Light- and redox-gated molecular brakes consisting of a pentiptycene rotor and an indole pad

Article abstract of DOI:10.1002/jccs.201400035

Two photochemically and electrochemically active alkenes 3Me and 3An containing pentiptycene and indole groups have been synthesized and investigated as light and/or redox-gated molecular brakes. The pentiptycene group functions as the four-bladed rotor, the indole group as the brake pad, and the vinylene group as the switch module. The E configuration corresponds to the brake-off state, in which the rotation of the rotor is free with a rotation rate of 10⁸-10⁹ at ambient temperature according to DFT calculations. The Z configuration corresponds to the brake-on state, in which the rotation rate is decreased to 10¹-10², depending on the N-substituent of indole, according to line-shape analysis of variable temperature ¹³C NMR spectra. The overall braking effect reaches a factor of 10⁶-10⁸. While the combined photochemical E → Z and electrochemical Z → E switching has a higher capacity than the two-way photochemical switching in the case of 3Me, the switching capacity are comparable for the two methods in 3An. The results also show that photochemical E-Z isomerization is much more reliable than the electrochemical counterpart, as the stability of the redox intermediates plays a critical role in determining the robustness of the molecular brakes via electrochemical switching. The photochemical and/or electrochemical switching between the E and Z isomers of two alkenes substituted with both pentiptycene and indole groups results in a change of the rotation rate as large as 10⁶-10⁸ fold for the pentiptycene group about the pentiptycene-vinylene C-C bond, corresponding to a new generation of light- and redox-gated molecular brakes.

Full text of DOI:10.1002/jccs.201400035

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Invited Paper
Light- and Redox-Gated Molecular Brakes Consisting of a Pentiptycene Rotor and an
Indole Pad
a
a
a,b
a,
Chen-Yi Kao, I-Tsun Lee, Ch. Prabhakar and Jye-Shane Yang *
Department of Chemistry, National Taiwan University, Taipei, Taiwan 10617, R.O.C.
a
b
Department of Chemistry, National Institute of Technology, Kurukshetra-136119, India
(Received: Jan. 22, 2014; Accepted: Feb. 24, 2014; Published Online: Mar. 17, 2014; DOI: 10.1002/jccs.201400035)
Two photochemically and electrochemically active alkenes 3Me and 3An containing pentiptycene and
indole groups have been synthesized and investigated as light and/or redox-gated molecular brakes. The
pentiptycene group functions as the four-bladed rotor, the indole group as the brake pad, and the vinylene
group as the switch module. The E configuration corresponds to the brake-off state, in which the rotation
8
9
of the rotor is free with a rotation rate of 10 -10 at ambient temperature according to DFT calculations.
1
2
The Z configuration corresponds to the brake-on state, in which the rotation rate is decreased to 10 -10 ,
1
3
depending on the N-substituent of indole, according to line-shape analysis of variable temperature
C
6
8
NMR spectra. The overall braking effect reaches a factor of 10 -10 . While the combined photochemical E
Z and electrochemical Z ® E switching has a higher capacity than the two-way photochemical switch-
®
ing in the case of 3Me, the switching capacity are comparable for the two methods in 3An. The results also
show that photochemical E-Z isomerization is much more reliable than the electrochemical counterpart, as
the stability of the redox intermediates plays a critical role in determining the robustness of the molecular
brakes via electrochemical switching.
Keywords: Pentiptycene; Indole; Molecular machine; Molecular device.
INTRODUCTION
useful strategy toward high switching efficiency between
2
2
“
Molecular machine” is a bottom-up approach to-
two neutral mechanical states (Fig. 1a). The pentiptycene
group in 1 serves as a 4-bladed rotor and the indanone
group as the brake pad. The rotation rate for the rotor in E-1
and Z-1 differs by 500 fold, corresponding to brake-off and
brake-on states. The photochemical E-1 ® Z-1 switching is
as high as 89% and the electrochemical Z-1 ® E-1 switch-
ing recovers 96% of the E isomer, leading to a high effi-
ciency (i.e., 85% reversibility) of device operation. The
electrochemical switching involves radical anions as tran-
sient intermediates rather than one of the mechanical states.
Thus, the stability of the radical anions, presumably lo-
cated in the enone moiety, is important in determining the
robustness of the system. The switching efficiency for 1 is
largely improved as compared to the light-gated analogues
ward miniaturization of machines to improve the efficiency
1
-7
and precision of works. Scientists have successfully con-
structed prototyped molecular devices that undergo me-
chanical responses to external stimuli, such as photons,
electrons and chemicals. Depending on the types of me-
chanical responses, these devices are dubbed as molecular
4
,8-14
15-19
20-25
26-34
shuttles,
motors,
brakes
and many others
in
analogy to the motion performed by macroscopic ma-
chines. Regarding the operation of molecular devices, the
switching efficiency between mechanical states is associ-
ated with the nature of energy source. While systems using
chemical fuels generally have the merit of high switching
efficiency, a serious drawback for chemicals-fuelled sys-
tems is the accumulation of chemical waste during opera-
tions. Light and electrical energy are free of this problem,
2R (R = H, OMe, NO
2
, i-Pr, and t-Bu), which possess an ef-
ficiency of 20-46%, depending on the substituents on the
2
6,27
23,24
as the energy is dissipated by heat.
However, the
brake pad (Fig. 1b).
However, the braking effect is 6 or-
switching operation is generally less efficient with light
and is less stable with electrochemical redox gating. There-
fore, strategies that overcome these deficiencies are de-
sired.
ders of magnitude larger for 2 than 1.
