Blue-green iridium(III) emitter and comprehensive photophysical elucidation of heteroleptic cyclometalated iridium(III) complexes

Article abstract of DOI:10.1021/ic500070s

Synthesis and photophysical properties of the highly emissive complex [Ir(Fppy)₂(dmb)]⁺ are reported along with those of additional heteroleptic cyclometalated Ir(III) complexes, [Ir(ppy) ₂(NN)](PF₆): FppyH = 2-(2,4-difluorophenyl)pyridine; ppyH = 2-phenylpyridine; NN = 4,4′-dimethyl-2,2′-bipyridine (dmb), 1,10-phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (Ph ₂phen). TD-DFT calculations and Franck-Condon emission spectral band shape analyses show that the broad and structureless emission from [Ir(Fppy)₂(dmb)]⁺ in acetonitrile at 298 K mainly arises from a triplet metal-to-ligand charge-transfer excited state, ³MLCT_Ir(ppy)-NN. The emission maximum varies systematically with variations in electron-donating or -withdrawing substituents on both the NN and the Xppy ligands, and emission efficiencies are high, with an impressive φ ≈ 1 for [Ir(Fppy)₂(dmb)]⁺. At 77 K in propionitrile/butyronitrile (4/5, v/v), emission from [Ir(Fppy) ₂(dmb)]⁺ is narrow and highly structured consistent with a triplet ligand-centered transition (³LC_NN) and an inversion in excited-state ordering between the ³MLCT _Ir(ppy)-NN and ³LC_NN states. In a semirigid film of the poly(ethyleneglycol)dimethacrylate with nine ethylene glycol spacers, PEG-DMA550, emission from [Ir(Fppy)₂(dmb)]⁺ is MLCT-based. The thermal sensitivity of the photophysical properties of this excited state points to a possible application as a temperature sensor in addition to its more known use in light-emitting devices.

Full text of DOI:10.1021/ic500070s

Article
pubs.acs.org/IC
Blue-Green Iridium(III) Emitter and Comprehensive Photophysical
Elucidation of Heteroleptic Cyclometalated Iridium(III) Complexes
Kassio P. S. Zanoni,† Bruna K. Kariyazaki,† Akitaka Ito,‡,§ M. Kyle Brennaman,‡ Thomas J. Meyer,‡
and Neyde Y. Murakami Iha*,†
†Laboratory of Photochemistry and Energy Conversion, Instituto de Química, Universidade de Sao Paulo − USP, Av. Prof. Lineu
̃
Prestes, 748, 05508-900, Sao Paulo, SP, Brazil
̃
‡Department of Chemistry, University of North Carolina at Chapel Hill − UNC, 123 South Road, Chapel Hill, North Carolina
27599-3290, United States
§Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585,
Japan
S
* Supporting Information
ABSTRACT: Synthesis and photophysical properties of the
highly emissive complex [Ir(Fppy)2(dmb)]+ are reported along
with those of additional heteroleptic cyclometalated Ir(III)
complexes, [Ir(ppy)2(NN)](PF6): FppyH = 2-(2,4-
difluorophenyl)pyridine; ppyH = 2-phenylpyridine; NN =
4,4′-dimethyl-2,2′-bipyridine (dmb), 1,10-phenanthroline
(phen), or 4,7-diphenyl-1,10-phenanthroline (Ph2phen). TD-
DFT calculations and Franck−Condon emission spectral band
shape analyses show that the broad and structureless emission
from [Ir(Fppy)2(dmb)]+ in acetonitrile at 298 K mainly arises
from a triplet metal-to-ligand charge-transfer excited state,
3MLCTIr(ppy)→NN. The emission maximum varies systemati-
cally with variations in electron-donating or -withdrawing
substituents on both the NN and the Xppy ligands, and emission efficiencies are high, with an impressive ϕ ≈ 1 for
[Ir(Fppy)2(dmb)]+. At 77 K in propionitrile/butyronitrile (4/5, v/v), emission from [Ir(Fppy)2(dmb)]+ is narrow and highly
structured consistent with a triplet ligand-centered transition (3LCNN) and an inversion in excited-state ordering between the
3
3MLCTIr(ppy)→NN and LCNN states. In a semirigid film of the poly(ethyleneglycol)dimethacrylate with nine ethylene glycol
spacers, PEG-DMA550, emission from [Ir(Fppy)2(dmb)]+ is MLCT-based. The thermal sensitivity of the photophysical
properties of this excited state points to a possible application as a temperature sensor in addition to its more known use in light-
emitting devices.
states (3MLCT) with ligand-centered (3LC) or mixed
INTRODUCTION
■
3MLCT/3LC emissions being also possible depending on the
Stable, emitting coordination compounds with tunable
absorption and emission spectra and redox potentials have
drawn worldwide attention due to their potential use in
complex and conditions.10a−f Heteroleptic 2-phenylpyridyl
Ir(III) complexes, [Ir(ppy)2(NN)]+, have high photochemical
molecular devices.1 In particular, Iridium(III) complexes have
and thermal stabilities, and the coordination chemistry is
found application in oxygen sensors,2 DNA intercalators,3
extensively based on a large number of ligands. Synthetic
luminescent biological probes,4 CO2 and water reduction,1c,k,5
variations have been used to achieve comprehensive emission
temperature sensors,6 with the most appealing application as
tuning over the entire visible region by incorporation of
emitters in organic light-emitting diodes (OLEDs)1b,k,7 and
electron-donating or -withdrawing groups on both cyclo-
light-emitting electrochemical cells (LECs).1c,8
metalated ppy and ancillary NN through their effect in
The photophysical properties of cyclometalated Ir(III)
stabilizing or destabilizing the donor and acceptor orbi-
complexes are strongly influenced by spin−orbit coupling
tals.1c,10c,e,11 Excited-state properties of these complexes are
exerted by the Ir(III) core. For example, this leads to high
also sensitive to the microenvironments around the complex
triplet emission quantum yields due to the highly mixed spin
character of the emitting excited states.1c,i,9 The results of
(i.e., solvent polarity, rigidity, temperature) because of their
density functional theory (DFT) and time-dependent DFT
(TD-DFT) studies show that their relatively short-lived
emissions occur mainly from metal-to-ligand charge-transfer
Received: January 10, 2014
Published: March 31, 2014
© 2014 American Chemical Society
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charge-transfer character and asymmetric structure,12 with
innumerous possibilities of emission colors.
