A luminescent coordination polymer based on bisterpyridyl ligand containing o-carborane: Two tunable emission modes

Article abstract of DOI:10.1039/c1dt90011k

We designed a novel ditopic bisterpyridyl ligand containing o-carborane that can construct a coordination polymer by complexation with metal ions. Through the use of Sonogashira-Hagihara coupling, the desired ligand molecule was successfully synthesized.

Full text of DOI:10.1039/c1dt90011k

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PAPER
A luminescent coordination polymer based on bisterpyridyl ligand containing
o-carborane: two tunable emission modes
Kenta Kokado and Yoshiki Chujo*
Received 9th August 2010, Accepted 6th December 2010
DOI: 10.1039/c1dt90011k
We designed a novel ditopic bisterpyridyl ligand containing o-carborane that can construct a
coordination polymer by complexation with metal ions. Through the use of Sonogashira–Hagihara
coupling, the desired ligand molecule was successfully synthesized. Addition of Zn(II) ions rapidly
underwent the generation of a fluorescent coordination polymer, which was confirmed by 1H NMR,
UV-vis, and fluorescent titration experiments. Furthermore, the electronic structure of the bisterpyridyl
ligand molecules was drastically changed upon the complexation with metal ions. The obtained
coordination polymer showed light blue emission derived from the intraligand charge transfer (ILCT)
state, whereas a bare ligand molecule exhibited yellowish-green aggregation-induced emission (AIE) in
a poor solvent such as water, because of the variable C–C bond in o-carborane.
formation.6 Therein, excitation of electrons of the highest occupied
molecular orbital (HOMO) lying on p-phenylene-ethynylene moi-
eties resulted in intramolecular charge transfer to the antibonding
Introduction
Coordination polymers have gained significant attention in the
past decade as intelligent materials, since various combinations of
orbital of the C–C bond in the o-carborane cluster and subsequent
concentration and/or metal–ligand interaction allow us to control
fluorescence quenching by the variable C–C bond in the solution
the condition of self-assemblies.1 A large number of studies have
been reported on the construction of supramolecular architectures
utilizing metal–ligand coordination, i.e., kinetically labile but
thermally stable bonds.2 Among a variety of ligands employed for
syntheses of coordinative polymers, 2,2¢:6¢,2¢¢-terpyridine is one of
the most promising chelating ligands because of the extraordinary
binding affinity toward most transition metal ions due to dp →
pp* bonding, together with the strong chelating effect.3 Further-
more, the introduction of the terpyridine unit into p-conjugated
molecules and successive complexation with particular metal
ions (e.g. Zn(II)) lead to prominent photophysical properties.4
For instance, the emission color of terpyridine substituted oligo-
phenylenevinylene (OPV) derivatives bathochromically shifted
upon the coordination to Zn(II), indicating the conversion to
intraligand charge transfer (ILCT) state between the coordination
site and chromophore.5 On the basis of this finding, the com-
plexation of the terpyridine unit incorporated in p-conjugated
molecules would easily enable manipulating the emission colors
from luminescence of the chromophore to that of the ILCT state.
Recently, we have reported the alternating polymers
with o-carborane (1,2-dicarbadodecaborane) and p-phenylene-
ethynylene sequences, and the electron-withdrawing nature of
o-carborane successfully provided unique aggregation-induced
emission (AIE) in water dispersion; i.e., non-emissive species in the
dissolved state are induced to emit intensely by aggregate or film
state.7 This fact prompted us to explore color tuning of emission
by complexation of bisterpyridyl ligand containing o-carborane;
i.e., bisterpyridyl ligand exhibiting AIE in a poor solvent should
switch its emission mode to ILCT state by the complexation with
metal ions (Fig. 1). In this article, we describe here the synthesis
of a novel ditopic terpyridyl ligand linked by o-carborane moiety,
and optical properties of the coordination polymers obtained by
chelating bisterpyridyl ligand with Zn(II) ion.
Fig.
1
Formation of polymeric structure by the addition of
Zn(OAc)2·2H2O to TPYOCB.
