Thermally induced defluorination during a mer to fac transformation of a blue-green phosphorescent cyclometalated iridium(III) complex

Article abstract of DOI:10.1021/ic201655n

The new homoleptic tris-cyclometalated [Ir(C^λN) ₃] complexes mer-8, fac-8, and fac-9 incorporating γ-carboline ligands are reported. Reaction of 3-(2,4-difluorophenyl)-5-(2-ethylhexyl)- pyrido[4,3-b]indole 6 with iridium(III) chloride under standard cyclometalating conditions gave the homoleptic complex mer-8 in 63% yield. The X-ray crystal structure of mer-8 is described. The Ir-C and Ir-N bonds show the expected bond length alternations for the differing trans influence of phenyl and pyridyl ligands. mer-8 quantitatively isomerized to fac-8 upon irradiation with UV light. However, heating mer-8 at 290 °C in glycerol led to an unusual regioselective loss of one fluorine atom from each of the ligands, yielding fac-9 in 58% yield. fac-8 is thermally very stable: no decomposition was observed when fac-8 was heated in glycerol at 290 °C for 48 h. The γ-carboline system of fac-8 enhances thermal stability compared to the pyridyl analogue fac-Ir(46dfppy)₃10, which decomposes extensively upon being heated in glycerol at 290 °C for 2 h. Complexes mer-8, fac-8, and fac-9 are emitters of blue-green light (λ_max^em = 477, 476, and 494 nm, respectively). The triplet lifetimes for fac-8 and fac-9 are ～4.5 μs at room temperature; solution φ_PL values are 0.31 and 0.22, respectively.

Full text of DOI:10.1021/ic201655n

Article
pubs.acs.org/IC
Thermally Induced Defluorination during a mer to fac
Transformation of a Blue-Green Phosphorescent Cyclometalated
Iridium(III) Complex
Yonghao Zheng, Andrei S. Batsanov, Robert M. Edkins, Andrew Beeby, and Martin R. Bryce*
Department of Chemistry, Durham University, Durham DH1 3LE, U.K.
S
* Supporting Information
ABSTRACT: The new homoleptic tris-cyclometalated [Ir-
(C∧N)3] complexes mer-8, fac-8, and fac-9 incorporating γ-
carboline ligands are reported. Reaction of 3-(2,4-difluor-
ophenyl)-5-(2-ethylhexyl)-pyrido[4,3-b]indole 6 with iridium-
(III) chloride under standard cyclometalating conditions gave
the homoleptic complex mer-8 in 63% yield. The X-ray crystal
structure of mer-8 is described. The Ir−C and Ir−N bonds
show the expected bond length alternations for the differing
trans influence of phenyl and pyridyl ligands. mer-8
quantitatively isomerized to fac-8 upon irradiation with UV
light. However, heating mer-8 at 290 °C in glycerol led to an unusual regioselective loss of one fluorine atom from each of the
ligands, yielding fac-9 in 58% yield. fac-8 is thermally very stable: no decomposition was observed when fac-8 was heated in
glycerol at 290 °C for 48 h. The γ-carboline system of fac-8 enhances thermal stability compared to the pyridyl analogue fac-
Ir(46dfppy)3 10, which decomposes extensively upon being heated in glycerol at 290 °C for 2 h. Complexes mer-8, fac-8, and fac-
9 are emitters of blue-green light (λmaxem = 477, 476, and 494 nm, respectively). The triplet lifetimes for fac-8 and fac-9 are ∼4.5
μs at room temperature; solution ΦPL values are 0.31 and 0.22, respectively.
phosphorescence.16−20 For example, the attachment of
electron-withdrawing substituents, notably fluorine, to the
INTRODUCTION
Luminescent transition metal complexes are employed in a
■
phenyl ring of ppy ligands is an established strategy for
diverse range of applications, notably as phosphorescent
emitters for organic light-emitting displays (OLEDs)1−6 and
hypsochromically shifting the emission by lowering the HOMO
energy, resulting in an increased HOMO−LUMO gap.21−27
Electron-donating groups on the phenyl ring and electron-
withdrawing groups on the pyridyl moiety, or an extended
ligand chromophore, bathochromically shift the emission.28
Efficient blue emitters remain challenging targets.
