Closed-Shell and Open-Shell 2D Nanographenes - Polyarenes - page 223

Scheme 9 Different synthetic methods to stable bisanthenes [ 82 - 84 ]

from closed-shell to open-shell in a series of homologues of Kekul´ PAHs, anthenes
represent an excellent prototype since the radicals will have fixed positions on the
zigzag edges in order to draw the maximum sextet benzenoid rings, which mini-
mize the influence of migration and delocalization. Therefore, the discussion of
biradical character can be concentrated on the energy balance between the formal
loss of the double bond and the aromatic stabilization, which provides more
intriguing properties and functionalities for the anthene family. Moreover, special
and intriguing properties derived from open-shell characters provide new potential
functionalities in materials science.
The first member of anthenes, bisanthene, was firstly reported by Clar and the
improved synthesis was recently developed by Bock's and our groups. The parent
bisanthene was a blue color compound with low stability owing to its high-lying
HOMO energy level, and the most reactive sites are the meso -positions of the
zigzag edges which readily undergo addition reaction with the singlet oxygen in air.
Therefore, it is essential to do modifications on the zigzag edges in order to obtain
stable and soluble bisanthene derivatives to allow further study and applications.
Systematic research was conducted in our group to stabilize bisanthene by
different strategies as shown in Scheme 9 . The first approach is to introduce
electron-withdrawing imide groups onto the zigzag edges, which was proven to
be an effective concept to stabilize extended PAHs [ 82 ]. The bisanthene bisimide
compound 100 was achieved from precursor 99 by base-promoted cyclization
reaction in moderate yield, and it displayed good stability with no significant
change in solution exposed to air. Moreover, as was the case in other systems,
100 exhibited a red shift of 170 nm at the absorption maximum compared with
parent 96, indicating that 100 can be used a promising candidate for NIR absorbing
materials.
An alternative approach was synchronously developed in our group by means of
meso -substitution with aryl or alkyne substituents to block the reactive sites [ 83 ].
Based on this consideration, three meso -substituted bisanthenes 102a-102c were
prepared by nucleophilic addition of aryl or alkyne Grignard reagent to the