Noncovalent binding of sensitizers for lanthanide(III) luminescence in an EDTA-bis(β-cyclodextrin) ligand

Article abstract of DOI:10.1021/ja017025y

EDTA-linked β-cyclodextrin dimer 3 was synthesized from EDTA bis(anhydride) 1 and mono-(propylamino)-appended β-cyclodextrin 2. p-tert-Butylbenzoate 5, bound by the β-cyclodextrin cavities of 3 with an association constant of 10⁴ M^-1 in water, acts as a sensitizer for the Eu^III and Tb^III complexes of 3. Luminescence spectroscopy, microcalorimetry, and Gd^III-induced NMR relaxation rate measurements prove that 3 forms a 1:2 complex with 5 and that one of the β-cyclodextrin-bound sensitizers coordinates to the EDTA-encapsulated Ln^III ion. The Eu^III complex of 3 forms strong 1:1 complexes (K ≈ 10⁷ M^-1) with bis(propylamido adamantyl)-functionalized biphenyl sensitizers 7 and 8 in water. Both β-cyclodextrins of 3 are involved in the binding of these guests. The amide functionality adjacent to the biphenyl unit in 7 and 8 coordinates to the EDTA-encapsulated Ln^III ion. For these biphenyl-based antennae both binding to β-cyclodextrin and coordination to the Ln^III center are crucial for efficient sensitization.

Full text of DOI:10.1021/ja017025y

Published on Web 02/07/2002
Noncovalent Binding of Sensitizers for Lanthanide(III)
Luminescence in an EDTA-bis(â-cyclodextrin) Ligand
Jasper J. Michels, Jurriaan Huskens,* and David N. Reinhoudt*
Contribution from the Laboratory of Supramolecular Chemistry and Technology, MESA+
Research Institute, UniVersity of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
Received September 7, 2001. Revised Manuscript Received December 5, 2001
Abstract: EDTA-linked â-cyclodextrin dimer 3 was synthesized from EDTA bis(anhydride) 1 and mono-
(propylamino)-appended â-cyclodextrin 2. p-tert-Butylbenzoate 5, bound by the â-cyclodextrin cavities of
3 with an association constant of 104 M-1 in water, acts as a sensitizer for the EuIII and TbIII complexes of
3. Luminescence spectroscopy, microcalorimetry, and GdIII-induced NMR relaxation rate measurements
prove that 3 forms a 1:2 complex with 5 and that one of the â-cyclodextrin-bound sensitizers coordinates
to the EDTA-encapsulated LnIII ion. The EuIII complex of 3 forms strong 1:1 complexes (K ≈ 107 M-1) with
bis(propylamido adamantyl)-functionalized biphenyl sensitizers 7 and 8 in water. Both â-cyclodextrins of 3
are involved in the binding of these guests. The amide functionality adjacent to the biphenyl unit in 7 and
8 coordinates to the EDTA-encapsulated LnIII ion. For these biphenyl-based antennae both binding to
â-cyclodextrin and coordination to the LnIII center are crucial for efficient sensitization.
strongly,7b but is less commonly used in combination with a
covalently linked chromophore.
Introduction
Most trivalent lanthanide (LnIII) ions have long-lived lumi-
nescent 4f states which exhibit sharp emission bands at
characteristic wavelengths ranging from green (TbIII) to near-
infrared light (NdIII, ErIII). This renders LnIII ions interesting
for application in fluoroimmunoassays1-4 and optical amplifica-
tion.1,5 Because of the forbidden nature of the optical intra-4f
transitions, LnIII ions are usually excited via organic antenna
chromophores (sensitizers). This energy transfer process usually
involves an electron exchange (Dexter) mechanism which
requires a physical overlap of the antenna orbitals and the LnIII
4f shells.6 Hence, the effectiveness is strongly distance depend-
ent and limits practical LnIII-antenna distances to <5 Å. A
common strategy is to link the chromophore covalently to a
multidentate LnIII-complexing ligand such as diethylenetri-
aminepentaacetate (DTPA),7 tetraazacyclododecanetetraacetate
(DOTA),8 or terpyridyl9 and terphenyl10 derivatives. Ethylene-
diaminetetraacetate (EDTA) is well known to complex LnIII ions
â-Cyclodextrin derivatives covalently functionalized with a
crown ether,11,12 DOTA,13 or DTPA14 moiety to complex EuIII
or TbIII have been exploited as luminescent sensing molecules
for the detection of aromatic guests. The noncovalent capture
of a guest sensitizer in the â-cyclodextrin cavity enables
sensitized lanthanide luminescence and, thus, detection of the
guest. The guest binding of the crown ether-based systems is
decreased relative to â-cyclodextrin because of an increased
polarity of the â-cyclodextrin cavity due to the charge of the
crown-complexed LnIII ion. The DOTA and DTPA ligands
effectively shield the LnIII ion from water because they occupy
(almost) all coordination sites. This reduces the quenching of
the lanthanide luminescence by the OH vibrations of the water
molecules, but also prohibits coordination of a sensitizer to the
LnIII center. This leads to a relatively inefficient energy
transfer.13,15
Ideally, one would like a lanthanide ion to be well shielded
but still with a few coordination sites free for coordination of
a sensitizer. The smaller EDTA, in comparison to DTPA and
DOTA, seems suitable for this purpose. In general, lanthanides
prefer hard donor atoms such as oxygen; therefore, carboxylates
* To whom correspondence should be addressed. Phone: +31 53
4892980. Fax: +31 53 4894645. E-mail: smct@ct.utwente.nl.
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Latva, M. HelV. Chim. Acta 1996, 79, 789.
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2056 VOL. 124, NO. 9, 2002 J. AM. CHEM. SOC.
10.1021/ja017025y CCC: $22.00 © 2002 American Chemical Society

Noncovalent Binding of Luminescence Sensitizers
A R T I C L E S
which the solution was diluted with CH2Cl2 and washed with 0.1 M
HCl and brine. The organic layer was dried over Na2SO4. The product
was purified over silica gel using EtOAc/EtOH/H2O ) 6/2/1 as the
eluent (Rf ) 0.5) and used as such in the deprotection step. The
TBDMS-protected precursor of 3 (262 mg, 0.062 mmol) was dissolved
in THF at room temperature, and 1.25 mL (1.23 mmol) of a 1 M
solution of tetrabutylammonium fluoride (TBAF) in THF was added.
The reaction mixture was refluxed (66 °C) overnight. The solvent was
evaporated in vacuo, and the residue was diluted with water. The
aqueous layer was washed twice with diethyl ether. The excess of TBAF
was removed by eluting the product four times over an Amberlite MB3
mixed H+/OH- ion exchange column. EDTA-based â-cyclodextrin
dimer 3 was obtained in 127 mg yield (48% overall) as a white solid.
1H NMR (D2O): δ 5.08 (d, 2H, J ) 3.3 Hz, H1 (CD) spacer attached),
4.94 (d, 12H, J ) 3.6 Hz, H1 (CD) free glucose ring), 3.90 (t, 2H, J
) 9.3 Hz, H3 (CD) spacer attached), 3.84 (t, 12H, J ) 9.3 Hz, H3
(CD) free glucose ring), 3.75-3.60 (m, 46H), 3.60-3.33 (m, 36H),
3.26 (t, 4H, J ) 6.0 Hz, NCH2CH2CH3), 3.18 (s, 4H, NCH2CH2N),
1.72 (t, 4H, J ) 6.3 Hz, OCH2CH2). 13C NMR (D2O, ref CH3OD): δ
172.0, 168.5, 101.7, 99.9, 81.4, 81.0, 80.1, 72.9, 72.3, 71.9, 71.6,
69.8, 60.1, 56.3, 51.2, 36.4, 28.3. MALDI-TOF-MS Calcd for
seem appropriate as the coordinating site on the sensitizer.
