On the reversible photoisomerization of spiropyran-modified zeolite l single crystals

Full text of DOI:10.1002/cphc.200900860

DOI: 10.1002/cphc.200900860
On the Reversible Photoisomerization of Spiropyran-Modified Zeolite L
Single Crystals
R. Q. Albuquerque,[a] J. Kꢀhni,[b] P. Belser,[b] and L. De Cola*[a]
Photoisomerization of spiropyrans has been the subject of
many interesting studies in a wide number of fields, such as
sensors for cations,[1] sensors of pH,[2] switchable surfaces,[3]
lasing effect,[4] among many others. This molecule can be pho-
toisomerized upon UV irradiation, giving rise to the zwitterion-
ic merocyanine (MC) form, which dramatically changes its hy-
drophilicity and photophysical properties (see Scheme 1). Irra-
Zeolite L is a porous aluminosilicate material consisting of
thousands of one-dimensional nanochannels parallel to each
other, which run throughout the long axis of the crystal.[8] Zeo-
lite L can be used not only as luminescent devices,[9] in bio-
imaging,[10] or in photodynamic treatments,[11] but also in many
other applications.[12] This class of crystals can still be function-
alized on glass, indium–tin oxide or other substrates and can
be synthesized from 50 nm up to 20 mm,[13] which may be stra-
tegic, depending on the application. SP-functionalized zeolite L
crystals with encapsulated dyes may be an interesting option
to make new dual emitters, which can be used in bio-imaging
experiments, even without taking advantage of energy transfer
between encapsulated dyes and SP. Because cells may exhibit
auto-fluorescence, a system having two different emissions at
different times allows for distinguishing between the auto-fluo-
rescence and the luminescence of the dual-emitters, simply by
direct visualization of the sample in the microscope.
Scheme 1. Spiropyran (SP) undergoes photoisomeration upon UV irradiation
to form the zwiterionic merocyanine (MC) form. MC can be then back-iso-
merized to the SP form by visible irradiation or heating. The succinimid
group attached at the nitrogen allows binding the spiropyran to amino
groups.
The advantages of using zeolite L intead of, for example,
silica nanoparticles, are: 1) the size of the zeolite L crystals can
be synthetically tuned from 50 nm to 20 mm, in order to be
used in different experiments; 2) the luminescence of the en-
capsulated dyes, which can be highly anisotropic, is also
strong because the inserted dyes are in the monomeric form
and are completely isolated from the outer bio-environment;
3) zeolite L has been already shown to be easily functionalized
with targeting groups, enabling them to attach to different
living organisms.[11]
diation in the visible region or heating brings back the spiro-
pyran (SP) to the MC form. Solvent polarity and pH have been
already shown to strongly influence the properties of spiropyr-
ans.[5] SP does not absorb in the visible range, while the mero-
cyanine, which absorbs at ca 570 nm (in DMSO), exhibits red
fluorescence and its lifetime is typically about 260 ps.[6]
Recently the use of spiropyrans in the field of dual emission
for bio-imaging has been reported, where silica nanoparticles
containing spiropyran and a green-emitting perylene derivative
were shown to exhibit green or red emission depending on
the UV/Vis irradiation.[7] Such systems may become even more
efficient if the dyes embedded inside the porous silica nano-
particles aggregate as little as possible, as well as if the dyes
are completely isolated from the biosystem being labeled. This
problem may be addressed by combining spiropyrans with
versatile porous materials, such as zeolite L, which are known
to encapsulate organic dyes in the monomeric form and to
protect them against the outer chemical environment.
Herein we show for the first time the reversible photoisome-
rization of a spiropyran derivative (at the single crystal level)
functionalized at the channel entrances of zeolite L and discuss
the use of such single crystals as dual emitters in the field of
bio-imaging. In a first step zeolite L can be loaded with a
green-emitting dye (pyronine) and afterwards its outer surface
can be functionalized with a spiropyran derivative. This system
is also an interesting model for sensors based on energy trans-
fer, because the spectral overlap necessary to make a Fçrster-
type energy transfer from the encapsulated dye to the spiro-
pyran (or vice versa, depending on the dye) strongly depends
on the form of the spiropyran (SP or MC), which may be tuned
by UV/Vis irradiation of the system.
[a] Dr. R. Q. Albuquerque, Prof. L. De Cola
Physikalisches Institut and Center for Nanotechnology (CeNTech)
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Heisenbergstrasse 11, 48149 Mꢁnster (Germany)
Fax: (+49) 251-980-2834
The ninhydrin test used to quantify the number of amino
groups functionalized at the outer surface of the 1 mm long
zeolite L crystals indicates that there are ca 5.23ꢁ10ꢀ8 mol NH2
groups in 1 mg of zeoliteꢀNH2. This value is 75 times larger
than the expected one, which would have been obtained
given 100% NH2 coverage of zeolite L. The maximum number
of amino groups expected to cover the zeolite L crystals was
roughly estimated by considering a perfect cylinder with
length/diameter of 1 mm/800 nm, whereas the size distribution
E-mail: decola@uni-muenster.de
[b] Dr. J. Kꢁhni, Prof. P. Belser
Institute of Inorganic Chemistry
University of Fribourg, CH-1700 Fribourg (Switzerland)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.200900860.
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ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
575

