Synthesis and optical properties of diaza- and tetraazatetracenes

Article abstract of DOI:10.1002/chem.201103227

A series of functionalized diaza- and tetraazatetracenes was synthesized, either by condensation of an aromatic diamine with an ortho-quinone/ diethyloxalate followed by chlorination with POCl₃ to give diazatetracenes or by palladium-catalyzed

Full text of DOI:10.1002/chem.201103227

FULL PAPER
DOI: 10.1002/chem.201103227
Synthesis and Optical Properties of Diaza- and Tetraazatetracenes
Benjamin D. Lindner,[a] Jens U. Engelhart,[a] Michaela Mꢀrken,[a] Olena Tverskoy,[a]
Anthony L. Appleton,[b] Frank Rominger,[a] Kenneth I. Hardcastle,[c]
Markus Enders,[d] and Uwe H. F. Bunz*[a]
Abstract: A series of functionalized
diaza- and tetraazatetracenes was syn-
thesized, either by condensation of an
aromatic diamine with an ortho-qui-
none/diethyloxalate followed by chlori-
nation with POCl3 to give diazatetra-
cenes or by palladium-catalyzed cou-
pling of a phenylenediamine with vari-
ous 2,3-dichloroquinoxalines to give
tetraazatetracenes (after oxidation with
MnO2). Representative examples in-
cluded halogenated and nitrated deri-
AHCTUNGTRENNvGUN atives. The optical properties of these
azatetracenes were discussed with re-
spect to their molecular structures and
substitution patterns. The diazatetra-
cenes and tetraazatetracenes formed
two different groups that had signifi-
cantly different electronic structures
and properties. Furthermore, 1,2,3,4-
tetrafluoro-6,11-bis((triisopropylsilyl)-
AHCTUNGTRENNeGUN thynyl)benzo[b]phenazine was synthe-
sized, which is the first reported fluori-
nated diazatetracene. Single-crystal X-
ray analysis of this compound is report-
ed.
Keywords: alkynes · diamines · het-
erocycles · palladium · tetracenes
Introduction
successfully utilized as the semiconducting layers in thin-
film transistors, as demonstrated by Miao et al. and
others.[8–11]
Herein, we report different synthetic routes to alkynylated
“Anthony-like” diaza- and tetraazaacenes and investigate
the influence of their specific molecular structures on their
optical properties.[1] Larger azaacenes and N,N-dihydrooli-
goazaacenes have been known since the end of the 19th cen-
tury.[2] However, there are only a few reports of their syn-
thesis and practically no procedures that allow the systemat-
ic access to a range of their functionalized derivatives. With
the advent and promise of organic electronics, this class of
compounds has experienced a bit of a “gold rush”, and sev-
eral groups are now researching afresh their synthesis.[3–11]
Azaacenes promise to be electron-transporting complements
of acenes and therefore are of potential importance in or-
ganic-electronic applications. A few azaacenes have been
Nevertheless, when compared to other classes of substan-
ces, there is a dearth of substituted azaacenes and only few
methods for their synthesis. In most cases, the azaacene core
was constructed by using the classic condensation of an
ortho-quinone (or an aromatic ortho-dihydroxy compound)
with an aromatic ortho-diamine. Whilst exploiting this ap-
proach, we have developed a palladium-catalyzed Buch-
wald–Hartwig-type coupling reaction for the synthesis of
larger heteroacenes.[12] Herein, we report our synthetic ef-
forts towards preparing substituted azatetracenes and the in-
fluence of molecular structure on their optical/electronic
properties.
Results and Discussion
[a] Dipl. Chem. B. D. Lindner, Dipl. Chem. J. U. Engelhart, M. Mꢀrken,
O. Tverskoy, Dr. F. Rominger, Prof. U. H. F. Bunz
Organisch-Chemisches Institut
Synthesis: We performed the condensation of anthracene di-
amine 1 with diethyloxalate to give the intermediate naph-
thoquinoxalinedione, which, upon reaction with POCl3, fur-
nished the target compound (2) in 26% overall yield
(Scheme 1).[3d] Diaminonaphthalene 4 reacted with ortho-
quinone and ortho-chloranil to afford the corresponding di-
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: uwe.bunz@oci.uni-heidelberg.de
[b] Dr. A. L. Appleton
School of Chemistry and Biochemistry
Georgia Institute of Technology
901 Atlantic Drive, Atlanta GA, 30332 (USA)
AHCTUNGTRENNUNG
azatetracenes 3a and 3b, respectively.[3a,d] When compound
4 was treated with ortho-fluoranil, we isolated two com-
pounds following column chromatography on silica gel: the
desired tetrafluorodiazatetracene (3c) but also a second
compound (Scheme 2).[13] When we performed the conden-
sation reaction in EtOH, the electrophilic compound 3c re-
acted with the solvent in an nucleophilic-aromatic-substitu-
tion reaction to give compound 5 in 24% yield. This result
was not too surprising, as highly fluorinated ring compounds
[c] Dr. K. I. Hardcastle
Department of Chemistry, Emory University
1515 Dickey Drive, Atlanta, GA 30322-1003 (USA)
[d] Prof. M. Enders
Anorganisch-Chemisches Institut
Ruprecht-Karls-Universitꢀt Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103227.
