Tuning energy transfer in switchable donor-acceptor systems

Article abstract of DOI:10.1039/b719095f

The synthesis and characterisation of a coumarin-dithienylcyclopentene- coumarin symmetric triad (CSC) and a perylene bisimide-dithienylcyclopentene- coumarin asymmetric triad (PSC) are reported. In both triads the switching function of the photochromic dithienylcyclopentene unit is retained. For CSC an overall 50% quenching of the coumarin fluorescence is observed upon ring-closure of the dithienylcyclopentene component, which, taken together with the low PSS (<70%), indicates that energy transfer quenching of the coumarin component by the dithienylcyclopentene in the closed state is efficient. Upon ring opening of the dithienylcyclopentene unit the coumarin emission is restored fully. The PSC triad shows efficient energy transfer from the coumarin to the perylene bisimide unit when the dithienylcyclopentene unit is in the open state. When the dithienylcyclopentene is in the closed (PSS) state a 60% decrease in sensitized perylene bisimide emission intensity is observed due to competitive quenching of the coumarin excited state and partial quenching of the perylene excited state by the closed dithienylcyclopentene unit. This modulation of energy transfer is reversible over several cycles for both the symmetric and asymmetric tri-component systems. The Royal Society of Chemistry.

Full text of DOI:10.1039/b719095f

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Tuning energy transfer in switchable donor–acceptor systems†
Johannes H. Hurenkamp, Jaap J. D. de Jong, Wesley R. Browne, Jan H. van Esch*‡ and Ben L. Feringa*
Received 11th December 2007, Accepted 23rd January 2008
First published as an Advance Article on the web 21st February 2008
DOI: 10.1039/b719095f
The synthesis and characterisation of a coumarin–dithienylcyclopentene–coumarin symmetric triad
(CSC) and a perylene bisimide–dithienylcyclopentene–coumarin asymmetric triad (PSC) are reported.
In both triads the switching function of the photochromic dithienylcyclopentene unit is retained. For
CSC an overall 50% quenching of the coumarin fluorescence is observed upon ring-closure of the
dithienylcyclopentene component, which, taken together with the low PSS (<70%), indicates that
energy transfer quenching of the coumarin component by the dithienylcyclopentene in the closed state
is efficient. Upon ring opening of the dithienylcyclopentene unit the coumarin emission is restored fully.
The PSC triad shows efficient energy transfer from the coumarin to the perylene bisimide unit when the
dithienylcyclopentene unit is in the open state. When the dithienylcyclopentene is in the closed (PSS)
state a 60% decrease in sensitized perylene bisimide emission intensity is observed due to competitive
quenching of the coumarin excited state and partial quenching of the perylene excited state by the
closed dithienylcyclopentene unit. This modulation of energy transfer is reversible over several cycles
for both the symmetric and asymmetric tri-component systems.
arrangement of donor–acceptor units. Both approaches have seen
considerable success in achieving efficient energy transfer and in
Introduction
Energy (ET) and electron (EnT) transfer between molecular enti-
furthering our understanding of the physical basis of, e.g., Fo¨rster
resonance energy transfer (FRET).8
ties is of continuing interest in the development of molecular based
photonic systems, including photovoltaics,1 molecular electronics2
and sensor technologies.3 The excellent efficiency in energy and
electron transfer between donor and acceptor components in the
photosynthetic apparatuses of plants and bacteria is achieved
through the optimal supramolecular spatial arrangement and
energetic matching of donor and acceptor units.4 Achieving such
control represents a considerable challenge in synthetic systems,
where the tight organization exerted by nature through membrane
and protein structures is not present a priori.4,5
Efficient energy transfer in synthetic donor acceptor systems can
be achieved either by through-bond (superexchange) interactions
or by through-space energy transfer. Depending on the mechanism
of through-space energy transfer (e.g., Dexter6 or Fo¨rster7 energy
transfer) between donor and acceptor chromophoric units, the
efficiency is dependent on the absorption cross-section of the
energy acceptor and its overlap with the fluorescence spectrum
of the donor unit and also on their spatial arrangements. The
electronic properties of the donor and acceptor units can be
tuned synthetically to achieve an optimal energetic overlap.
However, control over the spatial and orientational arrangement
between components is often more difficult to achieve. Overall the
approaches taken to control this latter aspect can be divided into
two groups—i.e. the covalent and non-covalent (supramolecular)
Previously, we have shown that efficient energy transfer can be
achieved in a coumarin donor–perylene bisimide acceptor based
system.9,10 In this tetra-coumarin–perylene bisimide system, we
demonstrated that energy transfer with an efficiency of >95%
could be achieved with good stability even under conditions
of high near-UV irradiation flux, without requiring a through-
bond interaction between the donor and acceptor components.
Furthermore, careful matching of the energetics of the donor and
acceptor units avoided potentially deleterious competing electron
transfer processes. However, energy transfer in this molecule,
although efficient, is not subject to post-synthetic control, i.e.
the energy transfer efficiency from the donor coumarin units to
the perylene bisimide acceptor cannot be altered or modulated
reversibly after synthesis.
Controlling energy transfer post-synthetically, e.g., by chang-
ing the direction and efficiency of the process, represents an
interesting, albeit considerable, challenge. The incorporation of
an addressable component into supramolecular systems capable
of attenuating energy transfer from an energy donor to an
energy acceptor would allow for modulation of the emission
output of the donor–acceptor system. Control over fluorescence
intensity has been demonstrated by several groups, and, typically,
this is achieved through quenching of the fluorescence of the
chromophore by an additional unit,11 whose electronic structure
can be changed upon external perturbation, e.g., by photo-12,13 or
electrochemical14 switching, pH change,3,15 or by disconnection of
a quenching unit.16
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. E-mail:
j.h.vanesch@tudelft.nl, b.l.feringa@rug.nl; Fax: 0031-50-3634296;
Tel: 0031-50-3634235
† Electronic supplementary information (ESI) available: UV–Vis and
emission spectra following irradiation. See DOI: 10.1039/b719095f
‡ Present address: Delft ChemTech, Technical University Delft,
Julianalaan 136, 2628 BL, Delft, The Netherlands
Dithienylcyclopentene switches, which belong to a class of
photochromic switches that show potential as photoswitchable flu-
orescence quenching units, are suitable candidates to impart func-
tionality in these donor–acceptor systems. These photochromic
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switches undergo a photochemical cyclization reaction upon
irradiation with UV light, which is reversible upon irradiation
with visible light (Fig. 1). Dithienylcyclopentenes show sufficient
stability, good to very good photostationary states (PSS) and
the two states have very different absorption spectra, which are
both thermally stable.17 The synthetic routes, which are available,18
facilitate attachment of substituents (in this case chromophores).
