Synthesis and electronic properties of conjugated pentacene dimers

Article abstract of DOI:10.1021/ol801886h

(Chemical Equation Presented) Conjugated pentacene dimers 1-3 were synthesized in two steps from readily available precursors. Noteworthy is the initial step, which assembles five independent fragments to form the carbon-rich molecular framework. Solution

Full text of DOI:10.1021/ol801886h

ORGANIC
LETTERS
2008
Vol. 10, No. 21
4779-4782
Synthesis and Electronic Properties of
Conjugated Pentacene Dimers
Dan Lehnherr,† Jianbo Gao,‡ Frank A. Hegmann,*,‡ and Rik R. Tykwinski*,†
Department of Chemistry, UniVersity of Alberta, Edmonton, AB T6G 2G2, Canada,
and Department of Physics, UniVersity of Alberta, Edmonton, AB T6G 2G7, Canada
rik.tykwinski@ualberta.ca; hegmann@phys.ualberta.ca
Received August 12, 2008
ABSTRACT
Conjugated pentacene dimers 1-3 were synthesized in two steps from readily available precursors. Noteworthy is the initial step, which
assembles five independent fragments to form the carbon-rich molecular framework. Solution-cast films of these materials are air stable.
Photocurrent measurements for solution-deposited thin films show that dimer 3 exhibits photoconductive gain >10.
Organic semiconducting materials offer the potential to
revolutionize optoelectronics, including applications in pho-
tovoltaic cells, chemical sensors, thin film transistors, as well
as devices based on nanowires.1 In an effort to provide
materials with improved performance for such goals, poly-
acenes have been a favorite target, building from the well-
established electronic attributes of anthracene and pentacene.1
One of the biggest breakthroughs in this area has been the
development of versatile synthetic protocols to functionalize
the pentacene core, and seminal work by Anthony and others
have established that such syntheses can provide pentacene
derivatives with technologically relevant properties.1,2
An obvious challenge to further improving the overall
usefulness of pentacene-based materials would be the forma-
tion of oligomers and polymers that might show enhanced
properties. Oligomers also provide an excellent opportunity
for assessing structure-property relationships, via compari-
sons to the corresponding monomeric analogues. Finally,
oligomers and polymers also often facilitate the solution-
state deposition of high-quality thin films. Despite these
attractive features, pentacene-based oligo- and polymers
have, for the most part, remained unrealized until quite
recently.3 On the other hand, the study of anthracene
oligomers has demonstrated improved conduction versus the
corresponding monomers.4 This suggests that the formation
of conjugated pentacene oligomers should also afford materi-
(2) (a) Sheraw, C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. AdV.
Mater. 2003, 15, 2009–2011. (b) Wolak, M. A.; Delcamp, J.; Landis, C. A.;
Lane, P. A.; Anthony, J.; Kafafi, Z. AdV. Funct. Mater. 2006, 16, 1943–
1949. (c) Park, J. G.; Vasic, R.; Brooks, J. S.; Anthony, J. E. J. Appl. Phys.
2006, 100, 044511. (d) Park, J. G.; Vasic, R.; Brooks, J. S.; Anthony, J. E.
J. Low Temp. Phys. 2006, 142, 391–396. (e) Park, S. K.; Anthony, J. E.;
Jackson, T. N. IEEE Electron DeVice Lett. 2007, 28, 877–879. (f) Dickey,
K. C.; Subramanian, S.; Anthony, J. E.; Han, L.-H.; Chen, S.; Loo, Y.-L.
Appl. Phys. Lett. 2007, 90, 244103. (g) Allard, S.; Forster, M.; Souharce,
B.; Thiem, H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070–4098.
† Department of Chemistry.
‡ Department of Physics.
(1) (a) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004,
104, 4891–4945. (b) Anthony, J. E. Chem. ReV. 2006, 106, 5028–5048. (c)
Anthony, J. E. Angew. Chem., Int. Ed. 2008, 47, 452–483. (d) Briseno,
A. L.; Mannsfeld, S. C. B.; Jenekhe, S. A.; Bao, Z.; Xia, Y. Mater. Today
2008, 11 (4), 38–47.
10.1021/ol801886h CCC: $40.75
Published on Web 09/30/2008
2008 American Chemical Society

