Chemistry Reference
In-Depth Information
energies, folding and twisting of homomerous and heteromerous BAEs by system-
atically varying the bridges using a series of six different functional groups X,Y and
a polycyclic system [
2
]. Likewise, the conformational space and dynamic stereo-
chemistry of five representative homomerous BAEs has been studied in detail with
PM3 [
3
]. Recently the availability of sufficient computational power and develop-
ment of efficient algorithms made ab initio electronic structure calculations of
BAEs feasible. Density functional theory (DFT) including hybrid density func-
tionals overcame the systematic bias of single-determinant Hartree-Fock against
twisted conformations [
141
]. This allows a theoretical understanding of, e.g.,
thermochromism, the singlet-triplet gap in the highly twisted bis-dibenzo[
a,i
]
fluprenylidene [
45
], and the prediction of BAE properties based on DFT calcula-
tions. Recently experimental studies of BAEs often also include DFT calculations
to enhance the understanding of the observed phenomena, e.g., [
59
,
67
,
76
,
78
,
177
,
178
,
204
]. NMR spectra of the 1,1
0
-difluoro-derivatives of bifluorenylidene (2)
show through-space coupling across the fjords, in particular
J
(F,F) in the
Z
-diaste-
reomer and
J
(F,H) in the
E
-diastereomer [
205
-
207
]. This highlights the close
proximity of the bucking atoms. The
Z
-diastereomer has a higher equilibrium
concentration than the
E
-diastereomer, indicating a lower energy. DFT calculations
revealed that this is due to a combination of steric strain and electrostatic interac-
tions [
205
]. The calculated structures are twisted with additional
anti
- and
syn
-
pyramidalization of the
Z
- and
E
-diastereomers, respectively, in agreement with
symmetry considerations (see below, Fig.
17
). DFT calculations also helped to
elucidate the mechanism of the unexpected rearrangement in the course of a
Peterson olefination, in which, instead of the target - the angularly annelated
benzo[
a
]bifluorenylidene (28) - its linearly annelated constitutional isomer benzo
[
b
]fluorenylidene (29) was formed [
208
].
28
29
2.8 Applications
The thermochromic, piezochromic, photochromic, and electrochromic properties of
BAEs call for applications as molecular switches and functional materials.
