Synthesis and crystal structure and optical properties of fluorenic-core oligomers

Article abstract of DOI:10.1039/b109089p

Fluorenic core oligomers, displaying interesting photoluminescence properties, have been synthesized by an organometallic route. The crystal and molecular structure of a fluorene derivative and three homologous oligomers have been studied. The spiro-deriv

Full text of DOI:10.1039/b109089p

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Synthesis and crystal structure and optical properties of
fluorenic-core oligomers
a
Silvia Destri, Mariacecilia Pasini, Chiara Botta, William Porzio,* Fabio Bertini
a
a
a
a
b
and Luciano Marchio`
a
Istituto di Chimica delle Macromolecole del C.N.R., via E.Bassini 15, 20133 Milano, Italy.
E-mail: rx@icm.mi.cnr.it
Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica,
Universit a` di Parma, Parco Area delle Scienze, I-43100 Parma
b
Received 5th October 2001, Accepted 24th January 2002
First published as an Advance Article on the web 4th March 2002
Fluorenic core oligomers, displaying interesting photoluminescence properties, have been synthesized by an
organometallic route. The crystal and molecular structure of a fluorene derivative and three homologous
oligomers have been studied. The spiro-derivative of dibromofluorene shows a very strong interaction between
the H and Br atoms. The thienyl-terminated oligomer with a spiro-substituent adopts a particular herringbone
arrangement with overlap of only the end-thienyl residues, conformationally disordered. The crystal of the
all-phenyl derivative consists of the packing of discrete molecules cofacially arranged. The molecule with linear
alkyl chains on the 9-position of the fluorene core and thienyl rings as end-groups exhibits polymorphism and
its crystal structure, tetragonal, is very peculiar because of the loose packing allowing a good separation among
adjacent thienyl rings.
The absorption and emission properties of this series were investigated and the electroluminescence of
single-layer devices was characterized. The photoluminescence properties of the molecules are strongly
dependent on their structural organization, displaying a red-shift of the emission from the amorphous phase,
to the crystalline structure, to the aggregated form.
A comparison of the packing of these oligomers and oligothienylenes fully accounts for the optical properties
of the reported molecules.
emitting in blue region. Specifically, the introduction of a
spiro-bridge to connect conjugated molecules can minimize
the parallel aggregation and increase the PL intensity.
Introduction
1
0
The relevant optoelectronic properties of conjugated oligo-
mers, either thienylene or phenylene based, used in the
fabrication of devices, and their role as suitable model com-
pounds to understand the properties of related polymers are
With this in mind, here we report on the preparation of
differently substituted oligomers, obtained by the addition of
2-thienyl, phenyl and p-cyanophenyl groups to a fluorene core,
namely linear side chains (molecule 6 in Scheme 1) linked to
fluorene as a protecting group or a spiro-cyclopentane residue
at the same positions (molecules 7, 8, 9 respectively in
Scheme 1). The aim of this work is to relate the structural
features of different solid state organizations to the optical
properties of the molecules as a function of their different
chemical structure. In particular, we analyse the variations of
the photoluminescence properties of the molecules going from
the solution to different solid state arrangements.
1
–3
well known and documented.
Specifically because of the
remarkable dependence of these properties on supramolecular
organization, the determination of both the molecular packing
and conformations constitutes a crucial point in understanding
the electronic properties, hence improving the performance of
2
devices developed. In view of this, fluorene and polyfluorenes
have been receiving great attention especially as active layers
5
in light emitting diode (LED) prototypes
7
components in liquid crystal (LC) devices.
4
,6
or back-light
A thorough knowledge of the relationship between the
crystal packing and the resulting charge transport and optical
properties addresses the strategy of increasing either the charge
carrier mobility in field effect transistors (FET) and LED or
photoluminescence (PL) quantum yield in the latter displays.
The recent study of thiophene–fluorene oligomers and their
9
polyesters is directed towards this goal. To pursue this
Results and discussion
8
Synthesis
Organometallic coupling reactions have been employed to
prepare 2,7-di(2-thienyl)-9,9-dihexylfluorene 6, 2’,7’-di(2-
thienyl)spiro[cyclopentane-1,9’-fluorene] 7, 2’,7’-diphenylspiro-
[cyclopentane-1,9’-fluorene] 8 and 2’,7’-bis(4-cyanophenyl)spiro-
[cyclopentane-1,9’-fluorene] 9, as described in Scheme 1. The
acidic protons in the 9-position of fluorene were removed by
objective a very detailed characterization of intermolecular
interactions is required. On the other hand the insertion of
various substituents in the molecular main-frame strongly
modifies both the solid state arrangement and the optical
properties.
1
1
In fact copolymers of fluorene with a great number of
comonomers ranging from benzene, stilbene to anthracene and
spiro-bridge substituted fluorene have been prepared in order
lithiation followed by alkylation with two equivalents of
bromohexane to give 1, while reaction with 1,4-dibromobutane
afforded the cyclopentane derivative 3. Bromination following
resulted in
6
12
the literature procedure with bromine and FeCl
to avoid aggregate formation during the operation of LEDs all
3
9
24
J. Mater. Chem., 2002, 12, 924–933
This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b109089p

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Scheme 1
the formation of the dibromides 2 and 4 in high yields.
1
Structure of 4{
3
Both dibromides gave Stille cross-coupling products with
-(tributylstannyl)thiophene catalysed by Pd(PPh ) in refluxing
toluene, affording 2,7-di(2-thienyl)-9,9-dihexylfluorene 6 and
The bond distances and bond angles are as expected and
supplied as supplementary material. The ORTEP drawing of
2
3
4
4
with labels is shown in Fig. 1.
The geometrical features of the present structure (bond
distances, angles and torsions) are clearly comparable with
2
’,7’-di(2-thienyl)spiro[cyclopentane-1,9’-fluorene] respec-
7
tively. The spirofluorene derivative 4 is more reactive with
respect to the corresponding compound 2; in fact after
17
those observed in 2,7-dibromo-9,9-dioctylfluorene. In both
1
0 hours almost all of 4 had reacted while 2 needed a whole
˚
cases the fluorene moiety is planar within 0.02 A. The
cyclopentane ring adopts an envelope conformation displaying
an angle of 144u between the [C(14)–C(15)–C(16)–C(17)] and
day to accomplish the reaction. The yields are comparable
with that reported in the literature for the preparation of
a similar compound i.e. 2,7-di(2-thienyl)-9,9-didecylfluorene
[C(9)–C(14)–C(17)] planes. Moreover the cyclopentane moiety
1
4
3 4 2 3
with Pd(PPh ) –PdCl (PPh ) 1 : 1 as catalysts. As a side
and the fluorenyl residue are nearly orthogonal. However, a
different packing arrangement is expected in view of the dioctyl
versus cyclopentane substitutions.