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The coupling of a dichloropyrazine with an ortho-diamine
in the presence of pyridine as a base to obtain fluoflavine
derivatives, as reported by Guirado et al., was unsuccessful
in our case.[17] In contrast, our recently developed palladi-
um-catalyzed coupling conditions for heteroacenes gave sat-
isfactory-to-good yields.[3f,h] During the course of this work,
we re-tried the synthesis of compound 6. However, the at-
tempted coupling of compound 4 to 2,3-dichloropyrazine
only resulted in trace amounts of compound 6, which was
oxidized by MnO2, but not into a tetraazatetracene; rather,
it was oxidized into the yellow bis-amide 7 (Scheme 3). Sur-
Scheme 1. Synthesis of diazaacene 2 and a structural comparison with
known diazaacenes 3a and 3b.[3a,d]
Scheme 2. Synthesis of partially fluorinated diazatetracenes 3c and 5.
Scheme 3. Trace synthesis of compounds 6 and 7. dba=dibenzylideneace-
tone.
easily undergo nucleophilic substitution, particularly with al-
cohols.[14] The position of the ethoxy substituent was de-
duced from the doublet splitting of the OCH2 moiety in the
1H NMR spectrum of compound 5, which was due to the
coupling with the single F atom at the ortho position.[15] To
the best of our knowledge, compounds 3c and 5 are among
the first reported fluorinated azaacenes.[11b,16]
prisingly, the Pd-catalyzed coupling approach worked signifi-
cantly better for the synthesis of isomeric tetraazacenes 10
(Scheme 4). This class of compounds has been known since
1968, when Kuhn, Skrabal, and Fischer first synthesized the
unsubstituted tetraazatetracene and discussed its electronic
properties.[18] Starting from known benzothiadiazoles 8a and
Scheme 4. Synthesis of the tetrazaacenes 11.
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Synthesis and Properties of Diaza- and Tetraazatetracenes
FULL PAPER
quinoxalines.[3f–h,12] N,N-Dihydrotetraazatetracenes 10a–10h
were obtained in 35–65% yield; oxidation of these some-
what-air-sensitive intermediates by MnO2 furnished tetraa-
zatetracenes 11a–11h in 43–92% yield as analytically pure
samples after column chromatography on silica gel and crys-
tallization. The coupling yields of TIPS-substituted 11 were
very similar to those of the TES-substituted azaacenes, even
though the oxidation of the TIPS-ethynyl-substituted deriva-
tives of compound 10 was, on average, less efficient. These
improved coupling results, when compared to the synthesis
of 6, were perhaps owing to the enhanced oxidative stability
of the formed N,N-dihydrotetraazacenes (10a–10h), which
apparently did not poison the coupling catalyst in its Pd2+
form as they were probably not able to reduce it into Pd0.[3f]
Optical properties and quantum chemical calculations:
Figure 1 shows a photograph of solutions of the diazaacenes
(top and middle rows) and of the tetraazaacenes (bottom
row). All of the solutions were deeply colored and the dia-
zaacene solutions were also quite fluorescent. Both the ab-
sorption and emission colors were dependent upon the spe-
cific structure of the azatetracene. The tetraazaacenes were
non-fluorescent, but also formed red-to-purple-colored solu-
tions. The absorption and emission spectra of the diazaa-
cenes are shown in Figure 2. As expected, small Stokes
shifts were observed, which were testament to the rigid mo-
lecular frameworks and typical of acene-type structures. We
also observed the vibronic fine-structure of the long-wave-
length a-band.
Figure 1. Photographs of solutions of compounds 2, 3a–3c, and 5 in
CH2Cl2 in daylight (top row) and under a hand-held black light with illu-
mination at 365 nm (middle row); bottom row: photographs of solutions
of compounds 11a, 11d, 11g, and 11j in daylight.
