A convergent synthesis of (diphenylvinyl)benzene (DPVB) star-shaped compounds with tunable redox, photo- and electroluminescent properties

Article abstract of DOI:10.1039/b707496d

In this paper we describe a convenient and convergent approach to the synthesis of novel and highly luminescent (diphenylvinyl)benzene (DPVB)-based conjugated star-shaped molecules, which can be used as emitting dyes in doped OLEDs. The versatility of this synthetic route allows tuning of the HOMO-LUMO energy levels of the materials by means of the introduction of different electroactive peripheral groups. Two related star-shaped dyes have been prepared, one with amine and one with alkoxy end groups, and have been characterized using optical spectroscopy and cyclic voltammetry. The energy levels of the dyes are found to lie within the band gap of a polyfluorene (PF) derivative such that energy transfer from the photoexcited PF to the dyes takes place. In OLEDs that contain 1 wt% of dye in PF, the emission is predominantly from the dye. Although the EL spectra are similar for the two dyes, other device characteristics differ greatly. OLEDs using the amine-terminated dye have a luminance that is a few orders of magnitude lower than that based on the alkoxy-terminated dye. The differences are explained by the positioning of the HOMO and LUMO levels of the two dyes compared to those of PF. The dye with the amine end group can act as a deep trap for holes, reducing charge transport in the film. The Royal Society of Chemistry.

Full text of DOI:10.1039/b707496d

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www.rsc.org/materials | Journal of Materials Chemistry
A convergent synthesis of (diphenylvinyl)benzene (DPVB) star-shaped
compounds with tunable redox, photo- and electroluminescent properties
Rafael Go´mez,ab Dirk Veldman,b Bea M. W. Langeveld,c Jose´ L. Segura*a and Rene´ A. J. Janssen*b
Received 17th May 2007, Accepted 3rd July 2007
First published as an Advance Article on the web 23rd July 2007
DOI: 10.1039/b707496d
In this paper we describe a convenient and convergent approach to the synthesis of novel and
highly luminescent (diphenylvinyl)benzene (DPVB)-based conjugated star-shaped molecules,
which can be used as emitting dyes in doped OLEDs. The versatility of this synthetic route allows
tuning of the HOMO–LUMO energy levels of the materials by means of the introduction of
different electroactive peripheral groups. Two related star-shaped dyes have been prepared, one
with amine and one with alkoxy end groups, and have been characterized using optical
spectroscopy and cyclic voltammetry. The energy levels of the dyes are found to lie within the
band gap of a polyfluorene (PF) derivative such that energy transfer from the photoexcited PF to
the dyes takes place. In OLEDs that contain 1 wt% of dye in PF, the emission is predominantly from
the dye. Although the EL spectra are similar for the two dyes, other device characteristics differ
greatly. OLEDs using the amine-terminated dye have a luminance that is a few orders of magnitude
lower than that based on the alkoxy-terminated dye. The differences are explained by the
positioning of the HOMO and LUMO levels of the two dyes compared to those of PF. The dye with
the amine end group can act as a deep trap for holes, reducing charge transport in the film.
heterocycles, leading to highly efficient OLEDs covering a
wide range of emission wavelengths.8
Introduction
Organic electroluminescent materials have been the subject of
Another approach to fine tune the emission properties of
intense research for lighting, back lights, and full-color flat-
PFs is the use of a dopant with enhanced color purity. Thus,
panel displays due to their balance of low cost and high
dye-doped OLEDs have evolved as an option to improve the
efficiency.1 Although the electroluminescence (EL) of organic
electrooptical properties of polymer OLEDs in which doping
compounds was first demonstrated by Bernanose in 1953,2 it
of the device with a small amount of a fluorescent dye can lead
was not significantly developed until 1987 when Tang and
to significant changes in the color of luminescence and an
improvement in the device properties.9 In these types of
Van Slyke constructed a device employing aluminium tris(8-
hydroxyquinoline) as the electroluminescent active layer.3 In
devices the luminescent properties of the dopants determine
particular, the use of semiconducting polymers in organic
the emission characteristics of the device, whereas the trans-
electroluminescent devices (OLEDs) since the report by Friend
port properties of the host primarily impact on the electrical
characteristics of the device.10 An efficient host–guest system is
et al. of the first poly(p-phenylenevinylene) (PPV)-based
OLED,4 has been responsible for the rapid development of
usually fabricated using highly radiative guest molecules with
the field. Ever since this discovery, many conjugated polymers
have been thoroughly investigated regarding their application
in OLEDs. Among them, polyfluorene (PF) derivatives emerge
as one of the most relevant electroluminescent materials due to
their high photoluminescence (PL) and electroluminescence
(EL) quantum yields and good chemical and thermal
stabilities. Furthermore, facile methods for functionalizing
the C9 position of the fluorene unit offer the possibility of
tuning the optoelectronic properties of PF and to obtain
materials with enhanced solubility and film forming ability.5–7
Possibly, the most successful synthetic strategy to tune the
energy levels is copolymerization of fluorene with e.g. aromatic
the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) levels lying inside the
HOMO–LUMO gap of the host.11 This situation allows
energy transfer from the singlet exciton state from the host to
the guest, thus leading to emission solely or predominantly
from the dye, even at very low dye-loading levels.9,12 The dye
acts as an emissive trap for the excitons formed in the device. It
is a clear case that appropriate engineering of the energy levels
of the guest material must be accomplished to obtain useful
materials. In relation to that, it is well known from device
studies on organic host–guest systems that the introduction of
low concentrations of dyes in a conducting polymer also
strongly affects the charge carrier mobilities if they act as
electron or hole traps.13 That is, if the HOMO (LUMO) level
of the dopant is above (below) that of the host, holes
(electrons) get trapped on the dyes, a process that can strongly
affect the charge carrier mobility. The reduction of the charge
carrier mobility is strongly dependent on the amount of
disorder in the host, the concentration of charge carriers, the
aDepartamento de Qu´ımica Orga´nica, Facultad de Qu´ımica, Universidad
Complutense de Madrid, E-28040, Madrid, Spain.
E-mail: segura@quim.ucm.es; Tel: +34-91-3945142
bMolecular Materials and Nanosystems, Eindhoven University of
Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
cTNO Science and Industry, P.O. Box 6235, 5600 HE Eindhoven,
The Netherlands. E-mail: r.a.j.janssen@tue.nl; Tel: +31-40-2473597
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concentration of guest and for a large part on the energy
difference between the host and guest energy levels as has been
modeled extensively.14
band gap of the polymer host which hampers charge transport,
in contrast to the dye with the alkoxy end groups that shows
much higher luminance.
The requirements for a good guest material are thus
stringent: the HOMO and LUMO levels need to be within
the gap of the host, but they may not lie too deep in the
HOMO–LUMO gap, as that may cause charge carrier
trapping. The versatility of our synthetic route, allowing the
introduction of different peripheral groups, enables adjust-
ment of the energy levels of the dyes to those of the host.
On the basis of their unique properties, monodisperse
dendritic materials, e.g. starburst molecules, have emerged as
attractive candidates for optoelectronic applications as they
combine a well-defined architecture and conjugation length
with good processability.15 Remarkably, dendrimers offer an
appealing approach for the rational design of the materials, as
it is possible to assign different functional properties to each
component of the macromolecular structure. Moreover, not
only do dendrimers allow the control of the optical, electrical
or processing characteristics by the core, dendrons or surface
groups, but also the dendritic shell decreases the interactions
between the neighboring molecules or solvent and the core and
the formation of excimers (i.e. intermolecular excitations) or
aggregates, so that emissive excitons can be shielded from the
environment through the periphery of the dendrimer, thus
reducing parasitic perturbations such as solvation effects in the
solid phase or luminescence quenching.16 Dendritic macro-
molecules have been blended into organic light emitting diodes
(OLEDs) as either hole transporting components, electron
transporting components, or even as single component
emitters.17 So far, dendritic analogues of the most important
conjugated polymeric materials, namely poly(p-phenylene-
ethynylene), poly(p-phenylene), poly(p-phenylenevinylene)
and polythiophene derivatives, have already been prepared
as monodisperse macromolecular alternatives to their parent
polymers.18
Results and discussion
Synthesis
The convergent synthetic approach for the synthesis of
the starburst molecules 17 and 18 (Fig. 1), combining a
triphenylethene-based stilbenoid skeleton, an array of elec-
tron-donating dialkylamine or trialkoxy groups and bearing
solubilizing alkyl chains on the peripheral positions is based on
an olefination reaction between the benzene trisphosphonate
16 and dendritic branches functionalized with aldehyde groups
(Scheme 4; see later). Therefore, the first target molecules for
our synthetic workup are (diphenylvinyl)benzenes (DPVBs),
i.e. triphenylethenes, containing phenylenevinylene-based den-
drons carrying aldehyde functionalities at their focal point.
Synthesis of DPVB derivatives. To profit from the advant-
ages reported for convergent synthetic approaches to star-
shaped molecules, e.g. dendrimers,22 we have developed the
key building blocks 4 and 5 (Scheme 1). These versatile
intermediates allow the introduction of the desired DPVB
structural feature into the dendritic branches. Moreover,
the aldehyde groups in 4 and 5 can be used to introduce
different electroactive peripheral moieties by efficient Horner–
Wadsworth–Emmons coupling reactions which increase the
conjugation length of the arms and allow tailoring of the
energy levels of these starburst molecules. In addition to
the aldehyde functionalities, the presence of another functional
group in these DPVB building blocks, i.e. a bromine atom in
4 or a nitrile group in 5, enables us to obtain the suitable
aldehyde-functionalized dendrons.
