Vacuum-processable ladder-type oligophenylenes for organic-inorganic hybrid structures: Synthesis, optical and electrochemical properties upon increasing planarization as well as thin film growth

Article abstract of DOI:10.1039/c2jm15868j

A novel synthetic route to even-numbered ladder-type oligo(p-phenylene)s (LOPPs) carrying no solubilizing groups to facilitate vacuum-processing is presented. The influence of increasing bridging adjacent phenylene units on the optical and electrochemical properties is discussed in the series of p-sexiphenyl 6P, terfluorene 3F, and ladder-type sexiphenyl L6P. The influence of the extension of the π-system is taken into consideration as well. Furthermore it is shown that highly ordered thin films of L6P on alumina surfaces can be prepared by organic molecular beam deposition (OMBD).

Full text of DOI:10.1039/c2jm15868j

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PAPER
Vacuum-processable ladder-type oligophenylenes for organic–inorganic
hybrid structures: synthesis, optical and electrochemical properties upon
increasing planarization as well as thin film growth
a
Bjorn Kobin, Lutz Grubert, Sylke Blumstengel, Fritz Henneberger and Stefan Hecht
a
b
b
a
*
*
*
€
Received 14th November 2011, Accepted 12th December 2011
DOI: 10.1039/c2jm15868j
A novel synthetic route to even-numbered ladder-type oligo(p-phenylene)s (LOPPs) carrying no
solubilizing groups to facilitate vacuum-processing is presented. The influence of increasing bridging
adjacent phenylene units on the optical and electrochemical properties is discussed in the series of
p-sexiphenyl 6P, terfluorene 3F, and ladder-type sexiphenyl L6P. The influence of the extension of the
p-system is taken into consideration as well. Furthermore it is shown that highly ordered thin films of
L6P on alumina surfaces can be prepared by organic molecular beam deposition (OMBD).
‘‘fruitfly’’ of organic electronics. Besides studies on the applica-
tion in organic light-emitting diodes (OLEDs)4,5 there are many
reports about its crystallographic structures on a variety of
metallic and dielectric surfaces,6 notably also on the electronic
structure at the interface with ZnO.2 Furthermore, non-radiative
energy transfer between ZnO and a spiro-derivative of 6P has
been demonstrated.7 However, due to its intrinsic flexibility, in
particular its rotational degrees of freedom along the aryl–aryl
connections, oligo- and poly(p-phenylene)s in general display
rather broad absorption and emission spectra and a significant
Stokes shift. To overcome these restrictions, we focused on
bridged oligo(p-phenylene)s, so-called ladder oligo(p-phenylene)
s (LOPPs), which due to their fixed planar geometry should
exhibit the desired optical features. Note that to date a large
number of typically odd-numbered oligomers as well as polymers
of that type has been synthesized and studies of their optical and
electronic properties as well as their applicability in OLEDs,
photovoltaic cells, and electronic materials have been described
in the literature,8 yet all these examples necessitate long alkyl
chains to provide sufficient solubility for solution processing.
However, the significant weight bestowed by the side chains
prohibits their evaporation without thermal degradation and
therefore it is not surprising that thus far there is no report on
vacuum-processable LOPPs.9 Furthermore, note that even-
numbered oligomers have been synthesized only up to four
phenylene units. Herein, we disclose a new synthetic route to
even-numbered LOPPs, designed specifically for vacuum-based
deposition techniques. We describe both the synthesis and
characterization with regard to their optical and electrochemical
properties in solution as well as the growth of thin films on
alumina surfaces by organic molecular beam deposition
(OMBD). These molecules should enable the realization of
the targeted organic–inorganic semiconductor hybrid structures
to allow for efficient exciton and charge transfer across the
organic–inorganic interface.
1. Introduction
Optoelectronic devices incorporating organic semiconducting
materials are heavily investigated to achieve higher device effi-
ciencies and longer durability in combination with their light
weight, flexibility, and potentially low cost.1 The integration of
organic and inorganic semiconductor materials to unite specific
favorable properties of both material classes is particularly
promising. To achieve proper interaction of both semiconductor
materials in order to promote either energy or electron transfer
between the two device components, the energy level alignment
at the interface and hence the organization of the molecular
building blocks is crucial.2 In order to gain a detailed under-
standing of these important processes it is essential to construct
well-defined inorganic–organic hybrid structures. Such model
systems can be realized by thin-film growth of organic molecules
on semiconductor surfaces by vapor deposition or molecular
beam deposition techniques.3 The most important prerequisites
for the molecular building blocks to be employed are therefore
their thermal stability beyond the evaporation temperatures.
