Luminescent europium(III) complexes based on tridentate isoquinoline ligands with extremely high quantum yield

Abstract

Here, we report two europium(III) complexes (Eu1 and Eu2) with high photoluminescence quantum yield, based on tridentate isoquinoline derivatives (HL1 and HL2). The single crystal structures of Eu1 and Eu2 show that three ligands coordinate with an Eu(III) ion through diphenylphosphoryl oxygen, pyridyl nitrogen and carboxyl oxygen atoms (O^N^O) with no solvent in the first coordination sphere. An appropriate triplet energy level (about 20 000 cm⁻¹) ensures efficient energy transfer from the ligands to the Eu(III) ion. As a result, the two complexes exhibited significantly high quantum yields in the solid state (94% and 87% for Eu1 and Eu2, respectively) with excitation bands extending to 350 nm.

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Introduction

On account of the unique photophysical properties of f–f transitions, such as characteristic narrow-band spectra and long luminescence lifetimes, inorganic rare earth compounds (oxides or fluorides) have been widely used as trichromatic phosphors and up-conversion imaging agents.^1,2 But direct excitation from the Laporte forbidden f–f transitions results in very low absorption intensities (<3 M⁻¹ cm⁻¹).³ Alternatively, using organic ligands with high absorption coefficients to sensitize the emission of lanthanide ions can reduce the consumption of lanthanide materials, and show great potential in applications such as organic light emitting diodes (OLEDs),⁴ bio-imaging,⁵ pH sensing,^6,7 and light conversion for photovoltaic devices.^8,9

High photoluminescence quantum yield (PLQY) is indispensable for lanthanide complexes to be used in the above applications, which requires appropriate design of the ligand structure. On the one hand, the ligands should have appropriate triplet energy levels for efficient energy transfer and on the other hand, they need to afford saturated coordination environments to protect the lanthanide ions from quenching by the O–H and N–H oscillators originating from coordinated solvents.¹⁰ A variety of luminescent lanthanide complexes have been synthesized, such as those based on β-diketonate derivatives,¹¹ pyridyl derivatives,¹² and dipyridyl derivatives.¹³ Recently, we have reported the use of a diphenylphosphoryl (DPPO) group to construct a series of highly emissive lanthanide(III) complexes.^12,14 Among them, the Eu(III) complex based on 6-(diphenylphosphoryl)picolinic acid (denoted as Eu3 in this work) displayed very high PLQY in the solid state (81%). But the short excitation band (<280 nm) of the complex limits its further application in bio-imaging and light conversion for photovoltaic devices.

In this work, aiming at extending the excitation wavelength while maintaining the high PLQY of the complex, two isoquinoline derivatives, 3-diphenylphosphoryl-1-carboxyisoquinoline (HL1) and 1-diphenylphosphoryl-3-carboxyisoquinoline (HL2), were designed with an enlarged π conjugation system and reserved (O^N^O) coordination sites. Their corresponding Eu(III) complexes Eu1 and Eu2 were synthesized and characterized by X-ray diffraction (Scheme 1). The absorption spectra showed a large red-shift of the absorption edge for Eu1 and Eu2 compared with that of Eu3. Besides, Eu1 and Eu2 displayed bright red emissions under UV-A excitation (edge to 350 nm) with PLQY/lifetime values of 94%/2.18 ms and 87%/2.32 ms in the solid state, respectively. Sm(III), Nd(III), and Yb(III) complexes based on HL1 were also prepared and they showed favorable photophysical properties.

Scheme 1 Chemical structures of Eu1, Eu2 and Eu3.

Experimental

Materials and methods

All the reagents and solvents are commercially available and used without additional purification except when otherwise noted.

¹H, ¹³C, and ³¹P NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. Both ¹³C and ³¹P NMR spectra were obtained as proton-decoupled data. Electrospray ionization mass spectra (ESI-MS) were recorded in positive or negative ion mode on a Bruker Apex IV Fourier transform ion cyclotron resonance mass spectrometer. Elemental analyses (C, H, and N) were performed with a VARIO elemental analyzer from Elementar Analysensysteme GmbH. Thermo-gravimetric analyses were carried out using Q600SDT instruments at an elevation temperature rate of 10 °C min⁻¹ under a 100 ml min⁻¹ nitrogen flow. All the lanthanide complex samples were dried at 120 °C for 6 hours and then placed under ambient conditions for 1 day before EA and TGA measurements. Single crystals of Eu1 and Eu2 were obtained by slow evaporation of their methanol/dichloromethane solutions. The single crystal data were collected using a RAPID-S image plate diffractometer, and determined and refined using the SHELXL program package.

