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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Red Luminescent Eu(III) Coordination Bricks Excited on Blue LED Chip Toru Koizuka,†,‡ Kei Yanagisawa,‡ Yuichi Hirai,‡ Yuichi Kitagawa,§ Takayuki Nakanishi,§ Koji Fushimi,§ and Yasuchika Hasegawa*,§ †

Semiconductor Development Division, Nichia Corporation, 1-1 Tatsumi, Anan, Tokushima 774-0001, Japan Graduate School of Chemical Sciences and Engineering and §Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Hokkaido, Japan

‡

S Supporting Information *

ABSTRACT: Three types of red luminescent Eu(III) complexes with Schiff base and hfa ligands (hfa: hexafluoroacetylacetonate), mononuclear [Eu(hfa)2(OAc)(salen)2] (OAc: acetate anion, salen: N,N′-bis(salicylidene)ethylenediamine), brick-type [Eu2 (hfa) 4 (OAc) 2(salbn)2 ] (salbn: N,N′-bis(salicylidene)-1,4-butanediamine), and polynuclear [Eu(hfa) 2 (OAc)(salhen)] n (salhen: N,N′-bis(salicylidene)-1,6-hexanediamine) are reported for white light-emitting diode (LED) devices. Among these complexes, brick-type [Eu2(hfa)4(OAc)2(salbn)2] excited by blue light (460 nm) exhibits the photosensitized quantum yield (Φπ−π* = 47%) and remarkably high efficiency of sensitization (ηsens = 96%). The efficiency of sensitization is caused by the excited state based on ligand−ligand interaction between the Schiff base and hfa ligands in Eu(III) complexes. To fabricate LED devices, the red luminescent [Eu2(hfa)4(OAc)2(salbn)2] was mounted on an InGaN blue LED chip.

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INTRODUCTION White light-emitting diodes (white LEDs) are a key device in recent industrial applications for general lighting and backlight displays as an environmentally friendly light source in the 21st century.1−3 White LEDs are generally fabricated as one of two types. Type A combines green and red phosphors with a blue LED, while Type B combines red, green, and blue phosphors with a UV LED (Figure 1).4−9 At present, photodegradation of encapsulated and packaging materials (silicone, epoxy, polyester, and polyamide derivatives) is of concern in the LED applications. 10 Qiu and co-workers reported the degradation of silicone and epoxy molding compounds under UV irradiation.11 Lin and co-workers pointed out the importance of photodegradation of bisphenol-type resin in UV LEDs.12 For these reasons, white LED combining green and red phosphors with blue LEDs have been mainly commercialized as practical light sources. The developments of brilliant green and red phosphors excited by a blue LED (wavelength: 450−470 nm) is required for fabrication of highefficiency white LED devices. For the red phosphor, the narrow 4f−4f transition of Eu(III) ion (5D0 → 7F2: 615 nm, full width at half-maximum (fwhm) = 15 nm) is a promising candidate for backlight display devices, although broad 4f−5d transitions of Eu(II) ions in inorganic matrices (e.g, CaAlSiN3:Eu) have been applied for general lighting. Europium(III) ion, however, has a small absorption coefficient (ε < 1 M−1 cm−1 at 465 nm).13 To sensitize the © XXXX American Chemical Society

luminescence of lanthanide(III) ions, various organic chromophores such as β-diketonates and pyridine-based ligands in Eu(III) complexes have been developed.14−21 These organic chromophores with π-conjugation systems are referred to as light-harvesting antenna ligands. In a previous study, Gong reported a red luminescent Eu(III) complex containing βdiketonate ligands with carbazole units excited by blue LED (emission quantum yield = 16%).22 Expanded π-conjugation in β-diketonate ligands promotes energy back transfer from Eu(III) ion to the triplet state of the ligand, which results in a decrease of the emission quantum yield.23,24 The triplet state level of the β-diketonate ligand with carbazole units is estimated to be 18 800 cm−1, which is close to the emitting levels in Eu(III) ion (17 000 cm−1).22,25 Photosensitized units with a higher triplet state (≥19 000 cm−1) should be effective for suppressing of the energy back transfer. We reported strongly luminescent Eu(III) complexes and coordination polymers composed of low vibrational hfa ligands (hfa: hexafluoroacetylacetonate, small nonradiatve rate constants = knr, emission quantum yields > 70%).26,27 The Eu(III) complexes with hfa ligands suppress the energy back transfer (triplet state ≥ 22 400 cm−1), which promotes efficient photosensitization and high emission quantum yields. The hfa ligand in Eu(III) complex absorbs the UV light (absorbance band < 400 nm), which is not Received: March 26, 2018

