DOI: 10.1002/chem.201504453
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Photodegradation
Exploration of the Photodegradation of Naphtho[2,3-g]
quinoxalines and Pyrazino[2,3-b]phenazines
Nicole Kolmer-Anderl,[a] Andreas Kolmer,[b] Christina M. Thiele,*[b] and Matthias Rehahn*[a]
Abstract: Nitrogen-containing polycyclic aromatic hydrocar-
bons are very attractive compounds for organic electronics
applications. Their low-lying LUMO energies points towards
a potential use as n-type semiconductors. Furthermore, they
are expected to be more stable under ambient conditions,
which is very important for the formation of semiconducting
films, where materials with high purity are needed. In this
study, the syntheses of naphtho[2,3-g]quinoxalines and pyra-
zino[2,3-b]phenazines is presented by using reaction condi-
tions, that provide the desired products in high yields, high
purity and without time-consuming purification steps. The
HOMO and LUMO energies of the compounds are investigat-
ed by cyclic voltammetry and UV/Vis spectroscopy and their
dependency on the nitrogen content and the terminal sub-
stituents are examined. The photostability and the degrada-
tion pathways of the naphtho[2,3-g]quinoxalines and pyrazi-
no[2,3-b]phenazines are explored by NMR spectroscopy of ir-
radiated samples affirming the large influence of the nitro-
gen atoms in the acene core on the degradation process
during the irradiation. Finally, by identifying the degrada-
tions products of 2,3-dimethylnaphtho[2,3-g]quinoxaline it is
possible to track down the most reactive position in the
compound and, by blocking this position with nitrogen, to
strongly increase the photostability.
merisation occurs during irradiation without oxygen.[5] Howev-
er, this reactivity especially of soluble pentacene and tetracene
derivatives is a disadvantage in terms of organic field effect
transistor applications were highly purified materials are
needed to achieve good performances. Hence, the sensibility
towards oxidation of these compounds is a disadvantage with
regard to the fabrication of stable organic field effect transis-
tors, especially because quinone compounds are known to act
as electronic traps.[6] That is why new organic semiconductors
with an increased durability towards photooxidation and low-
lying LUMO energies, that are suitable for n-type semiconduc-
tor applications, are needed. For this reason nitrogen-contain-
ing acenes are one focus of current research.[7]
Introduction
Polycyclic aromatic hydrocarbons, like tetracene and penta-
cene and their derivatives, are of special interest due to their
big potential as semiconductors in organic electronic applica-
tions, like organic field effect transistors (OFETs).[1] Especially
pentacene as excellent p-type semiconductor and its deriva-
tives are well studied also in terms of reducing LUMO energies
by adding electron-withdrawing substituents to generate n-
type semiconductors.[2] But compared to the number of known
organic p-type semiconductors only few organic semiconduc-
tors with n-type behaviour and good transistor properties are
known, although they are needed to create, for example, com-
plementary electronic circuits.[3]
Theoretical studies have already shown that increasing the
number of nitrogen atoms in the acene core is an efficient tool
to increase the electron affinity, which is advantageous for
stable n-type behaviour of organic semiconductors.[8] In addi-
tion, these materials exhibit low LUMO energies, which are
necessary for n-type semiconductors, too. Furthermore there
are several representatives that show good n-type properties
in OFET applications.[9] UV/Vis measurements indicate that the
nitrogen atoms increase the stability of acenes towards the un-
desirable oxidation process due to its higher electron affini-
ty.[9,10]
Unfortunately, polycyclic aromatic hydrocarbons are also
known for the photochemical reactions they undergo during
irradiation. On the one hand endoperoxide formation is report-
ed for pentacene and tetracene and several derivatives when
irradiated in the presence of oxygen.[4] On the other hand di-
[a] Dr. N. Kolmer-Anderl, Prof. Dr. M. Rehahn
Ernst-Berl-Institut für Technische und Makromolekulare Chemie
Technische Universität Darmstadt
Alarich-Weiss-Straße 4, 64287 Darmstadt (Germany)
In this paper, we report the synthesis of naphtho[2,3-g]qui-
noxalines and pyrazino[2,3-b]phenazines and investigate the
influence of the nitrogen content in the tetracene core on the
LUMO energies and the stability towards photooxidation. For
this purpose the degradation process is studied by NMR spec-
troscopy and the chemical shifts of products and intermediates
are calculated to verify the proposed decomposition pathways.
[b] Dr. A. Kolmer, Prof. Dr. C. M. Thiele
Clemens-Schçpf-Institut für Organische Chemie und Biochemie
Technische Universität Darmstadt
Alarich-Weiss-Straße 4, 64287 Darmstadt (Germany)
Supporting information for this article is available on the WWW under
Chem. Eur. J. 2016, 22, 5277 – 5287
5277
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