Self-assembled triple-stranded lanthanide dimetallic helicates with a ditopic ligand derived from bis(benzimidazole)pyridine and featuring an (4-isothiocyanatophenyl)ethynyl substituent

Article abstract of DOI:10.1002/1522-2675(200207)85:7<1915::AID-HLCA1915>3.0.CO;2-N

Bis(2-{6-(diethylcarbamoyl)-4-[(4-isothiocyanatophenyl) ethynyl]pyridin-2′-yl}-]-1-ethylbenzimidazol-5-yl)-methane (L^G) reacts with trivalent lanthanide ions in acetonitrile to yield triple-stranded dimetallic helicates [Ln₂(L^G)₃]⁶⁺. ¹H-NMR Data point to the helicates being the only species formed under stoichiometric conditions and having a time-averaged D₃ symmetry on the NMR time scale. The photophysical properties of L^G and its helicates are discussed with respect to the closely related ligands L^B, L^E, and their complexes, two ligands devoid of the isothiocyanatophenylethynyl substituent. The quantum yield of the ligand fluorescence is three times smaller compared to L^E, while that of the Eu^III-centered luminescence (1.1%) is three times larger. On the other hand, the luminescence of Tb^III is not sensitized by L^G. This is explained in terms of energy differences between the singlet and triplet states on one hand, and between the 0-phonon transition of the triplet state and the excited metal ion states on the other. This work demonstrates that bulky substituents in the 4-position of the pyridine ring do not prevent the formation of triple-stranded helicates, opening the way for luminescent probes that can easily be coupled to biological materials.

Full text of DOI:10.1002/1522-2675(200207)85:7<1915::AID-HLCA1915>3.0.CO;2-N

Helvetica Chimica Acta Vol. 85 (2002)
1915
Self-Assembled Triple-Stranded Lanthanide Dimetallic Helicates with a
Ditopic Ligand Derived from Bis(benzimidazole)pyridine and Featuring an
(4-Isothiocyanatophenyl)ethynyl Substituent
by Raphae»l Tripiera), Marcel Hollensteina), Mourad Elhabiria), Anne-Sophie Chauvina), Gae»l Zucchia),
Claude Piguetb), and Jean-Claude G. B¸nzli*a)
a) Swiss Federal Institute of Technology Lausanne, Institute of Molecular and Biological Chemistry, BCH 1402,
CH-1015 Lausanne (e-Mail: jean-claude.bunzli@epfl.ch)
b) Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, 30 quai E. Ansermet,
CH-1211 Geneva 4
Bis(2-{6-(diethylcarbamoyl)-4-[(4-isothiocyanatophenyl)ethynyl]pyridin-2'-yl}-1-ethylbenzimidazol-5-yl)-
methane (LG) reacts with trivalent lanthanide ions in acetonitrile to yield triple-stranded dimetallic helicates
.
[Ln2(LG)3]6‡ 1H-NMR Data point to the helicates being the only species formed under stoichiometric
conditions and having a time-averaged D3 symmetry on the NMR time scale. The photophysical properties of LG
and its helicates are discussed with respect to the closely related ligands LB, LE, and their complexes, two ligands
devoid of the isothiocyanatophenylethynyl substituent. The quantum yield of the ligand fluorescence is three
times smaller compared to LE, while that of the EuIII-centered luminescence (1.1%) is three times larger. On the
other hand, the luminescence of TbIII is not sensitized by LG. This is explained in terms of energy differences
between the singlet and triplet states on one hand, and between the 0-phonon transition of the triplet state and
the excited metal ion states on the other. This work demonstrates that bulky substituents in the 4-position of the
pyridine ring do not prevent the formation of triple-stranded helicates, opening the way for luminescent probes
that can easily be coupled to biological materials.
Introduction. Supramolecular functional architectures containing lanthanide ions
receive a sustained interest, especially in the field of medicine, because the optical and
magnetic properties arising from the shielded 4f electronic configurations are easily
traceable, therefore, providing suitable diagnostic [1][2], therapeutic [3][4], or
analytical tools [5][6]. One challenge associated with the development of such
molecular edifices lies in the accurate control of the LnIII inner coordination sphere in
order to reach large thermodynamic stability and kinetic inertness without altering the
specific properties of the metal ion. In the particular case of photophysical properties,
additional features have to be incorporated in the receptor in order to achieve efficient
sensitization of the luminescent lanthanide ions [7]. In recent years, we have focused
part of our research efforts on the design of segmental ligands featuring aromatic
tridentate and/or bidentate coordination units and coded for the self-assembly of 4f-4f
[8] or 4f-3d [9][10] dimetallic helicates. We have shown that the coordination sphere of
the LnIII ions can be controlled by semirigid aromatic tridentate ligands such as
bis(benzimidazole)pyridines [11][12]. The covalent connection of two such units leads,
for example, to the formation of ditopic ligand LA (Scheme 1), forming stable
dimetallic helicates [Ln2(LA)3]6‡ in MeCN in which the ligand strands are wrapped
around two 9-coordinate LnIII ions with tricapped trigonal prismatic coordination
geometry [13][14]. The self-assembly process results in the formation of protective

1916
Helvetica Chimica Acta Vol. 85 (2002)
environments for the metal ions, with the ligand strands wrapped tightly around them
and held together by weak interstrand p-p stacking interactions [14]. To increase the
resistance of the molecular edifices toward hydrolysis, we have modified the terminal
subunits of ligand LA by introducing dialkylamide groups (cf. LB) [15]. Further
hydrolysis of the amide functions resulted in a dicarboxylic ligand LC inducing the self-
assembly of neutral and highly stable [Ln2(LC-2 H)3] helicates in H2O [16]. The use of
magnetic (e.g., in magnetic resonance imaging [2]) or luminescent (e.g., in fluoroim-
munoassays [5]) lanthanide-containing probes frequently requires a coupling with the
biological material of interest, for instance, monoclonal antibodies. Such bioconjugates
are often obtained by reacting the amine functions of the antibody with isothiocyanate
(SCN) groups located on the metal chelate and held at a reasonable distance from the
active site of the probe [17]. In this paper, we build on our recent work in which halogen
substituents were introduced at C(4) of the pyridine moieties of LB to yield the reactive
synthons LE and LF [18]. The latter are used to introduce a (4-isothiocyanatophenyl)-
ethynyl substituent at C(4) of pyridine (cf. LG), a group known for its easy coupling
with amino functions of antibodies [5], oligonucleotides and proteins in the case of
DNA labelling [19][20]. The aim of the paper is to test the influence of these relatively
bulky groups on the wrapping process around the lanthanide ion and on the
photophysical properties of the resulting assemblies.
Results and Discussion. Synthesis and Solution Structure of Ligand LG. Bis(2-{6-
(diethylcarbamoyl)-4-[(4-isothiocyanatophenyl)ethynyl]pyridin-2-yl}-1-ethyl
benz-
imidazol-5-yl)methane (LG) was obtained in good yield according to the previously
described two-step procedure, relying on a modified Phillips coupling reaction as the
key step in achieving the desired benzimidazole units. One of the starting materials is

