Tuning of the excitation wavelength in Eu3+-aminophenyl based polyfluorinated β-diketonate complexes: a red-emitting Eu3+-complex encapsulated in a silica/polymer hybrid material excited by blue light

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Tuning of excitation wavelength in Eu³⁺-aminophenyl based

polyfluorinated β-diketonate complexes: Red-emitting Eu³⁺-

complex encapsulated in silica/polymer hybrid material excited

by blue light†

Received 00th January 20xx,

Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

T. V. Usha Gangan^a,band M. L. P. Reddy^a,b

*

www.rsc.org/

We describe herein the synthesis, characterization and photophysical properties of a series of europium complexes based

on three aminophenyl based polyfluorinated β-diketonates, namely, 1-(4-aminophenyl)-4,4,5,5,5-pentafluoro-3-

hydroxypent-2-en-1-one,

(diphenylamino)phenyl)-4,4,5,5,5-pentafluoro-3-hydroxypent-2-en-1-one

bis(diphenylphosphino)-9,9-dimethylxanthene oxide. The results demonstrated that the triphenylamine based

1-(4-(dimethylamino)phenyl)-4,4,5,5,5-pentafluoro-3-hydroxypent-2-en-1-one

and 1-(4-

and an ancillary

ligand, 4,5-

polyfluorinated Eu³⁺-β-diketonate complexes dramatically red-shifted the excitation maximum to visible region (λ_ex,

=

max

400 nm) with impressive quantum yield (40 %) as compared to the simple Eu³⁺-aminophenyl-β-diketonate complexes (λ_ex,

= 370 nm). This can be explained on the basis of conjugation between nitrogen lone pair electrons and the phenyl π-

max

electrons in the β-diketonate ligand system. On the other hand, the electron-donating dimethylamino group (Hammett

constant: σ_p= -0.83) containing Eu³⁺-β-diketonate complexes moderately shifted the excitation maximum in the UV region

from 370 to 380 nm as compared to unsubstituted aminophenyl (Hammett constant: σ_p= -0.66) Eu³⁺complexes. The

displacement of water molecules in aminophenyl based Eu³⁺-β-diketonate binary complexes by a rigid phosphine oxide

ligand richly enhances the photoluminescence quantum yields as well as the excited state lifetime values of the

corresponding ternary complexes. As an integral part of this work, hybrid materials have been developed through sol-gel

route by encapsulating a ternary Eu³⁺compound in a silica/polymer hybrid for high performamnce luminescence

applications. In addition, a bright red-emitting diode was fabricated by coating the designed hybrid material onto a 400

nm emitting InGaN chip and examined the photoluminescence. Notably, the current study clearly highlights that the

developed triphenylamine based Eu³⁺-β-diketonate complex is an interesting red-emitting material excited by blue light

and therefore may find potential application in the fields of biological and material sciences.

Introduction

indirect excitation of the metal center. This indirect excitation,

also known as antenna effect, takes advantage of the

The unique photoluminescence properties of Eu³⁺complexes

have aroused tremendous interest for decades owing to their

outstanding potential applications in medical diagnostics and

organic light emitting diodes.^1-4The shielding of the f orbitals

by the 5s²and 5p⁶closed shells results in narrow line-like

emissions of optically pure colors with long radiative lifetimes.

However, the f-f transition that result in light emission from

the lanthanide ions are both spin-and parity-forbidden which,

in turn, mandates the use of antenna molecules for the

coordinated ligands in the sense that energy transfer from the

ligand-centered excited states to the metal center results in

lanthanide ion luminescence.⁴The β-diketonate ligand class is

emerging as one of the important antenna molecules in terms

of high harvest emissions because of the effectiveness of the

energy transfer from this ligand to the Ln³⁺ion.⁵

Unfortunately, the excitation window appears to be

limited to the near-UV region in many of the Eu³⁺-β-diketonate

complexes due to the energy constraints posed by the

photophysics of sensitized Eu³⁺luminescence, as well

documented by Reinhoudt and coworkers.⁶Therefore, one of

the challenges in this field is to develop luminescent Eu³⁺

complexes that can be excited by visible light and this field has

become more important because of the increasing demand for

less harmful reagents in life sciences and also low voltage

driven pure red emitters in optoelectronic applications.

Indeed, some of the recent literature reports demonstrated

^aAcSIR-Academy of Scientific & Innovative Reasearch, CSIR-NIIST Campus,

Thiruvananthapuram, India

^bMaterials Science and Technology Division, National Institute for Interdisciplinary

Science and Technology (NIIST), Council of Scientific and Industrial Research (CSIR),

Thiruvananthapuram-695 019, India, E-mail: mlpreddy55@gmail.com

†

Electronic Supplementary Information (ESI) available: ¹H NMR, ¹³C NMR spectra of

the ligands, thermogravimetric curves for complexes and gels, lifetime decay

profiles, photochemical stability, FT-IR spectra of the hybrid materials, UV-visible

absorption spectra of the hybrid materials in solid state and XRD patterns of the

hybrid materials.

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that the excitation window can be shifted to visible region in examined the photoluminescence properties. Additionally, a

Eu³⁺-β-diketonate complexes by appropriate molecular bright red emitting diode was fabricated by coating the

engineering of the ligand systems with suitably expanded

π

-

luminescent hybrid material onto a 400 nm emitting InGaN

conjugation in the complex molecules.^7-9However, some of chip and investigated the photoluminescence.

the Eu³⁺-β-diketonate complexes reported exhibit poor

quantum yields.^7,8a,b

Triphenylamine derivatives are widely used as hole-

transporting materials in Organic Light Emitting Diodes (OLEDs)

due to their high charge mobility, light-harvesting unit and

high thermal stability.¹⁰It is well documented that the

replacement of C–H bonds in a β-diketonate ligand with lower-

energy C–F oscillators is able to lower the vibration energy of

the ligand, which decreases the energy loss caused by ligand

vibration and enhances the emission intensity of the

lanthanide ion. Further, due to the heavy-atom effect, which

facilitates intersystem crossing, the lanthanide-centered

luminescent properties are enhanced.¹¹Based on the above

Scheme 1 Structures of the β-diketonate ligands.

considerations, we conceive of a strategy that simultaneously

incorporates highly conjugated triphenylamine and

polyfluorinated alkyl groups into the β-diketonate ligand,

Experimental Section

expecting to obtain the resultant ligands possessing high

luminescence efficiency and photochemical stability under

Materials and instrumentation

visible light excitation upon coordination with trivalent

The following chemicals were acquired commercially and used

lanthanides. Thus, a series of novel aminophenyl based β-

diketonate ligands (Scheme 1), namely, 1-(4-aminophenyl)-

4,4,5,5,5-pentafluoro-3-hydroxypent-2-en-1-one (HAPFP), 1-

(4-(dimethylamino)phenyl)-4,4,5,5,5-pentafluoro-3-

without further purification: Europium(III) nitrate hexahydrate,

99.9% (Alfa-Aesar); gadolinium(III) nitrate hexahydrate, 99.9%

(Sigma-Aldrich); triphenylamine, 98% (Sigma-Aldrich); 4-

aminoacetophenone, 99% (Alfa-Aesar); sodium hydride 60%

hydroxypent-2-en-1-one

(HDMAPFP)

and

1-(4-

dispersion

in

mineral

oil

98%

(Sigma-Aldrich);

ethyl

4,5-

(diphenylamino)phenyl)-4,4,5,5,5-pentafluoro-3-hydroxypent-

2-en-1-one (HDPAPFP) have been synthesized and utilized for

the construction of Eu³⁺-β-diketonate coordination compounds

in the presence and absence of an ancillary ligand, 4,5-

bis(diphenylphosphino)-9,9-dimethylxanthene oxide (DDXPO)

with a view to shift the excitation window to the visible region.

