Luminescent ethynyl-pyrene liquid crystals and gels for optoelectronic devices

Article abstract of DOI:10.1021/ja908061q

Two functional ethynyl-pyrene derivatives have been designed and synthesized by di- and tetrasubstitutions of bromo pyrene derivatives with N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)benzamide fragments. The photoluminescence wavelength of the pyrene cor

Full text of DOI:10.1021/ja908061q

Published on Web 11/18/2009
Luminescent Ethynyl-Pyrene Liquid Crystals and Gels for
Optoelectronic Devices
Ste´phane Diring,† Franck Camerel,† Bertrand Donnio,‡ Thierry Dintzer,§
Stefano Toffanin,| Raffaella Capelli,| Michele Muccini,*,| and Raymond Ziessel*,†
Laboratoire de Chimie Organique et Spectroscopies AVance´es (LCOSA), UMR 7515,
CNRS-ECPM, UniVersite´ de Strasbourg, 25 Rue Becquerel, 67087 Strasbourg Cedex 2, France,
Institut de Physique et Chimie des Mate´riaux de Strasbourg (IPCMS), UMR 7504,
CNRS-UniVersite´ de Strasbourg, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France,
Laboratoire des Mate´riaux, Surface et Proce´de´s pour la Catalyse (LMSPC), UMR 7515,
CNRS-ECPM, UniVersite´ de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France,
and Consiglio Nazionale delle Ricerche (CNR), Istituto per lo Studio dei Materiali
Nanostrutturati (ISMN), Via P. Gobetti 101, I-40129 Bologna, Italy
Received September 22, 2009; E-mail: ziessel@unistra.fr; m.muccini@bo.ismn.cnr.it
Abstract: Two functional ethynyl-pyrene derivatives have been designed and synthesized by di- and tetra-
substitutions of bromo pyrene derivatives with N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)benzamide
fragments. The photoluminescence wavelength of the pyrene core can be tuned by the substitution pattern
and the state of matter (solid, solution, gel, or liquid crystal). The disubstituted pyrene derivative 1 is not
mesomorphic but produces robust and highly fluorescent gels in DMF, toluene, and cyclohexane. The
well-defined fibers and ropes of the gel states were characterized by SEM and laser scanning confocal
microscopy, and extended over several micrometers. The gels were integrated as active layers in field-
effect transistors, which provided good bulk electron and hole charge mobilities as well as light emission
generation. The tetra-substituted pyrene derivative is not a gelator but displays a stable liquid crystalline
phase with 2D hexagonal symmetry between 20 and 200 °C. The pronounced luminescence properties of
the mesophase allow one to observe original mesophase textures with flower-like patterns directly by
fluorescence microscopy without crossed-polarizers.
Introduction
several hydrogenated carbon chains disposed around the outside
edge of the core. This specific organization is auspicious for
Intensive research has focused on the development of
functional soft materials through molecular self-assembly.
Landmark examples of hierarchical assemblies include micelles,
liposomes, microcapsules, dendrimers, colloidal particles, gels,
and liquid crystalline materials.1 Among these soft materials,
supramolecular gels and liquid crystalline materials are of
particular interest because they can allow to produce one-
dimensional stacks of functional molecules and are easily
processable without using technologically advanced techniques.
On the one hand, discotic liquid crystals possess the ability to
self-organize into highly anisotropic and ordered structures such
as columns. Molecules forming discotic liquid crystals have,
in most cases, a structure that consists of a flat central core with
charge separation along the stacks.2 On the other hand, gel
formation by low molecular weight organic gelator (LMWOGs)
is induced, in most cases, by the formation of a 3D network of
entangled supramolecular fibers. Self-organized one-dimensional
assemblies offer great potentials as molecular wires in opto-
electronic applications.3 A particular case demonstrates the
potential of thienylvinylene anthracene organogels to provide
single-nanofiber transistors.4
Along these lines, self-assembly of π-conjugated molecules
into linear structures has attracted much attention because of
their potential applications in photonics and optoelectronics, such
as field-effect transistors (FETs),5 organic light emitting diodes
(OLEDs),6 organic light emitting transistors (OLETs),7 and solar
cells.8 One-dimensional molecular assemblies can be constructed
† LCOSA, ECPM.
‡ IPCMS.
§ LMSPC, ECPM.
| CNR, ISMN.
(2) Schimdt-Mende, L.; Fechtenlo¨tter, A.; Mu¨llen, K.; Moons, E.; Friend,
H.; MacKenzie, J. D. Science 2001, 293, 1119.
(1) (a) Hudson, S. D.; Jung, H.-T.; Percec, V.; Cho, W.-D.; Johansson,
G.; Ungar, G.; Balagurusamy, V. S. K. Science 1997, 278, 449. (b)
Kukula, H.; Schlaad, H.; Antonietti, M.; Fo¨rster, S. J. Am. Chem. Soc.
2002, 124, 1658. (c) Tedeschi, C.; Li, L.; Mo¨hwald, H.; Spitz, C.;
von Seggern, D.; Menzel, R.; Kirstein, S. J. Am. Chem. Soc. 2004,
126, 3218. (d) Li, L.; Mo¨hwald, H. Angew. Chem., Int. Ed. 2004, 43,
360. (e) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning,
A. P. H. J. Chem. ReV. 2005, 105, 1491. (f) O’Neill, M.; Kelly, S. M.
AdV. Mater. 2003, 15, 1135. (g) Terech, P.; Weiss, R. G. Chem. ReV.
1997, 97, 3133. (h) Estroff, L. A.; Hamilton, A. D. Chem. ReV. 2004,
104, 1201. (i) Tschierske, C. Chem. Soc. ReV. 2007, 36, 1930.
(3) (a) Yuen, M.-Y.; Roy, V. A. L.; Lu, W.; Kui, S. C. F.; Tong, G. S. M.;
So, M.-H.; Chui, S. S.-Y.; Muccini, M.; Ning, J. Q.; Xu, S. J.; Che,
C.-M. Angew. Chem., Int. Ed. 2008, 47, 9895. (b) Hirst, A. R.; Escuder,
B.; Miravet, J. F.; Smith, D. K. Angew. Chem., Int. Ed. 2008, 47,
8002. (c) Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. ReV.
