In situ synthesis and luminescence characteristics of complexes of europium(III) with 4,6-diacetylresorcinol

Article abstract of DOI:10.1016/j.materresbull.2007.07.040

The complexes of europium(III) with 4,6-diacetylresorcinol (H₂DAR) and a co-ligand (phen, bpy or 2,2′-bipyridine N,N′-dioxide (2,2′-bpyO₂)) were in situ synthesized in silica matrix via a two-step gel process. The formation of complexes in silica gel was confirmed by the luminescence excitation spectra. The silica gels that contain in situ synthesized europium complexes exhibit the characteristic emission bands of the Eu(III). The results show that there are two ways to enhance the emission intensity of the Eu(III): (i) synthesize the complex in silica matrix and (ii) synthesize the complex with a co-ligand, which coordinates with Eu(III) in the composite system and can efficiently transfer the energy from 4,6-diacetylresorcinol to the Eu(III). The order of the luminescence intensities of the complexes is: Eu₂(DAR)₃(phen)₂-(sol-gel) > Eu₂(DAR)₃(2,2′-bpyO₂)₂-(sol-gel) > Eu₂(DAR)₃ (bpy)₂-(sol-gel) > Eu₂(DAR)₃-(sol-gel) > pure Eu₂(DAR)₃·4H₂O.

Full text of DOI:10.1016/j.materresbull.2007.07.040

Materials Research Bulletin 43 (2008) 2397–2402
www.elsevier.com/locate/matresbu
In situ synthesis and luminescence characteristics of complexes of
europium(III) with 4,6-diacetylresorcinol
a
a
a
a
Sheng-Li Liu a,b, , Chun-Lin Wen , Chun Chen , Su-Su Qi , En-Xiang Liang
*
a School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
b Hunan Provincial Key Laboratory of QSAR/QSPR, Hunan University of Science and Technology, Xiangtan 411201, China
Received 2 June 2007; received in revised form 15 July 2007; accepted 26 July 2007
Available online 2 August 2007
Abstract
The complexes of europium(III) with 4,6-diacetylresorcinol (H2DAR) and a co-ligand (phen, bpy or 2,20-bipyridine N,N0-
dioxide (2,20-bpyO2)) were in situ synthesized in silica matrix via a two-step gel process. The formation of complexes in silica gel
was confirmed by the luminescence excitation spectra. The silica gels that contain in situ synthesized europium complexes exhibit
the characteristic emission bands of the Eu(III). The results show that there are two ways to enhance the emission intensity of the
Eu(III): (i) synthesize the complex in silica matrix and (ii) synthesize the complex with a co-ligand, which coordinates with Eu(III)
in the composite system and can efficiently transfer the energy from 4,6-diacetylresorcinol to the Eu(III). The order of the
luminescence intensities of the complexes is: Eu2(DAR)3(phen)2-(sol–gel) > Eu2(DAR)3(2,20-bpyO2)2-(sol–gel) > Eu2(DAR)3
(bpy)2-(sol–gel) > Eu2(DAR)3-(sol–gel) > pure Eu2(DAR)3ꢀ4H2O.
# 2007 Published by Elsevier Ltd.
Keywords: A. Optical materials; B. Sol–gel chemistry; D. Luminescence
1. Introduction
Materials containing lanthanide ions have been used as phosphors and laser materials because of their sharp,
intensely luminescent f–f electronic transitions. In particular, a number of lanthanide complexes display a bright and
narrow lanthanide ion emission.
In such a process, the quantities that contribute to the luminescence intensity are as follows: (i) the intensity of the
ligand absorption, (ii) the efficiency of the ligand-to-metal energy transfer, and (iii) the efficiency of the metal
luminescence [1].
There are several ways to increase the intensity of the lanthanide luminescence exploiting the antenna effect: (i)
Find a suitable ligand that can transfer the energy to the metal ion efficiently. (ii) Synthesize the complex with a
co-ligands, e.g., phen, bpy, or 2,20-bipyridine N,N0-dioxide, which shows intense absorption band in the UV region due
to p–p* transition. In order to eliminate O–H oscillators in the short range and reduce the nonradiative decay from
the excited state of Ln(III), the coordination sphere should be supplemented with a co-ligand such as phen, bpy, or
* Corresponding author at: School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201,
China.
E-mail address: slliu@hnust.cn (S.-L. Liu).
0025-5408/$ – see front matter # 2007 Published by Elsevier Ltd.
doi:10.1016/j.materresbull.2007.07.040

