Deuteration isotope effect on nonradiative transition of fac-tris (2-phenylpyridinato) iridium (III) complexes

Article abstract of DOI:10.1016/j.cplett.2010.03.084

Deuterated 2-phenylpyridine and coordinated iridium(III) complexes Ir(ppy-h₈)_n(ppy-d₈)_3-n (n = 0, 1, 2, 3) have been prepared. Both the relative and absolute phosphorescent quantum yields and the emission decay

Full text of DOI:10.1016/j.cplett.2010.03.084

Chemical Physics Letters 491 (2010) 199–202
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Deuteration isotope effect on nonradiative transition of fac-tris
(
2-phenylpyridinato) iridium (III) complexes
a
a
b
a,*
Taichi Abe , Akira Miyazawa , Hideo Konno , Yuji Kawanishi
a
Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan
Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1, Higashi, Tsukuba,
b
Ibaraki 305-8565, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Deuterated 2-phenylpyridine and coordinated iridium(III) complexes Ir(ppy-h ) (ppy-d )
have been prepared. Both the relative and absolute phosphorescent quantum yields and the emission
decay times of dilute solutions have been studied around room temperature. Phosphorescent quantum
8 3ꢀn
(n = 0, 1, 2, 3)
8
n
Received 28 December 2009
In final form 30 March 2010
Available online 2 April 2010
yields of fully deuterated Ir(ppy-d
than that of normal complex Ir(ppy-h
hand solvent deuteration has almost no effect. Nonradiative rate constants show little temperature depen-
)
8 3
have ranged from 0.80 to 0.93 in several solvents, which are higher
8 3
) . Ligand deuteration increases the quantum yield. On the other
5
ꢀ1
5 ꢀ1
dence for the both complexes (knr = 1.4 ± 0.3 ꢁ 10 s and 0.95 ± 0.2 ꢁ 10 s ) at 273–308 K.
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
the probability of vibronic coupling between the electronically ex-
cited complex and vibrational modes [22]. Although the deutera-
tion effect on phosphorescent metal complexes seem to be
poorer than that of fluorescent molecules [22], investigations on
the candidate materials in OLEDs will afford further information
for better understanding [20].
Cyclometallated iridium(III) complexes have been widely stud-
ied for decades because of their intriguing photophysical proper-
ties and the good prospects they offer for developing a new field
of applications in organic light-emitting diodes (OLEDs) [1–6].
Ir(ppy-h
8
)
3
(ppyH-h
9
: 2-phenylpyridine) is a well-known complex
Photophysical study of deuterated iridium(III) complex
þ
because of its prominent features, which include a relatively short-
lived excited state, a high luminescence quantum yield and clear
green luminescence. This complex can be used as a phosphorescent
emitter in OLEDs and its emission mechanism has also been stud-
ied theoretically [6–13]. To investigate the excited state properties
and relaxation processes, the deuteration of organic molecules and
transition metal complexes have been studied [14–20]. The deuter-
ation of polypyridyl ruthenium(II) and rare earth complexes
IrCl
2
ðbpy-d
8
Þ
was first reported from Watts et al. [24]. The excited
2
state lifetime, which showed temperature dependence from 77 to
354 K, was increased by deuteration of the ligands. Deuteration
seems to contribute in the decay kinetics that is a dual emission
from the thermally equilibrated charge transfer and ligand field
excited states to the ground state. Although cyclometallated irid-
ium(III) complexes have drawn attention for their phosphorescent
character, there have been few reports on the isotope effect of
them. Therefore, in this study we measured luminescence spectra
and decay times to examine the nature of the radiative and nonra-
pushes up their luminescent quantum yield (
U) and excited state
lifetime ( ) [21,22]. The ligand deuteration of a complex [Ru(bpy-
s
2
+
0
d
)
8 3
]
(bpy = 2,2 -bipyridine) results in an approximately 20% in-
crease in its , in addition solvent deuteration doubles [15]. Yanag-
ida et al. have realized an enhanced with polymer doped
8 3 8 3
diative processes of Ir(ppy-h ) and Ir(ppy-d ) . Both the ligand
and solvent deuteration isotope effects of a dilute solution are dis-
cussed (see Fig. 1).
