Micromolding of a highly fluorescent reticular coordination polymer: Solvent-mediated reconfigurable polymerization in a soft lithographic mold

Article abstract of DOI:10.1002/anie.201000096

(Figure presented) The fluorescent stripes: Coordination polymerization of pyrldlne-based ligands and zinc or silver ions was controlled by soft lithographic mlcromolding In capillaries. The polymer patterns (see picture) that are produced are highly fluorescent and supramolecularly structured.

Full text of DOI:10.1002/anie.201000096

Angewandte
Chemie
DOI: 10.1002/anie.201000096
Micropatterning
Micromolding of a Highly Fluorescent Reticular Coordination
Polymer: Solvent-Mediated Reconfigurable Polymerization in a
Soft Lithographic Mold**
Youngmin You, Hoichang Yang, Jong Won Chung, Jong H. Kim, Yunoh Jung, and
Soo Young Park*
Construction of ordered molecular superstructures is of
central importance to the performance of photonic/electronic
devices, in which molecular stacking modes contribute
profoundly to charge carrier generation, carrier mobility,[1]
and fluorescence emission[2] behavior. Although enormous
efforts have been applied based on supramolecular
approaches, the method often suffers from bare practicability
in respect to spatial regularity and density, large area
production, and reproducibility. Novel materials and innova-
tive processing methods that favor high-fidelity superstruc-
ture formation are thus in great demand.
Coordination polymers have the fascinating benefits of
versatility with respect to the range of possible combinatorial
ensembles of metal and ligands,[3] easy implementation of the
chemistry between metal and ligands,[4] and advanced func-
tionalities afforded by metal and ligand interactions.[5] More
importantly, coordination polymers can be designed to
produce ordered supramolecular structures, the formation
of which is driven by the coordination geometry around the
metal ion.[6] Although reticular structures based on coordi-
nation geometry have been widely exploited,[7] the kinetically
labile nature of coordination bonds is gaining increasing
interest.[8] Lehn and co-workers recently demonstrated ligand
exchange and reshuffling at a contact between two different
coordination polymers.[9] Coordination polymerization, there-
fore, drives construction of the thermodynamically most-
favored structure because lability permits sampling of several
possible geometries through the reversible reorganization of
coordination bonds. Therefore, it was envisioned that coor-
dination polymerization may be employed for the reprodu-
cible construction of highly ordered molecular superstruc-
tures.
By taking advantage of the chemical reconfigurability of
coordination polymers, we demonstrate herein a facile and
efficient patterning method using a coordination polymer
with a highly fluorescent reticular superstructure (Scheme 1).
Preformed coordination polymers can be depolymerized in a
strongly coordinating solvent, and then allowed to flow into
microchannels by capillary forces. Inside the capillaries,
controlled repolymerization is facilitated through selective
absorption of the solvent by the mold, leading to the
formation of a patterned coordination polymer. Patterning
(that is, allocation and macroscale structural geometry) is
implemented by a soft lithographic method, whilst the
molecular superstructure is determined by the coordination
geometry. The method of micromolding in capillaries
(MIMIC)[10] was employed herein to enable the simultaneous
patterning and formation of supramolecular structures by
reconfigurable coordination polymerization. Micromolded
coordination polymers were characterized by two-dimen-
sional (2D) grazing-incidence X-ray diffraction (GIXD),
which identified the three-dimensional (3D) reticular super-
[*] Dr. Y. You, Dr. J. W. Chung, J. H. Kim, Y. Jung, Prof. Dr. S. Y. Park
Center for Supramolecular Optoelectronic Materials
WCU Hybrid Materials Program
and
Department of Materials Science and Engineering
Seoul National University
San 56-1, Shilim-dong, Gwanak-gu, Seoul 151-744 (Korea)
Fax: (+82)2-886-8331
E-mail: parksy@snu.ac.kr
Homepage: http://csom.snu.ac.kr/
Prof. Dr. H. Yang
Department of Advanced Fiber Engineering, Inha University
253 Yonghyun-dong, Nam-gu, Incheon 402-751 (Korea)
[**] This work was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science, and Technology (CRI;
RIAMIAM0209 (0417-20090011)), and the US department of
Energy, Office of Science, Office of Basic Energy Sciences (DE-AC02-
98CH10886).
Scheme 1. Structure of the coordination polymer and conceptual
representation of the coordinative patterning by soft-lithography-driven
coordination polymerization.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201000096.
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structure. This unique superstructure yielded bright fluores-
cence in the solid state. These results provide valuable tools
for achieving high fidelity control in molecular superstructure
fabrication and a reliable nanofabrication method. In partic-
ular, the solution processing used in this technique may
facilitate fabrication of a broad array of devices that utilize
coordination polymers.