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S.-L. Liu et al. / Materials Research Bulletin 43 (2008) 2397–2402
Fig. 1. Schematic presentation of H2DAR, phen, bpy, and 2,20-bpyO2.
2,20-bipyridine N,N0-dioxide. Quenching water molecules in the coordination sphere of lanthanide ion can be removed
and therefore the luminescence intensity of the complex be enhanced. Moreover, the co-ligand coordinating with
Ln(III) in the composite system might play an important role in the process of energy absorption and efficient transfer
to the chelated Ln(III). And (iii) synthesize the complex by in situ technique via silica gel process, this process being a
potentially attractive means of synthesizing novel luminescent materials. The incorporation of lanthanide complexes,
particularly europium(III) complexes [2–5], has been investigated in detail. Xerogels doped with europium(III)
complexes have been shown to exhibit substantially improved luminescence characteristics with respect to
comparable materials containing the simple metal.
In this paper, the complexes of europium(III) with 4,6-diacetylresorcinol (H2DAR), whose low triplet state energies
are suitable for the luminescence of Eu(III), and phen, bpy or 2,20-bipyridine N,N0-dioxide (2,20-bpyO2), whose
structures were shown in Fig. 1, were synthesized and incorporated in silica matrix by in situ technique via a gel
process, and the luminescence properties of the complexes in silica matrix were also discussed in detail.
2. Experimental
2.1. Synthesis
Tetraethyloxysilane (TEOS, AR, Kermel), Eu2O3 (Eu2O3/TREO = 99.99, Rknerc), phen (AR, Kermel), bpy (AR,
Kermel); H2DAR and 2,20-bpyO2 were synthesized after Refs. [6,7], the others reagents used were of analytical
grade.
Europium(III) chloride solution was prepared as follows: 0.5 mmol of europium(III) oxide was added into 4 ml of
concentrated hydrochloric acid (36%) in a beaker and digested on a steam bath until it was completely dissolved.
Surplus HCl was removed by evaporation and the resulting europium(III) chloride was dissolved in 10 ml anhydrous
alcohol.
2.1.1. Synthesis of Eu2DAR3ꢀ4H2O
Eu2DAR3ꢀ4H2O was prepared as follows: 3 mmol of H2DAR was dissolved in 95% ethanol and a proper NaOH
solution with ethanol (0.5 mol/l) was added to the above solution. EuCl3 solution (2 mmol) was added dropwise to the
solution of H2DAR with stirring at 50–60 8C. The deposit appeared obviously in the solution. After that, the reactant
solution was stirred for 4 h at the same temperature, then filtered and washed with 80% ethanol solution
(V
:V(EtOH) = 1:4) until no Clꢁ was found in the washed solution. The product was vacuum dried for 6 h at 80 8C,
ðH OÞ
2
and the complex was obtained. Anal. Calcd. for Eu2(DAR)3ꢀ4H2O: C, 37.83; H, 3.39; Eu, 31.91; found: C, 37.80; H,
3.19; Eu, 31.86.
2.1.2. Synthesis of the complexes in silica matrix
The solution used to prepare the silica xerogel consists of 1 mol of TEOS, 4 mol of ethanol and 4 mol of distilled
water. A small amount of HCl is added to the solution to promote hydrolysis. H2DAR and phen with a proper
proportion of EuCl3 (nEu: nH DAR: nphen = 1:1.5:1) were added. pH was controlled within the acid region (about 2–3).
2
The mixture was stirred at room temperature for several hours in order to ensure the full hydrolysis of TEOS.
Finally, a proper amount of hexamethylenetetramine ((CH2)6N4) was added to accelerate the polycondensation
reaction and the mixture was stirred for another 10 min. The resulting transparent solution was poured into a plastic

