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Controlled polarized luminescence of smectic lanthanide complexes
Accepted Manuscript Controlled polarized luminescence of smectic lanthanide complexes Аndrey А. Knyazev, А.S. Krupin, Benoît Heinrich, Bertrand Donnio, Yuriy G. Galyametdinov PII:
S0143-7208(17)31357-8
DOI:
10.1016/j.dyepig.2017.08.018
Reference:
DYPI 6180
To appear in:
Dyes and Pigments
Received Date: 14 June 2017 Revised Date:
9 August 2017
Accepted Date: 9 August 2017
Please cite this article as: Knyazev АА, Krupin АS, Heinrich Benoî, Donnio B, Galyametdinov YG, Controlled polarized luminescence of smectic lanthanide complexes, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.08.018. 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 proof before it is published in its final 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.
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Controlled polarized luminescence of smectic lanthanide complexes Аndrey А. Knyazev,a А. S. Krupin,a Benoît Heinrich,b Bertrand Donnio,*,b Yuriy G. Galyametdinov*,a,c a
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Physical and Colloid Chemistry Department, Kazan National Research Technological University 420015, Kazan, Russia. b Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS-Université de Strasbourg (UMR 7504), France. E-mail*: [email protected] c Kazan Physical-Technical Institute, RAS, 420029, Kazan, Russia. E-mail*: [email protected]
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Abstract A new series of β-diketonate lanthanide adducts with 5,5'-di(heptadecyl)-2,2'-bipyridine, showing a smectic thermotropic mesomorphism, has been synthesized. The peculiar thermodynamic dependence of the phase transitions with the lanthanide ion is described, and the liquid crystalline properties of the series have been analyzed by SAXS. The luminescence in the solid state for the Eu(III), Sm(III), Yb(III), Er(III) and Nd(III) adducts was investigated. The enhancement of the luminescence by the orientation of the LC domains through film shear deformation is demonstrated.
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Keywords: Lanthanidomesogens, smectic mesophase, photoluminescence, glassy state, polarized
1. Introduction
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In the last decade, there has been considerable activity in the field of lanthanidecontaining liquid crystals (lanthanidomesogens) [1,2,3,4]. Due to their high coordination number (6 ≤ CN ≤ 12), the resulting coordination polyhedra in 4f lanthanide complexes exhibit different geometries than from those traditionally obtained with transition 3d and 4d metal complexes [5], with, for example, nine-coordinate tricapped trigonal prismatic or eight-coordinate square antiprismatic arrangements around the metal ion [6]. These rather unpredictable threedimensional coordination geometries render therefore trivalent lanthanide metal ions more challenging for introduction into thermotropic liquid crystals than transition metal ions [7,8], but give several new attractive opportunities for investigating new structure-mesomorphism relationships [1-4]. The most successful strategy in designing lanthanidomesogens has mainly consisted in coordinating various mesomorphic ligands, wrapping the trivalent ion [1], but mesomorphism could also be induced by the right combination of non-mesomorphic ligands. The first examples of the emergence of liquid crystallinity in lanthanidomesogens with nonmesomorphic ligand were found with lanthanide Schiff’s base complexes with the stoichiometry [Ln(LH)2LX2] (where LH is the non mesomorphic salicylaldimine Schiff’s base, and X is the counterion) and with some Lewis base adduct of tris(β-diketonates ([La(dk)3LH]), respectively [9,10]. 1
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Some elements of the lanthanide series show strong luminescence [11,12,13,14,15] and interesting magnetic properties [16,17]. The emission observed for these materials is metalcentered between energy levels within the 4f shell of the trivalent lanthanide ion. Organic ligands coordinating a lanthanide ion (for example, β-diketonates, Lewis bases) ensure the transfer of the excitation energy onto the emissive ion (antenna effect) [18,19]. The two main advantages of luminescence in trivalent lanthanide ions is: i) their narrow emission lines of a high color purity [20,21,22], and ii) the facile tuning of the emission wavelength by the proper choice of the lanthanide ion: red emission is obtained with Eu(III), green emission with Tb(III), blue emission with Tm(III), orange emission with Sm(III), and near-infrared emission with Nd(III), Er(III) and Yb(III) ions [23]. The mesomorphic properties within this family of compounds bring the anisotropy of the magnetic properties [24,25,26,27,28,29], allowing controlled orientation of magnetic domains, and optical properties, allowing one to create luminescent materials that emit polarized monochromatic light [30,31] in the mesophases. Several other studies on the influence of the luminescence as a function of the orientation of the liquid crystals [32,33,34], LC polymers [35,36], zeolites [37,38,39,40] and lanthanide complexes [41,42,43,44] have been reported and proved very promising to control luminescence properties. Thus, the synthesis and understanding behavior of liquid crystals based on lanthanide ions are timely and pressing challenges for the development of new optoelectronic materials. This study presents the synthesis of a new series of β-diketonate lanthanide adducts with 5,5'-di(heptadecyl)-2,2'-bipyridine (bpy17-17) as co-ligand, which shows thermotropic mesomorphism with a rare example of smectic phases induction in β-diketonates adducts. Only few successful samples of complexes with Lewis bases are known [45,46,47,48,49]. The peculiar thermodynamic dependence of the phase transitions with the lanthanide ions is described, and the liquid crystalline properties of the series have been analyzed by DSC, POM and SAXS. The luminescence in the solid state was investigated for the Eu(III), Sm(III), Yb(III), Er(III) and Nd(III) adducts, and enhancement of the luminescence could be achieved by the orientation of the LC domains through film shear deformation.
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2. Results and Discussion
Synthesis and characterization
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An almost complete series of lanthanidomesogens, namely tris-[1-(4-(4pentylcyclohexyl)phenyl)-3-(thiophen-2-yl)propane-1,3-dionato]-[5,5’-di(heptadecyl)-2,2’bipyridine] lanthanum, of the type [Ln(CPDK5-Th)3(bpy17-17)], where Ln = La(III), Nd(III), Eu(III), Sm(III), Gd(III), Tb(III), Ho(III), Er(III), Yb(III), Lu(III), has been synthesized (Figure 1).
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Figure 1. Structure of the lanthanide(III) β-diketonate complexes [Ln(CPDK5-Th)3(bpy1717)], with Ln = La(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Ho(III), Er(III), Yb(III), Lu(III).
