Two Luminescent Molecular Hybrids Composed of Bridged Eu(III)-β

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1484–1489

Articles Two Luminescent Molecular Hybrids Composed of Bridged Eu(III)-β-Diketone Chelates Covalently Trapped in Silica and Titanate Gels Bing Yan* and Qian-Ming Wang Department of Chemistry, Tongji UniVersity, Shanghai 200092, China ReceiVed June 19, 2006; ReVised Manuscript ReceiVed October 31, 2007

ABSTRACT: In the context, a kind of β-diketone, dibenzoylmethane (DBM) was first grafted with the coupling reagent 3-(triethoxysilyl)-propyl isocyanate (TESPIC), and the as-derived monomers were introduced into inorganic network through powerful covalent bonds. Subsequently, two novel chemically bonded hybrid materials, Eu(III)-modified dibenzoylmethane with silica and titanate hosts, were prepared and characterized by IR, phosphorescence, fluorescence spectroscopy, time-resolved photoluminescence spectroscopy, and scanning electron microscopy (SEM). The above spectroscopic data indicate that the modified DBM could sensitize Eu(III) ions to exhibit attracting red luminescence in the two respective hosts. It is noted that the covalently bonded silicate hybrid material presents a stronger red/orange intensity ratio, longer lifetimes, and higher quantum efficiency than a covalently bonded titanate one and europium complex of DBM, suggesting that the silica network is more suitable for the emissions of covalently bonded composites than titanium oxide. Introduction Extensive research has been developed recently in lanthanidecontaining hybrid materials due to their excellent optical properties,1 and their specific functions make them widely applicable in various fields such as photonic crystal,2,3 optical glasses,4 and fluorescent or laser systems.5 In the few past decades, Eu(III) complexes especially β-diketones have been intensively studied owing to their inherent sharp emission peaks and high quantum efficiency.6,7a,b Among the above Eu(III)-βdiketones complexes, Eu-dibenzoylmethanate complexes show distinguished and desirable photo- or electro-luminescence properties. In general, rare earth complexes can be incorporated into the silica matrices during the processing of the sol–gel approach. However, such a doped strategy cannot solve the problem of the quenching effect of luminescent centers because the high energy vibration brought about by the surrounding hydroxyl groups and weak interactions (such as hydrogen bonding, van der Waals forces, or electrostatic forces) mainly functionalize between organic and inorganic components.8 Moreover, inhomogeneous dispersion of two phases and leaching of the photoactive molecules often occur in this sort of hybrid material for which the concentration of complex is also greatly reduced. Therefore, another appealing method that has emerged concerns the covalently bonded hybrids with increasing chemical stability, and the as-derived molecular-based hybrid materials exhibit a monophasic appearance even at a high * To whom correspondence should be addressed. E-mail: [email protected].

concentration of lanthanide complexes9–24 for they belong to complex molecular network systems. Several preceding reports have been carried out on the complexes of lanthanide-pyridinedicarboxylic acid or their derivatives, and the feasibility of the dicarboxylic acid system has been firmly proven.13 Zhang and his co-workers started to modify 1,10-phenanthroline and 2,2′bipyridine to prepare the predicted molecular-level hybrid materials.9,10 Our research team recently put more emphasis on the coordination behavior of rare earth ions and have now modified active amino groups, hydroxyl groups carboxyl groups with coupling reagents,14–17 as “molecular bridges” which cannot only chelate to rare earth ions but also bond a silica host with an alkoxysilane group. For the sake of further optimizing the light output and investigating chemical reactivity of carbanions, we first selected dibenzoylmethane (DBM) and considered that it was partly acidic and its R-hydrogen (the hydrogen atom in the methylene) can be extracted by base because of the polarization of C-H bonds by the adjacent carbonyl groups. Our original idea is to prohibit the degree of freedom for the active molecule so as to reduce the contribution of vibration movements to optical dephasing. In view of the red emissions of europium-β-diketones and their outstanding luminescent yields, we were motivated to modify DBM with the electrophilic reagent 3-(triethoxysilyl)-propyl isocyanate (TESPIC), and the objective of the work was to use DBM as molecular platforms to assemble groups of covalent bond ligands capable of sensitizing Eu3+ ions, thus increasing its thermal or photostability. In this report, totally different from previous

10.1021/cg0603725 CCC: $40.75  2008 American Chemical Society Published on Web 04/11/2008

Hybrids of Bridged Eu(III)-β-Diketone Chelates

amino carboxyl or hydroxyl moiety research, we attempt to modify active methylene of DBM by TESPIC bearing a trialkoxysilyl group as a functionalized organic ligand (DBMSi) under a sufficient basic environment; then we designed a covalently bonded hybrid inorganic–organic system that incorporated europium nitrate.

