Brilliant Sm, Eu, Tb, and Dy chiral lanthanide complexes with strong circularly polarized luminescence

Article abstract of DOI:10.1021/ja064902x

The synthesis, characterization, and luminescent behavior of trivalent Sm, Eu, Dy, and Tb complexes of two enantiomeric, octadentate, chiral, 2-hydroxyisophthalamide ligands are reported. These complexes are highly luminescent in solution. Functionalizati

Full text of DOI:10.1021/ja064902x

Published on Web 12/15/2006
Brilliant Sm, Eu, Tb, and Dy Chiral Lanthanide Complexes
with Strong Circularly Polarized Luminescence
Ste´phane Petoud,†,| Gilles Muller,‡ Evan G. Moore,† Jide Xu,† Jurek Sokolnicki,§
James P. Riehl,§ Uyen N. Le,‡ Seth M. Cohen,†, and Kenneth N. Raymond*,†
Contribution from the Department of Chemistry, UniVersity of California,
Berkeley, California 94720, Department of Chemistry, San Jose´ State UniVersity,
One Washington Square, San Jose´, California, 95192, and Department of Chemistry,
UniVersity of Minnesota, Duluth, Minnesota 55812
Received July 10, 2006; E-mail: raymond@socrates.berkeley.edu
Abstract: The synthesis, characterization, and luminescent behavior of trivalent Sm, Eu, Dy, and Tb
complexes of two enantiomeric, octadentate, chiral, 2-hydroxyisophthalamide ligands are reported. These
complexes are highly luminescent in solution. Functionalization of the achiral parent ligand with a chiral
1-phenylethylamine substituent on the open face of the complex in close proximity to the metal center
yields complexes with strong circularly polarized luminescence (CPL) activity. This appears to be the first
example of a system utilizing the same ligand architecture to sensitize four different lanthanide cations
and display CPL activity. The luminescence dissymmetry factor, glum, recorded for the Eu(III) complex is
one of the highest values reported, and this is the first time the CPL effect has been demonstrated for a
Sm(III) complex with a chiral ligand. The combination of high luminescence intensity with CPL activity
should enable new bioanalytical applications of macromolecules in chiral environments.
Introduction
combine both the general sensitivity of fluorescence with the
advantage of high specificity for CPL signals. Due to their long
The structures of proteins, nucleic acids, and other organic
molecules are essential information for an understanding of their
function in vivo and hence critical for biochemistry, biology,
and molecular biology. The ability to identify different protein
conformations is one example of this need, since protein folding
is strongly determinant in subsequent function. Recent pertinent
examples include the role of a malfolded cellular prion protein
in transmissible spongiform encephalopathies1 (e.g., BSE or
“mad cow disease”) and the influence of different protein
conformations on subsequent spider silk properties.2
emission lifetimes and sharp emission bands, luminescent
complexes, primarily based on Tb(III) and Eu(III), have proven
to be very good luminescence probes for CPL applications. As
examples, the conformational changes in calcium and iron
binding proteins have been investigated using Tb(III) and Eu-
(III) ions as structural probes, a proof of principle that
measurements such as these provide highly specific local
information on the chiral environment of the lanthanide metal
ion.5,6 This information is thus a crucial complement to that
obtained through the analogous but much less sensitive chiro-
optical absorption technique, circular dichroism (CD), which
is often used to determine macromolecular information such as
a protein’s secondary and tertiary structure.7,8
Circularly polarized luminescence (CPL) is one of the most
sensitive techniques that can be used to determine a protein’s
conformation in solution, using a luminescent probe that reports
on its chiral environment.3,4,23 Ideally, this technique should
† University of California, Berkeley.
(11) Parker, D. Chem. Soc. ReV. 2004, 33, 156.
‡ San Jose´ State University.
(12) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science
1998, 281 (5385), 2013.
§ University of Minnesota, Duluth.
(13) Bu¨nzli, J.-C. G., Choppin, G. R., Eds.; Lanthanide Probes in Life, Chemical
and Earth Sciences: Theory and Practice; Elsevier: Amsterdam, 1989.
(14) Mathis, G. J. Biomol. Screening 1999, 4, 309.
(15) Sabbatini, N.; Guardigli, M.; Lehn, J. M. Coord. Chem. ReV. 1993, 123,
201.
(16) Petoud, S.; Bu¨nzli, J.-C. G.; Glanzman, T.; Piguet, C.; Xiang, Q.; Thummel,
R. P. J. Lumin. 1999, 82, 69.
(17) Brittain, H. G.; Richardson, F. S.; Martin, R. B. J. Am. Chem. Soc. 1976,
98, 8255.
| Current address: Department of Chemistry, Chevron Science Center,
219 Parkman Ave., Pittsburgh, PA 15260.
Current address: Department of Chemistry and Biochemistry, Uni-
versity of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093.
(1) Lasmezas, C. I.; Deslys, J.-P.; Robain, O.; Jaegly, A.; Beringue, V.; Peyrin,
J.-M.; Fournier, J.-G.; Hauw, J.-J.; Rossier, J.; Dormont, D. Science 1997,
275, 402.
(2) Jiang, P.; Shen, L.; Yang, K.; Ran, D.; Wang, J.; Guo, C. Dongwuxue Zazhi
2003, 38, 10.
(18) Bu¨nzli, J.-C. G.; Piguet, C. Chem. ReV. 2002, 102, 1897.
(19) Petoud, S.; Cohen, S. M.; Bu¨nzli, J.-C. G.; Raymond, K. N. J. Am. Chem.
Soc. 2003, 125, 13324.
(3) Coruh, N.; Hilmes, G. L.; Riehl, J. P. J. Lumin. 1988, 40-41, 227.