Thus, Horner–Wadsworth–Emmons olefination between
4,49-dicyanobenzophenone (1)23 and dimethyl-4-bromobenzyl-
phosphonate (2)24 yields the corresponding DPVB moiety
in 4-[29-di(40-dicyanophenyl)vinyl]bromobenzene (3). The
generation of aldehyde groups in the DPVB moiety 3 can
now be easily accomplished by further selective reduction
of the nitrile groups using diisobutylaluminium hydride
(DIBAL-H), to yield dialdehyde 4 in good yield.
Taking advantage of their blue light emission, (diphenyl-
vinyl)benzene (DPVB) derivatives have been recently
employed in the fabrication of OLEDs. The arylene analog
(diphenylvinyl)biphenyl (DPVBi), which exhibits intense
fluorescence in the blue region, has been utilized for the
realization of blue- and white-light-emitting diodes with high
luminous efficiency in doped configurations.19 Besides, the
DPVB structural feature has also been incorporated into
semiconducting conjugated polymers20 and, recently, Yoshida
et al. have employed palladium-catalyzed chemistry to access
triarylethene derivatives.21
Synthesis of functionalized dendritic branches. The presence
of two aldehyde groups in 4 makes it possible to use a further
olefination reaction to (i) increase the conjugation length of
the peripheral branches and (ii) introduce different electro-
active units (Scheme 2), which in turn has a strong influence
on the properties of the materials. Thus, a Wittig reaction
between 4 and 4-(N,N-dibutylaminobenzyl)triphenylphospho-
nium iodide (6)25 was used to prepare the extended conjugated
system 7. This approach required an additional isomerization
of the reaction mixture to the all-E isomer via reflux in
p-xylene containing a catalytic trace of iodine. Because of the
good solubility of these compounds, stemming mainly from
the flexible outer chains, we could monitor this aspect by high-
resolution NMR. In particular, a 3J value of ca. 16 Hz for the
vinylic protons relates to the (E,E) configuration. Moreover,
In this paper we describe a novel convergent approach to the
synthesis of (diphenylvinyl)benzene (DPVB)-based conjugated
dendrimers (17 and 18, Fig. 1). The introduction of different
electroactive peripheral groups creates the possibility of tuning
the HOMO and LUMO levels of these materials and thus their
redox behavior, absorption and emission properties. We have
been able to match the energy levels of these dyes with those of
a PF derivative (Fig. 1) such that energy transfer from the PF
to the dyes takes place. We have prepared dye-doped polymer
OLEDs that clearly demonstrate that the device characteristics
depend greatly on the different end-groups. The dye with the
amine functionality has energy levels that are deeper within the
1
the H-NMR spectrum of 7 recorded after completion of the
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Fig. 1 Structures of the star-shaped DPVB molecules 17 and 18 and of the polymer host (PF).
isomerization process is simpler than that of the initial
isomeric mixture. Horner–Wadsworth–Emmons olefination is
a convenient alternative to circumvent the extra conversion
step.26 Thus, the (E,E) isomer of 7 could be directly obtained
by reacting 4 with dimethyl-4-(N,N-dibutylamino)benzylphos-
phonate (9) (Scheme 2), synthesized from the corresponding
benzyl alcohol 8 by treatment with elemental iodine and
trimethylphosphite. In this way, the Z configuration could not
be detected in the 1H-NMR spectrum of 7, only superimposed
AB spin patterns between 7.0 and 6.9 ppm were observed for
the olefinic protons. Besides the above-mentioned presence of
3
the characteristic vicinal coupling for Jtrans of ca. 16 Hz, the
Scheme 1
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Scheme 2
presence in the FTIR spectra of a band around 960 cm21
further supports the (E,E) disposition of the double bonds. It is
also worth mentioning the coincidence of the spectroscopic
data of compound 7, either obtained via Wittig or Horner–
Wadsworth–Emmons reaction.
(Scheme 3). In contrast to the poor direct conversion of 7
into the corresponding cyano derivative 10 by a Rosenmund
von Braun reaction (vide supra), when 13 is heated to reflux in
DMF in the presence of copper cyanide and a catalytic amount
of sodium iodide, compound 14 is smoothly obtained in
a good 72% yield. Similarly to the dibutylamino-substituted
In order to generate a tri(styrylbenzene) core in the star-
shaped final molecules,27 an aldehyde focal group must be
introduced in the dendritic branches. Thus, the Rosenmund
von Braun reaction of the bromo derivative 7, namely, the
treatment with copper cyanide in dry N,N-dimethylformamide
in the presence of a catalytic amount of sodium iodide, gave
the corresponding cyano derivative 10, although only in low
(28%) yield (Scheme 2). The low yield contrasts the other
Rosenmund von Braun reactions, which usually proceed
cleanly and with better yields (vide infra) and might be
explained in terms of some secondary reaction of the labile
dialkylamino groups of 7 with the organometallic species
generated during the Rosenmund reaction.28
In order to circumvent this limitation and obtain reasonable
amounts of 10, a novel approach was envisaged. Thus, a novel
Rosenmund von Braun reaction, but this time using the
bromo derivative 4 as substrate, yields the cyano derivative 5
(Scheme 1) in good yield.
Subsequent
Horner–Wadsworth–Emmons
olefination
between 5 and phosphonate 9 straightforwardly affords cyano
derivative 10 with good yield (Scheme 2), whose further
reduction with DIBAL-H leads, finally, to the desired
aldehyde-functionalized branch 11.
In parallel, the synthesis of trialkoxy-susbstituted dendritic
branches was carried out following a similar procedure
Scheme 3
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Table 1 Maxima of absorbance (S1 r S0) and emission (S1 A S0) in
dichloromethane (DCM), toluene (TOL), and dispersed in a thin film
of PF (both PL and EL)
S1 r S0/nm
S1 A S0/nm
S1 A S0/nm
DCM
TOL
DCM
TOL
PF (PL)
PF (EL)
17
18
a
397
392 (sh)a
396
395 (sh)a
585
504
513
503
518
485
515
493
(sh) represents the shoulder of the absorbance spectrum at low
energy.
Scheme 4
3
series, compounds 13–15 also display a value for the Jtrans
olefinic coupling of ca. 16 Hz. As it has been observed
for other stilbenoid star-shaped structures bearing trialkoxy-
benzenes at the periphery the overlapping AB double bond
patterns are highly degenerated into the direction of A2
singlets.29
Optical properties in solution
UV-vis absorption spectra of compounds 17 and 18 along with
those of reference compounds 7 and 13 were recorded in
toluene and dichloromethane. There is only a minor influence
of the solvent (Table 1) and the absorption spectra recorded in
dichloromethane are shown in Fig. 2. The absorption spectrum
of 17 shows a p-p* absorption band with a maximum at 397 nm
and an onset around 480 nm, whereas compound 18 has an
absorption maximum at 350 nm with a shoulder at 392 nm and
an onset around 450 nm.
Synthesis of starburst molecules. Horner–Wadsworth–
Emmons reaction between phosphonate 1630 and aldehydes
11 and 15 afforded dendrimers 17 and 18 in fairly good yields
(66 and 62%), in spite of the strongly hampered environment
(Scheme 4).
The conjugated system of the dendrimers can be regarded
mainly as that of the branches plus the central benzene ring
considering the restriction to the conjugation imposed by the
meta substitution pattern of the central benzene ring, as is
already observed for other benzene-centered dendrimers meta-
substituted with three distyrylbenzene chromophores.32 Thus
the longest conjugated fragment in molecules 17 and 18 is
expected to be a linear tetra(p-phenylenevinylene) (OPV4)
system. Comparison of the absorption spectra of 17 and 18
with those of similarly end-substituted OPV structures shows
that the shoulders in the spectra of 17 and 18 correspond well
The presence of the peripheral dibutylamino and dodecyloxy
chains, although they could reduce charge carrier mobility, is
necessary to provide a good solubility to compounds 17 and
18, and therefore appropriate spectroscopic, electrochemical
and electronic characterizations could be carried out. The
disappearance of the aldehyde peak and increased integration
values in the 1H-NMR spectra of these starburst structures
prove the successful coupling reaction between the respective
aldehyde functionalized branches and the core-bearing moiety.
1H-NMR spectra of compounds 17 and 18 showed the correct
ratio of aliphatic and aromatic protons, and although the
lack of symmetry of the peripheral vinyl-substituted DPVB
moieties renders complex NMR spectra, most characteristic
signals in the 1H-NMR spectra could be assigned.31 In the
FTIR spectra of 17 and 18 the strong carbonyl bands around
1700 cm21 of the aldehyde precursors have disappeared. The
most interesting band is the wagging vibration of the trans-
configured double bonds, which is located around 960 cm21
and supports the proposed structures. The matrix-assisted
laser desorption time-of-flight (MALDI-TOF) technique
provided an excellent tool for the determination of the
molecular masses of 17 and 18, determining the absence of
dendrimers which contain only one or two dendrons, thus it
can be used for the control of the last reaction step. Final
evidence concerning the purity of dendrimers 17 and 18 is
given by elemental analyses, which reveal mass percentages
that are in accordance with the expected values.
to the absorption maxima of end-capped OPV4-amine (lmax
=
419 nm)33 and OPV4-alkoxy (lmax = 383 nm),34 labeled ‘‘A’’
and ‘‘B’’ in Fig. 2. The absorption maxima of 17 and 18
correspond to those of reference compounds 7 and 13 which
have a maximum conjugation length equivalent to OPV3.
