Article pubs.acs.org/IC

Efficient Layers of Emitting Ternary Lanthanide Complexes for Fabricating Red, Green, and Yellow OLEDs Zubair Ahmed and Khalid Iftikhar* Lanthanide Research Laboratory, Department of Chemistry, Jamia Millia Islamia, New Delhi 110 025, India S Supporting Information *

ABSTRACT: A series of novel nona- and octacoordinate highly volatile and luminescent complexes, [Eu(hfaa)3(indazole)3] and [Ln(hfaa)3(indazole)2] (Ln = Tb, Dy, and Lu), were synthesized using a monoanionic bidentate hexafluoroacetylacetone (hfaa−) and a neutral monodentate indazole ligand. The X-ray diffraction analyses of their singlecrystals indicate that the complexes are mononuclear. The Eu complex is nonacoordinate and has a distorted monocapped square antiprismatic structure whereas the terbium and dysprosium complexes are octacoordinate and possess a trigonal bicapped prism geometry. The indazole units are involved in π−π stacking interaction and N−H···F hydrogen bonding with the fluorine atoms of hfaa−. The photophysical studies of indazole and the complexes show that the triplet states are at the appropriate positions and make ligand-to-metal energy transfer process efficient. A strong protective shield is provided by the coordination of three hfaa− moieties (which have low frequency C−F vibrational oscillators), and two/three ancillary indazole ligands around these metal ions ascribe higher quantum yields and longer radiative life times (ΦEu = 69% ± 10, 989 ± 1 μs, ΦTb = 33% ± 10, 546 ± 1 μs, and ΦDy = 2.5% ± 10, 13.6 ± 1 μs) to these novel compounds. The emission from europium, terbium, and dysprosium are, respectively, red, green, and yellow. Finally, these compounds were used, as emitting layers, to fabricate electroluminescent devices of their respective colors. The best devices are found with the following structure: ITO/ CuPc (15 nm)/[Eu complex]:CBP or [Tb complex]:CBP or [Dy complex]:CBP (80 nm)/BCP (25 nm)/AlQ (30 nm)/LiF (1 nm)/Al (100 nm), which indicates an improved EL performance for the Eu device over the Eu devices reported in the literature. The ligand, indazole, is a good sensitizer for trivalent europium, terbium, and dysprosium ions. It together with hfaa− plays an important role in fabricating OLEDs, especially, processed at low temperature.

■

INTRODUCTION There is an intense drive at the moment toward the design and synthesis of the luminescent lanthanide complexes because they are very useful in biomedical luminescent imaging,1 optical fiber lasers, 2 sensors, 3 and organic light emitting devices (OLEDs).4−7 The luminescence from the lanthanide complexes is unique as they give sharp emission with high color purity and long radiative lifetimes that are the result of well-shielded inner f-orbitals by the outermost s and p subshell. Moreover, the emission color can be easily tuned over the visible spectrum simply by changing lanthanide ions, for example, Eu3+ (red), Tb3+ (green), Sm3+ (pink/orange), and Dy3+ (yellow/white). Due to the spin and parity forbidden nature and very low absorption coefficients of f−f transitions,8 the luminescence from lanthanides is generally obtained by indirect sensitization using organic chromophore with high molar absorption coefficient (so-called antenna effect).9 The β-diketones and their lanthanide complexes are extensively explored because they give very volatile, thermodynamically stable, and highly luminescent complexes, which are essential conditions for the fabrication of OLEDs.4,10 The excited states of β-diketones having π−π* transition in the UV © XXXX American Chemical Society

region possess appropriate energy levels for effective and efficient sensitization of lanthanide ions. Moreover, fluorination of β-diketones can alter the photophysical properties of lanthanide β-diketone complexes by altering the energy of the triplet state (ET) of the β-diketones.10 Being coordinatively unsaturated11 the tris-lanthanide β-diketonates are very reactive and coordinate readily with incoming ancillary ligands having oxygen/nitrogen donor atom(s) to achieve coordinative saturation. These coordinatively saturated lanthanide βdiketone complexes are more luminescent since they quench radiationless transitions. Besides β-diketones, the indazole is a rigid heteroaromatic ligand which acts mainly as a monodentate protonated ligand in metal complexes by coordinating through its pyridine-ring nitrogen whereas the pyrrole-ring nitrogen usually gives rise to interesting molecular packing and arrangements in the solid12 state by involving hydrogen bond(s) formation. Moreover, the interesting results on the complexes of indazole derivatives12−14 are tempting enough to investigate the ternary tris(β-DK) Received: July 20, 2015

A

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

capillary method and a DSC analyzer (DSC 6220 SIINT, Japan) were used for determining the melting points of the complexes. A PerkinElmer spectrum RX I FT-IR spectrophotometer was employed for obtaining IR spectra in the range 4000−400 cm−1 as a KBr disc. The ESI-MS spectra in positive ion mode were recorded on Waters Micromass Q-T mass spectrometer. The thermal analyses of the complexes were carried out under a dinitrogen atmosphere with a heating rate of 10 °C/min on Exstar 6000 TGA/DTA machine from SIINT, Japan. The electronic spectra (200−1100 nm) were recorded on a PerkinElmer Lambda-40 spectrophotometer in 1 cm3 stoppered quartz cell of 1 cm path length in the concentration range between 2 × 10−5 and 5 × 10−5 M. The NMR spectra of indazole and the complexes, in CDCl3, were recorded on a BRUKER AVANCE II 400 NMR spectrometer by dissolving a sufficient amount of the samples (6−10 mg in 0.50 mL CDCl3). A JY SPEX CCD3000 spectrometer was used to obtain electroluminescence (EL) spectra. A Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a calibrated silicon photodiode was used to measure current−luminance−voltage properties. Steady state emission and excitation spectra were recorded on a Horiba-Jobin Yvon Fluorolog 3-22 spectrofluorimeter equipped with a 450 W xenon lamp as the excitation source and an R-928P Hamamatsu photomultiplier tube as detector. An Edinburgh FLS920 fluorescence spectrometer equipped with a Hamamatsu R5509-72 supercooled photomultiplier tube at 193 K and a TM300 emission monochromator with NIR grating blazed at 1000 nm was used for recording photoluminescence spectra in the NIR region. The calibration curve supplied with the instrument was used to get the corrected spectra. A Voigt function was chosen, by using Peak Fit v 4.12 (Jandel Software, Inc.) to fit the peaks in order to determine the peak center maximum, full width at half-maximum (fwhm or peak width), and peak area. The relative quantum yields (Φs’s) of the sensitized Ln(III) emission of the complexes, in the visible region, were measured in chloroform at room temperature and are cited relative to a reference solution of quinine bisulfate in 1 N H2SO4 (η = 1.338, Φr = 54.6%)23 with an experimental error of 10%. The compound, [Yb(TTA)3(H2O)2] in toluene (η = 1.4964, Φr = 0.35%),24 was used as the standard for NIR emitting Dy(III) ion (estimated error ±10%). The relative quantum yield was calculated using the equation23

complexes of lanthanide with unsubstituted indazole ligand which could also fulfill the criterion for the formation of important molecular assemblies that can be useful as emissive materials. It is assumed that these complexes could bear the following important features: (i) π−π stacking interactions in the indazole units and (ii) the hydrogen bond between pyrazole nitrogen of the indazole and fluorine atoms of β-diketone (the N−H···F). In the field of OLEDs, the luminescent lanthanide tris(βDKs) complexes are emerging as potential materials since (i) the line-like emission is achieved from lanthanide based materials due to 4f−4f transitions arising from the Ln(III) emitting state, and (ii) the excitation of Ln(III) via intramolecular energy transfer from the triplet state, as well as relaxation from the singlet to the triplet state, of the ligands via intersystem crossing results in utilization of both singlet and triplet exciton energy produced by electron−hole recombination for emission. Although the intersystem crossing efficiency is not 100%, the lanthanide complexes’ electroluminescence efficiency could theoretically reach 100%4 or at least exceed those of commonly used organic fluorescent materials to a larger extent. The Eu(III),4,15,16 Tb(III),15−18 and Dy(III)19 complex based electroluminescent devices are of great significance since they give sharp line-like emission band of, respectively, red, green, and white/yellow. In an effort to improve OLED performance, researchers have designed and synthesized a large number of lanthanide complexes by incorporating certain modifications (for example using charge-transport groups containing ligands20) and doping.10,20−22 Although several devices are reported in the literature, the search for better electroluminescent materials consisting of Ln(III) for fabricating new devices is still needed at the present time. In this Article, we report a modified method to synthesize novel [Ln (hfaa)3(indazole)n] complexes (Ln = Eu, Gd, Tb, Dy, and Lu; hfaa is the anion of hexafluoroacetylacetone; n = 3 for Eu and 2 for other metals) using indazole ligand, at room temperature. The photophysical properties of indazole and these crystallographically characterized monometallic complexes of europium, terbium, and dysprosium have been analyzed both as a solid and in solution. Finally, these novel complexes, by virtue of being more volatile and having higher luminescence efficiency, have been employed as emitting layers, to fabricate red (Eu), green (Tb), and yellow (Dy) light emitting devices. The synthesized complexes have requisite properties required for the fabrication of OLEDs, for instance, higher luminescence efficiency, higher volatility, light weight, good transparency, and easy thermal evaporation without decomposition.

