Cationic Bis-cyclometallated Iridium (III) phenanthroline complexes with pendant fluorenyl substituents: Synthesis, redox, photophysical properties and light-emitting cells

Article abstract of DOI:10.1002/chem.200700308

We report the synthesis, characterisation, photophysical and electrochemical properties of a series of cationic cyclometallated Ir ^III complexes of general formula [Ir(ppy)₂(phen)]PF ₆ (PPy = 2-phenylpyridine, phen = a substituted phenanthroline). A feature of these complexes is that the phen ligands are substituted with one or two 9,9-dihexylfluorenyl substituents to provide extended π conjugation, for example, the 3-[2-(9,9-dihexylfluorenyl)]phenanthroline and 3,8-bis[2-(9,9- dihexylfluorenyl)]phenanthroline ligands afford complexes 6 and 9, respectively. A single-crystal X-ray diffraction study of a related complex 18 containing the 3,8-bis(4-iodophenyl)phenanthroline ligand, revealed an octahedral coordination of the Ir atom, in which the metallated C atoms of the ppy ligands occupy cis positions. The complexes 6 and 9 displayed reversible oxidation waves in cyclic voltammetric studies (E_1/2^ox = + 1.18 and +1.20 V, respectively, versus Ag/Ag⁺ in CH₂Cl₂) assigned to the metal-centred Ir^III/Ir^IV couple. The complexes exhibit strong absorption in the UV region in solution spectra, due to spin-allowed ligand-centred (LC) ¹π-π* transitions; moderately intense bands occur at approximately 360-390 nm which are red-shifted with increased ligand length. The photoluminescence spectra of all the complexes were characterised by a broad band at λ_max≈595 nm assigned to a combination of ³MLCT and ³π→π* states. The long emission lifetimes (in the microsecond time-scale) are indicative of phosphorescence: the increased ligand conjugation length in complexes 9 and 17 leads to increased lifetimes for the complexes (τ = 2.56 and 2.57 μs in MeCN, respectively) compared to monofluorenyl analogues 6 and 15 (τ = 1.43 and 1.39 μs, respectively). DFT calculations of the geometries and electronic structures of complexes 6′, 9′ (for both singlet ground state (S₀) and triplet first excited (T₁) states) and 18 have been performed. In the singlet ground state (S₀) HOMO orbitals in the complexes are spread between the Ir atom and benzene rings of the phenylpyridine ligand, whereas the LUMO is mainly located on the phenanthroline ligand. Analysis of orbital localisations for the first excited (T₁) state have been performed and compared with spectroscopic data. Spin-coated light-emitting cells (LECs) have been fabricated with the device structures ITO/PEDOT:PSS/Ir complex/Al, or Ba capped with Al (ITO = indium tin oxide, PEDOT = poly(3,4-ethylene-dioxythiophene), PSS = poly(styrene) sulfonate). A maximum brightness efficiency of 9 cd A^-1 has been attained at a bias of 9 V for 17 with a Ba/Al cathode. The devices operated in air with no reduction in efficiency after storage for one week in air.

Full text of DOI:10.1002/chem.200700308

FULL PAPER
DOI: 10.1002/chem.200700308
Cationic Bis-cyclometallated Iridium(III) Phenanthroline Complexes with
N
Pendant Fluorenyl Substituents: Synthesis, Redox, Photophysical Properties
and Light-Emitting Cells
Xianshun Zeng,[a] Mustafa Tavasli,[a] Igor F. Perepichka,[a] Andrei S. Batsanov,[a]
[b]
Martin R. Bryce,*[a] Chien-Jung Chiang,[b] Carsten Rothe,[b] and Andrew P. Monkman
Abstract: We report the synthesis, char-
acterisation, photophysical and electro-
chemical properties of a series of cat-
ionic cyclometallated IrIII complexes of
complexes exhibit strong absorption in
the UV region in solution spectra, due
to spin-allowed ligand-centred (LC)
1p–p* transitions; moderately intense
bands occur at approximately 360–
390 nm which are red-shifted with in-
creased ligand length. The photolumi-
nescence spectra of all the complexes
were characterised by a broad band at
lmax ꢀ595 nm assigned to a combina-
structures of complexes 6’, 9’ (for both
singlet ground state (S0) and triplet
first excited (T1) states) and 18 have
been performed. In the singlet ground
state (S0) HOMO orbitals in the com-
plexes are spread between the Ir atom
and benzene rings of the phenylpyri-
dine ligand, whereas the LUMO is
mainly located on the phenanthroline
ligand. Analysis of orbital localisations
for the first excited (T1) state have
been performed and compared with
spectroscopic data. Spin-coated light-
emitting cells (LECs) have been fabri-
cated with the device structures ITO/
PEDOT:PSS/Ir complex/Al, or Ba
capped with Al (ITO=indium tin
general formula [Ir
N
N
(ppy=2-phenylpyridine, phen=a sub-
stituted phenanthroline). A feature of
these complexes is that the phen li-
gands are substituted with one or two
9,9-dihexylfluorenyl substituents to
provide extended p conjugation, for ex-
ample, the 3-[2-(9,9-dihexylfluorenyl)]-
phenanthroline and 3,8-bis[2-(9,9-di-
hexylfluorenyl)]phenanthroline ligands
afford complexes 6 and 9, respectively.
