Dual role of a di-urethanesil hybrid doped with europium β-diketonate complexes containing either...

PAPER www.rsc.org/materials | Journal of Materials Chemistry

Dual role of a di-urethanesil hybrid doped with europium b-diketonatecomplexes containing either water ligands or a bulky chelating ligand†

Mariana Fernandes,a Sonia S. Nobre,b Maria Cristina Goncalves,a Ana Charas,c Jorge Morgado,c

Rute A. S. Ferreira,*b Luıs D. Carlosb and Veronica de Zea Bermudez*a

Received 31st July 2008, Accepted 13th November 2008

First published as an Advance Article on the web 19th December 2008

DOI: 10.1039/b813155d

In the present work we report the unusual role played by a sol-gel derived di-urethane cross-linked

poly(oxyethylene) (POE)/siloxane (di-urethanesil, d-Ut(600)) hybrid matrix in the immobilization of

the b-diketonate aquocomplex Eu(btfac)3(H2O)2 (btfac� is 4,4,4-trifluoro-1-phenyl-2,4-butanedionate)

and in the quasi preservation in the hybrid of the 5D0 quantum efficiency q(5D0) value displayed by this

complex in the isolated state (0.12 versus 0.18, respectively). We demonstrate that the d-Ut(600)

framework acts as an inert (although optically active) support towards Eu(btfac)3(H2O)2, enabling Eu3+

sensitization by the btfac� ligands and energy transfer between the hybrid host excited states and the

lanthanide intra-4f6 levels, while the number of Eu3+-coordinated water molecules remains constant.

We also provide evidence that the incorporation of the Eu(btfac)3phen (phen is 1,10-phenantroline)

complex into the same hybrid matrix is disadvantageous from the 5D0 quantum efficiency standpoint,

because of the severe steric hindrance that emerges between the POE chains of the host structure and

the bulky phen molecules, leading to the expulsion of these ligands from the Eu3+ first coordination

sphere and to their replacement by two water molecules. As a consequence, embedding of

Eu(btfac)3phen in d-Ut(600) results in a significant reduction of the q(5D0) value of the isolated complex

(from 0.47 to 0.16). The behaviour observed in the presence of this bidentate chelating ligand correlates

well with the practically identical q(5D0) values and number of Eu3+-coordinated water molecules (ca. 2)

estimated for the two hybrids.

Introduction

Photonic materials lend themselves to technological application

in a wide variety of domains, including optical communications,

aerospace, sensing, lighting, computing and bio-medicine.

Traditionally photonic devices have been produced from semi-

conductors, glasses or polymers.

Owing to their unique luminescent features, in particular

narrow emitting bands and therefore quasi monochromatic

behaviour, lanthanide ions have been used extensively in this

context. Lanthanide ions exhibit, however, low luminescence

intensity owing to the fact that the 4f–4f transitions are parity

forbidden. One of the most successful strategies that has been

employed to overcome this drawback is the complexation of

the lanthanide ions through coordination to organic ligands

aDepartment of Chemistry, CQ-VR, University of Tras-os-Montes e AltoDouro, 5001-801 Vila Real, Portugal. E-mail: [email protected]; Fax:+351 259 350480; Tel: +351 259 350253bDepartment of Physics, CICECO, University of Aveiro, 3810-193 Aveiro,Portugal. E-mail: [email protected]; Fax: +351 234 378197; Tel: +351 234378103cInstituto de Telecomunicacoes, Department of Chemical Engineering,Instituto Superior Tecnico, 1049-001 Lisboa, Portugal

† Electronic supplementary information (ESI) available: FT-IR spectraof the Eu(btfac)3L complexes: (a) L ¼ (H2O)2 and (b) L ¼ phen; SEMmicrograph of the d-Ut(600)200Eu(btfac)3(H2O)2 di-urethanesil; XRDpatterns of the d-Ut(600)n Eu(btfac)3L di-urethanesils: A: L ¼ (H2O)2:(a, black line) n ¼ 400; (b, red line) n ¼ 200; B: L ¼ phen: (a, blackline) n ¼ 400; (b, blue line) n ¼ 200. See DOI: 10.1039/b813155d

This journal is ª The Royal Society of Chemistry 2009

(so-called ‘‘antennae’’1,2) acting as sensitisers.3–6 These ligands

absorb UV light and transfer the energy efficiently to the central

lanthanide ion (ligand-to-metal intramolecular energy transfer),

which then undergoes the corresponding radiative emitting

process.

The design of room temperature highly luminescent lantha-

nide complexes depends strongly on the efficiency of energy

transfer and on the concentration of the quenching species

present (e.g., OH groups that lead to non-radiative processes).

Apart from quantum efficiency, other critical issues, such as

solubility, volatility, photodegradability and thermal stability,

have severely limited the practical application of lanthanide

complexes. In the past few years it has been recognized that the

combination of the versatile sol-gel synthetic route7 with the

organic/inorganic hybrid concept8 allows these drawbacks to be

successfully circumvented. Firstly, the sol-gel process allows to

process, at mild conditions (moderate temperatures), homoge-

neous, transparent, easily shaped, thermally and mechanically

stable materials from a wide variety of cheap, pure and available

precursors. Secondly, the sol-gel chemistry enables the combi-

nation of organic and inorganic components, and as a conse-

quence tuning the properties of the final hybrid material. The

addition of lanthanide complexes to sol-gel derived organic/

inorganic hybrid materials is thus extremely attractive, because

the hybrid networks offer multifunctionality and tailored

features from the nanometric to the millimetric length scales.8–10

This combined strategy has resulted in the development

of several luminescent systems based on host di-urea and

J. Mater. Chem., 2009, 19, 733–742 | 733

Fig. 1 Absorption spectra of the Eu(btfac)3L complexes: (a) L ¼ (H2O)2

and (b) L ¼ phen.

di-urethane cross-linked poly(oxyethylene) (POE)/siloxane hybrid

matrices (di-ureasils11 and di-urethanesils,12 respectively).13

Although various Eu3+-based complexes have been embedded in

the di-ureasil media,14–19 only one study has concerned a POE/

siloxane di-urethanesil network.20 The framework employed in

the latter work, represented by the notation d-Ut(600) (Ut

represents the urethane linkage and 600 is the average molecular

weight of the poly(ethylene glycol) precursor in gmol�1, corre-

sponding to about 13 oxyethylene repeat units) was doped with

the Eu(tta)3phen (tta� is thenoyltrifluoracetonate and phen is

1,10-phenanthroline) complex.

Some of these studies17,19 suggested that the use of an aquo-

complex may be of the utmost interest.

