Molecular-engineered [Ir(Fppy)2(Mepic)] towards efficient blue-emission

Article abstract of DOI:10.1039/c5nj01352f

A novel Ir(III) complex, [Ir(Fppy)₂(Mepic)] (Fppy = 2-(2,4-difluorophenyl)pyridinate; Mepic = 3-methylpyridine-2-carboxylate) was engineered to show a blue phosphorescence with an impressive unitary quantum yield in our search for luminescent metal complexes for low cost displays and efficient lighting devices. Application of the Golden rule through Franck-Condon spectral analyses along with TD-DFT calculations and comparisons to the [Ir(Fppy)₂(pic)] complex have provided insights on the mixing extension between charge-transfer (CT) and ligand-centered (LC) excited states in the emissive lowest-energy triplet transition for different media. The relatively narrow and vibronically-structured emission of [Ir(Fppy)₂(Mepic)] at 298 K mainly arises from a spin-orbit-induced mixed ³MLCT-LC_{Ir(Fppy)→Fppy}/³LLCT_Fppy→Mepic excited state, with an enhanced MLCT character because of the methyl addition to pic. In frozen media at 77 K, the CT contribution is extinguished, leading to a pure LC_Fppy emission, with a similar emissive spectral profile to the free ligand, FppyH. The efficient blue emission shows that the complex is suitable for high-performance light-emitting devices and for thermo- and rigidity-sensors.

Full text of DOI:10.1039/c5nj01352f

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MOLECULARꢀENGINEERED [Ir(Fppy)2(Mepic)] TOWARDS EFFICIENT
BLUEꢀEMISSION
Kassio P.S. Zanoni 1, Akitaka Ito 2 and Neyde Y. Murakami Iha 1*
1Laboratory of Photochemistry and Energy Conversion, Departamento de Química
Fundamental, Instituto de Química, Universidade de São Paulo – USP, Av. Prof. Lineu
Prestes, 748, 05508ꢀ900, São Paulo, SP, Brazil; Eꢀmail: neydeiha@iq.usp.br,
2Department of Chemistry, Graduate School of Science, Osaka City University, 3ꢀ3ꢀ138
Sugimoto, Sumiyoshiꢀku, Osaka 558ꢀ8585, Japan.
Keywords: Iridium Complex; Molecular Engineering; CIE Coordinates; TDꢀDFT;
FranckꢀCondon Analysis.
ABSTRACT
A novel Ir(III) complex, [Ir(Fppy)2(Mepic)] (Fppy = 2ꢀ(2,4ꢀdifluorophenyl)pyridinate;
Mepic
=
3ꢀmethylpyridineꢀ2ꢀcarboxylate) was engineered to show
a
blue
phosphorescence with an impressive unitary quantum yield in our search for
luminescent metal complexes for low cost displays and efficient lighting devices.
Application of the Golden Rule through FranckꢀCondon spectral analyses along with
TDꢀDFT calculations and comparisons to the [Ir(Fppy)2(pic)] complex have provided
insights on the mixing extension between chargeꢀtransfer (CT) and ligandꢀcentered (LC)
excited states in the emissive lowestꢀenergy triplet transition for different media. The
relatively narrow and vibronicallyꢀstructured emission of [Ir(Fppy)2(Mepic)] at 298 K
3
mainly arises from a spin–orbitꢀinduced mixed MLCTꢀLCIr(Fppy)→Fppy/3LLCTFppy→Mepic
excited state, with an enhanced MLCT character because of the methyl addition to pic.
In a frozen media at 77 K, the CT contribution is extinguished, leading to a pure LCFppy
emission, with similar emissive spectral profile to the free ligand, FppyH. The efficient
1

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blue emission shows that the complex is suitable for highꢀperformance lightꢀemitting
devices and for thermoꢀ and rigidityꢀsensors.
TABLE OF CONTENTS ENTRY
TABLE OF CONTENTS SYNOPSIS
A detailed photophysical investigation of [Ir(Fppy)2(Mepic)] by TDꢀDFT and Frankꢀ
Condon emission band shape analyses revealed a succeeded molecular engineering
towards an intensified blue emission, with phosphorescence from a spin–orbitꢀinduced
3
mixed MLCTꢀLCIr(Fppy)→Fppy/3LLCTFppy→Mepic excited state and an impressive unitary
quantum yield.
