Chemistry Reference
In-Depth Information
53
54
Fig. 26 Aryl substituted heptacenes 53 and 54 [
58
]
As expected, the 7,16-di(
p
-
tert
-butylphenyl)heptacene 53 decomposed rapidly
in solution in an ambient atmosphere in the presence of light, while the four
additional
o,o
-dimethylphenyl substituents in 54 enhance the stability somewhat
[
58
]. The half-life in the presence of light was not given, but appears to lie in the
region of 10 min for this derivative from the displayed time dependence of acene
absorption. The solids have “
significant lifetimes
”[
58
] and could be studied by
LDI-TOF-MS. In addition to the molecular ion M
+
, the less substituted derivative
53 also showed the presence of the oxygen adduct (M
+
+ 32) and its fragment
(M
+
+ 16) [
58
].
3.2.3 Thioaryl Substituted Heptacenes
An approach to acene stabilization different to silylethynyl substituents was dis-
covered by the group of Miller. In an investigation of the stability of pentacenes,
Kaur et al. [
59
] identified thioalkyl and thioaryl groups as being superior to
silylethynyl groups in stabilizing the aromatic core with respect to endoperoxide
formation. The Miller group subsequently investigated the influence of thioaryl
groups on the stability of the heptacene core [
58
]. The synthesis (Fig.
27
) of two
thioaryl substituted heptacenes was achieved by double Diels-Alder reactions of
naphthoquinone and transient
o
-quinodimethane generated from 55. The resulting
heptacenetetraone 56 is either treated with 2,6-dimethylphenyllithium or reduced
with NaBH
4
. The resulting OH groups are removed in the usual way using SnCl
2
.
In agreement with the earlier observations at the pentacene core, the two (
p
-
tert
-
butylphenyl)thiophenyl substituents in 57 increase the stability of the heptacene
towards oxidation [
58
]. An even larger increase in stability is observed with the
presence of four
o,o
-dimethylphenyl groups as in 58 [
58
]. This heptacene derivative
is stable for weeks in the solid state, for 1-2 days in solution in the dark, and for
several hours under ambient conditions in the presence of laboratory light. The
heptacene 58 could be fully characterized by NMR, UV/vis-NIR, fluorescence, and
LDI-TOF-MS [
58
]. By the latter technique the absence of oxygenated derivatives