s
U
neodymium(III) complexes by eliminating C–H vibrational oscilla-
tors [23]. A reduction in the number of C–H oscillators by the deu-
2
. Experiments
teration and/or fluorination of Nd(HFA-h
hexafluoroacetylacetone and PMMA: poly(methylmethacrylate)]
greatly increases to = 0.7 for Nd(HFA-d /P-FiPMA [P-FiPMA:
poly(hexafluoroisopropylmethacrylate)] from < 0.01 for Nd-
HFA-h /PMMA. These enhancements of and are caused by
the diminished nonradiative processes that result from reducing
1 3 2
) in PMMA [HFA-h :
General: UV–vis absorption spectra were measured by Hewlett–
U
1 3
)
Packard 8453 diode array spectrometer. The luminescence spectra
were recorded with a spectrometer (HORIBA JOBIN YVON-SPEX
Fluorolog-3) to determine the relative luminescence quantum
U
(
1
)
3
U
s
3
yield of Ir(ppy) . The excitation wavelength was set at 370 nm
for every measurement. ESI-MS measurements were carried out
1
on a Thermo Quest FINNIGAN AQA. H NMR spectra were mea-
*
Corresponding author. Fax: +81 29 861 6296.
E-mail address: yuji-kawanishi@aist.go.jp (Y. Kawanishi).
sured by a Bruker Avance 400 spectrometer (external standard
0
009-2614/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2010.03.084

2
00
T. Abe et al. / Chemical Physics Letters 491 (2010) 199–202
(
3 2
400 MHz, d(CD ) SO) 8.11, 7.77, 7.73, 7.46, 7.10, 6.78, 6.66, 6.63;
FT-IR (wavenumber of peaks) 2277, 2251, 1566, 1548, 1529,
1
8
502, 1401, 1349, 1328, 1297, 1260, 1234, 1021, 986, 974, 850,
22, 757, 636, 577.
3
. Result and discussion
3.1. Synthesis of deuterated compounds
8 n 8
Fig. 1. Structure of Ir(ppy-h ) (ppy-d )3ꢀn (n = 0, 1, 2, 3).
Deuterated 2-phenylpyridine-d
exchange reactions with deuterium oxide (D
der and platinum on carbon or on aluminum oxide at 180 °C for
4 h as a modified literature procedure [28–30]. One of our advan-
tages in preparing deuterated ligands is to reduce the use of deute-
rium oxide by stepwise deuteration reaction. Since the deuterium
content rate of the desired product depends on the protium-to-
deuterium molar ratio (%) in the reaction system, we first run a
reaction with [D] ꢃ 90% and enriched by a second reaction with
the same volume of fresh deuterium oxide; this will lead the final
deuterium content rate to [D] ꢃ 99% and reduce the use of deute-
rium oxide by 5 times. A series of iridium complexes Ir(ppy-
9
was prepared by catalytic H–D
2
O), aluminum pow-
TMS = 0.00 ppm). Emission lifetimes were evaluated by the least
square method on a PC for luminescence time courses recorded
after a laser pulse (Nd:YAG laser 355 nm, 1 mJ/pulse). The detec-
tion system consisted of a photomultiplier H3186 (Hamamatsu,
rise time 2 ns) directly coupled to a monochromator HR320 (Jo-
bin–Yvon 1200 g/mm). The signal from the PM was directly ana-
lyzed by an oscilloscope TDS310 (Tektronix) working in average
mode. Absolute luminescence quantum yields were measured by
a Hamamatsu photonics k.k. C9920-02 system. Relative phospho-
rescence quantum yields were determined with using ethanol
2
solution of rhodamine6G (UFL = 0.95 at 298 K) as an external stan-
8 n 8
h ) (ppy-d )3ꢀn (n = 0, 1, 2, 3) were synthesized by microwave as-
sisted reactions [27]. The advantage of the microwave technique
dard [25]. All solutions for luminescence measurements were pre-
ꢀ6
pared using the optical dilute method (1.0 ꢁ 10
to 1.0 ꢁ
mol/l) and degassed by the freeze–pump–thaw method
26]. A microwave apparatus CEM Discover (2.45 GHz frequency,
ꢀ
7
is the reduction of reaction time. Differently from the preparation
1
0
2
+
3 3
of [Ru(bpy) ] , Ir(ppy) requires dehydrogenation processes for
[
complexation and long reaction time may cause de-deuteration
of the ligands. Thus, microwave assisted reaction is a preferred ap-
CEM Corporation, NC, USA) and WMO1000S (TOKYO RIKAKIKAI
CO, LTD) were used for small (within 3 mL) and large scale synthe-
ses, respectively.
