Photochemical characterization of biomimetic molecular switches

Article abstract of DOI:10.1016/j.tet.2011.07.070

The performance of fluorenylidene-pyrroline (FPs) and N-alkylated fluorenylidene-pyrroline (NAFPs) derivatives for their use as light-driven molecular switches has been studied. Both types of compounds share fast and controllable photoisomerization. Other

Full text of DOI:10.1016/j.tet.2011.07.070

Tetrahedron 67 (2011) 7570e7574
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Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Photochemical characterization of biomimetic molecular switches
*
Laura Rivado-Casas, Marina Blanco-Lomas, Pedro J. Campos, Diego Sampedro
~
Departamento de Química, Universidad de La Rioja, Grupo de Síntesis Química de La Rioja, Unidad Asociada al C.S.I.C., Madre de Dios, 51, 26006 Logrono, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The performance of fluorenylideneepyrroline (FPs) and N-alkylated fluorenylideneepyrroline (NAFPs)
derivatives for their use as light-driven molecular switches has been studied. Both types of compounds
share fast and controllable photoisomerization. Other competitive reaction pathways that could lead to
low efficiency have been considered. Only weak fluorescence was measured and high photostability was
found when irradiating these compounds for long times, together with high photoisomerization quan-
tum yields. NAFPs are capable of using visible light, which could be useful for practical applications.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 29 June 2011
Accepted 21 July 2011
Available online 28 July 2011
Keywords:
Molecular switches
Photochemistry
Photoisomerization
Sigmatropic rearrangement
Retinal
1. Introduction
a photoreceptor protein (a visual pigment), which is located in the
rod visual cells responsible for twilight vision. Rh has 11-cis retinal
The synthesis and applications of molecular switches based on
photochemical E/Z isomerizations have received much attention in
recent years.1,2 Among this type of compounds, azobenzene de-
rivatives have been extensively used for very different uses in
several contexts to convert light energy into ‘mechanical’ motion at
the molecular level.3,4 Thus, azobenzene-based switches have been
employed to control ion complexation,5 to modify electronic
properties,6 or to trigger folding/unfolding of oligopeptides7,8 and
other biological applications.9,10 Although, their unique properties
as effective switches, azobenzene derivatives are just one type of
compounds capable of performing controlled E/Z isomerization.
Consequently, the preparation and characterization of novel
switches that differ from azobenzene in properties, such as size,
polarity and isomerization mechanism represents an attractive
research target. The discovery of new or alternative building blocks
could expand the applicability of the switch concept to different
and increasingly complex molecular environments. A use of
the above principle led to the preparation of chiral diarylidenes,
featuring a single isomerizable C]C bond. These systems afford
light-driven molecular rotation along the central double bond
and the chiral framework imposes a preferential direction of
isomerization.11,12
as its chromophore, which is embedded inside a single peptide
transmembrane protein called opsin. In Rh a selective photo-
isomerization of the 11-cis chromophore (PSB11) occurs via evo-
lution of a single
p/p
* excited state (S1) that survives for only
150 fs and yields, upon decay, the all-trans ground state (S0)
product with a 67% quantum yield.14 These properties make Rh an
excellent starting point for the design of E/Z switches. However it
should be noted that irradiation of PSB11 in solution features an
unselective isomerization and a picosecond excited state lifetime.14
Thus, the search for artificial Rh-mimetic molecules capable of
maintaining similar properties in solution seems an attractive
research target. In this context, theoretical calculations can also be
helpful in order to get some insight into the features needed for
efficient switches based on the PSB11.15
Using this strategy, it has been recently reported the synthesis16
of an N-alkylated indanylideneepyrroline (NAIP, Fig. 1) Schiff base
that displays, in methanol solution, excited state properties similar
to those of Rh-embedded PSB11.17 These switches display absorp-
tion maxima in the near-UV region. Clearly, the design and syn-
thesis of switches capable of absorbing in the visible constitute
a major target. Bearing this idea in mind we designed a new type of
switch based in the PSB11 where the indanylidene unit of NAIPs
One of the most remarkable examples in Nature of a molecular
motor is the retinal chromophore of rhodopsin (Rh), which suffers
a cisetrans photoisomerization during the process of vision.13 Rh is
was replaced for moieties displaying a more expanded p-system.