However, complexation of a carboxylate requires the removal
of a water molecule of the first coordination sphere and is
intrinsically weak, especially in the presence of another shielding
ligand such as EDTA. Therefore, we designed a ligand with
cyclodextrin binding sites to complex the organic sensitizer and
bring it in close proximity to the LnIII ion so that the increase
in effective molarity may lead to efficient coordination to a
vacant coordination site.
Here we report the synthesis and guest binding of an EDTA-
based â-cyclodextrin dimer and its EuIII and TbIII complexes.
The EDTA ligand provides six donor atoms and does not
saturate the coordination sites on the LnIII ion. Consequently,
sensitizing guests having both hydrophobic binding sites for
â-cyclodextrin and a LnIII-coordinating functionality are of
particular interest. Complexation of small and large sensitizers
was studied. Luminescence spectroscopy, microcalorimetry, and
GdIII-induced 13C relaxation rate enhancement measurements
were performed to study the complexes and to probe possible
coordination to the LnIII center. Such a ditopic mode of binding16
is especially the subject of investigation as this may lead to the
development of cyclodextrin systems with both strong binding
and efficient energy transfer. Furthermore, covalent and non-
covalent combinations of lanthanide complexes and cyclodex-
trins are of interest for magnetic resonance imaging (MRI)
applications as they combine a good thermodynamic stability
of LnIII complexation with a high molecular weight and thus
high relaxivity.17
C
100H166O76N4: m/z ) 2639.0. Found: m/z ) 2662.9 ([M + Na]+).
Anal. Calcd for C100H166O76N4‚5H2O: C, 43.99; H, 6.50; N, 2.05.
Found: C, 43.96; H, 6.20; N, 2.05.
Bis(1-propylammonium) Salt of EDTA-bis(propylamide) (4).
EDTA-bis(N-propylamide) bis(propylammonium) salt 4 was synthe-
sized analogous to a literature procedure22 by dropwise addition of a
solution of EDTA bisanhydride 1 in CH2Cl2 to a CH2Cl2 solution
containing a large excess of n-propylamine and subsequent evaporation
in vacuo. The product was obtained as a colorless oil. 1H NMR (CD3-
OD): δ 3.20-3.14 (m, 12H, OdCCH2 + NCH2CH2CH3), 2.85 (t, 4H,
J ) 7.5 Hz, NCH2CH2CH3), 2.72 (s, 4H, NCH2CH2N), 1.67 (sx, 4H,
J ) 7.6 Hz, CH2CH3), 1.54 (sx, 4H, J ) 7.4 Hz, CH2CH3), 1.00 (t,
6H, J ) 7.5 Hz, CH3), 0.92 (t, 6H, J ) 7.4 Hz, CH3). 13C NMR (CD3-
OD): δ 178.7, 174.2, 60.9, 60.2, 54.5, 42.4, 42.0, 23.7, 22.3, 11.9,
11.2. FAB-MS Calcd for C16H30N4O6: m/z ) 374.2. Found: m/z )
375.2 ([M + H]+), 373.1 ([M - H]-).
Experimental Section
General. Chemicals were obtained from commercial sources and
used as such. â-Cyclodextrin was dried in vacuo at 80 °C in the presence
of P2O5 for at least 5 h before use. Solvents were dried using standard
laboratory procedures. NMR spectra were recorded using a Varian Inova
300 NMR spectrometer. 1H NMR chemical shifts (300 MHz) are given
relative to residual CHCl3 (7.25 ppm), CHD2OD (3.30 ppm), DMSO-
Biphenyl-4,4′-dicarboxylic acid-bis(1-adamantanemethylamide)
(7). Biphenyl-4,4′-dicarboxylic acid (47 mg, 0.20 mmol) was stirred
overnight in an excess of SOCl2 at 80 °C. The excess of thionyl chloride
was removed by evaporation in vacuo, and the product was redissolved
in CH2Cl2. A solution of 1-adamantanemethylamine 11 (67 mg, 0.41
mmol) and Et3N (57 µL, 0.57 mmol) in CH2Cl2 was added dropwise
at room temperature. The solution was stirred for 30 min at room
temperature. Subsequently, the turbid reaction mixture was filtered
through a Millipore filter (L 0.23 µm) and purified over silica gel using
CH2Cl2:MeOH ) 20:1 as the eluent (Rf ) 0.4). Product 7 was obtained
h,d5 (2.50 ppm), or HDO (4.65 ppm) unless mentioned otherwise. 13
C
chemical shifts (75 MHz) are given relative to CDCl3 (77.0 ppm), CD3-
OD (49.0 ppm), or DMSO-d6 (39.5 ppm) unless mentioned otherwise.
Mass spectra were recorded with a Finnigan MAT 90 spectrometer
using m-nitrobenzyl alcohol (NBA) as the matrix. Elemental analyses
were carried out with a model 1106 Carlo-Erbu Strumentazione
elemental analyzer. Molecular modeling was performed using
HYPERCHEM.
EDTA-Based â-Cyclodextrin Dimer (3). A solution of EDTA
bisanhydride 1 (2.6 mg, 0.10 mmol) in CH2Cl2 was added dropwise to
a solution of TBDMS-protected mono(2-O-aminopropyl)-â-cyclodextrin
221 (410 mg, 0.21 mmol) and Et3N (29 µL, 0.021 mmol) in CH2Cl2 at
room temperature. The reaction mixture was stirred for 30 min, after
1
in 54 mg yield (50%) as a white solid. H NMR (DMSO-d6): δ 8.36
(t, 2H, J ) 6.5 Hz, NHCdO), 7.96 (d, 4H, J ) 8.7 Hz, ArH), 7.81 (d,
4H, J ) 8.4 Hz, ArH), 3.01 (d, 4H, J ) 6.3 Hz, NCH2), 1.93 (s, 6H,
adamantyl CHCH2), 1.63 (br q, 12H, J ) 11.6 Hz, adamantyl CHCH2),
1.51 (d, 12H, J ) 2.1 Hz, adamantyl CHCH2). 13C NMR (DMSO-d6):
δ 166.3, 141.6, 134.2, 128.1, 126.7, 50.6, 36.6, 34.4, 27.8. FAB-MS
Calcd for C36H44N2O2: m/z ) 536.4. Found: m/z ) 537.4 ([M + H]+).
Anal. Calcd for C36H44N2O2‚0.1MeOH: C, 80.30; H, 8.29; N, 5.19.
Found: C, 80.02; H, 8.19; N, 5.49.
(16) (a) Willner, I.; Goren, Z. J. Chem. Soc., Chem. Commun. 1983, 1469. (b)
Tabushi, I.; Kuroda, Y. J. Am. Chem. Soc. 1984, 106, 4580.
(17) (a) Aime, S.; Botta, M.; Frullano, L.; Geninatti Crich, S.; Giovenzana, G.
B.; Pagliarin, R.; Palmisano, G.; Sisti, M. Chem.-Eur. J. 1999, 5, 1253.
