Impact of the use of sterically congested Ir(iii) complexes on the performance of light-emitting electrochemical cells

Article abstract of DOI:10.1039/c8tc01130c

The synthesis, structural and optoelectronic characterization of a family of sterically congested cyclometalated cationic Ir(iii) complexes of the form [Ir(C^N)₂(dtBubpy)]PF₆ (with dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine and C^N = a cyclometalating ligand decorated at the 4-position of the pyridine ring and/or the 3-position of the phenyl ring with a range of sterically bulky substituents) are reported. This family of complexes is compared to the unsubstituted analogue complex R1 bearing 2-phenylpyridinato as cyclometalating ligand. The impact of sterically bulky substituents on the C^N ligands on both the solid state photophysics and light-emitting electrochemical cell (LEEC) device performance is investigated. X-ray diffraction analysis of complexes 1a, R2, 2a, and 1b show an increasing internuclear distance in the solid state, within these four complexes. Emission studies in solution and neat film show that the chosen substituents essentially do not impact the emission energy. The photoluminescence quantum yields (Φ_PL) are in the same range (Φ_PL ～ 25-31%), except for 1b, which shows a lower Φ_PL of 12%. All complexes exhibit similar monoexponential emission lifetimes in the submicrosecond regime. LEECs based on R1, 1a, 1b and R2 were fabricated, showing yellow luminescence and moderate efficiencies and lifetimes. The arguably best performing LEEC device, showing the highest luminance (737 cd m^-2), current efficiency (7.4 cd A^-1) and EQE (2.6%), employed emitter 1a.

Full text of DOI:10.1039/c8tc01130c

View Article Online
View Journal
Journal of
Materials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. Hierlinger, E.
Trzop, L. Toupet, J. Ávila, H. J. Bolink, V. GUERCHAIS and E. Zysman-Colman, J. Mater. Chem. C, 2018,
DOI: 10.1039/C8TC01130C.
This is an Accepted Manuscript, which has been through the
Volume
4 Number 1 7 January 2016 Pages 1–224
Royal Society of Chemistry peer review process and has been
accepted for publication.
Journal of
Materials Chemistry C
Accepted Manuscripts are published online shortly after
Materials for optical, magnetic and electronic devices
www.rsc.org/MaterialsC
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

Page 1 of 35
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC01130C
Impact of the Use of Sterically Congested
Cyclometalated Ligands on the Optoelectronic
Properties and Device Performances In Light-
Emitting Electrochemical Cells of Cationic
Iridium(III) Complexes
Claus Hierlinger,a,b Elzbieta Trzop,c Loïc Toupet,c Jorge Ávila,d Henk J. Bolink,d Véronique
Guerchais*a Eli Zysman-Colman*b
a Univ Rennes, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) – UMR 6226, F-
35000 Rennes, France. *E-mail: veronique.guerchais@univ-rennes1.fr
b Organic Semiconductor Centre, EaStCHEM School of Chemistry, University of St
Andrews, St Andrews, Fife, KY16 9ST, UK. *E-mail: eli.zysman-colman@st-andrews.ac.uk;
Web: http://www.zysman-colman.com
c
Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, F-35000 Rennes,
France
c Instituto de Ciencia Molecular, Universidad de Valencia, C/J. Beltran 2, 46980 Paterna,
Spain
Abstract
The synthesis, structural and optoelectronic characterization of a family of sterically congested
cyclometalated cationic Ir(III) complexes of the form [Ir(C^N)2(dtBubpy)]PF6 (with dtBubpy
1

Journal of Materials Chemistry C
Page 2 of 35
View Article Online
= 4,4ʹ-di-tert-butyl-2,2ʹ-bipyridine and C^N = a cyclometalating ligand decoratDeOdI: 1a0t.10th39e/C48T-C01130C
position of the pyridine ring and/or the 3-position of the phenyl ring with a range of sterically
bulky substituents) are reported. This family of complexes is compared to the unsubstituted
analogue complex R1 bearing 2-phenylpyridinato as cyclometalating ligand. The impact of the
use the sterics on the C^N ligands on both the solid state photophysics and light-emitting
electrochemical cell (LEEC) device performance is investigated. X-ray diffraction analysis of
complexes 1a, R2, 2a, and 1b show an increasing internuclear distance in the solid state, within
these four complexes. Emission studies in solution and neat film show that the chosen
substituents essentially do not impact the emission energy. The photoluminescence quantum
yields (ΦPL) are in the same range (ΦPL ~ 25 – 31%), except for 1b, which shows a lower ΦPL
of 12%. All complexes exhibit similar monoexponential emission lifetimes in the
submicrosecond regime. LEECs based on R1, 1a, 1b and R2 were fabricated, showing yellow
luminescence and moderate efficiencies and lifetimes. The arguably best performing LEEC
device, showing the highest luminance (737 cd m-2), current efficiency (7.4 cd A-1) and EQE
(2.6%), employed emitter 1a.
Introduction
Iridium(III) complexes have a remarkable combination of photophysical properties,1
including high photoluminescence quantum yields, PL, wide colour tenability, relatively short-
lived phosphorescence lifetimes, PL, that make them the go-to emitter class in solid-state
electroluminescence (EL) devices, such as organic light emitting diodes (OLEDs)2 and in light
emitting electrochemical cells (LEECs).3 LEECs possess a much simpler design, with fewer
layers, compared to OLEDs,4 as they typically employ an ionic emissive material that is dually
responsible for charge mobility and the emission of light within the device. The most popular
2

Page 3 of 35
Journal of Materials Chemistry C
View Article Online
and widely studied class of emitters for LEECs is the heteroleptic cationic Ir(III)DcOoI:m10.p10l3e9x/Co8TfC01130C
the form [Ir(C^N)2(N^N)]PF6 (C^N is a cyclometalating ligand, N^N is a diimine ancillary
ligand).
Unlike in OLEDs where the emissive compound is present in only small concentrations
as a dopant within a higher bandgap host, in LEECs the emissive layer is frequently composed
either of a homogenous layer of emitter molecules or the emitter is the major component within
the layer. As a consequence, one issue that can limit device performance is excited state self-
quenching during device operation.5 A strategy to circumvent this issue is to decorate the
complexes with bulky, hydrophobic substituents that serve to increase the intermolecular
distance while simultaneously hindering the disadvantageous attack of small molecules.6 Chart
1 shows representative literature examples of iridium complexes (R1-R9) bearing bulky
substituents that have been used as emitters in light-emitting electrochemical cells.
3

