / 433 10682 / 0 / 415 1.069 1.047 R1 = 0.0306, wR2 = 0.0670 R1 = 0.0480, wR2 = 0.0926 R1 =0.0268, wR2 = 0.0643 R1 = 0.0376, wR2 = 0.0848 1.411 and -0.839 0.713 and -0.688

Index ranges Reflection collected/ unique Completeness to  = 25.242 (%) Max. and min. Transmission Data / restraints / parameters Goodness–of–fit on F2 R indices (all data) Final R indices [I > 2(I)] Larg. peak & hole (Å–3)

3. Results and Discussion

3.1. Characterization of the Ln(dpm)3(2-pyr) (Ln = Eu, Tb) complexes Microanalyses

and

molar

conductivity

data

suggest

the

formation

of

Ln(dpm)3(2-pyr) complexes, with Ln = Eu, Tb. Their molar conductance in nitromethane indicates that they act as neutral complexes, suggesting that the three dpm and one 2-pyr ligands are linked to the metal ion. The

IR

spectroscopy

at

room

temperature

for

Eu(dpm)3(2-pyr)

and

Tb(dpm)3(2-pyr) are similar and can be divided into four parts: (a) peaks between 1400 and 1650 cm–1 represent the vibrational modes and stretching modes of the C−O and C−C bonds in the ligand ring structure; (b) peaks between 1300 and 1400 cm–1 and those between 900 and 1100 cm–1 are due to the CH3 groups in the tertiary butyl groups; (c) peaks between 1100 and 1300 cm–1 and those between 700 and 900 cm–1 can be assigned to various vibrational modes of the C−C(CH3)3 bond; (d) peaks around 800 cm–1 represent the out-of-plane bending modes of the C−H bond between the two carbonyl groups. In addition, FTIR spectra exhibit strong bands around 1600 cm –1 assigned to ν(C=O) coupled to ν(C=C) of the dpm, suggesting that this ligand is coordinated to the Ln(III) ion in chelating form. The ν(C=O) stretching band and aliphatic C–H stretching of

8

CH3 group at 2900 cm–1 of 2-pyr cannot be clearly distinguished from the ν(C=O) of the dpm ligand, as they occur in the same region. TG/DTG and DTA curves of the compounds were recorded at heating rate of 10 oC min–1, from 30 to 750 ºC in dynamic air atmosphere. The DTA curves exhibit three effects for the precursors Tb(dpm)3 and Eu(dpm)3, being two endothermic and one exothermic for Tb(dpm)3 and one endothermic and two exothermic for Eu(dpm)3 (Figure 1). The first endothermic peak at 107 °C for the Tb(dpm)3 correspond to the dissociation of the originally dimeric complex, Tb2(dpm)6 [42]. Although this peak has not been identified for the Eu(dpm)3 precursor, it has been reported that in the solid state this complex is also found in dimeric form, Eu2(dpm)6 [6,13,42-44]. The second endothermic peak at 182 °C for Tb(dpm)3 and first endothermic peak at 190 °C for Eu(dpm)3 are related to the evaporation and partial decomposition of the samples with a small exothermic peak at 275 °C for the Eu(dpm)3. The last large peak is exothermic and represents decomposition of the precursors by oxidation (Figure 1). TG/DTG and DTA curves for Ln(dpm)3(2-pyr) complexes are similar to each other, but with differences in peak temperatures. The DTA curves exhibit three effects being two endothermic and one exothermic for both complexes (Figure 1). The first peak at 109 ºC and 70 ºC for Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr), respectively, are related to the melting of the complexes. The second endothermic peak at 181 °C and 156 ºC for Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr), respectively, are related to evaporation and partial decomposition of the complexes (Figure 1). The last peak at 393 ºC and 289 ºC for Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr), respectively, are exothermic and represents decomposition of the complexes by oxidation. The coordination of the 2-pyr ligand slightly affects the thermal properties of the starting complexes. Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr) complexes (Figure 1) evaporate at lower temperatures than Eu(dpm)3 and Tb(dpm)3 starting complexes. The same behavior was observed for the weight loss curves of Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr) complexes, which are slightly shifted to the low temperature region compared to the TG curves of Eu(dpm)3 and Tb(dpm)3.

