Novel self-assembled 2D networks based on zinc metal ion co-ordination: Synthesis and comparative study with 3D networks

Article abstract of DOI:10.1039/c3ra47377e

The synthesis of linear and trigonal terpyridine bearing molecules (tpys) and their self-assembly to form novel extended self-assembled 2D networks of trigonal tpys with linear tpys through zinc metal ion (Zn²⁺) co-ordination is reported. The r

Full text of DOI:10.1039/c3ra47377e

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Novel self-assembled 2D networks based on zinc
metal ion co-ordination: synthesis and comparative
study with 3D networks†
Cite this: RSC Adv., 2014, 4, 17680
a
b
a
a
b
Deepa Rajwar, Xinfeng Liu, Zheng Bang Lim, Sung Ju Cho, Shi Chen,
c
d
a
b
Jens M. H. Thomas, Abbie Trewin, Yeng Ming Lam, Tze Chien Sum*
and Andrew C. Grimsdale*a
The synthesis of linear and trigonal terpyridine bearing molecules (tpys) and their self-assembly to form
novel extended self-assembled 2D networks of trigonal tpys with linear tpys through zinc metal ion
2+
2+
(
Zn ) co-ordination is reported. The resulting Zn co-ordination networks have been characterized by
means of X-ray photoelectron spectroscopy (XPS), small angle X-ray scattering (SAXS), BET, and
photophysical methods. The presence of some short range order in these networks has been shown by
the SAXS results and these results have been analyzed with the help of molecular modelling studies.
2+
These metallo-supramolecular Zn networks revealed the influence of the metal ion on the thermal and
optical properties of the synthesized metallo-supramolecular assemblies, similar to the results previously
reported for 1D and 3D self-assembled metallo-supramolecular materials. Moreover, these networks
have shown high luminescence with a long fluorescence lifetime and good thermal stabilities. The
monolayer of such ordered networks can be used as a template to build hierarchical nanostructures.
These hierarchical nanostructures could be used as active components in electronic devices and as
templates for the formation of hybrid organic–inorganic nanomaterials.
Received 6th December 2013
Accepted 28th March 2014
DOI: 10.1039/c3ra47377e
www.rsc.org/advances
interactions). Self-assembly rapidly and easily generates large
and complex “supramolecules” (ensembles of individual
Introduction
Ordered 1D, 2D and 3D structures formed through self- molecules) from easily available building blocks with maximum
assembly of organic molecules by non-covalent processes such efficiency. Synthesis of supramolecular networks from these
as metal–ligand co-ordination offer considerable prospects for building blocks becomes straightforward due to the thermo-
advances in nanoscience and progress in nanotechnology. The dynamically driven self-correcting ability of non-covalent

eld of supramolecular electronics has progressed rapidly in interactions. Metallo-supramolecular networks have attracted a
1–4
recent years. Supramolecular electronics is one of the most lot of interest, not only because of the higher strength of this
promising “bottom-up” techniques in nanoelectronics of p- non-covalent interaction in comparison to other non-covalent
conjugated molecules with a device length of 5–100 nm. interactions (such as hydrogen-bonding) but also for the self-
Supramolecular p-conjugated ‘oligomers’ have attracted much healing ability and reversibility of these networks due to their
attention in last few decades due to the reversibility of the non- dynamic nature. Co-ordination driven self-assembly, gives a
covalent interactions (hydrogen bonds, co-ordination bonds, better control over the rational design of supramolecular
hydrophobic interactions, electrostatic, and van der Waals networks by taking advantage of the predictable nature of the
metal–ligand co-ordination and ability of ligands to program
5,6
directionality.
a
School of Materials Science and Engineering, Nanyang Technological University,
Singapore. E-mail: ACGrimsdale@ntu.edu.sg; Fax: +65-6790 9081; Tel: +65-6790
Thus, this strategy of metal–ligand co-ordination represents
an alternative method for “bottom-up” technique in nano-
electronics by synthetic approach for designing molecules of
6
728
b
Division of Physics and Applied Physics, School of Physical and Mathematical
Sciences, Nanyang Technological University, Singapore. E-mail: tzechien@ntu.edu. required dimensions of few nanometers. For instance, a wide
sg; Fax: +65-6316 2971; Tel: +65-6795 7981
range of 1D, 2D and 3D systems formed by co-ordination
c
7–12
Institute of Integrative Biology, University of Liverpool, Liverpool, L69 7ZB, UK
bonding have been widely explored in past years.
Hence,
d
Department of Chemistry, Lancaster University, Bailrigg, Lancaster, LA1 4YB, UK
these supramolecular co-ordination networks have found a lot
of prospective to be utilized for different nanotechnological
applications. For example, supramolecular co-ordination
networks formed by Fe metal ion co-ordination have been
†
Electronic supplementary information (ESI) available: NMR, CV, TGA, DSC, TEM
and mass spectral data for new compounds and networks. Structures, PL proles
and PL data for S1, S3 and S4, molecular model for 1, 3, N1, and N3. See DOI:
1
0.1039/c3ra47377e
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10
2+
reported with excellent magnetic properties. Ru co-ordina- importance as models for the advancement of new metal-
2
+
tion complexes with terpyridine ligands have been used in the lopolymers. The low cost and nontoxicity of Zn ions has made
13
building of polymer wires on gold. Zinc metal ion complexes them a promising candidate for making highly developed co-
14–18
2+
are very well known for their optoelectronic properties
and ordination networks. The other advantage of using Zn ions for
are potential candidates for many optoelectronic devices. In co-ordination is that metal to ligand charge transfer (MLCT)
another application, non-linear optical properties of zinc metal processes do not occur in such systems, because of the full d10
19
2+
ion co-ordination networks have been reported recently.
electron conguration of the Zn ions. Hence there is only the
Terpyridines were proposed as building blocks for supra- possibility of intraligand (IL) charge transfer in these systems.
2
0,21
molecular chemistry decades ago.
Still there remains much High PL quantum yields and EL performance have been
30
2+
scope for development in the supramolecular chemistry of ter- reported using such systems. Winter et al. have used Zn as a
pyridines and much research work has been performed in this template to assemble organic building block into polymer
1
1,13,18,22–27
18

eld.
Transition metal-complexes of p-conjugated bis- chains through coordination to chelating terpyridyl units.
0
0
00
and tris(terpyridine) have high prospective to be used as new These linear oligomers with tpy (2,2 :6 ,2 -terpyridine) end-
functional materials in electronic and optoelectronic devices. groups self-assembled with Zn to give luminescent polymers.