Chart 1. List of Chemical Structures for Investigated Ir(III)
Complexes and PEG-DMA550
Ligand and solvent variations have been used to develop the
systematics of excited-state emission from this class of Ir(III)
complexes. Further developments will benefit from a deeper
understanding of the nature of the excited states and their
emissive properties. In earlier work, we reported on the use of
Re(I) complexes in PVK-based OLEDs.1j,l More recently, we
proceeded to Ir(III) emitters, with the use of [Ir(ppy)2(dmb)]+,
ppyH = 2-phenylpyridine and dmb = 4,4′-dimethyl-2,2′-
bipyridine, as the active layer in a LEC for the first time.13
This has motivated the engineering of new highly emissive
compounds with appropriate characteristics for light-emitting
devices and to obtain the synthetic control needed to tune
excited-state properties. For this purpose, use of TD-DFT to
provide insight into both electronic and molecular structure can
be helpful. Quantitative tools for excited-state evaluation are
also available by interpretation of excited-state properties
including application of Franck−Condon (FC) analysis to
emission spectra, which, through application of time-dependent
perturbation theory and the Fermi “Golden Rule”, provides
quantitative understanding of time-dependent excited-state
processes and has been applied to nonradiative decay in a
series of Re(I), Os(II), and Ru(II) polypyridyl complexes.12d,14
In this Article, we report the synthesis of a blue-green highly
emissive heteroleptic Ir(III) complex, [Ir(Fppy)2(dmb)]+,
FppyH = 2-(2,4-difluorophenyl)pyridine, and its photophysical
properties along with those of an additional three complexes of
the type [Ir(ppy)2(NN)]+, NN = dmb, 1,10-phenanthroline
(phen), or 4,7-diphenyl-1,10-phenanthroline (Ph2phen), in
different media and temperatures. [Ir(Fppy)2(dmb)]+ is novel
in exhibiting an intense blue-green emission. Its emission
properties, and those of the series of Ir(III) complexes, have
been analyzed by Franck−Condon emission spectral fitting and
TD-DFT calculations on the excited states. The effect of ligand
substituents and medium effects on excited-state properties are
rationalized systematically.
ophosphate, was filtered and washed with water and recrystallized in
dichloromethane/n-pentane. The solid was dried under vacuum to
obtain 467 mg (0.52 mmol), 40% yield, of pure product. Anal. Calcd
for IrC34H24N4F10P: C, 45.29%; H, 2.68%; N, 6.21%. Found: C,
1
45.51%; H, 2.68%; N, 6.17%. H NMR (500 MHz, CD2Cl2) δ/ppm:
8.33 (m, 4H), 7.83 (m, 4H), 7.50 (dd, J = 5.8; 0.8 Hz, 2H), 7.29 (dd, J
= 5.8, 0.8 Hz, 2H), 7.04 (ddd, J = 7.6, 5.8, 1.4 Hz, 2H), 6.59 (ddd, J =
12.5, 9.2, 2.4 Hz, 2H), 5.74 (dd, J = 8.3, 2.4 Hz, 2H), 1.53 (s, 6H).
[Ir(ppy)2(dmb)][PF6] (Ir2). IrCl3·H2O (Aldrich) (230 mg, 0.77
mmol) and ppyH (245 μL, 1.67 mmol) were heated at reflux for 8 h in
ethylene glycol monoethyl ether (10 mL) and water (6 mL) to
produce the μ-chloro-bridged cyclometalated Ir(III) precursor. After
being cooled to room temperature, dmb (145 mg, 0.80 mmol) was
added to the mixture and refluxed again for 20 h. After addition of
NH4PF6, the yellow precipitate was filtered, washed, and recrystallized
in dichloromethane/n-pentane, to obtain 420 mg (0.51 mmol), 65%
yield, of pure product. Anal. Calcd for IrC34H28N4F6P: C, 49.21%; H,
EXPERIMENTAL SECTION
■
All chemicals and solvents used for synthesis were purchased from
Sigma-Aldrich or Synth, and were used as supplied. Spectroscopic or
HPLC grade acetonitrile, propionitrile, and butyronitrile were used as
supplied. For electrochemical measurements, acetonitrile was distilled
prior to use.
1
3.40%; N, 6.75%. Found: C, 49.10%; H, 3.43%; N, 6.62%. H NMR
(200 MHz, CD2Cl2) δ/ppm: 8.29 (s, 2H), 7.95 (d, J = 7.7 Hz, 2H),
7.77 (m, 6H), 7.50 (d, J = 5.8 Hz, 2H), 7.23 (d, J = 5.5 Hz, 2H), 7.00
(m, 6H), 6.31 (d, J = 7.5 Hz, 2H), 1.55 (s, 6H).
1H NMR spectra were recorded in a DRX500 (500 MHz) or in an
AC200 (200 MHz) Bruker Avance spectrometer, using CD2Cl2,
CDCl3, or CD3CN as solvent. The residual solvent signals were
employed as internal standards. Elemental analysis data were obtained
on a Perkin-Elmer CHN 2400.
[Ir(ppy)2(phen)][PF6] (Ir3). IrCl3·H2O (Aldrich) (190 mg, 0.63
mmol) and ppyH (205 μL, 1.40 mmol) were heated at reflux for 8 h in
ethylene glycol monoethyl ether (7.5 mL) and water (4.5 mL). Next,
phen (120 mg, 0.67 mmol) was added to the mixture, which was
heated at reflux again for 20 h. After precipitation with NH4PF6 and
recrystallization in dichloromethane/n-pentane, 250 mg (0.29 mmol),
45% yield, of pure yellow product was obtained. Anal. Calcd for
IrC34H24N4F6P: C, 49.45%; H, 2.93%; N, 6.78%. Found: C, 49.63%;
H, 3.07%; N, 6.51%. 1H NMR (200 MHz, CD2Cl2) δ/ppm: 8.66 (d, J
= 8.0 Hz, 2H), 8.38 (d, J = 4.0 Hz, 2H), 8.24 (s, 2H), 7.75 (m, 10H),
7.35 (d, J = 5.2 Hz, 2H), 7.00 (m, 4H), 6.40 (d, J = 6.6 Hz, 2H).