INTRODUCTION
Worldwide concerns for increasing energy demand significantly spur the
scientific endeavor for novel efficient lightening technologies. One of the most
successful, lowꢀconsumption and ecoꢀfriendly technologies are the organic lightꢀ
emitting diodes (OLEDs), typically prepared by vacuum processes in a multilayered
structure.1ꢀ6 In the emitting layer of OLEDs, both host matrix and guest emissive species
play important roles. The former promotes formation of excited state of the guest
species by energy transfer. Since the innovative work by Forrest and Thompson in
1998,7 phosphorescent transition metal complexes have been broadly utilized as guests
in order to achieve high external conversion efficiencies, since their emitting triplet
states harvest both singlet (25 %) and triplet excitation (75 %).2,6,7,8 In special, Ir(III)
2

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complexes are the most prominent due to their strong structural and electronic
interactions between ligand(s) and the metal center with one of the highest spin–orbit
coupling (SOC) constant (i.e. ξIr = 4430 cm–1).5,9,10 These facts lead to remarkable and
unique features, such as excellent thermal and photochemical stability and the tunability
of absorption and emission spectra as well as redox potentials.11ꢀ14
Several works have emerged concerning the efficiency and color versatility of
Ir(III)ꢀemitters.1,2,4,10 The luminescence of cyclometalated Ir(III) complexes is best
ascribed as pertaining to an admixture of MLCT and LC states.1,15,16 Two of the main
synthetic strategies to increase quantum yields of Ir(III) emitters10 are to limit the
emission within only one appropriate ligand17,18,19 and induce a larger mixing between
MLCT and LC states.20,21,22 The addition of a methyl group to the Fppy moiety of
[Ir(Fppy)2(NN)] complexes, for example, led to increases in radiative rates by enhancing
the MLCT character in the emissive state, yet no major impacts in their emission
energies were observed.23
In our previous work,22 emission properties of a series of heteroleptic Ir(III)
complexes, [Ir(Xppy)2(NN)]+ (Xppy = cyclometalated ligands; NN = diimine ligands),
with luminescence from orange to blueꢀgreen, were quantitatively evaluated to provide
3
3
the extent of coupling between MLCTIr(ppy)→NN and LCNN states, which drastically
enhanced the emission quantum yield. One of these complexes, [Ir(ppy)2(dmb)]+ (ppy =
2ꢀphenylpyridinate; dmb = 4,4'ꢀdimethylꢀ2,2'ꢀbipyridine), was also successfully used as
the active layer in a light emitting electrochemical cell (LEC).24
In this work, a blue lightꢀemissive Ir(III) complex, [Ir(Fppy)2(Mepic)],
FIrMepic (Fppy = 2ꢀ(2,4ꢀdifluorophenyl)pyridinate and Mepic = 3ꢀmethylpyridineꢀ2ꢀ
carboxylate), was engineered by introduction of a methyl group to the picolinate moiety
of the [Ir(Fppy)2(pic)], FIrpic, archetypalꢀblueꢀemitter25 to enhance the MLCT
3

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character in the triplet emissive state. The intense phosphorescence of the designed
complex and the distinct photophysical properties are discussed in details, with results
supported by experimental and theoretical techniques, such as stateꢀofꢀtheꢀart Franckꢀ
Condon analyses for the emission spectra and timeꢀdependent density function theory
(TDꢀDFT) calculations.
EXPERIMENTAL SECTION
All chemicals and solvents for synthesis were purchased from SigmaꢀAldrich or
Synth, and used as supplied. HPLC grade acetonitrile, propionitrile or butyronitrile were
used for the spectroscopic and photophysical measurements. Compounds merꢀ
[Ir(Fppy)2(dmb)][PF6] and facꢀ[Ir(ppy)3], employed as emission standards for the
emission quantum yield measurement, were synthesized as previously described,22,26
while [Ru(bpy)3]Cl2 was purchased from SigmaꢀAldrich. The standard complex FIrpic,
without the methyl group, was synthesized following the same methodology for
FIrMepic described below.
Synthesis of bis[2ꢀ(2,4ꢀdifluorophenyl)pyridinateꢀC2,N](3ꢀMethylpyridineꢀ2ꢀ
carboxylateꢀO,N)iridium(III) (FIrMepic)
IrCl3·H2O (400 mg, 1.34 mmol) and 2ꢀ(2,4ꢀdifluorophenyl)pyridine (FppyH,
450 ꢁL, 3.01 mmol) were dissolved in a 3/1 (v/v) mixture of ethylene glycol monoethyl
ether/water (20 mL) and heated at reflux with stirring for 10 h. After cooling to room
temperature, 3ꢀmethylpyridineꢀ2ꢀcarboxylic acid (MepicH, 200 mg, 1.46 mmol) and
Na2CO3 (204 mg) were added to the mixture, which was refluxed again for 15 h,
leading to an orange solution. The product precipitated upon cooling was collected by
filtration, and washed three times with ultraꢀpure water then diethyl ether. The yellow
solid was recrystallized by dissolving the solid in ethyleneglycol monoethylether and
precipitated adding water slowly. The product, Chart 1, was filtrated, washed with
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diethyl ether and dried under vacuum to obtain 252 mg of pure product (0.36 mmol,
53% yield).
Chart 1. Chemical structure of FIrMepic.
Elemental analysis data were obtained on a PerkinꢀElmer CHN 2400: Calcd for
IrC29H18N3O2F4·2H2O: C, 46.67 %; H, 2.98 %; N, 5.64 %. Found: C, 46.61 %; H, 2.95
%; N, 5.68. The 1H NMR spectrum (Figure S1 and Table S1 in supporting information)
was recorded in a 500 MHz Bruker AIII, using CD3CN as solvent. The residual solvent
signal was employed as an internal standard. δ/ppm: 8.61 (1H, d, J = 5.8 Hz), 8.30 (1H,
d, J = 8.5 Hz), 8.22 (1H, d, J = 8.3 Hz), 7.91 (1H, t, J = 7.6 Hz), 7.86 (1H, t, J = 7.7
Hz), 7.78 (1H, d; J = 7.6 Hz), 7.63 (1H, d, J = 5.3 Hz), 7.60 (1H, d, J = 5.8 Hz), 7.27
(2H, m), 7.12 (1H, t, J = 5.8 Hz), 6.52 (2H, m), 5.76 (1H, dd, J = 8.9, 2.4 Hz), 5.55 (1H,
dd, J = 8.9, 2.4 Hz), 2.72 (3H, s).
