Influence of Eu(III) Complexes Structural Anisotropy on Luminescence

May 8, 2017 - The results of experimental data confirmed theoretical prediction about significant influence introduced in molecular structure long ter...
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Influence of Eu(III) Complexes Structural Anisotropy on Luminescence of Doped Conjugated Polymer Blends Andrey A. Knyazev,*,† Maksim E. Karyakin,† Aleksandr S. Krupin,† Ksenia A. Romanova,† and Yuriy G. Galyametdinov†,‡ †

Physical and Colloid Chemistry Department, Kazan National Research Technological University, 420015 Kazan, Russia Kazan E. K. Zavoisky Physical-Technical Institute, Russian Academy of Sciences, 420029 Kazan, Russia

‡

S Supporting Information *

ABSTRACT: Europium(III) complexes that are the anisometric analogues of a well-known Eu(DBM)3Phen (DBM = dibenzoylmethane; Phen = 1,10-phenanthroline) complex were synthesized. Quantum-chemical calculations of the excited states of new Eu(III) complexes and some conjugated polymers showed that poly(N-vinylcarbazole) provides the most efficient energy transfer in polymer films doped with synthesized Eu(III) complexes. These new composite films were prepared, and their luminescence properties were studied. The results of experimental data confirmed theoretical prediction about significant influence introduced in molecular structure long terminal alkyl chains and cyclohexane on the luminescence efficiency of “Ln(III)−polymer” composite. As the result a substantial contribution of complex anisometry to the luminescence efficiency of composite was revealed due to the increase of the complex limit concentration before self-quenching more than twofold.

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INTRODUCTION Coordination compounds of lanthanides(III) with specific structure are an interesting class of substances that can be used as efficient luminescent materials for various applications.1−3 Their main property and advantage is the luminescence effect provided by the Ln(III) ion and not by organic ligands. In such compounds, a central ion possesses poor absorptive capacity, so excitation energy is transferred through surrounding organic ligands (“antenna effect”). An ion, therefore, makes a predominant contribution to the luminescence spectrum of a substance, while all the other parameters depend on ligands.4,5 Considerable efforts in this research area were focused on the development of new ligands with various chromophores that increased the efficiency of energy transfer to an emitting ion and influenced the performance of luminescent materials. Derivatives of β-diketones are among the most promising and well-tested chromophores with high energy transfer efficiency.6−8 Over the recent years, there is accelerating change of focus in this research field: from the synthesis of new compounds to their inclusion into matrices of various structures and dimensions (organic, inorganic, or mixed organic−inorganic ones). Polymers are the ideal candidates for such systems, as they possess an attractive range of technological properties such as mechanical strength, flexibility, processability, and low cost.9,10 There are, however, limits for the application of conducting polymers doped by Ln(III) as emitting materials due to their reduced luminescence efficiency. One of the reasons is the luminescence self-quenching effect emerging with © 2017 American Chemical Society

the increase of concentration, as complexes tend to crystallize and aggregate. The maximum concentration of a complex in polymer blends (PVK, CN-PPP, PHB, PFO, etc.) does not exceed 20 wt %.11−15 A polymer emission effect is also observed for such blends in addition to the emission of a complex, thus indicating incomplete energy transfer from a polymer to a complex and the resulting reduction of luminescence efficiency. A possible solution for the problem of crystalline defects in films is the use of anisometric lanthanide complexes with long hydrocarbon substituents that inhibit crystallization and aggregation.16,17 It allows increasing a concentration threshold for emitting ions and, thus, providing maximum luminescence efficiency for a composite material. Enhanced solubility of such complexes in organic solvents and their ability to mix with conjugated polymers can be achieved through providing their structural similarity (an anisotropic form and long hydrocarbon substituents at the ends of molecules). Such advantages allow creating highly homogeneous thin-film materials by the modern nanotechnology methods. There are several approaches to the selection of a polymer providing best opportunities for efficient energy transfer to a complex. The most popular approach is the comparison of the relative positions of excited levels for a polymer and the ligands surrounding the central ion in a lanthanide complex.11,12,14 Quantum chemistry is suitable for preliminary structural modeling of ligand environment of Ln(III) complexes and Received: November 24, 2016 Published: May 8, 2017 6067