2007, 36, 1902. (d) Shimizu, Y.; Oikawa, K.; Nakayama, K.-I.;
Guillon, D. J. Mater. Chem. 2007, 17, 4223. (e) Kumar, S. Chem.
Soc. ReV. 2006, 35, 83.
(4) Hong, J.-P.; Um, M.-C.; Nam, S.-R.; Hong, J.-I.; Lee, S. Chem.
Commun. 2009, 310.
9
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J. AM. CHEM. SOC. 2009, 131, 18177–18185 18177

A R T I C L E S
Diring et al.
through weak intermolecular interactions such as hydrogen
bonding, π-π stacking, electrostatic interactions, van der Waals
forces, and solvophobic interactions. As a matter of fact, the
electronic performances in such electronic devices rely on the
intermolecular order in the active layer.
to stabilize molecular organizations over larger temperature
ranges and to favor the emergence of molecular self-assemblies,
for example, organo-gels or liquid crystals.16 For this purpose,
we introduced amide functions between the ethynyl-pyrene core
and the promesogenic gallic substituents to stabilize the liquid-
crystalline phases over large temperature ranges and to favor
the emergence of robust organic gels.
Pyrene is a prototypical molecule, which has high fluores-
cence quantum yield in solution and shows efficient excimer
emission.9 Pyrene and its derivatives have been extensively
applied as biological probes,10 in photonic devices,11 and as
liquid crystalline materials.12 Despite its wide use, the fact that
the absorption and emission properties of pyrene cores are
confined to the UV region constitutes a major drawback.
Recently, it has been demonstrated that the absorption and
emission characteristics of pyrene can be bathochromatically
tuned in the visible region by extending the π-conjugation upon
introduction of ethynyl functions.13 Pyrene can adequately be
functionalized in the 1,3,6,8 positions providing di- or tetra-
substitutions with ethynyl derivatives producing compounds in
a controlled manner.13 Introduction of promesogenic side groups
has allowed the synthesis of fluorescent mesomorphic matter,
and a stimuli responsive liquid crystalline material in which
changes in the photoluminescence emission were triggered by
a shear-induced phase transition.14,15
Results and Discussion
Synthesis. The target molecules were prepared by a conver-
gent protocol using 1,6-dibromopyrene17 or 1,3,6,8-tetrabro-
mopyrene18 and N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)-
benzamide with catalytic amounts of palladium(0) (Scheme 1).
The latter terminal alkyne was synthesized in two steps from
3,4,5-tris(hexadecyloxy)benzoic acid chloride and 4-trimethyl-
silylethynylaniline (see the Supporting Information). Compounds
1 and 2 were stable and obtained in good yields and high scales.
1
Their molecular structures and purity were assigned using H,
13C NMR, mass spectroscopy, and elemental analysis.
Spectroscopic Characterizations in Solution and in the
Solid State. The absorption spectra of compound 1 in diluted
dichloromethane solutions show two main absorption band peaks
at 431 and 411 nm (Table 1). The absorption bands located at
higher energies (307, 295, and 250 nm) are likely due to π-π*
and n-π* transitions localized on the alkoxyphenyl fragments
(Figure 1a). The absorption spectrum of compound 2 is similar
but shows a pronounced bathochromic shift of the main
absorption band peaks now located at 482 and 458 nm (Figure
1b). Exciting the low energy absorption bands of compound 1
leads to strong emissions at 445 nm (0-0 transition) and 473
nm (0-1 transition) with a quantum yield of 95% (Table 1 and
Figure 1a). The fluorescence spectrum shows relatively good
mirror symmetry with respect to the lowest energy absorption
transitions, confirming that the same optical transitions are
involved in both absorption and emission processes. The small
Stokes shift observed is due to substantial invariance of the
molecular geometric structure in the ground (S0) and the first
excited (S1) states, while the perfect match between the
excitation and absorption spectra points to the efficient radiative
deactivation of the excited electronic state. The fluorescence
In addition, despite their exceptional luminescence properties,
ethynyl-pyrene derivatives have not been used up to now to
provide robust and highly luminescent low molecular weight
gelators. The present contribution fills this gap and uses these
organo-gels in field-effect optoelectronic devices. To reach the
goal, we used hydrogen-bonding functions such as amide groups
(5) (a) van de Craats, A. M.; Stutzmann, N.; Bunk, O.; Nielsen, N. N.;
Watson, M.; Mu¨llen, K.; Chanzy, H. D.; Sirringhaus, H.; Friend, R. H.
AdV. Mater. 2003, 15, 495. (b) Pisula, W.; Menon, A.; Stepputat, M.;
Lieberwirth, I.; kolb, U.; Tracz, A.; Sirringhaus, H.; Pakula, T.; Mu¨llen,
K. AdV. Mater. 2005, 17, 684. (c) Kitamura, T.; Nakaso, S.; Mizoshita,
N.; Tochigi, Y.; Shimomura, T.; Moriyama, M.; Ito, K.; Kato, T. J. Am.
Chem. Soc. 2005, 127, 14769.
(6) (a) Mitschke, U.; Ba¨uerle, P. J. Mater. Chem. 2000, 10, 1471. (b)
Hassheider, T.; Benning, S. A.; Kitzerow, H.-S.; Achard, M.-F.; Bock,
H. Angew. Chem., Int. Ed. 2001, 40, 2060.
(7) (a) Muccini, M. Nature Materials, 2006, 5, 605. (b) Loi, M. A.; Da
Como, E.; Dinelli, F.; Murgia, M.; Zamboni, R.; Biscarini, F.; Muccini,
M. Nat. Mater. 2005, 4, 81.
(8) (a) Greeg, B. A.; Fox, M. A.; Bard, A. J. J. Phys. Chem. 1990, 94,
1586. (b) Fox, A. M.; Grant, J. V.; Melamed, D.; Torimoto, T.; Liu,
C.-Y.; Bard, A. J. Chem. Mater. 1998, 10, 1771. (c) Kubo, W.; Kambe,
S.; Nakade, S.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S.
