Purely heterometallic lanthanide(III) macrocycles through controlled assembly of disulfide bonds for dual color emission

Article abstract of DOI:10.1021/ja109157g

Lanthanide complexes based on bis(amides) of die thy lenetria minepentaace tic acid with thiol functionalities are modified with 2,2′-dipyridyl disulfide to give activated complexes that can selectively react with thiol-functionalized complexes to form heterometallic lanthanide macrocycles. The preparation and full characterization of the polyaminocarboxylate ligands N,N″-bis-[p-thiophenyl(aminocarbonyl)]diethylenetriamine-N, N′, N″-triacetic acid (H₃L^X) and the activated N, NPrime;-bis[p-(pyridyldithio)-[phenyl(aminocarbonyl)]]diethylenetriamine-N, N′, N″-triacetic acid (H₃L^y) and the complexes LaL^x, NdL^x, SmL^x, EuL^x, GdL ^x, DyL^x, TbL^x, ErL^x, and YbL ^x are reported. The luminescence properties of the LnL^x complexes emitting in the visible (where Ln = Dy³⁺, Tb³⁺, Eu³⁺, and Sm³⁺) are examined by steady-state and time-resolved photoluminescence, and the triplet state energy level of GdL ^x was estimated to be 24 100 cm^-1 from the 0-0 band of the 77 K phosphorescence spectrum. Near-infrared emission was detected for the NdL^x, YbL^x, and ErL^x complexes, demonstrating the versatility of the thiophenol chromophore. The assembly of purely heterometallic EuTbL^x₂ macrocycles by reaction of EuL ^x with TbI^y was followed by UV-vis absorption spectroscopy, monitoring the characteristic absorption peak of pyridyl-2-thione at 353 nm. Analysis of the solution by mass spectrometry reveals the formation of purely heterometallic macrocycle EuTbL^x₂. This is in contrast with the results obtained by dynamic self-assembly under oxidative conditions, where we observe a statistical mixture of macrocyclic complexes of Eu₂L^x₂, Tb₂L^x ₂, and EuTbL^x₂. The EuTbL^x ₂ macrocycle displays dual color emission, incorporating the characteristicf-ftransitions of Eu³⁺ and Tb³⁺. Investigation into the time-resolved photophysical properties of EuTbL ^x₂ reveals energy transfer from Tb³⁺ to Eu ³⁺, facilitated by the different conformations of the macrocycle in solution.

Full text of DOI:10.1021/ja109157g

ARTICLE
pubs.acs.org/JACS
Purely Heterometallic Lanthanide(III) Macrocycles through Controlled
Assembly of Disulfide Bonds for Dual Color Emission
David J. Lewis, Peter B. Glover, Melissa C. Solomons, and Zoe Pikramenou*
School of Chemistry, The University of Birmingham, Edgbaston B15 2TT, United Kingdom
S Supporting Information
b
ABSTRACT: Lanthanide complexes based on bis(amides) of
diethylenetriaminepentaacetic acid with thiol functionalities are
modified with 2,20-dipyridyl disulfide to give activated complexes
that can selectively react with thiol-functionalized complexes to
form heterometallic lanthanide macrocycles. The preparation and full characterization of the polyaminocarboxylate ligands N,N00-bis-
[p-thiophenyl(aminocarbonyl)]diethylenetriamine-N,N0,N00-triacetic acid (H3Lx) and the activated N,N00-bis[p-(pyridyldithio)-
[phenyl(aminocarbonyl)]]diethylenetriamine-N,N0,N00-triacetic acid (H3Ly) and the complexes LaLx, NdLx, SmLx, EuLx, GdLx,
DyLx, TbLx, ErLx, and YbLx are reported. The luminescence properties of the LnLx complexes emitting in the visible (where Ln =
Dy3þ, Tb3þ, Eu3þ, and Sm3þ) are examined by steady-state and time-resolved photoluminescence, and the triplet state energy level
of GdLx was estimated to be 24 100 cm-1 from the 0-0 band of the 77 K phosphorescence spectrum. Near-infrared emission was
detected for the NdLx, YbLx, and ErLx complexes, demonstrating the versatility of the thiophenol chromophore. The assembly of
purely heterometallic EuTbLx macrocycles by reaction of EuLx with TbLy was followed by UV-vis absorption spectroscopy,
2
monitoring the characteristic absorption peak of pyridyl-2-thione at 353 nm. Analysis of the solution by mass spectrometry reveals
the formation of purely heterometallic macrocycle EuTbLx . This is in contrast with the results obtained by dynamic self-assembly
2
under oxidative conditions, where we observe a statistical mixture of macrocyclic complexes of Eu2Lx , Tb2Lx , and EuTbLx . The
2
2
2
EuTbLx2 macrocycle displays dual color emission, incorporating the characteristic f-f transitions of Eu3þ and Tb3þ. Investigation
into the time-resolved photophysical properties of EuTbLx2 reveals energy transfer from Tb3þ to Eu3þ, facilitated by the different
conformations of the macrocycle in solution.
’ INTRODUCTION
synthesize heterometallic complexes containing two or more dif-
ferent lanthanides, and many attempts using macrocyclic struc-
tures have led to statistical mixtures.8,9
The approaches for heterometallic complexes adopted to
date10-17 rely on either self-assembly by size selectivity of the
lanthanides or on “statistical crystallisation”. The first approach is
based on appropriate ligand design with multidentate pockets
that are commensurate with lanthanide size, while the second
strategy relies on stepwise formation and reactions of stable
complexes.
The design of multifunctional probes is important in molec-
ular multimodal imaging applications where different detection
techniques are employed to achieve high sensitivity and spatial
resolution.1 For example, recent advances highlight the dual
detection advantages of employing a combination of magnetic
resonance imaging (MRI) or X-ray techniques with lumines-
cence to image cells.2-5 Ratiometric luminescent probes that use
different emission colors for analysis of complex biological samples
are also widely used as an approach to multiple signal detection.6,7
Ideally it is desirable that the output signals are generated by
probes incorporated into a single molecular entity to achieve
consistent, targeted delivery of the probe. Lanthanides are widely
used as luminescent probes with the wavelength of the light
emission to be determined by the choice of the lanthanide ion,
whether it is a visible (Sm3þ, Eu3þ, Tb3þ, and Dy3þ) or near-
infrared emitter (Nd3þ, Er3þ, and Yb3þ) and with large Stokes
shift between excitation and emission light. The wide spectrum
of emission and the narrow patterned spectra make them ideal
candidates for ratiometric probes. Additionally, the coordination
preferences of the emissive lanthanide ions are very similar to
Gd3þ, which is known for its activity as a contrast agent in MRI.
