synthesis, structural, thermal and optical studies of rare earth coordinated complex: tb(sal)3phen

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Materials Chemistry and Physics 130 (2011) 1351– 1356

Contents lists available at SciVerse ScienceDirect

Materials Chemistry and Physics

j ourna l ho me pag e: www.elsev ier .com/ locate /matchemphys

ynthesis, structural, thermal and optical studies of rare earth coordinatedomplex: Tb(Sal)3Phen

agandeep Kaur, Y. Dwivedi, S.B. Rai ∗

aser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India

r t i c l e i n f o

rticle history:eceived 13 September 2010eceived in revised form 30 August 2011ccepted 11 September 2011

ACS:5.75.Fg7.64.k−8.47.Jc4.70.Km3.50.−J

a b s t r a c t

Complexes of salicylic acid (Sal) and 1,10-phenanthroline (Phen) were synthesized coordinated withterbium ion (Tb3+) in crystalline phases. The structural characterizations of the lanthanide complex weremade using FT-IR, NMR (1H and 13C) and XRD techniques. These measurements confirm the formation ofTb(Sal)3Phen complex structure. The thermal aspects of the complex were examined using DTA and TGAtechniques. An enhancement in luminescence intensity of Tb3+ ion bands were observed in Tb(Sal)3Phencomplex as compared to TbCl3 crystals on 355 nm laser excitation. Enhancement is reported due tothe efficient energy transfer process from Sal to Tb3+ ions. This is also confirmed by the time resolvedphotoluminescence spectroscopy with increase in lifetime of Tb3+ ions due to encapsulation in Sal/Phennetwork. Our system in itself can be a deserving candidate for luminescent solar collector material whencoupled with solar cells.

8.35.bm

eywords:rystal growthourier transform infrared spectroscopyFTIR)hotoluminescence spectroscopy

© 2011 Elsevier B.V. All rights reserved.

. Introduction

A surge of investigations have been carried out during recentears to develop rare earth coordinated complexes due toheir interesting photophysical properties. These materials areound suitable for numerous applications in electroluminescentnd photoluminescent devices, novel optical displays, detectors,elecommunications and also in biological probing, etc. They havehe striking advantages of stability, cost-effectiveness, ecologicalcceptability and bulk manufacturing. Along with, these can also beood candidates for the enhancement of amplified field emissionn advanced photonic systems [1–4].

The coordination of lanthanide ions (Ln3+) with organic ligands�-diketone and carboxylates) provide an extra stability to the

olecular structure, sheath to Ln3+ ions from higher lattice vibra-ions and an efficient sensitization for Ln3+ ions which intensifieshe luminescence intensity [4,5]. These organic ligands predomi-

∗ Corresponding author. Tel.: +91 542 2307308; fax: +91 542 2368390.E-mail address: sbrai49@yahoo.co.in (S.B. Rai).

254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2011.09.028

to Ln3+ ions is facilitated. The organic sensitization of Ln3+ ions hasbeen studied extensively for their utility in LED applications and inlaser systems [6]. Terbium possess large yield in green region andsuitable for fluoroimmunoassays, DNA and RNA labeling and struc-tural probes [7,8]. Direct excitation of 5D4 emitting state of Tb3+ iongives rise to a weak luminescence due to low absorbance and nar-row bands. Their luminescence can be increased by exploiting theenergy transfer from a suitable organic ligand or donor. The com-plex of Tb with �-diketonate is not only used as the green emitterbut also a low price energy model for ultraviolet-excited white LEDs[9]. Salicylic acid (Sal), also known as ortho-hydroxybenzoic acid isone of the most efficient aromatic carboxylic acid with monocliniccrystal structure. It fulfills the coordination numbers of rare-earthcomplexes. Because of its excellent coordination ability to rare-earth ions and the ability of sensitizing the luminescence of Ln3+

ions, salicylic acid had been applied to many ternary rare-earth sys-tems [10–12]. In view of this, we considered the Sal/Phen complex avery interesting chelating ligand for the terbium ions. The additionof 1,10-phenanthroline (Phen) molecule as a synergetic shieldingligand will give extra rigidity and stability to the structure [10,11].

