Synthesis of a water-soluble iridium(III) complex with pH and metal cation sensitive photoluminescence

Article abstract of DOI:10.1246/cl.2006.720

A water-soluble iridium complex with bulky ligands was synthesized. Photoluminescence intensities of this iridium complex were stronger in basic aqueous solutions than that in organic solvents, and they were found to depend on pH and additive metal ions i

Full text of DOI:10.1246/cl.2006.720

720
Chemistry Letters Vol.35, No.7 (2006)
Synthesis of a Water-soluble Iridium(III) Complex
with pH and Metal Cation Sensitive Photoluminescence
Koji Konishi, Hiroyasu Yamaguchi, and Akira Haradaꢀ
Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043
(Received February 9, 2006; CL-060173; E-mail: harada@chem.sci.osaka-u.ac.jp)
A water-soluble iridium complex with bulky ligands was
Complex 1 has three-dimensional blocking ligands around
the iridium center. Therefore, photoluminescence quenching
by self-assembling of these complexes can be inhibited. The sol-
ubility of this complex to water should be improved by introduc-
ing carboxyl groups to the ligands.
synthesized. Photoluminescence intensities of this iridium com-
plex were stronger in basic aqueous solutions than that in organic
solvents, and they were found to depend on pH and additive
metal ions in its aqueous solution.
The UV–vis absorption spectrum of 1 can be divided into
two regions; the metal-to-ligand charge-transfer (MLCT) band
as shoulder peaks over 330 nm and the ꢀ–ꢀꢀ transition of the
bulky ligands at 270 nm. The difference of UV–vis spectra
was not observed in basic aqueous solutions and DMF.
The photoluminescence intensity of the complex 1 was
monitored in DMF or basic aqueous solution (Figure 1). We
found that the photoluminescence intensities were stronger in
Iridium complexes have recently received much attention as
photoluminescent materials1 because of their long luminescence
lifetime and strong phosphorescence intensity in organic sol-
vents and on films. Such complexes with various kinds of li-
gands2 and dendritic structures3 have been designed and report-
ed. However, there are few examples of iridium complexes
which can emit light efficiently at high brightness in water.4
When the iridium complexes can be dissolved in water, they
have a potential to be used as a photoluminescent probe. We
report here synthesis of a new water-soluble iridium complex
and its photoluminescence property.
The water-soluble iridium complex with 30,50-di(4-carbox-
ylphenyl)-3-(2-pyridyl)biphenyl-4-yl ligands (1) was synthesized
as shown Scheme 1. The ligand molecule was synthesized by
Suzuki coupling reaction.5 We obtained the methyl ester deriv-
ative of the iridium complex by the reaction of the ligand with
iridium chloride trihydrate. Finally, the methyl ester groups
in the derivative were hydrolyzed to give the water-soluble
iridium complex 1. This complex was characterized by 1H NMR,
13C NMR, FT-IR, MALDI-TOF MS, and elemental analysis.
The details for the synthetic procedures of this complex were
described in Supporting Information. This synthetic procedure
could lead to facial and meridional isomers of complex 1. The
product was found to be the racemic mixture (Figure S1).
1.2
basic aqueous
1
0.8
0.6
0.4
0.2
0
solution
DMF
450
500
550
600
650
700
Wavelength /nm
Figure 1. Solvent dependence of 1 on photoluminescence spec-
tra in basic aqueous solution (pH ¼ 10) and DMF. PL intensities
were normalized by absorbance 0.31 at 360 nm ([1] = 1:0 ꢂ
10ꢃ5 M).
N
O
O
MeO
MeO
Br
(A)
(B)
Br
(B)
O
O
O
Br
B
MeO
O
MeO
Br
Br
O
O B
O
N
OMe
O
OMe
N
N
Ir
3
Ir
KOH/H2O
(C)
X
Y
O
O
X =
Y =
X
Y
OMe
OH
3
1
Scheme 1. (A) PdCl2(dppf) (dppf = 1,10-bis(diphenylphosphino)ferrocene), KOAc, bis(pinacolato)diboron, DMSO, 80 ꢁC, Ar. (B)
Tetrakis(triphenylphosphine)palladium(0), aqueous sodium carbonate, ethanol, toluene, reflux, Ar. (C) Iridium(III) chloride trihydrate,
water, 2-ethoxyethanol, heat then silver trifluoromethanesulfonate, 130 ꢁC, Ar.
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.7 (2006)
721
1.2
1
1.2
1
None
Na+
0.8
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
0
Li+, K+, Rb+, Cs+
Mg2+
Cu2+, Hg2+
7
8
9
10
11
12
450
500
550
600
650
700
pH
Wavelength /nm
Figure 2. Correlation between pH and photoluminescence
intensities of 1 in 0.13 M phosphate buffer [KH2PO4(aq)/Na2-
HPO4(aq), pH 7–9] and 0.13 M carbonate buffer [Na2CO3(aq)/
