Preparation and luminescence properties of covalent linking of luminescent ternary europium complexes on periodic mesoporous organosilica

Microporous and Mesoporous Materials 116 (2008) 28–35

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Preparation and luminescence properties of covalent linking of luminescent ternary europium complexes on periodic mesoporous organosilica Xianmin Guo a,b, Xiaomei Wang a,b, Hongjie Zhang a,*, Lianshe Fu c, Huadong Guo a,b, Jiangbo Yu a, L.D. Carlos c, Kuiyue Yang a a

State Key Laboratory of Rare Earth Resource Utilizations, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Graduate School of the Chinese Academy of Sciences, Changchun 130022, PR China c Department of Physics, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b

a r t i c l e

i n f o

Article history: Received 12 December 2007 Received in revised form 9 March 2008 Accepted 12 March 2008 Available online 18 March 2008 Keywords: Organic/inorganic hybrids Periodic mesoporous organosilica Lanthanide complex Photoluminescence

a b s t r a c t A novel periodic mesoporous organosilica (PMO) covalently grafting with phen (phen-PMO, phen = 1, 10phenanthroline) was synthesized via a co-condensation of 1,2-bis(triethoxysilyl)ethane (BTESE) and 5, 6bis(N-3-(triethoxysilyl)propyl)ureyl-1, 10-phenanthroline (phen-Si) using supramolecular polyoxyethylene (10) stearyl ether (Brij 76) surfactant as template (under acidic conditions). Accordingly, a series of PMO materials (PMOs) containing Eu(tta)3phen (denoted as Eu(tta)3phen-PMO, tta = 2-thenoyltrifluoroacetone) were synthesized by impregnation of Eu(tta)3  2H2O into phen-PMO through a ligand exchange reaction. For comparison, Euphen-PMO was also prepared using the same approach except EuCl3  6H2O instead of Eu(tta)3  2H2O. The mesostructures of the PMO materials were characterized by XRD, N2 adsorption–desorption and TEM measurements. The results showed that during the surfactant extraction process, the chelating organic ligand structure was preserved, which was confirmed by Fourier transform infrared (FTIR) and 29Si CP–MAS NMR spectroscopies. Under UV irradiation, Eu(tta)3phen-PMO exhibited the characteristic emission of Eu3+ ions. Based on the emission spectra, the experiment intensity parameters for Eu(tta)3phen, Euphen-PMO and Eu(tta)3phen-PMO were calculated according to Judd–Ofelt theory. Compared to the pure complex, the resulting hybrid material exhibited better thermal stability and similar emission quantum efficiency, demonstrated by thermogravimetric analysis and luminescence characterization, respectively. The hybrid material Eu(tta)3phen-PMO showed higher emission quantum efficiency than that of Euphen-PMO, indicating that tta is an efficient sensitizer for the luminescence of central Eu3+ ions. Ó 2008 Elsevier Inc. All rights reserved.

1. Introduction Lanthanide complexes are a useful class of luminophores since they exhibit high quantum efficiency, narrow-band emission and high color purity under ultraviolet excitation, through protecting metal ions from vibrational quenching and increasing light absorption cross section by the well-known ‘‘antenna effect” [1–6]. However, they have so far been excluded from practical applications as phosphor devices mainly due to their poor thermal stabilities and low mechanical strength [7,8]. Many lanthanide complexes have been incorporated into solid matrices, such as sol–gel-derived hybrid materials, using low-temperature soft-chemistry processes to circumvent this shortcoming [9–14]. Incorporation of lanthanide complexes into these matrices has not only improved the photoand thermal stabilities of the complexes, but also avoided the self-quenching, due to the concentration effect. Up to now, lantha* Corresponding author. E-mail address: [email protected] (H. Zhang). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.03.007

nide complexes can be incorporated into mesoporous matrices, such as MCM-41, HMS or SBA-15 either by simple doping method (no strong bonding between the phases) or by covalent bond grafting technique because of their unique properties, for example, large, controllable pore sizes and high surface areas [9a,15,16]. Compared to the pure lanthanide complexes, the obtained hybrid materials show stronger luminescent intensities and higher quantum efficiencies [11g,11h,16b]. A new class of mesostructured silica materials, designated as the periodic mesoporous organosilica (PMO) materials (PMOs) were reported in the literature in 1999 by three different independently working research groups [17]. PMO are prepared by the controlled hydrolysis and condensation of organosilanes precursors (R0 O)3-Si-R-Si-(OR0 )3 with two trialkoxysilyl groups bridged by an organic group (R), which permits the stoichiometric incorporation of organic groups into the silica network. This route confirms that PMO possesses a uniform distribution of organic and inorganic moieties at the molecular level within the framework, leading to the homogeneous distribution of organic functional

X. Guo et al. / Microporous and Mesoporous Materials 116 (2008) 28–35

groups as compared with those obtained by the simple doping method or covalent bond grafting approach [18]. Moreover, compared to the mesoporous silica materials, PMO present better aging, hydrothermal and mechanical stabilities as well as unique physical and chemical properties [19,20]. In recent years, many organic groups have been covalently grafted into the network of PMO silica matrices in the presence of aliphatic, aromatic, and heterocyclic surfactants [21]. However, to the best of our knowledge, very few lanthanide complexes have been incorporated inside the framework of PMO [22]. In this work, we first synthesized a new compound 5,6-bis(N-3-(triethoxysilyl)propyl)ureyl-1, 10-phenanthroline (phen-Si) with double roles: both as a precursor for the synthesis of PMO and as an organic ligand 1, 10-phenanthroline (phen) for the rare earth complex, followed by synthesis of PMO containing rare earth ligand (phenPMO) by co-condensation of 1,2-bis(triethoxysilyl)ethane (BTESE) and phen-Si using polyoxyethylene (10) stearyl ether (Brij 76) surfactant as template under acidic conditions. Finally, the PMO hybrid material covalently grafting with europium complex Eu(tta)3phen (tta = 2-thenoyltrifluoroacetone) using covalentlybonded heterocyclic ligand phen group to the silica framework, named as Eu(tta)3phen-PMO, was successfully prepared by impregnation of the binary europium complex Eu(tta)3  2H2O into phen-PMO ethanol solution through a ligand exchange reaction. The main objective of this work is to prepare new PMOs covalently grafted with europium complex and to investigate their structures and luminescent properties.