Photophysical measurements
UVꢀVis absorption spectrum was recorded on an Agilent 8453 diode array
spectrophotometer. Steadyꢀstate emission spectra were recorded in a PC1 photonꢀ
counting spectrofluorimeter (ISS) with a photomultiplier based, photonꢀcounting
detector with detector sensitivity correction. Emission decays were recorded on a
ChronosBH timeꢀresolved fluorometer (ISS) using a diode laser (λex = 378 nm,
frequency = 20 kHz, ISS) as an excitation light source. The absorbances at the
excitation wavelength of sample solutions were set between 0.1 and 0.2 in fourꢀ
polishedꢀface quartz cuvettes with 1.000 cm optical path length. Solutions were
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deoxygenated with argon for at least 10 minutes prior to measurement. For 77 K
experiments, samples were prepared in a mixture of propionitrile and butyronitrile (4/5,
v/v) (prop:but) in quartz tubes inserted into a quartz Dewar flask containing liquid N2.
The emission quantum yields (φ) for FIrMepic and FIrpic in acetonitrile at 298
K were calculated by the absolute method applying the methodology developed by
Friend et al27 using a BaSO4ꢀcoated integration sphere, Edinburgh 1ꢀMꢀ2 (in the
University of North Carolina), as well as by the relative method using Equation 1. For
the latter one, acetonitrile solutions of three different reference compounds were
employed as emission standards: a) merꢀ[Ir(Fppy)2(dmb)][PF6] (
= 0.094 28, λex = 400 nm, with a
= 0.99 29, λex = 370 nm, with a 389 nm long
φ
= 0.96 22, λex = 375
nm, with a 389 nm long pass filter); b) [Ru(bpy)3]Cl2 (
φ
420 nm long pass filter); c) facꢀ[Ir(ppy)3] (
φ
pass filter). The results using the integrating sphere and the three references gave
excellent agreement.
ꢊ
ꢅ
ꢇꢈꢉ
ꢆꢇ
ꢀꢁꢂ = ꢀꢂꢃꢄ
(1)
ꢅ
ꢊ
ꢆꢇ
ꢇꢈꢉ
ꢀꢁꢂ
= Emission quantum yield for the sample;
ꢀꢂꢃꢄ
=
Emission quantum yield for the reference in the same solvent;
ꢋꢁꢂ
= Absorbance of the sample at the excitation wavelength;
ꢋꢂꢃꢄ
= Absorbance of the reference at the excitation wavelength;
ꢌ = Integral of the sample phosphorescence spectrum;
ꢁꢂ
ꢌꢂꢃꢄ
=
Integral of the reference phosphorescence spectrum.
Cyclic Voltammetry measurements were performed with a Gamry Interface
1000 potentiostat. The threeꢀelectrode setup consisted of a platinum working electrode
(0.5 cm2), a platinum coiledꢀwire counterꢀelectrode and a silver wire pseudoꢀreference.
Tetraꢀnꢀbutylammonium hexafluorophosphate (50 mmol Lꢀ1) was employed as a
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supporting electrolyte in acetonitrile and ferrocene (0.3 mmol Lꢀ1) as an internal
standard (ꢍꢎꢑꢏ/ꢎꢏ = +0.64 V vs. NHE).
ꢐ
Theoretical calculations
Molecular orbital calculations for the complex were conducted with a Gaussian
09W software.30 Optimization of the groundꢀstate geometry was performed by using
DFT with the B3LYP functional. The LanL2DZ31ꢀ33 and 6ꢀ31G(d,p)34,35 basis sets were
used to treat iridium and all other atoms, respectively. TDꢀDFT calculations were then
performed to estimate energies and oscillator strengths of the lowestꢀenergy 40 singlet
and 5 triplet transitions. All the calculations were carried out in acetonitrile as solvent
by using a Polarizable Continuum Model (PCM).36,37 Electron density populations were
plotted using GaussView 5.0.38 The simulated absorption spectrum was generated using
the same software.
FranckꢀCondon analyses for emission spectra
FranckꢀCondon band shape analyses for emission spectra were carried out on the
basis of Equations 2 and 3, which include two and three vibrational acceptor mode(s),
respectively, as averages of the multiple modes coupled to the transition between the
emitting excited state and the ground state.39ꢀ43 Prior to the spectral fitting analysis, the
number of photons at a given wavelength were corrected to the wavenumber scale by
using the relationship,44 I(ꢒꢓ) = I(λ) × λ2.
ꢥ
ꢥ
ꢞ
ꢡ
ꢡ
ꢤ
ꢢ
ꢍꢑ − ꢙꢚℏꢛꢚ − ꢙꢜℏꢛꢜ
ꢍꢑ
ꢠꢚ
ꢠꢜ
ꢕ ꢖ
ꢔ ꢒꢓ = ꢗ ꢗ ꢘ
ꢝ ꢟ
ꢣ ꢟ
ꢣ
ꢙꢚ!
ꢙꢜ!
ꢡ
ꢦꢑ ꢡ ꢦꢑ
ꢤ
ꢢ
ꢱ
× expꢧ−4ln2 ꢘꢨꢩꢪꢫ ꢭꢡ
ꢤꢝ ꢲ
(2)
(3)
ℏꢮ ꢭꢡ ℏꢮ
ꢬ
ꢢ
ꢢ
ꢤ
ꢨꢩ
ꢯ/ꢰ
ꢥ
ꢥ
ꢥ
ꢞ
ꢡ
ꢡ
ꢡ
ꢤ
ꢢ
ꢴ
ꢍꢑ − ꢙꢚℏꢛꢚ − ꢙꢳℏꢛꢳ − ꢙꢜℏꢛꢜ
ꢍꢑ
ꢠꢚ
ꢠꢳ
ꢠꢜ
ꢕ ꢖ
ꢔ ꢒꢓ = ꢗ ꢗ ꢗ ꢘ
ꢝ ꢟ
ꢣ ꢟ
ꢣ ꢟ
ꢣ
ꢙꢚ!
ꢙꢳ!
ꢙꢜ!
