A MALDI-TOF MS study of lanthanide(III)-cored poly(phenylenevinylene) dendrimers

Article abstract of DOI:10.1002/jms.1534

An extensive study by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) of some first-generation and second-generation lanthanide(III)-cored poly(phenylenevinylene) dendrimers is described. The complexes were ob

Full text of DOI:10.1002/jms.1534

Research Article
Received: 9 September 2008
Accepted: 29 October 2008
Published online in Wiley Interscience: 3 December 2008
(www.interscience.com) DOI 10.1002/jms.1534
A MALDI-TOF MS study of lanthanide(III)-cored
poly(phenylenevinylene) dendrimers
˜
Joaquín C. García-Martínez, Carmen Atienza, Milagros de la Pena,
∗
´
´
Ana C. Rodrigo, Juan Tejeda and Julian Rodríguez-Lopez
An extensive study by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) of some
first-generation and second-generation lanthanide(III)-cored poly(phenylenevinylene) dendrimers is described. The complexes
were obtained by self-assembly of suitably functionalized carboxylate dendrons around the lanthanide ion (La3+, Er3+). Fourier
transform infrared (FT-IR) spectroscopy gave reasonable evidence for the proposed structures. However, MS was used to
ascertain unequivocally the complex formation. The most reliable results were found in the negative reflector mode, using
2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile(DCTB)asmatrix.Well-definedandhighlyresolvedbase
peaks corresponding to negative ions of [Gn4La]− and [Gn4Er]− were found in all cases, with an excellent match between the
theoretical and observed isotope distributions. However, the 3 : 1 stoichiometry used in the synthesis guarantees an empirical
c
formula Gn3Ln for the complexes. Copyright ꢀ 2008 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: dendrimers; poly(phenylenevinylene); lanthanide complexes; MALDI-TOF
Introduction
due to the starting dendron obtained instead.[9,17,18] The structure
is too weak to withstand the conditions of the mass analyses.
Thus, the complete elucidation of the structure of these
Ln(III)-cored dendrimers is still a challenge, and further work is
necessary in order to overcome this issue. Typically, they are
depicted in the literature as three dendrons that are probably
assembled as carboxylate ligands surrounding the lanthanide
core.[9–11,13–18] Some studies with different poly(benzyl ether)
dendrons indicate that the co-ordination structure depends on
the type and generation of the dendron as well as the lanthanide
ion.[19]
As part of our research program aimed at the construction
of π-conjugated dendritic architectures,[20] we prepared some
first-generation and second-generation lanthanide(III)-cored den-
drimers with a poly(phenylenevinylene) backbone (Fig. 1). The
ligand-exchange reaction of a lanthanide triacetate with the ap-
propriate acid dendron afforded the desired complexes in good
yields (Experimental). FT-IR spectroscopy gave reasonable evi-
dence for the proposed structures. Thus, the carbonyl absorptions
associated with the precursor acid dendrons, ca. 1690 cm−1, were
absent in the spectra of the corresponding Ln3+-cored dendrimer
complexes. In this paper, we present a MALDI-TOF MS study that
provides further evidence for complex formation, confirming the
occurrence of dendritic ligand–lanthanide interactions.
Luminescent lanthanide complexes have attracted a great deal of
attentionbecauseof theiracademicinterestandpotentialutilityin
a wide variety of photonic applications such as planar waveguide
amplifiers, plastic lasers, light-emitting diodes, and luminescent
probes.[1–8] The encapsulation of luminescent lanthanide (III) ions
(Ln3+) within luminescent dendrimers can lead to systems capable
of shielding the Ln3+ ion from a non-radiative environment,
and efficiently transferring excited energy from the peripheral
chromophores to the central Ln3+ core. Thus, Fre´chet[9,10] and
Kawa[11] reported the site-isolation and antenna effects of Eu3+
,
Tb3+, and Er3+-cored dendrimer complexes obtained by self-
assembly of suitably functionalized carboxylate aryl ether-type
dendrons around the lanthanide ion. Kim and co-workers used
dendritic luminescent ligands based on metalloporphyrin,[12–14]
naphthalene,[15,16] and 9,10-diphenylanthracene[17] units that also
bear aryl ether-type dendrons.
