Formation of gels and liquid crystals induced by Pt...Pt and π-π* interactions in luminescent σ-alkynyl platinum(II) terpyridine complexes

Article abstract of DOI:10.1002/anie.200604012

(Figure Presented) Well organized! A new class of phosphorescent materials was obtained by the coupling of trialkylgallate-functionalized alkynes to a planar Pt^II terpyridine unit. Pt...Pt interactions in these complexes lead to the formation of highly colored organogels and columnar liquid crystals (see picture). Amide groups linking the gallate and alkyne units provide additional stabilization of the structures through hydrogen bonding.

Full text of DOI:10.1002/anie.200604012

Angewandte
Chemie
DOI: 10.1002/anie.200604012
Self-Assembly Processes
Formation of Gels and Liquid Crystals Induced by Pt···Pt and p–p*
Interactions in Luminescent s-Alkynyl Platinum(II) Terpyridine
Complexes**
Franck Camerel, Raymond Ziessel,* Bertrand Donnio, Cyril Bourgogne, Daniel Guillon,
Marc Schmutz, Cristian Iacovita, and Jean-Pierre Bucher
Luminescent platinum–terpyridine or orthometalated bipyr-
idine complexes have engendered widespread interest as
functional materials, in particular stemming from their
intense phosphorescence in the visible region of the electro-
magnetic spectrum.[1] These stable, neutral or ionic complexes
have been widely used as electroluminescent thin films or as
dopants in organic light-emitting devices (OLEDs),[2] as DNA
intercalators,[3] and molecular probes for biological macro-
molecules.[4] In some cases, the ligand design,[5] the solvent,[6]
or the use of soluble polymers induces metal···metal and p–p
stacking interactions, which allow the tuning of the optical
properties over a large spectral range.[7] Such intermolecular
features are most useful for the engineering of liquid crystals
and organogelators, for which intermolecular interactions are
critical.[8]
by the alkynyl tethers, most of these complexes exhibit only
lamellar (SmA) and nematic mesophases. Also, the absence
of p-accepting chromophores results in non-luminescent
materials, which notably limits their practical utilization in
energy-conversion devices (OLEDs, solar cells, etc.). While
phosphorescent organogels of quinolinol platinum(II) com-
plexes have recently been characterized, there was no
indication of metal···metal interactions that might provide
an additional mechanism for the control of their properties.[11]
Thus, the construction of well-organized phosphorescent
architectures remains an important objective, with the
essential features being the need to incorporate 1) heavy
metals to favor spin–orbit coupling for phosphorescence,
2) ligand tailoring to facilitate intermolecular interaction
through hydrogen bonding (e.g. amide vectors) or p–p
interactions through polyaromatic and/or polyimine frag-
ments, and 3) d8-transition metals which are known to favor
square-planar structures, thus facilitating metal–metal inter-
actions. Here, we disclose our results on organogels and
mesomorphic materials obtained through such a strategy.
Syntheses of the pivotal ethynyl platforms 1a–c were
based on gallate-substituted derivatives with methyl, dodecyl
(C12H25), or phytol-like (C20H41) substituents.[12] The final
Rodlike metal alkynyl complexes of PdII, PtII, RhI, and
HgI exhibit thermotropic liquid-crystalline properties,[9] and
metal–poly(yne) polymers form lyotropic liquid crystals.[10]
However, as a result of the shape and anisotropy introduced
[*] Dr. F. Camerel, Dr. R. Ziessel
Laboratoire de Chimie MolØculaire
ECPM, UMR 7509
CNRS – UniversitØ Louis Pasteur
25 rue Becquerel, 67087 Strasbourg Cedex 02 (France)
Fax: (+33)3-9024-2689
E-mail: ziessel@chimie.u-strasbg.fr
Dr. B. Donnio, Dr. C. Bourgogne, Dr. D. Guillon, C. Iacovita,
Prof. J.-P. Bucher
Institut de Physique et Chimie des MatØriaux de Strasbourg
(IPCMS)
Groupe des MatØriaux Organiques (GMO), UMR 7504
CNRS – UniversitØ Louis Pasteur
23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2 (France)
Dr. M. Schmutz
Institut Charles Sadron
CNRS – UPR 22
6 rue Boussingault, 67083 Strasbourg (France)
complexes were prepared by a cross-coupling reaction under
anaerobic conditions between the terminal ethynyl deriva-
tives and [(terpy)PtCl]BF4 (terpy = 2,2’:6’,6’’-terpyridine).[13]
The reaction is catalyzed by CuI (ca. 6 mol%), and triethyl-
amine is required to quench the nascent HCl. The deep-red
complexes 2a–c were obtained in good yields after column
[**] This work was supportedby the Centre National de la Recherche
Scientifique and the Minist›re de la Recherche et des Nouvelles
Technologies. The authors thank Professor Jack Harrowfield(ISIS
Strasbourg) for carefully reading and commenting on this manu-
script prior to submission. We also thank Thomas Dombray for his
help with the synthesis of the gallate derivatives with phytol-like
substituents andStØphane Diring for his contribution in the
graphical design. We also warmly thank Johnson Matthey plc. for the
loan of precious metals.