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The synthesis of both the ligands is known and has already been reported elsewhere [48,50]. The general method for the synthesis of the lanthanide complexes [Ln(CPDK5Th)3(bpy17-17)] is straightforward and briefly described now: 0.115 g (0.3 mmol) of 1-(4-(4pentylcyclohexyl)phenyl)-3-(thiophen-2-yl)propane-1,3-dione (CPDK5-Th), 0.063 g (0.1 mmol) of 5,5'-diheptadecyl-2,2'-bipyridine (bpy17-17) and 0.017 g (0.3 mmol) of KOH were dissolved in 20 mL of 96% ethanol, and the mixture stirred and heated to 70°C. Then, was followed the drop wise addition of a solution of La(III)Cl3.5H2O 0.034 g (0.1 mmol) in ethanol (1.5 mL), and the mixture stirred for 5 more min at 70°C. The precipitate was filtered, purified in ethanol and then dried in vacuum above P2O5. All the complexes were fully characterized by elemental analysis and mass spectrometry, and the 1H NMR was performed for the La(III) complex. La(CPDK51 Th)3bpy17-17: Yield: 0.132 g (69%), m.p.: 200°C. H NMR (400 MHz, CDCl3): δ 8.45–8.53 (m, 2H, Pyr H3), 8.23-8.27 (m, 2H, Pyr H6), 7.86–7.90 (m, 6H, Ph), 7.78–7.84 (m, 3H, Th), 7.61– 7.65 (m, 3H, Th), 7.57-7.60 (m, 2H, Pyr H4), 7.28–7.36 (m, 6H, Ph), 7.15–7.18 (m, 3H, Th), 6.42-6.55(m, 3H, CH-CO); 2.61-2.67 (m, 4H, CH2–Pyr), 2.49–2.59 (m, 3H, CH-Ph), 1.88–1.95 (m, 12H, С6H10), 0.99–1.67 (m, 99H, CH2, С6H10), 0.83–0.96 (m, 15H, CH3). Found (%): C, 71.02; H, 9.00; N, 1.34; S, 5.10; La, 7.10. C116H163N2O6S3La. Calcd (%): C, 72.69; H, 8.57; N, 1.46; S, 5.02; La, 7.25. ESI-MS (m/z): 1939.4 (M + Na)+. Nd(CPDK5-Th)3bpy17-17: Yield: 0.135 g (70%), m.p.: 230°C. Found (%): C, 72.03; H, 8.98; N, 1.40; S, 5.02; Nd, 7.50. C116H163N2O6S3Nd. Calcd (%): C, 72.49; H, 8.55; N, 1.46; S, 5.00; Nd, 7.50. ESI-MS (m/z): 1944.7 (M + Na)+. Sm(CPDK5-Th)3bpy17-17: Yield: 0.132 g (68%), m.p.: 228°C. Found (%): C, 71.71; H, 9.05; N, 1.45; S, 4.80; Sm, 7.50. C116H163N2O6S3Sm. Calcd (%): C, 72.26; H, 8.52; N, 1.45; S, 4.99; Sm, 7.80. ESI-MS (m/z): 1950.8 (M + Na)+. Eu(CPDK5-Th)3bpy17-17: Yield: 0.130 g (67%), m.p.: 234°C. Found (%): C, 71.66; H, 8.99; N, 1.35; S, 4.75; Eu, 7.75. C116H163N2O6S3Eu. Calcd (%): C, 72.20; H, 8.51; N, 1.45; S, 4.98; Eu, 7.87. ESI-MS (m/z): 1952.4 (M + Na)+. Gd(CPDK5-Th)3bpy17-17: Yield: 0.129 g (66%), m.p.: 233°C. Found (%): C, 71.55; H, 8.98; N, 1.35; S, 4.76; Gd, 8.00. C116H163N2O6S3Gd. Calcd (%): C, 72.00; H, 8.49; N, 1.45; S, 4.97; Gd, 8.13. ESI-MS (m/z): 1957.7 (M + Na)+. Tb(CPDK5-Th)3bpy17-17: Yield: 0.132 g (70%), m.p.: 236°C. Found (%): C, 71.45; H, 9.02; N, 1.28; S, 4.73; Tb, 8.05. C116H163N2O6S3Tb. Calcd (%): C, 71.94; H, 8.48; N, 1.45; S, 4.97; Tb, 8.21. ESI-MS (m/z): 1959.4 (M + Na)+. Ho(CPDK5-Th)3bpy17-17: Yield: 0.125 g (64%), m.p.: 236°C. Found (%): C, 3
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71.26; H, 8.88; N, 1.31; S, 4.65; Ho, 8.15. C116H163N2O6S3Ho. Calcd (%): C, 71.72; H, 8.46; N, 1.44; S, 4.95; Ho, 8.49. ESI-MS (m/z): 1965.4 (M + Na)+. Er(CPDK5-Th)3bpy17-17: Yield: 0.122 g (63%), m.p.: 232°C. Found (%): C, 71.15; H, 8.95; N, 1.26; S, 4.75; Er, 8.50. C116H163N2O6S3Er. Calcd (%): C, 71.63; H, 8.45; N, 1.44; S, 4.95; Er, 8.60. ESI-MS (m/z): 1967.7 (M + Na)+. Yb(CPDK5-Th)3bpy17-17: Yield: 0.125 g (64%), m.p.: 226°C. Found (%): C, 71.00; H, 8.86; N, 1.19; S, 4.70; Yb, 8.55. C116H163N2O6S3Yb. Calcd (%): C, 71.42; H, 8.42; N, 1.44; S, 4.93; Yb, 8.87. ESI-MS (m/z): 1973.5 (M + Na)+. Lu(CPDK5-Th)3bpy17-17: Yield: 0.127 g (65%), m.p.: 222°C. Found (%): C, 70.88; H, 8.82; N, 1.23; S, 4.66; Lu, 8.75. C116H163N2O6S3Lu. Calcd (%): C, 71.35; H, 8.41; N, 1.43; S, 4.93; Lu, 8.96. ESI-MS (m/z): 1975.4 (M + Na)+. Mesomorphism
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All lanthanide complexes are mesomorphic, despite that both β-diketone 1-(4-(4pentylcyclohexyl)phenyl)-3-(thiophen-2-yl)propane-1,3-dione [50] and 5,5'-diheptadecyl-2,2'bipyridine [51,52] ligands are themselves not liquid-crystalline, melting directly into the isotropic liquid at 104 °C and 82 °C, respectively. The thermal behavior of the Ln(III) adducts is summarized in Table S1 and in the phase diagram shown in Figure 2. The melting points (determined during the first heating treatment) of the complexes do not depend much on the size of the Ln(III) ion, but still nevertheless show a zig-zag variation along the series from the neodymium up to the erbium derivatives, oscillating between ca 140-160°C. The clearing temperatures of the Ln(III) complexes (Table S1, determined on further heating) do not depend on the size of the ion, and they isotropize at around ca 222-236°C, except for the La(III) derivative, which clears at 200°C. The mesophase was identified as an untitled smectic phase, smectic A (SmA) for all but the La(III) adduct, which instead shows likely the more ordered smectic B phase (SmB), as deduced from hot-stage polarizing optical microscopy. Upon cooling from the isotropic melt, typical bâtonnets of SmA-like phase were formed (Figure 3). These bâtonnets coalesce to give rise to a fan-shaped focal conic textures (Figure 4). On further cooling below room temperature, the Ln (III) complexes do not crystallize but the mesophase freezes into a glass. No decomposition of the samples was detected in the entire mesomorphic range, and the smectic phase of the Ln(III) complexes is thermally stable and exists over a wide temperature range of ca. 80°C (Table S1).