Herein, we demonstrate that these ligands are easily

accessible, readily coordinate to europium, and efficiently

sensitize its luminescence when excited under visible light. The

origin of “amino conjugation effect” on the emission as well as

other excited state properties in these complexes has also

been elucidated and discussed.

pentafluoropropionate,

bis(diphenylphosphino)-9,9-dimethylxanthene, 97% (Sigma-

Aldrich); iodomethane, 98% (Alfa-Aesar); Tetraethyl

orthosilicate (TEOS), 98% (Sigma-Aldrich). All the other

chemicals used were of analytical reagent grade without

further purification.

Elemental analyses were performed with an Elementar -

vario MICRO cube elemental analyzer.

A Perkin-Elmer

Spectrum two FT-IR spectrometer using KBr was used to obtain

the IR spectral data and a Bruker 500 MHz NMR spectrometer

was used to record the ¹H NMR (500 MHz), ¹³C NMR (125.7

MHz) and ³¹P NMR (202.44 MHz) spectra of the new

compounds in chloroform-d solution. The chemical shifts are

reported in parts per million relative to tetramethylsilane,

SiMe₄for ¹H NMR and ¹³C NMR spectra and with respect to

85% phosphoric acid for ³¹P NMR spectra. Electro spray

ionization (ESI) mass spectra were recorded on a Thermo

Scientific Exactive Benchtop LC/MS Orbitrap Mass

Spectrometer. Matrix assisted laser desorption ionization time-

of-flight (MALDI-TOF) mass spectra were recorded on KRATOS

Due to their poor thermal resistivity, moisture sensitivity

and feeble mechanical strength, the lanthanide complexes are

difficult to directly utilize as luminescence sources in many

optoelectronic applications. These inherent problems can be

solved by encapsulating the lanthanide luminescent complexes

in suitable solid matrixes including polymers,^2a,4d,8c,12sol-gel

silica,^2a,3c,4d,13mesoporous materials^{2a,4d,5g,8c,14}and even

carbon nanotubes.^8d,15The potential utility of these materials

depend on exploiting the synergy between the excellent

luminescence features of lanthanides and the intrinsic

characteristics of sol-gel derived hybrid materials. These

materials may find promising applications such as light

emitting devices, active wave guides and biomedical actuators

and sensors. In order to enhance the luminescent properties

and improve the stability of the lanthanide complexes, herein,

a novel ternary Eu³⁺luminescent complex has been embedded

into silica/polymer hybrid material, characterized and

analytical spectrometer (Shimadzu Inc.)

and the

thermogravimetric analyses were performed on a TG/DTA-

6200 (SII Nano Technology Inc., Japan). Scanning electron

microscopy (SEM) of EuC-PMMA-Gel and EuC-Gel were

performed on a ZEISS EVO-18 Cryo-SEM instrument. X-ray

powder patterns (XRD) were recorded in the 2

70 using Cu-K_αradiation (Philips X’pert Pro). The molar

absorption coefficient ( ) of the ligands were measured in THF

θ range of 10-

°

ε

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solution on a UV-vis spectrophotometer (Shimadzu, UV-2450). mixture was stirred at 60

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°

C for 1 day, cooled at room

The photoluminescence (PL) spectra were recorded on a Spex- temperature and quenched with a mixture of ice and water.

Fluorolog FL22 spectrofluorimeter equipped with a double The product was ﬁltered and washed with water to afford the

grating 0.22 m Spex 1680 monochromator and a 450W Xe compound as a white solid. Yield: 82%. ¹H NMR (CDCl₃, 500

lamp as the excitation source operating in the front face mode. MHz) δ (ppm): 7.87 (d, 2H, J = 8.5 Hz), 6.65 (d, 2H, J = 8.5 Hz),

The lifetime measurements were carried out at room 3.06 (s, 6H), 2.51 (s, 3H). ¹³C NMR (CDCl₃, 125.7 MHz) δ (ppm):

temperature using a Spex 1040D phosphorimeter. The overall 196.47, 153.17, 130.53, 125.33, 129.60, 110.59, 40.02, 25.98.

quantum yield (Φ

_overall) was measured using an integrating m/z = 164 (M+H)⁺.

sphere in a SPEX Fluorolog spectrofluorimeter.

Synthesis of 1-(4-(diphenylamino)phenyl)ethanone

.

Acetyl

The PL quantum yields of thin films (

Φ

_overall) were chloride (4.08 mmol) taken in dichloromethane solution was

determined using a calibrated integrating sphere system. A Xe- added dropwise to a slurry of zinc chloride (4.08 mmol)

arc lamp was used to excite the thin film samples that were suspended in a solution of triphenylamine (4.08 mmol). After

placed in the sphere. All samples were prepared by drop the addition was completed, the mixture was heated under

casting the material placed between two glass cover slips. The reflux for 24 h. The reaction mixture was then poured into

quantum yields were determined by comparing the spectral cold, dilute hydrochloric acid. The organic layer was separated,

intensities of the lamp and the sample emission as reported in washed with water until the wash water was neutral, and then

the literature.¹⁶Using this experimental setup and the dried over anhydrous sodium sulphate. The resulting residue

integrating sphere system, the solid state fluorescence was purified by column chromatography using ethyl

quantum yield of a thin film of the standard green OLED acetate/hexane (10:90), thereby affording the desired product

material tris-8-hydroxyquinolinolato aluminium (Alq₃) was as a yellow solid. Yield: 60%. ¹H NMR (CDCl₃, 500 MHz) δ

determined to be 0.40, which is consistent with previously (ppm): 7.79 (d, 2H, J = 10 Hz), 7.30 (m, 4H), 7.14 (m, 6H), 6.98

reported values.¹⁷Several measurements were carried out for (d, 2H, J = 9 Hz), 2.53 (s, 3H). ¹³C NMR (CDCl₃, 125.7 MHz) δ

each sample, so that the presented value corresponds to the (ppm): 196.48, 152.17, 146.51, 129.88, 129.60, 125.96, 124.63,

arithmetic mean value. The estimated error for the quantum 119.68, 26.23. m/z = 288 (M+H)⁺.

yields is (± 10%).

Scheme 2 Synthetic procedure for the ligands.

Synthetic procedures

The ligands 1-(4-aminophenyl)-4,4,5,5,5-pentafluoro-3-

hydroxypent-2-en-1-one (HAPFP), 1-(4-

Synthesis of the ligands. A modified method of typical Claisen

condensation procedure is used for the synthesis of β-

diketonate ligands. The corresponding ketone (1.0 mmol) and

ethyl pentafluoropropionate (1.0 mmol) were added to 20 mL

(dimethylamino)phenyl)-4,4,5,5,5-pentafluoro-3-hydroxypent-

2-en-1-one (HDMAPFP) and 1-(4-(diphenylamino)phenyl)-

of dry tetrahydrofuran (THF) and stirred for 10 min at 0 °C in

an ice bath. To this reaction mixture, sodium hydride (60%)

was added in an inert atmosphere and stirred for 30 min

followed by further stirring at 65 °C for 24 h. To the resulting

solution, 2 M HCl (25 mL) was added, and extracted thrice with

dichloromethane (3 × 25 mL). The organic layer was separated

and dried over Na₂SO₄, and the solvent was evaporated. The

4,4,5,5,5-pentafluoro-3-hydroxypent-2-en-1-one

(HDPAPFP)

were synthesized according to the method described in

scheme 2.

