Structural, Thermal, and Fluorescence Properties of Eu(DBM)3Phenx

Sep 27, 2010 - Corresponding author. Phone/Fax: 91-542-230-7308; 91-542-2369889. E-mail: [email protected], [email protected]., †. Institute of ...
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Structural, Thermal, and Fluorescence Properties of Eu(DBM)3Phenx Complex Doped in PMMA A. K. Singh,† S. K. Singh,‡ H. Mishra,*,§ R. Prakash,† and S. B. Rai*,‡ School of Materials Science and Technology, Institute of Technology, Banaras Hindu UniVersity, Varanasi 221005, India, Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu UniVersity, Varanasi 221005, India, and Department of Physics, Mahila Maha Vidyalaya, Banaras Hindu UniVersity, Varanasi 221005, India ReceiVed: June 1, 2010; ReVised Manuscript ReceiVed: August 14, 2010

Tris (dibenzoylmethanido) (o-phenanthroline)-europium(III) complexes [Eu(DBM)3(Phen)x] have been synthesized, and their structural, thermal, and fluorescence properties have been investigated both as an independent complex and also after embedding it in poly(methyl methacrylate) (PMMA) polymer matrix. X-ray analysis reveals an unusual crystalline to amorphous transformation in structure of the Eu(DBM)3Phenx complexes when x exceeds 1.0 (mole %). Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) analysis depict the interaction between the Eu(DBM)3Phenx complex and the PMMA matrix. The fluorescence intensity of Eu3+ ion in the Eu(DBM)3Phen1 · 5 complex is an order of magnitude larger compared to the Eu(DBM)3 · 2H2O complex. In addition to the transition 5D0 f 7F2, which has a rise time of 83 µs, transitions from the 5D1 and 5D2 levels are also observed, along with several ligand field splittings in the Eu(DBM)3Phen1 · 5 complex. Appearance of a rise time (∼0.76 ns) in the decay of the fluorescence from DBM in the presence of Phen and a simultaneous large decrease in the fluorescence intensity (both the singlet and triplet emission) of DBM along with a decrease in fluorescence decay time, in the presence of Eu3+, indicates a cascading excitation energy transfer from Phen to DBM and finally to the 5D0 state of Eu3+ ion in Eu(DBM)3Phen1.5 through short-range excitation energy transfer. A systematic and significant improvement in the thermal and fluorescence properties of the complex is observed with an increase in the mole concentration of Phen. The maximum fluorescence is observed for Eu(DBM)3Phen1 · 5, which is a new composition as compared with the usual composition reported earlier. This finding indicates the possibility for the development of brighter luminescent lanthanide photonic materials. 1. Introduction Study of fluorescence enhancement of lanthanide ions in the presence of aromatic molecules is of special interest because lanthanide ion complexes with aromatic molecules are frequently used as structural and functional probes in biological systems and have technological applications in lasers, sensors, electroluminescence displays, light emitting diodes, etc.1-16 Free lanthanide ions have a low absorbance in rather narrow bands, and their direct excitation yields only a weak luminescence. This luminescence can be strengthened if a stable complex is formed with a suitable organic ligand, which can transfer its excitation energy to the rare earth ion. The emission intensity in these complexes is strongly dependent on the efficiency of the ligand absorption, ligand-to-lanthanide energy transfer, and lanthanide emission.6 The incorporation of rare earth complexes in a polymer matrix has been found to be of enormous interest, because such a composite material possesses advantages of the luminescence characteristics of rare earth ions with excellent mechanical properties, lightweight, good transparency, low temperature processability, etc.4-6 PMMA is one of the best studied polymer matrixes for the above purpose, as it has a low optical absor* Corresponding author. Phone/Fax: 91-542-230-7308; 91-542-2369889. E-mail: [email protected], [email protected]. † Institute of Technology. ‡ Department of Physics. § Mahila Maha Vidyalaya. Present address: Institute of Fluorescence, UMBI, University of Maryland, Baltimore, MD 21201.

bance, has a low cost, is easily synthesized, and its refractive index can be tailored with molecular weight. Lanthanide β-diketonate complexes have been most extensively investigated for their luminescence properties.10-13 There are mainly three types of lanthanide(III) β-diketonate complexes named as tris complexes, Lewis base adducts of the tris complexes, and tetrakis complexes. Complexes of europium in its 3+ oxidation state with β-diketone ligands have been extensively studied systems in PMMA matrix,4-6 since the intense hypersensitive transition of Eu3+ (5D0 f 7F2) at 612 nm lies in the low loss window region of the PMMA.4 The tris (β-diketonates) have three β-diketonate ligands for each rareearth ion, and they can be represented by the general formula [R(β-diketonate)3]. As the coordination sphere of the rare-earth ion is unsaturated in these six-coordinated complexes, the rareearth ion also needs a zero charged ligand (Lewis bases) such as triophenylphosphine oxide (topo) or 1,10-phenanthroline (Phen), to coordinate for structural stability.10 These additional ligands can reduce the nonradiative decay and thus strongly enhance the fluorescence intensity of the complexes.10 In addition to this, it may also improve the structural, thermal, and optical properties of the materials. However, the precise mechanism for this improvement is not yet well understood. Sager et al.17 investigated the influence of different substituents on the β-diketonate ligands on the intramolecular energy transfer. Although the europium(III) β-diketonate complexes often show intense luminescence, the intensities are strongly