(b) Fatin-Rouge, N.; To´th, EÄ .; Perret, D.; Backer, R. H.; Merbach, A. E.;
Bu¨nzli, J.-C. G. J. Am. Chem. Soc. 2000, 122, 10810. (c) Skinner, P. J.;
Beeby, A.; Dickins, R. S.; Parker, D.; Aime, S.; Botta, M. J. Chem. Soc.,
Perkin Trans. 2 2000, 1329. (d) Zitha-Bovens, E.; Van Bekkum, H.; Peters,
J. A.; Geraldes, C. F. G. C. Eur. J. Inorg. Chem. 1999, 287.
Biphenyl-4,4′-dicarboxylic acid-bis(1-adamantylcarboxylic acid
3-amidopropyl-amide) (8). Biphenyl-4,4′-dicarboxylic acid (50 mg,
0.21 mmol) and amine 12 (98 mg, 0.41 mmol; see below) were reacted
analogous to the preparation of 7, after which the mixture was washed
with 0.1 M HCl and brine. The organic layer was dried over Na2SO4.
The product was purified over silica gel using CH2Cl2:MeOH ) 9:1
(18) Canet, D.; Levy, G. C.; Peat, I. R. J. Magn. Reson. 1975, 18, 199.
(19) Although a luminescence increase was observed in H2O upon addition of
biphenyl guests 7 and 8, titrations of 7-10 were carried out in D2O. The
higher signal-to-noise ratio obtained in D2O allowed more accurate
determination of association constants at the low experimental host
concentrations.
(20) No difference in the value for τobs was obtained when the LnIII ion was
excited directly or via the sensitizer (λex ) 280 nm). This confirms that
energy transfer is fast (microsecond region) as compared to the lanthanide
luminescence decay (millisecond region).
(21) Nelissen, H. F. M.; Schut, A. F. J.; Venema, F.; Feiters, M. C.; Nolte, R.
J. M. Chem. Commun. 2000, 577.
(22) Coates, J.; Sammes, P. G.; West, R. M. J. Chem. Soc., Perkin Trans. 2
1996, 1283.
9
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A R T I C L E S
Michels et al.
as the eluent (Rf ) 0.6). Product 8 was obtained in 86 mg yield (60%)
as a white solid. 1H NMR (CDCl3): δ 7.96 (d, 4H, J ) 8.7 Hz, ArH),
7.69 (d, 4H, J ) 8.4 Hz, ArH), 7.51 (t, 2H, J ) 6.3 Hz, NHCdO),
6.21 (t, 2H, J ) 6.5 Hz, NHCdO), 3.46 (q, 4H, J ) 6.0 Hz, NCH2),
3.38 (q, 4H, J ) 6.1 Hz, NCH2), 2.05 (s, 6H, adamantyl CHCH2),
1.88 (d, 12H, J ) 2.4 Hz, adamantyl CHCH2), 1.77-1.67 (m, 16H,
NCH2CH2 + adamantyl CHCH2). 13C NMR (CDCl3): δ 179.2, 167.4,
142.9, 133.7, 127.7, 127.1, 40.7, 39.2, 36.4, 35.9, 35.5, 29.9, 28.1.
FAB-MS Calcd for C42H54N4O4: m/z ) 678.4. Found: m/z ) 679.1
([M + H]+). Anal. Calcd for C42H54N4O4‚0.46CHCl3: C, 69.50; H,
7.48; N, 7.63. Found: C, 69.21; H, 7.31; N, 7.61.
(D2O, ref CH3OD): δ 40.7, 38.6, 37.0, 35.9, 27.8, 26.9. FAB-MS Calcd
for C14H24N2O: m/z ) 236.2. Found: m/z ) 237.3 ([M + H]+). Anal.
Calcd for C16H25N2O3F3: C, 54.84; H, 7.21; N, 8.00. Found: C, 54.79;
H, 7.13; N, 8.31.
EuIII and TbIII Complexes of 3. All measurements were carried
out at pH 7 in doubly distilled water (Q2) at 25 °C, unless mentioned
otherwise. The lanthanide(III) complexes of EDTA-based â-cyclodex-
trin dimer 3 were prepared by adding a solution of EuCl3 or TbCl3 in
water to a solution of 3, adjusting the pH to 7 with NaOH. A slight
excess of 3 relative to LnIII (1.03:1) was used, ensuring quantitative
complexation of the lanthanide ion.
13C NMR Relaxation Time Measurements. GdIII-induced 13C
relaxation rate enhancement measurements were performed on a Varian
Unity WB NMR spectrometer at 100.6 MHz using the inversion
recovery pulse sequence. The longitudinal 13C relaxation times were
calculated using a nonlinear least squares three parameter curve fitting
routine.18 To a solution containing a constant high concentration of
benzoate 5 (0.5 M), an increasing amount of 3‚GdIII was added
determining the longitudinal 13C NMR relaxation times (T1)i of 5 after
each addition. Values for FL ()[3‚GdIII]/[5]) of 0, 5.0 × 10-5, 1.0 ×
10-4, 1.9 × 10-4, and 5.0 × 10-4 were applied.
Biphenyl-4,4′-dicarboxylic acid-bis(1-adamantanemethylsulfon-
amide) (9). A solution of biphenyl-4,4′-disulfonyl chloride (125 mg,
0.36) in acetonitrile was added dropwise to a solution of 1-adaman-
tanemethylamine 11 (124 mg, 0.76 mmol) and Et3N (106 µL, 0.76
mmol) in acetonitrile at room temperature. Upon the addition a white
precipitate formed, which was collected by filtration. The white solid
was washed twice with acetonitrile and subsequently dried in vacuo.
The product 9 was obtained in 183 mg yield (83%). 1H NMR (DMSO-
d6): δ 7.95 (d, 4H, J ) 9.0 Hz, ArH), 7.89 (d, 4H, J ) 8.4 Hz, ArH),
7.57 (br, 2H, SNH), 2.41 (br s, 4H, NCH2), 1.91 (br s, 6H, adamantyl
CHCH2), 1.61 (q, 12H, J ) 13.0 Hz, adamantyl CHCH2), 1.42 (s, 12H,
adamantyl CHCH2). 13C NMR (DMSO-d6): δ 142.1, 140.7, 127.9,
127.3, 54.4, 39.6, 36.5, 33.0, 27.7. FAB-MS Calcd for C34H44N2O4S2:
m/z ) 608.3. Found: m/z ) 609.3 ([M + H]+). Anal. Calcd for
C34H44N2O4S2: C, 67.07; H, 7.28; N, 4.60; S 10.53. Found: C, 66.95;
H, 7.30; N, 4.81; S, 10.39.
Photophysical Studies. Luminescence titrations were carried out
using an Edinburgh Analytical Instruments FS900 fluorescence spec-
trophotometer. Absorption spectra were recorded on a Hewlett-Packard
8452A diode array spectrophotometer. For all excitation wavelengths
used the optical density remained below 0.1, as determined by UV-
vis spectroscopy. For the titrations of p-tert-butylbenzoate 5 to the EuIII
and TbIII complexes of EDTA-based â-cyclodextrin dimer 3, aliquots
of a 11.5 mM solution of 5 in H2O were added to 3.0 mL of the 0.1
mM 3‚LnIII complex in H2O in a quartz cuvette, recording the
luminescence spectrum between 550 and 725 nm or 450 and 650 nm,
for 3‚EuIII or 3‚TbIII, respectively (λex ) 280 nm). For the titrations of
biphenyl compounds 7-10 to the EuIII complex of 3, 0.45 mM stock
solutions of 7-10 in DMSO were prepared. Aliquots of this solution
were added to a quartz cuvette containing 2.75 mL of a 6.0 µM solution
of the 3‚EuIII complex in D2O,19 recording the EuIII luminescence
spectrum between 550 and 725 nm (λex ) 285 nm). During the titration,
the amount of DMSO present in the solution remained below 9%, which
is assumed not to affect the guest binding strength of â-cyclodextrin.
