Multicolour self-assembled fluorene co-oligomers: From molecules to the solid state via white-light-emitting organogels

Article abstract of DOI:10.1002/chem.200900620

Five fluorene-based co-oligomers have been prepared to study their self-assembly in a wide range of concentrations, from dilute solutions to the solid state. Subtle changes to the chemical structures, introduced to tune the emission colours over the entir

Full text of DOI:10.1002/chem.200900620

FULL PAPER
DOI: 10.1002/chem.200900620
Multicolour Self-Assembled Fluorene Co-Oligomers: From Molecules to the
Solid State via White-Light-Emitting Organogels
Robert Abbel,[a] Rob van der Weegen,[a] Wojciech Pisula,[b] Mathieu Surin,[c]
Philippe Leclꢀre,[a, c] Roberto Lazzaroni,[c] E. W. Meijer,[a] and
Albertus P. H. J. Schenning*[a]
Abstract: Five fluorene-based co-oligo-
mers have been prepared to study their
self-assembly in a wide range of con-
centrations, from dilute solutions to the
solid state. Subtle changes to the chem-
ical structures, introduced to tune the
emission colours over the entire visible
range, induce strong differences in ag-
gregation behaviour. Only two of the
fluorescent co-oligomer derivatives
self-assemble to form soluble fibrils
from which fluorescent organogels
emerge at higher concentrations. In
contrast, the other compounds form
precipitates. Mixed fluorescent co-oli-
gomer systems exhibit partial energy
transfer, which allows the creation of
white-light-emitting gels. Finally,
a
mechanism for the hierarchical self-as-
sembly of this class of materials is pro-
posed based on experimental results
and molecular modelling calculations.
Keywords:
energy transfer
·
fluorene · gels · liquid crystals ·
self-assembly
Introduction
sion colours that span the entire visible spectrum and reach
even into the near infrared region.[5] Despite these promis-
ing prospects, only a few studies that have focussed on fluo-
rene-based co-oligomers and their supramolecular chemistry
have so far been published.[6] Such studies are interesting
because they allow the effect of the chemical structure and
the supramolecular state on, for example, the optical proper-
ties to be systematically investigated.
p-Conjugated polymers and oligomers play a prominent role
in organic electronic devices.[1] The macroscopic properties
of these devices strongly depend on the chemical structure
and the morphologies of the p-conjugated materials used
and consequently a lot of research has focussed on the influ-
ence of these two factors.[2] Among the most studied classes
of p-conjugated materials are polyfluorene and its copoly-
mers.[1e,3] They are known to exhibit a richly varied phase
behaviour and supramolecular chemistry.[4] Additionally,
they allow easy band-gap tuning over a wide range when ap-
propriate co-monomers are used,[3c,d] which results in emis-
Recently we reported the synthesis and aqueous self-as-
sembly of a set of fluorene co-oligomers equipped with hy-
drophilic tetra(ethylene glycol) wedges that form fluorescent
nanoparticles with tuneable emission colours.[6e] Herein we
describe a series of five fluorene-based (co)oligomers (1–5;
Scheme 1) equipped with 3,4,5-tridodecylbenzamide wedges
at the periphery that form self-assembled structures in an
apolar solvent. The conjugated parts of the molecules con-
sist of two chiral dialkylfluorene units linked by a central ar-
omatic moiety (fluorene, naphthalene, quinoxaline, benzo-
thiadiazole or thienopyrazine) that was chosen to cover a
wide range of electron-accepting capability to vary the ab-
sorption and emission wavelengths with only minor changes
in the shape and size of the molecules. Amide linkers be-
tween the chromophores and the wedges were chosen to in-
troduce hydrogen bonding as a non-covalent ordering inter-
action. Our fluorene-based (co)oligomers can be classified
as so-called polycatenars, which are known to exhibit a rich
variety of thermotropic and lyotropic liquid crystalline (LC)
mesophases.[7,8] We have studied the self-assembly of the
[a] Dr. R. Abbel, R. van der Weegen, Dr. P. Leclꢀre,
Prof. Dr. E. W. Meijer, Dr. A. P. H. J. Schenning
Laboratory of Organic and Macromolecular Chemistry
Eindhoven University of Technology
P.O. Box 513, 5600 MB Eindhoven (The Netherlands)
Fax : (+31)40-245-1036
E-mail: a.p.h.j.schenning@tue.nl
[b] Dr. W. Pisula
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
[c] Dr. M. Surin, Dr. P. Leclꢀre, Prof. Dr. R. Lazzaroni
Chimie des Matꢁriaux Nouveaux, Universitꢁ de Mons-Hainaut
20, Place du Parc 7000 Mons (Belgium)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900620.
Chem. Eur. J. 2009, 15, 9737 – 9746
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9737

elevated temperatures, whereas
the transition temperatures of 5
were intermediate between the
values obtained for 1 and 2–4.
Upon cooling from the isotrop-
ic melt, most of the samples
showed significant undercooling
even at low cooling rates, which
is a sign of crystallisation rather
than the formation of a thermo-
tropic LC phase.[16] This conclu-
sion was further supported by
the high enthalpies of melting
(45–75 kJmolÀ1), which are
Scheme 1. Chemical structures of oligofluorenes 1–5.
polycatenars 1–5 as a function of concentration going from
dilute solution to the solid state. In dilute solution at low
temperatures, some of these oligomers exhibited the so-
called b-phase-type organisation. In concentrated solutions,
a lyotropic organogel is formed in which the emission colour
can be tuned by energy transfer to yield white emissive gels.