Helvetica Chimica Acta Vol. 85 (2002)
1917
the analogous 4-halogenated compound (LE or LF, X ˆ Cl or Br) synthesized according
to the improved procedure we have recently proposed [21]. The halogen functions at
C(4) of the pyridine were first substituted by (4-aminophenyl)ethyne moieties
(Scheme 1) according to a modification of Heck×s coupling reaction [22] reported by
Sonogashira et al. [23]. (4-Aminophenyl)ethyne itself was synthesized via a literature
Scheme 1
i) [PdCl2(PPh3)2], CuI, Et3N. ii) NaOH. iii) 1, [PdCl2(PPh3)2], CuI, Et3N/THF. iv) DPT, CH2Cl2.

1918
Helvetica Chimica Acta Vol. 85 (2002)
procedure [24] in a somewhat better yield (54 instead of 43%), similar to the yield
reported by Mongin and Gossauer (57%) [25]. Our choice of this method over the
more efficient one proposed by Lavastre et al. (71%) [26] was dictated by the milder
reaction conditions and lower cost of the starting materials. The NH2 functions were
subsequently transformed into SCN groups with di(pyridin-2-yl)thiocarbonate (DPT),
a reagent far less toxic than thiophosgene. The reactivities of the two halogen
substituents analogues were compared and, surprisingly, it appeared that the
brominated synthon LF reacts less readily than chlorinated LE, the yields being 28
and 39%, respectively. This may be due to poor solubility of LF under the conditions
used. The yield obtained with the chlorinated derivative is in the range of those
reported for similar coupling reactions [27]. The IR spectrum of LG presents an intense
CˆO stretching band at 1636 cmÀ1 arising from the amide group, a value close to that
obtained for the CˆO groups of LB. These observations point to the environment of the
amide group being probably not much modified by the 4-substitution of the pyridine
unit. A broad band at 2039 cmÀ1 is assigned to the antisymmetric stretching of the SCN
groups.
The solution structure of LG in CDCl3 was investigated by 1H- (Fig. 1 and Table 1)
1
1
and 13C-NMR spectroscopy, including H-1H COSY and H-13C (HSQC) experiments
that allowed us to assign all observed signals. The spectra are typical of a species with
C2v symmetry (14peaks for the 50 protons of the molecule and 25 peaks for the 57 C-
atoms two pairs of signals overlap accidentally. To assess the arrangement of the
terdentate coordinating units, ROESY (Rotating frame Overhauser Enhancement
SpectroscopY) experiments were carried out on a solution of LG in CDCl3. Along with
Fig. 1. 1H-NMR Spectrum of [La2(LG)3]6‡ (CDCl3, 400 MHz, 295 K)

Helvetica Chimica Acta Vol. 85 (2002)
1919
Table 1. 1H-NMR Chemical Shifts (d, with respect to TMS) for LG (CDCl3) and Chemical-Shift Differences
Dd ˆ dcompl À dlig [ppm] for [Ln2(LG )3]6‡ (Ln ˆ Eu, La, (CD3CN), at 295 K and 400 MHz)
HÀC(3)
HÀC(4')
HÀC(5)
HÀC(5''),
HÀC(7'')
HÀC(7')
HÀC(4''),
HÀC(8'')
HÀC(6')
LG
d/ppm
mult.
8.51
d, 1 H
À 3.25
s, 1 H
7.70
s, 1 H
5.19
7.59
d, 1 H
À 2.09
s, 1 H
7.53
d, 2 H
À 0.05
7.37
d, 1 H
0.51
d, 1 H
8.7
7.247.22
s, 2 H
0.09
s, 1 H
À 0.74
d, 1 H
8.5
Eu
Dd/ppma)
mult.
s, 1 H
dd, 2 H
2.1, 6.6
À 0.040.06
dd, 2 H
1.9, 6.48.4
dd, 2 H
2.1, 6.8
À 0.01
J/Hz
Dd/ppm
mult.
La
À 0.30
d, 1 H
0.8
À 1.69
s, 1 H
0.02
d, 1 H
0.8
‡ 0.02
d, 1 H
dd, 2 H
d, 1 H
J/Hz
1.9, 6.4 7.8
CH2(b)
HÀC(10')
HÀC(7)
HÀC(9)
HÀC(11')
HÀC(8)
HÀC(10)
LG
d/ppm
mult.
Dd/ppm
mult.
4.30
s, 1 H
0.28
4.76
q, 2 H
À 1.06
m, 2 H
3.62
q, 2 H
0.45
3.38
q, 2 H
À 0.41
m, 2 H
1.45
t, 3 H
2.11
t, 3 H
5.8
1.29
t, 3 H
À 0.58
t, 3 H
7.1
1.10
t, 3 H
À 0.71
t, 3 H
7.0
Eu
s, 1 H
m, 2 H
J/Hz
La
Dd/ppm
mult.
J/Hz
À 0.38
s, 1 H
À 0.02
m, 2 H
À 0.11
m, 2 H
À 0.39
m,2 H
0.06
t, 3 H
7.0
À 0.28
t, 3 H
7.1
À 0.18
t, 3 H
6.8
a) Dd ˆ dcomplex À dligand
.
the results of a COSY investigation, this experiment clearly points to a transoid
conformation of the terminal pyridines with respect to the benzimidazole moieties.
NOE Effects are found between the bridging CH2(b) H-atoms and HÀC(4'), HÀC(6')
as well as between HÀC(10') and HÀC(7'). No such effect is detected between
HÀC(10') and the aromatic H-atoms of the pyridine unit. The C2v symmetry and the
transoid conformation are further supported by the molecular-mechanics optimization
performed with the program HyperChem [28] (Fig. 2).
Formation of the Dimetallic Helicates. Reaction of stoichiometric amounts of LG
and Ln(ClO4)3 ¥ x H2O (Ln ˆ La, Eu, Tb) in an MeCN/CH2Cl2 mixture gave pale yellow
solutions, from which the complexes [Ln2(LG)3](ClO4)6 ¥ n H2O (n ˆ 3 6) were
recrystallized in 65 70% yields. The IR spectra of these helicates present a number
of identifying bands including i) an intense CˆO stretching vibration at 1581 cmÀ1, red
shifted by 55 cmÀ1 with respect to LG and indicating a strong LnIII-amide interaction, ii)
intense stretching vibrations at 2088 and 2208 cmÀ1 arising from the isothiocyanate and
CꢀC groups, respectively, the vibration mode of the SCN groups being blue-shifted by
À1
4 9 cm with respect to LG, iii) a broad band at 3400 cmÀ1 arising from interstitial H2O
molecules, and iv) vibrations from ionic perchlorate at 1090 and 624cm À1
.
1
The H-NMR spectra of the LaIII and EuIII helicates (Table 1) show an A2 spin
system for the bridging CH2 group, which is typical for two C2-related homotopic
protons, while the AB spin system observed for CH2(10') arises from two diastereotopic
protons, thus indicating a time-averaged D3 symmetry in solution on the NMR time-
scale. As previously observed with LA, LB, and (LC-2 H)2À the signal of the H-atoms at
C(4') is the most shifted upon complexation because wrapping the ligands around the
metal ions brings them i) into the shielding region of the neighboring benzimidazole

