Driving unidirectional molecular rotary motors with visible light by intra- and intermolecular energy transfer from palladium porphyrin

Article abstract of DOI:10.1021/ja306986g

Driving molecular rotary motors using visible light (530-550 nm) instead of UV light was achieved using palladium tetraphenylporphyrin as a triplet sensitizer. Visible light driven rotation was confirmed by UV/vis absorption, circular dichroism and ¹

Full text of DOI:10.1021/ja306986g

Article
pubs.acs.org/JACS
Driving Unidirectional Molecular Rotary Motors with Visible Light by
Intra- And Intermolecular Energy Transfer from Palladium Porphyrin
Arjen Cnossen, Lili Hou, Michael M. Pollard,‡ Philana V. Wesenhagen, Wesley R. Browne,*
and Ben L. Feringa*
Centre for Systems Chemistry, Stratingh Institute for Chemistry and Zernike Institute for Advanced Materials, Faculty of
Mathematics and Natural Sciences, University of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands
S
* Supporting Information
ABSTRACT: Driving molecular rotary motors using visible
light (530−550 nm) instead of UV light was achieved using
palladium tetraphenylporphyrin as a triplet sensitizer. Visible
light driven rotation was confirmed by UV/vis absorption,
circular dichroism and 1H NMR spectroscopy and the rotation
was confirmed to be unidirectional and with similar photosta-
tionary states, despite proceeding via a triplet instead of a
singlet excited state of the molecular motor. Energy transfer
proceeds in both inter- and intramolecular fashion from the
triplet state of the porphyrin to the motor. Stern Volmer plots
show that the rate of intermolecular quenching of the porphyrin excited state by the molecular motor is diffusion-controlled.
chromophores can be affected by crosstalk. For example, when
four azobenzenes were attached to the meso-positions of a
porphyrin, it was found that E-Z photoisomerization no longer
took place.22 A recent study by Tour showed that the
incorporation of a second generation molecular motor into a
molecular car equipped with wheels derived from fullerenes
leads to complete quenching of the photochemistry of the
motor by the fullerenes.23
Motivated by the rich literature on the photoisomerization of
stilbene and related olefin chromophores,24,25 we considered
using an exogenous chromophore to drive the molecular motor
function. In this way, visible light that is not (directly) absorbed
by the molecular motor can be used to drive the rotary cycle by
employing, for example, a porphyrin chromophore to harvest
and relay photochemical energy (Scheme 1).26,27 A number of
examples of metalloporphyrins that can sensitize the isomer-
INTRODUCTION
■
Light driven molecular switches and, more recently, linear and
rotary motors show promise in future nanosystems and devices
based on integrated molecular components.1−5 Near- and mid-
UV light is typically required to operate most photoactive
molecular systems, such as azobenzene6,7 and diarylethene8,9
switches and overcrowded alkene-based motors.10,11 Red and
near-IR light-driven molecular components are, however, highly
desirable, in expanding for instance their application to
biological systems, where UV light can trigger undesirable
responses, including cellular apoptosis. Shifting to longer
wavelength also reduces irradiation-induced damage to soft
material devices based on organic molecules. In certain cases,
this has been achieved by shifting the absorption spectrum to
the visible region through the use of substituents12 or Lewis
acids.13 However, to provide sufficient energy to drive
photochemical reactions with red light, typically it is expected
that two-photon absorption would be required.14 One solution
is sensitization of switches through energy transfer from
appended chromophores to induce isomerization. An example
in nature is found in the exogenous chromophore used to
trigger rhodopsin with long wavelength light, used by deep-sea
fish for vision.15,16 Some examples in the recent literature
illustrate the use of sensitized photochemistry in azoben-
zenes,17,18 diarylethene switching19 and in a catenane-based
molecular rotor.20 Stoddart, Balzani, Credi and co-workers used
an energy transfer relay to drive a rotaxane-based linear
molecular motor with visible light.21
Scheme 1. Rotation of a Molecular Motor Driven by Energy
Transfer from a Sensitizer That Absorbs Visible Light
The incorporation of molecular motors into more complex
systems is desirable as it allows for advanced functions to be
driven with external stimuli. However, the dynamic properties
in systems combining molecular switches or motors with other
Received: July 17, 2012
Published: October 4, 2012
© 2012 American Chemical Society
17613
dx.doi.org/10.1021/ja306986g | J. Am. Chem. Soc. 2012, 134, 17613−17619

Journal of the American Chemical Society
Article
ization of stilbene and its derivatives by irradiating at
wavelengths longer than the lowest absorption of stilbene
have been reported.28−30 In the present study, both inter- and
intramolecular energy transfer as a means of driving a rotary
motor was examined.
to play a role.36 If the photoisomerization proceeds via a triplet
mechanism, the photochemistry might take place over a longer
time scale, but since the thermal helix inversion of 1 has a half-
life of several minutes,31 the thermal step will still be rate
limiting.
