Blue-emitting cationic iridium(III) complexes featuring pyridylpyrimidine ligands and their use in sky-blue electroluminescent devices

Article abstract of DOI:10.1039/c7tc03110f

The synthesis and structural and photophysical characterisation of four novel, cationic iridium(iii) complexes are reported. These complexes were designed to emit in the blue region of the visible spectrum without employing sp² carbon-fluorine bonds, which have been shown to be electrochemically unstable. Two different C^∧N (where C^∧N is a bidentate cyclometalating ligand possessing a nitrogen-carbon chelate) ligands [5-(4-methylpyridin-2-yl)-2,4-dimethoxypyrimidine (Mepypyrm) and 5-(5-(trifluoromethyl)pyridine-2-yl)-2,4-dimethoxypyrimidine (CF₃pypyrm)] combine electron-withdrawing pyrimidyl nitrogen atoms (in a para relationship with respect to the metal) with methoxy groups in a meta relationship with respect to the metal, which both inductively withdraw electron density from the metal centre, stabilizing the highest occupied molecular orbital. The result is highly efficient (Φ_PL = 73-81%) green to blue (λ_PL = 446-515 nm) emission for complexes 1-4 in MeCN solution. Complex 1 exhibits a broad, unstructured charge transfer (CT) emission profile, while complexes 2-4 exhibit structured, vibronic emission profiles. Density Functional Theory (DFT) calculations corroborate these findings with spin density calculations predicting a T₁ state that is metal-to-ligand and ligand-to-ligand (C^∧N to N^∧N) charge transfer (³MLCT/³LLCT) in nature for complex 1, while complexes 2-4 are predicted to exhibit ligand-centred (³LC) states with spin density localised exclusively on the C^∧N ligands. These complexes were used as emitters in sky-blue and blue-green light-emitting electrochemical cells (LEECs). The bluest of these devices (CIE: 0.23, 0.39) is among the bluest reported for any iridium-based LEEC. It is noteworthy that although the electroluminescence intensity decreases rapidly with time (t_1/2 = 0.1-20 min), as is typical of blue-green LEECs, for devices L1, L3 and L4 we have observed for the first time that this decay occurs without an accompanying red-shift in the CIE coordinates over time, implying that the emitter does not undergo any chemical degradation process in the non-doped zones of the device.

Full text of DOI:10.1039/c7tc03110f

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Baranoff and E. Zysman-Colman, J. Mater. Chem. C, 2017, DOI: 10.1039/C7TC03110F.
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C7TC03110F
Blue-Emitting Cationic Iridium(III) Complexes Featuring Pyridylpyrimidine
Cyclometalating Ligands and their Use in Sky-Blue and Blue-Green Light-Emitting
Electrochemical Cells (LEECs) and Organic Light-Emitting Diodes (OLEDs)
Adam F. Henwood,a Amlan K. Pal,a David B. Cordes,a Alexandra M. Z. Slawin,a Tomas W.
Rees,b Cristina Momblona,c Azin Babaei,c Antonio Pertegás,c Enrique Orti, c Henk J.
Bolink,c* Etienne Baranoff,b* Eli Zysman-Colmana*
a Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St
Andrews, St Andrews, Fife, KY16 9ST, UK, Fax: +44-1334 463808; Tel: +44-1334 463826;
*E-mail: eli.zysman-colman@st-andrews.ac.uk;
URL: http://www.zysman-colman.com
b School of Chemistry, University of Birmingham, Edgbaston, B15 2TT, Birmingham, UK
c Instituto de Ciencia Molecular, Universidad de Valencia, C/J. Beltran 2, 46980 Paterna,
Spain
Abstract. The synthesis, structural and photophysical characterisation of four novel,
cationic iridium(III) complexes is reported. These complexes were designed to emit in the
blue region of the visible spectrum, without employing sp2 carbon-fluorine bonds, which
have been shown to be electrochemically unstable. Two different C^N (where C^N is a
bidentate cyclometalating ligand possessing an nitrogen-carbon chelate) ligands [5-(4-
methylpyridin-2-yl)-2,4-dimethoxypyrimidine
(Mepypyrm)
and
5-(5-
(trifluoromethyl)pyridine-2-yl)-2,4-dimethoxypyrimidine (CF3pypyrm)] combine electron-
withdrawing pyrimidyl nitrogen atoms (in a para relationship with respect to the metal) with
methoxy groups in meta relationship with respect to the metal, which both inductively
withdraw electron density from the metal centre, stabilizing the highest occupied molecular
orbital. The result is highly efficient (ΦPL = 73 – 81%) green to blue (λPL = 446 – 515 nm)
emission for complexes 1 – 4 in MeCN solution. Complex 1 exhibits a broad, unstructured
charge transfer (CT) emission profile, while complexes 2 – 4 exhibit structured, vibronic
emission profiles. Density Functional Theory (DFT) calculations corroborate these findings,
1

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with spin density calculations predicting a T1 state that is metal-to-ligand and ligand-to-ligand
(C^N to N^N) charge transfer (3MLCT/3LLCT) in nature for complex 1, while complexes 2 –
4 are predicted to exhibit ligand-centred (3LC) states with spin density localised exclusively
on the C^N ligands. These complexes were used as emitters in sky-blue and blue-green light-
emitting electrochemical cells (LEECs). The bluest of these devices (CIE: 0.23, 0.39) is
among the bluest reported for any iridium-based LEEC. Noteworthy is that although the
electroluminescence intensity decreases rapidly with time (t1/2 = 0.1 – 20 min), as is typical of
blue-green LEECs, for devices L1, L3 and L4 we have observed for the first time that this
decay occurs without an accompanying red-shift in the CIE coordinates over time, implying
that the emitter does not undergo any chemical degradation processes in the non-doped zones
of the device.
