Polyelectrolyte-induced self-assembly of positively charged alkynylplatinum (II)-terpyridyl complexes in aqueous media

Article abstract of DOI:10.1002/chem.200800276

Polyelectrolytes carrying multiple negative charges were found to induce the aggregation and self-assembly of the positively charged platinum(II)- terpyridyl complexes in aqueous media. The aggregation and self-assembly of the complexes were driven by electrostatic interactions between the polymer and the complex, and by terpyridine ligand π-π stacking and platinum-platinum (metal-metal) interactions. As a result, remarkable UV/ Vis and emission spectral changes were observed. The spectroscopic property changes were related to the structural properties of the metal complexes as well as the polyelectrolytes. The induced self-assembly of the platinum complexes was also strongly affected by the solution properties of the aqueous media, for example, the solution pH, ionic strength, and the percentage of organic solvent added.

Full text of DOI:10.1002/chem.200800276

FULL PAPER
DOI: 10.1002/chem.200800276
Polyelectrolyte-Induced Self-Assembly of Positively Charged
Alkynylplatinum(II)–Terpyridyl Complexes in Aqueous Media
Cong Yu, Kenneth Hoi-Yiu Chan, Keith Man-Chung Wong, and
Vivian Wing-Wah Yam*[a]
Abstract: Polyelectrolytes carrying
multiple negative charges were found
to induce the aggregation and self-as-
sembly of the positively charged plati-
num(II)–terpyridyl complexes in aque-
ous media. The aggregation and self-as-
sembly of the complexes were driven
by electrostatic interactions between
the polymer and the complex, and by
terpyridine ligand p–p stacking and
platinum–platinum (metal–metal) in-
teractions. As a result, remarkable UV/
Vis and emission spectral changes were
observed. The spectroscopic property
changes were related to the structural
properties of the metal complexes as
well as the polyelectrolytes. The in-
duced self-assembly of the platinum
complexes was also strongly affected
by the solution properties of the aque-
ous media, for example, the solution
pH, ionic strength, and the percentage
of organic solvent added.
Keywords: noncovalent
interac-
tions · platinum · polyelectrolytes ·
self-assembly
scopy
· UV/Vis spectro-
Introduction
the supramolecular ionic assembly of moieties containing
opposite charges, most of the work reported is confined to
the use of inorganic ions or chromophoric ions that are or-
ganic in nature. Assembly of metal–ligand chromophoric
units from polyelectrolytes is extremely scarce.[1,2]
Platinum(II)–polypyridyl complexes belong to a particu-
larly interesting class of organometallic chromophoric com-
plexes, and they are square planar. These complexes are
known for many years to possess a strong tendency to ar-
range themselves into linear chain or oligomeric structures
in the solid state, many of which involve metal–metal inter-
actions and aromatic ligand p–p stacking interactions. De-
pending on the extent of these interactions, these complexes
often exhibit different colors.[3–13]
We recently synthesized a number of platinum(II)–terpyr-
idyl complexes, and interesting solid-state as well as solution
spectroscopic properties, including induced aggregation
under various conditions, have been observed.[14–18] Herein
we present a detailed study of the properties of polyelectro-
lyte-induced self-assembly of several organometallic platinu-
m(II) complexes in aqueous media. Triflate salts of several
water-soluble alkynylplatinum(II)–terpyridine complexes
with different cationic charges and ligands of different hy-
drophobicity have been synthesized (1–4), and tested for
their self-assembly properties in the presence of various
anionic polyelectrolytes. The UV/Vis and emission spectral
changes were studied and correlated to the nature of the
complexes, polymer, and the media. It is envisaged that such
Polymers containing charged groups, known as polyelectro-
lytes, are among the most important classes of macromole-
cules. Common examples of polyelectrolytes include a large
number of biopolymers, for example, proteins, nucleic acids,
and many types of synthetic polymers. They have been ex-
tensively studied over the past several decades, due both to
scientific interest in understanding their behavior as well as
the enormous potential in various applications, in diverse
fields such as chemistry, biology, materials science, medicine,
physics, and nanotechnology. Polyelectrolytes have been fab-
ricated into multilayers, self-assemblies, thin films, and nano-
structures, and have been used for electronic and photonic
applications; for separation and sensor developments; for
biomedical applications, including controlled drug delivery,
gene therapy; and the development of new biomaterials.[1]
Despite growing interest in the use of polyelectrolytes for
[a] Dr. C. Yu, Dr. K. H.-Y. Chan, Dr. K. M.-C. Wong,
Prof. Dr. V. W.-W. Yam
Center for Carbon-Rich Molecular and
Nanoscale Metal-Based Materials Research
Department of Chemistry and
HKU-CAS Joint Laboratory on New Materials
The University of Hong Kong, Pokfulam Road
Hong Kong (P. R. China)
Fax : (+852)2857-1586
E-mail: wwyam@hku.hk
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nate) and poly(4-styrene sulfonate) in their acid forms carry
the strongly acidic sulfonic acid groups, whereas polyacry-
late in its acid form carries the mildly acidic carboxylic acid
functions. Poly(4-styrene sulfonate), which contains a phenyl
ring, is more hydrophobic than the other two polymers.
