Rapid synthesis of new emitting Ir(III) polypyridine complexes

Article abstract of DOI:10.1246/cl.2000.1206

A microwave-assisted synthesis of photosensitive iridium(III) polypyridine complexes has been developed. The spectroscopic and electrochemical properties of the newly prepared complexes were studied. These complexes were characterized by intense phosphorescence emission from 505 nm to 630 nm.

Full text of DOI:10.1246/cl.2000.1206

1206
Chemistry Letters 2000
Rapid Synthesis of New Emitting Ir(III) Polypyridine Complexes
Naokazu Yoshikawa,Yoshitaka Masuda, and Takeko Matsumura-Inoue*
Department of Chemistry, Nara University of Education, Takabatake-cho, Nara 630-8528
(Received July 31, 2000; CL-000722)
A microwave-assisted synthesis of photosensitive iridium(III)
polypyridine complexes has been developed. The spectroscopic
and electrochemical properties of the newly prepared complexes
were studied. These complexes were characterized by intense
phosphorescence emission from 505 nm to 630 nm.
Transition metal complexes with polypyridine ligands have
attracted much attention for their photoinitiated energy transfer
and redox properties.1–3 In particular, iridium polypyridine
complexes have been the subject of recent studies.3–5 However,
the synthesis of iridium polypyridine complexes is time con-
suming and laborious for purification and thus the yield is very
low. Here, we report a rapid synthesis of Ir(III) complexes by
microwave irradiation and the spectroscopic and electrochemi-
cal properties of the newly prepared complexes. The synthetic
procedures are given in Scheme 1.
A typical procedure of the preparation of polypyridine
complexes is as follows. The precursor (NH4)3[IrCl6]·H2O
(0.238 g, 0.5 mmol) and 2,2'-bipyridine (0.234 g, 1.5 mmol)
were suspended in ethylene glycol (15 mL). Under purging
nitrogen atmosphere, the suspension was subjected to reflux for
15 min in the microwave oven6 (RR-12AF, 500 W, Mitsubisi
Denki co. Ltd) with a frequency of 2450 MHz activates the OH
group of ethylene glycol, and then cooled to room temperature.
The saturated aqueous solution of KPF6 (20 mL) was added for
precipitation. The precipitate was filtrated and recrystallized
from acetonitrile / diethyl ether to afford [Ir(bpy)3](PF6)3 in a
yield of 84%. The other complexes were also prepared by the
procedures similar to those for [Ir(bpy)3](PF6)3.7 The abbrevia-
tions of these complexes and ligands are as follows:
[Ir(bpy)3](PF6)3 : (1A), 2,2'-bipyridine: bpy, [Ir(dmbpy)3](PF6)3
: (2A), 4,4'-dimetyl-2,2'-bipyridine: dmbpy, [Ir(dpbpy)3](PF6)3 :
(3A), 4,4'-diphenyl-2,2'-bipyridine: dpbpy, [Ir(phen)3](PF6)3 :
(4A), 1,10-phenanthroline: phen, [Ir (dpphen)3](PF6)3 : (5A),
4,7-diphenyl-1,10-phenanthroline: dpphen, [Ir(bqn)3](PF6)3 :
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1207
(6A), 2,2'-biquinoline: bqn.
The spectroscopic and electrochemical properties of the
newly synthesized complexes are summarized in Table 1. The
absorption spectra in acetonitrile give the intensive absorption
bands below 300 nm (ε in the range 104–105 M–1cm–1), which
are assigned to π–π* transition of polypyridine ligands in the
complexes, and the shoulder at 378–465 nm, which corresponds
to a mixed π–π* and metal-to-ligand charge transfer transition
(MLCT).8, 9
The complexes exhibit an intense luminescence (excitation
at 318 nm) around 505–630 nm in acetonitrile solution at room
temperature, which can be assigned to a triplet π–π* emission
3+
(Table 1).10 In particular, the emission intensity of Ir(dpbpy)32+