chromatography and crystallization from
a mixture of
dichloromethane/methanol and further fully characterized
by NMR and FTIR spectroscopy, ESI mass spectrometry, and
elemental analysis.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Compounds 2a–c exhibit strong solvatochromism. Addi-
3LLCT state and that at 830 nm in pure dodecane is assigned
to a MMLCT state. On the basis of the assumption that the
3
tion of methanol (7% v/v) to the deep-red solution of 2b in
dichloromethane (lmax = 487 nm) results in the development
of a yellow color, reflected in a hypsochromic shift of 17 nm in
lmax. In dodecane, a deep-green solution forms with the
appearance of a new, strong absorption band at 644 nm, while
the original intense metal-to-ligand charge-transfer (MLCT)
band is bathochromically shifted by 11 nm (Figure 1a). An
unusual dark-green color found in apolar dodecane could be
caused by aggregation of the molecules favoring Pt···Pt, Pt···p,
and p–p* stacking interactions, we were led to investigate the
aggregation properties, in particular, their tendency to form
gels. It was our aim to provide a simple entry to soft materials
in which the local organization is driven by Pt···Pt and p–p*
interactions.
Whereas complex 2a is insoluble in alkanes, 2c is highly
soluble and no gels were produced under standard conditions.
Compound 2b, however, was found to be a good gelator of
dodecane (minimum gelation concentration, MGC =
7.5 mmolLÀ1). Dark-green gels (Figure 2a) displaying a
Figure 2. a) Gel obtainedwith 2b in dodecane (8.1 mmolLÀ1). The gel
does not flow when the test tube is turned upside down. b) TEM
image of a diluted gel of compound 2b in dodecane
(c=0.08 mmolLÀ1) showing an interconnectednetwork of fibers.
c) Zoom showing a fiber with the smallest observeddiameter (2 nm;
indicated by arrow). d) Zoom of intertwined bundles of fibers.
thermally reversible sol-to-gel phase transition were obtained
from hot dodecane solutions. Modulation of the UV/Vis and
static emission properties is feasible as a function of temper-
ature to provide a deep-red liquid above 608C that turns back
to the deep-green gelatinous state upon cooling to room
temperature (Figure S2 in the Supporting Information).
Figure 1. a) Absorption spectra of compound 2b in dodecane
(c=1.9”10À4 molLÀ1; green line), in dichloromethane
(c=1.9”10À4 mol LÀ1; redline), andin dichloromethane with 7% v/v
methanol (c=1.01”10À4 molLÀ1; yellow line). The inset shows a
photograph of the corresponding solutions from left to right. b) Emis-
sion spectrum (lex =482 nm) of compound 2b in dodecane
(c=1.9”10À4 molLÀ1) upon addition (% v/v) of a mixture of methanol
(25% v/v) in dichloromethane.