Such an organogel[9,10] has recently been reported for an
oligo(p-phenylenevinylene) system.[11]
Results and Discussion
Synthesis: The fluorene-based oligomers 1–5 were prepared
in a similar manner to that described earlier for fluorene-
based bolaamphiphiles (Scheme 2).[6e] Suzuki reaction of the
dibromide co-monomer derivatives[4e,12] (Br–Ar–Br, Ar=
fluorene, naphthalene, quinoxaline, benzothiadiazole or
thienopyrazine, respectively) with an aminofluorene boronic
ester[13] yielded the bis(aminofluorene) oligomers. Subse-
quent coupling of the resulting diamino chromophores with
tris(dodecyloxy)benzoic acid chloride prepared in situ from
the corresponding acid[14] led to the products 1–5. All five
compounds were purified by recycling GPC[15] and fully
characterised by 1H and 13C NMR and IR spectroscopies,
UV/Vis and photoluminescent (PL) spectrophotometry,
mass spectrometry and elemental analysis (see the Support-
ing Information).
Scheme 2. Synthesis of 1–5. Reagents and conditions: 1) dioxane/H2O,
Na2CO3, [PdACHTNUGTRNEN(UG PPh3)4], reflux, 3 d, 43-65%; 2) CH2Cl2, oxalyl chloride,
room temperature, overnight, product used in situ; 3) CH2Cl2, Et3N,
room temperature, 6 h, 52–83%.
about one order of magnitude higher than the values usually
found for LC isotropic transitions in polycatenars.[7] Slow
cooling below the isotropisation temperatures resulted in
spherulitic or fan-like textures that could not be deformed
by applying pressure, as was shown by POM. We assume
that the phase transitions observed at lower temperatures
are due to changes from one crystalline structure to another.
Presumably, the arrangements of molecules in these phases
are very similar because no changes are evident from the
POM or X-Ray scattering measurements (see below).
Solid-state phase behaviour: The phase behaviour of the dif-
ferent oligomers in the solid state was elucidated by differ-
ential scanning calorimetry (DSC), polarised optical micro-
scopy (POM), variable-temperature infrared (VT-IR) spec-
troscopy and two-dimensional wide-angle X-ray scattering
(2D-WAXS). DSC revealed that at room temperature 1–5
are crystalline solids that melt by several phase transitions
upon heating (Figure 1, Table 1 and the Supporting Informa-
tion). The temperatures of the first transition and the iso-
tropisation both depended on the chemical structures and
were the lowest for 1, which has the highest density of flexi-
ble side chains. When the two fluorene units were connected
by a benzene-type ring, as in 2–4, melting occurred at more
The role of hydrogen bonding in the solid state was eluci-
dated by VT-IR spectroscopy (see the Supporting Informa-
tion). At room temperature, the vibrations arising from
amide functionalities revealed hydrogen bonding (nNH
=
9738
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 9737 – 9746

Self-Assembled Fluorene Co-Oligomers
FULL PAPER
sion will be limited to the example of 1 and a comparison
with the other compounds will be provided thereafter.
The appearance of a high number of reflections in the
2D-WAXS pattern for 1 indicated a well-ordered bulk struc-
ture (Figure 2). After annealing at elevated temperatures
Figure 2. 2D-WAXS pattern for 1 recorded at 308C a) prior to and
b) after annealing at 1258C. On the left the extruded filament is assigned
by the three planes a, b and c. Plane c is oriented along the extrusion di-
rection, whereas a and b are distributed perpendicular to the filament
axis. In (b), Millerꢃs indexes are assigned to the intralamellar correlations,
and the white arrows indicate off-meridional reflections.
(1258C), new higher-order reflections were observed that in-
dicated an enhanced order. However, the supramolecular
organisation did not change qualitatively. The scattering
angle of the equatorial reflections yielded an orthogonal
unit cell with d spacings of a=2.10 and b=4.74 nm. The
unit cell parameter a is related to the distance orthogonal to
the chromophore long axis, whereas b correlates to the dis-
tance parallel to it. The equatorial position and the aniso-
tropic shape of these reflections indicate alignment along
the extrusion direction of the fibres. The meridional reflec-
tions (related to the c axis) revealed a period of 0.5 nm
Figure 1. a) DSC trace (heating/cooling rate 108CminÀ1
; second run;
endo up) of 1. b) Magnification of the transitions in the lower-tempera-
ture region. c) POM image (crossed polars) of 1 cooled from the isotrop-
ic melt at 1598C at a rate of 28CminÀ1
.
along the extrusion direction.
A stacking distance of
0.44 nm between the molecules was determined from the
positions of the off-meridional reflections. This value corre-
sponds well with the values typically found for p-stacked
polyfluorenes.[18] Additional off-meridional reflections relat-
ed to a d spacing of 1.0 nm suggest a more complex packing.
Interestingly, this spacing of 1.0 nm is twice the distance of
0.5 nm between individual molecules and has therefore been
attributed to a structural organisation in which every second
molecule is in an identical positional order, which suggests
an alternating arrangement of the molecules in the unit cell
(see below).[19]
Table 1. Solid-state characteristics of 1–5.
Compound
Transition temperatures [8C]
Orthorhombic unit cell
(Enthalpies[a] [kJmolÀ1])
a [nm]
b [nm]
1
2
3
4
5
31, 47 (13.2),[b] 134 (46.9)
80 (23.8), 219 (69.1)
74 (33.0), 223 (75.3)
2.10
1.85
1.80
1.76
n.d.[c]
4.74
4.44
4.89
4.85
n.d.[c]
82 (25.6), 223 (57.6)
43, 50 (21.6),[b] 186 (53.6)
[a] Only transitions with DH>2 kJmolÀ1 are considered. [b] These phase
transitions were not sufficiently resolved to be integrated separately.