Indeed the molecular packing, viewed along the c axis (see
Fig. 2) consists of alternating sheets of molecules, forming
angles close to 85u and linked together by the hydrogen–
bromine interactions, as also observed in 1,2,3,4 tetra-
bromobenzene crystals. In fact the H(16) and Br(2) normal-
˚
ized distance is of 2.91 A is significantly shorter than any
18
analogous distance in tetrabromobenzene polymorphs.
Other intermolecular contacts involving both Br–C and C–C
atoms exceed the sum of commonly accepted van der Waals
radii. Hence the key factor keeping the molecular sheets
together is the formation of strong H Br interactions (see
Fig. 2).
reaction, the self-coupling of the tin compound took place.
The target compound 8 was synthesized by Pd catalysed coupl-
ing of 2’,7’-dibromospiro[cyclopentane-1,9’-fluorene] 4 with
phenylzinc chloride. 4 could be transformed into bis(dioxabo-
rolane) derivative 5 via lithiation in THF and quenching
with
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane.
1
5
We slightly modified the literature procedure in order to
avoid both the alkylation of the fluorene core and the for-
mation of the mono(dioxaborolane) derivative. To accomplish
a suitable compromise an accurate check of temperature and
time of both the two steps of the reaction had to be adopted.
18
1
6
The Suzuki reaction of 5 with 4-bromobenzonitrile in
toluene and aqueous Na CO gave in high yield the target
oligomer with two cyano groups 9.
2
3
…
In the following the crystal and molecular structures of 4, 6, 7
and 8 are reported. The detailed study of the structure of 4
is justified by the presence of a fluorenic core; in contrast the
structure of 8 could be solved only at an approximate level,
due to the crystal quality (see Experimental section).
{CCDC reference number 158135. See http://www.rsc.org/suppdata/
jm/b1/b109089p/ for crystallographic files in .cif or other electronic
format.
J. Mater. Chem., 2002, 12, 924–933
925

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Fig. 1 ORTEP drawing of 4, with numbering scheme, the ellipsoid
probability is 30%.
Fig. 2 Unit cell content of 4, as viewed along the c axis.
Structure of 6{
observed in the XRD powder patterns, where only the peaks
belonging to these directions are detected. All in all, the
molecules tend to stay with their conjugated residues
perpendicular to the substrate.
This molecule was studied by DSC, optical microscopy and
XRD, and showed polymorphism.
In Fig. 3 DSC scans of 6 are reported. Two endothermic
peaks are clearly observed in the first heating run with onsets at
The crystal structure of 6 consists of the packing of discrete
cross-like units formed by the essentially planar conjugated
backbone and by two perpendicular hexyl chains. The ORTEP
drawing with labels is shown in Fig. 4. Bond distances, bond
angles, and torsions, supplied as supplementary material, are
2
1
7
2 uC and 113 uC. Corresponding enthalpies are y3 mJ mol
21
and y93 mJ mol . In the second run (cooling rate
2
1
2
slow crystallization rate; in fact the subsequent heating (rate 20
5 uC min ) no exothermic peak is detectable, indicating a
17
quite comparable with those of 7 and dioctylfluorene. In fact
the hexyl-chain conformation is identical to that observed in
the latter crystal, and also the fluorene moiety is planar within
2
1
uC min ) shows a small endothermic peak above 115 uC. The
optical microscopy observations are in agreement with the
above results: indeed some brightness appears near 80 uC
followed by complete clearing at 122 uC. Hence, the first peak
is attributed to a conformation transition of the hexyl chain,
as the enthalpy involved is comparable to that calculated for a
˚
.02 A, while the two thiophene residues, in syn conformations
0
with respect to the fluorene moiety, are bent by almost 5u. The
˚
interaction between S and H(2) (short distance of 2.72 A)
allows for a planar conformation of fluorene and of the
adjacent thiophene rings.
The crystal packing, reported in Fig. 5 as viewed along the
c-axis, is composed of a network of molecules related by a four-
fold alternating axis, tilted clockwise from the a-axis by y25u.
Each molecule, standing on a two-fold axis, is related to the
nearest through contacts of the thiophene moiety with both a
1
9
trans to gauche transition in a hydrocarbon chain, while the
second peak is attributed to the melting point.
Temperature XRD analysis on 6 powders corroborates the
suggestion of chain disordering, since above 80 uC the pattern is
quite similar to that observed at room temperature, except for a
huge peak broadening, while above 130 uC only a broad peak of
the amorphous phase is detected.
From crystal structure analysis (see Table 1 for details) it is
possible to perform line profile analyses of three intense low
angle peaks, belonging to the [112], [121] and [202] directions
respectively; then an attribution of this mesophase is allowed,
since the sudden reduction of the crystallite dimensions after
˚
˚
this transition (to about half their values, from 700 A to 350 A
on the average) together with the very low enthalpy involved
(
‘
see above) indicate that the new phase is typically a
‘conformationally disordered crystal’’, as already observed
2
0
in both organic molecules and polymers. In such phases the
molecules maintain the same crystal packing and unit cell
dimensions, but different conformations of the side chains,
owing to the relatively loose arrangement (see below).
Films cast from different solvents on to polar substrates tend
to slowly crystallize into a 3D phase, with some preferred
orientation along different crystallographic directions, as
{CCDC reference number 158136. See http://www.rsc.org/suppdata/
jm/b1/b109089p/ for crystallographic files in .cif or other electronic
format.
2
1
Fig. 3 DSC scans of 6 a) heating run at 20 uC min , b) cooling run at
21 21
5 uC min , c) second heating run at 20 uC min
.