A remarkable enhancement of the q(5D0) value of

Eu(tta)3(H2O)2 from 0.27 to 0.74 was achieved upon incorpo-

ration of this aquocomplex into the di-ureasil network d-U(600)

(U represents the urea linkage and 600 is the average molecular

weight of the diamine precursor in gmol�1, corresponding to

about 8.5 oxyethylene repeat units).17 This result was correlated

with the fact that doping was accompanied by the displacement

of the two water ligands from the Eu3+ local environment, and

their subsequent replacement by ether oxygen atoms of the host

POE chains.17

The Eu(tta)3(H2O)2 complex was also introduced into a co-

condensed d-U(900)/d-U(600) (900 gmol�1 corresponds to about

15.5 oxyethylene repeat units) di-ureasil-type hybrid network.19

For a sample with n ¼ 400 (n, composition, is the ratio of oxy-

ethylene moieties per Eu3+ ion), whilst a slight reduction (5–30%)

in the 5D0 radiative paths occurred with respect to those of the

isolated complex, the 5D0 non-radiative channels decreased

substantially (78%).19 The latter effect, essentially ascribed to the

removal of two water molecules from the Eu3+ first coordination

sphere of the complex, was considered to be the main effect

responsible for the increase of the q(5D0) value from 0.2717 in the

isolated complex to 0.57 in the doped hybrid sample.19

More recently, we demonstrated that the addition of

Eu(tta)3(H2O)2 to a poly(3-caprolactone) (PCL(530))/siloxane

di-urethanesil biohybrid host yields an effective protecting cage

that efficiently encapsulates the emitting centres and reduces

luminescence quenching.21 The results obtained indicated that

one (two) carbonyl oxygen atom(s) of the PCL(530) chains of the

host matrix entered the Eu3+ coordination shell, thus replacing

the labile water molecule(s). The significant increase of the D0

quantum efficiency q(5D0) value of the doped hybrid with respect

to that of Eu(tta)3(H2O)2 (0.44 versus 0.29, respectively) revealed

that complex anchoring to the PCL(530)-based host structure

contributed to enhance the q(5D0) value.21

In the present work this attractive synthesis strategy, which

relies on the addition of lanthanide b-diketonate aquocomplexes

to polymer/siloxane hybrid networks possessing available cation

coordinating sites—present in the cross-links and/or in the

polymer chains—that enable in situ formation of new b-diketo-

nate complexes devoid of water ligands and improved optical

features, has been explored in the context of our investigation of

the di-urethanesils. We have doped the d-Ut(600) matrix with

two different concentrations of the Eu(btfac)3(H2O)2 and

Eu(btfac)3phen (btfac� is 4,4,4-trifluoro-1-phenyl-2,4-butanedi-

onate) complexes and have determined the morphology, struc-

ture, thermal characteristics and photoluminescent features of

734 | J. Mater. Chem., 2009, 19, 733–742

the resulting hybrid materials. We note that, since various

different acronyms may be found in the literature for this b-

diketonate ligand, we have decided to adopt here the designation

suggested by Binnemans.22 We also emphasize that the

Eu(btfac)3phen complex has been proposed for the production of

organic/inorganic white-light phosphors.23

Results and discussion

Characterization of the complexes

The ultraviolet-visible (UV-VIS) spectra of the Eu(btfac)3(H2O)2

and Eu(btfac)3phen complexes, depicted in Fig. 1, exhibit broad

bands with high molar absorption coefficients. In both cases the

lowest-energy absorption maxima occurs near 324 nm (Fig. 1). In

the spectrum of the aquocomplex this feature dominates the

whole spectral interval (Fig. 1(a)). This common band corre-

sponds to ligand singlet-to-singlet transitions and is thus asso-

ciated with the btfac� ligand.24–26 In addition, the absorption

spectra of the two complexes display another feature at 255 and

262 nm, respectively (Fig. 1), also associated with intraligand p–

p* transitions.25,26 In the case of Eu(btfac)3phen this feature

receives the contribution of the phen ligand which absorbs in the

same spectral region (not shown).

The HOMO and LUMO energy levels of Eu(btfac)3phen were

calculated combining cyclic voltammetry (CV) data and the

value of the lowest energy absorption edge retrieved from the

UV-VIS absorption spectrum. The oxidation and reduction

potentials of the complex, obtained from CV, allowed us to

determine its ionization energy (IE) and electron affinity (EA)

values, using the following set of three equations:27

IE (eV) ¼ Eonsetox + 4.35 (1)

E(HOMO) ¼ �IE (2)

EA (eV) ¼ Eonsetred + 4.35 (3)

It is worth noting that the IE and EA are the relevant energy

levels for charge injection when a complex is used in light-emit-

ting devices. The HOMO energy level of the Eu(btfac)3phen

complex derived from equations (1)–(3) was �5.27 eV. The


Scheme 1 Structure of the Eu(btfac)3L complexes: A: L ¼ (H2O)2 and B:

L ¼ phen.

LUMO energy level was determined by means of the following

equation:

E (LUMO) ¼ E (HOMO) + DEopt (4)

where DEopt is the optical energy gap that corresponds to the

lowest energy absorption edge of the UV-VIS absorption spec-

trum (approximately 400 nm, see Fig. 1). The value deduced for

the LUMO energy of Eu(btfac)3phen was �2.06 eV.

The thermal gravimetric analysis (TGA) curve of Eu(btfa-

c)3(H2O)2 displays a mass loss of ca. 5% with onset at about

93 �C (Fig. 2A), that confirms the release of two water molecules

from the first coordination sphere of the Eu3+ ion in the complex.

The sample then starts to decompose gradually. Beyond ca.

240 �C the degradation rate increases significantly (Fig. 2A). In

the differential scanning calorimetry (DSC) thermogram of the

aquocomplex (Fig. 2A) two endotherms are evident: a peak

centred at 134 �C (onset at approximately 100 �C), associated

with the removal of water molecules from the lanthanide coor-

dination sphere, and a shoulder at 140 �C, which is very likely

due to the fusion of the complex. We note that its melting point

has been reported to be situated at 107–110 or 148–150 or

93.6 �C.22 The TGA curve of the Eu(btfac)3phen complex proves

that this compound is thermally stable up to 260 �C (Fig. 2B),

a finding that confirms that the replacement of the water mole-

cules by a chelating ligand, such as phen, is clearly beneficial. The

sharp and intense peak seen in the DSC thermogram of Fig. 2B

indicates that Eu(btfac)3phen melts at 200 �C, in accordance with

literature data.22

To further investigate the chemical composition of the

lanthanide coordination sphere of the two complexes we used

Fourier transform infrared (FT-IR) spectroscopy. The pair of

characteristic C]O and C]C stretching bands of coordinated

b-diketonates are found in the FT-IR spectra of Eu(btfa-

c)3(H2O)2 and Eu(btfac)3phen (Fig. S1 of the ESI†) at 1608 and

1531 cm�1 and at 1613 and 1530 cm�1, respectively.28,29 As

expected, a broad band at 3386 cm�1, due to the O–H stretching

mode of the water ligands is present in the FT-IR spectrum of the

aquocomplex, but absent in that of the Eu(btfac)3phen complex.

Fig. 2 DSC (left axis) and TGA (right axis) curves of the Eu(btfac)3L

complexes: A: L ¼ (H2O)2 and B: L ¼ phen.


The observed shift of the C–N stretching mode from 1643 cm�1 in

the free phen ligand to about 1620 cm�1 in the Eu(btfca)3phen

complex allowed us to confirm that the substitution of the water

ligands of Eu(btfac)3(H2O)2 by the phen ligand had been

successful.28

Scheme 1A and 1B show the structure of the Eu(btfac)3(H2O)2

and Eu(btfca)3phen complexes, respectively, derived from the

TGA, DSC and FT-IR results.