10.1021/jp1050063  2010 American Chemical Society Published on Web 09/27/2010

Properties of Eu(DBM)3Phenx Complex Doped in PMMA dependent on the type of β-diketone and on the nature of the complex. Of course, the luminescence intensity is dpendent on the quantum yield of the luminescent transition as well as on the amount of incident radiation, absorbed by the complex, its temperature, etc. Further, the efficiency of energy transfer from the singlet and triplet states of the organic ligand to the lanthanide ion would be proportional to the overlap between the ligand emission and lanthanide absorption.18-20 The luminescence intensity in such systems is related to the anisotropy around the lanthanide(III) ion. The present work is mainly dedicated to unraveling the properties of the Eu(DBM)3Phenx complex doped in PMMA with different mole ratios of Phen (x ) 0.0, 0.5, 1.0, and 1.5 mol %). A systematic and significant improvement in the thermal stability and fluorescence properties is observed with increasing mole concentration of Phen. The maximum fluorescence is observed for Eu(DBM)3Phen1 · 5, which is a new composition as compared to the usual composition reported earlier. The intensification of the luminescence is explained as involving a cascading of excitation energy from the ligand (donor) to the Eu3+ (acceptor) ion. In the first step, the incident light excites the singlet state of the ligand molecule (Phen, DBM) which decays quickly (in picoseconds) to the triplet state. In a second step, a dipole-quadrupole interaction (Dexter exchange interaction) transfers the excitation energy of these ligands (both from triplet and singlet state) to the Eu3+ at the rate 109-104 s-1. Since the complex is trapped in the free volume of the polymer, such systems can serve as good luminescent solar collecting (LSC) materials.21 2. Experimental Section 2.1. Materials. Europium(III) oxide (99.95%, Aldrich), dibenzoylmethane (98%, SRL), 1,10-phenanthroline (99.5%, Loba Chemie Pvt. Ltd.), ethanol (Changshu Yangyuan chemical, China), sodium hydroxide (Qualigens, 99%), PMMA (Mw 70000, Himedia), hydrochloric acid, and chloroform (Merck, 99.9%) have been used. 2.2. Sample Preparation. Europium chloride was prepared by dissolving the europium oxide in hydrochloric acid. The Eu(DBM)3Phenx complexes were prepared by the method reported in ref 22 with a little modification. Dibenzoylmethane (6 mmol), 1,10-phenanthroline (0-4 mmol depending on composition), and 8 mL of 1N sodium hydroxide were dissolved well in 25 mL of ethanol. To this solution, 2 mmol of EuCl3 · 6H2O in 10 mL of distilled water was added slowly while constant stirring of the mixture was maintained for better reaction. During this process, a light yellow colored precipitate of Eu(DBM)3Phenx complexes was formed. The above complexes were purified by recrystallization in a mixture of ethanol and dichloromethane. Samples were dried at ∼40 °C temperature in a vacuum oven for over 24 h. In the complex, the positively charged Eu3+ ion makes a covalent bond with the negatively charged DBM central ligand and the zero charged ligand 1,10-phenanthroline attached with europium through weak interaction as reported in the literature.2 In order to verify the purity of various synthesized complexes, we have analyzed the carbon and nitrogen content in the complexes with pure ligand molecules. Since nitrogen is associated with the Phen molecule, determining the nitrogen content in the complexes is an essential probe for identifying the various newly synthesized complexes. The results of elemental analysis are given in Table 1. The elemental analysis data of ligand molecules (DBM and Phen) shows that they have adequate purity. In the case of Eu(DBM)3Phen0.5 · 0.5H2O and Eu(DBM)3Phen1.0, it is observed

J. Phys. Chem. B, Vol. 114, No. 41, 2010 13043 TABLE 1: Elemental Analysis Data of the Eu(DBM)3Phenx Complex experimental sample

C (%)

DBM (C15H12O2) Phen (monohydrated) (C12H10N2O) Eu(DBM)3 · 2H2O (EuC45H40O8) Eu(DBM)3Phen0.5 · 0.5H2O (EuC51H41O6.5N) Eu(DBM)3Phen (EuC57H44O6N2) Eu(DBM)3Phen1 · 5 (EuC63H48O6N3)

80.34 72.71

N (%)

calculated C (%)

N (%)

80.37 14.13

62.79

72.94

14.40

62.63

66.30

1.51

66.54

1.42

68.12

2.79

67.26

2.51

69.30

3.83

68.50

3.01

SCHEME 1: Molecular Structure of the Eu(Phen)3 · 2H2O Complex

SCHEME 2: Molecular Structure of the Eu(DBM)3 · 2H2O Complex

SCHEME 3: Molecular Structure of the Eu(DBM)3Phen1.0 Complex

that it has slightly less nitrogen content than estimated by taking account of eight coordination for europium. This suggests that there is still some vacant coordination of europium in the complex which may be fulfilled by the water molecules. Similar findings were also reported by Ahmed et al.23 for the Eu(DBM)3Phen1.0 complex. Further, our experimental elemental analysis data shows that in the case of Eu(DBM)3Phen1.5 the association of nitrogen content with europium is slightly higher than required for eight coordination of europium in Eu(DBM)3Phen1.5. This may be due to some excess Phen association with the europium, as the calculated coordination for Eu(DBM)3Phen1.0 is reported as 8.13;24 also, the coordination of europium is not very strict and varies with the various environments.24,25 Schemes 1, 2, and 3 show the Eu(Phen)3 · 2H2O, Eu(DBM)3 · 2H2O, and Eu(DBM)3Phen1.0 complexes, respectively. These synthesized complexes were used further for various types of studies. The doped PMMA films were prepared simply by blending the Eu(DBM)3Phenx complex (0.5 wt %) in a fixed amount of PMMA using chloroform as solvent. After the blending, equal amounts of the solutions were kept in