DOI: 10.1021/acs.inorgchem.6b02825 Inorg. Chem. 2017, 56, 6067−6075

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The optimized structures of Eu(III) complexes in the ground state are shown in Figure 1. Structural parameters of the studied compounds are in the typical range for Eu(III) complexes with similar coordination environment. Geometry optimization of the Eu(DBM)3Phen complex in the ground state showed no significant changes compared with the experimental data from the Cambridge Structural Database (Supporting Information, Figure S1 and Table S1). The presence of a heavy atom and a large number of atoms in the structure of the studied Eu(III) complexes complicates the quantum-chemical calculations. A special semiempirical model Sparkle Model for the calculation of Lanthanide Complexes (SMLC) was developed by de Andrade and coworkers.18,19 This method places a point charge instead of Ln(III) ion in the center of a repulsive spherical potential and can be used for quite accurate calculations of equilibrium geometries of Ln(III) complexes in the ground state. It can also be used for calculations of the excited states, but the accuracy of the obtained data can be quite poor. The more reliable way is to use ab initio methods based on CASSCF and different variations of perturbation theory, but the complicity of the studied systems turns these methods into very time- and resource-consuming instruments. In the majority of the publications, the excitation energies in Ln(III) compounds are obtained by density functional theory (DFT) and time-dependent (TD) DFT methods.20,21 In this work, we also used the DFT method for the calculations of the excited states. It has been confirmed by experimental and theoretical works20−24 that the lowest triplet levels of the ligands play a key role in the energy transfer processes in Ln(III) compounds. The highest values of the quantum yields can be achieved when the excited state of the ligand can transfer excitation to the excited level of Ln(III) with higher energy than the emitting level because of the accompanying back transfer. The back energy transfer from the resonance level of the central ion to the triplet level of the ligand can make a great contribution to the intramolecular transfer. The phosphorescence process in the central ion that occurs with a spin flip (for example 5D0→7FJ in Eu(III)) is relatively slow, and ions have an opportunity to direct excitation back to the ligand. When the transfer occurs to higher level (e.g., 5D1 in Eu(III)) the nonradiative relaxation occurs much faster than back transfer because of a small gap between the sublevels of the excited ion multiplets (5D1→5D0 in Eu(III)). After such relaxation the resonance between the triplet levels of the ligand and the central ion is broken, and the back transfer becomes impossible. Different ligands in Ln(III) complexes can achieve resonance with different multiplet levels of the ion and transfer excitation energy by independent pathways, for example, as in one of the studied Eu(III) complex (Table 1). According to the results of quantum-chemical simulations of Eu(CPDK3‑Ph)3Bpy17−1 the excited triplet states of the ligands notably differ in energy. The triplet level of β-diketone CPDK3‑Ph matches the 5D1 (2.359 eV)25,26 sublevel of Eu(III) multiplet, while Bpy17−1 has the higher triplet level and transfers excitation to the 5D2 (2.667 eV) sublevel. The process with Bpy17−1 ligand can include additional energy losses caused by interligand excitation transfer or back transfer that decrease the total Eu(III) pumping. Thus, the energy transfer in this complex can occur by stepwise pathway: the excitation is transferred from one ligand (Bpy17−1) to another (CPDK3‑Ph) before it finally goes to the resonant level of the Eu(III) ion.

the structure of conjugated polymers for the tailor-made synthesis of respective composite materials with predicted high luminescence efficiency. Such a modeling makes it possible to find the optimal recipe for a composite, while no literature data have been found for the effect of anisotropic geometry of molecules on their intermolecular interactions in a composite film, concentration quenching, and, thus, the luminescence efficiency. This research, therefore, consists of two main parts: theoretical modeling of a Ln(III) complex−polymer composite and the comparison of theoretical quantum-chemical modeling results with the experimental data on the evaluation of the influence of intermolecular interactions (arising from introduction of long terminal substituents into the structure of ligands) on the luminescence efficiency of composite materials with Ln(III) complexes and conjugated polymers. Anisometric alternatives of the Eu(DBM)3Phen (DBM = dibenzoylmethane; Phen = 1,10-phenanthroline) complex were synthesized to achieve this research purpose. Quantum-chemical modeling of the equilibrium geometry of complexes and conjugated polymers, and the energies of their lower excited states, resulted in selection of poly(N-vinylcarbazole) (PVK) as the polymer providing optimal energy transfer. Polymer films were made from PVK doped with synthesized anisometric Eu(III) complexes; the comprehensive analysis of their luminescence properties was performed. The anisotropy of the doped complex was revealed to make a considerable contribution to the luminescence intensity of a composite due to more than double increase of its concentration limit.

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RESULTS AND DISCUSSION New Eu(III) complexes with anisometric geometry were synthesized (Chart 1); such complexes have not been Chart 1. Chemical Structures of Eu(III) Complexes

previously described in literature. The following compounds were used as the ligands: β-diketone-1-phenyl-3-(4-(4propylcyclohexyl)phenyl)propane-1,3-dione, which is an anisometric analogue of the well-known β-diketone DBM, and various Lewis bases. The first stage of this research was quantum-chemical modeling of the equilibrium geometry and the positions of excited levels of Eu(III) complexes to estimate intramolecular transfer efficiency. 6068

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Figure 1. Optimized ground-state structures and geometry parameters of the studied Eu(III) complexes.