UV-Vis absorption spectra were measured on a PerkinElmer Lambda 35 spectrometer. Photoluminescence spectra and decay curves of the lanthanide complexes, and low temperature phosphorescence spectra of the gadolinium complexes were recorded on an Edinburgh FLS 980 spectrometer. Absolute PLQYs of the complexes were measured on a Hamamatsu C9920-02 PLQY measurement system (for emissions in the visible region) or an Edinburgh FLS980 with an integrating sphere (for emissions in the NIR region). The photophysical properties of the complexes were obtained in CH₂Cl₂ solutions (1 × 10⁻⁵ M) and in the solid states. All measurements were carried out at room temperature except the phosphorescence experiments at 77 K.

Because of the low absorption intensity of the f–f transition, the intrinsic quantum yield (Φ^Eu_Eu) of the Eu(III) complex cannot be determined experimentally, but can be estimated from eqn (1).

Φ^Eu_Eu = τ_obs/τ_rad (1)

The radiative lifetime (τ_rad) of Eu (⁵D₀) can be calculated using eqn (2),¹⁵ in which n represents the refractive index of the medium (1.46 for dichloromethane), A_MD,0 is the spontaneous emission probability for the ⁵D₀ → ⁷F₁ transition under vacuum (14.65 s⁻¹), I_tot/I_MD is the ratio of the total integrated intensity of the corrected Eu(III) emission spectrum to the integrated intensity of the magnetic dipole ⁵D₀ → ⁷F₁ transition.

1/τ_rad = A_MD,0 × n³ × (I_tot/I_MD) (2)

Sensitization efficiency (η_sens) can be determined using eqn (3), where Φ_tot is the PLQY of the complex.

Φ_tot = Φ^Eu_Eu × η_sens (3)

Synthetic procedures

Synthesis of 1-methoxycarbonylisoquinoline (2). In a dried 500 mL round bottom flask, 1-isoquinolinecarboxylic acid (5.00 g, 28.8 mmol) was dissolved in 250 mL methanol. 12.5 mL of concentrated H₂SO₄ was slowly added to the above solution. After refluxing for 24 h, the solution was allowed to cool to room temperature and 35.0 g NaHCO₃ was partially added to neutralize the solution. The precipitate was filtered out and the solvent was then removed under reduced pressure. To the residue 100 mL water was added and it was extracted with 100 mL CH₂Cl₂. The organic extracts were washed with water (3 × 50 mL) and dried over anhydrous Na₂SO₄. CH₂Cl₂ was then removed under reduced pressure to afford a clear colorless oil (4.6 g, 87%). ¹H NMR (400 MHz, CDCl₃) δ 8.85 (d, J = 7.9 Hz, 1H), 8.64 (d, J = 5.5 Hz, 1H), 7.86 (dd, J = 19.5, 6.8 Hz, 2H), 7.70 (m, 2H), 4.11 (s, 3H); ESI-MS (m/z): calcd for C₁₁H₉NO₂, 187.1; found, 188.1 (M + H⁺), 210.1(M + Na⁺).

Synthesis of 1-methoxycarbonylisoquinoline N-oxide (3). 3-Chloroperoxybenzoic acid (10 g, 49 mmol, 2.0 equiv., 85% w/w) was added to 75 mL dichloromethane solution of 2 (4.6 g, 25 mmol, 1.0 equiv.). The mixture was stirred at room temperature for 15 h and filtered. The filtrate was washed with sat. NaHCO₃ (2 × 100 mL) and water (2 × 100 mL). The organic extracts were dried over anhydrous Na₂SO₄ and the solvent was removed under reduced pressure to give an orange oil (5.0 g, 99%). ¹H NMR (400 MHz, CDCl₃) δ 8.17 (d, J = 7.1 Hz, 1H), 7.83 (d, J = 7.8 Hz, 1H), 7.73 (d, J = 7.1 Hz, 1H), 7.70–7.59 (m, 3H), 4.15 (s, 3H); ESI-MS (m/z): calcd for C₁₁H₉NO₃, 203.1; found, 204.1 (M + H⁺), 226.0 (M + Na⁺).