ꢡ
ꢦꢑ ꢡ ꢦꢑ ꢡ ꢦꢑ
ꢴ ꢤ
ꢢ
ꢱ
× expꢧ−4ln2 ꢘꢨꢩꢪꢫ ꢭꢡ
ꢤꢝ ꢲ
ℏꢮ ꢭꢡ ℏꢮ ꢭꢡ ℏꢮ
ꢬ
ꢢ
ꢢ
ꢴ
ꢴ
ꢤ
ꢨꢩ
ꢯ/ꢰ
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In these equations, I(ꢒꢓ) is the emission intensity at the energy ꢒꢓ (in cm–1). E0 is
the energy gap between the zeroth vibrational levels in the ground and excited states.
ħωH, ħωM and ħωL are the quantum spacings for averaged highꢀ, mediumꢀ and lowꢀ
frequency vibrational modes, respectively.39 SH, SM and SL are the associated electronꢀ
vibrational coupling constants or HuangꢀRhys factors,45 related to structural differences
between excited and ground states along the displacement normal coordinates of the
coupled average highꢀ, mediumꢀ and lowꢀfrequency vibrational modes, respectively.
ꢒꢓ1/2 is the full width at halfꢀmaximum (fwhm) for an individual vibronic line.46,47 In the
fitting procedure, E0, ħω, S and ꢒꢓ1/2 were optimized with a leastꢀsquares minimization
routine with application of a Generalized Reduced Gradient (GRG2) algorithm.48 The
summation was carried out from v* = 0 in the excited state to levels v = 0→10 (for 2ꢀ
mode fit) or 0→5 (for 3ꢀmode fit) in the ground state.
RESULTS AND DISCUSSION
Absorption
The electronic absorption spectrum of FIrMepic in acetonitrile at 298 K is
shown in Figure 1, along with the simulated spectrum, which are similar to those of
parent complexes, [Ir(Fppy)2(dmb)]+ or FIrpic.22,25,49 The major band around 253 nm
can be ascribed to a ligandꢀcentered (LC) ππ* transition in the Fppy ligand(s). Broad
and relatively weak absorptions observed in the longer wavelength region are an overlap
of LC, metalꢀtoꢀligand charge transfer (MLCT) and ligandꢀtoꢀligand charge transfer
(LLCT) mixedꢀspin transitions as a direct consequence of the strong SOC effect exerted
by the Ir(III) core:1,9,10,15,22 for instance, the less intense lowestꢀlying band around 460
nm is ascribed to a spinꢀforbidden triplet transition. Although it is difficult to assign the
nature of this featureless absorption, TDꢀDFT calculations and photophysical properties
(i.e., τ, knr and kr) lead to a mixed MLCT/LC excited state, which is Fppyꢀrelated since
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Mepicꢀrelated ones lye at higher energies.51 This mixed MLCT/LC excited state
corresponds to an intramolecular charge transfer transition from a dπIr(Fppy) to a π*Fppy
ꢀ
orbital enabled by the strong quasiꢀcovalent bond between Ir(III) and C atoms in the
Fppy ligands.
Figure 1. Electronic absorption spectrum of FIrMepic (black ―) and the simulated
spectrum by TDꢀDFT calculations (grey •••), both in acetonitrile at 298 K.
TDꢀDFT calculated energies and oscillator strengths for singlet transitions of
FIrMepic are correlated with the experimental absorption spectrum, Figure 1, by
assuming full widths at halfꢀmaximum of the transitions to be 0.230 eV and shifting the
spectrum to higherꢀenergy by 5%. Slight deviations observed in the lowꢀenergy region
are due to the contributions of spinꢀorbitꢀallowed triplet transitions which are not taken
in account in the simulation.
Table 1 summarizes related molecular orbitals (MOs) for singlet (S1–S10) and
triplet (T1) transitions, with energies corrected by the 5% factor. The three lowestꢀ
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energy singlet transitions (S1, S2 and S3) result from several MOs mainly related to the
highestꢀenergy occupied MO (HOMO), characterized as dπIr(Fppy). From electron density
populations and contours in Table 2 and Figure 2, the lowestꢀenergy unoccupied MO
(LUMO) and LUMO+2 are best characterized as π*ꢀorbitals of the Fppy ligand and
LUMO+1 is of the Mepic ligand. Therefore, S1 to S3 transitions are mainly ascribed to
MLCTIr→Fppy and MLCTIr→Mepic having partial mixing with LCFppy and LLCTFppy→Mepic
,
respectively. The nature of HOMO for FIrMepic is very similar to those of the related
complexes, [Ir(Fppy)2(dmb)]+ and FIrpic.22,25,49 The S4 to S7 transitions are mainly
originated from HOMO–1 (dπIr(Fppy)) and mixedꢀMOs.
The calculated S1 energy (ES1 = 3.156 eV) for FIrMepic is in veryꢀgood
agreement to the experimental electrochemical data in acetonitrile (ꢀEREDOX = 3.2 V),
from oxidation and reduction potentials around 1.5 and ꢀ1.7 V vs. NHE, respectively,
EREDOX = 3.21 V).25
similarly to FIrpic (1.6 and ꢀ1.7 V,
ꢀ
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Table 1. Energies and contributions of individual MOs for transitions in the FIrMepic
complex.
