Tb3+âEu3+ Energy Transfer in Mixed-Lanthanide-Organic

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Tb3+#Eu3+ Energy Transfer in Mixed-Lanthanide-Organic Frameworks Marcelo Oliveira Rodrigues, José Diogo Lisboa Dutra, Luiz Antônio de Oliveira Nunes, Gilberto F. De Sa, W. M. De Azevedo, Patricia Silva, Filipe A. Almeida Paz, Ricardo Oliveira Freire, and Severino Alves Junior J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3054789 • Publication Date (Web): 28 Aug 2012 Downloaded from http://pubs.acs.org on September 3, 2012

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The Journal of Physical Chemistry

Tb3+→Eu3+ Energy Transfer in MixedLanthanide-Organic Frameworks Marcelo O. Rodrigues1,6*, José Diogo Lisboa Dutra,2 Luiz Antônio O. Nunes3, Gilberto F. de Sá4, Walter M. de Azevedo4, Patrícia Silva5, Filipe A. Almeida Paz5, Ricardo Oliveira Freire2, Severino A. Júnior 4* 1

Instituto de Química, Universidade de Brasília, 70910-900, Brasília – DF, Brazil. 2

3

Departmento de Química, UFS, 49100-000, São Cristóvão - SE, Brazil

Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970 São Carlos – SP, Brazil 4 5 6

Departamento de Química Fundamental, UFPE, 50590-470, Recife - PE, Brazil.

Departamento de Química, Universidade de Aveiro, CICECO, 3810-193 Aveiro, Portugal.

Programa de Pós Graduação em Química, DQF—UFPE, UFPE, 50590-470, Recife - PE, Brazil

TITLE RUNNING HEAD: Corresponding author footnote: * Prof. Dr. Marcelo O. Rodrigues ([email protected]) Instituto de Química, Universidade de Brasília, 70910-900, Brasília – DF, Brazil. Tel. +55 61 3107-3876; Fax: +55 61 3273-4149 *Prof. Dr. Severino Alves Júnior ([email protected]) Departamento de Química Fundamental, UFPE, 50670-901, Recife, PE, Brazil. Tel. +55 81 2126-7475; Fax: +55 81 2126-8442.

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ABSTRACT :

In this work, we report a theoretical and experimental investigation of the energy transfer mechanism in two isotypical 2D coordination polymers, ∞[(Tb1-xEux)(DPA)(HDPA)], where H2DPA is

pyridine

2,6-dicarboxylic

∞[(Tb0.95Eu0.05)(DPA)(HDPA)]

acid

and

and

x

=

0.05

or

∞[(Tb0.5Eu0.5)(DPA)(HDPA)],

0.50.

Emission

spectra

of

(1) and (2), show that the high

quenching effect on Tb3+ emission caused by Eu3+ ion, indicates an efficient Tb3+→Eu3+ energy transfer (ET). The kET of Tb3+→ Eu3+ ET and rise rates (kr) of Eu3+ as function of temperature for (1) are in same order of magnitude, indicating that the sensitization of the Eu3+ 5D0 level is highly fed by ET from the 5D4 level of Tb3+ ion. The ηET, and R0 values vary in the 67-79% and 7.15-7.93 Å ranges. Hence, Tb3+ is enabled to transfer efficiently to Eu3+ that can occupy the possible sites at 6.32 and 6.75 Å. For (2) the ET processes occur on average with ηET and R0 of 97% and 31 Å respectively. Consequently, Tb3+ ion is enabled to transfer energy to Eu3+ localized at different layers. The theoretical model developed by Malta was implemented aiming to insert more insights about the dominant mechanisms involved in the ET between lanthanides ions. Calculated single Tb3+→ Eu3+ ET are in the three order of magnitude inferior of those experimentally, however, it can be explained by the theoretical model which does not consider the role of phonon assistance in the Ln3+→ Ln3+ ET processes. In addition, the Tb3+→ Eu3+ ET processes are predominantly governed by dipole-dipole (d—d) and dipole-quadrupole (d—q) mechanisms.

