Making nonsymmetrical bricks: Synthesis of insoluble dipolar sexiphenyls

Article abstract of DOI:10.1021/ol5009087

A versatile synthesis of nonsymmetrical, terminally substituted p-sexiphenyl (6P) derivatives has been developed. The synthesis makes use of a nonsymmetrical starting material as well as modular functionalization using Suzuki cross-coupling to yield a soluble precursor, which finally is converted to the insoluble target 6P derivatives. These derivatives display similar electronic and optical properties to the parent 6P, yet the permanent dipole along their molecular axis allows for tuning of their self-assembly on various substrate surfaces.

Full text of DOI:10.1021/ol5009087

Letter
pubs.acs.org/OrgLett
Making Nonsymmetrical Bricks: Synthesis of Insoluble Dipolar
Sexiphenyls
Yves Garmshausen, Jutta Schwarz, Jana Hildebrandt, Bjorn Kobin, Michael Patzel, and Stefan Hecht*
̈
̈
Department of Chemistry & IRIS Adlershof, Humboldt-Universitat zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany
̈
S
* Supporting Information
ABSTRACT: A versatile synthesis of nonsymmetrical, terminally substituted p-sexiphenyl (6P) derivatives has been developed.
The synthesis makes use of a nonsymmetrical starting material as well as modular functionalization using Suzuki cross-coupling
to yield a soluble precursor, which finally is converted to the insoluble target 6P derivatives. These derivatives display similar
electronic and optical properties to the parent 6P, yet the permanent dipole along their molecular axis allows for tuning of their
self-assembly on various substrate surfaces.
ny optoelectronic device consists of several components
self-assembly behavior on various substrate surfaces and
Aresponsible for specific functions, such as injecting and therefore provide an effective means to tune interfacial
energetics.3
ejecting charges, transporting holes and electrons, absorbing
The desired optical and electronic properties of the organic
semiconductor require both sufficient oligomer length and
coupling between the repeat units to ensure for efficient π-
conjugation.4 However, in the series of unsubstituted oligo(p-
phenylene)s the solubility decreases rapidly with the length of
the p-phenylene chain.5 Introduction of solubilizing side chains
is problematic as they can cause an increased phenyl−phenyl
twist and hence lead to decoupling of the repeat units.6
Furthermore, such long alkyl side chains increase the molecular
weight considerably, therefore precluding vacuum-processing of
the organic material by thermal evaporation.7 Due to the
challenging preparation and purification primarily caused by the
low solubility, only shorter dipolar oligomers such as p-
quarterphenylenes8,9 and hydroxyl p-quinquiphenyl10 have
been synthesized thus far. With regard to 6P, which was first
prepared by Pummerer and Bittner as early as in 1924,11 only a
few terminally substituted derivatives are known; however, all
of them are symmetrical. To meet this challenge, we developed
a new modular synthetic route involving a nonsymmetric,
soluble precursor, which after desired terminal functionalization
is converted to the dipolar, insoluble 6P in the final step of the
synthesis.
and emitting photons, among others, and the interface between
these different materials is of utmost importance for the overall
device performance. Among the various interfaces, the one
between organic molecules and inorganic solid surfaces is
highly relevant yet the control over its structure and energetics
remains challenging. To engineer this interface, one can either
tailor the solid substrate surface or the organic molecule. While
the first approach is somewhat limited to specific surfaces and
their reconstructions, tuning the molecular structure seems to
offer endless opportunities. However, there are few privileged
small molecules that have received a lot of attention and they
typically consist of planar and rigid, extended π-systems.1
Among the best studied molecules is p-sexiphenyl (6P),
which is known to grow in well-ordered thin films on surfaces,
such as KCl(001), GaAs(001), mica(001), TiO2(110),
Au(111), or Al(111).2 Options to control the self-assembly
and therefore properties of the first interfacial layers have been
rather limited, in particular to different deposition techniques
and rates as well as sample temperatures.2 Although substituted
6P derivatives should grant the desired control over self-
assembly, such compounds have thus far remained elusive due
to the lack of solubility precluding their synthesis. Here, we
describe the synthesis of various 6P derivatives with different
substituents on their termini. Such 6P derivatives having a
permanent dipole along their molecular axis display distinct
Received: March 27, 2014
© XXXX American Chemical Society
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Our synthetic efforts have been inspired by the work of
Mayer and Schiffner, who in 1932 discovered the synthetic
access to p-phenylenes by addition of a phenyl Grignard
reagent to cyclohexane-1,4-dione followed by elimination of
water from the formed diol and aromatization.12 This strategy
was rediscovered three decades later, when Kern et al.
