Phosphorescent, terdentate, liquid-crystalline complexes of platinum(II): Stimulus-dependent emission

Article abstract of DOI:10.1002/anie.200802101

Liquid crystals shining bright. A highly efficient platinum(II) luminophore is rendered liquid crystalline using a simple and flexible synthetic approach. Ordering in the liquid-crystalline state allows monomer emission when the characteristic for the material is exciplex-like emission. More than that, emission characteristics are subject to tribological control, with the initial state re-obtained by thermal cycling. (Figure Presented).

Full text of DOI:10.1002/anie.200802101

Communications
DOI: 10.1002/anie.200802101
Metallomesogens
Phosphorescent, Terdentate, Liquid-Crystalline Complexes of
Platinum(II): Stimulus-Dependent Emission**
Valery N. Kozhevnikov,* Bertrand Donnio, and Duncan W. Bruce*
In memory of Naomi Hoshino-Miyajima
In addition to their widespread use in electro-optic displays,
liquid crystals have huge potential for application in a range
of scientific and technological areas.[1] For example, in
molecular electronics, their ability to self-organize sponta-
neously into well-defined structures gives improved morphol-
ogies, high charge-carrier mobilities, and orientation-depen-
dent effects such as polarized emission. Luminescent liquid
crystals have the potential for use in organic light-emitting
diodes (OLEDs). Although early examples were based on
purely organic materials,[2] in recent years, luminescent
metallomesogens have been reported.[3]
Indeed, phosphorescent metal complexes, in particular
bis(N,C)-coordinated IrIII [4] and N,C,N-coordinated PtII,[5] are
the best performing emitters for use in OLEDs[6,7] and the
incorporation of these luminophores into liquid-crystal sys-
tems is of interest. The square-planar geometry of PtII makes
it easier than IrIII to adapt for liquid-crystal design. Thus,
Venkatesan et al. reported columnar mesophases from suit-
ably substituted PtII complexes of 2-phenyl- and 2-thienylpyr-
idines, where the disc-like motif was generated primarily
through the use of hexacatenar b-diketonates.[8] Damm
et al.[9] and Ghedini et al.[10] have also reported luminescent
complexes of platinum(II) and palladium(II), respectively,
based on orthometallated ligands, while Camerel et al.[11] and
Cardolaccia et al.[12] reported luminescent, gel-forming PtII
complexes of terpyridines and alkynes, respectively.
allows ready access to materials disubstituted in the pyridine
5-positions. The synthesis is applied to the design of com-
plexes that represent the first examples of liquid crystals
based on this excellent PtII luminophore.
The methodology is adapted from one developed earlier
for the synthesis of pyridines[14] and, in particular, of 2,2’:6’,2’’-
terpyridines.[15] Recently, we described the use of this method-
ology in the preparation of mesomorphic phenyltriazines and
phenylpyridines[16] as well as inherently mesomorphic terpyr-
idines.[17] The design strategy adopted herein required the
preparation of hexacatenar systems with half-disc geometries
by functionalization of the 5-positions in pyridine rings with
3,4,5-trialkoxyphenyl fragments. Thus, the ligands were
prepared[14] by condensing the hydrozone oxime precursor
(1-1) with isophthalylaldehyde to give the di-N-oxide of
triazine 2-1 (Scheme 1). This was reduced to the triazine itself
Our attention was caught by the terdentate N,C,N-
coordinated PtII complexes of 1,3-dipyridylbenzenes for
which Williams and co-workers recently reported exception-
ally high quantum efficiencies.[13] Herein, we describe a new
synthetic route to functionalized 1,3-di(2-pyridyl)benzenes
that does not use cross-coupling methodologies and which
[*] Dr. V. N. Kozhevnikov, Prof. Dr. D. W. Bruce
Department of Chemistry
University of York
Scheme 1. Synthesis of ligands 3-n and 4-n, where n is the carbon
chain length of the alkyl substituents on Ar.
Heslington, York, YO10 5DD (UK)
Fax: (+44)1904-432-561
E-mail: db519@york.ac.uk
vk503@york.ac.uk
before being converted into precursors 3-1 and 4-1 in a retro
Diels–Alder reaction using either 2,5-norbornadiene or 1-
morpholinocyclopentene, respectively. Long alkyl chains
were then introduced by demethylation followed by O-
alkylation. The final complexes (5 and 6, Figure 1) were then
obtained by reacting the finished ligands with K2[PtCl4] in
AcOH.
