A luminescent steroid-based organogel: ON-OFF photoswitching by dopant interplay and templated synthesis of fluorescent nanoparticles

Article abstract of DOI:10.1039/c2jm34891h

The modulation of the fluorescence emission of a perylene dye between ON and OFF states was achieved with high efficiency by means of a diarylethene photochromic compound in an organogel medium. Fluorescent gels were prepared from a steroidal low molecular weight organogelator doped with the perylene fluorophore. The compatibility of the photochromic compound in this medium and the non-destructive readout capability were demonstrated. The dye doped organogel was further used as a template and an innovative method was employed to grow fluorescent silica spherical nanoparticles with physically and chemically admixed perylene dye.

Full text of DOI:10.1039/c2jm34891h

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COMMUNICATION
A luminescent steroid-based organogel: ON–OFF photoswitching by dopant
interplay and templated synthesis of fluorescent nanoparticles†
a
Valeria C. Edelsztein, Elizabeth A. Jares-Erijman, Klaus Mullen, Pablo H. Di Chenna
a
c
b
*
€
a
*
and Carla C. Spagnuolo
Received 24th July 2012, Accepted 12th September 2012
DOI: 10.1039/c2jm34891h
The morphology of the silica nanoparticles so obtained could be
controlled by choosing the reaction conditions. In the absence of a
catalyst the slow polymerization leads to templated nanotubes of
silica while the fast, base catalysed polymerization rendered silica
nanospheres with high aspect ratio homogeneity.
The modulation of the fluorescence emission of a perylene dye
between ON and OFF states was achieved with high efficiency by
means of a diarylethene photochromic compound in an organogel
medium. Fluorescent gels were prepared from a steroidal low
molecular weight organogelator doped with the perylene fluo-
rophore. The compatibility of the photochromic compound in this
medium and the non-destructive readout capability were demon-
strated. The dye doped organogel was further used as a template and
an innovative method was employed to grow fluorescent silica
spherical nanoparticles with physically and chemically admixed
perylene dye.
On the other hand, perylene dyes are unique fluorophores due to
their optical properties as well as high thermal and photochemical
stability.13 These ultrastable dyes have found applications as organic
super-conductors,14 photovoltaic cells15 and fluorescent bioprobes,16
among others. Some perylene structures have been successfully
modified in order to get self-assembled organogels.17,18 In this study
we work with the tetraphenoxylated perylene diimide derivative
(PDIPhO, Fig. 1).
Photochromic compounds of the family of diarylethenes (DAEs)
have been widely studied during the last few decades as memory
materials, switching components in molecular devices and supra-
molecular systems.19–21 DAEs have remarkable features such as high
thermal stability and high fatigue resistance, which make them
valuable material as optical components, especially as ON/OFF
switches for the emission of fluorophores.22–26
Materials science has found a promising new area of research in the
development of low-molecular mass organogelators (LMOGs).1,2
The potential applications of such well-defined structures include
catalysis, drug delivery, molecular electronics, and so forth.3–7 Among
the LMOGs, steroid analogues with gelating properties have been
used to prepare different soft functional materials.8–11 Recently,
Edelsztein et al. have reported on the synthesis of a novel non-cho-
lesteryl steroid LMOG (1).12 This organogelating pregnane steroid
possesses a rigid and asymmetric structure due to the presence of a
6b,19 oxygen bridge (acting as a hydrogen bond acceptor) and a
complementary hydrogen bond donor (hydroxyl group) at position
5a. These functionalities together with the presence of a 3b-tert-
butyldimethylsilyl ether moiety covalently linked at C-3 make the
structure unique among the steroidal LMOGs. The gels of LMOG 1
have been used for the in situ sol–gel polymerization of tetraethyl
orthosilicate (TEOS).
In this article we present a set of interesting results arising from
experiments designed to explore the versatility and potentiality of
the organogel (OG) from LMOG 1 as a functional material. These
investigations include special optical properties such as ON–OFF
aCIHIDECAR-CONICET, Facultad de Ciencias Exactas y Naturales,
Universidad de Buenos Aires, Dto. Quꢀımica Orgaꢀnica, 3er piso Pabelloꢀn
II, Cdad. Universitaria, Buenos Aires, Argentina. E-mail: carlacs@qo.
fcen.uba.ar; Fax: +54 1145763346; Tel: +54 1145763346
bUMYMFOR(CONICET-UBA), Facultad de Ciencias Exactas y Natura-
ꢀ
les, Universidad de Buenos Aires, Dto. Quımica Organica, 3er piso Pabe-
llon II, Cdad. Universitaria, Buenos Aires, Argentina. E-mail: dichenna@
ꢀ
ꢀ
qo.fcen.uba.ar; Fax: +54 1145763346; Tel: +54 1145763346
cMax Planck Institute for Polymer Research, Ackermannweg 10, D-55128,
Mainz, Germany
† Electronic supplementary information (ESI) available: Synthesis of
LMOG 1 and DAE. Description of steady-state and time resolved
experiments. See DOI: 10.1039/c2jm34891h
Fig. 1 Chemical structures of PDIPhO, LMOG 1 and DAE.