synthesized methyl and methoxy side chain substituted p-
phenylenes up to p-octiphenyl5 and rejuvenated by Itami and
co-workers in their elegant synthesis of cyclic oligo(p-
phenylene)s.13 For this purpose, the latter authors introduced
methyloxymethyl groups to stabilize their nonplanar inter-
mediates. An alternative elegant route again involving a soluble
precursor has been developed by Unroe and Reinhardt leading
to unsubstituted (and hence symmetrical) oligo(p-phenylene)s
up to p-octiphenyl.14
Retrosynthetically, the target nonsymmetrical 6P compounds
were disected into two terminally functionalized biphenyl units,
one phenylene extension, and the original solubilizing cyclo-
hexane fragment (Scheme 1). The latter (shown in red, see
Scheme 1) was envisioned to originate from commercially
available monoketal-protected cyclohexa-1,4-dione 1, circum-
venting a desymmetrization in a statistical fashion (to avoid
potentially cumbersome purification). Initial attachment of one
terminal biphenyl unit (green) was followed by introduction of
the phenyl spacer (black) carrying a halogen functionality for
subsequent connection to the other terminal biphenyl unit
(carrying the substituent R′). The resulting 6P precursors
should display sufficient solubility due to the central cyclo-
hexane kink, thereby allowing for facile chromatographic
purification. The final sequence of deprotection, elimination,
and oxidation leads to aromatization of the former saturated
cyclohexane fragment and installs the last missing phenylene
unit. Isolation of the formed 6P derivatives is enabled by both
their low solubility (allowing for removal of all other soluble
byproducts by extensive washing) as well as their crystallinity
and thermal stability (allowing for recrystallization and final
sublimation).
Our actual syntheses (Scheme 2) involved addition of
lithiated biphenyls 2 or 3, followed by acidic ketal removal and
subsequent MOM-protection of the tertiary alcohol to yield
intermediates 4 and 5, respectively. Nucleophilic addition of the
monolithiated dibromobenzene 6 followed again by MOM-
protection gave access to compounds 7 and 8, which carry a
bromine substituent for further functionalization. These
advanced intermediates (7 and 8) were prepared on a 2 g
scale and readily functionalized by Suzuki cross-coupling with
various biphenyl boronic acids 9−12 to install the second
terminus and yield the nonsymmetric 6P precursors 13−17.
Because of their excellent solubility in a variety of organic
solvents, such as ethyl acetate, methylene chloride, THF,
among others, precursors 13−17 were readily purified by
column chromatography. The final deprotection−elimination−
oxidation sequence was performed in one pot using NaHSO4·
H2O at 150 °C with p-chloranil as the oxidant, according to a
procedure developed by Itami and co-workers.15,16 The green
intermediate elimination product proved somewhat resistant to
undergo final aromatization, presumably due to its low
solubility even at 150 °C in a DMSO/xylene mixture.5,17
However, when raising the temperature to 210 °C with 1,2,4-
trichlorobenzene as solvent and using p-chloranil or air as the
oxidant complete aromatization was accomplished. After being
cooled to room temperature, the products were collected by
simple filtration and purified by crystallization from 1,2,4-
trichlorobenzene and final gradient sublimation.