(4) Riehl, J. P.; Richardson, F. S. Chem. ReV. 1986, 86, 1.
(5) Coruh, N.; Riehl, J. P. Biochemistry 1992, 31, 7970.
(6) Abdollahi, S.; Harris, W. R.; Riehl, J. P. J. Phys. Chem. 1996, 100, 1950.
(7) Mulkerrin, M. G. Spectrosc. Methods Determ. Protein Struct. Solution 1996,
5.
(20) Wagnon, B. K.; Jackels, S. C. Inorg. Chem. 1989, 28, 1923.
(21) Crosby, G. A.; Demas, J. N. J. Phys. Chem. 1971, 75, 991.
(22) Meech, S. R.; Phillips, D. J. Photochem. 1983, 23, 193.
(23) Riehl, J. P.; Muller, G. Circularly Polarized Luminescence Spectroscopy
from Lanthanide Systems. In Handbook on the Physics and Chemistry of
Rare Earths; Gschneidner, K. A., Bu¨nzli, J.-C. G., Pecharsky, V. K., Eds.;
North-Holland Publishing Company: Amsterdam, 2005; Vol. 34, Chapter
220, p 289.
(8) Kelly, S. M.; Jess, T. J.; Price, N. C. Biochim. Biophys. Acta 2005, 1751,
119.
(9) Bu¨nzli, J.-C. G. Acc. Chem. Res. 2006, 39, 53.
(10) Bu¨nzli, J.-C. G.; Piguet, C. Chem. Soc. ReV. 2005, 34, 1048.
9
10.1021/ja064902x CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 77-83
77

A R T I C L E S
Petoud et al.
Due to the lanthanides’ electronic structure, their complexes
have unique optical properties, including luminescence lifetimes
that range from micro- to milliseconds, and sharp emission bands
whose width at half-height (fwhm) rarely exceed 10 nm.9-11
This is much narrower than the typically broad fluorescence
arising from organic molecule or Cd/Se nanoparticles.12 As a
result of these unique properties, well-designed lanthanide
complexes can be used as luminescent probes to analyze
biological problems, where a targeted analyte may be present
in a matrix that can also contain a large number of other
fluorescent molecules (i.e., high background autofluorescence).
Similarly, discrimination of the relevant fluorescent signal from
that present in biological media (e.g., fluorescence from amino
acids such as tyrosine or tryptophan) is readily achieved by time
gating (a time-resolved measurement).13 Indeed, the combination
of temporal and spectral discrimination properties have led to
the development of time-resolved fluoroimmunoassays with very
high sensitivities,14 without the need for tedious prior purification
of the sample which is useful when a very large number of
samples must be screened.
Because the lanthanide f-f transitions are Laporte forbidden,13
the molar absorptivity is very low. The typical strategy to
circumvent this is to increase the luminescence excitation by
coordinating the luminescent lanthanide ion to a chromophore,
which acts as an antenna to effectively transfer light energy to
the metal.15,16 For example, in biological systems, amino acids
such as tryptophan have been utilized for this purpose, although
in that case the binding environment is not optimized for the
lanthanide cation.6,17 In order to form luminescent lanthanide
complexes with CPL activity, chiral complexes have been
developed where the coordination geometry around the lan-
thanide is well-defined and controllable, which enables fine-
tuning of the photophysical properties.18
match the time scale of the experiment (microseconds to
milliseconds depending on the nature of the lanthanide cation).
Hence, any type of exchange between the complex and the
environment in the luminescence time scale should be avoided.
Ideally, a rigid, nonfluxional complex, with a saturated coor-
dination sphere, should best satisfy this latter condition.
We have reported the properties of a new family of lanthanide
complexes formed with ligands incorporating 2-hydroxyisoph-
thalamide as the chelating unit.19 Upon coordination, these
complexes demonstrate several superior luminescence properties,
including highly efficient transfer of electronic excitation from
the ligand to metal center and sufficient protection of the
lanthanide metal ion against nonradiative deactivation from the
environment (e.g., solvent molecules). Indeed, the quantum yield
of the Tb(III) complex in H2O, 61%, is the highest value yet
reported for a luminescent metal complex with high stability in
aqueous solution. In addition, these newly developed ligands
allow the sensitization of four different trivalent lanthanides
emitting in the visible (Sm, Eu, Tb, and Dy) with the same
ligand. Herein, we have extended the superior luminescence
properties of these complexes to CPL activity by the addition
of a chiral substituent to the open face of an octadentate,
tetrapodal ligand framework, utilizing the 2-hydroxyisophthala-
mide chelating unit as the sensitizer. This strategy results in
the formation of enantiomerically pure complexes with strong
circular dichroism (CD) activity. Significantly, these complexes
are also strongly luminescent, emitting in the visible region,
and display strong CPL activity. As a result of their brightness
and improved signal-to-noise ratio, these complexes present new
possibilities for increased sensitivity of assays based on CPL
measurements. While the Tb(III) complex reported here is the
most fluorescent, the strongest CPL effect has been observed
for the Eu(III) complex, with one of the highest ∆I values
reported to date. In addition, the Dy(III) and Sm(III) complexes
are also luminescent and have significant CPL activity, a unique
feature which enables us to have four different luminescent
probes that can be easily discriminated from each other. Since
the sharp emission bands do not appreciably overlap, multiplex
detection is possible.
