Binding of europium complex to polymerizable macrocyclic molecules and its optical properties

Optical Materials 29 (2007) 1367–1374 www.elsevier.com/locate/optmat

Binding of europium complex to polymerizable macrocyclic molecules and its optical properties Rahmat Hidayat a

a,b,*

, Okihiro Sugihara a, Masaaki Tsuchimori c, Manabu Kagami c, Tadatomi Nishikubo d, Toshikuni Kaino a

Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan b 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, Japan d Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi Kanagawa-Ku, Yokohama, Japan Received 23 February 2006; accepted 18 June 2006 Available online 4 October 2006

Abstract Study on the incorporation of trivalent europium (Eu3+) complex into polymerizable macrocylic molecules, namely calixarene and calixresorcinarene derivatives, has been carried out. Broadening of hypersensitive luminescence peak was observed in solution and polymer containing a compound of europium complex and calixresorcinarene monomer, which is ascribed to ligand field splitting due to anisotropic local field at the Eu3+ ion site. From 1H NMR spectroscopy, it is found that the chemical shift originated from the moiety at 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 binding does 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 significant 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 of applications such as electronic displays, lasers, optical amplifiers, and molecular sensing in biochemistry. For some applications, lanthanide ions should be attached to host molecules or incorporated into a suitable matrix or medium. In the field of photonic and communications, particularly in the use of lanthanide ions as light amplifying

*

Corresponding author. E-mail addresses: rahmat@fi.itb.ac.id (R. Hidayat), kaino@tagen. tohoku.ac.jp (T. Kaino). 0925-3467/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2006.06.016

medium, the ions are usually incorporated into host molecular crystals or glass, such as fluoride and silicate glasses. However, high temperature required in glass processing limits diversity in developments of optical devices based on lanthanide-doped glass. In order to overcome this limitation, there are numerous efforts to incorporate lanthanide ions in polymers or organic/inorganic hybrid matrixes because of their attractive preparation routes offering easiness and flexibility in fabrication methods. Incorporation these lanthanide complexes into organic polymers, organic–inorganic hybrid materials such as organically modified silicate (ormosil) matrices, ureasils xerogels, and other functionalised silica, and their applications for optical devices have been much reported [1–5].

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In order to incorporate lanthanide ions into those host materials, the ions are usually complexed with organic ligands, such as b-diketones, pyridine and carboxylates, which can increase the solubility and may also function as a light antenna or sensitizing group [6–13]. Lanthanide ions can be also complexed with ligands having macrocylic or cavitand shapes, such as crown ethers, cryptates, polydentate hemisperands and calixarenes [14–20]. Some kinds of those macrocyclic ligands are interesting as they are capable to capture lanthanide ions by inclusion phenomenon depending on the cavity size and attached functional groups. Unfortunately, the absorption band of these ligands appears in deep violet region so there are very limited light sources for photo-excitation available today. Recently, polymerizable macrocyclic molecules such as calixarene and calixresorcinarene polymers have been extensively developed as new kind of resists for photo- or electron-beam lithography [21]. Acrylate or epoxy moieties are attached to the lower or upper rims of these macrocyclics. Our group has successfully demonstrated the fabrication of calixarene acrylate waveguides by means of one- and two-photon polymerization techniques [22]. We are currently interested in developing optical waveguides using these macrocyclic polymers that incorporating lanthanide complexes. When an interaction between lanthanide ions and macrocylic monomers are established either by the formation of coordination bond, inclusion phenomenon, or other kind of non-bonding interactions, the ions will be distributed homogeneously with appropriate separation distance from each other. Consequently, complex aggregation and interaction between neighboring ions could be suppressed even at high doping concentration. In this paper, we present our study on binding of trivalent europium (Eu3+) ions to macrocyclic monomers and polymers, and correlated change in optical properties due to this binding. We have carried out optical and FT-NMR spectroscopy for this study.