A single-crystal X-ray diffraction study
of a related complex 18 containing the
3,8-bis(4-iodophenyl)phenanthroline
ligand, revealed an octahedral coordi-
nation of the Ir atom, in which the
metallated C atoms of the ppy ligands
occupy cis positions. The complexes 6
and 9 displayed reversible oxidation
waves in cyclic voltammetric studies
(Eo1=x2 =+1.18 and +1.20 V, respectively,
versus Ag/Ag+ in CH2Cl2) assigned to
the metal-centred IrIII/IrIV couple. The
3
3
tion of MLCT and p!p* states. The
long emission lifetimes (in the micro-
second time-scale) are indicative of
phosphorescence: the increased ligand
conjugation length in complexes 9 and
17 leads to increased lifetimes for the
complexes (t=2.56 and 2.57 ms in
MeCN, respectively) compared to mon-
ofluorenyl analogues 6 and 15 (t=1.43
and 1.39 ms, respectively). DFT calcula-
tions of the geometries and electronic
oxide,
PEDOT=poly(3,4-ethylene-
(styrene)
dioxythiophene), PSS=poly
ACHTREUNG
sulfonate). A maximum brightness effi-
ciency of 9 cdAꢁ1 has been attained at
a bias of 9 V for 17 with a Ba/Al cath-
ode. The devices operated in air with
no reduction in efficiency after storage
for one week in air.
Keywords:
N
N
cyclometalation · iridium · lumines-
cence
·
organic light-emitting
devices
[a] Dr. X. Zeng, Dr. M. Tavasli, Dr. I. F. Perepichka,+ Dr. A. S. Batsanov,
Prof. M. R. Bryce
Supporting information for this article is available on the WWW
under http://www.chemistry.org or from the author and includes: gen-
eral details of instrumentation and procedures; synthetic and charac-
terisation details for all new compounds; crystal structure data for
complex 18; additional absorption and emission spectra; AFM image
of complex 17 on a glass/ITO/PEDOT-PSS substrate; EL spectra for
complex 9 and luminance and luminous efficiency data for an LEC of
9 as a function of time; atomic coordinates for the geometry optimi-
sations.
Department of Chemistry, Durham University
Durham DH1 3LE (UK)
Fax : (+44)191-384-4737
E-mail: m.r.bryce@durham.ac.uk
[b] C.-J. Chiang, Dr. C. Rothe, Prof. A. P. Monkman
Department of Physics, Durham University
Durham DH1 3LE (UK)
[+] Current address: Centre for Materials Science
Faculty of Science and Technology
University of Central Lancashire, Preston, PR12HE (UK)
Chem. Eur. J. 2008, 14, 933 – 943
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933

Introduction
plored.[10] From this viewpoint, we now describe the synthe-
sis and photophysical properties of the new cationic iridium
complexes 6, 9, 15 and 17 (Schemes 1–4, respectively),
which comprise two cyclometallating C^N ppy ligands and
one N^N phenanthroline ligand. Phenanthroline was chosen
as a variant to 2,2-bipyridine as it is known to form stable
complexes with iridium.[8d,9c,11] 9,9-Dihexylfluorene units
were attached to the phenanthroline unit to enhance organic
solubility and hydrophobicity and to extend the ligand p
system, which should be beneficial in reducing non-radiative
intermolecular charge recombination due to steric effects, as
recently noted by Bolink and co-workers for an analogous
4,7-diphenylphenanthroline complex.[11d] Additionally, in
complexes 15 and 17 carbazole units were attached because
of their known high stability, processability and hole-trans-
porting properties.[12] We[13] and others[14] have recently re-
ported the photophysical properties of neutral cyclometal-
lated IrIII complexes which incorporate fluorene units into
the ligands. A charged complex has been used by Wong and
co-workers as a phosphorescent dopant in a multilayer
OLED.[14d]
Luminescent transition-metal complexes are of great inter-
est as multifunctional chromophores for advanced optoelec-
tronic applications.[1] In particular, a broad selection of li-
gands, principally based upon 2-phenylpyridine (ppy),[2]
have been utilised to form phosphorescent bis-[3] and tris-cy-
clometallated[4] neutral iridium
(III) complexes. The photo-
N
physical properties of these phosphors can be tuned by vary-
ing the ligand[5] or its substituent groups and by the use of
additional ancillary ligands.[6] Complexes of this type are
components in electrophosphorescent organic/polymer light-
emitting devices (O/PLEDs) where they generally function
as an emitting guest in a blend with a host material.[7] The
electrogenerated singlet and triplet excited states mix by
spin–orbit coupling and all the triplet excited states contrib-
ute to light emission. Additionally, the triplet state lifetime
is usually shortened, which helps to suppress triplet–triplet
annihilation. The key properties of transition-metal com-
plexes are their high stability, high luminescence quantum
yields, short excited state lifetime and tuneable emission
energy.
Ionic chromophores have attracted attention recently as
structural variants of the more widely studied neutral com-
plexes. A selection of heteroleptic charged Ir species com-
prising two cyclometallating
Results and Discussion
Synthesis of ligands and their iridium complexes: The syn-
thesis of complexes 6 and 9 is shown in Schemes 1 and 2, re-
spectively.
ppy ligands and a neutral di-
imine ligand (usually 2,2-bipyri-
dine) of generic formula
3-Bromo- and 3,8-dibromo-1,10-phenanthroline,[15] 2 and
7, respectively, were the precursors to the new substituted li-
gands. For the first series of complexes, the Suzuki–Miyaura
cross-coupling reactions[16] with the fluorenylboronic ester
3[17] proceeded in good yields under standard conditions
(C^N)2Ir
(N^N)+Xꢁ (typically
A
X=PF6) 1 have been studied.[8]
Analogues with different cyclo-
metallating ligands (e.g., 1-phe-
nylpyrazole, benzoquinoline, 2-
([Pd(PPh3)4], Na2CO3 (aq), toluene, 888C) to give the phe-
A
thienylpyridine and 1-phenylisoquinoline) are also known.[9]
The N^N ligand can serve to improve complex stability
compared to tris-cyclometallated systems and the presence
of counterions may be beneficial for device applications.