[Ir(ppy)2(Ph2phen)][PF6] (Ir4). IrCl3·H2O (Aldrich) (160 mg, 0.54
mmol) and ppyH (175 μL, 1.20 mmol) were heated at reflux for 8 h in
ethylene glycol monoethyl ether (7.5 mL) and water (4.5 mL). After
being cooled to room temperature, Ph2phen (440 mg, 1.32 mmol) was
added to the mixture and refluxed again for 20 h, and the compound
was precipitated as PF6 salt and recrystallized in dichloromethane/n-
pentane to obtain 240 mg (0.25 mmol), 47% yield, of pure orange-
yellow product. Anal. Calcd for IrC46H32N4F6P: C, 56.49%; H, 3.30%;
Syntheses of Complexes. The novel complex [Ir(Fppy)2(dmb)]+,
−
Ir1, was prepared and isolated as its PF6 salt by the well-established
Nonoyama procedure,15 with synthesis of the μ-chloro-bridged
cyclometalated Ir(III) dimer as an intermediate, and followed by
introduction of the dmb ligand.13b,c,16 Complexes [Ir(ppy)2(NN)]+,
NN = dmb (Ir2), phen (Ir3), and Ph2phen (Ir4), were synthesized by a
procedure similar to one reported in the literature.8d,10f,12c,13,16,17
Chart 1 summarizes their chemical structures.
[Ir(Fppy)2(dmb)][PF6] (Ir1). IrCl3·H2O (Aldrich) (395 mg, 1.32
mmol) and FppyH (450 μL, 3.01 mmol) were dissolved in a 5/3 (v/v)
mixture of ethylene glycol monoethyl ether/water (20 mL) and heated
at reflux with stirring for 15 h. After being cooled to room
temperature, dmb (242 mg, 1.33 mmol) was added to the mixture,
which was refluxed again for 22 h. Upon cooling, the mixture was
washed with diethyl ether. The greenish yellow precipitate, obtained
after slow addition of aqueous solution of ammonium hexafluor-
1
N, 5.73%. Found: C, 56.65%; H, 3.51%; N, 5.70%. H NMR (200
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MHz, CDCl3) δ/ppm: 8.32 (d, J = 5.3 Hz, 2H), 8.18 (s, 2H), 7.94 (d,
J = 8.1 Hz, 2H), 7.74 (m, 6H), 7.58 (m, 12H), 7.06 (m, 6H), 6.43 (d, J
= 6.9 Hz, 2H).
and the solvent are treated classically and included in the band widths
of individual vibronic transitions. Prior to the spectral fitting analysis,
the number of photons at a given wavelength were corrected to the
PEG-DMA550 Film Preparation. Poly(ethyleneglycol)-
dimethacrylate (PEG-DMA550) films, with nine ethylene-glycol
spacers, Chart 1, containing Ir(III) complexes were prepared similarly
to the procedure reported in the literature.18 Complexes were
dissolved in PEG-DMA550 fluids with 2,2′-azobis(2,4-dimethylvaler-
onitrile), DuPont, 1 wt %, as an initiator in 1.0 cm path length glass
cuvettes, which were sealed with rubber septa and heated overnight at
50 °C under vacuum to yield optically transparent films. The
concentration of Ir(III) complexes was kept within the range 25−35
μmol L−1.
wavenumber scale by using the relationship, I(ν
̃
) = I(λ) × λ2.25
3
∞
νM
⎛
⎜
⎝
⎞ ⎛
⎞
E0 − νMℏωM
SM
I(ν)
̃
= ∑
⎟ ⎜
⎟
⎠
E0
νM!
⎠ ⎝
νM=0
2
⎡
⎢
⎣
⎤
⎛
⎝
⎞
⎠
̃
ν − E0 + νMℏωM
⎢
⎥
⎜
⎜
⎟
⎟
exp −4 ln 2
⎥
⎦
ν1/2
̃
(2)
(3)
4
∞
∞
νM
νL
⎛
⎜
⎝
⎞ ⎛
⎟ ⎜
⎞⎛
⎟⎜
⎞
⎟
⎠
E0 − νMℏωM − νLℏωL
SM
νM!
SL
ν !
Photophysical Measurements. UV−vis absorption spectra were
recorded on a Hewlett-Packard 8453 diode array spectrophotometer.
Steady-state and time-resolved emission spectra were recorded on a
ISS-PC1 photon-counting spectrofluorometer or an Edinburgh
Instruments FLS920 time-correlated single photon counting emission
spectrometer equipped with a Hamamatsu R928P photomultiplier
tube (PMT). A xenon lamp (λex = 365 nm, 4 nm bandwidth, with a
389 nm long pass filter) or an ISS laser (λex = 378 nm, frequency = 20
kHz) were used with the ISS-PC1 as excitation light source for steady-
state or time-resolved measurements, respectively. For experiments in
the FLS920, a xenon lamp (λex = 365 nm, 5 nm bandwidth, with a 395
nm long pass filter) or a pulsed Edinburgh Instruments EPLED-360
LED laser (λex = 369 nm, frequency = 20 kHz) was employed.
Emission intensity of steady-state spectra was corrected for system
spectral response. For 298 K measurements, the absorbance of sample
solutions in acetonitrile was set between 0.1 and 0.3 in a four polished
face cuvette with 1.000 cm optical path length. Solutions were
deoxygenated with argon for at least 10 min prior to measurement. For
77 K experiments, samples were prepared in a mixture of
propionitrile/butyronitrile 4/5 (v/v) in cylindrical quartz tubes, 0.4
cm radius, and were inserted into a Dewar flask containing liquid N2.
Emission quantum yields (ϕ) of complexes in acetonitrile at 298 K
were measured applying the methodology earlier reported by Friend et
al.19 using an BaSO4-coated integration sphere, model 1-M-2
(Edinburgh), with samples positioned at its center, as well as
calculated from eq 1 using [Ru(bpy)3][PF6] in the same solvent as
the reference (λex = 420 nm, 5 nm bandwidth, with a 435 nm long pass
filter). The results of both techniques gave excellent agreement.