Crystal Growth & Design, Vol. 8, No. 5, 2008 1485 Scheme 1. Predicted Structure of Europium Centered Covalently Bonded Hybrids

Experimental Section Chemicals and Procedures. 3-(Triethoxysilyl)-propyl isocyanate was provided by Lancaster Synthesis Ltd. The solvents used were purified by common methods. Other starting reagents were used as received. A typical procedure for the preparation of DBM-Si was as follows: 1 mmol of dibenzoylmethane was first dissolved in refluxing anhydrous tetrahydrofuran (THF) by stirring, and 2 mmol of NaH (0.08 g, 60%) was added to the solution. Two hours later, 2.5 mmol (0.56 g) of 3-(triethoxysilyl)-propyl isocyanate was then put into the solution by dipping for half an hour. The whole mixture was refluxed at 65 °C under argon for 8 h. After isolation and purification of the sample, a yellow oil DBM-Si was furnished. IR: -CONH- 1704 cm-1, -(CH2)32921 cm-1, Si-O 1073 cm-1. Anal. Calcd. for C35H54O10N2Si2 (719.01): C 58.5, H 7.52, N 3.90; Found: C 58.4, H 7.41, N 3.88. 1H NMR (CDCl3) C35H54O10N2Si2: δ 8.05(2H,t), 7.58(4H,m), 7.18(2H, s), 6.91(2H, s), 6.60(2H, brs), 3.82(12H, m CH2(OEt)), 3.62(2H,brs), 3.16(2H,m), 1.86(6H,m), 1.61(2H, m), 1.22(14H,m, CH3(OEt)), 0.63(4H,t). The sol–gel derived hybrid containing Eu(III) ions was prepared as follows: DBM-Si was dissolved in dimethylformamide (DMF) with stirring. A stoichiometric amount of Eu(NO3)3 · 6H2O was added to the solution. After two hours, a corresponding amount of tetraethoxysilane (TEOS) was added in the reaction solution. Then one drop of diluted hydrochloric acid was put into it to promote hydrolysis. The mole ratio of RE(NO3)3 · 6H2O/DBM-Si/TEOS/H2O was 1:3:12:48. The mixture was agitated magnetically to achieve a single phase in a covered Teflon beaker, and then it was aged at 80 °C until the onset of gelation, which occurred within 5 days. The gels were collected as monolithic bulks and ground as solid powder materials (named as Hybrid I) for the photophysical studies (see Scheme 1). Using a similar method, TEOS was replaced by tetrabutyl titanate, and Hybrid II was prepared. However, different from slow sol–gel processes of silicon alkoxides, the hydrolysis and polycondensation reactions of titanium alkoxides are rather fast and often produce an opaque precipitate. We finally adopted Qiu’s25 and Lantelme’s26 research strategy and used concentrated inorganic acid such as HCl to give a clear sol to transparent bulk materials. In addition, we prepared Eu-DBM binary complexes and Eu-DBM-doped TiO2 for comparison purposes. The concentration of Eu(III) ions in Hybrid I and II were measured under the traditional complexometric titration method.27 Hybrids were through nitration and titrated with EDTA solution, using a buffer (pH 5.8) and xylenol-orange as an indicator. The contents of Eu(III) ions in Hybrid I and II were 6.9 and 6.3%, respectively, which were in agreement with the stoichiometry [Eu(DBM-Si)3] · 12SiO2/12TiO2 (Anal. Calcd: 6.85 for Si; 6.21 for Ti, %) (see Scheme 1). Measurements. All measurements were completed under room temperature except that phosphorescence spectra (5 × 10-4 mol · L-1 THF solution) were measured under 77 K. 1H NMR spectra were recorded in CDCl3 on a Bruker AVANCE-500 spectrometer with tetramethylsilane (TMS) as an internal reference. Elemental analyses (C, H, N) were determined with an Elementar Cario EL elemental analyzer. IR spectra were recorded with a Nicolet Nexus 912 AO446 spectrophotometer (KBr pellet), 4000–400 cm-1 region. Ultraviolet absorption spectra of these powder samples (5 × 10-4 mol · L-1 THF solution) were recorded with an Agilent 8453 spectrophotometer. Fluorescence–excitation and emission spectra were obtained on a Perkin-Elmer LS-55 spectrophotometer. Luminescent lifetimes for hybrid materials were obtained with an Edinburgh Instruments FLS 920 phosphorimeter using a 450 W xenon lamp as an excitation source (pulse width, 3 µs). The microstructures were checked by scanning electronic microscopy (SEM, Philips XL-30).

