www.elsevier.com/locate/optmat

Optical Materials 29 (2007) 1367–1374

Binding of europium complex to polymerizable macrocyclicmolecules and its optical properties

Rahmat Hidayat a,b,*, Okihiro Sugihara a, Masaaki Tsuchimori c, Manabu Kagami c,Tadatomi Nishikubo d, Toshikuni Kaino a

a Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japanb Department of Physics, Faculty of Science and Mathematics, Bandung Institute of Technology, Jl. Ganesha 10, Bandung, Indonesia

c Toyota Central R&D Labs. Inc., 41-1 Yokomichi, Nagakute, Aichi-Gun, Aichi, Japand Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi Kanagawa-Ku, Yokohama, Japan

Received 23 February 2006; accepted 18 June 2006Available online 4 October 2006

Abstract

Study on the incorporation of trivalent europium (Eu3+) complex into polymerizable macrocylic molecules, namely calixarene andcalixresorcinarene derivatives, has been carried out. Broadening of hypersensitive luminescence peak was observed in solution and poly-mer containing a compound of europium complex and calixresorcinarene monomer, which is ascribed to ligand field splitting due toanisotropic local field at the Eu3+ ion site. From 1H NMR spectroscopy, it is found that the chemical shift originated from the moietyat the upper rim of calixresorcinarene is remarkably downfield shifted. Pseudocontact shift analysis suggests that binding between Eu3+

ion and calixresorcinarene monomer is formed at the Eu3+ second coordination sphere. These experimental results show that the bindingdoes not alter nephelauxetic effect of the Eu3+ first coordination shell, but only slightly change coordination structure and symmetry.Nevertheless, the binding results in longer luminescence lifetime in comparison to the case of pure europium complex indicating signif-icant reduction of non-radiative decay.� 2006 Elsevier B.V. All rights reserved.

PACS: 81.05.Lg; 33.50.Dq; 33.25.+k

Keywords: Europium complex; Macrocyclic molecule; Calixarene; Calixresorcinarene; Polymer; Luminescence; Coordination structure; Ligand field;Chemical shift

1. Introduction

Trivalent lanthanide ions are used in wide range ofapplications such as electronic displays, lasers, opticalamplifiers, and molecular sensing in biochemistry. Forsome applications, lanthanide ions should be attached tohost molecules or incorporated into a suitable matrix ormedium. In the field of photonic and communications, par-ticularly in the use of lanthanide ions as light amplifying

0925-3467/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2006.06.016

* Corresponding author.E-mail addresses: rahmat@fi.itb.ac.id (R. Hidayat), kaino@tagen.

tohoku.ac.jp (T. Kaino).

medium, the ions are usually incorporated into host molec-ular crystals or glass, such as fluoride and silicate glasses.However, high temperature required in glass processinglimits diversity in developments of optical devices basedon lanthanide-doped glass. In order to overcome this limi-tation, there are numerous efforts to incorporate lantha-nide ions in polymers or organic/inorganic hybridmatrixes because of their attractive preparation routesoffering easiness and flexibility in fabrication methods.

Incorporation these lanthanide complexes into organicpolymers, organic–inorganic hybrid materials such asorganically modified silicate (ormosil) matrices, ureasilsxerogels, and other functionalised silica, and their applica-tions for optical devices have been much reported [1–5].

HC

4

O O

H2CH2C

O CH2

Et H2C CH2

OH2C

Et

CH3

CH2

R

OCOCH=CH2

8

ba

Fig. 1. Molecular structures of (a) CRA[4]Ox and (b) BUCA[8]Ac withR = t-Bu, or BOCA[8]Ac with R = t-Bu and t-Oc, where Bu = butyl andOc = octyl.

1368 R. Hidayat et al. / Optical Materials 29 (2007) 1367–1374

In order to incorporate lanthanide ions into those hostmaterials, the ions are usually complexed with organicligands, such as b-diketones, pyridine and carboxylates,which can increase the solubility and may also functionas a light antenna or sensitizing group [6–13]. Lanthanideions can be also complexed with ligands having macrocylicor cavitand shapes, such as crown ethers, cryptates, polyd-entate hemisperands and calixarenes [14–20]. Some kindsof those macrocyclic ligands are interesting as they arecapable to capture lanthanide ions by inclusion phenome-non depending on the cavity size and attached functionalgroups. Unfortunately, the absorption band of theseligands appears in deep violet region so there are very lim-ited light sources for photo-excitation available today.