9
26
J. Mater. Chem., 2002, 12, 924–933

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a
Table 1 Crystal data of the molecules, refinement conditions and experimental details
4
6
7
8
Chemical formula
Formula weight
Space group
C
378.1
nu61/Pbca
11.746(4)
11.892(3)
20.568(7)
17
H
14Br
2
C
486.8
nu88/I4
16.563(1)
16.563(1)
20.657(1)
32
H
38
S
2
C
384.57
nu 29/Pca2
12.666 (4)
21.383 (9)
7.351 (1)
25
H
20
S
2
C
372.51
nu4/P2
12.192 (8)
7.217 (3)
13.224 (6)
117.23(5)
1034.6(17)
2
1.20
29 24
H
1
/a
1
1
˚
a/A
˚
b/A
˚
c/A
b/u
3
˚
V/A
2873.0(16)
8
1.748
5666.9(5)
8
1.14
1990.9(16)
4
1.28
Z
D
2
3
calc/g cm
Radiation
Cu Ka (1.54184 l)
70.00
1488
293
Mo Ka (0.71073 l)
2
1
m/cm
F(000)
Temperature/K
20.6
2144
223
26.7
808
293
0.7
396
293
Anomalous dispersion
Parameters refined
All non-hydrogen atoms
186
189
209
286
Unweighted agreement factor
Weighted agreement factor
Factor including unobs. data
Esd of obs. of unit weight
Refinement
Minimization function
Least-squares weights
Largest shifts/s
0.033
0.065
0.070
0.041
0.094
0.091
1.32
0.061
0.16
0.19
0.062
0.121
0.166
Full-matrix least-squares
2
Sw(|F
o c
2| 2 |F2 |)
2
o o
4F /s (F )
0.002
0.002
0.17(3)
20.17(3)
Pale yellow
Needle
0.001
0.47(7)
20.27(7)
Pale yellow
Prism
0.11
0.17(4)
20.16(4)
White
Prism
0.2 6 0.15 6 0.1
Enraf-Nonius CAD4
3
˚
High peak in final diff. map/e A
˚
Low peak in final diff. map/e A
0.33(3)
20.37(3)
White
Prism
0.25 6 0.15 6 0.12
Enraf-Nonius CAD4
Lorentz-polarization
3
Crystal color
Crystal shape
Crystal dimensions/mm
3
0.2 6 0.15 6 0.1
SMART CCD
0.33 6 0.20 6 0.07
Enraf-Nonius CAD4
Instrument
Corrections
Empirical absorption (from 1.00 to 1.01 on I)
Extinction (coefficient ~ 0.0000004)
Maximum h/u
Unobserved data
Reflections included
69.9
23.5
1084
967 F
27
1111
1232 with F
25
1015
959 with F
2
2
o
2
)
2
o
2
o
2
o
2
1617w2.0s(F
o
)
w 2.0 s(F
o
w 2.0 s(F
)
w2.0 s(F
o
)
Intensity measurements
Monochromator
Attenuator
Graphite crystal, incident beam
Zr foil, factor 17.0
2.0 mm horizontal
Detector aperture
2.0 to 2.8 mm horizontal
4
v–2h
.0 mm vertical
Scan type
Scan rate
Scan width/deg
a
2
1
1–20u min (in omega)
1.0 1 0.350 tan h
Where only one value is indicated, it refers to all the crystals.
hexyl chain parallel and an aromatic residue perpendicular
(
edge to face contact), involving C or S atoms at distances over
˚
.76 A. This relatively loose packing agrees with the low density
3
value (1.17 g cm ), as compared with the density found in
2
3
2
3
crystals of the analogous 7 (1.28 g cm ).
On the other hand, in the solvent accessible void (near 20.09,
.01, 20.09) no relevant electronic density residue could be
0
observed in the final difference Fourier calculation, see Table 1.
Fig. 5 Packing of the crystal of 6 , as viewed along the c axis. The
ellipsoid probability is 25%.
Fig. 4 ORTEP drawing of 6 with labels, the ellipsoid probability is 30%.
J. Mater. Chem., 2002, 12, 924–933
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Fig. 7 Molecular packing of 7, as viewed along the short axis.
Fig. 6 ORTEP drawing of 7 and related labelling scheme. The
probability of thermal ellipsoids is 30%.
Structure of 7§
The bond distances and bond angles of 7 are as expected and
supplied as supplementary material. The ORTEP drawing is
shown in Fig. 6.
Crystals of thiophene based oligomers, especially tetramers,
often display statistical disorder in the end-thiophene groups,
2
1
as first reported by Barbarella et al. Also in this case such a
disorder is present, i.e. the two rings predominantly assume a
syn conformation with respect to the central cyclopentadienyl
ring, which means a 180u rotation around the C(4)–C(5) and
C(14)–C(17) bonds, so that atoms S(1) and S(2) replace atoms
C(3) and C(18), respectively. However, since these end-residues
are non-equivalent, owing to the lack of any intramolecular
symmetry, the occupation factors have been refined to differ-
ent values, namely 0.79 and 0.63 for atoms S(1) and S(2),
respectively, with the corresponding occupancy factors of
Fig. 8 ORTEP drawing of 8 and related labelling scheme. The
probability of thermal ellipsoids is 25%.
1
The molecule crystallizes in the P2 space group and displays
a high thermal motion at the end-phenyl rings at room
temperature, in fact the average thermal parameters of such
C-atoms together with those of the cyclopentane residue are
larger than the fluorene ones, moreover a remarkable mosaic
spreading in the crystal was observed; indeed the crystallization
into a single crystal was critical.
0
.21 and 0.37 for atoms C(3) and C(18), respectively.
The bond distances and bond angles of the fluorenyl residue,
slightly concave, resemble those found in other parent com-
1
7
pounds and 4, while the bent cyclopentane moiety shows
the values expected considering the thermal motion. The
thienyl rings present values largely affected by the statistical
disorder, but attempts to refine the two sites by adopting a rigid
On the other hand, in crystals of phenylene based oligomers,
4
the molecules display non-planar backbone conformations,
2
and typical torsion angles close to 40u, to accommodate the
steric hindrance of adjacent phenylene hydrogens; moreover
the exceptional librational motion of the rings, particularly the
end ones, determines a complete conformational disorder at
2
2
body approach, using the geometry of oligothiophenes, did
not yield significant improvements. The two thiophene rings
2
5
˚
˚
are planar within 0.022 A and 0.012 A respectively. Both end-
thiophene rings are bent on the same side with respect to
the fluorenyl mean-plane by over 24u. Also the cyclopentane
moiety adopts a bent conformation, as expected.
room temperature.
The bond distances and angles of the fluorene residue are
comparable with those observed either in other parent com-
1
7
pounds or in crystals of 7, while the cyclopentane moiety
geometry is affected by the large thermal motion. The end-
phenyl rings consequently lie out of the molecular plane of
the fluorene residue by y30u [C(14)–C(19)] and y35u [C(20)–
C(25)] respectively. The cyclopentane moiety adopts a bent
conformation as observed in the crystal structure of 7, although
with very high thermal parameters.
The molecular packing, shown in Fig. 7, consists of adjacent
rows of molecules strictly aligned along the b-axis and forming
a herringbone arrangement. Each molecule forms an angle of
over 65u with respect to the adjacent one. The intermolecular
˚
short contacts, close to 3.40 A, involve atoms affected by
statistical disorder, specifically atoms S(1) with C(4) (2 2 x, 1 2
y, ½ 1 z) and atoms S(2) with C(14) and C(17) (2 2 x, 2 y,½ 1
The molecular packing, shown in Fig. 9 as viewed along the
short axis, consists of overlapped layers of molecules slightly
shifted relative to each other, giving thus rise to a cofacial
arrangement. The intermolecular contacts, involving carbon
2
2,23
z). Unlike the case of the oligothiophene series,
this
peculiar packing limits p-overlap to end-thiophene moieties
of adjacent molecules (see Fig. 7). This observation assumes
relevance with respect to both intermolecular charge transport
and energy transfer, as shown in recent studies on thiophene–
8
phenylene oligomers, in order to understand the optical
properties of these molecules.