Electronic absorption and emission properties: The UV/Vis
spectra of complexes 1–4 (30 mm) in a water–acetonitrile
mixture (5 mm Tris-HCl, 10 mm NaCl, pH 7.5, 40% CH3CN)
showed an intense band at 300–360 nm and a less intense
band at 360–500 nm, typical of that found for alkynylplati-
num(II)–terpyridine complexes (Figure 1). The electronic
studies would provide a fundamental understanding of the
various factors that govern the self-assembly of the metal
complexes and the interplay of electrostatic and hydropho-
bic interactions between the polyelectrolytes and the metal
complexes, which may serve as model systems for the under-
standing of the interactions between the metal complexes
and various anionic biomolecules.
Figure 1. UV/Vis spectra of 30 mm of complexes 1 (c), 2 (b), 3
(g) and 4 (d) in buffer–CH3CN solvent mixture (Medium: 5 mm
Tris-HCl, 10 mm NaCl, pH 7.5, 40% CH3CN).
Results and Discussion
Four platinum(II)–alkynyl complexes 1–4 were synthesized
absorption spectral data are summarized in Table 1. Based
on our previous work[14–18] and other related studies,[3–13] the
high-energy bands of complexes 1–4 are tentatively assigned
as intraligand p!p* transitions of the terpyridine and al-
kynyl ligand, while the lower energy bands are assigned as
by the reaction of Sonogashira coupling and characterized
1
by H NMR and IR spectroscopy, MS, and elemental analy-
ses. Functionalization of the platinum(II)–terpyridyl–alkynyl
complexes through the attachment of a hydroxymethyl func-
tional group at the butadiynyl end in 1 and trimethylammo-
nium groups to give dicationic species in 2–4 has rendered
these complexes soluble in water. These complexes with dif-
ferent charges and degree of hydrophobicity were designed
and employed for the present study. Of the four complexes,
complex 3, which contains an extra phenyl ring, is the most
hydrophobic. For complexes 2–4, the positively charged tri-
methylammonium group is furthest away from the platinum
metal center and the terpyridine ligand in 3, while complex
4, which contains only one alkynyl unit, has the positively
charged trimethylammonium group closest in distance from
the platinum metal center and the terpyridine ligand.
Table 1. Photophysical data of complexes 1–4 in solution state.
Absorption, lmax [nm] (e [dm3 molꢀ1 cmꢀ1])[a]
Emission
lmax [nm][b]
1
2
3
4
238 (33665), 267 (19315), 278 (18090), 287 (22540),
313 (10765), 329 (12905), 339 (12805), 405 (4550)
237 (31240), 256 (22190), 277 (17940), 286 (24725),
308 (10110), 330 (12805), 340 (11225), 391 (4665)
242 (32720), 274 (40235), 285 (40480), 308 (12380),
327 (12270), 340 (14385), 417 (5145)
788
742
796
[c]
233 (27975), 254 (23725), 274 (21055), 283 (31930),
306 (10980), 330 (14070), 346 (11255), 378 (3590)
–
[a] Measured in water–acetonitrile mixture (5 mm Tris-HCl, 10 mm NaCl,
pH 7.5, 40% CH3CN) with 30 mm complexes. [b] Measured in buffer
(5 mm Tris-HCl, 10 mm NaCl, pH 7.5) with 0.15 mm complexes. [c] Non-
emissive
Three polymers of different nature were also selected for
the present investigation, namely polyacrylate, poly(vinyl
sulfonate), and poly(4-styrene sulfonate). Poly(vinyl sulfo-
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Alkynylplatinum(II)–Terpyridyl Complexes
FULL PAPER
admixtures of metal-to-ligand charge-transfer (MLCT;
band at around 800 nm, with the intensity becoming stron-
ger with decreasing CH3CN percentage. In pure buffer, be-
sides complex 1, complex 3 (30 mm) also showed a very
weak emission band at around 800 nm. In addition, when a
more concentrated complex solution was tested (0.15 mm),
complexes 1–3 clearly showed emission bands at 788, 742,
and 796 nm, respectively, while complex 4 remained non-
emissive (Table 1). Based on the UV/Vis absorption studies,
the emission bands, which appear in the NIR region, are
tentatively assigned as triplet MMLCT emission,[14–18] result-
ing from the background self-aggregation of the platinu-
m(II) complexes.
The influence of the solution ionic strength on the com-
plex background self-aggregation in a pure aqueous buffer
solution was also studied. The metal complex concentration
was kept at 30 mm and the NaCl concentration was varied.