Characterization of the hybrids

In agreement with the terminology adopted in previous publi-

cations,12,30,31 the di-urethanesil samples examined here were

identified using the notation d-Ut(600)nEu(btfac)3(H2O)2 and d-

Ut(600)nEu(btfac)3phen.

The di-urethanesil matrix displays a non-porous texture.32 No

porosity is detected either in the samples doped with the

Eu(btfac)3(H2O)2 and Eu(btfac)3phen complexes (Fig. S2 of the

ESI†). A comparison of the scanning electronic microscopy

(SEM) micrographs of the doped xerogels with that of the non-

doped framework32 leads us to conclude that the guest complexes

are homogeneously dispersed within the hybrid framework.

We may infer from the TGA data reproduced in Fig. 3A that

the hybrid materials synthesized are thermally stable up to about

150 �C. At higher temperature decomposition takes place at

a regular weight loss rate. The DSC thermogram of d-

Ut(600)200Eu(btfac)3phen displays a weak endothermic event

centred at approximately 200 �C (Fig. 3B), a proof of the pres-

ence of a minor proportion of free Eu(btfac)3phen complex at

this composition. In contrast, the DSC curves of the Eu(btfa-

c)3(H2O)2-based samples and that of d-Ut(600)400Eu(btfac)3phen

prove that these materials are entirely amorphous.

The X-ray diffraction (XRD) patterns of the d-

Ut(600)400Eu(btfac)3(H2O)2, d-Ut(600)200Eu(btfac)3(H2O)2 and

d-Ut(600)400Eu(btfac)3phen di-urethanesils corroborate the

DSC results (Fig. S3 of the ESI†). The apparent contradiction

found in the XRD pattern of d-Ut(600)200Eu(btfac)3phen,

which, unlike DSC data, suggests that the material is amorphous,

is probably a consequence of the fact that the proportion of

crystallinity present in this xerogel lies below the limit of detec-

tion of the XRD equipment employed. All the diffractograms

display a broad peak located near 21� (Fig. S3 of the ESI†)

associated with the coherent diffraction of the siliceous

domains.33 The structural unit distance calculated by means of

the Bragg law is ca. 4.24 A.

J. Mater. Chem., 2009, 19, 733–742 | 735

Fig. 4 Excitation spectra monitored at 612.0 nm for: A: Eu(btfa-

c)3(H2O)2 (a, blue line) and Eu(btfac)3phen (b, black line) complexes and

B: d-Ut(600)400Eu(btfac)3(H2O)2 (a, black line) and d-Ut(600)400Eu(bt-

fac)3phen (b, red line) di-urethanesils. (*) and (**) denote the intra-4f6 self-

absorptions 7F0 / 5L6, 5D3, respectively. The inset shows the excitation

spectrum of the d-Ut(600)nEu(btfac)3phen hybrid monitored at 530 nm.

Fig. 3 A: TGA curves of the d-Ut(600)nEu(btfac)3L di-urethanesils: (a,

thick line) L¼ (H2O)2 and n ¼ 400; (b, dash line) L¼ (H2O)2 and n ¼ 200;

(c, dot line) L ¼ phen and n ¼ 400; (d, thin line) L ¼ phen and n ¼ 200. B:

DSC curves of the d-Ut(600)nEu(btfac)3L di-urethanesils: (a) L ¼ (H2O)2

and n ¼ 400; (b) L¼ (H2O)2 and n ¼ 200; (c) L ¼ phen and n ¼ 400; (d) L

¼ phen and n ¼ 200.

Fig. 4A compares the excitation spectrum of the Eu(btfa-

c)3(H2O)2 complex with that of Eu(btfac)3phen monitored within

the 5D0 / 7F2 transition. The spectra are composed of a large

band with at least four components around 265, 290, 325 and

380 nm and of a series of Eu3+ intra-4f6 lines ascribed to the7F0,1 / 5D2,1 transitions. Two self-absorptions assigned to the7F0 / 5L6, 5D3 transitions are superimposed over the broad

band. Comparison of the two spectra allows detecting changes in

the relative intensity of the excitation components. In particular,

for the aquocomplex the most intense component peaks at 380

nm, whereas for Eu(btfac)3phen the most intense feature appears

around 265 nm. Moreover, the relative intensity of the Eu3+

intra-4f6 lines is higher for the Eu(btfac)3(H2O)2 complex. These

differences indicate that the replacement of the water molecules

by the phen ligand impacts on the Eu3+ excitation paths,

favouring the sensitization process with respect to direct intra-4f6

excitation. Except for the lowest excitation energy component

(380 nm), the remaining components resemble those observed in

the UV/VIS absorption spectra (Fig. 1), being therefore ascribed

to ligand singlet-to-singlet transitions associated with the btfac�

ligand and to intraligand p–p* transitions.24–26

At n ¼ 400 the incorporation of the complexes into the di-

urethanesil hybrid matrix modifies the relative intensity of the

btfac� related components. While for d-Ut(600)400Eu(btfac)3-

phen a significant reduction in the relative intensity of the Eu3+

736 | J. Mater. Chem., 2009, 19, 733–742

intra-4f6 lines results, for the di-urethanesil doped with the

aquocomplex, in an increase in their relative contribution to the

excitation spectra (Fig. 4B), indicating for the former hybrid an

increase in the Eu3+ sensitization process relative to that found in

the isolated complex. Furthermore, a broad band in the high-

wavelength side of the excitation spectra (not present in the

excitation spectra of the isolated complexes shown in Fig. 4A) is

detected in both hybrids. This band may be ascribed to the

contribution of the hybrid host excited states. This suggestion is

reinforced by the excitation spectrum monitored within the

hybrid host typical emission band (530 nm),13,20 as we will

demonstrate in detail next. As shown in the inset of Fig. 4B, such

a spectrum displays a large broad band between 380 and 520 nm.

Fig. 5 compares the emission features of the complexes under

excitation into the ligands (325 nm) and via intra-4f6 levels (5D2,

465 nm). All the spectra display the typical Eu3+ 5D0 / 7F0-4

transitions. The variation in the excitation energy induces for

both complexes changes in the relative intensity of the Stark

components, a result that suggests a large distribution of

similar Eu3+ local environments. A comparison of the spectra

of the two complexes allows the detection of changes in the

energy and relative intensity of the emission lines. In particular,

the energy value (E00) for the non-degenerated 5D0 / 7F0

transition is 17260.5 � 0.1 cm�1 for Eu(btfac)3(H2O)2 and

17249.1 � 0.2 cm�1 in the case of Eu(btfac)3phen. The higher


Fig. 6 A: Emission spectra of the d-Ut(600)400Eu(btfac)3phen di-ure-

thanesil under different excitation wavelengths (1) 365 nm (red line) (2)

395 nm (blue line), (3) 420 nm (black line) and (4) 450 nm (green line). The

inset shows the emission of the d-Ut(600)nEu(btfac)3(H2O)2 under

395 nm. B: Magnification of the 5D0 / 7F0-4 transitions for d-

Ut(600)400Eu(btfac)3(H2O)2 (a) and for d-Ut(600)400Eu(btfac)3phen (b)

under 350 and 395 nm excitation wavelengths, respectively.