Structure of 8}
The bond distances and bond angles of 8 are as expected and
supplied as supplementary material. The ORTEP drawing with
labels is shown in Fig. 8.
§
CCDC reference number 158137. See http://www.rsc.org/suppdata/jm/
b1/b109089p/ for crystallographic files in .cif or other electronic format.
CCDC reference number 158138. See http://www.rsc.org/suppdata/
}
jm/b1/b109089p/ for crystallographic files in .cif or other electronic
format.
Fig. 9 Packing arrangement of 8, as viewed along the b axis.
9
28
J. Mater. Chem., 2002, 12, 924–933

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˚
atoms are larger than 3.55 A. However the cofacial arrange-
ment, although partially shifted, assumed in this crystal, can gua-
8
rantee an efficient p-overlap of the adjacent faced molecules.
Optical studies
The influence of the addition of electron donation (thienyl
group) or electron withdrawal (benzonitrile group) to/from the
fluorene core on the absorption and emission properties is
clearly shown in Table 2, where the absorption and emission
maxima of CHCl3 and THF solutions respectively of the
oligomers are reported. The effect of thienyl substitution on a
fluorene core is a clear red-shift with respect to the phenyl
addition, while a p-nitrilo group produces a blue-shift with
respect to 8, especially for the emission. This observation is
accounted for considering that the electron withdrawing
mesomeric effect of the cyano group is more effective in the
excited planar state of the molecule.
Table 2 also reports the photoluminescence quantum yields
PL-QY) values for diluted THF solutions, the energy gap
EG) values for cast films, the corresponding oxidation
(
(
potentials and electronic affinities calculated according to ref.
6. The QY measurements are carried out in very dilute
2
2
5
26
(y10 M) solutions, particularly for 9 (10 M) because of its
strong tendency to aggregate producing a sharp decrease of PL
intensity even in 10 M solution (QY # 8%).
Absorption and PL investigations are performed in the solid
state for cast films and single crystals, for 6, 7, and 8; PL of 7
and 8 vacuum evaporated films are also measured.
2
5
Fig. 10 Optical absorption and emission spectra of 8. Top: diluted
THF solution; middle: cast films (solid line) and single crystal (dotted
line); bottom: vacuum evaporated film as prepared (solid line) and after
10 days ageing (dotted line).
While all the molecules display noticeable PL in solution, in
solid state a selective reduction is detected, particularly for 9.
In Fig. 10 the optical spectra of 8, which is treated first as it
has the lowest PL in the series, are reported. The absorption
spectrum of the cast film displays a long wavelength shoulder at
about 350 nm, that is not present in the spectrum of the
solution. The PL spectra of the cast films are quite different
from those of the solution. In fact, while for the solution a
structured spectrum, peaking at 352 nm, is observed, the solid
state samples display an unstructured, red-shifted band,
peaking at 404 nm (cast film) and 420 nm (single crystal).
The PL spectrum of the vacuum evaporated films, with a
maximum at 380 nm, shows a smaller red-shift, with respect to
the solution spectrum, which increases in time by leaving the
evaporated film in air or under nitrogen atmosphere, at room
temperature. Moreover, as shown in Fig. 10, by ageing the film,
the PL intensity decreases and a long wavelength component at
about 540 nm appears.
PL spectra only slightly red-shifted with respect to the
solutions. Moreover, after heating (140 uC) and fast quenching
of the cast film of 6, both its absorption and structured PL
component show exactly the same shape and spectral position
observed in the diluted solution spectra.
The emission of the single crystals is red-shifted with respect
to the PL of the cast and evaporated films, for all three
molecules. In this respect, it should be mentioned that,
8
following Cornil et al., the tight crystal packing of molecules
In Fig. 11 and Fig. 12 the optical spectra of 7 and 6
respectively are reported. The introduction of thienyl end-
groups instead of phenyls induces a red-shift of the absorption
and emission spectra of 6 and 7 solutions, with respect to those
of 8. On the other hand, the change of the alkyl substituent at
the 9-position gives no modification of the optical properties.
In fact, the spectra of diluted solutions of 6 and 7 are very
similar. Indeed 7 solutions show a higher tendency to aggre-
gate at high concentrations and, as can be seen in Fig. 11,
2
2
concentrated (10
M) solutions of molecule 7 display
absorption and PL spectra similar to those of the cast films.
Cast films of both 6 and 7 oligomers show absorption and
Table 2 Optical and electronic data for the oligomers
Absorption/ Emission/ PL-QY EG/
nm
E
V
ox
/
EA/
eV
Molecules nm
(%)
eV
a
Fluorene
260
353
354
326
319
290
382
382
353
323
—
65
55
45
60
—
3.05
3.11
3.16
3.03
—
0.85
0.8
1.05
—
—
2.2
2.09
2.29
—
6
7
8
9
a
Fig. 11 Optical absorption and emission spectra of 7. Top: diluted
(solid line) and concentrated (dotted line) THF solution excited at
340 nm and 410 nm, respectively; middle: cast film excited at 340 nm
(solid line) and 400 nm (dotted line) and cast film after degradation
(dashed line); bottom: vacuum evaporated film (solid line) and single
crystal (dotted line).
Cyclohexane solution.
J. Mater. Chem., 2002, 12, 924–933
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different tendencies towards crystallization/aggregation of the
molecules, which prevent any quantitative measurement.
On the basis of these findings some insights on the nature
of aggregates can be suggested, namely a few molecules (two
or three) approaching face to face, therefore realising efficient
p-overlap, unlike the 3D structure (see above). Such short-
2
9
range order, undetectable by structural investigations, is
likely developed from amorphous regions and is responsible
for the low energy emission. This organization is different
from both that of amorphous regions, where no translational
order is achieved, and that of crystalline regions, where long
2
9
range order is detected.
In conclusion, in the solid state quite different absorption
and PL spectra are observed, depending on the aggregation
type attained and on the ageing of the samples. Different PL
emissions are assigned to contributions from the amorphous,
the crystalline and the aggregated components of the samples
indicating a progressive shift towards lower energy by passing
from disordered (partially amorphous) cast films to crystalline
samples, and from crystalline films to aggregated samples.
While films of 6 display a high tendency to self-crystallization,
cast films of 7 , as well as cast and evaporated films of 8, appear
to be more sensitive to aggregation effects spontaneously
occurring by ageing the samples.