These counterions are mobile under the influence of an ap-
plied bias leading to accumula-
nanthroline–fluorene conjugates 4 and 8, respectively. The
charged complexes 6 and 9 were then obtained by reaction
of these ligands with the dimeric species [{Ir(ppy)2Cl}2] 5,
A
followed by anion exchange by using KPF6, based on litera-
ture precedents.[18]
tion of negative ionic charge
near one electrode and deple-
tion near the other electrode.
This ionic space charge creates
high electric fields at the elec-
trodes which increases electron-
ic charge injection into the
metal complexes. Thus operat-
ing voltages can be quite low
and air-stable electrodes can be
used.
The effect on the redox and
optoelectronic properties of
changing the structure of the li-
gands (especially the N^N
ligand) around
a charged Ir
Scheme 1. Reagents and conditions: i) compounds 2+3, [Pd
A
centre remains largely unex- (77% yield); ii) compounds 4+5, CH2Cl2, MeOH, reflux, 24 h; then KPF6, 208C, 1 h (73% yield).
934
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Chem. Eur. J. 2008, 14, 933 – 943

Bis-cyclometallated Iridium(III) Phenanthroline Complexes
A
FULL PAPER
line ligands 14 and 16 were sub-
sequently obtained from 13, b y
analogy with reactions of
shown in Schemes 1 and 2.
3
Crystal structure of complex
18: None of the complexes 6, 9,
15 or 17 gave crystals suitable
for X-ray structure analysis.
However, the crystal structure
of the related complex 18 was
determined to confirm the
mode of coordination of the
phenylpyridine and phenan-
throline ligands in this series.
The synthesis of 18 is described
in the Supporting Information.
Scheme 2. Reagents and conditions: i) compounds 7 (1 equiv), 3 (2.0 equiv), [Pd(PPh3)4], 1m Na2CO3 (aq), tol-
uene, 888C, 48 h (77% yield); ii) compounds 8 (0.1 equiv), 5 (0.05 equiv), CH2Cl2, MeOH, reflux, 24 h; then
KPF6, 208C, 1 h (90% yield).
A
The second series of ligands 14 and 16 and their com-
plexes 15 and 17 (Schemes 3 and 4) have pendant carbazole
units which were attached to the fluorene unit by twofold al-
kylation at the C9 position of 2-iodofluorene 10 by using N-
(6-bromohexyl)carbazole 11 under basic conditions to afford
12 in 88% yield. Compound 12 was then converted to the
boronic ester derivative 13 in 71% yield. The phenanthro-
The asymmetric unit compris-
es one complex cation, one-
hexafluorophosphate
anion
(Figure 1) and two acetone
molecules (not shown), one of
which forms a short O1···I1 con-
tact of 3.27(1) Š and the other
is chaotically disordered. The
coordination geometry of iridi-
um is similar to that found in
previously studied diamino-
bis(phenylpyridine)iridium(III)
N
type cations, where the diamino
ligands were various bipyridine
derivatives[8c,e,19,20] and dipyri-
do[3,2-a:2’,3’-c]phenazine
(dppz).[21] To our knowledge, no
1,10-phenanthroline complex of
this type has been characterised
crystallographically. The Ir
atom in 18 adopts a distorted
octahedral coordination. As
ꢁ
usual, the two Ir C bonds are
cis to each other and trans to
ꢁ
the Ir N
are longer than the mutually
A
Scheme 3. Reagents and conditions: i) compounds 10+11, NaOH (aq), THF, [18]crown-6, reflux, 36 h (88%
ꢁ
trans Ir N
than 0.1 Š, due to a trans influ-
A
yield); ii) compound 12, nBuLi, THF, ꢁ788C, 5 h; then B
pounds 13+7, [Pd(PPh3)4], 1m Na2CO3 (aq), THF, toluene, 888C, 72 h (79% yield); iv) compounds 14+5,
CH2Cl2, MeOH, reflux, 24 h; then KPF6, 208C, 1 h (81% yield).
A
ACHTREUNG
ence. In bipyridine-bis(phenyl-
Chem. Eur. J. 2008, 14, 933 – 943
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www.chemeurj.org
935

M. R. Bryce et al.
ꢁ
tral C C bond by up to 108 (and in the case of 2,9-diphenyl-
4,7-di-tert-butylbipyridine ligand,[8c] by as much as 338).
Clearly, phenanthroline (and dppz) ligands lack this flexibili-
ty. Possibly for this reason, the phenanthroline ligand “bite”
angle (N-Ir-N) in 18 is somewhat wider than those typical
for bpy ligands (75.2–76.78), although it remains smaller
than the N-Ir-C “bite” angles of the ppy ligands.
Solution electrochemical studies: Cyclic voltammetric (CV)
studies of iridium complexes were carried out in dichloro-
methane solution with 0.1m tetra-n-butylammonium hexa-
fluorophosphate (Bu4NPF6) as the supporting electrolyte.