Heterometallic lanthanide complexes are therefore popular as they
are potentially able to incorporate luminescent properties with
magnetic ones by judicious choice of metal centers. However, due
to their similar coordination preferences, it is a challenge to
H3Lx and H3Ly (Scheme 1) were designed to provide a hard
base binding core for lanthanide complexation, via three carbox-
ylate and two amide oxygen donors and three backbone nitrogen
donors. Two pendant amide arms provide versatile thiophenol
groups for binding to soft metals or forming disulfide bonds.
Previously, we have reported lanthanide complexes of functional
DTPA-bis(amides) as luminescent sensors,18 taking part in the
self-assembled formation of dinuclear lanthanide complexes,19 in
assembly with platinum moieties,20 and assembled on lumines-
cent gold nanoparticles.21
In this paper we introduce an activated thiol approach in the
thiol-disulfide exchange chemistry for the solution assembly of
LnLn0Lx2 macrocycles. We report the synthesis and characterization
Received: October 12, 2010
Published: December 23, 2010
r
2010 American Chemical Society
1033
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|

Journal of the American Chemical Society
ARTICLE
Scheme 1. Structures of H3Lx, H3Ly, and EuTbLx
2
of H3Lx, H3Ly, and LnLx and the photophysical properties of the
lanthanide complexes together with the properties of the hetero-
metallic EuTbLx2 (Scheme 1).
’ EXPERIMENTAL SECTION
General. All chemicals were purchased from Acros and Sigma-
Aldrich. Solvents were purchased from Sigma-Aldrich or Fisher.
Deuterated solvents were purchased from Goss Scientific or Sigma-
Aldrich and used as received. HPLC-grade solvents were used in
photophysical studies. Water was deionized by use of an Elga Option 3
water purifier. All ligand and complex synthetic procedures were carried
1
out under dinitrogen with degassed anhydrous solvents. H and 13C-
{1H} polarization enhancement nurtured during attached nucleus
testing (PENDANT) NMR spectra were obtained on Br€uker AC 300,
AV 300, AMX 400, AV 400 or DRX 500 spectrometers. Electrospray
mass spectra were recorded on a Micromass LC-TOF machine. Ele-
mental analyses were recorded on a Carlo Erba EA1110 simultaneous
CHN elemental analyzer at the University of Birmingham or by the
CHN Microanalysis Service, School of Pharmacy, University of London.
Photophysical Studies. Steady-state luminescence experiments
were carried out on a Photon Technology Instruments fluorescence
system equipped a 75 W xenon arc lamp as the illumination source;
single excitation and emission monochromators, the latter with grating
blazed at 500 nm; a Hamamatsu R928 photomultiplier tube (PMT) for
visible detection; and a Judson Technologies J 23 InGaAs photodiode
cooled by thermoelectric cooling unit for near-infrared detection. The
NIR signals were amplified by use of a Stanford Research Systems SR510
lock-in amplifier coupled to a PTI OC-4000 optical chopper. Spectra
were recorded by use of PTI Felix fluorescence analysis software. The
77 K emission spectrum was recorded by use of a quartz electron param-
agnetic resonance (EPR) tube fitted with a Young's tap. The peak with
the asterisk in Figure 1 is attributed to scattering of the excitation light.
Quantum yields were measured by the optically dilute relative
method22 by use of the equation
Figure 1. Emissionspectrum of GdLx in methanol at 77 K, λexc = 290 nm
(*, scattering light).
(0.5 mol dm-3) was used as standard for Tb3þ and Dy3þ quantum yield
3
measurements.24
Luminescence lifetimes in the visible were recorded by use of a
Continuum Surelite I Nd:YAG laser (10 Hz) as the excitation source,
with the 355 nm harmonic. Data were recorded on a LeCroy 9350AM
500 MHz oscilloscope as an average of 500 laser pulses, analyzed by use
of Kaleidagraph software and fitted by a nonlinear least-squares iterative
technique (Marquardt-Levenberg algorithm).25
UV-vis absorption spectra were recorded on a Shimadzu UV-
3101PC UV-vis-NIR spectrometer.
Diethylenetriamine-N,N0,N00-triacetic-N,N00-dianhydride
[DTPA-bis(anhydride)]. By a modification to the reported procedure,26
acetic anhydride (65.0 mL, 0.69 mol) was added dropwise to a stirring
suspension of diethylenetriamine-N,N,N0,N00,N00-pentaacetic acid (H5DTPA,
48.0 g, 0.12 mol) in anhydrous pyridine (50 mL) at 65 °C. Stirring was
continued for a further 5 h while the temperature was maintained, after
which time a light brown precipitate was collected by filtration. The
precipitate was washed with acetic anhydride (200 mL) and brought to
reflux in acetonitrile (250 mL) for 1 h. The resulting cream-colored
precipitate was then isolated by filtration and washed with acetonitrile
(2 ꢀ 50 mL) and diethyl ether (2 ꢀ 20 mL) to afford the title compound
(37.7 g, 88%). All characterization data were in agreement with that previ-
ously reported by Wang et al.27 1H NMR (300 MHz, D2O), δ ppm: 3.97
(8H, s, NCH2CO), 3.62 (2H, s, NCH2CO2H), 3.46 (4H, t, 3J = 6.2 Hz,
NCH2CH2N), 3.16 (4H, t, 3J= 6.2 Hz, NCH2CH2N). 13 C{1H} PENDANT
NMR (75 MHz, D2O), δ ppm: 173 (CO2H), 170 (O—CdO),
120.9 (NCH2CO2H), 56.3 (NCH2COO), 52.6 (NCH2CH2N), 49.9
(NCH2CH2N).
!