Their combined DH values are therefore given. [c] n.d.=not determined.
The distribution of reflections in the 2D-WAXS patterns
for compounds 2–4 (see the Supporting Information) shows
a lamellar organisation identical to that found for 1. In all
cases, orthorhombic unit cells were determined (Table 1).
The spacing along the stacking direction could only be de-
termined for 2 and 3 and was identical to that of 1. For 2,
the corresponding reflections were too diffuse to allow a
quantitative evaluation.
Based on the WAXS and VT-IR data we have proposed a
supramolecular organisation in which the molecules of 1–4
stack on top of each other in a quasi-1D lamellar superstruc-
ture (see the Supporting Information). This organisation is
3298, nC O =1643, amide II=1536 and amide III=
=
1221 cmÀ1). Upon heating, shifts in the absorptions were ob-
served that are characteristic of decreasing hydrogen-bond
strength,[17] whereas free amide bonds were found in the iso-
tropic melt.
2D-WAXS experiments were performed by using extrud-
ed filaments containing aligned molecules of 1–4. The struc-
tural analysis revealed a qualitatively similar supramolecular
organisation in the bulk for all compounds, with differences
only in their degrees of internal order. Therefore, the discus-
Chem. Eur. J. 2009, 15, 9737 – 9746
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9739

A. P. H. J. Schenning et al.
caused by the combined effects of several non-covalent in-
teractions, such as p stacking, hydrogen bonding and micro-
phase separation between the aromatic and aliphatic moiet-
ies. The spacing of 0.5 nm along the c axis corresponds to
the intralamellar period and the additional meridional re-
flections of 1.0 nm indicate an alternating or shifted arrange-
ment of the chromophores. This arrangement is presumably
necessary to reduce steric repulsion between the branched
alkyl side chains attached to the fluorene units (see below).
visible regions of their absorption spectra. When excited at
their respective absorption maxima, the compounds emit
light covering the whole visible range, from blue (1, 2) to
green (3) and yellow (4) through to red (5), with moderate-
to-high quantum yields. These spectral features qualitatively
resemble the properties of their parent polymers[21] and
show that it is possible to easily modulate the emission
wavelength over the entire visible spectrum by using fluo-
rene co-oligomers.[6c]
Optical properties in the molecularly dissolved state: All
compounds easily dissolve in organic solvents of low polari-
ty, such as THF, toluene, dichloromethane and chloroform.
As an example of a good solvent, the spectral properties in
chloroform (Figure 3 and Table 2) will be discussed. The
compounds show maxima in different parts of the UV and
Self-assembly in an apolar solvent: Methylcyclohexane
(MCH) is known to be a poor solvent for poly(dialkylfluor-
ene)s,[22] which is why it was chosen to study the self-assem-
bly of 1–5. At sufficiently low concentrations (10 mm and
below) the compounds were molecularly dissolved in MCH
at room temperature. Although none of the absorption spec-
tra changed significantly on changing the solvent from
chloroform to MCH, the emission maxima of 1–5 shifted to
the blue (<10 nm for 1 and 2 and ca. 30 nm for 3–5,
Table 3), most likely due to solvatochromism.[23] These sol-
Table 3. Spectral properties of dilute solutions of 1–5 in MCH (in the mm
range) at room temperature.
Compound
UV absorption
lmax [nm]
Photoluminescence
e [mÀ1 cmÀ1
]
lmax [nm]
fPL [%]
1
2
3
4
5
362
338
331, 389
337, 431
366, 528
123000
83200
85200, 34600
93300, 36400
72400, 12900
402, 424
420
483
529
649
84[a]
60[a]
48[a]
71[b]
35[c]
Reference compounds for quantum yield determination (see ref. [20]):
[a] Quinine bisulfate in 1n H2SO4. [b] N,N’-Bis(pentylhexyl)perylenebis-
imide in dichloromethane. [c] Rhodamine 101 in ethanol.
vent effects were significantly larger for those compounds
containing a more polar central unit. This might be due to
the greater differences in the solvation of the ground and
excited states when a more polar solvent is used. Remarka-
bly, the choice of solvent also had a strong influence on
some of the fluorescence quantum yields, which increased
by up to a factor of 10 in MCH for 1. No circular or linear
dichroic (CD or LD) effects were found in dilute (<10 mm)
solutions at room temperature.
Figure 3. a) UV/Vis absorption and b) photoluminescence spectra of
dilute (mm range) solutions of 1–5 in chloroform. The PL spectra are
weighted according to the respective quantum yields.
Because the molecules were molecularly dissolved at
room temperature, we slowly cooled the solutions of 1–5 to
induce self-assembly. Upon cooling, spectral changes were
observed in both the absorption and emission spectra (Figure
4a,b). Precipitation was observed for compounds 2–4, which
also had the highest melting temperatures in the bulk (see
above).[24] Both observations may reflect distinct differences
in the p-stacking abilities in the molecules. Because this pre-
cipitation precluded a reproducible investigation, we will
confine the following discussion to compounds 1 and 5. In
the case of 1, sharpening of the p–p* absorption band and
the emergence of a distinct vibronic splitting pattern was ob-
served. Furthermore, the peak maxima were shifted to the
red, whereas the onset of absorption hardly changed. Simi-
Table 2. Spectral properties of dilute solutions of 1–5 in chloroform (in
the mm range) at room temperature.
