Synthesis, photophysical, and device properties of novel dendrimers based on a fluorene hexabenzocoronene (FHBC) core

Article abstract of DOI:10.1021/ol8029164

The synthesis of easily functionalized and highly soluble fluorene-containing hexabenzocoronenes (FHBC) has been achieved in high yield at a gram scale. Conjugated triarylamine oligomers were coupled to the FHBC cores via Buchwald-Hartwig coupling, and th

Full text of DOI:10.1021/ol8029164

ORGANIC
LETTERS
2009
Vol. 11, No. 4
975-978
Synthesis, Photophysical, and Device
Properties of Novel Dendrimers Based
on a Fluorene-Hexabenzocoronene
(FHBC) Core
Wallace W. H. Wong,*,† David J. Jones,† Chao Yan,† Scott E. Watkins,‡
Simon King,§ Saif A. Haque,§ Xiaoming Wen,† Kenneth P. Ghiggino,† and
Andrew B. Holmes†
School of Chemistry, Bio21 Institute, UniVersity of Melbourne, Vic. 3010, Australia,
CSIRO Molecular and Health Technologies, Ian Wark Laboratory, Clayton South,
Vic. 3169, Australia, and Department of Chemistry, Imperial College London,
Exhibition Road, London, SW7 2AZ, United Kingdom
wwhwong@unimelb.edu.au
Received December 18, 2008
ABSTRACT
The synthesis of easily functionalized and highly soluble fluorene-containing hexabenzocoronenes (FHBC) has been achieved in high yield at
a gram scale. Conjugated triarylamine oligomers were coupled to the FHBC cores via Buchwald-Hartwig coupling, and the photophysical
properties of resulting dendritic materials were examined by ultrafast laser spectroscopic techniques. Efficient quenching of the triarylamine
oligomer fluorescence was observed paving the way for the inclusion of these materials in bulk heterojunction solar cells. In preliminary
studies, solar cell devices with external quantum efficiencies above 5% have been fabricated.
Hexabenzocoronene (HBC) is a planar aromatic molecule
consisting of 13 fused six-membered rings (Figure 1).1 It
belongs to the family of polycyclic aromatic hydrocarbons
consisting of flat disklike cores. HBC and its derivatives have
been shown to self-assemble into columnar structures giving
rise to ordered morphology in films.2,3 This property is
potentially very useful in bulk heterojunction solar cells
where the active layer consists of an electron and a hole
transport material usually blended together in a random
fashion. The self-assembly of materials into ordered struc-
tures in a bulk heterojunction could increase the efficiency
of the photovoltaic device by facilitating charge separation
(2) (a) Destrade, C.; Nyugen, T. H.; Gasparoux, H.; Malthete, J.; Levelut,
A. M. Mol. Cryst. Liq. Cryst. 1981, 71, 111. (b) Carfanga, C.; Roviello,
A.; Sirigu, A. Mol. Cryst. Liq. Cryst. 1995, 122, 151. (c) Raja, K. S.;
Ramakrishnan, S.; Raghunathan, V. A. Chem. Mater. 1997, 9, 1630. (d)
Zamir, S.; Singer, D.; Spielberg, N.; Wachtel, E. J.; Zimmermann, H.;
Poupko, R.; Luz, Z. Mol. Cryst. Liq. Cryst. 1996, 21, 39.
† University of Melbourne.
‡ CSIRO.
(3) (a) Ito, S.; Wehmeier, M.; Diedrich Brand, C.; Ku¨bel, C.; Epsch,
R.; Rabe, J. P.; Mu¨llen, K. Chem.-Eur. J. 2000, 6, 4327. (b) Kastler, M.;
Pisula, W.; Wasserfallen, D.; Pakula, T.; Mu¨llen, K. J. Am. Chem. Soc.
2005, 127, 4286.
§ Imperial College London.
(1) Wu, J.; Pisula, W.; Mu¨llen, K. Chem. ReV. 2007, 107, 718. Herwig,
P. T.; Kayser, C. W.; Mu¨llen, K.; Spiess, H. W. AdV. Mater. 1996, 8, 510.
10.1021/ol8029164 CCC: $40.75
Published on Web 01/22/2009
2009 American Chemical Society