In our previous studies, motors such as 1 operated by the
direct absorption of a photon to excite the molecules to a
singlet excited state, followed by relaxation to a twisted state
(1p*) that may either decay forward to give the thermally
unstable isomer 1b or backward to give the starting alkene 1a
(Scheme 2).31,32 By using a triplet sensitizer, a lower lying
An important aspect in this approach is whether the rotation
of the motor remains unidirectional. If the photoisomerization
via the triplet state proceeds analogously to that of the singlet
process, it is expected that overall the rotation will still be
unidirectional. Indeed, even though the photoisomerization is
reversible, the subsequent thermal helix inversion is essentially
one-way, which ensures that the rotation can only operate in
one overall direction (Scheme 4). However, one can envision
that in the triplet excited state other processes can take place,
which could allow ill-defined interconversion of the stable or
unstable forms.
Scheme 2. Photoisomerization of 1a to 1b Driven by UV
Light
Scheme 4. Excitation of Palladium Tetraphenylporphyrin
with Visible Light Followed by Efficient Intersystem
Crossing Generates the Triplet Porphyrin Excited State
a
triplet excited state of the molecular motor can be accessed.
Following this excitation pathway, the molecule can adopt a
twisted state (3p*).
Taking this approach is nontrivial, however, since in addition
to competing electron-transfer processes, achieving equally
favorable photostationary states is challenging.34 This twisted
state does not necessarily have the same geometry as that
reached on the singlet excited state surface, and in principle
could have a different preference for relaxing toward the
unstable or the stable form (Scheme 3).35
Scheme 3. Schematic Energy Profile along the Rotation
Reaction Coordinate of an Asymmetrically Substituted
Alkene33
a
Intermolecular energy transfer to the molecular motor drives
photoisomerization of the central olefinic bond. Thermal helix
inversion of the unstable form completes one half rotation.
RESULTS AND DISCUSSION
■
Molecular Design. We selected palladium tetraphenylpor-
phyrin (PdTPP) as a sensitizer to investigate the possibility of
harnessing longer wavelength light to drive the rotary cycle of 1
through intermolecular energy transfer. This chromophore is
well suited for the purpose because it has a relatively long-lived
triplet state lifetime (up to 2 ms)37 and has a strong absorption
at wavelengths longer than those of the motor, allowing for
selective excitation of PdTPP in the presence of 1. With a
triplet energy of 178 kJ/mol,37 the excited state energy of
3[PdTPP] is significantly lower than that of the singlet excited
state of motor 1 (∼300 kJ/mol). The triplet excited state
energy of motor 1 is unknown; however, it can be estimated
using time-dependent density functional theory (TD-DFT)
calculations. TD-DFT calculations on the B3LYP/6-31G-
(d,p)38−40 level of theory gives a value of 182 kJ/mol, which
is comparable with that of PdTPP.41
The photostationary state that is obtained depends on the
efficiency of the excitation and the decay ratio. If there is a large
energy difference between the triplet excited state of the stable
and the unstable forms, the energy transfer efficiency to both
forms will be different. Furthermore, if the excited state has a
preference for decay to either the stable or the unstable form, it
will influence the photostationary state obtained also.
The overall rate of the rotation of the molecular motor
depends on the rate of the thermal helix inversion step, because
it is much slower than the photoisomerization step, although at
high rates the irradiation intensity and concentration also start
In addition to intermolecular energy transfer42 from the
sensitizer and the molecular motor, a covalent system was
17614
dx.doi.org/10.1021/ja306986g | J. Am. Chem. Soc. 2012, 134, 17613−17619

Journal of the American Chemical Society
Article
Scheme 5. Synthesis of TPP Motor Conjugates 12 and 13
designed to allow for intramolecular energy transfer, to increase
the effective molarity and thus the efficiency of energy transfer.
