Journal of Luminescence 137 (2013) 269–273

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Journal of Luminescence

0022-23

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Measurement and model calculation of the temperature dependenceof ligand-to-metal energy transfer rates in lanthanide complexes

Wagner M. Faustino a,n, Luiz A. Nunes b, Idelma A.A. Terra b, Maria Claudia F.C. Felinto c, Hermi F. Brito d,Oscar L. Malta e

a Departamento de Quımica, Universidade Federal da Paraıba, 58051-970, Jo~ao Pessoa PB, Brazilb Instituto de Fısica, Universidade de S ~ao Paulo, 13560-970 S ~ao Carlos SP, Brazilc IPEN, 05505-800 S ~ao Paulo SP, Brazild Instituto de Quımica, Universidade de S ~ao Paulo, 05508-900 S~ao Paulo SP, Brazile Departamento de Quımica Fundamental, Universidade Federal de Pernambuco, 50670-901, Recife PE, Brazil

a r t i c l e i n f o

Article history:

Received 12 September 2012

Received in revised form

7 January 2013

Accepted 9 January 2013Available online 18 January 2013

keywords:

Lanthanide coordination compounds

Energy transfer

Transients

13/$ - see front matter & 2013 Elsevier B.V. A

x.doi.org/10.1016/j.jlumin.2013.01.008

esponding author. Tel.: þ55 83 3216 7591;

ail address: wmfaustino@yahoo.com.br (W.M

a b s t r a c t

Temperature dependent transient curves of excited levels of a model Eu3þ complex have been

measured for the first time. A coincidence between the temperature dependent rise time of the 5D0

emitting level and decay time of the 5D1 excited level in the [Eu(tta)3(H2O)2] complex has been found,

which unambiguously proves the T1-5D1-

5D0 sensitization pathway. A theoretical approach for the

temperature dependent energy transfer rates has been successfully applied to the rationalization of the

experimental data.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Intramolecular energy transfer is a process of paramountimportance in describing the luminescence behavior of lantha-nide coordination compounds. Emission quantum yields of 4f–4ftransitions can be considerably increased when the compound isexcited through the ligands, instead of direct excitation of thelanthanide 4f states. This phenomenon was first rationalized byWeissman in 1942 in terms of ligand-to-metal intramolecularenergy transfer in trivalent europium coordinated by organicligands [1]. Since then, the enormous amount of data appearingin the literature [2–7] on the luminescence properties of this typeof compound has been interpreted in terms of this process. Thetheoretical challenge of describing intramolecular energy transferprocesses that occur between a ligand and a lanthanide ion inluminescent complexes, including selection rules, was first trea-ted in detail in references [5,8]. Expressions for transfer ratescorresponding to the so-called direct Coulomb and exchangemechanisms have been obtained, from which selection rulescould be derived [9].

Trivalent europium is extremely useful in studying and testingintramolecular energy transfer models due to the special char-acteristics of its energy level structure. The energies of the excited

ll rights reserved.

fax: þ55 83 3216 7437.

. Faustino).

5DJ manifolds (J¼0–4) of the 4f6 configuration are sufficientlywell separated from the ground state 7FJ manifolds (J¼0–6) andusually present appropriate resonance conditions with excitedstates of the ligands. In this case most of the experimental datahas been interpreted in terms of energy transfer from the lowesttriplet states of the ligands (T1) to the 5DJ manifolds, particularlythe 5D0 and 5D1 excited states, following absorption from thesinglet ground state (S0) to either the lowest excited singlet state(S1) or the 5L6 4f level that after populate the T1 state in theligands via respectively intersystem crossing or energy trans-fer[1–6], even though direct intramolecular energy transfer fromS1 to excited states of the lanthanide is also possible in some cases[10]. The experimental measurements to get information on thetransfer channels have been based either on the behavior of the T1

phosphorescence, from gadolinium compounds with large energygap (Gd3þ: 7P7/2-

8S7/2�32000 cm�1), mainly, and/or the rise-times of the luminescence transient curves of the lanthanideemitting levels. The interpretation of the results becomes ratherdelicate when mixed ligands, having close excited states, arecoordinated to the lanthanide ion. Since both the transient andthe steady-state regime emission quantum yield is the result of abalance between absorption, decay rates (radiative and non-radiative) and intramolecular energy transfer rates, the treatmentof an appropriate system of rate equations for each case canindicate which energy transfer channels are actually operative.This presumes the knowledge of energy transfer mechanisms(transfer rate values) and the associated selection rules, an aspect

′′′ψ′′′ψ

ψ ψ

∗φ

φ

∗φ

φ

Fig. 1. Configurational coordinate diagrams for direct and reverse thermally

activated ligand-to-metal energy transfer processes.