Despite these attractive features, there is a limited number
of lanthanide based luminescent probes suitable for practical
CPL applications in solution. The reason is the weak CPL signal
intensity for the complexes described in the literature, due to a
combination of limitations imposed by key requirements which
must be fulfilled in order to obtain efficient luminescent
complexes with detectable CPL signals in solution. First, the
limited absorption of lanthanide cations must be overcome by
coordinating a ligand that is able to harvest significant amounts
of UV or visible light and then efficiently transfer this electronic
energy to the lanthanide metal ion. Second, since the excited
states of the lanthanide cations can be easily deactivated through
nonradiative interaction with their chemical environment (cou-
pling to vibrational modes of solvent molecules or of the ligand),
the lanthanide must be protected from this quenching environ-
ment when bound to the ligand. This implies that the ligand
must fulfill the high coordination number requirement of the
lanthanide cation by providing an adequate number of donor
atoms (typically between 8 and 12 in solution). Third, the
ensuing complex must have sufficient thermodynamic stability
and/or kinetic inertness at the appropriate concentration to
interact with, and report on, the macromolecule of interest, since
dissociation would lead to loss of the antenna effect and a
subsequent decrease in luminescence. Fourth, to observe an
unambiguous CPL signal it is preferable that only a single
enantiomer of the chiral complex be present in solution or, more
specifically, the existence of only one of these enantiomers must
Experimental Section
Synthesis. N,N,N′,N′-Tetrakis(2-aminoethyl)ethane-1,2-diamine (H22)
was prepared according to a literature procedure.20 Unless stated
otherwise, all other reagents were purchased from commercial suppliers
and used as supplied.
2-Methoxy-bis(2-mercaptothiazolide)isophthalic Acid. 2-Meth-
oxyisophthalic acid (20 mmol), 2-mercaptothiazoline (40 mmol), and
a catalytic amount of 4-dimethylaminopyridine (20 mg) were dissolved
in 150 mL of CH2Cl2 under a nitrogen atmosphere. To this was added
1,3-dicyclohexylcarbodiimide (40 mmol), and the solution gradually
became yellow in color. After stirring for 5 h, the reaction mixture
was filtered, and the filtrate evaporated to dryness to afford a yellow
oil. Recrystallization from hot ethyl acetate gave a bright yellow
microcrystalline solid. Yield: 55%. IR (film from CDCl3) ν 1229, 1684,
1
2955 cm-1. H NMR (300 MHz, CDCl3, 25 °C): δ 3.42 (t, J ) 7.3
Hz, 4H, CH2), 3.89 (s, 3H, CH3), 4.60 (t, J ) 7.3 Hz, 4H, CH2), 7.13
(t, J ) 7.7 Hz, 1H, ArH), 7.43 (d, J ) 7.6 Hz, 2H, ArH). 13C NMR
(400 MHz, CDCl3, 25 °C): δ 29.2, 55.6, 62.9, 123.1, 128.1, 131.9,
154.7, 167.1, 200.8. Anal. Calcd (Found) for C15H14N2O3S4‚0.25H2O:
C, 44.70 (44.75); H, 3.63 (3.53); N, 6.95 (6.82) %.
Me4(H22IAM)(2-mercaptothiazolide)4. N,N,N′,N′-Tetrakis(2-ami-
noethyl)ethane-1,2-diamine (H22) (2.8 mmol) dissolved in 150 mL of
9
78 J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007

Highly Luminescent Ln Complexes with CPL Activity
A R T I C L E S
CH2Cl2 was added dropwise to a solution of excess 2-methoxy-bis(2-
mercaptothiazolide)isophthalic acid in 600 mL of CH2Cl2. The reaction
mixture was stirred for ca. 24 h and then evaporated to dryness yielding
a yellow oil which was purified by flash chromatography on silica gel
(0-15% MeOH in CH2Cl2 gradient). The solvent was evaporated to
give the product as a yellow foam. Yield: 83.8%. IR (film from CH2-
Cl2) ν 1522, 1653, 2942 cm-1. 1H NMR (500 MHz, CDCl3, 25 °C): δ
2.67 (s, 4H, CH2), 2.71 (t, J ) 6.2 Hz, 8H, CH2), 2.71 (t, J ) 6.2 Hz,
8H, CH2), 3.39 (br t, 8H, CH2), 3.48 (s, J ) 5.8 Hz, 8H, CH2), 3.76 (s,
12H, OCH3), 4.59 (t, J ) 7.2 Hz, 8H, CH2), 7.14 (t, J ) 7.7 Hz, 4H,
ArH), 7.35 (d, J ) 5.8 Hz, 4H, ArH), 7.79 (br t, 4H, NH), 7.97 (d, J
) 6.0 Hz, 4H, ArH). 13C NMR (500 MHz, CDCl3, 25 °C): δ 29.1,
37.9 50.6, 53.5, 55.7, 63.1, 124.3, 127.2, 129.1, 132.0, 133.9, 155.6,
164.9, 167.3, 201.4. Anal. Calcd (Found) for C58H64N10O12S8‚2CH2-
Cl2: C, 47.43 (47.45); H, 4.51 (4.52); N, 9.22 (9.54) %.
(99.999%), and Dy (99.99+%) using the following general procedure.
TotheligandH4R(+)BnMeH22IAM‚HBr‚3H2OorH4S(-)BnMeH22IAM‚
HBr‚2H2O (20.0 µmol) in MeOH (ca. 7 mL) was added ca. 1.05 equiv
of LnCl3‚6H2O. The resulting clear solution was heated to reflux
temperature, and 10 equiv of sym-collidine (ca. 30 µL) was added to
deprotonate the phenolic oxygens and ensure complexation. This
solution was held at reflux temperature for 4 h and then allowed to
cool to room temperature. Slow addition of distilled H2O dropwise (ca.
1-5 mL) induced precipitation of the desired complexes as white solids.
