A detailed investigation of light-harvesting efficiency of blue color emitting divergent iridium dendrimers with peripheral phenylcarbazole units

Article abstract of DOI:10.1039/c3cp53797h

The increase in phosphorescence efficiency was estimated by the energy transfer mechanism for divergent iridium dendrimers with peripheral phenylcarbazole units. A series of Ir-core/phenylcarbazole-end dendrons of the type, Ir(dfppy)₂(pic-Cz

Full text of DOI:10.1039/c3cp53797h

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A detailed investigation of light-harvesting efficiency
of blue color emitting divergent iridium dendrimers
with peripheral phenylcarbazole units†
Cite this: Phys.Chem.Chem.Phys.,
2014, 16, 4510
Yang-Jin Cho, Kyung-Ryang Wee, Ho-Jin Son, Dae Won Cho* and
Sang Ook Kang*
The increase in phosphorescence efficiency was estimated by the energy transfer mechanism for divergent
iridium dendrimers with peripheral phenylcarbazole units. A series of Ir-core/phenylcarbazole-end
dendrons of the type, Ir(dfppy)2(pic-Czn) (Gn, n = 0, 1, 2, and 3), was synthesized, where dfppy, pic, and
Czn (n = 2, 4, and 8) are 2-(4,6-difluorophenyl)pyridine, picolinate substituted with Czn at the 3-position,
and 4-(9-carbazolyl)phenyldendrons connected with 3,5-di(methyleneoxy)benzyloxy branches,
respectively. Selective excitation of the Czn units of G1–G3 resulted in >90% quenching of the Cz
fluorescence accompanied by the growth of phosphorescence from the Ir(dfppy)2(pic) core as a
consequence of energy transfer from the excited-singlet Czn chromophore to the core. The rate
constants of energy transfer were determined by steady-state and transient spectroscopic measurements to
be 4.32 ꢀ 109 sꢁ1 (G1), 2.37 ꢀ 109 sꢁ1 (G2), and 1.46 ꢀ 109 sꢁ1 (G3), which were in good agreement with
Received 6th September 2013,
Accepted 27th October 2013
those calculated using the Forster model. The phosphorescence enhancements were 157 (G1), 213 (G2), and
DOI: 10.1039/c3cp53797h
¨
264% (G3) when compared to the phosphorescence of the core Ir(dfppy)2(pic-Ph2) (G0), in which pic-Ph2 is
www.rsc.org/pccp
3-(3,5-dibenzyloxybenzyl)picolinate.
material purity, respectively.14–16 Of particular interest are the
dendrimers that contain both host and dopant units in a single
Introduction
Phosphorescent transition metal complexes have attracted molecular array facilitating efficient energy transfer. For the diver-
much attention due to their ability to maximize light emitting gent iridium-cored dendrimers,17 systematic generation growth
efficiency by fully utilizing triplet energy for radiation.1,2 High correlates well with the energy transfer efficiency when the number
phosphorescence efficiency comes from the large triplet popula- of host end groups varies depending on the choice of end group
tion through the strong spin–orbital coupling induced by heavy carrying branches. Since end-on host units surround the iridium
transition metal.3,4 Ir3+ complexes have been recognized as pro- core, the photophysical process propagates in a radial manner; in
mising candidates for phosphorescence organic light emitting general one can expect that electronically excited end-on host
diodes due to their outstanding emission properties such as high groups can be quenched either by the energy transfer or the electron
quantum yields of luminescence, photo stability, and color tuning transfer to the central iridium. The final emission efficiency at the
by electronic control of the main ligand, as well as variation of the iridium center is determined by the energy transfer from the
ancillary ligand.5–13
peripheries and through the branches and finally to the iridium.
Dendrimers are highly branched macromolecules. Their mono- Consequently, the choice of end group as well as branch can be
dispersive nature suggests that they can offer many advantageous detrimental to high efficiency. Furthermore, Ir-dendrimers are
properties for optoelectronic applications that neither small generally used in order to prevent destructive excited-state inter-
molecules nor polymers can solely provide in terms of ideal actions between emitting phosphors, like triplet–triplet annihilation,
material specification such as thermal or oxidation stability and especially under high current density.18–20 Dendrimers can shield
the Ir3+-complex core to weaken the oxygen quenching effect.21
Department of Advanced Materials Chemistry, Korea University, Sejong Campus,
Sejong 339-700, Korea. E-mail: sangok@korea.ac.kr
Carbazole (Cz)-based dendrons have been used as effective
charge transporting moieties for iridium-based phosphorescent
emitters.22,23 Dendrimers have proved to be successful materials
for solution-processible phosphorescent organic light emitting
diodes (OLEDs).24–35 Divergent iridium dendrimers are classified
into two types: one with engagement of rigid branching units
† Electronic supplementary information (ESI) available: Absorption and emission
spectra of carbazole dendrons, lifetimes of phosphorescence of dendrimers, the
energy minimized geometries of Gn (n = 1, 2, and 3), energy levels and isodensity
plots for selected occupied and unoccupied molecular orbitals of G1. See DOI:
10.1039/c3cp53797h
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was done with silica gel 60G (particle size 5–40 mm, Merck Co.).
1H and 13C NMR spectra were recorded on a Varian Mercury 300
spectrometer at 300.1 and 75.4 MHz, respectively. Elemental
analyses were carried out using a Carlo Erba Instruments
CHNS-O EA 1108 analyzer. High-resolution tandem mass spectro-
metry (Jeol LTD JMS-HX 110/110A) and MALDI-TOF mass spectro-
scopy (ABI Voyager STR) were performed by the Ochang branch of
the Korean Basic Science Institute.
Spectroscopic measurements
Steady-state absorption and fluorescence spectra were measured
using a UV-vis spectrophotometer (Shimadzu, UV-3101PC) and a
fluorescence spectrophotometer (Varian, Cary Eclipse), respec-
tively. Time-resolved fluorescence spectra were measured using a
single photon counting method with a streak scope (Hamamatsu
Photonics, C10627-03) equipped with a polychromator (Acton
Research, SP2300).39 An ultrashort laser pulse was generated
with a Ti:sapphire oscillator (Coherent, Vitesse, FWHM 100 fs)
pumped with a diode-pumped solid-state laser (Coherent, Verdi).
High-power (1.5 mJ) pulses were generated using a Ti:sapphire
regenerative amplifier (Coherent, Libra, 1 kHz). For excitation of
the sample, the output of the Ti:sapphire regenerative amplifier
was converted to 440 nm using an optical parametric amplifier
(Coherent, TOPAS). The instrument response function was also
determined by measuring the scattered laser light to analyze the
temporal profile. This method provides a time resolution of
approximately 20 ps after the deconvolution procedure. The
temporal emission profiles were well fitted to a single-exponential
function. The residuals for each system were less than 1.1. The
phosphorescence spectra at 77 K were obtained by photoexcitation
with the 355 nm pulses from a nanosecond Q-switched Nd:YAG
laser (Continuum, Surelite II-10, pulse width of 10 ns FWHM). The
phosphorescence spectra were collected using a monochromator
(DongWooOptron, Monora 500i) equipped with an intensified
charge-coupled device (ICCD, Ando, iStar). The temporal profiles
were measured using a monochromator equipped with a photo-
multiplier (Zolix Instruments Co., CR 131) and a digital oscilloscope
(Tektronix, TDS-784D).
