Chemistry Reference
In-Depth Information
Application of BAEs as photochemical memory in a computer was proposed in
1956 [
143
]. However, until recently organic materials have not been sufficiently
reliable as erasable memory [
209
]. The topic of molecular switches has been
reviewed [
18
,
209
-
211
].
Photo- and electroswitching in BAEs is based on the fact that photoexcitations as
well as oxidation or reduction depletes the
π
-bonding HOMO and/or occupies the
π
*
anti
-bonding LUMO. In BAEs with
anti
-folded ground state this inverts the
equilibrium between
anti
-folded and twisted conformations, leading to a rapid
conformational change. Potential energy surfaces for ground and excited states
and/or anionic/cationic forms have been reported, e.g., in [
16
,
76
,
202
]. After return
to the electronic ground state of the neutral molecule, the reverse conformational
change requires a thermally activated crossing of the barrier for conformational
isomerization. Hence the life time of the twisted conformation depends on the
temperature and high barriers are advantageous for stable switches.
Several types of molecular switches derived from BAEs have been developed. A
donor substituted bianthrone-derivative (30) has been studied as a molecular switch
involving conformational changes stabilized by charge transfer [
15
,
16
,
212
,
213
]. Bianthrone (3) as single molecule junction between silver electrodes revealed
two states with different conductance and hysteretic ON and OFF switching
[
214
]. Hexabenzo[7,7
0
]fulvalene (31) can be switched by photo- and thermal
anti
/
syn
isomerization, both in very high yields [
76
].
O
CH
3
X
S
CH
3
S
R
1
R
2
CH
3
S
O
30
31
32
Feringa et al. took advantage of the inherent chirality of substituted bistricyclic
aromatic enes and related overcrowded dissymmetric enes and devised chiral
optical molecular switches based on photochemical and thermal
E
,
Z
isomerizations
[
17
,
18
,
210
,
215
]. The systems included the disubstituted BAEs 32 [
188
,
216
]. The
benzo[
a
]-annelated polycyclic aromatic ene 33 showed stereospecific thermal and
photochemical
P-trans
to
M-cis
isomerizations [
187
,
217
]. Irradiating an initially
racemic sample of 34 with either left or right circular polarized light (CPL)
photochemically generated an enantiomeric excess of the (
M
)- or (
P
)-enantiomer,
respectively [
218
]. Linear polarized light led to racemization. By doping a nematic
