Design of tailored multi-charged phosphorus surface-block dendrimers

Article abstract of DOI:10.1039/b610632n

Polycationic or neutral dendrons of generation 1 to 3 bearing protonated (or not) amino end groups and a vinyl group at the focal point as well as polyanionic or neutral dendrons of generation 1 to 3 decorated with carboxylate or carboxylic groups on thei

Full text of DOI:10.1039/b610632n

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Design of tailored multi-charged phosphorus surface-block dendrimersw
a
a
a
´
´
Valerie Maraval, Alexandrine Maraval, Gregory Spataro, Anne-Marie
Caminade,*a Jean-Pierre Majoral,*a Dong Ha Kimbc and Wolfgang Knoll*c
Received (in Montpellier, France) 21st July 2006, Accepted 31st August 2006
First published as an Advance Article on the web 20th September 2006
DOI: 10.1039/b610632n
Polycationic or neutral dendrons of generation 1 to 3 bearing protonated (or not) amino end
groups and a vinyl group at the focal point as well as polyanionic or neutral dendrons of
generation 1 to 3 decorated with carboxylate or carboxylic groups on their surface and exhibiting
a vinyl group at the core were prepared. Addition of ethylenediamine to the vinyl core of
dendrons having amino end groups allowed the synthesis of uniquely tailored water-soluble
phosphorus bis-dendrons via core to core assembling of dendrons having a vinyl core. In
preliminary experiments layer-by-layer deposition of bis-dendrons on silica surface was
demonstrated.
(silica, quartz. . .) via layer-by-layer deposition7 as well as for
Introduction
obtaining nanotubes8 and microcapsules,9 walls of these un-
precedented materials being constituted of alternate poly-
cationic and polyanionic layers. Water soluble polycationic
dendrons were also found very useful: they allow the forma-
tion of dendronized nanolatexes with remarkable properties.10
These results obtained with hydrosoluble phosphorus den-
drimers or dendrons prompted us to try to design water
soluble phosphorus bis-dendrons11 (surface-block dendri-
mers12) with one of the two dendrons bearing anionic groups
in order to ensure solubility in water and easy surface
modification (glass, quartz, nanotubes, nanocapsules, nano-
particles. . .) via electrostatic interactions, the other one bear-
ing easily quaternizable amino end groups or any type of
functional groups which will bring new properties for the
considered surfaces and/or nanomaterials.
Dendrimers are symmetrical and spherical macromolecules,
comprising a relative dense shell and composed of a core,
branching sites, and terminal groups that usually form a well-
defined structure.1 Dendrons, also called dendritic wedges
differ from dendrimers by the presence of one (or more)
functional group(s) located at the core, these functional
groups being generally different from the end groups located
at the surface. Such a diversity in the nature and the location
of functional groups offers many possibilities to build original
dendritic macromolecules as already exemplified by us2,3 and
by others,1 therefore enhancing in principle the use of such
nano-objects in different fields ranging from biology to mate-
rial sciences.
Indeed core–core coupling reactions between phosphorus
containing dendrons or core-surface couplings involving both
dendrons and dendrimers led for example to the formation of
surface-block dendrimers I,3 or layer-block dendrimers II.4
Direct grafting of dendrons within the cascade structure of
dendrimers gives rise via ‘‘core–interior’’ coupling reactions
to other unprecedented dendritic macromolecules III5
(Scheme 1).
We report here the preparation of multifunctionalized
dendrons of generation 1 to 3, their core to core assembly
giving rise to unique tailored water soluble phosphorus bis-
dendrons and preliminary results concerning their use for layer
by layer deposition.
Aiming at proposing original properties and applications of
dendrons and dendrimers we focused first our attention to the
preparation of water soluble phosphorus dendrimers which
were found suitable candidates for a number of applications in
biology.6 Polycationic and polyanionic dendrimers also
proved in our hands to be useful for surface modification
a Laboratoire de Chimie de Coordination du CNRS, 205 route de
Narbonne, 31077 Toulouse Cedex 4, France. E-mail: caminade@
lcc-toulouse.fr, majoral@lcc-toulouse.fr; Fax: þ33 (0)5 61 55 30 03
b Division of Nano-sciences and Department of Chemistry Ewha
Womans University II-I Daehyun-Dong, Seodaemun-Gu, Seoul
120-750, Korea
c Max Planck Institute for Polymer Research, Ackermannweg 10,
55128 Mainz, Germany. E-mail: knoll@mpip-mainz.mpg.de; Fax:
þ49 (0)6131 379 360
w Electronic supplementary information (ESI) available: characteriza-
tion of all compounds (31P, 1H and 13C NMR data). See DOI: 10.1039/
b610632n
Scheme 1 Some examples of dendritic architectures built from den-
drons.