■

Qs Qr

= (A r )(ηs2)(IS)/(A S)(ηr2)(Ir)

(1)

where all notations have their usual connotation. The refractive index is assumed to be equivalent to that of the pure solvent (η = 1.45 for chloroform). The quantum yields of the complexes in the solid state were measured at room temperature according to the following expression, described by Bril et al.25 Qx =

1 − rst A × x × Q st 1 − rx A st

(2)

where rx and rst are the diffuse reflectance of the complexes and of the standard phosphor, respectively, and Qst is the quantum yield of the standard phosphor. Ax and Ast represent the area under the emission spectra of the complex and the standard, respectively. In order to have absolute intensity values, BaSO4 was used as a reflecting standard. Pyrene was employed as the standard phosphor, the emission spectrum of which comprises an intense broad band peaking around 472 nm, with a constant Qst value (61%) for an excitation wavelength of 313 nm.26 Three measurements were carried out for each sample, so that the presented Qx value corresponds to the arithmetic mean value. The errors in the quantum yield values in this method were estimated within ±10%.27 The cyclic voltammetry (CV) experiments were carried out on a CHI600C electrochemical workstation using the conventional three-electrode configuration consisting of a platinum working electrode, a platinum wire auxiliary electrode, and a silver-wire pseudoreference electrode. The cyclic voltammogram was obtained in chloroform solution using tetrabutylammonium hexafluorophosphate

EXPERIMENTAL SECTION

Materials. The lanthanide oxides (Eu, Gd, Tb, Dy, and Lu, 99.9%, Leico Chem.) were converted to the corresponding chlorides. Hexafluoroacetylacetone, Hhfaa (MTM Lancaster); indazole (KochLight); 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, BCP (98%, Acros); N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′diamine, TPD (99%, Aldrich); tris(8-hydroxyquinolinato) aluminum, Alq3 (the electron-transport material), and copper phthalocyanine (the hole-injecting material), CuPc, 99% (eLight Corporation); and 4,4′bis(carbazole-9-yl)biphenyl, CBP (Lumthech Corporation), were used as received. The solvents used in this study were either AR or spectroscopic grade. Methods. The C, H, and N contents were analyzed at the Department of Chemistry, B.H. U., Varansi. Both the conventional B

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

V Z density (calcd) abs coeff F(000) cryst size θ range for data collection index ranges reflns collected indep reflns completeness to θ = 25.00° abs corr max and min transm refinement method data/restraints/params GOF on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole

empirical formula fw T wavelength cryst syst space group unit cell dimensions

C36H21EuF18N6O6 1127.55 298(2) K 0.710 73 Å monoclinic P21/n a = 11.9959(17) Å α = 90° b = 18.959(3) Å β = 103.200(7)° c = 19.390(3) Å γ = 90° 4293.4(11) Å3 4 1.744 Mg/m3 1.590 mm−1 2208 0.16 × 0.13 × 0.10 mm3 1.52−25.00° −14 ≤ h ≤ 14, −22 ≤ k ≤ 22, −23 ≤ l ≤ 22 47 229 7562 [R(int) = 0.0271] 99.9% semiempirical from equivalents 0.8572 and 0.7850 full-matrix least-squares on F2 7562/0/604 1.098 R1 = 0.0419, wR2 = 0.1182 R1 = 0.0541, wR2 = 0.1310 1.026 and −0.611 e Å−3

Table 1. Crystal Data and Structure Refinement for Eu, Tb, and Dy Complexes C29H15F18N4O6Tb 1016.37 298(2) K 0.710 73 Å triclinic P1̅ a = 9.7278(3) Å α = 103.022(2)° b = 11.4273(4) Å β = 95.802(2)° c = 17.6049(7) Å γ = 98.031(2)° 1870.07(11) Å3 2 1.805 Mg/m3 2.026 mm−1 984 0.22 × 0.16 × 0.12 mm3 1.20−25.00° −11 ≤ h ≤ 11, −13 ≤ k ≤ 13, −20 ≤ l ≤ 17 26 327 6588 [R(int) = 0.0902] 100.0% semiempirical from equivalents 0.7931 and 0.6642 full-matrix least-squares on F2 6588/0/511 1.051 R1 = 0.0489, wR2 = 0.1375 R1 = 0.0574, wR2 = 0.1470 0.889 and −1.381 e Å−3

C29H15DyF18N4O6 1019.95 298(2) K 0.710 73 Å orthorhombic Pbca a = 20.6001(15) Å α = 90° b = 20.8819(15) Å β = 90° c = 35.748(3) Å γ = 90° 15 377.7(19) Å3 16 1.762 Mg/m3 2.075 mm−1 7888 0.16 × 0.13 × 0.12 mm3 1.14−25.00° −24 ≤ h ≤ 22, −24 ≤ k ≤ 24, −42 ≤ l ≤ 42 117 238 13 546 [R(int) = 0.0399] 100.0% semiempirical from equivalents 0.7888 and 0.7325 full-matrix least-squares on F2 13 546/24/1046 1.069 R1 = 0.0448, wR2 = 0.1272 R1 = 0.0793, wR2 = 0.1477 0.777 and −0.597 e Å−3

Inorganic Chemistry Article

C

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (NBu4PF6) (0.1 M) as the supporting electrolyte at scan rate of 0.1 V s−1. Ferrocene/ferricenium (Fc/Fc+) was used as internal reference during the measurement. X-ray Structure Determination. Slow evaporation of hexane solutions of the Eu, Tb, and Dy complexes gave very good quality single-crystals. These crystals were mounted on a capillary. X-ray diffraction studies of the crystal were carried out on a BRUKER AXS SMARTAPEX diffractometer with a CCD area detector (KR) 0.710 73 Å, with graphite monochromator.28 Frames were collected at T = 293 K by ω, φ, and 2θ-rotation at 10 s per frame with SAINT.29 The measured intensities were reduced to F2 and corrected for absorption with SADABS.28 Structure solution, refinement, and data output were carried out with the SHELXTL program.30 Non-hydrogen atoms were refined anisotropically. C−H hydrogen atoms were placed in geometrically calculated positions by using a riding model. O−H hydrogen atoms were localized by difference Fourier maps and refined in subsequent refinement cycles. Images were created with the Diamond program.31 Crystallographic and refinement data are summarized in Tables 1 and 2. Preparation of EL Devices. For the fabrication of the EL devices, the organic and emitting layers were sequentially deposited under high vacuum environment (≤5.0 × 10−5 Pa) by thermal evaporation onto ITO glass substrates with a speed of 0.1−0.3 nm/s whereas LiF/Al were evaporated with the speed of 0.01/0.5 nm s−1 under a similar vacuum. The quartz crystal monitors were used which monitored the evaporation speed of the individual materials and thickness of the deposited layers. Synthesis. An in situ route was adopted for the syntheses of the complexes. The synthesis given here for [Eu(hfaa)3(indazole)3] (Scheme 1) will represent all the complexes. An ethanol (5 mL) solution of Hhfaa (1.486 g, 7.1 mmol) was added to 25% aqueous ammonia solution (0.53 mL, 0.122 g, 7.1 mmol). This mixture was kept in a 50 mL stoppered conical flask for half an hour until all ammonia vapors dissolved. It gave NH4(hfaa), ammonium salt of hexafluoroacetylacetone. This NH4(hfaa) solution was mixed with 5 mL of ethanol solution of EuCl3·6H2O (0.868 g, 2.37 mmol) and 15 mL of ethanol solution of indazole (1.415 g, 7.11 mmol). The stoichiometry between EuCl3·6H2O, Hhfaa, and indazole was maintained as 1:3:3. This mixture was stirred for 7 h at room temperature. During stirring a white precipitate was formed, and it was, repeatedly, filtered off. The filtrate was concentrated and left for slow evaporation at room temperature. After 3 days, light yellow crystals appeared. These were filtered off and washed repeatedly with CCl4. Final recrystallization was done from hexane. The stoichiometric ratio between metal chloride, Hhfaa, and indazole was kept as 1:3:2 for the syntheses of Gd, Tb, Dy, and Lu complexes. [Eu(hfaa)3(indazole)3]. Color: light yellow. Yield: 83%. Anal. found: C, 38.44; H, 1.83; N, 7.40. C36H21F18O6N6 Eu requires C, 38.35; H, 1.87; N, 7.45%. TOF MS−ES+: m/z 1128.52, [Eu(hfaa)3(indazole)3 + H]+ (100%). Mp 140 °C. 1H NMR (500 MHz, CDCl3, 298 K): (δ) ppm, 15.23 (br, 3H, N−H),12.93 (s, 3H, H-1), 11.21 (s, 3H, H-5), 8.61 and 8.81 (s, 6H, H-4 and H-2), 6.39 (s, 3H, H-3), 3.63 (S, 3H, C−H). FT-IR data (cm−1): 3409 (vs), 2366 (w), 1656 (vs), 1507 (vs), 1376 (w), 1211 (vs), 1144 (vs), 950 (s), 798 (s), 747 (s), 659 (vs), 584 (s), 517 (w). [Gd(hfaa)3(indazole)2]. Color: white. Yield: 80%. Anal. found: C, 34.29; H, 1.58; N, 5.55. C29H15F18O6N4Gd requires C, 34.33; H, 1.49; N, 5.52%. TOF MS−ES+: m/z 1015.7, [Gd(hfaa)3(indazole)2 + H]+ (41%). Mp 133 °C. FT-IR data (cm−1): 3409 (s), 2367 (w), 1658 (s), 1509 (s), 1375 (w), 1213 (s), 1146 (s), 953 (s), 793 (s), 745 (s), 661 (s), 582 (s), 516 (w), 503 (w). [Tb(hfaa)3(indazole)2]. Color: white. Yield: 81%. Anal. found: C, 34.33; H, 1.66; N, 5.48. C29H15F18O6N4Tb requires C, 34.27; H, 1.49; N, 5.51%. TOF MS−ES+: m/z 1017.1, [Tb(hfaa)3(indazole)2 + H]+ (100%). Mp 134 °C. 1H NMR (500 MHz, CDCl3, 298 K): (δ) ppm, −240.32 (br, 2H, H-1), −190.21 (br, 2H, H-5), −152.13 (br, 2H, H3), −40.11 and −20.23 (s, 4H, H-4 and H-2), 149.16 (S, 3H, C−H). FT-IR data (cm−1): 3410 (vs), 2369 (w), 1648 (s), 1493(vs), 1370 (w), 1205 (s), 1150 (s), 947 (w), 793 (w), 741 (w), 653 (w).