To construct molecular brakes with both high switch-
ing efficiency and large braking effect, we have combined
the merits of 1 and 2 and designed a new generation of mo-
lecular brake 3Me and 3An, where the N-substituted indole
group is redox-active and the oxidized intermediate, a radi-
We recently demonstrated with the molecular brake 1
that a combination of light and electrical energy could be a
*
Corresponding author. Tel: +886233661649; Fax: +886233668681; Email: jsyang@ntu.edu.tw
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Invited Paper
Kao et al.
Scheme 1 Retrosynthetic analysis of 3Me and 3An
formylindole 7 and methyl iodide via the Wittig reaction
followed by N-methylation or N-acylation.
Scheme 2 illustrates the synthesis of the 3-vinyl-
indoles 4 and 5. An attempt to prepare the N-methyl-3-
vinylindole 4 by N-methylation followed by Wittig reac-
tion encountered two problems. One is the difficulty of pu-
rification of the intermediate compound 8, presumably due
to its high polarity, and the other is the poor stability of 4,
which easily decomposed in the air. We attributed the insta-
bility of 4 to the electron-donating character of the methyl
group, which increases the basicity of the pyrrole ring and
Fig. 1. Three prototypes of pentiptycene-derived mo-
lecular brakes operated by using photonic and/
or electric energies: (a) photochemical E ® Z
and electrochemical Z ® E (mode II), (b) two-
way photochemical switching (mode I), and (c)
dual modes of I and II.
3
8
thus the reactivity of the vinyl group. Therefore, we intro-
duced the electron-withdrawing tert-butyl carbonyl group
to 7 with the reagents di-tert-butyl dicarbonate and DMAP.
The product 9 was then reacted with methyl ylide to form 5,
which can be readily purified by column chromatography.
3
5,36
cal cation, can serve as a switching module (Fig. 1c).
The results show that the braking effect in 3Me and 3An
6
8
reaches a factor of 10 -10 , comparable to the case of 2, and
the switching could be operated by light alone (mode I, as
in Fig. 1b) or by combined light and electrical energy
Scheme 2 Synthesis of the vinylindoles 4 and 5
(
mode II, as in Fig. 1a). The switching efficiency of the
mode II is larger than that of the mode I. While the mode-I
switching efficiency is similar to that in 2, the mode-II effi-
ciency is lower than that observed for 1. The capacity of
electrochemical switching in terms of driving forces and
stability of redox intermediates are also discussed.
RESULTS AND DISCUSSION
t
Synthesis. The retrosynthetic analysis of 3Me and
In the presence of Pd
2
(dba)
3
and P( Bu)
3
, the reaction
3
3
An is shown in Scheme 1. The molecular brakes 3Me and
An are composed of a pentiptycene rotor and an indole
between compounds 5 and 6 formed the desired p-conju-
gated backbone (Scheme 3). After removing the Boc group
with trifluoroacetic acid, the resulting compound 11 was
pad, which were linked by a vinylene at the C3 position of
the indole and at the central phenylene of the pentiptycene.
This could be achieved by the Heck coupling between
N
converted to the target compounds 3Me and 3An by S 2 re-
action with methyl iodide and by C-N cross coupling reac-
tion with 4-iodoanisole, respectively. The Z isomers Z-
3Me and Z-3An were obtained from photoisomerization of
the corresponding E isomers under irradiation of 365 nm
3
-vinylindoles (4 and 5) and the bromo-substituted penti-
3
7
ptycene 6, a known compound. The 3-vinylindoles could
be in turn prepared from the commercially available 3-
508
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Light- and Redox-Gated Molecular Brakes
1
3
Scheme 3 Synthesis of 3Me and 3An
Z-3Me. The VT C NMR and the simulated spectra of
Z-3Me and Z-3An are shown in Fig. 3 and Fig. 4, respec-
tively, and pertinent kinetic parameters, including the acti-
‡
‡
vation energy (E ), enthalpy (DH ), entropy (DS ) and the
a
‡
free energy of activation (DG ), from Arrhenius and Eyring
plots (Figure S9-11) are listed in Table 1. Table 1 also
shows the coalescence temperature (T
c
) for the C1, C1’and
C4, C4’ signals of Z-3Me and Z-3An and the correspond-
‡
ing energy barrier (DG ). The results show that the rotor in
c
-
1
Z-3An (13.5 kcal mol ) encounters a larger activation en-
-
1
ergy than that in Z-3Me (9.8 kcal mol ), which is consis-
tent with the bulkier anisole vs methyl group. The dramatic
‡
difference in the entropy (DS ) factor between Z-3Me and
light. The separation between the E and Z isomers required
a preparative HPLC apparatus.
Rotation Kinetics in Brake-on States. The rotation
kinetics of the pentiptycene rotor in Z-3Me and Z-3An has
been investigated by variable-temperature (VT) NMR
spectroscopy. On the basis of a series of 2D NMR experi-
ments, including NOESY, ROESY, COSY and HSQC (Fig-
ure S1-S8), the proton and carbon signals in the pentipty-
1
13
cene peripheral phenylene region of the H and C NMR
spectra determined at 243 K can be unambiguously as-
signed (Fig. 2). The line-shape analysis was carried out
Fig. 3. (a) Experimental and (b) simulated VT 13C
NMR spectra in the region of signals for the
pentiptycene peripheral phenylene of Z-3Me
1
3
1
with the C NMR rather than the H NMR spectra, because
the proton signals have serious overlapping in the case of
(
6
DMF-d , 125 MHz). Values of temperature (T
-
1
in K) and interconversion rate (k in s ) between
the two isoenergetic states are also given for ev-
ery trace.