Dr. B. Donnio
CNRS-UniversitØ Louis Pasteur (UMR 7504)
Institut de Physique et Chimie des MatØriaux de Strasbourg
23 rue du Loess BP 43, 67034 Strasbourg Cedex 2 (France)
[**] This work was supported by the EU and the EPSRC. This paper is
dedicated to the memory of a dear friend and colleague, Dr Naomi
Hoshino-Miyajima.
The ligands themselves were not liquid-crystalline and
melted directly to the isotropic fluid between 30 and 808C.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802101.
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Figure 1. Structure of the new, phosphorescent platinum(II) complexes
5-n and 6-n.
Table 1: Thermal behavior for the new platinum(II) complexes.
Compound
Transition[a]
T [8C]
DH [kJmolÀ1
]
DS [JKÀ1 molÀ1
]
5-4
5-6
Cr–Iso
254
126
187
149
167
80
91
151
81
–
–
6
20
5
21
26
8
39
61
10
39
14
25
26
12
19
Cr–Colr
Colr–Iso
Cr–Colr
Colr–Iso
Cr–Cr’
Cr’–Colr
Colr–Iso
Cr–Cr’
Cr’–Colr
Colr–Iso
Colh’–Colh
Colh–Iso
Cr–Colh’
Colh’–Colh
Colh–Iso
2.4
9.1
1.9
9.2
9.2
3.0
16.6
21.5
3.8
16.6
5.9
12.3
7.8
5.0
9.2
5-8
5-10
5-12
92
Figure 2. X-Ray diffraction pattern for a) 5-10 at 1208C and b) 6-10 at
1808C.
151
145
220
33
143
206
6-10
6-12
phase is rectangular (Colr), belonging to plane group c2mm
with a = 59.08 and b = 38.25 Š. Three, broad, wide-angle
reflections at 3.4, 4.3, and 4.7 Š are assigned to aromatic–
aromatic separation, Pt···Pt separation, and alkyl chain
separation, respectively. If unit density is assumed, then it
can further be calculated that a columnar slice of 4.3 Š
thickness with a ring–ring separation of 3.4 Š contains
approximately two molecules with antiparallel arrangement,
also consistent with the proposed Pt–Pt separation (see
Supporting Information). Contact preparations between 5-10
and other homologues confirm Colr symmetry throughout the
series (in the process of contact preparation, two samples are
brought together in their liquid crystal phase where the phase
identity for one sample is known. If the two are co-miscible,
then the phase of the second material is identified. If they are
not, then no information is obtained).
Complex 6-10 however clearly shows hexagonal symmetry
(Colh, a = 43.44 Š). In this case, a d(001) reflection is seen at
3.7 Š, a result of the greater steric demands of the fused
cyclopentene ring, whereas dPt-Pt and the molten alkyl chains
give rise to a broad reflection in the approximate range 178
< 2q < 208. The breadth of the wide-angle reflection makes
structural predictions more difficult, but the data show that at
unit density, between two and three molecules are contained
[a] The abbreviations are explained in the text.
However, coordination to PtII induced mesomorphism, sum-
marized in Table 1. Thus for complexes 5-n, the shortest-chain
homologue (5-4) simply melted to the isotropic state (Iso) at
2548C and no monotropic phases were seen on cooling. All of
the other homologues, with peripheral chains from hexyloxy
to dodecyloxy showed a columnar phase with a range of up to
618C. For complexes 6-n containing cyclopentene rings, two
mesomorphic homologues are reported, 6-10 and 6-12. The
optical textures (see Supporting Information) confirmed that
the mesophases were columnar. Comparison of the transition
temperatures between the two series shows that longer chains
are required in series 6-n before mesomorphism is observed
and that both the crystal (Cr) and liquid-crystal phases are
stabilized strongly by the introduction of the cyclopentene
group.
The mesophase symmetry for two derivatives, 5-10 and 6-
10, was examined by X-ray diffraction (Figure 2). The data
(see Supporting Information) show clearly that, in 5-10, the
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within a slice of column between 3.3 and 5 Š thick. The lower-
reveals structured monomer emission at 575 nm and 624 nm
(Figure 3a,inset shows the luminescence color). Figure 3b
however shows the photomicrograph obtained when the same
complex was cooled rapidly directly from the isotropic melt.