Table 2. Selected Bond Lengths and Bond Angles of the Complexes N(1)−Eu(1) N(2)−Eu(1) N(3)−Eu(1) O(1)−Eu(1) O(2)−Eu(1) O(3)−Eu(1) O(4)−Eu(1) O(5)−Eu(1) O(6)−Eu(1) N(1)−Tb(1) N(2)−Tb(1) O(1)−Tb(1) O(2)−Tb(1) O(3)−Tb(1) O(4)−Tb(1) O(5)−Tb(1) O(6)−Tb(1) N(1)−Dy(1)/ N(5)−Dy(2) N(2)−Dy(1)/ N(7)−Dy(2) O(1)−Dy(1)/ O(7)−Dy(2) O(2)−Dy(1)/ O(8)−Dy(2) O(3)−Dy(1)/ O(9)−Dy(2) O(4)−Dy(1)/ O(10)−Dy(2) O(5)−Dy(1)/ O(11)−Dy(2) O(6)−Dy(1)/ O(12)−Dy(2)

[Eu(hfaa)3(indazole)3] 2.602(5) O(1)−Eu(1)− O(2) 2.574(4) O(6)−Eu(1)− O(5) 2.606(4) O(3)−Eu(1)− O(4) 2.383(3) N(2)−Eu(1)− N(1) 2.451(3) N(2)−Eu(1)− N(3) 2.436(4) N(1)−Eu(1)− N(3) 2.472(4) 2.455(4) 2.422(4) [Tb(hfaa)3(indazole)2] 2.525(5) O(5)−Tb(1)− O(6) 2.515(6) O(3)−Tb(1)− O(4) 2.353(4) O(1)−Tb(1)− O(2) 2.392(4) N(2)−Tb(1)− N(1) 2.334(4) 2.372(5) 2.359(4) 2.359(4) [Dy(hfaa)3(indazole)2] 2.518(5)/2.509(5) O(4)−Dy(1)− O(3) 2.517(6)/2.486(5) O(5)−Dy(1)− O(6) 2.338(5)/2.315(5) O(1)−Dy(1)− O(2) 2.360(4)/2.352(4) N(2)−Dy(1)− N(1) 2.330(6)/2.330(5) O(10)− Dy(2)− O(11) 2.327(5)/2.331(4) O(11)− Dy(2)− O(12) 2.316(5)/2.340(4) O(7)−Dy(2)− O(8) 2.338(5)/2.343(4) N(7)−Dy(2)− N(5)

68.85(12) 68.45(13) 69.12(12) 81.41(14) 79.20(14) 140.62(14)

70.63(16) 71.68(15) 72.16(14) 143.67(19)

71.19(19) 71.35(17) 72.51(16) 145.4(2) 122.66(16) 74.20(18) 71.49(15) 142.03(18)

[Dy(hfaa)3(indazole)2]. Color: white. Yield: 80%. Anal. found: C, 34.26; H, 1.46; N, 5.60. C29H15F18O6N4Dy requires C, 34.15; H, 1.48; N, 5.49%. TOF MS−ES+: m/z 1021.5, [Dy(hfaa)3(indazole)2 + H]+ (100%). Mp 137 °C. 1H NMR (500 MHz, CDCl3, 298 K): (δ) ppm, −200.12 (br, 2H, H-1), −141.02 (br, 2H, H-5), −73.51 (br, 2H, H-3), −27.01 and −37.34 (s, 4H, H-4 and H-2), 160.01 (s, 3H, C−H). FTIR data (cm−1): 3418 (s), 2371 (w), 1658 (s), 1487 (s), 1369 (w), 1214 (s), 1152 (vs), 949 (s), 869 (w), 803 (s), 745 (w), 657 (s), 583 (s), 483(s). [Lu(hfaa)3(indazole)2]. Color: white. Yield: 82%. Anal. found: C, 33.81; H, 1.55; N, 5.46. C29H15F18O6N4Lu requires C, 33.73; H, 1.46; N, 5.43%. TOF MS−ES+: m/z 1033.41, [Lu(hfaa)3(indazole)2 + H]+ (62%). Mp 139 °C. 1H NMR (500 MHz, CDCl3, 298 K): (δ) ppm, 11.06 (br, 2H, N−H), 7.87 (s, 2H, H-1), 7.71 (d, 2H, H-5), 7.46 (d, 4H, H-3 and H-4), 7.20 (q, 2H, H-2), and 6.19 (s, 3H, C−H). FT-IR data (cm−1): 3420 (vs), 2375 (w), 1660 (s), 1494(s), 1372 (w), 1215 (s), 1156 (s), 952 (w), 796 (w), 745 (w), 658 (w). D

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of Complexes

Figure 1. TG/DTA plots of (1) [Eu(hfaa)3(indazole)3], (2) [Tb(hfaa)3(indazole)2], and (3) [Dy(hfaa)3(indazole)2].

■

RESULTS AND DISCUSSION

were allowed to react in situ, in a single pot. To our surprise the above reaction did not yield the anticipated seven-coordinate complexes, in spite of a smaller available indazole amount (Ln:indazole ratio was 1:1), and our assumption did not turn out to be true; instead, Eu gave nonacoordinate [Eu(hfaa)3-

Synthesis and Characterization. In an effort to synthesize low molecular symmetry seven-coordinate lanthanide complexes, ethanol solutions of Hhfaa, indazole, and lanthanide chlorides, and 25% aqueous NH3 solution (3:1:1:3 mol ratio) E

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 2. (a) Molecular structure of Eu complex (b) showing π−π stacking and N−H···F interactions. (c) Coordination geometry around Eu(III) ion. Thermal ellipsoids are drawn at the 30% probability level.

Figure 3. (a) Molecular packing of Tb complex showing N−H···F interactions (b) coordination geometry around Tb(III) ion. Thermal ellipsoids are drown at the 30% probability level.

shows 99.8% weight loss in one step (Figure 1), and DTA displays an endothermic peak between 180 and 200 °C for this transformation. Another peak at lower temperature, in the DTA curve, represents melting points (140 °C, Eu; 134 °C, Tb; 137 °C, Dy). The higher melting points of Dy and Eu complexes could be due to π−π stacking interactions. The volatilization begins around 180 °C and is complete by 200 °C. The thermal behavior of these complexes reflects their possible use as emissive materials managed by thermal evaporation, especially at lower temperature. The compositions of the complexes and their deposited vapor have similar composition. The IR spectrum13a of free indazole, in the solid state, displays a very strong, broad absorption band with maximum at 3181 cm−1 for the N−H stretching mode. The N−H stretching mode has been shifted to higher frequency, in the complexes, and appears as a relatively less strong sharp band (Figure S1 in SI). The observed shift and relatively less strong nature of the N−H band suggests interaction of ring nitrogen (indazole) with lanthanide ion in these complexes.13b The bands typical of lanthanide tris(β-DKs)32 (CO, CC, and CF3 stretching

(indazole)3] while Tb, Dy, and Lu yielded octacoordinate [Ln(hfaa)3(indazole)2] complexes. The initial yield was very low (between 28% and 35%) compared to the expected ones since three and two indazole units were consumed per [Eu(hfaa)3] and [Ln(hfaa)3] (terbium, dysprosium, and lutetium) units, respectively. The poor yield of the complexes prompted syntheses of these complexes using the lanthanide chloride, Hhfaa, NH4OH, and indazole in 1:3:3:3 mol ratio, in the case of Eu (3 indazole units per Eu), and 1:3:3:2 mol ratio, in the cases of Gd, Tb, Dy, and Lu atoms (2 indazole units per atoms). It improved the yield from 28% to 83%. During stirring of reaction mixtures, a white precipitate of NH4Cl appeared which does not melt up to 300 °C and was soluble in water. The formation of white precipitate, NH4Cl, authenticates success of the reaction. Europium being larger in size facilitates coordination of three indazole units in addition to three hfaa moieties which lead to the formation of nonacoordinate complex. The thermograms of nona- and octacoordinate complexes reflect that these complexes are volatile in nature. The TGA F

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 4. (a) Molecular structure of Dy complex with coordination geometry around Dy1 and Dy2, and (b) N−H···F interactions and π−π stacking. Thermal ellipsoids are drawn at the 30% probability level.

mode33) are also present with their positions shifted in the complexes. The stoichiometry of the complexes was also ascertained by the electrospray ionization mass spectrometry (ESI-MS). The ESI-MS spectra of the complexes in chloroform show the intact molecular peaks for the species, [M + H]+, with m/z values of 1128.52, 1015.7, 1017.1, 1021.5, and 1015.7, confirming the presence of three indazole units in Eu complex and two units in Gd, Tb, Dy, and Lu complexes together with three hfaa molecules (Figures S2−4 in SI). Moreover, no m/z ratio/peak corresponding to chelate, [Ln(hfaa)3 + H]+, was observed which indicates that the indazole molecules are attached strongly to the Ln(III) ion. It is important to note that CF3 groups in fluorinated β-diketonate complexes can be easily split which allows the possibility of fluorine atoms migration, and therefore, their ESI-MS spectra are quite complicated.34 The mass spectra of the complexes also show several other peaks which may be due to the formation of different ionic species, arising from various vapor phase processes like fragmentation, ligand exchange, and redistribution and could be supported by the reports available in literature.35 Crystal Structure. Slow evaporation of hexane solutions of the Eu, Tb, and Dy complexes gave very good quality singlecrystals which were then subjected to X-ray diffraction for their structure determination. The crystal data, and selected bond length and angles, obtained from single-crystal X-ray analyses are summarized in Tables 1 and 2. The single-crystal structures of the complexes are displayed in Figures 2−4. The complexes contain two independent moieties of [Eu(hfaa)3(indazole)3] and [Tb(hfaa)3(indazole)2]/[Dy(hfaa)3(indazole)2] in one unit cell. The metal ions (Eu, Tb and Dy) are well-separated in their crystal lattice with Eu···Eu, Tb···Tb, and Dy···Dy distances of 10.6, 8.02, and 10.3 Å, respectively. These distances are either equal (Tb···Tb) or larger (Eu···Eu and Dy···Dy) than the minimum distance of 8.0 Å. This is considered an optimum distance and is an advantageous aspect which minimizes energy migration between metal ions resulting in effective emission from the lanthanides.11f The Eu(III) complex is nonacoordinate and acquires a distorted monocapped square antiprism structure. It is coordinated to three N atoms of indazoles and six oxygen atoms of three hexafluoroacetylacetone moieties,