The blue shift in 17 and 18 with respect to OPV4-amine and
OPV4-alkoxy can be feasibly explained in terms of structural
Synthesis of the polyfluorene host material. Copolymeriza-
tion of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolyl)-9,9-
bis[4-(3,7-dimethyloctyloxy)phenyl]fluorene and 2,7-dibromo-
9,9-bis(3,7-dimethyloctyl)fluorene by Suzuki reaction and
end-capping
with
4,4,5,5-tetramethyl-1,3,2-dioxaborolyl-
Fig. 2 UV-vis spectra of dichloromethane solutions of dendrimers 17
and 18 along with references 7 and 13. ‘‘B’’ and ‘‘A’’ indicate the
absorption maxima of OPV4-alkoxy (ref. 34) and OPV4-amine (ref. 33)
respectively.
benzene afforded poly(9,9-bis[4-(3,7-dimethyloctyloxy)phenyl]
fluorene-alt-9,9-bis(3,7-dimethyloctyl)fluorene) (PF, Fig. 1) in
good yield.
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Table 2 Redox potentials (onsets) in V vs. Ag/AgCl in the indicated
,
solvents: TBAPF6 (0.1 M) solutions, 20 uC, scan rate of 200 mV s21
Pt disk and wire as working and counter electrode
Eox,o (DCM)
Ered,o (THF)
17
18
PF
a
+0.43
+0.94
+1.10
21.89
21.83
,22.1a
In ortho-dichlorobenzene: no reduction wave was detected up to
22.1 V.
against ferrocene/ferrocenium (+0.31 V). For comparison,
reference compounds 7 and 13 were also investigated.
The dibutylamino-substituted dendrimer 17 exhibits
a
chemically quasi-reversible oxidation wave with
a peak
potential of EP = +0.51 and an onset of Eox,o = +0.43 V
ox
Fig. 3 Normalized photoluminescence (PL) spectra of dendrimers 17
(squares) and 18 (circles) in toluene (open symbols) and dichloro-
methane (solid symbols) excited at 370 nm.
vs. Ag/AgCl in DCM. The low oxidation potential is a result of
the electron rich dibutylaminophenyl moieties. For reference
compound 7 the oxidation potential is anodically shifted
0.24 V and it also shows a similar quasi-reversible wave
distortions generated by the bulky peripheral dodecyloxy and
dibutylamino chains which induce different conformations of
the same molecule.35 Possibly the structural distortion is
around the CLC core of the triarylethene moiety, because in
other extended p-systems comprising this moiety, a non-planar
structure is present around the CLC core as a result of the
sterically congested environment.21b
(EP = +0.75).
ox
On the other hand, for compound 18 only a single broad
irreversible oxidation wave could be detected at a significantly
higher potential than for 17, with an onset value of Eox,o
=
+0.94 V vs. Ag/AgCl in DCM and Eox,o = +1.05 V for its
reference compound 13. The lower oxidation potentials of 17
and 18 compared to their reference compounds 7 and 13 can be
explained by the enhanced electron donating ability of their
more extended conjugated system.
The dilute-solution photoluminescence (PL) of dendrimers
17 and 18 in toluene and dichloromethane is shown in Fig. 3.
In toluene (er = 2.38)36 the dendrimers emit green light, with
broad emission bands and maxima at lmax = 513 and 503 nm,
respectively (Table 1). The more polar (er = 8.93)36 dichloro-
methane causes a red shift of 72 nm of the PL spectrum of 17
due to the presence of the dialkylamino peripheral groups.
These electron-rich groups may induce intramolecular charge
transfer (CT) upon photoexcitation, followed by a reorganiza-
tion of the solvent which stabilizes the CT state resulting in
a bathochromic shift of the fluorescence.37,38 The emission
maximum of dye 18 hardly shifts upon going from toluene to
dichloromethane which confirms that the observed effect is
induced by the electron donating amines in dye 17.
The reduction potentials of compounds 17 and 18 were
determined in THF. Both dendrimers showed irreversible
reduction potentials with onset values of Ered,o = 21.89 and
21.83 V respectively. The dendrimers are both more electron
accepting than a non-substituted linear OPV4 which shows a
reduction potential of 22.13 V vs. Ag/AgCl.40
The absolute positions of the HOMO and LUMO levels
were estimated from the onset of the oxidation and reduction
potentials, respectively, assuming an energy of 4.7 eV below
vacuum for the Ag/AgCl electrode (Table 2).41 The energy of
the LUMO of PF is however difficult to estimate because we
were not able to observe a reduction wave of PF within our
solvent window (up to 22.1 V). The reduction potential has
however been determined in a film by Janietz et al.42 and from
this they estimated the LUMO to be at 2.1 eV below vacuum,
however the large discrepancy between the electrochemical
(3.7 eV) and the optical (2.95 eV) gap was already reported
by these authors; the large difference is striking and likely
a lower value for the LUMO level is more appropriate.7c,d
Takeda et al.43 have found that the reduction potential of a
poly(dihexylfluorene) in THF is in equilibrium with that of
pyrene and found a value of 22.65 V vs. Fc/Fc+, which would
lead to a LUMO energy of about 22.35 eV vs. vacuum. The
HOMO and LUMO levels of dyes 17 and 18 are depicted
in Fig. 4 with errors ¡0.1 eV together with that of PF
(LUMO at 22.1 up to 22.4 eV) and the work functions of the
electrodes.
The absolute photoluminescence quantum yields (WF) were
determined by comparison with 9,10-diphenylanthracene in
cyclohexane (WF = 0.90)39 and are considerably higher in
toluene (WF = 0.40 and 0.44) than in dichloromethane (0.33
and 0.10 for dye 17 and 18 respectively). It is surprising that
the quantum yield of the CT-like emission of 17 in dichloro-
methane is as high as 0.33, because the CT character usually
facilitates non-radiative decay.37
Energy levels
In order to estimate the positions of the HOMO (highest
occupied molecular orbital) and LUMO (lowest unoccupied
molecular orbital) levels, we carried out cyclic voltammetry
investigations in dichloromethane (DCM) and tetrahydro-
furan (THF) solutions, using a platinum disk and wire as
working and counter electrodes, respectively, Ag/AgCl as
reference electrode and tetrabutylammonium hexafluorophos-
phate (TBAPF6, 0.1 M) as supporting electrolyte (Table 2). All
potentials are expressed vs. Ag/AgCl, which was calibrated
As mentioned before, it is
a prerequisite for device
characteristics that the HOMO and LUMO levels of the dye
lie within the gap of PF, because PF acts as the hole and the
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Fig. 5 Normalized absorbance and PL (top) and EL (bottom) spectra
of 1% by weight of dyes 17 and 18 in a PF film compared to a film of
pristine PF. For the PL spectra the films were excited at 385 nm.
the red shifted absorption spectrum of dye 17 relative to that
of dye 18 providing the former a better overlap with the PF
emission spectrum.
The relative PL quantum yields of the films containing 1%
of dye 17 and dye 18 are, respectively, 0.80 and 0.85 compared
to the pristine polymer film, which means that there are only
minor losses compared to the pristine PF film. The decrease
(20% for dye 17 and 15% for dye 18) in the PL quantum yield
can presumably be explained by the higher absolute PL
quantum yield of PF than for the dyes, as for poly(dialkyl-
fluorenes) PL quantum yields of WF . 0.5 have been reported
in the solid state,45 which is higher than what we have found
for dyes 17 and 18 in solution (40 and 44%).
Fig. 4 (Top) Schematic representation of the electronic levels. The
HOMO and LUMO levels of the polymer host PF and dyes 17 and 18
are estimated from CV measurements; the LUMO level of PF was
taken from the literature (from 22.1 up to 22.4 eV vs. vacuum).42,43
The black bands show the error margins (¡0.10 eV). (Bottom) OLED
device architecture. Dopant dyes 17 and 18 were dispersed in the guest
PF layer at a mass ratio of 1%. Typical layer thicknesses: ITO/
PEDOT:PSS (100 nm)/active layer (80 nm)/Ba (5 nm)/Al (100 nm).
electron transporter and therefore both the holes and the
electrons must be transferred from the polymer to the dye
molecules after injection from the electrodes. From Fig. 4 we
can conclude that dyes 17 and 18 fulfill these requirements.
Thus upon mixing PF with the dyes and photoexciting the
host, most of the energy is transferred to the dyes, even more
efficient for dye 17 than for dye 18, with only a minor loss in
PL quantum yields.