Materials: Deuterium oxide (99.9 atom%) was purchased from
ISOTEC™. Other reagents were purchased from commercial suppli-
ers and used without further purification. Ir(ppy-h ) was prepared
8 3
by a microwave assisted method according to a literature proce-
dure [27].
proach. Fig. 2 shows the FT-IR spectrum of fac-Ir(ppy-h
8 3
) and Ir(p-
py-d . The highest energy of observed absorption bands is shifted
8 3
)
ꢀ1
from 3050 to 2280 cm by deuteration. The low frequency shift
should reduce extent of vibronic coupling and thus energy dissipa-
3
tion process of excited Ir(ppy) . It has been pointed out that energy
reduction in the fingerprint region such as CCC and CCH modes can
2
3
þ
also contribute to the deactivation processes of Ruðbpy-d Þ
n
9 2 9
2-Phenylpyridine-d : A mixture of D O 40 g (2 mol), ppyH-h
[
31,32]. Recently, Raman and infrared absorption energies and
5
.0 g (32 mmol), 200 mg of Pt/C (5 wt.%, 0.25 mol%) and 200 mg
+
2 2 2 2
intensities of [Ru(bpy) (CN) ] and [Ir(bpy) (CN) ]
have been
of Al powder (200 mesh, 7.4 mmol) were set in a stainless reactor
which was sealed and then heated with stirring at 180 °C for 24 h.
After cooling to room temperature, the suspension was filtered and
washed by acetone. The filtrate was extracted by dichloromethane
and the solvents of collected organic phase were evaporated.
Resulting liquid was reacted again with the same procedure except
calculated based on the density functional theory [33]. By follow-
ing the provided information, we have assigned the experimentally
ꢀ1
given low energy modes (
8 3
m < 1500 cm ) of Ir(ppy-h ) and Ir(ppy-
8 3
d ) . The vibrational frequencies of the components are listed in
Table 1 accompanied by relevant data. Both the deuterated com-
plexes show lower energies than their corresponding normal com-
plexes. Since several modes often contribute together in the low
energy frequencies, the extent of energy decrements by deutera-
tion is not simply same in each absorption. Larger overtone factors
results in poorer vibronic coupling between the states so that
2 3
Pt/Al O was used instead of Pt/C. After taking the processes as
well as the first reaction, the crude product was distilled at 70 °C
under reduced pressure (<1 mm Hg) to obtain 4.0 g of colorless li-
quid (yield 80%). Deuterium content rate was determined to be
1
over 95% in average by H NMR analysis with using 1,2-dibromo-
ethane as an external standard. ESI-MS (relative abundance%)
+
1
1
63 (20), 164 (100), 165 (10), (MH );
3 2
H NMR ((CD ) CO
1
00
4
00 MHz) d(ppm); 8.67, 8.13, 7.93, 7.86, 7.49, 7.43, 7.31 (every
peak was observed as singlet); FT-IR (wavenumber of peaks)
2
1
272, 2253, 2201, 2103, 1549, 1525, 1411, 1366, 1333, 1307,
274, 1230, 995, 847, 821, 736, 598, 580, 562, 546.