is more than one hundred times as large as that for Ru(bpy)3
(excitation at 318 nm). The range of π–π* absorption band
energy is 2.67 eV to 3.28 eV, while that of emission band ener-
gy is 1.97 eV to 2.36 eV. The complex Ir(bqn)33+ with elec-
tron-withdrawing ligands shows a phosphorescence emission
and a large red-shift to 630 nm. It is noticed that Ir polypyri-
dine complex shows intense emission with various colors, such
as yellow for Ir(dpphen)33+, red for Ir(bqn)33+, which can be
applied to photosensitizers.
Cyclic voltammograms of these complexes show three or
four reduction waves corresponding to the reduction of
polypyridine ligands in the complexes from 0 to –2.0 V. The
reduction and oxidation potentials of iridium(III) bis-bipyridine
complexes are nearly equal to those of iridium(III) tris-bipyri-
dine complexes. Thus, these trends are different from those of
Ru polypyridine complexes.11
References and Notes
1
2
3
4
P. I. Djurovich and R. J. Watt, Inorg. Chem., 32, 4681
(1993).
I. Ortmans, P. Didier, and A. Kirsch-De Mesmaeker, Inorg.
Chem., 34, 3695 (1995).
G. Calgero, G. Giuffrida, S. Serroni, V. Ricevuto, and S.
Campagna, Inorg. Chem., 34, 541 (1995).
A. Mamo, I. Stefio, M. P. Parisi, A. Credi, M. Venturi, C.
DiPietro, and S. Campagana, Inorg. Chem., 36, 5947
(1997).
The reduction and oxidation potentials are changed by the
electronic donor or acceptor properties of ligands.12
Ir(dmbpy)33+, a complex with an electron-donating ligand, give
smost negative reduction potential among the complexes listed
in Table 1. On the other hand, Ir(bqn)33+, a complex with an
electron-withdrawing ligand exhibits most positive reduction
5
6
7
L. M. Vogler, B. Scott, and K. J. Brewer, Inorg. Chem., 32,
898 (1993).
T. Matsumura-Inoue, M. Tanabe, T. Minami, and T.
Ohashi, Chem. Lett., 1994, 2443.
Anal. Found: C, 53.08; H, 3.07; N, 5.55%. Calcd for
C72H48N6IrP3F18: C, 53.23; H, 2.96; N, 5.17%. Found: C,
48.96; H, 3.46; N, 5.14%. Calcd for C66H48N6IrP3F18: C,
51.06; H, 3.09; N, 5.42%. Found: C, 37.09; H, 3.13; N,
7.17%. Calcd for C36H36N6IrP3F18: C, 36.27; H, 3.02; N,
7.05%. Found: C, 44.57; H, 2.56; N, 5.75%. Calcd for
C54H36N6IrP3F18: C, 46.44; H, 2.58; N, 6.02%.
M. F. Finlayson, P. C. Ford, and R. J. Watts, J. Phys.
Chem., 90, 3916 (1986).
3+
potentials. The oxidation waves of Ir(phen)33+, Ir(dpphen)3
,
and Ir(bqn)33+ at 2.05 V, 1.99 V and 2.03 V respectively, are
nearly equal to those of the free ligands, and thus the oxidation
process corresponds to the removal of an electron from the
lumo π orbital of ligands in the complexes. The oxidation-
potentials of Ir(bpy)3 and Ir(dmbpy)33+ are more negative
3+
(+0.2 V) than those of the free ligands, which suggests the weak
contribution to ligand π orbitals. It is confirmed that an elec-
tron is removed from the π orbital of ligand in the oxidation
process, while in the reduction process three or four electrons
are added one by one to π* orbitals of the ligand.
8
9
V. T. Coombe, G. A. Heath, A. J. MacKenzie, and L. J.
Yellowlees, Inorg. Chem., 23, 3423 (1984).
The correlation between π–π* absorption band energy (Eab)
and redox potentials (∆E) shows a linear relationship with a
slope of unity (Figure 1), which indicates that the potential dif-
ference between the first reduction and the oxidation peaks is
consistent with the absorption band energy.
10 R. J. Watts, Inorg. Chem., 20, 2302 (1981).
11 T. Matsumura-Inoue, J. Electroanal. Chem., 209, 135,
(1986).
12 X. Xiao, T. Matsumura-Inoue, and S. Mizutani, Chem.
Lett., 1997, 241.