als with unique and enhanced electronic properties. Herein,
we report on the development of a stepwise synthetic route
that provides stable, conjugated pentacene dimers 1-3
(Scheme 2). Furthermore, a preliminary study of semicon-
Scheme 2. Synthesis of Pentacene Dimers 1-3
Scheme 1. Synthesis of Model Compound 4
ducting properties of these compounds has been carried out
through the analysis of photoconductive gain in comparison
to model compounds 4 (Scheme 1) and 5 (Figure 1).
acenequinone. A general stepwise approach to unsymmetrical
pentacene chromophores has recently been reported and is
demonstrated in Scheme 1 for the synthesis of model
compound 4.6 In this protocol, the reaction of 6,13-
pentacenequinone with 1 equiv of lithiated triisopropylsily-
lacetylene (formed via reaction with HexLi) gave mono-
addition product 6 in excellent yield.7,8 The conversion of 6
to diol 7, however, required the use of excess lithiated
phenylacetylene due to the presence of the acidic hydroxy
group (which quenched 1 equiv of the acetylide). Unfortu-
nately, this procedure is not amenable to the formation of a
conjugated dimer. Clearly, the reaction of monoadduct 6 with
a diacetylide is problematic given the presence of a hydroxy
group, which would only partially quench an incoming
diacetylide via protonation. Furthermore, the use of excess
Figure 1.
Molecular structure of model compound 5.5
The synthesis of conjugated pentacene dimers necessitated
a method of desymmetrization of the core of 6,13-pent-
(3) (a) Lehnherr, D.; Tykwinski, R. R. Org. Lett. 2007, 9, 4583–4586.
(b) Okamoto, T.; Bao, Z. J. Am. Chem. Soc. 2007, 129, 1308–1309. (c)
Tokito, S.; Weinfurtner, K.-H.; Fujikawa, H.; Tsutsui, T.; Taga, Y. Proc.
SPIE-Int. Soc. Opt. Eng. 2001, 4105, 69–74.
(5) For the initial synthesis and study of 5, see: Anthony, J. E.; Brooks,
J. S.; Eaton, D. L.; Parkin, S. R. J. Am. Chem. Soc. 2001, 123, 9482–9483.
(6) Lehnherr, D.; McDonald, R.; Tykwinski, R. R. Org. Lett. 2008, 10,
4163-4166.
(4) (a) Ito, K.; Suzuki, T.; Sakamoto, Y.; Kubota, D.; Inoue, Y.; Sato,
F.; Tokito, S. Angew. Chem., Int. Ed. 2003, 42, 1159–1162. (b) Merlo,
J. A.; Newman, C. R.; Gerlach, C. P.; Kelley, T. W.; Muyres, D. V.; Fritz,
S. E.; Toney, M. F.; Frisbie, C. D. J. Am. Chem. Soc. 2005, 127, 3997–
4009. (c) Kashiwagi, K.; Yasuda, T.; Tsutsui, T. Chem. Lett. 2007, 36,
1194–1195.
(7) Boudebous, A.; Constable, E. C.; Housecroft, C. E.; Neuburger, M.;
Schaffner, S. Acta Crystallogr. Sect. C 2006, 62, o243–o245.
(8) See Supporting Information for details.
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Org. Lett., Vol. 10, No. 21, 2008