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Figure 2. Temperature phase (black) and enthalpies (red) diagrams of the Ln(III) β-diketonate complexes[Ln(CPDK5-Th)3(bpy17-17)]. Abbreviations: Cr = crystalline phase; SmA/B = smectic A/B phase; I = isotropic liquid.
Figure 3. Formation of bâtonnets on cooling from the isotropic melt at 220°C (top), and focal conic-like texture of the smectic A phase at 160 °C, obtained on further cooling (down) of [Sm(CPDK5-Th)3(bpy17-17)] (500×magnification).
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By DSC (Figure 2), it was found that the enthalpy of the Cr→Sm transition (1st heating) decreases with the increasing of the ordinal number of the lanthanide and corresponding decrease in the ion radius. Thus, for light lanthanide ions complexes such as La(III), Nd(III), Sm(III), Eu(III), the enthalpy of about 10 J/g is typical for lanthanidomesogens [53] and the transition comes down to melting from partly crystallized state. All these complexes freeze into a glass on cooling, but partly crystallize before melting on subsequent heating. For the Ho(III), Er(III), and Lu(III) complexes, melting peaks could on the contrary not be identified and the fluid mesophases were therefore directly obtained from initial glassy amorphous or pseudomesomorphous-like states. A possible reason of these different behaviors is the minor changes of the coordination sphere modifying the molecular packing in the crystalline state. Upon cooling down to the ambient temperature, most DSC curves (Figure 4) exhibit deviation from linearity roughly between 50 and 100°C, which likely corresponds to a broad glass transition. On cooling, the mesophases are frozen in a room temperature glassy state preserving the supramolecular packing.
Figure 4. Representative DSC curves: Eu(CPDK5-Th)3(bpy17-17) (top) and Lu(CPDK5-Th)3(bpy1717) (bottom). In contrast, the clearing enthalpies (Sm→I transition) hardly change within the series (8.1±1.5 kJ.mol-1, Figure 2), with two exceptions, and suggests that the molecular organization in 6
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the smectic phase is not influenced by the nature of the Ln(III) ion. Notable exceptions to this finding are the somewhat larger discrepancy for the Gd(III) derivative (5.9 kJ.mol-1), and especially the significantly higher clearing enthalpy of the La(III) complex (11.8 kJ.mol-1). The specificity of the La(III) adduct is also manifested in the associated transition temperature of 200°C, while the clearing temperatures of the rest of the series range between 222 and 236°C. Small-angle X-ray scattering was performed for the La(III), Sm(III), Tb(III) and Lu(III) complexes, and confirmed a local-range structured pristine state (Figure S1) melting to a single lamellar phase (Figure 5) for all the terms of the series. Above melting, the three latter complexes give rise to SAXS patterns with one strong and one weak sharp, equidistant smallangle reflections (001) and (002), arising from the lamellar structure, and with broad wide-angle scattering contributions from lateral distances in the molten state of the aliphatic chains (hch), of the promesogenic parts, CPDK and bpy17-17, (hpMes) and of the entire complexes (distinct broad maximum hcom around 10 Å, only distinguishable for the Lu(III) complex). These structural features comply with a SmA phase, in consistency with optical microscopy. Regarding the La(III) complex, the SAXS pattern deviate from this description by the more numerous and intense lamellar higher-order reflections (extending to (004) instead of (002)) and by the appearance of a semi-diffuse signal hcom, attributed to the intermediate-range correlated arrangement of the entire complexes (hcom= 10-11 Å, associated correlation length (Scherrer): ξ = 100-150 Å). This regular and compact molecular packing layers explains that the layer spacing d is constant throughout the smectic range and that the molecular area Amol (obtained from the ratio of calculated molecular volume and periodicity d) is in agreement with the estimated overall cross-section of the complex (Figure 6). All these structural features fit a hexatic SmBlike structure as occurring in simple calamitic systems [54], except that the symmetry of the intermediate-range correlated lattice might not be hexagonal if the average complex shape deviates sufficiently from a cylindrical. The thermal behavior of the three other derivatives is on the contrary typical for SmA: the distribution of the main molecular axes around layer normal is indeed inherent to the liquid-like lateral packing and explains the expanded Amol-values (exceeding in this case the overall cross-sections by 10-15%) and the further decrease of d at higher temperature.
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Figure 5. Representative SAXS patterns of Ln(CPDK5-Th)3(bpy17-17) in the smectic phases (T = 160°C; Ln(III) = La(III), Sm(III), Tb(III), Lu(III)).
Figure 6. Variations with temperature of the layer spacing (d) and molecular area (Amol) in the smectic phases of Ln(CPDK5-Th)3(bpy17-17) complexes (Ln (symbols): La(III) (squares), Sm(III) 8
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(circles), Tb(III) (up triangles), Lu(III) (down triangles)). Lines are linear fits, used as guides for the eyes.
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These SAXS results further support the deductions made from DSC: SAXS patterns recorded in the room temperature pristine state only display a broad scattering signal from liquid-like lateral distances and a broadened small-angle peak from lamellar periodicities of 3334 Å (Sm(III), Tb(III), Lu(III)) or 37 Å (La(III)), correlated over ca. 20 nm (Sm(III), Tb(III), Lu(III)) or 10 nm (La(III)). The pristine state is thus an amorphous state close to a SmA, except that the layering is not long-range correlated as in the mesophase, but extends to few lamellae only (ca. 6 for Sm(III), Tb(III) et Lu(III), and ca. 2-3 for La(III)). Photophysical properties
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The luminescence spectra for the Nd(III), Sm(III), Eu(III), Gd(III), Er(III), and Yb(III) complexes were recorded in the solid state. Sm(III) and Eu(III) ions are known for their strong red photoluminescence, whereas Nd(III), Er(III) and Yb(III) are good infrared emitters. We used the adduct Gd(CPDK5-Th)3bpy17-17 to study the changes of the photophysical processes in the ligand environment of the Ln(III) ion under UV irradiation (Figure 7). Because of the large energy gap (≈32,000 cm-1) between the ground state 8S7/2 and the first excited state 6P7/2 of the Gd(III) ion, there is no ligand-to-metal energy transfer in the Gd(CPDK5-Th)3bpy17-17 and only the ligand luminescence was registered. Since the atomic number and ionic radius of the other Ln(III) and Gd(III) ions are almost identical, it was expected that the replacement of the other Ln(III) ions by Gd(III) would not change the intersystem crossing efficiency in the ligands.
Figure 7. Luminescence spectrum of [Gd(CPDK5-Th)3bpy17-17) in the solid state, at 77 K.
Since the phosphorescence emission band corresponds to the jump from the phosphorescent state to a number of vibrational states of the ground state, the band at 540 nm corresponds to the green side of the phosphorescence spectra. Thus, the triplet level for CPDK5-1 -1 Th was deduced to be at 18520 cm , which is a little lower than that of TTA anion (20325 cm , 3.14 eV). Therefore ligands can effectively transfer energy only to the Nd(III), Eu(III), Sm(III), Er(III), and Yb(III) ions as shown in the Jablonski diagram (Figure 8). 9
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` Figure 8. Jablonski energy diagram of emissive levels of Ln(III) ions and triplet states of the ligand.
