Functional mesoporous aluminosilicate nanoparticles as host material to fabricate photo-switchable polymer films

Article abstract of DOI:10.1039/c0jm03986a

Precisely nano-sized aluminosilicate particles (average diameter size 50 to 80 nm) with tailor-made internal surface polarity are suitable as host material for photochromic dyes of the chromene type in order to produce optical transparent photoswitchable polymer films. The sophisticated host material does accomplish two important requirements for a technical application, appropriate internal space for the light induced reversible switching process of the adsorbed dye and suppression of disturbing chemical reactions. The adjustment of the internal surface polarity of the host material was achieved by silane reagents which could be observed by means of specific solvatochromic probes. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0jm03986a

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PAPER
Functional mesoporous aluminosilicate nanoparticles as host material to
fabricate photo-switchable polymer films†
a
b
c
b
c
d
Ingolf Kahle, Oliver Tr €o ber, Sabine Trentsch, Hannes Richter, Bernd Gr €u nler, Steffen Hemeltjen,
e
e
a
Maik Schlesinger, Michael Mehring and Stefan Spange*
Received 18th November 2010, Accepted 24th January 2011
DOI: 10.1039/c0jm03986a
Precisely nano-sized aluminosilicate particles (average diameter size 50 to 80 nm) with tailor-made
internal surface polarity are suitable as host material for photochromic dyes of the chromene type in
order to produce optical transparent photoswitchable polymer films. The sophisticated host material
does accomplish two important requirements for a technical application, appropriate internal space for
the light induced reversible switching process of the adsorbed dye and suppression of disturbing
chemical reactions. The adjustment of the internal surface polarity of the host material was achieved by
silane reagents which could be observed by means of specific solvatochromic probes.
Introduction
The most important point is to combine two requirements,
providing appropriate space for the switching process and pro-
tecting the dye against contaminants, in an optimal way for an
application. For this reason most of the previously reported
attempts did not ensure a satisfying photochromic behaviour,
such as strong difference in both UV/Vis absorption intensity
and energy between the different photoinduced isomers. A slow
rate of the photoswitching reaction or a limited lightfastness is
due to photodegradation processes which is caused for example
by a strong acidity or polarity of the used matrices. Additionally,
there are problems with side reactions like protonation of the
dyes or with competition between direct (colourless samples
become coloured upon irradiation) and reverse photochromism
Organic photochromic compounds are characterized by the
ability to change their optical and physical properties by irradi-
ation with ultraviolet or visible light. Because of these effects,
there is a wide interest in such materials since they offer a variety
of applications like optical waveguides and shutters, light
modulators, optical storage media, ophthalmic lenses, and
1–5
photochromic coatings for double glazed windows.
Unfortunately, photochromic dyes which are suitable for
technical applications are often sensitive to oxygen, water,
carbon dioxide, traces of acids, and other environmental
contaminants. For this reason the photochromic molecules need
to be protected against external influences by incorporation into
a rigid solid matrix under perpetuation of their photophysical
properties. Various attempts have been made to enhance the
photostability and flexibility of such dyes in an appropriate
environment, e.g. embedding in inorganic gels and matrices or
(
bleaching of the samples upon irradiation) that is often observed
11–15
in inorganic matrices.
Nevertheless, inorganic host materials
such as zeolites or related molecular sieves offer many advan-
tages compared to organic polymers relating to their thermal
stability, mechanical robustness, and chemical inertness.
9–21
organic polymers.
The objective of this work is to demonstrate the feasibility for
the synthesis of photochromic host–guest materials based on
mesoporous aluminosilicates and their application as pigments for
the fabricationof photoswitchable polymer films. Forthispurpose
the particle size of the host materials must be in the dimension
below 200 nm to avoid light scattering and light intensity gradi-
ents. This concept is depicted in Scheme 1. Several para-phenyl
substituted naphthopyrans (3,3-diphenyl-[3H]-naphtho-[2,1b]-
pyran (1), 3,3-di-(p-methoxyphenyl)-[3H]-[2,1b]-pyran (2), and
3,3-di-(p-N,N-dimethylaminophenyl)-[3H]-[2,1b]-pyran (3)) are
a
Department of Polymer Chemistry, Chemnitz University of Technology,
Strasse der Nationen 62, Chemnitz, 09111, Germany. E-mail: stefan.