Introduction. Light-emitting electrochemical cells (LEECs) are simple solid-state lighting
devices comprised of only one or two organic layers sandwiched between two electrodes.1
The emissive layer is constituted of charged compounds that are typically either a conjugated
fluorescent polymer with an ionic salt additive,1b, 2 or a phosphorescent ionic transition metal
complex (iTMC),3 although more recently quantum dot4 and perovskite5 based devices have
emerged as well. The iTMCs have been the most widely studied class of emissive materials
as the phosphorescent nature of the emitter enables all the excitons generated in the device to
be converted into light. Among iTMCs, iridium(III) complexes of the form [Ir(C^N)2(N^N)]+
(where C^N is a bidentate cyclometalating ligand such as 2-phenylpyridinato, ppy, and N^N
is a bidentate diimine ligand such as 2,2’-bipyridine, bpy) are the most popular, due to their
short phosphorescence lifetimes, wide range of accessible colours, high photoluminescence
quantum yields and high photo- and chemostabilities.3a, 3b
2

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Upon application of an external bias in the LEEC there is a redistribution of the ions in the
emissive layer, lowering the barrier to charge injection and facilitating light emission.6 The
smaller charge injection barrier thereby permits the use of air-stable electrodes to operate the
LEEC while the charged nature of the emissive layer mitigates the need for additional charge
transport layers that are normally required for conventional organic light-emitting devices
(OLEDs). Thus, these devices are structurally simple and can be fabricated using solution-
processing techniques onto a variety of substrates, making them an attractive cost-effective
technology for large area displays and lighting.1b, 7
For all the potential of LEECs, the biggest limiting factors impeding their widespread
adoption so far are: (1) their inferior stability compared to OLEDs and (2) the near complete
absence of blue emitters. So far, the bluest iTMC LEEC reported8, as determined by the
device’s Commission Internationale de l’Éclairage (CIE) x, y coordinates (CIE: 0.20, 0.28)
does not come close to the ideal blue CIE coordinates (CIE: 0.14, 0.08)9 and shows poor
external quantum efficiency (EQE = 0.28%). Higher device efficiencies (EQE = 3.4 – 7.6%)
have been reported, but these values are limited to blue-green devices with significantly red-
shifted CIE y coordinates (CIEx = 0.20 – 0.25; CIEy = 0.40 – 0.46), meaning that they cannot
be considered as ‘true-blue’ devices.10 Among devices reported with electroluminescence
maxima less than 500 nm, the longest device lifetime reported is only 2.17 h.11
One of the most common strategies for blue-shifting the emission of cationic iridium
complexes is to modify the cyclometalating ring of the C^N ligands with electron-
withdrawing fluorine atoms [making ligands like 2-(2,4-difluorophenyl)pyridine, dFppy12]
that stabilise the HOMO orbitals localised on the phenyl ring and the metal atom. However, it
has been demonstrated that the Caryl-F bonds on the C^N ligands are inherently unstable in
3

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the device, with defluorination a known degradation pathway in both LEECs and OLEDs.13
Thus, there is interest in designing C^N ligands that can mediate a blue-shift in the emission
of the iridium complex without the presence of Caryl-F bonds. One promising avenue is to
replace the phenyl ring of the C^N ligands with an electron-poor heterocycle. Incorporation
of a nitrogen atom within the phenyl ring of the C^N ligands, such as with 2’,6’-difluoro-
2,3’-bipyridine,14 provided enhanced blue-shifted character of the complex to compared to
dFppy. These complexes utilise the electron-withdrawing properties of the cyclometalated
pyridine to stabilise the HOMO energy in concert with the fluorine atoms on the pyridine
ring. The fluorine atoms can be replaced with ambivalent methoxy groups (inductively
electron-withdrawing when meta-substituted, σm = +0.12, and electron-donating when para-
substituted, σp = –0.27)15 that blue-shift the emission when situated in the 2’,6’-positions of
the cyclometalating ring, and provide an effective fluorine-free alternative to the
difluorophenyl moiety of dFppy as a strategy for blue-shifting the emission of these
complexes.16
Other fluorine-free cyclometalating heterocycles have also recently been explored. For
example,
cyclometalated
1-methyl-2-(2’-pyridyl)pyridinium
and
1-methyl-3-(2’-
pyridyl)pyridinium ligands have been employed to generate blue-green cationic iridium
complexes.17 Cyclometalated pyridylpyrimidines, which incorporate two nitrogen atoms into
the cyclometalating ligand framework, have also been shown to be effective at blue-shifting
the emission of neutral iridium complexes but so far no cationic examples have been
reported.18
Four complexes were investigated in this study, employing combinations of four different
ligands (Chart 1). For the C^N ligands, the same cyclometalating pyrimidyl ring (2,4-
4

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dimethoxypyrimidine) was chosen, wherein the non-coordinating carbon atoms of the
pyrimidine ring were substituted with methoxy groups. The pyridyl ring of the C^N ligand
was varied either to contain a methyl group para to the metal centre [5-(4-methylpyridin-2-
yl)-2,4-dimethoxypyrimidine, Mepypyrm], or to contain a trifluoromethyl group para to the
pyrimidyl ring [5-(5-(trifluoromethyl)pyridine-2-yl)-2,4-dimethoxypyrimidine, CF3pypyrm].
For the N^N ligands, 4,4’-di-tert-butyl-2,2’-bipyridine (dtBubpy) and the rigid biimidazole
ligand 1,1’-(α,α’-o-xylylene)-2,2’-biimidazole (o-Xylbiim) were investigated. The dtBubpy
was used as a reference ancillary ligand, as it is one of the most commonly used N^N ligands
for LEECs, forming complexes in combination with both Mepypyrm (complex 1) or
CF3pypyrm (complex 2). We also employed o-Xylbiim in combination with Mepypyrm
(complex 3) and CF3pypyrm (complex 4), as we have previously shown that it is effective at
blue-shifting the emission properties of [Ir(C^N)2(N^N)]+ complexes while also enhancing
their photoluminescence quantum yields, ΦPL.19
Chart 1. Complexes synthesised and characterised in this study.
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Given the paucity of cationic iridium complexes bearing cyclometalated heterocycles, it is
helpful to compare complexes 1 – 4 to reference complexes with well-characterised
photophysical properties. Complex R1 uses the most commonly used C^N ligand, dFppy, to
blue-shift the emission of iridium(III) complexes and thus serves as a benchmark complex.
