Construction of supramolecular pyrene-modified metallacycles via coordination-driven self-assembly and their spectroscopic behavior

Article abstract of DOI:10.1021/om301108s

The design and self-assembly of novel multipyrene hexagonal metallacycles via coordination-driven self-assembly is described. By employing newly designed 120 dipyridine donor and di-Pt(II) acceptor linkers substituted with pyrene, a variety of tris- and hexakis(pyrene) hexagonal metallacycles with well-defined shape and size were prepared via [3 + 3] and [6 + 6] self-assembly, respectively, under mild conditions in high yields. The structures of these novel metallacycles were well characterized by multinuclear NMR (³¹P and ¹H) spectroscopy, cold-spray ionization time-of-flight mass spectrometry (CSI-TOF-MS), electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), and elemental analysis. The shape and size of all hexagonal metallacycles were investigated by the PM6 semiempirical molecular orbital method. The preliminary study of their spectroscopic behavior was also carried out. It was found that these pyrene-modified metallacycles displayed different optical behaviors, which might be caused by the structural effects.

Full text of DOI:10.1021/om301108s

Article
pubs.acs.org/Organometallics
Construction of Supramolecular Pyrene-Modified Metallacycles via
Coordination-Driven Self-Assembly and Their Spectroscopic
Behavior
Nai-Wei Wu,† Jing Zhang,† Deji Ciren,‡ Qing Han,† Li-Jun Chen,† Lin Xu,*,† and Hai-Bo Yang*,†
†Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University,
3663 North Zhongshan Road, Shanghai 200062, People’s Republic of China
‡Department of Public Teaching, Tibet Agricultural and Animal Husbandry College, 8 Xueyuan Road, Linzhi, Tibet 860000, People’s
Republic of China
S
* Supporting Information
ABSTRACT: The design and self-assembly of novel multi-
pyrene hexagonal metallacycles via coordination-driven self-
assembly is described. By employing newly designed 120°
dipyridine donor and di-Pt(II) acceptor linkers substituted
with pyrene, a variety of tris- and hexakis(pyrene) hexagonal
metallacycles with well-defined shape and size were prepared
via [3 + 3] and [6 + 6] self-assembly, respectively, under mild
conditions in high yields. The structures of these novel
metallacycles were well characterized by multinuclear NMR
(31P and 1H) spectroscopy, cold-spray ionization time-of-flight mass spectrometry (CSI-TOF-MS), electrospray ionization time-
of-flight mass spectrometry (ESI-TOF-MS), and elemental analysis. The shape and size of all hexagonal metallacycles were
investigated by the PM6 semiempirical molecular orbital method. The preliminary study of their spectroscopic behavior was also
carried out. It was found that these pyrene-modified metallacycles displayed different optical behaviors, which might be caused by
the structural effects.
cated metallosupramolecular structures. In comparison to the
traditional covalent synthesis strategy, the coordination-driven
self-assembly protocol offers considerable synthetic advantages
such as fewer steps, fast and facile construction of the final
products, and inherently self-correcting and defect-free
assembly. Furthermore, as the rigid molecular precursors
present a number of different locations where functional
moieties can be attached, a variety of functionalized polygons
have been successfully designed and prepared, which can be
potentially employed as precursors of electronic, catalytic, and
photophysical materials or used for molecular recognition and
encapsulation.12 For example, we have demonstrated that the
introduction of functional groups, such as crown ether,
INTRODUCTION
■
Self-assembly is an essential process for many key activities of
living cells, such as the formation of proteins (protein folding),
nucleic acid complexes, and plasma membranes, cytoskeleton
assembly, etc.1 Generally, biological systems are entirely driven
by self-assembly and capable of performing physiological
activities by taking advantage of a range of reversible
noncovalent interactions such as hydrogen bonding, charge−
charge, donor−acceptor, π−π stacking, and van der Waals.2
Additionally, many examples have demonstrated that nature
utilizes the self-assembly approach to construct biologically
relevant materials, in which the function of the self-assembled
whole is greater than the sum of its individual parts.3 Inspired
by nature, chemists have devoted to utilizing noncovalent
interactions to build abiological self-assembly, which has not
only furthered our understanding of the self-assembly process
itself but also opened the door to the construction of a variety
of complicated supramolecular architectures.4
Among various self-assembly protocols, coordination-driven
self-assembly has emerged as a powerful tool to construct
polygons and polyhedra with well-defined shapes, sizes, and
geometries,2d,5 since the metal−ligand bonds are highly
directional and relatively strong. During the past few years,
the groups of Stang,6 Fujita,7 Mirkin,8 Raymond,9 Lehn,10 and
others11 have extensively explored the directional-bonding
coordination-driven approach to the self-assembly of compli-
́
ferrocene, and Frechet-type dendrons, at the vertex of 120°
building blocks enabled the construction of novel function-
alized two-dimensional metallacycles.13
The list of polygons synthesized to date includes the
molecular square, rectangle, triangle, pentagon, and hexagon.14
Among these polygons, the hexagon has attracted considerable
attention, since it is one of the most common patterns in
nature, such as the honeycomb structure of a beehive and the
micrograph of a snowflake.15 Thus, there has been an increasing
interest in the construction of supramolecular hexagonal
Received: November 18, 2012
© XXXX American Chemical Society
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architectures through self-assembly in recent years.16 According
to the “directional bonding” and “symmetry interaction”
models,5a,b the shape of an individual 2-D hexagon is usually
determined by the value of the turning angle within its angular
components. Moreover, the hexagonal supramolecular systems
can be self-assembled in two different ways. For example,
discrete hexagonal entities of the type A2 L2 can be self-
coordination-driven self-assembly, respectively. It should be
noted that, by synthesizing a diverse library of donor and
acceptor building blocks, two complementary strategies have
been investigated in order to construct “isomeric” supra-
molecular multipyrene hexagons possessing the same geometry.