Energy*
(Wavelength)
Excited State
Transition
3.156 eV
(393 nm)
3.224 eV
(385 nm)
3.271 eV
(379 nm)
HOMO–0 → LUMO+0 (61 %)
HOMO–0 → LUMO+1 (39 %)
HOMO–0 → LUMO+1 (80 %)
HOMO–0 → LUMO+2 (20 %)
HOMO–0 → LUMO+2 (82 %)
HOMO–0 → LUMO+0 (18 %)
HOMO–1 → LUMO+0 (40 %)
HOMO–1 → LUMO+1 (36 %)
HOMO–1 → LUMO+2 (24 %)
HOMO–1 → LUMO+0 (81 %)
HOMO–2 → LUMO+1 (19 %)
HOMO–1 → LUMO+2 (56 %)
HOMO–0 → LUMO+3 (44 %)
HOMO–0 → LUMO+3 (81 %)
HOMO–1 → LUMO+1 (19 %)
HOMO–2 → LUMO+0 (40 %)
HOMO–4 → LUMO+0 (24 %)
HOMO–3 → LUMO+0 (23 %)
HOMO–2 → LUMO+1 (13 %)
S1
S2
S3
3.632 eV
(341 nm)
S4
3.681 eV
(337 nm)
3.768 eV
(329 nm)
3.790 eV
(327 nm)
S5
S6
S7
3.872 eV
(320 nm)
S8
S9
3.943 eV
(314 nm)
HOMO– → LUMO+4 (100%)%)
HOMO–2 → LUMO+2 (20 %)
HOMO–0 → LUMO+4 (19 %)
HOMO–2 → LUMO+0 (18 %)
HOMO–4 → LUMO+2 (13 %)
HOMO–3 → LUMO+2 (11 %)
HOMO–3 → LUMO+0 (10 %)
HOMO–4 → LUMO+0 (09 %)
3.960 eV
(313 nm)
S10
HOMO–0 → LUMO+0 (53 %)
HOMO–0 → LUMO+1 (34 %)
HOMO–3 → LUMO+2 (13 %)
2.830 eV
(438 nm)
T1
*Energy values were shifted to higherꢀenergy by 5%.
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Table 2. MO electron density populations for S1 to S7 and T1 transitions in the FIrMepic
complex.
Contribution / %
Molecular Orbital
Ir(III)
Fppy
Mepic
93.68
2.97
LUMO+2
LUMO+1
LUMO
2.43
3.89
5.10
91.93
90.39
64.11
88.27
35.27
65.09
43.59
27.18
65.83
68.61
5.97
3.64
LUMO+3
LUMO+2
LUMO+1
LUMO
2.78
33.11
6.54
5.19
5.29
59.44
32.20
6.19
2.71
HOMO
50.22
56.66
29.73
23.09
HOMOꢀ1
HOMOꢀ2
HOMOꢀ3
16.16
4.44
8.30
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Figure 2. MO contours (0.03 eÅ–3) related to S1 and T1 transitions in FIrMepic.
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Emission at 298 K
Excitation of FIrMepic in acetonitrile at 298 K leads to singlet (and triplet) excited
states, followed by internal conversion and intersystem crossing to give the lowestꢀlying triplet
state (T1). MOs of T1 differ from those of the relevant singlet state, Figure 2 and Table 2. The
excitation spectrum, Figure 3, is almost identical to the absorption spectrum in the 270–465 nm
region, Figure 1, and indicative of an efficient formation of the T1 state. Deviations in the 230–
270 nm region are due to the inner filter effect.
Figure 3. Excitation (black ••••; λem = 474 nm) and emission spectra (blue ―; λex = 365 nm) for
[Ir(Fppy)2(Mepic)] in acetonitrile at 298 K.
Deactivation from T1 occurs with intense blue phosphorescence and the emission
spectrum is relatively narrow with weak vibronic modes, Figure 3. The emission maximum at
474 nm in acetonitrile, Table 3, is almost independent of the polarity of the surrounding medium,
as shown in Figure S2 in supporting information. The methyl group in the pic moiety has a little
influence on λmax (compared to FIrpic) and only plays a major role in the emissive constant, as
discussed in following paragraphs.
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1
The uncoordinated FppyH ligand exhibits a weak ππ*Fppyꢀfluorescence at 298 K (λmax
=
=
3
307 nm), whereas the ππ*Fppyꢀphosphoresnce is only observed at frozen media at 77 K, (λmax
455 nm), Figure S7 in supporting information. The structured and mediaꢀindependent emission
for the FIrMepic could indicate a ππ*ꢀbased character of T1 state related to Fppy ligands. In this
assumption, the heavy Ir(III) core acts solely on eliminating the forbidden character of
3
intersystem crossings, which leads to the phosphorescence from the lowestꢀlying ππ*Fppy
channel. However, the impressively high emission quantum yield (φ ~ 1) and the relatively longꢀ
lived emission (τ = 0.76 ꢁs) for the complex at 298 K, Table 3, differs significantly from those
3
for pure ππ* phosphorescence in same conditions and are analogous to pure MLCT
emissions.15,43,52ꢀ54 Therefore, from the distinct emission properties, it is more appropriate to
assume a mixed MLCTꢀLC character (Fppyꢀbased) for the T1 state of FIrMepic, harvested by
singlet and triplet relaxations, similarly to FIrpic (Figure S8 in supporting information).
Table 3. Emission properties of FIrMepic, FIrpic and the free FppyH ligand in different media
at 298 and 77 K.