9

0.2

m / %

-0.6 -0.8

40

0.3

-1.0 20

80

Tb(dpm)3(2-Pyr)

-1.2

-2

60

-4

-0.003

-0.004

-6 20

-1.4

0 -0.3

-0.005

-8

-1.6 200

300

400

500

600

0

700

100

200

Temperature / C 0.1

600

700

0.6

Eu(dpm)3(2-pyr)

80 -0.2 -0.3 -0.4 -0.5

1.0

0.0

100

-0.6

20

TG DTG DTA

0.4 0.2 0.0 -0.2 -0.4

-0.7

-0.6

-0.8

-0.8

m / %

60

-0.1

DTA / V.mg

m / %

DTA /V.mg

-1

TG DTG DTA

0.8

-1

Eu(dpm)3

0.0

-1

80

40

0.5

500

1.0

DTG / mg.s

100

2.5

1.5

400

Temperature / C

4.0

3.0

300

o

0

60

-0.4

-0.6 40 -0.8 20 -1.0

0.0

0 0

100

200

300

400

500

600

o

Temperature / C

700

-0.2

-1

100

DTG / mg.s

0

2.0

-0.002

40

0.0

3.5

-0.001

TG DTG DTA

0

-1

-0.4 -1

-1

DTA / V.mg

60

2

-1

TG DTG DTA

0.9

100 0.000

-0.2

DTG / mg.s

Tb(dpm)3

80

DTA / V.mg

1.2

0.6

0.001 4

0.0

DTG / mg.s

100

m / %

1.5

0 0

100

200

300

400

500

600

700

0

Temperature / C

Figure 1. TG/DTG and DTA curves of the Eu(III) and Tb(III) complexes recorded at heating rate of 10 oC min–1, from 30 to 750 ºC in dynamic air atmosphere.

3.2. X-ray crystallography The molecular plot of Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr) are shown in Figures 2 and 3 respectively. Selected bond lengths and angles around the metal are given in Table 2. Ln(dpm)3(2-pyr), where Ln = Eu and Tb, crystallize in the space group Pī with one complex in the asymmetric unit for both compounds (Figure 2 and 3). Both compounds present similar structures, but different from their precursors, Ln(dpm)3 [44]. The Ln(III) ion is seven-coordinated by six O atoms of the three chelating dpm ligands and one O atom of the 2-pyrrodilidone ligand. The coordination polyhedron of the Eu(III) center can be described as a distorted monocapped trigonal prism with the O11, O12, O21 and O1, O22, O32 atoms occupying the trigonal planes and O31 atom occupying the monocapped position. The distortion of the geometry can be caused by the presence of bidentate ligand connecting both trigonal planes and a trigonal plan to the capped position 10

(Table 2). Similar coordination also was observed in Tb(III) ion (Figure 3), with the O1, O3, O7 and O4, O5, O6 atoms occupying the trigonal planes and O2 atom occupying the monocapped position. This coordination mode is commonly observed in [Ln(dpm)3L] complexes [44]. In relation to the supramolecular arrangement, the hydrogen bonds involving lactams lead to formation of dimmers between the complexes in solid state. This interaction is observed in both compounds. Eu(dpm)3(2-pyr) complex: [N1–H1⋯O1i (i = -x+2,-y+1,-z+1);

d(N1⋯O1

=

2.970(3)

Å;

angle(N1H⋯O1)

=

167.6o]

and

Tb(dpm)3(2-pyr) complex: [N1–H1⋯O7i (i = -x,-y+1,-z+1); d(N1⋯O7 = 3.025(5) Å; angle(N1H⋯O1) = 171.0o].

Figure 2. (A) Molecular structure of Eu(dpm)3(2-pyr) at 50% probability level. Hydrogen atoms were omitted for clarity. (B) Coordination polyhedron of the Eu(III) ion in Eu(dpm)3(2-pyr).

(A)

(B)

11

Figure 3. (A) Molecular structure of Tb(dpm)3(2-pyr) at 25% probability level. Hydrogen atoms were omitted for clarity. (B) Coordination polyhedron of the Tb(III) ion in Tb(dpm)3(2-pyr). Table 2. Selected bond lengths (Å) for Ln(dpm)3(2-pyr) (Ln = Eu, Tb). Eu(dpm)3(2-pyr) Eu1–O1 Eu1– O11 Eu1– O12 Eu1– O21 Eu1– O22 Eu1– O31 Eu1–O32 O21–Eu1–O11 O21–Eu1–O32 O11–Eu1–O32 O21–Eu1–O22 O11–Eu1–O22 O32–Eu1–O22 O21–Eu1–O31 O11–Eu1–O31 O32–Eu1–O31 O22–Eu1–O31 O21–Eu1–O12 O11–Eu1–O12 O32–Eu1–O12 O22–Eu1–O12 O31–Eu1–O12 O21–Eu1–O1 O11–Eu1–O1 O32–Eu1–O1 O22–Eu1–O1 O31–Eu1–O1 O12–Eu1–O1

Bond (Å) 2.422(2) 2.315(2) 2.369(18) 2.3089(19) 2.329(2) 2.3543(19) 2.3213(19) Angles (o) 91.03(7) 148.09(7) 98.99(8) 72.10(7) 157.59(7) 88.84(7) 80.23(7) 79.60(7) 72.03(7) 82.98(8) 77.80(7) 72.17(7) 134.11(7) 116.66(7) 143.58(7) 121.86(7) 121.84(7) 77.79(7) 80.26(8) 145.60(7) 70.63(7)