A similar reasoning has been applied in the construction of
nanoscopic cages such as cuboctahedra, truncated tetrahedra,
and supramolecular M3L2 prisms.21 Moreover, it was found that
a combination of the complementary donor units and acceptors
allowed for the formation of novel multipyrene hexagons with
precisely controlled shapes, sizes, and geometries via coordina-
tion-driven self-assembly. The structures of all multipyrene
hexagonal metallacycles were characterized by multinuclear
NMR (1H and 31P), CSI-TOF-MS and ESI-TOF-MS, and
elemental analysis. Their primary spectroscopic behavior has
been investigated.
6
6
assembled from a combination of six shape-defining and
directing corner units A2 with six appropriate linear linker units
L2. An alternative route for the assembly of hexagonal
supermolecular systems involves the combination of two
complementary ditopic building blocks, A2 and X2, each
incorporating 120° angles between the active coordination
sites, allowing for the formation of hexagonal structures of type
A2 X2 .
3
3
Pyrene is one of most useful fluorophores, which has
received significant attention because of its interesting optical
properties such as relatively long excited-state lifetimes and
exceptional distinction of the fluorescence bands for monomers
and excimers.17 Recently, the incorporation of multiple pyrene
subunits into a single scaffold to construct multipyrene
materials, such as polymers and dendrimers, has evolved to
be one of the most attractive subjects within materials
science.18 It provides an efficient approach to amplify and
modulate the fluorescence response because the multipyrene
materials often absorb light more effectively and usually exhibit
excimer fluorescence. However, the preparation of multipyrene
polymers and dendrimers often requires considerable synthetic
effort and can be plagued by low yields and largely amorphous
final structures. Thus, the introduction of multiple pyrene
groups onto/into discrete supramolecular systems in a
controlled manner is still a major challenge.
As mentioned above, previous studies indicated that
coordination-driven self-assembly featured considerable syn-
thetic advantages to construct multifunctional assemblies. By
employing such an approach, many functionalized assemblies,
such as tris(crown ether) hexagons and multiferrocenyl
hexagons, have been successfully prepared.13c,19b Moreover,
we have previously realized well-controlled self-assembly on a
large scale based on platinum−acetylide complexes as well.20
Inspired by these successful examples, we envisioned that the
construction of multipyrene functionalized hexgons would be
realized via the coordination-driven self-assembly strategy,
which would avoid elaborate synthesis work. More importantly,
this strategy allows for precise control over the shape and size
of the final construction as well as the distribution and total
number of incorporated pyrene moieties.
RESULTS AND DISCUSSION
■
Synthesis of 120° Pyrene-Modified Acceptor Ligand 5
and Donor Ligand 6. The linear geometry of the alkynyl unit
together with its π-unsaturated nature have made the rigid
alkynyl moieties ideal building blocks for the construction of
molecular wires, organometallic oligomers, and polymeric
materials.22 Herein we utilized the coupling reaction of trans-
[PtI2(PEt3)2] or 4-bromopyridine with C−H bonds in alkynes
as the key steps in the synthesis of novel 120° building units
(Figure 1). The new 120° pyrene-modified ligands 5 and 6 can
be easily synthesized in a few steps, as shown in Scheme 1. The
functional pyrene moiety was introduced by a Sonogashira
coupling reaction of compound 1 with 1-bromopyrene,
affording 2 in a good yield (93%). The key intermediate 3
was obtained by the deprotection reaction of 2 with K2CO3.
