Bright blue phosphorescence from cationic bis-cyclometalated iridium(III) isocyanide complexes

Article abstract of DOI:10.1021/ic202297h

We report new bis-cyclometalated cationic iridium(III) complexes [(CN) ₂Ir(CN-tert-Bu)₂](CF₃SO₃) that have tert-butyl isocyanides as neutral auxiliary ligands and 2-phenylpyridine or 2-(4'- fluorophenyl)-R-pyridines (where R is 4-methoxy, 4-tert-butyl, or 5-trifluoromethyl) as CN ligands. The complexes are white or pale yellow solids that show irreversible reduction and oxidation processes and have a large electrochemical gap of 3.58-3.83 V. They emit blue or bluegreen phosphorescence in liquid/solid solutions from a cyclometalatingligand- centered excited state. Their emission spectra show vibronic structure with the highest-energy luminescence peak at 440-459 nm. The corresponding quantum yields and observed excitedstate lifetimes are up to 76% and 46 μs, respectively, and the calculated radiative lifetimes are in the range of 46-82 μs. In solution, the photophysical properties of the complexes are solvent-independent, and their emission color is tuned by variation of the substituents in the cyclometalating ligand. For most of the complexes, an emission color red shift occurs in going from solution to neat solids. However, the shift is minimal for the complexes with bulky tert-butyl or trifluoromethyl groups on the cyclometalating ligands that prevent aggregation. We report the first example of an iridium(III) isocyanide complex that emits blue phosphorescence not only in solution but also as a neat solid.

Full text of DOI:10.1021/ic202297h

Article
pubs.acs.org/IC
Bright Blue Phosphorescence from Cationic Bis-Cyclometalated
Iridium(III) Isocyanide Complexes
Nail M. Shavaleev,*,† Filippo Monti,‡ Ruben D. Costa,§,⊥ Rosario Scopelliti,† Henk J. Bolink,§
́
Enrique Ortí,§ Gianluca Accorsi,‡ Nicola Armaroli,*,‡ Etienne Baranoff,†,∥ Michael Gratzel,†
̈
and Mohammad K. Nazeeruddin*,†
†
́
Laboratory of Photonics and Interfaces, Ecole Polytechnique Fed
́
er
́
ale de Lausanne, CH-1015 Lausanne, Switzerland
Consiglio Nazionale delle Ricerche, Via P. Gobetti
‡Molecular Photoscience Group, Istituto per la Sintesi Organica e la Fotoreattivita,
101, 40129, Bologna, Italy
§Instituto de Ciencia Molecular, Universidad de Valencia, 46980 Paterna, Spain
̀
S
* Supporting Information
ABSTRACT: We report new bis-cyclometalated cationic iridium(III)
complexes [(C∧N)2Ir(CN-tert-Bu)2](CF3SO3) that have tert-butyl
isocyanides as neutral auxiliary ligands and 2-phenylpyridine or 2-(4′-
fluorophenyl)-R-pyridines (where R is 4-methoxy, 4-tert-butyl, or
5-trifluoromethyl) as C∧N ligands. The complexes are white or pale
yellow solids that show irreversible reduction and oxidation processes and
have a large electrochemical gap of 3.58−3.83 V. They emit blue or blue-
green phosphorescence in liquid/solid solutions from a cyclometalating-
ligand-centered excited state. Their emission spectra show vibronic
structure with the highest-energy luminescence peak at 440−459 nm. The corresponding quantum yields and observed excited-
state lifetimes are up to 76% and 46 μs, respectively, and the calculated radiative lifetimes are in the range of 46−82 μs. In
solution, the photophysical properties of the complexes are solvent-independent, and their emission color is tuned by variation of
the substituents in the cyclometalating ligand. For most of the complexes, an emission color red shift occurs in going from
solution to neat solids. However, the shift is minimal for the complexes with bulky tert-butyl or trifluoromethyl groups on the
cyclometalating ligands that prevent aggregation. We report the first example of an iridium(III) isocyanide complex that emits
blue phosphorescence not only in solution but also as a neat solid.
problems encountered with these complexes are (a) low phos-
phorescence quantum yields that result from quenching of
high-energy emissive excited states by d−d metal states and
(b) red-shift and quenching of phosphorescence in going from
solutions to neat solids caused by aggregation and triplet−triplet
interaction.
Recently, it was reported that neutral and cationic Ir(III)
complexes with alkyl isocyanides show blue to green phos-
phorescence in solution (Chart 1).6,10,43−46 However, their solid-
state photophysical properties, which are of importance for
practical applications, were only briefly mentioned.10 Here,
we present new bis-cyclometalated, cationic Ir(III) complexes,
[(C∧N)2Ir(CN-tert-C4H9)2](CF3SO3), with 2-phenylpyridines
(C∧N) and strong-field, nonchromophoric neutral tert-butyl
isocyanides (1−5 in Scheme 1). To tune the photophysical
properties of the complexes, we modified the cyclometalating
2-phenylpyridine ligands with electron-withdrawing (4′-F and
5-CF3), electron-donating (4-OCH3), and bulky (4-tert-butyl and
5-CF3) substituents. As a result, we developed cationic iridium(III)
isocyanide complexes that emit phosphorescence at higher
INTRODUCTION
■
Highly phosphorescent cationic iridium(III) complexes
[(C∧N)2Ir(L)n]+, where C∧N is a cyclometalating ligand and
L is a neutral auxiliary ligand,1−3 are widely used as triplet
emitters in luminescent sensors4−6 and switches,7 organic light-
emitting diodes (OLED),8−11 and light-emitting electro-
chemical cells (LEC).12−32 They are also developed for applica-
tion as nonlinear optical materials,33 donor−acceptor chromo-
phores with long-lived charge-separated states,34 and sensitizer
dyes for solar cells.35 The photophysical properties of these
complexes are determined by the strong spin−orbit coupling
provided by the Ir(III) and by the triplet-emitting excited state
of ligand-centered (LC) or charge-transfer (CT) nature.2 Key
advantages of these complexes are the ease of synthetic
modification, the facile variation of emission colors from blue to
near-infrared, the high phosphorescence quantum yields, and
the relatively short excited-state lifetimes.1−3,36−39
We are particularly interested in high-energy-emitting Ir(III)
complexes because they are required for generation of
blue9,23−28 and white9,29−31 light in electroluminescent devices.