The present study is focused to get better insight into the struc-tural and thermal properties of Tb(Sal)3Phen hybrid complex andto study the coupling of excited states of Sal/Phen complex (donor)with 5D4 state of Tb3+ ion (acceptor) through energy transfer

1 try and Physics 130 (2011) 1351– 1356

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rocess. The spectral study of our rare earth coordinated complexoupled with a suitable solar cell is good candidate as lumines-ent solar collecting material. FT-IR, NMR, DTA and XRD techniquesonfirm the formation of complex while the absorption, excitationnd emission spectroscopy reveals the photo-physical properties ofhe complex. Furthermore, we intended to seek into the modifica-ions induced by the Sal/Phen in time resolved photoluminescenceroperties of Tb(Sal)3Phen complex.

. Experimental

Terbium oxide and Salicylic acid used are 99.9% pure while 1,10-phenanthrolines 99.5% pure (all purchased from Sigma–Aldrich) and used without further purifica-ion. Standard stock solution (0.01 M) of Tb3+ was prepared by dissolving a knownmount of Tb4O7 in hydrochloric acid. The solution was divided into two parts:ne part was kept aside to allow formation of white TbCl3·6H2O crystals and thether part was used for the formation of the complex. The rare earth coordinatedomplex was prepared by mixing the ethanolic solution of salicylic acid and 1,10-henanthroline to the remaining part of previously prepared solution of terbiumhloride in 3:1:1 ratio. Mixture thus prepared was allowed to dry in air at roomemperature for 10 h. Brownish crystals of Tb(Sal)3Phen complex were grown in theample.

The absorption spectra were recorded using a JASCO V-670 absorption spec-rophotometer in the wavelength range of 200–600 nm. Infrared spectra of theomplexes were measured in the spectral region 400–2000 cm−1 in transmit-ance mode by dispersing and palletizing the sample in KBr at 2.0 cm−1 resolutionsing FT-IR spectrometer [PerkinElmer RX-1]. Baseline corrections were introducedhenever needed. NMR spectra of the samples were recorded using FTNMR-JOELL300 system at 80 Hz. X-ray diffraction (XRD) patterns were recorded using 18 kWotating anode (Cu) based Rigaku powder diffractometer fitted with a graphiteonochromator. Differential thermal analyses (DTA) and thermogravimetric anal-

sis (TGA) techniques were used to explore the thermal properties of the asynthesized complex. These measurements were performed on Pyris Diamond Ther-al analyzer at a fixed heating rate of 10 ◦C min−1 under N2-gas atmosphere. For

xcitation and emission spectra, a Spectrofluorometer [Flouromax-4, Horiba Jobinvon] was used. The dynamics of luminescence were examined using the third har-onic of pulsed Nd:YAG laser [Spitlight-600, Innolas, Germany] having pulse width

ns, as an excitation source. The overall response time of our detection system was1 �s. The collected signal was fed to 150 MHz digital oscilloscope (model no. HM507, Hameg Instruments) to record the decay curve. Lifetime of the radiative levelsas estimated by fitting the decay curve in an exponential function. The detector

ffset was removed before fitting and the data were normalized to the initial voltage.

able 1he most probable vibrational assignment of few experimentally observed finger printrequencies and the relative intensities are also given using DFT/B3LYP/6–31 ++G*.