Complex R2, like complexes 2 and 4, has a -CF3 substituent incorporated into the pyridine
ring of the C^N ligands [2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine, dFCF3ppy] and
thus is useful to understand the influence of this substituent. Complex R3 is structurally very
similar to 1 except the cyclometalating heterocycle is a pyridine and not a pyrimidine (2’,6’-
dimethoxy-4-methyl-2,3’-bipyridine, 5-Mepypy). The comparison between R3 and 1
provides a basis to understand the effects of the additional nitrogen atom contained within the
pyrimidine ring on the optoelectronic properties of the complex. Finally, complex R4, a very
efficient blue emitter recently reported by us,19b contains the o-Xylbiim N^N ligand (λPL
=
459 nm, ΦPL = 90% in MeCN). The C^N ligand for R4, [2-(2,4-difluorophenyl)-4-
mesitylpyridine, dFMesppy], is electronically indistinct from dFppy, since the orthogonal
conformation of the mesityl ring effectively deconjugates this group from the C^N
chromophore; we and others have shown that the mesityl group can enhance the ΦPL of the
complex.19b, 20
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Chart 2. Reference complexes for this study.
Results and Discussion.
Synthesis.
The two C^N ligands were each synthesised in two steps. Lithiation of 5-bromo-2,4-
dimethoxypyrimidine with n-BuLi and quenching with trimethylborate afforded, after
hydrolysis with HCl, the corresponding boronic acid. Both C^N ligands were then obtained
through a Suzuki-Miyaura21 cross-coupling reaction with the appropriate substituted
halopyridine. The o-Xylbiim N^N ligand was synthesised as previously reported.19a Synthesis
of the chloro-bridged dimers proceeded by refluxing the C^N ligand in 2-ethoxyethanol with
22
[Ir(COD)(ꢀ-Cl)]2 as the iridium source.15a,
Complexes 1-4 were obtained following
cleavage of the isolated crude dimers with an excess of N^N ligand in refluxing DCM/MeOH
solution. The anion metathesis step was initially carried out using NH4PF6, but to avoid
7

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protonation of the pyrimidine rings on the complexes (vide infra) KPF6 was used
subsequently instead.
a
Scheme 1. Synthesis of C^N ligands. Reagents and conditions: i) n-BuLi (1.3 equiv., 2.5 M
b
in hexanes), THF, N2, -78 °C, 1 h; ii) B(OMe)3 (1.5 equiv.), rt, 16 h; iii) HCl, 16 h, rt.
K2CO3 (3.0 equiv.), 1,4-dioxane/water (2:1 v/v), Pd(PPh3)4 (5 mol%), 80 °C, 16 h.
Scheme 2. Synthesis of 1 – 4. Reagents and conditions: a 2-EtOC2H4OH, 110 °C, N2, 3 h. b i)
CH2Cl2/MeOH (1:1 v/v), 40 °C, 19 h, N2; ii) Excess KPF6 (aq).
Structural Characterisation.
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13
All complexes were comprehensively characterised by 1H, C and, for complexes 2 and 4,
19F NMR spectroscopy, along with high resolution mass spectrometry (HRMS) and elemental
analysis (EA). Additionally, the structures of 1 – 4 were determined by single crystal X-ray
diffraction.
Figure 1. X-ray crystal structures of complexes 1 – 4, thermal ellipsoids are drawn at the 50
% probability level. Counterions, C-H hydrogen atoms and non-hydrogen bonding solvent
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molecules have been removed for clarity. Atom labelling: hydrogen (light grey), carbon
(grey), nitrogen (blue), oxygen (red), fluorine (yellow), iridium (dark blue). Hydrogen
bonding of the protonated pyrimidine rings in 3 and 4 is denoted with a purple line.
In the solid-state, all four complexes show the expected distorted octahedral geometry
23
about the iridium centre,19b,
with two C^N ligands coordinating through the pyridyl
nitrogen atoms in a mutually trans configuration and the cyclometalating carbon atoms of the
pyrimidine rings mutually cis to each other. The coordination sphere is completed by
coordination through the nitrogen atoms of the N^N ligands (Figure 1). We were surprised to
observe that in contrast to complexes 1 and 2, the crystal structures of 3 and 4 are not
monocationic. In both cases one of the pyrimidyl nitrogen atoms of each of the complexes is
protonated, which is hypothesized to arise from the initial use of NH4PF6 as the anion
-
metathesis reagent. In the case of 3, the complex is a dication (3-H+) with two PF6 anions
present for charge balance and a hydrogen bond [N···N 2.14(6) Å] between the protonated
pyrimidine ring and an acetonitrile solvent molecule. Complex 4 crystallises as a dimeric pair
of protonated (4-H+) and non-protonated (4) complexes, which form a tight hydrogen bond
[N···N 1.98(5) Å] between the protonated pyrimidine ring of 4-H+ and the non-protonated
-
pyrimidine ring of 4. Thus, in the crystal structure there are three PF6 anions for charge
balance: one for each cyclometalated complex, and one for the additional proton.
Table 1. Selected structural parameters for complexes 1 – 4.
Complex
Bond Length (Å)
Ir-NC^N Ir-NN^N
Bond Angle (o)
N-Ir-C N-Ir-N
Ir-C
1.993(19) 2.030(19) 2.134(16) 79.5(8) 75.6(6)
2.014(17) 1.949(19) 2.146(14) 79.5(8)
1
1.997(15) 1.98(2)
1.950(17) 2.04(2)
2.138(13) 80.7(8) 76.6(5)
2.119(15) 80.7(8)
2
1.968(7) 2.049(6) 2.109(6) 79.9(3) 75.5(2)
1.998(5) 2.059(6) 2.153(4) 78.9(3)
1.980(6) 2.051(5) 2.099(5) 80.2(2) 75.85(19)
3
4-H+
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1.970(6) 2.040(5) 2.138(5) 79.9(2)
1.964(6) 2.029(5) 2.118(5) 79.8(2) 75.6(2)
1.989(6) 2.035(5) 2.136(5) 79.7(2)
4
For complexes 1 and 2, an unusual structural feature is observed, where only one of the Ir-
CC^N bond lengths is shorter than the Ir-NC^N bond lengths from the same C^N ligand (Table
1). This is not typical of cyclometalated iridium complexes as the Ir-CC^N bonds are generally
considered to be stronger than the Ir-NC^N bonds, and thus they would all be expected to be
shorter. By contrast, complexes 3-H+, 4 and 4-H+ show more conventional behaviour.