2. Experimental 2.1. Materials BTESE, 3-(triethoxysilyl)-propyl isocyanate (ICPTES), Brij 76 and phen were used as received. Ethanol (EtOH) was distilled after desiccation with calcium oxide. Europium (III) chloride solution was obtained by reaction of europium oxide (Eu2O3, 99.99%) with hydrochloric acid (HCl) and dissolved in EtOH. The molar concentration of EuCl3  6H2O was determined by EDTA volumetric method. All reagents were of analytic grade and purchased from Beijing Chemical Factory except BTESE, ICPTES and Brij 76 from Aldrich. 2.2. Synthesis of pure PMO and PMO grafting with phen (phen-PMO) Three steps were involved in the synthesis of phen-PMO. In the first step, 5,6-diamino-1, 10-phenanthroline (designated as phen(NH2)2) was synthesized according to the method described in reference [23]. In the second step, 5,6-bis(N-3-(triethoxysilyl)propyl)ureyl-1, 10-phenanthroline (phen-Si) was prepared by the reaction of phen-(NH2)2 and ICPTES in EtOH using a method similar to one described in literature [24], and characterized by proton NMR spectroscopy. 1H NMR (DMSO,400 MHz) d (ppm):11.82 (2H, s, NH), 8.99 (2H, dd, ArH), 8.72 (2H, dd, ArH), 7.79 (2H, m, ArH), 5.90 (2H, s, NH), 3.75 (12H, q, OCH2), 3.03 (4H, q, NCH2), 1.46 (4H, quint, CH2), 1.14 (18H, t, CH3), 0.53 (2H, t, SiCH2). In the third step, pure PMO and phen-PMO were synthesized: 0.5 g of Brij 76 was dissolved in 2.5 g of distilled water and 10 mL of 2 M HCl solution at 55 °C. Then a mixture of BTESE and phen-Si was added into the above solution dropwise under vigorous stirring. The molar composition was xphen-Si: (1  x)BTESE : 0.29Brij 76: 8HCl: 242H2O, where the value of x (the molar ratio of phen-Si/(phenSi + BTESE)) is 0, 0.02, 0.05, 0.10 and 0.15, respectively. The mixture was stirred at 55 °C for 24 h. The obtained product was recovered by filtration, washed thoroughly with copious amounts of deionized water, and air-dried at room temperature. The surfactant was removed by Soxhlet extraction with ethanol for 24 h to obtain

29

both the pure PMO (x = 0) and the hybrid materials, which were named as phen(x)-PMO (x = 0.02, 0.05, 0.10 and 0.15). 2.3. Synthesis of Eu(tta)3phen-PMO and Euphen-PMO Binary europium complex Eu(tta)3  2H2O was synthesized using the methods described in the literature [9a,12]. A certain amount of Eu(tta)3  2H2O was dissolved in ethanol, then phen(0.10)-PMO was added into the above solution, where the molar ratio of Eu(tta)3  2H2O: phen(0.10)-PMO = 2: 1. The mixture was stirred for 12 h under reflux. The precipitate was filtrated and washed with copious acetone in order to remove the excess of Eu(tta)3  2H2O until the filtrate did not exhibit any red fluorescence under UV irradiation. The resulting Eu(tta)3phen(0.10)-PMO hybrid material was dried at 60 °C under vacuum for 10 h. The synthesis procedure of Euphen-PMO was similar to that of Eu(tta)3phen(0.10)-PMO except EuCl3 instead of Eu(tta)3  2H2O. Both hybrid materials were obtained as outlined in Scheme 1. According to the literature [25], we estimate the possible coordination structures of the Eu3+ ions in Euphen-PMO and Eu(tta)3phen-PMO materials, which were also charted in Scheme 1. 2.4. Characterization The small-angle powder X-ray diffraction patterns (XRD) were recorded in the 2h range of 0.50°–5.0° with Rigaku-D/max 2500 diffractometer using Cu Ka radiation (k = 1.5418 Å, 40 kV and 200 mA) at a step width of 0.02°. Fourier transform infrared spectra (FTIR) were measured on a Bruker Vertex 70 Spectrophotometer within the wavenumber range 4000–400 cm1 using KBr pressed pellet technique. Solid-state 29Si magic-angle spinning (MAS) NMR spectra were recorded at 79.46 MHz using a Bruker Avance 400 spectrometer. The chemical shifts were quoted in ppm from tetramethylsilane (TMS). Transmission electron microscopy (TEM) observation was made on a JEM-100CX II at an acceleration voltage of 200 kV. Nitrogen (N2) adsorption–desorption isotherms were performed at 196 °C using a Nova l000 analyzer with nitrogen. The samples were outgassed in the degas port of the adsorption analyzer overnight at 120 °C prior to measurements. Specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) model and pore size distributions were evaluated from the desorption branches of the nitrogen isotherms using the Barrett– Joyner–Halenda (BJH) model. Pore volumes were estimated at a relative pressure of 0.98 (P/P0), assuming full surface saturated with nitrogen. Thermogravimetric analysis (TGA) was performed on an SDT 2960 analyzer (Shimadzu, Japan) from 30 to 800 °C at a heating speed of 10 °C/min. The fluorescence excitation and emission spectra were recorded at room temperature with a Hitachi F-4500 spectrophotometer equipped with a 150 W Xenon lamp as an excitation source. Luminescence lifetimes were measured with a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using different wave-number lasers (pulse width = 4 ns) as the excitation source (Continuum Sunlite OPO).

