Phosphorescent cationic iridium(III) complexes bearing a nonconjugated six-membered chelating ancillary ligand: a strategy for tuning the emission towards the blue

Article abstract of DOI:10.1039/c8dt00467f

This study concerns an assessment of the impact of the interruption of the electronic crosstalk between the pyridine rings in the ancillary ligand on the photoluminescence properties of the corresponding iridium(iii) complexes. Two new cationic Ir(iii) co

Full text of DOI:10.1039/c8dt00467f

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Phosphorescent Cationic Iridium(III)
Complexes Bearing a Nonconjugated Six-
Membered Chelating Ancillary Ligand: A
Strategy for Tuning the Emission Towards
the Blue
Claus Hierlinger,a,b David B. Cordes,b Alexandra M. Z. Slawin,b Denis Jacquemin*c
Véronique Guerchais*a Eli Zysman-Colman*,b
a Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, Fꢀ
35000 Rennes, France. Eꢀmail: veronique.guerchais@univꢀrennes1.fr
b Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St
Andrews, St Andrews, Fife, KY16 9ST, UK. Eꢀmail : eli.zysmanꢀcolman@stꢀandrews.ac.uk;
Web: http://www.zysmanꢀcolman.com
c UMR CNRS 6230, Université de Nantes, CEISAM, 2 rue de la Houssinière, 44322 Nantes
Cedex 3, France. Eꢀmail: Denis.Jacquemin@univꢀnantes.fr
1

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Abstract
This study concerns an assessment of the impact of the interruption of the electronic crosstalk
between the pyridine rings in the ancillary ligand on the photoluminescence properties of the
corresponding iridium(III) complexes. Two new cationic Ir(III) complexes,
[Ir(dFmesppy)2(pmdp)]PF6, 1, and [Ir(mesppy)2(pmdp)]PF6, 2, [where dFmesppy is 2ꢀ(2,4ꢀ
difluorophenyl)ꢀ4ꢀmesitylpyridinato, mesppy is 4ꢀmesitylꢀ2ꢀphenylpyridinato and pmdp is
2,2'ꢀ(phenylmethine)dipyridine, L1] possessing sterically congested cyclometalating ligands
combined with the nonconjugated diimine ancillary N^N ligand are reported and their
solutionꢀstate and thin film photophysical properties analyzed by both experimental and
theoretical methods. The crystal structure of 1 confirms the formation of a sixꢀmembered
chelate ring by the N^N ligand and illustrates the pseudoꢀaxial configuration of the phenyl
substituents. Upon photoexcitation in acetonitrile, both complexes exhibit a ligandꢀcentered
emission profile in the blueꢀgreen region of the visible spectrum. A significant blueꢀshift is
observed in solution at room temperature compared to the analogous reference Ir(III)
complexes (R1 and R2) bearing 4,4'ꢀdiꢀtertꢀbutylꢀ2,2'ꢀbipyridine (dtBubpy) as the N^N
ligand. The computational investigation demonstrates that the HOMO is mainly centered on
the metal and on both cyclometalating aryl rings of the C^N ligands, whereas the LUMO is
principally localized on the pyridyl rings of the C^N ligands. The photoluminescence
quantum yield is reduced compared to the reference complexes, a probable consequence of
the greater flexibility of the ancillary ligand.
Introduction
Iridium(III) complexes are attractive phosphors because of their generally high
photoluminescence quantum yields, ΦPL, their relatively short emission state lifetimes, τPL,
and their facile and wide emission color tunability as a function of ligand identity.1 In
2

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electroluminescent devices such as organic light emitting diodes (OLEDs)2 and lightꢀemitting
electrochemical cells (LEECs),3 blue emissive materials are critical components for fullꢀcolor
displays and for the generation of white light in the context of solidꢀstate lighting.4 Charged
complexes are particularly germane for LEECs. Typically, heteroleptic cationic Ir(III)
complexes of the form [Ir(C^N)2(N^N)]+ consist of two cyclometalating C^N ligands, often
based on a 2ꢀphenylpyridinato (ppy) scaffold, and one fiveꢀmembered chelating diimine N^N
ancillary ligand such as 2,2'ꢀbipyridine (bpy), 1,10ꢀphenanthroline (phen) or their derivatives.