Tb(dpm)3(2-pyr) Tb1-O1 Tb1-O2 Tb1-O3 Tb1-O4 Tb1-O5 Tb1-O6 Tb1-O7 O1-Tb1-O2 O1-Tb1-O3 O1-Tb1-O4 O1-Tb1-O5 O1-Tb1-O5 O1-Tb1-O7 O2-Tb1-O3 O2-Tb1-O4 O2-Tb1-O5 O2-Tb1-O6 O2-Tb1-O7 O3-Tb1-O4 O3-Tb1-O5 O3-Tb1-O6 O3-Tb1-O7 O4-Tb1-O5 O4-Tb1-O6 O4-Tb1-O7 O5-Tb1-O6 O5-Tb1-O7 O6-Tb1-O7

Bond (Å) 2.301(2) 2.318(3) 2.298(3) 2.291(3) 2.296(2) 2.351(2) 2.398(2) Angles (o) 72.95(8) 87.23(9) 147.73(9) 98.70(8) 135.44(8) 77.36(9) 83.95(10) 79.75(10) 79.43(9) 142.26(8) 147.27(9) 72.88(9) 159.80(9) 116.07(9) 81.13(9) 92.79(9) 76.79(9) 122.51(10) 72.69(8) 118.97(9) 70.22(9)

3.3. Luminescence of the Eu(III) complexes The excitation spectrum of the Eu(III) complexes in the solid state were recorded from 260 to 600 nm at 82 K and at room temperature (300 K), under emission of the hypersensitive 5D07F2 transition at 612 nm. The Eu(dpm)3 and Eu(dpm)3(2-pyr) complexes were compared under the same conditions (Figure 4) and showed characteristic narrow absorption bands of the Eu(III) ion assigned to the transitions 7

F0→5D0 (~ 578 nm), 7F0→5D1 (~ 524 nm, ~ 532 nm), 7F0→5D2 (~ 463nm), and 7F0→5D3 12

(~ 435 nm). The 7F0→5L6 (~ 394 nm), 7F0→5G2, 5G4 and 5G6 (~ 374 nm) transitions are partially or totally overlapped by the ligand’s 1π → 1π* transition. The excitation bands assigned to the 7F0→5D3 and 7F0→5L6 transitions show weak relative intensities at room temperature for both Eu(dpm)3 and Eu(dpm)3(2-pyr). This result is in accordance with previous studies showing that the Eu(dpm)3 complex presents very weak luminescence intensity at 300 K because the LMCT mechanism, i.e., the dpm → Eu(III) charge transfer states efficiently reduces the Eu(III) emitting 5D0 level population and also promotes an efficient non-luminescent pathway for its decay [5,6,12,23,30,31,43,45]. However, the excitation spectrum monitoring the Eu(5D07F2) transition for the Eu(dpm)3(2-pyr) complex present the typical ligand’s 1π → 1π* broad band which can be observed from approximately 280 to 400 nm, while for Eu(dpm)3 the intensity absorption of the ligand’s 1π → 1π* band abruptly decreases above 330 nm up to approximately 400 nm. This feature is significantly noticeable on the spectra obtained at 82 K. Also, the excitation spectrum of Eu(dpm)3(2-pyr) obtained at 82 K shows the 7F0→5L6 transition with a higher absorption intensity than that observed for Eu(dpm)3 (Figure 4), indicating that the 5D0 level is populated more efficiently from the 5L6 level in the Eu(dpm)3(2-pyr) complex. This result suggests an augmentation of the LMCT energy of Eu(dpm)3(2-pyr),

1

7

7 5 F0 L6

82 K 1

5 F0 L6

7

(  *)

(1) Eu(dpm)3

5 F0 L6

7

300 K

(2) Eu(dpm)3(2-pyr) 300K

7

Intensity / a.u.

(2) Eu(dpm)3(2-pyr) 1

7F 5D0 0

1

(  *)

7F 5D1 0

82 K

7F 5D2 0

(1) Eu(dpm)3

5 F0 D3

5 F0 L6

reducing this 3π* de-excitation mechanism via LMCT in this complex [6].

1

1

(  *)

270

300

330

360

390

420

450

480

510

540

570

600

 / nm 13

Figure 4. Excitation spectra of (1) Eu(dpm)3 and (2) Eu(dpm)3(2-pyr), recorded at 82 and 300 K, with emission monitored on the Eu(III) 5D0→7F2 transition at 612 nm.

Figure 5 shows the emission spectra of the Eu(III) complexes in the solid state which were recorded from 550 to 720 nm at 82 K and at room temperature (300 K) under excitation at 335 nm. These spectra consist of sharp bands assigned to the intraconfigurational 5D07FJ transitions (J = 0-4) being the hypersensitive 5D07F2 transition the dominating one. As previously mentioned, the Eu(dpm)3 complex presents a very weak luminescence intensity at 300 K because, at this temperature, the LMCT states provide a mechanism which reduces the luminescence intensity and the Eu(III) 5D0 lifetime. However, at 82 K these states do not suppress and quench the luminescence efficiently, so the emission spectrum next to liquid nitrogen temperature is better resolved. Regarding Eu(dpm)3(2-pyr), there is a significant change in the shape of the 5

D07F2 band compared to the same transition of Eu(dpm)3. This is due to the insertion

of the 2-pyr ligand, which distorted the crystalline field of the precursor complex, Eu(dpm)3, causing a difference in the emission intensity of the complexes.

