Photochromic fluorescent diarylethene nanocrystals grown in sol-gel thin films

Article abstract of DOI:10.1016/j.dyepig.2010.03.017

Nanocrystals of two diarylethene compounds, 1,2-bis(2-ethyl-5-phenyl-3- thienyl)perfluorocyclopentene (D1) and 1,2-bis(3-methyl-2-thienyl)ethene (D2), were grown in sol-gel thin films and their optical properties (fluorescence and photo-conversion) were c

Full text of DOI:10.1016/j.dyepig.2010.03.017

Dyes and Pigments 89 (2011) 241e245
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photochromic fluorescent diarylethene nanocrystals grown in solegel thin films
Nathalie Sanz-Menez , Virginie Monnier , Isabelle Colombier , Patrice L. Baldeck ,*,
a
a
b
b
c
a
Masahiro Irie , Alain Ibanez
Institut Néel, CNRS and Université Joseph Fourier, B166, F-38042, Grenoble Cedex 9, France
Laboratoire de Spectrométrie Physique, Université Joseph Fourier, CNRS (UMR 5588), BP8, F-38402, Saint Martin d’Hères Cedex, France
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 January 2010
Received in revised form
Nanocrystals of two diarylethene compounds, 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene
(
D1) and 1,2-bis(3-methyl-2-thienyl)ethene (D2), were grown in solegel thin films and their optical
properties (fluorescence and photo-conversion) were compared to that of diarylethene microcrystals.
Ó 2010 Elsevier Ltd. All rights reserved.
11 March 2010
Accepted 16 March 2010
Available online 30 March 2010
Keywords:
Nanocrystal
Photochromic
Diarylethene
Solegel films
Fluorescence
1. Introduction
allows a better localization of excitation volume and a higher
discrimination against the surrounding background [11].
Organic photochromic materials have attracted much attention
because of their potential applications to rewritable optical
memory media and photo-optical switches [1]. Among various
types of photochromic dyes, diarylethene derivatives are the most
promising molecules for these applications because they undergo
ring-closing and ring-opening reactions when irradiated with UV
and visible light with an excellent thermal stability of both isomers
and a high degree of fatigue resistance [2e5]. Moreover, when they
are included in supramolecular assemblies, they can exhibit a large
number of interesting properties: for instance, photoregulation of
luminescence when they are combined to tetraazaporphyrins [6],
photoregulation of Lewis acidity when a boron atom is incorpo-
rated in the photochromic backbone [7] and in general multiple
photoswitching when they are associated to polymers, chiral
molecules, liquid crystals or molecular magnets [8,9]. Thus, the
potential applications offered by the diarylethenes cover a large
field: optical data storage, optical limiters, switches, logic gates,
molecular wires or sensors [10]. Photochromic diarylethenes
exhibiting large two-photon cross sections are now particularly
investigated for 3D optical storage. Indeed, two-photon absorption
However, to design optical devices, photochromic reactions
should take place in the solid state, such as dyes dispersed in
polymer matrices [12], in amorphous solids [13] or in single crystals
[14e16]. Irie and co-workers demonstrated that using single crys-
tals for photochromism was the most favourable approach because
side fatigue reactions were strongly suppressed and the reaction
efficiency was found to be extremely high [17]. Moreover, various
diarylethene derivatives are found to undergo reversible photo-
chromic reactions in the single-crystalline phase [18e20]. Despite
of these major advantanges, the industrial development of molec-
ular crystals is limited due to delicate growths and shaping (slicing,
polishing..) for the design of optical devices.
In order to overcome the basic mechanical drawbacks of organic
crystals, we engineered new hybrid organic-inorganic materials
through a simple and generic preparation of stable organic nano-
crystals grown in solegel thin films [21e23]. We chose the high
versatility of the solegel chemistry based on silicon alkoxide
precursors and also its ability to obtain stable inorganic host-
matrices close to the room temperature. Thus, solegel process can be
involved in the shaping (dispersion, grafting, nanocrystallization.)
of a wide variety of organic fluorophores. It has been extensively
shown that the solegel chemistry can be used to encapsulate organic
molecules in inorganic matrices, bulk or thin films [24]. In all these
previous works, dyes were dispersed within or grafted onto solegel
*
Corresponding author. Tel.: þ33 4 76 51 47 82; fax: þ33 4 76 63 55 95.