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The luminescence spectra of the Nd(III), Sm(III), Eu(III), erbium(III), and Yb(III) complexes were recorded in the solid state. The emission spectrum of [Eu(CPDK5-Th)3bpy17-17] is shown in Figure 9. The Eu(III) compound showed a strong red photoluminescence upon irradiation with ultraviolet light. The luminescence spectra were measured upon excitation at 408 nm.
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Figure 9. Room-temperature luminescence spectrum of [Eu(CPDK5-Th)3bpy17-17] and [Sm(CPDK5-Th)3bpy17-17] in the solid state (the excitation wavelength was 408 nm).
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The narrow peaks observed in the luminescence spectrum are various transitions between the excited 5D0 level and the different J-levels of the ground term 7F (7FJ, J = 0–4 in the spectrum shown in Figure 8). No transition starting from the 5D1 level could be seen. The most intense line is the hypersensitive transition 5D0→7F2 around 613 nm. The intensity ratio I(5D0→7F2)/I(5D0→7F1) is 17.7 (integrationcalculated in nm or in cm–1 with λ2 correction). Values larger than 10 are typical for Eu(III) β-diketonate complexes [55]. The observed luminescence lifetime was determined from measurement of the luminescence decay curve and was found to be 0.141 ms at room temperature. The decay curve was a single exponential curve. This supports the fact that only one type of Eu(III) site is present. The Sm(III) compound showed an orange photoluminescence upon irradiation with ultraviolet light (Figure 9). The luminescence spectra were measured upon excitation at 408 nm. The Sm(III) complex shows emission bands at 565 nm, 605 nm, 650 nm and 701 nm which can be assigned to 4G5/2 → 6HJ transitions, where J = 5/2, 7/2 9/2, and 11/2, respectively. However, the transition of 4G5/2 → 6H9/2 is the most dominant. The observed luminescence lifetime was determined from measurement of the luminescence decay curve and was found to be 0.014 ms at room temperature. The decay curve was a single exponential curve. This supports the fact that only one type of Sm(III) site is present. The Sm(III) compound showed characteristic bands at 887, 909, 927, 952 and 1036 nm which are corresponding to transitions between the excited 4G5/2 level and the different 6FJ levels of the ground term (J = 1/2, 3/2, 5/2, 7/2 и 9/2) (Figure 10). The peaks at 1130, 1210 and 1300 nm correspond to the second harmonic of transitions between the excited 4G5/2 level and the different 6HJ levels of the ground multiplet (J = 5/2, 7/2, 9/2).
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Figure 10. Room-temperature luminescence spectra of (a) [Sm(CPDK5-Th)3bpy17-17], (b) [Yb(CPDK5-Th)3bpy17-17], (c) [Er(CPDK5-Th)3bpy17-17] and (d) [Nd(CPDK5-Th)3bpy17-17] in the solid state (the excitation wavelength was 408 nm).
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The emission spectrum of [Yb(CPDK5-Th)3bpy17-17] consists of several intense lines at 970 nm, corresponding to the 2F5/2→2F7/2 transition (Figure 10). Only one line could be observed in the luminescence spectrum of the Er(III) compound: the 4I13/2→4I15/2 line at 1530 nm in the near-infrared part of the spectrum (Figure 10). The emission spectrum of [Nd(CPDK5-Th)3bpy174 4 17] consists of three intense lines at 877, 1058 and 1337nm, corresponding to the F3/2→ IJ transition (J=9/2, 11/2, 13/2) (Figure 10). The intensity of the luminescence could moreover be tuned by the orientation of the LC domains through shear deformation in the SmA mesophase (at 200 °C). The [Eu(CPDK5Th)3bpy17-17] and [Sm(CPDK5-Th)3bpy17-17] complexes were oriented in the mesophase at 200°C between two quartz plates using a shear deformation. The orientation was visually controlled by the texture observed under a microscope. Once oriented, the samples were placed on a cooled metal plate (maintained at 0°C) to be quickly frozen down to room temperature in the glassy state. The corresponding polarized luminescence spectra of [Eu(CPDK5-Th)3bpy17-17] and [Sm(CPDK5-Th)3bpy17-17] are presented in Figures 11-12.
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Figure 11. Luminescence spectra of thin films between quartz plates (non-oriented and oriented) [Eu(CPDK5-Th)3bpy17-17] and [Sm(CPDK5-Th)3bpy17-17].
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The luminescence spectra reveal increase of the emission intensities in the oriented films compared to that of the non-oriented ones: by ca. 50 % for the frozen smectic A phase of [Eu(CPDK5-Th)3bpy17-17] and by 24% for [Sm(CPDK5-Th)3bpy17-17] (Figure 11). This luminescence enhancement primarily likely originates from the alignment of the optical axes of the LC domains in the oriented films, which reduces scattering and improves the transparency. However the alignment of the domains also likely combines with a somewhat modified average orientation of the molecules in the films, which could in turn contribute to the differences in the luminescence intensities. The luminescence spectra of the two samples with parallel and perpendicular alignment relative to the polarization plane of an excitation source (Figure 12) were also recorded. The excitation light beam (408 nm) was unpolarized. The samples were placed in the sample compartment of the spectrofluorimeter in such a position that the luminescence could be measured in the transmission mode (scheme, Figure 12). A polarizer could be placed at the exit of the monochromator. The polarization direction was parallel (para) to the alignment layer in the films. To eliminate any influence of the polarization by the instrument, the aligned film was turned by +90° to the orthogonal position (ortho) and compared. It can be seen that it is also 13
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possible to tune the luminescence intensity by twofold for the Eu(III) complex, and by ca 60% for the Sm(III) complex.
Figure 12. Scheme of polarized luminescence of oriented thin films between quartz plates and results for [Eu(CPDK5-Th)3bpy17-17] and [Sm(CPDK5-Th)3bpy17-17] films. 3. Conclusions 14
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An almost complete series of β-diketonate Ln(III) adducts with the non-mesomorphous 5,5'-di(heptadecyl)-2,2'-bipyridine (bpy17-17) ligand was prepared. The complexes exhibit a broad range thermally stable smectic phase (SmA or SmB), and upon cooling till ambient temperatures, are vitrified with preservation of the supramolecular packing adopted in the mesophase. The ligands were selected from the energy levels (Jablonsky diagram) and provided efficient energy transfer to Nd(III), Sm(III), Eu(III), Er(III), and Yb(III) ions. Red, green and NIR effective luminophores were obtained. The presence of LC properties in lanthanide derivatives combined with their optical behavior allowed us to demonstrate the possibility of polarized luminescence intensity and frequency tuning by changing of relative molecular orientation and type of Ln(III) ion.
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Supporting Information Table with complete data of the mesomorphic behavior, pristine SAXS patterns, volumetric data, and experimental equipements.
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Acknowledgements The liquid crystalline properties were studied in the frame of the RFBR grant №17-0300258a. AAK thanks grant of the President of the Russian Federation, No. MD-6102.2016.3 for supporting synthesis of samples. BD and BH thank the CNRS and the University of Strasbourg for support.