Heavy transition metal ions, such as Pt , Ru , Os , and Ir
have been explored thoroughly in the past.
electron-poor square-planar divalent Pt and Pd ions have of 2D metallo-supramolecular networks of Zn ions with ter-
been widely used in conjunction with electron-rich nitrogen pyridine ligands and a comparison of their optoelectronic
containing moieties in the self-assembly process.
2
+
2
+
2+
2+
2+
Moving one step forward towards achieving ordered and
In particular, extended networks, this paper mainly focuses on the synthesis
20,24,28
2
+
2+
2+
25,29
12,19
Metal ions properties with the 3D networks previously reported by us.
with lled electron shells have recently achieved much To create metallo-supramolecular networks, monomers with
Fig. 1 Molecular structure of basic building blocks for 2D self-assembled networks.
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self-assembling end-groups are mandatory. In the previous
work in this eld, the focus has been on linear and tetrahedral
2
+
Zn terpyridine networks. By systematic examination on their
photophysical properties together with their structural charac-
terization, it has been concluded that by changing conjugated
backbone on the terpyridine units it is possible to modify the
optoelectronic properties of these networks. Strong emission in
the solid state indicates that these materials correspond to the
capable candidates with respect to their potential applications
Scheme 2 Synthetic route to molecule 6.
in optoelectronic devices such as OLEDs. Hence it was antici- NMR and its molecular weight was conrmed by high resolu-
2
+
pated that the Zn based materials might be made solution tion mass spectrometry (HRMS). 2D self-assembled networks of
processable by the attachment of more solubilizing groups to p- molecule 1 were made through zinc metal ion co-ordination.
2
+
conjugated terpyridines. These complexes could be very prom-
ising candidate for solar cell applications. In extension of this N1 was obtained by reuxing its 2D monomers 1 with zinc
work, the present investigations aim the design of zinc metal co- acetate (Zn(OAc) ) at the stoichiometric ratio of 1 : 1.5, in NMP
ordinated 2D networks made up of trigonal and linear p- solutions followed by anion exchange using an excess of
conjugated terpyridines with respect to their possibility in the NH PF . Although 2D monomer 1 is soluble in common organic
eld of optoelectronic device applications. Various bis(terpyr- solvents (e.g. ether, DCM, CHCl , THF, etc.), the 2D networks
Formation of 2D Zn co-ordinated self-assembled network
2
4
6

3
idine) derivatives were used in the synthesis of self-assembled N1–3 are not only insoluble in those common organic solvents
2
+
supramolecular 2D networks by Zn metal co-ordination with but also in highly polar aprotic solvents (e.g. DMSO, DMF, NMP,
tpy ligands. Photophysical properties with respect to their acetonitrile etc.) at room temperature. This behaviour is
device applications along with their thermal, and electro- believed to be caused by the highly cross linked or inter-
chemical properties, as well as ordering in their self-assembled penetrated structure of the 2D networks. Therefore, all the
networks, are discussed, with comparisons made to the similar unwanted byproducts (including any unreacted starting mate-
1
2,19
3
D networks previously made by us.
The chemical structures rial 1) were washed away using ether to get the nal network N1
2
+
of basic building blocks for 2D self-assembled networks are in solid form. The formation of new 2D Zn co-ordinated self-
shown in Fig. 1.
assembled supramolecular network N1 from trigonal tpy
molecule 1 is shown in Scheme 5.
Using the method previously developed by us for making
Results and discussion
Synthesis
12
extended 3D networks, extended networks N2 and N3 were
formed using the p-type or n-type conjugated tpy linker units 3
and 2 as shown in Scheme 6. 2,5-Bisdodecyloxy-p-phenylene has
been employed as the p-type linker (3) in N3. Its being linear
and electron rich, it is a good electron donor. In the other
extended network N2, 2,1,3-benzothiadiazole (benzo[c][1,2,5]
thiadiazole) has been used as the n-type linker (2) as it is known
to be a good electron acceptor. The same insoluble behaviour
was found for the 2D extended networks N2 and N3. Networks
N2 and N3 were obtained by reuxing 2D network N1 with zinc
Boronation of molecule 4 through Suzuki coupling reaction
gave the key intermediate core molecule 5 in a moderate yield.
Synthesis scheme of this trigonal core molecule 5 is outlined
in Scheme 1. In the next step towards synthesis of molecule 1, in
the presence of Pd(0) catalyst, compound 9 was Suzuki-coupled
with triborane 5 to produce compound 1 as an off white solid.
The synthetic steps involved in the preparation of key inter-
mediate molecule 9 and new 2D p-type core molecule 1 for 2D
self-assembly are outlined in Schemes 2–4. The structure of this
2
acetate Zn(OAc) at the stoichiometric ratio of 1 : 1.5 in NMP
solution to break the network, followed by inserting the linear
1
13
novel trigonal-tpy molecule 1 was conrmed by H NMR and
C
Scheme 1 Synthetic route to trigonal molecule 5.
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Scheme 3 Synthetic route to key intermediate molecules 7, 8 and 9.
Scheme 4 Synthesis scheme of 2D supramolecule 1.
bis(tpy) monomers 2 and 3 in between the 2D molecules,
The concept of reversible self-assembly was demonstrated in
respectively, to form the 2D extended networks N2 and N3 aer these networks which conrmed our concept for their forma-
anion exchange using an excess of NH PF . Aer forming tion. By adding more metal ions into the 2D network N1, co-
4
6
networks N2 and N3, any residual 2D starting monomer (1) and ordination complexes can be dissociated again in favour of an
the linear bis(tpy) monomers (2 and 3) were washed away with open form with each zinc ion only attached to one tpy unit and
ether.
its other co-ordination sites presumably saturated by solvent
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2
+
Study of presence of Zn ion in 2D network structures
X-ray photoelectron spectroscopy (XPS) studies were performed
in order to achieve an insight into the network structures. XPS
measurements were performed with monochromatic Al-K alpha
radiation (1486.7 eV) to conrm the existence of the zinc metal-
ions and to achieve more data in favour of the proposed network
structures. XPS spectra of the zinc complexes on carbon tape
substrates are shown in Fig. 2. The presence of Zn 2p photo-
emission peaks in N1, N2 and N3 conrms the Zn–ligand
2+
Scheme 5 Synthesis of one component 2D Zn co-ordinated self- coordination. In contrast, no Zn 2p peak is present in starting
assembled supramolecular network N1.
material 1. In addition, the O 1s, C 1s, N 1s and F 1s peaks (may
ꢁ
be due to PF
6
counterions) are also shown in widescans,
indicating a successful complexation of the terpyridine units.