Salicylic acid

Modes ofvibration

Theoretical Experimentalfrequency (cm−1)

Frequency(cm−1)

Relativeintensity (%)

�opph 536.17 0.23 535

�as(CCC) 740.34 1.45 740

�s(CCH) 862.99 2.48 862

�COH 878.27 0.91 877

�opphH 985.46 0.36 985

C–Cstr 1048.46 19.33 1049

�C–OH 1087.33 23.59 1087

�Ar–OH 1281.69 44.58 1295

�ipOH,H + �C–C 1338.36 16.22 1338

�ph 1358.21 11.63

�ipphH + �C–C 1484.12 14.85 1485

�(ring) + (CH)w 1515.10 31.57 –

�(ring) + �(CC) 1524.61 6.54 –

�C C 1624.06 17.30 1624

�C C 1667.83 27.50 1665

�C O 1665.95 100 –

�as(OH) 3171.98 1.67 –

�as(CH) 3189.74 3.48 –

�as(CH) 3200.47 2.53 –

�s(ch) 3237.76 1.589 3237

�oh(oh) 3673.55 57.11 –�oh(cooh) 3746.75 13.78

: stretching, �: deformation, w: wagging, superscript (ip) denotes in plane and (op) dntisymmetric.

Fig. 1. FTIR spectra of salicylic acid, 1,10-phenanthroline and Tb(Sal)3Phen complex.

3. Results and discussion

3.1. Structural characterization

3.1.1. Fourier transform infrared (FT-IR) analysisVibrational structure of the complex and its ingredients was

studied by recording infrared absorption spectra of salicylic acid,1,10-phenonthroline and Tb(Sal)3Phen complex in the range of400–2000 cm−1 in solid form (Fig. 1). The FT-IR spectra of the sal-icylic acid displays the expected strong characteristic absorptionpeaks �Ar-OH (1295 cm−1), �O-H (1485 cm−1), �C O (1665 cm−1).The bands around 3100–3000 cm−1 are assigned to CH stretch[13]. The bands �OH (COOH, 2597 cm−1), �OH

COOH (intermolec-

vibrations of salicylic acid and 1,10-phenantholine. The theoretically calculated

1,10-Phenanthroline

Modes ofvibration

Theoretical Experimentalfrequency (cm−1)

Frequency(cm−1)

Relativeintensity (%)

�(HCN) 520.193 0.86 520�(CH) 740.19 6.18 733�(ring) 769.556 44.19 769�ch 870.118 8.26 850�C–C 1413.91 0.11 1413�C–C + �C C 1484.067 2.19 1484�ip

H + �C C 1535.29 11.35 –�C C 1539.647 62.52 1507�C C 1592.65 21.05 1589�C C + �as(C N) 1635.411 16.45 1619�C C + �s(C N) 1649.219 13.11 1647�C C 1660.179 15.72 –�as(NC–H) 3140.79 37.72 –�s(NC–H) 3141.32 64.99 –�as(C–H) 3155.21 2.09 –�as(C–H) 3163.28 27.97 –�as(C–H) 3164.00 2.70 –�as(C–H) 3174.656 64.89 –�as(C–H) 3190.523 100 –�s(C–H) 3190.872 –

enotes out of (ph) denotes phenyl group, (s) symmetric and plane, subscript (as)

G. Kaur et al. / Materials Chemistry and Physics 130 (2011) 1351– 1356 1353

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ig. 2. 1H NMR spectra of salicylic acid, 1,10-phenanthroline and Tb(Sal)3Phen com-lex in DMSO solution and their respective structures.

pectrum broad band observed at 3237 cm−1 can be assigned to thearboxylic group which is intra-molecularly bonded [15].

The vibrational frequencies of the characteristic absorptions(C–H) (733 and 850 cm−1), �C C (1507, 1589, and 1619 cm−1)nd �C N (1647 cm−1) were assigned belonging to phenanthrolineolecule. The validity of the assignment for the three compounds

ave also been verified by theoretical calculations carried out usingensity Functional Theory (DFT) using Becke’s three parameterybrid exchange function with the LYP (Lee, Yang and Parr) correla-ion function standard B3LYP/6-311G* basis set. The experimentalrequencies of finger print vibrations and the theoretically cal-ulated frequencies and their relative intensities alongwith theirespective assignments are given in Table 1.