2. Experimental Europium (Eu3+) ion complexed with dibenzoylmethane (DBM) used in this study was prepared by a similar method as reported in many literatures [1,9,10]. In order to promote interaction or coordination bond formation between the ions and macrocyclic monomers, which are commonly large and bulky molecules, the europium ions were only partially chelated, namely EubmLn where b is deprotonated DBM and L is Cl with m = 2 and n = 1. As a typical reaction procedure, the reaction was carried out 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 stirred for over than 1 h at room temperature and then collected by filtration. The product, hereafter just referred simply as Eub2L, was drained with water, dried and then dissolved in chloroform. The product was reprecipitated from

a

Et H 2 H2C C O CH2 O

H2 Et CH2 C O H C O 2 H C CH3 4

b

R

CH2

8

OCOCH=CH2

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

ethanol/chloroform mixture solution by slow evaporation using rotary evaporator.1 The product (Eub2L) was mixed with macrocylic monomers in chloroform or dichloroethane, and then stirred at temperature of about 80 C. Fig. 1 shows the molecular structure of macrocyclic monomers used in this study, namely calixresorcinarene containing oxetane groups (CRA[4]Ox) and p-tert-(bu)calix[8]arene containing acrylate groups (BUCA[8]Ac with R = t-Butyl and BOCA[8]Ac with R = t-Butyl and t-Octyl in arbitrary position). The compound was reprecipitated in hexane/chloroform mixture solution by the same method as mentioned above. Polymers containing these complexes were prepared by thermal initiated radical polymerization. Eub2L/ BOCA[8]Ac compound, acrylate monomer and initiator was dissolved in chloroform with a certain weight percentage ratio. Acrylate monomer used here was triethylene glycol dimethacrylate (TGD) or methyl methacrylate (MMA). As thermal radical initiator, a,a 0 -azoisobutyronitrile (AIBN) or Lauroyl peroxide has been used. The polymerization was carried out in an oven, with temperature kept at about 85 C for AIBN initiator and 65 C for LP initiator, for over than one night. For preparing thin film polymers, the monomer solution was spin-cast or cast on appropriate substrate (glass, quartz or Si wafer) prior polymerization and the oven was filled with N2 gas. CRA[4]Ox can be polymerized by cationic polymerization, however, Eub2L was found decomposed as the cationic polymerization progressed. Accordingly, polymer containing Eub2L/ CRA[4]Ox compound were obtained by polymerizing MMA 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. The fluorescence spectra were measured using a photomultichannel analyzer (Hamamatsu PMA-11) and a N2 laser or blue (405 nm) diode laser as the excitation light source. A pre-amplified Si PIN photodiode connected to an oscilloscope was used to measure the fluorescence decays. A N2 laser or Nd-YAG (THG) laser was used as the excitation light source. The 1H NMR spectra were measured

1

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

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

5.0x10

4

4.5x10

4

4.0x10

4

3.5x10

4

-1

3.0x10

4

-1

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

2.5x10

4

2.0x10

4

1.5x10

4

1.0x10

4

2

The disk shaped polymer sample was prepared by thermal polymerization 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 total weight of Eub2L is about 33.3 mg. After polymerization, the polymer was removed from the glass cell. The polymer has shape of 12.5 mm in diameter and 5 mm in height averagely. The top and bottom sides of this polymer were polished with high fine grade of polishing paper in order to minimize surface roughness and thus light scattering during the absorption spectroscopy measurement. The height of the disk after polishing is 3.4 mm. The concentration of Eub2L was calculated from the mole number of Eub2L molecules divided by the polymer volume and then multiplied with Avogadro number.