Due to low conductivity of three networks, a few eVs charging is
observed and calibrated using C 1s and other photoemission
peaks. The binding energy of Zn 2p in N1, N2 and N3 located at
1020.5 eV (N2) and 1021.3 eV (N1 and N3). The difference in
binding energy could be either due to small residue charging or
related to the different side chains of three networks.
Molecular organization studies of 2D network structures
Small angle X-ray scattering (SAXS) was performed to study the
molecular organization. SAXS for N1, N2 and N3 in the solid
state are compared. As can be seen from Fig. 3, N1 has three
ꢁ
1
clear broad peaks that centre at scattering vector q ¼ 0.50 nm
,
ꢁ
1
ꢁ1
1
.24 nm and 2.46 nm respectively. The peak at scattering
ꢁ1
ꢁ1
vector q ¼ 0.50 nm ranges between 0.40 and 0.70 nm . This
corresponds to a distance of 12.6 nm with a range between 9.0
ꢁ
1
and 15.7 nm. The peak at scattering vector q ¼ 1.24 nm ranges
ꢁ1
between 1.10 and 1.50 nm . This corresponds to a distance of
.0 nm with a range between 4.0 and 5.7 nm. The peak at
5
ꢁ1
scattering vector q ¼ 2.46 nm exhibits a greater intensity and
is relatively less broad than the other two peaks. It ranges
ꢁ1
between 2.00 and 3.10 nm . This corresponds to a distance of
2.5 nm with a range between 2.0 and 3.1 nm.
N2 has two very broad weak peaks centred at scattering
ꢁ
1
ꢁ1
vector q ¼ 0.38 nm and 2.09 nm respectively. The peak at
ꢁ1
scattering vector q ¼ 0.38 nm ranges between 0.3 and 0.5
ꢁ
1
nm . This corresponds to a distance of 1.65 nm with a range
between 12.6 and 20.9 nm. The peak at scattering vector q ¼
ꢁ
1
ꢁ1
2
.09 nm ranges between 1.00 and 3.10 nm . This corre-
sponds to a distance of 3.0 nm with a range between 2.0 and 6.3
nm. N3 shows one broad but clear peak centred at scattering
ꢁ1
ꢁ1
2+
vector q ¼ 2.66 nm and ranges between 2.00 and 3.00 nm
This corresponds to a distance of 2.3 nm with a range between
.1 and 3.0 nm. N1 shows three clear, although relatively broad,
peaks compared to N2 and N3, which have fewer peaks that are
.
Scheme 6 Synthesis of 2D Zn co-ordinated self-assembled two
component extended supramolecular networks N2 and N3.
2
molecules. Disassembly of N1 was followed by re-assembly to both weaker and broader. This suggests that N1 has a more
form extended 2D networks N2 and N3. 2D network N1 was only dened structure compared to N2 and N3. This reects the
ꢀ
partially soluble in the NMP solution at 105 C and the colour of single component self-assembly composition of N1, in contrast
solution was light brown at that moment, aer adding 1.5 to N2 and N3, which are composed of two different basic
equivalents of Zn(OAc) in NMP, the insoluble solid gradually building blocks and will contain a statistically random combi-
2
dissolved in the solution and the colour of solution gradually nation of each. Due to the insolubility of the networks we could
changed to light yellow before adding the linear bis(tpy) linkers. not form lms for study by STM or AFM as has been recently
2
+
31
All these novel 2D Zn co-ordinated self-assembled networks done for sheets of a similar network made by Schl u¨ ter et al.
N1–3 were obtained in moderate yields (>50%).
High resolution TEM images were obtained from deposits of N1
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Fig. 2 XPS spectra of 1 and its zinc complexes N1, N2 and N3 on a carbon tape. Wide-scan spectra and zinc 2p region.
Fig. 4 Nitrogen adsorption–desorption isotherms of 2D networks
N1–3 measured at 77.3 K.
Fig. 3 Powder SAXS diffractogram of 2D networks N1–3.
sample size. These materials are thus believed to be highly
interpenetrating networks, and hence no porosity was found, in
2
+
these 2D Zn co-ordinated self-assembled networks. Similar
(
see ESI†) on carbon coated copper grids but were unable to
12
results were reported for the 3D zinc metal ion co-ordinated
networks with similar tpy branching units previously made by
us. Shortening or removal of the long side chains is anticipated
to solve this problem.
determine any structural data on the networks from them. Even
with the use of electron energy loss spectroscopy (EELS) we were
unable to see any peaks corresponding to the presence of zinc
ions, which we attribute to the low concentration of the zinc in
these networks.
Structural modelling of 2D self-assembled networks
Porosity studies
Atomistic models for network materials have previously shown
2
+
The porosity and the surface area of the 2D Zn co-ordinated that the node-strut topology is highly inuential on the result-
self-assembled networks N1–3 were investigated by sorption ing structural properties of the network and their ability to pack
analysis using nitrogen. The results are shown in Fig. 4 and to ll space efficiently.
32,33,36
Increased exibility and confor-
summarized in Table S1 in the ESI.† In comparison to the mational freedom of the network building blocks allow the
covalently bonded organic microporous networks based on networks to interpenetrate more freely and hence pack to ll
3
2–35
33,36a
tetraphenylmethane reported earlier,
the surface areas of more of the available space.
2
ꢁ1
these networks are basically small (<2.1 m g ). All the values
Firstly, atomistic models of the building blocks, 1 and 3, and
are well within the experimental error of the equipment. The their respective one-component and two-component extended
negative dip for these networks is believed to be due to small 3D self-assembled networks were generated. Models of 1 and 3
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3
7
ꢀ
ꢀ
were fully optimised using the COMPASS force eld in the ligands for 1. These range between 103 and 138 compared to
ꢀ
˚
Forcite module of Materials Studio 5.0 (ref. 38) with charges the ideal of 120 , and is 2 A out of plane. The geometry of the
calculated using the Gasteiger method and are shown in Fig. 5. optimised model for 3 shows a planar rigid molecule with the
Assessing the optimised geometry of model of 1 shows that the tpy-N–centroid–tpy-N angle being 177 .
39
ꢀ
structure deviates from the idealised planar form. There are
Using these idealised building units, we have rationalised
three angles that can be dened between the centroid of the the SAXS data discussed above. For N1, there are four distinct
central phenyl and the central nitrogen atom of two of the tpy repeat units available. These are, in order of dimension, (i) the
Fig. 5 The optimised geometry of (a) 1 and (b) 3 and idealised hexagons and broken hexagons for (c) N1 and (d) N3 respectively showing the four
types of repeating distances potentially present within the extended network. The red rod inset in (a) represents 1 in the extended networks. The
2+
blue rod represents 3 in N3. The black spheres represent the linking Zn
.
