Chemical Physics Letters 485 (2010) 309–314

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Structural and optical properties of Sm(DBM)3Phen doped in poly(methylmethacrylate) (PMMA): An evidence for cascading energy transfer process

A.K. Singh a, S.K. Singh b, R. Prakash a, S.B. Rai b,*

a School of Materials Science and Technology, Institute of Technology, Banaras Hindu University, Varanasi 221 005, Indiab Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221 005, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 October 2009In final form 17 December 2009Available online 23 December 2009

0009-2614/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.cplett.2009.12.057

* Corresponding author. Fax: +91 542 2369889.E-mail address: sbrai49@yahoo.co.in (S.B. Rai).

Sm(DBM)3Phen complexes have been synthesized and studied in detail. X-ray diffraction analysis ofSm(DBM)3Phen complex reveals crystalline behavior. An enhancement in the fluorescence intensity ascompared to the Sm(Phen)3�2H2O and Sm(DBM)3�2H2O samples has been observed. Stark splitting ofthe Sm3+ emission bands has also been observed. The fluorescence decay time for Sm3+ (4G5/2 ?

6H9/2

transition) is seen to increase from 221.6 ls in Sm3+ to 232.6 ls in presence of Sm(DBM)3�2H2O and to260.6 ls in Sm(DBM)3Phen. Energy transfer from excited Phen to DBM and finally from DBM to Sm3+

is proposed as the cause of the above observations.� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Recent years have witnessed an enormous increase in efforts toimprove the fluorescence intensity of lanthanide ions in differenthosts in view of their various applications [1–6]. The organic moi-eties of the polymers may be useful hosts since they are compati-ble with many real systems. However, lanthanides by themselvesoffer very weak absorption and poor quantum yields. The fluores-cence of the rare earths can be enhanced by synthesizing a stablecomplex with a suitable organic ligand [3], which transfers its exci-tation energy to the rare earth ion.

Rare earth complexes with lanthanides in the 3+ oxidationstate, with b-diketon ligand have been intensively studied in poly-methyl methacrylate (PMMA) matrix [1–9]. The reason is thestrong absorption of b-diketones [such as Dibenzoylmethane(DBM), Trifluroacetoacetylnaphtalene (TFAcAcN), Dithienylpro-pandione (DTPD)] in the 300–400 nm region and the suitabilityof PMMA as the host matrix [10–12]. For structural stability oneneeds in addition to a negatively charged ligand, neutral ligandsuch as triophenylphosphine oxide or 1,10-phenanthroline (Phen),called the ‘synergetic ligand’. A synergetic ligand can reduce therate of nonradiative decays and thus strongly enhance the fluores-cence intensity of the complex [13].

Luminescent materials with a narrow deep-red emission arestrongly preferred for high-quality display devices. Sm3+ is onesuch material which offers a deep-red emission with high colorpurity. In the present work Sm3+ ion is used as a luminescent cen-ter. The synthesis of Sm(DBM)3Phen complex and its structural and

ll rights reserved.

optical properties are studied. The complex is then doped in aPMMA polymer film and the resulting film has also been investi-gated. Various techniques e.g. X-ray diffraction (XRD), Fouriertransform infrared (FTIR) spectroscopy, UV/Visible absorptionspectroscopy, steady state and time domain fluorescence havebeen used. The dynamics of fluorescence enhancement have alsobeen discussed.

2. Experimental

2.1. Materials

Samarium oxide (99.95%, Aldrich), dibenzoyl methane (DBM)(98%, SRL), 1,10-phenanthroline (Phen) (monohydrated, 99.5%,Loba Chemie Pvt. Ltd.), ethanol (Changshu Yangyuan chemical, Chi-na), sodium hydroxide (Qualigens, 99%), PMMA (Mw. 15 000,Himedia), hydrochloric acid and chloroform (Merck, 99.9%) havebeen used. Samarium chloride was prepared by dissolving samar-ium oxide into hydrochloric acid.

2.2. Preparation

The Sm(DBM)3Phen complexes were prepared by the methodreported by Melby et al. [14] with a little modification. DBM(6 mmoles), Phen (2 mmoles), and 8 ml of 1 N sodium hydroxidewere well dissolved in 25 ml of ethanol. To this solution, 2 mmolesof SmCl3�6H2O dissolved in 10 ml of water was added slowly whilea constant stirring of the mixture was maintained for better reac-tion. During this process a light yellow precipitate ofSm(DBM)3Phen was formed. Sm(Phen)3�2H2O and Sm(DBM)3�2H2Owere also prepared by same procedure with obvious changes in the

Fig. 1. Powder X-ray diffraction (XRD) patterns of Sm(DBM)3Phen, Eu(DBM)3Phencomplex and Sm(DBM)3Phen doped in PMMA.

