Light-emitting electrochemical cells based on a supramolecularly-caged phenanthroline-based iridium complex

Article abstract of DOI:10.1039/c0cc05084a

The complex [Ir(ppy)₂(pphen)][PF₆] (Hppy = 2-phenylpyridine, pphen = 2-phenyl-1,10-phenanthroline) has been prepared and evaluated as an electroluminescent component for light-emitting electrochemical cells (LECs). Like in analogous

Full text of DOI:10.1039/c0cc05084a

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
ChemComm
Cite this: Chem. Commun., 2011, 47, 3207–3209
www.rsc.org/chemcomm
COMMUNICATION
Light-emitting electrochemical cells based on a supramolecularly-caged
phenanthroline-based iridium complexw
a
a
a
b
b
´
Ruben D. Costa, Enrique Ortı, Henk J. Bolink,* Stefan Graber, Catherine E. Housecroft
´
and Edwin C. Constable*b
Received 21st November 2010, Accepted 13th January 2011
DOI: 10.1039/c0cc05084a
The complex [Ir(ppy)2(pphen)][PF6] (Hppy = 2-phenylpyridine,
pphen = 2-phenyl-1,10-phenanthroline) has been prepared and
evaluated as an electroluminescent component for light-emitting
electrochemical cells (LECs). Like in analogous LECs using
bpy-based iridium(III) complexes a significant enhancement of
the device stability is observed.
intramolecular interaction is believed to be the reason for
the higher stability of these LECs.
Attempts to increase the stability through the use of 6,60-
diphenyl-2,20-bipyridine (dpbpy) were disappointing as no
improvement in the LEC stability was observed.3c This was
attributed to the distortion of the bpy ligand planarity due to
the double intramolecular p-stacking interaction which facilitates
the population of the metal-centred (3MC) excited states. In this
communication we extend the concept of supramolecular cage
formation to an ionic iridium(III) complex [Ir(ppy)2(pphen)][PF6]
(1) incorporating the pphen (pphen = 2-phenyl-1,10-phenan-
throline) ligand. The phen metal-binding domain is of interest as
it is rigidly planar and leads to complexes with greater stability
than 2,20-bipyridine. In complexes of pphen we expect larger
HOMO–LUMO gaps as its LUMO level is higher in energy than
the LUMO of the bpy ligand.4
Light-emitting electrochemical cells (LECs) based on ionic
transition-metal complexes (iTMCs) are the simplest type of
devices for lighting applications. They consist of an
amorphous film of an ionic active compound between two
electrodes.1 LECs have some disadvantages with respect to
organic light-emitting diodes (OLEDs), such as longer turn-on
times, lower lifetimes and efficiencies, but offer the advantage
of using a less reactive cathode material (aluminium instead of
calcium or magnesium) and, hence, do not require stringent
protection from environmental oxygen and water.
The reaction of pphen with [(ppy)2Ir(m-Cl)2Ir(ppy)2]5 in
CH2Cl2 at reflux followed by anion exchange yielded
[Ir(ppy)2(pphen)][PF6] in 80% yield after purification. The
highest mass peaks in the electrospray mass spectrum
(m/z 757.1) were consistent with the [Ir(ppy)2(pphen)]+ ion.
The principal problem of LECs is their limited stability,
which mainly originates from the degradation of the iTMC
active component in the emissive layer.2 Recently, we have
reported a breakthrough in the stability of iTMC-containing
LECs and achieved lifetimes of thousands of hours using
supramolecularly-caged bipyridine-based ionic iridium(III)
complexes as the single active component.3 The long lifetimes
of these devices establish LECs as a viable alternative to
OLED technology for lighting applications. In these
complexes, [IrL2(pbpy)]+ (pbpy = 6-phenyl-2,20-bipyridine;
HL = 2-phenylpyridine or Hppy and 3,5-dimethyl-1-phenyl-
pyrazole or Hdmppz), the phenyl ring of the pbpy ligand
forms face-to-face p-stacks with the cyclometallated phenyl
ring of the L ligands, resulting in supramolecular cage formation
in both the ground and the excited states that makes the
nucleophilic attack at the Ir(III) core more difficult. This
1
The solution H and 13C NMR spectra were also in accord
with this cation (see the ESIw for details).