Specifically, we focused on switches where the indanylidene unit
has been replaced by a fluorenylidene unit. Through the prepara-
tion of a library of fluorenylideneepyrroline (FPs) and N-alkylated
fluorenylideneepyrroline switches (NAFPs, Fig. 1), we showed that
it is possible to obtain biomimetic light-driven switches where the
* Corresponding author. E-mail address: diego.sampedro@unirioja.es(D. Sampedro).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.07.070

L. Rivado-Casas et al. / Tetrahedron 67 (2011) 7570e7574
7571
absorption maximum of the unsubstituted system was red-shifted
2. Results
by >50 nm with respect to that of the NAIPs. We also showed that,
as it happened with NAIP switches, FPs and NAFPs undergo Z/E
and E/Z photoisomerization displaying a photomodulable sta-
tionary state.18,19
2.1. Excited state multiplicity
Our first step was to determine the nature of the excited state
involved in the photoisomerization. This is relevant not only to
explore alternative reactive pathways, but also to study the lumi-
nescence of these compounds. In order to do this, several runs of an
experiment with three different samples of a solution of an FP
switch were performed. The samples in every run were irradiated
at the same time using a merry-go-round photochemical reactor
(see Experimental section for details). A solution of 1 (R1¼EtO,
R2¼Ph, R3¼Me, R4¼CO2Me, R5¼H, Fig. 2) in acetonitrile was used as
blank, while two more solutions were prepared with oxygen-
saturated solvent and 5 equiv of cis-piperylene, respectively, as
triplet quenchers. After irradiating for 5 min all three samples for
every run afforded the same mixture of isomers composed by
77ꢁ1% of the starting isomer 1 and 23ꢁ1% of the photoisomer 10.
Thus, in all three cases, including the samples containing triplet
quenchers, the photoisomerization proceeded equally. This sug-
gests an excited state of singlet nature. This is not surprising due to
the fact that the photoisomerization of the model reference for the
design of NAFP switches, the PSB11, also takes place in the singlet
potential energy surface, as it happens with related compounds.20
Fig. 1. NAIP and NAFP switches.
In our previous study,18 we proposed the use of FPs and NAFPs as
potentially practical molecular switches. We also compared the
performance of both types of compounds. Thus, NAFPs feature
a strong absorption in the visible that make them quite useful when
low-energy irradiation is needed. Due to this, clean and cheap
sunlight could be used as the energy source. Even more, photo-
isomerization of NAFPs is four times faster than FPs. Although
NAFPs present some promising features, a complete study of the
photochemical behavior is necessary before more complex appli-
cations could be envisaged. This is especially important in the case
of a new molecular switch, but it should be noted that in many
cases different compounds are proposed and used as molecular
switches without a complete understanding of the competitive
pathways hindering their role as effective switches. Before a certain
compound could be defined and used as a good molecular switch,
one should be aware of different processes that could lead to
a decrease in the efficiency. Thus, it is important not only to de-
termine the switching capacity of new compounds, but also to
quantitatively establish the relevance of other processes that could
eventually lead to a poor performance as a molecular switch. For
instance, luminescence, low quantum yields of isomerization, slow
isomerization, low photostability or side-reactions yielding by-
products could turn a promising candidate into a compound
without practical use. Thus, herein we report our study on the
competitive processes that could thwart the practical uses of FPs
and NAFPs.
Fig. 2. Photochemical isomerization of FP and NAFP switches.
2.2. Fluorescence measurements
The E/Z isomerization process for FP and NAFP derivatives has
been previously reported.18 The progress of the photoreaction
starting from pure samples of any isomer can be easily followed by
1H NMR to yield equivalent mixtures (see Supplementary data). The
competitive thermal isomerization takes place in a much longer
time scale (more than 15 h at 50 ꢀC). Several competitive pathways
were explored in order to decide the proficiency of FPs and NAFPs
as molecular switches. First, an efficient luminescent deactivation
was considered. If fluorescence or phosphorescence is quantita-
tively important, only a fraction of the molecules will be able to use
the light energy to isomerize. In second place, photostability of FPs
and NAFPs was checked by irradiating the samples well beyond the
photostationary state. Also, as a measure of the efficiency of the
switching process, the isomerization quantum yield at low con-
versions was measured. Finally, the effect of different wavelengths
in the switching process was checked.
Once assessed the nature of the excited state involved in the
photoisomerization, we aimed for the study of deactivation pro-
cesses that could diminish the efficiency of the FP switches. We
measured the emission and excitation spectra of a 3ꢂ10ꢃ5 M so-
lution of 1 in deoxygenated acetonitrile at 298 K. Only a weak
emission band centered at 430 nm was obtained when exciting
from 270 nm to 350 nm (see Experimental section for details). The
fluorescence lifetime was found to be 4.30ꢂ10ꢃ9 s. In order to de-
termine the fluorescence quantum yield, a solution of trans-stil-
bene in deoxygenated hexane was used as standard of fluorescence.