However, the characterization of these self-assembled com-
plexes is not easy due to their fragile nature, which results from
the weak non-covalent ionic interaction between the Ln3+ ion
and the dendritic carboxylate sub-units. Experimental evidence
for the proposed structures is usually provided by techniques
such as Fourier transform infrared (FT-IR) and elemental analysis.
Unfortunately, the paramagnetic properties of the lanthanide ion
preclude conventional nuclear magnetic resonance (NMR) studies.
Laser light scattering allows the estimation of the assembly size,[9]
while size exclusion chromatography (SEC) was found to be in-
appropriate for these fragile complexes. Similarly, matrix-assisted
laser desorption/ionization time-of-flight (MALDI-TOF), electro-
spray (ESI), or fast atom bombardment (FAB) mass spectrometry
(MS)donotgiverisetoasignalcorrespondingtotheLn3+-complex
in a variety of matrices, with complex spectra including the signal
∗
´
´
´
´
Correspondence to: Julian Rodríguez-Lopez, Area de Química Organica,
´
Facultad de Química, Universidad de Castilla-La Mancha, Avda. Camilo Jose
Cela, 10, 13071-Ciudad Real, Spain. E-mail: julian.rodriguez@uclm.es
´
´
Area de Química Organica, Facultad de Química, Universidad de Castilla-La
´
Mancha, Avda. Camilo Jose Cela, 10, 13071-Ciudad Real, Spain
c
J. Mass. Spectrom. 2009, 44, 613–620
Copyright ꢀ 2008 John Wiley & Sons, Ltd.

J. C. García-Martínez et al.
Scheme 1. Synthesis of first-generation and second-generation poly(phenylenevinylene) acid dendrons.
Experimental
dard II (covered mass range: 700–3200 Da) and Protein Calibration
Standard I (covered mass range: ∼5000–17500 Da) from Care
(Bruker) for both positive and negative mode. Calculated and
experimental values are referred to base peaks.
General
In air-sensitive and moisture-sensitive reactions all glassware
was flame dried and cooled under Ar. O-dichlorobenzene
(ODCB) was dried over CaH2, filtered, distilled under re-
duced pressure, and stored over molecular sieves (4 Å). In-
frared (IR) spectra were recorded on a Shimadzu Prestige 21
FT-IR spectrophotometer equipped with an attenuated total
reflectance (ATR) sample unit. Mass spectra were measured
by an Autoflex II time-of-flight (TOF)/TOF Bruker spectrometer
(Bremen, Germany). trans-3-indoleacrylic acid (IAA), α-cyano-4-
hydroxycinnamic acid (HCCA), 2-mercaptobenzothiazole (MBT),
1,8,9-trihydroxyanthracene (Dithranol), 2,5-dihydroxybenzoic
acid (DHB) and 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-
enylidene]malononitrile (DCTB) were used as matrix materials.
Samples co-crystallized with matrices on the probe were ion-
ized by a nitrogen laser pulse (337 nm) and accelerated under
20 kV with time-delayed extraction before entering the TOF mass
spectrometer. Matrix (20 mg/mL) and sample (2 mg/mL) were dis-
solved in tetrahydrofuran (THF). The ratio of matrix solution to
sample was 100 : 1. For all matrices, 5 µl mixture of matrix and
sample was applied to a MALDI-TOF MS probe and air-dried. The
laser intensity was attenuated appropriately to obtain the best
signal-to-noise ratio and isotopic resolution for the matrix used.
ExternalcalibrationwasperformedusingPeptideCalibrationStan-
The synthesis of the acid dendrons was performed in a
convergent manner following the methodology outlined in
Scheme 1, which is based in our previous experience in the
synthesis of poly(phenylenevinylene) dendrons.[20]
Diphosphonates
1 and 2 were obtained by Arbuzov
reaction of methyl 3,5-bis(bromomethyl)benzoate and 1,3-
bis(bromomethyl)-5-iodobenzene, respectively, with triethyl
phosphite following a standard methodology.[21–23]
(E,E)-3,5-Bis(4-hexyloxystyryl)benzoic acid (3)
To a stirred solution of diphosphonate 1 (1 g, 2.29 mmol) and
4-hexyloxybenzaldehyde (945 mg, 4.58 mmol) in anhydrous THF
(20 mL) under argon was added potassium tert-butoxide (642 mg,
5.72 mmol) in small portions. The mixture was stirred at room
temperature for 3 h. After hydrolysis with 1M KOH (5 mL), the
mixture was heated under reflux for an additional 4 h and acidified
with concentrated HCl. The precipitated dendron was isolated by
filtration and purified by washing with hot ethanol. Yield 75%.
