Compounds 1aꢀc have lower LUMO energy levels
than the reported silylethynylated monoazapentacene,
diazapentacenes12 and tetraazapentacenes3,8 that do not
have extra substituents. In particular, 1b has the lowest
LUMO energy level among all the known N-heteroacenes
including those that have extra electron-withdrawing sub-
stituents, such as fluorine and nitro groups.13 The record
low LUMO energy levels of 1aꢀc are in agreement with
the earlier conclusion that the LUMO energy level of
N-heteropentacene goes down with the increasing number
of unsaturated N atoms,2 particularly in the internal rings
of pentacene.3 From the optical gap and LUMO energy
level, the energy level of highest occupied molecular orbital
(HOMO) was estimated as ꢀ6.14 eV for 1a, ꢀ6.33 eV for
1b and ꢀ6.28 eV for 1c. The very close LUMO energy
levels and slightly different HOMO energy levels of 1aꢀc
suggest that CꢀC triple bonds have different contributions
to the HOMO and LUMO. This is in agreement with the
calculated HOMOs, which are delocalized to CꢀC triple
bonds, and the calculated LUMOs, which are almost
localized on the hexaazapentacene backbone.14 On the other
hand, the cyclic voltammograms of 2aꢀc (shown in the
Supporting Information) are very similar, exhibiting
one quasi-reversible oxidation wave and one irreversible
reduction wave at ca. 0.8 and ꢀ1.5 V vs ferrocenium/ferrocene,
respectively. From these potentials, the HOMO and LUMO
energy levels of 2aꢀc were estimated as ca. ꢀ5.6 and ꢀ3.3 eV,
respectively.11
The low LUMO energy levels of 1aꢀc suggest that they
may function as n-type semiconductors although the in-
stability in the solid state limits their potential applica-
tions.15 However, our attempts of fabricating thin film
transistors of 1aꢀc appeared unsuccessful because these
compounds decomposed during thermal evaporation and
formed disordered films with poor morphology during
solution-based process. On the other hand, the relatively
high LUMO energy levels and relatively low HOMO energy
levels of 2aꢀc suggest that these compounds are not suitable
candidates for either n-type or p-type semiconductors.
The very low LUMO energy levels suggest that1aꢀc can
function as oxidants in organic reactions. To test the
oxidizing ability of these hexaazapentacenes, 1a was
selected as a representative to oxidize dihydroanthracene
in both stoichiometric and catalytic reactions. As shown
in Table 1 (entry 2), dihydroanthracene was completely
oxidized by 1 equiv of 1a to anthracene after 16 h. Because
the reduction product of 1a in this reaction is 2a, which can
Table 1. Oxidation of 9,10-Dihydroanthracene by 1a and PbO2
productsb
entry
conditionsa
1a (1 equiv), 2 h
A (%)
AQ (%)
1
2
3
4
5
6
55
100
22
89
6
0
0
1a (1 equiv), 16 h
1a (0.3 equiv), PbO2 (30 equiv), 2 h
1a (0.3 equiv), PbO2 (30 equiv), 29 h
PbO2 (30 equiv), 16 h
0
0
3
PbO2 (30 equiv), 29 h
14
13
a The reactions were carried out in CDCl3 at roomtemperature. b The
yield was determined by 1H NMR.
be oxidized back to 1a by PbO2, it is possible to oxidize
dihydroanthracene with 1a in a catalytic manner with
PbO2 as the second oxidant. When 0.3 equiv of 1a and
30 equiv of PbO2 were used as the oxidant, dihydroan-
thracene was oxidized to anthracene in a yield of 89% as
shown in Table 2 (entry 4). In control experiments, PbO2 itself
oxidized dihydroanthracene slowly to not only anthracene
but also anthraquinone, which was, however, not observed
from the crude products of the reactions using 1a as the
oxidant. The mechanism for this dehydrogenation reaction
by 1a presumably involves a transfer of hydride or a hydrogen
atom from the benzyl group to the pyrazine nitrogen as
suggested by its two reversible one-electron reductions.
With adjacent pyrazine and dihydropyrazine rings that
are not shielded by the bulky triisopropylsilyl groups,
molecules of 2aꢀc can in principle form intermolecular
hydrogen bonds with each other. To study such hydrogen
bonds, single crystals of 2a and 2b were grown from
solutions in ethyl acetate and subjected to X-ray crystal-
lographic analysis. Shown in Figure 3a is the crystal
structure of 2a, which exhibits a one-dimensional face-to-
face π-stacking with head-to-tail arrangement. It is found
that neighboring molecules of 2a stack with each other
with two slightly different arrangements alternating in one
stack. One arrangement has the two π-faces separated by
˚
3.35 A and the two silicon atoms of neighboring molecules
˚
separatedby 7.42A. Incomparison, the otherarrangement
˚
(12) Liu, Y.-Y.; Song, C.-L.; Zeng, W.-J.; Zhou, K.-G.; Shi, Z.-F.;
Ma, C.-B.; Yang, F.; Zhang, H.-L.; Gong, X. J. Am. Chem. Soc. 2010,
132, 16349–16351.
has two π-faces separated by 3.26 A and the two silicon
˚
atoms of neighboring molecules separated by 13.75 A,
which is accompanied with a shift along the long axis of
pentacene backbone. Intermolecular hydrogen bonds are
not observed between the π-stacks of 2a since the NꢀH
moiety of each molecule of 2a is somehow blocked by the
triisopropyl groups of neighboring molecule in the same
stack. Unlike its isomer 2a, 2b exhibits intermolecular hydro-
gen bonds but lacks πꢀπ interactions in its crystal structure.
As shown in Figure 3b, molecules of 2b form a hydrogen-
bonded ribbon, which has a shape of “X” as viewed along the
(13) For the structures and energy levels of these silylethynylated
N-heteropentacenes as well as the references, see the Supporting
Information.
(14) The frontier molecular orbitals of 1aꢀc and 2aꢀc were calcu-
lated with the Gaussian 09 software package using simplified model
molecules, which have smaller trimethylsilyl groups replacing the tri-
isopropylsilyl groups. The geometries of these model molecules were
first optimized at the B3LYP level of density functional theory (DFT)
with the 6-31G(d, p) basis set, and the HOMO and LUMO were then
calculated with the 6-311þþG(d, p) basis set.
ꢀ
(15) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Bredas,
J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436–4451.
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Org. Lett., Vol. 14, No. 16, 2012