The emission energy is normally tuned through substituent decoration about these ligands,
with typically electronꢀwithdrawing groups attached to the C^N ligands and electronꢀdonating
groups incorporated onto the N^N ligand. These changes in electronic properties of the
different ligands are used in concert to increase the HOMOꢀLUMO gap, and by extension the
energy of the emissive triplet state. Much less attention has been devoted to the impact of
changing the chelate ring size of either ligand type on the emission energy, particularly in the
context of the incorporation of an sp3 carbon spacer between the coordinating rings, breaking
their conjugation. Bidentate chelating C^N ligands forming sixꢀmembered rings remain rare
and can be assigned into one of two categories, whether they contain conjugated5 or
nonconjugated6 chelating ligands. Recently, we reported7 a series of tripodal C^N^C ligands
based on 2ꢀbenzhydrylpyridine that can form three sixꢀmembered chelate rings through a
double CꢀH bond activation when coordinated to the iridium center. We also investigated the
impact of a conformationally flexible C^N ligand within a family of [Ir(bnpy)(N^N)]PF6
complexes; employing the nonconjugated sixꢀmembered chelating benzylpyridinato (bnpy) as
the C^N ligand.8 Depending on the nature of the N^N ligand, we observed phosphorescence
ranging from yellow to red and marked variations of the ratio of the conformers.
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The use of sixꢀmembered chelate ancillary ligands on cationic iridium(III) complexes
is more common though there are only a handful of reports for this category as well (Chart 1).
Examples include the use of a di(pyridinꢀ2ꢀyl)methane (dpm) that incorporates a methylene
spacer to interrupt the πꢀconjugation9 of the ligand such as [Ir(ppy)2(dpm)]PF6; unfortunately,
no photophysics was reported for this complex. Other studies have focused on the
functionalization of this methylene bridge. For instance, the complexes [Ir(ppy)2(dpyOHꢀ
R)]Cl [where R = H and CH2CN, giving dpyOHꢀR as di(pyridinꢀ2ꢀyl)methanol and 3ꢀ
hydroxyꢀ3,3ꢀdi(pyridineꢀ2ꢀyl)propanenitrile, respectively] have been investigated.10 The
effect of successfully interrupting the direct electronic crosstalk between the coordination
moieties was demonstrated by comparing the photophysical properties of
[Ir(ppy)2(dpyOH)]Cl (with λPL = 477, 507 and 547 nm, ΦPL = 10% in MeCN) and the
reference complex [Ir(ppy)2(bpy)]PF6 (λPL = 602 nm, ΦPL = 9% in MeCN);11 a large blueꢀshift
of 125 nm (4353 cmꢀ1) was indeed observed. The complex [Ir(ppy)2(dpyOHꢀCH2CN)]Cl
(with λPL = 535 nm, ΦPL = 49% in MeCN) is surprisingly a brighter emitter than the
structurally related complexes shown in Chart 1, exhibiting predominantly MLCT emission.
The two complexes [Ir(ppy)2(dpyꢀR)]Cl [where R = O and NꢀNH2, giving dpyꢀR as diꢀ2ꢀ
pyridylketone and 2,2’ꢀ(hydrazonomethylene)dipyridine, respectively] are very poor emitters
in acetonitrile, with ΦPL < 0.5%. The former exhibits an unstructured emission centered at 678
nm, whereas the latter displays a blueꢀshifted, structured emission profile (λPL = 480, 510
nm).10
4

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PF6
Cl
Cl
PF6
NH
N
Ir
N
N
Ir
N
N
Ir
N
N
Ir
N
N
N
N
N
R
N
N
N
N
R
OH
[Ir(ppy)2(dpm)]PF6
J. Organomet. Chem., 2016, 808, 122
[Ir(ppy)2(dpyOH-R)]Cl
[Ir(ppy)2(dpa)]PF6
Inorg. Chim. Acta, 2006, 359, 4144
[Ir(ppy)2(dpy-R)]Cl
41
Dalton Trans., 2012, , 1065
41
Dalton Trans., 2012, , 1065
R = H, CH2CN
R = O, N-NH2
NBu4
PF6
PF6
PF6
N
Ir
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Ir
N
Ir
N
Ir
S
O
n
N
N
N
N
N
N
N
[Ir(ppy)2(b-trz)]NBu4
56
[Ir(ppy)2(bpm)]PF6
Eur. J. Inorg. Chem., 2012, 3209 Inorg. Chem., 2013, , 10292
[Ir(ppy)2(dmdiim)]PF6
[Ir(ppy)2(N^N)]PF6
56
Inorg. Chem., 2017, , 10584
52
Inorg. Chem., 2017, , 15110
N^N = di(pyridin-2-yl)sulfane, n = 0
N^N = 2,2'-sulfinyldipyridine, n = 1
N^N = 2,2'-sulfonyldipyridine,
n = 2
Chart 1. Structural representation of Iridium(III) complexes bearing conjugated and
nonconjugated sixꢀmembered chelate ancillary ligands reported in the literature.