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Highlights: 1. Smectic lanthanidomesogens; 2. Liquid-crystal-enhanced photoluminescence; 3. Polarized luminescence;
S0143-7208(17)31357-8
DOI:
10.1016/j.dyepig.2017.08.018
Reference:
DYPI 6180
To appear in:
Dyes and Pigments
Received Date: 14 June 2017 Revised Date:
9 August 2017
Accepted Date: 9 August 2017
Please cite this article as: Knyazev АА, Krupin АS, Heinrich Benoî, Donnio B, Galyametdinov YG, Controlled polarized luminescence of smectic lanthanide complexes, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.08.018. 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 proof before it is published in its final 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.
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Controlled polarized luminescence of smectic lanthanide complexes Аndrey А. Knyazev,a А. S. Krupin,a Benoît Heinrich,b Bertrand Donnio,*,b Yuriy G. Galyametdinov*,a,c a
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Physical and Colloid Chemistry Department, Kazan National Research Technological University 420015, Kazan, Russia. b Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS-Université de Strasbourg (UMR 7504), France. E-mail*: [email protected] c Kazan Physical-Technical Institute, RAS, 420029, Kazan, Russia. E-mail*: [email protected]
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Abstract A new series of β-diketonate lanthanide adducts with 5,5'-di(heptadecyl)-2,2'-bipyridine, showing a smectic thermotropic mesomorphism, has been synthesized. The peculiar thermodynamic dependence of the phase transitions with the lanthanide ion is described, and the liquid crystalline properties of the series have been analyzed by SAXS. The luminescence in the solid state for the Eu(III), Sm(III), Yb(III), Er(III) and Nd(III) adducts was investigated. The enhancement of the luminescence by the orientation of the LC domains through film shear deformation is demonstrated.
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Keywords: Lanthanidomesogens, smectic mesophase, photoluminescence, glassy state, polarized
1. Introduction
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In the last decade, there has been considerable activity in the field of lanthanidecontaining liquid crystals (lanthanidomesogens) [1,2,3,4]. Due to their high coordination number (6 ≤ CN ≤ 12), the resulting coordination polyhedra in 4f lanthanide complexes exhibit different geometries than from those traditionally obtained with transition 3d and 4d metal complexes [5], with, for example, nine-coordinate tricapped trigonal prismatic or eight-coordinate square antiprismatic arrangements around the metal ion [6]. These rather unpredictable threedimensional coordination geometries render therefore trivalent lanthanide metal ions more challenging for introduction into thermotropic liquid crystals than transition metal ions [7,8], but give several new attractive opportunities for investigating new structure-mesomorphism relationships [1-4]. The most successful strategy in designing lanthanidomesogens has mainly consisted in coordinating various mesomorphic ligands, wrapping the trivalent ion [1], but mesomorphism could also be induced by the right combination of non-mesomorphic ligands. The first examples of the emergence of liquid crystallinity in lanthanidomesogens with nonmesomorphic ligand were found with lanthanide Schiff’s base complexes with the stoichiometry [Ln(LH)2LX2] (where LH is the non mesomorphic salicylaldimine Schiff’s base, and X is the counterion) and with some Lewis base adduct of tris(β-diketonates ([La(dk)3LH]), respectively [9,10]. 1
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Some elements of the lanthanide series show strong luminescence [11,12,13,14,15] and interesting magnetic properties [16,17]. The emission observed for these materials is metalcentered between energy levels within the 4f shell of the trivalent lanthanide ion. Organic ligands coordinating a lanthanide ion (for example, β-diketonates, Lewis bases) ensure the transfer of the excitation energy onto the emissive ion (antenna effect) [18,19]. The two main advantages of luminescence in trivalent lanthanide ions is: i) their narrow emission lines of a high color purity [20,21,22], and ii) the facile tuning of the emission wavelength by the proper choice of the lanthanide ion: red emission is obtained with Eu(III), green emission with Tb(III), blue emission with Tm(III), orange emission with Sm(III), and near-infrared emission with Nd(III), Er(III) and Yb(III) ions [23]. The mesomorphic properties within this family of compounds bring the anisotropy of the magnetic properties [24,25,26,27,28,29], allowing controlled orientation of magnetic domains, and optical properties, allowing one to create luminescent materials that emit polarized monochromatic light [30,31] in the mesophases. Several other studies on the influence of the luminescence as a function of the orientation of the liquid crystals [32,33,34], LC polymers [35,36], zeolites [37,38,39,40] and lanthanide complexes [41,42,43,44] have been reported and proved very promising to control luminescence properties. Thus, the synthesis and understanding behavior of liquid crystals based on lanthanide ions are timely and pressing challenges for the development of new optoelectronic materials. This study presents the synthesis of a new series of β-diketonate lanthanide adducts with 5,5'-di(heptadecyl)-2,2'-bipyridine (bpy17-17) as co-ligand, which shows thermotropic mesomorphism with a rare example of smectic phases induction in β-diketonates adducts. Only few successful samples of complexes with Lewis bases are known [45,46,47,48,49]. The peculiar thermodynamic dependence of the phase transitions with the lanthanide ions is described, and the liquid crystalline properties of the series have been analyzed by DSC, POM and SAXS. The luminescence in the solid state was investigated for the Eu(III), Sm(III), Yb(III), Er(III) and Nd(III) adducts, and enhancement of the luminescence could be achieved by the orientation of the LC domains through film shear deformation.
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2. Results and Discussion
Synthesis and characterization
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An almost complete series of lanthanidomesogens, namely tris-[1-(4-(4pentylcyclohexyl)phenyl)-3-(thiophen-2-yl)propane-1,3-dionato]-[5,5’-di(heptadecyl)-2,2’bipyridine] lanthanum, of the type [Ln(CPDK5-Th)3(bpy17-17)], where Ln = La(III), Nd(III), Eu(III), Sm(III), Gd(III), Tb(III), Ho(III), Er(III), Yb(III), Lu(III), has been synthesized (Figure 1).
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Figure 1. Structure of the lanthanide(III) β-diketonate complexes [Ln(CPDK5-Th)3(bpy1717)], with Ln = La(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Ho(III), Er(III), Yb(III), Lu(III).