ꢀ
ꢁꢀ
ꢁ
ꢀ
ꢁ
2
ArðλrÞ
IrðλrÞ
nx
nr2
Dx
Dr
Φx ¼ Φr
AxðλxÞ IxðλxÞ
where Φx is the quantum yield of the lanthanide complex, Φr is the
quantum yield of reference, A(λ) is the absorbance of a sample at
excitation wavelength λ, I(λ) is the relative intensity of excitation light at
wavelength λ, n is the refractive index of the solvent in question, and D is
the integrated area of the emission spectrum for sample and reference
(corrected for PMT response). Ruthenium(II) tris(bipyridine) dichloride
in aerated water23 was used as standard for Eu3þ and Sm3þ quantum
yield measurement. Quinine sulfate in aerated aqueous sulfuric acid
1034
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Journal of the American Chemical Society
ARTICLE
N,N00-Bis[p-thiophenyl(aminocarbonyl)]diethylenetriamine-
N,N0,N00-triacetic Acid (H3Lx). 4-Aminothiophenol (2.00 g, 16.0
mmol) was added to a stirred suspension of DTPA-bis(anhydride) (1.02
g, 2.8 mmol) in anhydrous pyridine (10 mL). The mixture was stirred
under nitrogen for 24 h at room temperature and the slightly cloudy
yellow suspension was filtered. The volume of the solution was reduced
in vacuo to give a thick yellow oil. Water (10 mL) was added to dissolve
the oil, and the crude ligand was isolated by lowering the pH to 3 by
dropwise addition of concentrated HCl. The aqueous layer was decanted
and the isolated hygroscopic solid was washed with water (2 ꢀ 25 mL)
and acetonitrile (2 ꢀ 25 mL) and then stirred in acetonitrile (2 ꢀ
50 mL) until a light yellow powder had formed. The powder was
collected by suction filtration and washed with acetonitrile (2 ꢀ 10 mL)
and diethyl ether (2 ꢀ 20 mL). Hydrazine monohydrate (3.0 mL, 3.1 g,
60.0 mmol) was then added dropwise to a solution of the crude product
in pyridine (20 mL). The solvent was removed under vacuum, and the
resulting oil was dissolved in water (10 mL) and filtered to yield a clear
solution. The solution was acidified to pH 3 by dropwise addition of
HCl. The white solid formed was isolated and washed with acetonitrile
(2 ꢀ 25 mL) and then stirred in acetonitrile (2 ꢀ 50 mL) until a white
powder had formed. The powder was collected by filtration and washed
with acetonitrile (2 ꢀ 10 mL) and diethyl ether (2 ꢀ 20 mL) and then
dried under vacuum to afford H3Lx (0.67 g, 40%). 1H NMR (300 MHz,
MeOH-d4), δ ppm: 7.40 (4H, d, 3J = 8.6 Hz, Hf); 7.13 (4H, d, 3J = 8.6
Hz, Hg); 4.12 (2H, s, Ha); 3.62 (4H, s, Hd); 3.59 (4H, s, He); 3.49 (4H, t,
3J = 5.2 Hz, Hc); 3.22 (4H, t, 3J = 5.2 Hz, Hb). 13C {1H} PENDANT
NMR (75 MHz, MeOH-d4), δ ppm: 172.8 (CO2H, C6), 169.6 (CONH,
C8), 168.1 (CO2H, C1), 135.1 (ArCNHCO, C9), 129.0 (ArCH, C10),
126.0 (ArCSH, C12), 120.5 (ArCH, C11), 57.5 (CH2, C5), 54.6 (CH2,
C7), 53.8 (CH2, C2), 53.3 (CH2, C4), 49.5 (CH2, C3). MS (ES-TOFþ):
m/z 608 {M þ H}þ, 630 {M þ Na}þ. HRMS (ES-TOF) calcd for C26-
H32N5O8S2: 606.1692. Found: 606.1697. Anal. Calcd for C26H33N5-
O8S2(N2H6Cl2)0.6(H2O)2: C 44.2, H 5.8, N 12.3. Found: C 44.4, H 5.9,
N 12.4. The large excess of hydrazine used in the reaction left some of
dihydrochloride after acidic work-up. UV-vis (MeOH) λmax, nm (log
ε) 265 (4.5).
for NdC26H31N5O8S2: 747.0691. Found: 747.0701. UV-vis (MeOH):
λmax, nm (log ε) 265 (4.4).
SmLx. Yield 81%. MS (ES-TOFþ) m/z: 756 {M þ H}þ, 779 {M þ
Na}þ. HRMS (ES-TOF) calcd for SmC26H31N5O8S2: 757.0811.
Found: 757.0806. Anal. Calcd. for SmC26H31N5O8S2KCl(H2O): C:
36.8, H 3.9, N 8.3. Found: C 36.8, H 4.3, N 8.2. UV-vis (MeOH)
λmax, nm (log ε): 266 (4.5).
EuLx. Yield 86%. MS (ES-TOFþ) m/z: 758 {M þ H}þ, 794 {M þ
K}þ. HRMS (ES-TOF) calcd for EuC26H31N5O8S2: 758.0826. Found:
758.0833. UV-vis (MeOH) λmax, nm (log ε): 266 (4.4).
GdLx. Yield 71%. MS (ES-TOFþ) m/z: 763 {MþH}þ. HRMS (ES-
TOF) calcd for GdC26H31N5O8S2: 763.0855. Found: 763.0848. UV-
vis (MeOH) λmax, nm (log ε): 266 (4.3).
TbLx. Yield 86%. MS (ES-TOFþ) m/z: 764 {M þ H}þ. HRMS (ES-
TOF) calcd for TbC26H31N5O8S2: 764.0868. Found: 764.0877. UV-
vis (MeOH) λmax, nm (log ε): 266 (4.6).
DyLx. Yield 65%. MS (ES-TOFþ) m/z: 767 {M þ H}þ, 791 {M þ
Na}þ, 821 {M þ K}þ. HRMS (ES-TOF) calcd for DyC26H31N5O8S2:
769.0906. Found: 769.0921. UV-vis (MeOH) λmax, nm(logε):266(4.4).
ErLx. Yield 27%. MS (ES-TOFþ): m/z 771 {M þ Na}þ, 787 {M þ
K}þ. HRMS (ES-TOF) calcd for NdC26H31N5O9S2: 771.0917. Found:
771.0913. UV-vis (MeOH): λmax, nm (log ε) 265 (4.4).
YbLx. Yield 27%. MS (ES-TOFþ): m/z 779 {M þ H}þ, 802 {M þ
Na}þ, 817 {M þ K}þ. HRMS (ES-TOF) calcd for YbC26H31N5O8S2:
