Hierarchical multicolor nano-pixel matrices formed by coordinating luminescent metal ions to a conjugated poly(4′-octyl-2′,6′-bispyrazoyl pyridine) film via contact printing

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Abstract We introduce a cost-effective, yet feasible reactive printing approach namely, “coordination chemistry at the conjugated ligand polymer surface”. By using a contact printing technique we selectively fabricated high resolution nano-pixel configurations consisting of red, blue and green (R,G,B) colors arranged in a hierarchical three-dimensional fashion. For this, we have synthesized a novel blue emitting conjugated ligand polymer, [poly(4′-octyl-2′,6′-bispyrazoyl pyridine)] ("Equation missing" ~ 7–8 KDa) having a tridentate ligand in its repeating unit which forms a thin film prone of binding metal ions. On top of this ligand polymer film, a layer of high-resolution cross-stripes (width ca. 800 nm) was printed comprised of Eu and Tb ions over a large area ~ 100 × 100 μm 2 . The final woodpile-like assembly produced a new pixel group consisting of B; BG; BR; and BGR (White) colors. The area of the white emitting square is ca. 0.64 μm 2 . The patterned layers that make up the pixels are very thin and thus this new technique might find applications in flexible light emitting devices.

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SYNTHESIS AND

PROCESSING

ELECTRONIC DEVICES

Received
5 November 2014

Accepted
16 January 2015

Published
12 February 2015

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R.C.

[(chandrasekar100@](mailto:chandrasekar100@yahoo.com&semi; rcsc&commat;uohyd.ernet.in)

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## Hierarchical multicolor nano-pixel matrices formed by coordinating luminescent metal ions to a conjugated poly(49-octyl-29,69-bispyrazoyl pyridine) film via contact printing

Supratim Basak 1, Md Ahamad Mohiddon 1, Martin Baumgarten 2, Klaus Mu¨llen 2

& Rajadurai Chandrasekar 1

1Functional Molecular Nano-/Micro-Solids Laboratory, School of Chemistry, University of Hyderabad, Prof. C. R. Rao Road,
Hyderabad – 500046, India, 2 Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany.

We introduce a cost-effective, yet feasible reactive printing approach namely, ‘‘coordination chemistry at the
conjugated ligand polymer surface’’. By using a contact printing technique we selectively fabricated high
resolution nano-pixel configurations consisting of red, blue and green (R,G,B) colors arranged in a
hierarchical three-dimensional fashion. For this, we have synthesized a novel blue emitting conjugated
ligand polymer, [poly(49-octyl-29,69-bispyrazoyl pyridine)] (Mn, 7–8 KDa) having a tridentate ligand in
its repeating unit which forms a thin film prone of binding metal ions. On top of this ligand polymer film, a
layer of high-resolution cross-stripes (width ca. 800 nm) was printed comprised of Eu and Tb ions over a
large area, 100 3 100 mm 2 . The final woodpile-like assembly produced a new pixel group consisting of B;
BG; BR; and BGR (White) colors. The area of the white emitting square is ca. 0.64 mm 2 . The patterned layers
that make up the pixels are very thin and thus this new technique might find applications in flexible light
emitting devices.

odern technologies aiming to excel the performance of multimedia display devices continue to be in
great demand 1,2 . One of the innovative solutions is the development of technological know-how to
achieve multicolor lights from the emissive organic layer composed of well-ordered thin matrix pixels
# M
down to nano regime 3,4 . However, the construction (deposition and patterning) of color matrix pixels in a nano
scale geometric layout is still a challenging and pertinent task. In commercial devices, both red, green, blue (RGB)
and RGB white (RGBW) pixels are typically used with the cross section in the range of several microns. Compared
to devices having traditional RGB stripe pixels, the inclusion of white pixels in the RGBW matrix boosts the
brightness of the images and consumes less power with increasing contrast 5 . In order to realize extreme high
resolution in the display devices, increasing the number of pixels by reducing the pixel sizes is an innovative and
demanding strategy.