Recently, polymerizable macrocyclic molecules such ascalixarene and calixresorcinarene polymers have beenextensively developed as new kind of resists for photo- orelectron-beam lithography [21]. Acrylate or epoxy moietiesare attached to the lower or upper rims of these macrocyc-lics. Our group has successfully demonstrated the fabri-cation of calixarene acrylate waveguides by means ofone- and two-photon polymerization techniques [22]. Weare currently interested in developing optical waveguidesusing these macrocyclic polymers that incorporating lan-thanide complexes. When an interaction between lantha-nide ions and macrocylic monomers are established eitherby the formation of coordination bond, inclusion phenom-enon, or other kind of non-bonding interactions, the ionswill be distributed homogeneously with appropriate sepa-ration distance from each other. Consequently, complexaggregation and interaction between neighboring ionscould be suppressed even at high doping concentration.In this paper, we present our study on binding of trivalenteuropium (Eu3+) ions to macrocyclic monomers and poly-mers, and correlated change in optical properties due tothis binding. We have carried out optical and FT-NMRspectroscopy for this study.

1 From elemental analysis, the average composition was found asfollow: C = 55.7%, H = 3.6%, and Cl = 6.0%. The expected compositionis C = 57.3%, H = 4.0%, and Cl = 5.4%.

2. Experimental

Europium (Eu3+) ion complexed with dibenzoylmethane(DBM) used in this study was prepared by a similarmethod as reported in many literatures [1,9,10]. In orderto promote interaction or coordination bond formationbetween the ions and macrocyclic monomers, which arecommonly large and bulky molecules, the europium ionswere only partially chelated, namely EubmLn where b isdeprotonated DBM and L is Cl with m = 2 and n = 1.As a typical reaction procedure, the reaction was carriedout by mixing EuCl3 (172 mg) in ethanol solution (10 ml)containing DBM (298 mg) and NaOH (2/3 ml of 2 M aq.solution) in an Erlenmeyer flask. The solution was stirredfor over than 1 h at room temperature and then collectedby filtration. The product, hereafter just referred simplyas Eub2L, was drained with water, dried and then dissolvedin chloroform. The product was reprecipitated from

ethanol/chloroform mixture solution by slow evaporationusing rotary evaporator.1

The product (Eub2L) was mixed with macrocylic mono-mers in chloroform or dichloroethane, and then stirred attemperature of about 80 �C. Fig. 1 shows the molecularstructure of macrocyclic monomers used in this study,namely calixresorcinarene containing oxetane groups(CRA[4]Ox) and p-tert-(bu)calix[8]arene containing acry-late groups (BUCA[8]Ac with R = t-Butyl and BOCA[8]Acwith R = t-Butyl and t-Octyl in arbitrary position). Thecompound was reprecipitated in hexane/chloroform mix-ture solution by the same method as mentioned above.Polymers containing these complexes were preparedby thermal initiated radical polymerization. Eub2L/BOCA[8]Ac compound, acrylate monomer and initiatorwas dissolved in chloroform with a certain weight percent-age ratio. Acrylate monomer used here was triethylene gly-col dimethacrylate (TGD) or methyl methacrylate (MMA).As thermal radical initiator, a,a 0-azoisobutyronitrile(AIBN) or Lauroyl peroxide has been used. The polymer-ization was carried out in an oven, with temperature keptat about 85 �C for AIBN initiator and 65 �C for LP initia-tor, for over than one night. For preparing thin film poly-mers, the monomer solution was spin-cast or cast onappropriate substrate (glass, quartz or Si wafer) prior poly-merization and the oven was filled with N2 gas. CRA[4]Oxcan be polymerized by cationic polymerization, however,Eub2L was found decomposed as the cationic polymeriza-tion progressed. Accordingly, polymer containing Eub2L/CRA[4]Ox compound were obtained by polymerizingMMA and BOCA[8]Ac polymers doped with Eub2L/CRA[4]Ox inside.

Absorption spectra were measured by using a UV–Vis–NIR (Shimadzu UV-3100PC) spectrophotometer. Thefluorescence spectra were measured using a photomulti-channel analyzer (Hamamatsu PMA-11) and a N2 laseror blue (405 nm) diode laser as the excitation light source.A pre-amplified Si PIN photodiode connected to an oscil-loscope was used to measure the fluorescence decays. AN2 laser or Nd-YAG (THG) laser was used as the excita-tion light source. The 1H NMR spectra were measured

4.5x104

5.0x104

4

5x104

CRA[4]Ox

R. Hidayat et al. / Optical Materials 29 (2007) 1367–1374 1369

by using a 400 MHz 1H FT-NMR (Lambda 400 OxfordInstrument) spectrometer.

200 250 300 350 400 450 500 550 6000.0

5.0x103

1.0x104

1.5x104

2.0x104

2.5x104

3.0x104

3.5x104

4.0x104

DBM Euβ2L Euβ2L/CRA[4]Ox

ε (L

mol

-1cm

-1)

Wavelength (nm)

200 300 400 500 6000

1x104

2x104

3x104

4x10

Fig. 2a. Absorption spectra of dilute solution of Eub2L and Eub2L/CRA[4]Ox mixture in chloroform. Inset: Absorption spectrum of dilutesolution CRA[4]Ox in chloroform.