Synthesis of 1-(4-(dimethylamino)phenyl)ethanone. To a DMF

solution of 4-aminoacetophenone (7.40 mmol) iodomethane

(17 mmol) and K₂CO₃(17 mmol) were added. The resultant

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crude product thus obtained was then purified by column (where Ln = Eu³⁺, Gd³⁺) (1.0 mmol) in 2 mL water was added

chromatography on silica gel with mixture of ethyl acetate and drop wise and stirred for 12 h at room temperature. The

hexane (5:95 for HAPFP, 2:98 for HDMAPFP and HDPAPFP) as resultant precipitate formed was filtered off, washed with

the eluent to get the product.

water and dried. The products were purified by

recrystallization from chloroform solution and used for further

1-(4-aminophenyl)-4,4,5,5,5-pentafluoro-3-hydroxypent-2-

en-1-one (HAPFP). Yield: 60%. Elemental analysis (%): analysis and photophysical studies. Efforts to grow single

calculated for C₁₁H₈F₅NO₂(281.18): C 46.99, H 2.87, N 4.98; crystals of complexes were not fruitful. The synthesis method

1

Found: C 46.74, H 2.84, N 4.76. H NMR (CDCl₃, 500 MHz) δ is described in Scheme 3.

(ppm): 15.28 (broad, enol–OH), 8.17 (s, 2H, NH₂), 8.04 (d, 2H, J Eu(APFP)₃(H₂O)₂(1). Elemental analysis (%): calculated for

= 9 Hz), 7.79 (d, 2H, J = 8.5 Hz), 6.64 (S, 1H). ¹³C NMR (125.7 C₃₃H₂₅F₁₅N₃O₈Eu (1029.06): C 38.54, H 2.45, N 4.09; Found: C

MHz, CDCl₃) δ (ppm): 196.68, 183.78, 156.39, 139.88, 132.16 38.35, H 2.53, N 3.93. IR (KBr) ν_max(cm^-1): 3435, 3320, 1712,

127.21, 117.21, 112.14, 96.82, 77.26-76.75 (CDCl₃). IR (KBr) 1523, 1328, 1216, 1039, 695. m/z

ν_max(cm^-1): 3346 (N-H), 1703, 1534, 1333, 1209, 1037, 794. [Eu(APFP)₃+Na+1]⁺.

=

1016.28

m/z = 281 (M)⁺.

Eu(DMAPFP)₃(H₂O)₂

(2). Elemental analysis (%): calculated for

1-(4-(dimethylamino)phenyl)-4,4,5,5,5-pentafluoro-3-

C₃₉H₃₇F₁₅N₃O₈Eu (1113.16): C 42.10, H 3.35, N 3.78; Found: C

hydroxypent-2-en-1-one (HDMAPFP). Yield: 65%. Elemental 42.25, H 3.33, N 3.87. IR (KBr) ν_max(cm^-1): 3443, 2927, 1594,

analysis (%): calculated for C₁₃H₁₂F₅NO₂(309.23): C 50.49, H 1370, 1323, 1281, 1014, 671. m/z

1

=

1094.44

3.91, N 4.53; Found: C 50.64, H 4.06, N 4.58. H NMR (CDCl₃, [Eu(DMAPFP)₃(H₂O)]⁺.

500 MHz) δ (ppm): 15.97 (broad, enol–OH), 7.86 (d, 2H, J = 9.5 Eu(DPAPFP)₃(H₂O)₂(3). Elemental analysis (%): calculated for

Hz), 6.68 (d, 2H, J = 10 Hz), 6.48 (s, 1H), 3.11 (S, 6H). ¹³C NMR C₆₉H₄₉F₁₅N₃O₈Eu (1485.25): C 55.80, H 3.33, N 2.83; Found: C

(CDCl₃, 125.7 MHz) δ (ppm): 185.68, 175.78, 154.39, 133.88, 56.01, H 3.45, N 2.91. IR (KBr) ν_max(cm^-1): 3437, 3049, 1614,

130.20, 127.21, 119.21, 111.14, 91.82, 40.02, 77.28-76.78 1583, 1492, 1327, 1275, 1013, 678. m/z

(CDCl₃). IR (KBr) ν_max(cm^-1): 2925, 1586, 1377, 1329, 1229, [Eu(DPAPFP)₃-H]⁺.

= 1447.33

1011, 741. m/z = 310.08 (M+H)⁺.

Gd(APFP)₃(H₂O)₂(7). Elemental analysis (%): calculated for

C₃₃H₂₅F₁₅N₃O₈Gd (1034.06): C 38.34, H 2.44. N 4.06; Found: C

1-(4-(diphenylamino)phenyl)-4,4,5,5,5-pentafluoro-3-

hydroxypent-2-en-1-one (HDPAPFP). Yield: 85%. Elemental 38.20, H 2.18, N 3.91. IR (KBr) ν_max(cm^-1): 3435, 3321, 1711,

analysis (%): calculated for C₂₃H₁₆F₅NO₂(433.37): C 63.74, H 1523, 1328, 1216, 1039, 695. m/z = 1016.48 [Gd(APFP)₃(H₂O)]⁺.

1

3.72, N 3.23; Found: C 63.90, H 3.84, N 3.35. H NMR (CDCl₃, Gd(DMAPFP)₃(H₂O)₂(8). Elemental analysis (%): calculated for

500 MHz) δ (ppm): 15.71 (broad, enol–OH), 7.78 (d, 2H, J = 9 C₃₉H₃₇F₁₅N₃O₈Gd (1118.16): C 41.90, H 3.35. N 3.76; Found: C

Hz), 7.35 (m, 4H), 7.20 (m, 6H), 6.98 (d, 2H, J = 8.5 Hz), 6.50 (s, 42.06, H 3.26, N 3.68. IR (KBr) ν_max(cm^-1): 3443, 2927, 1595,

1H) (Fig. S1). ¹³C NMR (CDCl₃, 125.7 MHz) δ (ppm): 189.63, 1370, 1323, 1281, 1014, 671. m/z

185.74, 153.39, 145.83, 130.10, 129.88, 129.79, 129.48, [Gd(DMAPFP)₃(H₂O)₂+Na]⁺.

=

1141.89

126.39, 125.42, 123.80, 119.06, 92.45, 77.27-76.76 (CDCl₃). IR Gd(DPAPFP)₃(H₂O)₂(9). Elemental analysis (%): calculated for

(KBr) ν_max(cm^-1): 3059, 1609, 1585, 1490, 1331, 1265, 1016, C₇₁H₅₅F₁₅N₃O₈Gd (1454.23): C 55.61, H 3.31. N 2.82; Found: C

697. m/z = 434.22 (M+H)⁺.