Table 1. Vertical Energies (eV) in Eu(III) Complexes in the Ground-State and Optimized-Triplet Geometries Compared to the Experimental Data complex Eu(CPDK3‑Ph)3Phen

localization of excitation

multiplicity

ΔEtheor, eV

CPDK3‑Ph

1 3 1 3 1 3 1 3 1 3 1 3 1 3 1 3

3.428 2.403 3.847 2.655 3.381 2.372 3.679 2.852 3.478 2.463 3.300 2.381 3.408 2.605 3.577 2.655

Phen Eu(CPDK3‑Ph)3Bpy17−1

CPDK3‑Ph Bpy17−1

Eu(CPDK3‑Ph)3(TPPO)2

CPDK3‑Ph TPPO

Eu(DBM)3Phen

DBM Phen

a

ΔEexp, eV 2.380 2.68422 2.380 2.870a 22 2.380 2.35027 2.678,28 2.54629 2.68422

The triplet excited state for 2,2′-bipyridine (Bpy) ligand without alkyl substituents CH3− and C17H35−.

level of the ligands (DBM and Phen) to the 5D1 sublevel of Eu(III) multiplet. It has been demonstrated by experimental works22,24 that the highest quantum yields can be obtained for Eu(III) complexes with ligands in which triplet levels differ from the 5D0 sublevel of Eu(III) by 0.310−0.434 eV, and for Tb(III) complexes ΔE = T1(ligand) − 5D4(Tb(III)) = 0.310− 0.496 eV. Thus, for Eu(III) complexes the values of the triplet levels should lie in the range of 2.451−2.637 eV. The ligands of Eu(DBM)3Phen satisfy this empirical rule that explains the sufficient emission efficiency of this complex and its usage in luminescence materials. The luminescence efficiency of blends based on Ln(III) complexes and conjugated polymers is defined by the relative position of the excited states of the polymer matrix, the ligands, and the emitting ion. In emitting layer the polymer matrix can produce two types of excitons (by the electron and hole capture): singlet and triplet. Triplet excitation can be transferred nonradiatively to the triplet excited levels of the ligands. Then intramolecular energy transfer occurs to the resonance excited levels of Ln(III) ion, which after internal conversion can emit a photon. Efficiency of the emitting layer

Excited-state geometries of the complexes are represented in Figures S2−S5 in the Supporting Information. The optimization of the triplet excited states led to the structural changes in the polymer and in the ligands in the Eu(III) complexes that carried the excitation. To reduce computational costs the long alkyl substituents were excluded from the structures of the complexes, since alkyl-saturated substituents do not strongly influence photophysical properties of the complexes. The ligand triphenylphosphine oxide (TPPO) in Eu(CPDK3‑Ph)3(TPPO)2 has the lowest triplet excited state among the studied ligands and can transfer excitation to the lowest 5D0 (2.141 eV) multiplet. This pathway as has been discussed before can result in critical energy losses due to the back transfer. By contrast in Eu(CPDK3‑Ph)3Phen complex βdiketone and Phen has triplet levels that match the same 5D1 multiplet that is more energetically favorable and allows to assume that this complex will have the most complete energy transfer and the highest luminescence efficiency. Eu(DBM)3Phen was used as a model substance with absolute quantum yield of 4.9%.30 In this complex excitation can also been transferred only by one pathway: from triplet 6069

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were taken from experimental data.25,26 PVK provides the effective energy transfer to the ligand environment of the complex, since the triplet excited level of PVK locates above excited levels of β-diketone, Phen, Bpy17−1, and TPPO. Other investigated polymers cannot transfer excitation energy to neighbor organic ligands because of their low excited states and can act only as a matrix for uniform distribution of phosphorescence Eu(III) emitters to prevent concentration quenching. Preparation of Eu(CPDK3‑Ph)3Phen and PVK films is the most preferable method for the calculation purposes, as such films provide the most efficient energy transfer. According to the quantum-chemical calculations, there are similar values of excited levels for the CPDK3‑Ph ligand in the synthesized complex and the commercial DBM. The influence of alkyl substituents on the adsorptive capacity of Eu(III) complexes is insignificant.35 The authors, however, suppose that these substituents can make a considerable contribution to the intermolecular organization in the films, exerting influence on intermolecular interactions and, therefore, on optical properties. This assumption received the experimental confirmation. Luminescence spectra of complexes in their solutions and films were studied. The films with the same width (100 ± 2 nm) were obtained by the rotation spraying (spin-coating) method from 1 × 10−3 mol/L toluene solutions. The luminescence spectra of the solutions of the complexes (Figure 3) confirmed the results of quantum-chemical calculations. Solutions of complexes have the same maximum of absorbance (360 nm) at the excitation wavelength (Supporting Information, Figure S7). The maximum luminescence intensity was demonstrated by the Eu(CPDK3‑Ph)3Phen solution and corresponds to a wavelength of 613 nm at excitation wavelength 360 nm. Different behavior, however, was shown by the films (Figure 4). The comparison of excitation spectra of the solutions and films reveals a significant spectral broadening and hypsochromic shift of the maxima toward short-wavelength region in the films presumably as a result of the formation of agglomerates.36,37 The most intensive luminescence was demonstrated by the amorphous Eu(CPDK3‑Ph)3Bpy17−1 complex, which is a probable indication of intermolecular organization processes occurring in the films and the decreasing amount of crystalline defects. The maximum excitation of the polymer corresponds to the peak at a wavelength of 345 nm. On the next stage the nanosized films (∼100 ± 2 nm) were prepared on the basis of a conjugated conducting polymer PVK doped with complexes with various weight percentages (0− 100%) with 10% steps. The selection criterion for these components was the maximum possible overlap of the polymer emission and the complex excitation spectra (Figure 4) providing more complete polymer-to-complex energy transfer. The presence of alkyl chains in the dopant molecules and anisotropic form of such molecules resulted in their good solubility in organic solvents and good mixing with the PVK polymer due to certain structural similarities. Figure 5 demonstrates the dependencies of luminescence intensities at the 613 nm emission wavelength on the concentration of the complex (per mole of an emitting substance) for the excitation at 345 nm. It is shown that the Eu(DBM)3Phen luminescence intensity in the film drops with the increase of the complex content. This effect is a result of the facts of concentration luminescence quenching and the resulting growth of defects in a film.38 To confirm this assumption, the films with different content of