These were easily collected by vacuum filtration and dried to give
typical yields of ca. 80%. Elemental analysis for [LnR(+)BnMeH22IAM]-
Cl‚xH2O. Ln ) Tb, x ) 6; Calcd (Found) C, 55.48 (55.71); H, 5.66
(5.51); N, 8.74 (8.70) %. Ln ) Eu, x ) 4; Calcd (Found) C, 57.01
(57.01); H, 5.56 (5.55); N, 8.98 (8.92) %. Ln ) Dy, x ) 6; Calcd
(Found) C, 55.36 (55.43); H, 5.65 (5.43); N, 8.72 (8.69) %. Ln ) Sm,
x ) 6; Calcd (Found) C, 55.78 (55.85); H, 5.69 (5.57); N, 8.79 (8.91)
%. Elemental analysis for [LnS(-)BnMeH22IAM]Cl‚xH2O. Ln ) Tb,
x ) 6; Calcd (Found) C, 56.76 (56.50); H, 5.54 (5.52); N, 8.94 (8.66)
%. Ln ) Eu, x ) 4; Calcd (Found) C, 57.01 (57.33); H, 5.56 (5.69);
N, 8.98 (9.00) %. These complexes are highly soluble in MeOH, EtOH,
DMSO, and DMF.
Physical Methods. UV-visible absorption spectra were recorded
on Perkin-Elmer Lambda 9 and Lambda 19 double beam absorption
spectrometers using quartz cells of 0.10 and 1.00 cm path lengths.
Circular dichroism spectra were recorded on Jasco J-810 and Jasco
J-710 instruments. Emission spectra were acquired on a HORIBA Jobin
Yvon IBH FluoroLog-3 spectrofluorimeter, equipped with three slit
double grating excitation and emission monochromators with disper-
sions of 2.1 nm/mm (1200 grooves/mm). Spectra were reference
corrected for both the excitation light source variation (lamp and grating)
and the emission spectral response (detector and grating). Quantum
yields were determined by the optically dilute method21 using the
following equation:
Me4R(+)BnMeH22IAM. Me4(H22IAM)(2-mercaptothiazolide)4 (1.5
g, 1.1 mmol) was dissolved in 30 mL of CHCl3, to which R(+)-R-
methylbenzylamine (0.5 g, 4.1 mmol) was added slowly as a solution
in 5 mL of CH2Cl2. The mixture was stirred under a nitrogen atmosphere
for 24 h, by which time TLC analysis indicated the reaction was
complete. The reaction mixture was extracted with 5% citric acid to
remove any remaining free amine and then loaded directly onto silica
gel for purification by flash chromatography with a 0-8% MeOH in
CH2Cl2 gradient. Removal of solvent gave the product as a beige foam.
Yield: 72%. IR (film from CDCl3) ν 1652, 2938, 3283, 3379 cm-1
.
1H NMR (500 MHz, CDCl3, 25 °C): δ 1.57 (d, J ) 7.0 Hz, 12H,
CH3), 2.68 (br m, 12H, CH2), 3.41 (br m, 8H, CH2), 3.67 (s, 12H,
CH3), 5.30 (m, 4H, CH), 7.07 (br t, 4H, ArH), 7.14-7.40 (br, m, 20H,
ArH), 7.64 (d, 4H, J ) 5.5 Hz, ArH), 7.82 (br, s, 12 H, ArH and NH).
13C NMR (125 MHz, CDCl3, 25 °C): δ 21.6, 37.7, 48.7, 52.8, 53.2,
62.7, 124.1, 125.8, 126.9, 127.0, 128.2, 128.4, 132.7, 133.0, 142.8,
155.1, 164.2, 165.0.
Me4S(-)BnMeH22IAM. This compound was prepared by the same
procedure as that for Me4R(+)BnMeH22IAM, except S(-)R-methyl-
benzylamine was used instead of the R(+)R-methylbenzylamine
enantiomer. Separation and purification were performed as described
2
Qx/Qr ) [Ar(λr)/Ax(λx)][I(λr)/I(λx)][nx /nr2][Dx/Dr]
Here A is the absorbance at the excitation wavelength (λ), I is the
intensity of the excitation light at the same wavelength, n is the
refractive index, and D is the integrated luminescence intensity. The
subscripts “x” and “r” refer to the sample and reference, respectively.
In this case, quinine sulfate in 1.0 N sulfuric acid was used as the
reference (Qr ) 0.546).22
1
above. The H and 13C NMR were essentially identical to those of
Me4R(+)BnMeH22IAM.
H4R(+)BnMeH22IAM. Me4R(+)BnMeH22IAM was dissolved in
40 mL of dry, degassed CH2Cl2, and BBr3 was introduced to the solution
under a nitrogen atmosphere via syringe. The resulting yellow slurry
was stirred for 48 h. The slurry was slowly quenched with MeOH,
after which the reaction mixture was diluted with 100 mL of water.
