Emissive metallomesogens based on 2-phenylpyridine complexes of iridium(III)

Article abstract of DOI:10.1021/ja201245s

Preparation of Ir^III complexes using anisotropic 2,5-di(4-alkoxyphenyl)pyridine ligands leads to emissive, liquid-crystalline complexes containing bound Cl and dimethyl sulfoxide. Using analogous poly(alkoxy) ligands allows the preparation of bis(2-phenylpyridine)iridium(III) acac complexes, which are also mesomorphic. The observation of liquid crystallinity in octahedral complexes of this type is without precedent.(Figure Presented)

Full text of DOI:10.1021/ja201245s

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Emissive Metallomesogens Based on 2-Phenylpyridine Complexes of
Iridium(III)
Amedeo Santoro,† Anton M. Prokhorov,† Valery N. Kozhevnikov,† Adrian C. Whitwood,† Bertrand Donnio,‡
J. A. Gareth Williams,§ and Duncan W. Bruce*,†
†Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K.
‡CNRS-Universitꢀe de Strasbourg (UMR 7504), Institut de Physique et Chimie des Matꢀeriaux de Strasbourg, 23 rue du Loess BP 43,
67034 Strasbourg Cedex 2, France
§Department of Chemistry, University of Durham, Durham DH1 3LE, U.K.
S Supporting Information
b
the emission wavelength of the parent system may be tuned over
ABSTRACT: Preparation of IrIII complexes using aniso-
tropic 2,5-di(4-alkoxyphenyl)pyridine ligands leads to emis-
sive, liquid-crystalline complexes containing bound Cl and
dimethyl sulfoxide. Using analogous poly(alkoxy) ligands
allows the preparation of bis(2-phenylpyridine)iridium(III)
acac complexes, which are also mesomorphic. The observa-
tion of liquid crystallinity in octahedral complexes of this
type is without precedent.
a wide range of wavelengths.4,5
ight emission in organic light-emitting diode (OLED) de-
L
vices arises from recombination of injected charges in a
process analogous to the decay of an electronically excited state
in a molecular system. The excited state arises when the electro-
des inject an electron into the LUMO and a hole into the
HOMO, which means that there are no spin constraints on the
nature of that excited state. Statistically then, triplet states are
formed with 3 times the probability of singlet states, which places
an inherent limit of 25% on the efficiency of materials where spin
constraints allow emission (recombination) only from the singlet
state. However, in complexes of heavy transition elements, there
is extensive spinꢀorbit coupling, which can modify the spin
selection rules and allow intersystem crossing from the singlet to
the triplet state and emission from the triplet manifold, increasing
the theoretical efficiency of emission to 100%.1
However, by far the most widely studied metal complexes for
OLED applications are those based on the bis(2-phenylpyridine)
iridium(III) chromophore. Tris(2-phenylpyridine)iridium(III)
was first reported in 1985,6 and since then, its derivatives have
been studied widely7 and have indeed been applied in commer-
cial OLED displays following initial reports of their efficacy in
device configurations.8 As this is a well-studied material that has
been used in commercial mixtures, the design and preparation of
derivatives with liquid-crystalline properties is of great interest.
However, this represents a not inconsiderable challenge, for
while it is in general straightforward to realize liquid-crystalline
materials from square-planar complexes such as those of PtII, it is
an altogether different challenge with octahedral complexes.9
In considering how such materials can be designed, one must
recognize the fact that the chromophoric unit is the bis(2-
phenylpyridine)iridium(III) moiety, which means that liquid
crystals must be realized either by modification of this ligand or
by modification of the coligand(s), often an acetylacetonate,
occupying the two remaining coordination sites. Previous work
on octahedral metallomesogens has tended to show that in order
to achieve an anisotropic complex, it is normally necessary either
to have highly anisotropic ligands10 or to use several highly
substituted ligands11 [see the examples in the Supporting
Information (SI)]. However, Szerb et al.12 recently reported
With regard to desirable modifications of the components of
OLED displays, the introduction of liquid crystallinity has certain
attractions. Because liquid-crystal phases are ordered rather than
amorphous, greater charge-carrier mobility can result, while if it is
the emitting material that has liquid-crystal properties, then the
emission can be polarized.
We recently described the preparation and photophysical
properties of metallomesogens of PtII, which are emissive. Thus,
we reported the modification of 1,3-bis(2-pyridyl)benzene li-
gands to give complexes (1-n) with columnar phases, where the
emission wavelength could be controlled using the liquid-crystal
mesophase.2 We then described 2,5-di(4-alkoxyphenyl)pyridine
complexes of PtII, (2-n), which showed nematic and smectic A
phases and were very efficient emitters (Φlum = 57%).3 Both of
these systems are attractive, as previous studies have shown that
Received: February 9, 2011
Published: March 17, 2011
r
2011 American Chemical Society
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Chart 1. Molecular Structure of Complex 3
Chart 3. Absorption Spectrum [at 298 K in CH2Cl2 (black
line)] and Emission Spectra [at 298 K in CH2Cl2 (red line)
and 77 K in EPA glass (blue line)] of Complex 5 Containing
Dicatenar Ligands
Chart 2. Optical Textures of Complexes (a) 5 (155 °C on
Cooling) and (b) 7 (43 °C on Cooling)
Chart 4. X-ray Diffraction Pattern for 3 at 120 °C
the mesomorphic, cationic iridium(III) complexes [Ir(ppy)2-
(bipy0)][PF6], wherein the liquid-crystal properties were driven
by bipy0, a bis(3,4,5-trialkoxybenzoyl) ester of 4,40-dihydroxy-
