Strong circularly polarized luminescence from highly emissive terbium complexes in aqueous solution

Article abstract of DOI:10.1002/ejic.201000309

Two luminescent terbium(III) complexes have been prepared from chiral ligands containing 2-hydroxyisophthalamide (IAM) antenna chromophores and their non-polarized and circularly-polarized luminescence properties have been studied. These tetradentate liga

Full text of DOI:10.1002/ejic.201000309

FULL PAPER
DOI: 10.1002/ejic.201000309
Strong Circularly Polarized Luminescence from Highly Emissive Terbium
Complexes in Aqueous Solution
Amanda P. S. Samuel,[a] Jamie L. Lunkley,[b] Gilles Muller,[b] and Kenneth N. Raymond*[a]
Keywords: Lanthanides / Terbium / Luminescence / Circularly polarized luminescence (CPL) / Chirality
Two luminescent terbium(III) complexes have been prepared
from chiral ligands containing 2-hydroxyisophthalamide
(IAM) antenna chromophores and their non-polarized and
circularly-polarized luminescence properties have been
studied. These tetradentate ligands, which form 2:1 ligand/
TbIII complexes, utilize diaminocyclohexane (cyLI) and di-
nescence lifetime measurements in H2O and D2O indicate
that while cyLI-Tb exists as a single species in solution,
dpenLI-Tb exists as two species: a monohydrate complex
with one H2O molecule directly bound to the TbIII ion and a
complex with no water molecules in the inner coordination
sphere. Both cyLI-Tb and dpenLI-Tb display increased CPL
phenylethylenediamine (dpenLI) backbones, which we rea- activity compared to previously reported TbIII complexes
soned would impart conformational rigidity and result in TbIII
complexes that display both large luminescence quantum
yield (Φ) values and strong circularly polarized luminescence
(CPL) activities. Both TbIII complexes are highly emissive,
with Φ values of 0.32 (dpenLI-Tb) and 0.60 (cyLI-Tb). Lumi-
made with chiral IAM ligands. The CPL measurements also
provide additional confirmation of the presence of a single
emissive species in solution in the case of cyLI-Tb, and mul-
tiple emissive species in the case of dpenLI-Tb.
solution reported to date (Φ ≈ 0.60).[9] While chiral IAM
ligands afford CPL-active TbIII complexes that retain the
brightness of the parent achiral forms, their CPL activity is
relatively modest. CPL activity is commonly reported as the
Introduction
Circularly polarized luminescence (CPL), the emission
analog of circular dichroism (CD), combines the sensitivity
of luminescence techniques with the specificity of chirop-
tical spectroscopy.[1–7] The luminescence of lanthanide com-
plexes is especially sensitive to changes in coordination ge-
ometry and molecular environment, and as such, CPL-
active lanthanide complexes are excellent candidates for lu-
minescent probes that can report on their chiral surround-
ings.[8] To maximize the sensitivity of a luminescent lantha-
nide-based chiroptical probe the complex should possess
both a large luminescence quantum yield and strong CPL
activity. Additionally, solubility and stability of the LnIII
complex in aqueous solution is very important for practical
applications. Such systems, however, remain elusive; often
complexes are optimized in respect to either emission inten-
sity or CPL activity, though not both (see below). A system
that combines a large luminescence quantum yield with
strong CPL activity in aqueous solution would represent a
significant advance in the field of LnIII-based CPL.
luminescence dissymmetry factor, glum, defined as glum
=
2(IL – IR)/(IL + IR), where IL and IR are the intensities of
left- and right-polarized emission respectively. Among chi-
ral TbIII-IAM complexes with large Φ values, |glum| values
Յ 0.078 are observed.[10,11] As a strategy to increase CPL
activity we designed chiral IAM ligands that are more rigid
than the previously reported tetrapodal octadenate ligands
since CPL activity had been shown to increase with the con-
formational rigidity of the complex.[12,13] These ligands,
cyLI-IAM and dpenLI-IAM (Figure 1), utilize diaminocy-
clohexane and diphenylethylenediamine backbones respec-
tively. Tetradentate IAM ligands of this type form 2:1 li-
gand/LnIII (ML2) complexes,[14] analogous to the previously
reported octadentate ligands that form 1:1 ligand/LnIII
complexes. Tetradendate ligands offer the advantage of be-
We have previously shown that TbIII complexes of 2-
hydroxyisophthalamide (IAM)-based ligands are exception-
ally bright, displaying some of the highest luminescence
quantum yield (Φ) values of LnIII complexes in aqueous
[a] Department of Chemistry, University of California
Berkeley, CA 94720-1460, USA
Fax: +1-510-486-5283
E-mail: Raymond@socrates.berkeley.edu
[b] Department of Chemistry, San José State University
San José, CA 95192-0101
Figure 1. Chemical structures of cyLI- and dpenLI-IAM ligands.