A

DOI: 10.1021/acs.inorgchem.8b00805 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

anediamine) to obtain red phosphors under blue LED excitation (Figure 2). Their geometrical structures were characterized using X-ray single-crystal analysis. The emission quantum yields of [Eu(hfa)2(OAc)(salen)2], [Eu2(hfa)4(OAc)2(salbn)2], and [Eu(hfa)2(OAc)(salhen)]n excited at 460 nm were calculated to be 27%, 47%, and 16%, respectively. The brick-type [Eu2(hfa)4(OAc)2(salbn)2] showed bright luminescence excited at 460 nm with a high efficiency of sensitization (ηsens = 96%). The highly efficient luminescence of the Eu(III) complex with salbn ligands is based on LLI between the Schiff base and hfa ligands. Red luminescence was also successfully observed from [Eu2(hfa)4(OAc)2(salbn)2] on an InGaN blue chip LED device. In this study, the efficient red luminescence of the brick-type Eu(III) complex with blue LED was achieved for the first time.

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EXPERIMENTAL SECTION

Materials. Europium(III) acetate n-hydrate (99.9%) was purchased from Wako Pure Chemical Industries Ltd. Hexafluoroacetylacetone, 1,4-butanediamine, 1,6-diaminohexane, salicylaldehyde, and N,N′bis(salicylidene)ethylenediamine were obtained from Tokyo Chemical Industry Co., Ltd. All other solvents were reagent grade and used without further purification. Apparatus. 1H NMR (400 MHz) spectra were obtained using a JEOL ECS400 spectrometer; CD2Cl2 (δH = 5.32 ppm) was used as internal reference. Elemental analyses were performed on an Exeter Analytical CE440. Infrared spectra were recorded with a JASCO FTIR4600 spectrometer. Preparation of N,N′-Bis(salicylidene)-1,4-butanediamine (salbn). Salicylaldehyde (2.4 g, 20.0 mmol) and 1,4-diaminobutane (0.88 g, 10.0 mmol) were dissolved in ethanol (40 mL). The solution was refluxed for 2 h with stirring. The reaction solution was cooled at 0 °C. The resulting precipitate was separated by filtration, washed with cool ethanol, and dried under vacuum. The yellow powder was obtained. Yield: 2.6 g (88%); 1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 13.47 (broad, OH), 8.37 (s, 2H; imine-H), 7.31−7.26 (m, 4H; Ar), 6.91− 6.87 (m, 4H; Ar), 3.64 (t, 4H; N−CH2), 1.80 (m, 4H; −CH2) ppm. IR (KBr): 2946−2849, 1634, 1284 cm−1. Anal. Calcd (%) for C18H20N2O2: C, 72.95; H, 6.80; N, 9.45. Found: C, 73.11; H, 6.80; N, 9.46. Preparation of N,N′-Bis(salicylidene)-1,6-hexanediamine (salhen). Salicylaldehyde (4.9 g, 40.0 mmol) and 1,6-diaminohexane (2.3 g, 20.0 mmol) were dissolved in ethanol (60 mL). The solution was refluxed for 2 h with stirring. The reaction solution was cooled at 0 °C. The resulting precipitate was separated by filtration, washed with cool ethanol, and dried under vacuum. The yellow powder was obtained.