Journal of Materials Chemistry C
Page 4 of 35
View Article Online
DOI: 10.1039/C8TC01130C
Chart 1. Representative literature examples of iridium complexes bearing bulky substituents.
A successful molecular design that was applied to charge-neutral Ir(III) complexes for
OLEDs was the incorporation of mesityl substituents at the C(4) position of the pyridyl ring of
the ppy ligands in [Ir(dFmesppy)2(pic)] (dFmesppyH = 2-(4,6-Difluorophenyl)-4-(mesityl)-
pyridine and pic = picolinate). An increase in the photoluminescence quantum yield, PL, was
observed, which also translated into enhanced external quantum efficiencies of the OLED
devices compared to that employing the well-known [Ir(dFppy)2(pic)], FIrpic (dFppyH = 2-
(4,6-difluorophenyl)pyridine).7 The enhancement in EQE compared to the FIrpic-based devices
was attributed to reduced concentration quenching and increased solubility in organic solvents
and therefore better dispersion during the fabrication of the solution-processed device. Notably,
the mesityl group adopts an orthogonal conformation relative to the pyridine ring, resulting in
4

Page 5 of 35
Journal of Materials Chemistry C
View Article Online
a disruption of the conjugation between these two aryl groups, minimizing the impact on the
DOI: 10.1039/C8TC01130C
emission energy. Such an approach has been applied to cationic iridium complexes by us, where
we have observed an enhancement of solution- and solid-state PL compared to the non-
decorated analog complex.8
A comparison of R1 and R2 of the form [Ir(C^N)2(dtBubpy)]PF6 (where C^N is ppy or
mesppy for R1 and R2, respectively and dtBubpy is 4,4ʹ-di-tert-butyl-2,2ʹ-bipyridine) reveals
the impact of the mesityl group on the photophysical properties in cationic complexes.8a In
CH2Cl2, the same emission maximum (PL = 577 nm) is observed for both complexes while the

PL is moderately enhanced for the mesitylated complex (PL = 35 and 40% for R1 and R2,
respectively). The LEECs fabricated with R1,8a showed a lifetime of t1/2 = >1300 h and an EQE
of 2.5%. Surprisingly and in contrast to the above-mentioned impact of the mesityl group, for
LEECs based on R2 there was a reduction in the device lifetime (t1/2 = 0.6 h) and EQE (1.4%)
upon incorporation of the mesityl groups on the C^N ligands. However, with R2 faster response
times could be achieved in devices compared to those based on R1, as the mesityl groups induce
a more efficient electronic communication and recombination within the device, which results
in faster turn-on times. The related complex [Ir(dPhPy)2(bpy)]PF6, R3 (where dPhPy is 2,4-
diphenylpyridine and bpy is 2,2ʹ-bipyridine) bears a phenyl group on the 4-position of the
pyridine of the C^N ligand in lieu of a mesityl group and contains a bpy N^N ligand in lieu of
the dtBubpy ligand.9 In CH2Cl2, complex R3 exhibits yellow phosphorescence (em = 598 nm)
with a PL of 21% . A single-layer LEEC based on R3 containing a lithium salt additive showed
improved device performance compared to the unsubstituted analogue. The device with R3
displayed a short response time (ton = 5 min), favorable lifetime (extrapolated lifetime calculated
at 100 cd m-2, t1/2L100 of 3800 h), and a peak luminance of 5500 cd m-2 (with ton = 191 min,
t1/2L100 = 4752 h and a peak luminance of 2753 cd m-2 for the unsubstituted analog). The authors
5

Journal of Materials Chemistry C
Page 6 of 35
View Article Online
ascribed the improved LEEC performance to the bulky, hydrophobic nature of the phenyl
DOI: 10.1039/C8TC01130C
substituent, which they asserted impeded self-quenching pathways of the Ir(III) complex.
Another analogous complex is [Ir(Phppy)2(bpy)]PF6, R4 (where Phppy is 2-([1,1'-biphenyl]-3-
yl)pyridine), bearing a phenyl group trans to the Ir-CC^N bond. Complex R4 exhibits a yellow-
orange emission with a PL of 13% in CH2Cl2. LEECs based on R4 are likewise highly stable
with t1/2 = 2800 h and also are very bright with a maximum luminance of 1024 cd m-2.10
The use of the sterically congested the dtBubpy in R611 coupled with dFppy ligands
contributed to a record maximum external quantum efficiency in the LEEC of 15%,11a
significantly higher than that reported for the LEECs with R1, with EQE values ranging from
0.6 - 5.0%.8a, 12 LEECs fabricated with R512b incorporating a bulky ancillary ligand (5,5ʹ-
diphenyl-2,2ʹ-bipyridine, dpbpy) showed long device lifetimes of 110 h (LEEC operating at 4
V), particularly compared to [Ir(ppy)2(dtBubpy)]PF6, R1 (1.3 h). Using R7,5 which contained
the 4,5-diaza-9,9ʹ-spirobifluorene ancillary ligand, resulted in LEECs with a maximum external
quantum efficiencies of 7.1% at a device lifetimes of 12 h and luminance value of 52 cd m-2.
Very stable LEECs have been fabricated employing emissive complexes where there is a
phenyl ring positioned adjacent to one of the coordinating nitrogen atoms of the ancillary ligand
as exemplified in [Ir(dmppz)2(pbpy)]PF6, R8, (where dmppz is 3,5-dimethyl-1-phenylpyrazole
and pbpy is 6-phenyl-2,2ʹ-bipyridine).13 The phenyl ring of the pbpy forms a face-to-face π-
stacking with the pyrazole unit of the C^N ligand, which insulates the complex from
adventitious attack from small molecules.14 High stability LEECs were fabricated with the
complex [Ir(ppy)2(Meppbpy)]PF6, R9, (where Meppbpy is 4-(3,5-dimethoxyphenyl)-6-phenyl-
2,2’-bipyridine) with a device lifetime of over 950 h and high luminance and current efficiency
6