Chart 1 Molecular structures of Gn (n = 0, 1, 2, and 3).
such as alkene and alkyne connectors, and the other with
flexible branches, such as connecting with alkyl or alkoxy
groups. The flexible branches are generally used in order to
secure the synthetic versatility.36 It has also been reported that
energy transfer in Ir3+-complex dendrimers with Cz dendrons
occurs at efficiencies greater than 90% from the excited-state
Cz dendron to the iridium core.37 However, most dendrimer
systems studied only dealt with energy transfer efficiency or in
most cases discussed the final emission enhancement. There
has been no quantitative analysis between energy transfer and
harvesting efficiency to show any correlation amongst iridium
dendrimers.38 In fact, the mechanism for the actual light-
harvesting process has not been understood in detail. In this
regard, it is important to analyze the relationship between the
energy transfer mechanism and light-harvesting efficiency.
In this study, we synthesized new dendritic complexes (Ir(dfppy)2-
(pic-Cz2) (G1), Ir(dfppy)2(pic-Cz4) (G2), and Ir(dfppy)2(pic-Cz8) (G3)),
together with Cz-free reference complex Ir(dfppy)2(pic-Ph2) (G0),
as shown in Chart 1, where dfppy, pic, and Czn are 2-(4,6-
difluorophenyl)pyridine, picolinate and 4-(9-carbazolyl)phenyl-
carbazole dendrons (n = 2, 4 and 8), respectively. The Ir-complex
core is connected with Czn units by 3,5-di(methyleneoxy)benzyloxy
branches. Herein, Cz works as the energy donor after light
absorption, whereas the Ir-complex core acts as the energy
acceptor. For quantitative kinetic analysis of the energy transfer,
we carried out steady-state and time-resolved spectroscopic
measurements.
Electrochemistry measurements
The cyclic voltammetry (CV) experiments were performed using
a BAS 100 electrochemical analyzer and a three-electrode cell
system comprised of a glassy carbon disk (dia. 3 mm), a platinum
disk (dia. 1.6 mm) electrode as the working electrode, and a
platinum wire and Ag/AgNO3 as the counter and reference
Experimental
General procedures
Starting materials were purchased from Aldrich Chemical Co. electrodes, respectively, operated at a scan rate of 0.1 V sꢁ1
.
and TCI Co. and were used without further purification. Reac- Freshly distilled, degassed CH2Cl2 was used as the solvent with
tions were carried out under a dried nitrogen (N2) atmosphere. 0.1 M tetrabutylammonium perchlorate (TBAP) as the supporting
Solvents were dried according to standard procedures and electrolyte. The potential values were calibrated with respect to
freshly distilled before use. Dichloromethane (CH2Cl2), aceto- the Fc+/Fc (Fc = ferrocene) redox couple.
nitrile (ACN), and dimethylformamide (DMF) were distilled over
Theoretical calculations
calcium hydride under a N2 atmosphere and stored over mole-
cular sieves. Tetrahydrofuran (THF) and 2-methyltetrahydrofuran Full geometry optimizations of the dendrimers in their singlet
(2-MeTHF) were distilled over sodium/benzophenone. Reactions ground state were performed with density functional theory
were monitored with thin layer chromatography (TLC) using (DFT) using the B3LYP40 exchange correlation functional (Becke’s
commercial TLC plates (Merck Co.). Column chromatography three parameter (B3) gradient-corrected exchange functional
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combined with the correlation Lee–Yang–Parr (LYP) functional),
On the other hand, the oxidation of dendrimers (G1–G3) gives
with the relativistic effective core potential and basis set LanL2DZ41 an irreversible wave at around B0.8 V, revealing significantly
for the iridium and the 6-31G*42 basis set for the remaining atoms. different behavior from the reversible oxidation of the Ir3+ complex
No symmetry constraints were applied during the geometry core (G0) occurring at a similar potential (at 0.87 V). The electro-
optimizations, which were carried out with the Gaussian 09 chemical oxidation of the latter compound and the related Ir3+
package.43 The nature of the stationary points location was heteroleptic complexes has been assigned to the metal-centered
further checked by computations of harmonic vibrational frequen- Ir3+/4+ redox process.47–50 The CVs for G1, G2, and G3 showed
cies at the same level of theory. All isodensity plots (isodensity additional oxidation peaks at B0.80 and B1.05 V upon scanning
contour = 0.03 a.u.) of the frontier orbitals were visualized by toward positive potential and two cathodic peaks at B0.72 and
Chem3D Ultra program. The excitation energies and oscillator B0.48 V upon reverse scan. This electrochemical behavior was
strengths for the lowest 30 singlet–singlet and triplet–triplet attributed to the oxidation of the Cz and 3,5-bis(benzyloxy)phenyl
transitions at the optimized geometry in the ground state were units, since identical oxidation potentials were reported for
obtained in time-dependent DFT (TD-DFT) calculations using 1,3-bis(benzyloxy)benzene51 and N-alkylated Cz,52–54 respectively.
the same basis set and functional as for the ground state.
In accordance with this assignment, all dendrimers revealed two
anodic peaks and two cathodic peaks at very similar potentials,
and current efficiencies were higher with the greater number of
the Cz and 3,5-bis(benzyloxy)phenyl units in the dendrons.
It is well-established that the bridge unit of dibenzoxyphenyl
exhibits irreversible one-electron oxidation behavior at around
1.0–1.5 V.51 Further, the electrochemical properties of N-substituted
Cz exhibit a unique behavior of quasi-reversible, one anodic
and two cathodic peaks upon electrochemical oxidation, which
has been interpreted in terms of the initial one-electron oxida-
tion followed by oxidative coupling to generate dimers and
polymers susceptible to oxidation at lower potentials.52–54
Therefore, we assumed that the electrochemical behaviors of
dendrimers are basically the sum of their constituent units
(Ir3+ core, bridge unit of 3,5-bis(benzyloxy)phenyl, and carbazoles),
indicating negligible interaction of the constituent units.
Results and discussion
Electrochemistry
Fig. 1 shows CV traces of G0, G1, G2, and G3 in CH2Cl2 solution,
from which their oxidation and reduction potentials versus Fc+/Fc
were estimated, and are shown in Table 1. All the complexes
exhibit one reversible reduction wave at approximately ꢁ2.5 V.
It has been reported that electrochemical reduction processes
of the related Ir(III) complexes proceed on the ligand-centered
orbitals, mainly from the fluorinated phenylpyridine ligand.44–46
DFT calculations on G1 support that the first reduction potential
is due to the reduction of the dfppy ligand, based on the analysis
of lowest unoccupied molecular orbital (LUMO).
Steady-state absorption and emission spectra of Cz-OH and G0
Cz-OH exhibits two absorption bands at 292 nm (1La
’
1A
transition) and 339 nm (1Lb ’ A transition) characteristic of
Cz compounds as shown in Fig. 2.55–57 On the other hand,
G0 shows a strong absorption band at 230–350 nm, assigned
as spin-allowed ligand-centered 1p–p* transitions, and broad
absorption bands between 350 and 445 nm, attributed to the
1
Fig. 1 Cyclic voltammograms of G0, G1, G2 and G3 taken at a sweep rate
of 0.1 V sꢁ1 for 0.5 mM CH2Cl2 solution containing 0.1 M TBAP.