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The synthesis of both the ligands is known and has already been reported elsewhere [48,50]. The general method for the synthesis of the lanthanide complexes [Ln(CPDK5Th)3(bpy17-17)] is straightforward and briefly described now: 0.115 g (0.3 mmol) of 1-(4-(4pentylcyclohexyl)phenyl)-3-(thiophen-2-yl)propane-1,3-dione (CPDK5-Th), 0.063 g (0.1 mmol) of 5,5'-diheptadecyl-2,2'-bipyridine (bpy17-17) and 0.017 g (0.3 mmol) of KOH were dissolved in 20 mL of 96% ethanol, and the mixture stirred and heated to 70°C. Then, was followed the drop wise addition of a solution of La(III)Cl3.5H2O 0.034 g (0.1 mmol) in ethanol (1.5 mL), and the mixture stirred for 5 more min at 70°C. The precipitate was filtered, purified in ethanol and then dried in vacuum above P2O5. All the complexes were fully characterized by elemental analysis and mass spectrometry, and the 1H NMR was performed for the La(III) complex. La(CPDK51 Th)3bpy17-17: Yield: 0.132 g (69%), m.p.: 200°C. H NMR (400 MHz, CDCl3): δ 8.45–8.53 (m, 2H, Pyr H3), 8.23-8.27 (m, 2H, Pyr H6), 7.86–7.90 (m, 6H, Ph), 7.78–7.84 (m, 3H, Th), 7.61– 7.65 (m, 3H, Th), 7.57-7.60 (m, 2H, Pyr H4), 7.28–7.36 (m, 6H, Ph), 7.15–7.18 (m, 3H, Th), 6.42-6.55(m, 3H, CH-CO); 2.61-2.67 (m, 4H, CH2–Pyr), 2.49–2.59 (m, 3H, CH-Ph), 1.88–1.95 (m, 12H, С6H10), 0.99–1.67 (m, 99H, CH2, С6H10), 0.83–0.96 (m, 15H, CH3). Found (%): C, 71.02; H, 9.00; N, 1.34; S, 5.10; La, 7.10. C116H163N2O6S3La. Calcd (%): C, 72.69; H, 8.57; N, 1.46; S, 5.02; La, 7.25. ESI-MS (m/z): 1939.4 (M + Na)+. Nd(CPDK5-Th)3bpy17-17: Yield: 0.135 g (70%), m.p.: 230°C. Found (%): C, 72.03; H, 8.98; N, 1.40; S, 5.02; Nd, 7.50. C116H163N2O6S3Nd. Calcd (%): C, 72.49; H, 8.55; N, 1.46; S, 5.00; Nd, 7.50. ESI-MS (m/z): 1944.7 (M + Na)+. Sm(CPDK5-Th)3bpy17-17: Yield: 0.132 g (68%), m.p.: 228°C. Found (%): C, 71.71; H, 9.05; N, 1.45; S, 4.80; Sm, 7.50. C116H163N2O6S3Sm. Calcd (%): C, 72.26; H, 8.52; N, 1.45; S, 4.99; Sm, 7.80. ESI-MS (m/z): 1950.8 (M + Na)+. Eu(CPDK5-Th)3bpy17-17: Yield: 0.130 g (67%), m.p.: 234°C. Found (%): C, 71.66; H, 8.99; N, 1.35; S, 4.75; Eu, 7.75. C116H163N2O6S3Eu. Calcd (%): C, 72.20; H, 8.51; N, 1.45; S, 4.98; Eu, 7.87. ESI-MS (m/z): 1952.4 (M + Na)+. Gd(CPDK5-Th)3bpy17-17: Yield: 0.129 g (66%), m.p.: 233°C. Found (%): C, 71.55; H, 8.98; N, 1.35; S, 4.76; Gd, 8.00. C116H163N2O6S3Gd. Calcd (%): C, 72.00; H, 8.49; N, 1.45; S, 4.97; Gd, 8.13. ESI-MS (m/z): 1957.7 (M + Na)+. Tb(CPDK5-Th)3bpy17-17: Yield: 0.132 g (70%), m.p.: 236°C. Found (%): C, 71.45; H, 9.02; N, 1.28; S, 4.73; Tb, 8.05. C116H163N2O6S3Tb. Calcd (%): C, 71.94; H, 8.48; N, 1.45; S, 4.97; Tb, 8.21. ESI-MS (m/z): 1959.4 (M + Na)+. Ho(CPDK5-Th)3bpy17-17: Yield: 0.125 g (64%), m.p.: 236°C. Found (%): C, 3
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71.26; H, 8.88; N, 1.31; S, 4.65; Ho, 8.15. C116H163N2O6S3Ho. Calcd (%): C, 71.72; H, 8.46; N, 1.44; S, 4.95; Ho, 8.49. ESI-MS (m/z): 1965.4 (M + Na)+. Er(CPDK5-Th)3bpy17-17: Yield: 0.122 g (63%), m.p.: 232°C. Found (%): C, 71.15; H, 8.95; N, 1.26; S, 4.75; Er, 8.50. C116H163N2O6S3Er. Calcd (%): C, 71.63; H, 8.45; N, 1.44; S, 4.95; Er, 8.60. ESI-MS (m/z): 1967.7 (M + Na)+. Yb(CPDK5-Th)3bpy17-17: Yield: 0.125 g (64%), m.p.: 226°C. Found (%): C, 71.00; H, 8.86; N, 1.19; S, 4.70; Yb, 8.55. C116H163N2O6S3Yb. Calcd (%): C, 71.42; H, 8.42; N, 1.44; S, 4.93; Yb, 8.87. ESI-MS (m/z): 1973.5 (M + Na)+. Lu(CPDK5-Th)3bpy17-17: Yield: 0.127 g (65%), m.p.: 222°C. Found (%): C, 70.88; H, 8.82; N, 1.23; S, 4.66; Lu, 8.75. C116H163N2O6S3Lu. Calcd (%): C, 71.35; H, 8.41; N, 1.43; S, 4.93; Lu, 8.96. ESI-MS (m/z): 1975.4 (M + Na)+. Mesomorphism
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All lanthanide complexes are mesomorphic, despite that both β-diketone 1-(4-(4pentylcyclohexyl)phenyl)-3-(thiophen-2-yl)propane-1,3-dione [50] and 5,5'-diheptadecyl-2,2'bipyridine [51,52] ligands are themselves not liquid-crystalline, melting directly into the isotropic liquid at 104 °C and 82 °C, respectively. The thermal behavior of the Ln(III) adducts is summarized in Table S1 and in the phase diagram shown in Figure 2. The melting points (determined during the first heating treatment) of the complexes do not depend much on the size of the Ln(III) ion, but still nevertheless show a zig-zag variation along the series from the neodymium up to the erbium derivatives, oscillating between ca 140-160°C. The clearing temperatures of the Ln(III) complexes (Table S1, determined on further heating) do not depend on the size of the ion, and they isotropize at around ca 222-236°C, except for the La(III) derivative, which clears at 200°C. The mesophase was identified as an untitled smectic phase, smectic A (SmA) for all but the La(III) adduct, which instead shows likely the more ordered smectic B phase (SmB), as deduced from hot-stage polarizing optical microscopy. Upon cooling from the isotropic melt, typical bâtonnets of SmA-like phase were formed (Figure 3). These bâtonnets coalesce to give rise to a fan-shaped focal conic textures (Figure 4). On further cooling below room temperature, the Ln (III) complexes do not crystallize but the mesophase freezes into a glass. No decomposition of the samples was detected in the entire mesomorphic range, and the smectic phase of the Ln(III) complexes is thermally stable and exists over a wide temperature range of ca. 80°C (Table S1).
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Figure 2. Temperature phase (black) and enthalpies (red) diagrams of the Ln(III) β-diketonate complexes[Ln(CPDK5-Th)3(bpy17-17)]. Abbreviations: Cr = crystalline phase; SmA/B = smectic A/B phase; I = isotropic liquid.