779.1003. Found: 779.0991. UV-vis (MeOH): λmax, nm(logε) 265(4.4).
N,N00-Bis[p-(pyridyldithio)[phenyl(aminocarbonyl)]]diethyl-
enetriamine-N,N0,N00-triacetic Acid (H3Ly). H3Lx (0.30 g, 5.0
mmol) was dissolved in methanol (112 mL). The resulting solution
was then added dropwise over a period of 20 min to a vigorously stirred
solution of 2,20-pyridyl disulfide (0.54 g, 25.0 mmol) in methanol (30 mL),
and the mixture was stirred for a further 30 min, yielding a strongly
colored yellow solution. The solvent was then removed under vacuum at
40 °C, giving a yellow oil that was redissolved in methanol (2 mL), and
to which was added acetonitrile (50 mL) to precipitate crude H3Ly,
which was stirred for 16 h at room temperature. The product was
isolated by filtration and washed with acetonitrile (2 ꢀ 20 mL) and
diethyl ether (10 mL) to yield H3Ly as an initially hygroscopic cream-
colored powder, which was dried under vacuum and stored under a
nitrogen atmosphere (0.17 g, 42%). 1H NMR (300 MHz, MeOH-d4), δ
ppm: 8.41 (2H, m, Hk), 7.77 (4H, m, Hh,I), 7.44 (4H, d, 3J = 8.8 Hz, Hf),
7.31 (4H, d, 3J = 8.8 Hz, Hg), 7.14 (2H, m, Hj), 4.31 (2H, s, Ha), 3.64
(4H, s, Hd), 3.59 (4H, s, He), 3.52 (4H, t, 3J = 5.1 Hz, Hc), 3.22 (4H, t,
3J = 5.1 Hz, Hb). 13C {1H} PENDANT NMR (75 MHz, MeOH-d4), δ
ppm: 175.2 (COOH, C6), 172.5 (CONH, C5), 170.3 (COOH, C1),
161.6 (C, C13), 151.2 (CH, C17), 140.2 (CH, C15), 140.1 (C, C9), 133.0
(C, C12), 131.1 (CH, C10), 123.5 (CH, C14), 123.0 (CH, C11), 122.2
(CH, C16) 59.8 (CH2, C5), 56.9 (CH2, C7), 55.9 (CH2, C4), 55.7 (CH2,
C2), 51.8 (CH2, C3). MS (ES-TOFþ) m/z: 826 {M þ H}þ, 848 {M þ
Na}þ. HRMS (ES-TOF) calcd for C36H39N7O8S4Na: 848.1641. Found:
848.1657. Anal. Calcd for C36H39N7O8S4(N2H6Cl2)0.25(H2O)0.6: C
50.1, H 4.9, N 12.2. Found: C 49.8, H 5.0, N 11.9. UV-vis (MeOH)
λmax, nm (log ε): 262 (4.5), 285(sh) (4.4).
Preparation of LnLx (where Ln = La3þ, Nd3þ, Sm3þ, Eu3þ
,
Gd3þ, Dy3þ, Tb3þ, Er3þ, and Yb3þ). Two procedures, A and B,
were followed. (A) H3Lx (0.25 g, 0.40 mmol) was added to a solution of
LnCl3 xH2O (0.40 mmol) in degassed methanol (3 mL) and sonicated
3
until all the ligand had dissolved. Acetonitrile (30 mL) was added to the
solution yielding the complex as a white precipitate, which was collected
by filtration under N2, washed with acetonitrile and diethyl ether, and
then dried in vacuo. (B) H3Lx (0.20 g, 0.33 mmol) was dissolved in
degassed methanol (5 mL). To the stirred solution was added KOH
(0.055 g, 0.99 mmol) as a methanolic solution (5 mL), and the mixture
was stirred for 10 min. The solvent was then removed under vacuum to
yield the tripotassium salt K3Lx in the reaction vessel, which was redis-
solved in degassed deionized water (5 mL). To the aqueous solution was
added LnCl3 6H2O (1:1 Ln3þ:K3Lx, masses calculated as appropriate
3
for each lanthanide salt). This caused an immediate precipitation of a
white solid, which was isolated by suction filtration and washed with
acetone (2 ꢀ 10 mL) and diethyl ether (5 mL) and then dried under
vacuum to yield LnLx.
’ RESULTS AND DISCUSSION
1
LaLx. Yield 83%. H NMR (300 MHz, MeOH-d4), δ ppm: 7.60-
Synthesis and Characterization of H3Lx and H3Ly. H3Lx
was prepared from the reaction of 4-aminothiophenol with
DTPA-bis(anhydride) in anhydrous pyridine (Scheme 2). The
dual acylation proceeded smoothly to form the desired product
in respectable yield and high purity. Hydrazine monohydrate was
used midsynthesis to reduce any disulfides in the crude product
to thiols, under mild conditions.
7.10 (8H, ArH), 4.50-2.30 (18H, aliphatic CH2). 13C {1H} PENDANT
NMR (75 MHz, MeOH-d4), δ ppm: 182.6-180.5 (CO2H, CONH),
136.2-134.3 (ArC), 130.5, 130.3 (ArCH), 128.6 (ArC), 124.1, 122.8,
122.0 (ArCH), 62.8-51.3 (CH2). MS (ES-TOFþ) m/z: 744 {M þ
H}þ, 766 {M þ Na}þ. HRMS (ES-TOF) calcd for LaC26H31N5O8S2:
744.0685. Found: 744.0678. UV-vis (MeOH) λmax, nm (log ε): 266
(4.4).
1
The 300 MHz H NMR spectrum of H3Lx in methanol-d4
NdLx. Yield 53-80%. MS (ESþ): m/z 749 [M þ H]þ, 771 [M þ
Na]þ. MS (ES-TOFþ): m/z 789 {M þ K}þ. HRMS (ES-TOF) calcd
shows resonances in the aromatic and aliphatic regions of the
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Journal of the American Chemical Society
ARTICLE
Scheme 2. Synthetic Route for H3Lx and H3Ly
Scheme 3. Chemical Structure of LnLx
spectrum (Supporting Information). Although there are several
reports of NMR spectra of DTPA-bis(amide) derivatives,27-34,42,35
few of them have observed the detailed features. Our results agree
with the characteristic backbone features for substituted DTPA-
bis(amides). The aromatic protons Hf and Hg appear at 7.40 and
7.13 ppm, respectively, as 4H doublets (AB system). In the
aliphatic region the central CH2 protons Ha appear as a unique
2H singlet at 4.12 ppm. The protons Hd and He, which corres-
pond to the amide and terminal carboxylic acid “arms”, appear as
two separate 4H singlets, with the carboxylic acid appended Hd
appearing slightly further downfield at 3.62 ppm compared to the
amide-bound He, which appear at 3.59 ppm due to the slightly
more electron-withdrawing carboxylic acid group compared to
the amide. The ethylenic backbone protons Hb and Hc appear at
3.49 and 3.22 ppm, respectively, manifesting themselves as two
separate 4H triplets. The 75 MHz 13C {1H} PENDANT NMR
spectrum of H3Lx in methanol-d4 shows 12 carbon resonances at
the expected chemical shifts. The aromatic region from 110 to
150 ppm displays four signals, two C-H and two quaternary
carbon resonances, consistent with the thiophenol ring systems
appended (Supporting Information). The UV-vis absorption
spectrum of H3Lx in methanol (Supporting Information) shows
a single π-π* band with λmax = 265 nm and a molar absorptivity
coefficient of log ε = 4.5.
species, showing {M þ H}þ and {M þ Na}þ peaks at m/z 826
and 843 respectively with high-resolution spectra confirming the
mass of the ligand to be within 2 ppm of the calculated value. The
UV-vis absorption spectrum of H3Ly (Supporting Information)
gives further evidence to the aromatic modification with a new
shoulder appearing at 285 nm, which is attributed to the π-π*
absorption of the pyridyl group,38 superimposed upon the
original π-π* absorption of the amidophenyl group.