55.78, H 3.43, N 2.98. IR (KBr) ν_max(cm^-1): 3433, 3054, 1615,

4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene

oxide 1584, 1492, 1327, 1276, 1013, 698. m/z = 1022.30

(DDXPO).

The corresponding phosphine (5.0 mmol) was [Gd(DPAPFP)₂]^+.

dissolved in 10 mL of 1,4-dioxane solution, to which 1.0 mL of Synthesis of Eu³⁺complexes 4-6. Ternary Eu³⁺complexes were

30% H₂O₂(10.5 mmol) was added drop wise with vigorous prepared by stirring equimolar solutions of corresponding

stirring. The resultant mixture was then stirred for 2 h and binary complexes and DDXPO in CHCl₃solution for 12 h at 70

then 10 mL of water was added to the reaction mixture to °C. The products were isolated by solvent evaporation and

arrest the reaction. The mixture was extracted with 3 × 30 mL puriﬁed by recrystallization from a chloroform mixture. The

of dichloromethane. The oily phase was then washed with 2 × synthesis procedure is illustrated in scheme 4.

30 mL of water to remove 1,4-dioxane. The dichloromethane Eu(APFP)₃(DDXPO) (4). Elemental analysis (%): calculated for

layer was dried with Na₂SO₄. The solvent was removed in C₇₂H₅₃O₉F₁₅N₃P₂Eu (1603.22): C 53.94, H 3.33, N 2.62; Found: C

vacuo. The product was recrystallized from dichloromethane. 54.06, H 3.44, N 2.65. IR (KBr) ν_max(cm^-1): 3060, 1713, 1527,

Yield: 95%. Elemental analysis (%): calculated for C₃₉H₃₂O₃P₂1333, 1217, 1179, 1019, 696. m/z

=

1345.83

(610.18): C 76.71, H 5.28; Found: C 76.52, H 5.40. ¹H NMR [Eu(APFP)₂(DDXPO)+Na]⁺. ³¹P NMR (CDCl₃, 202.44 MHz) δ

(CDCl₃, 500 MHz) δ (ppm): 7.61 (d, 2 H, J = 5 Hz), 7.41 (q, 12 H, (ppm): -77.55.

J = 20 Hz), 7.30 (q, 8 H, J = 20 Hz), 6.99 (m, 2 H), 6.80–6.76 (q, 2 Eu(DMAPFP)₃(DDXPO) (5). Elemental analysis (%): calculated

H, J = 20 Hz), 1.70 (s, 6 H). ³¹P NMR (CDCl₃, 202.44 MHz) δ for C₇₈H₆₅O₉F₁₅N₃P₂Eu (1687.32): C 54.52, H 3.88, N 2.49;

(ppm): 30.97. IR (KBr) ν_max(cm^-1): 1727, 1670, 1436, 1401, Found: C 54.39, H 4.01, N 2.45. IR (KBr) ν_max(cm^-1): 3061, 1596,

1229, 1190, 1114, 875, 785, 746, 719, 694. m/z = 611.31 1506, 1403, 1330, 1274, 1179, 1014, 676. m/z = 1402.20

(M+H)⁺.

[Eu(DMAPFP)₂(DDXPO)+Na+1]⁺. ³¹P NMR (CDCl₃, 202.44 MHz)

Synthesis of binary complexes. To a solution of β-diketonate δ (ppm): -82.80.

ligand (3.0 mmol) in ethanol, NaOH (3.0 mmol) in water was Eu(DPAPFP)₃(DDXPO) (6). Elemental analysis (%): calculated

added and stirred for 5 min. To this mixture, Ln(NO₃)₃∙6(H₂O) for C₁₀₈H₇₇O₉F₁₅N₃P₂Eu (2059.67): C 62.98, H 3.77, N 2.04;

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Found: C 63.02, H 3.92, N 2.11. IR (KBr) ν_max(cm^-1): 3061, 1622,

1588, 1524, 1492, 1327, 1274, 1178, 1013, 696. m/z = 1628.93

[Eu(DPAPFP)₂(DDXPO)]⁺. ³¹P NMR (CDCl₃, 202.44 MHz) δ

(ppm): -86.87.

Results and discussion

Synthesis and characterization of lanthanide complexes

Scheme 2 summarizes the protocols used for the synthesis of

various β-diketonates that are used in the present study. The

β-diketonates (HAPFP, HDMAPFP and HDPAPFP) were

obtained as yellow solids in 60-85% yields by Claisen

condensation of the corresponding ketones with ethyl

pentafluoropropionate in the presence of a strong base NaH.

Synthesis of Eu(DPAPFP)₃(DDXPO)-Gel [EuC-Gel]. Tetraethyl

orthosilicate (TEOS) was first mixed with ethanol, and then

HCl-acidified water (pH = 2) was added to the above mixture

under magnetic stirring to initiate the hydrolysis and

condensation reaction. The molar ratio of TEOS/ethanol/H₂O

was maintained at 1:4:4. A transparent sol was obtained. After

Detailed characterisation of the synthesized ligands was

1

performed by H NMR, ¹³C NMR (Fig. S1 to S6 in the ESI

†

NMR, FT-IR and mass spectroscopic (ESI-MS) methods, as well

), ³¹P

stirring for

containing an appropriate amount of the ternary complex

Eu(DPAPFP)₃(DDXPO) (the mass ratio of Eu³⁺complex (

)/SiO₂

3 h, N,N-dimethylformamide (DMF) solution

1

as by elemental analysis. H NMR analysis shows that the β-

6

was 3:10) was added to the sol. The mixture was stirred at

room temperature for 4 h. The gel was allowed to stand for 5

diketonate compounds mainly exist as enol form in chloroform

solutions. The chelating ligand 4,5-bis(diphenylphosphino)-9,9-

dimethylxanthene oxide (DDXPO) was synthesized according

days and then dried at 45 °C. The gel was collected as

monolithic bulks and ground into powdered material for the

e,f

to the previous literature report.⁵The synthesis procedures

for the Ln³⁺(Eu³⁺and Gd³⁺) complexes are depicted in Schemes

photophysical studies.

Synthesis of Polymer-Eu(DPAPFP)₃(DDXPO)-Gel (EuC-PMMA-

Gel). Similar to the EuC-Gel preparation, TEOS was first mixed

with ethanol. Then HCl-acidified water (pH = 2) was added to

start the hydrolysis and condensation reaction. The molar ratio

of TEOS/ethanol/H₂O was 1:4:4. After stirring for 3 h, DMF

solution containing the complex Eu(DPAPFP)₃(DDXPO) and

polymer (PMMA) were added to the sol. The mass ratio of Eu³⁺

complex/PMMA/SiO₂was 3:4:10. The mixture was stirred for

4h at room temperature to make sure uniform mixing and

complete hydrolysis, and then placed in a sealed container,

3

and 4, respectively. The lanthanide complexes were

characterized by FT-IR, MALDI-TOF and elemental analysis. The

elemental analysis and MALDI-TOF studies of Ln³⁺complexes

(1-9

) revealed that the central Ln³⁺ion is coordinated to three

β-diketonate ligands. In the case of ternary complexes (4-6),

one molecule of the bidentate phosphine oxide, DDXPO, is also

present in the coordination sphere, which is confirmed by FT-

IR analysis. Shift in the P=O stretching frequency of DDXPO

1190 cm^-1to 1179 cm^-1in ternary complexes

4-6 confirms the

participation of phosphoryl oxygen in the complex formation

with the metal ion. This has been further confirmed from the

³¹P NMR spectra of the complexes

4-6 that the P=O resonances

which was kept at 45 °C until the precursor solution was

converted into a monolithic gel and ground into powdered

material for the photophysical studies.