14

Intensity (a.u)

7

D0  F4

5

7

D0  F3

5

5

7 5

5

7

D0  F0

82 K

D0  F1

7

D0  F2

(2) Eu(dpm)3(2-Pyr)

(1) Eu(dpm)3 82 K

(2) Eu(dpm)3(2-Pyr) 300 K

(1) Eu(dpm)3 300 K

550

575

600

625

 nm

650

675

700

Figure 5. Eu(dpm)3 (1) and Eu(dpm)3(2-pyr) (2) emission spectra, at 82 K and 300 K, recorded from 550 to 720 nm under excitation at 335 nm.

Experimental Judd–Ofelt intensity parameters for the Eu(III) ion in the complexes were determined from the emission spectrum by the following expression [5]:

 

4e2 3 A0 3 c3 

7

FJ U (  ) 5 D0

(1)

2

where A0 are the coefficients of spontaneous emission,  is the Lorentz local field correction term that is given by   n  n2  2  / 9 with the refractive index n = 1.5 and 2

7

FJ U (  ) 5 D0

2

are the squared reduced matrix elements, whose values are 0.0032 and

0.0023 for the 5D0 → 7F2 and 5D0 → 7F4 transitions, respectively [5]. In order to evaluate the luminescence efficiency of the Eu(III) complexes, the intrinsic quantum efficiency () of the Eu(III) ion was determined. The quantum efficiency of the 5D0 emitting level () for the Eu(III) complexes are presented in Table 3 and were calculated from the ratio Arad/Atotal, where Arad and Atotal 15

are radiative and total rates assigned to the decay processes of the 5D0 emitting level, respectively. In this case, the Arad values were obtained by summing the radiative spontaneous coefficients due to the 5D0→7FJ transitions, while Atotal were determined from the lifetime values of the 5D0 emitting level () by considering the reciprocal relation between these properties (Atotal = 1/). Further specific details about these parameters are reported in the literature [5]. The experimental data reveal that Eu(dpm)3(2-pyr) present higher luminescence quantum efficiency than Eu(dpm)3. Since only slight changes occur in the radiative coefficient (Arad), the values of  are mainly associated to changes in the non-radiative rates (Anrad). Table 3 presents the experimental intensity parameters for Eu(dpm)3 and Eu(dpm)3(2-pyr). The experimental intensity parameters Ω2 and Ω4 of Eu(dpm)3 present different values from those of Eu(dpm)3(2-pyr), indicating that Eu(III) ions are in different chemical environments (Table 3). According to the literature, the Ω2 value is the most influenced by small angular changes in the local geometry. This effect, together with changes in the ligating atom polarizability (), has been used to rationalize the hypersensitivity of certain 4f–4f transitions to changes in the chemical environment [45]. Luminescence decay curves of Eu(dpm)3 and Eu(dpm)3(2-pyr) were recorded at 82 and 300 K under excitation at 335 nm and emission monitored at 612 nm. Both luminescence decay curves were well adjusted to a single exponential function from which it was possible to obtain the lifetime (τ) values of the 5D0 emitting level (Table 3). Table 3. Experimental values of intensity parameters (R02 and Ω), radiative (Arad) and non-radiative (Anrad) rates, lifetimes (τ) and emission quantum efficiencies (η) of the 5D0 emitting level determined for complexes Eu(dpm)3 and Eu(dpm)3(2-pyr) at 82 and 300 K. Eu(dpm)3 Optical data –20

Ω2 (10 cm ) Ω4 (10–20 cm2) Arad (s–1) Anrad (s–1) Atot (s–1) τ (ms) η (%) 2

82 K 27.98 8.18 1082.11 1280.84 2362.95 0.4232 45.8

300 K 27.39 8.04 1065.5 20721.0 21786.5 0.0459 4.9

Eu(dpm)3(2-pyr) 82 K 22.16 8.82 887.3 632.9 1520.2 0.6578 58.4

300 K 22.52 9.3 910.0 3656.2 4566.2 0.2190 19.9

The lifetime observed for Eu(dpm)3 at 300 K (τ = 0.0459 ms) is almost 10 times lower than that observed at 82 K (τ = 0.4232 ms), indicating that there is a significant 16