In energy transfer processes, the distance and orientation
between the donor and acceptor is important.43 In the covalent
system employed (Scheme 5), a flexible, nonconjugated linker
is used, which precludes through-bond interactions, with an
ester formation as the final coupling step. Substitution on this
position of the motor has no effect on the thermal helix
inversion, although the photostationary state is affected by
electron donating or withdrawing substituents.44 In a different
molecular motor, the effect of substitution with large alkyl
groups was also reported to be minimal,45 so we do not expect
a significant difference in behavior between the motor unit in
12 and compound 1.
and 13C NMR spectroscopy and HR-MS. For control
experiments, free-base analogue 13 was also synthesized by
coupling motor 9 to 11.
Photoisomerization by Intermolecular Energy Trans-
fer. Sensitization of the photoisomerization of 1 by collisional
energy transfer from PdTPP was studied in 1,2-dichloroethane
at 293 K. The UV/vis absorption spectra of 1a and the PSS
mixture of 1a and 1b obtained after irradiation at 365 nm is
shown in Figure 1. The absorption spectra of 1 overlap partially
with the absorption spectrum of PdTPP, with the Soret band of
the porphyrin dominating the absorption spectrum.
Synthesis. The synthesis of target motor 9 with an ethylene
glycol spacer for attachment of the porphyrin is shown in
Scheme 5. Starting from 1-methoxynaphthalene 3, ketone 4 was
synthesized in a one pot Friedel−Crafts acylation/Nazarov
cyclization in moderate yield. Deprotection of the phenol could
be carried out conveniently by heating with pyridine hydro-
chloride. Purification of the resulting naphthol 5 was somewhat
hampered by poor solubility; however, the crude product could
be used directly in the next step. Alkylation of 5 with tert-butyl
chloroacetate gave 6 in good yield. Conversion of the ketone
moiety to a thioketone was effected by Lawesson’s reagent.
Thioketone 7 was found to be stable toward column
chromatography and could be stored for several days at
ambient conditions without noticeable degradation. The
sterically overcrowded olefinic bond was introduced by a
Barton-Kellogg coupling between 7 and 9-diazofluorene. A
mixture of olefin 8 and the corresponding episulfide was
obtained, which was not separated but instead treated with
triphenylphosphine yielding functionalized motor 8 in 65%
Figure 1. UV/vis absorption spectra of 1a (full line), the PSS mixture
of 1a and 1b obtained after irradiation at 365 nm (dashed line) and
PdTPP (dotted line).
Irradiation of the porphyrin at 420 nm would also lead to
direct excitation and isomerization of 1. Hence, irradiation was
carried out at a wavelength resonant with one of the Q-bands
(indicated by the arrow in Figure 1) of the porphyrin, at longer
wavelength than the absorption of both the stable and the
unstable forms of 1. Excitation at 530−550 nm was achieved by
the use of a visible light source with an appropriate band-pass
filter or a 532 nm pulsed laser.
An argon purged solution of 1 alone and a 1:1 mixture of
PdTPP and 1 (5 μM) were irradiated at 532 nm (pulsed laser, 6
ns, 10 Hz). A red shift in the absorption was observed,
corresponding to the photoisomerization of motor 1 (Figure
2), as observed before upon direct irradiation of 1 with UV
light. The change in absorption is minor and partially obscured
by the overlap with the porphyrin absorption, but is
nevertheless reproducible. In contrast, when a solution of 1
1
yield. The H NMR spectrum of 9 shows the typical splitting
pattern of the protons on the cyclopentene ring. Further
characterization was performed with 13C NMR spectroscopy
and high-resolution mass spectrometry (HR-MS). Attempts to
hydrolyze the ester functionality in 8 for subsequent coupling
to an alcohol substituted porphyrin failed under both acidic and
basic conditions. However, reduction of the ester to an alcohol
using LiAlH4 proceeded in good yield. Acid functionalized
porphyrin 10 was synthesized in three steps to allow coupling
to 9 (see Supporting Information). The alcohol moiety in 9 was
reacted with 10 in a carbodiimide-mediated esterification to
1
give 12 in 88% yield. Compound 12 was characterized by H
17615
dx.doi.org/10.1021/ja306986g | J. Am. Chem. Soc. 2012, 134, 17613−17619

Journal of the American Chemical Society
Article
not invert fully, even upon prolonged irradiation, which can be
rationalized by a relatively low PSS; from the decrease in signal
intensity, a ratio 1a/1b of about 75:25 can be estimated. Upon
heating to 40 °C for 20 min, the spectral changes in both the
UV/vis absorption and the CD spectra are reversed.
Unidirectionality of the Sensitized Rotation Process.
