“Butterfly Wings” StabilizeHeptacene
Elias C. Rüdiger,
and Uwe H. F. Bunz*
Abstract: The synthesis of bisalkynylated derivatives of
tetrabenzo[a,c,p,r]heptacene and tetrabenzo[a,c,l,n]penta-
cene via two-/fourfold Stille reactions involving a9-stan-
nafluorene andsuitable tetrabromoacenes is reported.
These triphenylene-“winged” heptacenes are surprisingly
stable and maintain asignificant portion of the electronic
properties of heptacenes.
The larger acenes (> tetracene),first described by Clar,
importantsemiconductors, yet sensitive materials. As aconse-
quence, only unsubstitutedpentacenehas found widespread
application in organic electronics, whereby unsubstituted hep-
tacene even though prepared by Bettinger, is too sensitiveto
be usefulfor applications.
Much later,Anthony et al. came up
with arobustscheme to functionalize pentacenebyattaching
triisopropylsilylethynyl-groupsatthe 6,13-positions of penta-
cene, dramatically stabilizing the pentacene nucleus.
concept was extended to hexaceneand heptacene, but even
extremelybulky silyl groups provide only moderate stability
Recently,others have developed additional con-
cepts for stabilization of larger acenoarenes.
Herein, we com-
bine the aforementionedapproaches with our previously es-
tablished benzannulation strategy
by two or four attached
benzene rings, leadingtoarrow- or butterfly-shaped aromatic
hydrocarbons. While we sacrifice some of the “acene-ness”, we
gain stability in the formed hexacenes and heptacenes.
undergo twofold or fourfold Stille reactions to deliver acenes
1–4 in reasonable to excellent yields (Scheme 2). Well-soluble
1–3 are stable in solutionand in the solid-state under air;but-
terfly heptacenes 4a–c remain stable for aweek in solution
under ambient conditions and for months in the solid-state
(see the Supporting Information, Figure S2 fordetails). Al-
though the introduction of bulky silyl substituents, such as
tri(sec-butyl)silyl and tricyclopentylsilyl at the backbonecenter
this wasnot observed for 4b and
c,indicating that the dominating intermolecularinteraction is
p-stacking of the sterically accessible triphenylene “wings”. As
aconsequence, even the solubility of 4c is only very modest
Figure 1and Table 1show the opticaldata of the prepared
butterfly- and arrow-shaped acenes, and as acomparison, the
absorption maxima of the acene compounds A–C (Scheme 1).
The addition of asecond triphenylene “wing” to 2 inducesa
pronounced red shift of 1210 cm
in 4a–c,shiftingits absorp-
tion maximumto760 nm. The “true” acenes A–C show absorp-
and 835 nm,
whereas 3 absorbsat556 nm and 4a–c at 752 nm. Compound
1 exhibits an absorption maximum at 585 nm. The incorpora-
tion of one or two triphenylene units decreased the acene
character of the butterfly- or arrow-type arenes,but by far not
to the amount expected, because 4 showed asimilarabsorp-
tion maximumtohexacene B and not to pentacene A.Hence,
the two outmost benzene rings in 4 do enlargethe p-system
—bothofthem together display the same effect as one regu-
larly annulatedbenzene ring.
This was further illustrated by the experimentally estimated
positions of the frontier molecular orbital (FMO) levels, in good
agreement with DFT calculations (B3LYP/6–311 ++G**,
Table 1): Butterfly-type heptacenes 4a–c exhibit similarenergy
Scheme1.Previously reported,centrally stabilized acenes.
[a] M. Müller,E.C.Rüdiger,S.Koser,O.Tverskoy,Dr. F. Rominger, Dr.F.Hinkel,
Dr.J.Freudenberg, Prof. Dr.U.H.F.Bunz
Organisch-Chemisches Institut, Ruprecht-Karls-UniversitätHeidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
[b] Dr.F.Hinkel,Prof. Dr.U.H.F.Bunz
Centre of Advanced Materials
Im Neuenheimer Feld 225, 69120 Heidelberg (Germany)
InnovationLab, Speyerer Strasse 4, 69115 Heidelberg (Germany)
Supporting information and the ORCID number(s) for the author(s) of this
articlecan be foundunder https://doi.org/10.1002/chem.201801079.
Chem. Eur.J.2018, 24,8087 –8091 2018 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim8087