Chemistry Reference
In-Depth Information
4
X
5
4a
10a
3
6
A
E
B
2
7
9
9a
8a
X = Y: homomerous
1
8
fjord
fjord
1'
8'
9a'
8a'
9'
2'
7'
C
F
D
X ¹ Y: heteromerous
3'
Y
1
6'
4a'
10a'
4'
5'
Fig. 1 General structure of bistricyclic aromatic enes (1) and atom labeling
1
Introduction
1.1 Presentation of BAEs
Bistricyclic aromatic enes (BAEs 1) (Fig.
1
)[
1
-
3
] have fascinated chemists since
the bright red hydrocarbon bifluorenylidene (2) was first synthesized in 1875 [
4
]
and thermochromism, piezochromism, and photochromism have been revealed in
bianthrone (3)[
5
,
6
] and dixanthylene (4)[
7
,
8
]. BAEs (1) are defined by the
general structure shown in Fig.
1
and consist of two tricyclic moieties that are
connected at the central positions C
9
and C
9
0
by a double bond. The intramolecular
overcrowding [
9
,
10
] in the fjord regions on both sides of the central double bond
imposes non-planarity on the
-conjugated structures and leads to interesting new
molecular properties that may be tuned by variation of the bridges X and Y that
close the central rings, by substitutions, or by fusing of additional rings leading to
the more general class of polycyclic aromatic enes (PAEs) [
2
,
11
-
14
]. The color
changes in thermo-, piezo-, electro-, and photochromism in BAEs are due to
interconversions between different non-planar conformations and triggered by
thermal, mechanical (pressure), electrochemical, or optical (UV-vis) stimuli,
respectively. More recently, BAEs and related molecules have been developed as
functional molecular materials, including molecular switches and molecular motors
[
15
-
19
]. Furthermore, the proximity of the carbon atoms across fjord regions
facilitates cyclizations, which has been exploited in the synthesis of larger polycy-
clic systems including bowl-shaped fullerene fragments [
20
-
25
].
π
O
O
O
O
2
4
3