Figure 3. Formation of bâtonnets on cooling from the isotropic melt at 220°C (top), and focal conic-like texture of the smectic A phase at 160 °C, obtained on further cooling (down) of [Sm(CPDK5-Th)3(bpy17-17)] (500×magnification).
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By DSC (Figure 2), it was found that the enthalpy of the Cr→Sm transition (1st heating) decreases with the increasing of the ordinal number of the lanthanide and corresponding decrease in the ion radius. Thus, for light lanthanide ions complexes such as La(III), Nd(III), Sm(III), Eu(III), the enthalpy of about 10 J/g is typical for lanthanidomesogens [53] and the transition comes down to melting from partly crystallized state. All these complexes freeze into a glass on cooling, but partly crystallize before melting on subsequent heating. For the Ho(III), Er(III), and Lu(III) complexes, melting peaks could on the contrary not be identified and the fluid mesophases were therefore directly obtained from initial glassy amorphous or pseudomesomorphous-like states. A possible reason of these different behaviors is the minor changes of the coordination sphere modifying the molecular packing in the crystalline state. Upon cooling down to the ambient temperature, most DSC curves (Figure 4) exhibit deviation from linearity roughly between 50 and 100°C, which likely corresponds to a broad glass transition. On cooling, the mesophases are frozen in a room temperature glassy state preserving the supramolecular packing.
Figure 4. Representative DSC curves: Eu(CPDK5-Th)3(bpy17-17) (top) and Lu(CPDK5-Th)3(bpy1717) (bottom). In contrast, the clearing enthalpies (Sm→I transition) hardly change within the series (8.1±1.5 kJ.mol-1, Figure 2), with two exceptions, and suggests that the molecular organization in 6
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the smectic phase is not influenced by the nature of the Ln(III) ion. Notable exceptions to this finding are the somewhat larger discrepancy for the Gd(III) derivative (5.9 kJ.mol-1), and especially the significantly higher clearing enthalpy of the La(III) complex (11.8 kJ.mol-1). The specificity of the La(III) adduct is also manifested in the associated transition temperature of 200°C, while the clearing temperatures of the rest of the series range between 222 and 236°C. Small-angle X-ray scattering was performed for the La(III), Sm(III), Tb(III) and Lu(III) complexes, and confirmed a local-range structured pristine state (Figure S1) melting to a single lamellar phase (Figure 5) for all the terms of the series. Above melting, the three latter complexes give rise to SAXS patterns with one strong and one weak sharp, equidistant smallangle reflections (001) and (002), arising from the lamellar structure, and with broad wide-angle scattering contributions from lateral distances in the molten state of the aliphatic chains (hch), of the promesogenic parts, CPDK and bpy17-17, (hpMes) and of the entire complexes (distinct broad maximum hcom around 10 Å, only distinguishable for the Lu(III) complex). These structural features comply with a SmA phase, in consistency with optical microscopy. Regarding the La(III) complex, the SAXS pattern deviate from this description by the more numerous and intense lamellar higher-order reflections (extending to (004) instead of (002)) and by the appearance of a semi-diffuse signal hcom, attributed to the intermediate-range correlated arrangement of the entire complexes (hcom= 10-11 Å, associated correlation length (Scherrer): ξ = 100-150 Å). This regular and compact molecular packing layers explains that the layer spacing d is constant throughout the smectic range and that the molecular area Amol (obtained from the ratio of calculated molecular volume and periodicity d) is in agreement with the estimated overall cross-section of the complex (Figure 6). All these structural features fit a hexatic SmBlike structure as occurring in simple calamitic systems [54], except that the symmetry of the intermediate-range correlated lattice might not be hexagonal if the average complex shape deviates sufficiently from a cylindrical. The thermal behavior of the three other derivatives is on the contrary typical for SmA: the distribution of the main molecular axes around layer normal is indeed inherent to the liquid-like lateral packing and explains the expanded Amol-values (exceeding in this case the overall cross-sections by 10-15%) and the further decrease of d at higher temperature.
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Figure 5. Representative SAXS patterns of Ln(CPDK5-Th)3(bpy17-17) in the smectic phases (T = 160°C; Ln(III) = La(III), Sm(III), Tb(III), Lu(III)).
Figure 6. Variations with temperature of the layer spacing (d) and molecular area (Amol) in the smectic phases of Ln(CPDK5-Th)3(bpy17-17) complexes (Ln (symbols): La(III) (squares), Sm(III) 8
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(circles), Tb(III) (up triangles), Lu(III) (down triangles)). Lines are linear fits, used as guides for the eyes.
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These SAXS results further support the deductions made from DSC: SAXS patterns recorded in the room temperature pristine state only display a broad scattering signal from liquid-like lateral distances and a broadened small-angle peak from lamellar periodicities of 3334 Å (Sm(III), Tb(III), Lu(III)) or 37 Å (La(III)), correlated over ca. 20 nm (Sm(III), Tb(III), Lu(III)) or 10 nm (La(III)). The pristine state is thus an amorphous state close to a SmA, except that the layering is not long-range correlated as in the mesophase, but extends to few lamellae only (ca. 6 for Sm(III), Tb(III) et Lu(III), and ca. 2-3 for La(III)). Photophysical properties
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The luminescence spectra for the Nd(III), Sm(III), Eu(III), Gd(III), Er(III), and Yb(III) complexes were recorded in the solid state. Sm(III) and Eu(III) ions are known for their strong red photoluminescence, whereas Nd(III), Er(III) and Yb(III) are good infrared emitters. We used the adduct Gd(CPDK5-Th)3bpy17-17 to study the changes of the photophysical processes in the ligand environment of the Ln(III) ion under UV irradiation (Figure 7). Because of the large energy gap (≈32,000 cm-1) between the ground state 8S7/2 and the first excited state 6P7/2 of the Gd(III) ion, there is no ligand-to-metal energy transfer in the Gd(CPDK5-Th)3bpy17-17 and only the ligand luminescence was registered. Since the atomic number and ionic radius of the other Ln(III) and Gd(III) ions are almost identical, it was expected that the replacement of the other Ln(III) ions by Gd(III) would not change the intersystem crossing efficiency in the ligands.
Figure 7. Luminescence spectrum of [Gd(CPDK5-Th)3bpy17-17) in the solid state, at 77 K.
Since the phosphorescence emission band corresponds to the jump from the phosphorescent state to a number of vibrational states of the ground state, the band at 540 nm corresponds to the green side of the phosphorescence spectra. Thus, the triplet level for CPDK5-1 -1 Th was deduced to be at 18520 cm , which is a little lower than that of TTA anion (20325 cm , 3.14 eV). Therefore ligands can effectively transfer energy only to the Nd(III), Eu(III), Sm(III), Er(III), and Yb(III) ions as shown in the Jablonski diagram (Figure 8). 9
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` Figure 8. Jablonski energy diagram of emissive levels of Ln(III) ions and triplet states of the ligand.