while the octacoordinate terbium and dysprosium complexes are coordinated to two N atoms of two indazole units and six oxygen atoms of three hexafluoroacetylacetone moieties making the polyhedra as LnO6N2 (Ln = Tb and Dy) with a bicapped trigonal prism geometry. The Eu and Dy complexes are stabilized by the π−π stacking interactions between indazole groups with an interplanar distance of 4.02 and 3.96 Å, respectively; as a consequence, the melting points of the two complexes are higher (Figures 2 and 4). In contrast, this interaction is not observed in the terbium complex which could be due to perpendicular positioning of indazole units. The overall stability of the complexes is reinforced by the presence of hydrogen bonds between the hydrogen atom of the N−H group of indazoles and fluorine atoms of hfaa. The distances for these interactions (N−H···F) are 2.476, 2.601, and 2.406 Å for Eu, Tb, and Dy, respectively. Recently a similar interaction was reported for a europium complex of an indazole derived ligand.36 Aside from the stability, N−H···F (hfaa−) hydrogen bonding protects Ln(III) ions from vibrations of free high energy N−H oscillators, a very important key factor for enhanced luminescence from the lanthanide complexes.37 These interactions, either π−π stacking or N−H···F hydrogen bonding, are similar to those reported for the lanthanide complexes based on a C2-symmetric cyclen scaffold.36 The Ln−O (hfaa) and Ln−N (indazole) average bond lengths, of the complexes, are 2.440(3) and 2.594(5) Å, Eu(III); 2.362(4) and 2.520(5) Å, Tb(III); and 2.334(4) and 2.497(5) Å, Dy(III), respectively. A comparison of the Eu− O(hfaa) average bond length of the present Eu complex with the average bond length of other ternary europium tris(β-DKs) complexes, [Eu(fod)3(phen)],38 [Eu(tta)2(terpyridine-carboxylate)],10a [Eu(DBM)3·dmbp·H2O],39 [Eu(tta)3(phen)],11c and [Eu(DPM)3(terpy)],11j reported in the literature reveals that Eu−O(hfaa) bond of the [Eu(hfaa)3(indazole)3] is slightly longer (0.06−0.09 Å). However, the Eu−O bond length is in perfect agreement with a closely related ternary complex of [Eu(hfaa)3].40 Similarly, the Tb−O(β-diket) and Dy−O(βdiket) bond lengths in Tb and Dy complexes are longer than the reported Tb−O and Dy−O bond length in [Tb(tta)2(terpyridine-carboxylate)],10a [Dy(bfa)3phen],41 and {[Dy(acac)3(NIT-2Py)]·0.5NIT-2Py}/{[Dy(tfa)3(NIT-2Py)]· G

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 5. 1H NMR spectra of (A) [Lu(hfaa)3(indazole)2] (inset X: higher resolution of region from 7.0 to 8.0 ppm (δ)), (B) [Eu(hfaa)3(indazole)3], and (C) [Tb(hfaa)3(indazole)2] complexes in CDCl3 at 298 K.

0.5C7H16}.42 Moreover, these distances are in agreement with those of other hfaa complexes.11l The longer Ln−O (hfaa) bond length in the present complexes compared to those of the above-mentioned complexes is the result of a stronger negative inductive effect of the six fluorine atoms on two terminals of

hfaa. The average Eu−N(indazole) bond length is either slightly shorter or equal to the Eu−N bond length in phen, terpyridine-carboxylate, dmbp, terpy, and NIT-2Py complexes mentioned above (fod, tta, DBM, DPM, bfa, acac, and tfa are the anions of 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octaH

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 6. (A) Excitation and emission spectra of complexes in chloroform and in the solid state at room temperature. (B) Excitation wavelength (λex) = 340 nm, monitored at 5D0 → 7F2 of Eu, 5D4 → 7F5 of Tb, and 4F9/2 → 6H13/2 of Dy. Concentration = 5 × 10−5 M. (C) Images of crystals of the complexes under UV light.

dione, thenoyltrifluoro acetylacetone, dibenzoylmethane, dipivaloylmethane, 4,4,4-trifluoro-1-phenyl-1,3-butanedione, acetylacetone, and trifluoroacetylacetone, respectively; dmbp = 4,4′dimethyl-2,2′-bipyridinate, terpy = 2,2′:6′,2″-terpyridine, NIT2Py = 2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl- 3oxide). The shorter Ln−N (indazole) bond length in the present complexes possibly will be an advantage for energy transfer from the triplet state of the ligands to the emitting level of the metal ion, effectively. The extents of deviation, from the ideal arrangement of monocapped square antiprism (MSAP) and bicapped trigonal prism (BCTP), of these structures were estimated by evaluating S,43a the shape-measure parameter using the following equation43 S = min Figure 7. Emission spectrum of Dy complex in chloroform at room temperature in NIR region: λex = 400 nm, monitored at 4F9/2 → 6F5/2 of Dy; concentration = 5 × 10−5 M.

⎡⎛ 1 ⎞ m ⎤ ⎢⎜ ⎟ ∑ (δi − θi)2 ⎥ ⎢⎣⎝ m ⎠ i = 1 ⎥⎦

(3)

where m is the number of possible edges (m = 21/18 for the MSAP/BCTP), δi is the obtained dihedral angle between planes along the ith edge (angle between normal lines of I

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 8. Energy diagram showing lanthanide excited states and their position with respect to the triplet states of hfaa and indazole sensitizers.

Table 3. Photophysical Parameters of [Eu(hfaa)3(indazole)3] and [Ln(hfaa)3(indazole)2] (Ln = Tb and Dy) in Solution and Solid State transition (cm−1)

fwhm (nm)a

KRAD (s−1)b

KNR (s−1)b

τobs (μs)

Qrel (%)c

D0 → 7F2 (16 255) D4 →7F6 (18 381) 4 F9/2 → 6H13/2 (17 761) 5 D0 → 7F2 (16 257) 5 D4 → 7F6 (18 382) 4 F9/2 → 6H13/2 (17 764)

6.8 9.3 5.4 6.5 9.1 5.1

6.77 × 102 5.67 × 102 1.91 × 103 6.92 × 102 5.91 × 102 2.09 × 103

3.34 × 102 2.65 × 103 3.83 × 105 3.11× 102 1.20 × 103 7.17 × 104

989 546 13.6 997 558 13.9

67 31 2.6 69 33 2.9

compd [Eu(hfaa)3(indazole)3]d [Tb(hfaa)3(indazole)2]d [Dy(hfaa)3(indazole)2]d [Eu(hfaa)3(indazole)3]e [Tb(hfaa)3(indazole)2]e [Dy(hfaa)3(indazole)2]e

5 5

a

Full width at half-maximum of emission peak {fwhm (nm)}. bRadiative rate (KRAD) and nonradiative rate (KNR) were determined using equations KRAD = Qrel/τobs and KNR = 1/ τobs − KRAD. cRelative quantum yield calculated using eqs 1 and 2. dSolution (chloroform). eSolid state.

adjacent faces), and θi is the dihedral angle for the ideal MSAP/ BCTP structure.19 A distorted nonacoordinate polyhedron may be characterized by comparing it to two polytopal nonacoordinate polyhedra: (i) tricapped trigonal prism (D3h) and (ii) monocapped square antiprismatic (C4v). The data analysis (Table S1 in Supporting Information) suggests a distorted MSAP structure for the Eu complex (Figure S5 in SI).

Moreover, the lower S value for the Dy complex as compared to the Tb complex indicates a more distorted BCTP structure of the latter. NMR Spectra. The solutions used to obtain 1H NMR spectra of the complexes were sufficiently concentrated (6−10 mg in 0.50 mL CDCl3). The NMR spectrum of the free indazole displays six resonances of equal intensity. The resonances appearing at 10.65 (broad singlet) and 8.10 (sharp singlet) ppm (δ) are assigned to N−H and H-1 protons of the ligand, respectively. The signals due to the benzene ring appear at 7.73 (d; H-5), 7.48 (d; H-2), 7.35 (t; H-4), and 7.15 (t; H-3). The NMR of the diamagnetic [Lu(hfaa)3(indazole)2] displays six signals due to indazole and one for the methine proton of the β-diketone which resonate at 6.19 ppm (δ) as a sharp singlet (Figure 5). These protons resonate in the intensity ratio of 2:2:2:2:2:2:3. The signals due to coordinated indazole are observed at 11.06 (bs; N−H), 7.87 (s; H-1), 7.71 (d; H-5), 7.46 (H-3 and H-4), and 7.20 (quartet; H-2) ppm (δ). The resonances of coordinated indazole are substantially shifted in the complex as compared to their positions in the free ligand. The intensity ratio of 3:12 between the methine (hfaa) and indazole resonances confirms that Lu(III) is coordinated to

S(D3h) = 7.67, S(C4v) = 6.48

For the octacoordinate Tb/Dy structures, the following values of shape measured are found: S(D4d ) = 13.88/13.41, S(D2d ) = 14.95/14.42, S(C2v) = 11.71/11.33

The lower value of S for C2v shows that the coordination polyhedron around Tb and Dy ions is closest to an idealized BCTP structure. It is worth mentioning that a majority of the reported octacoordinate compounds acquire square antiprismatic geometry11e,l,c,44 while present Tb/Dy complexes adopt BCTP geometry. It could be due to the presence of freely movable Ln−N (indazole) bonds which provide a highly distorted coordination environment around the Tb/Dy ion. J

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 9. Molecular structures and device configuration.

two molecules of indazole and three moieties of hexafluoroacetylacetone and has acquired an eight-coordinate structure. The 1H NMR spectra of complexes containing paramagnetic metal ions are more interesting because paramagnetic metal ions induce huge upfield or downfield shifts of the resonances of the coordinated diamagnetic ligands. The NMR spectrum of the [Eu(hfaa)3(indazole)3] displays seven signals, one due to hfaa and six due to indazole (Figure 5B). The resonances of indazole are substantially downfield shifted as compared to their positions in the diamagnetic Lu complex (Figure 5A), and the 3:3:3:3:3:3:3 intensity ratio substantiates coordination of three indazole and three hfaa units making the europium nonacoordinate. Moreover, a sharp singlet appearing at 3.61 ppm (δ) whose intensity is equivalent to 3 protons is due to C−H protons of the β-diketone and the resonances at 15.39 (bs; N−H), 12.91 (s; H-1), 11.19 (s; H-5), 9.0(s; H-3), 8.5 (s; H-4), and 6.23 (s; H-2) ppm (δ) are assigned to coordinated

Table 4. Electroluminescence Parameters of the Devices with Different Structures device

Vona (V)

Vmaxb (V)

Jc (mA/cm2)

Ld (cd/m2)

ηe (cd/A)