310 A.K. Singh et al. / Chemical Physics Letters 485 (2010) 309–314

composition. The precipitate was washed with distilled water andwas kept at �40 �C in a vacuum oven for over 24 h. Doped PMMAfilms were prepared simply by blending the Sm(DBM)3Phen com-plex so formed and PMMA in a chloroform solvent. After that,the solutions were cast in covered petri dishes for controlled evap-oration for overnight. Finally, blend films were dried in vacuumoven at 70 �C for removal of the remaining solvents.

2.3. Characterization

X-ray diffraction (XRD) patterns were recorded using 18 kWrotating anode (Cu) based (Rigaku, Japan) powder diffractometeroperating in the Bragg-Brentano geometry and fitted with a graph-ite monochromator in the diffracted beam. Data were recordedfrom 2h = 5� to 60� at a scanning rate of 4�/min at 6 kW energy.Thermogravimetric analyses (TGA) were done with Mettler TGAat a heating rate of 20 �C/min. (under the nitrogen environment).The UV/Visible absorption spectra of the different samples were re-corded in the 200–800 nm region using a Perkin Elmer, Lambda 25spectrophotometer. Perkin Elmer, RXI double beam Fourier trans-form infrared (FTIR) spectrophotometer was used for the infraredstudies. The fluorescence spectra were recorded in the 400–1000 nm region using fluorescence spectrometer [Horiba (Flouro-max-4)]. Fluorescence decay for 4G5/2 ? 6H9/2 transition (649 nm)of Sm3+ was also studied (at room temperature) using 355 nmwavelength from a pulsed Nd:YAG laser (Innolas, Spitlight 600).Data were acquired using an oscilloscope (analog digital scope-HM1507) with software SP107 and analyzed using ORIGIN

� 8.0 pro-gram. The decay time was determined using a non-linear leastsquares fit. Differentiation between the mono-exponential andbi-exponential fits was made on the basis of v2 values, standarddeviations and weighted residuals. Intensity decay curves so ob-tained were fitted to the exponential

Iðs; tÞ ¼ a1 expð�t=s1Þ � a2 expð�t=s2Þ

where s1, s2 are the shorter and longer lifetime components, respec-tively, and a1, a2 are the corresponding amplitudes. It was alsonoted that one of the amplitude terms [a1 or a2] is negative whichis indicative of a rise time prior to the decay.

3. Results and discussion

3.1. X-ray diffraction (XRD) analysis

X-ray diffraction pattern of the Sm(DBM)3Phen complex showsthe well defined crystalline behavior similar to that of Eu(DBM)3Phen studied earlier [2] (see Fig. 1). The XRD pattern ofSm(DBM)3Phen doped in PMMA shows only the amorphous peaksof PMMA suggesting that the complex molecules are homoge-neously distributed in the PMMA matrix and the crystalline peaksof the complex are not detectable due to its small concentrations.

3.2. Fourier transform infrared (FTIR) spectrum and analysis

FTIR spectra of pure PMMA, pure Sm(DBM)3Phen complex, andthe complex doped in PMMA have been recorded in the 400–4000 cm�1 region to determine the miscibility of this complex inthe PMMA (see supporting information Fig. S1). Peaks at990 cm�1, 1384 cm�1, 2952 cm�1 and 2994 cm�1 are easily as-signed to the characteristic bands of pure PMMA namely C–O–Csymmetric stretching, O–CH3 deformation, CH3 asymmetric stretchand O–CH2 asymmetric stretch, respectively. An intense and sharppeak at 1733 cm�1 is due to the carbonyl group present in PMMA.One observes that all the PMMA peaks are shifted towards lowerwave number side due to the influence of the ligand molecules.

The absorption peaks also become sharper in the presence of thecomplex signifying an ordering of the PMMA molecule.