φ ∗
Compound
Media
Acetonitrile
τ
kr
(105 s–1)
13
knr
(104 s–1)
2.6
λmax
(nm)
474
(ꢁs)
0.77
298 K
0.98 ± 0.02a
0.96 ± 0.06b
0.98 ± 0.05c
FIrMepic
0.96 ± 0.05e
**
prop:but
298 K
77 K
298 K
472
456
470
0.61
1.97
0.74
ꢀꢀ
ꢀꢀ
11
ꢀꢀ
ꢀꢀ
2.3
**
Acetonitrile
0.80 ± 0.03c
FIrpic
0.83 ± 0.07d
**
prop:but
Acetonitrile
prop:but
77 K
298 K
77 K
456
307
455
1.95
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
ꢀꢀ
**
FppyH
< 0.005
**
1.90
*Quantum yields were measured by the relative method against standard compounds ꢀ merꢀ[Ir(Fppy)2(dmb)][PF6] 22
(a), [Ru(bpy)3]Cl2 28 (b), facꢀ[Ir(ppy)3] 29 (c), or the FIrMepic sample itself (d) ꢀ or by the absolute method using an
integrating sphere (e).
**Not measured.
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Emission quantum yields for FIrMepic, Table 3, were measured by the relative method
against three different emission standards to verify the impressive obtained values: a) merꢀ
22
29
[Ir(Fppy)2(dmb)][PF6] ; b) [Ru(bpy)3]Cl2 28; c) facꢀ[Ir(ppy)3] . Data obtained using the three
standards are in excellent agreement among them and also to the absolute value, measured using
an integrating sphere. Indeed, FIrMepic shows an unitary quantum yield, higher than FIrpic,
and the electronic origins for the emissionꢀenhancement are explained in following paragraphs.
Here, it is worth reinforcing that the
φ values for FIrpic obtained in this work (0.83 and 0.80) are
considerably higher compared to previews measurements (0.63 55 or 0.6111 in the same solvent).
This value is very accurate considering it was obtained using two moreꢀappropriate reference
compounds (with similar λem,
λexc and φ 56), whose quantum yields were also previously
measured by an integrating sphere. The use of the reevaluated value can avoid misleading
comparisons in future works.
It is clear that the T1 state of FIrMepic mainly deactivates through a radiative process (kr),
Table 3, rather than the nonradiative one (knr), both calculated by Equations 4a and 4b.
ꢵ
ꢶ
ꢀ = ꢵ ꢭꢵ
(4a)
(4b)
ꢶ
ꢷꢶ
ꢹ
ꢸ = ꢵ ꢭꢵ
ꢶ
ꢷꢶ
kr is related to the square of the transition dipole moment upon radiative deactivation
from the mixedꢀT1 state,1,22,50 resultant from the SOCꢀpermutation between closeꢀ and lowꢀlying
excited states of different nature.1,9,10,22,50 The remarkable kr value (13 × 105 s–1) for FIrMepic is
an experimental proof of the mixedꢀnature for T1 with a major role of a strongerꢀdipole MLCT
counterpart, similarly to that of [Ir(Fppy)2(dmb)]+ (kr = 15 × 105 s–1).22
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A mixed character for T1 at 298 K is also supported by TDꢀDFT calculations. As
summarized in Table 1, the lowestꢀlying triplet excited state is ascribed to an interaction between
three different transitions, HOMO → LUMO (53%), HOMO → LUMO+1 (34%) and HOMOꢀ3
→ LUMO+2 (13%). The first two are similar in nature (both tripletꢀLUMO and tripletꢀLUMO+1
are π*Fppy related, Table 2 and Figure 2), possessing totally 87% contribution in the mixed
3
character, thus the emitting excited state of FIrMepic is best characterized as MLCTꢀ
LCIr(Fppy)→Fppy. The third one is assigned to a 3LLCTFppy→Mepic transition, πFppyπ*Mepic related, and
has a minor contribution (13%) to the emitting T1 state.
As summarized in Table 2, the observed krꢀconstant for FIrMepic is higher than FIrpic
(by 2 × 105 s–1) as a consequence of the methyl addition, although the electronic origin is not
obvious and relies on SOCꢀinduced permutations1,10,50 theoretically corroborated by TDꢀDFT, as
described hereafter. For FIrpic, the absence of electronꢀdonating methyl group in the pic moiety
leads to a more stabilized π*pic (tripletꢀLUMO+2) hence a smaller energy difference between
3LLCTFppy→pic and the lowꢀlying 3MLCTꢀLCIr(Fppy)→Fppy states. This smaller ꢀE results in a more
effective mixing between the MLCTꢀLC and LLCT counterparts, decreasing the (strongerꢀ
dipoleꢀ) MLCT character (80%, Tables S3 and S5 in supporting information) in T1. Not only the
lower MLCT influence but also the LLCT enhancement is hazardous to kr by weakening of Irꢀ
ligand bonds.19
TDꢀDFT calculations were also conducted for two other possible isomers (4ꢀ and 5ꢀ
Mepic), to analyze theoretically the impact of the methyl position in the resultant T1 admixture
based on the proposed SOCꢀinduced electronic mechanism. Their electron density populations
and contours are summarized in Tables S6ꢀS9 and Figures S10ꢀS12 in supporting information.
The calculated excited state properties of the 6ꢀMepic isomer (with a reported
φ
value lower than
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FIrpic17,18) are not compared, since the methyl group distorts the complex structure by a steric
hindrance.
A comparison of orbital energies among the three isomers, Tables S6ꢀS9 in Supporting
Information, indicates a lower value for the 3ꢀMepic complex (–1.802 eV), which comes from an
orbital node exactly at the 3ꢀposition that leads to a less effective electron donating effect.