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ꢀ
ꢀ
distance between the molecular centroid of 1 and a coordinated 158 and 180 through out-of plane and in-plane molecular
2
+
˚
˚
Zn , (ii) the molecular centroid – molecular centroid, and bends with a distance of between 0 A and 2 A out of the
nally, depending upon the topology of the network either (iii) molecular plane. This demonstrates the high degree of exi-

the molecular centroid – molecular centroid across a hexagonal bility and conformational freedom available to both 1 and 3
repeat unit, if the network has a 2D graphite-like honeycomb network building blocks.
2
+
2+
structure, or (iv) a Zn to opposite Zn across a partial
We then turned our attention to the automated generation of
hexagonal ring, if the structure has a 3D structure. For N3, the a 3D self-assembled network structure. The 3D self-assembled
two-component self-assembly results in a number of types of networks were broken into fragment building units; these are
repeat units being available. These are the based on the same not necessarily chemically representative of the actual mono-
repeat units as identied for N1, but for each distinct repeat meric reactants but enable us to automate the building process.
unit of N1, there are now a number of different combinations For N1, we used two basic building units, Fragments A and B,
available for N3, particularly for repeat unit type (iii) where there shown in Fig. S.11(a) and (c) respectively in ESI.† For Fragment
will be multiple combinations possible. This variation in the A (based on 1), a library of 21 different conformers is available.
build units results in the likelihood that these types will not be This includes the idealised optimised structure and an addi-
observed in SAXS measurements with only the build units of tional 20 structures generated by sampling the two MD simu-
type (i) now being regular repeat units. These repeat units are lations. The MD simulations were sampled every 100 000 steps
identied in Fig. 5(c) and (d) for network N1 and N3 and the geometry of the resulting structures were optimised
respectively.
before the tpy end groups were removed. The remaining three
A hexagonal and partial hexagon ring for both networks were unsaturated carbon atoms of the terminal phenyl groups were
generated for N1 and N3, shown in Fig. 5(c) and (d) for network assigned as bonding atoms. The distance between the bonding
˚
N1 and N3 respectively and were fully optimised using the atoms and the molecular centroid is approximately 14.0 A.
3
7
2+
COMPASS force eld in the Forcite module of Materials Studio Fragment B is generated from the Zn and the tpy coordinating
ꢁ
5
.0 (ref. 38) with charges calculated using the Gasteiger ligands with the two PF
method. For N1, the average distance between repeat units (i) the Zn -tpy and PF
6
counter ions. The geometry of both
39
2+
ꢁ
6
were fully optimised with DFT, using
40
41
42
and (ii) is 2.0 nm and 4.0 nm respectively, correlating well to the NWChem 6.1, a def2-tzvp basis set, B3LYP functional and
43
observed peaks which have corresponding distances centred on incorporating the Grimme dispersion correction. A hydrogen
.5 nm and 5.0 nm. For the idealised hexagon ring the type (iii) atom is removed from the two central rings of each tpy group
2
distance is 8.0 nm, which does not correlate well with the nal and the remaining unsaturated carbons were allocated as
peak observed in the SAXS data centred at 12.6 nm. For the bonding atoms. The bonding atom – bonding atom distance is
ꢁ
˚
partial hexagon ring, the type (iii) distance is 12.0 nm, which 9.7 A. The incorporation of two PF
6
groups within Fragment C
correlates well with the nal peak observed in the SAXS dif- ensures that these counter ions are accounted for within the
fractogram centred at 12.6 nm. This suggests that hexagon rings network generation process. For N3, constructed from 1 and 3,
are not a signicant structural component of N1. For N3, the we included an additional Fragment C (shown in Fig. S.11(b) in
type (i) average distances measure 2.0 nm (same as for N1) and ESI†), which is generated from a library of 5 conformers which
2.4 nm, which correspond well with the peak centred at a include the idealised structure and an additional four created
distance of 2.3 nm. No further peaks are observed in the SAXS by sampling the MD simulation of 3, similarly to 1. The terminal
diffractogram for N3, suggesting that the structural compo- tpy groups were removed and the remaining unsaturated
nents of type (ii), (iii) or (iv) and are not repeating in dimension. carbons on the terminal phenyl groups are allocated as bonding
This also suggests that hexagon ring formation is not a signif- atoms. The distance between the bonding atoms of Fragment C
˚
icant structural component of N3. The lack of a hexagon ring is 11.5 A. Representative structures for Fragments A, B, and C
component in both N1 and N3 further suggests that the are shown in Fig. S.11(c)–(e)† respectively with the bonding
building blocks, 1 and 3 are not rigid structurally-directing atoms highlighted in yellow. For Fragments A and C, Fig. S.11
units but have some conformational exibility.
To fully explore the conformational freedom of the two for each fragment.
building blocks, we have used molecular dynamic simulation at A Python code was used to automate the generation of the
00 K and 298 K using the Forcite module, the COMPASS force self-assembled network. The code builds the network following
eld and charges assigned using the Gasteiger method and with pre-set rules for a series of building steps. The rst step is a Seed
and S.12† shows an overlay of the library of structures available
1

an NPT ensemble. For each simulation, a total of 2 ns simula- step where one or more fragments are added to a simulation cell
tion time with a 1.0 fs time step was used. The angles and out-of- of pre-set dimensions. We assign rules as to which Fragments
plane distance are monitored for the nal 1 ns of each simu- are permitted to bond as follows; for network N1, Fragment A
ꢀ
lation for 1 and 3. For 1, the three angles range between 64 and can bond to Fragment B, but not A to A or B to B, similarly for
ꢀ
1
74 . One angle and the out-of-plane distance is monitored network N3. Fragment A can bond to B but not A or C and
throughout the 298 K MD simulation showing an average angle Fragment C can bond to B but not C or A. A Build step bonds a
ꢀ
˚
of 111 ꢂ 18 and an average out-of-plane distance of 6.4 ꢂ 1.2 A. randomly selected additional fragment to a randomly selected
It is noticeable that during the 298 K MD simulation the existing fragment or partially built network. The fragment is
extended side chains wrap inwards towards the centre of the bonded with idealised bond lengths, although the torsional
molecule. For 3, the tpy–centroid–tpy angle ranges between angles of the fragments about the bond are random. Additional
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bonds between fragments are only permitted if the bond might be expected from the two dimensionality of Fragment C,
geometry ts within the specied geometrical parameters, the whereas those of N1 extend in all three dimensions. We see a
bond distance is within a set range and no non-bonding atoms wide range of different combinations of Fragment A and C
˚
are closer than 1 A. This ensures that the initial generation of leading to a very wide range of structural components. The only
the network is geometrically sensible.
repeat units observed in the randomised clusters of N3 are
We generated an idealised cluster for each network N1 and those of type (i). This accounts for the absence of any peaks
N3. This enables us to assess the presence of the idealised observed that do not correlate to type (i) distances within the
structural components and their respective dimensions within SAXS diffractogram. Each cluster shows an extended structure
an extended structure. The idealised clusters are generated with no evidence of order. The networks oen turn in on
exclusively from the idealised models of 1 and 3, and do not
include any of the structures obtained from the MD simulation.