Table 1 Oxidation and reduction potentials of dendritic Ir3+ complexes
Oxidation (V)
Reduction (V)
a
b
ox
b
a
red
Compounds Epa1, Epa2
Epc1, Epc2
E1/2
Epc1
Epa1
E1/2
G0
G1
G2
G3
0.81
0.87
0.84
—
—
ꢁ2.51 ꢁ2.45 ꢁ2.48
ꢁ2.48 ꢁ2.40 ꢁ2.44
ꢁ2.50 ꢁ2.44 ꢁ2.47
ꢁ2.51 ꢁ2.45 ꢁ2.48
0.82, 1.06 0.49, 0.74
0.78, 1.03 0.47, 0.70
0.79, 1.04 0.46, 0.71
—
Fig. 2 UV-vis absorption and normalized photoluminescence spectra of
G0 and Cz-OH for 4 mM solution: fluorescence of Cz-OH in CH2Cl2 at
298 K and phosphorescence of Cz-OH and G0 in 2-MeTHF at 77 K.
a
b
Epa = anodic peak potential. Epc = cathodic peak potential. In
CH2Cl2.
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metal-to-ligand charge transfer (1MLCT) transitions. A noticeable in the ground state. Another important point of the G0 absorption
feature of the G0 spectrum is the long end-tail absorption extending spectrum is the good overlap with the fluorescence emission
to 470 nm. This wavelength is well overlapped with the onset spectrum of Cz-OH in the region of 330–450 nm, which ensures
wavelength (B440 nm) of phosphorescence, suggesting that 3MLCT Forster-type energy transfer from the excited-singlet Cz chromo-
¨
transitions might be involved in the end-tail absorption.58–61
phore to the Ir complex core (Table 2).
On the other hand, the phosphorescence of Cz-OH was
The emission spectrum of G0 at 298 K shows characteristic
measured at 400–550 nm in 2-MeTHF at 77 K, which is not MLCT emission bands at around 470 nm (Fig. 3b), which are
observed at room temperature. The phosphorescence spectrum considerably broader than that observed at 77 K without a
of G0 in 2-MeTHF at 77 K exhibits a vibrational structure. The significant red shift. For Gn, excitation at 290 nm gave a weak
strong vibrational band at 460 nm is related to the electronic emission at around 350 nm and a much stronger one at
0–0 transition from the lowest triplet state to the ground state. B470 nm, which are attributable to the fluorescence from the
Cz unit and the phosphorescence emitted from the Ir-complex
Steady-state absorption and emission spectra of Gn (n = 1, 2 and 3)
core, respectively. Crucially, the excitation of the Cz chromophore
in Gn at 290 nm results in the dominant appearance of the Ir-core
phosphorescence at B470 nm while the Cz fluorescence at
B350 nm is markedly quenched compared with that of Czn-OH
when measured under identical experimental conditions (Fig. 3b).
Furthermore, the phosphorescence intensities of Gn measured at
a fixed concentration are significantly higher than that of G0 and
increase with the increase of the dendrimer generation, which
demonstrates the occurrence of efficient energy transfer from the
excited-singlet state of Czn to the Ir-core associated with the light-
harvesting function of the Cz dendron.
Fig. 3a shows the absorption spectra of G0 and Gn (n = 1, 2, and 3).
The absorption spectra of Gn in the range 250–350 nm reveal the
features very similar to those of Czn-OH (Fig. S1 in ESI†). The
absorbance at 293, 327, and 340 nm is proportionally increased
with the number of Cz units in the dendron. It should be noted
that the spectra of Gn (n = 1, 2 and 3) show neither spectral
broadening, nor shift of each absorption band independently of
the dendrimer generation, and can be reproduced by the sum of
G0 and Czn, indicating that there is no significant electronic
interaction between the Czn donor and the Ir-complex acceptor
Emission lifetimes
The fluorescence lifetimes of Czn-OH (Cz2-OH, Cz4-OH, and
Cz8-OH) are all identical at 6.1 ns, whereas the Cz fluorescence
of Gn (n = 1, 2, and 3) shows markedly reduced lifetimes of 0.22,
0.40, and 0.62 ns in the order of generation (Fig. 4) accompanied
byo9% contributions of longer lifetime components (B6 ns),
which are almost identical to those of Czn-OH. The longer compo-
nent arises from the unquenched residual fraction of excited Cz, in
parallel with the weak fluorescence observed in the steady-state
emission spectra. On the other hand, the phosphorescence life-
times of Gn are commonly B1.0 ms, irrespective of the dendrimer
generation, as well as of the excitation wavelength, 309 nm for the
Cz excitation or 416 nm for the MLCT excitation of the Ir core, as
shown in Table 3, and Fig. S2 in ESI.† This means that the final
emitting state is the 3MLCT state of the Ir-complex core.
Quenching efficiency (energy transfer
efficiency)
Steady-state spectroscopic method (method 1)
The donor (Czn) and acceptor (Ir-complex) pairs in Gn are
separated with no electronic interaction, as revealed from the
absorption spectra of Gn. The fixed distance of their separation
confirms the dynamic nature of the observed fluorescence
quenching. The fluorescence quenching efficiency (eq) can be
calculated using the following eqn (1);
ꢀ
ꢁ
ID
ID0
eqð%Þ ¼ 1 ꢁ a ꢀ
ꢀ 100
(1)
Fig. 3 Absorption (a) and photoluminescence spectra (b) of 4.0 mM
solutions of G0 and Gn (n = 1, 2, and 3) in N2 saturated CH2Cl2 at 298 K.
Inset in figure (a) are magnified spectra using 82 mM solution. The emission
spectra in (b) were taken by excitation at 290 nm.
where I0D and ID are integrated fluorescence intensities in
the absence and presence of the Ir-core acceptor, respectively,
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Table 2 Absorption (labs) and emission (lem) wavelengths and absorption coefficients of G0 and Gn
At RTa
At 77 Kb
Compounds
labs (nm) (e ꢀ 103 (cmꢁ1
M
ꢁ1))
lem (nm)
lem (nm)
G0
G1
G2
G3
290 (28.3), 327 (14.9), 340 (9.7), 378 (5.1), 457 (0.6)
473, 494
460, 493
458, 491
458, 491
458, 491
290 (72.9), 293 (75.8), 327 (22.1), 340 (18.9) 378 (5.2), 457 (0.6)
290 (112.8), 293 (120.4), 327 (30.9), 340 (28.2), 378 (5.4), 457 (0.6)
290 (198.7), 293 (213.5), 327 (46.9), 340 (46.7), 378 (5.1), 457 (0.6)
350, 365, 471, 496
350, 365, 470, 499
352, 365, 471, 498
a
b
In Ar saturated CH2Cl2. In 2-MeTHF.
in Table 4. The energy transfer from the excited singlet Cz
(1Cz*) state to the excited singlet of the ligand is difficult,
because the excited state of the dfppy-ligand lies above
the 1Cz* state. On the other hand, the MLCT states of the
Ir-complex lie below the 1Cz* state and the fluorescence of Cz is
well overlapped with the absorption of the Ir-complex core in
the range of 340–450 nm. Therefore, we suggest that the energy
transfer process takes place through the low-lying MLCT states
of the Ir-complex.
Time-resolved spectroscopic method (method 2)
Based on the fluorescence decay-time constants, the quenching
efficiencies of 1Cz* were determined using the following
eqn (2);62
Fig. 4 Fluorescence decay profiles of Gn and Cz2-OH in CH2Cl2 mon-
kD
tAD
tD
itored at 360 nm. Excitation wavelength is 309 nm.
eqð%Þ ¼
ꢀ 100 ¼ 1 ꢁ
ꢀ 100
(2)
kD þ kAD
where kD and kAD indicate the Cz fluorescence rate constants in
the absence and presence of the acceptor (Ir-complex), respec-
tively. The eq of Gn are 96, 94 and 90% in order of generation, as
listed in Table 4. These values are quite similar to the results
determined by method 1 using the steady-state spectral data.