KEYWORDS:

Lanthanides,

Energy

Transfer,

Luminescence,

Spectroscopy,

Metal-Organic

Frameworks.


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Introduction The research of lanthanides compounds grew considerably since the 90´s, when Lehn proposed the use of these compounds as Light Converting Molecular Devices (LCMDs).1 Historically, many researchers envisage this moment as a landmark in lanthanides chemistry2 because Lehn´s proposal was followed by the discovery of new technological applications of lanthanides luminescence, that encompass not only inorganic phosphors but also optical probes in immunoassays, electroluminescent materials, sensors, markers, etc.3-11 Development of materials containing, simultaneously, more than one type of Ln3+ ion into the structure open up not only the possibility of obtaining the luminescence over diverse regions of the visible and near-infrared spectrum,12-15 but also allows the investigating of the energy transfer (ET) process between different optical centers.16 The comprehension of non-radiative energy transfer among lanthanide ions is fundamentally important for prediction of new optic properties and the consequent development of efficient phosphors for displays and lighting, optical markers, lasers and solar cells.13,1723

However, several aspects such as the concentration effect of donor and acceptor species, 4f—4f

transitions intensities, energy integral overlaps, coupled wavefunctions and differentiation among the ET mechanisms are not fully understood yet. 24 In the last years, our research3 group has been developing several theoretical approaches which predict very well several spectroscopic properties for lanthanide compounds such as singlet and triplet k

energy positions, electronic spectra of lanthanide complexes, ligand field parameters, B q , 4f - 4f intensity parameters, Ωλ (λ = 2, 4 and 6), energy transfer rates, WET, between the lanthanide trivalent ions and the ligand, quantum yields and luminescence efficiencies.25-33 Following our interest, a new theoretical methodology was developed aiming to provide more insights in the Ln3+→Ln3+ energy transfer process, with emphasis in electric multipolar, Dipole—Dipole

(d—d), dipole—quadrupole

(d—q), quadrupole —quadrupole (q—q) and Exchange (Ex).24


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When the Eu3+ ion is used as energy receptor from Tb3+, it is common to observe that the luminescence from Eu3+ is strongly sensitized to the detriment of Tb3+ emission.34,35 Recent reports have reported the importance of modulation of ET between Tb3+ and Eu3+ ions aiming to obtain solid whiteemitting materials and probes based in luminescent switching.36-39 Although various examples of Lanthanide-Organic Frameworks (LnMOF) based on mixed lanthanide ions have been reported, the ionion energy transfer process has not been investigated in details.12,37,38,40 In this work, we report a complementary experimental and theoretical investigation of the ET mechanism between Tb3+ 5D4 →Eu3+ in two isotypical 2D Lanthanide-Organic Frameworks, ∞[(Tb0.95Eu0.05)(DPA)(HDPA)] ∞[(Tb0.5Eu0.5)(DPA)(HDPA)],

and

herein designated as (1) and (2) respectively (where H2DPA is pyridine

2,6-dicarboxylic acid). These materials were selected as model for this study due to the eﬃcient antenna eﬀect of DPA ligands and the high emission of Ln3+ ion corroborated by the lack of solvent molecules directly coordinated to the metal center.

Experimental details Synthesis of ∞[(Tb1-xEux)(DPA)(HDPA)] A mixture of pyridine-2,6-dicarboxylic acid, H2DPA, (0.7 mmol), Ln(NO3)3.6.H2O, 0.35 mmol, (Ln = Tb3+ and Eu3+ in appropriate ratios, 0.3325/0.0175 and 0.175/0.175 mmol for (1) and (2) respectively) and H2O (ca. 4 mL), was placed in a 10 mL IntelliVent reactor which was placed inside a CEM Focused Microwave™ Synthesis System Discover S-Class equipment. Reactions took place at 160ºC during 10 minutes, under constant magnetic stirring. The final materials were obtained in a yield of ca. 90% after washing with water, acetone and the air-dried. General Instrumentation The emission spectra of Stokes photoluminescence were achieved through standard optical setups. The visible signal went through a 0.30 m Thermo Jarrel Ash 82497 monochromator, and collected by photomultipler tube (R928). Room temperature photoluminescence were acquired upon excitation at 325 nm of a He-Cd laser. The measurements of rise times and emission decay curves were carried out with excitation at 355nm from a Nd:YAG laser (Continuun-Surelite SLII-10). The signal was filtered by ACS Paragon Plus Environment