A solution of 1 in deoxygenated acetonitrile was also prepared and
the emission spectra for both compounds were measured using
an excitation wavelength of 290 nm. Under these conditions,
a fluorescence quantum yield of 0.06 (ꢁ0.02) was found. This val-
ue shows that deactivation through fluorescence does not effec-
tively compete with the isomerization process. In fact, this result

7572
L. Rivado-Casas et al. / Tetrahedron 67 (2011) 7570e7574
emphasizes the adequate design of these switches as only a small
fraction of the light energy is wasted in the radiative decay.
irradiated a pure sample of 5 with a medium-pressure Hg lamp
and without a Pyrex filter. Compound 5 has a band maximum
located at 270 nm (Fig. 3). However, no back-reaction was found
and 4 could not be detected in the reaction crude. We also checked
the thermal back-reaction by heating 5 in refluxing toluene for 2
days. Again, formation of 4 was not detected and 5 could be
recovered unaltered. Thus, formation of 3 and 5 is not reversible
and it could be used to synthesize pyrrole derivatives using more
adequate substrates.
2.3. Photostability
Next, we explored the photostability of FPs and NAFPs. In order
to be useful, compounds acting as switches should be photostable
so as to allow as many photocycles as possible to take place. Thus,
we checked the photostability of these compounds by irradiating
them well beyond the photostationary state. Although in a typical
photoisomerization (see Experimental section for details) the
photostationary state is reached within 0.5 h using a 125 W
medium-pressure Hg lamp, after irradiating 2 (R1¼EtO, R2¼Ph,
R3¼Me, R4¼H, R5¼CO2Me, Scheme 1) for 100 h, no decomposition
was observed by 1H NMR, as only the two isomers could be
detected. Thus, FPs showed again good features as switches for
practical applications. With the aim of checking the limits of the
photoisomerization, we irradiated even longer periods of time us-
ing a 400 W medium-pressure Hg lamp. After more than 150 h,
new signals were found in the 1H NMR spectrum. This new com-
pound could be characterized as 3 (Scheme 1).
Fig. 3. UV spectra for 4 and 5.
2.4. Isomerization quantum yield
Once studied the competitive pathways that could decrease the
competence for the switching process in FPs and NAFPs, we aimed
for a direct measurement of their efficiency. Thus, we evaluated the
isomerization quantum yield of 1 (FP) and 6 (NAFP, R1¼EtO, R2¼Ph,
R3¼Me, R4¼CO2Me, R5¼H, R6¼Me, Fig. 2) using trans-azobenzene
as actinometer (see Experimental section for details). For the FP 1,
we obtained an average value of 0.6 (ꢁ0.1) after several runs. For
the NAFP 6 a value of 0.5(ꢁ0.1) was found. It should be noted that
these values for the quantum yield are comparable to those found
for the PSB11 in vivo (0.67 for Rh)13,14 and bigger than those
reported for biarylidenes (0.07e0.55).11 Thus, both FPs and NAFPs
can efficiently use the light energy into the switching process and
show very promising features for their use in practical applications.
Scheme 1. Rearrangement of 2 and 4 to yield 3 and 5.
Formation of 3 can be explained by a photochemically allowed
[1,3]sigmatropic rearrangement. Although this reaction clearly does
not affect the use of FPs as switches due to the different time scale of
the two processes (0.5 h with a 125 W lamp vs more than 150 h
with a 400 W lamp) it could be of some interest in other context.
Thus, we also checked the photostability of NAFPs by irradiating 4
(R1¼EtO, R2¼Ph, R3¼Me, R4¼H, R5¼H, R6¼Me, Scheme 1) for long
times using a 400 W Hg lamp. After ca. 160 h compound 5, the
rearranged product analogue to 3, was also found and, in this case, it
could be obtained in 98% after recrystallization. Thus, as it has been
seen in the preliminary studies of FPs and NAFPs,18 the methylated
analogues react faster. This transformation presents no practical use
as it takes place in these compounds, due to the long irradiation
times needed to yield the rearranged products. In fact, the systems
were designed to minimize competitive reactions. However, the
same methodology could be used for the synthesis of pyrrole
derivatives using compounds where the energy could be effec-
tively employed in the rearrangement. For instance, analogues of 5
have been recently used in the synthesis of merocyanine dyes,21
act as direct precursors of stable cyclic alkylaminocarbenes22 and
take part on the structure of certain alkaloids,23 just to mention
some applications. However, regarding the photoisomerization
process of FPs and NAFPs, this rearrangement could be considered
negligible as both types of compounds present considerable
photostability. The process shown in Scheme 1 could be due to
photochromism, either type P or T. In order to check this, we
2.5. Photostationary state composition
In order to further evaluate the potential use of these com-
pounds, we explored the effect different experimental conditions
could cause in the photostationary state (PSS). Once assessed the
lack of competitive reaction paths, the use of FPs and NAFPs as
switches could be affected by the experimental conditions. Spe-
cifically, the composition of the photostationary state is relevant
when it comes to practical applications as it will determine the
macroscopic modification induced for the switch once in-
corporated in a complex system. For FPs the photostationary state is
composed by a mixture of ca. 45:55 (1:10) as shown by the value
found for 1 (see Experimental section for details, Table 1).