The FTIR spectrum of Tb(Sal)3Phen complex gives importantnformation regarding the structural changes occurred due to inter-ction between Tb3+, Sal and Phen. In the formed complex theands are either shifted, broadened or their relative intensity getshanged with respect to their parent species. A weak absorptionand belonging to Tb-O appeared near 414 cm−1. The character-

stic absorption bands of C O and C–O bands belonging to theree carboxylic acids ligands of Sal at 1665 and 1750 cm−1 disap-ear in the complex while the characteristic absorption peaks, i.e.as(COOH) (1580 cm−1) and �s(COOH) (1377 cm−1) shift to 1592nd 1383 cm−1 in the complex. Moreover, �OH(COOH) stretchingibration appeared shifted to lower energy which suggested thathe oxygen atoms of carbonyl group of conjugated carboxylate wasoordinated to Tb3+ ions through the oxygen atom [16]. Evidence ofhemical bond formation between Tb3+ ions and the nitrogen atomss confirmed as the vibrational band at 1588, 850 cm−1, assignedo C N and CH out-of-plane deformation [17] in phenanthrolinepectrum, is shifted to 1557.5, 847 cm−1 in the [Tb(Sal)3(Phen)]omplex. This indicates that nitrogen atoms of phenanthroline areoordinating Tb3+ in complex [18].

.1.2. Nuclear magnetic resonance (NMR) spectroscopy1H and 13C NMR spectra were recorded for the samples

n standard DMSO solution for detailed structural investigation. comparison of 1H NMR spectrum of salicylic acid, 1, 10-henanthroline and Tb(Sal)3Phen complex is portrayed in Fig. 2.henanthroline and salicylic acid molecules contained eight and

Fig. 3. X-ray diffraction pattern of Tb(Sal)3Phen complex.

four equivalent protons, respectively. Peaks corresponding to Ar–Hare shifted downfield in case of Tb coordinated complex. Further-more the peaks due to –COOH at 13.4 ppm and –OH at 11.3 ppmof salicylic acid are observed. Amongst these, the peak at 13.4 dueto COOH proton disappear in Tb(Sal)3Phen complex, which showsthe coordination of Tb3+ ions with the carbonyl of salicylic acid.This clearly indicates that the aromatic hydrogen is responsible forcausing a downfield shift. The substantial changes in the peak posi-tions observed are due to complexation of Tb ions. The shifting isdue to electron-spin relaxation time and magnetic anisotropy ofTb3+ ion [19,20]. The chemical shifts observed in 13C NMR spec-tra of phenanthroline and the complex shows that both nitrogensof the phenanthroline are coordinating in the coordination com-pounds [21]. The detailed peak analyses of the compounds are asfollows:

Salicylic acid: 1H NMR (DMSO-d6, 300 MHz): 6.96 (3H, d,j = 1.28 Hz), 7.52 (4H, t, j = 1.28 Hz), 6.91 (5H, t, j = 2.57 Hz), 7.80 (6H,d, j = 1.29 Hz), 11.33 (bs, C2OH, j = 1 Hz), 13.40 (bs, COOH); 13C NMR(DMSO-d6): 113.1 (C1), 161.22 (C2), 117.1 (C4), 135.69 (C3), 119.21(C2), 130.28 (C1).

1,10-Phenanthroline: 1H NMR (DMSO-d6, 300 MHz): 7.79 (8H,3H, dd, j = 1.04 Hz), 7.99 (5H, 6H, s, j = 1.04 Hz), 8.50 (4H, 7H, dd,j = 1.04 Hz), 9.10 (9H, 2H, t, j = 1 Hz); 13C NMR (DMSO-d6): 126.38(C3), 128.50 (C5), 135.90 (C2), 143.01 (C4), 147.81 (C12).

Tb(Sal)3Phen complex: 1H NMR (DMSO-d6, 300 MHz): 6.77 (3H,s, j = 3 Hz), 7.31 (4H, s, j = 1.46 Hz), 7.67 (6H, d, j = 1.47 Hz), 7.93(13H,14H, s, j = 1.99 Hz), 8.68 (10H, 15H, d, j = 1.01 Hz), 8.96 (8H,17H, s, j = 1.00 Hz); 13C NMR (DMSO-d6): 117.77 (C4), 120.00 (C2),130.00 (C1), 136.73 (C8).where s: singlet, d: doublet, t: triplet, dd:doublet of doublets.