Results
Synthesis
The symmetric triad, consisting of two coumarin donors and a
central dithienylcyclopentene unit, was prepared to demonstrate
the quenching concept for the selected donor and switchable-
acceptor chromophores (Scheme 1). It was decided to use
amides in combination with piperazines as a ‘molecular resistor’
(i.e. similar to the use of adamantanes20a or ethers25 in other
multicomponent systems) to construct a symmetric triad, in
which the coumarin chromophore is electronically decoupled from
the dithienylcyclopentene. This approach was used successfully
already in building a tetra-coumarin–perylene bisimide system.10
The amide coupling method used here to construct the substituted
dithienylcyclopentene photochromic switching units has been
employed successfully in the synthesis of switchable gelator
systems also.26
Fig. 1 Schematic representation of the ring-open and ring-closed state of
a dithienylcyclopentene switch.
Indeed, dithienylcyclopentenes have been employed successfully
already as fluorescence quenchers, examples of which have been
reported by the groups of Lehn,19 Irie,20 Tian21 and Branda.22
However, control of energy transfer efficiency between an energy
donor and acceptor pair by a third, addressable, component allows
for more versatile control of excited state properties, such as the
triad system reported by Walz et al.23 This approach to switchable
triad systems has been demonstrated in systems involving electron
transfer also,24 however, the present work focuses on through-
space energy transfer. In this contribution we report a covalently
linked donor–switch–acceptor triad based on a coumarin (donor),
a dithienylethene (switch) and a perylene bisimide (acceptor) unit.
The energy transfer between the donor and acceptor units can
be redirected by photochemical isomerization of the central
‘switching’ unit. In the open form, the dithienylcyclopentene acts
as a photophysically innocent bridging unit. In the closed form
it acts to quench the emission of the coumarin donor and, to a
lesser extent, the perylene bisimide acceptor, thereby modulating
the luminescence output of the perylene bisimide unit (Fig. 2),
by reducing the efficiency of energy transfer from the coumarin
donor to the perylene acceptor.
Scheme 1 Synthesis of CSC 5: (A) CDI, CH2Cl2, RT (92%); (B)
CF3COOH, CH2Cl2 (quant.); (C) 1. CDMT, NMM, CH2Cl2, 0 ◦C, 2.
NMM, 3, (16%).
7-Methoxycoumarin-3-acetic acid 1 was coupled to mono
Boc protected piperazine using the amide coupling reagent 1,1ꢀ-
carbonyldiimidazole (CDI),27 which allows for straightforward
workup and subsequent purification by column chromatography.
The coumarin N-Boc-piperazine 2 was deprotected subsequently
with trifluoroacetic acid (TFA). Two equivalents of the free amine
coumarin piperazine 3 were coupled to the dicarboxylic acid switch
4 using 2-chloro-4,6-dimethoxy-1,3,5-triazine (CDMT), a reagent,
which has been found to give good yields in combination with
the dicarboxylic acid switch,26 and N-methyl-morpholine (NMM)
in CH2Cl2 to yield the pure coumarin–switch–coumarin triad
(CSC) 5 in 16% yield (non-optimized) after purification by column
chromatography (Scheme 1).
A switch-acceptor unit was constructed, using an amine sub-
stituted perylene bisimide 9 (Scheme 2), which can be coupled
to the dithienylcyclopentene diacid 4. Perylene bisbutylimide 628
was saponified partially to provide a mixture of the perylene-
bis-anhydride and the perylene-mono-imide-monoanhydride (∼2
: 1, respectively, as determined by 1H NMR spectroscopy).29
The mixture was condensed with 4-amino-1-Boc-piperidine,30 to
yield a mixture of substituted perylene bisimides with either two
piperidines or one butyl and one piperidine group at the imide
positions. The mixture was separated chromatographically pro-
viding the mono N-Boc-piperidine mono butyl perylene bisimide
8 in 8.6% yield from the perylene bis-butylimide 6 (Scheme 2).
Attempts to mono substitute the diacid dithienylcyclopentene
4, with the coumarin piperazine 3 followed by coupling to
Fig. 2 Schematic representation of a donor–switch–acceptor triad in two
different states: (a) the switch is in the open State 1: excitation of the donor
(D), energy transfer to the acceptor (A), followed by sensitized emission.
(b) The switch is in the closed State 2: when D is excited the energy is
quenched by the switch and sensitized emission is prevented.
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Electronic and photochemical properties
Coumarin–switch–coumarin triad (CSC) 5. The absorption
spectrum of CSC 5 in the open and closed (i.e. at the photostation-
ary state (PSS) obtained by irradiation at kexc = 312 nm, PSS312 nm
)
form are shown in Fig. 3. The spectra show features of both
coumarin and open or closed switch and the maxima correspond
closely to those of the model compounds indicating that no direct,
or through-bond, electronic communication between the switch
and the coumarin is present (Fig. 3 and Table 1). Although
at 298 K, irradiation at kexc = 312 nm resulted in 10–20%
decomposition per cycle, at 220 K, photochromic switching was
fully reversible.
Scheme 2 Synthesis of mono piperidine perylene bisimide 9: (A) KOH,
H2O, i-PrOH, 15 h (yield n.d.); (B) 4-amino-1-Boc-piperidine, toluene,
120 ◦C, 24 h (8.6% from 6); (C) CF3COOH, CH2Cl2 (quant.).
the perylene bisimide 9 were unsuccessful due to difficulties in
purification of the mono acid following the first coupling step.
Therefore, it was decided to use a one step procedure with
equimolar amounts of the chromophores followed by isolation
of the desired compound by column chromatography. First one
equivalent of the perylene-mono-piperidine-mono-butyl 8 was
deprotected using TFA and the product 9 (1 equivalent) was
coupled, together with the coumarin piperazine 3 (1 equivalent)
to the diacid dithienylcyclopentene photochromic switch 4 (1
equivalent) to give, in addition to the homo-coupling products,
the target perylene–switch–coumarin triad (PSC) 10 in 4.4% yield
(non-optimized, Scheme 3).
Fig. 3 Absorption spectra of CSC 5 open (—), CSC PSS312 nm (---)
irradiated at 220 K with k312 nm, and coumarin model 2 are shown ( · · · ).
The spectra were recorded at RT in CH2Cl2.
Comparison of the difference spectrum obtained by subtraction
of the spectra of 5 in the open form and in the PSS312 nm with the
difference spectrum of the closed form of the model compound 11
shows the similarity between the two systems, confirming that the
change in absorption observed upon irradiation at 220 K31 is due
to photochemical ring closure and that the photochromism of the
dithienylcyclopentene is retained in the triad (Fig. 4).
CSC 5 can undergo several ring opening–closing cycles with a
near complete recovery in the absorption spectrum of the open
form after each cycle (see ESI†).32
The fluorescence spectrum of the open form of 5 (k ∼390 nm),
is identical to that of the free coumarin (Fig. 5). As for absorption
spectroscopy, photochemical ring closure results in changes to
the luminescence properties of 5. Irradiation at k312 nm results in a
decrease in the intensity of the characteristic coumarin emission by
Scheme 3 Synthesis of PSC 10: (A) CDMT, NMM, CH2Cl2, 0 ◦C; (B)
NMM, 1 eq 3, 1 eq 9, 24 h.
The model compound piperidine–dithienylcyclopentene–
piperidine (pipSpip 11, Fig. 4, right) was prepared by simi-
lar methods as for CSC 5. All compounds were purified by
column chromatography and characterized with 1H and 13C
NMR spectroscopy and (MALDI-TOF) mass spectrometry (see
experimental section for details).