A more widely studied sixꢀmembered chelate N^N ligand is di(pyridinꢀ2ꢀyl)amine
12
(dpa).9,
With [Ir(ppy)2(dpa)]PF6 [λPL = 483, 514 (sh) nm, ΦPL = 43% in CH2Cl2] a
significant blueꢀshift and increase in ΦPL can be observed with respect to [Ir(ppy)2(bpy)]PF6,
which is a result of the presence of the electronꢀdonating central amine.12a Using sulfurꢀ
bridged sixꢀmembered chelate N^N ligands [di(pyridineꢀ2ꢀyl)sulfane and its oxidized
derivatives], the emission energy could be tuned as a function of the oxidation state of the
central sulfur atom.13 Blueꢀgreen ligandꢀcentered (3LC) emission was observed when the
sulfur was in the +2 ([Ir(ppy)2(di(pyridinꢀ2ꢀyl)sulfane)]PF6, with λPL = 478, 510, 548 (sh) nm,
ΦPL = 4% in CH2Cl2) or +4 oxidation states ([Ir(ppy)2(2,2'ꢀsulfinyldipyridine)]PF6, with λPL
=
478, 510, 548 (sh) nm, ΦPL = 1% in CH2Cl2). Through oxidation of the sulfur atom to the +6
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oxidation state ([Ir(ppy)2(2,2'ꢀsulfonyldipyridine)]PF6, with λPL = 552 nm, ΦPL = 3% in
CH2Cl2) a redꢀshift and green emission of 3MLCT character were observed.
Examples of nonconjugated sixꢀmembered chelate ancillary rings employing
coordinating heterocycles other than pyridine include those using bis(tetrazolate),14
bis(pyrazole)15 or a bisꢀNHC (see Chart 1).16 In each of these examples, skyꢀblue emission
was observed as a function of the nature of the more σꢀdonating heterocycle as well as the
presence of the methylene bridge, with photoluminescence quantum yields in MeCN of 75%,
21% and 38%, respectively. Recently, Chi and coꢀworkers reported a nonplanar tetradentate
N^N^N^N chelate bearing a pyrazole unit and a nonconjugated tripodallyꢀarranged
terpyridine, which can coordinate to iridium forming a sixꢀmembered ring.17 They obtained
efficient, skyꢀblueꢀemitting OLEDs by using this complex as the dopant emitter. In each of
these literature examples the methylene spacer disrupts the conjugation across the
coordinating moieties, enabling a blueꢀshifted emission. The strongly donating character of
the coordinating heterocycle also contributed to the blueꢀtoꢀskyꢀblue emission of these
complexes.
In an ongoing effort in the ZysmanꢀColman group to develop charged blueꢀemitting
iridium(III) complexes for solutionꢀprocessed OLEDs and LEECs, we investigated the
coordination of the nonconjugated diimine 2,2'ꢀ(phenylmethine)dipyridine (pmdp, L1, Chart
2) to iridium as an N^N ancillary ligand, in combination with either 2ꢀ(2,4ꢀdifluorophenyl)ꢀ4ꢀ
mesitylpyridinato (dFmesppy) or 4ꢀmesitylꢀ2ꢀphenylpyridine (mesppy) as C^N ligands,
resulting in the formation of the complexes [Ir(dFmesppy)2(pmdp)]PF6, 1, and
[Ir(mesppy)2(pmdp)]PF6, 2, respectively. The mesityl group was incorporated onto the C^N
ligands to increase the solubility of the resultant complexes in organic solvents without
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significantly affecting their emission energy, due to the nearlyꢀorthogonal conformation
between the mesityl substituent and the pyridine of the C^N ligands, thereby disrupting
conjugation;18 indeed, the unsubstituted ppy analog [Ir(ppy)2(pmdp)]Cl showed very poor
solubility in organic solvents (such as CH2Cl2, CHCl3, MeOH, MeCN, DMSO) and its
characterization in solution was not successful. The impact of the use of the pmdp ligand is
studied through comparison with two reference complexes R118a and R219 bearing the same
C^N ligands and 4,4'ꢀdiꢀtertꢀbutylꢀ2,2'ꢀbipyridine (dtBubpy) as the conjugated N^N ligand.
The photophysical properties of these complexes are corroborated by density functional
theory (DFT) and timeꢀdependent DFT (TDꢀDFT) investigations.
R
R
R
R
PF6
H
PF6
N
Ir
N
N
Ir
N
N
N
N
N
R
R
R
R
N
N
L1
R = F, 1
R = H, 2
R = F, R1
R2
R = H,
Chart 2. Structural representation of 2,2'ꢀ(phenylmethine)dipyridine (pmdp, L1), used as N^N
ancillary ligand in this study and complexes 1 and 2 and their reference complexes R118a and
R2,19ꢀ20 respectively.