spange@chemie.tu-chemnitz.de; Fax: + 49 (0) 371 531 21239; Tel: +49
(
b
0) 371 531 21230
Fraunhofer Institute for Ceramic Technologies and Systems IKTS,
Hermsdorf branch of the institute, Michael-Faraday-Straße 1,
Hermsdorf, 07629, Germany
c
INNOVENT e. V., Technology Development, Pr u€ ssingstr. 27 B, Jena,
0
d
7745, Germany
Department of physical Chemistry, Chemnitz University of Technology,
Straße der Nationen 62, Chemnitz, 09111, Germany
6,34
used as photochromic materials in this work.
e
Department of Coordination Chemistry, Chemnitz University of
Technology, Straße der Nationen 62, Chemnitz, 09111, Germany
The photochromism of these types of dyes involves the
3
photoinduced cleavage of the central C(sp )–oxygen bond of the
†
section, synthesis, characterization and AM
materials. See DOI: 10.1039/c0jm03986a
Electronic supplementary information (ESI) available: Experimental
pyran ring which leads to an equilibrium between the closed
(colourless) and open (coloured) forms (Scheme 2). This
1
calculations of used
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molecular structure of 1, 2, and 3 inside the supercages of the
13
1
faujasites was evidenced by DRIFT and C-{ H}-CP-MAS
NMR spectroscopy (see ESI†). Relating to the two different
MCM materials, a common adsorption process of the dyes from
solution was used.
The acidity and polarity of the host material are of great
importance for the photochromic behaviour of the used dyes.
Thus, the investigation of ‘‘surface polarity’’ of the Y zeolites and
MCM 41 as well as MCM 48 nano-particles was carried out
25–28
using an established procedure.
For the measurement of the
hydrogen bond donating ability (a) of the surface, the a-sensitive
solvatochromic probe bis-(1,10-phenanthroline)-cis-dicyano-
iron(II) was adsorbed onto the solid particles and from the
position of the UV/Vis absorption band of the adsorbed dye the
a values were calculated using LSER (linear solvation energy
relationships). The surface polarity parameters were referenced
Scheme 1 Concept for the synthesis of dye-loaded aluminosilica host
materials which are suitable as photoswitchable pigment particles for
production of functional transparent polymer films.
29
to the Kamlet–Taft solvent parameter set.
The a values of HY (a ¼1.71), DAY (a ¼1.43), MCM 41
(
a ¼1.53) and MCM 48 (a ¼1.21) are large compared to pure
silica (a ¼1) and in the expected order for those types of alu-
minosilica materials, which indicate a strong Brønsted acidity.
The monoprotonated cationic form of 1 (lmax ¼570 nm), 2
30
(
lmax ¼600 nm), and 3 (l
¼640 nm) respectively is generated
max
by reaction of acidic surface protons of MCM 41 with the oxygen
atom of the pyran ring resulting in a significant shift of the
Fig. 1a and b).
The acid–base behaviour of 1, 2, and 3 with aluminosilica
materials is comparable with that in liquid acidic media
(
see ESI†).
Consequently, it is important to adjust appropriately the
Scheme 2 Structure of naphthopyran dye and the photoinduced
internal acidity and polarity of the host materials to enable
the desired photochromic behaviour of the guest molecules. The
post-functionalisation of MCM 41 or MCM 48 with hexame-
thyldisilazane (HMDS) is suitable to reduce the acidity strength
in such an extent that the adsorbed dye becomes not ring-cleaved
upon adsorption (Fig. 2a). The HMDS-postfunctionalised
MCM host materials are denoted as MCM-S. The acidity of
MCM 41 and MCM 48 decreases significantly to about a ¼1 by
chemical surface modification using HMDS.
stereoisomers of the open form.
transition from the spiro form to the zwitterionic or quinoidal
ꢀ
forms needs approximately 90 rotation of one half of the
7,8
molecule.
Results and discussions
As inorganic host materials Y zeolites (protonated form HY and
dealuminated form DAY), MCM 41, and MCM 48 which differ
in their Si/Al-ratio as well as pore-size and pore-volume were
synthesized. The effects of both, host-confinement and acidity on
the photoswitching behaviour have been investigated. The
synthesis of the MCM 41 as well as MCM 48 nano-particles was
The UV/Vis absorption bands of the neutral charged open
forms of the dyes 1, 2, and 3 occur at 450 nm, 495 nm, and 550
nm, respectively (see Fig. 2b).
