Chloride ion impact on materials for light-emitting electrochemical cells

Article abstract of DOI:10.1039/c3dt53229a

Small quantities of Cl^- ions result in dramatic reductions in the performance of ionic transition metal complexes in light-emitting electrochemical cells. Strong ion-pairing between aromatic protons and chloride has been established in both the

Full text of DOI:10.1039/c3dt53229a

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Chloride ion impact on materials for light-emitting
electrochemical cells†
Gabriel E. Schneider,a Henk J. Bolink,b,c Edwin C. Constable,*a Cathrin D. Ertl,a
Catherine E. Housecroft,a Antonio Pertegàs,b,c Jennifer A. Zampese,a
Andreas Kanitz,d Florian Kesslerd and Sebastian B. Meierd
Cite this: Dalton Trans., 2014, 43,
1961
Received 15th November 2013,
Accepted 27th November 2013
DOI: 10.1039/c3dt53229a
www.rsc.org/dalton
Small quantities of Cl− ions result in dramatic reductions in the
performance of ionic transition metal complexes in light-emitting
electrochemical cells. Strong ion-pairing between aromatic protons
and chloride has been established in both the solid state and
solution. X-ray structural determination of 2{[Ir(ppy)2(bpy)][Cl]}·
2CH2Cl2·[H3O]·Cl reveals the unusual nature of an impurity
encountered in the preparation of [Ir(ppy)2(bpy)][PF6].
High performance electronic devices have traditionally been
prepared by vapour deposition methods involving reactions
of ultrapure volatile compounds to generate solid state
materials.1 Solution casting offers an alternative fabrication
method in which preformed materials are deposited from the
solution phase.2 Materials of interest are usually refined to
chemical, but not electronic, standards of purity. In this com-
munication we report the dramatic effect of trace components
on the device performance of ionic iridium emitters in light-
emitting electrochemical cells (LECs) and describe an unusual
interaction of chloride with such complexes.
We routinely prepare the complex [Ir(ppy)2(bpy)][PF6] (Hppy =
2-phenylpyridine, bpy = 2,2′-bipyridine), which gives long-lived
LECs,3 by the reaction of [Ir2(ppy)4Cl2] with bpy in MeOH
under microwave heating, followed by precipitation with
excess [NH4][PF6] and multiple chromatographic purification
(Scheme 1). The preparation is readily upscaled to gram
Scheme 1 Synthesis of [Ir(ppy)2(bpy)][PF6] with labelling for NMR
spectroscopic assignments: (i) bpy, MeOH, microwave, 2 h, 120 °C;
(ii) [NH4][PF6].
quantities, but we have found batch-to-batch variability
between materials of apparently similar chemical constitution
and purity when evaluated in a solid-state LEC (see Fig. 1).4
The device configuration is similar to that previously reported.3
A
layer of poly(3,4-ethylenedioxythiophene): polystyrene-
sulfonate (PEDOT:PSS) (100 nm) was deposited on top of a
patterned indium tin oxide (ITO) substrate to increase the device
preparation yield. The complex [Ir(ppy)2(bpy)][PF6] together
with 1-butyl-3-methylimidazolium hexafluoridophosphate
[BMIM][PF6] at a molar ratio of 4 : 1 was then spin-coated from
MeCN solution to yield an 80 nm thick layer. The ionic liquid
was added to reduce the turn-on time of the device.5 The
device was completed by the deposition of 80 nm aluminium
as the top electrode contact.
aDepartment of Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel,
Switzerland. E-mail: edwin.constable@unibas.ch
The 1H NMR spectra of the materials used for the two
devices in Fig. 1 were at first sight identical, but a detailed ana-
lysis revealed that the signal assigned to proton E3 was shifted
by about 0.5 Hz between the two samples (Fig. S1†). Sub-
sequent elemental analysis indicated that the poorly perform-
bInstituto de Ciencia Molecular, Universidad de Valencia, Catedrático José Beltrán 2,
Paterna, E-46980, Spain. E-mail: henk.bolink@uv.es
cFundació General de la Universitat de Valencia (FGUV), PO Box 22085, Valencia,
Spain
dSiemens AG, Corporate Technology CT RTC MAT IEC-DE, Günther-Scharowsky-Str. 1,
91058 Erlangen, Germany
†Electronic supplementary information (ESI) available: Fig. S1: 1H NMR signals ing materials had typically 1% Cl− and 99% [PF6]− as counter-
for HE3 and HB3 in batches 1 and 2 of materials; Fig. S2: 19F NMR spectra
ion. Further purification of the poorly performing material by
for addition of [nBu4N]Cl to [Ir(ppy)2(bpy)][PF6]. Synthesis of chloride-free
precipitation with [NH4][PF6] and Ag[PF6] (to precipitate any
[Ir(ppy)2(bpy)][PF6] and of [Ir(msppy)2(6-Phbpy)][PF6] and [Ir(msppy)2(6-Phbpy)][Cl].