Table 1 Absorption and emission spectra of CSC 5 and PSC 10 open and PSS312 nm
Compound
Absorptiona kmax/nm (103 e/cm−1 M−1
)
Emissiona kmax/nm
2
6
11
11
5
5
10
10
Coumarin pip Boc
Perylene bisimide butyl
pipSpip open
322(18.3)
393
608
266(40.6), 286(49.6), 451(16.7), 539(26.7), 577(43.1)
239(18.0)
236(13.6), 487(29.2)
321 (31.7)
323 (33.3), 493 (4.0)
267 (50.2), 286 (54.2), 452 (13.4), 540 (22.4), 579 (36.2)
267 (47.5), 286 (52.7), 453 (14.4), 540 (23.2), 579 (36.7)
b
pipSpip PSS312 nm
CSC open
CSC PSS312 nm
394
395
391, 612
391, 613
b
PSC open
PSC PSS312 nm
b
a Measurements taken in CH2Cl2 at RT. b At RT after irradiation with k312 nm light at 220 K to PSS.
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closed form of 5 by HPLC was unsuccessful. However, for the
model compound 11, separation was achieved and a ratio of 30%
open–70% closed was determined for PSS312 nm.
34 This value can be
taken as the upper limit for CSC 5 (and for PSC 10, vide infra) since
the PSS is dependent on both the quantum yield for ring opening
and closing, and the relative absorption cross-section of the open
and closed forms atk312 nm. Sincethedithienylcyclopentene switch is
electronically decoupled from the coumarin in 5 the quantum yield
of ring opening and of ring closing are not likely to be significantly
different between 5 and 11. However, the coumarin components
of 5 absorb at k312 nm and energy transfer from the coumarin to the
closed dithienylcyclopentene component will increase the effective
absorptivity of the closed dithienylcyclopentene at this wavelength
in comparison to 11. This will serve to change the photostationary
state of 5 at k312 nm in favour of the open state in comparison to 11.
Fig. 4 Left: the UV–Vis difference spectrum obtained by subtraction of
the spectrum of the CSC 5 PSS312 nm state from the spectrum of the CSC
5 open form recorded at 298 K; the PSS was obtained by irradiation with
k312 nm at 220 K. Right: the difference spectrum obtained by subtraction
of the spectrum of the pipSpip 11 PSS312 nm state from the spectrum of
the pipSpip 11 open form recorded at 298 K; the PSS was obtained by
irradiation with k312 nm at 220 K.
Perylene–switch–coumarin triad (PSC) 10. The absorption
and emission spectra of the PSC triad are shown in Fig. 6 and
Fig. 7, respectively. The absorption spectrum of PSC 10 correlates
closely with the spectrum of a 1 : 1 : 1 mixture of the perylene
bisimide butyl 6, pipSpip 11 and coumarin-pip-Boc 2. The near
perfect overlap of the kmax of the perylene bisimide and coumarin
components indicates that the amide based bridging units do
not allow for significant through-bond electronic communication
between the three units or perturbation of the electronic structure
of the individual components.
Fig. 5 Left: absorption spectra of 5 open (—) and CSC 5 PSS312 nm (---),
and fluorescence spectra of 5 open ( · · · ) and 5 PSS312 nm (-·-). Right: effect
of ring closing and subsequent opening on the fluorescence at k = 394 nm
(ring closing with kexc = 312 nm at 220 K and opening with k >400 nm at
298 K). Spectra recorded in CH2Cl2 at 298 K.
50%, which is reversed by irradiation at k > 400 nm. This change
is not observed when opening and closing a 2 : 1 mixture of the
coumarin model 2 and 11, thus excluding trivial or radiative energy
transfer being responsible for the changes seen in the symmetric
triad 5.
The absence of a change in the absorption of the coumarin band
in the spectrum of 5 upon ring closure of the dithienylcyclopentene
switching unit indicates that the ring closing does not perturb the
electronic structure of the coumarin moieties. Hence the decrease
in emission intensity can be assigned to energy transfer quenching
by the dithienylcyclopentene unit in the closed state. Indeed the
absorption spectrum of the closed dithienylethene unit shows good
spectral overlap with the emission spectrum of the coumarin; a
prerequisite for energy transfer (Fig. 5).
Fig. 6 Absorption spectra of PSC 10 (—) and a 1 : 1 : 1 mixture of the
individual components 2, 6 and 11 (---) recorded in CH2Cl2 at 298 K.
In the open state, the emission of 5 (kexc = 337 nm, kem
=
420 nm) decays monoexponentially with a fluorescence lifetime of
1.1 ns. At the PSS312 nm, in which a mixture of the open and closed
form of the switch are present, it is no longer possible to fit the
emission decay with a monoexponential function. A biexponential
fit provided the expected decay of the coumarin emission 1.1 ns
and a second cross-correlated component, i.e. a component with
a decay lifetime less than the resolution of the instrument (500
ps).33 This latter component can be attributed to emission from
the coumarin in the closed form of 5 in which efficient energy
transfer results in the coumarin emission lifetime being limited
by the rate of energy transfer from the excited coumarin to the
dithienylcyclopentene unit in the closed state.10
Fig. 7 Left: emission spectra of 10 (—) and of the model mixture 1 : 1 : 1
of 2, 6 and 11 (---) irradiated at k322 nm at RT in CH2Cl2; spectra were
corrected for absorption at the excitation wavelength. Right: excitation
spectra of PSC 10 (—) and of the model mixture (a 1 : 1 : 1 ratio of 2, 6
and 11) (---) monitored at k620 nm at RT in CH2Cl2.35
The degree of quenching of coumarin fluorescence observed (in
this case 50% for 5 at the PSS) is dependent on the open–closed
ratio at the PSS state. Separation of a mixture of the open and
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In contrast to the absorption spectra, the emission spectra of the
PSC triad 10 and of a 1 : 1 : 1 mixture of the separate components
show considerable differences. When the 1 : 1 : 1 mixture is
excited at k322 nm, the emission spectrum shows the characteristic
emissions of the coumarin (kem = 393 nm) and perylene bisimide
(kem = 609 nm) components. The excitation spectra recorded at the
maxima of the coumarin and perylene bisimide emissions show
the characteristic shapes of the coumarin and perylene bisimide
absorption spectra, respectively. For the PSC triad 10, excitation at
k322 nm shows emission characteristic of the coumarin and perylene
bisimide components also, however, the coumarin emission is very
weak when compared with the 1 : 1 : 1 mixture and the perylene
bisimide emission is more intense (Fig. 7, left). That this is due
to energy transfer from the coumarin to the perylene bisimide
components is apparent from the excitation spectra recorded at
the emission maximum of the perylene bisimide component. The
excitation spectrum of PSC 10 shows that a large contribution
to the emission at k620 nm originates from an absorption with a
maximum at k ∼325 nm, the kmax of the coumarin absorption.
This contribution is not seen in the excitation spectrum of the
1 : 1 : 1 mixture. This confirms that in 10 in the open state, efficient
intramolecular energy transfer from the coumarin to the perylene
bisimide takes place (Fig. 7, right).
efficiency of quenching of the perylene excited state by the
dithienylethene unit is ca. 55–65%.