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tions, i.e. 2 and 3, incorporating also a primary amine at the
core and tertiary amines on the surface. This implies first the
preparation of dendrons of generation 2, 1-[G2] and 3, 1-[G3]
bearing respectively, 4 and 8 terminal P(S)Cl2 groups. For this
purpose the growing of 1-[G1] is pursued using our classical
method of synthesis of dendrimers whose first steps are
described above, i.e. nucleophilic substitution of P–Cl bonds
of 1-[G1] with 4-hydroxybenzaldehyde followed by condensa-
tion reaction with the phosphorhydrazide H2NN(CH3)P(S)Cl2
leading to the dendron 1-[G2], which in turn is treated with
hydroxybenzaldehyde and H2NN(CH3)P(S)Cl2 to give 1-[G3].
The formation of the dendritic wedge 2-[G2] from 1-[G2]
involves first the addition of 8 equiv. of the diamine
H2N(CH2)2NEt2 which leads directly to the expected polyca-
tion bearing 8 terminal protonated amino groups on the
surface. A marked shielding effect is observed in 31P NMR
for the signal of the phosphorus which undergoes the reaction,
from 63.3 to 73.3 ppm. The presence of the unreacted vinyl
Results and discussion
Synthesis of dendrons having amine/ammonium end groups
The first reaction carried out to synthesize these dendrons is a
Staudinger reaction between diphenylvinylphosphine and the
azide N3P(S)(OC6H4CHO)2 leading to the dialdehyde 1
(Scheme 2). Condensation of 1 with the phosphorhydrazide
H2NN(CH3)P(S)Cl2 affords the first generation dendron
1-[G1]4 which is further condensed with the diamine
H2N(CH2)2NEt2 to give directly the tetraammonium dendron
2-[G1]. Hydrogen chloride formed during this process is quan-
titatively trapped by the terminal tertiary amines. Such an
experiment can be easily followed by 31P NMR, the grafting of
four equivalents of diamine onto 1-[G1] inducing a large
modification of the chemical shift corresponding to the term-
inal phosphorus groups (from 63.5 to 73.4 ppm). It must be
emphasized that the use of a stoichiometric amount of amine
prevents Michael type additions at the vinyl core, as shown by
the absence of modification of the chemical shift correspond-
ing to the PQN group at the core (see later). Addition of
sodium hydroxide 1 M to 2-[G1] led to the neutral dendron 3-
[G1]. This reaction is characterized in 31P NMR by a signal
corresponding to external PQS groups at 71.4 ppm. The last
step leading to the targeted dendron 4-[G1] suitable for the
synthesis of bis-dendrons via core–core coupling reactions, is a
Michael type addition involving the vinyl group located at the
1
group is indicated by H NMR (ABCX system) and by 13C
NMR (d ¼ 138.1 ppm for CH2Q). The next step consists of
the treatment of 2-[G2] with sodium hydroxide giving the
neutral dendron 3-[G2] which in turn is treated with a large
excess of ethylenediamine to afford, via a Michael type reac-
tion, the dendron 4-[G2], bearing a primary amine at the
focal point and 8 tertiary amino groups on the outer shell
(Scheme 3).
Similarly dendrons 2-[G3], 3-[G3], and 4-[G3] are grown from
1-[G3] using the above method. All the steps are monitored by
31P, 1H, and 13C NMR. As an example the formation of 4-[G3]
from 3-[G3] is indicated in 31P NMR by a slight deshielding
effect for the signal due to the PQS unit of the PQN–PQS
linkage from 56.4 (3-[G3]) to 55.9 (4-[G3]) ppm, and a deshield-
ing effect from 15.0 to 21.2 ppm for the signal due to the
PQN unit. A marked shielding is also observed for the
CH2Q transformed in NCH2– in 13C NMR from 136.8
to 42.3 ppm.
core of 3-[G1] and
a large excess of ethylenediamine
H2N(CH2)2NH2. The formation of the dendron 4-[G1] is
monitored by NMR: 31P NMR shows two doublets at 21.0
and 55.8 ppm (2Jpp ¼ 32.3 Hz) and one singlet (external
P(S)(NH(CH2)2NEt2)2 groups) at 71.5 ppm while disappear-
ance of the signals due to the vinyl groups is easily observed in
1H NMR.