J. Phys. Chem. B 2003, 107, 4374.
(13) (a) Maeda, H.; Maeda, T.; Mizuno, K.; Fujimoto, K. Chem.-Eur. J.
2006, 12, 824. (b) Venkataramana, G.; Sankararaman, S. Org. Lett.
2006, 8, 2739. (c) Venkataramana, G.; Sankararaman, S. Eur. J. Org.
Chem. 2005, 4162. (d) Zhao, Z.; Li, J.-H.; Chen, X.; Lu, P.; Yang, Y.
Org. Lett. 2008, 10, 3041. (e) Kim, H. M.; Lee, Y. O.; Lim, C. S.;
Kim, J. S.; Cho, B. R. J. Org. Chem. 2008, 73, 5127. (f) Leroy-Lhez,
S.; Fages, F. Eur. J. Org. Chem. 2005, 2684. (g) Xing, Y.; Xu, X.;
Zhang, P.; Tian, W.; Yu, G.; Lu, P.; Liu, Y.; Zhu, D. Chem. Phys.
Lett. 2005, 408, 169. (h) Zhao, Z.; Xu, X.; Jiang, Z.; Lu, P.; Yu, G.;
Liu, Y. J. Org. Chem. 2007, 72, 8345. (i) Xiao, J.; Li, J.; Li, C.; Huang,
C.; Li, Y.; Cui, S.; Wang, S.; Liu, H. Tetrahedron Lett. 2008, 49,
2656. (j) Shimizu, H.; Fujimoto, K.; Furusyo, M.; Maeda, H.; Nanai,
Y.; Mizuno, K.; Inouye, M. J. Org. Chem. 2007, 72, 1530. (k) Xiao,
J.; Xu, J.; Cui, S.; Liu, H.; Wang, S.; Li, Y. Org. Lett. 2008, 10, 645.
(l) Bernhardt, S.; Kastler, M.; Enkelmann, V.; Baumgarten, M.; Mu¨llen,
K. Chem.-Eur. J. 2006, 12, 6117.
(9) Fo¨rster, T.; Kasper, K. Z. Electrochem. 1955, 59, 976.
(10) (a) Okamoto, A.; Kanatani, K.; Saito, I. J. Am. Chem. Soc. 2004, 126,
4820. (b) Fujimoto, K.; Shimizu, H.; Inouye, M. J. Org. Chem. 2004,
69, 3271. (c) Han, M. K.; Lin, P.; Paek, D.; Harvey, J. J.; Fuior, E.;
Knutson, J. R. Biochemistry 2002, 41, 3468–3476. (d) Langenegger,
S. M.; Ha¨ner, R. Chem. Commun. 2004, 2792.
(11) (a) Otsubo, T.; Aso, Y.; Takamiya, K. J. Mater. Chem. 2002, 12, 2565.
(b) Jia, W.-L.; McCormick, T.; Liu, Q.-D.; Fukutani, H.; Motala, M.;
Wang, R.-Y.; Tao, Y.; Wang, S. J. Mater. Chem. 2004, 14, 3344. (c)
Tang, C.; Liu, F.; Xia, Y.-J.; Lin, J.; Xie, L.-H.; Zhong, G.-Y.; Fan,
Q.-L.; Huang, W. Org. Electron. 2006, 7, 155–162. (d) Oh, H.-Y.;
Lee, C.; Lee, S. Org. Electron. 2009, 10, 13.
(12) (a) Benning, S. A.; Hassheider, T.; Keuker-Baumann, S.; Bock, H.;
Della Sala, F.; Frauenheim, T.; Kitzerow, H.-S. Liq. Cryst. 2001, 28,
1105. (b) Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiy-
anovskaya, I.; Singer, K. D.; Balagurusamy, V. S. K.; Heiney, P. A.;
Schnell, I.; Rapp, A.; Spiess, H.-W.; Hudson, S. D.; Duan, H. Nature
2002, 419, 384. (c) de Halleux, V.; Calbert, J. P.; Brocorens, P.; Cornil,
J.; Declercq, J. P.; Bre´das, J. L.; Geerts, Y. AdV. Funct. Mater. 2004,
14, 649. (d) Kamikawa, Y.; Kato, T. Org. Lett. 2006, 8, 2463. (e)
Kim, Y. H.; Yoon, D. K.; Lee, E. H.; Koan, Y. K.; Jung, H.-T. J.
Phys. Chem. B 2006, 110, 20836. (f) Sienkowska, M. J.; Monobe,
H.; Kaszynski, P.; Shimizu, Y. J. Mater. Chem. 2007, 17, 1392. (g)
Sienkowska, M. J.; Farrar, J. M.; Zhang, F.; Kusuma, S.; Heiney, P. A.;
Kaszynski, P. J. Mater. Chem. 2007, 17, 1399.
(14) (a) Sagara, Y.; Kato, T. Angew. Chem., Int. Ed. 2008, 47, 5175. (b)
Kamikawa, Y.; Kato, T. Langmuir 2007, 23, 274.
(15) Hayer, A.; de Halleux, V.; Ko¨lher, A.; El-Garoughy, A.; Meijer, E. W.;
Barbera, J.; Tant, J.; Lein, J.; Lehmann, M.; Gierschner, J.; Cornil, J.;
Geerts, Y. H. J. Am. Chem. Soc. 2006, 110, 7653.
(16) Ziessel, R.; Camerel, F.; Donnio, B. Chem. Rec. 2009, 9, 1.
(17) Grimshaw, J.; Grimshaw, J. T. J. Chem. Soc., Perkin Trans. 1 1972,
1622.
(18) Kim, H. M.; Lee, Y. O.; Lim, C. S.; Kim, J. S.; Cho, B. R. J. Org.
Chem. 2008, 73, 5127.
