Azulene-moiety-based ligand for the efficient sensitization of four near-infrared luminescent lanthanide cations: Nd3+, Er3+, Tm3+, and Yb3+

Article abstract of DOI:10.1002/chem.200701068

The ML₄ complexes formed by reaction between the bidentate azulene-based ligand diethyl 2-hydroxyazulene-1,3-dicarboxylate (HAz) and several lanthanide cations (Pr³⁺, Nd³⁺, Gd³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, and Lu ³⁺) have been synthesized and characterized by elemental analysis, FT-IR vibrational spectroscopy and electrospray ionization mass spectroscopy, Spectrophotometric titrations have revealed that four Az ligands react with one lanthanide cation to form the ML₄ complex in solution. Studies of the luminescence properties of these ML₄ complexes demonstrated that Az is an efficient sensitizer for four different near-infrared emitting lanthanide cations (Nd³⁺, Er³⁺, Tm³⁺, and Yb ³⁺); the resulting complexes have high quantum yield values in CH₃CN. The near-infrared emission arising from Tm³⁺ is especially interesting for biologic imaging and bioanalytical applications since biological systems have minimal interaction with photons at this wavelength. Hydration numbers, representing the number of water molecules bound to the lanthanide cations, were obtained through luminescence lifetime measurements and indicated that no molecules of water/solvent are bound to the lanthanide cation in the ML₄ complex in solution. The four coordinated ligands protect well the central luminescent lanthanide cation against non-radiative deactivation from solvent molecules.

Full text of DOI:10.1002/chem.200701068

DOI: 10.1002/chem.200701068
Azulene-Moiety-Based Ligand for the Efficient Sensitization of Four Near-
Infrared Luminescent Lanthanide Cations: Nd3+, Er3+, Tm3+, and Yb3+
Jian Zhang and StØphane Petoud*[a]
Abstract: The ML4 complexes formed
by reaction between the bidentate azu-
lene-based ligand diethyl 2-hydroxya-
complexes demonstrated that Azꢀ is an
efficient sensitizer for four different
near-infrared emitting lanthanide cat-
ions (Nd3+,Er 3+,Tm 3+,and Yb 3+);
the resulting complexes have high
quantum yield values in CH3CN. The
near-infrared emission arising from
Tm3+ is especially interesting for bio-
logic imaging and bioanalytical applica-
tions since biological systems have min-
imal interaction with photons at this
wavelength. Hydration numbers,repre-
senting the number of water molecules
bound to the lanthanide cations,were
obtained through luminescence lifetime
measurements and indicated that no
molecules of water/solvent are bound
to the lanthanide cation in the ML4
complex in solution. The four coordi-
nated ligands protect well the central
luminescent lanthanide cation against
non-radiative deactivation from solvent
molecules.
zulene-1,3-dicarboxylate (HAz) and
3+
several lanthanide cations (Pr3+,Nd
,
Gd3+,Ho 3+,Er 3+,Tm 3+,Yb 3+,and
Lu3+) have been synthesized and char-
acterized by elemental analysis,FT-IR
vibrational spectroscopy and electro-
spray ionization mass spectroscopy.
Spectrophotometric titrations have re-
vealed that four Azꢀ ligands react with
one lanthanide cation to form the ML4
complex in solution. Studies of the lu-
minescence properties of these ML4
Keywords: azulenes · lanthanides ·
luminescence · near-infrared spec-
troscopy · sensitizers
Introduction
communication where the electronic structure of lanthanide
3+
ions such as Er3+,Nd
and Ho3+ can be used as the active
The development of near-infrared (NIR) emitting lantha-
nide complexes in both aqueous and non-aqueous media
(with Ln3+ =Nd3+,Yb 3+,Er 3+,Ho 3+,Tm 3+,and Pr 3+) is an
increasingly active research area in recent years.[1–23] As an
example,NIR emission is advantageous for biological appli-
cations because i) NIR photons scatter less than visible pho-
tons for improved biological imaging resolution),[24] ii) bio-
logical systems have low native autofluorescence in the NIR
domain for better signal-to-noise ratio and corresponding
detection sensitivity,[25] and iii) biological tissues are almost
material for optical amplifiers of NIR signals.[29,30]
The luminescence signals of lanthanide ions originate
from their f–f electronic transitions within the partially filled
4f orbitals. The electronic levels of lanthanides are hardly
affected by their surrounding environment because of the
shielding effect of the 4f electrons by the filled 5s and 5p or-
bitals.[1,2] These transitions are spin-forbidden,resulting in
emissions that appear as sharp line-like bands,a useful ad-
vantage for spectral discrimination between signals of the
sample from the background fluorescence for improved
signal-to-noise ratio and detection sensitivity. Free lantha-
nide cations have low absorption coefficients and long in-
trinsic luminescence lifetimes (“radiative lifetimes” from mi-
croseconds to milliseconds).[1,2] In order to generate suffi-
cient emission signal for sensitive detection,lanthanide cat-
ions need to be sensitized with a suitable antenna that pos-
sesses the appropriate electronic structure[31] and efficiently
protected from high energy vibrations to prevent the non-ra-
diative deactivation of the lanthanide luminescence. The
sensitization process of luminescent lanthanide cations in co-
ordination complexes formed with chromophoric ligands
can usually be described as resulting from the excitation of
transparent to NIR light (NIR photons travel deep (several
centimeters) in tissues,organs or organisms).
[25,26]
NIR lan-
thanide luminescence can also be advantageously used in
NIR organic light-emitting diode technology[27,28] and tele-
[a] J. Zhang,Prof. Dr. S. Petoud
Department of Chemistry,University of Pittsburgh
219 Parkman Ave.,Pittsburgh,PA 15260 (USA)
Fax : (+1)412-624-8611
E-mail: spetoud@pitt.edu
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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Chem. Eur. J. 2008, 14,1264 – 1272

FULL PAPER
the sensitizer (antenna) by absorption of photons through
the singlet states of the chromophoric group,followed by
the internal ligand-centered energy conversion via intersys-
tem crossing from the singlet to the triplet state. The energy
is then transferred from the triplet state of the ligand to the
accepting levels of the lanthanide ions. Energy transfer from
the singlet state to the lanthanide is also possible.[17,32,33] Be-
cause the accepting energy levels of NIR emitting lantha-
nide ions are generally located at lower energy compared
with visible emitting lanthanides,we hypothesized that effi-
cient sensitizing ligands for NIR emitting lanthanide cations
should have a triplet state located at relatively lower energy.
As reference,we have previously studied the tropolonate
(Trop-) ligand which possesses a triplet state located at an
energy of 17200 cmꢀ1 (580 nm) for the sensitization of sever-
al NIR emitting lanthanide ions.[34]
Scheme 2. Structure of the ligand HAz and proposed bidentate coordina-
tion mode of Azꢀ with lanthanide ion.
Azulene is a non-benzenoid aromatic molecule; its name
is derived from the Spanish word “azul”,meaning blue. It is
a dark blue crystalline solid used in many cosmetics. Azu-
lene is an isomer of naphthalene but its photophysical prop-
erties are significantly different. Its structure consists of a
cyclopentadiene ring fused with a cycloheptatriene ring,and
can be therefore considered as a fusion product of a 4p elec-
trons cyclopentadienide anion which is aromatic and the cor-
responding aromatic 6p electrons tropylium cation
(Scheme 1). Due to this particular electronic structure,azu-
Scheme 3. Synthesis of ligand HAz.
lyzed and quantified. The ligand Azꢀ has been demonstrated
to act as an efficient sensitizer for several different lantha-
nide ions emitting in the near-infrared: Yb3+,Nd 3+,Er 3+,as
well as Tm3+. It is worth noting that Tm3+ luminescence
arising from a complex in solution is rarely reported,and lu-
minescence is especially appealing for bioimaging,since its
main NIR emission band is located around 800 nm,which
corresponds to an absorption minimum for water and tis-
sues.[25,26]
Scheme 1. Azulene and its polar resonance molecular structure.