are shifted upfield compared to the free ligand, indicating the

involvement of phosphoryl oxygen in coordination with the

Eu³⁺ion. The broad absorption band noted in the region 3000

– 3500 cm^-1of the FT-IR spectra of the complexes (

), points to the existence of water molecules in the

coordination sphere. On the other hand, the absence of this

broad band in complexes inferred that the water

1-3 and 7-

9

4–6

molecules have been displaced successfully by the chelating

phosphine oxide ligand. The carbonyl stretching frequency of

the β-diketonate ligands, HAPFP (1703 cm^-1), HDMAPFP (1586

cm^-1), HDPAPFP (1609 cm^-1) is shifted to higher wave numbers

in

¹in

in

1

–

9

(1712 cm^-1in

1

; 1594 cm^-1in

2

; 1614 cm^-1in

, 1711 cm^-1in

and 1615 cm^-1in

9), thus confirming the coordination of

3

; 1713 cm^-

4

; 1596 cm^-1in

5

; 1622 cm^-1in

6

7

; 1595 cm^-1

8

the carbonyl oxygen to the Ln³⁺ions.

Scheme 3 Synthesis of the Ln³⁺(Ln = Eu, Gd) binary complexes.

The thermal behaviour of the Ln³⁺complexes were

investigated by means of thermogravimetric analysis (TGA)

under a nitrogen atmosphere and the results are given in the

Fig. S7 in ESI.

data that complexes

about 3% in the first step (90 to 140

the elimination of the coordinated water molecules. On the

other hand, complexes are more stable than the precursor

samples and do not show decomposition up to 250 C. The

†

It is clear from the thermogravimetric analysis

and undergoes a mass loss of

C), which corresponds to

1-

3

7-9

°

4-6

1

-3

°

total weight loss occurred in the thermal analysis of all these

complexes are much higher than that calculated values for the

non-volatile lanthanide(III) oxide, indicating the partial

Scheme 4 Synthesis of the Eu³⁺ternary complexes 4-6.

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sublimation of these complexes under atmospheric pressure,

which is commonly observed in the case of fluorinated β-

f

diketonate complexes.⁵

Electronic spectra of the aminophenyl based β-diketonate ligands

and their corresponding Eu³⁺complexes (1-6)

The absorption spectra of the free ligands and the

corresponding Eu³⁺complexes (

1-6) have been recorded in THF

solution (c = 5 × 10^-6M) at 298 K and are depicted in Figs. 1

and 2, respectively. The absorption profiles of the Eu³⁺

complexes in THF solution are found to be similar to that of

the ligands, indicating that the singlet excited states of the

ligand is not significantly affected by complexation to the Eu³⁺

ion. The main absorption band of all the β-diketonate ligands

(singlet-singlet n-π^*enolic transition: 290-380 nm in HAPFP

with λ_max= 350 nm; 340-470 nm in HDMAPFP with λ_max= 420

nm; 340-490 nm in HDPAPFP with λ_max= 425 nm), on

Fig. 2 UV-visible absorption spectra of complexes 1-6 in THF (c = 5 × 10^-6M).

complexation with Eu³⁺ion, underwent a hypsochromic shift (

10 nm in and 35-45 nm in and 38-45 nm in and

), while the short wavelength band (

π^*transition of

aromatic moiety of the β-diketonate ligand: 250-305 nm in

and ; 275-325 nm in and ; 275-340 nm in and ) remains

∼

Table 1 Molar absorption coefficient for the ligands and their corresponding Eu³⁺

1

4;

∼

2

5;

∼

3

complexes

6

π-

Compounds

λ

_max(nm)

ε (L mol^-1cm^-1)

1

4

2

5

3

6

HAPFP

350

420

425

340

385

398

340

375

380

2.40 × 10⁴

2.96 × 10⁴

3.08 × 10⁴

7.30 × 10⁴

8.91 × 10⁴

9.32 × 10⁴

7.45 × 10⁴

8.90 × 10⁴

9.28 × 10⁴

unaffected. This indicates that the electron density on the

diketonate moiety has been perturbed by the imposed

negative charge of deprotonation and presence of Lewis acidic

metal centre, while the aromatic part retains the same

strength.⁵The electronic transitions of the aromatic moiety

of the β-diketonate ligand (peak at ca. 250-320 nm) and the

chelated phosphine oxide (peak at ca. 250-300 nm) units are

overlapped. The presence of the ancillary ligand DDXPO not

only enhances the absorption intensity but also satisfies the

high coordination number of the central Eu³⁺ion and thus

improves the coordination and thermal stabilities of ternary

complexes. The extinction coefficients (Table 1) of the

HDMAPFP

HDPAPFP

Eu(APFP)₃(H₂O)₂

(1)

Eu(DMAPFP)₃(H₂O)₂(2)

f,18

Eu(DPAPFP)₃(H₂O)₂

Eu(APFP)₃(DDXPO)

(3)

(4)

Eu(DMAPFP)₃(DDXPO) (5)

Eu(DPAPFP)₃(DDXPO) (6)

Solid-state photophysical properties of Eu³⁺complexes 1-6

The solid-state normalized excitation spectra of the Eu³⁺binary

complexes (1-6) are about three times higher compared to

(

1-

3) and ternary complexes (

4-6) recorded at 298 K are

that of the free ligand, in line with the formation of 3:1 (ligand

: metal). These features point to the ligand being an adequate

light-harvesting chromophore for the sensitization of Eu³⁺

displayed in Figs. 3 and 4, respectively. The excitation profiles

5

were recorded by monitoring the intense D₀

transition of the Eu³⁺ion. The excitation spectra of the

compounds ( ) exhibit a broad band between 250 - 500 nm,

which is attributable to the

π^*transition of the coordinated

→

⁷F₂(612 nm)

a-g,8

luminescence.⁵

1-6

π

-

ligands. The absence of any absorption bands due to the f-f

transitions of the Eu³⁺ion proves that luminescence

sensitization via excitation of the ligand is effective. The

replacement of hydrogen ions in aminophenyl based β-

diketonate ligand with a highly conjugated phenyl moieties

significantly influences the

molecules of the Eu³⁺

and

window to the visible region ( _ex,max= 400 nm), with important

π

-conjugation in the complex

(3

6) and shifts the excitation

λ

applications in biomedical analysis and lighting devices.^1-4The

introduction of N-phenyl substituent in aminophenyl based β-

diketonate ligand results in considerable bathochromic shifts

in the excitation wavelength (∼30 nm in 3 and 6), suggesting

the substantial interactions between the N-phenyl and

aminophenyl β-diketonate moieties. Indeed, it has been

suggested for triphenylamine and its derivatives, there is a

strong conjugation between the nitrogen lone pair electrons

Fig. 1 UV-visible absorption spectra of the ligands HAPFP, HDMAPFP, HDPAPFP and

DDXPO in THF (c = 5 × 10^-6M).

and the phenyl

π

-electrons, and that the whole molecule is a

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Fig. 3 Normalized excitation spectra of Eu³⁺binary complexes 1-3 in solid state.