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a monochromator collected by a photomultiplier and processed by a digital oscilloscope (Hewlett Packard 54501A 100 MHZ). From these curves the rise and life time values were obtained as those for which the emission intensity drops by a factor 1/e. X-ray powder diffraction analyses were performed at room temperature, using a Bruker D8 Advanced with DaVinci design equipped with a LynxEye Linear Position Sensitive Detector and a Copper (Cu) sealed tube (λkα1 = 1.5404 Å, λkα2 = 1.5444 Å, Iα2/Iα1 = 0.5). Intensity data were collected in step scanning mode, in the range from 5 to 50° (2θ), with a step size of 0.01°, Soller slit with 2.5° of divergence, 0.5° scattering slit and 0.6 mm receiving slit. The Rietveld refinement for (1) and (2) were performed with the software GSAS/EXPGUI, using as starting premise the atomic coordinates of the structural model previously reported.41 The preferential orientation was corrected using spherical harmonic model (sixth order) proposed by Jarvinen, the peak profile was adjusted by Thompson-CoxHastings function modified by Young and Desai (pV-TCHZ),42 surface roughness correction was refined by Pitschke function and background was fitted by an eighth-degree

shifted Chebyshev

polynomial function. In the final runs, the following parameters were refined: scale factor, background and absorption coefficients, spherical harmonic, unit-cell parameters and pV-TCHZ correction for asymmetric parameters. Theoretical Methodology Methodology for a Single Tb→Eu Energy Transfer Process Theoretical rates for a single Tb→Eu energy transfer process and the contributions of DipoleDipole (Wd-d), Dipole-Quadrupole (Wd-q), and Quadrupole-Quadrupole (Wq-q)

mechanisms were

calculated by Malta’s methodology.24 This method is based on following Kushida’s expressions that do not include the shielding effects for the energy transfer mechanisms:43-45

(1 − σ 1D ) 2 (1 − σ 1A ) 2 4π e 4  Wd −d =  Ω DK ψ D J D U ( K ) ψ D* J D* * 6 ∑ JD JA 3h R  K 2  ×  ∑ Ω KA ψ A J A U ( K ) ψ A* J A*  F  K 

[ ][ ]

2

  

(1)


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Wd − q × r

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(1 − σ 1D ) 2 (1 − σ 2A ) 2 2π e 4  =  Ω DK ψ D J D U ( K ) ψ D* J D* * 8 ∑ JD JA h R  K

2

[ ][ ]

2 2 A

Wq − q =

f C2 f

2

× ψ *A J A* U ( 2 ) ψ A J A

2

× ψ D J D U ( 2 ) ψ D* J D*

2

(2)

F

(1 − σ 2D ) 2 (1 − σ 2A ) 2 28π e 4 × r2 * 10 JD JA 5h R

[ ][ ]

  

2

r2

D

× ψ *A J A* U ( 2 ) ψ A J A

2

2 A

f C2 f

4

(3)

F

were the subscript indexes D and A are donor and acceptor species, [J] is

2(J) +1, R is donor

acceptor distance, r 2 represents the radial integrals between the a lanthanide ion and the electrons of 4f orbitals,46 (1 - σ) are shielding factors of the lanthanide ions, ψ J U ( k ) ψ * J *

2

element of the tensor operators U(l) from intermediate coupling, and

are reduced matrix

l Cλ l

are reduced matrix

elements of the Racah’s tensor operators.47 σ2, σ4 and σ6 values were calculated by Edvardsson and Klintenberg,48 σ1 was obtained from Equation 4 proposed by Malta:24