The UV/Vis absorption bands of the naphthopyrans signifi-
cantly differ whether the dyes are adsorbed on the external
surface or encapsulated inside the Y-zeolites (see Fig. 3). Both,
the monoprotonated cationic and the zwitterionic or rather
23,24
carried out using a template assisted sol–gel procedure.
By
means of this process, it is possible to adjust the macroscopic
particle size of the host systems in the order of 50–80 nm.
The dimensions of the windows of Y zeolites (0.74 nm) and of
the mesoporous MCM materials (2–3 nm) are different.
Accordingly, two synthetic strategies were used to incorporate
chromene dyes into the host cage of these materials. For Y
zeolites, the ‘‘ship in the bottle reaction’’ (SIBOR) is indispens-
able because the pores of the Y zeolite cannot be entered by the
dye molecules due to their large size. The question arises whether
it is possible to apply SIBOR for chromene dyes (with size
dimension from 1.1 to 1.4 nm depending on the nature of the
para substituent; for model calculations see ESI†) to Y zeolites
Fig. 1 (a) Interactions between the open coloured form and functional
groups of a silica surface; (b) UV/Vis absorption spectra of monop-
rotonated cationic forms of 1, 2, and 3 within MCM 41particles.
(
with a cage diameter of 1.3 nm). The formation of the target
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084 | J. Mater. Chem., 2011, 21, 5083–5088
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Fig. 2 (a) Schematic illustration of surface modification of silica surface
with HMDS; (b) UV/Vis absorption spectra of 1, 2, and 3 in MCM 41S
after UV irradiation.
Fig. 4 UV/Vis induced spectral evolution of MCM 41S containing 1
upon irradiation with a xenon lamp (290 to 900 nm), and time-dependent
photocolouration and bleaching (inset).
k1
k2
A ƒƒ ƒƒ !B ƒƒ ƒƒ !
ƒ
ƒƒƒ
C
kꢂ1
with A ¼ open forms except zwitterionic cis-isomer
B ¼ zwitterionic cis-isomer
C ¼ closed spiro-form
Fig. 3 UV/Vis absorption spectra of 2 and 3 synthesized in the pores of
The curves were fitted using the OriginLab data analysis and
graphing software (OriginPRO 8.1). As can be seen in Fig. 5,
a bi-exponential fit accurately models the measured decay curve
of the samples.
DAY zeolite and when absorbed on the external surface of DAY zeolite.
quinoidal forms are observed inside the pores. This behaviour is
explained by different acidity and polarity parameters of the
The decay can be described by the following general equation:
30,31
internal and external surface of Y zeolites.
Unfortunately, the
chromene-loaded faujasite materials do not show any photo-
I(t) ¼I
1
ꢃexp (ꢂs
1
t) + I
2
ꢃexp (ꢂs
2
t) + I
N
(1)
chromic effect upon irradiation. The cage of the Y zeolite (ꢁ0.3
3
ꢂ1
with I , I , I , s and s ¼fitting parameters.
cm g ) is too narrow for the switching process of the photo-
chromic dyes. Additionally, the zwitterionic form is stabilized
through the strong hydrogen bond donating ability and dipo-
larity of the host cage. Therefore, the dye-loaded Y zeolites are
not suitable as photoswitchable pigments.
1
2
N
1
2
It is not possible to obtain the rate constants from the fitted
data as it is impossible to determine three rate constants from the
equation of a second-order exponential decay. Thus, the
½
weighted-average half-lives (t ) were calculated from these fits in
order to be able to make comparative statements about the
switching behaviour of the dyes (see eqn (2)). The calculated half-
lives are displayed in Table 1.
It is important to note that only the dye-loaded MCM 41S and
MCM 48S host materials show the aspired photochromic
behaviour. The photoswitching process takes place within
seconds as shown in Fig. 4, while decolourisation occurs imme-
diately after irradiation has been finished.
No significant reduction of the absorption intensity was
observed during the first five irradiation cycles of the chromene-
loaded MCM-S materials.
