A pentiptycene-derived molecular brake: Photochemical Ea→Z and electrochemical Za→E switching of an enone module

Article abstract of DOI:10.1002/chem.201002132

The synthesis and brakelike performance of a new molecular system (1) consisting of a pentiptycene rotor and a 2-methyleneindanone brake are reported. The rotation kinetics of the rotor was probed by both variableerature ¹H and ¹³C N

Full text of DOI:10.1002/chem.201002132

FULL PAPER
DOI: 10.1002/chem.201002132
A Pentiptycene-Derived Molecular Brake: Photochemical E!Z and
Electrochemical Z!E Switching of an Enone Module
Ying-Chen Chen,[a] Wei-Ting Sun,[a] Hsiu-Feng Lu,[b] Ito Chao,*[b] Guan-Jhih Huang,[a]
Ying-Chih Lin,[a] Shou-Ling Huang,[a] Hsin-Hau Huang,[a] Yan-Duo Lin,[a] and
Jye-Shane Yang*[a]
Abstract: The synthesis and brakelike
performance of new molecular
system (1) consisting of a pentiptycene
rotor and 2-methyleneindanone
in the Z form ((Z)-1) than in the E
form ((E)-1), rotation of the rotor is
slowed down 500-fold at room temper-
ature (298 K) on going from (E)-1 to
(Z)-1, corresponding to the brake-off
and brake-on states, respectively. The
(E)-1!(Z)-1 photoisomerization in
acetonitrile is efficient and reaches an
(E)-1/(Z)-1 ratio of 11:89 in the photo-
stationary state upon excitation at
290 nm, attributable to a much larger
isomerization quantum efficiency for
(E)-1 versus (Z)-1. An efficient (Z)-
1!(E)-1 isomerization (96%) was also
achieved by electrochemical treatment
through the radical anionic intermedi-
ates. Consequently, the reversibility of
the E–Z switching of 1 is as high as
85%. The repeated E–Z switching of 1
with alternating photochemical and
electrochemical treatments is also dem-
onstrated.
a
a
brake are reported. The rotation kinet-
ics of the rotor was probed by both var-
iable-temperature 1H and 13C NMR
spectroscopy and DFT calculations,
and the switching between the brake-
on and brake-off states was conducted
by a combination of photochemical
and electrochemical isomerization. Be-
cause of the greater steric hindrance
between the rotor and the brake units
Keywords: conformation analysis ·
electrochemistry · isomerization ·
molecular devices · photochemistry
Introduction
shuttles, motors, brakes, and many others.[1–13] Undoubtedly,
the state-to-state switching should be as selective and effi-
cient as possible. In addition, light and electrical energy are
cleaner energy sources than chemicals, because they are free
of the problem of the accumulation of chemical waste
during operation.[2] A clean and highly efficient switching
mode is in particular demand for operating molecular ma-
chines on a single-molecule level or in a nanosized domain.
The cis/trans (Z–E) photoisomerization is one of the most
important switching modes in light-driven molecular ma-
chines.[4–6] Because of the comparable photoactivity for the
E and Z isomers in most cases, the switching direction and
efficiency are determined mainly by their relative molar ab-
sorptivity at the excitation wavelengths. Ideally, each of the
isomers can be selectively excited to reach a quantitative
conversion. However, this is generally not the case due to
the spectral overlap of the isomers, and therefore examples
of high efficiency (e.g., >85%) for both the E!Z and Z!
E photoisomerizations are rare.[6]
Artificial molecular machinery is an important subject in
the areas of nanoscience and nanotechnology.[1,2] A common
feature for all prototypes of molecular machines is the pres-
ence of distinct mechanical states (dynamic or static) that
can be switched or biased by external stimuli (energy input)
such as light, electrical energy, chemicals, and heat. The ma-
chinelike movements associated with the switching processes
perform the designated work, leading to so-called molecular
[a] Y.-C. Chen, W.-T. Sun, G.-J. Huang, Prof. Y.-C. Lin, S.-L. Huang,
H.-H. Huang, Dr. Y.-D. Lin, Prof. J.-S. Yang
Department of Chemistry, National Taiwan University
Taipei 10617 (Taiwan)
Fax : (+886)223636359
E-mail: jsyang@ntu.edu.tw
[b] Dr. H.-F. Lu, Prof. I. Chao
Institute of Chemistry, Academia Sinica
Taipei 11529 (Taiwan)
Several redox centers have been employed for the con-
struction of electrochemically driven molecular machines.
Fax : (+886)227831237
E-mail: ichao@chem.sinica.edu.tw
Cꢀ [9]
Examples include CuI/CuII,[7] NiII/NiIII/NiIV,[8] C60/C60
,
tet-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002132.