(1 − σ k ) = ρ ( 2 / β ) k +1

(4)

where β is a number very close to 1, ρ is the shielding factor expressed in terms of the radial overlap integral between the 4f sub-shell and valence shell of a ligand atom in the first coordination sphere and its typical value in lanthanide compounds is 0.05. Considering k = 1, it is possible to obtain the σ1 value. The F value presents in Equation (1), (2) and (3) arises from the overlap between the bands of donor emission and acceptor absorption. F was estimated from Equation (5):49


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 1 ln 2 1 F=  2 π h γ D γ A  hγ D     1 × exp  4  

2

  1   +   γ h   A

2

   ln 2   

−1

2

  2   ∆  − ln 2    2 h γ   D  1     ln 2 +    hγ D   

2∆ ln 2 ( hγ D ) 2  1  2   h γ  A 

(5)

where h γ is the band width at half-height and ∆ is the difference between donor and acceptor transition energies involved in the process. The typical F values for an even of lanthanides ions are in the 10-12 - 10-13 erg-1 range. All values for numerical estimative of ET process between two lanthanide ions are collected in Table 1. INSERT TABLE 1.

Results and Discussion Because the ionic radii of Tb3+ and Eu3+ ions are very similar small variations on cell parameters of the materials were expected. Figures S1 and S2 show the final Rietveld refinement plots for (1) and (2), which unequivocally show that the compounds herein reported and studied are indeed identical to those previously reported.41,50 The supramolecular structure of the materials consists of infinite twodimensional (2D) square-grid-type layers interconnected via offset π―π stacking. Figure 1 depicts the …

structure of the materials along of b axis, emphasizing the nearest Ln3+.. Ln3+ intermetallic distances. For each Ln3+ ion there are 14 nearest Ln3+ neighbors in the same layer, whose distances range between ca. 6.32 and 11,00 Å. The adjacent layer, each metal center is surrounded by 4 nearest neighbor within 10 Å, on average.

INSERT FIGURE 1 ACS Paragon Plus Environment

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Figure 2 shows the normalized steady-state emission spectra of (1) and (2) recorded in the 480720 nm range upon excitation at 325 nm at 300K. The spectra display the typical narrow bands corresponding to the centered Tb3+ 5D4→7FJ and Eu3+ 5D0→7FJ transitions. It is important to note that the high quenching effect over the Tb3+ emission caused by Eu3+ even with just 5%, indicates an efficient Tb3+→Eu3+ ET.

INSERT FIGURE 2

The Eu3+ emission bands are particularly influenced by the chemical environment of the first coordination sphere. The relative intensity and splitting of the respective transitions can, thus, be used as probe of symmetry sites.32,51 The (1) and (2) steady-state emission spectra show three well-defined Stark components attributed to Eu3+ 5D0→7F1 transition, indicating a low symmetry around the metal ions. The selection rules for the electric dipole transition indicate that the 5D0→7F0 is only observed if the point symmetry of Eu3+ is either Cnv, Cn or Cs. According with crystallographic studies, the substitution of Tb3+ sites by Eu3+ does not cause structural changes, consequently, the Ln3+ ions are localized in a coordination environment with local symmetry C1.41 Therefore, the analysis of the local lanthanide ion chemical environment provided by emission spectroscopy is in good agreement with the X-ray diffraction data. In accordance with the Förster-Dexter theory, the basic condition to occur the ET among two species is the spectral overlap of the acceptor absorption and the donor emission.52 In this case the requirement for ET is completely fulfilled around 540 nm and 590 nm where the transitions 5D4→7F5 and the 5D4→7F4 of Tb3+ emission overlap, respectively,

7

F1→5D1 and 7F0→5D0 of Eu3+ absorption

(Figure 3). Indeed, it is well know that at low temperatures the 7F1 manifold is poorly populated, while the population of 7F2 state can be ignored, therefore the Tb3+→Eu3+ ET 5

occurs especially through

D0.53,54 At temperatures above 100K, the ET channels have been increased by thermal population of ACS Paragon Plus Environment

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the

The Journal of Physical Chemistry 7

F1 and 7F2 levels, hence, large values of constants rates, kET, and efficiency, ηET, have been

expected. It is important to note that the participation of forbidden Eu3+ transitions 5D0→7F0

in the

ET process is intimately attributed with the local C1 symmetry of the Ln3+ sites.