Fig. 5 298 K emission spectra of Eu³⁺complexes 1-6 in solid state.

No ligand-based emission is noted in the region 400-550 nm,

indicating an efficient ligand-to-metal energy transfer process.

The five narrow emission peaks centered at 579, 592, 612,

5

652, and 701 nm, are assigned to D₀

⁷F₃and D₀

→ → →

⁷F₀, D₀⁷F₁, ⁵D₀

⁷F₂, ⁵D₀

→

⁷F₄transitions, respectively. Among

5

the peaks, the intense ⁵D₀

→

⁷F₂peak points to a highly

polarizable chemical environment around the Eu³⁺ion and is

responsible for the red emission.^5,8,21Moreover, the presence

5

of single and sharp peak in the region of the D₀

→

⁷F₀at 579

nm, indicates the existence of single chemical environment

around the Eu³⁺ion of point group symmetry C_s, C_nor C_nv.^5,8,22

It can be noted from the emission spectra that the

luminescence intensity of the ternary complexes

significantly enhanced (7-12 fold) as compared to the

precursor complexes ( ) by the displacement of water

(4-6)

1-3

Fig. 4 Normalized excitation spectra of Eu³⁺ternary complexes 4-6 in solid state.

molecules from the coordination sphere by the rigid chelating

phosphine oxide, DDXPO.

new chromophore with characteristic absorption and

excitation profiles.¹⁰

b

,19

Here the resonance effect plays major

role compared to electron donating effect in shifting the

excitation maximum. On the other hand, the presence of

electron-donating dimethylamino group in Eu³⁺complexes

2

and

region (λ_ex,max= 380 nm) as compared to the parent Eu³⁺

complexes and

_ex,max= 370 nm).^18aThe donating capacity

5 moderately red shifted the excitation window in the UV

1

4 (λ

can be described by Hammett substituent constant, σ_pwhich

represents a situation where the substituent is directly

attached with the reaction centre in an electron demanding

state. The Hammett constants, σ_pfor N(CH₃)₂is -0.83 and for

NH₂σ_p= -0.66, clearly explains the observed behaviour.²⁰

The ambient-temperature emission spectra of Eu³⁺

complexes

maxima (

complexes

1

-

6

excited at their corresponding excitation

_ex= 370 nm for complexes and _ex= 380 nm for

and λ_ex= 400 nm for complexes and

λ

1

4; λ

Fig. 6 ⁵D₀decay profiles for complexes 3 and 6 (solid-state) where emission monitored

2

5

;

3

6)

exhibit characteristic narrow emission bands arising from the around 612 nm. The straight lines are the best fits (r²= 0.999) considering single-

5

→

⁷F_J(J = 0-4) transitions of the Eu³⁺

exponential behavior.

intra-configurational D₀

ion (Fig. 5).

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Table 2 The radiative (A_RAD) and non-radiative (A_NR) decay rates, 5D₀lifetime (

τ

_obs), intrinsic quantum Yield (

Φ

_Ln, %), energy transfer efficiency (

Φ

_sens, %), and overall quantum yield

(Φ

_overall, %) for complexes 1-6 in the solid state

A_RAD

(s^-1)

A_NR

(s^-1)

τ_obs

(ꢀs)

Φ_Ln

(%)

29

10

19

63

59

63

Φ_sens

(%)

41

Φ_overall

(%)

Compounds

Eu(APFP)₃(H₂O)₂

(1)

(2)

(3)

(4)

1082

963

2649

8667

6416

481

275 ± 2

105 ± 3

129 ± 3

800 ± 3

498 ± 2

551 ± 1

12 ± 1

1 ± 0.1

3 ± 0.3

60 ± 6

25 ± 2

40 ± 4

Eu(DMAPFP)₃(H₂O)₂

Eu(DPAPFP)₃(H₂O)₂

Eu(APFP)₃(DDXPO)

13

1505

786

17

95

Eu(DMAPFP)₃(DDXPO) (5)

Eu(DPAPFP)₃(DDXPO) (6)

1189

1152

826

42

676

63

The luminescence decay times (τ_obs

diketonate complexes

were measured at room absorption intensity.⁴

)

for Eu³⁺-β- experimentally up on direct f-f excitation because of very low

c,e,21

Therefore, the radiative lifetimes of

(

1

-

6

)

temperature using an excitation wavelength that maximizes Eu³⁺(⁵D₀) have been calculated from eqn 2, where

n

the Eu³⁺emission intensity and monitored by the most intense represents the refractive index (1.5) of the medium. A_MD,0is

emission line at 612 nm. The lifetime profiles for all the the spontaneous emission probability for the D₀/⁷F₁transition

5

complexes are fitted with single exponentials, thus indicating in vacuo (14.65 s^-1), and I_tot/I_MDsignifies the ratio of the total

the presence of only one emissive Eu³⁺centre. Typical decay integrated intensity of the corrected Eu³⁺emission spectrum

profiles for complexes

1

-

6

are displayed in Figs. 6, S8 and S9,

†

to the integrated intensity of the magnetic dipole ⁵D₀/⁷F₁

respectively. The corresponding lifetime values are transition:²⁵

summarized in Table 2. The shorter ⁵D₀lifetimes noted for Eu³⁺

1/τ_rad= A_MD,0× n³× (I_tot/I_MD

)

complexes

decay channels associated with vibronic coupling on account

of the presence of water molecules in the coordination The intrinsic quantum yield for the designed Eu³⁺-β-diketonate

spheres of these complexes.^5,8,23These values are essentially complexes (

) have been estimated from the ratio τ_obs/τ_rad

temperature dependent, with τ_obs(at 77 K τ_obs= 362, 422 and and the pertinent values are listed in Table 2. The overall

339 s in respectively) approximately 2 to 4 fold quantum yields ( _overall), radiative (A_RAD) and non-radiative

enhancements in the case of complexes , while going from (A_NR) decay rates and energy transfer efficiencies ( _overall) are

298 to 77 K, thereby reflecting the presence of thermally also presented in Table 2. The substitution of water molecules

activated deactivation processes. This effect has been well in [Eu(DPAPFP)₃(H₂O)₂] by the chelating ligand (DDXPO) leads

documented for several other hydrated Eu³⁺-β-diketonate to 4-fold enhancement in the D₀lifetime (129 to 551

complexes.^8,24On the other hand, the lifetime values of 13-fold enhancement in the solid state quantum yield (3 to 40

1-3

may be due to the dominant non-radiative

(2)

1-6

ꢀ

1-

3,

Φ

1-3

Φ

3

5

ꢀ

s) and

complexes 4-6 have been significantly enhanced (3-5 times) as %). Similarly, in the case of 1-(4 (dimethylamino)phenyl)-

compared to their corresponding binary complexes. This may 4,4,5,5,5-pentafluoro-3-hydroxypent-2-en-1-one based Eu³⁺

be due to the dramatic decrease of non-radiative decay rates complexes, the displacement of water molecules with DDXPO

as compared to their precursor complexes. However, the in [Eu(DMAPFP)₃(H₂O)₂]

lifetime values of the ternary complexes ( ) are almost lifetime (105 to 498 s) and overall quantum yields (1 to 25 %).