Compound 3 was then reacted with 4 equiv of trans-
[PtI2(PEt3)2] to give diiodometal complex 4. Subsequent
halogen abstraction with AgNO3 resulted in the isolation of
bis(nitrate) salt 5 in reasonable yield (66%). The second
Sonogashira coupling reaction of compound 3 with 4-
bromopyridine in the presence of Pd(PPh3)4 afforded the
desired pyrene-modified 120° donor ligand 6 in 45% yield.
The molecular structures of the newly designed precursors 5
and 6 were well characterized by using multiple-nuclei NMR
(1H, 31P, and 13C), mass spectrometry, and elemental analysis.
The 31P{1H} NMR spectrum of diplatimum acceptor 5
displayed a singlet at 20.6 ppm, accompanied by flanking
195Pt satellites (Figure S4B in the Supporting Information).
Single crystals of precursor 4 that were suitable for X-ray
diffraction were grown by slow evaporation of the precursor in
dichloromethane/methanol solution (3/1 v/v) at ambient
temperature for 2−3 days. An ORTEP of complex 4 (Figure 2)
showed that all of the atoms (except for the triethylphosphine
ligands) approximately lay in the same plane. The platinum
atoms in compound 4 were found to adopt a slightly distorted
trans square planar geometry (C−Pt−I and P−Pt−P bond
angles of 175.8 and 173.4°, respectively), which might be
caused by the steric demands of the bulky triethylphosphine
ligands. It also showed that precursor 4 was indeed a suitable
candidate for a 120° building unit, with the angle between the
two platinum coordination planes being approximately 124°.
The distance between the two Pt centers in precursor 4 was
∼1.0 nm.
Herein, we employ the newly designed 120° building blocks
(5 and 6) decorated with one pyrene functional group (Figure
1) with the corresponding building blocks to construct a new
family of multipyrene hexagons via [3 + 3] and [6 + 6]
Self-Assembly of Pyrene-Acceptor Ligand 5 into
Hexagonal Tris(pyrene) Metallacycle 8a and Hexakis-
(pyrene) Metallacycle 8b. According to the “directional
Figure 1. Molecular structure of pyrene-modified ligands 5 and 6.
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Scheme 1. Synthesis of 120° Pyrene-Modified Ligands 5 and 6
Scheme 2. Coordination-Driven Self-Assembly of
Multipyrene Hexagons 8a,b
Figure 2. Crystal structure of precursor 4. Selected bond lengths (Å)
and angles (deg): Pt(1)−C(26) = 1.952, Pt(1)−I(1) = 2.648, Pt(1)−
P(1) = 2.309, Pt(1)−P(2) = 2.307, C(25)−C(26) = 1.187; P(2)−
Pt(1)−P(1) = 173.44, C(26)−Pt(1)−I(1) = 175.8, P(2)−Pt(1)−I(1)
= 91.07, P(1)−Pt(1)−I(1) = 94.95, C(26)−Pt(1)−P(2) = 90.1,
C(26)−Pt(1)−P(1) = 83.7.
donor 7a via [3 + 3] self-assembly in a 1:1 stoichiometric ratio
in an acetone/water mixed solvent (10/1 v/v) for 8 h. A more
dramatic extension of functionalization by means of coordina-
tion-driven self-assembly is manifested in the construction of
molecular hexagons with six pyrenes, where each pyrene moiety
is on each of the vertices of the hexagon. The hexakis(pyrene)
hexagonal metallacycle 8b was prepared by mixing the di-Pt(II)
acceptor ligand 5 with 180° linear donor 7b via [6 + 6] self-
assembly by employing a procedure similar to that in the
construction of 8a.
bonding” model and the “symmetry interaction” model,5a,b the
shape of an individual 2-D polygon is usually determined by the
value of the turning angle within its angular components. As
discussed above, hexagonal assemblies can be self-assembled
from a combination of three 120° building blocks with three
complementary 120° building blocks or six 120° building
blocks with linear building blocks. With the 120° pyrene-
modified acceptor precursor 5 in hand, the self-assembly of two
multipyrene hexagons of different sizes and with different
numbers of pyrene groups were investigated. As shown in
Scheme 2, the hexagonal tris(pyrene) metallacycle 8a was
prepared by mixing the di-Pt(II) acceptor ligand 5 with 120°
The self-assembly progress was easily monitored with
1
31P{1H} and H NMR (Figures 3 and 4; see sections 2 and 3
in the Supporting Information). The 31P {1H} NMR spectra of
8a,b displayed a sharp singlet (17.3 ppm for 8a and 17.2 ppm
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resolved, and they agree very well with their theoretical
distribution. Given the significantly larger molecular weight
(10446 Da) of 8b, it is more difficult to get strong mass signals
even under CSI-TOF-MS conditions. With considerable efforts,
the peak at m/z 1512.42 was observed in the CSI-TOF-MS
spectrum, which corresponds to the [M − 6PF6]6+ charge state.