Page 7 of 35
Journal of Materials Chemistry C
-2
-1
15
View Article Online
DOI: 10.1039/C8TC01130C
(183 cd m and 8.2 cd A , respectively). In the majority of cases, decorating the complex
with bulky, hydrophobic substituents greatly improves the stability of LEECs.
These previous findings prompted us to design a family of cationic Ir(III) complexes of the
form [Ir(C^N)2(dtBubpy)]PF6 in which the sterics about the C^N ligands are systematically
modified via a combination of decoration at the 4-position of the pyridine ring and/or the 3-
position of the phenyl ring in order to ascertain how the steric requirements affect the
photophysics of the complexes and the corresponding LEEC device performance (Figure 1).
Complexes R1 and R2 were included into this study to serve as references.
7

Journal of Materials Chemistry C
Page 8 of 35
View Article Online
DOI: 10.1039/C8TC01130C
Figure 1. Proligands and Ir(III) complexes under investigation in this study.
Results and Discussion
Synthesis of the C^N ligands
The family of substituted 2-phenylpyridine (ppyH) cyclometalating C^N ligands bearing R1
(at the 4-pyridine position) and R2 (at the 4-phenyl position) can be divided into two classes: 4-
8

Page 9 of 35
Journal of Materials Chemistry C
View Article Online
substituted 2-phenylpyridine derivatives L1a, LR1, L2a and L3a [with R1 = tert-butyl (tBu),
DOI: 10.1039/C8TC01130C
mesityl (mes), manisyl (man) – 4-methoxy-2,6-dimethylphenyl, and 3,5-di-tert-butylphenyl
(dtBuphen), respectively, and R2 = H], and the substituted arylpyridine L1b (where R1 = R2 =
tert-butyl). The ligand LR1 is the unsubstituted ppyH (Figure 1).
Compounds L1a and L1b are accessible as colourless oils from 4-tert-butylpyridine via a
Minisci reaction16 in moderate yields (45 and 46%, respectively), while LR1 was commercially
available. For LR2, L2a and L3a, a two-step Suzuki-Miyaura cross-coupling strategy was
adapted from 2-chloro-4-iodopyridine (Scheme 1).8c The mono-arylated 2-chloro-4-substituted
compounds ClR2, Cl2a and Cl3a were isolated in excellent yields (94-96%) after purification
by column chromatography. Key to the high yields is the use of excess R1B(OH)2, which led to
easier purification of the intermediate 4-aryl-2-chloropyridines. A second Suzuki-Miyaura
reaction with PhB(OH)2 afforded LR1, L2a and L3a in excellent yields (86-90%) (Scheme 1).
Scheme 1. Synthesis scheme for compounds LR1, L2a and L3a (with R1 = mes, man and
a
b
dtBphen, respectively). R1B(OH)2, K2CO3, Pd(PPh3)4, dioxane/H2O (4/1), 95 °C, 3 d;
PhB(OH)2, K2CO3, Pd(PPh3)4, dioxane/H2O (4/1), 95 °C, 18 h.
Synthesis of the Ir(III) complexes
9

Journal of Materials Chemistry C
Page 10 of 35
View Article Online
The target complexes were synthesised via a two-step literature procedure (Scheme 2). The
DOI: 10.1039/C8TC01130C
.
C^N ligands were each first reacted with IrCl3 6H2O to afford the corresponding [Ir(C^N)2Cl]2
dimers in excellent yields (79-89%).17 These intermediate dimers were then reacted with
dtBubpy in the presence of AgPF6 to afford the desired heteroleptic complexes as yellow solids
in generally excellent yields (79-92%) as their hexafluorophosphate salts.18 Through the
systematic increase of steric bulk of the cyclometalated ligands, the corresponding
[Ir(C^N)2Cl]2 dimers as well as the cationic complexes show increased solubility in organic
solvents such as CH2Cl2, CHCl3 and MeCN compared to [Ir(ppy)2Cl]2 and the reference
complex R1. Recrystallization from pentane/diethyl ether gave pure R1, whereas for the other
complexes (1a, 1b, R2, 2a and 3a) column chromatography on silica (eluent: 5% MeOH in
CH2Cl2) was required. Following chromatographic purification, a second anion exchange
-
reaction with NH4PF6 was conducted to ensure that the sole counterion was PF6 .
Scheme 2. Synthesis of target complexes. a IrCl3.6H2O, 2-ethoxtyethanol/H2O (3/1), 125 °C,
24 h. b 1) 4,4’-di-tert-butyl-2,2’-bipyridine, AgPF6, 1,2-dichloroethane, 85 °C, 2h. 2) r.t., 12h.
10

Page 11 of 35
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC01130C
X-ray structural studies
Single crystals of sufficient quality of 1a, 1b, R2 and 2a were grown from CH2Cl2/Et2O.
̅
The determined space groups are P1 (for 1a) and P21 (for 1b, R2 and 2a). The crystal structure
of R2 has been previously reported; however, in the previous report the crystallization
conditions were different.8a The crystal structure of 1a, 1b, R2 and 2a are presented in Figure
2.
Figure 2. Representation of the crystal structure of 1a, 1b, R2 and 2a. The hydrogen atoms,
-
solvent molecules, minor disorder, as well as the counter ion PF6 are omitted for clarity.
All complexes show the expected distorted octahedral geometry around the Ir centre
with the pyridyl units of the C^N ligands mutually trans to each other, as is commonly seen in
other cationic Ir(III) complexes of the form [Ir(C^N)2(N^N)]PF6.1c, 19 The bond lengths and
bond angles are as expected for this class of iridium complex. The torsion angles based on the
angle between the planes of the R1 aryl and the pyridine rings of the two C^N ligands in R2 are
78.32° and 88.70° and in 2a are 81.58° and 89.97°, which are similar to those previously
reported for arylated C^N ligands in charged complexes,8b-d but are notably larger to those
11