Such a strategy of synthesis of dendron 4-[G1] of generation
1 was applied for the obtaining of dendrons of higher genera-
Synthesis of dendrons having carboxylic acid/carboxylate end
groups
Having in hand dendrons of generation 1, 2, and 3 with
terminal amino groups and a primary amine at the core we
further investigated the possibility to prepare dendrons keep-
ing a vinyl group on the focal point and presenting carboxylic
or carboxylate end groups.
The preparation of the simplest dendron of this type i.e.
5-[G1] involves first the treatment of 1-[G1] with 4-hydroxy-
benzaldehyde in the presence of caesium carbonate then a
condensation reaction with hydrazine benzoic acid H2NNH
C6H4COOH leading to the dendron 5-[G1] decorated with 4
carboxylic end groups. Such a grafting reaction is character-
ized by the disappearance of the signal corresponding to the
CHO groups in 1H NMR and IR on behalf of new signals due
to the incorporation of both hydrazine and carboxylic groups.
The most characteristic data is the presence of a singlet at
10.86 ppm in 1H NMR and at 167.2 ppm in 13C NMR, due to
the presence of carboxylic groups. Further addition of sodium
hydride leads to the salt 6-[G1], a water soluble dendron of
generation 1 (Scheme 4).
Scheme 2 Synthesis of the dendrons 4-[G1] bearing a primary amine
at the focal point and 4 tertiary amino end groups.
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Scheme 3 Synthesis of dendrons of generation 2 (2-[G2], 3-[G2], 4-[G2]) and 3 (2-[G3], 3-[G3], 4-[G3]).
The same sequence of reactions was applied to dendrons of
generation 2 bearing 8 aldehyde end groups and generation
3 (16 aldehyde end groups). Water-soluble dendrons 6-[G2]
and 6-[G3], respectively were thus obtained and fully charac-
terized (see experimental section). In all of these experiments,
4-[G1], and 6-[G1] (generation 1) on the one hand, and 4-[G2]
and 6-[G2] (generation 2) on the other. Surface-block dendri-
mers 4-[G1]-6-[G1], and 4-[G2]-6-[G2] were thus obtained in
90% yield.
The completion of the reaction was monitored mainly by
31P NMR which pointed out significant shifts concerning
PQNQPQS groups owing to dendron 6-[Gn]. As an example
the 31P signal of the PQN group of PQN–PQS units in 4-[G2]
(terminal amino groups) and 6-[G2] (terminal carboxylate
1
reactions were monitored by 31P, H and 13C NMR.
Synthesis of polyanionic bis-dendrons
Cross coupling reactions between dendrons bearing vinyl
groups and amino groups at the core were attempted with
the goal to prepare tailored water-soluble phosphorus surface-
block dendrimers. Such a coupling involves a Michael type
addition as shown in Scheme 5.
2
groups) at 21.2 (d, Jpp ¼ 33.7 Hz) for 4-[G2] and 14.7 (d,
2Jpp ¼ 28.0 Hz) ppm for 6-[G2] disappears on behalf of
2
2
new signals at 21.6 (d, Jpp ¼ 31.9 Hz) and 21.7 (d, Jpp
¼
30.9 Hz), respectively. A slight change is observed for the
two 31P signals of the PQS group of the PQNQPQS
Typically the reaction proceeds at 70 1C in a THF/water 1/1
solution containing each dendron partner. The first experi-
ments were conducted with dendrons of similar generation i.e.
2
units moving from 55.8 (d, Jpp ¼ 33.7 Hz) for 4-[G2] and
55.1 (d, 2Jpp ¼ 28.0 Hz) for 6-[G2] to 55.6 (d, 2Jpp ¼ 31.9 Hz),
Scheme 4 Preparation of polyanionic dendrons of generation 1 to 3: 6-[G1], 6-[G2], 6-[G3].
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Scheme 5 Strategy of synthesis of symmetrical bis-dendrons (4-[G1]-6-[G1], 4-[G2]-6-[G2]).
