Chemistry Reference
In-Depth Information
solutions [
71
]. The AIE luminogens tetraphenylethene (10) and dixanthylene (4)
differ significantly in their conformational spaces. While 10 is highly twisted about
the formal single bonds connecting the phenyl rings to the central ethene group [
83
,
242
,
243
], 4 is yellow and adopts an
anti
-folded conformation as the global
minimum [
70
,
71
].
Bifluorenylidene and its derivatives have been investigated as a new generation
of acceptor compounds in bulk heterojunctions in organic photovoltaic devices
[
244
,
245
]. Bifluorenylidene anions are stabilized by steric “twist”-strain relief and
gain in aromaticity. Higher open circuit voltage and short circuit currents could be
achieved due to the higher LUMO, better overlap with the solar spectrum, and
higher electron charge carrier mobility.
Further applications of BAEs and related polycyclic systems will hopefully be
forthcoming.
3 Symmetry Analysis in Stereochemistry: General Theory
Symmetry is a powerful tool in analyzing and classifying molecular conformations
and the mechanisms of dynamic stereochemistry. In particular, symmetry consid-
erations can be very efficient in analyzing the dynamic stereochemistry of flexible
molecules, complexes, and clusters and in predicting possible mechanisms and
pathways for isomerizations, conformational rearrangements, and enantiomer-
izations. Furthermore, prior knowledge about the symmetry is extremely helpful
in calculations searching for a transition state (TS).
3.1 Symmetry Operators and Permutation-Inversion
Operators
In analyzing the symmetry of molecules, symmetry is usually thought of in terms of
rotations about molecule-fixed axes, inversion at the center of mass, reflections at
planes, and rotoreflections. This applies to rigid molecules, i.e., molecules that
never depart far from their symmetric reference configuration [
246
]. In this context
a reference configuration or
molecular framework
may be visualized as a rigid ball
and stick model with labeled atoms [
247
]. However, e.g., high resolution gas-phase
spectroscopy also required symmetry analysis of molecules with large amplitude
and/or tunneling motions, i.e., molecules interconverting between several molecu-
lar frameworks [
246
,
247
]. Likewise, the symmetry analysis of the dynamic
stereochemistry of non-rigid molecules may include systems more complex than
a single (reference) conformation and its point group. The dynamic stereochemistry
of BAEs with twisted,
anti
-folded, and
syn
-folded conformations is a case in point.
For these purposes it is more convenient to describe symmetry based on permuta-
tions of identical nuclei and a laboratory-fixed inversion [
246
,
247
]. In the same