Electroluminescence (EL) was not observed in 6, 7 and 8
single-layer devices using ITO and Al as electrodes. Even the
use of Ca instead of Al as cathode did not allow the observation
of any emission from a device with a spin coated film of 7
as active layer. On the contrary the ITO/6/Ca–Al device yielded
a green emission, at high applied voltages (20–30 V) with
Fig. 12 Optical absorption and emission spectra of 6. Top: diluted
THF solution; middle: cast film (solid line) and single crystal (dotted
line); bottom: PL (solid line) and EL (dashed line) of spin coated film
(see text).
7
and 8 should induce a drastic quenching of the photo-
2
3
22
low current densities (0.8–2 10 A cm ) with a maximum
luminescence, hence the grain-boundary defects of the small
single crystals, here studied, could likely act as emission
2
2
efficiency of about 4 6 10 %. The spectral position of
electroluminescence of 6 is reported in Fig. 12 together with the
PL obtained from the same film. The low energy position of
the EL spectrum can be attributed to aggregate emission of the
material at the metal interface, where electron–hole recombi-
nation takes place. In fact the spectral shape of the PL obtained
by exciting the film from the ITO interface of the device, after
device operation, resembles that of the crystalline phase with an
additional low energy component (similar to that observed in
the PL after thermal annealing or ageing of the films). The film
region at the cathode interface can undergo softening by
heating during metal deposition, see DSC analysis. The reason
for the low currents observed can be found in the high tendency
of films of 6 to crystallize. Indeed, considering the structural
results, it is expected that the peculiar packing of crystalline 6
does not favour an efficient charge transport. All in all, the EL
study indicates that, even though the work function of the
electrodes matches well with both ionization potential and
electronic affinity of the oligomers (see Table 2 and ref. 30) and
fluorene derivatives are expected easily to inject electrons from
2
centers. In contrast, the crystal packing of molecules of 6,
7
˚
where the distance between adjacent molecules is close to 8 A,
should allow for intrinsic emission. However, to definitely
assess the intrinsic PL-QY of the molecules an accurate
measurement on single crystals is in progress.
The presence of a broad low energy tail in the PL spectra of
the cast films suggests that both the amorphous (structured
blue component of the spectra, similar to the solution) and the
crystalline phase (broad low energy shoulder, similar to the
single crystal) are present in the cast films. In particular, for
cast films of 6, the intensity of the low energy component
increases, with respect to the structured blue component, after
only few days ageing, indicating a strong tendency to self-
crystallization of this molecule. This phenomenon clearly
appears in thin spin-coated and cast films of 6 which lose their
transparency in about 1–2 days.
The PL of the cast films of 6 and 7 display an additional low
energy broad component in the 420–520 nm region, which can
be selectively obtained by exciting at wavelengths above 400 nm,
and is particularly evident in films of 7 (see Fig. 11). This
component is also observed in concentrated solutions of 7 (see
Fig. 11), disappearing on successive dilution. The same broad
component is observed to increase in intensity, with respect
to the structured component, in cast films after prolonged
exposure to light and air, or after thermal annealing in
vacuum or in air. In Fig. 11 the PL spectrum of a cast film of 7
is shown for a sample heated in air under light exposure. This
low energy component is also observed in evaporated films,
displaying a well structured emission with peaks at 485 and
31
Ca rather than holes from ITO electrodes, the high tendency
of 6 to crystallize reduces the charge mobility in the material
thus limiting the potential of this material in LED fabrication.
Conclusions
Fluorene core oligomers with thienyl and phenyl end groups
have been synthesized via organometallic synthesis, then
characterized both optically and structurally. The noticeable
PL of all the compounds in solution is modulated in solid state.
A comparison among the crystal structures of three oligomers
possessing different crystal packings can be performed by
considering the mean distances calculated between the closest
faced conjugated residues, which vary from 4.9 A˚ , in crystal 8,
to 5.1 A˚ , in 7, up to about 8 A˚ , in 6. Such facing between
5
chemical degradation as no signals attributable to oxygen
20 nm. In these samples we do not detect any evidence for
1
derivatives could be detected either in H NMR or FTIR
analysis. This low energy component seems thus to be
associated with aggregation rather than degradation effects.
Similar observations on polyfluorenes have been reported by
adjacent molecules is more efficient for 8, while in 7 a displaced
herringbone type of arrangement is observed and in 6 a very
far shifted facing is detected. Indeed many factors contribute
5
,28
Miller et al. and have been attributed to aggregate
formation. PL-QY of cast films are not reported due to the
9
30
J. Mater. Chem., 2002, 12, 924–933

View Article Online
5
–8
to the solid state PL-QY,
but the packing features are
pressure to give a pale yellow oil. Recrystallization in hexane
afforded 0.88 g of white crystals (yield 75%). Mp 67.5–68.5 uC;
MS(EI): m/z ~ 492 [M ]; FTIR (cm ): 1468, 1070, 880, 810;
insufficient to establish a ranking in the series; in this respect
further studies have to be performed.
1
21
1
The dependence of the optical properties of the three
molecules on their state of aggregation and its time evolution
have been reported. In diluted solutions, the PL-QY values
range from 45% to 65%. Absorption and emission spectra of
thienyl-residue containing molecules are red-shifted. In the
oligomer having linear alkyl chains as substituents, a quite fast
progressive crystallization takes place, while for molecule 7 the
solid state arrangement does not appreciably modify in short
times at room temperature. A progressive red-shift of the PL
is observed in the solid state by passing from amorphous
films (cast films or vacuum evaporated films) to crystals, to
aggregated films. Freshly prepared vacuum evaporated films of
3
H-NMR (270 MHz, CDCl , ppm): 7.52 (d, 4 and 5 protons,
J ~ 6.6 Hz,), 7.45 (dd, 3 and 6 protons, J ~ 6.6 Hz, J
o
~
o
m
m
1.7 Hz), 7.44 (d, protons 1 and 8, J ~ 1.7 Hz), 1.94–1.88 (m,
4H, a methylene protons), 1.19–1.03 (m, 12H), 0.78 (t, 6H,
methyl protons, J ~ 6.91 Hz), 0.64–0.52 (m, 4H, b methylene
protons).