and transport. By using HBC as the core, the aim is to
synthesize planar conjugated dendrimers with the option of
incorporating electron and hole transport materials as well
as dyes for use in organic bulk heterojunction photovoltaic
devices (Figure 1).
the Suzuki-Miyaura coupling of the key asymmetric 9,9-
dioctylfluorene synthon with hexabromophenylbenzene fol-
lowed by iodination and oxidative cyclization with iron
trichloride (Scheme 1).
Scheme 1. Synthesis of FHBC Core 1
Figure 1. Self-assembly of HBC derivatives into columnar struc-
tures3 and proposed assembly into thin films with ordered morphol-
ogy.
FHBC core 1 was highly soluble in most organic solvents
and can be isolated in gram quantities in high yield. The
2-fold and 4-fold symmetric bis-FHBC cores 2 and 3 were
also obtained in the gram scale in high yield through a series
of Suzuki-Miyaura coupling and Diels-Alder reactions
(Schemes 2 and 3). Cooling of warm dichloromethane
The chemistry of hexabenzocoronenes has been well
established by the group of Mu¨llen in the past decade.1,3,4
Many HBC derivatives with alkyl substituents have been
reported. Some derivatives have been shown to π-π stack
in the solid state by X-ray crystallography,5 while others were
identified by AFM imaging and a variety of spectroscopic
techniques to assemble into columnar structures.3 Extended
HBC derivatives have also been synthesized, and graphitic
sheets of over 400 carbon atoms have been isolated and
identified.6 The group of Aida has reported an amphiphilic
HBC system which has been shown to assemble into
nanotube structures.7 However, HBC derivatives with con-
jugated oligomeric substituents have not been reported. The
primary challenge in the synthesis of these systems is the
incorporation of the conjugated substituents as well as alkyl
groups for solubility. As such, 9,9-dialkylfluorenes are ideal
building blocks in conjugated oligomer-HBC hybrid sys-
tems. In this paper, the synthesis and photophysical properties
of novel, planar, conjugated dendrimers based on a 9,9-
dioctylfluorenyl hexabenzocoronene (FHBC) core are re-
ported.
Scheme 2. Synthesis of FHBC Core 2
Three FHBC cores were synthesized in this study. The
6-fold symmetric hexakis-FHBC core 1 was obtained through
(4) Wu, J.; Grimsdale, A. C.; Mu¨llen, K. J. Chem. Mater. 2005, 15, 41
.
(5) Herwig, P. T.; Enkelmann, V.; Schmelz, O.; Mu¨llen, K. Chem.-Eur.
J. 2000, 6, 1834.
(6) Simpson, C. D.; Mattersteig, G.; Martin, K.; Gherghel, L.; Bauer,
R. E.; Ra¨der, H. J.; Mu¨llen, K. J. Am. Chem. Soc. 2004, 126, 3139.
(7) (a) Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.; Ichihara, H.;
Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Science 2004,
304, 1481. (b) Yamamoto, Y.; Fukushima, T.; Saeki, A.; Seki, S.; Tagawa,
S.; Ishii, N.; Aida, T. J. Am. Chem. Soc. 2007, 129, 9276. (c) Mynar, J. L.;
Yamamoto, T.; Kosaka, A.; Fukushima, T.; Ishii, N.; Aida, T. J. Am. Chem.
Soc. 2008, 130, 1530.
solutions of FHBC cores 2 and 3 gave yellow crystalline
solids which were collected by filtration.
With the FHBC cores in hand, electron and hole transport
materials as well as dyes can be attached through the iodo-
976
Org. Lett., Vol. 11, No. 4, 2009