Compounds
Absorption
lmax [nm]
Photoluminescence
e [mÀ1 cmÀ1
]
lmax [nm]
fPL [%]
1
2
3
4
5
366
342
332, 385
338, 427
367, 520
123000
89100
91200, 36300
79400, 30900
70800, 11200
405, 426
423
515
563
675
8[a]
10[a]
55[a]
70[b]
10[c]
Reference compounds for quantum yield determination (see ref. [20]):
[a] Quinine bisulfate in 1n H2SO4. [b] N,N’-Bis(pentylhexyl)perylenebis-
imide in dichloromethane. [c] Rhodamine 101 in ethanol.
9740
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 9737 – 9746

Self-Assembled Fluorene Co-Oligomers
FULL PAPER
(Figure 4e). Again CD and LD
effects were observed upon
cooling (see the Supporting In-
formation),
which
suggests
comparable aggregation and
conformation changes to those
of 1.
At a concentration of 2.5 mm,
1 and 5 formed stable blue and
red fluorescent organogels at
room temperature, whereas
precipitation was found for all
other compounds. The gels ex-
hibited birefringence viewed
under crossed polars, which
points to a lyotropic organisa-
tion (Figure 5). In the case of 1,
upon increasing the concentra-
tion from 0.25 to 5 mm, a low-
energy shoulder in the absorp-
tion spectrum and a redshift of
around 5 nm in the photolumi-
nescence spectrum were ob-
served. Remarkably, compared
with a dilute solution at room
temperature, neither the ab-
sorption nor the fluorescence
spectrum of 5 was significantly
affected on increasing the con-
centration, even though a gel
was formed. In the case of 1,
the fraction of aggregated mol-
ecules could be estimated from
the intensity of the low-energy
shoulder relative to that in the
fully aggregated state (Fig-
ure 6a; see the Supporting In-
formation). This reveals that at
Figure 4. a,b) Temperature dependence of the UV/Vis absorption and PL spectra of 1 in MCH (7 mm for ab-
sorption, 0.7 mm for PL). c,d,e) The corresponding spectra for 5 (21 mm for absorption, 7 mm for PL). The
arrows in the absorption spectra indicate spectral changes upon cooling from room temperature to À908C. Ex-
citation wavelengths: lexc =366 nm (1) and 520 nm (5).
larly, the photoluminescent spectrum of 1 was also shifted to
the red (Dlꢀ10 nm between 25 and À808C) and its vibronic
fine structures sharpened. In addition, CD and LD effects
were observed upon cooling to À108C (see the Supporting
Information), which suggests that elongated aligned aggre-
gates were formed.[25] The absorption and fluorescence spec-
tra at low temperatures are similar to the optical properties
reported for polyfluorenes present in the so-called b phase.
In this phase, which is usually only found for polymers with
unbranched alkyl substituents,[22b,26] the fluorene repeat units
are more planarised with respect to each other in an alter-
nating arrangement. Our results suggest that this planarisa-
tion phenomenon takes place concomitantly with aggrega-
tion.[27]
room temperature the gelation process took place in a
rather narrow concentration window (between 0.4 and
2 mm) with hardly any aggregated species in more dilute sol-
utions and almost full aggregation under more concentrated
conditions. A similar transition was observed by concentra-
tion-dependent IR spectroscopy at room temperature, which
reveals the presence of hydrogen bonds between the amide
functionalities that were broken at a concentration below
1 mm (Figure 6a; see the Supporting Information).[28]
Variable-temperature (VT) UV/Vis absorption spectros-
copy showed that the gels formed by 1 could be melted (Fig-
ure 6b). At elevated temperatures, the absorption spectra
closely resembled those recorded in dilute solutions at room
temperature. This observation demonstrates that heating
allows the transformation of gels into the molecularly dis-
solved state. Remarkably, even at very low temperature gra-
dients (28ChÀ1), the curves for cooling and heating did not
overlap, which suggests that the gelation process takes place
under kinetic rather than thermodynamic conditions.[29]
The main absorption band of 5 (at 320–400 nm) shows a
very similar temperature dependence to that found for 1,
whereas less pronounced changes were evident from its
lowest-energy band (Figure 4c,d). Although the fluorescence
spectra sharpened somewhat, no shift occurred upon cooling
Chem. Eur. J. 2009, 15, 9737 – 9746
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9741

A. P. H. J. Schenning et al.
Figure 6. a) Concentration dependence of the fraction of aggregated 1 at
room temperature (208C) determined by UV/Vis spectrophotometry (&)
and IR spectroscopy (&). b) VT-UV/Vis absorption (cooling rates
108ChÀ1 (solid line) and 28ChÀ1 (dotted line)) of 1 (concentration=
500 mm, monitored at l=410 nm).
Figure 5. a,b) POM images (crossed polars) and photographs under UV
illumination (365 nm) of organogels (2.5 mm) formed by 1 and 5 in
MCH. c,d) Normalised absorption and fluorescence spectra of 1 and 5 in
the gel state (2.5 mm; solid line) and in dilute solution in MCH (0.7 (1)
and 7 mm (5); dashed line) at room temperature.
Atomic force microscopy measurements of thin deposits
prepared by drop casting from solutions of 1 or 5 in MCH
on glass substrates (AFM) showed a fibrillar morphology
with ribbons of a constant thickness of around 20 nm and a
height of 4.3 nm (Figure 7).[30] In the case of 1, additional
clustering into thicker structures and overlap between cross-
ing fibres were observed. When the gels were spin-coated,
the fibres did not cluster, but aligned themselves.