200 300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Abs

orba

nce

(arb

. uni

t)

Wavelength (nm)

7F0 → 5D1

7F0 → 5D2

Fig. 2b. Absorption spectrum a disk shaped bulk sample of Eub2L/CRA[4]Ox compound embedded in BOCA[8]/MMA polymer.

3. Results and discussion

Fig. 2a shows the absorption spectra measured fromdilute solution of Eub2L (1.5 lmol/L) and Eub2L/CRA[4]Ox mixture (4.0 lmol/L) in chloroform representedin molar absorption coefficient (L mol�1 cm�1). TheEub2L/CRA[4]Ox compound is 1:1 in molar ratio. Thespectra show a broad absorption band peaked at 348 nm,which is originated from p-p* excitation of the DBMligands. The peak is shifted about 5 nm to longer wave-length in comparison to the DBM spectrum and the bandtail is extending up to 420 nm because of higher p orbitalsdelocalization in Eub2L. The molar absorption coefficientis in order of 104 L mol�1 cm�1, which is typical of dipoleallowed transition as commonly found in organic ligands.Absorption band peaked at 275 nm is originated fromCRA[4]Ox as indicated in the inset of the Fig. 2a. Thisabsorption band is well separated from the absorptionband of Eub2L. It is clear that in this dilute solution,CRAOx is transparent enough so that Eub2L can appro-priately absorb the excitation light.

Sample polymers with high concentration of Eub2L wereprepared to evaluate the absorption cross-sections for f–fdirect transitions, i.e. 7F0! 5D1 and 7F0! 5D2. Fig. 2bshows the absorption spectrum measured from a disk shapedpolymer bulk with size of 12.5 mm in diameter and 3.4 mmin thickness. This polymer was prepared from monomersolution consisting of MMA, BOCA[8]Ac and Eub2L/CRA[4]Ox, with ratio 4:1:1 in weight percentage, wherethe weight percentage ratio of Eub2L/CRA[4]Ox is 1:2.The molecular weight of CRAOx and Eub2L are 1330 and632 g/mol, respectively. The ratio is therefore approximatelyequivalent to molar ratio of 1:1, which is corresponding toca. 0.5 · 1020 cm�3 of Eu(III) ions.2 Optical transitions7F0! 5D2 and 7F0! 5D1 were observed at 466 nm and534 nm, respectively. From this absorption spectrum, themolar absorption coefficient (e) for 7F0! 5D2 transitionwas calculated to be 11.8 L mol�1 cm�1, or equivalent toabsorption cross-section (r) of 4.5 · 10�20 cm2. This valueindicates that these macrocyclic monomers are suitable forpolymer matrix of this complex. However, as indicated by

2 The disk shaped polymer sample was prepared by thermal polymer-ization of 400 mg of MMA, 100 mg of BOCA[8]Ac and 100 mg of Eub2L/CRAOx with addition of about 20 mg of thermal initiator in a glass tube.Since the weight ratio of Eub2L/CRAOx used in this case is 1:2, the totalweight of Eub2L is about 33.3 mg. After polymerization, the polymer wasremoved from the glass cell. The polymer has shape of 12.5 mm indiameter and 5 mm in height averagely. The top and bottom sides of thispolymer were polished with high fine grade of polishing paper in order tominimize surface roughness and thus light scattering during the absorptionspectroscopy measurement. The height of the disk after polishing is3.4 mm. The concentration of Eub2L was calculated from the molenumber of Eub2L molecules divided by the polymer volume and thenmultiplied with Avogadro number.

its absorption edge extending up to 500 nm, it is essentialto significantly reduce the attenuation loss at this opticalwavelength regime, which is probably caused by high lightscattering loss.

Fig. 3(a) shows the luminescence spectra of pure Eub2Lin chloroform, Eub2L/CRA[4]Ox compound and the samecompound embedded in BOCA[8]Ac/MMA polymer. Theluminescence peaks are originated from 5D0! 7FJ transi-tions with J values as indicated in the figure. The lumines-cence can be originated from magnetic dipole transition or/and forced electric dipole transitions according to Judd–Ofelt theory, where the spontaneous emission rate for thosetransitions is given by [23–25]

AðJ ; J 0Þ ¼ 8p2e2

mck2JJ 0

nðn2 þ 22Þ9

f ed þ n3f md

� �ð1Þ

where kJJ 0 is emission wavelength for transition betweeninitial manifold j(S,L)Ji and final manifold j(S,L)J 0i, f ed

is oscillator strength for forced dipole transition

540 560 580 600 620 640 660 680 700 7200.0

0.2

0.4

0.6

0.8

1.0 Euβ2L (in CHCl3) Euβ2L/CRA[4]Ox

(powder) Euβ2L/CRA[4]Ox

(polymer)

PL In

tens

ity (a

rb. u

nits

)