1920
Helvetica Chimica Acta Vol. 85 (2002)
Fig. 2. Optimized geometry of LG obtained by molecular mechanics
ring and ii) close to the metal centers, a crucial effect for pseudo-contact shifts with
paramagnetic EuIII [14][15]. The pyridine HÀC(3) and HÀC(5) also undergo
substantial shifts as a result of i) the coordination of the N-atom to the metal ions
and ii) the conformational change from a transoid to a cisoid arrangement of the two
ligand arms, as confirmed by the NOE effect detected between HÀC(10') and
HÀC(3). A comparison with the NMR spectra of [Ln2(LB)3]6‡ and [Ln2(LE)3]6‡ leads
to the conclusion that the complexes with LB, LE, and LG adopt a similar helical
conformation, whatever the nature and bulkiness of the substituent at C(4) of the
pyridine. Moreover, it appears that the ionic-radius contraction does not modify
substantially the overall structure of the helicates. In the helicates with LB and LE, the
protons CH2(7), CH2(9), and CH2(10') are diastereotopic at room temperature
[15][18] (six signals observed), due to a blocked D3 triple helical structure. In the case
of LG, the corresponding signals overlap indicating a less different micro-environment
and slightly different wrapping of the ligands in [Ln2(LG)3]6‡ (Table 1). On the other
hand, the aromatic protons of the (4-isothiocyanatophenyl)ethynyl substituent are only
slightly influenced upon complexation (Dd ꢁ 0.07 ppm, Eu helicate with LG). It is
noteworthy that a more pronounced effect was observed with triple-helical mono-
nuclear complexes with 4-[(4-aminophenylenyl)ethynyl]pyridine-2,6-dicarboxylic acid,
[LnL3]3‡ [29].
To find out whether other species form during the self-assembly process, we have
titrated LG with Eu(ClO4)3 ¥ x H2O and monitored the resulting solutions by 1H-NMR
for ratios R ˆ [EuIII]tot/[LG]tot between 0.25 and 1. For R ꢂ 0.5, the spectra show the
presence of the free ligand and of the 2 :3 complex, but no other species could be
detected. When R reaches 0.67, the signals are identical to those observed for the
isolated complex [Eu2(LG)3]6‡. This experiment, therefore, unambiguously indicates
the formation of the thermodynamically stable 2 :3 helicate by strict self-assembly,

Helvetica Chimica Acta Vol. 85 (2002)
1921
under stoichiometric conditions and high ligand concentration. When R > 0.67, other
signals appear that may tentatively be assigned to a 2 :2 species [15].
Photophysical Properties in Solution: Ligand-Centered Transitions. The absorption
spectrum of LG in MeCN at room temperature presents two broad bands in the range
25 45000 cmÀ1, which were assigned to p ! p* transitions (Fig. 3 and Table 2). The
band centered at 31570 cmÀ1 is comprised of four components and is typical of an
extended aromatic system. Comparison with the spectra of similar and model
molecules (L1 and L2; Scheme 2) leads to the following assignment. The components
at 28310 and 30400 cmÀ1 correspond to transitions mainly centered onto the pyridine
and benzimidazole units, while the component at 31570 cmÀ1 appears to be typical of
the (4-isothiocyanatophenyl)ethynyl group. With respect to LE, the envelope of the first
absorption band is slightly shifted towards higher energies. On the other hand, in frozen
MeCN solution, the energy of the singlet state of LG is red-shifted by ca. 4000 cmÀ1
compared to LE, while the 0-phonon component of the triplet state is less sensitive to
the substitution at C(4) of the pyridine, its energy being 20090 cmÀ1, compared to 21200
and 21280 cmÀ1 for LE and LB, respectively. It has been suggested that an efficient
intersystem crossing occurs, when the energy difference between the singlet and triplet
states amounts to ca. 5000 cmÀ1 [30]. This is an important factor in the sensitization of
lanthanide ions, since the ligand-to-metal energy-transfer process will be more efficient
3
when the triplet-state population is larger. For LE, DE(1pp* À pp*) ꢁ 4700 cmÀ1 [18]
(taken as the energy difference between the maximum of the fluorescence band and the
0-phonon component of the triplet state), while this value is smaller for both LB (ca.
3000 cmÀ1) [15] and LG (ca. 2090 cmÀ1), resulting in a quite weak triplet-state emission,
which is seen only at low temperature. Other differences in the photophysical
properties of LE and LG lie in their fluorescence quantum yields and in the lifetimes of
Fig. 3. Absorption spectra of ligands LE, LG, L1 and L2 (7 8 ¥ 10À7 m in MeCN at 295 K).