3. Results and discussion 3.1. XRD patterns and TEM The XRD patterns for phen(x)-PMO (x = 0.02, 0.05, 0.10 and 0.15) shown in Fig. 1 indicate the formation of well-ordered twodimensional hexagonal structure. With increasing the concentration of phen-Si, the intensity of the (1 0 0) reflection decreases gradually indicating that the ordered assembly of the mesostructure is disturbed upon incorporating the chelate organosilane precursor. This is probably due to the different ratio of hydrolysis and

30

X. Guo et al. / Microporous and Mesoporous Materials 116 (2008) 28–35

(EtO)3Si(CH2)3HNCOHN

NHCONH(CH2)3Si(OEt)3

Brij 76 H+/H2O

+

(EtO)3Si-CH2CH2-Si(OEt)3

N

BTESE

N

phen-Si as-synthesized phen-PMO surfactant removal

3+

= Eu or Eu(tta)3

EtOH

EuCl3

Eu(tta)3·2H2O Euphen-PMO or Eu(tta)3phen-PMO

phen-PMO

O

O

CH2-CH2-CH2-Si-O-Si-CH2-CH2-CH2 O

O

NH O=C NH

N

Cl Cl

NH C=O NH

N

O O -O-Si-(CH2)3-NH-C-HN

Eu N Cl OH N 2

NH O=C NH

NH

-O-Si-(CH2)3-NH-C-HN O O

CH2-CH2-CH2-Si-O-Si-CH2-CH2-CH2 O

O

O-C Eu

O

C=O NH

O

O

S N N

CH2 O-C CF3

3

Eu(tta)3phen-PMO

Euphen-PMO

Intensity (a.u.)

Scheme 1. Schematic diagram of incorporation of europium complex into the framework of PMO via a co-condensation of 1,2-bis(triethoxysilyl)ethane (BTESE) and 5,6bis(N-3-(triethoxysilyl)propyl-1, 10-phenanthroline(phen-Si)) using convalent bond.

a b c d 1

2

3

4

5

o

2θ ( ) Fig. 1. XRD patterns of phen(x)-PMO, x = 0.02 (a), 0.05 (b), 0.10 (c) and 0.15 (d).

polycondensation of the two organosilanes changing then the structure of the hybrids. The corresponding unit cell parameter a0 (the internal pore diameter plus one pore wall thickness) can p be calculated according to the formula a0 = 2d1 0 0/ 3, where d1 0 0 is obtained from 2h angle of the first reflection peak in the XRD pattern by Bragg’s equation: 2d1 0 0sinh = k (k = 1.5418 Å for the Cu Ka line). The values of d1 0 0 and a0 for the materials at different stages during the preparation process are listed in Table 1. As it can be seen, with increasing the concentration of phen-Si, the peak of the (1 0 0) reflection shifts noticeably to lower angle and the a0 value increases correspondingly, which proves that more content of phen-Si has been covalently-bonded with the framework of periodic mesoporous organosilicas. The above results suggest that an optimum molar ratio x (phen-Si/(phen-Si + BTESE)) = 0.10 can be employed to synthesize phen-PMO with a high loading of chelated organic content and also a good mesostructure. Accordingly, phen(0.10)-PMO was selected as the candidate for the preparation

31

X. Guo et al. / Microporous and Mesoporous Materials 116 (2008) 28–35 Table 1 Structural parameters of phen(0.02)-PMO, phen(0.05)-PMO and phen(0.10)-PMO Samples

d1 0 0 (nm)

a0 a (nm)

db (nm)

tc (nm)

SBET (m2 g1)

V (cm3 g1)

Phen(0.02)-PMO Phen(0.05)-PMO Phen(0.10)-PMO

5.66 6.50 7.18

6.54 7.51 8.29

3.01 3.27 3.56

3.53 4.24 4.73

1034 706 591

0.91 0.65 0.62

d1 0 0: (1 0 0) spacing; a0: cell parameter; d: pore diameter; t: wall thickness; SBET: BET surface area; V: pore volume. p a Calculated using the equation a0 = 2d1 0 0/ 3. b Pore size distribution by desorption branch (BJH). c Calculated by a0  d.

3.2. Nitrogen adsorption–desorption measurements Fig. 3 presents the N2 adsorption–desorption isotherms for phen(0.02)-PMO (a), phen(0.05)-PMO (b) and phen(0.10)-PMO (c), and Fig. 4 displays the pore size distributions for phen(0.10)PMO (a) and Eu(tta)3phen(0.10)-PMO (b). The structural parameters of the materials are listed in Table 1. All these samples exhibit Type IV isotherm curves with H1-type hysteresis loops at low relative pressure, characteristic of mesoporous materials according to the IUPAC classification, which is related to the capillary condensation of nitrogen within the pores [26]. A sharp adsorption step in the P/P0 region of 0.3–0.6 from the two branches of adsorption–desorption isotherm curves suggests that all samples possess a well-defined array of regular mesopores. The specific surface areas and the pore size distributions were calculated by the BET method and BJH model, respectively. The values of estimated pore diameter (d), wall thick (t), BET surface area (SBET), and pore volume (V) are summarized in Table 1. It can be clearly seen that the SBET and V decrease from 1034 m2 g1 and 0.91 cm3 g1 for phen(0.02)-PMO to 591 m2 g1 and 0.62 cm3 g1 for phen(0.10)PMO, respectively. However, with increasing the content of phen-Si, d changes slightly from 3.01 to 3.56 nm, whereas t increases markedly from 3.53 to 4.73 nm, proving that the chelating organic ligand phen has been successfully grafted into the framework of the periodic mesoporous organosilicas, which is consistent with the XRD results. Compared the pore distribution of

a

600 -1

Adsorption volume (cm g )

b

3

of periodic mesoporous organosilica materials covalently-bonded with Eu(tta)3  2H2O (Eu(tta)3phen-PMO) and EuCl3 (EuphenPMO). From Fig. 2, it can be seen that the framework hexagonal ordering has been still conserved very well upon introducing Eu(tta)3  2H2O into the pore wall of periodic mesoporous organosilica. The distance between the centers of the mesopores is estimated to be ca. 7 nm, which is in good agreement with the value calculating from the corresponding XRD data.