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the covered Petri dishes of the same dimension carefully to keep the thickness for all the complexes approximately the same. After air drying, the samples were kept in a vacuum oven at ∼55 °C overnight to remove the trapped solvent molecules present inside the sample. As-prepared films were further used for fluorescence and time domain analysis. 2.3. Instrumentation. X-ray diffraction (XRD) patterns were recorded using an 18 kW Cu rotating anode based (Rigaku, Japan) powder diffractometer operating in the Bragg-Brantano geometry and fitted with a graphite monochromator in the diffracted beam. Data were acquired from 2θ ) 5° to 60° at a scanning rate of 4°/min at 6 kW energy. Thermal analysis of pure Eu(DBM)3Phenx complexes and the doped PMMA film was carried out using a thermogravimetric analyzer (TGA) (Mettler/DSC1) and differential scanning calorimetry (DSC) (Mettler-Toledo 823) under a nitrogen atmosphere at heating rates of 20 and 10 °C/min, respectively. The UV/vis absorption spectra of different samples were measured in the 200-800 nm range (Perkin-Elmer, Lambda 25), and the infrared (IR) absorption spectral measurements were carried out in the 400-4000 cm-1 region using a Perkin-Elmer, RXI double beam Fourier transform infrared (FTIR) spectrophotometer with a resolution of 2 cm-1. The photoluminescence studies were done using a 355 nm wavelength from a 7 ns pulsed Nd:YAG laser (Innolas, Spitlight 600, Germany). The fluorescence spectra were recorded in the 400-900 nm region using a fluorescence spectrometer (QE65000, Ocean Optics, USA) and a Flouromax-4 (Horiba Jobin Yvon) spectrophotometer. Fluorescence decays (of donor) were measured using a fluorescence lifetime system Flouromax4, employing the time-correlated single photon counting (TCSPC) technique, with a TBX picosecond detection module. The excitation source was a pulsed LED source of wavelength 282 and 372 nm having a maximum repetition rate of 1.0 MHz and a pulse duration of 1.1 ns (fwhm). The intensity decays were analyzed by decay analysis software (DAS), version 6.4. The fluorescence decay time measurement of (acceptor) Eu3+ at 5D0 f 7F2 transition (612 nm) was carried out at room temperature using the 355 nm wavelength of a laser (data were acquired using an oscilloscope (analog digital scope-HM1507) with software SP107). The decay time was determined using the nonlinear least-squares fit method. Care was taken in data analysis to differentiate between the monoexponential and biexponential fits by judging the χ2 values, standard deviations, and weighted residuals. Intensity decay curves so obtained were fitted to the exponential

I(τ, t) ) R1 exp(-t/τ1) - R2 exp(-t/τ2) where τ1 and τ2 are the shorter and longer lifetime components, respectively, and R1 and R2 are the corresponding amplitudes. The appearance of negative amplitude is indicative of a rise time due to some excited state processes. 3. Results and Discussion 3.1. XRD Measurement Analysis. Powder XRD spectra of all four rare earth complexes Eu(DBM)3 · 2H2O, Eu(DBM)3Phen0.5 · 0.5H2O, Eu(DBM)3Phen1.0, and Eu(DBM)3Phen1.5 along with DBM, Phen, and EuCl3 · 6H2O were carried out as given in Figure 1 to understand the interaction among the three components, namely, EuCl3 · 6H2O, DBM, and Phen, and to know their crystalline behavior with different mole ratios of Phen. The XRD pattern of Eu(DBM)3 · 2H2O has been indexed with the reference of EuCl3 · 6H2O and DBM. In Figure 1(iv), the

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Figure 1. Powder XRD patterns of (i) DBM, (ii) Phen, (iii) EuCl3 · 6H2O, (iv) Eu(DBM)3 · 2H2O, (v) Eu(DBM)3Phen0.5 · 0.5H2O, (vi) Eu(DBM)3Phen1.0, and (vii) Eu(DBM)3Phen1 · 5 complexes.

symbols E, D, P, and N refer to the peaks of EuCl3 · 6H2O, DBM, Phen, and new unindexed peaks, respectively. From the XRD patterns, it is clear that all three constituent phases (i-iii) of the complexes are well crystalline in nature which is also supported by the DSC data given later. The peak positions of the Eu(DBM)3Phen1.0 well match with the reported one in the literature2 except for a few new peaks at higher 2θ values. The characteristic peaks of the complex lie at 2θ ∼ 31.7, 45.4, and 56.4° positions in which the first two are mainly due to Eu and the later one is the new peak which may be generated due to increased cell size. The uppermost XRD pattern in Figure 1 corresponds to the Eu(DBM)3Phen1.5, which is surprisingly amorphous in nature with the three characteristic crystalline peaks of the complex. This is probably due to excess Phen associated with the Eu3+ (the excess Phen association in the Eu(DBM)3Phen1 · 5 complex was also observed by our elemental analysis, as discussed in the Experimental Section) which may change its crystalline behavior. This Phen composition in the Eu(DBM)3Phenx complex depicts a change in crystalline structure, which strongly influences the thermal and optical properties of the complex and doped PMMA films. XRD patterns of pure PMMA and Eu(DBM)3Phenx complex doped PMMA film (Figure S1, Supporting Information) shows only amorphous peaks of PMMA, suggesting that the complex molecule is homogeneously distributed in the PMMA matrix and the crystalline peaks of the complex molecule are not detectable due to their lower concentration. 3.2. Thermal Measurement Analysis. To study the thermal behavior of the pure and doped complexes, DSC and TGA

Properties of Eu(DBM)3Phenx Complex Doped in PMMA

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Figure 3. TGA thermograms of (i) Eu(Phen)3 · 2H2O, (ii) Eu(DBM)3 · 2H2O, (iii) Eu(DBM)3Phen1.0, and (iv) Eu(DBM)3Phen1 · 5 complexes. Inset I1 shows TGA thermograms of DBM (solid line) and Phen (dashed line), and inset I2 shows TGA thermograms of PMMA beads (black), PMMA film (red), and PMMA + 0.5% (wt/wt) Eu(DBM)3Phen1.5.