Results and Discussion
The ancillary ligand pmdp, L1, was obtained in 40% yield as a beige solid following a
modified procedure21 wherein 2ꢀbenzylpyridine was treated with nꢀBuLi at ꢀ78 °C and
subsequently reacted with 2ꢀfluoropyridine under SNAr conditions. Complexes 1 and 2 were
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obtained as their hexafluorophosphate salts in a twoꢀstep synthesis following our previously
reported protocol.18a After column chromatography on silica (eluent: 0 – 8% MeOH in
CH2Cl2) followed by an ion exchange with aqueous NH4PF6 and recrystallization, complexes
1 and 2 were isolated as yellow solids in excellent yields (81% and 89%, respectively) as their
hexafluorophosphate salts. Solutionꢀstate NMR spectroscopy in CDCl3 revealed the
orientation of the phenyl ring on L1 to be in a pseudoꢀaxial configuration. The complexes
13
were characterized by 1H, C and 31P NMR spectroscopy and, for 1, 19F NMR spectroscopy;
ESIꢀHR mass spectrometry, elemental analysis, and melting point determination (see Figures
S1−S12 in the Electronic Supporting Information, ESI†, for NMR and ESIꢀHR mass spectra).
Crystal Structure
Single crystals of sufficient quality of 1 were grown from vapor diffusion of a CH2Cl2
solution of the complex with hexane acting as the antiꢀsolvent. The structure of 1 was
determined by singleꢀcrystal Xꢀray diffraction (see Figure 1, and Table S1 in the ESI†).
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Figure 1. Solidꢀstate structure of complex 1, thermal ellipsoids are drawn at the 50%
ꢀ
probability level. Hydrogen atoms, PF6 counterion and solvent molecules are omitted for
clarity.
Complex 1 shows a distorted octahedral coordination environment around the iridium
with the two N atoms of the C^N ligands in the typical trans configuration. The IrꢀCC^N bond
lengths of [2.005(4) and 2.006(4) Å] and the IrꢀNC^N bond lengths [2.050(4) and 2.057(4) Å]
are, as expected, in the same range as the average bond lengths in R1 [IrꢀCC^N = 2.000 Å and
IrꢀNC^N = 2.035 Å]. The IrꢀNN^N bonds [2.197(4) and 2.201(4) Å], are notably longer than
both those found in R1 (average IrꢀNN^N = 2.125 Å),18a and those found in a related complex,
[Ir(ppy)2(dpa)]PF6, where the ancillary ligand forms a nonconjugated sixꢀmembered chelate
ring (average IrꢀNN^N = 2.171 Å).
The bite angles of the C^N ligands in 1 are 80.25(17)° and 80.47(17)°, which are in
the same range as the corresponding bond angles in R1 (average CC^NꢀIrꢀNC^N = 80.8°]). The
bite angle of the ancillary ligand in 1 is 87.96(13)°, which is slightly increased compared to
that in [Ir(ppy)2(dpa)]PF6 [86.0(2)°].12a As expected, compared to the bite angle of the
ancillary ligand found in R1 [76.2(4)°], a significant enlargement can be observed. The angles
between the planes of mesityl ring and the pyridine of the C^N ligands in 1 are 71.4(2)° and
78.8(2)°, which are slightly smaller than analogous interꢀplanar angles in R1 [84.5° and
85.0°], while being larger than those found in the racemic form of R2 [57.3°], but falling
between the angles found in the enantiopure forms of R2 [74.9° and 89.4°].18a, 19
An interesting feature revealed by the crystal structure of 1 is the geometry of the N^N
ligand, which has the phenyl substituent in a pseudoꢀaxial configuration. Additionally, the
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pyridines of the ligand are affected by two conflicting preferences: that of the iridium centre
for an octahedral coordination geometry, and that of the methine carbon for a tetrahedral
geometry. This results in the pyridines adopting a splayed Vꢀshape with respect to the methine
[CpyꢀCmethineꢀCpy angle 117.3(4)°], the planes of the pyridine rings inclined at 39.2(2)° to each
other, and angled such that the iridium centre does not sit in the same plane as either of these
rings.
Electrochemical Properties
The electrochemical behavior of 1 and 2 was evaluated by cyclic voltammetry (CV)
and differential pulse voltammetry (DPV) in deaerated MeCN solution at 298 K, at a scan rate
of 100 mV s−1, using Fc/Fc+ as the internal reference and referenced with respect to SCE (0.38
V vs. SCE).22 The voltammograms are depicted in Figure 2 and the electrochemistry data are
found in Table 1.