Table 1
Photostationary states and thermal equilibria for FPs and NAFPs
1 (FP)
6 (NAFP)
h
D
n
45:55
62:38
57:43
59:41

L. Rivado-Casas et al. / Tetrahedron 67 (2011) 7570e7574
7573
Interestingly, in the analogue NAFP the main isomer found in the
PSS is different with a mixture of 57:43 (6:60) (Table 1). Also, the
equilibrium between isomers 1 and 10 can be displaced from the
PSS after heating at 50 ꢀC for several hours. Thus, the main isomer
can be easily modified by choosing between light and heat. How-
ever, in the case of the mixture of 6 and 60, the composition remains
almost unaltered. In this case, FPs show a more versatile behavior
than NAFPs as different stimuli can afford a different main com-
ponent of the mixture. This could be important in practical appli-
cations in which on/off switching cycles are necessary.
We also checked the effect of the solvent in the PSS. Irradiation
on both FP 1 and NAFP 6 in acetonitrile and chloroform slightly
affected the relative rate constants18 but the PSS remained the same
within the experimental error. Finally, we explored the wavelength
dependence in the photoisomerization of 6. This factor could allow
for an easy and fast change in the PSS under a very simple modi-
fication in the reaction conditions. Using a monochromatic light,
we followed the photoisomerization process at 330 and 433 nm
(see Experimental section for details). We chose those wavelengths
spectrometer. 1H NMR samples were internally referenced to TMS
(0.00 ppm).125 W or 400 W medium-pressure Hg lamps were used
for the photoisomerization. The synthesis of 1, 2, 4, and 6 has been
already reported.18
4.2. Typical procedure for the irradiation
A solution for each compound was prepared and irradiated at
room temperature under Ar atmosphere through Pyrex glass with
a medium pressure-mercury lamp (125 W). The isomerization re-
action was followed by 1H NMR.
4.3. Excited state multiplicity
Three different solutions 0.005 M of 1 in acetonitrile were ir-
radiated in quartz tubes with a medium-pressure Hg lamp using
a merry-go-round reactor. One of the solutions was used as a blank.
In the other two, a triplet quencher was added. In one case the
solvent was saturated with O2 by bubbling for 15 min. In the other
case, 5 equiv of cis-piperylene were added. The three solutions
were irradiated at the same time for 5 min. After that, the product
composition was analyzed by 1H NMR. Several runs were per-
formed to ensure experimental liability.
because they have the same extinction coefficient (
3 ¼1060) at both
sides of the absorption band maximum. Under these conditions,
solutions of the same concentration will absorb exactly the same
amount of light. We found again the same composition of the PSS
(57% of 6 and 43% of 60) for the irradiation at these two wavelengths
for 6 h. This is not surprising as 6 and 60 have almost identical UV
spectra and are almost equally stable. However, this fact empha-
sizes the usefulness of these compounds under low-energy irra-
diation, as the use of quite long wavelengths affords the same
results than isomerization under more energetic conditions.
4.4. Fluorescence measurements
Luminescence spectra, fluorescence lifetime and fluorescence
quantum yield were recorded at room temperature with a spec-
trofluorimeter and lifetime data were fitted using the spectrofluo-
rimeter software. Quartz cuvettes (1.0 cm path length) were used
for the measurements. In order to determine the fluorescence
quantum yield, a solution of trans-stilbene in deoxygenated hexane
was used as standard of fluorescence. A solution of 1 in de-
oxygenated acetonitrile was used for the fluorescence lifetime and
quantum yield measurements see Supplementary data for spectra.
In the last case, a solution of trans-stilbene in hexane was used as
standard as its absorption and emission spectra are similar to 1.24
The emission spectra for both compounds were measured using
an excitation wavelength of 290 nm.