A new aggregation-induced enhanced emission (AIEE)-
type[11] bridging ligand, 4,4’-di(4-pyridyl)cyanostilbene, was
readily synthesized in three steps and fully characterized
using spectroscopic identification methods (see the Exper-
imental Section). Coordination polymerization of the metal
ion with this ligand was carried out by the slow diffusion of a
metal ion solution (Zn(ClO4)2·6H2O in MeOH, 5 mm) that
was carefully layered over the top surface of a ligand solution
(CHCl3, 5 mm). The bilayer solution was left undisturbed for
one day to give a yellow suspension, which was filtered,
triturated, and thoroughly washed with CHCl3 and MeOH to
remove residual components. The polymer was almost
insoluble in common organic solvents, precluding the use of
common characterization methods, including GPC (DMF +
LiBr or DMF + H3PO4), 1H NMR, or viscometry. Instead,
concurrent XPS peaks corresponding to Zn 2p, C 1s, and N 1s
in the polymer powder indicated that zinc ions were success-
fully incorporated into the coordination polymer. Quantita-
tive analysis of the XPS spectra afforded a ratio of zinc to
ligand that was slightly larger than 1:2 (Supporting Informa-
tion, Figure S1). This ratio implies that the polymer contained
2D or 3D alternating (zinc–ligand) backbone structures.[12]
We conjecture that the weakly coordinating perchlorate ion
allowed formation of this multidimensional structure. Powder
XRD studies showed a set of diffractions with d spacings of
21.43, 15.35, and 10.88 ꢀ (Supporting Information, Fig-
ure S2), which were consistent with the structure proposed
by the XPS result. Fluorescence intensity of the coordination
polymer powder decreased by a factor of about 10À2 with
increasing temperature (Supporting Information, Figure S3),
which reversibly recovered at room temperature, whereas the
ligand powder showed no temperature-dependent change in
fluorescence intensity.
The ligand was coordinated through the 4-pyridyl moiety,
which was structurally equivalent to the pyridine solvent
competing with the ligand for coordination to zinc. Therefore,
depolymerization and repolymerization were expected when
an excess of pyridine was added or removed, respectively. An
aliquot of pyridine completely solvated the polymer powder
to give a clear solution (ca. 102 mgmLÀ1). 1H NMR spectra of
the polymer were taken in the presence and absence of
[D5]pyridine, which revealed appearance of displaced free
ligands as [D5]pyridine was added (Supporting Information,
Figure S4). The fluorescent aggregates were recovered from
the solution by evaporating the pyridine. Interestingly, during
slow evaporation, the polymer tended to fuse (Supporting
Information, Figure S5), which was similar to previous
observations.[8m] The reassembly induced by removing pyri-
dine solvent was indicative of the thermodynamic sampling of
the coordination polymer. We combined the depolymeriza-
tion and repolymerization method with the standard MIMIC
method to achieve spatial patterning and supramolecular
Figure 1. a) Optical microscopy (scale bar=100 mm) and b) FE-SEM
images (scale bar: 10 mm) of the patterns. c) Optical microscopy
image of a single line pattern, collected under cross-polarized con-
ditions. Rotation of a patterned wire showed birefringence at every 45
degree rotational interval with respect to the polarizer. d),e) Fluores-
cence images of the patterns obtained using an image restoration
microscope (emission cut-off filters: 480 nm (d), 580 nm (e), scale
bar: 40 mm). f) Fluorescence intensity profile along the green arrow
indicated in (e).
structure formation. Typically, 30 mg of the coordination
polymer powder was dissolved in 200 mL pyridine. The
solution was introduced into rectangular capillaries in con-
formal contact with a patterned poly(dimethylsiloxane)
(PDMS, Sylgard 184, Dow Corning) mold and a glass
substrate. The PDMS slowly swelled at the inner walls of
the fully filled capillaries by the absorption of pyridine.[13]
After polymerization, the mold was peeled from the substrate
to yield patterned coordination polymer wires (4 mm ꢁ 2 mm
width ꢁ height; Figure 1a). The feature size and density of the
patterns were governed by PDMS molds. The MIMIC
method is versatile in that filling the capillary is rarely limited
by the nature of the substrate.[14] During the reconfigurable
coordination polymerization in the MIMIC mold, removal of
pyridine was critical for the formation of ordered coordina-
tion structures.
Field-emission scanning electron microscope (FE-SEM)
images showed that the outer surfaces of the patterns are
nearly flawless, and grooves or shrinkages were minimal
(Figure 1b). Atomic force microscopy further characterized
the well-defined pattern morphology (Supporting Informa-
tion, Figure S6). The fabricated patterns showed strong
alternative birefringence under cross-polarized illumination
conditions, indicating that the patterns had a pronounced
directional order (Figure 1c). The MIMIC-fabricated coordi-
nation polymer patterns were highly fluorescent, and no
background fluorescence emission was observed in the spaces
between the patterns (Figure 1d,e). Fluorescence mapping
across the patterns revealed alternating on/off fluorescence
signals (Figure 1 f).
