Chemistry Reference
In-Depth Information
C22C
C21C
C20C
C19C
Si3C
Si2C
Si1C
C23C
C24C
Si4C
C18C
C17C
C16C
C26C
C25C
C2C
C1C
C3C
C4C
C5C
C6C
C15C
C7C
C13C
C14C
C8C
C11C
C12C
C9C
C10C
Fig. 20 X-Ray crystal structure of heptacene 40c. Reprinted with permission from [
37
]. Copyright
2005 American Chemical Society
A logical way of extending the lifetime of a heptacene is to increase the number
of stabilizing groups. This approach was followed by Wudl's group [
56
] and by Qu
and Chi [
57
]. The former group introduced four additional phenyl groups and
reduced the size of the silyl groups by using TIPS. The synthesis started from
2,6-dibromo-anthraquinone 41 by introduction of the two TIPS-ethynyl groups
(Fig.
21
).
The double aryne generated from 42 by LiTMP induced HBr elimination was
used in Diels-Alder reactions with diphenylisobenzofurane [
56
]. Reductive deox-
ygenation of the product 43 was achieved with Zn dust at 100
C. The heptacene 44
crystallized upon cooling of the reaction mixture [
56
]. Single crystal X-ray crystal-
lography revealed that heptacene 44 shows an edge-to-face herringbone packing
motif (Fig.
22
) without
ˀ
-
ˀ
interactions. The molecules have an essentially planar
heptacene backbone. The crystals are remarkably stable as they do not decompose
within 21 days when exposed to laboratory atmosphere and light [
56
].
The acene could still be detected in solutions that were exposed to air for 41 h
[
56
]. The oxidation of the heptacene results in a change of color of the solution from
brown to orange. Based on UV/vis,
1
H-NMR, and FAB-MS data, the authors
suggested that the products of oxidation are endoperoxides, resulting from addition
to the 6,17 or 8,15 positions (Fig.
23
). These are the most reactive sites next to the
7,16 positions that are protected by the silylethynyl groups. Remarkably, dimeriza-
tion of heptacene 44 was not observed [
56
]. This indicates that the four additional
phenyl rings are sufficiently bulky to prevent dimerization. Thus, a combination of