Colorless solid. Mp 154–156 ◦C. 1H-NMR (CDCl3, 500 MHz) δ: 0.92
(t, 6H, J = 7.0 Hz, 2xCH3), 1.36 (m, 8H, 4xCH2), 1.48 (m, 4H, 2xCH2),
1.80 (m, 4H, 2xCH2), 3.99 (t, 4H, J = 7.0 Hz, 2xOCH2), 6.91 (A of ABq,
c
www.interscience.wiley.com/journal/jms
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2009, 44, 613–620

Ln(III)-cored poly(phenylenevinylene) dendrimers
ArH), 10.82 (s, 1H, COOH). 13C-NMR and DEPT (THF-d8, 125 MHz)
δ: 167.5 (COOH), 160.2 (C), 139.6 (C), 139.3 (C), 138.8 (C), 132.9
(C), 131.0 (C), 130.8 (CH), 129.7 (CH), 128.8 (CH), 128.8 (CH), 128.5
(CH), 127.7 (CH), 126.8 (CH), 124.8 (CH), 124.1 (CH), 115.4 (CH), 68.6
(OCH2), 32.6 (CH2), 30.3 (CH2), 26.7 (CH2), 23.5 (CH2), 14.4 (CH3).
FT-IR (ATR) ν: 2400–3100 (broad), 1686 (C O), 1605, 1584, 1508,
1248, 1173, 955 cm−1. MALDI-TOF (dithranol) m/z 1135.3 (M+·).
Anal. calculated for C79H90O6: C, 83.56; H, 7.99. Found: C, 83.27; H,
7.95.
Preparation of the lanthanide(III)-cored poly(phenyleneviny-
lene) dendrimers
The Ln(III)-cored poly(phenylenevinylene) dendrimers were pre-
pared according to the procedure previously described in the
literature.[9] A stoichiometric mixture of the anhydrous lanthanide
triacetate, Ln(OAc)3, and the appropriate acid dendron (1 : 3 molar
ratio) were refluxed in ODCB. The acetic acid generated by the
exchangereactionwasdistilledoffwithODCBcontinuouslyduring
the reaction for 3 h followed by final evaporation to dryness under
reduced pressure.
G13La : Yield 97%. Brown solid. FT-IR (ATR) ν: 1605, 1508, 1450,
1393, 1244, 1173, 1028, 845, 814 cm−1. MALDI-TOF m/z 2241.1
([G14La]−). (Although La3+ is a diamagnetic ion, the NMR
spectra of the La3+-complexes described here also showed
very broad signals, avoiding their characterization by this
technique.)
G13Er : Yield 98%. Yellow-brown solid. FT-IR (ATR) ν: 1605,
1524, 1508, 1450, 1404, 1426, 1172, 1028, 959, 845, 814 cm−1
MALDI-TOF m/z 2269.1 ([G14Er]−).
.
G23La : Yield: 98%. Pale yellow solid. FT-IR (ATR) ν: 1605, 1584,
1506, 1244, 1171, 1028, 957 cm−1. MALDI-TOF m/z 4676.3
([G24La]−).
G23Er : Yield 98%. Pale yellow solid. FT-IR (ATR) ν: 1605, 1584,
1506, 1454, 1393, 1244, 1171, 955 cm−1. MALDI-TOFm/z 4703.8
([G24Er]−).
Figure 1. First-generation and second-generation lanthanide(III)-cored
poly(phenylenevinylene) dendrimers.
