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a transformation of the radical to the corresponding hydroxyphenalenone derivative, a radical synthetic
precursor. A considerable decrease of the calculated SOMO-LUMO energy gap (0
.
24
→
0
.
09 eV) as well
as a lowering of SOMO energy level (
−
2
.
99
→−
3
.
27 eV) in comparison with those of the
6OPO
may
influence this instability to sunlight.
3.8.4 Redox-based spin diversity
The redox activities of the
OPO
systems were investigated by cyclic voltammetry measurements. Each
of the anionic salts shows two-stage one-electron redox waves (Figure 3.39).
36,37a
All of the redox processes
for the
TB4OPO
and
TB6OPO
systems are reversible, while the oxidation wave (
E
p
OX
) from the anion
to the neutral radical of
p
-methoxyphenyl derivative of
3OPO
system,
MP3OPO
, is irreversible. This is
probably due to the
dimerization of
3OPO
system (
vide supra
). Furthermore, the reduction potentials
from the neutral radical to the anion in the
OPO
systems are similar to those of tetracyanoquinodimethane
(TCNQ) and
p
-chloranil, typical electron acceptor molecules, indicating high electron accepting abilities.
In these experimental conditions, all the chemical species except for the
3OPO
neutral radical possess
high stabilities.
Chemical generation of the radical dianions of
MP3OPO
,
TB4OPO
,and
TB6OPO
systems and inves-
tigations of their electronic structures have been conducted. Treatment of the neutral radicals with alkali
metals such as potassium and sodium give the radical dianions as stable chemical species in degassed
sealed conditions even at room temperature (Figure 3.40). EPR
σ
1
H-ENDOR/TRIPLE measurements show
totally different hfccs from those of the corresponding neutral radicals, experimentally revealing the “redox-
based spin diversity” nature of these species (
vide supra
).
36,37a,38
Quantum chemical calculations and the
“MO-based VB method” (molecular orbital-based valence bond method)
37b
also corroborate these views
with contributing weights of any possible resonance structures. In order to observe these unique electronic
properties in other systems, there are two major necessities: (1) high stabilities in each oxidation state, and
(2) a significant difference in the orbital symmetry between two frontier orbitals. For these limitations,
not only polycyclic aromatic hydrocarbons, such as naphthalene and anthracene, but also the multistage
amphoteric redox systems based on the phenalenyl (
vide infra
) are excluded.
49
This novel concept attracts
much attention in terms of the topological symmetry of spin density distribution versus charge fluctuations
and stabilization effect of the
OPO
systems.
36,37a,38
/
E
1/2
red
= -2.35 V
E
p
ox
=-0.11V
(a)
OMe
E
1/2
red
=-2.43V
Et
4
N
+
E
1/2
ox
= -0.08 V
Et
4
N
+
(b)
-
Et
4
N
+
O
O
O
-
E
1/2
ox
=-0.30V
E
1/2
red
=-2.39V
O
O
O
(c)
MP3OPO
-
TB4OPO
-
TB6OPO
-
0
-1
-2
VvsFc/Fc
+
Figure 3.39
Cyclic voltammograms for (a) Et
4
N
+
·
MP3OPO
−
(5mM), (b) Et
4
N
+
·
TB4OPO
−
(3mM), and
(c)Et
4
N
+
·
TB6OPO
−
(10mM)inacetonitrilesolutionsat300K.
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