Fig. 9 Emission spectra of PSC 10 in the open state (solid line), after
irradiation at k312 nm to form the PSS312 nm state (dashed line) and after
visible (>450 nm) irradiation to reform the open state (dotted line). All
traces were recorded by excitation at k450 nm in CH2Cl2 at RT.
A decrease of 30% in the intensity of the perylene bisimide
fluorescence of 10 in its PSS312 nm is observed (Fig. 10) when excited
at the kmax of the absorption of the coumarin (k = 322 nm). This
could indicate that energy transfer from the coumarin unit is
less efficient in the closed state. This decrease, however, is not
accompanied by a concomitant increase in the fluorescence of the
coumarin component. Hence, the decrease in perylene bisimide
emission intensity is due to the introduction of an alternative
quenching pathway for the coumarin component, and not a
decrease in the overall efficiency of quenching of the coumarin
excited state in the triad.
Absorption and emission spectroscopy of 10 at the PSS312 nm
.
The changes in the absorption spectrum on irradiation to the PSS
at 312 nm are minor, due to the relatively low molar absorptivity
of the switching unit compared with those of the coumarin and
perylene bisimide components (Fig. 8). The changes are more
apparent in the difference spectra and are very similar to those
observed for the model switching unit 11 (Fig. 4 right),36 and
confirms that the dithienylethene unit retains its photochromic
behaviour in the PSC triad 10. As for 5, ring opening and closing
of the switching unit of 10 can be performed over several cycles
(see ESI†).
Fig. 8 Left: PSC 10 open (—) and PSS312 nm (---) irradiated at 220 K37
with k312 nm in CH2Cl2, spectra recorded at RT. Right: difference spectrum
for PSC 10 at t = 0 min and PSS after t = 10 min irradiation at k312 nm at
220 K (see Fig. 4 for comparison with 5 and 11).
Fig. 10 Emission spectra of PSC 10 open (—) and PSS312 nm (---) irradiated
at 220 K with 312 nm in CH2Cl2. All traces were recorded by excitation at
322 nm in CH2Cl2 at RT.
The absence of a large overlap of the absorption of the
dithienylethene component in the closed state and the emission
spectrum of the perylene unit would suggest that energy transfer
between the two units would be inefficient. However, comparison
of the emission spectra of the perylene unit in the open and
PSS312 nm states (under direct excitation i.e. kexc 450 nm, Fig. 9)
shows that the perylene bisimide emission is quenched significantly
(21%) upon ring closing. Taking the low photostationary state
(i.e. <70% closed form) into account, in the closed state the
The reduction in the intensity of the perylene bisimide com-
ponent is reversed upon irradiation at k >400 nm and the
closing–opening cycle can be repeated several times with limited
degradation (<8% per cycle, Fig. 11). The reversibility of the
changes in emission spectrum of 10 confirms that the effect is
due to the opening and closing of the dithienylethene switch
component of the triad. A steady increase of the coumarin
fluorescence (kmax = 391 nm) is observed with each cycle, which
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is assigned to photodegradation of the perylene bisimide unit.
Indeed, when irradiating at k254 nm, rapid decomposition of the
perylene bisimide with a near complete recovery of the expected
emission intensity of the coumarin component is observed (see
ESI†).
of the switch does not perturb the energy of the emissive excited
state of the perylene, i.e. the radiative and non-radiative decay
rates are unaffected.
Fluorescence decay traces were recorded for 10 in the open,
PSS312 nm and reopened state (Fig. 12). Irradiation of the open
state of 10 to the PSS312 nm (closed) state results in a decrease in
emission intensity, however the emission decay lifetimes measured
in either state are unaffected.
Fig. 12 TCSPC spectra of PSC 10 PSS312 nm irradiated with k322 nm light and
fluorescence decay counts measured at 615 nm. All traces were recorded
over the same acquisition time to enable comparison of the signal intensity,
top (black): PSC 10 open, bottom (dark grey): PSC PSS312 nm by irradiation
with k312 nm light for 4 min, and middle (light grey): PSC 10 open after
irradiating the PSS312 nm form with k >400 nm light for 20 min, all traces
recorded in CH2Cl2 at RT.
This indicates that even though there is less energy being
transferred to the perylene bisimide acceptor from the coumarin
donor, this is not caused by a change in the electronic structure
of the perylene bisimide. Upon irradiation with k >400 nm to
reopen the dithienylcyclopentene switch component, the intensity
of the fluorescence increases again, recovering to nearly its original
intensity, as observed by emission spectroscopy (Fig. 12).
Fig. 11 Above: emission spectra of switching cycles for PSC 10 from PSC
open to PSC 10 closed PSS, irradiated with, respectively, k312 nm at 220 K
and k >400 nm light at RT in CH2Cl2, compensated for absorption. Below:
fluorescence intensity at k614 nm plotted for three switching cycles of PSC
10 using k312 nm light at 220 K to close and k >400 nm at RT to open,
measurements were performed in CH2Cl2 and spectra were recorded at
RT.
Discussion
The fluorescence decay kinetics for the triad show considerable
differences, when comparing the open state and PSS312 nm. In the
open form, the emission decay of 10 (recorded at kem = 420 nm) is
biexponential, with a slow component of ∼2.0 ns, assigned to the
fluorescence decay of free coumarin, and a short (cross-correlated)
component, which is attributed to energy transfer (between the
coumarin and perylene bisimide unit) rate limited fluorescence
decay of the coumarin component of the triad. This decay time
is comparable with that observed in the tetra-coumarin–perylene
The symmetric CSC triad 5
In the symmetric triad CSC 5, the ability of the dithienylcy-
clopentene unit to switch between an open and closed state is
apparent from the appearance of the characteristic absorption
band of the closed state in the visible region (kmax ∼500 nm)
upon UV irradiation and its subsequent disappearance upon
irradiation with visible light. The decrease in the fluorescence
intensity of the coumarin components observed upon ring closing
of the dithienylcyclopentene unit can be assigned to energy
transfer quenching of the excited coumarin unit by the closed
dithienylcyclopentene on the basis of an absence of such an effect
in the 2 : 1 mixture of 2 and 11 and the biexponential nature of
the emission decay upon ring closure. The incomplete quenching
(∼50%) of the fluorescence of the coumarin components at
the PSS312 nm is not indicative of inefficient quenching, however,
but reflects the low photostationary state achievable for the
bisimide system reported earlier.9,10 Compound 10, at the PSS312 nm
,
shows biexponential decay kinetics with similar lifetimes and
contributions as in the open state.
The emission decay kinetics of 10 in the open state, recorded at
kem 615 nm (i.e. the kmax of the emission of the perylene bisimide
component), show monoexponential decay kinetics with a decay
lifetime of ∼7 ns, which corresponds closely to the lifetime of
the perylene bisimide model compound 6. The same value was
observed for the PSC 10 PSS312 nm, indicating that the closed form
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dithienylcyclopentene unit, which is less than 70% in favour of
the closed form. This is supported by the fluorescence lifetime
decay traces where, for the open form, strictly monoexponential
behaviour is observed with a lifetime very similar to the free
coumarin component, whereas at the PSS312 nm, the fluorescence
decay is no longer monoexponential but shows two distinct
contributions—a component identical to that observed in the
open state and a cross-correlated component with a lifetime
considerably less than the instrument resolution (i.e. <500 ps)
(Fig. 13).