[10]
rathiafulvalene (TTF)/TTF +/TTF2+
,
methyl viologen
C
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1193

+
[11]
(MV2+)/MV
,
naphthalimide/naphthalimideꢀ,[12] and fer-
Whereas (E)-2 was obtained in a good yield (86%) by heat-
ing the two carbonyl components in acidic ethanol solu-
tions,[15] the synthesis of (E)-1 was base-promoted and re-
quired the assistance of microwave irradiation and the KF/
Al2O3 catalyst[16,17] to reach a high yield (82%). For compar-
ison, the yield for (E)-1 drops to 37% with the same re-
agents when heated for 16 h. The corresponding Z isomers
can be obtained through photoisomerization of the E iso-
mers in solutions. For the purposes of good solubility, THF
was adopted as the solvent. A 352 nm light source was em-
ployed. This wavelength corresponds to the onset of the ab-
sorption profile of (E)-1 in THF (Figure S1, Supporting In-
formation), so that deep penetration of the light to the opti-
cally dense solutions (1–1.5 mm)
C
rocene/ferrocene+.[13] In these cases, the different redox
states correspond to different mechanical states, and the ma-
chinelike movements are thermodynamically driven in re-
sponse to a change in the redox states. To our knowledge, a
molecular machine that employs electrochemical reactions
as a switching mode between two neutral and non-redox-re-
lated mechanical states has yet to be demonstrated.
Herein we report a new approach toward the efficient
control of molecular machines by a combination of direc-
tional photochemical E!Z and electrochemical Z!E isom-
ACHTUNGTRENNUNGerization, as demonstrated by the molecular brake system 1
(Scheme 1). The two stereoisomers (E)-1 and (Z)-1 corre-
is allowed. Although the wave-
length of 352 nm is near the ab-
sorption maximum of (E)-2
(Figure S2), it gave
a larger
(Z)-2/(E)-2 ratio in the photo-
stationary state (PSS) than the
light of longer wavelength (vide
infra). It should also be noted
that the use of shorter wave-
length light led to a lower yield
due to the formation of uniden-
tified by-products.
DFT calculations: The thermo-
chemistry of (E)-1 and (Z)-1
was evaluated by DFT calcula-
tions at the BMK/6-311+G**//
B3LYP/6-31G* level. We have
shown previously[5] that this
level of theory reproduces the
experimental results of pentip-
Scheme 1. Synthesis of the E and Z isomers of 1 and 2. The numerical labels for protons and carbons are for
discussion of the VT NMR spectra. The symbol f denotes the dihedral angle between the two phenylene
planes.
spond to the brake-off and brake-on mechanical states, re-
spectively. The E–Z switching results in a 500-fold change in
the Brownian rotation rate of the pentiptycene group (the
rotor) at room temperature due to the different extent of
steric interaction with the indanone unit (the brake compo-
nent) in the two stereoisomers: the smaller steric demand of
the methylene versus the carbonyl groups in the indanone
moiety leads to a lower barrier for the rotation of the rotor
in (E)-1 than in (Z)-1. In conjunction with the behavior of
model compounds (E)-2 and (Z)-2 (Scheme 1), it is shown
that the pentiptycene group in 1 plays a critical role in both
the photochemical and electrochemical switching processes.
tycene-based molecular brakes quite well. To expedite the
calculations, the octyloxy group on the pentiptycene rotor
was replaced by a methoxy group. Each isomer of (E)-1 and
(Z)-1 was found to have two optimized ground-state con-
formers, which differ merely in the syn or anti orientation of
the two groups, methoxy and 2-methyleneindanone, with re-
spect to the central phenylene ring of the pentiptycene
rotor. The difference in energy between the two conformers
in each case is less than 0.7 kcalmolꢀ1 (Table S1 in the Sup-
porting Information). Thus, the reported free energy at
298 K is estimated on the basis of the free energies of the
two conformers with consideration of the Boltzmann distri-
bution, rather than the lowest energy structures of the
ground and transition states. The results suggest that (E)-1
is more stable than (Z)-1 by 5.86 kcalmolꢀ1 free energy.
This is consistent with the fact that (Z)-1 was not observed
in the products of the aldol condensation reaction. The con-
jugation effect appears to be unimportant in accounting for
their relative stability, because in the lowest-energy ground-
state structures (Figure 1), the dihedral angle (f, defined in
Scheme 1) between the two phenylene rings along the long
molecular axis is larger in (E)-1 (ꢁ808) than in (Z)-1
Results and Discussion
Synthesis: The synthesis of 1 and 2 is outlined in Scheme 1.