4

5x10

CRA[4]Ox

4

4x10

4

3x10

4

2x10

4

1x10 0 200

300

400

500

600

DBM Euβ2L Euβ2L/CRA[4]Ox

3

5.0x10 0.0 200

250

300

350 400 450 500 Wavelength (nm)

550

600

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

4.0 3.5 Absorbance (arb. unit)

Fig. 2a shows the absorption spectra measured from dilute solution of Eub2L (1.5 lmol/L) and Eub2L/ CRA[4]Ox mixture (4.0 lmol/L) in chloroform represented in molar absorption coefficient (L mol1 cm1). The Eub2L/CRA[4]Ox compound is 1:1 in molar ratio. The spectra show a broad absorption band peaked at 348 nm, which is originated from p-p* excitation of the DBM ligands. The peak is shifted about 5 nm to longer wavelength in comparison to the DBM spectrum and the band tail is extending up to 420 nm because of higher p orbitals delocalization in Eub2L. The molar absorption coefficient is in order of 104 L mol1 cm1, which is typical of dipole allowed transition as commonly found in organic ligands. Absorption band peaked at 275 nm is originated from CRA[4]Ox as indicated in the inset of the Fig. 2a. This absorption band is well separated from the absorption band of Eub2L. It is clear that in this dilute solution, CRAOx is transparent enough so that Eub2L can appropriately absorb the excitation light. Sample polymers with high concentration of Eub2L were prepared to evaluate the absorption cross-sections for f–f direct transitions, i.e. 7F0 ! 5D1 and 7F0 ! 5D2. Fig. 2b shows the absorption spectrum measured from a disk shaped polymer bulk with size of 12.5 mm in diameter and 3.4 mm in thickness. This polymer was prepared from monomer solution consisting of MMA, BOCA[8]Ac and Eub2L/ CRA[4]Ox, with ratio 4:1:1 in weight percentage, where the weight percentage ratio of Eub2L/CRA[4]Ox is 1:2. The molecular weight of CRAOx and Eub2L are 1330 and 632 g/mol, respectively. The ratio is therefore approximately equivalent to molar ratio of 1:1, which is corresponding to ca. 0.5 · 1020 cm3 of Eu(III) ions.2 Optical transitions 7 F0 ! 5D2 and 7F0 ! 5D1 were observed at 466 nm and 534 nm, respectively. From this absorption spectrum, the molar absorption coefficient (e) for 7F0 ! 5D2 transition was calculated to be 11.8 L mol1 cm1, or equivalent to absorption cross-section (r) of 4.5 · 1020 cm2. This value indicates that these macrocyclic monomers are suitable for polymer matrix of this complex. However, as indicated by

ε (L mol cm )

3. Results and discussion

1369

3.0 7

2.5

F0 → 5D1

2.0 1.5 7

1.0

F0 → 5D2

0.5 0.0 200

300

400

500

600

700

800

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

its absorption edge extending up to 500 nm, it is essential to significantly reduce the attenuation loss at this optical wavelength regime, which is probably caused by high light scattering loss. Fig. 3(a) shows the luminescence spectra of pure Eub2L in chloroform, Eub2L/CRA[4]Ox compound and the same compound embedded in BOCA[8]Ac/MMA polymer. The luminescence peaks are originated from 5D0 ! 7FJ transitions with J values as indicated in the figure. The luminescence can be originated from magnetic dipole transition or/ and forced electric dipole transitions according to Judd– Ofelt theory, where the spontaneous emission rate for those transitions is given by [23–25]   8p2 e2 nðn2 þ 22 Þ ed 3 md f AðJ ; J 0 Þ ¼ þ n f ð1Þ 9 mck2JJ 0 where kJJ 0 is emission wavelength for transition between initial manifold j(S, L)Ji and final manifold j(S, L)J 0 i, f ed is oscillator strength for forced dipole transition

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0.8

Eu β 2L (in CHCl3) Eu β 2L/CRA[4]Ox (powder) Eu β 2L/CRA[4]Ox (polymer)

0.6

D1 D0

7

J=2

F6 7

7

0.4

1.0

5 5

PL Intensity (arb. units)

PL Intensity (arb. units)

1.0

F0

F2

J=4

0.2

J=0

J=1

J=3

0.8 0.6 0.4 0.2 0.0

0.0 540 560 580 600 620 640 660 680 700 720 Wavelength (nm)