Blue-phosphorescent bis-cyclometalated cationic Ir(III) complexes
with various neutral ligands such as diimines,22−27,40 carbenes,28,32
carbon monoxide,41 and phosphines41,42 are known. The common
Received: October 24, 2011
Published: January 26, 2012
© 2012 American Chemical Society
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procedures gave access to the required 2-chloropyridines
Chart 1. Previously Reported Cationic Iridium(III)
Isocyanide Complexes6,10,43−45
(Scheme 3).47,48 The cyclometalated precursors [(C∧N)2Ir(μ-Cl)]2
a
Scheme 3. Synthesis of 2-Chloropyridines
a
Reaction conditions: (a) H2O2, acetic acid, under air, 80 °C;
(b) POCl3, “dry finger”, under air, 130 °C; (c) NaOCH3, dry DMF,
under Ar, RT.
a
Scheme 1. Synthesis of New Ir(III) Complexes
were made by reaction of IrCl3·3H2O with 2-phenylpyridines in
2-ethoxyethanol/water (Scheme 1).49
Five new complexes, [(C∧N)2Ir(CN-tert-C4H9)2]CF3SO3
(1−5), were prepared in higher than 60% yield by reaction
of a cyclometalated Ir(III) precursor [(C∧N)2Ir(μ-Cl)]2 with
silver triflate (to remove the chloride anion)50 and with an
excess of tert-butyl isocyanide (Scheme 1).43 The complexes were
purified by column chromatography (on silica) and recrystalliza-
tion; they were characterized by elemental analysis, ESI+ MS, and
1
NMR spectroscopy. H and 19F NMR spectra of 1−5 show a
1
single set of signals for the constituent ligands; integration of H
peaks confirms the 1:1 ratio of the C∧N to CN-tert-C4H9 ligands;
19F NMR confirms the presence of F group and of CF3SO3−
counteranion; and ESI+ TOF mass spectra show the peak of
[(C∧N)2Ir(CN-tert-C4H9)2]+. The complexes are air- and moisture-
stable solids that are soluble in polar organic solvents.
a
Structural Characterization. Figure 1 shows the X-ray
molecular structure of 4. Two independent molecules of the
complex with similar structural features are present in the unit
cell. The metal ion is in a distorted octahedral [(C∧N)2Ir(C)2]+
coordination environment with the two pyridine nitrogen
atoms in trans position. The Ir−C bonds with the tert-butyl iso-
cyanides are shorter (on the average) and show a wider varia-
tion of length (Table 1) compared with the Ir−C bonds with
the cyclometalating ligand. For the tert-butyl isocyanides, the
Ir−CN angle is more linear and shows less variation than the
CN−C angle (Table 1).
Reaction conditions: (a) cyclometalating ligand, 2-ethoxyethanol/
water (3/1), under Ar, 120 °C; (b) under Ar, RT (i) AgCF3SO3,
CH3OH/CH2Cl2; (ii) tert-butyl isocyanide, CH2Cl2.
energies and with higher efficiencies than do the known analogs
(Chart 1),6,10,43−46 and we report the first example of an
iridium(III) isocyanide complex that is blue-phosphorescent
as a neat solid.
RESULTS AND DISCUSSION
■
Synthesis. The 2-phenylpyridine ligands were prepared by
Suzuki−Miyaura coupling of 4-fluorophenylboronic acid with
substituted 2-halopyridines (Scheme 2). Modified literature
The cyclometalating ligands are nearly planar: the dihedral
angle between their constituent rings is less than 10°, and the
methoxy group is in-plane with the pyridyl ring. The complexes
do not participate in face-to-face π−π stacking. Intermetallic
communication is likely to be negligible because of the long Ir−
Ir distance [8.8662(15) Å].
a
Scheme 2. Synthesis of 2-Phenylpyridines
The main structural parameters of 4 (Table 1) are similar to
those reported for two of the complexes shown in Chart 1 [R4′ =
CH343 or C(O)H6]; of note is that all three structures have two
independent molecules in the unit cell.
Electrochemistry. The electrochemical properties of 1−5
were studied by cyclic voltammetry in acetonitrile and DMF
(Figure 2 and Figure S1 in the Supporting Information). All
observed redox processes were irreversible, and their reduc-
tion potentials (relative to Fc+/Fc)51 were solvent-independent
(Table 2).
a
Reaction conditions: (a) K2CO3, Pd(PPh3)4, THF/water (3/1),
under Ar, 85 °C.
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Figure 2. Cyclic voltammograms of 1−5 in CH3CN (0.1 M NBu4PF6,
100 mV/s). The unit on the vertical axis is 20 μA. The cyclic
voltammograms of 1−5 in DMF are shown in the Supporting
Information.
indicate that the reduction of 1−5 is localized on the pyridyl
ring, whereas oxidation takes place on the Ir−phenyl fragment.