Cl− present as AgCl) followed by chromatography gave material
Crystallographic data for 2{[Ir(ppy)2(bpy)][Cl]}·2CH2Cl2·[H3O]·Cl and [Ir(msppy)2-
with optimal performance, confirming that the presence of small
(6-Phbpy)][Cl]. CCDC 959828 and 971737. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3dt53229a
amounts of Cl− has a profound effect on the device properties as
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Non-linear least-squares fitting of the 1H chemical shift
of E3 using WinEQNMR28 yielded a log K value of 2.24
(R = 0.001789) for this equilibrium, favouring the formation of
the chloride ion-pair. Similar small shifts, of the order of
δ 0.7 ppm are also observed in the 19F NMR spectrum upon
the addition of [nBu4N]Cl to [Ir(ppy)2(bpy)][PF6] (Fig. S2†); the
31P NMR spectrum is unchanged. As expected, the equilibrium
is also affected by the addition of additional [PF6]− (Fig. 3) and
on this basis, the chemical shift of the E3 proton can be used
as a sensitive qualitative and quantitative probe for the pres-
ence of Cl− in supposedly pure samples of [Ir(ppy)2(bpy)][PF6].
For a CD2Cl2 solution, the value of δE3 as a function of the
percentage of [Ir(ppy)2(bpy)]Cl versus [Ir(ppy)2(bpy)][PF6] is
given by:
Fig. 1 Performance of two batches of [Ir(ppy)2(bpy)][PF6] with
a
similar chemical history incorporated in a LEC device (ITO/PEDOT:PSS/
[Ir(ppy)2(bpy)][PF6] + [BMIM][PF6] (4 : 1)/Al). The performance of batch 2 is
significantly inferior due to the presence of small amounts of chloride as
counterion.
2
δE3 ¼ 8:505 þ fðꢁ0:3289 ꢂ 10ꢁ5Þð%ClÞ þ 0:007843ð%ClÞg
Not unexpectedly, the photophysical properties of the
complex depend on the counterion and pure samples of
[Ir(ppy)2(bpy)][PF6] and [Ir(ppy)2(bpy)][Cl] were studied in the
solid state and in MeCN solution (to avoid CH2Cl2 as a poten-
tial source of Cl−). In solution, the pure complexes and those
smaller counter-ions have been shown to alter the LEC
dynamics.6,7 The ESI† provides a protocol for the preparation of
chloride-free material with reproducible device performance.
In order to understand the effect of chloride ion, we have
performed 1H NMR titrations of [nBu4N]Cl into CD2Cl2 solu-
tions of pure [Ir(ppy)2(bpy)][PF6] (Fig. 2). The signal due to
proton E3 (the 3 and 3′ protons of the 2,2′-bipyridine ligand)
exhibited dramatic shifts to higher frequency upon the
addition of [nBu4N]Cl, from δ 8.505 to 9.326 ppm after the
addition of >10 equivalents. Of the remaining protons, only
the resonance for proton E4 showed a shift and this was very
much less than that observed for proton E3.
with mixed counter-ions exhibited the same properties (λem
=
600 nm, ϕ = 0.13), whereas in the solid state, [Ir(ppy)2(bpy)][PF6]
and [Ir(ppy)2(bpy)]Cl exhibited different emission maxima
(λem = 510 nm and 540 nm respectively) although the quantum
yields were similar (ϕ = 0.16).