15000

15500

16000

16500

17000

17500

-1

Wavenumber (cm ) 0.10

Euβ2L/CRA[4]Ox

1.0

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

PL Intensity (arb. units)

PL Intensity (arb. units)

o

(polymer) at 77 K

0.8 0.6 0.4 0.2 0.0 540 560 580 600 620 640 660 680 700 720 Wavelength (nm)

0.08

5 5

0.06

D0

D0

7

7

F0

F1

0.04 0.02 0.00 16600

16800

17000

17200

17400

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 in BOCA[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 I00 and I01 lines.

f ed

8p2 mc ¼ 3hkJJ 0

X k¼2;4;6

Xk

  !  ðS; LÞJ jU k jðS; LÞJ 0  2J þ 1

ð2Þ

and f md is oscillator strength for forced dipole transition f md ¼

2p2 ðjhðS; LÞJ jL þ 2SjðS; LÞJ 0 ijÞ hmc

ð3Þ

with the labels used in those equations are universal labels as found elsewhere [23–25]. The forced dipole transition depends on the Judd–Ofelt intensity parameter Xk (k = 2, 4, 6), which reflects the odd-symmetry crystal/ligand field terms and the strength of state admixing of opposite parity. Consequently, the forced dipole transitions are thus strongly depends on the local environment, that is, the coordination structure and chemical properties of the ligands. The intensity parameter Xk can be calculated theoretically or estimated experimentally from absorption spectrum. The magnetic dipole transition depends only on the quantum number of the initial and final states; accordingly, it is independent of local environment. The observed luminescence spectrum shows typical characteristics of most b-diketonate europium complexes with the appearance of intense hypersensitive I02 (5D0 ! 7F2)

line peaked at around 610 nm. The hypersensitive transition is usually associated with large value of Judd–Ofelt intensity parameter X2 caused by the absence of inversion symmetry in coordination structure or on other words, anisotropic local environment [6,10,13,26,27]. In the case of low symmetry and highly anisotropic ligand/crystal field, the intensity of this hypersensitive peak is much larger than that of other transition lines. Moreover, because the maximum number of this ligand field splitting for J manifold is (2J + 1), energy level splitting into maximum of five Stark levels in this hypersensitive line can be expected. Fig. 3(b) shows the curve fitting of the hypersensitive line of Eub2L/CRA[4]Ox polymer with three Gaussian functions clarifying that the peak is composed of three peaks (with their spectral shape parameters as indicated in Table 1), suggesting three Stark levels for 7F2 level. In this case, the ligand field in Eub2L/CRA[4]Ox compound are not able to completely remove the degeneracy of 7F2 level. The ligand field in Eub2L/CRA[4]Ox compound nevertheless exists with much lower symmetry than that in pure Eub2L. This may also related to extensive distortion of coordination structure in Eub2L/CRA[4]Ox compound rather than in pure Eub2L. The splitting of this I02 line therefore indi-

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

Transition line

Peak

5

D0 ! 7F0

17,268 cm1 (579 nm)

Line width

v.w.

5

D0 ! 7F1

16,750 cm1 16,900 cm1 17,030 cm1

v.w. v.w. v.w.

5

D0 ! 7F2

16,333 cm1 (612.3 nm) 16,211 cm1 (616.9 nm) 16,045 cm1 (623.2 nm)

3.9 nm (103 cm1) 7.0 nm (182 cm1) 4.5 nm (117 cm1)