Experimental
Materials
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto
University, Katsura, Nishikyo-ku, Kyoto, 615-8510, Japan. E-mail: chujo@
chujo.synchem.kyoto-u.ac.jp; Fax: +81-75-383-2605; Tel: +81-75-383-2604
All synthetic procedures were performed under argon atmo-
sphere. Unless stated otherwise, all reagents were obtained from
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commercial sources and used without further purification.
Tetrahydrofuran (THF) and triethylamine were purified using a
two-column solid-state purification system (Glass Contour Sol-
vent System, Joerg Meyer, Irvine, CA). 1,2-Bis(4-iodophenyl)-o-
carborane6a and 4¢-(4-ethynylphenyl)-2,2¢:6¢,2¢¢-terpyridine8 were
synthesized and characterized according to the literature.
every addition, UV-vis and fluorescence spectra were recorded
after 5 min (lex = 315 nm).
1H NMR titration experiments
To obtain the desired metal-to-ligand ratio in the mixed solvent of
CDCl3–CD3OD (60 : 40), a solution of Zn(OAc)2·2H2O in CDCl3–
CD3OD (60 : 40, 12.5 mM) was added to a solution of TPYOCB in
CDCl3–CD3OD (60 : 40, 8.3 mM). After each addition, 1H NMR
spectra were recorded at 25 ◦C.
Measurements
1H (400 MHz), 13C (100 MHz) and 11B (128 MHz) NMR
measurements were recorded on a JEOL JNM-EX400 instrument.
1H and 13C NMR spectra used 0.05% tetramethylsilane (TMS)
as an internal standard and 11B NMR spectra were referenced
externally to BF3·Et2O at room temperature. UV-vis spectra
were recorded on a Shimadzu UV-3600 spectrophotometer at
room temperature. Fluorescence emission spectra and absolute
quantum yield by integrating sphere method were recorded on
a HORIBA JOBAN YVON Fluoromax-4 spectrofluorometer.
Dynamic light scattering (DLS) measurements were performed
by an Otsuka FPR-1000 system. Cyclic voltammetry measurement
was conducted with a BAS CV-50W Electrochemical Analyzer.
Results and discussions
Synthesis of TPYOCB
We chose bis(4-phenylethynylphenyl)-o-carborane as an AIE unit
and a core p-conjugated system, because the unit was proven
to exhibit AIE property with relatively high quantum yield in
our previous study.6a To obtain the molecules having ILCT
property, the terpyridine ring should be directly attached to the
p-conjugated system for electronic communications between o-
carborane and terpyridine. Therefore, the ditopic ligand TPYOCB
was synthesized via Sonogashira–Hagihara coupling reaction
between bis(4-iodophenyl)-o-carborane and 4¢-(4-ethynylphenyl)-
2,2¢:6¢,2¢¢-terpyridine (Scheme 1), which were prepared by litera-
ture method,6a,8 in excellent yield (85%). TPYOCB was soluble in
common organic solvents, such as THF, chloroform, DMF, and so
Synthesis
Synthesis of TPYOCB. 1,2-Bis(4-iodophenyl)-o-carborane
(548 mg, 1.00 mmol), 4¢-(4-ethynylphenyl)-2,2¢:6¢,2¢¢-terpyridine
(733 mg, 2.20 mmol), Pd(PPh3)4 (58 mg, 50 mmol), and CuI
(2.40 mg, 50 mmol) were dissolved in anhydrous THF (10 mL)
and triethylamine (5 mL), and the mixture was deaerated. After
the mixture was refluxed for 24 h, CHCl3 (50 mL) was added,
and washed with aqueous NH3 solution (10%, 100 mL), distilled
water (100 mL ¥ 2), and brine (100 mL ¥ 2). The organic layer
was dried over MgSO4, filtered, and evaporated. The residue
was chromatographed on alumina (eluent: CHCl3/triethylamine =
90/10) to afford TPYOCB as a white powder (845 mg, 85%). 1H
NMR (400 MHz, CDCl3): d (ppm) 8.73 (s, 4H), 8.72 (d, 4H, J =
4.63 Hz), 8.66 (d, 4H, J = 7.80 Hz), 7.89 (d, 4H, J = 8.28 Hz), 7.87
(td, 4H, J = 7.74, 1.87 Hz), 7.62 (d, 4H, J = 8.28 Hz), 7.44 (d, 4H,
J = 8.77 Hz), 7.35 (d, 4H, J = 8.77 Hz), 7.35 (ddd, 4H, J = 7.49,
4.69, 1.16 Hz), 3.70–1.66 (m, 10H). 13C NMR (100 MHz, CDCl3):
d (ppm) 156.1, 149.2, 149.1, 138.7, 136.9, 132.2, 131.5, 130.6,
on. The structure of TPYOCB was characterized by 1H NMR, 13
C
NMR, and 11B NMR spectroscopies. In the 1H NMR spectra, the