Fig. 4. (a) Experimental and (b) simulated VT 13C
NMR spectra in the region of signals for the
pentiptycene peripheral phenylene of Z-3An
Fig. 2. H (500 MHz) and 13C (125 MHz) NMR spectra
1
of the pentiptycene peripheral phenylene re-
gion of (a) Z-3Me and (b) Z-3An in DMF-d
6
at
6
(DMF-d , 125 MHz). Values of temperature (T
-
1
2
43 K. The alphabets and Arabic numerals de-
in K) and interconversion rate (k in s ) between
the two isoenergetic states are also given for ev-
ery trace.
note the corresponding protons and carbon at-
oms labeled in Figure 1.
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13
Table 1. VT C NMR data and activation parameters for the rotation of the pentiptycene rotator in Z-3Me and Z-3An
‡
DG c
‡
‡
‡
DG (298 K)
Tc
Ea
DH
DS
k
(293K)
Nuclei
logA
-
1
- 1
- 1
- 1
- 1
- 1
[
K]
[kcal mol ] [kcal mol ]
[kcal mol ] [kcal mol ] [kcal mol ]
(s )
Z-3Me
C(1,1’)
C(4,4’)
263
263
313
13.4
13.4
15.3
9.8 ± 0.3
9.5 ± 0.5
13.5 ± 0.5
9.8 ± 0.3
9.5 ± 0.4
11.6 ± 0.4
9.2 ± 0.3
9.0 ± 0.5
13.0 ± 0.5
-15.7 ± 1.2
-16.7 ± 1.7
-7.6 ± 1.7
13.9 ± 0.3
13.9 ± 0.7
15.2 ± 0.7
301
297
24
Z-3An C(1,1’;4,4’)
Z-3An is noticed, indicating that the shape of brake pad
could significantly modify the rotation kinetics.
styrylindole moiety in both E-3Me and Z-3Me. The dihed-
o
ral angle (j, defined in Fig. 1c) is ~50 for E-3Me, and it in-
o
Rotation Kinetics in Brake-off States. Unlike the
two sets of signals for the pentiptycene methine nuclei in
Z-3Me and Z-3An, only one set of signals could be ob-
served for the NMR spectra of E-3Me and E-3An even at
creases to 72 for Z-3Me. The methoxy group could be in
syn or anti orientation of the vinylindole group with respect
to the central ring of pentiptycene. Nevertheless, the syn
and anti conformers are nearly isoenergetic. In the case of
Z-3Me, rotation about the indole-vinylene C-C bond also
leads to two distinct conformers, in which the one (Z-
2
23 K, indicating a fast rotation of the rotor. A quantitative
description of the rotation rates for the rotor in the E iso-
mers could be achieved by DFT calculations. In this con-
text, DFT calculation at the BMK/6-311+G (d, p)//B3LYP/
3
Me-a, Fig. 5b) with the N-methyl group pointing toward
the U-shaped cavity of the pentitpycene rotor is lower by
6
-31G (d) level was carried out for E-3Me. To simplify the
-1
~
4.0 kcal mol than the one (Z-3Me-b, Fig. 5b) with the
calculation, the octyloxy group was replaced by a methoxy
group. To justify the results of the DFT calculations, the
corresponding calculation on Z-3Me was also performed
and the results were compared with the experimental data
evaluated by VT NMR.
indolyl 6-membered ring facing the U-cavity of pentipty-
cene. The rotational transition states possess a nearly pla-
o
nar geometry for the styryl moiety (i.e., j ~ 0 ) for all cases.
The calculated rotation barriers are 4.76 and 14.79 kcal
-1
mol for E-3Me and Z-3Me, respectively.
Fig. 5 shows the optimized ground-state (GS) and
transition-state (TS) structures of E-3Me and Z-3Me along
the rotation coordinates. It is unexpected that E-3Me and
Z-3Me are essentially isoenergetic with an energy differ-
The DFT-derived rotation barriers for Z-3Me agree
-1
satisfactorily with the experimental data (13.9 kcal mol ),
which justifies the method of DFT calculations. A differ-
‡
-1
ence of the rotational barrier (DDG = 10.03 kcal mol ) be-
tween the E and Z isomers of 3Me indicates a change of ro-
tation rate in 7-8 order of magnitude. Such a braking per-
formance is comparable to the cases of compound 2R (R =
-
1
ence of only 0.02 kcal mol in the ground state. Because of
the bulky pentiptycene group, it is not coplanar with the
2
4
OMe).
Fig. 5. DFT-derived ground state (GS) and the transi-
tion state (TS) structures of (a) E-3Me and (b)
Z-3Me along the rotation coordinate about the
aryl-vinyl bond. The units of labeled distances
and angles are Å and degree, respectively.
Fig. 6. The UV absorption spectra of E and Z-isomers
of 3Me and 3An in CH Cl
2
2
.
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Light- and Redox-Gated Molecular Brakes
Photochemical Switching. The electronic absorp-
tion spectra of 3Me and 3An in dichloromethane are shown
in Fig. 6. The large similarity of absorption spectra between
Table 2. Ratio of the E and Z isomers of 3Me and 3An at the
photostationary state at different excitation wavelengths
determined by HPLC analysis
lex (nm)
375
370
290
250
3
Me and 3An in either the E or the Z form indicates that the
N-substituent plays a little role in the electronic transitions.
According to equation 1, the molar fraction of the E and Z
isomers at the photostationary state (pss) was determined
by their relative absorptivity at the irradiation wavelength
and their relative isomerization quantum yields. Therefore,
the optimum E ® Z and Z ® E switching should be con-
ducted at longer and shorter wavelengths, respectively.