1922
Helvetica Chimica Acta Vol. 85 (2002)
Table 2. Ligand-Centered Absorption and Emission Properties of the Ligand LG and [Ln2(LG )3]6‡ (Ln ˆ La,
Eu, Tb). Data for LB and LE are taken from [15] and [18], respectively.
Compound
E(p ! p*)/cmÀ1a
32200
)
E(1pp*)/cmÀ1b
)
E(3pp*)/cmÀ1b
)
t(3pp*)/msb)
Æ 0.2
LB
24330c)
19940 (sh)c)
20040b)
4
.2
41320 (sh)
21280 (sh)c)
16810 (sh)
18000 (sh)
18960
LE
29420 (sh)
30840
40160 (sh)
25880
22170
670 Æ 20
171 Æ 3
21200
LG
28310 (sh)
30400
17400 (sh)
18470
31570
20090
32750 (sh)
40140 (sh)
28420
34120 (sh)
40120 (sh)
28340
34140 (sh)
43460 (sh)
28390
34210 (sh)
43460 (sh)
[La2(LG )3]6‡
[Eu2(LG )3]6‡
[Tb2(LG )3]6‡
22420
20620
21050
18380
18900
19800
537 Æ 30
d
d
)
)
16250
16950
17940
570 Æ 50
L1
L2
30260
31730
33740 (sh)
40000 (sh)
30270
23550
23240
18420c)
19600c)
20780c)
560 Æ 10c)
19100c)
20190c)
21280c)
159 Æ 4c)
40170 (sh)
a) Electronic spectral data at 295 K in MeCN; energies are given for the maximum of the band envelope (sh:
d
shoulder). b) Luminescence data in MeCN at 77 K. c) Solid-state luminescence data at 77 K. ) 3pp*
Luminescence quenched by transfer to the Eu ion. Values in italics: maximum of the absorption bands.
their triplet states. In MeCN, the absolute fluorescence quantum yield of LG amounts to
24%, compared to 92% for LE [18], and at 77 K the t(3pp*) lifetime is concomitantly
smaller (171 ms) than for LE (670 ms) [18]. A first conclusion is that nonradiative
deactivation pathways are more effective in LG compared to LE probably because of the
more-extended p-system of the former.
Photophysical Properties in Solution: [Ln2(LG)3]6‡ Helicates (Ln ˆ La, Eu, Tb). In
MeCN, the dimetallic helicates display three absorption bands centered around 28400,
34000, and 43500 cmÀ1 (Table 2). The 30400-cmÀ1 absorption band of uncomplexed LG
split upon complexation, one component appearing at lower energy (ca. 2000 cmÀ1,
similar to the shift observed for [Ln2(LB)3]6‡ [15]), while the other component
undergoes an hypsochromic shift of ca. 3000 cmÀ1.
Upon ligand excitation, one broad fluorescence band is seen for all the investigated
1
complexes. In MeCN at 77 K, the energy shift of the pp* state with respect to
uncomplexed LG amounts to 250, 1550, and 1120 cmÀ1 for La, Eu, and Tb, respectively.

Helvetica Chimica Acta Vol. 85 (2002)
1923
Scheme 2
i) MeCN, NaHCO3, r.t.
This shift is much smaller than those observed for the helicates prepared from LE (3150
to 3640 cmÀ1 from La to Lu), consistent with the fairly different electronic structure of
the two ligands. A bathochromic shift is observed for the 0-phonon component of the
3pp* emission band at 77 K: 290 and 2150 cmÀ1 for La and Tb, respectively. The triplet-
state emission is not seen for the Eu helicate, but, instead, the characteristic metal-
5
7
3
centered D0 ! FJ emission lines appear, pointing to an efficient pp* state-to-metal
ion energy transfer. On the other hand, TbIII luminescence is not sensitized (Fig. 4)
because the energy of the triplet state (17940 cmÀ1) is smaller than the energy of the 5D4
level (ca. 21000 cmÀ1). The lifetime of the ligand pp* states is much larger in the
3
helicates, which, besides the heavy-atom effect, points to the ligand conformation being
rigidified upon complexation.

1924
Helvetica Chimica Acta Vol. 85 (2002)
Fig. 4. Phosphorescence spectra of [Ln2(LG)3]6‡ (Ln ˆ Eu, Tb) in frozen MeCN at 77K (nÄexc ˆ 25640 (Eu) and
25455 (Tb) cmÀ1
)
The absolute quantum yield of [Eu2(LG)3]6‡ has been measured in MeCN solution.
It amounts to 1.1%, a value three times larger than the ones reported for the helicates
with LB (0.35%) [15] and LE (0.45%) [18]. The overall sensitization of the lanthanide
ion luminescence is proportional to the product of the efficiencies of intersystem
3
crossing and of the pp*-to-LnIII transfer. The former energy-transfer process is not
very efficient, mainly due to a small energy difference between the singlet and triplet
state (ca. 2090 cmÀ1 for LG and 2620 cmÀ1 for [La2(LG)3]6‡). The second process may be
1
quite complex, involving several metal-ion excited states (and the ligand pp* state)
[31] and depending on i) the relative energy of the ligand and metal-ion excited states,
and ii) the donor-acceptor distance and the orientation of the chromophore (donor)
with respect to the metal ion (acceptor) [32]. In our case, the energetic factor,
5
DE(3pp* À D0), is not favorable: as a rule of thumb, an energy difference of ca. 2000
2500 cmÀ1 is considered to be optimum, since it minimizes back-transfer processes
5
[30][33]. Substitution at C(4) brings DE(3pp* À D0) from ca. 3700 cmÀ1 for LB
(measured on the Gd helicate) to ca. 3220 cmÀ1 for LE (measured on the Gd helicate)
and to only ca. 2600 cmÀ1 (measured on the La helicate) for LG. Therefore, the larger
quantum yield of [Eu2(LG)3]6‡ arises either from better orientation of the chromo-
phoric antenna or from reduced nonradiative deactivation processes. For EuIII
compounds, photoelectron-transfer processes and ligand-to-metal charge-transfer
5
state(s) must also be taken into account. The luminescence lifetime of the D0(Eu)
level gives some clue to this problem. First, it is quite large at room temperature, e.g.,
2.02 Æ 0.01 ms in the solid state and 1.62 Æ 0.01 ms in MeCN; therefore, reflecting the
absence of high-energy oscillators (e.g., OH) in the inner coordination sphere of the
metal ion. The latter value is similar to those found for the helicates with LB (1.67 ms
[15]) and LE (1.60 ms [18]), illustrating that, in solution, the EuIII coordination sphere is
comparable for the three helicates, the helical wrapping of the ligand strands providing
good protection of the metal ions against solvent interaction. The temperature