Fig. 5 Emission spectra of the Eu(btfac)3L complexes excited at 325 nm

(black lines) and 465 nm (red lines): (a) L ¼ (H2O)2 (a) and (b) L ¼ phen.

The insets show a magnification of the 5D0 / 7F0 transition.

energy found for the aquocomplex indicates that the Eu–

ligands bonds have a less covalent character relative to the

situation found in the phen-based complex.34,35 This assump-

tion is based on the fact that the energy of the 5D0 / 7F0

transition is related to the so-called nephelauxetic effect, in

which the red-shift observed for d-d and f-f energy differences,

with respect to the free ion, is related to a decrease in the

values of the Slater integrals and spin–orbit coupling para-

meter.34–37 The high value found for the full width at half

maximum (fwhm) of the 5D0 / 7F0 transition (fwhm00) (24.5 �0.1 cm�1) reinforces the presence of a large distribution of Eu3+

local environments in both complexes.

The hybrid’s band has been already reported for the non-

doped d-Ut(600) hybrid and for other amide/amide functional-

ised similar organic-inorganic hybrids, being ascribed to elec-

tron/hole recombination occurring in the amide/amide cross-

linkages and within the siliceous domains, whose energy deviates

to the red as the excitation wavelength increases.38–40 The relative

intensity between the hybrids’ component and that of the Eu3+

lines depends on the excitation wavelength and on the type of

complex: for the phen-containing di-urethanesil (excitation

wavelengths between 300–395 nm) the Eu3+ emission is more

intense, whereas for the hybrid sample doped with the aquo-

complex the broad band’s intensity is higher than that of the5D0 / 7F0-4 transitions in the 330–450 nm excitation interval

(inset of Fig. 6A). These observations suggest a higher efficiency

for the energy transfer between the hybrid host excited states and

the intra-4f6 levels.

Fig. 6B shows high resolution emission spectra focused on the5D0 / 7F0-4 transitions for the d-Ut(600)400Eu(btfac)3(H2O)2

and d-Ut(600)400Eu(btfac)3phen materials. The spectra are

similar and do not depend on the excitation wavelength, indi-

cating that similarly to the situation found for the complexes the

Eu3+ ions occupy the same average local environment in each

hybrid. Taking into account the Stark splitting of the 5D0 /7F1-4

transitions (particularly evident for the 5D0 / 7F1,2 transitions

with 3 and 4 components, respectively) and the higher intensity of

the 5D0 /7F2 transition, the Eu3+ local group is characterized by

a low symmetry group without an inversion centre. The complex

incorporation into the hybrids induces an increase in fwhm


relative to that found in the isolated complexes, which was

recently attributed to changes outside the coordination poly-

hedron.14 For instance, the fwhm00 increased to around 32.8 �1.0 cm�1, which is 25% higher than the value found in the isolated

complexes. To get further knowledge on the Eu3+-local changes

after the Eu3+ incorporation into the hybrids, the E00 was esti-

mated leading to approximately the same values for the two

hybrids, namely 17263.4 � 0.6 cm�1 and 17260.3 � 0.2 cm�1 for d-

Ut(600)400Eu(btfac)3(H2O)2 and d-Ut(600)400Eu(btfac)3phen,

respectively. Comparing these E00 values with those of the

isolated complexes, we note changes only in the case of d-

Ut(600)400Eu(btfac)3phen. In particular, the E00 value is deviated

to the blue, relative to that found for the isolated complex,

pointing out modifications in the Eu3+ local coordination towards

a less covalent environment.

The 5D0 lifetime values of the complexes and of the hybrids

were acquired by monitoring the 5D0 / 7F2 transition under

direct intra-4f6 excitation (5L6, 395 nm, 5D2, 465 nm). All the

curves are well described by a single exponential function

yielding to the 5D0 lifetime values gathered in Table 1. The

aquocomplex displays a smaller 5D0 lifetime value relative to that

of the phen-containing complex, whereas the 5D0 lifetime value

for the two doped hybrids is the same, similarly to the situation

found for the energy of the 5D0 / 7F0 transition. The observed

J. Mater. Chem., 2009, 19, 733–742 | 737

Table 1 5D0 lifetime (t, ms), radiative (kr, ms�1) and non-radiative (knr, ms�1) transition probabilities, quantum efficiency (q(5D0)) and number of watermolecules (nw) coordinated to Eu3+ for the isolated complexes and for the corresponding d-Ut(600)-based di-urethanesils

Sample t kr knr q(5D0) nw

Eu(btfac)3(H2O)2 0.419 � 0.002 0.428 1.959 0.18 2.2 � 0.1Eu(btfac)3phen 0.823 � 0.004 0.564 0.652 0.47 0.4 � 0.1d-Ut(600)400 Eu(btfac)3(H2O)2 0.391 � 0.002 0.304 2.253 0.12 2.5 � 0.1d-Ut(600)400 Eu(btfac)3phen 0.392 � 0.006 0.410 2.148 0.16 2.4 � 0.1

Fig. 7 FT-IR spectra of d-Ut(600), d-Ut(600)400Eu(btfac)3(H2O)2 and

d-Ut(600)400Eu(btfac)3phen di-urethanesils in the ‘‘amide I’’ region. A:

Curve-fitting results. B: Integral intensity of the different spectral

components resolved. The lines are guides to the eyes.

reduction of the 5D0 lifetime values after incorporation into the

hybrid media will be discussed next.

The emission features of the complexes and of the hybrids

were further quantified and compared through the estimation

of the 5D0 radiative (kr) and non-radiative (knr) transition

probabilities and of the quantum efficiency (q(5D0)).

Assuming that only non-radiative and radiative processes are

involved in the depopulation of the 5D0 state, q(5D0) may be

defined as:

qð5D0Þ ¼kr

kr þ knr(5)

The emission intensity (I) taken as the integrated intensity S of

the emission curves, for the 5D0 /7F0-4 transitions, is expressed

by:

Ii/j ¼ hui/jAi/jNi h Si/j (6)

where i and j represent the initial (5D0) and final (7F0-4) levels,

respectively, hui/j is the transition energy, Ai/j corresponds to

Einstein’s coefficient of spontaneous emission and Ni is the

population of the 5D0 emitting level.41

The radiative contribution may be calculated from the

relative intensities of the 5D0 /7F0-4. The branching ratio for the

5D0 / 7F5,6 transitions must be neglected due to their poor re-

lative intensity with respect to that of the remaining 5D0 /7F0-4

transitions. Their influence on the depopulation of the 5D0

excited state can therefore be ignored. Since the 5D0 / 7F1

transition can be considered as a reference, due to its dipolar

magnetic nature, kr can be calculated as:

kr ¼ A0/1

hu0/1

S0/1

X4

J¼0

S0�J

hu0�J

(7)

where A0-1 is the Einstein’s coefficient of spontaneous emission

between the 5D0 and the 7F1 Stark levels, in vacuum A0-1 ¼ 14.65

s�1.42 An effective refractive index of 1.5 was used leading to

A(5D0 /7F1) z 50 s�1.

