Viologen-based dendritic macromolecular asterisks: Synthesis and interplay with gold nanoparticles

Article abstract of DOI:10.1039/c4cc02392g

The viologen-skeleton reacts with a hydrazine-terminated cyclotriphosphazene core to provide novel dendritic macromolecular asterisks that efficiently exchange, deliver and stabilize gold nanoparticles for up to eight months. the Partner Organisations 201

Full text of DOI:10.1039/c4cc02392g

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Viologen-based dendritic macromolecular
asterisks: synthesis and interplay with gold
nanoparticles†
Cite this: Chem. Commun., 2014,
50, 6981
Received 1st April 2014,
Accepted 7th May 2014
Nadia Katir,a Abdelkrim El Kadib,*a Vincent Colliere,b Jean Pierre Majoral*b and
`
Mosto Bousminaac
DOI: 10.1039/c4cc02392g
www.rsc.org/chemcomm
The viologen-skeleton reacts with a hydrazine-terminated cyclo- asterisks incorporating three types of phosphorus units: a
triphosphazene core to provide novel dendritic macromolecular cyclotriphosphazene core, phosphonates end groups and hexa-
asterisks that efficiently exchange, deliver and stabilize gold nano- fluorophosphate as counteranions. Interestingly, standard organic
particles for up to eight months.
reactions and easy work-up procedures provide a library of mono-,
bis-, tris- and tetrakis-viologen-based molecular and macro-
Owing to their various properties, 4,40-bipyridinium derivatives molecular building blocks that can react with a functional cyclotri-
better known as viologens constitute staple building blocks in phosphazene core to generate photo-reactive phosphorus–viologen
electro-chemistry as well as in coordination and supramolecular macromolecular asterisks. These soft scaffolds were used here
chemistry.1 Thanks to their cationic character and their ability to efficiently stabilize nanosized gold particles.
to undergo two successive one-electron reversible reductions,
The first step in constructing these architectures consists in
viologen-based macromolecular systems have applications in the synthesis of a viologen terminated with aldehyde on one
several fields, including medicinal chemistry and materials side and a phosphonate on the other side. Thereafter, a set of
science, mainly as interacting charges with cells and living 4,40-pyridin-pyridinium featuring terminal phosphonates (P1
organisms,2 reducing agents for active metal particles,3 switch- and P3) or aldehyde extremities (P5 and P7) that are prone to
able ionic liquids,4 photo-and electrochromic materials5 and as single alkylation were isolated in good yields (71 to 89%).
mediators for charge-induced energy transfer in photovoltaic Alkylation of P1 and P3 by 4-(methylbromo)benzaldehyde
devices.6 Their introduction into classical linear polymers and followed by ionic exchange with NH4PF6 affords mono-viologen
dendrimers offers additional opportunities in imparting specific 1V and bis-viologen 2V in good yields (73–75%) as their hexa-
properties to these macromolecular structures. Dendrimers are fluorophosphate salts. Selective mono-alkylation of P3 by 1,4-bis-
of particular interest due their well-designed and ramified bromomethylbenzene provides bromo-ended P4 (yield: 58%)
structure, their nanometric size, periodically alternated diver- that can condensate to mono-substituted viologens P5 and P7.
ging and well-separated branches and the presence of multi- This quaternization followed by ionic exchange enables quanti-
functional end groups.7
tative access to 3V (89%) and 4V (84%) as their hexafluoro-
The incorporation of the viologen skeleton within the frame- phosphate salts (Scheme 1). These novel precursors, presenting
work of dendritic nanostructures8 has been shown to bring specific good solubility in acetone and acetonitrile, were unambiguously
1
photo- and bio-activity relationship. However, to the best of our characterized by H, 13C and 31P NMR and mass spectroscopy
knowledge no report is available on viologen macromolecular (see S1 and S2, ESI†).
asterisks9 assembled around an inorganic core.10
Owing to their reactive aldehyde extremity, nV (n = 1 to 4)
Herein, we report a novel approach allowing the design of easily react with hexahydrazidocyclotriphosphazene used as
such new ambiphilic viologen-based dendritic-macromolecular a core11 to provide quantitatively D1V, D2V, D3V and D4V
(Scheme 2). 31P NMR shows two signals: a single symmetrical
a
signal of the (PQN)3 ring at 17 ppm (instead of the one
observed at 29 ppm for the starting core)11 and a second signal
at around 23–25 ppm assignable to the peripheral phosphonate
of the viologen framework. Combining these results with those
of 1H and 13C NMR – in which the disappearance of aldehydes
has been ascertained – unequivocally demonstrates that the
ring core and the viologen asterisks are chemically linked via a