Clearly, the organic material should possess the desired opto-
electronic properties, such as high absorption coefficients and
luminescence efficiencies, narrow absorption and luminescence
bands as well as a small Stokes shift to enable efficient exciton
migration within the organic layer. Furthermore, the organic
component should resist any type of optically or electrochemi-
cally induced degradation.
One of the prime organic semiconductors, meeting many but
not all of the aforementioned conditions, is p-sexiphenyl 6P—the
aDepartment of Chemistry, Humboldt-Universit
Brook-Taylor-Str. 2, 12489 Berlin, Germany. E-mail: sh@chemie.
hu-berlin.de
bInstitut fu€r Physik, Humboldt-Universita€t zu Berlin, Newtonstrasse 15,
12489 Berlin, Germany. E-mail: sylke.blumstengel@physik.hu-berlin.de;
fh@physik.hu-berlin.de
a€t zu Berlin,
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2. Results and discussion
2.1 Synthesis
Our synthesis to even-numbered LOPPs is based on a central
fluorene unit, which is connected to two terminal aromatic
moieties via cross-coupling reactions. The bridging, leading to an
extended aromatic system and hence extremely low solubility,
represents the penultimate step of the sequence. The functional
groups to induce the bridging reaction are incorporated in the
central building block and hence access to the highly function-
alized fluorene derivative 5 (Scheme 1) is required.
Since attempts to direct functionalization of 2,7-dibromo-9,9-
dimethylfluorene in its 3- and 6-position were not successful,
phenanthrene-9,10-dione 1 was chosen as the starting material as
it is regioselectively brominated using bromine and dibenzoyl
peroxide in nitrobenzene.10 Ring contraction of 3,6-dibromo-
phenanthrene-9,10-dione followed by decarboxylation using
aqueous KOH and KMnO4 provided 3,6-dibromofluorenone,11
which was reduced via a Wolff–Kishner reduction12 and the
formed 3,6-dibromofluorene was subsequently methylated
employing CH3I and KOtBu13 to provide 3,6-dibromo-9,9-
dimethylfluorene 2.14 Two-fold lithiation using n-butyl lithium
and subsequent reaction with chloromethyl methyl ether (MOM-
Cl) gave 3 in 75% yield. Note that other electrophiles, such as
dimethyl carbonate or acetone, gave lower yields. Diether 3 was
brominated with N-bromosuccinimide in propylene carbonate15
and the formed 2,7-dibrominated fluorene 4 was oxidized16 to the
corresponding diester building block 5.
The core unit 5 was reacted with 9,9-dimethylfluorene-2-
boronic acid, which was readily generated by lithiation of 6
followed by reaction with tributylborate (Scheme 2, top), in
a Suzuki cross-coupling reaction employing catalytic amounts of
Pd(PPh3)4 and Na2CO3 in a biphasic water–tetrahydrofuran
(THF) solvent mixture.17 Diester 7 was treated with excess
CH3MgI to yield diol 8. Final bridging via intramolecular Frie-
del–Crafts alkylation18 provides ladder-type sexiphenyl L6P,
readily visible by its characteristic blue fluorescence appearing
within a few seconds of the reaction. Purification of the final
product L6P was achieved by precipitation, extensive washing
of the solid, and final sublimation. Interestingly, the solubility
of L6P in CHCl3 and CH2Cl2 is much better as compared to
p-sexiphenyl 6P enabling characterization by standard solution
techniques, such as NMR, UV/vis absorption and fluorescence
measurement as well as cyclic voltammetry (CV). It appears that
Scheme 2 Synthesis of ladder oligomers L4P and L6P.
the methyl groups at the sp3-hybridized bridging carbon atoms
projecting out of plane prevent efficient p–p stacking, at least to
some degree.
The synthesis of the ladder-type quarterphenyl L4P was carried
out following an analogous route but employing phenylboronic
acid (Scheme 2, bottom). Intermediate diester 10 was reacted with
CH3MgI to afford diol 11, which was converted to L4P via
two-fold intramolecular Friedel–Crafts alkylation. Purification of
L4P was achieved by recrystallization and sublimation. For
comparison purposes, bifluorene 2F19 was synthesized by cross-
coupling 6 with its corresponding boronic acid while terfluorene
3F was synthesized according to literature procedures.20
2.2 Optical and electrochemical properties
The optical and electrochemical behavior of the newly synthe-
sized oligomers was investigated in solution, in particular to
determine the influence of increasing rigidity, i.e. loss of rota-
tional degrees of freedom, in a series ranging from 6P over 3F to
Scheme 1 Synthesis of highly functionalized fluorene 5 (TEBAC ¼
triethyl benzyl ammoniumchloride).
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L6P as well as the effect of extending the p-system in even-
numbered LOPPs in a series ranging from 1F over L4P to L6P
(Scheme 3).