Figure 7. AFM height images of 1 drop cast (a,b) and spin-coated (c)
from solutions in MCH and 5 (d) drop cast from MCH solution. The
height scale is 25 nm (a,b,c) or 10 nm (d), respectively.
9742
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 9737 – 9746

Self-Assembled Fluorene Co-Oligomers
FULL PAPER
To relate the structures observed by AFM to those pres-
ent in solution and in the gel, the fluorescence, CD and LD
spectra of films of 1 on glass substrates were recorded and
revealed features that were comparable to those found in
MCH solutions upon cooling (see the Supporting Informa-
tion). This finding suggests that a similar supramolecular or-
ganisation is present in both cases. Thermal annealing of
these films above the melting temperature of 1 intensified
the spectral features that indicate aggregation, pointing to
reorganisation processes that lead to a higher degree of in-
ternal order. Similar fluorescence spectra were also obtained
when 1 was melted from the solid state.
Interestingly, white emissive organogels could be prepared
by partial energy transfer in a similar approach to that re-
ported earlier for bolaamphiphilic fluorene co-oligomers,[6e]
hydrogen-bonded supramolecular p-conjugated oligomers[13]
and oligo(p-phenylenevinylene) organogels.[11] Mixing small
amounts of 3 (green), 4 (yellow) and 5 (red) into a gel of 1
(molar ratio 1/3/4/5=4.5:0.7:0.2:0.1) in a similar fashion to
that described for the bolaamphiphilic fluorene co-oligomer
analogues[6e] resulted in partial energy transfer from the
blue-emitting donor to the embedded energy acceptors
upon excitation of 1 (Figure 8). The fluorescence lifetime of
1 in the MCH gel revealed a biexponential decay with life-
times of t=0.93 (94.5%) and 1.81 ns (5.5%), whereas for
the white-light-emitting gel shorter decay times were ob-
served with t=0.28 (95.4%) and 1.10 ns (5.6%), respective-
ly (Figure 8b). This decrease in the fluorescence lifetime of
1 shows that the emission is quenched by approximately a
factor of three in the mixed gel, which indicates energy
transfer. Under crossed polars and as a spin-coated film, this
white emissive gel had an appearance similar to the two gels
formed by 1 and 5 (see the Supporting Information).
Figure 8. a) PL spectrum of a gel containing 1 (4.5 mm) and 3–5 (excita-
tion wavelength: lexc =375 nm). b) PL lifetime–decay (lexc =400 nm)
monitored at 415 nm of a gel containing 1 and 1 and 3–5. c) Photograph
of the same mixture under UV illumination (lexc =365 nm; molar ratio
1/3/4/5=4.5:0.7:0.2:0.1).
tween the most stable conformation and a fully co-planar ar-
rangement. In the case of the fluorene co-oligomers 3–5,
similar small torsion angles were found ranging from 34 to
Molecular modelling: Molecular mechanics (MM) and mo-
lecular dynamics (MD) calculations were carried out to pro-
vide some insight into the dif-
ferences in the self-assembly of
1–5. MM calculations indicate
that in the case of 1, neighbour-
ing fluorene units are preferen-
tially oriented with the methyl-
ene bridges in an anti confor-
mation (Figure 9 and the Sup-
porting Information). This ar-
rangement is more stable by
18 kcalmolÀ1 than the syn con-
formation, which is disfavoured
by the steric hindrance between
the alkyl groups. In agreement
with earlier results on poly-
fluorene,[31] a relatively small
tilt angle (368) is found between
the fluorene units. The torsion
barrier between two neighbour-
ing fluorene units is estimated
Figure 9. Calculated single molecule conformations of 1–5 in a vacuum. The 3,4,5-tridodecylbenzamide wedges
to be less than 2 kcalmolÀ1 be- and alkyl chains have been omitted for clarity.
Chem. Eur. J. 2009, 15, 9737 – 9746
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9743

A. P. H. J. Schenning et al.
40o (Figure 9 and the Support-
ing Information). Remarkably,
the naphthalene–fluorene co-
oligomer (2) has an energy min-
imum associated with a torsion
angle of approximately 718 be-
tween the two units. The naph-
thalene co-monomer is there-
fore almost perpendicular to
the conjugated plane of the flu-
orene units. Looking at the con-
formations, compounds 1 and 5
are linear conjugated mole-
cules: the most stable confor-
mations correspond to those in
which the bridging carbon atom
of one fluorene unit (C-9) is
anti to C-9 of the neighbouring
fluorene unit in the case of 1 or
anti to the sulfur atom of the
neighbouring thiophene unit in
the case of 5 (Figure 9).[18] In
contrast, the para substitution
of the phenylene core in 2, 3
and 4 leads to curved conform-
ers (i.e., with a banana shape).
The differences in the shapes of
the molecules (almost linear for
Figure 10. Snapshot at the end of the MD run showing the calculated supramolecular packing of a cluster of 1.
Fluorenes are displayed in black, alkyl groups in grey and hydrogen bonds by dotted lines.
1 and 5 versus slightly curved for 2, 3 and 4, see the Sup-
porting Information) and in the torsion angles between
neighbouring units (less than 408 for 1 and 5 and greater
than or equal to 408 for 2, 3 and 4) may explain the differen-
ces in the aggregation between the different compounds:
while 1 and 5 lead to fibres in solution, precipitation occurs
with the oligomers 2–4.
drance of alkyl substituents at C-9, hydrogen-bonding and
p–p interactions with the alkyl chains displaying a Y-shaped
configuration with respect to the fluorene planes.