Helvetica Chimica Acta Vol. 85 (2002)
1925
dependence of the lifetime is also similar for the three helicates: for anhydrous solid-
state samples of [Eu2(LB)3]6‡ and [Eu2(LE)3]6‡, t(5D0) decreases from 2 ms (77 K) to
1.6 ms (295 K) [15], and from 2.2 (77 K) to 1.5 ms (295 K), respectively [18], while a
decrease from 1.8 (77 K) to 1.6 ms (295 K) is observed for [Eu2(LG)3]6‡ in MeCN.
Therefore, temperature-dependent deactivation processes are similar for the three
5
helical molecular edifices. This is surprising in view of the small DE(3pp À D0) value
for the LG helicate. The larger quantum yield observed for [Eu2(LG)3]6‡ has, therefore,
to be explained by a more favorable positioning of the chromophore.
Photophysical Properties of [Eu2(LG)3]6‡ in the Solid State. We have taken
advantage of the EuIII luminescence to probe the environment of the metal ion, despite
the relative broadness of the emission bands. Upon excitation through the ligand 1pp*
7
state at 295 K, the emission spectrum displays the characteristic Eu(5D0 ! Fj)
transitions. The crystal-field splitting of the spectrum can be interpreted in terms of a
coordination polyhedron derived from D3 symmetry. The extremely weak and broad
7
5D0 ! F0 transition is consistent with the fact that it is symmetry forbidden in the D3
5
7
symmetry point group. The D0 ! F1 transition displays three components (Table 3),
5
7
5
7
corresponding to D0(A1) ! F1(A2) and to a D0(A1) ! F1(split E-sublevel), which
are both allowed in D3 symmetry; the splitting amounts to 43 cmÀ1, reflecting a
relatively large distortion from D3 symmetry. For comparison, splittings in the range
20 30 cmÀ1 and 60 75 cmÀ1 were observed for the helicates with LB and LE,
5
7
respectively. The D0 ! F2 transition is the most intense (1.9 times the intensity of
5
7
the D0 ! F1 transition) and displays five bands; two doublets are assigned to the
allowed transitions A1 ! E in D3 symmetry (splittings of 27 and 29 cmÀ1). Analysis of
7
the 5D0 ! F4 transition shows six components, two appearing as closely spaced
doublets (three transitions to E sublevels and one to an A2 sublevel are allowed in D3):
the less intense singlet at 14146 cmÀ1 is assigned to the A1 ! A2 transition, and the
broad one at 14401 cmÀ1 to one A1 ! E transition. The two doublets are assigned to two
A1 ! E transitions (splitting of 41 and 38 cmÀ1). In conclusion, the solid-state structure
of the Eu helicate is similar to those determined for the complexes with LB and LE, and
comparable to the solution structure unravelled by the NMR investigation.
Table 3. Electronic Sublevels ([cmÀ1], origin: 5D0) of the Eu(7Fj) Manifold (J ˆ 1 4 ) in [Eu2(LG )3]6‡ as
Determined from Emission Spectra in the Solid State at 295 K
5D0
7F1
7F2
7F3
7F4
17222
319
380
423
981
1833
2690
2731
2821
2987
3025
3076
1008
1050
1076
1107
Conclusions and Perspectives.
The introduction of (4-isothiocyanatophenyl)-
ethynyl groups at C(4) of the pyridine rings of ligand LB is easily achieved, and the
resulting functionalized ditopic and hexadentate ligand LG is obtained in reasonable
overall yield, 38%, as calculated from LE and 1, itself obtained in 38% yield [21]. Under

1926
Helvetica Chimica Acta Vol. 85 (2002)
stoichiometric conditions, LG self-assembles with lanthanide perchlorates in MeCN to
yield dimetallic triple-stranded helicates [Ln2(LG)3]6‡ in 65 71% yield. NMR Spectra
show the structures of these molecular edifices in comparison to those of the analogous
helicates with LA [14], LB [15], and LE [18]. We, thus, deduce that the bulky substituents
at C(4) of pyridine do not prevent helical wrapping of the ligand strands around the
metal ions. The electron-withdrawing substituents provide a moderate antenna effect
for EuIII, with a threefold increase in the quantum yield of the metal-centered
luminescence as compared with helicates with LB and LE. On the other hand, the (4-
isothiocyanatophenyl)ethynyl substituents induce a bathochromic shift of the ligand
triplet state that lies at an energy too low to sensitize the luminescence of TbIII.
In conclusion, we have demonstrated that the ligands LA G represent a versatile
class of ditopic receptors, amenable to easy substitution while retaining their
coordination properties for the self-assembly of triple-stranded lanthanide-containing
helicates. The choice of introducing two equivalent binding points in LG has mainly
been dictated by a simpler synthetic chore and because a symmetrical ligand generates
simple NMR spectra. In view of the results obtained, work is in progress to improve the
sensitization properties of these receptors, to replace the diethylamide function by
carboxylic groups, as well as to introduce a single isothiocyanate group and test the
coupling reactions of the helicates with biological material.
Experimental Part
General. MeCN, CH2Cl2, DMF, and Et3N were distilled from CaH2; SOCl2 was distilled from elemental S.
Silica gel (Merck 60, 0.04 0.06 mm) was used for prep. column chromatography (CC). Other products were
purchased from Fluka AG (Buchs, Switzerland) and used without further purification, unless otherwise stated.
Spectroscopic and Analytical Measurements. Electronic spectra in the UV/VIS range were recorded at 208
with a Perkin-Elmer Lambda 7 spectrometer in 1.0- and 0.1-cm quartz cells. IR Spectra were obtained from KBr
pellets with a Mattson a-Centauri FT-IR spectrometer. ES-MS spectra of the complexes were measured on a
Finnigan SSQ-710C spectrometer on 10À4 m solns. in MeCN; the cap. temp. was set to 2008 and the acceleration
potential to 4.5 kV. 1H- and 13C-NMR spectra were recorded at 258 on Bruker AM-360 or Bruker AVANCE 400-
DRX spectrometers. Chemical shifts are reported in ppm with respect to TMS; J values are given in Hz. Solns.
1
for the titration of LG by Eu(ClO4)3 ¥ x H2O, monitored by H-NMR were prepared as follows. An amount of
20 mg of LG (20.6 mmol) were dissolved in 3 ml of anh. CH2Cl2 and 21.1 mg of Eu(ClO4)3 ¥ x H2O were dissolved
in 10 ml of anh. MeCN. The EuIII concentration (3.84¥ 10 À3 m) was determined by complexometric titration with
EDTA in the presence of urotropine and xylene orange [34]. Solns. with EuIII/LG ratios of 1.00, 0.66, 0.50, and
0.25 were prepared, stirred at r.t. for 2 h and evaporated to dryness. The solid residues were dissolved in 10 ml of
dry CD3CN in a glove-box under N2.
The exper. procedures for high-resolution (HR), laser-excited luminescence studies have been published
previously [15]. Emission spectra are corrected for the instrumental functions, but HR excitation spectra are
not. Quantum yields of the ligand-centered emission were measured relative to quinine sulfate in dil. acidic
solution (abs quantum yield: 0.546) [35]. Quantum yields of the metal-centered emission were determined as
described in [36] at excitation wavelengths where i) the Lambert-Beer law is obeyed and ii) the absorption of the
reference [Eu(terpy)3]3‡ (abs. quantum yield 0.013) closely matches that of the sample. Ligand-excitation and
emission spectra were recorded on a Perkin-Elmer LS-50B spectrometer equipped for low-temp. (77 K)
measurements. Elemental analyses were performed by Dr. H. Eder (Microchemical Laboratory, University of
Geneva).
The molecular-mechanics optimization has been carried out first by means of a MM ‡ method (RMS
gradient ˆ 0.01) followed by a semi-empirical method based on the AM1 Hamiltonian (RMS gradient ˆ 0.1).
Syntheses. 4-Ethynylbenzenamine (1). 2-methylbut-3-yn-2-ol (4.61 g, 55 mmol), CuI (0.17 g, 0.91 mmol),
and [PdCl2(PPh3)2] (0.65 g, 0.91 mmol) were introduced in a mixture of freshly distilled Et3N/THF 1:1 (85 ml).
The soln. was degassed with N2 during 15 min at r.t. Then, 4-iodoaniline (10 g, 46 mmol) was added under N2,