On the basis of NMR and FT-IR analysis, the most probable struc-ture of the lanthanide coordinated Tb(Sal)3Phen complex is shownin Fig. 2.

3.1.3. Powder X-ray diffraction analysisIn order to understand the nature of the coordinated complex,

powder XRD was done and shown in Fig. 3. Salicylic acid cor-responds to monoclinic cell (a = 11.52 ± 0.0.12, b = 11.22 ± 0.011,c = 4.92 ± 0.005 A, and = 91◦ ± 2′) with space group P21/C [22]while 1,10-phenanthroline corresponds to monophase rhombo-hedral crystalline structure (a = 17.80, c = 8.522 A, and = = �)with space group P3 or P32 [23]. The XRD pattern of Tb(Sal)3Phencomplex shows new peaks with disappearance of the individual

1354 G. Kaur et al. / Materials Chemistry and Physics 130 (2011) 1351– 1356

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� → �* and n → �* transitions as well. This reveals that the organicligands are within coordination sphere of the Ln3+ ions and couldeffectively sensitize them.

ig. 4. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) ofhe salicylic acid and Tb(Sal)3Phen complex.

ew coordinated complex completely different from the individualngredients.

.2. Thermal analysis

Fig. 4 shows the thermogravimetry (TG) and differential thermalnalysis (DTA) curves of Salicylic acid and Tb(Sal)3Phen complex.he first two peaks at 59 and 100 ◦C in DTA plot of Salicylic acidhows the evaporation of crystal water as it occurs at comparativelyow temperature. Similarly the first weight loss on TG curves occursrom 59 to 100 ◦C the percentage of weight loss were 3% and 4%,espectively which was coinciding with the loss in water moleculesn acid. One of the other three endothermic peaks at 134 ◦C is dueo melting while the rest two broad peaks (232 and 300 ◦C) areorresponding to the evaporation of Salicylic acid. In TG curve theajor mass loss occurs in the temperature range of 185–275 ◦Cith the loss of 62% of the material and the second mass reduction

f 38.2% takes place in the range of 275–320 ◦C [25].It is observed that the thermal decomposition of the

b(Sal)3Phen complex has three steps ranging from 150 to 270,70 to 363 and 363–412 ◦C. The first step (<270 ◦C) has a mass lossf 64.9% is primarily assigned to the vaporization of solvated waterrom the complex. The rest of steps (>270 ◦C) have a mass loss of2.7% and 12.8%, respectively, corresponding to the decompositionf three salicylate and one phenanthroline ligands.

.3. Optical characterization

.3.1. UV–vis absorption spectraUV–vis absorption spectra of TbCl3 and Tb3+ ion coordinated

al/Phen complex dispersed in PVA were recorded in the range of00–520 nm (Fig. 5). Few weak bands were observed at 269, 315,41, 367 and 486 nm corresponding to the electronic transitionsrom ground state 7F6 to different excited states 5I5, 5H7, 5L6, 5L10nd 5D4 of Tb3+, respectively.

UV–vis absorption spectrum of the Tb(Sal)3Phen complex dopedVA film consists of bands at 238 and 305 nm corresponding to2← S0 and S1← S0 transitions of Sal and S1← S0 transition of Phent 254 nm superimposed to the low intensity broad PVA absorptionands. Thus for convenience, PVA absorption is taken in back-round. A broad band at 269 nm is basically composed of �* ← �ransition of the aromatic ring of Phen and 5I5← 7F6 transition of

Fig. 5. Absorption spectra of TbCl3 and Tb(Sal)3Phen by dispersing the crystals inPVA.

ions without Sal/Phen. The enhancement in the extinction is due tothe organic-lanthanide coordination and consequently due to theencapsulation effect. Due to formation of complex, Tb3+ ions are nomore directly bonded with PVA, now the coordination of Tb3+ ionwith the Sal/Phen complex which provides more rigid environmentto Tb3+ and prevents it from the vibrations of the host molecule,which improves the transition probability of the transitions.

