Chemistry Reference
In-Depth Information
py
N
NN
N
O
O
py
+
16
17
18
O
O
O
1. OsO
4
,
NMO
2. PhI(OAc)
2
1. NaBH
4
2. POCl
3
19
20
Fig. 6 Synthesis of monoketone precursor 20 of hexacene [
32
]
Very recently, the synthesis of hexacene from a bridged monoketone precursor
20 was reported by Watanabe et al. [
32
]. Using Diels-Alder chemistry, the
monoketone 20 was constructed from the isonaphthofulvene precursor 16 and
anthracene-1,4-dione 17 (Fig.
6
). After reduction and aromatization of the quinone
18 to give 19, the double bond is oxidatively cleaved to yield 20.
The photochemical decomposition (
30 nm) of 20 in THF solution
produced hexacene (Fig.
7
) as identified by the typical vibrational progression of
the
p
band. However, before quantitative conversion of the precursor was achieved,
hexacene decomposed under the irradiation conditions. This observation is similar
to that reported by Mondal et al. [
29
] using their
λ ¼
365
-diketone 15.
Thermal decomposition of the bridged monoketone precursor 20 in the solid
state, on the other hand, produced green-blue hexacene in a rather clean reaction, as
indicated by cross-polarization magic angle spinning
13
C NMR as well as by IR
spectroscopy (Fig.
7
)[
32
]. Watanabe et al. [
32
] succeeded in growing crystals of
hexacene by vapor deposition. These were suitable for single-crystal X-ray crystal-
lography. Hexacene crystallizes in the triclinic space group P-1, and the molecules
pack in a herringbone motif similar to pentacene (Fig.
8
)[
32
].
The authors investigated the chemical stability of solid hexacene in some detail
[
32
]. They reported that hexacene is stable towards air in the dark both in the solid
state and as a thin film. Irradiation of the thin film in air resulted in partial
photodecomposition, as evidenced by a new UV/vis band at 430 nm and by
detection of an endoperoxide by MALDI-TOF mass spectrometry [
32
]. However,
even after 300 min of irradiation, substantial amounts of hexacene (about 70% of
the original UV/vis intensity) remained. Watanabe et al. [
32
] explained the obser-
vation of a significantly increased stability of hexacene in the solid state compared
to hexacene isolated in a PMMA matrix by the accessibility of the hexacene
molecules. In the solid state predominantly the surface of the sample is oxidized,
while in the PMMA matrix the single hexacene molecules can react with diffusing
α