For all four complexes, upon increasing the NaCl concentra-
tion, the UV/Vis spectra showed an increase in the intensity
of the complex MMLCT bands, indicating an enhancement
of background aggregation of the complexes (complex 1
precipitated at around 0.5m NaCl concentration, while solu-
tions of complexes 2–4 remained clear at 1m NaCl concen-
tration). Figure 2 shows the UV/Vis absorption spectra of 2
dp(Pt)!p*
C
E=25575 cmꢀ1)>1(lmax=405 nm; E=24691cm ꢀ1)>3(lmax
417 nm; E=23981cm ꢀ1) is in line with the order of the in-
creasing electron-richness of the alkynyl ligands. Complex 4,
which contains a positively charged electron-withdrawing
trimethylammonium group closest to the platinum(II)–ter-
pyridyl unit, possesses the least electron-rich alkynyl ligand,
giving rise to the highest energy MLCT/LLCT transition.
The lower electronic absorption energies of 2 than 4 and of
3 than 2 are also consistent with the increasing distance of
the trimethylammonium moiety on going from 4 to 2 to 3,
which gave rise to the decreasing electron-withdrawing in-
ductive effect of the +NMe3 group. A better p-donating
ability of 3 is also anticipated with the presence of the
phenyl moiety. Complex 1, with the absence of the positively
charged trimethylammonium unit on the alkynyl ligand,
showed a lower energy MLCT/LLCT transition than 2, as
the neutral non-cationic hydroxymethylbutadiynyl ligand
should behave as a better donor ligand.
When the percentage of acetonitrile in the solvent mix-
ture was reduced, the UV/Vis spectra of complexes 1–4
(30 mm) all showed a decrease in the intensity of the low-
energy MLCT/LLCT band and the emergence of a new
lower energy absorption shoulder in the region of approxi-
mately 480–550 nm. Concentration-dependence studies of
complexes 1–4 indicate that Beerꢁs law is not obeyed in the
low-energy absorption region at high complex concentra-
tions. With reference to our previous studies,[14–18] the emer-
gence of the low-energy absorption shoulder or tail origi-
nates from
a
metal–metal-to-ligand charge-transfer
(MMLCT) transition as a result of close Pt–Pt contacts and
p–p interactions in solution. The UV/Vis studies thus
strongly suggest that these complexes show a tendency to
exhibit ground-state complex aggregation in aqueous solu-
tion. This is not surprising; since the aromatic terpyridine
ligand is relatively hydrophobic, and in addition, the planar
structure of the complexes also facilitates their intermolecu-
lar stacking interactions. Since complex 1 has only one posi-
tive charge and thus is a monocationic complex, the electro-
static repulsive force between the complex molecules is the
smallest, and so it has the strongest tendency of showing a
background self-aggregation. The significantly tailed UV/Vis
spectrum with noticeable absorption spanning all the way to
about 600 nm is a strong indication of the background ag-
gregation. Complex 4, on the other hand, is dicationic and,
in addition, the trimethylammonium charged group is posi-
tioned closest to the hydrophobic terpyridine moiety; hence
there exists the strongest electrostatic repulsive force among
the complex molecules that gives rise to the weakest tenden-
cy for background self-association to occur.
*
Figure 2. UV/Vis absorption spectra of 2 in the absence ( ) and in the
~
presence (?) of 1m NaCl, and 3 in the absence ( ) and in the presence
( ) of 1m NaCl in buffer (5 mm Tris, 10 mm NaCl, pH 7.5) solution.
&
and 3 in the absence and in the presence of 1m NaCl in
buffer solution. Emission studies also revealed that with the
increase of the NaCl concentration, complex 3 showed quite
clear MMLCT band intensity enhancement, while complex
1 showed small MMLCT band intensity enhancement; com-
plexes 2 and 4 did not show the appearance of the MMLCT
emission band. These results indicate that the addition of
NaCl increased the tendency of complex p–p hydrophobic
stacking interactions, causing an increase in the background
aggregation. Thus the effect of NaCl is exactly opposite to
that of adding acetonitrile. The tendency for the complex to
undergo background self-aggregation is also consistent with
the nature and properties of the complexes as discussed ear-
lier (vide supra).
Corresponding emission studies of complex 1 (30 mm) in a
water–acetonitrile mixture (5 mm Tris, 10 mm NaCl, pH 7.5,
0–40% CH3CN) showed the appearance of a weak emission
Polyelectrolyte-induced aggregation studies: When 30 mm of
complex 1 was mixed with 180 mm of the polymers in a pure
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V. W.-W. Yam et al.
aqueous buffer solution (5 mm Tris-HCl, 10 mm NaCl,
pH 7.5), drastic UV/Vis spectral changes were observed (see
Figure 3). With polyacrylate and poly(vinyl sulfonate), new
rene sulfonate) was the least effective. For example, when
poly(vinyl sulfonate) was mixed with complexes 1–3, pro-
nounced aggregate formation was observed, as reflected by
the growth of the aggregate MMLCT absorption, with UV/
Vis absorption energy changes between the monomer and
the aggregate species of 6575, 6883, and 7118 cmꢀ1, whereas
for poly(4-styrene sulfonate), the corresponding values were
5534, 5575, and 5930 cmꢀ1 for complexes 1–3, respectively.