The peak was isotopically resolved and agrees with the
theoretical distribution, although overlap with the signal of a
very minor unknown fragment, which could not be attributed
to any alternative polygonal supramolecules, was also observed.
Self-Assembly of Pyrene-Donor Ligand 6 into Hex-
agonal Tris(pyrene) Metallacycle 10a and Hexakis-
(pyrene) Metallacycle 10b. As mentioned above, by
employing a diverse library of donor and acceptor building
blocks, the construction of “isomeric” supramolecular multi-
pyrene hexagons possessing the same geometry can be realized
by two complementary strategies. Thus, from the 120° pyrene-
modified donor precursor 6, another series of multipyrene
hexagons were also obtained.
Figure 3. 31P{1H} NMR spectra (161.9 MHz, 298 K) of the 120° di-
Pt(II) acceptor 5 in CD2Cl2 (A), the [3 + 3] hexagon 8a in acetone-d6
(B), and the [6 + 6] hexagon 8b in acetone-d6 (C).
As shown in Scheme 3, the [3 + 3] molecular hexagon 10a
was prepared by mixing the donor ligand 6 with 120° di-Pt(II)
Scheme 3. Coordination-Driven Self-Assembly of
Multipyrene Hexagons 10a,b
1
Figure 4. Partial H NMR spectra (400 MHz, 298 K) of the 120°
donor ligand 7a in CD2Cl2 (a), [3 + 3] hexagon 8a in acetone-d6 (b),
180° donor ligand 7b in CDCl3 (c), and [6 + 6] hexagon 8b in
acetone-d6 (d).
acceptor 9a in a 1:1 stoichiometric ratio in aqueous acetone
(acetone/water, 10/1 v/v) overnight. The PF6 salts of 10a were
synthesized by dissolving the yellow NO3 salts of 10a in
acetone/water and adding saturated aqueous solution of KPF6
to precipitate the product, which was collected by vacuum
filtration. 31P{1H} NMR analysis (Figure 5A) of the reaction
mixture of 10a is consistent with the formation of asingle,
highly symmetric species, as indicated by the appearance of a
sharp singlet (ca. 14.6 ppm) with concomitant 195Pt satellites,
shifted upfield by ca. 5.1 ppm in comparison to the signal for
the starting platinum acceptor 9a. This change, as well as the
decrease in coupling of the flanking 195Pt satellites (ΔJ = −199
Hz), is consistent with back-donation from the platinum atoms.
for 8b), which suggests the formation of a discrete, highly
symmetric supramolecular assembly. It was found that the peak
is shifted upfield from the starting platinum acceptors 5 by
approximately 3.3 and 3.4 ppm, respectively. This change, as
well as a decrease in coupling of flanking 195Pt satellites (ΔJPPt
= −161.9 Hz for 8a,b), is consistent with back-donation from
the platinum atoms. Additionally, the protons of the pyridine
rings (α-H and β-H) exhibited shifts 0.2−0.4 ppm downfield,
resulting from the loss of electron density upon coordination of
the pyridine N atom with the Pt(II) metal center.
Mass spectrometric investigation of multipyrene hexagonal
metallacycles was performed by the CSI-TOF-MS technique,
which allows the assemblies to remain intact during the
ionization process in order to obtain the high resolution
required for unambiguous determination of individual charge
states.23 In the CSI-TOF-MS spectrum of 8a (Figure S10 in the
Supporting Information), for example, peaks at m/z 1636.39
and 1191.09, related to [M − 3PF6]3+ and [M − 4PF6]4+,
respectively, were observed. These peaks were isotopically
1
In the H NMR spectrum of 10a, the hydrogen atoms of the
pyridine rings exhibited obvious shifts downfield (α-H, 0.48
ppm; β-H, 0.60 ppm) due to the loss of electron density that
occurs upon coordination of the pyridine N atom with the
Pt(II) metal center (Figure 6).
As indicated in Scheme 3, the [6 + 6] hexagon 10b can be
self-assembled from six 180° acceptor units 9b and six of the
120° pyrene donor 6, each derivatized with one pyrene moiety.
D
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for the starting acceptor ligand 9b (δ 19.5 ppm). As expected,
1
in the H NMR spectra, the α-H of the pyridine rings shifted
slightly from 8.6 ppm in the free donors to 8.7 ppm in the
assemblies and β-H exhibited approximately 0.4 ppm downfield
shifts (Figure 6) because of the loss of electron density that
occurs upon coordination of the pyridine N atom with the
Pt(II) metal center. All of the multiple NMR analyses above are
in agreement with the formation of discrete and highly
symmetric multipyrene hexagonal metallacycles.
