An antilock molecular braking system

Article abstract of DOI:10.1021/ol3018193

A light-driven molecular brake displaying an antilock function is constructed by introducing a nonradiative photoinduced electron transfer (PET) decay channel to compete with the trans (brake-off) → cis (brake-on) photoisomerization. A fast release of the brake can be achieved by deactivating the PET process through addition of protons. The cycle of irradiation- protonation-irradiation-deprotonation conducts the brake function and mimics the antilock braking system (ABS) of vehicles.

Full text of DOI:10.1021/ol3018193

ORGANIC
LETTERS
2012
Vol. 14, No. 16
4154–4157
An Antilock Molecular Braking System
Wei-Ting Sun,† Shou-Ling Huang,† Hsuan-Hsiao Yao,‡ I-Chia Chen,*,‡ Ying-Chih Lin,†
and Jye-Shane Yang*,†
Department of Chemistry, National Taiwan University, Taipei, Taiwan 10617, and
Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 30013
icchen@mx.nthu.edu.tw; jsyang@ntu.edu.tw
Received July 3, 2012
ABSTRACT
A light-driven molecular brake displaying an antilock function is constructed by introducing a nonradiative photoinduced electron transfer (PET)
decay channel to compete with the trans (brake-off) f cis (brake-on) photoisomerization. A fast release of the brake can be achieved by
deactivating the PET process through addition of protons. The cycle of irradiationꢀprotonationꢀirradiationꢀdeprotonation conducts the brake
function and mimics the antilock braking system (ABS) of vehicles.
Photoinduced electron transfer (PET) plays a crucial role
in both biological and chemical systems. In photosynthetic
reaction centers,1 cascade PET converts the photon energy
to chemical energy. In photovoltaic devices,2 PET leads
to charge separation and creates electric power. Many
intensity-based photoluminescent sensors rely on PET to
quench the excited state.3 For moleculardevices, PET could
drive a linear or rotary motion.4 We demonstrate herein
a new application of PET, namely, to perform an antilock
function in photon-driven molecular brakes that mimics
the antilock braking system (ABS) equipped in vehicles.
The development of molecular machinery systems is an
important subject in nanoscience and nanotechnology.5
Effective control of linear or rotary molecular motions is
a prerequsite toward artificial molecular machines.6 Many
prototypes of controlled motions on the molecular level
† National Taiwan University.
‡ National Tsing Hua University.
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r
10.1021/ol3018193
Published on Web 08/02/2012
2012 American Chemical Society