The oxidation potentials of the complexes 6, 9, 15 and 17
and ferrocene as an internal reference (Fc/Fc+ couple:
Eo1=x2 =+0.46 V versus SCE[23]) versus Ag/Ag+ are summar-
ised in Table 1 and the CV traces are shown in Figure 2. The
ox
1=2
Table 1. Summary of the half-wave oxidation potentials (E [V]) ob-
tained by cyclic voltammetry for complexes 6, 9, 15, 17, N-hexylcarbazole
and ferrocene versus Ag/Ag+.[a]
ox
1=2
ox
pa
ox
pc
Compound
E
{E , E } [DE, mV] versus Ag/Ag+
6
9
15
+1.18 [75]
+1.20 [76]
{1.06, 0.98}
17
1) {1.06, 0.57} 2) 1.08[94]
{1.05, 0.67}
0.28 [65]
Scheme 4. Reagents and conditions: i) compounds 7+13 (2 equiv), [Pd-
(PPh3)4], 1m Na2CO3 (aq), THF, toluene, 888C, 48 h (84% yield); ii) com-
pounds 16+5, CH2Cl2, MeOH, reflux, 24 h; then KPF6, 208C, 1 h (92%
N-hexylcarbazole
ferrocene
[a] 0.1m Bu4NPF6 in dichloromethane, scan rate=100 mVsꢁ1
ACHTREUNG
.
yield).
Figure 2. Cyclic voltammograms of complexes 6, 9, 15, 17 and N-hexylcar-
Figure 1. Cation and anion in the crystal of 18·2Me2CO (50% probability
thermal ellipsoids).
bazole in
a solution of 0.1m Bu4NPF6 dichloromethane, scan rate
100 mVsꢁ1, with ferrocene (Fc) as an internal reference.
ꢁ
ꢁ
pyridine) complexes the Ir C distances of 1.99–2.04 and Ir
CV traces of the complexes 6 and 9 both displayed chemi-
cally reversible oxidation waves (at Eo1=x2 =+1.18 and
+1.20 V, respectively). The increase in the ligand length by
one fluorene unit (from 6 to 9) produced only a small effect
(20 mV positive shift) on the oxidation potential of 9 which
is understandable from DFT calculations, according to
which the HOMO energies of both complexes are close, and
HOMO orbitals are spread between the iridium atom and
benzene rings of ppy ligands, with no coefficients on the flu-
orene-phenathroline ligand (see theoretical section below).
ꢁ
N
N
ACHTREUNG
ACHTREUNG
complexes, but may increase to 2.22–2.26 Š for 2,9-substitut-
ed bpy ligands. Interestingly, the dppz complex[21] shows a
ꢁ
ꢁ
ꢁ
uniform lengthening of Ir C, Ir N
G
N
bonds, averaging 2.07(2), 2.10(2) and 2.17(2) Š, respectively,
although the precision of the latter structure is not high. The
bpy ligand has substantial conformational flexibility;[22] in
the complexes mentioned above it is twisted around the cen-
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Chem. Eur. J. 2008, 14, 933 – 943

Bis-cyclometallated Iridium(III) Phenanthroline Complexes
A
FULL PAPER
Only a minor effect of the structure of the phenanthroline
ligand on the oxidation potentials of its iridium complexes
has been observed before,[9a] which is understandable as the
oxidation processes are mostly associated with the bis-cyclo-
metallated phenyl-Ir moiety and therefore are assigned to
the pure metal-centred IrIII/IrIV couple.[24] The CVs of the
carbazole-containing complexes 15 and 17 are different
from complexes 6 and 9 and more complicated. They exhibit
irreversible oxidation waves in which the anodic peaks are
dominated by the Epa of the N-hexylcarbazole unit which
the red (ca. 390 nm) in complexes 9 and 17 with the increase
in the ligand length. This red shift is similar to the absorp-
tion of charged iridium complexes containing poly(9,9-dio-
ctylfluorene)[26] and also the increase in the extinction coeffi-
cients with increased conjugation length indicates that this
region (from 360 to 450 nm) is mainly dominated by the
!
ligand S1 S0 transitions. This absorption region shows a
minor solvatochromic shift to shorter wavelengths with in-
creased solvent polarity, for example, lmax for complexes 9
and 17 are 387 and 390 nm, respectively (in toluene) and
380 nm (in acetonitrile). Finally, as evident from their low
energy and low extinction coefficients, the weak bands at
around 400 nm and those extending towards the red are as-
signed to both spin-allowed metal-to-ligand charge-transfer
(1MLCT) transitions (eꢀ(1–2)”104 mꢁ1 cmꢁ1) and spin–
orbit-coupling-enhanced 3LC and 3MLCT transitions
(shoulders at 468 nm, with eꢀ(1–2)”103 mꢁ1 cmꢁ1). These
features are consistent with the literature data for related
iridium complexes.[19] It is also noteworthy that extinction
coefficients increased as the conjugation length increased.
However, the increase for the carbazole-containing com-
plexes (e.g., from 15 to 17) was less pronounced than the in-
crease from 6 to 9.
occurs at +1.05 V. Carbazoles, such as 1,4-bis(carbazolyl)-
ox
benzene (E =+1.04 V)[25] are known to have irreversible
pa
oxidation potentials. This irreversibility would explain the
ox
pc
re-reduction cathodic peak (E ) observed at +0.67 V for N-
hexylcarbazole and at +0.57 V for 17. This process overlaps
the second quasi-reversible oxidation process in 15 and 17,
which is assigned to a metal-centred IrIII/IrIV couple.