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Luminescent Ethynyl-Pyrene Liquid Crystals and Gels
A R T I C L E S
Scheme 1a
a Reagents and conditions: (i) 1,6-dibromopyrene, N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)benzamide (2.2 equiv), n-propylamine, [Pd(PPh3)4], 60
°C, overnight, 74%; (ii) 1,3,6,8-tetrabromopyrene, N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)benzamide (6 equiv), piperidine, [Pd(PPh3)4], 90 °C, 20 h,
54%.
Table 1. Optical Data Measured in Dichloromethane Solution at
at 502 and 670 nm for 1 and 2, respectively, as compared to
the emission maxima measured in diluted solutions. For 2, the
298 K
b
krb
(108 s-1
knr
large red-shift together with the broad and featureless emission
profile is likely due to the formation of excimers in the solid
state.9,15 In the solid state, the more red-shifted PL emission
spectra of compound 2 (166 nm at peak maximum) with respect
to compounds 1 (57 nm at peak maximum) likely suggest that
the molecular arrangement of the bisubstituted compound 1 leads
to weaker intermolecular interactions that prevent the formation
of excimers. Indeed, the more resolved PL spectrum of
compound 1 in the solid state corroborates this hypothesis.
a
ΦF
λabs (nm)
ε (M-1 cm-1
)
λF (nm)
τF (ns)
)
(108 s-1
)
1
2
431
411
307
295
250
482
458
362
293
59 500
62 500
60 800
54 300
38 700
51 000
48 200
93 400
69 000
445
473
0.95
1.1
8.64
4.04
0.45
0.71
504
539
0.85
2.1
Gelation Properties. Gelation of organic solvents by low
molecular weight gelators is typified by the formation of a stable
three-dimensional, noncovalent network structure through in-
tricate intermolecular interactions, such as hydrogen bonding,
π-π stacking, and hydrophobic effect with the solvent. The
presence of a large aromatic core and amide functions on the
ambipolar compounds 1 and 2 led us to probe their gelation
abilities in various polar or nonpolar, protic or nonprotic solvents
(see Table S1). For gelation tests, 5 mg of compound was mixed
with 0.2 mL of solvent in a septum-capped vial. The mixture
was heated until the solid was completely dissolved. The
resulting clear solution was cooled in air to room temperature
and left overnight to complete the gelation. The gel formation
criteria used was to verify the absence of flowing of the solution
when the test tube was turned upside-down at room temperature.
Compound 2 was found to be deprived of gelation ability, and
precipitation always occurs in the tested solvent upon cooling
after solubilization at elevated temperatures. The presence of
four donor-acceptor functions in 2 invariably leads to greater
intermolecular interactions and to better aggregation capabilities
leading to a higher tendency toward precipitation. On the
contrary, a subtle balance between precipitation and solubili-
zation was found with compound 1. The bisamide pyrene
compound was found to be able to gel solvents such as
cyclohexane, toluene, and dimethylformamide. Transparent gels
were formed in cyclohexane, and turbid gels were formed in
toluene and DMF at 25 °C. In each case, the complete volume
of solvent is immobilized and can support its own weight
without collapsing as shown in Figure 2. The organogels
exhibited thermally reversible sol to gel phase transitions. When
a Determined in dichloromethane solution (c ≈ 1.10-6 M) using
Rhodamine 6G as reference (ΦF ) 0.78 in water, λexc ) 488 nm).19 All
ΦF values are corrected for changes in refractive index. b Calculated
using the following equations: kr ) ΦF/τF, knr ) (1 - ΦF)/τF, assuming
that the emitting state is produced with unit quantum efficiency.
decay profiles can be well-fitted by a single exponential curve,
with a fluorescence lifetime of 1.1 ns.
Excitation of compound 2 in the lowest energy bands also
leads to an intense emission centered at 504 and 539 nm with
a quantum yield of 85% (Figure 1b). The symmetry of emission
band, the fluorescence decay profile (which can be well-fitted
by a single exponential curve with fluorescence lifetime of 2.1
ns), together with the small Stokes shift observed are in
accordance with an individual singlet excited state. The emission
is red-shifted by ∼60 nm, as compared to compound 1 (Table
1). This bathochromic shift suggests an efficient delocalization
in the excited state. Introduction of two ethynyl fragments on
the pyrene unit leads to a bathochromic shift of ∼20 nm as
compared to a naked pyrene (λem ) 420 nm),9 and introduction
of four ethynyl fragments leads to a bathochromic shift of almost
100 nm. Extension of the molecular size with the introduction
of conjugated ethynyl-phenyl fragments can efficiently tune
the luminescence properties from the UV to the visible region.
Emission properties of compounds 1 and 2 in the solid state
were also evaluated directly on the powders thanks to a
spectrofluorimeter equipped with an optical fiber Figure 1
(dotted lines). Fluorescence spectra are red-shifted with maxima
(19) Olmsted, J. J. Phys. Chem. 1979, 83, 2581.
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A R T I C L E S
Diring et al.
Figure 1. (a) For 1, absorption, emission (λex ) 390 nm), and excitation (λem ) 473 nm) spectra in dichloromethane (c ) 1.0 × 10-6 mol L-1) and emission
spectrum of the solid at room temperature (λex ) 420 nm). (b) For 2, absorption, emission (λex ) 430 nm), and excitation (λem ) 538 nm) spectra in
dichloromethane (c ) 0.5 × 10-6 mol L-1) and emission spectrum of the solid at room temperature (λex ) 450 nm).
Figure 3. Emission spectra of compound 1 in cyclohexane, toluene, and
DMF gels (c ) 25 g L-1) (λex ) 450 nm).
extended toward the higher wavelengths. The shape and the full-
width-at-half-maximum (fwhm) of the emission band indicate
that in cyclohexane favorable π-π interactions between the
pyrene core and the solvent can induce higher electronic
Figure 2. (a) Gelation test of the compound 1 in DMF (5 mg/0.2 mL). (b)
Same gel observed upon UV irradiation at 365 nm.
stabilization in the fiber formation process.