E-mail address: patrice.baldeck@ujf-grenoble.fr (P.L. Baldeck).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.03.017

242
N. Sanz-Menez et al. / Dyes and Pigments 89 (2011) 241e245
networks [25]. Our process is based on the confined nucleation and
growth of organic crystals in the pores of solegel glasses. The
crystalline nature of these nanoparticles was previously evidenced
for different dyes by differential scanning calorimetry [26] (presence
of a melting point), by advanced spectroscopy techniques [27] and
by electron microscopy [28]. These nanocomposite films not only
combine the optical properties of organic crystals (luminescence,
NLO properties, photochromism [22,29,30]) with those of amor-
phous inorganic materials (high stability, transparency, convenient
processing and shaping), but also the advantages of the crystalline
phase. Indeed the small size of the nanocrystals leads to the prep-
aration of non-scattering materials for visible and IR wavelengths;
secondly, through the dye nanocrystallisation, the chemical stability
and photostability of organic phases are also improved in compar-
ison to isolated molecules dispersed in solutions or in solid matrices
such as solegel or polymers. Finally, because a nanocrystal is an
a 150 W xenon lamp with a power density of 2 mW/cm2 applied
during 30 s. The lamp was associated to a glass filter of 100 nm
bandwidth, centered at 335 nm. The corresponding absorption
spectra were recorded with a Lambda-9 PerkineElmer spectro-
photometer. Spatially resolved fluorescence experiments on an
individual nanocrystal were performed using a Zeiss Axiovert 200
microscope in which an optical fibber of 50 mm diameter joined the
optical image plane of an ꢁ100 objective (numerical aperture 1.3)
to an Ocean Optic spectrometer. D2 crystals and solegel films were
irradiated with a mercury lamp filtered at the wavelength of
2
365 nm (4 mW/cm on sample). They were probed with an unpo-
larized halogen lamp of sufficiently low energy to avoid the photo-
induced back reaction. The whole spectrum is recorded within less
than 2 s after the end of the irradiation process. The spectral
window is [400e800 nm] and the point spread function is 250 nm.
4
8
aggregation of about 10 to 10 organic molecules organized in
a crystalline network, the light excitation can be spread in these
nanoparticles that can induce a behaviour of single fluorescent
emitter [22]. This paper concerns the preparation and the photo-
physical properties of two types of diarylethene nanocrystals grown
in solegel films.
3. Results and discussion
We define by D1A the open-ring isomer of D1 and by D1B the
closed-ring one (Fig. 1, top scheme). Same notation was used for D2
Fig. 1, bottom scheme). D1 was found to undergo a thermally
(
reversible photochromic reaction in solution as well as in the
single-crystalline phase [20]. Upon irradiation with ultraviolet
light, solution containing D1A molecules and single-crystal D1A
turned blue along with the formation of closed-ring isomer D1B
2
. Experimental section
We selected two organic dyes, 1,2-bis(2-ethyl-5-phenyl-3-
(Fig. 1, top scheme). The blue solution and single crystal returned
thienyl)perfluorocyclopentene and 1,2-bis(3-methyl-2-thienyl)
ethene respectively named D1 and D2 in this paper which exhibit
photochromic properties in the crystalline state. The organic
synthesis of these two diarylethenes was previously described
colourless by irradiation with visible light (
l
> 500 nm) or by
ꢀ
heating above 100 C. The bleaching is due to the cycloreversion of
D1B to D1A. The D2 single crystal undergoes the same photo-
chromic reaction (Fig. 1, bottom scheme) in the single-crystalline
phase and emits fluorescence only from the open-ring form isomer
D2A. Upon ultraviolet light irradiation, the colourless solution or
the corresponding crystal change to yellow D2B. Thus, the formed
close-ring isomer is non-fluorescent.
[
19,20].
D1 and D2 nanocrystals grown in solegel thin films were
obtained by spin-coating. We first prepared solutions containing
solvent, dye, silicon alkoxides and water, which are inserted in
airtight flasks. In this work, solutions were obtained from an
equimolar mixture of two alkoxide precursors: TMOS (tetrame-
thoxysilane) and MTMOS (methyltrimethoxysilane). The sols were
prepared under neutral conditions (pH z 7) to avoid protonation
that can induce an optical properties changing of organic mole-
cules. The process was a one-step hydrolysis and condensation of
Fig. 2 shows optical transmission images of D1 nanocrystals in
solegel films. Before irradiation (Fig. 2a), nanocrystals appeared
colourless. D1 nanocrystals turned clearly blue upon irradiation
with 335 nm light (Fig. 2b). In previous work, Irie and co-workers
[
20] showed that light could penetrate the crystal as deep as
00 m and change the color. To confirm this photo-induced
conversion in nanocrystals, absorption spectra of solegel films
Fig. 2c) were recorded. The absorption maximum in the visible
4
m
ꢀ
alkoxide precursors, which was performed during 15 h at 80 C for
ꢀ
D1 and 15 h at 60 C for D2 with one water molecule per alkoxide
(
function eOR (h ¼ [H O]:[eOR] ¼ 1). THF (analytical grade) was
2
region was found at 650 nm. The residual absorption found in this
used to dissolve the organic dyes and to mix the hydrolysis water
with alkoxide precursors, the molar ratio [solvent]:[alkoxide], s,
was equal to 5. Heating allowed to rapidly dissolve the organic
powder and to enhance the hydrolysis and condensation kinetics
leading to the formation of silicate chains dispersed in the solution.