Complexes 1−5 have a wide electrochemical gap of more
than 3.58 V (ΔE = Eox − Ered; Table 2). The electrochemical
gap increases when electron-donating and electron-withdrawing
groups are attached to the pyridyl and phenyl rings of the
cyclometalating ligand, respectively, and it reaches a maximum
of 3.83 V for 4. To our knowledge, this is the highest value
reported for the electrochemical gap of a cationic, bis-cyclo-
metalated Ir(III) complex;43 it is slightly higher than the ΔE =
3.81 V of the Ir(III) pyridine−carbene complex [(N,C4′-2-(2′,6′-
difluoro-3′-pyridyl)pyridyl)2Ir(3-methyl-1-(4′-methyl-2′-pyridyl)-
benzimidazol-2-ylidene)](PF6).32
Infrared Spectroscopy. The infrared absorption spectra of
1−5 were measured in neat films obtained by evaporation of di-
chloromethane solutions. Complexes 1−5 show two infrared ab-
sorption peaks of equal intensity in the range of 2170−2201 cm−1
and separated by about 20 cm−1 that are characteristic of
stretching vibrations of the tert-butyl isocyanide CN bond
(Table 2 and Figure S2 in the Supporting Information).52,53
The increase in frequency of the CN stretching vibrations in
going from the free ligand (2137 cm−1)54 to 1−5 suggests a
weak π backbonding from Ir(III) to the tert-butyl isocyanides
(the alkyl isocyanide ligands are known to act as strong σ-donors
and weak π-acceptors in the complexes with metals in high
oxidation states).52,53
We note that the frequencies of the infrared peaks are lower
for complexes with lower oxidation potentials (Table 2 and
Figure 3). This trend reflects either a stronger π backbonding
from Ir(III) to the tert-butyl isocyanides or a weaker σ-donation
from the tert-butyl isocyanides to Ir(III) in the easier-to-oxidize
(more electron-rich) complexes.52
Electronic Absorption Spectroscopy. The complexes are
white (3, 4) or pale yellow (1, 2, 5) solids and give nearly
colorless solutions in dichloromethane and acetonitrile. Their
electronic spectra in these solvents show intense bands below
300 nm with molar absorption coefficients ε = (10−50) ×
103 M−1 cm−1 originating from π−π* electronic transitions of
the cyclometalating ligands (Figures S3−S7 and Table S2 in the
Supporting Information). At lower energy, the spectra feature a
Figure 1. Structure of 4 (50% probability ellipsoids; only one of the
two independent molecules shown; H atoms, triflate anion, and the
disorder within the tert-butyl groups omitted; ORTEP). Heteroatom
color codes: O, red; N, blue; F, green; Ir(1), black.
a
Table 1. X-ray Structural Parameters of 4
bond lengths (Å)
angles (deg)
b
b
∧
C N
isocyanide
isocyanide
Ir−C
Ir−N
Ir−C
Ir−CN
CN−C
Ir(1)
Ir(2)
2.053(6)
2.062(6)
2.049(6)
2.066(6)
2.058
2.060(5)
2.049(5)
2.077(6)
2.052(6)
2.060
2.015(6)
2.025(7)
2.033(6)
2.058(7)
2.033
175.7(6)
178.2(6)
173.4(6)
176.4(6)
175.9
176.5(7)
172.6(7)
169.3(7)
173.1(7)
172.9
c
avg
d
Δ
0.017
0.028
0.043
4.8
7.2
a
Two independent molecules of the complex, labeled as Ir(1) and
Ir(2), are present in the unit cell. Each row corresponds to one ligand.
b
tert-Butyl isocyanide in trans position to the carbon atom of the C∧N
c
d
ligand in the same row. Averaged value. Difference between the
minimum and the maximum values.
The first oxidation of the nonsubstituted complex 1 occurs at
1.23 V. The electron-withdrawing 4′-F substituent in the phenyl
ring increases the oxidation potential to 1.42 V (2). Modifi-
cation of the pyridyl ring in 2 with the electron-withdrawing
5-CF3 group increases the oxidation potential by 200 mV (5),
whereas electron donors facilitate the oxidation by 50 mV
(4-tert-butyl, 3; 4-OCH3, 4).
The first reduction of 1 occurs at −2.38 V. Fluorination of
the phenyl ring facilitates the reduction by 40 mV (2). Modi-
fication of the pyridyl ring in 2 with an electron-withdrawing
5-CF3 group shifts the reduction potential positively by 410/380 mV,
whereas an electron donor shifts the reduction potential nega-
tively by 50/90 mV (4-tert-butyl) and 100/120 mV (4-OCH3)
(the values stated for the potential shifts refer to DMF/CH3CN
solvents).
The trends observed in the redox potentials (Table 2)
together with the results of theoretical calculations (see below)
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Table 2. Infrared Absorption, Electrochemical Properties, and DFT-Calculated HOMO and LUMO Energies of Ir(III)
Complexes
a
c
,
d
c
,
e
f
g
R
Eox, V
Ered, V
ΔE, V
calculated energy, eV
b
h
complex
Ph
Py
IR, cm−1
CH3CN
CH3CN
DMF
CH3CN
EHOMO
ELUMO
EΔ
1
2
3
4
5
H
H
2193, 2170
2198, 2176
2196, 2174
2195, 2173
2201, 2183
1.23
1.42
1.37
1.37
1.62
−2.38
−2.34
−2.43
−2.46
−1.96
−2.38
−2.34
−2.39
−2.44
3.61
3.76
3.80
3.83
3.58
−6.06
−6.18
−6.17
−6.19
−6.41
−1.73
−1.80
−1.72
−1.69
−2.25
4.33
4.38
4.45
4.50
4.16
4′-F
4′-F
4′-F
4′-F
H
4-tert-Bu
4-OCH3
5-CF3
−1.93
a
b
Substituents on the phenyl (Ph) or pyridyl (Py) rings of the cyclometalating ligand. Infrared absorption bands of solid films of the neat complex in
c
the region of the CN stretching vibrations. All redox processes were irreversible. Relative to Fc+/Fc. Estimated errors: 50 mV. On glassy carbon
d
working electrode, in the presence of 0.1 M (NBu4)PF6, at scan rate 100 mV/s. Oxidation peak potentials. In DMF, the oxidation peak of 1−5 was
e
f
g
outside of the electrochemical window of the solvent (>1.0 V relative to Fc+/Fc). Reduction peak potentials. ΔE = Eox − Ered. DFT calculations at
h
the B3LYP/(6-31G**+LANL2DZ) level in CH3CN. EΔ = ELUMO − EHOMO
.
Luminescence of Liquid Solutions. Upon excitation with
near-UV light, the complexes exhibit blue (2−4) or blue-green
(1, 5) luminescence in both dichloromethane and acetonitrile
solutions. The luminescence spectra at room temperature show
a vibronic structure with the highest-energy peak (λem, 0−0
transition) at 440−459 nm; the vibronic structure becomes
better resolved at 77 K (Figures 5 and 6). The emission color of
1−5 (i.e., the position of the maxima and the relative intensities
of the peaks in the spectrum) is solvent- and temperature-
independent (Figure 6 and Table 3).