Mass spectrometric studies of both assemblies were
performed using the CSI-TOF-MS and ESI-TOF-MS techni-
ques. The CSI-TOF-MS spectra of hexagon 10a exhibited two
charged states at m/z 1233.4 and 957.4, corresponding to [M −
4PF6]4+ and [M −5PF6]5+, respectively. These peaks were
isotopically resolved, and they agree well with their theoretical
distribution (Figure S11 in the Supporting Information).
Moreover, in the ESI-TOF-MS spectra of 10b, a peak at m/z
1940.52 corresponding to [M − 5PF6]5+ was observed. This
peak was isotopically resolved, and it is in good agreement with
its theoretical distribution. Thus, the obtained analysis data
ensure that only the [3 + 3] and [6 + 6] multipyrene hexagons
10a,b, respectively, were formed in each self-assembly.
Molecular Modeling Studies. Unfortunately, all attempts
to grow X-ray-quality single crystals of hexagonal multipyrene
metallacycles 8a,b and 10a,b have proven to be unsuccessful to
date. Thus, the PM6 semiempirical molecular orbital method
was employed to optimize the geometry of all multipyrene
metallacycles. The optimized structure of each multipyrene
hexagon featured a very similar, roughly planar hexagonal ring
decorated with multiple pyrene groups at the exterior surface
(Figure 7). Moreover, the sizes of these novel hexagonal
metallacycles were determined as well. For instance, the [3 + 3]
hexagons 8a and 10a have internal radii of approximately 1.4
and 1.5 nm, respectively, while the [6 + 6] hexagons 8b and
10b have larger internal radii of 2.1 and 2.5 nm, respectively.
Additionally, it was found that the [3 + 3] hexagons 8a and 10a
have external radii of approximately 2.8 and 2.9 nm,
Figure 5. 31P{1H} NMR spectra (161.9 MHz, acetone-d6, 298 K) of
the 120° di-Pt(II) acceptor 9a and the [3 + 3] hexagon 10a (A) and
31P{1H} NMR spectra (400 MHz, acetone-d6, 298 K) of the 180° di-
Pt(II) acceptor 9b and the self-assembled [6 + 6] hexagon 10b (B).
1
Figure 6. Partial H NMR spectra (400 MHz, 298 K) of the pyrene-
modified 120° donor ligand 6 in CDCl3 (a), [3 + 3] hexagon 10a in
acetone-d6 (b), and [6 + 6] hexagon 10b in acetone-d6 (c).
The reaction mixture was stirred at ambient temperature for 4 h
to yield a cloudy yellow solution. As a result of the coordination
of the pyridine moiety with the Pt center, only one sharp 31P
NMR peak at 13.3 ppm was observed for 10b (Figure 5B),
shifted upfield by almost 6.2 ppm in comparison with the signal
Figure 7. Simulated molecular models of the pyrene-modified [3 + 3]
hexagon 8a (A), the [6 + 6] hexagon 8b (B), the [3 + 3] hexagon 10a
(C), and the [6 + 6] hexagon 10b (D).
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Figure 8. Emission spectra of 5 (10−5 M), 6 (10−5 M), 8a (0.33 × 10−5 M), 8b (0.17 × 10−5 M), 10a (0.33 × 10−5 M), and 10b (0.17 × 10−5 M) in
CH2Cl2.
Figure 9. Normalized absorbance and emission titration spectra of 10a (a, c) and 10b (b, d) in CH2Cl2.
respectively, while larger external diameters of 3.5 and 3.9 nm
for the [6 + 6] hexagons 8b and 10b were observed. This
indicated that the size of hexagonal cavities in the final
assemblies can be controlled by using different sizes of donors
and acceptors.
avoids the time-consuming procedures and lower yields often
encountered in covalent synthetic protocols. Furthermore, the
synthesis is straightforward and the yield is quantitative, which
eliminates the need for purification.
