Synthesis and fluorescent properties of lanthanide complex covalently bonded to porous silica monoliths

Journal of Alloys and Compounds 490 (2010) 264–269

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Synthesis and fluorescent properties of lanthanide complex covalently bonded to porous silica monoliths Yu Chen a , Qi Chen a,b,∗ , Li Song b , Hui-ping Li b , Feng-zhen Hou b a Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China b Department of Inorganic Materials, East China University of Science and Technology, Shanghai 200237, China

a r t i c l e

i n f o

Article history: Received 30 June 2009 Received in revised form 29 September 2009 Accepted 30 September 2009 Available online 6 October 2009 Keywords: Schiff base Meso-structured Luminescence Emission quantum efficiency

a b s t r a c t Schiff base synthesized by APTES (3-aminopropyltriethoxysilane) and salicylaldehyde was used as the ligand for coordination with Eu3+ ions. It was covalently attached with meso-structured monoliths via post-synthetic method. The assembly materials were characterized by infrared spectra, CHN elemental analysis, BET analysis and luminescence measurement. The results showed that assembly material had much weaker emission intensity of the ligand, which indicated that energy transfer from ligand to Eu3+ in the assembly became more efficient than that in the neat complex. Additionally, Eu-assembly sample exhibited much longer lifetime and higher emission quantum efficiency of Eu3+ than those of the neat complex. © 2009 Published by Elsevier B.V.

1. Introduction Lanthanide complexes are of great importance as they have strong absorption ability in the UV region and generate luminescence in the visible region by the “antenna effect” [1], which is defined as the ion-centered luminescence originates from the intramolecular energy transfer through the excited state of the ligand to the emitting level of the lanthanide ions. But poor thermal, chemical and photo stability limited their practical use. It is widely believed that the complexes need to be incorporated into stable rigid matrices to solve these problems. Various rigid matrices like polymer [2,3], sol–gel silica [4,5], organic–inorganic hybrids [6,7], layered double hydroxides [8] and highly ordered mesoporous sieves [9–12] have been used for encapsulation of different kinds of lanthanide complexes. Those lanthanide complex containing luminescent hybrids can be divided into two categories, which is based on the interaction between host and guest molecular [13]. One is named as Class I, which usually describes materials with weak interaction (hydrogen bonding, van der Waals force) between different phases. The other is known as Class II, which is

defined as hybrid materials with strong interaction (usually covalent bonds) between different phases. In recent years, it is found that lanthanide complexes incorporated in rigid matrices via covalent bonds usually exhibited complementary properties [14–18]. Therefore, the purpose of this work is to develop luminescent hybrids by which lanthanide complexes are covalently connected to the matrix instead of only embedding in the host by weak physical interaction. In previous work, we developed meso-structured monoliths as host materials for the incorporation of lanthanide complex [19]. In this paper, Schiff base complex synthesized by APTES (3-aminopropyltriethoxysilane) and salicylaldehyde was used to covalently attach to meso-structured silica monoliths. It was found that the assembly material had stronger emission from Eu3+ and the emission of Schiff base ligand became much weaker. Also, Eu-assembly showed dramatic increase of luminescence lifetime, stimulated emission cross-section (5 D0 → 7 F2 transition) and emission quantum efficiency comparing with those of the neat complex. As Schiff base is able to interact with DNA [20–22], this assembly material has great potentials in biological application. 2. Experimental

∗ Corresponding author at: Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China. Tel.: +86 21 64253466; fax: +86 21 64250882. E-mail address: [email protected] (Q. Chen). 0925-8388/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.jallcom.2009.09.173

2.1. Synthesis of mesoporous monoliths The method to obtain the meso-structured monoliths is same way described in previous work [19].

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Scheme 1. (a) The synthesis process of Schiff base complex. (b) Chemical structure of Eu-Schiff base complex powder.

2.2. Synthesis of lanthanide (Ln = Eu3+ ) Schiff base complex powder The Schiff base ligand was synthesized by adding APTES (3-aminopropyltriethoxysilane) and salicylaldehyde (in the molar ratio of 1:1) in the ethanol solution and stirring for 2 h. EuCl3 ethanol solution was added into the above solution stirring for about 8 h. The yellow precipitates appeared and were obtained by filtration. Upon filtration, the yellow precipitates were washed by ethanol several times, and dried at 50 ◦ C. The synthesis process and chemical structure of Eu-Schiff base complex powder are shown in Scheme 1a and b, respectively. It can be seen that the complex is a kind of oligomer formed by condensation reaction of –Si(OC2 H5 )3 group. The element analysis showed that C% H% N% and Eu% of the obtained compound were 37.12%, 4.68%, 4.09% and 15.53%, respectively. That was consistent with the predicted formula (EuC30 H33 O7.5 N3 Si3 Cl3 ·4H2 O)n . The calculation value of C%, H%, N% and Eu% are 37.13%, 4.22%, 4.33% and 15.67%. TG analysis showed 7.15% weight loss of Eu-complex at range of 50–150 ◦ C, which was also consistent with the weight percentage of crystal water of Eu-complexes (7.43%).