Nano-pixel matrices composed of light emitting molecules or polymers can be created by using top-down
lithographic patterning techniques using X-ray 6,7, electron beam 8, and photolithography 9, although none of them
allows an economical scaling up. On the contrary, a soft lithography procedure allows a rapid patterning of large
surface areas with multiple luminescent components. Therefore, soft lithography techniques 10, such as micro
contact (m-CP) and micro transfer printing, have become most feasible tools for the fabrication of OLEDs 11,12 . To
pattern light emitting polymers, CP is the preferred technique owing to the possibility to cover large surface areas
with submicro-scale spatial resolution and elimination of vacuum processes. In this method, the polymer ink is
transferred to the surface using a rigid or elastomeric mold. The use of conjugated polymer blends to achieve
active layers suffers from phase separation due to incompatibility of dissimilar polymer mixtures 13 . Moreover,
controlling the length and direction of the phase separated polymer mixture to form a high resolution pattern is
quite difficult. In order to achieve high spatial resolution, the deterioration of pattern features by lateral diffusion
of ink after the transfer of the ink on to the surfaces must be prevented. Existing solutions include using (i) reactive

Figure 1 | Fabrication Strategy. Graphical representation of a hierarchical reactive contact printing process leading to multicolor matrix pixels.

m-CP 15, (ii) a polymer ink that interacts strongly with the surfaces
through electrostatic interactions 14, and (iii) heavy weight dendrimers
to suppress lateral diffusion 16, but none of them have been implemented for light emitting applications. From this viewpoint an original
chemical protocol is needed to develop lithographically patterned
stable multicolor nano-pixels. To the best of our knowledge there is
still no reactive m-CP methodology known based on ‘‘coordination
chemistry at the conjugated ligand polymer film surface’’ 17,18 .

We envisaged the application of a versatile CP strategy for the
reactive modification of a layer of blue emissive conjugated ligand
polymer by selective coordination with luminescent metal ions (Red:
Eu 31 and Green: Tb 31 ). Such a technique would allow the creation of
high resolution multicolor geometric layouts which include white
(layered RGB) pixels down to nano domain. To implement this strategy, the development of a novel synthetic procedure for polymerization of an organic ligand molecule is necessary. It is known that the
tridentate 2,6-bispyrazoyl pyridine (bpp) unit can bind hexacoordinated lanthanide complexes to form photoluminescent complexes
with coordination number nine 19 . In order to exploit bpp ligand
based conjugated polymer film as a platform to print photoluminescent metal ions to generate multicolor nano-pixels, we have homo

polymerized 49-octyl-29,69-bispyrazoyl pyridine ligand 18 . In the resultant polymer the bpp core helps to coordinate photoluminescent
metals while octyl chain units provide easy solution processability.

In this letter we report a unique method to reactively coordinate
photoluminescent metal ions to a novel blue emitting poly(49-octyl29,69-bispyrazoyl pyridine) (Poly) film surface through the CP technique (Fig. 1). The surface of the blue emitting polymer is cross stripe
patterned by coordinating red luminescent Eu(III) and green luminescent Tb(III) inks (at a right angle 90u) to obtain a well ordered
woodpile like multicolor matrix pixel arrangements. The final woodpile structure consists of four different areas which differ in their
chemical composition and thereby form a remarkable four color
pixel group consisting of : (i) blue: from the emitting polymer, (ii)
purple: from the mixing of polymer and Eu(tta)3 emissions, (iii) pink:
from the mixing of polymer and Tb(acac)3 emissions and (iv) white:
from the mixing of polymer, Eu(tta)3 and Tb(acac)3 emissions.

Results and Discussions
Syntheses of monomer, dimer, and homopolymer of bpp. At first, to
probe the effect of conjugation on the optical properties of the
homopolymer (Poly), n-mers such as monomer (M), and dimer (D)

Figure 2 | Polymer Synthesis Syntheses of monomer (M) dimer (D) and of 4 octyl 2 6 di(1H pyrazol 1 yl)pyridine polymer (Poly)

Figure 3 | Single crystal structure of dimer (D) at 293 K. (a), top view. (b), side view. Hydrogen atoms are omitted for clarity (nitrogen atoms are shown
in blue color).