methyl-2,20-bipyridine.
We therefore decided to commence an investigation using the
2,5-diphenylpyridine derivatives employed in the preparation of
complexes 2-n. Thus, we first prepared the di-μ-chlorodiiridium-
(III) dimer directly from the ligand and hydrated IrCl3; the dimer
was cleaved with dimethyl sulfoxide to give 3 and then reacted
with Naþacacꢀ to give 4.
molecules (omitted from Chart 1 for clarity) and showed the
expected trans geometry of the two pyridyl nitrogens. The two
IrꢀN distances were identical at 2.070(3) Å, as were the two
IrꢀC bond lengths [IrꢀC trans to Cl = 2.016(3) Å; IrꢀC trans
to S = 2.027(3) Å] when the estimated standard deviations are
taken into account. Interestingly, the crystal structure of the
analogous acac complex of unsubstituted 2,5-diphenylpyridine
has been determined,14 and no significant differences between
the two structures were found.
However, complex 3 is not stable over time, so cationic
complex 5 was prepared by reaction of the di-μ-chloro dimer
with AgPF6 in MeCN.15 This complex is stable and also liquid-
crystalline, showing a columnar mesophase between 145 and
163 °C (the optical texture is shown in Chart 2a). The complex is
photoluminescent in the green region of the spectrum (Chart 3;
Φlum = 1.3% and τ = 1.75 μs in CH2Cl2 at 298 K).
In liquid-crystal terms, the fact that 3 is mesomorphic is
remarkable, and there are no precedents in the literature for
structures of this kind that form liquid-crystal mesophases. The
higher-temperature mesophase of 3 is of a centered rectangular
type (c2mm, a = 60.92 Å, b = 43.1 Å; indexed in Chart 4), which
could be indicative of either a columnar rectangular phase or a
Complex 4 was emissive (λem = 540 nm; Φlum = 15%) but not
liquid-crystalline, whereas, surprisingly, complex 3 showed both
a lamellar and columnar rectangular phases. Complex 3 was also
emissive (λem = 580 nm) but inefficiently so (Φlum = 0.5%). We
were able to grow single crystals of an analogue of 3 with methoxy
chains, for which X-ray diffraction (XRD) analysis gave the
structure shown in Chart 1.13 The complex crystallized in space
group Pbcn along with some disordered CHCl3 solvent
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Chart 5. Polycatenar 2,5-Diphenylpyridine Ligands
Chart 7. X-ray Diffraction Pattern of Complex 7
Chart 6. Structures of Complexes 6ꢀ8
However, the reaction of a pentacatenar 2-phenylpyridine with
hydrated IrCl3 led to dimer 7 (Chart 6), which showed a
columnar hexagonal liquid-crystal phase from room temperature
(where it already existed in the mesophase) to 75 °C, at which
temperature it cleared. The optical texture is shown in Chart 2b
and the XRD pattern in Chart 7 (a = 27.72 Å). Unusually for such
a chloro-bridged dimer, 7 is photoluminescent, albeit rather
weakly so (λmax = 570 nm, Φlum = 0.8%, τ = 5.1 μs; see the
spectra in the SI).
Cleavage of this dimer to give the corresponding acac mono-
mer led to a glassy complex with no liquid-crystal properties, as
had been observed for the dicatenar analogue 4. Conversely, use
of a hexacatenar 2-phenylpyridine gave a di-μ-chloro dimer that
was not liquid-crystalline, but in this case its corresponding
monomeric acac complex (8) was, and a columnar hexagonal
liquid-crystal phase was observed between 31 and 66 °C (texture
in Chart 3b, XRD pattern in the SI; a = 27.54 Å). This complex
was quite strongly luminescent in the orange region of the
spectrum (λmax = 582 nm, Φlum = 9.1%). Clearly then, the
observation of liquid crystallinity is a fine balance relating to the
number and disposition of terminal alkoxy chains on the
2-phenylpyridine ligand, as is the efficiency of emission, control
of which requires more detailed investigation. Nonetheless, use
of these ligands has led to liquid-crystalline analogues of highly
desirable luminescent iridium(III) complexes with mesophases
just above room temperature, offering an exciting prospect for
future development.
ribbon phase. In common with assignments recently made by us
for mesomorphic complexes of platinum(III),16 we prefer the
ribbon phase, as the complexes are not those classically asso-
ciated with the formation of columnar phases.
Despite the unprecedented mesomorphism found in 3 and 5,
the cationic nature of complex 5 would render it incompatible
with conventional OLED architectures that require charge-
neutral complexes, while the instability of 3 would also make it
unsuitable. We therefore turned our attention to the induction of
liquid-crystalline properties in neutral, inert, heteroleptic tris-
(chelate) complexes analogous to 4.
Since the simple two-chain ligand used in complex 4 did not
induce mesomorphism, polycatenar 2,5-diphenylpyridines con-
taining four, five, or six terminal alkoxy chains were prepared
(Chart 5).17 Such an approach appeals to the design of highly
substituted, mesomorphic tris(acac)metal(III) complexes, as
mentioned above and reported in ref 11.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data (CIF),
b
structure and preparation of ligands, further photophysical data,
and selected optical spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Using these ligands produced some very interesting results in
that we were eventually able to demonstrate liquid-crystal
properties not only in a monomeric complex with acac as the
coligand but also in the precursor di-μ-chloro dimers. Thus, use
of the tetracatenar ligand led (via the di-μ-chloro dimer) to
complex 6, which, while not a liquid crystal, was a very bright
orange emitter (λmax = 586 nm, Φlum = 50%, τ = 5.2 μs).
Corresponding Author
db519@york.ac.uk
’ ACKNOWLEDGMENT
Funding from the University of York (A.S.), the EU (V.N.K.),
and the U.K. EPSRC (A.M.P., V.N.K.) is gratefully acknowledged.
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