Experimental Section
lene appears to be a versatile organic fragment with both an
electron-rich five-membered ring that could act as a poten-
tial electron density donor and an electron-deficient seven-
membered ring that could act as a potential electron density
acceptor. Due to this rich electronic character,azulene and
its derivatives have been attracting a growing interest in var-
ious areas of molecular materials,such as charge transfer
complexes,[35,36] conducting polymers,[37,38] liquid crystals,[39,40]
Materials: All reagents were used as received,unless otherwise stated.
Tropolone,thionyl chloride,diethyl malonate,LnCl 3·nH2O (Ln=Nd,Gd,
Er,Tm,and Yb,99.9% or 99.99%, n=6 or 7 depending on the Ln),and
KOH standardized solution in methanol (0.100 N) were purchased from
Aldrich. All deuterated NMR solvents were purchased from Cambridge
Isotope Labs and used as received. KGd
A
to our published procedure.[34]
Methods: Infrared spectra were recorded on a Perkin-Elmer Spectrum
BX FT-IR instrument. Samples were prepared with a drop of dichloro-
methane solution,and evaporated to dryness on KBr pellets. Elemental
analyses were performed by Atlantic Microlab,Inc. (Norcross,Georgia).
1H NMR spectra were recorded on a Bruker DPX-300 spectrometer at
300 MHz. MS-EI and MS-ESI were measured on a Micromass Autospec
and Agilent HP 1100 series LC-MSD instruments respectively. UV/Vis
absorption spectra were recorded on a Perkin-Elmer Lambda 19 spectro-
photometer.
anion receptor/sensors,[41,42]
optoelectronic molecular
switches,[43] as well as nonlinear optical (NLO) material.[44–46]
For the sensitization of NIR emitting lanthanide cations,
the azulene moiety is interesting since it possesses a triplet
state located at significantly lower energy (13600 cmꢀ1),[47]
in comparison to the tropolonate ligand (17200 cmꢀ1). It is
therefore hypothesized that azulene would have better sen-
sitizing efficiency for NIR lanthanides since its triplet state
is located at lower energy,closer to the energy of accepting
levels of several NIR emitting lanthanide cations. In this
work,we have designed and synthesized a new bidentate
ligand that incorporates the azulene moiety (Scheme 2): di-
ethyl 2-hydroxyazulene-1,3-dicarboxylate (HAz; synthesized
according to Scheme 3),for the coordination and sensitiza-
tion of NIR emitting lanthanide cations. The synthesis of the
ligand and several of its lanthanide complexes are reported
in this paper. The luminescent properties have been ana-
Spectrophotometric titration: Spectrophotometric titrations were per-
formed with a Perkin–Elmer Lambda 19 spectrophotometer connected
to an external computer. All titrations were performed in a 1.00 cm ther-
mostated (25.0ꢁ0.18C) cuvette in CH3OH or CH3CN at constant ionic
strength m=0.01m (tetrabutylammonium perchlorate). In a typical ex-
periment,2.00 mL of a ligand solution in CH 3OH (initial total ligand
concentration 1”10ꢀ5 m) was titrated with LnCl3 solutions,(stock solution
concentration: 2”10ꢀ5 m in CH3OH). The kinetic of formation of the
complexes was tested to take place within one minute by monitoring the
changes through UV/Vis absorption spectra. For the titration experi-
ments,two minutes after each addition of aliquot of the lanthanide salt
stock solution to the ligand solution,the UV/Vis spectrum of the result-
Chem. Eur. J. 2008, 14,1264 – 1272
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
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1265

S. Petoud and J. Zhang
ing solution was recorded. Factor analysis and mathematical treatment of
the spectrophotometric data were performed with the SPECFIT pro-
gram.[48]
solution became cloudy due to the formation of the precipitate of the po-
tassium salt of the deprotonated ligand. Methanol (10 mL) was added to
the solution which was then heated until complete dissolution of the pre-
cipitates. LnCl3·nH2O (0.01 mmol) (Ln=Pr,Nd,Gd,Ho,Er,Tm,Yb and
Lu) in methanol (10 mL) was added to the resulting solution. This solu-
tion was stirred overnight and the resulting yellow precipitate was col-
lected by filtration,washed three times with methanol and dried in
vacuum over P2O5 for 48 h.
Luminescence measurements: Emission and excitation spectra were mea-
sured using a JY Horiba Fluorolog-322 spectrofluorometer equipped
with a Hamamatsu R928 detector for the visible domain and an Electro-
Optical Systems,Inc. DSS-IGA020 L detector for the NIR domain. The
NIR luminescence relative quantum yields were measured by using KYb-
A
Data for KPr(Az)4: 39.4 mg,59% isolated yield; IR (KBr): n˜ = 2978
(w),1683 (s, nC O),1615 (s, nC O),1489 (s),1454 (m),1212 (m),1147 (s),
corrected for the instrumental function for both excitation and emission.
Values were calculated using the following equation:
=
=
803 cmꢀ1 (m); ESI-MS (CH2Cl2 negative mode): m/z: 1289.2 [M(Az)4]ꢀ;
elemental analysis calcd (%) for C64H60O20PrK·CH3OH (1361.22): C
57.35,H 4.74; found: C 57.42,H 4.49.
2
ArðlrÞ IðlrÞ
Fx
Fr
hx Dx
¼
2
Data for KNd(Az)4: 42.3 mg,64% isolated yield; IR (KBr): n˜ = 2977
(w),1686 (s, nC O),1615 (s, nC O),1490 (s),1454 (m),1212 (m),1148 (s),
AxðlxÞ IðlxÞ hr Dr
=
=
803 cmꢀ1 (m); ESI-MS (CH2Cl2 negative mode): m/z: 1289.2 [M(Az)4]ꢀ;
elemental analysis calcd (%) for C64H60O20NdK·CH3OH (1364.55): C
57.21,H 4.73; found: C 57.45,H 4.54.
where subscript r stands for the reference and x for the sample; A is the
absorbance at the excitation wavelength, I is the intensity of the excita-
tion light at the same wavelength, h is the refractive index (h=1.478 in
DMSO, h=1.344 in acetonitrile, h=1.342 in CD3CN),and D is the mea-
sured integrated luminescence intensity. The luminescence lifetime meas-
urements were performed by excitation of solutions in 1 cm quartz cells
using a Nd/YAG Continuum Powerlite 8010 Laser at 354 nm (third har-
monic) as excitation source. Emission was collected at a right angle to
the excitation beam,the emission wavelength selected with a Spectral
Products CM 110 1/8 meter monochromator. The signal was monitored
by a cooled photomultiplier (Hamamatsu R316–2) coupled to a 500 MHz
bandpass digital oscilloscope (Tektronix TDS 754D). The signals to be
treated (at least 15000 points resolution for each trace) were averaged
from at least 500 individual decay curves. Luminescence decay curves
were imported into Origin 7.0 scientific data analysis software. The decay
curves were analyzed using the Advanced Fitting Tool module. Reported
luminescence lifetimes are averages of at least three independent deter-
minations.