=
Infrared spectroscopy revealed that all C O and NH
functions are mutually hydrogen bonded in the gel (stretching
vibrations at 1648 and 3379 cmÀ1, respectively). These results
are in line with similar observations made with 4-methyl-3,5-
diacylaminophenyl platforms equipped with 2,2’:6’,2’’-terpyr-
idine ligands.[12a]
unusually strong near-infrared emission is observed at 830 nm
on irradiation of the dilute dodecane solution at 644 or
487 nm, and the excitation spectrum perfectly matches the
absorption spectrum over the whole absorption range. Upon
progressive addition of a mixture of methanol (25% v/v) in
dichloromethane to the dodecane solution, the phosphores-
cent band at 850 nm gradually disappears and is replaced by
an emission band at 635 nm (Figure 1b). This latter emission
band is characteristic of that found for a solution in methanol/
dichloromethane only. The strong absorption at 487 nm
Images obtained from transmission electron microscopy
(TEM) experiments on a diluted gel of compound 2b in
dodecane (0.08 mmolLÀ1) dried onto a carbon-coated copper
grid revealed the presence of an extended network of
interlocked fibers responsible for the immobilization of the
solvent (Figure 2b). The contrast of the structures is directly
related to the platinum backbone, as no staining and no metal
shadowing was applied to the sample before observation.
Thinner fibers with a width of 2 nm (Figure 2c), in close
agreement with the molecular dimension determined by
molecular modeling (see below), partially intertwine to form
bundles of fibers with a diameter of over 4 nm, thus creating
interconnected filaments (Figure 2d). At higher concentra-
tions, no significant variation of the mean diameter is found
but a higher number of interconnections are observed. The
formation of these elongated fibers indicates that the self-
ꢀ
involves mixed dp(Pt)!p*(terpy) MLCT and p(C )!p*
(terpy) ligand-to-ligand charge-transfer (LLCT) transi-
tions.[14] Likewise, the intense absorption at 644 nm may be
assigned to a metal–metal–ligand charge-transfer (MMLCT)
transition.[6] The intense emission observed at 644 nm in
3
methanol/dichloromethane is assigned to a mixed MLCT/
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À
assembly of compound 2b is driven by strong directional
intermolecular interactions such as hydrogen bonding
(observed by FTIR) and Pt···Pt interactions (observed by
UV/Vis and phosphorescence spectroscopy).
Interestingly, in the solid state, complex 2c is liquid-
crystalline over a large temperature range. All the recorded
X-ray diffraction (XRD) measurements (between 20 and
2008C) are identical and show several sharp and intense
sharp wide-angle signal (h0). The BF4 anions are located
between the complexes. Snapshots of the MD experiment
account for the excellent molecular packing of 2c in the
hexagonal columnar (Colh) phase as shown in Figure 4. The
size of the central rigid electron-rich core is around 2 nm, a
value that is very close to the smallest width of a fiber as
determined for 2b by TEM in dilute gels.
small-angle reflections with reciprocal spacings in the ratios
pffiffiffi pffiffiffi pffiffiffi pffiffiffi pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffi
1: 3: 4: 7: 9: 12: 13: 16: 19: 21 (Figure 3). These ten
Figure 4. Snapshot showing the molecular self-assembly of 2c into
columns (polar central core shown in red, tetrafluoroborate counterion
shown in blue, platinum andboron atoms shown in violet, andchains
shown in green). Left: top view; right: side view (two cells). The circle
has a diameter of 2 nm.
Figure 3. X-ray diffractogram of the Colh phase of 2c recorded at
T=1608C.
Finally, the organization of these fibrillar superstructures
on well-defined crystallographic surfaces such as mica and
highly ordered pyrolytic graphite (HOPG) was also inves-
tigated (note that the molecules were deposited on amor-
phous carbon films for the TEM investigations). A diluted gel
(0.08 mmolLÀ1) of 2b was deposited by drop-casting on a
mica substrate. As shown in Figure 5a, fibers with no
apparent orientational order and an average height of 2 nm
(see cross-section of Figure 5b) are clearly visible.