Results and Discussion Figure 1 gives the IR spectra of DBM (A) and DBM-Si (B). From A to B, we could observe that the vibration of -CH2- at

3060 cm-1 (A) was taken place by a strong broadband located at around 2921 cm-1 (B) which designates the three methylene groups of 3-(triethoxysilyl)-propyl isocyanate. Furthermore, the bands centered at 3386 cm-1 correspond to the stretching vibration of grafted -NH groups. In addition, the peaks at 1562 and 1509 cm-1 were ascribed to the bending vibration of N-H groups corresponding to the “amide II” mode. Several new peaks at 1778 and 1704 cm-1 are due to the C)O absorptions of TESPIC, indicating that hydrogen bonded C)O groups were largely connected to each other in terms of Carlos’s research22 about the grafted “amide I” mode. In conclusion, TESPIC was successfully grafted onto the CH2 groups of the coupling agent. Besides this, NO3also participates in the coordination to Ln3+, showing two absorption bands for stretching vibrations of N)O (1472.70, 1468.28 cm-1) and NO2 (1288.65, 1282.16 cm-1), which suggest that the coordination between Ln3+ ions and NO3- ions belong to a bidentate chelation effect. We measured the phosphorescence spectra of Gd-DBM (A) and GdDBM-Si (B) using the fact that Gd3+ ion does not have energy levels in the visible part of the electromagnetic spectrum and therefore is an ion suitable to check the excited energy levels of the ligands (Figure 2). Because the shortest wavelength of the phosphorescence emission band at 492 nm corresponded to be the 0–0 transition of Gd-DBM, the lowest triplet state energy

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Figure 1. IR spectra of DBM (A) and DBM-Si (B).

Figure 2. Phosphorescence spectra of DBM and DBM-Si at 77 K.

of DBM can be determined to be 20320 cm-1, which is in agreement with the data from ref 28. The low temperature phosphorescence spectrum of Gd-DBM-Si shows a broadband of maximum peak at 500 nm without clear splits as Gd-DBM. The triplet state energy of DBM-Si can also be estimated to be around 20000 cm-1 caused by the modification with TESPIC. According to Sato’s result and energy transfer mechanism,28–31 an optimal energy difference should exist between the triplet position of modified bridging ligands such as DBM-Si and the emissive energy level Ln3+; the larger and the smaller ∆E (triplet energy levels of DBM-Si to Ln3+) will decrease the luminescence properties of rare earth complexes. On the basis of our previous work,32 we considered that DBM-Si will be suitable for the emission levels of Eu3+ ions. Figure 3 shows the excitation and emission spectra of Eu covalently bonded silica hybrid material. For excitation spec-

Figure 3. Excitation and emission spectra of Eu-containing silicate hybrid material (Hybrid I).

trum, a broad absorption band is in the range of 200-500 nm, suggesting the effective absorption of the Eu-DBM system. Among the strong band at around the 250–450 nm range that can be attributed to the charge transfer state of Eu-O formed between Eu3+ and ligand dibenzoylmethane unit, some weak narrow lines located at 400 nm are probably due to the transitions within 4f6 configurations of Eu3+. In the region of between 200 and 250 nm, the absorptions of Si-O host lattice can also be observed. Anyway, it was clearly proven that efficient energy transfer from DBM-Si to Eu(III) has occurred. As far as excitation spectra are concerned, Hybrid I excited by 412 nm exhibit characteristic emissions of Eu ions. Five narrow emission peaks centered at 582, 590, 612, 649, and 699 nm, assigned to 5D0 f 7F0, 5D0 f 7F1, 5D0 f 7F2, 5D0 f 7F3, and