The combined results of WAXS, AFM, optical spectrosco-
py and molecular modelling have allowed us to propose a
self-assembly process for 1 as a function of the concentra-
tion (Figure 11). Whereas in very dilute solutions, molecular-
ly dissolved species are predominant, increasing the concen-
tration causes aggregation, first into one-dimensional fibres.
The fluorene repeat units adopt a planarised conformation
in these aggregates. With increasing length, the fibres entan-
gle and cluster into ribbons, thereby forming a lyotropic or-
ganogel.
To gain more insight into the supramolecular structure of
the fluorene derivatives, MD calculations were carried out
on clusters of 1 and compared with the data obtained from
X-ray diffraction and AFM measurements. The fluorene
units of adjacent molecules tend to be in a face-to-face con-
figuration at a distance of around 0.5 nm, which is in agree-
ment with the results of the XRD measurements. In terms
of packing configurations, the neighbouring fluorenes are
packed in an alternating orientation,[18b] that is, with the C-9
atoms pointing in opposite directions, and with a slight shift
along the molecular axis. Such a configuration leads to the
doubling of the distance between equivalent sites along the
p stack (Figure 10). Note that the length of the fully extend-
ed molecule 1 with alkyl groups is around 6.8 nm (with a
conjugated terfluorene core of 2.35 nm). This is larger than
the b unit cell spacing observed in the XRD experiments,
which suggests interdigitation of the alkyl groups in the
solid state. Moreover, the molecular modelling studies sug-
gest that the additional reflection peak along the a axis (at
1.0 nm) is likely to originate from the “flipped” configura-
tion of the face-to-face fluorene units.[18b,32] This specific
packing originates from the balance between the steric hin-
Conclusions
Five fluorene-based polycatenars have been synthesised that
exhibit emission over the entire visible spectrum. Two of
these fluorene co-oligomers self-assemble in a manner that
is dependent on the concentration. The solid-state phase be-
haviour is qualitatively uniform and governed by the com-
bined effects of several supramolecular interactions, most
notably hydrogen bonding, p stacking and microphase sepa-
ration due to mutual repulsion of alkyl chains and the aro-
matic backbones. The self-assembly in methylcyclohexane,
an apolar solvent, was sensitive to apparently minor changes
in the chemical structures. Depending on the ability of the
aromatic parts to form stacks, either precipitation or the for-
9744
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 9737 – 9746

Self-Assembled Fluorene Co-Oligomers
FULL PAPER
Acknowledgements
The authors thank Prof. Klaus Mꢄllen for kindly providing the X-ray
setup at the MPI in Mainz, Martin Wolffs for the fluorescence lifetime
measurements and Henk Eding for analytical measurements. M.S. and
P.L. are “Chargꢁ de Recherches” and “Chercheur Qualifiꢁ” of the F.R.S.-
FNRS (Belgium), respectively. The authors gratefully acknowledge the
Netherlands Organization of Scientific Research, Chemical Sciences
(NWO-CW) for financial support.
[1] a) S. Allard, M. Forster, B. Souharce, H. Thiem, U. Scherf, Angew.
Chem. 2008, 120, 4138; Angew. Chem. Int. Ed. 2008, 47, 4070; b) J.
Roncali, P. Leriche, A. Cravino, Adv. Mater. 2007, 19, 2045; c) C. D.
Dimitrakopoulos, P. R. L. Malenfant, Adv. Mater. 2002, 14, 99;
d) S. R. Forrest, Nature 2004, 428, 911; e) S. A. Chen, H. H. Lu,
C. W. Huang, Adv. Polym. Sci. 2008, 212, 49.
[2] See, for example: a) V. Coropceanu, J. Cornil, D. A. da Silva Filho,
Y. Olivier, R. Silbey, J.-L. Brꢁdas, Chem. Rev. 2007, 107, 926; b) H.
Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard,
B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W.
Meijer, P. Herwig, D. M. de Leeuw, Nature 1999, 401, 685; c) P.
Chen, G. Yang, T. Liu, T. Li, M. Wang, W. Huang, Polym. Int. 2006,
55, 473.
[3] a) U. Scherf, E. J. W. List, Adv. Mater. 2002, 14, 477; b) D. Neher,
Macromol. Rapid Commun. 2001, 22, 1365; c) M. T. Bernius, M. In-
basekaran, J. OꢃBrien, W. Wu, Adv. Mater. 2000, 12, 1737; d) M. In-
basekaran, E. Woo, W. Wu, M. Bernius, L. Wujkowski, Synth. Met.
2000, 111, 397.
[4] For fluorene homopolymers, see: a) M. Knaapila, M. J. Winokur,
Adv. Polym. Sci. 2008, 212, 227; b) S. H. Chen, A. C. Su, S. A. Chen,
J. Phys. Chem. B 2005, 109, 10067; c) S. H. Chen, A. C. Su, C. H. Su,
S. A. Chen, J. Phys. Chem. B 2006, 110, 4007; for fluorene copoly-
mers, see: d) C. L. Donley, J. Zaumseil, J. W. Andreasen, M. M.
Nielsen, H. Sirringhaus, R. H. Friend, J.-S. Kim, J. Am. Chem. Soc.
2005, 127, 12890; e) R. Abbel, A. P. H. J. Schenning, E. W. Meijer,
Macromolecules 2008, 41, 7497.