In solution, batches of 1 and 2 prepared using NH4PF6 as the anion metathesis reagent gave
unusually broad and featureless 1H NMR spectra, which were attributed to the protonation of
the pyrimidine rings, a feature observed in the crystal structures of 3 and 4. Figure 2 shows a
1
comparison of the H NMR spectra of batches of complex 2 prepared using NH4PF6 and
KPF6, showing the sharper features present for the batch prepared using KPF6.
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1
Figure 2. Aromatic region of the H NMR spectra for 2 in CD3CN prepared using NH4PF6
(top) and KPF6 (bottom) as the anion metathesis reagent.
In order to ensure valid comparison across the series of complexes, all photophysical
measurements were carried out on samples prepared using KPF6. Elemental analysis
confirmed they are in their monocationic forms.
1
The H NMR spectra of non-protonated samples of complexes 3 and 4 are broadened, as a
result of the expected fluxional motion of the o-Xylbiim ligand, which is slow on the NMR
timescale. Heating the samples resulted in simplification of the spectra due to the dynamic
pseudo C2-symmetric geometry (Figures S15 and S17 in the ESI). Eyring analysis on the
barrier to inversion for 3 and 4 gave similar activation energies to each other (ꢁG‡ = 82.9 and
12

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83.2 kJ mol-1 for complexes 3 and 4, respectively) as well as to previously reported
complexes (ꢁG‡ = 72.2 – 82.3 kJ mol-1).19
Electrochemical Properties:
Electrochemical measurements on 1 – 4 were carried out in MeCN. The cyclic voltammetry
(CV) traces are shown in Figure 3 while the relevant electrochemical data is given in Table 2.
The first oxidation wave in 1 was found to be quasi-reversible while those of 2-4 were found
to be irreversible. DFT calculations point to HOMOs that reside on both the Ir(III) ion and
the aryl ring of the C^N ligands. Thus, the oxidation of all the complexes is inferred as the
removal of an electron from the admixture of the Ir(III) metal centre and the pyrimidine rings
of the C^N ligands (Figure 4). The oxidation potential for 1 [Epa(ox) = 1.53 V] is cathodically
shifted compared to 2 [Epa(ox) = 1.70 V] indicating that the -CF3 group is exerting a stabilising
influence on the HOMO of 2. This is consistent with the analogous comparison of complexes
(ox)
(ox)
R1 [E1/2 = 1.60 V] and R2 [E1/2 = 1.69 V] though the effect is more pronounced in 2
compared to 1 due to the concomitant removal of the electron-donating methyl group, which
is not present in R1. The higher energy calculated for the HOMO of 1 (EHOMO = -5.93 eV)
compared to that of 2 (EHOMO = -6.25 eV) is in good agreement with the measured 0.17 V
cathodic shift of the energy of the oxidation wave of 1 with respect to that of 2 (Table 2). A
slightly less positive oxidation potential (difference of 0.07 V) is observed for 1 compared to
R1, which indicates that dFppy stabilises the HOMO in a similar manner to the Mepypyrm
C^N ligand, with the difference of oxidation potential being attributed to the presence of the
electron donating methyl group on the pyridine. Indeed, similar small differences in the
oxidation potential have been found in changing the C^N ligands in heteroleptic iridium
complexes from dFppy to dFMeppy where the only difference is the presence of a methyl
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group at the 4-position of the pyridine ring.24 There is only a small difference in the oxidation
potential measured for R3 [E1/2(ox) = 1.51 V] compared to 1, demonstrating that the additional
nitrogen atom in the pyrimidine has only a modest influence on the HOMO. In contrast to 1
(ox)
and R1, complex 2 has a virtually identical oxidation potential to R2 [E1/2
= 1.69 V],
demonstrating the importance of the influence of the -CF3 substitution in the C^N ligand
design.
Table 2. Relevant electrochemical data for complexes 1-4.a
Cmpd Epa(ox) Epc(ox) Epc(red)
(V) (V) (V)
Epa(red) EHOMO ELUMO
(V)
EHOMO ELUMO
(eV) c (eV) c
-5.93 -2.33
-6.25 -2.48
-5.81 -1.55
-6.12 -2.00
ꢁEredox
(V)
ǀ
Ε
LUMO-HOMOǀ
(eV) c
3.60
3.77
4.26
4.12
----
(eV) b (eV) b
1.53 d 1.73 d -1.39 e
-1.45 e
-5.95 -3.03
-6.12 -3.04
-5.88 -2.42
-6.07 -2.68
-6.02 -3.06
-6.11 -3.05
-5.93 -3.01
-5.79 -2.43
2.92
3.08
3.46
3.39
2.96
3.06
2.92
3.36
1
2
3
4
1.70 f
1.46 f
1.65 f
–
–
–
–
–
–
–
-1.38 e
-2.00 f
-1.74 f
-1.36
-1.37
-1.41
-1.99
-1.45 e
–
–
–
–
–
–
R119b 1.60 g
R225 1.69 g
R316a 1.51 g
R419b 1.37 g
----
----
----
----
----
----
----
----
----
----
----
a Measurements were carried out in MeCN at a scan rate of 100 mV s-1 with Fc/Fc+ employed
as an internal standard, and data are reported vs SCE (Fc/Fc+ = 0.38 V in MeCN).25 Reported
potentials are referenced to the Fc/Fc+ redox couple. EHOMO/LUMO = -[Eox/red
+ 4.8]
b
vs Fc/Fc+
eV.25 c DFT calculated energy in eV. d Quasi-reversible redox wave. e Reversible redox wave.
f Irreversible redox wave. g Reported refer to E1/2 values.
