Structural and photophysical studies on ternary Sm(III), Nd(III), Yb(III), Er(III) complexes containing pyridyltriazole ligands

Graphical abstract

The structure of two new ancillary triazole’s type ligands and properties of Sm(III), Nd(III), Yb(III), Er(III) complexes based on them are characterized. The factors affecting the intensity and character of the luminescence of the Ln(III) are discussed.

Abstract

Two bidentate pyridine-triazole ligands (3-(pyridine-2-yl)-5-phenyl-1,2,4-triazole (L1) and 5-phenyl-2-(2′-pyridyl)-7,8-benzo-6,5-dihydro-1,3,6-triazaindolizine (L2)), have been synthesized and used for Ln(Dbm)₃ (Ln = Sm(III), Nd(III), Yb(III) and Er(III)) coordination. The structures of the ligands and resulting Sm(III) complex were determined in the solid state by X-ray diffraction. The title complexes were characterized by UV, fluorescent, IR-spectroscopy and thermogravimetric and elemental analyses. Photophysical studies on the Ln(III) complexes were carried out showing luminescence in the region typical for Ln(III). The effect of various factors on the enhancement luminescence of complexes is discussed.

1. Introduction

Luminescent lanthanide complexes have attracted much attention in last decades because of their unique photophysical properties, such as sharp and long-lived emission in the visible and near-IR region [1]. The main attention of researchers in this area is focused on Tb(III) and Eu(III) ions, whose complexes exhibit strong ion luminescence in the red and green parts of visible range respectively. This might well be used for the design of electroluminescent devices [2]. Complexes of Sm(III), and Dy(III), which also emit in the visible region, were given less attention because their luminescence intensities are, in comparison, not high. Recently, interest towards the compounds of the lanthanides, which emit IR energy, has increased. This is mainly due to potential applications in telecommunication network as optical signal amplifier, and in biological and medical systems as potential luminescence probes [3,4].

However, the f–f transitions are spin- and parity-forbidden thus requiring the use of organic ligands that possess a reasonably large molar absorption cross section to indirectly excite the metal centre. β-Diketonates, carboxylates, and pyrazolonates type of ligands are most commonly used for Ln(III) ions complexation [5,6]. Six oxygen atoms from three such ligands are not sufficient to complete the coordination number requirement of a lanthanide ion (typically 8 or 9). Thus, the tris(complex) usually contains water or other solvent molecules in the first coordination sphere, which effectively quench luminescence. A convenient method of displacement of water molecules is the introduction of different neutral donor ligands into a molecular complex of Ln(III). The most efficient is the reaction with chelating heterocycles, such as 1,10-phenanthroline or 2,2′-dipyridyl derivatives [7]. Such ligands on the one hand displace the OH-oscillators and the other enhances the absorption and transfer of excitation energy to the lanthanide ion.

One of the promising routes of application of Ln(III) complexes is the production of electroluminescent devices [1]. For efficient electroluminescence Ln(III) complexes require several additional important factors, including high carrier transporting property [8]. It is well known that Ln(III) complexes show low carrier transporting property themselves usually. One of the ways to improve such property is the introduction of hole- and electron-transporting groups (e.g. carbazole, oxadiazole) on anionic or neutral ligands. This idea is well-established and several OLED devices were described regarding such method [9]. It was found recently that pyridyl derivatives of triazole are promising electron transporting materials [10]. In this regard, a fruitful area of research is based on the use of triazole derivatives as ancillary ligands lanthanides complexes. However, before our investigations activities on using triazole derivatives for creating luminescent lanthanide complexes the number of paper in this topic was very few. In the previous investigation we have found that use of triazole’s derivatives as neutral ligand in the Eu(III) complexes yields an increase in the intensity of the luminescence by several times [11]. In this report we described the structural properties of two pyridyltriazole derivatives and structure and luminescent properties of Ln(Dbm)₃ (Ln(III)–Sm(III), Nd(III), Yb(III), Er(III)) complexes based on it (Fig. 1).

Fig. 1.

Molecular structures and abbreviations for the used ligands and complexes.