c

400

200

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) Fig. 3. N2 adsorption–desorption isotherms phen(0.05)-PMO (b) and phen(0.10)-PMO (c).

of

phen(0.02)-PMO

(a),

phen(0.10)-PMO with that of Eu(tta)3phen(0.10)-PMO, the corresponding pore sizes have scarcely any change, further supporting the conclusion that Eu(tta)3  2H2O is covalently coordinated with phen into the wall of PMO. Moreover, compared to phen(0.10)PMO, a less-pronounced decrease in both SBET and V occurs for Eu(tta)3phen(0.10)-PMO (SBET decreases from 591 to 535 m2 g1 and V from 0.62 to 0.59 cm3 g1, the latter data for Eu(tta)3phen(0.10)-PMO are not shown in Table 1). All the above results confirm that Eu(tta)3phen has mainly been formed into the framework of PMO. 3.3. FTIR spectra The FTIR spectra of as-synthesized PMO with template (a), assynthesized phen(0.10)-PMO (b) and surfactant-extracted phen(0.10)-PMO (c) are displayed in Fig. 5. Bands located at

Fig. 2. TEM images of Eu(tta)3phen(0.10)-PMO recorded along the [1 0 0] (a) and [1 1 0] (b) zone axes.

32

X. Guo et al. / Microporous and Mesoporous Materials 116 (2008) 28–35

extraction procedure. Meanwhile, contrasting (b) with (c), the intensity of the absorption bands due to m(C–H) vibrations and ds(C–H) vibrations decreases, which indicates that the surfactant has been removed.

a b

3

-1

Pore volume ( cm g )

3

3.4. Solid-state

1

2

4

6

8

10

Pore diameter (nm) Fig. 4. BJH pore distribution Eu(tta)3phen(0.10)-PMO (b).

isotherms

of

phen(0.10)-PMO

(a)

and

1030

1032, 1096 cm1 with the shoulder at 1161 cm1 (asymmetric vibrations mas, Si–O–Si), 802, 799 cm1 (stretching vibrations ms, Si–O–Si), and 450 cm1 (in-plane bending vibrations d, Si–O–Si) present the formation of the C–Si–O–Si framework [16b]. In addition, the COC stretching vibrations in the Brij 76 also appears in the region of 1200–950 cm1. The peaks at 2924 cm1 and 1385 cm1 are assigned to m(C–H) vibrations and ds(C–H) vibrations of the surfactant and the bridge silsesquioxane precursor, respectively. The broad band peaking around 3420–3435 cm1 corresponds to hydrogen bonded OH groups and/or water molecules. The bands at 1270 cm1 and 1413 cm1 are attributed to out-plane rocking x(Si–C) and d(Si–C), respectively. Compared with (a) and (b), the new peaks appears at 1548, 1488, 1461, 1435 cm1 indicates that phen-Si was incorporated into all the PMO materials. The broad band in the 1780–1580 cm1 region is mainly from the vibration of ‘‘amide I” from NHC(@O)NH in the phen-Si, suggesting that the chelate organic ligand has been grafted into PMO via a covalent bond. It should be noted that the vibration from water adsorbed in the materials also appears as a relatively sharp peak in this region [27]. In addition, the above peaks can also be observed in the spectrum of surfactant-extracted phen-PMO, which confirms that the phen group in the framework remains intact during the surfactant

Si MAS NMR spectrum

3.5. Thermogravimetric analysis Thermogravimetric analyses of Eu(tta)3phen and Eu(tta)3phen(0.10)-PMO were carried out in nitrogen atmosphere from ca. 30 to 700 or 800 °C, including the weight loss (TGA) and the corresponding derivative weight loss (DrTGA) curves, Fig. 7. The pure Eu(tta)3phen complex in nitrogen atmosphere shows only a weight loss of ca. 54% between 300 to 400 °C with a maximum weight loss at 352 °C. Whereas Eu(tta)3phen(0.10)-PMO in nitrogen atmosphere presents a weight loss of ca. 4.5% below 130 °C, which is due to the physically adsorbed water and residue ethanol. This is followed by a weight loss of ca. 15% between 130 and 430 °C with a maximal weight loss at 360 °C, corresponding to the thermal decomposition of incompletely removed surfactant, probably a small amount of the matrix of bridged ethylene and also evaporation of the chemically bonded water [28]. A weight loss of ca. 17.0% centered at 526 °C can be attributed to the decomposition of ethylene and organic europium complex bridged inside the framework. Compared to the above decomposition temperature of the pure Eu(tta)3phen complex centered at about 350 °C [29], the thermal stability of europium complex has been improved upon covalently grafted to the framework of PMO.

