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among them only the endo–anti–endo and endo–syn–endo isomers
were observed as a result of the well-known endo-effect. The
molar ratio of syn and anti isomers can be estimated by the corre-
sponding aromatic 1H NMR signals in the mixture, that is, singlet at
d 8.59 and 8.54, respectively, and was found to be about 3/1. Aro-
matization of the central anthracene moiety was accomplished by
a reduction/dehydration sequence, that is, by reducing the car-
bonyl groups to hydroxyls in 7 with sodium borohydride, followed
by dehydration with phosphoryl chloride in the presence of pyri-
dine to yield 8. The syn and anti isomers of 8 can be separated by
silica gel chromatography.10 The three aromatic 1H NMR signals
of anti-8 appeared at d 7.91 and 8.40, which are very close to those
of syn-8 at d 7.84 and 8.19. The overall yield of 8 (two isomers)
from 4 was about 65%. Compound 8 can be used for the prepara-
tion of 1,2,3,4,8,9,10,11-octachloropentacene (10), which has been
predicted to be a potential n-type FET material.11 The chlorine
atoms were then stripped off by a reduction of sodium in the pres-
ence of t-butanol. However, under such a condition the central aro-
matic ring was also reduced to yield compound 9. The aromaticity
can be regenerated by the oxidation with dichlorodicyanoquinone
(DDQ). The 1H NMR spectrum of 3 is similar to that of 8, whereas
the hydrogen atoms on the double bonds appear at d 6.75 for both
anti and syn isomers.12 The yield of conversion from 8 to 3 was
about 15%.
aromatic chromophore of each pentacene derivative. The absorp-
tion maximum of compound 3 at 276 nm is significantly red-
shifted with respect to that of 1 at 244 nm. This is because the
main chromophore in 3 is anthracene, which possesses a narrower
band gap than the naphthalene moiety in 1. The absorption maxi-
mum of 8 is close to that of 3, indicating a rather mild influence
induced by the presence of chlorine atoms on the non-aromatic
moieties. For both 3 and 8, the weak 1A?1La bands at 300–
380 nm with vibronic progressions nearly overlap with each other.
After acidic hydrolysis, the standard pentacene absorption pattern
can be clearly identified by both the strong 1B band and a very
characteristic 1La band. The 1B band of pentacene appears at
300 nm, and that of 10 at 318 nm. The long wavelength 1La band
of pentacene is located at 496–576 nm with characteristic vibronic
sub-levels. The corresponding band of octachloropentacene is at
477–595 nm, which is slightly red shifted with respect to that of
pentacene (inset in Fig. 2).
The low thermal stability of 2 rendered it unsuitable for pro-
cessing at ambient temperature. In an earlier report, Mackenzie
has reported the production of benzene derivatives through a ther-
mal dissociation of the corresponding norbornadienone ketals.14 In
these reactions, an alkene and a carbon dioxide were extruded via
a cycloreversion process. By directly heating 3, pentacene was
produced with a nearly quantitative yield. A TGA curve (Fig. 3)
indicated that the dissociation started at 215 °C, while the
percentage of weight loss corresponded correctly to the sum of
ethylene and carbon dioxide (Scheme 2). The formation of penta-
cene was confirmed also by its absorption spectrum, which was
identical to the authentic sample. An analogous fragmentation
was also found for the octachloride derivative 8, from which octa-
chloropentacene 10 was obtained by heating at 285 °C. Both penta-
cene and octachloropentacene were obtained in high purity as
indicated by their TGA profiles.
The photolysis of 3 by 366 nm excitation in the degassed THF
was also investigated. At the early irradiation period, compound
3 seemed to expel one site of fragment, forming a tetracene deriv-
ative, as supported by the observation of the tetracene relevant
vibronic progressive peaks at 420–485 nm. However, upon extend-
ing the photolysis time, the tetracene-like absorption bands grad-
ually decreased, while no absorption spectra ascribed to
pentacene was observed. The associated photoreaction seems to
be complicated, and thus is not further pursued.
The solubility of 3 in most organic solvents is considerably high-
er than that of 1. For example, in dichloromethane, the solubility
for syn and anti forms of 3 were estimated to be 77 and 10 mg/
mL, respectively, compared to 0.7 mg/mL of 1. An OTFT device
was made by spin-coating a toluene solution of 3 on the surface
The stereochemistry of the anti-isomer of 3 was confirmed by
X-ray crystal diffraction analysis.13 The crystal was found to be tri-
ꢀ
clinic in the space group Pı with a = 5.9680(6) Å, b = 8.6125(9) Å,
c = 13.6272(14) Å,
Solvent molecules (dichloromethane) are incorporated in the crys-
tal packing, and therefore hinder the intermolecular stacking.
a = 76.360(2)°, b = 84.030(2)°, c = 82.906(2)°.
p–p
A drawing of the crystal structure is shown in Figure 1, whereas an
anti arrangement of the two ketal groups can be clearly justified.
The central aromatic region is flat, and exhibits typical absorption
characteristics of the anthracene moiety.
Hydrolysis of the ketal groups of 3 in an attempt to collect dike-
tone 2 by the standard protocol under acidic conditions was not
successful due to the unstable nature of 2. A spontaneous de-car-
bonylation of 2 happened at ambient condition and pentacene
was obtained directly from 3. For example, when ketal 3 was trea-
ted with a heterogeneous mixture of ferric trichloride and silica gel
in dichloromethane, pure pentacene was obtained after filtering off
the solids and drying. A similar hydrolysis could also be proceeded
onto octachloride 8, yielding octachloropentacene 10 as dark pur-
ple solids.
The transformation can be well elaborated via the absorption
spectra depicted in Figure 2. A distinctive strong absorption that
appeared at 240–320 nm is attributed to the 1A?1B transition of
Figure 1. Crystal structure of the anti-isomer of compound 3. Numbering scheme was arbitrarily chosen.