Both absorption (labs) and fluorescence emission (lem) maxima
are bathochromically shifted with increasing degree of bridging,
i.e. going from 6P via 3F to L6P (Fig. 1, top). This is in good
agreement with the fact that bridging forces adjacent phenylene
units into coplanarity hence leading to an increased effective
p-conjugation. In addition, the maximum extinction coefficients
3max are significantly increased upon bridging, i.e. it is more than
doubled when going from 4P to L4P and again from 3F to L6P
as well as more than tripled going from 6P to L6P (Table 1). The
absorption and excitation spectra of 3F and 6P,21 respectively,
are rather broad and unstructured when compared to L6P. In
addition to having a very high extinction coefficient 3max the
absorption maximum of L6P is very sharp, in particular on the
low-energy edge. The degree of bridging has perhaps the most
profound effect on the Stokes shift (D~n), i.e. the energy offset
between absorption and emission maxima, which is significantly
decreased when going from 6P (4700 cmꢀ1) via 3F (2000 cmꢀ1) to
L6P (300 cmꢀ1 amounting to 0.04 eV only). These observations
are readily explained by the fact that oligo(p-phenylene)s can
freely rotate about the phenylene–phenylene bonds in the ground
state in solution, leading to absorption of various rotamers and
hence spectral broadening. Upon excitation the unbridged oligo
(p-phenylene)s can undergo relaxation on their excited state
potential energy surface by adopting a more planar conforma-
tion,22 hence giving rise to a large Stokes shift. In contrast, the
geometry of the rigidified L6P in its ground and excited states is
almost identical leading to a vanishing Stokes shift, while 3F
represents a somewhat intermediate case. Furthermore, rigidifi-
cation in L6P is reflected by the clearly visible vibronic coupling.
In this regard, analysis of the absorption spectra of the bridged
series, i.e. 1F, L4P, and L6P (Fig. 1, bottom), shows that in
all cases the 0,0-transition is favored. The distance between the
Fig. 1 Spectral properties of p-phenylenes in solution: absorption (Abs),
excitation (Exc), and fluorescence (Flu) spectra of 6P, 3F, and L6P in
CH2Cl2 (top). Absorption spectra of 1F, L4P, and L6P in CH2Cl2
(bottom).
In addition to the optical properties, the electrochemical
behavior of the newly synthesized LOPPs has been investigated
and compared to their respective compounds. The cyclo-
voltammograms of 3F (Fig. 2, top) and L6P (Fig. 2, bottom)
show that both compounds can reversibly be oxidized in two
successive one-electron steps to their respective radical cations
and dications. The bridged structure in L6P facilitates the first
oxidation event, as it occurs at a 300 mV lower oxidation
potential as compared to 3F (Table 2). Furthermore, the
enhanced p-conjugation and therefore stronger coupling lead to
0,0- and 0,1-transitions varies between 1250 and 1450 cmꢀ1
.
Importantly, the fluorescence quantum yield (F) of both L6P
and 3F is close to unity.23 With increasing size of the p-system,
the quantum yields seem to increase only slightly (Table 1).
Table 1 Optical properties of the investigated p-phenylenes in solutiona
labs/nm
3max/Mꢀ1 cmꢀ1
lem/nm
F
D~n/cmꢀ1
6P
3F
320b
349
407
369
301
295
55 000c
90 000
190 000
100 000
16 000
41 000
377
393
412
374
303
0.93c
0.99
4700
2000
300
360
220
L6P
L4P
1F
0.92
0.67
0.53–0.80d
0.89
4Pe
a
b
c
In CH2Cl2 at 25 ꢁC. Maximum of excitation spectrum. Values
according to ref. 24. Values for non-methylated fluorene according to
d
ref. 25 and 26. e Values according to ref. 27.
Scheme 3 Characterized compounds and variation of their structural
parameters.
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Fig. 3 Spectral change of L6P during electrochemical oxidation:
formation of radical cation and dication (c ¼ 8 ꢂ 10ꢀ5 Mꢀ1 in CH2Cl2).
Recorded in a 1 mm quartz cuvette using a platinum grid electrode, 0.1 M
Bu4NPF6, dE/dt ¼ 10 mV sꢀ1, 0 / 1.3 / 0 V vs. Ag/AgNO3, spectra
recorded every 7.3 mV.
Fig. 2 Cyclovoltammogram of 3F (top) and L6P (bottom) in CH2Cl2
(0.1 M Bu4NPF6), dE/dt ¼ 1 V sꢀ1
.
comparing the resulting films with drop-cast films (of varying
thickness). Inspection of the optical absorption and emission
spectra of L6P (Fig. 4) shows the characteristic features already
discussed for the corresponding solution spectra (see Fig. 1).