for solid-state lighting.7 In this regard, cyclometalated iridium-
(III) complexes have received special attention as dopants for
harvesting the otherwise nonemissive triplet states formed in
OLEDs.8−10 The complexes are charge neutral and generally
have good chemical and photochemical stability, combined
with highly efficient emission from triplet metal-to-ligand
charge-transfer (MLCT) states. Tris-homoleptic Ir(III) com-
plexes [Ir(C∧N)3], where C∧N is a monoanionic bidentate
ligand [e.g., 2-phenylpyridine (ppy)], have been extensively
studied. Two stereoisomers can occur, designated facial (fac)
and meridional (mer). The fac and mer isomers are the
thermodynamically and kinetically controlled products, respec-
tively. Generally, the mer isomer has a red-shifted emission, a
significantly decreased quantum efficiency, and a shorter
emission lifetime compared to those of the fac isomer, all of
which are considered undesirable. The mer isomers are not
widely studied: they can usually be isomerized to the fac isomer
thermally (above ∼200 °C) or by irradiation with UV
light.11−15
Studies on carbazolyl-Ir derivatives are very limited. Our
group29,30 and Ho et al.31 have recently established that fac-
Ir(III) complexes of 3-pyridylcarbazole ligands, e.g., 1 (Chart
1), are emitters of green light with good stability and high
photoluminescence quantum yields. We sought to explore new
analogues, and TD-DFT calculations (B3LYP/3-21G*/
LANL2DZ level) were used to guide the choice of molecules
for synthesis. We were attracted to the 2-phenyl-γ-carboline
system 2, which is isomeric with 1, for the following reasons. (i)
The TD-DFT calculations suggested that the T1 ← S0
transition of 2 (2.53 eV) is similar in energy to that of 1
(2.63 eV) and should, therefore, retain the green emission
observed for 1,29−31 whereas for isomeric systems, the
comparable transitions are lower in energy (see Chart 2 and
Results and Discussion). (ii) System 2 is synthetically accessible
in a few steps, and functionalization of the cyclometalated
The HOMO of Ir(III) complexes consists principally of a
mixture of phenyl π-orbitals and Ir d-orbitals, whereas the
LUMO is predominantly located on ppy, especially the pyridyl
π-orbitals. Structural changes in the skeletal and substituent
groups of the cyclometalating ligand lead to color tuning of
Received: August 1, 2011
Published: December 19, 2011
© 2011 American Chemical Society
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Chart 1. Structures of Complexes 1, 2, and 8−10
Chart 2. Structures of the Parent Isomeric Complexes Used in the DFT Calculations
and 700 MHz spectrometers. Chemical shifts are referenced to
tetramethylsilane [Si(CH3)4] at 0.00 ppm. Melting points were
determined in open-ended capillaries using a Stuart Scientific SMP3
melting point apparatus at a ramping rate of 5 °C/min and are
uncorrected. Mass spectra were recorded on a Waters Xevo OTofMS
instrument with an ASAP probe, a Thermoquest Trace instrument, or
a Thermo-Finnigan DSQ instrument. Elemental analyses were
performed on a CE-400 elemental analyzer. Cyclic voltammograms
were recorded at a scan rate of 100 mV/s at room temperature using
an airtight single-compartment three-electrode cell equipped with a Pt
disk working electrode, a Pt wire counter electrode, and a Pt wire
pseudoreference electrode. The cell was connected to a computer-
controlled Autolab PG-STAT 30 potentiostat. The solutions contained
the complex and n-Bu4NPF6 (0.1 M) as the supporting electrolyte in
dichloromethane. All potentials are reported with reference to an
internal standard of the decamethylferrocene/decamethylferrocenium
phenyl ring of 2 should allow the emission color to be tuned by
analogy with ppy analogues discussed above.
In this Article, we focus on complexes 8 and 9, which are
derivatives of 2 with fluorinated phenyl rings. The following
findings are the most interesting aspects of this work. (i) mer-8
can be cleanly converted into fac-8 photochemically, but not
thermally. (ii) Under thermal conditions, mer-8 undergoes
regioselective loss of one fluorine atom from each ligand to
yield fac-9 in good yield. (iii) The complexes are efficient
emitters of blue-green light. (iv) Improved thermal stability is
imparted by the γ-carboline moiety of fac-8, compared to the
pyridyl analogue, fac-Ir(46dfppy)3 10.
EXPERIMENTAL SECTION
■
+
couple (FcMe10/FcMe10 = 0.00 V).