Figure 1. Phosphor-converted white LED: Type A blue LED with red and green phosphors, Type B UV LED with red, green, and blue phosphors.

excited using the blue light (λ: 450−470 nm). The πconjugated antenna ligands with higher triplet states and absorption bands in the blue light region are thus desirable for A-type white LEDs (Figure 1). Here, we focus on a new combination of photosensitized Schiff base and low vibrational hfa ligands to achieve a strong luminescent Eu(III) complex under blue light irradiation.28−32 The energy level of triplet state in the salen-type Schiff base ligand (salen: N,N′-bis(salicylidene)ethylenediamine) is estimated to be more than 20 000 cm−1, which is larger than that of the 5D0 state in the Eu(III) ion (emitting level ≅ 17 000 cm−1).25,32 A specific excited state based on ligand−ligand interaction (LLI) between the Schiff base and hfa ligands in the Eu(III) complex is expected to provide bright luminescence excited at 460 nm (blue light). Three types of Eu(III) complexes were prepared with Schiff base and hfa ligands, mononuclear [Eu(hfa)2(OAc)(salen)2] (OAc: acetate anion), brick-type [Eu 2 (hfa) 4 (OAc) 2 (salbn) 2 ] (salbn: N,N′-bis(salicylidene)-1,4-butanediamine), and polynuclear [Eu(hfa)2(OAc)(salhen)]n (salhen: N,N′-bis(salicylidene)-1,6-hex-

Figure 2. Chemical structure of (a) [Eu(hfa)2(OAc)(salen)2], (b) [Eu2(hfa)4(OAc)2(salbn)2], and (c) [Eu(hfa)2(OAc)(salhen)]n. B

DOI: 10.1021/acs.inorgchem.8b00805 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Yield: 6.0 g (92%); 1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 13.57 (broad, OH), 8.35 (s, 2H; imine-H), 7.31−7.25 (m, 4H; Ar), 6.91− 6.86 (m, 4H; Ar), 3.59 (t, 4H; N−CH2), 1.71 (m, 4H; −CH2), 1.44 (m, 4H; −CH2) ppm. IR (KBr): 2934−2859, 1634, 1281 cm−1. Anal. Calcd (%) for C20H24N2O2: C, 74.05; H, 7.48; N, 8.63. Found: C, 74.14; H, 7.50; N, 8.62. Preparation of [Eu(hfa)2(OAc)(salen)2]. Europium acetate nhydrate (0.50 g, 1.4 mmol) was dissolved in ethanol (40 mL). A solution of hfa (0.90 g, 4.3 mmol) was added to the solution. Salen (0.77 g, 2.9 mmol) was added to the solution and stirred until dissolved. After the solution was stirred at room temperature for 24 h, the precipitates were separated by filtration, washed with water several times, and dried under vacuum. The yellow powder was obtained. Recrystallization from ethanol (80 vol %) and dichloromethane (20 vol %) gave yellow needle crystals of the europium complex. [Eu(hfa)2(OAc)(salen)2]. Yield: 0.93 g (56%); Anal. Calcd (%) for C44H37EuF12N4O10: C, 45.49; H, 3.21; N, 4.82. Found: C, 45.70; H, 3.17; N, 4.79. IR (KBr): 2954−2852, 1653, 1630, 1610, 1538, 1280, 1256, 1218−1202, 1146 cm−1. Preparation of [Eu2(hfa)4(OAc)2(salbn)2]. Europium acetate nhydrate (1.0 g, 2.9 mmol) was dissolved in ethanol (60 mL). A solution of hfa (1.2 g, 5.8 mmol) was added to the solution. After the solution was stirred at room temperature for 1 h, salbn (0.85 g 2.9 mmol) was added in the solution. After the solution stirred at room temperature for 24 h, the precipitates were separated by filtration and washed with diethyl ether. The precipitates were filtered and dried under vacuum. The yellow powder was obtained. Recrystallization from tetrahydrofuran (THF; 40 vol %) and hexane (60 vol %) gave yellow block crystals of the europium complex. [Eu2(hfa)4(OAc)2(salbn)2]. Yield: 1.8 g (34%); Anal. Calcd (%) for C60H50Eu2F24N4O16: C, 39.10; H, 2.73; N, 3.04. Found: C, 39.25; H, 2.62; N, 2.97. IR (KBr): 2980−2863, 1654, 1610, 1550, 1478, 1255, 1211, 1146 cm−1. Preparation of [Eu(hfa)2(OAc)(salhen)]n. Europium acetate nhydrate (5.0 g, 14 mmol) was dissolved in water (40 mL). A solution of hfa (9.0 g, 43 mmol) was added to the solution. After the solution was stirred at room temperature for 4 h, precipitates were separated by filtration and dried under vacuum. White solid was obtained. Prepared white solid (0.8 g, 1.0 mmol) was dissolved in diethyl ether (40 mL). Salhen (0.65 g, 2.0 mmol) was added to the solution. After the solution was stirred at room temperature for 24 h, the precipitates were filtered and dried under vacuum. The yellow powder was obtained. Recrystallization from ethanol (60 vol %) and dichloromethane (40 vol %) gave yellow plate crystals of the europium complex. [Eu(hfa)2(OAc)(salhen)]n: Yield: 0.90 g (95%); Anal. Calcd (%) for C32H29EuF12N2O8: C, 40.48; H, 3.08; N, 2.95. Found: C, 40.34; H, 2.96; N, 2.88. IR (KBr): 2950−2860, 1653, 1610, 1539, 1479, 1254, 1202, 1143 cm−1. Optical Measurements. Kubelka−Munk spectra of the Eu(III) complexes were measured using a JASCO V-670 spectrometer with an integrating sphere unit. Kubelka−Munk function is given as (1 − R ∞)2 k , R ∞ = R ∞ ,sample/R ∞ ,std = s 2R ∞