can be varied by selection of the polymers that suit the particular ligands and by searching the ligands that will provide the lossless energy transfer to Ln(III) ion. On the next stage the quantum-chemical calculations of some conjugated polymers (Chart 2) that are often used in Chart 2. Chemical Structures of Polymers

optoelectronics for red luminophores were performed. The selected polymers were PVK, poly(9,9-di-n-octylfluorenyl-2,7diyl) (PFO), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), and poly[(9,9-dihexylfluoren-2,7diyl)-alt-(2,5-dimethyl-1,4-phenylene)] (PF-DMB).11,12,14 It included the simulation of equilibrium geometries of their monomers in the ground state, optimization of their excitedstates geometries (Figure S6 in the Supporting Information), and single-point calculations of the excited states. In spite of the numerous literature data on the optical characteristics of conjugated polymers the quantum-chemical calculations can help one to solve problematic experimental question. For example, in ref 31, authors confirm that the PVK polymer has the triplet state at the higher energy than it was demonstrated previously in many experimental works.32−34 Authors proved that PVK will not quench the luminescence of heavy metal dopants and can be used as a host. Our quantumchemical calculations of the lowest triplet excited state of PVK (Table 2) are in agreement with experimental results of ref 31 Table 2. Vertical Energies (eV) in Polymers in the GroundState and Optimized-Triplet Geometries Compared to the Experimental Data polymer PVK PFO MEH-PPV PF-DMB

multiplicity

ΔEtheor, eV

1 3 1 3 1 3 1 3

4.100 3.030 3.593 2.330 2.259 1.350 3.932 2.280

ΔEexp, eV 3.00031 2.15033

and confirm the higher position of the PVK triplet. Calculations show that this polymer is one of the most energetically convenient polymers for the studied Eu(III) complexes and acts as an efficient cosensitizer of the Eu(III) luminescence. Energy-level diagrams for the Eu(CPDK3‑Ph)3Bpy17−1−PVK and Eu(CPDK3‑Ph)3Phen−PVK systems are shown in Figure 2a,b, respectively. Since the 4f shell of Eu(III) is shielded from the influence of the environment by 5s and 5p shells excited levels of Eu(III) do not depend on the nature of ligands and 6070

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Figure 2. Calculated lowest triplet T1 and singlet S1 levels (eV) of polymer and (a) Eu(CPDK3Ph)3Bpy17−1 and (b) Eu(CPDK3‑Ph)3Phen relative to the experimental multiplet levels of Eu(III) with radiative (solid arrows) and nonradiative transitions (dotted arrows) and energy-transfer pathways (bold dotted arrows).

Figure 3. Excitation and luminescence spectra of the solutions of the Eu(III) complexes with a concentration of 1 × 10−5 mol/L in toluene.

Figure 4. Excitation and luminescence spectra of the films of the Eu(III) complexes and conjugated polymer PVK.

ual complex, the 3.9-fold growth of per mole emission intensity was achieved. The summary concentration of “complex + polymer” in the blends was constant 1.6 mg/mL. Therefore, obtained decrease of the concentration of the emitting Eu(III) complex resulted in the decrease of the relative quantum yield (Table 3). However, this decrease is more significant for the blends of the Eu(DBM)3Phen complex due to the less effective energy transfer from PVK in comparison with the blends with Eu(CPDK3‑Ph)3Phen and Eu(CPDK3‑Ph)3Bpy17−1. The studied films had similar values of luminescence lifetimes (Table 3) that is well-described in the literature.13,39,40 The values of intrinsic quantum yields for the blends of Eu(CPDK3‑Ph)3Bpy17−1 and

Eu(CPDK3‑Ph)3Phen and Eu(DBM)3Phen were analyzed by atomic force microscopy (Figure 6). As shown in Figure 6, the films containing Eu(CPDK3‑Ph)3Phen have a more uniform surface and less surface roughness. The films containing Eu(DBM)3Phen is not uniform and have a bigger agglomerates of Eu(DBM)3Phen. The luminescence intensity increases notably in the films with Eu(CPDK3‑Ph)3(TPPO)2, Eu(CPDK3‑Ph)3Bpy17−1, and Eu(CPDK3Ph)3Phen, and energy transfer processes supersede concentration quenching in the films with the content of complexes up to 40 wt %. The maximum energy transfer was demonstrated by the films with 30 wt % of the Eu(CPDK3‑Ph)3Bpy17−1 complex. In comparison with the individ6071

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Figure 5. Dependence of the luminescence intensity at 613 nm per a complex mole in a blend for the excitation at 345 nm.