The idealised cluster for N3 was generated following the ideal-
ised alternating 1 and 3 building pattern. The clusters were
generated by an initial Seed step with one randomly chosen
fragment and then a series of Build steps following the bonding
rules. Each cluster Fragment is capped with either an uncoor-
dinated tpy or a H atom. Aer generation, the geometry of the
cluster is optimised using the Fast Inertial Relaxation Engine
44
(
FIRE) rigid body minimiser within HOOMD-blue. The frag-
ment build units are treated as rigid bodies so that only the
bonds between fragments are optimised. The Intermolecular
forces, bond and angle parameters are taken from the PCFF
45
force eld. Intermolecular forces are described using a Len-
2
+
46
nard-Jones potential with additional Zn parameters. Charges
2
+
are calculated using the Gasteiger method and QEq for the Zn
39
coordinated molecule in fragment B. The idealised clusters for
N1 and N3 are shown in Fig. S.12(a) and (b)† respectively. There
are structural features of type (iv), as described above, present in
both idealised networks N1 and N3. For N1, the average is 11
nm and range between 10.0 nm and 14.4 nm, in good agree-
ment with the range observed in the SAXs diffractogram for the
peak centred at 12.6 nm with a range between 9 nm and 15.7
nm. For N3, the average is 16 nm and range between 19 nm and
14 nm. This does not correlate with any peaks in the diffracto-
gram for N3. There are no structural components of type (iii)
due to the automated network generation method. For N1, the
type (i) and type (ii) are similar to the values obtained for the
hexagon ring and for the partial ring. For the cluster model of
idealised N3, the repeating structural components of type (ii)
are well dened at 5.0 nm. We would expect to see peaks that
correspond to both these distances in the SAXS diffractogram.
These peaks are not present, with only peaks corresponding to
the type (i) structural components present. This suggests that
the regular alternating building unit model for N3 does not
represent the real structure. An alternative model for N3 can be
generated based on a randomised combination of the building
block Fragments A and C.
Finally, we have constructed cluster models that include the
structural exibility observed in the MD simulations and
randomised combination for N3. For N1 and N3 we have
included the full library of structures for Fragment A and C. For
network N3 the choice is random between Fragment A and C
with an equal probability for either Fragment. A representative
cluster is shown in Fig. 6(a) and (b) for N1 and N3 respectively
also showing the underlying net and fragment combination.
Five clusters were generated for each network and are shown in
Fig. 6 Extended networks of (a) N1 and (b) N3 generated through an
automated method with random choice of building block. Right – the
network showing all atoms and, left – the underlying network with 1
and 3 represented as red and blue rods respectively. The black spheres
2+
ꢁ
represent the linking Zn . The PF counter ions are omitted for
6
Fig. S.13.† The clusters of network N3 are largely planar, as clarity.
17688 | RSC Adv., 2014, 4, 17680–17693
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Table 1 5% weight loss temperature (T5d), glass transition temperature
(T ) and melting temperature (T ) of 1 and 2D networks N1–3
g m
a
ꢀ
b
ꢀ
c
ꢀ
Networks
T
5d ( C)
T
g
( C)
T ( C)
m
1
310
374
390
386
115
—d
d
d
N1
N2
N3
—
—
d
d
—
—
d
d
—
—
a
b
T
5d ¼ 5% weight loss temperature.
T
g
¼ glass transition temperature.
c
d
T
m
¼ melting temperature. Relevant data is not found.
themselves, suggesting that network interpenetration is likely.
This suggests that an amorphous, efficiently packed solid will
be formed with no porosity. This is in agreement with the
porosity study discussed above.
Fig. 8 Solid state photoluminescence spectra of core molecules 1, 2,
3
and their 2D self-assembled networks N1–3.
Thermal behaviour of 2D self-assembled networks
Thermogravimetric analysis (TGA) plots and Differential Scan-
ning Calorimetry (DSC) plots of core molecule 1 and its 2D
networks N1–3 are shown in the ESI.† T5d, T and T of 2D
g m
networks N1–3 are listed in Table 1. According to Table 1, 2D
zinc metal ion co-ordinated self-assembled networks N1–3 have
slightly higher T5d compared to their respective starting mate-
rials 1. Besides that, N1–3 are generally more thermally stable
compared to 1 as more than 50% of materials of N1–3 remain
synthesized co-ordination networks N1–3 is clearly indicated by
the observed red shis in the recorded emission spectra as
shown in Fig. 8. Furthermore, the emission spectra of the dis-
cussed systems (N1–3) were measured in solid state at room
temperature. Molecules 1, 2 and 3 were emissive and the
resulting Zn co-ordination networks N1–3 were emissive too.
The solid state optical properties of the networks can be
adjusted by inserting different types of linkers into the extended
networks. This aspect might be explained by the fact that the
synthesized metallo-supramolecular networks tend to aggregate
mainly due to the conjugated and rigid structure of the spacers
2
+
ꢀ
2+
aer heating to 900 C. Thus, Zn co-ordination helped to
2
+
improve the thermal stabilities of these Zn co-ordinated
networks N1–3. But no phase transitions (T and T ) were
observed in the DSC measurements of N1–3. Also, these 2D Zn
co-ordinated self-assembled networks N1–3 are not crystalline
as no signicant exothermic peaks (T ) have been observed
g
m
2
+
(2 and 3) and as well as core molecule 1. Fig. 8 shows the
emission spectra of core molecule 1 (emission peak at 436 nm)
c
2
+
compared to the Zn co-ordination networks N1–3 demon-
strating the red shi of 82, 233, and 115 nm respectively. N2
shows red uorescence under a UV lamp, as shown in Fig. 7, in
comparison to the networks N1 and N3 both of which show
yellow uorescence under a UV lamp. This again shows the
during the heating cycles in their DSC plots.