The luminescence of 1−5 is quenched by oxygen; therefore,
photophysical measurements in 10−5 M liquid solutions were
carried out under argon. Under these conditions, 1−5 display
luminescence quantum yields (Φ) of 32−76% and observed
excited-state lifetimes (τ) of 25−46 μs (Table 3). The
luminescence decays are single-exponential functions suggest-
ing the presence of one emissive center in liquid solution. The
calculated radiative lifetimes of the excited states (τrad = τ/Φ,
which are the ideal lifetimes in the absence of radiationless
processes) are in the range of 46−82 μs and are solvent-
independent (Table 3).
Figure 3. Correlation between the wavenumber of the two infrared
transitions associated with the stretching vibrations of the CN bond
(shown in black and red) and the oxidation potential (in CH3CN;
irreversible process) of 1−5. IR spectra were recorded for thin films of
the neat complexes. Empty circles: data from this work (Table 2).
Filled circles: literature data for [(N,C2′-(2-para-tolylpyridyl))2Ir(CN-
tert-Bu)2](CF3SO3).43
composite band with a main maximum at 318−354 nm and ε =
(8−14) × 103 M−1 cm−1 that we assign to the (Ir−phenyl)-to-
pyridyl charge transfer (CT) transition (Figure 4 and Table 3).2
The luminescence spectra of 1−5 show vibronic structure
and are solvent-independent, whereas the radiative lifetimes are
in the range of tens of microseconds. At the same time, the
substituents in the cyclometalating ligands determine the
emission color of 1−5: a blue shift is achieved by the 4′-F in
the phenyl or the 4-OCH3 in the pyridyl; a red shift is achieved
by the 5-CF3 in the pyridyl (Table 3). All of these findings
suggest that 1−5 emit from a predominantly cyclometalating-
ligand-centered triplet excited state of π−π* nature.2,43,44 In
1−5, the shorter wavelength of λem corresponds to the shorter
wavelength of the CT absorption band maximum and to the
larger electrochemical gap (Tables 2−4).
Luminescence of Solid Solutions. The photophysical
properties of 1−5 were also studied in solid solutions in poly-
(methyl methacrylate) (PMMA) under air in the form of drop-
cast films. In PMMA, 1−5 show blue or blue-green luminescence.
The luminescence spectra, quantum yields (35−45%), and life-
times (13−30 μs; biexponential decays) of 1−5 in PMMA are
similar to those measured in 10−5 M liquid solutions (Table 3
and Figure 6). It is a remarkable result, considering that the
PMMA films had a relatively high concentration of the complex
(1 wt %) and were exposed to air. We therefore conclude that
1−5 in PMMA do not form aggregates and are not quenched
by oxygen.
Figure 4. Absorption spectra of 1−5 in CH2Cl2. The unit on the
vertical axis is 2 × 103 M−1 cm−1. The spectra are shifted with respect
to one another along the vertical axis by 6 × 103 M−1 cm−1. Additional
absorption spectra are shown in the Supporting Information.
The blue-shift of the CT band maximum by up to 6 nm in aceto-
nitrile with respect to dichloromethane (Table 3) suggests that the
ground state of 1−5 is more polar than the excited state.
Luminescence of Solid Complexes. In neat solids under
air (spin-coated films), the complexes emit blue (3), green
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Table 3. Photophysical Properties of Ir(III) Complexes
a
b
c
d
e
,
f
g
h
complex
medium
CH3CN
CH2Cl2
λabs, nm (ε, 103 M−1 cm−1)
λem, nm
Φ, %
τ, μs
τrad, μs
T1, eV
1
341 (7.5)
347 (7.9)
453
455
452
455
456
447
449
447
449
456
447
449
445
448
451
440
440
436
440
444
458
459
455
458
463
52
45
32
25
62
56
3.15
3.22
3.27
3.35
3.10
CH2Cl2, 77 K
PMMA
86 (20%), 28 (80%)
35
14 (25%), 27 (75%)
solid
4.4
59
0.6 (44%), 2.0 (56%)
2
3
4
5
CH3CN
CH2Cl2
CH2Cl2, 77 K
PMMA
327 (10)
332 (11)
46
78
77
48
37
80 (50%), 20 (50%)
40
13 (38%), 30 (62%)
solid
2.6
0.8 (43%), 3.8 (57%)
CH3CN
CH2Cl2
CH2Cl2, 77 K
PMMA
326 (12)
328 (13)
55
50
40
73
66
33
120 (42%), 35 (58%)
39
14 (32%), 29 (68%)
solid
6.2
1.5 (70%), 4.1 (30%)
CH3CN
CH2Cl2
CH2Cl2, 77 K
PMMA
318 (13)
322 (14)
44
32
36
82
81
26
50
35
14 (37%), 30 (63%)
1.3 (49%), 2.3 (51%)
35
solid
4.1
CH3CN
CH2Cl2
CH2Cl2, 77 K
PMMA
348 (11)
354 (12)
76
69
46
48
33
44 (24%), 23 (76%)
13 (78%), 25 (22%)
2.0 (92%), 4.5 (8%)
45
solid
8.4
a
Acetonitrile or dichloromethane solution at room temperature under air (absorption) or under argon (emission, 10−5 M); frozen dichloromethane
b
solution at 77 K; PMMA film (1 wt % of the complex) or neat solid complex at room temperature under air. Lowest energy absorption band.
c
d
e
f
Highest-energy vibronic luminescence peak (0−0 transition). λexc = 330 nm. λexc = 330 or 373 nm. The luminescence decay is a biexponential
function in solid samples; this observation may reflect the presence of different conformers of the complex in solid matrix (dichloromethane at 77 K;
PMMA) or oxygen quenching/aggregate formation (nea1t0complex). Biexponential luminescence decay was previously reported for neat films of
g
h
[(N,C2′-(2-para-tolylpyridyl))2Ir(CN-tert-Bu)2](CF3SO3). Calculated radiative lifetime: τrad = τ/Φ. Energy of the first triplet state with respect to
the ground singlet state S0 (adiabatic energy difference) from DFT calculations in CH3CN.
Figure 5. Corrected and normalized luminescence spectra of 1−5 at
room temperature in dichloromethane solution (blue trace) and in
neat solid film (red trace). The luminescence spectra in acetonitrile
and PMMA are almost identical to the spectra in dichloromethane and
are shown in Figure 6 and in the Supporting Information.