Ir(ppy-d
IrCl O) 35 mg (0.1 mmol), 2-phenylpyridine-d
ꢂ3H
1 mmol) and 1 mL of ethylene glycol were set in a 10 mL glass ves-
8
)
3
:
A
mixture of iridium trichloride hydrate
(
(
3
2
9
165 mg
0
00
1
sel. The reaction was carried out by microwave irradiation at
00 °C for 30 min. After cooling to room temperature, 1 M aqueous
2
hydrochloric acid was added to the solution. The precipitated prod-
uct was filtered, washed with 1 M HCl (aq) followed by water. The
collected crude product was reacted with 165 mg of 2-phenylpyr-
idine-d
9
in ethylene glycol 1 mL by microwave irradiation; and this
0
3
500
3000
2500
wavenumber (cm )
2000
1500
1000
was repeated twice. The desired product was purified by a silica gel
column chromatography with dichloromethane as an eluent (yield
-1
5
5 mg 81%). ESI-MS (relative abundance %) 675 (20), 676 (50), 677
Fig. 2. FT-IR spectra of Ir(ppy-d
under nitrogen atmosphere at room temperature.
8 3 8 3
) (top) and Ir(ppy-h ) (bottom) of KBr tablet
+
1
(
80), 678 (90), 679 (100), 680 (30), 681 (10), (MH ); H NMR

T. Abe et al. / Chemical Physics Letters 491 (2010) 199–202
201
Table 1
Selected vibration modes and frequencies of infrared and Raman peaks.
Types of vibrationa,b
[Ir(bpy-h
]+b
Ir(ppy)
c
[Ru(bpy)
(ꢀh
a
8 2
) (CN)
2
3
3 2
]Cl
d
e
I
Ram
m
m
(ꢀh
8
)
m
(ꢀd
8
)
m
8
)
m
(ꢀd
8
)
e
1255e
m
m
m
(C–C), d(CH)
(C–C), d(CH),
100.8
317.3
14.1–51.0
32.4
1325
1324
1279–1285
1110
1059
1310
1299
1261
1124
1058
1260
1234
1021
872
1320
1310
1264
c
c
m
(C–N)
1227
e
c
1020e
(C–N),
m
(C–C), d(CH)
c
d(CH),
d(CH),
m
m
(C–C),
(C–C),
a
a
(CCC)
(CCC)
1121
867
e
851e
31.2
850
1067
a
b
c
Ref. [31].
Ref. [33].
ꢀ1
Infrared absorption frequency (cm ).
Calculated Raman intensity in Ref. [33].
Resonance Raman frequency (cm ).
d
e
ꢀ1
depression of the thermal relaxation of the excited Ir(ppy)
plausible.
3
is
3
in detail, we consider the nonradiative pathways of Ir(ppy) in
possible distributions, namely nonradiative relaxation through a
thermal transition to a metal-centered MC state, intermolecular
quenching through energy transfer via solvents, and intramolecu-
lar deactivation as rapid horizontal transitions from excited to
ground state electronic surfaces. Here, the ligand field parameter
3.2. Luminescence and photophysical properties of Ir(ppy)
3
The luminescence peak wavelength of the Ir(ppy-h
py-d were located at 523 nm with the identical waveform at
93 K in acetonitrile-h (Table 2). A slight bathochromic shift was
8 3
) and Ir(p-
3
of Ir(ppy) is sufficiently large and hence the MC state is inaccessi-
8 3
)
ble from the lowest excited state around room temperature [5]. To
discuss the inter- and intramolecular deactivation processes, we
measured the temperature dependent luminescence lifetime of
Ir(ppy-h ) and Ir(ppy-d ) in acetonitrile-h /d from 273 to
8 3 8 3 3 3
308 K, as summarized in Table 3. Both the complexes Ir(ppy-h8)3
2
3
observed compared with that of toluene solution (kmax = 511 nm),
tetrahydrofuran solution (kmax = 514 nm) and dichloromethane
solution (kmax = 516 nm). The energy shift behavior was similar
2
3
þ
to ionic RuðbpyÞ [34], suggesting that emission energy of neutral
and Ir(ppy-d
and = 2.1 s (2.07–2.13
showed a slight increase in
temperature dependence. Moreover, the result of deuterated solu-