Wavelength (nm)

J=0 J=1 J=3

J=27F0

7F6

5D0

5D1

7F2

J=4

15000 15500 16000 16500 17000 175000.0

0.2

0.4

0.6

0.8

1.0

PL In

tens

ity (a

rb. u

nits

)

Wavenumber (cm-1)

540 560 580 600 620 640 660 680 700 7200.0

0.2

0.4

0.6

0.8

1.0 Euβ2L/CRA[4]Ox (polymer) at 77 oK

PL In

tens

ity (a

rb. u

nits

)

Wavelength (nm)16600 16800 17000 17200 17400

0.00

0.02

0.04

0.06

0.08

0.10

5D0

7F05D0

7F1

Euβ2L in CHCl3 Euβ2L/CRA[4]Ox (polymer)

PL

Inte

nsity

(ar

b. u

nits

)

Wavenumber (cm-1)

Fig. 3. (a) Normalized luminescence spectra of Eub2L in chloroform (dot line), Eub2L/CRA[4]Ox powder (dash line) and Eub2L/CRA[4]Ox embedded inBOCA[8]A/acrylate polymer (solid line); (b) curve fitting of the luminescence spectrum of Eub2L/CRA[4]Ox (polymer) with three Gaussian functions; (c)normalized luminescence spectra of Eub2L/CRA[4]Ox (polymer) at 77 K; (d) different spectral shapes of I0�0 and I0�1 lines.

1370 R. Hidayat et al. / Optical Materials 29 (2007) 1367–1374

f ed ¼ 8p2mc3hkJJ 0

Xk¼2;4;6

XkðS; LÞJ jU kjðS;LÞJ 0� ��� ��

2J þ 1

!ð2Þ

and f md is oscillator strength for forced dipole transition

f md ¼ 2p2

hmcðS; LÞJ jLþ 2SjðS; LÞJ 0h ij jð Þ ð3Þ

with the labels used in those equations are universal labelsas found elsewhere [23–25]. The forced dipole transition de-pends on the Judd–Ofelt intensity parameter Xk (k =2,4,6), which reflects the odd-symmetry crystal/ligand fieldterms and the strength of state admixing of opposite parity.Consequently, the forced dipole transitions are thusstrongly depends on the local environment, that is, thecoordination structure and chemical properties of the li-gands. The intensity parameter Xk can be calculated theo-retically or estimated experimentally from absorptionspectrum. The magnetic dipole transition depends onlyon the quantum number of the initial and final states;accordingly, it is independent of local environment.

The observed luminescence spectrum shows typical char-acteristics of most b-diketonate europium complexes withthe appearance of intense hypersensitive I0�2 (5D0! 7F2)

line peaked at around 610 nm. The hypersensitive transitionis usually associated with large value of Judd–Ofelt intensityparameter X2 caused by the absence of inversion symmetryin coordination structure or on other words, anisotropiclocal environment [6,10,13,26,27]. In the case of low sym-metry and highly anisotropic ligand/crystal field, the inten-sity of this hypersensitive peak is much larger than that ofother transition lines. Moreover, because the maximumnumber of this ligand field splitting for J manifold is(2J + 1), energy level splitting into maximum of five Starklevels in this hypersensitive line can be expected. Fig. 3(b)shows the curve fitting of the hypersensitive line ofEub2L/CRA[4]Ox polymer with three Gaussian functionsclarifying that the peak is composed of three peaks (withtheir spectral shape parameters as indicated in Table 1),suggesting three Stark levels for 7F2 level. In this case, theligand field in Eub2L/CRA[4]Ox compound are not ableto completely remove the degeneracy of 7F2 level. Theligand field in Eub2L/CRA[4]Ox compound neverthelessexists with much lower symmetry than that in pure Eub2L.This may also related to extensive distortion of coordina-tion structure in Eub2L/CRA[4]Ox compound rather thanin pure Eub2L. The splitting of this I0�2 line therefore indi-

Table 1Luminescence spectral shape parameters of Eub2L/CRA[4]Ox (see the textfor details).

Transition line Peak Line width Intensity

5D0! 7F0 17,268 cm�1 v.w.(579 nm)

5D0! 7F1 16,750 cm�1 v.w.16,900 cm�1 v.w.17,030 cm�1 v.w.

5D0! 7F2 16,333 cm�1 3.9 nm s(612.3 nm) (103 cm�1)16,211 cm�1 7.0 nm m(616.9 nm) (182 cm�1)16,045 cm�1 4.5 nm w(623.2 nm) (117 cm�1)

s, m, w, and v.w. are strong, medium, weak, and very weak, respectively.

0.0 0.2 0.4 0.6 0.8 1.00.01

0.1

1 Euβ2L/CRA[4]Ox

in acrylate polymer Euβ2L in acrylate polymer

Lum

ines

cenc

e In

tens

ity (a

rb. u

nits

)

Time (ms)

Fig. 4. Luminescence decay of Eub2L (dot line) and Eub2L/CRA[4]Oxembedded in acrylate polymers (dash line). The thin solid lines representthe fitting lines of the corresponding decays (refer the text for detail).