channels could dynamically abort the isomerization and
thus increase the braking time. This phenomenon resem-
bles the ABS function that increases the braking distance
for vehicles on loose surfaces.12 To prove this concept,
we have introduced an amino group as in 1 to invoke a
nonradiativePET processtothe electronically excited state
of the stilbene moiety, given the fact that a tertiary amino
group is a potential electron donor for excited stilbenes.13
The PET process could compete but not inhibit the
isomerization reaction of the brake-off state (t-1) toward
the brake-on (t-1) state so that formation of a complete
brake state is delayed. Such a PET-based ABS function
could in principle be deactivated by protonation of the
amino group to perform the simple brake function as in the
case of 2.
Scheme 1. TransꢀCis Photoisomerization of 1 and 2a
a The numeric labels for protons and carbons are for discussion of the
NMR spectra.
have been reported and dubbed as shuttles, elevators,
brakes, and many others, in analogy to the operation of
the corresponding macroscopic objects.5,7ꢀ10 The energy
sources (fuels) for the operation are versatile, including
chemicals, light (photons), and electric or thermal energy.
A combination of different energy sources could improve
the operation efficiency8 and/or perform multiple motion
control9 or functions.10 We report herein a photon-driven
molecular brake (1) having a proton-gated antilock func-
tion; namely, the brake performance is driven by UV light,
but the brake operation is gated by the proton-responsive
amino group (Scheme 1).
Scheme 2. Synthesis of t-1 and c-1
The antilock braking system 1 was designed from
the recently reported photon-gated molecular brake 2.11
The trans and cis forms of 2 (t-2 and c-2) correspond to the
brake-off and brake-on states with a rotation rate of 109
and 102 sꢀ1, respectively, for the pentiptycene rotor at
room temperature. The 107-fold braking effect can be
further improved by increasing the size of the styryl
brake component through adding bulky substituents to
the phenyl group.11 The brake operation is also effective,
as the switching from the brake-off (t-2) to the brake-on
(c-2) state is nearly quantitative. Since the photoinduced
trans f cis isomerization is operating the brake function,
the presence of other competing nonradiative decay
The synthesis of 1 is shown in Scheme 2. The known
compound iodo-substituted pentiptycene phenol 314 un-
derwent a typical SN2 reaction with the hydrochloride salt
of 2-chloro-N,N-dimethyl-ethylamine15 to afford 4 in an
excellent yield (88%). The trans isomer of 1 (t-1) was then
prepared from the Heck reaction between styrene and 4.
The cis isomer c-1 was obtained on irradiation of t-1 in
THF at 365 nm.
The rotational barriers and activation parameters for
the pentiptycene rotor in c-1 have been evaluated by
variable-temperature (VT) NMR spectroscopy. Detailed
VT NMR spectra (Figures S1ꢀS5) and peak assignments
are supplied as Supporting Information. The rate constant
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Org. Lett., Vol. 14, No. 16, 2012
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(k) of rotation deduced by line shape analysis of theꢀC1(1)
and C(4) signals of the VT 13C NMR spectra is 670 s at
293 K (Figure 1). The k value is very close to that (686 sꢀ1
)
for the parent brake system c-2 at 298 K.11 The activation
barriers (Ea = 10.6 ( 0.6 kcal molꢀ1) and the enthalpy
(ΔH‡ = 10.1( 0.5 kcal molꢀ1) and entropy (ΔS‡ = ꢀ11.8
( 0.2 cal molꢀ1
K
ꢀ1) of activation derived from the
Arrhenius and Eyring plots (Figures S6 and S7) also
resemble those of c-2.11 Evidently, the amino substituent
imposes a negligible effect on the rotation kinetics of the
pentiptycene rotor. The rotation rate for the pentiptycene
rotorint-1isalsoexpectedtobe the sameasthat(∼109 sꢀ1
)
Figure 2. (a) UV absorption spectra of t-1 and c-1 in the absence
(solid lines) and presence (dash lines) of 10 equiv of HCl in
acetonitrile, and (b) transient absorption spectra of t-1 in
acetonitrile at 0.5ꢀ300 ps (λex = 266 nm).
in t-2, which was evaluated by density functional theory
calculations.11 Thus, the braking system 1 retains the
excellent performance of 2 with a 107-fold braking effect.
More information about the PET reaction is obtained
from time-resolved spectroscopy. The fluorescence life-
times (τf) for t-1 and t-2 in acetonitrile are 77 and 88 ps,
respectively.17 Provided that t-1 and t-2 correspond to the
PET and PET-free systems, the rate constant (kPET,
)
intra
for the proposed intramolecular PET is estimated to be
1.6ꢁ 109 sꢀ1 (i.e., (77ps)ꢀ1 ꢀ (88ps)ꢀ1). Onthe basisof Φtc
and τf, the rateconstants (ktc =Φtc/τf) for the t-1f c-1and
the t-2 f c-2 conversions are 1.4 ꢁ 109 and 5.5 ꢁ 109 sꢀ1
,
respectively. The difference in the ktc of t-1 vs t-2 (i.e., 4.1 ꢁ
109 sꢀ1) is larger than kPET, intra, which indicates that not
only the intramolecular PET but also the intermolecular
PET takes place in diminshing the Φtc. This can be under-
stood by the fact that the measurement of τf is in dilute
solutions (1 ꢁ 10ꢀ5 M) but that of Φtc is under 100-
fold more concentrated solutions. Intermolecular PET
between t-2 and triethylamine has been investigated. The
SternꢀVolmer plot (Figure S8) gives rise to a quenching
Figure 1. VT 13C NMR (left, 125 MHz) and simulated (right)
spectra of c-1 in CD2Cl2 at the region of C(1) and C(4) (see
Scheme 1 for labels). Values of temperature (T, K) and inter-
conversion rate (k, sꢀ1) between the two isoenergetic states
(i.e., 180° rotation) are also given for each trace.
constant of2.0 ꢁ 1010
s
ꢀ1, indicating a diffusion-controlled
PET process.
Another piece of evidence for the occurrence of PET in
t-1 is provided by the transient absorption spectra. Within
2 ps after excitation of t-1 at 266 nm, an absorption band
near 600 nm is detected (Figure 2b). The anion radical
of the parent trans-stilbene was reported to have an
absorption maximum near 500 nm.13a,18 The absorption
maximum is red-shifted for substituted stilbenes.19 The
observed signal for t-1 can thus be attributed to the anion
radical of the pentiptycene-derived stilbene.