Data for KGd(Az)4: 39.2 mg,58% isolated yield; IR (KBr): n˜ = 2978
(w),1683 (s, nC O),1616 (s, nC O),1494 (s),1455 (m),1213 (m),1151 (s),
=
=
803 cmꢀ1 (m); ESI-MS (CH2Cl2 negative mode): m/z: 1306.2 [M(Az)4]ꢀ;
elemental analysis calcd (%) for C64H60O20GdK·CH3OH (1377.56): C
56.67,H 4.68; found: C 56.45,H 4.39.
Data for KHo(Az)4: 55.3 mg,82% isolated yield; IR (KBr): n˜ = 2978
(w),1683 (s, nC O),1616 (s, nC O),1495 (s),1471 (s),1456 (m),1213 (m),
=
=
ꢀ1
1152 (s),803 cm
(m); ESI-MS (CH2Cl2 negative mode): m/z: 1313.2
[M(Az)4]ꢀ; elemental analysis calcd (%) for C64H60O20HoK·CH3OH
(1385.24): C 56.36,H 4.66; found: C 56.24,H 4.43.
Data for KEr(Az)4: 47.4 mg,70% isolated yield; IR (KBr): n˜ = 2978
(w),1683 (s, nC O),1616 (s, nC O),1495 (s),1471 (s),1455 (m),1213 (m),
=
=
ꢀ1
1151 (s),803 cm
(m); ESI-MS (CH2Cl2 negative mode): m/z: 1316.2
[M(Az)4]ꢀ; elemental analysis calcd (%) for C64H60O20ErK·CH3OH
(1387.57): C 56.26,H 4.65; found: C 56.26,H 4.41.
2-Chlorocyclohepta-2,4,6-trien-1-one: The title compound was synthe-
sized according to the method described by Brettle et al.[49] and Doering
et al.[50] Tropolone (1.00 g,8.20 mmol) was dissolved in dry benzene
(25 mL) to give a colorless solution. Thionyl chloride (1.07 g,8.99 mmol)
was then added,which immediately give a white precipitate of tropolone
hydrogen chloride; the precipitate dissolved after heating under reflux
for 1.5 h to afford a dark-red solution. Excess thionyl chloride and ben-
zene were evaporated and the brown residue was washed with hexane
and evaporated. After chromatography (silica gel,35% hexanes/ethyl
acetate),2-chlorotropone was obtained as a white solid (1.07 g,94%).
Data for KTm(Az)4: 51.7 mg,76% isolated yield; IR (KBr): n˜ = 2978
(w),1683 (s, nC O),1616 (s, nC O),1495 (s),1481 (s),1456 (m),1213 (m),
=
=
ꢀ1
1153 (s),803 cm
(m); ESI-MS (CH2Cl2 negative mode): m/z: 1317.2
[M(Az)4]ꢀ; elemental analysis calcd (%) for C64H60O20TmK·CH3OH
(1389.24): C 56.20,H 4.64; found: C 56.18,H 4.38.
Data for KYb(Az)4: 45.0 mg,66% isolated yield; ESI-MS (CH Cl2 nega-
2
tive mode): m/z: 1322.2 [M(Az)4]ꢀ; IR (KBr): n˜ = 2976 (w),1685 (s, nC
=
O),1620 (s, nC O),1495 (s),1481 (m),1455 (m),1213 (m),1152 (s),
=
803 cmꢀ1 (m); elemental analysis calcd (%) for C64H60O20YbK·CH3OH
(1393.35): C 56.03,H 4.63; found: C 56.14,H 4.30.
1
M.p. 64–658C; H NMR (CD3COCD3,300 MHz): d = 7.93 (d, J=9.3 Hz,
Data for KLu(Az)4: 44.3 mg,65% isolated yield; IR (KBr): n˜ = 2978
(w),1683 (s, nC O),1622 (s, nC O),1495 (s),1480 (s),1455 (m),1213 (m),
1H),7.41–7.34 (m,1H),7.25–7.04 ppm (m,3H); IR (KBr):
n˜ = 3568
=
=
(nO-H),3215 ( nC-H),1603 ( nC O),1591 ( nC O),1544,1477,1458,1413,1361,
=
=
ꢀ1
1152 (s),803 cm
(m); ESI-MS (CH2Cl2 negative mode): m/z: 1323.2
1306,1242,1204,1076,994,957,897,778,749 cm
ꢀ1; EI-MS: m/z: calcd
[M(Az)4]ꢀ; elemental analysis calcd (%) for C64H60O20LuK·CH3OH
(1395.28): C 55.95,H 4.62; found: C 56.02,H 4.49.
for 140.002893; found: 140.003216 [M]+.
Diethyl 2-hydroxyazulene-1,3-dicarboxylate: The title compound was
synthesized according to the method described by Brettle et al.[49] To a
sodium ethoxide solution prepared from sodium (700 mg) and absolute
ethanol (50 mL),diethyl malonate (2.4 g) and 2-chlorotropone (700 mg)
were added,and the mixture was allowed to stand for 72 h at room tem-
perature. The reaction mixture turned into a gelatinous orange mass.
Water was then added to this suspension,and the sodium salt of the tar-
geted compound precipitated out. After collection by filtration,the pre-
cipitate was redissolved in glacial acetic acid. This solution was diluted
with water and extracted with chloroform. The solvent was then evapo-
rated and the residue was recrystallized from ethanol to give orange-
yellow needles (720 mg,50%). M.p. 95 8C; 1H NMR (CD3COCD3,
300 MHz): d = 1.45 (t, J=7.2 Hz,6H),4.48 (q, J=7.2 Hz,4H),7.84–
Results and Discussion
Formation of ML4 complexes: The lanthanide complexes
were prepared by mixing for 15 h the deprotonated ligand
with stoichiometric amounts of the lanthanide chloride in
methanol at room temperature. The result of the elemental
analysis suggests the formation of complexes with KLn(Az)4
as molecular formula for all the lanthanide complexes stud-
ied (Ln=Pr,Nd,Gd,Ho,Er,Tm,Yb,Lu). This indicates
that only one of the C=O groups of the ligand is coordinat-
ed to lanthanide cation forming a complex with ML4 formu-
la (the molecule acting as bidentate ligand,as shown in
Scheme 2). This indication was confirmed by the FT-IR
analysis. The IR absorption spectra of both the free ligand
7.93 (m,3H),9.44–9.47 (m,2H),11.68 ppm (s,1H); IR (KBr):
n˜ = 2978,
1671 (nC O),1650 ( nC O),1597,1533,1476,1435,1333,1284,1204,1176,
=
=
1032,799,735 cm ꢀ1; EI-MS: m/z: calcd for 288.099774; found: 288.100211
[M]+.
Lanthanide complexes: To a solution of ligand diethyl 2-hydroxyazulene-
1,3-dicarboxylate (115.2 mg, 0.04 mmol) in MeOH (10 mL) was added
KOH (0.04 mmol) in methanol (0.100m) under stirring. The initially clear
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Near-Infrared Luminescent Lanthanide Cations
FULL PAPER
and the complex have two different C=O stretching bands
with one common and one different vibration frequency
(Figure 1). The presence of two different C=O vibrations in
Figure 2. Bottom: ES-MS spectrum in negative mode at a concentration
of ca. 10ꢀ4 m in CH3CN/CH2Cl2 (top left,prediction of the isotopic distri-
bution of the ML4 peak; top right: the zoomed region of the experimen-
tal ML4 peak).