Synthesis and Characterization of LnLx (where Ln = La3þ
,
Nd3þ, Sm3þ, Eu3þ, Gd3þ, Dy3þ, Tb3þ, Er3þ, and Yb3þ). The
lanthanide(III) complexes of H3Lx (Scheme 3) were synthesized
either via preparation of the tripotassium salt K3Lx or via direct
reaction and isolation with the corresponding lanthanide chloride.
Positive-mode electrospray ionization time-of-flight mass
spectrometry (ES-TOF) was used to identify LnLx. Mass spectra
of the lanthanide complexes showed the correct {M þ H}þ peak
in all cases, with small amounts of either the sodium or potassium
adducts, which is a result of the ionization conditions. The
characteristic isotope patterns of the complex peaks in the mass
H3Ly was synthesized by reaction of H3Lx with 5 equiv of 2,20-
pyridyl disulfide (Scheme 1). Reaction was evidenced by the
appearance of a strong yellow color corresponding to the libera-
tion of the chromophore. The reaction was run at a relatively low
concentration with good mixing to avoid generation of sym-
x
metric H6L2 and polymeric species (by competing thiol-
disulfide exchange). NMR spectroscopy was used to monitor
1
the transformation of H3Lx to H3Ly. The H NMR spectrum
(Supporting Information) gave evidence of reaction by the
appearance of pyridyl ring resonances in the aromatic region at
8.41, 7.77, and 7.14 ppm, integrating exactly for 2H, 4H, and 2H.
We have assigned these resonances as Hk, Hh/Hi, and Hj,
respectively, on the basis of previously reported pyridyl sulfide
chemical shifts.36,37 The formation of the two disulfide bonds is
evidenced by the shift of the proton Hg from 7.13 ppm in H3Lx to
7.31 ppm in H3Ly. The 75 MHz 13C {1H} PENDANT NMR
spectrum of H3Ly (Supporting Information) gave further verifi-
cation of ligand formation with the appearance of resonances
corresponding to C13, C17, C15, C14, and C16 at 161.6, 151.2,
140.2, 123.5, and 122.6 ppm, respectively. The latter assignments
were made by correlation with the 13C {1H} PENDANT NMR
spectrum of the 2,20-pyridyl disulfide starting material. Positive-
mode electrospray mass spectrometry identified H3Ly as the sole
spectra can be used as a signature of the incorporation of Ln3þ
;
all complexes displayed the correct isotope patterns. Electrospray
ionization high-resolution mass spectrometry was used to obtain
the exact monoisotopic masses of LnLx. The high-resolution
mass of the complexes matched the theoretical mass within (5
ppm in all cases.
As a result of its diamagnetic nature, it was possible to further
study LaLx by NMR spectroscopy. The 300 MHz H NMR
1
spectrum of LaLx in methanol-d4 displays two distinct areas of
resonances (Supporting Information). The aromatic region of
the spectrum shows manifold peaks ranging from 7.60 to 7.10
ppm. This manifold corresponds to the aromatic CH protons on
the amidothiophenol ring system and integrates for eight pro-
tons. The aliphatic region displays the same pseudomultiplet
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Journal of the American Chemical Society
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appearance with broad resonances ranging from 4.50 to 2.30
ppm; these correspond to the aliphatic CH2 protons on the
DTPA backbone and integrate for 18H in total if integration of
the methanol solvent signal is subtracted. The resonances are
broadened33,35,39 by the presence of rapidly interchanging rota-
mers of the ligand backbone surrounding the lanthanide metal
center in solution. In the 75 MHz 13C{1H} PENDANT NMR
spectrum of LaLx (Supporting Information), resonances are
observed in the expected regions: carboxylic acid and amide
resonances between 190 and 160 ppm, aromatic resonances
between 140 and 110 ppm, and aliphatic resonances between 70
and 45 ppm.
energy levels, and the reasons for this are two-fold: (i) The heavy
Gd3þ ion increases the rate of intersystem crossing from singlet
to triplet (the so-called “internal heavy atom effect”) in the
ligand, and (ii) Gd3þ has no accessible energy levels into which
the ligand can subsequently transfer energy. Hence, radiative
decay from an excited triplet state becomes the predominant
photophysical pathway. GdLx exhibits ligand phosphorescence
as a broad band centered at 440 nm after excitation at 290 nm in
degassed methanol at 77 K (Figure 1). The spectrum clearly
shows the vibrational structure of the phosphorescence centered
at 440 nm.
The triplet-state energy level was calculated to be 24 100 cm-1
from the (0-0) transition, comparable with the triplet energy
levels observed for benzene (29 500 cm-1) and aminobenzene
(24 800 cm-1) in polar solvents.40 Calculation of the triplet
energy of GdLx shows that the level is well-positioned for energy
transfer into a range of visible-emitting lanthanide luminescent
states,41,42 including those of Eu3þ (5D0, 17 300 cm-1), Sm3þ
(4G5/2, 17 900 cm-1), Tb3þ(5D4, 20 500 cm-1), and Dy3þ
(4F9/2, 21 100 cm-1), and yet acting as donor for the NIR-
emitting states of Nd3þ (11 200 cm-1), Yb3þ (10 300 cm-1),
and Er3þ (6500 cm-1). The excitation spectrum of TbLx
monitored at the strongest f-f transition at 545 nm (5D4 f
7F5) shows a broad band with λmax at 266 nm (Figure 2). This
closely resembles the profile of the absorption spectrum, which
demonstrates that an energy transfer from the two phenyl groups
sensitizes the lanthanide luminescence exhibited by TbLx. Simi-
lar excitation profiles are observed by monitoring emission of
EuLx at 614 nm (5D0 f 7F2), SmLx at 596 nm (4G5/2 f 6H7/2),
and DyLx at 484 nm (4F9/2 f 6H15/2).
We used the GdLx complex to probe the triplet energy level of
the aromatic thiophenyl sensitizer to assess its efficiency in the
energy-transfer process to the lanthanides. Gadolinium(III)
complexes are a popular choice for elucidating ligand triplet
The emission spectra of all four visible emitting complexes
show typical narrow-band lanthanide luminescence after excita-
tion into the ligand-based phenyl π-π* absorption band at 266
nm (Figure 3). Irradiation of SmLx with UV light at 266 nm leads
to pink emission assigned to the 4G5/2 f 6HJ transitions of Sm3þ
5
7
9
(where J = /2, /2, and /2). EuLx, TbLx, and DyLx are also
emissive, with the characteristic bands observed for each complex
after excitation at 266 nm. EuLx displays red emission that is
Figure 2. Absorption (dashed line) and excitation spectra with λem
=
545 nm (solid line) of TbLx. The excitation spectrum is corrected for
lamp response.