The values found for q(5D0), kr and knr are listed in Table 1.

Comparison of the q values estimated for the two complexes

reveals an increase from 0.18 to 0.47 when the labile water

molecules of the aquocomplex are replaced by the chelating phen

ligand, due to an increase in the kr value and a decrease of the knrvalue.

The incorporation of the complexes into the hybrids leads to

poor q(5D0) values relative to those found in the isolated

complexes (Table 1). However, the most significant result

retrieved from Table 1 is undoubtedly the fact that the q(5D0)

values are unexpectedly similar for the two hybrids. The decrease

738 | J. Mater. Chem., 2009, 19, 733–742

in the 5D0 quantum efficiency is motivated by a decrease in the krvalue and essentially to an increase in the knr value. Although

such an effect has already been observed for other b-diketonate

complexes incorporated into organic-inorganic hybrids,21,43,44,45

it has not been yet explained.

Seeking an explanation for the q(5D0) values exhibited by the

hybrid samples with respect to the corresponding isolated

complexes, we decided to inspect the ‘‘amide I’’46 (or simply the

carbonyl stretching) region of the FT-IR spectra of the complex-

doped di-urethanesil samples. Curve-fitting procedures per-

formed in the 1800–1650 cm�1 interval allowed us to resolve the


band profile of the d-Ut(600) hybrid into three main distinct

components located at about 1752, 1721 and 1697 cm�1

(Fig. 7A(a)). The weak 1752 cm�1 band is ascribed to the pres-

ence of ‘‘free’’ (i.e., non-hydrogen-bonded) carbonyl groups of

the urethane linkages of d-Ut(600).47 The 1721 and 1697 cm�1

components are associated with the occurrence of hydrogen-

bonded POE/urethane and urethane/urethane aggregates,

respectively.47

The plot of Fig. 7B shows that, despite the extremely low

concentration of Eu3+ in the hybrid material with n ¼ 400 (see

Table 2 of the Experimental section), the inclusion of

Eu(btfac)3phen within d-Ut(600) alters the ‘‘amide I’’ band of

this host hybrid, leading to band redistribution. This finding may

be correlated with modifications in the chemical environment of

the urethane carbonyl groups of d-Ut(600) in the presence of this

complex. No other significant spectral changes are detected,

however, in the ‘‘amide I’’ region of d-Ut(600)400Eu(btfac)3phen,

in particular the emergence of a new Eu3+ coordination-sensitive

absorption band, usually located below 1690 cm�1, and typically

produced by the di-ureasils doped with europium complexes14,15

and mono47/di-urethanesils12,30 doped with europium salts. Close

analysis of the plot of Fig. 7B allows the inference that upon

addition of Eu(btfac)3phen, the proportion of ‘‘free’’ carbonyl

groups is reduced at the expense of the formation of POE/

urethane aggregates. Nevertheless, the fraction of urethane/

urethane aggregates remains essentially unchanged (Fig. 7B).

This means that the addition of Eu(btfac)3phen to d-Ut(600)

promotes the interaction between the POE chains and the ‘‘free’’

carbonyl groups of the hybrid matrix, but does not induce the

rupture of any hydrogen-bonded aggregates of the matrix.

Therefore the poor q(5D0) value of the d-Ut(600)400Eu(btfac)3-

phen sample (Table 1) may simply be a consequence of the

bulkiness of the phen ligand. In fact it is very likely that due to

severe steric constraints arising from the formation of the new

POE/urethane aggregates, the Eu3+-phen distance increases

significantly. In such a case, several possibilities would remain for

the emitting centres: they might either bond to water molecules

or to urethane carbonyl oxygen atoms or to ether oxygen atoms

of the POE chains. Considering that the urethane cross-links and

the polymer segments of this hybrid are deeply involved in the

process of formation of POE/urethane aggregates, the hypothesis

of the establishment of Eu3+/O]C or/and Eu3+/OCC bonds,

respectively, seems to be highly improbable. Hence, water

Table 2 Experimental details of the synthesis of the d-Ut(600)-based di-urethanesils

d-Ut(600)nEu(btfac)3L

L (H2O)2 phen

n 400 200 400 200

m(PEG) (g) 1.04 1.05 1.01 1.03v(ICPTES) (mL) 852 865 831 846v(CH3CH2OH) (mL) 815 828 795 809v(H2O) (mL) 93 95 91 93m(complex) (g) 0.04485 0.08645 0.05444 0.11081Si/Eu (molmol�1) 64 34 60 30Si/Eu (gg�1) 12 6 11 6


molecules will presumably replace the phen ligand in the hybrid

system to maintain the Eu3+ coordination number constant.

In contrast, the inclusion of the aquocomplex in the di-ure-

thanesil medium appears to have practically no consequences on

the hydrogen-bonded array of d-Ut(600) and on the population

of ‘‘free’’ urethane groups (Fig. 7B). Therefore, these spectral

data suggest that the Eu(btfac)3(H2O)2 complex does not interact

at all with d-Ut(600). This might explain why the q(5D0) value of

Eu(btfac)3(H2O)2 suffers practically no change upon incorpora-

tion into the d-Ut(600) structure (Table 1).

The photoluminescence data provide full support of the

conclusions retrieved from the ‘‘amide I’’ region. The number of

water molecules (nw) coordinated to Eu3+ calculated using the

empirical formula of Supkowski and Horrocks:48

nw ¼ 1.11 � [kexp � kr � 0.31] (8)

where kexp¼(t)�1 yielded nw values for the aquocomplex and for

Eu(btfac)3phen of approximately 2 and 0, respectively (Table 1),

thus in perfect agreement with the structures proposed in Scheme

1. For the corresponding hybrid samples approximately 2 water

molecules were estimated in both cases (Table 1). The latter

finding lends support to the claim that in the case of the aquo-

complex the d-Ut(600) structure acts merely as an inert support.

In addition, it may be interpreted as an indication that in the case

of the di-urethanesil doped with Eu(btfac)3phen the composition

of the coordination sphere of the Eu3+ ions is deeply modified as

a result of the significant displacement suffered by the bidentate

phen ligand away from the emitting centre and its concomitant

replacement by two water molecules. Considering the well-

known strong chelating ability of the phen ligand, this unex-

pected withdrawal from the Eu3+ chemical environment must be

related to the occurrence of severe steric effects involving the

bulky phen molecules and the POE chains.

Experimental

Materials

Europium chloride (EuCl3.6H2O, Aldrich, 99.99%), 4,4,4-trifluoro-

1-phenyl-1,3-butanedione (Hbtfac, Aldrich, 99%), 1,10-phenan-

throline (phen, Aldrich, +99%,), 3-isocyanatepropyltriethoxysilane

(ICPTES, Fluka, 95%), acetonitrile (CH3CN, Aldrich, HPLC

grade), tetrabutylammonium tetrafluoroborate (TBABF4, Aldrich

99%) and potassium bromide (KBr, Merck, spectroscopic grade)

were used as received. Poly(ethylene glycol) (PEG(600), Aldrich,

95%, MW z 600 gmol�1), ethanol (CH3CH2OH, Panreac, PA

grade) and tetrahydrofurane (THF; Merck, PA grade) were stored

over molecular sieves. High purity distilled water was used in all

experiments.