Scheme 1. Retrosynthetic Analysis of Nonsymmetric 6P
Derivatives
Substituents were chosen according to several criteria, most
notably, (i) to provide electronic variation ranging from
electron-donating to strongly electron-accepting groups
(based on inductive and resonance effects), (ii) to aid
characterization of the compounds’ self-assembly structures
(for example by XPS and NEXAFS), and (iii) to be compatible
with the reaction conditions throughout the synthetic sequence.
The last requirement precluded the introduction of strong
tertiary amine donors, since both aniline as well as N,N-
dimethylaniline (as model compounds) decomposed when
exposed to the high-temperature, strongly acidic aromatization
conditions. Hence, a methoxy group was chosen as donor while
chloro, trifluoromethyl, and cyano groups were selected as
acceptor moieties (of increasing strength). In a first set of
monofunctional 6P derivatives, the one and only substituent
(−OMe, −Cl, or −CN) was introduced late in the sequence by
Suzuki cross-coupling with the appropriate biphenyl boronic
acids. In a second set of 6P derivatives, targeting donor−6P−
acceptor architectures, the methoxy donor was introduced in
the beginning of the sequence as it is compatible with the
subsequently employed organolithium reagents. The opposite,
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electron ionization), which in combination with the proven
structure of the purified precursors provides sound evidence for
the structural identity of the target 6P derivatives. Additional
proof stems first of all from the observed (and expected)
drastically lowered solubility during the aromatization step and
from the excitation and emission spectra showing the presence
of the characteristic 6P optical transition as well as the IR data.
The latter proved to be particularly insightful for 6P-CN and
MeO-6P-CN as the characteristic CN stretching mode can
be observed. Interestingly, donor-substitution gave rise to an
increased wavenumber (MeO-6P-CN: 2231.7 cm−1 as
compared to 6P-CN: 2228.1 cm−1), which surprisingly is
opposite to the expected decreased stretching vibration.19
Despite their low solubility,20 the optical properties of the 6P
derivatives, in particular their optical gaps, could be evaluated
since the compounds are very emissive, and therefore, even at
extremely low concentrations, fluorescence spectra could be
recorded. Note that in contrast to fluorescence spectra,
absorption spectra could not be obtained due to the insufficient
optical density. Instead, the intense fluorescence emission
enabled the corresponding excitation spectra to be obtained
(Figure 1). Furthermore, note that fluorescence emission and
Scheme 2. Synthesis of New p-Sexiphenyl Derivatives
Figure 1. Fluorescence emission (right) and corresponding excitation
(left) spectra of 6P-OMe (blue line), 6P-CN (green line), and MeO-
6P-CN (red line) at extremely low concentration (<5·10−6 M) in
CHCl3 at 25 °C. All spectra have been normalized to the same
intensities.
excitation spectroscopy proved to be the analytical method of
choice to monitor the completeness of the aromatization
sequence, because the not completely aromatized intermediate
dihydro-6P derivatives, present after deprotection and 2-fold
water elimination, can be detected even in traces due to their
red-shifted emission.
The detailed spectroscopic data (Figure 1 as well as Table S1
in the Supporting Information) show that substitution with
donors and/or acceptors causes a bathochromic shift of both
the fluorescence excitation as well as emission spectra when
compared to the parent unsubstituted p-sexiphenyl. The more
pronounced red-shift of acceptor-substituted 6P-CN as
compared to donor-substituted 6P-OMe can be explained by
the slightly more extended π-system of the cyano group. Not
unexpectedly, the combination of both donor and acceptor
moieties in the case of MeO-6P-CN results in the largest
electron-withdrawing groups (−CF3 or −CN) were introduced
afterward again by employing mild Suzuki cross-coupling
conditions. In all cases, the final deprotection−elimination−
oxidation steps could successfully be performed and yielded the
desired mono- and difunctional p-sexiphenyls.