2
and 56.1 (d, Jpp ¼ 30.9 Hz) ppm for 4-[G2]-6-[G2]. Comple-
Therefore such a strategy allowed us to couple dendrons of
the same generation. Similarly this methodology was applied
to the coupling of dendrons of different sizes and generation
(Scheme 6). Unsymmetrical multi-charged phosphorus bis-
tion of the reaction is also monitored by H and 13C NMR
1
which show the complete disappearance of the signals due to
the vinyl groups.
Scheme 6 Chemical structure of unsymmetrical bis-dendrons of generation 1, 2 or 3.
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Fig. 1 Schematic diagram for the preparation of a bis-dendron multilayer on a silica surface.
dendrons 4-[G1]-6-[G2], 4-[G2]-6-[G1], 4-[G3]-6-[G1], 4-[G1]-6-
Conclusion
[G3], and 4-[G3]-6-[G2], were thus prepared for the first time
and fully characterized (see Supplementary Informationw).
Different neutral, cationic or anionic phosphorus-containing
dendrons have been prepared. The nature of functional groups
located at the core of these dendrons allows their grafting via a
Michael type reaction and the formation of a large variety of
original bis-dendrons constituted of dendrons of different size
and generation and decorated with carboxylate groups and
protonated (or not) or alkylated (or not) tertiary amino
groups.
Layer by layer deposition
Layer by layer deposition of polycationic and polyanionic
dendrimers allowed us to prepare well defined nanotubes and
microcapsules with walls entirely formed with alternate layers
of these dendrimers.8,9 Electrostatic interactions play a key
role in such a construction and allow for the formation of
stable new nanomaterials. It was tempting to see if such a
deposition could be performed using not two kinds of den-
drimers (or polymers) but using only one kind of building
block, that is to say the water-soluble bis-dendrons reported in
this study. Of course, in a first approach experiments have to
be done on a planar surface in order to facilitate the char-
acterization of the deposition and to prove the feasibility of
the concept.
Preliminary experiments clearly indicate that these macro-
molecules are perfect nano-objects for attractive surface mod-
ification via layer-by-layer deposition. Work is in progress to
study the properties of the multilayers formed in these condi-
tions and to extend such a new strategy for various functio-
nalisation of nanomaterials such as nanotubes, nanocapsules,
nanoparticles, etc. Use of these amphiphilic surface-block
dendrimers for the construction of a variety of supramolecular
structures via self assembly is also under current investigation.
In a preliminary experiment some of these symmetrical bis-
dendrons were deposited on a silica surface precoated with a
monolayer of 3-APDMES (3-aminopropyl dimethylethoxy
silane) in order to ensure a positive surface. The schematic
diagram of construction of multilayers starting from bis-
dendrons 4-[G1]-6-[G1] or 4-[G2]-6-[G2] is shown in Fig. 1.
Deposition of the first layer took place via strong electrostatic
interaction between the positively charged silica surface and
the polyanionic part (6-[G1]) of bis-dendrons 4-[G1]-6-[G1]
(Fig. 1a). The neutral surface thus obtained was quaternized
using methyl iodide as alkylating reagent (Fig. 1b). The
resulting positive surface was then coated with the bis-dendron
4-[G1]-6-[G1] (Fig. 1c). The bis-dendron thin film was built by
reiteration of quaternization and adsorption sequences up to
deposition of 4 layers. A similar process was used starting with
bis-dendrons of generation 2, i.e. 4-[G2]-6-[G2]. UV-visible
absorbance, SPRS (surface plasmon resonance spectroscopy)
measurements allowed characterization of these controlled
associations of monolayers which were also characterized by
AFM.13
Experimental
General
All manipulations were carried out with standard high vacuum
and dry-argon techniques. The solvents were freshly dried and
distilled (THF and ether over sodium/benzophenone, pentane
and CH2Cl2 over phosphorus pentoxide, toluene over so-
dium). Classical 1H, 13C, 31P NMR spectra were recorded
with Bruker AC 200, AC 250, DPX 300 or AMX 400 spectro-
meters. References for NMR chemical shifts are 85% H3PO4
for 31P NMR, SiMe4 for 1H and 13C NMR. The attribution of
13C NMR signals has been done using Jmod, two-dimensional
HMBC, and HMQC, Broad Band or CW 31P decoupling
experiments when necessary.