8b, reduction with lithium aluminum hydride afforded com-
pounds 9a and 9b in almost quantitative yield; both were
competent coupling partners when reacted in the presence
The absorption maxima of the diazaacenes were in the
of [PdACHTUNGTRENNUNG(dba)2] and ligand L1 to give substituted 2,3-dichloro-
range 572–620 nm, in accordance with our recent results.[3d]
Figure 2. a) Normalized absorption spectra of compounds 2, 3a–3c, and 5 in n-hexane. b) normalized emission spectra of compounds 2, 3a–3c, and 5 in
n-hexane. 3a: lmax absorption 572 nm (2.17 eV), lmax emission 577 nm; 2: 586 nm (2.12 eV), 590 nm; 5: 588 nm (2.11 eV), 605 nm; 3c: 602 nm (2.06 eV),
616 nm; 3b: 620 nm (2.00 eV), 627 nm.
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U. H. F. Bunz et al.
These compounds (2, 3c, and 5)
bridged the color gap between
compounds 3a and 3b. Quan-
tum-chemical
computational
studies (Figure 3) on model
compounds (primed numbers,
TMS instead of TIPS) corrobo-
rate this trend. Diazaacenes 3c’
and 3b’ had almost identically
lying frontier molecular orbi-
tals. Surprisingly, the net effect
of fluorine substitution in com-
pound 3c’ was quite similar to
that of chlorination in com-
pound 3b’. The combination of
the ÀI and the +M effects of
the fluorine atoms in compound
3c’ was similar to the electronic
effect that chlorine substitution
exerted upon the diazatetra-
cene core in compound 3b’.
Figure 3. Quantum-chemical calculations (B3LYP 6-311+G**, Spartan ’10) for compounds 3a’, 2’, 5’, 3c’, and
3b’. The gaps were overestimated by about 0.1–0.15 eV.
However, the quantum-chem-
ical calculations at this level of
theory (B3LYP 6-311+G**)
did not include configuration
interactions, and therefore over-
estimated the optical gap by ap-
proximately 0.1–0.15 eV, which
was acceptable. Contrary to the
diazaacenes, tetraazaacenes 11
were
all
non-fluorescent.
Figure 4 shows the absorption
spectra of TIPS-substituted tet-
raazacenes 11. Noticeably, the
vibronic fine structures of com-
pounds 11a, 11d, 11g, and 11j
were considerably less distinct
than those observed in the dia-
zatetracenes. In addition, the
middle peak in the UV/Vis
Figure 4. Absorption spectra of compounds 11a, 11g, 11d, and 11j in n-hexane. 11a: 548 nm (2.27 eV); 11g:
spectra of compounds 11 was 574 nm (2.16 eV); 11d: 590 nm (2.10 eV); 11j: 598 nm (2.08 eV).
the most-intense, which was
also contrary to the features ob-
served for the diazatetracenes. When comparing the diaza-
and tetraazatetracenes (see below), the structure of the
frontier molecular orbitals suggested that the latter com-
pounds had more of an internal-charge-transfer (ICT) char-
acter. It was unclear whether this property was due to the
position of the alkyne groups (central versus on the terminal
benzene). The ICT character also led to the ground- and ex-
cited-state structures being somewhat different, and a de-
creased Franck–Condon-overlap of the two states. There-
fore, the broadened absorption bands and the loss of fluo-
rescence were two sides of the same coin. We will test this
hypothesis in the future by making diazaacenes in which the
terminal rings contain alkyne substituents. Quantum-chemi-
cal calculations on the TMS-substituted model systems (11c,
11 f, 11i, 11l) showed that the LUMOs of compound 11
were typically lower lying than those of the diazaacenes.
The introduction of two additional nitrogen atoms had an
effect similar to that of the tetrafluorination or the tetra-
chlorination of one of the outer rings in terms of the elec-
tronic effect on the position of the LUMOs in azaacenes. In-
terestingly, the quantum-chemical calculations on com-
pounds 11 reproduced the optical gaps better than those
performed on the diazaacenes. The reason for this result is
unclear.
The diazatetracenes and the tetraazatetracenes were
structurally most similar; however, their optical properties
were quite different. In particular, the lack of fluorescence
in compounds 11 was surprising. The reason for this behav-
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Chem. Eur. J. 2012, 18, 4627 – 4633

Synthesis and Properties of Diaza- and Tetraazatetracenes
FULL PAPER
ior could be due to the orbital
structure of the diazaacenes
versus that of the tetraazaa-
cenes. In the latter case, that is,
in compounds 11, the HOMOs
and the LUMOs were spatially
quite disjointed for all of the in-
vestigated derivatives.[19] There-
fore, the frontier molecular or-
bitals occupied different parts
of the molecule (Figure 3,
Figure 5). In the excited states
of tetraazaacenes 11, there was
presumably some ICT, so the
initially formed excited states
relaxed and changed their ge-
ometry. Apparently, this relaxa-
tion happened to such an
extent that nonradiative decay
was effective in all of the deriv-
atives of compound 11. The un-
usually poorly resolved optical
bands underscored this point.