A central question in driving photoisomerization by triplet
sensitization rather than direct excitation is whether or not the
unidirectionality of the rotary motor function is retained.
Previously, the unidirectionality of the rotary motor function of
1 under direct excitation (365 nm) was established by CD and
1H NMR spectroscopy.31 To be able to distinguish the four
distinct steps in the rotary cycle of the motor, a methoxy
substituent was introduced in the lower half. Compound 2 was
synthesized according to literature procedures,31 analogous to
1, and the E and Z isomers were separated by column
chromatography.
A 1:1 mixture of PdTPP and 2 (18 mM) was irradiated at
−40 °C in toluene-d8 at 546 nm ( 5 nm). The initial isomer E-
2a of the molecular motor was converted to the thermally
unstable Z-2b (Scheme 4, step 1). Characteristic changes in the
1H NMR spectrum are the shift of the alkyl signals and the
appearance of a signal of the methoxy group at 2.6 ppm (Figure
5). At the PSS, the ratio of E-2a to Z-2b was 64:36. Warming
Figure 2. UV/vis absorption spectra of a mixture of 1 and PdTPP (full
line) and the same mixture after irradiation at 532 nm (dashed line).
Inset: expansion of the 350−400 nm region.
only was irradiated under the same conditions no changes were
observed in the UV/vis absorption spectrum, confirming that
the photochemistry of 1 cannot be driven directly at 532 nm.
CD spectroscopy was employed to further characterize the
photoisomerization of 1. Enantiomerically pure (S)-1 was
obtained by preparative chiral stationary phase HPLC. The
absolute configuration was assigned based on comparison of
the CD spectrum to the spectrum predicted by TD-DFT
calculations on the B3LYP/6-31G(d,p) level of theory (Figure
3).
Figure 3. CD spectrum measured for 1 (full line) in dichloromethane
and normalized calculated CD spectrum for (S)-1 (dashed line). It
should be noted that the spectrum is solvent dependent as can be seen
by comparison with the spectrum of 1 in hexane.31.
1
Figure 5. Partial H NMR spectrum (−40 °C, toluene-d8) of (a) a
mixture of PdTPP and E-2a before irradiation, (b) mixture of E-2a and
Z-2b obtained upon irradiation at 546 nm at −40 °C, and (c) mixture
of E-2a and Z-2a obtained upon warming to 40 °C for 20 min. * =
ethanol.
Upon irradiation at 532 nm of a 1:1 mixture of (S)-1 and
PdTPP in chloroform, the major bands in the CD spectrum
start to decrease in intensity, which implies the formation of the
unstable form with opposite helicity (Figure 4). The bands do
the mixture at 40 °C for 20 min resulted in the conversion of Z-
2b to Z-2a (Scheme 4, step 2). The alkyl signals recover at their
original positions, while the intensity of the new methoxy signal
remains unchanged, indicating the formation of the Z-2a
isomer.
A similar series of experiments was performed with Z-2a
(Figure 6). When a mixture of Z-2a and PdTPP was irradiated
at 546 nm, the signal of the alkyl protons shifted, indicating the
formation of thermally unstable E-2b (Scheme 4, step 3).
Likewise, the signal of the methoxy group at 2.6 ppm decreases
with an increase in the intensity of the methoxy signal (3.4
ppm) of E-2b (Figure 6). At the PSS, the ratio of Z-2a to E-2b
was 65:35. Upon warming, the alkyl signals shift back to their
original position, while the intensity of the methoxy signal
remains the same (Scheme 4, step 4). This shows that all of the
thermally unstable form generated by photoisomerization in
Figure 4. CD spectra of a mixture of 1 and PdTPP (full line) and the
same mixture after irradiation at 532 nm (dashed line) and subsequent
heating to 40 °C for 20 min (dotted line). Note that the spectrum
after heating is slightly more intense due to a small change in
concentration during heating.
17616
dx.doi.org/10.1021/ja306986g | J. Am. Chem. Soc. 2012, 134, 17613−17619

Journal of the American Chemical Society
Article
Figure 7. UV/vis absorption spectrum of 12a (full line) and the
mixture of 12a and 12b obtained after irradiation at 532 nm (dashed
line). Inset: Molar absorptivity at 460 nm of a solution of 12
alternately irradiated at 532 nm for 10 s and left in the dark at rt for 20
min.
1
Figure 6. Partial H NMR spectrum (−40 °C, toluene-d8) of (a) a
mixture of PdTPP and Z-2a before irradiation, (b) mixture of Z-2a and
E-2b obtained upon irradiation at 546 nm at −40 °C, and (c) mixture
of Z-2a and E-2a obtained upon warming to 40 °C for 20 min.