temperature dependence, in agreement with the results reported by Berry et al [43]. This temperature dependence is due to the activation energy needed to promote the decay of the 5D0 state through the LMCT state [43], and is strongly correlated to the luminescence self-quenching promoted by the LMCT state. In Eu(dpm)3, the LMTC state is located close to the 5D0 level, so it acts as an efficient nonradiative channel that plays an important role in depopulating the 5D0 emitting level at 300 K [6]. Since the LMCT state quenches the Eu(III) 5D0 excited state more efficiently at 300 K than at 82 K, consequently higher the temperature, lower are both the lifetime and the luminescence. On the other hand, the lifetime value of the 5D0 emitting level observed for Eu(dpm)3(2-pyr) recorded at 300 K (τ = 0.2190 ms) is only three times lower than that observed at 82 K (τ = 0.6578 ms), indicating that there is a considerably lower temperature dependence. In addition, Eu(dpm)3(2-pyr) has higher emission quantum efficiency (η) than Eu(dpm)3 at both 82 and 300 K, which also suggests a less efficient LMCT state. These results are in accordance to those previously proposed by Simas et al, which observed that breaking the coordination symmetry, by the inserting of different ligands, may enhance Eu(III) luminescence and increase the Eu(III) 5D0 lifetime [46,47]. According to Faustino et al [12], the luminescence of Eu(III) complexes is a function of the energy position between the LMCT states and the ligand’s 3π*. If the energy of the LMCT states is much higher than the 3π*, the complex present a strong luminescence. However, if the energy of the LMCT states is considerably lower than 3π*, the emission quantum efficiency is almost null, which is the case of Eu(dpm)3, while if the gap between the energies of the LMCT states and the 3π* is small, the complex presents a weak luminescence. Since the Eu(dpm)3(2-pyr) complex have higher emission quantum efficiency when compared to Eu(dpm)3, but still presented a relatively weak luminescence, this result indicates that, despite the increase in the energy of the LMCT states, the 3π* and the LMCT states still have close energies and that the 3π*→LMCT process has not been fully suppressed.

3.4. Luminescence of the Tb(III) complexes The Tb(dpm)3 and Tb(dpm)3(2-pyr) coordination compounds presented strong green luminescence emission because the dpm and 2-pyr ligands act as a good “antenna” for the Tb(III) ion. The ligands absorb photons strongly and then transfers this excitation energy 17

to the Tb(III) ion, which then emits its typical green luminescent radiation. The excitation spectra (Figure 6) recorded under emission monitored on the 5D4 → 7F5 transition at 545 nm, recorded at 82 and 300 K, exhibit strong absorption bands in the range of 270–350 nm ascribed to the 1π → 1π* transition. In addition to this band, two absorption bands at 370 and 470 nm were assigned to the 7F6 → 5D3 and 7F6 → 5D4 transitions of the Tb(III) ion. No significant difference was observed between the excitation spectra of Tb(dpm)3 and Tb(dpm)3(2-pyr) recorded at 82 and 300 K. These data indicate that the ligand → Tb(III) intramolecular energy transfer occurs by a similar mechanism at both temperatures. It is important to mention that the LMCT states from organic ligands to Tb(III) ion is located at higher energies (˃ 60000 cm–1), much above the ligand´s 3π* (23530 cm–1 for dpm); therefore, it cannot self-quench (suppress) the luminescence in these coordination compounds [6].

(4) Tb(dpm)3(2-Pyr) 82 K

(4) Tb(dpm)3(2-Pyr)

(3) Tb(dpm)3

1 1 (  *)

300 K

225

250

275

300

325

350

5 7 Tb( F6 G6)

300 K

5 7 Tb( F6 L10)

Intensity (a.u)

82 K

5

5 7 Tb( F6 L10)

(3) Tb(dpm)3

7 Tb( F6 G6)

1 1 (  *)

375

400

425

450

 / nm

Figure 6. Excitation spectra of (3) Tb(dpm)3 and (4) Tb(dpm)3(2-pyr), recorded at 82 and 300 K, with emission monitored on the Tb(III) 5D4→7F5 transition at 545 nm.

18

The photoluminescence emission spectra obtained for Tb(dpm)3 and Tb(dpm)3(2-pyr) are shown in Figure 7. The luminescence emission narrow bands around 489, 544, 584, 619, 647, 666, and 676 nm are assigned to the intraconfigurational 5D4→7FJ transitions of the Tb(III) ion, with J = 6, 5, 4, 3, 2, 1 and 0 respectively [3]. The emission spectra of the Tb(III) complexes in solid state containing dpm and 2-pyr ligands, and recorded in the range of 460 to 700 nm, at 82 and 300 K, showed a similar profiles. However, the emission spectra at 300 K is less resolved compared to the spectra at 82 K due to increased non-luminescent energy loss at higher temperatures, such as molecular vibrations and Tb(III) → 3π* back-transfer. The absence of a phosphorescence broad emission band in the range between 400 and 600 nm coming from the ligands indicates that the intramolecular energy transfer from the dpm and 2-pyr ligands to the Tb(III) ion is very efficient at both temperatures.