The larger spectral shifts of the aggregate MMLCT absorp-
tion from the corresponding monomeric MLCT/LLCT ab-
sorption in poly(vinyl sulfonate) than poly(4-styrene sulfo-
nate) are indicative of a stronger metal–metal and p–p
stacking interactions in the complex aggregates of the
former, as the difference in the energy of the monomer and
aggregate/oligomer absorption is a measure of the strength
of these metal–metal and p–p interactions. It is likely that
the stronger hydrophobic interactions between the more hy-
drophobic poly(4-styrene sulfonate) and the complex would
reduce the tendency of the complex to undergo self-assem-
bly. It is also interesting to note that in all cases complex 3
gave the largest UV/Vis band energy changes between the
monomer and the aggregates regardless of the polymer used
(6858, 7118, and 5930 cmꢀ1 for polyacrylate, poly(vinyl sul-
fonate), and poly(4-styrene sulfonate), respectively), pre-
sumably because of its stronger hydrophobic characters.
The appearance of new emission bands upon mixing of
complexes with various polyelectrolytes was observed. For
the mixtures of 30 mm of complexes 1–4 with 180 mm of poly-
acrylate, new emission bands with peak maxima at 795, 764,
812, and 668 nm, respectively, were found (Figure 4). Similar
Figure 3. UV/Vis absorption spectra of 30 mm of 1 (top) and 2 (bottom)
&
in the absence of polymers ( ) and in the presence of the 180 mm of poly-
~
*
acrylate ( ), poly(vinyl sulfonate) ( ), and poly(4-styrene sulfonate) (?)
in buffer (5 mm Tris, 10 mm NaCl, pH 7.5) solution.
UV/Vis bands with peak maxima appeared at 550 nm (E=
18181 cmꢀ1) and 552 nm (E=18116 cmꢀ1), respectively, in
the electronic absorption spectra of 1. With poly(4-styrene
sulfonate), a weak broad absorption shoulder at longer
wavelength (ca. 522 nm; E=19157 cmꢀ1) with no clear band
maximum was observed. Similar UV/Vis spectral changes
were generally observed with complexes 2 and 3 in the pres-
ence of the polymers, while complex 4 also gave similar
changes in the UV/Vis absorption patterns, albeit considera-
bly smaller changes were observed. The UV/Vis spectral
changes of complex 2 in the presence of various polymers
are shown in Figure 3.
With reference to our previous work and other related
studies, the new UV/Vis bands are assigned as MMLCT
transitions.[14–18] The results clearly show that upon mixing of
the complex with the polymer, electrostatic interactions be-
tween the complex and the polymer induced the self-assem-
bly of the complex molecules through metal–metal and p–p
stacking interactions. The properties of the complex self-as-
sembly are apparently related to the properties of the com-
plex molecules and the nature of the polymers. In general,
more evident UV/Vis spectral changes typical of the forma-
tion of self-assembled aggregates were observed in com-
plexes 1–3, whereas for complex 4, the UV/Vis changes
were far less significant. For example, when the complexes
were mixed with polyacrylate, the shifts in energy to the red
from the complex monomer MLCT/LLCT absorption band
to the corresponding complex aggregate absorption band
were 6509, 6120, 6858, and 2005 cmꢀ1 for complexes 1–4, re-
spectively.[19] For the same complex, poly(vinyl sulfonate)
gave better performance than polyacrylate in inducing the
self-assembly of the complex molecules, while poly(4-sty-
~
&
*
Figure 4. Normalized emission spectra of 30 mm of 1 ( ), 2 ( ), 3 ( ) and
4 (?) mixed with 180 mm of polyacrylate in buffer (5 mm Tris, 10 mm
NaCl, pH 7.5) solution.