Materials, Synthesis, and Characterization. All reactions were
conducted under a blanket of argon that was dried by being passed
through a column of phosphorus pentoxide. All commercial chemicals
were used without further purification unless otherwise stated.
Solvents were dried through an HPLC column on an Innovative
Technology Inc. solvent purification system. Column chromatography
was conducted using 40−60 μm mesh silica. NMR spectra were
recorded on Bruker Avance 400 MHz or Varian VNMRS 500, 600,
Absorption spectra were recorded using a Unicam UV2-100
spectrometer operated with Unicam Vision in 1 cm path-length
quartz cells. Excitation and emission photoluminescence spectra were
recorded on a Horiba Jobin Yvon SPEX Fluorolog FL3-22
spectrofluorometer. Samples were held in quartz fluorescence cuvettes
(l = 1 cm × 1 cm) and degassed by repeated freeze−pump−thaw
cycles. Solutions had A values of 0.10−0.15 at 400 nm to minimize
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a
Scheme 1. Synthesis of mer-8, fac-8, and fac-9
a
Reagents and conditions: (i) I2, N-methylpiperazine, n-BuLi, THF, −78 to −23 °C, 60% yield; (ii) 2,4-difluorophenylacetylene, Pd(PPh3)4, CuI,
NEt3, 20 °C, 90% yield; (iii) t-BuNH2, toluene, 100 °C, 60% yield; (iv) IrCl3·3H2O, 2-ethoxyethanol, water, 120 °C; (v) 6, ethylene glycol,
acetylacetone, NEt3, 190 °C, ∼62% yield for step iv, based on IrCl3·3H2O, ∼63% for step v, based on 7; (vi) glycerol, 290 °C, 58% yield; (vii) hν
(365 nm), CD2Cl2, 100% yield.
inner filter effects. PLQYs were measured using the integrating sphere
technique.32 For photoluminescence lifetime measurements, samples
were prepared and degassed in the same way as described for steady-
state photoluminescence measurements. In a homemade setup, a N2
laser (337 nm, 10 μJ, 10 Hz) was used as an excitation source.
Emission was detected in a 90° geometry by a photomultiplier tube
(Hamamatsu R928) as a function of time, selecting a wavelength close
to the peak emission by way of a monochromator (Horiba Jobin Yvon
Triax 320) with a 0.1−2.0 nm bandpass. The signal was averaged and
converted to a digital signal by a digital storage oscilloscope (Tetronix
TDS 340). The data were fitted to single-exponential functions of the
form I(t) = I0 exp(−t/τ).
m), −112.84 (1F, br). HRMS (FTMS + ESI): calcd for
C75H75F6193IrN6 1366.5587, found 1366.5579.
Complex mer-8. A mixture of 6 (0.60 g, 1.5 mmol), iridium
chloride trihydrate (0.24 g, 0.67 mmol), and 2-ethoxyethanol and
water [10 mL, 3:1 (v/v)] was stirred at 120 °C overnight. The
precipitate was collected by suction filtration, and the intermediate
yellow solid 7 was dried (0.42 g, ∼62% based on IrCl3·3H2O). Anal.
Calcd for C100H100N8F8Cl2Ir2: C, 59.42; H, 4.99; N, 5.54. Found: C,
59.18; H, 5.11; N, 5.35. 1H NMR (600 MHz, CDCl3): δ 9.74 (4H, s),
8.11 (4H, s), 7.36−7.26 (4H, m), 7.22 (4H, m), 7.07 (4H, m), 6.91−
6.73 (4H, m), 6.37−6.11 (4H, m), 5.29−5.13 (4H, m), 3.91−3.67
(4H, m), 3.51 (4H, m), 1.79 (4H, m), 1.40−1.10 (32H, m), 0.99−0.57
(24H, m). 19F NMR (564 MHz, CDCl3): δ −110.05 to −110.86 (4F,
m), −112.08 (4F, dd, J = 27.9, 15.5 Hz). MS (MALDI+): m/z 2020.8
(M+, 100%). A mixture of 6 (0.12 g, 3.1 mmol), 7 (0.15 g, 0.07
mmol), ethylene glycol (10 mL), acetylacetone (0.1 mL), and NEt3
(0.1 mL) was stirred at 190 °C overnight. The precipitate was
collected by suction filtration. The crude product was purified by
column chromatography [SiO2, 2:3 (v/v) DCM/hexane eluent],
yielding mer-8 (0.12 g, ∼63% based on 7) as a yellow solid. Crystals
for X-ray analysis were grown by slow cooling of a solution of mer-8 in
DCM and hexane [1:2 (v/v)]. Anal. Calcd for C75H75F6IrN6: C, 65.91;
Figure 1. 19F NMR spectroscopic monitoring of the photochemical
conversion of mer-8 to fac-8.