I(ν)
̃
= ∑ ∑
E0
⎠
⎝
⎠
⎝
L
νM=0 νL=0
2
⎡
⎤
⎛
⎞
ν
̃
− E0 + νMℏωM + νLℏωL
⎢
⎥
⎜
⎜
⎟
⎟
× exp −4 ln 2
⎢
⎥
ν1/2
̃
⎝
⎠
⎣
⎦
̃ ̃
In these equations, I(ν) is the emission intensity at the energy ν
(cm−1). E0 is the energy gap between the zeroth vibrational levels in
the ground and excited states. ℏωM and ℏωL are the quantum spacings
for averaged medium- and low-frequency vibrational modes,
respectively.12d SM and SL are the associated electron-vibrational
coupling constants or Huang−Rhys factors,26 related to structural
differences between excited and ground states along the displacement
normal coordinates of the coupled average medium- and low-
frequency vibrational modes, respectively. ν1/2
half-maximum (fwhm) for an individual vibronic line.27
In the fitting procedure, E0, ℏω, S, and ν1/2 were optimized with a
̃
is the full width at
̃
least-squares minimization routine with application of a Generalized
Reduced Gradient (GRG2) algorithm.28 The summation was carried
out from v* = 0 in the excited state to levels v = 0→10 in the ground
state.
RESULTS AND DISCUSSION
■
Absorption Spectra. Absorption spectra of complexes Ir1
and Ir2, Figure 1, exhibit an intense band around 255 nm with
high molar extinction coefficients (ε ≈ 5 × 104 L mol−1 cm−1)
arising from a ππ* transition at the ancillary dmb ligand (i.e.,
ligand centered, LCdmb). Intense bands at 260 and 270 nm for
Ir3 and Ir4 can be assigned to LCphen and LCph2phen transitions,
respectively. Xppy ππ* states are expected to appear at higher
energies than for the ancillary NN ligands due to their
P Aref
Ir
ϕIr = ϕ
ref P AIr
(1)
ref
where ϕIr is the emission quantum yield of the sample; ϕref is the
emission quantum yield of the reference (0.095 in acetonitrile20); Ar is
the absorbance of the sample at the excitation wavelength; Aref is the
absorbance of the reference at the excitation wavelength; PIr is the
integral of the sample phosphorescence spectrum; and Pref is the
integral of the reference phosphorescence spectrum.
Theoretical Calculations. Electron density calculations for the
series of complexes were conducted with Gaussian 09W software.21
Optimization of ground-state structures was performed by using DFT
with the B3LYP functional. The LanL2DZ22a−c and 6-31G(d,p)22d,e
basis sets were used to treat iridium and all other atoms, respectively.
TD-DFT calculations were then performed to estimate energies and
oscillator strengths of the lowest energy ten singlet and five triplet
transitions for all complexes. Calculations were carried out in
acetonitrile as solvent by using a Polarizable Continuum Model
(PCM). Electron density populations were plotted using GaussView
5.0.23
Franck−Condon Analyses for Emission Spectra. Franck−
Condon band shape analysis for emission spectrum provides
information about contributing vibrational modes and structural
changes between the emitting excited state and the ground
state.12d,14a,24 In the versions in eq 2 or 3, one or two vibrational
acceptor mode(s), respectively, are included. They are averages of the
multiple modes coupled to the transition between the emitting excited
state and the ground state. Contributions from low-frequency modes
Figure 1. Absorption spectra of complexes Ir1 (black −), Ir2 (blue
− − −), Ir3 (green · · ·), and Ir4 (red − · − ·) in acetonitrile with an
enhanced scale shown in the inset.
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destabilization upon cyclometalation with Ir(III).29 In the
lower-energy region (λ > 280 nm), absorption spectra are
featureless with an overlap of transitions ascribed to mixed spin
character excited states due to strong spin−orbit coupling
effect,9 with ξIr = 4430 cm−1.30 However, bands from 320 to
440 nm can be assigned to largely spin-allowed metal-to-ligand
charge-transfer (MLCT) transitions, with ε values in the range
(2 − 10) × 103 L mol−1 cm−1. The lowest-lying absorption
from 440 to 480 nm (ε ≈ 0.8 × 103 L mol−1 cm−1) is
tentatively ascribed to the lowest-lying spin-forbidden tran-
sition, T1, assumed to be the lowest 3MLCT state. The
relatively high intensity for this nominally spin-forbidden
transition is due to spin−orbit coupling, which also facilitates
intersystem crossing between nominally singlet and triplet
states. Furthermore, Ir(III) dd transitions are typically at much
higher energies.5c
In addition, although not observable in absorption spectra,
there is experimental evidence (τ, knr and kr, and spectral
feature changes with rigidity and temperature (T)) for an
energetically close-lying, ligand-based ππ* excited state, 3LC.1c,i
As suggested in Scheme 1, it is presumably mixed with the
Scheme 1. Orbital Description of the Lowest Excited State,
3
3
T1, Illustrating Mixing between MLCT(1) and LC, for a >
Figure 2. Emission spectra (A) and decay profiles (B) for complexes
Ir1 (black), Ir2 (blue), Ir3 (green), and Ir4 (red) in acetonitrile at 298
b
K.
Table 1. Excited-State Parameters of the Investigated
Complexes in Acetonitrile at 298 K
complex λ/nm
ϕ
τ/μs
kr/105 s−1 knr/105 s−1
15 0.6 0.1
Ir1
Ir2
Ir3
Ir4
522
580
590
602
0.96 0.10
0.23 0.02
0.27 0.03
0.29 0.03
0.66 0.03
0.31 0.02
0.36 0.02
0.43 0.02
2
7.4 0.8
7.5 0.8
6.7 0.7
25
20
17
3
2
2
stabilizing electron-withdrawing fluoro groups in the cyclo-
metalated ppy ligand framework. A red-shift of the emission is
observed with an increase in the electron-withdrawing ability of
the substituents on the diimine ligands. These qualitative trends
are consistent with the results of the TD-DFT calculations
summarized in the following section.
lowest-lying 3MLCT(1) state resulting in mixed orbital
character for the lowest excited state, T1, with ΨT1
=
aΨ3MLCT + bΨ3LC as the wave function description.1c,i The
contribution of each individual state in T1 is given by a and b,
Remarkably, the emission quantum yield for Ir1 is ∼1, which
makes it distinctive as the most strongly emissive complex in
the heteroleptic [Ir(Xppy)2(NN)]+ series. Its emission lifetime
(τ = 0.66 μs) is relatively short despite the high emission
efficiency as observed for other Ir(III) complexes.1i Complex
Ir4 exhibits a longer emission lifetime (τ = 0.43 μs) than those
for Ir3 (τ = 0.36 μs) and Ir2 (τ = 0.31 μs) with emission
quantum yields of 0.29, 0.27, and 0.23, respectively. The
relatively short “triplet” lifetimes for all complexes arise from
spin−orbit coupling-induced mixing of singlet spin character
into the excited state, which, in turn, enhances mixing with the
ground state. For practical applications, these are suitable
photophysical characteristics for light-emitting devices, for
which τ should be short enough to allow rapid repopulation of
the emitting excited state with ϕ at least 0.20 for sufficient light
output, and Ir2 was recently applied for the first time in the
active layer of a LEC.13
3
3
respectively, for MLCT and LC. In the diagram, emission
from complexes occurs at ET and ΔE is the zero order energy
difference between the unmixed states 3MLCT and 3LC.