Photoluminescence in the solid state
Light emitting diodes (LEDs)
Thin films of the following composition were prepared on
quartz substrates: one consisting of only PF and two films of
PF containing 1% by weight of either dye 17 or 18. The
normalized PL spectra of these films upon photoexcitation at
385 nm are shown in Fig. 5. PF has an emission maximum at
422 nm. When 1% of either dye 17 or 18 is mixed with the
polymer, energy transfer takes place from PF to the dyes,
which then emit with PL maxima at 518 and 485 nm, for 17
and 18 respectively (Table 1). The PL spectra of 17 and 18
show small shifts (+5 nm and 220 nm) in comparison to the
spectra in toluene solution. Similar batho- and hypsochromic
shifts in the solid state were found for other OPV-containing
molecules with alkoxy and amine functionalities.44
LEDs were prepared with active layers consisting of pristine
PF or PF containing 1% by weight of either dye 17 or 18.
The device structure is shown in Fig. 4 (right). The electro-
luminescence (EL) spectra (Fig. 5) obtained from these devices
show that in EL there is a higher contribution from the dye
emission under electrical excitation (EL) than under photo-
excitation (PL). The enhanced contribution of the dyes can be
explained by the different mechanisms of exciting PF in PL
and EL. Upon photoexcitation, energy transfer takes place
from PF to the dye mostly by Fo¨rster energy transfer, whereas
in EL the same process (Fo¨rster energy transfer) can take place
after a hole and an electron have reached the same PF
chain and additionally in EL the holes and electrons that are
transported through the device can be trapped on a dye
molecule. The second contribution may lead to enhanced
emission from the dyes in EL as both the HOMO and the
LUMO levels of the dyes are within the gap of PF. Like
in PL, the PF emission is further reduced in the blend with
dye 17 than in the blend with dye 18. We note that for PF : 17
the EL spectrum is almost free of host contribution, while
the PF emission is readily detected in the PF : 18 blend (Fig. 5).
At 422 nm the emission intensity of the dyes with respect to
their maximum is ,1% (Fig. 3) and it is therefore possible to
determine the PL quenching in the dye-doped films by
comparing the emission intensity at that wavelength with that
of a pristine PF film. This reveals that for the blend with dye
17 the PF emission is 8.3 times quenched whereas in the case of
dye 18 the PF emission is 2.9 times quenched. Thus energy
transfer takes place from the polymer to both dyes, but more
effectively for the blend with dye 17. This can be explained by
4280 | J. Mater. Chem., 2007, 17, 4274–4288
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Table 3 Electroluminescence properties of OLED devices composed
of a PF layer without and with 1 wt% of dye
Added
dye
Layer
thickness, d
Turn-on
V/V
Lmax
/
Effmax/
lm W21, V
cd m22, Va
—
17
18
a
60 nm
88 nm
80 nm
3.2
8.0
3.5
330, 8.0
89, 12.0
825, 8.0
0.045, 3.5
0.08, 12
0.19, 4.5
The devices have not been characterized above the indicated
voltage, however L is still increasing with V.
indicating that not all emission is originating from the dye.
Dye 18 is hampering the current less, whereas the luminance is
still moderate.47,48 The performance (in lm W21) of these
dendrimers in polymer LEDs (Table 3) is similar to that of
some fluorescent guest molecules in polymer host matrices.47,49
Fig. 6 Current density (left) and luminance (right) versus bias voltage
for the LED devices: pristine PF (triangles), PF + 1% dye 17 (squares)
and PF + 1% dye 18 (circles).
Conclusions
In Fig. 6 the current density and luminance dependence on
the applied bias voltage (J–V and L–V) for the EL devices are
presented. The turn-on voltage (defined at 0.1 cd m22) is much
larger for PF : 17 (8.0 V) than for PF (3.2 V) and PF : 18
(3.5 V). This indicates that for the blend with dye 17 there is a
significant imbalance in the charge transport. Likely, both the
holes and the electrons are trapped by dye 17, because there is
an offset of the HOMO levels of PF and 17 of at least 0.45 eV,
and of the LUMO levels of at least 0.30 eV, which limits the
transport of holes and electrons within the device. This can be
compared to the study of Pai et al.,13a who showed that
addition of very small amounts (2%) of N,N9-diphenyl-N,N9-
bis(3-methylpheny1)-[l,l9-biphenyl]-4,49-diamine (TPD) to a
film of poly(N-vinylcarbazole) (PVK) reduces the hole
mobility by three orders of magnitude. The difference in
ionization potential between TPD and PVK is 0.5–0.6 eV. The
situation is clearly different for dye 18: here the electrons are
likely also to be trapped as the difference between the LUMO
levels is at least 0.35 eV, causing a strong reduction of the
electron mobility. However, the holes are less deeply trapped,
as the difference between the HOMO levels is only 0–0.20 eV.
Such a small difference likely has a much lower effect on the
hole mobility.14 Therefore, the holes may be transported to the
trapped electrons and then recombine on dye 18, leading to
electroluminescence.
In this paper we have described a convergent synthetic
approach to the synthesis of two highly luminescent (diphenyl-
vinyl)benzene (DPVB)-based conjugated dendrimers with
dialkylamine (17) or trialkoxy (18) peripheral groups that
emit green light. The star-shaped DPVBs have been used as
dopant emitters in OLEDs using a polyfluorene (PF) host. The
introduction of different electroactive peripheral groups allows
tuning of the HOMO and LUMO energy levels of the dyes to
lie within the band gap of PF. Accordingly, photoluminescence
experiments reveal that the excited state energy can be
transferred from PF to each of the two dyes. Likewise, in the
corresponding dye-doped polymer OLEDs the emission
originates predominantly from the dyes but is more pure for
PF : 17 than for PF : 18 (Fig. 5). The device characteristics,
however, differ more significantly and show a much higher
efficiency for the OLED containing the alkoxy- than for the
amine-functionalized dye (Fig. 6). The PF : 17 device suffers
from reduced charge transport in the electroactive layer, as
both the holes and the electrons are trapped on the dye. For
dye 18 the HOMO level is more than 0.25 eV closer to that of
the polymer and the hole mobility in the device is therefore
much less hampered by the introduction of that dye.
We have thus shown the importance of energy level
alignment in fluorescent host–guest OLEDs to find the best
compromise between large offsets (as in PF : 17) that may
cause transport limitations but pure dye emission and, on the
other hand, smaller offsets that offer improved transport but
may lead to residual emission of the host (as in PF : 18).
Regarding the luminance–voltage curves it must be noted
that a direct comparison between the devices with pristine PF
and with dyes 17 and 18 is not straightforward, because the
luminance as given in candela contains a multiplication by the
eye-sensitivity (the CIE 1924 photopic V(l)). Therefore, as
the human eye is much more sensitive for green light than for
blue light, the EL spectra of PF : 17 and PF : 18 would give a
higher luminance for the same amount of photons than an EL
spectrum of PF.
Experimental
Materials
4,49-Dicyanobenzophenone (1),23 dimethyl-4-bromobenzyl-
phosphonate (2),24 4-(N,N-dibutylaminobenzyl)triphenyl-
phosphonium iodide (6),25 N,N-dibutylaminobenzalcohol
(8),50 dimethyl 4,5,6-tridodecyloxybenzylphosphonate (12),51
The luminance that is obtained for the polyfluorene is
similar to values found for other purely blue emitting
polyfluorenes (y300 cd m22 at 8 V).46 For PF : 17 the
electroluminescence is predominantly originating from the dye
(Fig. 5). However, almost no current flows, whereas the
number of electrical excitations on dye 17 is severely limited.
As a result the device performance is disappointing. The
luminance for the blended PF : 18 is less pure in color,
1,3,5-[tris(dimethyl-phosphonatemethyl)]benzene
(16),30
2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolyl)-9,9-bis[4-(3,7-
dimethyloctyloxy)phenyl]fluorene52 and 2,7-dibromo-9,9-
bis(3,7-dimethyloctyl)-fluorene53 were prepared following
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previously reported synthetic procedures. All other chemicals
were purchased from Aldrich and used as received without
further purification unless otherwise specified. Column
chromatography was performed on Merck Kieselgel 60 silica
gel (230–240 mesh). Thin layer chromatography was carried
out on Merck silica gel F-254 flexible TLC plates. Solvents and
reagents were dried by usual methods prior to use and typically
used under inert gas atmosphere.
isopropanol baths, respectively. Then the glass/ITO substrates
were treated in
a UV/O3 photoreactor and remaining
dust particles were blown away with ionized nitrogen. As the
first layer in the devices a layer of poly(3,4-ethylenedioxy-
thiophene) : poly(styrenesulfonate) (PEDOT : PSS, 1 : 6 dis-
persion in water, HC Starck electronic grade Baytron1 P VP
AI4083,), filtered through a 0.45 mm hydrophilic PVDF filter,
was introduced by spin coating using a Sass Microtel RC8
Gyres spin coater (100 nm). PEDOT : PSS layers were
annealed at 200 uC during two minutes. Solutions of polymer,
with or without dye, in chlorobenzene were filtered through
a 0.45 mm PTFE filter and spin-coated on top of the
PEDOT : PSS layer, also using a Suss MicroTec RC8 Gyrset
spin coater (80 nm). In a nitrogen atmosphere inside a glove
box ([O2],[H2O] , 1 ppm) metal cathodes were applied by
vacuum evaporation through a mask, 5 nm Ba capped with
100 nm Al.