Interestingly, a diluted dodecane gel of compound 2b
deposited by spin-coating on freshly cleaved HOPG leads to
self-assembly of molecular fibers that follow mainly the
preferential crystallographic directions of the underlying
HOPG substrate (Figure 5c). The threefold symmetry of
the segment pattern is particularly visible at intercepts where
the fibers form angles of 608 and 1208 to each other. The cross-
sections show the presence of two types of fibers correspond-
ing to two different molecular heights, namely (2.0 ꢁ 0.1) nm
and (4.0 ꢁ 0.3) nm (Figure 5d). The 2-nm high fibers can be
considered to be made of single molecular layers that lie flat
on the surface (Figure S1 in the Supporting Information). The
4-nm high fibers are made of two such piled up layers, with
their alkyl chains still lying parallel to the surface. As can be
seen from the width determined by atomic force microscopy
(AFM) measurements, the fibers show a lateral extension of
several tens of nanometers. As the full lateral extension of a
single molecular wire is only about 3 nm, this result leads us to
the conclusion that the fibers are made of monolayer and
bilayer stripes that involve several molecular wires in the
lateral xy direction.
small-angle peaks are most readily assigned as the (10), (11),
(20), (21), (30), (22), (31), (40), (32), and (41) reflections, of a
hexagonal 2D lattice with the parameter a = 42.00 Š (at T=
1608C; see Table S1 in the Supporting Information). The
presence of these higher-order features indicates that the
columnar structure is very well developed and is extended
over large distances. In the wide-angle region, a broad halo,
designated as hch, at 4.9–5.1 Š and another isolated sharp
signal at 3.33 Š (h0) are observed. These scattering signals can
respectively be assigned to the liquid-like order of the
aliphatic chains in a molten state (hch), to molecular stacking
within the columns (h0), and also to short metal–metal
distances as observed in the crystalline structures of related
compounds.[5,6,15]
Comparison of the stacking periodicity along the colum-
nar axis and that along the molecular disc normal (h0 and h,
respectively) indicates that the molecules are stacked almost
perpendicular to the columnar axis (see Table S1 in the
Supporting Information).[16] Furthermore, the molecules
likely stack in an alternated fashion in order to fill the
available space between neighbors. This is also indicated by
molecular dynamics (MD) simulations in which a column
model was built from a periodic cylindrical cell with a 13.3-Š
height (4 ” 3.33 Š, corresponding to four stacked molecules),
in keeping with the experimental X-ray data. These calcu-
lations show a good space-filling of the available volume as
well as the enhancement of the microsegregation over the
entire simulation experiment time, contributing to the
cohesion of the columnar structure. Moreover, the model
showed that the cohesion of the columns is further reinforced
by Pt–Pt and Pt–alkyne interactions, as also revealed by the
The key feature of the present molecular design is the
introduction of a flat Pt(terpy) luminophore to a 3,5-
diacylamido framework that bears gallate-substituted deriv-
atives. Such highly colored complexes exhibit three out-
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Communications
137; b) D. R. McMillin, J. J.
Moore, Coord. Chem. Rev.
2002, 229, 113 – 121.
[2] W. Lu, B.-X. Mi, M. C. W.
Chan, Z. Hui, C.-M. Che, N.
Zhu, S.-T. Lee, J. Am. Chem.
Soc. 2004, 126, 4958 – 4971.
[3] G. Arena, L. Monsu Scolaro,
R. F. Pasternack, R. Romeo,
Inorg. Chem. 1995, 34, 2994 –
3002.
[4] E. M.A. Ratilla, H. M. Broth-
ers, N. M. Kostic, J. Am. Chem.
Soc. 1987, 109, 4592 – 4599.
[5] W. Lu, M. C. W. Chan, N. Zhu,
C.-M. Che, C. Li, Z. Hui, J.
Am. Chem. Soc. 2004, 126,
7639 – 7651.
[6] V. W.-W. Yam, K. M.-C. Wong,
N. Zhu, J. Am. Chem. Soc.
2002, 124, 6506 – 6507.
[7] C. Yu, K. M.-C. Wong, K. H.-
Y. Chan, V. W.-W. Yam,
Angew. Chem. 2005, 117, 801 –
804; Angew. Chem. Int. Ed.
2005, 44, 791 – 794.
[8] a) J. L. Sessler, W. B. Callaway,
S. P. Dudek, R. W. Date, D. W.
Bruce, Inorg. Chem. 2004, 43,
6650 – 6653; b) T. Cardinaels, J.
Ramaekers, D. Guillon, B.
Donnio, K. Binnemans, J. Am.
Chem. Soc. 2005, 127, 17602 –
17603; c) S. Xiao, M. Myers, Q.
Miao, S. Sanaur, K. Pang, M. L.