Intensity

s m w

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

cates a binding of Eub2L to CRA[4]Ox. At this stage, however, there is still not enough evident to conclude what kind of bonding or interaction which promote this binding of Eub2L to CRA[4]Ox. The red-shift of the observed I00 (5D0 ! 7F0) line with respect to the I00 line for Eu3+ ion gas has been accounted for the nephelauxetic effect, which is related to the influence of the ligand covalency in the Eu3+ first coordination shell leading to minute alternation on electrostatic distribution and spin–orbit parameters of the ion in comparison to the free ion gas [3,28–31]. Accordingly, the red-shift of I00 line could be used as an indicator of the bonding strength of Eu3+ ion to ligand. Based on this phenomenological effect, Carlos and coworkers have successfully showed the correlation between the reduction of the redshift of I00 line and the lowering of the covalency degree of the Eu3+ first coordination shell [3]. As evident in Fig. 3(a), the I00 line in our samples was found at the same wavelength of about 579 nm (17,268 cm1), which is redshifted about 106 cm1 from that of calculated for Eu3+ ion gas. This red-shift value is close to the theoretical values for the fluorinated b-diketones (e.g. trifluorophenylbutanedione (btfa)) complexes calculated by Malta et al., where overlap polarizability as a quantitative measure of covalency has been introduced into their calculation [31]. No change of I00 line wavelengths indicate the same nephelauxetic effect in all samples studied here. That is, the binding of Eu3+ ion to CRA[4]Ox does not alter the covalency of the Eu3+ first coordination shell. As also evident in Fig. 3(c), even though the temperature has been lowered down to 77 K, the luminescence spectrum for Eub2L/ CRA[4]Ox polymer is almost unchanged with the same position of I00 line as that observed in the room temperature spectrum. Eu3+ ions are fixed firmly in the matrixes with the covalency of the Eu3+ first coordination shell is unaffected by temperature. Another important feature from the luminescence spectra is I01 (5D0 ! 7F1) line. The observed I01 line could be originated from forced electric dipole transition and/or allowed magnetic dipole transition which is independent

of local environment. The ratio I02/I01 therefore can be also used as local environment indicator. As displayed in Fig. 3(d), it is clear that the I01 shape for Eub2L/ CRA[4]Ox is broader than that for pure Eub2L in solution. This I01 seems to be composed of three peaks, namely a magnetic dipole transition at 16,900 cm1 and forced electric dipole transitions split at 16,750 and 17,030 cm1. Using the peak at 16,900 cm1 as a reference peak, the splitting is accompanied with the increase of I00 line intensity indicating more anisotropic local environment in Eub2L/CRA[4]Ox compound, which is in agreement with the broadening/splitting of I02 line as mentioned above. Fig. 4 shows the luminescence decays (in normalized scale) of pure-Eub2L powder and Eub2L/CRA[4]Ox embedded in acrylate polymers excited by Nd-YAG (THG, 355 nm) laser at room temperature. For both samples, the decay is composed of two components, the short (s1) and long (s2) decay components. The decay time, or so-called the measured lifetime, is related to radiative (kr) and non-radiative decay (knr) by the relationship s = (kr + knr)1. The short decay component is therefore associated to the ions which are subjected to high non-radiative decay via molecular vibrations or multi-phonon relaxations. The long decay is associated to the ions that experience less non-radiative decay. The short (s1) and long (s1) decay time constants and the corresponding intensity factors (A1 and A2) for the decays shown in Fig. 4 are listed in Table 2, which are determined by curve fitting the decays with two-exponential decay function: IðtÞ ¼ I 0 ðA1 expðt=s1 Þ þ A2 expðt=s2 ÞÞ:

ð4Þ

It is found that the short decay time constant s1 are almost the same for both samples, whereas the long decay time constant s2 for Eub2L/CR[4]Ox is much longer than Eub2L case. The long decay component increase two times from 0.26 ms to 0.49 ms upon mixing with CRA[4]Ox. The intensitie ratios A1/A2 indicate that the short decay component is more dominant in Eub2L sample, whereas it is opposite in Eub2L/CRA[4]Ox sample. The binding of Luminescence Intensity (arb. units)