Luminescence lifetime measurements of the 3‚EuIII and 3‚TbIII com-
plexes were performed in duplo using an Edinburgh Analytical
Instruments LP900 fluorescence spectrophotometer monitoring the
luminescence decay upon direct excitation of the LnIII ion20 (λex ) 393
and 484 nm for 3‚EuIII and 3‚TbIII, respectively). The luminescence
emission was monitored at 617 nm for 3‚EuIII and 545 nm for 3‚TbIII.
Biphenyl-4,4′-dicarboxylic acid-bis(1-adamantylcarboxylic acid
3-amidopropyl-sulfonamide) (10). A mixture of free amine 12 (138
mg, 0.58 mmol; see below) and Et3N (81 mL, 0.58 mmol) in acetonitrile
was added dropwise to a solution of biphenyl-4,4′-disulfonyl chloride
(98 mg, 0.28) in acetonitrile at room temperature. The reaction mixture
was stirred for 30 min, after which the solvent was evaporated, and
the remaining white solid was recrystallized twice from MeOH. Product
1
10 was obtained in 147 mg yield (70%) as a white solid. H NMR
(DMSO-d6): δ 7.96 (d, 4H, J ) 8.4 Hz, ArH), 7.88 (d, 4H, J ) 8.7
Hz, ArH), 7.60 (br, 2H, SNH), 7.30 (t, 2H, J ) 5.7 Hz, NHCdO),
3.01 (q, 4H, J ) 5.7 Hz, NCH2), 2.71 (t, 4H, J ) 7.1 Hz, NCH2), 1.87
(s, 6H, adamantyl CHCH2), 1.64 (d, 12H, J ) 3.0 Hz, adamantyl
CHCH2), 1.59-1.47 (m, 16H, NCH2CH2 + adamantyl CHCH2). 13C
NMR (DMSO-d6): δ 177.1, 142.1, 140.0, 127.8, 127.4, 40.4, 39.8,
38.7, 36.1, 35.9, 29.2, 27.6. FAB-MS Calcd for C40H54N4O6S2: m/z )
750.4. Found: m/z
) 751.4 ([M +
H]+). Anal. Calcd for
C40H54N4O6S2: C, 63.97; H, 7.25; N, 7.46; S, 8.54. Found: C, 63.89;
H, 7.34; N, 7.48; S, 8.61.
1-Adamantylcarboxylic acid 3-aminopropylamide (12). A solution
of 1-adamantanecarboxylic acid (1.18 g, 6.55 mmol) and a large excess
of (COCl)2 in CH2Cl2 was stirred for 4 h at 40 °C. The mixture was
evaporated to dryness in vacuo, and CH2Cl2 was added to the remaining
white solid. A mixture of N-Boc-1,3-diaminopropane (1.15 g, 6.60
mmol) and Et3N (1.0 mL, 7.21 mmol) in CH2Cl2 was added dropwise
at room temperature. The solution was stirred for 30 min at room
temperature. Subsequently, the reaction mixture was washed with 0.01
M HCl and brine. The organic layer was dried over MgSO4. The product
was purified over silica gel using CH2Cl2:MeOH ) 25:1 as the eluent.
Trifluoroacetic acid (1.2 mL, 15.5 mmol) was added to a solution of
the N-Boc-protected precursor of 12 (524 mg, 1.55 mmol) in CH2Cl2.
The reaction mixture was stirred overnight at room temperature. The
solvent was evaporated in vacuo leaving a colorless oil. The residue
was repeatedly dissolved in CH2Cl2/toluene 2:1 (v/v) and evaporated
to dryness in vacuo until precipitation of colorless crystals was observed.
The TFA salt of 12 was obtained as a white solid in 511 mg yield
(89%). 1H NMR (D2O): δ 3.17 (t, 2H, J ) 6.6 Hz, CH2NCdO), 2.85
(t, 2H, J ) 7.7 Hz, CH2NH2), 1.90 (s, 3H, adamantyl CHCH2), 1.74
(qn, 2H, J ) 7.2 Hz, NCH2CH2), 1.69 (d, 6H, J ) 2.1 Hz, adamantyl
CHCH2), 1.59 (br q, 6H, J ) 12.1 Hz, adamantyl CHCH2). 13C NMR
Calorimetric Titrations. Calorimetric measurements were carried
out using a Microcal VP-ITC microcalorimeter with a cell volume of
1.4115 mL. For studying the complexation of p-tert-butylbenzoate 5
to the free EDTA-based â-cyclodextrin dimer 3 and the 3‚EuIII complex,
aliquots of a 20.0 mM solution of 5 in the buret were added to a 0.97
mM solution of 3 or 3‚EuIII in the calorimetric cell, monitoring the
heat change after each addition. For the accurate determination of
binding constants, 5 was also titrated to 3‚EuIII under more dilute
conditions: 2.0 mM of 5 (titrant) to 0.10 mM of 3‚EuIII (cell). For
studying the complexation of adamantyl derivative 12 to â-cyclodextrin,
aliquots of a 5.4 mM solution of â-cyclodextrin were added to a 0.53
mM solution of the TFA salt of 12. Dilution experiments showed that
at the experimental concentrations none of the species described above
showed significant aggregation behavior in water nor (de)protonation.
Results and Discussion
Synthesis. EDTA-based â-cyclodextrin dimer 3 was synthe-
sized in two steps starting from the commercially available
EDTA bisanhydride 1 and â-cyclodextrin derivative 2 (Scheme
1). The latter was synthesized according to a literature proce-
9
2058 J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002

Noncovalent Binding of Luminescence Sensitizers
A R T I C L E S
Scheme 1
group. This may allow coordination to the LnIII center of the
3‚LnIII complex and thus lead to efficient energy transfer for
sensitized luminescence. p-tert-Butylbenzyl alcohol 6, that
complexes as strong to â-cyclodextrin as 5 but lacks a LnIII-
coordinating site, was used as a reference. The triplet state
energy of 6 was assumed to be comparable to that of 5 and
therefore suitable as a sensitizer for both EuIII and TbIII
luminescence.
Chart 1
Because of the presence of two cyclodextrin moieties in 3,
we also envisaged using both cavities for the binding of an
antenna molecule to reach close proximity between the LnIII
ion and the sensitizer. Chart 1 shows four biphenyl-based
sensitizers (7-10), equipped with two adamantyl units as
binding sites for â-cyclodextrin. Sensitizers with different spacer
lengths between the biphenyl moiety and the adamantyl groups
were prepared. Biphenyl derivatives have triplet state energies23
typically in the range 21 000-23 000 cm-1 and are therefore
suitable antennae for the sensitization of EuIII luminescence.
dure.21 Two equivalents of 2 were reacted with EDTA bisan-
hydride 1 in the presence of triethylamine in dichloromethane.
After acidic workup and purification by silica column chroma-
tography, the TBDMS-protected precursor of EDTA-based
â-cyclodextrin dimer 3 was obtained. Subsequent deprotection
using tetrabutylammonium fluoride (TBAF) in refluxing THF
and removal of the excess of TBAF by ion exchange chroma-
tography yielded the water-soluble host 3.
EDTA-bis(N-propylamide) 4 (Scheme 1) was used as a
reference compound and synthesized analogous to a literature
procedure22 by reaction of EDTA bisanhydride 1 with 1-ami-
nopropane in dichloromethane. The product was obtained as
the bis(propylammonium) salt after removal of the solvent.
