Phosphorescent mesomorphic dyads based on tetraacetylethane complexes of iridium(III)

Article abstract of DOI:10.1002/anie.201105212

A good combination: Dimeric iridium(III) luminophores with tetraacetylethane bridges led to liquid-crystalline materials with very high photoluminescence quantum yields (see picture). The same design principle allowed for the preparation of heteronuclear iridium(III)-platinum(II) dyads, which were also luminescent.

Full text of DOI:10.1002/anie.201105212

Angewandte
Chemie
DOI: 10.1002/anie.201105212
Luminescent Metallomesogens
Phosphorescent Mesomorphic Dyads Based on Tetraacetylethane
Complexes of Iridium(III)**
Anton M. Prokhorov, Amedeo Santoro, J. A. Gareth Williams, and Duncan W. Bruce*
Dedicated to Professor Hubert Le Bozec on the occasion of his 60th birthday
Complexes of iridium and platinum with aromatic ligands are
of interest as emissive materials owing to their high spin–orbit
coupling constants, which can promote phosphorescence from
triplet states, a process that is otherwise formally forbidden.
They are particularly relevant to organic light-emitting diode
(OLED) technology, where the combination of charges leads
to a surplus of triplet to singlet states, although their
application in areas such as bioimaging[1] and sensing[2] have
also received recent significant attention. Incorporation of
emissive complexes can raise the maximum internal efficiency
of an OLED to 100% by promoting triplet emission.[3,4]
Thompson and co-workers have shown that the bis(2-
phenylpyridine)iridium(III) chromophore is a very effective
triplet emitter, the physical properties of which make it
suitable for incorporation into devices,[5] and the emission
characteristics of which can be readily tuned through
substitution of the ligands.[6]
based on a tetracatenar ligand was found to be highly emissive
(F = 50%) but not liquid-crystalline, whereas the di-m-chloro
dimer, 2, of the same ligand was liquid-crystalline, but
scarcely emissive (F < 1%; Figure 1).
To add the property of liquid crystallinity to emissive
materials[7] is also of interest, for the ordered state of the
molecules in the different mesophases suggests an improved
pathway for the transport of charge carriers, whereas certain
mesophases offer the possibility of polarized emission. We
have reported liquid-crystalline Pt(II) phosphor compounds
based on 1,3-di(2-pyridyl)benzene ligands that form columnar
phases,[8] whereas those based on extended 2,5-diphenylpyr-
idines show nematic and SmA phases with very high photo-
luminescent quantum yields.[9]
More recently and following a report of a luminescent,
ionic, liquid-crystalline complex of Ir(III),[10] we reported the
first example of a charge-neutral, luminescent, mesogenic
complex of Ir(III) based on a hexacatenar phenylpyridine
(ppy) ligand, with acetylacetonate (acac) as an ancillary
ligand (1a).[11] In the same study, a related complex (1b)
Figure 1. Examples of emissive Ir(III)acac monomer 1 and mesomor-
phic di-m-chloro dimer 2.
The results led to an assumption that the liquid crystal-
linity of 2 was somehow induced by the dichloro bridge which
kept the two metal centers distant to create a molecular motif
capable of self-organisation into a columnar phase. To test
this idea and extend the system further, an article by Ma et al.
was persuasive, in which 1,1,2,2-tetraacetylethane (tae) was
used for the assembly of emissive Pt(II) dyads based mainly
on substituted 2-phenylpyridines, for example, 3 (Figure 2).[12]
It was, therefore, of interest to consider the tae ligand as a
linker in dinuclear Ir(III) dyads based on polycatenar
diphenylpyridines, to see if the increased separation of the
two metal centers offered by tae compared to bridging Cl
ligands can enhance the liquid crystallinity, while retaining the
intense emission associated with monomeric complexes. The
strategy also offers the possibility of exploring mesogenic
heterodinuclear dyads: Ir(III)–Pt(II) complexes based on the
[*] Dr. A. M. Prokhorov, Dr. A. Santoro, Prof. D. W. Bruce
Department of Chemistry, University of York
Heslington, York YO10 5DD (UK)
E-mail: duncan.bruce@york.ac.uk
Dr. J. A. G. Williams
Department of Chemistry, University of Durham
Durham DH1 3LE (UK)
Dr. A. M. Prokhorov
Department of Organic Chemistry, Ural Federal University
Mira 19, Ekaterinburg 620002 (Russia)
[**] Support from the EPSRC, the University of York and Johnson
Matthey is acknowledged gratefully.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105212.