S.-L. Liu et al. / Materials Research Bulletin 43 (2008) 2397–2402
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box and sealed, and then placed in a drying oven at a temperature of 40 8C. Several hours later, transparent monolithic
wet gel was obtained and aged for 1 day and then dried at 60 8C for 2 days followed by perforating the cap of the plastic
box [4]. The Eu2(DAR)3phen2 gel (the nominal doped concentration is 0.14 mol%) was stored in a desiccator for
further measurements. Other gels (Eu2DAR3, Eu2DAR3(bpy)2 and Eu2DAR3(2,20-bpyO2)2) were prepared by a
method similar to the above process.
2.2. Spectroscopy measurements
The fluorescence excitation and emission spectra were recorded at room temperature on a Hitach F4500
Spectrofluorometer; IR spectra were obtained in 4000–400 cmꢁ1 region using a Perkin-Elmer Spectrum One
Spectrophotometer with KBr pellet technique. Fluorescence lifetimes were measured using an EMG 201 MSC quasi-
5
7
molecular 308 nm laser by monitoring the D0 ! F2 emission line of Eu(III) at room temperature.
3. Results and discussion
(CH2)6N4 was used as a generator of hydroxyl groups to adjust the pH value for the formation of gelation and the
complexes. Hydroxyl groups are gradually released through the reaction between water and (CH2)6N4 with an increase
of temperature [8]. That can provide a suitable environment for the formation of europium complexes.
3.1. FTIR analysis
Table 1 gives the main bands and their assignments of the IR spectra of pure Eu2DAR3ꢀ4H2O, pure silica gel, and
silica gel containing Eu2DAR3. For the pure complex, the broad band in the region 3500–3100 cmꢁ1 is due to the
coordination with water. The rocking mode of the coordinated water was observed around 800 cmꢁ1. The bands at
1312 and 1323 cmꢁ1 are assigned to the phenolic C–O stretching mode of the ligand. Compared with the ligand, these
shifts to higher frequencies in the spectrum of the chelated complex are due to its coordination to europium(III)
through the oxygen atom of the phenolic group and it appeared as a singlet or a doublet [9]. Furthermore, the
absorption in the region 1182 cmꢁ1 due to O–H was not observed in the infrared spectrum of the complex, suggesting
that the deprotonation of the ligand occurs before their coordination to europium metal. Likewise, carbonyl stretching
frequency in the ligand appearing at 1659 cmꢁ1 has a small decrease by 39 cmꢁ1 in the complex. This indicates that
the carbonyl oxygen is involved in the coordination with the metal ion. A characteristic feature of the IR spectrum of
the complex is the appearance of a prominent band in the region 482 cmꢁ1. From a comparison with the ligand
spectrum, we have tentatively attributed this prominent band to the M–O vibrational mode. Such bands have been
observed in acetylacetonate complexes [10] and other o-hydroxyarylcarbonyl compounds [11]. Based on these
observations the ligand is chelated to the europium(III) via the acetyl and phenolic oxygens, H2DAR is concluded to be
a bis-bidentate donor, a similar structure that coordinated with metal ions had been confirmed by Shyamala and
Jayatyagaraju [12] and Takano et al. [13].
From these data, it can be seen that the silica gel containing Eu2DAR3 exhibits almost the same absorption peaks as
the pure silica gel, and the characteristic absorption peaks belonging to Eu2DAR3 are not observed. It may be
considered that the dopant concentration of the luminescent complex in the silica gel is too low to be detected. On the
other hand, the results probably imply that the vibrations of the ligand of Eu(III) have been restricted to some extent by
the surrounding gel matrix. This further indicates that the molecular vibration of organic ligand is limited by the rigid
structure of silica gel matrix [14].
Table 1
Infrared spectral bands in cmꢁ1
Compound
Main absorption bands and their assignments (cmꢁ1
2850, 2920 (nO–H), 1659 (nC O), 1312, 1323 (nC–O
)
H2DAR
)
Pure Eu2(DAR)3ꢀ4H2O
1620 (nC O), 1325 (nC–O), 482 (dM–O)
3424 (nH–O–H), 1656 (nO–H), 1082 (nSi–O–Si), 960 (nSi–OH), 800 (nO–Si–O), 462 (dO–Si–O
3436 (nH–O–H), 1641 (nO–H), 1079 (nSi–O–Si), 956 (nSi–OH), 797 (nO–Si–O), 462 (dO–Si–O
Pure silica gel
)
)
Eu2(DAR)3-(sol–gel)