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fluorescence photoswitching achieved by dopant interplay in gels of
LMOG 1 and the characterization of fluorescent silica nanospheres
prepared from the luminescent organogel. We report on the pho-
tophysical studies of a donor–acceptor system comprising a perylene
dye PDIPhO and a DAE derivative27 as dopants in an optically
transparent organogel of LMOG 1 in TEOS and n-hexane. We
studied the changes in the optical properties of the perylene dye in
the gel, the dependence of fluorescence emission with temperature,
the influence of the dopant on the morphology of the gel and the
modulation of the emission of the dye by the photochromic
compound, through steady state and time resolved fluorescence
techniques. The study of such a switching system, especially the
photochromic behavior of DAE in the organogel matrix, is pre-
sented as a proof of principle for the development of an organo-
gelator composed of a perylene derivative covalently bound to 1, to
build sensor systems and other photonic devices based on soft
materials. We include in this communication a related application of
the fluorescent organogel as a template for the preparation of
emissive inorganic nanoparticles. This type of nano-structures have
found extensive use in bioanalysis and biodetection and could be
easily miniaturized for portable devices.28 We envision future
investigations on this gel–nanoparticle transformation with lumi-
nescent organogels highly pre-organized, e.g. with covalently
bound dye.
Fig. 2 Variation of the fluorescence intensity with temperature for a
PDIPhO doped TEOS gel.
For the subsequent optical experiments we decided to work with
OG from TEOS because it is far more stable than the OG from
n-hexane, given the volatility of this solvent and the lower Tg.
For the OG 1% from TEOS doped with PDIPhO a value of steady
state anisotropy (rss) of 0.045 was determined while it was 0.00 for the
dye in solution, at 610 nm. When the gel was heated from 10 ꢀC to
100 ꢀC, rss decreased, indicating higher mobility of PDIPhO in the sol
phase compared to that in the gel phase (Fig. S9†).
The structure and morphology of the doped organogel
Photochromism in the organogel
LMOG 1 was synthesized according to ref. 12 (see also the ESI†).
Perylene doped organogels from TEOS and n-hexane were prepared
by heating the suspensions until the solid dissolved and the solutions
were cooled to room temperature. The morphology of the perylene
doped organogel was analyzed by SEM observations of the xerogel
fibers obtained from n-hexane. The images showed an entangled
fibrillar network identical to that of the xerogel of 1 with fibers of the
same length and width (Fig. S18†). This observation indicates that
perylene does not intercalate between the steroid molecules and must
be distributed in the liquid phase that is immobilized within the 3D
self-assembled fibrillar structure of the gel.
The absorption spectra of the open and closed form of DAE were not
affected by its inclusion in the gel matrix of the OG from TEOS
(Fig. S10†). Upon irradiation of the gel using UV light (band pass
filter of 340 nm) the photochromic conversion was observed and an
absorption band around 560 nm appeared, which corresponds to the
closed form of DAE along with a change in the colour of the gel from
colourless to purple. Subsequent irradiation with visible light (long
pass filter of 550 nm) decoloured the gel raising the absorption band
of the open form at 313 nm.
Modulation of the fluorescence emission of the perylene
dye
Optical properties of the dye-doped organogel
An OG 1% from TEOS doped with PDIPhO and DAE was prepared
to study the quenching of the fluorescence emission of the dye by the
presence of DAE in its closed form. The photochromic compound
was embedded in the gel in its open form. Once the gel was formed,
the sample was irradiated with UV light for 180 seconds. Increasing
the irradiation time led to an increase in the amount of DAE closed
form, with an absorption maximum at 560 nm, thus matching the
emission of perylene. As a consequence an efficient overlap takes
place yielding a high degree of quenching. The loss of fluorescence
emission at 590 nm was 98.6%. As is shown in Fig. 3, the emission
intensity can be gradually switched off by increasing the irradiation
time with UV light. Once the OFF state was reached, the gel was
irradiated with visible light for 30 seconds to recover the full emission
intensity. This UV-Vis irradiation cycle between the two isomers of
the DAE compound was repeated more than 20 times rendering
reproducible ON–OFF states (Fig. 4).