The changes in the UV/vis absorption spectrum upon
irradiation are somewhat obscured by overlap of the strong
Soret band of the porphyrin. However, with CD spectroscopy
characteristic spectral changes are also expected in the 250−350
nm region, where the porphyrin has negligible absorption.
Indeed, when a solution of (R)-12a is irradiated at 532 nm, a
distinct decrease in the intensity of the CD signal is observed
(Figure 8). As was the case for 1, complete inversion is not
steps 1 and 3 is converted to the expected thermally stable form
and that the rotation is unidirectional. During the isomerization
steps, the same changes in helicity were observed for
compound 2 as for compound 1 (see Supporting Information).
Photoisomerization by Intramolecular Energy Trans-
fer. In motor-porphyrin hybrid 12, the energy transfer is
expected to take place in an intramolecular fashion to drive the
photoisomerization of the motor (Scheme 6). Because the
Scheme 6. Expected Isomerization Behavior of 12
Figure 8. CD spectra of 12a (full line) and the mixture of 12a and 12b
obtained after 10 s irradiation at 532 nm (dashed line) and the mixture
after the thermal step (dotted line). Note that the opposite
stereoisomer is used compared with 1 (Figure 4).
observed, presumably due to a relatively modest PSS (vide
infra). The observed spectral changes are fully reversible when
thermal helix inversion is allowed to take place by heating to 40
°C for 20 min. If the palladium-free porphyrin 13 is used
instead of 12, no changes are observed in either the UV/vis
absorption or the CD spectrum. Prolonged irradiation led to
irreversible changes in the CD spectrum, which are attributed
to degradation (see Supporting Information).
At 138 kJ/mol, the triplet excited state energy of free-base
tetraphenylporphyrin (H2TPP) is considerably lower than that
of both PdTPP and motor 1, so triplet energy transfer is not
expected in this case.46 Energy transfer from the porphyrin in
the singlet excited state cannot take place because it is too low
in energy relative to the singlet excited state of 1. These results
indicate that the energy transfer in the case of the palladated
porphyrin goes via triplet energy transfer.
motor is linked covalently to the sensitizer, the effective
molarity will be high and energy transfer should proceed more
efficiently than in the intermolecular case.
When irradiated with visible light (532 nm, 6 ns, 10 Hz) for
10 s, the photostationary state of an argon purged solution of
12 was reached, and this resulted in an increase in the
absorbance between 450 and 500 nm with a concomitant minor
red-shift of the shoulder around 375 nm (Figure 7), similar to
what was observed for 1. The isomerization was performed
repeatedly: the same sample was irradiated at 532 nm for 10 s
and left in the dark for 20 min for four cycles, with the change
in absorbance monitored at 460 nm (Figure 7, inset). The
excellent repeatability of the UV/vis absorption spectral
changes of 12 indicates that the photoisomerization of the
motor part is reversible and degradation does not occur.
The photostationary state for the sensitized isomerization
between 12a and thermally unstable 12b was determined by 1H
NMR spectroscopy. A solution of 12a in toluene-d8 was
irradiated (532 nm) at −40 °C until further changes were not
17617
dx.doi.org/10.1021/ja306986g | J. Am. Chem. Soc. 2012, 134, 17613−17619

Journal of the American Chemical Society
Article
observed. By comparing the integrals of the absorptions of 12a
and 12b in the alkyl region, the photostationary state ratio 12a/
12b was determined to be 67:33. This is similar to the 57:43
ratio obtained under direct irradiation (365 nm) for a methoxy-
substituted analogue of 1,44 and to the ratio obtained for the
intermolecular process (vide supra), with the slight difference
being within experimental uncertainty.
A more important parameter, however, is the photochemical
quantum yield for the visible light-driven photoisomerization.
This was determined to be 0.11 0.02 (using the reference
potassium reineckate as a standard).47,48 This is close to the
quantum yield obtained for direct excitation, 0.14, showing that
similar efficiency can be reached using visible light.49
Luminescence Lifetime and Quenching. The results of
the irradiation experiments using nonmetalated motor−
porphyrin hybrid 13 implied that the rotation upon visible
light irradiation proceeds via triplet energy transfer. To
demonstrate this conclusively, the luminescence lifetimes of
the porphyrins were measured. Steady-state emission spectra of
PdTPP, a mixture of PdTPP and 1 with a ratio of 1:2.5 and
motor−PdTPP hybrid 12 in 1,2-dichloroethane under an argon
atmosphere were recorded (Figure 9). PdTPP, irradiated with
PdTPP was not influenced by the presence of the molecular
motor. This confirms that energy transfer is not via singlet
energy transfer but via triplet energy transfer. As a control, the
steady-state fluorescence of H2TPP was compared to that of
13; singlet energy transfer was not observed and there was no
change in the fluorescence quantum yield (see Supporting
Information).