W.M. Faustino et al. / Journal of Luminescence 137 (2013) 269–273270

that has not been taken into consideration in these experimentalworks reported in the literature. Furthermore, the dependenceof intramolecular energy rates with temperature has beenoverlooked.

The aim of the present work is to report for the first time – tothe best of our knowledge – a direct measurement of temperaturedependent intramolecular energy transfer rates in a lanthanidecomplex. The ligand T1 triplet to the 5D1 state of the Eu3þ ionenergy transfer rate was measured by using time resolvedspectroscopy in the nano- and micro-second scales. The experi-ment was planned with the objective of testing predictions madeby the theoretical approach developed previously concerningenergy transfer mechanisms, selection rules and temperaturedependence of transfer rates [8,11]. The complex [Eu(tta)3(H2O)2](tta¼thenoyltrifluoroacetonate) was chosen as a model systemdue to the following reasons: (i) only one type of organic ligand ispresent; (ii) the complex has been characterized spectroscopicallyand theoretical studies on coordination geometry optimizationand on electronic structure have been carried out previously [5];and (iii) the tta S1 singlet state is energetically distant from theEu3þ 5D0 and 5D1 states (47000 cm�1), while the triplet T1

barycenter is located below the 5D2 and slightly above the 5D1

state (�400 cm�1).

2. Experimental section

The [Eu(tta)3(H2O)2] complex was prepared by the methoddescribed by Charles and Ohlmann [12]. The C and H contentswere obtained via conventional elemental analysis and the Eu3þ

content via complexometric titration with EDTA. The percentagesfound (calculated) were C: 33.88 (33.85); H: 1.88 (1.89); Eu: 17.76(17.85), corresponding to the general formula C24H16F9O8S3Eu.

The luminescence spectra in the visible range from 520 to630 nm were obtained using a laser HeCd (Kimmon/IK5652R-G)at 325 nm as the excitation source and were taken at differenttemperatures (between 10 and 300 K). The luminescence signalswere dispersed by a double monochromator (0.85 m, SPEX 1403)equipped with a photomultiplier tube (Hamamatsu/R928), ampli-fied by a lock-in amplifier (Stanford Research systems/SR 830DSP) and recorded by a computer.

The luminescence transient measurements were performed byusing the third harmonic (355 nm) of a Nd-YAG laser (Surelite I/Continumm, 10 Hz, 5 ns) as the excitation source. The transientsignals were dispersed by the same double monochromatorequipped with a photomultiplier tube and were recorded with adigital oscilloscope (TekTronix/TDS380) at temperatures from 10to 300 K. The temporal resolution of the electronic module wasabout 10 ns. The temperature dependent data were measured byusing a Janis Helium Flux Cryostat.

3. Theoretical approach

According to Fermi’s golden rule, the energy transfer rate froman excited donor state Dn to an excited acceptor An, is given by

WET ¼2p_

9/AnD Hj jADnS92F ð1Þ

where the matrix element corresponds to either direct orexchange electronic interactions between donor and acceptorstates, and the temperature dependent factor F corresponds totheir spectral overlap (resonance condition). In the case investi-gated here the donor state corresponds to a ligand centered statelabeled as 9fnS, and the acceptor state corresponds to a lantha-nide centered 4f state labeled as 9c

0

J0

M0

S as indicated in Fig. 1. Ifonly the direct coulomb interaction is operative the energy

transfer rate is given by [8,9]

WCET ¼

e2SLF

Gð2Jþ1Þ

Xl ¼ 2,4,6

Oe:dl

R6L

þðlþ1Þ/rlS2/3JCðlÞJ3S2

ð1�slÞ2

ðRlþ2L Þ

2

!/a0J0JUðlÞJaJS2

ð2Þ

where SL stands for the dipole strength of the intraligand transi-tion 93pnS-91pS, G stands for degeneracy of the donor state, andOe:d:

l (l¼2, 4 and 6) stands for the forced electric dipole contribu-tion to the so-called 4f–4f intensity parameters described by theJudd–Ofelt theory [13,14]. The donor–acceptor distance, RL (oftenbetween 3 and 5 A), is taken as the distance from the lanthanidenucleus to the barycenter of the ligand electronic state involved inthe energy transfer process. The eigenfunctions are taken in theintermediate coupling scheme, in which J is assumed as a goodquantum number, approximately. The lanthanide 4f radial inte-grals /rKS and reduced matrix elements are available in theliterature ([8] and references therein).