2. Experimental

2.1. Materials and methods

All reagents and solvents employed were commercially available and used as received without further purification. Ln(Dbm)₃·2H₂O, L1, L2 were prepared according to the literature [12,13]. Elemental analyses of C, H, and N were performed with a Perkin–Elmer 240 C analyser. IR spectra were measured with a Nicolet Nexus 470 FTIR spectrometer with KBr pellets in the range 4000–400 cm^–1. TG–DTA studies were carried out with a Paulik–Paulik–Erdey Q-derivatograph. Absorption spectra were recorded on a Perkin-Elmer Lambda-9 UV–Vis–NIR spectrophotometer. Solid-state excitation and fluorescence spectra were recorded on a Horiba Jobin–Yvon Fluorolog FL-3-22 spectrofluorometer equipped with a 450 W Xe lamp. Quantum yields were determined under ligand excitation on the same instrument, and using absolute method using a home-modified integrating sphere [14].

2.2. Synthesis

Title complexes were obtained by the general procedure given in the following. Ln(Dbm)₃·2H₂O (0.5 mmol) were dissolved in 10 ml of anhydrous MeOH. Solution of 0.5 mmol of related triazole’s ligands in 5 ml of MeOH was added dropwise into Ln(Dbm)₃·2H₂O solutions. The resulting mixture was stirred and refluxed for 3 h. After cooling overnight crystalline precipitate was formed, collected by vacuum filtration and washed with methanol to provide light-yellow solid. The resulting solid was air-dried and recrystallized from MeOH. Single crystals of 2 fit for X-ray diffraction analysis were picked out for measurement.

For 1. Anal. Calc. C₅₈H₄₃SmO₆N₄ C, 66.79; H, 4.16; N, 5.37; found C, 66.52; H, 4.01; N, 5.51. IR (KBr, cm⁻¹): ν = 3061 (m), 1596 (s), 1549 (s), 1519 (s), 1478 (s), 1455 (s), 1403 (s), 1308 (m), 1221 (m), 1068 (m), 725 (m), 608 (m), 508 (w).

For 2. Anal. Calc. for C₆₅H₄₈SmO₆N₅ C, 68.12; H, 4.22; N, 6.11; found C, 68.20; H, 4.54; N, 6.25. IR (KBr, cm⁻¹): ν = 3058 (w), 1614 (s), 1594 (s), 1548 (s), 1518 (s), 1478 (s), 1456 (s), 1409 (s), 1309 (m), 1284 (m), 1064 (m), 720 (m), 608 (m), 503 (w).

For 3. Anal. Calc. for: C₅₈H₄₃NdO₆N₄ C, 67.23; H, 4.18; N, 5.41; found C, 67.41; H, 4.39; N, 5.38. IR (KBr, cm⁻¹): ν = 3064 (m), 1595 (s), 1551 (s), 1519 (s), 1478 (s), 1456 (s), 1406 (s), 1308 (m), 1222 (m), 1068 (m), 721 (m), 611 (m), 509 (w).

For 4. Anal. Calc. for: C₆₅H₄₈NdO₆N₅ C, 68.52; H, 4.25; N, 6.15; found C, 68.77; H, 4.63; N, 6.07. IR (KBr, cm⁻¹): ν = 3058 (w), 1615 (s), 1595 (s), 1549 (s), 1518 (s), 1478 (s), 1456 (s), 1407 (s), 1311 (m), 1282 (m), 1064 (m), 722 (m), 608 (m), 507 (w).

For 5. Anal. Calc. for: C₅₈H₄₃YbO₆N₄ C, 65.41; H, 4.07; N, 5.26; found C, 65.38; H, 4.51; N, 5.14. IR (KBr, cm⁻¹): ν = 3059 (m), 1596 (s), 1551 (s), 1519 (s), 1475 (s), 1454 (s), 1403 (s), 1306 (m), 1222 (m), 1068 (m), 721 (m), 611 (m), 508 (w).