The single-crystal diffraction experiment was conducted on a three-
circle Bruker diffractometer with a SMART 6000 CCD area detector,
using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) and
Cryostream 700 (Oxford Cryosystems) open-flow N2 cryostats.
Crystal data: C75H75IrF6N6·xC6H14 (tentatively x ≈ 1/2), M =
1409.7, T = 120 K, triclinic, space group P1 (No. 2), a = 12.329(1)
̅
Å, b = 15.423(1) Å, c = 18.341(2) Å, α = 106.96(1)°, β = 95.64(1)°, γ
= 91.03(1)°, V = 3315.6(5) Å3, Z = 2, Dc = 1.412 g/cm3, μ = 2.08
mm−1, 32728 reflections with 2θ ≤ 50°, 11671 unique reflections, Rint
= 0.081, R1 = 0.049 [8266 data with I ≥ 2σ(I)], wR2(F2) = 0.117 (all
data). The structure was determined by Patterson methods and refined
by full-matrix least-squares against F2 of all data, using SHELXTL
version 6.1233 and OLEX2.34 Structural data in CIF format is available
as Supporting Information or from the Cambridge Structural Database
(CCDC-835332).
1
H, 5.53; N, 6.15. Found: C, 66.11; H, 5.63; N, 5.98. H NMR (500
MHz, CDCl3): δ 8.74 (1H, t, J = 3.0 Hz), 8.68 (1H, d, J = 7.4 Hz),
8.41 (1H, s), 8.28 (2H, s), 8.16 (1H, s), 7.83 (1H, t, J = 7.1 Hz), 7.79−
7.71 (1H, m), 7.54−7.33 (8H, m), 7.23 (1H, dd, J = 14.2, 6.7 Hz),
7.03 (1H, t, J = 7.6 Hz), 6.63 (1H, s), 6.53 (2H, dd, J = 22.8, 13.0 Hz),
6.48−6.40 (1H, m), 6.09 (1H, t, J = 7.3 Hz), 5.94−5.80 (1H, m),
4.29−4.08 (6H, m), 2.07 (3H, d, J = 8.9 Hz), 1.50−1.13 (24H, m),
1.07−0.78 (18H, m). 19F NMR (470 MHz, CDCl3): δ −110.59 (1F,
br) −111.06 (1F, dd, J = 15.8, 7.5 Hz), −111.26 (1F, br) −111.50 (2F,
Complex fac-8. mer-8 (4.3 mg, 3.1 μmol) was dissolved in
CD2Cl2 (0.5 mL) in a Young’s tap NMR tube and degassed three
times by freeze−pump−thaw cycles. The sample was irradiated by two
LEDs at 365 nm (∼100 mW each) positioned 2 cm from the sample
for a total period of 10 h with intermittent agitation. The conversion
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was monitored by 19F NMR until the isomerization to fac-8 was
judged to have reached completion (100% yield by NMR). A yellow
solid was obtained by removing the solvent. Anal. Calcd for
C75H75F6IrN6·CH2Cl2: C, 62.87; H, 5.35; N, 5.79. Found: C, 62.83;
H, 5.80; N, 5.43. 1H NMR (700 MHz, CD2Cl2): δ 8.43 (3H, d, J = 3.5
Hz), 8.21 (3H, m), 7.50 (3H, m), 7.45 (6H, m), 7.05 (3H, m), 6.40
(6H, m), 4.27 (6H, m), 2.13 (3H, m), 1.52−1.24 (24H, m), 0.99 (9H,
dt, J = 28.4, 7.4 Hz), 0.88 (9H, dt, J = 24.9, 7.3 Hz). 19F NMR (658
MHz, CD2Cl2): δ −111.51 (3F, m), −111.95 (3F, quintet, J = 6.6 Hz).