were synthesized. M was prepared by our reported procedure starting
from a low cost citrazinic acid in seven synthetic steps 18 . For the
preparation of D, a controlled iodination of M was performed to get
monoiodinated (1) (24%) and diiodinated (2) (99%) compounds
(Fig. 2) 18 . Afterwards compound 1 was homocoupled under Suzuki
conditions using bis(pinacolato)diboron, Pd(PPh3)4 and K2CO3 to get
dimeric bpp (D). The homopolymer of bpp (Poly) was synthesized via a
Yamamoto reaction of 2 using the 2-(4-iodo-1H-pyrazol-1-yl)-4-octyl6-(1H-pyrazol-1-yl)pyridine (1) as an end-capper (Fig. 2). The
polymerization reaction was performed using Ni(COD)2/COD/
bipyridine catalysts in a mixture of dimethylformamide and toluene
by stirring at 80uC. At the end of the reaction an excess of
compound 1 was added with the objective to maximize end-capping
of polymer. The number averaged molecular weight Mn of the polymer
as revealed by gel permeation chromatography analysis is in the range
of 7–8 KDa (number of repeating units; navg 5 22). The thermal
decomposition temperature (TD) with 5% weight loss of the polymer
is 380uC (Fig. S1).

Solid state structure of model dimer compound. The single crystal
structure of the dimer (D) revealed that the orientation of the two
bpp units was planar with a 0u torsion angle between the two
connecting (C7–C79) pyrazole rings (See Fig. 3). This observation
is indicative of the expected efficient electronic conjugation amongst
the bpp repeating units of the homopolymer (Poly).

Optical properties of monomer, dimer and polymer in solution
and solid state. The photo physical characteristics of the monomer,
dimer, and polymer were studied both in CHCl3 solution and in the
solid state; all spectroscopic data are summarized in Table S1
(supporting information). The solution state absorbance and
emission spectra of M, D and the Poly are shown in fig. 4a. In
solution state M, D and Poly exhibited absorption maxima at 301/
268, 317/253, 316/258 nm due to p-p* transitions and emission
maxima at 330, 344, 366 nm, respectively. The polymer showed a
low energy emission maximum at 366 nm because of increased
electronic conjugation leading to a larger band gap of ca. 3.38 eV
compared to monomer. In the solid state the absorption spectra of M,
D and Poly showed slightly red shifted broad band maxima
positioned at 301, 320, and 323 nm, respectively (Fig. 4b). Further
the solid state fluorescence emissions of M, D and Poly displayed
largely red shifted emission maxima at 351, 361 and 390 nm
compared to their solution states indicating solid state aggregation
and pronounced interchain interactions 21 .

Woodpile-like contact printing on the polymer surface with
Eu(III) and Tb(III) complexes using coordination interactions.
As the synthesized polymer comprised a series of tridentate ligands
for metal coordination and also forms fine blue emitting films, the
thin polymer film was employed as a reactive template for CP. To
perform CP elastomeric polydimethylsiloxane (PDMS) stamps

Figure 4 | Optical Studies Absorbance and emission spectra of monomer (M) dimer (D) and polymer (Poly) in (a) solution and (b) solid states

Figure 5 | Patterning of Eu(III) and Tb(III) inks coordinatively on a polymer surface. Raman/PL images of Eu(tta)3 printed stripes on the polymer
surface: (a), False color Image of 1635 cm 21 Raman peak corresponding to blue PL of polymer. (b), False color Image of the red PL peak (610–620 nm)
from stripes. (c), Combined image of (a) and (b). (d)), Spectra of Eu(tta)3 complexed pattern on polymer surface, polymer and Eu(tta)3?3H2O, the
marker peaks are shown in violet and magenta color for polymer and Eu(tta)3, respectively. PL images of the Tb(acac)3 pattern on polymer surface: (e),
False color Image of a peak in the region between 376–472 nm corresponding to blue PL of polymer. (f), False color Image of the green PL peak in the
region between 540–550 nm from stripes shown in green color. (g), Combined image of (e) and (f). (h), Spectra of Tb(acac)3 complexed pattern on
polymer surface, polymer and Tb(acac)3?H2O, the marker peaks are shown in violet and green color for polymer and Tb(acac)3?H2O, respectively.

