Chemistry Reference
In-Depth Information
conformations of 21 and 22 by a small margin, while the folded conformation is
preferred in 23. On the other hand, the thermochromism of bianthrone (3)is
suppressed by dicyanomethylene bridges in 24 and 25 due to additional
overcrowding at the cyano groups in the twisted conformation [
72
].
O
O
O
N
N
21
22
23
N
N
N
N
C
C
C
C
CH
3
O
N
N
O
CH
3
C
C
O
N
N
24
25
26
27
The conformational transformation into the twisted B formmay also be triggered
by electrochemical reduction or oxidation. Electron transfer leads to a large con-
formational change because the relative stability of the
anti
-folded and twisted
conformations is reversed in the anions of, e.g., bianthrone (3) and xanthylidene-
anthrone (26)[
174
-
178
]. The stability of the B form in the radical anion and
dianion of bianthrone and the reversibility of the reduction/oxidation BB
B
2
,
furnished another argument in favor of identifying B as the twisted conformation
[
114
]. Electrochemical studies of lucigenin, the dication of
N
,
N
0
-dimethyl-
biacridane (27), indicated analogous conformational changes upon conversion
between the cationic and neutral forms [
179
]. The dianions and dications of
bifluorenylidene (2) are also highly twisted [
50
,
180
]. Moreover, the electronic
and structural properties show that the dianions and the dications of bifluore-
nylidene (2), fluorenylidene-phenalene, and (
E
)-and (
Z
)-biphenalenylidene (6, 7)
form a continuum of aromaticity/antiaromaticity [
50
,
180
].