HRMS (MS AP+): calcd for [C75H75F6193IrN6] 1366.5587, found
1366.5583.
yellow solid. Mass spectrometry and elemental analysis were
consistent with homoleptic Ir(III) complex 8. NMR spectra
(Figure 1 and Figures S3 and S4 of the Supporting
Information) clearly showed that the asymmetric mer isomer
had been isolated: five peaks are present in the 19F NMR
spectrum, and the characteristically complex aromatic region is
observed in the 1H NMR spectrum. The X-ray crystal structure
of mer-8 is shown in Figure 2.
Attempts to convert mer-8 into fac-8 using standard thermal
conditions11−15,21 were unsuccessful. Heating mer-8 in glycerol
at temperatures of ≤265 °C for 18 h resulted in quantitative
recovery of mer-8. At higher temperatures (270−290 °C for 18
h), partial defluorination occurred and a mixture of unreacted
mer-8 and fac-9 was obtained. Heating mer-8 in glycerol at 290
°C for 72 h gave pure fac-9 in 58% yield. No other complex
Complex fac-9. A mixture of mer-8 (0.06 g, 0.04 mmol) and
glycerol (10 mL) was stirred at 290 °C for 72 h. The residue was
extracted with dichloromethane (3 × 10 mL). The organic layers were
combined, dried (MgSO4), and filtered. The yellow solid that was
obtained after removal of the solvent was purified by column
chromatography [SiO2, 3:7 (v/v) DCM/EtOAc eluent], yielding fac-9
(0.03 g, 58%) as a yellow solid. Anal. Calcd for C75H78F3IrN6: C,
1
could be isolated. The structure of fac-9 was established by H
and 19F NMR spectra, which showed the C3 symmetry of the
fac isomer, high-resolution mass spectrometry, and elemental
1
68.62; H, 5.99; N, 6.40. Found: C, 68.37; H, 6.05; N, 6.30. H NMR
(600 MHz, CD2Cl2): δ 8.14 (3H, d, J = 10.5 Hz), 7.79 (3H, s), 7.72
(3H, dd, J = 8.3, 5.7 Hz), 7.45−7.38 (3H, m), 7.36−7.28 (6H, m),
6.99−6.89 (3H, m), 6.53 (3H, td, J = 8.7, 2.7 Hz), 6.44 (3H, d, J =
10.2 Hz), 4.18 (6H, d, J = 7.6 Hz), 2.05 (3H, dd, J = 12.3, 6.6 Hz),
1.43−1.14 (24H, m), 0.85 (18H, m). 19F NMR (564 MHz, CD2Cl2): δ
−114.2 (3F, s). HRMS (FTMS + ESI): calcd for [C75H78F3193IrN6 +
H+] 1313.5948, found 1313.5927.
Computational Data. All computations were conducted with the
Gaussian 09 package.35 The model geometries from various starting
conformers were fully optimized with the B3LYP/3-21G*/LANL2DZ
basis set. Using this functional and basis set, the TD-DFT data were
obtained.
1
analysis. It is clear from the H and 19F NMR spectra (Figures
S7 and S8 of the Supporting Information) that the loss of
fluorine is regiospecific for each ligand. In particular, the COSY
spectrum (Figure S9 of the Supporting Information) shows
three hydrogen environments on each ligand that possess no
three-bond coupling to a neighboring hydrogen (one on the
fluorophenyl ring and two on the pyridyl ring) (see the
Supporting Information). This confirms that the fluorine at C4
is retained. If the fluorine at C6 had been retained, there would
be only two hydrogen environments (those on the pyridyl ring)
with no three-bond coupling. The preferential loss of the
fluorine (as fluoride) from C6 rather than C4 of the phenyl
rings of mer-8 can be explained by the enhanced electrophilicity
of C6, compared to that of C4, due to the electron-withdrawing
pyridine ring at the ortho C1 position. The high thermal
stability of fac-8 (see below) provides evidence that
defluorination occurs from the mer isomer, not the fac isomer
of 8, although the detailed mechanism is unclear.