Table 2 reports the E:Z ratio of 3Me and 3An in di-
chloromethane at pss under irradiation at 375, 370, 290,
and 250 nm. While ~ 90% of Z-3Me could be obtained
upon irradiation of E-3Me at 370 or 375 nm light, the best
recovery of E-3Me is only of 50% upon irradiation of the
mixture at 250 nm. Compound 3An displays a similar E:Z
ratio to that of 3Me at pss upon irradiation at the same
wavelengths. The predicted overall photochemical E-Z
switching efficiencies are 41% and 42% for 3Me and 3An,
respectively. Such a switching efficiency is similar to the
cases of 2 (20-46%).
[E-3Me]:[Z-3Me]
9:91
6:94
10:90
8:92
43:57
45:55
50:50
48:52
[
E-3An]:[Z-3An]
shown in Fig. 7 and Fig. 8, respectively. The results indi-
cate that photobleaching has occurred for 3Me but it is less
significant for 3An. Photobleaching was more severe when
the 250 instead of the 290-nm light was employed for the Z
®
E switching. The lower photostability for 3Me vs 3An
might be associated with its lower oxidation potential (vide
infra), which facilitates charge transfer in the excited state.
Electrochemical Switching. To improve the Z ® E
switching efficiency, we have investigated the electro-
chemical method with the concept illustrated by equation
2
. It is reasoned that one-electron oxidation of the indole
+
·
group of the Z isomer would lead to a radical cation Z , in
which the double bond character of the vinyl group is ex-
pected to be decreased and thus an equilibrium favoring the
more stable E-isomers could be achieved. Unfortunately,
the CV curves show that the first anodic reaction is essen-
tially irreversible for both 3Me and 3An (Fig. 9), indicating
a high activity of the oxidized intermediates. To minimize
the undesired reactions and thus the decomposition of these
[
E]/[Z]PSS = (e
Z
F
ZE)/(e
E
F
EZ
)
(1)
The spectral changes with alternating 370- or 375-
and 290-nm irradiation of 3Me and 3An for five cycles are
-
1
intermediates, we employed a high scan rate of 200 mVs
Fig. 7. UV absorption spectra of E-3Me (curve a) and
Z-3Me (curve d) and the photostationary states
under 370 nm irradiation (curve c) and 290 nm
Fig. 8. UV absorption spectra of E-3An (curve a) and
Z-3An (curve d) and the photostationary states
under 375 nm irradiation (curve c) and 290 nm
(curve b). Inset shows the change of the absorp-
tivity at 320 nm starting from E-3An for 5
switching cycles.
(
curve b). Inset shows the change of the absorp-
tivity at 320 nm starting from E-3Me for 5
switching cycles.
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for the potential sweep in the Z ® E switching to shorten
3An under continued operation of photochemical E ® Z
and electrochemical Z ® E switching was also investigated
by UV spectroscopy. As shown in Fig. 11, both systems
have poor stability; the absorbance at 320 nm decreases to a
significant extent after each cycle of photo- and electro-
chemical treatment (see the inset in Figure 11). Since the
two-way photochemical switching is more robust for both
3Me and 3An (vide supra), the poor stability observed for
the combined photochemical and electrochemical switch-
ing operation could be mainly attributed to the latter pro-
cess, which is consistent with the irreversible CV behavior.
It is possible that the nonplanar p-conjugated backbone in
3Me and 3An might localize radical cation in the indole
the lifetime of the oxidized intermediates. The applied po-
+
tential range is 0.0-0.51 V and 0.0-0.60 V (vs. Fc/Fc ) for
Z-3Me and Z-3An, respectively. On the basis of UV-Vis
spectra analysis (Fig. 10), the yields of E-3Me and E-3An
are 73% (80 scans) and 50% (100 scans), respectively.
These yields are significantly lower than that for the elec-
2
2
trochemical Z-1 ® E-1 switching (96%). The lower
yields for 3Me and 3An could be in part attributed to the
smaller thermodynamic driving forces for the equilibrium
between the Z and E isomers. On the basis of DFT calcula-
tions on 3Me, the E and Z isomers are nearly isoenergetic
in the neutral form (vide supra), and the Z ® E driving
-
1
39
force becomes somewhat larger (3.28 kcal mol ) in the cat-
ion radical state. Such a driving force is lower than the case
group, which possesses a substantial chemical reactivity.
The N-substituent appears to have little effect on the stabil-
-
1
of 1, in which the E isomer is 5.86 kcal mol lower than the
ity of the radical cation states.
2
2
Z isomer even in the neutral form. As a result of the small
deriving force for the electrochemical Z ® E isomeriza-
tion, the highest switching efficiency with photochemical
E ® Z and electrochemical Z ® E isomerization is only
CONCLUSIONS
While E-Z isomerization could be conducted either
photochemically or electrochemically, this work demon-
6
3% for 3Me and 44% for 3An.
-
-
-
e
×+
×+
+ e
Z ¾¾®[Z] „ [E] ¾ ¾¾ ®E
(2)
The overall stability of molecular brakes 3Me and
Fig. 10. The UV absorption spectra of (a) Z-3Me and
(
b) Z-3An before and after CV scans (80 cy-
+
Fig. 9. The anode redox waves of (a) 3Me and (b) 3An
in CH Cl with electrolyte 0.1 M Bu NPF at a
scan of 100 mVs .
cles in 0.0-0.51 V vs Fc/Fc for Z-3Me and
100 cycles in 0.0-0.60 V vs. Fc/Fc for Z-
+
2
2
4
6
-
1
3An).