Synthesis of 3-chloro-1-methoxycarbonylisoquinoline (4). In a dried round bottom flask, 3 (5.0 g, 25 mmol) was dissolved in 50 mL POCl₃ and the mixture was heated to reflux. After refluxing for 5 h the mixture was allowed to cool down and the solvent was then removed under reduced pressure. To the residue 50 mL NaOH solution (10% w/w) was added and it was extracted with CH₂Cl₂ (3 × 50 mL). The organic extracts were dried over anhydrous Na₂SO₄. The solvent was removed under reduced pressure and the residue was purified by column chromatography (silica gel, petroleum ether/dichloromethane = 1/2, v/v) to afford a white solid (3.7 g, 40%). ¹H NMR (400 MHz, CDCl₃) δ 8.76 (d, J = 8.6 Hz, 1H), 7.90 (s, 1H), 7.83 (d, J = 8.2 Hz, 1H), 7.76 (t, J = 7.5 Hz, 1H), 7.68 (t, J = 7.6 Hz, 1H), 4.09 (s, 3H). ESI-MS (m/z): calcd for C₁₁H₈ClNO, 221.0; found, 222.0(M + H⁺), 244.0 (M + Na⁺), 260.0(M + K⁺).

Synthesis of 3-diphenylphosphoryl-1-methoxycarbonylisoquinoline (5). 4 (0.50 g, 2.3 mmol, 1.0 equiv), K₂CO₃ (0.61 g, 4.5 mmol, 2.0 equiv), NiCl₂(dppp) (0.12 g, 0.23 mmol, 0.10 equiv) (dppp=1,3-bis(diphenylohosphino)propane), Diphenylphosphine oxide (0.91 g, 4.5 mmol, 2.0 equiv.) and toluene (40 mL) were added into a dried 100 mL round bottom flask. The flask was evacuated under vacuum and backfilled with N₂ three times, and the reaction mixture was heated to reflux. After refluxing for 18 h the mixture was allowed to cool to room temperature. The mixture was filtered, and then the solvent was removed under reduced pressure. The residue obtained was purified by column chromatography (silica gel, ethyl acetate/dichloromethane = 1/2) to afford a white solid (0.62 g, 71%). ¹H NMR (400 MHz, CDCl₃) δ 8.90 (d, J = 7.0 Hz, 1H), 8.66 (t, J = 7.3 Hz, 1H), 8.08–7.99 (m, 4H), 7.85–7.75 (m, 2H), 7.56–7.40 (m, 6H), 4.07 (s, 3H); ³¹P NMR (162 MHz, CDCl₃) δ 19.06 (s); ESI-MS (m/z): calcd for C₂₃H₁₈NO₃P, 387.1; found, 388.1 (M + H⁺), 410.1 (M + Na⁺), 426.1 (M + K⁺).

Synthesis of 3-diphenylphosphoryl-1-carboxyisoquinoline (HL1). In a 100 mL round bottom flask, 5 (0.53 g, 1.4 mmol) was dispersed in tetramethylammonium hydroxide aqueous solution (10 mL, 10%, w/w). The reaction mixture was heated to reflux. After refluxing for 1 h, the reaction mixture was cooled to room temperature. The solution pH was then adjusted with concentrated HCl to 2–3 and a white solid product precipitated. The mixture was filtered to afford a white solid (0.47 g, 93%). ¹H NMR (400 MHz, DMSO) δ 13.99 (s, 1H), 8.84 (d, J = 7.1 Hz, 1H), 8.51 (d, J = 8.2 Hz, 1H), 8.33 (d, J = 8.0 Hz, 1H), 8.03–7.81 (m, 6H), 7.69–7.49 (m, 6H). ³¹P NMR (162 MHz, DMSO) δ 17.95 (s). ¹³C NMR (400 MHz, DMSO) δ 167.56 (s), 152.15 (d), 147.99 (s), 146.68 (s), 135.87 (s), 133.51 (s), 132.53 (s), 132.38 (s), 132.00 (s), 131.40 (s), 129.46 (d), 128.98 (d), 126.43 (s), 125.58 (s). ESI-MS (m/z): calcd for C₂₂H₁₆NO₃P, 373.1; found, 374.1 (M + H⁺), 396.1 (M + Na⁺). Anal. calcd: C 70.78, N 3.75, H 4.32; found, C 70.56, N 3.78, H 4.35.

