Chemistry Reference
In-Depth Information
Fig. 1 Representative structures of PAHs with zigzag edges only (a), with armchair edges only
(b), and with both zigzag and armchair edges (c)
delocalized
-electron systems and charge fluctuation, as well as the possibility of
using them as molecule-based functional materials [
1
].
Rectangular-shaped nanographenes are featured by coexistence of zigzag and
armchair edges with a typical Kekul
´
structure. Their structures can be formulated
by cycles in a monocyclic system symbolizing the six
p
p
-electrons in benzene rings
(benzenoid aromatic sextets) according to Clar's aromatic sextet rule [
2
]. Therefore
the determination of stability can be simplified by counting the number of sextet
rings for a given system, that is, the more sextet rings the more stable the system.
Interestingly, recent theoretical and experimental work indicates that for rectangu-
lar PAHs with more than two zigzag edges, such as anthenes and periacenes, a
remarkably open-shell biradical character will emerge when the conjugation is
extended to some point (
n
2 for periacenes) which
originates from a narrowed bandgap and stabilization contribution of forming
more Clar's sextet rings in the biradical form (Fig.
1
)[
3
].
Nanographenes with only armchair edges can also be termed “all-benzenoid
PAHs” as their structures can be represented as “full” aromatic sextet rings without
additional double bonds. It is commonly accepted that the bandgap and stability of
PAHs are dependent not only on molecular size but also on the edge structures.
Although this class of PAHs possesses large size and extended conjugation, they
show high stability and relatively large bandgap due to the existence of the large
number of aromatic sextet rings.
Different functionalities and applications are expected for all three types of
nanographenes according to their respective intrinsic properties. For example,
0 for anthenes and
m
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