observations were also identified by using poly(vinyl sulfo-
nate) and poly(4-styrene sulfonate), and the corresponding
emission spectra of 1 and 3 in the presence of the polymers,
together with the emission spectra of their monomeric
forms are depicted in Figure 5. As described earlier, these
newly appeared emission bands are assigned as MMLCT
triplet emission.[14–18] It is interesting to note that the relative
positions of the emission bands appear to be related to the
properties of the complex molecules. For the mixtures of the
polymers with complexes 2–4, complex 3 in all cases studied
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Alkynylplatinum(II)–Terpyridyl Complexes
FULL PAPER
Effect of organic solvent: As mentioned above, the addition
of acetonitrile can reduce hydrophobic p–p stacking interac-
tions between the complex molecules and help to minimize
the complex background self-aggregation. Our results show
that the addition of acetonitrile can also effectively reduce
the nonspecific hydrophobic interactions between the com-
plex molecules and the polymer. For complex binding to
poly(4-styrene sulfonate), in a pure aqueous buffer solution,
only weak complex self-assembly was observed as shown in
Figure 3. With the addition of 40% of acetonitrile, the back-
ground aggregation was suppressed, and more importantly,
the performance of poly(4-styrene sulfonate)-induced com-
plex self-assembly was found to improve significantly for
complexes 1–3 as a result of the reduced nonspecific hydro-
phobic interactions. Much increased MLCT/LLCT UV/Vis
absorption band intensity and clear band maximum were
both observed. It is also interesting to note that since the hy-
drophobic interactions were largely suppressed, unlike in
the pure aqueous buffer solution, in which complex 3 gave
the largest UV/Vis band-energy changes between the mono-
mer and the aggregates, complexes 1–3 all gave similar per-
formance (energy difference: 6866, 6778, and 6590 cmꢀ1 for
complexes 1–3, respectively).
Figure 5. Normalized emission spectra of 30 mm of 1 (top) and 3 (bottom)
&
in the absence of polymers ( ) in buffer–CH3CN solvent mixture and in
~
*
the presence of the 180 mm of polyacrylate ( ), poly(vinyl sulfonate) ( ),
and poly(4-styrene sulfonate) (?) in buffer (5 mm Tris, 10 mm NaCl,
pH 7.5) solution.
gave an emission band at the longest wavelength (with peak
maxima at 812 (12315), 815 (12270), and 777 nm
(12870 cmꢀ1) for polyacrylate, poly(vinyl sulfonate), and
poly(4-styrene sulfonate) respectively), whereas complex 4
gave an emission band at the shortest wavelength (with
peak maxima at ca. 668 (14970), 663 (15083), 639 nm
(15649 cmꢀ1) for polyacrylate, poly(vinyl sulfonate), and
poly(4-styrene sulfonate) respectively), which is consistent
with the relative MLCT/LLCT UV/Vis absorption trends of
these complex assemblies. As mentioned above, the degree
of conjugation of the electron-donating alkynyl group and
the strength of the electron-withdrawing positively charged
trimethylammonium group determine the emission energy
and consequently their relative band positions. Complex 4,
which has a trimethylammonium group separated by only
one alkynyl attached to platinum, is the least electron-rich
complex, whereas complex 3, with an aryl-substituted alkyn-
yl group, is the most electron-rich complex. In addition, as
the electron-withdrawing trimethylammonium group moves
further away from complexes 4 to 2 to 3, the electron-with-
drawing effect becomes less important. As a result, complex
4 gave an emission band of the highest energy, while com-
plex 3 gave an emission band of the lowest energy. It is also
interesting to note that complex 3 gave the highest energy
MMLCT emission (780 nm) arising from aggregate forma-
tion upon addition of poly(4-styrene sulfonate) when com-
pared with the other anionic polymers, polyacrylate
(812 nm) and poly(vinyl sulfonate) (816 nm), as shown in
Figure 5 (bottom). The occurrence of the MMLCT emission
band at higher energy is indicative of a weaker tendency for
the complex cations to stack with each other to form self-as-
sembly. The reduced tendency of complex stacking is ascri-
bed to the hydrophobic interactions between the relatively
hydrophobic planar styryl groups and the more hydrophobic
complex 3 bearing an aryl group on the alkynyl unit.
Effect of polyelectrolyte concentration: The influence of the
ratio of polymer to complex on the self-assembly properties
of the complexes was studied. It was observed that addition
of 0.5–1.5 times the amount of polyacrylate (based on car-
boxylate unit concentration) to a solution of complex 1 at a
concentration of 30 mm gave rise to a slightly turbid solution,
with strong light-scattering in both the UV/Vis and emission
spectra. It is likely that with such relatively small amounts
of polymer added, the negative charges on the polymer
were largely neutralized upon binding to the metal complex,
and since there would not be enough electrostatic repulsive
forces between the polymer molecules for their dispersion,
aggregation and precipitation resulted. In contrast, with a
larger excess (3–48 times) of polyacrylate added, both the
UV/Vis and emission spectra showed the formation of low-
energy MMLCT absorption and emission bands, indicative
of excellent complex self-assembly. It is interesting to note
that the spectral changes are relatively insensitive to a varia-
tion in the amount of polymer added, indicating that in this
polyacrylate concentration range the self-assembly of com-
plex 1 took place to a significant extent and was not sensi-
tive to the amount of polymer added. Similar results were
obtained for complexes 2 and 3. For complex 4, the
3MMLCT emission of complex aggregates gave the best re-
sults under the conditions of 3–6 times the amount of poly-
3
acrylate added, with a significant decrease of the MMLCT
emission intensity upon further increase of the polymer con-
centration, and the almost complete disappearance of the
emission band when 24 times the amount of polyacrylate
were added.