R. Hidayat et al. / Optical Materials 29 (2007) 1367–1374 1371

cates a binding of Eub2L to CRA[4]Ox. At this stage, how-ever, there is still not enough evident to conclude what kindof bonding or interaction which promote this binding ofEub2L to CRA[4]Ox.

The red-shift of the observed I0�0 (5D0! 7F0) line withrespect to the I0�0 line for Eu3+ ion gas has been accountedfor the nephelauxetic effect, which is related to the influenceof the ligand covalency in the Eu3+ first coordination shellleading to minute alternation on electrostatic distributionand spin–orbit parameters of the ion in comparison tothe free ion gas [3,28–31]. Accordingly, the red-shift ofI0�0 line could be used as an indicator of the bondingstrength of Eu3+ ion to ligand. Based on this phenomeno-logical effect, Carlos and coworkers have successfullyshowed the correlation between the reduction of the red-shift of I0�0 line and the lowering of the covalency degreeof the Eu3+ first coordination shell [3]. As evident inFig. 3(a), the I0�0 line in our samples was found at the samewavelength of about 579 nm (17,268 cm�1), which is red-shifted about �106 cm�1 from that of calculated forEu3+ ion gas. This red-shift value is close to the theoreticalvalues for the fluorinated b-diketones (e.g. trifluorophe-nylbutanedione (btfa)) complexes calculated by Maltaet al., where overlap polarizability as a quantitative mea-sure of covalency has been introduced into their calculation[31]. No change of I0�0 line wavelengths indicate the samenephelauxetic effect in all samples studied here. That is, thebinding of Eu3+ ion to CRA[4]Ox does not alter the cova-lency of the Eu3+ first coordination shell. As also evident inFig. 3(c), even though the temperature has been lowereddown to 77 K, the luminescence spectrum for Eub2L/CRA[4]Ox polymer is almost unchanged with the sameposition of I0�0 line as that observed in the room tempera-ture spectrum. Eu3+ ions are fixed firmly in the matrixeswith the covalency of the Eu3+ first coordination shell isunaffected by temperature.

Another important feature from the luminescence spec-tra is I0�1 (5D0! 7F1) line. The observed I0�1 line could beoriginated from forced electric dipole transition and/orallowed magnetic dipole transition which is independent

of local environment. The ratio I0�2/I0�1 therefore can bealso used as local environment indicator. As displayed inFig. 3(d), it is clear that the I0�1 shape for Eub2L/CRA[4]Ox is broader than that for pure Eub2L in solution.This I0�1 seems to be composed of three peaks, namely amagnetic dipole transition at 16,900 cm�1 and forced elec-tric dipole transitions split at 16,750 and 17,030 cm�1.Using the peak at 16,900 cm�1 as a reference peak, thesplitting is accompanied with the increase of I0�0 line inten-sity indicating more anisotropic local environment inEub2L/CRA[4]Ox compound, which is in agreement withthe broadening/splitting of I0�2 line as mentioned above.

Fig. 4 shows the luminescence decays (in normalizedscale) of pure-Eub2L powder and Eub2L/CRA[4]Oxembedded in acrylate polymers excited by Nd-YAG(THG, 355 nm) laser at room temperature. For both sam-ples, the decay is composed of two components, the short(s1) and long (s2) decay components. The decay time, orso-called the measured lifetime, is related to radiative (kr)and non-radiative decay (knr) by the relationships = (kr + knr)

�1. The short decay component is thereforeassociated to the ions which are subjected to high non-radi-ative decay via molecular vibrations or multi-phononrelaxations. The long decay is associated to the ions thatexperience less non-radiative decay. The short (s1) and long(s1) decay time constants and the corresponding intensityfactors (A1 and A2) for the decays shown in Fig. 4 are listedin Table 2, which are determined by curve fitting the decayswith two-exponential decay function:

IðtÞ ¼ I0ðA1 expðt=s1Þ þ A2 expðt=s2ÞÞ: ð4Þ

It is found that the short decay time constant s1 are almostthe same for both samples, whereas the long decay timeconstant s2 for Eub2L/CR[4]Ox is much longer than Eub2Lcase. The long decay component increase two times from0.26 ms to 0.49 ms upon mixing with CRA[4]Ox. Theintensitie ratios A1/A2 indicate that the short decay compo-nent is more dominant in Eub2L sample, whereas it isopposite in Eub2L/CRA[4]Ox sample. The binding of

Table 2Decay parameters obtained from curve fitting of the experimental decaylines in Fig. 4

Eub2L Eub2L/CRA[4]Ox

A1 0.39 0.26s1 0.10 ms 0.10 msA2 0.33 0.47s2 0.26 ms 0.49 msA1/A2 1.18 0.55

1372 R. Hidayat et al. / Optical Materials 29 (2007) 1367–1374

Eu3+ to CRA[4]Ox thus significantly suppressed non-radi-ative decay and improve luminescence efficiency. It seemsto be similar case as the improvement of luminescencequantum efficiency in b-diketonate europium complexesafter the replacement of water in the first coordination shellby phenanthroline.