The triplet state energy of p-tert-butylbenzoate 5 (Chart 1)
was assumed comparable to that of 4-methylbenzoic acid (ET
) 26 880 cm-1),23 which is high enough to sensitize EuIII and
TbIII luminescence.24 The energies of the receiving 5D0 and 5D1
states of EuIII are 17 500 and 19 000 cm-1, respectively. The
receiving 5D4 state of TbIII is at 20 500 cm-1. Antenna molecule
5 has both a guest site (the p-tert-butylphenyl moiety) for
binding to â-cyclodextrin25 and a LnIII-coordinating carboxylate
Conversion of 4,4′-biphenyldicarboxylic acid into the corre-
sponding diacid chloride and subsequent reaction with an amine-
appended adamantane (11 or 12) yielded the bis(adamantyl)-
biphenyl derivatives 7 and 8, respectively. N-Propylamino-
appended adamantane 12 was prepared by coupling N-Boc-1,3-
diaminopropane to 1-adamantylcarbonyl chloride and subsequent
removal of the Boc protecting group with trifluoroacetic acid
(TFA). Sulfonamides 9 and 10 were obtained by reaction of 11
or 12 with 4,4′-biphenyldisulfonyl chloride.
Binding of p-tert-Butylbenzoate. When EDTA-based â-cy-
clodextrin dimer 3 was added to a solution of EuCl3 in H2O at
pH 7, the EuIII luminescence emission spectrum showed
significant changes upon direct excitation of the EuIII ion at 393
nm. First, the overall EuIII luminescence intensity increased upon
addition of 3, owing to displacement of water molecules from
the first coordination sphere of the EuIII ion upon complexation
by the EDTA unit. Second, the relative intensity of the emission
band centered around 615 nm, corresponding to the hypersensi-
5
7
tive (electric dipole) D0 f F2 transition, increased as this
transition is favored upon desymmetrization of the first coor-
dination sphere by complexation to the EDTA unit.26
(23) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,
2nd ed.; Marcel Dekker: New York, 1993.
(24) The triplet energy of the sensitizer should be at least 2000 cm-1 higher
than the receiving luminescent state(s) of the LnIII ion for irreversible energy
transfer to occur. See: Sato, S.; Wada, M. Bull. Chem. Soc. Jpn. 1970, 43,
1955.
(25) Hofler, Th.; Wenz, G. J. Inclusion Phenom. 1996, 25, 81.
9
J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002 2059

A R T I C L E S
Michels et al.
Figure 1. Increase in luminescence intensity of 3‚EuIII (left) and 3‚TbIII (right) in H2O upon addition of p-tert-butylbenzoate 5 (λex ) 280 nm). The
intensities are given relative to the intensity of 3‚LnIII in the absence of sensitizer at 617 and 545 nm for 3‚EuIII and 3‚TbIII, respectively. Inset: luminescence
intensities, monitored at 617 and 545 nm, plotted as a function of the [5]/[3‚LnIII] ratio; the lines represent the fit to a 1:1 binding model.
Figure 2. Luminescence emission spectra of a 1.0 × 10-4 M solution of 3‚EuIII (left, dashed line) and 4‚EuIII (right, dashed line) in H2O and after the
addition of 0.86 equiv of p-tert-butylbenzyl alcohol 6 (left, solid line) or p-tert-butylbenzoate 5 (right, solid line). Excitation of 5 and 6 occurred at 250 and
265 nm, respectively.
The luminescence intensities of the 3‚EuIII or 3‚TbIII com-
plexes in H2O strongly increased upon addition of p-tert-
butylbenzoate 5 and excitation at 280 nm (Figure 1). Maximum
amplifications of factors 110 and 95 were achieved for 3‚EuIII
and 3‚TbIII, respectively. When sensitizer 5 was excited at 250
nm, the luminescence intensity could even be raised by a factor
of 350 for 3‚EuIII and 310 for 3‚TbIII. This is 8 times more than
the amplification obtained for a previously reported â-cyclo-
dextrin-DTPA-TbIII couple using biphenyl as the sensitizer.15
The excitation spectra of 3‚EuIII (λem ) 617 nm) and 3‚TbIII
(λem ) 545 nm) in the presence of 5 showed the same spectral
features (shape and position of the bands) as the absorption
spectrum of 5. These observations show that 5 acts as a sensitizer
for EuIII and TbIII luminescence.
The insets of Figure 1 show the data points obtained from
the titrations of 3‚EuIII (left; monitored at 617 nm) and 3‚TbIII
(right; monitored at 545 nm) with 5 and the fits to a 1:1 binding
model. An apparent 1:1 association constant of 2.9 × 104 M-1
was obtained for both the 3‚EuIII‚5 and the 3‚TbIII‚5 complexes.
p-tert-Butylbenzyl alcohol 6, which has a comparable binding
affinity to that of â-cyclodextrin but lacks a LnIII-coordinating
functionality, shows much less effective sensitization of the
luminescence of 3‚EuIII (Figure 2, left). The luminescence
intensity of 3‚EuIII was increased only by a factor 2 upon
addition of 0.86 equiv of 6. The luminescence intensity of the
europium(III) complex of EDTA-bis(propylamide) 4, which
lacks the â-cyclodextrin cavities, was enhanced by a factor of
3.3 upon addition of 0.86 equiv of sensitizer 5 (Figure 2, right).
Depite the fact that EDTA-based â-cyclodextrin dimer 3 has
two binding sites for p-tert-butylbenzoate 5,25 the complex
stoichiometry inferred from the luminescence titrations is
apparently 1:1. Complexation of p-tert-butylbenzoate 5 by 3
and 3‚EuIII was also studied by microcalorimetry. Figure 3
depicts the (exothermic) heat profiles obtained from the
calorimetric titrations of the LnIII free â-cyclodextrin dimer 3
(left) and europium(III) complex 3‚EuIII (right) with p-tert-butyl
benzoate 5 in water. In contrast to the luminescence experiments,
the inflection points of the binding isotherms indicate a 1:2 (host:
guest) stoichiometry.27
Table 1 shows the thermodynamic parameters for the com-
plexation of p-tert-butylbenzoate 5 to 3 and to 3‚EuIII. The
stability constants for the first complexation step of p-tert-
butylbenzoate to 3 and 3‚EuIII (K1 ) 1.3 × 104 and 5.0 × 104
M-1, respectively) are comparable to those of the strongest
complexes of previously reported systems in which a LnIII-
complexing unit is covalently functionalized with â-cyclodex-
trin.13,14 The binding strength of p-tert-butylbenzoate 5 by
EDTA-based â-cyclodextrin dimer 3 is comparable to the
complexation by native â-cyclodextrin.25 The stability of the
3‚EuIII‚5 complex is enhanced by the presence of the EuIII ion,
which indicates coordination of 5 to the LnIII center.
The complexation of the p-tert-butylbenzyl part of 5 by the
â-cyclodextrin cavities of 3 or 3‚EuIII is largely exothermic and
results in exothermic overall heat changes and binding enthal-
pies. The binding enthalpy for 3‚EuIII is less exothermic than
(27) For the titration of 5 to â-cyclodextrin dimer 3, the data points were fitted
accurately to a 1:2 model assuming independent binding sites (K1 ) 4K2
and ∆H1 ) ∆H2). The data obtained for the complexation of 5 to 3‚EuIII
gave an accurate fit only to a model assuming nonequivalent binding sites.
(26) (a) Kirby, A. F.; Foster, D.; Richardson, F. S. Chem. Phys. Lett. 1983, 95,
507. (b) Kirby, A. F.; Richardson, F. S. J. Phys. Chem. 1983, 87, 2544.
9
2060 J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002

Noncovalent Binding of Luminescence Sensitizers
A R T I C L E S
Figure 3. Heat evolved per injection plotted against the [5]/[3] ratio for the calorimetric titrations of p-tert-butylbenzoate 5 to 3 (left) and 3‚EuIII (right) in
water.