As shown in our previous study, in an organic solvent mix-
ture, when the amount of base added was kept constant, ad-
dition of large excess of poly(acrylic acid) resulted in a sig-
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V. W.-W. Yam et al.
nificant decrease of the complex self-assembly, as a result of
the “diluted” average number of negatively charged carbox-
ylate functional groups on each polymer chain.[17] In an
aqueous buffer solution, the situation is quite different.
Since the solution pH was kept at a constant value of 7.5,
addition of uncharged poly(acrylic acid) is equivalent to the
addition of negatively charged polyacrylate, and the “dilu-
tion” effect is largely reduced. However, for complex 4,
since it has the weakest tendency to aggregate and self-as-
semble among the four complexes studied as a result of the
strong repulsive forces that exist between the positively
charged complex molecules, addition of a large excess of
polyacrylate results in a stronger tendency to “dilute” the
average number of complex molecules that would bind to
each polymer chain, leading to a reduced 3MMLCT emis-
sion.
gate, as a result of the increased strength of the hydrophobic
interactions. Interestingly, a concomitant decrease in the
MMLCT emission band intensity occurred (Figure 6), which
Effect of solution buffer pH: The influence of the solution
buffer pH on the self-assembly properties of the metal com-
plexes was also investigated. For polyacrylate, since the car-
boxylic acid functionality is only mildly acidic, a large de-
crease of the buffer pH reduces the percentage of the ion-
ized functional groups on each polymer chain, and thus in-
fluences the electrostatic binding of the positively charged
complexes to the polymer chain and their self-assembly. It
was found that for the mixture of 30 mm of complex 1 and
180 mm of polyacrylate, a decrease of the buffer pH from 7.5
to 6.0 and 5.0 caused only small UV/Vis spectral changes,
suggesting that the assembly properties of the complex were
little influenced in this pH region. Further decrease of the
buffer pH to 4.0 and 3.0 caused the solution to turn turbid,
indicating precipitation of the metal-complex-bound poly-
mer, probably as a result of the reduced net negative charg-
es on each polymer chain. At pH 2.0, the negative charges
on the polymer were largely removed, and the solution ap-
peared turbid, because of the decreased solubility of the po-
lymer. However, with the addition of 20% acetonitrile, a
clear yellow solution was obtained, with little complex ag-
gregation observed. Upon bringing the solution back to
basic conditions, both the UV/Vis and emission spectra
showed the revival of complex assembly, as indicated by the
observation of the low-energy MMLCT absorption and
emission bands. Poly(vinyl sulfonate), on the other hand,
was less susceptible to a change in pH since the sulfonic
acid moiety is strongly acidic. The polymer-induced complex
assembly was found to be little influenced by the buffer pH.
At pH 2.0, a mixture of 30 mm of complex 1 and 180 mm of
poly(vinyl sulfonate) still showed excellent complex self-as-
sembly.
Figure 6. Emission spectra of 30 mm of complex 2 + 180 mm of polyacry-
late, 5 mm Tris-HCl, 10 mm NaCl, pH 7.5, with increasing concentration
of NaCl added. Inset: plot of emission intensity at 771nm as a function
of NaCl concentration.
indicates that the bound complex molecules were gradually
replaced by sodium ions. With 0.34m NaCl added, the com-
plex molecules were completely replaced by sodium ions,
and a clear yellow solution was obtained. Our results also
show that because of the weaker tendency of complex 4 to
aggregate, upon mixing of it with polyacrylate, the MMLCT
emission band was completely gone with a total of only
0.1m NaCl added.
Conclusions
In the present study, several polymers carrying multiple neg-
atively charged functional groups have been shown to
induce the aggregation and self-assembly of the positively
charged water-soluble alkynylplatinum(II)–terpyridyl com-
plexes in an aqueous medium. The driving forces for the in-
duced aggregation and self-assembly were the result of elec-
trostatic binding of the complex molecules to the polymer,
metal–metal and ligand p–p stacking interactions, which
gave rise to remarkable UV/Vis and emission spectral
changes. The spectral changes were found to be related to
the properties of the complexes as well as the polymers. The
induced complex self-assembly was also influenced by the
properties of the aqueous medium, for example, ionic
strength, buffer pH, and the percentage of organic solvent
added. The current approach could be potentially utilized to
detect and characterize various charged polymers, as dem-
onstrated by our parallel research work on single-stranded
nucleic acids.[20] Similar to pyrene, which has been utilized
to probe polymer conformational changes,[1] it is envisaged
that our complex molecules could also be used for probing
conformational changes in polymers through the observa-
Effect of solution ionic strength: The effect of the ionic
strength of the buffered solution on the polyelectrolyte-in-
duced complex self-assembly was also studied. For a mixture
of 30 mm of complex 2 and 180 mm of polyacrylate, UV/Vis
spectral studies showed an increased background light-scat-
tering as the concentration of NaCl was increased, indicating
that the complex-bound polymer started to gradually aggre-
3
tion of the MMLCT emission. An additional advantage lies
4582
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 4577 – 4584

Alkynylplatinum(II)–Terpyridyl Complexes
FULL PAPER
Synthesis of platinum(II) complexes
in the fact that besides the appearance of new emission
bands, the significant UV/Vis spectral changes that give rise
to visual color changes could also be utilized for the probing
of polymer conformation studies.