Synthesis

To synthesize Eu(btfac)3(H2O)2 (Scheme 1A)49 a mass of 0.6477 g

(2.99 mmol) of Hbtfac was dissolved in 15 mL of CH3CH2OH.

The pH of this solution was adjusted to 6–7 by adding an

appropriate amount of an aqueous NaOH solution (1 M). An

aqueous solution of EuCl3$6H2O (0.3603 g, 9.8 � 10�4 mol in

5 mL of water) was then added dropwise to the ethanolic solution

of Hbtfac. The addition of 100 mL of water on heating at 60 �C

J. Mater. Chem., 2009, 19, 733–742 | 739

for 120 minutes followed. The resulting solution was left in the

fume cupboard until precipitation. The yellow solid obtained was

filtered off, washed with water and dried in a vacuum desiccator

at room temperature. Yield: 21%. % Anal. calcd.: C: 43.21, H:

2.64; Found: C: 43.21, H: 2.49.

To synthesize Eu(btfac)3phen (Scheme 1B)50 a mass of 0.6562 g

(3 mmol) of Hbtfac and 0.1807 g, phen (1 mmol) was dissolved in

15 mL of ethanol. The pH of this solution was adjusted to 6–7 by

adding an appropriate amount of an aqueous NaOH solution

(1 M). An aqueous solution of EuCl3.6H2O (0.3575 g, 9.75 �10�4 mol in 5 mL of water) was then added dropwise to the

ethanolic solution of Hbtfac and phen on heating at 60 �C for 120

minutes. The resulting solution was left in the fume cupboard

until precipitation. The yellow solid obtained was filtered off,

washed with water and dried in a vacuum desiccator at room

temperature. Yield: 24%. % Anal. calcd: C: 51.59, H: 2.66, N:

2.87; Found: C: 51.52, H: 2.40, N: 3.01.

The doped di-urethanesil hybrid matrices were prepared by the

conventional sol-gel method, according to the two-step experi-

mental procedure described in detail previously.20 The experi-

mental details of the synthesis of the doped samples are presented

in Table 2. Transparent monolithic xerogels were thus formed. A

non-doped di-urethanesil sample was also prepared.

‡ PeakFit is a product of Jandel Corporation, 2591 Rerner Boulevard,San Rafael, CA 94901, USA

Characterization

The elemental analyses of the complexes were performed at

Centro de Apoio Cientıfico e Tecnoloxico a Investigacion

CACTI-University of Vigo (Spain).

The absorption spectra of the complexes were acquired in the

700–200 nm range with a ultraviolet/visible UV/VIS Spectronic

Genexys 2PCC spectrophotometer. The solvent employed was

CH3CH2OH.

CV studies were performed in a three-electrode cell consisting

of a platinum working electrode, a platinum counter electrode

and a saturated calomel as a reference electrode, using a scan rate

of 50 mV/s (Solartron potentiostat 1285). The redox couple

ferrocene/ferricenium ion (Fc/Fc+) was used as an external

standard. Its energy, with respect to the vacuum level, defined as

zero, was estimated to be �4.8 eV.27,51 The complexes were dis-

solved in a solution of CH3CN containing 0.2 M TBABF4. The

electrolyte was purged with nitrogen (N2) and the measurements

were made under an inert atmosphere. The oxidation and

reduction potentials were deduced from the intersection of two

curves, one corresponding to the onset potential and the other to

the residual current of the voltammogram.

Samples for thermogravimetric studies were transferred to

open platinum pans and analyzed using a TA Instruments Q50

thermobalance at a heating rate of 10 �Cmin�1 using dried N2 as

the purging gas (40 mL min�1). Prior to measurement, the

samples were vacuum-dried over phosphorus pentoxide (P2O5).

A DSC131 Setaram differential scanning calorimeter was used

to determine the thermal behaviour of the complexes and of the

di-urethanesil hybrids. A mass of 10–30 mg of the finely ground

powder sample (complex) or film (xerogel) was placed in a 30 mL

aluminium can and stored in a desiccator over P2O5 for 1 week at

room temperature under vacuum. After this drying treatment the

can was hermetically sealed and the thermograms were recorded.

Each sample was heated from 25 to 300 �C at 10 �Cmin�1.

740 | J. Mater. Chem., 2009, 19, 733–742

The purge gas used was high purity N2 supplied at a constant

35 mL min�1 flow rate.

The FT-IR spectra were acquired at room temperature on

a Unicam FT-IR spectrophotometer. The spectra were collected

in the 4000–500 cm�1 range by averaging 64 scans at a resolution

of 4 cm�1. About 2 mg of each complex was mixed with 175 mg of

KBr finely ground and pressed into pellets. Prior to recording the

spectra the complexes were stored under vacuum for about 24

hours at approximately 80 �C to reduce the levels of adsorbed

water. To evaluate broad band envelopes and to identify

underlying spectral components, the iterative least-squares

curve-fitting procedure in the PeakFit‡ software was employed.

The best fit of the experimental data was obtained by varying the

frequency, bandwidth and intensity of the bands and by using

a Gaussian shape. A linear baseline correction with a tolerance of

0.2% was employed. The standard errors of the curve-fitting

procedure were less than 0.002.

XRD measurements were performed at room temperature

with a PANalytical X’Pert Pro equipped with a X’Celerator

detector using monochromated CuKa radiation (l ¼ 1.541 A)

over the 2q range between 10 and 70�. The di-urethanesil

samples, analyzed as films, were not submitted to any thermal

pre-treatment.

To evaluate the morphology of the samples, SEM micrographs

were obtained using a SEM/ESEM-FEI Quanta 400 scanning

electron microscope at high acceleration voltage (25 kV).

The samples were coated with a thin layer of carbon by ion

sputtering.

The photoluminescence spectra were recorded at room

temperature with a modular double grating excitation spectro-

fluorimeter with a TRIAX 320 emission monochromator (Flu-

orolog-3, Jobin Yvon-Spex) coupled to a R928 Hamamatsu

photomultiplier, using the front face acquisition mode. The

excitation source was a 450W Xe arc lamp. The emission spectra

were corrected for detection and optical spectral response of the

spectrofluorimeter and the excitation spectra were corrected for

the spectral distribution of the lamp intensity using a photodiode

reference detector. The lifetime measurements were acquired

with the setup described for the luminescence spectra using

a pulsed Xe-Hg lamp (6 ms pulse at half width and 20–30 ms tail).

Conclusions

In the present work we studied the morphological, structural and

thermal properties of hybrids composed of the short chain di-

urethanesil d-Ut(600) host framework and the Eu(btfac)3(H2O)2

and Eu(btfac)3phen complexes. Xerogels with compositions

n ¼ 400 and 200 were prepared. The samples produced are

transparent, non-porous, homogeneous and thermally stable

up to about 150 �C. Except for the Eu(btfac)3phen-doped di-

urethanesil with n ¼ 200, where a minor proportion of free

complex was detected, the remaining materials are amorphous.