UV−Visible Absorption and Fluorescence Studies of
the Building Blocks and Metallacycles. The pyrene moiety
is one of the most useful fluorophores due to its efficiency in
excimer formation and subsequent changes in its emission
properties. With pyrene-containing building blocks 5 and 6
along with the novel multipyrene hexagons 8a,b and 10a,b in
hand, an investigation of their spectroscopic behavior was
carried out. As shown in Figure S12 (Supporting Information),
the absorption spectrum of di-Pt(II) acceptor 5 (1.0 × 10−5 M)
Thus the obtained analysis data including the singularity of
each 31P NMR signal ensures that the desired multipyrene
hexagons 8a,b and 10a,b were formed in the self-assembly. The
1
sharp NMR signals in both the 31P{1H} and H NMR spectra
along with the solubility of these species ruled out the
formation of oligomers. This means that multiple pyrene
derivatives with predesigned shapes and sizes can be easily
prepared through coordination-driven self-assembly, which
F
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in CH2Cl2 exhibited broad and structureless bands around
230−290 and 300−400 nm. On the basis of a previous
spectroscopic invesitgation on platinum−acetylide complexs,24
the low-energy absorption bands were described as an
admixture of intraligand (IL) (π−π*(CCR)) and metal-to-
ligand charge transfer (MLCT) (d(Pt) π−π*(CCR))
transitions. Moreover, the higher energy absorption bands
were likely due to π−π* transitions localized on the pyrene
ring. Likewise, dipyridyl donor 6 (1.0 × 10−5 M) in CH2Cl2
exhibited a similar pattern, which was dominated by the π−π*
transition of aromatic pyrene rings and intraligand (IL) charge
transfer.25 The fluorescence emission of 5 and 6 with the
ethynyl−pyrene moieties appeared at λmax 405 nm with a
shoulder band at 425 nm in CH2Cl2 (Figure 8). This result
corresponded to the monomer emission of the pyrene
chromophore. In comparison to those of the typical pyrene-
containing compounds without ethynylene groups, in which the
emission peaks were at 375 and 395 nm,26 the fluorescence
emission spectra of 5 and 6 were red-shifted about 30 nm. The
bathochromic shift of the emission wavelength might be due to
the extended π-conjugation systems resulting from the
introduction of an ethynylene group.27
In comparison to the precursor 5, the absorption coefficients
(ε) of assemblies 8a,b were enhanced, since the effective
concentration of the pyrene moiety increased.28 A similar
feature was found in the assemblies 10a,b (Table S1 in the
Supporting Information). However, complexes 8a (ΦF = 0.030)
and 8b (ΦF = 0.013) have a quantum yield much lower than
that of compound 5 (ΦF = 0.87). Similarly, complexes 10a (ΦF
= 0.0097) and 10b (ΦF = 0.0032) featured a quantum yield
much lower than that of their precursor 6 (ΦF = 0.12). The
fluorescence quenching of these assembles might be caused by
the aggregation of themselves in CH2Cl2.29
It should be noted that these novel multipyrene hexagons
displayed dramatically different emission behaviors. The
assemblies 8a,b showed weak emissions at 405 and 425 nm
(Figure 8a). Notably, there was almost no emission band
beyond 500 nm, indicating the absence of excimer formation of
pyrene.30 Interestingly, different from 8a,b, which showed
typical monomeric emission of the pyrene, metallacycles 10a
(0.33 × 10−5 M) and 10b (0.17 × 10−5 M) in CH2Cl2 displayed
a longer fluorescence emission band at λmax 550 nm (Figure
8b), which corresponded to the excimer emission of the pyrene
chromophore.31 It should be noted that at the same
concentration as that of 10a,b, metallacycles 8a (0.33 × 10−5
M) and 8b (0.17 × 10−5 M) were not able to form excimers,
which might be attributed to the relatively lower charge
densities of 8a,b in comparison to that of 10a,b32 (Table S2 in
the Supporting Information). This means that the combination
mode of building blocks can influence the excimer formation of
the resulting pyrene-containing self-assemblies.
The effects of concentration on the absorption and
fluorescence emission of 10a,b are shown in Figure 9. When
the concentrations of 10a and 10b were increased, a decrease of
the absorbance at around 285 nm with a concomitant increase
of the absorbance around 325 nm was observed. Likewise, a
significant decrease of the emission at 405 nm and a
hypochromic shift (145 nm) of the emission spectra were
observed as well, which were in agreement with the formation
of pyrene excimers.33 Notably, dipyridyl donor 6 featured a
concentration-dependent emission in CH2Cl2 (Figure S13,
Supporting Information) as well. For example, it was found that
the high-energy emission spectrum with a maximum of 393 nm
at 1.0 × 10−9 M disappeared, with the emission maximum red-
shifted to 530 nm at 3.0 × 10−2 M. It should be noted that, in
comparison to metallacycles 10a,b, the formation of pyrene
excimers of 6 was observed at much higher concentration,
which is indicative of the notion that the metallacycles 10a,b
have a tendency stronger than that of 6 to form excimers under
similar conditions. These results indicated that structural effects
played important roles during the formation of pyrene
excimers.