The morphology of the gel structure was examined by
scanning electron microscopy (SEM). Figure 4 shows the
electron micrographs of xerogels deposited on silicon wafer from
compound 1 in cyclohexane, toluene, and DMF. The micro-
morphology of the gels is strongly dependent on the nature of
the gelating solvent. In cyclohexane, the xerogel appeared as a
3D network of interlocked thin fibers with diameter in the range
40-70 nm extending over micrometers (Figure 4a). The mean
diameter observed for the fibers (∼55 nm) is larger than the
one expected for a single molecular chain, thereby suggesting
that the xerogels are constituted of superbundles resulting from
the intertwining of several molecular chains. In toluene, a similar
3D network of fibers extending over several tens of micrometers
can be observed, but the diameter of the formed one-dimensional
assemblies is larger and is in the range 50-110 nm with a mean
diameter of 70 nm (Figure 4b). The larger size of the fibers
observed in the toluene gels accounts for the highest robustness
observed upon UV irradiation, these gels appear strongly
luminescent (Figure 2b).
Spectroscopic measurements show that the emission proper-
ties of the gel depend on the nature of the solvent used. In DMF,
the gels display a strong structured emission band centered at
500 nm (Figure 3). The 0-1 vibronic peak (500 nm) appears
more intense than the 0-0 vibronic peak (475 nm). The position
of the emission band, which is slightly red-shifted by 27 nm as
compared to the emission band in diluted solutions of compound
1, together with a well-resolved emission profile indicates the
relatively weak interactions among molecules. In toluene gels,
a well-structured emission centered at 497 nm is also observed.
Moreover, in this solvent, the 0-0 vibronic peak is more intense
than in DMF, and the 0-2 is clearly observed at 479 nm.
Interestingly, in cyclohexane gels, the emission band that is still
centered at 501 nm is less structured, broader, and more
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Luminescent Ethynyl-Pyrene Liquid Crystals and Gels
A R T I C L E S
to the molecular dimensions, and they are themselves formed
by the intertwining of several molecular chains (10-50 elemen-
tary fibers per rope).
The role of hydrogen bonding in the formation of the observed
fibers was ascertained by infrared spectroscopy in cyclohexane,
toluene, and DMF gels (25 g L-1). A single CdO stretching
band was observed at 1644, 1643, and 1659 cm-1, in cyclo-
hexane, toluene, and DMF, respectively. Furthermore, a large
NH stretching vibration at 3261 cm-1 in cyclohexane, 3265 cm-1
in toluene, and 3332 cm-1 in DMF was observed in the gelled
solvents. These spectroscopic data are an unambiguous signature
of hydrogen-bonded amides inducing a hydrogen-bonded net-
work, in line with the aggregation of the ligands into elongated
fibers (vide supra). It can also be supposed that the presence of
π-π interactions among pyrene cores, highlighted by the
fluorescence measurements, and/or among phenyl subunits might
add additional stabilization interactions. Gel formation in DMF
can be viewed as a three-step hierarchical supramolecular
assembly. First, the molecules are assembled, through hydrogen
bonding, π-π interactions, and van der Waals interactions of
the paraffin chains, into one-dimensional extended assemblies,
and then these molecular chains intertwine together to form the
elementary fibers with a mean diameter around 120 nm.
Ultimately these elementary fibers further intertwine to form
large ropes of larger diameters. The formation of these large
rope-like assemblies in the DMF gels accounts for their higher
robustness and opacity.
Laser Scanning Confocal Microscopy of the Gel of Compound
1. Laser scanning confocal microscope (LSCM) images obtained
from partially dissolved xerogel in DMF drop-cast on a
microscope coverslip are presented in Figure 5. The images
reveal a dense 3D network of greenish fluorescent cylindrical
strands, which is fairly related to the SEM images. The fiber
entanglement presents an irregular distribution of diameters and
lengths. Even though the in-plane resolution of the LSCM (∼200
nm) is lower than that of the SEM, we can distinguish at a
smaller field of view (Figure 5b) that the fibrous structure turned
out to be bundles of intertwining fibers with smaller mean
diameters.
To gain information on the photophysical properties of the
pyrene-derivative fibers at the nano scale, localized PL spec-
troscopy was performed in the regions that are delimited by
2.5 × 2.5 µm2-wide squares in Figure 5a and b. In particular,
we aim at studying a large fiber bundle in an assembly of
entangled fibers (Figure 5a) and an isolated bundle of intertwined
fibers (Figure 5b).
In both cases, the spectra profile and peak positions are nearly
invariant regardless of the spatial position of the PL measure-
ments (Figure 6). In particular, a well-defined vibronic progres-
sion is present with almost five detectable peaks. With respect
to the solution PL spectrum (Figure 1a), the vibronic progression
is dominated by the 0-1 transition, the emission broadens, and
the vibronic energy difference maintains invariant (∼0.17 eV).
Moreover, the 0-1 vibronic peak wavelength does not change,
but the vibronic progression (especially the 0-2 peak) is much
more resolved as compared to the diluted solution.
Figure 4. SEM images of xerogels of compound 1 in (a) cyclohexane, (b)
toluene, and (c) dimethylformamide deposited on silicon wafer (c ) 25 g
L-1).
of the toluene gels as compared to cyclohexane gels. In DMF,
the morphology of the gel freeze-dried on silicon wafer appeared
completely different. Gel formation in DMF arises from the
formation of a dense network of entangled fibers with diameter
in the range 500-1000 nm, which is 1 order of magnitude higher
than the two previous gels in cyclohexane and in toluene (Figure
4c). A closer look reveals that these large fibers in DMF are
formed by smaller elementary fibers (L ) 100-150 nm) (Figure
S1). The large fibers look like ropes and are formed by
the aggregation/intertwining of smaller size elementary fibers.
The mean diameter of the elementary fibers is large as compared
Differently from other pyrene-derivative molecules in the
aggregate state,20,21 the emission of compound 1 arranged in
fibers cannot be simulated successfully by the superimposition
(20) Winnik, F. M. Chem. ReV. 1993, 93, 587.