k nr =

Φ4f − 4f =

(3)

τ kr = obs k r + k nr τrad

(4) −1

where AMD,0 has a value of 14.65 s , which is the spontaneous emission probability for the 5D0−7F1 transition in vacuo, n is the refractive index of silicone resin (1.5), and (Itot/IMD) is the ratio of the total area of the Eu(III) emission bands to the area of the 5D0−7F1 band.33 Emission lifetimes (τobs) were determined using a Q-switched Nd:YAG laser pulsed at 355 nm (Spectra Physics, INDI-50, fwhm = 5 ns, λ = 1064 nm) and a photomultiplier tube (Hamamatsu Photonics, R5108, response time = 1.1 ns). The Nd:YAG laser response was displayed on a digital oscilloscope (Sony Tektronix, TDS3052, 500 MHz) synchronized to the single-pulse excitation. Emission lifetimes were estimated by the slope of logarithmic plots of the decay profiles. The photosensitized emission quantum yields (Φπ−π*) were measured using a JASCO F-6300-H spectrometer equipped with an integrating sphere unit (JASCO ILF-533, φ = 100 mm). The efficiency of sensitization (ηsens) was given by

ηsens =

Φπ − π * Φ4f − 4f

(5)

Computational Details. Time-dependent density functional theory (TD-DFT) calculations were performed with the Gaussian 09 program.34−37 The initial structures of Schiff base and hfa ligands were determined from the crystallographic data of the Eu(III) complexes. Single-Crystal X-ray Crystallography. The crystal of the complexes was mounted on a MiTeGen micromesh using paraffin oil. All measurements were performed on a Rigaku R-AXIS RAPID diffractometer using graphite monochromated Mo Kα radiation at −150 °C. An empirical absorption correction was applied, and the data were corrected for Lorentz and polarization effects. The structures were solved by direct methods using SHELXS-9738 and expanded using Fourier techniques. Structural refinements were performed by the full-matrix least-squares techniques. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using riding model. Fabrication of LED Package. An InGaN blue LED as a side-view type of LED package (Nichia Corporation) was fabricated for liquidcrystal display. Silicone resin with 7 wt % [Eu2(hfa)4(OAc)2(salbn)2] was used for molding resin on LED chip. The resin including Eu(III) complex was degassed and poured into the LED package. After the resin was injected, the LED was heated at 150 °C for 5 h to harden the resins.