Solution-state photophysical properties
Absorption: The absorption spectra of ligands 4, 8, 14 and
16 in toluene show a red shift for the bifluorene derivatives
8 and 16 (lmax =347 nm) compared to the monofluorene an-
alogues 4 and 14 (lmax =330 nm) (see Supporting Informa-
tion, Figure S1). The absorption spectra of complexes 6, 9,
15 and 17 were recorded both in toluene (Figure 3) and in
Emission: The photoluminescence (PL) spectra of com-
plexes 6, 9, 15 and 17 obtained from degassed solutions in
toluene at room temperature (excitation at 390 nm) are
shown in Figure 4. The increase in the emission intensities
Figure 3. The absorption spectra for complexes 6, 9, 15 and 17 in toluene.
Inset: expansion of the 450–600 nm region.
Figure 4. Normalised photoluminescence (PL) spectra for complexes 6, 9,
15 and 17 in degassed toluene, excited at 390 nm.
acetonitrile solution (see Supporting Information, Fig-
ure S2). In acetonitrile all of these complexes showed in-
tense peaks below l=300 nm, which are hidden in toluene
upon degassing, even though spectral profiles remained un-
changed, indicated the presence of long-lived phosphores-
cent species which are susceptible to oxygen quenching.[4,14a]
The PL spectra of all complexes are characterised by a
broad and structureless emission spectrum centred around
lmax ꢀ595 nm. An increase in conjugation length of the phe-
nanthroline ligand from one fluorene unit in 6 and 15 to two
in 9 and 17 shifted the emission maximum to the red (by
only 5 nm). A similar effect has been previously reporte-
1
by the solvent cut-off, and are assigned to spin-allowed p–
p* intraligand (1IL, C^N and N^N) transitions. At l>
300 nm, both in toluene and acetonitrile, less intense absorp-
tion features were seen. First, at around l=340 nm two
well-resolved absorption peaks were observed in complexes
15 and 17, and were attributed to the presence of the carba-
zole units. Second, broad absorption bands at lꢀ360 nm
were observed for complexes 6 and 15, and were shifted to
Chem. Eur. J. 2008, 14, 933 – 943
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937

M. R. Bryce et al.
d[13a,c,14c] as the conjugation length increased. However, when
Theoretical studies: The geometries and electronic struc-
tures of complexes 6, 9 and 18 were calculated by using the
PL
compared with the parent complex, (ppy)2Ir
G
=
max
579 nm, ppy=2-phenylpyridine, phen=1,10-phenanthro-
line)[8d] significant red shifts were observed (15 nm for 6 and
15, 20 nm for 9 and 17, respectively) consistent with extend-
ed conjugation involving the p systems of the phenanthro-
line and fluorene units. Red shifts in emission maxima were
also observed by Zhao and co-workers[9b] with increasing
conjugation length of diimine ligands. The PL spectra of the
complexes in degassed acetonitrile solutions were also inves-
tigated (see Supporting Information, Figure S2) and a blue
shift (7–10 nm) in the emission maxima (586 nm) was ob-
served for 9 and 17, compared to those of 6 and 15, respec-
tively.
density functional theory (DFT) method at the B3LYP/
(6-
31G(d,p)+LANL2DZ) level. To decrease the computation
AHCTREUNG
time calculations were performed for analogues of 6 and 9,
in which CH3 substituents replaced the C6H13 chains (de-
noted as 6’ and 9’, respectively). The geometries of both the
singlet ground state (S0) and the lowest triplet state (T1)
were fully optimised without imposing any symmetry restric-
tion and constraints. Selected bond lengths and angles in-
volving the Ir atom are given in Table 2, together with X-ray
diffraction data for 18. DFT calculations for a gas-phase
ꢁ
structure correspond well with the Ir C bond lengths ob-
served in a crystal of solvate 18·2Me2CO, whereas the calcu-
ꢁ
lated Ir N lengths are somewhat longer (by ꢀ0.03–0.04 Š
ACHTREUNG ACHTREUNG
A red shift (15–20 nm) in emission and also a large
3
Stokesꢁ shift (125 nm)[7b] between lmax for MLCT absorption
for Ir N
(3,4) and ꢀ0.07 Š for Ir CN
(1,2) Table 2). All
ꢁ
ꢁ
and emission spectra may indicate that the intraligand (3IL)
3p!p* state contributes to the emission. Given the broad
and structureless characteristics of the emission spectra, the
sensitivity to the medium and the relatively long lifetimes
observed (in the microsecond time scale; see below),
3MLCT states also contribute to the emission. Hence it can
be concluded that the emission originates from mixed
3MLCT/3p!p* states. This is in agreement with an observa-
tion by Lowry and co-workers[9c] that a library of iridium
complexes containing both cyclometallating and aromatic
diimine ligands had emissions from mixed excited states.
compounds adopt a geometry with orthogonal bonds be-
tween the Ir atom and the carbon atoms of ppy ligands (C3-
ꢁ
Ir-C4 close to 908), whereas C N bonds between the Ir and
nitrogen atoms of the ppy ligands form N3-Ir-N4 angles
close to 1808 (Table 2).