Journal of Materials Chemistry C
Page 12 of 35
View Article Online
found in R3 for which an average torsion angle of 26° between the phenyl and DthOeI: 1p0.y10r3i9d/Cin8TeC01130C
rings of the C^N ligands is observed.20 The steric hindrance provided by the substituents on
the C^N ligands modulates the inter-nuclear distance in the solid state. The shortest inter-
iridium distance between adjacent complexes is approximately 7.24 Å in 1a and increases
significantly in R2 and 2a (~10.55 and 10.97 Å, respectively). A similar Ir…Ir distance is
observed when both R1 and R2 are tert-butyl groups in 1b (~10.86 Å). In R1 the distance 7.897
Å is notably higher with respect to 1a. The inter-iridium distance for R3 is with 8.87 Å,20 which
is significantly shorter compare to the distance in R2 and 2a.
Cyclic Voltammetry
The electrochemical behaviour of each of the complexes was investigated by cyclic
voltammetry (CV) in deaerated MeCN solution at 298 K with n-Bu4NPF6 as the supporting
electrolyte at a scan rate of 100 mV s-1 and using ferrocene/ferrocenuim (Fc/Fc+) as the internal
reference. All potentials are referenced with respect to SCE (Fc/Fc+ = 0.38 V in MeCN).21 The
cyclic voltammograms are shown in Figure 3 and the electrochemistry data are given in Table
1.
12

Page 13 of 35
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC01130C
Figure 3. Cyclic voltammograms of 1a–5a measured in a deaerated MeCN with 0.1 M
nBu4NPF6 at a scan rate of 100 mV s-1 in the positive scan direction and Fc/Fc+ as the internal
standard, and are referenced with respect to SCE (Fc/Fc+ = 0.38 V in MeCN).21
Table 1. Electrochemical data and orbital energies for the investigated complexesa
풐풙
ퟏ/ퟐ
푬풓ퟏ풆/풅ퟐ / V
푬푯푶푴푶b / eV 푬푳푼푴푶b / eV 휟푬풓풆풅풐풙 / V
푬
/ V
휟푬풑 / mV
휟푬풑 / mV
R1
1a
1b
R2
2a
3a
1.13
1.13
1.15
1.35
1.29
1.21
90
180
70
-1.61
-1.54
-1.52
-1.37
-1.43
-1.49
70
100
90
-5.93
-5.93
-5.95
-6.16
-6.10
-6.01
-3.20
-3.26
-3.29
-3.43
-3.38
-3.37
2.74
2.67
2.67
2.72
2.72
2.70
90
80
90
70
100
120
13

Journal of Materials Chemistry C
Page 14 of 35
a Measurements were carried out in degassed MeCN at a scan rate of 100 mV s-1 with
View Article Online
DOI: 10.1039/C8TC01130C
Fc/Fc+ used as the internal reference and referenced with respect to SCE (Fc/Fc+ = 0.38 V in
b
MeCN).21 EHOMO/LUMO = -[Eox/red vs. Fc/Fc+ + 4.8] eV.22
Each complex displays a quasi-reversible oxidation wave attributed to an admixture of the
Ir(III)/Ir(IV) redox couple and contributions from the aryl ring of C^N ligands.10 The addition
of tert-butyl groups in 1a and 1b renders the oxidation somewhat less reversible compared to
the reference R1 though the oxidation potentials remain essentially unchanged, despite the
inductively electron-donating character of this substituent. Upon addition of aryl groups to the
4-pyridyl position, the oxidation waves are anodically shifted, a reflection of the moderately
strong electron-withdrawing character of these groups. This anodic shifting is somewhat
mitigated by the conjugation present between the 3,5-di-tert-butylphenyl substituent and the
coordinating pyridine in 3a, while in R2 and 2a the aryl groups are oriented perpendicular as a
function of the o-methyl substituents on the arene; the magnitude of the inductively electron-
withdrawing character of the aryl group is further modulated by the presence of the more
strongly electron-donating methoxy substituent in 2a (Eox = 1.29 V) compared to the methyl
substituent in R2 (Eox = 1.35 V). In CH2Cl2, it has been previously shown that R2 has an
oxidation wave at 1.14 V.8d Thus, the solvent has a unusually large influence on the oxidation
potential of this complex. All complexes display a single quasi-reversible reduction wave
within the electrochemical window of MeCN. Despite the consistent assignment that the
reduction is localized on the ancillary dtBubpy ligand, the reduction potential varies over a
moderately large range from -1.37 V for R2 to -1.61 V for R1. In CH2Cl2 R2 exhibits an
irreversible reduction at -1.15 V.18 The most inductively electron-withdrawing substituents on
the C^N ligands induce the greatest shift to less negative potentials in R2 and 2a, this via
modulation of the electron density on the iridium centre. Analogous to that observed for the
14

Page 15 of 35
Journal of Materials Chemistry C
View Article Online
oxidation potentials, this effect is counteracted by the increased conjugation prDeOseI: n10t.10in39/C38aT.C01130C
Surprisingly, despite the electron-donating nature of the tert-butyl groups, the reduction
potential is not further cathodically shifted, but instead is -1.54 V for 1a and -1.52 V for 1b.
UV-Vis absorption spectroscopy
The absorption spectra for all complexes recorded in aerated MeCN at 298 K are shown in
Figure 4 and the data summarized in Table 2.
Figure 4. UV-Vis absorption spectra recorded at 298 K in aerated acetonitrile.
Table 2. UV-vis absorption data of the investigated Ir(III) complexes

