Full Paper
thermal helix inversion can be predicted with reasonable accu-
ꢀ
1
racy (within several kJmol ) through the use of density func-
[31,44,48,53,55]
tional theory (DFT) at the B3LYP/6-31G(d,p) level.
Indeed, this method has been utilized to design new motors in
silico by prediction of the helix inversion energy barriers of
motors prior to their synthesis to determine whether their ro-
tation rates would be of the desired order of magnitude. How-
ever, to date a method has not been available that can accu-
rately predict the energy barrier of the potentially competing
TEZI. The reason for the lack of a suitable computational
method for investigating the TEZI is that the transition state
(TS) involved cannot be accurately described by single deter-
minant methods such as Hartree–Fock and Kohn–Sham DFT.
For this reason, a multideterminant method has to be em-
ployed instead. One popular multideterminant method poten-
tially capable of providing an accurate description is the
CASSCF theory in which the orbitals of interest are treated as
in a full configuration interaction (CI) calculation (the active
Scheme 1. General scheme of photochemical E–Z isomerization and thermal
helix inversion of second-generation molecular motors.
spect to the original configuration. The strain can subsequently
be reduced by a thermally activated isomerization in which
(usually) the upper half moves around the lower half, again re-
sulting in an inversion of the helicity. In the resulting stable
isomer, the upper half has undergone a 1808 rotation with re-
spect to the lower half (see Scheme 1, in the case R=H, the
symmetry in the lower half causes the initial and final states to
be chemically identical). In theory, it is possible that the ther-
mal isomerization of the metastable state follows an alterna-
tive and competing pathway other than thermal helix inver-
sion (THI). Structurally similar stilbene switches are able to un-
dergo thermal E–Z isomerization (TEZI) from cis to trans, al-
though the activation energy for this process usually exceeds
[56]
space). Such an approach can be very effective in describing
the ‘static correlation’ that often cannot be accounted for ef-
fectively by a single determinant method. Unfortunately, the
use of CASSCF is limited by the number of orbitals that can be
treated in this way, since full CI calculations are computational-
ly demanding. In practice, the size of a complete active space
is often limited to 14 electrons in 14 orbitals to keep the calcu-
ꢀ
1 [49,50]
1
50 kJmol .
For some overcrowded alkenes though, this
[57]
barrier has been observed to be significantly lower due to the
steric strain in the minimum energy configurations, thus forc-
ing the double bond far from planarity. As an example, bis-flu-
orenylidenes exhibit activation energies for the TEZI of
lation manageable. This poses a challenge as the p system
of a molecular motor is typically much larger. As such, if
CASSCF or a related theory is to be used for studies on molec-
ular motors, it is necessary to limit the number of CSFs in such
a way that the accuracy is not significantly impaired.
ꢀ
1 [51,52]
~
105 kJmol .
For second-generation motors, in order to
positively identify the outcome of the thermal isomerization of
the photochemically generated metastable state the lower half
has to be desymmetrized. The two-step process starting from
the stable-(Z) state will then lead either to the opposite isomer
stable-(E) of the initial configuration, which is indicative of
a THI, or back to the initial stable isomer stable-(Z), thus indi-
cating a reversible switching process by a TEZI (Scheme 2).
Previously, research on molecular motors has to a large
Herein, we report on the switching behavior of four second-
generation overcrowded alkenes, namely 1–4 (Scheme 3). Their
photochemical and thermal isomerization processes have been
studied by various analytical methods, while the thermal iso-
merization processes are also studied by computational meth-
ods. A strategy for studying the TEZI by computational chemis-
try will also be presented. We will show that the metastable
isomers of 1–4 are able to undergo thermal isomerization
through both the THI and TEZI pathways. Finally, we will dem-
onstrate that 1–4 exhibit properties that make them highly
useful bistable switches, such as high selectivity, low switching
fatigue, and high thermal stability.
[31,48,53–55]
extent been supported by computational chemistry.
For example, it has been shown that the energy barrier of the
Results and Discussion
Design
As mentioned above, the bridging units (X and Y, see
Scheme 1) included in the rings connected by the tetrasubsti-
tuted alkene play an important role in the structure’s flexibility,
thermal stability, and switching properties. Previous studies on
overcrowded alkenes with symmetrical lower halves have
shown the effect of the size of the rings connected to the
bridging alkene bond on the activation barrier of the thermal
[48]
relaxation step. In particular, the combination of a five-mem-
bered ring in the lower half (fluorene) with a sulfur- or oxygen-
containing six-membered ring in the upper half (Scheme 1,
Scheme 2. General scheme for photochemical and thermal behavior (TEZI
vs. THI) of desymmetrized overcrowded alkenes stable-(Z) and metastable-
(
E).
&
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Chem. Eur. J. 2016, 22, 1 – 11
2
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