Synthesis of Eu(L1)₃ (Eu1). In a 10 mL round bottom flask, HL1 (0.100 g, 0.268 mmol, 3.00 equiv.) and NaOH (10.7 mg, 0.268 mmol, 3.00 equiv.) were dissolved in 1 mL methanol. At room temperature, to the mixture, EuCl₃·6H₂O (33.0 mg, 0.0893 mmol, 1.00 equiv.) methanol solution (1 mL) was added and a white solid product precipitated immediately. The mixture was filtered to afford a white solid (90 mg, 80%). ESI-MS (m/z): calcd for C₆₆H₄₅EuN₃O₉P₃, 1269.2; found, 1270.2 (M + H⁺). Anal. calcd for C₆₆H₄₅EuN₃O₉P₃·4.7H₂O: C 58.56, N 3.10, H 4.05; found, C 58.66, N 3.04, H 3.84.

Synthesis of Gd(L1)₃ (Gd1). Gd1 was prepared by a similar method to that used for synthesizing Eu1. Gd1 was obtained as a white solid product (75 mg, 66%). ESI-MS (m/z): calcd for C₆₆H₄₅GdN₃O₉P₃, 1274.2; found, 1275.2 (M + H⁺). Anal. calcd for C₆₆H₄₅GdN₃O₉P₃·4H₂O: C 58.88, N 3.12, H 3.97; found, C 58.37, N 3.08, H 3.94.

Synthesis of Sm(L1)₃ (Sm1). Sm1 was prepared by a similar method to that used for synthesizing Eu1. Sm1 was obtained as a white solid product (53 mg, 47%). ESI-MS (m/z): calcd for C₆₆H₄₅N₃O₉P₃Sm, 1268.2; found, 1291.2 (M + Na⁺). Anal. calcd for C₆₆H₄₅N₃O₉P₃Sm·4.2H₂O: C 58.95, N 3.12, H 4.02; found, C 58.71, N 3.01, H 3.89.

Synthesis of Nd(L1)₃ (Nd1). Nd1 was prepared by a similar method to that used for synthesizing Eu1. Nd1 was obtained as a purple solid product (56 mg, 44%). ESI-MS (m/z): calcd for C₆₆H₄₅N₃NdO₉P₃, 1260.2; found, 1283.2 (M + Na⁺). Anal. calcd for C₆₆H₄₅N₃NdO₉P₃·5H₂O: C 58.66, N 3.11, H 4.10; found, C 58.95, N 3.04, H 3.96.

Synthesis of Yb(L1)₃ (Yb1). Yb1 was prepared by a similar method to that used for synthesizing Eu1. Yb1 was obtained as a white solid product (56 mg, 44%). ESI-MS (m/z): calcd for C₆₆H₄₅N₃O₉P₃Yb, 1290.2; found, 1313.2 (M + Na⁺). Anal. calcd for C₆₆H₄₅N₃O₉P₃Yb·3.5H₂O: C 58.58, N 3.11, H 3.87; found, C 59.05, N 3.07, H 3.64.

Synthesis of 3-carboxyisoquinoline N-oxide (7). In a dried 100 mL round bottom flask, 3-carboxyisoquinoline (2.0 g, 12 mmol, 1.0 equiv.) and urea hydrogen peroxide (UHP) (2.1 g, 23 mmol, 2.0 equiv.) were added in anhydrous CH₂Cl₂ (50 mL). The mixture was cooled to 0 °C, and trifluoroacetic anhydride (TFAA) (3.5 mL, 23 mmol, 2.0 equiv.) was added dropwise. After maintaining at 0 °C for 30 min, the mixture was allowed to warm to room temperature and stirred for 12 h. A saturated K₂S₂O₈ aqueous solution (80 mL) was added, and then extracted with CH₂Cl₂ (4 × 80 mL). The combined organic extracts were dried over Na₂SO₄, and the solvent was removed under reduced pressure. The obtained residue was washed with Et₂O followed by filtration and dried under vacuum to afford a white solid (2.2 g, 100%). ¹H NMR (400 MHz, CDCl₃) δ 8.97 (d, J = 1.6 Hz, 2H), 8.07 (dd, J = 8.2, 4.9 Hz, 1H), 8.00–7.94 (m, 1H), 7.92–7.84 (m, 2H). ESI-MS (m/z): calcd for C₁₀H₇NO₃, 189.0; found, 190.1 (M + H⁺), 212.0 (M + Na⁺).