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3343
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A. P. S. Samuel, J. L. Lunkley, G. Muller, K. N. Raymond
FULL PAPER
ing more readily synthesized than higher denticity ligands 1,2-diaminocyclohexane were each added to 1 over 36 h to
and consequently by utilizing tetradentate ligands we are produce the mono-substituted species, 2a and 2b, respec-
able to easily investigate how small structural changes in tively. These two species were each then combined with
the ligand scaffolds influence the photophysical behavior of methylamine to yield 3a and 3b. The methyl-protected li-
the resulting TbIII complexes. Herein we report the synthe- gands were then both deprotected using BBr3 to yield the
ses of cyLI- and dpenLI-IAM and their TbIII complexes final ligands. The TbIII ML2 complexes (here referred to as
along with their absorption and luminescence properties in cyLI-Tb and dpenLI-Tb) used for the spectroscopic mea-
the presence of both circularly-polarized and non-polarized surements were prepared in situ by combining 1 equiv. of
light.
TbCl3 (in 1  HCl) with 2 equiv. of ligand (in a basic aque-
ous solution) in 0.1  Tris buffered H2O (pH = 7.4). The
complexes were characterized using mass spectrometry
(ES–) in addition to the optical techniques that are de-
scribed in the upcoming sections. The ability to rely on
these optical techniques is especially valuable in the study
of TbIII-IAM complexes since 1H NMR spectroscopy is
problematic because of signal broadening caused by the
paramagnetic TbIII center.[16]
Results and Discussion
CyLI- and dpenLI-IAM were synthesized following the
procedure shown in Scheme 1. By starting with the dithia-
zolide precursor 1[15] one is able to substitute both ends of
the molecule with two distinct amines. Dilute solutions of
(1R,2R)-(+)-1,2-diphenylethylenediamine and (1R,2R)-(+)-
The absorption spectra of cyLI-Tb and dpenLI-Tb dis-
play broad transitions similar to those previously observed
for Tb-IAM complexes in aqueous solution, which are at-
tributed to π-π* transitions in the IAM chromophore (Fig-
ure 2, a).[9,17] As with other Tb-IAM complexes, the broad
absorption bands of cyLI-Tb and dpenLI-Tb allow for exci-
tation with wavelengths up to ca. 390 nm, which is more
favorable for practical biological applications compared to
many other high quantum yield TbIII complexes that re-
quire more damaging higher energy excitation wave-
lengths.[18,19] Additionally, both cyLI-Tb and dpenLI-Tb
show CD activity as expected (Figure 2, b). The CD spectra
show strong Cotton effects[20] for both complexes between
300–380 nm.
Figure 2. Absorption spectra (2a) and CD spectra (2b) of cyLI-Tb
(solid) and dpenLI-Tb (dashed) in aqueous solution [C = 10–5 ,
0.1  Tris (pH = 7.4)] at 298 K.
Scheme 1. Syntheses of cyLI- and dpenLI-IAM ligands.
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Highly Emissive Terbium Complexes in Aqueous Solution
To determine the efficiency of TbIII sensitization, the tained for cyLI-Tb and for one of the two dpenLI-Tb spe-
emission behavior of cyLI-Tb and dpenLI-Tb was investi- cies. The other dpenLI-Tb species, however, has approxi-
gated. A summary of the luminescence quantum yield and mately one water molecule in the inner coordination sphere,
lifetime values is given in Table 1. The luminescence spectra which quenches TbIII emission and contributes to the low
display the characteristic bands corresponding to transi- luminescence quantum yield observed for dpenLI-Tb. The
tions from the 5D4 electronic level to the 7FJ (J = 0–6) overall quantum yield for dpenLI-Tb is the weighted sum
manifold of TbIII (Figure 3). The two complexes show slight of the quantum yields of the q = 0 and q = 1 species. Since
differences in the relative intensities and fine structures of the latter experiences quenching due to bound water, it has
the peaks, which points to variation in the coordination en- a lower quantum yield than a q = 0 species.