INSERT FIGURE 3

Figure 4 exhibits the lifetimes as a function of temperature for (1) obtained upon excitation at 355 nm while monitoring the 5D4 and 5D0 emissions of Tb3+ and Eu3+ ions.

INSERT FIGURE 4

The fluorescence lifetimes of Eu3+ and Tb3+ ions are reduced by the temperature increases, due to the effect of multi-phonon relaxations. Similarly, the same effect is responsible for the reduction of the emission lifetimes of both ions in (2) whose values are measured at 6, 77 and 300 K (See Table S1 in the ESI). The Tb3+→ Eu3+ ET

produce a considerable decrease in the decay times of Tb3+, on average

27 % compared with the pure Tb3+ compound (See Figures 11S and 12S in the ESI). However, the curves decay exponentially even in the presence of the acceptor Eu3+ ion. In conjunction with the intermetallic distance and the fact that the Tb3+ ions find themselves in same crystal domain, the Tb3+→ Tb3+ energy diffusion becomes very rapid.55,56 Consequently the ET rates for randomly distributed Tb3+→ Eu3+ pairs are averaged out, justifying a single exponential decay behavior of Tb3+.57

In

contrast to the case of slow donor—donor energy migration is expected that the decay curve of the donor presents a non-exponential behavior at short time domains caused by direct ET from the donor to the nearest acceptor neighbors and an exponential component at long times due to energy diffusion among donors.58,59 In terms of the Eu3+ 5D0 lifetimes measured at 300 K, an increase of the decay emission was observed to 1.60 ms for (1) in comparison with 1.35 ms reported for the Eu3+ pure ACS Paragon Plus Environment

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compound.41

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For (2) a substantial increase (1.40 ms) was not observed. It was justified by the

concentration quenching effect. The kET and ηET, of Tb3+→ Eu3+ ET may be estimated from Equations (6)and (7):60,61

k ET = τ 1−1 − τ 0−1

(6)

τ 1−1 − τ 0−1 = τ 1−1

(7)

η ET

were τ0 and τ1 are fluorescence lifetimes monitoring 5D4 emission of Tb3+ ion at 545 nm of [Tb(DPA)(HDPA)] and [(Tb1-xEux)(DPA)(HDPA)] respectively. Considering an electric dipolo—electric dipolo interaction between Tb3+ and Eu3+ ions and that the effect of the lanthanide contraction is insignificant, the critical transfer distance was estimated from Equation (8):11

k ET

where R is the

3 R  = K 2τ 0−1  0  2  R 

6

(8)

Ln3+ ions pair distance (6.32 Å), K2 is the orientation factor (K2=2/3 in

random orientation) and R0 is the critical transfer distance. The decay curve of Eu3+ cation in (1) displays rise and decay fractions which indicates a slow cooperative processes feeding the emitting levels. Rise times for the Eu3+ ion as a function of temperature in (1) were estimated from Equation (9):62

 − (t − t 0 )   n = n0 + n1 1 − exp τ r   

(9)


10

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where n0, n1, t0 and τr, are the populations of the emitting state 5D0, populations of 5D1 excited state, oscilloscope delay and decay times for 5D1→5D0 process, respectively. The behavior of the lifetime of Tb3+ and rise time of Eu3+ ions deserves some explanations. While fluorescence lifetimes give information about radiative and non-radiative decay processes, ET rates and efficiency of the Tb3+—Eu3+ interaction, on the other hand, the rise time curves of Eu3+ have implicit details of the ET mechanism, simply because they probe the distribution of photon populations on the excited states during the energy migration between ligands and both Ln3+ ions.63 Figure 5 shows the data related to kET of the Tb3+→ Eu3+ ET and rise rates (kr) of Eu3+ ion as a function of temperature for compound (1).