Compounds (E)-1 and (E)-2 are prepared through crossed
aldol condensation reactions between 1-indanone and the
corresponding benzaldehydes 3 and 4. The synthesis of the
pentiptycene carboxaldehyde 3 has been reported previous-
ly,[14] and the benzaldehyde 4 is commercially available.
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A Pentiptycene-Derived Molecular Brake
FULL PAPER
plays an important role in the relative energies of the con-
formers of 1.
Variable-temperature NMR experiments: The rotational
barriers and activation parameters of (E)-1 and (Z)-1 were
also evaluated by variable-temperature (VT) NMR spectros-
copy. Figure 2 shows the pentiptycene peripheral phenylene
1
region of the H and 13C NMR spectra of (E)-1 and (Z)-1 in
CD2Cl2 at 253 K. The signals can be assigned unambiguously
on the basis of a series of 2D NMR experiments, including
COSY, HSQC, NOESY, and ROESY (Figures S5–S12 in the
Supporting Information). The presence of two sets of signals
(Figures 2a and 2b) for (Z)-1 but not for (E)-1 confirms
that the former possesses a larger rotational barrier than the
latter. Nevertheless, we observed decoalescence of the sig-
1
nals (e.g., H1 and C6) in both the H and 13C NMR spectra
for (E)-1 upon cooling to a temperature near 213 K. In the
case of (Z)-1, rotation of the rotor is already arrested on the
NMR timescale at 253 K. Thus, the temperature was raised
to reach coalescence of the signals (e.g., nuclei H4 and C3
1
at 293 K). Line shape analyses of both the H and 13C VT
NMR spectra (Figures 2c and 2d) provide the rotation rate
constants (k), and thus the activation barriers (Ea), and the
enthalpy (DH°) and entropy (DS°) of activation through Ar-
rhenius and Eyring plots (Figures S13–S17 in the Supporting
Information). These data, along with the free energy of acti-
vation at 298 K, are summarized in Table 1. The observed
Figure 1. DFT-derived lowest-energy ground-state (GS) and transition-
ꢀ
state (TS) structures for the rotation about the aryl–vinyl C C bond in
a) (E)-1 and b) (Z)-1. The units of labeled distances and angles are ꢁ
and degrees, respectively.
difference in DG° (3.8 kcalmolꢀ1) between (E)-1 and (Z)-
298
1 is of the same order of magnitude as that predicted by
DFT calculations (vide supra), and suggests a 500-fold dif-
ference in the rotation rate at 298 K. Because of the similar
(ꢁ608). In fact, if f were set to 608 for (E)-1, one of the
methylene H atoms of the indanone moiety would have ex-
tremely close contact with a nearby H atom of pentiptycene
(<1.9 ꢁ). A coplanar p-conjugated backbone (i.e., f=08)
in 1 would also encounter severe steric congestion between
the pentiptycene bridgehead nuclei and the methylene ((E)-
1) or the carbonyl groups ((Z)-1) of the indanone, and these
conformations correspond to the transition states of rotation
DS° values for (E)-1 and (Z)-1, their difference in DG°
298
mainly results from the enthalpic term. This is consistent
with the rigidity of the brake component, which lacks inter-
nal rotational freedoms.
Photochemical (E)-1!(Z)-1 switching: With the different
rotation rates for the rotor in (E)-1 (fast rotation) and (Z)-1
(slow rotation), compound 1 behaves as a molecular brake
in which the carbonyl group serves as the brake latch
(Figure 3). To switch between the brake-off and brake-on
states, we have to conduct E–Z isomerization reactions. This
was carried out in acetonitrile (0.5 mm), as a quick compari-
son between CH2Cl2, THF, and acetonitrile showed that the
latter is superior in the switching efficiency for both the
photochemical (E)-1!(Z)-1 and the reverse electrochemical
(Z)-1!(E)-1 switching. The details are described in the fol-
lowing.
Due to the nearly overlapped absorption spectra for (E)-1
and (Z)-1 (Figure 4), the selective excitation of either (E)-1
or (Z)-1 is essentially impossible. However, to our surprise,
the photochemical (E)-1!(Z)-1 switching in acetonitrile
with excitation at 290 nm is as high as 89% in PSS according
to HPLC analysis (Figure 5a).[18] This must indicate that the
isomerization quantum yield for (E)-1!(Z)-1 (FEZ) is much
larger than that for (Z)-1!(E)-1 (FZE) according to Equa-
tion (1), for which the molar ratio of (Z)-1/(E)-1 in PSS
ꢀ
about the aryl–vinyl C C bond (Figure 1). The calculated
free-energy rotational barriers at 298 K (DG°298) are 11.86
and 14.38 kcalmolꢀ1 for (E)-1 and (Z)-1, respectively, which
predicts a 102-fold difference in rotation rate (DDG°
=
298
2.52 kcalmolꢀ1).