broadening peaks observed at 3.70–1.60 ppm were assigned to the
presence of the o-carborane structure, and the sharp peaks in the
aromatic region were attributed to signals of the protons on the ter-
pyridine ring and the phenyl group on o-carborane and terpyridine
(vide infra). The 11B NMR spectrum also showed the broad peaks
at around -2 to -11 ppm assignable to the borons of o-carborane
cluster. These results suggest that the efficient palladium-coupling
reaction was performed with the monomer bis(4-iodophenyl)-o-
carborane and 4¢-(4-ethynylphenyl)-2,2¢:6¢,2¢¢-terpyridine, in other
words, the ditopic ligand TPYOCB with anticipated structure was
successfully obtained.
130.3,127.3, 125.5, 123.9, 123.2, 121.3, 118.6, 91.8, 89.2, 84.8. 11
B
NMR (128 MHz, CDCl3): d (ppm) -2.8, -10.1. HRMS (FAB)
calcd for C60H46N6B10: m/z 960.4715, found: m/z 960.4742%.
Synthesis of Zn complex. To a solution of ditopic ligand
TPYOCB (24 mg, 25 mmol) in CHCl3 (10 mL), a methanol solution
of Zn(OAc)2·2H2O (10 mM, 2.5 mL, 25 mmol) was added and the
solution was stirred for 1 h at room temperature. After the reaction,
excess NH4PF6 was added into the solution. The resulting solution
was poured into ether, and the precipitate was collected as light
yellow solids. Yields: 83%.
Scheme 1 Synthesis of novel ditopic ligand TPYOCB.
Metal ion complexation of TPYOCB
The ditopic ligand TPYOCB underwent complexation with
Zn(II) ion (Scheme 2). The complexation of TPYOCB resulted
in characteristic signal changes in the 1H NMR spectra as
shown in Fig. 2. The signals of the terpyridyl unit shifted to
lower field on Zn(II) complex formation, whereas those of the
bis(4-phenylethynylphenyl)-o-carborane moiety were essentially
UV-vis and fluorescence titration experiments
UV-vis and fluorescence titrations were performed at 25 ◦C as
usual host titrations by dissolving TPYOCB (1.0 ¥ 10-5 M) in
10 mL of CHCl3–CH3OH (60 : 40) and adding a 5.0 ¥ 10-4
M
solution of Zn(OAc)2·2H2O in CH3OH in steps of 10 mL. After
1920 | Dalton Trans., 2011, 40, 1919–1923
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UV-vis and fluorescence spectroscopies
To further characterize the complexation of TPYOCB with Zn(II)
ion, UV-vis and fluorescent titration experiments were also carried
out to confirm the supramolecular structure. The absorption max-
ima of TPYOCB (314 nm) bathochromically shifted compared
to that of bis(4-phenylethynylphenyl)-o-carborane (293 nm),6a
indicating the extension of electron delocalization by the direct
attachment of the terpyridine ring to p-conjugated system. Upon
addition of Zn(II) to TPYOCB in a mixed solvent of chloroform
and methanol (60 : 40) as shown in Fig. 3(a), the increase in
the absorption peak at 334 nm was observed, while that at
314 nm was decreased, with its isosbestic point at 323 nm. The
enhancement of absorption maximum at 334 nm is attributable to
terpyridine and Zn(II) coordinative interactions, and the change
in absorption spectra rapidly leveled off above 1.0 equivalent of
Zn(II) as shown in the inset in Fig. 3(a). Zn(II) titration with
TPYOCB shows a gradual increase in the emission intensity with
25 nm red shift of the emission maxima (Fig. 3(b), 440 nm →
465 nm). The absolute fluorescence quantum yield was 110 times
improved (UF = 0.0016 → 0.18) upon Zn(II) addition (Fig. 3(b)
inset), and it also showed plateau with more than 1.0 equivalent
of Zn (II). The equilibrium constant (K1) for the first binding
event is 1.5 ¥ 106 M-1, determined by the following emission
changes at peak wavelength. These facts obviously indicate that
the formation of terpyridine–Zn(II) complex is responsible for
“turn-on” fluorescence derived from the ILCT state. On the
other hand, bare TPYOCB exhibits AIE property because of
the distribution of the C–C antibonding orbital of o-carborane.