This sol ageing was necessary to control the sol viscosity, around
10e20 cps, to obtain high quality films, to ensure the control of
the solegel coating thickness and the particle growth and to avoid
the nanocrystal coalescence. The resulting sols, which were stable
during several weeks, were deposited at room temperature by
spin-coating onto thin microscope slides with a rotation speed of
4
000 rpm during 10 s. Thus, films of around 0.5 mm thick were
prepared. These nanocomposite coatings were then stabilized by
ꢀ
annealing at 80 C. Dye concentration, expressed as the molar
ꢂ3
ratio d ¼ [dye]:[Si] z 5 ꢁ10 , where [Si] is the concentration of
alkoxide molécules, depends on the dye solubility in THF, the
nature of the host matrix and on particles sizes which are necessary
to be achieved. The latter was targeted here between 200 and
5
00 nm in diameter to favour nanocrystals characterizations by
epifluorescence microscopy.
All the optical characterizations were done at room tempera-
ture. UV irradiation on D1 solegel films was performed using
Fig. 1. Photochromic reactions for 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclo-
pentene (D1A-D1B) and for 1,2-bis(3-methyl-2-thienyl)ethene (D2A-D2B).

N. Sanz-Menez et al. / Dyes and Pigments 89 (2011) 241e245
243
Fig. 2. Optical transmission images of D1 nanocrystals grown in solegel film (a) before irradiation, (b) after irradiation and (c) corresponding absorption spectra.
region before irradiation was due to spontaneous irradiation during
samples preparation. The absorption apex was the same as that of
D1B registered for single crystal [20]. This confirms that the blue
color is due to the formation of D1B isomer nanocrystals. Further-
more, solegel films returned colourless by irradiation with visible
light (l > 500 nm). Absorption spectra of D2 nanocrystals are not
presented because of their small optical densities in the blue, that
were to difficult to measure with our spectrometer.
D2 individual nanocrystals in solegel film were observed by
epifluorescence microscopy (Fig. 3a). When excited at 365 nm, the
open-ring isomer D2A in single crystal is fluorescent, and the close-
ring D2B is not fluorescent [18,19]. Thus, we can assume the
formation of D2A fluorescent isomer nanocrystals in solegel
matrix. Particles size distribution was found to be between 200 nm
and 350 nm. The fluorescence spectrum of one individual D2
nanocrystal was compared with that of one single D2 microcrystal
(Fig. 3b). The fluorescence maximum was observed at 525 nm for
the individual nanocrystal. A slight shift to longer wavelengths was
thus observed compared to the fluorescence maximum of the
single microcrystal (515 nm). This is due to the matrix polarity
effect in the case of D2 nanocrystals in solegel film. To confirm this
result, the fluorescence maximum of the D2 molecule dissolved in
THF, which is the polar solvent used in the preparation of the
solegel thin films, was also measured (Fig. 3b). In this case, the apex
was also found at 525 nm. For comparison, the fluorescence
maximum in 3-methylpentane (non-polar solvent) was measured
at 500 nm [18]. These results are consistent with the fluorescence
red-shift of the open-ring isomer D2A when the environment
Fig. 3. (a) Fluorescence image of D2 nanocrystals in solegel film and (b) normalized fluorescence spectra of an individual nanocrystal compared to the fluorescence emission of
a single microcrystal and of D2 powder dissolved in THF solution.