(ox)
As was observed for 1 and 2, the oxidation potential of 3 [Epa = 1.46 V] is cathodically
shifted compared to 4 [Epa(ox) = 1.65 V]. Similar to the trends predicted for 1 and 2, the DFT
predicted HOMO energies of 3 (EHOMO = -5.81 eV) and 4 (EHOMO = -6.12 eV) are in line with
their oxidation potentials where there is good agreement with the measured 0.19 V cathodic
shift of the energy of the oxidation wave of 3 with respect to that of 4 (Table 1). Both the o-
Xylbiim complexes 3 and 4 show marginally cathodically shifted oxidation waves compared
to their corresponding dtBubpy analogues 1 and 2, which is a consequence of the electron-
releasing nature of the biimidazole N^N ligand. This behaviour is corroborated by the DFT
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calculated energies of HOMOs of 1 vs. 3 and 2 vs. 4 (Table 1). A similar comparison exists
with complexes R1 and R4 [E1/2(ox) = 1.37 V].
Figure 3. CV traces of complexes 1 – 4 in MeCN solution, reported versus SCE (Fc/Fc+ =
0.38 V in MeCN).26 Scan rates were at 100 mV s-1 and are in the positive scan direction.
Determining trends for the observed reduction potentials is less straightforward. For
complexes 1 and 2, a reversible reduction at virtually the same potential [Epc(red) = -1.39 V for
1 and -1.38 V for 2] was attributed to reduction of the dtBubpy ligand, an assignment
supported by DFT calculations. By contrast, the LUMO is predicted by DFT to be on the
biimidazole (biim) part of the o-Xylbiim ligand for 3 and on the CF3pypyrm moiety for 4
(Figure 4). The destabilisation of the LUMO of complex 1 (ELUMO = -2.33 eV) explains the
(red)
small cathodic shift of 10 mV measured for its first reduction wave [Epc
= -1.39 V] in
comparison to those of complex 2 [ELUMO = -2.48 eV; Epc(red) = -1.38 V]. Complexes R1 – R3
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likewise show reversible reduction waves in a similar range [E1/2(red) = -1.36 V for R1, -1.37
for R2 and -1.41 for R3]. For complexes 3 and 4, the reduction waves are observed at
(red)
significantly lower potentials [Epc
= -2.00 V for 3, -1.74 V for 4] and are irreversible in
nature. The nature of the reductions in 3 and 4 can be inferred by scanning beyond the
reduction potentials of the dtBubpy ligand for complexes 1 and 2 (Figure S18 in the ESI).
Complex 1 exhibits a second irreversible reduction at the same potential [Epc(red) = -2.00 V] as
the reduction for 3, which strongly suggests reduction of the pyridine ring of the C^N ligand.
However, it is worth noting that the reduction wave in R4, which was attributed to reduction
of the o-Xylbiim ligand, is also at almost the same potential [E1/2(red) = -1.99 V]. It is therefore
plausible that the reduction wave of 3 is due to the reduction of the N^N ligand, which is
supported by the DFT calculations, where the LUMO is predicted to be on the biim moiety of
the o-Xylbiim ligand (Figure 4). For complex 4, there are at least two observable reduction
processes that can be attributed to either or both the reduction of the o-Xylbiim ligand and/or
the pyridyl ring of the C^N ligand. A similar set of multi-electron reductions are observed for
(red)
complex 2 [Epc
= -1.86 V], and these are anodically shifted compared to the second
reduction observed for 1. This behaviour mirrors the anodic shift observed in the first
reduction of 4 compared to 3 and is consistent with direct reduction of a more electron
deficient CF3-substituted pyridine ring, an assignment corroborated by DFT calculations. The
predicted destabilisation of LUMO (ELUMO = -1.55 eV) explains the measured cathodic shift
(red)
of 260 mV in the CV of 3 [Epc(red) = -2.00 V] compared to that of 4 [ELUMO = -2.00 eV; Epc
(red)
= -1.74 V]. Finally, complex R2 is reported to have a second reduction [E1/2
= -1.68 V]
that was attributed to the reduction of the dFCF3ppy ligand in a similar regime to the first
reduction of 4. The electrochemically observed redox gap, ꢁEredox, follows the order of 1
(2.92 V) < 2 (3.08 V) < 4 (3.39 V) < 3 (3.46 V) and this trend is also in good agreement with
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the DFT predicted HOMO-LUMO energy gap, ǀ
(4.12 eV) < 3 (4.26 eV) (Table 2).
ΕHOMO-LUMOǀ; 1 (3.60 eV) < 2 (3.77 eV) < 4
Figure 4: Calculated frontier MO energies of [1]+, [2]+, [3]+ and [4]+, obtained from DFT
[B3LYP/SBKJC-VDZ for Ir(III) and 6-31G** for C, H, N, O, F] with CPCM(MeCN) and 0.5
eV threshold of degeneracy (orbitals with an isocontour value of 0.03). Kohn-Sham MOs of
[1]+, [2]+, [3]+ and [4]+ are also shown.
UV-Vis Absorption.
The UV-vis absorption spectra for 1 – 4 are shown in Figure 5 and the molar absorptivity
data is given in Table 3. The predicted transitions obtained by TD-DFT calculations are
tabulated for each complex in Tables S2-S5. In the high-energy region of the spectrum (250 –
325 nm), π-π* transitions for all complexes dominate and the features of the spectra are
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determined by the nature of the N^N ligand. The principal π-π* bands for complexes 1 and 2
are blue-shifted (λabs = 264 nm for 1 and 261 nm for 2, ε ~ 60,000 M–1 cm–1) and more
absorptive than for 3 and 4 (λabs = 268 nm for 3 and 276 nm for 4, ε ~ 40,000 M–1 cm–1),
which is in accordance with what was observed previously between complexes containing
dtBubpy or o-Xylbiim as the N^N ligand. This trend is perfectly in line with the TD-DFT
predicted principal π-π* transition energies for complexes 1 (259 nm) and 2 (255 nm)
compared to those of complexes 3 (262 nm) and 4 (277 nm). Excepting complex 4, these
transitions consist of primarily of π-π* transitions localized on the C^N ligand mixed, to a
small degree, with some ligand-to-ligand charge transfer from the C^N ligands to the N^N
ligand, and Ir(dπ) to both C^N and the ancillary ligands (metal-to-ligand charge transfer,
1MLCT). In addition, two bands are observed for 1 and 2 (λabs = 300 and 311 nm for 1 and
297 and 311 nm for 2) that are not present for 3 and 4, which suggest that these bands involve
uniquely the dtBubpy N^N ligand. However, TD-DFT calculations predict that these
1
1
transitions are mostly LC transitions on the C^N ligands with some MLCT Ir(dπ) to C^N
transition; the 1MLCT character is more dominant for 2 (Tables S2 and S3).