Comparison of the emission spectra of the open and PSS312 nm
state of PSC 10 shows an intensity decrease of the perylene
bisimide emission, which is not accompanied by a proportional
increase in emission from the coumarin component. Hence,
energy transfer from the coumarin donor is still taking place,
but is directed elsewhere. Considering 5, it is most likely that
energy transfer from the coumarin to the closed form of the
dithienylcyclopentene unit is taking place. The decrease in
emission intensity of 10 upon irradiation to the PSS312 nm state
is ∼30% (Fig. 10), however, this does not take into account the
direct excitation of the perylene bisimide. Surprisingly, in the
closed state however, not only the coumarin emission is quenched
but the perylene bisimide excited state is also partly quenched by
the dithienylethene. This is unexpected since the overlap of the
dithienylethene absorption spectrum and the emission spectrum
of the perylene bisimide is negligible.
Overall the modulation of the fluorescence by photochromic
switching can be attributed to several possible effects. The ring-
closing of the switching unit has two effects, one is an increase in
the rigidity of the triad, thereby increasing the average distance be-
tween coumarin-donor and perylene bisimide-acceptor. Secondly,
by ring closing, a new, low energy chromophore is formed, which
allows for competitive (with the perylene bisimide) energy transfer
quenching of the coumarin emission, as observed for 5. That the
latter mechanism is most likely to be the major effect is confirmed
by the absence of a proportional increase in emission intensity of
the coumarin component in the PSS312 nm state.
In the open state of 10 it is not possible to quench the coumarin
excited state via the dithienylcyclopentene (open switch) since its
lowest excited state lies higher in energy than that of the coumarin
(vide supra), and the rate of fluorescence decay of the coumarin is
limited by the rate of energy transfer to the perylene bisimide unit
(Fig. 14).
Fig. 13 Energy level diagram of the spectroscopic processes observed in
the open and closed state of 5 (C = coumarin, S = dithienylcyclopentene).
The biexponential decay at the PSS312 nm reflects the presence of
both the open form and closed form of the symmetric triad in
solution. Nevertheless, it is clear that the dithienylcyclopentene
component can act as a switchable efficient ‘energy sink’ for the
coumarin components.
The asymmetric PSC triad 10
The ability to quench the fluorescence of the coumarin by the
dithienylcyclopentene component in the closed state but not the
open state can be used to modulate the emission output of
coumarin–perylene bisimide based donor–acceptor systems.9,10 In
the present report the dithienylcyclopentene unit is incorporated
between the energy donating coumarin unit and the energy
accepting perylene bisimide unit, i.e. in the triad 10.
The absorption spectrum of PSC 10 is almost identical to
that of the 1 : 1 : 1 mixture of 2, 6 and 11 indicating that the
amide units employed to link the three components together do
not facilitate ground state electronic communication and that
the covalent tethering of the individual components does not
result in a perturbation of their electronic properties. Indeed the
photochemistry of 11, with respect to ring opening and closing of
the dithienylcyclopentene unit, is retained in 10. With respect to
luminescence, for 10 the covalent attachment of the chromophores
does not affect the spectral shape compared with the emission spec-
tra of the separate components. However, the relative intensities
of each emission component show substantial changes.
When the emission spectrum of the open form of PSC 10 is
compared to that of the model mixture, two aspects are notable.
First it is evident that the intensity of the coumarin fluorescence in
the model mixture is much higher then in the emission spectrum
of 10. Secondly the intensity of the perylene bisimide acceptor
fluorescence is increased significantly in the emission spectrum
of 10 compared with 6. This shows that there is intramolecular
energy transfer from the coumarin-donor to the perylene bisimide-
acceptor only in the triad system and not in the solution of the
mixture of the separate component units.
Fig. 14 Energy level diagram of the spectroscopic processes observed in
the open and closed state of PSC 10 (P = perylene bisimide, C = coumarin,
S = dithienylcyclopentene switch).
In the closed state of 10 there are additional competing energy
transfer and decay processes (Fig. 14). From previous results it
is known that the energy transfer from the coumarin donor to
the perylene bisimide acceptor (k1) is a fast process (i.e. ∼11 ps,
see ref. 10). For k2, energy transfer from the coumarin to the
closed dithienylcyclopentene, to be competitive, must be of the
same order of magnitude or faster, which in this case is probable
since a ∼60% decrease in fluorescence intensity is observed upon
irradiation to the PSS312 nm of 10. Once energy has been transferred
to the closed dithienylcyclopentene, it can dissipate through a fast
non-radiative decay path (k3) or be transferred to the perylene
bisimide (k4). Energy transfer to the perylene bisimide through
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the Fo¨rster mechanism is unlikely since the Ufl for the closed
dithienylcyclopentene is low26 and a high quantum yield is a
prerequisite for efficient FRET. Nevertheless it is clear from Fig. 9
that the dithienylethene unit in the closed state can itself quench
the emission of the perylene bisimide component (k−4), albeit
with low efficiency (55–65%), implying that the proximity of the
perylene and closed dithienylethene components is sufficient to
allow for energy transfer to take place. This means that energy
transferred to the closed dithienylcyclopentene dissipates through
a fast non-radiative decay pathway. Thus in the closed state the
dithienylcyclopentene component provides a fast and efficient
route to quench the coumarin emission as was observed for 5
and as an inefficient route able to quench the perylene emission
only partially.
The present system is comparable to the system of Walz
et al., who have used energy transfer quenching to influence
intramolecular energy transfer in a triad molecule.23 In that system
and in contrast to 10, the photochromic switch (a fulgimide)
is closed and provides the lowest energy state of the system. It
is, therefore, able to quench both of the chromophores’ excited
states (i.e. anthracene and coumarin) and acts as an energy sink
for all intramolecular processes. For the present system (i.e. 10),
the lowest excited state of the photochromic switch in the closed
state lies between the donor coumarin and the perylene bisimide
acceptor excited states (Fig. 14). Hence it is able to quench the
excited state of the coumarin efficiently, however, the perylene
bisimide excited state is quenched only partially.
relative to TMS. CI and EI mass spectra were recorded on a JEOL
JMS-600 mass spectrometer in the scan range of m/z 50–1000
with an acquisition time between 300 and 900 ms and a potential
between 30 and 70 V. MALDI-TOF spectra were recorded on an
Applied Biosystems Voyager-DE Pro. UV–Vis absorption spectra
(accuracy 2 nm) were recorded on a Hewlett-Packard UV/Vis
8453 spectrometer. Fluorescence measurements were performed
on a SPF-500C (SLM Aminco) or a Jobin-Yvon Fluorolog 3–
22 spectrofluorimeter. Luminescence lifetime measurements were
obtained using an Edinburgh Analytical Instruments (EAI) time-
correlated single-photon counting apparatus (TCSPC) comprised
of two model J-yA monochromators (emission and excitation),
a single photon photomultiplier detection system model 5300,
and a F900 nanosecond flashlamp (N2 filled at 1.1 atm pressure,
40 kHz) interfaced with a personal computer via a Norland MCA
card. A 400 nm cut off filter was used in emission to attenuate
scatter of the excitation light (337 nm). Data correlation and
manipulation was carried out using EAI F900 software version
5.1.3. Emission lifetimes were calculated using asingle-exponential
fitting function, Levenberg-Marquardt algorithm with iterative
deconvolution (Edinburgh instruments F900 software). The re-
duced v2 and residual plots were used to judge the quality of the
fits. Lifetimes are 5%.