Synthesis of 1-diphenylphosphoryl-3-carboxyisoquinoline (HL2). To an anhydrous CH₂Cl₂ (100 mL) solution of 3-carboxyisoquinoline N-oxide (2.0 g, 11 mmol, 1.0 equiv.) was added isobutyric chloride (3.4 g, 32 mmol, 3.0 equiv.) under a N₂ atmosphere. The resulting solution was stirred for 10 min at room temperature. Then ethyl diphenylphosphinite (3.7 g, 16 mmol, 1.5 equiv.) was added and the solution was stirred for another 10 min at room temperature. The mixture was extracted with tetramethylammonium hydroxide aqueous solution (TMAOH, 10%, w/w, 7 × 30 mL). To the combined aqueous solution extracts, concentrated HCl was added until the pH was adjusted to 2 and a white solid product precipitated. The mixture was filtered to afford a white solid (1.7 g, 45%). ¹H NMR (400 MHz, DMSO) δ 13.34 (s, 1H), 9.48 (d, J = 8.2 Hz, 1H), 8.79 (d, J = 1.0 Hz, 1H), 8.32 (d, J = 7.9 Hz, 1H), 8.03–7.83 (m, 6H), 7.65–7.46 (m, 6H). ¹³C NMR (400 MHz, DMSO) δ 166.34 (s), 156.67 (s), 155.40 (s), 141.33 (d), 136.38 (s), 134.03 (s), 133.00 (s), 132.30 (s), 132.03 (s), 131.01 (s), 129.82 (s), 129.03 (s), 126.69 (s), 126.26 (s). ³¹P NMR (162 MHz, DMSO) δ 22.20(s). ESI-MS (m/z): calcd for C₂₂H₁₆NO₃P, 373.1; found, 374.1(M + H⁺), 396.1 (M + Na⁺). Anal. calcd: C 70.78, N 3.75, H 4.32; found, C 70.67, N 3.61, H 4.21.

Synthesis of Eu(L2)₃ (Eu2). In a 10 mL round bottom flask, HL2 (200.0 mg, 0.536 mmol, 3.00 equiv.) and NaOH (21.4 mg, 0.536 mmol, 3.00 equiv.) were dissolved in 1 mL methanol. At room temperature, to the mixture, EuCl₃·6H₂O (65.4 mg, 0.179 mmol, 1.00 equiv.) methanol solution (1 mL) was added and a white solid product precipitated immediately. The mixture was filtered to afford a white solid (205 mg, 90.5%). ESI-MS (m/z): calcd for C₆₆H₄₅EuN₃O₉P₃ 1269.2, found 1270.2 (M + H⁺), 1292.2 (M + Na⁺). Anal. calcd for C₆₆H₄₅EuN₃O₉P₃·1.9H₂O: C 60.83, N 3.22, H 3.77; found, C 60.57, N 3.17, H 3.62.

Synthesis of Gd(L2)₃ (Gd2). In a 10 mL round bottom flask, HL2 (100.0 mg, 0.268 mmol, 3.00 equiv.) and NaOH (10.7 mg, 0.268 mmol, 3.00 equiv.) were dissolved in 1 mL methanol. At room temperature, to the mixture, GdCl₃·6H₂O (33.2 mg, 0.0893 mmol, 1.00 equiv.) methanol solution (1 mL) was added and a white solid product precipitated immediately. The mixture was filtered to afford Gd(L2)₃ as a white solid (103 mg, 93.2%). ESI-MS (m/z): calcd for C₆₆H₄₅GdN₃O₉P₃, 1274.2; found, 1275.2 (M + H⁺). Anal. calcd for C₆₆H₄₅GdN₃O₉P₃·2H₂O: C 60.50, N 3.21, H 3.77; found, C 60.70, N 3.24, H 3.70.

