Solvent-dependent aggregation behavior of a new Ru(ii)-polypyridyl based metallosurfactant

Article abstract of DOI:10.1039/c1cc13957f

Variation of the solvent polarity leads to the formation of vesicles and reverse vesicles of a newly synthesized amphiphilic Ru(ii)-polypyridyl complex.

Full text of DOI:10.1039/c1cc13957f

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 11074–11076
www.rsc.org/chemcomm
COMMUNICATION
Solvent-dependent aggregation behavior of a new Ru(II)-polypyridyl
based metallosurfactantw
Prasenjit Mahato,a Sukdeb Saha,a Sipra Choudhury*b and Amitava Das*a
Received 2nd July 2011, Accepted 10th August 2011
DOI: 10.1039/c1cc13957f
Variation of the solvent polarity leads to the formation of
vesicles and reverse vesicles of a newly synthesized amphiphilic
Ru(II)-polypyridyl complex.
on the use of such metallosurfactants for the formation of
vesicles or reverse vesicles are uncommon.13 Such structures
are of immense importance for their application potential as
photodynamic therapeutic reagents. Elsevier and Eiser have
two earlier reports, which reveal the formation of inverted
derivatives;13 however, to
Amphiphilic molecules self-assemble to generate various
interesting structures that differ in nature, size and shape.
The nature of these self-assembled structures varies between
miceller/reverse miceller structures, lamellae, ellipsoids, disks,
cylinders and vesicles/reverse vesicles depending on the solvent
environment, molecular architecture and concentration.1,2 Among
these various forms, vesicles composed of a spherical bilayer of
amphiphiles containing an aqueous core have drawn much
attention from a fundamental structural perspective, as well as
for their potential applications in modeling cell membranes,3
drug and gene delivery,4 as nanoreactors,5 templating nano-
materials,6 gelation7 and molecular recognition.2a Reverse
vesicles are composed of a reverse bilayer shell that encapsulates
solvent of low polarity, where hydrophobic portions of the
precursor amphiphiles are exposed to the low polar solvent
both in the core and in the exterior.8 These are of great funda-
mental interest because they have many impending applications
like normal vesicles, e.g., encapsulation and controlled delivery of
hydrophobic solutes.9 To fully realize the promising aspects
mentioned above, a detailed understanding of the structural
features of normal vesicles and reverse vesicles is necessary.
However, studies in which metalloaggregates have been used
for vesicle formation are scarce in the literature and more
importantly these aggregates have been studied mainly in
aqueous solution.10 Thus, studies with reverse vesicle formation
in non-polar organic solvents are even scarcer.11 Complexes
2+
micelles/vesicles using Ru(bpy)3
the best of our knowledge there is no report on normal
vesicles. In this communication, we are reporting a new
metallosurfactant (1) using the Ru(II)-polypyridyl core as the
hydrophilic head group with each bpy-ligand functionalized
with a hydrophobic C18 hydrocarbon tail (Scheme 1). This
complex is found to form normal vesicles in water and reverse
vesicles in non-polar solvents like toluene and cyclohexane—
an observation that is unique in the literature. These vesicles
were characterised by dynamic light scattering (DLS), static
light scattering (SLS), atomic force microscopy (AFM) and
transmission electron microscopy (TEM) studies.
Complex 1 (Ru(L3)3Cl2) was synthesized by reacting L3 and
RuCl3ꢀ3H2O in appropriate mole ratio in a mixed solvent
(ethanol–dioxan, 1 : 1, v/v) medium at 90 1C and the desired
purity was achieved by column chromatography.w Complex 1
was expected to exhibit aggregation behavior in polar as well
as in nonpolar solvents due to its amphiphilic nature. It was
found to form aggregates on dropwise addition of water to the
methanol solution of 1 and the final solvent combination was
adjusted to CH3OH : H2O = 1 : 9 (v/v) (B10ꢁ6 M). DLS
studies with this showed aggregates having distribution of
varying hydrodynamic diameters, with a mean diameter (Dh)
of 230 nm (Polydispersity index (PDI) = 0.18) (Fig. 1A). The
larger hydrodynamic diameter revealed that these aggregates
2+
containing the Ru(bpy)3 (where bpy = 2,20-bipyridine) unit
as the building block are of wide interest due to their distinct
coordination geometry, stability in the ground and in the
photoexcited states, and rich redox/photophysical properties.12
Though, reports on the aggregation behavior of amphiphilic
complexes containing Ru(bpy)32+ as the polar head group and
an aliphatic long chain as the lipophilic tail are there, examples
a Central Salt & Marine Chemicals Research Institute (CSIR),
Bhavnagar, 364002 Gujarat, India. E-mail: amitava@csmcri.org;
Fax: +91 278-2567562
b Chemistry Division, Bhabha Atomic Research Center Trombay,
Mumbai 400085, India. E-mail: sipra@barc.gov.in
w Electronic supplementary information (ESI) available: Synthesis of 1,
DLS/SLS data, AFM and TEM images. See DOI: 10.1039/c1cc13957f
Scheme 1 Molecular structure of compound 1 and cartoon represen-
tation of aggregates in different solvents.
c
11074 Chem. Commun., 2011, 47, 11074–11076
This journal is The Royal Society of Chemistry 2011

View Article Online
Fig. 1 Size distribution of 1 in (A) methanol–water (1 : 9; v/v),
(B) toluene and (C) cyclohexane medium. Insets: Guinier plot
obtained from SLS studies in respective solvent medium.