The intra- and intermolecular energy transfer were further
studied by means of time-resolved emission spectroscopy. The
phosphorescence decay curves of PdTPP, a 1:2.5 mixture of
PdTPP and 1, and 12 were recorded at 710 nm (Table 1, see
also Supporting Information). The lifetimes were fitted using
first-order exponential decay kinetics. The phosphorescence
lifetime was reduced from 34 μs for PdTPP to 9 μs for the
mixture, and to 0.41 μs for 12, which confirms that quenching
of PdTPP triplet excited state occurs. As expected, the covalent
system 12 shows more efficient energy transfer than the
mixture of PdTPP and 1.
The phosphorescence lifetime of PdTPP in the presence of
the motor at concentrations between 0.0 and 0.5 mM in
(argon-saturated) 1,2-dichloroethane was determined and the
data is shown in the form of a Stern−Volmer plot (τ0/τ versus
motor concentration, Figure 10). A linear fit yielded a
Figure 9. Emission spectra of PdTPP (2 × 10−5 M, full line), a 1:2.5
mixture of PdTPP and 1 (PdTPP 2 × 10−5 M, 1 5 × 10−5 M, dashed
Figure 10. Stern−Volmer plot for quenching of PdTPP by 1 (points)
and linear fit (red line).
line), and 12 (2 × 10−5 M, dotted line) under Ar atmosphere, λexc
=
532 nm.
bimolecular quenching rate constant kq of 1.8 × 109 M−1 s−1
3
for the quenching of PdTPP* by 1. The diffusion-controlled
visible light (532 nm), gives fluorescence at 610 nm and
phosphorescence at 710 nm.50 The phosphorescence quantum
yield for PdTPP is significantly reduced by the molecular
motor, both in an inter- and intramolecular fashion, which is
attributed to the energy transfer from PdTPP to the motor
(Table 1). The quenching of phosphorescence in 12 was
greater than in the mixture of 1 and PdTPP under the same
conditions. In contrast, the fluorescence quantum yield for
rate constant in 1,2-dichloroethane is approximately 8.9 × 109
M−1 s−1 at 25 °C.51 This indicates that energy transfer is
diffusion controlled in the present intermolecular system also.
CONCLUSION
■
In conclusion, the photoisomerization in second generation
molecular motors can be driven by visible light when a suitable
triplet sensitizer is employed. Energy transfer from the triplet
excited state of PdTPP to the motor leads to the formation of
the unstable form, which subsequently undergoes thermal helix
inversion resulting in rotation of the upper half relative to the
lower half. The photoisomerization was characterized by UV/
Table 1. Photophysical Properties of PdTPP and 3 in 1,2-
a
dichloroethane
fluor. λmax
(nm)
phosphor. λmax (nm)
{τT in μs}
b
b
Φf(%)
Φp (%)
1
vis, CD and H NMR spectroscopy and it was found that the
PdTPP
PdTPP:1
12
610
610
610
710 (36 4)
710 (9 1)
710 (0.41)
∼0.01
∼0.01
∼0.01
0.5 0.1
∼0.05
<0.04
sensitized photoisomerization proceeds similar to photo-
isomerization by direct irradiation; the main difference is that
the PSS is decreased to a minor extent with respect to the
unstable form. Energy transfer from the porphyrin to the motor
takes place conveniently in an intermolecular fashion to drive
the photoisomerization. However, covalent linking of the motor
to the porphyrin increases the efficiency substantially.
a
All the measurement were performed in Ar-saturated environment.
Fluorescence quantum yield Φf, phosphorescence quantum yield Φp
b
and triplet lifetime τT. The quantum yields were determined using
[Ru(bpy)3] (PF6)2 in water as a reference (Φ = 0.028).
17618
dx.doi.org/10.1021/ja306986g | J. Am. Chem. Soc. 2012, 134, 17613−17619

Journal of the American Chemical Society
Article
(18) Sakamoto, R.; Kume, S.; Sugimoto, M.; Nishihara, H. Chem.