(21) Da Como, E.; Loi, M. A.; Murgia, M.; Zamboni, R.; Muccini, M.
J. Am. Chem. Soc. 2006, 128, 4277.
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A R T I C L E S
Diring et al.
Figure 5. 80 × 80 µm2 (a) and 25 × 25 µm2 (b) LCSM images of xerogels of compound 1 in DMF deposited on a microscope coverslip. Excitation
wavelength 408 nm and 60× oil-immersion microscope objective.
the PL localized spectrum of the isolated bundle (Figure 6, curve
b) shows a redistribution of the oscillator strengths within the
vibronic progression and a broadened emission.
So the spectroscopic fingerprints of localized PL spectra
correlated to the morphological features of the LSCM images
highlight the role of the single fiber bundle structural order and
that of the interactions among different bundles.
Electronic Properties in OFET Devices of the Gel Formed
with Compound 1. To evaluate the electronic properties of the
pyrene-derivative xerogel in a technologically appealing elec-
tronic device structure, top-contact/bottom gate organic field-
effect transistors (OFETs)25 with compound 1 as active layer
were fabricated using a spin-coated film of the fibers obtained
from DMF gels (see experimental section in the Supporting
Information).
Figure 6. Localized PL spectra of the xerogel of compound 1 in DMF
deposited on a microscope coverslip. The spectra labeled as (a) and (b) are
collected from the 2.5 × 2.5 µm2-wide regions in images reported in Figure
5a and b, respectively.
The collected locus electrical characteristics highlight that a
good bulky transport for both electrons and holes takes place
from source to drain electrodes (Figure 7). Indeed, the output
characteristics (not reported here) reveal an almost negligible
modulation of the drain-source current due to the application
of a gate voltage.
From the structural and morphological investigations, a
continuum of micrometer-long fibers can likely connect source
and drain electrodes, allowing charge transport across fiber
bundles without the formation of a charge accumulation layer.
Moreover, it was observed that increasing the quantity of
material forming the active layer led to an increase of the current
flowing from source to drain electrodes as it is expected because
the spin-coated film is much more uniform and a higher amount
of percolating paths are present.
of a monomer band with a vibronic progression similar to the
one of the solution PL spectrum together with broad, structure-
less, and red-shifted excimer bands. Because the progression is
dominated by the 0-1 line with a slight fwhm reduction with
respect to the solution,22,23 we hypothesize that pyrene moieties
tend to form H-type aggregates with π-π stack along the fiber
axis. In particular, it is likely that the presence of hydrogen
bonding among the molecular units allows the fiber to arrange
in a suitable 3D architecture that prevents efficient excimer
formation. Indeed, it is well-known that excimer formation
depends upon the distance between the facing molecules.24
Considering the PL spectrum of a fiber bundle in an assembly
of entangled fibers (Figure 6, curve a), the well-resolved vibronic
progression shows that the alkyl spacer gelificators localized at
the rim of the fiber columnar structure inhibit the interaction
among fibers.
The light emission collected in the high source-drain voltage
region can be attributed to a well-known diode-like process26
of exciton formation and radiative recombination in the proxim-
ity of the metal electrodes, which is typically present in field-
effect devices based on luminescent active materials.
Nonetheless, we cannot conclude that the inhomogeneous
broadening of the wide-field PL spectrum of the xerogel in DMF
(Figure 3) is due only to interbundle structural disorder because
The lack of field effect in the top-contact OFET devices could
be favored by the inhomogeneous coverage of the metallic
(25) (a) Cicoria, F.; Santato, C.; Dinelli, F.; Murgia, M.; Loi, M. A.;
Biscarini, F.; Zamboni, R.; Heremans, P.; Muccini, M. AdV. Funct.
Mater. 2005, 15, 375. (b) Gomes, H. L.; Stallinga, P.; Dinelli, F.;
Murgia, M.; Biscarini, F.; De Leeuw, D. M.; Muccini, M.; Mu¨llen,
K. Polym. AdV. Technol. 2005, 16, 227. (c) Xu, Z.-X.; Roy, V. A. L.;
Stallinga, P.; Muccini, M.; Xiang, H.-F.; Che, C.-M. Appl. Phys. Lett.
2007, 90, 223509.
(22) Oelkrug, D.; Egelhaaf, H.-J.; Gierschner, J.; Tompert, A. Synth. Met.
1996, 76, 249.
(23) Muccini, M.; Lunedei, E.; Bree, A.; Horowitz, G.; Garnier, F.; Taliani,
C. J. Chem. Phys. 1998, 108, 7327.
(24) Jun, E. J.; Won, H. N.; Kim, J. S.; Leec, K.-H.; Yoon, J. Tetrahedron
Lett. 2006, 47, 4577.
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Luminescent Ethynyl-Pyrene Liquid Crystals and Gels
A R T I C L E S
Figure 8. Diffraction pattern of compound 2 at T ) 120 °C.
heating curves and at 205 and 19 °C on the cooling curves
(Figure S3). Above 218 °C, 2 is a isotropic fluid, and the
material appears completely dark under crossed polarizers. On
cooling, the material becomes birefringent below 205 °C, and
a texture readily develops under crossed-polarizers. The ob-
served texture, which is characterized by dark, homeotropically
aligned regions and birefringent filaments (chromosome-like
motifs), indicates the formation of an optically uniaxial columnar
phase (Figure 10a and Figure S4). Shearing converts the
structured texture into an unspecific, homogeneous birefringent
texture, which indicates the fluid-like nature of the material.
Below 19 °C, no clear textural change is observed, and the
texture is hardly damaged by shearing. The high enthalpy value
measured on the DSC curves associated with this transition
indicates the formation of a crystalline phase below 19 °C.