These observed luminescence characteristics mentionedabove suggest two possibilities. The first, the binding is pro-moted by the coordination of CRA[4]Ox by Eu3+ ion at thefirst coordination shell but with much weaker nephelauxeticeffect in comparison to that caused by DBM ligand. Thesecond, the binding of Eub2L to CRA[4]Ox merely disturbthe structure and symmetry of complex coordinationbecause the binding is promoted by a non-covalent coordi-nation bonding and the binding site is located at the outer offirst coordination shell. It is therefore necessary to know thecoordination structure in this compound. X-ray single-crys-tal diffraction analysis is usually used to predict the coordi-nation structure. Unfortunately, attempts to findcoordination structure of Eub2L/CR[4]Ox were failedbecause of amorphous property of the compound. In thiswork, we used NMR spectroscopy to verify the binding ofEub2L to CRA[4]Ox. It is, however, not able to give infor-

-5.4 -5.2 -5.0 -4.8 -4.6 -4.40

10

20

δH (p

Inte

nsity

(arb

. uni

t)

-5.4 -5.2 -5.0 -4.8 -4.6 -4.40

10

20

30

δH (

Inte

nsity

(arb

. uni

t)

b

c

c

b

Fig. 5. Proton (1H) NMR spectra of (a) pure CRA[4]O

mation about coordination structure as detail as obtainedfrom X-ray diffraction measurement. In NMR spectros-copy, because europium ion is a paramagnetic element, inthe presence of external magnetic field, the ion producesinduced magnetic field and increases the local magneticfield. Consequently, a proton (1H) located nearby may sensestronger local magnetic field and become more de-shielded.Shifting of the chemical shift (DdH) of that proton, so-calledlanthanide induce shift (LIS), due to this de-shielding effectcan be seen in NMR spectrum.

In general, there are two kinds of LIS, namely contact(Fermi) and pseudocontact (dipolar) shifts. Contact shiftis a trough bond effect which is effective only when theligand is coordinated in the first coordination shell.Pseudocontact shifts, on the other hand, is a through spaceeffect which is dominant when the ligand is coordinated inthe second coordination shell. The shifting of the chemicalshift due to pseudocontact shift (in ppm) is given by

DdPCH ðr; hÞ ¼

Dvzz

2NA

3 cos2 h� 1

r3

� �þ

Dvxy

2NA

sin2 h cos 2/r3

� �ð5Þ

where NA is Avogadro number, and r (A), h and / are thepolar coordinates of the nearest proton with respect tolanthanide ion as the center of coordinate system [32–34].The anisotropy of the paramagnetic susceptibility ofthe lanthanide ions is represented by two principal valuesof paramagnetic susceptibility tensor, vzz and vxy. Thepseudocontact shift is therefore very sensitivity to the coor-dination structure and the orientation of the ion.

The NMR spectrum of Eub2L/BOCA[8]Ac mixture (notshown here) was found to be simply overlapping spectrum

-4.2 -4.0 -3.8 -3.6 -3.4 -3.2pm)

-4.2 -4.0 -3.8 -3.6 -3.4 -3.2ppm)

b

a

a

HC

4

O O

H2CH2C

O CH2

Et H2C CH2

OH2C

Et

CH3

a

c b

x and (b) Eub2L/CRA[4]Ox compound in CDCl3.

0 2 4 6 8 10

−5

−3

−1

1

3

5

r (Å)

150°

90°120°

135° 180°

ΔδH

PC (

ppm

)

3

2

1

1

2

3

2π

43π π

θ

ΔδH

PC (

ppm

)

θx

y

z

φ

Fig. 6. (a) Pseudocontact shift DdPCH as function of distance rl for various h = 90�, 120�, 135�, 150�, and 180� at / = 45�. (b) Pseudocontact shift DdPC

H asfunction of angle h at r = 5 A and / = 45�. Inset: The coordinate system used in the calculations.