The bleaching kinetics were measured for samples containing
1
0 wt% of dye by monitoring the absorption of the different
samples vs. time at the maximum of the absorption band. The
open-form isomers contribute to the UV/Vis absorption band in
the visible region and absorb in the UV around 300 to 330 nm,
while the closed spiro-form does absorb only in the UV region.
The quinoidal and zwitterionic forms are in equilibrium but
thermal relaxation to the closed spiro-form is only possible via
the zwitterionic cis-isomer. Thus, the thermal relaxation reaction
can be described as follows:
Fig. 5 Loss in intensity of absorption at 495 nm over time for irradiated
dye 2 (10 wt%) in MCM 48S (- experimental data and — fit).
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Table 1 Calculated half-lives from fitting parameters for the post-
functionalised samples loaded with 10 wt% of dyes 1, 2 and 3
expected at higher angles of 2q. Those reflections were not
detected until loadings up to 20 wt%. Reflections of low intensity
caused by dye crystals are observed using samples with loadings
about 30 wt%. Thus, it was shown that the dye is preferentially
located inside the pores for host–guest materials with low to
moderate loadings. Similar results were reported for related
host–guest systems such as spirooxazine molecules adsorbed in
Sample
t
½,1/s
t½,2/s
t
½,average/s
MCM 41S-1
MCM 41S-2
MCM 41S-3
MCM 48S-1
MCM 48S-2
MCM 48S-3
9.77
4.72
27.87
6.70
4.41
30.35
13.89
825.90
20.96
12.22
23.39
11.39
681.55
10.97
6.76
18
SBA-15.
28.84
231.33
123.51
Noteworthy, only a limited number of very specific dyes
32,33
undergo photoswitching in their crystalline state.
An efficient
photoswitching reaction that is essential for the dye’s application
can only occur when the dye is dissolved or dispersed in a matrix
system.
ðI
1
t
½;1 þI
2
2
t
½;2Þwith t
ln 2
ln 2
t
½;average
¼
½;1 ¼ s and t½;2 ¼ s
(2)
ðI
1
þI
Þ
1
2
In order to transport the dye-loaded mesoporous nano-
particles in an applicable form, their incorporation into a trans-
parent polymer film was enforced. For this purpose, the MCM
41S and MCM 48S particles loaded with dyes 1 and 2 were
extruded into a transparent polyethylene (PE) film. The proce-
dure is suitable to produce transparent PE films which can be
used as test material. In addition, pure dyes were also extruded
into PE films to be able to confirm an improvement of perfor-
mances. After 2 days the films which were loaded solely with pure
dyes became turbid and did not show any photochromic
behaviour while the polyethylene films doped with dye-loaded
aluminosilica particles were still transparent and photochromic
after several months. Obviously a host material is necessary to
prevent a diffusion of the dyes out of the PE films. It is important
to note that the fast photoswitching behaviour is only slightly
affected by the extrusion of those pigments. Weighted-average
half-lives of 64.32 s and 48.83 s for films containing MCM 41S
loaded with 1 and 2 respectively show slower bleaching kinetics
but a similar photochromic behaviour as the host–guest mate-
rials for itself.
Except the samples which were loaded with dye 3 all other
dye-loaded MCM-S samples show very fast kinetics (nearly
tenfold faster) as compared with previous reported photo-
17–19
chromic hybrid materials.
Reason for the slow fading rates of
dye 3 is the two strong electron donating dimethylaminogroups
due to their stabilising effect of the open zwitterionic form and
22
a higher sterical requirement for the photoswitching process.
Even if the molecular size of dye 2 is larger than dye 1 it shows
faster fading rates. Since the ring closure is only possible from the
zwitterionic cis-isomer an explanation for the faster kinetics of 2
is the stabilising effect of the zwitterionic form by the electron
donating methoxy groups and therefore a more possible back
reaction to the closed form. But in contrast to dye 3 this stabil-
ising effect is obviously not strong enough to hinder the ring
closure. Additionally, the dye loaded MCM 48S samples show
faster kinetics than the dye loaded MCM 41S samples. The
structure of MCM 48 consists of two independent and intricately
interwoven networks of mesoporous channels, while the struc-
ture of MCM 41 consists of hexagonal packing of one-dimen-
sional channels. This may lead to less steric hindrance of the
ring-closing process.