Small-angle X-ray diffraction for compound 2 confirmed the
POM observations and DSC experiments. At low temperatures,
the X-ray patterns exhibit Bragg reflections, which sharpen and
become more intense on continuing heating (above 80 °C), until
the isotropization temperature is reached, where all of the sharp
reflections collapsed. The XRD pattern recorded at 120 °C, taken
as a representative example, is shown in Figure 8. It is
characterized by the presence of seven sharp small-angle X-ray
reflections, with reciprocal spacings in the ratio 1:ꢀ3:2:ꢀ7:3:
ꢀ12:ꢀ13. These features are most readily assigned as the (10),
(11), (20), (21), (30), (22), and (31) reflections of a hexagonal
lattice with a parameter a ) 51.7 Å (Table S3). In addition to
these fine reflections, four other, but diffuse, scatterings were
also observed. The most intense one, denoted as hch, is connected
to the molten aliphatic chains, in liquid-like conformation. The
other three diffuse scatterings were measured as h (3.6 Å), h′
(7.2 Å), and h′′ (14.2 Å, h′′ overlapping with the small angle
reflections), corresponding, respectively, to intermolecular π-π
stacking and modulation along the main axis (h′ ) 2h, h′′ )
4h). The same X-ray patterns were obtained for the other
temperatures, confirming the presence of one single Colh phase
throughout the temperature range. In addition, the lattice
parameter of the mesophase is quasi constant with temperature
(a ) 50-52 Å), implying little molecular rearrangement within
the columns, and that the variation of the molecular volume in
this temperature range is solely governed by that of the aliphatic
chains. The column formation results from the one-dimensional
stacking of these nearly disk-like molecules on top of each other
(one molecule per 3.0 Å, see supporting information) with a
continuous alternation of the molecular plane tilt, generating a
pseudohelix. This conformation is further facilitated owing to
the cross-like molecular shape, which permits the lateral arms
Figure 7. Locus characteristics and corresponding electroluminescence (EL)
of a top-contact and bottom-gate FET in p polarization (a) and n polarization
(b) having pyrene-derivative fibers from compound 1 as the active layer.
contacts on the organic film. Therefore, to prevent this issue
and check the field-effect response, bottom-contact/bottom-
gate OFETs with n-doped silicon substrate as a gate electrode,
a 300 nm thick silicon dioxide layer as a gate dielectric,
prepatterned gold source and drain electrodes, and a spin-coated
active layer were realized. Also, in this case, no modulation of
the source-drain current from the applied gate voltage was
detected.
We note that, given the bulk electrical conductivity, the optical
emission properties, as well as the morphological arrangement
of fibers in the xerogel, this material could likely be used for
the realization of fiber-based organic light-emitting diodes.
However, we do not exclude a priori that compound 1 may be
implemented in the realization of field-effect devices by means
of a proper treatment of the dielectric surface aimed at inducing
a suitable packing of the molecular units, which in principle
can enhance the charge accumulation and the field-effect charge
flow.4
Mesomorphic Behavior. Thermal behavior of the compounds
1 and 2 was investigated by polarizing optical microscopy
(POM) and differential scanning calorimetry (DSC), and the
transition temperatures and enthalpies are gathered in Table S2
and Figure S2. The hexacatenar compound 1 is not mesomorphic
and melts directly into the isotropic liquid above 177 °C.
DSC traces of the cross-like compound 2 display two
reversible thermal transitions centered at 27 and 218 °C on the
(26) (a) Santato, C.; Capelli, R.; Loi, M. A.; Murgia, M.; Cicoria, F.; Roy,
V. A. L.; Stallinga, P.; Zamboni, R.; Rost, C.; Karg, S. F.; Muccini,
M. Synth. Met. 2004, 146, 329. (b) Rost, C.; Karg, S.; Riess, W.; Loi,
M. A.; Murgia, M.; Muccini, M. Synth. Met. 2004, 146, 237.
9
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A R T I C L E S
Diring et al.
Figure 9. View of the molecular packing obtained after dynamic simulation of the columnar tilted arrangement.
to occupy the available space then generated (interdigitation),
and to interact through hydrogen bond interactions. Molecular
dynamic simulation confirms that 4 molecules of pyrene stacked
one on top of the other in a helicoidal fashion (see the
Supporting Information and Figures S5 and S6) with a periodic-
ity of 12.2 Å. Such a helical stacking of the molecules in the
column could account for the modulations observed in the X-ray
pattern, and h′ and h′′ could therefore be related to the existence
of a superlattice periodicity in the direction of the stacks of
molecules (Figure 9). Of course, such a helicoidal stacking is
short-range and not correlated between the columns.
Textural Characterizations of the Colh Phase of Compound 2
by Luminescence Microscopy. Recently, it has been demonstrated
that luminescence properties of liquid crystalline phases can be
exploited to probe the molecular organization in thin films at
the macroscopic scale by fluorescence microscopy.27,28 For these
reasons, the emission properties of compound 2 in the various
material states detected by DSC and XRD were evaluated with
the help of a fluorescence microscope equipped with a heating
stage.
Compound 2 appears highly luminescent in the solid state
upon UV irradiation in the range 300-350 nm (Figure S7). The
observed red luminescence confirms the measurements per-
formed by fluorescence spectroscopy on solid powder and
testifies the formation of aggregated species in the solid material.
The emission spectrum recorded on the solid powder at room
temperature displays a broad and unstructured emission band
centered at 675 nm (Figure 1b). At temperatures above 218 °C,
the material is in the isotropic state, and all of the molecules
are randomly distributed. The luminescence appears orange and
completely uniform inside the fluid material (Figure S8).