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of ligand 6, shown in Scheme 1,
followed a literature precedent for other pyrido[4,3-b]indole
derivatives36 and proceeded in 32% overall yield. Ligand 6
Photoisomerization of mer-8 was successfully achieved by
irradiation of mer-8 in degassed CD2Cl2 in an NMR tube with
UV light (365 nm). The conversion to fac-8 was quantitative as
1
determined by H and 19F NMR analysis. Figure 1 shows the
evolution of the 19F NMR spectrum upon photolysis. This is
consistent with a report by Thompson et al. of clean
photochemical isomerization of mer-tris(4,6-difluorophenyl)-
pyridinato-N,C2-iridium(III) [mer-Ir(46dfppy)3] to the fac
isomer 10 (Chart 1), a reaction that could not be achieved
thermally in refluxing glycerol.21 De Cola et al. also reported
the high thermal stability of mer isomers of fluorinated Ir(ppy)3
systems: for example, only mer-tris[2-(3′,4′,6′-trifluorophenyl)-
pyridinato]-N,C2-iridium(III) [mer-Ir(F3ppy)3] was obtained at
250 °C.24 No defluorinated products were reported in these
studies.21,24 It appears, therefore, to be a general feature that
fluorinated mer cyclometalates do not isomerize as easily as
nonfluorinated analogues. We assume that the mer-8 to fac-8
isomerism is a unimolecular process, based on literature
precedents.14
The thermally induced defluorination of mer-8 merits further
discussion as fluorinated aryl groups are widely used to shift the
emission of cyclometalated Ir complexes to the blue portion of
the spectrum.21−24 Holmes et al. noted that there may be
drawbacks to this strategy as the high electronegativity of
fluorine could make the ligands electrochemically reactive in
OLEDs.39 More recently, high-performance liquid chromatog-
raphy coupled with mass spectrometry (HPLC−MS) showed
Figure 2. X-ray molecular structure of mer-8 at 120 K, showing only
one conformation of the disordered side chains. Thermal ellipsoids are
drawn at the 50% probability level. Hydrogen atoms have been
omitted for the sake of clarity.
incorporates a difluorophenyl substituent to shift the emission
toward the blue portion of the spectrum. The N-2-ethylhexyl
substituent was chosen to impart solubility to the final
products. Ligand 6 was reacted with iridium(III) chloride
under standard conditions37,38 (step iv) for forming a bridged
μ-dichloro diiridium C∧N ligand complex 7. This intermediate
was reacted without further purification with a mixture of 6,
acetylacetone, and NEt3 in ethylene glycol at 190 °C to give a
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Figure 3. Normalized absorption and emission spectra (left) of ligand 6 and complexes mer-8, fac-8, and fac-9. Absorption spectra (right) with molar
extinction coefficients. Spectra were recorded in a degassed dichloromethane solution.
Table 1. Photophysical and Solution Electrochemical Data
Table 2. B3LYP/3-21G*/LANL2DZ TD-DFT Calculated
Lowest-Energy Transitions from the Ground State to the
Lowest Triplet States
abs
em
λmax
λ
PLQY,
τ
,
τ
Eox
1/2
f
a
max ab
ac
,
Pad
0 e
,
complex
(nm)
(nm)
ΦPL
(μs)
(μs)
(V)
a
mer-8
fac-8
fac-9
278
280
272
477, 512
476, 511
494, 529
−
0.31
−
4.5
−
14.5
0.90
0.95
0.71
compd triplet state
excitation
energy (eV) [λ (nm)]
1′
T1
T2
T3
T1
T2
T3
T1
LUMO ← HOMO
2.63 (471)
2.65 (469)
2.67 (465)
2.53 (490)
2.57 (482)
2.57 (482)
2.78 (446)
0.22
4.3
19.5
LUMO + 1 ← HOMO
LUMO + 2 ← HOMO
LUMO ← HOMO
a
Data obtained in a degassed dichloromethane solution at 20 °C.
Excitation wavelength of 380 nm. Measured using an integrating
sphere; estimated error of 10%. Estimated error of 5%.