(5) Coumarin–switch–coumarin (CSC) triad
Diacid 4 (200 mg, 0.58 mmol) was suspended in CH2Cl2 (20 ml)
and placed in an ice bath. Subsequently N-methylmorpholine
(0.13 ml, 1.21 mmol) was added whereupon the solid dissolved. 2-
Chloro-4,6-dimethoxytriazine (192 mg, 1.16 mmol) was added and
the reaction mixture was stirred for 4 h at 0 ◦C, after which another
two equivalents of N-methylmorpholine (0.13 ml, 1.21 mmol) were
added followed by 3 (350 mg, 1.16 mmol). Stirring was continued
Conclusions
In the present contribution, two energy transfer donor–acceptor
systems are reported. In the first system, the function of the
dithienylcyclopentene-based photochromic switching unit as a
molecular switch, i.e. turning coumarin emission on and off, is
demonstrated. In the asymmetric PSC triad system 10 we have
demonstrated that energy transfer efficiency in a donor–acceptor
system can be addressed through modulation of the energy transfer
quenching abilities of a photoactive unit. The low PSS achievable
(<70%) and the poor photostability at room temperature in
the present system, both related to intrinsic properties of the
dithienylcyclopentene unit, will be addressed in further studies.
Nevertheless, the synthetic approach taken enables connection
of the photoactive units covalently without loss of molecular
function. These combined observations show that it is possible to
build a molecular triad that allows for modulation of the energy
transfer in a donor–acceptor system by introducing a switchable
selective quencher of the donor unit, thereby allowing control of
the emission output.
◦
for 1 h at 0 C, and overnight at room temperature. CH2Cl2 (50
ml) was added and the solution was washed with, respectively,
1 M aq. HCl (2 × 20 ml), brine (1 × 20 ml), saturated aqueous
bicarbonate solution (1 × 20 ml) and H2O (1 × 20 ml). The organic
phase was dried over Na2SO4 and the solvent was removed in
vacuo. The resulting solid crude product was purified using column
chromatography (2% MeOH in CH2Cl2, SiO2), yielding the light
1
yellow solid (85 mg, 0.093 mmol, 16%). H NMR (400 MHz,
CDCl3) d = 7.63 (s, 2H), 7.32 (d, J = 8.6 Hz, 2H), 6.82–6.74 (m,
6H), 3.82 (s, 6H), 3.62 (s, 16H), 3.58 (s, 4H), 2.75 (t, J = 7.4 Hz,
4H), 2.10 (s, 6H), 2.09–1.97 (m, 2H) ppm. 13C NMR (101 MHz,
CDCl3) d = 168.5 (s), 163.2 (s), 162.3 (s), 161.8 (s), 155.0 (s), 141.8
(d), 139.2 (s), 135.3 (s), 135.2 (s), 132.3 (s), 130.7 (d), 128.5 (d),
119.4 (s), 112.8 (s), 112.5 (d), 100.4 (d), 55.6 (q), 45.8 (t), 41.7 (t),
37.6 (t), 34.1 (t), 23.0 (t), 14.3 (q) ppm. MALDI-TOF MS (MW
= 916.28) m/z = 916.46 [M+].
Experimental
(8) Mono butyl mono N-Boc-piperidine perylene bisimide
Uvasol-grade solvents (Merck) were employed for all spectro-
scopic measurements. All reagents employed in synthetic pro-
cedures were of reagent grade or better, and used as received
unless stated otherwise. N-Boc-piperazine,38 2,9 3,9 426 and 639 were
prepared according to literature. 1H NMR spectra were recorded
at 400 MHz; 13C NMR spectra at 101 MHz. All spectra were
recorded at ambient temperature, with the residual proton signals
of the solvent as an internal reference. Chemical shifts are reported
Partial saponification of N,Nꢀ-dibutyl-1,6,7,12-tetra(4-tert-
butylphenoxy)perylene-3,4,9,10-tetracarboxylic acid bisimide 6
(3.3 g, 3.0 mmol) was carried out with KOH (75 g, 134 mmol) in
a mixture of isopropyl alcohol (500 mL) and H2O (50 mL) under
a dinitrogen atmosphere by stirring at reflux for 13 h, followed
by separation of the basic aqueous layer. The organic layer was
poured onto an aqueous 10% HCl (1 l) solution and left overnight,
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during which the color changed from green to orange to dark red.
Filtration and thorough washing and drying, yielded a mixture of
perylene bisanhydride and perylene mono butylimide in a ratio
133.5, 133.1, 133.0, 132.6, 131.2, 130.2, 128.7, 126.9, 122.8, 122.71,
120.9, 120.5, 120.3, 120.0, 119.9, 119.7, 119.6, 119.6, 119.5, 113.2,
112.7, 100.7, 55.9, 51.8, 46.1, 42.1, 40.6, 38.2, 37.9, 34.6, 34.4,
31.7, 30.4, 28.7, 23.3, 20.6, 14.7, 14.4, 14.0 ppm. MALDI-TOF
MS (MW = 1735.71) m/z = 1735.89 [M+].
1
of ∼2 : 1 (3.3 g) as determined by H NMR spectroscopy. This
mixture was heated at reflux under a dinitrogen atmosphere in dry
toluene (330 ml) with 4-amino-1-Boc-piperidine (2 g, 10 mmol)
for 3 d. The solvent was evaporated and the remaining crude
product was purified by column chromatography (0.5% MeOH in
CH2Cl2, SiO2) providing the mono butyl mono N-Boc-piperidine
(11) PipSpip
11 was synthesized using a procedure similar to CSC 5 using
piperidine as amine and starting from 4 (200 mg, 0.58 mmol).
Purification provided pipSpip as a cream solid (42 mg, 0.087 mmol,
1
perylene bisimide as a red solid (315 mg, 0.26 mmol, 8.6%). H
NMR (400 MHz, CDCl3) d = 8.22 (s, 2H), 8.20 (s, 2H), 7.26–7.20
(m, 8H), 6.87–6.79 (m, 8H), 5.18–5.02 (m, 1H), 4.42–4.05 (m,
3H), 2.91–2.66 (m, 1H), 2.66 (dq, J = 11.8, 3.5 Hz, 2H), 1.72–1.57
(m, 4H), 1.45 (s, 9H), 1.39 (dd, J = 15.1, 7.5 Hz, 2H), 1.29 (s,
36H), 0.94 (t, J = 7.3, 7.3 Hz, 3H) ppm. 13C NMR (101 MHz,
CDCl3) d = 163.7 (s), 163.4 (s), 155.9 (s), 155.8 (s), 154.4 (s),
152.8 (s), 152.8 (s), 147.2 (s), 132.9 (s), 132.7 (s), 126.6 (d), 122.6
(s), 122.4 (s), 120.5 (s), 120.4 (s), 119.9 (d), 119.8 (d), 119.4 (s),
119.4 (s), 119.3 (d), 119.2 (d), 79.4 (s), 52.0 (d), 44.3 (t), 43.5 (t),
40.3 (t), 34.3 (t), 31.4 (q), 30.1 (t), 28.4 (q), 20.3 (t), 13.7 (q) ppm.