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

1371

1

Euβ2L/CRA[4]Ox in acrylate polymer Euβ2L in acrylate polymer

0.1

0.01 0.0

0.2

0.4

0.6

0.8

1.0

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

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

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

A1 s1 A2 s2 A1/A2

Eub2L

Eub2L/CRA[4]Ox

0.39 0.10 ms 0.33 0.26 ms 1.18

0.26 0.10 ms 0.47 0.49 ms 0.55

Eu3+ to CRA[4]Ox thus significantly suppressed non-radiative decay and improve luminescence efficiency. It seems to be similar case as the improvement of luminescence quantum efficiency in b-diketonate europium complexes after the replacement of water in the first coordination shell by phenanthroline. These observed luminescence characteristics mentioned above suggest two possibilities. The first, the binding is promoted by the coordination of CRA[4]Ox by Eu3+ ion at the first coordination shell but with much weaker nephelauxetic effect in comparison to that caused by DBM ligand. The second, the binding of Eub2L to CRA[4]Ox merely disturb the structure and symmetry of complex coordination because the binding is promoted by a non-covalent coordination bonding and the binding site is located at the outer of first coordination shell. It is therefore necessary to know the coordination structure in this compound. X-ray single-crystal diffraction analysis is usually used to predict the coordination structure. Unfortunately, attempts to find coordination structure of Eub2L/CR[4]Ox were failed because of amorphous property of the compound. In this work, we used NMR spectroscopy to verify the binding of Eub2L to CRA[4]Ox. It is, however, not able to give infor-

Intensity (arb. unit)

c

Intensity (arb. unit)

b

mation about coordination structure as detail as obtained from X-ray diffraction measurement. In NMR spectroscopy, because europium ion is a paramagnetic element, in the presence of external magnetic field, the ion produces induced magnetic field and increases the local magnetic field. Consequently, a proton (1H) located nearby may sense stronger local magnetic field and become more de-shielded. Shifting of the chemical shift (DdH) of that proton, so-called lanthanide induce shift (LIS), due to this de-shielding effect can be seen in NMR spectrum. In general, there are two kinds of LIS, namely contact (Fermi) and pseudocontact (dipolar) shifts. Contact shift is a trough bond effect which is effective only when the ligand is coordinated in the first coordination shell. Pseudocontact shifts, on the other hand, is a through space effect which is dominant when the ligand is coordinated in the second coordination shell. The shifting of the chemical shift due to pseudocontact shift (in ppm) is given by     Dvxy sin2 h cos 2/ Dvzz 3 cos2 h  1 PC DdH ðr; hÞ ¼ þ r3 r3 2N A 2N A ð5Þ ˚ ), h and / are the where NA is Avogadro number, and r (A polar coordinates of the nearest proton with respect to lanthanide ion as the center of coordinate system [32–34]. The anisotropy of the paramagnetic susceptibility of the lanthanide ions is represented by two principal values of paramagnetic susceptibility tensor, vzz and vxy. The pseudocontact shift is therefore very sensitivity to the coordination structure and the orientation of the ion. The NMR spectrum of Eub2L/BOCA[8]Ac mixture (not shown here) was found to be simply overlapping spectrum

a

30

b

20 10 0 -5.4

-5.2

-5.0

-4.8

-4.6

20

-4.4 -4.2 δH (ppm)

a c

10

-4.0

-3.8

Et H 2 H2C C O CH2 O

-3.6

-3.4

bH

-3.2

Et 2 CH2 C O HC O 2

c

H C CH3 4

b

a 0 -5.4

-5.2

-5.0

-4.8

-4.6

-4.4 -4.2 δH (ppm)

-4.0

-3.8

-3.6

-3.4

-3.2

Fig. 5. Proton (1H) NMR spectra of (a) pure CRA[4]Ox and (b) Eub2L/CRA[4]Ox compound in CDCl3.