Eur. J. 2009, 15, 1429−1439.
Using excitation at longer wavelengths than 546 nm is in
principle possible. However, energy transfer to motor 1
requires that the triplet excited state energy of the sensitizer
be higher than that of 1. Considering the triplet excited state
energies of PdTPP (178 kJ/mol) and H2TPP (138 kJ/mol), the
current limit is between 672 and 872 nm. An alternative
approach is to use motor structures with lower triplet excited
state energies. Ultimately, driving the photoisomerization of
overcrowded alkene-based motors with IR light would expand
their use in biological systems.
(19) Brayshaw, S. K.; Schiffers, S.; Stevenson, A. J.; Teat, S. J.;
Warren, M. R.; Bennett, R. D.; Sazanovich, I. V.; Buckley, A. R.;
Weinstein, J. A.; Raithby, P. R. Chem.Eur. J. 2011, 17, 4385−4395.
(20) Leigh, D. A.; Wong, J. K. Y.; Dehez, F.; Zerbetto, F. Nature
2003, 424, 174−179.
́
(21) Balzani, V.; Clemente-Leon, M.; Credi, A.; Ferrer, B.; Venturi,
M.; Flood, A. H.; Stoddart, J. F. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
1178−1183.
(22) Hunter, C. A.; Sarson, L. D. Tetrahedron Lett. 1996, 37, 699−
702.
(23) Morin, J.-F.; Shirai, Y.; Tour, J. M. Org. Lett. 2006, 8, 1713−
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, compound characterization data,
control experiments, and computational details. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
1716.
S
(24) Hammond, G. S.; Saltiel, J.; Lamola, A. A.; Turro, N. J.;
Bradshaw, J. S.; Cowan, D. O.; Counsell, R. C.; Vogt, V.; Dalton, C. J.
Am. Chem. Soc. 1964, 86, 3197−3217.
(25) Benson, R.; Williams, D. F. J. Phys. Chem. 1977, 81, 215−220.
(26) Lukas, A. S.; Wasielewski, M. R. In Molecular Switches; Feringa,
B. L., Ed.; Wiley-VCH: Weinheim, 2001; pp 1−35.
(27) Gust, D.; Moore, T. A.; Moore, A. L. Chem. Commun. 2006,
1169−1178.
AUTHOR INFORMATION
Corresponding Author
w.r.browne@rug.nl; b.l.feringa@rug.nl
Notes
■
(28) Mercer-Smith, J. A.; Whitten, D. G. J. Am. Chem. Soc. 1978, 100,
2620−2625.
The authors declare no competing financial interest.
(29) Brink, M.; Wennerstrom, O. J. Photochem. Photobiol., A 2001,
̈
143, 201−208.
ACKNOWLEDGMENTS
(30) Mollerstedt, H.; Wennerstrom, O. J. Photochem. Photobiol., A
̈
̈
■
2001, 139, 37−43.
This work was supported by the European Research Council
(Advanced Investigator grant 227897; A.C., B.L.F.), The
Netherlands Organization for Scientific Research (VIDI grant
700.57.428, W.R.B.) and the Ubbo Emmius fund (L.H.).
(31) Vicario, J.; Meetsma, A.; Feringa, B. L. Chem. Commun. 2005,
5910−5912.
(32) Vicario, J.; Walko, M.; Meetsma, A.; Feringa, B. L. J. Am. Chem.
Soc. 2006, 128, 5127−5135.
(33) Turro, N. J. Modern Molecular Photochemistry; University
Science Books: Sausolito, CA, 1991; pp 473−481.
(34) Wang, J.; Kulago, A.; Browne, W. R.; Feringa, B. L. J. Am. Chem.
Soc. 2010, 132, 4191−4196.
DEDICATION
■
‡In Memoriam; Febuary 27, 2010
(35) Arai, T.; Tokumaru, K. Chem. Rev. 1993, 93, 23−39.
(36) Klok, M.; Browne, W. R.; Feringa, B. L. Phys. Chem. Chem. Phys.
2009, 11, 9124−9131.
REFERENCES
■
(1) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007,
46, 72−191.
(2) Coskun, A.; Banaszak, M.; Astumian, R. D.; Stoddart, J. F.;
Grzybowski, B. A. Chem. Soc. Rev. 2012, 41, 19−30.
(3) Faiz, J. A.; Heitz, V.; Sauvage, J. P. Chem. Soc. Rev. 2009, 38, 422−
442.