vironments experienced by the TbIII ions in each com-
The CPL spectra of cyLI-Tb and dpenLI-Tb are plotted
plex.[21] This difference in coordination environment is sup- in Figure 4 for the magnetic dipole allowed D4 Ǟ7F5 tran-
ported by the luminescence quantum yield (Φ) values for sition. The glum values are summarized in Table 2. The CPL
the two complexes: cyLI-Tb has a quantum yield of 0.60, signals provide additional confirmation that stable chiral
while dpenLI-Tb has a quantum yield of 0.32. The cyLI- species are present in solution. Both spectra display several
Tb value is consistent with the high quantum yield values peaks corresponding to crystal-field splitting; the difference
observed for TbIII complexes with octadentate IAM li- in sign, shape and magnitude of the CPL signal are consis-
5
gands.[9,11,17]
tent with the fact that the two systems do not exhibit the
same crystal field structure in solution. The CPL signal ex-
hibited by cyLI-Tb is similar whether via direct excitation
of the TbIII ion (λex = 488 nm) or indirect excitation via the
ligand absorption bands (λex = 341 nm), which indicates
that the same species in solution is responsible for the CPL
activity detected. Additionally, the glum values obtained for
this complex are the same upon excitation with right-,
left-, and plane-polarized light, which is consistent with the
presence of only one species in solution;[23] had the solution
contained a mixture of diastereomers, the CPL signal would
have been dependent on the polarization of the excitation
beam.[24] In contrast, the glum values obtained for dpenLI-
Tb are dependent on the polarization of the excitation
beam and on whether direct or indirect excitation is used,
which indicates that more than one species in solution is
responsible for the CPL signal detected. These results sup-
port the lifetime data, which indicate the presence of two
emitting species in solution that differ in hydration number
and therefore also coordination environment.
Table 1. Photophysical properties of the TbIII complexes [10–5
(Tris buffer, pH 7.4, λex = 340 nm)].

Complex
QY (Φ) τ (r.t.), ms
q[a] τ (77 K), ms[b]
H2O
D2O
H2O
D2O
cyLI-Tb
dpenLI-Tb
0.60
0.32
1.66
1.51
1.84
1.68
0
0
1.77
1.89
2.05
2.21
(84%) (88%)
0.550 1.68
(16%) (12%)
(37%)
1.89
(63%)
(37%)
2.21
(63%)
1
[a] Calculated from r.t. values using q = 5ϫ(1/τH2O – 1/τD2O
–
0.06).[21] [b] Contained 10% (v/v) glycerol.
Figure 3. Luminescence spectra of cyLI-Tb (solid) and dpenLI-Tb
(dashed) in aqueous solution (0.1  Tris, pH = 7.4) at 298 K nor-
malized to the intensity of the J = 5 peak.
To further investigate this difference in emission intensity
the luminescence lifetimes were measured both in H2O and
D2O at room temperature (r.t.) and at 77 K (Table 1).
Mono-exponential lifetime decays are exhibited by cyLI-
Tb, while dpenLI-Tb exhibits bi-exponential decays, indi-
cating that for the former, only one luminescent species is
present in solution, while for the latter, two luminescent
species are present. The number of bound water molecules
(q) was estimated for each of the complexes based on the
lifetimes measured at room temperature using the formula
Figure 4. CPL (top) and total luminescence spectra (bottom) for
the 5D4 Ǟ7F5 transition for cyLI-Tb (solid) and dpenLI-Tb
developed by Beeby et al.,[22] and q values of zero were ob- (dashed) in aqueous solution.
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A. P. S. Samuel, J. L. Lunkley, G. Muller, K. N. Raymond
FULL PAPER
ppm. 13C NMR (100 MHz, [D6]DMSO): δ = 29.7, 53.6, 57.5, 64.2,
118.7, 120.2, 126.9, 128.3, 131.5, 136.6, 142.4, 158.6, 167.2, 175.6,
201.8 ppm. MS (FAB+): m/z = 771 [MH+].
Table 2. Summary of CPL data for cyLI-Tb and dpenLI-Tb.