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Figure 2. Cyclic voltammetry (in solid line) and differential pulse voltammetry (in dotted line)
carried out in degassed MeCN at a scan rate of 100 mV sꢀ1, referenced to SCE (Fc/Fc+ = 0.38
V vs. SCE).22
Table 1: Selected electrochemical properties of complex 1 and 2 and their reference
complexes R1 and R2.
Electrochemistrya
ꢀꢉꢊꢋꢊc / eV ꢀꢌꢍꢋꢊc / eV
ꢀꢀꢅ / mV ꢀꢀꢆꢇꢈꢃꢄb / V
ꢃꢄ
ꢆꢇꢈ
ꢁ/ꢂ
ꢀ
/ V
ꢀꢀꢅ / mV
ꢀ
/ V
ꢁ/ꢂ
1.79
1.36
80
n.d.d
ꢀ
n.d. d
ꢀ6.21
ꢀ5.78
ꢀ
1
2
78
ꢀ1.90e,
ꢀ2.15,
ꢀ2.42,
ꢀ,
120,
98
3.26
ꢀ2.52
R1f
R2g
1.59
1.17
ꢀ
ꢀ
ꢀ1.36
ꢀ
ꢀ
2.95
2.32
ꢀ6.01
ꢀ
ꢀ3.06
ꢀ
ꢀ1.15e
a
in degassed MeCN at a scan rate of 100 mV sꢀ1 with Fc/Fc+ as internal reference, and
referenced with respect to SCE (Fc/Fc+ = 0.38 V in MeCN);22 ꢁEredox is the difference
(V) between first oxidation and first reduction potentials; EHOMO/LUMO = ꢀ[Eox/red vs
c
Fc/Fc+ + 4.8] eV;23 d not detectable; e irreversible; f from ref. 18a; g in CH2Cl2 from ref 20.
Both complexes exhibit a quasiꢀreversible single electron oxidation, which can be
attributed to the Ir(III)/(IV) redox couple with contributions from the C^N ligands.24 Complex
1, bearing the dFmesppy C^N ligands, displays a notably more positive oxidation potential
(1.79 V) than 2 (1.36 V) due to the presence of the electronꢀwithdrawing fluorine atoms, a
trend that can be also seen in the comparison of R1 (1.59 V) and R2 (1.17 V in CH2Cl2). Both
1 and 2 are more difficult to oxidize compared to their respective reference complexes R1 and
R2, demonstrating that the lessꢀconjugated pmdp ligand influences less strongly the oxidation
potential of the complex than the more πꢀaccepting dtBubpy ligand used in the reference
complexes. Upon scanning to negative potential, surprisingly no reduction wave is observed
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for complex 1. Complex 2 exhibits three reduction waves, with the first being irreversible (ꢀ
1.90 V), while the second (ꢀ2.15 V) and third (ꢀ2.42 V) being quasiꢀreversible. Compared to
R2 (ꢀ1.15 V in CH2Cl2), the first reduction wave of 2 is significantly shifted to a more
negative potential (by 0.75 V), reflecting the disruption of the conjugation of the N^N ligand,
making the reduction more difficult. Based on a comparison with the electrochemistry
reported by Thompson et al.15b for the related complex [Ir(tpy)2(pz3CH)]CF3SO3 [where tpy is
2ꢀparaꢀtolylpyridinato and pz3CH is η2−tri(1
Hꢀpyrazolꢀ3ꢀyl)methane], the first two reduction
waves are the result of successive reductions of the pyridyl rings of the two C^N ligands
while the third reduction wave corresponds to the reduction of the ancillary ligand.
Photophysical Properties
To study the impact of the interruption of the electronic communication between the
pyridine rings within the ancillary ligand L1, we investigated the photophysical properties of
1 and 2. UVꢀvis absorption spectra for 1 and 2 are shown in Figure 3 with the data
summarized in Table S2 in the ESI†.
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Figure 3. UVꢀvis absorption and photoluminescence spectra of 1 and 2 in MeCN at 298 K.
Complexes 1 and 2 both exhibit similar absorption profiles to their respective
reference complexes R1 and R2. High intensity bands below 270 nm (ε on the order of 39 –
45 × 103 Mꢀ1cmꢀ1) are observed for 1 and 2 and are assigned as ligandꢀcentered π–π*
transitions, which is a typical feature for associated complexes of the form of
[Ir(C^N)2(N^N)]+.25 Moderately intense bands (ε on the order of 9 – 20 × 103 Mꢀ1cmꢀ1) in the
region of 310 – 345 nm are assigned to a combination of spinꢀallowed singlet metalꢀtoꢀligand
and ligandꢀtoꢀligand charger transfer (1MLCT/1LLCT) transitions, and appear as a shoulder.