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Light- and Redox-Gated Molecular Brakes
EXPERIMENTAL
Methods: NMR spectra were acquired on a Bruker DMX
5
00 or Bruker AVIII 400 spectrometer using 5 mm gradient TBI
1
13
and TBO probes, respectively. The chemical shifts for H and
C
1
spectra were referenced to the signals of chloroform-d
1
(d( H) =
1
3
1
13
7
1
7
.24 and d( C) = 77.00), or DMF-d (d( H) = 8.01 and d( C) =
62.70). COSY, NOESY, ROESY and HSQC spectra were mea-
sured using pulse sequences in the Bruker software package. In
the case of variable-temperature measurements, actual sample
1
temperature was well calibrated by H signals of ethylene glycol
and methanol so that the temperature error was assured to be
within ±1 K. Signal acquisition was begun after a sufficient tem-
perature equilibration time (10-15 minutes). The fitting of dy-
namic NMR line shapes at different temperatures was performed
with the Topspin 2.0 program, Bruker BioSpin Group. Infrared
spectra were recorded on a Nicolet Magna-IR 550 Spectrometer
Series II. UV/Vis absorption spectra were measured on a Varian
Cary300 Bio-type at room temperature. Photoswitching experi-
-
5
ments were conducted on N
a 75-W Xe arc lamp and monochromator. The cyclic voltammetry
CV) data were recorded on a CHI612B electrochemical analyzer
2
-bubbled solutions (10 M) by using
(
at room temperature, and a glassy carbon electrode served as the
working electrode. The substrates were 0.5 mM in dichloro-
4 6
methane containing 0.1 M Bu NPF as supporting electrolyte.
Fig. 11. The UV absorption spectra of (a) E-3Me and
(
3
b) E-3An before (curve a) and after 370- and
75-nm light irradiation (curve b) and fol-
The potentials were calibrated against SCE by the addition of
+
ferrocene (Fc) as an internal standard, taking E
versus SCE in CH Cl
carried out in a 1 mm quartz cell with N
containing 0.5 mM substrate and 0.1 M Bu
0
(Fc/Fc ) = 0.52 V
lowed by 80 and 100 CV scans (curve c) and
then with irradiation by 370- and 375-nm light
(
2
2
. The bulk electrochemical oxidation was
-bubbled DCM solutions
NPF as electrolyte. A
2
curve d), respectively. Inset shows the
change of the absorptivity at 320 nm starting
4
6
from E-isomer for 3.5 and 3 switching cycles.
Pt grid was used as the working electrode, a Pt wire as the counter
electrode, and an Ag wire as the reference electrode. DFT calcula-
4
0
strates that the reliability of the latter is much more sensi-
tive to the chemical structures. For 3Me and 3An, the radi-
cal cations generated by electrochemical oxidation are re-
active and thus the electrochemcial Z®E switching is un-
reliable, leading to compound decomposition. In contrast,
the photochemical Z ® E isomerization is much more ro-
bust. Therefore, molecular brakes 3Me and 3An, which
possess a pronounced brake performance with a difference
in rotation as large as 7-8 orders of magnitude between the
brake-on and brake-off states, are better gated by light
rather than by the dual fuels of light and electrical energy.
Although photon-electron dual-fuel system remains to be
attractive in constructing highly efficient molecular de-
vices, the chemical stability of the redox state is of prime
factor for consideration in the design of new molecular de-
vices.
tions were performed with the Gaussian 09 program. Relative
energies were calculated at B3LYP/6-31g* level and BMK/6-
4
1
311+G (d, p)// B3LYP/6-31G (d) level. Free energy values at
BMK level 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.
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 synthe-
sis were reagent grade. Detailed synthetic procedures and struc-
tural characterization data for are provided below. It should be
4
2
43
44
noted that compounds 5, 8, and 9 are known compounds.
Compound 5: A solution of PPh MeI (1.0 g, 2.5 mmol) in
dry 10 mL of THF was cooled to 0 °C under nitrogen atmosphere.
3
J. Chin. Chem. Soc. 2014, 61, 507-516
© 2014 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
513

Invited Paper
Kao et al.
-
1
n-butyllithium (1.7 mL, 1.6 M in hexane) was injected drop. The
reaction mixture was stirred at room temperature for 1 h and then
injected the solution of compound 9 (0.5 g, 2.0 mmol, dissolved in
(KBr): 2978, 2937, 2814, 2736, 1742, 1679, 1556, 1359 cm .
Compound 10: Compound 6 (0.32 g, 0.50 mmol) and
Tris(dibenzylidene-acetone)dipalladium (90 mg, 0.10 mmol) was
dissolved in 3.0 mL of dry DMF in nitrogen atmosphere, and then
tri-tert-butylphosphine (2.4 mL, 0.05 M in toluene) and the solu-
tion of compound 7 (0.22 g, 0.9 mmol dissolved in 2.0 mL of
triethylamine) was injected. The reaction mixture was heated to
90 °C, and stirred for 16 h. After cooled to room temperature, the
reaction mixture was extracted with dichloromethane and brine.