Eu(CPDK3‑Ph)3Phen are higher than value of individual complex instead of the Eu(DBM)3Phen blends. The intrinsic quantum yield of the film containing 30 wt % of the complex Eu(CPDK3‑Ph)3Bpy17−1 is 59%, whereas the quantum yield of individual complex is 42%.

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CONCLUSION Anisometric analogues of the Eu(DBM)3Phen complex have been studied to show that the geometry of a complex may exert considerable influence on the luminescence properties of the blends including a conjugated polymer and a Ln(III) complex. It is shown that in blends of PVK and some studied complexes the emission intensity per a complex mole increased by 3.9 times, and intrinsic quantum yield increased by 40% in comparison with individual compounds. The presence of alkyl and cyclohexane substituents in the structure of βdiketone ligands, in contrast to commercial DBM, hinders crystallization, inhibits formation of defects in the structure of films, and has better dispersibility in the doped PVK films.

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EXPERIMENTAL SECTION

General Information. 1H NMR spectra were recorded with Bruker Avance 300 and Bruker Avance 400 spectrometers (operating at 300 and 400 MHz). CHN elemental microanalysis was performed on a CE Instruments EA-1110. Europium elemental microanalysis was performed on an X-ray fluorescence Bruker S8 TIGER. The films of the complexes and polymer blends were prepared by spin coating on a Spin coater WS-650 MZ-23NPP Laurel from toluene solution. The 20 × 20 mm quartz glass was used as a substrate for the films. The absence of crystallization was confirmed by using a polarization microscope Olympus BX-51. Film thicknesses were measured using a KLA-Tencor P-10 profilometer. The relief of the film surface was examined with a scanning probe microscope Solvent P47 Pro manufactured by NT-MDT semicontact method. The excitation and luminescence spectra and lifetimes (excitation wavelength = 345 nm) were measured by a Varian Cary Eclipse spectrofluorimeter. Quantum Yields. The intrinsic quantum yield of Eu(III) is thus obtained from φLn = τ/τr, where τ is the observed luminescence lifetime of Eu(III). The empirical radiative lifetime (τr) of Eu(III) is determined 1/τr = AMDn3(Itot/IMD), assuming that the dipole strength of the magnetic dipole transition Eu(III) (5D0→7F1) is independent of the environment, where AMD = 14.65 s−1 is the spontaneous emission probability of the 5D0→7F1 transition in vacuum, n is the refractive index of the medium, and Itot/IMD is the ratio of the total emission area to the area of the 5D0→7F1 transition. The refractive index of the europium complex was measured on a spin-coated thin film deposited on a quartz plate at room temperature by a WVASE32TM

Figure 6. A 20 × 20 μm AFM image of thin films Eu/PVK with different content of europium complexes. (left) Eu(CPDK3‑Ph)3 Phen (right) Eu(DBM)3Phen. Ellipsometer. The values of n determined by ellipsometry are 1.6615 for Eu(CPDK3‑Ph)3Bpy17−1 and 1.6026 for the PVK-based blend containing 30 wt % Eu(CPDK3‑Ph)3Bpy17−1 at 611.7 nm. Previously, we checked that the luminescence spectrum of this film was identical to the spectrum of the sample prepared of quartz plates. The relative quantum yield φ of films of complexes was calculated from the relation:41 φ = φstd × [Astd(λstd)/Aunk(λunk)] × [Istd(λstd)/Iunk(λunk)] × [Dunk/Dstd], where the subscripts std and unk indicate the standard and unknown sample, A(λ) corresponds to the absorbance of the films 6072