Figure 5. Quantum-chemical calculations (B3LYP 6-311+G**, Spartan ’10) for compounds 11c, 11 f, 11i, and
11l.
Single crystal analysis: Unfortu-
nately, we were not able to
obtain single crystals of com-
pounds 11, but we were able to
obtain suitable crystals of com-
pounds 3b and 3c, and there-
fore we were able to investigate
the difference in the packing of
these consanguine structures.
For a comparison of their crys-
tal lattices and cell parameters
with that of unsubstituted com-
pound 3a, see Table 1.[3a]
Both 3b and 3c formed
brick-wall-type packing motifs
that appeared to be superficial-
ly similar. However, compound
Figure 6. a–c) Packing of compound 3c and d–f) of compound 3b.
3b crystallized in a monoclinic cell, whilst compound 3c was
triclinic, with different axis lengths and angles. Also, their
molecular packing motifs differed. In compound 3c
(Figure 6), the molecules formed 1D, offset stacks, in which
the fluorinated parts of two successive molecules were
stacked on top of the pyrazine unit of the other molecule,
with a small plane-to-plane-distance of 3.25 ꢂ. This struc-
ture was surprising, because when benzene and hexafluoro-
benzene were co-crystallized, there was a strong interaction
between the perfluorinated and benzene rings. This expect-
ed packing of the fluorinated above the benzene-like ring of
an adjacent molecule did not happen for compound 3c. In-
stead, centrosymmetric sandwiches formed and packed into
the stacks described above. These stacks were partially inter-
digitated through the terminal benzene rings with the neigh-
boring ones (nonbonding distance: 3.37 ꢂ), so that an inter-
Table 1. Crystal-structure data of compounds 3a–3c.
Compound
Space group
a [ꢂ]
b [ꢂ]
c [ꢂ]
a [8]
b [8]
g [8]
V [ꢂ]
Z
¯
3a
3b
3c
P1
7.3968(7)
34.94(1)
7.6248(4)
13.893(2)
15.039(6)
14.128(2)
18.428(2)
14.898(6)
17.729(1)
112.065(7)
90.00
70.291(3)
95.624(7)
104.591(7)
81.942(1)
90.530(8)
90.00
82.755(2)
1744.31
7576.52
1773.77
2[a]
8
2
C2/c
¯
P1
[a] See reference [3a].
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U. H. F. Bunz et al.
Figure 7. Close contacts in the packing of compound 3b. The ABCD layer structure is shown in two views.
esting tightly packed 2D brick wall resulted for compound
3c. In the single-crystal structure of compound 3b, the for-
mation of centrosymmetric dimers was also observed, in
which the chlorinated rings stacked on top of the pyrazine
rings of the successive molecules. Contrary to fluorinated
compound 3c, this motif did not repeat itself in the forma-
tion of ABABAB stacks, but rather an additional double
layer (C,D) of centrosymmetric dimers was observed, so
that a repeating ABCDABCD layer sequence resulted
(Figure 7). The average overall perpendicular distance be-
tween the layers of acene 3b was 3.50 ꢂ. In the related com-
pounds (3a and 3c), the analogous layer-to-layer distances
were 3.36 and 3.27 ꢂ respectively.
HOMOs of the azaacenes profited less from the electroneg-
ative stabilization, primarily because the orbital coefficients
of the HOMOs were small or disappeared at the locus of
the substituent. Overall, our approach has fundamentally
broadened the number and scope of substituted azatetra-
cenes available. Powerful methods beyond the classic con-
densation procedures are now available to quickly and effi-
ciently grow this class of compounds.[3f,h]
Acknowledgements
U.B., J.E., and B.L. thank the Deutsche Forschungsgemeinschaft for their
generous support (DFGBu-771/7-1).
Overall, the structures of compounds 3a (H) and 3c (F)
were almost identical, whilst the packing of compound 3b
(Cl) was different from both of them. Therefore, we note
that halogenated quinoxaline subunits exerted a command-
ing influence on the overall packing motifs of the azaacenes.
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Received: October 12, 2011
Published online: February 16, 2012
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