5
.0 mL of dry THF). The reaction mixture was stirred another 16 h
at room temperature, and then quenched with water, washed with
NH Cl and brine, and extracted with dichloromethane. The or-
ganic layer was dried over anhydrous MgSO , and removed sol-
4
4
vent with reduced pressure. Column chromatography with di-
chloromethane/hexane (v/v, 30/70) gave the pure product as col-
orless oil with a yield of 91%. The product is easily decomposed
4
The organic layer was dried over anhydrous MgSO , and removed
1
in atmosphere. H NMR (400 MHz, CDCl
3
) d: 1.66 (s, 9H), 5.31
the solvent with reduced pressure. Column chromatography with
(
dd, J = 11.2 Hz, 1.2 Hz, 1H), 5.79 (dd, J = 17.6 Hz and 1.2 Hz,
dichloromethane/hexane (v/v, 30/ 70) gave compound 10 as white
1
1
H), 6.80 (dd, J = 17.6 Hz, 11.2 Hz, 1H), 7.26 (ddd, J = 7.6 Hz, 7.6
solid with a yield of 72%. m.p. = 213-214 °C. H NMR (400 MHz,
Hz and 1.2 Hz, 1H), 7.33 (ddd, J = 6.8 Hz, 6.8 Hz and 1.2 Hz, 1H),
3
CDCl ) d: 0.94 (t, J = 7.0 Hz, 3H), 1.37-1.49 (m, 8H), 1.67 (quin,
1
3
7
.61 (s, 1H), 7.78 (d, J = 7.2 Hz, 1H), 8.16 (d, J = 8.0 Hz, 1H);
C
J = 7.6 Hz, 2H), 1.76 (s, 9H), 1.98-2.05 (tt, J = 6.8 and 7.6 Hz,
2H), 3.94 (t, J = 6.8 Hz, 2H), 5.70 (s, 2H), 5.74 (s, 1H), 6.79 (d, J =
16.4 Hz, 1H), 6.90-6.93 (m, 8H) 7.26-7.28 (m, 4H), 7.30-7.32 (m,
4H), 7.41-7.48 (m, 2H), 7.55 (d, J = 16.4 Hz, 1H), 7.91 (s, 1H),
3
NMR (100 MHz, CDCl ) d: 28.2, 83.8, 114.4, 115.3, 119.2, 120.0,
1
2
22.9, 124.0, 124.6, 128.1, 128.7, 135.9, 149.6; IR (KBr): 2970,
-
1
391, 1733, 1636 cm .
Compound 8: Indole-3-carboxaldehyde 7 (1.0 g, 6.9
mmol) was dissolved in 10 mL of THF. Sodium hydride (0.50 g,
2.5 mmol; 60% suspension in paraffin oil) was slowly added into
1
3
7.96 (d, J = 8.0 Hz, 1H), 8.27 (d, J = 7.2 Hz, 1H); C NMR (100
MHz, CDCl ) d: 14.2, 22.8, 26.5, 28.3, 29.4, 29.6, 30.6, 31.9,
3
1
48.2, 51.2, 76.1, 84.3, 115.7, 119.0, 119.9, 123.3, 123.5, 123.6,
123.8, 124.4, 125.0, 125.1, 126.6, 127.3, 129.1, 134.9, 136.1,
142.2, 125.3, 145.6, 148.9, 149.8; IR (KBr): 3066, 3018, 2927,
the solution in room temperature and stirred for 10 minutes.
Methyl iodide (4.5 g, 7.1 mmol) was then slowly injected into the
reaction mixture with iced bath, and stirred in room temperature
-
1
+
1735, 1456, 1369 cm ; FAB-HRMS calcd. for C57
799.4025, found 799.4025.
3
H53NO (M )
4
for 2 h. The reaction mixture was washed with NH OH and brine.
Column chromatography with EA/hexane (v/v, 30/70) gave the
Compound 11: Compound 10 (0.30 g, 0.38 mmol) was dis-
solved in 20 mL of dichloromethane, and trifluoroacetic acid (3.0
g, 2.0 mL, 26 mmol) was injected into the solution with iced bath.
The reaction mixture was stirred in room temperature for 3 h, and
then quenched by water, extracted with dichloromethane. Fresh
column with dichloromethane and recrystallized with methanol
pure product as white solid with a yield of 66%. m.p. = 67-68 °C
1
H NMR (400 MHz, CDCl
.65 (s, 1H), 8.29 (d, J = 6.8 Hz, 1H), 9.97 (s, 1H); C NMR (100
MHz, CDCl ) d: 33.67, 109.80, 118.13, 122.05, 122.93, 124.02,
25.31, 137.88, 139.07, 184.36.
Compound 9: Indole-3-carboxaldehyde 7 (1.0 g, 6.9
3
) d: 3.85 (s, 3H), 7.29-7.35 (m, 3H),
1
3
7
3
1
gave the product as white solid with a yield of 76%. m.p. = 155-
1
mmol) was dissolved in 20 mL of acetonitrile, and di-tert-butyl
dicarbonate (1.5 g, 6.9 mmol) and 4-dimethylaminopyridine (50
mg, 0.41 mmol) were added into the solution under ice bath. The
solution was stirred at room temperature for 2 h. After the aceto-
nitrile was dried with reduced pressure, the reaction mixture was
dissolved in 20 mL of dichloromethane and extracted with water
157 °C. H NMR (400 MHz, CDCl
3
) d: 0.94 (t, J = 7.0 Hz, 3H),
1.35-1.50 (m, 8H), 1.67 (quin, J = 7.6 Hz, 2H), 1.98-2.05 (quin, J
= 7.0 Hz, 2H), 3.93 (t, J = 6.8 Hz, 2H), 5.69 (s, 2H), 5.80 (s, 1H),
6.86 (d, J = 16.8 Hz, 1H), 6.90-6.92 (m, 8H), 7.26-7.28 (m, 4H),
7.30-7.32 (m, 4H), 7.34-7.36 (m, 2H), 7.45 (d, J = 16.8 Hz, 1H),
1
3
7.50-7.53 (m, 2H), 8.10 (m, 1H), 8.34 (br, 1H); C NMR (100
MHz, CDCl ) d: 14.2, 22.7, 26.5, 29.4, 29.6, 30.6, 31.9, 48.3,
1.2, 76.1, 111.6, 115.7, 120.1, 120.9, 123.0, 123.4, 123.5, 123.6,
25.0, 125.1, 125.8, 127.5, 129.0, 134.7, 137.0, 142.1, 145.5,
4
and brine. The organic layer was dried over anhydrous MgSO ,
3
5
1
1
and the solvent was removed with reduced pressure. The residue
was purified by recrystallize with hexane to give compound 9 as
-
1
1
45.8, 148.6; IR (KBr): 3419, 3064, 3018, 2925, 2854, 1458 cm .