The mixture was boiled for 5 h, and then the hot water was decanted
away from an oily residue remaining in the flask. The residue was
dissolved in CH2Cl2, dried with MgSO4, and filtered. The filtrate was
evaporated to dryness to give an off-white solid. Yield: 85%. Elemental
analysis: Found (Calcd) C, 61.66 (61.87); H, 5.99 (6.10); N, 9.73 (9.75)
Luminescence lifetimes were determined on two different instru-
ments. The first was a HORIBA Jobin Yvon IBH FluoroLog-3
spectrofluorimeter, adapted for time-correlated single photon counting
(TCSPC) and multichannel scaling (MCS) measurements. A sub-
microsecond Xenon flash lamp (Jobin Yvon, 5000XeF) or pulsed LED
(IBH, NanoLED-16) was used as the light source, coupled to a double
grating excitation monochromator for spectral selection. In the former
case, the input pulse energy (100 nF discharge capacitance) was ca. 50
mJ, yielding an optical pulse duration of less than 300 ns at fwhm,
while, for the pulsed LED system, the peak output at 340 nm had a
fwhm of 10 nm and a pulse duration of ca. 800 ps. A thermoelectrically
cooled single photon detection module (HORIBA Jobin Yvon IBH,
TBX-04-D) incorporating a fast risetime PMT, wide bandwidth
preamplifier, and picosecond constant fraction discriminator was used
as the detector. Signals were acquired using an IBH DataStation Hub
photon counting module, and data analysis was performed using the
commercially available DAS 6 decay analysis software package from
HORIBA Jobin Yvon IBH. Goodness of fit was assessed by minimizing
the reduced chi squared function, ø2, and a visual inspection of the
weighted residuals.
1
% for C74H80N10O12‚HBr‚3H2O. H NMR (400 MHz, DMSO-d6, 25
°C): δ 1.47 (d, J ) 6.8 Hz, 12H, CH3), 3.47 (br s, 4H, CH2), 3.74 (br
s, 12H, CH2), 5.16 (br m, 4H, CH), 6.92 (t, J ) 7.6 Hz, 4H, ArH),
7.28 (br m, 20H, ArH), 7.97 (d, J ) 7.2 Hz, 4H, ArH), 8.16 (d, J )
7.6 Hz, 4H, ArH), 8.91 (br t, 4H, ArH), 9.18 (d, J ) 7.6 Hz, 4H, NH).
13C NMR (125 MHz, DMSO-d6, 25 °C): δ 22.5, 29.9, 34.9, 49.1, 52.7,
117.4, 118.6, 119.3, 126.5, 127.3, 128.8, 133.0, 134.4, 144.3, 160.2,
167.5, 167.8. Anal. Calcd (Found) for C74H80N10O12‚HBr‚3H2O: C,
61.87 (61.66); H, 6.10 (5.99); N, 9.75 (9.73) %.
H4S(-)BnMeH22IAM. This compound was prepared using the
same deprotection procedure as that for Me4R(+)BnMeH22IAM except
1
starting with the S(-) isomer. The H and 13C NMR are essentially
identical to that of H4R(+)BnMeH22IAM. Anal. Calcd (Found) for
C74H82N10O10‚HBr‚2H2O: C, 62.66 (62.56); H, 6.04 (5.91); N, 9.87
(9.67) %.
[LnR(+)BnMeH22IAM]Cl‚xH2O and [LnS(-)BnMeH22IAM]-
Cl‚xH2O. Luminescent complexes were isolated in their protonated
forms with Ln (AR purity) ) Sm (99.99+ %), Eu (99.99+%), Tb
Alternately, a home-built instrument incorporating either a pulsed
N2 laser (Oriel, 79110) at 337.1 nm or the third harmonic of a Nd:
YAG (Continuum, Powerlite, 8010) at 354.7 nm was used as the
excitation source. Emission was collected perpendicular to the excitation
beam, and spectral selection was achieved using either a single or double
9
J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007 79

A R T I C L E S
Petoud et al.
Chart 1. Synthetic Scheme and Structure of Ligands and Complexes
grating monochromator (Spectral Products, CM 110 or Spex, FL1005,
respectively). A Hamamatsu R-928 PMT was used as the detector, and
signals were acquired and digitized with a Tektronix TDS754D digital
oscilloscope. Data fitting was performed by nonlinear least-squares
analysis with Origin 7.
In both cases, each trace contained at least 10 000 points, and the
reported lifetime values result from at least three independent measure-
ments. Typical sample concentrations for both absorption and fluores-
cence measurements were ca. 10-4-10-6 M, and 1.0 cm cells in quartz
suprasil or equivalent were used for all measurements.
Circularly polarized luminescence and total luminescence spectra
were recorded on an instrument according to literature procedures,23
operating in a differential photon-counting mode. The light source for
excitation was a continuous wave 450 W xenon arc lamp from a Spex
FluoroLog-2 spectrofluorimeter, equipped with excitation and emission
monochromators with dispersions of 4 nm/mm (SPEX, 1681B). CPL
measurements were performed at 295 K in MeOH or DMF with analyte
concentrations of ca. 10-3 M.
(III)) and is attributed to deprotonation and metal coordination
by the phenols.24 A similar effect has been observed25 with other
2-hydroxyisophthalic acid derivatives upon complexation
with beryllium, and the absorption bands of the complexes
reported here are very similar to those reported previously19
for the parent octadentate 2-hydroxyisophthalamide ligand
(H22IAM)(Chart 1). Circular dichroism (CD) measurements
indicate that both the free ligands and their complexes are chiral
in methanol solution. In particular, the ligand does not appear
to have epimerized when placed in solution or when the reaction
mixture was heated in the presence of base during the synthesis
of the complexes.26 Furthermore, the two enantiomeric R(+)
and S(-) forms of the complexes are mirror images in their
observed CD spectra, as illustrated in Figure 1 for the Tb(III)
complexes.
For both the R(+) or S(-) enantiomeric forms of the
complex, the UV/visible absorption and CD spectra of the
different lanthanide complexes do not differ significantly,
indicating the ligand centered electronic structure of the four
complexes formed with the different lanthanide cations are
similar. This is expected, since the metal ion electrons of the
f-orbitals are almost uninvolved in chemical bonding. Thus the
only major structural change between the complexes of this
series is the varying size of the central metal ions. By analogy
to the behavior of the parent [Ln(H22IAM)] complexes,19 the
monoprotonated neutral species is likely predominant in solution.