When the geometries of the ground state (S0) and the
lowest excited state (T1) are compared, relatively small ge-
ometry changes are observed. In the T1 state, slight shorten-
ꢁ
ing of Ir N distances is observed, compared to the singlet
ꢁ
ground state (S0), whereas Ir C bond lengths and C-Ir-C
and N-Ir-N angles remain almost unchanged. The benzene
Phosphorescence lifetimes and
photoluminescence
quantum
yields: Time-resolved lumines-
cence decay measurements for
all complexes were performed
at 298 K in degassed solutions
in acetonitrile (OD<0.7 at
lex =355 nm). The phosphores-
cence decays collected at the
emission maximum for each
complex show first-order kinet-
ics, with decay times of t=1.34
(6), 2.56 (9), 1.39 (15) and
2.76 ms (17). The increase in the
ligand conjugation length in
complexes 9 and 17 clearly in-
duces an increased phosphores-
cence lifetime of the complexes.
Table 2. Selected bond lengths and angles for complexes 6’, 9’, and 18 from DFT B3LYP/
LANL2DZ) calculations and from X-ray crystallography (for 18).
N
C
Compound
(state)[a]
Bond length [Š]
AHCTREUNG
ꢁ
Ir N1
ꢁ
Ir N2
ꢁ
Ir N3
ꢁ
Ir N4
ꢁ
Ir C3
ꢁ
18, X-ray
18 (S0)
6’ (S0)
9’ (S0)
6’ (T1)
9’ (T1)
2.153(6)
2.228
2.226
2.227
2.214
2.208
2.153(7)
2.228
2.227
2.227
2.210
2.208
2.051(7)
2.084
2.082
2.082
2.080
2.078
2.044(6)
2.083
2.084
2.082
2.079
2.078
2.013(8)
2.021
2.021
2.021
2.023
2.020
Phosphorescent
quantum
yields (FPL) were measured at
room temperature in thorough-
ly degassed toluene solutions
Compound
Angles [8]
Dihedral angles in
ppy ligand [8]
A
C3-Ir-C4
N3-Ir-N4
N1-Ir-N2
C3-C-C-N3
C4-C-C-N4
(ODꢀ0.1)[27] by using Ir
A
18, X-ray
18 (S0)
6’ (S0)
9’ (S0)
6’ (T1)
9’ (T1)
91.0(3)
90.4
90.5
90.7
90.7
172.4(3)
173.6
173.6
173.6
174.1
174.5
77.5(3)
75.6
75.6
75.7
76.4
2(1)
1.3
ꢁ1.3
ꢁ1.3
ꢁ0.7
ꢁ1.2
6(1)
1.5
ꢁ1.4
ꢁ1.3
ꢁ1.5
ꢁ1.2
(FPL =0.40 in toluene)[28] as the
standard. The FPL for the com-
plexes also show a modest in-
crease with the ligand conjuga-
tion length from 0.08 (6 and 15)
to 0.11 (9 and 17).
91.2
76.5
[a] The geometries of complexes 6’ and 9’ were optimised at both the singlet ground state (S0) and the lowest
triplet state (T1). The S0 state was more stable in both cases.
938
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Chem. Eur. J. 2008, 14, 933 – 943

Bis-cyclometallated Iridium(III) Phenanthroline Complexes
A
FULL PAPER
and pyridine rings in the ppy ligands are essentially coplanar
in both S0 and T1 states (Table 2).
Figure 5 shows the atomic orbital composition calculated
for the frontier molecular orbitals of complexes 6’ and 9’ in
the S0 state. For both complexes the HOMOs (that is, the or-
S0 is calculated at the optimised geometry of T1 (by analogy
to ref. [11d]) the values (1.76 eV (702 nm) for 6’ and 1.85 eV
(670 nm) for 9’) correspond better to the red edges of the
emission rather to their maxima (Figure 4).
Examination of the change in the total charge distribution
between the iridium atom and
ligands in complexes 6’ and 9’
from the S0 ground state to the
first excited T1 state is summar-
ised in Table 3. In both cases
the positive charge on the Ir
centre remains almost constant
(with only minor increase by
about +0.03–0.05), which indi-
cates that the excited state has
little MLCT character. Similar
small changes in the charges on
the ligands are observed. The
overall charges on the two ppy
ligands increase in negative
charge (by about ꢁ0.04–0.02)
whereas the positive charge on
the phen/Fl ligand (Fl=fluo-
rene) is increased by +0.02–
0.03, indicating a net transfer of
electron density between the
ppy and phen/Fl ligands.
An orbital energy diagram
for the triplets (T1) of 6’ and 9’
is shown in Figure 6 and locali-
sation of the orbitals is shown
in Figure 7. For both complexes
the higher single occupied orbi-
Figure 5. Isocontour plots (0.04 ebohrꢁ3) calculated for the HOMO and the LUMO orbitals of complexes 6’
and 9’.
bitals into which holes are injected, and which are involved
in the hole transport process in light-emitting electrochemi-
cal cell (LEC) operation; see below) are distributed be-
tween the Ir atom and the benzene rings of the ppy ligands.
The LUMOs (the orbitals into which electrons are injected
and transported in the LEC) are mainly located on the pyri-
dine rings of the phenanthroline ligand, with only minor ex-
tension to the outer fluorene moieties. Extension of the con-
jugation length (6’!9’) results in a decrease of both HOMO
and LUMO orbital energies (by 0.14 and 0.18 eV, respec-
tively) with only a minor decrease in the HOMO–LUMO
gap (0.04 eV). The location of the frontier orbitals in both 6’
and 9’ supports the HOMO–LUMO transition being a mix-
Table 3. Comparison of charges[a] on iridium atom and ligands in com-
plexes 6’ and 9’ for optimised geometries of the singlet (S0) and triplet
(T1) states.