Figure 2. DSC thermograms of rare earth complexes (i) Eu(DBM)3 · 2H2O, (ii) Eu(DBM)3Phen0.5 · 0.5H2O, (iii) Eu(DBM)3Phen1.0, and (iv) Eu(DBM)3Phen1 · 5 after quenching at 200 °C (the inset shows the DSC thermograms of pure DBM and Phen).

measurements have been carried out. In order to check the specific interaction of various constituents in the complex, a second DSC run was performed. Before the second DSC runs, the samples were heated up to 200 °C and then quenched to room temperature. The DSC thermograms of the rare earth complexes (i) Eu(DBM)3 · 2H2O, (ii) Eu(DBM)3Phen0.5 · 0.5H2O, (iii) Eu(DBM)3Phen1.0, and (iv) Eu(DBM)3Phen1.5 are shown in Figure 2. The inset in Figure 2 shows sharp melting peaks in the DSC thermograms at 71 and 122 °C for DBM and Phen, respectively, indicating their crystalline nature. It is observed that as the mole ratio of Phen in the complex is increased the glass transition temperature is also increased systematically, indicating the specific interaction in the complex. This is due to the higher transition temperature of Phen. Further DSC thermograms of different rare earth complexes doped in PMMA show only the glass transition temperature of PMMA (no signature of the glass transition temperature of the complex is observed probably due to its lower concentration) which decreases up to ∼4 °C from the pristine PMMA, indicating interaction between the complex molecules and PMMA host (Figure S2, Supporting Information). Thermogravimetric analysis of Phen, DBM, PMMA, Eu(Phen)3 · 2H2O, Eu(DBM)3 · 2H2O, Eu(DBM)3Phen1.0, and Eu(DBM)3Phen1 · 5 has been carried out (see Figure 3) to explore their degradation behavior as well as water content in complexes. Inset I1 to Figure 3 shows that the DBM molecule is anhydrous, whereas Phen is monohydrated (shows ∼9% wt loss at ∼140 °C). The TGA thermogram of Eu(DBM)3 · 2H2O and Eu(Phen)3 · 2H2O shows ∼4 and ∼6% wt loss at 140 °C, which is attributed to the disassociation of two water molecules from Eu(DBM)3 · 2H2O and Eu(Phen)3 · 2H2O. It is remarkable that the thermal stability of Eu(DBM)3Phen1.0 increases significantly as compared to Eu(DBM)3 · 2H2O. Further, improvement in

degradation temperature was observed for Eu(DBM)3Phen1.5 with respect to Eu(DBM)3Phen1.0. This suggests that the Phen content increases in the complex as the thermal stability increases. Inset I2 to Figure 3 shows the TGA thermograms for PMMA (in bead and in film prepared by solution casting). It is observed that the PMMA bead degrades at ∼345 °C (for 5% wt loss); on the other hand, solution cast PMMA shows ∼10% wt loss at ∼165 °C, which is attributed to the entrapped solvent removal from the polymer matrix. In the case of complexes doped in polymer, we observed the same degradation behavior as in solution cast PMMA. Thus, TGA studies infer that the Eu(DBM)3 · 2H2O and Eu(Phen)3 · 2H2O complexes are dihydrated. Further, for doped polymer films, we observed only the intrinsic features of the PMMA host which was expected for 0.5% complex loading in the polymer. 3.3. FTIR Measurement Analysis. FTIR spectra of pure Eu(DBM)3Phenx (x ) 0.0, 0.5, 1.0, and 1.5) complexes and doped in PMMA have been recorded in the 400-4000 cm-1 region to understand the extent of miscibility of the complexes in PMMA. Figure S3a in the Supporting Information shows the FTIR spectra of the pure Eu(DBM)3Phen1.0 and pure Eu(DBM)3Phen1.5 complexes, while Figure S3b shows the spectra for pure PMMA and Eu(DBM)3Phen1.5 (0.5 wt %) complex doped in PMMA. The FTIR spectra of the Eu(DBM)3Phenx (x ) 1.0 and 1.5) show no significant dependence on the concentration variation of Phen except for a decrease in the intensity of different peaks. This decrease in intensity of vibrational bands indicates the decrease in nonradiative transition. In Figure S3b of the Supporting Information, the characteristic bands of pure PMMA, i.e., C-O-C symmetric stretching, O-CH3 deformation, CH3 asymmetric stretch, and O-CH2 asymmetric stretch, occur at 990, 1384, 2952, and 2994 cm-1, respectively. PMMA which is a saturated polymeric ester gives rise to an intense and sharp peak at 1733 cm-1 due to a carbonyl group. Two things can be noticed from these observations: First, all the PMMA peaks are shifted toward the lower wavenumber side, showing the influence of the ligand molecules on the microenvironment of the PMMA host and the mutual interaction between them. Second, the absorption peaks become sharper after the addition of the complex. This signifies the alignment of a larger number of molecules having the same

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Figure 4. UV/vis absorption spectra of (i) DBM, (ii) Eu(DBM)3 · 2H2O, (iii) Eu(DBM)3Phen1.0, (iv) Eu(DBM)3Phen1.5, (v) Phen, and (vi) Eu (0.5% wt/wt) doped in PMMA film (film thickness ∼20 µm) (the inset shows the absorption spectra of these compounds in 1.2 × 10-7 M chloroform solution).