The fluorescence emission maximum of PDIPhO in the OG from
TEOS matched exactly the spectrum of the dye in solution while it
was slightly shifted in the gel from n-hexane for comparison (Fig. S2
and S3†). The fluorescence intensity was similar in the OG compared
to that in solution, suggesting that no aggregate formation of the dye
takes place in the gel medium. The fluorescence lifetime of PDIPhO
was determined to be 5.82 ns in solution and 5.77 ns in the neat OG.
The fluorescence intensity of the perylene doped OG from TEOS
decreased 35% when the temperature was raised from 10 ꢀC to 100
ꢀ
ꢀC (Fig. 2). Tg was estimated to be 68 C from this measurement
which is in accordance with the value determined from the DSC
experiment (72 ꢀC (ref. 12)). This experiment was also performed for
a gel of LMOG 1 1% in n-hexane with similar results: Tg (fluo.) ¼
45 ꢀC and Tg (DSC) ¼ 52 ꢀC.
The variation of the excitation spectrum with the temperature was
also measured and no change was observed. This suggests that
PDIPhO exists as monomers in the gel state similar to the sol state.
A rather similar system can be tracked in the literature where a
diarylethene compound is used to modulate the fluorescence emission
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Fig. 5 Non-destructive fluorescence readout capability of both open
(B)- and closed (C)-DAE isomers.
Fig. 3 Decrease of the fluorescence intensity of PDIPhO and fluores-
cence lifetime changes (inset) upon increasing irradiation times of the
DAE open form with UV light.
Templated synthesis of fluorescent silica nanoparticles
Organogels from LMOG 1 have been previously used as a template
for the synthesis of high aspect ratio inorganic materials as silica
nanotubes and nanospheres.12 Fluorescent dye-doped silica nano-
particles have demonstrated excellent potential for use in advanced
bioanalysis to facilitate deeper understanding of biology and medi-
cine at the molecular level. In particular, silica nanoparticles have
been applied to diagnostic and therapeutic applications in cancer and
gene/drug delivery.24 Integration of fluorescent dye-doped silica
nanoparticles (FSNPs) with functional molecules using various
surface modification techniques can bring about great improvements
in bioapplications such as selective recognition, sensitive imaging, and
reporting.
Regarding this aspect, we aimed at the preparation of fluorescent
silica nanospheres by the in situ sol–gel polymerization process using
the fluorescent organogels of LMOG 1 and PDIPhO described
previously. Gels of LMOG 1 from TEOS doped with PDIPhO
produced, after 5 days of base catalyzed sol–gel polymerization
conditions, emissive spherical particles of silica with homogeneous
size (200–300 nm) (Fig. 6a). The nanoparticles were exhaustively
washed with methanol until no fluorescence emission was observed at
610 nm in the supernatant.
Fig. 4 ON–OFF photoswitching of the fluorescence of PDIPhO in
the gel.
of an organogel but in this case the organogelator itself has an
emissive behaviour, with absorption and emission activity in the UV
and Vis region, respectively.29 A significant quenching is also
demonstrated although no further confirmation on the energy
transfer mechanism is reported, such as time-resolved determinations.
To investigate in our system whether the quenching was due to a
radiative or non-radiative energy transfer process, we performed the
quenching of the fluorescence by increasing the irradiation time with
UV light and measured the fluorescence lifetime (s) of PDIPhO in the
gel. As can be seen from Fig. 3 (inset), s is practically unmodified thus
indicating the prevalence of a trivial radiative energy transfer between
PDIPhO and DAE (closed form).
Non-destructive fluorescence readout
The fluorescence intensity at 590 nm did not change under contin-
uous irradiation for more than 2 hours within the fluorimeter
(4 mW cmꢁ2) (Fig. 5).
A very promising non-destructive readout capability is achieved in
this system and the fluorescence emission modulation in the gel is
good enough to achieve an efficient logic operation.
Fig. 6 SEM images of silica nanoparticles prepared by template
synthesis from organogel. (a) PDIPhO doped silica Np; (b) PDI-APTES
silica Np.
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Fig. 7 PDI-APTES.
