Closed-Shell and Open-Shell 2D Nanographenes - Polyarenes - page 215

Fig. 20 Structures and energy diagrams for monoradicals 33a, 68, and 69 [ 65 ]

3 2D Nanographenes with Both Zigzag Edges

and Armchair Edges

The rectangular fragments of graphene possess the structural character of coexis-
tence of both zigzag and armchair edges (Fig. 1 ). In contrast to the above-
mentioned all zigzag edged open-shell PAHs, 2D aromatic systems falling into
this category can all be depicted as closed-shell Kekul´ structures, and the presence
of armchair edges provides enhanced stability. Meanwhile, the existence of zigzag
edges leads to a narrowed bandgap, which endows them with attracting properties
such as charge transporting capability and long wavelength absorption and emis-
sion. For systems with more than two zigzag edges, a crossover from closed-shell
all-Kekul ´ form to open-shell biradical form in the ground state will take place with
the extension in conjugation, due to the decrease in bandgap which facilitates the
double excitation and the aromatic stabilization of the open-shell structure (forma-
tion of more sextet benzenoid rings for biradical form). There are several basic units
of this class of finite nanographenes. Rylenes are those with two zigzag edges and
extended conjugation along the armchair edges, or peri -fused naphthalenes.
Anthenes are those with three zigzag edges and extended armchair edges, or peri -
fused anthracenes. Periacenes are those with two armchair edges and extended
zigzag edges. More important, the coexistence of both zigzag and armchair edges
results in a combination of moderate stability and intriguing electronic properties,
making these PAHs highly favorable for applications in dye chemistry and
materials science. The examples of using rylenes and anthenes as semiconductors
and dyes are already abundant while more are expected, and the recent advances in
open-shell systems provide new insights in the field of magnetic materials and
nonlinear optics. In the following section, three types of 2D aromatic networks,
namely rylenes, anthenes, and periacenes, will be summarized and discussed.