The CHN elemental analysis, carried on VarioEL III analyzer, determined the C%, H%, and N% of the complex. Also, the amount of Eu3+ was performed by complex on metric titration with EDTA. Infrared spectra were recorded on TJ27030A infrared spectrophotometer. Wide angle XRD spectrum was recorded by Rigaku (D/MAX 2550 VB/PC) diffractometer (40 kV/100 mA) over the 2 range of 10–80◦ .

2.3. Synthesis of assembly materials When the yellow precipitates appeared (mentioned above), the minimal amount of DMF was added into the mixture to dissolve the complexes. Then, the mesostructured silica monoliths were immersed into the solution and oscillated at room temperature for 24 h. The amount of silica and Eu3+ used for synthesis of the assembly was in the molar ratio of 0.037:1. Scheme 2 is presented to show the process of attachment. 2.4. Naming of samples The Schiff base complexes powder is named as Eu-C, and the assembly samples are named as Eu-A. 2.5. Characterization Adsorption and desorption isotherms were measured with a Micromeritics surface area and porosimetry system analyzer (ASAP 2010N). Specific surface area was evaluated using BET method. Pore size distribution was calculated in the way of BJH algorithm. Fluorescence measurements at room temperature were recorded by an Edinburgh Instrument (FLS920) spectrofluorimeter with a double grating monochromator, and a 450 W xenon lamp as excitation source.

Scheme 2. The synthesis of the assembly material.

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and 0.39 cm3 g−1 . Decrease in BET surface area and pore volume of assembly sample is due to the presence of organic ligands. 3.2. Textual analysis of Eu-Schiff base complex and the corresponding assembly

Fig. 1. Nitrogen adsorption and desorption isotherms of meso-structured silica and Eu-A.

3. Results and discussion 3.1. BET analysis of meso-structured silica monolith and assembly sample Nitrogen adsorption–desorption isotherms and pore size distribution (PSD) of meso-structured silica monolith and Eu-A are shown in Figs. 1 and 2. It can be seen that both samples have the similar Type IV isotherms with Type H3 loops [23], which indicate that the structure of mesopores did not change after covalently attaching with organic ligands. The variation of PSD implies that the Eu-Schiff base complex was successfully encapsulated in the channels of mesopores by post-synthetic method. The BET surface area and pore volume of meso-structured silica are 436 m2 g−1 and 0.69 cm3 g−1 , and those of Eu-A sample are down to 137 m2 g−1

Fig. 2. Pore size distribution curves of meso-structured silica and Eu-A.

IR spectra of free ligand (a), Eu-C (b), meso-structured silica (c) and assembly Eu-A (d) are shown in Fig. 3a. It can be seen that the vibration of C N group of the free ligand is at 1637 cm−1 , while the C N group vibration of Eu-complexes is at 1655 cm−1 (shown in Fig. 3b). The shifts indicate that Eu3+ ions form coordination bond with C N group. It is also observed that phenol group (shown in Fig. 3c) shifts from 1287 to 1296 cm−1 , which implies that the coordination bond is formed between Eu3+ and phenol group. In the case of assembly sample, the absorption bands at 1030–1250 cm−1 range belong to the asymmetric Si–O stretching vibration modes, whereas the band at 796 cm−1 can be attributed to the symmetric Si–O stretching vibration. The absorption peak at 470 cm−1 is the Si–O–Si bending vibration. The vibration of surface –OH groups (at 953 cm−1 ) becomes very weak and is almost not observed. That indicates that –OH groups are mostly consumed during the attaching process, and the Schiff base is covalently attached to the silica matrix. The presence of absorption bands at 3000–2900 cm−1 is attributed to the stretching vibration band of –CH2 – and –CH–. The strong absorption band at about 3400 cm−1 is the evidence of presence of water molecular. Additionally, wide angle X-ray spectrum of the assembly is presented Fig. 3d. It can be seen that the sample has characteristic peak (around 22.4◦ ) of amorphous silica materials, which is the evidence of formation of silica network. That also indicates the formation of covalently bonded hybrid as no other peak related to organic phase is detected. 3.3. Luminescent measurement 3.3.1. Excitation and emission spectra of Eu-C and Eu-A The excitation spectra of Eu-Schiff base complexes and its corresponding assembly samples are shown in Fig. 4. In the case of Eu-complex, the band at about 374 nm is attributed to the electric transition of the organic ligand, and the strong band ranging from 390 to 470 nm is related to electric transition of ligand enhanced by f–f transition of Eu3+ (7 F0 → 5 L6 , 7 F0 → 5 D3 and 7 F0 → 5 D2 transitions of Eu3+ are at 396, 416 and 466 nm). While the excitation spectrum of assembly sample exhibits blue shift phenomenon comparing with the neat complexes, which indicates that the lanthanide complexes are dispersed in the channels of mesopores [24]. The emission spectrum (shown in Fig. 5a) of Eu-complex shows the characteristic bands assigned to 5 D0 → 7 Fj (j = 0–4) transitions, among which 5 D0 → 7 F2 transition is the dominant band. The band ranging from 460 to 560 nm is attributed to the emission of Schiff base ligand. Interestingly, the spectrum of assembly sample (Eu-A) only exhibits the dominant bands attributing to 5 D0 → 7 Fj (j = 0–4) transitions, as the emission band of ligand is too weak to be observed. It indicates that the energy transfer from ligand to Eu3+ becomes more efficient than that in the corresponding neat complex. 3.3.2. Radiative properties, emission quantum efficiency and Judd–Ofelt analysis Detailed information regarding radiative process is obtained by the following method. Based on the emission spectra and the observed luminescence lifetime of the 5 D0 emitting level, the emission quantum efficiency (0 ) of Eu3+ in the samples can be calculated according to Ref. [25]. The 5 D0 radiative decay rates (ARAD ) for the Eu3+ complexes are calculated by summing over the radiative rates A0J for each 5 D0 → 7 Fj emission transition. The A0j rates (see Eq. (1)) are calculated from the integrated intensity