82 K

5 7 D4  F1

5 7 D4  F 3

5 7 D4  F 4

5 7 D4  F6

5 7 D4  F5

(4) Tb(dpm)3(2-Pyr)

(3) Tb(dpm)3

Intensity (a.u)

82 K

(4)Tb(dpm)3(2-Pyr) 300 K

(3) Tb(dpm)3 300 K

475

500

525

550

575

600

625

650

675

700

 / nm

Figure 7. Tb(dpm)3 (3) and Tb(dpm)3(2-pyr) (4) emission spectra, at 82 and 300 K, recorded from 460 to 700 nm under excitation at 335 nm.

19

The luminescence lifetime of the Tb(III) emitter 5D4 level was previously obtained for the Tb(dpm)3 complex at 77 and 300 K, and it was observed a smaller temperature dependence compared to the Eu(III) 5D0 level of Eu(dpm)3. The values obtained were 0.821 ms at 77 K and 0.763 ms at 300 K. Since the LMCT state do not acts as a 5D4 suppressor in the Tb(dpm)3 complex, the temperature dependence is due to the energy back-transfer from the 5D4 level to the ligand’s 3π* state. Although this channel also acts as a luminescence suppressor, it is considerably less efficient than the LMCT states present in the Eu(dpm)3 complex. The lifetimes of the Tb(III) 5D4 level was also determined for Tb(dpm)3(2-pyr) at 82 and 300 K, and the values obtained were 1.035 and 0.895 ms, respectively.

3.5. PALS and DBARLS measurements

Positron annihilation lifetime (PALS) and Doppler broadening annihilation radiation lineshape (DBARLS) spectroscopies parameters, obtained for the Eu(dpm)3, Eu(dpm)3(2-pyr), Tb(dpm)3 and Tb(dpm)3(2-pyr) complexes, are shown in Table 4. For our purposes, only o-Ps lifetimes (τ4 and τ3) and relative formation probabilities (I4 and I3) are relevant. PALS and DBARL parameters for Eu(dpm)3 and Tb(dpm)3 were previously obtained and are also shown elsewhere [23,25,30,31]. Table 4 – PALS (τ3 and I3) and DBARL (FWHM) parameters, at 295 K, for Eu(dpm)3, Eu(dpm)3(2-pyr), Tb(dpm)3 and Tb(dpm)3(2-pyr). PALS analysis with three or four components, 1 fixed at 0.120 ns.

Compound

3 / ns

I3 / %

4 / ns

I4 / %

FWHM / keV

Eu(dpm)3 Eu(dpm)3(2-pyr) Tb(dpm)3 Tb(dpm)3(2-pyr)

2.20 ± 0.55 1.82 ± 0.06 1.40 ± 0.05 1.22 ± 0.08

2.4 ± 0.2 15.3 ± 0.6 39.6 ± 3.4 16.5 ± 0.5

− 3.6 ± 0.7 3.7 ± 0.4 4.6 ± 0.4

− 4.3 ± 1.4 6.4 ± 1.2 6.8 ± 0.8

2.98 ± 0.02 2.89 ± 0.01 2.70 ± 0.01 2.75 ± 0.01

PALS results presented in Table 4 show that all complexes present significant Ps formation probability (I3 > 5%), except for the Eu(dpm)3 complex. Regarding Tb(dpm)3, Tb(dpm)3(2-pyr) and Eu(dpm)3(2-pyr), the spectra treatment with 4 components provided 20

a better reduced chi square (χ2) when compared to a 3 component treatment, since the Ln(dpm)3 complexes, except for Eu(dpm)3, presents o-Ps trapped in vacancies, which increases its lifetime and makes the four component treatment more appropriate [25,41]. However, since Eu(dpm)3 present a very low o-Ps intensity, no τ4 and I4 parameters is obtained from its spectra treatment. The Tb(dpm)3(2-pyr) and Eu(dpm)3(2-pyr) complexes form dimmers via formation of hydrogen bonds between two Ln(dpm)3(2-pyr) molecules, and the space between these dimmers can explain the long-lived o-Ps. Since both I3 and I4 are related to o-Ps formation, for the sake of simplicity, from here on we will refer to Io-Ps as the sum of I3 and I4. DBARL results, which are given in terms of the full width at half-maximum (FWHM) of the annihilation line, confirmed those of PALS, evidencing that all complexes present higher Ps formation probabilities than Eu(dpm)3. Narrower FWHM are related to high Ps formation probabilities, p-Ps formation more specifically, while larger FWHM are due to free positron annihilation, i.e., very low Ps formation probability. As an example, for gamma high pure Ge (HPGe) detector with energy resolution around 1.25 keV at the 514 keV gamma ray of the Sr-85, compounds presenting FWHM larger than 2.83 keV do not form Ps or has very low Ps formation probability. Significantly narrower FWHM (between 2.44 and 2.66 keV) is characteristic of high Ps formation probabilities [48]. It is not surprising that detectors with different resolutions provide different FWHM, so the FWHM values presented in Table 4 vary from those obtained by Marques-Netto et al [48], but for our purposes, the comparison between the FWHM of the complexes here studied is enough. Regarding Eu(dpm)3(2-pyr), the results obtained (Table 4) are very interesting, since both PALS and DBARL results indicate that Eu(dpm)3(2-pyr) present a considerably higher Ps formation probability than Eu(dpm)3. The vast majority of Eu(III) complexes present low Ps formation probability and, to the best of our knowledge, Ps formation in Eu(III) complexes was observed only once by Faustino et al [28]. PALS and optical results then indicate that, comparing to Eu(dpm)3, the insertion of the 2-pyr ligand caused an increase in the energy of the LMCT states, resulting in higher τ and η of the 5