Fig. 3(c) (red line) shows apparent AIE behavior of TPYOCB,
with its emission maximum at 509 nm, caused by decrease of
solubility and aggregate formation. In the mixed solvent of
Scheme 2 Zn(II) ion complexation of TPYOCB.
Fig. 2 1H NMR spectra at different ligand/metal ion ratios for the ditopic
ligand TPYOCB and Zn(OAc)2·2H2O in CDCl3–CD3OD (60 : 40, 5 mM
of TPYOCB).
identical. For example, Zn(II) addition induced gradual downfield
shift of signals of the H3, H3¢, H4, and H5 position of the
terpyridine ring, and signals of phenyl rings bonded to o-carborane
(7.33 and 7.44 ppm) or 4¢ position of terpyridine (7.61 and
7.87 ppm) stayed at around the same position during the titration.
Upon addition of Zn(II), the 1H NMR signals became very broad,
indicating clear signs of polymerization. The protons on the
terpyridine unit showed greater broadening of signals with more
Zn(II) addition, while those on the farther aromatic ring from
the terpyridine unit exhibited sharper signals. The addition of 1.0
equivalent of Zn(II) should generate the longest polymer chain,
and this expectation is apparently reflected in the broadness of
the signals. The signals were sharpened by the addition of 2.0
equivalents of Zn(II), displaying a predominance of 1 : 1 open-
form complex.
TPYOCB and its Zn(II) complex exhibited reversible reduction
peaks around -1.64 and -2.20, respectively, in cathodic scans
up to -3.5 V. These peaks are attributed to the reduction of
terpyridine-based moieties. As a result of complexation, there
was a decline in reduction potential at about 0.6 V, and the
reduction potential of the complex was quite similar to the
previous reports.3f ,4 The oxidation peak in anodic scan up to 1.0 V
of the complex was absent, because of the difficulty in oxidation
of d10 Zn ion species. According to the reported equation,9 the
lowest unoccupied molecular orbital (LUMO) energy levels of
TPYOCB and the Zn complex were estimated from the onset
reduction potentials as -2.40 and -2.96 eV, respectively. These
findings indicate that the bisterpyridyl ligand TPYOCB underwent
effective complexation with Zn(II) ion, i.e., consequent formation
of expected coordination polymer, and the electronic structure of
TPYOCB drastically changed upon the complexation with Zn(II).
Fig. 3 (a) UV-vis and (b) fluorescence spectra obtained by the titra-
tion of TPYOCB in CHCl3–CH3OH (60 : 40; 1.0 ¥ 10-5 M) with
Zn(OAc)2·2H2O. The inset shows (a) the molar extinction coefficient
at 334 nm and (b) fluorescence quantum yield as a function of
Zn(II)/TPYOCB ratio. (c) Fluorescence spectra of TPYOCB (red) and
the Zn complex (black) in CHCl3–CH3OH (60 : 40; 1.0 ¥ 10-5 M, bold
line) and mixed solvent of THF–H2O (1 : 99, 1 ¥ 10-5 M, thin line), (d)
CIE (Commission Internationale de L’Eclairage) 1931 (x,y) chromaticity
diagram of TPYOCB and the Zn complex from the fluorescence spectra.
