Photoresponsive supramolecular organometallic nanosheets induced by Pt II...PtII and C-H...π interactions

Article abstract of DOI:10.1002/anie.200905678

It's a flat world: Charge-neutral pincertype cyclometalated platinum (I I) aryl acetylides can be assembled into quasi-twodimensional nanosheets through bilateral intermolecular noncovalent interactions (see picture). These nanosheets can be further self-

Full text of DOI:10.1002/anie.200905678

Angewandte
Chemie
DOI: 10.1002/anie.200905678
Nanostructures
Photoresponsive Supramolecular Organometallic Nanosheets Induced
II
II
ꢀ
by Pt ···Pt and C H···p Interactions**
Yong Chen, Kai Li, Wei Lu,* Stephen Sin-Yin Chui, Chun-Wah Ma, and Chi-Ming Che*
While graphene, a carbon-based two-dimensional nanomate-
rials, has received an upsurge of interest,[1] self-assembly of
small organic and organometallic molecules into 2D nano-
structures could also be harnessed to develop new classes of
functional supramolecular nanomaterials.[2] In principle,
quasi-2D lamellae or nanosheets are planar structures
having a thickness less than 100 nm and lateral dimensions a
few orders of magnitude greater than their thickness. Control
over the bilateral intermolecular noncovalent interactions is
anticipated to organize small molecules into regular 2D
nanostructures, which has been a formidable challenge yet to
be achieved. Recently, Shelnutt and co-workers obtained
discrete porphyrin nanosheets reprecipitated from their
solutions;[3] Sathish and co-workers constructed hexagonal
C60 nanosheets using a liquid–liquid interfacial precipitation
method;[4] the groups of Yao[5] and Hu[6] prepared single-
crystalline nanosheets of polycyclic aromatics using a surfac-
tant-assisted reprecipitation and a physical vapor transporting
method, respectively; and Zhang and co-workers suggested
that molecules with intramolecular charge-transfer dipole
moments could be grown into quasi-2D nanostructures.[7]
Moreover, some amphiphiles and organogelators were
found to self-organize into sheet-like nanostructures in
contact with solvents.[8] Despite these advances, template-
and surfactant-free synthesis of free-standing, crystalline, and
optoelectronically active nanosheets from small molecules
remains elusive.
applications and light-energy conversion reactions.[9] Inter-
molecular PtII···PtII interactions play a key role in the close
packing of molecules. To date, quasi-one-dimensional nano-
wires have been obtained by self-organization of organo-
platinum(II) complexes, presumably owing to the fact that
molecular propagation is faster along the PtII···PtII chain axis
than in the lateral directions. We and others have taken
advantage of this anisotropic growth mechanism to assemble
cationic organoplatinum(II) salts into nanostructures with
luminescent, semiconducting, liquid crystalline, and gelating
properties.[10] Herein we report free-standing and crystalline
nanosheets self-assembled from neutral pincer-type[11] cyclo-
metalated platinum(II) aryl acetylides. Indeed, intermolecu-
[12]
lar PtII···PtII and C H···p(C C) interactions in an orthog-
onal configuration account for the quasi-2D molecular
organization herein. Notably, these organometallic nano-
sheets luminesce in the red to near infrared (NIR) region, and
their electronic conductivity can be modulated by visible-light
irradiation.
ꢀ
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The charge-neutral organoplatinum(II) complexes herein,
1
2
ꢁ
namely [4-R -(N^C^N)PtC CC6H4-4’-R ] (1–8, N^CH^N =
1,5-bis(2’-pyridyl)benzene), were prepared in quantitative
yields by stirring the corresponding precursor, [4-R1-
(N^C^N)PtCl] (R1 = CF3,[10g] H,[13a] or CH3[13b]), and an
excess of aryl acetylene in methanol in the presence of
NaOH for one day at room temperature.[14] It is notable that
when R1 is CF3, the solids of complexes 1–6 display various
colors, depending on the substituent R2 on the aryl acetylide
ligand (Scheme 1). When R2 is an electron-withdrawing group
(CF3 for 5 and NO2 for 6), an electron-donating group (OCH3
for 3 and N(CH3)2 for 4), or a relatively neutral group (H for 1
and F for 2), the solid is bright yellow, deep red, or dark green
in color, respectively. When R1 is H or CH3, no such color
contrast was observed upon varying R2 from electron-with-
drawing to -donating groups; the solids are yellow in color, as
depicted for complexes 7 and 8 in Scheme 1.