INSERT FIGURE 5

Between 6 and 120 K the values of kET are noticeable increased due to multi-phonon intermediation. In the 120-300K range the rates were gradually reduced. This can be explained due to energy gap among the Tb3+ and Eu3+ levels, 5D4 and 5D1, which is of 1300 cm-1, thus small enough to allow the thermally activated back ET processes, typical of this temperature range. The kr values agree well with those reported for related Tb3+/Eu3+ compounds.60 Interestingly, kr and kET are in same order of magnitude, indicating that the sensitization of the Eu3+ 5D0 level is highly affected by ET from 5D4 of the Tb3+ ion. The ηET, and R0 values vary between 67-79% and in between 7.15-7.93 Å respectively. Hence, the Tb3+ ion is enabled to transfer efficiently to the Eu3+ ions which can occupy the possible sites at 6.3 and 6.75 Å. In case of (2), ET processes occur on average with ηET and R0 of 97% and 31 Å respectively. Consequently, Tb3+ ion is enabled to transfer energy to Eu3+ centers localized in different layers. The percolation approach has been considered an interesting model to describe the energy migration in lanthanides materials.64 Considering that at low concentration the Eu3+ ions are arranged in the LnMOF matrix as segregated micro-regions (small “clusters”) or fully surrounded by Tb3+ ion, the ACS Paragon Plus Environment

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Tb3+→Eu3+ET occurs only among nearest-neighbors. At high concentration,

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the Eu3+ ions are

predominantly arranged in “clusters”, therefore, only those ions in ideal distances are apt to be excited by Tb3+, while the Eu3+ ions out of this criteria are exclusively excited via ligands or Eu3+→Eu3+ energy diffusion. In this context, the sample (1) is more adequate than (2) to access the impact of the energy transferred from Tb3+ over Eu3+ luminescence, because the negative effect of concentration quenching, typical of Eu3+ materials, are minimized. In this stage of discussion, it is important to make clear that the values of

kET obtained from

Equation (9) represent an average of Tb3+→ Eu3+ ET. The model represented by (1) has only a handful of Ln3+ sites, 5.0%, randomly occupied by acceptor species (Eu3+ ions), and a distribution of distinct populations of Tb3+ ions (donor specie) discriminated by the interaction levels with acceptors or other donors. Therefore, measuring the magnitude of a single Tb3+→ Eu3+ ET can be regarded as an impossible task, because each population of Tb3+ ions within the effective interaction sphere of the Eu3+ ion is characterized by local ET process. Other important question is to measure the contributions of the mechanisms dipole—dipole

(d—d), dipole—quadrupole (d—q), quadrupole —quadrupole (q—q) and

exchange (Ex) in energy transfer processes. Malta´s model was applied aiming to insert new insights about the dominant mechanisms involved in the ET between lanthanides ions. For the calculation were considered R as the shortest Tb3+—Eu3+ distances ca. 6.3 Å, and the ET channels Tb3+/5D4→ Eu3+/5D1 and Tb3+/5D4→ Eu3+/5D0. ET rate values for a single Tb3+→ Eu3+ process (Table 2) are in the three orders of magnitude inferior to those obtained experimentally, 10 to 20 s-1 and 5500 s-1 respectively. This difference may be designated to the theoretical model which does not consider the role of phonon assistance in the Tb3+→ Eu3+ ET. Additionally, the results suggest for channels investigated that the ET process are predominantly governed by d—d and d—q mechanisms.

INSERT TABLE 2


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Conclusions This work reports an experimental and theoretical approach to the energy ET between Tb3+ 5D4 →Eu3+ in two isotypical 2D Lanthanide-Organic Frameworks ([(Tb1-xEux)(DPA)(HDPA)],

where

H2DPA is pyridine 2,6-dicarboxylic acid and x = 0.05, and 0.50 mol respectively). Steady-state emission spectra of the samples [(Tb0.95Eu0.05)(DPA)(HDPA)] and [(Tb0.5Eu0.5)(DPA)(HDPA show a high quenching effect over Tb3+ emission caused by Eu3+ ion even with just 5%, indicating an efficient Tb3+→Eu3+ ET. The Tb3+→ Eu3+ ET rates (kET) for (1) and (2) and rise rates (kr) of the Eu3+ ion for (1) were acquired at different temperatures. The Tb3+ ion in (1) is enabled to transfer efficiently (average 73%) to the Eu3+ ions that can occupy the possible sites at ca.6.3 and 6.75 Å, while for (2) the energy transfer occurs on average with ηET and R0 of 97% and ca. 31 Å respectively. kr and kET for (1) are in same order of magnitude, indicating that the ET from 5D4 of Tb3+ ion

contributing

to the

sensitization of the Eu3+ 5D0 level is highly affected by energy transfer from 5D4 of Tb3+ ion. The role of the dipole—dipole