Fig. 3 Unstained TEM images of
1
in (A) methanol : water
2+
(1 : 9; v/v), (B) toluene and in (C) cyclohexane. Inset (A): UO2
stained TEM image in methanol : water (1 : 9; v/v) showing cell wall
thickness. Inset (C): magnified unstained TEM image showing cell wall
thickness.
were vesicles not micelles, as for micelles the diameter should
be in the order of 3–7 nm.14
Static light scattering (SLS) measurements were also performed
in methanol–water medium (1 : 9, v/v) with varying angles
from 301–1501 and radius of gyration (Rg) was evaluated from
the slope of the Guinier plot (inset, Fig. 1A).15 w The average
Rg value obtained for these aggregates is 116.6 nm and the
Rg/Rh ratio (where Rh = 1/2 Dh) is 1.01, which is very close to
the value for a spherical vesicle.16
form smaller spheres as observed from the TEM images also
support the vesicular nature of these spherical structures.w
Complex 1 with the Ru(bpy)32+-core as the polar head
group and three symmetrical hydrophobic tail groups is
expected to adopt a cone shape geometry.13b Such cone shaped
complexes are expected to form inverted micelles or vesicles in
nonpolar medium. We checked such a possibility in toluene
and cyclohexane. The solubility of 1 in toluene was found to
be higher than in cyclohexane, which could be ascribed to the
higher polarity of toluene. In the midst of the apolar environ-
ment the apolar aliphatic chains are expected to be exposed to
the solvent medium with the hydrophilic headgroup situated at
the core of the bilayers that face each other (Scheme 1). This is
Vesicle formation was also confirmed by AFM studies.
Fig. 2A shows the morphology of the self-assembled vesicles
and this enabled us to calculate the diameter of the collapsed dry
aggregates. AFM images further revealed that the aggregates
were ellipsoid instead of spherical. The deposited droplet over
the mica surface did not undergo instant drying; instead there
was a moving drying front which allowed these vesicles to
orient along its direction and accounts for the ellipsoid shape.
AFM cross-sectional analysis also revealed that the length,
width, aspect ratio and diameter to height ratios are in the
order of 174–465 nm, 108–347 nm, 1.25–1.6 and 3.9–5.7
respectively.w The diameters of these vesicles are thus larger
than the heights of the vesicles, which indicates flattening upon
being transferred from solution to the mica surface.w Removal
of solvent molecules from the hollow spheres during the drying
process and the high local force applied by the AFM tip could
have also contributed to this. Thus these structures are robust,
yet soft enough to deform upon drying.
2+
further supported by the fact that Ru(bpy)3 derivatives are
not soluble in apolar solvents like toluene and cyclohexane.
The DLS study of a freshly prepared solution of 1 (B10ꢁ6 M)
in toluene and cyclohexane revealed that mean Dh for these
molecular aggregates are 308 nm (PDI = 0.35) and 229 nm
(PDI = 0.19), respectively (Fig. 1B and C). Rg values obtained
from SLS experiments for these aggregates are 166.3 and
121.4 nm (inset, Fig. 1B and C). Thus the Rg/Rh values are
1.08 and 1.06 in toluene and cyclohexane, respectively. These
values are very close to unity and corroborate the formation
of reverse vesicles.17 Smaller size of the reverse vesicles in
cyclohexane could be accounted if one considers that in media
of lower polarity, the polar head set to tighter inverted vesicles.
While toluene, being more polar among these, is expected to
penetrate the vesicular membrane better and cause enhance-
ment of the vesicle size. The mean hydrodynamic diameters of
complex 1 in toluene and in toluene–methanol mixture of
varying proportion (e.g. 25 : 1 and 25 : 4 (v/v)) were found to
be 308, 61 and 39 nm respectively.w This further confirms the
proposition that with the increase in polarity of the medium
the interaction of the amphiphiles becomes more flexible and
thus the rigidity of the inverted vesicles decreases, and finally
they become soluble in the mixture of the solvents.