Complex
cyLI-Tb
λex
Transition
5D4 Ǟ7F5
λ [nm]
glum
341 nm
542.0
548.6
542.4
545.4
551.0
+0.20
–0.063
+0.16
–0.082
+0.043
3a (Representative Procedure): To
a solution of 2a (0.60 g,
0.89 mmol) in 15 mL of CH2Cl2 was added 1.0 mL of methylamine
(40% aq. soln.). The reaction mixture was stirred at room tempera-
ture for 6 h and was then washed with 1  NaOH. The resulting
off-white residue was applied to a silica column and the product
dpenLI-Tb
343 nm
5D4 Ǟ7F5
1
was eluted with 4% MeOH/96% CH2Cl2; yield 0.402 g (91%). H
NMR (400 MHz, [D6]DMSO): δ = 1.30 (m, 2 H, CHH), 1.44 (m,
2 H, CHH), 1.71 (m, 2 H, CHH), 1.94 (m, 2 H, CHH), 2.76 (d, J
= 5 Hz, 6 H, NHCH3), 3.70 (s, 6 H, OCH3), 3.89 (m, 2 H,
CH2CHNH), 7.17 (t, J = 8 Hz, 2 H, ArH), 7.53 (m, 4 H, ArH),
8.21 (m, 4 H, NH) ppm. 13C NMR (100 MHz, [D6]DMSO): δ =
24.4, 26.2, 31.7, 52.3, 62.4, 123.4, 130.2, 130.4, 131.0, 131.1, 154.8,
165.4, 166.1 ppm. MS (FAB+): m/z = 497 [MH+].
Conclusions
Tetradentate IAM ligands yield highly luminescent TbIII
complexes with large glum values in aqueous solution at
physiologically relevant pH, which may be attributed to the
increased rigidity of the ligand frameworks. Varying the chi-
ral amine that serves as the ligand backbone was found to
impact solvent access to the TbIII center, which was deter-
mined through luminescence lifetime, luminescence quan-
tum yield and CPL measurements. The CyLI-Tb complex
is particularly interesting, since it has an extremely high
1
3b: H NMR (400 MHz, [D6]DMSO): δ = 2.81 (d, J = 4 Hz, 6 H,
NHCH3), 3.80 (s, 6 H, OCH3), 5.51 (m, 2 H, CHNH), 6.95 (t, J =
8 Hz, 2 H, ArH), 7.16 (m, 10 H, ArH), 7.85 (dd, J = 8, 2 Hz, 2 H,
ArH), 7.89 (dd, J = 8, 2 Hz, 2 H, ArH), 8.80 (q, J = 5 Hz, 2 H,
NHCH3), 9.21 (m, 2 H, NH) ppm. 13C NMR (100 MHz, [D6]-
DMSO): δ = 26.0, 57.5, 64.2, 116.7, 118.1, 120.4, 126.8, 127.3,
quantum yield, a glum value over twofold larger than the 127.5, 130.5, 132.4, 138.6, 159.2, 165.6, 168.3 ppm. MS (FAB+):
m/z = 595 [MH+].
largest observed for a comparably emissive Tb-IAM com-
plex, and consists of a single emitting species in solution.
cyLI-IAM (Representative Procedure): A suspension of 3a (0.325 g,
0.66 mmol) in 30 mL of CH2Cl2 was cooled to –78 °C and 1.0 mL
(10.6 mmol) of BBr3 was added. The reaction mixture was warmed
to room temperature and stirred for 48 h. The volatiles were re-
moved under vacuum and a few milliliters of 1  HCl were added,
causing the product to precipitate out of solution. The product
was recrystallized from hot H2O; yield 0.229 g (74%). 1H NMR
(400 MHz, [D6]DMSO): δ = 1.31 (m, 2 H, CHH), 1.48 (m, 2 H,
CHH), 1.73 (m, 2 H, CHH), 1.98 (m, 2 H, CHH), 2.81 (d, J =
4 Hz, 6 H, NHCH3), 3.94 (m, 2 H, CH2CHNH), 6.93 (t, J = 8 Hz,
2 H, ArH), 7.91 (t, J = 9 Hz, 4 H, ArH), 8.61 (m, 2 H,
Experimental Section
General Methods: All chemicals were obtained from commercial
1
suppliers and used without further purification. H and 13C NMR
spectra, elemental analyses, and mass spectra were obtained at the
corresponding analytical facility in the College of Chemistry, Uni-
versity of California, Berkeley.