Spiro[cyclopentane-1,9’-fluorene] (3)
4 ml (6.3 mmol) of n-butyllithium were added dropwise during
15 min to a stirred solution of 0.5 g (3 mmol) of fluorene in
THF (14 ml) at 278 uC in the dark; the colour of the solution
became orange. The mixture was stirred at 278 uC for 45 min
and then diluted with THF (14 ml), and then a solution of
0.65 g (3 mmol) of 1,4-dibromobutane in THF (3 ml) was
added dropwise during 10 min. The solution was allowed to
warm to room temperature, then it was stirred for 4 h. The
mixture was poured into water, which caused bleaching of the
colour, and extracted with ether. The organic layers were
washed with brine and dried over magnesium sulfate; the
solvent was removed by a rotary evaporator. The crude white
solid was washed with EtOH to provide 0.65 g (99%) of the title
compound as a white waxy solid. Mp 88–90 uC; MS (EI): m/z
7
and 8 display spectra similar to those of amorphous samples
and are quite sensitive to aggregation effects during time. The
features of the amorphous component mainly are observed in
cast or spin coated films from solution of 6 and 7. In contrast,
cast films of 8 give rise to an emission similar to that of the
crystalline phase, due to both crystal defects and large mosaic
spread. The study of a single layer LED device of 6 indicates
that the strong tendency of the film to crystallize induces very
low charge mobility giving electroluminescence spectra typical
of the aggregated material at the metal interface.
1
21
1
~
220 [M ]; FTIR (cm ): 1468, 760, 732; H-NMR (270 MHz,
Experimental
CDCl , ppm) 7.71–7.68 (m, protons 4 and 5 which are non-
3
equivalent), 7.45–7.41 (m, protons 1 and 8), 7.32–7.28 (m, 4H,
protons 2,3,6 and 7), 2.17–2.04 (m, 8H, aliphatic protons).
Materials
Fluorene (Aldrich), bromine (Merck), butyllithium 1.6 M in
hexane and tert-butyllithium 1.7 M in pentane, 1,4-dibromo-
butane and 1-bromohexane, 4-bromobenzonitrile (Aldrich)
and Pd(PPh ) (Janssen) were used as received. All the solvents
2’,7’-Dibromospiro[cyclopentane-1,9’-fluorene] (4)
3
4
The bromination was carried out in the dark. 3.2 mg
tetrahydrofuran (THF), diethyl ether, toluene, were freshly
distilled. 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
obtained from Aldrich was stored under nitrogen.
(
0.017 mmol) of ferric chloride and 0.17 ml (3.3 mmol) of
bromine were added to a stirred solution of 0.34 g (1.57 mmol)
of spiro[cyclopentane-1,9’-fluorene] in CHCl (2.4 ml) at 0 uC.
3
The solution was allowed to warm to room temperature and
stirred for 4.30 h. The resulting slurry was poured into water
and washed with sodium thiosulfate until the colour dis-
appeared. The organic layer was dried over magnesium sulfate
to give of a pale yellow oil which was recrystallized in hexane to
afford 0.5 g of white crystals (yield 85%). Mp 164.5–166.5 uC;
9
,9-Dihexylfluorene (1)
7
.87 ml (12.6 mmol) of n-butyllithium were added dropwise
during 20 min to a stirred solution of fluorene 1 g (6 mmol)
in THF (14 ml) at 278 uC; the colour of the solution became
red-orange. The mixture was stirred at 278 uC for 45 min and
1
21
MS (EI): m/z ~ 378 [M ]; FTIR (cm ): 1468, 1212, 886, 820;
1
.94 ml (13.8 mmol) of hexyl bromide in THF (3 ml) were
1
H-NMR (270 MHz, CDCl , ppm) 7.53 (d, protons 1 and 8,
3
added dropwise during 20 min. The solution was allowed to
warm to room temperature and stirred for 3 h. The mixture was
poured into water, which bleached the colour, and extracted
with ether. The organic extracts were washed with brine and
dried over magnesium sulfate, and the solvent was removed
by a rotary evaporator to provide 1.98 g (yield 99%) of the
title compound as a white, deliquescent solid. Mp 30–32 uC;
m o
J ~ 1.5 Hz), 7.51 (d, protons 4 and 5, J ~ 8.1 Hz), 7.44 (dd,
protons 3 and 6, J ~ 8.1 Hz, Jm ~ 1.5 Hz), 2.16–2.03 (m, 8H,
o
aliphatic protons).
2’,7’-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)spiro[cyclopentane-1,9’-fluorene] (5)
1
21
MS(EI): m/z ~ 334 [M ]; FTIR (cm ): 1450, 760, 730;
1
To a solution of 200 mg (0.52 mmol) of 2’,7’-dibromospiro-
H-NMR (500 MHz, CDCl , ppm) 7.79 (dd, 4 and 5 protons,
3
[cyclopentane-1,9’-fluorene] in dry THF (8 ml) at 278 uC
J
7
o
~ 6.8 Hz, J
.0 Hz, Jm ~ 1.1 Hz), 7.39 (overlapped triplets, protons 2, 3
~ 7.5 Hz), 2.08–205 (m, 4H, a methylene protons),
.23–1.14 (m, 12H), 0.86 (t, 6H, methyl protons, J ~ 7.17 Hz),
m o
~1 Hz), 7.43 (dd, 1 and 8 protons, J ~
was added dropwise n-butyllithium (0.7 ml, 0.52 mmol). The
greenish blue mixture was stirred at this temperature during
and 6, 7, J
1
0
o
1
h and 30 min, then 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (298 mg, 1.82 mmol) was added by syringe, and
the resulting solution was stirred at 278 uC for 1 h. In that time
the colour changed from brown through red to orange, then
the bath was left to reach room temperature very slowly under
stirring during 24 h. The pale yellow mixture was poured into
water and extracted with ether. The organic layer was washed
.77 (m, 4H, b methylene protons).
2
,7-Dibromo-9,9-dihexylfluorene (2)
The reaction was carried out in the dark to avoid bromination
of the aliphatic chains. To a stirred solution of 9,9-dihexyl-
fluorene 0.79 g (2.38 mmol) in CHCl
(
3
(3.6 ml) at 0 uC, 4.86 g
0.03 mmol) of ferric chloride and 0.25 ml (4.98 mmol) of
4
with water, brine and dried over MgSO . After removal of
the solvent, preparative TLC (hexane : ethyl acetate, 93 : 7 as
eluent) gave 38 mg of the title compound as a pale yellow solid
bromine were added. The red solution was allowed to warm to
room temperature and stirred for 3 h. The resulting slurry was
poured into water and washed with sodium thiosulfate until
the colour disappeared. The organic layer was dried over
magnesium sulfate and the solvent was removed under reduced
1
1
(yield 15%) MS (EI): m/z ~ 472 [M ]; H-NMR (500 MHz,
CDCl , ppm) 7.86 (s, protons 1 and 8), 7.79 (d, protons 4 and 5,
J ~ 7.52 Hz), 7.72 (d, protons 3 and 6, J ~ 7.52 Hz,), 2.17–
3
o
o
2.10 (m, 8H, aliphatic protons), 1.36 (s, 24H, methyl protons).
