Chemistry Reference
In-Depth Information
Fig. 3 X-Ray crystal structures: (a) Front and (d) side views of 12,(b) front and (e) side views of
13, and (c) front and (f) side views of 14. Crystal structure data from [
30
,
31
] was gathered from
the Cambridge Structural Database (
CSD
)
The
1
H NMR spectra of these compounds are consistent with the pattern seen in
lower angular phenylenes [
19
,
32
]. The terminal benzenoid rings are the most
aromatic, having the most deshielded protons, whereas the penultimate ring has the
most shielded protons and therefore the most cyclohexatrienic character. In the case of
heliphenes 13-15, where the termini begin to overlap, only those protons lying over
the terminal (aromatic) ring are shielded. The protons that reside over the penultimate
ring are actually slightly deshielded. Crystal structures demonstrating the helical turn
and overlapping termini of heliphenes 12 through 14 are shown in Fig.
3
.
Fewer reports have focused on the development of synthetic methods to access
materials containing aromatic segments larger than one benzene ring. Taking
advantage of the ability of palladium to catalyze the [2
2] cycloaddition
of arynes, the synthesis of tris(benzocyclobutadieno)triphenylene 17 and related
phenylene-containing PAHs, 18 and 19, was reported [
33
]. Key to this strategy was
the synthesis of a 2,3-dehydrobiphenylene precursor 16 from the appropriate 2,3-
bis-(trimethylsilyl)biphenylene as demonstrated in Scheme
5
. The aryne was then
generated by addition of the fluoride ion and followed by either trimerization to
give 17 or cyclization with dimethylacetylene dicarboxylate (DMAD) and the
appropriate Pd(0) source to give 18 or 19.
þ
2
þ
2.2 Photophysical Properties of [N]Phenylenes
Examination of photophysical properties of [N]phenylenes of a variety of
topologies can also be useful in probing relative levels of conjugation within each
chromophore. Consistent with the earlier discussion of relative resonance energies,
calculations predict larger HOMO-LUMO gaps for zigzag [N]phenylenes than for
the linear isomer due to increased bond alternation [
34
].
The photophysical behavior of biphenylene (20) and both the linear (21)
and angular
(22)
isomers of
[3]phenylene were well-studied prior
to the