having rectangular shape arrays were prepared from the master (NTMDT AFM test gratings model TGZ3) by the established
procedures 22 . In view of the polarity of the ink molecules and the
wettability of the PDMS stamp, a 151 mixed solution of ethylene
glycol and acetone was used to prepare inks containing Eu(III) and
Tb(III) metal ions. Ethylene glycol makes the ink viscous and acetone
adequately wet the PDMS surface facilitating surface coordination
reaction at the interfaces between stamp and polymer layer to form
high resolution stripes. Hence the PDMS surface does not have to be
activated by oxidation through an oxygen plasma treatment. The red
luminescent ink was prepared by dissolving 1.5 mg of Eu(tta)3 in
1 mL of mixed solution. At first a thin film of ligand polymer layer
was generated on a clean silica substrate by spin coating (at

1000 rpm/10 sec/6000 rpm/10 sec) of a CHCl3 solution of
polymer (c, 4 mg/mL). The stamp was carefully wetted with the
ink solution and the excess solution was removed by using a filter
paper. Then the stamp coated with ink was printed on the thin
polymer film substrate and subsequently a weight of 20 g (area: 2
3 1.5 cm metal block) was kept on top of the stamp to make a
uniform smooth contact between the bottom of the stamp and the
polymer substrate. After an overnight reaction the stamp was lifted
off very carefully to avoid any mechanical damage.

The chemical composition of the patterns was confirmed by performing micro Raman/photoluminescence (PL) spectroscopy and
imaging on a laser confocal microscope (LCM) equipped with a
532 nm neodymium doped yttrium aluminium garnet (Nd/YAG)

Figure 6 | EDS data of the patterns formed on polymer film exhibiting a) Eu and b) Tb peaks, respectively. The inset show their corresponding TEM
images of Eu(tta)3 and Tb(acac)3patterns formed from a polymer film coated on carbon coated TEM grids.

and 355 nm ultra violet (UV) diode lasers. For the local laser excitation of the sample containing Eu(III) patterns on the polymer, a
532 nm visible laser was used. The collected signals from the sample
were filtered using a 532 nm long pass edge filter (LPEF) and sent to a
CCD detector. In the sample, the polymer has no direct electronic
absorption in the visible wavelength range, while the Eu(tta)3complex
has a slight absorption in this range. As a result a clear Raman signal
from the polymer and red PL signal (f-f transitions) from the Eu(tta)3
patterns was obtained and subsequently used for 2D mapping.
Imaging of the high intensity Raman signal of the polymer appeared
at 583 nm (1635 cm 21 ; C 5 C stretch) is shown in blue color corresponding to the pure blue emitting polymer film (Fig. 5a and 5d).
PL Imaging of the 610–625 nm signal region ( 5 D0 R 7 F2) from the red
emitting Eu(III) exhibited the formation of micro stripes at a regular
1.5 mm intervals on the polymer surface (red color, Fig. 5b and 5d).
To see the mixed colors, the two images (Fig. 5a and 5b) were superimposed and shown in fig. 5c. Here, the blue areas indicate the
polymer and the pink stripes (red 1 blue color mixing) correspond
to Eu(tta)3 stripes, their interface [Poly(Eu(tta)3)n] and unreacted
polymer as well.

Using the same protocol a series of green emitting Tb(acac)3
stripes at regular gaps was printed with high accuracy on the
reactive polymer surface. The green ink solution was prepared
by dissolving 2.12 mg of Tb(acac)3 in a 151 mixture of acetone
and ethylene glycol solution. To identify the composition and
resolution of the Tb(acac)3 stripes formed on the polymer surface,
a UV laser (355 nm) and CCD connected to LCM was used to
carry out spectroscopy and imaging studies. As Tb(III) emission
starts from 480 nm and extends up to 628 nm, the signal from
376 nm to 464 nm was used for the polymer imaging (Fig. 5h).
The high intensity PL peak at 545 nm ( 5 D4 R 7 F5) corresponding
to Tb(III) was used to imaging Tb(III) stripes. The overlay of the
polymer and Tb(III) images (Fig. 5e and 5f) reveals the formation
of cyan emitting stripe pattern on the blue emitting polymer surface (Fig. 5g).