The sensitivity of the E3 protons to the presence of Cl−
suggests a highly specific interaction of chloride ion with
these positions of the 2,2′-bipyridine rings. This position is
relatively acidic and has previously been observed to undergo
deuterium exchange in the complex [Ru(bpy)3]2+ in basic con-
ditions9 and is also the site of orthometallation in iridium10 as
well as nickel, palladium and platinum11 2,2′-bipyridine com-
plexes. As this manuscript was being prepared, related obser-
vations for specific interactions of chloride with the 3- and
3′-protons in [Ru(bpy)3]2+ derivatives were described.12 We
have observed analogous shifting of the E3 proton signals in a
wide range of [Ir(C^N)2(N^N)]+ complexes, both in CD2Cl2 and
CD3CN solutions. For example, in CD3CN, protons E3 and F3†
in [Ir(msppy)2(6-Phbpy)][PF6] resonate at δ 8.56 and 8.60 ppm,
compared to δ 8.65 and 8.68 ppm in [Ir(msppy)2(6-Phbpy)]Cl
Dichloromethane is a poorly solvating solvent in which rela-
tively tight ion-pairing between cation and anion is expected
and we have analysed the data according to the equilibrium:
f½IrðppyÞ2ðbpyÞꢀ½PF6ꢀg þ Clꢁ Ð f½IrðppyÞ2ðbpyÞꢀClg þ ½PF6ꢀꢁ
Fig. 2 The effect of adding 1.1 equivalents of [nBu4N]Cl in 0.1 aliquots
to a CD2Cl2 solution of [Ir(ppy)2(bpy)][PF6] (0.02 mmol of complex in
0.75 mL CD2Cl2). Only the resonance assigned to proton E3 is signifi-
cantly affected and a limiting shift of δ 9.326 ppm is observed after
adding >10 equivalents of [nBu4N]Cl. See Scheme 1 for labelling.
Fig. 3 Room temperature 400 MHz 1H NMR spectra of CD2Cl2 solu-
tions of (a) pure [Ir(ppy)2(bpy)][PF6], (b) [Ir(ppy)2(bpy)][PF6] with added
[nBu4N]Cl, and (c) after the addition of [nBu4N][PF6] to solution (b).
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(Hmsppy
=
2-(4-methylsulfonylphenyl)pyridine, 6-Phbpy
=
occupancy 0.5. The O⋯Cl separation of 2.818(6) Å is similar
6-phenyl-2,2′-bipyridine).† While significant, the shift in signals to, but shorter than, that observed in the crystal structure of
is less dramatic in CD3CN than in the less polar CD2Cl2. [H3O]Cl (2.95(1) Å).14 Single crystals of [Ir(msppy)2(6-Phbpy)][Cl]
The 13C NMR spectra† of [Ir(msppy)2(6-Phbpy)][PF6] and grew after diffusion of Et2O into a MeCN solution of the
[Ir(msppy)2(6-Phbpy)][Cl] show no significant differences. To complex. Fig. 4b shows the structure of the ion pair containing
understand the origin of the NMR spectroscopic changes, we the Δ-[Ir(msppy)2(6-Phbpy)]+ cation; both enantiomers are
determined the solid state structures of [Ir(msppy)2(6-Phbpy)]Cl present in the lattice. The octahedral coordination environ-
(as a representative bulk-sample chloride salt) and of the ment of atom Ir1 is similar to that in [Ir(ppy)2(bpy)]+, and
chloride-containing material present in gram-scale syntheses bond parameters are given in the caption to Fig. 4b. The Cl−
of [Ir(ppy)2(bpy)][PF6].
anion is again bound by the bpy 3- and 3′-protons (H4a and
Single crystals† of 2{[Ir(ppy)2(bpy)][Cl]}·2CH2Cl2·[H3O]·Cl H7a in Fig. 4b) with CH⋯Cl distances of 2.62 and 2.80 Å,
were obtained from solutions containing [Ir(ppy)2(bpy)][PF6] respectively, with an H4a–Cl1–H7a angle of 46.0°. Although
and [Ir(ppy)2(bpy)]Cl. The structure of one of the two enan- the CSD (v. 5.34 with three updates, Conquest v. 1.1515) con-
tiomeric {[Ir(ppy)2(bpy)]Cl} units in the lattice is shown in tains 429 examples of metal-bound 2,2′-bipyridine with Cl
Fig. 4. The [Ir(ppy)2(bpy)]+ cation closely resembles that in (either Cl− or covalently bonded Cl) hydrogen-bonded between
[Ir(ppy)2(bpy)][PF6]3 with the metal ion in a slightly distorted the 3- and 3′-protons, only nine of these are iridium com-
octahedral environment and with the expected trans-arrange- plexes; of these, only two involve Cl− ion16,17 as opposed to, for
ment of ppy− N-donor atoms. The ppy− ligands are near- example, a Cl–M unit of an adjacent molecule.