2a 2b 3a 3b 4a 4b 5a 5b 5c

9.1 7.3 10.2 8.1 12.4 10.1 5.9 7.8 8.6

16.3 15.2 18.2 16.3 19.1 18.4 15.2 16.8 17.7

202 183 206 198 229 200 198 183 195

832 1152 302 352 258 296 1750 693 326

2.52 3.81 1.41 1.98 0.39 0.51 4.19 2.01 0.66

a

Turn on voltage. bVoltage at maximum brightness. cCurrent density. Brightness. eCurrent efficiency.

d

K

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 10. Current density−voltage−brightness (J−V−B) curve of devices: 5a, 5b, and 5c. Figure 11. EL spectra of devices 2a, 3a, and 4a at different operating voltages.

indazole. The NMR spectra of [Tb(hfaa)3(indazole)2] (Figure 5C) and [Dy(hfaa)3(indazole)2] (Figure S6 in SI) display six signals, one due to hfaa and five due to indazole. The relative intensity of the signals (3:2:2:2:2:2) authenticates the presence of two indazole units and three molecules of β-diketone in the complexes, making the metal ions (Tb and Dy) octacoordinate. The N−H protons’ signal could not be detected in the region between 300 and −300 ppm. The absence of the N−H signal could be related to enhanced relaxation of these protons by virtue of higher magnetic moment of Tb (μTb = 9.72) and Dy (μDy = 10.65). The Tb and Dy induced huge upfield paramagnetic shifts which are larger for Tb. The proton resonances in [Tb(hfaa)3(indazole)2] are spread between −242 and 148 ppm (Figure 5C). In the case of [Dy(hfaa)3(indazole)2], the H-1 protons of indazoles resonate at much higher field side of the TMS, −197.54 ppm (δ), while the C−H protons of hfaa suffered downfield shift and resonate at 160.6 ppm (δ), covering a range of ∼282 ppm (δ) (Figure S6 in SI). It is important to mention that in a given complex the signals of indazole and hfaa have been shifted in opposite directions. For example, in Tb and Dy complexes indazole protons resonate to

Figure 12. EL spectra of devices 2b, 3b, and 4b.

the higher field side of TMS whereas C−H protons of hfaa appear at much lower field relative to Me4Si. The reverse of this is noted for the Eu complex. For Tb and Dy complexes the upfield shift is highest for H-1 proton and reveals that the L

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 13. Energy band diagram of the devices.

of [Gd(hfaa)3(indazole)2] recorded at 77 K (λex = 388 nm) is in the visible region (Figure S9 in SI). The spectrum shows two maxima centered at 21 930 and 22 935 cm−1. Since the maximum at 21 930 cm−1 corresponds to ET(0−0)(hfaa)44 and therefore, the other maxima at 22 935 cm−1 could safely be assigned to energy of the triplet state of indazole, ET(0−0)(indazole). The emission transitions 5D0 → 7Fj(j=0,1,2,3,4) of Eu(III) are responsible for the characteristic red luminescence from this metal (Figure 6). The intense red luminescence could be related to a more asymmetric and fully saturated coordination environment (EuO6N3) around this metal ion.45 Although the emission intensity of the magnetic-dipole 5D0 → 7F1 transition does not depend on the chemical environment around the metal, even then splitting of this transition into many stark components tells about the site symmetry around Eu(III) ion.46 The presence of stark components in this transition, of the present Eu complex, favors a single low symmetry site around Eu ion. Moreover, the intensity ratio (ηEu)47 of the transitions, 5 D0 → 7F2/5D0 → 7F1, for [Eu(hfaa)3(indazole)3] is 11.40. The value is very high and suggests a highly low symmetric coordination environment47 around the Eu ion. Additionally, a single peak for the 5D0 → 7F0 (17 267 cm−1) transition favors there being only one type of chemical environment around the Eu ion in a unit cell.48 For an efficient energy transfer from the ligands to the Eu(III) ion, the optimum energy difference,49 ΔE (3π π* 5 D0), between the triplet state of the ligands and emitting level of Eu should be >2500 cm−1. For the present system these differences are {ET(0−0)(hfaa) − 5D0} and {ET(0−0)(indazole) − 5 D0}, 4430 and 5435 cm−1, respectively. These energy differences suggest efficient transfer of energy from both hfaa− and indazole to the europium ion. The PLQ11l values for the [Eu(hfaa)3(indazole)3] complex are 67% and 69% in solution and in the solid state, respectively. However, the PLQ for [Eu(hfaa)3(H2O)2] is only 26%11l which is much lower than the PLQ of the present complex. It is evident that the replacement of water molecules from the coordination sphere of [Eu(hfaa)3(H2O)2] by the indazoles leads to an approximately 26-fold enhancement in quantum efficiency. It could be related to the check in the depletion of energy by removing the high frequency O−H oscillators which are very active in nonradiative deactivation of energy. The above discussions reveal that indazole is quite efficient in populating the emitting level of Eu(III) and appears as a potential source/material for europium sensitization. Furthermore, the PLQy of the Eu complex is significantly higher than those reported for [Eu(hfaa)3(bpyO2)]11f (35% solution and 40% solid); [Eu-

dipolar shift depends upon the distance between the metal ion and the resonating nucleus and decreases with increasing distance of the proton from the metal. This and opposite direction shifts of β-diketone and indazole, in a given complex, signify that the shifts are dipolar in nature. Absorption Spectroscopy. The UV−vis spectra of the uncoordinated ligands show a strong 1π−π* transition (Figure S7 in SI). The maxima of these singlet-to-singlet electronic excitations appear at 294 (hfaa) and 275 (indazole) with high ϵ values (ϵhfaa = 6.7 × 103 M−1 cm−1 and ϵindazole = 7.7 × 103 M−1 cm−1), in chloroform (5 × 10−3 M). The higher ϵ values of ligands ascertain that these have strong light absorbing property. The electronic spectra of the complexes display a strong band with a maximum at 375 nm (Figure S8 in SI). It is essentially the overlapping absorption band of the two ligands. The combined ligands’ absorption band in the complexes are red-shifted (nephelauxetic effect) by about 56 nm from 294 to 350 nm in the case of indazole and 75 nm from 275 to 350 nm in the case of hfaa and proves coordination of the ligands. The more than 3-fold higher values of the molar extinction coefficients of the complexes (ϵEu = 2.81 × 104 M−1 cm−1, ϵTb = 2.31 × 104 M−1 cm−1, and ϵDy = 2.41 × 104 M−1 cm−1) in comparison with the ligands confirm coordination of three hfaa and two or more indazoles to the metal ions. Excitation and Emission Spectra. The overlapping of excitation and absorption spectra of the complexes is a mechanistic approach for transfer of energy from ligands to metal center. For the present complexes, the excitation spectra, obtained in CHCl3 solution, display good overlap with their absorption spectra in the UV− vis region, and, therefore, suggest energy transfer from hfaa/indazole to Ln ions. We have also recorded the excitation spectra of the solid samples where they cover relatively greater spectral range and account for an effective transfer of energy in the solid state, too (Figure 6). The Tb and Dy complexes display, in their excitation spectra, broad bands while Eu displays several weak intraconfigurational f−f transitions at 453 nm (22 075 cm−1, 5D2 ← 7F0,1) and 537 nm (18 621 cm−1, 5D1 ← 7F0,1) together with the broad band consequently demonstrating that the excitation of ligands is effective for sensitization of emission from these complexes. Furthermore, it is observed that the absorption band of the ligands (hfaa or indazole) and excitation band of the complexes overlap effectively, which indicates that both the ligands hfaa and indazole are efficient in transference of energy to the trivalent lanthanides. The energy of the triplet state of hexafluoroacetylacetone,44 {ET(0−0)(hfaa)}, is 21 930 cm−1. In order to find out the energy of the triplet state of indazole, the phosphorescence spectrum M

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

differences between the energy levels {ET(0−0)(hfaa) − 4F9/2 = 1100 cm−1} and {ET(0−0)(indazole) − 4F9/2 = 2105 cm−1} for the present Dy(III) system appear appropriate since the quantum yield noted for the present dysprosium complex is much higher than those available for other Dy complexes.41,54 The transitions are weak in the NIR region (Figure 8) since some of the energy from the triplet state of the ligands is also transferred to states lower in energy than the emitting 4F9/2 level of dysprosium, and therefore, the PLQy in NIR region is 0.0041% only which is quite low.54 Decay Curves. The emission decay curves, measured at 300 K, are monoexponential and show that the complexes have only one luminescent site (Figure S10 in SI). The emission lifetimes were estimated from the slope of the logarithmic plot of decay profiles (Table 3). It is worth mentioning that the emission lifetime of the levels 5D0 {989 μs; [Eu(hfaa)3(indazole)3]} and 5 D4 {546 μs; [Tb(hfaa)3(indazole)2]} show significant improvement over the parent hydrated chelate, [Eu(hfaa)3(H2O)2]11l (700 μs) and [Tb(hfaa)3(H2O)2]11l (530 μs). Furthermore, the emission lifetime of 5D0, 5D4, and 4F9/2 states of Eu, Tb, and Dy systems are lengthier than those of similar ternary lanthanide β-DKs complexes, [Eu(hfaa) 3 (bpyO 2 )] 1 1 j (700 μs), [Eu(NTA) 3 bpy]/[Eu(NTA)3phen]50 (620/662 μs), [Eu(PBI)3bpy]11d (978 μs), [Eu(tta)3phen]58 (700 μs), and [Eu(btfa)3phen]59 (210 μs), [Eu(tta) 2 (terpyridine-carboxylate)] 10a (790 μs), [Eu(fod)3(phen)]38 (850 μs), Eu(TTA)3Phen (710 μs), [Tb(hfaa)3(bpyO2)]11f (∼5 × 10−5 μs), [Tb(tta)2(terpyridinecarboxylate)]10a (340 μs), [Dy(PM)3(TP)2]19 (7.24 μs), and [Dy(bfa)3phen]41 (0.77 μs). It is important to mention that the metal ions in these complexes are coordinatively saturated by the coordination of three hfaa− moieties (which have low vibrational frequency C F oscillators) and ancillary indazole ligands (with suitable triplet state). The coordinatively saturated environment around Ln(III) ions protects them from the outer environment, and the outcome of this is higher quantum yields and longer emission radiative lifetimes.10a,11d−g,l The emission spectra of the solid complexes were also studied in order to see the impact of solvent (the high frequency vibrational oscillator, C−H, of chloroform) on emission from these complexes (Figure 6). It is important to mention that the emission transitions of the complexes, in the solid state, are shifted to higher wavelength, a red shift (nephelauxetic effect), and appear with enhanced intensity and larger stark splitting. The red shift of the maxima with enhanced intensity and larger stark splitting suggests higher degree of covalency in the solid complexes and greater asymmetry of the field around the lanthanide in the solid state. A relatively less asymmetric field surrounding the lanthanides together with high frequency C−H vibrational oscillators (which are quite effective in dissipating energy through radiation less transitions) of the chloroform are collectively responsible in lowering the emission transition probabilities which in turn decreases PLQy in solution. It reflects that optically pure colored clear emission could be achieved in solid state. EL Properties. In order to study the electrochemical behavior of the complexes, 12 devices with different structures were designed and fabricated using present complexes as emitting layers (Figure 9). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the complexes were measured by cyclic voltammetry (CV) to realize the charge injection/transport