Orbitally, the MLCT and LC excited states share a common π*
acceptor with the hole in either the lowest dπ orbital on the
metal or a π orbital on a ligand with the excited-state
3
3
configurations, dπ5π*1 for MLCT(1) and π1π*1 for LC. The
orbital origin of the mixing between states in T1 is through
dπ5−π mixing with partial oxidation of the ligands by electron
donation to the metal induced by the electronic effect on the
dπ levels of the diimine ligand.
Emission Spectra and Photophysical Properties at
298 K. Emission spectra for the Ir(III) complexes in
acetonitrile at 298 K, Figure 2A, are broad and structureless,
independent of the excitation wavelength consistent with
MLCT emission, with T1 more influenced by the lowest
MLCT triplet, 3MLCT(1).1i,31 Emission parameters are
summarized in Table 1. Emission maxima vary with the nature
of the substituents with emission from Ir1 (λ = 522 nm) blue-
shifted relative to the other three due to the presence of
The quantities ϕ and τ are related to the rate constants for
radiative (kr) and nonradiative (knr) excited-state decay as
shown in eqs 4a and 4b. From its quantum yield, decay of Ir1
excited state is dominated by kr. For excited states with similar
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Table 2. Calculated Energies and Oscillator Strengths for Lowest-Energy Singlet (S1) and Triplet (T1) Transitions
complex
excited state
transition
energy (wavelength)
oscillator strength
Ir1
S1
HOMO→LUMO
2.863 eV (433 nm)
2.826 eV (439 nm)
0.0002
T1
HOMO→LUMO (85%)
HOMO−6→LUMO (15%)
HOMO→LUMO
Ir2
Ir3
Ir4
S1
T1
S1
T1
S1
T1
2.608 eV (475 nm)
2.581 eV (480 nm)
2.577 eV (481 nm)
2.547 eV (487 nm)
2.545 eV (487 nm)
2.511 eV (494 nm)
0.0002
0.0003
0.0004
HOMO→LUMO
HOMO→LUMO
HOMO→LUMO
HOMO→LUMO
HOMO→LUMO
Figure 3. Chemical structures and molecular orbital contours for the main molecular orbitals of investigated Ir(III) complexes.
character, kr varies as the square of the transition dipole
kr for complex Ir1 (15 × 105 s−1) relative to kr ≈ 7.4 × 105 s−1
3
moment, μ, and the cube of the average emission energy, ∼ET :
for the other three complexes is remarkable. Although the
electronic origin of the effect is not obvious, an experimentally
grounded explanation for the twice higher kr is a distinct
excited-state character for the Ir1 complex, with pronounced
kr ∝ μ2ET3 .1i,12d kr is also closely related to the spin−orbit
coupling of the emissive excited state, which comes from a
mathematical formalism that relates kr to the inverse of the
energy difference between two perturbing excited states (kr ∝
3
mixing between MLCT and 3LC states as depicted in Scheme
1i
3
−ΔE), for example, 3MLCT and LC. The enhanced value of
1, differing from the pure 3MLCT emission for the other three
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Table 3. Electron Density Populations of Molecular Orbitals Defining S1 and Emissive T1 States for Complexes Ir1−Ir4
Ir1
Ir2
Ir3
Ir4
orbital
iridium
Fppy
dmb
iridium
ppy
dmb
iridium
ppy
phen
iridium
ppy
Ph2phen
LUMO(triplet)
LUMO(singlet)
HOMO
3.41
3.29
2.98
2.26
93.61
94.45
3.20
3.30
3.31
2.47
2.22
94.23
94.47
3.62
3.81
3.89
2.55
2.48
93.64
93.63
3.65
3.74
3.94
2.34
2.30
93.92
93.76
3.86
46.09
2.26
50.71
1.72
47.71
48.67
47.10
49.25
47.15
48.99
HOMO−6
96.02
Figure 4. (A) Orbital energies calculated by B3LYP/LanL2DZ (Ir) and B3LYP/6-31(d,p) (CHNF) and (B) redox potentials for the series Ir1−Ir4,
from cyclic voltammetry measurements; see Figure S3 in the Supporting Information.
Table 4. Redox Potentials for Metal-Based Oxidation (Ir4+/
complexes. Variations in knr in the series are discussed in a later
section.
Ir3+) and Ligand-Based Reduction (NN0/NN−) versus NHE
in Acetonitrile, ∼0.5 mmol L−1, at Room Temperature from
kr
a
Cyclic Voltammetry Measurements
ϕ =
kr + knr
(4a)
complex
E1/2(NN0/NN−)/V
E1/2(Ir4+/Ir3+)/V
1
Ir1
Ir2
Ir3
Ir4
−1.15
−1.22
−1.14
−1.09
1.80
1.47
1.48
1.49
τ =
kr + knr
(4b)
Time-Dependent Density Functional Theory Calcu-
lations. Relevant energies and oscillator strengths for the
lowest-energy singlet (S1) and triplet (T1) transitions are
summarized in Table 2. The results of calculations on the first
ten singlet and five triplet transitions are also listed in the
Supporting Information, Tables S2−S5. As shown in
Supporting Information Figure S2, calculated energies and
oscillator strengths for the singlet transitions agreed very well
with experimental absorption spectra for all four complexes.