Results and discussion

Syntheses of ligands and complexes

For the preparation of the ligand HL1, a five-step synthetic route was used (Scheme 2). The C–P bond was formed by the cross-coupling of 4 with diphenylphosphine oxide, catalyzed by Ni(dppp)Cl₂ in the presence of K₂CO₃.¹⁶ The ligand HL2 was synthesized in three steps similar to the method of synthesizing quinolin-2-ylphosphonates by deoxygenative phosphorylation of the corresponding N-oxides (Scheme 3).¹⁷ First, 3-carboxyisoquinoline was treated with urea hydrogen peroxide (UHP) and trifluoroacetic anhydride (TFAA) in CH₂Cl₂ to afford the N-oxide 7 without further purification. Then 7 was isopropylcarbonylated by using isobutyric chloride to give an isoquinolinium salt 8. An Arbuzov reaction of 8 and the ethyl diphenylphosphinite was conducted, followed by the elimination of isobutyric acid to afford the final product HL2 (the assumed reaction mechanism is shown in Scheme S1†).

Scheme 2 Synthetic route of HL1: (i) CH₃OH, H₂SO₄, reflux; (ii) m-CPBA, CH₂Cl₂, RT; (iii) POCl₃, reflux; (iv) Ph₂P(O)H, K₂CO₃, NiCl₂(dppp), toluene, reflux; and (v) TMAOH, reflux; concentrated HCl.

Scheme 3 Synthetic route of HL2: (i) UHP, TFAA, 0 °C, RT; (ii) ⁱPrCOCl, RT; and (iii) Ph₂POEt, RT.

The lanthanide complexes were prepared by the reaction of the corresponding deprotonated ligands and one-third equivalents of lanthanide chloride salt in methanol.

Crystal structure characterization

Crystal structures of Eu1 and Eu2. Details of crystal data and data collection parameters for Eu1 and Eu2 are listed in Table 1. The crystal structures of Eu1 and Eu2 are shown in Fig. 1. The Eu(III) ion is coordinated by three tridentate(O^N^O) ligands with a coordination number of 9. No solvent molecules coordinate with the Eu(III) ion directly. The coordination environment of the Eu(III) ions is best categorized to a tricapped trigonal prism by estimating the degree of the distortion of the title compounds using the “shape measure” criterion (Table S1 and Fig. S7†).^18–20 The Eu(III) ion and a coordinated ligand form two five-membered rings, and the P=O bonds of the diphenylphosphoryl group and the carboxyl group are found to be located nearly in the same plane with the isoquinoline segment (the dihedral angles are 4.93° and 9.88° for Eu1, and 6.37° and 8.32° for Eu2). The selected bond lengths for Eu1 and Eu2 are listed in Table 2. The average coordination bond length of both Eu1 and Eu2 is 2.49 Å, being suitable for energy transfer from the ligands to the centered ion. Eu1 crystallized in the trigonal space group P3, featuring a chiral trimeric structure, which contained one Λ complex and two Δ enantiomers in one repeating unit (Fig. S8†). Eu2 crystallizes in the triclinic space group P1̄ and consists of centrosymmetric complexes. The difference between the packing structures of Eu1 and Eu2 may be due to the different aspects of the isoquinoline moiety.

Fig. 1 Perspective view of the crystal structures of Eu1 (a) and Eu2 (b) at a 50% probability level. Solvent molecules and all hydrogen atoms are omitted for clarity. Element colors: C, gray; H, white; Eu, green; N, blue; O, red; and P, orange.

Table 1 Crystallographic data for Eu1 and Eu2

Compound	Eu1	Eu2
Empirical formula	C66H45EuN3O9P3	C71H65EuN3O14P3
Formula weight/g mol^−1	1268.92	1429.13
Crystal size/mm	0.22 × 0.18 × 0.16	0.32 × 0.28 × 0.26
Temperature/K	113	113
Crystal system	Trigonal	Triclinic
Space group	P3	P1̄
a/Å	19.351(4)	11.452(2)
b/Å	19.351(4)	15.657(3)
c/Å	13.859(2)	20.993(4)
α/°	90.00	77.41(3)
β/°	90.00	89.17(3)
γ/°	120.00	75.35(3)
V/Å^3	4494.6(18)	3551.0(14)
Z	3	2
D calcd/g cm^−3	1.406	1.337
F(000)	1926	1464
Absorption coefficient/mm^−1	1.186	1.014
Data/restraints/parameters	13 646/1/739	16 850/0/840
Goodness-of-fit on F^2	1.033	1.061
R 1/wR2 [I > 2σ(I)]	0.0389/0.0993	0.0338/0.0902
R 1/wR2 (all data)	0.0413/0.1005	0.0377/0.0926