drimers obtained from the FHBC cores all have similar
absorption spectra with maxima at 375 nm. The fluorescence
spectra of the films clearly showed the quenching of the
triarylamine fluorescence in the blends and for the conjugated
dendrimers. The hexakis(fluorenyl) HBC core 1 quenched
the fluorescence of triarylamine 29 most efficiently (complete
quenching for a 1:1 blend), while the fluorescence of 29 was
partially quenched for FHBC cores 2 and 3. Fluorescence
decay data confirmed the steady state quenching results (see
Supporting Information). No fluorescence attributed to the
triarylamine was observed in all three dendrimers, but a weak
exciplex emission at ∼540 nm was identified. This is most
prominent in dendrimer 32.
Scheme 3. Synthesis of FHBC Core 3
The dendrimers were examined using laser spectroscopic
techniques. Transient absorption data were collected using
a highly sensitive microsecond absorption system under N2
or air at room temperature. Figure 2 shows the decay of a
aryl functionality using a range of coupling reactions. As a
proof of principle, a triarylamine oligomer 29 was coupled
to the FHBC cores. Buchwald-Hartwig coupling of oligomer
29 with the FHBC cores gave the three dendritic products
in high yield (Scheme 4). All novel compounds were
Scheme 4
.
Synthesis of the FHBC Dendrimers
Figure 2. Transient spectroscopic kinetics of core 1 (upper graph)
and FHBC dendrimer 30 (lower graph) recorded using a probe light
of 750 nm wavelength. In both cases, the lifetime is reduced in the
presence of air (dashed red trace) compared with a pure N2
environment (solid black trace). The inset shows the transient
spectral feature observed in core 1 (open red squares) and the FHBC
dendrimer 30 (full black squares) at a time 90 µs after excitation
for pure N2 environments.
1
characterized using H and 13C spectroscopy, mass spec-
trometry, and elemental analysis.
broad transient feature (see inset), whose peak is centered
around 750 nm, observed in core 1 and the FHBC dendrimer
30. The decrease in lifetime of this feature observed in an
air atmosphere is indicative of quenching of triplet excited
states due to a spin-allowed transition to the triplet ground
state of molecular oxygen. As with HBCs studied elsewhere,
there is no evidence of phosphorescence from these materials
at room temperature by time-correlated single photon count-
The compatibility of the FHBC cores and triarylamine hole
transport material 29 was examined by fluorescence quench-
ing studies. Thin films of FHBC cores and 29 and their 1:1
blends as well as the corresponding dendrimers were
spincoated onto glass slides (20 mg/mL toluene solution at
2000 rpm). FHBC core 1 has an absorption maximum at
390 nm, while cores 2 and 3 have maxima at 368 and 366
nm, respectively (see Supporting Information). The den-
Org. Lett., Vol. 11, No. 4, 2009
977