orientation. This means that, after the addition of the complex in PMMA host, the molecules are more ordered. 3.4. Steady State Optical Measurement Analysis. (i) UV/ Visible Absorption Measurements. UV/vis absorption spectra of (i) DBM, (ii) Eu(DBM)3 · 2H2O, (iii) Eu(DBM)3Phen1.0, (iv) Eu(DBM)3Phen1.5, (v) Phen, and (vi) Eu (0.5% wt/wt) doped in PMMA (film thickness ∼20 µm) and in 1.2 × 10-7 M chloroform solution were measured and are shown in Figure 4 and its inset, respectively. DBM in chloroform shows two electronic transitions with absorption maxima at 267 and 342 nm having an optical density of 0.30 and 0.87, respectively, which corresponds to S0 f S2 and S0 f S1 transitions, respectively, which are of π f π* in nature.7 On the other hand, when DBM compound is doped in PMMA host, it shows a structural absorption spectrum. This is due to the rigidity of the polymer matrix which had confined the movement of the molecule in the matrix. Further, Phen in PMMA host shows a peak at 266 nm and a hump at 276 nm. The absorption spectrum of the europium-DBM complex, i.e., Eu(DBM)3 · 2H2O in chloroform, shows a slight red shift in the 342 nm band of DBM. However, in PMMA, it shows a significant red shift of 6 nm. Interestingly, the optical density of the DBM absorption band reduces drastically (1.06 to 0.38 in polymer and 0.87 to 0.49 in solution) in the presence of Eu3+, suggesting a new complex molecule formation in the ground state. On addition of Phen in Eu(DBM)3 · 2H2O, the absorption band corresponding to the S0 f S2 transition of Eu(DBM)3Phenx complexes shows a red shift of 2-3 nm in the case of polymer host and 5-6 nm in the case of solution. This is due to the association of Phen in the complex molecule whose absorption band lies toward higher wavelength. The optical density corresponding to the S0 f S1 transition of DBM decreases with increasing mole concentration of Phen in the complex molecule (0.87 to 0.36, 0.87 to 0.23 and 1.06 to 0.25, 1.06 to 0.15 for x ) 1.0 and x ) 1.5 for solution and polymer host, respectively). In addition, 3-4 nm red shift is observed in this band; the red shift in the S0 f S1 band in these complex molecule formations in solution and further red shift and enhancement of structural

Figure 5. Fluorescence spectra of (i) Phen, (ii) DBM, (iii) Eu(DBM)3 · 2H2O, (iv) Eu(DBM)3Phen1.0, (v) Eu(DBM)3Phen1 · 5, and (vi) Eu(Phen)3 · 2H2O doped in PMMA (0.5% wt/wt) on excitation with λex ) 266 nm (a) donor and (b) acceptor doped in PMMA (insets I1, I2, and I3 show the fluorescence spectra obtained by pulse laser excitation).

spectral features in PMMA indicates the greater stabilization of the rigid complex compound, which reduces the loss of energy through vibrational modes. (ii) Fluorescence Emission and Excitation Measurements. A systematic investigation of different constituent ions/ molecules and different complexes doped in PMMA has been carried out to understand the mechanism of fluorescence enhancement in detail and is described as follows. EuCl3 · 6H2O (concentration 0.5 wt %) doped PMMA film shows very weak emission (negligible) from Eu3+ ion at 612 nm on 266 and 355 nm laser pulse excitation. This is due to weak absorption of the rare earth ions. Pure Phen doped in PMMA film (thickness ∼20 µm) with a concentration of 0.5 wt % when excited by the corresponding absorption maximum at 266 nm gives a Stokes shifted intense blue emission band at 375 nm, as shown in Figure 5(i). However, when the

Properties of Eu(DBM)3Phenx Complex Doped in PMMA Eu(Phen)3 · 2H2O (0.5 wt %) complex doped in PMMA (thickness ∼20 µm) is excited with 266 nm, the characteristic emission band of Phen with reduced intensity and sharpness shifted toward the higher wavelength side is observed (Figure 5(vi)). Apart from this, by zooming the spectrum, one can see a kink at 612 nm (same in intensity as in pure Eu3+). The reduced fluorescence intensity of Phen along with appearance of more structured emission in the presence of Eu3+ ion indicate a stable complex formation of Eu(Phen)3 · 2H2O, but no enhancement in Eu3+ fluorescence indicates that there is no direct excited state energy transfer from the singlet or triplet of Phen to Eu3+. Further, when pure DBM (concentration 0.5 wt %) doped in PMMA film (thickness ∼20 µm) is excited with the corresponding absorption frequency λex ) 266 nm (S0 f S2 transition) or λex ) 355 nm (S0 f S1 transition), it gives only a Stokes shifted (S1 f S0) very weak broad emission band with a peak at 412 nm, as shown in Figure 5(ii). However, when the Eu(DBM)3 · 2H2O (0.5 wt %) complex doped in PMMA (thickness ∼20 µm) is excited by the corresponding absorption band 266 or 355 nm, a fluorescence quenching is observed in DBM emission along with fluorescence enhancement in Eu3+ emission (about 5 times), as shown in Figure 5(iii). This suggests an excited state energy transfer from the singlet state of DBM to europium ions. Further, Eu(DBM)3Phenx (0.5 wt %) complexes doped in PMMA (thickness ∼20 µm) excited with 266, 355, and 380 nm show an enhancement in the fluorescence intensity of europium (∼10 times in Eu(DBM)3Phen1 · 5 with respect to Eu(DBM)3 · 2H2O for all three excitation wavelengths), while the fluorescence intensity corresponding to the DBM and Phen decreases drastically, as shown in Figure 5(iv and v), on excitation with 266 nm (a similar pattern is observed with 355 and 380 nm also). This suggests that Phen plays a significant role with DBM which cause the fluorescence intensity of Eu3+ largely to enhance, although direct energy transfer from Phen to Eu3+ does not take place. All seven bands arising from 5D0 f 7Fj, viz., 5D0 f 7F0, 5D0 f 7F1, 5D0 f 7F2, 5D0 f 7F3, 5D0 f 7F4, 5D0 f 7F5, and 5D0 f 7F6, appear in the present case. We also observe Stark splitting (both with lamp and with laser excitation) in some of these bands due to ligand field. Normally, only a few bands of Eu3+ are seen in the 5D0 f 7Fj transition in glass/crystal or complexes.26-28 Stark splitting is more clear (inset I2 to Figure 5) when excitation is with λ ) 355 nm. Some weak transitions from the higher energy levels (i.e., 5D1 and 5 D2) to the 7Fj are also observed in the emission spectrum on 355 nm laser excitation (see inset I1 in Figure 5). These weak bands also show Stark splitting due to the ligand field.28 The 5 D0 f 7F2 transition at 612 nm is a hypersensitive electric dipole transition, and its intensity dominates over all other transitions. The much stronger intensity of this band indicates that the ligand field surrounding the Eu3+ ion is highly polarizable.26 The 5D0 f 7F1 transition is used as a reference to judge the environmental asymmetry of the rare earth because it is allowed by the magnetic dipole transition and its intensity is independent of the environment, whereas the hypersensitive transition strongly depends on it. The intensity ratio (R) of the 5D0 f 7F2 to 5D0 f 7F1 transition (also called the monochromaticity) is commonly used as a measure of the rare earth site symmetry.5 The lower value of R signifies higher symmetry and vice versa. The R value for Eu3+ doped glass varies from 0.90 to 7.04 in different hosts.28 Liang et al5 found a value of R ∼ 10 of Eu(DBM)3Phen1 doped in PMMA. However, in the present case, we find the value of R ∼ 20 of Eu(DBM)3Phen1.5 doped PMMA which indicates more asymmetry in the vicinity of the Eu3+ ion in