b
c
2′
d
LUMO + 1 ← HOMO
LUMO + 2 ← HOMO
LUMO + 2 ← HOMO
LUMO ← HOMO
e
Calculated using the relationship τ0 = τP × ΦT/ΦP and assuming
f
ΦT = 1. Redox data were obtained in a dichloromethane solution and
mer-8′
are reported vs FcMe10/FcMe10+ (FcMe10/FcMe10+ = −0.59 V vs Fc/
Fc+).46
T2
LUMO + 2 ← HOMO
LUMO + 1 ← HOMO
LUMO ← HOMO − 1
LUMO ← HOMO
2.82 (440)
T3
T1
T2
T3
T1
T2
T3
T1
T2
T3
T1
T2
2.85 (436)
2.77 (447)
2.79 (445)
2.79 (445)
2.62 (472)
2.65 (467)
2.65 (467)
2.40 (516)
2.43 (510)
2.44 (509)
2.26 (549)
2.27 (545)
fac-8′
fac-9′
11
LUMO + 1 ← HOMO
LUMO + 2 ← HOMO
LUMO ← HOMO
LUMO + 2 ← HOMO
LUMO + 1 ← HOMO
LUMO ← HOMO
LUMO + 1 ← HOMO
LUMO + 2 ← HOMO
LUMO ← HOMO
12
LUMO + 1 ← HOMO
LUMO ← HOMO − 1
LUMO + 2 ← HOMO
LUMO ← HOMO − 2
T3
2.27 (545)
Figure 4. Normalized emission spectra of fac-8 and fac-10 in a
degassed dichloromethane solution.
a
Dominant component(s).
290 °C for 2 h led to extensive decomposition to a multitude of
products (TLC evidence) that could not be isolated. We note
that Thompson et al. reported that mer-10 did not isomerize
when refluxed in glycerol for 24 h, which the authors ascribed
to a large kinetic barrier that needed to be overcome for this
derivative. It was not stated if any products were detected.21
These results establish that the γ-carboline system of fac-8
imparts additional thermal stability to the complexes, compared
to the pyridyl analogue fac-10.
X-ray Crystal Structure of mer-8. The asymmetric unit
comprises one molecule. The iridium atom adopts a mer-
octahedral coordination (Figure 2) with three C∧N-chelating
ligands. In agreement with the rules of trans influence,21 the Ir−
N1 and Ir−N3 bonds (in trans positions with respect to each
that the heteroleptic complex bis(4,6-difluorophenyl)-
pyridinato-N,C2-iridium(picolinate) (FIrpic), which is a stand-
ard sky-blue phosphor,40,41 decomposes during the manufac-
ture of OLED (i.e., vacuum sublimation) and operation of the
device to give a complex mixture of defluorinated products.42,43
In our work, we have isolated a single defluorinated product
(fac-9) from a homoleptic complex (mer-8) under controlled
thermal conditions. This observation establishes that defluori-
nation may be a more general problem with Ir complexes
containing multifluorinated ligands.
In contrast to mer-8, fac-8 is thermally very stable. No
decomposition was observed upon refluxing fac-8 in glycerol at
290 °C for 48 h. However, fac-Ir(46dfppy)3 1021 (Chart 1) is
unstable under these conditions: heating fac-10 in glycerol at
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Figure 5. HOMO and LUMO orbital plots for mer-8′ and fac-8′ (C, gray; H, white; Ir, turquoise; N, blue; F, sky blue).
45
em
other) are shorter [2.065(4) and 2.050(4) Å, respectively] than
the Ir−N2 bond [2.145(6) Å], which is trans to the 1.978(6) Å
Ir−C3 bond, whereas the latter is shorter than Ir−C1 and Ir−
C2 bonds [2.040(7) and 2.069(7) Å, respectively], which are
trans to each other.
10 λmax values of 480 nm in chloroform and 468 nm in 2-
methyltetrahydrofuran21). The electron donating effect of the
carboline nitrogen at position 4 of the pyridyl ring of fac-8,
which should raise the LUMO and blue shift the emission,12
apparently outweighs the effects of extending the π-system,
which should red shift the emission.28
Photophysical and Electrochemical Properties. The
absorption and emission spectra of mer-8, fac-8, and fac-9 and
the absorption spectrum of ligand 6 in an oxygen-free
dichloromethane solution are shown in Figure 3, and the
data are listed in Table 1. The complexes show strong
absorption bands in the 250−300 nm region that are assigned18
to ligand-centered π−π* transitions and closely resemble the
absorption spectrum of the free ligand 6. The complexes show
absorption bands with lower extinction coefficients in the range
of 350−450 nm that are ascribed to singlet and triplet metal-to-
The photoluminescence quantum yield (PLQY) and lifetime
data (observed and pure radiative) for fac-8 and fac-9 are listed
in Table 1. These data could not be reliably obtained for mer-8
because of its partial conversion to fac-8 during the experiment,
observed by changes in the emission spectrum. The observed
lifetimes of fac-8 and fac-9 (τP = 4.5 and 4.3 μs, respectively)