244
N. Sanz-Menez et al. / Dyes and Pigments 89 (2011) 241e245
Fig. 4. Evolution of fluorescence signal during coloring-bleaching steps of (a) one D2 microcrystal and (b) one individual D2 nanocrystal.
polarity increases. The evolution of the fluorescence emission
during coloring and bleaching cycles for one single D2A micro-
crystal and an individual D2A nanocrystal were studied (Fig. 4). The
mercury lamp was used to excite the nanocrystal fluorescence as
well to induce the photochromic reaction. During the coloring step,
nanocrystals or single crystals were irradiated by UV light, and then
were bleached with visible light. The fluorescence decrease is due
to the conversion of D2A isomer to non-fluorescent closed-ring
isomer (D2B) in single-crystalline phase. Indeed, upon irradiation
with 365 nm light, colourless crystals of D2A form turn yellow and
are not any more fluorescent [19]. In the case of the single micro-
crystal, the fluorescence emission reached a plateau (Fig. 4a)
around 34 (a.u.) and the maximum of intensity was of 50 (a.u), thus
we obtain a conversion rate of D2A to D2B isomer of 30%, in the
single crystal. This is similar to the 20% decrease obtained previ-
ously by CW light irradiation, and to the 50% decrease obtained by
confocal laser microscopy [19]. Fig. 4b confirms that the photo-
effect is also present in the nanocrystalline phase and is reversible.
Indeed, D2A was found to undergo a reversible photochromic
reaction in nanocrystals as well as in the single microcrystalline
phase as previously shown. In this case, the fluorescence decrease
was close to 40%. The recovery time is much faster than the writing
time. Thus, the ring-opening process is much more efficient than
the ring-closing process.
properties of the silicate thin films compared to brittle molecular
crystals and better dye photostability through its crystallization
referred to dispersed molecules in solegel or polymer thin films.
Furthermore, size effect of nanocrystals on photochromic proper-
ties would lead to interesting filtering properties or three dimen-
sional optical recording developments.
References
[1] Irie M. Photo-reactive materials for ultrahigh-density optical memory.
Amsterdam: Elsevier; 1994.
[
2] Hanazawa M, Sumiya R, Horikawa Y, Irie M. Thermally irreversible photo-
chromic systems. Reversible photocyclization of 1,2-bis (2-methylbenzo[b]
thiophen-3-yl)perfluorocyclocoalkene derivatives. Journal of the Chemical
Society, Chemical Communications; 1992:206e7.
[3] Irie M. Diarylethenes for memories and switches. Chemical Reviews
2000;100:1685e716.
[4] Matsuda K, Irie M. Diarylethene as a photoswitching unit. Journal of Photo-
chemistry and Photobiology C 2004;5:169e82.
[
5] Uchida K, Nakayama Y, Irie M. Thermally irreversible photochromic systems.
Reversible photocyclization of 1,2-Bis(benzo[b]thiophen-3-yl)ethene deriva-
tives. Bulletin of the Chemical Society of Japan 1990;63:1311e5.
[6] Tian H, Chen B, Tu H, Müllen K. Novel bisthienylethene-based photochromic
tetraazaporphyrin with photoregulating luminescence. Advanced Materials
2002;14:918e23.
[
[
7] Lemieux V, Spantulescu MD, Baldridge KK, Branda NR. Modulating the lewis
acidity of boron using a photoswitch. Angewandte Chemistry International
Edition 2007;47:5034e7.
8] Tian H, Wang S. Photochromic bisthienylethene as multi-function switches.
Chemical Communications; 2007:781e92.
The photo-conversion speed measured for nanocrystals is lower
than for free microcrystals. Indeed, for nanocrystals, the confine-
ment effect of the solegel matrix may delay the photochromic
reaction of diarylethene molecules. This confinement is due to
strong hydrogen bonds formed between the dye molecules of the
nanocrystal surface with solegel matrix, particularly between
fluorine atoms and silanol groups (CeF/HeOeSi hydrogen bonds).
[9] Wang S, Shen W, Feng Y, Tian H. A multiple switching bisthienylethene and its
photochromic fluorescent organogelator. Chemical Communications; 2006:
1497e9.
[
[
10] Tian H, Feng Y. Next step of photochromic switches? Journal of Materials
Chemistry 2008;18:1617e22.
11] Pawlicki M, Collins HA, Denning RG, Anderson HL. Two-photon absorption
and the design of two-photon dyes. Angewandte Chemistry International
Edition 2009;48:3244e66.
[
[
[
12] Tanio N, Irie M. Photooptical switching of polymer film waveguide containing
photochromic diarylethenes. Japanese Journal of Applied Physics 1994;
4
. Conclusions
33:1550e3.
13] Kawai T, Fukuda N, Gröschl D, Kobatake S, Irie M. Refractive index change of
dithienylethene in bulk amorphous solid phase. Japanese Journal of Applied
Physics 1999;38:L1194e6.
14] Bucar DK, MacGillivray LR. Preparation and reactivity of nanocrystalline
cocrystals formed via sonocrystallization. Journal of the American Chemical
Society 2007;129:32e3.