1161

3

450

c

T 802

2924

b

a

The solid-state 29Si MAS NMR spectrum for the bridged Eu(tta)3phen(0.10)-PMO is displayed in Fig. 6. The spectrum exhibits only three signals at ca. 53.8, 57.6 and 63.9 ppm, corresponding to the organosiloxane groups „SiOSiR(OH)2 (T1), („SiO)2SiROH (T2) and („SiO)3SiR (T3), respectively, where R is the bridging organic moiety. In order to study in detail the degree of hydrolysis/condensation of bridge organic groups, the spectral deconvolution are carried out using the ORIGINÒ package, from which the relative integrated intensities of the organosiloxane groups have been obtained. The condensation degree was calculated to be around 90% using the expression (%T1 + 2%T2 + 3%T3)/3, indicating that the hydrolysis/condensation of the bridge organic groups in the order mesoporous structure is almost complete. No characteristic peaks of the various siloxane Qm [Qm = Si(OSi)m(OH)4m, m = 2–4] appear, which confirms that the C–Si bonds of the precursors are stable and do not cleave in the whole synthesis process.

1096

a

Absorbance

29

2

T

b

1

T

2

c 4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

-40

-60

-80

-100

Chemical shift (ppm) Fig. 5. FTIR spectra for as-synthesized phen(0)-PMO (a) as-synthesized phen(0.10)PMO (b) and surfactant-extracted phen(0.10)-PMO (c).

Fig. 6. Solid-state

29

Si MAS NMR spectrum of Eu(tta)3phen(0.10)-PMO.

33

X. Guo et al. / Microporous and Mesoporous Materials 116 (2008) 28–35

F2

0.004

7

1.0

250

300

350

D0

600

5

5

500

D0

400

7

7

300

Temperature (oC ) Fig. 7. TGA and DrTGA curves of Eu(tta)3phen (a and a0 , respectively) and of Eu(tta)3phen(0.10)-PMO (b and b0 , respectively).

450

D0

200

5

100

400

Wavelength (nm)

7

-0.016

0.4

200

F4

a

F3

-0.012

a'

0.5

F1

0.6

7

-0.008

5

b

F0

0.7

D0

TGA (%)

-0.004

DrTGA % (min-1)

0.8

Intensity (a.u.)

b'

Intensity (a.u.)

5

D0

0.000

0.9

340 nm

615 nm

580

600

620

640

660

680

700

720

740

Wavelength (nm)

325 nm

F1

Intensity (a.u.)

5

D0

7

615 nm

D0

7

200

5

Intensity (a.u.)

250

300

350

400

D0

D0

7

7

F4

F3

Wavelength (nm)

580

600

620

640

5

5

The excitation and emission spectra of Eu(tta)3phen(0.10)-PMO (a) and Euphen(0.10)-PMO (b) are presented in Fig. 8. The excitation spectra (room temperature) were obtained by monitoring the most intense Eu3+ emission wavelength (615 nm). The excitation and emission spectra of the Eu(tta)3phen pure complex can be seen in the literature elsewhere [16b]. The excitation spectrum exhibits a broad excitation band from 200 to 500 nm (kmax = 383 nm), which can be attributed to transitions from the ground state (p) S0 to the excited level (p*) S1 of the organic ligands, while the emission spectrum displays the characteristic Eu3+5 D0 ? 7F04 intra-4f6 transitions [16b]. Compared to the pure Eu(tta)3phen complex, the excitation spectra of Eu(tta)3phen(0.10)-PMO and Euphen(0.10)-PMO (Fig. 8 insets) become narrower and the maximum excitation wavelength shifts from 383 to 340 and 325 nm, respectively. The blue-shift of the excitation band after the introduction of the Eu3+ ions into PMO is attributed to a hypsochromic effect, which results from the change in polarity of the environment surrounding the Eu3+ cations in the periodic mesoporous organosilicas [15c,16b]. After ligand-mediated excitation at 340 nm, the emission spectrum of Eu(tta)3phen(0.10)-PMO clearly shows the emission bands centered at 578, 590, 611, 651, and 700 nm, corresponding to 5D0 ? 7F04 transitions, respectively. The full width at half maximum (fwhm) of 5 D0 ? 7F1 is 115 cm1 for Eu(tta)3phen, 257 cm1 for Eu(tta)3phen(0.10)-PMO and 344 cm1 for Euphen(0.10)-PMO. The increased fwhm in Eu(tta)3phen(0.10)-PMO and Euphen(0.10)-PMO suggests the presence of more than one component, in agreement with the results from the lifetime measurements which will be discussed in the following part. The difference in the excitation and emission spectra of Eu(tta)3phen and Eu(tta)3phen(0.10)-PMO shows that the environment of the periodic mesoporous organosilicas affects the energy transfer in the complex. In the case of Euphen(0.10)-PMO, the corresponding emission spectrum contains not only the typical emission of Eu3+ ions but also a broad emission band in the blue spectral region from 420 to 570 nm. Under UV (365 nm) irradiation, the resulting Eu(tta)3phen(0.10)-PMO hybrid exhibits stronger luminescence than Euphen(0.10)-PMO, which indicates that the b-diketone tta is a more efficient sensitizer for the Eu3+ luminescence than phen and that the presence of tta in PMO matrix improves the luminescence properties of the Eu(tta)3phen(0.10)-PMO resulting hybrids. This conclusion can also be supported calculating the 5D0 the quantum efficiency. The 5D0 decay curves (monitored at the intensity maximum of the 5D0 ? 7F2 transition) of Eu(tta)3phen(0.10)-PMO and Euphen(0.10)-PMO exhibit a bi-exponential character:

F2

3.6. Luminescence properties

660

680

700

720

740

Wavelength (nm) Fig. 8. Room temperature emission spectra of Eu(tta)3phen(0.10)-PMO (a, kex = 340 nm) and Euphen(0.10)-PMO (b, kex = 325 nm) in the solids. The excitation spectra are shown in the insets.