The eq of high generation dendrons slightly decreased because
of the longer distance between Cz and the Ir-core.
Table 3 Fluorescence lifetime (tf) of Cz and phosphorescence lifetime
(tp) of Ir-core in CH2Cl2
d
tfa (ns)
tp (ms)
tp (ms)
at 360 nm
at 500 nm, rt at 470 nm, 77 K
Czn-OH
Ir(dfppy)2(pic)
G0
G1
G2
G3
6.1
—
—
—
0.97
—
2.80
2.85
2.82
2.84
2.92
1.01b, 0.99c
0.99a, 0.98c
1.07a, 0.96c
0.22 (100%)
0.40 (100%)
Light-harvesting efficiency
0.62 (91%), 6.0 (9%) 0.98a, 0.94c
The phosphorescence of Gn is considerably enhanced com-
pared with that of G1, as shown in Fig. 3. The actual enhance-
ment could be calculated by the ratio of (IP/I0P) ꢀ100, where
IP and I0P are integrated phosphorescence intensities of the
Ir-complex in the absence and presence of donors, respectively,
in N2 saturated CH2Cl2. As listed in Table 4, the phosphorescence
of the Ir-complex core is enhanced by 157 (G1), 213 (G2) and
264% (G3) through the light-harvesting process.
a
b
c
d
lex = 309 nm.
lex = 355 nm. lex = 416 nm. In 2-MeTHF and at 77 K.
Error is less than ꢂ2%.
and the factor
a (=ADA/AD) denotes the light-absorption
contribution of the acceptor to the overall absorption at the
excitation wavelength. The quenching efficiencies of Czn in Gn
are 98, 95 and 92% in the order of dendron generation, as listed
The light-harvesting process depends on the energy transfer
efficiency and the emission quantum yield of the donor. There-
fore, the light-harvesting efficiency (eLH) is given by the following
relation:
Table 4 Quenching efficiency (eq) of Cz in dendrons determined by
steady-state emission (method 1) and a time-resolved spectroscopic
technique (method 2). The phosphorescence harvesting efficiency (eh) of
the Ir-complex cores in Gn (n = 1, 2, and 3) dendrons
e
LH(%) = (1 + n ꢀ eq ꢀ ff) ꢀ 100
(3)
where eq is the energy transfer efficiency (0.96–0.90), ff the
fluorescence quantum yield (0.3) of the donor (Cz), which is
measured in this study using 9,10-diphenylanthracene as a
standard, and n the number of Cz in dendrons. The expected
light-harvesting efficiencies show good agreement with experi-
mental values.
eq, Cz (%), method 1
eq, Cz (%), method 2
e
LH, Ir-corea (%)
G1
G2
G3
98
95
92
96
94
90
157 (158)
213 (213)
264 (316)
a
Using reference with G0. The parenthesis values are determined by
eqn (3) using the data of method 2.
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¨
Rate constant of Forster-type energy transfer
average distances between Cz and dfppy (RDA are 16, 19 and 21 Å)
for Gn, which can be calculated from the energy minimized
geometries for many computer-generated conformers using the
semi-empirical calculation (PM3 method) (Fig. S3, ESI†).65
Using these average values, the calculated energy-transfer
rate constants (kET) from excited Cz to the core are 4.95 ꢀ 109,
2.70 ꢀ 109, and 1.48 ꢀ 109 sꢁ1 for each generation of dendron,
which showed excellent agreement with experimental results,
as listed in Table 5. The quenching efficiencies and the rate
constants for energy transfer showed the dependency on the
center-to-center distances between the donor and acceptor in
dendrons.
The emission-lifetime measurements clearly demonstrated that
the excited-singlet Cz chromophore is quenched by the core at
rate constants (kq) of 4.39 ꢀ 109 (G1), 2.34 ꢀ 109 (G2) and 1.45 ꢀ
109 sꢁ1 (G3), respectively (Table 5). The non-radiative rate
constant for Cz-OH can be easily calculated according to the
equation, knr = (1 ꢁ ff) ꢀ kf, using the lifetimes of Cz-OH and
their fluorescence quantum yields (0.3) measured in this study,
giving a value of (knr = 1.11 ꢀ 108 sꢁ1). The knr value for Czn-OH
is quite similar to a reported value for the intersystem crossing
(kisc = 1.24 ꢀ 108 sꢁ1) of phenylcarbazole.63 The excited singlet
state of Cz seems to be deactivated non-radiatively through the
intersystem crossing process. However, the observed kq values
for Cz of Gn are over ten-times larger than that of kisc. Thus, in
Gn, the intersystem-crossing process of excited-singlet Cz can-
not compete with the quenching of excited-singlet Cz by the
Ir-complex core. Consequently, the fluorescence quenching
occurs exclusively by energy transfer from the excited-singlet
state of Cz to the Ir core with little participation of triplet Cz.
The energy-transfer rate constants can be calculated using
the following relationship (eqn (4)):
Transient absorption study
In order to confirm the nature of the excited state dynamics
of Gn, we performed nanosecond laser flash photolysis experi-
ments at room temperature. The transient absorption (TA)
difference spectra were collected at 500 ns-delay after excitation
at 309 nm. Fig. 5 shows the TA spectra of G0 and Gn in
Ar-saturated CH2Cl2 measured under identical conditions.
The TA spectra obtained for G0 exhibit the typical features of
a charge transfer state of the Ir-complex core. The strong and
sharp TA band at 420 nm can be assigned the characteristic of
the formation of a ligand radical anion, which may be formu-
lated as Ir4+(dfppyꢃꢁ1/2)2(pic-Ph) via the 3MLCT process.66 On
the other hand, a weaker broad absorption band in the longer
wavelength region can be assigned to T1–Tn transitions.66
The TA decay time for Gn well matches the emission decay
ꢀ
ꢁ
1
1
kET
¼
(4)
tD ð1=fETÞ ꢁ 1
where tD is the donor fluorescence lifetime in the absence of
the acceptor (Table 3) and fET is the energy transfer efficiency,
which is applied with quenching efficiencies of Cz for each
64
¨
generation (Table 4). The Forster model is appropriate to
interpret the rate constant for energy transfer given as
ꢀ
ꢁ
ð1
fDk2 9000 ln 10
kET
¼
J; J ¼
FDðlÞeðlÞl4dl
(5)
tDR6 128p5Nn4
0
where N is Avogadro’s number, k2 the orientation factor of the
transition dipole moments, fD the donor quantum yield,
J the overlap integral of the donor emission spectrum with
the acceptor absorption spectrum, n the index of refraction
of the solvent, tD the donor lifetime in the absence of the
acceptor, and R the distance between the donor and acceptor
¨
moieties. In the Forster model, all of the variables in the
expression except R are either constants or can be determined
experimentally. We used the following values for the para-
meters in eqn (5): k2 = 2/3, f = 0.3, J = 2.63 ꢀ 10ꢁ14 Mꢁ1 cm3,
n = 1.4246, and tD = 6.3 ns. For R values, an accurate determi-
nation is crucial, because the expression depends on the sixth
power of R. However, the dendron should take variable dis-
tances between donors and acceptor. Therefore, we adopted the
Table 5 Experimental and calculated energy transfer rate constants (kET
)
for energy transfer from the excited-singlet Cz unit to the Ir-complex core,
and average distances between donor and acceptor estimated by semi-
empirical calculations
Compounds
KET (exp) (sꢁ1
)
KET (cal) (sꢁ1
)
R (Å)
3.93 ꢀ 109
2.57 ꢀ 109
1.48 ꢀ 109
4.95 ꢀ 109
2.70 ꢀ 109
1.48 ꢀ 109
16
19
21
Fig. 5 Transient absorption spectra of G0, G1, G2, and G3 measured at
500 ns delay after excitation in Ar-saturated CH2Cl2 as obtained by laser
flash photolysis techniques. Excitation wavelength is 309 nm.