2D GIXD measurements (incident beam angle = 0.58)
provided essential information about the polymer super-
structure (Figure 2). Strong diffraction peaks were observed
along both the out-of-plane and in-plane directions, indicating
good ordering. A single series of Bragg peaks along the out-
of-plane direction corresponded to the (00l) crystal planes,
with d spacings of 21.13, 10.59, 6.14, 5.425, and 4.363 ꢀ
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Figure 3. Photoluminescence spectra of the ligand solution (10 mm,
CH2Cl2, black curve), its nanoparticle suspension (THF/water=1:9
(v/v), 10 mm, green), and the AgI coordination polymer (blue) and ZnII
coordination polymer suspension (10 mgLÀ1, CHCl3, red). Magnifica-
tions shown as dashed lines.
Of interest was the large enhancement in fluorescence
intensity that accompanied coordination polymerization
(Figure 3). In contrast with the virtually nonfluorescent
ligand solution (photoluminescence quantum yield, PLQY=
0.00086, CH2Cl2) and ligand nanoparticle suspension[11a]
(PLQY= 0.0071, THF/water= 1:9 v/v) of ligand, the PLQY
of the zinc coordination polymer increased to 0.61. An
unusual enhancement of fluorescence by a factor of 700 was
attributed to the cumulative effects of a suppressed non-
fluorescent n–p* transition, locked intramolecular motion,
and reduced interchromophoric contacts in the polymer
structure, all of which arose from coordination interactions.
In fact, both a bathochromic shift and an increase in
fluorescence intensity, by a factor of 10, were observed
when the ligand solution was treated with hydrochloric acid.
Fluorescence of the coordination polymer also exhibited a
positive slope in the Lippert–Mataga plot, which was not
observed in the ligand solution (Supporting Information,
Figure S7). This behavior, together with the diamagnetic
character of zinc(II) ions, suggested that the coordination by
the lone pair electrons on the pyridine promoted a new
efficient fluorescence transition. Radiationless vibronic
motions appear to be repressed by the coordination bond
formation. Finally, the 3D cubic structure of the coordination
polymer sufficiently separated the ligands, such that inter-
chromophoric contacts were minimized. For comparison, we
prepared a silver(I) coordination polymer that would be
anticipated to be linear owing to the coordination geometry
around the silver(I) center. Indeed, FE-SEM images of this
polymer demonstrated the formation of nanobelts (Support-
ing Information, Figure S8) in which polymer chains were
mutually aligned along their long axis. This alignment was
Figure 2. a,b) 2D GIXD of the cast region (a) and coordination poly-
mer patterns (b). The peaks marked with an asterisk in (b) represent
X-ray reflections of the (0k0)+(00l) planes.) c) 1D out-of-plane (black
line) and in-plane (red line) X-ray profiles extracted from (b). d) Repre-
sentation of the molecular superstructure of the patterns. Distances
indicated by black arrows: 21.13 ꢀ. Zinc atoms and counteranions are
omitted for clarity.
(calculated peak positions at qz = 0.2974, 0.5933, 0.8759,
1.1581, and 1.4402 ꢀÀ1 in Figure 2), which indicates the
presence of periodic layers connected by vertically coordi-
nated ligand pillars. In particular, the peak observed at
21.13 ꢀ corresponds to the length of the ligand (determined
quantum-chemically, optimized using B3LYP/6-31G**). In-
plane d spacings of (0k0) were identified at the same positions
as those of the out-of plane peaks. Therefore, the entire
coordinated polymer framework assumed a primitive cubic
structure along the primary axes. The 2D GIXD results,
together with the XPS and powder XRD results, led us to
conclude that the molecular superstructure was composed of
a cubic structure in which ligands and zinc ions occupied rods
and connecting points, respectively (Figure 2d).