y ¼ A1 expðt=sf Þ þ A2 expðt=ss Þ þ y0

ð1Þ

where A1 and A2 are the pre-exponential factors obtained from the curve fitting, and sf and ss stand for the lifetimes for the fast term and slow term, respectively. The values of sf and ss are 0.15 (in 51%) and 0.43 ms (in 49%) for Eu(tta)3phen(0.10)-PMO, 0.01 (in 61%) and 0.24 ms (in 39%) for Euphen(0.10)-PMO. The average lifetime (sav) of 0.36 and 0.24 ms of Eu3+ ion for Eu(tta)3phen(0.10)PMO and Euphen(0.10)-PMO, respectively, can be calculated using the following equation [30]: sav ¼ ðA1 s2f þ A2 s2s Þ=ðA1 s f þ A2 ss Þ

ð2Þ

On the other hand, the decay curve of the pure complex exhibits a single exponential profile with a lifetime of 0.70 ms [16b]. The different decay curves of the pure complex and hybrid materials confirm that the Eu3+ ions are located in different environments (on average). The corresponding 5D0 emission quantum efficiencies (q) were estimated by the formula [11b,11g,11h]: q ¼ kr =ðkr þ knr Þ

ð3Þ

where kr and knr denote the radiative and non-radiative probability constants, respectively. Accordingly, the number of water molecules

34

X. Guo et al. / Microporous and Mesoporous Materials 116 (2008) 28–35

Table 2 Photoluminescence data of Eu(tta)3phen(0.10)-PMO and Euphen(0.10)-PMO in solids Samples a

Eu(tta)3phen Eu(tta)3phen(0.10)-PMO Euphen(0.10)-PMO

X2 (1020 cm2)

X4 (1020 cm2)

s (ms)

q (%)

nw

13.87 24.81 7.09

0.33 0.83 0.37

0.70 0.36 0.24

30.5 27.9 6.7

0.8 1.9 3.9

Xk (k = 2 and 4): experimental intensity parameters; s: lifetime and q: emission quantum efficiency; nw: number of water molecules coordinated to the Eu3+ ions. a Values are from literature [16b].

coordinated to the Eu3+ ions (nw) can also be estimated from kexp (kexp = 1/s) and kr by the empirical formula [31] nw ¼ 1:1ðkexp  kr  0:31Þ

ð4Þ

The data for the q and nw are listed in Table 2. From Table 2 we can discern that the 5D0 lifetime in Eu(tta)3phen(0.10)-PMO is longer than that in Euphen(0.10)-PMO, indicating that tta can replace part of the water molecules coordinated to the Eu3+ ions which results in stronger luminescence intensity. The shorter lifetime of Eu(tta)3phen-PMO, compared with that of pure Eu(tta)3phen, can be ascribed to a possible quenching by the silanol groups of the PMO matrix (and/or other OH groups from water molecules). The experimental intensity parameters (Xk, k = 2 and 4) were calculated from the emission spectra of Eu(tta)3phen(0.10)-PMO and Euphen(0.10)-PMO based on the 5D0 ? 7F2,4 electronic transitions and the 5D0 ? 7F1 magnetic dipole allowed transition as the reference. The radiative emission rates Arad (J) are estimated by the equation [5,32,33]  E2  X D 4e2 x3 1   Arad ðJÞ ¼ Xk 5 D0 U ðkÞ 7 F J v 3 3 hc 2J þ 1 k where k = 2 and 4, e is the electronic charge, x is the angular frequency of the transition, ⁄ is Planck’s constant over 2p, c is the velocity of light, v is the Lorentz local field correction that is given by n(n2 + 2)2/9 with the refraction index n = 1.5, and  E2  D   5 D0 U ðkÞ 7 F J are the square reduced matrix elements whose values are 0.0032 and 0.0023 for J = 2 and 4, respectively. The results are listed in Table 2. The X6 parameter was not determined since the 5D0 ? 7F6 transition could not be experimentally observed. Consequently, their influence on the depopulation of the 5D0 excited state is neglected, and the radiative contribution is estimated based only on the relative intensities of the 5D0 ? 7F04 transitions. It is noticed that the value of the X2 intensity parameter of Eu(tta)3phen(0.10)-PMO is higher than that of Euphen(0.10)-PMO, which might be interpreted as a consequence of the hypersensitive behavior of the 5D0 ? 7F2 transition. Therefore, the dynamic coupling mechanism is quite operative, suggesting that the Eu3+ ion is in a highly polarizable chemical environment in Eu(tta)3phen(0.10)-PMO.

thermal stability. Since the europium complexes with b-diketone usually show higher luminescence intensity, higher emission quantum efficiency and poorer thermal stabilities than those with carboxylic acid and heterocyclic ligands, the improvement of the thermal stability of the europium complexes with b-diketone ligand in PMO hybrid materials is quite important to their potential optical applications. Acknowledgments The authors are grateful to the financial aids from the National Natural Science Foundation of China (Grant Nos. 20490210, 206301040 and 20602035) and the MOST of China (Grant Nos. 2006CB601103, 2006DFA42610). L.S.F. thanks Fundacão para a Ciência e Tecnologia (Portuguese agency) for post-doctoral grant (SFRH/BPD/5657/2001). References [1] [2] [3] [4] [5] [6] [7] [8] [9]

[10]

[11]

4. Conclusions Europium complex was covalently grafted into the framework of PMO via a co-condensation of BTESE and phen-Si under acidic conditions. In the hybrid materials, the chelating organic ligand structure was preserved, which was confirmed by FTIR and 29Si CP–MAS NMR spectroscopies. The resultant Eu(tta)3phen-PMO hybrid exhibits the characteristic emission of Eu3+ ions under UV irradiation. Compared to Euphen-PMO, Eu(tta)3phen-PMO shows higher emission quantum efficiency (27.9% for Eu(tta)3phen(0.10)-PMO and 6.7% for Euphen(0.10)-PMO) and longer lifetime (0.36 ms for Eu(tta)3phen(0.10)-PMO and 0.24 ms for Euphen(0.10)-PMO). In the meanwhile, compared to pure Eu(tta)3phen, Eu(tta)3phenPMO also displays a similar emission quantum efficiency and better