The fluorescence spectrum of the silica nanoparticles showed an
emission band at 610 nm (Fig. S20†). The perylene dye successfully
survived the conditions of the polymerization process. Schneider and
Mu reported that alkoxy-silane modified perylene derivatives that
could attach covalently to the silica matrix during the polymerization
of TEOS enhance the solubility resulting in a higher concentration of
the dye as well as its photostability.30 As a first approximation to
covalently bound fluorophore-silica nanoparticles, a simple alkoxy-
silane-PDI (PDI-APTES, Fig. 7) was prepared from 3,4:9,10-per-
ylenetetracarboxylic dianhydride and APTES, which as expected had
lower solubility than the phenoxy-substituted perylene-
tetracarboxydiimides. However, its solubility sufficed to incorporate
it homogeneously in the sol–gel process under the same conditions as
for the physically admixed PDI-PhO.
Fig. 8 LSCM images of silica nanoparticles. (a) PDI-APTES silica Np;
(b) PDI-PhO silica Np. Left: fluorescence; right: transmission.
Morphologically, the nanoparticles obtained with the covalently
attached dye are bigger (300–600 nm). After successive washings
with methanol/ethanol, the particles showed an emission peak at
610 nm (Fig. S22†). Table 1 lists the main photophysical properties
of both fluorescent nanoparticles in comparison with the free dyes.
The fluorescence lifetimes of the dyes in the nanoparticles, whether
covalently bound or physically admixed, had almost no change
compared with the free dyes in solution. On the other hand, the
steady state anisotropy (rss) rendered significant values for the dyes
in the inorganic matrix, which reflects the difference in polarization
between bound and non-bound states. This property has
potential applications to use the nanoparticle as a probe for studying
local viscosity in different media. Both types of fluorescent
nanoparticles with perylene dye physically or chemically attached
were also observed in a confocal microscope. Fig. 8 shows images of
these nanospheres adsorbed onto a microscope slide. It can be seen
that for doped nanoparticles, the image shows darker nanoparticles
accounting for the loss of fluorescent molecules during the
washes (Fig. 8b). In the case of PDI-APTES Nps the higher
number of bright particles indicates the presence of covalent
bonded PDI molecules that could not be eliminated by washing the
material.
preparation along with variations of the precursors of the inorganic
matrix in order to achieve smaller nanoparticles.
Conclusions
We have established a simple photoswitchable system with ultrastable
optical components such as a perylene dye and a diarylethene
photochromic compound, integrated in an organogel matrix. A
highly efficient energy transfer process occurs between PDIPhO and
DAE included in the organogel. The ON–OFF photoswitching
process is performed by alternative cycles of irradiation of the gel with
UV-Vis light. The organogel based on LMOG 1 offers a suitable
medium for optical applications. These results provide the proof of
principle for further studies on more controlled and complex systems.
The preparation of a compound which combines perylene dye linked
covalently to the steroid fragment is currently under progress. Such a
compound will be studied as a single organogelator molecule in order
to develop soft materials with optoelectronic functions. We have
established a method for the preparation of doped and covalently
bound perylene-silica nanoparticles by the in situ sol–gel polymeri-
zation of TEOS gels of LMOG 1. The nanospheres so obtained were
fluorescent and stable proving the presence of the fluorophore unit in
the nanoparticles. These results are the first step toward the prepa-
ration of functionalized, highly fluorescent photostable nano-
materials from organogels of LMOG 1 with promising biomedical
and biotechnological applications.
The biocompatibility of silica makes them promising for in vivo
imaging applications. A further increase of the dye concentration in
the nanoparticle could be reached by adding substituents at the bay
position of perylene-3,4:9,10-tetracarboxydiimides, which is known
to enhance even more the solubility. Such a derivative is under
Acknowledgements
This work is dedicated to Elizabeth Jares, in her memory and in
acknowledgment of her passion for life and work, inspired leadership
and unfailing support. We thank Francisco Guaimas for the
Table 1 Photophysical properties of silica nanoparticles
lmax/nm
s/ns
rss
ꢀ
microscopy images and Guillermo Menendez for advice on time
PDI-APTES
PDIPhO
PDIPhO doped Np
PDI-APTES Np
567
615
611
611
4.3
5.8
5.5
4.9
0.00
0.00
0.36
0.20
resolved measurements. The authors are grateful to ANPCyT
(Argentina), CONICET (Argentina), University of Buenos Aires and
Max Planck Society for financial support. V.C.E. is also grateful to
CONICET for a post-doctoral fellowship.
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16 K. Peneva, G. Mihov, F. Nolde, S. Rocha, J.-i. Hotta,
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