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S.-L. Liu et al. / Materials Research Bulletin 43 (2008) 2397–2402
Fig. 2. The excitation spectra of (a) pure Eu2DAR3ꢀ4H2O and (b) Eu2DAR3 in silica gel.
3.2. Luminescence properties
Gd(III) complex was selected as a model complex for the determination of the triplet state energies of the organic
ligand owing to their high phosphorescence–fluorescence ratio compared to those of other Ln(III) complexes and
Gd(III) can sensitize the phosphorescence emission of ligands. The phosphorescence spectrum of the gadolinium
complex in ethanol with H2DAR was measured, and the triplet state of H2DAR can be determined to be 21,739 cmꢁ1
based on the maximum phosphorescence bands at 460 nm. The energy differences between the triplet state of H2DAR
and the resonance energy level of Eu(III) (5D0, 17,265 cmꢁ1) can be calculated to be 4474 cmꢁ1. They are all indeed
5
sufficient to efficiently populate the D0 luminescent state of Eu(III).
Fig. 2 shows the excitation spectra of (a) pure Eu2DAR3ꢀ4H2O and (b) Eu2DAR3 in silica gel. Either of the
excitation spectra consists of a very broad band instead of a characteristic narrow band of Eu(III) (395 nm), which
indicates that the europium complex has been synthesized by an in situ technique via silica gel process. Compared to
that of pure complex, the maximum excitation wavelength of the complex in silica gel shows a small blue-shift
changing from 400 to 353 nm, which implies that different compositions may exist in the two samples. For pure
Eu2DAR3, the surrounding environment of the Eu(III) is homogeneous, so the excitation spectrum consists of a
symmetric and broad band ranging from 250 to 450 nm. However, silica gel is a non-crystalline substance with a
porous microstructure, so the excitation spectrum becomes an asymmetric band, and the decrease of local symmetry
for Eu(III) also should be proved from the increase of the ratio of the emission intensity of the peak at 610 nm (electric
5
7
5
7
dipole D0 ! F2 transition) and 589 nm (magnetic dipole D0 ! F1 transition).
The emission spectra of the complexes consist of four main peaks at 578 nm (5D0 ! F0), 589 nm (5D0 ! F1),
7
7
610 nm (5D0 ! F2) and 650 nm (5D0 ! F3), and the emission at 610 nm is the strongest. From Fig. 3(1) we can see
that the emission intensity of Eu2DAR3 in silica gel (the nominal doped concentration is 0.14 mol%) is four times
greater than that of pure Eu2DAR3ꢀ4H2O. Synthesis of the complexes by an in situ technique via silica gel process can
replace some or all of the H2O molecules or OH groups bound to the europium ion, and the emission intensity is thus
increased by the more efficient energy transfer from the organic system and the reduction of the OH vibration that can
quench the luminescence of Eu(III).
7
7
The lifetimes of 5D0 excited state for the complexes were measured by time-resolved spectroscopy, the
lifetime valves were calculated through the mono-exponential decay method [15], and the results are given in
Table 2.
The emission quantum yield q for the Eu2(DAR)3(phen)2-(sol–gel), Eu2(DAR)3(2,20-bpyO2)2-(sol–gel),
Eu2(DAR)3(bpy)2-(sol–gel) and Eu2(DAR)3-(sol–gel) were obtained following the procedure described in Ref.
[16]. The q value is defined as the ratio between the number of photons emitted by the Eu(III) and the number of
photons absorbed by the ligands. According to the method developed by Bril [17] at Philips Research Laboratories, the
q value for a given sample can be determined by comparison with standard phosphors, whose quantum yields have