Results and Discussion
4H, J = 8.5 Hz, ArH), 7.01 (A of ABq, 2H, J = 17.0 Hz, 2xCH ), 7.18
(B of ABq, 2H, J = 17.0 Hz, 2xCH ), 7.48 (B of ABq, 4H, J = 8.5 Hz,
ArH), 7.78 (br s, 1H, ArH), 8.09 (br s, 2H, ArH). 13C-NMR and DEPT
(CDCl3, 125 MHz) δ: 171.2 (COOH), 159.2 (C), 138.5 (C), 129.9 (C),
129.8 (CH), 129.4 (C), 129.0 (CH), 127.9 (CH), 126.3 (CH), 125.1 (CH),
114.8 (CH), 68.1 (OCH2), 31.6 (CH2), 29.2 (CH2), 25.7 (CH2), 22.6
(CH2), 14.0 (CH3). FT-IR (ATR) ν: 2400–3100 (broad), 1693 (C O),
Selection of the matrix
DCTB produced good quality spectra in positive and negative
modes and was chosen for further studies. The relative softness of
this aprotic matrix compared to other polar matrices is evidenced
by a highly unlikely impossible protonation of the analytes and
lower laser power requirements. These advantages make DCTB a
suitable matrix for metal complex characterization.[24] Polar acid
matrices, such as IAA and HCCA, are not specifically designed
to promote the formation of radical ions but to give protonated
ions, and consequently these cause the complex to split during
the mass spectrometric measurement. Indeed, the experiments
performedwiththesematricesyieldedMALDIspectrathatshowed,
in either positive or negative ion mode, exclusively the peak
1508, 1248, 1173 cm−1. MALDI-TOF (dithranol) m/z 526.3 (M+·
,
100). Anal. calculated for C35H42O4: C, 79.81; H, 8.04. Found: C,
79.59; H, 8.02.
(E,E,E,E)-3,5-Bis-[3,5-bis(4-hexyloxystyryl)styryl]benzoic acid
(6)
This compound was prepared from diphosphonate 1 and first-
generation dendritic aldehyde 5 following a similar procedure
as described above. The product was purified by washing with
hot ethyl acetate. Yield 70%. Colourless solid. 1H-NMR (THF-d8,
500 MHz) δ: 0.93 (t, 12H, J = 7.0 Hz, 4xCH3), 1.38 (m, 16H, 8xCH2),
1.50 (m, 8H, 4xCH2), 1.80 (m, 8H, 4xCH2), 3.99 (t, 8H, J = 7.0 Hz,
4xOCH2), 6.91 (A of ABq, 8H, J = 8.5 Hz, ArH), 7.10 (A of ABq, 4H,
J = 16.5 Hz, 4xCH ), 7.26 (B of ABq, 4H, J = 16.5 Hz, 4xCH ), 7.41
(s, 4H, 4xCH ), 7.51 (B of ABq, 8H, J = 8.5 Hz, ArH), 7.60 (br s, 2H,
ArH), 7.68 (br s, 4H, ArH), 8.06 (br s, 1H, ArH), 8.17 (d, 2H, J = 1.5 Hz,
corresponding to the dendritic free fragment. A similar finding
[9]
was previously reported by Frechet et al. and clearly indicates
dissociation of the complex. On the other hand, dithranol, MBT
and DHB, which are typically used as matrices for desorption-
ionization of synthetic polymers and small molecules,[25] always
gave lower quality spectra when compared with DCTB. The main
peak again corresponded to the free ligand ion, although other
peaks, fragments and matrix-related ions were also observed. The
resolution and signal-to-noise ratio of the base peak were lower,
´
c
J. Mass. Spectrom. 2009, 44, 613–620
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/jms

J. C. García-Martínez et al.
Figure 2. Negative reflector mode spectra of Ln3+-cored first-generation dendrimers G13Ln with expanded regions comparing the theoretical isotope
distribution for the complex anion [G14Ln]− (on the left) with the data acquired (on the right).
c
www.interscience.wiley.com/journal/jms
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2009, 44, 613–620

Ln(III)-cored poly(phenylenevinylene) dendrimers
mode shows the formation of complexes between dendrons and
Ln3+, the poor quality and low peak resolution rule out the use of
thismodeasadefinitivetechniquetocharacterizethecompounds.
Negative ion mode
In a similar way to the positive mode, the Ln3+-cored dendritic
complexes did not show the radical ion [M]•− in the negative
reflector mode. However, well-defined and highly resolved base
peaks were found in all cases. The MALDI-TOF spectrum of the
Er3+-cored first-generation dendrimer, G13Er, in DCTB is shown in
Fig. 2 (top). The spectrum is easy to interpret with only one peak
at m/z = 2269.1. This is the only signal observed – there are no
fragmentationsormatrix-relatedionsatlowerm/zvaluesorsignals
correspondingtoasample/matrixadductathigherm/z values.The
lackofotherionsalsogivesanindicationofthepurityofthesample.