2a: (2-Methoxy-1,3-phenylene)bis[(2-thioxothiazolidin-3-yl)meth-
anone][15] (1) (5.0 g, 12.5 mmol) was dissolved in 10 mL of CH2Cl2.
CH2CHNH), 8.69 (m, 2 H, NHCH3), 14.71 (s, 2 H, OH) ppm. 13
C
NMR (100 MHz, [D6]DMSO): δ = 24.4, 26.1, 31.5, 52.4, 117.9,
118.0, 118.7, 132.2, 132.7, 159.3, 166.6, 167.7 ppm. MS (FAB+):
m/z = 469 [MH+]. Anal. calcd. (found) for C24H28N4O6·0.5H2O:
C, 60.36 (60.15); H, 6.12 (6.06); N, 11.73 (11.42).
A
250 mL solution of (1R,2R)-(+)-1,2-diaminocyclohexane
(0.128 g, 1.1 mmol) in a 95:5 CH2Cl2/MeOH solution was can-
nulated into the solution of 1 over 36 h. The solvents were then
removed under vacuum and the reaction mixture was dissolved in
CH2Cl2 and washed with 1  NaOH. The product was purified by
silica gel chromatography (2% MeOH/98% CH2Cl2); yield 0.620 g
(84%). 1H NMR (400 MHz, CDCl3): δ = 1.35 (m, 4 H, CH2), 1.74
(m, 2 H, CHH), 2.15 (m, 2 H, CHH), 3.35 (m, 4 H, NCH2CH2S),
3.76 (s, 6 H, OCH3), 3.98 (m, 2 H, CH2CHN), 4.55 (m, 4 H,
NCH2CH2S), 7.07 (t, J = 8 Hz, 2 H, ArH), 7.30 (dd, J = 5, 2 Hz,
2 H, ArH), 7.60 (d, J = 8 Hz, 2 H, NH), 7.89 (dd, J = 8, 2 Hz, 2
H, ArH) ppm. 13C NMR (100 MHz, CDCl3): δ = 24.8, 29.2, 32.6,
53.4, 55.7, 63.1, 124.1, 127.1, 129.1, 132.1, 134.0, 155.7, 165.0,
167.4, 201.3 ppm. MS (FAB+): m/z = 673 [MH+].
cyLI-Tb: [C48H52N8O12Tb]–, MS (ES–): m/z = 1091.2 [M–].
1
dpenLI-IAM: H NMR (400 MHz, [D6]DMSO): δ = 2.83 (d, J =
4 Hz, 6 H, NHCH3), 5.60 (m, 2 H, CHNH), 6.97 (t, J = 8 Hz, 2
H, ArH), 7.20 (m, 10 H, ArH), 7.88 (dd, J = 8, 2 Hz, 2 H, ArH),
7.96 (dd, J = 8, 2 Hz, 2 H, ArH), 8.86 (q, J = 5 Hz, 2 H, NHCH3),
9.25 (m, 2 H, NH), 14.67 (s, 2 H, OH) ppm. 13C NMR (100 MHz,
[D6]DMSO): δ = 26.1, 57.5, 116.9, 118.2, 119.9, 127.2, 127.6, 128.0,
131.5, 133.4, 139.6, 159.3, 165.8, 168.6 ppm. MS (FAB+): m/z =
567 [MH+]. Anal. calcd. (found) for C32H30N4O6·0.5H2O·MeOH:
C, 65.66 (65.46); H, 5.18 (5.35); N, 9.28 (9.48).
2b: Compound 1[15] (7.96 g, 20.0 mmol) was dissolved in 10 mL of
CH2Cl2. A 250 mL solution of (1R,2R)-(+)-1,2-diphenylethylenedi-
amine (0.425 g, 2.0 mmol) in a 95:5 CH2Cl2/MeOH solution was
cannulated into the solution of 1 over 36 h. The solvents were then
removed under vacuum and the reaction mixture was dissolved in
CH2Cl2 and washed with 1  NaOH. The product was purified by
silica gel chromatography (15% EtOAc/85% CH2Cl2); yield 0.470 g
dpenLI-Tb: [C64H56N8O12Tb]–, MS (ES–): m/z = 1287.3 [M–].