D0 emitting level and also an increase in Io-Ps . Since luminescence self-quenching by the

LMCT state is an intramolecular phenomena, these results are a strong evidence of the participation of electronic excited states in the Ps formation, as previously proposed by Ito et al [49,50] and Mogensen [51]. Recently, we have proposed a kinetic scheme called 21

cybotactic correlated system kinetic mechanism (CCSKM) which also consider that the Ps formation is intermediated by electronic excited states formed inside the final positron spur/blob and, in the case of Ln(III) complexes, is related with ligand’s excited states and all electronic intramolecular and intermolecular processes which are able to change its population, such as ligand → Ln(III) and Ln(III) → Eu(III) energy transfer, or ligand → LMCT and Ln(III) → LMCT states charge transfer [23,30]. According to the CCSKM mechanism, an epithermal or resonant positron collides with the ligand’s electronic cloud, exciting an electron and forming a cybotactic correlate system (CCS) composed by the colliding positron and the excited molecule. The Ps is then formed due to the interaction of the positron and the ligand’s excited electron in the CCS, {e+*…L*Ln3+}. As shown in Figure 8, the low energy LMCT states promotes a path for the decay of the Eu(III) 5D1 and 5D0 excited states, self-quenching (suppressing) the luminescence and the ligand’s 3π* excited state, and consequently also of the CCS, preventing Ps formation. Low energy LMCT states can also promote the decay of the 1π* excited state and Ps formation may also occur through the interaction of a positron with an electron in this ligand’s excited state. However, for the sake of simplicity, this process was omitted. 30

Energy (x 103cm-1)

25

3

dpm

3

dpm

20

LMCT

5

D1

5

D0

LMCT

Ps

15

e+*

612 nm

335 nm

10

e+** 7

e+**

5

F6

7

F4

7

S0

0

7

Eu(dpm)3

7 7

F2

F0

7

3+

335 nm

F5

F3

F1

Eu levels

S0

Eu(dpm)3(2-pyr)

Figure 8 Partial energy level diagram for the relevant photophysical processes associated with photoluminescence and Ps formation in Eu(dpm)3 and Eu(dpm)3(2-pyr) complexes.

22

Comparing the Ps formation between Eu(dpm)3 and Eu(dpm)3(2-pyr), the observed Io-Ps increase and FWHM reduction can be explained by an increase in the energy of the LMCT states, reducing its capacity to promote the decay of the ligand’s 3π* excited state and the CCS decomposition, which is in accordance with luminescence results. However, although it was observed a reduction in the efficiency of the Ligand → Eu(III) charge transfer process, the LMCTS is still able to promote the decay of the 3π* excited state to some extent. The luminescence intensity increased with the insertion of the 2-pyr ligand, but the Eu(dpm)3(2-pyr) complex is still not highly luminescent. Also, the Io-Ps observed is considerably lower than those of the Ln(dpm)3 complexes, which present high energy LMCT states (Io-Ps ~ 40%) [23,25,30]. PALS and DBARL results also indicate that the luminescence increase is directly related to a less efficient Ligand → Eu(III) charge transfer process and not solely due to a relaxation of the Laporte rule (electronic f-f transitions in lanthanide complexes are forbidden in centrosymmetric molecules), so that complexes with lower symmetry will present higher emission quantum efficiencies (η), as proposed by Simas et al [46,47]. Regarding the Tb(III) complexes, since the Tb(dpm)3 complex present high energy LMCT states, which consequently have very low efficiency in promoting the decay of the ligand’s 3π* excited state, it presents a considerably high Io-Ps and lower FWHM compared to Eu(dpm)3. The same relation between the energy of the LMCT states and Ps formation intensity was observed for all Ln(dpm)3, which have LMCT states in the UV region and present high Io-Ps (> 30%), such as Gd(dpm)3 and Sm(dpm)3, for example [23,25,30]. Surprisingly, the Tb(dpm)3(2-pyr) complex present a considerably lower Io-Ps and higher FWHM than Tb(dpm)3, as shown in Table 4. This is a very interesting result, since the insertion of the 2-pyr ligand promoted an increase in the relative Ps formation in the Eu(III) complex, while the opposite behavior was observed for the Tb(III) complex. The processes that led to the Io-Ps reduction in the Tb(dpm)3(2-pyr) complex compared to Tb(dpm)3 are not well understood yet. A possible explanation is that the insertion of 2-pyr reduces the efficiency of the Tb(III) → Ligand energy back-transfer, which is a process that lead to the reformation of the primary Ps precursor, CCS = {e+…L*Tb3+}, resulting in a lower Ps formation. A possible way to evaluate the accuracy of this proposal is to study the energy of the 3π* performing luminescence measurements on Gd(dpm)3(2-pyr). The resonance between two energy levels increase 23