Figure 3. Normalized emission spectra of (a) TbLx (solid line) and EuLx (dashed line) in methanol, λexc = 266 nm, and (b) SmLx (solid line) and DyLx
(dashed line) in methanol, λexc = 266 nm.
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Table 1. Luminescence Lifetimes and Number of
Inner-Sphere Methanol Molecules for LnLx
LnLx
τMeOH(ms)
τMeOD(ms)
m
EuLx
TbLx
SmLx
DyLx
0.84
1.43
1.75
1.98
1.3 ( 0.5
1.6 ( 0.5
1.5 ( 0.5
0.014
0.018
0.025
0.024
assigned to the 5D0 f 7FJ transitions of Eu3þ (J = 0, 1, 2, 3, and
4) at λ = 578, 590, 613, 650, and 695 nm, respectively. Excitation
of TbLx at 266 nm leads to green emission from the 5D4 f 7FJ
transitions of Tb3þ (J = 6, 5, 4, 3, 2, 1, and 0) at λ = 487, 545, 585,
621, 650, 668, and 680 nm, respectively. Similarly, through-
ligand sensitization of DyLx leads to typical yellow Dy3þ emis-
sion from the 4F9/2 f 6HJ transitions (J = 15/2, 13/2, and 11/2) at
λ = 484, 577, and 660 nm, respectively.
The spectra of the isolated complexes agree with those
obtained when the complexes were formed in situ with addition
of equimolar solution of the lanthanide chloride into solution of
H3Lx. The solutions formed in situ showed the characteristic
complex peaks in the electrospray mass spectra, in agreement
with the isolated complexes.
The rate of decay of each luminescent state was measured by
time-resolved luminescence spectroscopy and the lifetimes are
reported in Table 1. In all cases, the measurement of luminescence
lifetimes led to monoexponential fittings of the decay curve, indi-
cating the presence of a single lanthanide coordination environment.
Calculation of the number of inner-sphere methanol mole-
cules,43 m, by use of eq 1 led to an average of 1.5 ( 0.5 per LnLx:
Figure 4. Emission spectra of NdLx (solid line), YbLx (dashed line),
and ErLx (dotted line) in methanol-d1, λexc = 265 nm.
values take into account the efficiency of the overall energy-
transfer process from the phenyl sensitizer to the lanthanide ion,
affected by the rather high level of the triplet state of the sensitizer
(24 100 cm-1), responsible for the energy transfer, in compar-
ison with the lanthanide excited state; the sensitizer's distance
from the luminescent center; and the presence of other deactiva-
tion processes originating from charge-transfer states or other
nonradiative states and vibronic coupling. Clearly, the phenyl
sensitizer may not an have ideally matched triplet state with the
lanthanide, and also the heavy atom effect may not be as efficient
in promoting the population of the triplet state due to the
distance between the lanthanide center and the sensitizer. For
DyLx and SmLx, the quantum yields were measured to be 0.03%
and 0.02%, respectively; the low values are not surprising since
their luminescence signal is affected by additional quenching mecha-
nisms as aforementioned in the discussion of the lifetimes.48
The triplet state of the 4-amidothiophenol sensitizer also
proved suitable for sensitization of a range of NIR-emitting
lanthanide ions. Luminescence spectra of NdLx, ErLx, and YbLx
after excitation into the thiophenol π-π* transition displayed
typical narrow-band emission in the NIR region (Figure 4).
Irradiation of NdLx at 265 nm leads to characteristic emission
bands at 890, 1064, and 1340 nm, which correspond to the
4F3/2 f 4IJ (J = 9/2, 11/2, and 13/2) f-f transitions of the Nd3þ
ion. Similarly, excitation of YbLx at 265 nm leads to a single
m ¼ AðkMeOH - kMeOD
Þ
ð1Þ
where A is an experimentally determined lanthanide-specific
quenching constant that reflects the susceptibility to radiationless
vibrational decay from hydroxyl oscillators for a certain lantha-
nide ion (A = 8.4, 2.1, and 0.05 ms for Tb3þ, Eu3þ, and
Sm3þ 43,44
and k is the rate of luminescence decay (reciprocal
)
luminescence lifetime), per millisecond, in deuterated and non-
deuterated solvents. The noninteger values obtained for LnLx
reflect both the quenching contribution of outer-sphere solvent
molecules and the possibility of some molecules bearing one or
two coordinated solvent molecules. The reported crystal struc-
tures of the ethyl-34 and benzyl-32 substituted DTPA-bis(amide)
complexes show one solvent molecule to be bound on the
lanthanide center. The lower values of the luminescence lifetimes
for SmLx and DyLx are attributed to the additional deactivation
pathways of their excited states. The vibrational deactivation
of the luminescent state by hydroxyl and amide oscillators
(νOH = 3300-3500 cm-1, νNH = 3100-3300 cm-1) is particularly
efficient due to the small energy gap between the luminescent-
state and ground-state manifolds: those lanthanides with smaller
energy gaps (e.g., Dy3þ and Sm3þ) are quenched more efficiently
than for lanthanides with larger energy gaps (e.g., Eu3þ and
Tb3þ) due to better vibronic coupling as dictated by the energy
gap law and Franck-Condon factors.45 The shorter lifetime of
EuLx compared to TbLx is attributed to the presence of an ligand
to metal charge transfer (LMCT) state in the easily reduced
europium complex (E0 = -0.35 V) from the carboxylate ligands
that quench the europium excited state.46,47
2
2
emission band at 993 nm, corresponding to the F5/2 f F7/2
transition of the Yb3þ ion. Excitation of ErLx at 265 nm also leads
to NIR emission, with a single band centered at 1535 nm,
assigned to the 4I13/2 f 4I15/2 transition of the Er3þ ion. These
steady-state luminescence experiments were performed in deute-
rated methanol-d1 to avoid quenching by hydroxyl oscillators.48,49
To confirm that the transfer of energy from the thiophenol
antenna chromophore to the lanthanide ion was responsible for
the luminescence observed, excitation spectra were recorded for
all the NIR-emitting complexes. Monitoring of YbLx, NdLx, and
ErLx at their strongest emission bands (980, 1060, and 1530 nm,
respectively) led to excitation spectra with profiles similar to
those of the UV-vis absorption spectra (Supporting In-
formation). It is worthwhile noting that, in contrast to the
excitation spectra recorded for the visible-emitting complexes,
the bathochromic wavelengths of the π-π* absorption band are
relatively more effective in sensitizing the emission from the
lanthanide ion, and as a result the excitation band is much broader in
The luminescence quantum yields of TbLx and EuLx in
methanol were found to be 0.32% and 0.04%, respectively. The
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Scheme 4. Heterometallic Macrocycle Assembly in Solution by Use of Activated Thiols
the cases of the NIR-emitting complexes than for the visible
complexes.