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Figure 6. Excitation spectra of (i) Eu(DBM)3 · 2H2O, (ii) Eu(DBM)3Phen1.0, (iii) Eu(DBM)3Phen1.5, and (iv) EuCl3 · 6H2O (0.5% wt/wt) doped in PMMA (film thickness ∼20 µm).

PMMA matrix when the Phen concentration is increased from 1 to 1.5. This high value of R for the complex suggests that the above developed material has potential to be used as an UV radiation sensor and in other strong fluorescent devices. The maximum fluorescence is observed by 380 nm excitation (shown in Figure S4 in the Supporting Information) for all the complexes doped in PMMA. This corresponds to the maximum excitation for all the complexes doped PMMA film at 380 nm, as is also clear from the excitation spectra shown in Figure 6. Interestingly, we observe a week excitation band for rare earth complexes at 540 nm. Since it is too weak, it is not observed in the absorption spectra. However, this observation is well supported by the emission spectra, as shown in inset I3 of Figure 5. This weak band in the excitation spectra might be n f π* of the complex. Further, maximum fluorescence intensity is observed for x ) 1.5 mol (Figure 5v); the intensity decreases upon increasing the Phen further, suggesting the stable complex formation at x ) 1.5 mol. We have also studied the fluorescence of Eu(DBM)3Phen1 · 5 for different concentrations in PMMA. The maximum fluorescence is observed at 1.0 wt % concentration of the complex. It shows a concentration quenching at higher concentration level. The overlapped absorption and emission spectra of Phen and DBM are shown in Figure 7a and Eu and DBM doped in PMMA (film thickness ∼20 µm) in Figure 7b, respectively. From the figure, it is clear that the emission spectrum of Phen overlaps the absorption spectrum of DBM substantially, while the absorption spectrum of Eu overlaps with the emission spectrum of DBM (Figure 7b), suggesting that, when Phen is excited, it transfers its energy to DBM and consequently DBM transfers its energy to the europium ions through the cascade energy transfer process in the Eu(DBM)3Phenx complex. The critical transfer distance (R0A) between DBM (donor) and Eu3+ (acceptor) is found to be about ∼3.7 A0, while it is ∼10A0 between the Phen (donor) and DBM (acceptor) system, calculated by the relation

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Figure 7. (a) Absorption spectra of (i) Phen and (ii) DBM and emission spectra of (iii) Phen and (iv) DBM (0.5% wt /wt) doped in PMMA (film thickness ∼20 µm). (b) Plot showing the overlap between the absorption spectrum of Eu3+ and emission spectrum of DBM (0.5% wt/wt) doped in PMMA (film thickness ∼20 µm).

R0A6 ) 5.86 × 10-25Ωns-4φD(A0)6

φD is the quantum yield of the donor emission (0.1 for DBM and 0.3 for Phen), “ns” is the refractive index of the medium (PMMA, ns ) 1.49), and Ω is the overlap integral between donor emission and acceptor absorption in units of cm3/M which can be calculated using the equation9

Ω)

∫

FD(ν¯ )εA(ν¯ ) ν¯ 4

dν

Figure 8. Decay curve of the 5D0 f 7F2 excited state of Eu3+ in (a) (i) EuCl3 · 6H2O and (ii) Eu(Phen)3 · 2H2O and (b) (i) Eu(DBM)3 · 2H2O and (ii) Eu(DBM)3Phen1 · 5 (0.5% wt/wt) doped in PMMA (film thickness ∼20 µm).

εA(ν) here is the molar extinction coefficient at wavenumber ν, and FD(ν) is the donor fluorescence intensity (normalized to unit area in the wavenumber scale). Further, the luminescence quantum yield (φ) calculated by the ratio of measured τ and calculated radiative lifetime τrad of Eu3+ ion27 is given in Table 2. As can be seen from the table, φ of Eu3+ increases in the presence of ligands in PMMA. The value of critical transfer distance indicates cascade energy transfers from the excited state of Phen to the excited state of DBM and from the excited triplet state of DBM to the 5D0 state of Eu3+ take place through shortrange interaction. 3.5. Time Domain Measurement Analysis. At this stage, it is clear that Eu3+ (acceptor) is getting sensitized by DBM (donor). Sensitization may occur from the excited singlet or triplet state of DBM or from both. To get dynamical information of fluorescence enhancement, time domain measurements of DBM and Eu3+ ion have been carried out in different complexes, as given in Table 2. The donor and acceptor dynamics are given below: (a) Acceptor Decay Dynamics. In the Eu3+ ion doped PMMA film, the decay is monoexponential at 612 nm and has a characteristic decay time of 227 µs [Figure 8a(i)]. On the other hand, the decay data of the Eu3+ ion at 612 nm in the presence of DBM have been found to fit best with the decay function,