Table 1. Thermodynamic Parameters of the Complexation of
p-tert-Butylbenzoate 5 to 3 and 3‚EuIII, As Determined by
Microcalorimetry
Table 2. Luminescence Lifetimes in H2O and D2O and the
Number of Water Molecules in the First Coordination Sphere, n
(Eq 1), of the LnIII Complexes of 3, in the Absence and Presence
of 5 (λem ) 617 and 545 nm, λex ) 393 and 484 nm for EuIII and
TbIII, Respectively)
stoichiometry
(host:guest)
K
(M
∆G°
(kcal/mol)
∆H°
(kcal/mol)
T∆S°
(kcal/mol)
-1
host
)
III
Ln complex
τ
H O (ms)
τ
D O (ms)
na
2
2
3
1:1
1:2
1:1
1:2
1.3 × 104
3.2 × 103
5.0 × 104
1.1 × 104
-5.6
-4.8
-6.4
-5.5
-5.1
-5.1
-4.1
-4.4
0.5
-0.3
2.3
3‚EuIII
0.29
0.93
0.33
0.98
2.2
2.9
2.2
2.6
2.9
2.9
2.4
2.5
3‚TbIII
3‚EuIII
3‚EuIII‚52
3‚TbIII‚52
1.1
a n ( 0.5 or (1.5 for EuIII and TbIII, respectively.
that for 3, which supports coordination of 5 to the EuIII center
of 3‚EuIII upon binding by a â-cyclodextrin cavity, as the
coordination of carboxylates to lanthanide(III) ions is known
to be slightly endothermic.28 Also the increased binding entropy
for 3‚EuIII as compared to that of 3 supports coordination, as
LnIII-coordinated water is liberated upon binding of the car-
boxylate sensitizer. In conclusion, all thermodynamic parameters
(K, ∆H°, and T∆S°) indicate the coordination of the carboxylate
group of 5 to the LnIII center in the 3‚LnIII‚5 complex.
the number of coordination sites occupied by other ligands, an
empirical relationship has been derived (eq 1).29
n ) q(1/τH O - 1/τD O - kcorr
)
(1)
2
2
Here, q is a constant with a value that depends on the lanthanide
ion; q is 1.05 and 4.2 ms for EuIII and TbIII complexes,
respectively. τH O and τD O are the luminescence lifetimes of
2
2
The luminescence data for 3‚EuIII and 3‚TbIII (Figure 1) were
also fitted well to a 1:2 model using the values obtained for K1
and K2 from microcalorimetry. Fixing these numbers in the
fitting procedure while independently varying the luminescence
intensities of the complexes led to a luminescence increase of
only about 10% upon complexation of the second sensitizer 5
to both LnIII complexes. This explains why the luminescence
data fit equally well to a 1:1 binding model. This finding
suggests that one cyclodextrin-bound sensitizer quantitatively
coordinates to the lanthanide and that energy transfer almost
exclusively occurs upon coordination.
Ditopic binding of guests in cyclodextrin derivatives has been
shown before,16 but the novelty here lies in the fact that it is
used for efficient energy transfer from the guest to the lanthanide
center. Thus, the cyclodextrin unit is used for efficient com-
plexation, while the carboxylate ensures a close enough proxim-
ity of the guest to the LnIII ion. The microcalorimetry experi-
ments described above show that the intrinsic interaction of the
coordination of sensitizer 5 to the LnIII center is weak (in the
order of 1-10 M-1). Still, coordination of one sensitizer 5 takes
place quantitatively because the cyclodextrins of 3 increase the
effective concentration of 5 in the vicinity of the LnIII center.
To estimate the number of water molecules (n) in the first
coordination sphere of a lanthanide(III) ion, and thus indirectly
the LnIII complexes in H2O and D2O, and kcorr (0.25 and 0.05
ms-1 for EuIII and TbIII, respectively) corrects for closely
diffusing second sphere water molecules.
The luminescence lifetimes τH O and τD O (Table 2) for the
2
2
3‚EuIII and 3‚TbIII complexes were determined in the absence
and presence of an excess of sensitizer 5 using time-resolved
luminescence spectroscopy. Substitution of the lifetimes in eq
1 showed that, in the absence of sensitizer, the water coordina-
tion number is three, indicating the occupation of six of the, in
total, nine LnIII coordination sites by the EDTA unit, that is,
via the two nitrogens and all four carboxylic oxygens, similar
to that observed for LnIII complexes of native EDTA.30 Upon
complexation of 5, effectively 0.5 water molecule31 is displaced
from the first coordination sphere of the LnIII center, indicating
coordination of only one sensitizer to the LnIII center. The fact
that ∆n < 1 can be attributed to expansion of the first
coordination sphere due to electrostatic repulsion between
multiple negatively charged ligands (in this case the EDTA
moiety of 3 and carboxylate 5).30,32
(29) (a) Kropp, J. L.; Windsor, M. W. J. Chem. Phys. 1965, 42, 1599. (b)
Horrocks, W. DeW., Jr.; Sudnick, D. R. Acc. Chem. Res. 1981, 14, 384.
(c) Holz, R. C.; Chang, C. A.; Horrocks, W. DeW., Jr. Inorg. Chem. 1991,
30, 3270. (d) Beeby, A.; Clarkson, J. M.; Dickins, R. S.; Faulkner, S.;
Parker, D.; Royle, L.; de Souza, A. S.; Williams, J. A. G.; Woods, M. J.
Chem. Soc., Perkin Trans. 2 1999, 493.
(30) Huskens, J.; Kennedy, A. D.; Peters, J. A.; Van Bekkum, H. J. Am. Chem.
Soc. 1995, 117, 375.
(31) This value is not corrected for the fact that the luminescence quenching by
the amide protons of the EDTA ligand is canceled in D2O due to proton-
deuteron exchange.
(28) (a) Huskens, J.; van Bekkum, H.; Peters, J. A.; Choppin, G. R. Inorg. Chim.
Acta 1996, 245, 51. (b) Martell, A. E.; Smith, R. M. Critical Stability
Constants; Plenum Press: New York, Vol. 3, 1977; Vol. 5, 1982; Vol. 6,
1989.
9
J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002 2061

A R T I C L E S
Michels et al.
group.28a Hence, Ri is a weighted average of the enhancements
of the individual modes. Normalized values for the individual
relaxation rate enhancement contributions for the coordination
modes (Rb,i and Rm,i) can then be calculated with eqs 2 and 3
using an initial estimate for Rm,1, the rate enhancement of the
nucleus closest to the metal ion with the ligand in the
monodentate coordination mode.
Rm,i ) (rm,1/rm,i)6 × Rm,1
Rb,i ) (rm,1/rb,i)6 × Rm,1
(2)
(3)
The fractions of complexed carboxylate ligand in the mono-
and bidentate coordination mode fm and fb ()1 - fm) can be
obtained by fitting the experimental values for Ri to eq 4 using
Figure 4. Longitudinal 13C NMR relaxation rate plotted as a function of
the [3‚GdIII]/[5] ratio for all carbon atoms of 5.
Table 3. Experimental and Calculated 3‚GdIII-Induced 13C
Relaxation Rate Enhancements of Carboxylate 5, and Estimates
for the Ci-GdIII Distances for Two Different Coordination Modes,
Based on Molecular Modeling
R
m,1, fm, and the nonspecific relaxation rate enhancement34c Rinter
as independently varied fitting parameters. The fractions of 5
Ri ) fmRm,i + (1 - fm)Rb,i + Rinter (4)
R (103 s-1
)
R (103 s-1
)
r (Å)
i
r (Å)
i
i
i
Ci
experimental
calculated
bidentate
monodentate
coordinating to the GdIII ion in a monodentate and bidentate
fashion were found to be 73% and 27%, respectively, with Rm,1
) 23.6 × 103 s-1 and Rinter ) 0.6 × 103 s-1. It shows that the
majority binds in a monodentate fashion.