For comparison, the model compounds (E)-2 and (Z)-2
were also studied. Unlike 1, the ground states of (E)-2 and
(Z)-2 adopt a coplanar geometry and the rotational transi-
tion states exhibit a perpendicular orientation of the two
phenylene rings (Figure S3 in the Supporting Information).
The argument of a twisted versus planar p-conjugated back-
bone in 1 versus 2 is indeed supported by the significantly
blue-shifted UV/Vis absorption spectra (vide infra). The E
isomer remains more stable than the Z isomer in 2, but the
free-energy difference becomes smaller (3.59 kcalmolꢀ1)
compared to that of 1. In addition, the rotational barriers
for (E)-2 and (Z)-2 are similar (4.80 and 5.03 kcalmolꢀ1, re-
spectively), and are much lower than those for 1. Evidently,
the steric effect imposed by the bulky pentiptycene group
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1195

I. Chao, J.-S. Yang et al.
Figure 2. Pentiptycene peripheral phenylene (blade of the rotor) region of the a) 1H NMR and b) 13C NMR spectra of (E)-1 and (Z)-1 in CD2Cl2 at
1
253 K. c) VT H NMR and d) VT 13C NMR spectra of (E)-1 (H1 and C6) and (Z)-1 (H4, C2, and C3) in CD2Cl2, for experimental data (left) and simulated
data (right). Values of temperature (T, K) and interconversion rate (k, sꢀ1) between the two isoenergetic states are also given for each trace. See
1
Scheme 1 for nuclei labeling and Figure S4 in the Supporting Information for more complete VT H NMR spectra for (E)-1.
1
Table 1. VT H NMR and 13C NMR data and activation parameters for the rotation of the pentiptycene rotor in (E)-1 and (Z)-1.[a]
Nuclei[b]
Tc[c]
[K]
DG°
Ea
logA
DH°
DS°
DG°
k(298K)
[sꢀ1
c
(298K)
[kcalmolꢀ1
[kcalmolꢀ1
]
[kcalmolꢀ1
]
[kcalmolꢀ1
]
[calmolꢀ1 Kꢀ1
]
]
N
]
(E)-1
(Z)-1
H (1,1’)[d]
C (6,6’)[e]
213
203
293
283, 293
10.2
10.4
15.0
15.0,15.5
8.3ꢃ0.1
8.3ꢃ0.3
10.6ꢃ0.1
10.7ꢃ0.3
11.0ꢃ0.4
10.2ꢃ0.6
7.8ꢃ0.1
7.9ꢃ0.3
ꢀ12.0ꢃ0.6
ꢀ11.0ꢃ1.2
ꢀ10.1ꢃ1.8
ꢀ13.9ꢃ2.6
11.4ꢃ0.2
11.1ꢃ0.4
15.1ꢃ0.8
15.0ꢃ1.1
20567
–
H (4,4’)[f]
12.6ꢃ0.5
11.4ꢃ0.7
12.1ꢃ0.5
10.8ꢃ0.7
37
42
C (2,2’; 3,3’)[f]
[a] Recorded in CD2Cl2. [b] For line shape analysis. See Scheme 1 for the nuclei labels. [c] Coalescence temperature. [d] Temperature range 193–293 K,
11 data points. [e] Temperature range 183–243 K, seven data points. [f] Temperature range 263–293 K, four data points.
tion wavelengths (lex) but also by the relative values of FEZ
and FZE
.
½ðZÞ-1ꢂ=½ðEÞ-1ꢂPSS ¼ ðeEFEZÞ=ðeZFZE
Þ
ð1Þ
On the basis of eE/eZ =1.07 and [(Z)-1]/[(E)-1]PSS =8.09 at
lex =290 nm (Figure 4), a FEZ/FZE ratio of 7.56 is predicted.
This is indeed borne out by the observed quantum yields of
FEZ =0.53 and FZE =0.08.[19] Such a large ratio of FEZ/FZE
observed for 1 is approaching so-called one-way photoACHTUNGTRENNUNGisom-
Figure 3. Schematic representation of the brake function of 1.
AHCTUNGTREGeNNNU rization. A few examples of one-way photoisomerization
for alkenes, either E!Z or Z!E, have been reported.[10–22]
The origins of the small or negligible isomerization quantum
efficiency for one of the isomers could be either kinetic (i.e.,
([(Z)-1]/[(E)-1]PSS) is determined not only by the relative
molar absorptivity of (E)-1 (eE) and (Z)-1 (eZ) at the excita-
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A Pentiptycene-Derived Molecular Brake
FULL PAPER
bonyl group, a scenario similar to the transition state for the
aryl–vinyl rotation of (Z)-1 (Figure 1b). The subsequent C=
C torsion would also result in a sterically hindered state re-
sembling the transition structure for the aryl–vinyl rotation
of (E)-1. These steric interactions could abort the isomeriza-
tion, causing a low FZE value.