J. Mater. Chem., 2002, 12, 924–933 931

View Article Online
2
,7-Di(2-thienyl)-9,9-dihexylfluorene (6)
protons). Elemental analysis for C H (%): calculated C
2
9
24
9
3.51; H 6.49; found C 93.43; H 6.54.
2
,7-Dibromo-9,9-dihexylfluorene (0.19 g, 0.33 mmol), Pd-
(
3 4
PPh ) (44 mg, 0.038 mmol) and 2-tributylstannylthiophene
(
0.32 g, 0.85 mmol) were dissolved in 3 ml of dry toluene. The
2
’,7’-Bis(4-cyanophenyl)spiro[cyclopentane-1,9’-fluorene] (9)
reaction was performed following the procedure reported
above for 2’,7’-di(2-thienyl)spiro[cyclopentane-1,9’-fluorene]
except for the recrystallization from toluene–hexane mixture
giving the title compound as a pale yellow solid (yield 57%). Mp
4-Bromobenzonitrile (29.5 mg, 0.19 mmol), 2’,7’-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)spiro[cyclopentane-1,9’-
fluorene] (38 mg, 0.08 mmol) and Pd(PPh ) were dissolved in a
3
4
1
1
1
22 uC; MS (EI): m/z ~ 498 [M ]; H-NMR (270 MHz, CDCl
ppm) 7.68 (d, protons 4 and 5, J ~ 7.9 Hz), 7.60 (dd, protons
and 6, J ~ 7.9 Hz, Jm ~ 1.6 Hz), 7.56 (d, protons 1 and 8,
3
,
2 3
mixture of toluene (2 ml) and aqueous 2 M Na CO (1.4 ml)
o
under nitrogen atmosphere. The solution was refluxed during
25 h, than poured into slightly acid (HCl) water and extracted
more times with ether. The collected organic phase was dried
over Na SO and the solvent removed. Purification was per-
3
J
o
m
~ 1.6 Hz), 7.38 (dd, 2H, 3’ thienyl proton, J3’,4’ ~ 3.6 Hz,
J3’,5’ ~ 1.1 Hz), 7.29 (dd, 2H, 5’ thienyl proton, J5’,4’ ~ 5.1 Hz,
3’,5’ ~ 1.1 Hz), 7.1 (dd, 2H, 4’ thienyl proton, J4’,5’ ~ 5.1 Hz,
J4 ~ 3.6 Hz), 2.04–1.98 (m, 4H, a methylene protons), 1.38–
2
4
J
formed on preparative TLC (hexane : ethyl acetate, 93 : 7 as
eluent) yielding a white solid (60%). Mp 343–344 uC; MS
’,3’
1
APCI): m/z ~ 423 [M1H] ; H-NMR (500 MHz, CDCl
1
1
.21 (m, 4H, c methylene protons), 0.91 (t, 6H, methyl protons,
J ~ 7.21 Hz), 0.83–0.69 (m, 4H, b methylene protons).
Elemental analysis for C33 (%): calculated C 79.45; H
.68; S 12.86; found C 79.52; H 7.73; S 12.45.
(
3
,
ppm) 7.83 (d, protons 4 and 5, J ~ 7.9 Hz), 7.75 (s, 8H, end-
o
38 2
H S
phenyl protons), 7.65 (d, protons 1 and 8, J
dd, protons 3 and 6, J ~ 7.9 Hz, J ~ 1.4 Hz), 2.21 (s, 8H,
aliphatic protons). Elemental analysis for C H N (%):
m
~ 1.4 Hz), 7.59
7
(
o
m
3
1
22
2
2
’,7’-Di(2-thienyl)spiro[cyclopentane-1,9’-fluorene] (7)
calculated C 88.12; H 5.25; N 6.63; found C 88.09; H 5.22;
N 6.65.
2’,7’-Dibromospiro[cyclopentane-1,9’-fluorene] (0.15 g, 0.4 mmol)
and Pd(PPh ) (45 mg, 0.04 mmol) were dissolved in 3 ml of
dry toluene. After the addition of 2-tributylstannylthiophene
0.32 g, 0.85 mmol) the solution was heated at 55 uC for 1 h
3
4
Methods
(
and at 105 uC overnight, then it was poured into 5 ml of 2 M
HCl. The aqueous layer was extracted many times with toluene.
All the organic phases were collected, neutralized with
Mass spectrometric measurements were performed on a
Hewlett Packard 5985 B GC-MS instrument equipped with a
DB-5 MS (30 m) capillary column. Mass spectral data were
obtained under the following conditions: ionizing energy,
70 eV; ion source temperature, 200 uC; GC/MS transfer line
3 2 4
NaHCO and water, then dried over Na SO . The crude
solid resulting from solvent removal was purified by flash
chromatography using heptane : dichloromethane (7 : 3)
giving the title compound as a pale yellow solid (yield 72%).
2
1
temperature, 280 uC; scanning rate 1.2 scans s over the mass
range m/z ~ 33–520. Molecules of 9, introduced by direct
injection, were detected with a Finnigan MAT LCQ ion trap
1
21
Mp 181.7–182.7 uC; MS (EI): m/z ~ 384 [M ]; FTIR (cm
)
596, 1468, 1414, 888, 816, 760, 700; H-NMR (270 MHz,
1
1
1
dTCE, ppm) 7.69 (d, 2H, protons 4 and 5, J
mass spectrometer operating in APCI(1) mode. H-NMR
spectra were recorded on either a Bruker DMX 500 or a 270
MHz spectrometer. Absorption UV–vis spectra were obtained
o
~ 7.9 Hz), 7.59
~ 1.1 Hz), 7.64 (s,
(
dd, 2H, protons 3 and 6, J
o
~ 7.9 Hz, J
m
2
3
H, protons 1 and 8), 7.38 (d, 2H, 3’ thienyl proton, J3’, 4’
~
~ 5.0 Hz), 7.12
3
either in CHCl or in cyclohexane using a Varian-Cary 2400
.7 Hz), 7.31 (d, 2H, 5’ thienyl proton, J5
’, 4’
instrument. Fluorescence emission spectra were obtained from
THF solutions with the help of a SPEX 270M polychromator
equipped with a cooled CCD detector by exciting with a Xenon
lamp. PL-QY in solution was measured by using quinine
(
2
dd, 2H, 4’ thienyl proton, J4’, 5’ ~ 5.0 Hz, J4’, 3’ ~ 3.7 Hz),
.20–2.14 (m, 8H, aliphatic protons). Elemental analysis for
(%): calculated C 78.08; H 5.24; S 16.68; found C
25 20 2
C H S
7
8.15; H 5.29; S 16.49.
25
sulfate as a reference, in diluted solution (v10 M).
FTIR-spectra were recorded on a Bruker IFS 48 instrument.