Figure 1. FT-IR spectra of the ligand HAz (c) and of the K[Yb(Az)]
complex (g).
3+
the free ligand can be explained by the formation of a hy-
drogen bond between the OH group with one of the C=O
group,which results in the red shift of the C =O stretching
band. Similarly,the presence of different types of C =O vi-
brations in the coordinated ligand can be explained by one
of the two C=O of the ligand being coordinated to the Ln3+
cation. The bands located at higher energy that appears at
1682 and 1686 cmꢀ1 can be assigned to the ligand and to the
Yb3+ complex respectively. This vibration frequency can be
assigned to free C=O groups,which are not involved in the
formation of hydrogen or coordination bonds. The vibration
band located at lower wavenumber in the Yb complex
(1620 cmꢀ1) is red-shifted by about 22 cmꢀ1 compared to the
value measured for the free ligand (1642 cmꢀ1),confirming
the formation of a coordination bond between this C=O and
the lanthanide cation.
Er3+,Tm
and Yb3+ cations. Spectra were collected vary-
ing the metal to ligand ratio in both CH3CN and methanol
solvents at constant ionic strength. UV/Vis spectra recorded
during the spectrophotometric titration of a solution of the
deprotonated ligand Azꢀ by Tm3+ in CH3CN are depicted
as example in the Figure 3. For all the four cations,a
smooth evolution of the absorption spectra for Ln/Azꢀ in
the range 0.1–1.0 with a single sharp endpoint for Ln/Azꢀ =
4.0 has been observed by monitoring the absorbance at
480 nm (Figure 3).
The software SPECFIT[48] was used to analyze the experi-
mental data. Factor analysis indicated the presence of five
independent colored species in solution. The data obtained
from the titrations with the different cations were best fitted
with a model where four complexes are successfully formed
in solution: ML,ML ,ML and ML4 and satisfactory stabili-
2
3
Electrospray mass spectroscopy (ES-MS) measurements
provide an insight on the nature of the species of complexes
formed in solution. In the negative ion mode,the molecular
peak corresponding to the [Ln(Az)4]ꢀ anion is observed.
The presence of signal assigned to the free ligand indicates
partial dissociation of the complexes in these experimental
conditions. The ES-MS spectrum of the Nd3+ complex is de-
picted as an example in Figure 2. It is important to point out
that the relative abundance reflected by intensities in the
MS spectra can not be directly used to interpret the relative
amount of the species of complexes present in solution but
allow to conclude the presence of the [Ln(Az)4]ꢀ species.
ty constants were obtained using this model. Convergence
of the fitting process to calculate logK4 values was only pos-
sible if the values of logK1,log K2 and logK3 were fixed to
values of 9,8 and 7,respectively,for the titrations per-
formed in both MeOH and CH3CN solvents. This could be
explained by the high stability of the ML1,ML and ML3
2
species formed in solution. A complementary explanation
could be the strong correlation of the UV/Vis spectra of the
individual species as indicated by the spectra of the individ-
ual ML1,ML 2,ML and ML4 species calculated with the
3
help of the Specfit software (see Figure S1,Supporting In-
formation). logK4 stability constant values of 4.5ꢁ0.3,4.7 ꢁ
0.3,4.5 ꢁ0.3 and 4.1ꢁ0.3 were obtained after fitting for
3+
Spectrophotometric titration: It has been shown by elemen-
tal analysis that the composition of the complexes corre-
sponds to ML4 when isolated in the solid state. To investi-
gate the number and nature of the species formed between
the ligand Azꢀ and the lanthanide cations in solution,UV/
Nd3+,Er 3+,Tm
and Yb3+ complex,respectively,in
CH3OH,and 5.5 ꢁ0.3,5.4 ꢁ0.3,4.9 ꢁ0.3,4.9 ꢁ0.3 were ob-
tained from measurements in CH3CN. The overall higher
values of logK4 obtained in CH3CN can be explained by this
solvent being less coordinating than MeOH which posses an
oxygen hard Lewis donor,resulting in lower competition be-
Vis spectrophotometric titrations were performed with Nd3+
,
Chem. Eur. J. 2008, 14,1264 – 1272
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1267

S. Petoud and J. Zhang
rameters (CAChe)[51] also indicate that it is possible for four
ligands to coordinate a central lanthanide cation. The quali-
tative analysis of the modeled structure indicates that these
four ligands provide a good protection of the lanthanide
cation from non-radiative deactivations from the environ-
ment.
Photophysical properties of the ligand HAz: The UV/Vis
absorption spectrum of the free ligand HAz in CH3CN is re-
ported in Figure 5. This spectrum indicates the presence of
several electronic transitions with apparent maxima at 237,
261 and 311 nm with a shoulder located at 300 nm. There
are two additional broader bands located at lower energy
with an apparent maxima at 355 and 430 nm,which can be
assigned to S0–S2 and S0–S1 transitions respectively (see
below for the attribution of these bands).
Figure 3. Top: Series of UV/Vis absorption spectra collected during the
spectrophotometric titration of the deprotonated ligand Azꢀ in CH3CN
solution with Tm3+. (Total ligand concentration: 3.42”10ꢀ5 molLꢀ1; ali-
quots of TmCl3 in CH3CN were successively added (up to 1:1 metal to
ligand ratio); T=25.0ꢁ0.18C; 0.01m tetrabutylammonium perchlorate
for ionic strength control). Bottom: Variations of observed molar extinc-
tions at 484 nm in the titrations obtained with four lanthanide cations.
Figure 5. UV/Vis spectrum of the ligand HAz in CH3CN (2.3”
10ꢀ5 molLꢀ1 at room temperature). Inset: Zoomed region of the absorp-
tion band centered at 430 nm.
In contrast to the blue color of azulene,the ligand HAz is
yellowish orange. It is known that the electronic structure
and photophysical properties of derivatives of azulene are
affected by their substituents.[52] This perturbation of the
molecular orbitals of the azulene moiety can be rationalized
by a qualitative pictorial description of the low lying molec-
ular orbitals (Figure 6).[53] The HOMO,LUMO,and
LUMO+1 orbitals of azulene,which correspond to S 0, S1
and S2 levels,respectively,are depicted in the Figure 6.
For azulene,the small overlap between HOMO and
LUMO leads to a transition located at lower energy of the
S1 state. The large electron–electron correlation energy for
the LUMO+1 and HOMO considerably raises the energy
of the second singlet excited state (S2 level of azulene). As a
tween the ligand and this solvent. In addition,the titration
spectra show a smooth evolution of the absorption spectra
at 484 nm for Ln/Azꢀ in the range 0.1–1.0 with a single
sharp end point for Ln/Azꢀ =1:4 (Figure 3). This series of
results lead to the conclusion that four different lanthanide
complexes ML1,ML ,ML and ML4 are successfully formed
2
3
in MeOH solution. The values of logK4 for the ML4 com-
plexes are all comprised within the experimental error. A
molecular modeling calculation (Figure 4) using MM3 pa-
result,the S (raised in electronic energy level) and S1 (low-
2
ered in electronic energy levels) couple together to create a
unusually large gap resulting the so called “blue window”
(360–450 nm) of azulene,[53] which provides to this com-
pound its blue color.