Electrochemical (Z)-1!(E)-1 switching: The advantage of a
low FZE value in the photochemical (E)-1!(Z)-1 switching
precludes the use of light as an efficient switching method
for the reverse process, (Z)-1!(E)-1. An alternative isomer-
ization pathway is thus required. We proposed that this
might be achieved electrochemically, because it is known
that many radical ions of diarylethenes undergo the Z!E
isomerization efficiently.[24] This phenomenon can be under-
stood by considering a reduced double-bond character for
the vinyl group in the radical ionic states, as is the case in
the electronically excited state for photoisomerization; thus,
the equilibrium of the isomers is driven toward the more
stable E isomer. Because of the electron-accepting enone
group in 1, the proposed redox-induced (Z)-1!(E)-1 isom-
Figure 4. UV/Vis absorption spectra for (E)-1, (Z)-1, (E)-2, and (Z)-2 in
acetonitrile.
AHCTUNGTRENNUNG
Cꢀ
Cꢀ
ðZÞ-1 þ eꢀ ! ½ðZÞ-1ꢂ ꢀ+½ðEÞ-1ꢂ ! ðEÞ-1 þ eꢀ
ð2Þ
(
This rationale is indeed promising in view of the revers-
AHCTUNGTRENNUNG
Cꢀ
Cꢀ
termediates [(E)-1] and [(Z)-1] are sufficiently stable
under the experimental conditions, and have a relative sta-
bility similar to that in their neutral forms, which is ꢁ5 kcal
molꢀ1 more stable for (E)-1 than (Z)-1 based on DFT calcu-
lations (vide supra). Consequently, the equilibrium of the
radical anions should be driven toward the E isomer, as
shown in Equation (2).
Figure 5. HPLC chromatograms of a) (E)-1 before (trace I) and after
(trace II) irradiation with 290 nm light, b) (Z)-1 before (trace III) and
after (trace IV) electrochemical treatment (see text for details), and
c) (E)-1 (trace 0) after alternating 290 nm irradiation (odd-numbered
traces) and electrochemical treatment (even-numbered traces). The
system is operated in acetonitrile solution.
a high torsional barrier or the presence of ultrafast deactiva-
tion routes)[17] or thermodynamic (i.e., unfavorable equilibri-
um to the twisted excited state).[21] In view of the fact that
mutual (E)-2!(Z)-2 and (Z)-2!(E)-2 photoisomerization
reactions occur with quantum yields of 0.50 and 0.23, respec-
tively,[23] we can conclude that the inefficient (Z)-1!(E)-1
photoisomerization is associated with the bulky pentipty-
cene group. A tentative explanation for the distinct FEZ and
FZE values is based on the different dihedral angles f be-
tween the pentiptycene central ring and the 2-methylenein-
danone group (Scheme 1). The large f value (808) in (E)-1
might lead to an excited state more localized in the 2-meth-
yleneindanone group (the red part in Figure 3), and thus
direct rotation about the C=C bond is feasible. Excitation of
(Z)-1 might lead to a planarization relaxation of the p-con-
jugated benzylideneindanone backbone due to a smaller f
(608), but this process would encounter severe steric conges-
tion between the bridgehead hydrogen atoms and the car-
Figure 6. CV curves of a) (E)-1 and (Z)-1 and b) (E)-2 and (Z)-2 in ace-
tonitrile (0.3 mm, 0.1m tetrabutyl ammonium tetrafluoroborate) recorded
at 100 mVsꢀ1
.
The electrochemical (Z)-1!(E)-1 switching experiment
was conducted in N2-bubbled anhydrous acetonitrile con-
taining 0.5 mm (Z)-1 and 0.1m Bu4NPF6 electrolyte, in a
1 mm quartz cell equipped with a platinum grid, a platinum
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I. Chao, J.-S. Yang et al.
wire, and a silver metal wire as the working, counter, and
reference electrodes, respectively. To minimize possible in-
termolecular side reactions (e.g., dimerization) of the inter-
mediates [(Z)-1] and [(E)-1] , we employed potential
pulses to keep a low concentration and short duration of the
intermediates in solution. The voltage was switched between
0 and ꢀ1.55 V (the reduction peak potential vs. SCE), with
10 s and 20 s dwell time for the reduction and oxidation pro-
cesses, respectively (Figure 7a). An (E)-1/(Z)-1 ratio of 96:4
was achieved with around 11 pulses (Figure 5b).