Substitution in the 4ꢀposition is the most effective (ꢀ1,768 eV), since the electron donation occurs
in the inverse direction of the Ir(Fppy)ꢀtoꢀMepic charge transfer, as similarly observed for the
Ruꢀtoꢀdmb charge transfer in the [Ru(dmb)3]2+ complex.57ꢀ59
3
In terms of the final admixture in the T1 transition, the LLCTFppy→Mepic (HOMOꢀ3 →
LUMO+2) influence increases from 3ꢀMepic to 4ꢀMepic complexes (from 13% to 15%),
decreasing the contribution of the MLCT counterparts (85% for 4ꢀMepic), Table S10 in
supporting information. The MLCT character is further decreased in the 5ꢀMepic complex (69%)
by an influence of a fourth transition, LCFppy related (15%), in the excited state mixing, Table
S11 in supporting information. Therefore, based on this proposed electronic effect, it is also
possible to speculate that the 4ꢀMepic isomer will lead to a higher emissive constant compared to
the nonꢀsubstituted FIrPic, since it also possesses a higher MLCT/LLCT ratio, yet only the 3ꢀ
Mepic one has been synthesized.
As for nonꢀradiative processes, knr is slightly higher for FIrMepic (2.6 × 104 s–1) in
comparison to FIrpic (2.3 × 104 s–1, here obtained using the new reevaluated
φ value) as a
consequence of its slightly lowerꢀenergy emission, in accordance to the energy gapꢀlaw that
predicts higher knr constants for decreased emission energies (ln knr is linear to ꢀEem).22,39
Nevertheless, the much larger increase in kr, as discussed above, plays a major role in the
resultant enhanced quantum yield.
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Humans can perceive colors by a trichromatic stimuli on the virtual cortex, quantified by
the Comission Internationale d’Eclairage (CIE) in three matching functions or spectral sensitivity
curves, ̅x(λ), ̅y(λ) and ̅
z(λ).60,61 The x and y CIE coordinates for FIrMepic, FIrpic and
[Ir(Fppy)2(dmb)]+ were calculated by Equations 5a and 5b from their X, Y and Z tristimulus,
Equations 6a to 6c, using photoluminescence spectral data in acetonitrile at 298 K. The
numerical values of the CIE standard ̅x(λ), ̅y(λ) and ̅
z(λ), are available as freeꢀaccess tables.62
ꢻ
ꢺ = ꢻꢭꢼꢭꢽ
(5a)
(5b)
(6a)
(6b)
(6c)
ꢼ
ꢾ = ꢻꢭꢼꢭꢽ
ꢿ = ꣂꣃꢑ ꢔꢕꣁꢖꢺ
̅ꢕꣁꢖdꣁ
ꣀ
꣄ꣃꢑ
ꣅ = ꣂꣃꢑ ꢔꢕꣁꢖ꣆ꢕꣁꢖdꣁ
ꣀ
꣄ꣃꢑ
꣇ = ꣂꣃꢑ ꢔꢕꣁꢖ꣈
̅ꢕꣁꢖdꣁ
ꣀ
꣄ꣃꢑ
Figure 4. Color space chromaticity diagram with CIE coordinates for the emission of FIrMepic,
FIrpic and [Ir(Fppy)2(dmb)]+ in acetonitrile at 298 K.
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The emissive deactivation of FIrMepic in acetonitrile at 298 K is skyꢀblue (x = 0.14; y =
0.29), with CIE coordinates practically identical to the standard blue emissive FIrpic (x = 0.14; y
= 0.26) in same conditions, Figure 4. The remarkable
φ strenghs this complex to be employed as
a standard tripletꢀemitter and could also be suitable for the pursuit of whiteꢀlight emitting
devices. These results also show our progress from our previously reported bluishꢀgreen
[Ir(Fppy)2(dmb)]+ (x = 0.26; y = 0.52) towards emission in a very attractive and highly desired
color even at room temperature.
Emission at 77 K
As for the emission of FIrMepic and FIrpic in prop:but frozen glasses at 77 K, their
spectra are narrower and shifts to higherꢀenergy in comparison to the 298 K one, Figure 5. Their
band shapes resemble the phosphorescence from uncoordinated FppyH at the same temperature
3
and solvent as well as their emission lifetimes are identical, leading to a pure LCFppy emission
character for both complexes at 77 K. The medium freezing is also followed by an intensity
increase for the FIrpic complex arising from the temperature dependence of the energy gap
law.22,39
Figure 5. Emission spectra at 77 K (λex = 365 nm) for FIrMepic (black ―), FIrpic (grey ―)
and FppyH (cyan ―) in prop:but frozen glasses.
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FranckꢀCondon analyses for emission spectra
In order to quantitatively evaluate the excitedꢀstate character and vibrational progressions
of the emitting T1 state in different media and temperatures, FranckꢀCondon analyses for
emission spectra were conducted by application of the 2ꢀ or 3ꢀaverage mode approximation,
Equations 2 or 3. Fitted spectra for FIrMepic are shown in Figure 6, together with the
experimentally obtained ones, and the fitting parameters are summarized in Table 4.
Figure 6. Experimental (solid grey areas) and fitted emission spectra for FIrMepic and FppyH
by using Equations 2 (•••) or 3 (―) and parameters summarized in Table 4.
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Table 4. Spectral fitting parameters for FIrMepic, FIrpic and FppyH at 298 (in acetonitrile) and 77 K (in prop:but).
T / K
Compound
FIrMepic
FIrpic
Number of
E0
ꢒꢓ1/2
ħωH
/ cm–1
2240
2250
2380
ħωM
ħωL
SH
SM
SL
R
vibrational modes
298
2
3
2
21300
21300
21400
1110
1110
1080
ꢀꢀ
990
980
0.15
0.15
0.15
ꢀꢀ
1.09
1.09
1.00
0.9996
0.9996
0.9989
1390
--
0.01
--
1070
3
3
3
3
21420
21880
21920
21820
1040
450
430
490
2460
1470
1450
1470
1320
1010
1010
1000
980
470
490
480
0.12
0.59
0.60
0.58
0.28
0.38
0.37
0.38
0.79
0.45
0.39
0.48
0.9990
0.9969
0.9960
0.9974
77
FIrMepic
Firpic
FppyH
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The emission spectra of both complexes in nitriles at 298 K were adequately reproduced
(correlation factor R ≥ 0.9995) by accounting highꢀ (ħωH) and lowꢀfrequency (ħωL) averaged
vibrational modes, Equation 2. The FIrMepic spectrum is almost independent (SM ~ 0.01) to the
mediumꢀfrequency mode (ħωM) obtained by the fitting using Equation 3.