For 6. Anal. Calc. for: C₆₅H₄₈YbO₆N₅ C, 66.83; H, 4.14; N, 5.99; found C, 66.71; H, 4.69; N, 5.87. IR (KBr, cm⁻¹): ν = 3059 (w), 1614 (s), 1595 (s), 1551 (s), 1521 (s), 1478 (s), 1456 (s), 1407 (s), 1310 (m), 1284 (m), 1064 (m), 724 (m), 608 (m), 509 (w).

For 7. Anal. Calc. for: C₅₉H₄₇ErO₇N₄ Calc. C, 64.94; H, 4.34; N, 5.13; found C, 65.02; H, 4.27; N 5.24. IR (KBr, cm⁻¹): ν = 3061 (m), 1596 (s), 1550 (s), 1519 (s), 1478 (s), 1458 (s), 1407 (s), 1311 (m), 1225 (m), 1068 (m), 721 (m), 607 (m), 506 (w).

For 8. Anal. Calc. for: C₆₅H₄₈ErO₆N₅ C, 67.16; H, 4.16; N, 6.02; found C, 67.34; H, 4.02; N, 6.15. IR (KBr, cm⁻¹): ν = 3061 (w), 1615 (s), 1595 (s), 1548 (s), 1523 (s), 1477 (s), 1456 (s), 1411 (s), 1312 (m), 1284 (m), 1065 (m), 724 (m), 609 (m), 507 (w).

2.3. X-ray crystallography

An experimental array of reflection was obtained by the standard method [15] on a Bruker SMART APEX II automated diffractometer equipped with a CCD detector and a monochromatic radiation source (Mo Kα radiation, λ = 0.71073 Å). The structures of L1, L2 and 2 were solved by a direct method and refined in the full-matrix anisotropic approximation for all non-hydrogen atoms. Hydrogen atoms of the carbon-containing ligands were geometrically generated and refined in the riding model. The calculations were performed using the shelx-97 program package. The crystallographic parameters and X-ray diffraction experimental-parameters are given in Table 1.

Table 1.

Crystallographic data for L1, L2 and 2.

Parameter	L1	L2	2
Formula	C13H10N4	C20H15N5	C65H49N5O6Sm
Formula weight	222.25	325.37	1146.44
Crystal dimensions (mm)	0.23 × 0.08 × 0.08	0.33 × 0.14 × 0.12	0.14 × 0.09 × 0.05
T (K)	173	173	173
Crystal system	monoclinic	monoclinic	orthorhombic
Space group	C2/c	P21/c	P212121
Unit cell dimensions
а (Å) b (Å) c (Å) β (°) V (Å3) Z	21.713(7) 4.1548(12) 23.567(7) 96.374(6) 2112.9(11) 8	11.1828(16) 8.8266(13) 16.647(2) 107.894(2) 1563.7(4) 4	9.7971(8) 20.4870(18) 25.634(2) 5145.0(8) 4
dcalc. (g сm−3)	1.397	1.382	1.480
μ (mm−1)	0.09	0.09	1.20
F (0 0 0)	928	680	2336
θmax (°)	2.4–29.5	2.6–31.1	2.2–23.1
Index ranges	−10 ⩽ h ⩽ 26,	−15 ⩽ h ⩽ 15,	−11 ⩽ h ⩽ 12,
	−5 ⩽ k ⩽ 5,	−12 ⩽ k ⩽ 12,	−26 ⩽ k ⩽ 9,
	−29 ⩽ l ⩽ 21	−23 ⩽ l ⩽ 23	−33 ⩽ l ⩽ 23
Reflections measured/independent	4361/2125	18 432/4761	20 996/11 549
R(I > 2σ (I))	R1 = 0.048, wR2 = 0.176	R1 = 0.049, wR2 = 0.139	R1 = 0.043, wR2 = 0.112
Residual electron density (max/min) (е/Å3)	0.28/−0.31	0.40/−0.34	0.54/−0.88

3. Results and discussion

3.1. Syntheses and general descriptions

Ligand L1 was obtained as a light yellow solid in high yield by the reaction of 2-CN-pyridine and benzoic acid’s hydrazide according to the literature [12]. Ligand L2 was synthesized by the condensation of benzaldehyde and 3-(2-aminophenyl)-5-(pyridin-2-yl)-1,2,4-triazole in dry alcohol. The lanthanide complexes Ln(Dbm)₃L were obtained as light yellow crystals in moderate yield by a reaction of the related triazole and Ln(Dbm)₃·2H₂O (Scheme 1).