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Fig. 3. (a): IR spectra of free ligand (a), Eu-C (b), meso-structured silica (c) and assembly Eu-A (d); (b): IR spectra of free ligand, Eu-C at the range of 1800–1400 cm−1 ; (c): IR spectra of free ligand, Eu-C at the range of 1400–1200 cm−1 ; (d): WAXRD spectrum of Eu-A.

ratios between the 5 D0 → 7 Fj and 5 D0 → 7 F1 emission transitions and the value of magnetic-dipole transition rate A01 is estimated to be 50 s−1 [26]. A0j = A01

I0j 01 I01 0j

(1)

The non-radiative rates ATNON are obtained from the calculated rates and the experimental decay rates by the following Eq. (2).

ATRAD

1 = ATTOT = ATRAD + ATNON T where  T stands for the 5 D0 decay time at temperature T.

(2)

The emission quantum efficiency (0 ) of the emitting 5 D0 level is defined and calculated by the below Eq. (3).

0 =

ATRAD ATRAD

+ ATNON

= ATRAD obs

(3)

Therefore, the emission quantum efficiency (0 ) is obtained from the observed luminescence lifetime ( obs ) and the radiative transition rates (Arad ), which are shown in Table 1. Other vital parameters of the radiative properties are the fluorescence branching ratio (ˇ) and the stimulated emission cross-section () of different transitions, which are calculated by the following two

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equations and presented in Table 2. ˇ0j = 0j =

A0j

(4)

ATOT 4P 8 cn2 eff

A0j

(5)

where P is the peak wavelength of the emission band (j = 0–4), and

eff is determined by the following equation.

eff =

Fig. 4. Excitation spectra of Eu-C and Eu-A (monitored under 613 nm). Table 1 Comparison of radiative, non-radiative rates, emission quantum efficiency, decay lifetimes, J-O parameters and parameter R2/1 .

Eu-C Eu-A

Arad (s−1 )

Anon-rad (s−1 )

0 (%)

 (ms)

˝2

˝4

R2/1

457 581

7119 1687

6.35 25.62

0.132 0.441

9.31 13.13

5.40 7.22

5.96 10.72

Arad , Anon-rad , 0 and  stand for radiative rates, non-radiative rates, quantum efficiency and lifetime. Parameter ˝2 and ˝4 are in the units of 10−20 cm2 , R02 stands for the ratio of 5 D0 → 7 F0 /5 D0 → 7 F2 , R2/1 is the ratio of 5 D0 → 7 F2 /5 D0 → 7 F1 . Table 2 Comparison of fluorescence branching ratio, effective bandwidth (nm) and stimulated emission cross-section (10−22 cm2 ) of sample Eu-C and Eu-A of 5 D0 → 7 Fj (j = 0–4) transitions. 5

Eu-C (ˇ%) Eu-A (ˇ%) Eu-C (eff ) Eu-A (eff ) Eu-C () Eu-A ()

D0 → 7 F0

2.66 1.05 4.99 2.73 1.61 1.48

5

D0 → 7 F1

10.95 8.60 15.56 13.66 2.30 2.67

5

D0 → 7 F2

60.97 67.58 15.11 10.34 15.33 31.42

5

D0 → 7 F3

8.17 4.64 18.32 9.79 2.13 2.95

5

D0 → 7 F4

17.25 18.13 16.85 9.61 6.61 15.79

ˇ%, eff and  stand for the fluorescence branching ratio, effective bandwidth stimulated emission cross-section, respectively.