R. Hidayat et al. / Optical Materials 29 (2007) 1367–1374 1373

of Eub2L and BOCA[8]Ac spectra. In contrast, the NMRspectra of Eub2L/CRA[4]Ox mixture is notably differentwith that of the pure CRA[4]Ox spectrum as evident inFig. 5. In case of pure CRA[4]Ox, three band centered at�3.7, �4.2 and �4.6 ppm were observed and assigned tochemical shift originated from proton at methylene (CH2)(b), methine (CH) (a), and oxetane (CH2) group (c), respec-tively, as indicated in Fig. 5. In Eub2L/CRA[4]Ox mixture,the b band is noticeably downfield shifted while the a and c

bands are almost in the same position. The average dH

shifting (DdH) is about �1.0 ppm. In the present case,bulky structure of DBM and upper rim groups hindersclose contact between europium and macrocyclic mono-mers, and the appearance of dH shifting in Fig. 5 can beassigned to pseudocontact shift originating from the bind-ing of Eub2L to CRA{4}Ox in the proximity of methylene(b) group at the upper rim of CRA[4]Ox.

In order to clarify this experimental result, the pseudo-contact shift given by Eq. (5) is analyzed and comparedto the experimental DdH value. Due to the lack of molecu-lar geometry for both the complex and CRA[4]Ox at thepresent stage, we consider a very simple situation whereone Eu3+ complex is located on the top of the CRA[4]Oxwith the main paramagnetic (z-) axis of the ion is perpen-dicular to the cavity. Pseudocontact shift DdPC

H was plottedin Fig. 6(a) as function of distance r for various h = 90�,120�, 135�, 150� and 180�, whereas / = 45�. The molar sus-ceptibility vzz and vxy values were adopted from Table 5 inRef. [32], namely 0.67 · 10�3 and 1.4 · 10�3 cm3 mol�1,respectively. The intersections between the experimentalvalue DdH = � 1.0 (horizontal line) and the DdPC

H curvesfor h = 90� and 120� are indicated by · sign in the figure.From this result, it is probable that Eub2L is located nearthe center or cavity of CRA[4]Ox which is separated fromthe nearest proton at the upper rim of about 5 A. Thisvalue is nevertheless almost two times larger than the firstcoordination sphere radius [32–34] so that the oxygen atthe upper rim of CRA[4]Ox should be coordinated at thesecond coordination sphere. The shifting of chemical shiftðDdPC

H Þ may continuously changes with h as indicated in

Fig. 6(b), whereas the experimental NMR data showsdownfield shifting only at a certain magnitude of DdH

indicating the presence of a preferential coordinationstructure.

4. Conclusions

Binding of partially chelated europium complex (Eub2L)ions with macrocyclic molecules and its correlation to lumi-nescence characteristics have been clarified. The lumines-cence of Eub2L/CRA[4]Ox compound exhibits broaderhypersensitive peak (I0�2 line) due to crystal/ligand fieldsplitting. The number of spitting and the intensity ratios(I0�2/I0�1) indicate a slight change on their coordinationstructure and symmetry. The I0�0 line indicates that thebinding does not induce additional nephelauxetic effect inthe Eu3+ first coordination shell. Slight decrease of I0�2/I0�0 ratio with splitting I0�2 line may reflect an increasein anisotropic at the Eu3+ ion. Luminescence decay wasfound much longer in Eub2L/CRA[4]Ox than those of pureEub2L, which may be due to suppression of non-radiativedecay via vibration or multi-phonon relaxation. Theseexperimental results imply two possibilities, either the bind-ing produces insignificant nephelauxetic effect compared tothat caused by DBM ligands, or the binding is promotedby a non-covalent coordination bonding at the outer offirst coordination shell. The 1H FT-NMR spectroscopyand pseudocontact shift analysis show the possibility ofbinding site at the second coordination shell, supportingthe second possibility. The bonding or interaction whichpromote this binding, however, cannot be completely clar-ified at this stage. Coordination of oxygens located at theupper rims of CRA[4]Ox or other weak interaction suchas Van der Walls interaction could be the underlying mech-anism for the establishment of this coordination, but moredetail studies on their molecular and electronic structuresare still required for further clarification. This work alsodemonstrates that binding lanthanide ions to bulky macro-cyclic monomer can minimize parasitic process such asnon-radiative decay.

1374 R. Hidayat et al. / Optical Materials 29 (2007) 1367–1374

Acknowledgement

One of us (R.H.) gratefully acknowledges the fellowshipsupport from Tohoku University COE Program on GiantMolecules and Complex Systems.

References

[1] T. Kobayashi et al., Appl. Phys. Lett. 71 (1997) 2421.[2] T. Jin, S. Tsutsumi, Y. Deguchi, K. Machida, G. Adachi, J.

Electrochem. Soc. 143 (1996) 3333;X. Fan, X. Wua, M. Wanga, J. Qiub, Y. Kawamoto, Mater. Lett. 58(2004) 2217;X. Fan et al., Mater. Sci. Eng. B 100 (2003) 147.