ing.8 Triplet state formation and, potentially, triplet energy
migration may thus occur in the compact HBC core as-
semblies. If the triplet state energy is such that it precludes
charge separation processes, then triplet state formation could
be a limitation on the efficiency of photovoltaic devices since
it would represent a potential loss of extracted current.
The HOMO energy levels of the FHBC cores 1 and 2 and
dendrimers 30 and 31 were measured using electrochemical
techniques. Cyclic voltammograms of these compounds were
-
recorded in toluene solution with 0.1 M Bu4N+BF4 as
electrolyte. Both onsets of oxidation for FHBC cores 1 and
2 are at 1.0 V vs ferrocene/ferrocenium, while the oxidation
onsets for dendrimers 30 and 31 are at -0.1 V. This means
the HOMO levels of the FHBC cores and the dendrimers
are -5.8 and -4.7 eV, respectively. The optical band gaps
of all three dendrimers obtained from their thin film UV-
vis spectra are approximately 2.6 eV. These energy levels
confirm that the FHBC dendrimers are an appropriate match
with an electron acceptor, such as [6,6]-phenyl-C61-butyric
acid methyl ester (C60 PCBM) with LUMO at -3.7 eV and
HOMO at -6.1 eV, for use in organic solar cells.9
Figure 3
. IPCE spectra of devices containing dendrimer 30 with
C60 PCBM (black line) and C70 PCBM (red line). Device structure
is ITO/PEDOT:PSS (38 nm)/dendrimer 30:PCBM 1:2 (40-60 nm)/
Ca (20 nm)/Al (100 nm).
In preliminary device studies, bulk heterojunction solar
cells with a device structure of ITO/PEDOT:PSS (38 nm)/
active layer (40-60 nm)/Ca (20 nm)/Al (100 nm) were
fabricated. The devices were tested with an Oriel solar
simulator fitted with a 1000 W Xe lamp filtered to give an
output of 100 mW/cm2 at AM 1.5. The active layer of the
device consists of a blend of one of the dendrimers and C60
PCBM in ratios of 1:2 or 1:4. The performance of the devices
with the three dendrimers is similar reaching Voc ) 0.64 V,
Jsc ) 0.68 mA/cm2, fill factor ) 0.30, and power conversion
efficiency ) 0.13% (see Supporting Information for all
device data). It is worthy of note that no annealing was
carried out on any of the devices, and this is the subject of
ongoing studies. Devices consisting of the dendrimers and
the C70 analogue of C60 PCBM were also fabricated. It has
been shown that C70 can provide better device performance
because of its superior optical absorption profile.10 In a
and under low power conditions (1 mW/cm2) to prevent
device degradation. All devices measured in this study are
stable under a simulated solar spectrum of 100 mW/cm2 at
AM 1.5.
In summary, a series of easily functionalized fluorenyl-
hexabenzocoronene (FHBC) compounds have been synthe-
sized in high yield at a gram scale. Triarylamine oligomers
were attached to these FHBC moieties via Buchwald-Hartwig
coupling leading to a series of electron-donating dendritic
materials. In association with electron-accepting fullerene
derivatives, bulk heterojunction solar cells were fabricated
with external quantum efficiencies over 5%. The assembly
of these FHBC compounds into columnar structures in thin
films are currently under investigation. The effect of thermal
or solvent annealing on solar cell performance is also under
investigation. Further studies are underway to extend the
series of FHBC compounds by attaching different organic
electronic materials to their periphery.
dendrimer-fullerene blend ratio of 1:2, devices with Voc
)
0.66 V, Jsc ) 1.0 mA/cm2, fill factor ) 0.34, and power
conversion efficiency ) 0.22% were measured. A compari-
son of the IPCE spectra of the C60 and C70 devices clearly
shows the contribution of C70 to the photocurrent (Figure
3). The performance of these solar cell devices are either
better than or comparable to that reported in the literature
for devices containing HBC derivatives.11 In previous reports,
the devices were typically tested using monochromatic light
Acknowledgment. We thank the Australian Research
Council (ARC, DP0451189, DP0877325), the Victorian
Government Department of Primary Industries (ETIS), the
Australian Government Department of Innovation, Industry,
Science and Research (ICOSC, CG10059), the Common-
wealth Scientific and Industrial Research Organisation
(CSIRO), the Victorian Endowment for Science, Knowledge
and Innovation (VESKI), University of Melbourne and
DAAD/Go8 exchange scheme for generous financial support.
S.A.H. acknowledges EPSRC, BP-Solar, ESF-SONS2 pro-
gram, and the Royal Society for financial support.
(8) El Hamaoui, B.; Laquai, F.; Baluschev, S.; Wu, J.; Mu¨llen, K. Synth.
Met. 2006, 156, 1182.
(9) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater.
2001, 11, 15.
(10) Wienk, M. M.; Kroon, J. M.; Verhees, W. J.; Knol, J.; Hummelen,
J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Int. Ed. 2003, 42,
3371.
(11) (a) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.;
Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (b) Schmidtke,
J. P.; Friend, R. H.; Kastler, M.; Mu¨llen, K. J. Chem. Phys. 2006, 124,
174704. (c) Li, J.; Kastler, M.; Pisula, W.; Robertson, J. W. F.; Wasserfallen,
D.; Grimsdale, A. C.; Wu, J.; Mu¨llen, K. AdV. Funct. Mater. 2007, 17,
2528. (d) Jung, J.; Rybak, A.; Slazak, A.; Bialecki, S.; Miskiewicz, P.;
Glowacki, I.; Ulanski, J.; Rosselli, S.; Yasuda, A.; Nelles, G.; Tomovic´,
Z.; Watson, M. D.; Mu¨llen, K. Synth. Met. 2005, 155, 150.
Supporting Information Available: Experimental pro-
cedures, full spectroscopic data for all new compounds, and
device data. This material is available free of charge via the
Internet at http://pubs.acs.org.
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