MALDI-TOF MS (MW = 1221.6) m/z = 1221.6 [M+].
1
15%). H NMR (400 MHz, CDCl3) d = 6.84 (s, 2H), 3.59–3.49
(m, 8H), 2.76 (t, J = 7.4 Hz, 4H), 2.05 (s, 6H), 2.09–1.97 (m, 2H),
1.70–1.59 (m, 4H), 1.59–1.47 (m, 8H) ppm. 13C NMR (101 MHz,
CDCl3) d = 163.0 (s), 138.1 (s), 135.1 (s), 134.9 (s), 133.4 (s),
129.9 (d), 37.9 (t), 26.0 (t), 24.6 (t), 22.9 (t), 14.3 (q) ppm. MS(EI)
for C27H34N2O2S2 m/z 482 [M+], HRMS calcd for C27H34N2O2S2:
482.2061, found: 482.2073.
Acknowledgements
The research was financially supported by the Zernike Institute
for Advanced Materials (JHH, JJDdJ) and Nanoned (WRB). The
authors acknowledge T. Tiemersma-Wegman for help with the
HPLC analysis. Prof. J. G. Vos and Dr W. Henry (Dublin City
University) are thanked for their generous access to time correlated
single photon counting facilities.
General deprotection method for BOC protected amines 2 and 8
The Boc protected amine was stirred in a mixture of 1 : 1 CH2Cl2–
CF3COOH for 4 h. An equal volume of water was added and the
mixture was neutralized by addition of solid NaHCO3, after which
the aqueous layer was separated and the organic layer washed with
a saturated NaHCO3 solution (aq). The organic layer was dried
over Na2SO4 and solvent removed in vacuo. The product was used
in subsequent steps without further purification.
Notes and references
1 B. O’Regan and M. Graetzel, Nature, 1991, 353, 737; A. Hagfeldt and
M. Graetzel, Acc. Chem. Res., 2000, 33, 269.
(10) Perylene–switch–coumarin (PSC) triad
2 M. R. Wasielewski, J. Org. Chem., 2006, 71, 5051; F. D. Lewis, R. L.
Letsinger and M. R. Wasielewski, Acc. Chem. Res., 2001, 34, 159; M. R.
Wasielewski, Chem. Rev., 1992, 92, 435.
3 A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley,
C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97,
1515; J. F. Callan, A. P. de Silva and D. C. Magri, Tetrahedron, 2005,
61, 8551.
4 X. Hu, A. Damjanovic, T. Ritz and K. Schulten, Proc. Natl. Acad. Sci.
U. S. A., 1998, 95, 5935; G. McDermott, S. M. Prince, A. A. Freer,
A. M. Hawthornthwaite-Lawless, M. Z. Papiz, R. J. Cogdell and N. W.
Isaacs, Nature, 1995, 374, 517; R. J. Cogdell, A. T. Gardiner, A. W.
Roszak, C. J. Law, J. Southall and N. W. Isaacs, Photosynth. Res., 2004,
81, 207; C. J. Law, A. W. Roszak, J. Southall, A. T. Gardiner, N. W.
Isaacs and R. J. Cogdell, Mol. Membr. Biol., 2004, 21, 183.
5 D. Gust, T. A. Moore and A. L. Moore, Chem. Commun., 2006, 1169.
6 N. J. Turro, Modern Molecular Photochemistry, University Science
Books, Sausalito, 1991.
7 B. W. Van Der Meer, G. Coker III and S.-Y. S. Chen, Resonance Energy
Transfer Theory and Data, VCH, Weinheim, 1994.
8 G. D. Scholes, Annu. Rev. Phys. Chem., 2003, 54, 57.
9 J. H. Hurenkamp, W. R. Browne, R. Augulis, A. Pugzlys, P. H. M. van
Loosdrecht, J. H. van Esch and B. L. Feringa, Org. Biomol. Chem.,
2007, 5, 3354.
10 R. Augulis, A. Pugzˇlys, J. H. Hurenkamp, B. L. Feringa, J. H. van Esch
and P. H. M. van Loosdrecht, J. Phys. Chem., 2008, in press.
11 K. Matsuda and M. Irie, J. Photochem. Photobiol., C, 2004, 5, 169;
F. M. Raymo and M. Tomasulo, Chem. Soc. Rev., 2005, 34, 327; F. M.
Raymo and M. Tomasulo, J. Phys. Chem. A, 2005, 109, 7343; F. M.
Raymo and M. Tomasulo, Chem.–Eur. J., 2006, 12, 3186.
12 P. Belser, L. De Cola, F. Hartl, V. Adamo, B. Bozic, Y. Chriqui, V. M.
Iyer, R. T. F. Jukes, J. Kuhni, M. Querol, S. Roma and N. Salluce, Adv.
Funct. Mater., 2006, 16, 195.
Diacid 4 (93 mg, 0.27 mmol) was suspended in CH2Cl2 (20 ml)
and placed in an ice bath. Subsequently N-methylmorpholine
(0.06 ml, 0.56 mmol) was added whereupon the solid dissolved.
2-Chloro-4,6-dimethoxytriazine (98 mg, 0.56 mmol) was added
and the reaction mixture stirred for 4 h at 0 ◦C, after which
another two equivalents of N-methylmorpholine (0.06 ml, 0.56
mmol) were added followed by the deprotected mono butyl mono
N-Boc-piperidine perylene bisimide 9 (300 mg, 0.27 mmo◦l) and 3
(81 mg, 0.27 mmol). Stirring was continued for 1 h at 0 C, and
overnight at room temperature. CH2Cl2 (50 ml) was added and the
solution was washed with, respectively, 1 M aq. HCl (2 × 20 ml),
brine (1 × 20 ml), saturated aqueous bicarbonate solution (1 × 20
ml) and H2O (1 × 20 ml). The organic phase was dried on Na2SO4
and the solvent was evaporated. The resulting solid crude product
was purified using column chromatography (2% MeOH in CH2Cl2,
SiO2), providing a dark red solid (20 mg, 0.012 mmol, 4.4%). 1H
NMR (400 MHz, CDCl3) d = 8.20 (s, 2H), 8.19 (s, 2H), 7.61 (s,
1H), 7.29 (d, J = 9.2 Hz, 1H), 7.23 (d, J = 8.9 Hz, 8H), 6.94 (s, 1H),
6.85–6.76 (m, 11H), 5.27–5.17 (m, 1H), 4.48 (s, 2H), 4.13–4.07 (m,
2H), 3.82 (s, 3H), 3.66–3.52 (m, 8H), 2.94 (s, 2H), 2.83–2.62 (m,
6H), 2.15 (s, 2H), 2.02 (s, 2H), 2.09–1.98 (m, 1H), 1.61 (s, 3H),
1.76–1.59 (m, 4H), 1.47–1.32 (m, 2H), 1.29 (s, 18H), 1.82 (s, 18H),
0.93 (t, J = 7.3 Hz, 3H) ppm. 13C NMR (101 MHz, CDCl3) d =
168.8, 164.0, 163.6, 163.6, 163.3, 162.5, 162.1, 156.3, 156.1, 155.4,
153.1, 153.0, 147.6, 142.0, 139.6, 138.7, 135.6, 135.6, 135.5, 135.5,
1276 | Org. Biomol. Chem., 2008, 6, 1268–1277
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13 R. A. van Delden, N. P. M. Huck, J. J. Piet, J. M. Warman, S. C. J.