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Figure 5. UV-vis absorption spectra of complexes 1 – 4 in MeCN solution.
At lower energies, the trends are reversed, with the absorption features insensitive to the
nature of the N^N ligand but strongly affected by the nature of the C^N ligand. Complexes 1
and 3 have a pair of poorly resolved absorption bands (λabs = 335 and 362 nm for 1 and 339
and 369 nm for 3). Complexes 2 and 4 have a similar pair of absorption bands but these
bands are well resolved and red-shifted (λabs = 350 and 384 nm for 2 and 352 and 388 nm for
4). The similarity of the absorption spectra suggests that these bands are constituted of
transitions localised on the C^N ligands. However, their much lower absorptivity values
compared to the higher energy absorption bands implies that they are not likely to be
comprised of significant π-π* transitions. Instead, the lower absorptivities of these bands for
the four complexes is indicative of 1MLCT charge transfer contributions from the metal to the
pyridine of the C^N ligand, as well as possibly from the methoxy substituents into the pyridyl
1
rings (intra-ligand charge transfer, ILCT). However, TD-DFT calculations point to a
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1
1
predominant LCC^N transition mixed with some MLCT from Ir(dπ) to the C^N ligands for
complexes 1 and 4. Likewise, the transitions at 357 nm and 339 nm of 2 and 3, respectively,
1
1
mainly possess LCC^N character with again some MLCT character from Ir(dπ) to the C^N
ligands. The transition at 380 nm in 2 involves mainly a ligand-to-ligand charge transfer
1
(1LLCT) contribution from the C^N ligands to dtBubpy, along with MLCT from Ir(dπ) to
the N^N ligand (Tables S3 and S4). The transition at 369 nm in 3 is rather complicated and
consists mostly of an 1LLCT from the C^N ligands to the o-Xylbiim ligand and 1MLCT from
Ir(dπ) to both the ligands (Table S4). The red-shift observed for complexes 2 and 4
corroborates this charge transfer assertion, due to stabilisation of the orbitals on the pyridyl
ring by the electron-withdrawing -CF3 group, and is in accordance with an anodic shift in the
second reduction potentials of 2 compared to 1 and the first reduction potential of 4 compared
to 3. Although the principal bands for complexes 1 and 3 are blue-shifted, there is weak
absorption present in these two complexes beyond the onset of absorption for complexes 2
and 4, suggesting that the lowest energy transition is in fact higher in energy for complexes 2
and 4 than for 1 and 3.
Table 3. Absorption maxima and their corresponding molar absorptivities for complexes 1-4.a
λ
abs (nm) [ε (× 104 M-1 cm-1)]
Complex
264 [6.49], 300 [3.45], 311(sh) [3.08], 335 [1.71], 361 [0.90]
261 [5.61], 275(sh) [4.63], 297(sh) [3.10], 311 [2.50], 350 [1.53], 384 [0.69]
268 [3.94], 322(sh) [1.46], 339 [1.23], 369 [0.41]
1
2
3
4
276 [4.05], 315(sh) [1.84], 352 [1.16], 388 [0.42]
a Measurements were carried out in MeCN.
Emission Spectroscopy.
The photophysical properties of these complexes were studied in MeCN solution at 298 K.
Their emission profiles are shown in Figure 6, and the relevant photophysical data are given
in Table 4. Complex 1 is a green emitter, with broad, unstructured emission that is
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characteristic of a mixed charge transfer state between both the metal to the N^N ligand
(3MLCT) and the C^N ligands to the N^N ligand (3LLCT). The mixed charge transfer nature
of the emission spectrum of 1 is also supported by the DFT predicted T1 spin density
distribution, where the spin density was found to be well distributed over the entire complex,
(Figure 7). The photophysical properties of 1 (λPL = 515 nm, ΦPL = 81%) are remarkably
similar to R1 (λPL = 515 nm, ΦPL = 72%) and R3 (λPL = 517 nm, ΦPL = 53%), suggesting that
the additional nitrogen in the pyrimidyl ring in this instance does not have a significant
influence on the energy of the T1 state.
Figure 6. Normalised emission spectra for complexes 1 – 4 in deaerated MeCN solution. λexc:
360 nm. Inset: MeCN solutions of complexes 1 – 4 illuminated under UV (365 nm)
irradiation.
3
By contrast, 2 displays structured LC emission localized on the C^N ligands, that is
significantly blue-shifted (λPL = 454, 481 nm, ΦPL = 77%) compared to 1. The 3LC nature of
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the emission is further supported by the predicted spin-density distribution. Analogously,
complex R2 is blue-shifted (λPL = 470 nm, ΦPL = 68%) compared to complex R1,
demonstrating that the -CF3 group exerts a significant blue-shifting effect on the emission
energy. However, the blue-shifted emission of 2 compared to R2 implies a synergistic blue-
shifting effect between the -CF3 moiety and the pyrimidine compared to the dFCF3ppy
ligand.