These are blueꢀshifted by 31 nm (2879 cmꢀ1) for 1 compared to 2, due to the electronꢀ
withdrawing fluorine atoms present in the former. At lower energies both complexes exhibit
low intensity bands (ε on the order of 0.5 – 6 × 10ꢀ3 Mꢀ1cmꢀ1) in the region of 360ꢀ450 nm that
are attributed to a combination of spinꢀforbidden 3MLCT/3LLCT transitions. These
assignments are corroborated by theory (vide infra).
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The photoluminescence properties of 1 and 2 were investigated at 298 K in degassed
MeCN (Figure 3), as polymethyl methacrylate (PMMA) doped films (5 wt% of complex in
PMMA) and as spinꢀcoated neat films (Figure 4a). The spectra of 1 and 2 in a 2ꢀ
methyltetrahydrofuran (2ꢀMeTHF) glass at 77 K are depicted in Figure 4b. The photophysical
data of 1 and 2 and R1 and R2 are summarized in Table 2.
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Figure 4. a) Photoluminescence spectra of 1 and 2 recorded in PMMA doped films (5 wt% of
complex in PMMA) in solid lines and in neat film (spinꢀcoated from 2ꢀmethoxyethanol) in
dotted lines (λexc = 360 nm); b) Photoluminescence spectra of complexes 1 and 2 recorded in
2ꢀMeTHF glass at 77 K (λexc = 360 nm).
Table 2. Photophysical properties of 1 and 2 and their reference complexes R1 and R2.
MeCNa
PMMA Filmb
Neat Filmc
Glassd
e
f
g
e
h
g
e
h
g
e
g
λPL ΦPL
τPL
/ ns
λPL ΦPL
τPL
λPL ΦPL
τPL
λPL
τPL
/ ns
/ nm / %
/ nm / %
/ ns
/ nm / %
/ ns
/ nm
455,
325 (5%) 464,
46 1260 (35%) 490,
3260 (60%) 527
75 (12%)
460,
30
186 (15%) 459,
445 (85%) 488
20 (1%)
2920 (99%)
21 263 (46%) 487,
1
488
755 (42%)
516
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473,
162 (4%) 485,
26 770 (31%) 514,
2000 (65%) 581
71 (33%)
481,
509
95 (16%) 478,
206 (84%) 512
200 (1%)
2800 (99%)
11
9
210 (63%) 508,
2
1195 (4%)
539 ,
474,
1370
390 (68%)
1230 (34%)
R1i
R2j
515 80
577 40
97
ꢀ
16300 508
54
ꢀ
ꢀ
ꢀ
502
478,
ꢀ 516,
550
25 (6%)
18 211 (42%)
672 (52%)
757
ꢀ
ꢀ
a In deaerated MeCN at 298 K; b at 298 K, spinꢀcoated from a 2ꢀmethoxyethanol solution of 5 wt% of
c
the complex in PMMA on a pristine quartz substrate;
at 298 K, spinꢀcoated from a 2ꢀ
d
e
f
methoxyethanol solution; in 2ꢀMeTHF at 77 K; λexc = 360 nm; Quinine sulfate used as the
reference (ΦPL = 54.6% in 0.5 M H2SO4 at 298 K);26 g λexc = 378 nm; h Measured using an integrating
sphere; i from ref. 18a; j in CH2Cl2 from ref 19ꢀ20
.
Upon photoexcitation at 360 nm in MeCN, 1 and 2 show structured emission profiles,
indicative of an emission with a significant ligandꢀcentered character (see below for spin
densities), with maxima at 460 and 480 nm for 1, and 481 and 509 nm for 2, the former being
more intense in both cases. The emission maxima of 1 is blueꢀshifted by 55 nm (2322 cmꢀ1)
compared to that of R1 (λPL = 515 nm),18a which itself presents an unstructured mixed chargeꢀ
transfer emission profile. The same trend is observed when comparing the emission of 2 to R2
(λPL = 577 nm in CH2Cl2).19ꢀ20 Comparison of the photophysical properties of 2 with the
archetype complex [Ir(ppy)2(bpy)]PF6 (λPL = 602 nm, ΦPL = 9%) reveals an even more
pronounced blueꢀshift (ꢀ = 121 nm, 4179 cmꢀ1).11 A comparison with the structurally related
complex [Ir(ppy)2(dpyOHꢀH)]Cl (with λPL = 477, 507 nm, ΦPL = 10% in MeCN) reveals an
essentially similar photophysical profile.10 A final comparison of the MeCN emission profiles
of 1 with [Ir(dFppy)2(oꢀxylbiim)]PF6 (where oꢀxylbiim = 1,1ʹꢀ(α,αʹꢀoꢀxylylene)ꢀ2,2ʹꢀ
biimidazole) reveals that both complexes show a similar LC emission with the maximum of
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the latter at 459 nm, but with a ΦPL of 90%. Therefore, the magnitude of the effect of breaking
the conjugation in the N^N ligand is similar to the use of one of the most electronꢀdonating
ancillary ligands.18a
The photoluminescence quantum yield (ΦPL,MeCN) of 1 is 30%, which is notably higher
than that of 2 (ΦPL,MeCN = 11%), a trend also observed in R1 and R2. Compared to their
reference complexes R1 (ΦPL = 80%) and R2 (ΦPL = 40%), 1 and 2 show much lower ΦPL
values, which can be rationalized by the flexibility of the N^N ancillary ligand leading to an
increased nonꢀradiative decay pathway. Both complexes exhibit biꢀexponential emission
lifetimes, τPL, in the subꢀmicrosecond regime.
