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Fig. 13 Structures of phenylene-containing graphitic materials: (a) biphenylene sheet, (b)
biphenylene nanoribbons with armchair and zigzag edge morphology, (c) biphenylene nanotubes,
and (d) biphenylene dimer. Reproduced with permission from [
79
]. Copyright 2010 American
Chemical Society
and C
60
, involves a sequence of electrocyclic reactions and the eventual extrusion
of 2 equiv. nitrogen gas (Scheme
16
).
Other graphitic materials containing phenylenes, such as nanoribbons, nanosheets,
and nanotubes, can also be imagined and have been subjects of theoretical interest
(Fig.
13
)[
79
]. The biphenylene nanoribbons (BNR, Fig.
13b
) were dubbed either
armchair or zigzag depending on the edge morphology, analogous to graphene
nanoribbons (GNR). For the armchair BNR, calculations predict the band gap will
decrease monotonically with the width of the ribbon, whereas zigzag BNR are
predicted to have metallic character for all but the narrowest ribbon. Spin-polarized
calculations predict both edge configurations to be diamagnetic, unlike GNR for
which the zigzag morphology is predicted to have interesting magnetic properties
[
80
]. Both armchair and zigzag biphenylene nanotubes are predicted to possess
metallic character.
3 Diaryl[a,e]pentalenes
Another commonly studied formally antiaromatic moiety is the 8
-electron bicy-
clic pentalene molecule (Fig.
14a
)[
81
]. Pentalene is locked in a planar geometry,
unlike cyclooctatetraene, an 8
p
-electron system that adopts a boat geometry to
reduce antiaromaticity. Unsubstituted pentalene is highly reactive and dimerizes at
temperatures above
p
100
C[
82
-
84
]. The 10
-electron dianion of pentalene,
however, is a stable and well-characterized species [
85
-
87
].
p