abs / nm ( / M-1 cm-1)a
15

Journal of Materials Chemistry C
Page 16 of 35
View Article Online
DOI: 10.1039/C8TC01130C
255 (56 853), 286 (34 421), 309 (28 841),
329 (16 244), 408 (3 757), 466 (663)
R1
1a
1b
R2
2a
3a
258 (55 115), 309 (22 533), 323 (12 442),
340 (10 817), 374 (7 397), 410 (3 976), 448 (941)
272 (53 237), 310 (22 321), 329 (12 109), 377 (7477), 397
(5 357), 410 (4 408), 429 (2 455), 470 (837)
267 (54 540), 310 (24 832), 331 (13 504),
344 (12 153), 385 (6 752), 419 (3 826), 471 (675)
265 (53 788), 296 (33 350), 310 (28 049),
343 (14 195), 385 (7 269), 419 (4 019), 467 (855)
272 (56 407), 311 (29 765), 327 (23 945),
350 (17 179), 402 (6 388), 439 (2 460), 472 (993)
a recorded in aerated MeCN at 298 K.
The acquired absorption data for R1,23 1b24 and R28d match those previously reported.
For all complexes, the electronic absorption spectra show intense ( on the order of 5.5 × 104
1
M-1 cm-1) high-energy absorption bands between 255 - 272 nm that are assigned to π–π*
ligand-centred (1LC) transitions on both the C^N and N^N ligands. With respect to R1, complex
3a shows a modest red-shift of the 1LC band that is due to the greater conjugation of the 3,5-
di-tert-butylphenyl ring with the pyridine while a similar red-shifting compared to R1 of this
band is observed for 1b, also due to LUMO stabilization of the C^N ligands. The profiles of
the low-energy bands are found to be insensitive to the nature of the substituents on the C^N
ligands. All complexes show moderately intense absorption bands in the range of 300 to 340
nm, which are assigned to spin-allowed mixed metal-to-ligand charge transfer transitions
(1MLCT) and ligand-to-ligand charge transfer transitions (1LLCT). Weaker absorption bands
are observed beyond 400 nm, tailing to 490 nm. These bands are assigned to spin-forbidden
(3MLCT/3LLCT) transitions.25
16

Page 17 of 35
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC01130C
Photophysical Properties
Solution-state Photophysical Behavior
The photoluminescence (PL) spectra in degassed MeCN are shown in Figure 5a and the
data are summarized in Table 3.
17

Journal of Materials Chemistry C
Page 18 of 35
View Article Online
DOI: 10.1039/C8TC01130C
Figure 5. Emission spectra. a) recorded at 298 K in deaerated MeCN, with λexc = 400 nm, b)
dropcast thin films at 298 K, with λexc = 400 nm.
Table 3. Photophysical data of 1a-5a.
MeCN
Neat film
a
a
a,b
c
e
e,f
g
PL
τPL
ΦPL
kr × 10-5 knrd × 10-5
PL
τPL
ΦPL
/ nm
/ ns
/ %
/ s-1
/ s-1
/ nm
/ ns
/ %
5 (11.7%)
592
581
31
26
3.61
13.60
578
29 (30.6%)
180 (57.7%)
23
R1
1a
9 (13.7%)
40 (33.2%)
200 (53.1%)
597
497
3.42
16.70
579
40
18

Page 19 of 35
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C8TC01130C
8 (7.7%)
611
592
588
598
250
643
623
501
12
28
29
25
3.20
2.95
3.05
3.39
36.80
12.60
13.00
16.57
565
565
564
570
43 (23.4%)
225 (68.9%)
27
15
16
40
1b
R2
2a
3a
10 (16.4%)
42 (37.5%)
171 (46.1%)
10 (14.4%)
42 (35.5%)
179 (50.1%)
8 (13.8%)
36 (34.3%)
183 (51.9%)
.
a recorded at 298 K in deaerated MeCN, with exc = 400 nm; b [Ru(bpy)3]Cl2 6H2O was used
as reference (ΦPL = 0.04 in 10-5 M aerated H2O);26 c kr = τPL / ΦPL. d knr = 1/(τPL - kr) = [(1 -
e
f
ΦPL)/ τPL]; values refer to dropcast thin films at 298 K, with exc = 400 nm; with
g
contribution of component in parentheses; values refer to dropcast thin films at 298 K
using an integration sphere under an N2 environment.
Upon excitation at 400 nm, all complexes show moderately intense yellow emission
over a narrow range between 588 to 611 nm and show a broad and unstructured profile,
indicative of an emission with mixed CT character. An increasing red-shifting of the emission
spectra is observed upon addition of tert-butyl groups (5 nm, 140 cm-1 for R1 to 1a and 19 nm,
530 cm-1 for 1a to 1b). This trend is also observed for the LC band in the absorption spectra
and correlates as well with a slightly smaller Eredox gaps. The effect of the addition of electron-
donating groups on the phenyl rings of the C^N ligands has been shown to induce a red-shift of
the emission.23a A red-shift of 6 nm (169 cm-1) in the emission spectrum is also observed for 3a
compared to R1, which is due to stabilization of the triplet state as a result of the increased
conjugation within the C^N ligands. Given the orthogonal conformation of the aryl groups in
R2 and 2a there is expectedly no significant change in the emission energy, consistent with
19

Journal of Materials Chemistry C
Page 20 of 35
previous studies.8b, 8d This design approach permits a modulation of the bulkiness of the
View Article Online
DOI: 10.1039/C8TC01130C
resulting complexes without substantially affecting the emission energy.
The photoluminescence quantum yield (PL) values of all complexes are approximately
in the same range (ΦPL ~ 25 – 31%), except for notably red-shifted 1b, which shows a lower
ΦPL of 12%. This finding is a logical consequence of the energy gap law, which states that the
non-radiative decay rate increases with decreasing emission energy.27 These values are
comparable to those obtained for related cationic iridium complexes emitting in the same
energy region.3d, 5, 8a, 23a, 24, 28 The emission decays are monoexponential and the lifetimes (PL)
are all in the submicrosecond regime. The addition of increasing numbers of tert-butyl groups
on the C^N ligands result in a decrease in the PL values, a consequence of an increase in the
non-radiative decay rate, knr. Over the series of substituted complexes, the radiative decay rate,
kr, remains similar, though lower than the reference complex R1.
Emission Studies in Neat Films
In order to emulate the emissive layer in the LEEC devices, neat films of all complexes
were prepared by drop-casting a dichloromethane solution onto a glass substrate, which was
then dried under vacuum. The emission spectra of the neat films were recorded at 298 K open
to air (Figure 5b and Table 3). All complexes display blue-shifted, broad and unstructured
emission spectra (~ 30 nm, 820 cm-1) compared to the MeCN solution measurements. The
absence of any red-shifting indicates that there is no excimer formation. Multi-exponential
emission decay kinetics are observed for all complexes, with the longest component in each
being significantly shorter than the PL found in MeCN solution. Photoluminescence quantum
yields for R1, R2 and 2a, remain essentially unchanged compared to solution measurements
20