The obtained E0 for FIrMepic at 298 K (21300 cmꢀ1, or 2.64 eV) is in good agreement to
the TDꢀDFTꢀcalculated T1 energy and it fairly correlates to ꢀEREDOX (3.19 V) in a factor of 0.83,
very similar to [Ir(Xppy)2(NN)]+ complexes (0.82),22 differing from unity because the emission
energy also accounts intersystem crossings and contributions from outer and innerꢀsphere
reorganization energies. These are characteristic features of most emitters with a main MLCT
nature.22,63ꢀ68
The ꢒꢓ1/2 (or fwhm) of an individual vibronic line is proportional to the square root of the
solvent reorganization energy, with higher values for CT emissions.39 fwhm obtained for 298 K
FIrMepic emissions (~ 1100 cm–1) arise from the mixed CTꢀLC nature, since values are lower
22
than pure CT states (~2580 cm–1 ꢀ on the basis of twoꢀmode fit ), yet not as small as pure LC
ones (e.g. the free FppyH ligand, ~ 490 cm–1). For FIrpic, the lower MLCT character in the
mixedꢀT1 state results in a slight decrease in fwhm (~ 1040 cm–1).
The mediumꢀ and lowꢀfrequency modes obtained from the spectral fittings at 298 K (ħωM
~ 1400 cm–1 and ħωL ~ 980 cm–1) can be ascribed to skeletal modes in the Fppy ligand since
similar frequency modes are obtained from the spectral simulation for FppyH (at 77 K). These
results are also clear evidences of the Fppyꢀlocalized T1 transition in both complexes, as obtained
from TDꢀDFT. The nature of ħωH is unclear, possibly a picꢀskeletal mode associated to the lower
3LLCTFppy→Mepic extent in T1, since ħωH is the most responsive to the addition of the vibrationꢀ
restrictive methylꢀgroup, with a decrease in frequency from 2460 to 2250 cm–1.
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The spectra of both complexes and the free ligand in the frozen media at 77 K are
satisfactorily reproduced by the 3ꢀaverage mode approximation, Equation 3, with quite similar
parameters, Table 4. The results clearly demonstrate that the emitting T1 state of both complexes
at 77 K are purely ππ*Fppy in character (i.e., 3LCFppy) with no extent of MLCT or LLCT mixings,
leading to spectral resemblances and comparable emission lifetimes, Table 3. Therefore, by
freezing the medium from 298 to 77 K, the mixing of chargeꢀtransfer and LC excited states is
extinguished by destabilization of CT states owing to pronounced thermoꢀ and rigidochromic
effects, in contrast to relatively unaffected ππ* excited states.
CONCLUSIONS
The FIrMepic complex was successfully engineered to present desired photophysical
features, specially blue luminescence from a distinctꢀnature excited state. Emission spectrum
studies, along with theoretical calculations, have proven a 3MLCTꢀLCIr(Fppy)→Fppy/LLCTFppy→Mepic
mixed character for the emissive T1 state of FIrMepic and FIrpic complexes. Mixedꢀnature
emissive T1 states are enabled by a strong spin–orbit coupling of the Ir(III) core along with the
quasiꢀcovalent bond between Ir(III) and C atoms.
The detailed photophysical study presented here stands as a proofꢀofꢀconcept example of
the interplay between different excited states and SOCꢀpermutations in Ir(III) compounds.
Distinct emissiveꢀstates lead to veryꢀefficient blue phosphorescence (CIE ~ x = 0.14; y = 0.29)
for both complexes at 298 K, yet the methylated FIrMepic complex shows an increased
emissive constant, with unitary quantum yield. The electronic origin of this methylꢀinduced
improvement is because of a larger influence of the MLCT counterpart in T1, based on SOCꢀ
induced permutations corroborated by TDꢀDFT calculations. The influence of the CTꢀ
counterpart is extinguished by freezing the medium at 77 K due to a pronounced rigidochromic
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effect, which leads to a smaller bandwidth in a vibronicallyꢀstructured spectrum, very similar to
that of the free FppyH ligand.
The improved photophysical features of FIrMepic, in special the unitary quantum yield,
makes it a strong candidate for veryꢀefficient lightꢀemitting technologies, such as phosphorescent
temperatureꢀ and rigidityꢀsensors, and especially in blueꢀlight emitting devices.
Furthermore, by a judicious deviceꢀdesign, whiteꢀlightꢀemitting systems can be
developed by employing FIrMepic and orange/red Ir(III) complexes within the x > 0.45 CIE
region (for example, recently published ones69ꢀ72) in their active layers.
ASSOCIATED CONTENTS
1
Supporting Information. H NMR spectrum of FIrMepic, complete TDꢀDFT data, emission
spectra in different media, emission decays and time resolved emission spectrum. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* Eꢀmail: neydeiha@iq.usp.br
Author Contributions
All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
AKNOWLEDGEMENTS
This work is supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP),
and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). We thank to
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Bruna K. Kariyazaki for the initial synthesis of the complex and Dr. M. Kyle Brennaman, from
the University of North Carolina at Chapel Hill, for quantum yield absolute measurements.
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