3.3. Thermogravimetric (TGA) analysis

Thermogravimetric analysis of Phen, DBM, PMMA,Sm(Phen)3�2H2O, Sm(DBM)3�2H2O, Sm(DBM)3Phen and the com-plex doped in PMMA have been carried out (see Fig. 2) to exploretheir degradation behavior as well as water content in complexes.Fig. 2a and b evident that DBM molecule is anhydrous, whereasPhen is monohydrated (shows �9 wt.% loss at �150 �C) molecule.TGA thermogram of Sm(DBM)3�2H2O and Sm(Phen)3�2H2O shows�4% and �6 wt.% loss at 150 �C which is attributed to the disasso-ciation of two water molecules from Sm(DBM)3�2H2O andSm(Phen)3�2H2O (calculated wt. loss for two water molecule re-moval from Sm(DBM)3�2H2O and Sm(Phen)3�2H2O are �4.1%,�4.9%, respectively). It is interesting to note that TGA curve ofSm(DBM)3Phen shows anhydrous nature with improved thermalstability. Inset to Fig. 2a shows the TGA thermograms for PMMA(in bead and in film prepared by solution casting). It is observedthe PMMA bead degrades at �345 �C (with 5 wt.% loss) on theother hand solution cast PMMA shows �10 wt.% loss at �165 �Cwhich is attributed to the solvent removal from the polymer ma-trix. In case of complexes doped in polymer (Fig. 2) we observedsame degradation behavior as in solution cast PMMA. Thus, ourTGA studies infer that the Sm(DBM)3�2H2O and Sm(Phen)3�2H2Ocomplexes are dihydrated. Further, for doped polymer films we ob-served only the intrinsic features of PMMA host which was ex-pected for 1% complex loading in the polymer.

3.4. Steady state optical measurement and its analysis

3.4.1. UV/Visible absorption and excitation measurements:UV/Visible absorption spectra of (i) SmCl3�6H2O, (ii) DBM, (iii)

Phen and (iv) Sm(DBM)3Phen [all in 1.2 � 10�7 molar chloroformsolution] were recorded (see Fig. 3). SmCl3�6H2O has no discreteabsorption in UV/Visible region. DBM on the other hand showstwo distinct transitions with absorption maxima at 269 nm and342 nm having optical densities 0.17 and 0.57, respectively. Thesepeaks correspond to absorption from the ground p state to the first

Fig. 2. Thermogravimetric analysis of (a) (i) Phen, (ii) Sm(Phen)3�2H2O and (iii) Sm(Phen)3�2H2O doped PMMA (b) (i) DBM, (ii) Sm(DBM)3.2H2O and (iii) Sm(DBM)3�2H2Odoped PMMA (c) (i) Sm(DBM)3Phen and (ii) Sm(DBM)3Phen doped PMMA. Inset to the figure shows the TGA thermogram of PMMA bead (upper) and film (lower) prepared bysolvent casting.

Fig. 3. UV/Visible absorption spectra of (i) SmCl3�6H2O, (ii) DBM, (iii) Phen, and (iv)Sm(DBM)3Phen (in 1.2 � 10�7 molar chloroform solution). Inset to the figure showsthe excitation spectra of (1 wt.%/wt.%) Sm(DBM)3Phen doped in PMMA by moni-toring fluorescence at 649 nm.

A.K. Singh et al. / Chemical Physics Letters 485 (2010) 309–314 311

two lowest energy singlet excited states (S2 S0 and S1 S0).Phen shows its electronic absorption maxima at 274 nm whichcorresponds to the S1 S0 transition. Absorption spectrum ofSm(DBM)3Phen complex shows a slight red shift in the positionof the two bands of DBM. It appears that the weak band at269 nm of DBM gets overlapped by the intense peak of Phen inthe complex molecule. Interestingly, the optical density of theDBM absorption band at 342 nm is reduced from 0.57 to 0.39 inthe presence of Sm3+, indicating the formation of a new complexin the ground state. The inset to Fig. 3 shows the excitation spec-trum of the complex doped in PMMA observed by monitoringthe fluorescence at 649 nm. The maximum excitation for the com-plex doped PMMA film is seen at 400 nm. A similar red shift of theexcitation band from the UV/Visible absorption band has also beenreported by Lou et al. [6] in case of Eu(DBM)3Phen doped in PMMAfilms.

3.4.2. Fluorescence emission and its analysis3.4.2.1. 266 nanometer pulse excitation and cascade energy transferprocess. Fluorescence excited by a pulse laser emitting 266 nm for

various molecules doped in the PMMA host has been studied tounderstand the mechanism for the enhancement of the fluores-cence intensity. Fig. 4 shows the fluorescence spectra of the differ-ent species in 300–750 nm region. SmCl3�6H2O (concentration1 wt.%) doped PMMA film shows a very weak emission (negligible)centered at 649 nm due to Sm3+ ion (Fig. 4vi). Pure Phen doped inPMMA film (concentration 1.0 wt.%) emits an intense blue band at376 nm (Fig. 4i). However, when Sm(Phen)3�2H2O (1 wt.% concen-tration) doped in PMMA film is excited with 266 nm, the character-istic emission band of Phen (at 376 nm) becomes sharper andshows a decrease in intensity (Fig. 4ii) but the emission band at649 nm due to Sm3+ does not show any change. This suggests thatno energy transfer takes place from Phen to Sm3+ ions.