To investigate the stripes formed on polymer surface by using
TEM, a carbon coated grid (mesh size : 200) was spin coated with
the polymer followed by CP with Eu(tta)3 and Tb(acac)3 inks, individually. Bright-field TEM investigations of the Eu(tta)3 and
Tb(acac)3 coordinated m-CP patterns clearly displayed, formation
of a series of 1D stripes corresponding to the metal complexed polymer (dark gray areas) and pure polymer (light gray areas) (insets of
Fig. 6a and b, respectively). Additionally, the energy dispersive X-ray
spectroscopic analyses (EDS) clearly demonstrated the presence of
Eu and Tb complexes in the dark patterns displaying characteristic
peaks (Fig 6a and b respectively)

After successful formation of metal coordination driven stripes on
2D polymer surface, we thought to extend the CP method to build
wood-pile like stacked (yet very thin) 3D cross-stripes comprising
Eu(tta)3 and Tb(acac)3 complexed polymer regions displaying multiple color pixel groups. It was also anticipated that this 3D patterning
may generate square shaped white color emissive areas within each
pixel group, due to overlay of three primary colors, blue from polymer, red and green from Eu(III) and Tb(III) ions, respectively. For
this, a sample containing dried stripes of Tb(acac)3 coordinated onto
a polymer a film was used as a substrate.

To create cross-patterns, at first the direction of the Tb(III) stripes
was determined by using a confocal optical microscope.
Subsequently, over the Tb(III) patterned polymer film, another
stamp inked with Eu(tta)3 was printed at an angle of,90u. TEM
and FESEM examinations of the pattern formed on a carbon coated
TEM grid clearly demonstrated the formation of cross stripes, comprising Eu(III) and Tb(III), patterned over a large surface area
(Fig. 7a and c, respectively). Figure 7b shows a close up view of the
multicolor emitting cross stripes. The approximate pixel areas of the
white square (Tb 1 Eu 1 Poly), pink rectangle (Tb 1 Poly) or purple
rectangle (Eu 1 Poly) and the polymer square are ca 0.64 mm 2,
1.76 mm 2, and 4.84 mm 2, respectively (Fig. 7b). Selective high resolution energy dispersive spectroscopic (EDS) line mapping of the
Eu(III) and Tb(III) stripes exhibited corresponding metal ion peaks
in the respective spectra (Fig. S4 b and c). Further, the EDS mapping
of the whole pink square area (Tb 1 Eu 1 Poly) indicated the
presence of both Eu(III) and Tb(III) ions, confirming the overall
printing resolution of the cross pattern (Fig. S4 d–f). To verify the
emissive properties of Eu(III) and Tb(III) cross-patterns on the polymer film, a selected area of the film was scanned by using a 355 nm
laser and the obtained spectrum showed emission from 400–700 nm
(Fig. 7d). The corresponding CIE diagram has values of x 5 0.28 and
y 5 0.34 which is very close to white light, the pure polymer displayed
the values in the blue region, x 5 0.20 and y 5 0.23 (Fig. 7e).

Conclusions
In this work we have introduced an unprecedented coordination
chemistry approach to create useful multicolor matrix pixel group
below,3 mm resolution using the conventional contact printing
technique. To implement our strategy, we have synthesized a novel
homopolymer containing tridentate bispyrazolylpyridine repeating
units. The polymer thin film spin coated on a clean glass slide acts as a
surface for reactive contact printing of luminescent metal ions via
coordination chemistry. We have been able to create high resolution
pixels consist of four colors (primary colors and white) in the nano/
micro space from the cross stripes We foresee that this innovative

Figure 7 | Hierarchical multicolor matrix pixels. (a) and (b), TEM image of Eu(tta)3 and Tb(acac)3 cross stripes formed on polymer (Poly) surface, in
low and high resolutions, respectively. (c), FESEM image of the cross stripes. (d), Average emission spectrum of cross stripes obtained from an area of 13
3 3 mm 2 using a 355 nm UV laser scanning. (e), The corresponding CIE diagram is shown (black square) together with polymer (black circle).

fabrication methodology to create multi-colour light emissive layers
might be of interest for applications such as LEDs 23–25 .