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THF–H2O (1 : 99, poor solvent for TPYOCB), UF rises to 0.29,
which is 150 times higher than that of bare TPYOCB in CHCl3–
CH3OH (60 : 40, good solvent for TPYOCB, UF = 0.0016), and
into the origin of the electronic transitions for the ligand and
complex. The calculated absorption wavelength of TPYOCB with
oscillator strength maxima f max was 329.24 nm (f max = 1.7123)
which is corresponding to the transition from HOMO to LUMO +
1. In the case of the Zn complex, since HOMO to HOMO - 9 are
mainly occupied by d electrons of Zn(II) ion whose transition
to p orbital on the ligand is almost forbidden (f ª 0), the
wavelength with oscillator strength maxima f max was 344.38 nm
(f max = 1.2784) corresponding to the transition from HOMO - 10
to LUMO + 1 (Fig. 4). These results support the observation in
optical measurements described above, and clearly suggest that
the difference in the direction of intramolecular charge transfer
between TPYOCB and the Zn complex is responsible for the two
tunable emission modes, as predicted in Fig. 1.
the size of the aggregates reached 46.7
3.6 nm, measured
by dynamic light scattering (DLS). Interestingly, though bare
TPYOCB exhibits almost no fluorescence in common organic
solvents, there are two distinct ways to enhance the emission,
i.e., complexation with Zn(II) ion (ILCT) or aggregate formation
(AIE). Each method efficiently augmented the fluorescence inten-
sity, whereas the final emission color significantly differs from one
another. The obtained emission maxima and CIE coordinates (x,y)
were 465 nm and (0.28,0.34) for ILCT, and 509 nm and (0.43,0.45)
for AIE, respectively (Fig. 3(d)). These data can point out that two
tunable emissionmodes individuallygenerate two explicit emission
colors thanks to the AIE molecule with intramolecular charge
transfer profile derived from the electron-withdrawing nature of
o-carborane.
Conclusions
In summary, a novel ditopic bisterpyridyl ligand containing o-
carborane was successfully synthesized, and it efficiently un-
derwent complexation with metal ions, thus the construction
DFT calculations
In order to provide effective understanding for their emission-
switching property between ILCT and AIE, the electronic states of
TPYOCB and its Zn(II) (2 : 1) complex ([Zn2(TPYOCB)](OAc)2)
were investigated by theoretical calculations performed with
the Gaussian 03 suit program using density-functional theory
(DFT) method at the B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p)
level.10 The lowest unoccupied molecular orbital (LUMO) of
TPYOCB is mainly located on the phenylene-ethynylene unit with
distribution of electron in the o-carborane cluster, while that of
the Zn complex has more distribution in the terpyridine unit
instead of o-carborane due to the electron-withdrawing character
of terpyridine as a result of complexation. Additionally, the
calculated LUMO level of TPYOCB and the Zn complex was
-2.12 eV and -2.77 eV, respectively (Fig. 4), which showed good
correlation with the electrochemical data (-2.40 eV and -2.96
eV, respectively). Next, time-dependent (TD) SCF calculations
using DFT method were carried out to provide further insight
1
of supramolecular structure. H NMR, UV-vis, and fluorescent
titration data clearly display smooth complexation of the obtained
bisterpyridine molecule with Zn(II) ions. Furthermore, a bare
ligand molecule exhibited aggregation-induced emission (AIE) in
a poor solvent such as water, and coordination to metal ions caused
switching of the emission mode to the intraligand charge transfer
(ILCT) state, which was also supported by DFT calculations. As
a result, the emission color was drastically turned from green-
yellow to light blue due to the change of emission modes upon
the complexation. These two convertible emission modes and
the reversible coordination chemistry may find utility in sensory
materials or stimuli-responsive fluorophores.
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This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 1919–1923 | 1923
©