s at 298 K; 578 s in On the other hand, a 3-fold enhancement in the D₀lifetime

(275 to 800 s) and 5-fold enhancement in the overall

In order to quantify the ability of the ligands designed to quantum yield (12 to 60 %) has been noted in Eu³⁺complex (

1-(4-aminophenyl)-4,4,5,5,5-pentafluoro-3-

2

significantly enhances the ⁵D₀

4

-6

ꢀ

5

independent of temperature (τ_obs= 551

ꢀ

77 K).

ꢀ

4

)

sensitize the luminescence of Eu³⁺, and to draw conclusions containing

about the relationship between the structure and the hydroxypent-2-en-1-one β-diketonate in the presence of

photophysical properties, it was appropriate to analyze the DDXPO.

emission in terms of eqn 1, where Φ_overalland Φ_Lnrepresent

the ligand-sensitized and intrinsic luminescence quantum Energy transfer between ligands and Eu³⁺

yields of Eu³⁺; Φ_sensrepresents the efficiency of the ligand-to-

metal energy transfer and τ_obs/τ_radare the observed and the To demonstrate the energy transfer process of the derived

b-e,2

radiative lifetimes of Eu³⁺(⁵D₀):⁴

Eu³⁺complexes, the energy levels of relevant electronic states

of the newly synthesized ligands have been calculated. The

singlet (S₁) energy levels of the designed β-diketonate and the

chelating phosphine oxide ligands were estimated by referring

to the upper wavelengths of the UV-vis absorption edges of

the corresponding Gd³⁺complexes (Fig. 7).

(1)

Φ_overall= Φ_sens× Φ_Ln= Φ_sens× (ꢀ_obs/ꢀ_rad

)

The intrinsic quantum yields of Eu³⁺could not be determined

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processes are efficient for these ligands. Based on the above

observations the photoluminescence mechanism for the

derived Eu³⁺complexes is proposed to involve a ligand

sensitized luminescence process (antenna effect).^5,8,28The

typical energy level diagram for complex

6 and the plausible

energy transfer pathways are shown in Fig. 8.

Fig. 7 UV-vis absorption spectra at 298 K (left), and 77 K phosphorescence spectra

(right) of the complexes 7-9 in THF (c = 5 × 10^-6M).

The pertinent S₁levels of HAPFP, HDMAPFP, HDPAPFP and

DDXPO are found to be 25000 (400 nm), 23981 (416 nm),

23148 (432 nm) and 31850 cm^-1(313 nm), respectively. The

triplet (T₁) energy levels were calculated from the position of

the highest energy band in the corresponding

phosphorescence spectra of the Gd³⁺complexes (Fig. 7).²⁶

Accordingly, the T₁of the ligands HAPFP, HDMAPFP, HDPAPFP

and DDXPO were found to be 21505 (465 nm), 20833 (480

nm), 20202 (495 nm) and 23470 cm^-1(426 nm), respectively.

All these values are entirely consistent with the above

discussions that the replacement of hydrogen atoms with the

Fig. 8 Schematic energy level diagram and energy transfer processes for the complex 6.

S₁represents the first excited singlet state and T₁represents the first excited triplet

state.

The photostability of the Eu(DPAPFP)₃DDXPO complex

The photostability of the visible light sensitized Eu³⁺complex

was investigated by means of measuring the luminescence

intensity at 612 nm excited at 400 nm at definite time intervals

(10 min) for 3 h and the results are shown in Fig. S10.† The

results demonstrated that the luminescence intensity of the

complex is almost the same even after 3 h of irradiation. This

indicates the photostability of the Eu³⁺complex.

6

electron donating methyl or π-conjugated phenyl groups in

aminophenyl based β-diketonate ligands have profound

effects on the singlet and triplet states of these ligands. The

energy gaps between the Eu³⁺core (⁵D₀

∼

17250 cm^-1) and the

donor ligand’s T₁levels turns out to be 4255, 3583, 2952 and

6220 cm^-1for HAPFP, HDMAPFP, HDPAPFP and DDXPO,

respectively. The triplet energy levels of the ligands appears at

appreciably higher energy than that of the ⁵D₀state of Eu³⁺,

thus indicating that the newly developed β-diketonate ligands

can act as an efficient sensitizers for the Eu³⁺ion.²⁶On the

other hand, the ⁵D₁state Eu³⁺(18800 cm^-1) is found to be

Synthesis, characterization and luminescence studies of hybrid

materials EuC-Gel and EuC-PMMA-Gel

The photographs of the designed hybrid gel materials are

displayed in Fig. 9. The

FT-IR

spectra

of

the

Eu(DPAPFP)₃(DDXPO) doped hybrid materials EuC-Gel and EuC-

closer to the T₁of HDPAPFP (ꢁ

E (T₁-⁵D₁) = 1402 cm^-1), which

can lead to thermally assisted back energy transfer.²⁷However,

the T₁level of HDMAPFP and HAPFP are situated slightly above

5

the ⁵D₁state of Eu³⁺. The D₂of Eu³⁺emitting state (21200 cm^-

¹) is higher than the T₁states of the β-diketonate ligands,

which can lead to the thermally assisted back-energy transfer

from the central core.²⁷It is also noticed that the energy gap

between the S₁and T₁levels are 3495, 3148, 2946 and 8380

cm^-1for HAPFP, HDMAPFP, HDPAPFP and DDXPO, respectively.

Also in diphenylamino substituted ligand, the singlet-triplet

energy gap is lowered compared to the other ligands as

explained before. According to Reinhoudt’s empirical rule,⁶the

Fig. 9 Photographs of the EuC-PMMA-Gel (left) and EuC-Gel (right) before (a)

and after (b) UV irradiation.

intersystem crossing process becomes effective when ꢁE (S₁-

T₁) is around 5000 cm^-1and hence the intersystem crossing

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PMMA-Gel are shown in Fig. S11.† The formation of Si-O-Si the major electronic transition (from 439 to 447 nm) occurs in

framework in the hybrid materials were confirmed by the hybrid materials, which indicates that the electronic

peaks observed around 1077 cm^-1_as, Si-O-Si), 787 cm^-1

_s, Si- distributions of the system have changed when the complexes

O-Si) and 454 cm^-1

, Si-O-Si) and in both the hybrid materials are embedded in the matrixes and the perturbation is induced

(

ν

(ν

(

δ

d,f

which are characteristic of the trialkoxysilyl function.^{13 ,29}The by the silanol groups in the hybrid materials.

absorption band noted at 1622 cm^-1for the both the gels can

The excitation and emission spectra of the isolated

be assigned to the vibration C=O group of the ligand. A new hybrid materials as solids at room temperature are illustrated

band at 1736 cm^-1noted in the IR spectra of EuC-PMMA-Gel, in Fig. 11. The excitation spectra of the hybrid materials, which

ascribed to the C=O vibrations of PMMA. The above results were obtained by monitoring at 612 nm, exhibit a broad

suggest the incorporation of the polymer into hybrid material, excitation band between 300 and 515 nm. This band can be

which may bring small changes via coordinating with the assigned to

π-π* electronic transition of the ligands. In the

central Eu³⁺ions.