Significantly, the lower energy absorption band for all of the
complexes is located at ca. 350 nm. This band is quite broad
and gives good excitation of the complexes with wavelengths
of up to ca. 390 nm. For practical applications, the low energy
of this excitation wavelength has the advantage of being less
harmful towards biological material, whereas most of the
Results
Ligand Design and Synthesis. Octadentate ligands formed
with four 2-hydroxyisophthalamide chelating groups tethered
by a flexible alkylamino backbone such as H22IAM have proven
to be efficient ligands for the coordination of luminescent
lanthanide cations.19 The methyl group of the amide group on
the open face of the ligand has been replaced, yielding the two
homochiral enantiomeric R(+) or S(-) forms of a chiral benzyl
methyl substituent. This then gives an enantiomerically pure
chiral form of the parent complex for CPL applications. This
design strategy minimizes the distance between the stereogenic
centers and the lanthanide cation and should maximize the
chirality of the excited-state environment for the complex and
subsequent CPL effects.
UV-Visible Absorption and Circular Dichroism. The
absorption spectra of the ligands are dominated by an intense
transition with an apparent maximum at ca. 316 nm which upon
complexation is red-shifted by ca. 30 nm, with a new absorption
band at ca. 350 nm. This change is essentially invariant for each
of the lanthanide complexes (Sm(III), Eu(III), Tb(III), and Dy-
(24) Kanamori, D.; Furukawa, A.; Okamura, T.-A.; Yamamoto, H.; Ueyama,
N. Org. Biomol. Chem. 2005, 3, 1453.
(25) Keizer, T. S.; Sauer, N. N.; McCleskey, T. M. J. Am. Chem. Soc. 2004,
126, 9484.
(26) Paul, J. M.; Potter, G. A. (Chiroscience Limited, UK). Racemization of
chiral amines in the presence of metal hydroxides, PCT Int. Appl., WO
9721662, 1997.
9
80 J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007

Highly Luminescent Ln Complexes with CPL Activity
A R T I C L E S
Figure 3. Normalized emission spectra (λexc ) 350 nm) for 1 × 10-6
M
Figure 1. UV-visible absorption (lower curve, right axis) and CD spectra
solutions of the [LnR(+)BnMeH22IAM] complexes in MeOH with Ln )
(upper curves, left axis, (‚ ‚ ‚)
) [TbR(+)BnMeH22IAM], (s) )
Tb, Eu, Dy, Sm.
[TbS(-)BnMeH22IAM]) for 10-5 M solutions of the complexes in MeOH
at 298 K.
Table 1. Summary of Photophysical Parameters for the
[LnR(+)BnMeH22IAM] Complexes in MeOHa
quantum
luminescence
lifetime,
complex
λ
exc, nm
yield,
Φ, %
τ, µs
[TbR(+)BnMeH22IAM]
[EuR(+)BnMeH22IAM]
[DyR(+)BnMe22IAM]
[SmR(+)BnMeH22IAM]
354
347
350
350
63.0
2.3
1.3
0.8
1271 ( 118 (1494)
784 ( 99 (1403)
18 ( 3
17 ( 2
a Luminescence lifetimes and quantum yield values are reported here
with an error of (15%. Numbers in parentheses refer to measurements in
deuterated solvent.
overlap with transitions of the other complexes. Hence, as was
reported19 for the parent octadentate H22IAM complexes, the
chiral derivatives reported here also allow for the sensitization
of four different lanthanide complexes, in this case each active
for CPL.
Luminescence efficiency has been assessed by quantum yield
determinations upon ligand excitation for each of the differing
complexes, with results as summarized for the R(+) form in
Table 1. These values are quite similar to those previously
reported19 for the parent lanthanide complexes formed with the
2-hydroxyisophthalamide moieties as the chelating group, in
particular for Tb(III), indicating the chiral substituent has only
a slight effect on the photophysical properties of the ensuing
complex. However, as evident from the emission spectra (Figure
3), a significant amount of ligand emission centered at ca. 410
nm is observed for the Eu, Sm, and Dy chiral derivatives,
indicating the intramolecular ligand-to-lanthanide energy transfer
is incomplete, in contrast to the parent H22IAM derivatives19
and the Tb(III) complex reported here. This observation may
also account for the slightly diminished quantum yield values
observed here, although these complexes can still be considered
as the brightest complexes with CPL activity. These intense
signals are a significant advantage for practical bioanalytical
applications, as it will allow for enhanced sensitivity to be
obtained by improving the signal-to-noise ratio.
Figure 2. Photograph of ca. 10-4 M solutions of the free ligand (far left)
and the [LnR(+)BnMeH22IAM] complexes in MeOH with Ln ) Tb, Eu,
Dy, Sm from left to right, respectively, upon excitation by a standard
laboratory UV lamp (λexc) 365 nm).
lanthanide complexes described in the literature to date have
excitation bands located at significantly higher energy, typically
between 300 and 330 nm.27,28
Emission Spectra and Luminescence Lifetimes. Upon UV
irradiation, solutions of the free ligands (ca. 10-4 M) emit a
strong blue luminescence (apparent λmax at ca. 410 nm) that
can be easily detected with the naked eye. However, upon
complexation this emission is replaced by the typical lumines-
cence color arising from the corresponding lanthanide cation
(red for Eu(III), green for Tb(III), yellow/gray for Dy(III), and
pink-purple for Sm(III)). These colors can also be easily
visualized using a standard laboratory UV lamp (λex ≈ 365 nm,
Figure 2), and no loss in signal intensity over a period of several
weeks was observed, indicating good stability of the complexes.