G1
G2
G3
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time constant of the Ir-complex core, indicating that the which is also true for L + 2 and L + 3. On the other hand, L + 1 is
transient spectra are associated with the emissive MLCT state. mainly populated on the ancillary ligand part. The orbitals
The similarity between the transient spectra of G0 and those of from L + 1 to L + 5 are also associated with the dfppy and
Gn suggests that the lowest excited state (emissive state) is picolinate ligands around the Ir-atom, which are lower excited
MLCT in character.
states, whereas L + 6 and L + 7 orbitals are located at higher
energy states attributable to the Cz and 3,5-bis(benzyloxy)-
phenyl branch, respectively (see diagrams in Fig. S4, ESI†).67
Table 6 lists the typical electronic transitions with significant
oscillator strengths ( f > 0.001 on singlet, lowest four transitions
on triplet) produced by TD-DFT calculations. The calculated
results in Table 6 imply that the observed absorption bands of
G1 at 350–450 nm involve the transitions from H ꢁ 3, H ꢁ 5,
and H ꢁ 7 to LUMO – L + 2. These transitions apparently
possess mixed characters of [dp(Ir) - p*dfppy or pic] MLCT and
[pdfppy or pic - p*pic or dfppy] ligand-to-ligand charge transfer.
Among them, the transitions from H ꢁ 3 to LUMO or L + 2
and from H ꢁ 5 to LUMO or L + 1 with a dominant triplet
MLCT character are expected to have a major impact on the
400–500 nm band.
Density functional calculations
It is now widely accepted that DFT/TD-DFT calculations are useful
for obtaining insights into different electronic states related to the
relevant absorption and emission processes. Following accurate
geometry optimization, we modeled the electronic structure of G1.
As shown in Fig. 6, the highest occupied molecular orbital
(HOMO, H) consists principally of a Cz unit. Similar features
can be observed for the other occupied MOs, H ꢁ 1 and H ꢁ 2,
whereas the H ꢁ 3, H ꢁ 5 and H ꢁ 7 orbitals consist principally
of the dfppy p-orbitals with a substantial contribution of
IrIII d-orbitals, as shown in Fig. S4 (ESI†). The H ꢁ 4 orbital is
populated on 3,5-bis(benzyloxy)phenyl branches. The first
oxidation of G1 can be assigned as the oxidation of Cz units,
because the higher occupied orbitals, such as HOMO – H ꢁ 2,
are related to Cz. By contrast, LUMO (L) is largely localized on
the main ligand with little ancillary ligand orbital character,
Conclusions
We have synthesized the series of Ir(dfppy)2(pic-Czn) dendrimers.
The light-harvesting process and photophysical properties of
Fig. 6 Energy levels and isodensity plots for selected occupied and
unoccupied molecular orbitals of G1 obtained by DFT calculations.
Fig. 7 Schematic energy diagram for Ir(dfppy)2(pic-Czn) dendrimers.
Table 6 Selected oscillator strength (f) and calculated electronic transitions of G1
No.
l (nm)
f
Transition (contribution)
Assignmenta
1
2
3
4
5
6
7
8
398
387
377
374
372
367
360
358
348
335
326
324
323
0.0049
0.0054
0.0228
0.0549
0.0106
0.038
H ꢁ 3 - LUMO (56%), H ꢁ 3 - L + 1 (27%)
H ꢁ 5 - LUMO (67%), H ꢁ 5 - L + 1 (14%)
H ꢁ 5 - L + 1 (42%), H ꢁ 7 - L + 1 (33%)
H ꢁ 7 - LUMO (45%), H ꢁ 5 - LUMO (14%), H ꢁ 3 - L + 2 (14%)
H ꢁ 3 - L + 2 (72%)
MLCT
MLCT
ML0CT
MLCT
LL0CT
MLCT
MLCT
MLCT
MLCT
MLCT
MLCT
MLCT
MLCT
H ꢁ 7 - L + 1 (31%), H ꢁ 7 - LUMO (19%)
H ꢁ 9 - L + 1 (14%), H ꢁ 8 - LUMO (14%)
H ꢁ 5 - L + 2 (77%)
0.017
0.0023
0.0327
0.0014
0.0019
0.0077
0.005
9
H ꢁ 7 - L + 2 (82%)
10
11
12
13
H ꢁ 5 - L + 12 (25%), H ꢁ 3 - L + 12 (46%)
H ꢁ 7 - L + 12 (25%), H ꢁ 5 - L + 12 (24%), H ꢁ 3 - L + 12 (27%)
H ꢁ 5 - L + 3 (95%)
H ꢁ 9 - LUMO (38%), H ꢁ 8 - LUMO (43%)
a
M = Ir(III), L = dfppy, and L0 = pic.
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dendrimers have been determined. The Cz-dendrons act as 127.6, 105.8, 101.4, 70.2, 65.4. HRMS (FAB): m/z 320.1410 (calcd
extremely efficient light-harvesting antennas capable of trans- for [M + H]+ 320.1412). Anal. Calcd for C21H20O3: C, 78.73; H,
ferring light energy through space to their Ir-core complex. The 6.29. Found: C, 78.70; H, 6.31.
fluorescence of Cz overlapped well with the MLCT absorption
Ph2-Br. Triphenylphosphine (6.7 g, 26 mmol) in dry CH2Cl2
band of the Ir-complex. The energy transfer can be explained by (24 ml) was added to bromine (1.3 mL, 26 mmol) dropwise at
¨
the Forster model as shown in Fig. 7. The energy transfer 0 1C and stirred for 15 min. The salt was dissolved in dry CH2Cl2
efficiency (>90%) was found to be quantitative for generation, and added to a solution of Ph2-OH (4.1 g, 13 mmol) in dry
with a slight decrease observed for the high generations, which CH2Cl2 (20 ml) via a cannula. The solution was then allowed to
was determined by the steady-state and the time-resolved warm to room temperature and was stirred for 1 h. The reaction
spectroscopic methods. The energy transfer rate constants was quenched by adding water, and the aqueous layer was
¨
calculated by the Forster model showed good agreement with extracted using CH2Cl2. The combined organic phase was dried
the experimental values. The CV result and DFT calculation using MgSO4, and the solvent was removed by reduced pressure.
imply the occurrence of energy transfer from the Cz-unit to the The product was purified by column chromatography on silica
Ir-core complex. Most importantly, actual enhancement of gel (eluent: CH2Cl2–n-hexane (v/v = 1 : 5) and then CH2Cl2) to
1
light-harvesting is strongly dependent on key factors such as give a white solid in 90% yield. H NMR (300.1 MHz, CDCl3):
the number of Cz-units, the fluorescence quantum yield of Cz d 7.44–7.35 (m, 10H), 6.65 (s, 2H), 6.56 (s, 1H), 5.03 (s, 4H), 4.42
and the energy transfer efficiency. As a result, the light harvest- (s, 2H). 13C NMR (75.4 MHz, CDCl3): d 160.1, 139.8, 136.6, 128.7,
ing efficiencies of 157 (G1), 213 (G2), and 264% (G3), are 128.7, 128.1, 127.6, 127.0, 108.2, 102.2, 70.2, 33.8. HRMS (FAB):
measured, and showed good agreement with expected values.
m/z 382.0571 (calcd for [M + H]+ 382.0568). Anal. Calcd for
C21H19BrO2: C, 65.81; H, 5.00. Found: C, 65.79; H, 4.99.