tion were = 1.8 s (1.77–1.82 s) and = 2.1 s (2.06–2.10 s),
8
)
3
in acetonitrile-h
s), respectively. The deuterated sample
; however, neither sample exhibited
3
were s = 1.8 ls (1.76–1.79 ls)
3
Ir(ppy) is also affected by solvents polarity. The luminescence
s
l
l
quantum yield determined for Ir(ppy-h
ment method was 0.87 ± 0.05 in toluene, whereas that of Ir(ppy-
was 0.93 ± 0.05. These results are in good agreement with
the recently reported Ir(ppy-h PL = 89 ± 3 (%) in dichlorometh-
ane [35]. It has been brought up that absolute method is a useful
technique to obtain [36], and we measured them in the same
fashion. Deuterated Ir(ppy-d exhibited = 0.80 ± 0.03, 0.90 ±
.04 and 0.92 ± 0.04 which was slightly larger than Ir(ppy-h in
8 3
) by a relative measure-
s
8 3
d )
s
l
l
s
l
l
8 3
) : U
thus no deuteration solvent effect was observed. It is worth noting
that the lack of temperature and solvent deuteration dependence is
U
3
unique for Ir(ppy) since this behavior contrasts with that of ruthe-
8
)
3
U
nium(II) and osmium(II) complexes [37,38]. It has been reported
0
8 3
)
2
3
þ
that the knr of OsðbpyÞ increases with temperature as a result
each entry of Table 2. To interpret this difference, we compared
the corresponding rate constants. The nonradiative rate constant
(knr) of the deuterated complex in each solution was considerably
reduced from that of the normal complex. Exchanging the C–H for
C–D vibrational oscillators reduced the nonradiative deactivation
as a result of reduced probability of vibronic coupling between
the states and this is attributed to the ligand deuteration effect
of activated intermolecular quenching, such as an energy transfer
from the excited complex molecule to the solvent [38]. Solvent
deuteration suppresses this interaction between the energy donor
and the acceptor, therefore the isotope effect is observed in the
enhancement of the
of temperature and solvent deuteration indicates that there is
weak solvent-chromophore quenching of excited Ir(ppy) by en-
U and s. The fact that the knr is independent
of Ir(ppy)
controlling the
ger than knr in this Ir(ppy)
dependence of is attributable to the k
vent contributes less to the knr. Thus, it is not reasonable to con-
sider strong quenching process model in terms of the
3
. By contrast, the k
and because it is about an order magnitude lar-
system. As regards this role, the solvent
, in other words, the sol-
r
is more likely to play a key role in
3
ergy transfer processes. Fig. 3 clearly shows that only the ligand
deuteration effect is observed as the logarithm of knr decrease from
U
s
3
1
1.8 ± 0.2 to 11.4 ± 0.2. Accordingly, we interpret this to mean that
intramolecular deactivation is a main pathway for the nonradiative
processes of Ir(ppy)
The luminescence quantum yield and excited state lifetime of
partially deuterated Ir(ppy-h (ppy-d ) and Ir(ppy-h )(ppy-d
ligands, were
U
r
3
.
a
3
interaction between excited Ir(ppy) and solvent. To discuss this
8
)
2
8
8
8 2
) ,
9 9
which were prepared by combining ppyH/D-h /d
Table 2
8 3 8 3
Photophysical properties of Ir(ppy-h ) and Ir(ppy-d ) .
k
nm
max
U
s
l
k
10
r
k
10
nr
Solvent
Table 3
5
ꢀ1
5 ꢀ1
s
s
s
3 3
Luminescence lifetime of Ir(ppy-h8)3 and Ir(ppy-d8)3 in acetonitrile-h /d .