[5] a) Y. Zhu, R. D. Champion, S. A. Jenekhe, Macromolecules 2006, 39,
8712; b) W.-Y. Lee, K.-F. Chen, T.-F. Wang, C.-C. Chueh, W.-C.
Chen, C.-S. Tuan, J.-L. Lin, Macromol. Chem. Phys. 2007, 208, 1919.
[6] a) A. C. A. Chen, S. W. Culligan, Y. Geng, S. H. Chen, K. P. Klubek,
K. M. Vaeth, C. W. Tang, Adv. Mater. 2004, 16, 783; b) Y. Geng,
A. C. A. Chen, J. J. Ou, S. H. Chen, K. Klubek, K. M. Vaeth, C. W.
Tang, Chem. Mater. 2003, 15, 4352; c) M. Surin, P. Sonar, A. C.
Grimsdale, K. Mꢄllen, S. De Feyter, S. Habuchi, S. Sarzi, E. Braek-
en, A. Ver Heyen, M. Van der Auweraer, F. C. De Schryver, M. Cav-
allini, J. F. Moulin, F. Biscarini, C. Femoni, R. Lazzaroni, P. Leclꢀre,
J. Mater. Chem. 2007, 17, 728; d) N. Li, X. Zhang, Y. Geng, Z. Su,
Macromol. Chem. Phys. 2008, 209, 1806; e) R. Abbel, R. van der
Weegen, E. W. Meijer, A. P. H. J. Schenning, Chem. Commun. 2009,
1697.
Figure 11. Proposed self-assembly mechanism of 1 in solution in MCH
and its concentration dependence at room temperature.
mation of lyotropic organogels were observed. Interestingly,
blue and red luminescent gels were obtained at high concen-
trations, whereas white emissive organogels were obtained
by mixing different fluorene-based oligomers. This shows
that in the self-assembled state the emission colour can be
tuned by partial energy transfer in a modular approach.
The supramolecular chemistry of these closely related
conjugated oligomers strongly depends on apparently small
differences in their chemical structures. Their varying aggre-
gation behaviour in solution suggests that similarly subtle
adaptations in conjugated polymers might have a compara-
bly strong influence. This could have important implications
for solution-processed organic electronic devices; chemical
modifications to adjust the electronic properties of the ma-
terials might unintentionally also alter their self-assembly
behaviour.
[7] a) H.-T. Nguyen, C. Destrade, J. MalthÞte, Adv. Mater. 1997, 9, 375;
b) M. Gharbia, A. Gharbi, H.-T. Nguyen, J. MalthÞte, Curr. Opin.
Colloid Interface Sci. 2002, 7, 312.
[8] a) K. Raꢅs, M. Daoud, M. Gharbia, A. Gharbi, H. T. H.-T. Nguyen,
ChemPhysChem 2001, 2, 45; b) E. Gorecka, D. Pociecha, J. Matras-
zek, J. Mieczkowski, Y. Shimbo, Y. Takanishi, H. Takezoe, Phys.
Rev. E 2006, 73, 031704; c) B. Donnio, B. Heinrich, H. Allouchi, J.
Kain, S. Diele, D. Guillon, D. W. Bruce, J. Am. Chem. Soc. 2004,
126, 15258; d) T. Yasuda, K. Kishimoto, T. Kato, Chem. Commun.
2006, 3399; e) E. Peeters, P. A. van Hal, S. C. J. Meskers, R. A. J.
Janssen, E. W. Meijer, Chem. Eur. J. 2002, 8, 4470; f) A. I. Smirnova,
D. W. Bruce, J. Mater. Chem. 2006, 16, 4299; g) A. I. Smirnova,
D. W. Bruce, Chem. Commun. 2002, 176; h) S. H. Seo, G. N. Tew,
J. Y. Chang, Soft Matter 2006, 2, 886.
Experimental Section
Full experimental details for this paper are given in the Supporting Infor-
mation.
[9] a) A. Ajayaghosh, V. K. Praveen, Acc. Chem. Res. 2007, 40, 644;
b) S. J. George, A. Ajayaghosh, Chem. Eur. J. 2005, 11, 3217; c) Y.
Kamikawa, T. Kato, Langmuir 2007, 23, 274; d) X-Q. Li, X. Zhang,
S. Ghosh, F. Wꢄrthner, Chem. Eur. J. 2008, 14, 8074; e) H. Jintoku,
Chem. Eur. J. 2009, 15, 9737 – 9746
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
9745

A. P. H. J. Schenning et al.
T. Sagawa, T. Sawada, M. Takafuji, H. Hachisako, H. Ihara, Tetrahe-
dron Lett. 2008, 49, 3987.
[19] Although VT-IR proves the presence of hydrogen bonds at room
temperature, no information about these interactions can be extract-
ed from the WAXS data.
[10] a) A. Ajayaghosh, C. Vijayakumar, V. K. Praveen, S. S. Babu, R.
Varghese, J. Am. Chem. Soc. 2006, 128, 7174; b) K. Sugiyasu, N.
Fujita, S. Shinkai, Angew. Chem. 2004, 116, 1249; Angew. Chem. Int.
Ed. 2004, 43, 1229; c) T. Shu, J. Wu, M. Lu, L. Chen, T. Yi, F. Li, C.
Huang, J. Mater. Chem. 2008, 18, 886; d) A. G. L. Olive, A. del
Guerzo, C. Belin, J. Reichwagen, H. Hopf, J.-P. Desvergne, Res.
Chem. Intermed. 2008, 34, 137; e) K. Sugiyasu, N. Fujita, M. Takeu-
chi, S. Yamada, S. Shinkai, Org. Biomol. Chem. 2003, 1, 895; f) J.-P.