As shown in Figure 3, inspection of the emission spectra for
the R(+) complexes in solution reveals the typically sharp
luminescence bands, characteristic of the different lanthanide
cations. Furthermore, it can be seen that each lanthanide complex
possesses at least one transition that does not significantly
In addition to steady state measurements, the luminescence
lifetimes of the different lanthanide complexes were determined.
The resultant monoexponential decays for the R(+) form as
summarized in Table 1. As expected, the values obtained for
the S(-) enantiomeric form were identical within the experi-
mental error. We also note the luminescence lifetimes were
strongly influenced by the water content of the MeOH solvent,
(27) Yamada, T.; Shinoda, S.; Sugimoto, H.; Uenishi, J.-I.; Tsukube, H. Inorg.
Chem. 2003, 42, 7932.
(28) Li, M.; Selvin, P. R. J. Am. Chem. Soc. 1995, 117, 8132.
9
J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007 81

A R T I C L E S
Petoud et al.
and this effect was particularly evident in the case of Eu(III),
which is known to be more sensitive to the presence of OH
oscillators.13 Nonetheless, the relatively long luminescence
lifetimes are an indication that the octadentate ligand scaffold
provides a significant level of protection from nonradiative
deactivation of the lanthanide cations by the solvent molecules.
For the R(+) enantiomeric form of the Tb(III) and Eu(III)
complexes, the luminescence lifetime measurements were also
performed in MeOD, which allowed an estimate of the number
of bound solvent molecules to be made using the empirical
Horrock’s relationship29
n ) A(kH - kD)
where kD and kH refer to the radiative decay rates (1/τ) observed
in the deuterated and non-deuterated solvent, respectively, and
A is an empirical constant evaluated as 8.4 ms-1 and 2.1 ms-1
for Tb(III) and Eu(III). The resulting n parameter was evaluated
to be 1.0 ((0.5) and 1.2 ((0.5), respectively, suggesting a single
bound solvent molecule, which would also account for the
diminished quantum yields, in particular for the Eu(III) com-
plexes, when compared to the parent H22IAM complex.
However, these values may also be an overestimate, since, as
discussed elsewhere by Parker et. al.,30 the original Horrocks
relationship does not account for the effect of proximate NH
and CH oscillators or OH oscillators not directly coordinated
to the metal ion.
Figure 4. Total luminescence (lower curves, right axis) and CPL spectra
(upper curves, left axis, (‚ ‚ ‚)
) [LnR(+)BnMeH22IAM], (s) )
[LnS(-)BnMeH22IAM]) for 10-3 M solutions of the Ln ) Eu(III) (top)
and Ln ) Tb(III) (bottom) complexes in MeOH (λexc ) 371-376 nm) at
298 K.
Circularly Polarized Luminescence. The CPL spectra for
the magnetic dipole allowed transitions of each of the com-
plexes, [LnR(+)BnMeH22IAM], Ln ) Eu, Tb, Sm, and Dy,
and [LnS(-)BnMeH22IAM], Ln ) Eu, Tb, were measured
following excitation at a λex of 371-376 nm in MeOH at 295
K. For Eu(III), Tb(III), and Dy(III), the transitions of interest
5
7
5
7
are the D0 f F1 (ca. 594 nm), D4 f F5 (ca. 536 nm), and
4F9/2 f H11/2 (ca. 669 nm), respectively, while, for Sm(III),
6
there are two strong magnetically dipole allowed transitions,
4
6
4
6
namely G5/2 f H7/2 (ca. 565 nm) and G5/2 f H5/2 (ca. 597
nm). Of the several sharp emission lines shown in Figure 3,
these wavelengths are particularly well suited for CPL measure-
ments since these transitions satisfy the magnetic dipole selection
rule, ∆J ) 0, (1 (except 0 T 0), where it is predicted the CPL
signal should be large.31
As shown in Figure 4, for Eu(III) and Tb(III), respectively,
and in Figure 5, for Sm(III), CPL signals are readily apparent
confirming the presence of a chiral emitting species and also
display good complementarity between the R(+) and S(-)
enantimoeric forms of the former complexes. All of the
complexes examined exhibit relatively strong CPL activity.
However, the magnitude of the luminescence dissymmetry
factor, glum, which may be defined as
IL - IR
∆I
Figure 5. Total luminescence (lower curves, right axis) and CPL spectra
glum
)
)
4
6
4
6
(upper curves, left axis) of the G5/2 f H5/2 and G5/2 f H7/2 transitions
for a 10-3 M solution of the [Sm(R(+)BnMeH22IAM)] complex in MeOH
(λexc ) 371-376 nm) at 298 K.
1
1
I
(IL + IR)
2
2
where IL and IR refer to the intensity of left and right circularly
polarized light, respectively, is quite different for the four metal
centers, with results as summarized in Table 2.
In addition to the magnitude of the CPL signal, the line shape
of the observed spectra also differs significantly for the four
lanthanide complexes, as each CPL spectrum displays several
(29) Horrocks, W. D., Jr.; Sudnick, D. R. Acc. Chem. Res. 1981, 14, 384.
(30) Beeby, A.; Clarkson, I. M.; Dickins, R. S.; Faulkner, S.; Parker, D.; Royle,
L.; de Sousa, A. S.; Williams, J. A. G.; Woods, M. J. Chem. Soc., Perkin
Trans. 2 1999, 3, 493.
(31) Richardson, F. S. Inorg. Chem. 1980, 19, 2806.