(4) Balzani, V.; Credi, A.; Venturi, M. Chem. Soc. Rev. 2009, 38,
1542−1550.
(5) Molecular Switches; Feringa, B. L., Browne, W. R., Eds.; Wiley-
VCH: Weinheim, 2011.
(37) Eastwood, D.; Gouterman, M. J. Mol. Spectrosc. 1970, 35, 359−
375.
(38) Becke, A. D. J. Chem. Phys. 1993, 98, 1372−1377.
(39) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789.
(40) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys.
1980, 72, 650−654.
(41) These calculations are known to have a significant margin in the
determination of the energy of electronically excited states, see:
Grimme, S.; Neese, F. J. Chem. Phys. 2007, 127, 154116.
(42) For a review on inter- and intermolecular energy transfer, see
Raymo, F. M.; Tomasulo, M. Chem. Soc. Rev. 2005, 34, 327−336.
(43) See for example: Aratani, N.; Kim, D.; Osuka, A. Acc. Chem. Res.
2009, 42, 1922−1934.
(6) Dhammika Bandara, H. M.; Burdette, S. C. Chem. Soc. Rev. 2012,
41, 1809−1825.
(7) Barrett, C. J.; Mamiya, J.; Yager, K. G.; Ikeda, T. Soft Matter 2007,
3, 1249−1261.
(8) Matsuda, K.; Irie, M. J. Photochem. Photobiol., C 2004, 5, 169−
182.
(44) Pollard, M. M.; Wesenhagen, P. V.; Pijper, D.; Feringa, B. L.
Org. Biomol. Chem. 2008, 6, 1605−1612.
(9) Raymo, F. M.; Tomasulo, M. Chem. Soc. Rev. 2005, 34, 327−336.
(10) Feringa, B. L. J. Org. Chem. 2007, 72, 6635−6652.
(11) Pollard, M. M.; Klok, M.; Pijper, D.; Feringa, B. L. Adv. Funct.
Mater. 2007, 17, 718−729.
(12) Beharry, A. A.; Sadovski, O.; Woolley, G. A. J. Am. Chem. Soc.
2011, 133, 19684−19687.
(13) Yang, Y.; Hughes, R. P.; Aprahamian, I. J. Am. Chem. Soc. 2012,
134, 15221−15224.
(45) Caroli, G.; Kwit, M. G.; Feringa, B. L. Tetrahedron 2008, 64,
5956−5962.
(46) Darwent, J. R.; Douglas, P.; Harriman, A.; Porter, G.; Richoux,
M.-C. Coord. Chem. Rev. 1982, 44, 83−126.
(47) Handbook of Photochemistry; Montalti, M., Credi, A., Prodi, L.,
Gandolfi, M. T., Eds.; CRC Press: Boca Raton, FL, 2006; pp 609−610.
(48) Szychlinski, J.; Bilski, P.; Martuszewski, K.; Blazejowski, J.
Analyst 1989, 114, 739−741.
(14) Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L.
Angew. Chem., Int. Ed. 2009, 48, 3244−3266.
(15) Douglas, R. H.; Partridge, J. C.; Dulai, K. S.; Hunt, D. M.;
Mullineaux, C. W.; Hynninen, P. H. Vision Res. 1999, 39, 2817−2832.
(16) Okada, T.; Ernst, O. P.; Palczewski, K.; Hofmann, K. P. Trends
Biochem. Sci. 2001, 26, 318−324.
(49) Conyard, J.; Addison, K.; Heisler, I. A.; Cnossen, A.; Browne, W.
R.; Feringa, B. L.; Meech, S. R. Nat. Chem. 2012, 4, 547−551.
(50) Rogers, J. E.; Nguyen, K. A.; Hufnagle, D. C.; Mclean, D. G.; Su,
W.; Gossett, K. M.; Burke, A. R.; Vinogradov, S. A.; Pachter, R.; Fleitz,
P. A. J. Phys. Chem. A 2003, 107, 11331−11339.
(51) Handbook of Photochemistry; Montalti, M.. Credi, A.. Prodi, L..
Gandolfi, M. T., Eds.; CRC Press: Boca Raton, FL, 2006; p 424.
(17) Venkataramani, S.; Jana, U.; Dommaschk, M.; Sonnichsen, F.
D.; Tuczek, F.; Herges, S. Science 2011, 331, 445−448.
̈
17619
dx.doi.org/10.1021/ja306986g | J. Am. Chem. Soc. 2012, 134, 17613−17619