This peak corresponds to the negative ion of [G14Er]− (calculated
m/z = 2269.1). The presence of multi-isotopic elements means
the isotope distribution is very distinguishable, almost like a finger
print, and this proved to be an excellent match with the theoretical
distribution, thus providing strong evidence for the formation of
[G14Er]−.
Figure 3. MALDI-TOF negative reflector mode spectrum of a physical
mixture prepared by mixing G13Er with the first-generation acid dendron
Gꢁ1 using DCTB as the matrix.
especially when DHB was used, because greater laser power was
needed to acquire the spectra (see Supporting information, Fig.
S11-S12). Thus, all figures presented in this paper relate to spectra
obtained using DCTB as the matrix.
A MALDI-TOF MS/MS experiment in negative mode on the base
peak was also performed with the intention of confirming the
composition of the formed species (see Supporting information,
Fig. S18). The MS/MS spectrum showed a few peaks with an
isotopic pattern that indicated the presence of an Er atom
in the fragments. It is important to highlight that a peak at
m/z = 1744.5 corresponding to the species [G13Er]•− (calculated
m/z = 1743.8) was observed, i.e. loss of a dendron from the
complex [G14Er]−. Moreover, the presence of the dendritic unit
on the structure was definitively confirmed by the existence of
fragmentation peaks associated with ligand loss: i.e. [G1]− (m/z =
525.3), [G12]− (m/z = 1051.6) and [G13]− (m/z = 1577.7).
The formation of [G14Er]− can again be explained in terms
of the tendency of the lanthanide metal ions to have a high
co-ordination environment, which during the desorption process
incorporate other dendritic ligands to reach a stable co-ordination
number of eight. The same peak was observed when dithranol or
MBT were used as matrices, but in these cases its intensity was
lower.Otherpeaksrelatedtofragmentationsormatrixinteractions
also appeared in the spectrum, thus complicating the study.
Analogous results were found for the La3+-cored first-
generation dendrimer G13La. The MALDI-TOF spectrum showed
a base peak corresponding to [G14La]− (m/z = 2241.1, calcu-
lated m/z = 2241.1) and an excellent match for the isotope
distribution of the complex anion (Fig. 2, bottom). Analysis of the
fragments in the MS/MS spectrum revealed the presence of the
dendritic ligand coordinated to the metal, also becoming visible
a peak at m/z = 1716.6 corresponding to [G13La]•− (calculated
m/z = 1715.8) (see Supporting information, Fig. S19).
Positive ion mode
When the Ln3+-cored dendritic complexes were analysed in the
positive ion extraction mode, peaks corresponding to radical ion
[M]•+ or protonated [M + H]+ forms were not observed and
nor were single alkali metal adducts [M + Na]+ or [M + Ag]+
when a cationization reagent was added. A number of signals
above the molecular mass of the compounds were found instead.
For instance, the spectrum of the Er3+-cored first-generation
dendrimer, G13Er, showed three main peaks (m/z = 4706.2,
6448.3, 8199.7) and these can be attributed to the species
[G18Er3]+ (calculated m/z = 4707.2), [G111Er4]+ (calculated
m/z = 6451.0) and [G114Er5]+ (calculated m/z = 8195.9),
respectively (see Supporting information, Figs. S13-S15). The
tendency of lanthanide ions towards a high co-ordination
environment may explain the formation of adducts during the
desorption process. When a cationization reagent such as sodium
iodide was added to the matrix/sample mixture, the spectrum
only showed peaks corresponding to the substitution of one
Er3+ atom by three Na+ in the form [G12Na3]+ (m/z = 1120.4),
among other combinations. Smaller peaks close to the base peak
at higher m/z values were also found and these could be related
to an interaction between the matrix and the adducts (Supporting
information, Fig. S16). SimilarspectrawereobtainedwiththeLa3+
-
cored first-generation dendrimer G13La (Supporting information,
Fig. S17).
In order to clarify if the species [G14Ln]− are really formed
during the mass analysis, we registered the MALDI-TOF spectrum
of a physical mixture prepared by mixing G13Er with a different,
although structurally related, acid dendron Gꢁ1. Together with
the signal associated to [G14Er]−, several peaks corresponding
to crossover species [G13Gꢁ1Er]−, [G12Gꢁ12Er]− and [G1Gꢁ13Er]−
were also observed (Fig. 3). Interestingly, the signal intensities of
the crossover species became more noteworthy as the amount of
Gꢁ1 increased in the mixture.