Characterization
Melting points were measured with an Electrothermal melting
point apparatus and are uncorrected. FTIR spectra were
recorded as KBr pellets in a Shimadzu FTIR 8300 spectro-
meter. NMR spectra were recorded on a Bruker AC-200,
Avance 300 or AMX-400 apparatus as noted, and the chemical
shifts were reported relative to tetramethylsilane (TMS) at
0.0 ppm (for 1H-NMR) and CDCl3 at 77.16 ppm (for
13C-NMR). The splitting patterns are designated as follows:
s (singlet), d (doublet), m (multiplet) and b (broad). Mass
spectra were recorded with a Varian Saturn 2000 GC-MS and
with a MALDI-TOF MS Bruker Reflex 2 (dithranol as
matrix). Elemental analyses were performed on a Perkin-Elmer
EA 2400.
The OLEDs were characterized by a low-noise single-
channel DC power source that can act as a voltage source
or current source and as a voltage meter or current meter
(Keithley 2400, Keithley Instruments). In a typical character-
ization of a LED the voltage is increased stepwise from 22.0 V
to a preset positive voltage and back. Light from the diode is
coupled into a silicon photodiode with a photopic filter (UDT
Sensors Inc., PIN-10AP) and read out by a picoampmeter
(Keithley 6485, Keithley Instruments). Calibration of the
photodiode was performed with a luminance meter (Minolta
LS-110). For recording the electroluminescence spectra a
fiber-coupled spectrograph (Avantes PC-2000) was used. The
spectrometer was calibrated in wavelength and intensity.
Electrochemistry
Cyclic voltammetry experiments were performed with a
computer controlled EG & G PAR 273 potentiostat in a
three-electrode single-compartment cell (5 ml). The platinum
working electrode consisted of a platinum wire sealed in a soft
glass tube with a surface of A = 0.785 mm2, which was polished
down to 0.5 mm with Buehler polishing paste prior to use in
order to obtain reproducible surfaces. The counter electrode
consisted of a platinum wire and the reference electrode was a
Ag/AgCl secondary electrode. All potentials were internally
referenced to the ferrocene/ferrocenium (Fc/Fc+) couple which
was located at +0.31 V vs. Ag/AgCl. For the measurements,
concentrations of 5 6 1023 mol l21 of the electroactive species
were used in freshly distilled and deaerated dichloromethane
(Lichrosolv, Merck) and 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6, Fluka) which was twice recrystal-
lized from ethanol and dried under vacuum prior to use.
4-[2,2-Bis(4-cyanophenyl)ethenyl]bromobenzene (3)
To a solution of 4,49-dicyanobenzophenone (1) (150 mg,
0.64 mmol) and dimethyl 4-bromobenzylphosphonate (2)
(214 mg, 0.77 mmol) in 15 ml of dry tetrahydrofuran,
potassium tert-butoxide (86 mg, 0.77 mmol) is added portion-
wise under argon atmosphere. The resulting mixture is heated
at reflux for 4 hours, cooled to room temperature and
methanol and water are added. The mixture is extracted with
dichloromethane and the combined organic extracts dried over
anhydrous magnesium sulfate. After vacuum evaporation of
the solvent, the residue is purified by column chromatography
(silica gel, dichloromethane) to afford 4-[2,2-bis(4-cyanophe-
nyl)ethenyl]bromobenzene (3) (192 mg, 78%) as a white solid
(Found: C, 68.17; H, 3.35; N, 7.24. C22H13BrN2 requires: C,
68.59%; H, 3.40%; N, 7.27%); n (KBr)/cm21 2230, 2222, 1595,
1558, 1485, 1074, 848, and 827; mp 97 uC; dH (200 MHz,
CDCl3, Me4Si) 7.67 (AA9BB9 system, 2H), 7.63 and 7.35
(AA9BB9 system, 4H), 7.35–7.32 (m, 4H), 7.04 (s, 1H, Hvinyl),
6.86 (AA9BB9 system, 2H); dC (50 MHz, CDCl3, Me4Si)
146.07, 143.66, 139.75, 134.36, 132.65, 132.17, 131.46, 130.99,
130.91, 127.91, 122.12, 111.95 (CN), 111.52 (CN); m/z (EI) 385
(M+, 100), 305 (91).
UV-vis absorption and photoluminescence
UV-vis absorption and fluorescence spectra were recorded
with a Perkin-Elmer Lambda 900 spectrometer and an
Edinburgh Instruments FS920 double-monochromator lumi-
nescence spectrometer using a Peltier-cooled red-sensitive
photomultiplier, respectively. The fluorescence spectra were
corrected for the optical density of the excitation beam and for
the detection sensitivity of the photomultiplier. Thin films were
spin cast from chloroform on quartz substrates.
Preparation and characterization of light-emitting diodes
4-[2,2-Bis(4-formylphenyl)ethenyl]bromobenzene (4)
OLEDs were fabricated under clean room conditions.
Glass substrates with patterned (well defined) indium–tin
oxide (ITO) areas were cleaned by scrubbing with a soap
solution, followed by ultrasonic treatments using acetone and
To a refluxing solution of 4-[2,2-bis(4-cyanophenyl)ethenyl]-
bromobenzene (3) (1.0 g, 2.60 mmol) in 15 of dry dichloro-
methane, a diisobutylaluminium hydride dichloromethane
4282 | J. Mater. Chem., 2007, 17, 4274–4288
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solution (1 M, 5.2 ml, 5.2 mmol) is added dropwise during a
period of 2 hours under argon atmosphere. The solution is
allowed to react at reflux for 18 hours and cooled to room
temperature. Methanol is then carefully added until the
reaction crude becomes a thick suspension. After careful
addition of water, the reaction crude is treated with
concentrated hydrochloric acid until complete solution and
extracted with chloroform. The combined organic extracts are
then washed with water and dried over anhydrous magnesium
sulfate. After vacuum evaporation of the solvent, the remain-
ing residue is purified by column chromatography (silica gel,
dichloromethane) to afford 4-[2,2-bis(4-formylphenyl)ethenyl]-
bromobenzene (4) (681 mg, 67%) as a white solid (Found: C,
66.77%; H, 3.45%. C22H15BrO2 requires: C, 67.53%; H,
3.86%); n (KBr)/cm21 2922, 2817, 2729, 1701, 1604, 1591,
1562, 1207 and 1172; mp 136 uC; dH (200 MHz, CDCl3, Me4Si)
10.06 (s, 1H, –CHO), 10.03 (s, 1H, –CHO), 7.89 (AA9BB9
system, 2H), 7.85 (AA9BB9 system, 2H), 7.43 and 7.36
(AA9BB9 system, 4H), 7.30 (AA9BB9 system, 2H), 7.08 (s,
1H, Hvinyl), 6.89 (AA9BB9 system, 2H); dC (50 MHz, CDCl3,
Me4Si) 191.56 (CLO), 191.47 (CLO), 148.06, 145.79, 135.83,
135.02, 131.51, 131.16, 131.10, 130.62, 130.31, 129.87, 128.11,
121.95; m/z (EI) 391 (M+, 100), 282 (40), 253 (58), 126 (33).
90%) as a colorless oil (Found: C, 62.74%; H, 9.15%; N, 4.16%.
C17H30NO3P requires: C, 62.36%; H, 9.23%; N, 4.28%); n
(KBr)/cm21 2955, 2933, 2871, 1616, 1464, 1367, 1256, 1190,
1059 and 1032; dH (200 MHz, CDCl3, Me4Si) 7.09 (AA9BB9
system, 1H), 7.08 (AA9BB9 system, 1H), 6.57 (AA9BB9 system,
2H), 3.70 (s, 3H, –OCH3), 3.65 (s, 3H, –OCH3), 3.27 (t, 4H, J
7.76 Hz, –NCH2–), 3.06 (d, 2H, J 20.70 Hz, –CH2P), 1.62–1.47
(m, 4H, –CH2–), 1.31–1.22 (m, 4H, –CH2–), 0.94 (t, 6H, J
7.12 Hz, –CH3); dC (50 MHz, CDCl3, Me4Si) 147.10, 130.30 (d,
JCP 6.50 Hz), 116.76 (d, JCP 10.00 Hz), 111.96 (d, JCP 2.75 Hz),
52.76 (–O–CH3), 52.62 (–O–CH3), 50.77 (–N–CH2–), 31.46 (d,
–P–CH2–, JCP 138.2 Hz), 29.26, 20.24, 13.88; m/z (EI) 327
(M+, 24), 284 (100), 242 (61), 118 (38).
Bromo derivative 7
Via Wittig reaction. To a refluxing and well-stirred suspen-
sion of 4-[2,2-bis(4-formylphenyl)ethenyl]bromobenzene (4)
(234 mg, 0.60 mmol) and 4-(N,N-dibutylaminobenzyl)triphe-
nylphosphonium iodide (6) (1.09 g, 1.80 mmol) in 45 ml of
anhydrous tetrahydrofuran, potassium tert-butoxide (200 mg,
1.8 mmol) is added portionwise under argon atmosphere. After
24 hours the reaction crude is allowed to cool to room
temperature and methanol and water are added. The resulting
mixture is extracted with chloroform and the combined
organic extracts are dried over anhydrous magnesium sulfate.