The spherical nature of these vesicles was further confirmed
by TEM studies. A small drop of the methanol–water (1 : 9; v/v)
solution was deposited on a carbon coated copper grid and air
dried. Fig. 3A shows the 2D projection of the polydispersed
spherical vesicles. These vesicles were in the 100–400 nm
diameter range and belong to medium size vesicles.2a These
images, on staining with 1% uranyl nitrate solution, showed
a darker cell wall with a thickness of 7–8 nm (Fig. 3A).
The length of the fully extended individual molecule of 1 is
B3.5 nm and this suggests that the vesicle wall is a bilayer
and these vesicles are unilamellar and hollow in nature. Again
fusion of two nearby spheres and fission of the large spheres to
Results of the AFM experiments with cast films on mica
confirm that complex 1 forms aggregates in toluene and
cyclohexane solutions. The size of the reverse vesicles obtained
from the toluene solution was found to vary between 270 and
530 nm in length and 170 and 320 nm in width, while the
aspect ratio varied from 1.35–1.56 (Fig. 2).w Cross-section
analysis of the typical structures revealed large ratios of
diameter to height (4.4–6.8). In the case of cyclohexane
solution, length, width, aspect ratio and diameter to height
ratios are in the order of 134–333 nm, 117–280 nm, 1.2–1.5
and 3.7–6.7, respectively.w Thus AFM results confirmed that
Fig. 2 AFM images of cast films deposited on mica of
1 in
(A) methanol : water (1 : 9; v/v), (B) toluene and in (C) cyclohexane.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 11074–11076 11075

View Article Online
different analytical techniques.w Complex 1 formed vesicles
in polar aqueous-methanol (9 : 1; v/v) medium and reverse
vesicles in nonpolar medium like toluene and cyclohexane.
These vesicles and reverse vesicles were characterised by DLS,
SLS, AFM and TEM studies. AFM and TEM confirmed the
spherical and hollow nature of these aggregates. Aggregate
formation was also confirmed by dye encapsulation studies in
respective solutions.
Fig. 4 Emission spectra of absorption matched rhodamine B (A) in
methanol–water (1 : 9, v/v) solution and in the presence of vesicles;
(B) in water and in the presence of reverse vesicles in toluene solution
of complex 1.
This work was supported by the DST and CSIR, India. P.M.
and S.S. acknowledge CSIR for Senior Research Fellowship.
Notes and references
the reverse vesicles formed in both toluene and cyclohexane
had flattened shape with rounded edges and the hollow feature
of the original inverted vesicle. The reverse vesicles appeared
with an ellipsoidal instead of a spherical shape, as during the
drying process of the deposited droplet, solvents were not
dried instantly and there was a moving drying front that
allowed these reverse vesicles to orient along its direction
and accounts for the ellipsoid shape. These reverse vesicles
were found to be stable even in the dry state and sizes thus
obtained from AFM studies matched well with those of the
DLS study.
1 (a) Y. Tan, K. Xu, Y. Li, S. Sunc and P. Wang, Chem. Commun.,
2010, 46, 4523; (b) X. Zhang and C. Wang, Chem. Soc. Rev., 2011,
40, 94; (c) S. I. Ali, J. P. A. Heuts and A. M. van Herk,
Soft Matter, 2011, 7, 5382; (d) M. Abkarian, E. Loiseauab and
G. Massiera, Soft Matter, 2011, 7, 4610; (e) A. Ghosh,
S. Choudhury and A. Das, Chem.–Asian J., 2010, 5, 352;
(f) Vesicles, Surfactant Science Series, ed. M. Rosoff, Marcel
Dekker, New York, 1996, vol. 62.
2 (a) J. Voskuhl and B. J. Ravoo, Chem. Soc. Rev., 2009, 38, 495;
(b) Y. Wang, H. Xu and X. Zhang, Adv. Mater., 2009, 21, 2849;
(c) P. Walde, K. Cosentino, H. Engel and P. Stano, ChemBioChem,
2010, 11, 848; (d) H. Asanuma, H. Li, T. Nakanishi and
H. Mc¸ hwald, Chem.–Eur. J., 2010, 16, 9330.
3 J. Fendler, Membrane Mimetic Chemistry, Wiley, New York, 1983.
4 (a) Liposomes: From Biophysics to Therapeutics, ed. M. J. Ostro,
Dekker, New York, 1987; (b) M. L. Immordino, F. Dosio and
L. Cattel, Nanomedicine, 2006, 1, 297; (c) E. H. Boris,
M. Winterhalter, P. M. Frederik, J. J. Vallner and D. D. Lasic,
Adv. Drug Delivery Rev., 1997, 24, 165.
5 (a) G. Hamasaka, T. Muto and Y. Uozumi, Angew. Chem., Int.
Ed., 2011, 50, 4876; (b) I. Stanish, D. A. Lowy, C. W. Hung and
A. Singh, Adv. Mater., 2005, 17, 1194; (c) F. Axthelm, O. Casse,
W. H. Koppenol, T. Nauser, W. Meier and C. G. Palivan, J. Phys.
Chem. B, 2008, 112, 8211.