The emission energies and profiles of 1 and 2 in 5 wt% PMMAꢀdoped films are not
significantly changed compared to those in MeCN. The photoluminescence quantum yields of
the PMMAꢀfilms are increased (ΦPL,PMMA = 46 and 26% for 1 and 2, respectively) compared
to the solutionꢀstate measurements. This behavior is also observed in R1 and is mainly
attributed to a reduction in knr due to the expected reduction of the conformational motions of
both the mesityl groups, and in the case of 1, the N^N ligand. Both complexes exhibit a threeꢀ
component emission decay in the subꢀmicrosecond regime in doped films, with the longest
component significantly longer than the corresponding long component of τPL in MeCN. In
spinꢀcoated neat films the structured emission profiles are likewise not significantly changed
compared from those in MeCN; however, they exhibit a more pronounced shoulder at 527 and
581 nm, respectively. Compared to the neat films, the PMMAꢀdoped films show a slight blueꢀ
shift [λPL,PMMA = 459 (sh), 488 nm and λPL,PMMA = 478, 512 (sh) nm for 1 and 2, respectively].
The photoluminescence quantum yields of the neat films (ΦPL, Neat) are lower (ΦPL,Neat = 21
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and 9% for 1 and 2, respectively) than those seen in the solutionꢀstate measurements. The
decrease in ΦPL in neat films was also observed in R1 and R2 and can be explained by πꢀ
stacking intermolecular interactions between mesityl rings on adjacent complexes, providing
an avenue for aggregationꢀcaused quenching. Both complexes exhibit a threeꢀcomponent
emission decay in the subꢀmicrosecond regime in neat films, significantly shorter than in
doped PMMA films. There is no significant shifting in the emission energy at low
temperature compared to measurements at 298 K, reflecting that the emission remains ligandꢀ
centered under both sets of conditions. The emission decay profiles for both 1 and 2 are
biexponential in the glass, with an expected much longer emission lifetime compared to those
in MeCN.
Theoretical calculations
To gain more insights into the nature of the excitedꢀstates in both 1 and 2, we have
performed DFT and TDꢀDFT calculations in acetonitrile (see the ESI† for details). First, for 1
the DFT optimized geometry present IrꢀCC^N, IrꢀNC^N and IrꢀNN^N bond lengths of 2.003 and
2.003 Å, 2.058 and 2.065 Å, and 2.223 and 2.242 Å, respectively. These values are close to
the ones obtained in the crystal (vide supra) with a mean absolute deviation of 0.018 Å. The
bite angles are 80.2° and 80.5° for the C^N ligands, and 86.9° for the ancillary ligand, are
likewise close to their experimental counterparts. This indicates that the selected theoretical
protocol is physically sound for the considered complexes. The DFT calculations indicate that
when going from 1 to 2, the energy of the HOMO increases by 0.34 eV, which is rather
consistent with the electrochemical value (0.43 eV, see Table 1), whereas the energy of the
LUMO is shifted to higher energy by 0.11 eV, resulting in a HOMOꢀLUMO gap that is
smaller by 0.23 eV in the fluorineꢀfree complex. As can be seen in Figure 5, the HOMO is
mainly centered on the metal and the cyclometalating aryl rings of the C^N ligands, whereas
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the LUMO is principally localized on the pyridyl rings of the C^N ligand that is the furthest
away from the ancillary phenyl ring. This holds for both compounds, so that the observed
electrochemical differences are mainly due to the inductive effects of the fluorine atoms and
not to a change in shape of the frontier orbitals. These MO topologies are also consistent with
the fact that the energy of the HOMO significantly varies from 1 to 2, whereas the LUMO
energy is less affected.
Figure 5. Representation of the six frontier orbitals of 1 (top) and 2 (bottom). A contour
threshold of 0.03 au is used, and the hydrogen atoms have been omitted for clarity.