(NTA)3bpy]50 (51%), [Eu(hfa)3(DPPPO)]51 (61%); [PtLEu(hfaa)3]40 (38%) and [Eu(tta)2(terpyridine-carboxylate)]10a (66% solid; 54% THF; 23% methanol; 62% PMMA; 60% PVA), {bpyO2 = 2,2′-bipyridine-N,N′-dioxide, NTA = 1-(2naphthoyl)-3,3,3-trifluoroacetonate, DPPPO = {bis[o(diphenylphosphoryl)phenyl]phenylphosphane}. The Tb complex displays characteristic green luminescence due to 5D4 → 7FJ=6−0 transitions (Figure 6). The transition 5D4 → 7F6 is reasonably profound to the surroundings (ligands and solvents) and coordination geometry of the complex (TbO6N2), whereas the transition 5D4 → 7F5 is used as the standard for evaluating variations in the ratios of the emission intensities prompted by the ligands.11l,52 The intensity ratios for various emission transitions of [Tb(hfaa)3(H2O)2],11l are f4−6 (0.22), f4−5 (1.00), f4−4 (0.11), f4−3 (0.08), and f4−2 (0.08). These ratios calculated for [Tb(hfaa)3(indazole)2] are f4−6 (0.39), f4−5 (1.00), f4−4 (0.15), f4−3 (0.07), and f4−2 (0.06). It is evident that replacement of two water molecules by two indazole units results in approximately 2-fold enhancement in the intensity of the 5D4 → 7F6 transition. Moreover, the optimum energy difference49 for effective transfer of energy from the triplet state of the ligand and emitting state of terbium, ET(0−0) (organic ligands) − E (5D4) state (Tb), should be 2400 ± 300 cm−1. However, for the present Tb complex, the energy difference, ET(0−0) (indazole) − E 5D4 (Tb), of ∼2505 cm−1, is appropriate for transfer of energy while ET(0−0) (hfaa) − 5D4 (Tb) is much less (∼1500 cm−1) than recommended. It results in poor transfer of energy and additionally would favor back energy transfer from Tb(III) ion to hfaa, which would be detrimental for effective emission. In order to confirm back energy transfer, the emission spectrum of the complex was recorded at 77 K. At low temperature emission lifetime is increased from 546 μs (room temperature) to 788 μs. The PLQy (31%) of the present complex is higher than the PLQy of [Tb(hfaa)3(H2O)2]11l (27%). It corroborates with earlier observation. The PLQy of [Tb(hfaa)3(indazole2)] is considerably larger than the [Tb(hfaa)3(bpyO2)] (0.75%),11f [Tb(h fa a) 3 ( b p y ) ] ( 3 . 4 % 5 3 a a n d 29 % 5 3 b ) , an d [ T b (tfac)2(terpyridine-carboxylate) ] (13%).10a Reports on emission of β-diketone complexes of dysprosium (both in the vis and NIR regions) are scanty;41,54 however, the luminescence from Dy(III) is of much significance since emission from its complexes, in the visible range, is white/ yellow. The visible region emission from [Dy(hfaa)3(indazole)2] consists of two transitions, (i) 4F9/2 → 6H15/2 (magnetic-dipole) and (ii) 4F9/2 → 6H13/2 (hypersensitive, electric-dipole) (Figure 6). The emission intensity ratio of these transitions is very high (3.7) and suggests that this complex lacks a center of symmetry.55 The electric-dipole transition, 4 F9/2 → 6H13/2, is very peculiar, since it is probed to determine coordination symmetry around Dy(III) systems.56 A very high intensity of this transition could be related to the bicapped trigonal prism geometry (BCTP) of the [Dy(hfaa)3(indazole)2] complex. The PLQy of the complex is 2.8% which is much higher than those reported for [Dy(bfa)3phen]41 (0.2%) and [Dy(hfa)3(bpy)] (0.4%53a and 1.7%53b). The NIR region of the emission spectrum of [Dy(hfaa)3(indazole)2] (Figure 7) displays crystal-field fine structures (stark splitting) in some of the peaks which indicates that Dy(III) ion occupies well-defined crystallographic sites in the complex.57 The optimum energy difference required for effective energy transfer from ligand triplet state to the emitting level of dysprosium is not known with certainty.55 The N

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

complexes, since it is directly proportional to PL. The ΦEL is related to the ΦPL by the relationship62,63

properties. The internal standard used was the redox couple ion, Fc/Fc+ (ferrocene/ferricenium). The CV scan of the chloroform solution of the complexes showed two adjoining oxidation peaks. The intersection point of the two tangents, drawn at the rising and background currents, was used to find out the onset potential (Figure 11 in SI). The highest occupied molecular orbital/lowest unoccupied molecular orbital levels of the complexes were determined by following equation.60

ΦEL = ηoutηe/h ΦPL

ηout and ηe/h are the fraction of emitted photons coupled out of the device and efficiency of excitons in the emitting layer, respectively. The EL efficiencies of the present devices are in the order 5a > 5b > 5c. For the devices 2a, 3a, and 4a, the EL spectra show characteristic emission lines of Eu, Tb, and Dy ions as well as a strong emission band between 500 and 520 nm and a weak peak at 400 nm, respectively, due to BCP and TDP layers (Figure 11). It is interesting to note that the emission observed from BCP is very strong at low voltage and become weak as the voltage rises, whereas the opposite trend is observed for TDP emission (Figure 11). These behaviors of the layers could be attributed to shifting in the electron/hole recombination. It reflects that, in the recombination zone, the charge carriers are not adequately balanced. Since the holes’ mobility is relatively higher at low voltages, as compared to electrons, the remaining holes move toward the BCP layer and get combined there with electrons to form few excitons which results in a weak emission from BCP. However, with increasing voltage the electrons’ mobility also increases.64 An increasing voltage shifts the recombination zone slowly toward the interface of emitting and TDP layers, and therefore, emission is observed from both TDP and Ln(III) ions. In contrast, for the devices 2b, 3b, and 4b, obtained by replacement of TDP layers by CuPc layer, no emission was observed, except from the emitting layers (complexes) (Figure 12). It is believed that the relatively low lying HOMO level (5.2 eV) of CuPc as compared to that of TDP (5.4 eV) is responsible for the decrease in hole injection rate to the emitting layer as well as to the BCP layer which leads to well-adjusted and more confined charge carriers within the emitting layer, in the presence of CuPc layer (Figure 13). As a result, the devices 2b, 3b, and 4b are more efficient than the devices 2a, 3a, and 4a, respectively. On the other hand, the devices 5a, 5b, and 5c are superior, in terms of stability and efficiency, to all devices under this study. This could be due to an increase in thickness of the emitting layer and BCP layer that provides larger recombination zone and, therefore, decrease in the quenching probability as well as the total current density within devices.

E HOMO = −(1.4 ± 0.1) × (qVCV ) − (4.6 ± 0.08) eV

The ELUMO was determined by subtracting the singlet energy gap (Eg) from the EHOMO level. In the case of [Eu(hfaa)3(indazole)3], the HOMO and LUMO levels were calculated and are −2.9 and −5.7 eV, respectively, while these energy levels (HOMO and LUMO) obtained for Tb and Dy complexes are similar to those of the Eu complex. On the other hand, the HOMO and LUMO levels of CBP layer are −2.8 and −6.1 eV, respectively, which are very close to HOMO and LUMO energy levels of the complexes and would ease to confine the charge on the layer which is emitting. Furthermore, the triplet state energies (ET) of CBP and BCP layers (2.6 and 2.5 eV, respectively) are higher than the ET (−2.4 eV) of the complexes. It would be an added feature which will help in trapping the charge as well as in properly retaining excitons within the layers which emit. The EL and PL spectra of the devices 1a and 1b are similar in shape and display characteristic emission peaks of Eu and Tb ions. These devices begin illumination at the voltage (turn on voltage, Von) of 10 and 12 V, respectively, and achieve maximum brightness (Lmax) of 42 and 28 cd/m2 at the voltages 17 and 19 V. For the devices 2a and 2b, the Lmax of 832 and 1152 cd/m2 were attained at 16.3 and 15.2 V with Von 9.1 and 7.3 V, respectively (Table 4). It is worth mentioning that the device 2b starts functioning at low voltage and is brighter with better productivity, compared to device 2a. It could be due to well-balanced charge injection and transportation within device 2b, since the carrier injection/transportation ability of each layer is related to turn on voltage.61 Furthermore, the [Eu(hfaa)3(indazole)3] complex shows intermolecular π−π interactions between indazole units which facilitate retention of charge within the device, and could be an important factor in improving the EL efficiencies. The devices (3a and 3b) based on the Tb complex showed the Lmax of 302 and 352 cd/m2 at 18.2 and 16.3 V with Von of 10.2 and 8.1 V, respectively. While the Lmax values of 258 and 296 cd/m2 were obtained for the [Dy(hfaa)3(indazole)2] based devices (4a and 4b), at 19.1 and 18.4 V, respectively. It is important to mention that a significant improvement, in performance of the devices, is observed when the device structure is changed from two layered to four layered. In order to make devices more efficient, some further modifications were made in four layered structure, like variation in thickness of layers, since the device efficiency is strongly affected by change in thickness of the emitting layer and BCP layer.19 For this reason, the devices 5a, 5b, and 5c were fabricated in which the thickness of emitting layer and BCP layer were increased to 80 and 25 nm, respectively. Interestingly, distinctive improvement in brightness is found for these three devices with Lmax of 1750, 693, and 326 cd/m2, achieved at 15.8, 16.8, and 17.7 V with current efficiencies of 4.19, 2.01, and 0.66 cd/A, respectively (Figure 10). It is important to mention that the efficiency of the electroluminescence depends on photoluminescence efficiency of the

