Chemistry Reference
In-Depth Information
Fig. 5 Interdependence of
the descriptors
E
/
Z
,
anti
/
syn
, and
R
/
S
anti
-folded
syn
-folded
R'
R'
E
R'
R'
R
R
R
R
RS'
SR'
RR'
SS'
R'
R'
R
Z
R'
R'
R
R
RR'
SS'
RS'
SR'
conformations are still overcrowded, although the intramolecular overcrowding
is reduced to a permissible degree [
1
-
3
]. The bridges X and Y play an important
role in the relative stability of the three conformations, leading in favorable cases
to thermochromism [
2
]. In X-ray crystal structures a combination of twisting
and folding is often observed, although usually one mode is by far the dominant
one [
1
,
28
]. The molecular structures of leading examples determined by X-ray
crystallography are shown in Fig.
6
[
46
].
The red bifluorenylidene (2), the smallest overcrowded bistricyclic aromatic ene
(1, X,Y: —), adopts a twisted conformation (t) in the crystal structure [
47
,
48
]. The
two fluorenylidene moieties are nearly planar and the molecule is twisted about the
C
9
C
9
0
bond by
32-33
. The central five-membered rings introduce a bias
towards planarity of the tricyclic moieties, while concomitantly rendering them
sterically less demanding, especially in twisted conformations [
2
]. Bifluore-
nylidenes and their putative constitutional isomers (
E
)- and (
Z
)-biphenalenylidene
(6 and 7)[
49
,
50
] are the only homomerous BAEs with twisted global minimum
conformations [
47
,
51
-
54
]. The cove regions in
E
-biphenalenylidene (6) resemble
the rim motif in [4]helicene (benzo[
c
]phenanthrene), while (
Z
)-biphenalenylidene
(7) has a bay and a fjord region on the two sides of the central double bond. The
non-planarity of bifluorenylidene (2) was inferred in 1935 on the basis of the dipole
moment of its 2,2
0
-difluoro-derivative [
55
].
The yellow bianthrone (3) with central six-membered rings and bridges X,Y
¼
ω
¼
¼
C
¼
Ois
anti
-folded (a): the tricyclic moieties are folded in opposite directions and
have boat-shaped central rings [
56
,
57
]. According to the first reliable X-ray
structure by Apgar and Wasserman quoted in [
1
], the tricyclic moieties are folded
in opposite directions (
anti
) by A-B
40
. Recently crystal structure
studies at low temperature, at high temperatures, and at high pressures confirmed
this structure of the yellow form [
58
-
60
]. The results regarding the solid state
colored form(s) at high temperature and high pressure are discussed in Sect.
2.5
The colorless (tetrabenzo[7,7
0
]fulvalene) (8) with central seven-membered rings
(1, X,Y: HC
¼
C-D
¼
CH) has two conformational isomers that were characterized by
X-ray crystallography: an
anti
-folded and a
syn
-folded polymorph [
61
]. In the
syn
-folded conformation (s) both tricyclic moieties are folded in the same direction
and the CH groups in positions 1, 1
0
, 8, and 8
0
are pointing towards each other.
¼