and the spectral profiles are indicative of phosphorescence from
a mixture of ligand-centered and MLCT excited states. These
lifetimes are relatively long compared to that of fac-Ir(ppy)3
(1.9 μs in 2-methyltetrahydrofuran)21 and those of fac-Ir(F3-
ppy)3 and fac-Ir(F4-ppy)3 (1.6 and 2.3 μs, respectively, in
degassed dichloromethane).24
3
ligand charge-transfer (1MLCT and MLCT) states, following
literature precedent18 and the calculations of Hay.44 The
emission of the complexes is in the blue-green region. The
following trends in this series of complexes can be seen. (i) The
em
The electrochemical properties of complexes mer-8, fac-8,
and fac-9 were examined by cyclic voltammetry in a
dichloromethane solution. The complexes showed reversible
oxidation waves assigned to the Ir(III)/Ir(IV) couple (Table
1). No additional ligand-centered oxidation waves were seen on
scanning to 2.0 V. No reduction waves were observed on
scanning between 0 and −2.0 V. mer isomers are generally
easier to oxidize for reasons explained previously.21 The lower
λmax values for mer-8 and fac-8 are essentially the same,
although for mer-8 the lower-energy vibronic band is more
intense relative to the analogous band for fac-8. This is a subtle
difference, as seen previously,24 whereas significant broadening
of the emission is seen in the mer isomer for other fac−mer
pairs.21 (ii) The λmaxem of fac-9 is significantly red-shifted (by 18
nm, i.e., 770 cm−1) compared to that of fac-8 because of the
loss of a fluorine substituent from each ligand in fac-9. The
emission from fac-9 is visibly greener compared to that of mer-8
oxidation potential of mer-8 compared to that of fac-8 (ΔEox
1/2
em
= 50 mV) is consistent with data for other pairs of mer and fac
and fac-8. (iii) The λmax of fac-8 (476 nm in DCM) is blue-
shifted compared to that of fac-10 [484 nm (Figure 4)] (cf. fac-
isomers.21,24 Similarly, as expected, the loss of a fluorine atom
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from each ligand lowers the oxidation potential of fac-9 by 240
mV compared to that of fac-8.
profiles of fac-8 and fac-9; cyclic voltammograms of mer-8, fac-
8, and fac-9; and computational data. This material is available
free of charge via the Internet at http://pubs.acs.org.
DFT Calculations. To guide our synthetic targets in this
work, we conducted B3LYP/3-21G*/LANL2DZ density func-
tional theory (DFT) calculations on fac-1′, fac-2′, and the
isomeric ring systems fac-11 and fac-12 (Chart 2). To reduce
computation time, the N-methyl analogues of fac-1 and fac-2
(fac-1′ and fac-2′, respectively) were calculated. The calculations
suggested that the T1 ← S0 transition of 2′ (2.53 eV) is similar
in energy to that of 1′ (2.63 eV) and should, therefore, preserve
the green emission observed for 1,29−31 whereas for isomeric
systems 11 and 12, the comparable transitions have lower
energies (2.40 and 2.26 eV, respectively). These data and those
for mer-8′, fac-8′, and fac-9′ are listed in Table 2. It can be seen
that attachment of fluorine atoms to the phenyl rings of the
ligands increases the HOMO−LUMO gap, leading to the
observed blue-shifted emission.
The HOMO and LUMO surfaces of mer-8′ and fac-8′ are
shown in Figure 5. For both isomers, the HOMO is located on
the iridium and difluorophenyl units. The LUMO is
predominantly localized on one of the carboline ligands of
mer-8′, whereas it is distributed equally among the three
carboline ligands of fac-8′. The HOMO−LUMO surfaces for
fac-9′ (Figure S16 of the Supporting Information) are similar to
those for fac-8′. These data are consistent with the HOMO−
LUMO distributions in related Ir(C∧N)3 complexes.21,44
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: m.r.bryce@durham.ac.uk.
ACKNOWLEDGMENTS
■
We thank Durham University (Y.Z. and R.M.E.) and Thorn
Lighting (Y.Z.) for funding. R.M.E. is funded by a Durham
Doctoral Fellowship.
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CONCLUSIONS
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New homoleptic tris-cyclometalated [Ir(C∧N)3] complexes
have been synthesized and characterized using a range of
techniques. mer-8 can be cleanly isomerized to fac-8 under
photochemical conditions. However, under thermal conditions
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