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1:9). Found, %: C 82.64; H 8.15. C24H28O2. Calculated, %: C 82.72; H 8.10. 5-Heptadecyl-5′-methyl-2,2′-bipyridine. Yield 72%. mp 82 °C. MS (ESI) m/z: 410 (M+). Anal. Calcd for C28H44N2 (Mw = 408.66 g mol−1): C, 82.29; H, 10.85; N, 6.85%. Found: C, 82.21; H, 10.88; N, 6.84%. 1H NMR (300 MHz, CDCl3): δ 0.86−0.90 (m, 3H, CH3), 1.20−1.31 (m, 28H, CH2), 1.57−1.70 (m, 2H, CH2CH2−Pyr), 2.35 (s, 3H, CH3−Pyr), 2.63 (t, 2H, CH2−Pyr, J = 7.3 Hz), 7.60 (dd, 2H, H4, J1 = 2.0 Hz, J2 = 1.9 Hz), 8.24−8.30 (m, 2H, H3), 8.45 (s, 2H, H6). Synthesis of complexes tris[1-phenyl-3-(4-(4-propylcyclohexyl)phenyl)propane-1,3-dione]-mono[1,10-phenantroline]europium (Eu(CPDK3‑Ph)3Phen), tris[1-phenyl-3-(4-(4-propylcyclohexyl)phenyl)propane-1,3-dione]-bis[triphenylphosphine oxide]europium Eu(CPDK3‑Ph)3(TPPO)2 and tris[1-phenyl-3-(4-(4propylcyclohexyl)phenyl)propane-1,3-dione]-[5-heptadecyl-5′-methyl2,2′-bipyridine]europium Eu(CPDK3‑Ph)3Bpy17−1 was performed by the procedure described in literature.44−47 Tris[1-phenyl-3-(4-(4-propylcyclohexyl)phenyl)propane-1,3dione]-[Lewis base]europium: General Procedure. An alcoholic solution 0.04 mmol of EuCl3·6H2O was slowly dropped into a stirred hot alcoholic solution containing 0.12 mmol of β-diketone, 0.04 mmol of 1,10-phenanthroline (5-heptadecyl-5′-methyl-2,2′-bipyridine or 0.08 mmoltriphenylphosphine oxide), and 0.125 mmol of KOH. The light yellow precipitate that formed was isolated by hot filtration, washed with hot alcohol, and dried under vacuum. Further, the product was dissolved in toluene, and the obtained solution was filtered off and evaporated to dryness under vacuum. Complex Eu(CPDK3‑Ph)3Phen. Yield: 59% (0.087 g). mp 142 °C. Anal. Calcd for C84H89N2O6Eu (%): C, 73.40; H, 6.53; N, 2.04; Eu, 11.06. Found (%):C, 73.01; H, 6.88; N, 2.12; Eu, 11.00. Complex Eu(CPDK3‑Ph)3(TPPO)2. Yield: 64% (0.096 g). mp 159 °C. Anal. Calcd for C108H111O8P2Eu (%): C, 74.08; H, 6.39; Eu, 8.68. Found (%):C, 73.52; H, 6.81; Eu, 8.50. Complex Eu(CPDK3‑Ph)3bpy17−1. Yield: 48% (0.072 g). mp 182 °C. Anal. Calcd for C100H125N2O6Eu (%): C, 74.92; H, 7.86; N, 1.75; Eu, 9.48; Found (%):C, 74.56; H, 8.19; N, 1.63; Eu, 9.50. Theoretical Calculations. The presence of a large number of long alkyl substituents in the structure of the studied anisometric Eu(III) complexes does not allow one to prepare a single crystal for X-ray diffraction analyses and obtain a crystal structure. Therefore, quantumchemical simulation is the only instrument for the study of the structure and certain properties of anisometric Eu(III) compounds. Quantum-chemical calculations of the equilibrium geometry of Eu(III) complexes and the polymers in the ground state were performed in a gas phase using the Priroda 06 software48,49 by the DFT method with the PBE exchange correlation functional.50 The relativistic effects of Eu(III) ions were considered by using the rL11 relativistic basis set. The rL1 basis set was applied for other atoms51 (analogues of the cc-pVDZ and cc-pCVDZ correlation-consistent double-ζ basis sets of Dunning). Calculations were performed without symmetry constraints. The geometries of the coordination polyhedron of complexes were adopted from the Cambridge Structural Database, which contains the X-ray diffraction analysis data for similar compounds without alkyl substituents.52−54 The isomer with a crosswise arrangement of β-diketones in the complexes was chosen for calculations. According to our previous work55 the arrangement of ligands when long alkyl substitutes do not sterically hinder each other has the lowest energy. Excited-state calculations were performed using the Firefly 8.1.0 software56,57 and included the stage of optimization of the excited-state geometries. Since the inner 4f shell of Eu(III) is located near the atomic nuclei the excited states of the ion slightly depend on the influence of the environment. Therefore, the scalar relativistic 4f-incore pseudopotential ECP52MWB with the associated valence basis set was used for Eu(III),58,59 and 6-31G(d,p) basis set was used for other light atoms. Excited-state calculations of polymers were performed by TDDFT method with the PBE functional. Optimized excited-state structures of Eu(III) complexes with triplet localization on separate ligands were obtained by CIS method, since TDDFT failed to obtain the triplet state localization on each of the ligands in