There are three substituents on the ligand HAz,two C =O
groups and one OH group. The C=O group is a conjugative,
electron-withdrawing substituent and it generates two op-
posing effects on the electron densities at C-1 and C-3 of
Figure 4. Two different views of the [Yb(Az)4]ꢀ complex created with
molecular modeling (MM3 parameters).
1268
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Chem. Eur. J. 2008, 14,1264 – 1272

Near-Infrared Luminescent Lanthanide Cations
FULL PAPER
UV/Vis spectrum red-shifts from 428 nm (in the free ligand)
to 452 nm in the complex with an increased molar absorptiv-
ity. This can be attributed to the perturbation of the elec-
tronic structure of the ligand upon metal ion coordination,
making this transition more allowed compared to the free
ligand.
The fluorescence of the free ligand HAz is faint. No phos-
phorescence of the free ligand was detectable at 77 K. How-
ever,it has been possible to record the steady-state fluores-
cence and time-resolved phosphorescence bands arising
from the ligand bound to Gd3+ in the KGd(Az)4 complex
(Figure 7). There is no ligand to lanthanide energy transfer
in this complex because the Gd3+ electronic levels are too
high in energy to accept energy from the singlet and/or trip-
let states of the ligand. The location of these bands in both
types of spectra is helpful to identify the energy of the sin-
glet and triplet states of the ligands bound to lanthanide
metal ions. The fluorescence spectrum obtained upon excita-
tion of the ligand at room temperature resulted in the pres-
ence of a broad ligand-centered band that is assigned to the
S1–S0 transition with an apparent maximum of the electronic
envelope at 18700 cmꢀ1 (536 nm). In time-resolved mode,
when a delay (0.01 ms) is applied after the excitation flash
to record the phosphorescence spectrum,the fluorescence
band disappears and is replaced by a phosphorescence
Figure 6. a) Probability of location of the electron in the HOMO,LUMO
and LUMO+1 of azulene;[53] b) schematic illustration of effects of differ-
ent substituents on the energies of electronic levels of azulene.
the azulene moiety HOMO to stabilize or destabilize the
molecular orbitals. It has been found that the aldehyde
group causes a blue shift of the S1 state because of its induc-
tive electron withdrawing effect,but causes red shift of the
S2 state due to the conjugation effect.[52] On the other hand,
the hydroxyl group at C-2,which is a resonance donating
group,will destabilize the LUMO orbital of azulene. This
will lead to an increased S0–S1 gap and to a corresponding
blue shift,which explains why the energy position of the
maximum of the S0–S1 transition of 1,3-dicarboxaldehyde
azulene is at approximately 500 nm (20000 cmꢀ1),while it is
around 428 nm for our ligand. The red-shift of the band cor-
responding to S2 results in the diminished transmission of
blue light (near 400 nm),and a simultaneous increase in
transmittance of red/yellow light,therefore,the ligand is
orange.
signal containing three bands with apparent maxima located
ꢀ1
at 15700 cmꢀ1 (635 nm),14306 cm
(699 nm) and
12700 cmꢀ1 (788 nm). A similar pattern is also observed for
the KGd(Trop)4 complex (Table 1 and Figure S3,Supporting
AHCTREUNG
Table 1. Comparison of singlet and triplet state energies (cmꢀ1) for the
Tropolonate and Azꢀ ligands.
Coordinated ligand-centered luminescence in KGd(Az)4:
The UV/Vis absorption spectra of the different lanthanide
complexes studied in this work were measured in CH3CN at
room temperature (Figure S2 in Supporting Information).
They are similar to each other since the energy location of
these bands all reflect the electronic structure of the ligands
bound to a central lanthanide cation. The spectrum of the
Gd3+ complex is shown in Figure 7 as an example. Com-
pared with the free ligand,the p!p* transition band in the
KGd(Az)4
KGd(Trop)4
A
singlet[a]
triplet[b]
18700
14300
23500
15600
15700
12700
17200
14300
[a] Steady state fluorescence spectra recorded at 298 K (10ꢀ5 molLꢀ1 in
CH3CN and DMSO for KGd(Az)4 and KGd(Trop)4,respectively).
[b] Time-resolved phosphorescence spectra recorded on solid samples at
77 K.
AHCTREUNG
Information),which can be attributed to the partial similari-
ty between the electronic structures of these two molecules.
This set of bands is attributed to the phosphorescence aris-
ing from the triplet state of the bound ligand. In comparison
to the energy of the triplet state reported for azulene
13600 cmꢀ1 (733 nm),the triplet state of the ligand is signifi-
cantly blue-shifted if we consider the maxima of the bands
located at 15700 and 14300 cmꢀ1. This is due to the presence
of the two substituents,which are required for the coordina-
tion of the lanthanide cations. Nevertheless,the triplet state
of this ligand is still located at relatively low energy com-
pared with the tropolonate ligand and most other ligands
used for sensitization of NIR emitting lanthanide cations.
Figure 7. UV/Vis absorption (c),normalized fluorescence ( c) and
phosphorescence (g) spectra of KGd(Az)4 in CH3CN (UV/Vis: room
temperature; fluorescence: M=1.2”10ꢀ5 molꢀ1 Lꢀ1, lex =400 nm,298 K;
phosphorescence: lex =400 nm,77 K,delay time 0.01 ms,gate time
20 ms).
Sensitization of lanthanide-centered near-infrared emission:
Remarkably,the Az ligands can sensitize four different NIR
Chem. Eur. J. 2008, 14,1264 – 1272
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www.chemeurj.org
1269

S. Petoud and J. Zhang
emitting Ln3+ (Ln=Yb,Nd,Er,and Tm) in Ln(Az)
plexes in CH3CN at room temperature. The luminescence
spectra of these four different lanthanide complexes are de-
picted in Figure 8. Through excitation of the absorption
com-
Table 2. Absolute quantum yield values of lanthanide complexes at
298 K in CH3CN and CD3CN at different excitation wavelength.
4
Ln
Yb
Wavelength [nm]
CH3CN[a]
CD3CN[a]
322[a]
375[b]
447[c]
322[a]
383[b]
449[c]
322[a]
387[b]
447[c]
320[a]
380[b]
446[c]
2.7(2)”10ꢀ2
3.8(2)”10ꢀ2
3.2(2)”10ꢀ2
4.0(2)”10ꢀ3
4.5(3)”10ꢀ3
3.7(2)”10ꢀ3
2.0(1)”10ꢀ4
2.1(1)”10ꢀ4
1.8(1)”10ꢀ4
4.3(2)”10ꢀ5
5.9(2)”10ꢀ5
4.9(2)”10ꢀ5
3.1(2)”10ꢀ2
4.2(2)”10ꢀ2
3.6(2)”10ꢀ2
4.7(4)”10ꢀ3
5.3(3)”10ꢀ3
4.9(2)”10ꢀ3
2.2(1)”10ꢀ4
2.4(1)”10ꢀ4
2.2(1)”10ꢀ4
5.1(1)”10ꢀ5
6.6(1)”10ꢀ5
6.5(2)”10ꢀ5
Nd
Er
Tm
[a] 1.7”10ꢀ5 molLꢀ1. [b] 1”10ꢀ4 molLꢀ1. [c] 2”10ꢀ4 molLꢀ1
.
(Figure S2,Supporting Information). Notably this observa-
tion was possible for all four NIR emitting lanthanide cat-
ions sensitized by Azꢀ despite the fact that the luminescent
Figure 8. Normalized excitation (dashed line) and emission spectra (plain
line) of four different lanthanide complexes 10ꢀ5 molꢀ1 Lꢀ1 in CH3CN
(lex =380 nm).