Meskers, H. P. J. M. Dekkers and B. L. Feringa, J. Am. Chem. Soc.,
2003, 125, 15659; N. P. M. Huck and B. L. Feringa, J. Chem. Soc.,
Chem. Commun., 1995, 1095.
14 W. R. Browne, M. M. Pollard, B. de Lange, A. Meetsma and B. L.
Feringa, J. Am. Chem. Soc., 2006, 128, 12412.
28 F. Wu¨rthner, C. Thalacker, S. Diele and C. Tschierske, Chem.–Eur. J.,
2001, 7, 2245; R. Iden and G. Seybold, (BASF AG), Ger. Pat. Appl.,
DE 3434059 A1, 1985 (Chem. Abstr., 1985, 103, 38696q); D. Dotcheva,
M. Klapper and K. Mu¨llen, Macromol. Chem. Phys., 1994, 195, 1905.
29 F. Wu¨rthner, A. Sautter, D. Schmid and P. Weber, Chem.–Eur. J., 2001,
7, 894.
15 S. Abad, M. Kluciar, M. A. Miranda and U. Pischel, J. Org. Chem.,
2005, 70, 10565.
16 T. Komatsu, K. Kikuchi, H. Takakusa, K. Hanaoka, T. Ueno, M.
Kamiya, Y. Urano and T. Nagano, J. Am. Chem. Soc., 2006, 128,
15946.
30 F. Wu¨rthner, B. Hanke, M. Lysetska, G. Lambright and G. Harms,
Org. Lett., 2005, 7, 967.
31 Irradiation was carried out at 220 K to suppress bimolecular reactions,
in particular interference by traces of water.
32 Some degradation is visible per cycle, with the most significant decrease
in intensity of the absorption of the closed state at k = 493 nm after the
first cycle, after which the intensity is seen to stabilize.
33 Cross-correlated: the lifetime of the process is less than the FWHM of
the excitation pulse.
17 H. Tian and S. J. Yang, Chem. Soc. Rev., 2004, 33, 85; M. Irie, Chem.
Rev., 2000, 100, 1685.
18 L. N. Lucas, J. J. D. de Jong, J. H. van Esch, R. M. Kellogg and B. L.
Feringa, Eur. J. Org. Chem., 2003, 1, 155.
19 A. Fernandez-Acebes and J. M. Lehn, Chem.–Eur. J., 1999, 5,
3285; A. Fernandez-Acebes and J. M. Lehn, Adv. Mater., 1998, 10,
1519.
20 M. Irie, T. Fukaminato, T. Sasaki, N. Tamai and T. Kawai, Nature,
2002, 420, 759; T. Fukaminato, T. Sasaki, T. Kawai, N. Tamai and
M. Irie, J. Am. Chem. Soc., 2004, 126, 14843; L. Giordano, T. M.
Jovin, M. Irie and E. A. Jares-Erijman, J. Am. Chem. Soc., 2002, 124,
7481.
34 HPLC separation of pipSpip 11 was performed on an Alltech Econo-
sphere Silica 10 lm column using n-heptane–2-propanol 95 : 5 and a
flow of 1.0 ml min−1. The retention times for the open and closed form
were 29.4 and 32.3 min, respectively. The PSS was determined at k =
304 nm, an isoabsorptive point for both forms.
35 The sharp features between k = 450 and 500 nm are instrumental
artifacts; the spectra are uncorrected for variations in lamp output
intensity.
21 L. Sun, S. Wang and H. Tian, Chem. Lett., 2007, 36, 250.
22 T. B. Norsten and N. R. Branda, J. Am. Chem. Soc., 2001, 123,
1784.
23 J. Walz, K. Ulrich, H. Port, H. C. Wolf, J. Wonner and F. Effenberger,
Chem. Phys. Lett., 1993, 213, 321.
36 The feature at k ∼600 nm is caused by sensitivity of the perylene unit
to solvent polarity. Cooling freezes out residual water present in the
CH2Cl2, thereby decreasing solvent polarity and causing a small shift
in the perylene absorption after the second measurement, which as a
result causes this spectral distortion.
24 A. Hartschuh, I. B. Ramsteiner, H. Port, J. M. Endtner and F.
Effenberger, J. Lumin., 2004, 108, 1; J. M. Endtner, F. Effenberger,
A. Hartschuh and H. Port, J. Am. Chem. Soc., 2000, 122, 3037; P. A.
Liddell, G. Kodis, A. L. Moore, T. A. Moore and D. Gust, J. Am. Chem.
Soc., 2002, 124, 7668.
25 A. Adronov, S. L. Gilat, J. M. J. Fre´chet, K. Ohta, F. V. R. Neuwahl
and G. R. Fleming, J. Am. Chem. Soc., 2000, 122, 1175.
26 J. J. D. de Jong, L. N. Lucas, R. M. Kellogg, J. H. van Esch and
B. L. Feringa, Science, 2004, 304, 278; L. N. Lucas, PhD Thesis,
Towards Photoresponsive Supramolecular Materials 2001, University
of Groningen, ISBN 90-367-1528-8.
37 Irradiation of PSC 10 at room temperature resulted in significant
photodegradation of the PSC triad 10 (see ESI†). However, at 220 K,
degradation is suppressed and switching of the dithienylcyclopentene
component is observed.
38 E. A. A. Walle´n, J. A. M. Christiaans, E. M. Jarho, M. M. Forsberg, J. I.
Vena¨la¨inen, P. T. Ma¨nnisto¨ and J. Gynther, J. Med. Chem., 2003, 46,
4543; L. A. Carpino, E. M. E. Mansour, C. H. Cheng, J. R. Williams,
R. MacDonald, J. Knapczyk and M. Carman, J. Org. Chem., 1983, 48,
661.
39 G. Seybold and G. Wagenblast, Dyes Pigm., 1989, 11, 303; G. Seybold,
A. Stange, (BASF AG), Ger. Pat., DE 35 45 004, 1987 (Chem. Abstr.,
1988, 108, 77134c).
27 G. Anderson and R. Paul, J. Am. Chem. Soc., 1958, 80, 4423.
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