[12] [13] [14] [15]

S.I. Weissman, J. Chem. Phys. 10 (1942) 214. S. Sato, M. Wada, Bull. Chem. Soc. Jpn. 43 (1970) 1955. N. Sabbatini, M. Guardingli, J.M. Lehn, Coord. Chem. Rev. 123 (1993) 201. V. Bekiari, P. Lianos, Adv. Mater. 10 (1998) 1455. G.F. De Sá, O.L. Malta, C. De Mello Donegá, A.M. Simas, R.L. Longo, P.A. SantaCruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165. R. Reisfeld, Struct. Bond. 106 (2004) 209. T. Jin, S. Tsutsumi, Y. Deguchi, K. Machida, G. Adachi, J. Electrochem. Soc. 142 (1995) L195. D.W. Dong, S.C. Jiang, Y.F. Men, X.L. Ji, B.Z. Jiang, Adv. Mater. 12 (2000) 646. (a) K. Binnemans, P. Lenaerts, K. Driesen, C. Görller-Walrand, J. Mater. Chem. 14 (2004) 191; (b) K. Driesen, R. Van Deun, C. Görller-Walrand, K. Binnemans, Chem. Mater. 16 (2004) 1531; (c) P. Lenaerts, A. Storms, J. Mullens, J. D’Haen, C. Görller-Walrand, K. Binnemans, K. Driesen, Chem. Mater. 17 (2005) 5194. (a) H.R. Li, J. Lin, H.J. Zhang, L.S. Fu, Q.G. Meng, S.B. Wang, Chem. Mater. 14 (2002) 3651; (b) F.Y. Liu, L.S. Fu, J. Wang, Z. Liu, H.R. Li, H.J. Zhang, Thin Solid Films 419 (2002) 178; (c) L.N. Sun, H.J. Zhang, L.S. Fu, F.Y. Liu, Q.G. Meng, C.Y. Peng, J.B. Yu, Adv. Funct. Mater. 15 (2005) 1041. (a) M.M. Silva, V. de Zea Bermudez, L.D. Carlos, A.P. Passos de Almeida, M.J. Smith, J. Mater. Chem. 9 (1999) 1735; (b) L.D. Carlos, Y. Messaddeq, H.F. Brito, R.A. Sá Ferreira, V. de Zea Bermudez, S.J.L. Ribeiro, Adv. Mater. 12 (2000) 594; (c) R.A. Sá Ferreira, L.D. Carlos, R.R. Goncalves, S.J.L. Ribeiro, V. de Zea Bermudez, Chem. Mater. 13 (2001) 2991; (d) V. de Zea Bermudez, R.A. Sá Ferreira, L.D. Carlos, C. Molina, K. Dahmouche, S.J.L. Ribeiro, J. Phys. Chem. B 105 (2001) 3378; (e) L.D. Carlos, R.A. Sá Ferreira, J.P. Rainho, V. de Zea Bermudez, Adv. Func. Mater. 12 (2002) 819; (f) P.P. Lima, R.A. Sá Ferreira, R.O. Freire, F.A. Almeida Paz, L.S. Fu, S. Alves Jr., L.D. Carlos, O.L. Malta, ChemPhysChem 7 (2006) 735; (g) M. Fernandes, V. de Zea Bermudez, R.A. Sá Ferreira, L.D. Carlos, A. Charas, J. Morgado, M.M. Silva, M.J. Smith, Chem. Mater. 19 (2007) 3892; (h) C. Molina, K. Dahmouche, Y. Messadeq, S.J.L. Ribeiro, M.A.P. Silva, V. de Zea Bermudez, L.D. Carlos, J. Lumin. 104 (2003) 93. L.R. Melby, N.J. Rose, E. Abramson, J.C. Caris, J. Am. Chem. Soc. 86 (1964) 5117. C. Franville, D. Zambon, R. Mahiou, Chem. Mater. 12 (2000) 428. F. Embert, A. Mehdi, C. Reye, R.J.P. Corriu, Chem. Mater. 13 (2001) 4542. (a) Q.H. Xu, L.S. Li, X.S. Liu, R.R. Xu, Chem. Mater. 14 (2002) 549; (b) Q.H. Xu, W.J. Dong, H.W. Li, L.S. Li, S.H. Feng, R.R. Xu, Solid State Sci. 5 (2003) 777; (c) Q.G. Meng, P. Boutinaud, A.-C. Franville, H.J. Zhang, R. Mahiou, Micropor. Mesopor. Mater. 65 (2003) 127; (d) M.H. Bartl, B.J. Scott, H.C. Huang, G. Wirnsberger, A. Popitsch, B.F. Chmelka, G.D. Stucky, Chem. Comm. (2002) 2474;