Regardless of the method for film formation, the spectral shapes
are rather similar reproducing all absorption and emission
maxima. However, it appears that in the case of the vacuum-
deposited film some bands in the absorption spectrum are more
pronounced, in particular the maximum at ca. 350 nm, while the
long-wavelength edge of the emission is significantly broadened.
We attribute these changes to higher degree of crystallinity in the
vacuum-deposited samples as compared to the ones cast from
solution. This is further supported by AFM measurements (see
below). Importantly, these initial results suggest that the ladder
oligomer L6P can successively be vacuum-deposited without
undergoing thermal decomposition.
a
a greater separation of the two oxidation peaks (DEp ) in L6P
(Table 2). The same trends are observed when comparing 2F with
its bridged analogue L4P. Therefore, the completely bridged
structures become more electron-rich, which could in part be
attributed to their smaller HOMO–LUMO gap, and their radical
cations are more stabilized by the enhanced p-conjugation as
compared to their dications. Furthermore, the number of
reversible one-electron oxidation steps of LOPPs as a function of
the number of phenylene units is basically the same as reported
for oligo(p-fluorene)s.28,29 Thus, whereas the oxidation of 1F is
irreversible, L4P forms a stable radical cation yet undergoes an
irreversible second electron transfer, and L6P exhibits two
reversible oxidation steps.
While performing the cyclovoltammetric measurements with
L6P UV/vis absorption spectra were recorded (Fig. 3). The
formation of the radical cation L6P+_ is indicated by a newly
formed absorption band maximum located at 581 nm. Moving to
higher potential the absorption band of the dication L6P2+ can be
observed with a maximum at 1084 nm. Note that the spectral
shape of the three species remains rather similar as all three
compounds display three maxima, due to vibronic transitions,
and the distance between the first and second absorption
Thin films of L6P were prepared on Al2O3 (0001) substrates by
OMBD in an UHV (5 ꢂ 10ꢀ9 Torr) deposition chamber. The
deposition rate as measured by a quartz microbalance was 0.1
nm minꢀ1. The substrate was kept at room temperature. The
morphology of a L6P submonolayer at a coverage of 0.27 is
depicted in the AFM image of Fig. 5. Extended flat molecular
maximum remains at about 1450–1500 cmꢀ1
.
2.3 Thin film growth
Initial studies were carried out by evaporating L6P onto quartz
slides using conventional sublimation apparatus and
a
Table 2 Oxidation potentials of investigated p-phenylenesa
Epa1/V
Epa2/V
DEpa/V
3F
0.793
0.492
0.687
1.276b
0.894
1.009
0.778
1.193b
—
0.216
0.286
0.506
—
L6P
L4P
1F
2F
1.248b
0.354
a
Potentials vs. Fc/Fc+ in CH2Cl2 (0.1 M Bu4NPF6), dE/dt ¼ 1 V sꢀ1
.
Fig. 4 Absorption (Abs) and photoluminescence (PL) spectra of thin
films of L6P on quartz substrates, either prepared by vacuum deposition
or drop-casting from CH2Cl2 solution.
b
Irreversible oxidation.
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semiconductors, such hybrid inorganic–organic structures,
merging the advantageous properties of both material classes
promise exciting new phenomena to be unraveled and exploited
for optoelectronic and photonic applications.
4. Experimental section
Absorption measurements were carried out on a Varian Cary 50
Bio UV/vis spectrometer in 1 cm quartz cuvettes. Fluorescence
spectra were obtained on a Varian Cary Eclipse spectrofluo-
rimeter. Solutions of a concentration of about 10ꢀ6 mol Lꢀ1 (for
dimethylfluorene about 10ꢀ5 mol Lꢀ1) were used for the
absorption measurements. Perylene (F ¼ 94%, ref. 25) was used
as references for the measurement of the fluorescence quantum
yields. It was dissolved in cyclohexane, the other products in
CH2Cl2. The solutions were diluted 1 by 30 for the measurement
of fluorescence spectra. The reference solution was degassed with
argon before carrying out the measurements. Electrochemical
measurements were carried out using a HEKA-Elektronik
PG310 potentiostat or an Autolab PGSTAT128N and an Ava
Spec-2048x14 spectrometer equipped with an Avalight-DH-S-
1
BAL lamp. H-NMR and 13C-NMR spectra were referenced to
7.26 ppm and 77.16 ppm, respectively, for CDCl3 and 5.32 ppm
and 53.8 ppm, respectively, for CD2Cl2. Thin films of L6P were
grown in an OMBD system by CreaPhys. The AFM image was
recorded in tapping mode with a Nanoscope 3a controller,
Veeco, USA.