9
82 J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007

Highly Luminescent Ln Complexes with CPL Activity
A R T I C L E S
Table 2. Summary of CPL Results for the [Ln(BnMeH22IAM)]
Complexes
four different luminescent lanthanide cations (Sm(III), Eu(III),
Tb(III), and Dy(III)), each of which with a characteristic
emission band in the visible region and each of which with
activity for CPL measurements (suitable for chiral multiplex
conditions). This is also the first report of a Sm(III) complex
formed with a chiral ligand that has CPL activity.
While the Eu(III) complex displays one of the highest CPL
intensities described in the literature, its overall quantum yield
is relatively modest. In contrast, the Tb(III) complex, which
has a much greater quantum yield, displays a weaker CPL effect.
Generally, these two effects will self-compensate, allowing the
complexes to have a good compromise between luminescence
intensity and CPL activity.
electronic
transition
wavelength
(nm)
complex
glum
[TbR(+)BnMeH22IAM]
[TbS(-)BnMeH22IAM]
[EuR(+)BnMeH22IAM]
[EuS(-)BnMeH22IAM]
[DyR(+)BnMeH22IAM]
[SmR(+)BnMeH22IAM]
5D4 f 7F5
543
543
596
596
669
565
597
+0.044
-0.048
+0.298
-0.294
+0.013
-0.027
-0.028
5D4 f 7F5
5D0 f 7F1
5D0 f 7F1
4F9/2 f 6H11/2
5G5/2 f 6H7/2
4G5/2 f 6H5/2
peaks corresponding to crystal-field splitting of the electronic
level. For Eu(III), the splitting of the 7FJ levels is less important
than those for the analogous Sm(III), Dy(III), or Tb(III)
complexes, and simpler spectra are obtained which are more
amenable to structural interpretation. In particular, it is of interest
to note that at least four distinguishable peaks are seen in the
Eu(III) 5D0 f 7F1 transition (Figure 4), whereas, under tetragonal
or lower symmetries, a maximum of three crystal field bands
(2J + 1) should be observed. As a consequence, it can be
concluded that there exists more than one conformation of this
complex present in MeOH solution. For comparison, the CPL
spectrum of the [EuR(+)BnMeH22IAM] complex was also
measured in DMF solution (Figure S1, Supporting Information)
but yielded essentially identical spectra to those obtained in
MeOH.
The significant CPL activity resulting from the Sm(III) and
Dy(III) compounds reported here is promising for further CPL
studies, and it appears this is the first reported observation of
CPL activity from a Sm(III) complex formed with a chiral
ligand. The addition of these two luminescent probes is an
exciting prospect for multiplex measurements based on the
sensitivity of these lanthanide complexes. While most of the
CPL reports in the literature involving Dy(III) and Sm(III) ions
refer to the measurement of racemic complexes with achiral
ligands, there is one report of CPL for a chiral DOTA-based
ligand Dy(III) complex with a larger CPL dissymmetry factor
(glum ) -0.41 at 667 nm) than reported here.32 However, for
Eu(III), the reported glum values reported herein are some of
the highest ever observed, and the CPL signal from the Tb(III)
complexes, while not as large, are also easily measurable. The
stronger signal from the Eu(III) complex as compared to the
Tb(III) compound is in accordance with previously observed
behavior from other complexes with approximately C4 sym-
metry.23
The complexes described here have luminescent lifetimes
ranging from the microsecond to millisecond time scale. This
feature facilitates the appropriate selection of a complex with a
luminescence lifetime corresponding to the type of experiment
or assay to be performed, which will in itself be dependent on
the kinetics of the biological event taking place in solution.
Last, it is pertinent to note the existence of several coordina-
tion environments about the central metal ion for the [LnR(+)-
BnMeH22IAM] complexes, as confirmed by CPL and high-
resolution emission spectra for the Eu(III) complex which
5
indicates that the emission signal corresponding to the D0 f
7Fo electronic transition is not a unique signal. This is indicative
of an environment around the lanthanide cation in which several
chiral environments may be present due to some remaining
flexibility of the ligand. Evidently, the coordination geometry
of the ligand is not perfectly optimized for the lanthanide cation.
Our efforts now focus on tighter control over the lanthanide
binding to prevent different coordination modes and hence
further improve the strength of the CPL signal. Similarly, the
incorporation of a more water soluble chiral group is ongoing
to facilitate the use of these systems in aqueous systems.
Acknowledgment. This research was supported by the
Director, Office of Energy Research, Office of Basic Energy
Sciences, Chemical Sciences Division, United States Department
of Energy under Contract DE-AC03-76F00098. We acknowl-
edge the Swiss National Science Foundation, the Leenards
Foundation, and the Novartis Foundations for postdoctoral
fellowships to S.P. G.M. thanks the National Institute of Health
(NIH) Minority Biomedical Research Support (MBRS) Program
for its support (2S06 GM008192-24AI). The authors wish to
thank Ms. Francoise Muller for assistance in making CPL
measurements reported in this work. This technology is licensed
to Lumiphore, Inc., in which some of the authors have a
financial interest.
Conclusions
We report chiral, octadentate, 2-hydroxyisophthalamide ligands
whose lanthanide complexes have good luminescence properties
in solution and strong CPL activity. This is apparently the first
system where the same ligand architecture is able to sensitize
Supporting Information Available: A comparison of ob-
served CPL effects for [EuR(+)BnMeH22IAM] in MeOH and
DMF solution. This material is available free of charge via the
Internet at http://pubs.acs.org.
(32) Dickins, R. S.; Howard, J. A. K.; Maupin, C. L.; Moloney, J. M.; Parker,
D.; Riehl, J. P.; Siligardi, G.; Williams, J. A. G. Chem. Eur. J. 1999, 5,
1095.
JA064902X
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J. AM. CHEM. SOC. VOL. 129, NO. 1, 2007 83