Conclusion
We have demonstrated the first example of combined light-
driven E!Z and electrically driven Z!E isomerization to
achieve a high switching efficiency in the operation of a mo-
lecular brake. Efficient control of the mechanical states of
artificial molecular machinery using light and/or electrical
energy has the advantages of clean energy input, time and
power economy, and maximum work performance. These
merits are particularly critical for constructing nanosized
molecular machinery or integrated molecular devices.
Cꢀ
Cꢀ
Experimental Section
Methods: NMR spectra were acquired on a Bruker DMX 500 or Bruker
AVIII 400 spectrometer using 5 mm gradient TBI and TBO probes, re-
spectively. The chemical shifts for 1H and 13C NMR spectra were refer-
enced to the signals of tetramethylsilane (d(1H)=0 and
dACHTUNGTRENNUNG
(13C)=0).
Single-pulse spectra were recorded using a 308 pulse and a suitable delay
time (2 s and 6 s for 1H and 13C respectively). Other spectra (COSY,
NOESY, ROESY, and HSQC) were measured using pulse sequences in
the Bruker software package. In the case of variable-temperature mea-
Figure 7. Potential programs for redox-induced a) (Z)-1!(E)-1 and
b) (Z)-2!(E)-2 isomerization in acetonitrile. Two potential pulses are
depicted for each case.
AHCTUNGTRENNUNG
surements, the actual sample temperature was well calibrated by 1H sig-
nals of ethylene glycol and methanol, so that the temperature error was
within ꢃ1 K. Signal acquisition was begun after a sufficient temperature
equilibration time (10–15 min). The fitting of dynamic NMR line shapes
at different temperatures was performed with the Topspin 2.0 program,
Bruker BioSpin Group. Infrared spectra were recorded on a Varian 640-
IR FTIR spectrometer. UV/Vis absorption spectra were measured on a
Varian Cary300 Bio-type at room temperature. Quantum yields of photo-
isomerization were measured on optically dense N2-bubbled solutions
(10ꢀ3 m) at 313 nm using a 75 W Xe arc lamp and monochromator. trans-
4-(Phenylamino)stilbene was used as a reference standard (Ftc =0.34 in
dichloromethane).[19] Photoisomerization quantum yield measurements
were conducted in N2-bubbled THF/acetonitrile (v/v=1/4) solutions
(0.5 mm) at selected wavelengths using a 75 W Xe arc lamp and mono-
chromator. The extent of photoisomerization (<10%) was determined
using HPLC analysis (Waters 600 Controller and 996 photodiode array
detector) without back-reaction corrections. The reproducibility error
was <10% of the average. The cyclic voltammetry (CV) data were re-
corded on a CHI612B electrochemical analyzer at room temperature
(23ꢃ18C), and a glassy carbon electrode served as the working elec-
trode. The substrates were 0.3 mm in acetonitrile containing 0.1m
Bu4NPF6 as supporting electrolyte. The potentials were calibrated against
SCE by the addition of ferrocene (Fc) as an internal standard, taking E0-
We believe that steric shielding of the enone redox center
by the bulky pentiptycene group plays an important role in
the stability of the radical anion intermediates. Indeed, this
is confirmed by comparison with the model compound 2.
The cathodic CV curve for (Z)-2 is irreversible (Figure 6b),
and unidentified by-products quickly built up when (Z)-2
was subjected to the same electrochemical treatments as
(Z)-1. Nevertheless, a new potential program (Figure 7b)
with intermittent cyclic voltammetric scans between 0 and
ꢀ1.45 V for (Z)-2 was able to overcome this problem and
allow a 95% conversion of (Z)-2!(E)-2 (ꢁ8 scans). The
fact that (E)-1 and (E)-2 can also be reduced under the ap-
plied potentials (Figure 6) but survive in the final stage of
electrochemical treatments supports the conclusion that the
equilibrium in the radical anions favors the E isomer
[Eq. (2)].
These results (an 89% (E)-1!(Z)-1 and a 96% (Z)-1!
(E)-1 isomerization) account for the overall efficiency of
85% for the operation of the molecular brake (see Support-
ing Information for a motion picture for this concept). Fig-
ure 5c shows five cycles of an alternating forward photo-
ACHTUNGTRENNUNGisomACHTUNGTRENNUNGerization and backward electrochemical isomerization
of (E)-1 in acetonitrile. The small amount of fatigue (<5%)
of the system demonstrates that electrochemical E–Z isom-
AHCTUNGTRENNUNG
(Fc/Fc+)=0.52 V versus SCE in CH2Cl2. The electrochemical reduction
experiments were carried out in a 1 mm quartz cell with N2-bubbled
MeCN solutions containing 0.5 mm substrate and 0.1m Bu4NPF6 as elec-
trolyte. A Pt grid was used as the working electrode, a Pt wire as the
counter electrode, and an Ag wire as the reference electrode. The extent
of electrochemical isomerization was also based on HPLC analysis. The
DFT calculations were performed with the Gaussian03 program.[25] Free-
energy values were estimated by combining the single-point energies at
the BMK/6–311+G** level and the thermal corrections for Gibbs free
energy calculated from frequency analyses at the B3LYP/6–31G* level[26]
in the gas phase with the consideration of the Boltzmann distribution of
the possible conformations of the substituents.