1 Ip



I() d

(6)

where IP is the peak intensity of the emission band. Luminescence lifetime profiles of both samples are also shown in Fig. 6 and Table 1. As both decay profiles cannot be well fitted by monoexponential decay curves, the life time  was determined by the following expression [10]:

 tI(t)dt = 

(7)

I(t)dt

It can be observed that the emission quantum efficiency of EuA increases dramatically comparing with that of Eu-C. When the lanthanide complexes are covalently attached to meso-structured silica, the rigid matrix and nano-scale mesopores restrict the vibration of the lanthanide complex, which leads to the decrease of its non-radiative rates. Also, the assembly (Eu-A) shows the increasing fluorescence branching ratio of 5 D0 → 7 F2 transition and higher value of stimulated emission cross-section comparing with the neat complex, which makes this assembly a good candidate for biological probe materials. Important Judd–Ofelt’s experimental intensity parameters ˝2 and ˝4 are calculated by using the following equation [27,28] (shown in Table 1). A0j =

 2 4e2 ω3 (2J + 1)−1 ˝ 7 Fj ||U () ||5 D0  3 3c

(8)



A0j (calculated by the way described in Eq. (1)) is the Einstein coefficient, the spontaneous emission probability of an electric

Fig. 5. (a) Emission spectra of Eu-C and Eu-A (excited by 374 nm). (b) Magnified emission spectra of 5 D0 → 7 F2 transition of Eu-C and Eu-A.

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transition, the larger ˝2 value of Eu-A indicates the stronger covalency degree. The other reason is low crystal field symmetry of the coordinating environment. It is known that the effect of the ligand perturbation produces Stark splitting of the 7 FJ levels of Eu3+ and the maximum splitting number of every J level is 2J + 1. The number of sublevels and bands is affected by the symmetry of Eu3+ . Stark splitting of 5 D0 → 7 F2 emission of Eu-C and Eu-A is shown in Fig. 5b. More splitting numbers are observed in the spectrum of Eu-C, which proves that Eu3+ ions lie in low symmetry coordinating environment. 4. Conclusions In this paper, Eu-Schiff base complex and its assembly were measured by combination of techniques. It was found that more efficient energy transfer from ligand to Eu3+ took place in the assembly materials. Eu-complex assembly sample showed much longer lifetime, higher ˝2 value and emission quantum efficiency compared with those of the neat complex. Acknowledgement The authors would like to thank the financial support by Shanghai Leading Academic Discipline Project (project number: B502). Fig. 6. Luminescence lifetime profiles of Eu-C and Eu-A.

dipole transition between 5 D0 and 7 Fj . Where  = 2 and 4, J is the total angular momentum of the ground state,  is the Plank’s constant, ω (ω = 2 c) [(cm−1 ) is the average transition energy] is the frequency of the transition, and is the Lorentz local field correction term which is determined by n((n2 + 2)2 )/9 with the refraction index n = 1.5. 7 Fj ||U() ||5 D0 2 are the square reduced matrix elements whose values are 0.0032 and 0.0023 for j = 2 and 4, respectively, which are gained from Ref. [3]. The intensity of ˝2 (Eu3+ ), hypersensitive 5 D0 → 7 F2 transition, is closely connected to covalency between Eu3+ and the surrounding ligands. Table 1 shows that the covalency degree of Eu-A became much stronger comparing with Eu-C, which is related to the changes of Eu3+ ion chemical environment. Intensity parameter ˝4 is believed to be related to the bulk properties of lanthanide based hosts, but there is no theoretical predication for this sensibility to macroscopic properties [29]. 3.3.3. Parameter R2/1 (5 D0 → 7 F2 /5 D0 → 7 F1 ) The emission intensity of electron transitions of 5 D0 → 7 F2 , which is attributed to the forced electric dipole transition, is hypersensitive to the coordinating environment. Meanwhile, the intensities of transitions of 5 D0 → 7 F1 , which is attributed to the magnetic-dipole transition, are almost not affected by surrounding environment. R2/1 (the ratio of intensity of 5 D0 → 7 F2 to 5 D0 → 7 F1 ) provides valuable information about changes of the symmetry at site of Eu3+ and its covalence degree [30]. Therefore, the changes of crystal field symmetry of the coordinating environment and covalency of chemical bond with lanthanide ions lead to the variations of R2/1 . From Table 1, it can be observed that Eu-A has higher R2/1 value than Eu-C, which can be justified by following two reasons. One is the increasing covalency degree of chemical bond with Eu3+ . As the dominant emission of Eu3+ at the visible wave region is 5 D0 → 7 F2

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