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RESULTS AND DISCUSSION Coordination Structures. Three Eu(III) complexes were synthesized by the chelation of europium acetate (Eu(OAc)3· nH2O) with hfa and Schiff base ligands (salen, salbn, and salhen) at room temperature. (Figure 2). Single crystals of [Eu(hfa)2(OAc)(salen)2] and [Eu(hfa)2(OAc)(salhen)]n were prepared by recrystallization from ethanol and dichloromethane. A single crystal of [Eu2(hfa)4(OAc)2(salbn)2] was also obtained by recrystallization from THF and hexane. Crystallographic data and perspective views of Eu(III) complexes with hfa and Schiff base ligands are shown in Table S1 and Figure 3.39 The perspective views of each Eu(III) complexes show eight-coordinated structures. The three complexes are coordinated by one anionic OAc molecule, two hfa ligands, and two Schiff base ligands in the Eu(III) coordination unit. The selected Eu−O coordination bond lengths of the three Eu(III) complexes are described in Table S2. The average Eu−O distances of the Eu-Schiff base ligand in

(1)

where k is the absorption coefficient of sample, s is the scattering coefficient of sample, and R∞ is the diffuse reflectance of the sample with respect to BaSO4. The R∞,sample (powder of Eu(III) complexes) were measured without BaSO4. Emission and excitation spectra for ligands, Gd(III) complexes, and Eu(III) complexes were recorded on a Fluorolog-3 spectrofluorometer and corrected for the detector system (HORIBA). Radiative rate constant (kr) and nonradiative rate constant (knr) of Eu(III) complexes were calculated using the following equations: ⎛I ⎞ 1 k r = AMD,0n3⎜ tot ⎟ = ⎝ IMD ⎠ τrad

1 1 − τobs τrad

(2) C

DOI: 10.1021/acs.inorgchem.8b00805 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Average Distance of Eu−O Bond Length Eu-Schiff (Å)

Eu-hfa (Å)

Eu-OAc (Å)

2.296 2.281 2.267 2.281

2.426 2.431 2.425 2.427

2.436 2.473 2.447 2.452

[Eu(hfa)2(OAc)(salen)2] [Eu2(hfa)4(OAc)2(salbn)2] [Eu(hfa)2(OAc)(salhen)]n average

complexes, intermolecular CH/F and CH/π interactions were observed. Characteristic intermolecular π−π interactions were also identified in the brick-type [Eu2(hfa)4(OAc)2(salbn)2] complex, which promote a tight packing structure in a solid (Figure 3d, Figure S1). On the basis of the crystal data, we calculated the continuous shape measures factor S to estimate the degree of distortion of the coordination structure in the first coordination sphere. The S value is given by N

S = min

∑k |Q k − pk |2 N

∑k |Q k − Q 0|2

× 100 (6)

where N is the number of vertices, Qk is the vertices of an actual structure, Q0 is the center of mass of an actual structure, and Pk is the vertices of an ideal structure.40,41 Eight-coordinated lanthanide complexes generally exhibit trigonal dodecahedron (TDH), square antiprism (SAP), or biaugmented trigonal prism (BTP) structures, according to evaluation of the S values (Table S3). In [Eu2(hfa)4(OAc)2(salbn)2], the STDH value (1.609) was smaller than that of SSAP (3.653) and SBTP (2.196). The geometrical structure of [Eu2(hfa)4(OAc)2(salbn)2] is categorized as a distorted TDH. The S values of mononuclear [Eu(hfa)2(OAc)(salen)2] are estimated to be 2.571 (SSAP), 2.144 (STDH), and 2.491 (SBTP), respectively. The S values of polynuclear [Eu(hfa)2(OAc)(salhen)]n were also calculated to be 2.112 (SSAP), 2.141 (STDH) and 1.861 (SBTP), respectively. We cannot find a significant difference between SSAP, STDH, and SBTP in mononuclear [Eu(hfa)2(OAc)(salen)2] and polynuclear [Eu(hfa)2(OAc)(salhen)]n. Photophysical Properties. The emission and excitation spectra of Eu(III) complexes in solid state are shown in Figure 4. The emission bands at ∼578, 592, 613, 650, and 698 nm were assigned to be 4f−4f transitions of Eu(III) ions (5D0−7FJ: J = 0, 1, 2, 3, and 4, respectively). The emission spectra were normalized with respect to the spectral area of the magneticdipole transition at ∼592 nm (Eu: 5D0−7F1). In the excitation