Cz2-Br. The synthesis of Cz2-OH has been described earlier.69
Synthetic details
All complexes were synthesized following the synthetic route The reaction of triphenylphosphine (3.7 g, 14 mmol) and bromine
shown in Scheme 1. All procedures were carried out in an inert (0.7 mL, 14 mmol) in minimal CH2Cl2 with a solution of Cz2-OH
atmosphere. Cyclometalated IrIII-chloro-bridged dimers of gen- (4.6 g, 7 mmol) in CH2Cl2 afforded a white solid in 80% yield.
eral formula [(dfppy)2Ir(m-Cl)]2 were synthesized by the method 1H NMR (300.1 MHz, CDCl3): d 8.17 (d, 4H), 7.69 (d, 4H), 7.62
reported by Nonoyama.68
(d, 4H), 7.45–7.41 (m, 8H), 7.31 (t, 4H), 6.77 (s, 2H), 6.71 (s, 1H),
Ph2-OMe. A mixture of benzyl bromide (7.2 g, 42 mmol), 5.19 (s, 4H), 4.49 (s, 2H). 13C NMR (75.4 MHz, CDCl3): d 161.3,
methyl-3,5-dihydroxybenzoate (3.5 g, 21 mmol), and potassium 141.0, 140.3, 137.8, 136.0, 129.3, 127.5, 126.2, 123.6, 120.6, 120.3,
carbonate (11.6 g, 84 mmol) was dissolved in dry ACN (50 ml). 110.0, 108.5, 102.5, 70.0, 33.7. HRMS (FAB): m/z 712.1721 (calcd
The reaction mixture was heated at 85 1C for 15 h. After cooling for [M + H]+ 712.1725). Anal. Calcd for C45H33BrN2O2: C, 75.73; H,
to room temperature, the reaction mixture was poured into 4.66; N, 3.93. Found: C, 75.91; H, 4.68; N, 3.92.
water. The organic phase was extracted with CH2Cl2 and dried
Cz4-OMe. Cz2-Br (4.0 g, 5.6 mmol) and methyl 3,5-dihydroxy-
using MgSO4. After removal of the solvents under reduced benzoate (0.47 g, 2.8 mmol) were reacted with potassium
pressure, the residue was purified by column chromatography carbonate (22.4 g, 3 mmol) in ACN/THF (30 ml/10 ml). The crude
(CC) on silica gel (eluent: CH2Cl2–n-hexane (v/v = 2 : 1)) to product was separated by column chromatography on silica gel
1
provide a white solid product in 76% yield. H NMR (300.1 MHz, (eluent: CH2Cl2–n-hexane (v/v = 2:1)) to give a white powder in
CDCl3): d 7.30–7.45 (m, 12H), 6.81 (s, 1H), 5.08 (s, 4H), 3.91 (s, 3H). 91% yield. 1H NMR (300.1 MHz, CDCl3): d 8.17 (d, 8H), 7.71
13C NMR (75.4 MHz, CDCl3): d 166.8, 159.8, 136.5, 132.1, 128.7, (d, 8H), 7.67 (d, 8H), 7.49–7.44 (m, 18H), 7.36 (t, 8H) 6.91 (s, 1H),
128.2, 127.7, 108.4, 107.3, 70.4, 52.4. HRMS (FAB): m/z 348.1366 6.82 (s, 4H), 6.74 (s, 2H), 5.19 (s, 8H), 5.11 (s, 4H), 3.91 (s, 3H). 13
C
(calcd for [M + H]+ 348.1362). Anal. Calcd for C22H20O4: C, 75.84; NMR (75.4 MHz, CDCl3): d 166.5, 160.4, 160.0, 141.0, 139.4, 137.7,
H, 5.79. Found: C, 75.85; H, 5.74. 136.1, 132.4, 129.4, 127.4, 126.2, 123.6, 120.6, 120.3, 110.0, 108.7,
Ph2-OH. Ph2-OH was prepared according to the general 107.5, 106.8, 102.0, 70.4, 69.9, 52.6. HRMS (FAB): m/z 1432.5341
method for LiAlH4 reduction of aromatic esters to benzylic (calcd for [M + H]+ 1432.5350). Anal. Calcd for C98H72N4O8: C,
alcohols. Ph2-OMe (6.2 g, 18 mmol) was added dropwise to a 82.10; H, 5.06; N, 3.91. Found: C, 82.41; H, 5.08; N, 3.90.
suspension of LiAlH4 (0.81 g, 21 mmol) in dry THF (30 ml)
Cz4-OH. The reaction of Cz4-OMe (3.7 g, 2.6 mmol) with
at 0 1C. The suspension was warmed to room temperature and LiAlH4 (0.13 g, 3.3 mmol) in 30 ml of THF afforded a white solid
stirred until the reaction was completed as indicated by TLC. in 88% yield. The crude product was separated by column
The reaction was quenched by adding water with an ice bath. chromatography on silica gel (eluent: CH2Cl2). 1H NMR
It was stirred at room temperature for 15 min, and extracted with (300.1 MHz, CDCl3): d 8.19 (d, 8H), 7.69 (d, 8H), 7.67 (d, 8H),
CH2Cl2. The organic layers were combined, washed with water, 7.46–7.42 (m, 16H), 7.33 (t, 8H), 6.85 (s, 4H), 6.76 (s, 2H), 6.73
dried under MgSO4 and filtered. The filtrate was concentrated (s, 2H), 5.18 (s, 8H), 5.09 (s, 4H), 4.67 (s, 2H), 4.29 (s, 1H).
using a rotary evaporator under reduced pressure. The residue 13C NMR (75.4 MHz, CDCl3): d 160.4, 141.0, 139.8, 137.8, 136.1,
was purified by CC on silica gel (eluent: CH2Cl2) to give a white 129.4, 127.4, 126.2, 123.7, 120.6, 120.3, 110.0, 106.8, 101.9, 70.2,
solid in 74% yield. 1H NMR (300.1 MHz, CDCl3): d 7.44–7.33 70.0, 65.5. MALDI-TOF: m/z 1404.53 (calcd for [M + H]+
(m, 10H), 6.63 (s, 2H), 6.55 (s, 1H), 5.04 (s, 4H), 4.63 (s, 2H). 1404.54). Anal. Calcd for C97H72N4O7: C, 82.88; H, 5.16; N,
13C NMR (75.4 MHz, CDCl3): d 160.2, 143.5, 136.9, 128.7, 128.1, 3.99. Found: C, 83.16; H, 5.18; N, 3.98.
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Scheme 1 Synthetic route of Ir(dfppy)2(pic-Ph2) and Ir(dfppy)(pic-Czn) (n = 2, 4, and 8). (a) K2CO3, ACN, 85 1C, 15 h; (b) LiAlH4, THF, 0 1C; (c) PPh3, Br2,
CH2Cl2, 0 1C, 1 h; (d) LiAlH4, THF, room temperature, 12 h; (e) 1/2 IrCl3ꢄH2O, 2-ethoxyethanol : H2O (v/v = 3 : 1), 120 1C; (f) Na2CO3, 2-ethoxyethanol,
140 1C; (g) Czn-Br or Ph2-Br, K2CO3, ACN:THF (v/v = 3 : 1), 85 1C.