The extremely poor solubility of the target compounds
excludes conventional characterization techniques relying on
reasonably concentrated solutions of the analyte, most notably
NMR spectroscopy.18 Hence, all 6P derivatives were
characterized by high-resolution mass spectrometry (using
C
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Letter
(9) Wallmann, I.; Schnakenburg, G.; Lutzen, A. Synthesis 2009, 79−
̈
bathochromic shift. The corresponding optical gap amounts to
3.39 eV (onset) as compared to 3.46 eV for the unsubstituted
p-sexiphenyl. While methoxy or chloro substitution does not
affect the vibronic fine structure of the emission spectra typical
for the parent unsubstituted p-sexiphenyl,21 introduction of
cyano or trifluoromethyl groups leads to suppression of these
vibronic features. Note that although the structural identity of
the investigated 6P derivatives has been established (vide
supra), their low solubilities and relatively high molecular
weights preclude liquid and gas chromatography to estimate
their purity, which could only be assessed via elemental
analysis.22
In summary, we have developed a synthetic route to
previously inaccessible dipolar p-sexiphenyl derivatives. Cur-
rently, we are investigating the influence of their dipolar
structure on their self-assembly behavior at various solid
substrate surfaces23 and the resulting interfacial energy level
alignment.3,24
84.
(10) Yamaguchi, I.; Goto, K.; Sato, M. Tetrahedron 2009, 65, 3645−
3652.
(11) Pummerer, R.; Bittner, K. Chem. Ber. 1924, 57, 84−88.
(12) Mayer, F.; Schiffner, R. Ber. Dtsch. Chem. Ges. 1932, 65, 1337−
1338.
(13) Takaba, H.; Omachi, H.; Yamamoto, Y.; Bouffard, J.; Itami, K.
Angew. Chem., Int. Ed. 2009, 48, 6112−6116.
(14) Unroe, M. R.; Reinhardt, B. A. Synthesis 1987, 981−986.
(15) Omachi, H.; Matsuura, S.; Segawa, Y.; Itami, K. Angew. Chem.,
Int. Ed. 2010, 49, 10202−10205.
(16) Matsui, K.; Segawa, Y.; Itami, K. Org. Lett. 2012, 14, 1888−
1891.
(17) Bouffard, J.; Watanabe, M.; Takaba, H.; Itami, K. Macromolecules
2010, 43, 1425−1429.
1
(18) Exemplarily, we have obtained an H-NMR spectrum of a solid
sample of 6P-OMe (see the Supporting Information).
(19) Deady, L. R.; Katritzky, A. R.; Shanks, R. A.; Topsom, R. D.
Spectrochim. Acta 1973, 29A, 115−121.
(20) For MeO-6P-CF3 the solubility in CHCl3 was determined to be
less than 20 mg/L, corresponding to an approximate concentration of
10−5 M at saturation. The value is in agreement with the solubility of
parent 6P in toluene that has been determined to be <10 mg/L in ref
5a.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details including synthesis and characterization
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
(21) Heimel, G.; Daghofer, M.; Gierschner, J.; List, E. J. W.;
Grimsdale, A. C.; Mullen, K.; Beljonne, D.; Bred
́
as, J.-L.; Zojer, E. J.
̈
Chem. Phys. 2005, 122, 054501-1−054501-11.
(22) Elemental analysis gave satisfactory results for all investigated 6P
derivatives, except MeO-6P-CN.
AUTHOR INFORMATION
■
(23) For an example of 6P, see: Blumstengel, S.; Glowatzki, H.;
Sadofev, S.; Koch, N.; Kowarik, S.; Rabe, J. P.; Henneberger, F. Phys.
Chem. Chem. Phys. 2010, 12, 11642−11646.
(24) Duhm, S.; Heimel, G.; Salzmann, I.; Glowatzki, H.; Johnson, R.
L.; Vollmer, A.; Rabe, J. P.; Koch, N. Nat. Mater. 2008, 7, 326−332.
Corresponding Author
*E-mail: sh@chemie.hu-berlin.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Gudrun Scholz and Dr. Kerstin Scheurell for
measuring the solid-state NMR of 6P-OMe. Generous support
by the German Research Foundation (DFG via SFB 951) and
the Helmholtz Association (via Helmholtz Energy Alliance) is
gratefully acknowledged. BASF AG, Bayer Industry Services,
and Sasol Germany are thanked for generous donations of
chemicals.
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