Figure 6. Corrected and normalized room-temperature luminescence
spectra of 3 in acetonitrile (ACN), dichloromethane (DCM, also
shown at 77 K), and PMMA and as a solid neat film. The luminescence
spectra for the other complexes are shown in Figure 5 and in the
Supporting Information.
(1, 4, 5), or yellow (2) luminescence. For most of these com-
plexes, a red shift of the emission color occurs in going from
solutions to neat solids. It results from the red shift (by up to 9
nm), broadening, and change of the relative intensities of the
luminescence peaks (Table 3 and Figure 5). In solutions, the
0−0 transition (λem = 440−459 nm) either dominates the emis-
sion spectrum or contributes significantly to it. In neat solids, how-
ever, the intensity of 0−0 transition is low (except for 3), and the
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a
Table 4. Variation of Experimentally Measured and DFT-Calculated Parameters of Ir(III) Complexes with Respect to 1
Δ(P) = P(complex) − P(1), eV
complex
Δ(Eox)
Δ(EHOMO
)
Δ(Ered
)
Δ(ELUMO
)
Δ(ΔE)
Δ(EΔ)
Δ(Eabs
)
Δ(Eem
)
Δ(T1)
1
2
3
4
5
0.00
+0.19
+0.14
+0.14
+0.39
0.00
−0.12
−0.11
−0.13
−0.35
0.00
+0.04
−0.05
−0.08
+0.42
0.00
−0.07
+0.01
+0.04
−0.52
0.00
+0.15
+0.19
+0.22
−0.03
0.00
+0.05
+0.12
+0.17
−0.17
0.00
+0.16
+0.17
+0.26
−0.07
0.00
+0.04
+0.04
+0.08
−0.03
0.00
+0.07
+0.12
+0.20
−0.05
a
The nonsubstituted complex 1 was arbitrarily chosen as a benchmark. The data for calculations were taken from Tables 2 and 3. Spectroscopic
parameters were converted from the wavelength (λabs, λem; nm) to the energy (Eabs, Eem; eV) scale. For all of the experimentally measured
parameters, the values for CH3CN solutions were used. The definitions of the parameters are the same as in Tables 2 and 3.
long-wavelength peaks (λ > 475 nm) are dominant. The quench-
ing by oxygen and the self-quenching caused by aggregation and
triplet−triplet interaction are responsible for the low quantum
yields (<8.4%) and short lifetimes (<5 μs; biexponential decays) of
the neat complexes 1−5 under air.
cyclometalating ligands together with a small contribution from
d-orbitals of Ir(III) (Figure 7). The electron-withdrawing
In order to reduce aggregation, which red-shifts and quenches
the emission of Ir(III) complexes in neat solids, the ligands are
often modified with bulky spectator groups.15,18,31 In 1−5, the
bulky tert-butyl isocyanide ligands alone are not sufficient to
prevent aggregation. For example, 4 is a blue emitter in solu-
tion, but a pale green emitter in neat solid (Figure 5). However,
the complexes that have additional bulky groups in the cyclo-
metalating ligands to further reduce aggregation (4-tert-butyl, 3;
5-CF3, 5) show the smallest red shift of color and the highest
luminescence quantum yields (6.2−8.4%) in neat solids (Table 3
and Figure 5). For example, the emission spectrum of 3 remains
the same under all studied conditions (Figure 6), and 3 is the first
iridium(III) isocyanide complex to show blue-phosphorescence in
neat solid.
−3
Figure 7. (a) Electron density contours (0.05 e bohr ) calculated for the
HOMO and LUMO of 1. (b) Spin density distribution (0.005 e bohr−3)
calculated for the triplet emitting excited state (T1) of 1 resulting from
the HOMO-to-LUMO monoexcitation.
Overview of Photophysical Properties. Review of the
literature shows that 1−5 emit phosphorescence at shorter
wavelengths and with higher quantum yields than do the
reported [(C∧N)2Ir(C)2]+ and [(C∧N)2Ir(C∧C)]+ analogs in all
media.6,10,28,41,43−45 Up to now, the highest-energy [(C∧N)2Ir-
groups (4′-F or 5-CF3) in the cyclometalating ligands stabilize
the HOMO of 1−5 (Table 2).
The lowest-unoccupied molecular orbital (LUMO) mainly
resides on the pyridyl ring of the cyclometalating ligand (Figure 7).
Therefore, modification of the pyridyl ring in 2 changes the
LUMO energy: an electron-donor (4-tert-butyl or 4-OCH3) de-
stabilizes it by 0.08−0.11 eV; an electron-withdrawing 5-CF3
group strongly stabilizes it by 0.45 eV (Table 2).
(isocyanide)2]+ emitters had λem = 444 nm/Φ = 16%43 or λem
=
458−461 nm/Φ < 38%6,43 in solution and λem = 458 nm/Φ =
3.7% in neat films.10 For dicarbene complexes [(C∧N)2Ir(C∧C)]+,
λem = 452 nm/Φ < 35% in solution and λem = 458−460 nm/Φ <
6% in neat films.28 For carbonyl complexes [(C∧N)2Ir(CO)2]+,
λem = 441−451 nm/Φ < 9.7% in solution.41
The trends in the calculated HOMO and LUMO energies of
1−5 are in agreement with the trends in the measured oxida-
tion and reduction potentials: the lower (more negative) EHOMO
and ELUMO energies correspond to the higher (more positive) Eox
and Ered potentials, respectively (Tables 2 and 4). At the same
time, the larger calculated HOMO−LUMO energy gap
corresponds to a larger electrochemical gap (Tables 2 and 4).
To gain insight into the photophysical properties of 1−5, we
optimized the geometry of the lowest-energy triplet excited
state (T1) by using the spin-unrestricted UB3LYP approach.
The electron promotion associated with the excitation to T1
shortens the Ir(III) bonds to the cyclometalating ligands by
0.01−0.05 Å and stretches the Ir(III) bonds to the tert-butyl
isocyanide ligands by 0.05 Å. The unpaired-electron spin-
density distribution calculated for T1 perfectly matches the
topology of the HOMO-to-LUMO monoexcitation. Therefore,
excitation to T1 involves an electron promotion from the
HOMO (Ir−phenyl fragment) to the LUMO (pyridyl ring). All
of the complexes have a similar spin-density distribution that is
mainly localized on the C∧N ligands together with a small con-
tribution (about 0.13e) from Ir. Therefore, T1 can be described
as a cyclometalating-ligand-centered triplet state, with a small
The advantage of 1−5 is that they do not require poly-
fluorinated cyclometalating ligands to achieve blue phospho-
rescence, unlike the reported analogs28,41,43 or the extensively
studied diimine [(C∧N)2Ir(N∧N)]+ complexes22−27,40 [the prob-
lem with electron-deficient polyfluorinated ligands is that they
have reactive C(sp2)−F bonds55].