Comparison of optical properties of 2D and 3D self-assembled
networks
Photophysical properties of 2D self-assembled networks N1–3 effect of the inserted n-type linker 2 which has a lower band gap
are in agreement with the observations for the terpyridine and acts as sole emissive unit to produce red uorescence in
2
+
ligands and their 3D Zn complexes discussed previously. network N2, whereas, network N3 which has the p-type linker 3
Fig. 7(a) and (b) shows the appearance of self-assembled shows similar emission to network N1. Hence it is apparent that
2
+
networks with naked eyes and under UV lamp respectively. The aer Zn ion complexation with tpy ligands, the position of the
2
+
presence of Zn ions in the molecular structure of the emission peak changes signicantly.
2
+
Moreover, it was expected that the Zn tpy connectivity will
increase the electron delocalization on the conjugated and rigid
spacers, which can be observed by red shi in the PL spectra of
N1–3. The reason behind the red shi in the emission spectrum
is the increase in conjugation, which will increase the number
of delocalized electrons in the system. Finally, the longer the
value of conjugation length, the lower will be the energy gap
between closest energy levels and the longer will be the
absorption wavelength, and so the emission wavelength too.
Femtosecond time-resolved photoluminescence dynamics of 1
and its self-assembled networks N1–3 and comparison with
previously reported 3D networks has been performed. Funda-
mentally, uorescence is a property of electronically excited
Fig. 7 (a) Solid appearance and (b) solid fluorescence of 2D networks
N1–3.
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2, which has a lower band gap. Hence the uorescence lifetime
of these networks are also lower compared to the results
obtained for N1, S1, N3 and S3 which were formed by the p-type
linker units.
Experimental
Palladium tetrakis (triphenylphosphine) Pd(PPh
3
)
4
was
purchased from Strem Chemicals. Other reagents and solvents
were purchased from Alfa Aesar and Fisher Scientic and were
used as received. All reactions were carried out under an inert
N
2
atmosphere. Analytical thin layer chromatography (TLC) was
performed on aluminum sheets pre-coated with silica gel (with
uorescent indicator 254 nm). Visualization was accomplished

with UV light. Purication of reaction mixtures using column
chromatography was done on silica gel 60 Geduran (Merck).
1
13
Nuclear magnetic resonance (NMR) spectra ( H and C) were
recorded on a Bruker Advance 400 spectrometer, using
magnetic eld 400 MHz. Common solvents used in the NMR
Fig. 9 Time-resolved fluorescence decay of 1 and its self-assembled
networks N1–3 in solid state.
characterization were deuterated chloroform (CDCl
3
) and DCM
(
CD Cl ) and. Coupling constants (J) are reported as Hertz (Hz).
2
2
NMR splitting patterns are designated as s, singlet; d, doublet;
dd doublet of doublets, t, triplet; and m, multiplet. Mass spectra
were recorded using a Shimadzu Axima Matrix-assisted laser
desorption/ionization time-of-ight (MALDI-TOF) mass spec-
a
Table 2 Lifetimes and pre-exponential factors from the fitting
Networks
A
1
s
1
(ns)
A
2
s
1
(ns)
saverage (ns)
1
0.56
0.63
0.60
0.46
0.10 ꢂ 0.01
0.17 ꢂ 0.01
0.19 ꢂ 0.01
0.61 ꢂ 0.01
0.44
0.37
0.40
0.54
0.53 ꢂ 0.02
2.16 ꢂ 0.02
0.84 ꢂ 0.01
2.75 ꢂ 0.02
0.45 ꢂ 0.06 trometer. Thermogravimetric analysis (TGA) experiments were
N1
N2
N3
1.92 ꢂ 0.07
0.68 ꢂ 0.04
2.41 ꢂ 0.06
ꢀ
performed on a TA TGA-Q500 with a dynamic heat rate (10 C
ꢁ1
ꢁ1
min ) under nitrogen atmosphere (50 mL min ) in the
ꢀ
temperature range 25–800 C. Differential scanning calorimetry
a
2
2
An average lifetime, s
a
, was also calculated using s
a
¼ (A
1 1 2 2
s + A s )/
(
DSC) measurements were performed on a TA DSC-Q10 with
(
A
1
s
1
+ A ) for comparison.
2 2
s
ꢁ1
ꢀ
dynamic heating and cooling rate (5 C min ) under nitrogen
atmosphere (50 mL min ) in the temperature range 0–200 C.
ꢁ
1
ꢀ
states. Thus, uorescence mechanisms can be better under- Synthesis of 1,3,5-tris(4-iodophenyl)benzene (4)
stood by time-resolved uorescence spectroscopy. To obtain the
47
Compound 4 was synthesized as reported previously.
dynamics of the PL spectra, the uorescence lifetimes of
2
+
compound 1 and its Zn self-assembled networks N1–3 were
measured. The uorescence decay behavior in the presence of
Synthesis of triboronate (5)
2
+
4 (684.09 mg, 1 mmol), bis(pinacolato)diborane (1.15 g, 4.5
mmol), potassium acetate (1.18 g, 12 mmol) and PdCl (dppf)
65.85 mg, 0.09 mmol) were dissolved in anhydrous DMSO (15
Zn is shown in Fig. 9, with the exponential t results. Table 2
shows the tted lifetimes and the pre-exponential factors. For
comparison, we have also calculated the average lifetime.
Comparatively, molecule 1 was found to show a shorter lifetime
compared to N1, N2 and N3. An increase in the uorescence
lifetime of networks N1–3 was observed aer zinc meal ion co-
ordination of trigonal and linear terpyridine moiety in 1, 2 and
2
(
mL) and then the solution was degassed under N
2
atmosphere.
ꢀ
The resulting solution was stirred at 80 C for 12 h, cooled to RT,
and then poured into ice water. The extraction of the resulting
and water. Finally the
. The solvent was
evaporated with rotary evaporator; the residue was washed with
reaction solution was done with CHCl
3
CHCl layers were dried over anhydrous MgSO
3
4
3. Similar results were observed of networks S1 (similar to N1),
S3 (similar to N3) and S4 (similar to N2) made up of a tetragonal
cold hexane. Then the product was puried by recrystallization
from MeOH–THF to afford 5 as an off-white solid (350 mg,
tpy molecule (having similar branching units to trigonal tpy
12
molecule 1) previously reported by us (see ESI†). The values of
uorescent life times are given in Table S1 in the ESI.† These
1
5
1%). H NMR (400 MHz, CDCl
3
): d 7.93 (d, J ¼ 8.2 Hz, 2H), 7.82

(s, 1H), 7.71 (d, J ¼ 8.2 Hz, 2H), 1.37 (s, 12H).
results further suggest that networks with similar units have
similar lifetime values. Enhancement in the lifetime aer zinc
metal ion co-ordination may be attributed to the increase in
Synthesis of intermediates 6, 7 and 8
conjugation aer metal complexation as observed with a red 2,5-Dibromobenzene-1,4-diol (2,5-dibromohydroquinone) was
4
8
shi in the PL spectra too. These results also suggest a complete dialkylated to form 1,4-dibromo-2,5-bis(dodecyloxy)benzene 6.