If diamagnetic contributions are assumed to be negligible,
the relaxation rates can be related to the distance ri between
nucleus i and the GdIII ion via38,39
1
2
3
4
5
6
7
53.12
8.72
5.00
1.58
1.32
0.81
0.74
53.12
8.72
4.98
1.71
1.19
0.81
0.75
2.83
4.23
5.11
6.47
7.09
8.62
9.38
2.50
3.76
4.63
4.53, 6.00
5.81, 7.01
6.97
8.37
8.57, 9.20, 9.23
2.50
GdIII - O
Rm,i ) C/ri6
(5)
Further evidence for coordination of the carboxylate group
of 5 to LnIII in the 3‚LnIII‚52 complex was obtained using GdIII-
induced 13C NMR relaxation rate enhancement measurements,
a technique which previously has been successfully applied to
elucidate LnIII complex geometries28a,33 and allows application
with
2
µ0
2
C )
γ2µ 2â2τc
eff
(6)
( )
5 4π
of LnIII ions in MRI.34-37 Figure 4 depicts the longitudinal 13
C
Here, µ0/4π is the magnetic permeability of a vacuum, µeff is
the effective magnetic moment of GdIII, γ is the magnetogyric
ratio, â is the Bohr magneton, and τc is the correlation time.
For small LnIII-carboxylate complexes (MW ≈ 400), C has been
NMR relaxation rates (1/T1)i of sensitizer 5 as a function of the
[3‚GdIII]/[5] ratio (FL). The GdIII-induced relaxation rate en-
hancement Ri for the carbon atoms of 5 is obtained by dividing
the slopes of the lines in Figure 4 by the number of coordinating
sensitizers.28a,34c Only one sensitizer coordinates, indicated by
the microcalorimetric and luminescence titrations.
The values for Ri of the carbon atoms of carboxylate 5 are
listed in Table 3. Relaxation rate enhancement is most pro-
nounced for the carboxylic carbon atom (C1) of 5. Ri decreases
strongly for the other carbon atoms of 5 in the order indicated
in Table 3, which proves coordination of the sensitizer to the
GdIII center. Table 3 also lists molecular modeling estimates
for the distances ri between the carbon atoms of benzoate 5
and the GdIII center in the complex, calculated for bidentate
(rb,i) and monodentate (rm,i) coordination of the carboxylate
estimated to be about 1.1 × 107 Å6 s-1 28a
.
The rotational
correlation time τR for the present case can be estimated from
the Debye-Stokes-Einstein equation (eq 7).
4πa3η
3kT
τR )
(7)
Here, a is the hydrodynamic radius of the complex, and η is
the viscosity of the solvent. Equation 7 shows that τR correlates
linearly with the volume, and thus the molecular weight, of the
complex. Since under the conditions employed here τc ≈ τR,34c
C is estimated to be 8 × 107 Å6 s-1 for the 3‚GdIII‚52 complex
(MW ≈ 3000). Therefore, Rm,1 was estimated to be 28 × 103
s-1, which corresponds well to the observed Rm,1 value and thus
to quantitative coordination of one carboxylate group to the GdIII
center.
Both luminescence and NMR data strongly point to the lack
of coordination of the second guest molecule to the LnIII center.
Most likely, this is due to both steric and electrostatic reasons.
It would require bringing both cyclodextrin cavities close to
the EDTA unit, as well as expelling an extra water molecule
(32) Na¨slund, J.; Lindqvist-Reis, P.; Persson, I.; Sandstro¨m, M. Inorg. Chem.
2000, 39, 4006.
(33) (a) Peters, J. A.; Kieboom, A. P. G. Recl. TraV. Chim. Pays-Bas 1983,
102, 381. (b) Vijverberg, C. A. M.; Peters, J. A.; Kieboom, A. P. G.; van
Bekkum, H. Tetrahedron 1986, 42, 167. (c) van Westrenen, J.; Peters, J.
A.; Kieboom, A. P. G.; van Bekkum, H. J. Chem. Soc., Dalton Trans.
1988, 2723.
(34) (a) Lauffer, R. B. Chem. ReV. 1987, 87, 901. (b) Parker, D. Chem. Soc.
ReV. 1990, 19, 271. (c) Peters, J. A.; Huskens, J.; Raber, D. J. Prog. Nucl.
Magn. Reson. Spectrosc. 1996, 28, 283.
(35) Fatin-Rouge, N.; To´th, EÄ .; Perret, D.; Backer, R. H.; Merbach, A. E.; Bu¨nzli,
J.-C. G. J. Am. Chem. Soc. 2000, 122, 10810.
(36) Andre´, J. P.; To´th, EÄ .; Fischer, H.; Seelig, A.; Ma¨cke, H. R.; Merbach, A.
E. Chem.-Eur. J. 1999, 5, 2977.
(37) Anelli, P. L.; Bertini, I.; Fragai, M.; Lattuada, L.; Luchinat, C.; Parigi, G.
Eur. J. Inorg. Chem. 2000, 625.
(38) Bloembergen, N. J. Chem. Phys. 1957, 27, 572.
(39) Solomon, I. Phys. ReV. 1955, 99, 559.
9
2062 J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002

Noncovalent Binding of Luminescence Sensitizers
A R T I C L E S
Figure 5. Increase in luminescence intensity of 3‚EuIII in D2O upon addition of adamantyl-appended biphenyl sensitizers 7 (left) and 8 (right) (λex ) 285
nm). The intensities are given relative to the intensity of 3‚EuIII at 615 nm in the absence of sensitizer. Inset: luminescence intensity at 615 nm, plotted
against the [7]/[3‚EuIII] (left) and [8]/[3‚EuIII] (right) ratios; the lines represent the fit to a 1:1 binding model.
from the first coordination sphere of the LnIII ion, which
becomes progressively more difficult. Furthermore, the first
coordination sphere becomes more negatively charged, thus
repelling the coordination of another guest molecule. As shown
above, the coordination of the first carboxylate is already weak,
and binding of the second is therefore probably too weak to be
observed, even though the complexation by the second cyclo-
dextrin cavity is efficient and increases the effective molarity
of the guest.
Using the Stern-Volmer equation (eq 8), an estimate of the
energy transfer rate can be calculated if deactivation of the
antenna triplet state via oxygen quenching is assumed to be the
only process competing with energy transfer. Ideox and Iox are
the 3‚EuIII complex in water had a strong effect on the
luminescence intensity (Figure 5, left and right, respectively)
upon excitation of 7 or 8 at 285 nm. Amides 7 and 8 act as
sensitizers for EuIII luminescence as proven by the luminescence
increase and by the fact that the absorbance spectra of 7 and 8
show the same spectral features (shape and position of the bands)
as the excitation spectra of the 3‚EuIII‚7 and 3‚EuIII‚8 complexes
(λem ) 617 nm).