Figure 5. a) AFM tapping mode image (2.15 mm”2.15 mm) of a dried diluted gel of 2b in dodecane
(c=0.08 mmolLÀ1) deposited on mica. b) Cross-section along the blue bar shown in part (a). c) AFM
tapping mode image (5 mm”5 mm) of a diluted gel of compound 2b in dodecane (c=0.08 mmolLÀ1
deposited on HOPG. d) Cross-section along the blue bar shown in part (c).
)
Steigerwald,
C.
Nuckolls,
standing features: 1) relative ease of synthesis and high
Angew. Chem. 2005, 117, 7556 – 7560; Angew. Chem. Int. Ed.
2005, 44, 7390 – 7394; d) J. Wu, J. Li, U. Kolb, K. Müllen, Chem.
Commun. 2006, 48 – 50; e) L.A. Estroff, A. D. Hamilton, Chem.
Rev. 2004, 104, 1201 – 1218; f) N. M. Sangeetha, U. Maitra,
Chem. Soc. Rev. 2005, 34, 821 – 836.
stability; 2) strong solvatochromism and phosphorescence
depending on the state of matter (solution or gel); and 3) the
formation of nanofibril structures or extended columnar
mesophases by switching from linear chains in 2b to branched
chains in 2c. TEM and AFM studies confirm the presence of
linear fibrils with lengths of several hundred nanometers in
dodecane gels, with their 2-nm wide core consistent with
complexes that strongly interacting through metal···metal (at
ca. 3.6 Š) and metal···acetylide (at ca. 3.8 Š) contacts. The
deep-green color and the near-infrared emission are testi-
mony for highly aggregated complexes and effective Pt···Pt
interactions. Additionally, amide functions are engaged in
hydrogen bonding, which accounts for additional stabilization
of the entire edifice. Fiber alignment on oriented polymer
substrates would be auspicious for charge-separation devices.
Studies along these lines are currently in progress.
[9] “Metallomesogens ”: B. Donnio, D. Guillon, D. W. Bruce, R.
Deschenaux in Comprehensive Coordination Chemistry II: From
Biology to Nanotechnology, Vol. 7 (Eds.: J. A. McCleverty, T. J.
Meyer, M. Fujita, A. Powell), Elsevier, Oxford, 2003, chap. 7.9,
pp. 357 – 627.
[10] S. Takahashi, E. Murata, M. Kariya, K. Sonogashira, N.
Hagihara, Macromolecules 1979, 12, 1016 – 1018.
[11] M. Shirakawa, N. Fujita, T. Tani, K. Kanko, S. Shinkai, Chem.
Commun. 2005, 4149 – 4151.
[12] a) F. Camerel, B. Donnio, C. Bourgogne, M. Schmutz, D.
Guillon, P. Davidson, R. Ziessel, Chem. Eur. J. 2006, 12, 4261 –
4274; b) C.-Y. Liu, A. Fechtenkꢀtter, M. D. Watson, K. Müllen,
A. J. Bard, Chem. Mater. 2003, 15, 124 – 130.
[13] R. Büchner, J. S. Field, R. J. Haines, C. T. Cunningham, D. R.
McMillin, Inorg. Chem. 1997, 36, 3952 – 3956.
[14] F. Hua, S. Kinayyigit, J. R. Cable, F. N. Castellano, Inorg. Chem.
2005, 44, 471 – 473, and references therein.
[15] R. Hayoun, D. K. Zhong, A. L. Rheingold, L. H. Doerrer, Inorg.
Chem. 2006, 45, 6120 – 6122.
[16] a) D. Guillon, Struct. Bonding (Berlin) 1999, 95, 41 – 82; b) F.
Morale, R. W. Date, D. Guillon, D. W. Bruce, R. L. Finn, C.
Wilson, A. J. Blake, M. Schrꢀder, B. Donnio, Chem. Eur. J. 2003,
9, 2484 – 2501.
Received: September 29, 2006
Published online: December 15, 2006
Keywords: liquidcrystals · nitrogen heterocycles · organogels ·
.
platinum · self-assembly
[1] a) M. Hissler, J. E. McGarrah, W. B. Connick, D. K. Geiger, S. D.
Cummings, R. Eisenberg, Coord. Chem. Rev. 2000, 208, 115 –
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