(d—d), dipole—quadrupole (d—q), quadrupole —quadrupole (q—q) and

exchange (Ex) mechanisms in energy transfer processes were estimated by Malta´s model. ET rate values for a single Tb3+→ Eu3+ process are inferior of those experimentally obtained, however, it may be justified by the theoretical model which does not consider the role of phonon assistance in the Tb3+→ Eu3+ ET. The energy transfer channels herein investigated are predominantly governed by

d—d and

d—q mechanisms.

Acknowledgment: The authors gratefully acknowledge CNPq (INCT/INAMI and RH-INCT/INAMI), PRONEX-FACEPE-CNPq (APQ-0859-1.06/08) and FCT (PTDC/QUI-QUI/098098/2008—FCOMP01-0124-FEDER-010785) for its financial support and CETENE and LNLS (proposal D10A-XRD2— 10164) for providing the facilities. We also wish to thank prof. Dr, Petrus Santa Cruz that provides the SPECTRA LUX Software. This software freely available through of the email [email protected]. Supporting Information Available: Lifetimes curves of (1), (2) and [Tb(DPA)(HDPA)] . Additional drawings showing FT-IR, TGA/DTG, Decay curves, emission spectra. CIE diagram and X-ray powder patterns. This material is available free of charge via the Internet at http://pubs.acs.org. ACS Paragon Plus Environment

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Tables Table 1 – Numerical estimative of the ET process. λ= 2

λ= 4

λ= 6

l Cλ l

-1.366

1.128

-1.27

rλ

(Eu3+)

0.9175

2.0200

9.0390

rλ

(Tb3+)

0.8220

1.6510

6.8520

σλ (Eu3+)

0.502

0.0190

-0.0308

σλ (Tb3+)

0.486

0.0193

-0.0300

σ1 (Tb3+) = σ1 (Eu3+) = 0.900

Table 2 - Calculated values of single Tb3+→Eu3+energy transfer for channels Tb3+/5D4→ Eu3+/5D1 and Tb3+/5D4→ Eu3+/5D1 and contributions of the mechanisms dipole—dipole (d—d), dipole—quadrupole (d—q), quadrupole —quadrupole (q—q) and Exchange (Ex). d–d d–q q–q Ex

Tb3+/5D4→ Eu3+/5D1

Tb3+/5D4→ Eu3+/5D1

12.136

21.229

13.208

19.457

5.434

8.005

6.313 x 10-16

3.681 x 10-16


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Figure Captions Figure 1: Schematic representation of the ∞[(Tb1-xEux)(DPA)(HDPA)] material, showing Ln3+….Ln3+ intermetallic distances among the nearest neighbors (8.38Å along of b axis). Hydrogen atoms were omitted to clarify. Figure 2: Steady-state emission spectra acquired at room temperature of the compounds (1) and (2) upon excitation at 325 nm. Tb3+ and Eu3+ transitions are represented in green and red respectively. Figure 3: Comparison of the excitation and emission lines of Eu3+ (solid red line) and Tb3+ (solid green line) ions. Shaded regions indicate the ion-ion energy transfer channels.

Figure 4: Decay times as a function of temperature for (1) upon excitation at 355nm while monitoring: (a) 5D4 of Tb3+ at 542 nm; (b) 5D0 of Eu3+ at 616 nm. Figure 5: Temperature dependence of the Tb3+→Eu3+ energy transfer constant for (1).


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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TOC


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Tb3+âEu3+ Energy Transfer in Mixed-Lanthanide-Organic

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