ACHTUNGTRENNUNG
Materials: Anhydrous THF and MeCN were obtained from the solvent
purifier SPBT-103 of LC Technology Solutions Inc., equipped with an
SP-505 column. The moisture content was less than 10 ppm. All the other
solvents and materials for synthesis were reagent grade. Detailed synthet-
ic procedures and structural characterization data for compounds 1 and 2
are provided below.
1198
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 1193 – 1200

A Pentiptycene-Derived Molecular Brake
FULL PAPER
Compound (E)-1: Supported KF catalysts (10 wt%) were prepared by
impregnation of supports (neutral Al2O3, 9 g) to incipient wetness with
solutions of KF (1 g) in methanol (2 mL)/water (2 mL). The slurry was
stirred for 2 h and then the solvent was evaporated on a rotary vacuum
evaporator. Afterwards, the catalyst was dried at 1508C for 24 h, and
14.1, 22.6, 26.0, 29.1, 29.2, 29.3, 31.8, 35.4, 68.0, 114.0, 124.1, 125.7, 127.3,
127.6, 132.3, 133.3, 133.9, 139.5, 140.8, 148.6, 160.6, 192.0 ppm; IR (KBr):
n˜ =1677, 1594, 1182, 972, 737 cmꢀ1 HRMS (ESI+): m/z: calcd for
;
C52H44O2H+: 349.2168 [M++H]; found: 349.2151.
then stored in
a A mixture of aldehyde 3 (0.080 g,
desiccator.[16]
0.14 mmol) and indanone (0.016 g, 0.15 mmol), sodium acetate (0.024 g,
0.30 mmol), supported KF catalysts (1.0 g of 10 wt%; also containing
0.14 mmol of KF), and CH2Cl2 (20 mL) was stirred for 30 min. The solu-
tion was concentrated under reduced pressure, and then the solvent was
evaporated off under vacuum to obtain a powdered dry solid. The reac-
tants adsorbed onto the solid surface. The solid was mixed with acetic an-
hydride (5 mL) in a round-bottomed flask, and the mixture was irradiat-
ed in a microwave oven at 550 W for 15 min. After cooling, the reaction
mixture was dissolved in CH2Cl2 and H2O. The organic layer was washed
with H2O and then dried over anhydrous MgSO4, and the filtrate was
concentrated under reduced pressure. Column chromatography with
CH2Cl2/hexane (1:2) as eluent afforded (E)-1 as a pure pale yellow solid.
Yield: 82%; m.p. 197–1998C; 1H NMR (500 MHz, CD2Cl2): d=0.97 (t,
J=7.0 Hz, 3H), 1.40–1.42 (m, 4H), 1.46–1.58 (m, 4H), 1.73 (quint, J=
7.6 Hz, 2H), 2.07 (quint, J=7.6 Hz, 2H), 3.32 (s, 2H), 4.00 (t, J=6.8 Hz,
2H), 5.38 (s, 2H), 5.76 (s, 2H), 6.90–6.97 (m, 8H), 7.21 (d, J=7.0 Hz,
4H), 7.36 (d, J=7.0 Hz, 4H), 7.39 (d, J=7.6 Hz,1H), 7.53 (t, J=7.3 Hz,
8H), 7.66 (t, J=7.3 Hz, 1H), 8.02 (s, 1H), 8.04 ppm (s, 1H); 13C NMR
(125 MHz, CD2Cl2): d=14.4, 14.5, 23.2, 23.3, 27.0, 29.0, 30.2, 30.3, 31.2,
31.6, 32.2, 32.5, 48.7, 51.9, 76.8, 124.0, 124.2, 124.9, 125.7, 125.8, 127.1,
128.4, 131.5, 135.6, 136.0, 139.2, 142.4, 142.5, 145.7, 145.9, 150.0, 150.3,
193.4 ppm; IR (KBr): n˜ =3068, 3021, 1721, 1650, 1460, 742 cmꢀ1; FAB-
HRMS: calcd for C52H44O2: 700.3341; found: 700.3346.
Acknowledgements
Financial support for this research was provided by the National Science
Council and Academia Sinica of Taiwan (ROC). The computing time
granted by the National Center for High-Performance Computing and
the Computing Center of Academia Sinica is acknowledged.