Hybrids of Bridged Eu(III)-β-Diketone Chelates

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Figure 4. Excitation and emission spectra of Eu-containing titanate hybrid material (Hybrid II). Table 1. The Luminescence Efficiency and Lifetimes of the Solid Covalent Hybrids systems -1 a

ν00 (cm ) ν01 (cm-1)a ν02 (cm-1)a ν03 (cm-1)a ν04 (cm-1)a I00b I01b I02b I03b I04b I02/I01b,c A00 (s-1) A01 (s-1) A02 (s-1) A03 (s-1) A04 (s-1) τ (ms)d Arad (s-1) τexp-1 (s-1) Anrad (s-1) η (%)

complex

Hybrid I

Hybrid II

17182 16949 16340 15408 14306 4.2 26.9 301 1.15 0.48 11.2 7.76 50 580.3 2.35 1.06 0.69 641.5 1449.0 807.5 44.3

17182 16949 16340 15408 14306 3.6 18.9 232 0.90 0.56 12.3 9.55 50 637.9 2.62 1.33 1.04 701.4 961.5 260.1 73.0

17197 16949 16340 15408 14306 1.4 8.0 46.5 0.21 0.17 5.81 8.87 50 286.3 1.44 1.26 0.95 347.9 1052.6 704.7 33.0

a The energies of the 5D0 f 7FJ transitions (ν0J). b The integrated intensity of the 5D0 f 7FJ emission curves. c The emission intensity ratio of red/orange. d For 5D0 f 7F2 transition of Eu3+.

5

D0 f 7F4 transitions, respectively. Among the peaks, the emission at 612 nm from the 5D0 f 7F2 induced electronic dipole transition is the strongest, suggesting the chemical environment around Eu(III) ions does not have an inversion center.33,34 Besides, a green emission band at 535 nm occurs, corresponding to the high energy transition of 5D1 f 7F1. Similar to Eu covalently boned silica hybrid materials, the luminescent spectra of Eu covalently boned titanate hybrid material are shown in Figure 4. A wide range excitation spectra cover the ultraviolet to visible blue region, indicating the whole hybrid system possesses the effective absorption. The absorptions for the host lattice of the Si-O inorganic network exists at the short wavelength ultraviolet region in the range of 200-250 nm.35 The most dominant excitation band from 250 to 500 nm can be ascribed to the Eu-O charge transfer band (CTB) derived from Eu-DBM-Si molecular units. Besides this, some weak excitation lines exist that overlap with CTB, which can be attributed to the f-f transition to Eu3+. It is noteworthy that the ligand emission background completely vanished in the emission spectra of

Hybrid II under the excitation of 387 nm, and only the characteristic emissions of Eu ions can be found; 582, 590, 612, 650, and 699 nm, respectively, corresponded to 5D0 f 7 FJ (J ) 0–4) transitions. On the other hand, the profile of the high-energy side of the emission spectrum of Figure 4 (500–550 nm) suggests the presence of a broad weak emission overlapped the Eu3+ lines, which may be related to the hybrid Ti-O host. It is more interesting that the luminescence intensity of Hybrid I is higher than that of Hybrid II, and we predicted that two reasons may contribute to the improved luminescence. One is that silica backbone brought a more favorable asymmetry microenvironment for polarization of Eu(III) ions than titanates. The additional reason is that the too fast speed of hydrolysis and polycondensation for Ti(C4H9O)4 triggers phase separation between organic and inorganic components to a certain degree, thus increasing the possibility of radiativeless processes. We regard that the lowest triplet state of the ligands (given by phosphorescence spectra, 20 000 cm-1) is higher than the emission level of 5D1 of Eu(III) ions (535 nm, 18 691 cm-1), and energy mostly transfers from the triplet state of DBM-Si to the 5D1 level followed by the 5D0 levels of Eu(III). Further, we compared the red/orange intensity ratio of Eu3+ for the two covalently bonded hybrids and the Eu-DBM complex; it can be found that the red/orange intensity ratio of Hybrid I are much higher than that of Hybrid II, suggesting that the red emission property for silica hybrids is more excellent than the titanate one and the Si-O network is more suitable for the red luminescence of Eu3+. To further investigate the luminescence efficiency and lifetimes of the covalent hybrids, we also synthesized Eu-DBM binary complex and compared it with Hybrid I. Their luminescent decay curves (figures not shown) were all single exponential (ln(S(t)/S0) ) -k1t ) -t/τ), indicating that all the Eu3+ ions occupy the same average coordination environment. The respective lifetimes were given in the following Table 1. It was found that Hybrids I and II present longer luminescent lifetimes than the pure europium complex of DBM, which indicates that a rigid Si-O and Ti-O polymeric network restricts the vibration of DBM ligands considerably and so improves the corresponding luminescence stability. Furthermore, we determined the emission quantum efficiencies of the 5D0 europium ion excited-state for Eu3+ hybrids on the basis of the emission spectra and lifetimes of the 5D0 emitting level using the four main equation according to refs 36- 43. The detailed principle and method was adopted as ref 44, and the data are shown in Table 1.