Cz4-Br. The reaction of triphenylphosphine (1.2 g, 4.5 mmol) (eluent: CH2Cl2–n-hexane (v/v = 2 : 1)) to give a white powder in
1
and bromine (0.2 ml, 4.5 mmol) in CH2Cl2 (15 ml) with a 62% yield. H NMR (300.1 MHz, CDCl3): d 8.21 (d, 16H), 7.67
solution of Cz4-OH (3.1 g, 2.2 mmol) in CH2Cl2 afforded a white (d, 16H), 7.61 (d, 16H), 7.50–7.42 (m, 32H), 7.35 (t, 16H), 6.9
solid in 94% yield. The reaction mixture was purified by column (s, 2H), 6.87 (s, 7H), 6.85 (s, 4H), 6.75 (s, 8H), 5.16 (s, 16H), 5.11
chromatography on silica gel (eluent: CH2Cl2–n-hexane (v/v = (s, 8H), 5.07 (s, 4H), 3.94 (s, 3H). 13C NMR (75.4 MHz, CDCl3):
1 : 1)). 1H NMR (300.1 MHz, CDCl3): d 8.20 (d, 8H), 7.71 (d, 8H), d 166.9, 160.5, 160.0, 141.0, 139.8, 137.8, 130.5, 129.4, 127.3,
7.67 (d, 8H), 7.49–7.44 (m, 16H), 7.36 (t, 8H), 6.86 (s, 4H), 6.78 126.2, 123.7, 120.7, 120.6, 120.4, 120.1, 117.7, 110.1, 106.8,
(s, 2H), 6.77 (s, 2H), 6.71 (s, 1H), 5.20 (s, 8H), 5.08 (s, 4H), 4.47 102.0, 70.3, 69.9, 52.6. MALDI-TOF: m/z 2941.03 (calcd for
(s, 2H). 13C NMR (75.4 MHz, CDCl3): d 160.4, 160.3, 141.0, 139.6, [M + H]+ 2941.10). Anal. Calcd for C202H148N8O16: C, 82.43; H,
137.8, 136.1, 129.4, 127.4, 126.3, 123.7, 120.6, 120.3, 110.0, 108.5, 5.07; N, 3.81. Found: C, 82.80; H, 5.09; N, 3.80.
106.9, 102.6, 102.0, 70.3, 70.0, 33.9. MALDI-TOF: m/z 1466.44
(calcd for [M + H]+ 1466.46). Anal. Calcd for C97H71BrN4O6: C, (0.03 g, 0.7 mmol) in 10 ml of THF afforded a white solid in 88%
79.33; H, 4.87; N, 3.82. Found: C, 79.56; H, 4.88; N, 3.81. yield. The crude product was separated by column chromato-
Cz8-OH. The reaction of Cz8-OMe (1.1 g, 0.4 mmol) with LiAlH4
Cz8-OMe. Cz4-Br (1.7 g, 1.1 mmol) and methyl 3,5-dihydroxy- graphy on silica gel (eluent: CH2Cl2). 1H NMR (300.1 MHz, CDCl3):
benzoate (0.1 g, 0.6 mmol) were reacted with potassium carbon- d 8.17 (d, 16H), 7.70 (d, 16H), 7.62 (d, 16H), 7.46–7.41 (m, 32H),
ate (0.6 g, 4.6 mmol) in ACN/THF (30 ml/10 ml). The crude 7.31 (t, 16H), 6.76 (s, 14H), 6.70 (s, 7H), 5.28 (s, 1H), 5.19 (s, 28H),
product was separated by column chromatography on silica gel 4.73 (d, 2H). 13C NMR (75.4 MHz, CDCl3): d 160.4, 144.0, 141.0,
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137.7, 136.2, 129.3, 127.5, 126.2, 123.6, 120.6, 120.3, 110.0, (calcd for [M + H]+ 1013.21). Anal. Calcd for C49H34F4IrN3O5: C,
106.1, 101.6, 70.0, 68.2, 65.5. MALDI-TOF: m/z 2913.07 (calcd 58.10; H, 3.38; N, 4.15. Found: C, 47.47; H, 2.28; N, 5.89.
for 2913.11). Anal. Calcd for C201H148N8O15: C, 82.81; H, 5.12; N,
3.84. Found: C, 83.05; H, 5.13; N, 3.83.
Ir(dfppy)2(pic-Cz2). This compound was purified by column
chromatography on silica gel eluting with CH2Cl2 to remove the
Cz8-Br. The reaction of triphenylphosphine (0.6 g, 2.4 mmol) Cz2-Br and then CH2Cl2–ethylacetate (v/v = 1 : 1) to provide
and bromine (0.12 ml, 2.4 mmol) in CH2Cl2 (5 ml) with a the desired product as a white solid in 80% yield. 1H NMR
solution of Cz4-OH (3.5 g, 1.2 mmol) in CH2Cl2 afforded a white (300.1 MHz, CDCl3): d 8.83 (d, 1H), 8.20 (t, 2H), 8.13 (d, 3H),
solid in 70% yield. The residue was purified by column chromato- 7.66 (d, 6H), 7.55 (d, 4H), 7.43–7.48 (m, 2H), 7.39 (t, 9H), 7.28
graphy on silica gel (eluent: CH2Cl2–n-hexane (v/v = 1 :1)) to give a (t, 5H), 7.09–7.14 (m, 2H), 7.05 (s, 2H), 6.89 (s, 1H), 6.68 (s, 1H),
1
white solid. H NMR (300.1 MHz, CDCl3): d 8.12 (d, 16H), 7.62 6.44 (t, 1H), 6.35 (t, 1H), 5.79 (d, 1H), 5.53 (d, 1H), 5.28 (s, 2H),
(d, 16H), 7.55 (m, 16H), 7.24–7.37 (m, 48H), 6.61–6.77 (m, 19H), 5.18 (s, 4H). 13C NMR (75.4 MHz, CDCl3): d 171.4, 164.6, 162.8,
6.43–6.48 (m, 2H), 4.99–5.16 (m, 28H), 4.25 (s, 2H). 13C NMR 162.3, 160.5, 158.8, 153.0, 149.0, 148.1, 141.3, 141.2, 138.6,
(75.4 MHz, CDCl3): d 160.4, 141.0, 139.8, 136.1, 130.4, 129.4, 138.4, 137.6, 136.6, 129.6, 127.2, 126.2, 124.17, 123.6, 122.5,
127.4, 126.1, 123.5, 120.3, 110.0, 106.8, 101.9, 70.3, 69.9. MALDI- 120.6, 120.2, 114.6, 110.0, 105.7, 102.1, 98.0, 71.0, 70.0. HRMS
TOF: m/z 2974.98 (calcd for [M + H]+ 2975.02). Anal. Calcd for (FAB): m/z 1343.3216 (calcd for [M + H]+ 1343.3221). Anal. Calcd
C201H147BrN8O14: C, 81.06; H, 4.97; N, 3.76. Found: C, 81.40; H, for C73H48F4IrN5O5: C, 65.27; H, 3.60; N, 5.21. Found: C, 65.48;
4.99; N, 3.75.
H, 3.61; N, 5.19.