Scheme 1.

Synthetic route to the Ln(III) complexes.

The molar ratio of lanthanide ion, Dbm and investigated L was determined to be 1:3:1. The TGA–DTA analyses of all the Ln(III) complexes were carried out and no solvent molecules were found. Complexes are stable on air till 290–345 °C. Single crystals of L1, L2 and 2 were grown by recrystallization from ethanol (L1) and methanol (L2 and 2). Considering the photophysical properties, the elemental and thermogravimetric analyses, IR spectroscopy, complexes 1, 3, 5, 7 and 2, 4, 6, 8 have a similar molecular structure.

3.2. Crystal structures of triazole ligands

The structures of the ligands L1 and L2 were determined by X-ray crystallography. 3-Phenyl-5-(pyridin-2-yl)-1,2,4-triazole – L1 crystallizes in monoclinic symmetry with C2/c space group. The ligand L1 is nearly planar with the available lone pairs of the pyridyl and triazole nitrogen atoms in the directions suitable for chelate forming (Fig. 2 a). The angles between the planes defined by the triazole cycle and the benzene rings or pyridyl groups are 4.45° and 14.66°, respectively. The bond’s lengths of triazole molecule are the same with the tabulated values. It is well known that 1,2,4-triazole derivatives have three NH-tautomeric forms [13]. As clearly seen from Fig. 1 3-(pyridine-2-yl)-5-phenyl-1,2,4-triazole exists in IH-tautomeric form. The weak H-bonds to the pyridyl and triazole rings (N2–H…pyridyl, intermolecular contacts have H…A 2.07 Å) lead to form hydrogen bond bridging dimer of title molecules in crystall lattice unit (Fig. 2b). The vacant triazole’s nitrogen atom has no close intermolecular interactions.

Fig. 2.

(a) Molecular structure of the 5-(pyridine-2-yl)-3-phenyl-1,2,4-triazole (b) the molecule packs via H-bonds in crystalline lattice.

The molecular structure of L2 has been determined before, however by spectroscopic methods only. Here we describe the molecular structure of L2 determined by X-ray analysis. 5-Phenyl-2-(2′-pyridiyl)-7,8-benzo-6,5-dihydro-1,3,6-triazaindolizine – L2 crystallizes in monoclinic symmetry with P2₁/c space group. As shown in the previous paper [13] such derivatives can exist in two isomeric forms: azomethine and dihydrotriazaindolizine cyclic forms. Fig. 3 clearly shows that the title triazole is in a cyclic form in solid, which is consistent with our earlier studies in solution. The triazole, iminophenyl, and pyridyl cycles lie almost in the same plane, and the phenyl fragment is turned relatively to the iminophenyl ring by 74.8°. In the title compound, pyridyl and triazole groups are situated at orientations suitable for chelate forming. The N(2)–N(3) bond (1.3568(15) Å) is somewhat shorter than the standard ordinary bond N–N (1.451 Å), which can be explained by the delocalization of double bond in the triazole fragment. Packing in the crystal lattice stabilized primarily by intramolecular hydrogen bonds with the hydrogen atom of secondary amine group and nitrogen atom of pyridine cycle (N1–H…pyridyl N5, intermolecular contact have H…N5 2.249 Å). The formation of hydrogen bonds leads to the formation of zigzag chains extending along the axis b.

Fig. 3.

Molecular structure of the 5-phenyl-2-(2′-pyridiyl)-7,8-benzo-6,5-dihydro-1,3,6-triazaindolizine.

Therefore the results of X-ray studies of the title triazoles show that such heterocyclic systems are potential chelating ligands. It is well known that for the displacement of water molecules from the coordination sphere of fluorescent lanthanide complexes chelating ligands such as 1,10-phenanthroline or 2,2′-dipyridyl are used. Using L1 and L2 ligands as an alternative to 1,10-phenanthroline is promising, not only in terms of water displacement but also to improve the light absorbance properties of such complexes as riazole derivatives actively absorb in the UV and visible regions.