The ultimate goal of this investigation was to elucidate the

behaviour of the Eu(btfac)3(H2O)2 complex within the d-Ut(600)

medium and in particular to determine if, similarly to the situa-

tion observed in analogue hybrid systems previously analyzed by


our group (di-ureasils17,19 and di-urethanesils21), the coordinating

functionalities of the hybrid host could act as complex anchoring

sites and thus play an active role in the enhancement of the 5D0

quantum efficiency q(5D0) of the isolated aquocomplex. In the

case of d-Ut(600) this complex immobilization process was

expected to involve bonding of the Eu3+ ions to either the

carbonyl oxygen atoms of the urethane cross-linkages or the

ether oxygen atoms of the POE chains or both, and the simul-

taneous release of one(two) labile water ligand(s) of the

aquocomplex. For comparison purposes, we also examined d-

Ut(600)-based samples doped with the corresponding b-diketo-

nate complex containing a phen ligand instead of the water

ligands. The photoluminescence and FT-IR spectroscopic

analyses were focused on samples with n ¼ 400.

As a result of the lack of coordinative interaction between the

guest aquocomplex and the host hybrid matrix, the q(5D0) and nwvalues of Eu(btfac)3(H2O)2 remain practically unaltered upon

incorporation into the d-Ut(600) structure (from 0.18 to 0.12 and

from 2.2 � 0.1 to 2.5 � 0.1, respectively). These findings lead us

to suppose that the coordinating ability of the urethane carbonyl

oxygen atoms and of the ether oxygen atoms of the POE chains

of d-Ut(600) is not strong enough to displace the labile water

ligands of Eu(btfac)3(H2O)2. Consequently, d-Ut(600) plays the

mere role of support in the presence of this complex, although

ensuring the maintenance of the optical features in the isolated

state.

Doping d-Ut(600) with Eu(btfac)3phen is clearly disadvanta-

geous, as a significant reduction of the q(5D0) value of the iso-

lated complex occurs (from 0.47 to 0.16). Intense repulsive forces

originating from severe steric interactions taking place between

the POE chains and the bulky phen molecules are thought to be

responsible for the unusual displacement of the strong bidentate

chelating phen ligand from the Eu3+ first coordination shell and

its subsequent replacement by two water molecules (nw ¼ 2.4 �0.1 for d-Ut(600)400Eu(btfac)3phen). This uncommon effect

may be interpreted as an indication that in both d-Ut(600)-based

materials the local environment of the Eu3+ ions is grosso modo

the same. Not surprisingly, the q(5D0) values of both hybrids are

similar. The decrease of q(5D0) upon doping was correlated in

d-Ut(600)400Eu(btfac)(H2O)2 and d-Ut(600)400Eu(btfac)3phen

with a decrease in the radiative paths and essentially to an

increase in the non-radiative channels.

In terms of complex anchoring ability, the behaviour of

d-Ut(600) contrasts deeply with that of the PCL(530)/siloxane di-

urethanesil,21 in which the polymer carbonyl oxygen atoms

govern the complex immobilization. It also differs markedly

from that of the structurally analogous d-U(600) di-ureasil

matrix, whose POE chains dictate the in situ complex forma-

tion.17 It is thus evident that the nature of the coordinating atoms

of the cross-links and polymer chains are key parameters in the

whole process. However, the study carried out with the co-

condensed d-U(900)/d-U(600) di-ureasil-type hybrid network19

suggests that the length of the polymer chains of the hybrid

structure may also influence it.

In conclusion, the use of aquocomplexes may be attractive for

optical purposes, as long as an adequate host hybrid matrix and

appropriate ligands are employed. This choice will determine the

balance of radiative and non-radiative channels of the 5D0 level

and will consequently affect the magnitude of q(5D0).


Acknowledgements

The support of Fundacao para a Ciencia e Tecnologia (FCT)

(PTDC/CTM/72093/2006) is gratefully acknowledged. M. Fer-

nandes and S. Nobre thank FCT for grants (SFRH/BD/38530/

2007 and SFRH/BD/28739/2006, respectively). The authors

would like to express their gratitude to H. D. Burrows, of

the Department of Chemistry of the University of Coimbra

(Portugal), for helpful discussions.

References

1 J.-M. Lehn, Angew. Chem. Int. Ed. Engl., 1990, 29, 1304.2 G. F. de Sa, O. L. Malta, C. de Mello Donega, A. M. Simas,

R. L. Longo, P. A. Santa-Cruz and E. F. da Silva Jr., Coord. Chem.Rev., 2000, 196, 165.

3 E. B. van der Tol, H. J. van Ramesdonk, J. W. Verhoeven,F. J. Steemers, E. G. Kerver, W. Verboom and D. N. Reinhoudt,Chem. Eur. J., 1998, 4, 2315.

4 J.-C. Bunzli and C. Piguet, Chem. Soc. Rev., 2005, 34, 1048.5 N. S. Baek, Y. H. Kim, S.-G. Roh, B. K. Kwak and H. K. Kim, Adv.Funct. Mater., 2006, 16, 1873.

6 M. H. V. Werts, R. H. Woudenberg, P. G. Emmerink, R. van Gassel,J. W. Hofstraat and J. W. Verhoeven, Angew. Chem. Intl. Ed., 2000,39, 4542.

7 C. J. Brinker and G. W. Scherer, in Sol-Gel Science: The Physics andChemistry of Sol-Gel Processing, Academic Press, San Diego, CA,1990.

8 P. Gomez-Romero and C. Sanchez, in Functional Hybrid Materials,Wiley Interscience, New York, 2003.

9 P. Escribano, B. Julian-Lopez, J. Planelles-Arago, E. Cordoncillo,B. Viana and C. Sanchez, J. Mater. Chem., 2008, 18, 23.

10 C. Sanchez, B. Julian, P. Bellevile and M. Popall, J. Mater. Chem.,2005, 15, 3559.

11 M. Armand, C. Poinsignon, J.-Y. Sanchez and V. de Zea Bermudez,U. S. Patent 5 283 310, 1993.

12 M. C. Goncalves, N. J. O. Silva, V. de Zea Bermudez, R. A. SaFerreira, L. D. Carlos, K. Dahmouche, C. V. Santilli,D. Ostrovskii, I. C. Correia Vilela and A. F. Craievich, J. Phys.Chem. B, 2005, 109, 20093.

13 L. D. Carlos, R. A. Sa Ferreira, V. de Zea Bermudez, in HybridMaterials - Synthesis, Characterization and Aplication, ed. G.Kickelbick, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim,2007, (ch. 9), pp. 337–400.

14 P. P. Lima, R. A. Sa Ferreira, R. O. Freire, F. A. A. Paz, L. Fu,S. Alves Jr, L. D. Carlos and O. Malta, Chem. Phys. Chem, 2006, 7,735.

15 L. Fu, R. A. Sa Ferreira, N. J. O. Silva, A. J. Fernandes, P. Ribeiro-Claro, I. S. Goncalves, V. de Zea Bermudez and L. D. Carlos,J. Mater. Chem., 2005, 15, 3117.

16 L. Fu, R. A. Sa Ferreira, S. S. Nobre, L. D. Carlos and J. Rocha,J. Lumin., 2006, 122–123, 265.

17 C. Molina, K. Dahmouche, K. Y. Messadeq, S. J. L. Ribeiro,M. A. P. Silva, V. de Zea Bermudez and L. D. Carlos, J. Lumin.,2003, 93, 104.