3.3.2. Excitation spectraThe excitation spectra of TbCl3 and Tb(Sal)3Phen complex cor-

responding to 5D4→ 7F5 (544 nm) transition of Tb3+ ion wererecorded to confirm the possibility of energy transfer from sali-cylate ligand to Tb3+ ions and are shown in Fig. 6.

The excitation spectrum of TbCl3 sample shows few weak bandsat 341, 351, 358, 369, 377 and 487 nm wavelengths correspond-ing to absorption of Tb3± ion. But, the excitation spectrum ofTb(Sal)3Phen complex is largely dominated by the ligand bands.In this case, addition of Sal/Phen improves the excitation intensityof Tb3± ion which clearly shows an efficient antenna effect.

An asymmetric excitation broad band centered at 311 nm isattributed to n → �* transition of salicylate ligand and an adjacentsmall intensity band at 246 nm is due to Phen. The appearance ofthe bands corresponding to the Tb3+ ion emission clearly indicatethe effective sensitization for Tb3+ ions by salicylate ligand through

Fig. 6. Excitation spectra of TbCl3 and Tb(Sal)3Phen corresponding to 5D4→ 7F5

(544 nm) transition of Tb3+ ion.

G. Kaur et al. / Materials Chemistry and Physics 130 (2011) 1351– 1356 1355

Fig. 7. Photoluminescence spectrum of TbCl3 and Tb(Sal)3Phen complex on exci-tlo

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ation with 355 nm showing different transitions of Tb3+ ions emanating from 5D4

evel. The visual snapshots of the Tb(Sal)3Phen crystal on excitation with and with-ut 355 nm laser radiation.

.3.3. Photoluminescence on 355 nm excitationThe photoluminescence spectra of TbCl3 and Tb(Sal)3Phen

omplexes were recorded using 355 nm laser radiation and theesultant spectra were shown in Fig. 7. The 355 nm (∼28170 cm−1)aser photon partially resonates with 5L9 level (∼28,490 cm−1) ofb3+ ion and n → �* transition of salicylate ligand.

The spectra of TbCl3 exhibits characteristic emission peakst 487, 544, 583 and 618 nm of Tb3+ ions emanating from 5D4evel to various levels of ground state. The 5D4→ 7F5 transition544 nm) amongst these is the brightest one. The mechanism ofbserved transitions can be explained as follows. Initially 355 nm∼28,170 cm−1) photon promotes Tb3+ ions to 5L9 level. The excitedb3+ ions relax non-radiatively from 5L9→ 5D3 level. The emissionsrom 5D3 level to lower lying states (7Fi; i = 3, 4, 5, and 6) are notisible, because the energy gap (∼4500 cm−1) between 5D3 and 5D4evels is comparable to the highest lattice phonon vibrations of –OH

hich induce rapid non-radiative relaxation. Further, the energyf 5D3→ 7F0 and 5D4→ 7F6 transition is analogous hence a pos-ible cross relaxation process might also occur [26]. The possibleathway of the cross relaxation process is:

b3+(5D3) + Tb3+(7F6) → Tb3+ + (5D4) + Tb3+(7D0)

Excited Tb3+ ions in 5D4 level relax radiatively to different lowying levels [7Fj; (j = 6, 5, 4, 3)] and emit, observed colors.

When Tb(Sal)3Phen complex was excited with 355 nm pho-on, it shows 12 fold enhancement in emission intensities of Tb3+

ands alongwith a broad band [370–475 nm] emission centered at15 nm corresponding to salicylate ligand. A large increase in inte-rated emission intensity signifies an effective energy transfer fromrganic ligands to coordinated Tb3+ ion.