3+
Yb3+,Nd 3+,Er 3+,and Tm
cations have different energies
of accepting levels. It can be concluded that the excitation
has the same efficiency for all three absorbance bands for
all four lanthanide cations as the quantum yield values do
not depend on the excitation wavelengths.
3+
bands of the coordinated ligand,the solution of Yb
com-
plex displays a NIR emission band ranging from 976 to
2
2
1028 nm,which is assigned to the F5/2 ! F7/2 transition. For
the Nd3+ complex,emission bands were observed at 895,
1051,and 1323 nm and are attributed to the transitions from
The quantum yield of the Yb3+ complex is especially
large with values of 3.8% in CH3CN and 4.2% in CD3CN,
compared with the value reported for other Yb3+ complexes
in solution,such as 3.2 and 2.4% reported for a monopor-
phyrinate complex in CH2Cl2,[11] 1.9% for our tropolonate
complex in DMSO,[34] 0.5% for a terphenyl-based complex
in DMSO,[54] 1.8% for a bimetallic helicate in D2O,[55] or
0.45% for fluorexon complex in D2O.[56] Although smaller,
the quantum yield of the Nd3+ complex (0.45% in CH3CN
and 0.53% in CD3CN) is also high compared with other
Nd3+ complexes with 0.2% for monoporphyrinate com-
plexes in CH2Cl2,[11] 0.21% for tropolonate Nd3+ complex in
DMSO,[34] 0.038% for the Nd-fluorexon complex in D2O,[56]
0.075% for 8-hydroxyquinolinate-based Nd3+ complex in
D2O.[23]
4
the 4F3/2 level to the 4I9/2, I11/2 and 4I13/2 sublevels respectively.
For Er3+ complex,the emission band observed at 1524 nm
4
4
can be assigned to the transition from the I13/2 level to I15/2
level. Tm3+ emission is not frequently observed for lantha-
nide complexes in solution. In CH3CN solution,two transi-
tions were observed at 803 and 1487 nm. They are assigned
to 3F4
!
3H6 and 3F4
!
3H4 transitions respectively. The
3+
emission wavelengths of the Nd3+,Yb
and Tm3+ com-
plexes are in a spectral range well suited for bioanalytical
assays (minimization of the absorption by the biological
system).[25] As shown in Figure 8,there is only slight overlap
between these emission spectra,allowing the potential for
multiplex measurements. Attempts to generate Ho3+ and
Pr3+ luminescence upon ligand L excitation were unsuccess-
ful.
To quantify the intramolecular ligand to metal ion energy
transfer efficiency in the different lanthanide complexes,lu-
minescence quantum yields were measured in CH3CN and
The luminescent lifetimes have been recorded in order to
evaluate the coordination environment around the lantha-
nide cations in these complexes in solution and to quantify
the degree of protection against non-radiative deactivation
provided by the four Azꢀ ligands around the central lantha-
nide cation. The luminescence decays of the Yb3+ and Nd3+
complexes in different solvents were measured and fit as
monoexponential decays in all the studied solvents
(Table 3). This indicates that a unique and consistent coordi-
nation environment is present around the lanthanide in the
complex.
CD3CN upon ligand excitation using KYb(Trop)4 as refer-
R
ence.[34] Three different excitation wavelengths correspond-
ing to three maxima of the absorption spectra at 320,380
and 447 nm,respectively,were used systematically. The re-
sults are reported in Table 2. The quantum yield values re-
corded with different excitation wavelengths are all within
the experimental error (5–17%). Therefore,it is suggested
that a unique path of energy is used for the sensitization of
all the lanthanide cations studied by the chromophoric
ligand. This assumption is supported by the fact that all the
excitation spectra are similar to each other (Figure 8) and
are well matched to the corresponding absorption spectra
Table 3. Luminescence lifetimes (ms) of [Nd(Az)4]ꢀ and [Yb(Az)4]ꢀ com-
plexes in different solvents (298 K,2”10 ꢀ5 molLꢀ1).
MeOH
CD3OD
CH3CN
CD3CN
Yb3+
Nd3+
12.01ꢁ0.07
0.37ꢁ0.01
33.71ꢁ0.03
1.33ꢁ0.01
24.61ꢁ0.01
1.85ꢁ0.01
32.81ꢁ0.12
2.68ꢁ0.01
1270
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Chem. Eur. J. 2008, 14,1264 – 1272

Near-Infrared Luminescent Lanthanide Cations
FULL PAPER
The comparison between the luminescence lifetimes in
deuterated and non-deuterated solvents can be used to
quantify the number of solvent molecules coordinated to
the lanthanide cations in solution. The calculation of the hy-
dration/solvation number is obtained by using an empirical
Equation (1). This formula was first developed by Horrocks
et al.[57,58] In this formula, q is the number of water mole-
cules bound to lanthanide ion in the first sphere of coordi-
nation; kH and kD are the rate constants of excited states of
lanthanide ion in H2O and D2O,respectively. A is a propor-
tionality constant related to the sensitivity of the lanthanide
ion to vibrational quenching by OH oscillators, B is the cor-
rection factor for outer sphere water molecules.
Conclusion
In summary,lanthanide complexes [Ln(Az) ]ꢀ formed by re-
4
action of one lanthanide cation with four azulene-based li-
gands,HAz,have been isolated and characterized in solid
state and in solution. Their photophysical properties have
been analyzed,with a specific interest for the sensitization
of near-infrared lanthanide cations. It was hypothesized that,
due to its triplet state located at lower energy compared to
the tropolonate ligand previously studied in our group,the
HAz ligand would be more efficient in sensitizing several
lanthanide cations.
Spectrophotometric titrations indicated that four Azꢀ
react successively with one lanthanide cation to form a ML4
type of complex in solution. The investigation of the photo-
physical properties of [Ln(Az)4]ꢀ shows that Azꢀ sensitize
efficiently four different NIR emitting lanthanide cations,
Nd3+,Er 3+,Yb 3+,and Tm 3+. The analysis of luminescence
lifetimes of the lanthanide cations recorded in deuterated
and non-deuterated solvents indicated that no solvent mole-
cules are bound to the lanthanide cation in the [Ln(Az)4]ꢀ
complexes formed with Ln=Nd3+ and Yb3+. This efficient
protection of the lanthanide cation against non-radiative de-
activations results from the coordination the four ligands
Azꢀ to the central lanthanide cation. As a result,the lumi-
nescence lifetimes of the Nd3+ and Yb3+ complexes are sig-
nificantly longer in comparison to the values reported in the
literature for most near-infrared emitting lanthanide com-
plexes in solution. The quantum yields of the Nd3+ and
Yb3+ complexes are among the highest values reported for
NIR emitting lanthanide complexes in solution,which we at-
tribute to a combination of efficient ligand to lanthanide
energy transfer and good protection of the lanthanide cat-
q ¼ AðkHꢀkDÞꢀB
ð1Þ
More recently,Beeby et al. modified this formula for the de-
termination of q for Yb3+ and Nd3+ complexes in MeOH
solution,[59,60] in which A=0.29 ms (Nd3+) or 2 ms (Yb3+),
B=0.4 (Nd3+) or 0.1 (Yb3+), kH =1/tH and kD =1/tD are
given in msꢀ1.
Applying these formulas, q values of ꢀ0.05 and 0.16 were
calculated for the Yb3+ and Nd3+ complexes,respectively.