NaHCO3(aq), pH 9–11] ([1] = 1:0 ꢂ 10ꢃ5 M). PL intensities
were respectively normalized by absorbance at 360 nm.
Figure 3. Changes of photoluminescence intensities of 1 by
adding various metal chlorides in 0.13 M carbonate buffer
[Na2CO3(aq)/NaHCO3(aq)] including 0.27 M metal ion ([1] =
1:0 ꢂ 10ꢃ5 M). PL intensities were normalized by absorbance
0.31 at 360 nm. In these measurements, pH was constant at 10.
basic aqueous solution (ꢀ ¼ 0:11) than that in DMF (ꢀ ¼
0:028).6 The photoluminescence lifetime of complex 1 was
0.82 ms in basic aqueous solution. It was found to be 4.5 times
longer than that in DMF solution (0.18 ms). These results indi-
cate that the chromophores are isolated each other by electrostat-
ic repulsion due to the dissociation of protons from carboxylic
acids in the ligands at higher pH.
EcoChemistry’’ of Osaka University. The present work is par-
tially supported by the Grant-in-Aid for Scientific Research
(KAKENHI) in Priority Area ‘‘Molecular Nano Dynamics’’ from
Ministry of Education, Culture, Sports, Science and Technology.
References and Notes
1
2
C. W. Tang, S. A. Van Slyke, Appl. Phys. Lett. 1987, 51, 913.
a) S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq,
R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau, M. E.
Thompson, Inorg. Chem. 2001, 40, 1704. b) S. Lamansky,
P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C.
Adachi, P. E. Burrows, S. R. Forrest, M. E. Thompson,
J. Am. Chem. Soc. 2001, 123, 4304. c) A. B. Tamayo, B.
D. Alleyne, P. I. Djurovich, S. Lamansky, I. Tsyba, N. N.
Ho, R. Bau, M. E. Thompson, J. Am. Chem. Soc. 2003,
125, 7377.
The pH dependence of 1 on photoluminescence spectra was
studied. The correlation between pH and the photoluminescence
intensity of 1 was shown in Figure 2. The emission intensity
increased with an increase in pH. The drastic change of the in-
tensity occurred around pH 9. It was suggested that the change
of the emission intensity of complex 1 was in accord with the
degree of ionization of carboxylic acid in the ligand (Figure S2).
Photoluminescence spectra of 1 in the presence of various
kinds of metal chlorides were measured. As shown in Figure 3,
the photoluminescence intensity of 1 was quenched by adding
metal ions. The degrees of quenching by the addition of divalent
metal cations (Mg2þ, Cu2þ, and Hg2þ) were larger than that of
monovalent alkali metal cations (Liþ, Naþ, Kþ, Rbþ, and
Csþ). In the case of monovalent metal ions, it was suggested that
the photoluminescence of 1 was quenched by the exchanges
between protons of carboxylic acids and metal ions. While, the
chelate formation could be considered in the case of divalent
metal ions to give a larger photoluminescence quenching.
In conclusion, a water-soluble iridium complex was synthe-
sized, and its photophysical properties were investigated. We
found that photoluminescence intensities of the iridium complex
1 were stronger in basic aqueous solution than that in organic
solvents, and they depend on pH and additive metal ions in
aqueous solutions. The luminescent probe based on 1 can be ap-
plied for sensing toxic heavy metal ions such as lead, cadmium,
and mercury.
3
4
a) S.-C. Lo, E. B. Namdas, P. L. Burn, I. D. W. Samuel,
Macromolecules 2003, 36, 9721. b) E. B. Namdas, A.
Ruseckas, I. D. W. Samuel, S.-C. Lo, P. L. Burn, J. Phys.
Chem. B 2004, 108, 1570.
a) M. Licini, J. A. G. Williams, Chem. Commun. 1999, 1943.
b) M.-L. Ho, F.-M. Hwang, P.-N. Chen, Y.-H. Hu, Y.-M.
Cheng, K.-S. Chen, G.-H. Lee, Y. Chi, P.-T. Chou, Org.
Biomol. Chem. 2006, 4, 98.
5
6
T. Ishiyama, M. Murata, N. Miyaura, J. Org. Chem. 1995,
60, 7508.
Photoluminescence quantum yields were measured by a
relative method using quinine sulfate in 0.1 M sulfuric acid,
which has a photoluminescence quantum yield of 0.55,
as a standard. The quantum yield (ꢀIr) of the iridium
complex was calculated by the following equation: ꢀIr ¼
2
ꢀstdðAstdPIr=AIrPstdÞðnDIr=nDstÞ , where Astd and AIr are the
absorbance of the standard and the iridium complex, respec-
tively. Pstd and PIr are the corresponding relative integrated
photoluminescence intensities, and n is the refractive index
of the solvent.
The authors express their special thanks for the Center
of Excellence (21COE) program ‘‘Creation of Integrated
Published on the web (Advance View) June 3, 2006; doi:10.1246/cl.2006.720