Helvetica Chimica Acta Vol. 85 (2002)
1927
and the mixture was heated at 408 during 3 h. The mixture was filtered, and the filtrate was evaporated under
vacuum. The residue was dissolved in Et2O (100 ml), washed with H2O (3 Â 50 ml) and dried (Na2SO4) during
2 h. After solvent evaporation, the brown solid obtained was dried under vacuum (10À2 Torr) during 1 h. Toluene
was added (30 ml) along with 6 NaOH pellets. The mixture was refluxed during 3 h. After completion of the
reaction (TLC; AcOEt/hexane 1:1, Rf (4-ethynylbenzenamine) 0.31), the mixture was filtered, and toluene was
evaporated. The resulting black oil was triturated with hexane, and the deep purple solid obtained was
crystallized from Et2O/hexane 1:3 (25 ml) to give a pale yellow solid (2.9 g, 54%). IR (KBr): 1513, 1592
1
(CCaryl), 1618 (ArNH2), 2230 (CꢀCH), 3388, 3487 (ArNH2). H-NMR (CDCl3): 2.91 (s, CH); 3.77 (s, NH2);
6.55 (d, 3J ˆ 9.48, 2 H); 7.22 (d, 3J ˆ 9.48, 2 H). 13C-NMR (CDCl3): 79.9; 85.2; 112.6; 116.1; 134.9; 146.4. ESI-MS
‡
(MeOH): 118.1 ([M ‡ H] ). Anal. calc. for C8H7N (117.15): C 82.02, H 6.02, N 11.96; found: C 82.32, H 6.24,
N 12.19.
Bis(2-{6-(diethylcarbamoyl)-4-[(4-aminophenyl)ethynyl]pyridin-2-yl}-1-ethylbenzimidazol-5-yl)methane
(2). The synthon LE was synthesized as described in [18][21]; 370 mg of LE (0.45 mmol), CuI (7 mg,
0.036 mmol), and [PdCl2(PPh3)2] (13 mg, 0.018 mmol) were introduced in a 1:1 mixture of freshly distilled
Et3N/THF (15 ml). The soln. was degassed during 15 min, and 1 (160 mg, 1.36 mmol, 3 eqs) was added under N2,
and the mixture was further refluxed during 4h. After solvent evaporation, CH 2Cl2 (50 ml) was added, and the
soln. was washed with H2O (3 Â 50 ml). The org. phase was dried (Na2SO4) during 1 h, filtered, evaporated, and
dried under vacuum (508, 10À2 Torr). The brown solid obtained was purified by CC (Al2O3; CH2Cl2/MeOH 99 :1
to 94:6) to yield 155 mg (39%) of 2. Brown pale solid. IR (KBr): 1543, 1589 (Ar), 1618 (ArNH2), 1636 (CˆO),
2184(C ꢀC), 2871, 2930 (CH2), 2970 (Me), 3487 (ArNH2). 1H-NMR (CDCl3): 1.12 (t, 3J ˆ 7.15, Me(11')); 1.32
(t, 3J ˆ 7.12, 3 H, Me(8) or Me(10)); 1.40 (s, 2 NH2); 1.45 (t, 3J ˆ 7.12, 3 H, Me(8) or Me(10)); 3.48 (q, 3J ˆ 7.22,
1HÀC(7) or 1 HÀC(9)); 3.66 (q, 3J ˆ 6.99, 1 HÀC(7) or 1 HÀC(9)); 4.33 (s, 1 H, CH2(b)); 4.84 (q, 3J ˆ 6.82,
CH2(10')); 7.21 (s, HÀC(6')); 7.26 (s, HÀC(4''), HÀC(8'')); 7.39 (br. d, HÀC(7')); 7.61 (br. d, HÀC(5''),
HÀC(7'')); 7.63 (br. d, HÀC(5)); 7.72 (s, HÀC(4')); 8.53 (br. d, HÀC(3)). 13C-NMR (CDCl3): 15.1 (C(8) or
C(10)); 15.6 (C(8) or C(10)); 17.1 (C(11')); 40.9 (C(9) or C(7)); 41.8 (C(10')); 43.2 (CH2(b)); 44.9 (C(9) or
C(7)); 112.6 (C(7')); 121.9 (C(4')); 124.8 (C(5)); 126.2 (C(4''), C(8'')); 127.3 (C(6')); 128.4(C(3)); 134.8 (C(5 '',
C(7'')); 88.9, 95.1 (CꢀC); 114.2 (2 C); 132.8 (2 C); 135.9, 137.4, 138.6, 144.2, 151.5 (quat. C); 167.6 (CˆO). ESI-
‡
MS (MeOH): 888.1 ([M ‡ H] ).
Bis(2-{6-(diethylcarbamoyl)-4-[(4-isothiocyanatophenyl)ethynyl]pyridin-2-yl}-1-ethylbenzimidazol-5-yl)-
methane (LG). Compound 2 (100 mg, 0.11 mmol) was dissolved in anh. CH2Cl2, di(pyridin-2-yl) thiocarbonate
(76.64mg, 0.33 mmol, 3 equiv.) was added, the soln. was stirred at r.t. overnight and refluxed during 3 h. Solvent
evaporation yielded a brown solid, which was recrystallized from a mixture of AcOEt/hexane 40 :60 to give
104.8 mg (98%) of LG. Yellow-brown powder. IR (KBr):31542, 1589 (Ar), 1636 (CˆO), 2036 (NˆCˆS), 2184
(CꢀC), 2871, 2930 (CH2), 2970 (Me). 1H-NMR (CDCl3): 1.10 (t, 3J ˆ 7.16, Me(11')); 1.29 (t, 3J ˆ 7.14, 3 H,
Me(8) or Me(10)); 1.45 (t, 3J ˆ 7.14, 3 H, Me(8) or Me(10)); 3.38 (q, 3J ˆ 7.21, 1 HÀC(7) or 1 HÀC(9)); 3.62
(q, 3J ˆ 6.84, 2 H, 1 HÀC(7) or 1 HÀC(9)); 4.30 (s, 1 H, CH2(b)); 4.76 (q, 3J ˆ 6.72, CH2(10')); 7.22
(s, HÀC(6')); 7.24( s, HÀC(4''), HÀC(8'')); 7.37 (d, 3J ˆ 8.44, HÀC(7')); 7.53 (d, 3J ˆ 8.44, HÀC(5''),
HÀC(7'')); 7.59 (d, 4J ˆ 1.48, HÀC(5)); 7.70 (s, HÀC(4')); 8.51 (d, 4J ˆ 1.48, HÀC(3)). 13C-NMR (CDCl3):
15.0 (C(8) or C(10)); 15.5 (C(8) or C(10)); 16.1 (C(11')); 40.3 (C(9) or C(7)); 41.3 (C(10')); 42.9 (CH2(b)); 43.5
(C(9) or C(7)); 110.8 (C(7')); 120.8 (C(4')); 124.4 (C(5)); 125.9 (C(4''), C(8'')); 126.7 (C(6')); 127.4(C(3)); 133.9
(C(5''), C(7'')); 88.6, 94.8 (CꢀC); 113.1 (2 C); 132.9 (2 C); 135.6, 137.3, 138.0, 143.7, 150.5 (quat. C); 168.6
‡
(CˆO); 155.4(N ˆCˆS). ESI-MS (MeOH): 486.5 ([M ‡ 2 H]2‡), 971.4([ M ‡ H] ). Anal. calc. for
C57H50N10O2S2 (971.22): C 70.49, H 5.19, N 14.42; found: C 70.68, H 4.98, N 14.65.
Dimethyl 4-[(4-Isothiocyanatophenyl)ethynyl)]pyridine-2,6-dicarboxylate (L1). Dimethyl 4-[(4-aminophen-
yl)ethynyl]pyridine-2,6-dicarboxylate (0.25 g, 0.81 mmol), synthesized according to a known procedure [29],
was dissolved in dry CH2Cl2 (100 ml) and di(pyridin-2-yl) thiocarbonate (0.19 g, 0.81 mmol) was added, and the
soln. was stirred overnight at r.t. Solvent evaporation yielded an orange oil, which was purified by CC (CH2Cl2
100%): 0.27 g (94%) of L1. TLC (CH2Cl2/MeOH 95 :5): Rf 0.71. IR (KBr): 2084(br.), 1719 (br.), 1507 (br.).
1H-NMR: 8.35 (s, HÀC(3), HÀC(5)); 7.55 (d, J ˆ 9.8, HÀC(3'), HÀC(5')); 7.24( d, J ˆ 9.8, HÀC(2'),
‡
HÀC(6')); 4.03 (s, 2 Me). ESI-MS (MeOH): 352.6 (M ).
Dimethyl 4-[(4-{N'-[5-(Methoxycarbonyl)pentyl]thioureido}phenyl)ethynyl]pyridine-2,6-dicarboxylate
(L2). Compound L1 (50 mg, 16 mmol) and methyl 6-aminocaproate (29 mg, 0.16 mmol; as its hydrochloride)
were dissolved in 20 ml of MeCN; 13.5 mg (0.16 mmol) NaHCO3 were added, and the mixture was stirred 12 h
at r.t. before being evaporated and redissolved in CH2Cl2. The resultant soln. was washed with 2 Â 50 ml of H2O,
dried (Na2SO4), and filtered. The yellow solid was dried (508/10À2 Torr) to yield 68 mg (0.12 mmol, 85%) of L2.
‡
ESI-MS (MeOH): 498.1 ([M ‡ H] ). 1H-NMR (CDCl3): 1.38, 1.63, 2.32 (m, 2 H, 4H, 2 H, aliph. CH 2); 3.65