expected to increase the interchromophoric contacts.[15]
A
smaller PLQY (0.33) of the silver(I) coordination polymer
emphasized the importance of the 3D reticular superstructure
in the zinc(II) coordination polymer in achieving high
fluorescence efficiency.[16]
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Communications
the patterns was initially attempted using an instrument equipped
with an integrated sphere; however, acquired values had large error
ranges. Photoluminescence quantum yields (PLQYs) were calculated
relatively using quinine sulfate (0.58; 0.1m aq. H2SO4) and 9,10-
diphenylanthracene (0.82; benzene) as standards. The polymer was
thoroughly triturated and suspended in CHCl3 (< 10 mgLÀ1) for all
optical measurement, otherwise mentioned. The photoluminescence
measurements for organic suspensions were performed as descri-
bed.[11a] X-ray photoelectron spectroscopy (XPS) was conducted
using an AXIS, KRATOS with pelletized coordination polymer
mounted on a SiO2 substrate. 2D GIXD experiments were performed
at the beam line X21 at the National Synchrotron Light Source
(NSLS) at Brookhaven National Laboratory (Upton, NY, USA). The
sample was mounted on a two-axis goniometer atop of an X-Z stage,
and the scattered intensity was recorded by a 2D detector. The
incident beam angle was about 0.58 for all the 2D GIXD patterns. The
morphology of the patterned wires was observed in non-contact mode
(scan rate and size were 0.2 Hz and 50 ꢁ 50 mm2, respectively) by an
XE-150 atomic force microscope (AFM), PSIA. An image restoration
microscope (DeltaVision RT, AppliedPrecision) was used to record
fluorescence images of the patterns. FE-SEM images were collected
using a JSM-6330F (JEOL) instrument. Wide-angle X-ray diffracto-
grams were acquired in reflection mode using nickel-filtered Cu KR
radiation on a D8 Advance (Bruker) instrument equipped with a
point detector operating at 40 kV and 40 mA.
In summary, we have described a novel method for
fabricating highly luminescent supramolecular patterns. The
combined use of soft lithography and coordination polymer-
ization permitted the control and synchronization over
allocation, macroscale structural geometry, and interior
superstructures. A reticular cubic structure was revealed by
2D GIXD, and the coordinative structure was investigated by
various characterization methods. Owing to the cubic
arrangement of the ligands involved in the coordination
network, high intensity fluorescence was achieved without
suffering from typical interchromophoric quenching. This
approach holds a great potential for reliable nanoscopic
fabrication that requires controlled integration of functional
molecules.
Experimental Section
4-(4-(Cyanomethyl)phenyl)pyridine: Isopropanol (18 mL), (4-(cya-
nomethyl)phenyl)boronic acid (2.0 g, 12 mmol), and 2m aqueous
K2CO3 (24 mL) were added to a solution of 4-bromopyridine
hydrochloride (2.0 g, 10 mmol) in toluene (18 mL). The solution
was stirred for 30 min under a nitrogen atmosphere. After the
addition of tetrakis(triphenylphosphine)palladium(0) (0.35 g,
0.30 mmol), the mixture was heated to reflux for 7 h. The cooled
solution was poured onto water (200 mL) and extracted three times
with ethyl acetate (200 mL). The combined organic layer was dried
over magnesium sulfate and evaporated under vacuum. Finally, silica
gel column chromatography (n-hexane/EtOAc = 1:2) yielded a white
powder (1.9 g, 10 mmol) in a quantitative yield. 1H NMR (300 MHz,
CDCl3): d = 3.83 (s, 1H), 7.47 (d, J = 8.4 Hz, 2H), 7.50 (d, J = 6.2 Hz,
2H), 7.66 (d, J = 8.3 Hz, 2H), 8.68 ppm (d, J = 6.2 Hz, 2H); 13C NMR
(125 MHz, CDCl3): d = 23.63, 117.65, 121.82, 127.99, 128.97, 131.16,
138.21, 147.83, 150.32 ppm; Elemental analysis (%) calcd for
C13H10N2: C 80.39, H 5.19, N 14.42; found: C 80.21, H 5.28, N 14.51.
4-(4-Formylphenyl)pyridine: The same synthetic procedure used
for 4-(4-(cyanomethyl)phenyl)pyridine was applied, except using (4-
formylphenyl)boronic acid (2.0 g, 13 mmol) instead of (4-(cyanome-
thyl)phenyl)boronic acid. Yield = 84% (1.4 g). 1H NMR (300 MHz,
CDCl3): d = 7.59 (d, J = 6.2 Hz, 2H), 7.81 (d, J = 8.3 Hz, 2H), 8.02 (d,
J = 8.2 Hz, 2H), 8.74 (d, J = 6.2 Hz, 2H), 10.10 ppm (s, 1H); 13C NMR
(125 MHz, CDCl3): d = 122.07, 127.98, 130.65, 136.77, 144.04, 147.50,
150.38, 191.78 ppm; Elemental analysis (%) calcd for C12H9NO:
C 78.67, H 4.95, N 7.65; found: C 78.49, H 5.09, N 7.68.
Received: January 7, 2010
Published online: April 16, 2010
Keywords: coordination polymers · fluorescence ·
.
micromolding · nanostructures · soft lithography
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[16] An insightful referee suggested that
a heavy-atom effect
contribution for silver may account for the relatively small
PLQY.
Angew. Chem. Int. Ed. 2010, 49, 3757 –3761
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