18 P. P. Lima, S. A. Junior, O. L. Malta, L. D. Carlos, R. A. Sa Ferreira,R. Pavithran and M. L. P. Reddy, Eur. J. Inorg. Chem., 2006, 19,3923.

19 M. Fernandes, V. de Zea Bermudez, R. A. Sa Ferreira, L. D. Carlosand N. V. Martins, J. Lumin., 2008, 128, 205.

20 M. Fernandes, M. C. Goncalves, V. de Zea Bermudez, R. A. SaFerreira, L. D. Carlos, A. Charas and J. Morgado, J. AlloysCompds., 2008, 451, 201.

21 M. Fernandes, V. de Zea Bermudez, R. A. Sa Ferreira, L. D. Carlos,A. Charas, J. Morgado, M. M. Silva and M. J. Smith, Chem. Mater.,2007, 19, 3892.

22 K. Binnemans, Handbook on the Physics and Chemistry of RareEarths, Elsevier, Amsterdam, 2005, vol. 34, (ch. 225), pp.107–225.

23 K. M. Lee, K. W. Cheah, B. I. An, M. I. Gong and Y. L. Liu, Appl.Phys. A, 2005, 80, 337.

24 F. R. G. Silva, J. F. S. Menezes, G. B. Rocha, S. Alves, H. F. Brito,R. L. Longo and O. L. Malta, J. Alloys Compds., 2000, 303–304, 364.

J. Mater. Chem., 2009, 19, 733–742 | 741

https://www.researchgate.net/publication/289213373_Handbook_on_the_Physics_and_Chemistry_of_Rare_Earths?el=1_x_8&enrichId=rgreq-b140784e1623a3cf7c374baea537aa7e-XXX&enrichSource=Y292ZXJQYWdlOzIyODA5Mjc1MDtBUzoxNjI4NTIzODQwMjY2MjRAMTQxNTgzODQ0MjQ4Mg==

https://www.researchgate.net/publication/289213373_Handbook_on_the_Physics_and_Chemistry_of_Rare_Earths?el=1_x_8&enrichId=rgreq-b140784e1623a3cf7c374baea537aa7e-XXX&enrichSource=Y292ZXJQYWdlOzIyODA5Mjc1MDtBUzoxNjI4NTIzODQwMjY2MjRAMTQxNTgzODQ0MjQ4Mg==

25 H. J. Batista, A. V. M. de Andrade, R. L. Longo, A. M. Simas,G. F. de Sa and L. C. Thompson, J. Lumin., 1997, 72–74, 159.

26 G. F. de Sa, S. Alves Jr., B. J. P. da Silva and E. F. da Silva Jr., Opt.Mater., 1998, 11, 23.

27 M. Al-Ibrahim, H.-K. Roth, U. Zhokhavets, G. Gobsch andS. Sensfuss, Sol. Energy Mater. Sol. Cells, 2005, 13, 85.

28 Z. Jablonski, L. Rychlowska-Himmel and M. Dyrek, Spectrochim.Acta, 1978, 35A, 1297.

29 S.-L. Ma, W.-X. Zhu, G.-H. Huang, D.-Q. Yuan and X. Yan, J. Mol.Struct., 2003, 646, 89.

30 M. C. Goncalves, V. de Zea Bermudez, R. A. Sa Ferreira, L. D. Carlos,D. Ostrovskii and J. Rocha, Chem. Mater., 2004, 16, 2530.

31 M. C. Goncalves, V. de Zea Bermudez, R. A. Sa Ferreira,D. Ostrovskii and L. D. Carlos, Proceedings of the MaterialsResearch Society, Boston, 2005, 847, p.EE.13.1.1.

32 M. C. Goncalves, PhD Thesis, University of Tras-os-Montes e AltoDouro, Vila Real (Portugal), 2007.

33 L. D. Carlos, V. de Zea Bermudez, R. A. Sa Ferreira, L. Marques andM. Assuncao, Chem. Mater., 1999, 11, 58113.

34 O. L. Malta, H. J. Batista and L. D. Carlos, Chem. Phys., 2002,282, 21.

35 L. D. Carlos, O. L. Malta and R. Q. Albuquerque, Chem. Phys. Lett.,2005, 415, 238.

36 L. D. Carlos and A. L. L. Videira, J. Chem. Phys., 1994, 101, 8827.37 C. K. Jørgensen, Prog. Inorg. Chem., 1962, 4, 73.38 L. D. Carlos, R. A. Sa Ferreira, V. de Zea Bermudez and

S. J. L. Ribeiro, Adv. Funct. Mater., 2001, 11, 111.

742 | J. Mater. Chem., 2009, 19, 733–742

39 L. D. Carlos, R. A. Sa Ferreira, R. N. Pereira, M. Assuncao and V. deZea Bermudez, J. Chem. Phys. B., 2004, 108, 14924.

40 S. S. Nobre, P. P. Lima, L. Mafra, R. A. Sa Ferreira, R. O. Freire,L. Fu, U. Pischel, V. de Zea Bermudez, O. L. Malta andL. D. Carlos, J. Phys. Chem. C, 2007, 111, 3275.

41 L. D. Carlos, Y. Messaddeq, H. F. Brito, R. A. Sa Ferreira, V. de ZeaBermudez and S. J. L. Ribeiro, Adv. Mater., 2000, 12, 594.

42 M. H. V. Werts, R. T. F. Jukes and J. W. Verhoeven, Phys. Chem.Chem. Phys., 2002, 4, 1542.

43 P. Lenaerts, A. Storms, J. Mullens, J. D’Haen, C. Gorller-Walrand,K. Binnemans and K. Driesen, Chem. Mater., 2005, 17, 5194.

44 P. Lenaerts, C. Gorller-Walrand and K. Binnemans, J. Lumin., 2006,117, 163.

45 P. Lenaerts, K. Driesen, R. Van Deun and K. Binnemans, Chem.Mater., 2005, 17, 2148.

46 D. J. Skrovanek, S. E. Howe, P. C. Painter and M. M. Coleman,Macromolecules, 1985, 18, 1676.

47 V. de Zea Bermudez, D. Ostrovskii, M. C. Goncalves, L. D. Carlos,R. A. Sa Ferreira, L. Reis and P. Jacobsson, Phys. Chem. Chem.Phys., 2004, 6, 638.

48 R. M. Supkowski and W. de Horrocks, Inorg. Chim. Acta, 2002,340, 44.

49 R. G. Charles and R. C. Olmann, J. Inorg. Nucl. Chem., 1965, 27, 25.50 L. R. Melby, N. J. Rose, E. Abramson and J. C. Caris, J. Am. Chem.

Soc., 1964, 86, 5117.51 H. Pommerehne, W. Guss Vestweber, R. F. Mahrt, H. Bassler,

M. Porsh and J. Daub, Adv. Mater., 1995, 7, 551.


Dual role of a di-urethanesil hybrid doped with europium β-diketonate complexes containing either...

Documents

Transcript of Dual role of a di-urethanesil hybrid doped with europium β-diketonate complexes containing either...