Generally, the peaks were broad, isotopically unresolved and
with intensities that were highly dependent on the matrix/sample
ratio. This fact supports the idea of a recombination of species
during the desorption process and makes the experiment very
irreproducible and unsuitable for an unequivocal characterization.
When the La3+- or Er3+-cored second-generation dendrimers
G23Ln were analysed, the formation of adducts was very difficult
due to steric hindrance and, for instance, peaks associated with
the species [G28Er3]+ (m/z = 9578.1) and [G211Er4]+ (m/z =
13 149.1) were not found. Although the positive ion extraction
Alternatively, mass analyses in negative linear mode were
also performed for G13Ln complexes. In a comparable manner
c
J. Mass. Spectrom. 2009, 44, 613–620
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/jms

J. C. García-Martínez et al.
Figure 4. Negative reflectormode spectraof Ln3+-cored second-generation dendrimers G23Ln with expanded regions comparingthe theoreticalisotope
distribution for the complex anion [G24Ln]− (on the left) with the data acquired (on the right).
c
www.interscience.wiley.com/journal/jms
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2009, 44, 613–620

Ln(III)-cored poly(phenylenevinylene) dendrimers
to the positive mode, the spectra showed peaks at higher
m/z values, which can be attributed to higher co-ordination
polynuclear species, containing 7, 10 and even 13 monodendrons:
i.e. [G17Ln2]−, [G110Ln3]− and [G113Ln4]−, respectively (see
Supporting information, Fig. S20-S21). These peaks appeared
muchmoreintenseintheanalysisofaphysicalmixturepreparedby
mixing G13Ln with the corresponding acid dendron G1. All these
results strongly support the formation of the higher co-ordination
species during the mass analysis.
Acknowledgements
This work was funded by the Spanish Ministerio de Educacio´n
y Ciencia (projects NAN2004-08843-C05-02 and CTQ2006-08871)
and the Junta de Comunidades de Castilla-La Mancha (projects
PCI08-0033 and PAI07-0007-1001). A.C.R. and J.C.G.-M. also thank
the Spanish MEC for a predoctoral fellowship and financial support
through the Ramon y Cajal program, respectively.
When a mixture of the lanthanide triacetate Ln(OAc)3 and the
first-generation acid dendron 3 in a 1 : 4 molar ratio was refluxed in
ODCB under the same experimental conditions described above,
the analysis of the final product showed unreacted free acid. The
same result was obtained in the treatment of G13Ln with an
equimolar amount of the acid dendron 3. These two experiments
guarantee an empirical formula of G13Ln for the complexes, i.e. a
fourth ligand is not incorporated under the reaction conditions.
MALDI-TOF MS in negative reflector mode was also useful
to study the second-generation complexes. The spectra of the
Ln3+-cored dendrimer G23Ln are shown in Fig. 4. Comparable
outcomeswereobtainedunderthesameexperimentalconditions.
Base peaks corresponding to [G24Er]− (m/z = 4703.8) and
[G24La]− (m/z = 4676.3) were found along with a few others
less significant (calculated values are [G24Er]− m/z = 4704.6 and
[G24La]− m/z = 4676.6). Once again, the excellent agreement
between the theoretical and observed isotope distribution
confirms the formation of a complex between the dendrons
and the lanthanide metal ion.
References
[1] H. K. Kim, S. G. Roh, K.-S. Hong, J.-W. Ka, N. S. Baek, J. B. Oh,
M. K. Nah, Y. H. Cha, J. Ko. Novel lanthanide-cored supramolecular
systems with highly efficient light-harvesting dendritic arrays
towards tomorrow’s information technology. Macromolecular
Research 2003, 11, 133.
[2] H. K. Kim, J. B. Oh, N. S. Baek, S.-G. Roh, M. K. Nah, Y. H. Kim. Recent
progress in luminescent lanthanide complexes for advanced
photonics applications. BulletinoftheKoreanChemicalSociety 2005,
26, 201.
[3] E. Desurvire. Erbium-Doped Fiber Amplifiers: Principles and
Applications. Wiley: New York, 1994.