1928
Helvetica Chimica Acta Vol. 85 (2002)
(s, MeOCOÀC(8'')); 4.03 (s, 2 MeOCO(py)); 6.31 (t, 3J ˆ 5.14, HÀN(3'')); 7.27 (d, 3J ˆ 8.44, HÀC(2'),
HÀC(6')); 7.59 (d, 3J ˆ 8.8, HÀC(3'), HÀC(5')); 7.95 (br. s, HÀN(1'')); 8.34( s, HÀC(3), HÀC(5)).
13C-NMR (CDCl3): 25.00, 26.95, 29.14, 34.40 (aliph. CH2); 46.00 (1 MeOCO(py)); 52.23 (MeOCOÀC(8''));
54.05 (1 MeOCO(py)); 86.86, 96.38 (CꢀC); 124.72 (C(2'), C(6')); 130.27 (C(3), C(5)); 134.81 (C(3'), C(5'));
119.99, 138.51, 149.15 (quat. C); 165.35 (CˆO); 181.06 (CˆS).
Preparation of the Complexes. The perchlorate salts Ln(ClO4)3 ¥ x H2O (Ln ˆ La, Eu, Tb, x ˆ 2 8) were
√
prepared from the corresponding oxides (Rhone-Poulenc, 99.99%) according to the method in [37]. Caution:
perchlorate salts combined with org. ligands are potentially explosive and should be handled in small quantities
and with the necessary precautions [38].
[Ln2(LG)3][ClO4]6 ¥ n H2O (Ln ˆ La, Eu, Tb). A 23-mmol amount of Ln(ClO4)3 ¥ n H2O in dry MeCN
(1.5 ml) was slowly added to a soln. of LG in dry CH2Cl2 (1 ml). After stirring at r.t. for 4h, the soln. was
evaporated, the solid residue redissolved in MeCN (1 ml), the soln. was filtered, and Et2O diffused into the soln.
for 3 d. The resulting pale brown microcrystalline aggregates were collected by filtration and dried to give the
complexes [Ln2(LG)3](ClO4)6 ¥ n H2O (Ln ˆ La, Eu, Tb) in 65 71% yields. The complexes for NMR
measurements were prepared by the same procedure but not recrystallized. Elemental analyses are consistent
with the above formulation and n ˆ 4(La), 6 (Eu) and 3 (Tb). Anal. calc. for C 171H160Cl6La2N30O35S6 (3878.26):
C 52.96, H 4.16, N 10.83; found: C 53.23, H 4.01, N 11.18. Anal. calc. for C171H162Cl6Eu2N30O36S6 (3922.39):
C 52.36, H 4.16, N 10.71; found: C 52.09, H 4.31, N 11.01. Anal. calc. for C171H154Cl6N30O32S6Tb2 (3864.25):
C 53.15, H 4.02, N 10.87; found: C 52.98, H 4.32, N 10.65.
This work is supported through grants from the Swiss National Science Foundation and the Swiss Office for
¡
¬
Education and Science (COSTaction D18). We thank Dr. Fabiana GonÁalves and Veronique Foiret for their help
in recording the luminescence data.
REFERENCES
[1] V. W. W. Yam, K. K. W. Lo, Coord. Chem. Rev. 1999, 184, 157.
[2] P. Caravan, J. J. Ellison, T. J. McMurry, R. B. Lauffer, Chem. Rev. 1999, 99, 2293.
[3] J. L. Sessler, W. C. Dow, D. Oconnor, A. Harriman, G. Hemmi, T. D. Mody, R. A. Miller, F. Qing, S. Springs,
K. Woodbur, S. W. Young, J. Alloys Compd. 1997, 249, 146.
[4] Z. J. Guo, P. J. Sadler, Angew. Chem., Int. Ed. 1999, 38, 1513.
[5] I. Hemmil‰, T. Stahlberg, P. Mottram in −Bioanalytical Applications of Labelling Technologies×, Wallac Oy,
Turku, 1995.
[6] D. Parker, Coord. Chem. Rev. 2000, 205, 109.
¬
[7] J.-C. G. B¸nzli, N. Andre, M. Elhabiri, G. Muller, C. Piguet, J. Alloys Compd. 2000, 303/304, 66.
[8] C. Piguet, J.-C. G. B¸nzli, Chem. Soc. Rev. 1999, 28, 34 7.
[9] C. Piguet, C. Edder, S. Rigault, G. Bernardinelli, J.-C. G. B¸nzli, G. Hopfgartner, J. Chem. Soc., Dalton
Trans. 2000, 3999.
[10] C. Edder, C. Piguet, J.-C. G. B¸nzli, G. Hopfgartner, Chem. Eur. J. 2001, 7, 3014.
[11] S. Petoud, J.-C. G. B¸nzli, F. Renaud, C. Piguet, K. J. Schenk, G. Hopfgartner, Inorg. Chem. 1997, 36, 5750.
[12] G. Muller, J.-C. B¸nzli, K. J. Schenk, C. Piguet, G. Hopfgartner, Inorg. Chem. 2001, 40, 2642.
[13] G. Bernardinelli, C. Piguet, A. F. Williams, Angew. Chem., Int. Ed. 1992, 31, 1622.
[14] C. Piguet, J.-C. G. B¸nzli, G. Bernardinelli, G. Hopfgartner, A. F. Williams, J. Am. Chem. Soc. 1993, 115,
8197.
[15] N. Martin, J.-C. G. B¸nzli, V. McKee, C. Piguet, G. Hopfgartner, Inorg. Chem. 1998, 37, 577.
[16] M. Elhabiri, R. Scopelliti, J.-C. G. B¸nzli, C. Piguet, J. Am. Chem. Soc. 1999, 121, 10747.
[17] C. C. Wu, M. W. Brechbiel, R. W. Kosak, O. A. Gansow, Bioorg. Med. Chem. Lett. 1994, 4, 449.
[18] C. Platas, M. Elhabiri, M. Hollenstein, J.-C. G. B¸nzli, C. Piguet, J. Chem. Soc., Dalton Trans. 2000, 2031.
[19] J. Hovinen, H. Hakala, Org. Lett. 2001, 3, 2473.
[20] K. K. W. Lo, D. C. M. Ng, W. K. Hui, K.-K. Cheung, J. Chem. Soc., Dalton Trans. 2001, 2634.
[21] A.-S. Chauvin, R. Tripier, J.-C. G. B¸nzli, Tetrahedron Lett. 2001, 42, 3089.
[22] H. A. Dieck, F. R. Heck, J. Organomet. Chem. 1975, 93, 259.
[23] K. Sonogashira, Y. Tohda, N. Hagihara, Tetrahedron Lett. 1975, 50, 4467.
[24] H. Takalo, J. Kankare, E. H‰nninen, Acta Chem. Scand. 1988, B42, 448.
[25] O. Mongin, A. Gossauer, Tetrahedron 1997, 53, 6835.
[26] O. Lavastre, L. Olivier, P. H. Dixneuf, S. Sibandhit, Tetrahedron 1996, 52, 5495.