UV/Vis Absorption, Circular Dichroism, Emission, and Circularly
Polarized Luminescence Spectra. General Methods: Absorption
spectra were recorded on a Cary 300 UV/Vis spectrophotometer
using a 1-cm quartz cell. Emission spectra were recorded on a
FluoroLog-3 (JobinYvon) fluorimeter using a 1-cm Supracil quartz
δ = 3.30 (m, 4
H, luminescence cell (room-temperature measurements). The TbIII
(30%). 1H NMR (400 MHz, CDCl3):
NCH2CH2S), 3.86 (s, 6 H, OCH3), 4.52 (m, 4 H, NCH2CH2S) 5.50 complexes (10 m) were prepared in situ in 0.1  Tris-buffered H2O
(m, 2 H, CHNH), 6.95 (t, J = 8 Hz, 2 H, ArH), 7.16 (m, 10 H,
ArH), 7.85 (dd, J = 8, 2 Hz, 2 H, ArH), 7.89 (dd, J = 8, 2 Hz, 2
(pH 7.4). The complexes were characterized using mass spectrome-
try (ES–). Quantum yields were determined by the optically dilute
H, ArH), 8.80 (q, J = 5 Hz, 2 H, NHCH3), 9.21 (m, 2 H, NH) method[25] using the following equation:
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Highly Emissive Terbium Complexes in Aqueous Solution
Qx/Qr = [Ar(λr)/Ax(λx)][I(lr)/I(lx)][nx2/nr2][Dx/Dr]
[10] S. Petoud, G. Muller, E. G. Moore, J. Xu, J. Sokolnicki, J. P.
Riehl, U. N. Le, S. M. Cohen, K. N. Raymond, J. Am. Chem.
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[11] M. Seitz, E. G. Moore, A. J. Ingram, G. Muller, K. N. Ray-
mond, J. Am. Chem. Soc. 2007, 129, 15468–15470.
[12] R. S. Dickins, J. A. K. Howard, C. W. Lehmann, J. M. Mo-
loney, D. Parker, R. D. Peacock, Angew. Chem. Int. Ed. Engl.
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[13] J. L. Lunkley, D. Shirotani, K. Yamanari, S. Kaizaki, G.
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[14] A. P. S. Samuel, J. Xu, K. N. Raymond, Inorg. Chem. 2009, 48,
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where 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 intensity. Quinine sulfate
in 1.0  sulfuric acid was used as the reference (Qr = 0.546).[26]
Circularly polarized luminescence and total luminescence spectra
were recorded on an instrument according to literature pro-
cedures,[23,24,27] operating in a differential photon-counting mode.
CPL measurements were performed at 295 K in H2O (0.1  Tris,
pH = 7.4) with analyte concentrations of 10–4 to 10–5 .
[15] S. M. Cohen, B. O’Sullivan, K. N. Raymond, Inorg. Chem.
2000, 39, 4339–4346.
Acknowledgments
[16] Attempts to characterize IAM-TbIII complexes using 1H NMR
spectroscopy yielded spectra with no distinct resonances that
could be assigned unambiguously to IAM protons and thus
these results were uninformative.
This research is supported by the Director of the U. S. Department
of Energy at LBNL, Office of Science, Office of Basic Energy Sci-
ences, and the Division of Chemical Sciences, Geosciences, and
Biosciences (Contract No. DE-AC02-05CH11231. G. M. thanks
the National Institute of Health, Minority Biomedical Research
Support (1 SC3 GM089589-01) and the Henry Dreyfus Teacher-
Scholar Award for financial support.
[17] A. P. S. Samuel, E. G. Moore, M. Melchior, J. Xu, K. N. Ray-
mond, Inorg. Chem. 2007, 46, 7535–7544.
[18] H. Takalo, V.-M. Mukkala, L. Meriö, J.-C. Rodríguez-Ubis, R.
Seadno, O. Juanes, E. Brunet, Helv. Chim. Acta 1997, 80, 372–
387.
[19] M. Xiao, P. R. Selvin, J. Am. Chem. Soc. 2001, 123, 7067–7073.
[20] G. Snatzke, in: Circular Dichroism, 2nd ed. (Eds.: N. Berova,
K. Nakanishi, R. W. Woody), Wiley-VCH, New York, 2000.
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Received: March 17, 2010
Published Online: June 2, 2010
Eur. J. Inorg. Chem. 2010, 3343–3347
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