The kinetics of films doped with dye-loaded MCM 48S
particles are about twofold slower than the unextruded pigments
but still fast (half-lives of 27.23 s for 1 and 12.83 s for 2). Possibly,
an amount of polymer entered the pores of the host materials
during extrusion and thus impaired the switching reaction of the
dyes slightly.
The dye-loaded MCM 48S materials were investigated by
nitrogen sorption experiments and XRD scattering to show
whether larger crystalline portions of the dye are present outside
or inside the pores (Fig. 6).
The pore volume of the dye-loaded MCM samples decreases
with increasing the quantity of adsorbed dye (for MCM-48S
Images of the dye-loaded MCM-S powders and the resulting
PE film materials are shown in Fig. 7.
3
ꢂ1
3
ꢂ1
from 0.88 cm g (0 wt%) to 0.28 cm g (20 wt%)) while the
pore size distribution is just altered about 7% referring to the 20
wt% sample of MCM 48S. The XRD measurements support the
results obtained by the nitrogen sorption experiments. XRD
reflections corresponding to those of the pure dye crystals are
UV/Vis light-irradiated sections of the film reversibly switch to
the target colour.
Fig. 7 Representative images of photochromic host–guest hybrid-
material MCM-41S loaded with dyes 1 (a), 2 (b), and polyethylene-films
containing MCM-41S loaded with dye 1 (upper film (c)) and dye 2
(bottom film (c)). The right sections of the hybrid films have been irra-
diated with UV-light.
Fig. 6 (a) Nitrogen sorption measurements of MCM 48S loaded with
0
wt%, 10 wt%, and 20 wt% of 1 (top down); (b) Respective XRD-
patterns.
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086 | J. Mater. Chem., 2011, 21, 5083–5088
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The precursor gel had the following molar composition
Experimental
SiO : Na O : H O : CTAB : Al O : TPAOH ¼10 : 1 : 1660 :
2
2
2
2
3
Materials
1
.2 : 0.05 : 1.6.
The Y zeolites were purchased from the S u€ dchemie Company.
0
1
,1-Diphenyl-2-propyn-1-ol (98%; ACROS), 4,4 -dimethoxy-
Functionalization of MCM 41 and MCM 48
,1,1,3,3,3-Hexamethyldisilazane (HMDS) was used for the
benzophenone (>98%; Lancaster), 2-naphthol (98%; ACROS),
0
1
4
,4 -diaminobenzophenone (>96%; Merck), pyridinium-4-tolue-
silylation of the mesoporous materials. For this purpose an
ꢀ
nesulfonate (PPTS; >99%; Merck) were used without further
purification. All used solvents were redistilled over appropriate
drying agents prior to use.
amount of 1 g calcined sample was heated at 120 C for 2 h under
vacuum to remove water adsorbed in the pores of the material.
1
5 ml toluene was added to the powder under nitrogen atmo-
sphere. 4 mmol of HMDS was then added and the mixture was
refluxed for 4 h. The powder was filtered off, washed with abs.
ethanol, and dried at RT under vacuum (5 mbar) for 8 h. The
amount of trimethylsilyl groups on MCM materials was 2.14
Synthesis of photochromic naphthopyrans
3
,3-Diphenyl-[3H]-naphtho-[2,1b]pyran (1), 3,3-di-(p-methoxy-
phenyl)-[3H]-naphtho-[2,1b]pyran (2), and 3,3-di-(p-dimethy-
laminophenyl)-[3H]-naphtho[2,1b]pyran (3) were synthesized
through condensation of 2-naphthol and the particular prop-
argyl alcohol in the presence of PPTS (5 mol%) and trimethyl
orthoformate (2 eq.) in 1,2-dichloroethane according to the
ꢂ1
mmol g which was calculated from C-content measured by
elemental analysis.
Impregnation with photochromic dye
34
ꢀ
literature.
The MCM materials were dried at 120 C under vacuum for 3 h.
Afterwards the host materials were impregnated with the
synthesized photochromic dyes from solution.