Figure 3. Perspective views of (a) [Eu(hfa)2(OAc)(salen)2], (b) [Eu2(hfa)4(OAc)2(salbn)2], (c) [Eu(hfa)2(OAc)(salhen)]n, and (d) intermolecular π−π interactions (blue line) of [Eu2(hfa)4(OAc)2(salbn)2] with the coordination geometry of each Eu(III) ion showing 50% probability displacement ellipsoids. Hydrogen atoms were omitted for clarity.

mononuclear [Eu(hfa) 2 (OAc)(salen) 2 ], brick-type [Eu2(hfa)4(OAc)2(salbn)2], and polynuclear [Eu(hfa)2(OAc)(salhen)]n were determined to be 2.296, 2.281, and 2.267 Å, respectively, which are shorter than those of Eu-hfa (2.427 Å) and Eu-OAc (2.452 Å) (Table 1). These results indicate that Eu(III) complexes with OAc, hfa, and Schiff base ligands have asymmetric coordination geometry with different Eu−O bond lengths. According to the specific interactions in Eu(III)

Figure 4. Excitation and emission spectra of (a) [Eu(hfa)2(OAc)(salen)2] (blue line), (b) [Eu2(hfa)4(OAc)2(salbn)2] (red line), and (c) [Eu(hfa)2(OAc)(salhen)]n (green line) excited at 460 nm in solid state. D

DOI: 10.1021/acs.inorgchem.8b00805 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Photophysical Properties of Eu(III) Complexes Φπ−π*a (%) [Eu(hfa)2(OAc)(salen)2] [Eu2(hfa)4(OAc)2(salbn)2] [Eu(hfa)2(OAc)(salhen)]n

27 47 16

Φ4f−4fb (%) 37 49 30

ηsensb (%)

krb (s−1)

knrb (s−1)

τobsc (ms)

75 96 55

8.4 × 10 8.7 × 102 8.8 × 102

1.4 × 10 9.3 × 102 2.1 × 103

0.44 0.56 0.34

2

3

a Φπ−π* of the Eu(III) complexes were measured by excitation at 460 nm. bFrom calculation using eqs 2−5. cτobs of the Eu(III) complexes were measured by excitation at 355 nm.