0
0
0 00
modication in the energy levels aer zinc metal ion complex- 4 -(4-Bromophenyl)-2,2 :6 2 -terpyridine
7
was synthesized
ation. Networks N2 and S4 were formed by the n-type linker unit through the reaction of 4-bromobenzaldehyde and
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49
ꢀ
C under N
2
-acetylpyridine. The enolate of 2-acetylpyridine was produced
2 4 6
atmosphere for 24 h. An excess of NH PF was then
by KOH under mild conditions, followed by an aldol conden- added into the hot solution and stirring was continued for 30
sation and a Michael addition. The intermediate, soluble min. The resulting reaction solution was added slowly into
diketone, was then permitted to form the central pyridine ring MeOH for precipitation. The precipitate was then ltered off
with an aqueous ammonia nitrogen source. Finally by borona- and washed with MeOH. The precipitate was again dissolved in
5
0
0
tion through a Miyaura reaction 7 was converted to 4 -(4- NMP followed by reprecipitation of the solid by adding ether.
0
0
00
pinacolatoboronphenyl)-2,2 :6 ,2 -terpyridine 8.
The precipitate was dried under vacuum for 24 h to give N1 as a
yellow solid (27 mg, 74%).
Synthesis of linear tpy compound (9)
2
+
Synthesis of Zn self-assembled network of 1 with 2 (N2)
6
0
(10 g, 16 mmol), 8 (4.0 g, 9.20 mmol), and Pd(PPh
.55 mmol) were degassed using N
CO (2 M, 30 mL) were then injected into Zn acetate dihydrate (2.18 mg, 0.008 mmol) in NMP (2 mL)
the RBF. The reaction mixture was then stirred in the dark at was added dropwise into the resulting solution. The reaction
3 4
) (638 mg,
ꢀ
2
in a RBF. Degassed, THF (90 N1 (15 mg, 0.005 mmol) in NMP (10 mL) was heated to 105 C.
2
+
mL) and aqueous K
2
3
ꢀ ꢀ
0 C for 24 h. Aer completion of the reaction (monitoring with mixture was stirred at 105 C under N for 1 h. 2 (9.63 mg,
2
8
TLC), the reaction mixture was cooled to RT and THF was 0.0075 mmol, 1.5 times N1 moles) in NMP (10 mL) was then
removed using a rotary evaporator. The reaction mixture was added dropwise and the resulting reaction mixture was stirred
ꢀ
then extracted with CH
MgSO
2
Cl
2
. Organic layer was dried over at 105 C under N
2
for 24 h. An excess of NH
4
PF
6
was then added
4
; solvent was removed by rotary evaporator. Finally the into the hot reaction mixture and stirring was continued for 30
compound was puried by column chromatography on alumina min. The resulting reaction solution was added slowly into
eluting with 10% EA in hexane. The product 9 was obtained as a MeOH for precipitation. The precipitate was then ltered off
1
off-white solid (4 g, 53%). H NMR (400 MHz, CDCl ): d 8.98 (s, and washed with MeOH. The precipitate was again dissolved in
3
1
H), 8.77 (t, J ¼ 8.0 Hz, 2H), 8.03–7.94 (m, 2H), 7.69 (d, J ¼ 8.0 NMP followed by reprecipitation of the solid by adding ether.
Hz, 1H), 7.41 (t, J ¼ 6.0 Hz, 1H), 4.03 (t, J ¼ 6.0 Hz, 1H), 3.900 (t, J The precipitate was dried under vacuum for 24 h to give N2 as a
6.0 Hz, 1H), 1.84 (t, J ¼ 8 Hz, 1H), 1.71 (t, J ¼ 8 Hz, 1H), 1.51 (t, red solid (14 mg, 62%).
J ¼ 8 Hz, 1H), 1.26–1.19 (br, 16H), 0.89–0.81 (m, 3H). Anal. calcd
for C51 66BrN : C, 73.54; H, 7.99; Br, 9.59; N, 5.04; O, 3.84%. Synthesis of Zn -self-assembled network of 1 with 3 (N3)
Found: C, 73.80; H, 10.63; N, 4.97%. HR ES -TOF MS: calculated
¼
2
+
H
3 2
O
+
ꢀ
N1 (15 mg, 0.005 mmol) in NMP (10 mL) was heated to 105 C.
+
+
m/z: 832.4417 (M ), found: m/z 832.4398 (M ).
2+
Zn acetate dihydrate (2.2 mg, 0.0075 mmol) in NMP (2 mL)
was added dropwise into the reaction mixture. The reaction
Synthesis of trigonal tpy compound (1)
ꢀ
mixture was stirred at 105 C under N
2
for 1 h. 3 (7.2 mg, 0.0075
5
(0.10 mmol, 68 mg), 9 (0.6 mmol, 500 mg) and Pd(PPh
3
)
4
mmol, 1.5 times N1 moles) in NMP (10 mL) was then added
(
0.010 mmol, 12 mg) were put into RBF and degassed using N
2
.
dropwise and the resulting reaction mixture was stirred at
ꢀ
Aqueous K CO (2 M, 20 mL) and toluene (35 mL) were degassed 105 C under N for 24 h. An excess of NH PF was then added
2
3
2
4
6
using N and then injected into the RBF. The reaction mixture into the hot reaction mixture and stirring was continued for 30
2
ꢀ
was then stirred in the dark at 90 C for 72 h and monitored by min. The resulting reaction solution was added slowly into
TLC. The reaction mixture was cooled to RT and toluene was MeOH for precipitation. The precipitate was then ltered off
removed using a rotary evaporator. The resulting reaction and washed with MeOH. Precipitate was again dissolved in
solution was then extracted with CHCl
3
and dried over MgSO
4
.
NMP followed by reprecipitation of the solid by adding ether.