[3] L.D. Carlos, R.A. Sa Ferreira, V. de Zea Bermudez, C. Molina, L.A.Bueno, S.J.L. Ribeiro, Phys. Rev. B 60 (1999) 10042;V.Z. Bermudez, R.A. Sa Ferreira, L.D. Carlos, C. Molina, K.Dahmouche, S.J.L. Ribeiro, J. Phys. Chem. B 105 (2001) 3378;C. Molina, K. Dahmouche, Y. Messaddeq, S.J.L. Ribeiro, M.A.P.Silva, V. de Zea Bermudez, L.D. Carlos, J. Lumin. 104 (2003) 93.

[4] R. Reisfeld, T. Saraidarov, M. Gaft, M. Pietraszkiewicz, O.Pietraszkiewicz, S. Bianketti, Opt. Mater. 24 (2003) 1.

[5] E.J. Nassar et al., Mater. Res. 4 (2001) 18;E.J. Nassar, O.A. Serra, I.L.V. Rosa, J. Alloys Compd. 250 (1997) 380.

[6] L.D. Carlos, C. De Mello Donega, R.Q. Albuquerque, S. Alves Jr.,J.F.S. Menezes, O.L. Malta, Mol. Phys. 101 (2003) 1037.

[7] C.R. De Silva, J. Wang, M.D. Carducci, S.A. Rajapakshe, Z. Zheng,Inorg. Chim. Acta 357 (2004) 630.

[8] G.F. de Sa, O.L. Malta, C. de Mello Donega, A.M. Simas, R.L.Longo, P.A. Santa-Cruz, E.F. da Silva Jr., Coord. Chem. Rev. 196(2000) 165;O.L. Malta et al., Chem. Phys. Lett. 282 (1998) 233;R.Q. Albuquerque, Chem. Phys. Lett. 331 (2000) 519.

[9] S. Bruck, M. Hilder, P.C. Junk, U.H. Kynast, Inorg. Chem.Commun. 3 (2000) 666.

[10] B.L. An, M.L. Gong, K.W. Cheah, J.M. Zhang, K.F. Li, Chem.Phys. Lett. 385 (2004) 345.

[11] S. Rigault, C. Piguet, J.C.G. Bunzli, J. Chem. Soc., Dalton Trans.(2000) 2045;V. Zolin et al., J. Alloys Compd. 341 (2002) 376.

[12] P.R. Selvin, J. Jancarik, M. Li, L.W. Hung, Inorg. Chem. 35 (1996)700.

[13] G. Vincentini, L.B. Zinner, J. Zukerman-Schpector, K. Zinner,Coord. Chem. Rev. 196 (2000) 353.

[14] L.J. Charbonniere et al., J. Chem. Soc., Dalton Trans. (1998) 505;J.C.G. Bunzli et al., J. Alloys Compd. 303–304 (2000) 66.

[15] S.S. Braga et al., J. Phys. Chem. B 106 (2002) 11430.[16] L.H. Slooff et al., J. Appl. Phys. 91 (2002) 3955.[17] P. Thuery, M. Nierlich, J. Vicens, H. Takemura, Polyhedron 19

(2000) 2673.[18] J. Plutnar et al., Inorg. Chim. Acta 335 (2002) 27.[19] M. Pietraszkiewicz et al., J. Alloys Compd. 380 (2004) 241.[20] J. Vicens, V. Bohmer (Eds.), Calixarenes: A Versatile Class of

Macrocyclic Compounds, Kluwer Academic Publishers, Dordrecht,1991.

[21] T. Nishikubo et al., Polym. J. 35 (2003) 213.[22] Y. Yamagishi et al., Proc. SPIE 4991 (2003) 414;

K. Komatsu et al., Polym. Prepr. – Japan 53 (2004) 4583.[23] B.R. Judd, Phys. Rev. B 127 (1962) 750.[24] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511;

G.S. Ofelt, J. Chem. Phys. 38 (1963) 2171.[25] Richard C. Powell, Physics of Solid State Materials, Springer-Verlag,

New York, 1998.[26] M.C.F. Cunha et al., Coord. Chem. Rev. 119 (1992) 1.[27] J.A. Koningstein, Phys. Rev. A 136 (1964) 717.[28] C.K. Jørgensen, Prog. Inorg. Chem. 4 (1962) 73;

D.J. Newman, J. Phys. Chem. Solids 34 (1973) 541;L.D. Carlos, A.L.L. Videira, J. Chem. Phys. 101 (1994).

[29] S.T. Frey, W.De.W. Horrocks Jr., Inorg. Chim. Acta 229 (1995) 383.[30] F. Gao, S. Zhang, J. Phys. Chem. Solids 58 (1997) 1991.[31] O.L. Malta, H.J. Batista, L.D. Carlos, Chem. Phys. 282 (2002) 21.[32] J.W.M. de Boer et al., J. Magn. Reson. 25 (1977) 455.[33] C.F.G.C. Geraldes et al., J. Alloys Compd. 323–324 (2001) 824.[34] J. Ren, S. Zhang, A.D. Sherry, C.F.G.C. Geraldes, Inorg. Chim. Acta

339 (2002) 273.