As shown in Table 2, the lowest-energy spin-allowed
transitions for all four complexes to give S1 are HOMO →
LUMO transitions. From Figure 3 and Table 3, the HOMO is
largely dπIr mixed with the πXppy, and the LUMO is largely a π*
orbital on the diimine ligand, with S1 mainly MLCT in
character with a minor contribution of a ligand-to-ligand
charge-transfer (LLCT) transition. For convenience, in further
a
Figure S3 in the Supporting Information.
that changes in the HOMO caused by substituent changes in
the ppy framework influence oxidation of Ir(III) complexes,
while variations in the LUMO are dictated by the diimine
acceptor ligand.
The next two lowest-lying singlet excited states (S2 and S3,
respectively) are ascribed to HOMO → LUMO+1 and HOMO
→ LUMO+2 transitions, Tables S2−S6 in the Supporting
Information. They are mainly related to ppy MLCT and IL for
Ir1 and Ir2, with the electron density in both LUMO+1 and
LUMO+2 residing significantly on the cyclo- metalating ppy
ligands. For Ir3 and Ir4, LUMO+1 is largely π*NN and S2 is an
1MLCTIr(ppy)→NN state, while S3 is related to a ppy MLCT/IL
transition with LUMO+2 mainly π*Xppy. TDDFT results
diverge for the four complexes after the fourth lowest-lying
singlet excited state (S4), with transitions ascribed to MLCT/
LLCT, from both Ir(III) and ppy to the NN ligand, or
intraligand ppy excited states.
discussions this excited state is referred to as 1MLCTIr(ppy)→NN
.
As shown in Figure 4A, LUMO energies decrease through the
series from Ir2 to Ir4 due to the enhanced increasing electron-
withdrawing capability of the NN ligand. A significant change in
the HOMO energy for Ir1 is observed due to the electron-
withdrawing F substituents on the ppy ligand.
Variations in calculated S1 energies are also in good
agreement with relative redox potentials in acetonitrile, Table
4 and Figure 4B. Complexes Ir2−Ir4 exhibit a reversible one-
electron oxidation at ∼1.48 V vs NHE, presumably from the
expected Ir4+/Ir3+ couple, which shifts anodically for complex
Ir1 with the oxidation appearing at 1.80 V. Reversible, ligand-
based reduction appears at −1.22, −1.14, and −1.09 V for
complexes Ir2, Ir3, and Ir4, respectively. These values show
In the calculations, the triplet T1 states for complexes Ir2−
Ir4 follow the same trend of S1, arising from formally spin-
forbidden MLCT transitions, which gain allowedness through
spin−orbit coupling. They are lowest-lying and are the origin of
the observed emissions. However, exclusively for Ir1, the T1
transition is 15% HOMO−6, localized in the π orbital of the
dmb ligand, and, therefore, based on the calculations, T1 for Ir1
is of mixed MLCTIr(ppy)→NN/3LCNN character, with a = 0.85
3
and b = 0.15.
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Emission Spectral Fitting at Room Temperature.
Results and Applications. Franck−Condon analyses for
emission spectra at room temperature in acetonitrile were
conducted by application of the single average mode
approximation and eq 2. Experimental spectra are compared
to calculated ones in Figure 5. The agreement between
with an average of a series of C−C/C−N stretching modes in
the NN acceptor ligands33 and with results obtained earlier for
related MLCT dπ6 polypyridyl emitters.14b,24a The electron-
vibrational coupling constant or Huang−Rhys factor, SM, is a
measure of the extent of the skeletal structural change in the
acceptor ligand and increases with energy gap, E0, as found
earlier for MLCT excited states of Os(II)34 and Re(I)
polypyridyl complexes.32c Therefore, FC analysis supports
aforementioned discussions that T1 character is largely
3MLCTIr(ppy)→NN at 298 K.
In the limit of applicability of the single mode approximation
and in the weak vibrational coupling limit, with E0 ≫ SMℏωM
and ℏωM ≫ kBT, the rate constant for nonradiative decay is
given by eq 5. In eq 5, the Franck−Condon weighted density of
states, ln[FC(calc)], can be calculated by using the parameters
from emission spectral fitting and eqs 6 and 7.12d,34
ln knr = ln β0 + ln[FC(calc)
]
(5)
⎡
⎤
⎥
Figure 5. Experimental (colored solid curves) and simulated emission
spectra (black broken curves) by use of eq 2 with the parameters listed
in Table 5 in acetonitrile at 298 K.
γ E0
ℏωM
ℏωME0
1
ln[FC(calc)] = − ln
2
0
− S
−
⎢
M
⎣(1000 cm−1)2 ⎦
2
Δν0,1/2
̃
(γ + 1)2
experimental and calculated spectra was excellent with high
correlation coefficients in all cases, with R2 ≥ 0.9998. Spectral
fitting parameters obtained from the fits (E0, S, and ℏωM) are
listed in Table 5.
(
)
0
ℏωM
+
(6)
16 ln 2
⎛
⎞
E0
γ = ln
− 1
⎜
⎟
⎠
0
S ℏω
⎝
(7)
(8)
M
M
Table 5. Emission Spectral Fitting Parameters in Acetonitrile
at 298 K
Vk2
2π
ℏ
β0 =
complex
E0/cm−1
ν1/2
̃
/cm−1
ℏωM/cm−1
SM
ln[FC(calc)]
(1000 cm−1)
Ir1
Ir2
Ir3
Ir4
19 920
17 570
17 230
16 820
2350
2810
2740
2730
1380
1390
1380
1340
1.34
0.93
0.79
0.71
−21.31
−20.33
−21.66
−22.61
The quantity β0 includes the vibrationally induced electronic
coupling matrix element, Vk, which mixes the initial and final
electronic states, eq 8. Calculated ln[FC(calc)] values are listed in
Table 5, and a plot of ln knr versus ln[FC(calc)] is shown in
Figure 7.
As shown in Figure 6, E0 values in Table 5 are proportional
to the difference in redox potentials between oxidation,
Figure 7. Plot of ln knr versus ln[FC(calc)] from the knr values in Table 1
and the results of emission spectral fitting in Table 5 in acetonitrile at
room temperature.
Figure 6. Plot of E0 from emission spectral fitting vs (E1/2(Ir4+/Ir3+) −
E1/2(NN0/NN−)) in acetonitrile at room temperature.