Solvent was removed with rotary evaporator and the pure The precipitate was dried under vacuum for 24 h to give N3 as a
compound was obtained by column chromatography on yellow solid (12 mg, 55%).
alumina eluting with 25% EA in hexane. 1 was obtained as an
1
off-white solid (90 mg, 35%). H NMR (400 MHz, CD Cl ): d 8.85 Spectroscopic characterizations
2
2
(
s, 1H), 8.73 (dd, J ¼ 11.4, 6.1 Hz, 2H), 8.05–7.72 (m, 12H), 7.70–
For the measurements of photoluminescence (PL) spectra, a cw
He–Cd laser emitting at 325 nm was used as the excitation
source and the signals were dispersed by a 750 mm mono-
chromator combined with suitable lters, and detected by a
photomultiplier using the standard lock-in amplier technique.
Time-resolved photoluminescence (TRPL) spectroscopy was
performed using 325 nm laser pulses generated from an optical
parametric amplier parametric amplier (Light Conversion
TOPAS) that was pumped by a 1 kHz regenerative amplier
7
5
0
1
1
5
2
.48 (m, 1H), 7.46–7.32 (m, 3H), 7.17–7.09 (m, 2H), 4.02 (d, J ¼
.8 Hz, 2H), 1.75 (s, 2H), 1.42 (s, 2H), 1.24 (d, J ¼ 23.0 Hz, 16H),
1
3
2 2
.84–0.80 (m, 3H). C NMR (101 MHz, CD Cl ): d 156.76,
56.64, 151.04, 149.77, 130.79, 130.69, 130.60, 128.38, 128.04,
27.45, 127.35, 124.47, 121.71, 119.10, 116.62, 116.46, 70.27,
4.00, 35.21, 32.49, 32.17, 30.23, 30.21, 29.93, 26.75, 25.82,
+
3.23, 14.45, 11.76. HR ES -TOF MS: calculated m/z: 2562.6750
+
+
(
M ), found: m/z 2562.6750 (M ).
(
Coherent Legend; center wavelength: 800 nm; pulse width: 150
2
+
Synthesis of Zn -self-assembled network of 1 (N1)
fs; power: 1 mJ per pulse), which was in turn seeded by an 80
To a solution of 1 (25 mg, 0.010 mmol) in NMP (10 mL), zinc(II) MHz Coherent Vitesse oscillator. The TRPL signals were
acetate dihydrate (4.2 mg, 0.015 mmol) in NMP (2 mL) was collected in a typical backscattering geometry by a pair of lenses
ꢁ
1
added dropwise. The resulting solution was then stirred at 105 to a DK240 1/4 m monochromator with 300 g mm grating,
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and the temporal evolution of the PL was resolved by an
Optronis Optoscope streak camera system. The streak camera
system has an ultimate temporal resolution of ꢃ10 ps when
operated at the shortest time window of 330 ps.
6 T. Chen, G.-B. Pan, H. Wettach, M. Fritzsche, S. H ¨o ger,
L.-J. Wan, H.-B. Yang, B. H. Northrop and P. J. Stang, J. Am.
Chem. Soc., 2010, 132, 1328–1333.
7 R. Chakrabarty, P. S. Mukherjee and P. J. Stang, Chem. Rev.,
2011, 111, 6810–6918.
8
9
T. Chen, G.-B. Pan, H. Wettach, M. Fritzsche, S. H ¨o ger,
L.-J. Wan, H.-B. Yang, B. H. Northrop and P. J. Stang, J. Am.
Chem. Soc., 2010, 132, 1328–1333.
O. Ivasenko, J. M. MacLeod, K. Y. Chernichenko,
E. S. Balenkova, R. V. Shpanchenko, V. G. Nenajdenko,
F. Rosei and D. F. Perepichka, Chem. Commun., 2009, 1192.
0 M. Ruben, U. Ziener, J. M. Lehn, V. Ksenofontov, P. G u¨ tlich
and G. Vaughan, Chem.–Eur. J., 2004, 11, 94–100.
1 T. Bauer, A. Schl u¨ ter and J. Sakamoto, Synlett, 2010, 877–880.
2 Z. B. Lim, H. Li, S. Sun, J. Y. Lek, A. Trewin, Y. M. Lam and
A. C. Grimsdale, J. Mater. Chem., 2012, 22, 6218.
Conclusions
2
+
Synthesis and characterization of various Zn terpyridine 2D
self-assembled networks were described in this work. Their
thermal and photophysical properties were studied in detail
and compared to the constituent chelating terpyridine ligands.
These 2D co-ordination networks are found to be thermally
stable as compared to their starting materials, especially at very
high temperature due to the effect of zinc metal ion co-
ordination.
1
1
1
2
+
2
D Zn supramolecular networks were compared to the 3D
2
+
2+ 13 Y. Ohba, K. Kanaizuka, M. Murata and H. Nishihara,
Macromol. Symp., 2006, 235, 31–38.
Zn networks in order to understand the inuence of the Zn
ions on the photophysical properties of the designed structures.
The synthesized 2D Zn co-ordination networks revealed
marked red shis in the emission spectra like the 3D Zn
networks made up of similar building blocks. It was observed
that the presence of the Zn ions in the synthesized model
complexes revealed interesting spectroscopic properties, in
particular, enhanced uorescence and uorescence life time.
Thus, from the experimental results it was concluded that the
characteristics of the basic building units are considerably
inuencing the photophysical properties of the Zn co-ordi-
nation networks. Such Zn systems could be potentially inter-
esting for the construction of optoelectronic devices.
2
+
14 L. J. Liang, X. J. Zhao and C. Z. Huang, Analyst, 2012, 137,
2
+
953.
1
1
1
5 X. Zhou, X. Jin, D. Li and X. Wu, Chem. Commun., 2011, 47,
3921.
6 X. Chen, Q. Zhou, Y. Cheng, Y. Geng, D. Ma, Z. Xie and
L. Wang, J. Lumin., 2007, 126, 81–90.
7 S.-H. Hwang, C. N. Mooreeld, P. Wang, J.-Y. Kim, S.-W. Lee
and G. R. Newkome, Inorg. Chim. Acta, 2007, 360, 1780–1784.
2
+
2
+
18 A. Winter, C. Friebe, M. Chiper, M. D. Hager and
2
+
U. S. Schubert, J. Polym. Sci., Part A: Polym. Chem., 2009,
47, 4083–4098.
1
9 T. He, D. Rajwar, L. Ma, Y. Wang, Z. B. Lim, A. C. Grimsdale
and H. Sun, Appl. Phys. Lett., 2012, 101, 213302.
Acknowledgements
20 D. W. Fink and W. E. Ohnesorge, J. Phys. Chem., 1970, 74, 72–
7.
7
We acknowledge the nancial support, including the award of a
Research Student Scholarship to D.R., from the Singapore
Ministry of Education through the Academic Research Fund
Tier 1 (SUG 40/06 and RG19/07) and also from the National
Research Foundation through Competitive Research Project 5-
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