Our generic process used to prepare nanocrystals of organic
dyes in solegel thin films was successfully extended to diary-
lethenes compounds which are photochromic in the crystalline
phase. Thus, we obtained nanocrystals of D1 and D2 molecules
grown in solegel thin films. The solegel host matrix was found to
maintain photochromism properties of nanocrystals. We clearly
attempted to identify nanocrystals under their D1A, D1B, D2A and
D2B forms depending on lightening conditions. Indeed, upon UV
irradiation, D1A and D1B nanocrystals in solegel films underwent
reversible photochromic reactions. On the other hand, the host
matrix plays the crucial role of a one-step and easy shaping of
optical devices compared to large organic crystals while keeping
their photochromic properties. This advantage is also coupled to
the strong improvement of dye stability: higher mechanical
[15] Irie M, Lifka T, Kobatake S, Kato N. Photochromism of 1,2-bis(2-methyl-5-
phenyl-3-thienyl)perfluorocyclopentene in a single-crystalline phase. Journal
of the American Chemical Society 2000;122:4871e6.
[
16] Irie M, Uchida K, Eriguchi T, Tsuzuki H. Photochromism of single crystalline
diarylethenes. Chemistry Letters; 1995:899e900.
17] Shibata K, Muto K, Kobatake S, Irie M. Photocyclization/cycloreversion
quantum yields of diarylethenes in single crystals. Journal of Physical
Chemistry A 2002;2002:209e14.
[
[18] Fukaminato T, Kawai T, Kobatake S, Irie M. Fluorescence of photochromic
1
1
,2-bis(3-methyl-2-thienyl)ethene. Journal of Physical Chemistry B 2003;
07:8372e7.
[
19] Fukaminato T, Kobatake S, Kawai T, Irie M. Three-dimensional erasable optical
memory using a photochromic diarylethene single crystal as the recording

N. Sanz-Menez et al. / Dyes and Pigments 89 (2011) 241e245
245
medium. Proceedings of the Japan Academy, Series B, Physical and Biological
Sciences 2001;77:30e5.
[25] Gomez-Romero P, Sanchez C. Functional hybrid materials. Wiley-VCH Verlag;
2004.
[
20] Kobatake S, Shibata K, Uchida K, Irie M. Photochromism of 1,2-bis(2-ethyl-
[26] Sanz N, Boudet A, Ibanez A. Melting behavior of organic nanocrystals
grown in solegel matrices. Journal of Nanoparticle Research 2002;
4:99e105.
[27] Brasselet S, LeFloc’h V, Treussart F, Roch JF, Zyss J, Botzung E, et al. In situ
diagnostics of the crystalline nature of single organic nanocrystals by
nonlinear microscopy. Physical Review Letters 2004;92:207401e2.
[28] Monnier V, Bacia M, Putaux JL, Ibanez A. TEM characterization of organic
nanocrystals grown in solegel thin films. Journal of Nanoparticle Research
2008;10:129e39.
[29] Sanz N, Ibanez A, Morel Y, Baldeck PL. Organic nanocrystals grown in gel
glasses for optical-power-limiting applications. Applied Physics Letters
2001;78:2569e71.
[30] Spagnoli S, Block D, Botzung-Appert E, Colombier I, Baldeck PL, Ibanez A, et al.
Photochromism of spiropyran nanocrystals embedded in solegel matrices.
Journal of Physical Chemistry B 2005;109:8587e91.
5
-phenyl-3-thienyl)perfluorocyclopentene in a single-crystalline phase.
Conrotatory thermal cycloreversion of the closed-ring isomer. Journal of the
American Chemical Society 2000;122:12135e41.
[
[
21] Ibanez A, Maximov S, Guiu A, Chaillout C, Baldeck PL. Controlled nano-
crystallization of organic molecules in solegel glasses. Advanced Materials
1998;10:1540e3.
22] Monnier V, Sanz N, Botzung-Appert E, Bacia M, Ibanez A. Confined nucleation
and growth of organic nanocrystals in solegel matrices. Journal of Materials
Chemistry 2006;16:1401e9.
[
23] Sanz N, Gaillot AC, Baldeck PL, Ibanez A. Organic nanocrystals grown in
solegel coatings. Journal of Materials Chemistry 2000;10:2723e6.
24] Avnir D, Levy D, Reisfeld R. The nature of the silica cage as reflected by
spectral changes and enhanced photostability of trapped rhodamine 6G.
Journal of Physical Chemistry 1984;88:5957e62.
[