Methods
1H and 13C NMR spectroscopic data were recorded on a Bruker DPX 400 spectrometer; chemical shifts (d) are expressed in parts per million relative to solvent proton
as internal standard (CDCl3-d1 5 7.26 ppm). LC mass spectrometry was performed
on Shimadzu LCMS-2010A mass spectrometer. IR spectra were recorded on JASCO
FT/IR-5300 or Nicolet 5700 FT-IR. Elemental analysis was recorded on a Thermo
Finnigan Flash EA 1112 analyzer. Spin coating was done on a glass substrate using
Laurell TECHNOLOGIES CORPORATION Model WS-400B-6NPP/LITE/8K Spincoater. UV-Visible absorption and fluorescence spectra were recorded on a spectrophotometer. The solid and liquid state absorbance spectra were collected from a
Shimadzu UV-3600 spectrometer and a spectrofluorimeter (Horiba, JobinYvon)
respectively. For fluorescence quantum yield measurements, solution of ligands in
DCM was matched optically at an absorbing wavelength and then the quantum yield

of quinine-sulfate in 0.1 M H2SO4 (Wf 5 0.57, at 22uC) was used as reference. The
molecular weights were determined by PSS-Win GPC (PSS) (pump: alliance GPC
2000) GPC equipped with an RI detector using a PL gel MIXED-B column (particle
size: 10 mm, dimensions: 0.8 3 30 cm) calibrated against polystyrene standards. For
thin-layer chromatography (TLC), silica gel plates Merck 60 F254 were used and
compounds were visualized by irradiation with UV light.

Atomic Force Microscopy Studies. AFM studies were carried out on NT-MDT
Model Solver Pro M microscope using a class 2R laser of 650 nm wavelength having
maximum output of 1 mW. All calculations and image processing were carried out by
using NOVA 1.0.26.1443 software provided by the manufacturer. The images were
recorded in a semi-contact mode using a super sharp silicon cantilever (NSG
10_DLC) with a diamond like carbon tip (NT-MDT, Moscow). The dimension of the
tip is as follows: cantilever length 5 100 (65) mm, cantilever width 35 (65) mm, and
cantilever thickness 5 1.7–2.3 mm, resonance frequency 5 190–325 kHz, force
constant 5 5.5–22.5 N/m, chip size 5 3.6 3 1.6 3 0.4 mm, reflective side 5 Au, tip
height 5 10 20 mm and DLC tip curvature radius 5 1 3 nm

Confocal Raman micro spectroscopy Studies. Raman spectra of the samples were
recorded on a Wi-Tec alpha 200 confocal Raman spectrometer equipped with a
Peltier-cooled CCD detector. Using a 600 grooves/mm grating BLZ 5 500 nm, the
accumulation was 10 and integration time was typically 0.5000 s. A He-Ne laser
operating at 633 nm was used as an excitation source for the Raman scattering. All
measurements were performed in air.

PL imaging of stripes. The imaging for the Eu(tta)3complexed polymer were carried
out using a backscattering (reflection mode, B-LCOM) laser confocal optical
microscope of the WiTec alpha 300 SNOM instrument. The Nd:YAG laser operating
at 532 nm (maximum output power is 40 mW) was used for the excitation of the
stripes (spot size,680 nm). In the backscattering mode experiment, a 100 3
objective lens was used for the local excitation and for the collection of Raman and PL
photons from polymer and Eu(tta)3 complex respectively. The collected signal was
sent to a CCD detector through a multimode optical fiber of diameter 100 mm (core).
The spectrum was recorded using a 532 nm long pass edge filter (LPEF) and a 300
grooves/mm grating BLZ 5 500 nm, with a 0.1 s integration time. All measurements
were carried out under ambient conditions.