Figure 2
. Spectral data for functionalized pentacenes. (a) Solution-state UV-vis absorption spectra of dimers 1-3 and 4-5 (CH2Cl2).
Inset: expansion of low energy absorption region (450-750 nm). (b) Solid-state UV-vis absorption spectra. (c) Photocurrent yield.
diacetylide, as in the transformation of 6 to 7, would not
solve this problem. Thus, a one-pot protocol was explored
that circumvented the isolation and derivatization of 6.
Lithiated triisopropylsilylacetylene was added to a suspension
of 6,13-pentacenequinone in THF to form intermediate 6′.
This reaction mixture was subsequently added to a solution
of lithiated phenylacetylene in THF, and following workup
and purification by column chromatography, diol 7 was
isolated in excellent yield. Aromatization of 7 using
SnCl2·2H2O then gave 4. Compound 4 is highly soluble in,
for example, CH2Cl2, CHCl3, and THF, and it can be
manipulated under normal laboratory conditions in solution
or in the solid state with minimal decomposition.
With a viable one-pot procedure in hand, the synthesis of
dimers 1-3 followed a similar route. Lithiated trialkylsilyl-
acetylene was added to a suspension of 6,13-pentacene-
quinone in THF and provided either intermediate 6′ or 8. A
solution of this intermediate was then added to the appropri-
ate dilithiated 1,4-diethynylbenzene to complete the dimer-
ization process and afforded pentacene tetraols 9-11 in
17-39% yield. While the isolated yields for 9-11 were
moderate to low, it is worth noting that, in a single step,
four new carbon-carbon bonds have been formed through
the reaction of five individual precursors (equivalent to
64-79% yield per bond forming event). Finally, Sn(II)-
mediated reduction of 9-11 afforded conjugated pentacene
dimers 1-3 in good yields of 72-77%. All dimers could
be stored as solids for months under refrigeration in the
presence of both air and water without any observable
decomposition. Additionally, solutions of these materials
could be handled under normal ambient laboratory conditions
for several hours with minimal decomposition.
with n-hexyl3Si groups to give 2 produced a dimer that is
soluble in a number of common solvents (e.g., CH2Cl2,
CHCl3, and THF). Dimer 3 represents an attempt to maintain
solubility through the use of a π-spacer functionalized with
two Me3Si groups while minimizing the potential of disrupt-
ing π-stacking interactions that might arise from the terminal
n-hexyl3Si groups.1,9 Surprisingly, 3 does not possess
enhanced solubility relative to 1, although the electronic
makeup of the molecule is altered in comparison to that of
1 and 2 (vide infra).10
UV-vis spectra of dimers 1-3 in CH2Cl2 have been
examined in comparison to 4 and 5 (Figure 2a). The λmax
for 4 (652 nm) is red-shifted versus 5 (643 nm), as expected
due to the increased conjugation length as a result of the
pendent phenyl ring. The absorptions for pentacene dimers
1-3 all show an additional red shift in comparison to 4,
with λmax values centered at ca. 680 nm. While the absorption
characteristics of 1 and 2 are similar, that of 3 shows
additional fine structure, perhaps as a result of the pendent
silicon groups on the central core.10 To probe the origin of
the difference in the solution-state UV-vis absorption profile
for 3 relative to 1 and 2, concentration dependency has been
investigated to rule out aggregation. Solutions of 3 in CH2Cl2
have been prepared from 1.7 × 10-5 to 5.5 × 10-7 M. No
change in λmax values is observed, and molar absorptivity
values (ε) confirmed that Beer’s Law is obeyed. Since neither
the qualitative shape of the absorption nor the ε values
change appreciably as a function of concentration, it is
concluded that the difference in the spectrum of dimer 3 is
not due to aggregation. Thus, subtle electronic effects arising
from the Me3Si groups on the π-spacer of dimer 3 are likely
the source of the differences in its UV-vis spectrum.10
Solid-state absorption spectra of thin films of 1, 2, 4, and
5 (Figure 2b) are red-shifted compared to their solution-state
spectra. The exception to this trend is dimer 3, which shows
a blue-shifted λmax (657 nm), with a value similar to that of
The thermal stability of pentacene dimers 1-3 was
evaluated by thermal gravimetric analysis (TGA) and dif-
ferential scanning calorimetry (DSC) in comparison to 4.
Although the TGA showed no significant weight loss (<5%)
below 410 °C for all derivatives, the DSC trace of 4 and
dimer 2 contained exothermic peaks at much lower temper-
atures, namely, 180 and 157 °C, respectively. Dimers 1 and
3 showed better thermal stability based on the DSC analyses,
with decomposition temperatures of 445 and 389 °C,
respectively.
(9) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4, 15–
18.
(10) Altered HOMO-LUMO levels of π-systems resulting from silyl
substitution are well documented: (a) Giordan, J. C. J. Am. Chem. Soc.
1983, 105, 6544–6546. (b) Gleiter, R.; Scha¨fer, W.; Sakurai, H. J. Am.
Chem. Soc. 1985, 107, 3046–3050. (c) Igawa, K.; Tomooka, K. Angew.
Chem., Int. Ed. 2006, 45, 232–234. (d) Yamaguchi, S.; Xu, C.; Okamoto,
T. Pure Appl. Chem. 2006, 78, 721–730. (e) Shao, G.; Orita, A.; Nishijima,
K.; Ishimaru, K.; Takezaki, M.; Wakamatsu, K.; Gleiter, R.; Otera, J. Chem.
Asian J. 2007, 2, 489–498.
Pentacene dimer 1 shows limited solubility in common
organic solvents. Formally replacing the i-Pr3Si groups of 1
Org. Lett., Vol. 10, No. 21, 2008
4781