X. Guo et al. / Microporous and Mesoporous Materials 116 (2008) 28–35 (e) S.X. Ge, N.Y. He, C. Yang, M. Gu, J.M. Cao, J. Nanosci. Nanotechnol. 5 (2005) 1305. [16] (a) H.R. Li, J. Lin, L.S. Fu, J.F. Guo, Q.G. Meng, F.Y. Liu, H.J. Zhang, Micropor. Mesopor. Mater. 55 (2002) 103; (b) C.Y. Peng, H.J. Zhang, J.B. Yu, Q.G. Meng, L.S. Fu, H.R. Li, L.N. Sun, X.M. Guo, J. Phys. Chem. B 109 (2005) 15278; (c) L.N. Sun, H.J. Zhang, C.Y. Peng, J.B. Yu, Q.G. Meng, L.S. Fu, F.Y. Liu, X.M. Guo, J. Phys. Chem. B 110 (2006) 7249; (d) L.N. Sun, J.B. Yu, H.J. Zhang, Q.G. Meng, E. Ma, C.Y. Peng, K.Y. Yang, Micropor. Mesopor. Mater. 98 (2007) 156; (e) S. Gago, J.A. Fernandes, J.P. Rainho, R.A. Sá Ferreira, M. Pillinger, A.A. Valente, T.M. Santos, L.D. Carlos, P.J.A. Ribeiro-Claro, I.S. Goncalves, Chem. Mater. 17 (2005) 5077; (f) S.M. Bruno, R.A. Sá Ferreira, L.D. Carlos, M. Pillinger, P. Ribeiro-Claro, I.S. Goncalves, Micropor. Mesopor. Mater., in press, doi:10.1016/j.micromeso. 2007.12.004. [17] (a) S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 121 (1999) 9611; (b) B.J. Melde, B.T. Hollande, C.F. Blanford, A. Stein, Chem. Mater. 11 (1999) 3302; (c) T. Asefa, M.J. MacLachlan, N. Coombs, G.A. Ozin, Nature 402 (1999) 867. [18] (a) T. Asefa, G.A. Ozin, H. Grondey, M. Kruk, M. Jaroniec, Stud. Surf. Sci. Cat. 141 (2002) 1; (b) S. Inagaki, Nanotechnology in Mesostructured Materials, in: Sang-Eon Park, Ryong Ryoo, Wha-Seung Ahn, Chul Wee Lee, Jong-San Chang (Eds.), Proceedings of the 3rd International Materials Symposium, Stud. Surf. Sci. Cat. 146 (2003) 1; (c) M.C. Burleigh, S. Jayasundera, C.W. Thomas, M.S. Spector, M.A. Markowitz, B.P. Gaber, Colloid Polym. Sci. 282 (2004) 728; (d) J. Morell, G. Wolter, M. Fröba, Chem. Mater. 17 (2005) 804; (e) W.H. Zhang, B. Daly, J. O’Callaghan, L. Zhang, J.L. Shi, C. Li, M.A. Morris, J.D. Holmes, Chem. Mater. 17 (2005) 6407; (f) M.A. Wahab, I. Imae, Y. Kawakami, C.S. Ha, Chem. Mater. 17 (2005) 2165; (g) B. Hatton, K. Landskron, W. Whitnall, D. Perovic, G.A. Ozin, Acc. Chem. Res. 38 (2005) 305;

[19]

[20] [21] [22] [23] [24] [25] [26]

[27] [28]

[29] [30] [31] [32]

[33]

35

(h) S.Z. Qiao, C.Z. Yu, Q.H. Hu, Y.G. Jin, X.F. Zhou, X.S. Zhao, G.Q. Lu, Micropor. Mesopor. Mater. 91 (2006) 59. (a) M.C. Burleigh, M.A. Markowitz, S. Jayasundera, M.S. Spector, C.W. Thomas, B.P. Gaber, J. Phys. Chem. B 107 (2003) 12628; (b) Q.H. Yang, J. Liu, J. Yang, L. Zhang, Z.C. Feng, J. Zhang, C. Li, Micropor. Mesopor. Mater. 77 (2005) 257. T. Asefa, M. Kruk, M.J. MacLachlan, N. Coombs, H. Grondey, M. Jaroniec, G.A. Ozin, J. Am. Chem. Soc. 123 (2001) 8520. D.M. Jiang, Q.H. Yang, H. Wang, G.R. Zhu, J. Yang, C. Li, J. Catal. 239 (2006) 65. E. Besson, A. Mehdi, C. Reye, R.J.P. Corriu, J. Mater. Chem. 16 (2006) 246. S. Bodige, F.M. Macdonnell, Tetrahedron Lett. 38 (1997) 8159. G.M. Kloster, S.P. Watton, Inorg. Chim. Acta 297 (2000) 156. J.G. Mao, Z.S. Jin, J.Z. Ni, Chin. J. Struct. Chem. 13 (1994) 377. (a) D.H. Everett, Pure Appl. Chem. 31 (1972) 577; (b) K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603; (c) S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982; (d) M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169. A. Dabrowski, M. Barczak, N.V. Stolyarchuk, I.V. Melnyk, Y.L. Zub, Adsorp. J. Inter. Adsorp. Soc. 11 (2005) 501. (a) M.C. Burleigh, M.A. Markowitz, S. Jayasundera, M.S. Spector, C.W. Thomas, B.P. Gaber, J. Phys. Chem. B 106 (2002) 9712; (b) M.P. Kapoor, N. Setoyama, Q.H. Yang, M. Ohashi, S. Inagaki, Langmuir 21 (2005) 443. H.H. Li, S. Inoue, K. Machida, G. Adachi, Chem. Mater. 11 (1999) 3171. (a) S. Buddhudu, M. Morita, S. Murakami, D. Rau, J. Lumin. 83–84 (1999) 199; (b) P.N. Minoofar, B.S. Dunn, J.I. Zink, J. Am. Chem. Soc. 127 (2005) 2656. R.M. Supkowski, W.D. Horrocks Jr., Inorg. Chim. Acta 340 (2002) 44. (a) B.R. Judd, Phys. Rev. 127 (1962) 750; (b) E.E.S. Teotonio, J.G.P. Espínola, H.F. Brito, O.L. Malta, S.F. Oliveria, D.L.A. de Foria, C.M.S. Izumi, Polyhedron 21 (2002) 1837. L.D. Carlos, R.A. Sá Ferreira, V. de Zea Bermudez, Organic–Inorganic Hybrid Materials and Nanocomposites, vol. 1, American Scientific Publishers, Morth Lewis Way, 2004, p. 353. Chapter 9.