Melting points were obtained by using either a Reichert
microscope coupled with a Mettler FP5 hot stage or a DSC
Perkin-Elmer PYRIS1 apparatus. All XRD experimental
details, structural resolution and refinement of the four crystal
structures are reported in Table 1. XRD patterns of powders
and films were obtained using a computer controlled Siemens
D-500 diffractometer equipped with an Anton-Paar camera for
variable temperature experiments under nitrogen atmosphere.
The structures were solved by direct methods using the
2
’,7’-Diphenylspiro[cyclopentane-1,9’-fluorene] (8)
To a solution of bromobenzene (0.1 g, 6.37 mmol) in diethyl
ether (8 ml), tert-butyllithium (13.3 mmol) dissolved in Et O
2
(
4 ml) was added dropwise during 15 min at 278 uC. The
solution was stirred at 278 uC for 1 h and transferred via
cannula into anhydrous zinc chloride (1.22 g, 8.95 mmol)
dissolved in THF (8 ml) at room temperature. The resulting
slurry was stirred for 1 h at room temperature, than was
3 4
transferred by cannula into a round flask containing Pd(PPh )
3
2
SIR88 program for 7 and 8 and SIR92 program for 4 and 6.
Refinement using full-matrix least-squares was carried out
keeping the hydrogen atoms fixed to the corresponding
(
13.7 mg, 0.012 mmol) and 2’,7’-dibromospiro[cyclopentane-
1
,9’-fluorene] (300 mg, 0.79 mmol) in THF (4.5 ml). The pale
yellow solution was heated to 55 uC and became brown during
stirring for 16 h; then it was cooled to room temperature and
poured into water. The aqueous layer was extracted three times
with CHCl
3
solid resulting from solvent removal was washed several times
with pentane then it was crystallized from toluene–ethanol
mixture giving 0.23 g of the title compound as a white solid
˚
C-atoms at 0.95 A, with the same isotropic thermal parameters
multiplied by 1.3. In view of its ease of crystallization, 8
displays twinned crystals and over more than 50 samples
examined, the only single crystal was of insufficient quality,
in fact a large mosaic dispersion was detected; this accounts for
the very approximate crystal structure obtained, see Table 2.
A list of the observed and calculated structure factors together
with torsion angles and anisotropic thermal vibration para-
meters is available on request to the authors.
3
and the collected organic layers were washed with
SO . The crude pale yellow
M HCl, water and dried over Na
2
4
1
(yield 77%). Mp 219–221 uC; MS (EI): m/z ~ 372 [M ]; FTIR
(cm ): 1596, 1464, 888, 816, 760, 700; H-NMR (500 MHz,
2
1
1
CD Cl , ppm) 7.78 (d, 2H, protons 4 and 5, J ~ 7.8 Hz), 7.69–
o
All reactions were carried out under an atmosphere of
purified inert gas, i.e. nitrogen. Flash chromatography was
performed on silica gel 60, 230–400 mesh ASTM (Merck or
Macherey-Nagel) column.
2
2
7
.65 (m, 6H), 7.59 (d, 2H, protons 3 and 6, J
dd, 4H, m-protons in phenyl ring, Jo ~ 7.52), 7.37 (t, 2H,
p-proton in phenyl ring, J ~ 7.22 Hz), 2.19 (s, 8H, aliphatic
o
~ 7.8 Hz), 7.47
(
o
9
32
J. Mater. Chem., 2002, 12, 924–933

View Article Online
Film and device preparation
Thin films of 6 and 7, up to 400 A thick, were prepared by
S. R. Ferrer, P. O. Jackson, S. M. Khan, L. May, M. O’Neill,
E. Nicholls, S. M. Kelly and J. C. Richards, Adv. Mater., 2000, 12,
971.
J. Cornil, D. A. dos Santos, X. Crispin, R. Silbey and J. L. Br e´ das,
J. Am. Chem. Soc., 1998, 120, 1289; J. Cornil, J. Ph. Calbert,
D. Beljonne, R. Silbey and J. L. Br e´ das, Adv. Mat, 2000, 12, 978;
J. Cornil, J. Ph. Calbert and J. L. Br e´ das, J. Am. Chem. Soc., 2001,
˚
2
7
vacuum deposition at 10
mbar, using a double filament
8
9
Knudsen cell, heating the crucible at the optimized evaporation
temperature of 185 uC and 175 uC respectively, after a slow
heating ramp suitably chosen to prevent overheating of the
1
23, 1250.
M. Bellet eˆ te, J. F. Morin, S. Beaupr e` , M. Ranger, M. Leclerc and
2
1
˚
source. The mean growing rates ranged from 3 A min to
2
1
˚
5
A min
Cast films of 6, 7, 8, 9 were obtained on quartz and ITO
at room temperature from THF, CHCl , cyclohexane and
.
G. Durocher, Macromolecules, 2001, 34, 2288.
10 N. Johansson, D. A. Dos Santos, S. Guo, J. Cornil, M. Fahlman,
J. Salbeck, H. Schenk, H. Arwin, J. L. Br e´ das and W. R. Salanek,
J. Chem. Phys., 1997, 107, 2542; R. W. Alder, K. R. Anderson,
P. A. Benjes, C. P. Butts, P. A. Kouitentis and A. G. Orpen, Chem.
Commun., 1998, 309; N. Johansson, J. Salbeck, J. Bauer,
F. Weiss o¨ rtel, P. Br o¨ ms, A. Andersson and W. R. Salaneck,
Adv. Mater., 1998, 10, 1136; W. Kreuder, D. Lupo, J. Salbeck,
H. Schenk and T. Stehlin, US Patent 5621131, 1997; D. Lupo,
J. Salbeck, H. Schenk, T. Stehlin and R. Stern, U. S. Patent
5840217, 1998; W. Yu, J. Pei and A. J. Heeger, Adv. Mater., 2000,
12, 828.
3
toluene. Subsequent thermal treatments on films of 6 were
carried out under nitrogen atmosphere, after three evacuation
cycles.
Spin coated films of 6 were prepared at 2000 rpm from
2
cyclohexane and toluene solution (8 mg cm ). Very thin films
3
(
Av 0.05) were used for PL measurements, while thicker films
were employed for device preparation. LEDs were prepared
by spin coating onto ITO (indium–tin-oxide) coated glass
substrates, by evaporating Ca and Al as top electrode at
vacuum v10 mbar and deposition rates of 150 A min
11
12
13
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Partial contributions to this work from both oriented project
MATSTA II of C.N.R. and project ‘‘Nanotechnologies’’ of
M.U.R.S.T. are kindly acknowledged. We also thank Professor
Lanfranchi for useful discussions. We are also indebted to
Professor F. De Martin of ‘‘Dipartimento di Chimica
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