The operation to reverse the brake-on c-1 state back to
the brake-off t-1 state was also investigated. The quantum
efficiency for the c-1 f t-1 conversion (Φct) is 0.20, which
is again lower than 0.37 for the c-2 f t-2 conversion.11 In
the presence of 10 equiv of HCl, the Φct increases to 0.45.
The transient absorption spectra for c-1 were also recorded
The photon-driven brake performance for t-1 was con-
ducted in acetonitrile. According to the UV absorption
spectra of t-1 and c-1 (Figure 2a), the wavelength 340 nm is
suitableforperforming the brakeoperation, asc-1has little
absorptivity at 340 nm. Indeed, the t-1 f c-1 conversion is
as high as 95% in the photostationary state. The quantum
efficiency for the t-1 f c-1 switching (Φtc) is 0.11, which is
much lower than that (0.48) for the reference system t-2
under the same conditions. The difference between t-1 and
t-2 reveals the effect of the amino group on Φtc. The amino
effect is deactivated by protons, as a high Φtc (0.49) is
recovered for t-1 in the presence of 10 equiv of HCl. The
relative efficiency for the t-1 f c-1 switching is shown in
Figure 3a. Whereas complete braking takes more than
175 min in the absence of protons (curve i), it takes ∼100 min
in the presence of protons (curve ii). It should be noted that
the presence of protons neither perturbs the absorption
profiles of both t-1 and c-1 (Figure 2a) nor induces the
t-1 f c-1 isomerization in the dark (curve iii of Figure 3a).
These observations indicate that PET occurs from the amino
group to the excited stilbene and is responsible for the
antilock function.
(17) The fluorescence lifetime is an average of a short major compo-
nent (62 ps, 92% for t-1 and 79 ps, 98% for t-2) and a long minor
component (250 ps, 8% for t-1 and 530 ps, 2% for t-2). The distinct
lifetimes for the two components are tentatively attributed to confor-
mers of different orientation for the amino group relative to the stilbene
chromophore.
(18) Levanon, H.; Neta, P. Chem. Phys. Lett. 1977, 48, 345–349.
(19) Lewis, F. D.; Wu, T.; Zhang, Y.; Letsinger, R. L.; Greenfield,
S. R.; Wasielewski, M. R. Science 1997, 277, 673–676.
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Figure 3. Plots of the fraction of t-1 against the irradiation time for (a) t-1 f c-1 at 340 nm, (b) c-1 f t-1 at 250 nm, and (c) t-1 f c-1 //
c-1/Hþ f t-1/Hþ // t-1 f c-1 // c-1/Hþ f t-1/Hþ // t-1 f c-1 at alternating 340 and 250 nm in acetonitrile. The substrate concentration
is 1.0 ꢁ 10ꢀ5 M. The black (i) and red (ii) curves are at 0.0 and 10.0 equiv of HCl, respectievely, and the blue (iii) curves are at 10.0 equiv
of HCl in the dark.
(Figure S9), and the spectral features are similar to the
case of t-1. Evidently, the PET process observed for t-1 is
also operated in c-1, and it could also be deactivated by
protonation of the amino group. As depicted in Figure 3b,
Scheme 3. Optimal Operation Procedures for the ABS Function
of the Molecular Brake 1
the relatively longer recovery time for t-1 in the neutral
(curve i) vs the protonated (curve ii) form is consistent with
our conclusion. The effective competition of PET to the
photoisomerization of cis-stilbene observed for the case
of c-1 is rather unusual, given the fact that excited state
quenching of cis-stilbene due to the torsion of the vinyl
CdC is ultrafast (∼1.3 ps).20 It appears that the bulky
pentiptycene group modifies the excited-state potential
energy surface for the isomerization process so that the
bases. Figure 3c shows 2.5 cycles of Scheme 3 starting from
PET process is kinetically compatible. The recovery of t-1
t-1. To shorten the experimental time, the photostationary
is ∼60% at 250 nm, a wavelength that corresponds to
states were not reached for each switching step. Each segment
of the curve in Figure 3c is similar to the corresponding
segments in Figure 3a and 3b, which is consistent with the
expected ABS operation. The operation cycle involves the
energy inputs of light, acid (HCl), and base (KOtBu).
In summary, we report the first example of an antilock
molecular braking system operated with photons and
chemicals (acids and bases). The antilock function results
from the competition of PET with the cisꢀtrans photo-
the highest ratio of the molar absorptivity of c-1 vs t-1
(Figure 2a). As in the case of t-1 f c-1, protons alone
cannot drive the c-1 f t-1 isomerization in the dark
(Figure 3b, curve iii). Because of the extremely weak
fluorescence, no attempts were made to analyze the decay
kinetics for c-1.
To mimic the ABS operation for the wheels of vehicles,
the PET process is desired only in the turn-on but not the
turn-off operation of the brake function. In other words,
isomerization and can be deactivated with protons. We
the PET process in c-1 should be deactivated for a fast
recovery of the brake-off t-1 state. Therefore, an optimal
operation of the ABS function can be achieved through the
cycle shown in Scheme 3. The fast rotation of the pentip-
believe that the rich molecular/supramolecular photo-
chemistry could further improve the function and perfor-
mance of light-gated molecular devices.
tycene rotor in t-1 is stopped in c-1 through the photon
(340 nm)-driven ABS operation. Protonation of c-1 (c-1/Hþ)
deactivates the ABS function and allows fast photon
(250 nm)-induced release of the brake to restore fast
rotation of the rotor (t-1/Hþ). The ABS function can be
turned on again by removing the protons in t-1/Hþ with
Acknowledgment. We thank the National Science
Council of Taiwan for financial support.
Supporting Information Available. Experimental meth-
ods, synthesis, 1D and 2D NMR spectra, the Arrhenius
and Eyring plots, complete VT 13C NMR spectra, and
a SternꢀVolmer plot. This material is available free of
charge via the Internet at http://pubs.acs.org.
(20) (a) Todd, D. C.; Jean, J. M.; Rosenthal, S. J.; Ruggiero, A. J.;
Yang, D.; Fleming, G. R. J. Chem. Phys. 1990, 93, 8658–8668. (b) Petek,
H.; Yoshihara, K.; Fujiwara, Y.; Lin, Z.; Penn, J. H.; Frederick, J. H.
J. Phys. Chem. 1990, 94, 7539–7541.
The authors declare no competing financial interest.
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