Chemistry Reference
In-Depth Information
with threefold symmetry, were reported in 2004 [
120
]. Due to the existence of five-
membered rings, 161 was nonplanar and therefore offered the possibility of subse-
quently making bowl-shaped molecules by flash vacuum pyrolysis. Both
compounds were characterized by MALDI-TOF spectroscopy, solid-state UV-vis
absorption, and Raman spectroscopy. The rectangular PAH 162 was synthesized
and for this
D
2h
symmetric compound, long flexible phytyl chains were required to
guarantee sufficient solubility in normal organic solvents for various character-
izations. Giant molecule 163 with 222 carbon atoms is by far the largest PAH
structure characterized to date, and the UV/vis spectrum of insoluble molecule 163
recorded as thin film shows absorption covering the complete visible range of the
electronic spectrum, with the absorption maximum at 765 nm. The broad and
unstructured pattern of the band can be explained by the large number of electronic
transitions taking place in this giant PAH 163 and strong aggregation in solids, thus
leading to the continuum-like appearance in the visible range [
121
].
One-dimensional graphitic ribbons 168 were synthesized under oxidative
cyclodehydrogenation conditions from the soluble branched polyphenylene precur-
sor 167 as shown in Scheme
19
[
122
]. Although the insolubility of 168 prevented
standard spectroscopic structure elucidation, the electronic properties were
investigated by solid-state UV-vis, Raman, and infrared spectroscopy. A wide
and featureless absorption band covering the visible range of the spectrum with
an absorption maximum of approximately 800 nm was observed, indicating the
highly extended conjugated framework. 168 represented the first example of a two-
dimensional highly conjugated all-benzenoid graphene ribbon.
As mentioned above, HBC can be viewed as “superbenzene” as it possesses the
same shape and symmetry as benzene but enlarged conjugation. In this regard, a series
of “superbenzene” analogues, including “superphenylene”, “supernaphthalene”,
“superphenalene”, and “supertriphenylene”, were prepared (Fig.
32
). A series of
oligomers up to the trimer of HBC 169 were prepared by either Yamamoto coupling
or oxidative cyclodehydrogenation from corresponding branched oligophenylenes
[
123
]. The UV-vis and fluorescence spectra of dimer and trimer are very similar to
individual HBC units, which is understandable considering the large torsion angle
between HBC units and small atomic orbital coefficients of the bridgehead carbons.
The synthesis of “supernaphthalene” 170a was firstly reported in 1998 utilizing the
AlCl
3
/CuCl
2
system as cyclodehydrogenation condition [
124
]. However, partially
fused species and chlorinated adducts arising from this method inevitably led to poor
reproducibility. Later, a derivative 170b carrying two
tert
-butyl groups was prepared in
a novel approach, in which the
tert
-butyl groups induced a significant deviation from
planarity of the aromatic backbone [
125
]. Together with the solubilizing effects of alkyl
chains, 170b exhibited high solubility and effective suppression of aggregation which
allowed the first recording of structure-rich UV-vis and a resolved
1
HNMRspectrum
for aromatic systems twice as large as HBC. Upon removal of
tert
-butyl groups, the
solubility decreased dramatically, accompanied with a loss of well-resolved band
structure and a bathochromic shift of the absorption spectrum due to the recovery of
planarity. PAHs with threefold symmetry, such as “superphenalene” 171 and “supertri-
phenylene” 172, were also achieved [
126
,
127
].