Square-planar platinum(II) complexes containing p-con-
jugated ligands are particularly attractive building blocks for
self-assembly reactions, because cofacial molecular aggrega-
tion can maximize the orbital interactions between two
p systems, thus leading to rich photophysical and photo-
chemical properties that can be harnessed for optoelectronic
[*] Dr. Y. Chen, K. Li, Dr. W. Lu, Dr. S. S.-Y. Chui, C.-W. Ma,
Prof. Dr. C.-M. Che
Department of Chemistry and
HKU-CAS Joint Laboratory on New Materials
The University of Hong Kong, Pokfulam Road, Hong Kong (China)
E-mail: luwei@hku.hk
cmche@hku.hk
[**] This work was supported by the “Nanotechnology Research
Program” of the University Development Fund of The University of
Hong Kong, and the Hong Kong Research Grants Council (HKU
7011/07P and 7008/09P). Y.C. thanks The University of Hong Kong
for a Postdoctoral Fellowship. Thanks go to Dr. Zong-Xiang Xu,
Frankie Yu-Fee Chan, and Amy Sui-Ling Wong for assistance in
device fabrication and TEM/SEM observations. We thank Dr.
Nianyong Zhu for solving the crystal structure of complex 7.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200905678.
Scheme 1. Chemical structures and solid-state colors of 1–8.
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Colors of organoplatinum(II) complexes in the solid state
2.646 ꢀ and contact angle of 163.68 have been identified
between every two adjacent molecules along the c axis.
In the crystal structure of 7, the molecules pack into a
dimeric structure in a head-to-tail fashion with an interplanar
distance of approximately 3.4 ꢀ and a shortest intermetal
separation of 4.886 ꢀ (see the Supporting Information), thus
indicating a prominent p–p interaction but not a PtII···PtII
interaction between the two paired molecules.
are usually associated with intermetal interactions.[15] Various
packing modes have been identified by X-ray crystallography
for crystals of complexes 1 (Figure 1), 4 (Figure S1 in the
Supporting Information), and 7 (Figure S2 in the Supporting
Information), which appear as green, red, and yellow solids,
respectively.[16] The salient features in the crystal structures of
both 1 and 4 are the intermolecular PtII···PtII/p–p and
ꢀ
ꢁ
C H···p(C C) interactions in an orthogonal configuration.
The reprecipitation method was adopted to prepare the
nanostructures. Complexes 1, 4, and 7 are used as represen-
tative examples in the following description. The complex was
dissolved in CH2Cl2 or THF to give a bright yellow solution
(ca. 6.0 mm, 0.5 mL). Addition of 5 mL n-hexane or cyclo-
hexane into the CH2Cl2 solution gave a green, red, or yellow
suspension for 1 (Figure 2a), 4 (Figure 2g), and 7, respec-
Figure 1. Crystal structure of 1. a) Perspective view and b,c) packing
diagrams. The three fluorine atoms of the CF3 group are disordered.
Dashed lines indicate intermolecular distances shorter than the sum
of van der Waals radii.
Complex 1 crystallizes in an orthorhombic lattice without
solvates. The acetylenic phenyl ring is perpendicular to the
(N^C^N)Pt plane, and the latter is coplanar with the
ab plane. Infinite PtII···PtII chains exist along the c axis, with
a uniform intermetal contact of 3.383 ꢀ (shorter than the sum
of van der Waals radii of 3.44 ꢀ) and a Pt-Pt-Pt angle of
172.858. Neighboring molecules are arranged in a staggered
fashion with a torsion angle of 49.48 around the Pt–Pt axis
ꢀ
(Figure 1b). Intermolecular C H···p interactions have been
discerned between every two adjacent molecules along the
b axis (Figure 1c), as indicated by a short contact of 2.691 ꢀ
between H15 of the (N^C^N) ligand and acetylenic C19 of
another molecule, and a C15-H15-C19 angle of 159.68. No
short intermolecular contacts could be identified along the
a axis.