Page 21 of 35
Journal of Materials Chemistry C
View Article Online
while those of 1a, 1b and 3a are enhanced, a function of reduced non-radiative decay in the
DOI: 10.1039/C8TC01130C
neat film.
LEEC Devices
LEECs were prepared and the electroluminescence properties of complexes all
complexes are shown in Figure 6a-c while Figure 6d describes the device architecture. The
LEEC data are summarized in Table 4. The devices were built on ITO-patterned substrates,
where poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer (80 nm)
was deposited by spin-coating. The LEEC active layer (100 nm) was deposited from MeCN by
spin-coating, which contained the emitting complex mixed with the ionic liquid (IL) 1-butyl-3-
methylimidazolium hexafluorophosphate, [Bmim][PF6], in a 4:1 molar ratio (complex:IL).
Aluminium was evaporated as a top contact electrode. For simplicity, the LEECs are referred
as DR1 to D3a containing complex R1 to 3a. LEECs were characterized by applying a bias and
monitoring the emitted light over time. Unlike LEECs measured at constant voltage (DC),
pulsed-current LEECs show faster response and improved device lifetime.29 The devices were
characterized under an inert atmosphere by applying an average pulsed current (1 kHz, 50%
duty cycle) of 100 A m-2.
21

Journal of Materials Chemistry C
Page 22 of 35
View Article Online
DOI: 10.1039/C8TC01130C
Figure 6. Luminance versus time (DR1 and D1a in (a) and D1b and DR2 in (b)) operated with
a pulsed current of 100 A m-2 (1KHz, 50% duty cycle and block wave). (c) Electroluminescence
spectrum. (d) LEEC architecture.
The yellow electroluminescence of the devices consists on one peak centred at 582 nm
for DR1 and 577-578nm for D1a, D1b and DR2 (see Figure 6c). The electroluminescence
spectra are narrower than the thin film photoluminescence spectra. The solubility of the D2a
and D3a was poor (<15 mg mL-1) in both MeCN and CH2Cl2, which led to poor morphology
quality of the spin-coated films and devices with high leakage currents. Even when the solution
is filtered (200 nm porous size), the saturated solutions form grains during the solvent
22

Page 23 of 35
Journal of Materials Chemistry C
View Article Online
evaporation by spin-coating, which was confirmed by optical microscopy (see Figure S25 in
DOI: 10.1039/C8TC01130C
the Supporting Information).
The LEECs DR1, D1a, D1b and DR2 exhibit instantaneous luminance that increases
rapidly leading to short turn-on time (ton) of less than one minute, which is defined as the time
to reach the maximum luminance, except DR1 where ton is 2400 s. The long ton is related with
a slow evolution of the charge current equilibrium. A slow growth of the doped regions delays
degradation and slows down the exciton-quenching rates.30 That would lead to more stable
devices. In fact, DR1 has the highest t1/2. The devices are bright, with maximum luminance
values in the same order of magnitude, between 350 and 750 cd m-2 across all devices. The
devices are moderately efficient, with a maximum efficacy of 7.4 cd A-1 in the case of D1a.
Lower efficiencies are achieved for D1b, DR1 and DR2, following the trend in the
photoluminescence quantum yield of the thin film. However, the LEECs differs in terms of
their stability, defined as time to reach half of the maximum luminance. DR1 has a lifetime
above 62 hours, which makes it the most stable device within this study, maintaining almost
constant the luminance over the time about 300 cd m-2. D1a has a higher luminance (737 cd m-
2) although it is achieved only during few seconds and then proceeds to drop to ~600 cd m-2,
lasting for only 14 h. D1b and DR2 present low stabilities. Initially D1b displays luminance
values around 350 cd m-2 but is stable for 1.3 h. DR2 shows higher turn-on luminance (~550
cd m-2) however, the emission drops fast and the t1/2 only 0.3 h. The result comparison with
previous works is complicated as the operational parameters are different.12 Pulse current is
used to improve the device stability compared to what is obtained using a constant voltage
driving. The limited stability of these devices relates in part to poor film-forming
characteristics, leading to a morphology with imperfections, such as tiny dots. This was in
particular the case for the films prepared with complexes 2a and 3a (SI). Even though such
23

Journal of Materials Chemistry C
Page 24 of 35
View Article Online
imperfections were not identified for the films of the other complexes it is likely due to the
DOI: 10.1039/C8TC01130C
similarity in the chemical structure that also here some film-forming issues are occurring, albeit
at a smaller scale and dimension. This would also lead to pathways where the degradation is
accelerated. Higher photoluminescence quantum yield values were published for similar
complexes.8a The described imperfections in the film formation can explain the lower observed
photoluminescence quantum yield. However, the obtained EQEs (1.2% – 2.6%) are in line with
the typical values for efficient yellow LEECs. Indeed, only a few examples with high EQE
(>6%) have been published for yellow/orange LEECs.3a, 3d, 31 Furthermore, the EQE’s appear
in line with the PLQY of the films, again highlighting the main issue is the film formation.
Table 4. Device performance of the LEEC: ITO/PEDOT:PSS/Complex:[Bmim][PF6]/Al
operated with a pulsed current of 100 A m-2 (1KHz, 50% duty cycle and block wave).
Device Lummax ton / s
t1/2 Efficacy PE
EQE
CIE (x, y)
EL
/ cd m-2
/ h / cd A-1 / lm W-1 / %
/ nm
432
737
353
557
2400
25
62
14
4.3
7.4
3.4
5.6
2.3
3.7
1.4
2.1
1.8
582
577
578
578
0.50, 0.52
0.49, 0.50
0.51, 0.51
0.47, 0.51
DR1
D1a
D1b
DR2
2.6
1.2
2
<5
1.3
0.3
54
b
c
aMaximum luminance. Time to reach maximum luminance. Time to reach one-half of the
d
e
maximum luminance. Maximum efficacy. Maximum power efficiency. fMaximum external
g
h
quantum efficiency. Wavelength emission in electroluminescence. CIE coordinates obtained
from the electroluminescence spectrum at maximum luminance.
Conclusions
24