Compound
(state)[b]
Charge on the fragment
AHCTREUNG
Ir
ppy
ppy
phen/Fl
6’ (S0)
6’ (T1)
+0.792
+0.820
+0.826
+0.795
+0.844
+0.863
ꢁ0.135
ꢁ0.150
ꢁ0.129
ꢁ0.144
ꢁ0.152
ꢁ0.172
ꢁ0.138
ꢁ0.164
ꢁ0.157
ꢁ0.144
ꢁ0.152
ꢁ0.172
+0.481
+0.495
+0.495
+0.494
+0.461
+0.480
6’ (ST0 1
9’ (S0)
9’ (T1)
)
9’ (ST0 1
)
[a] Mulliken atomic charges have been used in analysis. [b] S0 and T1 are
for optimised geometries, ST0 1 is for singlet state at T1-optimised geome-
try.
1
ture of MLCT and ligand-to-ligand charge transfer states.
The energy difference between T1 and S0 at the optimised
geometries is calculated to be similar for both complexes
(2.23 eV (554 nm) for 6’ and 2.19 eV (565 nm) for 9’). These
values are somewhat blue-shifted compared to the observed
emission maxima of the complexes (592 and 596 nm in tolu-
ene; and 585 and 584 nm in acetonitrile, for 6 and 9, respec-
tively; Figure 4 and Figure S3 in the Supporting Informa-
tion). When the vertical energy difference between T1 and
tal (h-SOMO) is delocalised between the phenanthroline
and fluorene moieties, with larger coefficients on the fluo-
rene compared to that for the LUMO orbitals in the S0 state
(compare Figures 5 and 7). There is a difference in the dis-
tribution of occupied orbitals (HOMO, HOMO-1, HOMO-
2) and l-SOMO between 6’ and 9’. Whereas 6’ shows a large
energy difference for the l-SOMO and HOMO(a) orbitals
Chem. Eur. J. 2008, 14, 933 – 943
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www.chemeurj.org
939

M. R. Bryce et al.
of 0.65 eV, these orbitals are quite close in energy (0.16 eV
difference) in 9’ (Figure 6). This results in a different locali-
sation of orbital coefficients for these complexes. In 6’ l-
SOMO is mainly localised on the fluorene moiety of the
phen/Fl ligand and HOMO(a) is spread between the Ir
atom and the benzene rings of two ppy ligands. The reverse
situation is observed for 9’: l-SOMO is spread between the
Ir atom and the benzene rings of two ppy ligands, whereas
HOMO(a) is localised on the fluorene moiety of the phen/
Fl ligand (Figure 7).
The DFT calculations suggest possible further design fea-
tures for this class of Ir complexes for LEC applications.
Considering the electron-rich character of the fluorene
moiety compared to phenanthroline (in the phen/Fl ligand),
and consequently the localisa-
Figure 6. Orbital energy diagram for triplet states of complexes 6’ and 9’
from UB3LYP/
metries).
A
E
tion of the LUMO in the S0
state predominantly on the
phen ring, it would be interest-
ing to study phen ligands with
electron-deficient end groups
on which the LUMO orbital
can be delocalised. The same is
valid when considering the lo-
calisation of l-SOMO orbitals
in the first excited T1 state. In-
troduction of electron-deficient
substituents in the benzene
rings of ppy ligands should in-
crease the HOMO coefficients
on the Ir centre, thus increasing
the contribution of MLCT com-
pared to LLCT transitions.
Light-emitting cells (LECs):
Spin-coated LECs have been
prepared from the iridium com-
plexes 6, 9, 15 and 17 with the
following device structures:
ITO/PEDOT:PSS/Ir complex/
Al, or Ba capped with Al
(ITO=indium
PEDOT=poly(3,4-ethylene-
dioxythiophene), PSS=poly-
(styrene) sulfonate). Further
tin
oxide,
AHCTREUNG
details are given in the Support-
ing Information. Broadly simi-
lar results were obtained for all
the complexes, the highest effi-
ciencies were obtained for 17
and these data are presented
below. Analogous data for an
LEC made from 9 are shown in
the Supporting Information,
Figures S5 and S6. An AFM
image of a film of complex 17
spin-coated from a solution in
toluene is shown in the Sup-
Figure 7. Isocontours plots (0.04 ebohrꢁ3) of higher and lower singly occupied molecular orbitals (h-SOMO
and l-SOMO) and HOMO (a electrons) orbitals for T1 equilibrium geometries of complexes 6’ and 9’.
940
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Chem. Eur. J. 2008, 14, 933 – 943

Bis-cyclometallated Iridium(III) Phenanthroline Complexes
A
FULL PAPER
porting Information (Figure S4). With <1 nm deviation, the
average surface roughness is extremely low for a spin-
coated organic material. It is notable that the solubilising
substituents enable such good quality films to be obtained
from a solution in toluene, rather than the more polar and
commonly used acetonitrile. Microcrystallites of complex 17
can be seen as regular needles, of approximately 15 nm
length, positioned perpendicular to the film surface.