3,6-Dibromophenanthrene-9,10-dione
The literature procedure10 was adapted to large scale synthesis:
phenanthrene-9,10-dione 1 (52 g, 250 mmol) and dibenzoyl
peroxide (2 g, 8.3 mmol) were dissolved in nitrobenzene (250
mL). An initial amount of bromine (14.5 g, 90 mmol) was added
to the mixture and heated to 120 ꢁC. When the formation of
gaseous HBr started, the remaining bromine (72 g, 450 mmol, in
sum 86.4 g, 540 mmol) was added dropwise. After heating for one
hour, the mixture was cooled and ethanol (250 mL) was added.
The precipitated product was filtered and washed with ethanol
until the washing solution turned colourless. After drying under
vacuum 83.4 g (228 mmol, 91% yield) 3,6-dibromophenanthrene-
Fig. 5 AFM image of L6P deposited on an Al2O3 (0001) surface (top)
and the corresponding height profile (bottom).
islands with a height corresponding approximately to the length
of the L6P are visible. Thus, the islands are comprised of upright
standing molecules. Such morphology is typical for molecules
crystallizing in a herringbone structure on chemically inert
substrates (such as sapphire). It was found also for 6P molecules
deposited on ZnO (0001)2 and TiO2.30
1
9,10-dione were obtained as an orange powder. H-NMR (500
MHz, CDCl3) d (ppm) 8.12 (d, J ¼ 1.8 Hz, 2H), 8.07 (d, J ¼ 8.3
Hz, 2H), 7.67 (dd, J ¼ 8.3, 1.7 Hz, 2H). 13C-NMR (126 MHz,
CDCl3) d (ppm) 179.0, 136.1, 133.6, 132.3, 130.0, 127.6.
3. Conclusions
A new synthetic route to yield even-numbered LOPPs has been
developed based on the use of a highly functionalized central
fluorene building block. Direct comparison with the structurally
related non-bridged oligo(p-phenylene)s as well as partially
bridged oligofluorenes shows that the newly synthesized LOPPs
exhibit outstanding photophysical properties, such as sharp and
intense optical transitions characterized by narrow absorption
bands and very small Stokes shifts as well as large extinction
coefficients and high fluorescence quantum yields. The origin of
the observed optical properties can clearly be linked to their
rigid structure maximizing p-conjugation. Furthermore, oxida-
tion of LOPPs is more facile as compared to their non-bridged
counterparts. Initial investigations demonstrate that these
materials are applicable to OMBD techniques allowing the
controlled generation of organic (ultra)thin films on solid
3,6-Dibromo-9-fluorenone
The literature procedure11 was used: KOH (101 g, 2.2 mol) was
dissolved in 150 mL of water and heated to 130 ꢁC. Then,
3,6-dibromophenanthrene-9,10-dione (61.9 g, 169 mmol) was
suspended in the solution. After stirring for 30 min, the mixture
turned black and very viscous. Within a period of 2 h KMnO4
(141.5 g, 895 mmol) was added carefully. The mixture was stirred
at 130 ꢁC for one hour and cooled to room temperature, followed
by neutralization with concentrated sulfuric acid. Sodium
bisulfite was added carefully to the (slightly acidic!) mixture until
it turned light yellow. The precipitate was filtered off and washed
with water. 3,6-Dibromo-9-fluorenone (41.1 g, 121.6 mmol, 72%
yield) was obtained as a light yellow powder. 1H-NMR (500
substrate
surfaces.
In
conjunction
with
inorganic
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MHz, CDCl3) d (ppm) 7.67 (d, J ¼ 1.6 Hz, 2H), 7.55 (d, J ¼ 7.8
Hz, 2H), 7.50 (dd, J ¼ 7.8, 1.6 Hz, 2H).
dimethylfluorene 3 (1.9 g, 6.7 mmol) was dissolved and NBS (2.4
g, 13.4 mmol) was added. The mixture was stirred for 16 h at 60
ꢁC. Then, water was added and the precipitate was filtered off.
Purification by column chromatography gave 2.5 g (5.7 mmol,
85% yield) 2,7-dibromo-3,6-bis(methoxymethyl)-9,9-dimethyl-
fluorene 4 as a colorless solid. 1H-NMR (500 MHz, CDCl3)
d (ppm) 7.82 (s, 2H), 7.57 (s, 2H), 4.59 (s, 4H), 3.53 (s, 6H), 1.46
(s, 6H). 13C-NMR (126 MHz, CDCl3) d (ppm) 154.4, 138.0,
136.6, 127.1, 121.8, 120.7, 74.3, 58.9, 47.1, 27.1.
3,6-Dibromofluorene
The literature procedure12 was modified concerning workup and
purification: 3,6-dibromo-9-fluorenone (33.8 g, 100 mmol) was
dispersed in 300 mL of triethylene glycol and hydrazine hydrate
(100%, 21.8 mL, 450 mmol) was added. While stirring overnight at
100 ꢁC the solution slowly turned clear. Then, KOH (33 g in 90 mL
ꢁ
of water) was added. Stirring was continued for 2 h at 130 C.