Ir(dfppy)2(pic). A mixture of [(dfppy)2Ir(m-Cl)]2 (0.48 g,
Ir(dfppy)2(pic-Cz4). This product was purified by column
0.4 mmol), 3-hydroxypicolinic acid (0.16 g, 1.2 mmol), and chromatography on silica gel eluting with CH2Cl2 to remove
sodium carbonate (0.33 g, 3.1 mmol) was dissolved in 2-ethoxy- the Cz4-Br and then CH2Cl2–ethylacetate (v/v = 1 : 4) to provide
ethanol (25 ml) and heated to 120 1C for 15 h. After cooling to the desired product as a white solid in 57% yield. 1H NMR
room temperature, the mixture was quenched with water. The (300.1 MHz, CDCl3): d 8.80 (d, 1H), 8.20 (t, 2H), 8.13 (s, 9H), 7.64
organic phase was extracted with CH2Cl2 and then dried using (d, 10H), 7.56 (d, 10H), 7.38–7.43 (m, 20H), 7.23–7.28 (m, 10H),
MgSO4. After the removal of solvents under reduced pressure, 6.98 (s, 2H), 6.80 (s, 4H), 6.44 (t, 1H), 6.33 (t, 1H), 5.79 (d, 1H),
the residue was purified by column chromatography on silica 5.55 (d, 1H), 5.15–5.18 (m, 12H), 5.04–5.09 (m, 2H). 13C NMR
gel (eluent: CH2Cl2–methanol (v/v = 100 : 1)) to give the desired (75.4 MHz, CDCl3): d 171.5, 165.9, 164.6, 160.4, 160.3, 158.7,
product (yield: 0.9 g, 65%). 1H NMR (300.1 MHz, CDCl3): d 153.0, 149.0, 148.1, 141.3, 140.9, 140.5, 139.8, 138.4, 137.7,
13.56 (s, 1H), 8.65 (d, 1H), 8.29 (d, 1H), 8.23 (d, 1H), 8.79 (t, 3H), 136.2, 129.3, 127.4, 126.2, 124.2, 123.6, 122.6, 120.6, 120.3,
7.46 (d, 1H), 7.42 (dd, 1H), 7.25(d, 2H), 7.21 (t, 1H), 7.02 (t, 1H), 114.7, 110.0, 106.9, 105.6, 102.1, 98.02, 70.2, 69.9. MALDI-
6.46 (t, 1H), 6.40 (t, 1H), 5.77 (d, 1H), 5.56 (d, 1H). 13C NMR TOF: m/z 2097.64 (calcd for [M + H]+ 2097.60). Anal. Calcd for
(75.4 MHz, CDCl3): d 177.7, 165.8, 164.3, 162.6, 162.3, 160.9,
C125H86F4IrN7O9: C, 71.55; H, 4.13; N, 4.67. Found: C, 71.70; H,
160.5, 152.3, 149.9, 148.8, 148.2, 139.8, 138.7, 138.6, 134.5, 4.14; N, 4.65.
130.2, 128.4, 127.5, 123.6, 123.5, 123.1, 123.0, 122.9, 122.7,
Ir(dfppy)2(pic-Cz8). This product was purified by column
114.8, 114.7, 98.6, 98.1. HRMS (FAB): m/z 711.0752 (calcd for chromatography on silica gel eluting with CH2Cl2 to remove
[M + H]+ 711.0757). Anal. Calcd for C28H16F4IrN3O3: C, 47.32; H, the Cz8-Br and then CH2Cl2–ethylacetate (1 : 5 v/v) to provide
2.27; N, 5.91. Found: C, 47.47; H, 2.28; N, 5.89.
the desired product as a white solid in 24% yield. 1H NMR
(300.1 MHz, CDCl3): d 8.77 (d, 1H), 8.10 (d, 18H), 7.60 (d, 20H),
7.52 (d, 20H), 7.35–7.42 (m, 40H), 7.23–7.25 (m, 20H), 6.62–6.66
(m, 9H), 6.41 (t, 1H), 6.29 (t, 1H), 5.75 (d, 1H), 5.51 (d, 1H), 5.01
General procedure for Ir(dfppy)2(pic-Ph2) or Ir(dfppy)2(pic-Czn)
(n = 2, 4, and 8) complexes
A mixture of Ph2-Br or Czn-Br (0.27 mmol), Ir(dfppy)2(pic) (bs, 20H), 4.95–5.00 (m, 10H). 13C NMR (75.4 MHz, CDCl3):
(0.27 mmol) and K2CO3 (0.3 g, 2.17 mmol) in ACN/THF d 171.4, 160.3, 148.1, 141.1, 140.9, 139.7, 139.6, 138.3, 137.6,
(12 ml/4 ml) was refluxed under a N2 environment for 12 h. 136.1, 130.4, 129.3, 127.3, 126.2, 123.5, 122.6, 120.5, 120.2,
After cooling to room temperature, the reaction mixture was 120.0, 109.9, 106.7, 105.5, 101.8, 70.2, 69.9. MALDI-TOF:
poured into water. The organic phase was extracted with CH2Cl2 m/z 3606.12 (calcd for [M + H]+ 3606.17). Anal. Calcd for
and dried under MgSO4. After removal of solvents under reduced
C229H162F4IrN11O17: C, 76.23; H, 4.53; N, 4.27. Found: C,
pressure, the residue was purified by silica gel column chromato- 76.55; H, 4.55; N, 4.25.
graphy to provide the desired product.
Ir(dfppy)2(pic-Ph2). This product was purified by column
chromatography on silica gel eluting with CH2Cl2 to remove
Abbreviations and full names
the Ph2-Br and then CH2Cl2–ethylacetate (v/v = 1: 1) to provide the
1
desired product as a solid in 76% yield. H NMR (300.1 MHz, Ph2-OMe
3,5-Bis-benzyloxybenzoic acid methyl ester
(3,5-Bis-benzyloxyphenyl)methanol
1,3-Bis-benzyloxy-5-bromomethylbenzene
9,90-(4,40-(5-(Bromomethyl)-1,3-phenylene)-
bis(oxy)bis(methylene)bis(4,1-phenylene))-
bis(9H-carbazole)
CDCl3): d 8.82 (d, 1H), 8.23 (t, 2H), 7.73–7.65 (m, 2H), 7.42–7.13 Ph2-OH
(m, 14H), 6.91 (t, 1H), 6.84 (d, 2H), 6.55 (s, 1H), 6.49–6.34 (m, 3H), Ph2-Br
5.78 (d, 1H), 5.44 (d, 1H), 5.30 (s, 2H), 5.03 (s, 4H). 13C NMR Cz2-Br
(75.4 MHz, CDCl3): d 171.2, 160.3, 158.6, 152.9, 152.8, 138.2,
136.7, 128.9, 128.6, 128.1, 127.8, 70.3. HRMS (FAB): m/z 1013.18
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Cz4-OMe
Cz4-OH
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(3,5-Bis(3,5-bis(4-(9H-carbazol-9-yl)benzyloxy)-
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Ir(dfppy)2(pic)
Iridium(III) bis[(4,6-difluorophenyl)-
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Ir(dfppy)2(pic-Ph2) Iridium(III) bis[(4,6-difluorophenyl)-
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¨
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Ir(dfppy)2(pic-Cz4) Iridium(III) bis[(4,6-difluorophenyl)-
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(4-(9H-carbazol-9-yl)benzyloxy)benzyloxy)-
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Ir(dfppy)2(pic-Cz8) Iridium(III) bis[(4,6-difluorophenyl)-
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Acknowledgements
This work was supported by the Fundamental Technology
Development Programs for the Future through the Korea
Institute of Science and Technology (KIST, No. 2E23752-13-038)
and the Korea Evaluation Institute of Industrial Technology (KEIT)
funded by the Ministry of Knowledge Economy (MKE, Korea)
through the Improving Competitiveness of Materials and Compo-
nents Industry Program (Grant number 10042682). We also
acknowledge the Chungcheong Institute for Regional Program
Evaluation (CIRPE, grant number A0046 00226).
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