Therefore, bis-cyclometalated, cationic iridium(III) isocyanide
complexes are promising blue-phosphorescent emitters in
terms of color and quantum efficiency in solution (2−4) and
in neat solid (3). However, their long radiative lifetimes may
cause high sensitivity to triplet−triplet interaction and oxygen
quenching and can limit their use in electroluminescent devices.
Theoretical Calculations. The molecular and electronic
structures of 1−5 were investigated by performing density
functional theory (DFT) calculations at the B3LYP/(6-31G**
+LANL2DZ) level. The fully optimized ground-state geom-
etries of 1−5 show a near-octahedral coordination of Ir(III)
and are in good agreement with the X-ray structure of 4.
Calculations show that the highest-occupied molecular orbital
(HOMO) of 1−5 is composed of phenyl π-orbitals from the two
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Inorganic Chemistry
Article
3
stated otherwise. The complexes were often isolated as oils that
solidified on standing. Further details are provided below.
contribution of MLCT state, and with no participation of the
tert-butyl isocyanides (Figure 7). The predominant ligand-
centered nature of the emitting T1 state explains the well-
resolved vibronic structure of the luminescence spectra of 1−5
(Figures 5 and 6). The calculated changes in the adiabatic
energy of T1 with respect to the ground state are in agreement
with the experimentally observed shifts in the emission energy
of 1−5 (Tables 3 and 4).
Complex 1. [(C N)2Ir(μ-Cl)]2 (C N = N,C2′-2-phenylpyridyl; 100 mg,
0.093 mmol), AgCF3SO3 (53 mg, 0.21 mmol), and tert-butyl isocyanide
(0.25 mL, 183 mg, 2.2 mmol) gave pale yellow glass: 159 mg (0.19 mmol,
100%). Anal. Calcd for C33H34F3IrN4O3S·H2O (MW 833.94): C, 47.53;
H, 4.35; N, 6.72. Found: C, 47.65; H, 4.34; N, 6.48. 1H NMR (400 MHz,
CD2Cl2): δ = 8.96−8.91 (m, 2H), 8.11−8.04 (m, 4H), 7.73 (dd, J = 7.6,
1.2 Hz, 2H), 7.44−7.36 (m, 2H), 7.06 (td, J = 7.2, 1.2 Hz, 2H), 6.94 (td,
J = 7.6, 1.2 Hz, 2H), 6.19 (ddd, J = 7.6, 1.2, 0.4 Hz, 2H), 1.36 (s, 18H,
CN-tert-C4H9) ppm. 19F NMR (376 MHz, CD2Cl2): δ = −78.9 (3F,
CF3SO3) ppm. ESI+ TOF MS: m/z 667.1 ({M − CF3SO3}+, 100%).
For the scaled-up procedure, [(C∧N)2Ir(μ-Cl)]2 (300 mg, 0.28 mmol,
in 30 mL of CH2Cl2 and 10 mL of CH3OH), AgCF3SO3 (170 mg,
0.66 mmol, in 5 mL of CH3OH), and tert-butyl isocyanide (1.5 mL,
1.1 g, 0.013 mol, in 30 mL of CH2Cl2) gave 369 mg (0.44 mmol, 79%)
of the product after chromatography (silica, 20 g). Complex 1 emits
bluish-green and yellow-green phosphorescence in CH2Cl2 solution
and as a powder, respectively.
∧
∧
Conclusions. We have developed bis-cyclometalated,
cationic iridium(III) isocyanide complexes 1−5 that in solution
show efficient blue or blue-green phosphorescence with the
highest-energy luminescence peak at 440−459 nm and with
quantum yields and excited-state lifetimes reaching 76% and 46 μs,
respectively. Complexes 1−5 emit from a cyclometalating-
ligand-centered triplet excited state, and their photophysical
properties are solvent-independent. For most of the com-
plexes, an emission color red shift occurs in going from
solution to the neat solid. However, the shift is minimal for
the complexes with bulky groups on the cyclometalating ligands
that prevent aggregation, and complex 3 is the first in its class of
compounds to show blue phosphorescence in neat solid. We
expect that the easy-to-make cationic bis-cyclometalated Ir(III)
isocyanide complexes will provide access to efficient phos-
phorescent emitters from deep-blue to near-infrared spectral
range by simple variation of the cyclometalating and isocyanide56
ligands.
Complex 2. [(C∧N)2Ir(μ-Cl)]2 (C∧N = N,C2′-2-(4′-fluorophenyl)-
pyridyl; 100 mg, 0.087 mmol), AgCF3SO3 (50 mg, 0.195 mmol), and
tert-butyl isocyanide (0.25 mL, 183 mg, 2.2 mmol) gave pale yellow
solid: 137 mg (0.161 mmol, 92%). Anal. Calcd for C33H32F5IrN4O3S
(MW 851.91): C, 46.53; H, 3.79; N, 6.58. Found: C, 46.34; H,
3.95; N, 6.22. 1H NMR (400 MHz, CD2Cl2): δ = 8.89 (d, J = 6.0 Hz,
2H), 8.13−8.05 (m, 2H), 8.01 (d, J = 8.4 Hz, 2H), 7.76 (dd, J = 8.8, 5.2
Hz, 2H), 7.45−7.38 (m, 2H), 6.78 (dt, J = 8.8, 2.4 Hz, 2H), 5.84 (dd, J =
8.8, 2.4 Hz, 2H), 1.39 (s, 18H, CN-tert-C4H9) ppm. 19F NMR (376 MHz,
CD2Cl2): δ = −78.8 (3F, CF3SO3), −109.0 (2F, Ph−F) ppm. ESI+ TOF
MS: m/z 703.1 ({M − CF3SO3}+, 100%). Complex 2 emits pale blue and
yellow phosphorescence in solution and as a powder, respectively.