4 (665 nm). This suggests an interruption of conjugation
between the two pentacenes and the aryl π-spacer, perhaps
a manifestation of steric demands of the Me3Si groups in
the solid-state packing of this molecule.
The emission properties of pentacene materials 1-5 have
been investigated as CH2Cl2 solutions. As a model com-
pound, 4 shows fluorescence with λmax,em ) 661 nm,
representing a small Stokes shift of only 9 nm. This emission
field.1,2 The photoconductive yield for 3 is greater than 10
electrons per absorbed photon, representing a bulk photo-
conductive gain >10 (i.e., for every photon absorbed, >10
charge carriers traverse the active region of the device).15
This efficiency places spin-cast films of dimer 3 within an
order of magnitude of thermally-deposited films of pristine
pentacene.16
A plausible explanation for the efficiency of 3, supported
by the UV-vis spectroscopic analysis, is that the central
Me3Si groups disrupt the planarity of the three aromatic
chromophores in the solid state and ultimately reduce the
anisotropy of the pentacene chromophores in the film leading
to improved efficiency. This is analogous to the improved
charge-carrier mobility that is observed in single crystals that
have 2-D slipped-stack arrangements vs 1-D stacks.17,18
is red-shifted in comparison to 5, which shows λmax,em
)
649 nm and an even smaller Stokes shift of only 6 nm. The
fluorescence quantum yield (ΦF) of 4 is ΦF ) 0.12,
comparable to 5 (ΦF ) 0.15), measured in CH2Cl2 relative
to cresyl violet perchlorate in MeOH (ΦF ) 0.67 at 5.9 ×
10-7 M).11 None of the pentacene dimers 1-3 show any
significant emission (ΦF < 0.01) in the range of 560-850
nm when measured under the same conditions.
In conclusion, a new synthetic route to stable, processable
pentacene dimers has been developed. The subsequent
characterization of photoconductive gain has demonstrated
the potential for enhancing electronic properties through
synthetic design. The methodology presented provides for
future optimization of optoelectronic properties through (a)
the use of alternate π-spacers, (b) variation of solubilizing
end-groups, and (c) formation of longer pentacene-based
oligomers. A full account of these efforts will be reported
in due course.
To explore the semiconducting properties of the newly
realized pentacene dimers, measurement of photoconductive
yield has been explored using a traditional monochromator
and lock-in technique (Figure 2c).12 Photocurrent yield is
defined as the ratio between the photogenerated charge-
carrier flow rate and the absorbed photon rate. When the
photocurrent yield exceeds unity, the material shows pho-
toconductive gain. It is worth noting that, to date, there have
been only a limited number of organic materials that have
shown photoconductive gain.13 Thin films of 1-5 (ca.
0.2-0.5 µm) for analysis have been obtained by solution-
state spin-casting from CHCl3.14 As established by AFM
analysis, films of 2 and 4 are quite homogeneous, while those
of 1 and 3 show a more varied morphology consistent with
microcrystalline domains.8 Photocurrent yield for dimer 1
(Figure 2c) is the same magnitude as pseudomonomer 4,
while dimer 2 has the lowest efficiency, presumably due to
the larger size of the pendent n-hexyl3Si groups that might
disrupt intermolecular interactions between pentacene chro-
mophores. Dimer 3 has the highest efficiency, outperforming
5, which has become somewhat of a benchmark in the
Acknowledgment. This work has been generously sup-
ported by the University of Alberta and the Natural Sciences
and Engineering Research Council of Canada (NSERC)
through the Discovery Grant program. We also acknowledge
the use of the University of Alberta NanoFab. D.L. thanks
NSERC (PGS-D), the Alberta Ingenuity Fund, the University
of Alberta, and the Killam Trusts for scholarship support.
Supporting Information Available: Experimental pro-
cedures, spectroscopic data for new compounds, and AFM
images of thin films. This material is available free of charge
via the Internet at http://pubs.acs.org.
(11) Isak, S. J.; Eyring, E. M. J. Phys. Chem. 1992, 96, 1738–1742.
(12) Lang, D. V.; Chi, X.; Siegrist, T.; Sergent, A. M.; Ramirez, A. P.
Phys. ReV. Lett. 2004, 93, 086802.
OL801886H
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1994, 64, 187–189. (b) Katsume, T.; Hiramoto, M.; Yokoyama, M. Appl.
Phys. Lett. 1996, 69, 3722–3724. (c) Da¨ubler, T. K.; Neher, D.; Rost, H.;
Ho¨rhold, H. H. Phys. ReV. B 1999, 59, 1964–1972. (d) Nakayama, K.-i.;
Hiramoto, M.; Yokoyama, M. J. Appl. Phys. 2000, 87, 3365–3369. (e)
Huang, J.; Yang, Y. Appl. Phys. Lett. 2007, 91, 203505. (f) Campbell, I. H.;
Crone, B. K. J. Appl. Phys. 2007, 101, 024502.
(15) For a discussion of the mechanisms of photoconductive gain in
organic materials, see ref 13f.
(16) Gao, J.; Hegmann, F. A., unpublished results.
(17) For acene solid-state packing terminology and its influence on
electronic properties, see ref 1b.
(18) It is also possible that the variations in film morphology between
the different samples also plays a role in efficiency, and this premise is
currently being explored.
(14) Due to reduced solubility, spin casting was done from a solution
dispersion of 3 in CHCl3. For AFM images of films used in the analysis of
photoconductive gain, see the Supporting Information.
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Org. Lett., Vol. 10, No. 21, 2008