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Figure 2. Example of a Pt(II)tae complex.
tae ligand have scarcely been considered as emitters, with a
single example in the patent literature.[13]
Thus, a series of dinuclear Ir(III) complexes was prepared
based on ppy derivatives, with tae as the bridging ligand. The
complexes, 5a–f, were obtained from a set of 2,5-diphenyl-
pyridines with different numbers of dodecyloxy chains, 4a–f
(Figure 3), by reaction with IrCl3·3H2O followed by cleavage
of the resulting di-m-chloro dimers with tae. The procedures
Figure 4. Structure of enantiomeric forms for complexes 5 with 5a as
exemplar.
Figure 3. Set of polycatenar diphenylpyridine ligands, 4 (R=C12H25).
Table 1: Thermal behavior and emission data of the Ir(III)tae complexes,
5.
used for the reactions were the same as for preparation of the
monomeric acac analogues, except that a stoichiometric
amount of tae was used to avoid the formation of a
monocoordinated complex with the tae ligand.
[{Ir(4)2}2tae][a]
Transition and T [8C]
Emission lmax [nm]
(Quantum yield F [%])[b]
5a (isomer 1)
5a (isomer 2)
5b (2:1)
Cr 153 Iso
Cr 157 Iso
541, 574 (58)
541, 574 (58)
Being derived from bis(chelate) structures, the iridium
centers are asymmetric and two asymmetric iridium centers
assembled in one molecule can produce two diastereomeric
forms: the meso form with LD-stereochemistry and the
racemate of the DD- and LL-enantiomers with identical
configurations at the metal (Figure 4).[14] In the case of di-m-
chloro dimers, only the racemic form is normally seen because
of steric hindrance,[15] while for the tae dimers, the iridium
centers are more distant and formation of both diastereomers
is possible. Indeed, 5a–f were formed as a mixture (shown by
1H NMR spectroscopy) of these two stereoisomers (Table 1)
which, in some cases (5a and 5d), could be separated by
column chromatography, although it did not prove possible to
identify which isomer was which.
Cr 74 Colh 131 Iso
Cr 82 Iso
Cr 79 Colh 126 Iso
Cr 63 Colh 95 Iso
Cr 17 Colh 61 Iso
Cr 130 Colh 147 Iso
586, 636 sh (55)
587, 637 sh (45)
588, 633 sh (43)
588, 633 sh (43)
582, 620 sh (38)
578, 625 sh (50)
5c (2:1)
5d (isomer 1)
5d (isomer 2)
5e (4:3)
5 f (4:3)
[a] Unless stated, the samples are mixtures of diastereoisomers
according to the ratios given in parentheses. [b] sh=shoulder.
The luminescence quantum yields, which are of the order
of 50%, are rather high compared to typical Ir(III)acac
complexes with simple ppy ligands,[16] and the luminescence
lifetimes, which are around 4 ms in CH2Cl2 at 298 K, are
slightly longer than those usually found for triplet-emitting
[Ir(NC)2(OO)] complexes emitting in this region (typically
1–2 ms).[17] Previously we noted elevated quantum yields and
longer lifetimes for Pt(II) complexes of 2,5-diarylpyridines,
compared to their parent and less substituted [Pt(ppy)(acac)]
analogues.[6] The trend was interpreted in terms of the
combined effects of reduced radiative and nonradiative
decay rate constants, owing to the delocalization of the
excited state over the aromatic ring at the 5-position of
All the dinuclear Ir(III) tae complexes synthesized are
intensely luminescent in solution at room temperature. Thus,
for 5a, the emission is in the green-yellow region, lmax
=
541 nm in CH2Cl2, while introduction of further alkoxy
groups shifts the emission in the other five complexes to
longer wavelengths, although there is little variation between
them, all displaying orange luminescence with lmax between
578 and 588 nm (Table 1, Figure 5). In those cases where the
racemate and meso forms could be separated from one
another, they were found to have indistinguishable photo-
physical properties.
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Angewandte
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different and sharp melting and clearing points are observed.