of the crystals, as well as the existence of terraces, were not
taken into account. Considering the approximations used in
this estimate, one concludes that the zeolite L surface is cov-
ered with amino groups 100%, as indeed suggested by Fig-
ure 2c. However, an excess of FMOC–APMS used in the amino
functionalization of the zeolite L, which would lead to some
cross-linked material on the surface of the crystals, cannot be
excluded, since it would give rise to a larger number of amino
groups detected in the ninhydrin test, as indeed observed.
The absorption spectrum of the synthesized SP in DMSO,
before and after UV irradiation, as well as the red emission of
the photo-generated MC, are shown in Figure 1. The solvent
DMSO was chosen because of its basic pH, which avoids the
protonation of the negatively charged oxygen of the MC form,
giving rise to an absorption spectrum peaked at 570 nm. The
MC species is subject to strong solvatochromism, a strong red
shift being observed upon increasing the polarity of the sol-
vent, which is due to the high hydrophilicity of that species. In-
terestingly, the SP shown in Scheme 1 has exhibited the rather
rare negative photochromism[14] when in H2O:EtOH mixed in a
1:1 ratio (v/v), where the coloured MC species is the most
stable form and is in fact spontaneously formed in the dark
(Figure S1, Supporting Information). This may be due to a
strong stabilization of the open form (MC) by hydrogen bonds
and/or protonation. Protonation, however, seems to be a sec-
ondary effect to explain the negative photochromism of SP in
the H2O/EtOH mixture, since solutions of SP in acidic solvents
like CH2Cl2, where protonation may occur, give rise to a posi-
tive photochromism, where the colourless SP is the most
stable form in the dark. After UV irradiation at 366 nm, excita-
tion of MC at 460 nm produces a red luminescence peaked at
ca 650 nm, whose intensity
Figure 1. Absorption (a) and emission (c) spectra of the spiropyran in
DMSO (10ꢀ4 m) at RT before and after UV irradiation at 366 nm. The relatively
noisy emission spectra were recorded upon excitation at 460 nm, where MC
weakly absorbs, in order to prevent the light-induced back-isomerization
MC!SP.
whose photoisomerization was also shown to be reversible, as
shown in Figure 2c. To the best of our knowledge, this is the
first time that a reversible photoisomerization of a spiropyran
at the single-crystal level is reported, as shown in Figure 2b.
Figure 3 shows the colour changes of zeolite L crystals
loaded with pyronine and functionalized with SP after irradia-
tion with UV. The luminescence was generated by continuously
exciting at 360 nm with the microscope excitation cube. Imme-
diately after UV irradiation, when SP is photoisomerized to MC,
slowly decreases as a result of
the thermally induced back-iso-
merization MC!SP (Figure 1).
Excitation at the main absorp-
tion band of MC (ca 570 nm)
also induces the back-isomeriza-
tion MC!SP.
The confocal picture of a SP-
functionalized zeolite L crystal is
shown in Figure 2a, which was
obtained upon excitation at
535 nm, after short exposure of
the crystals to the daylight. One
can see from the distribution of
the red emission of MC that the
4 mm long zeolite L crystals are
predominantly functionalized at
the channel entrances. The func-
tionalized single crystals exhibit
reversible photoisomerization, as
shown in Figure 2b. Functionali-
zation of the small zeolite L crys-
Figure 2. a) Confocal microscopy picture of a 4 mm long zeolite L crystal functionalized with SP, mostly localized at
the channel entrances. The red emission comes from the MC form (exc=535 nm) after exposing the crystals to
daylight. b) Epifluorescent microscopy pictures showing the reversible photoisomerization of a single crystal of
tals (method 2, Experimental
Section) gave rise to crystals
completely covered by MC, zeolite L functionalized with SP. c) The same as in (b), but using smaller crystals (ca. 1 mm long).
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ChemPhysChem 2010, 11, 575 – 578