Single crystals of [Ir(ppy)2(pphen)][PF6]ꢀ2H2O were grown
by slow diffusion of Et2O vapour into a CH2Cl2 solution of the
a Instituto de Ciencia Molecular, Universidad de Valencia,
E-46980 Paterna, Valencia, Spain. E-mail: henk.bolink@uv.es;
Fax: +34 96 354 3273; Tel: +34 96 354 4416
b Department of Chemistry, University of Basel, CH-4056 Basel,
Switzerland. E-mail: Edwin.constable@unibas.ch;
Fax: +41 61 267 1005; Tel: +41 61 267 1001
w Electronic supplementary information (ESI) available: Synthesis
and characterisation of [Ir(ppy)2(pphen)][PF6], device preparation
and characterisation and computational details. CCDC [CCDC
NUMBER(S)]. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc05084a
Fig. 1 (a) ORTEP representation of the structure of the [Ir(ppy)2-
(pphen)]+ cation present in [Ir(ppy)2(pphen)][PF6]ꢀ2H2O showing the
numbering scheme adopted; thermal ellipsoids are drawn at 40%
probability. Hydrogen atoms are omitted for clarity. (b) Representation
of the [Ir(ppy)2(pphen)]+ cation emphasising the face-to-face p-stacking
of the pendant phenyl group with one of the ppyꢁ ligands.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3207–3209 3207

View Article Online
complex. Fig. 1a depicts the structure of the [Ir(ppy)2(pphen)]+
cation. The Ir–Nppy (2.0405(17), 2.0460(18) A), Ir–Cppy
(2.0007(17), 2.014(2) A) and Ir–Npphen (2.1456(18), 2.2260(16) A)
distances closely resemble those previously reported for the
reference complex [Ir(ppy)2(phen)][PF6] (2) where phen is 1,10-
phenanthroline.4b The pendant phenyl group does not,
therefore, affect the coordination sphere of the Ir(III) core.
Fig. 1b illustrates the face-to-face p-stacking between the
pendant phenyl ring and the phenyl ring of one of the ppy
ligands; the interaction is in an optimal offset arrangement at a
separation (centroid-to-plane) of 3.31 A. This observation
confirms that the concept of supramolecular cage formation
can be extended to phenanthroline-based iridium(III)
complexes and, therefore, long-living LECs are expected.
The photophysical properties of 1 are similar to those of
complex 2. Both complexes show almost identical emissions in
MeCN solution (lexc = 355 nm) at B530 nm (Fig. S1, ESIw).
Furthermore, the photoluminescence quantum yields and the
excited-state lifetimes are similar for both complexes (Table S1,
ESIw). As expected, the photoluminescence spectra of
complexes 1 and 2 are blue-shifted compared to those
observed for the analogous bpy-based complexes.3a,4b
Fig. 2 Current density (circles) and luminance (triangles) versus time
at an applied bias of 3 V for LEC devices using 1 and 2 (top and
bottom, respectively) with an iTMC : IL molar ratio of 4 : 1.
Simple two-layer LECs were prepared to investigate the
electroluminescent properties of complex 1. Prior to the deposition
of the emissive layer, a thin layer (100 nm) of poly(3,4-ethylene-
dioxythiophene)–poly(styrenesulfonate) (PEDOT : PSS) was
spin-coated to increase the reproducibility of the devices. The
emissive layer contains complex 1 and small amounts of the ionic
liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate at
different molar ratios (1 : IL; 1 : 1 and 4 : 1). The IL is added to
reduce the turn-on time (ton) defined as the time to reach the
maximum luminance.6 Aluminium was used as the top electrode
contact (see ESIw for details). Reference LEC devices were also
prepared using complex 2.