In the insets of Figure 5 the EuIII luminescence intensity at
617 nm is plotted as a function of the [7]/[3‚EuIII] (Figure 5,
left) or [8]/[3‚EuIII] (Figure 5, right) ratios. A sharp inflection
point at a 1:1 host:guest ratio is observed for both titration
curves, indicative of 1:1 binding stoichiometry. Least-squares
fitting to a 1:1 binding model gave high stability constants (9.9
× 106 and >1.0 × 107 M-1 for the 3‚EuIII‚7 and 3‚EuIII‚8
complexes, respectively). These values are about 200 times
higher than that of the association constant for the binding of
monoadamantyl derivative 12 by native â-cyclodextrin in water
(K ) 5.4 × 104 M-1), as determined by microcalorimetry
(enthalpogram not shown). The high association constants and
the 1:1 binding stoichiometry suggest that both â-cyclodextrin
cavities of dimer 3 are involved in the complexation of
bisadamantyl-biphenyl sensitizers 7 and 8. The stabilities of the
3‚EuIII‚7 and 3‚EuIII‚8 complexes are >1000 times higher than
those of the complexes reported by Parker et al.13 and Nocera
et al.11,12,14,15 and are comparable to Breslow’s 1:1 complex of
a bisadamantyl-functionalized phosphate ester and a 2,2′-
bipyridyl-spaced â-cyclodextrin dimer.44
Ideox/Iox ) 1 + kdiffτT[O2]
(8)
the luminescence intensities in the absence and presence of
oxygen, kdiff is the diffusion-controlled quenching rate constant,
and τT is the lifetime of the triplet state of the sensitizer. If
deactivation of the antenna triplet is dominated by energy
transfer to the LnIII center, the energy transfer rate constant (kET)
can be calculated by
kET ≈ 1/τT
(9)
When kdiff and [O2] are assumed to be 1010 M-1 s-1 and 1.39
mM, respectively,23,40 it follows that kET,Eu and kET,Tb are both
in the order of 106 s-1 41
. This energy transfer rate is 1-2 orders
of magnitude lower than that previously reported.42,43 Still, the
energy transfer in the 3‚LnIII‚52 complexes is faster than that in
a â-cyclodextrin-DTPA-TbIII couple with biphenyl as the
sensitizer (kET,Tb ) 5.8 × 104 s-1).15
Apparently, the length of the spacers between the biphenyl
unit and the adamantyl groups does not affect the complexation
strength of the bisadamantyl guest by â-cyclodextrin dimer 3
significantly. The excitation spectra of 3‚EuIII (λem ) 617 nm)
recorded upon the addition of equimolar amounts of bisada-
mantyl-biphenyl sensitizers 7 and 8 in water show that also the
luminescence enhancing properties of the biphenyl sensitizers
are not significantly affected by the spacer length. This is
supported by the small difference in maximum intensity
amplification achieved for 7 and 8 (300 and 360 times the initial
intensity in the absence of sensitizer, see Figure 5).
Binding of Bisadamantyl Biphenyl Sensitizers. The binding
of bisadamantyl-appended biphenyl guests 7-10 to the EuIII
complex 3‚EuIII of EDTA-based â-cyclodextrin dimer 3 in D2O
was studied using luminescence spectroscopy.19 Because of the
very low solubility of 7-10 in water, stock solutions of these
guests were prepared in DMSO. Addition of adamantylamido-
appended biphenyl derivatives 7 and 8 to a 6.0 µM solution of
(40) The solubility of molecular oxygen in water at 20 °C and 1 bar is 1.39
mM, see: Handbook of Chemistry and Physics, 76th ed.; CRC Press: New
York, 1995; p 6-4.
In contrast to amides 7 and 8, addition of sulfonamides 9
and 10 to 3‚EuIII in D2O did not increase the luminescence
intensity. The fluorescence spectra of amide 8 and sulfonamide
10 showed that the singlet state energy of the sulfonamide
(41) The luminescence spectra in the absence and presence of oxygen were
recorded several times to obtain reliable values for the ratio Ideox/Iox
.
(42) Van der Tol, E. B.; Van Ramesdonk, H. J.; Verhoeven, J. W.; Steemers,
F. J.; Kerver, E. G.; Verboom, W.; Reinhoudt, D. N. Chem.-Eur. J. 1998,
4, 2315.
(43) Steemers, F. J.; Verboom, W.; Hofstraat, J. W.; Geurts, F. A. J.; Reinhoudt,
D. N. Tetrahedron Lett. 1998, 39, 7583.
(44) (a) Zhang, B.; Breslow, R. J. Am. Chem. Soc. 1993, 115, 9353. (b) Zhang,
B.; Breslow, R. J. Am. Chem. Soc. 1997, 119, 1676.
9
J. AM. CHEM. SOC. VOL. 124, NO. 9, 2002 2063

A R T I C L E S
Michels et al.
sensitizer 10 (31 500 cm-1) is comparable to that of the amide
sensitizer 8 (30 300 cm-1), both high enough to allow energy
transfer to the EuIII center, provided that the singlet-triplet
energy gap is small enough in both cases. This suggests that,
although sulfonamides 9 and 10 probably bind to the 3‚EuIII
complex via accommodation of the adamantyl units by the
â-cyclodextrin cavities, the biphenyl moiety is too far away from
the EuIII center for efficient energy transfer to occur.
Most likely, one of the amide functionalities adjacent to the
biphenyl moiety of 7 or 8 coordinates to the EuIII center of 3‚
EuIII, thus allowing energy transfer, whereas the sulfonamide
groups of 9 and 10 do not coordinate. It has been shown that
the coordination strength of amides to LnIII ions is comparable
to that of carboxylates,45 whereas sulfonamides coordinate
significantly weaker. The geometry of the 1:1 complex, in which
both â-cyclodextrins accommodate an adamantyl group, prob-
ably does not allow coordination of the amide functionalities
adjacent to the adamantyl units in 8 and 10. The lack of
coordination of the sulfonamide groups of 9 and 10 explains
the absence of the spacer length effect on the luminescence
enhancement of 7 and 8. These observations show that also for
large sensitizers binding to both â-cyclodextrins of the 3‚LnIII
complex, coordination (weak) of the antenna to the lanthanide-
(III) center is of utmost importance for efficient sensitization
of LnIII luminescence.
LnIII ion in the noncovalent binding of antenna molecules for
sensitized luminescence. This phenomenon requires the presence
of cyclodextrins on the host, a LnIII-coordinating site on the
antenna guest, and a free coordination site on the LnIII ion.
Despite the weak interaction between a carboxylate sensitizer
and the EDTA-encapsulated LnIII ion in water, quantitative
coordination takes place because the â-cyclodextrins increase
the effective concentration of sensitizer in the vicinity of the
LnIII center. Coordination of carboxylate sensitizer 5 occurs
predominantly in a monodentate fashion and ensures relatively
efficient energy transfer as compared to similar systems in which
the sensitizer does not coordinate to the LnIII ion.13,15
Very strong binding is achieved when both â-cyclodextrin
cavities of the host are involved in the complexation of a tailor-
made biphenyl sensitizer functionalized with two adamantyl
groups. The association constants found are at least 3 orders of
magnitude higher than earlier reported values for similar
systems.11-14 Also for these complexes coordination of the
sensitizer to the LnIII center is a prerequisite for efficient energy
transfer. This indicates that the restriction of motional freedom
of the two â-cyclodextrins of a host by complexing to the same
guest molecule does not bring the antenna moiety in close
enough proximity to the LnIII center.
Conclusions
Acknowledgment. The Netherlands Organization of Scientific
Research (NWO-CW) is gratefully acknowledged for financial
support. We thank Ms. Hanny Visser for performing the 13C
NMR relaxation rate enhancement measurements.
This work demonstrates that two â-cyclodextrins covalently
linked to an EDTA unit cooperate with a ligated luminescent
(45) (a) Bru¨cher, E.; Sherry, A. D. In The Chemistry of Contrast Agents in
Medical Magnetic Resonance Imaging; Merbach, A. E., To´th, EÄ ., Eds.;
Wiley: Weinheim, 2001. (b) Caravan, P.; Ellison, J. J.; McMurry, T. J.;
Lauffer, R. B. Chem. ReV. 1999, 99, 2293.
JA017025Y
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