Complex 4 crystallizes as thin plates with intrinsic
twinning problems. Nevertheless, we were able to solve the
crystal structure of 4 by direct methods based on a set of X-ray
diffraction data with 87% completeness (see the Supporting
Information for details). The crystal structure has a mono-
clinic space group with one half of a hexane solvate molecule
per three crystallographically independent molecules of the
complex. The (N^C^N)Pt planes of the these molecules are
nearly coplanar with the [10ꢀ1] crystal faces (see the
Supporting Information for diagrams). The intermetal dis-
tances between the two neighboring molecules are 3.229,
4.879, and 3.710 ꢀ. Similar to crystal 1, the intermolecular
Figure 2. Characterization of nanosheets of 1 (a–f) and 4 (g–l).
a,g) Photographs of dispersions in CH2Cl2/n-hexane (1:10 v/v).
b,c,h,i) SEM images at different magnifications and tilt angles.
d,e,j,k) TEM images and corresponding SAED patterns. The TEM
images have been rotated 928 so that the diffraction pattern coincides
in orientation with the image; f,l) Powder XRD patterns of thin films
prepared by drop-casting of the dispersions.
tively. The nanostructures precipitated from the solvent upon
long-term standing but can be re-dispersed upon shaking. No
color or morphological changes were observed after months
or even after boiling the suspensions.
Observations by scanning electron microscopy (SEM,
Figure 2b,c) and transmission electron microscopy (TEM,
Figure 2d) revealed that the as-prepared suspension of 1
contains 2D sheet-like nanostructures, irrespective of the
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C H···p(C C) interactions with a short contact distance of
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reprecipitation solvent system. Both the length and width of
the sheets are tens of micrometers, while the average thick-
ness is less than 50 nm, which has been further confirmed by
atomic force microscopy (AFM, Figure S3 in the Supporting
Information). Decreasing the complex concentration or
changing the good solvent from CH2Cl2 to THF during
reprecipitation can reduce the lateral dimensions while
keeping the thickness of the nanosheets in the nanometer
scale (Figure S4 in the Supporting Information). The selected
area electron diffraction (SAED) pattern of a single nano-
sheet of 1 shows sharp and ordered spots (Figure 2e), which
were successfully indexed according to the X-ray single-
crystal structure of 1. The d spacings of 9.59 and 3.40 ꢀ
correspond to the respective [020] and [002] Miller planes
found in the crystal structure of 1. By correlating the X-ray
and electron diffraction data, the growth directions of the
nanosheets coincide with the c and b axes, that is, the
The above-mentioned film of 1 displayed a low-energy
absorption band at lmax 720 nm and a near infrared (NIR)
emission at lmax 822 nm (Figure 3a). Both the low-energy
absorption and emission are associated with the extended
Figure 3. a) Absorption (dashed lines) and emission (solid lines)
spectra of complex 1 in film (black lines) and in CH2Cl2 solution (gray
lines). b) Normalized room-temperature solid-state emission spectra
of complexes 1 (solid line), 4 (dashed line), and 7 (dotted line).
directions of extended PtII···PtII and C H···p(C C) interac-
tions, respectively. We suggest that these two types of
intermolecular noncovalent interactions in an orthogonal
configuration dictate the quasi-2D anisotropic growth of
nanosheets. Complex 2 exhibits a similar capability to form
nanosheets (Figure S5 in the Supporting Information) as
complex 1, except that the d spacing (9.38 ꢀ) found for the
[020] planes of nanosheets of 2 is smaller than that of 1
(9.59 ꢀ).
ꢀ
ꢁ
PtII···PtII chains found in the crystal structure of 1.[8,9] In
contrast, a diluted solution of 1 in CH2Cl2 is almost colorless,
with the lowest absorption band at 387 nm; it luminesces with
an emission maximum at 486 nm (Figure 3a). Again com-
plexes 1, 4, and 7 are used to compare the spectroscopic
properties in solution and in the solid state. Upon excitation
at 365 nm, 1, 4, and 7 in diluted CH2Cl2 solutions (concen-
tration ca. 2.0 ꢁ 10ꢀ5 moldmꢀ3) at 298 K show intense emis-
sions with lmax (lifetime, quantum yield) at 486 (1.7 ms, 29%),
480 (3.8 ms, 8%), and 496 nm (2.6 ms, 52%), respectively.
These photophysical properties are typical for pincer-type
cyclometalated platinum(II) complexes.[13b,c] Powder samples
of 1 and 4 at room temperature show structureless emissions
at lmax 819 and 702 nm (Figure 3b), respectively, which can be
assigned to triplet metal-metal-to-ligand charge-transfer
(3MMLCT) excited states[9,10] associated with PtII···PtII inter-
actions found in the crystal structures of 1 and 4. A powder
sample of 7 at room temperature exhibits a broad emission
with lmax at 587 nm superimposed by vibronically structured
bands on the high-energy side (Figure 3b). We ascribe the
high-energy emission in the region of 480–550 nm to triplet
metal-to-ligand charge-transfer (3MLCT) excited states
[(5d)Pt!p*(cyclometalated ligand)] and the low-energy
band at lmax 587 nm to a triplet pp*excimer.