Complex 3. [(C∧N)2Ir(μ-Cl)]2 (C∧N = N,C2′-2-(4′-fluorophenyl)-4-
(tert-butyl)pyridyl; 100 mg, 0.073 mmol), AgCF3SO3 (42 mg, 0.163 mmol),
and tert-butyl isocyanide (0.3 mL, 220 mg, 2.6 mmol) gave white solid:
126 mg (0.131 mmol, 90%). During chromatography, brown and
yellow impurities preceded and followed the product, respectively, but
they were easily removed. Anal. Calcd for C41H48F5IrN4O3S (MW
964.12): C, 51.08; H, 5.02; N, 5.81. Found: C, 51.15; H, 5.06; N, 5.72.
1H NMR (400 MHz, CD2Cl2): δ = 8.74 (d, J = 6.4 Hz, 2H), 7.95
(d, J = 2.4 Hz, 2H), 7.77 (dd, J = 8.8, 5.2 Hz, 2H), 7.40 (dd, J = 6.0,
2.0 Hz, 2H), 6.77 (td, J = 8.8, 2.8 Hz, 2H), 5.84 (dd, J = 8.8, 2.4 Hz,
2H), 1.51 (s, 18H, Py−tert-C4H9), 1.39 (s, 18H, CN-tert-C4H9) ppm.
19F NMR (376 MHz, CD2Cl2): δ = −78.9 (3F, CF3SO3), −109.6 (2F,
Ph−F) ppm. ESI+ TOF MS: m/z 815.2 ({M − CF3SO3}+, 100%). Com-
plex 3 emits pale blue phosphorescence in solution and as a powder.
Complex 4. The reaction was performed with [(C∧N)2Ir(μ-Cl)]2
EXPERIMENTAL SECTION
■
General Information. Elemental analyses were performed by
Dr. E. Solari, Service for Elemental Analysis, Institute of Chemical Sciences
and Engineering (ISIC EPFL). H and 13C NMR spectra were re-
1
corded with a Bruker AV400 (400 MHz) spectrometer; 19F NMR
spectra with a Bruker AV200 (200 MHz) or a Bruker AVIII-400
(400 MHz) spectrometers. ESI+ and GC-EI+ mass spectra were recorded
with a Q-TOF Ultima (Waters) or TSQ7000 (Thermo Fisher) spectro-
meter (Mass-Spectroscopy Service, ISIC EPFL).
Purification, crystal growth, and handling of all compounds were
carried out under air. All products were stored in the dark. Chemicals
from commercial suppliers were used without purification. Chroma-
tography was performed on a column with an i.d. of 30 mm on silica
gel 60 (Fluka, Nr 60752). The progress of reactions and the elution of
products were followed on TLC plates (silica gel 60 F254 on aluminum
sheets, Merck).
∧
(C N = N,C2′-2-(4′-fluorophenyl)-4-(methoxy)pyridyl; 94 mg, 0.074 mmol),
Synthesis of Ir(III) Isocyanide Complexes 1−5. The structures
of 1−5 are shown in Scheme 1. The syntheses of 2-phenylpyridines
and [(C∧N)2Ir(μ-Cl)]2 complexes are described in the Supporting
Information. The reactions were performed under argon and in the
absence of light. The solvents were deoxygenated by bubbling with Ar,
but they were not dried. CAUTION: tert-butyl isocyanide is a foul-
smelling volatile liquidensure adequate ventilation! The cyclometalated
Ir(III) precursor [(C∧N)2Ir(μ-Cl)]2 was dissolved in CH2Cl2 (20 mL)
at RT. A solution of AgCF3SO3 (excess, Aldrich) in CH3OH (10 mL)
was added dropwise. A precipitate of AgCl immediately formed. The
reaction mixture was stirred (4 h) to give a yellow solution containing
the white precipitate. It was filtered through a paper filter to remove
AgCl, and evaporated to dryness (these operations were done under air).
The resulting Ir(III) bis-solvato complex50 was dissolved or suspended in
degassed CH2Cl2 (20 mL), and tert-butyl isocyanide (large excess,
Aldrich) was added. The reaction mixture became a solution, and its
color changed from yellow to pale yellow or colorless. It was stirred at
RT under argon for 96 h to give a pale yellow solution. It was directly
loaded on the chromatography column (silica, 15 g) to avoid rotor-
evaporating foul-smelling tert-butyl isocyanide. Elution with 0.5%
CH3OH in CH2Cl2 removed the impurities. Elution with 1.0−2.0%
CH3OH in CH2Cl2 recovered the product as pale yellow or colorless
eluate. Purification by chromatography gave the pure product, unless
AgCF3SO3 (42 mg, 0.163 mmol), and tert-butyl isocyanide (0.4 mL,
292 mg, 3.5 mmol). After chromatography, the product still con-
tained impurities. It was dissolved in CH2Cl2 (1 mL) and poured
into ether (15 mL) with vigorous stirring. The resulting white emul-
sion was diluted with hexane (10 mL) and sonicated to convert emulsion
into suspension. It was allowed to settle. The product was filtered and
washed with hexane. White solid: 85 mg (0.093 mmol; 63%). Anal. Calcd
for C35H36F5IrN4O5S (MW 911.96): C, 46.10; H, 3.98; N, 6.14. Found:
1
C, 46.04; H, 3.71; N, 6.24. H NMR (400 MHz, CD2Cl2): δ = 8.63 (d,
J = 6.8 Hz, 2H), 7.69 (dd, J = 8.8, 5.2 Hz, 2H), 7.41 (d, J = 2.8 Hz, 2H),
6.98 (dd, J = 6.8, 3.2 Hz, 2H), 6.76 (td, J = 8.4, 2.4 Hz, 2H), 5.92 (dd, J =
9.2, 2.8 Hz, 2H), 4.11 (s, 6H, Py−OCH3), 1.39 (s, 18H, CN-tert-C4H9)
ppm. 19F NMR (376 MHz, CD2Cl2): δ = −78.9 (3F, CF3SO3), −109.6
(2F, Ph−F) ppm. ESI+ TOF MS: m/z 763.1 ({M − CF3SO3}+, 100%).