CONCLUSION
■
In conclusion, the work presented here provided a simple and
highly efficient approach to the construction of multipyrene
hexagons possessing structurally well-defined cavities of varying
diameter and shape via coordination-driven self-assembly. The
covalent attachment of a pyrene to both 120° di-Pt(II) acceptor
and 120° bipyridyl donor units allowed for the formation of a
diverse library of new functionalized donor and acceptor
building blocks, from which a series of “isomeric” supra-
molecular multipyrene hexagons possessing the same geometry
were obtained via two complementary strategies. All of these
assemblies were characterized by multinuclear NMR (31P and
1H) spectroscopy, CSI-TOF-MS and ESI-TOF-MS, elemental
analysis, and molecular modeling. These resultant multipyrene
assemblies are unique in that the number and arrangement of
their pendant macrocycles can be precisely controlled, owing to
the conserved rigidity of their core 2-D polygonal scaffolds. The
primary spectroscopic behavior of these newly designed
assemblies was investigated. An interesting result was that
these novel multipyrene hexagons displayed dramatically
different emission behaviors, although they possessed the
same geometry, which indicated that structural effects played
important roles during the formation of pyrene excimers in this
study. All of these findings enrich the library of functionalized
supramolecular metallacycles and demonstrate again the power
and scope of the coordination-driven self-assembly.
EXPERIMENTAL SECTION
■
General Information. All solvents were dried according to
standard procedures, and all of them were degassed under N2 for 30
min before use. Reagents were used as purchased. All air-sensitive
reactions were carried out under an argon atmosphere. H NMR, 13C
1
NMR, and 31P NMR spectra were recorded on a Bruker 400 MHz
spectrometer (1H, 400 MHz; 13C, 100 MHz; 31P, 161.9 MHz) at 298
K. The 1H and 13C NMR chemical shifts are reported relative to
residual solvent signals, and 31P NMR resonances are referenced to an
internal standard sample of 85% H3PO4 (δ 0.0). Coupling constants
(J) are denoted in Hz and chemical shifts (δ) in ppm. Multiplicities are
denoted as follows: s = singlet, d = doublet, m = multiplet, br = broad.
Synthesis of Compound 5. Diiodine complex 4 (184 mg, 0.13
mmol) and AgNO3 (64 mg, 0.39 mmol) were placed in a 25 mL
Schlenk flask followed by 3 mL of dichloromethane. The reaction
mixture was stirred in the dark at room temperature for 24 h. A clear
solution with a heavy creamy precipitate resulted; the precipitate was
filtered off, and the solvent was removed in vacuo. The residue was
redissolved in a minimal amount of dichloromethane, and then diethyl
ether was carefully added to precipitate the product. The product 5
was obtained as a light yellow solid and dried in vacuo: yield 106 mg,
66%; 1H NMR (CD2Cl2, 400 MHz) δ 8.67 (d, J = 9.2 Hz, 1H), 8.29−
8.07 (m, 8H), 7.35 (s, 2H), 7.09 (s, 1H), 2.00−1.95 (m, 24H), 1.28−
1.20 (m, 36H); 31P NMR (CD2Cl2, 161.9 MHz) δ 8.58 (s, JPt−P
=
2470.6 Hz); MALDI HRMS m/z calcd for C52H72N2O6P4Pt2 1334.36
[M − H]+, found 1334.4. Anal. Calcd for C52H72N2O6P4Pt2: C, 46.78;
H, 5.44; N, 2.10. Found: C, 46.52; H, 5.59; N, 2.22.
G
dx.doi.org/10.1021/om301108s | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Synthesis of Compound 6. Under an atmosphere of nitrogen, 20
mL of THF and 20 mL of i-Pr2NH were added to a mixture of
compound 3 (494 mg, 1.41 mmol), 4-bromopyridine hydrochloride
(1.10 g, 5.64 mmol), Pd(PPh3)4 (81 mg, 0.070 mmol), and CuI (13
mg, 0.070 mmol) in a 100 mL Schlenk flask. The mixture was stirred at
65 °C for 12 h. After that, insoluble materials were filtered through
filter paper and the solvent was removed by evaporation on a rotary
evaporator. Column chromatography with dichloromethane/acetone
(5/1 v/v) as eluent afforded a yellow solid of compound 6 in a yield of
was added CD2Cl2 (0.7 mL) solvent, and the reaction solution was
then stirred at room temperature for 4 h to yield a cloudy yellow
solution. The solution was then transferred into an NMR tube to
1
collect H and 31P NMR spectra. The solid product was obtained by
removing the solvent under vacuum: yield 96%; yellow solid; 1H NMR
(CD2Cl2, 400 MHz) δ 8.71−8.69 (m, 30H), 8.33−8.02 (m, 66H), 7.87
(d, J = 5.6 Hz, 24H), 7.07 (s, 24H), 1.36 (br, 144H), 1.18−1.12 (m,
216H); 31P NMR (CD2Cl2, 161.9 MHz) δ 13.30 (s, JPt−P = 2736.1
Hz). Anal. Calcd for C420H504F36N12O36P24Pt12S12: C, 48.27; H, 4.86;
N, 1.61; found: C, 48.32; H, 4.98; N, 1.72.
