Chemistry Reference
In-Depth Information
folded (f) conformers could be characterized by X-ray crystallography
[
62
,
63
]. Crystals with twisted molecules are deep purple, while crystals with folded
molecules are yellow. This unambiguously establishes the connection between
color and conformation in BAEs [
63
]. In the twisted conformations, the central
double bond has a pure twist of
ω
¼
42
. In the folded conformations, the
20
), while
fluorenylidene moieties
are moderately folded (C-D
the
45
.
Substituents in the fjord regions (positions 1,1
0
,8 and 8
0
) substantially increase the
twisting in bifluorenylidenes (1, X,Y: —) and fluorenylidene-xanthenes (1,X:O,
Y: —), enhancing the twisting and deepening the color (see Table
1
).
The X-ray structures of representative BAEs and those analyzed in our laboratory
since the latest review [
3
] are summarized in Table
1
. Further examples of X-ray
structures, including details of the conformations, and geometrical parameters can be
found in several reviews [
1
,
2
,
28
]. Quite a few X-ray structures of BAEs and related
compounds have been published recently in the context of applications (see Sect.
2.8
).
¼
xanthylidene moieties are folded by A-B
2.4 BAE Variations
The degree of overcrowding in BAEs (1) may be reduced by removing the bridges
X and/or Y. The following two variations leading to open BAEs exist: (1) removal
of one bridge (X or Y) of 1 to give a BAE-1, i.e., a mono-bridged tetraarylethene,
e.g., 9-diphenylmethylene-9
H
-fluorene (9,X
¼
) and 9-diphenylmethylene-9
H
-
¼
xanthene (9,X
O); (2) removal of the two bridges of 1 to give a BAE-2, i.e.,
tetraarylethene, e.g., tetraphenylethene (10). Removal of the ring constraint (and of
π
-delocalization if the central ring had aromatic character) facilitates rotations of
the aryl groups about the formal single bonds to the central ene group. This
conformational flexibility reduces overcrowding in the highly non-planar ground
state, while rotation is hindered by steric overlap. The syntheses, molecular and
crystal structure, and NMR spectroscopic study of the following naphthologs of
BAE-1s have recently been reported: 9-[(di-1-naphthyl)methylene]-9
H
-fluorene
(11,X
O) [
81
]. The
DFT study of the conformational spaces of 11 and their constitutional isomers,
9-[(di-2-naphthyl)methylene]-9
H
-fluorene (12,X
¼
) and 9-[(di-1-naphthyl)methylene]-9
H
-xanthene (11,X
¼
¼
) and 9-[(di-2-naphthyl)
methylene]-9
H
-xanthene (12,X
O) was also reported [
81
]. These naphthologous
BAE-1 analogs of BAEs bifluorenylidene (2) and dixanthylene (4) adopt twisted
and folded-twisted conformations, respectively, both in the solid state and in
solution, relieving the steric strain by low ethylenic twist angles and high naphthyl
twist angles. The conformational preference is governed by the more efficient
conjugation between the naphthyl substituents and the twisted central C
¼
C bond
and by the overcrowding due to the
peri
-hydrogen atoms in the 1-naphthyl groups.
The synthesis and conformational space of the BAE-2 tetrakis(2-naphthyl)ethene
(13) have recently been reported [
82
,
83
] (see also the discussion of the AIE effect
¼