After evaporation of the solvent, the residue is purified by
column chromatography (silica gel, hexane–dichloromethane
8 : 2 v/v) to yield a mixture of isomers. This mixture is
suspended in 10 ml of p-xylene and heated to reflux for
48 hours, after addition of a catalytic amount of iodine. After
the reaction mixture is cooled, the crude was poured on top of
a short chromatography column and purified (silica gel,
hexane–dichloromethane 8 : 2), leading to pure trans,trans-7
(214 mg, 45%) as a yellow oil.
4-[2,2-Bis(4-formylphenyl)ethenyl]benzonitrile (5)
Under argon atmosphere, a mixture of 4-[2,2-bis(4-formyl-
phenyl)ethenyl]bromobenzene (4) (150 mg, 0.38 mmol), copper
cyanide (52 mg, 0.57 mmol) and a catalytic amount of sodium
iodide in 65 ml of anhydrous N,N-dimethylformamide is
heated at reflux for 72 hours. After cooling to room tempera-
ture, the reaction crude is poured into an aqueous ammonium
hydroxide (14 wt%) solution and repeatedly extracted with
chloroform. The combined organic extracts are then washed
with an aqueous hydrochloric acid (2 M) solution, with water
and dried over anhydrous magnesium sulfate. After the
solvent is stripped away under vacuum, the residue is purified
by column chromatography (silica gel, dichloromethane)
to afford 4-[2,2-bis(4-formylphenyl)ethenyl]benzonitrile (5)
(95 mg, 74%) as an off-white solid (Found: C, 81.52%; H,
4.35%; N, 4.02%. C23H15O2N requires: C, 81.88%; H, 4.48%;
N, 4.15%); n (KBr)/cm21 2924, 2821, 2729, 2225, 1701, 1601,
1558, 1172 and 833; mp 115 uC; dH (200 MHz, CDCl3, Me4Si)
10.06 (s, 1H, –CHO), 10.03 (s, 1H, –CHO), 7.90 (AA9BB9
system, 2H), 7.87 (AA9BB9 system, 2H), 7.45 (AA9BB9 system,
4H), 7.37–7.33 (m, 2H), 7.14 (s, 1H, Hvinyl), 7.11 (AA9BB9
system, 2H); dC (50 MHz, CDCl3, Me4Si) 191.61 (CLO),
150.23, 132.18, 131.10, 130.50, 130.24, 130.05, 128.46, 118.66,
111.30; m/z (EI) 337 (M+, 100), 280 (43).
Via Wittig–Horner reaction. To a refluxing and well-stirred
suspension of 4-[2,2-bis(4-formylphenyl)ethenyl]bromoben-
zene (4) (200 mg, 0.51 mmol) and dimethyl-4-(N,N-dibutyl-
amino)benzylphosphonate (9) (503 mg, 1.54 mmol) in 25 ml of
anhydrous tetrahydrofuran, potassium tert-butoxide (173 mg,
1.54 mmol) is added portionwise under argon atmosphere.
After 24 hours the reaction crude is allowed to cool to room
temperature and methanol and water are added. The resulting
mixture is extracted with chloroform and the combined
organic extracts are dried over anhydrous magnesium sulfate.
After evaporation of the solvent, the residue is purified by
column chromatography (silica gel, hexane–dichloromethane
8 ; 2 v/v) to afford bromo derivative 7 (320 mg, 79%) as a
bright-yellow oil (Found: C, 78.42%; H, 7.22%; N, 3.22%.
C52H61BrN2 requires: C, 78.66%; H, 7.74%; N, 3.53%); n
(KBr)/cm21 2955, 2928, 2870, 1592, 1554, 1365, 1180, 1145,
1108, 960 and 821; dH (400 MHz, CDCl3, Me4Si) 7.44
(AA9BB9 system, 2H), 7.41 and 7.14 (AA9BB9 system, 4H),
7.38 (AA9BB9 system, 2H), 7.30–7.25 (m, 4H), 7.24 (AA9BB9
system, 2H), 7.05 (d, 1H, Jtrans 16.36 Hz, –CHLCH–),
6.95 (AA9BB9 system, 2H), 6.88 (d, 1H, Jtrans 16.36 Hz,
–CHLCH–), 6.87 (s, 1H, Hvinyl), 6.86 (d, 2H, Jtrans 16.36 Hz,
–CHLCH–), 6.63 and 6.62 (AA9BB9 system, 4H), 3.29 (t, 8H,
Dimethyl 4-(N,N-dibutylamino)benzylphosphonate (9)
To a solution of N,N-dibutylaminobenzalcohol (8) (2.0 g,
8.51 mmol) in 12 ml of trimethylphosphite, iodine (2.18 g,
8.51 mmol) is added at 0 uC. The solution is stirred at 0 uC for
5 minutes and warmed at room temperature for 2 hours The
excess trimethylphosphite is evaporated under reduced pres-
sure and the residue purified by column chromatography
(silica gel, dichloromethane–ethyl acetate 9 : 1 v/v) to afford
dimethyl 4-(N,N-dibutylamino)benzylphosphonate (9) (2.5 g,
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J 7.03 Hz, –CH2–N–), 1.59–1.55 (m, 8H, –CH2–), 1.42–1.25
(m, 8H, –CH2–), 1.00–0.96 (m, 12H, –CH3); dC (100 MHz,
CDCl3, Me4Si) 147.58, 142.84, 140.94, 137.73, 137.66, 137.41,
136.31, 130.80, 130.35, 129.38, 128.93, 128.83, 127.66, 127.53,
125.90, 125.58, 125.42, 124.10, 122.84, 122.75, 120.04, 111.85,
111.30, 50.47 (–N–CH2–), 29.17, 20.05, 13.75; m/z (EI) 794
(M+, 100), 751 (55), 354 (34), 311 (45), 277 (48), 218 (71).
hydride dichloromethane solution (1 M, 1.5 ml, 1.5 mmol) is
added dropwise during a period of 1 hour under argon
atmosphere. The solution is allowed to react at reflux for
24 hours and cooled to room temperature. Methanol is then
carefully added until the reaction crude becomes a thick
suspension. After water is added, the reaction crude is treated
with concentrated hydrochloric acid until complete solution
and extracted with chloroform. The combined organic extracts
are then washed with water and dried over anhydrous
magnesium sulfate. After vacuum evaporation of the solvent,
the remaining residue is purified by column chromatography
(silica gel, hexane–dichloromethane 1 : 1 v/v) to afford aldehyde
11 (550 mg, 72%) as an orange oil (Found: C, 85.13%; H, 8.22%;
N, 3.55%. C53H62N2O requires: C, 85.67%; H, 8.41%; N, 3.77%);
n (KBr)/cm21 2958, 2924, 2870, 1701, 1591, 1524, 1364, 1181,
1142, 1110 and 959; dH (400 MHz, CDCl3, Me4Si) 9.83 (s, 1H,
–CHO), 7.66 (AA9BB9 system, 2H), 7.44 (AA9BB9 system, 2H),
7.43 (AA9BB9 system, 2H), 7.36–7.31 (m, 2H), 7.25 (AA9BB9
system, 2H), 7.23 (AA9BB9 system, 2H), 7.15 (AA9BB9 system,
2H), 7.07 (d, 1H, Jtrans 16.40 Hz, –CHLCH–), 7.05–6.98 (m, 2H),
6.89 (d, 2H, Jtrans 16.40 Hz, –CHLCH–), 6.87 (d, 1H, Jtrans
16.40 Hz, –CHLCH–), 6.56 and 6.64 (AA9BB9 system, 4H),
3.22 (t, 8H, J 7.00 Hz, –CH2–N–), 1.49 (bs, 8H, –CH2–), 1.34–
1.22 (m, 8H, –CH2–), 0.89 (t, 12H, J 7.37 Hz, –CH3); dC
(100 MHz, CDCl3, Me4Si) 191.37 (CLO), 147.61, 145.40, 143.87,
140.55, 138.14, 137.79, 137.39, 133.89, 130.35, 129.65, 129.16,
127.90, 127.54, 125.91, 125.53, 125.44, 124.00, 122.68, 122.60,
111.29, 50.46 (–N–CH2–), 29.16, 20.02, 13.68; m/z (EI) 743
(M+, 96), 699 (64), 366 (38), 328 (40), 286 (53), 218 (48), 57 (79),
43 (100).
Cyano derivative 10
Via Rosenmund von Braun reaction. Under argon atmo-
sphere, a mixture of bromo derivative 7 (580 mg, 0.73 mmol),
copper cyanide (97 mg, 1.10 mmol) and a catalytic amount of
sodium iodide in 60 ml of anhydrous N,N-dimethylformamide is
heated at reflux for 72 hours. After cooling to room temperature,
the reaction crude is poured into an aqueous ammonium
hydroxide (14 wt%) solution and repeatedly extracted with
chloroform. The combined organic extracts are washed with an
aqueous hydrochloric acid (2 M) solution, with water and dried
over anhydrous magnesium sulfate. After the solvent is
stripped away under vacuum, the residue is purified by column
chromatography (silica gel, hexane–dichloromethane 4 : 6 v/v) to
afford cyano derivative 10 (151 mg, 28%) as a yellow oil.