3.3. Crystal structures of complex 2

The structure of Sm(III) complex 2 was determined by single-crystal X-ray diffraction analysis. Complex crystallizes in orthorhombic symmetry with P2₁/c space group and the asymmetric unit consists of only discrete mononuclear complex without solvent. The drawing of 2 is depicted in Fig. 4.

Fig. 4.

Molecular structure of the complex 2. Selected bond lengths (Å) and angles (°). Sm1–O1 2.311 (4), Sm1–O3 2.347 (4), Sm1–O2 2.351 (4), Sm1–O5 2.367 (4), Sm1–O6 2.396 (4), Sm1–O4 2.403 (4), Sm1–N1 2.627 (4), Sm1–N5 2.727 (5), O1–Sm1–O2 71.02 (14), O3–Sm1–O4 72.46 (14), O5–Sm1–O6 69.81 (13), N1–Sm1–N5 62.96 (13).

As expected, Sm(III) ion is eight-coordinated surrounded by six oxygen atoms from three Dbm^- ligands and two nitrogen atoms from L2. The coordination geometry of the metal centre is best described as approximately square antiprismatic, given the geometric constraints imposed by the bite angles of the chelating ligand, with one square plane defined by О1/О2/О5/О6 (average deviation of these atoms from the best-fit mean plane through them 0.068 Å) and the other defined by О3/О4/N1/N5 (average deviation of these atoms from the best-fit mean plane through them 0.136 Å). Two square rings are rotated by an angle of 86.81° with respect to each other. The angle between these two mean planes is 1.12°. The Sm(III) is situated out of the plane of the Dbm^- ring, the angles between the planes defined by Sm–O–O and O–C–C–C–O atoms (where C and O atoms belong to a given 1,3-diketonato ring) are 24.45°, 13.81° and 27.84° for the three ligands. The Sm(III) ion also lies out of the plane of the L2 chelate ring, the angle between the planes is defined by Sm–N1–N5 and N1–N5–C5–C6 atoms being 22.31°. Only one of the three Dbm ligands is relatively planar which is found in “trans” position to L2 in the coordination sphere of Sm(III).

The bond lengths between the samarium ion and the oxygen atoms of Dbm^- are varied from 2.311 to 2.403 Å. This is within the expected range for Sm–O bond lengths in β-diketonates complexes [5]. Slight asymmetry is observed for the bond lengths Sm–N1 and Sm–N5 – 2.627 and 2.727 Å, respectively. The bond lengths of the carbonyl groups are almost the same, indicating that a strong conjugation exists in the chelate rings. Ligand L2 is coordinated in a cyclic molecular form. The structural parameters of L2 are comparable with those obtained for the free ligand. As one of the differences it should be noted that pyridyl ring is rotated in “trans” position to the iminophenyl group. The crystal lattice of Sm(Dbm)₃L2 is characterized by the absence of solvate molecules and additional hydrogen bonds so that packing is governed by van der Waals forces only. This is consistent with the softness of the crystals.

3.4. Photophysical properties

The diffuse reflectance of all complexes was recorded in solid state at room temperature. Spectra of the Ln(III) complexes within the spectral range of ligand-centred transitions were nearly identical, so, only the diffuse reflectance spectra of 1 and 2 complexes are inserted in Fig. 5 as examples.

Fig. 5.

Diffuse reflectance spectra of complexes 1 and 2.

The photophysical properties’ data of investigated compounds are presented in Table 2.

Table 2.

The most relevant photophysical properties of Ln(III) complexes at room temperature.