Here the domains are very small, which means that there is a
high concentration of (isotropic) grain boundaries; no mo-
nomer emission is observed, rather excimer-like emission
(660 nm; Figure 3b,inset shows the luminescence color). The
two different samples were investigated using X-ray diffrac-
tion, although in these experiments it was not possible exactly
to reproduce the rate of cooling in the purely optical
experiments. No difference was observed, but in fact all that
might have been expected would be a sharpening of the d(10)
and d(001) reflections to indicate different correlation
lengths. However, while by optical microscopy the observed
domains are small in the sample with the longer-wavelength
luminescence (Figure 3b), they are still large on a molecular
length scale and so even if we were to reproduce the
experiment precisely, it is unlikely that any difference would
be observed between the two samples represented in Figure 3.
The photophysical properties of PtII complexes are known
to be very sensitive to intermolecular association,[18] in
solution as well as in the crystalline state. The former relate
to dynamic processes while the latter are highly dependent on
the overall organization in the crystal lattice. Liquid crystals
present something of an intermediate situation, with mole-
cules organised dynamically into different mesophases. Thus,
in the columnar phase (Figure 3a), complexes are constrained
into an antiparallel arrangement in which they behave
independently (as monomers). However, in Figure 3b the
sample is dominated by isotropic grain boundaries in which
temperature phase, assigned as Colh’, shows the same
reflections as the Colh phase plus others that may be indexed
into plane group p6mm. Consistent with theory, the two
phases of the same symmetry are separated by a first-order
phase transition; the lattice parameter a increases to > 46 Š
at the transition. For 6-12, a melting point is observed on the
first heating scan, but cooling back to ambient temperature
leads to a glassy phase. Indeed, 6-10 does not show a melting
point and the material is obtained from solution in the Colh’
phase.
Complexes 5-n and 6-n are luminescent at room temper-
ature in dichloromethane solution and the emission spectra
(see Supporting Information) are reminiscent of those
reported for the parent complex.[11] However, the component
bands appear to be broader and quantum efficiencies lower
(f= 0.07), probably the result of more efficient non-emissive
vibronic deactivation. Compared to 5-n, complexes 6-n (with
the cyclopentene rings) show absorption and emission spectra
that are shifted hypsochromically, and emission quantum
yields that are generally greater (f= 0.12). This effect arises
because the steric effect of the cyclopentene rings does not
allow such efficient overlap of the pyridine rings with the
outer aromatic rings, raising the energy of the HOMO–
LUMO gap. Furthermore, the steric crowding (evident in a
related X-ray structure[17]) will reduce motional freedom and
hence vibronic deactivation.
In the present system, it has been observed that the same
sample of pure 5-6 can produce different emission colors
depending on the method of sample preparation—that is, the
thermal history. Thus, Figure 3a shows a photomicrograph of
the columnar mesophase of 5-6 in a glassy state. Large, well-
developed domains are seen obtained by cooling slowly from
the isotropic melt to 1708C and then rapidly to room
temperature. The emission spectrum (Figure 3a,bottom)
[
]
*
the complexes are free to adopt excimer-like structures
from which emission is observed.
Attention then turned to the production of thin films by
spin coating, where it was found that a pure film of 6-10
displayed only excimer-like emission (l = 660 nm; excitation
spectra in Supporting Information). However, after heating
the film to 1108C followed by cooling to room temperature,
there was a drastic change in emission color from the red of
the excimer to yellow, indicating simultaneous emission from
monomer and excimer (Figure 4). However, if the film is
subjected to mechanical disturbance (such as rubbing), the
red emission of the excimer returns. A further heat–cool cycle
re-establishes monomer emission. Thus, emission is under
tribological control and the initial state can readily be re-set,
suggesting re-usable, stimulus-responsive applications.
In summary, preparation of liquid-crystalline derivatives
of a highly efficient N,C,N-PtII luminophore has been
realized. Furthermore, it is found that emission in the
liquid-crystal phase is characteristic of the monomeric com-
plex, very different from the excimer-like emission that
normally characterizes non-liquid-crystalline analogues.[7]
However, it is also shown that the emission of pure films is
responsive both to method of preparation and tribological
stimulation, so that it is possible to move controllably
between monomer- and excimer-like states.
Figure 3. 5-6 at room temperature, Top: Photomicrographs (taken
between crossed polarisers), Bottom: emission spectra
(lexcitation =420 nm.), Inset:real samples of pure films sandwiched
between glass slides; a) fast cooled from the LC phase after the texture
is fully developed, b) fast cooled direct from the isotropic phase.
[*] Strictly, the term “excimer” applies only in solution. It is used here to
imply an excited dimer formed in a condensed phase.
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Experimental Section
All experimental data are found in the accompanying Supporting
Information.
Received: May 5, 2008
Published online: July 9, 2008
Keywords: liquid crystals · metallomesogens ·
.
phosphorescence · platinum
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