white powder with a yield of 97%. m.p. =114-116 °C. H NMR
+
FAB-HRMS calcd. for C52
99.3497.
Compound E-3Me: Compound 11 (82 mg, 0.12 mmol)
H
45NO (M ) 699.3501, found
(
400 MHz, CDCl
3
) d: 1.69 (s, 9H), 7.35 (ddd, J = 7.6 Hz, 7.6 Hz
6
and 1.2 Hz, 1H), 7.40 (ddd, J = 7.2 Hz, 7.2 Hz and 1.2 Hz, 1H),
8
1
1
.13 (d, J = 7.6 Hz, 1H), 8.21 (s, 1H), 8.27 (d, J = 7.2 Hz, 1H),
1
3
was dissolved in 4 mL of THF. Sodium hydride (10 mg, 0.15
mmol; 60% suspension in paraffin oil) was slowly added into the
0.09 (s, 1H); C NMR (100 MHz, CDCl
3
) d: 28.1, 85.6, 115.1,
21.6, 122.1, 124.6, 126.1, 126.1, 136.0, 136.4, 148.8, 15.7; IR
514
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J. Chin. Chem. Soc. 2014, 61, 507-516

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Light- and Redox-Gated Molecular Brakes
solution in room temperature and stirred for 10 minutes. Methyl
iodide (0.40 g, 0.28 mmol) was then slowly injected into the reac-
tion mixture with iced bath, and stirred at room temperature for 2
action mixture was extracted with dichloromethane and water.
4
The organic layer was dried with anhydrous MgSO , and concen-
trated under reduced pressure. Column chromatography with di-
chloromethane/hexane (v/v, 35/65) gave the pure product as
4
h. The reaction mixture was washed with NH OH and brine. Col-
1
umn chromatography with dichloromethane/hexane (v/v, 30/70)
white solid with a yield of 62%. m.p. = 266-268 °C. H NMR (400
gave the pure product as white solid with a yield of 89%. m.p. =
3
MHz, CDCl ) d 0.94 (t, J = 6.8 Hz, 3H), 1.37-1.49 (m, 8H), 1.67
1
2
3
28-230 °C. H NMR (400 MHz, CDCl
3
) d: 0.93 (t, J = 7.0 Hz,
(quin, J = 7.6 Hz, 2H), 1.98-2.06 (tt, J = 6.8 and 7.6 Hz, 2H), 3.94
(t, J = 6.8 Hz, 2H), 3.91 (s, 1H), 5.70 (s, 2H), 5.82 (s, 1H), 6.88-
6.94 (m, 9H), 7.09 (s, 1H), 7.12 (s, 1H), 7.27-7.32 (m, 8H), 7.35-
7.38 (m, 2H), 7.49 (d, J = 16.8 Hz, 1H), 7.52-7.57 (m, 3H), 7.61
H), 1.37-1.49 (m, 8H), 1.67 (quin, J = 7.2 Hz, 2H), 1.98-2.05 (tt,
J = 6.8 and 7.2 Hz, 2H), 3.93 (t, J = 6.8 Hz, 2H), 3.92 (s, 3H), 5.69
s, 1H), 5.79 (s, 2H), 6.84 (d, J = 16.4 Hz, 1H), 6.81-6.93 (m, 8H)
.25-7.27 (m, 4H), 7.29-7.32 (m, 4H), 7.35-7.46 (m, 5H), 8.09 (d,
(
1
3
7
3
(s, 1H), 8.13 (m, 1H); C NMR (100 MHz, CDCl ) 14.2, 22.8,
1
3
J = 7.6 Hz, 1H); C NMR (100 MHz, CDCl
3
) d: 14.2, 22.7, 26.5,
26.5, 29.4, 29.6, 30.6, 32.0, 48.3, 51.2, 55.7, 76.1, 111.0, 114.9,
115.7, 120.2, 121.0, 121.1, 123.1, 123.5, 123.6, 125.0, 125.1,
126.3, 126.8, 127.4, 127.5, 128.5,132.3, 134.8, 137.5, 142.1,
145.5, 145.8, 148.6, 158.7; IR (KBr): 3065, 3018, 2927, 2854,
2
1
1
2
7
9.4, 29.6, 30.6, 32.0, 48.3, 51.2, 76.0, 109.8, 114.1, 120.0, 120.1,
20.4, 122.5, 123.4, 123.6, 125.0, 126.3, 127.6, 128.3, 128.9,
34.7, 137.8, 142.1, 145.5, 145.8, 148.5; IR (KBr): 3065, 3018,
-
1
+
-1
+
924, 2853, 1459 cm ; FAB-HRMS calcd for C53
13.3658, found 713.3660.