Ir(ppy-h
8
)
3
3
511
0.87a
1.5
1.6
1.4
1.8
5.8
5.1
6.4
4.1
0.9
1.0
0.8
1.4
Toluene
s
l
b
5
5
5
14
19
23
0.84
0.89
0.74
Tetrahydrofuran
Dichloromethane
Acetonitrile
s
b
b
273 K 278 K 283 K 288 K 293 K 298 K 303 K 308 K
Ir(ppy-d
8
)
511
0.93a
1.6
1.8
1.7
2.1
5.8
5.1
5.5
3.9
0.4
0.6
0.5
0.9
Toluene
Ir(ppy-h8)3
b
CH CN 1.76
CD CN 1.78
1.76
1.81
1.77
1.77
1.77
1.80
1.78
1.82
1.79
1.79
1.79
1.80
1.77
1.81
5
5
5
14
19
23
0.90
Tetrahydrofuran
Dichloromethane
Acetonitrile
3
b
0.92
3
b
0.80
8 3
Ir(ppy-d )
a
CH
CD
3
CN 2.07
3
CN 2.10
2.09
2.07
2.09
2.09
2.13
2.06
2.13
2.10
2.07
2.09
2.13
2.10
2.12
2.08
Values obtained from relative quantum yield measurement.
Values obtained from absolute quantum yield measurements.
b

2
02
T. Abe et al. / Chemical Physics Letters 491 (2010) 199–202
as high as 0.9. We are presently in the pursuance of studies on
OLED devices with these deuterated iridium emitters and the re-
sults will be reported in elsewhere.
1
2
Acknowledgments
1
1.5
This research was supported financially by the project ‘Develop-
ment of Microspace and Nanospace Reaction Environment Tech-
nology for Functional Materials’ of the New Energy and Industrial
Technology Development Organization (NEDO), Japan.
1
1
Appendix A. Supplementary data
1
0.5
3
.2
3.3
3.4
1
3.5
3.6
3.7
/T (10-3 K-1)
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2010.03.084.
Fig. 3. Plot of the nonradiative rate constants versus temperature of Ir(ppy-h
8 3
) and
Ir(ppy-d in acetonitrile-h : h/j, and acetonitrile-d : d/s. Error regions are
represented by broken and dotted line for the normal complex and the deuterated
one, respectively.
8
)
3
3
3
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(
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Þ
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seems to be
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trapped by a vibronic coupling with hydrogen involved vibrational
modes. This result is similar to the ligand selective emission of
2
2ꢀn
þ
Ptðbpy-h
8
Þ ðbpy-d
8
Þ
(n = 0, 1, 2) which is led by the effective
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n
[
[
[
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intramolecular energy transfer at low temperature [21,40].
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[
[
[
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Ir(ppy)
rather than intermolecular quenching through solvents.
We have prepared a series of deuterated Ir(ppy-h (ppy-d
n = 0, 1, 2, 3) and studied the luminescence quantum yield and ex-
cited state lifetime of their solutions around room temperature.
The Ir(ppy-d was = 0.80, 0.90, 0.92, 0.93 in acetonitrile, tetra-
hydrofuran, dichloromethane and toluene, respectively, which was
slightly higher than that of Ir(ppy-h in each system. The ligand
deuteration isotope effect was observed as a result of the reduced
nr when the sample was fully deuterated. A comparison of the rate
constants for non-deuterated and fully deuterated Ir(ppy) showed
3
would be driven through intramolecular deactivation
8
)
n
)
8 3ꢀn
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(
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5
8
)
3
U
(
[
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8 3
)
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[
[
[38] A. Masuda, Y. Kaizu, Inorg. Chem. 37 (1998) 3371.
[
[
k
3
36] K. Suzuki et al., Phys. Chem. Chem. Phys. 11 (2009) 9850.
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that the knr was independent of temperature (273–308 K) and sol-
vent deuteration. Our observation of only the ligand deuteration
effect suggests that intramolecular deactivation is a main pathway
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40] W. Humbs, H. Yersin, Inorg. Chim. Acta 265 (1997) 139.
3
for the nonradiative process, and therefore Ir(ppy) would have a U