On the other hand, if only the exchange interaction is opera-tive the energy transfer rate is given by [8]

Wex ¼/4f 9LS4

ð2Jþ1Þ

8p3_

e2

R4L

/c0J0JSJcJS2X

m

9/j9X

j

mzðjÞ smðjÞ9jnSj2F

ð3Þ

where sm (m¼�1, 0, 1) is the spherical component of the spinoperator of electron j in the ligand, mz is the z-component of itsdipole operator and S is the total spin operator of the lanthanideion. Typical values of the squared matrix element of the coupleddipole and spin operators in Eq.(3) are in the range 10�34–10�36

(e.s.u.)2cm2 [8,9]. This coupled operator breaks down the usualselection rules in the singlet–singlet and singlet–triplet matrixelements. An important aspect in Eq. (2) is that shielding effectsproduced by the filled 5s and 5p sub-shells are taken into accountthrough the Sternheimer’s shielding factors sk,. These factors aretaken into account by the fact that the donor ligand state feelsthis shielding effect in a multipolar interaction, according to thepresent model, in contrast to the case of the exchange interaction,where the donor–acceptor overlap integral naturally includes thiseffect. An analytical formulation for these factors has been givenin Ref. [8].

The quantity /4f9LS is the radial overlap integral between the4f sub-shell and the ligand eigenfunction f * centered at adistance RL, as mentioned above. This overlap is certainly difficultto estimate. It should be small, less than 0.05, the typical value ofthe total overlap between 4f orbitals and the ligating atom.Within our purposes we have considered values between 0.04

525 535 545 555 5650.0

0.2

0.4

0.6

0.8

1.0

10K300K

Inte

nsity

(arb

. uni

t)

Wavelength (nm)

(x10)

570 580 590 600 610 620 6300.0

0.2

0.4

0.6

0.8

1.0

5 D0→

7 F1

5 D1→

7 F1

5 D1→

7 F2

5 D0→

7 F2

300K 10K

Inte

nsity

(arb

.uni

t)

Wavelength (nm)

5 D0→

7 F0

5 D1→

7 F0

(x3)

λexc.= 325nm

λexc.= 325nm

Fig. 2. Emission spectra of the [Eu(tta)3(H2O)2] complex recorded at 10 and 300K:

(a) 5D0-7F0–4 transitions and (b) 5D1-

7F0–2 transitions. The emission intensities of the5D0-

7FJ transitions are stronger than those arising from the 5D1 level by a factor of 103.

W.M. Faustino et al. / Journal of Luminescence 137 (2013) 269–273 271

and 0.03, which are reliably acceptable taking into account thatthe molecular ligand excited states are quite delocalized.

A point of high importance, as it can be derived from Eq. (3), isthe selection rule associated to the 5D0 and 5D1 levels of the Eu3þ

ion. According to the reduced matrix element of the lanthanidespin operator DJ¼0, 71 (J¼ J¼0 excluded). This means thatenergy transfer to the 5D0 level, the Eu3þ ion being initially inthe 7F0 ground state, by the exchange mechanism, is forbidden,even though this selection rule might be relaxed if the 7F1 level isthermally populated at high temperatures. Besides, this energytransfer process is also forbidden by the dipole–dipole anddipole–multipole mechanisms, according to the reduced matrixelements of the unit tensor operator U(l), as far as J-mixingeffects, which are small, are neglected.

The calculation of F factor is based on the methodologydescribed in Ref. [11] and starts from the energy level diagramsshown in Fig. 1. Energy conservation implies phonon creationwhen 9c

0

J0

M0

S is below the donor state, and phonon annihilationin the opposite case.

From these facts, and assuming a Boltzmann distribution forthe vibrational components of the electronic states, one mayshow that [11]

F �X

m,m’,n,n’

ð1�e�_ojn =kT Þe�m_ojn =kT/wjn

m 9wjm’S2ð1�e

�_oJ’=kTÞe�n’_o

J’=kT

/iJn9i

J’n’S2

ZgJ0

n0 ðEÞgjn

m ðEÞdEdE0 � dðDEþm_oj�m0_ojn�n0_oJ’þn_oJÞ

ð4Þ

where wjn

i , wji , iJi and iJ’

i are vibrational components, withfrequencies of, ojn , oJ and oJ’, corresponding to the electronicstates 9fS, 9fnS, 9cJMS and 9c

0

J0

M0

S, respectively, and DE is thezero-point energy difference between the donor and acceptorstates. gJ0

n0 ðEÞ and gjn

m ðEÞ are the band-shapes of the vibroniccomponents of the 4f and intraligand states, respectively.