`
Euro-Mediterranean University of Fez (UEMF), Fes-Shore, Route de Sidi harazem,
`
Fes, Morocco. E-mail: a.elkadib@ueuromed.org
b Laboratoire de Chimie de Coordination, Centre National de la Recherche
Scientifique, 205 route de Narbonne, 31077 Toulouse Cedex 4, France
c Hassan II Academy of Science and Technology, Avenue Mohammed VI,
10222 Rabat, Morocco
† Electronic supplementary information (ESI) available: Synthesis and characteri-
zations of precursors and materials. See DOI: 10.1039/c4cc02392g
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being more significant when comparing D1V and D2V (540 nm)
with D3V and D4V (495 to 507 nm) (see S5, ESI†). This hypso-
chromic effect might emanate from the difference in the supra-
molecular organization of these macromolecular asterisks, which
probably results in shielding or crowding the photo-responsive
viologen motifs in different electronic environments.12
Considering the ability of viologens to interact with metallic
species,3 the aim was to verify if the number of viologen units
grown in each asterisk accounts for the interplay with gold
particles (asterisk effect). To this end, D1V, D2V, D3V and D4V
were subjected to a stoichiometric reaction with HAuCl4 (with
respect to the number of viologen units in each dendrimer) and
the reaction was monitored by UV-visible spectroscopy. In the
absence of any reducing agent, besides the well-recognized
absorption band of the viologen motifs, a new band appeared
at 227 nm assignable to AuCl4ꢀ. Compared to the starting neutral
gold complex HAuCl4 that exhibits a broad band at 215 nm, the
new band is narrowed and 12 nm red shifted as a result of its
exchange with a viologen (formation of viologen+–AuCl4ꢀ). Upon
addition of NaBH4, a redshift in the viologen band is observed
(reaching 30 nm for D3V@Au and D4V@Au) and the clear yellow
solutions turned purplish red indicative of the formation of gold
nanoparticles (Fig. 1). This has been additionally ascertained by
the apparition of a broad surface plasmon resonance (SPR) band
centered at 523–535 nm, typical of gold nanoparticles.3c,13 The
intensity of the plasmonic band increased slightly after 4 days
and even continues to rise 6 days after. This indicates that
regardless of the number of molecular units in each branch,
viologen fragments act as anchoring sites to firstly exchange and
then smoothly transform molecular gold to nanosized particles.
The absence of the second SPR band at around 650 nm proves the
formation of isotropic particles thereby excluding any preferential
growth at the asterisk branching direction built from sequential
viologen units (anisotropic nanoparticles).3c
Scheme 1 Multi-step synthesis of 1V to 4V.
HRTEM analysis revealed nanosized particles without any
presence of bulky aggregates (Fig. 2 and S6, ESI†). The average
Scheme 2 Synthesis of D1V to D4V.
Fig. 1 Digital photos of DnV (left) and DnV@Au NPs (right).
hydrazido linkage [in 13C NMR, CQN resonates at 136–137 ppm
(d, 3JPC = 13 to 15 Hz); in IR:n (CQN) = 1638 cmꢀ1 (see S1 to S3,
ESI†)]. All these syntheses were notably performed using metal-
free routes, making them particularly adapted for biological
applications.
Expectedly, owing to the presence of viologens in these
dendritic substructures, UV-visible spectroscopic analysis shows
a broad band at 264 to 280 nm wavelength ascribed to p–p*
transition, with a hyperchromic effect (the intensity is proportional
to the number of the viologen units: see S4, ESI†). Fluorescence
Fig. 2 TEM images of D2V@Au, D3V@Au and D4V@Au. Onset: single
studies reveal a substantial shift in the fluorescence maxima, particle at higher magnification.
6982 | Chem. Commun., 2014, 50, 6981--6983
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Table 1 UV-visible and fluorescence measurements of DnV before and
after the reaction with HAuCl4ꢁ3H2O
(for up to eight months) nanosized gold particles (S9 in ESI†).
Considering the broad spectrum of utilization of viologen
motifs, the importance of phosphonate terminal groups in
biological applications2c and for hybrid material synthesis,14
and the outstanding catalytic and optical properties of gold
nanoparticles, promising applications can emerge from these
nanocomposite ‘‘ternary’’ hybrid systems.
AuNPsd
Viologen–PF6
D1V 282 (540)
Viologen+–AuCl4
Viologen@AuNPsc (nm)
ꢀ a,b
ꢀ a
282, 227
266, 227
264, 227
264, 226
289 (530)
269 (535)
293 (523)
294 (528)
45
8.5
7.5
7
D2V 266 (540)
D3V 264 (507)
D4V 265 (495)
a
b
UV-visible analysis (nm). The value between brackets correspond to
c
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NaBH4 (nm). (The value between brackets corresponds to the charac-
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measured using HRTEM.
d
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