spectra, characteristic broad bands at ∼450 nm were attributed to the combination band between the Schiff base and hfa ligands. Significantly broad excitation bands are useful for efficient luminescence from Eu(III) complexes under blue LED irradiation. All complexes also exhibit 4f−4f transition of Eu(III) at ∼532 nm in the excitation spectra. The diffuse reflectance spectra are shown in Figure S2. We found that the emission spectral shapes of the Eu(III) complexes are much different from those of previous reported eight-coordinated Eu(III) complexes (Figure S3). Generally, emission spectral shape of Eu(III) complex is dependent on the coordination geometry. The characteristic spectral shapes of [Eu(hfa)2(OAc)(salen)2], [Eu2(hfa)4(OAc)2(salbn)2], and [Eu(hfa)2(OAc)(salhen)]n are caused by asymmetric structures with different Eu−O bond lengths (Eu-Schiff: 2.28 Å, Eu-hfa: 2.43 Å, and Eu-OAC: 2.45 Å). To clarify packing structure interaction in crystal, we compared emission spectrum of [Eu2(hfa)4(OAc)2(salbn)2] in solid with that in solution state (Figure S4). The emission shape of [Eu2(hfa)4(OAc)2(salbn)2] in THF is much different from that in solid state. These results indicate that tight packing interactions in crystal influences the photophysical properties, directly. The luminescence decay curves of Eu(III) complexes in solid state revealed single exponential decays with millisecond-scale lifetimes. The emission lifetimes were determined from the slopes of the logarithmic plots of the profiles (Figure S5). The emission quantum yields of the 4f−4f transitions in Eu(III) complexes were calculated using the emission lifetime and radiative rate constant estimated from emission spectra. The photosensitized quantum yields excited at π−π* transitions were also measured using an integrating sphere. The photophysical properties (kr, knr, Φ4f‑4f, Φπ−π*, and ηsens) of [Eu(hfa)2(OAc)(salen)2], [Eu2(hfa)4(OAc)2(salbn)2], and [Eu(hfa)2(OAc)(salhen)]n are summarized in Table 2. The emission quantum yields Φ4f−4f value of [Eu(hfa)2(OAc)(salen)2], [Eu2(hfa)4(OAc)2(salbn)2], and [Eu(hfa)2(OAc)(salhen)]n are 37, 49, and 30%, respectively. The knr value of brick-type [Eu2(hfa)4(OAc)2(salbn)2] is smaller than that of [Eu(hfa)2(OAc)(salen)2] and [Eu(hfa)2(OAc)(salhen)]n. The tight packing structure of [Eu2(hfa)4(OAc)2(salbn)2] promotes suppression of the radiationless transitions via vibrational relaxation. The small knr in [Eu2(hfa)4(OAc)2(salbn)2] leads to high Φ4f−4f. The Φπ−π* value of [Eu2(hfa)4(OAc)2(salbn)2] (47%) is larger than those of [Eu(hfa)2(OAc)(salen)2] and [Eu(hfa)2(OAc)(salhen)]n. On the basis of the calculation of Φ4f−4f and Φπ−π*, Eu(III) complexes with the salen, salbn, and salhen promote highly efficient sensitization. In particular, the brick-type structure of [Eu2(hfa)4(OAc)2(salbn)2] shows a high efficiency of sensitization (ηsens = 96%). To reveal mechanism for the efficient sensitization of Eu(III) complexes with Schiff base and hfa ligands, we performed TDDFT calculations for hfa and Schiff base ligands (CAMB3LYP/6-31G+ (d, p), Figures 5, S6, and S7 and Tables S4 and S5).34−37 On the one hand, according to calculation using

Figure 5. (a) Molecular orbitals related to T1 state of hfa and Schiff base in [Eu2(hfa)4(OAc)2(salbn)2] based on the TD-DFT (CAMB3LYP/6-31G+ (d, p)) and (b) LLI image of brick-type [Eu2(hfa)4(OAc)2(salbn)2].

CAM-B3LYP/6-31G+ (d, p), we found that [Eu2(hfa)4(OAc)2(salbn)2] shows the lowest triplet state with LLI (Figure 5a). On the other hand, the orbitals related to lowest triplet state of [Eu(hfa)2(OAc)(salen)2] and [Eu(hfa)2(OAc)(salhen)]n do not exhibit LLI (Figure S6). We also observed T2 level transition; [Eu(hfa)2(OAc)(salen)2] shows LLI (Figure S7 and Table S5). To explain LLI experimentally, we measured UV−vis and phosphorescence spectra of salen, salbn, salhen, Gd(III) complexes, and Eu(III) complexes (Figures S8−S12). We observed new absorption edges at ∼500 nm in UV−vis spectra of Eu(III) complexes. The different shapes of phosphorescence spectra between Gd(III) complexes and ligands indicated the presence of triplet states with LLI. Trivedi and co-workers reported that nearinfrared lanthanide metallacrown complexes exhibited significantly high quantum yields based on efficient sensitization due to the intra- and/or interligand charge transfer.42 Liu and coworkers achieved high emission quantum yield of tetranuclear Eu(III) complex with combination of intraligand charge transfer (ILCT) sensitization.43 The new combination of Schiff base and hfa ligands in Eu(III) complexes led to the formation of the LLI, which results in high efficiency of sensitization. Finally, we demonstrated red luminescent [Eu2(hfa)4(OAc)2(salbn)2] was applied to an LED device (Figure 6). The LED operating at 20 mA exhibited bright pink color emission, as shown in Figure 6a. The radiant flux was 1.9 lm. The emission spectrum of the LED package is shown in Figure 6b. Blue emission (InGaN) and red emission bands (Eu(III) complex) were observed. The CIE 1931 chromaticity E