with the reduction of the energy gap between them, so it is expected that the insertion of the 2-pyr ligand promotes an increase in the 3π* energy, increasing the energy gap between the Tb(III) 5D4 and the ligand´s 3π* levels, reducing the efficiency of the Tb(III) → Ligand energy back-transfer. It is worth noticing that both Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr) have similar Io-Ps but different FWHM (Table 4), indicating that these complexes have ligand´s electrons with different amounts of movement. Since the ligands are the same, this difference in the electrons´ amount of movement indicates that Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr) have ligands with distinct electronic environments. Although the Eu(dpm)3(2-pyr) present an increase in the Ps formation probability compared to Eu(dpm)3, it still presents relatively low energy LMCT states, which promotes a somewhat efficient Ligand → Eu(III) charge transfer. On the other hand, the Tb(III) complexes do not present low energy LMCT states, and we proposed that the processes which leads to the decrease of the Ps formation probably is related to a less efficient Tb(III) → Ligand energy back transfer. Since Ligand → Ln(III) charge transfer and Ln(III) → Ligand energy back transfer are distinct intramolecular processes, they consequently result in different ligand electronic environment and may be related to the different FWHM obtained for the Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr).

4. Conclusion The coordination compounds, Ln(dpm)3(2-pyr), where Ln = Eu and Tb, were prepared and characterized by IR, TG and X-ray crystallography. The complexes present structural characteristics similar to each other, but different from their precursors, Ln(dpm)3. Optical spectroscopy results suggest that the photoluminescent properties of the Eu(III) complexes are dependent on the energy position of the lying-low LMCT state. Eu(dpm)3(2-pyr) present higher lifetimes (τ) and emission quantum efficiencies (η) of the 5

D0 emitting level, and lower temperature dependence of the Eu(III) 5D0 relaxation rate,

than Eu(dpm)3, indicating an increase in the LMCT energy of Eu(dpm)3(2-pyr) when compared to Eu(dpm)3. Therefore, the formation of Eu(III) complexes with different ligands demonstrated to be a viable way to increase the energy of the LMCT states and, consequently, enhance the luminescence of Eu(III) complexes. On the other hand, the 24

luminescence lifetime of the Tb(dpm)3(2-pyr) complex, when compared to its precursor, Tb(dpm)3, show that they are not heavily influenced by temperature since the LMCT state do not acts as 5D4 emitter level suppressor on Tb(III) complexes. Positron annihilation spectroscopies (PALS and DBARL) results are very interesting and relevant. We observed a considerably high positronium (Ps) relative formation probability (Io-Ps) in Eu(dpm)3(2-pyr), which is a remarkable result since the vast majority of Eu(III) complexes present very low Io-Ps. The results reinforce the correlation between positron annihilation and optical spectroscopies, indicating that low energy LMCT states promote Ps formation and luminescence suppression, and strongly corroborate the CCSKM, a recently proposed by our group Ps formation kinetic mechanism, which involves the participation of the ligand’s excited states in Ps formation. Regarding the Tb(III) complexes, Tb(dpm)3(2-pyr) presents a considerably lower Io-Ps than Tb(dpm)3. This is a very unexpected and interesting result since, in opposite to what was observed for the Eu(III) complexes, the insertion of the 2-pyr ligand promoted an Io-Ps reduction. The processes that led to the Io-Ps reduction in the Tb(dpm)3(2-pyr) complex compared to Tb(dpm)3 are not well understood yet. A possible explanation is that the insertion of 2-pyr reduces the efficiency of the Tb(III) → Ligand energy back-transfer, which promote the reformation of the CCS, the primary Ps precursor. However, more studies are needed to verify the validity of this hypothesis. Acknowledgments The authors express sincere thanks to the LabCri (DF/ICEx-UFMG) and X-ray Diffraction Laboratory (DQ-UFJF) by measurements of single crystal X-ray diffraction. This research is supported by grants CNPq, CAPES, IFES/Vitória and FAPEMIG.

Appendix A. Supplementary material CCDC 1821451 and 1553383 contain the supplementary crystallographic data for Eu(dpm)3(2-pyr) and Tb(dpm)3(2-pyr), respectively. The data can be obtained free of charge

from

The

Cambridge

Crystallographic

Data

Centre

via

www.ccdc.cam.ac.uk/structures.

References 25

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