Complex	UV–Vis (solid) λmax (nm)	Emissiona (solid) λem (nm)	Intensityb (Quant/s · 10−3	QY%c	Rel. enhance.d
Sm(Dbm)3·L1	282, 348	595 645	750 4709	1.8	1.5
Sm(Dbm)3·L2	286, 355	598 645	2080 17 700	5.1	7.1
Nd(Dbm)3·L1	283, 348	882 1064	45.9 105.9	0.3	3.5
Nd(Dbm)3·L2	286, 354	882 1063	73.9 190.9	0.45	6.3
Yb(Dbm)3·L1	283, 347	977 1037	917.6 591.2	0.9	6.5
Yb(Dbm)3·L2	288, 352	979 1017	1199.9 670.3	1.2	8.5
Er(Dbm)3·L1	282, 349	1540	21.9	–e	3.0
Er(Dbm)3·L2	288, 354	1540	20.6	–e	2.7

^aRepresentation of the most intense bands.

^bValue of the luminescence intensity reduced to the same conditions of measurement.

^сThe absolute quantum yield was calculated by QY = A/(R_st − R), where A is the area under the emission peak, R_st and R are the diffuse reflectance of the reflecting standard (BaSO₄) and of the sample, respectively. Estimated error on values of quantum yields is 10%.

^dIncreasing of the maximum intensity of luminescence Ln(Dbm)₃·2H₂O with the binding of L1 or L2.

^eDid not measure.

There are two main peaks at 282–288 and 347–355 nm respectively. The first peak is attributed to π–π^∗ transition of the triazole ligand and the low-energy band is the electronic transitions of Dbm^-.

The emission studies were carried out in the region typical for Ln(III) upon excitation of the ligand absorption bands. Using identical experimental settings (slit width, integration time, excitation wavelength, sampling interval), the direct comparison of the emission intensity of luminescence for the same lanthanide ion with different ancillary ligands was investigated.

3.4.1. Samarium

It is well known that ions Sm(III), together with Eu(III), Tb(III) and Dy(III) form a group whose complexes exhibit ion luminescence in visible region [1, a]. Since the energy of the triplet level of Dbm-anion is located below the resonance level of Tb(III) and Dy(III) ions complexes do not show emission [1]. It was shown previously that europium complexes with title ancillary ligands exhibit intensive red photoluminescence [11]. Luminescence of Sm(III) complexes is generally weaker than for Eu(III) due to smaller difference between the energies of the first excited level ⁴G_5/2 and the nearest lower-lying level ⁶F_11/2; i.e., the probability of nonradiative deactivation of the excited states of this ion is considerably higher. We observed pink-orange luminescence excited at 365 nm, for 1 and 2 complexes in the solid state (Fig. 6). The ligand-centred emission is not detected in solid, suggesting an efficient ligand-to-metal energy transfer process.

Fig. 6.

Corrected luminescence spectra of complex 1 – (2), complex 2 – (1) and Sm(Dbm)₃·2H₂O – (3) in the solid state at room temperature under excitation at 365 nm.

The three main peaks observed are 565, around 605, and 645 nm, assigned to the ⁴G_5/2 → ⁶H_J transitions, where J = 5/2, 7/2, 9/2 respectively, of the Sm(III) ion were detected in spectra. The transition ⁴G_5/2 → ⁶H_9/2 (electric-dipole transition) at around 646 nm is the most dominated. The ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_9/2 bands represent the three main components of fine structure, attributed to additional transitions resulting from Stark splitting of the term due to low symmetry of the complex [16]. The maximum emission intensities of electric-dipole transition (at 646 nm) by using L1 and L2 as extra ligands are enhanced as compared with Sm(Dbm)₃·2H₂O in 1.5 and 7.1 times, respectively. Due to difficulty to compare emission intensities between solid state samples since this intensity depends on the grinding and homogeneity of the sample and we measured the quantum yield which evaluates integrated intensity. The photoluminescence quantum yields of title complexes measured are 1.1% and 5.1% for 1 and 2, respectively. The value of the quantum yield of emission for complex 2 is comparable to the previously described complex Sm(Dbm)₃Phen [17].

3.4.2. Neodymium, erbium and ytterbium

The NIR luminescence properties of Nd(III), Yb(III), Er(III) complexes were investigated in the solid state at room temperature.

Excitation of the ligands’ absorption bands at 365 nm resulted in the emission spectra of the Er complexes 7 and 8. For the emission bands attributed to ⁴I_13/2 → ⁴I_15/2 transitions that cover large spectrum ranges extending from 1450 to 1620 nm and from 1455 to 1610 nm, respectively, both centred around 1540 nm. Unfortunately, the effect of binding ligands L1 and L2 appeared not high and overall intensity of luminescence was very low.