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The luminescence spectra of the Nd(III), Sm(III), Eu(III), erbium(III), and Yb(III) complexes were recorded in the solid state. The emission spectrum of [Eu(CPDK5-Th)3bpy17-17] is shown in Figure 9. The Eu(III) compound showed a strong red photoluminescence upon irradiation with ultraviolet light. The luminescence spectra were measured upon excitation at 408 nm.
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Figure 9. Room-temperature luminescence spectrum of [Eu(CPDK5-Th)3bpy17-17] and [Sm(CPDK5-Th)3bpy17-17] in the solid state (the excitation wavelength was 408 nm).
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The narrow peaks observed in the luminescence spectrum are various transitions between the excited 5D0 level and the different J-levels of the ground term 7F (7FJ, J = 0–4 in the spectrum shown in Figure 8). No transition starting from the 5D1 level could be seen. The most intense line is the hypersensitive transition 5D0→7F2 around 613 nm. The intensity ratio I(5D0→7F2)/I(5D0→7F1) is 17.7 (integrationcalculated in nm or in cm–1 with λ2 correction). Values larger than 10 are typical for Eu(III) β-diketonate complexes [55]. The observed luminescence lifetime was determined from measurement of the luminescence decay curve and was found to be 0.141 ms at room temperature. The decay curve was a single exponential curve. This supports the fact that only one type of Eu(III) site is present. The Sm(III) compound showed an orange photoluminescence upon irradiation with ultraviolet light (Figure 9). The luminescence spectra were measured upon excitation at 408 nm. The Sm(III) complex shows emission bands at 565 nm, 605 nm, 650 nm and 701 nm which can be assigned to 4G5/2 → 6HJ transitions, where J = 5/2, 7/2 9/2, and 11/2, respectively. However, the transition of 4G5/2 → 6H9/2 is the most dominant. The observed luminescence lifetime was determined from measurement of the luminescence decay curve and was found to be 0.014 ms at room temperature. The decay curve was a single exponential curve. This supports the fact that only one type of Sm(III) site is present. The Sm(III) compound showed characteristic bands at 887, 909, 927, 952 and 1036 nm which are corresponding to transitions between the excited 4G5/2 level and the different 6FJ levels of the ground term (J = 1/2, 3/2, 5/2, 7/2 и 9/2) (Figure 10). The peaks at 1130, 1210 and 1300 nm correspond to the second harmonic of transitions between the excited 4G5/2 level and the different 6HJ levels of the ground multiplet (J = 5/2, 7/2, 9/2).
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Figure 10. Room-temperature luminescence spectra of (a) [Sm(CPDK5-Th)3bpy17-17], (b) [Yb(CPDK5-Th)3bpy17-17], (c) [Er(CPDK5-Th)3bpy17-17] and (d) [Nd(CPDK5-Th)3bpy17-17] in the solid state (the excitation wavelength was 408 nm).
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The emission spectrum of [Yb(CPDK5-Th)3bpy17-17] consists of several intense lines at 970 nm, corresponding to the 2F5/2→2F7/2 transition (Figure 10). Only one line could be observed in the luminescence spectrum of the Er(III) compound: the 4I13/2→4I15/2 line at 1530 nm in the near-infrared part of the spectrum (Figure 10). The emission spectrum of [Nd(CPDK5-Th)3bpy174 4 17] consists of three intense lines at 877, 1058 and 1337nm, corresponding to the F3/2→ IJ transition (J=9/2, 11/2, 13/2) (Figure 10). The intensity of the luminescence could moreover be tuned by the orientation of the LC domains through shear deformation in the SmA mesophase (at 200 °C). The [Eu(CPDK5Th)3bpy17-17] and [Sm(CPDK5-Th)3bpy17-17] complexes were oriented in the mesophase at 200°C between two quartz plates using a shear deformation. The orientation was visually controlled by the texture observed under a microscope. Once oriented, the samples were placed on a cooled metal plate (maintained at 0°C) to be quickly frozen down to room temperature in the glassy state. The corresponding polarized luminescence spectra of [Eu(CPDK5-Th)3bpy17-17] and [Sm(CPDK5-Th)3bpy17-17] are presented in Figures 11-12.
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Figure 11. Luminescence spectra of thin films between quartz plates (non-oriented and oriented) [Eu(CPDK5-Th)3bpy17-17] and [Sm(CPDK5-Th)3bpy17-17].
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The luminescence spectra reveal increase of the emission intensities in the oriented films compared to that of the non-oriented ones: by ca. 50 % for the frozen smectic A phase of [Eu(CPDK5-Th)3bpy17-17] and by 24% for [Sm(CPDK5-Th)3bpy17-17] (Figure 11). This luminescence enhancement primarily likely originates from the alignment of the optical axes of the LC domains in the oriented films, which reduces scattering and improves the transparency. However the alignment of the domains also likely combines with a somewhat modified average orientation of the molecules in the films, which could in turn contribute to the differences in the luminescence intensities. The luminescence spectra of the two samples with parallel and perpendicular alignment relative to the polarization plane of an excitation source (Figure 12) were also recorded. The excitation light beam (408 nm) was unpolarized. The samples were placed in the sample compartment of the spectrofluorimeter in such a position that the luminescence could be measured in the transmission mode (scheme, Figure 12). A polarizer could be placed at the exit of the monochromator. The polarization direction was parallel (para) to the alignment layer in the films. To eliminate any influence of the polarization by the instrument, the aligned film was turned by +90° to the orthogonal position (ortho) and compared. It can be seen that it is also 13
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possible to tune the luminescence intensity by twofold for the Eu(III) complex, and by ca 60% for the Sm(III) complex.
Figure 12. Scheme of polarized luminescence of oriented thin films between quartz plates and results for [Eu(CPDK5-Th)3bpy17-17] and [Sm(CPDK5-Th)3bpy17-17] films. 3. Conclusions 14
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An almost complete series of β-diketonate Ln(III) adducts with the non-mesomorphous 5,5'-di(heptadecyl)-2,2'-bipyridine (bpy17-17) ligand was prepared. The complexes exhibit a broad range thermally stable smectic phase (SmA or SmB), and upon cooling till ambient temperatures, are vitrified with preservation of the supramolecular packing adopted in the mesophase. The ligands were selected from the energy levels (Jablonsky diagram) and provided efficient energy transfer to Nd(III), Sm(III), Eu(III), Er(III), and Yb(III) ions. Red, green and NIR effective luminophores were obtained. The presence of LC properties in lanthanide derivatives combined with their optical behavior allowed us to demonstrate the possibility of polarized luminescence intensity and frequency tuning by changing of relative molecular orientation and type of Ln(III) ion.
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Supporting Information Table with complete data of the mesomorphic behavior, pristine SAXS patterns, volumetric data, and experimental equipements.
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Acknowledgements The liquid crystalline properties were studied in the frame of the RFBR grant №17-0300258a. AAK thanks grant of the President of the Russian Federation, No. MD-6102.2016.3 for supporting synthesis of samples. BD and BH thank the CNRS and the University of Strasbourg for support.
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Highlights: 1. Smectic lanthanidomesogens; 2. Liquid-crystal-enhanced photoluminescence; 3. Polarized luminescence;