Complex 4 emits blue and pale-green phosphorescence in solution and as
a powder, respectively.
Complex 5. [(C∧N)2Ir(μ-Cl)]2 (C∧N = N,C2′-2-(4′-fluorophenyl)-5-
(trifluoromethyl)pyridyl; 100 mg, 0.071 mmol; its solution in CH2Cl2
may turn into suspension upon prolonged stirring), AgCF3SO3
(40 mg, 0.156 mmol), and tert-butyl isocyanide (0.3 mL, 220 mg,
2.6 mmol) gave pale yellow solid: 114 mg (0.115 mmol, 81%). Anal.
2269
dx.doi.org/10.1021/ic202297h | Inorg. Chem. 2012, 51, 2263−2271

Inorganic Chemistry
Article
Calcd for C35H30F11IrN4O3S (MW 987.90): C, 42.55; H, 3.06; N, 5.67.
Found: C, 42.66; H, 3.33; N, 5.57. 1H NMR (400 MHz, CD2Cl2): δ =
9.12 (s, 2H), 8.34 (dd, J = 8.4, 2.0 Hz, 2H), 8.21 (d, J = 8.8 Hz, 2H),
7.89 (dd, J = 8.8, 5.2 Hz, 2H), 6.87 (td, J = 8.4, 2.4 Hz, 2H), 5.80 (dd,
J = 8.8, 2.4 Hz, 2H), 1.45 (s, 18H, CN-tert-C4H9) ppm. 19F NMR
(376 MHz, CD2Cl2): δ = −62.6 (6F, Py−CF3), −78.9 (3F, CF3SO3),
−105.3 (2F, Ph−F) ppm. ESI+ TOF MS: m/z 839.1 ({M − CF3SO3}+,
100%). Complex 5 emits greenish-blue and bluish-green phosphorescence
in solution and as a powder, respectively.
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ASSOCIATED CONTENT
* Supporting Information
■
S
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Phys. A: Mater. Sci. Process. 2010, 100, 1035. (b) He, L.; Duan, L.;
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General information; synthesis and characterization of 2-chloro-
pyridines, 2-phenylpyridines, and [(C∧N)2Ir(μ-Cl)]2 com-
plexes; 1H, 13C, and 19F NMR spectra; full experimental
details for the X-ray, electrochemical, and spectroscopic mea-
surements and DFT calculations; crystallographic data (Table S1);
cyclic voltammograms in DMF (Figure S1); IR absorption spectra
(Figure S2); electronic absorption spectra (Table S2 and Figures
S3−S7); luminescence spectra (Table S3 and Figures S8−S11);
HOMO and LUMO energies (Figure S12); Cartesian coordinates
of the computed species; and CIF of the crystal structure of 4,
CCDC 857727. This material is available free of charge via the
Internet at http://pubs.acs.org.
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dez-Hernan
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AUTHOR INFORMATION
Corresponding Author
*Tel: +41 21 693 6124. Fax: +41 21 693 4111. E-mail addresses:
nail.shavaleev@epfl.ch; nicola.armaroli@cnr.it; mdkhaja.
nazeeruddin@epfl.ch.
■
(16) Su, H.-C.; Chen, H.-F.; Wu, C.-C.; Wong, K.-T. Chem.Asian J.
2008, 3, 1922.
(17) Graber, S.; Doyle, K.; Neuburger, M.; Housecroft, C. E.;
Constable, E. C.; Costa, R. D.; Ortí, E.; Repetto, D.; Bolink, H. J.
J. Am. Chem. Soc. 2008, 130, 14944.
(18) Rothe, C.; Chiang, C. J.; Jankus, V.; Abdullah, K.; Zeng, X. S.;
Jitchati, R.; Batsanov, A. S.; Bryce, M. R.; Monkman, A. P. Adv. Funct.
Mater. 2009, 19, 2038.
(19) Margapoti, E.; Shukla, V.; Valore, A.; Sharma, A.; Dragonetti, C.;
Kitts, C. C.; Roberto, D.; Murgia, M.; Ugo, R.; Muccini, M. J. Phys.
Chem. C 2009, 113, 12517.
Present Address
⊥Department of Chemistry and Pharmacy & Interdisciplinary
Center for Molecular Materials (ICMM), Friedrich-Alexander-
Universitat Erlangen-Nurnberg, Egerlandstrasse 3, 91058
̈
̈
Erlangen, Germany
∥School of Chemistry, University of Birmingham, Edgbaston,
B15 2TT, England.
(20) Costa, R. D.; Pertegas
2010, 22, 1288.
(21) Costa, R. D.; Ortí, E.; Bolink, H. J.; Graber, S.; Housecroft, C. E.;
Constable, E. C. Adv. Funct. Mater. 2010, 20, 1511.
(22) Bolink, H. J.; Cappelli, L.; Cheylan, S.; Coronado, E.; Costa, R. D.;
Lardies, N.; Nazeeruddin, M. K.; Ortí, E. J. Mater. Chem. 2007, 17, 5032.
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(23) Tamayo, A. B.; Garon, S.; Sajoto, T.; Djurovich, P. I.; Tsyba, I. M.;
Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 8723.
(24) Nazeeruddin, Md. K.; Wegh, R. T.; Zhou, Z.; Klein, C.; Wang,
Q.; De Angelis, F.; Fantacci, S.; Gratzel, M. Inorg. Chem. 2006, 45,
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, A.; Ortí, E.; Bolink, H. J. Chem. Mater.
ACKNOWLEDGMENTS
■
This work is supported by the European Union (CELLO,
Grant STRP 248043; https://www.cello-project.eu/), by the
Spanish Ministry of Science and Innovation (MICINN; Grants
MAT2011-24594, CSD2007-00010, and CTQ2009-08790),
and by the Italian National Research Council (CNR,
MACOL, PM.P04.010). R.D.C. acknowledges the support of
the MICINN and the Alexander von Humboldt Foundation.
M.K.N. thanks World Class University program, funded by the
Ministry of Education, Science and Technology through the
National Research Foundation of Korea (Grant No. R31-2008-
000-10035-0), Department of Material Chemistry, Korea
University, Chungnam, 339-700, Korea.
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