Via Wittig–Horner reaction. To a refluxing solution of
4-[2,2-bis(4-formylphenyl)ethenyl]benzonitrile (5) (230 mg,
0.68 mmol) and dimethyl 4-(N,N-dibutylamino)benzylphos-
phonate (9) (669 mg, 2.05 mmol) in 20 ml of anhydrous
tetrahydrofuran, potassium tert-butoxide (230 mg, 2.05 mmol)
is added portionwise under argon atmosphere. After 24 hours
the reaction crude is allowed to cool to room temperature and
methanol and water are added. The resulting mixture is
extracted with chloroform and the combined organic extracts
are dried over anhydrous magnesium sulfate. After evapora-
tion of the solvent, the residue is purified by column chromato-
graphy (silica gel, hexane–dichloromethane 3 : 7 v/v) to afford
cyano derivative 10 (387 mg, 77%) as a yellow oil. (Found: C,
86.06%; H, 8.52%; N, 5.46%. C53H61N3 requires: C, 86.01%;
H, 8.31%; N, 5.68%); n (KBr)/cm21 2955, 2927, 2870, 2222,
1590, 1519, 1365, 1218, 1180, 1142, 1110, 958, 818 and 801; dH
(400 MHz, CDCl3, Me4Si) 7.45 and 7.15 (AA9BB9 system,
4H), 7.43 and 7.38 (AA9BB9 system, 2H), 7.41 and 7.12
(AA9BB9 system, 4H), 7.29 (AA9BB9 system, 2H), 7.05–7.02
(m, 2H), 6.98 (AA9BB9 system, 2H), 6.96 (d, 1H, Jtrans 16.00 Hz,
–CHLCH–), 6.92 (d, 2H, Jtrans 16.00 Hz, –CHLCH–), 6.88 (d,
1H, Jtrans 16.00 Hz, –CHLCH–), 6.86 (AA9BB9 system, 1H),
6.63 and 6.62 (AA9BB9 system, 4H), 3.29 (t, 8H, J 7.03 Hz,
–CH2–N–), 1.57 (bs, 8H, –CH2–), 1.41–1.25 (m, 8H, –CH2–),
1.00–0.92 (m, 12H, –CH3); dC (100 MHz, CDCl3, Me4Si)
147.64, 145.63, 142.17, 140.29, 138.26, 137.91, 137.06, 131.43,
130.97, 130.92, 130.26, 129.61, 129.29, 129.21, 127.88, 127.58,
126.10, 125.97, 125.47, 124.83, 123.69, 122.55, 118.85, 111.30,
109.09, 50.78 (–N–CH2–), 29.49, 20.35, 14.00; m/z (EI) 739
(M+, 64), 696 (47), 654 (12), 612 (28), 593 (100), 569 (23), 550
(93), 508 (61), 327 (23), 218 (29).
Bromo derivative 13
To a well-stirred solution of 4-[2,2-bis(4-formylphenyl)ethe-
nyl]bromobenzene (4) (120 mg, 0.3 mmol) and dimethyl 4,5,6-
tridodecyloxybenzylphosphonate (12) (752 mg, 1.0 mmol) in
10 ml of dry tetrahydrofuran, potassium tert-butoxide (122 mg,
1.0 mmol) is added portionwise. The resulting mixture is
heated at reflux for 1 hour, cooled to room temperature and
methanol and water are added. The mixture is extracted with
dichloromethane and the combined organic extracts dried over
anhydrous magnesium sulfate. After vacuum evaporation of
the solvent, the residue is purified by column chromatography
(silica gel, hexane–dichloromethane 1 : 1 v/v) to afford bromo
derivative 13 (424 mg, 86%) as a yellowish solid (Found: C,
78.56%; H, 10.16%. C108H171BrO6 requires: C, 78.83%; H,
10.48%); mp 54 uC; n (KBr)/cm21 2920, 2850, 1578, 1507, 1467,
1431, 1120 and 955; dH (400 MHz, CDCl3, Me4Si) 7.47 and
7.17 (AA9BB9 system, 4H), 7.45 and 7.26 (AA9BB9 system,
4H), 7.31 and 6.93 (AA9BB9 system, 4H), 7.07 (d, 1H,
Jtrans 16.60 Hz, –CHLCH–), 7.04 (d, 1H, Jtrans 16.60 Hz,
–CHLCH–), 6.97 (d, 1H, Jtrans 16.60 Hz, –CHLCH–), 6.95 (d,
1H, Jtrans 16.60 Hz, –CHLCH–), 6.91 (s, 1H, Hvinyl), 6.72 (s,
2H), 6.72 (s, 2H), 4.06–3.97 (m, 12H, –OCH2–), 1.92–1.66 (m,
12H, –CH2–), 1.48 (b, 24H, –CH2–), 1.26 (b, 84H, –CH2–),
0.87 (t, 18H, J 6.10 Hz, –CH3); dC (100 MHz, CDCl3, Me4Si)
147.77, 142.02, 138.94, 138.49, 137.01, 136.85, 136.37, 132.45,
131.16, 131.08, 130.73, 129.32, 129.14, 127.98, 127.21, 127.09,
126.73, 126.25, 120.61, 105.33, 73.57 (–OCH2–), 69.24
Aldehyde 11
To a refluxing solution of cyano derivative 10 (762 mg,
1.03 mmol) in 35 of dry dichloromethane, a diisobutylaluminium
4284 | J. Mater. Chem., 2007, 17, 4274–4288
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(–OCH2–), 31.94, 30.37, 29.77, 29.72, 29.68, 29.45, 29.41,
29.38, 26.15, 22.71, 14.12; m/z (FAB) 1645 (M+).
(d, 1H, Jtrans 16.60 Hz, –CHLCH–), 6.72 (s, 4H), 4.08–3.94
(m, 12H, –OCH2–), 1.86–1.72 (m, 36H, –CH2–), 1.26 (bs, 84H,
–CH2–), 0.88 (t, 18H, J 6.54 Hz, –CH3); dC (125 MHz, CDCl3,
Me4Si) 191.61 (CLO), 153.36, 145.27, 143.87, 141.66, 138.62,
138.56, 137.48, 137.21, 134.39, 132.37, 132.35, 130.75, 130.00,
129.55, 129.48, 128.25, 127.07, 126.97, 126.76, 126.30, 105.35,
73.58 (–OCH2–), 69.26 (–OCH2–), 31.94, 30.37, 29.77, 29.72,
29.68, 29.46, 29.41, 29.38, 26.15, 22.71, 14.12; m/z (FAB)
1594 (M+).
Cyano derivative 14
Under argon atmosphere, a mixture of bromo derivative 13
(114 mg, 0.14 mmol), copper cyanide (14 mg, 0.21 mmol) and a
catalytic amount of sodium iodide in 30 ml of anhydrous
N,N-dimethylformamide is heated at reflux for 72 hours. After
cooling to room temperature, the reaction crude is poured into
an aqueous ammonium hydroxide (14 wt%) solution and
repeatedly extracted with chloroform. The combined organic
extracts are then washed with an aqueous hydrochloric acid
(2 M) solution, with water and dried over anhydrous
magnesium sulfate. After the solvent is stripped away under
vacuum, the residue is purified by column chromatography
(silica gel, hexane–dichloromethane 7 : 3 v/v) to afford cyano
derivative 14 (160 mg, 72%) as a yellowish solid (Found: C,
82.72%; H, 10.66%; N, 0.64%. C109H171NO6 requires: C:
82.26%; H: 10.83%; N: 0.88%); mp 66 uC; n (KBr)/cm21 2924,
2852, 2228, 1579, 1504, 1467, 1431, 1314, 1242, 1117, 963 and
830; dH (400 MHz, CDCl3, Me4Si) 7.48 and 7.32 (AA9BB9
system, 4H), 7.46 and 7.42 (AA9BB9 system, 4H), 7.42 and 7.02
(AA9BB9 system, 4H), 7.16 and 7.14 (AA9BB9 system, 2H),
7.06 (d, 2H, Jtrans 16.10 Hz, –CHLCH–), 6.99 (d, 2H, Jtrans
16.10 Hz, –CHLCH–), 6.94 (s, 1H, Hvinyl), 6.72 (s, 2H), 6.72
(s, 2H), 4.06–3.95 (m, 12H, –OCH2–), 1.86–1.75 (m, 12H,
–CH2–), 1.48 (b, 24H, –CH2–), 1.26 (b, 84H, –CH2–), 0.86 (t,
18H, J 6.54 Hz, –CH3); dC (100 MHz, CDCl3, Me4Si) 153.37,
145.55, 142.21, 141.41, 138.60, 138.29, 137.60, 137.34,
132.30, 131.78, 130.66, 129.94, 129.69, 129.53, 128.22, 126.96,
126.81, 126.31, 119.02, 109.74, 105.36, 73.58 (–OCH2–), 69.26
(–OCH2–), 31.94, 31.60, 30.38, 29.77, 29.72, 29.68, 29.46,
29.41, 29.38, 26.15, 22.71, 14.12; m/z (FAB) 1589 (M+).