Dimethyl-2,7-dibromo-9,9-dimethylfluorene-3,6-dicarboxylate (5)
After cooling to room temperature, the mixture was poured into
1.2 L of water and neutralized with HCl. The orange precipitate
was filtered off, dried and purified by sublimation (10ꢀ3 mbar,
heater at 220 ꢁC). 3,6-Dibromofluorene (20.2 g, 62.3 mmol, 62%
yield) was obtained as a nearly white solid. 1H-NMR (500 MHz,
CDCl3) d (ppm) 7.87 (d, J ¼ 1.7 Hz, 2H), 7.44 (dd, J ¼ 8.0, 1.8 Hz,
2H), 7.40 (d, J ¼ 8.0 Hz, 2H), 3.80 (s, 2H). 13C-NMR (126 MHz,
CDCl3) d (ppm) 142.8, 142.3, 130.4, 126.7, 123.5, 121.2, 36.4.
The literature procedure16 was adapted to substrate 4: in 10 mL
of CH2Cl2 2,7-dibromo-3,6-bis(methoxymethyl)-9,9-dimethyl-
fluorene 4 (0.16 g, 0.36 mmol), benzyltriethylammonium chloride
(TEBAC, 0.49 g, 2.16 mmol) and KMnO4 (0.34 g, 2.16 mmol)
were dissolved and stirred for 5 h at reflux. Then an aqueous
solution of sodium thiosulfate was added until the purple color
disappeared. The mixture was extracted with CH2Cl2, dried
(MgSO4) and the solvent was removed. Column chromato-
graphy (cyclohexane/ethyl acetate) gave 0.15 g (0.32 mmol,
89% yield) dimethyl 2,7-dibromo-9,9-dimethylfluorene-3,6-
3,6-Dibromo-9,9-dimethylfluorene (2)
The literature procedure13 was adapted to the substrate: 3,6-
dibromofluorene (17.8 g, 55 mmol) was dissolved in dry THF
1
dicarboxylate 5. H-NMR (500 MHz, CDCl3) d (ppm) 8.13 (s,
2H), 7.69 (s, 2H), 3.97 (s, 6H), 1.48 (s, 6H). 13C-NMR (126 MHz,
CDCl3) d (ppm) 166.7, 157.7, 136.9, 131.4, 129.2, 123.3, 121.5,
52.7, 47.8, 26.6.
ꢁ
(150 mL) and cooled to 0 C. KOtBu (18.5 g, 165 mmol), and
after stirring for 10 min, CH3I (10.3 mL, 165 mmol) were added.
The solution was allowed to warm to room temperature and it
was stirred overnight. Then water was added and the mixture was
extracted with ethyl acetate. The organic phase was dried with
MgSO4. Upon removing the solvent, the crude product remained
as an orange solid. Column chromatography (cyclohexane) gave
12.2 g (34.7 mmol, 63% yield) 3,6-dibromo-9,9-dimethylfluorene
2 as a white solid. 1H-NMR (500 MHz, CDCl3) d (ppm) 7.80 (d,
J ¼ 1.8 Hz, 2H), 7.45 (dd, J ¼ 8.0, 1.8 Hz, 2H), 7.29 (d, J ¼ 8.0
Hz, 2H), 1.45 (s, 6H). 13C-NMR (126 MHz, CDCl3) d (ppm)
152.7, 140.2, 130.9, 124.4, 123.6, 121.2, 46.8, 26.9.
2-Bromo-9,9-dimethylfluorene (6)
2-Bromofluorene (10 g, 4ꢁ0.8 mmol) was dissolved in 200 mL of
DMSO and cooled to 0 C. Then, CH3I (6.1 mL, 97.9 mmol),
TEBAC (0.46 g, 2 mmol) and 25 mL of 50% aq. NaOH were
added. After stirring for 30 min, the solution was poured into
water and extracted with ethyl acetate. The organic phase was
washed with HCl, dried with MgSO4 and the solvent was
removed. Column chromatography (petroleum ether/ethyl
acetate) gave 11 g (40.2 mmol, 99% yield) 2-bromo-9,9-dime-
3,6-Bis(methoxymethyl)-9,9-dimethylfluorene (3)
1
thylfluorene 6. H-NMR (300 MHz, CDCl3) d (ppm): 7.73–7.68
(m, 1H), 7.61–7.57 (m, 2H), 7.50–7.42 (m, 2H), 7.38–7.33
(m, 2H), 1.49 (s, 6H). 13C-NMR (75 MHz, CDCl3) d (ppm) 155.8,
153.4, 138.3, 138.2, 130.2, 127.8, 127.3, 126.2, 122.8, 121.5, 121.1,
120.2, 47.2, 27.1.