Controlled Assembly of Heterometallic LnLn0Lx2 Dimers.
We introduce an activated thiol in the thiol-disulfide exchange
chemistry for the solution assembly of LnLn0Lx macrocycles
2
(Scheme 4). Converse to the usual reversible mechanism of
thiol-disulfide exchange, observed in such biological processes
as protein folding and recently exploited in dynamic combina-
torial libraries,50-52 the activation reaction does not favor the
reverse reaction.53 This is due to the dominance of the 2-pyr-
idylthione tautomer at room temperature,52 which cannot be fed
back in to the reverse pathway as a thiol would. The release of the
2-pyridylthione chromophore leads to a rise in absorption at 353
nm in methanol. Utilizing this chemistry we were able to
assemble europium-terbium macrocycles in solution, monitor-
ing the controlled assembly by UV-vis spectroscopy, and using
electrospray mass spectrometry and luminescence spectroscopy
Figure 5. UV-vis absorption spectra of the solution containing EuLx
(25 μmol dm-3) with TbLy (25 μmol dm-3) at time intervals of 0, 5,
10, 15, 20, 25, and 30 min, where 0 min is the mixing of the two solutions.
Inset: Plot of absorbance at 353 nm vs time for the same experiment.
for their full solution characterization.
3
3
Heterometallic EuTbLx macrocycles (Scheme 4) were
2
formed in methanolic solution by reaction of a solution of EuLx
(25 μmol dm-3) and an equimolar solution of the activated
3
TbLy. The reaction was monitored by UV-vis absorption
spectroscopy at time intervals of 0, 5, 10, 15, 20, 25, and 30
min (Figure 5). The appearance of the 353 nm peak is char-
acteristic of the presence of the pyridyl-2-thione chromophore. A
plot of the increasing absorbance values at 353 nm versus time
shows a sharp increase followed by a shallow curve profile, which
plateaus when absorbance reaches about 0.15, demonstrating the
release of the pyridyl-2-thione and the binding event of EuLx and
TbLy (Figure 5, inset). This two-step profile agrees with a fast
initial binding followed by a slow reorganization step of macro-
cycle closure. Furthermore, the binding of the two lanthanide
complexes is confirmed by the observation of the isosbestic points.
The final absorbance of the solution at 353 nm is in agreement
with the release of 25 μmol dm-3 solution of 2-pyridylthione as
3
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Journal of the American Chemical Society
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calculated by its molar absorptivity, ε, at 353 nm (Supporting
Information). In this case the binding is clearly weaker and the
curve does not show the two-step profile. As a control experi-
ment, we also examined the reactivity of a solution of 25
heterometallic EuTbLx2 macrocycle (Figure 6a). No homome-
tallic species were observed in this mass spectrum. We examined
the statistical assembly of the species EuTbLx2 to contrast with
the controlled formation of the EuTbLx2 by reacting EuLx and
μmol dm-3 H3Ly with 6 equiv of 4-acetamidothiophenol, which
TbLx (25 μmol dm-3) under mild dimethyl sulfoxide co-oxidation
3
3
led to the displacement of 2-pyridylthione with the expected final
absorbance at 353 nm of around A = 0.3 (Supporting In-
formation). It is worth noting that the binding in this case is
weaker and does not show the two-step effect.
conditions (20% DMSO in methanol or N,N-dimethylformamide,
DMF) (Scheme 5).54 The mass spectral analysis of this solu-
tion indicated the presence of all three possible Eu2Lx ,
2
EuTbLx , and Tb2Lx2 macrocyclic species with peak manifolds
2
The final absorbance of the solution at 353 nm is in agreement
of the doubly charged {M þ 2H}2þ ions at m/z 756, 759, and
762, respectively (Figure 6b). The individual isotope patterns
serve to act as a signature of double lanthanide incorporation in
the macrocycles.
with the release of 25 μmol dm-3 solution of 2-pyridylthione as
3
calculated by its molar absorptivity, ε, at 353 nm (Supporting
Information). In this case the binding is clearly weaker and the
curve does not show the two-step profile. As a control experi-
ment we also examined the reactivity of a solution of 25
Solutions of EuTbLx show typical narrow-band lanthanide
2
luminescence upon excitation at 266 nm (Figure 7). Emission
bands originating from both the europium and terbium centers
are observed as expected for a complex incorporating both metal
centers. This leads to an interesting emission spectrum ranging
from 450 to 750 nm, which is attractive with respect to the
optoelectronic ability of the complex to be tuned to a desired
emission wavelength for certain applications in luminescent
sensing or in optical devices.
μmol dm-3 H3Ly with 6 equiv of 4-acetamidothiophenol which
3
led to the displacement of 2-pyridylthione with the expected final
absorbance at 353 nm of around A = 0.3 (Supporting In-
formation). It is worth noting that the binding in this case is
weaker and does not show the two-step effect.
Positive-mode electrospray mass spectrometry was used to
examine the formation of heterometallic macrocycles in solution
(Figure 6). The controlled assembly of EuTbLx , by our approach
In order to examine whether there is communication between
Tb3þ and Eu3þ centers in the form of photoinduced energy
transfer (the 5D4 state of Tb3þ lies 3200 cm-1 higher than the
2
as indicated in Scheme 4, gave a single peak manifold centered at
m/z 781 corresponding to the doubly charged {M þ H þ Na}2þ
Figure 6. ES-MSþ of macrocyclic lanthanide species prepared by (a)
controlled assembly of EuTbLx2 with the activated thiol approach or (b)
statistical assembly under mild oxidative conditions, leading to peaks for
Eu2Lx , EuTbLx , and Tb2Lx .
Figure 7. Emission spectrum of EuTbLx2 in methanol, λexc = 266 nm.
Bands arising from Tb3þ emission assigned to 5D4 f 7FJ (J = 6-3) are
indicated by daggers (†). Bands arising from Eu3þ emission assigned to
5D0 f 7FJ (J = 0-4) are indicated by asterisks (*).