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Table 2 Selected bond lengths (Å) of Eu1 and Eu2

Eu1	Bond lengths/Å	Eu2	Bond lengths/Å
Eu–O(C)	2.367	Eu–O(C)	2.394
Eu–N	2.669	Eu–N	2.664
Eu–O(P)	2.428	Eu–O(P)	2.403
Average	2.488	Average	2.487

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Photophysical properties

The UV-vis absorption spectra of Eu1 and Eu2 were investigated in dichloromethane solutions (1 × 10⁻⁵ M, Fig. 2). Two absorption bands could be observed, with one narrow band at about 240 nm with a molar absorption coefficient (ε) of 1.2 × 10⁵ M⁻¹ cm⁻¹ and the other weaker broad band between 280 nm and 350 nm with an ε of 1.0 × 10⁴ M⁻¹ cm⁻¹. Both bands originate from π → π^* transitions of the ligands. The singlet energy gap (E_g) of the ligands can be estimated from the edge of the absorption spectra of the complexes, calculated to be 29 000 cm⁻¹ and 28 600 cm⁻¹ for L1⁻¹ and L2⁻¹, respectively. For comparison, the absorption spectra of Eu3 are also shown in Fig. 2. Two absorption bands at 230 nm (ε = 4.5 × 10⁴ M⁻¹ cm⁻¹) and 270 nm (ε = 1.7 × 10⁴ M⁻¹ cm⁻¹) are observed for Eu3, and the E_g is calculated to be as high as 35 200 cm⁻¹.¹³ The expansion of the conjugating system in these isoquinoline derivatives decreases their singlet energy gap of more than 6200 cm⁻¹ compared with their pyridine analogue.

Fig. 2 UV-vis spectra of the Eu(III) complexes in CH₂Cl₂ solution (1 × 10⁻⁵ M) at room temperature.

The phosphorescence spectra of the corresponding gadolinium(III) complexes in the glass state at 77 K were measured to evaluate the triplet state energy levels (E_T) of HL1 and HL2 (Fig. S9†), which were obtained from the 0-phonon component of the phosphorescence spectra (0 → 0 transition, fitted with gaussian distributions).²¹ The E_T of HL1 and HL2 is 20 000 cm⁻¹ and 20 200 cm⁻¹, respectively, lying about 2500 cm⁻¹ higher than the ⁵D₀ energy level (17 500 cm⁻¹) of the Eu(III) ion, which is beneficial for efficient energy transfer.²²

The excitation and emission spectra of the title Eu(III) complexes in dichloromethane solution and in the solid state are shown in Fig. 3 and Fig. S10.† The excitation spectra were recorded by monitoring the ⁵D₀ → ⁷F₂ (615 nm) transition. Similar to the absorption spectra, the excitation spectra of Eu1 and Eu2 show a broad band with edge to 350 nm while that of Eu3 displays a band tailed to about 290 nm. Upon excitation, Eu1 and Eu2 displayed characteristic emissions with peaks at 580, 593, 615, 650 and 700 nm, which are attributed to the f–f transitions of ⁵D₀ → ⁷F_J with J = 0, 1, 2, 3 and 4, respectively. The emissions derived from the ligands are not observed, which indicates the effective energy transfer from the ligands to the Eu(III) ions. The emission spectra for Sm1, Nd1 and Yb1 were also measured and are shown in Fig. S11.†

Fig. 3 The excitation and emission spectra of the Eu(III) complexes in CH₂Cl₂ solution (1 × 10⁻⁵ M) at room temperature.

The luminescence lifetimes (τ_obs) of Eu1, Eu2, Sm1, and Yb1 were obtained in both dichloromethane solution and the solid state by fitting the luminescence decay curves (Fig. S12–S14†) and the results are shown in Table 3. Long lifetime values of 2.2/2.3 ms were observed for Eu1/Eu2 while Sm1 and Yb1 showed much shortened ones. The τ_obs of Nd1 was not able to be obtained due to weak emission intensity. The monoexponential lifetime indicates that the lanthanide ions are locate in a similar environment for each complex.