The XRD patterns from 10 to 70

are shown in Fig. S12,

materials are amorphous. The broad peaks noted in both the efficient. The hybrid materials showed characteristic narrow

hybrid materials centered at 22.98 and 23.04

, can be band emissions of Eu³⁺corresponding to the ⁵D₀⁷F_J(J = 0-4)

attributed to the siliceous backbone of the hybrids.¹³

emission spectra of the hybrid materials, only characteristic

°

of the hybrid materials emissions of Eu³⁺ions are noted, which indicates that the

†

exhibiting that the developed hybrid energy transfer from the ligands to the central Eu³⁺ions is

°

→

a

,29e,30

transitions.

Further, the absence of any crystalline regions in these hybrid

materials correlates the presence of host inorganic networks.

The above results clearly indicate that the hybrid materials still

hold disordered sequences even after the doping into PMMA

matrix, although the polymeric carbon chains of the polymers

are essentially regularly ordered. Homogeneous, spherical and

interconnected silica particles (∼ 500 nm) can be evidenced

from SEM investigations of EuC-Gel particles (Fig. 10). On the

other hand, the pronounced interconnected sphere like

structures in EuC-PMMA-Gel is hard to visualize, in the

presence of PMMA.³¹

Fig. 11 Room temperature excitation and emission spectra of hybrid materials, EuC-Gel

and EuC-PMMA-Gel (Inset photographs show the gels coated on a glass plate).

The five expected peaks of the luminescent spectra are well

resolved. The content of the Eu³⁺complex in hybrid materials

and its relative luminescence intensities are listed in Table 3. It

is interesting to note that the complex of a unit mass in the

silica/polymer matrix is found to be superior to that of

Fig. 10 SEM images of the hybrid materials (a) EuC-Gel and (b) EuC-PMMA-Gel.

The thermal stabilities of EuC-Gel and EuC-PMMA-Gel

developed hybrid materials were studied by TG and DTA

measurements and the results are given in Fig. S13.† The DTA

curve show the thermal stability of the Eu³⁺ternary complex

corresponding pure complex. The shortening of lifetimes (τ_obs

305 s in EuC-Gel and 361 s EuC-PMMA-Gel) in the hybrid

materials has been noticed as compared to precursor Eu³⁺

complex (τ_obs= 551 s) (Fig. 12). This might be due to the

=

ꢀ

[353

enhanced after incorporating into the hybrid material [414

in EuC-Gel decomposition]. It can be noted from the TG curve

of EuC-PMMA-Gel that PMMA began to decompose at 398 C.

Further, when the temperature reached 445 C, the polymer

°C for Eu(DPAPFP)₃(DDXPO) decomposition] has been

quenching of abundant O-H oscillators on the silica matrix

surface to absorbed complex, which can be clearly seen from

the high values of nonradiative decay rates of the hybrid

materials (A_NR).^29,32The overall quantum efficiencies of these

hybrid materials have been significantly decreased due to the

presence of high frequency oscillators in the hybrid materials.

In addition, in the present work, EuC-PMMA-Gel hybrid

material has been used as a phosphor to fabricate LED after

coating onto a 400 nm emitting InGaN chip and investigated

photoluminescence properties (Fig. 13). The LED fabricated

°

C

°

e

PMMA had departed from the hybrid material.²⁹The results

revealed that thermal stability of the hybrid material has

enhanced after loading into the hybrid material.

The solid-state absorption spectra of the complex

(Eu(DPAPFP)₃DDXPO) and the hybrid materials are displayed in

Fig. S14†. Compared to the Eu³⁺ternary complex, a red-shift of

10 | J. Name., 2012, 00, 1-3

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with a 400 nm emitting chip exhibited strong red-emission.

Thus, the results indicate that EuC-PMMA-Gel hybrid material

is an interesting red-emitting material excited by blue-light,

making it a potential candidate in many photonic applications

Conclusions

In conclusion, a series of new antenna complexes of Eu³⁺based

on aminophenyl based polyfluorinated β-diketonate ligands in

the absence and presence of a chelating phosphine oxide has

been developed. Most importantly, among the designed

complexes, Eu³⁺-triphenylamine β-diketonate ternary complex

b

without using UV radiation for excitation.⁷

exhibits intense red-emission under blue light excitation (λ_ex

400 nm) with an impressive quantum yield ( _overall= 40 %) and

⁵D₀lifetime values (τ_obs= 551

s). Suitably expanded π-

=

Φ

ꢀ

conjugation in the complex molecules successfully red-shifted

the excitation band of the Eu³⁺-β-diketonate complexes to the

visible region. Contrary to the above, the electron-donating

dimethylamino group (Hammett constant: σ_p

= -0.83)

containing Eu³⁺-β-diketonate complexes moderately shifted

the excitation maximum in the UV region from 370 to 380 nm

as compared to unsubstituted aminophenyl (Hammett

constant: σ_p= -0.66) Eu³⁺complexes. In order to improve the

thermal and mechanical stability, the derived highly

luminescent Eu³⁺complex has been embedded into a hybrid

silica/polymer material by sol-gel method. The resultant hybrid

material EuC-PMMA-Gel displays efficient unit mass

Fig. 12 ⁵D₀decay profiles for EuC-Gel and EuC-PMMA-Gel (solid-state) where emission

monitored around 612 nm. The straight lines are the best fits (r²= 0.999) considering

single-exponential behavior.

luminescence emission (3.41

stability (398

C) as compared to precursor ternary Eu³⁺

complex [unit mass luminescence emission = 8.02

10⁸

thermal stability = 353 C]. In addition, the InGaN LED

×

10⁹) and greater thermal

°

×

;

Table 3 Luminescence intensities, content of the Eu³⁺complex, radiative-nonradiative

decay rates and the photoluminescence quantum yields of Eu(DPAPFP)₃DDXPO, EuC-

PMMA-Gel and EuC-Gel samples

°

developed by coating the EuC-PMMA-Gel hybrid material

displays bright red-emission under blue light excitation. The

present results clearly demonstrate that Eu³⁺-triphenylamine

based β-diketonate complexes may find potential applications

in many optoelectronic technologies.

Compounds

Eu(DPAPFP)₃DDXPO EuC-Gel

EuC-PMMA-

Gel

Content of the

100

23.8

18.5

complex 6 (wt %)

Acknowledgements

Relative intensity

8.02 × 10⁸

4.37 × 10⁸

1.84 × 10⁹

6.31 × 10⁸

3.41 × 10⁹

of ⁵D₀→⁷F₂

One of the authors T. V. Usha Gangan also acknowledges UGC,

New Delhi for the award of Senior Research Fellowship and

AcSIR for Ph.D registration.

Unit mass

luminescence

intensity

A_RAD(s^-1)

A_NR(s^-1)

1152

676

1031

2294

305 ± 2

31

953

1849

361 ± 2

34

τ_obs(ꢀs)

551 ± 1

63

Φ

_Ln(%)

_sens(%)

_overall(%)

63

56

63

40 ± 4

18 ± 2

22 ± 2

Fig. 13 Emission spectra of the blue LED (left) and EuC-PMMA-Gel coated blue LED (right). Inset: photographs of the LEDs in working state.

J. Name., 2013, 00, 1-3 | 11

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Tuning of the excitation wavelength in Eu3+-aminophenyl based polyfluorinated β-diketonate complexes: a red-emitting Eu3+-complex encapsulated in a silica/polymer hybrid material excited by blue light

DOI: 10.1039/c5dt02371h

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