2
2
2
Scheme 5. Activated Thiol Assembly versus Statistical Assembly for Heterometallic Lanthanide Complexes
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Journal of the American Chemical Society
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5D0 of Eu3þ), we used time-resolved luminescence spectroscopy
by monitoring the 5D4 f 7F6 emission band of the Tb3þ ion as a
potential energy donor (Scheme 6). The energy transfer steps
ET1 and ET2 (Scheme 6) are the ones taking place from the
phenyl chromophore to the lanthanide and the processes are
known to occur on the picosecond time scale;55 hence they are
not anticipated to affect the intralanthanide energy transfer
step ET3. The luminescence lifetime of a methanolic solution of
EuTbLx2 was found to be 0.5 ms, reduced by two-thirds in
comparison with 1.5 ms in homometallic Tb2Lx . This deactiva-
Scheme 6. Simplified Energy Diagram of Relevant Energy Levels
2
That Participate in the Energy Transfer from Tb3þ to Eu3þ
tion of the luminescent 5D4 state in Tb3þ can only be attributed
to a photoinduced process taking place between terbium and
europium ions. The luminescence signal of EuLx was monitored
at the position of the 5D4 f7F6 transition of Tb3þ under time-
resolved conditions as a control experiment. As expected, no
emission signal was detected at this wavelength, which showed
that the shortened lifetime in EuTbLx2 at 487 nm has no contri-
bution from direct Eu3þ luminescence and must be caused purely
by a reduction of Tb3þ luminescent lifetime. Although electron
transfer may also be a possibility between metal centers, previous
studies on energy transfer between pairs of Eu3þ and Tb3þ ions in
free solution,56 as well as those encapsulated in ligand frame-
works57,58 and protein scaffolds,59 tend to agree that energy trans-
fer from Tb3þ to Eu3þ proceeds through nonradiative mecha-
nisms and it is the most favorable step; indeed, energy transfer
between lanthanide centers is commonly attributed to a dipole-
dipole interaction, which is anticipated in fundamental terms due
to the buried nature of the f orbitals in the lanthanide ions.
The energy-transfer efficiency (η) for the purely dipole-dipole,
through-space energy-transfer mechanism can be quantified58 by
eqs 2 and 3:
6
η ¼ 1=½1 þ ðR=R0Þ ꢁ
η ¼ 1 - ðτ=τ0Þ
ð2Þ
ð3Þ
Figure 8. Models of EuTbLx2 macrocycle: (a) in protracted pseudo-exo state with trans-disulfides and (b) in pseudo-endo state with cis-disulfide bridges.
Distances between lanthanide centers are given in angstroms.
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Journal of the American Chemical Society
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where R0 is the distance required for 50% energy transfer, R is the
actual distance between donor and acceptor, and τ and τ0 are the
lifetimes of luminescence of the donor in the presence and
absence of acceptor. The value for R0 usually lies between 8 and
10 Å for Eu3þ-Tb3þ pairs in helicates58 and is quoted as 8.31 Å
for the free ions in dimethyl sulfoxide solution.56 An MM2
used in further work for biological or surface-binding applica-
tions. Our versatile ligand design (H3Lx) and the use of activated
thiol chemistry (H3Ly) enabled us to produce a new bimetallic
photoactive molecule (EuTbLx ) from monofunctional photo-
2
active building blocks. The EuTbLx macrocycle displays two-
2
color emission with a weak energy-transfer process operating
between the lanthanide centers. The potential of the assembly of
mononuclear lanthanide complexes to binuclear systems under
mild conditions is currently being explored for the development
of multimodal and dual-color probes, opening new possibilities
for choosing metals with attractive magnetic or radioactive pro-
perties in conjunction with luminescent centers.
energy minimization60 of a model of EuTbLx led to a protracted
2
complex with an interlanthanide distance of 17.8 Å (Figure 8a).
Using this value of 17.8 Å for R and an average value of 9.0 Å for
R0 with eq 2, we obtain a theoretical energy-transfer efficiency of
η = 1.6%. However, use of eq 3 shows that from the lifetime
measurements we experimentally estimate an energy-transfer
efficiency of η = 66% when τ = 0.5 ms and τ0 = 1.5 ms. The
observed efficiency of energy transfer does not correlate, there-
fore, with the theoretical value. It can be deduced that either (i)
the energy-transfer mechanism is not dipole-dipole but some
kind of through-bond interaction requiring orbital overlap or
multipole interactions (the latter has been observed for free Eu3þ
and Tb3þ in solution61) or (ii) the energy transfer occurs by
molecular distortions of the ligand framework in solution,
reducing the interlanthanide distance R and hence facilitating
more efficient energy transfer. The first case is less likely, on the
basis of the poor orbital overlap of the ligand framework pres-
ented, as the disulfide bridges between the lanthanide centers act
as insulating spacers between the separate ligand π systems; the
buried nature of the 4f orbitals also somewhat precludes the
orbital overlap required for the electron-exchange type energy-
transfer mechanism. The second case must lead to the conclusion
that an alternative structure where the lanthanide centers are in
closer proximity must exist in solution; this alternative structure
would therefore be predominantly responsible for the energy-
transfer efficiencies observed. When the dihedral bond angles of
both C-S-S bonds are set to 104° (as per experimentally
observed disulfide bonds)62 and the rest of the minimization is
retained in the configuration calculated for the protracted state
(pseudo-exo-macrocycle), the result was a pseudo-endo-macro-
cycle with a cis-cis conformation of the disulfide bridges and an
interlanthanide distance of 7.8 Å (Figure 8b)
’ ASSOCIATED CONTENT
S
Supporting Information. Fourteen figures showing NMR
b
spectra of H3Lx, H3Ly, and LaLx; UV-vis absorption spectra of
H3Lx, H3Ly, and NdLx; excitation spectra of NdLx, YbLx, and
ErLx; emission spectra of statistically formed EuTbLx ; and
2
UV-vis absorption spectra monitoring thiol reactivity. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
z.pikramenou@bham.ac.uk
’ ACKNOWLEDGMENT
We thank BBSRC (D.J.L.) and EPSRC (M.C.S., P.B.G) for
funding ofthe studentships, together with the School of Chemistry
at the University of Birmingham. Some of the spectrometers used
in this research were obtained, through Birmingham Science City:
Innovative Uses for Advanced Materials in the Modern World
(West Midlands Centre for Advanced Materials Project 2), with
support from Advantage West Midlands (AWM) and partial
funding by the European Regional Development Fund (ERDF).
Calculation of the energy-transfer efficiency between the
lanthanide centers in the pseudo-endo-macrocycle model using
values of 7.8 Å for R and 9.0 Å for R0 with eq 2 leads to a value of
η = 70%. This efficiency is very similar to the value calculated
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Kenwright, A. M.; Faulkner, S. J. Am. Chem. Soc. 2009, 131, 9916–9917.
’ CONCLUSION
We have synthesized and characterized a novel and versatile
ligand, H3Lx, based on the DTPA-bis(amide) motif for lanthanide-
(III) complexation incorporating thiol pendant arms that can be
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Journal of the American Chemical Society
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