By considering that /iJn9iJ0

n0S¼dn,n0 , which is an approximationoften assumed in lanthanide spectroscopy, and that both para-bolas shown in Fig. 1 have approximately the same curvature, onemay show that

F �Xm,m’

ð1�e�_oj=kT Þe�m_oj=kT/wjn

m 9wjm’S2R

�X

n’

ð1�e�_o

J’=kTÞe�n’_o

J’=kTgJ’

n’ðEÞ

24

35gj

n

m ðEÞdEdE0

�dðDEþðm�m’Þ_ojÞ ð5Þ

In Eq. (5), the term in brackets corresponds to the band-shapefunction for the electronic transition 9c

0

J0

M0

S- 9cJMS, G4f(E),which might be approximated by a Gaussian function [11]. Thistype of band-shape was also chosen for each vibronic componentof the state 9fnS. Thus, it may be shown that

F � FX

i ¼ maxð0,npÞ

1�e�_oj=kT� �

e�i_oj=kT/wjn

i�np9wji S

2ð6Þ

where np �DE=_o0 phonons are created, if DEr0, or annihilated,if DEZ0, in the energy transfer process. F is an overlap integralbetween two Gaussian functions, and is given by [9,11]

F ¼ln 2ffiffiffiffipp

1

_2g4f gvib

f½ð1=_gvibÞ2þð1=_g4f Þ

2�ln 2g

1=2

�exp1

4

ðð2D=_g4f2Þln 2Þ2

½ð1=_gvibÞ2þð1=_g4f Þ

2� ln 2

�ðD=_g4f Þ2ln 2

!ð7Þ

where _g4f and _gvib are the widths at half-height, respectively, ofthe 4f state and the vibronic component of the intraligandtransition, and D is the dissonance between DE and np_oj.

The sum in Eq. (6) can be described as a function of the so-called Huang–Rhys parameters, S, and it may be shown that

F � FX

i ¼ maxð0,npÞ

e�S/2mþ1SðS/mSÞiðS/1þmSÞi�np

i!ði�npÞ!ð8Þ

where /mS is the average vibrational quantum number given by

/mS¼

Pm

me�m_o0=kT

Pm

e�m_o0=kTð9Þ

4. Results and discussion

Fig. 2(a) shows the emission spectrum of the [Eu(tta)3(H2O)2]complex recorded in the spectral range from 570 to 630 nm, atroom and liquid helium temperatures (300 and 10 K), underexcitation at 325 nm using a He–Cd pumping laser. This photo-luminescence spectrum consists of the characteristic narrowbands of the Eu3þ ion assigned to the intraconfigurationaltransitions 5D0-

7F0 (580 nm), 5D0-7F1 (590, 592 and 596 nm)

and 5D0-7F2 (612, 615, 617, 620 and 622 nm), exhibiting the

hypersensitive 5D0-7F2 transition around 612 nm as the most

prominent. Fig. 2(b) presents several much less intense emissionbands (by three orders of magnitude), assigned to the 5D1-

7F0

(526 nm), 5D1-7F1 (534 ) and 5D1-

7F2 (557 nm) transitions,

Fig. 3. Luminescence transients of the 5D0 level of the [Eu(tta)3(H2O)2] complex

recorded in the temperature interval from 10 to 300 K under excitation at 355 nm:

Decay (a) and rising (b) curves of the 5D0 level. The inset shows the plot of risetime

(ms) versus temperature (K).

Fig. 4. Luminescence transients of the 5D1 level of the [Eu(tta)3(H2O)2] complex

recorded in the temperature interval from 10 to 300 K: Decay (a) and rising

(b) curves of the 5D1 level. The inset presents the plot of risetime (ms) versus

temperature (K).

Fig. 5. Luminescence lifetimes of the 5D1 level and risetimes of the 5D0 level of the

[Eu(tta)3(H2O)2] complex as a function of temperature.

W.M. Faustino et al. / Journal of Luminescence 137 (2013) 269–273272

showing the strongest emission intensity at 534 nm. In addition, asharp peak is observed at 564 nm in the emission spectrumrecorded at 300 K. However, this peak is absent at low tempera-ture, indicating that, at room temperature, this transition isoriginated from an upper 5D1 Stark level thermally populated.