both lifetime (t1/2, time to reach half of the maximum luminance
value) and total energy (Etot) emitted up to the time the luminance
reaches 1/5th of the maximum value for a cell area of 3 mm2,2c is
significantly higher for LECs using 1 than those employing 2
independent of the IL concentration (see Table 1). For example, at
a molar ratio of 4 : 1, LECs using 1 reach values of 230 h and
1.71 J, which are three times higher than those measured in LECs
using 2. Although the stability of LECs using 1 is significantly
improved, the enhancement is, surprisingly, not as great as has
been observed in LECs using pbpy-based complexes. The modifi-
cation of the bpy ligand by attaching a phenyl group (pbpy ligand)
increases the stability values from 70 h and 2.0 J to 1300 h and
13.6 J in LECs with [Ir(ppy)2(bpy)][PF6] and [Ir(ppy)2(pbpy)][PF6]
complexes, respectively.3a,4b In an attempt to understand why 1
does not yield long-lived LECs, density functional theory (DFT)
calculations were performed at the B3LYP/(6-31G**/LANL2DZ)
level on the [Ir(ppy)2(phen)]+ and [Ir(ppy)2(pphen)]+ cations. The
geometries of the singlet ground state (S0), the emitting (T1)
and the metal-centred (3MC) triplet excited states were fully
optimized for both complexes (see ESIw for details). Calculations
of the S0 state predict the face-to-face p-stacking (centroid-to-
plane distance of 3.54 A) observed in the solid state for
[Ir(ppy)2(pphen)]+ (Fig. 1). The intramolecular pꢁp interac-
tion is preserved for the lowest triplet excited state T1 (3.58 A).
All LECs containing 1 or 2 show the same electroluminescence
(EL) spectra (Fig. S1, ESIw) with maximum emission at B578 nm
and CIE coordinates x = 0.5084, y = 0.4862 and x = 0.5174,
y = 0.4824, respectively.7 The EL maximum is therefore
red-shifted compared to the photoluminescence spectra of the
complexes. This unexpected shift is most likely not a simple
polarity effect as it has not been observed for devices using
bpy-based iridium(III) complexes. The behaviour resembles that
observed for analogous complexes incorporating the phenan-
throline ligand, for which the shifted emission was attributed to
a change in the nature of the emitting excited state due to the high
concentration of the complex in the active layer.4a–c
The evolution of the current density and the luminance over
time for LECs using complexes 1 and 2 mixed with IL at
different ratios are depicted in Fig. 2 and summarized in
Table 1. Independent of the iTMC : IL ratio, the current
density and the luminance profiles are similar, both increasing
slowly with time, until the maximum is reached, due to the
slow movement of the small counteranions to the interface of
the electrodes that assists the injection process.1a,8
Table 1 Performance of ITO/PEDOT : PSS/iTMC : IL/Al LEC
devices at an applied bias of 3 V
Luminancec/ Efficiency/
(iTMC : IL) tona/h t1/2b/h cd mꢁ2
cd Aꢁ1
Etotd/J
1 (1 : 1)
2 (1 : 1)
1 (4 : 1)
2 (4 : 1)
0.09
0.09
13
0.37
0.33
230
73
168
101
92
8.0
9.3
5.2
5.8
0.25
0.19
1.71
0.59
LECs using 1 and 2 show similar turn-on times (ton) and
efficiencies. The main difference between the two devices lies in
the absolute value of the observed luminance and stability. The
devices incorporating 1 have significantly higher luminance values
than those using 2 (see Table 1). The stability, when expressed as
6.4
63
a
b
Time to reach the maximum luminance value. Time to reach half of the
c
maximum luminance value. Maximum value. Total emitted energy.
d
c
3208 Chem. Commun., 2011, 47, 3207–3209
This journal is The Royal Society of Chemistry 2011

View Article Online
In conclusion, a new iridium(III) complex incorporating the
novel pphen ligand has been prepared that exhibits intra-
molecular p-stacking between a pendant phenyl group and a
cyclometallated ppy ligand. This interaction results in
supramolecular cage formation in both the ground and excited
states. When the new complex is used as the active component
in LECs, the device exhibits a significant enhancement
of stability compared to LECs using the reference [Ir(ppy)2-
(phen)][PF6] complex. Additionally, from a comparison with
device stabilities observed in LECs using ionic supramolecularly-
caged bpy-based iridium(III) complexes it appears that the
3
metal–ligand bond length in the MC states is an important
stability determining factor.