Further, pure DBM (concentration 1 wt.%) doped in PMMA filmon excitation with 266 nm radiation gives a weak and broad emis-sion band centered at 413 nm (Fig. 4iii). However, whenSm(DBM)3�2H2O (1 wt.%) doped in PMMA film is excited by266 nm, the fluorescence band corresponding to DBM decreasesin intensity (Fig. 4iv) while the intensity of the emission of Sm3+

is enhanced. This suggests that energy transfer takes from the ex-cited state of DBM to Sm3+ ions.

For Sm(DBM)3Phen (1 wt.%) complex doped in PMMA on excita-tion with 266 nm shows a very strong fluorescence from Sm3+

while the fluorescence from DBM and Phen decreases drastically(Fig. 4v). This suggests that Sm(DBM)3Phen is a quite stable com-plex which causes nonradiative decays less likely and therebyenhancing the fluorescence intensity.

The inset in Fig. 4 (I2) shows that the absorption spectrum ofDBM (B) and the emission spectrum of Phen (C) have considerableoverlap. It is thus likely that the excited Phen molecule transfers itsenergy to the DBM molecule which inturn transfers its excitationenergy to the Sm3+ ions in Sm(DBM)3Phen complex (see Fig. 5).From the energy level diagram shown in Fig. 5 it is clear that bothDBM and Phen can absorb the incident radiation at 266 nm and getexcited to their singlet states. Both the excited molecules (Phenand DBM) decay non-radiatively to their longer lived triplet states.There might be some transfer of excitation energy from the singletstate of Phen molecule to the singlet state of DBM, and a more sub-stantial transfer from the triplet state of the Phen to the DBM trip-let. The excited DBM molecule in its singlet/triplet state transfersits excitation energy to samarium ion. The excited Sm3+ ions inthe various state so excited rapidly relax to its 4G5/2 metastablestate, from which the various fluorescence bands 4G5/2 ?

6Hj (viz.4G5/2 ?

6H5/2, 4G5/2 ? 6H7/2, 4G5/2 ? 6H9/2, 4G5/2 ?6H11/2) are ob-

served. It is notable that while only few 4G5/2 ? 6Hj transitions ofSm3+ are seen in various glass/crystal or complex hosts [7,8,15–19]. One sees a number of bands corresponding to the 4G5/2

Fig. 4. Fluorescence spectra of (i) (1 wt.%/wt.%) Phen, (ii) (1 wt.%/wt.%)Sm(Phen)3�2H2O (iii) (1 wt.%/wt.%) DBM, (iv) (1 wt.%/wt.%) Sm(DBM)3�2H2O, (v)(1 wt.%/wt.%) Sm(DBM)3Phen and (vi) (1 wt.%/wt.%) SmCl3�6H2O doped in PMMA onexcitation with kex = 266 nm. Inset I1 shows the fluorescence spectra in 750–1000 nm region under 355 laser pulse excitation (10 times enhanced) and I2 showsabsorption (A, B) and emission spectra (C, D) of Phen and DBM, respectively.

312 A.K. Singh et al. / Chemical Physics Letters 485 (2010) 309–314

? 6Hj transitions. Some of the levels also show a Stark splitting dueto the electrostatic field of the ligands. Some bands involving thetransition 4F3/2 ? 6Hj are also seen indicating that the 4F3/2 levelof Sm3+ is also populated to some extent. The energy level diagramalso shows that while both singlet and triplet levels of DBM takespart in energy transfer to Sm3+ ions, no direct energy transfer takesplace from Phen to Sm3+ ion. The energy transfer is therefore a cas-cade transfer.

3.4.2.2. 355 nm pulse excitation. The fluorescence behavior on exci-tation of the sample by 355 nm radiation is similar to that dis-cussed above for 266 nm excitation. However, with 355 nm

excitation the Phen molecules are not directly excited and hencecan not have any effect on fluorescence enhancement whenSm(Phen)3�2H2O sample is considered. The singlet–singlet absorp-tion of DBM overlaps well with the 355 nm excitation and its ex-cited singlet state can populate the triplet state of the DBM andhence of Phen (see also Ref. [6]). Further triplet of Phen transfersits energy to the triplet of DBM. The singlet/triplet excited DBMtransfers energy to Sm3+ and is responsible for the enhancementof the fluorescence intensity.