Table 4. Relevant solution state and thin film photophysical data for complexes 1 – 4.a
Solution
Thin film
Complex λem (nm) b,c CIE (x,
kr
× knr
× λem (nm)f,
ΦPL
τe
ΦPL
g
f,
y)
105 s-1 105 s-1
(%) d
(%)
(ꢀs) e
g
515
0.20,
0.41
81
77
80
1.36
4.21
9.01
5.96
1.82
0.89
1.40
0.55
0.22
500
65
17
15
1
2
3
454 (1.00), 0.14,
481 (0.98) 0.19
446 (0.69), 0.16,
475 (1.00), 0.23
510 (0.83)
458 (0.90)
482 (1.00)
451
(0.66),
481
(1.00),
513(0.82)
486(0.95),
512 (1.00)
457 (1.00), 0.15,
73
4.77
1.53
0.57
9
4
483 (0.96)
515
0.23
-
-
-
-
72
68
53
90
1.36
2.30
1.30
5.29
2.96
4.08
4.11
2.06
1.39
3.62
0.46
R1
R2
R3
470
517
459, 487
2.19
R4
a
b
c
Measurements at 298 K in deaerated MeCN. λexc: 360 nm. Numbers in brackets denote
d
relative weightings for each emission band. Quinine sulfate used as the reference (ΦPL
=
54.6% in 0.5 M H2SO4 at 298 K).27 e λexc: 375 nm. f Thin film composition: complex:1-butyl-
3-methylimidazolium hexafluorophosphate 4:1 molar ratio. gλexc: 320 nm.
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Figure 7. Triplet spin density distributions of complexes 1-4, obtained from TD-DFT
[UB3LYP/SBKJC-VDZ for Ir(III) and 6-31g** for C, H, N, O, F] with CPCM(MeCN).
Contours at isovalue of 0.02.
3
While complex 1 showed emission from a mixed MLCT/3LLCT state, the use of the o-
Xylbiim ligand in 3 and 4 renders the N^N ligand non-chromophoric and thus the emission of
3
3 originates from a blue-shifted, highly vibronic LC state (λPL = 446, 475, 510 nm, ΦPL
=
80%). These 3LC assignments are corroborated by the C^N-localised spin density
distributions (Figure 7). The principal vibronic bands in 3 virtually coincide with those of
complex 2, but the relative intensities differ; for complex 3, the most intense band is at 475
nm, while the band at 446 nm appears as a less intense shoulder. In the case of 2, the
emission intensities of both bands are very similar, with the band at 454 nm only marginally
more intense than that at 481 nm. Finally, complex 3 has a third shoulder at 510 nm that is
almost totally suppressed in the case of 2. Thus, 2 appears bluer than 3 due to a smaller
contribution to the emission from the blue-green region of the spectrum. The blue-shift in
emission observed for 3 compared with 1 is not mirrored in the analogous comparison
between 4 and 2. In this instance, the emission profile of 4 (λPL = 457, 483 nm, ΦPL = 73%),
overlaps almost coincidentally with 2, albeit with a small red-shift in the former. It is
therefore apparent that when the -CF3 group is incorporated into the C^N ligand, the emission
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becomes totally localised on the cyclometalating ligand and the ancillary ligand exerts almost
no influence.
The relatively long excited state lifetimes (> 4 ꢀs) and small radiative constants kr (< 2 ×
3
105 s–1) further support the predominantly LC character of the emissive triplet state in
complexes 2-4. By contrast, complex 1 displays a shorter τe (1.36 ꢀs) and larger kr (5.96 ×
105 s–1), which is in line with a 3CT-based triplet excited state. The optoelectronic properties
of complex 2 are therefore remarkable: although the LUMO of the complex resides on the
N^N ligand (see electrochemistry section), which is expected to result in a dπ-π*N^N
MLCT/LLCT state of lower energy than the πC^N-π*C^N LC state, emission appears instead to
originate from the higher energy LC state.
We can take this consideration further if we consider the comparison of 1 with R1.
Photophysically, these complexes are almost indistinct with both exhibiting green emission
from
a
3MLCT/3LLCT state and showing similar excited state lifetime and
photoluminescence quantum yields. We can therefore conclude that in this instance the
optoelectronic properties exerted by the Mepypyrm C^N ligand on complex 1 compared to
the dFppy C^N ligand on complex R1 are essentially the same. By contrast, the analogous
comparison of the photophysical properties does not hold between complexes 2 (containing
the CF3pypyrm C^N ligand) and R2 (containing the dFCF3ppy C^N ligand). Complex R2 is
3
blue-shifted compared to R1 but nevertheless emits from a predominantly LLCT/3MLCT
state implicating a chromophoric contribution from the dtBubpy N^N ligand. The CF3pypyrm
C^N ligand, on the other hand, renders the dtBubpy ancillary ligand in complex 2 non-
chromophoric, which is highly unusual for iridium(III) complexes bearing dtBubpy N^N
ligands.
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This unusual feature of the CF3pypyrm C^N ligand is mirrored in complex 4, which has
virtually identical photophysical properties to complex 2 (similar emission maximum, profile,
ΦPL, τe) despite the presence of the o-Xylbiim ancillary ligand that has a much higher energy
LUMO compared to dtBubpy (see the more negative reduction potential of 4 compared to 2).
The reasons behind such unexpected behaviour are currently not understood but are most
likely due to uncommon branching of events at very short time scales (ns and below), which
is beyond the scope of this manuscript.
The highest energy emission maxima, E0-0, are blue-shifted and in the order 3 (22,421 cm-1,
446 nm) > 2 (22,026 cm-1, 454 nm) ~ 4 (21,882 cm-1, 457 nm) > 1 (19,417 cm-1, 515 nm)
(Table 4). The predicted emission maxima, EAE = E(T1)-E(S0) at the T1 optimized geometry
(adiabatic electronic emission) obtained by DFT calculations for 1-4 are, respectively, at 482,
441, 422 and 440 nm. These values match closely to those observed experimentally with
relative errors of 6.8%, 6.0%, 9.1% and 6.8%, respectively for complexes 1-4. Crucially,
these calculations predict essentially no change in emission energy between complexes 2 and
4, corroborating the feature observed for these complexes in solution. This overall trend is
generally mimicked by the measurements pertaining to the ground state, with a gradually
increasing HOMO-LUMO gap across the series calculated from the electrochemistry data
[ꢁEredox: 3 (3.46 V) > 4 (3.39 V) > 2 (3.08 V) > 1 (2.92 V)) and the DFT calculations [ǀΕLUMO-
HOMOǀ: 3 (4.26 eV) > 4 (4.12 eV) > 2 (3.77 eV) > 1 (3.60 eV)] (Table 2). However, these
values also predict a much larger energy difference between complexes 2 and 4, which is not
observed in the PL spectrum or predicted by the T1 spin density DFT calculations. This
highlights the difference in excited state vs ground sate measurements, as well as reflecting
the unusual photophysical features of the CF3pypyrm C^N ligand in the excited state.