■

CONCLUSION

The reaction of hydrated Ln(III) chlorides, Hhfaa, and indazole in the presence of ammonium hydroxide in one pot (in situ) gives rise to formation of [Eu(hfaa)3(indazole)3] and [Ln(hfaa)3(indazole)2] (Ln = Gd, Tb, Dy, and Lu) complexes. The Eu in the complex acquired distorted monocapped square antiprismatic structure and is nonacoordinated. The terbium and dysprosium complexes are octacoordinated and acquire trigonal bicapped prism geometry. The Eu and Dy complexes are stabilized by the π−π stacking interactions between indazole groups with an interplanar distance of 4.02 and 3.96 Å, respectively, while this staking is absent in the Tb complex due to perpendicular positioning of indazole units. The overall stability of the complexes is reinforced by the presence of hydrogen bonds between N−H group of indazoles and fluorine atoms of hfaa, N−H···F (hfaa−), which facilitates the photoluminescence and electroluminescence properties. The coordination environment, which consists of of three hfaa− and O

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(8) Quici, S.; Marzanni, G.; Forni, A.; Accorsi, G.; Barigelletti, F. Inorg. Chem. 2004, 43, 1294−1301. (9) Weissman, S. I. J. Chem. Phys. 1942, 10, 214−217. (10) (a) Andreiadis, S. A.; Gauthier, N.; Imbert, D.; Demadrille, R.; Pécaut, J.; Mazzanti, M. Inorg. Chem. 2013, 52, 14382−14390. (b) Zheng, Y.; Lin, J.; Liang, Y.; Lin, Q.; Yu, Y.; Meng, Q.; Zhou, Y.; Wang, S.; Wang, H.; Zhang, H. J. Mater. Chem. 2001, 11, 2615−2619. (11) (a) Bellusci, A.; Barberio, G.; Crispini, A. G.; Ghedini, M.; La Deda, M.; Pucci, D. Inorg. Chem. 2005, 44, 1818−1825. (b) Moudam, O.; Rowan, B. C.; Alamiry, M.; Richardson, P.; Richards, B. S.; Jones, A. C.; Robertson, N. Chem. Commun. 2009, 6649−6651. (c) Freund, C.; Porzio, W.; Giovanella, U.; Vignali, F.; Pasini, M.; Destri, S.; Mech, A.; Di Pietro, S.; Di Bari, L.; Mineo, P. Inorg. Chem. 2011, 50, 5417− 5429. (d) Biju, S.; Raj, D. B. A.; Reddy, M. L. P.; Kariuki, B. M. Inorg. Chem. 2006, 45, 10651−10660. (e) Zucchi, G.; Murugesan, V.; Tondelier, D.; Aldakov, D.; Jeon, T.; Yang, F.; Thuery, P.; Ephritikhine, M.; Geffroy, B. Inorg. Chem. 2011, 50, 4851−4856. (f) Eliseeva, S. V.; Pleshkov, D. N.; Lyssenko, K. A.; Lepnev, L. S.; Bünzli, J. C. G.; Kuzmina, N. P. Inorg. Chem. 2011, 50, 5137−5144. (g) Raj, D. B. A.; Francis, B.; Reddy, M. L. P.; Butorac, R. R.; Lynch, V. M.; Cowley, A. H. Inorg. Chem. 2010, 49, 9055−9063. (h) He, G. J.; Guo, D.; He, C.; Zhang, X. L.; Zhao, X. W.; Duan, C. Y. Angew. Chem., Int. Ed. 2009, 48, 6132−6135. (i) Zaim, A.; Nozary, H.; Guenee, L.; Besnard, C.; Lemonnier, J. F.; Petoud, S.; Piguet, C. Chem. - Eur. J. 2012, 18, 7155−7168. (j) Holz, R. C.; Thompson, L. C. Inorg. Chem. 1988, 27, 4640−4644. (k) Yang, C.; Fu, L.-M.; Wang, Y.; Zhang, J.-P.; Wong, W.-T.; Ai, X.-C.; Qiao, Y.-F.; Zou, B.-S.; Gui, L.-L. Angew. Chem., Int. Ed. 2004, 43, 5010−5013. (l) Eliseeva, S. V.; Kotova, O. V.; Gumy, F.; Semenov, S. N.; Kessler, V. G.; Lepnev, L. S.; Bünzli, J. C. G.; Kuzmina, N. P. J. Phys. Chem. A 2008, 112, 3614−3626. (m) Lemonnier, J. F.; Babel, L.; Guenee, L.; Mukherjee, P.; Waldeck, D. H.; Eliseeva, S. V.; Petoud, S.; Piguet, C. Angew. Chem., Int. Ed. 2012, 51, 11302−11305. (n) Stanley, J. M.; Zhu, X. J.; Yang, X. P.; Holliday, B. J. Inorg. Chem. 2010, 49, 2035−2037. (12) Boo, P. A.; Casas, J. S.; Casellato, U.; Couce, M. D.; Freijanes, E.; Graziani, R.; Salgado, B.; Russo, U.; Sordo, J. J. J. Organomet. Chem. 1997, 530, 141−148. (13) (a) Bigotto, A.; Zerbo, C. Spectrosc. Lett. 1990, 23, 65−75. (b) Thakur, P.; Chakravortty, V.; Dash, K. C. Polyhedron 1997, 16, 1417−1424. (14) (a) Su, C.-Y.; Kang, B.-S.; Liu, H.-Q.; Wang, Q.-G.; Chen, Z.-N.; Lu, Z.-L.; Tong, Y.-X.; Mak, T. C. W. Inorg. Chem. 1999, 38, 1374− 1375. (b) Su, C.-Y.; Kang, B.-S.; Liu, H.-Q.; Wang, Q.-G.; Mak, T. C. W. Chem. Commun. 1998, 1551−1552. (c) Oki, A. R.; Bommarreddy, P. R.; Zhang, H.; Hosmane, N. Inorg. Chim. Acta 1995, 231, 109−114. (15) Liang, F.; Zhou, Q.; Cheng, Y.; Wang, L.; Ma, D.; Jing, X.; Wang, F. Chem. Mater. 2003, 15, 1935−1937. (16) (a) Yu, J.; Zhou, L.; Zhang, H.; Zheng, Y.; Li, H.; Deng, R.; Peng, Z.; Li, Z. Inorg. Chem. 2005, 44, 1611−1618. (b) Huang, L.; Wang, K.-Z.; Huang, C.-H.; Li, F.-Y.; Huang, Y.-Y. J. J. Mater. Chem. 2001, 11, 790−793. (17) Bian, Z. Q.; Gao, D. Q.; Wang, K. Z.; Jin, L. P.; Huang, C. H. Thin Solid Films 2004, 460, 237−241. (18) Wang, R.; Song, D.; Seward, C.; Tao, Y.; Wang, S. Inorg. Chem. 2002, 41, 5187−5192. (19) Li, Z.-F.; Zhou, L.; Yu, J.-B.; Zhang, H.-J.; Deng, R.-P.; Peng, Z.P.; Guo, Z.-Y. J. Phys. Chem. C 2007, 111, 2295−2300. (20) (a) Sun, P. P.; Duan, J. P.; Shih, H. T.; Cheng, C. H. Appl. Phys. Lett. 2002, 81, 792−794. (b) Wang, J.; Wang, R.; Yang, J.; Zheng, Z.; Carducci, M. D.; Cayou, T.; Peyghambarian, N.; Jabbour, G. E. J. Am. Chem. Soc. 2001, 123, 6179−6180. (c) Sun, M.; Xin, H.; Wang, K. Z.; Zhang, Y. A.; Jin, L. P.; Huang, C. H. Chem. Commun. 2003, 702−703. (d) Liang, F. S.; Zhou, Q. G.; Cheng, Y. X.; Wang, L. X.; Ma, D. G.; Jing, X. B.; Wang, F. S. Chem. Mater. 2003, 15, 1935−1937. (21) (a) Adachi, C.; Baldo, M. A.; Forrest, S. R. J. Appl. Phys. 2000, 87, 8049−8055. (b) Noto, M.; Irie, K.; Era, M. Chem. Lett. 2001, 320− 321. (c) Fang, J. F.; Ma, D. G. Appl. Phys. Lett. 2003, 83, 4041−4043. (22) (a) Yu, J.; Zhou, L.; Zhang, H.; Zheng, Y.; Li, H.; Deng, R.; Peng, Z.; Li, Z. Inorg. Chem. 2005, 44, 1611−1618. (b) Canzler, T. W.;

indazoles, provides an effective shield around the Eu, Tb, and Dy that protects the ions from the outer environment (surrounding), and the outcome of this protection is reflected in the form of better luminescence (larger quantum yields) and longer radiative lifetimes. Finally, possible use of these compounds was explored in fabricating electroluminescent devices by making their emitting layers. The devices having CuPc layers showed better performance as compared to the devices with TDP layers. It corroborates with our earlier observation.53b Moreover, the best devices were found with the following structure: ITO/CuPc (15 nm)/[Eu complex]:CBP or [Tb complex]:CBP or [Dy complex]:CBP (80 nm)/BCP (25 nm)/AlQ (30 nm)/LiF (1 nm)/Al (100 nm). These were achieved by increasing thicknesses of the layer.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01630. All crystallographic data have been deposited with the Cambridge Crystallographic Centre. CCDC reference numbers 999605−999607 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033; e-mail: [email protected]). Additional figures and table (PDF) Crystallographic details (CIF) Crystallographic details (CIF) Crystallographic details (CIF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-11-26837297. Fax: +91-11-26980229/26982489. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Prof. Bachcha Singh, Department of Chemistry, Banaras Hindu University, Varanasi, for his help in getting the microanalyses, and Mr. Avatar Singh, SAIF, Panjab University, Chandigarh, for recording NMR of the complexes. The authors are thankful to Prof. U. P. Singh, Department of Chemistry, IIT Roorkee, for X- ray diffraction data. Part of this research is supported by the UGC Special Assistance Programme (DRSII) of the Department of Chemistry, Jamia Millia Islamia (No. F.540/8/DRS/2013/SAP-I), which is gratefully acknowledged.