DOI: 10.1021/acs.inorgchem.8b00805 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(salhen)]n, respectively. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +81 11 706 7114. E-mail: hasegaway@eng. hokudai.ac.jp.

Figure 6. (a) Luminescence image of L ED package ([Eu2(hfa)4(OAc)2(salbn)2]/InGaN chip) and (b) emission spectrum of LED package.

ORCID

Yuichi Kitagawa: 0000-0003-1487-2531 Takayuki Nakanishi: 0000-0003-3412-2842 Yasuchika Hasegawa: 0000-0002-6622-8011

xy-coordinates (x, y = 0.29, 0.11) were calculated using the emission spectrum. Gong and co-worker also fabricated 5 wt % Eu(III) complex containing β-diketonate ligands with carbazole units dispersed in silicone gel.22 They observed red luminescent Eu(III) complex excited by InGaN blue LED operating at 20 mA. The emission quantum yield of Eu(III) complex containing β-diketonate ligands with carbazole units was estimated to be 16% excited 460 nm. We consider that Eu(III) complex with Schiff base and hfa ligand promotes large emission quantum yields, resulting in successful red luminescence excited by blue LED chip. The new combination of salbn and hfa in a Eu(III) complex thus provided efficient luminescence under blue LED irradiation.

Present Address ∥

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank A. Suzuki of Frontier Chemistry Center “Laboratories for Future Creation” Project.

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REFERENCES

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CONCLUSION In this study, novel Eu(III) complexes were successfully synthesized that exhibited strong red luminescence excited by blue light irradiation (460 nm). With the LLI between the Schiff base and hfa ligands, high efficiency of sensitization and photosensitized luminescence of Eu(III) complexes excited at 460 nm were realized for the first time. The photosensitized quantum yields and efficiency of sensitization of brick-type [Eu2(hfa)4(OAc)2(salbn)2] were estimated to be 47% and 96%, respectively, which are the largest values reported to date. Bright red luminescence from [Eu2(hfa)4(OAc)2(salbn)2] was also demonstrated in a blue LED device. The results from this study are expected to open new fields in lighting materials and coordination and photophysical chemistry.

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Faculty of Engineering, Hokkaido University.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00805. Intermolecular CH/F interactions and CH/π interactions; diffuse reflectance spectra; emission spectra of [Eu2(hfa)4(OAc)2(salbn)2] and [Eu(hfa)3(tppo)2]; emission spectra of [Eu2(hfa)4(OAc)2(salbn)2] in solid state and THF solution; emission decay profiles; molecular orbitals rerated to the T1 and T2 state of Schiff base and hfa ligands in Eu(III) complexes; absorption spectra of [Gd(hfa)3(H2O)2], Eu(III) complexes, and ligands in THF solution; synthesis and phosphorescence spectra of Gd(III) complexes and ligands in solid state; calculated oscillator strengths (PDF) Accession Codes

CCDC 1824941, 1824944, and 1825354 contain the supplementary crystallographic data for [Eu(hfa)2(OAc)(salen)2], [Eu2(hfa)4(OAc)2(salbn)2], and [Eu(hfa)2(OAc)F

DOI: 10.1021/acs.inorgchem.8b00805 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.8b00805 Inorg. Chem. XXXX, XXX, XXX−XXX