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

1373

z 5

135°

150°

3

180°

2

1

−1 −3

0

2

4

6

8

10

r (Å) 120°

90°

−5

ΔδHPC (ppm)

ΔδHPC (ppm)

3

θ

1

π

2

3π

y

φ

x

4

π

θ

1 2 3

PC Fig. 6. (a) Pseudocontact shift DdPC H as function of distance rl for various h = 90, 120, 135, 150, and 180 at / = 45. (b) Pseudocontact shift DdH as ˚ and / = 45. Inset: The coordinate system used in the calculations. function of angle h at r = 5 A

of Eub2L and BOCA[8]Ac spectra. In contrast, the NMR spectra of Eub2L/CRA[4]Ox mixture is notably different with that of the pure CRA[4]Ox spectrum as evident in Fig. 5. In case of pure CRA[4]Ox, three band centered at 3.7, 4.2 and 4.6 ppm were observed and assigned to chemical shift originated from proton at methylene (CH2) (b), methine (CH) (a), and oxetane (CH2) group (c), respectively, 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 hinders close contact between europium and macrocyclic monomers, and the appearance of dH shifting in Fig. 5 can be assigned to pseudocontact shift originating from the binding 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 pseudocontact shift given by Eq. (5) is analyzed and compared to the experimental DdH value. Due to the lack of molecular geometry for both the complex and CRA[4]Ox at the present stage, we consider a very simple situation where one Eu3+ complex is located on the top of the CRA[4]Ox with the main paramagnetic (z-) axis of the ion is perpendicular to the cavity. Pseudocontact shift DdPC H was plotted in Fig. 6(a) as function of distance r for various h = 90, 120, 135, 150 and 180, whereas / = 45. The molar susceptibility vzz and vxy values were adopted from Table 5 in Ref. [32], namely 0.67 · 103 and 1.4 · 103 cm3 mol1, respectively. The intersections between the experimental value DdH =  1.0 (horizontal line) and the DdPC H curves for h = 90 and 120 are indicated by · sign in the figure. From this result, it is probable that Eub2L is located near the center or cavity of CRA[4]Ox which is separated from ˚ . This the nearest proton at the upper rim of about 5 A value is nevertheless almost two times larger than the first coordination sphere radius [32–34] so that the oxygen at the upper rim of CRA[4]Ox should be coordinated at the second coordination sphere. The shifting of chemical shift ðDdPC H Þ may continuously changes with h as indicated in

Fig. 6(b), whereas the experimental NMR data shows downfield shifting only at a certain magnitude of DdH indicating the presence of a preferential coordination structure. 4. Conclusions Binding of partially chelated europium complex (Eub2L) ions with macrocyclic molecules and its correlation to luminescence characteristics have been clarified. The luminescence of Eub2L/CRA[4]Ox compound exhibits broader hypersensitive peak (I02 line) due to crystal/ligand field splitting. The number of spitting and the intensity ratios (I02/I01) indicate a slight change on their coordination structure and symmetry. The I00 line indicates that the binding does not induce additional nephelauxetic effect in the Eu3+ first coordination shell. Slight decrease of I02/ I00 ratio with splitting I02 line may reflect an increase in anisotropic at the Eu3+ ion. Luminescence decay was found much longer in Eub2L/CRA[4]Ox than those of pure Eub2L, which may be due to suppression of non-radiative decay via vibration or multi-phonon relaxation. These experimental results imply two possibilities, either the binding produces insignificant nephelauxetic effect compared to that caused by DBM ligands, or the binding is promoted by a non-covalent coordination bonding at the outer of first coordination shell. The 1H FT-NMR spectroscopy and pseudocontact shift analysis show the possibility of binding site at the second coordination shell, supporting the second possibility. The bonding or interaction which promote this binding, however, cannot be completely clarified at this stage. Coordination of oxygens located at the upper rims of CRA[4]Ox or other weak interaction such as Van der Walls interaction could be the underlying mechanism for the establishment of this coordination, but more detail studies on their molecular and electronic structures are still required for further clarification. This work also demonstrates that binding lanthanide ions to bulky macrocyclic monomer can minimize parasitic process such as non-radiative decay.

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