Closed-Shell and Open-Shell 2D Nanographenes - Polyarenes - page 211

Fig. 16 Two synthetic approaches to the zethrene derivatives 46, and the structures of zethrene
diimide 48 and heptazethrene diimide 49 [ 51 - 54 ]
to construct the zethrene core by homoannulation reaction from the precursor 47
using a Pd catalyst system. The transformation yield could be enhanced by up to
73%, which was quite impressive considering the forming of zethrene core and the
introduction of substituents were realized in one step. At the same time, a novel
zethrene derivative with two carboxylic imide groups located along the longer
molecular axis (48) was synthesized by our group [ 53 ]. The introduction of
electron-withdrawing groups successfully enhanced stability by lowering the
HOMO energy level, and a 100-nm bathochromic shift in the absorption spectrum
was also observed. Notably, in contrast to the theoretical predictions, none of the
above-mentioned zethrene derivatives display open-shell behavior in the ground
state.
Inspired by the prediction that higher homologues of zethrene (e.g., heptazethrene
and octazethrene) will possess greater biradical character, our group finally
synthesized the first stable heptazethrene derivative, a heptazethrene diimide 49
(Fig. 16 ) after many tries [ 54 ]. On the basis of calculations, the energy of singlet
biradical form of 49 was located 5.8 and 7.5 kcal mol 1 lower than the closed-shell
quinoid and triplet form and thus defined a singlet biradical ground state. The 1 H
NMR spectra showed line broadening at room temperature and line sharpening at low
temperatures, due to the existence of thermally accessible triplet species. The UV-vis
absorption spectrum gave a lower energy band, and a HOMO-LUMO gap as small as
0.99 eV was determined by cyclic voltammetry, attributed to a biradical ground state.
It is noteworthy that compound 49 exhibited good stability in solution which resulted
from a synergetic effect of radical delocalization and electron-withdrawing effect of
the substituents.
Fusion of three phenalenyl moieties onto one benzene ring will lead to a
monoradical system 50a with a large delocalization of the electron over the
whole molecule (Fig. 17 )[ 55 , 56 ]. Although good stability is expected after kinetic
stabilization by tert -butyl groups, compound 50b was found to decompose in 1 day