Syntheses
Synthesis of dendrons bearing ammonium end groups. A
stoichiometric amount of N,N-diethylethylenediamine (n ¼
1, 1.619 mmol, 232 mL; n ¼ 2, 1.435 mmol, 205 mL; n ¼ 3,
1.678 mmol, 240 mL) was added dropwise by syringe with
strong stirring to a solution of dendron (n ¼ 1, 0.410 mmol,
0.350 g; n ¼ 2, 0.179 mmol, 0.330 g; n ¼ 3, 0.105 mmol,
0.400 g) in distilled THF (15 mL). The solvent was removed by
Such preliminary results validate for the first time our
assumption that these unique bis-dendrons can be advanta-
geously used for layer-by-layer deposition on a surface instead
of alternating between polycationic and polyanionic dendri-
mers.
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filtration after stirring overnight at room temperature. The
residue was washed with distilled THF and evaporated to
dryness to give the dendrons 2-[Gn] as white powders. Yields:
2-[G1] 95%; 2-[G2] 98%; 2-[G3] 96%.
Acknowledgements
Thanks are due to the Fonds Social Europe
´
en (grant to G.S.)
and to the CNRS-DFG program for financial support.
Synthesis of dendrons bearing amine end groups. A solution
of NaOH 1 M (n ¼ 1, 1.2 mL; n ¼ 2, 1.15 mL; n ¼ 3, 1.12 mL)
was added dropwise to a stirred solution of 0.400 g of 2-[Gn]
(n ¼ 1, 0.303 mmol; n ¼ 2, 0.144 mmol; n ¼ 3, 0.070 mmol) in
distilled water (30 mL). The precipitate was recovered by
centrifugation and dissolved in chloroform. Then, the organic
solution obtained was dried over Na2SO4, filtered and evapo-
rated to dryness to give 3-[Gn] as white powders. Yields: 3-[G1]
90%; 3-[G2] 95%; 3-[G3] 97%.
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solution of 0.300 g of 3-[Gn] (n ¼ 1, 0.256 mmol; n ¼ 2, 0.121
mmol; n ¼ 3, 0.059 mmol) in THF (20 mL) was added a large
excess of ethylenediamine (n ¼ 1, 0.102 mol, 6.8 mL; n ¼ 2,
48.426 mmol, 3.2 mL; n ¼ 3, 23.579 mmol, 1.6 mL). The
resulting mixture was stirred at room temperature for 3 hours.
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Yields: 4-[G1] 95%; 4-[G2] 97%; 4-[G3] 98%.
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Synthesis of dendrons bearing carboxylic acid end groups. To
a solution of 1-[G0n] (n ¼ 1–3) (obtained by reaction of
hydroxybenzaldehyde on 1-[Gn]) in THF were added 1.3x
equiv. (n ¼ 1, x ¼ 4; n ¼ 2, x ¼ 8; n ¼ 3, x ¼ 16) of
hydrazinobenzoic acid. A sufficient quantity of dry methanol
to obtain a homogeneous solution was then added and the
resulting solution was stirred overnight at room temperature.
Then, 0.3x equiv. of ScavengePore polyphenylethyloxybenzal-
dehyde was added and the mixture was smoothly stirred at
room temperature for 3 days. After filtration, the solution was
evaporated to dryness and the new dendron was obtained as a
pale orange powder. Yields: 5-[G1] 93%; 5-[G2] 95%; 5-[G3]
96%.
Synthesis of dendrons bearing carboxylate end groups. To a
solution of 5-[Gn] (n ¼ 1–3) in THF was added a stoichio-
metric amount of sodium hydride (n ¼ 1, 4 equiv.; n ¼ 2, 8
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´
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´
0
a solution of 4-[Gn] in THF was added 0.5 equiv. of 6-[Gn ].
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homogeneous solution was obtained. This mixture was heated
at 70 1C in a sealed flask during one week. Then, 0.5 equiv. of
ScavengePore polyphenylethyloxybenzaldehyde was added
and the resulting mixture was smoothly stirred during 2 days
at room temperature. After filtration, the solution was evapo-
rated to dryness, the bis-dendron was washed with ether :
pentane mixtures to give pale orange powders. Yields: 4-[G1]-
6-[G1] 87%; 4-[G2]-6-[G2] 90%; 4-[G1]-6-[G2] 93%; 4-[G2]-6-
[G1] 90%; 4-[G3]-6-[G1] 86%; 4-[G3]-6-[G2] 92%; 4-[G1]-6-[G3]
93%.
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13 Full details will be reported in a forthcoming paper.
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This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2006
1736 | New J. Chem., 2006, 30, 1731–1736