Star-shaped compound 17
To a refluxing and well-stirred suspension of aldehyde 11
(200 mg, 0.27 mmol) and 1,3,5-[tris(dimethylphosphonate)-
methyl]benzene (16) (38 mg, 0.09 mmol) in 20 ml of dry
tetrahydrofuran, potassium tert-butoxide (35 mg, 0.31 mmol)
is added portionwise under argon atmosphere. After 24 hours
the reaction crude is allowed to cool to room temperature and
methanol and water are added. The resulting mixture is
extracted with chloroform and the combined organic extracts
are dried over anhydrous magnesium sulfate. After evapora-
tion of the solvent, the residue is purified by column chromato-
graphy (silica gel, hexane–dichloromethane 6 ; 4 v/v) to yield
dendrimer 17 (136 mg, 66%) as a yellow solid (Found: C,
87.06%; H, 8.48%; N, 3.19%. C168H192N6 requires: C, 87.91%;
H, 8.43%; N, 3.66%); mp 56 uC; n (KBr)/cm21 2954, 2926,
2857, 1595, 1517, 1364, 1181, 1145, 1109, 956, 820 and 801; dH
(500 MHz, CDCl3, Me4Si) 7.46 (AA9BB9 system, 6H), 7.45
(AA9BB9 system, 6H), 7.41 (AA9BB9 system, 6H), 7.40
(AA9BB9 system, 6H), 7.39 (AA9BB9 system, 6H), 7.33
(AA9BB9 system, 6H), 7.32–7.20 (m, 6H), 7.10 (AA9BB9
system, 6H), 7.07 (s, 3H), 7.05 (d, 4H, Jtrans 16.40 Hz,
–CHLCH–), 6.97 (s, 3H, Hvinyl), 6.91 (d, 4H, Jtrans 16.40 Hz,
–CHLCH–), 6.87 (d, 10H, Jtrans 16.40 Hz, –CHLCH–), 6. 64
and 6.63 (AA9BB9 system, 6H), 6.63 (AA9BB9 system, 6H),
3.22 (t, 24H, J 7.00 Hz, –CH2–N–), 1.51 (bs, 24H, –CH2–),
1.38–1.23 (m, 24H, –CH2–), 0.88 (t, 36H, J 7.19 Hz, –CH3); dC
(125 MHz, CDCl3, Me4Si) 147.54, 142.09, 141.28, 138.24,
137.77, 137.43, 137.26, 136.94, 135.16, 130.45, 129.60, 128.77,
128.66, 128.60, 127.77, 127.65, 127.49, 126.69, 125.94,
125.89, 125.39, 124.19, 123.47, 123.00, 122.87, 111.32, 50.47
(–N–CH2–), 29.18, 20.04, 13.69; m/z (FAB) 2294 (M+).
Aldehyde 15
To a refluxing solution of cyano derivative 14 (450 mg,
0.25 mmol) in 20 of dry dichloromethane, a diisobutylalumi-
nium hydride dichloromethane solution (1 M, 0.37 ml,
0.37 mmol) is added dropwise during a period of 1 hour
under argon atmosphere. The solution is allowed to react at
reflux for 24 hours and cooled to room temperature. Methanol
is then carefully added until the reaction crude becomes a thick
suspension. After careful addition of water, the reaction crude
is treated with concentrated hydrochloric acid until complete
solution and extracted with chloroform. The combined organic
extracts are then washed with water and dried over anhydrous
magnesium sulfate. After vacuum evaporation of the solvent,
the remaining residue is purified by column chromatography
(silica gel, hexane–dichloromethane 1 : 1 v/v) to afford
aldehyde 15 (327 mg, 82%) as a yellowish solid (Found: C,
82.32%; H, 10.66%. C109H172O7 requires: C: 82.10%; H:
10.87%); mp 77 uC; n (KBr)/cm21 2920, 2851, 1696, 1580,
1504, 1467, 1431, 1342, 1240, 1120 and 960; dH (400 MHz,
CDCl3, Me4Si) 9.91 (s, 1H, –CHO), 7.66 (AA9BB9 system,
2H), 7.48 and 7.34 (AA9BB9 system, 4H), 7.47 (AA9BB9
system, 2H), 7.24–7.16 (m, 4H), 7.08 (d, 1H, Jtrans 16.36 Hz,
–CHLCH–), 7.06 (d, 1H, Jtrans 16.36 Hz, –CHLCH–), 7.02 (s,
1H, Hvinyl), 6.98 (d, 1H, Jtrans 16.60 Hz, –CHLCH–), 6.96
Star-shaped compound 18
To a refluxing and well-stirred suspension of aldehyde 15
(200 mg, 0.12 mmol) and 1,3,5-tris[(dimethylphosphonate)-
methyl]benzene (16) (19 mg, 0.04 mmol) in 25 ml of anhydrous
tetrahydrofuran, potassium tert-butoxide (14 mg, 0.12 mmol)
is added portionwise under argon atmosphere. After 24 hours
the reaction crude is allowed to cool to room temperature and
methanol and water are added. The resulting mixture is
extracted with chloroform and the combined organic extracts
dried over anhydrous magnesium sulfate. After evaporation of
the solvent, the residue is purified by column chromatography
(silica gel, hexane–dichloromethane 8 : 2 v/v) to afford
dendrimer 18 (120 mg, 62%) as a yellow oil (Found: C,
82.94%; H, 10.36%. C336H522O18 requires: C, 83.21%; H,
10.85%); n (KBr)/cm21 2921, 2852, 1578, 1504, 1466, 1430,
1341, 1232, 1114, 958 and 830; dH (500 MHz, CDCl3, Me4Si)
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2 A. Bernanose, J. Chim. Phys., 1953, 50, 64.
7.52 (AA9BB9 system, 6H), 7.48 (AA9BB9 system, 6H), 7.36
(AA9BB9 system, 6H), 7.35 (AA9BB9 system, 6H), 7.26
(AA9BB9 system, 6H), 7.12 (AA9BB9 system, 6H), 7.11
(s, 4H), 7.08–7.04 (m, 8H), 7.04 (d, 6H, Jtrans 16.00 Hz,
–CHLCH–), 7.00 (d, 6H, Jtrans 16.00 Hz, –CHLCH–), 6.76 (s,
6H), 6.75 (s, 6H), 4.06 (t, 12H, J 6.00 Hz, –OCH2–), 4.01 (t,
6H, J 6.00 Hz, –OCH2–), 4.00 (t, 6H, J 6.00 Hz, –OCH2–),
1.86–1.72 (m, 36H, –CH2–), 1.48 (bs, 72H, –CH2–), 1.26 (b,
198H, –CH2–), 0.86 (bs, 54H, –CH3); dC (125 MHz, CDCl3,
Me4Si) 153.70, 142.37, 142.06, 139.48, 138.47, 138.45, 138.09,
137.06, 136.83, 136.68, 135.64, 132.50, 130.89, 130.87, 129.98,
129.96, 129.14, 129.10, 129.04, 128.04, 128.02, 128.01, 127.16,
126.75, 126.73, 126.29, 126.26, 126.24, 126.23, 126.21, 105.70,
73.97 (–OCH2–), 69.64 (–OCH2–), 32.36, 32.34, 30.79, 30.77,
30.17, 30.15, 30.12, 30.07, 29.88, 29.85, 29.81, 29.78, 26.55,
23.10, 14.52; m/z (FAB) 4849 (M+).
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Polyfluorene (PF)
A mixture of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolyl)-
9,9-bis[4-(3,7-dimethyloctyloxy)phenyl]fluorene
(0.44
g,
0.5 mmol) and 2,7-dibromo-9,9-bis(3,7-dimethyloctyl)fluorene
(0.30 gram, 0.5 mmol) in 25 ml of toluene is allowed to stir at
room temperature till complete dissolution. Upon deaeration
and blanketing with argon, 2 mol% of tetrakis(triphenylphos-
phine)palladium(0) is added, after which 1.7 ml of a 20 wt%
tetraethylammonium hydroxide aqueous solution is added.
The mixture is allowed to reflux during 20 hours. Then 4,4,5,5-
tetramethyl-1,3,2-dioxaborolylbenzene (1.0 mmol, end capping
reagent) and some fresh catalyst are added, followed by
refluxing for another 16 hours. The reaction mixture is allowed
to cool to room temperature. Several washing steps with
aqueous sodium cyanide are performed to remove catalyst
residues. Afterwards the organic layer is dried and concen-
trated. The polymer is isolated after several fractionations and
precipitations, using tetrahydrofuran and methanol, respec-
tively. Yield: ca. 70% of polymer as white fibers. The polymers
1
1
are characterized by H-NMR and SEC. H-NMR: the poly-
mer composition conforms to the monomer feed ratio. SEC:
molecular weight Mw ca. 102 kg mol21, polydispersity 3.4.
Acknowledgements
We thank R. Kicken (Eindhoven University of Technology,
Dept. Macromol. and Organic Chemistry) for a generous gift
of polyfluorene. N. Kiggen (TNO Science and Industry) is
acknowledged for assistance on LED devices. We thank the
MCyT (Ref. CTQ2004-03760), Universidad Complutense de
Madrid (PR1/07-14894) and Comunidad de Madrid (CCG06-
UCM/PPQ-1196, Group nr. 910759) for financial support.
R. G. is indebted to the ‘‘Programa Ramo´n y Cajal’’ and the
Flores Valles Company. This work was supported by the EU
Integrated Project NAIMO (No NMP4-CT-2004-500355).
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