3,6-Dibromo-9,9-dimethylfluorene 2 (12 g, 34 mmol) was dis-
solved under argon in 200 mL of dry THF and the solution was
cooled to ꢀ78 ꢁC. n-BuLi (2.2 M in cyclohexane, 42 mL, 92
mmol) was added and the solution was stirred for 30 min. Then,
chloromethyl methyl ether (MOM-Cl, 8.5 mL, 112 mmol) was
added and stirring was continued for 30 min at ꢀ78 ꢁC and
overnight (solution turned clear after 10 min) at room temper-
ature. The mixture was poured into water. It was extracted with
ethyl acetate, dried (MgSO4) and the solvent was removed.
Column chromatography (petroleum ether (b.p. 40–60 ꢁC)/ethyl
acetate) gave 7.2 g (25.5 mmol, 75% yield) 3,6-bis(methoxy-
methyl)-9,9-dimethylfluorene 3. 1H-NMR (500 MHz, CDCl3)
d (ppm) 7.73 (d, J ¼ 0.7 Hz, 2H), 7.41 (d, J ¼ 7.7 Hz, 2H), 7.29
(dd, J ¼ 7.7, 1.3 Hz, 2H), 4.54 (s, 4H), 3.44 (s, 6H), 1.48 (s, 6H).
13C-NMR (126 MHz, CDCl3) d (ppm) 153.6, 139.5, 137.2, 127.1,
122.6, 119.7, 95.8, 75.1, 58.3, 46.7, 27.3.
Terfluorene (7)
General procedure for Suzuki coupling according to ref. 17: 2-
bromo-9,9-dimethylfluorene 6 (4.8 g 17.5 mmol) was dissolved
under argon atmosphere in 50 mL of dry THF and cooled to ꢀ78
ꢁC. Subsequently n-BuLi (2.2 M in cyclohexane, 8.9 mL, 19.6
mmol) was added and the mixture was stirred for 30 min. It
turned brown and milky. Upon adding tributylborate (5.7 mL,
21 mmol), the solution was stirred for one hour at room
temperature.
Meanwhile dimethyl 2,7-dibromo-9,9-dimethylfluorene-3,6-
dicarboxylate 5 (3.3 g, 7 mmol) was dissolved in 50 mL of THF
and degassed using argon. Then, Pd(PPh3)4 (0.65 g, 0.56 mmol)
was added. After stirring for 15 min, 40 mL of 2 M aq. sodium
carbonate solution was added and the temperature was raised
to 60 ꢁC.
2,7-Dibromo-3,6-bis(methoxymethyl)-9,9-dimethylfluorene (4)
The literature procedure15 was adapted to substrate 3: in 30 mL
of
propylene
carbonate
3,6-bis(methoxymethyl)-9,9-
4388 | J. Mater. Chem., 2012, 22, 4383–4390
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Both solutions were combined and stirred overnight at 60 ꢁC.
After cooling to room temperature, water was added and the
mixture was extracted with ethyl acetate. The organic phase was
dried (MgSO4) and the solvent was removed. The crude product
was first purified by column chromatography and then dissolved
in a mixture of CH2Cl2 and ethyl acetate. CH2Cl2 was removed
under reduced pressure and the product precipitated. Upon
suction and drying under vacuum 3.6 g (5.2 mmol, 74% yield) of
terfluorene 7 were obtained. 1H-NMR (500 MHz, CDCl3)
d (ppm) 8.26 (s, 2H), 7.81 (d, J ¼ 8.1 Hz, 2H), 7.80–7.77 (m, 2H),
7.56 (s, 2H), 7.50–7.46 (m, 2H), 7.46–7.42 (m, 4H), 7.41–7.33
(m, 4H), 3.67 (s, 6H), 1.61 (s, 6H), 1.56 (s, 12H). 13C-NMR (126
MHz, CDCl3) d (ppm) 169.9, 156.9, 153.9, 153.7, 142.5, 140.8,
139.0, 138.6, 137.4, 130.9, 127.5, 127.4, 127.2, 125.1, 123.0, 122.8,
122.0, 120.3, 120.0, 52.2, 47.8, 47.0, 27.4, 27.1.
8 (0.4 g, 0.57 mmol) was dissolved in 50 mL of CH2Cl2 and
BF3$THF (50% in THF, 1.9 mL, 8.5 mmol) was added. After
stirring for 30 min at room temperature, ethanol and aq.