E
1/2(Ir4+/Ir3+), and reduction, E1/2(NN0/NN−), a characteristic
feature of MLCT excited states.32 The observed linear slope is
less than unity, around 0.82, because emission energy includes
losses by intersystem crossing and contributions from outer-
and inner-sphere reorganization energies. Similar deviations
were also reported for series of MLCT emitters, Re(I), Ru(II),
and Os(II) complexes.32
For complexes Ir2−Ir4, ln knr increases with ln[FC(calc)], as
predicted by eq 5 consistent with decay from emissive T1 states
3
with common MLCTIr(ppy)→NN origins. For these complexes,
the commonly observed energy gap law relationship ln knr
∝
−Eem is not observed, and the more complete expression in eq
5 is required to reconcile the data because of the differences in
acceptor ligand through the series, with a different mix of ring
skeletal vibrations.
According to Table 5, the magnitude of the quantum spacing
for the average acceptor mode, ℏωM, is ∼1370 cm−1, consistent
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Given the high emission efficiency, knr for complex Ir1 is not
known accurately but, nonetheless, deviates markedly from the
trend shown by the other three. An additional difference in the
series appears in the ∼400 cm−1 decrease in bandwidth at half
4.35−6.21 μs range as shown in Figure 8 and much longer than
those at room temperature.
The results of multimode Franck−Condon analyses of the
spectra are summarized in Table 6 with fits compared to
experimental spectra in Figure 8. Spectra for complexes Ir2−
Ir4 were adequately reproduced (R2 ≥ 0.9934) by including
both low-frequency (ℏωL) and medium-frequency averaged
vibrational modes (ℏωM), eq 3. For these complexes, the values
of ℏωM and ℏωL derived from the fits are ∼1380 and ∼410
cm−1, consistent with earlier results of MLCT emission,14b,24a
and arise from acceptor ligand skeletal modes and low
frequency Ir−N stretching modes, respectively. These proper-
height, ν1/2
̃
, for Ir1, which, in the classical harmonic oscillator
limit, is related by eq 9 to the solvent reorganization energy
12d
including low frequency modes treated classically, (λo,L)1/2
.
The decrease in bandwidth is symptomatic of a decrease in the
extent of charge-transfer character in the emitting excited state,
consistent with its mixed MLCT/LC character.
ν1/2
= (16kBTλo,L ln 2)1/2
̃
(9)
ties and the decrease in ν1/2
from ∼2750 to ∼1200 cm−1 upon
̃
Emission Spectra at 77 K and Franck−Condon
Analyses. A characteristic feature of MLCT emission from
cooling are both consistent with emitting T1 excited states for
polypyridyl excited states is their sensitivity to medium.35
A
Ir2−Ir4 at 77 K that are mainly MLCTIr(ppy)→NN in character,
3
although with some extent of LCNN contribution.
pronounced rigid medium effect, “rigidochromic effec-
t”,1a,d,35a,b,36 exists between fluid and rigid media arising from
the inability of the surrounding medium dipoles to reorient to
change in dipole character between excited and ground states.
A temperature dependence arises in bandwidth as described in
eq 9 and band energy due to frequency changes in medium
librations.12d By contrast, ligand-centered ππ* excited states are
relatively unaffected.
Two-mode fitting could not satisfactorily reproduce the
narrow, structured 77 K spectrum of Ir1. Although the
simulation is not enough yet, better agreement was obtained
with a three-mode analysis including a medium-low-frequency
vibrational mode (ℏωML), eq 10. In the three-mode analysis,
the summation was carried out from v* = 0 in the excited state
to levels v = 0→5 in the ground state.
Emission spectra of the series Ir1−Ir4 at 77 K in
propionitrile/butyronitrile (4/5, v/v) are shown in Figure 8.
4
∞
∞
∞
⎛
⎞
⎟
⎠
E0 − νMℏωM − νMLℏωML − νLℏωL
I(ν)
̃
= ∑ ∑ ∑ ⎜
E0
⎝
ν
=0 ν =0 ν =0
M
ML
L
ν
ν
ν
M
ML
L
⎛
⎜
⎝
⎞⎛
⎟⎜
⎠⎝
⎞⎛
⎟⎜
⎠⎝
⎞
⎟
⎠
SM
SML
νML
SL
×
νM!
!
̃
ν !
L
2
⎡
⎤
⎥
⎛
⎞
⎠
ν
− E0 + νMℏωM + νMLℏωML + νLℏωL
⎢
⎜
⎜
⎟
⎟
× exp −4 ln 2
⎢
⎣
⎥
⎦
ν1/2
̃
⎝
(10)
Emission spectra of dmb and Fppy at 77 K were also fitted to
eq 10 with optimized parameters summarized in Table 6. The
ν1/2 and ℏω values for Ir1 and dmb are more comparable than
̃
those of Fppy, although even more satisfactory fits for dmb
could have been obtained with a five-mode analysis.
In contrast to Ir2−Ir4, the low-temperature spectrum of Ir1
is clearly indicative of emission from a lowest-lying T1 state that
is mainly triplet ππ*(dmb) in character, 3LCNN, with a relatively
small extent of MLCT contribution (Figure S5 in the
Supporting Information). Despite possessing ππ* character,
the Huang−Rhys factor for Ir1 is smaller than for the free dmb
ligand due to coordination to the metal and its influence on
ligand rigidity. Comparison with the room-temperature data
shows that an inversion in excited-state ordering occurs
between room temperature, with emission from
3MLCTIr(ppy)→NN, and 77 K, with emission from 3LCNN
lowest-lying. Related observations have been made for high-
energy MLCT state Re(I) polypyridyl-based emitters, where
temperature-dependent inversion of MLCT and ligand-based
ππ* states is observed.37
Figure 8. Experimental spectra (colored solid curves) and emission
lifetimes in propionitrile/butyronitrile (4/5, v/v) at 77 K. Black
broken curves represent simulated spectra of investigated complexes
by using eq 3 or 10 with the parameters listed in Table 6. Simulated
spectra of dmb and Fppy are shown in Figure S4 in the Supporting
Information.
The four complexes were also immobilized in PEG-DMA550
films, which are semirigid and optically transparent at room
temperature. The emission spectra of the complexes in PEG-
DMA550 fluid and film, before and after polymerization, are
shown in Figure 9.