However, if a mixture is obtained, (5b, 5e, 5 f) then both
transitions are slightly broader, occuring over 1–28C at most.
That 5a is not mesomorphic is not surprising giving the low
coverage of the dimer periphery by alkoxy chains and so it is
with the tetracatenar ligand 4b that mesomorphism is seen
first. However, the mesomorphism is a delicate function of the
number of chains as complex 5c is not mesomorphic, while
complex 5d is, yet both are based on pentacatenar ligands. In
comparing the transition temperatures as a function of ligand,
it is generally apparent that more chains lead to lower clearing
temperatures but, for example, comparison between 5b and
5 f and between 5c and 5d shows that their mutual disposition
is clearly important. Interestingly, the clearing points of the
two isomers of 5d are significantly different (by ca. 308C).
Melting points are not compared as in most cases they refer to
mixtures.
Figure 5. Emission spectra of dinuclear complexes 5a–f in CH2Cl2 at
298 K.
Importantly, the iridium complexes also show luminescent
behavior in the columnar mesophase, which is shown in
Figure 7. The data show the relative luminescence intensity
(measured at 590 nm, lex = 475 nm) for complex 5b as it is
pyridine. This lowers the metal character in the excited state
but also renders it less well-coupled to deactivating vibra-
tional modes, such as those associated with the acac ligand.
In contrast to the monomeric acac analogues, which
mostly are not liquid-crystalline, complexes 5b, 5d, 5e, and 5 f
are mesomorphic (Table 1), showing a columnar hexagonal
mesophase. The phases were readily identified by their
characteristic optical textures (Figure 6), best exemplified
for the meso isomer of 5d, where the very characteristic fern-
like features are seen. Apparently, the bridging tae ligand
optimizes the geometry of the molecule, making it more disc-
like and inducing mesomorphism in the same way as the
bridging chlorides in 2. If separation of the racemate and
meso-forms is possible, their transition temperatures are
Figure 7. Relative emission intensity of complex 5b as a function of
temperature.
heated from the solid state, through its mesophase and into
the isotropic liquid state. Note that we cannot rule out from
these preliminary experiments that the abrupt decrease in
intensity at TCol-Iso is due to the sample flowing out of the
illuminating beam.
The work has been extended towards heterodinuclear
Ir(III)–Pt(II) complexes (6) of the same type. A complex with
an iridium fragment [Ir(4b)2] and Pt(ppy) units connected by
the tae ligand was prepared from [Ir(4b)2(m-Cl)]2 and the
monomeric [Pt(ppy)(tae)] complex (Figure 8). This hetero-
dimeric complex is not mesomorphic and simply melted to the
isotropic state on heating. However, it exhibits intense
emission (lmax = 588 nm, F = 46%, t = 4.9 ms in CH2Cl2 at
298 K; Figure 8), similar to the homometallic Ir dimer of 4b.
Irrespective of the excitation wavelength, no significant Pt-
based emission was observed at room temperature, which
would be expected at around 485 nm (for the 0–0 band) based
on the properties of [Pt(ppy)(acac)]. Clearly, under these
conditions, energy transfer from the higher-energy Pt unit to
Figure 6. Optical micrographs (on cooling) of the selected complexes:
5d, isomer 1, 1008C (top left), 5d, isomer 2, 608C (top right), 5b,
mixture of diastereoisomers, 858C (bottom left), 5 f, mixture of
diastereoisomers, 1338C (bottom right).
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state localized on the bridge. Meanwhile, a new Pt2Ir triad
incorporating bis-NC-metallating phenylpyrimidine ligands
has a high luminescence quantum yield, but the pertinent
excited state is delocalized over the entire assembly, leading
to the emission spectrum being distinctly different from that
of the constituent units (Figure 9).[19]
In conclusion, this study shows how the use of tae-bridged
metal centers can allow both liquid-crystalline properties and
high triplet luminescence efficiences to be combined within a
single molecule, opening up new potential in the design of
light-emitting devices.
Received: July 25, 2011
Revised: October 17, 2011
Published online: November 15, 2011
Keywords: iridium · liquid crystals · metallomesogens ·
.
phosphorescence · platinum
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Figure 9. Emission spectra of the Pt–Ir dimer 6 a) in CH2Cl2 at 298 K
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