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Compound (Z)-1: A N2-bubbled solution of (E)-1 (0.015 g, 0.02 mmol) in
THF (20 mL) was irradiated with a photochemical reactor at 352 nm for
1 h. The solvent was removed under reduced pressure, and the resulting
solid was purified by preparative HPLC with EA/hexane (1:1.5) as eluent
to afford the pure yellow solid of (Z)-1. Yield: 81%; m.p. 296–2988C;
1H NMR (500 MHz, CD2Cl2): d=0.84–0.90 (m, 3H), 0.95–0.98 (m, 2H),
1.40–1.48 (m, 4H), 1.49–1.58 (m, 2H), 1.73 (quint, J=7.6 Hz, 2H), 2.08
(t, J=7.3 Hz, 2H), 4.00 (t, J=6.3 Hz, 2H), 4.26 (s, 2H), 5.37 (s, 1H), 5.72
(s, 1H), 6.88–6.95 (d, 8H), 7.08 (s, 2H), 7.21–7.33 (m, 7H), 7.51–7.50 (m,
1H), 7.72–7.74 (m, 2H), 7.79 ppm (d, J=7.5 Hz, 1H); 13C NMR
(125 MHz, CD2Cl2): d=14.5, 23.3, 27.0, 30.0, 30.2, 31.1, 32.5, 34.1, 48.6,
48.7, 49.1, 52.0, 76.8, 123.8, 124.0, 124.3, 124.4, 124.6, 125.4 125.6, 125.7,
125.8, 127.0, 128.2, 132.2, 135.1, 135.2, 139.1, 140.4, 142.9, 145.3, 145.6,
149.8, 150.0, 191.7 ppm; IR (KBr): n˜ =3065, 3017, 1702, 1637, 1459,
740 cmꢀ1
ACHTUNGTRENNUNG
;
HRMS (ESI+): m/z: calcd for C52H44O2Na+: 723.3241
[M++Na]; found: 723.3241.
Compound (E)-2: A mixture of 1-indanone (0.7 g, 6.5 mmol), aldehyde 4
(1.5 g, 6.4 mmol), HCl (0.1 mL, 37 wt%), and EtOH (25 mL) was heated
at 808C for a period of 18 h. The reaction mixture was cooled to RT to
give a yellow solid. The solid was filtered, washed with EtOH, and re-
crystallized in hexane/dichloromethane to provide 1.82 g of (E)-2. Yield:
86%; m.p. 88–908C; 1H NMR (400 MHz, CDCl3): d=0.88–0.91 (m, 3H),
1.29–1.35 (m, 8H), 1.43–1.48 (m, 2H), 1.80 (quint, J=6.7 Hz, 2H), 3.98–
4.01 (m, 2H), 6.95–6.97 (m, 2H), 7.40 (t, J=7.6 Hz, 2H), 7.53–7.64 (m,
5H), 7.89 ppm (d, J=7.6 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=14.1,
22.6, 26.0, 29.1, 29.2, 29.3, 31.8, 32.4, 68.1, 114.9, 124.2, 126.0, 127.5, 127.9,
132.2, 132.5, 133.9, 134.2, 138.2, 149.4, 160.5, 194.3 ppm; IR (KBr): n˜ =
3372, 1690, 1598, 1250, 1178, 732 cmꢀ1; HRMS (ESI+): m/z: calcd for
C52H44O2H+: 349.2168 [M++H]; found: 349.2147.
Compound (Z)-2: A N2-bubbled solution of (E)-2 (10 mg, 0.03 mmol) in
THF (20 mL) was irradiated with a photochemical reactor at 352 nm for
1 h. The solvent was removed under reduced pressure, and the resulting
solid was purified by flash chromatography with EtOAc/hexane (1:6) as
eluent to afford the pure pale yellow solid of (Z)-2. Yield: 69%; m.p. 81–
828C; 1H NMR (400 MHz, CDCl3): d=0.88–0.92 (m, 3H), 1.30–1.35 (m,
8H), 1.43–1.49 (m, 2H), 1.80 (quint, J=7.6 Hz, 2H), 2.05 (quint, J=
6.7 Hz, 2H), 3.87 (s, 2H), 4.01 (t, J=6.6 Hz, 2H), 6.91–6.93 (m, 3H),
7.38–7.42 (m, 1H), 7.46–7.48 (m, 1H), 7.55–7.59 (m, 1H), 7.85 (d, J=
7.6 Hz, 1H), 8.18–8.20 ppm (m, 2H); 13C NMR (100 MHz, CDCl3): d=
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A
Received: July 26, 2010
Published online: November 30, 2010
1200
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