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Chemistry Letters Vol.36, No.7 (2007)
Reversible Photon-mode Phosphorescence Switching of Heteroleptic
Cyclometalated Iridium(III) Complexes via Photochromic Bisthienylethene Switch
Linked to Ancillary Ligand
Insuk Lee,1;2 Youngmin You,1 Seon-Jeong Lim,1 and Soo Young Parkꢀ1
1School of Materials Science & Engineering, Seoul National University, San 56-1, Shillim-Dong, Kwanak-Gu, Seoul 151-744, Korea
2Samsung Electronics Co., Ltd., San 24, Nongseo-Dong, Giheung-Gu, Yongin, Gyeonggi-Do 446-711, Korea
(Received March 26, 2007; CL-070320; E-mail: parksy@snu.ac.kr)
A novel class of heteroleptic IrIII complexes covalently
F
F
F
F
F
F
F
F
F
F
F
F
N
Ir
N
365 nm
N
O
linked with photochromic bisthienylethene (BTE) switch
showed optically addressed modulation in blue, green, and
yellow phosphorescence. Additional enhancement of phospho-
rescence modulation was successfully achieved by the incorpo-
ration of multiple BTE switches or by the use of intermolecular
energy transfer.
O
S
S
S
S
> 515 nm
OH
OH
1c
1o
Open-form
Closed-form
F
IrIII bis(phenylpyridine-C,N2') picolinate
N
Ir
N
F
F
F
S
N
O
F
F
F
F
O
O
O
S
F
N
Ir
1a
F
N
N
O
Photochromic compounds have attracted remarkable atten-
tion because of their potential capability as the optical memory
media and optical switching devices.1 Among the various photo-
chromic compounds, 1,2-bisthienylethene (BTE) derivatives are
the most promising candidate for the practical application owing
to their high thermal stability, excellent photofatigue resistance,
and fairly high photocyclization quantum yield.2
O
O
N
Ir
N
F
F
F
N
O
F
F
O
O
O
S
S
S
F
F
O
O
O
O
S
S
2a
S
F
F
F
F
F
F
S
F
F
F
F
F
2a'
N
Ir
N
F
F
F
S
N
O
F
F
O
S
F
3a
S
Cyclometalated IrIII complexes are the most valuable
emitting materials for the highly efficient organic light emitting
diodes (OLEDs).3 Strong spin–orbit coupling induced by the
core IrIII metal provides relatively short phosphorescent lifetime,
which allows 100% of internal quantum efficiency, in principle.
In this regard, many different classes of IrIII complexes have
been synthesized and are extending their application to the
various interdisciplinary fields, such as oxygen sensing, biolog-
ical imaging, and ion sensing.4
Considering the novel class of photoluminescence memo-
ries and switches, very recently, bistable emission switching
of several metal complexes (metalloporphyrin,5a W complex,5b
Os complex,5c Ru complex,5c and Eu complex5d) incorporating
covalently linked BTE switches have been demonstrated. The
photochromic switching of highly phosphorescent IrIII complex
emission, however, has not been attempted yet in spite of its
technical significance. One of its potential applications, especial-
ly in polymer matrices, is related to the optically addressable
electrophosphorescent memory device, in which signal write/
erase processes are performed optically whereas the saved
information is read electrically. In this letter, we report on the
synthesis and phosphorescence switching behavior of a series
of photochromic IrIII complexes with ancillary ligands (LX)
tethering BTE unit via covalent bonds (IrIII(C^N)2(LX); 1a,
2a, 2a0, and 3a which show blue, green, and yellow phosphores-
cence, respectively: see Scheme 1).
Scheme 1.
chromic reaction and phosphorescence emission, which often
limits the reversible photochromism of the BTE switches.5a
In addition, the BTE-linked ancillary ligand was not expected
to alter the phosphorescence emission because it is known that
the triplet energy of the picolinate is sufficiently higher than
those of cyclometalating ligands (2-(2,4-difluorophenyl)pyri-
dine, 2-phenylpyridine, and 2-phenylbenzothiazole).7c,d From
the ꢀ-chloride-bridged IrIII dimers and BTE-linked picolinate
ligand, photochromic IrIII complexes 1a, 2a, 2a0, and 3a
were successfully synthesized under mild chelating condition.
Phosphorescence emission of IrIII complexes originating from
the metal to the cyclometalating ligand charge-transfer (MLCT)
state of IrIII complexes must overlap with the absorption band
of the closed-form BTE for the efficient energy transfer resulting
in the phosphorescence switching. When the solutions of open-
form isomers 1a, 2a, 2a0, and 3a (2:0 ꢁ 10ꢂ5 M in Ar-saturated
THF) were irradiated with 365-nm UV light for 5 min, the
closed-form isomers were effectively generated to reach the
photostationary state (PSS) (the maximum conversions were
90, 78, 90 (one closed-form 87 and two closed-forms 3), and
67%, respectively).2 As shown in Figure 1b, representatively,
a new broad absorption band in the visible region (450–
700 nm) appeared upon the photocyclization of the BTE unit
o!c
(ꢀpc
open-form state by exposure to the visible light (>515 nm,
¼ 0:28), which was reversibly restored to the initial
Synthesis was straightforward to give the photochromic
IrIII complexes with BTE unit.6 To a BTE unit, which was syn-
thesized by the literature method,7a,b an ancillary ligand (LX) of a
methyl picolinate was covalently attached via a propyloxyphen-
yl linkage in order to restrict the electronic delocalization in
the excited states. This is because the direct electronic coupling
generally brings about the unfavorable interference of photo-
c!o
ꢀpc
¼ 0:027).
The open-form isomers of IrIII complexes 1a, 2a, and 3a
were moderately phosphorescent (ꢀp ¼ 0:04, 0.08, and 0.03,
respectively) with the maximum emission peaks at 483, 513,
and 556 nm, respectively, when excited with 355 nm light.6
On the other hand, the complexes showed significantly reduced
Copyright ꢀ 2007 The Chemical Society of Japan