ꢁ
ꢁ
[Pt
(tpy)(C C-C CCH2NMe3)]
[OTf]2 (2): The complex was synthesized
ꢁ
ꢁ
[Me3SiC C-C CCH2NMe3]
ride (13 mg, 0.22 mmol) in methanol (30 mL) was heated to 508C for
30 min, after which [Pt(tpy)(CH3CN)][OTf]2 (71mg, 0.09 mmol) was
G
U
ACHTREUNG
added, and the resultant orange solution was stirred at 508C overnight.
Solvent was then removed under reduced pressure, and the residue
washed with a small amount of methanol and was then dissolved in ace-
tonitrile. Subsequent recrystallization by diffusion of diethyl ether vapor
into an acetonitrile solution of the product gave complex 2 as an orange
red solid. Yield: 45 mg (57%); 1H NMR (CD3CN): d=3.16 (d, J=
2.4 Hz, 9H; -NMe3), 4.30 (d, J=2.4 Hz, 2H; -CH2-), 7.77–7.82 (m, 2H;
Experimental Section
Materials and characterization: All reagents were used as received.
Poly(acrylic acid) was synthesized as previously described with Mw of
10971. Poly(vinylsulfonic acid sodium salt) with Mw of 4000–6000, poly-
(sodium 4-styrenesulfonate) with Mw of 70000, propargyl bromide, and
propargyl alcohol were obtained from Aldrich.
[17]
tpy), 8.25–8.49 (m, 7H; tpy), 9.03 ppm (d, J=5.6 Hz, 2H; tpy); IR
ꢀ1
ꢁ
ꢁ
(Nujol): n˜ =2217 (m; C C), 2083 (m; C C), 1157 cm (m; S=O); posi-
tive-ion FAB-MS: m/z: 698 [M+OTf]+; elemental analysis calcd (%) for
C25H22N4F6O6PtS2·2H2O: C 34.00, H 2.96, N 6.34; found: C 33.89, H 2.68,
N 6.24.
1H NMR spectra were recorded with a Bruker DPX-300 (300 MHz) or
Bruker AVANCE 400 (400 MHz) Fourier transform NMR spectrometer
at ambient temperature, with chemical shifts reported relative to tetra-
methylsilane. Positive FAB mass spectra were obtained by using a Finni-
gan MAT95 mass spectrometer. IR spectra were obtained as Nujol mulls
on KBr disks on a Bio-Rad FTS-7 Fourier transform infrared spectropho-
tometer (4000–400 cmꢀ1). Elemental analyses were performed on a Carlo
Erba 1106 elemental analyzer at the Institute of Chemistry, Chinese
Academy of Sciences. UV/Vis absorption spectra were recorded on a
Cary 50 (Varian) spectrophotometer equipped with a Xenon flash lamp.
Steady-state emission spectra were recorded using a Spex Fluorolog-2
Model F111 spectrofluorometer. The emission spectra were obtained
with an excitation wavelength of 400 nm and were not corrected for
PMT (photomultiplier tube) response. Standard quartz cuvettes with
1cm path length were used for all spectral measurements. Unless speci-
fied otherwise, a buffer solution containing 5 mm Tris-HCl and 10 mm
NaCl at pH 7.5 at ambient temperature was used throughout the current
investigation.
ꢁ
[Pt
N
[OTf]2 (3): Complex 3 was synthesized
A
N
(10 mL) containing triethylamine (4 mL), followed by the addition of
ꢁ
[HC C-C6H4CH2NMe3-4]
G
a
catalytic
amount of copper(I) iodide. The reaction mixture was stirred overnight
at room temperature, after which diethyl ether (100 mL) was added and
stirred for 10 min. The product was isolated by filtration, washed with di-
ethyl ether, and dried. Subsequent recrystallization by diffusion of diethyl
ether vapor into a solution of the product in acetonitrile gave complex 3
as a red solid. Yield: 146 mg (62%); H NMR (CD3CN): d=3.05 (s, 9H;
-NMe3), 4.43 (s, 2H; -CH2-), 7.49 (d, J=8.2 Hz, 2H; -C6H4-), 7.60 (d, J=
8.2 Hz, 2H; -C6H4-), 7.75 (t, J=5.8 Hz, 2H; tpy), 8.22–8.41(m, 7H; tpy),
ꢁ
9.09 ppm (d, J=5.8 Hz, 2H; tpy); IR (Nujol): n˜ =2120 (m; C C),
ꢁ
ꢁ
Synthesis of the metal complexes: [Pt
N
N
1153 cmꢀ1 (m; S=O); positive-ion FAB-MS: m/z: 750 [M+OTf]+; ele-
mental analysis calcd (%) for C29H26N4F6O6PtS2·H2O: C 37.95, H 3.08, N
6.10; found: C 37.76, H 3.09, N 5.95.