Page 25 of 35
Journal of Materials Chemistry C
View Article Online
In conclusion, we successfully synthesized and characterized a series of cationic Ir(III)
DOI: 10.1039/C8TC01130C
complexes of the form [Ir(C^N)2(dtBubpy)]PF6 incorporating bulky substituents on the C^N
ligand in order to develop new emissive materials for light-emitting electrochemical cells. The
syntheses are straightforward allowing the introduction of tert-butyl, mesityl, manisyl, and 3,5-
di-tert-butylphenyl groups into the C^N ligand. This systematic study revealed that the
solubility of the complexes in standard organic solvents increased. At high concentrations (>15
mg mL-1) complexes 2a and 3a did not, however, show good solubility in MeCN and DCM.
All complexes have been structurally characterised and the single crystal structure
determinations of complexes 1a, 1b, R2 and 3a confirm that by increasing the sterical
congestion of the substituent the inter-nuclear distance between adjacent complexes increased
in the solid state. All complexes show bright yellow luminescence in MeCN, with moderate
photoluminescence quantum yields. The yellow phosphorescence of the parent complex R1 is
retained, due to a twisted conformation of the bulky aromatic substituents (in R2, 2a and 3a) as
they do not extend the π-conjugated system of the C^N ligand. Neat film photoluminescence
quantum yields of up to 40% could be obtained. Complexes R1, 1a, 1b and R2 were
successfully employed in LEECs, showing yellow luminescence with moderate external
quantum efficiencies.
Experimental Section
Synthesis
The compounds ClR2 and LR2 where synthesised according previously reported procedures.8b
General procedure I (for L1a and L1b)
A solution of tert-butylpyridine (1.0 equiv.) in CH2Cl2 was stirred at room temperature.
Trifluoroacetic acid (1.0 equiv.) was added followed by phenylboronic acid (1.5 equiv.), water
(12 mL), a solution of silver(I) nitrate (0.2 equiv.) in 8 mL of water and potassium persulfate
25

Journal of Materials Chemistry C
Page 26 of 35
View Article Online
(3.0 equiv.). The solution was stirred vigorously for 6 h. The reaction was quenched with water
DOI: 10.1039/C8TC01130C
(10 mL), extracted with CH2Cl2 (50 mL) and washed with conc. aqueous NaHCO3 (10 mL).
After layer separation, the brown yellow organic phase was dried over MgSO4 and the solvent
was evaporated under vacuum leaving brown oil which was purified on silica (10% of EtOAc
in petroleum ether). The desired fractions were combined and reduced until dryness yielding
the desired compound as oil.
4-(tert-butyl)-2-phenylpyridine (L1a)
Compound L1a was prepared following the general procedure I and was obtained as yellow-
brown oil (0.351 g, 1.661 mmol). Yield: 45%, Rƒ: 0.32 (10% of EtOAc in petroleum ether on
silica). 1H NMR (400 MHz, CDCl3) δ 8.62 (d, 1H), 8.02 – 7.93 (m, 2H), 7.74 – 7.69 (m, 1H),
7.48 (q, J = 7.4 Hz, 2H), 7.41 (q, J = 7.4 Hz, 1H), 7.25 – 7.22 (m, 1H), 1.36 (s, 9H). 13C NMR
(101 MHz, CDCl3) δ 161.4, 156.3, 148.9, 140.2, 129.7, 129.4, 125.9, 118.5, 116.8, 33.7, 31.2.
HR-MS (FTMS+): [M-H]+ Calculated: (C15H17NH): 212.1434 Found: 212.1428. The
characterisation matches that previously reported.16
4-(tert-butyl)-2-(4-(tert-butyl)phenyl)pyridine (L1b)
Compound L1b was prepared following the general procedure I and was obtained as yellow-
brown oil (1.000 g, 3.739 mmol). Yield: 46%, Rƒ: 0.36 (10% EtOAc in petroleum ether on
silica). 1H NMR (400 MHz, CDCl3) δ 8.55 (d, 1H), 7.89 (d, J = 8.3 Hz, 2H), 7.68 (s, 1H), 7.49
(d, J = 8.3 Hz, 2H), 7.22 – 7.20 (m, 1H), 1.52 (s, 18H). The matches that previously reported.32
General procedure II (for Cl2a and Cl3a)
A mixture of 2-chloro-4-iodopyridine (1.00 equiv.), the corresponding arylboronic acid (1.30
equiv.), 2 M aqueous K2CO3 (5.00 equiv.) and 1,4-dioxane (35 mL) was degassed for 15 min.
Pd(Ph3)4 (0.04 equiv.) was added, and the reaction mixture was stirred at 95 °C for 3 days, then
26

Page 27 of 35
Journal of Materials Chemistry C
View Article Online
cooled to room temperature. Toluene (80 mL) was added. After layer separationDtOhI:e10o.1r0g39a/nC8icTC01130C
phase was washed with water (10 mL) and dried over MgSO4. The solvent was evaporated
leaving a residue which was purified by column chromatography (15% EtOAc in petroleum
ether on silica). The desired fractions were combined, and the solvent evaporated leaving the
title compound as solid.
2-chloro-4-(4-methoxy-2,6-dimethylphenyl)pyridine (Cl3a)
Compound Cl3a was prepared following the general procedure II and was obtained as a
colourless sticky oil (0.995 g, 4.016 mmol). Yield: 94%. Rf: 0.32 (10% EtOAc in petroleum
ether on silica). 1H NMR (400 MHz, Chloroform-d) δ = 8.44 (d, J=5.0, 1H), 7.15 (m, 1H), 7.04
(d, J=5.0, 1H), 6.95 (s, 2H), 2.33 (s, 3H), 2.00 (s, 6H). 13C NMR (101 MHz, CDCl3) δ = 159.2,
152.7, 151.8, 149.7, 136.6, 130.4, 125.5, 124.0, 113.1, 77.4, 77.2, 77.0, 76.7, 55.2, 20.9. HR-
MS (FTMS+): [M-H]+ Calculated: (C14H15ClNOH): 248.0837 Found: 248.0837. CHN:
Calcd. for C14H14ClNO: C, 67.88; H, 5.70; N, 5.65. Found: C, 67.76; H, 5.59; N, 5.57.
2-chloro-4-(3,5-di-tert-butylphenyl)pyridine (Cl3a)
Before following the general procedure II for Cl3a, (3,5-di-tert-butylphenyl)boronic acid
was prepared, adapted from a previously reported procedure.33
Cl3a was obtained as a red solid (1.613 g, 5.346 mmol). Yield: 96%. Rf: 0.40 (10 vol.%
1
EtOAc in petroleum ether on silica). Mp: 68 °C. H NMR (400 MHz, Chloroform-d) δ = 8.42
13
(d, 1H), 7.54 (d, 2H), 7.41 (m, 3H), 1.38 (s, 18H). C NMR (101 MHz, CDCl3) δ = 152.8,
152.0, 151.9, 149.8, 136.4, 123.8, 122.3, 121.4, 120.8, 35.0, 31.4. HR-MS (FTMS+): [M-H]+
Calculated: (C19H25ClNH): 302.1670 Found: 302.1668. CHN: Calcd. for C19H24ClN: C,
75.60; H, 8.01; N, 4.64. Found: C, 75.41; H, 7.93; N, 4.61.
27