Table 3 Photophysical and thermal properties of the lanthanide complexes

Complex	PLQY^a (%)		τ obs (ms)	τ rad (ms)	Φ ^EuEu	η sens	T d ^d (°C)
	Solid	Solution
a Photoluminescence quantum yield (PLQY) measured in the visible region using the Hamamatsu C9920-02 PLQY measurement system with an integrating sphere, Ex = 330 nm and 350 nm for solutions and solids, respectively. b PLQY measured in the NIR region using Edinburgh FLS980 with an integrating sphere, Ex = 350 nm. c Observed lifetime τobs and calculated radiative lifetime τrad in the solution. d Decomposition temperature determined by the loss of 5% desolvated weight.
Eu1	94	69	2.2	2.7	0.80	0.87	352
Eu2	87	62	2.3	3.0	0.76	0.82	438
Sm1	3.2	n.a.	0.11	n.a.	n.a.	n.a.	354
Nd1	n.a.	n.a.	n.a.	n.a.	n.a.	n.a.	354
Yb1	2.0^b	n.a.	0.048	n.a.	n.a.	n.a.	356

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The photoluminescence quantum yields (PLQYs) of Eu1 and Eu2 were determined and the results are shown in Table 3. Eu1 and Eu2 show extremely high PLQYs in the solid state (94% and 87%, respectively) and in CH₂Cl₂ solution (69% and 62%, respectively). We also calculated the intrinsic quantum yield of the Eu(III)-centered emission (Φ^Eu_Eu) and the sensitization efficiency (η_sens) of these complexes in solution according to the literature.¹⁵ Both complexes showed high sensitization efficiencies (over 80%) and intrinsic quantum yields (over 75%), benefiting from the appropriate energy level of the ligands and suppressed quenching pathways. Sm1 and Yb1 showed much lower PLQYs compared with Eu1 in the solid states, as these complexes showed NIR emissions which were easily quenched by the small energy oscillators such as the C–H vibration.

Thermal stability study

The thermo-gravimetric analyses (TGA) of Eu1, Eu2, Sm1, Nd1 and Yb1 were carried out to evaluate the thermal stability of the complexes (Fig. 4). The weight loss below 150 °C is attributed to the loss of the lattice solvent, which is consistent with the results of elemental analyses in the Experimental part. Taking Eu1 and Eu2 for example, the solvent weight loss values of the two complexes are 6.2% and 2.6%, respectively, which correspond to about 4.7 water molecules for one Eu1 and 1.9 water molecules for one Eu2. Anal. calcd for Eu1·4.7H₂O: C 58.56, N 3.10, H 4.05; found, C 58.66, N 3.04, H 3.84. Anal. calcd for Eu2·1.9H₂O: C 60.83, N 3.22, H 3.77; found, C 60.57, N 3.17, H 3.62. The decomposition temperature (T_d) for all the complexes are higher than 350 °C (Table 3), which benefits from the rigid structures of the tridentate isoquinoline ligands.

Fig. 4 TGA curves of Eu1, Eu2, Sm1, Nd1, and Yb1.

Conclusions

In summary, two structurally rigid europium(III) complexes based on diphenylphosphoryl modified tridentate isoquinoline derivatives were synthesized. Because isoquinoline has an enlarged π conjugation system compared with its pyridine analogue, broad band excitation tailed to 350 nm is obtained. The tridentate ligands present appropriate triplet energy levels for efficient intramolecular energy transfer and can saturate the coordination spheres of the Eu(III) ion when forming 3 : 1 complexes, thus avoiding quenching by solvents. Consequently, the title complexes display extremely high PLQYs in the solid state (94% and 87% for Eu1 and Eu2, respectively), showing that it is an effective way to construct highly luminescent lanthanide(III) complexes by employing diphenylphosphoryl tridentate ligands.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is supported through grants from the National Key R&D Program of China (Grant 2017YFA0205100), National Natural Science Foundation of China (Grant 21621061), Key Project of Science and Technology Plan of Beijing Education Commission (Grant KZ201910028038), and Natural Science Foundation of Beijing Municipality (Grant 2172017).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1814236 and 2009015. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0qi00894j

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