These near-zero values indicate that there are no water/sol-
vent molecules bound to the lanthanide ion in the first coor-
dination sphere for [Ln(Az)4]ꢀ in solution. This is consistent
for both the larger Nd3+ (Shannonꢁs effective ionic radius:
1.109 Š[61]) and smaller Yb3+ (Shannonꢁs effective ionic
radius: 0.985 Š[61]) cations. Therefore,we can conclude that
the four bidentate ligands efficiently protect Ln3+ cations
against nonradiative deactivations induced by solvent mole-
cules. The azulene complexes provide superior protection
compared with the ML4 tropolonate complexes,[34] where
one solvent molecule is bound to the central lanthanide
cation. It also confirms the results of elemental analysis that
four Azꢀ ligands are bound to the central lanthanide cation
in [Ln(Az)4]ꢀ providing a lanthanide coordination number
of 8. The luminescence lifetime obtained for the Yb3+ com-
plex in acetonitrile solution (24.61 ms) is fairly long com-
pared to the value obtained in MeOH (12.01 ms) and the
typical average value around 10 ms reported for most Yb3+
complexes described in the literature.[23,62,63] This lumines-
cence lifetime is similar to Yb3+ in CD3OD and CD3CN,an
indication that the non-radiative quenching is similar for the
Yb3+ complex in both solvents. On the other hand,the lumi-
nescence lifetimes of the Nd3+ complex in these two sol-
ions. The fact that the azulene based ligand can sensitize not
only Yb3+,Nd 3+,and Er 3+,but also Tm
indicates that it is
3+
a promising sensitizer for NIR lanthanide ion,potentially
suitable for biological imaging. A sensitizer that provides
the advantage of a unique excitation wavelength (unique in-
strumental source) to obtain four different emission wave-
lengths is an important advantage for multiplex assays. To
provide applicability to this system,the ligand design will be
modified to obtain solubility in water and to increase the
stability of the complex in solution by increasing denticity of
this ligand.
vents are very different,1.33 and 2.68 ms in CD3OD and
Acknowledgements
3+
CD3CN,respectively. This indicates that Nd
is more sensi-
tive to high energy oscillations than Yb3+. The main differ-
Funding was provided through the University of Pittsburgh and through
the National Science Foundation (award DBI0352346). J.Z. was support-
ed through an Andrew Mellon Predoctoral Fellowship.
ence between these two solvents is the O-D group in
ꢀ1
CD3OD,which has vibrational energy around 2500 cm
versus the CN group in CD3CN,which has a vibrational
,
energy of 2250 cmꢀ1. Nd3+ has more electronic states at high
energy than Yb3+,which increases the probability of
quenching.
[1] J.-C. G. Bünzli, Acc. Chem. Res. 2006, 39,53–61.
[2] J.-C. G. Bünzli,C. Piguet, Chem. Soc. Rev. 2005, 34,1048–1077.
[3] A. Beeby,R. S. Dickins,S. Faulkner,D. Parker,J. A. G. Williams,
Chem. Commun. 1997,1401–1402.
[4] C. L. Maupin,D. Parker,J. A. G. Williams,J. P. Riehl, J. Am. Chem.
Soc. 1998, 120,10563–10564.
Chem. Eur. J. 2008, 14,1264 – 1272
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
www.chemeurj.org
1271

S. Petoud and J. Zhang
[5] M. P. O. Wolbers,F. C. J. M. van Veggel,B. H. M. Snellink-Ruel,
[32] C. Yang,L.-M. Fu,Y. Wang,J.-P. Zhang,W.-T. Wong,X.-C. Ai,Y.-
F. Qiao,B.-S. Zou,L.-L. Gui, Angew. Chem. 2004, 116,5120–5123;
Angew. Chem. Int. Ed. 2004, 43,5010–5013.
J. W. Hofstraat,F. A. J. Guerts,D. N. Reinhoudt,
J. Chem. Soc.
Perkin Trans. 2 1998,2141–2150.
[33] G. A. Hebbink,S. I. Klink,L. Grave,P. G. B. Oude Alink,F. C. J. M.
Van Veggel, ChemPhysChem 2002, 3,1014–1018.
[6] A. Beeby,R. S. Dickins,S. FitzGerald,L. J. Govenlock,D. Parker,
J. A. G. Williams,C. L. Maupin,J. P. Riehl,G. Siligardi,
Commun. 2000,1183–1184.
Chem.
[34] J. Zhang,P. D. Badger,S. J. Geib,S. Petoud,
Angew. Chem. 2005,
117,2564–2568; Angew. Chem. Int. Ed. 2005, 44,2508–2512.
[35] J. E. Frey,A. M. Andrews,S. D. Combs,S. P. Edens,J. J. Puckett,
R. E. Seagle,L. A. Torreano, J. Org. Chem. 1992, 57,6460–6466.
[7] Y. Hasegawa,T. Ohkubo,K. Sogabe,Y. Kawamura,Y. Wada,N. Na-
kashima,S. Yanagida, Angew. Chem. 2000, 112,365–368; Angew.
Chem. Int. Ed. 2000, 39,357–360.
[8] M. H. V. Werts,R. H. Woudenberg,P. G. Emmerink,R. van Gassel,
J. W. Hofstraat,J. W. Verhoeven, Angew. Chem. 2000, 112,4716–
4718; Angew. Chem. Int. Ed. 2000, 39,4542–4544.
[36] S. Schmitt,M. Baumgarten,J. Simon,K. Hafner,
Angew. Chem.
1998, 110,1129–1133; Angew. Chem. Int. Ed. 1998, 37,1077–1081.
[37] F. Wang,Y.-H. Lai,M.-Y. Han,
3230.
Macromolecules 2004, 37,3222–
[9] D. Imbert,M. Cantuel,J.-C. G. Bünzli,G. Bernardinelli,C. Piguet,
Am. Chem. Soc. 2003, 125,15698–15699.
J.
[38] M. Porsch,G. Sigl-Seifert,J. Daub, Adv. Mater. 1997, 9,635–639.
[39] S. Ito,H. Inabe,N. Morita,K. Ohta,T. Kitamura,K. Imafuku,
Am. Chem. Soc. 2003, 125,1669–1680.
J.
J. Mater.
[10] G. A. Hebbink,L. Grave,L. A. Woldering,D. N. Reinhoudt,
F. C. J. M. van Veggel, J. Phys. Chem. A 2003, 107,2483–2491.
[11] T. J. Foley,B. S. Harrison,A. S. Knefely,K. A. Abboud,J. R. Rey-
[40] S. E. Estdale,R. Brettle,D. A. Dummur,C. M. Marson,
Chem. 1997, 7,391–401.
[41] T. Zielinski,M. Kedziorek,J. Jurczak, Tetrahedron Lett. 2005, 46,
6231–6234.
nolds,K. S. Schanze,J. M. Boncella, Inorg. Chem. 2003, 42,5023–
5032.
[12] S. J. A. Pope,B. J. Coe,S. Faulkner,E. V. Bichenkova,X. Yu,K. T.
Douglas, J. Am. Chem. Soc. 2004, 126,9490–9491.
[13] R. Van Deun,P. Fias,P. Nockemann,A. Schepers,T. N. Parac-Vogt,
K. Van Hecke,L. Van Meervelt,K. Binnemans, Inorg. Chem. 2004,
43,8461–8469.