Helvetica Chimica Acta Vol. 85 (2002)
1929
[27] J. J. Bozell, C. E. Vogt, J. Am. Chem. Soc. 1988, 110, 2655.
[28] HyperChem, release 5.0, HeperCube Inc., Waterloo, Ontario, Canada, 1996.
¬
[29] C. Platas, C. Piguet, N. Andre, J.-C. G. B¸nzli, J. Chem. Soc., Dalton Trans. 2001, 3084.
[30] F. J. Steemers, W. Verboom, D. N. Reinhoudt, E. B. Vandertol, J. W. Verhoeven, J. Am. Chem. Soc. 1995,
117, 9408.
[31] F. R. GonÁalves e Silva, R. L. Longo, O. L. Malta, C. Piguet, J.-C. G. B¸nzli, PhysChemChemPhys 2000, 2,
5400.
[32] G. Zucchi, R. Scopelliti, J.-C. G. B¸nzli, J. Chem. Soc., Dalton Trans. 2001, 1975.
[33] M. Latva, H. Takalo, V. M. Mukkala, C. Matachescu, J.-C. Rodriguez-Ubis, J. Kankare, J. Lumin. 1997, 75,
149.
[34] G. Schwarzenbach in −Complexometric Titrations×, Chapman & Hall, London, 1957.
[35] S. R. Meech, D. C. Phillips, J. Photochem. 1983, 23, 193.
[36] S. Petoud, J.-C. G. B¸nzli, T. Glanzman, C. Piguet, Q. Xiang, R. P. Thummel, J. Lumin. 1999, 82, 69.
[37] J. F. Desreux in −Lanthanide Probes in Life, Chemical and Earth Sciences. Theory and Practice×, Eds. J.-
C. G. B¸nzli, G. R. Choppin, Elsevier Science Publ. B.V., Amsterdam, 1989 Chapt. 2, p. 43.
[38] W. C. Wolsey, J. Chem. Educ. 1973, 50, A335.
Received January 24, 2002