I ) I0[R1 exp(-t/τ1) - R2 exp(-t/τ2)] where τ1 is found to be 672 µs with R1 ) 44.5 and τ2 ) 66 µs with R2 ) -25.2, as shown in Figure 8b(i). Here, R1 is the amplitude and τ1 is the fluorescence decay time of the Eu3+, while R2 and τ2 correspond to the amplitude and the rise time in the Eu3+ decay, respectively. The negative amplitude in this expression is indicative of an excited state reaction (the amplitudes R1 and R2 are generally found to be equal and opposite provided that there is no direct excitation of the acceptor). The appearance of 66 µs rise time in the decay curve of Eu3+ ion (acceptor) in the presence of DBM (donor), again, indicates that the nonradiative energy transfer from DBM to Eu3+ ion is from the triplet state. The rate of energy transfer from the triplet state of donor to acceptor is found to be 15.1 × 103 s-1. Energy transfer from the singlet state should show a rise time of nanoseconds (could not be noticed in this long time scale in the present setup). Nie et al.29 reported the decay time of functionalized DBM (with carbazol) and its europium

TABLE 2: Decay Data of Eu3+ in Different Complexes Doped in PMMA sample

τ1 (µs)

R1

τ2 (µs)

R2

φ

Eu(DBM)3Phen1 · 5/PMMA Eu(DBM)3Phen1.0/PMMA Eu(DBM)3 · 2H2O/PMMA Eu (Phen)3 · 2H2O/PMMA EuCl3 · 6H2O/PMMA

742 ( 4 727 ( 4 672 ( 4 290 ( 4 227 ( 3

47.40 ( 0.20 55.80 ( 0.20 44.50 ( 0.20 4.25 ( 0.02 0.96 ( 0.01

83 ( 2.0 78 ( 2.0 66 ( 2.0

-27.0 ( 0.2 -33.3 ( 0.3 -25.2 ( 0.3

0.47 0.45 0.42 0.18 0.14

Properties of Eu(DBM)3Phenx Complex Doped in PMMA

J. Phys. Chem. B, Vol. 114, No. 41, 2010 13049 where judged by a distribution of residuals and chi-square values for multiple exponential functions is given above Figure 9. The rise time in the presence of Phen indicates energy transfer from the excited singlet state of Phen to the singlet state of DBM. In the Eu(DBM)3Phen1 · 5 complex, decay of DBM quenched and decay started to obey the equation Foster32 decay function, as shown in Figure 9(iii).

[

I ) I0 exp -

( )]

t t - 2γDA τD τD

1/2

+ I0 exp(-t/τD)

where γDA ) CA/C0A. CA is the acceptor concentration, and C0A is the critical concentration (in M) given by

C0A )

Figure 9. Decay curve of DBM emission at 450 nm in PMMA (i) in the absence of Eu3+, (ii) in the presence of Phen, and (iii) in the Eu(DBM)3Phen1 · 5 complex. The distribution of residuals and chi-square values for double and triple exponential functions for decay of DBM in the presence of Phen in PMMA film and best fitted triple exponential and Foster function of DBM decay in the Eu(DBM)3Phen1 · 5 complex are given below (λex ) 372 nm).

complex in various solvents like dichloromethane solution; it was found to be 3.19 and 3.13 ns, respectively. Further, absence of any rise time in Eu3+ decay in the Eu(Phen)3 · 2H2O [Figure 8a(ii)] complex indicates no direct energy transfer takes place from the Phen triplet to Eu3+. An increase in the decay time of Eu3+ from 672 to 742 µs in the presence of Phen [Figure 8b(ii)] is due to a decrease in nonradiative channel because of its shielding in the Eu(DBM)3Phen1 · 5 network. Energy transfer from the triplet state of DBM to the lanthanide was also reported by Lou et al.6 and Song et.al.30 Similar enhancement in fluorescence of Tb3+ ion by energy transfer from salicilic acid (SA) doped in PVA polymer was observed by Mishra et al.19 and in the biphenyl ring in cyclodextrin (CD), in the micellar system observed by Rudzinski et al.31 It was observed that, both in the PVA and in the CD cavity, Tb3+ ion facilitates intersystem crossing (ISC) in SA and biphenyl and subsequently, energy is transferred to the 5D4 state of Tb3+ ion. They observed a 12 µs rise time along with a 1.6 ms decay time in the green luminescence at 545 nm of Tb3+ ion, and the rate of energy transfer was 8.3 × 104 s-1. (b) Donor Decay Dynamics. Pure DBM doped in PMMA shows biexponential decay along with a decay time of 3.5 and 12 ns when excited by corresponding absorption bands of 372 nm wavelength, as shown in Figure 9(i). The amplitudes of these decay components are ≈30 and ≈70%, respectively. However, in the presence of Phen in PMMA, decay of DBM fitted in triple exponential function and started to show a short rise time of nearly ∼0.76 ns along with decay times of 3.2 and 12.6 ns [ Figure 9(ii)]. The goodness of fitting of the collected decay