Table 3. Lifetime Values, Intrinsic, and Relative Quantum Yields of the PVK Films of the Eu(III) Complexes complex content in the blend, wt % 10 20 30 40 60 80 100 10 20 30 40 60 80 100 10 20 30 40 60 80 100

lifetime τ, μs

intrinsic quantum yield φLn,a %

Eu(CPDK3‑Ph)3Bpy17−1 407 28.6 487 41.0 493 59.0 468 52.9 456 47.8 491 44.1 501 42.0 Eu(CPDK3‑Ph)3Phen 412 30.0 404 35.9 427 42.2 406 44.2 436 43.2 455 42.9 438 42.1 Eu(DBM)3Phen 468 34.0 454 39.8 444 43.8 443 45.6 445 47.8 454 48.8 476 50.1

relative quantum yield φover,a % 4.7 7.8 11.0 10.9 11.7 13.7 15.2 6.1 8.3 11.4 14.1 15.5 18.8 20.9 3.1 4.5 7.2 10.6 16.6 19.6 24.4

a The estimated errors for ΦLn and Φover are ±5% and ±10%, respectively.

at the excitation wavelength λ, I(λ) is the intensity of the exciting beam (assumed to be equal for both measurements), and D is the integrated luminescence spectrum. The standard fluorophore for measurements was a thin film of poly(methyl methacrylate) (PMMA) codoped with 1 wt % Eu(tta)3(H2O)2 (tta - tris-thenoyltrifluoroacetate) with φstd = 45%.42 The parameters used for the calculations of the intrinsic and relative quantum yields of the Eu(III) complexes are provided in Tables S2 and S3 in Supporting Information. At 345 nm both the polymer and the complex absorb light with almost the same absorption intensity. Therefore, the absorption intensity of the blend remains practically unchanged, while the component ratio changes. To determine the experimental energy of the triplet state of CPDK3‑Ph ligand, the phosphorescence spectrum of the Gd(CPDK3Ph)3(H2O)2 complex was measured (Supporting Information, Figure S8). The phosphorescence spectra was obtained in the solid state at 77 K on an optical spectrometer based on a MDR-23 monochromator with time delay of 1 μs. An LGI-21 pulsed nitrogen laser (337 nm wavelength, 1.5 mW average output power, 10 ns pulse duration, 50 Hz repetition rate) was used as an excitation source. Synthetic Procedures. Europium(III) chloride hexahydrate (EuCl3·6H2O), tris(dibenzoylmethane) mono(1,10-phenanthroline) europium(III) (Eu(DBM)3Phen), polymer poly(9-vinylcarbazole) (PVK, MW ≈ 1 100 000), 1,10-phenanthroline, and triphenylphosphine oxide (TPPO) were purchased from Sigma-Aldrich. 1-Phenyl-3(4-(4-propylcyclohexyl)phenyl)propane-1,3-dione and 5-heptadecyl5′-methyl-2,2′-bipyridine were prepared by modification of a literature procedure.43−46 1-Phenyl-3-[4-(4-propylcyclohexyl)phenyl]propane-1,3-dione. Yield 53.5%. mp 103 °C. m/z: 347 (M+). 1H NMR (300 MHz, CDCl3): δ 0.91−0.96 (m, 3H, CH3), 1.06−1.56 (m, 9H, CH2), 1.88− 1.95 (m, 4H, C6H10), 2.51−2.60 (m, 1H, CHC6H4), 4.62 (s, 0.1H, keto = CH2), 6.86 (s, 0.9H, enol = CH), 7.33 (d, 2H, C6H5, J = 8.1 Hz), 7.47−7.59 (m, 3H, OCC6H5), 7.92 (d, 2H, C6H5, J = 8.4 Hz), 7.99 (d, 2H, C6H5, J = 9.0 Hz), 16.95 (s, 0.8H, enol OH; keto/enol = 6073

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Inorganic Chemistry the complex. The excited-state optimization showed the structural changes in the ligand that carried the excitation. Quantum-chemical calculations were made using the facilities of the Joint Supercomputer Center of Russian Academy of Sciences and the Supercomputing Center of Lomonosov, Moscow State University.60