Emission in PEG-DMA550 fluid at 298 K is broad and
structureless for all of the complexes, including Ir1, similar to
fluid acetonitrile at 298 K, while polymerization and film
In the frozen medium, band energy blue-shifts of ∼2700 cm−1
occur compared to room temperature in acetonitrile, consistent
with their MLCT character. These emissions are notably
intense with ϕ ≈ 1 and for Ir2−Ir4 are relatively broad with
similar band shapes. By contrast, emission from Ir1 is narrow
and highly structured and similar in band shape to emission
from the free dmb ligand at 77 K, Figure 8, with a ligand-
centered ππ* origin.29 Emission lifetimes at 77 K are in the
̃
formation result in only slight blue shifts (Δλ ≈ 16 nm; Δν ≈
520 cm−1). In even more rigid PEG-DMA550 films at 77 K,
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Table 6. Spectral Fitting Parameters for Ir1−Ir4 and for Fppy and dmb in Propionitrile/Butyronitrile (4/5, v/v) at 77 K
E0/cm−1
ν1/2
̃
/cm−1
ℏωM/cm−1
SM
ℏωML/cm−1
SML
ℏωL/cm−1
SL
Fppy
dmb
Ir1
23 790
23 400
22 220
20 160
20 090
19 610
570
570
480
1480
1520
1500
1420
1380
1340
1.76
1.81
1.22
1.36
1.18
1.13
850
980
980
1.79
1.21
0.61
286
530
490
410
400
410
1.08
0.58
0.76
1.18
1.11
1.35
Ir2
1260
1280
1110
Ir3
Ir4
Scheme 2. Illustration of Excited-State Inversion for T1 in
Ir1 in the Temperature Decrease from 298 to 77 K
Figure 9. Emission spectra in PEG-DMA500 fluid at 298 K (· · · ·) and
in PEG-DMA500 films at 298 K (− ·· − ··) and 77 K (−).
emission is structured with profiles similar to those observed in
propionitrile/butyronitrile (4/5, v/v) glass at 77 K. As in the
nitrile glass, for Ir1, the change in emission with temperature in
3
3
the film is consistent with the MLCTIr(ppy)→NN to LCNN
inversion observed in the nitrile glass at 77 K. These results are
consistent with important roles from both temperature and
rigidity in the inversion between states, which is illustrated in
Scheme 2.
comes from the high emission quantum yield, greatly decreased
magnitude for knr, and decreased magnitude of λo, the latter
consistent with diminished charge-transfer character in the
excited state. For this complex, cooling to 77 K results in an
inversion in excited-state order with 3MLCTIr(ppy)→NN > 3LCNN
as evidenced by the highly structured emission.
Scheme 2 illustrates two elements of importance in
describing excited-state structure in the series Ir1−Ir4. The
excited-state properties of complexes Ir2−Ir4 are dominated by
low-lying T1 states that can best be described as MLCT in
character with broad, structureless emissions. The properties of
these complexes vary systematically with emission energies and
are systematically tunable by ligand changes. Nonradiative
decay is satisfactorily accounted for by a quantitative version of
the energy gap law parametrized by the results of emission
spectral fitting, which includes variations in the acceptor ligand.
Emission at 298 K originates from states largely
3MLCTIr(ppy)→NN in character with little mixing with low-lying
3LCNN states (orbital coefficients in Scheme 2 are a ≈ 1, b ≈ 0).
At 77 K, the impact of the rigid medium effect on
3MLCTIr(ppy)→NN increases MLCT excited-state energies,
decreasing the gap to the lowest-lying 3LCNN states, presumably
CONCLUSIONS
■
Quantitative evaluation of spectra, along with TD-DFT
calculations, provided information about the extent of coupling
between 3MLCTIr(ppy)→NN and 3LCNN states of a series of
heteroleptic Ir(III) complexes, including the novel [Ir-
(Fppy)2(dmb)]+ complex. Their emission at 298 K is mainly
3MLCTIr(ppy)→NN and is energetically tunable by ligand
modification. [Ir(Fppy)2(dmb)]+ showed distinguished emis-
sion features due to a contribution of triplet ligand-centered
excited state of dmb (3LCdmb) in the emitting T1 state. All four
complexes exhibit intense emission, yet the more pronounced
mixed character of complex Ir1 at room temperature induces an
impressive high ϕ ≈ 1. The results exposed here show the
importance to anticipate the nature of the ground and excited
states and their interactions to describe the emission
phenomena. The great features provided by Franck−Condon
analyses showed that it is a powerful, yet under-utilized, tool
3
with enhanced mixing with the lower-lying ππ* LCNN states.
For Ir1, the lowest-lying state at room temperature is also
MLCT in character but with evidence for significant mixing
between MLCT and low-lying LC states with a = 0.85, >b =
0.15 in the wave function in Scheme 2. Evidence for mixing
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that underlies theory to understand and predict a diverse set of
phenomena, and provide quantitative basis into rates or relative
rates for radiative and nonradiative decays from spectroscopic
measurements.
The dramatic changes in emission characteristics with
temperature observed for these complexes may allow them to
be employed in temperature sensors similar to an earlier
observation by Fischer et al.6 Their intense emissions and color
variations also make them potential candidates for light-
emitting devices that cover a considerable range and are
appealing visually as shown in Figure 10.
ACKNOWLEDGMENTS
■
This work was supported by Fundaca
̧
o de Amparo a
̃
̀
Pesquisa
do Estado de Sao Paulo (FAPESP; 2011/10120-8 and 2012/
̃
15478-0), and Conselho Nacional de Desenvolvimento
́
́
Cientifico e Tecnologico (CNPq). We thank Dr. Manuel A.
Mendez for the electrochemical measurements. A.I. acknowl-
edges support from the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under Award Number
DE-FG02-06ER15788. M.K.B. acknowledge support from the
UNC Energy Frontier Research Center (EFRC) “Center for
Solar Fuels”, an EFRC funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences
under Award Number DE-SC0001011.
REFERENCES
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ASSOCIATED CONTENT
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S
* Supporting Information
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1H NMR spectrum of complex Ir1, complete TDDFT data and
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: neydeiha@iq.usp.br.
(8) (a) Parker, S. T.; Slinker, J. D.; Lowry, M. S.; Cox, M. P.;
Bernhard, S.; Malliaras, G. G. Chem. Mater. 2005, 17, 3187−3190.
Notes
(b) Bolink, H. J.; Cappelli, L.; Coronado, E.; Parham, A.; Stossel, P.
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The authors declare no competing financial interest.
Chem. Mater. 2006, 18, 2778−2780. (c) Nazeeruddin, M. K.; Wegh, R.
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