Desvergne, A. G. L. Olive, N. M. Sangeetha, J. Reichwagen, H.
Hopf, A. del Guerzo, Pure Appl. Chem. 2006, 78, 2333.
[11] C. Vijayakumar, V. K. Praveen„ A. Ajayaghosh, Adv. Mater. 2009,
21, 2059.
[12] a) M. J. Edelmann, J.-M. Raimundo, N. F. Utesch, F. Diederich, C.
Boudon, J.-P. Gisselbrecht, M. Gross, Helv. Chim. Acta 2002, 85,
2195; b) B. A. DaSilveira Neto, A. Sant’Ana Lopes, G. Ebeling, R. S.
GonÅalves, V. E. U. Costa, F. H. Quina, J. J. Dupont, Tetrahedron
2005, 61, 10975; c) D. D. Kenning, K. A. Mitchell, T. R. Calhoun,
M. R. Funfar, D. J. Sattler, S. C. Rasmussen, J. Org. Chem. 2002, 67,
9073.
[20] D. F. Eaton, Pure Appl. Chem. 1988, 60, 1107.
[21] a) W.-C. Wu, C.-L. Liu, W.-C. Chen, Polymer 2006, 47, 527; b) B.
Liu, W.-L. Yu, Y.-H. Lai, W. Huang, Chem. Mater. 2001, 13, 1984.
[22] a) M. Knaapila, F. B. Dias, V. M. Garamus, L. Almꢆsy, M. Torkkeli,
K. Leppꢇnen, F. Galbrecht, E. Preis, H. D. Burrows, U. Scherf, A. P.
Monkman, Macromolecules 2007, 40, 9398; b) M. Knaapila, V. M.
Garamus, F. B. Dias, L. Almꢆsy, F. Galbrecht, A. Charas, J. Morga-
do, H. D. Burrows, U. Scherf, A. P. Monkman, Macromolecules 2006,
39, 6505.
[23] P. Suppan, J. Photochem. Photobiol. A 1990, 50, 293.
[24] We have not determined the gelator solubility of our compounds,
see: A. R. Hirst, I. A. Coates, T. R. Boucheteau, J. F. Miravet, B. Es-
cuder, V. Castelleto, I. W. Hamley, D. K. Smith, J. Am. Chem. Soc.
2008, 130, 9113.
[25] In contrast, no LD but strong CD effects were found in aggregated
and partially precipitated solutions of 2, 3 and 4.
[26] H. Cheun, F. Galbrecht, B. S. Nehls, U. Scherf, M. J. Winokur, J.
Lumin. 2007, 122–123, 212.
[27] At present, our results do not allow us to distinguish whether aggre-
gation is induced by the conformational changes or vice versa.
[28] a) S. Boileau, L. Bouteiller, F. LauprÞtre, F. Lortie, New J. Chem.
2000, 24, 845; b) Y. Liu, Y. Li, L. Jiang, H. Gan, H. Liu, Y. Li, J.
Zhuang, F. Lu, D. Zhu, J. Org. Chem. 2004, 69, 9049.
[29] As the models most frequently used to describe self-assembly (iso-
desmic and cooperative model) both assume thermodynamically
controlled processes, our data do not allow application of these
models.
[30] In contrast, the three compounds 2, 3 and 4 gave aggregates that re-
sembled crystallite structures, in line with their inability to form or-
ganogels.
[31] S. Y. Hong, D. K. Kim, C. Y. Kim, R. Hoffmann, Macromolecules
2001, 34, 6474.
[13] R. Abbel, C. Grenier, M. J. Pouderooijen, J. W. Stouwdam,
P. E. L. G. Leclꢀre, R. P. Sijbesma, E. W. Meijer, A. P. H. J. Schen-
ning, J. Am. Chem. Soc. 2009, 131, 833.
[14] Procedure analogous to L. Brunsveld, H. Zhang, M. Glasbeek,
J. A. J. M. Vekemans, E. W. Meijer, J. Am. Chem. Soc. 2000, 122,
6175.
[15] R. Abbel, M. Wolffs, R. A. A. Bovee, J. L. J. van Dongen, X. Lou,
O. Henze, W. J. Feast, E. W. Meijer, A. P. H. J. Schenning, Adv.
Mater. 2009, 21, 597.
[16] S. Kumar, Liquid Crystals: Experimental Study of Physical Proper-
ties and Phase Transitions, Cambridge University Press, Cambridge,
2001.
[17] a) W. Geiger, Spectrochim. Acta 1966, 22, 495; b) W. Herrebout, K.
Clou, H. O. Desseyn, N. Blaton, Spectrochim. Acta Part A 2003, 59,
47.
[18] a) M. Surin, P. Sonar, A. C. Grimsdale, K. Mꢄllen, R. Lazzaroni, P.
Leclꢀre, Adv. Funct. Mater. 2005, 15, 1426; b) M. Surin, E. Henne-
bicq, C. Ego, D. Marsitzky, A. C. Grimsdale, K. Mꢄllen, J.-L.
Brꢁdas, R. Lazzaroni, P. Leclꢀre, Chem. Mater. 2004, 16, 994.
[32] Interestingly, the formation of the flipped configuration is favoured
by the presence of hydrogen bonds rather than the “shifted” confor-
mation, which is more stable in poly(dioctylfluorene) (see
ref. [18b]).
Received: March 9, 2009
Published online: August 18, 2009
9746
www.chemeurj.org
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 9737 – 9746