3.5. Time domain analysis

The time variation of the fluorescence has also been investi-gated for the emission band at 649 nm (excitation wavelength355 nm). Table 1 summarizes the data obtained from time domainanalysis. The fluorescence decay for the different samples is shownin Fig. 6. For Sm3+ ion doped PMMA film, the decay is mono-expo-nential and has a characteristic decay time of 221.6 ls (as given inTable 1). Similar decay behavior is also observed for Sm(Phen)3�2H2O doped PMMA. On the other hand, the decay data of theSm3+ ion in the presence of DBM is described by

I ¼ I0½a1 expð�t=s1Þ � a2 expð�t=s2Þ�;

where s1 is found to be 232.6 ls with a1 = 7.992 and s2 = 13.5 lswith a2 = �5.748. The negative amplitude of one of the terms in thisexpression is indicative of an excited state reaction (usually theamplitudes a1 and a2 are almost equal with opposite signs if no di-rect excitation of the acceptor is involved).

The 13.5 ls rise time seen in the decay of Sm3+ ion (acceptor)emission in the presence of DBM (donor) is indicative of the non-radiative energy transfer from the DBM triplet state (rate of energytransfer being 7.4 � 108 s�1) [6,20–22]. Any energy transfer from asinglet state would involve a rise time of the order of nanoseconds(which would not be detected in the present experiment). Nie et al.[23] have reported the contribution of the singlet state inEu(DBM)3Phen complex. Further, the measured increase in the lifetime of the excited state of the Sm3+ ions (�28 ls) observed inSm(DBM)3Phen complex shows that Phen also plays role in thefluorescence intensity enhancement. The luminescence quantumyield (/) was also estimated as the ratio of the measured lifetimes and the theoretical estimate of the life time srad (as reported inRef. [8]) of the same state (see Table 1). It is seen that / increaseswhen Sm ion is in the complex Sm(DBM)3Phen. The role of the trip-let state of the DBM is further enhanced because Inter-SystemCrossing (ISC) increases in the presence of the Sm (heavy ion effect[19]).

Fig. 7 shows the antenna effect in the complex molecules wherein the ligand molecules act as an antenna catching the incident UVradiation and transferring it efficiently to the metal (Sm3+) ionsthrough intra-molecular nonradiative energy transfer process.

4. Conclusions

In this Letter we have investigated the fluorescence fromSm(DBM)3Phen both in the free condition and also after doped inPMMA. It is found that this complex also crystallizes as theEu(DBM)3Phen complex. The FTIR spectrum of PMMA doped withSm(DBM)3Phen complex shows sharper bands, probably due tothe order introduced in doped PMMA. The fluorescence spectraof the Sm(DBM)3Phen (compared to the Sm(DBM)3.2H2O andSm(Phen)3.2H2O) complex doped PMMA shows an increased fluo-rescence intensity of the Sm3+ bands. The reason for this enhance-ment is attributed to a energy transfer from Phen to DBM to Sm.

Table 1Decay data of various samarium complex doped in PMMA.

Sample s1

(ls)a1 s2

(ls)a2 /

SmCl3�6H2O/PMMA 221.6 ± 0.4 0.211 ± 0.0001 – – 0.149Sm(Phen)3�2H2O/PMMA 222.6 ± 0.3 0.201 ± 0.002 – – 0.150Sm(DBM)3�2H2O/PMMA 232.6 ± 0.5 7.992 ± 0.002 13.5 ± 0.5 –5.748 ± 0.095 0.157Sm(DBM)3Phen/PMMA 260.6 ± 0.8 2.469 ± 0.005 33.7 ± 0.5 –1.509 ± 0.008 0.176

Fig. 6. Fluorescence decay curve of (i) SmCl3�6H2O, (ii) Sm(Phen)3�2H2O, (iii)Sm(DBM)3�2H2O, and (iv) Sm(DBM)3Phen (1 wt.%/wt.%) doped in PMMA onexcitation with 355 nm pulsed laser. Fig. 7. Schematic representation of antenna effect in Sm(DBM)3Phen complex.

Fig. 5. Energy level diagram representing the cascading energy transfer in Sm(DBM)3Phen complex.

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Acknowledgements

Mr. S.K. Singh thankfully acknowledges the financial assistanceas SRF from CSIR, New Delhi.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cplett.2009.12.057.

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