6 P. Anilkumar and M. Jayakannan, J. Phys. Chem. B, 2009,
113, 11614.
7 Z. L. Huang, J. Y. Hong, C. H. Chang and Y. M. Yang, Langmuir,
2010, 26, 2374.
8 (a) J. Zhang, Y.-F. Song, L. Cronin and T. Liu, Chem.–Eur. J.,
2010, 16, 11320.
More convincing information about the reverse vesicles was
obtained from TEM images. The mean diameters of the
aggregates obtained from TEM were 250 and 209 nm, respec-
tively, for toluene and cyclohexane solutions (Fig. 3B and C).
These values are lower than that of Dh values obtained from
the DLS method. This is understandable if we consider that
DLS determines the hydrodynamic diameter, while TEM
determines the radius of the collapsed dry aggregates. The
wall thickness of the reverse vesicle was found to be B7 nm for
both instances (Fig. 3B and C), which indicated that the
reverse vesicles formed by complex 1 either in toluene or in
cyclohexane were of bilayer thickness and these structures
were unilamellar.
9 (a) H. Kunieda, K. Nakamura, M. R. Infante and C. Solans, Adv.
Mater., 1992, 4, 291; (b) C. Boettcher, B. Schade and
J. H. Fuhrhop, Langmuir, 2001, 17, 873.
10 P. Garcia, P. Eaton, H. P. M. Geurts, M. Sousa, P. Gameiro,
M. C. Feiters, R. J. M. Nolte, E. Pereira and B. de Castro,
Langmuir, 2007, 23, 7951.
11 J. Zhang, Y.-F. Song, L. Cronin and T. Liu, Chem.–Eur. J., 2010,
16, 11320.
12 (a) V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni and
M. Venturi, Acc. Chem. Res., 1998, 31, 26; (b) S. A. McFarland
and N. S. Finney, Chem. Commun., 2003, 388; (c) C. Metcalfe and
J. A. Thomas, Chem. Soc. Rev., 2003, 32, 215.
In order to check the dye encapsulation property of these
vesicles and reverse vesicles, these aggregates were generated in
the presence of rhodamine B (B106 M). The emission property
of rhodamine B in a vesicle/reverse vesicle solution was
compared with that of a pure rhodamine B solution having
comparable absorbance value at l 551 nm. The emission
intensity recorded for rhodamine B with vesicles in methanol–
water medium and with reverse vesicle in toluene/cyclohexane
solution was found to be appreciably quenched as compared
to that of free rhodamine B in water solution (Fig. 4 and
ESIw). This luminescence quenching is typically the result of
the encapsulation of the dye molecules inside these aggregates,
which lead to an increase in local concentration and thus the
self-quenching process (Fig. 4 and ESIw).18 In reverse vesicles
the effect of quenching of the dye was much higher compared
to the vesicles. Presumably, in reverse vesicles the polar dye
molecules are trapped in the reverse bilayer and more organised
upon interaction with the polar head groups, whereas in
13 (a) D. Domınguez-Gutierrez, G. De Paoli, A. Guerrero-Martınez,
´ ´ ´
G. Ginocchietti, D. Ebeling, E. Eiser, L. De Cola and
C. J. Elsevier, J. Mater. Chem., 2008, 18, 2762;
(b) D. Domınguez-Gutierrez, M. Surtchev, E. Eiser and
´ ´
C. J. Elsevier, Nano Lett., 2006, 6, 145.
14 T. Dwars, E. Paetzold and G. Oehme, Angew. Chem., Int. Ed.,
2005, 44, 7174.
15 P. A. Hasan, S. N. Sawant, N. C. Bagkar and J. V. Yakhmi,
Langmuir, 2004, 20, 4874.
16 T. Liu, J. Am. Chem. Soc., 2002, 124, 10942.
17 J. Zhang, Y.-F. Song, L. Cronin and T. Liu, Chem.–Eur. J., 2010,
16, 11320.
vesicles the dye molecules are in the interior and less organized.
2+
In summary, the novel Ru(bpy)3 based symmetrical
amphiphilic complex 1, containing three –C18H37 chains at
equilateral positions, was synthesized and characterised by
18 (a) E. N. Savariar, S. V. Aathimanikandan and S. Thayumanavan,
J. Am. Chem. Soc., 2006, 128, 16224; (b) A. L. Plant, Photochem.
Photobiol., 1986, 44, 453; (c) A. Datta, D. Mandal, S. K. Pal and
K. Bhattacharyya, Chem. Phys. Lett., 1997, 278, 77.
c
11076 Chem. Commun., 2011, 47, 11074–11076
This journal is The Royal Society of Chemistry 2011