A0J ) A01(I0J/I01)(υ01/υ0J)

Arad )

∑ A0J

(1)

) A00 + A01 + A02 + A03 + A04 (2)

τ ) Arad-1 + Anrad-1

(3)

η ) Arad ⁄ (Arad + Anrad)

(4)

Here A0J is the experimental coefficient of spontaneous emissions (J ) 0, 1, 2, 3, 4) for the branching ratio for the 5D0 f 7F5,6 transitions can be neglected as they both are not detected experimentally, whose influence can be ignored in the depopulation of the 5D0 excited state.36–43 Among A01 is Einstein’s coefficient of spontaneous emission between the 5D0 and 7F1 energy levels. In a vacuum, A01 has a value of 14.65 s-1, and when an average index of refraction n equal to 1.506 was

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considered, the value of A01 can be determined to be approximately 50 s-1 (A01 ) n3A01 (vacuum))41–43 and as a reference to calculate the value of other A0J. I is the emission intensity and can be taken as the integrated intensity of the 5D0 f 7FJ emission bands.36,37 ν0J refers to the energy barycenter of each transition 0-J (J ) 0, 1, 2, 3, 4) and can be determined from the emission bands of Eu3+’s 5D0 f 7FJ emission transitions. Arad and Anrad mean the radiative transition rate and nonradiative transition rate, respectively, and Arad can be determined from the summation of A0J (eq 2). And then the luminescence quantum efficiency can be calculated from the luminescent lifetimes, radiative and nonradiative transition rates. From the data of Table 1, Hybrid I possesses a much higher luminescence quantum efficiency (73.0%) than Hybrid II (33.0%) and even higher than the pure europium complex of DBM (44.3%). This is in agreement with the result from the luminescent intensities and lifetimes. The covalently bonded inorganic Si-O framework is a benefit for the luminescent properties of Eu3+ by increasing the ratio of radiative transition for the larger red/orange ratio and longer lifetimes. The scanning electron images for Hybrid I (panel A), Hybrid II (panel B), and Eu-DBM-doped TiO2 material (panel C) are given in Figure 5. The silica host covalently bonded composites (panel A) seem to be very homogeneous and porous caused primarily by loss of solvent and alcohols. We could not observe distinct grain distributions in regard to the specific interesting DBM-Si ligands acting as a double functional bridge between inorganic matrices and organic components and combining both together. In Eu-titanates covalent hybrids (panel B), the uniform morphology also occurred, whereas the common doped titanates materials (panel C) showed obvious phase separation because the organic moieties were generally dispersed into the surface of host lattice without powerful chemical bond combinations. Conclusions In summary, we designed two molecular-based hybrid materials. Dibenzoylmethane has been modified with 3-(triethoxysilyl)-propyl isocyanate (TESPIC) and acted as a crosslinking intermediate, which are double roles. On the one hand, it can coordinate to lanthanide ions through carbonyl groups; on the other hand, the hydrolysis and polycondensation reactions among triethoxysilyl of DBM-Si and TEOS support the formation of Si-O-Si network structures for the same ethoxy group among them. Moreover, strong red luminescent systems with novel and unique microstructures were achieved. In particular, Eu-DBM-Si exhibits the high luminescent quantum efficiency for the large red/orange ratio and long lifetime. It is rather meaningful that β-diketones could be initially introduced in a matrix through covalent bonds that may heavily affect the photo or thermal stability of Eu-β-diketones complexes. In this case, it should be emphasized that Eu(III) ions are well shielded from their chemical environment, and the drawback of limited solubility of the lanthanide complexes could be largely increased when the complexes are covalently linked to the matrices. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20671072).

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