planar with angles between the least squares planes of the
phenyl and pyridine rings of 4.8 and 5.0°. The bpy ligand is
more distorted with an interannular angle of 8.7°. The Cl−
anion is chelated to the bpy 3- and 3′-protons (crystallographic
Conclusions
ally labelled H4a and H7a, Fig. 4a), consistent with solution
The presence of chloride can result in a dramatic reduction
NMR spectra, and forms hydrogen bonds13 to H4a and H7a
in the performance of the ionic transition metal complex
with CH⋯Cl distances of 2.79 and 2.87 Å, respectively,
[Ir(ppy)2(bpy)][PF6] used as the emissive component in LECs.
(C4⋯Cl1 = 3.627(4) and C7⋯Cl1 = 3.794(4) Å) and the angle
H4a–Cl1–H7a = 45.6°. This chloride ion is also hydrogen
bonded to the 4-proton of one bpy and the 5-proton of a ppy−
(see Scheme 1) of two different {[Ir(ppy)2(bpy)][Cl]} moieties
(CH⋯Cl, 2.86 and 2.68 Å, respectively). The [H3O]+Cl− unit is
disordered and has been modelled with the O and Cl atoms
(O10 and Cl10) in symmetry related sites, each of fractional
The solution 1H NMR chemical shift of the bpy 3- and
3′-protons is very sensitive to the presence of Cl− (both in
CD2Cl2 and more polar CD3CN), and in the solid state, Cl−
forms a strong chelating hydrogen bond with this position.
The tight ion-pairing in [Ir(C^N)2(N^N][Cl] means that Cl− is
easily carried through synthetic steps which start with
[Ir2(C^N)4Cl2] dimers. An HCl adduct of the salt [Ir(ppy)2(bpy)]Cl
was structurally characterized allowing clarification of the
nature of side-products isolated in gram-scale preparations of
[Ir(ppy)2(Xbpy)][PF6] materials.
Acknowledgements
We thank the Swiss National Science Foundation, University of
Basel, European Union (CELLO, STRP 248043) and European
Research Council (Advanced Grant 267816 LiLo) for financial
support. A.P. acknowledges the Spanish Ministry of Economy
and Competitiveness (MINECO) for an FPI grant. Liselotte
Siegfried, Dr Colin J. Martin and Dr Collin Morris (University
of Basel) are thanked for initial NMR titrations, help with
WinEQNMR2, and ESI-MS measurements, respectively.
Fig. 4 (a) The Δ-cation–Cl− ion pair of 2{[Ir(ppy)2(bpy)][Cl]}·
2CH2Cl2·[H3O]·[Cl] Selected bond parameters: Ir1–C11
Ir1–C22 = 2.012(3), Ir1–N4 = 2.042(3), Ir1–N3 = 2.050(3), Ir1–N2 =
2.135(3), Ir1–N1 2.142(3) Å; N1–Ir1–N2 76.5(1), N3–Ir1–C11
80.2(1), N4–Ir1–C22
80.4(1)°. (b) The Δ-cation–Cl− ion pair in
= 2.011(3),
=
=
=
Notes and references
=
[Ir(msppy)2(6-Phbpy)]Cl. Selected bond parameters: Ir1–C35 = 2.005(4),
Ir1–C23 = 2.034(4), Ir1–N1 = 2.119(3), Ir1–N2 = 2.207(3), Ir1–N3 = 2.050(3),
Ir1–N4 = 2.050(3) Å; N1–Ir1–N2 = 76.14(12), C23–Ir1–N3 = 80.33(14),
C35–Ir1–N4 = 80.43(14)°. Both are shown with 40% probability level
ellipsoids.
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