Observation of blue-shifted emission points to a disaggregation
process induced by the increase of the thermal motion at elevated
temperatures. It is interesting to note that the luminescence
remains pronounced even at high temperatures. The observed
orange emission (λ ) 625 nm), which is red-shifted as compared
to the emission of the isolated species in dichloromethane
solution (λ ) 504 and 539 nm), also indicates that local stacking
of the pyrene cores persists in the isotropic phase. Upon cooling
below 205 °C, the material enters in the liquid-crystalline state,
and the typical texture of the hexagonal columnar phase, with
birefringent chromosome-like domains and large homeotropic
regions (dark regions), is observed by classical POM (Figure
10a). The red luminescence observed (λem ) 675 nm) upon
irradiation at 300 < λex < 350 nm without any polarizer is not
uniform anymore, and domains with different emission can be
observed (Figure 10b). The emission of the different bright and
dark domains likely matches the texture observed in transmission
between crossed-polarizers (Figure 10c). Interestingly, the
chromosome-like motifs detected by POM are almost nonlu-
minescent, whereas, in place of the homeotropic domains,
flower-like highly luminescent motifs appear. The number of
petals of the flowers is always six, which perfectly reflects the
6-fold symmetry of the mesophase (Colh). In the homeotropic
domains, the columns are oriented perpendicular to the glass
surface, and they are arranged in a two-dimensional lattice of
hexagonal symmetry. The observation of luminescent six petals
flower-like motifs is a direct visualization of the symmetry of
the 2D organization of the columns. The sizes of the flower-
like motifs are in the range 2-8 µm, which is very large as
compared to the surface of a column. The flower-like domains
correspond approximatively to a two-dimensional assembly of
1 × 105 to 2 × 106 columns. Some flowers present a black
circular nonemissive area in the center, whereas some others
have a completely uniform luminescence. The origin of this
black circular area is not completely clear, but it can arise from
destructive emissive interferences or from a peculiar orientation
of the columnar axis with respect to the normal of the surface
or with respect to the direction of the excitation beam. The
luminescence texture observed reveals that the columns oriented
(27) (a) Camerel, F.; Bonardi, L.; Retailleau, P.; Ziessel, R. J. Am. Chem.
Soc. 2006, 128, 4548. (b) Camerel, F.; Bonardi, L.; Ulrich, G.;
Charbonnie`re, L.; Donnio, B.; Bourgogne, C.; Guillon, D.; Retailleau,
P.; Ziessel, R. Chem. Mater. 2006, 18, 5009. (c) Camerel, F.; Ulrich,
G.; Barbera, J.; Ziessel, R. Chem.-Eur. J. 2007, 13, 2189.
(28) Binnemans, K. J. Mater. Chem. 2009, 19, 448.
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Luminescent Ethynyl-Pyrene Liquid Crystals and Gels
A R T I C L E S
low molecular weight gelators, which gelled DMF, toluene, and
cyclohexane. The fluorescence is maintained in the gel, and the
xerogel is composed of interlocked thin fibers extending over
micrometer distances. Intertwining of these molecular chains
provide superbundles of larger size. Laser scanning confocal
microscopy confirms that the photoluminescence is not depend-
ent on the size of the superstructure of the fiber entanglement
and that the 3D architecture efficiently prevents excimer
formation. These robust gels were used as the active layer in
an organic field-effect transistor. Charge transport across the
fiber bundles is very effective without the formation of a charge
accumulation layer.
However, the tetrasubstituted pyrene core 2 does not give
organo-gelators, but a rather stable liquid crystalline material
whose temperature domain ranges from room temperature to
200 °C. Optical microscopy and XRD diffraction confirms the
formation a columnar mesophase with a hexagonal symmetry.
Textural characterization of the Colh mesophase was directly
observed by luminescence microscopy in the absence of cross-
polarizers. Highly fluorescent flower-like motifs having in all
cases 6 petals reflect the 6-fold symmetry of the Colh mesophase.
This remarkable luminescent texture allows one to determine
by a simple observation, the size of the domains, the nature of
the mesophase and its symmetry, and the molecular orientations
that lead to the more efficient luminescence properties. The blue-
shift of the emission observed between the mesophase and the
isotropic phase (red to orange) is in agreement with the breaking
up, at the isotropization point, of large stacks (columns) into
smaller stacks made of a few chromophores.
The charge transport and light emission measurements in
field-effect configuration demonstrate that bundles of entangled
fibers could be used as active material in optoelectronic devices.
In particular, it is envisaged that, given the bulky nature of the
charge transport, luminescent ethynyl-pyrene gels could be used
for the fabrication of OLED-like devices.
Acknowledgment. This work was jointly supported by the
University of Strasbourg through the European School of
Polymer and Materials Chemistry (ECPM) and by the Centre
National de la Recherche Scientifique (CNRS) of France. This
work was supported at Bologna by EU under project EU-FP6-
Marie Curie-035859 (BIMORE) and Grant Agreement Number
248052 (PHOTO-FET) and by Italian Ministry MIUR under
Projects FIRB-RBNE033KMA, FIRB-RBIP06JWBH (NODIS),
and FIRB RBIPO642YL (LUCI). We thank Dr. C. Bourgogne
for the modeling of the liquid crystalline structure, and Dr. Lo¨ıc
Charbonnie`re for his help in measuring the fluorescence quantum
yields.
Figure 10. Compound 2 drop-cast on a microscope coverslip and observed
by optical microscopy at 194 °C: (a) with a white light in transmission
between crossed-polarizers (symbolized by the cross in the corner of the
picture) (classical texture); (b) upon irradiation at 300 < λex < 350 nm
(emission texture without any polarizer); and (c) overlapping of (a) and
(b).
perpendicularly to the surface are more emissive than the
columns lying on the support surface (chromosome-like elon-
gated motifs). These results demonstrate that, at the molecular
scale, the excitation is more efficient when the electromagnetic
wave impinges perpendicularly to the pyrene-ethynyl core
rather than perpendicularly to the edge of the disk-like molecule.
These observations clearly demonstrate that the luminescence
properties are not uniform in the mesophase and strongly depend
on the orientation of the molecules and aggregates.
Supporting Information Available: Complete experimental
section including preparation and characterization of the syn-
thetic precursors, DSC traces of compounds 1 and 2, X-ray
crystal data of compound 2 at various temperatures and
treatments, solid-state and isotropic state emission for compound
2, additional SEM micrographs of the DMF xerogels for 1, and
molecular dynamic simulation of the mesophase and molecules
packing in the mesophase. This material is available free of
charge via the Internet at http://pubs.acs.org.
Conclusions
Pyrene substitution with rigid 4-ethynylphenylaminoacyl
derivatives provides fluorescent molecules with outstanding
features. The double substitution in compound 1 gives rise to
JA908061Q
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