TDꢀDFT calculations return several lowꢀlying triplet states, the lowest being located at
442 nm in 1 and 463 nm in 2. The lowest dipoleꢀallowed singlet excited states are computed
at 373 nm (f=0.054) in 1 and 399 nm (f=0.063) in 2, corresponding to a blueꢀshift of 26 nm
between the two complexes, in line of the experimental value (31 nm, vide supra) though the
computed wavelengths are slightly larger than their experimental counterparts. These singlet
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transitions are mainly ascribable to a HOMOꢀLUMO electronic promotion, and therefore
1
present a mixed MLCT/1LLCT character, L being the C^N ligand(s).; the N^N ligand not
being involved in this transition. In 1, the next singlet transitions presenting significant
oscillator strengths are located at 340 nm (f=0.012), 337 nm (f=0.025) and 329 nm (f=0.202).
These three absorptions mainly correspond to HOMOꢀ1 to LUMO+1, HOMOꢀ2 to LUMO
and HOMOꢀ2 to LUMO+1 electronic transitions, indicating that the first and the third present
a significant CT character towards the N^N ligand.
In 1, the DFT computed 0ꢀ0 phosphorescence wavelength is 476 nm, a value that takes
into account the zeroꢀpoint vibrational effects. In 2, the computed value is 501 nm, at lower
energy in agreement with experimental data. In both 1 and 2, the spin density plot of the
lowest triplet excitedꢀstate shows contributions from the metal and C^N ligand residing the
closest to the phenyl ring of the ancillary ligand, confirming the mixed nature of the emitting
state (Figure 6). We would therefore make the hypothesis that the interaction with the phenyl
ring of the ancillary ligand tends to stabilize the spin density on the closest C^N ligand.
Indeed, for 2, we have been able to locate a triplet state presenting a more uniform
delocalization of its density on both C^N ligands, but it is higher in energy than the one
represented in Figure 6.
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Figure 6. Representation of the spin density difference plot for 1 (left) and 2 (right). Contour
threshold: 0.0008 au.
Conclusions
Two new cationic blue and blueꢀgreenꢀemitting iridium complexes of the form
[Ir(C^N)2(N^N)]PF6 bearing mesitylated C^N ligands and a sixꢀmembered chelate methine
bridged N^N are reported. The ancillary ligand L1 (pmdp = 2,2'ꢀ(phenylmethine)dipyridine)
consists of two pyridyl rings, whose electronic crosstalk is disrupted by a phenyl substituted
methylene group. For both complexes we performed the synthesis, characterization and
optoelectronic properties. The crystal structure of 1 reveals that the phenyl substituent on the
ancillary ligand adopts a pseudoꢀaxial configuration. We have shown that by using such an
ancillary ligand a significant blueꢀshift in the emission is observed for 1 and 2 compared to
their reference complexes R1 and R2. The photoluminescence quantum yields were lower
compared to the reference complexes as a consequence of the increased fluxional motion of
the ancillary ligand. Photoluminescence studies were also performed in neat films and the
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PMMAꢀdoped films showing similar structured emission profiles. We have demonstrated that
employing the nonconjugated N^N ligand 2,2'ꢀ(phenylmethine)dipyridine, L1 is a successful
strategy to blueꢀshift the emission of Ir(III) complexes.
Acknowledgment
We thank Umicore AG for the gift of materials. We thank the EPSRC UK National
Mass Spectrometry Facility at Swansea University for analytical services. C.H. acknowledges
the Région Bretagne, France for funding. D.J. acknowledges the European Research Council
and the Région des Pays de la Loire for financial support in the framework of a Starting Grant
(Marches ꢀ 278845) and the LUMOMAT RFI project, respectively. This research used
resources of 1) the GENCIꢀCINES/IDRIS, 2) the CCIPL (Centre de Calcul Intensif des Pays
de Loire), 3) a local Troy cluster. E.Z.ꢀC. acknowledges the University of St Andrews and
EPSRC (EP/M02105X/1) for financial support.
Supporting information. NMR and mass spectra for L1 and 1-2, Supplementary
crystallographic, electrochemical and photophysical data, description of the computational
protocol and results. CIF file for the Xꢀray structure of 1 (CCDC: 1821381).
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TOC
Synopsis
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Blue and blueꢀgreenꢀ emitting cationic Ir(III) complexes bearing mesitylated C^N ligands and
a nonconjugated N^N ligand have been studied, and the properties of these complexes
contrasted to reference complexes bearing a conjugated N^N ligand to elucidate the impact of
both the chelate ring size and the breaking of conjugation in the ancillary ligand on the
optoelectronic properties of the complex.
25