[4] K. Kuriki, Y. Koike, Y. Okamoto. Plastic optical fiber lasers and
amplifiers containing lanthanide complexes. Chemical Reviews
2002, 102, 2347, DOI: 10.1021/cr010309g.
[5] L. H. Slooff, A. van Blaaderen, A. Polman, G. A. Hebbink, S. I. Klink,
F. C. J. M. van Veggel, D. N. Reinhoudt, J. W. Hofstraat. Rare-earth
doped polymers for planar optical amplifiers. Journal of Applied
Physics 2002, 91, 3955, DOI:10.1063/1.1454190.
[6] T. Oyamada, Y. Kawamura, T. Koyama, H. Sasabe, C. Adachi.
Formation of europium chelate complexes by vacuum co-
deposition and their application in organic light-emitting diodes.
Advanced Materials 2004, 16, 1082, DOI: 10.1002/adma.200400090.
[7] T.-S. Kang, B. S. Harrison, M. Bouguettaya, T. J. Foley, J. M. Boncella,
K. S. Schanze, J. R. Reynolds. Near-infrared light-emitting diodes
(LEDs) based on poly(phenylene)/Yb-tris(β-diketonate) complexes.
Advanced Functional Materials 2003, 13, 205, DOI:
10.1002/adfm.200390031.
[8] T.-S. Kang, B. S. Harrison, T. J. Foley, A. S. Knefely, J. M. Boncella,
J. R. Reynolds, K. S. Schanze. Near-infrared electroluminescence
from lanthanide tetraphenylporphyrin : polystyrene blends.
Advanced Materials 2003, 15, 1093, DOI: 10.1002/adma.200304692.
[9] M. Kawa, J. M. J. Fre´chet. Self-assembled lanthanide-cored
dendrimer complexes: enhancement of the luminescence
properties of lanthanide ions through site-isolation and
antenna effects. Chemistry of Materials 1998, 10, 286, DOI:
10.1021/cm970441q.
[10] M. Kawa, J. M. J. Fre´chet. Enhanced luminescence of lanthanide
within lanthanide-cored dendrimer complexes. Thin Solid Films
1998, 331, 259, DOI: 10.1016/S0040-6090(98)00928-6.
[11] M. Kawa, T. Takahagi. Improved antenna effect of terbium(III)-cored
dendrimer complex and green-luminescent hydrogel by radical
copolymerization. Chemistry of Materials 2004, 16, 2282, DOI:
10.1021/cm034873e.
[12] J.-W. Ka, H. K. Kim. One-pot synthesis of new functionalized
azacryptands from resorcinol derivatives for advanced
photonic materials. Tetrahedron Letters 2004, 45, 4519,
DOI:10.1016/j.tetlet.2004.04.051.
[13] J. B. Oh, Y. H. Kim, M. K. Nah, H. K. Kim. Inert and stable erbium(III)-
cored complexes based on metalloporphyrins bearing aryl-
ether dendron for optical amplification: synthesis and emission
enhancement. Journal of Luminescence 2005, 111, 255,
DOI:10.1016/j.jlumin.2004.10.006.
Conclusions
Regrettably, the structure elucidation of Ln3+-cored dendrimer
complexes obtained by self-assembly of suitably functionalized
carboxylate dendrons around lanthanide ions is quite elusive
because of their fragile nature. The strong paramagnetism of
lanthanides impedes the use of NMR techniques and, on the
other hand, the gathering of good single crystals for X-ray
diffraction is seldom possible. However, in this paper MALDI-TOF
MS was used to ascertain the formation of first-generation and
second-generation lanthanide(III)-cored poly(phenylenevinylene)
dendrimer complexes, confirming the occurrence of dendritic
ligand–lanthanide interactions. The most reliable results were
found in the negative reflector mode, using DCTB as matrix. Well-
defined and highly resolved base peaks attributed to the adducts
[Gn4Ln]− were found in all cases due to their high stability, with
an excellent match between the experimental and theoretical
isotope distributions. The observed species in the mass analysis
do indicate that dendritic complexes have been formed (FT-IR
results are also an evidence), but not that they comprise three
dendrons. However, the 3 : 1 stoichiometry used for their synthesis
guarantees an empirical formula of Gn3Ln. Identification of this
type of structure can be standardized and a specially tuned
spectrometer is not required. Identical results were obtained with
an Applied Biosystems 4700 Proteomics Analyzer.