Journal of Materials Chemistry C
Page 28 of 35
View Article Online
DOI: 10.1039/C8TC01130C
General procedure III (for L2a and L3a)
A mixture of the corresponding 2-chloro-4-arylpyridine (1.00 equiv.), phenylboronic acid
(1.50 equiv.), palladium (II) acetate (0.06 equiv.), PPh3 (0.26 equiv.), 2 M aqueous Na2CO3
(4.00 equiv.) and 1,2-dimethoxyethane (25 mL) was degassed for 15 min. The reaction mixture
was heated to reflux for 24 h under Ar atmosphere, and then cooled to room temperature.
CH2Cl2 (50 mL) was added. After layer separation the organic layer was washed with brine and
dried over MgSO4. The solvent was evaporated leaving a residue, which was purified by column
chromatography (10% petroleum ether in CH2Cl2). The desired fractions were combined, and
the solvent evaporated leaving the desired compound as a solid.
4-(4-methoxy-2,6-dimethylphenyl)-2-phenylpyridine (L2a)
Compound L2a was prepared according to the general procedure III and was obtained as a
colourless solid (0.420 g, 1.451 mmol). Yield: 89%. Rf: 0.25 (10 vol.% EtOAc in petroleum
ether on silica). Mp.: 57-58 °C. 1H NMR (400 MHz, Chloroform-d) δ = 8.70 (dd, J=4.9, 1H),
8.00 (m, 2H), 7.55 (s, 1H), 7.46 (m, 3H), 7.06 (dd, J=4.9, 1H), 6.72 (s, 2H), 3.93 (s, 3H), 2.21
(s, 6H). 13C NMR (101 MHz, CDCl3) δ = 158.9, 157.6, 150.2, 149.8, 139.3, 136.8, 131.9, 129.0,
128.7, 126.9, 123.6, 121.9, 113.0, 77.4, 77.0, 76.7, 55.2, 21.0. HR-MS (FTMS+): [M-H]+
Calculated: (C20H19NOH): 290.1539 Found: 290.1538. CHN: Calcd. for C20H19NO: C,
83.01; H, 6.62; N, 4.84. Found: C, 82.89; H, 6.56; N, 4.73.
4-(3,5-di-tert-butylphenyl)-2-phenylpyridine (L3a)
Compound L3a was prepared following the general procedure III and was obtained as a
colourless solid (0.751 g, 2.186 mmol). Yield: 85%. Rf: 0.22 (5 vol.% EtOAc in petroleum
1
ether on silica). Mp.: 72-75 °C H NMR (400 MHz, CDCl3) δ = 8.73 (d, 1H), 8.05 (d, 2H),
7.90 (s, 1H), 7.52 (m, 5H), 7.45 (d, 2H), 1.40 (s, 18H). 13C NMR (101 MHz, Chloroform-d) δ
28

Page 29 of 35
Journal of Materials Chemistry C
+
+
View Article Online
= 150.0, 129.0, 128.9, 127.2, 123.3, 121.6, 120.9, 119.3, 31.6. HR-MS (FTMS ): [M-H]
DOI: 10.1039/C8TC01130C
Calculated: (C25H29NH): 344.2371 Found: 344.2373. CHN: Calcd. for C25H29N: C, 87.41; H,
8.51; N, 4.08. Found: C, 87.31; H, 8.43; N, 4.14.
General procedure IV (for the Synthesis of target Complexes)
The iridium chloride (2.0 equiv.) and the corresponding C^N ligand (5.0 equiv.) were
suspended in a mixture of 2-ethoxyethanol/water (75/25). The mixture was refluxed under
stirring. After 24 h the mixture was allowed to cool to r.t. and distilled water was added. A
precipitate was observed. The solid was washed with Et2O, H2O and dried under vacuum to
give the intermediate [Ir(C^N)2Cl]2. A suspension of this dimer (1.0 equiv.), the ancillary ligand
4,4’-di-tert-butyl-2,2’-bipyridine (2.2 equiv.) and AgPF6 (2.5 equiv.) in 1,2-dichloroethane was
kept at reflux for 2 h and stirred at r.t. for another 16 h. The solvent was then evaporated leaving
a brown yellow residue, which was then placed in dichloromethane. A beige precipitate was
observed. After filtration, the filtrate was reduced till dryness under reduced pressure, leaving
a yellow-brown solid, which was purified on silica with dichloromethane and increasing
percentages of methanol (0% - 8%). The desired fractions were collected and reduced to
dryness, giving a yellow solid, which was suspended under stirring in a hexane/Et2O mixture
(1/1) for 4 h. The yellow precipitate was filtered off and dissolved in methanol. An aqueous
NH4PF6 solution was added dropwise resulting in a precipitate. The suspension was stirred
vigorously for 2 h and subsequently filtered. The solid was dissolved in CH2Cl2 and washed
with water. The layers were separated, and the organic layer was reduced to dryness, leaving a
yellow solid, which was recrystallised through vapor diffusion using CH2Cl2 as the solvent and
Et2O as the anti-solvent. After filtration, the target complexes were obtained as solids.
[Ir(2-phenylpyridinato)2(4,4’-di-tert-butyl-2,2’-bipyridine)]PF6 (R1)
29