Fig. 3 Top: electron density contours (0.03 e Bohrꢁ3) calculated for
the unoccupied eg molecular orbital of [Ir(ppy)2(phen)]+ showing
s-antibonding interactions along the vertical Nppy–Ir–Nppy axis.
Bottom: minimum-energy structures calculated for the 3MC states of
[Ir(ppy)2(phen)]+ (left) and [Ir(ppy)2(pphen)]+ (right). R1 and R2 are
the optimized Ir–Nppy bond length values.
We would like to thank the European Union (CELLO,
STRP 248043), the Spanish Ministry of Science and Innovation
(MICINN) (MAT2007-61584, CTQ2009-08790 and Consolider-
Ingenio CSD2007-00010), the Swiss National Science
Foundation, and the Swiss National Center of Competence
in Research ‘‘Nanoscale Science’’. R.D.C. acknowledges the
support of a FPU grant of the MICINN. Dr Silvia Schaffner is
acknowledged for the structural determination.
This makes the complex more robust in the excited emitting
state and reduces the possibility for ligand-exchange reactions
that would lead to the degradation of the complex.
Notes and references
3MC states result from the excitation of one electron from
the occupied t2g (dp) HOMO to the unoccupied eg (ds*)
orbitals of the metal9 and are assumed to be the origin of
complex instability in [Ru(bpy)3]2+ devices.2 These states are
characterized by s-antibonding interactions between the metal
and the nitrogens of the ppy ligands (Fig. 3, top). Therefore,
the population of the 3MC states leads to the elongation of the
Ir–Nppy bonds, which in the case of 2 evolve from typical
values of 2.08 A in S0 to 2.51 A as it is also found for
[Ir(ppy)2(bpy)]+,4b and to the virtual decoordination of the
two Nppy atoms (Fig. 3, bottom). The rupture of the metal–
ligand bonds and consequently the opening of the complex
1 (a) J. D. Slinker, J. Rivnay, J. S. Moskowitz, J. B. Parker, S. Bernhard,
H. D. Abruna and G. G. Malliaras, J. Mater. Chem., 2007, 17, 2976;
(b) J. Slinker, D. Bernards, P. L. Houston, H. D. Abruna, S. Bernhard
and G. G. Malliaras, Chem. Commun., 2003, 2392.
