Closed-Shell and Open-Shell 2D Nanographenes - Polyarenes - page 206

Fig. 10 Spin center transfer between neutral radical and zwitterionic radical [ 35 ]. Reprinted with
permission from [ 35 ]. Copyright 2002 John Wiley and Sons

This behavior was visualized by a color change from orange to dark green. These two
species were also characterized by ESR measurements. Moreover, the 100% inter-
conversion was controllable in a 50 K temperature range (243-293 K) from one
species to the other by controlling the ratio of dichloromethane and trifluoroethanol.
This unprecedented phenomenon is the first known spin switching phenomenon in a
purely organic open-shell molecular system [ 35 ].

2.1.2 Extended Systems Based on Bis- and Trisphenalenyls

Another interesting molecular design towards stable phenalenyl radicals is to link
two phenalenyl moieties with an aromatic bridge to produce singlet biradical
compounds having a Kekul ´ structure as a structural resonance. These systems
were initially studied by Murata and Nakasuji and further investigated by Kubo
et al . [ 36 ]. There are two factors that account for the enhanced stability of these
systems; one is the intrinsic delocalization of the phenalenyl moiety and the other is
the aromatic stabilization by recovery of one more Clar's aromatic sextet ring from
the quinoid form to the biradical form. The biradical character of these compounds
has a pronounced effect on their electronic, magnetic, and solid state properties, and
the moderate stability increased the likelihood of using them as materials. Compu-
tational calculation represents another powerful tool to disclose the open-shell
nature, and parameters such as biradical character index, crystal geometry,
exchange interaction, and spin density are very informative in exploring the biradi-
cal nature.
A series of bis(phenalenyl) compounds were prepared since 1983 when the first
member of this family, a pentalenodiphenalene (PDPL, 37), was synthesized by
Nakasuji et al. (Fig. 11a )[ 39 ]. This molecule exhibited amphoteric multistage
redox properties with a numerical sum E sum as small as 0.99 eV. However, the
neutral state of 37 was substantially destabilized by the electronic contribution of
the 8 -electron antiaromatic pentalene subunit. Subsequently, indacenodiphenalene

(IDPL, 38a) with one benzene ring fused in the center was developed, and various
substituents were introduced to improve the solubility [ 40 , 41 ]. These compounds
were featured by a singlet biradical character in the ground state. The line broadening

p