■

REFERENCES

(1) Parker, D.; Williams, J. A. G. J. Chem. Soc., Dalton Trans. 1996, 3613−3628. (2) Bradley, J. D. B.; Pollnau, M. Laser Photonics Rev. 2011, 5, 368− 403. (3) Sage, I.; Bourhill, G. J. Mater. Chem. 2001, 11, 231−245. (4) Kido, J.; Okamoto, Y. Chem. Rev. 2002, 102, 2357−2368. (5) Martín-Ramos, P.; Coya, C.; Alvarez, A. L.; Silva, M. R.; Zaldo, C.; Paixao, J. A.; Chamorro Posada, P.; Martín-Gil, J. J. Phys. Chem. C 2013, 117, 10020−10030. (6) Law, G.-L.; Kwok; Wong, W.-T.; Wong, K.-L.; Tanner, P. A. J. Phys. Chem. B 2007, 111, 10858−10861. (7) Law, G.-L.; Wong, K.-L.; Tam, H.-L.; Cheah, K.-W.; Wong, W.-T. Inorg. Chem. 2009, 48, 10492−10494. P

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Kido, J. Org. Electron. 2006, 7, 29−37. (c) Chen, Z.; Ding, F.; Hao, F.; Bian, Z.; Ding, B.; Zhu, Y.; Chen, F.; Huang, C. Org. Electron. 2009, 10, 939−947. (23) Melhuish, W. H. J. Phys. Chem. 1961, 65, 229−235. (24) Meshkova, S. B.; Topilova, Z. M.; Bolshoy, D. V.; Beltyukova, S. V.; Tsvirko, M. P.; Venchikov, V. Y. Acta Phys. Pol. A 1999, 95, 983− 990. (25) Bril, A.; De Jager-Veenis, A. W. J. Electrochem. Soc. 1976, 123, 396−398. (26) Melhuish, W. H. J. Opt. Soc. Am. 1964, 54, 183−186. (27) Mello Donega, C. D.; Alves, S., Jr.; de Sa, G. F. Chem. Commun. 1996, 1199−1200. (28) Bruker Analytical X-ray Systems. SMART: Bruker Molecular Analysis Research Tool, Version 5.618; Bruker AXS: Madison, WI, 2000. (29) Bruker Analytical X-ray Systems. SAINT-NT, Version 6.04; Bruker AXS: Madison, WI, 2001. (30) Bruker Analytical X-ray Systems. SHELXTL-NT, Version 6.10; Bruker AXS: Madison, WI, 2000. (31) Klaus, B. DIAMOND, Version 1.2c; University of Bonn: Bonn, Germany, 1999. (32) Nakanishi, K. Infrared Spectra and Structure of Organic Compounds (Russian Translation); Mir Publishers: Moscow, 1965. (33) Richardson, F. S.; Wagner, W. F.; Sands, D. E. J. Inorg. Nucl. Chem. 1968, 30, 1275−1284. (34) Fratini, A.; Swavey, S. Inorg. Chem. Commun. 2007, 10, 636− 638. (35) (a) Hunter, A. P.; Lees, A. M. J.; Platt, A. W. G. Polyhedron 2007, 26, 4865−4876. (b) Zhang, J.; Badger, P. D.; Geib, S. J.; Petoud, S. Inorg. Chem. 2007, 46, 6473−6482. (36) Liu, T.; Nonat, A.; Beyler, M.; Regueiro-Figueroa, M.; Nono, K. N.; Jeannin, O.; Camerel, F.; Debaene, F.; Cianferani-Sanglier, S.; Tripier, R.; Platas-Iglesias, C.; Charbonniere, L. J. Angew. Chem., Int. Ed. 2014, 53, 7259−7263. (37) (a) Comby, S.; Bünzli, J.-C. G. Lanthanide near-infrared luminescence in molecular probes and devices. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, J. A., Jr., Bünzli, J.C. G., Pecharsky, V., Eds.; Elsevier Science B.V.: Amsterdam, 2007; Vol. 37, Chapter 235 and references therein. (b) Yanagida, S.; Hasegawa, Y.; Murakoshi, K.; Wada, Y.; Nakashima, N.; Yamanaka, T. Coord. Chem. Rev. 1998, 171, 461−480. (38) dos Santos, E. R.; Freire, R. O.; da Costa, N. B., Jr.; Filipe, A.; Paz, A.; de Simone, C. A.; Junior, S. A.; Arau jo, A. A. S.; Nunes, L. A. O.; Mesquita, M. E. de; Rodrigues, M. O. J. Phys. Chem. A 2010, 114, 7928−7936. (39) Chen, X.-F.; Zhu, X. H.; Xu, Y.-H.; Raj, S. S. S.; Ozturk, S.; Fun, H.-K.; Mac, J.; You, X.-Z. J. Mater. Chem. 1999, 9, 2919−2922. (40) Ziessel, R.; Diring, S.; Kadjane, P.; Charbonnière, L.; Retailleau, P.; Philouze, C. Chem. - Asian J. 2007, 2, 975−982. (41) Feng, J.; Zhang, H.-J.; Song, S.-Y.; Li, Z.-F.; Sun, L.-N.; Xing, Y.; Guo, X.-M. J. Lumin. 2008, 128, 1957−1964. (42) Mei, X.-L.; Ma, Y.; Li, L.-C.; Liao, D.-Z. Dalton Trans. 2012, 41, 505−511. (43) (a) Xu, J.; Radkov, E.; Ziegler, M.; Raymond, K. N. Inorg. Chem. 2000, 39, 4156−4164. (b) Harada, T.; Nakano, Y.; Fujiki, M.; Naito, M.; Kawai, T.; Hasegawa, Y. Inorg. Chem. 2009, 48, 11242−11250. (44) Eliseeva, S. V.; Ryazanov, M.; Gumy, F.; Troyanov, S. I.; Bunzli, J.-C. G.; Kuzmina, N. P. Eur. J. Inorg. Chem. 2006, 2006, 4809−4820. (45) Sabbatini, N.; Guardigli, M.; Manet, I. In Handbook on the Physics and Chemistry of Rare Earths, 21; Gschneidner, K. A., Jr., Eyring, L., Eds.; Elsevier Science BV: Amsterdam, The Netherlands, 1996; pp 69−119. (46) Kang, J. G.; Kim, T. J. Bull. Korean Chem. Soc. 2005, 26, 1057− 1064. (47) Görller-Walrand, C.; Binnemans, K. Handbook on the Physics and Chemistry of Rare Earths, 25; Gschneidner, K. A., Jr., Eyring, L., Eds.; North-Holland: Amsterdam, 1998; Chapter 167, p 101. (48) Klink, S. I.; Grave, L.; Reinhoudt, D. N.; van Veggel, F. C. J. M.; Werts, M. H. V.; Geurts, F. A. J.; Hofstraat, J. W. J. Phys. Chem. A 2000, 104, 5457−5468.

(49) Latva, M.; Takalo, H.; Mukkala, V. M.; Matachescu, C.; Rodríguez-Ubis, U. J. C.; Kankare, J. J. Lumin. 1997, 75, 149−169. (50) Fu, L.; Sá Ferreira, R. A.; Silva, N. J. O.; Fernandes, J. A.; Ribeiro-Claro, P.; Goncüalves, I. S.; de Zea Bermudez, V.; Carlos, L. D. J. Mater. Chem. 2005, 15, 3117−3125. (51) Miyata, K.; Hasegawa, Y.; Kuramochi, Y.; Nakagawa, T.; Yokoo, T.; Kawai, T. Eur. J. Inorg. Chem. 2009, 2009, 4777−4785. (52) Biju, S.; Reddy, M. L. P.; Cowley, A. H.; Vasudevan, K. V. J. Mater. Chem. 2009, 19, 5179−5187. (53) (a) Wang, Y.-L.; Ma, Y.; Yang, X.; Tang, J.; Cheng, P.; Wang, Q.-L.; Li, L.-C.; Liao, D.-Z. Inorg. Chem. 2013, 52, 7380−7386. (b) Ahmed, Z.; Iftikhar, K. RSC Adv. 2014, 4, 63696−63711. (54) Quici, S.; Cavazzini, M.; Marzanni, G.; Accorsi, G.; Armaroli, N.; Ventura, B.; Barigelletti, F. Inorg. Chem. 2005, 44, 529−537. (55) Raju, G. S. R.; Park, J. Y.; Jung, H. C.; Yang, H. K.; Moon, B. K.; Jeong, J. H.; Kim, J. H. Opt. Mater. (Amsterdam, Neth.) 2009, 31, 1210−1214. (56) Gu, F.; Wang, S. F.; Lu, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. Langmuir 2004, 20, 3528−3531. (57) Stouwdam, J. W.; van Veggel, F. C. J. M. Nano Lett. 2002, 2, 733−737. (58) Peng, C.; Zhang, H.; Yu, J.; Meng, Q.; Fu, L.; Li, H.; Sun, L.; Guo, X. J. Phys. Chem. B 2005, 109, 15278−15287. (59) de Sa, G. F.; Malta, O. L.; de Mello Donega, C.; Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; da Silva, E. F., Jr. Coord. Chem. Rev. 2000, 196, 165−195. (60) Dandrade, B.; Datta, S.; Forrest, S.; Djurovich, P.; Polikarpov, E.; Thompson, M. Org. Electron. 2005, 6, 11−20. (61) Xu, H.; Yin, K.; Huang, W. J. Phys. Chem. C 2010, 114, 1674− 1683. (62) Kang, T. S.; Harrison, B. S.; Bouguettaya, M.; Foley, T. J.; Boncella, J. M.; Schanze, K. S.; Reynolds, J. R. Adv. Funct. Mater. 2003, 13, 205−210. (63) Bunzli, J. C. G.; Comby, S.; Chauvin, A. S.; Vandevyver, C. D. B. J. Rare Earths 2007, 25, 257−274. (64) (a) Kepler, R. G.; Beeson, P. M.; et al. Appl. Phys. Lett. 1995, 66, 3618−3620. (b) Chen, B.; Liu. Synth. Met. 1997, 91, 169−171.

Q

DOI: 10.1021/acs.inorgchem.5b01630 Inorg. Chem. XXXX, XXX, XXX−XXX