‘
‘Ship in the bottle reaction’’ of naphthopyrans
The required amount of naphthopyran was dissolved in
dichloromethane and added to the mesoporous powder under
vacuum. This mixture was shaken gently for 3 h. Capillary forces
draw the solution into the pores. The loaded materials were
filtered and dried under vacuum at RT for 3 days.
ꢀ
Previously the Y zeolites (1.00 g) were dried at 180 C under
vacuum (5 mbar) for 3 h. A solution of 2-naphthol in 1,2-
dichloroethane was added and the mixture was shaken for 5 h to
ensure the diffusion of the molecules inside the cages. After that
an equimolar portion of the particular propargyl alcohol in the
presence of trimethyl orthoformate was added and kept under
reflux for 5 h. At the end of the reaction, the solid was filtered and
extracted in a Soxhlet with ethanol and dichloromethane and
dried under vacuum for 3 days.
Extrusion of photochromic compounds in PE
The host–guest hybrid-materials MCM 41S and MCM 48S
loaded with dyes 1 and 2 were extruded in PE films on a micro-
extruder RC-0250 in a ratio of 1 : 50 (pigment : PE) at 170 to
ꢀ
1
80 C. Pure dyes (2 wt%) were extruded in PE films for
Synthesis of MCM 41
comparative experiments in the same way. The film thickness was
between 50 and 70 mm.
The mesoporous aluminosilicate was synthesized according to
23
the literature. The amount of 0.11 g aluminium nitrate was
dissolved in 29.69 g water. The amount of 0.73 g cetyl-
trimethylammonium bromide (CTAB) and 2.08 g tetraethoxy-
silane (TEOS) was added to the solution. The mixture was stirred
for one hour and 6.5 g ammonia solution (3% wt) was added
dropwise. After stirring for 48 hours at room temperature, the
products were collected by filtration, dried and calcined in air.
Characterization
XRD measurements were carried out on a STOE Stadi P
a
diffractometer using Cu K radiation.
ꢀ
Nitrogen physisorption isotherms were measured at ꢂ196 C
using a Sorptomatic 1990. Prior to the measurements, the
ꢀ
samples were dried under vacuum at 130 C for 2 h. The pore size
The gel molar composition was SiO
NH OH : H
O ¼1 : 0.025 : 0.2 : 1.77 : 200.
2 2 3
: Al O : CTAB :
distribution was calculated using the Dollimore/Heal method.
The Brunauer–Emmet–Teller (BET) model was applied to
evaluate the surface area.
4
2
Diffuse reflectance IR Fourier transform (DRIFT) spectra
were recorded using a BIO-RAD FTS 165 spectrometer with
a praying mantis device. The samples were mixed with dry KBr.
Pure potassium bromide was used as reference.
Synthesis of MCM 48
2
4
MCM-48 was synthesized according to the literature. The
amount of 1.38 g tetrapropylammonium hydroxide (TPAOH),
1
13
0
.34 g NaOH, and 0.14 g Al
2
(SO
4
)
3
$18H
H O. 8.86 g TEOS was added slowly under vigorous stirring. The
2
O was dissolved in 80 g
H and C spectra in solution were measured at room
temperature with a Bruker Avance 250 spectrometer to confirm
2
solution was stirred for further 90 minutes till the solution
became clear. Afterwards the gel was aged at room temperature
for 20 h. The aged solution was added to 48.9 g of a 3.8 weight
percentage aqueous solution of CTAB. The mixture was given in
the structure of the synthesized dyes. CDCl served as solvents
3
13
13
and the natural abundance C from CDCl3 was used as
internal references. Solid state NMR spectra were collected at
9.4 T on a Bruker Avance 400 spectrometer equipped with
double-tuned probes capable of MAS (magic angle spinning).
ꢀ
a steel autoclave for 48 h at 150 C. The product was filtered,
ꢀ
13
1
dried and calcined in air at 550 C.
C-{ H}-CP-MAS-NMR spectra were measured at 100.6 MHz
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in 4 mm standard zirconium oxide rotors (BRUKER) spinning
at 12.5 kHz. Cross-polarization with a contact time of 5 ms was
used to enhance the sensitivity. The recycle delay was 5 s. Spectra
were referenced externally to tetramethyl silane (TMS) as well as
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088 | J. Mater. Chem., 2011, 21, 5083–5088
This journal is ª The Royal Society of Chemistry 2011