2 (a) J. D. Slinker, J.-S. Kim, S. Flores-Torres, J. H. Delcamp,
H. D. Abruna, R. H. Friend and G. G. Malliaras, J. Mater. Chem.,
2007, 17, 76; (b) L. J. Soltzberg, J. Slinker, S. Flores-Torres,
D. Bernards, G. G. Malliaras, H. D. Abruna, J. S. Kim,
R. H. Friend, M. D. Kaplan and V. Goldberg, J. Am. Chem.
Soc., 2006, 128, 7761; (c) G. Kalyuzhny, M. Buda, J. McNeill,
P. Barbara and A. J. Bard, J. Am. Chem. Soc., 2003, 125, 6272;
(d) J. Van Hounte and R. J. Watts, J. Am. Chem. Soc., 1976, 98,
4853; (e) D. W. Thompson, J. F. Wishart, B. S. Brunschwig and
N. Sutin, J. Phys. Chem. A, 2001, 105, 8117.
3 (a) H. J. Bolink, E. Coronado, R. D. Costa, E. Ortı, M. Sessolo,
´
3
enhance the degradation of the complex in the excited MC
S. Graber, K. Doyle, M. Neuburger, C. E. Housecroft and
E. C. Constable, Adv. Mater., 2008, 20, 3910; (b) S. Graber,
K. Doyle, M. Neuburger, C. E. Housecroft, E. C. Constable,
states. It is well established in pbpy-based iridium(III)
complexes that the intramolecular p-stacking prevents the
lengthening of the Ir–Nppy bond of the ppy ligand involved
in that interaction.3 The pendant phenyl ring exerts a cage
effect that restricts the opening of the structure of the complex
in the excited 3MC state. This makes the complex more robust
reducing the possibility of degradation reactions and leads to
long-lived LEC devices.3 In contrast to the pbpy-based
complexes, the molecular structure calculated for 1 in the
3MC state is similar to that computed for 2 (Fig. 3, bottom).
The p-stacking interaction is present in 1 (3.71 A) but it does
not prevent the weakening of the Ir–Nppy bonds (B2.50 A)
and the opening of the complex. This is most likely the
explanation for the lower stability observed for LECs
using complex 1 compared with that obtained for devices
incorporating pbpy-based complexes. Additionally, the 3MC
states are calculated after geometry relaxation to lie approxi-
mately 0.46 eV above the lowest energy T1 state both for 1 and 2.
Hence, the probability of degradation reactions should be similar
in both complexes. However, the attached phenyl group is a clear
steric impediment to the entrance of nucleophiles, which explains
why the stability of LEC devices based on 1 is enhanced
compared to that observed in LECs using 2.
R. D. Costa, E. Ortı
´
Soc., 2008, 130, 14944; (c) R. D. Costa, E. Ortı
, D. Repetto and H. J. Bolink, J. Am. Chem.
, H. J. Bolink,
S. Graber, C. E. Housecroft, M. Neuburger, S. Schaffner and
´
E. C. Constable, Chem. Commun., 2009, 2029; (d) R. D. Costa,
E. Ortı
E. C. Constable, Adv. Funct. Mater., 2010, 20, 1511; (e) R. D. Costa,
E. Ortı, H. J. Bolink, S. Graber, C. E. Housecroft and E. C. Constable,
´
, H. J. Bolink, S. Graber, C. E. Housecroft and
´
J. Am. Chem. Soc., 2010, 132, 5978.
4 (a) H. J. Bolink, L. Cappelli, S. Cheylan, E. Coronado, R. D. Costa,
N. Lardie
´
5032; (b) R. D. Costa, E. Ortı
s, M. K. Nazeeruddin and E. Ortı
´
´
, J. Mater. Chem., 2007, 17,
, H. J. Bolink, S. Graber, S. Schaffner,
M. Neuburger, C. E. Housecroft and E. C. Constable, Adv. Funct.
Mater., 2009, 19, 3456; (c) R. D. Costa, P. M. Viruela, H. J. Bolink and
E. Ortı, THEOCHEM, 2009, 912, 21; (d) C. Dragonetti, L. Falciola,
´
P. Mussini, S. Righetto, D. Roberto, R. Ugo, A. Valore, F. De
Angelis, S. Fantacci, A. Sgamellotti, M. Ramon and M. Muccini,
Inorg. Chem., 2007, 46, 8533.
5 M. Nonoyama, Bull. Chem. Soc.Jpn., 1974, 47, 767.
6 (a) S. T. Parker, J. Slinker, M. S. Lowry, M. P. Cox, S. Bernhard
and G. G. Malliaras, Chem. Mater., 2005, 17, 3187; (b) R. D. Costa,
A. Pertegas, E. Ortı and H. J. Bolink, Chem. Mater., 2010, 22,
´ ´
1288.
7 CIE, 1933.
8 Q. Pei, G. Yu, C. Zhang, Y. Yang and A. J. Heeger, Science, 1995,
269, 1086.
9 F. Alary, J. L. Heully, L. Bijeire and P. Vicendo, Inorg. Chem., 2007,
46, 3154.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3207–3209 3209