For the effective radiative energy transfer, acceptor (Tb3+)nergy level should coincide with the donor emissive level. Theechanisms accountable for luminescence enhancement can be

nderstood in the light of these. The dimeric and the tautomericorms of Sal emit in UV/blue regions which operate as antenna lig-nd for Tb3+ ion. The 355 nm laser photon excites the unexcited Salo its singlet state (S1) where-from photo-tautomeric fluorescencet 415 nm takes place [3]. Also there exists a possibility that the

Fig. 8. The schematic energy level diagram explaining the luminescence processand the possible energy transfer pathways for the Tb(Sal)3Phen complex.

and Phen lie at ∼24,184 and ∼22,075 cm−1 [27], respectively andsuitable for sensitization to Tb3+ ion. Hence the long lived tripletlevels of organic compounds were effectively coupled with Tb3+

levels to transfer their energies to Tb3+ ions. We expect that thedirect coordination of the Sal to Tb3+ ion improves the energy-transfer efficiency due to reduced donor–acceptor distance. Theschematic energy level diagram explaining the luminescence pro-cess and the possible energy transfer pathways for the Tb(Sal)3Phencomplex is shown in Fig. 8. The intra-molecular energy transferefficiency calculated is 52%, using the relative intensity of fluores-cence of donor [28]. On excitation with 355 nm laser radiation,Tb(Sal)3Phen crystals are perceived to be strong white due tothe mixing of complementary colors: incident blue photon andgreen/yellow emission of Tb3+ ions. A visual comparison of as syn-thesized (with and without excitation of 355 nm laser) is givenin inset of Fig. 7. Thus we can infer that our organic–inorganichybrid complex finds its utility as luminescent solar collector mate-rials when coupled with suitable solar cells, i.e. aluminium galliumarsenic photovoltaic cell, with sensitivity range from 300 to 700 nm[3].

3.3.4. Time resolved photoluminescence spectroscopyTo investigate into the time resolved photoluminescence spec-

trum in more detail, the decay curves of 5D4→ 7F5 transition ofTb3+ ions in TbCl3 and Tb(Sal)3Phen crystals were recorded usingpulsed 355 nm laser radiation and are shown in Fig. 9. The decaycurves were fitted to following exponential function

I = I0[A1e(−t/�1) − A2e(−t/�2)] (1)

where A1 and A2 are the amplitudes, �1 and �2 are the decay-timeand the rise-time of the acceptor ion.

The values of �1 and �2 are found to be ∼524 �s and ∼16.9 �sfor Tb3+ ions in TbCl3 crystals. Initially 355 nm photon is absorbedin 5L9 level of Tb3+ ion from where it relaxes ultimately to 5D4 levelnon-radiatively to give visible transitions. The appearance of risetime is due to the slow feeding (slow internal conversion rate) from

1356 G. Kaur et al. / Materials Chemistry and

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f the Tb3+ ion in Sal/Phen network which prevents the Tb3+ ionrom H2O vibrations possessing phonon frequency higher than thendividual frequencies of Sal and Phen components. It is also an evi-ence of energy transfer from Sal/Phen complex to Tb3+ ions [29]. It

s worth noting that a small risetime of ∼17 �s associated with theecay curve of Tb3+ ions reduces at higher Tb3+ concentrations. Atigher concentrations ion-ion interaction between two proximateb3+ ions is improved which strengthens the cross-relaxation pro-ess. The fast cross-relaxation process [5D3→ 5D4] appears througheduction in rise time [30].

. Conclusions

Tb(Sal)3Phen complex has been synthesized and its structuralnd optical properties have been investigated. The spectral andhotophysical aspects of our rare earth coordinated complex reveal

ts utility as luminescent solar collector material. Vibrational spec-roscopy reveals the bond formation between the oxygen atomsf carboxylic group with Tb3+ ions. Similarly, nitrogen atoms of

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Physics 130 (2011) 1351– 1356

an efficient energy transfer process between the excited states ofSal/Phen to Tb3+ ions.

Acknowledgements

Authors are grateful to the AvH foundation, Germany for provid-ing pulsed Nd:YAG laser. One of the authors (G. Kaur) would like tothanks CSIR (India) for Senior Research Fellowship.

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