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Complex R4 is the bluest among the reference complexes (λPL = 459, 487 nm, ΦPL = 90%)
due to the adoption of the o-Xylbiim ligand as the N^N ligand. Although it is moderately
more emissive, complexes 2 and 4 are modestly blue-shifted in emission, showing that it is
possible to achieve significantly blue-shifted emission without employing C-Faryl bonds.
The complexes were also evaluated in light-emitting electrochemical cells, LEECs. In this
application the complexes are processed into amorphous thin films. In order to understand the
photoluminescence properties in an analogous environment, samples of 1-4 were fabricated
by depositing the complexes onto quartz substrates following the same composition and
coating conditions as were used for LEEC preparation. The neat thin-film photoluminescence
spectra for each complex 1-4 are displayed in Figure 8. All four emitters preserve the same
spectral shape as observed in acetonitrile solution, although the peak position is shifted for 1
and 4. The emission maximum of 1 is centred at 500 nm, slightly blue-shifted from what is
observed in MeCN. The structured emission profiles of 2 and 3 are essentially identical (λmax
= 458 and 482 nm for 2 and 451, 481 and 513 nm for 3) to those measured in MeCN. The
photoluminescence of 4 on the other hand is modestly red-shifted compared to that in
solution (λmax = 486 and 512 nm). Due to the fact that the photoluminescence spectra are
comparable between solution and thin film, it is not unexpected that 1 (3MLCT/3LLCT) has
the highest emission quantum yield in thin film (ΦPL = 65%) while 2, 3 and 4 (3LC) show
lower ΦPL values in thin film (ΦPL < 20%)
.
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1
2
3
4
400
500 600
Wavelength (nm)
700
Figure 8. Neat thin–film photoluminescence spectra for 1-4 (λexc = 320 nm).
Device Characterization
The electroluminescence properties of 1-4 were evaluated by preparing LEECs with a
sandwich architecture, as illustrated in Figure 9d. The devices were fabricated on an ITO-
patterned,
cleaned
glass
substrate,
where
first
a
poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer was deposited by spin-
coating. The LEEC active layer was deposited from an acetonitrile solution, which consisted
of the emitter mixed with the ionic liquid (IL) 1-butyl-3-methylimidazolium
hexafluorophosphate, [Bmim][PF6], in a 4:1 molar ratio (complex:IL). After deposition of the
active layer, aluminium was thermally evaporated as the top electrode.
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Figure 9. Time-dependence luminance under average pulsed current driving of 100 A m-2 for:
a) LEEC 1; b) LEEC 2, 3 and 4; c) Evolution of the driving voltage versus time for the
LEECs based on complexes 1-4; d) Schematic of the LEEC architecture.
For simplicity, the LEECs containing complexes 1-4 are referred to as L1-L4. The devices
were characterized under an inert atmosphere by applying a pulsed current (1 KHz, 50% duty
cycle). The resulting applied average current density was 100 A m-2 due to the duty cycle of
50%. LEECs driven by this operational mode typically show instantaneous luminance, while
the voltage needed to maintain the applied current drops rapidly over time up to reach a
minimum value. This behaviour is explained by taking into account the presence of ions,
which dissociate, move towards the electrodes, forming an electric double layer thereby
reducing the injection barrier and finally contributing to the formation of p and n
electrochemically doped regions within the emitting layer. The luminance and voltage versus
time is depicted in Figure 9 and key parameters are summarized in Table 5.
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Figure 10. a) Normalised electroluminescence spectra for L1-L4. b) Photographs of L1-L3
under operation. c) CIE coordinate chart.
All LEECs exhibit instantaneous luminance, although with different colour emission
(Figure 10). While initially L1, L3 and L4 exhibit blue-green emission, L2 emits sky-blue
light. The electroluminescence of L1, L3 and L4 consists of two peaks centred at 492 and
521 nm for L1, 486 and 553 nm for L3, 492 and 549 nm for L4. However, L2 presents a
maximum at 485 nm with a shoulder at 535 nm, which confirms the observed sky-blue colour
(CIE: 0.23, 0.39). Nevertheless, this value is among the bluest iTMC LEECs reported
employing ionic liquids, with the bluest only moderately bluer (CIE: 0.20, 0.34) than our
own.28
Table 4. Performance for EL devices prepared with the complexes 1-4.
a
b
c
d
e
CIEh (x,
y)
λELg (nm)
Device Lummax t50cd t1/2
Efficacymax PEmax
EQEmax
(cd m-2) (s) (min) (cd A-1)
(lm W- f (%)
1
)
0.34,
0.49
0.23,
0.39
0.31,
0.45
L1
L2
L3
L4
O1
1060
101
29
<5
<5
-
20
2
10.5
1.0
4.6
3.8
0.4
0.1
<0.1
2.1
492, 521
485,535
(sh)
486, 553
0.4
0.2
0.1
-
0.3
<0.1
<0.1
5.3
492 (sh), 0.32,
8
-
<0.1
549
490, 554
0.48
0.34,
0.46
72
-
5.7
a Maximum luminance. b Time to reach 50 cd m-2. c Time to reach one-half of the maximum
d
e
f
luminance. Maximum efficacy. Maximum power efficiency. Maximum external
quantum efficiency. Emission maximum in electroluminescence. CIE coordinates
obtained from the electroluminescence spectrum at the beginning of operation.
g
h
29