S.-L. Liu et al. / Materials Research Bulletin 43 (2008) 2397–2402
2401
Fig. 3. (1) The emission spectra of pure Eu2DAR3ꢀ4H2O and Eu2DAR3 in silica gel; (2) the emission spectra of the complexes in silica gel.
been previously determined by absolute measurements. This method provides absolute yields while avoiding absolute
measurements, which are in general complicated. The quantum yield qx of a sample is thus determined as follows:
ꢀ
ꢁꢀ
ꢁ
1 ꢁ rST
1 ꢁ rx
Dfx
qx ¼
qST
DfST
Table 2
The luminescence properties of the pure complex and the complexes in silica gel
Compound
lex (nm) lem (nm) Relative emission intensity
Lifetimes (t/ms) q (%)
7
7
7
7
5D0 ! F0 5D0 ! F1 5D0 ! F2 5D0 ! F3
Pure Eu2(DAR)3ꢀ4H2O
Eu2(DAR)3-(sol–gel)
Eu2(DAR)3phen2-(sol–gel)
Eu2(DAR)3(bpy)2-(sol–gel)
Eu2(DAR)3(2,20-bipyO2)2-(sol–gel) 345
400
353
364
352
611
609
610
610
610
1.2
7.6
52.3
11.5
25.6
0.5
4.8
37.1
8.5
15.5
60.6
544.9
106.5
311.8
0.5
1.0
6.3
1.3
4.4
0.269
0.526
1.205
0.635
0.758
4.8
10.2
15.4
10.5
12.1
25.1

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S.-L. Liu et al. / Materials Research Bulletin 43 (2008) 2397–2402
where rST and rx are the amount of exciting radiation reflected by the standard and by the sample, respectively, and qST
is the quantum yield of the standard phosphor. The terms DFx and DFST give the integrated photon flux (photons sꢁ1
)
for the sample and the standard phosphor, respectively. The standard in our case was sodium salicylate (Aldrich),
which has a broad emission band with a maximum at 450 nm and q = 60% at room temperature [17]. But our
experiment shows that the best-excited wavelength of the complexes is not the same as that of sodium salicylate, so the
q is not an absolutely quantum yield, it only can be used for making a comparison between each other.
When phen (bpy or 2,20-bpyO2) was added as a co-ligand, the luminescence intensity of the complex increased, as
can be seen in Fig. 3(2). Compared with bpy or 2,20-bpyO2, phen is a more suitable co-ligand.
The results imply that phen (bpy or 2,20-bpyO2), as a co-ligand, coordinates with Eu(III) in the composite system.
The co-ligand can eliminate O–H oscillators in the short range too, reduce the nonradiative decay from the excited
state of Eu(III), and enhance the luminescence intensity, the lifetime and the emission quantum yield. The order of
the luminescence intensities of the complexes is: Eu2(DAR)3(phen)2-(sol–gel) > Eu2(DAR)3(2,20-bpyO2)2-
(sol–gel) > Eu2(DAR)3(bpy)2-(sol–gel) > Eu2(DAR)3-(sol–gel). The reason may be that the conjugacy of phen is
bigger than others.
The luminescence properties of the complexes are summarized in Table 2.
Acknowledgements
The authors are thankful to the Education Committee of Hunan Province of China (02C465), Hunan Provincial Key
Discipline and Hunan Provincial General College Leader of Learning of China.
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