The imaging for the Tb(acac)3 complexed polymer was carried out using a transmission mode laser confocal optical microscope (T-LCOM) facility of the WiTec
alpha 300 SNOM instrument. UV laser operating at 355 nm (maximum output
power is 7 mW) was used for the local excitation of the stripes by upright microscope
using a 40 3 UV objective (NA:0.6). The collection of PL photons was carried out
through bottom microscope (kept below the sample) by using a 60 3 visible range
objective (NA:0.8). The collection was done in such a way that the illumination source
(403) and the collection unit (603) were kept at fixed confocal position and the
sample was kept in between the objectives on a piezo-scan stage. The collected PL
signal was sent to a CCD detector through a multimode optical fiber of diameter
100 mm (core). The PL spectrum was recorded using a 300 grooves/mm grating BLZ
5 500 nm, with a 0.31 s integration time. Scanning was done on a 30 3 30 mm 2 area.
Total 120 3 120 spectra were collected to form 120 lines and each line was composed
of 120 points to make a virtual image. All measurements were carried out under
ambient conditions. Imaging for the cross stripe was done for an area of 13 3 3 mm 2

using a 355 nm laser. The collected PL photons were used to form an average spectrum. Converting the spectrum to the CIE coordinates shows the X and Y values are
0.28 and 0.34 respectively which are very close to the white light (X 5 0.33, Y 5 0.33).

Solid state UV-Vis absorption and fluorescent studies. Diffused reflectance spectra
were recorded for M, and D and the reflectance was converted to absorbance by
following a Kubelka–Monk function 20 . For the reflectance measurement the solid
sample was mixed with BaSO4 and mounted on a sample chamber. For the
fluorescent measurement very thin KBr films of the sample M and D were prepared
and mounted diagonally in a cuvette. The polymer was readily processed into a
homogeneous film of good optical quality by spin-coating from CHCl3 at 6000 rpm
on clean quartz substrates. The polymer coated substrate was used for the direct
absorbance and fluorescent measurement.

Synthesis of dimer (D). In a 100 mL flask containing DMSO (20 ml), 2-(4-iodo-1Hpyrazol-1-yl)-4-octyl-6-(1H-pyrazol-1-yl)pyridine (1) (2.2 g, 4.869 mmol),
bis(pinacolato)diboron (1.24 g, 4.896 mmol), K2CO3 (2.030 g, 14.69 mmol), and
Pd(PPh3)4 (0.282 g, 5 mol %) were taken. The solution was stirred at 80uC under N2
atmosphere until the starting material was disappeared as monitored by TLC. The
catalyst was removed by filtration and the residue was washed with CHCl3. The
filtrate was cooled to room temperature and then washed with deionised water (5 3
50 mL). The organic layer was separated and dried over Na2SO4, filtered, and was
evaporated in vacuo. The obtained light yellow solid residue was washed with MeOH
(10 ml) to remove colored impurities to get white colour NMR pure compound of D.
Alternatively, purification of the yellow solid residue was also can be done by column
chromatography (253 n-Hexane:DCM) on activated silica gel to get an analytically
pure white color powder of D. (1.16 g, 74%yield). 1 H NMR (400 MHz, CDCl3, 25uC):
d8.62 (s, 2H), 8.55 (d, 2H, J 5 2.4 Hz), 7.76 (d, 4H), 7.73 (s, 2H), 7.65 (s, 2H),6.5 (m,
2H), 2.76–2.72 (t, 4H, J 5 8 Hz), 1.74–1.70 (t, 4H, J 5 8 Hz), 1.37–1.27 (m, 22H),
0.89–0.86 (6H). 13 C NMR (100 MHz, CDCl3, 25uC): d158.5, 150.1,149.8,142.3, 140.3,
127.3, 123.3, 115.5, 109.6, 109.4, 107.9, 35.9, 31.8, 30.2, 29.4, 29.3, 29.2, 22.6, 14.1. FTIR (KBr): 3128.8, 2958.9, 2926.0, 2849.3, 1742.5, 1715.1, 1621.9, 1567.1, 1528.8,
1452.1, 1391.8, 1260.3, 1189.0, 1046.6, 969.9, 915.1, 860.3, 789.0, 761.6, 646.6,
602.7 cm 21 ; UV/Vis (CHCl3): absorbance 5 lmax317 nm and emission lmax 5
344 nm; LCMS (m/z): calcd. for C38H48N10, 644.4063; found, 645.45; HRMS (m/z):

[M 1 Na] 1 calcd. for C38H48N10, 644.4063; found 667.3974; analysis (calcd., found for
C38H48N10) C (70.78, 70.61), H (7.50, 7.58), N (21.72, 21.62); CCDC No. 992513.
M.p.: 141uC.