[21]
[22]
(tpy)Cl]
[OTf],[23] and
A
ꢁ
ꢁ
ꢁ
C CC6H4CH2Br,
[Pt(tpy)(CH3CN)]
methods.
Me3SiC C-C CCH2Br,
[Pt
[14,24]
A
A
[OTf]2
were synthesized according to literature
ꢁ
[Pt
(tpy)(C C-CH2NMe3)]
U
plex 4 was similar to that for complex 3, except that [HC CCH2NMe3]-
Synthesis of functionalized alkynes
[OTf] (103 mg, 0.415 mmol) was used instead of [HC CC6H4
4]
[OTf] to give complex 4 as a yellow solid. Yield: 106 mg (62%);
ꢁ
ꢁ
[HC CC6H4CH2NMe3-4]
A
A
mixture of Me3SiC CC6H4CH2Br
1H NMR ([D6]DMSO): d=3.19 (s, 9H; -NMe3), 4.54 (s, 2H; -CH2-), 7.90
(t, J=5.6 Hz, 2H; tpy), 8.52 (t, J=7.8 Hz, 2H; tpy), 8.58–8.70 (m, 5H;
(72 mg, 0.27 mmol) and trimethylamine (5 mL) was stirred in diethyl
ether (5 mL) for 30 min, during which precipitates of the bromide salt
were obtained. This was filtered and washed with diethyl ether, and was
then dissolved in methanol and metathesized to the trifluoromethanesul-
fonic (triflate) salt with a saturated methanolic solution of silver triflate.
During the metathesis reaction, the protecting group of the acetylene
was also removed. Removal of the AgBr precipitate gave a colorless fil-
trate and the solvent was evaporated to dryness under reduced pressure.
Extraction of the residue with chloroform, followed by solvent removal
gave the desired product as a white solid. Yield: 70 mg (80%); 1H NMR
ꢁ
tpy), 9.04 ppm (d, J=5.6 Hz, 2H; tpy); IR (Nujol): n˜ =2145 (m; C C),
1160 cmꢀ1 (m; S=O); positive-ion ESI: m/z 262.6 [M]2+; elemental analy-
sis calcd (%) for C23H22N4F6O6PtS2·0.5CH3CN: C 34.15, H 2.81, N 7.47;
found: C 34.33, H 3.02, N 7.49.
ꢁ
(CD3OD): d=3.10 (s, 9H; -NMe3), 3.70 (s, 1H; -C CH), 4.52 (s, 2H;
-CH2-), 7.55 (d, J=8.3 Hz, 2H; -C6H4-), 7.62 ppm (d, J=8.3 Hz, 2H;
Acknowledgements
-C6H4-).
V.W.-W.Y. acknowledges receipt of the Distinguished Research Achieve-
ment Award from The University of Hong Kong. This work has been
supported by a Central Allocation Vote (CAV) Grant (Project No. HKU
2/05C) and a National Natural Science Foundation of China and the Re-
search Grants Council of Hong Kong Joint Research Scheme (NSFC-
RGC Project No. N HKU 737/06) from the Research Grants Council of
Hong Kong Special Administrative Region, P. R. China.
ꢁ
[HC CCH2NMe3]
ACHTREUNG
ꢁ
[HC CC6H4CH2NMe3-4]
ACHTREUNG
ꢁ
toluene; 1.25 g, 8.41 mmol) was used instead of Me3SiC CC6H4CH2Br to
ꢁ
give [HC CCH2NMe3]
[OTf] as a pale yellow solid. Yield: 1.64 g (79%);
1H NMR (CD3OD): d=3.26 (s, 9H; -NMe3), 3.55 (t, J=2.5 Hz, 1H;
-CH-), 4.39 (d, J=2.5 Hz, 2H; -CH2-).
ꢁ
ꢁ
[Me3SiC C-C CCH2NMe3][OTf]: The procedure was similar to that for
E
ꢁ
ꢁ
ꢁ
[HC CC6H4CH2NMe3-4]
A
ꢁ
(100 mg, 0.465 mmol) was used instead of Me3SiC CC6H4CH2Br to give
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ꢁ
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G
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The freshly prepared functionalized alkynes were immediately used for
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Chem. Eur. J. 2008, 14, 4577 – 4584
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: February 14, 2008
Published online: April 7, 2008
4584
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Chem. Eur. J. 2008, 14, 4577 – 4584