In contrast to the complexes of erbium luminescence intensity complexes of neodymium and ytterbium are higher. As seen in Table 2 the luminescence intensity of Yb(III) complexes 5, 6 is 6–12 times higher than for Nd(III) with the same ligands. This is due to the presence of several sublevels of the ground level of Nd(III) explaining the loss of their radiation energy.

The excitation spectra of the 3 and 4 complexes were obtained by monitoring the characteristic emission of the Nd(III) ion at 1063 nm. In the excitation spectra a broad band ranging from 270 to 410 nm is observed. The maximum excitation spectra show features similar to the reflectance spectra of the related complexes and are red-shifted by about 10–15 nm, thus indicating the indirect excitation by energy transfer from the ligand to the Nd(III) ion. Complexes 3 and 4 show the characteristic emission lines of Nd(III) at 882 nm, 1063 nm and 1340 nm, due to ⁴F_3/2 → ⁴I_J (J = 9/2, 11/2, 13/2) transitions upon ligand-mediated excitation at 365 nm (Fig. 7).

Fig. 7.

Corrected luminescence spectra of complex 3 – (2); complex 4 – (1); Nd(Dbm)₃·2H₂O – (3) in the solid state at room temperature under excitation at 365 nm.

The band at 1063 nm the most intense which is typical for β-diketonate Nd(III) complexes and potentially applicable for laser emission. Binding of L1 and L2 to complexes 3 and 4 leads to increasing luminescence by a factor of 3.5 and 6.3, respectively, comparable with Nd(Dbm)₃·2H₂O but overall quantum yield was not high and reached 0.3% and 0.45%, respectively. It is noteworthy to point to the splitting of bands in the luminescence spectrum of complex 4 (visible as a shoulder at 1063 nm) which indicates that the Nd(III) ion is situated in low-symmetry surroundings.

The emission spectra of the Yb(III) complexes 5 and 6 (Fig. 8) excitation at 365 nm, clearly show the characteristic emission bands of the Yb(III) ion at 977–978 nm. It should be noted that the extremely large splitting of spectral band in the complexes 5 and 6 are not commonly expected.

Fig. 8.

Corrected and normalized luminescence spectra of complex 5 – (2); complex 6 – (1); Yb(Dbm)₃·2H₂O – (3) in the solid state at room temperature under excitation at 365 nm.

Such Stark splitting is not typical for the luminescent Yb(III) complexes and can be associated only with the low symmetry of the complex molecule because one transition ²F_5/2 → ²F_7/2 is observed for Yb(III) [18]. In our opinion significant contribution to the low symmetry of the complexes affords asymmetric structure of ligands, especially L2. This fact leads not only to the splitting of bands in the luminescence spectrum, but also partially removes the forbiddance of f–f electronic transitions thereby increasing emissions [19]. Indeed as for the previous complexes introduction into the inner coordination sphere of L2 leads to more enhancement of emission intensity than using ligand L1. As clearly seen in Table 2 quantum yield decreases in the order 2 > 1, 4 > 3 and 6 > 5, so luminescence efficiency is also higher for the complexes with L2. Thus, L1 and especially L2 ligands can be recommended as a new class of ancillary ligands for enhancement of the luminescence.

4. Conclusions

We have successfully designed new chelate triazole’s ligands and Sm(III), Nd(III), Yb(III) and Er(III) complexes based on it. We studied the ion luminescence properties of prepared complexes. It was shown that title complexes have remarkable emission; high branching ratio should be noted, yielding comparable high emission quantum yields. The intensity and form of the luminescence spectra are clearly determined by the symmetry of the complexes. A significant factor influencing the luminescence intensity is the low symmetry of the complexes due to the asymmetry of ligands. Luminescent Ln(III) complexes appear to be a suitable candidate for enhancing colour rendition of LED devices (Sm), and to be used as materials in telecommunications, bio-analyses, and medical bioprobes (Nd, Er, Yb).