Figure 8 compares the thin film PL and the electrolumi-
nescence (EL) spectra of an LEC made from 17. The EL is
Figure 9. Luminance and luminous efficiency of an ITO/PEDOT:PSS/17/
Al LEC as a function of time at a bias of 9 V.
erature.[11d] On applying up to 4 V bias no emission was ob-
served from our devices even after several hours. Addition-
ally, when the diode was charged at a high bias and subse-
quently operated at 3–5 V no current flow was observed and
hence no emission was detected. Thus we conclude that the
bulky side groups obstruct the diffusion of the counterions
and it therefore takes longer to switch on the devices made
with the larger complexes. Once charged these devices re-
quire a higher bias voltage due to reduced charge mobility.
Furthermore, the bias needed to charge and drive our LECs
increased with the size and the number of the side chains.
The monofluorene complex 6 initially exhibits a higher cur-
rent which relatively slowly increases with time. In contrast,
the current of the more bulky difluorene complex 9 starts
from a lower value, but increases more steeply.
Figure 8. Comparison of solid-state photoluminescence (excitation wave-
length 355 nm) and electroluminescence spectra of complex 17 in the
device configuration ITO/PEDOT:PSS/17/Al.
Figure 10 shows the maximum brightness that can be
reached with LECs of 17 at reasonable efficiencies. To
red-shifted to lmax =570 nm (2.17 eV) compared to [Ir(ppy)3]
A
(540 nm, 2.4 eV). The EL of 17 is 40 meV red-shifted com-
pared to the PL, which may be explained by charge trapping
and subsequent recombination at sites which are lower in
energy. Very similar spectra were obtained for complexes 6,
9 and 15, consistent with substitution at the phenanthroline
unit having little effect on the HOMO and the LUMO
levels of the complexes, which is consistent with recent DFT
calculations by Tamayo and co-workers.[9a] Identical spectra
were obtained by using Ba/Al as the cathode (data not
shown) instead of Al only. It is notable that the thin-film
emission spectra are blue-shifted and better resolved than
the solution spectra (compare Figures 4 and 8). In solution,
the excited state of the complex, with its charge transfer
character, can be stabilised leading to broader and typically
red-shifted spectra, with longer lifetimes observed in more
polar solvents.
Figure 10. Luminance and luminous efficiency of ITO/PEDOT:PSS/17/Al
and Ba/Al LECs as a function of bias; data were taken immediately after
charging the devices for 20 min at 10 V.
Figure 9 shows the typical increase in brightness and
brightness efficiency as a function of time for a standard
LEC made from 17 and an aluminium cathode. The ob-
ꢁ
served time dependence reflects the diffusion of the PF6
obtain these data the diode was first charged and then
tested as a function of bias. By using an aluminium cathode
at approximately 14 V less than 1000 cdmꢁ2 was achieved
with a brightness efficiency of 7 cdAꢁ1. These devices were
tested in air without any hermetic sealing. The brightness ef-
counter ions through the film under the applied voltage. In
this particular case, about 40 minutes was needed to attain
the maximum brightness efficiency of 8 cdAꢁ1 by applying a
high bias of 9 V; lower voltages are typically used in the lit-
Chem. Eur. J. 2008, 14, 933 – 943
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www.chemeurj.org
941

M. R. Bryce et al.
ficiency was further increased to 9 cdAꢁ1 by using Ba/Al in-
stead of Al as the cathode, which proves that the electron
injection of the Al devices is not ohmic and that the charge
carrier balance can be modified to give further improve-
ments in device efficiency. The devices shown in Figure 10
operated in air with no reduction in efficiency after storage
for one week in air.
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Conclusion
Ionic cyclometallated IrIII complexes of general formula [Ir-
ꢁ
(ppy)
2(phen)]+PF6 (ppy=2-phenylpyridine, phen=a sub-
stituted phenanthroline) have been synthesised. The phen li-
gands are substituted with one or two 9,9-dihexylfluorenyl
substituents to provide extended p conjugation. The com-
plexes 6 and 9 displayed reversible waves in cyclic voltam-
metric studies assigned to the metal-centred IrIII/IrIV couple
(Eo1=x2 =+1.18 and +1.20 V, respectively, versus Ag/Ag+ in
CH2Cl2). The photoluminescence spectra of all the com-
plexes were characterised by a broad band at lmax ꢀ595 nm
3
3
assigned to a combination of MLCT and p!p* states. The
increased ligand conjugation length in complexes 9 and 17
leads to increased phosphorescence lifetimes for the com-
plexes (t=2.56 and 2.57 ms in MeCN, respectively) com-
pared to monofluorenyl analogues 6 and 15 (t=1.43 and
1.39 ms, respectively). DFT calculations have established
that in the singlet ground state (S0) HOMO orbitals in the
complexes are spread between the Ir atom and benzene
rings of the phenylpyridine ligand, whereas the LUMO is
mainly located on the phenanthroline ligand. Analysis of or-
bital localisations for the first excited (T1) state have been
performed, and compared with spectroscopic data. These
complexes possess very good solubility even in non-polar or-
ganic solvents and good film-forming properties. Spin-
coated light-emitting cells (LECs) with structures ITO/PE-
DOT:PSS/Ir complex/Al, or Ba capped with Al attain a
maximum brightness efficiency of 9 cdAꢁ1 at a bias of 9 V
for 17 with a Ba/Al cathode. The bias needed to charge and
drive our LECs increased with the size and the number of
the side chains. The devices operated in air with no reduc-
tion in efficiency after storage for one week in air. Further
structural modifications to this series of complexes will be
reported in due course. These are challenging targets with
potentially high rewards in electroluminescent device tech-
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Bis-cyclometallated Iridium(III) Phenanthroline Complexes
A
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