Fig. 3(a) presents the luminescence decay curves of theemitting 5D0 level of the [Eu(tta)3(H2O)2] complex recorded attemperature interval from 10 to 300 K, under excitation at355 nm. The 5D0 lifetimes are in the millisecond scale; actuallythey are less than one millisecond (�0.2 ms) due to the presenceof water molecules in the first coordination sphere, as expected.Fig. 3(b) shows the very initial part of the transient curves, in themicrosecond scale, of this emitting level at different tempera-tures. The 5D0 risetimes (shown in the inset) were obtained as thearea under these curves from the initial time until the timecorresponding to maximal emission intensity.

The transient luminescence curves of the 5D1 level of the[Eu(tta)3(H2O)2] complex, recorded at several temperatures(10–300 K), are shown in Fig. 4. The decay curves also deviatefrom single exponentials due to intramolecular energy transferprocesses [Fig. 4(a)]. The very initial part of the transient curves ofthe 5D1 level, from the area under which the risetimes wereobtained, are shown in Fig. 4(b). To the best of our knowledge, it isthe first time that direct measurements of these risetimes in aneuropium complex at different temperatures were carried out.

Fig. 5 shows both the risetime of the emitting 5D0 and thelifetime of the 5D1 level at different temperatures. For eachtemperature these characteristic times are practically the same,which indicates that the 5D0 is populated directly from the5D1 level.

The theoretical approach developed in the previous section hasbeen applied to the analysis of the experimental results shown

Fig. 6. Experimental and theoretical energy transfer rates from the triplet state

(T1) to the 5D1 level of the [Eu(tta)3(H2O)2] complex. The theoretical curves

correspond to different values of the overlap integral in Eq. 3.

W.M. Faustino et al. / Journal of Luminescence 137 (2013) 269–273 273

above. In our analysis, we have considered only the temperaturedependent energy transfer rates from the triplet states to the 5D1

level since our experimental results indicate that the 5D0 is notefficiently populated from any level different from the 5D1. Thephonon energy was assumed to be �400 cm�1, which is typicalfor the vibrational stretching Ln3þ

�O in diketonate complexes[11] and in the calculations we have assumed _gvibCT �50 cm�1.The Huang–Rhys parameter was evaluated as 6.25, by consideringthe band width for the triplet state �3000 cm�1. The experi-mental energy transfer rates T-5D1 were assumed to be theinverse of the risetime of the 5D1 level obtained from Fig. 4(b).Theoretical and experimental temperature dependent energy

transfer rates from the triplet state to the 5D1 level of the[Eu(tta)3(H2O)2] complex are shown in Fig. 6.

Good agreement between the theoretical and experimentalprofiles of the temperature dependent energy transfer rates fromthe triplet state to the 5D1 level of the [Eu(tta)3(H2O)2] complex isobtained. This agreement strongly indicates that the lumines-cence sensitization of the Eu3þ ion in the complex is dominatedby the T-5D1-

5D0 path. This mechanism, which is qualitativelydiscussed in many papers reporting on luminescent europiumcomplexes, was also recently corroborated by Ha-Thi et al. [15].

In conclusion, the temperature dependent transient curves ofthe 5D1 and 5D0 levels in a model Eu3þ complex, reported here,for the first time, have been used along with a theoreticalapproach for the temperature dependent energy transfer ratesto corroborate the T1-

5D1-5D0 luminescence sensitization

pathway.

References

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Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196 (2000) 165.[6] J.-C.G. Bunzli, A.-S. Chauvin, H.K. Kim, E. Deiters, S.V. Eliseeva, Coord. Chem.

Rev. 254 (2010) 2623.[7] K. Binnemans, Chem. Rev. 109 (2009) 4374.[8] O.L. Malta, J. Non-Cryst. Solids 354 (2008) 4770.[9] O.L. Malta, J. Lumin. 71 (1997) 229.

[10] M. Klainerman, J. Chem. Phys. 51 (1969) 2370.[11] W.M. Faustino, O.L. Malta, G.F. de Sa, Chem. Phys. Lett. 429 (2006) 595.[12] R.G. Charles., R.C. Ohlmann., J. Inorg. Nucl. Chem 27 (1965) 255.[13 ] B.R. Judd, Phys. Rev. 127 (1962) 750.[14] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511.[15] M.-H. Ha-Thi, J.A.A. Delaire, V. Michelet, I. Leray, J. Phys. Chem. A. 114 (2010)

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