Suspensions of 4 in CH2Cl2/n-hexane contained 2D plank-
like nanosheets with a thickness, width, and length of less than
100 nm, 1–4 mm, and tens of micrometers, respectively. Sharp
crystal facets with an angle of 1078 were observed in both
SEM and TEM images (Figure 2h–j). Sharp and ordered
spots in the SAED pattern (Figure 2k) of a single nanosheet
of 4 have been successfully indexed according to the X-ray
single-crystal structure of 4. The d spacings of 9.01 and 3.35 ꢀ
correspond to the respective [30ꢀ3] and [002] Miller planes
found in the crystal structure of 4. Similar to the study on
nanosheets of 1, complex 4 grew into a quasi-2D nano-
structure coinciding with the [10ꢀ1] zone and c axis, that is,
the direction for the extended PtII···PtII/p–p stacking and
ꢀ
ꢁ
C H···p(C C) interactions, respectively. Using the same
preparation protocol, complexes 3, 5, and 6 form quasi-1D
nanowires or nanofibers, whereas complexes 7 and 8 form
mixtures of nanoparticles and nanobelts (see the Supporting
Information for micrographs).
The nanosheets of complexes 1 and 4 can be transferred
onto a flat substrate to form layered structures. The drop-cast
films of 1 on a glass slide exhibit metallic luster, and the
powder X-ray diffraction (XRD) pattern of this film (Fig-
ure 2 f) shows only [h00] peaks at 2q = 5.59, 11.19, 16.81,
22.48, 28.20, 33.98, 39.88, and 45.868 (d = 15.80, 7.90, 5.27,
3.95, 3.16, 2.64, 2.26, and 1.98 ꢀ, respectively), thus indicating
a highly ordered alignment of molecules and the formation of
a layered structure. It is evident that the nanosheets of 1 are
preferentially oriented with bc planes of the crystal lattice
parallel to the substrate surface. The powder XRD pattern of
a drop-cast film of 4 shows only [0k0] and [0k1] (k is an even
number) peaks (Figure 2 l), thus indicating a preferred
orientation of the nanosheets of 4 with ac planes of the
crystal lattice parallel to the substrate surface.
We tested the charge-transporting properties of nano-
sheets of 1 using a bottom-contact field-effect transistor
(FET) configuration. A suspension was drop-cast onto a
prepatterned silicon wafer, thus making a couple of nano-
sheets to bridge interdigitated electrodes with a channel
length of approximately 6 mm (Figure 4a). The output char-
acteristics (Figure 4b) of this device revealed that these
nanosheets behave as a p-type, hole-transporting semicon-
ductor. A transient channel current was recorded with this
device at VDS = VG = ꢂ 40 V and with a 40 mWcmꢀ2 white
light switching on and off every five seconds in a 90 s period.
As shown in Figure 4c, the conductivity of these nanosheets
showed reversible responses towards light irradiation. Both
forward and backward currents were enhanced when the light
was on, indicating that carrier density of both electron and
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Figure 4. a) SEM image of a bottom-contact FET device with nano-
sheets of 1 as semiconducting materials. b) Output characteristics (IDS
vs. VDS). c) Transient measurement (VDS =VG =ꢂ40 V) of the FET
device. The transient channel current was recorded with a 40 mWcmꢀ2
light switching on and off every five seconds in a 90 second period.
IDS =drain–source current, VDS =drain–source voltage, VG =gate volt-
age.
hole are increased by photoexcitation of the organo-
platinum(II)-based nanomaterials. We tested a total of five
devices and found that the modulation of conductivity with
light was reproducible. Although the performance of these
photoresponsive transistors[17] has not been optimized, the
photoresponsive properties of the nanosheets are promising
for organometallic-based optoelectronics. No charge accu-
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finding is consistent with the charge-neutral nature of
complex 1 and is in contrast to the previously reported FET
device with cationic platinum(II) salts as semiconductor-
s.[10a,e,g]
In conclusion, charge-neutral organoplatinum(II) com-
plexes can self-assemble into quasi-2D nanostructures that
display near infrared phosphorescence and light-modulated
conductivity. The molecular organization in the nanosheets is
attributed to orthogonal PtII···PtII and C H···p(C C) inter-
actions. This work highlights that phosphorescent platinu-
m(II) complexes provide an entry to new classes of 2D
supramolecular nanomaterials with optoelectronic properties.
ꢀ
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Received: October 9, 2009
Published online: November 20, 2009
Keywords: luminescence · metal–metal interactions ·
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