Synthesis of polymer (Poly). Bis(cyclooctadiene)nickel(0) (0.342 g, 2.4 equiv),
cyclooctadiene (0.153 mL, d 5 882 mg/mL, 2.4 equiv), and 2,29-bipyridine
(195.48 mg, 2.4 equiv) were dissolved in a solution of dry toluene (5 mL) and dry
N,N-dimethylformamide (5 mL) in a Schlenk flask within a glove box. The mixture
was heated at 60uC with stirring under argon for 20 min to generate the catalyst, and
then a solution of the 2,6-bis(4-iodo-1H-pyrazol-1-yl)-4-octylpyridine 2 (300 mg,
0.521 mmol) in dry toluene (5 ml) was added. The reaction was heated at 80uC for 2
days. Then a toluene solution (4 ml) of 2-(4-iodo-1H-pyrazol-1-yl)-4-octyl-6-(1Hpyrazol-1-yl)pyridine (70.29 mg, 0.156 mmol) was added and the reaction mixture
was heated at 80uC for an additional 12 h The crude reaction mixture was then

poured into a mixed solution (100 mL) of methanol and concentrated hydrochloric
acid (151) and stirred for 12 h. The precipitated white solid was filtered and dissolved
in THF (10 ml) and added drop wise to methanol (200 ml). The resulting solid was
filtered off and subjected Soxhlet extraction for 2 d in acetone (130 ml) followed by
methanol (volume 130 ml). The insoluble solid residue was redissolved in THF
(10 ml) and precipitated from methanol (180 ml). The precipitate was washed with
methanol (10 ml) and air dried to get polymer in 126 mg yield. 1 H NMR (500 MHz,
CDCl3): d 9.16 (s), 8.65–8.85 (end group), 7.89 (s), 7.6–7.8 (end group), 7.46 (s), 6.5–
6.6 (m, end group), d 2.83–2.80 (m), d 1.77–1.30 (m), 0.91 (m); FTIR (KBr disc): 3408,
3107, 2959, 2926, 2860, 1616, 1573, 1529, 1447, 1392, 1184, 1041, 975, 921, 860, 789,
652, 603 cm 21 ; UV/Vis (CHCl3): absorbance 5 lmax 316 nm and emission lmax 5
366 nm; GPC (THF): Mn 5 7.39 kDa (number average), Mw 5 14.27 kDa (weight
average) and PDI 5 1.93 (poly dispersity index).

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Acknowledgments
RC thanks the Max Planck Society for granting Max Planck Partner Group with University
of Hyderabad (Grant No. MPGA POLY 0006). SB and AMM thank Max Planck Partner
Group with University of Hyderabad (UoH) for the research fellowships. RC thanks School
of Chemistry, UoH for FESEM and Confocal Raman microscope facilities and Centre for
Nanotechnology, UoH for SNOM and TEM facilities.

Author contributions

The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript. R.C. conceived the research idea. S.B.
performed synthesis and patterning work under the supervision of R.C., M.B. and K.M.. S.B.
and A.M.M. performed the imaging experiments under the supervision of R.C. and
involved in the data analysis and discussions. S.B., K.M. and R.C. wrote the manuscript.

Additional information
[Supplementary information accompanies this paper at http://www.nature.com/](http://www.nature.com/scientificreports)
[scientificreports](http://www.nature.com/scientificreports)

Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Basak, S., Mohiddon, M.A., Baumgarten, M., Mu¨llen, K. &
Chandrasekar, R. Hierarchical multicolor nano-pixel matrices formed by coordinating
luminescent metal ions to a conjugated poly(49-octyl-29,69-bispyrazoyl pyridine) film via
contact printing. Sci. Rep. 5, 8406; DOI:10.1038/srep08406 (2015).

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