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(11) Liu, Y.; Li, J. D.; Li, C.; Song, J. G.; Zhang, Y. L.; Peng, J. B.; Wang, X. Y.; Zhu, M. X.; Cao, Y.; Zhu, W. G. Highly efficient sharp red electroluminescence from europium complex-doped poly(9,9dioctylfluorene) devices. Chem. Phys. Lett. 2007, 433, 331−337. (12) Díaz-García, M. A.; Fernandez De Á vila, S.; Kuzyk, M. G. Energy transfer from organics to rare-earth complexes. Appl. Phys. Lett. 2002, 81, 3924−3926. (13) Biju, S.; Reddy, M. L. P.; Cowley, A. H.; Vasudevan, K. V. 3Phenyl-4-acyl-5-isoxazolonate complex of Tb3+ doped into poly-βhydroxybutyrate matrix as a promising light-conversion molecular device. J. Mater. Chem. 2009, 19, 5179−5187. (14) Liu, Y.; Wang, Y.; Guo, H.; Zhu, M.; Li, C.; Peng, J.; Zhu, W.; Cao, Y. Synthesis and optoelectronic characterization of a monochromic red-emitting europium(III) complex containing triphenylamine-functionalized phenanthroline. J. Phys. Chem. C 2011, 115, 4209−4216. (15) Liu, Y.; Wang, Y.; Li, C.; Huang, Y.; Dang, D.; Zhu, M.; Zhu, W.; Cao, Y. Red polymer light-emitting devices based on an oxadiazole-functionalized europium(III) complex. Mater. Chem. Phys. 2014, 143, 1265−1270. (16) Dzhabarov, V. I.; Knyazev, A. A.; Strelkov, M. V.; Molostova, E. Y.; Schustov, V. A.; Haase, W.; Galyametdinov, Yu. G. Tris(βdiketonates) lanthanum nematic adducts. Liq. Cryst. 2010, 37, 285− 291. (17) Lapaev, D. V.; Nikiforov, V. G.; Knyazev, A. A.; Dzhabarov, V. I.; Lobkov, V. S.; Salikhov, K. M.; Galyametdinov, Yu. G. Intramolecular energy transfer in mesogenic europium(III) adduct. Opt. Spectrosc. 2008, 104, 851−857. (18) de Andrade, A. V. M; da Costa, N. B., Jr.; Simas, A. M.; de Sá, G. F. Sparkle model for the quantum chemical AM1 calculation of europium complexes. Chem. Phys. Lett. 1994, 227, 349−353. (19) de Andrade, A. V. M; Longo, R. L.; Simas, A. M.; de Sá, G. F. Theoretical model for the prediction of electronic spectra of lanthanide complexes. J. Chem. Soc., Faraday Trans. 1996, 92, 1835− 1839. (20) Puntus, L. N.; Lyssenko, K. A.; Pekareva, I. S.; Bünzli, J.-C. G. Intermolecular interactions as actors in energy-transfer processes in lanthanide complexes with 2,2′-bipyridine. J. Phys. Chem. B 2009, 113, 9265−9277. (21) Gutierrez, F.; Rabbe, C.; Poteau, R.; Daudey, J. P. Theoretical study of Ln(III) complexes with polyaza-aromatic ligands: geometries of [LnL(H2O)n]3+ complexes and successes and failures of TDDFT. J. Phys. Chem. A 2005, 109, 4325−4330. (22) Faustino, W. M.; Malta, O. L.; Teotonio, E. E. S.; Brito, H. F.; Simas, A. M.; de Sá, G. F. Photoluminescence of europium(III) dithiocarbamate complexes: electronic structure, charge transfer and energy transfer. J. Phys. Chem. A 2006, 110, 2510−2516. (23) Crosby, G. A.; Whan, R. E.; Freeman, J. J. Spectroscopic studies of rare earth chelates. J. Phys. Chem. 1962, 66, 2493−2499. (24) Latva, M.; Takalo, H.; Mukkala, V.-M.; Matachescu, C.; Rodriguez-Ubis, J. C.; Kankare, J. Correlation between the lowest triplet state energy level of the ligand and lanthanide(III) luminescence quantum yield. J. Lumin. 1997, 75, 149−169. (25) Carnall, W. T.; Fields, P. R.; Rajnak, K. Electronic energy levels in the trivalent lanthanide aquo ions. I. Pr3+, Nd3+, Pm3+, Sm3+, Dy3+, Ho3+, Er3+ and Tm3+. J. Chem. Phys. 1968, 49, 4424−4442. (26) Kramida, A.; Ralchenko, Yu.; Reader, J. NIST ASD Team (2015). NIST Atomic Spectra Database, version 5.3, National Institute of Standards and Technology, Gaithersburg, MD, 2015. Available online at: https://www.nist.gov/pml/atomic-spectra-database. Accessed Nov 10, 2016. (27) Xin, H.; Shi, M.; Gao, X. C.; Huang, Y. Y.; Gong, Z. L.; Nie, D. B.; Cao, H.; Bian, Z. Q.; Li, F. Y.; Huang, C. H. The effect of different neutral ligands on photoluminescence and electroluminescence properties of ternary terbium complexes. J. Phys. Chem. B 2004, 108, 10796−10800. (28) Niyama, E.; Brito, H. F.; Cremona, M.; Teotonio, E. E. S.; Reyes, R.; Brito, G. E. S.; Felinto, M. C. F. C. Synthesis and spectroscopic behavior of highly luminescent Eu3+−dibenzoylmetha-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02825. Optimized ground- and excited-state structures of Eu(III) complexes and polymers, the absorption spectra of the Eu(III) complexes, the phosphorescence spectrum of the Gd(III) complex, and the parameters used for the calculations of the intrinsic and relative quantum yields of Eu(III) complexes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +7 843 231 41 77. E-mail: [email protected]. ORCID

Ksenia A. Romanova: 0000-0002-7654-3779 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by a grant of the President of the Russian Federation, No. MD-6102.2016.3. REFERENCES

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