3000 2π NR0A3 3/2

where R0A is the critical transfer distance and N is Avogadro’s number. The respective analyzed overlapped decay curves are shown in Figure 9. The goodness of fitting of the collected decay where judged by a distribution of residuals and chi-square values for multiple exponential functions and Foster functions are given below Figure 9. The longer decay component is thought to indicate the bounded molecules of the Eu(DBM)3Phen1.5 complex with PMMA, while the short decay component indicates the unbounded molecules of the same complex in PMMA. Both of the decay time values show quenching, but prominent quenching occurs in the short decay time component. This observation shows that energy transfer more facilitated from the singlet state of free DBM molecules to the f orbital of Eu3+ ion. Although the system is quite complicated, preliminary results indicate cascade excited state energy transfer from the Phen to DBM and DBM to Eu3+ ion both through slow and fast energy transfer from the singlet and triplet states of ligands to the f orbital of rare-earth ions. 3.6. Phosphorescence Measurements. Further, to understand the microsecond range of rise time in the decay of Eu3+ ion in the presence of a donor, triplet state emission of DBM and Phen was recorded and overlapped with spectra of a different sample complex and shown in Figure 10. On 375 nm excitation, DBM shows a broad phosphorescence band having a maximum at 500 nm [Figure 10(i)] in PMMA, while no phosphorescence is observed from Phen doped PMMA polymer film by corresponding excitation from the absorption (282 nm) band. Eu3+ ion doped polymer film shows very weak emission lines corresponding to 5D0 to 7Fj state by 375 nm excitation [Figure 10(ii)]. Strong enhancement in the emission intensity of Eu3+ ions along with a decrease of phosphorescence of DBM is observed in Eu(DBM)3 · 2H2O doped PMMA films, as shown in Figure 10(iii) and their normalized spectrum [Figure 10(iv)]. Further, ∼5-fold enhancement (compared to the Eu(DBM)3 · 2H2O complex) in phosphorescence of Eu3+ (5D0 to 7F2) has been observed for Eu(DBM)3Phen1.5, as shown in Figure 10(v). These results again indicate the excitation energy transfer from the triplet state of DBM to the 5D0 state of Eu3+. From these observations, we notice that there are two channels of energy transfer from the (DBM)3Phen1.5: from the (i) singlet state (n f π*) and (ii) triplet state of the complex. The presence of Eu3+ ion may enhance intersystem crossing (ISC) to the triplet state by the heavy ion effect.33 Schematically, it can be depicted by Scheme 3. The energy level diagram of Eu3+ and the possible energy transfer mechanism obtained from this result is shown in Figure 11. It is clear from the diagram that both the DBM

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Singh et al. transfers its energy to the different energy levels of europium ion. It is also evident from the figure that both the singlet and triplet levels of DBM transfer their energy to europium ions and no direct energy transfer takes place from Phen to europium, which is also well supported by our steady and time domain measurements. The different energy levels of the Eu3+ thus populated give nonradiative transitions, as shown in the energy level diagram. Digital photographs of the above synthesized complexes with pure ligand molecules after exposure of UV light are shown in Figure 11. It is evident from the figure that the ligand molecules do not show visual color, while a beautiful orange-red color is clearly visible in the presence of DBM in the complexes which becomes brighter with Phen content. These findings give an opportunity for the development of brighter luminescent solar collector (LSC) materials from polymer substrates which potentially could be developed in next generation low cost LSC polymer sheets. Work is currently underway in our laboratory in this regard.

Figure 10. Phosphorescence spectra of (i) DBM in PMMA, (ii) Eu3+ in PMMA, and (iii) Eu(DBM)3 · 2H2O in PMMA and (iv) normalized spectra of Eu(DBM)3 · 2H2O in PMMA and (v) Eu (DBM)3 Phen1 · 5 in PMMA (λex ) 375 nm).

4. Conclusions In this Article, we found that Eu(DBM)3Phen1 shows crystalline structure, but an increase of Phen concentration beyond 1.0 mol % showed an unusual crystalline to amorphous change in structure. Thermal analysis also showed the influence of Phen on the glass transition temperature of the complexes which increases with the increase of Phen concentration. Further, for Eu(DBM)3Phenx doped PMMA, the glass transition temperature decreases with the increase of Phen concentration. The FTIR spectrum of doped Eu(DBM)3Phenx complex showed sharpness in bands, due to the more ordered structure of doped PMMA in comparison to pure PMMA. The fluorescence spectra of the complex doped PMMA show a drastic enhancement in the fluorescence intensity of Eu3+ due to the effect of Phen. The quantum yield of Eu3+ ion is increased from 0.14 to 0.47 in the presence of ligands. The energy levels of Eu3+ are split into Stark components due to the geometrical field. Transitions were also observed from higher energy states of the Eu3+ rare earth ion, indicating the strong interaction among excited Phen, DBM, and the Eu3+ ions. Cascading energy transfer takes place from the singlet level of Phen to DBM and finally from both the singlet and triplet levels of DBM to Eu3+ through short-range interaction. The present study may help in finding suitable materials for luminescence solar collectors as well as other photonic devices. Acknowledgment. Financial assistance from CSIR and DST are gratefully acknowledged. Authors are thankful to Professor Dhananjai Pandey, School of Materials Science & Technology, Institute of Technology (B.H.U.) for fruitful discussion. Thanks are also due to SAIF, CDRI, Lucknow for providing C, H, N analysis data.

Figure 11. Energy level diagram for the cascade energy transfer mechanism in the Eu(DBM)3Phenx complex (inset: digital photographs of newly synthesized complexes with pure ligand molecules after exposure of UV light).

and Phen absorbed the energy on excitation at 266 nm and populated their excited singlet states. The exited Phen and DBM molecules undergo a nonradiative transition to their longer lived triplet state. The bold dotted lines show that the excited singlet state of Phen transfers its energy to the DBM and finally DBM

Supporting Information Available: XRD patterns and DSC thermograms of Eu(DBM)3Phenx complexes doped in PMMA films, FTIR spectra of pure Eu(DBM)3Phenx complex and after embedding it in PMMA films, and variation in fluorescence intensity of Eu3+ ions with different phenanthroline content in Eu(DBM)3Phenx complexes doped in PMMA films for excitation wavelengths of 266, 355, and 380 nm. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bonzanini, R.; Girotto, E. M.; Goncalves, M. C.; Ranovanovic, E.; Muniz, E. C.; Rubira, A. F. Polymer 2005, 46, 253.

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