H
47NO (M )
1512, 1459 cm ; FAB-HRMS calcd for C59
H
51NO
2
(M )
805.3920, found 805.3912.
Compound Z-3Me. E-3Me: (7.0 mg, 0.01 mmol) was
Compound Z-3An. E-3An (8.0 mg, 0.01 mmol) was
placed in a quartz tube, and dissolved in 20 mL of dichloro-
methane. Bubble the solution with nitrogen for 10 minutes, and ir-
radiate it with 365 nm lamps for 45 minutes. The solvent was
dried with reduced pressure. Purification is carried out by pre-
parative HPLC (dichloromethane/hexane = 20/80 as eluent; flow
rate = 10 mL/min). The pure product was given as white solid
placed in a quartz tube, and dissolved in 20 mL of dichloro-
methane. Bubble the solution with nitrogen for 10 minutes, and ir-
radiate it with 365 nm lamps for 45 minutes. The solvent is dried
with reduced pressure. Purification is carried out by preparative
HPLC (THF/hexane = 30/70 (v/v) as eluent; flow rate = 10 mL/
min). The pure product was white solid with a yield of 72%. m.p.
1
1
=
114-116 °C. H NMR (400 MHz, DMF-d
6
) d: 0.91 (t, J = 6.0 Hz,
with a yield of 76%. m.p. = 115-117 °C. H NMR (400 MHz,
3
H), 1.36-1.52 (m, 8H), 1.69 (quin, J = 6.0 Hz, 2H), 2.01-2.06 (tt,
J = 5.6 and 6.0 Hz, 2H), 2.82 (s, 3H), 4.07 (t, J = 5.6 Hz, 2H), 5.31
s, 1H), 5.61 (s, 2H), 5.89 (s, 2H), 5.40 (br, 4H), 6.72 (d, J = 9.2
Hz, 1H), 6.91 (br, 6H), 7.13 (t, J = 6.0 Hz, 1H), 7.20 (t, J = 5.6 Hz,
H), 7.31 (d, J = 6.8 Hz, 1H), 7.41-7.44 (m, 5H), 7.85 (d, J = 6.0
Hz, 1H). Because the coalescence temperature for Z-3Me is at
6
DMF-d ) d 0.93 (t, J = 5.6 Hz, 3H), 1.35-1.47 (m, 8H), 1.63 (quin,
J = 6.0 Hz, 2H), 1.97 (tt, J = 5.6 and 6.0 Hz, 2H), 3.76 (s, 3H), 3.93
(t, J = 5.6 Hz, 2H), 5.15 (s, 1H), 5.65 (s, 1H), 5.83 (s, 2H), 6.19 (d,
J = 7.2 Hz, 2H), 6.83 (br, 2H), 6.89 (d, J = 6.8 Hz, 1H), 6.94 (br,
4H), 7.21-7.24 (m, 2H), 7.25-7.29 (m, 1H), 7.34 (br, 8H), 7.50 (d,
(
1
1
3
J = 9.2 Hz, 1H), 7.97 (d, J = 5.2 Hz, 1H); C NMR (125 MHz,
DMF-d ) 14.3, 23.2, 26.8, 30.8, 32.5, 48.6, 51.8, 55.7, 76.7,
1
3
2
93 K, the integration is not reliable for broad signals. C NMR
6
(
125 MHz, DMF-d ) d: 14.3, 23.1, 26.8, 30.8, 32.3, 32.5, 48.6,
6
111.7, 113.4, 114.6, 119.3, 121.0, 121.1, 123.1, 123.7, 124.0,
124.4, 125.2, 125.5, 125.8, 127.8, 128.0, 129.1, 130.8, 131.9,
134.1, 135.8, 136.3, 142.9, 146.6, 149.3, 158.5; IR (KBr): 3065,
5
1
1
1.8, 76.5, 110.2, 111.6, 119.0, 120.1, 120.3, 122.4, 123.9, 124.3,
25.4 (2C), 125.5 (2C), 127.7, 128.4, 129.9, 136.2, 136.8, 142.9,
-
1
-1
+
46.5, 149.2; IR (KBr): 3064, 2923, 2852, 1633, 1458 cm .
3017, 2927, 2854, 1513, 1459, 1249 cm . HRMS (ESI ) calcd for
59 4
C H51NNaO 828.3817, found 828.3812.
+
FAB-HRMS calcd for C53
H
47NO (M ) 713.3658, found 713.3645.
Compound E-3An: Compound 11 (70 mg, 0.10 mmol) and
Tris(dibenzylidene-acetone)dipalladium (9.0 mg, 0.01 mmol)
was in nitrogen atmosphere, and tri-tert-butyl phosphine (0.30
mL, 0.05 M in toluene) was injected into the solution. The solu-
tion was heated to 110 °C and stirred for 10 minutes. The solution
of 4-iodoanisole (26 mg, 0.11 mmol dissolved in 1 mL toluene,
bubbled with nitrogen for 10 minutes) was injected and heated to
reflux, stirred for 16 h. After the solution was cooled to room tem-
perature, the solvent was removed with reduced pressure. The re-
ACKNOWLEDGEMENTS
Financial support for this research was provided by
the National Science Council of Taiwan. The authors thank
Prof. Ying-Chih Lin and Ms. Shou-Ling Huang (NTU) for
the support of VT NMR measurements. The computing
time granted by the National Center for High-Performance
Computing and the Computing Center of National Taiwan
University is acknowledged.
J. Chin. Chem. Soc. 2014, 61, 507-516
© 2014 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.jccs.wiley-vch.de
515

Invited Paper
Kao et al.
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