1
45%: Rf = 0.58 (dichloromethane/acetone, 5/1 v/v); mp 198 °C; H
NMR (CDCl3, 400 MHz) δ 8.66−8.63 (m, 5H), 8.27−8.04 (m, 8H),
7.91 (s, 2H), 7.75 (s, 1H), 7.45 (d, J = 4.8 Hz, 4H); 13C NMR
(CDCl3, 100 MHz) δ 149.74, 134.89, 134.23, 131.92, 131.51, 131.05,
130.82, 130.69, 129.56, 128.49, 128.39, 127.06, 126.23, 125.73, 125.46,
125.10, 124.64, 124.43, 124.26, 124.06, 123.03, 116.64, 92.77, 91.84,
90.58, 87.86; MS (EI) m/z (%) 504 (M+, 1.75); HRMS calcd for
C38H20N2 504.1626, found 504.1633.
ASSOCIATED CONTENT
* Supporting Information
Text, tables, figures, and a CIF file giving details of synthesis
and characterization of the compounds and supplementary
experimental data, as well as the crystal structure of 4. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
Self-Assembly of Hexagon 8a. A mixture of bipyridyl donor
ligand 7a (2.92 mg, 10.04 μmol) and pyrene-modified acceptor 5
(13.92 mg, 10.04 μmol) was placed in a glass vial. Then water (0.5
mL) and acetone (5.0 mL) were added into the bottle with continuous
stirring (10 min). The reaction mixture was stirred for 8 h at 55 °C,
upon which the starting materials completely dissolved and the
reaction mixture attained a light yellow color. The PF6 salts of 8a were
synthesized by dissolving the yellow NO3 salts of 8a in acetone/water
and adding a aqueous solution of KPF6 to precipitate the product,
AUTHOR INFORMATION
Corresponding Author
*E-mail: lxu@chem.ecnu.edu.cn (L.X.); hbyang@chem.ecnu.
edu.cn (H.-B.Y.).
Notes
■
1
which was collected by vacuum filtration: yield 98%; yellow solid; H
The authors declare no competing financial interest.
NMR (acetone-d6, 400 MHz) δ 9.07 (d, J = 5.2 Hz, 12H), 8.69 (d, J =
9.2 Hz, 3H), 8.40−8.13 (m, 27H), 7.98−7.82 (m, 18H), 7.69 (t, J =
7.6 Hz, 3H), 7.55 (s, 6H), 7.28 (s, 3H), 2.06−2.02 (m, 72H), 1.32−
ACKNOWLEDGMENTS
■
1.26 (m, 108H); 31P NMR (acetone-d6, 161.9 MHz) δ 17.32 (s, JPt−P
=
This work was financially supported by the NSFC/China (No.
21132005, 91027005, and 20902007), Shanghai Shuguang
Program (No. 09SG25), RFDP (No. 20100076110004) of
Higher Education of China, Fok Ying Tung Education
Foundation (No.131014), and the Program for New Century
Excellent Talents in University of Ministry of Education of
China (NCET-09-0358).
2852.0 Hz); CSI-TOF-MS [M − 3PF6]3+ 1636.39, [M − 4PF6]4+
1191.09, [M − 5PF6]5+ 923.84. Anal. Calcd for C216H252F36N6P18Pt6:
C, 48.54; H, 4.75; N, 1.57. Found: C, 48.23; H, 4.54; N, 1.77.
Self-Assembly of Hexagon 8b. A mixture of bipyridyl donor
precursor 7b (1.04 mg, 6.66 μmol) and pyrene-modified acceptor 5
(8.90 mg, 6.66 μmol) was placed in a glass vial. Then water (0.5 mL)
and acetone (5.0 mL) were added into the bottle with continuous
stirring (10 min). The reaction mixture was stirred overnight at 55 °C,
upon which starting materials completely dissolved and the reaction
mixture attained a light yellow color. The PF6 salt of 8b was
synthesized by dissolving the yellow NO3 salt of 8b in acetone/water
and adding a aqueous solution of KPF6 to precipitate the product,
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ligand 6 (3.00 mg, 5.94 μmol) and the 120° nitrate 9a (6.94 mg, 5.94
μmol) was placed in a glass vial. Then water (0.5 mL) and acetone
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1
was collected by vacuum filtration: yield 97%; yellow solid; H NMR
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C, 48.75; H, 5.19; N, 1.74.
Self-Assembly of Hexagon 10b. The dipyridyl donor ligand 6
(2.50 mg, 4.95 μmol) and the organoplatinum 180° acceptor 9b (6.13
mg, 4.95 μmol) were weighed accurately into a glass vial. In the vial
H
dx.doi.org/10.1021/om301108s | Organometallics XXXX, XXX, XXX−XXX

Organometallics
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