[14] J. B. Oh, M.-K. Nah, Y. H. Kim, M. S. Kang, J.-W. Ka, H. K. Kim. Er(III)-
cored complexes based on dendritic Pt(II)-porphyrin ligands:
synthesis, near-IR emission enhancement, and photophysical
studies. Advanced Functional Materials 2007, 17, 413, DOI:
10.1002/adfm.200600451.
Supporting Information
[15] N. S. Baek, M. K. Nah, Y. H. Kim, S.-G. Roh, H. K. Kim. Efficient energy
transfer pathways for the sensitization of lanthanide ions by
luminescent ligands in luminescent lanthanide complexes. Bulletin
of the Korean Chemical Society 2004, 25, 443.
Copies of FT-IR, 1H-NMR, 13C-NMR and additional MALDI-TOF
spectra for the described compounds may be found in the online
version of this article.
c
J. Mass. Spectrom. 2009, 44, 613–620
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/jms

J. C. García-Martínez et al.
[16] N. S. Baek, Y. H. Kim, H. K. Kim. Recent progress in erbium(III)-cored
complexes based on dendritic ligands for advanced photonics
applications. Journal of Nonlinear Optical Physics and Materials
2006, 15, 369, DOI: 10.1142/S0218863506003360.
[17] N. S. Baek, Y. H. Kim, S.-G. Roh, B. K. Kwak, H. K. Kim. The first
inert and photostable encapsulated lanthanide(III) complexes
based on dendritic 9,10-diphenylanthracene ligands: synthesis,
strong near-infrared emission enhancement, and photophysical
studies. Advanced Functional Materials 2006, 16, 1873, DOI:
10.1002/adfm.200500835.
[18] D. L. Stone, G. M. Dykes, D. K. Smith. Dendritic lanthanide
complexes and the effect of Lewis-acid encapsulation. Dalton
Transactions 2003, 3902, DOI: 10.1039/b309643b.
[19] T. Seto, M. Kawa, K. Sugiyama, M. Nomura. XAFS studies of Tb
or Eu cored dendrimer complexes with various properties of
luminescence. Journal of Synchrotron Radiation 2001, 8, 710, DOI:
10.1107/S0909049500015910.
[21] E. Díez-Barra, J. C. García-Martínez, J. Rodríguez-Lo´pez. Synthesis
of novel cross-conjugated dendritic fluorophores containing both
phenylenevinyleneandphenyleneethynylenemoieties.TheJournal
of Organic Chemistry 2003, 68, 832, DOI: 10.1021/jo026019x.
[22] K. H. Ducheˆne, F. Vo¨gtle. Ein kohlenwasserstoff-geru¨st mit zwo¨lf
identischen funktionellen gruppen: synthese einer dodeca-
wirtverbindung. Synthesis 1986, 659, DOI: 10.1055/s-1986-31738.
[23] E. Díez-Barra, J. C. García-Martínez, J. Guerra, V. Hornillos, S. Merino,
R. del Rey, R. I. Rodríguez-Curiel, J. Rodríguez-Lo´pez, P. Sa´nchez-
Verdu´, J. Tejeda, J. Tolosa. On the synthesis of heterocyclic
dendrons. Arkivoc 2002, V, 17.
[24] M. F. Wyatt, B. K. Stein, A. G. Brenton. Characterization of various
analytes using matrix-assisted laser desorption/ionization time-
of-flight mass spectrometry and 2-[(2E)-3-(4-tert-butylphenyl)-2-
methylprop-2-enylidene]malononitrile matrix. Analytical Chemistry
2006, 78, 199, DOI: 10.1021/ac050732f.
[25] F. Hillenkamp, J. Peter-Katalinic´ (eds). In MALDI MS: APracticalGuide
to Instrumentation, Method and Applications. Wiley-VCH: Weinheim,
2007.
[20] J. C. García-Martínez, E. Díez-Barra, J. Rodríguez-Lo´pez. Con-
jugated dendrimers with poly(phenylenevinylene) and
poly(phenyleneethynylene) scaffolds. Current Organic Synthesis
2008, 5, 267, DOI: 10.2174/157 017 908 785 133 438.
c
www.interscience.wiley.com/journal/jms
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
J. Mass. Spectrom. 2009, 44, 613–620