orange luminescence is produced, which is the sum of the
green luminescence of pyronine and the red luminescence of
MC. The green emission of pyronine radiatively isomerizes MC
back to the non-emissive SP form, which results in the increas-
ing green content in the emission of the samples upon going
from panels 1 to 4 (Figure 3).
nel entrances in the case of 4 mm long crystals, as shown by
the fluorescence microscopy pictures. The supramolecular
hybrid system has shown to have a potential to be used as
dual-emitters for bio-imaging, as nicely shown by the change
from reddish to green fluorescence with the time. The devel-
opment of SP-zeolite L systems for dual-emission and bio-sen-
sors is currently being investigated in our group.
Experimental Section
Spiropyran: Spiropyran (SP) 1’-(2-(carbosuccinimidyl-oxy)ethyl)-3’,3’-
dimethyl-6-nitrospiro[2H–1]benzopyran-2,2’-indoline (Scheme 1) has
been synthesized according to a procedure previously described.[15]
The compound was purified by usual chromatography techniques
and stored at 48C in the dark.
Zeolite L: Zeolite L was synthesized following a hydrothermal pro-
cedure described elsewhere.[16] Zeolite L crystals featuring 4 mm/
1 mm and 1 mm/800 nm length/diameter aspect ratios were used.
All the crystals were cation exchanged with K+ before any reaction.
Some of the zeolite L crystals were loaded with the green emitting
dye pyronine, by means of cation exchange from the solution.
Figure 3. Fluorescence microscopy pictures showing dual emission coming
from pyronine-loaded zeolite L crystals functionalized with SP (in air). The
samples were continuously excited at 360 nm, and then pictures 1 to 4 were
taken with time intervals of 1 min.
Amino-Functionalized Zeolite L: The 1 mm long zeolite L crystals
were completely amino-functionalized by following a well-known
procedure:[17] 9-fluorenylmethyl carbamate N-hydroxysuccinimidyl
esther (FMOC-NHS) reacts with (3-aminopropyl)methoxydimethylsi-
lane (APMS) to give FMOC-APMS, which then covalently binds to
the free OH groups present in the external surface of the zeolite L
crystals. The ninhydrin test was carried out to quantify the number
of amino groups present in the functionalized zeolites.
Fçrster resonant energy transfer (FRET) from pyronine to MC
could not be observed by means of emission spectra, perhaps
because the loading of the encapsulated dye was small (less
than 1%), which means that the distance between the encap-
sulated pyronine and the spiropyran was considerably larger
than the Fçrster radius R0. The use of a higher loading of the
encapsulated dye would in fact lead to smaller distances be-
tween the spiropyran and the pyronine, but on the other hand
would not lead to a system showing dual-emission properties,
since in this case only the extremely intense luminescence of
the pyronine would be observed, and this is out of the scope
of the present work.
SP-Functionalized Zeolite L: This was carried out following two dif-
ferent methods.
Method 1: Inside a teflon tube SP still activated with the succinimid
group reacted with a stoichiometric amount of aminopropylme-
thoxysilane (APMS) in freshly distilled CH2Cl2 for 1 h to give SP–
APMS (Scheme 2), following a similar procedure reported in the lit-
erature.[17] From this solution, a stoichiometric calculated amount
was taken and added to a teflon tube containing a previously soni-
cated suspension of 10 mg of 4 mm long zeolite L in n-hexane. The
calculated amount of SP–APMS was equivalent to the number of
channel entrances present in the amount used of zeolite L. After
15 min of sonication of the reactional mixture, a reflux was carried
out for 3 h. The functionalized crystals were then washed with
n-hexane and centrifuged several times and then stored in the
dark at 48C.
Method 2: 1 mm long zeolite L crystals were first amino-functional-
ized as described above. A solution of 10ꢀ4 m SP in freshly distilled
CH2Cl2 was slowly added to a stirring suspension of amino-modi-
fied K-exchanged zeolite L (1 mgmLꢀ1) in the same solvent and left
reacting for 1 hour at room temperature. The crystals were then
washed with CH2Cl2 and centrifuged several times, and afterwards
stored in the dark at 48C. This procedure was also made using
4 mm long zeolites previously loaded with pyronine in order to in-
vestigate dual-emission properties.
The results shown in Figure 3 open new possibilities for
using hybrid materials based on zeolite L as dual-emitters. It is
worth saying that the whole hybrid supramolecular system can
be further covered with a protective layer of silica,[9] which
may enable one to study dual emission using the functional-
ized zeolites not only in the external membrane surface, which
may be in contact with the physiological medium, but addi-
tionally inside the cells, where the high hydrophobicity would
make it difficult for SP to photoisomerize to the highly hydro-
philic MC species. A sensor for cations is another interesting
application for such hybrid systems. As an example, the closure
time of the SP ring has shown to have a t1/2 of 2.4 s in CH3CN,
while addition of a very small amount of Co2+ has increased
this time to 6.9 s (Figure S2).
Measurements: Absorption spectra were measured on a Varian
Cary 5000 double-beam UV/Vis–NIR spectrometer and baseline cor-
rected. Steady-state emission spectra were recorded on a HORIBA
Jobin–Yvon IBH FL-322 Fluorolog 3 spectrometer equipped with a
450 W xenon arc lamp. The fluorescence pictures of the zeolites
showing the reversible photoisomerization of SP were made with
the confocal microscope DM IRB (Leica), the samples being irradiat-
We showed for the first time a reversible photoisomerization
of a derivative of spiropyran at the level of a single crystal by
using zeolite L. The synthesized spiropyran exhibited rare neg-
ative photochromism when dissolved in H2O:EtOH 1:1. The spi-
ropyran was indeed functionalized predominantly at the chan-
ChemPhysChem 2010, 11, 575 – 578
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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577

Acknowledgements
RQA would like to acknowledge the Alexander von Humboldt
Foundation and SFB TR161 for financial support.
Keywords: photochemistry
crystals · spiropyran · zeolites
· photoisomerization · single
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Received: November 3, 2009
Published online on January 4, 2010
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