[14] A. P. Bassett,R. Van Deun,P. Nockemann,P. B. Glover,B. M. Kar-
iuki,K. Van Hecke,L. Van Meervelt,Z. Pikramenou, Inorg. Chem.
2005, 44,6140–6142.
[42] H. Salman,Y. Abraham,S. Tal,S. Meltzman,M. Kapon,N. Tessler,
S. Speiser,Y. Eichen, Eur. J. Org. Chem. 2005,2207–2212.
[43] T. Mrozek,H. Gorner,J. Daub, Chem. Eur. J. 2001, 7,1028–1040.
[44] P. G. Lacroix,I. Malfant,G. Iftime,A. C. Razus,K. Nakatani,J. A.
Delaire, Chem. Eur. J. 2000, 6,2599–2608.
[45] L. Cristian,I. Sasaki,P. G. Lacroix,B. Donnadieu,I. Asselberghs,K.
Clays,A. C. Razus, Chem. Mater. 2004, 16,3543–3551.
[46] C. Lambert,G. Noell,M. Zabel,F. Hampel,E. Schmaelzlin,C.
Braeuchle,K. Meerholz, Chem. Eur. J. 2003, 9,4232–4239.
[47] W. G. Herkstroeter, J. Am. Chem. Soc. 1975, 97,4161–4167.
[15] S. Banerjee,L. Huebner,M. D. Romanelli,G. A. Kumar,R. E.
Riman,T. J. Emge,J. G. Brennan,
15900–15906.
J. Am. Chem. Soc. 2005, 127,
[48] H. Gampp,M. Maeder,C. J. Meyer,A. D. Zuberbuehler,
Talanta
[16] S. Comby,D. Imbert,A.-S. Chauvin,J.-C. G. Bünzli,
Inorg. Chem.
1985, 32,1133–1139.
2006, 45,732–743.
[49] R. Brettle,D. A. Dunmur,S. Estdale,C. M. Marson, J. Mater. Chem.
[17] W.-K. Lo,W.-K. Wong,W.-Y. Wong,J. Guo,K.-T. Yeung,Y.-K.
Cheng,X. Yang,R. A. Jones, Inorg. Chem. 2006, 45,9315–9325.
[18] L. Bertolo,S. Tamburini,P. A. Vigato,W. Porzio,G. Macchi,F. Mei-
nardi, Eur. J. Inorg. Chem. 2006,2370–2376.
[19] L. N. Sun,J. B. Yu,G. L. Zheng,H. J. Zhang,Q. G. Meng,C. Y.
Peng,L. S. Fu,F. Y. Liu,Y. N. Yu, Eur. J. Inorg. Chem. 2006,3962–
3973.
1993, 3,327–331.
[50] W. v. E. Doering,L. H. Knox, J. Am. Chem. Soc. 1952, 74,5683–
5687.
[51] F. L. CAChe WorkSystem Pro Version 6.1.12.33,Fujitsu America
Inc,. USA, 2000–2004.
[52] G. Eber,F. Grueneis,S. Schneider,F. Doerr, Chem. Phys. Lett. 1974,
29,397–404.
[53] R. S. H. Liu,A. E. Asato, J. Photochem. Photobiol. C 2003, 4,179–
194.
[54] S. I. Klink,G. A. Hebbink,L. Grave,F. C. J. M. van Veggel,D. N.
[20] R. Van Deun,P. Fias,P. Nockemann,K. Van Hecke,L. Van Meer-
velt,K. Binnemans, Inorg. Chem. 2006, 45,10416–10418.
[21] T. Lazarides,M. A. H. Alamiry,H. Adams,S. J. A. Pope,S. Faulk-
ner,J. A. Weinstein,M. D. Ward, Dalton Trans. 2007,1484–1491.
[22] T. K. Ronson,T. Lazarides,H. Adams,S. J. A. Pope,D. Sykes,S.
Faulkner,S. J. Coles,M. B. Hursthouse,W. Clegg,R. W. Harrington,
M. D. Ward, Chem. Eur. J. 2006, 12,9299–9313.
Reinhoudt,L. H. Slooff,A. Polman,J. W. Hofstraat,
J. Appl. Phys.
1999, 86,1181–1185.
[55] F. R. Goncalves e Silva,O. L. Malta,C. Reinhard,H.-U. Güdel,C.
Piguet,J. E. Moser,J.-C. G. Bünzli,
1670–1677.
J. Phys. Chem. A 2002, 106,
[23] S. Comby,D. Imbert,C. Vandevyver,J.-C. G. Bünzli,
Chem. Eur. J.
[56] M. H. V. Werts,J. W. Verhoeven,J. W. Hofstraat,
433–439.
Perkin 2 2000,
2007, 13,936–944.
[24] S. Kim,Y. T. Lim,E. G. Soltesz,A. M. De Grand,J. Lee,A. Na-
kayama,J. A. Parker,T. Mihaljevic,R. G. Laurence,D. M. Dor,
L. H. Cohn,M. G. Bawendi,J. V. Frangioni, Nat. Biotechnol. 2004,
22,93–97.
[25] R. Weissleder,V. Ntziachristos, Nat. Med. 2003, 9,123–128.
[26] S. Stolik,J. A. Delgado,A. Perez,L. Anasagasti, J. Photochem. Pho-
tobiol. B 2000, 57,90–93.
[57] W. D. Horrocks,Jr., D. R. Sudnick, J. Am. Chem. Soc. 1979, 101,
334–340.
[58] W. D. Horrocks,Jr., D. R. Sudnick, Acc. Chem. Res. 1981, 14,384–
392.
[59] G. M. Davies,R. J. Aarons,G. R. Motson,J. C. Jeffery,H. Adams,S.
Faulkner,M. D. Ward, Dalton Trans. 2004,1136–1144.
[60] A. Beeby,B. P. Burton-Pye,S. Faulkner,G. R. Motson,J. C. Jeffery,
J. A. McCleverty,M. D. Ward, J. Chem. Soc. Dalton Trans. 2002,
1923–1928.
[27] T. Jüstel,D. U. Wiechert,C. Lau,D. Sendor,U. Kynast,
Adv. Funct.
Mater. 2001, 11,105–110.
[28] T.-S. Kang,B. S. Harrison,T. J. Foley,A. S. Knefely,J. M. Boncella,
J. R. Reynolds,K. S. Schanze, Adv. Mater. 2003, 15,1093–1097.
[29] L. H. Slooff,A. Polman,M. P. Oude Wolbers,F. C. J. M. van Veggel,
D. N. Reinhoudt,J. W. Hofstraat, J. Appl. Phys. 1998, 83,497–503.
[30] L. H. Slooff,A. van Blaaderen,A. Polman,G. A. Hebbink,S. I.
[61] R. D. Shannon, Acta Crystallogr. Sect. A 1976, 32,751–767.
[62] S. I. Klink,H. Keizer,F. C. J. M. van Veggel,
Angew. Chem. 2000,
112,4489–4491; Angew. Chem. Int. Ed. 2000, 39,4319–4321.
[63] N. M. Shavaleev,L. P. Moorcraft,S. J. A. Pope,Z. R. Bell,S. Faulk-
ner,M. D. Ward, Chem. Eur. J. 2003, 9,5283–5291.
Klink,F. C. J. M. Van Veggel,D. N. Reinhoudt,J. W. Hofstraat,
Appl. Phys. 2002, 91,3955–3980.
J.
Received: July 12,2007
[31] S. I. Weissman, J. Chem. Phys. 1942, 10,214–217.
Published online: November 28,2007
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