3. Conclusions
The design of molecular devices capable of performing con-
trollable movements can be done using natural molecules as
models. In this case the rhodopsin chromophore inspired the de-
sign of two types of molecular switches. Fluorenylideneepyrroline
(FPs) and N-alkylated fluorenylideneepyrroline (NAFPs) derivatives
show promising features for their use as light-driven molecular
switches. Both types of compounds share some characteristics that
make them capable of practical applications. Not only show fast and
controllable photoisomerization, but also other competitive re-
action pathways than could lead to low efficiency are at a mini-
mum. Only very weak fluorescence was measured and high
photostability was found when irradiating these compounds for
long times. Thus, light energy is mainly used in the photo-
isomerization process, which takes place with a quantum yield
higher than related compounds and comparable to those found in
the PSB11 in vivo. Although both FPs and NAFPs share most of the
advantages, they also differ in some features. For FPs the photo-
stationary state can be only slightly affected by the reaction con-
ditions, but using heat as the external stimuli the main isomer
found in the equilibrium is different. NAFPs share the same ratios of
ca. 1:1 when both heat and light are used as stimulus. This is
probably the main drawback of these switches as these values
would prevent complete exploitation of energy. Still, these values
can be modified once the switches are included in a more complex
system. However, NAFPs are capable of using low-energy visible
light, which could be very useful for practical applications in the
presence of a complex matrix and sunlight exploitation.
4.5. Isomerization quantum yield
The photoisomerization quantum yield for 1 and 6 was mea-
sured using trans-azobenzene as actinometer.25 A solution of the FP
or NAFP in acetonitrile was prepared in such a way that the ab-
sorption a 313 nm was the same for both solutions. Irradiation at
313 nm was performed using a monochromator with a 500 W Hg
arc lamp placed in a proper lamp housing, being the samples placed
in quartz cuvettes (1.0 cm path length). Solutions of the actinom-
eter were irradiated before and after the irradiation of the switches
to ensure the stability of the light source. Due to the short irradi-
ation times of this experiment, thermal isomerization of com-
pounds 1 and 6 was not considered as thermal isomerization
requires heating for longer times. Quantification of the samples was
performed by 1H-NMR using 1,3,5-trimethoxybenzene as standard.
4.6. Photostationary state composition
4. Experimental section
To measure the PSS at different wavelengths, solutions of 1 and
6 in different solvents were converted to 1/10 or 6/60 mixtures using
monochromatic light until no change in composition was observed
by 1H NMR. A 500 W Hg (Xe) lamp was used as light source. A water
filter was used to remove infrared radiation and wavelength was
selected with a monochromator.
4.1. General experimental methods
All commercially available materials were used without further
purification. NMR spectra were recorded on a 300 or 400 MHz NMR

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L. Rivado-Casas et al. / Tetrahedron 67 (2011) 7570e7574
4.7. Synthesis of 3 and 5
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A solution of 2 or 4 was prepared and irradiated at room tem-
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pressure-mercury lamp (400 W) for ca. 150 h. The reaction was
followed by 1H NMR and TLC until consumption of the starting
material. Compound 3 is obtained as an inseparable mixture. All
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ization was made by comparison of its spectroscopical data with
those of 5, but results from GC/MS and HRMS could be obtained
from the crude. GC/MS: 438 (Mþ1, 10), 323 (20), 104 (100). HRMS
(C29H28NO3) calculated 438.2069, obtained 438.2064 In the case of
5, the reaction proceeds almost quantitatively. The product can be
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d 8.48e8.47 (m, 1H), 7.39e7.21 (m, 9H),
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J¼6.97, 6.94, 6.94 Hz, 1H), 1.94 (dd, J¼4.81, 2.40 Hz, 1H), 1.73 (s, 3H),
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179.0, 161.7, 146.4, 143.3, 143.0, 142.6, 142.4, 141.2, 131.1, 130.8,
ꢀ
129.2, 129.1, 128.8, 128.7, 126.5, 126.4, 125.9, 125.5, 121.3, 121.2,
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trile):
l
222 nm
(
3
¼21823 Mꢃ1
cmꢃ1),
l
279 nm
ꢀ
ꢀ
Andruniow, T.; Ferre, N.; Basosi, R.; Olivucci, M. Angew. Chem., Int. Ed. 2007, 47,
(
3
¼11096 Mꢃ1 cmꢃ1). HRMS (C30H30NO3) calculated 452.2220,
414.
obtained 452.2228. Melting point (uncorrected) 157e159 ꢀC.
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Leonard, J.; Haacke, S.; Canizzo, A.; Chergui, M.; Zanirato, V.; Fusi, S.;
ꢀ
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We thank the Spanish MEC (CTQ2007-64197) for financial
ꢀ
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