NaHCO3 were added. The organic phase was concentrated and
the product was separated by centrifugation. The yellowish
precipitate was washed with ethyl acetate several times. Finally,
0.28 g (0.42 mmol, 75% yield) of the ladder-type sexiphenyl L6P
was obtained. 1H-NMR (500 MHz, CDCl3) d (ppm) 7.84 (s, 6H),
7.80–7.77 (m, 4H), 7.46 (d, J ¼ 7.3 Hz, 2H), 7.36 (td, J ¼ 7.4, 0.9
Hz, 2H), 7.31 (td, J ¼ 7.2, 0.9 Hz, 2H), 1.66 (s, 6H), 1.65 (s, 12H),
1.59 (s, 12H). 13C-NMR (126 MHz, CDCl3) d (ppm) 154.2,
153.8, 153.8, 153.7, 153.3, 139.7, 139.3, 139.0, 138.9, 138.6, 127.1,
127.0, 122.7, 119.8, 114.3, 114.0, 114.0, 113.9, 46.7, 46.5, 29.9,
27.9, 27.6.
Ladder-type quarterphenyl (L4P)
Dimethyl 2,7-diphenyl-9,9-dimethylfluorene-3,6-dicarboxylate
(10)
Starting with 2,7-diphenyl-3,6-bis(1,1-dimethylhydroxymethyl)-
9,9-dimethylfluorene 11 (0.07 g, 0.15 mmol), 0.03 g (0.07 mmol,
47% yield) of the ladder-type quarterphenyl L4P was obtained
Starting from bromobenzene 9 (0.47 g, 3 mmol) and dimethyl
2,7-dibromo-9,9-dimethylfluorene-3,6-dicarboxylate 5 (0.47 g,
1 mmol), 0.42 g (0.91 mmol, 91% yield) dimethyl 2,7-diphenyl-
9,9-dimethylfluorene-3,6-dicarboxylate 10 was obtained without
1
after recrystallization from ethyl acetate. H-NMR (500 MHz,
CDCl3) d (ppm) 7.83 (s, 2H), 7.79–7.76 (m, 4H), 7.46 (d, J ¼ 7.2
Hz, 2H), 7.36 (td, J ¼ 7.4, 1.2 Hz, 2H), 7.31 (td, J ¼ 7.3, 1.1 Hz,
2H), 1.62 (s, 6H), 1.58 (s, 12H). 13C-NMR (75 MHz, CDCl3)
d (ppm) 154.0, 153.6, 153.1, 139.5, 139.0, 138.5, 127.0, 126.9,
122.6, 119.7, 114.2, 113.9, 46.6, 46.3, 27.7, 27.4.
1
the above mentioned crystallization step. H-NMR (500 MHz,
CDCl3) d (ppm) 8.24 (s, 2H), 7.47–7.42 (m, 6H), 7.41–7.37 (m,
6H), 3.69 (s, 6H), 1.54 (s, 6H). 13C-NMR (126 MHz, CDCl3)
d (ppm) 169.2, 156.8, 142.5, 141.7, 137.3, 130.3, 128.4, 128.1,
127.3, 125.2, 121.9, 52.0, 47.6, 26.9.
Acknowledgements
Terfluorene diol (8)
Generous support by the German Research Foundation (DFG
via SFB 951) is gratefully acknowledged. Wacker Chemie AG,
BASF AG, Bayer Industry Services, and Sasol Germany are
thanked for generous donations of chemicals.
General procedure for the reaction of terfluorene diesters with
Grignard reagents to terfluorene diols: terfluorene 7 (0.9 g, 1.3
mmol) was dissolved in 50 mL of dry THF under argon and
CH3MgI (3 M in diethyl ether, 10.8 mL, 32.5 mmol) was added.
After refluxing for 2 h, water was added and the mixture was
extracted with diethyl ether. The organic phase was dried with
MgSO4 and the solvent was removed. Column chromatography
(cyclohexane/ethyl acetate) gave 0.40 g (0.58 mmol, 44% yield) of
terfluorene 8. 1H-NMR (500 MHz, CD2Cl2) d (ppm) 8.16 (s, 2H),
7.80–7.76 (m, 4H), 7.51–7.46 (m, 4H), 7.36 (m, 6H), 7.21 (s, 2H),
2.12 (s, 2H), 1.53 (s, 12H), 1.52 (s, 12H), 1.49 (s, 6H). 13C-NMR
(75 MHz, CD2Cl2) d (ppm) 154.2, 153.5, 152.0, 146.3, 143.7,
140.0, 139.2, 138.6, 138.3, 129.0, 127.7, 127.4, 126.6, 124.8, 123.0,
120.3, 119.5, 117.8, 74.4, 47.2, 46.8, 33.0, 27.4, 27.2.
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J. Mater. Chem., 2012, 22, 4383–4390 | 4389

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4390 | J. Mater. Chem., 2012, 22, 4383–4390
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